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TEXT-BOOK

OF THE

EMBRYOLOGY OF MAN AND MAMMALS

TEXT-BOOK

EMBRYOLOGY OF MAN AND MAMMALS

DB. OSCAE HEBT\Sia

Professor extraordinarius of Anatomy and Comparative Anatomy, Director oj'the II. Anatomical
Institute oj the Unicertity of Berlin

TRANSLATED FROM THE THIRD GERMAN EDITION

BY

EDWARD L. MARK, Pn.D,

Hei'sey Professoi' of Anatomy fittKirvard University

ir 339 giynm m tyt jt anb 2 gi

LONDON: SWAN SONNENSCHEIN & CO., LIM.

NEW YORK: THE MACMILLAN CO.

1899

FIRST EDITION, October 1892 ;
SECOND EDITION, January 1899.

s

TBANSLATOB'S PBEFACE.

THE rapidly increasing recognition of the importance of Embryology
in all morphological studies makes it desirable that the most valuable
text-books upon the subject, in whatever language, be made available
for those who are beginning its study. Although the English-reading
student already has at command a number of text-books upon this
subject, it is evident to any one familiar with HEHTWIG'S Lehrbuch der
Entwicklungsgeschichte des Menschen und der Wirbelthiere that this
work covers the field of Vertebrate Embryology in a more complete
and satisfactory way than any book heretofore published in English.

Two important objects to be accomplished in a text-book are :
first, a clear and methodical exposition of the well-established facts
of the science; and, secondly, such a presentation of unsettled
questions as shall stimulate the reader to further inquiry and re-
search. I believe it is far too common for the second of these aims
to be overlooked. The present work fulfils both requirements in an
eminent degree, and in its historical surveys exhibits an exceptional
fairness of treatment, notwithstanding the author has been one of
the foremost contestants in several of the fields reviewed. The
summaries which follow the discussions of the several topics serve a
useful purpose in directing attention to the more important conclu-
sions drawn from each subject.

I have aimed to give a clear and accurate reproduction of the
author's ideas ; while I have endeavored not always successfully
to avoid awkward renderings and German idioms, I have preferred
to err on the side of a too literal rather than a too liberal translation.
There are a few points that demand a brief explanation. The German
word Anlage has heretofore been variously rendered into English
by rudiment, origin, beginning, basis, foundation, etc., while some
writers, recognising the inadequacy of any of these words to express
the idea, have incorporated the German word itself in their English.

The Anlage of a structure is its beginning or its undifferentiated
state the object in a simple condition which is destined to be

VI TRANSLATORS PREFACE.

followed by a more complicated one. The use of rudiment in this
sense is undesirable, because, in the interest of scientific accuracy, it
is important to restrict its meaning, as in German, to a structure
which is not destined to become more complicated, but which may have
been, either ontogenetically or phylogenetically, even more highly
developed than it now is. Origin and beginning are abstract terms,
whereas Anlage is more frequently used in the concrete ; basis and
foundation (Grundlage) convey a wrong impression that of the sub-
stratum upon which the structure is erected. The need of a new
word, which shall be used in the sense of Anlage, is evident. I
suggest the adoption of an already existing word, -fundament, used
at present only in a sense with which the proposed usage will not
produce confusion. This word has been uniformly employed in the
present translation, and the reader will see how readily and naturally
it lends itself to this use. Fundament would thus bear the same
relation to foundation that Anlaye does to Grundlage.

I have also departed from authorised usage by sometimes employ-
ing for Bindeyewebe and iS'tiitzyewebe the term sustentative (in a
mechanical sense) tissue, instead of connective tissue. My reason
for this is the narrower meaning of connective as compared with
sustentative.

In deference to a custom still followed in Human Anatomy, the
author, in describing the relative positions of parts, has very generally
used anterior and posterior for dorsal and ventral, etc. Instead of
converting these expressions into terms which are independent of the
temporary position of the organism, as I should have preferred, it
has seemed better to indicate the direction by a bracketed word in
those cases where a misunderstanding was most likely to occur. It
has of course not been necessary to repeat this after each term of
direction, but only after the first one of a series, the reader's atten-
tion being thus sufficiently directed to the matter to prevent any
misconception.

The rapid advances in Embryology make it impossible for a book
two years old to be a faithful reflection of the science of to-day in all
its branches ; there are some topics in which even radical changes
must be recognised. I have thought best, however, to reproduce the
book as it left the hands of its author, and to content myself with
calling the reader's attention to some of the topics in which the most
important advances have been made, such as the metamerism of the
head, and the plan and metamorphoses of the vessels of the visceral
arches.

TRANSLATORS PREFACE. Vll

I am under very great obligations to my colleague, Dr. C. B.
Davenport, for kind assistance and valuable criticism, but for which
many defects of the translation would have been overlooked. 1 am
also indebted to Drs. T. G. Lee, H. B. Ward, and W. McM. Wood-
worth for aid in reading portions of the proof.

E. L. MARK.

CAMBRIDGE, MASS.

AUTHOR'S PREFACE

TO THE FIRST EDITION.

;< Die Entwickelungsgeschichte 1st der wahre Lichttrager f iir Untersuchungen
iiber organische Kbrper." C. E. v. BAEB, "Ueber Entwickelungsgeschichte
der Thiere " (Bd. I., S. 231).

THE Embryology of Animals, although one of the youngest shoots
of morphological research, has, nevertheless, grown up in the course
of sixty years, along with the cell-doctrine and that of the tissues, to
a vigoi-ous and stately tree. The comprehension of the structure of
organisms has been extended in a high degree by numerous develop-
mental investigations. The study of the human body has also derived
great advantage from the same. In the newer anatomical text-
books (GEGENBAUR, SCHWALBE) Embryology is receiving more and
more attention in the description of the separate systems of organs.
To what extent many things "may be more clearly and attractively
described in this manner is best shown by a comparison of the des-
qriptions of brain, eye, heart, etc., in the older and the more recent
anatomical text-books.

Although it is generally recognised that Embryology constitutes " a
foundation-stone of our comprehension of organic forms," neverthe-
less the attention which its importance warrants is not yet given to
it ; it is especially true that it has not become as extensively as it
should be a component of well-rounded medical and natural-history
instruction, to which it is indispensable. The cause of this is
perhaps in part to be sought in the fact that in student-circles the
study of Embryology is often held to be especially difficult and a
comprehension of it to be laborious. And thus many do not venture
into this apparently obscure realm.

But ought the development of an organism to be really more
difficult to comprehend than the complicated finished structure ?

To a certain extent this was the case at a time when the most
divergent and contradictory opinions prevailed concerning many of
the most important processes of development, such as the formation
of the germ-layers, the protovertebree, etc., which the lecturer had to

AUTHOR S PREFACE TO THE FIRST EDITION. IX

take into account, and when many processes were not yet understood
in their essence and their significance. But, thanks to the results of
Comparative Embryology, the number of the unintelligible processes
has been every year diminished, and in the same ratio the study of
Embryology even for the beginner has been rendered easier.

At least, it is not in any way an essential feature of the process
of development that it should be more difficult to understand than
the structure of the completed form. For every development begins
with a very simple condition, from which the more complicated is
gradually derived and by which it is explained.

Inasmuch as I have for twelve years pursued the study of Embry-
ology with especial interest, both in annually recurring academic
lectures and in a series of scientific investigations, the desire has
been awakened in me to acquire for Embryology a broader and more
secure foundation in education, and to procure for it admission into
larger circles of medical men and well-educated naturalists. As the
result of this there has come into existence the book which is before
us, in which the especial problem has been to make the complicated
structure of the human body more intelligible through the knowledge
of its development.

For the solution of this problem I have in the present text-book
placed the comparative method of investigation in the foreground. I
do not thereby find myself in any way in opposition to another
direction of embryological research, which places the objective point
in the physiological or mechanical explanation of the form of the
animal body. Such a direction I hold to be fully warranted, and I
believe that, instead of being opposed to a comparative-morphological
direction, it can be of the most permanent value to it in the solution
of its problems. One will find that I have here given full attention
to the mechanico-physiological explanation of forms. Compare the
sections on cell-division and Chapter IV., " General Discussion of the
Principles of Development," in which the laws of unlike growth and
the processes of the formation of folds and evaginations are treated.

In the presentation of the separate processes of development, in
the main the important things only have been selected, the sub-
sidiary left out, in order thus to make the introduction into
embryological study easier. In the case of fundamental theories
I have gone into their history extensively, because it is of great
interest, and under certain circumstances operates as a stimulus,
for one to see in what way the state of a scientific question for the
time being has been attained. In pending controversial questions

X AUTHORS PREFACE TO THE FIRST EDITION.

I have, it is true, employed chiefly as the foundation of my pre-
sentation the views which appear to me the most entitled to
acceptance, but have not left unmentioned opposing conceptions.

Numerous figures in the text, as well as some colored plates, will
contribute materially to the easier comprehension of the various
developmental processes.

I submit, then, this text-book to physicians and to students of
medicine and the natural sciences, with the desire that it may
promote and facilitate the study of Embryology in wider circles, and
that it may thereby contribute to a deeper insight into the structure
of our own bodies.

OSCAR HEETWIG.
JENA, October 1886.

AUTHOR'S PREFACE

TO THE SECOND EDITION.

THE friendly reception which the " Text-book of the Embryology
of Man and Mammals " has found, is an indication of the increased
interest which this branch of Morphology now meets with.

Even more than a year ago, after the first part of the text -book
appeared and while the second part was in the press, the necessity of
preparing a second edition became evident.

In this edition fundamental changes have not been undertaken ;
the text has, however, undergone an expansion in some places, owing
to the attention given to several works which have recently appeared.
This has been the case with the section on the first developmental
processes of the egg (WEISMANN, BLOCHMANN) ; that on the origin of
the vascular system (RABL, RUCKERT) ; that on the development of
the foetal membranes (DuvAL, OSBORN) ; and that on the human
placenta (KASTSCHENKO, WALDEYER, HUGE).

As the second part of the text-book has just appeared, it has been
possible to incorporate it in the second edition without alteration.

It has, furthermore, seemed to me expedient in the second edition
to distribute at the ends of the several chapters the synopses of the
literature, which in the first edition were brought together at the close
of the whole work. Finally, there has been added an index of
subjects, by which a more rapid orientation concerning the separate
topics will be facilitated ; this will increase the usefulness of the
work.

May the book in this form make for itself new friends, not only
among students of medicine and the natural sciences, but also with
all those who have a fondness for and a, comprehension of studies
in natural science,

OSCAR HERTWIG.
JENA, February 1888.

AUTHOR'S PREFACE

TO THE THIRD EDITION.

IN the two yeais which have elapsed since the appearance of the
second edition of this text-book, our knowledge of the embryology of
Vertebrates has experienced many important enrichments, thanks to
the numerous investigations which are annually published. There-
fore, as the problem of preparing a third edition of the text-book
confronted me, I was compelled to make extensive changes in
many places. Thus the second and third chapters, concerning the
processes of fertilisation and cleavage of the egg, have undergone
expansion, owing to the presentation of the important discoveries
which have been made on the the egg of Ascaris megalocephala. I
have given an entirely new wording to the ninth chapter on the
development of connective substance and blood, also to the
sections on the origin of the urinary organs and the development of
the peripheral nervous system, and, finally, to the account of the
development of the heart and the venous system. Also at other
places one will often recognise the hand of improvement.

The third edition has been essentially improved by the addition of
thirty new figures, which I have taken from the investigations of
VAN BENEDEN, BOVERI, DUVAL, FLEMMING, HERMANN, His, BORN,
GEGENBAUR, NAGEL, VAN WIJHE, GRAF SPEE, BONNET, and KEIBEL.
Through the friendliness of Professor VAN BENEDEN I was also put
in a position to employ for my text-book three figures out of his
hitherto unpublished extensive work on the development of the
germinal layers of the Rabbit. By means of the increase in the
number of figures I hope that I have been able to render still easier
the comprehension of many of the processes of development.

And so I close the preface to the third edition by expressing
my thanks to all those who have rendered me friendly aid, and
especially to the publisher, who in the further equipment of the
text-book has met my wishes with the greatest willingness.

OSCAE HERTWIG.
BERLIN, March 1890.

CONTENTS.

PAGE

INTRODUCTION 1

MANUALS AND TEXT-BOOKS 4

PART FIRST.

CHAPTER I.
DESCRIPTION OF THE SEXUAL PRODUCTS 7

THE EGG-CELL 7

THE SEMINAL FILAMENTS . . 19

Historical . . . . .23

SUMMARY . . .27

CHAPTER II.
THE PHENOMENA OF THE MATURATION OF THE EGG AND THE

PROCESS OF FERTILISATION 30

THE PHENOMENA OP MATURATION 30

Historical 35

THE PROCESS OP FERTILISATION 37

Historical 45

SUMMARY 46

CHAPTER III.

THE PROCESS OF CLEAVAGE 51

Historical 69

SUMMARY 72

CHAPTER IV.

GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT 76

CHAPTER V.
THE DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS

(GASTR^A-THEORY) 84

CHAPTER VI.

THE DEVELOPMENT OF THE TWO MIDDLE GERM -LAYERS

(CCELOM-THEORY) 106

SUMMARY 142

CHAPTER VII.
HISTORY OF THE GERM-LAYER THEORY 145

CHAPTER VIII.

DEVELOPMENT OF THE PRIMITIVE SEGMENTS . . .161

SUMMARY . 169

XIV CONTEXTS.

CHAPTEE IX. PAC;E

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD (THE

PARABLAST- AND MESENCHYME-THEORIES) . . .170

Historical 189

SUMMAEY 191

CHAPTEE X.

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY . 194
SUMMARY . 200

CHAFTEE XI.

THE F(ETAL MEMBRANES OF REPTILES AND BIRDS . . . 20(5
SUMMARY 220

CHAPTEE XII.

THE FCETAL MEMBRANES OF MAMMALS 221

SUMMARY . 238

CHAPTEE XIII.
THE FCETAL MEMBRANES OF MAN 241

(1) THE CHORION 248

(2) AMNION 250

(3) YOLK-SAC ---. . .251

(4) ., DECIDUJS . . ,. 252

(5) PLACENTA . . . . . ... 258

(6) UMBILICAL CORD .... . . . . .268

SUMMARY . 272

PART SECOND.

CHAPTEE XIV.
THE ORGANS OF THE INNER GERM-LAYER. THE ALIMENTARY

TUBE WITH ITS APPENDED ORGANS . . , . .281
I. THE FORMATION OF THE MOUTH, THE THROAT-, GILL-, OR

VISCERAL CLEFTS, AND THE ANUS 282

II. THE DIFFERENTIATION OF THE ALIMENTARY TUBE INTO

SEPARATE REGIONS, AND FORMATION OF THE MESENTERIES 295
III. THE DEVELOPMENT OF THE SEPARATE ORGANS OF THE ALI-
MENTARY TUBE 304

A. The Organs of the Oral Cavity : Tongue, Salivary Glands, and

Teeth 304

B. The Organs arising from the Pharynx 313

(1) The Thymus . 314

(2) Thyroid Gland 317

(3) Lungs and Larynx 320

C. The Glands of the Small Intestine ...... 324

(1) The Liver 324

(2) Pancreas . ' . 3b2

SUMMARY . 333

CONTENTS. XV

CHAPTEE XV. PAGE

THE ORGANS OF THE MIDDLE GERM-LAYEE . . . .341

I. THE DEVELOPMENT OF THE VOLUNTAEY MUSCULATURE . . 342

-i. The Primitive Segments of the Trunk , 342

S. Head-Segments 351

II. THE DEVELOPMENT OP THE URINARY AND SEXUAL ORGANS . 353

(a) The Pronephros and the Mesonephric Duct , 353

() Mesonephros (Wolffian Body) . . . . . 359

(c) Metanephros (Kidney) 367

(rf) Miillerian Duct . 369

(e) Germinal Epithelium 374

(/) Ovary .374

(#) Testis 382

(#) ,, Metamorphosis of the Different Fundaments of the Uro-

genital System into their Adult Condition .... 385

A. In the Male (Descensus testiculomm) .... 387

B. Female ( ovariorum) .... 393
(i) The Development of the External Sexual Parts . . . 397

III. THE DEVELOPMENT OP THE SUPRARENAL BODIES . . . 403
SUMMARY . 405

CHAPTER XVI.
THE ORGANS OF THE OUTER GERM-LAYER . . . . .416

I. THE DEVELOPMENT OF THE NERVOUS SYSTEM .... 416

A. The Development of the Central Nervous System . . . 416

(a) The Development of the Spinal Cord . . . .418
(V) Brain 421

(1) Metamorphosis of the fifth Brain-Vesicle . . . 427

(2) fourth ., 429

(3) third 430

(4) ., ,, second . . . 431
Development of the Pineal Gland (Epiphysis cerebri) 432

., Hypophysis (Pituitaiy Body) . 436

(5) ,, Fore-Brain Vesicle . . .439

B. The Development of the Peripheral Nervous System . . 449

(a) Spinal Ganglia , 449

(J) Peripheral Nerves .... 452

(c) Sympathetic System .... 462

SUMMARY 463

II. THE DEVELOPMENT OP THE SENSORY ORGANS 4C7

A. The Development of the Eye 467

(a) The Development of the Lens . . . . . . 471

(&) ,, Vitreous Body . . . .474

(c) Secondary Optic Cup and the

Coats of the Eye . . .476
(d~) ., Optic Nerve . . . .484

(e) Accessory Apparatus of the Eye 486

xvi CONTEXTS.

PAGE

SUMMARY 489

B. The Development of the Organ of Hearing .... 490

(a) The Development of the Otocyst into the Labyrinth . 491
(i) ., ,. Membranous Ear-Capsule into

the Bony Labyrinth and the
Perilyrcphatic Spaces . 498

(c) Middle and External Ear . . 505
SUMMARY 510

C. The Development of the Organ of Smell 511

SUMMARY 518

III. THE DEVELOPMENT OF THE SKIN AND ITS ACCESSORY ORGANS 520

(a) The Skin 520

(It) ,. Hair 522

(c) Nails 526

(d) Glands of the Skin 528

SUMMARY . 531

CHAPTER XVII.
THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME 538

I. THE DEVELOPMENT OF THE BLOOD-VESSEL SYSTEM . . . 542

A. The first Developmental Conditions of the Vascular System . 542

(a) Of the Heart 542

(b) Vitelline Circulation, Allantoic and Placental Circulation 549
S. The further Development of the Vascular System up to the

Mature Condition 553

(a) The Metamorphosis of the Tubular Heart into a Heart

with Chambers 553

(&) The Development of the Pericardial Sac and the Dia-
phragm 566

(c) Metamorphoses of the Arterial System .... 570

(d) ,, Venous .... 577
SUMMARY 588

II. THE DEVELOPMENT OP THE SKELETON 593

A. The Development of the Axial Skeleton 593

(a) The Development of the Vertebral Column . . . 596
(&) Head -Skeleton . . . .603

I. Bones of the Cranial Capsule 619

II. Visceral Skeleton 622

(c) Concerning the Relation of the Head-Skeleton to the

Trunk-Skeleton 627

B. The Development of the Skeleton of the Extremities . . 635

(a) Pectoral and Pelvic Girdles 638

(V) Skeleton of the Free Extremity 610

(c) Development of the Joints 644

SUMMARY 647

APPENDIX TO LITERATURE . . 658

INTRODUCTION.

THE history of the development of the individual, or Ontogeny
(Embryology), is the science of the growth of an organism ; it de-
scribes the morphological changes which an organism passes through
from its origin in the ovum up to its complete maturity, and presents
these in their natural connection. We can regard the fertilisation
of the egg-cell as the beginning of the process of development for
Vertebrates, as it also is for all the rest of the higher animals.

In giving an account of the changes of the egg-cell, which begin
with fertilisation, one may choose between two different methods.

According to one method a particular organism is made the basis
of the account, and one describes the changes which its germ under-
goes from the moment of fertilisation onward, from hour to hour,
and from day to day. It is in this way that the embryology of the
Chick has been worked out by C. E. VON BAER in his classical paper,
and by FOSTER AND BALFOUR in their " Elements of Embryology."
This method has the advantage that the reader acquires a view of
the total condition of an organism in the separate stages of its
development.

A book of that kind is especially suitable for such persons as
desire to acquaint themselves, by their own observation, with the
embryology of a single animal, as, for example, the Chick, by
repeating the investigations of others. It is, on the contrary, less
adapted to those who wish to acquire a connected view of the
development of the separate organs, as the eye, the heart, the brain,
etc. For the formation of these will of course be treated of at different
places in describing younger and older embryos. In order to procure
a general survey of the course of development of an organ, the
reader must consult various places in the text-book, and collect for
himself what relates to the subject.

For beginners, and for the needs of theoretical instruction in
Embryology, the second method commends itself, in which the separate
organs are considered in succession, each for itself, and the changes
which a single organ has to pass through during development are

1

Z INTRODUCTION.

set forth connectedly from beginning to end. It is in this way that
KOLLIKER'S " Embryology of Man and the Higher Animals" is written.

The second method is, moreover, the only one applicable when the
problem is to investigate in a comparative way the development of
several organisms, and to fill up the gaps which exist in our know-
ledge of one by that which we know concerning nearly related
animals. But it is precisely in this position that we find ourselves,
when we wish to acquire a survey of the development of the human
body. An account which should limit itself to that which we know
about Man would exhibit numerous and extensive gaps. For up to
the present the eye of man has not seen how the human ovum is
fertilised, how it divides, how the germ-layers are formed, or how
the establishment of the most important organs is effected. It is
especially the period of the first three weeks, during which the
greatest variety of fundamental processes of development take place,
concerning which we know next to nothing ; there is also little
prospect that a change will soon occur in this regard. The time
will therefore perhaps never come when a complete embryology of
Man in the strict sense of the word will be possible.

However, the existing gaps can be filled out in another manner,
and one which is entirely satisfactory. The study of the most widelv
differing Vertebrates teaches us that they are developed according
to a common plan, that the first processes of development agree
in all really important points, and that the differences which we
encounter here and there are produced by causes of a subordi-
nate kind, as, e.g., by the egg's possessing a greater or less amount
of yolk.

When we see that the establishment of the central nervous system,
of the eye, of the spinal column, of the viscera, etc., takes place in
Mammals on the whole just as it does in Amphibia, Birds, and
Reptiles, the conclusion is near at hand, and justified, that Man
also in his development is no exception to this general phenomenon.
Thus in the study of Embryology we are naturally led to the com-
parative method. What, owing to the nature of the difficulties, we
cannot learn directly about the development of Man, we seek to
deduce by the investigation of other Vertebrates.

In earlier decennia the Hen's egg was the favorite object, and it
is upon this that we possess the most numerous and most complete
series of observations. During the last twenty years research has
also been directed to Mammals, in the investigation of which the
greatest difficulties have to be surmounted, as well as to Reptiles,

INTRODUCTION. 3

Amphibia, Fishes, etc. Only through the observation of such various
objects has insight been acquired into many processes, which in their
essence remained unintelligible to us from the study of the Chick
alone. For it was thus that one first learned to distinguish the
important from the accessory and unimportant, and to understand
the laws of development in their generality.

In this text-book, therefore, I shall not confine myself to a single
object, such as the egg of the Hen or the Rabbit, but from more
general comparative standpoints shall endeavour to present what,
through extensive series of investigations, we have thus far recognised
as the rule in regard to the real nature of the processes of fertilisa-
tion and cleavage, the formation of the germ layers, etc.

However, let no one expect a text-book of comparative Embiyo-
logy. The purpose and the problem is first of all to learn to com-
prehend the development and the structure of the human body.
What we know about that has been placed before everything else,
and the embryology of the remaining Vertebrates has been cited, and,
as it were, fully utilised, only in so far" as was necessary for the
purpose indicated.

In the division of the embryological material proposed by us, ac-
cording to the separate systems of organs, there is a long series of
processes, with which the development begins, which do not permit
of an arrangement, because at the beginning the fundaments of
definite, afterwards differentiated organs, are not recognisable in the
germ. Before there is any formation of organs, the egg is divided
into numerous cells, and these then arrange themselves into a few
larger complexes, which have been called the germ-layers, or the
primitive organs of the embryo. Further, in the higher Verte-
brates there ai-e formed certain organs, which are useful only during
embryonic life, and are subsequently lost namely, the foetal mem-
branes and foetal appendages. All of the processes of that nature
we shall treat of connectedly, and by themselves. In accordance
with this, we can divide our theme into two main sections, the first
of which will deal with the initial processes of development and the
embryonic membranes, the second with the origin of the separate
systems of organs. In order to facilitate for the advanced a more
thorough study, and a penetration into embryological literature, a
survey of the more important original works is given at the close of
the separate chapters. On the other hand, text-books of Embryo-
logy may be mentioned in this place. [Compare also the larger
monographic works cited at the end of the book.]

MANUALS AND TEXT-BOOKS.

Valentin, G. Handbuch der Entwicklungsgeschichte des Menschen mit

vergleichender Kiicksicht der Entwicklung der Saugetbiere und Vogel.

Berlin 1845.
Bisehoff. Entwicklungsgeschichte der Saugethiere und des Menschen.

Leipzig 1842.

Rathke, EL Entwicklungsgeschichte der Wirbelthiere. Leipzig 1861.
Kolliker, A. Entwicklungsgeschichte des Menschen und der hoheren Thiere.

Academische Vortrage. Leipzig 1861. 2. ganz umgearbeitete Auflage.

Leipzig 1879.
Kolliker, A. Grundriss der Entwicklungsgeschichte des Menschen und der

hoheren Thiere. 2. Auflage. Leipzig 1884.
Schenk. Lehrbuch der vergleichenden Embryologie der Wirbelthiere. Wien

1874.
Haeckel, E. Anthropogenic oder Entwicklungsgeschichte des Menschen.

Leipzig 1874. Dritte Auflage. 1877.
Foster, M., and F. M. Balfour. The Elements of Embryology. Part I.

(Chick.) London 1874. 2nd edit, by Adam Sedgwick and Walter Heape

1883. German translation by Kleinenberg. Leipzig 1876.
His, W. Unsere Korperform und das physiologische Problem ihrer Ent-

stehung. Leipzig 1875.
Balfour, F. M. A Treatise on Comparative Embryology. London 1880, -81,

2 vols. German translation by Dr. C. Vetter. Jena 1881.
Romiti, G. Lezioni di embriogenia umana e comparata dei vertebrati. Siena

1881, -82, -88.

Preyer, W. Specielle Physiologic des Embryo. 1883, -84.
Hoffmann, C. K. Grondtrekken der vergelijkende Ontwikkelingsgeschie-

denis van de gewervelde Dieren. Leiden 1884.
Duval, M. Atlas d'Embryologie. Paris 1888.

PART FIRST.

CHAPTER I.
DESCRIPTION OF THE SEXUAL PRODUCTS.

EGG-CELL AND SEMEN-CELL.

IN most animals, and without exception in all Vertebrates, the
development of a new being can take place only when reproductive
elements, produced by two sexually different individuals, the egg
by the female, and the seminal corpuscle or seminal filament by the
male, are at the proper time brought into union as the result of
the procreative act.

The egg and the seminal filament are simple elementary parts or cells,
which are produced in special glandular organs, the egg-cells in the
ovary of the female, and the semen-cells in the testis of the male.
After the beginning of sexual maturity at definite periods, they
detach themselves within the sexual organs from their union with
the remaining cells of the body, and form, under suitable conditions
of development, among which the union of the two sexual cells is
the most important, the starting- point for a new organism.

First of all, therefore, we have to acquaint ourselves with the
peculiarities of the two kinds of sexual products.

1. The Egg-cell.

The egg is by far the largest cell of the animal body. At a time
when nothing was known of its cell-nature, its separate components
were given special names, which remain in use even at the present
time. The contents were called egg-yolk, or vitettus ; the cell- nucleus
was called vesicula germinativa, or germinative vesicle, discovered by
the physiologist PURKINJE ; the nucleai corpuscles, or nucleoli, were
called germinative spots, or maculce germinativce (WAGNER) ; and,
finally, the cell-membrane A?as called the yolk-membrane, or mem-
brana vitellina. All these parts vary in not unimportant ways from.

8

EMBRYOLOGY.

the ordinary condition of the protoplasm and nucleus of most animal
cells.

The vitellus (figs. 1 and 3 n.d) rarely appears homogeneous, mucila-
ginous, and translucent, like the protoplasm of most cells; it is
ordinarily opaque and coarsely granular. This results from the
fact that the egg-cell, during its development in the ovary, store s
up in itself nutritive materials, or reserve stuffs. These consist of
fat, of albuminous substances, and of mixtures of the two, and
are described, according to their form, as larger and smaller yolk-
spherules, yolk-plates, etc. Later, when the process of development
is in progress, they are gradually used up in the growth and for
the increase of the embryonic cells. The fundamental substance

of the egg, in which the reserve stuffs
just now referred to are imbedded, is
protoplasm, physiologically the most in-
teresting and important of substances,
because in it take place, as we infer
from many phenomena, the essential
life-processes.

We must therefore distinguish in
the yolk, in accordance with the sug-
gestion of VAN BENEDEN, (1) the egg-
protoplasm, and (2) the yolk-substance,
or deutoplasm, which is of a chemi-
cally different nature, and is stored
up in the former.
When the deposition of reserve materials takes place to a great
degree, the really essential substance, the egg-protoplasm, may
become almost entirely obscured by it (figs. 3, 4). The protoplasm
then fills up the small interstices between the closely packed yolk-
globules, yolk-cakes, or lamellae, as mortar does those between the
stones in masonry, and appears in sections only as a delicate net-
work, in the smaller and larger meshes of which lie the yolk-elements.
Only at the surface of the egg is the egg-plasm constantly present
as a thicker or thinner continuous cortical layer.

The germinative vesicle usually occupies the middle of the egg.
It is the largest nuclear structure in the animal body, and its
diameter generally increases with the size of the egg.

The germinative vesicle (figs. 1, 2) is separated from the yolk by
a firm membrane, which may often be distinctly demonstrated, and
which surrounds various included components : nuclear liquid (Kein-

Fig. 1. Immature egg from the ovary
of an Echinoderm. The large ger-
minative vesicle shows a germinative
dot, or nueleolus, in a network of
filaments, the nuclear network.

DESCRIPTION OF THE SEXUAL PRODUCTS.

saft), nuclear network, and nudeoli. The nuclear liquid is more
fluid than the yolk, in the fresh condition usually as clear as water,
and when coagulated by the addition of reagents, absorbs only
a little or no coloring matter. It is traversed by a network
of delicate filaments (kn), which attach themselves to the nuclear
membrane. In this network are enclosed nucleoli, or genninative
spots (kf), small, for- the most part spherical, homogeneous, lustrous
structures, which consist of a substance akin to protoplasm nuclear
substance or nuclein. Nudein is distinguishable from protoplasm
in addition to certain other chemical reactions especially by the
fact that it absorbs with great
avidity pigments such as car-
mine, haematoxylin, aniline,
etc., on account of which it has
also received from FLEMMING
the name chromatin.

The number of the nucleoli
in the germinative vesicles of
different animals is highly
variable, but it is tolerably
constant for each species;
sometimes there is only a
single nucleolus present
(fig. 1), sometimes there are
several or even very many of
them (fig. 2kf). Accordingly

one may with AUERBACH distinguish uninucleolar, plurinucleolar,
and multinucleolar germinative vesicles.

At their surfaces eggs are surrounded by protective envelopes, the
number and condition of which are exceedingly variable throughout
the animal kingdom as well as among Vertebrates. It is best to
divide them, as LUDWIG has done, according to their method of
origin, into two groups, into the primary and the secondary egg-
membranes. Primary egg-membranes are such as have been pro-
duced either by the egg itself or by the follicular cells within the ovary
and the egg-follicle. Those produced by the yolk of the egg are
called vitelline membrane ; those formed by the follicular epithelium,
chorion. All which take their origin outside of the ovary, as a
result of secretions on the part of the wall of the oviduct, are to be
designated as secondary egg-membranes.

In their details the eggs of the various species of animals differ

Fig. 2. Germinative vesicle of a Frog's egg that
is still small and immature. It shows very
numerous mostly peripheral germinative spots
(/), in a fine nuclear network (kn). m, Nu-
clear membrane.

10 EMBRYOLOGY.

from each other in a high degree, so that they must really be con-
sidered as the most characteristic for the species of all the kinds
of animal cells. Their size, which is due to a greater or less ac-
cumulation of deutoplasm, varies so extensively that in some species
the egg-cells can be only barely recognised as minute dots, whereas
in others they attain the considerable dimensions of a Hen's egg, or
even of an Ostrich's egg. The form is usually globular, more rarely
oval or cylindrical. Other variations arise from the method in
which protoplasm and deutoplasm are constituted and distributed
within the limits of the egg ; there are in addition the differences of
the finer structure of the germinative vesicle and the great variability
of the egg-membranes.

Some of these conditions are of great significance from their in-
fluence on the manner of subsequent development. They have been
employed as a basis for a classification of the various kinds of eggs.

It is most expedient to divide eggs into two chief groups, into
simple and into compound eggs, the first of which is divisible into
several sub-groups.

A. Simple Eggs.

Simple eggs are such as are developed in an ovary out of a single
germinal cell. The eggs of all the Vertebrates and most of the
Invertebrates belong to this group.

In this chief group there occur, according to the manner in which
protoplasm and deutoplasm are distributed within the egg, three
modifications, which are of very great importance in the determination
of the first processes of development.

In the simplest case the deutoplasm, which ordinarily is present
only to a limited amount in the correspondingly small egg, is more
or less uniformly distributed in the protoplasm (fig. 1). In other
cases there has arisen out of this original condition, in conjunction
with an increase in the bulk of the yolk-material, an inequality in
the distribution of the two egg-substances previously distinguished.
The egg-plasma has accumulated in greater abundance at certain
regions of the egg -territory, and the deutoplasma at otJier regions.
Consequently, a contrast has arisen between portions of the egg-cell
which are richer, and those which are poorer, in protoplasm. A
further accentuation of this contrast exercises an extraordinarily
broad and profound influence on the first processes of development,
which take place in the egg after fertilisation. That is to say,
the changes, which further on are embraced under the process of

DESCRIPTION OF THE SEXUAL PRODUCTS.

11

A.P

cleavage, make their appearance only at the region of the egg
which is richer in protoplasm, whereas the region which is more
voluminous and richer in deutoplasm remains apparently quite
unaltered, and is not divided up into cells. By this means the
contrast, which was already present in the unsegmented egg,
becomes during development disproportionately greater and more
obvious. The one part undergoes changes, is divided into cells, and
out of these produces the individual organs ; the other part remains
more or less unaltered, and is gradually employed as nutritive
material. Following the example of REICHERT, the part of the
yolk which is richer in protoplasm, and to which the developmen-
tal processes remain confined,
has been designated formative
yolk, and the other nutritive
yolk.

The unequal distribution of
formative yolk (vitellus forma-
tivus) and of nutritive yolk
(vitellus nutritivus) within the
egg is accomplished in two dif-
ferent ways.

In the one case (fig. 3) the
formative yolk is accumulated
at one pole of the egg as &flat
germ-disc (k.sch). Inasmuch as
its specific gravity is less than
that of the nutritive yolk (n.d)
collected at the opposite pole, it
is always directed upward, and

it spreads itself out on the yolk just like a drop of oil on water. In
this case, therefore, the egg has undergone a polar differentiation ;
when at rest it must always assume a definite position, owing to the
unequal weight of the two poles. The dissimilar poles are distin-
guished : the upper, lighter pole, with the germ-disc, as the animal
(A.P); the under, heavier and richer in yolk, as the vegetative pole.
(V.P). The polar differentiation of eggs is often encountered in
Vertebrates, and is especially prominent in the classes of Bony
Fishes, Reptiles, and Birds.

In the second case (fig. 4) the formative yolk (b.d) is accumulated
over the whole surface of the egg, and surrounds the centrally placed
nutritive yolk (n.d) as a uniformly thick, finely granular cortical

V.P

Fig. 3. Diagram of an egg with the nutritive
yolk in a polar position. The formative
yolk constitutes at the animal pole (4. P) a
germ-disc (k.sclt), in which the germinative
vesicle (i\6) is enclosed. The nutritive yolk
(n.d) fills the rest of the egg up to the
vegetative pole (V.P),

12

EMBRYOLOGY.

b.d

k.b

fig. 4. Diagram of an egg with the nutri-
tive yolk in the centre. The germinative
vesicle (fc.6) occupies the middle of the
nutritive yolk (n.d), which is enveloped
in a mantle of formative yolk (b.d).

layer. The egg exhibits central differentiation, and therefore does
not assume a constant position when at rest. As in the former case
the yolk was polar in position, so here it is central. Such a condition

is never encountered in Verte-
brates, but it is characteristic of
Arthropods.

In order to distinguish the three
modifications, BALFOUR has made
use of the expressions alecithal,
telolecithal, and centrolecithal. He
calls those eggs alecithal in which
the deutoplasm, in small amount,
is uniformly distributed through
the protoplasm ; telolecithal, those
in which it is accumulated at the
vegetative pole ; centrolecithal,
those in which the accumulation of
deutoplasm has taken place at the

centre. In what follows, we shall speak of (1) eggs with uniformly
distributed yolk, (2) eggs with polar deutoplasm, and (3) eggs with
central deutoplasm.

It is now expedient to illustrate what has just been said by typical
examples, and for this purpose the eggs of Mammals, Amphibia,
Birds, and Arthropods have been selected. We shall also frequently
recur to these in the presentation of the subsequent phases of develop-
ment.

The egg of Mammals and of Man is exceedingly small, since it mea-
sures on the average only 0'2 mm. in diameter. It is for this reason
that it was not discovered until the present century in 1827, by CARL
ERNST VON BAER. Previously the much larger GRAAFIAN follicle
of the ovary, in which the smaller true egg is enclosed, had been
erroneously taken for the latter. The Mammalian egg (fig. 5) con-
sists principally of a finely granular protoplasmic substance, which
contains dark, fat-like spherules and granules (deutoplasm), and
which is turbid and opaque in proportion to the amount of these.
The germinative vesicle (k.b) contains a large germinative dot (k.f),
located, together with a few smaller accessory dots, in a nuclear
network (k.n). The egg-membrane is called zona pdludda (z.p),
because it surrounds the yolk as a relatively thick and clear layer. It
is a primary membrane, for it is formed within the GRAAFIAN follicle,
by the follicular cells. Under high magnification the zona pellucida

DESCRIPTION OF THE SEXUAL PRODUCTS.

13

Fig, 5_ Egg from a Rabbit's follicle which was 2 mm. in diameter, after WALDEYEB. It is
surrounded by the zona pelhicida (z.p), on which there rest at one place follicular cells (/.z).
The yolk contains deutoplasmic granules (d). In the germinative vesicle (fc.ft) the nuclear
network (J-.re) is especially marked, and contains a large germinative dot (k.f).

(z.p) appears radially striate, since it is traversed by numerous pore-
canals, into which, as long as the egg remains in the GRAAFIAN follicle,
very fine projections of the follicular cells (f.z) penetrate. These
fuse with the egg-plasm, and are probably concerned in the nutrition
and growth of the contents of the egg. (RETZIUS.)

The human ovum is wonderfully like the egg of Mammals in size,
in the condition of its contents, and the nature of its membranes.
However, it always can be distinguished by means of special, though
trifling, characteristics, as the careful investigations of NAGEL have
shown. Whereas in the Rabbit lustrous, fat-like spherules render
the yolk cloudy, the human ovum retains its transparency during
all stages of development, so that one may recognise most ac-
curately all its structural details, even on the living object. The
yolk is divided into two layers. The inner layer contains principally
deutoplasm, which produces in this case, contrary to most of the
Mammals, only a slight cloudiness ; it consists in part of feebly
lustrous, in part of highly refractive fragments, some coarser, some
finer ; but it is not possible to recognise the mutual boundaries of

14 EMBRYOLOGY.

the individual components, as is the case in other Mammals and
lower animals, where one distinguishes with great ease granules
and distinct drops. The outer layer or peripheral zone of the yolk is
more finely granular and still more transparent than the central
part, and contains the germinative vesicle with a large germinative
dot, in which NAGEL was able to observe amoeboid motions. The
zona pellucida is remarkably broad ; it is striate, and is separated
from the yolk by a narrow (perivitelline) space. There are two or
three layers of follicular cells attached to the periphery of the egg
when it is set free from the GRAAFIAN follicle. The long diameters
of these cells are arranged in a radial direction around the egg, as
is general in Mammals, and it is due to this circumstance that they
have received the name corona radiata, introduced by BISCHOFF.
The human egg without the follicular epithelium measures, on the
average, 17 mm. in diameter.

The eggs of many Worms, Molluscs, Echinoderms, and Ccelenterates
agree with the Mammalian egg in their size, and in the method in
which protoplasm and deutoplasm are uniformly distributed through
the egg.

The eggs of Amphibia, which were cited as the second example,
form a transition from simple eggs, with uniform distribution of
yolk-material, to eggs with distinctly expressed and externally
recognisable polar differentiation. Already these have deposited in
themselves a large amount of deutoplasm, and have thereby acquired
a very considerable size. The Frog's egg, for example, is stuffed
full of closely compacted, fatty-looking yolk-lumps ^(Dotterschollen)
and yolk-plates. The egg protoplasm is in part distributed as a
network between the little yolk-plates ; in part it forms a thin
cortical layer at the surface of the egg. Upon closer examination
however, the beginning of a polar differentiation is most distinctly
i ecognisable even here. It manifests itself in this way : at one
pole, which at the same time appears black on account of a deposit
of superficial pigment, the yolk-plates are smaller and enveloped in
more abundant egg-plasm ; and also, nrobably as a consequence of
this, slight differences in specific gravity are distinguishable between
the pigmented and the unpigmented, or the animal and the vegetative,
halves of the egg.

The germinative vesicle (fig. 2) lies in the middle of the immature
egg, is exceedingly large, even visible to the naked eye, and multi-
nucleolar, inasmuch as there are a hundred or more large germinative
dots (kf) distributed immediate! v under the nuclear membrane.

DESCRIPTION OF THE SEXUAL PRODUCTS. 15

The envelopes exhibit, in comparison with the Mammalian egg, an
increase in number, for to the zona pellucida (zona radiata), which
is produced in the follicle, there is subsequently added still another,
a secondary envelope. This is a thick, viscid, gelatinous layer,
which is secreted by the wall of the oviduct, and which becomes
swollen in water.

The polar differentiation, taken, as it were, in the very process
of developing in the case of the Amphibia, is found sharply expressed
in our third example, the Bird 's egg.

In order to form a correct picture of the condition of the egg-cell
in the case of the Hen, or of any k.b k.sch

other bird, we must seek it while
still in the ovary, at the moment
when it has finished its growth,
and is ready to be set free from the
follicle. It is then ascertained that
only the spheroidal yolk, the so-
called yellow of the egg, which in
itself is an enormously large cell
(fig. 6a), is developed in the botryoidal ^V^T' 11 <y U ? J ?" ?"

/' taken from the ovary, k.sch, Gevmma-

OVary. It is enclosed in a thin but tive disc ; t.&, germinative vesicle ;

tolerably firm pellicle (d.k), the ^^J^^J^ y *
vitelline membrane, the rupture of

which is followed by an extrusion of the soft pulpy contents. By
careful examination one will discover upon the latter a small white
spot, the germinative disc (k.sch), or discits proligerus, also called scar
or cicatricula. It has a diameter of about 3 or 4 mm., and consists
of formative yolk, a finely granular protoplasm with small yolk-
spherules, which alone is involved in the process of cleavage. In
the flattened germinative disc is also found the germinative vesicle,
fig. 6a (k.b) and fig. 6b (a;), which is likewise somewhat flattened and
lenticular.

The remaining chief mass of the egg-cell is nutritive yolk, which
is composed of numberless yolk-spherules united by slight traces of
egg-plasm, as though by a cement. Information concerning its finer
structure is to be gained from thin sections through the hardened
egg, which should be cut perpendicularly to the germinative disc.
According to differences in staining and in elementary composition,
there are now to be distinguished the white and the yellow nutritive
yolk (fig. 6a).

The u-hite yolk (w.d) is present in the egg cell only in a small

16

EMBRYOLOGY.

quantity ; it forms a thin layer over the whole surface, the white
yolk-rind ; secondly, it is accumulated in somewhat greater quantity
under the germinative vesicle, for which it at the same time forms a
bed or cushion (PANDER'S nucleus) ; and, thirdly, from this region it

Fig. 6b. Section of the germ-disc of a mature ovarian Hen's egg still enclosed in the capsule,
after BALFOOR.

a, Connective-tissue capsule of the egg ; b, epithelium of the capsule, on the inside of which lies
the vitelline membrane reposing upon the egg ; c, granular substance of the germinative
disc ; w.y, white yolk, which passes imperceptibly into the finely granular substance of the
disc ; x, germinative vesicle enclosed in a distinct membrane, but shrivelled up ; y, space
originally occupied by the germinative vesicle, but made empty by its shrivelling up.

penetrates in the form of a mortar-pestle into the very centre of the
yellow yolk, where it terminates in a knob-like swelling (latebra,
PURKINJE). Upon boiling the egg, it is less coagulated, and remains
softer than the yellow yolk. In the coagulated condition the latter
discloses upon sections a lamellated condition, in that it consists of
smaller and larger spherical shells, which envelope the latebra.

The two kinds of yolk also differ from each other in respect to
the condition of their elementary particles. The yellow yolk
consists of soft plastic spherules (fig. 7 A) from 25 to 100 p. in
diameter, which acquire a punctate appearance from the presence
of numerous exceedingly minute granules. The elements of the
white yolk are for the most part smaller (fig. 7 B), and likewise
spherical, but contain one or several large highly refractive granules.

Fig. 7. Yolk-elements from the Fowl's egg, after BAI.FOUR. A, Yellow yolk ; B, white yolk.

At the boundary between the two kinds of yolk there are present
spherules which effect a transition between them.

The freshly laid Hen's egg (fig. 8) has a different appearance
from that of such an ovarian egg. This results from the fact that
there is deposited around the yolk, when it detaches itself from

DESCRIPTION OP THE SEXUAL PRODUCTS.

17

the ovary and is taken up by the oviduct, several secondary en-
velopes derived from the wall of the oviduct, viz., the white of the
egg, or the albumen, the shell-membrane, and the calcareous shell.
Each of these parts is formed in a special region of the Hen's oviduct.
The latter is divided into four regions : (1) A narrow ciliated
initial part, into which the liberated egg is received, and where it
is fertilised by the spermatozoa already accumulated there ; (2) a

y-y-

w.y.

eh.!.

Fig. 8. Diagrammatic longitudinal section of an unincubated Hen's egg, after ALLEN THOMSON.
(Somewhat altered.)

b.l. Germ-disc ; w.y. white yolk, which consists of a central flask -shaped mass and a number of
concentric layers surrounding the yellow yolk (y.y.) ; v.t. vitelline membrane ; x. a somewhat
fluid albuminous layer, which immediately envelopes the yolk ; w. albumen composed of
alternating layers of more and less fluid portions ; ch.l. chalazse ; a.ch. air chamber at the
blunt end of the egg simply a space between the two layers of the shell-membrane ; i.t.m.
inner, s.m. outer layer of the shell-membrane ; s. shell.

glandular region, covered with longitudinal furrows, from which
the albumen is secreted and spread around the yolk in a thick layer ;
(3) a somewhat enlarged part, covered with small villi, the cells
of which secrete calcareous salts, and thus cause the formation of
the shell ; (4) a short narrower region, through which the egg
passes rapidly, and without undergoing any further change, when
being deposited.

The envelopes furnished in succession by the oviduct have the
following composition :

The white of the egg, or albumen (w), is a mixture of several
materials : according to chemical analyses, it contains 12% albumen,

18 EMBRYOLOGY.

1'5% fat and other extractive materials, 0'5% salts (potassic chloride,
sodic chloride, sulphates, and phosphates), and 86% water. It
surrounds the yolk in several layers of varying consistency. There
is a layer quite closely investing the latter, which is firmer and
especially noteworthy because it is prolonged into two peculiar
spirally twisted cords, the chalazce (ch.l), which consist of a very
compact albuminous substance, and which make their way through
the albumen to the blunt and to the pointed poles of the egg

The albumen is enclosed by the thin but firm shell-membrane (s.m)
(membrana testae), which is composed of felted fibres. It may be
separated into two lamellae an outer, which is thicker and firmer,
and an inner, which is thinner and smooth. Soon after the egg is
laid the two layers separate from each other at the blunt pole, and
enclose between them a space filled with air (a.ch), the so-called
air-chamber, which continues to increase in size during incubation,
and is of importance for the respiration of the developing Chick.

Finally, the shell, or testa (), is in close contact with the shell-
membrane; it consists of an organic matrix (2%), in which 98% cal-
careous salts are deposited. It is porous, being traversed by small
canals, through which the atmospheric air may gain entrance to the
egg. The porosity of the calcareous shell is an absolute necessity for
the normal development of the egg, since the vital processes in the
protoplasm can take place only when there is a constant supply of
oxygen. If the porosity of the shell be destroyed, either by soaking
it in oil or closing its pores with varnish, the death of the incubated
egg ensues in a very short time.

B. Compound Eggs.

Compound eggs are found only in a few subdivisions of the
invertebrated animals, as in the Cestodes, Trematodes, etc. ; they
are noteworthy in this respect, that they are produced by the
union of numerous cells, which are formed in two different glands
of the sexual apparatus of the female, in the germarium and in
the vitellarium. In the germarium is developed the egg-cell in the
restricted sense. This is always very small, and consists almost
exclusively of egg-plasm. When this cell at its maturity is set free
from its surroundings and comes into the sexual outlets, it is obliged
to pass the opening of the vitellarium; here there are associated
with it a number of yolk-cells, which, owing to deposition of reserve
material in the protoplasm, appear turbid and coarsely granular,

DESCRIPTION OF THE SEXUAL PRODUCTS. 19

and which constitute the dower that is given by the maternal
organism to the developing germ on its way. Thereupon the whole
is enclosed in one or several secondary egg-membranes, and now
constitutes the compound egg, in which, however, the developmental
processes manifest themselves exclusively on the simple germ cell ;
it is that alone which is fertilised and segments, while the yolk-cells
gradually degenerate and are employed as nutritive material. Thus
in this case also, upon closer examination, the general law, that the
descenclent organism takes its origin from a single cell of the maternal
body, suffers no exception.

2. The Seminal Filaments.

In contrast with eggs, which are the largest cells of the animal
body, the sperm-cells or sperm-filaments (spermatozoa) are the
smallest elementary parts ; they are accumulated in great multitudes
in the seminal fluid of the male, but can be recog-
nised in it only by the aid of high magnification,
being, for the most part, slender motile filaments.
Inasmuch as every cell consists of at least two
parts, namely, nucleus and protoplasm, we must
look for these parts in this case also. We shall
take for description the spermatozoa of Man.

In Man the seminal filaments (fig. 9) are about
- 05 mm. long. One may distinguish as head (&)
a short but thick region, which marks the anterior

end, as tail a long thread-like appendage (*), and Kg. 9.-Mature sper-
matozoa of Kan,
between the two a so-called middle piece (m). seen in two dif-

The head (k) has the form of an oval plate, "?* p08itio " s -

Each consists of a

which is slightly excavated on both surfaces, head (t), a mid-
and is somewhat thinner toward the anterior end. 5 (r ' l)> and

13. il \S}

Seen from the side (JB) it presents a certain re-
semblance to a flattened pear. Chemically considered, it consists of
nuclear substance (nuclein or chromatin), as microchemical reactions
show. To the head is united, by means of a short part called the
middle piece (m), the long thread-like appendage (), which is com-
posed of protoplasm, and is best compared to a flagellum, because,
like the latter, it executes peculiar serpentine motions in virtue of
its contractile properties. By means of these motions the sper-
matozoon moves forwards in the seminal fluid with considerable
velocity.

20 EMBRYOLOGY.

The spermatozoa have often been designated and it seems
to us with entire justice as ciliate, or still better as flagellate,
cells.

The spermatozoa of the remaining Vertebrates have a similar
structure to that of Man ; on the whole, the diversity of form which
is encountered in the comparative study of the egg-cell in the animal
kingdom is wanting here.

That spermatozoa are in reality metamorphosed cells cannot be
more clearly demonstrated than by their development. According
to the extended observations of LA VALETTE and other?, each
spermatozoon is formed from a single seminal cell or speimatid, and,
to be more precise, the head is formed from the nucleus, the contractile
filament from the protoplasm.

The metamorphoses which take place in the development have
been investigated with the greatest detail by FLEMMING and
HERMANN in the case of Salamandra maculata, the spermatozoa of
which are characterised by their very great size. The individual
spermatozoon here consists of : (1) a very long head, which has the
form of a finely pointed skewer, and takes up stains with avidity ;
(2) a short cylindrical middle piece, which differs from the first part
in chemical properties also ; (3) the motile caudal filament, which in
the Salamander exhibits the additional peculiarity that it is provided
with a contractile undulating membrane. Of these three regions
the skewer-like head, and probably also the middle piece, arise from
the nucleus of the sperm atid, whereas the contractile filament is
differentiated out of the protoplasm. In the development of the
head the nucleus of the seminal cell is seen to become more and
more elongated (fig. 10 A, B); at first it takes the form of a pear
(fig. 10 A k) ; then it grows out into an elongated cone (fig. 10 B K),
the base of which serves as the point of attachment for the middle
piece (mst). The cone becomes elongated and narrowed into a rod
(fig. 11 A, -5), which is finally converted into the characteristic form
of a skewer. With this elongation of the nucleus the chromatic
network becomes more and more dense, and at last assumes a quite
compact and homogeneous condition, as in the mature spermatozoon.
The fundament (Anlage) of the middle piece (figs. 10, 11, A, B, mst)
makes its appearance early when the nucleus begins to elongate
at that end of the nucleus which was called its base, in the form of
a small oval body, which at first takes up stains like the head, but
afterwards loses this property. Its first appearance demands still
further elucidation.

DESCRIPTION OF THE SEXUAL PRODUCTS.

21

Why are the male sexual cells so small and thread-like, and so
differently constituted from the eggs ?

The dissimilarity between the male and the female sexual cells is
explained by the fact that a division of labor has arisen between the
two, inasmuch as they have adapted themselves to different missions.

Fig. 10 A and B. Initial stages of the metamorphosis
of the seminal cell into the seminal filament,
after HERMANN.

A, Seminal cell with pear-shaped nucleus ; B, seminal
cell with cone-shaped nucleus ; sz, seminal cell ; k,
nucleus with chromatin network, and uucleoli (n) ;
mst, body out of which the middle piece is developed ;
r, ring-like structure, which is in contact with the
middle piece, and is claimed to have relation to the
formation of the spiral membrane of the filament ;
/, caudal appendage of the seminal filament.

Fig. 11 A and B . Two terminal
stages in the metamorphosis
of the seminal cell into
the seminal filament, after
FLEMMING.

k, Nucleus, which has become
elongated to form the head
of the spermatozoon ; mst,
its middle piece ; /, its
caudal filament.

The female cell has assumed the function of supplying the substances
which are necessary for that nutrition and growth of the cell proto-
plasm which a rapid accomplishment of the process of development
demands. It has therefore, while in the ovary, stored up in itself
yolk-substance, reserve material, for the future ; and consequently
has become large and incapable of motion. But inasmuch as it
is necessary for the accomplishment of a process of development
that union with a second cell from another individual should take
place, and since non-motile bodies cannot unite, therefore the male
element has been suitably modified to meet this second requirement.

22 EMBRYOLOGY.

For the purpose of locomotion and in order to make possible the
union with the non-motile egg-cell, it has become metamorphosed
into a contractile filament, and has rid itself completely of all
substances, as, for example, yolk-material, which would interfere
with this principal requirement. At the same time it has assumed
the form best adapted for passing through the envelopes with
which, as a means of protection, the egg is surrounded, and for
penetrating the yolk.

The conditions especially in the vegetable kingdom confirm the
accuracy of this interpretation. There are plants of the lowest
forms in which the two copulating sexual cells are entirely alike,
both being small and motile ; and there are other related species in
which a gradual differentiation is brought about by the fact that
one of the cells becomes richer in yolk and incapable of motion,
while the other becomes smaller and more active. From this it is
evident that the stationary egg must now be sought out by the
migratory cell.

A few physiological statements may be in place in this connection.
In comparison with other cells of the animal body, and especially
in comparison with the eggs, the seminal filaments are characterised
by greater duration of Life and power of resistance, a fact which is
frequently of importance for the success of fertilisation. The mature
spermatozoa, after they are set free from their connection with
other cells, remain for months in the testes and vasa deferentia
without losing their fertilising power. They also appear to remain
active for a long time after having been introduced into the sexual
passages of the female, perhaps for several weeks in the case of Man.
For some animals this is demonstrable to a certainty. For example, it
is known that the semen of Bats remains alive in the uterus of the
female during the whole winter ; and in the case of the Fowl it is
known that fertilised eggs can be laid up to the eighteenth day after
the removal of the Cock.

In the presence of external influences semen shows itself to be
much more resistent than the egg-cell, which is easily injured or
killed. For example, when semen is frozen and then thawed out,
the motion of the seminal filaments comes back again. Many salts,
if they are employed not too strong, have no deleterious influence.
Narcotics in strong concentration, and when employed for a long time,
make the filaments motionless, without immediately killing them,
because after removal of the injurious substance they can be revived.

DESCRIPTION OF THE SEXUAL PRODUCTS. 23

Very weak alkaline solutions stimulate the motions of seminal
filaments ; on the contrary, acids, even when they are very dilute,
produce death. Accordingly the motion becomes more lively in all
animal fluids of alkaline reaction, whereas in acid solutions it soon
dies out.

HISTORY. The discovery that egg and seminal filament are simple cells is
of far-reaching import for the comprehension of the whole process of develop-
ment. In order to appreciate this to its full extent, it will be necessary to
make a digression into the historical field. Such a digression will acquaint us
with some fundamental transformations, which have affected our conception of
the essentials of developmental processes.

In the last century, and even in the beginning of the present, ideas about the
nature of the sexual products were very indistinct. The most distinguished
anatomists and physiologists were of opinion that eggs agreed in their structure
in every particular with the grown-up organism, and therefore that they
possessed from the beginning the same organs in the same position and con-
nection as the latter, only in an extraordinarily diminutive condition. But in-
asmuch as it was not possible, with the microscopes of the time, actually to see
and demonstrate in the eggs at the beginning of their development the assumed
organs, recourse was had to the hypothesis that the separate parts, such as
nervous system, glands, bones, etc., must be present, not only in a very diminu-
tive, but also in a transparent condition.

In order to make the process more intelligible, the origin of the blossoms of
plants from their buds was cited as an illustrative example. Just as already
in a small bud all the parts of the flower, such as stamens and coloured petals,
are enveloped by the green and still unopened sepals, just as the parts grow
in concealment and then suddenly expand into a blossom, so also in the de-
velopment of animals it was thought that the already present but small and
transparent parts grow, gradually expand, and become discernible. The doctrine
which has j ust been outlined was consequently called the Theory of unfolding,
or evolution. However, a more appropriate designation for it is the one intro-
duced during recent decsrmiajjreformation theory. For the characteristic
feature of this doctrine is, that at no instant of development is there anything
new formed, but rather that every part is present from the beginning, or is
preformed, and consequently that the very essence of development the he-
coming is denied. " There is no such thing as becoming ! " is the way it is
expressed in the " Elements of Physiology " by HALLER. " No part in the animal
body was formed before another ; all were created at the same time."

As the necessary consequence of a rigid adherence to the pref ormation theory,
it follows, and indeed was formulated by LEIBNITZ, HALLER, and others, that
in any germ the germs of all subsequent offspring must be established or
included, since the animal species are developed from one another in un-
interrupted sequence. In the extension of this box-within-box doctrine
{Einschachtelungslehre) its expounders went so far as to compute how many
human germs at the least were concentrated in the ovary of mother Eve, and
thereby arrived at the number 200,000 millions.

The evolution theory offered a point of attack for a scientific feud, inasmuch
as every individual among the higher organisms is developed by means of the
cooperation of two separate sexes. When, therefore, the seminal filament as

24 EMBRYOLOGY.

well as the animal egg became known, there soon arose the actively discussed
question, whether the egg or the seminal filament mas the preformed germ.
Decennium after decennium the antagonistic camps of the ovists and of the
Mimalculists stood opposed to each other. Those who followed the latter
thought they saw, with the aid of the magnifying glasses of the times, the
spermatozoa of man actually provided with a head, arms, and legs. The
animalculists recognised in the egg only a suitable nutritive soil, as it were,
which was necessary to the growth of the spermatozoon.

In the face of such doctrines there dawned a new period for Embryology,
when in 1759 CASPAR FRIEDRICH WOLFF in his doctor's dissertation opposed
the dogma of the evolution theory, and, casting aside preformation, laid down
the scientific principle that what one could not recognise by means of his
senses was certainly not present preformed in the germ. At the beginning, so he
maintained, tlie germ is nothing else than an unorganised material eliminated
from the sexual organs of the parent, which gradually becomes organised, but
only during the process of development, in consequence of fertilisation. Ac-
cording to WOLFF, the separate organs of the body differentiate themselves
one after another out of the hitherto undifferentiated germinal material. In
individual cases he endeavoured, even at this time, to determine more exactly,
by means of observations, the nature of the process. Thus C. F. WOLFF was
the founder of the doctrine of epigenesis, which, through the discoveries of the
present century, has proved to be the right one.*

WOLFF'S doctrine of unorganised germinal matter has been compelled since
then to give way to more profound knowledge, thanks to the improved optical
aids of recent times, and to the establishment of the cell-theory by SCHLEIDEN
and SCHWANN. A better insight into the elementary composition of animals
and plants was now acquired, and especially into the finer structure of the
sexual products, the egg-cell and the seminal filament.

So far as regards the egg-cell, a series of important works began with
PURKINJE'S investigation of the Hen's egg in 1825, in which the germinative
vesicle was described for the first time. This was soon (1827) followed by
C. E. v. BAER'S celebrated discovery of the Mammalian egg, which had been
hunted for, but always without success. Extensive and comparative investiga-
tions into the structure of the egg in the animal kingdom were published in
1836 by R. WAGNER, who also discovered at the same time in the germinative
vesicle the germinative dot (macula germinativa).

With the establishment of the cell-theory there naturally arose the qttestion
as to how far the egg was in its structure to be regarded as a cell, a question
which was for years answered in widely different ways, and which even now
from time to time is brought up for discussion in an altered form. Even at that
time SCHWANN, albeit with a certain reservation, expressed it as his opinion that
the egg was a cell, and the germinative vesicle its nucleus; but others, his co-
temporaries (BiscHOFF and others), regarded the germinative vesicle as a cell,

* Historical presentations of the theory of evolution and the theory of
epigenesis, which are worth the reading, have been given by A. KIRCHHOFF
in his interesting paper, " CASPAR FRIEDRICH WOLFF. Sein Leben und seine
Bedeutung fur die Lehre von der organischen Entwicklung." Jcnaische Zeit-
schrift fur Medicin und Naturmissenschaft, Bd. IV., Leipzig, 1868 ; and by W.
His, " Die Theorien der geschlecht lichen Zengung." Archiv fur Antkropologie,
Bd. IV. u. V.

DESCRIPTION OF THE SEXUAL PRODUCTS. 25

and the yolk as a mass of enveloping substance. A unanimity of views in this
matter was brought about only after the general conception of " cell " had
received in Histology a more precise definition. This was due especially to
more accurate knowledge of the processes of cell-formation gained through
the works of NAGELI, KOLLIKER, REMAK, LEYDIG, and others.

The interpretation of eggs with separate formative and nutritive yolk, and
with partial cleavage, occasioned especial difficulty. Two antagonistic views
in this matter have existed for a long time. According to one view, eggs with
polar nutritive yolk (the eggs of Reptiles, Birds, etc.) are compound structures,
which cannot be designated as simple cells. Only the formative yolk, together
with the germinative vesicle, is comparable with the Mammalian egg ; the
nutritive yolk, on the contrary, is something new, superposed upon the cell
from without, a product of the follicular epithelium. The spherules of the
white yolk are explained as uninuclear and multinuclear yolk-cells. The
formative and nutritive yolk together are comparable with the entire contents
of the GRAAFIAN vesicle of Mammals. H. MECKEL, ALLEN THOMSON,
ECKER, STRICKER, His, and others, have expressed themselves in favour of this
view with slight modifications in the details.

According to the opposite view of LEUCKART, KOLLIKER, GEGENBAUR,
HAECKEL, VAN BENEDEN, BALFOUB, and others, the Bird's egg is just as truly
a simple cell as the egg of a Mammal, and the comparison with a GBAAFIAN
follicle is to be rejected. The yolk never contains enclosed cells, but only
nutritive components. As KOLLIKER, especially in opposition to His, has
shown, the white-yolk spherules contain no structures comparable with genuine
cell-nuclei ; and therefore cannot be interpreted as cells. As GEGENBAUE
already in 1861 sharply formulated it : " The eggs of Vertebrates with partial
cleavage are on that account essentially no more compound structures than
those of the remaining Vertebrates; they are nothing else than enormous
cells peculiarly modified for special purposes, but which never surrender this
their real character." There would be no change in this interpretation, even
if it should prove to be that the yolk was formed in part from the follicular
epithelium, and was set free from the latter as a sort of secretion. In that
event we should have to do with a special method of nutrition of the egg, the
cell-nature of which cannot on that account be called in question.

Various components of the yolk have received special names. REICHERT
first distinguished as formative yolk the finely granular mass, which, in the
Bird's egg, contains the germinative vesicle, and forms the germ-disc, because
it alone undergoes the process of cleavage, and produces the embryo. The
other chief mass of the egg he called nutritive yolk, because it does not
break up into cells, and because subsequently, enclosed in a yolk-sac, it is
consumed as nutritive material. Afterwards His introduced for these the
names chief germ and accessory germ (Haupt- und Nebeiikeirn).

Whereas the nomenclature of REICHERT and His is applicable only to eggs
with polar arrangement of nutritive yolk, VAN BENEDEN (1870) has undertaken
the division of the substance of the egg from a more general standpoint. He
distinguishes between the protoplasmic matrix of the egg, in which, as in
every cell in general, the vital processes take place, and the reserve and
nutritive materials, which are stored up in the protoplasm in the form of
granules, plates, and balls, and which he designates as deutoplasm. Every
egg possesses both components, only in different proportions, in varied forms
and distribution. BALFOUR has selected this latter condition as a basis f ji

26 EMBRYOLOGY.

division ; and has consequently made the three groups of alecithal, telolecithal,
and centrolecithal eggs, for which I have selected the designation eggs with
little or uniformly distributed yolk, eggs with polar, and eggs with central
yolk.

In recent times investigation has been directed to the finer structure of the
germinative vesicle, in which KLEINENBERG (1872) was the first to observe a
special protoplasmic nuclear trestle (Kerngeriist) or nuclear network, which since
then has been shown by numerous researches to be a constant structure. In
the case of the germinative dot I have myself designated two chemically and
morphologically distinguishable substances as nuclein and paranuclein, the
investigations concerning the importance and the role of which in the develop,
ment of the egg are not yet concluded.

The history of the spermatozoa begins with the year 1677. A student in
Leyden, HAMM, in the microscopic examination of semen, saw the briskly
moving bodies, and communicated his observation to his teacher, the celebrated
microscopist LEEUWENHOECK, who instituted more accurate investigations,
and published them in several papers, which soon attracted general attention.
The sensation caused was all the greater because LEEUWENHOECK declared the
seminal fiiaments to be the preexisting germs of animals, and maintained that
at fertilisation they penetrated into the egg-cell and grew up in it. Thus
arose the school of animalculists.

After the refutation of the preformation theory, it was thought that no
importance was to be ascribed to the seminal filaments in fertilisation, it
being held that it was the seminal fluid that fertilised. Even during the first
four decennia of the present century, the seminal filaments were almost
universally held to be independent parasitic creatures (spermatozoa) com-
parable with the Infusoria. Even in JOH. MULLER'S " Physiology " (1833-40)
occurs this statement : " Whether the semen-animalcules are parasitic animals,
or animated elements of the animals in which they occur, cannot for the
present be answered with certainty."

The settlement of the question was accomplished by comparative histological
investigations of the semen in the animal kingdom, and by physiological
experiment.

In two essays " Beitrage zur Kenntniss der Geschlechtsverhaltnisse und
der Samenfliissigkeit wirbelloser Thiere," and " Bildung der Samenfaden in
Blaschen " KOLLIKER showed that in many animals, e.g., in the Polyps, the
semen consists of filaments only, the fluid being entirely absent ; and that in
addition the filaments are developed in cells, and consequently are themselves
elementary parts of animals. REICHERT discovered the same to be true in
Nematodes. By means of physiological experiment it was recognised that
seminal fluid with immature and motionless filaments, and likewise mature but
filtered semen, did not fertilise. This was decisive for the view that the
seminal filaments are the active part in fertilisation, and that the fluid, which
is added thereto in the case of the higher animals under complicated sexual
conditions, "can be regarded only as a menstruum for the seminal bodies
which is of subordinate physiological significance."

Since then our knowledge (1) of the finer structure, and (2) of the develop-
ment of the seminal filaments, has made further advances. So far as regards
the first point, we have learned, especially through the works of LA VALETTE
and SCHWEIGGER-SEIDEL, to distinguish between head, middle piece, and

DESCRIPTION OF THE SEXUAL PRODUCTS. 27

tail, and to know their different chemical and physical properties. The view
expressed by KOLLIKER, that ordinarily the seminal filaments were the
metamorphosed and elongated nuclei of the seminal cells, underwent a modifi-
cation. According to the researches of LA VALETTE, only the head of the
seminal filament arises from the nucleus, the tail, on the contrary, from the
protoplasm of the spermatid. Finally FLEMMING brought forward convincing
proof that it is only the chromatin of the nucleus that is metamorphosed into
the head of the seminal filament. Important investigations concerning the
development of the seminal filaments in various animals have recently baen
made by VAN BENEDEN ET JULIN, PLATNER, HERMANN, and others.

SUMMARY.

The most important results of this chapter may be briefly sum-
marised as follows :

1. Male and female sexual products are simple cells.

2. The seminal filaments are comparable to flagellate cells. They
are usually composed of three portions, head, middle piece, and
contractile filament.

3. The seminal filament is developed out of a single cell, the
spermatid ; the head, and probably also the middle piece, from the
nucleus ; the contractile filament from the protoplasm.

4. The egg-cell consists of egg-plasm and yolk-particles, which are
reserve material (deutoplasm), imbedded in it.

5. The quantity and distribution of the deutoplasm in the egg-cell
is subject to great variation, and exercises the greatest influence on
the course of the first processes of development.

(a) The deutoplasm is small in amount, and uniformly dis-
tributed in the egg-plasm.

(&) The deutoplasm is present in greater quantity, and, in
consequence of unequal distribution, is more densely
accumulated either at one pole of the egg or in its middle.
(Polar and central deutoplasm.)

(c) In eggs with polar deutoplasm (eggs with polar differentia-

tion) the pole with more abundant deutoplasmic contents
is designated as the vegetative, the opposite one as the
animal pole.

(d) In the case of eggs with polar differentiation, the more

abundant protoplasm of the animal pole may be sharply
differentiated as germ-disc (formative yolk) from, the
portion which is richer in deutoplasm (nutritive yolk).
The developmental processes take place only in the
formative yolk, while the nutritive yolk remains on the
whole passive.

28 EMBRYOLOGY.

6. Eggs may be divided into several groups and sub-groups ac-
cording to their development from cells of the ovary alone, or from
cells of the ovarium and vitellarium, as well as according to the
distribution of the deutoplasm, as exhibited in the following
scheme :

I. Simple eggs. (Development from cells of the ovary.)

A. Eggs with little deutoplasm uniformly distributed through

the egg (alecithal*). (Amphioxus, Mammals, Man.)

B. Eggs with abundant and unequally distributed deutoplasm.

(1) Eggs with polar differentiation (telolecithal), with deuto-

plasm having a polar position, with animal and
vegetative poles. (Cyclostomes, Amphibia.)

(2) Eggs with polar differentiation, which are distinguished

from the preceding sub-group by the fact that with
them there has been effected a still sharper segregation
into formative yolk (germ-disc) and nutritive yolk
into a part which is active during development and a
part that is passive. (Eggs having polar differentia-
tion with a germ-disc. Fishes, Reptiles, Birds.)

(3) Eggs having central differentiation with central deuto-

plasm (centrolecithal) and superficially distributed
formative yolk (blastema, Keimliaut}. (Arthropods.)

II. Compound eggs. (Double origin from cells of the ovarium

and vitellarium.)

LITERATURE.

Baer, C. E. von. De ovi mammalium et hominis genesi epistola. lApsiac

1827.
Beneden, Ed. van. Recherches sur la composition et la signification de

I'o3uf. Mem. cour. de 1'Acad. roy. Sci. de Belgique. T. XXXIV. 1870.
Bisehoff. Entwicklucgsgeschichte des Kanincheneies. 1842.
Flemming. Zellsubstanz, Kern- und Zelltheilung. Leipzig 1882.
Frommann, K. Das Ei. Realencyclopadie der gesammten Heilknnde. 2.

Auflage.
Gegenbaur, C. Ueber den Bau und die Entwicklung der Wirbelthiereier mit

partieller Dottertheilung. Archiv f. Anat. und Pbysiol. 1861.
Guldberg. Beitrag zur Kenntniss der Eierstockseier bei Echidna. Sitzungsb.

d. Jena. Gesellsch. (1885), p. 113.
Hensen. Die Pbysiologie der Zeugung. Hermann's Handbuch der Physio-

logie. Bd. VI. Theil II. Leipzig 1881.

* The translator has been accustomed for several years to use the word
homolecithal instead of alecithal, heterolecithal being employed as a coordinate
term to embrace telolecithal and centrolecithal eggs.

LITERATURE. 29

Hertwig, Oscar. Beitrage zur Kenntniss der Bildung. Befruchtung and

Theilung des thierischen Eies. Morphol. Jahrb. Bde I. III. IV. 1875,

-77, -78.
His, W. Untersuchungen iiber die erste Anlage des Wirbelthierleibes. I.

Die Entwicklung des Hiihnchens im Ei. Leipzig 1868.
Kleinenberg. Hydra. Leipzig 1S72.

Leuckart, R. Article " Zeugung v in Wagner's Handworterbnch der Physio-
logic, Bd. IV. 1853.
Leydig, Fr. Beitrage zur Kenntniss des thierischen Eies im unbefruchteten

Zustand. Zool. Jahrbucher. Abth. f. Anat. Bd. III. (1888), p. 287.
Ludwig, Hubert. Ueber die Eibildung im Thierreiche. Wiirzbwg 1874.
Nagel, W. Das menschliche Ei. Archiv f. mikr. Anat. Bd. XXXT. 1888.
Purkinje. Symbolae ad ovi avium historiam ante incubationem. Lipsiae

1825.
Retzius. Zur Kenntniss vom Bau des Eierstockeies und des Graaf'schen

Follikels. Hygiea Festband 2. 1889.
Schwann. Mikroskopische Untersuchunge i iiber die Uebereinstimmung in

der Structur und dem Wachsthum der Thiere und Pflanzen. 1839. Engl.

transl. by H. Smith. London 1847.
Thomson, Allen. Article " Ovum " in Todd's Cyclopaedia of Anatomy and

Physiology. Vol. X. 1859.

Wagner, R. Prodromus hist, generationis. Lipsiae 1836.
Waldeyer, W. Eierstock und Ei. Leipzig 1870.
Waldeyer, W. Eierstock u. Nebeneierstock. Strieker's Handbuch der

Lehre v. den Geweben. 1871. Engl. transl. Nem York 1872.

Benecke, B. Ueber Eeifung und Bef ruchtung des Eies bei den Fledermausen.

Zool. Anzeiger (1879), p. 304.
Beneden, Ed. van, et Charles Julin. La spermatogenese chez 1'Ascaride

megalocephale. Bull, de 1'Acad. roy. Sci. de Belgique. T. VII. (1884),

p. 312.
Eimer. Ueber die Fortpflanzung der Fledermause. Zool. Anzeiger (1879),

p. 425.
Engelmann. Ueber die Flimmerbewegung. Jena. Zeitschr. f. Med. und

Naturwiss. Bd. IV. (1868), p. 321.
Flemming, "W. Beitrage zur Kenntniss der Zelle und ihrer Lebenserschein

ungen. II. Theil. Archiv f. mikr. Anat. Bd. XVIII. 1880.
Flemming, W. Weitere Beobachtungen iiber die Entwicklung der Spermato-

somen bei Salamandra maculosa. Archiv f. mikr. Anat. Bd. XXXI

1888.
Hermann. Beitrage zur Histologie des Hodens. Archiv f. mikr. Anat. Bd.

XXXIV. 1889.
Hertwig, Oscr.r, und Richard Hertwig. Ueber den Befruchtungs- und

Theilungsvorgang des thierischen Eies unter dem Einfluss ausserer Agen-

tien. 1887.
Kolliker. Physiologische Stndien uber die Samenflussigkeit. Zeitschr. f.

wiss. Zoologie. Bd. VII. (1856), p. 201.
Kolliker. Beitrage zur Kenntniss der Geschlechtsverhaltnisse und der

Samenflussigkeit wirbelloser Thiere, etc. Berlin 1841.

30

EMBRYOLOGY.

Kolliker. Die Bildung der Samenfaden in Blaschen. Denkschr. d. Schweizer.

Gesellsch. f. Naturwiss. Bd. VIII. 1847.
Nussbaum, M. Ueber die Veranderungen der Geschlechtsproducte bis zur

Eifurchung. Archiv f. mikr. Anat. Bd. XXIII. 1884.
Beichert. Beitrag zur Entwickelungsgeschichte der Samenkorperchen bei

den Nematoden. Miiller's Archiv. 1847.
Schweigger-Seidel. Ueber die Samenkorperchen und ihre Entwicklung.

Archiv. f. mikr. Anat. Bd. I. 1865.
Valette St. George, von La. Article " Hoden," Strieker's Handbuch der

Lehre von den Geweben. Engl. trans. Nem York 1872.
Valette St. George, von La. Spermatologische Beitrage. Archiv f. mikr.

Anat. Bde. 25, 27, 28. 1885, -86.

Waldeyer. Bau und Entwicklung der Samenfaden. Anat. Anzeiger (1887),
345. (Full list of the literature on Spermatozoa.)

CHAPTER II.

THE PHENOMENA OF THE MATURATION OF THE EGG AND
THE PROCESS OF FERTILISATION.

1. The Phenomena of Maturation.

EGGS, such as have been described in the previous chapter, are
not yet capable of development, even if they have acquired the
normal size. Upon the addition of mature semen they remain
unfertilised. In order that they may be fertilised they must first
pass through a series of changes, which I shall group together as
the phenomena of maturation.

The maturation-phenomena begin with changes of the germinative
vesicle, which have been followed out the most carefully on the
small transparent eggs of invertebrated animals, such as the
Echinoderms and Nematodes (the maw-worm of the horse). The
germinative vesicle gradually moves from the middle of the egg
the egg of an Echinoderm may serve as the basis of the description
towards its surface, shrivels a little (fig. 12,4), in that fluid escapes
from it into the surrounding yolk, its nuclear membrane disappears,
and the germinative dot becomes indistinct and breaks up into small
fragments (fig. 12 B kf). During this degeneration of the germinative
vesicle a nuclear spindle (fig. 12 sp) is formed, as can be recognised
only after appropriate treatment with reagents ; there arises out of
parts of the germinative dot, or out of a part of the nuclear substance
of the germinative vesicle, a nuclear spindle (fig. 12 B sp), a form

MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 31

of the nucleus which one encounters in the animal and vegetable
kingdoms in stages preparatory to cell-division.

The nuclear spindle, the more precise structure of which will be
described later, in discussing the process of cleavage, pursues still
further the direction already taken by the germinative vesicle, until
it touches with its apex the surface of the yolk, where it assumes a
position with its long axis in the direction of a radius (fig. 137 8 p).
A genuine process of cell-division soon takes place here, which is to
be distinguished from the ordinary cell-division only by this, that
the two products of the division are of very unequal size. To be

;;-
VA v* "*,.-- TV t

">>>..;;.;. ./.^.ij -- Id

V' .'^;'"-.J";J'.\;;-V '

Fig. 12. Portions of eggs of Asterias glacialis. They show the degeneration of the genninative
vesicle.

In figure A it begins to shrivel, in that a protuberance of protoplasm (*), with a radial structure
inside of it, penetrates into its interior, aud dissolves the membrane at that point. The
germinative dot (kf) is still visible, but separated into two substances, nuclein Ou) and
paranuclein (pn).

In figure B the germinative vesicle (/t-6) is entirely shrivelled, its membrane is dissolved, and
only small fragments of the genninative dot (kf) remain. In the region of the protoplasmic
protuberance of figure A there is a nuclear spindle (p) in process of formation.

more exact, therefore, we have to do here with a cdl-budding. At
the place where the nuclear spindle touches the surface with one of
its extremities the yolk arches up into a small knob, into which
half of the spindle itself advances (fig. 13 //). The knob thereupon
becomes constricted at its base, and with the half of the spindle
from which subsequently a vesicular nucleus is again formed is
detached from the yolk as a very small cell (fig. 13 /// rk l ). Here-
upon exactly the same process is repeated, after the half of the
spindle which remains in the egg, without having previously entered
into the vesicular quiescent stage of the nucleus, has restored itself
to a complete spindle (fig. 13 IV).

There now lie close together on the surface of the yolk two
spherules, which consist of protoplasm and nucleus, and therefore
liave the value of small cells (fig. 13 V rk l , rk 2 ), and which are
often to be identified in an unaltered condition, even after the
egg has been divided into a number of cells. They were already

32

EMBRYOLOGY.

known in earlier times under the name of direction bodies, or
polar cells. They have acquired the latter name because, in the case
of eggs in which an animal pole is to be distinguished, they always
arise at that pole. After the conclusion of the second process of
budding, one half of the spindle, the other half of which was employed
in the formation of the second polar cell, is left in the cortical layer

/. 11.

fit'

**.'*'*.*'' ^

-. f,.v-

IV.

r.

Fig. 13. Formation of the polar cells in Asterias glacialis.

In figure /. the polar spindle (/>) has advanced to the surface of the egg. In figure //. there has
been formed a small elevation (rA -1 ), which receives a half of the spindle. In figure ///. the
elevation is constricted off, forming a polar cell (rA- 1 ). Out of the remaining half of the
previous spindle a second complete spin !le {sp) has arisen. In figure IV. there bulges forth
beneath the first polar cell a second elevation, which in figure V. has become constricted off
as the second polar cell (rA- 7 ). Out of the remainder of the spindle is developed (figure VI.)
the egg-nucleus (tk).

of the yolk (fig. 13 Fand VI ek). From this arises a new, small,
vesicular nucleus, which consists of a homogeneous, tolerably fluid
substance without distinctly segregated nucleoli, and attains a
diameter of about 13 //,. From the place of its foi-mation it usually
migrates slowly back again toward the middle of the egg (fig. 14 ek).
The nucleus of the mature egg (fig. 14 ek) has been designated by
me as Egg nucleus, by VAN BENEDEN as female pronucleus. It is not
to be confounded with the germinative vesicle of the unfertilised egg.
Compare the figures of the immature egg (fig. 15) and the mature
egg (fig. 14) of an Echinoderm, both of which are drawn with the
same magnification. The germinative vesicle is of very considerable
size, the egg-nucleus remarkably small : in the case of the former
one distinguishes a clearly developed nuclear membrane, a nuclear
network, and a nucleolus ; the latter is almost homogeneous, without

MATURATION OF THE EGG, AKD PROCESS OF FERTILISATION. 33

nucleolus, and not separated from the protoplasm by any fixed
membrane. Similar distinctions in the condition of the germinative
vesicle and the egg-nucleus recur throughout the animal kingdom.

The formation of polar cells, and the accompanying metamorphosis
of the germinative vesicle into such an extraordinarily reduced egg-
nucleus, is a phenomenon of very wide, probably, indeed, of general
occurrence. Polar cells have been observed throughout the Ccelen-
terates, Echinoderms, Worms, and Molluscs. In the ripening of the
eggs of Arthropods, according to the earlier observations, they
appeared never to be present; but recently they have been found in

Fig. 14.

Fig. 15.

Fig. 14. Mature egg of an rchinochrm, It encloses in the yolk the very small homogeneous

egg-nucleus (ek).
Fig. 15. Immature egg from the ovary of an Echinoderir t

numerous species by a number of observers, especially by BLOCHMANN
and WEISMANN. Among Vertebrates polar cells are always en-
countered in Cyclostomes and Mammals, whereas in Fishes and
Amphibia they have been identified only in some cases, and in Reptiles
and Birds not at all as yet. They arise either some time before or
else during fertilisation.

In the case of Mammals (Rabbit and Mouse) the process has been
very carefully investigated by VAN BENEDEN, and recently by TAFANI.
Severa. weeks before the rupture of the GRAAFIAN follicle the ger-
minative vesicle ascends to the surface of the egg ; some days before
that epoch it there disappears, and at the place where it disappeared
there are formed the egg-nucleus and, under the zona pellucida, one
or two (TAFANI) polar cells. The egg after it has escaped from the
ovary always exhibits egg-nucleus and polar cells.

Also in the case of Fishes, Amphibia, Reptiles, and Birds, whose

3

34 EMBRYOLOGY.

eggs are of considerable size and with few exceptions opaque, the
germinative vesicle, distinguished by its numerous nucleoli, undergoes
a regressive metamorphosis. As has been followed step by step in
Teleosts by OELLACHEE, and in Amphibia by the author, it always
ascends from the middle of the yolk to its surface, and in
fact without exception to its animal pole : in the case of the
frog (fig. 16 kb) this occurs many weeks before the beginning
of maturation. Here immediately under the vitelline membrane,
it becomes flattened to a disc-like body, being at the same time
somewhat shrunken. Further changes, which it is very difficult
to follow in detail, take place in a comparatively short time ;
these occur in the case of the Amphibia at the time when the

Tig. 16. Frog's egg in process of ripening.

The germinative vesicle (M>), with numerous germinative dots (/), lies quite at the surface of
the animal pole as a flattened lenticular body.

eggs are detached from the ovary. For if one examines eggs which
have already escaped into the abdominal cavity, or have entered the
oviduct, it is uniformly found that the germinative vesicle with its
dots has disappeared. In this case, too, there are subsequently
formed from a part of the chromatic substance of the germinativo
vesicle two polar cells and an egg-nucleus, as has been proved by the
fine investigations of HOFFMANN for some species of Teleosts, of
O. SCHULTZE for several Amphibia (Siredon, Triton), and of KAST-
SCHENKO for certain Selachians.

WEISMANN and BLOCHMANN have discovered a very interesting fact
in the Arthropods. In eggs, namely, which develop parthenogenetic-
ally (in summer eggs of Polyphemus, Bythotrephes, Moina, Leptodora,
and Daphnia, as well as in Aphidse) only a single polar cell is elimin-
ated, whereas in eggs which require fertilisation for their further
development there are always two formed. At present, however,
this contrast cannot be established as a general law. For PLATNEK
round that in the case of Liparis dispar there are formed in
parthenogenetic eggs, as well as in those which are fertilised, two
polar cells, the first of which again divides. BLOCHMANN ariived at
the same result from the investigation of unfertilised eggs of bees,
from which drones are developed.

MATURATION OP THE EGG, AND PROCESS OF FERTILISATION. 35

Although the researches on the phenomena of maturation of
the egg in animals still present numerous gaps, nevertheless it
can be regarded as already well-established, that eggs with a germi-
native vesicle are never capable of fertilisation, that the germinative
vesicle is without exception dissolved, and that there is formed out of
components of it (as regards the details there are still many processes
to be more carefully studied) a very small egg-nucleus. During the
metamorphosis there arise, probably ivithout exception, polar cells.

The polar differentiation of many eggs rich in yolk, which was
pointed out in the first chapter, may be brought into causal connection
with the phenomena of maturation. Without exception the animal
pole is the part of the egg-sphere to which the germinative vesicle
ascends, and where the polar cells are subsequently formed. That
the protoplasm is accumulated here in greater quantity is in part
referable to the fact that it comes to the surface of the egg along
with the nucleus, which most certainly furnishes a centre of attrac-
tion for the protoplasm.

The insight into the phenomena of the maturation of the egg, as they have been
connectedly presented in the preceding pages, has been acquired only by many
roundabout ways and after the removal of many misconceptions. As early as
the year 1825 PURKINJE, the discoverer of the germinative vesicle in the Hen's
egg, found that in eggs which were taken from the oviduct this vesicle had
disappeared, and from this concluded that it was ruptured by the contractions
of the oviduct, and that its contents (a lympha generatrix) were mingled with
the germ. Whence the name vesicula germinativa. Similar observations were
made on this and other objects by C. E. v. BAER, OELLACHER, GOETTE,
KLEINENBERG, KOWALEVSKY, REICHERT, and others. But on the other hand
the positive statements were made for many eggs (by JOH. MULLER for
Entoconcha mirabilis ; by LEYDIG, GEGENBAUR, and VAN BENEDEN for
Kotifers, Medusae, etc.) that the germinative vesicle did not disappear, but
remained and gave rise by direct division at the time of segmentation to the
daughter-nuclei.

There were therefore in previous decennia two opposing parties : the one
asserted the continuance of the germinative vesicle and its division during the
process of cleavage ; the other maintained that the egg-cell in its development
passed through a condition without nucleus, and again acquired a nucleus in
consequence of fertilisation.

The controversial points were cleared up by investigations which BttTSCHLl
and the author had undertaken at the same time.

I showed in my first " Beitrage zur Kenntniss der Bildung, Befruchtung
und Theilung des thierischen Eies," that in all the older writings there
had been no distinction made between the nucleus of the immature, the
mature, and the fertilised egg, but that these nuclei had been often confounded
and held to be identical, and I first established the differences between germi-
native vesicle, egg-nucleus, and cleavage-nucleus, the latter being the names
which were introduced by me. In addition I showed that the disappearance

36 EMBRYOLOGY.

of the germinative vesicle and the origin of the egg-nucleus preceded fertilisa-
tion, and thus I distinguished between the phenomena of maturation and
fertilisation of the egg-cell, which generally had been interchanged and con-
founded. I also endeavoured to make it probable that the egg-nucleus
descended from the germinative vesicle, and in fact from a nucleolus of the
vesicle, and defended the thesis that the egg during its maturation did not
pass through a non-nuclear condition. In this I fell into an error : I overlooked,
like all previous observers, the connection between the formation of the polar
cells and the disappearance of the germinative vesicle, a process which it was
the more difficult to establish in the object which I studied because it takes
place in the ovary.

The excellent investigations of BtJTSCHLi, which brought the changes of the
genninative vesicle into connection with the formation of the polar cells, now
made their appearance, supplementing my results. The polar cells were
discovered in the year 1848 by FR. MILLER and LOVEK, and were named by
the former directive vesicles (Richtungsblaschen), because they always lie at
the place where subsequently the first cleavage-furrow makes its appearance.
Their wide distribution in the animal kingdom had also been established by
many investigators ; BCTTSCHLI was the first, however, to direct attention to
the peculiar processes which take place in the yolk, in the interpretation of
which he, nevertheless, committed several errors. He maintained that the
whole germinative vesicle is converted into a spindle-shaped nucleus, which
moves to the surface, and, while becoming constricted in the middle, is thrust
outside by the contractions of the yolk in the form of two directive bodies.
By this process the egg became non-nuclear, and again acquired a nucleus
only in consequence of fertilisation.

In two further articles on the Formation, Fertilisation, and Cleavage of the
Animal-Egg, I modified the teachings of BttTSCHLi, and brought them into
unison with my previous investigations, inasmuch as I pointed out that
the germinative vesicle is not as such directly converted into the nuclear
spindle, but in part is dissolved ; that the spindle takes its origin from the
nuclear substance in a manner which it is very difficult to investigate ; that
the polar cells are formed, not by the elimination of the spindle, but by a
genuine process of division or budding ; that in consequence of this the egg is
not destitute of a nucleus even after the constricting off of the second polar
cell, but that the egg-nucleus arises from the half of the divided polar spindle
which remains in the yolk, and therefore, in its ultimate derivation, from
components of the germinative vesicle of the immature egg.

Soon afterwards BCTSCHLI also interpreted the development of the directive
bodies as cell-budding, likewise GIARD and also FOL, who has produced a
very extensive and thorough investigation on the phenomena of the maturation
of the egg in animals. Recently VAN BENEDEN, supported by researches on
Nematodes, has combatted the interpretation of the process as cell-budding;
however, BOVERI and O. Z ACH ARIAS, who have established a complete agreement
between the formation of directive bodies and the process of cell-division in
the case of the Nematodes also, are unable to subscribe to his conclusion in
this matter.

As a new advance is to be recorded the discovery by WEISMANN and by
BLOCHMANN, that in eggs which are developed parthenogenetically only a
single polar cell arises.

If the original obscurity on the morphological side, in which the phenomena

MATURATION OF THE EGG, AKD PROCESS OP FERTILISATION. 37

of the maturation of the egg were enveloped, has been in general cleared up,
the same is not the case if we inquire after its physiological meaning. That
the germinative vesicle undergoes a regressive metamorphosis into component
parts is easily comprehensible, for a firm membrane and a rich accumulation
of nucleoplasm certainly cannot be necessary to the interaction of protoplasm
and active nuclear substance in the processes of division. Its dissolution is,
as it were, the preliminary requirement for the renewed activity of the nuclear
contents. But what function shall one ascribe to the polar cells ?

Concerning this several hypotheses have been proposed.

BALFOUB, SEDGWICK MINOT, VAN BENEDEN, and others, are of opinion
that the immature egg, like every other cell, is originally hermaphroditic, and
that by the development of polar cells it rids itself of the male constituents of
its nucleus, which afterwards are replaced by fertilisation. BALFOUR thinks
that, if no polar cells were formed, parthenogenesis must normally occur.

WEISMANN, supported by his discovery in the case of eggs developing
parthenogenetically (p. 34), ascribes a different function to the first and the
second polar cells. He distinguishes in the germinative vesicle two different
kinds of plasma, which he designates ovogenetic and germinal plasma.
He maintains that by the formation of the first polar cell the ovogenetic
plasma is eliminated from the ovum ; by that of the second polar cell, half
of the germinal plasma. In the latter case the ejected germinal plasma must
be replaced by fertilisation.

These hypotheses appear to me upon closer examination to present many
vulnerable points. To me appears more promising an interpretation of
BtTSCHLi, who compares the egg, as had already often been done, to the
mother-cell of spermatozoa. Just as the latter gives rise to many spermatozoa,
so also the egg must have once possessed the capability of dividing itself into
many eggs. In the formation of the polar cells, which are eggs that have
become rudimentary, as it were, there has been preserved a trace of these
original conditions. Also BOVERI regards the polar cells as abortive eggs.
I have likewise always conceived of the conditions in this manner.

2. The Process of Fertilisation.

The union of egg-cell and spermatic cell is designated as the process
of fertilisation. This process is to be observed, sometimes with great
difficulty, sometimes with considerable ease, according to the choice of
the animal for experimentation. The investigator ordinarily en-
counters great difficulties in cases where the ripe eggs are not laid, but
where a part, if not the whole, of their development is effected within
the sexual ducts of the maternal organism. In such cases the fertili-
sation also must evidently take place in the ducts of the female sexual
apparatus, into which the semen is introduced in the act of
copulation.

An internal fertilisation takes place in nearly all Vertebrates
except the greater part of the Fishes and many Amphibia. Usually the
egg and the spermatozoa meet, in the case of Man and Mammals, in

38

EMBRYOLOGY.

the beginning of the oviduct ; likewise in the case of Birds they meet
in the first of the four regions previously (p. 17) distinguished, and
at a time when the yolk is not yet surrounded with its albuminous
envelope and calcareous shell.

In contrast to internal fertilisation stands external fertilisation,
which is the simpler and more primitive method, and which occurs in
the case of many Invertebrates that live in the water, as well as
ordinarily in Fishes and Amphibia. In this method, while male and
female keep near together, both kinds of sexual products, which arc
for the most part produced in great number, are evacuated directly
into the water, where fertilisation takes place outside of the maternal

tig. 17 A, B, C. Small portions of eggs of Asterias glacialis, after FOL.

The spermatozoa have already penetrated into the gelatinous envelope which covers the eggs. In
A there begins to be raised up a protuberance toward the most advanced spermatozoon. In
B the protuberance and spermatozoon have met. In C the spermatozoon has penetrated
into the egg. A vitelline membrane, with a crater-like orifice, has now been distinctly
formed.

organism. The whole procedure is therefore much more easily observ-
able. The experimenter has it within his power to effect fertilisation
artificially, and thus to determine precisely the point of time at which
egg and semen are to meet. He needs only to collect in a watch-glass
containing water ripe eggs from a female, likewise in a second watch-
glass ripe semen from a male, and then to mingle the two in a
suitable manner. In this way artificial fertilisation is extensively
practised in fish-breeding. For the purpose of scientific investigation
the selection of the particular species of animal is of the greatest
importance. It is manifest that animals with large opaque eggs do
not commend themselves, whereas those species are especially suit-
able whose eggs are so small and transparent that one can observe
them under the microscope with the highest powers, and at the same
time pass in review every least speck. Many species of Echinoderms

MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 39

are in this respect most excellent objects for investigation. Conse-
quently it was by means of them that an accurate insight into the
processes of fertilisation- was first secured. They may therefore serve
in the following account as the foundation of our description.

If ripe eggs with egg-nucleus are removed from the ovary into a
watch-glass containing sea -water, and a small quantity of seminal
fluid is added, a very uniform result is obtained, since in the course
of five minutes every one of many hundreds or thousands of eggs is
normally fertilised, as can be accurately observed by means of high
magnification.

Although spermatozoa attach themselves to the gelatinous envelope

Fig. 19.

Fig. 18. Fertilised egg of a Sea-urchin.

The head of the spermatozoon which penetrated has been converted into a sperm -nucleus (fr)

surrounded by a protoplasmic radiation, and has approached the egg-nucleus (ek).
Fig. 19. Fertilised egg of a Sea-urchin.
The sperm-nucleus (ak) and the egg-nucleus (ek) hare come close to each other, and both are

surrounded by a protoplasmic radiation.

of an egg in great numbers, many thousands of them when con-
centrated seminal fluid is employed, still only a single one of them
is concerned in fertilisation, and that is the one which by the lash-
like motion of its filament first approached the egg. Where it strikes
the surface of the egg with the point of its head the clear superficial
expanse of the egg-protoplasm is at once elevated into a small knob
that is often drawn out to a fine point, the so-called receptive promin-
ence (Empfdngnisshugel), or cone of attraction. At this place the
seminal filament, with pendulous motions of its caudal appendage,
bores its way into the egg (fig. 17 A, ). At the same time a fine
membrane (fig. 71 C] detaches itself from the yolk over the whole
surface, beginning at the cone, and becomes separated irom it by
an ever-increasing space. The space probably arises because, in
consequence of fertilisation, the egg-plasma contracts and presses-

40

EMBRYOLOGY.

out fluid (probably the nuclear fluid which was diffused after the
disappearance of the germinative vesicle).

The formation of a vitelline membrane is in so far of great signi-
ficance for the fertilisation, as it makes the penetration of another
male element impossible. No one of the other spermatozoa swing-
ing to and fro in the gelatinous envelope is able after that to get
into the fertilised egg.

The one which has penetrated thereupon undergoes a series of
changes. The contractile filament ceases to vibrate, and soon dis-
appears ; but out of the head which, as was previously stated, is
derived from the nucleus of a sperm-cell (spermatid), and consists of
nuclein there is soon developed a very small spheroidal or oval

corpuscle, which afterwards becomes
somewhat larger, the semen- or
sperm-nucleus (fig. 18 sk). This
slowly moves deeper into the yolk,
whereupon it exerts an influence
upon the surrounding protoplasm.
For the latter is arranged radially
around the sperm -nucleus (sk), so
that there is formed a radiate
figure, which is at first small, but
afterwards becomes more and more
sharply expressed and more ex-
tended.

Now an interesting phenomenon
begins to hold the attention of the observer (figs. 18, 19, 20). Egg-
nucleus and sperm-nucleus mutually attract each other, as it were,
and migrate through the yolk toward each other with increasing
velocity. The sperm-nucleus (sk), enveloped in its protoplasmic radia-
tion, changes place more rapidly than the egg-nucleus (ek). Soon the
two meet, either in, or at least near, the middle of the egg (fig. 19) ;
become surrounded by a common radiation, which now extends
through the whole yolk-substance ; are firmly juxtaposed, and then
mutually flattened at the surface of contact ; and finally fuse with
each other (fig. 20 fk). The product of their fusion is the first
cleavage-nucleus (fk), which undergoes the further alterations leading
to cell-division.

This whole interesting process of fertilisation has consumed in the

present object of investigation the short time of about ten minutes only.

The phenomena of fertilisation discovered in the Echinoderms were

Fig. 20. Egg of a Sea-urchin immediately
after the close of fertilisation. Egg-nucleus
and sperm-nucleus are fused to form the
cleavage-nucleus (fk), which occupies the
centre of a protoplasmic radiation.

MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 41

soon observed, either completely or at least partially, in numerous other
animals also in Coelenterates and Worms (NUSSBAUM, VAN BENEDEN,
CARNOY, ZACHARIAS, BOVERI, PLAINER), and in Molluscs and Verte-
brates. As regards the last, it has been possible to follow accurately
in the case of Petromyzon the penetration of a single spermatozoon
into the egg through a special preformed micropyle in the vitelline
membrane (CALBERLA, KUPFFER, BENECKE, and BOHM). Likewise in
the Amphibia, proof has been brought forward that after fertilisation
a sperm-nucleus is formed at the animal pole, and that, surrounded by
a pigmented area, derived from the cortex of the yolk, it moves to-
ward another more deeply imbedded nucleus (egg-nucleus), and fuses
with it (O. HERTWIG, BAMBEKE, BORN). In Mammals the fertilisa-
tion takes place in the beginning of the oviduct. Evidence has also
been produced in their case that after the liberation of the polar
cells two nuclei are temporarily to be seen in the egg-cells, and that
these unite in the centre of the egg to form the cleavage-nucleus
(VAN BENEDEN, TAFANI).

This is the proper place in which to mention briefly the so-called
micropyle. In many animals (Arthropods, Fishes, etc.) the eggs are
enclosed before they are fertilised in a thick firm envelope, which
is impenetrable for spermatozoa. Now, in order to make fertilisation
possible, there are found in these cases at a definite place on the egg-
membrane sometimes one, sometimes several, small openings (micro-
pyles), at which the spermatozoa accumulate in order to glide into
the interior of the egg.

The egg of Nematodes has for several years rightly played an
important role in the literature of the process of fertilisation. But
this is especially true for the egg of the Maw-worm of the Horse
(Ascaris megalocephala), which VAN BENEDEN has made the subject
of a celebrated monograph. It is an excellent object, in so far
as it not only can be had for study everywhere and at all seasons of
the year, but also allows one to follow step by step, in the most
accurate manner, the penetration and subsequent fate of the sper-
matozoon. Since, moreover, the process of fertilisation in Ascaris
megalocephala presents many peculiarities in its details, an extended
presentation of them is both warranted and desirable.

In the case of this Worm, in which the sexes are separate individuals,
there is a copulation, and the fertilisation of the egg takes place within
the sexual passages of the female. In one region, which is expanded
into a kind of uterus, mature spermatic bodies are met with in great
numbers. The appearance of these differs greatly from that which

42 EMBRYOLOGY.

the male seminal elements ordinarily present in the animal kingdom :
for they are apparently motionless ; are comparable in form to a cone,
a conical ball, or a thimble (fig. 21); and consist in part of a
granular substance (6), in part of a homogeneous lustrous substance
(/), and of a small spherical body of nuclear substance (fc), which is
imbedded in the granular substance at the base of the cone.

When the small naked eggs enter into the region designated as
uterus, fertilisation takes place at once. One spermatic body, which
can execute feeble amoeboid motions with its basal end (SCHNEIDER),
attaches itself to the surface of the yolk (fig. 22 sk). Where contact
with the egg first takes place, there is formed, exactly as in the
Echinoderms, a special cone of attraction. Here the spermatic
body, without essential change of form, gradually
glides deeper into the yolk, until it is completely
enclosed therein (fig. 23).

While the two sexual products are thus externally
fused, the egg itself is not yet ripe, because it still
possesses the germinative vesicle (fig. 22 kb), but

body of Ascaris ., ,, , ,,

megaiocephaia, now promptly begins to enter upon the matura-
after VAN BENE- tion stage by preparing to form the polar cells.

DEN.

i; Nucleus ; 6, base The germinative vesicle, which is of small size in

of the cone, by the case of the Maw-worm of the Horse, loses its

ment to the egg sharp delimitation from the yolk, moves toward

takes place; /, that surface of the egg which is opposite to the

lustrous substance . /, ,\ .

resembling fat. cone of attraction (ngs. 23, 24), and is gradually

converted into a nuclear spindle (sp), the origin
of which may be traced upon this object with considerable precision.
The most important part of the process consists in the formation,
out of the chromatic substance, of numerous short, rod-like pieces
(figs. 23, 24, cA), which form directly the chromatic elements of
the spindle, the chromosomes (WALDEYER). As in the case of the
Echinoderms, there then arise at the surface of the yolk two small
polar cells (fig. 25 pz); as in that case, a vesicular egg-nucleus
(fig. 25 ei} arises from the half of the second polar spindle which
remains in the peripheral portion of the yolk.

Meanwhile the spermatic body has moved farther and farther
from the place of its entrance into the egg (figs. 22, 23, sk), and
finally comes to lie in the middle of the yolk (fig. 24 sk), approxi-
mately in the position occupied by the germinative vesicle before its
migration to the surface. During this period the spermatic body
has gradually lost its original form and its sharp delimitation ; out

MATURATION OP THE EGG, AND PROCESS OF FERTILISATION. 43

of its nuclear substance, which was described as a small, deeply
stainable spherule, there arises a vesicular nucleus (fig. 25 sk), which
acquires the same size and condition as the egg-nucleus.

Fig. 22. Fig. 23.

Fig. 22. An egg of Ascaris megalocephala just fertilised, after VAX BENEDEN.
sk, Spermatic body, with nucleus, which has entered the egg ; /, fat-like substance of the
spermatic body ; kb, germinative vesicle.

Fig. 23. A stage of a fertilised egg of Ascaris megalocephala, somewhat older than that of

fig. 22, after VAN BENEDEN.
sic, Spermatic body, which has penetrated deeper into the cortex of the yolk ; sp, polar spindle

which has arisen from the germinative vesicle ; ch, chromosomes of the spindle.

After the rapid and continuous accomplishment of these processes,
the egg of the Worm usually enters on a longer or shorter period of

Fig. 25.

Fig. 24. A still older stage of development, following that of fig. 23, of the egg of Asoaris

megalocephala, after BOVERI.
sp, Polar spindle, which has ascended to the surface of the yolk ; ch, 2 x 4 chromosomes ;

sk, spermatic nucleus, which has migrated into the middle of the egg.
Fig. 25 Egg of Ascaris megalocephala in preparation for the process of cleavage, after

E. VAN BENEDEN.
pz, Two polar cells which have arisen from the polar spindle (p) of fig. 24 by a repetition

of the process of budding ; ei, egg-nucleus ; sk, spermatic nucleus already preparing to

divide ; ch, nuclear loops or chromosomes.

rest. It now presents (compare fig. 25, which represents a stage
already further developed) at its surface within the vitelline mem-
brane two polar cells (pz), and in its interior two large vesicular
nuclei, the spermatic nucleus (sk) and the egg-nucleus (ei), the

44 EMBRYOLOGY.

latter of which has come close up to the former, without, however,
fusing with it. A union of the male and female nuclear substances
into a common nuclear figure takes place in the case of the Maw-
worm, when the process of egg-cleavage is beginning.

The processes of fertilisation just described can be designated as
typical for the animal kingdom. But they appear to recur in exactly
the same manner throughout the vegetable kingdom also, as has
been shown by the thorough investigations of STEASBURGEK. We
are therefore in a better position now than formerly to advance a
theory of fertilisation based upon an important array of facts :

In fertilisation clearly demonstrable morphological processes take
place. Of these the important and essential one is tJie union of two
cell-nuclei which have arisen from different sexual cells, a female egg-
nucleus and a male spermatic nucleus. These contain the fructifying
nuclear substance, which is an organised body and comes into activity
as such in fertilisation.

Recently the attempt has been made to expand the fertilisation
theory into a theory of transmission. Important reasons may be urged,
as appearing to indicate that the fructifying substance is at the
same time the bearer of the transmissible peculiarities. The female
nuclear substance transmits the peculiarities of the mother, the male
nuclear substance the peculiarities of the father, to the nascent creature.
Perhaps there is in this theory a morphological basis for the fact
that offspring resemble both progenitors, and in general inherit from
both equally numerous peculiarities.

If we accept these two theories, the nucleus, which, despite its
constant presence, previously had to be described as a problematic
structure of unknown significance, acquires an important role in the
life of the cell. It seems to be the cell's especial organ of fertilisation
and transmission, inasmuch as there is stored within it a substance
(idioplasma of NAGELI) which is less subject to cell metastasis.

In connection with the consideration of the process of fertilisation
may be permitted a slight digression to the realm of pathological
phenomena.

As follows from numerous observations in both the animal and
vegetable kingdoms, in the normal course of fecundation only a single
spermatic filament penetrates into an egg, when the encountering
sexual cells are entirely healthy. But with an impaired condition of
the egg-cell, superfetation by means of two or more seminal filaments
(polyspermia) takes place.

Superfetation may be produced artificially, if by way of experiment

MATURATION OF THE EGG, AND PROCESS OF FERTILISATION. 45

one injures the egg-cell. This may be accomplished either by
exposing it temporarily to a lower or a higher temperature, and
thus producing cold-rigor or heat-rigor, or by affecting it with
chemical reagents, chloroforming it, or treating it with morphine,
strychnine, nicotine, quinine, etc., or by doing violence to it in a
mechanical way, such as shaking it. It is interesting to observe how,
with all of these means, the degree of superfetation is, to a certain
extent, proportional to the degree of the injury ; how, for example, a
small number of spermatozoa penetrate into eggs which have been
slightly affected with chloral, whereas a greater number penetrate
those which have been more strongly narcotised.

In all unfertilised eggs the whole course of development becomes
abnormal. But whether, as claimed in FOL'S hypothesis, the origin
of double and of multiple organisms is referable respectively to the
penetration of two and many spermatozoa, must still be regarded as
doubtful. Certainly the question suggested richly deserves to be still
more thoroughly tested experimentally.

HISTORY. The facts here given concerning the theory of fecundation are
acquisitions of very recent times. To omit the older hypotheses, it was
generally assumed up to the year 1875 that the spermatozoa penetrate in great
numbers into the substance of the egg, but that they there lose their activity
and become dissolved in the yolk.

I succeeded in my study of the eggs of Toxopneustes lividus in finding
an object in which all the internal phenomena of fertilisation may be
determined with ease and certainty, and in establishing (1) that inconsequence
of fertilisation the head of a spermatic filament surrounded by a stellate figure
makes its appearance in the cortex of the yolk, and is metamorphosed into a
small corpuscle, which I called spermatic nucleus : (2) that within ten minutes
egg-nucleus and spermatic nucleus copulate ; (3) that normally fertilisation is
accomplished by only a single spermatic filament, whereas in pathologically
altered eggs several spermatozoa may penetrate. I was therefore able at that
time to announce the proposition, that fertilisation depends upon the fusion of
two sexually differentiated cell-nuclei.

A few months later, VAN BENEDEN announced that in the case of Mammals
the segmentation-nucleus arises from the fusion of two nuclei, as had
previously been observed by AUEEBACH and BtfTSCHLi in the case of numerous
other objects, and expressed the conjecture that one of them, which has at
first a peripheral position, might in part result from the substance of the
spermatozoa, which, in great numbers, as he maintained, fuse and become
commingled with the cortical portion of the yolk. An advance was soon after
this made by FOL, who investigated with the greatest detail the eggs of
Echinoderms at the very moment of the penetration of a spermatic filament
into the egg, and discovered the formation of a cone of attraction. Since
then it has been established by means of numerous researches (those of
SELENKA, FOL, HERTWIG, CALBERLA, KUPFFER, NUSSBAUM, VAN BENEDEN.
EBEETH, FLEMMING, ZACHABIAS. BOVERI, PLATNEB, TAFANI, BOHM, an 1

46 EMBRYOLOGY.

others) that in other objects also, and in other branches of the animal kingdom,
the processes of fertilisation take place in essentially the same manner. At
the same time the comprehension of the processes of fertilisation was
essentially advanced, especially by the works of VAN BENEDEN on the egg
of Ascaris megalocephala, to which have been added the important investiga-
tions of BOVEEI and others on the same object. STEASBUBGEB has established
in a series of excellent researches the identity of the processes of fertilisation
in the animal and vegetable kingdoms.

Finally, the phenomena of fertilisation were utilised simultaneously by
STEASBUBGEB and myself for the foundation of a theoiy of heredity, in our
endeavor to prove what others (KEBEB, HAECKEL, HASSE) had previously
expressed as a conjecture that the male and the female nuclear substances
are the bearers of the peculiarities which are transmitted from parent to
offspring. KOLLIKEE, Roux, BAMBEKE, WEISMANN, VAN BENEDEN, BOVEBI,
and others have since expressed themselves in a similar manner.

SUMMARY.

1. At maturation the germinative vesicle gradually rises to the
animal pole of the egg, and thereby undergoes a regressive meta-
morphosis (degeneration of the nuclear membrane and the fibrous
network, mingling of the nuclear fluid Kernsaft with the proto-
plasm).

2. A nuclear spindle (polar spindle or direction-spindle) is de-
veloped out of remnants of the germinative vesicle, principally,
indeed, out of the substance of the germinative dot, which breaks
up into chromosomes.

3. At the place where the spindle encounters the surface of the
yolk with one of its ends, there are formed two polar cells or direction-
bodies (Richtungakorper) by means of a process of budding, which Is
repeated.

4. At the second budding, half of the nuclear spindle remains in
the cortex of the yolk, and is metamorphosed into the egg-nucleus
The egg is then ripe.

5. In the case of eggs which develop parthenogenetically (Arthro-
poda), ordinarily only one polar cell is formed.

6. At fertilisation only a single spermatozoon penetrates a sound
egg (formation of a cone (K attraction, detachment of a vitelline mem-
brane).

7. The head of the spermatozoon is converted into the spermatic
nucleus, around which the neighbouring protoplasmic particles are
radially arranged.

8. Egg-nucleus and spermatic nucleus migrate toward each other,
.and in mcst instances immediately fuse to form the segmentation-

LITEEATURE. 47

nucleus ; in many objects they remain for a considerable time near
each other, but not united, and only later are together metamorphosed
into the segmentation-spindle.

9. In some animals fertilisation of the egg takes place only after
completion of its maturation, but in others it is inaugurated at the
very beginning of maturation, so that the two phenomena overlap
each other.

10. Fertilisation theory. Fertilisation depends on the copulation
of two cell-nuclei, which are derived from a male cell and a female
cell.

11. Theory of heredity. The male and female nuclear substances
contained in the spermatic nucleus and the egg-nucleus are the
bearers of the peculiarities which are transmissible from parents to
their offspring.

LITERATURE.

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Beneden, E. van. La maturation de 1'oeuf, la fecondation, etc., des mammi-
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Beneden, E. van. Contributions a 1'histoire de la vesicule germinative, etc.
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Beneden, E. van. .Recherches sur la maturation de 1'oeuf, la fecondation et
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jBeneden, van, et Neyt. Nouvelles recherohes sur la fecondation et la
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Blochmann. Ueber die Eichtungskorper bei den Insecteneiern. Biol. Cen-
tralblatt. Bd. VII. 1887.

Blochmann. Ueber die Richtungskorper bei Insecteneiern. MorphoL Jahrb
Bd. XII. 1887, p. 544.

Blochmann. Ueber die Reifung der Eier bei Ameisen und Wespen. Fest-
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Med. Theil.

48 EMBRYOLOGY.

Blochmann. Ueber die Zahl der Eichtungskdrper bei befruchteten uni 1

unbefruchteten Bieueneiern. Morpholog. Jahrb. Bd. XV. 1889.
Bohm, A. Ueber Eeif ung und Befruchtung des Eies von Petromyzon. Archiv.

f. mikr. Anat. Bd. XXXII. 1888, p. 613.
Born. Ueber den Einfluss der Schwere auf das Froschei. Archiv f. mikr.

Anat. Bd. XXIV. 1885, p. 475.
Born. Weitere Beitrage zur Bastardirung zwischen den einheimischen Anureri.

Archiv f. mikr. Anat. Bd. XXVII. 1886, p. 192.
Boveri. Ueber die Bedeutung der Eichtungskorper. Sitzungsb. d. Gesellsch.

f. Morphol. u. Physiol. in Miinchen. Sitzung vom 16. Nov. 1886, p. 101.

Miinchener medic. Wochenschr. Jahrg. 33. Nr. 50.

Boveri. Ueber die Befruchtung der Eier von Ascaris megalocephala. Sit-
zungsb. d. Gesellsch. f. Morphol. u. Physiol. in Miinchen. Sitzung vom 3.

Mai, 1887, p. 71.
Boveri. Ueber den Antheil des Spermatozoons an der Theilung der Eier.

Sitzungsb. d. Gesellsch. f. Morphol. u. Physiol. in Miinchen. Bd. III.

1887, p. 151.
Boveri. Zellenstudien. Jena. Zeitschr. Bde. XXI. XXII. XXIV. 1887.

-88, -90.
Biitschli. Studien iiber die ersten Entwicklungsvorgange der Eizelle, Zell-

theilung u. Conjugation der Infusorien. Abhandl. d. Senckenberg. naturf.

Gesellsch. Bd. X. Frankfurt 1876.
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Eichtungskorperchen. Biol. Centralblatt. Bd. IV. 1884, pp. 5-12.
Biitschli. Entwicklungsgeschichtliche Beitrage. Zeitschr. f. wiss. Zoologie.

Bd. XXIX. 1877.
Calberla. Befruchtungsvorgang beim Ei von Petromyzon Planeri. Zeitschr.

f. wiss. Zoologie. Bd. XXX. 1878, p. 437.
Carnoy, J. B. La cytodierese de 1'oeuf. La vesicule germinative et les

globules polaires de 1'Ascaris megalocephala. 1886. And La Cellule.

T. III. 1887.
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und in der Vereinigung derselben mit dem Ei. Archiv f . d. ges. Physiol.

Bd. XXXIX. 1886.
Eberth. Die Befruchtung des thierischen Eies. Fortschritte der Medic.

Nr. 14. 1884.
Flemming, W. Ueber die Bildung von Eichtungsfiguren in Saugethiereiern

beim Untergang Graaf'scher Follikel. Archiv f. Anat. u. Physiol., Anat.

Abth. 1885.
Flemming, W. Ueber Bauverhaltnisse, Befruchtung u. erste Theilung der

thier. Eizelle. Biol. Centralblatt. Bd. III. 1884, pp. 641, 678.
Flemming, W. Beitrage zur Kenntniss der Zelle, etc. III. Theil. Arch. f.

mikr. Anat. Bd. XX. 1881.
Fol. Sur le commencement de 1'henogenie. Archives des ScL phys. et nat.

Geneve 1877.
Fol. Eecherches sur la fecondation et le commencement de 1'henogenie.

Mm. de la Soc. de Phys. et d'Hist. nat. Geneve 1879.
Frommann. Article " Befruchtung " in Eeal-Encyclopadie der gesammten

Heilkunde. 2 Aufl.
Giard, Alf. Note sur les premiers phenomenes du developpement de 1'oursin.

Coraptes rendus. LXXXIV. 1877.

LITERATURE. 49

Greeff, B. Ueber den Ban imd die Entwicklung der Echinodermen. Sit-

zungsb. d. Gesellsch. z. Beford. d gesammten Naturwiss. zu Marbur^

Nr. 5. 1876.

Hasse, C. Die Beziehungen der Morphologic zur Heilkunde. Leipzig 1879.
Henking. Ueber die Bild ung von Richtungskb'rpern in den Eiern der Insecten

und deren Schicksal. Nachr. d. kgl. Gesellsch. d. Wiss. zu Gottingen

Jahrg. 1888.
Hensen. Die Physiologic der Zeugung. Handbuch der Physiologie von

Hermann. Bd. VI. Theil II. 1881.

Hensen. Die Grundlagen der Vererbung. Landwirthsch. Jahrb. 14. 1885.
Hertwig, Oscar. Beitrage zur Kenntniss der Bildung, Befruchtung u.

Theilung des thier. Eies. Morphol. Jahrb. Bd. I. 1875.
Hertwig, Oscar. Beitrage, etc. II. Theil. Morphol. Jahrb. Bd. III.

1877, pp. 1-86.

Hertwig, Oscar. Weitere Beitrage, etc. Morphol. Jahrb. Bd. III. 1877.
Hertwig, Oscar. Beitrage zur Kenntniss, etc. Morphol. Jahrb. Bd. IV.

Heft 1 u. 2. 1878.
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vorgang des thierischen Eies unter dem Einfluss ausserer Agentien.

Jena 1887.
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Ei. Jena. Zeitschr. Bd. XXIV. 1890.
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Akad. v. Wetensch. Amsterdam. Deel XXI. 1881.
Hoffmann, C. K. Ueber den Ursprung und die Bedeutung der sogenannten

freien Kerne in dem Nahrungsdotter bei den Knochenfischen. Zeitschr. f.

wiss. Zoologie. Bd. XLVI. 1888.
Kastschenko. Zur Frage fiber die Herkunft der Dotterkerne im Selachierei.

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Zeitschr. f. wiss. Zoologie. Bd. XLII. 1885, pp. 1-46.
Kolliker. Das Karyoplasma und die Vererbung. Eine Kritik der Weis-

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Kultschitzky. Ueber die Eireifung und die Befrnchtungsvorgange bei

Ascaris marginata. Archiv f. mikr. Anat. Bd. XXXII. 1888.
Kultschitzky. Die Befruchtungsvorgiinge bei Ascaris megalocephala.

Archiv f. mikr. Anat. Bd. XXXI. 1888, p. 567.
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u. vulgnris. Sitzungsb. d. math. Classe. d. Akad. d. Wissensch. zu

Munchen, 1882, p. 608.
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Neunaugen. Konigsberg 1878.

Lovan, S. Beitrage zur Kenntniss der Entwicklung der Mollusca acepbala

4

50 EMBRYOLOGY.

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Im Auszuge iibersetzt. Stockholm 1879
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THE PROCESS OP CLEAVAGE. 51

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CHAPTER III.
THE PROCESS OF CLEAVAGE.

FERTILISATION is in most instances immediately followed by further
development, which begins with the division of the egg-cell the
simple elementary organism into an ever-increasing number of
small cells the process of cleavage. We shall begin the study of
cleavage with a very simple case, and here also choose as a foundation
for the presentation of the subject the egg of an Echinoderm and
the egg of the common Ascaris of the Horse.

In the living egg of the Echinoderm the cleavage-nucleus (fig. 26
fk], which arose from the fusion of egg-nucleus and spermatic
nucleus, is at first spheroidal, and lies exactly in the middle of the
egg, where it forms the centre of a radiation which affects the
whole yolk-mass ; but it soon begins to be slightly elongated, and
at the same time to become less and less distinct, so that with the
living object one might be misled into assuming that it had been
completely dissolved. Before this, very regular changas in the dis-
tribution and arrangement of the protoplasm around the nucleus
have taken place. The monocentric radiation resulting from fer-
tilisation is divided. The two newly formed radiations thereupon
move to the poles of the elongated nucleus. At first small and in-
significant, they rapidly extend, and finally each occupies a half of
the egg (fig. 27), and the rays of the two systems meet at a sharp
angle in the median plane of the egg.

Just in proportion as the two radiations become more distinct,
there arises, within the granular yolk, as the starting-point and

52 EMBRYOLOGY.

centre of the radiations, a figure, which may be appropriately com-
pared (fig. 27) with a dumb-bell. It arises by the accumulation of a
large amount of homogeneous protoplasm around the poles of the
elongating nucleus, forming the two ends of the dumb-bell ; the
poles may be regarded as if they were two centres of attraction.
The non-granular streak, representing the handle of the dumb-bell,
is the nucleus, which has meanwhile undergone a peculiar metamor-
phosis and has become indistinct.

A more accurate knowledge of the nuclear metamorphosis may be
got by employing suitable reagents and dyes. By means of inter-
mediate stages, which may be disregarded here, thero arises out of

Fig. 26.

fig. 26. Egg of a Sea-urchin immediately after the conclusion of fertilisation, fk, Cleavage-
nucleus.

Fig. 27. Egg of a Sea-urchin in preparation for division. The nucleus is no longer to be seen ;

there has arisen in its place a dumb-bell figure.
Both figures are drawn from the living object.

the vesicular nucleus the nuclear spindle (fig. 31 ), which is a
typical structure for cell-division throughout the organic world.
This (sp) consists of two substances, both of which, in my opinion,
are derived from the quiescent condition of the nucleus namely,
(1) of a non-chromatic substance, which does not show affinity for any
dyes, and (2) of the stainable nuclein or chromatin. The non-chromatic
substance forms extraordinarily fine, and therefore at times scarcely
discernible, " spindle-fibres" which are united into a bundle, and
give rise to a spindle by the convergence of their ends to points. The
chromatin, on the contrary, has assumed the form of small individual
granules or chromosomes, which correspond in number with the
spindle-fibres, and are so arranged that each granule adjoins a
spindle-fibre at its middle point. In its totality, therefore, it con-
stitutes at the middle of the spindle a plate composed of individual

THE PROCESS OP CLEAVAGE. 53

granules the nuclear plate of STRASBURGER. That which in the case
of the Sea-urchin ordinarily appears as a chromatic granule is found,
upon the employment of the highest magnifying powers, but
especially in the study of objects (fig. 28 A) more suitable for this
purpose, to be a small Y-shaped loop. The number of the loops or
chromosomes appears to be very definite, and subject to law for each
species of animal.

At the tips of the spindle there may be demonstrated, in addi-
tion, two special and exceedingly minute bodies, one of which
occupies the exact centre of each of the two previously mentioned
systems of rays ; they are, in fact, to be regarded as the cause of the

W 1

Fig. 28. Diagram of nuclear division, after RABL.

In figure A one sees the spindle, composed of delicate non-chromatic fibres, with the protoplasmic
radiations at its tips and the chromatic loops at its middle. The splitting of the filaments
of the latter has already taken place. In figure B the daughter-loops resulting from the
fission have moved apart in opposite directions. In figure Cthey begin to arrange themselves
in a regular manner into two groups of loops. In figure D the groups of daughter-loops lie
near the two poles of the spindle.

latter. Inasmuch as during the elongation of the nucleus they are
to be found at each of its two poles, they may be especially designated
as polar corpuscles [or centrosomes]. During the whole process of the
division of nucleus and cell-body, it appears as though a directing
influence belongs to the two polar corpuscles.

Important changes in the nuclear loops of the spindle take place
during later stages of the process of division. Each loop is split
lengthwise into two daughter-loops (fig. 28 A), as discovered by
FLEMMING and as confirmed since then by numerous other investi-
gators (STRASBURGER, HEUSER, VAN BENEDEN, EABL, and others).
These daughter-loops soon move apart toward the opposite ends of
the spindle (figs. 28 B, C; see also the explanation of the figures), and
approach very closely to the polar corpuscles at their tips (fig. 28 D)

Thus by a complicated process a division of the stainable nuclear
substance into similar halves is brought about. As the immediate

54

EMBRYOLOGY.

consequence of this the protoplasmic parts of the cell also begin at
this time to be divided into halves by means of the process of cleavage,
which is already recognisable externally. There is formed at the
surface of the egg (fig. 29 A), in a plane passing between the two
groups of loops through the middle of the spindle perpendicular
to its long axis, a circular furrow, which rapidly cuts deeper and
deeper into the substance of the egg, and in a short time divides
it into two equal parts. Each of these contains half of the spindle

Fig. 29 A. Egg of a Sea-ursiiin at the moment of division.

A circular furrow cuts into the yolk and halves it in a plane which is perpendicular to the
middle of the nuclear axis and to the long axis of the dumb-bell.

B. Egg of a Sea-urchin after its division into two cells.

In each resultant of the division a vesicular daughter-nucleus has arisen. The radial arrange-
ment of the protoplasm begins to become indistinct.
Both figures are drawn from the living object.

with half of the loops, half of the dumb-bell, and a protoplasmic
radiation.

The resulting halves of the egg, still surrounded in common by the
vitelline membrane, then closely apply to each other the surfaces
resulting from the division, and become so flattened that each one of
them forms approximately a hemisphere (fig. 20 E). Internally,
however, nucleus and protoplasm enter upon a brief transitory resting
stage. There is developed out of the half of the nuclear spindle
with its daughter-loops a vesicular homogeneous daughter-nucleus
like the first, but in the protoplasm the radial arrangement becomes
less and less distinct and at last entirely disappears.

The egg of the common Maw-worm of the Horse is also a very
instructive object for the study of the process of cleavage, as it was
for the study of fertilisation, for it allows a still deeper insight into
this process. As has already been stated, the egg-nucleus and the

THE PROCESS OF CLEAVAGE. 55

spermatic nucleus remain for a time separate, even after they have
approached each other. After a brief period of rest both of them
begin to exhibit simultaneously the changes which precede the for-
mation of the nuclear spindle. In each the chromatic substance is
metamorphosed into a fine thread, which is arranged within the
nuclear membrane in numerous windings. Each filament is there-
upon divided into two equally large coiled loops, the chromosomes
(fig. 25 ch). Now the two vesicular nuclei lose their delimitation
from the surrounding yolk, in which there arise at a little distance
from each other two polar corpuscles [centrosomes], surrounded by a
system of rays, which is at first faint, but subsequently becomes
more distinct. Between the two centrosomes, the method of whose
development no one has as yet succeeded in observing, there are
formed spindle-fibres, and the four loops (chromosomes), set free by
the dissolution of the two nuclear membranes, so arrange themselves
that they lie upon the outside of the spindle at its equator.

In the case of the egg of the Maw-worm, therefore, the union of the
two sexual nuclei, which terminates the act of fertilisation, takes
place only at the time of the metamorphosis to form the cleavage-
spindle, in which metamorphosis they take an equal share. In conse-
quence of this remarkable deviation from the ordinary course of the
process of fertilisation, VAN BENEDEN has been able to establish the
interesting and important fact that half of the chromosomes of the
first cleavage-spindle are derived from the egg-nucleus, and half from
the spermatic nucleus, and that consequently they may be distin-
guished as female and male chromosomes. Since in this instance, just
as in nuclear division ordinarily, the four loops are split lengthwise
and then move apart toward the two polar corpuscles (centrosomes),
there are formed two groups of four daughter-loops each, of which
two are of male origin and two of female. Each group is then meta-
morphosed into the quiescent nucleus of the daughter-cell. This
furnishes incontestable proof, that to each daughter-nucleus in each
half of the egg, which arises as the result of the first cleavage, there is
transmitted exactly the same amount of chromatic substance from the
egg-nucleus as from the spermatic nucleus.

The first division is followed after a brief period of rest by the
second, this by the third, the fourth, etc., during which are repeated
the same series of changes in nucleus and protoplasm that have just
been described. Thus in quick succession the 2 first daughter-cells
are divided into 4, these into 8, 16, 32, 64, etc. (fig. 30), until
there has resulted a large spheroidal mass, which has received the

56

EMBRYOLOGY.

name morula or mulberry-sphere, because the cells protrude as small
elevations at its surface.

During the second and third stages of cleavage there is easily
recognisable a rigidly observed order in the direction which the planes
of cleavage sustain to each other. The second plane of cleavage always
halves the first and cuts it perpendicularly ; the third plane, again, is
perpendicular to the first two, and passes through the middle of the
axis formed by their intersection. If one regards the ends of this
axis as the poles of the egg, the first two planes of division may be
designated as meridional, the third as equatorial.

This uniformity is caused by the mutual relation which subsists
between nucleus and protoplasm, in which connection the two follow-
ing laws are to be noted : (1) The plane of division always cuts the
X. axis of the spindle perpendicularly at its centre. (2) The position of

Fig. 30. Various stages of the process of cleavage, after GEOENBAUR.

the axis of the nuclear spindle in turn depends on the form and differ-
entiation of the protoplasmic body which envelops it, and in such a
manner that the two poles of the nucleus take the direction of the greatest
protoplasmic masses. Thus, for example, in a sphere in which the
protoplasm is uniformly distributed, the centrally situated spindle
may come to lie in any radius ; but in an ovoid protoplasmic body,
only in the longest diameter. In a circular protoplasmic disc the
nuclear axis lies parallel to its surface in any diameter whatever of
the circle, but in an oval disc, as before, in the longest diameter
only.

Let us return now, after these general remarks, to the case under
consideration. Each daughter-cell forms at the close of the first seg-
mentation a hemisphere. According to the rule, the daughter-spindle
cannot assume a position perpendicular to the flat surface of the
hemisphere, but must lie parallel to it, so that a division into two
quadrants must result. At the next segmentation the axis of the
spindle must coincide with the long axis of the quadrant, whereby
this becomes divided into two octants.

THE PROCESS OF CLEAVAGE. 57

There are some important deviations from the process of division
just described, which affect the form of the cleavage products, although
leaving unaltered the finer processes relating to the nucleus. The
deviations are induced, as \ve shall show more in detail in the in-
dividual cases, by the variation in the amount of deutoplasm contained
in the eggs, and by the previously described variability in its distribu-
tion. One may appropriately separate the various forms of the
process of cleavage into two classes, and each class into two sub-
classes, although the forms merge into one another by means of
transitional conditions.

To the first class we assign such eggs as are completely divided
into segments by the process of cleavage. The cleavage itself we
designate as total ; and according as the segments are of equal or un-
equal size, we distinguish as subdivisions equal cleavage and unequal
cleavage.

With total is contrasted partial cleavage. This occurs in the
case of eggs which are provided with very abundant deutoplasm,
and are consequently of considerable size, and in which, at the same
time, the previously described separation into formative yolk and
nutritive yolk has been distinctly established. In this case the for-
mative yolk alone undergoes a process of cleavage, whereas the chief
mass of the egg, the nutritive yolk, remains undivided, and in general
unaffected, by the processes of embryonic development ; hence the
name partial cleavage. This, in turn, is resolvable into the two sub-
types of discoidal and superficial cleavage, according as the forma-
tive yolk rests as a disc upon the nutritive yolk, or envelops the
latter as a thick cortical layer. REMAK has designated eggs with
total segmentation as holoblastic, those with partial segmentation as
meroblastic.

We may therefore present the following scheme of cleavage :

I. TYPE

Total cleavage :

(a) Equal cleavage \ Holoblastic eggs,

(ft) Unequal cleavage
II. TYPE

Partial cleavage :

(a) Discoidal cleavage J- Meroblastic eggs.
(ft) Superficial cleavage

I a ' Equal Cleavage.

In the general consideration of the process of cleavage we have
already become acquainted with the phenomena of equal segmenta-

58 EMBRYOLOGY.

tion. It remains to be added to what has been previously said, that
this type is most frequent in the case of Invertebrates, and is to be
encountered among Vertebrates only in the cases of Amphioxus and
Mammals. With the latter, however, there early appears a slight
difference in the size of the segments ; this has induced many
investigators to designate the cleavage of Amphioxus and Mammals
as unequal also. If I have not followed this suggestion, it is
because the differences are of a trivial nature, because the nucleus
in the egg-cell and also in its segments still occupies a central
position, and because the different methods of cleavage are in
general not sharply definable, but connected by transitional con-
ditions.

Concerning Amphioxus, HATSCHEK states that at the eight-cell stage
four smaller and four larger cell are to be distinguished, and that
from that time forward in all the subsequent stages there is to be
observed a difference in size, and that the process of cleavage takes
place in a manner similar to that which will be subsequently
described for the Frog's egg. The egg of the Rabbit, concerning
which we have the painstaking investigations of VAN BENEDEN,
divides at the very outset into two segments of slightly different
size ; moreover, from the third stage of division onward there occurs
a difference in the rapidity with which the divisions follow each
other in the different segments. After the four cleavage-spheres
have been divided into eight, there is a stage with twelve spheres ;
this is followed by another with sixteen, and afterwards another with
twenty-four.

P- Unequal Cleavage.

As a basis for the description of unequal cleavage we may employ
the Amphibian egg, the structure of which has already been con-
sidered. As soon as the egg of the Frog or Triton is deposited in
the water and is fertilised, and while the gelatinous envelope is
swelling up, its black pigmented hemisphere or animal half becomes
directed upward, because it contains more protoplasm and small
yolk-spherules, and is specifically lighter. The want of uniformity
in the distribution of the various components of the yolk also induces
an altered position of the segmentation-nucleus. Whereas the latter
assumes a central position in all cases in which the deutoplasm is
uniformly distributed, it invariably alters its location whenever
one half of the egg is richer in deutoplasm and the other richer in
protoplasm ; it then migrates into the more protoplasmic territory.

THE PROCESS OF CLEAVAGE. 59

Jn the case of the Frog's egg, consequently, we find it in the black
pigmented hemisphere, which is turned upward.

When in this case the nucleus prepares to divide, its axis can no
longer assume the position of any and every radius of the egg. In
consequence of the want of uniformity in the distribution of the
protoplasm, the nucleiis comes under the influence of the more
protoplasmic pigmented part, which rests on the more deutoplasmic
portion like an inverted cup, and, on account of its less specific
gravity, floats at the surface, and is spread out horizontally. But
in a horizontal protoplasmic disc the nuclear spindle comes to occupy
a horizontal position (fig. 31 A ap). Consequently the plane of
division must be formed in a vertical direction. A small furrow now

pr

tp

-, d

Fig. 31. Diagram of the division of the Frog's egg.

A, Stage of the first division. B, Stage of the third division. The four segments of the second
stage of division are beginning to be divided by an equatorial furrow into eight segments.
F, pigmented surface of the egg at the animal pole ; pr, the part of the egg which is richer
in protoplasm ; d, the part which is richer in deutoplasm ; sp, nuclear spindle.

begins to show itself at the animal pole first, because the latter is
more under the influence of the nuclear spindle, which lies nearer
to it, and because it contains more protoplasm, from which proceed
the phenomena of motion during division. The furrow gradually
deepens downward, and cuts through to the vegetative pole.

By the first act of division we get two hemispheres (fig. 32 2 ), each
of which is composed of a quadrant richer in protoplasm and directed
upward, and another poorer in protoplasm and directed downward.
By this means both the position of the nucleus and the direction of
its axis are again determined, when it prepares for the second
division. According to the rule previously laid down, the nucleus is
to be sought in the quadrant which contains the more protoplasm ;
the axis of the spindle must take a position parallel to the long
axis of the quadrant, and must therefore come to lie horizontally

60

EMBRYOLOGY.

The second plane of division is consequently, like the first, vertical,
and cuts the latter at right angles.

After the conclusion of the second segmentation the Amphibian
egg consists of four quadrants (fig. 32 4 ), which are separated from
one another by vertical planes of division and possess two dissimilar
poles, one richer in protoplasm, lighter, and directed upwards; the
other richer in yolk, heavier, and directed downwards. In the case of
equal segmentation we saw that at the stage of the third segmentation
the axis of the nuclear spindle becomes parallel to the long axis of
the quadrant. The same thing occurs here also, although in a some-
what modified manner. On account of the greater accumulation of
protoplasm in the upper half of the quadrant, the spindle cannot, as

fig. 32. Cleavage of Rana temporaria, after ECKER.

The numbers placed above the figures indicate the number of segments present in the corre-
ponding stage.

in the case of equal segmentation, lie in the middle of it, but must
lie nearer to the animal pole of the egg (fig. 31 B sp). Moreover, it
is exactly vertical, because the four quadrants of the Amphibian egg
are definitely oriented in space on account of the difference in specific
gravity of their halves. In consequence of this the third plane
of division -must be horizontal, and must also lie above the equator of
the egg-sphere more or less toward its animal pole (fig. 32 8 ). The
segments are very unlike both in size and composition ; and this is
the reason why this form of segmentation has been called unequal.
The four upper segments are smaller and contain less yolk, the four
lower ones are much larger and richer in yolk. They are also
distinguished from each other as animal cells and vegetative cells,
according to the poles near which they lie.

In the course of further development, the distinction between,
animal and vegetative cells constantly increases, for the richer_the
cells are in protoplasm the more quickly and the more frequently

THE PROCESS OF CLEAVAGE. 61

do they divide. At the fourth stage the 4 upper segments are first
divided^ by vertical furrows into 8, and then after an interval the
4 lower ones are divided in the same manner, so that the egg is now
composed of eight smaller and eight larger cells (fig. 32 16 ). After
a short resting stage the eight upper segments are again divided, this
time by a horizontal furrow, and somewhat later a similar furrow
divides the eight lower segments also (fig. 32 32 ). In the same
manner the 32 segments are divided into 64 (fig. 32 64 ). In the
stages which follow this, the divisions in the animal half of the egg
are still more accelerated relatively to those of the vegetative half.
While the 32 animal cells are divided into 128 segments by two
divisions which follow each other in quick succession, there are
still found in the lower half only 32 cells which are preparing
for cleavage. It thus comes to pass that, as the final result of the
process of cleavage, there exists a spheroidal mass of cells with entirely
dissimilar halves, an upper, animal half with small, pigmented
cells, and a vegetative half with larger, clear cells, containing more
abundant yolk.

From the nature of the progress of unequal cleavage, as well as
from a series of other phenomena, one may lay down a general law,
first formulated by BALFOUE, that the rapidity of cleavage is pro-
portional to the concentration of protoplasm in the segment. Cells
which are rich in protoplasm divide more rapidly than these in which
protoplasm is more scanty and deutoplasm more abundant.

As we have seen, the Frog's egg, by reason of the difference in
specific gravity between its animal and vegetative halves, by reason
of the heterogeneous pigmentation of its surface, by reason of the
unequal distribution of protoplasm and deutoplasm, and by reason of
the eccentric position of its nucleus, allows us to pass fixed and easily
determinable axes through its spherical body. On this account it is
an especially favourable object upon which to determine the question
whether the egg allows one to recognise in the position of its parts,
even before fertilisation, immediately after the same, and during the
process of cleavage, fixed relations to the organs of the fully developed
organism. This question has been tested by means of ingenious
experiments, especially by PFLUEGEE and Roux, by the latter in his
" Beitriige zur Entwicklungsmechanik des Embryo."

These have resulted in determining that the first cleavage plane of
the egg corresponds to the median plane of the embryo, so that it
separates the material of the right half of the body from that of the
left Secondly, according to Roux, the position of the head- and tail-

62

EMBRYOLOGY.

ends of the embryo may be determined in the fertilised egg. That
half of the egg, namely, through which the spermatic nucleus
migrates to reach the egg-nucleus, becomes the tail-end of the
embryo ; the opposite half becomes the head-end. Every egg,
however, can be fertilised in any meridian whatever, as was demon-
strable experimentally, and thereby the tail end of the embryo may
be located at any chosen position in the egg. Thirdly, the plane
in which the two sexual nuclei meet each other (copulation-plane)
corresponds with the first plane of segmentation.

II a - Partial Discoidal Cleavage.

The Hen's egg serves us as the classical example for the description
of discoidal segmentation. In this instance the whole process of

fig. 38. Surface view of the first stages of cleavage in the Hen's egg, after COSTE.
a, Border of the germ-disc ; 6, vertical furrow ; c, small central segment ; cl, large peripheral
segment.

cleavage takes place while the egg is still in the oviduct, during the
period in which the yolk is being surrounded by the albuminous
envelope and the calcareous shell. It results simply in a cleavage of
the germ-disc of formative yolk, whereas the greater part of the
egg, which contains the nutritive yolk, remains unsegmented, and
becomes subsequently enclosed in an appendage to the embryo, the
so-called yolk-sac, and is gradually consumed as nutritive material.
Just as in the case of the pigmented, animal half of the Frog's egg,
so also in the case of the Hen's egg, turn it in whatever direction
one will, the germ-dij-'c floats on top, because it is the lighter part.
As in the Frog's egg the first plane of cleavage is vertical and begins
at the animal pole, so in the case of the Hen's egg (fig. 33 A)
a small furrow (b) makes its appearance in the middle of the disc,
and advances from above downward in a vertical direction. But

THE PROCESS OF CLEAVAGE.

63

whereas in the case of the Frog's egg the first plane of cleavage cuts
through to the opposite pole,' in the case of the Hen's egg it divides
only the germ-disc into two similar segments, which like two buds
rest upon the undivided yolk-mass with a broad base, by means of
which they still have a physical connection with each other. Soon after
this, there is formed a second vertical furrow, which crosses the first
at right angles, and likewise remains limited to the germ-disc, which
is now divided into four segments (fig. 33 ).

Each of the four segments is again divided into halves by a radial
furrow. The segments thus formed correspond to sectors, which
meet in the centre of the germ-disc with pointed ends, and have

5 5 c

Fig. 34. Section through the germ-disc of the Hen's egg during the later stages of segmentation

after BALFOUE.
The section, which represents rather more than half the breadth of the blastoderm (the middle

line is at c), shows that the segments of the surface and of the centre of the disc are smaller

than those below and toward the periphery. At the border they are still very large. One of

the latter is indicated at a.
<t, Large peripheral cell ; 6, larger cells of the lower layers ; c, middle line of the blastoderm ;

e, boundary between the blastoderm and the white yolk, w.

their broad ends turned toward the periphery. The apex of each of
the segments is then cut oft* by a cross furrow, i.e., by one which is
parallel to the equator of the egg (fig. 33 (7), in consequence of which
there are formed smaller central (c) and larger peripheral (d) seg-
ments. Since from this time forward radial furrows and those that
are parallel to the equator make their appearance alternately, the germ-
disc is subdivided into more and more numerous segments, which are
so arranged that the smaller lie at the centre of the disc, therefore
immediately around the animal pole, the larger toward its periphery.
With the advancing cleavage the smaller segments are entirely con-
stricted off from the underlying yolk, whereas the larger peripheral
ones still remain 'at first in continuity with it (fig. 34). In this way
we finally get a disc of small embryonic cells, which, toward the
middle, are arranged in several superposed layers.

64 EMBRYOLOGY.

The layer of yolk which immediately adjoins the periphery of the
cellular disc, and which is very finely granular and especially rich in
protoplasm, still merits particular consideration, for in it lie isolated
nuclei (fig. 35 nx 1 }, the much-discussed yolk-nuclei or parablast-nuclei
(the " merocytes " of RUCKERT). In the case of the Chick they are
less striking than in Teleosts and Selachians, in which they have
been accurately investigated by BALFOUR, HOFFMANN, RUCKERT,
and KASTSCHENKO. Formerly these were held to arise spontaneously
(free formation of nuclei) in the yolk, an assumption which in itself
is very improbable, since, according to our present knowledge, the
free formation of nuclei does nob appear to occur anywhere in

~..A 7

asisa

i?

'^^ J ^^<^^^^SS^SS^o?^g>^^^^ eo 4w'

Fig. 35. Section through the germ-disc of a Pristiums embryo during segmentation, after

BALFOUR.
n, Nucleus; nx, modified nucleus prior to division; nx', modified nucleus in the yolk; f,

furrows which appear in the yolk adjacent to the germ-disc.

either animal or vegetable kingdom. Consequently the yolk-nuclei
are now rightly held to be derived from the cleavage-nuclei. They
are probably produced even at an early period, when the first-formed
segments, which remain, as we have seen, for a long time in connection
with the yolk, begin to be constricted off from the latter. This
probably takes place in the following manner : there arise in the
segments nuclear spindles, the halves of which go into the completely
isolated embryonic cells at the time of their separation from the
yolk, while the remaining halves go into the underlying yolk-layer,
and are there converted into vesicular yolk-nuclei.

Their number subsequently increases by means of indirect division,
as is established by the fact that in sections nuclear spindles have
been observed in the yolk-layer (fig. 35 nx').

While, on the one hand, there is an increase in the number of the
yolk-nuclei, so, on the other hand, there is also a diminution in their

THE PROCESS OF CLEAVAGE. 65

number, as is asserted by several authors (WALDEYER,
BALFOUR, etc.). This takes place by the constricting off of nuclei
and surrounding protoplasm, which go to enlarge the cellular disc.
We may, with WALDEYER, designate these as secondary cleavage-cells,
and regard the whole process as a kind of supplementary segmentation.

By means of this a part of the voluminous yolk-material continues.
to be gradually individualised into cells. These annex themselves to
the border of the germ-disc, which with their aid increases in extent
and grows over a continually increasing territory of the unsegmented
yolk-sphere. In still later stages of development, long after the
cellular germ-disc has been differentiated into the germ-layers, the
supplementary segmentation continues to go on at the margin of the
disc in the neighbouring yolk-mass, and to furnish new cell-material.
Therefore the layer which encloses the yolk-nuclei forms an important
connecting link between the segmented germ and the unsegmented
nutritive yolk; I shall come back to this subject later.

The appearance of merocytes and the supplementary cleavage
which proceeds from them are phenomena which are induced by the
vast accumulation of yolk-material, and which allow the latter to be
divided up into cells, even though the process is a slow one.

The eggs of Selachians (KASTSCHENKO, RUCKERT) deviate a little
from the usual method of partial cleavage in meroblastic eggs,
and in a manner which recalls to a certain extent the processes
of superficial cleavage, which are to be treated of later. The
cleavage-nucleus, namely, is divided into two nuclei, these again
into four and even a greater number, without an accompanying
division of the germ-disc into a corresponding number of segments.
In this case, therefore, there arises at first a multinuclear proto-
plasmic mass, a plasmodium, in which the nuclei are distributed at
regular intervals. Subsequently furrows appear, generally in great
numbers and all at once, by means of which the germ-disc becomes
divided into cells from the centre to the periphery. Some of the
nuclei always remain in the periphery outside the territory of
cleavage, here undergo further division, migrate out of the germ-
disc into the surrounding nutritive yolk, and constitute the yolk-
nuclei or merocytes. These cause and maintain in the yolk for
a long time the process of supplementary cleavage.

When we institute a comparison between partial and unequal
cleavage, for the descriptions of which we have made use of the eggs
of the Hen and the Frog, it is not difficult to derive the former
from the latter, and to find a cause for the origin of the former.

5

66 EMBRYOLOGY.

It is the same as that which produced unequal cleavage from
equal cleavage; it is the great accumulation of nutritive yolk,
the inequality in the distribution of the egg-substances which
goes hand in hand with it, and the alteration in the position
of the cleavage-nucleus. The process of differentiation, which
is still in a stage of transition in the case of the Frog's egg, is
carried to an extreme in the case of the Hen's egg. Protoplasmic
substance was already abundantly accumulated at the animal pole in
the former case, but in the latter it is still more concentrated, and
at the same time has become differentiated from the nutritive yolk
as a disc enclosing the segmentation-nucleus. The yolk, accumulated
to an enormous extent at the opposite pole, is, in consequence of this
separation, relatively poor in protoplasmic substance, which only
scantily fills the interstices between the large yolk- spheres.

Inasmuch as the phenomena of motion during the process of
division emanate from the protoplasm and nucleus, whereas the
deutoplasm remains passive, the active substance in the case of mero-
blastic eggs can no longer master the passive substance and cause it to
participate in the cleavage. Even in the case of the Frog's egg a
preponderance of the animal pole during cleavage is observable ;
within its territory the nucleus lies, the radial figures of the proto-
plasm appear, and the first and second planes of division begin to
arise, whereas they cut through at the vegetative pole last of all ;
moreover the process of division during the later stages takes place
there with greater rapidity, so that a distinction arises between th.v
smaller animal cells and the larger vegetative ones. In the case of
the Hen's egg, the preponderance of the animal pole is still further
increased, and the contrast with the vegetative pole is most sharply
expressed. The cleavage-furrows not only begin there, but they
remain restricted to the territory immediately surrounding it. Thus
we get on the one hand a disc composed of small animal cells, on the
other an immense undivided yolk-mass, which corresponds to the
larger vegetative cells of the Frog's egg. The yolk-nuclei enclosed in
the periphery of the germ-disc are equivalent to the nuclei of the
vegetative cells of the Frog's egg.

IP- Partial Superficial Cleavage.

The second sub-type of partial cleavage is prevalent in the phylum
of Arthropods, and occurs in centrolecithal eggs, where a central
yolk-mass is enclosed in a cortical layer of formative yolk. Manifold

THE PROCESS OF CLEAVAGE. 67

variations are possible here, as well as transitions to equal and un-
equal cleavage. When the course pursued is quite typical, the
segmentation-nucleus, surrounded by a mantle of protoplasm, lies in
the middle of the egg in the nutritive yolk ; here it is divided into
two daughter-nuclei, without the occurrence of a corresponding division
of the egg-cell. The daughter-nuclei, in turn, undergo division into
4, these into 8, 16, 32 nuclei, etc., while the egg as a whole still
remains unsegmented. Subsequently the nuclei move apart, the
greater number gradually migrate to the surface, and penetrate into
the protoplasmic cortical layer, where they arrange themselves at
uniform distances from each other. It is only at this stage that
the process of egg-segmentation takes place, for now the cortical layer
is divided into as many cells as there are nuclei in it, while the central
yolk remains undivided. The latter is therefore suddenly enclosed in
a sac formed of small cells a blastoderm (Keimhaut). Instead of
a polar (telolecithal) yolk, we have a central (centrolecithal) yolk.
Ordinarily yolk-nuclei or merocytes remain behind in the yolk, as in
the meroblastic eggs of Vertebrates.

Now that we have become acquainted with the various forms of the
process of segmentation, it will be expedient to dwell for a moment
on its results. According as the process of cleavage takes place
by one or the other of the four methods described, there arises
a mass of cells with corresponding characteristics. From equal
segmentation there arises a spherical germ with cells approximately
uniform in size (Amphioxus, Mammals) (fig. 30, p. 56) ; from un-
equal segmentation, as well as from discoidal, there is produced a
form of the germ with polar differentiation. This manifests itself in
the first case (Cyclostomes, Amphibia) in the production of small
cells at the animal pole and large yolk-laden elements at the opposite,
vegetative pole (fig. 32 64 , p. 60). In the other case (fig. 35, p. 64)
the vegetative pole is occupied by an unsegmented yolk-mass, in
which at definite regions nuclei are found (Fishes, Reptiles, and
Birds). Finally there is developed from superficial cleavage a germ
composed of a mantle of cells, which envelops an unsegmented yolk-
mass in which also there are nuclei (Arthropods).

The multicellular germ undergoes further changes, sometimes in
the earlier stages of the cleavage-process, sometimes only in the later
stages, in that a small, fluid-filled cleavage-cavity is developed in its
centre, by the separation of the embryonic cells. At first small, this

68

EMBRYOLOGY.

mass of cells. A
or mulberry -sphere

- dz

cavity increases more and more in size, so that the surface
of the whole germ is augmented, and the cells which were at

first central come to the
surface.

Different names have been
given to the solid and to the
hollow
morula

is spoken of as long as the
segmentation-cavity is either
wanting or only slightly de-
veloped. But when a larger
cavity has been formed, as
is almost always the case

Fig. 36. Blastula of Amphioxus, after HATSCHEK. toward the end of the

h, Segmentation -cavity : oz, animal cells : dz, cells , i

with abundant yolk cleavage-process, the germ

is called a blastula or blas-

tosphere (Keimblase). The latter in turn exhibits a four-fold
variation of form, according to the abundance of yolk in the
original egg and the method of the antecedent segmentation.

In the simplest case (fig. 36) the wall of the blastula is only one
layer thick ; the cells are of uniform size and cylindrical, and are
closely united to one another
to form an epithelium (many
of the lower animals, Am-
phioxus). In the case of
lower, aquatic animals the
blastulse at this stage aban-
don the egg-envelopes, and,
since their cylindrical cells
develop cilia at the surface,
swim about with rotating
motion in the water as ciliate
spheres or blastospheres.

In eggs with unequal seg-
mentation the blastula is
ordinarily formed of several
layers of cells, as in the case of the Frog and Triton, and at
the same time it exhibits in different regions different thicknesses
(fig. 37). At the animal pole the wall is thin ; at the vegetative
pole, on the contrary, it is so much thickened that an elevation,

fig. 37. Blastula of Triton tceniatus,
f h, Segmentation-cavity; rz, marginal zone ; dz, cells
with abundant yolk.

THE PROCESS OF CLEAVAGE.

69

composed of large yolk-cells, protrudes from this side far into the
cleavage-cavity, thus considerably diminishing it.

The eggs with partial discoidal segmentation (fig. 38) are modified
most of all, and are therefore scarcely to be recognised as blastulae.
In consequence of the immense accumulation of yolk on the ventral
(vegetative) side, the cleavage-cavity (B) is extraordinarily constricted,
and is still preserved only as a narrow fissure filled with albuminous
fluid. Dorsally its wall consists of the small embryonic cells (kz) result-
ing from the process of cleavage, which are accumulated in several
superposed layers; at the surface they join each other closely,
deeper they lie more loosely associated. The floor of the cleavage-
cavity is formed of a yolk-mass, scattered through which are
to be found the
yolk-nuclei or
merocytes (die),
which likewise
result from the
cleavage-p r o c e s s.
It is to be seen
that they are espe-
cially numerous at
the place of tran-
sition from the
germ-disc to the
yolk-mass.

dk

Fig. 38. Median section through a germ-disc of Pristiurus in the

blastula stage, after RUCKERT.
S, Cavity of the blastula ; kz, segmented germ ; dk, finely granular

yolk with yolk-nuclei.

This nucleated

yolk-mass very evidently corresponds to the large vegetative cells
which constitute the floor of the cleavage-cavity in the case of the
Amphibian egg (fig. 37).

In the case of superficial cleavage there is formed, strictly speaking,
no blastula, since the place where the segmentation-cavity should be
developed is filled with nutritive yolk. The latter either remains
unsegmented or is subsequently divided, as in the Insects, into in-
dividual yolk-cells.

The investigation and right comprehension of the process of cleavage have
been attended with manifold difficulties. A voluminous literature has arisen
on this subject. We limit ourselves to pointing out the most important dis-
coveries and the chief questions which have been discussed.

The first observations on the process of segmentation were made on the
Frog's egg. Aside from short statements by SWAMMEEDAM and ROSEL VON

70 EMBRYOLOGY.

ROSENHOF, it was PREVOST ET DUMAS who were the first to describe, in 1824,
the manner in which regular farrows arise on the Frog's egg, and how by
means of these the whole surface is divided into smaller and smaller areas.
According to the French investigators, the furrows were restricted to the sur-
face of the egg. However, only a few years later, Ruscoxi (1826) and C. E.
V. BAER recognised that the furrows visible at the surface correspond to
fissures which extend through the whole mass of the yolk, and divide it into
separate parts. Even in his time VON BAEB rightly characterised the whole
process of segmentation, in which he discerned the first impulse of life, as an
automatic division of the egg-cell, but subsequently he abandoned this, the
right path, since he sought for the meaning of division in the dictum : that
" all yolk-masses are subject to the influence of the fluid and volatile
components of the fertilising material."

In the next decennary there followed numerous discoveries of the process of
segmentation in other animals. During this period acquaintance was also
gained with partial segmentation. After RTJSCONI and VOGT had seen it in
the case of fish eggs, KO"LLIKEB gave, in the year 1844, the first detailed
description of it as seen in the eggs of Cephalopods, and four years later
COSTE described it in the Hen's egg.

The question of the significance of the cleavage-process has engaged the
earnest attention of investigators, and has given rise to many controversies.
The discussion first took a definite turn upon the establishment of the cell-
theory. The question was, to determine whether and in what manner cleav-
age was a process of cell-formation. Although there were already many
observations on the division of eggs, SCHWANN himself took no definite posi-
tion on this question. The views of other investigators were at variance for
years. There was a difference of opinion as to whether the egg or the ger-
minative vesicle was a cell, whether the segments resulting from cleavage
possessed a membrane or not, and whether these segments were to be regarded
as cells or not. In the earlier literature the germinative vesicle and the
nuclei of the cleavage-spheres were often designated as embryonic cells, and
the surrounding yolk-mass as an enveloping sphere. The difficulty of com-
prehending the process of segmentation was also aggravated by the false
doctrine of free cell-formation from an organic matrix the cytoblastema
founded by ScHWANX. It remained for a long time a controverted point
whether the tissue-cells of the adult organism were the direct descendants of
the segmentation-spheres, or whether they arose at a later period by means
of free cell-formation from cytoblastema. After NAGELI on the botanical
side had adopted the right course, it was the service of KOLLIKEE, EEICHEET,
REMAK, and LEYDIG to have paved the way to a comprehension of cleavage,
and to have shown that free cell-formation does not take place, but that all
cellular elements arise in uninterrupted sequence from the egg-cell.

As far as regards the different kinds of cleavage, KO'LLIKEE designated
them as total and partial. VAN BENEDEN has given in his " Recherches sur
la composition et la signification de 1'ceuf " a more exhaustive review of the
subject, and has also expounded in a clear way the signification of the
deutoplasm for the different kinds of cleavage. Subsequently HAECKEL mate-
rially simplified the categories of segmentation recognised by VAN BENEDEN,
and proposed in bis " Anthropogenic " and in his paper " Die Gastrula und die
Eif urchung " the classification of the methods of cleavage on which is based
the scheme previously given, and according to which total cleavage is divided

THE PROCESS OF CLEAVAGE. 71

into equal and unequal, and partial into discoidal and superficial. At the
same time HAECKEL endeavoured to derive the different methods of cleavage
from one another, and apropos of this directed attention to the important role
of the nutritive yolk.

The processes which take place within the yolk have eluded observation
and a correct interpretation even more than the external phenomena of cleav-
age, so that it is only in the most recent times that we have acquired a satis-
factory insight into them. It is true that the problem, as to what part the
nucleus plays in segmentation, has had the uninterrupted attention of investi-
gators, but without any solution having been found. For years there were in
the literature two opposing views : sometimes one of them, sometimes the
other, attained temporarily greater currency. According to one view which
was almost universally adopted by the botanists, and was defended on the
zoological side principally by REICHERT, and even recently by AUERBACH
the nucleus disappears before every division, and is dissolved, to be afterwards
formed anew in each daughter-segment; according to the other view the
nucleus, on the contrary, is not dissolved, but is constricted, becomes
dumb-bell-shaped, and is divided into halves, and thereby induces cell-division.
This view was taught especially by such zoologists and anatomists as C. E.
v. BAER, JOH. MULLER, KO'LLIKER, LEYDIG, GEGENBAUR, HAECKEL, VAN
BENEDEN, and others, who were supported by the observations which they
had made on transparent eggs of the lower animals.

Light was first thrown on the disputed question at the moment when suit-
able objects were studied with the aid of higher magnifications, and especially
with the employment of modern methods of preparation (fixing and staining
reagents).

The works of FOL, FLEMMING, SCHNEIDER, and AUERBACH on the cleavage
of the eggs of various animals mark a noteworthy advance. They still main-
tained, it is true, that the nucleus is dissolved at the time of cleavage, but they
gave a detailed and accurate description of the striking radiation which arises
in the yolk upon the disappearance of the nucleus, and which during the
constriction of the egg soon becomes visible in the region of the daughter-
nuclei.* SCHNEIDER observed parts of the spindle-stage.

Soon after this a more exact insight into the complicated and peculiar
nuclear changes was obtained by means of three investigations, which were
carried out independently and simultaneously on different objects, and were
published in rapid succession by BUTSCHLI, STRASBURGER, and the author.
It was definitely established by these observations that there is no dissolution
of the nucleus at the time of division, but a metamorphosis, such as has been
described in the preceding pages. At the same time I likewise proved that the
egg-nucleus is not a new formation, but is derived from parts of the germinative
vesicle. From this resulted the important doctrine that, just as ill cells, so also
all nuclei of the animal organism are derivatives in an uninterrupted sequence,
the one from the egg-cell and the other from its nucleus. (Omnis cellula e cellula,
omnis nucleus e nucleo.) Through these researches there was furnished for the

* Eadiating structures had already been observed in the yolk before this,
but in an incomplete manner, by different authors by GRUBE in the Hiru-
dinea, by DERBES and MEISSNER in the Sea-urchin, by GEGENBAUR in Sagitta,
by KROHN, KOWALEVSKY, and KUPPFER in Ascidians, by LEUCKART in Nema-
todes, by BALBIANI in Spiders, and by OELLACHER in the Trout.

72 EMBRYOLOGY.

first time a scheme of nuclear division and cell-division, which has since
proved to be correct in all essentials, even though it has undergone important
improvements and additions at the hands of FOL, FLBMMING, VAN BENEDEN,
and RABL.

FOL published an extended monographic investigation of the process of
cleavage, which he had observed in many invertebrated animals. FLEMMING,
starting with nuclear division in tissue-cells, distinguished with great acumen
the non-chromatic and the chromatic parts of the nuclear figure, the non-
stainable nuclear spindle-fibres, and the stainable nuclear filaments and loops,
which are located upon the surface of the former. He made the interesting
discovery concerning the latter, that they become split lengthwise. Ligbt
was soon thrown upon this peculiar phenomenon, when HEUSER, VAN BENEDEN,
and BABL, independently of each other, discovered that the halves of the split
filaments moved apart toward the poles of the nucleus, and furnished the
fundament for the daughter-nuclei. VAN BENEDEN at the same time made
the additional and important observation on the egg of Ascaris megalocephala,
that of the four chromatic loops, which are constantly to be observed in the
case of the cleavage-nucleus, two are derived from the chromatic substance
of the spermatic nucleus, the other two from the chromatic substance of the
egg-nucleus ; and that, in consequence of the longitudinal splitting, each
daughter-nucleus receives at the time of division two male and two female
nuclear loops. In addition there have appeared many other recent works
of value on the process of cleavage by NUSSBAUM, KABL, CAHNOY, BOVEEI,
PLAINER, and others.

Within the last few years PFLtrGER has endeavored to prove by interesting
experiments that gravitation exercises a determining influence on the position
of the planes of cleavage. BORN, Roux, and the author, on the contrary,
thought they were able to explain division from the organisation of the egg-
cell itself. In the author's article, " Welchen Einfluss iibt die Schwerkraft
auf die Theilung der Zellen ? " he recognised the causes which determine the
various directions of the planes of division, (1) in the distribution of the
lighter egg-plasm and the heavier deutoplasm, and (2) in the influence which
the spatial arrangement of the egg-plasm exercises on the position of the
nuclear spindle, and that which the position of the latter exercises upon the
direction of the plane of cleavage.

SUMMARY.

1. In the process of cleavage the internal and the external pheno-
mena of segmentation are to be distinguished from each other.

2. The internal phenomena of cleavage find expression in changes
(a) of the nucleus, (6) of the protoplasm.

3. The nucleus while in the process of division consists of a non-
chromatic and a chromatic nuclear figure. The non-chromatic figure
is a spindle composed of numerous fibres. The chromatic figure is
formed of bent, V-shaped nuclear filaments (chromosomes), which lie
upon the surface of the middle of the spindle. At the two ends of
the spindle there is found a special polar corpuscle [centrosome].

THE PROCESS OF CLEAVAGE. 73

4. The division of the nucleus takes place in the following manner :
the nuclear filaments split lengthwise, and their halves move apart
in opposite directions toward the ends of the spindle, and are there
converted into vesicular daughter-nuclei.

5. The protoplasm arranges itself around the ends of the spindle
in filaments having the form of a stellate figure (an aster), so that
a double radiation or an amphiaster arises in the egg.

6. The external phenomena of cleavage consist in the division of
the egg-contents into individual parts, the number of which corre-
sponds to that of the daughter-nuclei. They exhibit various modifica-
tions, which are dependent on the arrangement and distribution of
the egg-plasm and the deutopiasm, as is to be seen from the fol-
lowing scheme of segmentation.

Scheme of the Various Modifications of the Process

of Cleavage.
I. Total Cleavage. (Holoblastic eggs.)

The eggs, which for the most part are small, contain a small or
moderate amount of deutopiasm, and are completely divided into
daughter-cells.

1. Equal Cleavage.

This takes place in eggs with meagre and uniformly distributed
deutopiasm (alecithal). By the process of cleavage there are formed
segments which, in general, are of uniform size. (Amphioxus, Mam-
malia.)

2. Unequal Cleavage.

This occurs in eggs in which a more abundant deutopiasm is un-
equally distributed, being concentrated toward the vegetative pole,
and in which the cleavage-nucleus is located nearer the animal and
more protoplasmic pole. Usually the segments become unequal in
size only with and after the third act of division. (Cyclostomes,
Amphibia.)

n. Partial Cleavage. (Meroblastic eggs.)

The eggs, which are often very large, ordinarily contain con-
siderable quantities of deutopiasm. In consequence of the unequal
distribution of this, the egg-contents are separated into a formative
yolk, in which alone the process of cleavage is manifested, and a
nutritive yolk, which remains undivided, and is used up during
embryonic development for the growth of the organs.

74 EMBRYOLOGY.

1. Discoidal Cleavage.

This takes place in eggs with nutritive yolk in a polar position
The process of cleavage remains confined to the formative yolk
accumulated at the animal pole, which has the form of a disc and
contains only a small amount of deutoplasm. There is formed, con-
sequently, a cellular disc. (Fishes, Reptiles, Birds.)

2. Superficial Cleavage.

This occurs in the case of eggs with central yolk. In typical
cases the nucleus alone, which occupies the middle of the egg, under-
goes repeated division. The numerous daughter-nuclei which arise
in this manner migrate into the layer of protoplasm which invests
the central nutritive yolk, and the protoplasm is thereupon divided
into as many segments as there are nuclei lying in it. There is
formed a germ-membrane (Keimhaut). (Arthropods.)

7. Eggs with total cleavage are designated as holoblastic, eggs
with partial cleavage as meroblastic.

8. The direction and position of the first cleavage-plane are strictly
conformable to laws which are founded in the organisation of the
cell ; they are determined by the following three factors :

First factor. The cleavage-plane always divides the axis of the
nucleus which is preparing for division perpendicularly at its middle.

Second factor. The position of the axis of the nucleus during
division is dependent upon the form and differentiation of the en-
veloping protoplasm.

In a protoplasmic sphere the axis of the nuclear spindle, occupying
the centre of the sphere, can lie in the direction of any radius what-
ever ; but in an oval protoplasmic body, only in the longest diameter.
In a circular disc the nuclear axis lies parallel to its surface in
any diameter of the circle, but in an oval disc only in the longest
diameter.

Third factor. In the case of eggs of unequal segmentation, which,
in consequence of their unequally distributed, polar deutoplasm,
are geocentric, and therefore assume when in equilibrium a parti-
cular position, the first two planes of cleavage must be vertical, and
the third must be horizontal and placed above the equator of the
sphere.

LITERATURE. 75

LITERArURE.

In addition to the writings cited in the second chapter see :

Auerbach. Organologische Studien. Heft I. und Heft II. Breslau 1874.
Baer, C. E. von. Die Metamorphose des Eies der Batrachier. Miiller

Archiv. 1834.
Born, G. Ueber die Furchung des Eies bei Doppelbildungen. Breslauer

avztl. Zeitschr. 1887. Nr. 15.
Coste. Histoire generate et particuliere du developpement des corps organises.

18471859.
Flemming. Ueber die ersten Entwicklungserscheinungen am Ei der Teich-

muschel. Archiv f. mikr. Anat. Bd. X. p. 257. 1874.
Flemming. Beitrage zur Kenntniss der Zelle und ihrer Lebenserscheinungen.

Archiv f. mikr. Anat. Bd. XVI. p. 302. 1878.
Flemming. Neue Beitrage zur Kenntniss der Zelle. Archiv f. mikr. Anat.

Bd. XXIX. p. 389. 1887.
Fol, H. Die erste EntwicklungdesGeryonideneies. Jena. Zeitschr. Bd. VII.

1873.
Fol, H. Sur le developpement des Pteropodes. Archives de Zoologie exper

et gen. T. IV. and V. 1875-76.

Gasser. Eierstocksei u. Eileiterei des Vogels. Marburger Sitzungsb. 1884.
Haeckel, E. Die Gastrula und Eifurchung. Jena. Zeitschr. Bd. IX. 1875.
Heape, Walter. The Development of the Mole, the Ovarian Ovum, and

Segmentation of the Ovum. Quart. Jour. Micr. Sci. Vol. XXVI. pp. 157

174. Vol. XXVII. pp. 123-63. 1886.

Kblliker. Entwicklungsgeschichte der Cephalopoden. Zurich 1844.
Leydig, Fr. Die Dotterfurchung nach ihrem Vorkommen in d. Thierwelt

und nach ihrer Bedeutung. Oken's Isis. 1848.
Pfliiger, E. Ueber den Einfluss der Schwerkraf t auf die Theilung der Zellen.

Arch. f. d. ges. Physiol. Bd. XXXI. p. 311. 1883.
Pfliiger, E. 2. Abhandlung. Bd. XXXII. pp. 1-71. 1883.
Prevost et Dumas. 2me Mem. sur la Generation. Ann. des sci. nat.

T. II. pp. 100, 129. 1824.

Rabl. Ueber Zelltheilung. Morphol. Jahrb. Bd. X. p. 214. 1885.
Rauber, A. Furchung u. Achsenbildung bei Wirbelthieren. Zool. Anzeiger,

p. 461. 1883.
Rauber, A. Schwerkraftversnche an Forelleneiern. Berichte der naturf.

Gesellsch. zu Leipzig. 1884.
Reichert. Der Furchungsprocess und die sogenannte Zellenbildung urn

Inhaltsportionen. Miiller's Archiv. 1846.
Remak. Sur le developpement des animaux vertebras. Comptes rendus.

T. XXXV. p. 341. 1852.
Roux. Ueber die Zeit der Bestimmung der Hauptrichtungen des Frosch-

embryo. Leipzig 1883.

Roux. Ueber die Bedeutung der Kerntheilungsfiguren. Leipzig 1883.
Roux. Beitrage zur Entwicklungsmechanik des Embryo. Nr. 4. Archiv f.

mikr. Anat. Bd. XXIX. p. 157. 1887.
Roux. Die Entwicklungsmechanik der Organismen, eine anatomische Wis-

senschaft der Zukunft. Wien 1890.
Rusconi. Sur le developpement de la grenouille. Milan 1828.

76 EMBRYOLOGY.

Salensky, W. Befruchtung und Furchung des Sterlet-Eies. Zool. Anzeiger.

Nr. 11. 1878.
Sarasin, C. F. Reifung u. Furchung des Rcptilieneies. Arbeiten a. d.

zool.-zoot. Inst. Wurzburg. Bd. VI. p. 159. 1883.
Schneider. Untersuchungen iiber Plathelminthen. Jahrb. d. oberhessischen

Gesellsch. f. Natur- u. Heilkunde. 1873.
Strasburger. Zellbildung und Zelltheilung. 3. Aufl. Jena 1875.

CHAPTER IV.

GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOP-
MENT.

A SIMPLE principle has exclusively controlled the embryonic pro-
cesses hitherto considered. By means of the cleavage of the egg-
substance, or cell-division, alone the originally simple elementary
organism has been converted into a cell-colony. This presents the
simplest conceivable form, inasmuch as it is a hollow sphere, the
wall of which is composed of one or several layers of epithelial cells.
But the principle of cell-division is not adequate for the production,
out of this simple organism, of more complicated forms with dissimilar
organs, such as the adult animals are ; further progress in develop-
ment can be brought about from this time forward only by the
supervention of two other principles, which are likewise simple ;
namely, the principle of unequal growth in a cell-membrane, and
the principle of the division of labour, together with the histological
differentiation connected with it.

Let us consider first the principle of unequal growth. When in a
cell-membrane the individual elements continue to divide uniformly,
the result will be either a thickening or an increase in the surface of
the membrane. The former takes place when the plane of division
has the same direction as the surface of the membrane, the latter
when it is perpendicular to the surface. With the increase in the
extent of surface the cells which were at first present are uniformly
and gradually crowded apart by the introduction of the new daughter-
cells, inasmuch as they are soft and plastic, and are joined together
only by means of a soft cementing substance. Were we to assume
that only such a growth took place in the case of the blastula during
its further development, nothing else could come of it except an ever
larger and thicker- walled hollow sphere of cells.

GENERAL DISCUSSION OF THE PRINCIPLES OP DEVELOPMENT. 77

The operation of an unequal growth of the surface produces quite
another result. When in the middle of a membrane the cells of a
single group within a short time repeatedly undergo " division " by
vertical planes, they will be suddenly compelled to claim for themselves
much greater surface, and they will consequently exert a vigorous
pressure, due to growth, upon the cells in their vicinity, and will
tend to push them apart. But in this case a separation of contiguous
cells, such as takes place with gradual and uniformly distributed
interstitial growth, will be impossible ; for the surrounding cells,
remaining in a passive condition, will constitute, as it were, a rigid
frame, as His has expressed it, around the extending part, which, in
consequence of accelerated growth, demands an increased area. It
must therefore secure room for itself in another manner, and increase
its surface by abandoning the level of the passive part through
the formation of a fold in either one direction or the other. The
fold will be still further increased, and forced farther from the
original level, if the increased activity of the process of cell-division
in it continues. Thus by means of unequal growth there has now
arisen out of the originally uniform membrane a new recognisable
part, or a special organ.

When the folding membrane encloses a cavity, as is the case with
the blastula, there are two cases conceivable in the formation of folds.
In the first place, the membrane may be folded into the interior of
the body, a process which in embryology is called invagination or
involution. Secondly, there may arise by evagination a fold, which
projects free beyond the surface of the body.

In the first case numerous variations in the details are possible, so
that the most various organs, as, e.g., the glands of the animal body,
parts of the sensory organs, the central nervous system, etc., are
formed.

In the origin of glands a small circumscribed circular part of a
cellular membrane is infolded as a hollow cylinder (fig. 39 1 and 4 ),
towards the interior of the body, into the underlying tissue, and by
continuous growth may attain considerable length. The invagina-
tion develops into either the tubular or the alveolar form of gland
(FLEMMING). If the glandular sac possesses from its mouth to its
blind end nearly uniform dimensions, we have the simple tubular
gland (fig. 39 l ), the sweat glands of the skin, LIEBERKUHN'S glands
of the intestine. The alveolar form of gland differs from this in that
the invaginated sac does not simply increase in length, but expands
somewhat at its end (fig. 39 5 , db), while the other part remains

78

E5IBBYOLOGY.

narrow and tube-like and serves as its duct (a). More complicated

forms of glands arise, when the same processes to which the simple

glandular sac owes its origin are repeated on the wall of the sac

12 845 e when on a small

tract of it a more
vigorous growth
again takes place,
and a part begins
to grow out from
the main tube as a
lateral branch (fig.
39 2 and 6 ). By
numerous repetitions
of such evaginations,
the originally simple
tubular gland may
acquire the form of
a much - branched
tree, upon which we

Fig. 39, Diagram of the formation ->f glands.

1, Simple tubular gland; 2, branched tubular gland; 3,

branched tubular gland with anastomosing branches ;

4 and 5, simple alveolar glands ; a, duct ; db, vesicular

enlargement ; 6, branching alveolar gland.

distinguish tha part

formed first as trunk, and the parts which have arisen by outgrowths
from it as chief branches and branchlets of first, second third and
fourth order, according to their ages and correlated sizes. According
as the lateral outgrowths remain tubular or become enlarged at their
tips, there arise either the compound tubular
glands (fig. 39 2 ) (kidney, testis, liver), or the a b

compound alveolar glands (fig. 39 6 ) (sebaceous
glands of the skin, lungs, etc.).

Again, the invaginating part of an originally
flat membrane assumes other forms in the pro-
duction of sense organs and the central nervous
system. For example, the part of the organ of
hearing which bears the nerve terminations
the membranous labyrinth is developed out of
a small tract of the surface of the body, which
becomes depressed into a small pit (fig. 40) in
consequence of its acquiring an extraordinary
vigor in growth. The edges of the auditory
pit then grow toward one another, so that this is gradually con-
verted into a little sac, which still opens out at the surface of the
body by means of a narrow orifice only (fig. 40 a). Finally, the

Fig. 40. Diagram of the
formation of the audi-
tory vesicle.

a, Auditory pit ; 6, audi-
tory vesicle, which has
arisen by a process of
constriction, and still
remains connected with
the outer germ-layer by
means of a solid stalk
of epithelium.

GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 79

narrow orifice closes. Out of the auditory pit there has arisen a
closed auditory sac (6), which then detaches itself completely from its
parent tissue, the epithelium of the surface of the body. Afterwards,
simply by means of the unequal growth of its different regions, by
means of constrictions and various evaginations, it acquires such an
extraordinarily complicated form, that it has justly received the
name of membranous labyrinth, as will be shown in detail in another
chapter.

The development of the central nervous system may serve as
the last example of invagination. Spinal cord and brain take their
origin at an early epoch from the layer of epithelial cells which limits
the outer surface of the body of the embryo. A narrow band of this
epithelium lying along the axis of the back becomes thickened, and is
distinguished from the thinner part of the epithelium, which produces
the epidermis, as the medullary plate (fig. 41 A mp). Inasmuch as
the plate grows more rapidly than its surroundings, it becomes in-
folded into a gutter which is at first shallow, the medullary groove.
This becomes deeper as a result of further increase of substance. At
the same time the edges (fig. 41 B mf), which form the transition
from the curved medullary plate to the thinner part of the cellular
membrane, become slightly elevated above the surrounding parts, and
constitute the so-called medullary folds. Subsequently these grow
toward each other, and become so apposed that the furrow becomes
a tube, which still remains temporarily open to the outside by means
of a narrow longitudinal fissure. Finally, this fissure also disappears
(fig. 41 C) ; the edges of the folds grow together ; the closed medullary
tube (n), like the auditory vesicle, then detaches itself completely
along the line of fusion (suture) of the cell-membranes of which it
was originally a component part and becomes an entirely independent
organ ().

Let us now examine somewhat more closely the mechanism of the
fusion and detachment of the neural tube.

The two medullary folds are each composed of two layers, which
are continuous with each other at the edge of the fold, the thicker
medullary plate (mp), which lines the furrow or tube, and the thin-
ner epidermis (ep), which has either a more lateral or a more super-
ficial position. When, now, the folds come into contact, they fuse,
not only along a narrow edge, but over so extensive a tract that
epidermis is joined to epidermis, and that the edges of the medullary
plate are joined to each other. The medullary tube thus formed,
and the continuous sheet of epidermis that stretches across it, are by

80

EMBRYOLOGY.

means of an intermediary cell-mass still in continuity along the suture
produced by the concrescence. But a separation soon takes place

Fig. 41 Cross sections through the dorsal halves of three Triton larvae.

A, Cross section through an egg in which the medullary folds (tnf) begin to appear.

B, Cross section through an egg whose medullary furrow is nearly closed.

C, Cross section through an egg with closed neural tube and well-developed primitive segments.
mf, Medullary folds ; mp, medullary plate ; n, neural tube (spinal cord) ; ch, chorda ;

ep, epidermis, or corneal layer ; mk, middle germ-layer ; nut 1 , parietal, mV, visceral sub-
division of the middle germ-layer ; i'k, inner germ-layer ; ush, cavity of primitive segment.

along this line, inasmuch as the intermediary band of substance
becomes narrower and narrower, and one part of it unites with the-

GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 81

epidermis, while the other part is annexed to the medullary tube. Thus
in the formation of the suture processes of fusion and of separation
occur almost simultaneously, a condition which often recurs in the
case of other invaginations, as in the constricting off of the auditory
vesicle, the vesicle of the lens, etc.

The neural tube having once become independent is subsequently
segmented in manifold ways by the formation of foldings, in conse-
quence of inequalities in the rate of surface growth, especially in its
anterior enlarged portion, which becomes the brain. There are
formed out of this by means of four constrictions five brain-vesicles,
which lie in succession one after
another ; and of these the most an-
terior, which becomes the cerebrum
with its complicated furrows and con-
volutions of first, second, and third
order, serves as a classical example
when one desires to show how a
highly differentiated organ with com-
plicated morphological conditions may
originate by the simple process of
folding.

In addition to invagination the second
method in the formation of folds,
which depends upon a process of eva-
yination, plays a no less important
part in the determination of the
form of animal bodies, giving rise to
protuberances of the surface of the body, which may likewise
assume various forms (fig. 42). As a result of exuberant growths
of small circular territories of a cell-membrane there arise rod-
like elevations, resembling the papillae on the mucous membrane
of the tongue (c), or the fine villi (a) in the small intestine (villi
intestinales), which are so closely set that they give a velvety ap-
pearance to the surface of the mucous membrane of the intestine.
Just as the tubular glands may be abundantly branched, so tufted
villi are here and there developed out of simple villi, since local
accelerations of growth cause the budding-out of lateral branches of
a second, third, and fourth order (fig. 42 6). We recall the external
tufted gills of various larvae of Fishes and Amphibia, which project
out from the neck-region free into the water, or the villi of the
chorion in Mammals, which are characterised by still more numerous

6

Fig. 42. Diagram of the formation of

papillae and villi.
a, Simple papilla ; b, branched papilla

or tufted villus ; c, simple papilla,

the connective-tissue core of which

runs out into three points.

82 EMBRYOLOGY.

branchings. The formation of the limbs is also referable to such
n process of external budding.

When the growth of the membrane takes place along a line,
the free edges form ridges or folds directed outward, such as
the valves of KERKRING or the gill-plates on the gill-arches of
Fishes.

From the examples cited it is clearly to be seen how the greatest
variety of forms may be attained by the simple means of invagina-
tion and evagination alone. At the same time, the forms may be
modified by two processes of subordinate importance, by separations
and by fusions which affect the cell-layers. Vesicular and sac-like
cavities acquire openings by the thinning out of the wall at a place
where the vesicle or sac lies near the surface of the body, until there
is a breaking through of the separating partition. Thus in the
originally closed intestinal tube of Vertebrates there are formed the
mouth-opening and the anal opening, as well as the gill-clefts in
the neck-region.

The opposite process fusion is still more frequently to be
observed. It allows of a greater number of variations. We have
already seen how the edges of an invagination may come in
contact and fuse, as in the development of the auditory vesicle,
the intestinal canal, and the neural tube. But concrescence may
also take place over a greater extent of surface, when the facing sur-
faces of an invaginated membrane come more or less completely into
contact, and so unite with each other as to form a single cell-mem-
brane. Such a result ensues, for example, in the closure of the
embryonic gill-clefts, in the formation of the three semicircular
canals of the membranous labyrinth of the ear, or, as a pathological
process, in the concrescence of the surfaces of contact of serous
cavities. Moreover fusions may take place between sacs which come
in contact with their blind ends, as very often occurs in the com-
pound tubular glands (fig. 39 3 ). Of the numerous lateral branches
which sprout out from the tubule of a gland, some come in contact
at their ends with neighboring branches, fuse with them, and
establish an open communication with them by the giving way
of the cells at the place of contact. It is by this means that
branched forms of tubular glands pass into the net-like forms to
which the testis and the liver of Man belong.

In addition to the formation of folds in epithelial layers, which
under a great variety of modifications determine in general the
organisation of the animal body, there were mentioned, as a second

GENERAL DISCUSSION OF THE PRINCIPLES OF DEVELOPMENT. 83

developmental principle of fundamental significance, division of labor
and the histological differentiation associated with it. In order to
understand fully the significance of this principle in development,
we must proceed from the thesis that the life of all organic bodies
expresses itself in a series of various duties or functions. Organisms
take to themselves substances from without ; they incorporate in their
bodies that which is serviceable, and eliminate that which is not
(function of nutrition and metastasis) ; they can alter the form of
their bodies by contraction and extension (function of motion) ; they
are capable of reacting upon external stimuli (function of sensibility) ;
they possess the ability to bring forth new organisms of their own
kind (function of reproduction). In the lowest multicellular organisms
each of the individual parts discharges in the same manner as the
others the enumerated functions necessary for organic lif e ; but the
more highly an organism is developed, the more do we see that its
individual cells differentiate themselves for the duties of life, that
some assume the function of nutrition, others that of motion, others
that of sensibility, and still others that of reproduction, and that with
this division of labor is likewise joined a greater degree of com-
pleteness in the execution of the individual functions. The
development of a specialised duty likewise leads invariably to an
altered appearance of the cell : with the physiological division of
labor there always goes hand-in-hand a morphological or histological
differentiation.

Elementary parts which are especially concerned in the duties of
nutrition are distinguished as gland-cells ; again others, which have
developed the power of contractility to a greater extent, have
become muscle-cells, others nerve-cells, others sexual cells, etc. The
cells which are concerned in one and the same duty are for the most
part associated in groups, and constitute a special tissue.

Thus the study of the embryology of an organism embraces chiefly
two elements : one is the study of the development of form, the
second the study of histological differentiation. We may at the
same time add that in the case of the higher organisms the morpho-
logical changes are accomplished principally in the earlier stages of
development, and that the histological differentiation takes place in
the final stages.

A knowledge of these leading principles will materially facilitate
the comprehension of the further processes of development.

84

EMBRYOLOGY.

CHAPTER V.

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS.
(GASTR32A-THEORY.)

THE advances which are brought about during the next stages in
the development of the blastula depend primarily upon processes of
folding. By these means there arise larval forms, which are at first
composed of two, and afterwards of four epithelial membranes, or
germ-layers.

The larval form which is composed of two germ-layers is catted the
gastrula. It possesses an important developmental signification,
because, as HAECKEL has shown in his celebrated Gastrsea-Theory,
it is to be found in each of the six chief branches of the animal
kingdom, and thus furnishes a common starting-point from which
along diverging lines the separate animal forms may be derived.
As with blastulae, so in the case of the gastrula four different
kinds can be distinguished, according to the abundance and the
method of distribution of the yolk. Starting from a simple funda-
mental form, three further modifications have arisen, all of which,

with the exception
of a single one which
is characteristic of
many Arthropods,
are to be encoun-
tered within the
phylum of Verte-
brates.

The simplest and
most primitive form,
with the considera-
tion of which we
have to begin, is
found only in the
development of Am-
phioxus lanceolatus.
As has been previously shown, its blastula is composed of cylin-
drical cells, which are closely joined into a single-layered epithelium
(fig. 43). At one place, which may be designated as the vegetative pole

Fig. 43. Blastula of Amphioxus lanceolatus, after HATSCBTEK.
fh, Cleavage-cavity ; az, animal cells ; v z, vegetative cells ;
AP, animal pole ; VP, vegetative pole.

DEVELOPMENT OP THE TWO PRIMARY GERM-LAYERS.

85

ilc

ud

fig. 44. Gastrula of Amphioxus lanceolatns, after

HATSCHEK.
at, Outer germ-layer ; ik, inner germ-layer ; w,

blastopore, or mouth of archenteron (ud).

(FP), the cells (vz) are somewhat larger and more turbid, owing to
the yolk-granules lodged in them. The process of the formation of
the gastrula commences at this place. The vegetative surface begins
at first to be flattened, and
then to be pushed in toward
the middle of the sphere.
By the advance of the
invagination the depression
grows deeper and deeper,
while the cleavage-cavity be-
comes to the same degree
diminished in size. Finally,
the invaginated portion (fig.
44 ik) comes in contact with
the inner surface of the un-
invaginated portion (ak) of
the blastula, and completely
obliterates the cleavage-
cavity. As a result there has been formed out of the hollow
sphere with a single wall a cup-shaped germ with double walls
the gastrula.

The cavity of the gastrula, which results from the invagination and
is not to be confounded with the cleavage-cavity which it has sup-
planted, is the primitive intestine (archenteron) (ud), or the intestino-
body cavity (coelenteron). This opens to the outside through the
primitive mouth (mouth of the archenteron, blastopore) (u).

Inasmuch as the names primitive intestine and primitive mouth
might easily give rise to erroneous conceptions, let it be remarked, in
order to preclude from the start such an event, that the cavity and
its external opening which arise by this first invagination are not
equivalent to the intestine and mouth of the adult animal. The
archenteron of the germ, it is true, furnishes the fundament for the
intestinal tube, but there are also formed out of it a number of other
organs, the chief of which are the subsequently formed thoracic and
abdominal cavities. The future destination of the cavity will there-
fore be better expressed by the term " ctelenteron" Finally, the
primitive mouth is only an evanescent structure among vertebrated
animals; later it is closed and disappears without leaving a trace, while
the permanent or secondary mouth is an entirely new structure.

The two cell-layers of the cup, which are continuous with each
other at the edge of the blastopore, are called the two primary

86 EM.SIIYOLOGY.

germ-layers, and are distinguished according to their positions as the
outer (ak) and the inner (ik). Whereas in the blastula the individual
cells diifer only a little from one another, with the process of gastru-
lation a division ot labor begins to assert itself, a fact which may
be recognised in the case of the free-swimming larvae of Inver-
tebrates. The outer germ-layer (ak) (also called ectoblast or ectoderm)
serves as a covering for the body, is at the same time the organ of
sensation, and effects locomotion when cilia are developed from the
cells, as is the case with Amphioxus. The inner germ-layer (ik)
(entoblast or entoderm) lines the coelenteron and provides for nutri-
tion. The cell-layers thus stand in contrast to each other both as
regards position and function, since each has assumed a special duty.
In view of this fact they have been designated by C. E. VON BAER
as the two primitive organs of the animal body. They present us
with a very instructive, because very simple, illustration of the
manner in which two organs originate from a single fundament.
By invagination the undifferentiated cells of the surface of the
blastula are brought into different relations to the outer world, and
have consequently been compelled to follow different courses in their
development, and to adapt themselves to special duties corresponding
to the new relations.

The separation of the embryonic cell-material into the two primi-
tive organs of VON BAER is of decisive significance for the whole
subsequent course of the development of the individual cells. For a
very definite portion of all the ultimate organs of the body is refer-
able to each of the two primitive organs. In order to put this im-
portant condition in the proper light at once, let it be stated that the
outer germ-layer furnishes the epithelial covering of the body, the
epidermis with the glands and hair, the fundament of the nervous
system, and that part of the sense organs which is functionally most
important. On this account the older embryologists imposed upon it
the name of dermo-sensory layer. The inner germ-layer, on the
contrary, is converted into the remaining organs of the body into
the intestine with its glands, into the body-cavity, into the muscles,
etc. ; by far the greater mass of the body, therefore, is differentiated
out of it, and it has to pass through the most numerous and the most
trenchant metamorphoses.*

* The practice of distinguishing the outer and the inner germ-layers as animal
and vegetative, which was formerly in vogue and is followed even now, is not
proper, and ought therefore to be given up. For the transversely striped muscu-
lature of the body, which belongs to its animal organs, does not arise from

DEVELOPMENT OF THE TWO PRIMARY GERM -LAYERS.

87

Larval forms quite like that of Amphioxus have also been observed
in the case of Invertebrates belonging to the phyla of Ccelenterata,
Echinodermata, Yermes, and Brachiopoda. For the most part they
quit the egg-envelope, even in the gastrula stage, to swim about in
the water by means of their cilia ; and they can now take nutritive
substances small infusoria, algae, or remnants of larger animals
through the primitive mouth
into the digestive cavity, and
make use of them in the fur-
ther growth of their bodies.
Likewise the substances
which are not serviceable be-
cause indigestible are ejected
from the body through the
same orifice. In the case
of the higher animals the
ingestion of food is not only
impossible at this time, but
also superfluous, because the
egg and the embryonic cells
arising from it still contain
yolk-granule?, which are
gradually consumed.

The modifications which gastrulation undergoes in the Amphibia are
easily referable to the simpler conditions in Amphioxus. In the case
of the Water-Salamander, which is to serve as an illustration in
this description, one half of the blastula (fig. 45), which is called
the animal half, is thin-walled and composed of small cells, which
lie in two or three layers one above another, and in the case of
the Frog contain black pigment. The other, or vegetative half (dz),
exhibits a greatly thickened wall, composed of much larger, more
deutoplasmic, polygonal cells (dz), which, loosely associated in several
layers, cause a protuberance into the cavity (fK) of the blastula,
which is proportionally diminished in size. Where the differentiated
halves meet, a transition is effected by means of cells, forming what
GOETTE has designated marginal zone (rz). Inasmuch as the specific
gravity of the animal half is much less than that of the opposite
half, it is without exception directed upward in water. The former

the outer germ-layer, as, in consequence of false observations, was formerly
believed, but rather from the primary inner germ-layer, as has now been esta-
blished by many observations.

Tig, 45. Blastula of Triton teniatus.
/A, Cleavage-cavity ; dz, yolk-cells ; rz, marginal
zone.

88

EMBRYOLOGY.

fig. 46. "Egg of Triton, which is
developing into a gastrula, seen
from the surface.

tt, Primitive mouth (blastopore).

constitutes the thinner roof, the latter the highly thickened floor, of
the excentrically placed cleavage-cavity.

When the gastrula begins to be developed, the invagination
takes place on one side in the marginal zone (fig. 46 u), and is

distinguishable externally by means of
a sharp, afterwards horseshoe-shaped
furrow, which is bounded on one side
by small cells, which in the case of
the Frog contain black pigment, on
the other side by large unpigmented
elements. At the fissure-like blasto-
pore there are infolded into the interior
of the blastula (fig. 47 u) along its
dorsal lip (dl) small cells, along its
ventral lip (vl) the large deutoplasmio
elements of the vegetative half; the

former constitute the roof, the latter the floor, of the coelenteron (ud).
The latter appears in the first stages of the invagination simply as
a narrow fissure alongside the capacious cleavage-cavity (fJi) ; soon,
however, it causes a com-
plete obliteration of this
cavity, the f undus of the
invagination becoming
enlarged into a broad
sac, while the entrance
always remains narrow
and fissure-like. Since
the coelenteron of the
Amphibia was first ob-
served by the Italian
investigator, RUSCONI, it
is ordinarily mentioned
in the older writings as
RUSCONI'S digestive
cavity, and the blasto-
pore likewise as the
RUSCONIAN anus.

At the close of the process of invagination the whole yolk-mass, or
the vegetative half of the blastula, has been taken into the interior
to form the lining of the coelenteron, being at the same time over-
grown by a layer of small cells (fig. 48). In the case of the Frog the

v.d

dl

fJi

Fig. 47. Longitudinal [sagittal] section through an egg
of Triton at the beginning of gastrulation,

ak, Outer germ-layer ; ik, inner germ-layer ; fh, cleavage-
cavity ; ud, coelenteron ; u, blastopore ; dz, yolk-
cells ; dl and vl, dorsal and ventral lips of the
ccelenteron.

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS.

89

Tig. 48. Sagittal section through an egg of Triton after

the end of gastrulation.
at, ik, dz, dl, vl, ud, as in fig. 47 ; d, vitelline plug ;

mk, middle germ-layer.

whole surface of the germ, with the exception of a small place about
as large as the head of a pin, which corresponds to the blastopore,
now appears black, because the small cells are deeply pigmented. At
the place excepted a part of the unpigmented yolk-mass protrudes
through the blastopore
and closes the entrance to
it as if with a stopper (d),
by reason of which it
bears the significant name
of vitelline plug.

Of the two germ-layers
of the gastrula the outer
subsequently becomes re-
duced in thickness in the
case of the Water-Sala-
mander to a single layer
of regularly arranged
cylindrical cells, whereas
in the case of the Frog it
is composed of two or

three layers of small, in part cubical, deeply pigmented elements.
The inner germ-layer in the roof of the ccelenteron likewise consists of
small (in the Frog, pigmented) cells, but in the floor it is composed
of large yolk-cells, which, heaped together in many layers, pro-
duce an elevation 'that projects far into the crelenteron and partly
fills it. For this reason the gastrula in Amphibia is compelled
to adopt in water a definite position of rest, because the yolk-mass,
being the heavier part, always assumes the lowest position (fig. 48).

The germ of the Amphibia is already a bilaterally symmetrical
body. The thickened, yolk-containing wall of the gastrula becomes
the ventral side of the adult animal ; the opposite wall, or roof of
the coelenteron, becomes the dorsum. The blastopore indicates, as
the sequel shows, the posterior end, the opposite part the head-end.
There may therefore be passed through the gastrula a longitudinal,
a dorso-ventral, and a transverse axis, which correspond with the
axes of the adult animal. This bilateral symmetry, which appears
so early in the Amphibia, is solely attributable to the accumulation
of yolk-material, and to the piling up of it on the ventral side of the
crelenteron.

The development of Amphibia furnishes us with a transitional
condition, which is serviceable for the comprehension of the much

90

EMBRYOLOGY.

more highly altered form which the gastrula acquires in the case of
eggs with partial cleavage in the classes of Selachii, Teleosts, Reptiles,
and Birds.

The conditions are the most readily intelligible in the case of the
Selachians. That which we have described in the blastula of the
Amphibia as the roof of the cleavage-cavity is in the blastula of

the Selachians a
small disc of em-
bryonic cells (fig.
49 kz), continuous
at its margin with
the extraordi-
narily voluminous
yolk - mass (dk),
which contains
nuclei, although it
is not divided up
into cells. This
yolk-mass corre-
sponds to the
yolk-cells of the

Amphibia, and, like the latter, forms the floor of the cleavage-cavity
(). Germ- disc and yolk thus together constitute a sac with an

Fig. 49. Median section through a germ-disc of Pristiurus in the
blastula stage, after RIJCKERT. The posterior end of the
embryo lies at the right. B, Cleavage-cavity ; dk, yolk-nuclei ;
kz, germ-cells ; V and H, front and hind margins of the germ-
disc.

Fig. 50. Median section through a germ-disc of Pristiurus, in which the gastrular invagination

has begun, after RUCKERT.
ud, First rudiment of the coelenteron ; B, cleavage-cavity ; dk, yolk-nuclei ; fd, finely granular

yolk ; gd, coarsely granular yolk ; V and H, front and hind margins of the germ-disc.

almost obliterated cavity (J3), and with walls differing in thickness
and in differentiation. A very small part of the wall, the germ-disc,
consists of cells. The much larger and thicker portion is yolk-mass,
which in the vicinity of the cavity contains nuclei, but is not divided
into cells.

As in the Amphibia, so here, the gastrulation begins at what

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 91

is subsequently the hind end (H) of the embryo, at a region in the
zone of transition or margin of the germ-disc, in which the most
superficial cells have assumed the cylindrical form, and are closely
joined together (fig. 49). The margin of the disc is folded in
(fig. 50) toward the cleavage-cavity (S), so that a small ccelen-
teron (we), shown in the accompanying section, and a fissure-
like blastopore are distinctly recognisable. The neighboring yolk
also participates in the invagination, since in the territory of
the zone of transition the yolk-nuclei (dK), enveloped in protoplasm,
become detached from the yolk, grow into the cleavage-cavity along
with the invaginated cells, and contribute to the formation of the
inner germ-layer in a similar manner to that in which, in the case of
the Amphibia, the vegetative cells at the lower lip of the blastopore
are carried in with the invagination into the cleavage-cavity. The
cleavage-cavity (B) is being continually encroached upon by the in-
growth of the cells originally in its roof, which form a continuous
layer projecting from behind forward. Consequently in the Sela-
chians also the germ-disc becomes two-layered as the result of the
invagination. It lies so close upon the yolk, that the ccelenteron
appears at most as a fissure. Moreover, the invagination in the
Selachians does not remain limited to one region of the original
margin of the germ-disc, but soon stretches itself out over its whole
posterior perimeter. The blastopore then appears as a large semi-
circular or horseshoe-shaped fissure at the future posterior end of the
embryonic fundament.

The enormous volume of the yolk causes an important difference
between the gastrulation of the Selachii and that of the Amphibia.
In the case of the latter the mass of the yolk-cells was quite rapidly
carried in with the invagination, and employed in the formation of
the ventral wall of the coelenteron. In the Selachians the taking
up of the yolk into the interior of the body ensues only at a slow
rate (in a manner to be more accurately explained later), so that for
a long time only the dorsal side of the gastrula consists of two cell-
layers, whereas the ventral wall is formed by the yolk-mass.

The eggs of Teleosts are very nearly related to those of Selachians
in their whole method of development. The same cannot be said
to be true to the same extent for the eggs of Reptiles and
Birds. The latter, indeed, also belong to the meroblastic type,
since they have developed a large amount of yolk, and in consequence
undergo partial segmentation; but in the formation of the germ-
layers, they exhibit many peculiarities, so that they require a separate

92

EMBRYOLOGY.

treatment. In Birds and Reptiles the investigation is accompanied
with greater difficulties than in the Selachians. Particularly the
development of the germ-layers in the Chick, notwithstanding the
fact that the best investigators have given it their attention, has
for a long time been the subject of very divergent descriptions. At
the present moment, however, the main facts in the case have been
established for the Bird's egg also by the very recent and excellent
work of DUVAL, and upon this as a basis the gastrulation in Birds is
easily to be correlated with that of the Vertebrates hitherto described.
Since the Bird's egg has played such an important role in the history
of embryology, and has even been called a classical object for investiga-
tion, it appears necessary to go briefly into the conditions which it
presents in the gastrida-stage, and in connection therewith to consider
some of the important results drawn from the study of the eggs of
Reptiles.

The blastula arises and the germ-layers begin to be developed out
of it while the Bird's egg tarries in the terminal region of the
oviduct.

The blastula arises in a manner which was first correctly described
by DUVAL. When by the process of segmentation a small disc of

cells has been formed,
there appears in the
latter a narrow fissure,
the cleavage-cavity (fig.
51 fh\ and the cell-
material is separated
into an upper layer (dw)
and a lower layer (vw),
which are continuous
with each other at the
margin of the disc. The
upper layer consists of
fully isolated cleavage-
spheres, which are flattened at their surfaces of contact and arranged
into an epithelium-like layer. They correspond to the thin-walled
half of the blastula in Triton (fig. 45), which has already been
designated as the animal half. The lower layer is composed of
larger cleavage-spheres, which are still in great part continuous
by means of their lower halves with the white yolk (wd), which
is spread out beneath the germ-disc and is known as PANDER'S
nucleus. Yolk-nuclei (merocytes) are also found here in great

VW dw fh

fig. 51. Section through the germ-disc of a freshly laid

unfertilised Hen's egg, after DUTAL.
fh, Cleavage-cavity ; wd, white yolk ; vw, lower cell-layer ;

dw, upper cell-layer of the blastula.

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 93

numbers, especially around the whole periphery of the germ-disc.
Since they increase in number by nuclear division, and since
some of them, enveloped in protoplasm, become detached from the
yolk, they contribute to the continuous growth of the germ-disc, a
process which has already (p. 65) been described as supplementary
cleavage. The lower cell-layer, together with the whole yolk-maas
with its free nuclei, must be compared to the vegetative half of the
blastula of Triton (fig. 45 dz).

The gastrulation proceeds from the posterior margin of the germ-
disc, and begins even some time before the egg is laid. The study
of it is~~coupled with great difficulties, and demands, most of all,
that, in the investigation of the disc by means of sections, one should
be accurately informed concerning the position of its anterior and
posterior margins. The orientation is essentially facilitated by the
fact that, in the case of every Hen's egg, with rare exceptions, the
side toward which the front end of the embryo is directed can be
stated accurately before opening the shell. This results from the
following rule established by KUPFFER, ROLLER, GERLACH, and DUVAL

When one so places an egg in front of him that the blunt pole is
turned to the left, the more pointed one to the right, then a line
uniting the two poles divides the germ-disc into a half on the side
toward the observer, which becomes the hind end of the embryo, and
a forward half, which is developed into the head-end. By taking
into account this rule, one can establish a difference on the germ-
disc even during the process of cleavage. In the anterior region the
cleavage takes place more slowly than in the posterior half. Con-
sequently larger embryonic cells are found in front, smaller and
more numerous ones behind (OELLACHER, KOLLIKER, DUVAL).

The difference between anterior and posterior becomes more evident
at the beginning of gastrulation. If one now examines carefully the
thickened margin of the germ-disc (Eandwulst of German writers,
bourrelet blastodermique of DUVAL), it is seen that the disc is limited
in front and on the sides by a notched and indistinct boundary,
but behind, on the contrary, by a sharper contour. The latter
is caused by the fact that the marginal ridge, in consequence of a
more vigorous growth of the cells, has become thickened and more
opaque, and has assumed a whiter colour. It is distinctly recognisable
from its surroundings as a whitish crescentic figure (fig. 52 A ).
Often there is also observable in the crescent a narrow furrow, the
crescentic groove (Sichelrinne, ROLLER), by means of which the germ-
disc acquires a still sharper limitation behind.

94

EMBRYOLOGY.

DUVAL has proved by means of sections, part of which was made in
a transverse direction, and part in the sagittal, that the Bird's egg is
now in the gastrula stage. Especially instructive are the two median

Jd vl ud ak ik

n a

Fig. 52 A. The unincubated germ-disc of a Hen's egg, after ROLLER.

d, Yolk ; kick, germ-disc ; s, crescent ; V and H, anterior and posterior margins of the germ-disc.

B. The germ-disc of a Hen's egg during the first hours of incubation, after KOLLEK.
d, Yolk ; ksch, germ-disc ; Es, embryonal shield ; s, crescent ; sfc, knob of the crescent ; V and H,
anterior and posterior margins of the germ-disc.

sections, figs. 53 and 54. As is to be seen at once in fig. 53, which re-
presents the somewhat younger stage, the crescentic groove described
as occupying the posterior part of the marginal ridge (vl) is continued
in the form of a narrow fissure (ud). Whereas in the blastula stage

(fig. 51) the lower cell-
layer passed over con-
tinuously into the white

yolk, it is now sharply
separated from it as far
as the fissure extends.
In fig. 53 this separation
has been completed only
in the posterior half of
the germ-disc; in the
anterior half, on the con-
trary, embryonic cells
(dk) and yolk are still
continuous. However,
in the somewhat older

stage (fig. 54) the connection is terminated in this region also,
since the fissure (ud) has extended itself nearly to the anterior
margin of the disc (vr). In consequence of this process the part of
the white yolk which lies beneath the fissure has become destitute of
cells and nuclei, with the exception of the marginal territory, where,

Fig. 53 Longitudinal section through the germ-disc of an
unincubated egg of the Siskin (Carduelis spinus). after
DUVAL.

.(A-, Outer , ik, inner germ-layer ; wd, white yolk ; dk, yolk-
nuclei ; ud, coelenteron ; vl, anterior lip, hi, posterior lip
at the place of in vagination (crescentic groove or blastopore).

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS.

95

a
*1
| |
M : ~

IS

especially behind (hi} the crescentic groove, free nuclei are constantly
to be found keeping up the supplementary cleavage.

Owing to the appearance of the new fissure (subgerminal cavity)
(fig. 53 ud), the cleavage- cavity (fig. 51 fh) is almost completely
obliterated. The two cell-layers of the
blastula-stage (fig. 51 dw, vw), described as
lying one above and one below the cleavage-
cavity, have come close together (figs. 53
and 54), being separated from each other
by only a narrow fissure. In the upper
layer (ak) the cells have assumed a cubical,
and at a somewhat later stage a cylindrical,
form, and constitute a compact epithelial
membrane. The lower layer (ik) is composed
of larger roundish and loosely arranged cells
in several layers. The former is the primary
outer germ -layer, the latter the inner layer.
In the region of the posterior marginal
ridge (vl), where the cells are at the same
time engaged in more active proliferation,
the two layers are continuous with each
other.

The highly important processes, by means
of which are produced the conditions repre-
sented in figs. 53 and 54, present many points
of comparison with the gastrulation of the
Selachians and Amphibia. We can conceive
that the newly appearing fissure has arisen,
as in the case of the germ-disc of Pristiurus
(fig. 50), by an infolding, in such a way that,
as in the former case, cells grow inward from
the posterior marginal ridge ; and that at
the same time, at the de< -p part of the iu-
vagination, the cells which are originally
continuous with the yolk (fig. 53 dk} detach
themselves from the latter, and are employed for the increase of the
inner germ layer.

If this explanation is correct, the fissure (ltd) which now exists be-
tween the inner germ-layer and the floor of the yolk corresponds to
the coelenteron, as GOETTE and RAUBER have already remarked, and
as DUVAL has for the first time demonstrated ; moreover, the cres-

<*-

EMBRYOLOGY.

centic groove (fig. 52 ) corresponds to the blastopore ; the thickened
portion of the marginal ridge (fig. 53 vl) which lies in front of the
crescentic groove, -within whose territory the two primary germ-
layers are continuous with each other, is the anterior or dorsal lip of
the blastopore ; and the yolk (hi) which lies behind the crescentic
groove, and which at this early stage contains numerous free nuclei,
may be designated as the posterior or ventral lip of the blastopore.

The develop-

v ment of the

coelenteron is
the cause of
the gradual re-
duction of the
cleavage - cav-
ity, and of its
persisting only
as a narrow fis-
sure separating
the primary
germ-layers.

The points of
comp arisen
with the gas-
trula of Triton
(fig. 47) are
made evident
as soon as we

Fig. 55. Embryonic fundament of Lassrta agilis, after KUPFFER. 1 / f V,

hf, Area pellucida ; df, area opaca ; u, blastopore ; , crescent ; ex, em- r 6 p 1 a <7e t H 6
bryonic shield. V, anterior, H, posterior end. mass of

mass

cells with un-

segmented yolk, and imagine nuclei imbedded in the latter in the
region of the ventral lip of the blastopore.

Through the exposition given by DUVAL, it appears to me that the
contest concerning the origin of the two primary germ-layers in
Birds has been happily settled. For a long time there have existed
on this very question two irreconcilable views.

According to the older view, to which many investigators still cling,
the germ-disc which results from the process of cleavage is divided by
fission into an upper and a lower layer (PANDER, VON BAER, REMAK,
KOLLIKER, His, and others). According to the other one (HAECKEL,
GOETTE, RAUBER, DUVAL, and others), the lower layer has arisen by

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS. 97

an infolding. Only by means of the theory of infolding can be ex-
plained the different conditions of the anterior and posterior margins
of the germ-disc, the more active cell-growth in the territory of the
crescent, the existence of a crescentic groove, and the continuity
of the two primary germ-layers which is demonstrable in that
region. Only by means of this theory, finally, is the relation of
Birds to the lower classes of the Vertebrates made possible.

The discoveries which KUPFFER UND BEXECKE have made in their
investigations of Reptiles, which are so closely related to Birds, also
contribute to the elucidation of the pending controversy. In the case
of Lacerta agilis (fig. 55), Emys europsea, etc., there is found, as in
the case of the Hen at a corresponding stage of development, at the
boundary of the pellucid and opaque areas of the posterior end of
the germ-disc, an exuberant cell-growth in the form of a crescent ().
In the middle plane and slightly in front of this crescent there is
to be seen a small, transversely placed, fissure-like, opening (u), which
leads into a blind sac and is comparable to the crescentic groove.
KUPFFER rightly interprets the opening as the blastopore, which is
enclosed between an anterior and a posterior lip, and the cavity as
the ccelenteron. He also draws a comparison between the corre-
sponding structures in Birds and Reptiles.*

Let us now direct our attention to the succeeding developmental
stages of the germ-disc of the Chick. These consist, chiefly, in
a constant increase of the superficial extent of the disc.

In the freshly laid, unincubated egg (fig. 54) the outer germ-layer
(ak) is composed of a single sheet of closely united cylindrical cells ;
the inner layer (ik), on the contrary, consists of a two-layered to
three-layered bed of somewhat flattened elements, which are only
loosely associated.

Under the influence of incubation the superficial extension of the
germ-disc makes rapid advances (fig. 56). In this process the outer
germ-layer (ak) outstrips the inner, and terminates in a region of the

* In the interpretation of the manner in which the imagination takes place
in the case of the eggs of Reptiles and Birds, I differ from other investigators
who also maintain that a gastrulation takes place (GOETTE, HAECKEL,
RAUBER, BALFOUR, and others). They regard the whole margin of the germ-
disc as the blastopore, at which the outer germ-layer bends over to become
continuous with the inner layer. According to my interpretation, the invagina-
tion occurs at a small circumscribed place of the margin. The blastopore is
from the beginning surrounded by cells both on its anterior and its posterior lip.
The relation of the .blastopore as well as that of the germ-layers to the yolk
will be more fully dealt with hereafter.

7

EMBRYOLOGY.

yolk where the latter
has not yet undergone
division into entodermic
cells. In the form of
its cells it is, in every
respect, in sharp con-
trast with the inner
layer. While the ecto-
dermic cells (fig. 56 ak)
attain their greatest
height in the middle
of the germ-disc, they
gradually decrease in
height toward the mar-
gin, and undergo a
transition into cubical
and finally into flat-
tened elements (fig. 57).
The reverse is the case
with the inner germ-
layer ; the latter has
now become converted in
the middle of the germ-
disc (fig. 56 ik] into a
single layer of much
flattened scale-like cells,
which are closely united
into a thin membrane.
Toward the periphery
they become somewhat
larger and more poly-
gonal (fig. 57), and here,
at some distance inside
the free margin of the
outer germ-layer, they
become merged in the
white yolk (dw), which
is abundantly provided
with yolk-nuclei (dk) in
the region of the transi-
tion. This region of th3

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS.

99

,. i.'X**-*ioS5>ft6Jb,

yolk is designated as the yolk-wall (viteUine rampart). It serves
for the augmentation of the inner germ-layer, in that the free
nuclei increase in number by division, and keep up the process of
supplementary cleavage already mentioned.

During incubation the liquefaction of the yolk makes further pro-
gress (fig. 56) and leads to the formation of a depression (ud), which
continually increases in depth and breadth, and over which the germ-
disc arches like a watch-glass. Upon examination from the surface
its middle, as far as the fluid reaches under it, appears clear and
translucent, whereas the marginal area, which lies upon the opaque
yolk, appears dark. Such a distinction is still more observable when
one detaches the whole
germ-disc from the yolk,
for in the region of the
fluid-filled space the thin
and transparent germ-
layers come off easily and
clean from their substra-
tum, whereas at the rim,
from the point where the
inner germ-layer merges
with the yolk-wall out-
ward, turbid yolk-substance remains clinging to the germ disc. For
a long time the middle, clear, circular area has been designated
in embryology as the clear germinal area (area pellucida), and the
more cloudy, ring-like rim as the opaque germinal area (area opaca).

In the next chapter I shall treat more in extenso of the important
changes which take place up to the time when the egg is laid
and during the first hours of incubation in the vicinity of the
crescentic groove and the anterior lip of the blastopore, because they
are connected with the development of the middle germ-layer.

It is still more difficult than in the case of the Chick to interpret
in its details the development of the germ-layers in Mammals, and to
refer it back to the gastrulation of the other Vertebrates. Especial
service has been rendered through the painstaking investigation of
these conditions : in the earlier times by BISCHOFF, in later years by
HENSEN, LiEBERKtiHN, VAN BENEDEN, KOLLIKER, and HEAPE. The
object of investigation which has been made use of in this work, and
which we shall employ as the basis of our description, has usually
been the Babbit ; besides this, the Bat and the Mole have also been
employed.

Fig. 57. Section through tae margin of the germ-disc
of a Hen's egg that had been incubated for six
hours, after DUVAL.

ak, Outer genu-layer ; dz, yolk-cells ; rfi, yolk-nuclei ;
die, yolk- wall.

100

EMBRYOLOGY.

While the Mammalian egg is gradually impelled through the
oviduct toward the uterus by the ciliary motion of the epithelium, it
becomes converted by the cleavage process into a spherical mass of
small cells (fig. 58 .4). Then there arises within it, by the secretion
of a fluid, a small fissure-like cleavage-cavity (fig. 58 ). The germ
has consequently entered upon the vesicular or blastula stage. The
wall of the blastula, or vesicula blastodermica, is composed of a
single layer of polygonal cells, arranged, as has been known since
BISCHOFF'S works, in mosaic, with the exception of a small region,
where the wall, as in the case of the Amphibian blastula, is thickened
by an accumulation of somewhat more granular and darker cells,

Fig. 58. Optical sections of a Rabbit's egg in two stages immediately following cleavage, after
ED. v. BENEDEN. Copied from BALFOUR'S " Comparative Embryology."

A, Solid cell-mass resulting from cleavage.

B, Development of the blastula by the formation of a cleavage-cavity in the cell-mass. (According

to VAS BENEDEN'S interpretation, ep is epiblast ; ky, hypoblast ; bp, blastopore.)

which produce a knob-like elevation that projects far into the
cleavage-cavity.

A peculiarity preeminently characteristic of the further develop-
ment of Mammals is that here, as in no other Vertebrate, the
blastula increases enormously in size (fig. 59), by the accumulation
of fluid which contains much albumen and produces a granular
coagulum upon the addition of alcohol ; it soon acquires a diameter
of I/O mm. Of course, with these processes of growth the zona
pellucida is altered and distended into a thin membrane. A gela-
tinous layer (zp) already secreted by the oviduct envelops the
latter.

In Rabbits' eggs which are a millimetre in diameter the wall of
the blastula has become very thin. The mosaic- like cells arranged
in a single layer have become very much flattened. Also the knob

DEVELOPMENT OF THE TWO PRIMARY GERM-LAYERS.

101

of cells, which projects into the cleavage-cavity, has become meta-
morphosed and has spread itself out more and more in the
form of a disc-like plate, which is continuous at its attenuated
margins with the thin
wall of the blastula.
The further processes
of development take
place principally in
this plate. Its most
superficial cells are
flattened out to thin
scales, such as also
form the wall of the
blastula elsewhere ; its
remaining elements,
on the contrary, ar-
ranged in from two
to three superposed
layers, are larger and
richer in protoplasm.

Up to this time the
embryo of the Mammal
is in the blastula stage.
It still consists everywhere of a single germ-layer. For the view
which has been advanced by many persons, that the germ-disc in this

Fig. 59. Rabbit's egg, 70-90 hours after fertilisation, after

ED. v. BENEDEM. Copied from BALFOUR'S " Comparative
Embryology."

bv, Cavity of the blastula ; zp, [gelatinous layer surrounding
the] zona pellucida ; ep, hy, as in Fig. 58.

Fig. 60. Cross section through the almost circular germinal area of a Rabbif s egg 6 days and 9

hours old (diameter 0-8 mm.), after BALFOUR.
ale, Outer, ik, inner germ-layer. The section shows the peculiar character of the upper layer with

a certain number of flattened superficial cells. Only about half of the whole breadth of the

germinal area is represanted.

stage of development is already in the two-layered condition, and that
the outer layer of flat cells constitutes the outer germ-layer and the
more protoplasmic cells lying under it the inner germ-layer, is, in my
opinion, untenable. Opposed to this are, first, the fact that the flat-
tened and the thicker cell- layers are firmly joined together and
are not separated from each other even by the narrowest fissure,
and, secondly, the further course of the development.*

* Holding to this interpretation, I am of course also unable to agree with a
view of VAN BENEDEN'S, according to which the gastrulation takes place at the

102

EMBRYOLOGY.

Two germ-layers first appear in
which have already attained a diameter of
more than 1 mm. and are about five days
old. At the place where the cell plate pre-
viously lay, one sees by inspection from the
surface a whitish spot, which is at first
round, but later becomes oval or pear shaped.
It is generally designated at this stage as
area embryonalis, or as embryonic spot. It
consists of two germ-layers (fig. 60), which
are separated by a distinct fissure, and may
be detached from each other. The inner
germ-layer (ik) is a single sheet of greatly
flattened cells. The outer germ-layer (ak),
on the contrary, is considerably thicker, and
shows that it is composed of two sheets of
cells : (1) a deeper layer of cubical or round-
ish, larger elements, and (2) a superficial
layer of isolated flatter cells, which were first
accurately described by RAUBER, and which
have been named after him RAUBER'S layer.
Toward the margins of the embryonic spot
the outer layer becomes thinner and pos-
sesses only a single layer of cells ; these are
continuous with the large flattened elements
which, as we have seen, alone constitute the
greater part of the wall of the sac in the
blastula stage. The inner germ-layer is
at first developed on only a small part of
the wall of the sac at the embryonic spot
and its immediate vicinity; it terminates
with a free notched margin, where there
are to be found loosely associated amoeboid
cells, which by their increase in number and
migration probably cause the further growth

end of the first stages of cleavage. He interprets in the originally solid
sphere of cells (fig. 58 A) the darker and larger centrally located elements
(Ay) as entodenn, the layer of smaller and clearer cells (ep) surrounding the
latter as ectoderm, and a small vacuity in this investing layer as the blastopore
(bp). I, on the contrary, believe that the gastrolation takes place in the
manner described on page 104.

DEVELOPMENT OP THE TWO PRIMARY GERM-LAYERS. 103

of the layer. This on older eggs slowly spreads itself from the
embryonic spot toward the opposite pole, and thereby the whole
blastodermic vesicle gradually becomes two-layered. While this is
talcing place, changes also proceed at the embryonic spot, which has
become oval and somewhat larger. RAUBER'S layer disappears *
(fig. 61) ; the underlying cubical or spherical cells have become
cylindrical and more closely crowded together. Each of the primary
germ-layers is now composed of a single layer of cells.

The two accompanying figures, which represent in two different
positions a Rabbit's egg seven days old, will serve for the illustration
of these conditions. In looking down from above (fig. 62 A) one sees
the embryonic spot (ag), now become oval. It is produced exclusively
by a definitely limited thickening of the outer germ- layer, and indi-
cateiTtEe place at which the cells are cylindrical ; in that respect it
corresponds to the embryonic shield of reptilian and avian embryos,
and is not to be confounded with the cell-plate (fig. 59), which was
described as a thickening of throne-layered blastula! In looking at
it from the side (fig. 62 5Jone~can distinguish on the blastula three
regions : (1) the embryonic spot (ag); (2) a region which includes the
upper half of the vesicle and reaches to the line ge, in which the wall
is still composed of two layers, but in which the cells of both the
outer and inner germ-layers are very much flattened ; and (3) a third
portion lying below the line ge, where the wall is composed exclusively
of the outer germ-layer.

There now arises the important question, in what manner the two-
layered condition in Mammals arises out of the single-layered form.
One has reason to expect that gastrulation takes place here in
the same way as with the remaining Vertebrates, by means of an
invagination or an ingression of cells which proceeds from a definite
territory of the thickened cell-plate of the blastula ; in this con-
nection attention must be directed to the posterior end of the
embryonic spot.

When the embryonic spot has acquired a pear-shaped appearance
(fig. 63), there is at its posterior end a somewhat less transparent,
because thickened, place (hw), which KOLLIKER has designated
the terminal ridge (Endwulst). It is comparable with the opacity

* Two views are held concerning the manner in which RAUBER'S layer
disappears. According to BALFOUR and HEAPE, the flat cells become meta-
morphosed into cylindrical cells, which are interposed between the other
cylindrical cells ; according to KOLLIKER, on the contrary, they disintegrate
and disappear.

104

EMBRYOLOGY

at the posterior margin of the germ-disc of Reptiles and Birds, when
their gastrulation begins. An invagination proceeding from this

point, such as DUVAL has
established for the Chick,
is unfortunately not as
yet proven with sufficient
certainty in the case
of Mammals ; the origin
of the two-layered stage
is also still involved in
obscurity.

However, there are in
the literature some observa-
tions, which, fragmentary
as they are, appear to me
to be worthy of special
regard.

At the stage at which
the blastula has become
for a certain distance two-
layered (fig. 62), there has
been discovered by HEAPE
in the case of the Mole, by
SELENKA in the Opossum,
and by KEIBEL in the
Rabbit, at one place of
the embryonic spot (pro-
bably in the region just
described as terminal ridge),
a small opening (fig. 64 u),
which is possibly to be in-
terpreted as blastopore and
to be compared with the
crescentic groove of Birds,
Here the two primary germ-

'Tig. 62. Blastula of the Babbit 7 days old without the
outer egg- membranes. Length 4'4 mm. After
KOLLIKER. Magnified 10 diameters.

Seen in A from above, in B from the side.

ag, Embryonic spot (area embryonalis) ; ge, the line
up to which the blastula is two-layered.

layers are continuous with
each other, and from here, as well as from the primitive streak, the
middle germ-layer takes its origin. I assume that, beginning at
this place, the lower germ-layer has in a still earlier stage been
developed by an infolding of a small territory of the single-layered
blastula (fig. 59).

DEVELOPMENT OF THE TWO PRIMARY GEUM-J.AYERS.

105

I,

\

One circumstance is especially characteristic of the gastrulation of

Mammals : that the invaginating

membrane is not a closed blind sac,

but possesses a free margin, with

which it grows along on the inner

\ surface of the outer germ-layer,

i until it has completely lined the

/ blastodermic vesicle. The reader

will please compare with this the

statements on page 102. But the

absence of a ventral closure becomes

intelligible, when we imagine that

the yolk-mass^which constitutes in

meroblastic eggs or in Amphibian

eggs the floor of the ccelenteron,

has degenerated and wholly disap-
peared. In this case ccelenteron

and cleavage-cavity become one

and the Same, as is the case with

Mammals.

Moreover we are induced to as-
sume that in the eggs of Mammals a
regressive metamorphosis of origin-
ally abundant yolk-contents must have taken place, on account of
many phenomena in their development, which would be unintelligible

hio

H

. 63. Fear-shaped embryonic spot of a
Rabbit's egg 6 days and 18 hours old,
after KOLLIKER.

Short primitive streak ; hw, crescent-
shaped terminal ridge ; V, anterior,
H, posterior end.

ak

Fig. 64. Median section of the embryonic fundament of a Mole's egg through that part in

which the primitive streak has begun to be formed, after HEAPE.
u, Blastopore ; ak, outer, ik, inner germ-layer ; V, anterior, H, posterior end.

without this assumption. These phenomena will be considered more
at length in a subsequent chapter.

106 EMBRYOLOGY.

CHAPTER VI.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.
(CCELOM-THEORY.) *

AFTEK the completion of the gastrula stage the processes of develop-
ment become more and more complicated, so that the attention of the
observer from this time on must be directed to a series of changes
which take place at the same time and in various parts of the
embryo. For a transformation now ensues, due to the simultaneous
folding of both the inner and outer germ-layers, whereby four new
chief organs of the vertebrate body are called into existence. Out
of the inner primary germ-layer arise (1) the two middle germ-layers,
which enclose between them the body -cavity ; (2) the secondary en-
toderm or entoblast (Darmdriisenblatt), which lines the secondary
intestine of vertebrated animals ; and (3) the fundament of the axial
skeleton, the chorda dorsalis, or notochord. At the same time there
is developed from the outer germ-layer, as its only system of organs,
the fundament of the central nervous system. Since these four pro-
cesses in the development are in part most intimately involved in
one another, they cannot be separated in their treatment.

Here again we have to do with a problem which is one of the
most difficult in the embryology of vertebrated animals the
history of the development of the two middle germ-layers. Not-
withstanding a voluminous literature which has grown out of this
theme, there are many conditions, especially among the higher
classes of Vertebrata, which are not yet explained in an entirely
satisfactory manner. We shall therefore enter somewhat more
minutely into this topic, which, like the question as to the origin of
the two primary germ-layers, possesses a fundamental significance
for the comprehension of the organisation of Vertebrates.

The presentation of what follows will be essentially facilitated, if
we allow ourselves a short digression into the history of the develop-
ment of the Invertebrata, and take under consideration a case in which
the middle germ-layers and the body-cavity are established in a
manner similar to that which obtains in the case of Vertebrata,
but which is easier to investigate and to understand. Such an

* In figs. 66-89 the individual germ-layers are represented in different depths
of shade, so as to make their relations to one another more evident. The
middle germ-layer is darkest.

DEVELOPMENT OP THE TWO MIDDLE GERM-LAYERS.

107

example is presented to us in the development of arrow-worms
(Sagitta) or Chcetognatha, concerning which observations have been
published by KOWALEVSKY, BIJTSCHLI, and the author.

After the process of cleavage there arises a typical blastula, which
after some time is converted into a typical gastrula. While the
latter elongates, two folds of the inner germ-layer arise at the bottom
of the coelenteron, and grow up parallel to each other (fig. 65).

Fig. 65.

Fig. 65. A stage in the development of Sagitta, after KOWA.LEVSKY, from BALKODR'S

" Comparative Embryology."
Optical longitudinal section through a gastrula at the beginning of the formation of the

body-cavity.
7i), Mouth ; al, alimentary cavity ; pv, body-cavity ; bl.p, blastopore.

Fig. 66. Optical cross section through a larva of Sagitta.

The ooelenteroii is separated by means of two folds, which protrude from its ventral wall (V),

into the intestinal canal proper and the two lateral body-cavities (lit), all of which are still

in communication with one another on the dorsal side (Z>).
D, Dorsal side ; f, ventral side ; ak, outer, ilc, inner germ-layer ; mk 1 , parietal, ml?, visceral

middle layer ; Ih, body-cavity.

They grow larger and larger, and at the same time stretch over on to
the ventral wall of the larva. From here the free edges finally grow
on the one hand up to the dorsal wall, on the other up to the
blastopore, and thereby completely divide the coalenteron into a
middle and two lateral spaces (fig. 66 Ih), which for a time communi-
cate with each other near the blastopore and along the subsequent
dorsum (D) of the embryo. After a short time this communication
is lost ; the blastopore becomes closed, and the edges of the folds
fuse with the adjacent surfaces of the ccelenteron. Of the three
cavities the middle becomes that of the permanent intestinal tube, the
two lateral ones (IK) become those of the two body-cavity sacs which

108 EMBRYOLOGY.

separate the intestine from the wall of the body. They appropri-
ately take the name enteroccel, since they are formed from the coelen-
teron by a process of constriction, and are genetically distinguishable
from other cavities which arise in other animals between the wall, of
the intestine and that of the body by simple splitting, and to which
is given the name Jlssicoel or schizocozl.

By the process of infolding the number of the germ-layers in Sagitta
has been increased from two to three. The primary inner germ-layer
is thereby divided into (1) a cell-layer (ik) which lines the intestinal
tube, and (2) a cell-layer which serves to enclose the two body-cavities
(mk 1 and mk 2 ). The first is designated as the secondary inner germ-
layer or entoblast, the second as the middle germ-layer (mesoblast).
One part of the latter is adjacent to the
outer germ-layer, the other part to the
intestinal tube ; accordingly the division
is carried still further into a parietal
(mk 1 ) and a visceral layer (mk 2 ) of the meso-
blast. For the sake of brevity the former
may be called the parietal (mk 1 ), the latter
the visceral (mk 2 ) middle layer. Conse-
quently, one may now speak of two middle
' 67 ;- Dia f anunatic >> MO- g e r m -layers instead of one, the total number

taon through a young Sagitta. "

dM, Dorsal, vM, ventral mesen- of the germ-layers being, naturally, raised

tery; dh, intestinal cavity; by this from three to four.
Ih, body-cavity ; ak, outer, ik, *

inner germ-layer; mt 1 , parietal, In regard to the course of the further

mV, visceral middle layer (mid- ji - L i_ jiji 1-1

die eerm-iayers) development it may be stated that, while

the larva elongates into a worm-like body,

the .two body -sacs (fig. 67 Ih) are increased to a greater extent
than the intestinal tube (ah) which they embrace. They everywhere
crowd the latter away from the wall of the body, grow around it
from above and below, where their thin walls come into direct con-
tact. By the fusion of the two body-sacs along their surfaces of
contact there are formed two delicate membranes, a dorsal (dM)
and a ventral (vM) mesentery, by means of which the intestinal tube
is attached to the dorsal wall and to the ventral wall of the
trunk.

Processes very similar to those of Sagitta occur in the development
of Vertebrata also, but in the latter case they are combined with
the development of the neural tube and the chorda dorsalis. In the
presentation of these we shall proceed as in the foregoing chapter,
which treated of the formation of the gastrula, and consider separately

DEVELOPMENT OP THE TWO MIDDLE GERM -LAYERS.

109

the processes in Ampbioxus, Amphibia, Selachians, Birds, and Mam-
mals, since they differ somewhat from one another.

The history of the development of Amphioxus lanceolatus is very in-
structive. The gastrula elongates, whereby the ccelenteron is turned
a little towards the future dorsal surface, and here terminates in the
blastopore, which marks the future hind end of the worm-shaped
body. Then the dorsal surface becomes somewhat flattened; the
cells in this region increase in height, become cylindrical, and form
the medullary or neural plate (fig. 69 mp). By a slight infolding of
the latter, there arises a medullary groove, which forces downward
the roof of the

ccelenteron in ** dh us ' ush n ush mk m

the form of a
ridge (ch). At
the place where
the thickened
medullary
plate joins the
small -celled
part of the
outer germ-
layer, or the
horn-layer (hb),
an interruption

Fig. 68. Optical longitudinal [sagittal] section through an embryo of
Amphioxus with five primitive segments, after HATSCHEK.

V, Anterior, H, posterior end ; it, inner, ink, middle germ-layer ; dh,
intestinal cavity ; n, neural tube ; en, neurenteric canal ; it* 1 , first
primitive segment ; nth, cavity of primitive segment.

in the continu-

ity now takes place, and the epidermis grows over the curved
neural plate from both sides, until its halves meet in the middle
line and fuse. Thus there arises along the back of the embryo
(fig. 70) a canal, the lower wall of which is formed by the curved
medullary plate (mp), and the upper wall by the overgrowing epi-
dermis (&). It is only at a later stage that the medullary plate in
Amphioxus, lying under the epidermis, is converted into a neural tube
(fig. 72 n) by the bending up of its edges and their fusion. As the
fundament of the nervous system becomes differentiated, it extends
so far toward the posterior end of the embryo, that the blastopore,
which is located there, still falls within its territory, and with the
closure of the neural tube is included within the end of the latter
Tn this manner it occurs that neural tube and intestinal tube, as
KOWALEVSKY first observed, are now, by means of the blastopore,
in continuity (fig. 68 'en) at the posterior end~bFthe body. The two
together constitute a canal composed of two arms, the form of which

110

EMBRYOLOGY.

Fig. 69. Cross section of an Amphioxus embryo, in

which the first primitive segment is being formed,

after HATSCHEK.
at, Outer, ik, inner, ml; middle germ-layer ; hb,

epidermis ; mp, medullary plate ; ch, chorda ;

*, evagination of the coelenteron.

is comparable with a siphon. The upper arm, which is the neural
tube, continues, for a time, to open to the outside world at its

anterior end. The bent por-
tion of the siphon, or the
blastoporic region, by means
of which the neural and the
intestinal tube are united, is
called canalis neurentericus
(fig. 68 en), a structure which
we shall again encounter in
the development of the re-
maining Vertebrata.

Simultaneously with the
neural tube are developed
the two middle germ-layers
and the chorda dorsalis (figs.
69 and 70). At the front
end of the embryo there
arise in the roof of the

coelenteron close to each other two small evaginations, the body-sacs
(mk), which grow dorsally and laterally at either side of the
curved medullary groove.
These are slowly enlarged,
since the process of evagina-
tion progresses from the an-
terior toward the posterior
end of the larva, and finally
reaches the blastopore. The
narrow strip of the wall of
the coelenteron which is found
between them and separating
them (its limits marked by
two stars * * in figs. 69 and
70), and which lies under
the middle of the medullary
groove, represents the funda-
ment of the chorda (ch).

The primary inner germ-
layer therefore has now undergone division into four different parts :
(1) the fundament of the chorda (ch), (2) and (3) the cells (mk) which
line the two body-sacs (Ih) and represent the middle germ-layer, and

Fig. 70. Cross section of an Amphioxus embryo,
in which the fifth primitive segment is in
process of formation, after HATSCHEK.

ak, Outer, ik, inner, mk, middle germ layer ; mp,
medullary plat* ; ch, chorda ; *, evagination
of the crelenteron ; dh, intestinal cavity ; Ih,
body-cavity.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

Ill

(4) the remaining part, which, since it is destined to form the bounding
wall of the subsequent intestine (dh), is to be designated as permanent
entoderm (Darmdriisenblatt) (ik).

The succeeding processes of development have as their objective
point the detachment from one another, by means of constriction and
fusion, of the parts which are still in continuity, and the formation
of discrete cavities. The processes of constriction begin at the
anterior end of the embryo, and progress thence to the blastopore
(figs. 70 and 71). At first the body-sacs become deeper (fig. 70 Ih),

- Ik

Fig. 71.

Fig. 72.

Fig. 71. Cross section through an Amphiozus embryo with five well-developed primitive seg-
ments, after HATSOHEK.

a/t, Outer, iic, inner, r mk, middle germ-layer ; mp, medullary plate; ch, chorda; dh, intestinal
cavity ; Ih, body-cavity.

Fig. 72. Cross section through the middle of the body of an Amphioxus embryo with eleven

primitive segments, after HATSCHEK.
, Neural tube; us, primitive segment. For the meaning of the other letters see Fig. 71.

and then lose their connection with the main cavity (dh) by the close
apposition of the cells which surround the entrances to them (fig. 71}.
By this process the margin of the secondary entoderm (ik) comes to
abut directly on the margin of the chordal fundament (ch). The
latter has meanwhile also undergone changes ; the plate-lite funda-
ment has become so curved by the elevation of its lateral margins,
that there has arisen a deep chordal groove, which is open along its
ventral side. Subsequently the lateral walls of the groove come into
close contact, and are thereby converted into a solid rod of cells, which
temporarily shares in the closure of the roof of the secondary intestine,
and appears as a ridge-like thickening of the latter. Then the cell-
rod (ch) becomes detached (fig. 72) from the wall of the intestine ; the
latter now, for the first time, becomes completely closed in the form
of a tube. To effect this the margins of the entoderm, indicated in

112 EMBRYOLOGY.

fig. 70 by stars ( * *), grow toward each other under the chorda and
fuse into a median raphe.

The final result of all these processes is shown in the cross section
fig. 72 : the original ccelenteron has become divided into three cavities
into the ventral permanent intestine (dh), and into the two body-
cavities (IK), which are situated dorso-laterally to it, and which con-
tinue to increase in size. Between these there has been interpolated
the chorda (ch), upon which the intestine abuts below and the neural
tube (n) above. The cells which have been cut off from the crelen-
teron by constriction and which are more deeply shaded in figs. 69
to 72, and enclose the body-cavities (lit) constitute the middle
germ-layer (mk). The part which lies in contact with the outer
germ-layer (fig. 72) is recognisable as the parietal middle layer
(ink 1 ) ; the part which is in contact with the neural tube, chorda,
and intestine as the visceral middle layer (mk 2 ).

Inasmuch as the process of differentiation just described begins,
as has been already stated, at the front end of the embryo and
extends slowly step by step toward the hind end, by an examina-
tion of a series of sections one may follow the various stages of
metamorphosis on a single object.

In the description given I have presented the conditions as though
in Amphioxus there arose two simple body-sacs, one on either side
of the intestinal tube. The processes are, however, somewhat more
complicated, for in the case of the embryo of fig. 70 the body -sacs,
while increasing in size posteriorly, undergo further changes in the
anterior region, and through repeated infoldings are divided into
separate compartments, the primitive segments (us), which lie one
behind the other. I content myself with this statement, since for
didactic reasons I shall defer the treatment of the development of
the primitive segments until I come to a subsequent chapter.

; While in the case of Amphioxus lanceolatus there is no doubt but
.hat the body-cavity and the middle germ-layer are formed by an out-
docketing of the wall of the ccelenteron, opinions upon the origin of the
same parts in the case of the remaining Vertebrata are still very
divergent. This results, in the first place, from the fact that the in-
vestigation, which can be carried out only by means of serial sections,
is coupled with greater technical difficulties, and, secondly, because the
conditions are somewhat altered, owing to the greater abundance of
yolk in the eggs, and furnish less clear and intelligible views. Where
in the gastrula of Amphioxus a great cavity is present, we see in the
case of the remaining Vertebrates a great mass of yolk-material

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

113

collected, and the coelenteron more or less completely filled with it.
Consequently there are formed in the?e cases for the produc ion cf
the body-cavity no hollow evaginations, but solid cell-growths, in that
the parietal and the
visceral lamellce of the
middle germ-layer have
the surfaces which inAm-
phioxus bound the body-
cavity pressed together at
the beginning of the de-
velopment and separated dk
only at a rather late
stage. In order to make
easier the comprehen-
sion of the somewhat
dissimilar appearances
furnished by an inves-
tigation of the separate
classes of Vertebrates,
let us describe first, with

Fig. 73. Diagram to show the development of the middle
germ-layers and the body-cavity in Vertebrate.

Cross section of an embryo in front of the blastopore.

mp, Medullary plate; ch, fundament of the chorda; ale,
' outer, ik, inner germ-layer ; mk\ parietal, ml?, visceral
lamella of the middle germ-layer; d, yolk-mass; die,
volk-nuclei ; dh, intestinal cavity ; Ih, body-cavity.

the aid of two diagram-
matic figures, how, according to a series of investigations which I
have undertaken, the development of the middle germ-layer and

the body-cavity would take
place in the case of the
vertebrated animals.

One of the diagrams (fig.
73) represents a cross section
in front of the blastopore.
It exhibits the inner germ-
layer (ik) extensively thick-
ened on the ventral side by
the deposition of yolk (d). so
that the .coelenteron is re-
duced to a small cavity (dh).
In the roof of the coelenteron
there lies a single layer of
cells (ch), the fundament of

the chorda, characterised by their cylindrical form. On both sides
of it the inner germ -layer has developed evaginations,. the two
body-sacs (Ih), which have grown down some distance between

8

Fig. 74. Cross section or an An:phioxus embryo.

See explanation of Fig. 70.
ak, Outer, it, inner, mk, middle germ-layer; ch,

chorda.

114

EMBRYOLOGY.

the yolk-mass and the outer germ-layer. Then- wall (mk l and
is composed of small cubical or polygonal elements, shaded darker
in the diagram. The coelenteron is distinctly separated by means
of the two ccelenteric folds (* *) into a median or intestinal cavity
proper (dh), lying beneath the chordal fundament, and the two narrow
body-sacs (IK), which communicate with the former only by means
of narrow fissures (* *) at the right and left of the chordal funda-
ment. The figure is easily reducible to the preceding (p. 113) cross
section of an Amphioxus embryo (fig. 74), if we conceive the simple
epithelium on the ventral side of the latter thickened by an accumula-
tion of yolk, and the two
small body-sacs grown
down a certain distance
between yolk-mass and
outer germ-layer.

In the second dia-
grammatic cross section,
which is through the
blastopore (fig. 75), the
coelenteron (ud) is wholly
filled up with the yolk-
mass (d). The body-sacs
(IK) described in the first
diagram are to be seen
here also, as they crowd
themselves downwards
between yolk and outer
germ-layer. Their walls

are composed of small cells, and the outer or parietal layer (ink 1 )
merges into the outer germ-layer at the blastopore, while the inner
or visceral layer (mk 2 ) is continuous with the yolk-mass or the inner
germ-layer.

Were the conditions in Vertebrates such as the two diagrams
represent, there coiild no longer be any doubt in regard to them,
any more than in the case of Amphioxus, that the body-cavity is
developed out of two evaginations of the coelenteron, and that its
walls constitute the two middle germ-layers. But there is not a
single Vertebrate which presents such clear and convincing evidence.
The distinctness is everywhere diminished, most of all by the
fact that the parts which are to be interpreted as body-sacs no longer
enclose cavities, because their walls are firmly pressed together, in

Fig. 75. Diagram to show the development of the middle
germ-layers and the body-cavity in Vertebrata.

Cross section through the blastopore of an embryo.

u, Blastopore ; ud, ccelenteron ; Ih, body-cavity ; d, yolk ;
ak, outer germ-layer ; mk l , parietal, ink", visceral
lamella of the middle germ-layer.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 115

consequence of the fact that the greater collection of yolk requires
the space for itself. Consequently we find, in place of the body-sacs
exhibited in the diagram, solid masses of cells, for which it remains
to be established that they correspond to the sacs in position and
development.

lu order to see what condition would result in consequence of a
disappearance of the body-cavity, we will imagine that in the two
diagrams the parietal and the visceral layers of the body-sacs are
firmly pressed together. In the first diagram (fig. 73) we should
then have a mass several cells thick, which would be everywhere dis-
tinctly separated from the two germ-layers in between which it had
grown with the exception of the place indicated by a star, which
marks the entrance to the body-sac ; this is the important region
whence the evagination or the outgrowth of the middle germ-layer
from the inner layer has taken place. At this point the cell-mass is
continuous, on the one side with the fundament of the chorda, on
the other with the entoderm. In the second diagram (fig. 75) we
should likewise see the thick cell-mass everywhere isolated, except in
the vicinity of the blastopore, where a transition to the outer as well
as to the inner germ-layer takes place. If, in addition to this, we
should imagine that the two lips of the blastopore were here pressed
together from right to left, we should have in the middle of the
cross section a thick, many-layered cell-mass, which on both sides is
resolved into the three germ-layers, or, in other words, at the blasto-
pore all three germ-layers by their fusion meet together in a single,
mass of cells.

By careful investigation it is, in fact, demonstrable that similar
conditions to those which we have produced by changes in the
diagrams are found in the investigation of the several classes of
Vertebrates. For this purpose we must make sections through three
different regions of the embryo : (1) through the region in front of
the blastopore, (2) through the region of the blastopore itself, and
(3) behind it. The agreement appears most prominent in the develop-
ment of the Amphibia, among which the Tritons again furnish the
most instructive objects.

When in the case of Triton the gastrulation, with the accompany-
ing obliteration of the cleavage-cavity, is fully completed, the embryo
becomes slightly elongated; the future dorsal surface (fig. 76 Z>)
becomes flattened, and gives rise to a shallow furrow (r), which
stretches from the anterior to the posterior end nearly up to the
blastopore (u). The latter has now assumed the form of a longitu-

116

EMBRYOLOGY.

dinal fissure. A cross section made through the middle of the
embryo in front of the blastopore (fig. 77) corresponds in every
particular to our first diagram (fig. 73), if we conceive that the
body-cavity in this case has disappeared. The outer germ-layer (ak)
consists of a single sheet of cells, which on the back of the embryo
are cylindrical, but become shorter toward its ventral side. The
cells enclosed within the outer layer exhibit a differentiation in three
ways, and therefore are subsequently converted into three different

Fig. 76. 'Egg of Triton with distinctly developed medullary groove, seen from the blastopore,

53 hours after artificial fertilisation.
D, Dorsal, V, ventral region ; u, blastopore ; h, elevation between blastopore and medullary

groove (r) f, semicircular furrow, which encloses the blastoporal area ; dp, yolk-plug.

Fig. 77. Cross section of an egg of Triton with feebly expressed medullary groove.
ak, Outer, ik, inner germ-layer ; mk l , parietal, mir", visceral lamella of the middle germ-layer ;
ch, chorda ; dh, intestinal cavity ; D, dorsal, V, ventral.

organs into chorda, entoderm, and middle germ-layer. First, there
is to be found on the roof of the ccelenteron (dh) under the medullary
groove, even close up to the blastopore, a narrow band of long
cylindrical cells (ch) ; it corresponds in every respect to the funda-
ment of the chorda in our diagram (fig. 73 ch), and in the cross
section through Amphioxus (fig. 74 ch). Secondly, the fundament
of the chorda is flanked on either side by two bands (mk l , mk*) of
small oval cells, which extend downwards to 'Vhxnit the middle
of the lateral region of the embryo. They do not share in bounding
the coelenteron, since a third kind of cells (ik), large and rich in yolk,
lie along their inner surfaces. The latter begin at the margin of

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

117

ml?

ak

dz

the chorda! fundament as a single layer, become two layers thick
farther down, and thus merge into the more voluminous accumu-
lation of yolk-cells, which, in all Amphibian embryos, occupy the
ventral side and restrict the gastrula-cavity. They correspond, to
continue with our comparison, with the entoderm, whereas the
small-celled masses, which, starting from the fundament of the
chorda, have crowded themselves out between the entoderm and
the outer germ-layer, are comparable with the cells which in Am-
phioxus and in our diagram form the wall of the body-sacs, or the
middle germ-layer. The
conclusion is therefore jus-
tified and very obvious, .^sBS&r-i
that in Triton the two mid-
dle germ-layers have arisen
in the anterior territory of
the embryonic body by a
process of evagination at
both sides of the chordal
fundament, just as in Am-
phioxus, except that in one
case the evaginated cell-mass
contains a cavity, in the
other case none.

A cross section through
the blastopore of the Triton
embryo (fig. 78) is to be
compared with our second
diagram (fig. 75). The
hollow body-sacs of the latter correspond to the solid cell-bands,
which are the fundament of the middle germ-layer. Near the
blastopore (u) they are split into two lamellae. Of these the outer
(mk 1 ) merges, as in our diagram, into the inner layer of the blasto-
poric lip, and becomes continuous at the edge of the blastopore with
the outer germ-layer (ak) ; the inner lamella (mk 2 ), on the contrary,
is connected with the mass of yolk-cells (dz), which lies like a wall in
front of the blastopore and even projects into it as the RUSCONIAN
yolk-plug (dp).

Posteriorly to the blastopore, the middle germ-layer stretches itself
out for some distance, but here only as a single connected mass.

According to the region~:trom Which thtTmiddle germ-layer is de-
veloped, we may divide it into two portions, and call that part which

Tig. 78. Gross section through the blastopore of an
egg of Triton with feebly expressed medullary
groove.

ak, Outer, ik, inner germ-layer; m/fc 1 , parietal, mk",
visceral lamella of the middle germ-layer; u,
blastopore ; dz, yolk-cells ; dp, yolk-plug ; dh,
intestinal cavity.

118

EMBRYOLOGY.

is produced on both sides of the chorda the gastral jmespderm, and
that which arises from the blastopore the peristomal mesoderm
(RABL).

mp

a/fc

mfc 1

Ih

mV

ik

fig. 79. Three cross sections from a series through an egg on which the medullary ridges begin
to appear. The sections illustrate the development of the chorda out of the chordal
fundament, and the constricting off of the two halves of the middle germ-layer.

ak, Outer, ih, inner germ-layer ; mk l , parietal, ml?, visceral lamella of the middle germ-layer ;
mp, medullary plate ; inf, medullary folds ; ch, chorda ; Ih, body-cavity.

The further development of the fundaments of mesoderm, chorda,
and intestine, which subsequently become entirely separated from
one another at the places where they now remain in connection,
causes the agreement with the conditions found in Amphioxus to

K A -
. ^j^^vv t4$ ^

DEVELOPMENT OF THE TWO MIDDLE GEKM-LAYEKS. 119

appear in stronger relief. The process of separation is introduced /
by the curving of the chorda! plate, and its conversion into the
chordal groove (fig. 79 A ch). Inasmuch as it is continuous at its
edges with the parietal lamella of the middle germ-layer (mk 1 ), there
arise in the roof of the^coelenteron the two small chordal folds, which
enclose between them the chordal groove. Its free margins abut
directly upon the folded edge, where the visceral lamella of the
middle germ-layer (mk 2 ) bends around into the entoderm (ik) to
produce the ccelenteric fold.

In the next following stage (fig. 79 E) the thickened medullary
plate, consisting of long cylindrical cells, becomes distinctly marked
off from the now still smaller cubical elements of the ectoderm.
Meanwhile the middle germ-layer begins to detach itself from its
previous connections in the vicinity of the place of evagination ; the
parietal lamella becomes separated from the fundament of the
chorda, the visceral lamella from the entoderm, and thereupon their
detached edges become fused to each other. By means of this pro-
cess the fundament of the body-sac, or of the middle germ-layer,
becomes closed on all sides, and is separated from the other
germ-layers. At the same time the entoderm (ik) and the funda-
ment of the chorda (ch) have come into contact along then* free
margins, so that the chorda appears like a thickening of the ento-
derm, and for a time shares in bounding the intestinal cavity on the
dorsal side. This is changed by a second process of detachment.

The fundament of the chorda, now converted into a solid rod,
is gradually excluded from participation in lining the intestine
(fig. 79 G), by the fact that the halves of the entoderm (ik), composed
of large yolk-cells, grow toward each other underneath it, and fuse
in a median raphe.

The closure of the permanent intestine on the dorsal side, the con-
stricting off of the two body-sacs from the inner germ-layer, and the
origin q/ the chorda dor sails are therefore in Amphibia, as in Amphi-
oxus, processes which are most intimately related with one another.
Here, too, constricting off of the parts mentioned begins at the head-end
of the embryo, and advances slowly toward the posterior end, where
there exists for a long time a zone of growth, by means of which the
increase in the length of the body is effected. Soon after this, the
moment arrives when in the embryos of Triton the body-cavity
becomes visible. For after the detachment of the organs previously
mentioned is completed, the two middle germ-layers at the head-end
of the body, and on both sides of the chorda, separate from each

120

EMBRYOLOGY.

other, and thus cause to appear a right and a left body-cavity
(enterocoel), which, according to my interpretation, were not pre-
viously recognisable, simply on account of the intimate mutual

nK contact of their

walls.

Meanwhile the
medullary plate
has become con-
verted, by the
process of folding
already described,
into the neural
tube (fig. 80 me),
which lies beneath
the epidermis.
Since the neural

Fig. 80 longitudinal [sagittal] section through an advanced em-
bryo of Bombinator, after GOETTE.

m, Mouth ; an, anus ; I, liver ; ne, neureuteric canal ; me, medullary
tube ; ch, chorda ; pn, pineal gland.

tube subsequently

encloses the blastopore, and is thereby in communication with the
intestinal tube (as the preceding longitudinal section of an advanced
embryo of Bombinator most distinctly shows), it follows that there is
also in the Amphibia a structure (fig. 80 ne) corresponding to the
neurenteric canal of Amphioxus (compare fig. 68 en).

More fundamental differences in the development of the middle
germ-layer are
met with in
the eggs of
Fishes, Rep-
tiles, and Birds,
which are more
abundantly
provided with
nutritive yolk
and undergo
partial cleav-
age, and also

in the eggs of Mammals. However, the variations appear in these
cases to be of a subsidiary nature, whereas in the chief points the
unity of the developmental processes for all vertebrated animals tas
been the more firmly established the more accurately the individual
stages have been investigated by means of improved methods.

In the presentation of these difficult conditions, we shall describe

Fig. 81 A and B.-Two germ-discs of Hens' eggs in the first hours of

incubation, after KOLLER.
df, Area opaca; hf, area pellucida ; *, crescent; tic, cresceut-knob ;

Es, embryonic shield ; pr, primitive groove.

DEVELOPMENT OF THE TWO MIDDLE GERAI-LAYERS. 121

first the changes which may be recognised in viewing the germ-disc
from the surface, and to these shall add, secondly, the more im-
portant results acquired by series of cross sections.

At the posterior margin of the germ-disc of the Chick (fig. 81 A),
which consists of two layers lying on the yolk like a watch-glass, we
had distinguished not only a short time before incubation, but also
during the early hours of that process the crescent (s) and the
crescentic groove, and had learned to recognise that this was the
place from which the inner germ-layer arose by a process of folding
under.

When, during the first hours of incubation, the germ-layers grow
out farther on the yolk, the crescentic groove (fig. 81 .Z?) is con-
verted into the primitive groove (pr), a structure of far-reaching
significance.

The metamorphosis, according to the excellent researches of DUVAL,
takes place in the following manner : In the middle of the anterior
blastoporic lip, where the outer germ-layer bends over to become
continuous with the inner, there arises a small notch, which is
directed forwards (fig. 81 A sk) ; this gradually elongates into a
groove (fig. 81 .5), corresponding with the future longitudinal axis of
the embryo, and by the following method : the right and the left halves
of the [anterior] blastoporic lip, together with the part which bounds
the first notch, grow toward each other, and come in contact with
each other in the median plane, with the same rapidity with which the

disc increases in super-

A B c

ficial extent. For a time, ,. ..

therefore, the blastopore ,.-;;~~~-^ /.'''--"~,~,~^\*\

has the form of a short '/~'"~'\\ ',''/'* " V'. \ : :' ;'' "' ' : ';

longitudinal groove, i( \\'\ / V\Vv. ,//'/'.'

which, at its posterior N^^' '*^~-&:'-'

end, is beilt around into pig. 82. Diagrams to elucidate the formation of theprimi-
two Short transversely tive groove, after DUVAL.

The increasing size of the germ-disc in the course of the

placed crescentic horns development is indicated by dotted circular lines. The

(g). Finally these also heay y lines ^resent the crescentic groove, and the

primitive groove which arises from it by the fusion of
have disappeared ; they, the edges of the crescent.

too. have grow y n toward

each other, toward the median plane, and have thus contributed
largely to the posterior elongation of the primitive groove. By this
remarkable process of growth the whole blastopore is converted from
a ti ansverse fissure into a longitudinal one.

The accompanying diagrams (fig. 82) serve to illustrate this highly

122 EMBRYOLOGY.

important process. The increase which the germ-disc has undergone
during successive stages is indicated by dotted lines. The margin of
the fold, where the upper germ-layer passes over into the lower
layer, or the anterior lip of the blastopore, is denoted by a heavy
black line. In the figures A, B, C, one
observes how, with the increasing extent of
the germ-disc, the right and left halves of
the blastoporic lip come together in the

median plane
in ever-increas-
ing extent, and
form the primi-
tive groove.

In figs. 83
and 84 are pre-
sented instruc-
tive cross sec-
tions through
the primitive
groove in the
first stages of its
development.
The first shows
us the two lips
of the blasto-
pore (fig. 83 u),
separated by a
small space,
into which
there projects
from below a

small elevation (dp) of yolk-substance,
containing a number of nuclei (merocytes),
comparable with the RUSCONIAN yolk-plug
in the Amphibian larva (fig. 78 dp). At
the lips, the upper germ-layer, a single cell thick, bends around into
the lower germ-layer, composed of loosely associated cells. The
blastopore leads into the ccelenteron, which lies between yolk and
germ-disc. In fig. 84 the margins of the two folds have come into
close contact, and have fused to form the anterior part of the primi-
tive streak, above which the primitive groove is still to be found.

2 a ,-
HI rtS

t-3*

: *

CO _rt 2

. o

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

123

When the last remnant of the crescentic groove has been employed
for the elongation of the primitive groove, the margin of the germ-
disc, which continues all the time to spread itself out uniformly over
the yolk, exhibits everywhere one and the same condition ; it has
become at all points a circumcresceiice-margin, now that the in-
vaginatwn-margin has detached itself from it as primitive groove.

Fig. So

Fig. 85. Surface view of the area pellucida in the blastoderm of a Chick, soon after the

formation of the primitive groove, after BALFODE.
pr, Primitive streak with primitive groove ; a/, amniotic fold. The darker shading surrounding

the primitive streak indicates the extent of the mesoblast.

Fig. 86. Surface view of the area pellucida of a blastoderm of 18 hours, after BAUOUR.

The area opaca is omitted ; the pear-shaped outline marks the limit of the area pellucida. At the
place where the two medullary folds are continuous with each other there is to be seen a
short curved line, which represents the head-fold. In front of it there lies a second line
concentric with it, the beginning of the amniotic fold. A, Medullary folds ; me, medullary
fnrrow ; pr, primitive groove.

When subsequently the pellucid and opaque areas become more dis-
tinctly separated, the primitive groove comes to lie in the posterior
part of the pellucid area. By careful examination of a surface pre-
paration (figs. 85 and 86 pr), one sees that it is bounded, both on the
right side and on the left, by two small folds, which are derived
from the blastoporic lips, and which appear darker and more opaque
because the cells are multiplying rapidly and are more closely
crowded. Since the two primitive folds, or the two blastoporic lips,

124

EMBRYOLOGY.

mf

Fig. 87. Blastoderm of the Chick, incubated 33 hours,
after DUVAL.

The area pellucida (/</) is surrounded with a portion of the
opaque area (<Jf). The fundament of the nervous
system is nearly closed in front and segmented into
the three brain- vesicles hb\ A6", lib' ; behind, the
medullary furrow (mf) is still open. On either side
of the latter there are six primitive segments (us).
The posterior end of the embryonic fundament is
occupied by the primitive streak and the primitive
groove (pr).

the primitive streak and its head-process,

are closely in contact
at the bottom of the
groove, and indeed are in
places completely fused,
they together produce
in the pellucid area a
dark streak of sub-
stance, which is about a
millimetre long and 0*2
mm. broad. With the
earlier embryologists, to
whom it was already
known, we designate
this as the primitive
streak of the germ-disc.
In the vicinity of the
primitive streak there
are to be distinguished
in surface views, now
and during the following
stages of development,
some additional changes,
which are caused by the
beginnings of special or-
gans. In the first place,
there is to be seen in the
anterior region of the
area pellucida, and in
the direct continuation
of the primitive streak,
a narrow, dark streak of
cells, which has been
designated by KOLLIKEE
as the head-process of
the primitive streak,
and which gradually in-
creases in length. Se-
condly, there appears an
increasing opacity (fig.
85) in the vicinity of
which afterward stretches

DEVELOPMENT OF THE TWO MIDDLE GERM LAYERS. 125

out farther laterally : it is connected with the origin of the middle
germ-layer.

In a still later stage of development (fig. 86), at the beginning of
the second day of incubation, the first fundament of the central
nervous system makes its appearance in the anterior portion of the
germ-disc. Over the head-process there arise at some distance from
each other the two medullary folds (A), which are continuous with
each other at their anterior ends, and which bound the broad medul-
lary furrow (me) ; posteriorly they become less prominent, and they
here embrace between them the anterior end of the primitive streak
(pr). Medullary furrow (me) and primitive groove (pr) must not
be confounded with each other, as occurred in the earlier days of
embryology ; they are two entirely distinct and dissimilar structures,
which exist at the same time, and independently of each other, as
fig. 86 shows.

Primitive streak and primitive groove are preserved for a long
time without undergoing important changes (fig. 87 pr). They
always occupy the posterior end of the embryonic body, which is
characterised by its slightly differentiated condition even in stages
when the development of the separate organs of the body is already
in full progress. On the contrary, the embiyonic territory lying in
front of it, which is so small at the time of the appearance of the
head-process, becomes greatly elongated and, at the same time,
differentiated into the separate organs of the body. This process
of differentiation begins in front, and proceeds posteriorly toward
the primitive groove, just as in Amphioxus and the Amphibia.
The margins of the medullary folds come into contact with each
other and begin to fuse, forming the neural tube (A6 1 , hb 2 , hb z ,
mf), the fusion progressing from the head- toward the tail-end.
There are~~alsa to be recognised now in the interior of the body,
at either side of the r.eural tube, the protovertebrae or primitive
segments (us), which we shall investigate more minutely further
oni^ The number of these is constantly increased by the growth
which is taking place at the tail-end.

When a large number of primitive segments has arisen, the ,\
primitive groove begins on surface-views to disappear ; for it is sur-
rounded by the medullary folds, and inasmuch as these fuse here as
well as elsewhere, it is enclosed in the terminal part of the neural
tube. A notable COTuEtTori, and one of great importance for the
interpretation of the primitive groove, has been discovered at this
stage in the embryos of several species of Birds by GASSER, BRAUN,

126

EMBRYOLOGY.

JO*

me

ob-
still
dis-

HOFFMANN, and others. At the front end of the primitive groove a
narrow canal has arisen, which leads obliquely from the neural tube
under the entoderm, and unites the two in the same manner in which
the blastopore does in Amphioxus and the Amphibia. A diagram-
matic longitudinal section through the hind end of a Chick (fig. 88)
shows us this important union (n.e), which exactly corresponds to the

condition of
an Amphi-
bian embryo
presented in
fig. 80.

Such a
neurenteric
canal has
been
served
more
tinctly in
Selachians
and Reptiles
and at even
e a r 1 i e r
stages,
whereas in
Teleosts it

does not come to development on account of special subsidiary
conditions.*

The investigation of the embryonic fundaments of a Mammal fur-
nishes us with views quite similar to those respecting the Chick. When

* In Selachians the blastopore is very early enclosed within the medul-
lary folds, and then assumes the condition of a long-persisting canal-like
passage to the intestinal cavity through the floor of the medullary groove,
and later through that of the neural canal.

"In the case of Reptiles, the primitive streak is very short and triangular,
and in many species soon discloses, before other organs have been differentiated,
an opening at its anterior end which leads to the cavity under the germ-disc,
which is filled with yolk. Subsequently the opening is converted into a canal,
the wall of which is composed of cylindrical cells, and is in continuity above
with the outer germ-layer, and below with the inner germ-layer. Then the
medullary folds, which are being formed in front of the orifice, grow around
it ; the orifice now becomes a genuine neurenteric canal, which in many cases
appears to become obliterated even before the closure of the medullary tube,
but in other cases persists for a Ions; time.

Pig. 83. Diagrammatic longitudinal section through the posterior end of
an embryo Chick at the time of the formation of the allantois, after
BALFOCR.

The section shows that the neural tube (Sp.c) is continuous at its posterior
end with the post-anal intestine (p.a.g) by means of the neurenteric
canal (n.e). The latter traverses the remnant of the primitive streak
(pr), which is folded over on to the ventral side, ep, Outer germ-layer ;
ch, chorda ; hy, entoderm ; al, allaiitois ; me, middle germ-layer ; an,
the place where the anus will arise; am, amnion ; so, somatopleure ;
gp, splanchnopleure.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

127

the embryonic area has assumed an oval form, the opacity at the
posterior end, or the terminal ridge (fig. 63 hw), which was compared
with the crescent of the Bird, elongates into the primitive streak ;
the latter occupies the posterior half of the embryonic area (fig. 89
A pr), and exhibits a distinct groove, that is flanked by a right and a
left ridge-like fold. (Compare with this the Chick as shown in fig. 85.)

Fig, 89 A. Embryonic fundament of an 8-days Babbit, after KOLI.IKER.
arg, Fundament of the embryo ; pr, primitive streak.

Fig. 89 B. Vascular area (o) and embryonic fundament (.(/) of a 7-days Rabbit's egg, after

KOLLIKEK.
o, Vascular area (area opaca) ; ag, embryonic fundament ; pr, primitive groove ; rf, medullary

furrow.

Afterwards there appears in this instance, just as with the Chick, a
narrow opaque streak in the forward prolongation of the primitive
streak, its head-process, and this divides the anterior portion of
the germ into a right and a left half (fig. 90 kf). After some time
there are developed on both sides of the head-process the medullary
folds (fig. 89 .5), which bound the broad medullary furrow (rf}, and
which, by forming a bow at their anterior ends, become continuous
with each other ; but posteriorly they diverge somewhat from each
other, and embrace the primitive groove (pi'). This stage corresponds
to the condition of the Chick presented in fig. 86.

128

EMBRYOLOGY.

From this time forward the anterior part of the embryonic area
grows in length much more rapidly than the hind part with its
primitive groove ; the latter remains almost unaltered in Mammals
up to late stages of development, and then diminishes in length, not
only relatively, but also absolutely.

Fig. 91.

Fig. 90. Germ-disc of an embryo Babbit with primitive streak, after B. VAN BENEDEN.
?, Primitive streak ; kf, head-process ; Me, HEXSEN'S node ; en, canalis neurentericus.

Fig. 91. An embryo Rabbit with a part of the area pellucida 9 days after fertilisation,
Magnified 22 diameters. After KO'LLIKER.

op, Area pellucid* ; ao, area opaca ; h', medullary plate in the region of subsequent first brain-
vesicle ; h", the same in the region of the subsequent mid-brain, where the medullary furrow
(rf) exhibits a widening ; h'", the same in the region of the subsequent third brain-
vesicle ; hz, fundament of the heart ; stz, trunk zone (Stammzone) ; pz, parietal zone ; pr,
remnant of the primitive streak.

At the same time the embryonic area passes from the oval to a
pronounced guitar-shaped outline. Such an embryo is represented
in fig. 91. The primitive streak (pr) is to be seen at its posterior
end, partly embraced by the medullary folds (rf). The middle germ-
layer is already fully developed, and in the future neck-region three
pairs of primitive segments have already been differentiated at the
sides of the chorda.

Just as there has been up to this stage an agreement with Birds

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 129

and Reptiles in other points, so there also is in the existence of a
neurenteric canal. At a rather early stage there isjilready noticeable,
at the anterior end of the primitive streak, a small spot, at which,
in consequence of cell-proliferation, a large amount of material is
accumulated. It is known under the name of HENSEN'S node (fig 90
hk). This is important chiefly because a narrow canal, the canalis
neurentericus (en), passes through it, and leads from the outside into
the interior of the blastodermic vesicle The presence of this canal
has already been established by several investigators by VAN"
BENEDEN in the Eabbit and the Bat, by BONNET in the Sheep, by
HEAPE in the Mole, and by GRAF SPEE in a young human embryo.
The latter exhibited a still widely open medullary furrow. At the
beginning of the primitive groove there was a wide, roundish,
triangular orifice, which traversed the germ-disc, and was surrounded
by a ring-like elevation corresponding in position to HENSEN'S node.

I have dwelt upon the primitive streak more at length, and have
considered more in detail its first appearance and its topographic
relations to other organs, because from a developmental standpoint
it is a very important structure, and one the significance of which
is still much discussed. For it corresponds to the blastopore of the
lower Vertebrates, and is important as the region from which the
middle germ-layer takes its origin. While I postpone an exposition
of the grounds which warrant us in designating the primitive groove
as blastopore, I shall at once consider the development of the middle
germ-layer. Information concerning this is to be got from cross
sections, which should be made, as in the Amphibians, (1) in front
of the primitive groove, (2) in the region of the groove, and (3) back
of it, both in younger and older embryos.

In embryonic fundaments which have reached the stages repre-
sented in figs. 81 J3, 85, and 89, the middle germ-layer is already
begun in the immediate vicinity of the primitive groove, and causes
the opacity which appears upon both sides and in front of it. Cross
sections through the cephalic process of the primitive streak now
allow the establishment of a complete agreement in one fundamental
point between Amphioxus and the Amphibia on the one hand, and
Selachians, Reptiles, Birds, and Mammals on the other.

Along a narrow median streak, in the former groups in front of the
blastopore, in the latter in front of the primitive groove, the embryonic\
fundament is composed of only two germ-layers, of which the lower i<;
destined to become the chorda. At both sides of these regions the two-
layered condition passes abruptly in all Vertebrates into a three-layered

9

130

EMBRYOLOGY.

one, the outer germ-layer being followed by the middle layer, and this
by the inner germ-layer.

rnp

fig. 92 A and B. Cross sections through the germ-disc of a Selachian. Copy after BALFOUR'S

Monograph, PI. IV., Fig. 8a, and PI. IX., Fig. la.
Only the left half of section A is represented.
ak, Outer, ik, inner, mk, middle germ-layer ; ch, chorda ; -np, medullary plate ; d, yolk.

The conditions in detail assume in Selachians, Birds, and Mammals
the forms indicated by the accompanying figures (92-95).

In the Selachians the medullary fold is well marked in cross
sections (fig. 92 A mp). Beneath it there lies, as in Amphioxus and
Triton, only a single layer of~tall cylindrical cells (ch), the funda-
ment of the chorda ; laterally this merges into a many-layered mass
of small cells, which is soon divided by means of a fissure into two
distinctly separated lamellae into the middle layer (mk), composed
of small polygonal cells, and into the inner layer (ik), which here
consists of a single layer of tall columnar cells. At the point in-
dicated by a star, the fundament of the chorda and the middle

and inner germ-layers
are continuous with one
another. At a later
stage (fig. 92 B) a se-
paration of the three
fundaments takes place,
as in Triton, and we

ak -

tt

Fig. 93. Cross section through the blastoderm of a Chick
in -which the first traces of the chorda and the medullary
furrow are to be seen, after BALFOUR AND DEIGHTON.

section at the right of the fundament of the chorda is
not figured.

at, Outer, mk, middle, ik, inner germ-layer ; ch, fundament
of the chorda.

The section passes through the fundament of the chorda 6n " ave 0) a round

in front of the primitive streak. The part of the chordal rod (ch), which

has been formed by in-
folding in the manner
previously described ; (2)
at either side of it the small-celled mass of the middle germ-layer
(mk), divided into halves by the chorda ; (3) the inner germ-layer
(ik), the halves of which, separated in the previous stage, are
now growing under the chorda, and are about to fuse into a single
layer.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 131

A similar view is furnished by a cross section through the cephalic
process of the germ of the Chick (fig. 93). Under the outer germ-
layer there is found in the median plane, in front of the primitive
groove, only the fundament of the chorda (c/t) ; at the point indicated
by a star it is continued laterally into the small-celled middle germ-
layer, and into the entoderm, which is composed of a single layer of
very much flattened cells.

The same is true for cross sections of Mammals (fig. 94) in corre-
sponding stages of development. Thus, for example, the funda-
ment of the chorda (ch) in the cross section through the embryo of a
Mole figured by HEAPE is a single layer of cylindrical cells ; it has
already become curved into a chordal groove, such as has been repre-
sented in fig. 79 A for Triton. Laterally it is continuous with a
mass of small cells, which is resolved into two layers at the point

Fig. 94. Cross section through the embryonic area of a Mole which is in about the stage of the

Rabbit represented in Fig. 89 B. After HEAPE.
The section passes through the chordal groove (ch) somewhat farther forward than the section

represented in Fig. 97, which has encountered a region that is to be interpreted as the

blastopore.
ok, Outer, mk, middle, ik, inner germ-layer ; eh, fundament of the chorda.

indicated by a star : (1) into the middle germ-layer (mk), composed
of several layers of small cells; and (2) into the inner germ-layer,
which, as before, appears as a single layer of flattened cells (ik).

In a still more convincing manner VAN BENEDEN has shown, in his
investigations upon the development of Mammals, that conditions
exist in the formation of the middle germ-layer and of the body-
cavity in this class which agree with those in Amphibia. The cross
section (fig. 95) through the germ-disc of the Eabbit, taken from
his work, is especially convincing. It shows the fundament of the
chorda (ch) as a single layer of cylindrical cells, flanked on the right
and left by the middle and inner germ-layers. The middle germ-
layer consists of a parietal (mk 1 ) and a visceral (mk 2 ) lamella of flat
cells, the former of which is continuous with the fundament of the
chorda, while the latter bends around at the point indicated by a
star to become continuous with the single-layered epithelium of the

132

EMBRYOLOGY.

inner germ-layer (ik). The place where the bend occurs even pro-
trudes distinctly as a lip into the ccelenteron, as in the case of the
Amphibia. Except for these unions at the sides of the chordal

mk* ml?

Mi

ik

Pig. 95. Cross section through the germ-disc of an embryo Rabbit, after E. VAN BEKEDEN.
fc, Outer, ik, inner, ink, middle germ-layer ; mk 1 , parietal, mfc", visceral lamella of the middle
germ-layer ; ch, chorda.

fundament, the middle germ-layer is everywhere sharply separated
by a fissure from the other two germ-layers.*

Further agreement with the conditions which the investigation
of Triton has furnished is afforded by a series of cross sections
through the primitive streak the obliterated blastopore. In the case of
all Vertebrates, this is the only place in the whole embryonic area where
all three germ-layers, although for only a short distance, are fused toith
one another, and cannot be distinguished as separate layers, whereas at
the sides of this region they are separated by distinct fissures.

-,nk pr ps mk ok

Pig. 96. Cross section through the middle of the primitive streak of a Chick's germ-disc, which
is in the stage of development represented in Fig. 81 B. After ROLLER.

At some distance from the primitive groove is to be seen upon the left side of the figure in cross
section the marginal groove of His. Upon the right sifle it is as yet little developed.

ak, Outer, ik, inner, mk, middle germ-layer ; pr, primitive groove ; ps, primitive streak ;
gr, marginal groove.

Figure 96 represents a cross section through the embryonic area
of a Chick in which the primitive groove is distinctly developed,

* In the development of Mammals there has been observed at certain stages
under the fundament of the chorda a peculiar structure, the so-called chordal
canal, which is not found in the other classes of Vertebrates. I mention it
here only incidentally, because the publication of VAN BENEDEN'S investiga-
tions will doubtless furnish the desired explanation of its origin and signi-
ficance.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYEKS. 133

but in which no traces of the medullary folds are to be observed.
The outer germ-layer (ak) is composed of a single layer of tall
cylindrical cells, the inner germ-layer (ik) of a single sheet of
greatly flattened elements. In the space between the two there
penetrates at both sides of the primitive groove a mass of small cells
in many superposed layers, the middle germ-layer (mk). In the
region of the primitive groove (pr) this goes over continuously into
the outer germ-layer, the cells of which are here found in prolifera-
tion, whereas its lateral wings are separated from the outer layer by
a fissure. The lower germ-layer is drawn by ROLLER from whose
work the accompanying figure is taken as being everywhere a

Fig. 97. Cross section through the embryonic area of a Mole, which is in a stage corresponding

approximately with that of the Rabbit represented in Fig 89 3. After HEAPE.
The section passes through the primitive groove, somewhat behind the one represented in Fig. 94.
at, Outer, ik, inner, mi, middle germ-layer ; u, primitive groove.

separate sheet of flattened cells. It is clear, however, from other
drawings and descriptions by DUVAL, RABL, and others, as well as
from the accounts in regard to the similar development of Reptiles,
that for a certain distance underneath the primitive groove the
middle germ-layer is as little to be distinguished as a separate
structure from the lower as it is from the upper germ-layer.

Cross sections through the primitive groove of infl.nnTna.1ian
embryos are very instructive (tig. 97). According to HEAPE'S inves-
tigations on the Mole, the groove (u) cuts deeply into a mass of
small cells. At this place all three layers are fused together ; and
it is only laterally to this that they are separated by means of
a distinct fissure, and that each is distinguishable by its character-
istic kind of cells the outer (ak) by its tall, the inner (ik) by its
much-flattened, and the middle (mk) by its small, more spherical
or polygonal cells.

The conditions of the germ-disc of the Rabbit found by VAN
BENEDEN are especially distinct (fig. 98). At the deep incision

EMBRYOLOGY.

of the primitive groove (pr) all three germ-layers are joined to
one another for a certain distance by means of a common cells

Fig. 98. Cross section through the primitive groove (blastopore) of a Babbit's germ-disc, after

ED. VAN BENEDEN.
ak, Outer, ik, inner, mk, middle germ-layer ; mV, parietal, mk', visceral lamella of the middle

germ-layer ; ul, lateral lip of the blastopore ; pr, primitive groove.

mass. At the same time one may observe, with tolerable dis-
tinctness, how the outer germ-layer (ak) bends around into the
parietal middle layer (mk 1 ) at the primitive fold (ul), while the
visceral lamella (mk 2 ) is continuous with the entoderm (ik), which
is only one cell thick. Indeed, in embryos of Eabbits and Bats, VAN
BENEDEN in some cases observed between the primitive folds, or

fig 99. Cross section through a human germ-disc, with open medullary groove, in the

vicinity of the neurenteric canal (pr), after GRAF SPEE.
at, Outer, ik, inner germ-layer; mk 1 , parietal, mk~, visceral lamella of the middle germ-layer;

ul, lateral lip of the blastopore ; pr, primitive groove.

blastoporic lips, a structure corresponding to the yolk-plug of
Amphibia.

It is certainly of great general interest that the investigation of
an extraordinarily young human germ-disc at the hands of GRAF
SPEE has furnished a cross section (fig. 99) which is near enough

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 135

like the one of the Rabbit here figured to be mistaken for it. In
the case of the human embryo, one sees a deep-cutting primitive
groove, and at the easily recognisable blastoporic lip (ul) the bend-
ing over of the outer germ-layer (ak) into the parietal lamella (mk l ).
The visceral lamella (mk 2 ) is well separated from the latter for some
distance ; under the primitive groove it is merged with the inner
germ-layer, the edges of the potential folds of the two sides being
fused into a mass of cells, which forms the floor of the primitive
groove.

Finally an agreement with the development of the Amphibia is
not wanting in sections which are made through the embryonic
areas of Birds, Reptiles, and Mammals behind the primitive groove.
The middle germ-layer begins to spread itself out backward also,
not, however, as in the anterior part of the embryonic area, in
the form of paired fundaments, but rather as a single continuous
cell-mass. This outgrowth too is united to the two primary
germ-layers only in the region of the posterior end of the primi-
tive streak, being elsewhere distinctly separated from both of
them.

For the completion of the previous account, some statements
about the further growth of the middle germ-layer may now be
added, concerning which cross sections through embryos of various
ages afford evidence. The middle germ-layer spreads itself out
on all sides between the two primary germ-layers, farther and
farther from the place of its first formation the vicinity of the
primitive groove. At first it is limited to the fundament of the
embryo itself, then it makes its way into the area pellucida, and,
finally, it is encountered in the opaque area. Everywhere and
constantly in its extension it appears as an entirely independent
layer, at least two cells thick, which is separated from its surround-
ings by fissures. It is found to be united for a short distance with
the inner and outer germ -layers, but only at the primitive groove,
which persists for a long time, in older embryos even, as we havo
already learned from surface-views. Even in the stage when the
neurenteric canal traverses the primitive streak, and puts the
ccelenteric cavity (under the entoderm, fig. 100 hy} in communication
with the neural tube, we see the cellular lining of the canal and the
middle germ-layer fused, so that in this region a connection still
exists between all three germinal layers. Compare the accompany-
ing cross sections through embryos of Lacerta muralis.

After the statement of the actual conditions, the questions remain

136

EMBRYOLOGY.

to be answered : (1) What is the meaning of the primitive groove ?
(2) How is the middle germ-layer developed ?

In the interpretation of the primitive groove I place myself, as is
to be seen from what precedes, wholly on the side of those investi-
gators who, like BALFOUE, HATSCHEK, KUPFFER, HOFFMANN, VAN
BENEDEN, L. GERLACH, RUCKERT, and others, recognise in it a structure

equivalent to, but somewhat modi-
fied from, the blastopore of lower
Vertebrates, and who compare the
primitive folds to lateral blasto-
poric lips closely pressed together.
In my description of a previous
stage I have already designated
as blastopore the crescentic
groove of Birds (fig. 52 B s)
and the prostoma (fig. 55 u)
of Reptiles, because that is the
place where the lower germ-layer
is infolded. In my opinion both
grooves are identical structures,
which, by changes in position and
form, have been so evolved, the
one from the other, that the
fissure, which ivas at first trans-
verse, has become converted into a,
longitudinal one. For Reptiles
KUPFFER has established this to
a certainty. According to his figures in Emys Europsea, e.g., the
transverse depression (u) represented in fig. 101 A is converted at
a later stage into the form shown in the adjacent figure (101 B u).
For the Birds the investigations of DUVAL previously recounted
(p. 121, fig. 82) are convincing. There is also to be taken into
account the additional fact, that even as early as in the
Amphibia an exactly corresponding metamorphosis of the blasto-
pore takes place. As the accompanying cuts (fig. 101 C and D)
show, the blastopore of the Amphibian is, at its first appearance,
a transverse fissure (fig. 101 C u). Then it becomes circular, and
embraces with its lips a protruding portion of the otherwise
enclosed yolk-mass, the yolk-plug, becomes narrower, and is
continued forward into a longitudinal groove. Finally it appears
(fig. 101 D u) as a deep groove- situated at the end of the

Ay

Fig. 100. Cross sections through the posterior
end of a young embryo of Lacerta muralis,
after BALFOUR.

in figure A the neurenteric canal is cut length-
wise ; in figure B only an evagination of
it, which is directed backward. Since the
sections probably have not cut the chief
axis of the embryo perpendicularly, the
middle germ-layer is fused with the wall
of the canal only on the right side in figure
A, whereas in figure B the connection is
present on both sides.

te, Neurenteric cana ; ep, outer, mep, middle,
hy, lower germ-layer.

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS.

137

medullary furrow, with its small circular opening filled up with a
yolk-plug.

In addition there are three important considerations which may
be urged in support of the interpretation of the primitive groove as
blastopore.

First, the primitive streak, even when an open canal is wanting,
is the only place in the whole germ-disc where a connection between

Fig. 101. A and B. A portion of a younger and of an older embryonic fundament of Emys

Europaea, with the prostoma or blastopore (it), after KUPFFER.
til, Lip of the blastopore.
C and D. Two eggs of Triton taniatus seen from the blastopore, one 30 hours, the other 53 hours

after artificial fertilisation.
u, Blastopore ; h, elevation between blastopore and dorsal groove ; /, semicircular furrow, which

encloses the blastoporic area ; dp, yolk-plug.

all the germ-layers is constantly present, as at the Amphibian
blastopore.

Secondly, the chief organs of the body, such as the chorda, the
neural tube, and the primitive segments, are developed in front of
the primitive streak in the case of the higher Vertebrates, just as
they arise in front of the blastopore in Amphioxus and the Amphibia.
Both blastopore and primitive streak occupy the posterior end of the
body. The so-called cephalic process of the primitive streak is
nothing else than the first rudiment of the chorda.

Thirdly, one may still recognise in the openings canales neu-
renterici which have been pointed out in the primitive streak at an
earlier or later stage in its development, in the case of Birds, Reptiles,
and Mammals, an indication that an open communication has

138 EMBRYOLOGY.

existed here from the beginning between the inner and the outer
germ-layers ; further, that this communication has disappeared
through the fusion of the blastoporic lips, but that it can be in part
reestablished in consequence of more favorable processes of growth.
At the same time the neurenteric canal, in cases where it reappears
in the primitive streak, effects a very characteristic union between
the posterior ends of the neural and intestinal tubes, in exactly the
same manner in which the blastopore of Amphioxus, the Amphibia,
and the Selachii does (compare fig. 80 with fig. 88 n.e).

In the interpretation of the primitive groove as blastopore I am
compelled to oppose a somewhat different view. Certain investi-
gators (BALFOUE, RAUBER, and others) recognise in the primitive
groove and the crescentic groove of meroblastic eggs only a small
part of the blastopore ; they interpret as the major part of it the
region which is encircled by the whole rim of the germ-disc and is
occupied by the yolk-mass, and to which they give the name yolk-
blastopore.* According to their conception, as also according to
the original assumption of HAECKEL. the two-layered germ-disc is a
flattened-out gastrula, its blastoporic rim lying upon the yolk-
sphere, which gradually grows around the yolk, and finally takes
the latter wholly inside itself, just as if it were a ball of food. The
primitive groove is a small detached part of the blastopore, which is
connected with the development of the middle germ-layer. The two
parts become completely separated from each other, and are closed
at different times, each for itself, the yolk-blastopore often late, at
the pole of the yolk-sac which is opposite to the embryo.

Such an assumption of a double blastopore appears to me to be
untenable. / propose that only that place of the germ be designated as
blastopore at ivhich, as in the gastrulation of Amphioxus and the
Amphibia, there actually occurs an invagination of cells, by means of
which the cleavage-cavity is obliterated. Such a process takes place
in the Selachii only at the crescentic hinder part of the margin of
the germ-disc, in the Reptiles and Birds at the small place designated
as crescentic groove. It is also from this place alone that subse-
quently the development of the middle germ-layer proceeds.

The anterior margin of the germ-disc in Selachians, and, after the
conversion of the crescentic groove into the primitive groove, the whole

* RAUBER has suggested for the various regions which he assumes for the
blastopore the designations prostoma sulcatum longitud'male (primitive groove),
prostoma mlcatum falciforme (crescentic groove), and prostoma marginale
(yolk-blastopore).

DEVELOPMENT OF THE TWO MIDDLE GEEM-LAYERS.

139

tnargin of the germ-disc in Birds and Reptiles, have an entirely dif-
ferent signification. This margin exhibits a very different relationship
from that of the primitive streak or blastopore ; it is a peculiarity of
meroblastic eggs, which is most intimately associated with the origin
of partial cleavage. It indicates the place at which the segmented
portion of the germ meets the unsegmented portion the place at
which there lie in the yolk free nuclei, by means of which a supple-
mentary cleavage is kept up until late stages in the process of
development, until, in fact, the time when the two primary germ-
layers have been formed by means of the invagination which
occurs at. the blastopore. At the expense of the cell-material, which
is constantly being augmented by supplementary cleavage, the germ-
layers increase in extent at their place of transition into the yolk,
and thus gradually grow over the unsegmented part. Whereas at
the blastopore an invagination of cells already present takes place, there
ensues at the margin of the germ- disc a formation of new cells, and
thereby an increase of the marginal part and an overgrowth of the
yolk. I therefore propose for it the name circumcrescence-margin
of the yolk-sphere. There can be no such thing as a separate opening
or a yolk-blastopore, because the yolk is an organic part of the germ,

and is in continuity
with the segmented
part of it by means
of the layer which
contains the yolk-
nuclei.

If we would insti-
tute a comparison be-
tween animals with
meroblastic eggs and
the Amphi bia at a stage
when gastrulation is
not yet completed, then
the blastopore of the
Amphibia, which is
indicated by the letter
u in the accompanying
section through the

gastrula of a Triton (fig. 102), corresponds to the prostoma of Rep-
tiles, and to the crescentic and primitive grooves of Birds ; the still
exposed mass of yolk-cells corresponds to the yolk-material which is

Fig. 102.~Iongitudinal section through a gastrula of Triton.
ale, Outer, it, inner germ-layer ;fh, cleavage-cavity ; ud, ccel-

enteron; u, blastopore; dz, yolk-cells; dl, dorsal, vl,

ventral lip of the coslenteron.

140 EMBRYOLOGY.

not yet overgrown by germ-layers ; the place marked by a star, at
which in the Amphibia the transition from the small-celled layer
to the mass of yolk-cells occurs, or the marginal zone of GOETTE, is
comparable to the margin of circumcrescence in meroblastic eggs.

In the second place, the question arises : How is the middle germ-
layer of Vertebrates developed ? The answer is : By a process of
folding similar to that in the case of Amphioxus lanceolatus. This
answer is substantiated by the fact that the individual processes in
the development of the middle germ-layer may be correlated with
corresponding processes in Amphioxus.

In view of the fundamental importance of the matter, I formulate
in a synoptic and precise manner in six paragraphs the points in
reference to which it has been possible to establish an agreement in
all Vertebrates.

1. Before the chorda is formed, the germ in all Vertebrates is
composed of two layers in the region of a median streak which lies
in front of the blastopore and primitive groove. It is here composed
of the medullary plate and the fundament of the chorda, which then
shares in bounding the intestinal cavity.

2. At both sides of this median streak the germ is three-layered,
if we regard the middle germ-layer as a single one ; it is four-layered,
if we allow that the latter consists of a parietal and a visceral cell-
layer, which are originally pressed firmly together, and only later
actually separated by the appearance of the body-cavity.

3. In no Vertebrate do the middle germ-layers arise by fission,
either from the outer or the inner germ-layers, because they are
everywhere, except in a very limited region of the germ, sharply
separated from both by means of a fissure.

4. A connection of the middle germ-layers with the neighbouring
cell-layers takes place only : (a) at the blastopore or primitive groove,
where all four (or three) germ-layers are joined together, and (6) at
both sides of the fundament of the chorda.

5. One observes the first fundament of the middle germ-layers at
the region of the germ just mentioned, and sees it spread itself out
from here i.e., from the periphery of the blastopore or the primitive
groove, and from both sides of the fundament of the chorda
forward, backward, and ventrad or laterad. In front of the
blastopore it appears in the form of paired fundaments separated by
the fundament of the chorda ; behind the blastopore, on the contrary,
as a continuous structure.

6. While the chorda is being developed, the two paired fundaments

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 141

of the middle germ-layers detach themselves from the adjacent cell-
layers at the sides where their ingrowth took place, and at the same
time the halves of the permanent entoderm grow together, whereby
the dorsal closure of the intestine is effected.

In view of these facts there is only one explanation at which we
can arrive. If it is certain that the middle germ-layers do not
arise by a fission in loco from either of the primary germ-layers,
then their gradual spreading out from a definite region of the germ
can result only from an ingrowth of cells, which occurs from those
places where a connection with other cell-layers has been demon-
strated. The middle germ-layers draw the principal material for
their growth from cells which, at the blastopore or at the primitive
groove, migrate between the two primary germ-layers.

But this immigration of cells may be interpreted as a process of
infolding of the primary germ-layers, as in the case of Amphioxus.
In the method of the infolding there exists, it is true, one very
striking and apparently important difference between Amphioxus
and the remaining Vertebrates. In Amphioxus the middle germ-
layer arises as a hollow sac, by means of the folding of the inner
germ-layer in the remaining Vertebrates as a solid mass of cells.
This undeniable difference is, however, easily explained in the
following manner : In the solid fundaments of the middle germ-
layer a cavity is wanting, because the cellular walls of the sac are
from the beginning firmly pressed together, in consequence of the
yolk-mass which fills the coelenteron. In addition to other striking
agreements with the conditions in Amphioxus lanceolatus, there are
three points of view which in particular com mend this interpretation :

(1) In all vertebrated animals there early arises in the middle
germ-layer a fissure, which is surrounded by cells, often cubical or
cylindrical, having an epithelial arrangement. The parietal and
visceral layers then take the form of epithelial lamellae, as is to be
seen in an especially striking manner in the case of the Selachii at
a very early stage of development. (2) From these epithelial layers
there arise in the adult genuine epithelial membranes, like the
ciliated peritoneal epithelium of many Vertebrates, and, in addition,
glands that in many respects resemble the glands derived from
epithelial membranes [of the other germ-layers] (kidney, testis,
ovary). (3) The objection that the middle germ-layer of Verte-
brates arises as a single cell-mass, and therefore cannot be equi-
valent to two layers of epithelium, loses its weight with every one
who knows the numerous analogous phenomena of development

142 EMBRYOLOGY.

occurring elsewhere, in which organs that should be hollow are at
first developed as solid masses of cells. We shall hereafter cite as
such the solid fundament of the neural tube in Bony Fishes, many
sensory organs and the most of the glandular sacs, which latter
arise as solid buds of epithelial lamellae, and only later, when they
become functionally active, acquire a cavity by the separation of
their cells.

SUMMARY.
A. The blastula.

1. Out of the mass of cleavage-cells (morula) there is developed
in all Vertebrates a sac-like germ (blastula) with cleavage-cavity.

2. There are four different kinds of blast ulae in Vertebrates,
according to the amount and distribution of yolk.

(a) In Amphioxus the cleavage-cavity is very large, and its

wall consists of a single layer of cylindrical cells of
nearly uniform size.

(b) In Cyclostomes and Amphibia the cleavage-cavity is small :

one half of the wall of the blastula is thin, and composed
of one or several layers of small cells ; the other half is
considerably thickened, and formed of large yolk-cells
arranged in many superposed layers.

(c) In Fishes, Reptiles, and Birds (meroblastic eggs) the

cleavage-cavity is small and fissure-like or wanting.
Only its roof or dorsal wall consists of cells (germ-disc) ;
its floor or ventral wall, on the contrary, consists of the
yolk-mass which has not been divided into cells, but
which contains yolk-nuclei in the vicinity of the margin
of the germ-disc.

(d) In Mammals the cleavage-cavity is very spacious, and filled

with an albuminous fluid ; its wall is composed of a single
layer of greatly flattened hexagonal cells, with the
exception of a small thickened place, where larger cells
in several superposed layers cause an elevation which
projects into the cavity.

B. The cup-shaped larva or gastrula with two germ-layers,

1. There ss, formed out of the blastula, by the invagination of
a portion of its surface, a two-layered form, the beaker-larva or
gastrula.

2. The two layers of the double beaker are the outer and the

DEVELOPMENT OF THE TWO MIDDLE GERM-LAYERS. 143

inner germ-layer (ectoblast, entoblast) ; the fissure separating the
two layers is the obliterated cleavage-cavity ; the cavity resulting
from the invagination is the ccelenteron, its external opening the
primitive mouth (blastopore, prostoma, crescentic groove, primitive
groove).

3. The four kinds of gastrulse correspond to the four kinds of
blastulae.

(a) In Amphioxus the ccelenteron is wide, and each germ-
layer is made up of a single sheet of cylindrical cells.

(6) In Cyclostomes and Amphibia the mass of yolk-cells is
accumulated on the ventral wall of the ccelenteron in
the inner germ layer, and causes a protuberance, by
means of which the coelenteron is reduced to a fissure.

(c) In Fishes, Reptiles, and Birds the process of invagination

remains confined to the germ-disc, since the unsegmented
yolk, on account of its considerable volume, cannot be
made to share in the invagination. The germ-disc
becomes two-layered by means of an ingrowth of cells
at the crescentic groove (blastopore). The yolk acquires
a cellular boundary very slowly and at a late period ;
it is overgrown by the margin of the germ-disc,
when the supplementary cleavage (yolk-nuclei) takes
place.

The outer germ-layer spreads itself out and envelops
the yolk most rapidly ; then follows the inner, and finally
the middle layer.

(d) In Mammals the inner germ-layer is developed from the

thickened region of the blastula, probably by means of
an invagination, because at a later stage an orifice of
invagination, comparable with the primitive groove of
Birds, or a blastopore, can be demonstrated. At the
beginning of its development the inner germ-layer
terminates below in a free margin, so that the ccelen-
teron is for a time closed in on the ventral side by the
outer germ-layer only, a peculiarity which is comparable
with the conditions in Reptiles and Birds, if we conceive
the yolk-material to have disappeared in this instance
before it is completely surrounded by the inner germ-
layer.

4. In Vertebrates the gastrula presents a sharply expressed
trilateral symmetry, so that one can easil distinguish the future

144 EMBRYOLOGY.

head- and tail-ends, the future dorsal and ventral sides of the body.
The blastopore (crescentic groove, primitive groove) marks the
posterior end. The ventral side is characterised by being the place
where the segmented or unsegmented yolk-material comes to lie.

C. The embryo with four germ-layers and a body-cavity.

1. In all Vertebrates there are formed from the roof of the
coelenteron two lateral evaginations of the inner germ-layer, by
means of which the coelenteron is divided into a median cavity, the
secondary intestine, and two lateral cavities, the two body-sacs.

2. The primary inner germ-layer is resolved in consequence of
this process of evagination into three parts :

First, the epithelial lining of the intestinal tube (secondary

inner germ-layer Darm dr iisenblatt) .
Secondly, the epithelial lining of the body-cavity, or the middle

germ-layer, in which a parietal and a visceral layer are

distinguishable.
Thirdly, the chorda, which takes its origin from the portion of

the primary inner germ-layer which lies between the

lateral evaginations from the roof of the ccelenteron.

3. Two modifications of the process of evagination can be recog-
nised in the case of Vertebrates.

(a) In Amphioxus the evaginations are small, numerous, and
segmentally arranged; provided from the first with a
cavity ; and, beginning in the fundus of the coelenteron,
developed toward the blastopore.

(6) In the remaining Vertebrates, instead of hollow sacs, there
grow out from the inner germ-layer two solid masses of
cells :

(1) In the vicinity of the blastopore (primitive groove,

peristomal mesoblast).

(2) From here forward along the roof of the coelenteron,

at a slight distance from the median plane, at both
sides of the fundament of the chorda (gastral
mesoblast).

The paired fundaments spread themselves out from
their place of origin between the two primary germ-
layers farther forward and ventralward.

4. The three organs derived from the primary inner germ-layer
(middle germ-layer, fundament of the chorda, secondary inner germ-
layer) are separated from one another by constrictions.

HISTORY OF THE GERM -LAYER THEORY. 145

First, the body-sacs are detached from the fundament of the
chorda and the entoblast, whereupon the edges of the
parietal and visceral lamellae, thus set free, fuse with
each other.

Secondly, the fundament of the chorda is bent into a chordal
groove, and this is converted into a solid rod, which is
completely isolated from the entoblast.

Thirdly, the entoblast closes together into a tube with a dorsal
raphe.

5. The development of the three fundaments, as also that of
various other organs, begins at the head-end of the embryo, and
advances from here toward the blastopore, where for a long time a
continual formation of new parts and an increase in the longitudinal
growth of the body take place.

6. During the development of the middle germ-layer, the blasto-
pore of the Amphibians, Fishes, Reptiles, Birds, and Mammals has
been metamorphosed into a groove occupying the longitudinal axis
of the embryo (primitive groove of the higher Vertebrates).

7. The blastopore and the primitive groove in later stages of
development undergo degeneration, and are not converted into any
organ of the adult. (For the details of this, see Part II.)

8. Before their disappearance the blastopore and primitive groove
are surrounded by the medullary folds and taken into the terminal
part of the neural tube, whereby a direct communication between
neural tube and intestinal tube the neurenteric canal is effected.
The two organs, which communicate with each other for a long time,
are later separated by its closure.

CHAPTER VII.
HISTORY OF THE GERM-LATER THEORY.

THE fundamental facts of the sheet-like structure of the vertebrate
body, which have been treated of in the two preceding chapters, are
epitomised as the doctrine of the germ-layers, or the germ-layer
theory. Since this theory is of the most far-reaching significance
for the comprehension of the evolution of form in animals, and can
be placed side by side with the cell-theory as coequal with the latter,
I devote a separate chapter to its history.

10

146 EMBRYOLOGY.

The very earliest establishment of the germ-layer theory is asso-
ciated with the most celebrated names in the field of embryology :
CASPAR FRIEDRICH WOLFF, PANDER, and CARL ERNST VON BAER.

CASPAR FRIEDRICH WOLFF, the discoverer of the metamorphosis of
plants, who, even before GOETTE, had clearly and distinctly stated
that the various organs of the plant, as, for example, the separate
parts of the flower, have been developed by various modifications of
leaf-like fundaments, also established the metamorphosis of animals,
for which he endeavoured to found a similar law of development.

He showed in his important work on the formation of the
intestinal canal of the Chick, that it originally appeared in the egg
as a leaf-like structure, and that this afterwards became folded into
a groove, and finally converted into a tube.

He conjectured that the remaining systems of organs might arise
in a similar way, and appended to the account of the development of
the intestinal canal the significant assertion : " It appears as though
at different periods, and many times in succession, various systems
might become formed after one and the same type, and as if they
might be on that account similar to one another, even though they
are in reality different. The system which is first produced, which
is first to take on a specific form, is the nervous system. When
this is concluded, then the fleshy mass, which really makes up the
embryo, is formed after the same type ; then appears a third, the
vascular system, which certainly ... is not so unlike the first ones
that the form described as common to all systems could not be easily
recognised in it. After this follows the fourth, the intestinal canal,
which, again, is formed after the same type, and appears as a com-
pleted independent whole, similar to the first three."

WOLFF'S article, written in Latin, made _no impression on his
contemporaries ; it had to be rescued from oblivion by MECKEL,
who published a German translation of it in 1812. It was probably
by means of this translation that the attention of PANDER was
directed to WOLFF. PANDER, under the stimulus and direction of
his celebrated teacher, DOLLINGER, further developed the doctrine,
the germ of which was contained in WOLFF'S paper.

In his publication, " Beitrage zur Entwicklung des Hiihnchens
im Ei," issued in the year 1817, PANDER distinguished in the blasto-
derm, as early as the twelfth hour of incubation, two thin separable
lamellte as the serous layer and the mucous layer, and main-
tained that subsequently a third, the vascular layer, was developed
between them. " Whatever noteworthy may subsequently occur,"

HISTORY OP THE GERM-LAYEK THEORY. 147

he remarks, " it is never to be regarded as anything else than a
metamorphosis of the blastod&'m and its layers, endowed as they are
with an inexhaustible store of formative energy." A few years
later the germ-layer theory reached at the hands of CARL ERNST VON
BAER a preliminary completion, which served for some time. VON
BAER, likewise a pupil of DOLLINGER, had observed in Wiirzburg the
beginning of the investigations of his young friend, PANDER. In
laborious studies pursued for many years, BAER followed with
wonderful accuracy the origin of the germ-layers and their meta-
morphosis into the individual organs of the adult body, principally in
the case of the Chick, but also in the case of some other Vertebrates,
and recorded his investigations in his classical work, " Ueber Entwick-
lungsgeschichte der Thiere, Beobachtung und Reflexion," which is
unsurpassable both in observations and in its general standpoints.

BAER differs from PANDER in maintaining that each of the
two primary germ-layers, which he distinguishes as animal and
vegetative, subsequently divides into two sheets. The animal
germ-layer divides itself into dermal lamella and sarcous lamella
(Hautschicht, Fleischschicht), the vegetative into mucous lamelll
and vascular lamella, so that now four secondary germ-layers have
arisen. The individual organs are developed out of the germ-layers
by morphological and histological differentiation.

A further advance beyond that of BAER could not be attained
until, with the establishment of the cell-theory, entirely new points
of view were introduced into morphology and, with improved con-
struction in microscopes, methods of investigation were refined.
It is chiefly REMAK and KOLLIKER who have promoted the germ-
layer theory in this direction.

REMAK took in hand successfully in his noted investigations on
the development of Vertebrates the very important question, how
the originally similar cells of the germ-layers are related to the
tissues of the completed organs. He showed that out of the lowest
of the four germ-layers there proceed only the epithelial and glan-
dular cells of the intestinal tube and its appendages, that from the
uppermost layer the epithelial cells of the epidermis, the sensory
organs, and the nervous tissue arise, whereas the two middle layers
furnish the mechanically sustentative substances and the blood, tho
muscular tissue, and the urinary and sexual organs.

In regard to the manner in which the four secondary germ-layers
arise, REMAK differs from BAER. Out of the two primary germ"
layers he first makes a third one, the middle germ-layer, arise, and

148 EMBRYOLOGY.

indeed he derives it exclusively from the lower germ-layer by a
process of fission. He designates the three layers as the upper or
sensorial, the middle or motor-germinative, and the lower or trophic.
The four secondary germ-layers of VON BAER come into existence
subsequently by a repetition of the fission, whereby the middle germ-
layer is split, at least in its lateral portions (lateral plates), into the
dermo-fibrous layer and the intestino-fibrous layer (Hautfaser- und
Darmfaserblatt), between which arise the thoracic and body-cavities.

REMAK in his account approximates the true state of affairs, as
detailed in the preceding chapters, more nearly than VON BAER ;
however, both made the same mistake of interpreting the formation
of the germ-layers as always a process of disassociation or fis?ion.
That is also the rock on which were wrecked the researches of numer-
ous other investigators, who in the decennary succeeding REMAK
dealt with the important question of the origin of the germ-layers.
It was difficult to decide this question for the higher Vertebrates,
which have been most frequently investigated ; so that very contra-
dictory opinions were expressed relative to the development of the
middle layer whether it was exclusively from the lower (REMAK),
exclusively from the upper, or from both layers.

This question could be clearly understood only upon the establish-
ment of new general standpoints. These could be acquired only by
the comparative method, and by the study of lower Vertebrates and
the Invertebrates.

Two fundamental processes needed to be better comprehended:

(1) How are the two primary germ-layers developed ?

(2) How are the two middle germ-layers developed ?

By means of the comparative developmental method, one question
has been brought nearer to a solution in the gastrcea-theory, the other
in the coelom-theory.

In the study of the first problem, which was the earlier solved,
HUXLEY and KOWALEVSKY, HAECKEL and RAY LANKESTER, have
shown especial merit. They demonstrated, partly through anato-
mical, partly through embryological studies, that, with the exception
of the Protozoa, the body of every invertebrated animal is constructed
of layers, which may be compared with the primary germ-layers of
Vertebrates.

The highly gifted English zoologist HUXLEY distinguished as early
as the year 1849 two membranes in the Medusae, an outer and an
inner layer, out of which alone their bodies are constructed ; and at
the same time expressed the happy idea that physiologically they

HISTORY OF THE GERM-LAYER THEORY. 149

were equivalent to the serous and the mucous layers of BAER.
Soon after this (1853) ALLMAN introduced for the layers of the
Ccelenterates the names, which are now so much employed, ectoderm
and entoderm ; subsequently use was also made of these for designat-
ing the embryonic layers.

The germ-layer theory was promoted to a still greater degree by
the Russian zoologist KOWALEVSKY, who made us acquainted in
numerous excellent detailed investigations with a profusion of
important facts concerning the embryology of Worms, Ccelenterates,
Molluscs, Brachiopods, Tunicates, and Arthropods. He produced
evidence that in all the Invertebrates which he investigated two
germ-layers are formed at the beginning of development, and that
in almost all cases, when the process of cleavage is at an end, a
cellular sac arises, and that this, by the infolding of a part of the
wall, becomes converted into a double cup, the cavity of which,
enclosed by two germ-layers, communicates with the outside by
means of an opening. He succeeded in establishing the existence
of this very important cup-shaped larva (gastrula) in many branches
of the animal kingdom.

In this connection should be mentioned the services of several
other embryologists, who at a still earlier period had observed in
isolated cases the cup-shaped larva and its origin by means of
invagination. RUSCONI and REMAK had described the cup-shaped
larva of Amphibia, GEGENBAUR that of the Sagittse or arrow-worms,
MAX SCHULTZE that of Petromyzon.

Whereas KOWALEVSKY by his series of investigations enriched our
knowledge of material facts, HAECKEL first sought to utilise the
same for a general theory, since by the process of morphological
comparison he brought into association hitherto disconnected obser-
vations. Starting from the development and the anatomy of the
Sponges, he compared the layer-like structure of the embryos of all
animals with the layer-like structure of the Ccelenterates, and pro-
duced as the fruit of this study the celebrated gastrcea-theory, which,
attacked on many sides at the time of its publication, has now
found in its essential substance general acceptance, and has given
the impetus to numerous investigations. HAECKEL showed that in
the development of the various classes of animals from the Sponges up
to Man a single form of the germ makes its appearance, the gastrula,
which consists of two cell- layers, and that the two cell-layers of
the various embryonic forms are comparable to one another or
homologous. The gastrula in its simplest condition presents, as

150 EMBRYOLOGY.

he endeavored to establish, the form of a double cup with a
coelenteric cavity and a primitive mouth, but may be greatly
altered, as in the most of the Vertebrates, by the deposition of
yolk-material in the egg, so that the original fundamental form is
scarcely recognisable. Consequently he distinguished, according to
the kind of modification, different forms of the gastrula, as bell-
shaped, cap-shaped, disc-shaped, and vesicular yastrulce. He made
the various forms arise by a process of invagination from a still
simpler fundamental form, the blastula, which is the final result of
the cleavage process.*

HAECKEL published his excellent gastrsea-theory in two articles in
iheJenaischeZeitschri/t: (1) " Die Gastrseatheorie, die phylogenetische
Classification des Thierreichs, und die Homologie der Keimblatter,"
(2) " Nachtrage zur Gastraeatheorie."

At the same time with HAECKEL, KAY LANKESTER in England was
led to a similar theory, which he had worked out in a paper full of
new ideas : " On the Primitive Cell-layers of the Embryo as the Basis
of Genealogical Classification of Animals."

Both HAECKEL and LANKESTER failed to point out how the forma-
tion of the gastrula takes place in some of the divisions of Verte-
brates in Fishes, Reptiles, Birds, and Mammals. Essential service
in the establishment and explanation of numerous questions of detail,
which remained unsettled in the gastrcea-theory, has been rendered
by BALFOUR, VAN BENEDEN, GERLACH, GOETTE, HOFFMANN, ROLLER,
RAUBER, RUCKERT, SELENKA, DUVAL, and others.

Thus through HAECKEL'S gastrsea-theory the following points were
gradually cleared up : (1) The two primary germ-layers, which form
the foundation for the development of both Invertebrates and

* It should be here stated that even OKEX and C. ERNST v. BAER had
set forth, although in a very indefinite manner, the importance of the vesicular
form for the development of the animal body. OKEN was an opponent of the
germ-layer theory of WOLFF. In a criticism of PANDER'S investigations he
exclaimed with emphasis and a certain justice : " The facts cannot be so. The
body arises out of vesicles and never out of layers," and he added the very
pertinent remark : " It appears to me as if it had been entirely forgotten that
the yolk and the yolk-membrane, which is a vesicle, belong essentially to tlie
tody of the germ ; that the embryo does not swim upon it like a fish in the
water, nor lie upon it like a funnel on a cask."

In a similar manner BAER remarks, but without further expounding the
relation to the germ-layers : " Since the germ is the undeveloped animal itself,
one can affirm, not without reason, that the simple vesicular form is the
common fundamental form, out of which all animals are developed, not only
ideally, but historically."

HISTORY OF THE GERM-LAYER THEORY. 151

Vertebrates, arise, not through disassociation or fission, but through
infolding of an originally simple cell-layer.* (2) These are com-
parable with one another or homologous, because they are developed
according to the same process, and because the two fundamental
organs of the body, the layer which limits the body externally
(the ectoderm) and the layer which lines the digestive cavity (the
entoderm), arise from them. (3) The intestinal canal of all animals
arises by invagination.

In the question as to the development of the middle germ-layer
HAECKEL remained at the traditional standpoint, and inclined most
to C. E. VON BABE'S view that the parietal lamella arose by fission
from the outer primary layer, and the visceral lamella from the
inner germ-layer. Most embryologists, who worked on the develop-
ment of Vertebrates, entertained, on the contrary, REMAK'S view,
and made the whole middle germ-layer arise from the inner
by fission.

They regarded the body-cavity as a fissure in the middle germ-
layer, and compared it with other lymphatic spaces, such as occur in.
the connective tissue at various places in the body.

The correction of this view was undertaken by various persons
in the same manner as in the case of the primary germ-layers. By
detailed study of the formation of the germ-layers in the Chick
and Mammals, KOLLIKER found that the middle germ-layer did not
simply split itself off from the inner, but that it arose from a limited
region of the blastoderm, namely, from the primitive groove, where
the two primary germ -layers are continuous. He maintained that
from this region it grew out between the two primary germ-layers
as a solid cell-mass, and that subsequently the body-cavity appeared
in it by means of its fission into two layers. This was an essential
advance in the representation of the actual state of affairs.

But a deeper insight into these embryonic processes in Vertebrates
was first acquired in this case also through the study of Invertebrates,
especially through the important discoveries of METSCHNIKOFF and
KOWALEVSKY concerning the formation of the body-cavity in Echino-
derms, Balanoglossus, Chsetognathi, Brachiopods, and Amphioxus.
The former found that in the larvse of Echinoderms and in Torn aria,
the larva of Balanoglossus, the walls of the body-cavity are formed
from evaginations of the intestinal canal. But a still greater sensation

* It is still affirmed by several authors for certain Invertebrates that the
inner germ-layer develops, not by infolding, but by a splitting off or delamina-
tion from the outer germ-layer.

152 EMBRYOLOGY.

was created when KOWALEVSKY in 1871 published his " Embryology of
Sagitta," and showed how the ccelenteron of the gastrula was divided
by two folds into three cavities, into the secondary intestinal cavity
and into the body-cavities : this discovery was afterwards fully con-
firmed by the investigations of BtiTSCHLi and the author. After a
short interval, KOWALEVSKY'S account of the development of Sagitta
was followed by his work on Brachiopods, in which he again enriched
science with the new and important fact, that in this class also the
body-cavity was formed in the same way as in the case of the
Chsetognaths. This was followed by his fundamental work on
Amphioxus.

Through the important discoveries made on Invertebrates, HUXLEY,
LANKESTER, BALFOUR, my brother and I were stimulated to
theoretical speculations concerning the origin of the body-cavity
and the middle germ-layer in the animal kingdom.

HUXLEY distinguished three kinds of body-cavity according to their
origin : (1) an enteroccel, which arises as in Sagitta, etc., from evagi-
nations of the ccelenteron ; (2) a schizoccel, which is developed by
means of fission in a mesodermal connective substance lying between
the integument and the intestine ; (3) an epiccel, which is formed by
an invagination of the surface of the body like the perithoracic
space of the Tunicates. The last kind, HUXLEY thinks, may perhaps
correspond to the pleuroperitoneal cavities of the Vertebrates.

LANKESTER makes HUXLEY'S paper his starting-point. He gives
preference to the hypothesis of the common origin of the body-
cavity in all animals until decisive proof of diverse origins is
produced ; and, in fact, he makes the schizoccel arise out of the
enteroccel in the following manner. Evaginations of the coelenteron
have lost their lumen, and therefore are begun as solid cell-masses,
which only subsequently acquire a cavity. While LANKESTER in
this, as well as in a second publication, overlooks existing differences
in his effort to reduce everything to a single scheme, BALFOUR in
various essays takes more fully into account in his speculations the
actual condition of affairs ; he also limits himself chiefly to the
explanation of the conditions in Vertebrates. In investigating the
development of Selachians, he made the important discovery that
the middle germ-layer arises from the lateral margins of the primi-
tive mouth, and at first consists of two separate masses of cells,
which grow out forwards and laterally into the space between the
two primary germ-layers. Since in each cell-mass a separate cavity
soon makes its appearance, he designates the body-cavity as from the

HISTORY OF THE GERM-LAYER THEORY. 153

beginning a paired structure, and compares it to the body -sacs
which are developed in Invertebrates by evagination from the
ccelenteron. BALFOOR justly alleges that the originally solid con-
dition of the two fundaments can have no weight against his inter-
pretation, since in numerous instances organs which ought properly
to contain cavities are developed solid, and subsequently become
hollow, as, for example, in many Echinoderms one encounters solid
cell-masses in place of hollow evaginations of the coelenteron.

Led by theoretical considerations similar to those of the English
morphologists, my brother and I, by a thorough comparison of de-
velopmental and anatomical conditions, and with due regard to the
morphological and histological structure of organisms, then en-
deavored to bring to a solution this question of the day, the question
of the development of the body-cavity and the middle germ-layers,
by systematic investigations (published in " Studien zur Blatter-
theorie "), which extended over Invertebrates and Vertebrates.
The results of these series of investigations were published in two
articles : (1) in the " Ccelomtheorie, Versuch einer Erklarung des
mittleren Keimblattes," and (2) in the " Entwicklung des mittleren
Keimblattes der Wirbelthiere."

In the first paper, in order to prepare the way, we were compelled to
give the term germ-layer a more precise definition. We designated
as such a layer of embryonic cells which are arranged like an
epithelium and serve for the limitation of the surface* of the body.
At the close of segmentation there is only one germ-layer present;
namely, the epithelium of the blastula. The remaining germ-layers
arise from it by the processes of invagination and evagination. The
inner germ-layer is formed by means of gastrulation, the two middle
germ-layers by the formation of the body-cavities, in that two body-sacs
are evaginated from the coelenteron, and grow out between and separate
the two primary germ-layers. There are, in the first place, animals
which are formed of two germ-layers, and possess in their bodies only
one cavity, a ccelenteron, produced by invagination (Ccelenterata
and Pseudocoelia), and, secondly, animals with four germ-layers, a
secondary intestine, and a body-cavity derived from the coslenteron
an enterocoal. To the two-layered animals belong the Coelenterates
and the Pseudocrels, but all four-layered animals are Enteroccels.

From this standpoint we endeavored to prove that hitherto there
had been confused under the conception " middle germ-layer " two
things which are genetically, morphologically, and histologically
entirely different.

154: EMBRYOLOGY.

Besides the cell-layers which arose by invagination there had been
assigned to the middle germ-layer cells which detach themselves
individually from the primary germ-layers, and give rise between
the epithelial layers of the body to the sustentative substances, and
also to the blood, when such exists. Embryonic cells of that kind,
which are formed by emigration into the space surrounded by
the germ-layers, we named the mesenchymatic germ, and the tissue
produced from them mesenchyme. This occurs as well in two-
layered as in four-layered animals. In our opinion a sharp distinction
must be made between the formation of germ-layers, which is
correlated with the morphological differentiation of the body, and
the formation of mesenchyme, which will especially engage our
attention in one of the next chapters, if clearness and a uniform
principle are to be introduced into the whole germ-layer theory.

In the second article it was our aim to show that in the Vertebrates
a middle germ-layer is developed by infolding. For that purpose
the development of Amphibia, Fishes, Reptiles, Birds, and Mammals
was compared with the development of Amphioxus, and thus was
acquired the foundation upon which is based the account of the
development of the middle germ-layer given in the preceding chapter.

After the publication of these two papers, there appeared numerous
articles by VAN BENEDEN, DUVAL, HEAPE, HOFFMANN, K&LLIKER,
KOLLMANN, RABL, RUCKERT, STRAHL, WALDEYER, and others, through
which valuable facts concerning the development of the middle germ-
layer in the different classes of Vertebrates have been made known.
In some of these the chief points of view of the coelom-theory were in
general recognised as correct, attempts were made to modify details,
but especially was the question of the formation of the mesenchyme
of the Vertebrates actively discussed.

The mechanical principle oj the process oj development, by means of
which the germ-layers are formed, and out of these the separate organs,
is appreciated in its full significance by only a few, and in text-books
particularly has not been adequately presented.

Among the founders of the germ-layer theory, PANDER best com-
prehended this principle. " The blastoderm," he says in one place,
" forms, exclusively through the simple process of folding, the body
and the viscera of the animal. A delicate thread attaches itself as
the spinal cord to it, and scarcely has this taken place, when the
blastoderm sends the first folds, which themselves necessarily designate
the position of the spinal cord, as an envelope over the exquisite fila-

HISTORY OF THE GERM-LAYER THEORY. 155

merit, thus forming the first foundation of the body. Hereupon it
produces new folds, which, in contradistinction to the first, give shape
to the abdominal and thoracic cavities, together with their contents.
And for the third time it sends out folds to envelop in suitable
membranes the foetus, which is formed out of it and by means of it.
Therefore it need not surprise any one if, in the course of our
narration, so much is said about folds and envelopes." And in
order to avoid misunderstandings he adds in another place the
important statement that "wherever anything is said about the
folds of the skin, one is not to imagine a lifeless membrane, whose
mechanically produced folds would necessarily spread themselves over
the whole surface, without allowing themselves to be limited to a
definite space. The folds which cause the metamorphosis of the skin
are rather themselves of organic origin, and are produced at the
appropriate place, either through increase fa the size of the spherules
already present there, or through an accession of new spherules,
without the remaining part of the blastoderm being thereby altered."

PANDER'S successors have expressed themselves concerning the
mechanism of foldings much less clearly ; the most of them, indeed,
not at all. The whole doctrine was in fact condemned by RUDOLPH
WAGNER as positively erroneous. " It will occur to no one," he says
in his " Lehrbuch der Physiologie," " to imagine the three germ-
layers to be like the leaves of a book. No one will entertain the
mechanical conception that the embryo arose by a folding process of
these three layers."

After PANDER. LOTZE was the next to be occupied with the
" Mechanik der Gestaltbildung," as has been pointed out by EAUBER
in a meritorious history of this topic. He designates " unequal
growth " or " unequal vegetation " as the cause of the changes of
place, which in part only appear to be shaftings, out-pocketings,
invaginations, or extensions, but in part are actually such, being
brought about in this way by mechanical traction and pressure.

In very recent times His has prosecuted the study of embryology
from the mechanico-physiological standpoint more intensely than all
his predecessors, and has also particularly emphasised the signifi-
cance of the process of folding for the formation of the body. The
two principal writings of His in this connection are : " Unter-
suchungen iiber die erste Anlage des Wirbelthierleibes " (1868),
and " Unsere Korperform und das physiologische Problem ihrer
Entstehung " (1874). While I refer for details to the original papers,
I remark that, notwithstanding manifold agreements, I cannot

156 EMBRYOLOGY.

in important points assent to His's view. When, for example,
His (1874, p. 50) seeks to reduce the mechanics of form to the
simple problem of the form-changes in an unequally stretched
elastic plate, in my opinion he overlooks the fact that a plate com-
posed of cells, even if it possess elastic properties, is, nevertheless, a
much more complicated structure, and that the processes of folding
and evagination are primarily produced by the energy of the
growth of special groups of cells, and are therefore not to be com-
pared with the bendings and stretchings of elastic plates. As
PANDER has already emphatically stated, one is not to imagine in
the folding processes a lifeless membrane, but rather the folds are
themselves of organic derivation, called forth at the proper place by
a cell-multiplication at that place. For this reason, too, HAECKEL
in his polemic, " Ziele und Wege der heutigen Entwicklungs-
geschichte," has attacked this method of treating embryology,
introduced by His.

That the morphological differentiation of the animal body primarily
rests upon a process of folding of epithelial lamellse, my brother and
I have endeavored, by means of an abundant series of observations,
to demonstrate in a still more exhaustive manner than our pre-
decessors. In our " Studien zur Blattertheorie " we have, in the first
place, directed attention to the Ccelenterates as the animal organisms
in which the principle of the formation of folds is most clearly
shown throughout the whole organisation, even into details; and,
secondly, we have endeavored to establish for Vertebrates that
organs like the body-cavity, chorda, and primitive segments, which
it was claimed arose by a separating and splitting of cell-layers,
likewise come into existence through the typical process of foldings
and constriction.

Finally we have endeavored to point out a physiological cause
for the unequal growth of a cell-membrane, and have found such in
the Ccelenterates in the unlike functional activity of its various
regions. Parts of a membrane will grow more rapidly and must
become infolded, when in consequence of their position they are
called upon to accomplish more than neighboring regions.

In concluding this historical sketch attention should be called to
the fact that C. E. VON BAEE, in the general discussion of embryo-
logical processes, was the first to distinguish clearly between the
events of morphological differentiation, which take place in the
beginning of development, and those of histological differentiation,
which occur later.

LITERATURE. 157

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Jour. Micr. Sci. Vol. XIX. 1879.
Balfour. On the Structure and Homologies of the Germinal Layers of the

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;

158 EMBRYOLOGY.

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160 EMBRYOLOGY.

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DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 161

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CHAPTER VIII.
DEVELOPMENT OF THE PRIMITIVE SEGMENTS.

THE more one pursues the development of Vertebrates into later
stages, the more numerous become the changes which simultaneously
appear in the different regions of the embryonic body. We cannot
here undertake to describe step by step the processes which are
simultaneously accomplished, for by that method the presentation

11

162

EMBRYOLOGY.

would become fragmentary and the comprehension of the separate
processes would be made more difficult ; but it is necessary, in the
interest of a didactic method, to select from all the manifold pheno-
mena a single process of the development, and to follow it up until
it has come to a preliminary termination.

After the formation of the middle germ-layer two important
processes take place in the embryonic fundament. One process
leads to a division of the middle germ-layers into the two lateral

Fig. 103. Amphioxu* embryo -with five pairs of primitive segments in optical section, aftci

HATSCHEK.

A Seen from the side. B Seen from the dorsum.
In figure B are indicated the openings of the cavities of the primitive segments into the

intestinal cavity, which can be seen by deeper focussing. V, Anterior, H, posterior end ;

ok, outer, ik, inner, mk, middle germ-layer ; dh, intestinal cavity ; n, neural tube ;

en, neurenteric canal ; tw 1 , first primitive segment ; uth, cavity of primitive segment :

ud, ccelenteron.

plates and into two series of cuboidal bodies, which are situated at
the right and the left of the chorda, and which, under an erroneous
interpretation, were formerly called protovertebrce, but for which one
should now substitute exclusively the more accurate name primitive
segments [mesoblastic somites]. The other process, which occurs at
about the same time, at least in the case of the higher Vertebrates,
leads to the origin of those cells from which the sustentative sub-
stances and the blood of Vertebrates are derived.

DEVELOPMENT OF THE PRIMITIVE SEGMENTS.

163

In this chapter we shall take into consideration the formation of
the primitive segments first in the eggs of Amphioxus and the
Amphibians, and then in those of Fishes, Birds, and Mammals.

In Amphioxus the formation of the primitive segments is more
nearly simultaneous with the development of the middle germ-
layer than in the remaining Vertebrates. As soon as the two
ccelomic sacs begin to grow out from the ccelenteron at the front end
of the embryo, there begins a division of them into two rows of
small sacs lying one behind the other (tig. 103 A, B, us), and this
division proceeds from in front backwards. Here again we have
to do with a process of folding, which
repeats itself many times in the same
manner.

The wall of the groove-like coelomic
evagination, composed of cylindrical
cells, becomes, at a little distance from
its head-end, folded transversely to the
longitudinal axis of the embryo ; this
fold grows from above and from the
side downwards into the body- cavity;
in the same manner a second trans-
verse fold is soon formed on either
side of the body at a little distance
behind the first; behind the second
a third, a fourth, and so on, at the
same rate as that at which the em-
bryonal body elongates and the fun-
dament of the middle germ -layer

increases by the progress of the evagination toward the blasto-
pore.

In the embryo represented in fig. 103 five sacs may be counted on
either side of the body. The evagination is taking place at the
region marked mk ; it advances still farther toward the blastopore
and gives rise to a considerable series of primitive segments, the
number of which in a larva only twenty-four hours old has already
increased to about seventeen pairs. The primitive segments exhibit
at first an opening, by means of which their cavities (usK) are in
communication with the intestinal cavity. But these openings soon
begin to be closed in succession, by their margins growing toward
each other and then coalescing : this takes place ia the same sequence
as that in which the detachment of the parts takes place, from before

Fig. 104. Cross section through the
middle of the body of an Amphioxus
embryo with 11 primitive segments,
after HATSCHEK.

ak, Outer, ik, inner germ-layer ; wit 1 ,
parietal, ink 1 , visceral lamella of
the middle germ-layer; us, primi-
tive segment ; n, neural tube ; ch,
chorda ; IK, borly-eavity ; dh, intes-
tinal cavity.

164

EMBRYOLOGY.

backwards. At the same time the primitive segments (fig. 104}
gradually spread out both dorsally and ventrally, while their cells
increase in number and become altered in form. They grow upward
more and more at the side of the neural tube, which has meanwhile
detached itself completely from its matrix, the outer germ-layer.

TO/

Uth

A

Fig. 105 Two cross sections through a Triton embryo.

A, Cross section through the region of the trunk in which the neural tube is not yet closed an

the primitive eegments begin to be constricted off from the lateral plates.

B, Cross section throxigh the region of the trunk in which the neural tube is closed and the

primitive segments have been formed.

in/, Medullary folds ; mp, medullary plate ; n, neural tube ; ch, chorda ; ak, outer, ik, inner
germ-layer ; mi 1 , parietal, ?;tfc 2 , visceral middle layer ; dh, intestinal cavity ; Ih, body-cavity
uth, cavity of primitive segment ; rfz, yolk-cells.

Toward the ventral side they insert themselves between the secondary
intestine and the outer germ-layer.

Finally, it might be further mentioned here that at a still later
stage, as is to be seen on the right side of fig. 104, the dorsal portions
of the primitive segment are constricted off from the ventral. The
former lose their lumina and furnish the transversely striped

DEVELOPMENT OF THE PRIMITIVE SEGMENTS.

165

musculature of the body, but from the cavities of the latter originates
the real unsegmented body-cavity, since the partitions which at
first separate them become thinner, break through, and finally
disappear.

Similar processes take place in a somewhat modified manner in the
case of the remaining Vertebrates.

In the Tritons the middle germ-layer (fig. 105 A) becomes
thickened on both sides of the chorda (ch) and of the fundament of
the central nervous system (n), which is not yet closed into a tube,
and at the same time there appears a cavity (wsA) in its thickened
part, caused by the separation of the visceral and parietal lamellae.
The thickening is not produced by an increase in the number
of the layers of cells, but simply by the fact that the cells
increase in height and grow out into long cylinders, which are
arranged around the cavity like an epithelium. We distinguish
these thickened parts of the middle germ-layer, which lie on either
side of the chorda and the nervous system, as the primitive-segment
plates, from the lateral parts, or the lateral plates. In the territory
of the latter the cells are lower, and ordinarily there is no distinctly
marked cavity between visceral and parietal layer.

Whereas in Amphioxus the process of forming somites extends
itself over the whole of the middle germ-layer, in the case of the
Amphibians, and likewise all the re-
maining Vertebrates, it affects only
the part which is next to the chorda
and the neural tube, leaving the lateral
plates, on the contrary, untouched.
The segmentation begins at the head-
end, and proceeds slowly toward the
blastopore ; it is accomplished by fold-
ing and constricting off. The epithelial
lamella next to the neural tube and
the chorda, being composed of cylin-
drical cells, is raised up into small
transverse folds, which, separated from
each other by intervals of uniform size,
grow into the cavity of the primitive-
segment plate, and give rise to small sacs lying one behind the other
(fig. 106).

Soon afterwards each little sac is constricted off from the lateral
plates (fig. 105 A and B}. Consequently one now meets, both in

ale

Fig. 106. Frontal section through
the dorsum of an embryo Triton
with fully developed primitive seg-
ments.

One sees on both sides of the chorda
(ch) the primitive segments (us)
with their cavities (uih).

166 EMBRYOLOGY.

transverse and frontal sections at the right and left of chorda
and neural tube, cubical sacs the walls of which are formed of
cylindrical cells ; these sacs are everywhere surrounded by a fissure-
like space, and they enclose a small cavity (the primitive-segment
cavity), which is a derivative of the body-cavity. From the front
layer of the fold is produced the posterior wall of the newly formed
segment, from its posterior layer the front wall of the remnant of
the primitive-segment plate, or of the sac which is next to be con-
stricted off.

Of the Vertebrates which are developed out of meroblastic eggs, the
Selachians appear to exhibit most clearly the original mode of the
formation of primitive segments. A distinct body-cavity is formed on
either side of the trunk by the separation of the parietal and visceral
lamellae of the middle germ-layer (fig. 110). The dorsal portion of
the cavity, which flanks the neural tube, acquires thickened walls
(mp), and corresponds to the part previously designated as the
primitive-segment plate, which at the same time with the appear-
ance of the body-cavity begins to be divided into primitive segments.
In the anterior part of the body a series of transverse lines of
separation become visible (fig. 195 mp 1 ), the number of which is
continually increased toward the hind end of the body. For a
long time the cavities of the primitive segments, which are sepa-
rated from one another by these transverse furrows, remain in
communication ventrally with the common body-cavity by means
of narrow openings. One may therefore describe this state of
affairs by saying that the body-cavity is provided toward the back
of the embryo with a series of small sac-like evaginations, which lie
close together one after the other. Afterwards the primitive seg-
ments are entirely constricted off from the body-cavity, and then
their thickened walls come into close contact, and thus cause the
disappearance of the cavities of the segments (fig. Ill mp).

Whereas in the Selachians it is still evident that the formation of
the primitive segments depends upon folding and constricting off, the
process is obscured even to obliteration in the case of Reptiles, Birds,
and Mammals; this is referable simply to the fact that the two
lamellae of the middle germ-layer remain for a long time firmly
pressed together, only subsequently beginning to separate, and that
they are composed of several layers of small cells. The process of
folding arid constricting off appears here as a splitting up of a solid
cell-plate into small cubical blocks.

The part of the middle germ-layer that is next to the chorda and

DEVELOPMENT OF THE PRIMITIVE SEGMENTS.

167

neural tube appears in a cross section of a Chick embryo (fig. 107)
as a compact mass (Pv) consisting of many superposed small cells
which, as far as it is not divided up into separate blocks, is designated
as primitive-segment plate
or protovertebral plate. In
fig. 107 it is still connected

O

at the side by means of a
thin isthmus of cells with
the lateral plates, in whose
territory the middle germ-
layers are thinner and sepa-
rated from each other by a
fissure.

In observing the blasto-
germ from the surface the
region of the primitive-seg-
ment plates, as is to be seen
in the posterior part of a
nine-days-old Rabbit embryo
(fig. 108), appears darkerthan
the region of the lateral plate;
so that the two are . dis-
tinguished from each other ;
one is stem-zone (stz), the
other parietal zone (pz).

The development of the
primitive segments is ob-
servable in the Chick at the
beginning of the second day
of incubation, in the Rabbit
at about the eighth day.
Clear transverse streaks ap-
pear in the stem-zone at
some distance from the primi-
tive streak, about in the
middle of the embryonic

fundament, both on the right and the left of the chorda and neural
tube (fig. 108). They correspond to transverse fissures, by means
of which the primitive-segment plates are divided into the small
and solid cubical primitive segments (uw). In the nine-days-old*
Rabbit embryo represented in fig. 108 these plates are resolved in

168

EMBRYOLOGY.

./

front into eight pairs of primitive segments (uw), whereas in the
hind end of the embryonic area they still have the form of a con-
tinuous mass of cells, the
stem-zone (stz), which in sur-
face-views appears darker
than its surroundings.

In a somewhat more ad-
vanced stage the primitive
segment, which probably se-
cretes at the same time fluid,
develops in its interior, as
in the case of the Amphibia
and Selachii, a cavity, around
which the cells group them-
selves in a radial manner.
This cavity, too, is at first in
communication laterally with
the fissure of the body-cavity,
until the primitive segment

has been fully constricted

V-T1 .& ; ; / "V- 1 * II off

In Vertebrates, besides the
trunk-region, a part of the
head-region of the embryo is
also affected by this process
of segmentation which we
have been considering. We
must therefore speak in the
one case of head-segments,
and in the other of trunk-
segments. Up to the present
time the number and condi-
tion of the head-segments have ,
been made out (by BALFOUR,
MILNES MARSHALL, and VAN
WIJHE) most accurately for
the Selachians. In this in-
stance there are nine pairs of hollow head-segments. In the higher
Vertebrates such segments although fewer in number, have also
t been described; however, the less sharply differentiated structures
of the latter demand still further investigation.

fig. 108. Rabbit embryo of the ninth day, seen
from the dorsal side, after KOLLIKER. Magnified
21 diameters.

The stem-zone (stz) and the parietal zone (pz) are
to be distinguished. In the former 8 pairs of
primitive segments have been established at the
side of the chorda and neural tube.

up, Area pellucida ; r/, medullai-y groove ; vh, fore
brain ; ab, eye-vesicle ; mh, mid brain ; hh, hind
brain ; uw, primitive segment ; stz, stem-zone ;
pz, parietal zone ; h, heart ; ph, pericardial part
of the body-cavity ; vd, margin of the entrance to
the head-gut (vordere Darmpfoite), seen through
the overlying structures ; of, amniotic fold ; vo,
vena omphalomesenterica.

DEVELOPMENT OF THE PRIMITIVE SEGMENTS. 169

But, in any event, the accurate study of the earliest embryonic
segmentation of the body into a large number of metameres yields
this result of the highest importance for the general morphology of
the Vertebrate body, that the head not less than the trunk represents
a segmented portion of the body and has in no wise been produced
from a single primitive segment.

SUMMARY.

1. In Vertebrates the middle germ-layers immediately after
their origin are differentiated into several fundaments by processes
of folding and constricting off.

2. The process of differentiation in the middle germ-layer exhibits
two modifications.

(a) In Amphioxus the middle germ-layers are, at the time of

their first appearance, completely separated into primitive

segments lying one behind the other.
It is only later that each primitive segment is divided into a

dorsal portion (the real primitive segment) and a ventral

portion.
The dorsal portion, or primitive segment proper, furnishes the

transversely striped musculature of the trunk.
The ventral segments form the body-cavity, which is at first

segmented, but afterwards with the disappearance of the

partitions becomes a single cavity.
(6) In all other Vertebrates the fundaments of the middle

germ-layers are divided first into a dorsal and a ventral

region into the primitive-segment plates and the lateral

plates.
The lateral plate remains unsegmented. The body-cavity, which

becomes visible in it by separation of the parietal and

the visceral lamellae of the middle layer, is from the

beginning on each side of the body a single space.
The primitive-segment plate alone is divided into successive

primitive segments.

3. The segmentation of the middle germ-layers also extends over
the future head-region of the embryo. One therefore distinguishes

(a) Head-segments, the number of which amounts to nine ;

(b) Trunk-segments, the number of which is constantly being

increased during the development of the posterior trunk-
region.

170 EMBRYOLOGY.

CHAPTER IX.

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD.
(THE MESENCHYME-THEORY.)

WITH the question cf the origin of connective or mechanically sus-
tentative substance and blood we enter a very difficult field, the
cultivation of which has now been taken in tand successfully by many
persons. Here also we shall acquaint ourselves with a simple case
from the development of Invertebrates, before we begin with the
conditions in Vertebrates, which are more difficult to comprehend.

In Coelenterates and Echinoderms there is developed between the
germ-layers, which are composed of epithelial cells, a sustentative
tissue. It consists of a homogeneous jelly, in which are scattered a

Fig. 109. Two stages of development of Holothuria tubulosa, in optical section (after SELENKA),
from BALFOUR.

A, Blastosphere-stage at the end of cleavage.

B, Gastrula-stage.

mr, Micropyle ; fl, chorion ; s.c, segmentation-cavity, in which gelatinous substance is early
secreted as a gelatinous core ; bl, blastoderm ; ep, outer, hy, inner germ-layer ; ms,
amoaboid cells arising from the inner germ-layer ; a.e, welenteron (archenteron).

few isolated spheroidal or stellate cells, which are capable of changing
position by virtue of their amoeboid motion. It is usually developed
very early ; in the Echinoderms, for example, as early as the blastula-
stage (fig. 109).

Into the cavity of the blastula (A) a homogeneous soft substance, the
jelly-core (s.c), is secreted by the epithelial cells. Into this jelly there
migrate from the epithelium, and indeed from the particular region
which at the time of gastrulation is infolded (fig. 109 B) as the

DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD. 171

inner germ-layer (%), numerous cells (ms), which loose their epi-
thelial character, and send out processes in the manner of lymph-
corpuscles. They soon distribute themselves as migratory cells
everywhere in the jelly,

In the gastrula. -stage and subsequently, the cell-containing jelly
between the outer and the inner germ-layers represents a third sheet,
which is distinguished from the latter histologically, and, according
to the definition previously given, cannot be designated as a middle
germ-layer ; for by that definition we understand the term to be
limited to a sheet of embryonic cells, having an epithelial arrange-
ment and bounding a surface. The jelly-like sheet is a product oj
the germ-layers, which may be distinguished from them by the name
or intermed,iate layer (Zwischenblatt).

Once formed, the mesenchyme continues to grow as an independent
tissue, in that the cells which at first migrated into the jelly at a
definite stage of development, to which one may give the name
mesenchyme-germ, continue to increase uninterruptedly by means of
cell-division. In its growth it penetrates into all the interstices
which arise when the germ-layers, as happens in many Coelenterates,
produce the most complicated structures by the formation of folds and
evaginations ; it furnishes everywhere a support for the epithelial
layers which repose upon it. At the same time some of the mesen-
chyme-cells can alter their original histological character as simple
trophic or nutritive cells of the intermediate substance. Thus here
and there they differentiate contractile substance at their surface,
and become, as is to be seen in Ctenophores and Echinoderms, smooth
muscle-cells, the ends terminating either in one fine point, or
dividing themselves into several processes, as is more frequently the
case with Invertebrates.

In Vertebrates also, after the two primary germ-layers have arisen,
a process similar to that which we have just considered appears to
lead to the formation of connective tissue and blood, two tissues
which correspond morphologically and physiologically to the mesen-
chyme of Invertebrates.

In the first two editions of the " Lehrbuch " I set forth that the
whole mesenchyme-question in the Vertebrates was still in a nascent
condition, that the account therefore presented nothing final, but
bore in many respects the character of the provisional. Since that
time an essential advance has been made in this field. Thanks to
the investigations of HATSCHEK and RABL, of RUCKERT, ZIEGLEE, and
VAN WIJHE, we have acquired more accurate explanations concerning

i

172 EMBRYOLOGY.

the origin of the connective substances ; the question of the origin
of the vascular endothelium and of the blood, on the contrary, is
one that is less cleared up. This determines me to treat the two
questions separately in the following account.

A. The Origin of the Connective Tissues.

Selachian embryos appear to be the most suitable objects on
which to trace the origin of the connective substances. Here the
middle germ-layer serves as the matrix for the mesenchymatic tissue.
At the time when the primitive segment is still connected below with
the lateral_plateg,.and when the body-cavity is visible in the latter,
there appears a cell-growth at the lower border of each primitive
segment on the side which is directed toward the chorda. It is ordi-
narily designated as sclerotome. JH!t contains at first a small evagi-
nation of the body-cavity (fig. 258 A sJc). At the restricted place
.J designated, which is marked off from its surroundings, and jwhich
recurs on each primitive segment, cells in large numbers (fig. 110
sk) individually detach themselves from the epithelial layer, remove
by active migration from their place of origin, like the mesen-
chymatic cells of Invertebrates, and distribute themselves in the
space which is limited on the one side by the inner wall (mp)
of the primitive segment, and on the other by the chorda (ch)
and the neural tube (nr).

At the time of their appearance the amoeboid cells are separated
by only a small amount of inter-cellular substance: they increase
rapidly in number, and thereby soon crowd chorda, neural tube, and
primitive segment farther apart (fig. 111). The segmental arrange-
ment which the growths exhibit at their first appearance (fig. 195 Vr)
very early ceases to exist, since by their extension they become fused
together into a continuous sheet.

The mesenchyme, which thus grows forth out of the middle germ-
layer on both sides of the chorda, furnishes the foundation for the
whole axial skeleton ; it produces the skeletogenous tissue by the
growing toward each other and the fusion of the masses which are
formed on the right and left sides. As fig. Ill shows, the mesen-
chyme (sk) grows around the chorda (ch) both dorsally and ventrally,
and envelops it with a connective-tissue sheath, which is continually
becoming thicker. In the same manner it encloses the neural tube
(nr) and forms the membrana reuniens superior of the older embryo-
logists, the foundation out of which subsequently the connective-

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD.

173

tissue envelopes of the neural tube and the vertebral arches with
their ligaments are differentiated.

Conditions similar to those of Selachians are also to be observed,

Fig. 110.

Fig. 111.

Figs. 110 and 111. Diagrams of cross sections through younger and older Selachian embryos
to illustrate the development of the principal products of the middle germ-layer. After VAN
WIJHE, with some changes.

Fig. 110. Cross section through the region of the pronephros of an embryo, in which the
myotomes (mp) are in process of being constricted off.

Fig. 111. Cross section through a somewhat older embryo, in which the myotomes have just
been detached.

w, Neural tube ; ch, chorda ; ao, aorta ; >ch, subnotochordal rod ; mp. muscle-plate of the
primitive segment ; w, zone of growth, at which the muscle-plate bends over into the cutis-
plate (f'fi) ; rb, portion connecting the primitive segment with the [walls of the] body-cavity,
out of which are developed, among other things, the mesonephric tubules uk (fig. Ill) ;
xle, skeletogenous tissue, which arises as an outgrowth from the median wall of the con-
necting portion (rb) ; i~n, pronephros ; mk l , parietal, ml?, visceral middle layer, from the
walls of which mesenchyme is developed ; Ih, body-cavity ; it, entoderm ; h, cavity of the
primitive segment ; uk, mesonephric tubule, arisen from the connecting portion vb of the
diagram 110 ; uk 1 , place where the mesonephric tubule has detached itself from the primitive
segment ; v.g, mesonephric duct, with which the mesonephric tubule has united on the left
side : tr, union of the mesonephric tubule with the body -cavity (nephridial funnel) ; ma 1 ,
mes*, mesenchyme, which has arisen from the parietal and visceral lamellae of the middle
layer respectively.

although less distinctly, in Reptiles, Birds, and Mammals; they
have been described by REMAK, KOLLIKER, and others, and have been
brought into connection with the formation of the vertebral column.
The primitive segments, which are at first solid, soon acquire a
small cavity (fig. 116J, around which the cells are arranged into a

174 EMBRYOLOGY.

continuous epithelium. Then a part of the wall of the primitive
segment lying at its lower and median angle begins to grow with
oxtraordinary rapidity, and to furnish a mass of embryonic connective
tissue, which spreads itself around the chorda and neural tube in the
manner previously described. The dorsal and lateral parts of the
primitive segment (fig. 116 ms), which subsequently loses its cavity,
are not involved in this growth ; out of them arise principally the
fundaments of the trunk-musculature. This part is consequently
now distinguished as muscle-plate (ms).

Mesenchyme arises from three other places of the middle germ-
layer besides the primitive segments from the visceral lamella, from
the parietal lamella, and finally from that wall of the primitive
segment which is turned toward the epidermis and has been given
by RABL the name cutis-plate.^, Here also the conditions are best
followed in Selachii.

Individual cells migrate out from the visceral lamella (Darm-
faserblatt), which in early stages is composed partly of cubical,
partly of cylindrical cells (fig. 110 m& 2 ), and distribute themselves
upon the surface of the entodermic layer ; they are found at places
where no trace of a vessel is observable. They furnish the
mesenchyma of the intestinal wall, which is ever becoming more
abundant, and which is subsequently converted partly into connective
tissue, partly into the smooth muscle-cells of the tunica muscularis
(fig. Ill mes*).

A similar process is repeated in the parietal lamella (Haut-
t';tserl>latt). Emigrating cells produce between the epithelium of
The body-cavity and that of the epidermis an intermediate layer of
mesenchyme-cells (fig. 110 ink 1 , fig. Ill mes 1 ).

An important region for the production of connective tissue is,
finally, the cutis-plate, i.e., the epithelial layer of the original primi-
tive segment which is in contact with the epidermis (fig. 110 cp).
i pf^~ The process occurs here later than at the other places mentioned,
and begins with an active cell-growth, which gradually leads to a
complete disintegration of the epithelial lamella. " The disintegra-
tion," as RABL remarks, " proceeds in such a manner that the cells,
which hitherto exhibited an epithelial character, separate them-
selves from one another, and thereby lose their epithelial character."
It is probably from this part of the mesenchyme that the corium is
derived.

That the mesenchyme-cells scattered between the epithelial lam-
ellae are capable of executing extensive migrations, after the fashion

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 175

of migratory cells, is perhaps best shown in the investigation of
transparent embryos of Bony Fishes. " One sees distinctly," thus
WEXKEBACH describes it, "how the cells by means of amoeboid
motions, and of sometimes extraordinarily long protoplasmic pro-
cesses, move themselves about independently in the body of the em-
bryo and upon the yolk, which is not yet clothed with hypoblast,
ajid creep toward definite places, as if they acted voluntarily and
eonscicmsly. By virtue of this peculiarity, the mesenchyme-cells
actively penetrate into all larger and smaller fissures which exist
between the germ-layers and the fundaments of organs which have
arisen from them. Everywhere they form a filling and connecting
mass between these structures, which afterwards acquires a still
greater importance as the bearer of blood- and lymph-courses as well
as nerves.

In comparison with the earlier editions of the " Lebrbuch," I have here
given an essentially different presentation of the development of the mesen-
chyme. Formerly, supported by the investigations of His, WALDEYEB, KOLL-
MAXN, and others on meroblastic eggs, I thought it necessary to refer the
chief source of the mesenchyme to a limited territory of the germ, to the area
opaca, and made the cell-material arise by delamination from the entodermic
layer, especially from the yolk-wall. But now I asgujpft a. manifold nrigjnjTnm
various regions of the middle germ-layer. Thus I come back again to an in-
terpretationwEicErrhad already propounded as probable in " Die Coslomtheorie "
(p. 80) and " Die Entwickelung des mittleren Keimblattes " (p. 122), to the
interpretation, namely, that mesenchyme-germs in Vertebrates are perhaps
formed by an emigration of cells at several distinct places at the same time.
Whether this or that be the real mode, the essence of the mesenchyma-theory
is not thereby affected, for the essential part of that theory consists in this,
that it establishes in the earliest development of tissue a contrast between
the epithelial germ-layers and a packing tissue, produced by a dissolution of
the epithelial continuity, whicfi~spreads itself out between the germ-layers,
and soon appears as an independent structure.

Indeed, with this theory as a basis, it would not be surprising if the pro-
duction of mesenchymatic tissue should not be limited simply to the middle germ-
layer, and if the entoderm by the contribution of cell-material should participate
in its formation.

B. The Origin of the Vascular Endothelia and the Blood.

The question of the origin of the tissues represented in the above
heading is one of the most obscure in the realm of comparative
embryology. The very investigators who have endeavored most
recently and with the most reliable methods to elucidate this matter
do not hesitate to emphasise the uncertainty in the interpretation
of the conditions presented to them. Even the lowest Vertebrate,
Avhich is distinguished by the greater simplicity of its structure, and

176 EMBRYOLOGY.

by the greater ease with which all its processes of development are
understood, has failed us in this question. For HATSCHEK, who
knows the development of Amphioxus better than any one else, de-
signates the blood-vessels as the only system of organs concerning
which he was unable to arrive at a clear understanding.

Consequently in the field now to be examined there are many
views and observations which in part stand in the most direct
antagonism to each other. To give a comprehensive review of them
is not possible without the greatest fulness, which would be contrary
to the plan of this Text-book; I therefore limit myself, first, to
giving a survey of the various possibilities by which the origin of
the vessels and the blood might take place, and, secondly, to present-
ing a series of observations which have been made on Selachians,
Birds, and Mammals ; still it is always to be kept in mind that
much remains doubtful here, and that coming years may bring about
many a change in our interpretations.

According to one view, the vascular cavities are developed out of
fissure-like spaces between the germ-layers which remain unoccupied at
the time the fundament of the mesenchyme is produced. These cavities
acquire a boundary in this way : the neighboring mesenchyme-cells
begin to penetrate into them, and then unite into a vascular endo-
thelium. " The system of blood-vessels and that of lymphatic vessels,"
observes ZIEGLEE, " are produced in their first fundaments from
remnants of the primary body-cavity (the space between the primary
germ-layers), which at the general distribution of the formative
tissue (mesenchyma) remain behind as vessels, lacunae, or interstices,
and are enclosed by that tissue and incorporated in it." The formed
elements [corpuscles] arise at separate places in the blood-courses
by the growth and detachment of mesenchymatic cells.

According to another view, the vessels are constructed in this
manner : cells in the mesenchymatic tissue arrange themselves in
rows, and these cell-cords become hollowed out ; thereby the more
superficial cells furnish the endothelial wall, whereas the remaining
cells become blood-corpuscles. The blood-vessels are therefore nothing
else than cavities which have been secondarily produced in the
mesenchymatic tissues by means of their own cells. Both views
agree in this, that they cause the group of sustentative substances
to be brought into genetic connection with the blood, and the latter
to figure as a product of the metamorphosis of the mesenchyma.

Moreover, both views may present variations in the details,
according as they ascribe to the mesenchyme a different origin and

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 177

make it arise either out of the middle germ-layer alone, or out of
the entoblast alone, or by the migration of cells out of both layers
and their union into a single fundament. Still other variations
result from the first fundament of the blood-course being some-
times referred to a limited territory of the germ, sometimes to several
places. Thus, for the meroblastic eggs of B?rds, the area opaca is
designated by some observers as the place where vessels and blood
are first formed. From here they grow out as it were at first into the
embryonic body proper. The opposite is reported of Bony Fishes, in
which the first vessels, heart, aorta, caudal veins, and sub-intestinal
veins, together with blood -corpuscles, arise earliest in the embryonic
body itself, whereas they appear on the yolk only subsequently.
Finally, for the Selachians a local origin of the vessels is maintained
both for the area opaca and also for the embryonic body in the
restricted sense.

In opposition to the two views hitherto presented, a third view
assumes a separate origin for the connective substances on the one
hand, and for the vascular endothelium and the blood on the other.
Whereas the former are produced by the emigration of cells from the
middle germ-layer, the vascular endothelium is maintained to arise
from cells of the entoblast. It is held that an endothelial sac is
formed (perhaps by constriction) as an independent fundament,
which by budding gives rise to the whole vascular system.

After this brief survey of the various possibilities concerning the
origin of the blood-course, I turn to a description of certain con-
ditions, concerning the signification of which it must be admitted
that the views are also often very divergent.

The area opaca of the meroblastic eggs of Fishes, Reptiles, and
Birds has always played an important role in the literature on the
question of the origin of the blood. Notwithstanding the frequency
with which it has been investigated, the researches concerning it
cannot be regarded as concluded. It is from this standpoint that I
beg the reader to judge what follows.

In the case of the Chick, on which especially we shall base our
account, the opaque area is composed of only the two primary germ-
layers at the time when the middle germ-layer begins to be formed
from the region of the blastopore by the production of folds.

The outer germ-layer, as has already been described in Chapter V.,
has in general a simple structure, since it is composed of a single
layer of small cubical cells. The inner germ-layer (fig. 56 ik and
fig. 112), on the contrary, alters its condition the more we approach

12

178

EMBRYOLOGY.

the margin of the disc. In the area pellucida and in the immediately
surrounding parts it appears as a single layer of greatly flattened
cells, and is separated from the yolk-floor by a cavity filled with an
albuminous fluid ; in the opaque area it reposes directly upon the
yolk ; its cells here become higher, cubical, or polygonal, and finally
it terminates with a greatly thickened marginal zone, the previously
mentioned yolk- wall (dw). This is the important region of the germ
with which we now have especially to deal.

The yolk-wall consists in the Chick partly of embryonic cells,
which are separable from one another, partly of yolk-material

in which are enclosed
numerous large and
small nuclei enveloped
in protoplasm (the me-
rocytes), as at the final
stages of the process of
cleavage.

Such free nuclei have
also been demonstrated
with perfect certainty
in the marginal terri-
tory of the yolk during

the course of the formation of the germ-layers in Selachians,
Teleosts, and Reptiles (KUPFFEE, HOFFMANN, RUCKERT, STRAHL,
SWAEN).

The most accurate description of the yolk-nuclei has been given by
RUCKERT for the eggs of Selachians (fig. 113). They are present in
this case at the marginal portion of the germ-disc, embedded in the
yolk in not inconsiderable numbers, and are remarkable for their
size, sometimes reaching a diameter ten-fold as great as that of an
ordinary nucleus (k\ k*). From the protoplasm enveloping the
nucleus k* there proceeds a richly branched network of processes.
In the interstices of the net are lodged yolk-elements (d) in great
numbers, from the size of the ordinary yolk-plates down to the finest
granules. The former are often in process of disintegration. One
may conclude from this, as well as from other phenomena, that a
vigorous consumption of deutoplasm is taking place at the margin of
the germ. This deutoplasm is taken up as nutritive material by the
protoplasmic net surrounding the nucleus, and employed by means of
intra-cellular digestion for its growth. Consequently one also sees the
yolk-nuclei in active increase.

Tig. 118. Section through the margin of the germinal
disc of a Hen's egg incubated for six hours, after Du VAL.

<ilc, Outer germ-layer ; dz, yolk-cells ; dk, yolk-nuclei ;
dw, yolk-wall.

o

DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD. 179

Toward the surface of the yolk small clusters of nuclei (fig. 113 k)
arise out of the large deeper-lying yolk-nuclei. From these there
are finally produced genuine cells of the germ (z), by the small nuclei
surrounded by a layer of protoplasm detaching themselves from
the yolk, as it were by an act of supplementary cleavage. " Since the
merocytes thus on
the one hand un-
interruptedly take
up nutritive ma-
terial out of the
yolk, and on the
other continually
surrender it in the
form of cells to the
germ-layers of the
nascent embryo,
they present an
important link
between the latter

Fig. 113. Yolk-nuclei (merocytes) from Pristiurus, lying underneath
the germ-cavity B, after RUCKERT.

z, Embryonic cells ; k, superficial clear nuclei ; k l , deeper nuclei ;
k*, marginal nucjei rich in chromatin, largely freed from the
surrounding yolk, in order to show the processes of the proto-
plasmic mantle ; d, yolk-plates.

and the yolk."
(RUCKERT.)

The views of
investigators on

the significance

of the yolk- wall and of the merocytes enclosed in it are very divergent.
Indeed there is unanimity only in this, that the yolk-wall contributes
to the increase of the lower germ-layer by single cells becoming in-
dependent and attaching themselves at the margin to the elements
which already have an epithelial arrangement. On the other
hand it appears less certain how far the yolk-wall is concerned in
the formation of the blood. According to the observations of His,
DISSE, RAUBER, KOLLMANN, RUCKERT, SWAEN, GENSCH, HOFFMANN,
and others, it does share in this process during a limited period
of development in the case of Selachians, Teleosts, Reptiles, and
Birds.

In the Selachians the anterior margin of the germ-disc is the first
to be metamorphosed into a vascular zone. RUCKERT could find
here numerous and unequivocal indications that the previously
described peculiar cell-elements of the yolk (merocytes) provided
with large nuclei contribute to the formation of blood-islands, in
that they break up into clusters of small cells, detach themselves

180 EMBRYOLOGY.

from the yolk-containing part of the lower germ-layer, and become
differentiated on the one hand into the migratory cells of the first
blood-vessels, and on the other into the blood-corpuscles. RUCKERT
further maintains that the material destined for the production of
blood is su])plemente,d by means of cells freshly cleft off from the
yolk.

SWAEN remarks with the same positiveness, " Les premiers ilots
sanguins se developpent aux depens des elements de I'hypoblaste. . Ces
derniers constituent a la fin de ce developpement les parois de cavites
vasculaires closes et les cellules sanguines qui les rempli.ssent."
Likewise GENSCH makes the large cells in the yolk responsible for
the formation of the blood in the case of the Bony Fishes. HOFF-
MANN also finds in Reptiles that the blood and the endothelial
wall of the vessels, as well as the spindle-shaped cells which lie
between the vessels, are a product of the inner germ-layer, and
that they appear at definite places of the germ-disc at a time
when the middle germ-layer has not yet been formed in those
regions.

Finally, it is stated concerning the germ of the Chick that at the
end of the first day of incubation the cells in the yolk-wall have
become very numerous, through the multiplication of the nuclei
enclosed in the latter, and that afterwards the abundance of the
cells diminishes. For part of the cells which have been formed
by the active proliferation now detach themselves from the yolk-
wall, get into the space between the outer and inner germ-layers,
and there produce a third independent layer, which is continually
increasing in thickness, whereas the remaining part becomes modi-
fied into an epithelium of large cylindrical cells containing yolk-
granules. This middle layer is judged by several investigators to
be an independent fundament of the germ, and has in this sense
been described by His as parablast, by DISSE and others as vascular
layer, by RAUBER as desmohcemoblast, and by KOLLMANN as marginal
germ or acroblast.

All of these accounts need still more precise confirmation, since
they have often been called in question, even up to most recent
times. Thus KOLLIKER has always defended the position that
not only the connective substances, but also the vessels and the
blood, are products of the middle germ-layer, and are generated by it
in its'peripheral regions. KASTSCHENKO, in his study of the Selachii,
could not convince himself that the merocytes have special import-
ance in the formation of blood and vessels, but was not, however,

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 181

willing to deny it. So much the more positively do WENKEBACH
and ZIEGLER, on the strength of their investigations on Teleosts,
express themselves against the mode of blood-formation given by
GENSCH. According to ZIEGLER, the blood-corpuscles are developed
in the blood-vessels of the embryonic body itself. The free nuclei
of the yolk, the merocytes, on the contrary, it is maintained, do not
share in the formation of embryonic tissues, but, in adaptation to
the function of resorbing the yolk, undergo peculiar modifications,
which " cause the frequently affirmed but never proved production
of blood-corpuscles [by them] to appear improbable."

Under this condition of affairs, I must regard the question of the
source of the cell-layer in which, in the region of the opaque area,
the formation of blood takes place as not yet ready for final
judgment.

So far as regards the further changes, by means of which the
cell-layer under consideration is converted into connective substance
and blood, on the whole I subscribe, in this difficult field of in-
vestigation, to KOLLIKER'S representation.

At the end of the first day of incubation, the masses of cells which
lie between the inner and the outer germ-layers arrange themselves
in cylindrical or irregularly limited cords, which join themselves to-
gether into a close-meshed network ; they are the first fundaments
both of the vessels and also of their contents, the blood. In the
spaces of the net are to be found groups of indifferent cells, which
afterwards become embryonic connective tissue, and which are the
Substanzinseln (fig. 114) of authors.

At the beginning of the second day of incubation, the solid funda-
ments of the vessels become more distinct, in proportion as they
become bounded superficially by a special wall, and acquire
an internal cavity. The wall of the vessels is developed out of
the most superficial cells of the cords, and is composed during the
first days of incubation of a single layer of very much flattened
polygonal elements, on account of which the first vessels of the
embryo are often designated as endothelial tubes (fig. 114 and
fig. 115 gw).

The cavity of the vessel is probably formed by the penetration of
fluid into the originally solid cord from its surroundings, thus forming
the plasma of the blood, by which the cells are pressed apart and to
the sides. The cells then constitute here and there thickenings of
the wall, and project into the fluid-filled cavities as elevations of
loosely united spherical elements (fig. 114, Blood-islands). Conse-

182

EMBRYOLOGY.

Blood-island

Wall of blood-
vessel

quently the vessels which are just becoming permeable are very
irregular, since narrow places and wider ones, often provided

p- -- -WTWT :--*73 With evagiua '

tions, alternate

(fig. 114) with
one another,
an d since
the vessels
are sometimes
wholly excava-
ted, fluid-filled,
endotheli al
tubes, and
sometimes re-
main more or
less impassable,
owing to the
variously
formed cell ag-
gregates which
project from
the wall.

The aggrega-
tions of cells
themselves are
simply the
centres where
the formed com-
ponents of the
blood are pro-
duced . The
small spherical
nucleated cells,
which still en-
close dark yolk-
granules, be-
come at first

Blood-island

Blood-vessel

Wall of blood-
vessel

Substanzinseln
Blood-vessel

Fig. 114. A portion of the vascular area of the germ- disc of an embryo
Chick, in which 12 primitive segments are developed, after DISSE.

One sees the more darkly shaded blood-courses, in which lie the
" blood-islands," the centres whence the blood-corpuscles arise-
The clear spaces in the vascular network, the walls of which are
formed of flat endothelial cells, are the " substance-islands "
(Substanzinselii).

homoge n e o u s

by the dissolution of the latter, and then, owing to the formation
of the coloring matter of the blood in them, they take on a slightly
yellowish color, which gradually becomes more intense.

DEVELOPMENT OP CONNECTIVE SUBSTANCE AND BLOOD.

183

If one at this time examines a blastoderm which has been removed
from the yolk, the zone in which the formation of blood takes place
appears flecked with more or less intensely colored blood-red spots,
some of which are roundish, others elongated, and others branched.
The spots are known as the blood-points or blood-islands of the blasto-
derm (fig. 114). From these formative areas the superficial cells
now detach themselves and enter the blood-fluid as the isolated red
blood-corpuscles. Here, as well as in the blood-islands, they multiply
by means of cell-division, during which the nucleus is metamorphosed
into the well-known spindle-figure.

As REMAK first showed, divisions of blood-cells are to be observed
in the Chick in great numbers up to the sixth day of incubation,
whereas they later become more rare, and then wholly disappear.
Also in the case of Mammals and of Man (FoL) the first embryonic

Fig. 115. Cross section through a portion of the vascular area, after DISSE.

ale, Outer, it, inner germ-layer ; mi 1 , parietal, mk', visceral lamella of the middle germ-layer ;
Ih, extra-embryonic body-cavity ; gto, wall of blood-Tessel formed of endothelium ; bl, blood-
cells ; g, vessels.

blood-corpuscles, which are at this time provided as in the other Verte-
brates with a genuine cell-nucleus, possess the power of division.

In proportion as blood-corpuscles still further detach themselves
from the blood-points, the latter become smaller and smaller, and
finally disappear altogether ; but the vessels without exception then
contain, instead of a clear fluid, red blood with abundant formed
elements (fig. 115 bl).

Subsequently there occur changes in the Substanzinseln which lead
to the formation of embryonic connective substance. The germinal
cells, at first spheroidal, separate farther from one another, at the
same time secreting a homogeneous inter-cellular substance ; they
become stellate (fig. 116 sp), and send out processes by means of
which they are united into a network, which stretches all through
the gelatinous secretion ; other cells apply themselves to the endo-
thelial tubes of the vessels.

184

EMBRYOLOGY.

After the formation of vessels and blood is completed, the territory
of the area opaca, in which the processes just described take place,
is sharply delimited at its periphery (fig. 117) in all meroblastic eggs,
as well as in those of Mammals. For the close network of blood

vessels ends abruptly at its periphery in a broad, circular, marginal
vein (the vena or sinus terminalis, S.T.).

Beyond the sinus terminalis, there is formed on the yolk neither
blood nor blood-vessels. Nevertheless, the two primary germ-layers
spread themselves out laterally over the yolk still farther, the
outer layer more rapidly than the inner, until they have grown
entirelv around it.

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 185

We must therefore now distinguish iu the opaque area (Plate I.,
fig. 2, page 213) two ring-like areas, the vascular area (gh) and the
yolk-area (dK), area vasculosa and area vitellina. Since, moreover,

Fig 117. Diagram of the vascular system of the yolk-sac at the end of the third day of
incubation, after BALFOUR.

The whole blastoderm has been removed from the egg and is represented as seen from below.
Therefore what is really on the left appears on the right, and vice vcrsd. The part of the
area opaca in which the tine vascular network has been formed is sharply limited at the
periphery by the sinus tenninalis, and represents the vascular area ; outside of it lies the
yolk-area. The immediate vicinity of the embryo is destitute of a vascular network, and is
designated now, as at an earlier stage, by the name area pellucida.

H Heart; A A, aortic arches ; Ao, dorsal aorta , L.Of.A, left, R.Of.A, right vitelline artery;
S.T, sinus tenninalis ; L.Of, left, R.Of, right vitelline vein ; S. V, sinus venosus ; D.C, ductus
Cuvieri ; S.Ca. V, superior, V.Ca, inferior cardinal vein. The veins are drawn in outline,
the arteries in solid black.

the area pellucida is still recognisable, being traversed by only a few
chief trunks of blood-vessels leading to the embryo, the body of the
embryo is enclosed altogether by three zones or areas of the extra-
embryonic part of the germ-layers.

Up to the present we have pursued the formation of blood in the
opaque area. But how do the vessels in the body of the embryo

186 EMBRYOLOGY.

itself arise ? Here, too, the uncertainty of our present knowledge is
to be emphasised.

According to the representation of His, to which KOLLIKER also
adheres, and which the author himself has made the foundation of
his account in the first edition of this Text -hook, blood-vessels in the
embryo are not independently formed, but take their origin from
those already existing in the opaque area. According to His, the
germ of the blood and connective substances, originally a peripheral
fundament, makes its way from the opaque area at first into the
pellucid area, and from there into the body of the embryo itself,
and is distributed everywhere in the spaces between the epithelial
germ-layers and the products that have arisen by constriction from
them. Into the spaces migrate first of all amoeboid cells, which
send out in front of them branched processes ; on the heels of these
follow endothelial vascular shoots.

At variance with the teachings of His are noteworthy investiga-
tions of recent date, not only the previously mentioned accounts of
the manifold origin of the connective substances from the middlo
germ-layers, but also particularly the more recent observations con
cerning the independent origin of vessels and the endothelial sac of
the heart in the body of the embryo itself. (RUCKERT, ZIEGLER,
MAYER, RABL, KASTSCHENKO, and others.)

For Selachian embryos the question, whether the repository of
the material for the blood-vessels of the embryo is to be sought
exclusively on the nutritive yolk, is, as RUCKERT remarks, to be
answered definitely in the negative. The vessels arise in the embryo
itself within the territory of the mesenchyme, from cells which
are sometimes loosely, sometimes compactly arranged (RtiCKERT,
MAYER).

RUCKERT derives the cells that form the vessels from two different
sources, partly from the inner germ-layer of the yolk-wall, partly
from the adjoining mesoblast, and their double origin appears to
him a natural process of development, in so far as the two layers
which bound the first vessels also furnish the material for their walls.

To the same purport are the accounts concerning the formation
of the endothelial sac of the heart. At first it consists of a rather
irregular mass of cells, in which there appear separate cavities, that
gradually unite to form a single cardiac space. The cell-material
of the fundament of the heart is developed in situ (RtrcKERT, ZIEGLER,
MAYER, RABL, and of the earlier investigators GOTTE, BALFOUR,
HOFFMANN) from the wall of the bounding germ-layers; however,

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 187

uncertainty prevails as to whether the inner germ-layer alone,
or the middle, or both, are concerned in the production of the
fundament.

When once the first vessels have been formed, they grow further
independently, and continually give rise to new lateral branches by
means of a kind of budding process.

It can be observed that from the walls of vessels that are already
hollow, solid, slender sprouts go out, which are formed of spindle-
shaped cells, and by means of cross-branches join others to form a
network. The youngest and most delicate of these sprouts consist
of only a few cells arranged in a row, or indeed of only a single one,
which, reposing upon the endothelial tube like a knob, is drawn
out into a long protoplasmic filament. Into the solid sprout there
now projects from the already completed vessel a small evagination,
which gradually elongates and at the same time enlarges into a
tube, the wall of which is formed of the separated cells of the funda-
ment. The formation of blood-corpuscles no longer takes place in this
process, all the cells of the sprout being employed to form the wall of
the vessel. Since out of the vessels thus produced new sprouts
are formed, and so on, the fundaments of the vessels spread them-
selves out everywhere in the spaces between the germ-layers and
the organs which have by constrictions been formed from them.

There are, moreover, two different opinions about the manner in which the
sprouting takes place. Are the solid vascular shoots formed exclusively by
growth of cells in the wall of the endothelial tube, or do neighboring con-
nective-tissue cells take part in their formation ? While RABL holds to the
proposition that new vascular endothelia always take their origin from such as
are already in existence, KOLLIKER, MAYER, and R(JCKERT make statements
which appear to prove that the endothelial vascular tubes both continue to
grow by themselves alone, and also to elongate through the participation of
the connective-tissue cells of the surrounding tissue.

In the preceding pages we have endeavored to show in detail
how in Vertebrates the material of the cleavage- cells is differen-
tiated into the separate fundamental or primitive organs. As such
we must designate the outer and the inner germ-layers, the two
middle germ-layers, and the mesenchyme or intermediate layer.

In order properly to estimate at once the significance and the rdle
of these fundamental organs, we will glance at the final result of the
process of development propound the question, What organs and

188 EMBRYOLOGY.

tissues take their origin in the separate germ-layers and the mesen-
chyme ? A definite answer to this question is possible, except on a
few points concerning which the accounts of the different observers
are still contradictory, and which therefore will be indicated by a
mark of interrogation.

From the outer germ-layer arise : the epidermis, the epidermoidal
organs, such as hair and nails, the epithelial cells of the dermal
glands, the whole central nervous system with the spinal ganglia,
the peripheral nervous system (?), the epithelium of the sensory
organs (eye, ear, nose), and the lens of the eye.

The primary inner germ-layer is differentiated into :

1. The secondary inner germ-layer, or entoblast;

2. The middle germ -layers ;

3. The fundament of the chorda ;

4. The germ of the mesenehyme, which forms the intermediate
layer.

The entoblast (Darmdriisenblatt) furnishes the epithelial lining
of the whole intestinal canal and its glandular appendages (lung,
liver, pancreas), the epithelium of the urinary bladder, and the
taste buds.

The middle germ-layers undergo extremely various metamorphoses
after having been differentiated into primitive segments and lateral
plates.

From the primitive segments are derived the striated, voluntary
muscles of the body and a part of the mesenehyme.

From the lateral plates arise the epithelium of the pleuroperitoneal
cavity ; the epithelium of ovary and testis (primitive ova, mother-
cells of the spermatozoa) ; in general, the epithelial components of
the sexual glands and their ducts, as well as those of the kidney and
ureter ; and finally mesenchymatic tissue.

The fundament of the chorda becomes the chorda dorsalis, which in
the higher Vertebrates is reduced, during later stages of development,
to insignificant remnants.

The mesenchyme-germs, which produce the intermediate layer, un-
dergo manifold differentiations, for they spread themselves out in
the body between the epithelial components as the intermediate mass.
From them are derived . the multiform group of sustentative (con-
nective) tissues (mucous tissue, fibrillar connective tissue, cartilage,
bone), vessels (?) and blood (?), the lymphoid organs, the smooth,
involuntary muscles of the vessels, of the intestine, and of various
other organs.

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 189

HISTORY OF THE PABABLAST- AND MESENCHYME-THEORIES.

The older investigators, as, for example, KEMAK, grouped together all the
cells which are inserted between the two primary germ-layers under the
common name of the middle germ-layer, and assumed for them a common
origin. To this conception His opposed in the year 1868 in " Die erste Ent-
wicklung des Hubnchens im Ei " his "parablast-thewy," in which, influenced
principally by histogenetic considerations, he distinguished two fundaments
of different origin, an arckiblastic and a parablastlc.

As archiblastic fundament he designated the part of the middle germ-layer
which lies in the body of the embryo itself, the axial cord (Achsenstrang) and
the animal and vegetative muscle-plates, and he made them arise by de-
lamination from the primary germ-layers, and therefore ultimately from the
embryonic cleavage-cells.

He gave the name parablast to a peripheral fundament, lying originally outside
the embryo, wmcn is theliource of alfthlT connective substances, the blood and
tBe vascular endothelium, and which grows from the margin, or more speci-
fically from the opaque area, into the body between the archiblastic tissues.

The division of the middle germ-layer into archiblast (chief germ) and
parablast (accessory germ), proposed by His and carried out in several of his
writings, found at the time no approbation, and encountered decided and
successful opposition, especially on the part of HAECKEL, because the correct
views contained in the doctrine were obscured and covered up by peculiar
conceptions about the origin of the parablast. The parablast, it was claimed,
is not derived from the egg-cell, but from the white yolk, a product of the
granulosa-cells, which, according to the earlier teachings of His, penetrate
into the primordial ovum in great numbers and become the white yolk-cells
and the yellow spherules. But the granulosa-cells in turn, it was maintained,
arise from the connective tissue (leucocytes) of the mother ; consequently
after their migration into the egg they are capable of producing again
only connective tissue and blood.

His thought it \vas necessary to assume a fundamental difference between
chief germ and accessory germ ; the former alone had experienced the influence
of fertilisation, since it alone was descended fram cleavage-cells, whereas the
latter, since it issued from the white yolk (a derivative of the maternal con-
nective tissue), was " purely a maternal dower."

RAUBER, in a short communication, accepted the conclusions of His, in so
far as he also assumed a common origin for blood and connective tissue, a
special " baemo-desmoblast," but differed from him in that he derived them
from the cleavage-cells.

GOETTE (1874) is also to be mentioned in this connection, since he maintained
that the blood is developed out of yolk-cells, which break up into clusters of
smaller cells (Amphibia and Birds).

Proceeding from other standpoints, and induced by observations on In-
vertebrates, my brother and I were led in our Ccelom-Theory (1881) to a result
similar to that of His, namely, that two entirely different structures had been
hitherto embi aced under the expression middle germ-layer, and that it was
necessary to introduce in the place of the old indefinite conception two new
and more precise ones, " middle germ-layer in the restricted sense " and " mesen-
chyme-fferm." But our conception, notwithstanding many points of agree-
ment, took in detail a form very different from the doctrine of His.

190 EMBRYOLOGY.

All fundaments of the animal body are derived from embryonic cells, which
have been produced from the egg-cell by the process of cleavage. The dis-
tinction between middle germ-layer and mesenchyme-germ is to be sought
in another direction than in that indicated by His. TJie middle germ-layers
are sheets of embryonic cells, having an epithelial arrangement, which arise by
a process of folding from the inner germ-layer, just as the latter does by a fold-
ing of the blastula (compare the historical part of Chapter VII.). The mesen-
chymatic germ, on the contrary, embraces cells, which liave been individually
detached from epithelial union in the inner germ-layer, and furnish, tliefounda-
iwnyor connective substance and blood by spreading themselves out in the
system of spaces between the epithelial germ-layers.

After the appearance of the Ccelom-Theory, His entered again into an
explanation of his parablast-theory, and modified it in his paper, " Die Lehre
vom Bindesubstanzkeim," in so far as he no longer laid weight on the
question whether the fundament of the connective substance was derived from
the segmented or the unsegmented germ.

The theory of the double origin of the middle germ-layers, established by
His and by us in different ways, met with opposition on the part of KOLLIKER
who held to the older interpretation ; but by many others it was accepted ;
attempts were made further to confirm and also to modify it by KUPFFER,
DISSE, WALDEYER, KOLLMANN, HEAPE, and others, who defended the existence
of a special connective-tissue germ.

KTJPFFEB and his followers furnished important observations concerning
the presence of yolk-nuclei in a definite zone of the embryonic fundament, and
their relation to the formation of blood in Fishes and Reptiles.

HOFFMANN and RDCKERT showed that the yolk -nuclei do not arise by free
[spontaneous] formation of nuclei, but are descendants of the cleavage-nucleus.

DISSE investigated the germ-wall of the Hen's egg.

KOLLMANN named the cells which migrate out between the germ-layers
poreuts (Poreuten), and the whole fundament the acroblast.

Finally, WALDEYEB endeavored to derive the connective-tissue germ from
a special part of the cleavage-material, which he divided into an archiblast
and a parablast.

According to WALDEYER'S theory, the cleavage of the eggs of all those
animals in which there is any blood and connective substance does not take
place uniformly up to the end, but one must distinguish a irrimary and a
secondary cleavage. " The former divides the egg, so far as it is in any way
capable of cleavage, into a number of cells, which are ready for the production
of tissues. These then form the primary germ-layers. A remnant of im-
mature cleavage-cells (in the case of holoblastic eggs), or of egg-protoplasm,
which is not yet converted into the cell-form (in meroblastic eggs), is left
remaining. Neither the immature cells, nor the protoplasm still unconverted
into cells, enter for the present into the integrating condition of the germ-
layers. On the contrary, it is only afterwards that there is effected on this
material a further formation of cells, the secondary cleavage. The immature
cells of the holoblastic eggs, over-loaded with nutritive yolk, divide them-
selves, or, if one prefers, ' cleave ' themselves further, or the parts which are
most richly provided with protoplasm constrict themselves off from the
eggs, whereas the remnant of the nutritive material is consumed, the
unformed remnants of the protoplasm (germ-processes) of meroblastic eggs
become divided up into cells. The cell-material thus secondarily acquired

DEVELOPMENT OF CONNECTIVE SUBSTANCE AND BLOOD. 191

migrates in between the primary germ-layers, and becomes blood and connec-
tive substance."

According to the recent investigations of RABL, ZIEGLEB, VAN WIJHE,
RiiCKERT, and others, the mesenchyme is produced from various regions of
the middle germ-layer. A participation of the inner germ-layer in the forma-
tion of the blood-vessels is rendered probable.

SUMMARY.

1. Besides the four germ-layers, which have the form of
epithelial lamellae, special germs are developed in the higher
Vertebrates for the sustentative substances and the blood, the
mesenchyme-germs. The latter together make up the intermediate
layer.

2. The mesenchyme-germs arise by cells detaching themselves
from epithelial union with the germ-layers, and penetrating as
migratory cells into the fissure between the four germ -layers (the
remnant of the original cleavage-cavity) and spreading themselves ^ul
in this space.

3. Germ-layers and mesenchyme-germ (intermediate layer) ex-
hibit a difference in the method of their origin : the former are
developed by foldings of the wall of the blastula, the latter by emi-
gration of isolated cells from definite territories of the germ-layers.

4. Mesenchyme-germs arise from the wall of the primitive segment,
from the cutis-plate, and at certain regions of the parietal and
visceral lamellae of the middle germ-layer.

5. Blood-vessels are developed both in the_bodyzof the embryo
itself, in a manner which still remains to be accurately determined,
and also in the territory of the area opaca of Daeroblajtic_eggs^

6. The source of the cells from which the vessels~and blood of
the opaque area arise is at present a matter of controversy.

7. In the formation of vessels in the opaque area the following
phenomena are to be regarded :

(a) The embryonic cells of the intermediate layer arrange
themselves :

First into a network of cords, and
Secondly into the substance-islands (Substanzinseln).
(6) There are developed out of the cell-cords, at the same time
with the secretion of the fluid portions of the blood, the
endothelial wall of the primitive blood-vessels and their
cellular contents, the blood-corpuscles (blood-islands).
(c) The Substanzinseln become embryonic connective substance

192 EMBRYOLCGY.

(d) The place where blood-vessels and connective substance at

first arise in the opaque area is sharply limited at the
periphery by a circular vessel, the sinus terminalis.

(e) Since the outer and the inner germ-layers further con-

tinue to spread themselves out over the yolk after the
development of the intermediate layer, the body of the
embryo becomes surrounded by three areas :
First by the area pellucida,
Secondly by the vascular area ending in the sinus

terminalis,

Thirdly by the yolk-area, which is coextensive with
the margin of the overgrowth.

8. The red blood-corpuscles of all Vertebrates possess in the
earliest stages of development the power of increase by means
of division. The red bicod-corpuscles of Mammals have at this
time a nucleus.

9. The following table gives a survey of the fundamental organs
of the embryo, and the products of their further development :

I. Outer Germ-layer.

Epidermis, hair, nails, epithelium of dermal glands, central nervous
system, peripheral nervous system, epithelium of sensory organs, the
lens.

II. Primary Inner Germ-layer.

1. Entoblast, or secondary inner germ-layer.

Epithelium of the alimentary canal and its glands, epithelium
of urinary bladder.

2. Fundament of the chorda.

3. The middle germ-layers.

A. Primitive Segments.

Transversely striped, voluntary muscles of the body. Parts
of the mesenchyme.

B. Lateral Plates.

Epithelium of the pleuroperitoneal cavities, the sexual cells
and epithelial components of the sexual glands and their
outlets, epithelium of kidney and ureters. Parts of the
mesenchyme.

4. Mesenchyme- germ.

Group of the connective substances, blood-vessels and blood,
lymphoid organs, smooth involuntary muscles.

LITERATURE. 193

LITERATURE.
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Balfour. The Development of the Blood-vessels of the Chick. Quart Jour

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Gensch. Die Blutbildung auf dem Dottersack bei Knochenfischen. Archiv

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Hatschek. Ueber den Schichtenbau von Amphioxus. Anat. Anzeiger. 1888.
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Kollmann, J. Der Mesoblast und die Entwicklung der Gewebe bei Wirbel.

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13

194 EMBRYOLOGY.

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Bd. VIII. 1888.
Strahl. Die Anlage des Gefasssystems in der Keimscheibe von Lacerta agilis.

Sitzungsb. d. Gesellsch. z. Befb'rd. d. ges. Naturwiss. Marburg. 1883, p. 60.
Strahl. Die Dottersackwand und der Parablast der Eidechsen. Zeitschr. f.

wiss. Zoologie. Bd. XLV. 1887.
TJskow. Die Blutgefasskeime und deren Entwicklung bei einem Huhnerei.

M6m. de 1'Acad. imper. des Sci. St. Petersbourg. Se>. VII. T. XXXV.

Nr. 4. 1887.
Waldeyer. Archiblast und Parablast. Archiv f. mikr. Anat. Bd. XXII.

1883, pp. 1-77.
"Wenckebach. Beitrage zur Entwicklungsgeschichte der Knochenfischev

Archiv f. mikr. Anat. Bd. XXVIII. 1886, p. 225.
Ziegler. Der Ursprung der mesenchymatischen Gewebe bei den Selacbiern.

Archiv f . niikr. Anat. Bd. XXXII. 1888.
Ziegler. Die Entstehung des Blutes bei Knochenfischembryonen. Archiv f.

mikr. Anat. Bd. XXX. 1887.

CHAPTER X.
ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY.

AFTER having investigated in the preceding chapters the fundamental
organs of the body of vertebrated animals, or the germ-layers, and
their first important differentiations into neural tube, chorda, and
primitive segments, as well as the origin of the blood and connective
tissues, it will be our next undertaking to make ourselves acquainted
with the development of the external form of the body, and with the
development of the embryonic membranes, the latter being intimately
connected with the former.

There exists an extraordinary difference in these respects between the
lower and higher Vertebrates. When the embryo of an Amphioxus
has passed through the first processes of development, it elongates,
becomes pointed at both ends, and already possesses in the main
the worm-like or fish-like form of the adult animal. But the higher
we ascend in the series of Vertebrates, the more are the embryos,
when they attain the stage of development corresponding to the
Amphioxus embryo, unlike the adult animals: at this stage thev

J

assume very singular and strange forms, inasmuch as they become
surrounded by peculiar envelopes and are provided with various
appendages, which subsequently disappear.

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 195

The difference is referable, first of all, to the more or less extensive
accumulation of nutritive yolk, the significance of which for the
nascent organism is twofold.

From a physiological point of view, the nutritive yolk is a rich
source of energy which alone makes it possible for the embryological
processes to take place in uninterrupted sequence, until at length an
organism, with an already relatively high organisation, begins its
independent existence.

From a morphological point of view, on the other hand, the yolk plays
the role of ballast, which exerts a restrictive and modifying influence
on the direct and free development of those organs which are en-
trusted with the reception and elaboration of it. Even at the very
beginning of development we could see how the cleavage-process and
the formation of the germ-layers were retarded, altered, and to a certain
extent even suppressed by the presence of yolk. In what follows we
shall again have occasion to point out the same thing, how, owing
to the presence of yolk, the normal formation of the intestinal canal
and of the body can be attained only gradually and by a circuitous
process.

In the second place, the great difference which the embryos of
Vertebrates present is produced by the medium in which the eggs
undergo development. Eggs which, like those of water-inhabiting
Vertebrates, are deposited in the water, are developed in a more
simple and direct manner than those which, provided with a firm
shell, are laid upon the land, or than those which are enclosed in
the womb up to the time of the birth of the embryos.
In the two latter cases the growing organism attains its goal only
by very indirect ways. At the same time with the permanent organs
there are also developed others which have no significance for the
post-embryonic life, but which serve during the egg-stage of exist-
ence either for the protection of the soft, delicate, and easily injured
body, or for respiration, or for nutrition. These either undergo
regressive metamorphosis at the end of embryonic life, or are cast
off at birth as useless and unimportant structures. But inasmuch as
they are developed out of the germ-layers, they are also properly to
be regarded as belonging immediately to the nascent organism as
being its embryonic organs, and as such they too are to be treated in
morphological descriptions=

The extensive material which has to be mastered in this con-
nection I shall present grouped into two parts.

In the first part we shall inquire how the embryo overcomes the

196

EMBRYOLOGY.

obstacle which it encounters in the presence of the yolk and acquires
its ultimate form.

In the second and likewise more extensive part we must concern
ourselves more minutely with the embryonic enveloping structures
and appended organs, which subserve various purposes.

The collection of yolk-material disturbs the course of development
least in the case of the Amphibia. The latter therefore stand,
as it were, midway between Amphioxus with direct development

and the remaining Verte-
brates, and constitute a
transition between them.
In the Amphibia the yolk
shares in the process of
cleavage; after the close of
this process it is found ac-
cumulated for the most part
in the large yolk-cells which
form the floor of the blastula
(fig. 45) ; at the time of the
differentiation into germ-
layers it is taken up into the
ccelenteron, which it almost
completely fills (fig. 47); after
the formation of the body-
sacs the large yolk-cells lie
in a similar manner in the

ventral wall of the intestine proper (fig. 118 yk). Here they are in
part dissolved and employed for the growth of the remaining parts
of the body, in part they share directly in the formation of the
epithelium of the ventral wall of the intestine.

In consequence of the presence of the great accumulation of yolk-
cells, the Amphibian embryo acquires a shapeless condition at a time
when the Amphioxus larva has already become elongated and fish-
like. The bcdy, which is spherical during gastrulation, later becomes
egg-shaped, owing to .its elongation. Thereupon the head-end and
the tail-end begin to be established at the two poles as small eleva-
tions (figs. 118 and 80). The middle or trunk-part lying between
the latter becomes somewhat incurved along its dorsal region, in

fig. 118. Diagrammatic longitudinal section through
the embryo of a Frog, after GOETTE, from BALFOUR.

nc, Neural tube ; x, communication of the same with
blastopore and coalenteron (at) ; yb, yolk-cells ; m,
middle germ-layer. For the sake of simplicity the
outer germ-layer is represented ai if composed of
a single layer of cells.

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 197

which neural tube, chorda, and primitive segments are developed, so
that the cephalic and caudal elevations become joined by means of
a concave line. The ventral side of the trunk-region, on the con-
trary, is greatly swollen and bulges out ventrally and lateraDy like
a hernia, since it is filled with yolk-cells. This swelling is therefore
called the yolk-sac.

In the further progress of development the embryo continually
acquires a more fish-like shape. The anterior and the posterior
ends of the body, especially the latter, increase greatly in length,
and the middle of the trunk becomes thinner, for with the consump-
tion of the yolk-material the yolk-sac becomes smaller and finally
disappears altogether, its walls being incorporated into the ventral
wall of the intestine and that of the body.

The interferences in the normal course of development become greater
in the same ratio as the yolk increases in amount, as it does in the
case of the meroblastic eggs of Fishes, Reptiles, and Birds. With
the latter the yolk is no longer broken up into a mass of yolk-cells,
as in the case of the Amphibia ; it participates in the process of
cleavage, but only to. a slight extent, inasmuch as nuclei make their
way into the layer of yolk which is adjacent to the germ, and, sur-
rounded by protoplasm, continue to increase in number by division.
The gastrula-form is altered until it becomes unrecognisable; only_
a small part of its dorsal surface consists of cells, which are
arranged into the two primary germ-layers, whereas the whole
ventral side, where in the Amphibia the yolk-cells are found, is an
unsegmented yolk-mass.

Thus we acquire in the case of the Vertebrates mentioned a
peculiar condition ; the embryo, if we regard the yolk as not
belonging to the body, appears to be developed from layers that are
spread out flat instead of from a cup-like structure (Plate I., fig. 1,
page 213). Moreover we see even a greater distinction effected
between the dorsal and ventral surfaces of the egg during develop-
ment than was the case with the Amphibians. The fundaments of
all important organs, the nervous system, the chorda, the primitive
segments (Plate I., figs. 2, 8), are at first produced exclusively on the
former, whereas on the ventral side few and unimportant changes only
are to be observed. These consist principally in the extension of the
germ-layers, which spread out farther ventrally, grow over the yolk-
mass (Plate I., figs. 2-5), and form around it a closed sac consisting
of several layers. This circumcrescence of the unsegmented yolk by
the germ-layers is accomplished, on the whole, very slowly, the more

198

EMBRYOLOGY.

voluminous the accumulated yolk-material, the more time it requires :
thus, for example, in the case of Birds it is completed at a very late
stage of development, when the embryo has already attained a high
state of perfection (Plate I., fig. 5).

In the case of meroblastic eggs, the part of the germ-layers
on which the first fundaments of the organs (neural tube, chorda,
primitive segments, etc.) appear has been distinguished as the
embryonic area from the remaining part, or the extra-embryonic area.
The distinction is both fitting and necessary ; but the names might
have been more appropriate than " embryonic and extra-embryonic,"
since obviously everything that arises from the egg-cell, and con-
sequently even that

which originates in

the extra-embryonic
area, must be rec-
koned as belonging
to the embryo. The
differentiation into
two areas persists in
the course of further
development, and be-
comes expressed still
more sharply (fig.

Fig. 119. Advanced embryo of a Shark (Pristiurus), after 119). The embryonic
BALFOUR. , ,

Em, Embryo ; ds, yolk-sac ; st, stalk of the yolk-sac ; av, aiteria area > DV means Or the

vitellina ; w, vena vitellina. folding of its flattened

layers into tubes,

alone forms the elongated, fish-like body which all Vertebrates a'
first exhibit ; the extra-embryonic area, on the contrary, becomes
a sac filled with yolk (ds), which, like an enormous hernia, is united
to the embryo (Em) by means of a stalk (st) attached to its belly,
sometimes even while the embryo is still remarkably small.

We must now explain more minutely the details of the processes
of development which take place in this connection: first the
metamorphosis of the flattened embryonic area iato the fish-like
embryonal body, and secondly the formation of the yolk-sac.

In the presentation we shall adhere chiefly to the Hen's egg, but
for the time being we shall leave out of consideration the formation
of the embryonic membranes.

The body of the Chick is developed by a folding of the flattened
layers, and by the constricting off of the tubular structures thus formed

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY.

199

from the area pellucida. The beginning of the process of folding is
recognisable upon the surface of the blastoderm by means of
certain furrows, the marginal grooves (Grenzrinnen) of His. These
appear earlier in the anterior than in the posterior region of the
embryonic fundament, in correspondence with the law previously
enunciated, according to which
the anterior end of the body
anticipates in development the
posterior end.

At first that part of the
embryonic fundament which is
destined to become the head is
marked off by means of a cres-
centic groove (fig. 120). In the
case of the Chick this is indicated
during the first day of incubation,
at a time when the first trace
of the nervous system becomes
visible. It lies immediately in
front of the curved anterior end
of the medullary ridges, with its
concavity directed backward.

At a later stage the embryonic
area is marked off laterally. In

the Case of the embryo Seen from Fig. 120. Surface-view of the area pellucida of
/! i ii i i T_ a blastoderm of 18 hours, after BALFOCR.

the suriace in ng. 121, in which In frollt of the primitive groove (pr) lies the

the neural tube is already partly medullary furrow (me), with the medullary

ridges (A). These diverge behind and fade
out on either side in front of the primitive
groove ; anteriorly, on the contrary, they are
continuous with each other, and form an arch
behind a curved line, which represents the
anterior marginal groove. The second curved
line, lying in front of aud concentric with the
first, is the beginning of the amniotic fold.

closed and segmented into three
brain-vesicles, and in which six
pairs of primitive segments are
laid down, there may be re-
cognised at some distance from

these primitive segments two

dark streaks, the two lateral marginal grooves. They become
less distinct in passing from before backward, and wholly disappear
at the end of the primitive groove.

Finally, the tail-end of the embryo is marked off by the posterior
marginal groove, which like the anterior is crescentic, but has its
concavity directed toward the head.

In this manner a small part of the germ-layers, which alone is
required for the construction of the permanent body, is separated by a

200

EMBRYOLOGY.

Jtf

continuous marginal furrow from the much more extensive extra-
embryonic area, which
serves for the formation
df of evanescent organs like
the yolk-sac and the em-
bryonic membranes.

The marginal grooves
are formed by the infold-
ing of the outer germ -layer
and the parietal middle
layer, which are together
called the somatopleure, and
in such a manner that the
lidge of the original small
fold is directed downward
toward the yolk (Plate I.,
fig. 8 sf). The space en-
closed by the two folded
layers is the marginal
groove (gr). As we have
distinguished on the latter
several regions, which are
developed at different times,
so must we here distinguish
the corresponding folds,
and we consequently speak
of a head/old, a tail-
fold, and the two lateral
folds.

The head/old appears,
first of all, even on the
first, but more distinctly
on the second, day of in-
cubation. By means of
it the head-end of the
embryonal fundament is
formed and separated from
the extra-embryonic part
of the germ -layers. At

pr

Tig. 121. Blastoderm of the Chick, incubated 33 hours,
after DUVAL.

One sees the pellucid area, hf t surrounded by a portion
of the opaque area, df. The fundament of the nervous
system is closed anteriorly and segmented into three
brain-yesicles, W, hV, hb 3 ; behind, the medullary
fold mf is still open. On either side of it lie six
primitive segments, us. The posterior end of the
fundament of the embryo is occupied by the primitive
streak with the primitive groove, pr.

the moment of its origin it is turned directly downward toward tho
yolk; but the more it enlarges, whereby the anterior marginal

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 201

groove is deepened into a pit, the more its ridge is turned back-
wards.

Two diagrammatic longitudinal sections, one of which is shown in
fig. 122, the other on Plate I., fig. 11, may serve to illustrate this
process.

In fig. 122 there is shown, projecting above the otherwise smooth
flat surface of the germ-layers, a small protuberance, which encloses
the anterior end of the neural tube (N,C) and the simultaneously
forming intestinal tube (Z>), and which has arisen by the formation
of the fold F.So. The upper sheet of the fold, by directing itself

F'.So.

Fig. 122. Diagrammatic longitudinal section through the axis of an embryo Bird, after

BALFOUR.
The section represents the condition when the head-fold has begun, but the tail-fold is still

wanting.
F.So, Head-fold of the somatopleure ; F.Sp, head-fold of the splauchnopleure, forming at Sp the

lower wall of the front end of the mesenteron ; D, cavity of the fore gut ; pp, pleuroperitoneal

cavity ; Am, fundament of the anterior fold of the amnion ; N.C, neural tube; Ch, chorda;

A, S, C, outer, middle, inner germ-layer, everywhere distinguished by different shading;

Ht, heart.

backwards, furnishes the ventral wall of the cephalic elevation ; the
lower sheet forms the floor of the marginal groove.

In the second figure, in which there is represented a diagrammatic
longitudinal section through an older embryo, the head-fold (kf l ) has
extended still farther backward. The head has thereby become
longer, since its under surface has increased in consequence of the
advance in the process of folding.

Whoever desires to make this process, which is very important for
the comprehension of the construction of animal forms, clearer and
more intelligible, may do so with the help of an easily constructed
model. Let him stretch out his left hand on a table, and spread flat
over the back of it a cloth, which is to represent the blastoderm ;
then let him fold in the cloth with his right hand by tucking it a
little way under the points of his left fingers. The artificially pro-
duced fold corresponds to the head-fold previously described. The

202 EMBRYOLOGY.

points of the fingers, which by the tucking under of the cloth have
received a covering on their lower sides, and which project above
the otherwise flattened cloth, are comparable to the cephalic eleva-
tion. In addition we can represent the backward growth of the
head-fold by tucking the cloth still farther under the left fingers
toward the wrist.

The hinder end of the embryo develops in the same manner as the
front end, only somewhat later (compare fig. 11, Plate I.). Corre-
sponding to the posterior marginal groove (gr), the tail-fold is so formed
that its ridge is directed forward and that it grows toward the head-fold.

Where in surface-views of the blastoderm the lateral marginal
grooves are to be seen (fig. 121), one recognises on cross sections the
lateral folds (Plate I., fig. 8 sf). They grow at first directly from
above downwards, thus producing the lateral walls of the trunk.
Afterwards their margins bend somewhat toward the median plane
(Plate I., fig. 9 sf), thereby approaching each other, and in this way
gradually draw together to form a tube (Plate I., fig. 10). By their
infolding the trunk acquires its ventral wall.

In order to avoid misconceptions, let it be further remarked that
only at the beginning of their formation are head-, tail-, and lateral
folds somewhat separated from one another, but that when they
are more developed they are merged into one another, and thus are
only parts of a single fold, which encloses the fundament of the embryo
on all sides.

As the separate parts of this fold increase, they grow with their
bent margins from in front and from behind, from right and from
left, toward one another, and finally come near together in a small
territory, which corresponds approximately with the middle of the
surface of the embryo's belly, and is designated on the figure of the
cross section through this region (Plate I., fig. 10) by a ring-like line
(hn). Thus a small tubular body is formed (Plate I., fig. 3), which lies
upon the extra-embryonic area of the blastoderm and is united to it
by means of a hollow stalk (hn). The stalk marks the place where
the margins of the folds, growing toward one another from all sides,
have met, but a complete constricting off of the embryonic territory
from the extra-embryonic does not take place.

We can also represent these conditions, if, in the previously men-
tioned model, we in addition fold in the cloth that covers the tips
of the fingers along the sides of the hand and the wrist, and then
carry the circular fold thus artificially formed still farther under,
even to the middle of the palm. Then the cloth forms around the

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 203

hand a tubular sheath, which is continuous at one place by means
of a connecting cord with the flattened remaining portion of the
cloth.

A process similar to the externally visible one just described, by
which the lateral and ventral walls of the body are produced from
the sheet-like fundaments, takes place at the same time within the
embryo in the splanchnopleure. There are developed from it, as
from the somatopleure, an anterior, a posterior, and two lateral
intestinal folds.

First, at the time when the head is differentiated (fig. 122), the
part of the splanchnopleure corresponding to it (F.Sp.) is folded
together into a tube, the so-called cavity of the fore gut or head-gut (D).

The same process repeats itself on the third day of incubation at
the posterior end of the embryonal fundament, where, upon the
appearance of the caudal part (Plate I., fig. 11), there is formed
within it and out of the splanchnopleure the cavity of the hind gut.

Both parts of the intestine at first terminate with blind ends
directed toward the outer surface of the body. At the head-end
the mouth-opening is still wanting, at the posterior end the anus.
When, however, one raises the blastoderm with the nascent embryo
from the yolk, and examines it from the under side, the anterior
and posterior portions of the intestinal canal exhibit openings (vdpf
and hdpf}. through which one can look from the yolk-side into the
blind-ending cavities. One of these is called the anterior, the other
the posterior, intestinal portal or intestinal entrance (Plate I., fig. 11
vdpf and hdpf).

Between the two portals the middle region of the intestinal canal
remains for a long time as a leaf-like fundament. Then by its
becoming somewhat bent downwards (Plate I., figs. 9 and 2) there
arises under the chorda dorsalis an intestinal groove (dr), which lies
between fore and hind gut. Owing to the further increase of the
lateral intestinal folds (df), the groove becomes deeper and deeper,
and finally, by the approximation of the edges of the folds from in
front, from behind, and from both sides, becomes closed into a tube
in the same manner as the wall of the body.

At only one small place, which is indicated by the ring-like line
dn in Plate I., figs. 3 and 10, the folding and constricting-off process
is not completed, and here the intestinal tube too remains con-
tinuous, by means of a hollow stalk, with the extra-embryonic part
of the splanchnopleure, which encloses the yolk.

The part of the germ-layers which is not employed in the formation

204

EMBRYOLOGY.

of the embryo furnishes in the case of the Eeptiles and Birds the

yolk-sac and certain embryonic membranes. I shall speak of the

development of these in the next chapter.

The fate of the extra-embryonic area of the blastoderm in Fishes

is more simple, since there is formed from it only a sac for the

reception of the yolk.

Fig. 123 exhibits the embryo (Em) of a Selachian, which has

arisen by the 'infolding of a small area of the germ-layers in the

manner described for
the Chick. All the
remaining part of
the egg has become
a great yolk-sac (ds),
which is united with
the middle of the
belly by means of a
long stalk.

The Teleosts (Plate
I., fig. 6) show us
transitions from this

Fig. 123. Advanced embryo of a Shark (Pristiurus), after condition to One in

an, A Embryo ; dg, yolk-sac ; st, stalk of the yolk-sac ; av, arteria wblc51 the yolk-Sac,
v itellina ; m, vena vitellina. OS., in Amphibians,

is not separated by

a stalk from the mesenteron, but represents only a capacious
enlargement of the latter and of the belly- wall.

Let us now examine more carefully the structure of the yolk-sac.
As has been remarked already, all four of the germ-layers spread
themselves out one after another around the unsegmented yolk-mass
of meroblastic eggs (Plate I., figs. 6 and 7). As in the embryonal
body the two middle germ-layers separate from each other and allow
the body-cavity to appear between them, so, too, at a later stage
the same process occurs in the extra-embryonic area. Throughout
the region of the middle germ-layer there is formed a narrow
fissure, for which the name " extra-embryonic body-cavity," or
blastospheric ccelom (cavity of the blastoderm, KOLLIKER), would be
most suitable. It separates the envelope of the yolk into two layers,
of which the inner is the immediate continuation of the intestinal
wall (splanchnopleure), the outer, on the contrary, that of the body-
wall (somatopleure). Therefore, to be exact, we have before us a
double sac formed around the yolk, which we can distinguish as

ESTABLISHMENT OF THE EXTERNAL FORM OF THE BODY. 205

intestinal yolk-sac and dermal yolk-sac. The former is simply a
hernia-like evagination of the intestinal canal, and, like it, is
composed of three layers :

(1) The intestino-glandular layer (ik\ the entoblast or secondary
entoderm, which encloses the yolk ;

(2) The visceral middle layer, or the pleuroperitoneal epithelium
(mk 2 ) ; and

(3) The intermediate layer (Zwischenblatt), in which have been
developed the vitelline blood-vessels, which at the beginning of the
circulation of the blood have to conduct the liquefied nutritive
material from the yolk-sac to the places of embryonic growth.

The dermal yolk-sac is, as a continuation of the body- wall, likewise
composed of three layers the epidermis (&), the paiietal middle
layer (mk l ), and the connective-tissue intermediate substance
(Zwischensubstanz).

It has already been stated that the constricting-off of the yolk-sac
from the embryonal body is quite variable in extent, and can go so
far that the connection between the two is kept up only by means
of a narrow stalk. A more careful examination shows that in the
latter case the stalk itself is composed of two narrow tubes one
within the other (Plate I., fig. 7), of which the outer unites the
dermal yolk-sac (hs) to the ventral wall of the body, and the inner
the intestinal yolk-sac to the intestinal canal. The former is called
the dermal stalk, the latter the intestinal stalk (dn) or vitelline
duct, ductus vitello-intestinalis. The place of attachment of the
dermal stalk in the middle of the ventral surface of the embryo is
called the dermal navel (hn) ; the corresponding place of attachment
of the intestinal stalk to the wall of the intestine the intestinal
navel (dn). The embryonic body-cavity opens out between the two,
and is continuous with the fissure between dermal and intestinal
yolk-sac with the " extra-embryonic body-cavity " or the blasto-
spheric coelom (Mi 2 ).

The ultimate fate of the yolk-sac in the Fishes is the same as in
the Amphibia. It is still employed, even in the extreme case of the
Selachians, for the formation of the wall of the intestine and that
of the body. The more its contents are liquefied and absorbed,
the more the yolk-sac shrivels. When the intestinal yolk-sac has
become very small, it is drawn into~the body-cavity and finally
serves to close the intestinal navel, just as the dermal yolk-sac upon
its disappearance closes up the dermal navel. With^jthe^lower
Vertebrates a shedding of the embryonic parts has not yet come into

206 EMBRYOLOGY.

existence. The next chapter will explain what becomes of the
yolk-sac in the case of Reptiles and Birds.

SUMMARY.

1. In the case of Vertebrates whose eggs contain little yolk, the
embryo after the development of the germ-layers takes on an
elongated, fish-like form.

2. In eggs with abundant yolk the body of the vertebrated animal
is produced by only a small region of the germ-layers (the embryonic
fundament); the far greater extra-embryonic area is employed for
the formation of a yolk-sac and of embryonic membranes (the latter
only in Reptiles and Birds).

3. The separate layers of the embryonic fundament constrict them-
selves off from the extra-embryonic territory, and at the same time
become folded into tubes the somatopleure into the tubular body-
wall, the splanchnopleure into the intestinal tube (head-fold, tail-fold,
lateral folds, intestinal groove, intestinal fold).

4. The extra-embryonic territory of the germ-layers remains in
continuity with the two tubes by means of a stalk-like connection.

5. In Fishes the extra-embryonic territory of the germ-layers
becomes the yolk-sac, which is composed of two sacs, the intestinal
and the dermal yolk-sacs, separated from each other by a pro-
longation of the embryonal body-cavity.

6. The place where the dermal yolk-sac is attached to the belly-
wall of the embryo by a stalk-like prolongation is called the dermal
navel or umbilicus ; the corresponding place of attachment of the
intestinal yolk-sac to the middle of the intestinal canal is the
intestinal navel or umbilicus.

7. In Fishes the yolk-sac after resorption of the yolk-material,
accompanied by the phenomena of shrivelling, is employed for the
closure of the intestinal and dermal navels.

8. In Reptiles and Birds the extra-embryonic region furnishes,
in addition to the yolk-sac, several other embryonic membranes,
which complicate the development.

CHAPTER XI.

THE F(ETAL MEMBRANES OF REPTILES AND BIRDS.
As has already been stated, the course of development in all animals
which do not deposit their eggs in water in Reptiles, Birds, and
Mammals is unusually complicated, owing to the appearance of

THE F<ETAL MEMBRANES OF REPTILES AND BIRDS.

207

special egg-envelopes (embryonic or foetal membranes). Some of
them, according to their origin, are to be referred to the extra-
embryonic area of the germ-
layers, and indeed to that part
which in Fishes is employed for
the yolk-sac. They arise from
folds, which grow around the
embryo while it is still small,
and furnish a double envelope
for it.

The egg- envelopes (embryonic
membranes) of Reptiles and
Birds, which exhibit almost
identical conditions, and the
consideration of which we shall
take up first, are more simply
constituted than those of Mam-
mals. In the case of the former
there are associated with the
yolk-sac, in the possession of
which they agree with the
Amphibia and Fishes, three
additional embryonic appen-
dages, the amnion, the mem-
brana serosa (or briefly serosa),
and the allantois. They are
partly laid down at an early
period, at the time when the
embryonic body is converted

into tubes by the infolding of the germ-layers and is thereby con-
stricted off from the yolk-sac.

The Chick shall again serve as a basis for our description.

Fig. 124. Surface-view of the pellucid area of
a blastoderm of a Chick of 18 hours, after
BALFOUR.

In front of the primitive groove, pr, lies the
medullary furrow surrounded by the medullary
folds. Immediately in front of these one sees
a curved line, the head-fold, and in front of
it a second curved Hue running concentric
with it, the anterior fold of the aranion.

1. The Amnion, the Serosa, and the Yolk-Sac.

The amnion is a structure the appearance of which is recognisable
remarkably early in the Chick. At the time when one recognises
the semicircular head-fold at the anterior end of the incipient embryo
(fig. 124), by the growth of which the head of the embryo is marked
off, there is already present, at a short distance from it, a second fold
running parallel to it. This is the anterior fold of the amnion, a

208

EMBRYOLOGY.

product of the extra-embryonic part of the ectoderm and of the
parietal mesoderm united with it.

The two infoldings, which lie near to each other, have opposite

iver

F.So.

fig. 125 Diagrammatic longitudinal section through the axis of an embryo Bird, after

BALFOUR.
The section represents the condition when the head-fold is already formed, but the tail-fold is

still wanting.
F.So, Head-fold of the somatopleure ; F.Sp, head-fold of the splanchnopleure, forming at Sp

the floor of the anterior part of the intestine. For the remaining references see fig. 122,

p. 201.

directions (fig. 125). While the head-fold (F.So) advances with its
margin toward the yolk, the anterior fold of the amnion (Am), sepa-
rated from it by the marginal groove, rises externally above the

Fig. 126. Diagrammatic longitudinal section through the posterior end of an embryo Chick at

the time of the formation of the allantois, after BALFOUR.
ep, me, hy, Outer, middle, and inner germ-layers ; ch, chorda ; Sp.c, neural tube ; n.e, neurenteric

canal ; p.a.g, post-anal gut ; pr, remains of the primitive streak folded toward the ventral

side ; al, allantois ; an, point where the anus will be formed ; p.c, perivisceral cavity ;

am, amnion ; so, eomatopleure ; sp, splanchnopleure.

plane of the blastoderm. At the time when the head is being formed,
the amnion enlarges rather rapidly (Plate L, fig. 11 vaj), and grows over
and around the head in a cap-like fold, the rim of which is directed
backwards. At the end of the second day of incubation it already

THE FCETAL, MEMBRANES OF REPTILES AND BIRDS.

209

covers the anterior part of the head like a thin transparent veil, and
is therefore called the cephalic sheath.

In like manner, but at a somewhat later stage, there arise at the
tail-end and at both sides of the embryo the posterior and lateral
folds of the amnion. The posterior fold is still very inconspicuous even

at the time when the head is covered with the veil-like pellicle
(Plate I., fig. 11 haf). It enlarges slowly, and under the name of
caudal sheath covers over the posterior end of the body (fig. 126 am).
The lateral folds of the amnion are elevated externally to the lateral
marginal grooves (fig. 127 om), and project in the opposite direction
from those lateral folds by the bending in of which the lateral and
ventral walls of the embryo are produced. By this means the rim

14

210 EMBRYOLOGY.

of the fold is carried farther and farther from the splanchnopleure
(sp), which remains spread out flat over the yolk. In this way the
extra-embryonic part of the body-cavity, or the cavity of the blasto
derm (KOLLIKER), increases in extent in the vicinity of the embryo.
When the lateral folds of the amnion have grown up to the dorsal
surface of the embryo (Plate I., fig. 9 saf), they begin, by the bending
over of their edges medianwards, to form the so-called lateral
sheaths.

Inasmuch as the folds of the amnion, which are called by special
names, become, when they are in full development, continuous, and are
only parts of a single ring-like fold, the embryo eventually becomes
surrounded on all sides as though by a high wall. With further
enlargement, the amniotic sheaths then bend together over the back
of the embryo from in front and behind, and from the right and
the left (Plate I., figs. 2, 3, and 10, a/, vaf, haf), come together
with their edges in the median plane, and then fuse with each other
along a line, the amniotic suture, which closes from in front back-
wards (Plate I., fig. 10), except that at one very small place near
the tail-end the closing is interrupted for a considerable time, and
a small opening is preserved.

The fusion of the amniotic folds takes place in -the same manner
as the fusion of the medullary folds described on page 79. Each
fold (Plate I., figs. 3 and 10) consists of two layers, an inner and an
outer one, which are continuous at the margins of the folds, and are
separated by a fissure, which is a portion of the extra-embryonic
body-cavity. At the amniotic suture the corresponding layers of
the folds of both sides fuse, and hand in hand with this a separa-
tion of the inner from the outer layers takes place (Plate I., fig. 4).
As a result of this there have now arisen two envelopes over the
back of the embryo, an inner and an outer one, t^e amnion (A) and
the serosa (S).

The amnion is the product of the inner layer of the folds (Plate I.,
fig. 10 ifb). It forms a sac which immediately after its origin is
closely applied about the embryo, and which encloses a very small
amniotic cavity filled with fluid.

The serous membrane (serosa), which is derived from the outer
layer of the folds (a/6, Plate I., fig. 10), lies as a very delicate trans-
parent membrane closely applied to the amnion, and thus encloses
the embryo in still another envelope.

If we now glance back at the conditions described in the previous
chapter, and compare the development of Fishes with that of Reptiles

THE FCETAL MEMBRANES OF REPTILES AND BIRDS. 211

and Birds, it is to be seen that a considerable complication has arisen
in the case of the latter. Whereas in Fishes the extra-embryonic
area of the somatopleure becomes exclusively the dermal yolk-sac, in
Reptiles and Birds two sacs have arisen out of it by a process of
folding. The influences producing this folding appear to be clear.

Since the egg is enclosed in firmly applied envelopes, the embryonic /
body, when it is formed by the folding together of the germ-layers,
cannot rise from the yolk-sac ; .it therefore conies to lie in a depr
sion of the latter. There is the more reason for the occurrence of
this because the embryo at the beginning of development is exces-
sively small in comparison with the yolk, and because the yolk-layers
immediately underlying it become liquefied and absorbed. With
the sinking of the body into the yolk (Plate I., figs. 2 and 3), the
parts which in Fishes become the simple dermal yolk-sac (Plate I.,
figs. 6 and 7) fold in around it on all sides as amniotic folds, and
enclose it the more completely the deeper it sinks into the yolk.

The preceding account of the development of the amnion is made some-
what schematic in a single point. That is to say, the anterior fold of the
amnion is developed so early, that the middle germ-layer has not yet been
able to spread out as far as the anterior part of the embryonic area. The in-
folding, therefore, in this region involves only the outer and inner germ-layers,
which are still closely united. This condition is changed somewhat later,
when the middle germ-layer has grown into the region of the anterior fold of
the amnion, and has there split into a visceral and a parietal layer. The process
has not yet been followed out in detail in series of longitudinal sections. But
at all events we must assume that the entoblast, which is united with the
visceral middle layer, retracts from the anterior fold of the amnion and
again spreads out flat, as is represented in diagrammatic figure 11 (Plate I.). In
this manner the anterior amniotic fold, which in the meantime has become
greatly enlarged, now consists of the outer germ-layer and the parietal middle
layer, as is the case from the beginning with the subsequently arising posterior
and lateral folds of the amnion.

We now have to enter still more particularly upon the further
relations of amnion and serosa.

Up to the end of embryonic development the amniotic sac remains
in continuity with a small region on the ventral side of the embryo,
which is called the dermal umbilicus. In figs. 3, 4, 5, and 10
(Plate I.) this place is indicated by means of a circular line (hn).
Here the primitive layers of the body-wall are continuous with the
corresponding layers of the amnion, as, for instance, the epidermis of
the body with an epithelial layer lining the amniotic cavity. The
dermal umbilicus of Reptiles and Birds corresponds therefore with

212 EMBRYOLOGY.

the structure of the same name in embryo Fishes (Plate L, fig. 7 hn),
for it is at this point that the dermal yolk-sac is continuous by means
of its stem-like elongation with the walls of the belly. As in the
Fishes, it surrounds an opening (Plate I., figs. 7 and 5 hn) which unites
the portion of the body-cavity lying within the embryo (//t 1 ) with
the extra-embryonic part lying between the embryonic membranes
(tt 2 ). Furthermore, the stalk of the yolk-sac or vitelline duct,
which is continuous with the embryonic intestine, and which is
indicated in the above-mentioned figures of Plate I. by the small
circle dn, passes through the opening.

The amniotic sac affords an additional special advantage to the
embryos of Reptiles and Birds in that an albuminous saline fluid, the
liquor amnii, collects in its cavity. In it the delicate, easily injured
embryo composed of plastic cells floats, as it were, and is able to
move.

The amniotic sac is small at the beginning of its development, but
enlarges with each day of incubation, since it keeps pace with the
growth of the embryo and encloses a larger and larger amount of
amniotic fluid.

At the same time its wall becomes contractile. Certain cells in its
eomatic mesoderm develop into contractile fibres, which in the Chick
give rise to rhythmic movements from the fifth day of incubation
onward. One can observe these while the egg-shell remains intact,
if one holds the egg toward a source of bright light, and for this
purpose makes use of the ob'scope constructed by PREYER. In this
manner it can be determined that the amnion executes about ten
contractions in a minute, which, beginning at one pole, proceed to
the opposite end, like the contractions of a worm. Thus the amniotic
fluid is set in motion, and the embryo oscillates or rocks regularly
from one end to the other. The rocking of the embryo, as PKEYER
expresses it, becomes more and more obvious in the later days
of incubation, since the contractions of the amnion become more
energetic.

The serosa (S) is a wholly transparent, easily ruptured membrane,
which is closely applied to the vitelline membrane. It consists of two
thin cell-layers, which take their origin from the outer germ-layer
and the parietal middle layer, and like them are distinguished by
blue and red lines in the diagram. The serous membrane is origin-
ally present as a separate structure only in the region of the amnion
and of the embryo (Plate I., fig. 4), as far as the body-cavity is formed
in the middle germ-layer. It then enlarges to the same extent as the

h/af

face p. 213.

PLATE I

Swan Somenschein & Co.

THE FOETAL MEMBRANES OF REPTILES AND BIRDS. 213

yolk becomes overgrown and as the vascular area extends farther
downwards. Parietal and visceral middle layers separate more and
more from each other, until finally (in the Chick toward the end of
incubation) a separation results over the entire periphery of the yolk-
sphere. Figs. 3, 4, and 5, Plate I., show stages in this process. In
the last figure, which represents the condition on about the seventh
day of incubation, the extra-embryonic part of the body-cavity has
already become very considerable ; the serous envelope is, with the
exception of a small place at the vegetative pole of the yolk, every-
where formed as a separate structure.

In connection with this the wall of the yolk-sac also becomes
changed. Whereas at the beginning of the overgrowth it embraces
for a considerable distance all the germ-layers, after the separation
of the serosa it is composed exclusively of entoderm and the visceral
middle layer.

EXPLANATION OF THE FIGURES ON PLATE I.

Figs. 1-5 are diagrammatic representations of cross and longitudinal sections
through the Hen's egg at different stages of incubation. They are intended to
illustrate how the body of the Chick is developed out of the embryonic funda-
ment, and how the yolk-.ac, the amnion, the serosa, and the allantois arise out
of the extra-embryonic area of the germ-layers.

For the sake of clearness the embryonic fundament, and later the embryo,
are represented much too large in relation to the yolk.

In order more easily to distinguish the different parts from one another
different colors have been selected for them. The yolk is represented in
yellow, the entoderm green, the outer germ-layer blue, and the middle germ-
layer, together with the mesenchyme, red. The black dots indicate the limit
to which the outer and inner germ-layers have grown over the yolk in the '
different stages ; the red dots mark the boundary for the time being of the
middle germ-layer, which after the development of the blood-vessels ends in
the sinus terminalis.

The references apply to all of the figures.

ale, Outer germ-layer (blue). dg, Vitell