\ /

A

LABORATORY TEXT-BOOK

OF

EMBRYOLOGY

BY

CHARLES SEDGWICK MINOT, LL. D. (Yale and Toronto), D. Sc. (Oxford)

JAMES STILLMAN PROFESSOR OP COMPARATIVE ANATOMY IN THE HARVARD MEDICAL SCHOOL;
PRESIDENT OF THE BOSTON SOCIETY OF NATURAL HISTORY

SECOND EDITION, REVISED
WITH 262 ILLUSTRATIONS, CHIEFLY ORIGINAL

PHILADELPHIA

P. BLAKISTON'S SON & CO.

1012 WALNUT STREET
1910

COPYRIGHT, 1910
BY CHARLES SEDGWICK MINOT

BIOLOGY

LIBRARY

G

Printed by

The Maple Press

York, Pa.

TO

HENRY PICKERING BOWDITCH

AS A TOKEN OF

ADMIRATION AND LONG FRIENDSHIP THIS VOLUME

IS DEDICATED BY THE

AUTHOR

251988

PREFACE.

The accompanying volume is designed primarily for the use of students taking
a practical laboratory course in embryology. It is intended to direct the student's
attention to actual, original observations, to be made by himself, and to aid him in
drawing correct conclusions from those observations. By this plan, the student re-
peats and uses the actual methods by which embryblogical science has been built.
If he pursues such a course diligently, he will be able at the end of it to say that
he knows of his own knowledge. To attain this result is the ideal of laboratory
education.

In preparing .the new edition, advantage has been taken of the experience
gained with the use of the book by the author's classes, and of valuable sugges-
tions from many friends. The work has been so extensively revised that it may be
described as almost a new book. Chapters III to VI have been entirely recast and
rearranged so as to conform to the chronological order of development, an arrange-
ment which it is believed most readers will prefer to that adopted in the original
edition. Chapter II, on the early development of mammals, has been considerably
expanded; not with the object of giving a comprehensive treatment of the subject,
but rather with the intention of aiding the student to get, in connection with his
laboratory work, a connected story in his mind of the development of the principal
organs and systems of the body. Some new sections will be found also in Chap-
ter I. A considerable number of the figures are replaced by'new ones. The total
number of illustrations has increased from 218 to 262. With these changes it is
hoped that the second edition will deserve a continuance of the favor shown to
the original issue.

The author takes much pleasure in acknowledging gratefully the invaluable as-
sistance afforded him by members of his laboratory staff, and wishes to call at-
tention especially to the very admirable original illustrations which have been fur-
nished especially for the book by Drs. J. L. Bremer, F. P. Johnson, F. T. Lewis,
R. E. Scammon, and F. W. Thyng. Special mention must be made also of the
figures from models made by Messrs. W. W. Behlow, G. C. Coe, L. M. Fergu-
son, C. A. Hedblom, and A. R. Kilgore, students in the" embryological course at
the Harvard Medical School.

A large majority of the illustrations are from the Harvard Embryological Col-
lection, without which this work would not have been possible. The author requests
those who use this book to communicate to him any suggestions, which their
experience may lead to, for improving it.

CHARLES S. MINOT.

HARVARD MEDICAL SCHOOL, May 28, 1910.

vii

CONTENTS.

CHAPTER I. — GENERAL CONCEPTIONS, i

The Segmented Animals. Metamerism, 2

The Vertebrate Type of Structure, 2

The Principal Modifications of the Vertebrate Type, 7

Definition of Anlage, 9

A Summary of Embryological Development, 10

Cytomorphosis, 1 1

Comparison of Larval and Embryonic Types of Development, 16

Germ-layers, * 18

The Relations of Surface to Mass, 20

Classification of Glands, 23

The Law of Unequal Growth, 24

Germ-cells, 25

Sex, 27

The Theory of Heredity, 28

The Law of Recapitulation, 29

Arrest of Development, 31

CHAPTER II. — THE EARLY DEVELOPMENT OF MAMMALS, 33

The Spermatozoon, 33

The Fully Grown Ovum Before Maturation, 34

Ovulation, 35

The Maturation of the Ovum, . . . 36

Impregnation of the Ovum, 38

Segmentation of the Ovum, 42

The Blastodermic Vesicle, 45

The Embryonic Shield, 47

Growth of the Embryo and Separation of the Yolk, 49

Origin of the Mesoderm, 51

The Primitive Axis, ' 52

The Notochordal Canal, ; 53

The Notochord, * 55

The Ultimate Fate of the Notochord, 56

The Archenteron, 57

The Oral and Anal Plates, 58

The Digestive Canal, 59

The Yolk-sac, 63

The Origin of the Nervous System, 67

The Structure of the Medullary Canal, ' 69

Origin of Nerves, 72

ix

x CONTENTS.

PAGE

The Spinal Cord and Brain,

Plakodes,

The Nasal Pits and Olfactory Nerves, .

The Eye,

The Otocyst,

The Early History of the Mesoderm, .
Somatopleure and Splanchnopleure, . .

The Embryonic Ccelom,

The Mesenchyma,

The .Origin of the Blood-vessels and Blood, 9°

The Blood-corpuscles, •

The Origin of the Heart, 96

The Germinal Area, 9°

The Main Vessels of the Area Vasculosa, ... 97

The Aortic System, 99

The Venous System, 102

The Lymphatic System, I05

The Liver, I07

The Pancreas, I07

The Excretory Organs, Io8

The Urogenital Ducts, .no

The Allantois, IIX

The Trophoderm, "4

The Umbilical Cord, ......,;..,.. 115

The Chorion and Amnion, 1 1 7

CHAPTER III. — THE HUMAN EMBRYO, "8

Calculation of the Age of the Human Embryo, . . . . 118

The Classification of the Early Stages, 119

Hypothetical Development of the Blastodermic Vesicle in Primates, 122

Relations of the Embryo to the Uterus: the Two Stages, 124

Ovum of a Monkey in the Second Stage, 127

Human Embryo in the Second Stage, 128

The Embryo of a Gibbon in the Third Stage, 131

Human Embryo in th Fourth Stage with the Medullary Plate, 134

Human Embryo in the Fifth Stage with Open Medullary Groove, 136

Human Embryo in the Sixth Stage with Medullary Canal, . . . 137

Human Embryo in the Seventh Stage with One Gill-cleft Showing Externally, ... 1 40

Human Embryo in the Eighth Stage with Two Gill-clefts Showing Externally, 141

Human Embryo in the Ninth Stage with Three Gill-clefts Showing Externally, . . .- 143

Human Embryo in the Tenth Stage with Four Gill-clefts Showing Externally, 146

Human Embryo in the Eleventh Stage with the Cervical Sinus in Formation, 147

Human Embryos of the Fourth Week to the Fourth Month, 148

CHAPTER IV. — STUDY OF THE SEGMENTATION OF THE OVUM AND OF THE BLASTODERMIC YKSI-

CLE IN MAMMALS, ; 1 60

The Maturation, Fertilization, and Segmentation of the Ovum in White Mice, 160

Method of Obtaining Blastodermic Vesicles from the Rabbit, 166

Study of Rabbit Blastodermic Vesicles in Alcohol, 167

CHAPTER V.— STUDY OF YOUNG CHICK EMBRYOS, 1 74

Method of Obtaining Embryos, ; 1 74

CONTENTS. xi

PAGE

Embryo Chick with Eight Segments (About Twenty-eight hours' Incubation), 176

Embryo Chick with about Twenty-four Segments and Three Gill-clefts (About Forty-
six Hours' Incubation), 197

Embryo Chick with Twenty-eight Segments, 199

The Study of Transverse Sections, 199

Horizontal Section, 214

Histological Differentiation of the Chick Embryo with Three Gill-clefts, 216

CHAPTER VI. — STUDY OF PIG EMBRYOS, 219

Methods of Obtaining Embryos, 219

The Making of Serial Sections, 220

Selection of the Planes of Section and the Stages for Practical Study, 220

The Study of the External Form, 221

Pig Embryo of 7 . 5 mm., 221

Pig Embryo of 10 mm., ". 223

Pig Embryo of 15 mm., 225

Pig Embryo of 20 mm., 226

Pig Embryo of 7.8 mm. General Anatomy, 228

Pig Embryo of 12 mm. General Anatomy, 231

Pig Embryo of 6 mm. Studied in Sections, 246

Pig Embryo of 9 mm. Studied in Sections, 250

Pig Embryo of 12 mm. Studied in Sections', 259

Study of Transverse Sections, 261

Study of Sagittal Sections, 290

Study of Frontal Sections, 296

Pig Embryo of 17 mm. Study of Sections, 303

Frontal Section of the Umbilical Cord, 310

Pig Embryo of 20 mm. Study of Sections, 311

Transverse Sections 311

Sagittal Section, 322

Frontal Sections of the Head, 324

Pig Embryo of 24 mm. Study of Sections, 330

CHAPTER VII. — STUDY OF THE UTERUS AND THE FETAL APPENDAGES OF MAN, 339

Histology of the Uterus, 339

Menstruation, 339

The Pregnant Uterus: the Two Stages, 341

Human Uterus Three Months Pregnant, 343

Human Uterus Seven Months Pregnant, 345

Decid.ua Vera of the First Stage in Section, 346

Decidua Reflexa of the First Stage, 349

Decidua Vera and Chorion Laeve of the Second Stage, 350

The Placenta in Situ, 352

Decidua Serotina at Seven Months, 357

The Human Placenta, 359

Histology of the Human Chorion, 363

The Chorion with Trophoderm, 364

The Chorionic Villi, 367

The Structure of the Amnion, , 370

The Umbilical Cord, 372

The Structure of the Human Yolk-sac, 375

xii CONTENTS.

PAGE

CHAPTER VIII.— METHODS, 377

Measuring Length of Embryos, 377

Dissection of Embryos, 377

Methods of Hardening and Preserving, 377

Preservation in Alcohol, 380

Directions for Imbedding Specimens to be Microtomed, 380

Method of Mounting Paraffin Sections, 381

Methods of Staining, 381

Methods of Reconstruction, 385

Directions for Orienting Serial Sections of Embryos, 388

Microtomes, 389

INDEX, 393

TEXT-BOOK OF EMBRYOLOGY.

CHAPTER I.
GENERAL CONCEPTIONS.

The student of embryology should start with as clear and definite a concep-
tion as possible of what he is to gain from his pursuit of that science. If he is a
student of biology or of zoology, he must appreciate that knowledge of the laws
of development is an indispensable part of what he must master in order to
understand those sciences. He must appreciate that it is from the studies of the
embryologist that are derived our conceptions of the nature of sex, of heredity, of
variation, of differentiation, and many of our most important notions concerning
evolution, both of the individual and of the race. He will learn further that the
embryo illustrates to him with particular clearness the fundamental principles
of morphology. If he be a medical student, he will find in embryology first
of all the clue to the intelligent comprehension of the anatomy of the adult, a
comprehension which he can obtain in no other way, but he will also gain much
knowledge of direct practical value as to the embryo and as to the conditions in
the adult, acquaintance with which is invaluable in medical practice. And.
finally, he will find that it throws a vast light on pathology, both upon the prob-
lems of malformations and monstrosities, and also upon the whole question of
pathological change in the tissues.

The best study of embryology, therefore, is that which continually passes
beyond the direct observations to the conceptions which they justify and which
underlie many important branches of science which are related to, and in a large
part dependent upon, embryology.

The student ought to strive, accordingly, to pass from the direct observation
of the specimen to the generalizations, and accustom himself to regard always
each special preparation, which may be submitted, to his observation, as an illus-
tration of some general principle. To facilitate his reaching this result this
chapter offers a digest of some of the more important generalizations and funda-
mental laws of embryology.

•2 GENERAL CONCEPTIONS.

The Segmented Animals. Metamerism.

All vertebrates and certain invertebrates haye_-their bodies divided into a
series of parts, which begins in the region of the head and extends to the caudal
end of the body. These parts are called segments or metameres. In man each
vertebra corresponds to one segment. In the earthworm and other annelids
the segments are plainly marked on the outer surface of the body. Since all
segmented animals are bilaterally symmetrical, each segment is bilaterally sym-
metrical and may therefore be described as a paired structure. Embryology
has demonstrated that each segment arises as a pair of masses (somites] situated
between the digestive canal and the outer surface of the body. Each pair of
masses is formed from the middle germ-layer (mesoderm) exclusively (compare pages
84-85) and is known as "a primitive segment." The mesodermic somites are
to be considered the essential primary morphological segments, and in the course
of the development of the individual they produce adult metameric structures,
among which may be muscles, skeletal elements, excretory organs, etc., the whole
history of which depends upon their segmental origin. The nervous system and
to a considerable extent the blood-vessels exhibit a segmental arrangement, which
is usually regarded as a secondary correlation of these structures with the primary
mesodermic metamerism. The spinal nerves exemplify the correlation. In brief,
where the segmental organization exists, it dominates the anatomy alike of the embryo
and the adult; therefore the student of embryology should pay special attention
to the segmentation of the body in all its chief stages.

In regard to the bodily segmentation two general observations may be made :
First, the development^ of segments begins at the- cephalic end and progresses
tailward; hence so long as the development of segments continues various stages
of their differentiation may be found in a single embryo, the more advanced
stages being always cephalad from the less advanced. Second, there is a funda-
mental difference between the metamerism of vertebrates and that of annelids
and many other invertebrates, which consists in the unlike extent of the segments;
for the primitive segments of vertebrates are confined to the dorsal region of the
body, while in the other forms the segmentation extends from the start through the
dorsal and ventral regions both. It is probable that the segments of vertebrates
are homologous only with the dorsal part of the segments of annelids.

The Vertebrate Type of Structure.

When one traces the course of development of any vertebrate, one finds,
speaking in general terms, that the fundamental characteristics, which are more
or less common to all vertebrates, are those which first appear. Later, there
come in the secondary characteristics which distinguish one class from another,
and still later the subordinate characteristics by which the smaller subdivisions
of the vertebrate type become differentiated one from another. This statement,
however, is correct only if we add to it certain indispensable limitations. Every
embryo at every stage of its development is an individual of the particular genus

THE VERTEBRATE TYPE OF STRUCTURE. 3

and species to which it belongs. It has at every stage peculiarities which dis-
tinguish it from every other species. The embryos of allied forms resemble
one another more closely than do the embryos of forms which are only distantly
related to one another. The specific qualities of an embryo are, however, more
difficult to recognize than those of the -adult, and the student will be far more
impressed by the resemblances between embryos than by their differences. It
is owing to this very fact that the distinctive peculiarities of the species are not
accentuated in the embryo. We are able to derive from the embryos themselves
a series of conceptions which render it comparatively easy to perceive • the domi-
nant morphological features of the vertebrate type.

It will be convenient to put down six fundamental characteristics of the ver-
tebrate type as the most important, and to add to these six others which are
also fundamental, but perhaps less distinctive. This enumeration is necessarily
arbitrary, and can serve only to facilitate the work of the student. When his
knowledge deepens, he will be able to free himself from the limitations which
such a numerical classification may have put on his understanding of the matter.

A. The six most important characteristics are:

/ i. The pharynx and pharyngeal structures (gill-clefts, nerves, aortic arches,

heart).
./ 2. The notochord or structural axis.

3. Tubular central nervous system.

4. Limbs.

5. Position of mouth.

6. Division of the ccelom into:

(a) dorsal segmented part or cavities of the somites.

(b) ventral unsegmented part (splanchnocele), which is subdivided by
the septum transversum into a thoracic and an abdominal portion.

B. Other fundamental but less distinctive characteristics are:

7. Stomach, intestine, and mesentery.

8. Position of liver, and its relation to veins.

9. Wolffian tubules and ovotestis ( = urogenital ridge).

10. Urogenital ducts (Wolffian and Miillerian).

11. Special sense-organs (nose, eye, and ear).

12. Hypophysis.

The pig embryo illustrates all these characteristics, and we shall study the
ways in which the typical mammalian modifications of the type are gradually
evolved.

Let us now pass in review these twelve characteristics.

i. The pharynx is the cephalic portion of the digestive canal, and it acquires
in all vertebrates a somewhat complicated structure. This complication de-
pends primarily upon a series of lateral outgrowths from the pharynx which are
known by the name of gill-pouches. They are symmetrically arranged and there-

GENERAL CONCEPTIONS.

fore form pairs. They are designated by numbers, the pouch which lies nearest
to the mouth being called the first, the next the second, and so on. Among
the lower vertebrates the number of these gill-pouches varies from five to
perhaps nine pairs. In mammals there are always four distinct pairs. In
aquatic vertebrates the pouches acquire each an opening to the exterior at the
side of the neck, and are then designated as gill-clefts or branchial clefts. We
find that the position of the clefts determines the distribution of a series of the
most important of the cephalic nerves and the primitive distribution of the branches
of the aorta and of certain important muscles, hence the morphological features
of the pharynx have a profound influence upon the entire anatomy of the body
in that region. No similar pouches are formed from any other part of the di-
gestive canal. The pharynx also gives rise to the thyroid gland, the anlage of
which starts as an outgrowth from the median ventral side of the pharynx. The
entoderm of the third pair of gill-pouches produces the anlages of thymus glands,
and that of the fourth pair the anlages of the parathyroids.

2. The notochord is a rod of cells which extends nearly the entire length of
the embryo. It lies in the median plane, a little below the ventral edge of the
central nervous system. Its cephalic termination is always in the neighborhood
of the pituitary body. It may be considered the primitive structural axis of the
vertebrates. There are vertebrates in which it is the only structural axis ever pro-
duced, but in the great majority of vertebrates there is developed around the
notochord a series of skeletal elements which we know as vertebrae, and which
make a new structural axis in these forms. The notochord in these animals is
found to run through, the bodies of the vertebrae. The notochord diminishes in
size as we ascend the vertebrate series. It is of very considerable diameter in
the lowest fishes, smaller in amphibia and reptiles, and smallest of all in mam-
mals. In the lower forms it persists throughout life as a continuous rod. In
the higher forms it tends to become attenuated in the vertebral, expanded in the
intervertebral, regions, and in adult mammals persists only as a /series of dis-
connected thickenings (nuclei pulposi} between the vertebras.

3. The tubular central nervous system. This is found in vertebrates only, or
in animals which are closely related to vertebrates, so closely that b.y many
naturalists they are included in the same subkingdom. The hollow nervous system
is enlarged in the region of the head, the enlargement constituting the brain.
The rest of it is of smaller size and constitutes the spinal cord. That the brain
and spinal cord form the wall of a tube is one . of the fundamental conceptions
of anatomy.

4- The limbs. There are two pairs, which are lateral extensions of the sur-
face of the body and acquire in their interior a skeleton by which they are
supported and muscles by which they are moved. No structures in any invertebrate
animal are known to be homologous with vertebrate limbs.

5. The position of the mouth. The typical invertebrate mouth is surrounded

THE VERTEBRATE TYPE OF STRUCTURE. 5

by the nervous system. For instance, in insects or in the jointed worms (annelids)
there is a brain, so called, above the mouth, and a strand of nervous tissue
running down on either side of the body past the mouth to join the ganglion
on the lower side, thus completing a circumoral ring of nervous material through
which the oesophagus passes. In vertebrates, on the other hand, the mouth is
not enclosed by any cesophageal ring, and the entire nervous system is on one
side of the body and dorsal to the mouth.

6. The division of the primitive body-cavity. The body-cavity in the em-
bryo is known by the comprehensive name of the ccelom. It will not be possible
to acquire a clear idea of its division until the embryos are actually studied. It
forms many parts. Of these there are two dorsal series, one on each side of the
central nervous system, which form cavities of what we designate as the somites of
the body. There are also two large ventral divisions which extend from the
region of the head to that of the future pelvis, one division for each side of the
body. These two large parts are not divided into segments at all, though the
cavities of all of the segments are primitively connected with these two main
divisions. Comparatively early in^J;he development the two main cavities be-
come connected with one another,\so as to constitute a single cavity to which we
apply the name of splanchnocele. The splanchnocele surrounds the heart of
the embryo, where we recognize it as the pericardial cavity, and it extends through
the future abdominal region, where we recognize it as the abdominal cavity.
The pericardial and abdominal regions of the cavity are separated from one
another in the embryo by a broad transverse partition which bears the name
of septum transversum. This septum in mammals becomes in the adult the dia-
phragm. It is one of the most striking of all the morphological peculiarities
by which vertebrates are distinguished from invertebrates.

7. The stomach, intestine, and mesentery. The division of the digestive
tract of vertebrates into two fundamental parts, stomach and intestine, is very
characteristic. The stomach is not only an enlargement of the digestive canal,
but also may 'be distinguished from the intestine by its developing glands, which
are specific to it and unlike those of the intestine proper. The elongated oeso-
phagus occurs in the higher vertebrates only, and is • not a general characteristic
of the subkingd<3m. The mesentery by which the intestine is suspended to the
dorsal wall of the abdomen is the survival of the original partition by which the
two halves of the splanchnocele were separated from one another. The cavities
in the abdominal region come into communication with one another by the very
early disappearance of the partition on the ventral side of the intestine. But it
should be noted at once that a portion of this primitive ventral partition, or, as
we may call it, ventral mesentery, persists permanently in relation to the position
of the liver.

8. The position of the liver. The primitive large veins of the embryo pass
through the septum transversum, and it is by intercrescence with these veins, and

6 GENERAL CONCEPTIONS.

as an appendage to the septum itself, that the liver is developed, although it is
produced by a special local growth of the digestive canal.

9. The urogenital ridge. Out of a part of the primitive segments there are
developed excretory organs, and these, as they increase in size, form two pro-
tuberances on the dorsal side of the splanchnocele. Each protuberance is what
we know as the urogenital ridge, so named, first, on account of its form; and,
secondly, on account of its producing not only the excretory organs proper, but
also the genital glands.

10. The urogenital ducts. There is primitively a single duct for each uro-
genital ridge. This duct is commonly known as the Wolffian duct. A little later in
the history of the embryo there appears a second canal known as the Mullerian
duct, which is closely parallel to the first, but which has no connection with any of the
excretory apparatus, and is destined to serve later as the female genital duct. In no
invertebrate have we found anything certainly homologous with these two ducts.

11. Special sense-organs. These are the olfactory, the visual, the so-called
auditory organs, and the organs of the lateral line. We have to use the term
"so-called" in speaking of the auditory organ because we now know that the ear
in the lower vertebrates is not an organ of hearing, but an organ of balancing or
orientation, and it is only in the higher vertebrates that there is added to this
primitive function that of audition proper. It seems not improbable that many
invertebrate animals have sense-organs which are homologous with those of verte-
brates. Nevertheless, in the vertebrate type there are many peculiarities which are
distinctive, and these we shall best learn from a 'study of the actual development.
The sense-organs of the lateral line are highly developed important structures in
the Ichthyopsida, but apparently are not represented in mammals at any stage
of their development.

12. The hypophysis. The hypophysis is the embryological name applied to
the structure which we know in the adult as the anterior lobe of the pituitary
body. The posterior or infundibular lobe is a portion of the brain, but the an-
terior lobe is an outgrowth from the cavity of the mouth of the embryo. Com-
paratively early in the development of the individual this outgrowth becomes
entirely separated from the mouth-cavity (from the walls of which it arose), and
forms a closed vesicle. It exists in every known vertebrate animal, has been
much studied, but still remains an organ the significance of which we cannot ex-
plain. Its absolute persistency and the uniformity of its development indicate
that it is an organ of importance, but beyond that we can hardly go.

To these conceptions, the student should add the following comprehensive
morphological notions: The mammalian body may be defined as two tubes of
epithelium, one inside the other; the outer tube (epidermal or ectodermal) is very
irregular in its form; the inner tube (entodermal) is much smaller in diameter,
but much longer than the outer and has a number of branches (lung, pancreas,
etc.), and is placed within the ectodermal tube. Between these two tubes is the

PRINCIPAL MODIFICATIONS OF THE VERTEBRATE TYPE. . 7

very bulky mesoderm, which is divided by large cavities (abdominal and thoracic)
into two main layers, one of which is closely associated with the epidermis and
forms the body wall, the somatopleure of embryologists; the other joins with the
entoderm to complete the walls of the splanchnic viscera, and constitutes the
splanchnopleure of embryologists. The mesoderm is permeated by two sets of
cavities: (i) the heart and blood-vessels; (2) the lymphatic system. It is also
differentiated into numerous tissues (muscles, tendon, bone, etc.) , organs and the
internal parts of the urogenital system. The nervous system, although developed
from the ectoderm, is found separated from its site of origin, and completely en-
cased in mesoderm.

The Principal Modifications of the Vertebrate Type.

Our knowledge of human development being at the present time incom-
plete, it is often necessary to supplement that knowledge by reference to facts
of observation on the development of various vertebrates. Indeed, the best study
of human embryology includes more or less comparative work. We shall, therefore,
find frequent occasion to refer to the development of many vertebrate types.
Accordingly, in this section there are given definitions of the principal subdivisions
of the vertebrates to which we shall have occasion to refer.

From an embryological standpoint, vertebrates may be separated into two
main divisions, which are commonly designated as the Amniota and Anamniota,
distinguished by the presence or absence of the amnion, the amnion being a thin
membrane, which immediately surrounds the embryo in the higher forms. It
occurs in reptiles, birds, and mammals, which together constitute the Amniota.
It is absent in the fishes and amphibians, which therefore constitute the Anamniota.
These two divisions are also distinguished by other peculiarities. The higher forms
referred to all have the organ known as the allantois, an appendage of the embryo,
which is lacking in the lower forms. The comparative anatomist finds many
points of resemblance between the various classes of fishes, on the one hand, and
the amphibia, on the other, and indicates this relationship by the use of the term
Ichthyopsida, which means "fish-like." In our present classification the term
Ichthyopsida is synonymous with Anamniota. The comparative anatomist further
recognizes a close relationship between birds and reptiles, and puts these together
under the common designation of Sauropsida, or "reptile-like."

As regards the fishes, many classifications are more or less in vogue at the
present time. For the purposes of this book, the following names for the classes have
been adopted as names generally understood and sufficiently exact to meet our
needs: The lowest fishes are the hag-fishes and lampreys, constituting the group
of Marsipobranchs. Next comes the group comprising the sturgeon and its allies,
for which we have retained the old term of Ganoids. To these fishes the central
position in the system must be assigned, and it is probable that the higher fishes
are more or less directly descended from Ganoid-like forms. They fall into three

GENERAL CONCEPTIONS.

further classes, of which the largest and most varied is that of the bony fishes,
or Teleosts. Another class, known as the Elasmobranchs, comprises the sharks,
skates, rays, and electric fishes. The last class is known as the Dipnoi, or lung
fishes, which comprise only three living forms, the Ceratodus, living in Australia,
the Protopterus in Africa, and the Lepidosiren in South America.

The amphibia are divided into two classes, the Urodela, of which the newts
and salamanders are familiar examples, and the Anura, of which the frogs and
the toads are -the best known representatives. The two types are easily distin-
guished by the presence or absence, respectively, of the tail in the adult.

As to the reptiles, it is unnecessary to consider their classification, as we shall
not have much occasion to refer to them, our knowledge of their embryology
being very fragmentary at the present time, save for a rather extended series
of observations on the development of lizards. As regards birds, it may be noted
that embryologists have worked chiefly upon the chick, which iias been for a
century the classic object of embryological study. There are comparatively few
observations on the development of other species of birds.

Mammals are divided into three principal classes. Of these, the lowest is
that of the Monotremes, of which the only living representatives are found in
Australia and neighboring islands, a very few species concerning the develop-
ment of which very little is as yet known, but which are of importance, as they
resemble in certain respects the reptiles and assist us in drawing comparisons
between the reptilian and the mammalian types. Of this class, the Australian
duck-bill may be mentioned as typical.

The second class is that of the Marsupials, familiar to us in America
through the common opossum. In Australia there are many genera and species
of marsupials.

The third class comprises the majority of well-known mammals, and may
be termed the Placentalia, and, for embryological purposes, it is convenient to
consider the Placentalia as forming two principal subclasses, the animals with
claws and the animals with hoofs, the Unguiculates and the Ungulates. Of the
Unguiculates, we shall have occasion to refer to the Insect-Ivor a, of which the mole
may serve as a type; the Cheiroptera, or bats; the Rodents, including the rats.
guinea-pigs, rabbits, etc.; the Carnivora, cats, dogs, and allied animals; and, finally,
the Primates, which include the lemurs, monkeys, apes, and man.

Of the Ungulates, we shall have occasion to refer chiefly to the pig and the
sheep. The following table presents in their proper order those animals which
we shall have occasion to consider.

Annelida
Atriozoa

Tunicata (Ascidia)

Cephalochorda
Amphioxus

DEFINITION OF AN LACE. 9

Vertebrata

Anamniota (Anallantoidea)
Ichthyopsida
Pisces

Marsipobranchia (lampreys, etc.)
Ganoidea (sturgeon, etc.)
Teleostea (bony fishes)
Elasmobranchia (sharks, skates, etc.)
Dipnoi (lung-fishes)
Amphibia

Urodela (newts, salamanders, et:.)
Ansura (frogs, toads)
Amniota (Allantoidea)
Sauropsida

Reptilia (lizards, crocodiles, snakes, turtles, etc.)
Aves
Mammalia

Montotremata (duck-bill, etc.)
Marsupialia (opossum, kangaroo, etc.)
Placentalia

Unguiculate series

Insectivora (moles, etc.)
Cheiroptera (bats)

Rodentia (rats, rabbits, guinea-pigs, etc.)
Carnivora (cats, dogs, etc.)
Primata (lemurs, monkeys, apes, man)
Ungulate series
Ungulata

Artiodactyla (even-toed) (cattle, sheep, pig, deer, etc.)
Perissodactyla (uneven-toed) (horse, rhinoceros, etc.)

Of the invertebrate animals there will be little to be said. There are two
types of invertebrates which show relationship in their structure to true verte-
brates. One of these is the class of jointed worms, or Annelids; the other is the
class of Atriozoa, which comprises the subdivisions of Tunicata and of the Cepha-
lochorda. All of our observations on the development of this last type are based
on the one genus, Amphioxus, which will therefore be the name which, we shall
use whenever we have to refer to these animals.

Definition of Anlage.

There will be frequent occasion to use this word in a strictly technical sense.
It has been adopted from the German, as there is no satisfactory English equiva-
lent for it. The French use the word "ebauche" and the Italians " abozzo."
Primordium has been proposed as the Latin equivalent and is used by a few
American authors, but anlage is generally employed by both American and English
writers. "Anlage'' may be denned as follows: The first accumulation of cells
in the developing embryo recognizable as the commencement of a structure, organ,
or part.

10 GENERAL CONCEPTIONS.

A Summary of Embryological Development.

The following summary applies to what is known of vertebrates only. It I
would require some modifications to be applicable to the whole animal kingdom.
Each individual arises froni a single cell which is termed the impregnated or fer-
tilized ovum. From this all embryological study starts. The fertilized ovum
has its earlier history, since it is the product of the fusion of two sexual elements.
It is a living cell, and therefore contains protoplasm and nucleus. It is also
furnished with a certain amount of material known as yolk, which exists in
the form of separate granules imbedded in the protoplasm. This__y_olk js the
reserve food material, and by the. assimilation thereof the protoplasm of the
ovum can grow.

The first step in the development r, tht repeated division of the original cell
so that there is produced an increasing number- of cells. The earlier stages of this
cell multiplication are designated as the segmentation of the ovum. This name is
due to the fact that the process was 'first observed in the eggs of amphibia in the
early part of the last century, before the cell doctrine had been established. In
default of a better name, the separate cells into which the ovum divided were
called segments, for it was, of course, not known that they were cells. Although
this term is no longer appropriate, it is still universally used because of its con-
venience. There are two principal types of ovum known: in one type wre find
only a small amount of yolk material; in the other a very large amount. There
are ova known intermediate between these two types. When the ovum is of the
first type , the whole of it undergoes segmentation at once, and to such an ovum
the term holoblastic is applied. In the second type, on the contrary, we find that
the protoplasm tends to accumulate at one pole of the cell and the yolk granules
at the other. The protoplasmic portion exhibits a far more active cell division
than the yolk-bearing portion, so that the segmentation seems to take place exclu-
sively around one pole or part of the ovum, which is, therefore, designated as
\meroblastic. After the segmentation of the ovum the multiplication of the cells
continues, and they gradually arrange themselves in such a manner as to form
three distinct sheets or laminae, which are named "germ-layers.'" These layers
are designated: the outermost as Ectoderm, the innermost as Entoderm, and the
middle as Mesoderm* From an embryological point of view the importance
of these three primitive germ-layers cannot be over-emphasized. The principal
occupation of the student will be to familiarize himself with the appearance
of these layers and the modifications which they undergo, and the adult tissues
which are produced from them. They dominate every phase of development,
the form of every organ, the production of every tissue. Their importance is
so great that embryology might almost be defined as the science of germ-layers.

* Some English and occasionally Continental authors use other terms for the germ-layers, namely, for
ectoderm, epiblast; for entoderm, hypoblast; for mesoderm, mesoblast. I have preferred to maintain the older
terms which have been in almost universal use for a century.

CYTOMORPHOSIS. 11

The primitive germ-layers consist of very simple cells, and are themselves
at first extremely simple in their organization. The majority of the cells which
they contain undergo a greater or less degree of modification as development
progresses. This modification is termed differentiation, and is more fully con-
sidered in our next section, on Cytomorphosis. It is probable, however, that a
certain number of the cells very early in the development are set apart, preserving
the primitive character of their protoplasm and taking no share in the formation
of the tissues of the body. These cells, comparatively unmodified, are known as
the germ-cells; compare page 25 and the section on Heredity. As the remaining
cells form part of the body of the individual, they may be designated as somatic
cells. Besides the process of differentiation of the cells, we find that the production
of organs -is largely dependent upon the unequal growth of the germ-layers, one
part growing rapidly, another more slowly, so that the layers acquire, as the
embryo develops, a more or less complicated form, owing to the folding of the
layers. The "general principles which govern these important developments are
considered in the section upon the Relations of Surface to Mass.

Cytomorphosis.

This term is used to designate comprehensively all the structural modifica-
tions which cells or successive generations of cells may undergo, from the earliest
undifferentiated stage to their final destruction. It will be convenient, though
somewhat arbitrary, to distinguish four fundamental successive stages of cyto-
morphosis. These stages are (i) the undifferentiated stage; (2) the stage of
progressive differentiation, which itself often comprises many successive stages;

(3) the regressive stage or that during which degeneration or necrobiosis^ occurs;

(4) the stage of the removal of the dead material.

In the various parts of the body we find these stages to succeed one another
at varying rates, and there are always to be found in every living vertebrate
body a considerable number of cells which have passed through only a certain
differentiation and do not present any of the phenomena of degeneration or of death.
On the other hand, there are cells at every epoch of life after an early brief em-
bryonic period which degenerate and die off, although the life of the individual
is uninterrupted. At any given moment the body consists of cells which have
made unequal progress through the cytomorphic cycle.

i. The Undifferentiated Stage. — A fertilized ovum is an undifferentiated being,
although it has a very complex organization. As it has only one nucleus there
can be no variety of nuclei. The term "undifferentiated" therefore applies es-
pecially to the protoplasm, which never has any special structures or formed
parts, such as occur in the tissues and cells of the adult. It is, however, not uni-
form, but in many ova has distinct regional differences, which so far as hitherto
noted depend upon peculiarities in the masses and strands of protoplasm, and
upon the distribution of the yolk granules, of which there may be several kinds.

12 GENERAL CONCEPTIONS.

The uneven distribution of the yolk granules in the ova of mammals (compare
page 34) indicates that there are unlike regions, the morphological significance
of which, however, is not known yet.

Two views as to the constitution of the ovum in relation to the structures
which arise from it have been brought forward. According to one view, each
part of the ovum is predestined to form a definite part of the adult and cannot
form any other part. According to the other view, the ovum is homogeneous
in its essential lack of true differentiation, and any part of it may form any part
of the adult if given the requisite opportunity. The first view is known as the
mosaic Jheory, the egg being compared to a mosaic, and was founded by Wilhelm
Roux (1888). The second view is known as the theory of isotropism and was
founded by Ernst Pfliiger in 1884.

Pfliiger's theory of isotropism was based upon- his experiments on frogs' eggs.
Each egg has a small white area which normally, lies underneath, the larger,
darkly pigmented area of the egg alone showing from above. Out of the dark
area the back, with the nervous system and other parts, takes its origin. If the
eggs, freshly fertilized, are fastened with the white side up, then the white side pro-
duces an absolutely normal back and nervous system, normal as to form and func-
tion, though lacking the typical pigmentation. These observations were confirmed
by Born, who further discovered that the segmentation nucleus always rises
toward the upper side of the egg, and that the position of the nucleus determines
which part of the ovum shall become the dorsal side of the embryo. Another set of
experiments by Oskar Schultze demonstrated that both the unpigmented and the
pigmented sides of the same egg could be made to produce dorsal structures. An-
other class of experiments, which were first made by Hans Driesch, has revealed
that the earliest cells (segmentation spheres, blastomeres, or cleavage cells, as they
are variously called) produced by the ovum preserve the undifferentiated qualities
of the parent egg, and may develop in one way or another according to circumstances.
The egg of a sea-urchin divides into two cells, each of which multiplies and nor-
mally gives rise to half of the body of the animal. By somewhat violent shaking the
two cells may be artificially separated; each cell may then develop into a complete
larval sea-urchin, but of half the normal size only. Similar experiments have since
been made by several investigators, who have obtained like results with other
animals, vertebrate as well as invertebrate. Even more remarkable larvae have
been raised from blastomeres of the four-cell and eight-cell stages of segmentation,
producing larvae of one-fourth and one-eighth the normal size. Zoja claims to have
repeated the experiment successfully on the eggs of Clytia and to have obtained
one-sixteenth larvae.

Roux's mosaic theory was based on W. His's principle of the organ-forming
areas of the germ, or, as it has been also termed, the law of germinal localization.
His pointed out that in the normal course of development every spot in the
blastoderm corresponds to some future organ, and suggested that logically it is

C Y TOMORPHOSIS. 1 3

probable that every organ is represented by some region in the ovum itself. In
other words, although the organs of course do not exist preformed in the egg,
yet the material for them is there and prelocalized. Roux has developed this
conception and has compared the egg to a mosaic; each member of the mosaic
is assumed to undergo self-differentiation or its predestined development. Com-
plete verification of this theory has not been secured, but it has been demonstrated
that certain eggs of animals of several invertebrate orders contain substances,
which have an exact distribution, and which have a definite fixed relation to adult
structures. By putting some of these eggs in a centrifuge these substances may
have their distribution artificially changed. In eggs thus altered the substances
continue to transform themselves into their predestined structures, which conse-
quently appear displaced. By these experiments the mosaic theory has received
a limited confirmation.

At present it is impossible to reconcile the two theories of the constitution
of the ovum, but since both are based apparently on facts it seems probable that
they will, by wider knowledge, be fused into a single coherent conception.

Meanwhile the fact of most importance in cytomorphosis is, that the protoplasm
of the ovum is undifferentiated and lacks completely any variations of its stnu
comparable to those which we observe as characteristic in the cells of adult tis-
The potentialities of the ovum, on the other hand, are of course very great. Experi-
mental embryology is now endeavoring to ascertain what physiological causes ren-
der those potentialities active. From physiological embryology much is to be
expected.

2. Differentiation. — This may be defined as a process by which the structure of
the cells is modified, so that they become dissimilar by acquiring an organization
which adapts them to special functions. The cells which arise during the segmen-
tation of the ovum differ but slightly from one another. As development progresses
we find the cells change, some in one way, some in another, so that many kinds of
cells are produced, but of each kind we find a large number of cells. Each kind
of cell may be said, roughly speaking, to form a tissue for itself. Cells of each
tissue offer visible peculiarities by which they may be readily distinguished from one
another under the microscope. It thus appears that the production of tissues is the
main result of differentiation, so that this process of development may be fairly accu-
rately defined as equivalent to histogenesis. As to the factors which cause differen-
tiation, we have no satisfactory knowledge. We can, at present, only note the changes
when they acquire such magnitude as to become microscopically visible. As to the
physiological conditions which cause these changes we have almost no conceptions.
It is probable that the nucleus has a leading role to play, but our knowledge of
this role is too little advanced to permit a profitable discussion of the subject here.

The actual process of differentiation shows itself both in the protoplasm and in
the nucleus of the cell. The changes in the former are the more conspicuous, and
therefore the better known. The changes in the nucleus have still to be adequately

14 GENERAL CONCEPTIONS.

studied. The changes in the protoplasm are twofold: First, in the intimate struc-
ture of the protoplasm itself and in the size and disposition of its strands and fila-
ments; secondly, in the character of the various substances to be found imbedded
in the protoplasm. These two kinds of change are well illustrated, the first, by
the nerve-cells; the second, by the gland-cells, for instance, in the pancreas. The
student can easily see that the character of the protoplasm in the adult nerve-cell
differs profoundly from that of a cell from one of the embryonic germ-layers, hav-
ing become visibly much more complex. In the secretory cells of the pancreas the
zymogen granules are conspicuous; their distribution, uniform size, and refractile
qualities demonstrate, immediately their unlikeness to anything found in the embry-
onic cells. These granules are not protoplasm, but particles imbedded in the proto-
plasm or, as they may be called, enclosures.

The Law of Genetic Restriction. — Another fundamental idea, which it is most
important for the student to grasp, is that differentiation acts as a progressive
restriction upon the further development. Each successive stage of differentiation
puts a narrower limitation upon the possibility of further advance.

The range of possible changes at any given time is determined not merely by
the nature or kind, but also by the stage or degree of the previous differentiation.
The law of genetic restriction governs the entire ontogeny. In order to illustrate
it and to emphasize it, it will be profitable to consider a few illustrations from each
of the germ-layers. First, then, the ectoderm. This layer early separates into two
parts, one to form the nervous system, the second the epidermis; the nervous part
thereafter never forms epidermal structures, the epidermal part never forms a medul-
lary canal. The central nervous system retains in part a simple epithelial charac-
ter (ependyma proper), but most of its walls become nervous tissue; its cells pass
from the indifferent stage and become neuroglia cells or young nerve-cells (neuro-
blasts). Neuroglia cells never become anything else, and the nerve-cells are always
nerve-cells to the end. Next, as to the entoderm. Wherever in it specialization
takes place, as in the tonsil, thymus, thyroid, oesophagus, liver, or pancreas, each
territory of cells keeps its characteristics and never assumes those of another terri-
tory. Finally, as to the mesoderm. It is found very early to include in vertebrate
embryos four kinds of cells, of which the most numerous are the undifferentiated
cells, the other three kinds being the endothelium -of blood-vessels, red blood-cells,
and germ-cells. All of these are precociously specialized; they are few in number,
yet they are probably the parents of all the cells which are produced of their kind
each throughout life. Passing on to a later stage, we note that when a striated
muscle-fiber is produced a striated muscle-fiber it always remains, and it never
becomes anything else.

Two Types of Differentiation.— There are two distinct types of cell differentia-
tion which I think have not hitherto been clearly recognized or defined. For both
types the starting-point is the same— the undifferentiated embryonic cell. In one
type we find that, as the cells proliferate, a portion of them only undergoes differ-

CYTOMORPHOSIS. 15

entiation, and another portion remains more or less undifferentiated and retains
more or less fully the power of continued proliferation. The epidermis is a good
representative of this type. Its basal layer consists of embryonic cells, which
multiply; some of these cells move into the upper layers, enlarge, and differentiate
themselves into horny cells; others remain in the basal layer and continue to mul-
tiply. The progeny of a given basal epidermal cell do not all have the same fate,
but divide themselves into two kinds of cells, one kind retaining the ancestral
character, the other becoming something new and unlike the parent cell. Differ-
entiation according to the second type is characterized by its inclusion of all the
cells. This type has its culminating and most perfect illustration in the central
nervous system, where comparatively early in embryonic life all the cells become
specialized, and with the acquisition of specialization they forfeit their power of
multiplication— the neuroglia cells partly, the nerve-cells wholly.* The growth of
the brain after early stages depends not on the proliferation of cells, but chiefly
upon the increase in size of the individual cell. The correctness of this statement
is not affected, in my belief, by the fact that epithelial portions of the medullary
tube in comparatively late stages may be added to the nervous portion, the cells
multiplying rapidly, as we see at the growing edge of the young cerebellum. The
brain here grows by the addition of cells in the indifferent stage, but as soon as
these cells are differentiated they conform to the general law and divide no more
(neurones) or slowly (glia cells).

The importance to pathologists of a thorough knowledge of the genesis of the
tissues from their germ-layers can hardly be emphasized too strongly, for it is more
than probable that all pathological tissues are as strictly governed by the law of
genetic restriction as are the normal tissues.

3. Regression. — The use of this term does not imply that a cell can move
backward after differentiation into a stage of lower differentiation or into an un-
differentiated condition. So far as we know at present, such a change does not
occur, and we therefore look upon it as impossible. Regressive changes are very
unlike the constructive changes which appear in differentiation, for they are destruct-
ive. They fall into three main groups: first, changes of direct cell death; second,
necrobiosis or indirect cell death preceded by changes in cell structure; third, hyper-
trophic degeneration or indirect cell death preceded by growth and structural
change of the cell, often with nuclear proliferation. Direct cell death implies that
the cell loses its vitality, and, being dead, disintegrates; or, may be, is removed by
some means, chemical or phagocytic, before disintegration occurs. Necrobiosis and
hypertrophic degeneration are normal processes, which invariably occur in the
normal body and play an important role in its development. Without their occur-
rence on a large scale the normal round of human life would be impossible. The
student should • free himself from the unfortunate tradition that these processes are
exclusively pathological.

* With possibly very rare exceptions.

16 GENERAL CONCEPTIONS,

Correct notions on this subject are so important that a few illustrations may
be mentioned. Let us begin with necrobiosis. There are organs whose existence
is limited in time, such as the thymus and foetal kidney. These organs attain
their full differentiation, and thereafter most of their elements die off and finally
are resorbed, most of the organ disappearing. Another familiar illustration is
offered by hairs, which die and are shed. Cell death on a large scale is a com-
mon phenomenon of the tissues. It occurs in the cartilage both when the cartilage
is permanent and, even more conspicuously, when the cartilage gives way to bone,
the disintegration of the cartilage cells preceding the irruption of the bone-forming
tissues. It occurs among the gland-cells of the intestine, in the pregnant uterus,
and in all the tissues of human decidua reflexa. Degeneration in the stricter
sense of the ante-mortem and hypertrophic change of cell structure is also of wide-
spread occurrence in the healthy body. Perhaps no instance of this is more
familiar than the production of horny tissue in the epidermis or elsewhere. That
fatty degeneration takes place normally has long been taught, while mucoid and
colloid degeneration are so obviously normal that we commonly think of their
pathological occurrence as merely an exaggeration of a normal state. Hypertrophic
degeneration is an extremely common pathological process, but it also occurs as a
normal process, as, for example, in epidermal cornification, as just mentioned, and
very strikingly in the production of giant-cells (myeloplaxes, etc.), and on an astound-
ing scale in the uterine tissues during pregnancy in many, perhaps all, mammals.

4. The Removal of Cells. — The sloughing off of cells is one of the most familiar
phenomena, since it occurs incessantly over the epidermis and with hairs. Its part
in menstruation and its colossal role in the after-birth are known to all, and every
practitioner is accustomed to look for shed cells in urinary sediment. Large
numbers of cells are lost by the intestinal epithelium. The destruction of blood-
corpuscles is incessant, and we might greatly extend the list of these illustrations.
Owing to the enormous loss of cells to which the body is subject, there is provision
to make good this loss.- This provision is called "regeneration," and has been
dealt with in an enormous number of investigations. During embryonic life
regeneration plays a comparatively insignificant part, and we shall not have to deal
with it further.

Of the four stages of cytomorphosis, the second, or stage of differentiation, is
that which will principally claim our attention. But we cannot fully understand
the developmental processes unless we also have constantly in mind the normal
degeneration and death of cells, even in the embryo.

Comparison of Larval and Embryonic Types of Development.

We have seen in the preceding section that the first cells produced in develop-
ment from the ovum are undifferentiated, and are capable of development in many
and varied directions. The more they become specialized, the more their possi-
bilities of further varied development are decreased. It is thus obvious that the

COMPARISON OF LARVAL AND EMBRYONIC TYPES. 17

greater the number of cells of the undifferentiated type that can be produced, the
greater will be the number of elements which can be later differentiated. Hence,
the more the period for the production of undifferentiated cells is prolonged and
the commencement of differentiation postponed, the more complex may be the
degree of organization ultimately attainable.

It is convenient to designate the undifferentiated cells as they arise from the
segmentation of the ovum by the term "embryonic cells.1' The object of this
section is to point out that the larval type of development is less favorable for the
multiplication of embryonic cells than is the embryonic type; and, further, that the
embryonic type becomes more and more marked as we ascend in the animal
kingdom.

The Larval Type. — In the lower multicellular animals we encounter only larvae;
sponges, jellyfish, starfish, and worms all pass through their early stages as larvae.
Now, larvae are animal forms which have to obtain their own food and to protect
themselves against enemies. They are, therefore, provided with a variety of organs,
or, as we may say, with differentiated tissues which enable them to perform the
various physiological functions which "are necessary for the maintenance of their
existence. The differentiation of tissues comes in very early.

The Embryonic Type. — True embryos arise from eggs which contain a more or
less considerable amount of yolk or nutritive material, the presence of which renders
unnecessary any activity on the part of the embryo to obtain its food-supply; and
we find, moreover, that these embryos are protected by hard shells or other devices
from their enemies. Their only task is to pursue their own development. Under
these circumstances it is possible for the embryos to continue for a long time the
production of embryonic cells, and we observe that the beginning of the differen-
tiation proper is correspondingly postponed. The transition from the larval to the
embryonic type is very gradual. The yolk appears in the lower animals in small
quantities, increasing in some of the higher types, and attaining its maximum in
some of the highest. Since the embryo is dependent on the yolk, and since the
yolk exists only in the higher forms in sufficient quantities, 'it follows that fully
typical embryos can occur exclusively in the higher animal types.

In the mammalia the ovum contains a rather small quantity of yolk, yet the
mammals are the highest afiimals and develop most perfectly according to the em-
bryonic type. This peculiarity is due to the fact that two special physiological
devices have been evolved in the mammals to supply food to the developing em-
bryo. First, there is a special relation established between the embryo and the
uterus by means of a complicated adjustment of embryonic and uterine tissues,
which supplies nutrition to the embryo from the blood of the mother. Secondly,
there are the mammary glands, which also serve the same function. By these two
devices the embryo is even more completely freed from ' the necessity of seeking its
food and protecting itself than is the case with those forms, such as the birds or
elasmobranchs, in which the supply of food material is very large.

18 GENERAL CONCEPTIONS.

Germ-layers.

The germ-layers are the first groups of cells to arise as the result of the seg-
mentation of the ovum. They are three in number, and each forms a distinct sheet
or lamina. As stated on page 10, these three primitive layers are termed "ecto-
derm," "mesoderm," and "entoderm. " The ectoderm is the most external of the
three, and upon the outside of the body parts of the ectoderm remain permanently to
constitute the outside skin or epidermis. From its very position it necessarily is the
part of the body to come into relation with the external world, and accordingly we find
that its two great duties are to produce the protective covering of the body and the
apparatus for receiving and utilizing sensations; in other words, the chief sensory or-
gans and the nervous system. The entoderm, on the contrary, forms the internal
cavity of the digestive canal and its appendages. It therefore is concerned chiefly
with the production of the organs of digestion, and appears in the adult as the
epithelium of the digestive and respiratory organs and of the glands appended to
the digestive tract. The mesoderm, lying as it does between the other two layers,
is shut off by them from direct relation with the external world or with food-
matter, and is accordingly restricted to a series of internal functions, of which four
are especially important: (i) The function of circulation both of blood and lymph
through definite channels; (2) of excretion; (3) of movement; (4) of supporting the
body, especially the parts produced from the ectoderm and entoderm. It is from
the middle germ-layer, therefore, that the connective and skeletal tissues arise, that
the muscular tissues arise, that the excretory organs arise, and that the blood,
blood-vessels, and lymphatics arise.

The inner and outer germ-layers are primarily simple epithelial structures, con-
sisting each of a single layer of cells. This primitive characteristic is never wholly
obliterated and really controls all of the modifications which these two layers
undergo. The mesoderm, on the other hand, is primarily not epithelial, but mesen-
chymal. Mesenchyma consists of widely separated cells which form a continuous network of
protoplasm, the meshes of which are originally filled by a homogeneous intercellular
substance or matrix. The student will have frequent occasion in his practical work
to study it in its embryonic stages.

The Caelom. — The ccelom is the primitive body-cavity of the embryo. It arises
as a space in the mesoderm. Soon after this space has appeared we find that the
cells of the mesoderm, which bound it, assume an epithelial character; consequently
the mesoderm, after the coelom has appeared, consists of mesenchyma and of an
epithelial layer bounding the coelom. This epithelial layer is called the mesothelium.
The mesoderm, therefore, differs fundamentally from the ectoderm and entoderm
by this peculiarity, that it comprises both an epithelial and a non-epithelial portion.
Both portions play very important roles in the production of the various tissues and
organs of the body. There is another respect also in which the mesoderm differs
from the other germ-layers, for we find that it increases in volume and in com-
plexity as we asce»d from the lower to the higher types of animals, or as we. pass

GERM-LA YERS.

19

from the embryo toward the adult condition, more than does either the outer or
inner germ-layer.

The Specific Quality of the Germ-layers.— Each germ-layer has its specific and
exclusive function in the production of tissues, giving rise only to the tissues which
are proper to it and never to the tissues which are proper to either of the other
layers. We must, indeed, so far as - our present knowledge goes, regard most, at
least, of the cells in the germ-layers as originally wholly indifferent as individual
cells. But we must, nevertheless, not forget that, as members of a germ-layer,
their potential fate is already restricted. It is probable, if we could successfully
transplant an undifferentiated cell from one germ-layer to another, that it could
take part in the production of the tissues proper to that layer. But it is further
probable that this would be impossible after the differentiation of the cells in any
layer had fairly begun. After a cell has become definitively a member of one of
the germ-layers, it probably never migrates to join another layer. The accompany-
ing table presents the principal tissues classified according to the layers to which
they belong. There have been classifications of organs on the germ-layer basis
published before, but inasmuch as organs usually contain cells from two layers, we
get a more correct presentation of the actual genetic relationship by confining our
tabulation to the tissues. Blood-vessels and blood-cells arise very early, before the
clear separation of the mesoderm and entoderm has occurred. It is possible that they
are entodermal. With these two limitations the table presents our present knowledge.

(A) ECTODERMAL.

Epidermis.

a. epidermal appendages,

b. lens of eye.
Epithelium of

a. cornea,

b. olfactory chamber,

c. auditory organ,

d. mouth

(oral glands),
(enamel organ),
(hypophysis),

e. anus,

/. chorion,

fetal placenta,
g. amnion.
Nervous system.

a. brain,

optic nerve,
retina,

b. spinal cord,

c. ganglia,

d. neuraxons,

e. chromaffme cells.

CLASSIFICATION OF THE TISSUES.

(B) MESODERMAL.

1. Mesothelium.

a. epithelium of

peritoneum,
pericardium,
pleura,
urogenital organs,

b. striated muscles.

2. Mesenchyma.

a. blood-cells (red and white),

b. blood-vessels,

c. connective tissue,

cellular reticulum,
smooth muscle,
pseudo-endothelium,
fat cells,
pigment cells,

d. lymphatics,

e. spleen,

/. supporting tissues,

cartilage,

bone,
g. marrow of bone.

(C) ENTODERMAL.

1. Notochord.

2. Epithelium of

a. digestive tract,

oesophagus,

stomach,

liver,

pancreas,

small intestine,

yolk-sac,

large intestine,

caecum,

vermix,

rectum,

allantois (bladder),

b. pharynx,

Eustachian tube,

tonsils,

thymus,

parathyroids,'

thyroid,

c. respiratory tract,

larynx,

trachea,

lungs.

20 GENERAL CONCEPTIONS.

The Constitution of Organs. — Few organs are formed from a single germ-layer,
for as we find organs in the vertebrate body they usually consist of two parts, one
of which may be regarded as the part proper of the organ, upon which the per-
formance of its special function directly depends, and the accessory part, which
supplies the necessary physiological conditions for the functioning of the organ.
For example: in a salivary gland the actual work of secretion is performed by the
epithelial cells of the gland, but these cells cannot act unless they are supported
by connective tissue and supplied with blood and lymph, three conditions which
depend upon the mesoderm, and also supplied with nerves, a condition which
depends upon the ectoderm. By far the majority of organs have their functional
part produced from epithelium, and this epithelium may come either from the
original outer or inner germ-layer, as the case may be, or from the mesothelial
portion of the middle layer. But the organ, as a whole, requires for its comple-
tion the addition of other elements, as indicated in the example given. We find,
therefore, that there are no adult organs which are constituted solely by either the
ectoderm or entoderm, although there are organs the principal part of which may
come from one or the other of these germ-layers, but to complete the organ the
mesoderm must help. On the other hand, the mesoderm may form complete
organs by itself, or at least with no other aid from the other germ-layers than is
given by the supplying of nerve-fibers. Such purely mesodermal organs are illus-
trated by the spleen, the kidney, and the sexual glands.

The Relations of Surface to Mass.

However much the weight of an animal increases during its development, the
ratio of the free surface to the mass alters but slightly from the ratio established
when the embryo begins to take food from outside. It is only for convenience
that I express this law in this precise form; in reality, about it our knowledge is
scanty and our conceptions vague. According to a geometrical principle, when the
bulk of a body bounded by a simple surface increases, the surface enlarges less
than the mass— in the simplest case of a cube, the surface increases as the square,
the mass as the cube, of the diameter. If in a cube of unit diameter one unit
of surface bounds one unit of mass, then in a cube of three units diameter nine
units of surface will bound twenty-seven units of mass; the proportion in the first
cube is 1:1, in the second 1:3. To maintain the proper proportion in the embryo,
simple enlargement is insufficient, therefore the surface increases by becoming more
and more irregular. The irregularities are characteristic of each organ and part,
and may be either large or microscopic. They may be conveniently grouped under
two main heads — projections and invaginations.

Projections are illustrated by the limbs, filaments of the gills in fishes, the
villi of the intestine, folds of the stomach in ruminants, etc. In every case the
projection is covered by an epithelium and has a core of mesodermic tissue.

THE RELATIONS OF SURFACE TO MASS. 21

furaginations exist in much more varied form and play a principal part in
the differentiation of the animal body. They may be classified under four principal
heads: (i) dilatations; (2) diverticula; (3) glands; (4) vesicles. Dilatations have
considerable importance in embryology: the stomach, lungs, bladder, and uterus
arise as gradual dilatations of canals or tubes of originally nearly uniform diam-
eters. Diverticula^ in the sense of relatively large blind pouches, also form impor-
tant organs, such as the caecum and appendix vermiformis, or the gall-bladder;
these structures arise each as a blind outgrowth of a canal, the walls of which at
a certain point rapidly grow to form the pouch. Glands are, as first shown by
Johannes Miiller's classic researches, only small diverticula, which end blindly and
appear in an immense variety of modifications; the manifold types of glands are
discussed below in a separate paragraph; they constitute the largest class of organs
with which we have to deal. The glands are developed from epithelium and push
their way into the mesoderm upon which the epithelium rests, while in dilatations,
and in diverticula, .the epithelium and mesoderm expand together. Vesicles we call
those epithelial sacs which develop somewhat like glands by growing into the meso-
derm, but the mouth of the invagination closes by the coalescence of the epithelium,
thus shutting the cavity. The closed sac separates from the epithelium from which
it arose, and connective tissue grows between the two; the sac may then undergo
various modifications. The membranous labyrinth of the ear is developed from the
ectoderm in this way, as is also the lens of the eye. We might perhaps also class
the medullary canal under this head (cf . Chapter V) if we choose to consider it as
a vesicle so much lengthened that it has become a tube.

Glands. — A gland may be defined as a structure which produces material which
is discharged from the gland and used elsewhere to meet a physiological need. Ac-
cording to the nature of this material, we distinguish two fundamentally different types
of so-called glands. One of these we designate as the true glands, which produce
chemical substances which are thrown off from the cells producing them to constitute
the secretion of the gland, so that the cells themselves all remain in the gland. In
the second type the cells themselves are multiplied, so that the structure yields, as
it were, a crop of cells, which is removed from the site of origin and then utilized
physiologically. Glands of this type may be called cytogenic. Of the true glands
we may distinguish several sorts. The simplest kind consists of a single cell. Of
unicellular glands, the goblet cells are the most familiar type known in man. Most
true glands, however, comprise many cells and are -classed as multicellular. The
majority of these have an internal cavity, which may be simple or very complicated
in its form, but is always bounded by a layer of epithelium. They have in addi-
tion a canal, which leads from the cavity of the gland to an external opening,
and is called the duct. When the secretion is produced, the chemical substances
formed by the epithelial cells are discharged into the cavity of the gland and thence
flow through the duct to the outlet. In certain cases a remarkable modification
may occur by the obliteration of the duct, thus producing a so-called ductless epi-

22 GENERAL CONCEPTIONS.

thelial gland. In such structures the secretion can escape only by transfusion into
the blood or lymph.

The epithelial glands with ducts exhibit two main sorts of modification, for
they are either small structures which occur in great numbers, or they are larger
structures, each constituting a separate organ. Hence, we divide the glands into
simple glands and compound or organic glands. The simple glands are always
small and have one or several centers of growth, according as they are simple
tubes or slightly branching. Those of each kind are always very numerous, and
occur more or less together over considerable areas. Good illustrations of simple
glands are offered by the sweat glands and the intestinal glands. The compound
glands are of greater bulk. They are provided with a single main duct which is
more or less branched, each branch connecting finally with the secretory portion
proper of the organ, which portion may itself also be branched. Each gland fall-
ing in this division is a more or less complete organ by itself, receiving its special
blood supply and its special innervation — it is, in short, a clearly marked physio-
logical entity. A specially striking morphological modification in the structure of
compound glands is offered by the liver. In this organ, branches of the glands
unite and form an anastomosing gland structure, in connection with which we
observe that the branches of the gland are not associated with a development of
connective tissue and of blood capillaries between the epithelial elements of the
organ, as in other compound glands; but are associated, on the contrary, with the
presence of a sinusoidal circulation. We must therefore regard the liver as a type
by itself.

Another class of secreting organs may be termed false glands, as they never
have ducts at any stage of their development. Their chemical product is termed
an internal secretion, and is removed by transfusion into the blood. One division
of the false glands is derived from the growth of epithelium, while another division
arises by modifications in mesenchymal cells. As regards the glands of the cyto-
genic class, we have to deal with those which produce the free wandering cells, of
which the most familiar example is the white corpuscle of the blood; those which
produce the red corpuscles of the blood; and, finally, those which produce the
genital elements.

As the student proceeds in his study of embryology, he will have clear illustra-
tions of the development and morphology of all the various sorts of glands. He
will find it advantageous, as his acquaintance with glands increases, to consult the
classification of glands as presented in the following table, based upon the very
important morphological distinctions pointed out in the preceding paragraph.
Formerly the classification of glands was based upon relatively unimportant details
of their microscopic form, and not upon true morphological differences. Hence
the classification here proposed differs radically from those in vogue up to the
present time.

CLASSIFICATION OF GLANDS. 23

Classification of Glands.*

Class A. Unicellular.

Class B. True Glands, always developed with ducts.

Division i. Simple Glands (unifollicular or single glands).

a. Ectodermal.

1. Sweat glands.

2. Sebaceous glands.

3. Buccal glands.

b. Entodermal.

- i. (Esophageal.

2. Gastric.

3. Intestinal.

c. Mesothelial.

i. Uterine.

Division 2. Compound Glands (organic or true compound glands).
Type a, ductic epithelial branching (with capillary circulation).

1. Ectodermal.

Salivaries, tear gland, Harderian.
Mammary.

2. Entodermal.

Pancreas.

3. Mesothelial.

Appendicular glands of the urogenital system.
Type b, ductic anastomosing (with sinusoidal circulation).

1. Liver.

2. (Paraphysis ?)

Type c, ductless epithelial (with secondary obliteration of the duct).

1. Thyroid.

2. Hypophysal gland.

3. Infundibular gland.

4. Pineal gland (epiphysis).

Class C. False Glands, never developed with ducts.

Division i. Epithelioid Glands (? exclusively entodermal).

1. Parathyroid.

2. Islands of the pancreas.

3. Carotid.

4. Thymus.

Division 2. Mesenchymal ductless Glands.

1. Suprarenal cortex.

2. Coccygeal gland.

3. Interstitial cells of genital glands.
Class D. Cytogenic Glands.

Division i. Lymphaal structures; producing mesamceboids .

1. Lymph glands and follicles.

2. Hemolymph glands.

* The present table is a modification of the classification of glands proposed by the author in 1905 (Amer.
Journ. Anat., iv, 256). The principal change is in putting the cytogenic glands in a class by themselves, as should
have been done originally.

24 GENERAL CONCEPTIO.\S.

3. Spleen.

4. (?) Tonsils and thymus.
Division 2. Sdnguijactive organs.

i. Bone marrow.
Division 3. Genital glands.

1. Ovary.

2. Testis.

The Law of Unequal Growth.

The changing shapes of the embryo and the development of the irregularities-
projections and imaginations — which preserve the proper proportion between the
surface and the mass of the body, both depend upon the unequal growth of the
germ-layers, especially in superficies. The expansion of a germ-layer having the
epithelial type of structure* may take place by three means: (i) the multiplication
of the cells; (2) the flattening out of the cells; (3) enlargement of the cells. In
the early stage's of development the influence of the first two factors predominates;
during the later stages, especially after birth, the latter. Of the three factors, the
first is the most important.

The unequal multiplication of the cells in all embryonic epithelia is the funda-
mental factor of development, and we see it shaping the embryo, its organs, and
the parts of organs, before histological differentiation really begins. The distinct
areas and centers of growth which are necessary to develop the human body out
of the germ-layers are innumerable, and their distribution, limitations, and inter-
actions make up a large part of the subject-matter of embryology. At every turn
of our studies we encounter fresh illustrations. If in a limited area of a cellular
membrane there occurs a growth of expansion more rapid than in the neighboring
parts, then that area is, as it were, bounded by a fixed ring, and can, therefore,
find room for its own expansion only by rising above the level of the membrane;
thus, when in the embryonic region of the blastodermic vesicle the growth becomes
more rapid, the embryo begins to rise above the level of the vesicle; thus, when,
at a certain point of the surface of the embryo, a steady and long-continued
growth occurs, the limb appears, gradually lengthening out, and enlarges from a
small bud at first to a complete arm or leg. If the departure takes place the
other way, we have an imagination produced; thus for every hair of the skin and
for every gland of the intestine there is a separate center of growth.

The causes of the unequal growths are unknown. We have not even an
hypothesis to offer as to why one group of cells multiplies or expands faster than
another group of apparently similar cells close by in the same germ-layer. It is
no real explanation to say that it is the result of heredity, for that leaves us as
completely in the dark as ever as to the physiological factors at work in the devel-
oping individual.

The conception that the development of an animal depends fundamentally

* By this limitation \vc exclude' the mesenchyma hut not the epithelium.

GERM-CELLS. 25

upon the unequal expansion and consequent foldings and bendings of the germ-
yers was first suggested by the researches of C. F. Wolff on the development of
the intestine, and was more clearly recognized by Pander, who definitely asserted
that the formation of the embryo is effected by foldings of the germ-layers, and
the truth of Pander's view was conclusively demonstrated by C. E. von Baer in 1828.
In recent times His has studied the problem very intently, and in his memoir on
the chick discussed it minutely. In this memoir is to be found most of what
little we know of this aspect of embryological mechanics.

Germ-cells.

Recent investigations have made it probable that a few cells are set apart dur-
ing the period of segmentation to form the germ-cells. Their number is small;
they preserve for some time the appearance of segmentation spheres, as the cells
which are formed during the segmentation of the ovum are sometimes called.
They multiply very slowly during the earliest stages of development. A great
majority of the cells produced during segmentation lose the character of segmen-
tation spheres, and divide rapidly and repeatedly. They are termed somatic cells
and form the various tissues of the body. The germ-cells, on the contrary, seem
to multiply very slowly and never to become very numerous in the embryo. As
they multiply they separate from one another and become more or less completely
surrounded by tissue cells. They pursue their development, one is tempted to say,
independently of tissue formation and somewhat like foreign members of the body.
We put, accordingly, the germ-cells in a class by themselves in contrast to the
body or somatic cells.

Our actual knowledge of the history of the germ-cells is very incomplete. The
statements just made about them are based on observations on very few animals.
Their exact origin has been traced only in five vertebrates, three fishes, the teleosts
Cymatogaster and Micrometrus, the elasmobranch Squalus acanthias, and in the
frog and turtle. In these five forms the germ-cells arise during segmentation, and
remain more or less closely together, or segregated, during the earliest stages.
They then separate from one another and gradually migrate into the epithelium,
which covers the anlage of the genital gland and which thus becomes the so-called
"germinal epithelium."

The most accurate information we have refers to their development in the dog-
fish. In this species the germ-cells are delaminated from the entoderm together
with other cells of the mesoderm, and cannot, with our present knowledge, be dis-
tinguished from other mesodermic cells. They soon, however, become recognizable,
because while the majority of the mesodermic cells are passing into the second
stage (compare the section on Mesenchyma, page 89) these germ-cells change but
little, if at all, so that they can be recognized as something distinct from the neigh-
boring cells. For a short time they are found gathered into twro compact groups
(Fig. i, Germ-cells] symmetrically placed in the extra-embryonic region, but not far

26

GENERAL CONCEPTIONS.

from the embryo. The cells then break apart from one another and gradually
become separated, and migrate by unknown means, first over the wall of the intes-
tine, which has meanwhile been differentiated, then over the surface of the mesen-
tery into the anlage of the genital gland. During their entire migration they are
lodged in the mesothelium, and when they have reached their final destination they
are still in the mesothelium of the genital anlage, where they remain until finally
differentiated in the adult. The epithelium, with the germ-cells in their definite
position in it, is called the germinal epithelium (compare page 25). The germinal
epithelium has been observed in all vertebrates, but the origin of the germ-cells in

FIG. i. — SECTION ACROSS THE POSTERIOR PART OF AN EMBRYO DOG-FISH (SQUALUS ACANTHIAS). TRANSVERSE

SERIES 463, SECTION 147.

Ect. Ectoderm. Ent, Entoderm. Md, Medullary tube. Mes, Mesoderm. Nch, Notochord. x, Cellular strand

connecting the germ-cell cluster with the yolk.

mammals is entirely unknown. The hypothesis may be accepted, that they arise in a
manner essentially similar to that known in the dog-fish. For some of the theories
based on the known development of the germ-cells, see page 28.

The existence of the germinal epithelium has long been known, and its charac-
teristics have been described in many text-books. The germ-cells in the germinal
epithelium are known also by the names of "sex-cellsn and "primitive ova." The
transformation of these cells into true ova has been traced in a great many forms,
so that the transformation may be considered as demonstrated conclusively for all
vertebrate animals. It is further believed that the germ-cells also give rise to the
male elements, playing in the formation of the testes a role similar to that which

SEX. 27

they play in the ovary. The proof that the germ-cells are the exclusive parents of
the spermatozoa is difficult to obtain, but most embryologists regard the existing
proof as sufficient.

When a germ-cell is transformed into an ovum, it undergoes great enlargement,.
its nucleus is modified, the protoplasm is changed in appearance and becomes loaded
with yolk granules, and over the surface of the cell appear two membranes, an
inner very thin one, called the vitelline membrane, and an outer much thicker
one, known as the zona pellucida. (For a fuller description see page 34.) We
thus learn that the germ-cells preserve their resemblance to segmentation spheres
only during embryonic life. When they become ova, they pass through a series
of important changes in their organization. Since germ-cells also give rise to the
male elements, we must say further that in order to produce those elements the
germ-cells pass through another series of profound changes.

It is further known that in order to evolve the sexual elements, both male
and female, the cell which is to produce them divides twice, and in a special man-
ner, which we designate by the term "reduction division." This process is de-
scribed in all the recent text-books of cytology and histology. It does not fall
within the scope of this work, which deals with embryology in the strict sense only.

Sex.

The sex of an individual depends primarily upon the nature of the sexual
glands. The same two, right and left, parts produce the sexual glands in all verte-
brates. Each part originally is a limited area of the surface of the cephalad end
of the urogenital ridge (compare p. 6) and becomes either a testis or an ovary,
and, since the two sides develop alike, the individual is wholly male or female as
the case may be.

As an exceedingly rare anomaly, lateral hermaphroditism has been recorded. In
this anomaly there is a testis upon one side, an ovary on the other. This is the
only form of true hermaphroditism known to occur in the amniota.

Each sex is further distinguished by secondary sexual characteristics, in part such
as are immediately concerned with reproduction, like the uterus, mammary glands,
vas deferens, etc., in part such as are less directly connected with reproduction,
such as size, distribution of hair, etc. In the course of development the sexual
glands are clearly differentiated before the secondary sexual characteristics appear.
Hence arises the question, have the glands a causal relation to the secondary
characteristics ?

The hormone theory is the only one, available at present, to explain such a
causal relationship. It is known that various glands produce a so-called internal
secretion, which is distributed through the body probably by the medium of the
blood and acts upon structures quite remote from the organ producing the secre-
tion. Similar chemical products arise also from organs, which cannot be regarded
as glands in the usual sense. All of these secretions or products have received the

28 GENERAL CONCEPTIONS.

comprehensive name of hormones (Starling). Now, it has become probable that
the sexual glands produce hormones, which exert an effect upon other organs; for
example, the mammalian corpus luteum is believed to yield a hormone, which so
affects the uterus as to render it adapted to pregnancy. The hypothesis accord-
ingly naturally suggests itself that hormones from the sexual glands occasion the
development of the secondary sexual characteristics. It is well to bear in mind that
our .knowledge of hormones is still meager, and that the suggested hypothesis may
or may not prove valid.

Concerning the cause of sex, i. e., why one individual is male and another
female, we know very little. Nothing positive as to .the cause of sex in vertebrates
is known, though many speculations have been published. In certain insects,
however, it has been discovered that the sexes are distinguished by the females
having one more chromosome in each cell nucleus than the males.* This difference
is explained by the fact that there are two kinds of spermatozoa, one of which
contains an extra (accessory) chromosome. Those ova which at the time of fer-
tilization receive an accessory chromosome become females, those that do not,
males. At present we can add only that the important discoveries mentioned may
furnish the clue to solve the problem of the cause of sex.

The Theory of Heredity.

We owe to Moritz Nussbaum the theory of germinal continuity — the only
theory of heredity which seems tenable at the present time. According to this
theory, the germ-cells are set aside during the segmentation of the ovum and pre-
serve the essentially undifferentiated qualities of the protoplasm and nucleus of the
ovum, from the division of which they arise. Just as the cells formed during seg1
mentation are capable of producing the various tissues of the body, so the germ-
cells have and preserve this faculty. If we term the material of the original ovum
germ-plasm, we may say that this germ-plasm gives rise to the various tissue-
forming cells which make up the body. And by this very conversion into tissue
cells that germ-plasm is changed, and is no longer, as we have learned before,
capable of the full range of development. The germ-cells, on the contrary, do re-
main so capable and it is precisely in order to preserve this capacity that they hold
aloof from the formation of the body tissues and pursue their own independent
career. A portion of the germ-plasm of the parent ovum is, so to speak, short-
circuited into the genital elements which produce the offspring.

If we accept this view, we are forced to make the supplementary hypothesis
that the conspicuous complicated changes, by which the germ-cells are converted
into sexual elements, do not involve differentiation in the true sense— i. e., strictly "
comparable to that which we observe in the somatic cells. Although this hypothe-

* In other insects more complicated relations have been discovered. The relation of the chromosomes to sex
appears to be a complicated and difficult problem.

THE LAW OF RECAPITULATION. 29

sis seems a logical necessity of the theory of germinal continuity, we cannot at
present verify it by any observed facts.

The only other theory of heredity which has ever been seriously considered is
that of pangenesis, which was formulated by Darwin, whose words I quote: "But
besides this means of increase I assume that cells before their conversion into com-
pletely passive or 'form-material' throw off minute granules or atoms, which
circulate freely throughout the system, and when supplied with proper nutriment
multiply by self-division, subsequently becoming developed into cells, like those from
which they were derived. These granules, for the sake of distinctness, may be
called cell-gemmules, or as the cellular theory is not fully established, simply
gemmules. They are supposed to be transmitted from the parents to the off-
spring, and are generally developed in the generation which immediately succeeds,
but are often transmitted in a dormant state during many generations, and are
then developed."

Many modifications of this theory have been proposed by speculative writers,
and many different names have been bestowed upon the gemmules of Darwin ac-
cording to the fancy of each author and the particular set of qualities which he
attributed to these imaginary particles. Such views attained their culmination in
the set of elaborate and complicated hypotheses forming the doctrine of Weismann,
or so-called Weismannism, which was for a time widely and actively discussed. All
of these speculations have only an historical interest, having proved themselves,
from a scientific standpoint, to be absolutely barren.

The Law of Recapitulation.

This law, as commonly formulated, is that the development of the individual
recapitulates the development of the race, or, in other words, the ontogeny recapit-
ulates the phylogeny. This way of stating the law is in so far objectionable that it
presents the theoretical interpretation of the law rather than the actual generaliza-
tion of the facts. The essential datum upon which the law is based is, that the
embryo of a given animal has striking morphological resemblances to the adult
forms of lower allied types. Since the theory of evolution was established by Dar-
win this resemblance has been interpreted as due to the inheritance of ancestral
characters appearing in the embryo. The embryo is looked upon as the representa-
tive of the actual ancestor by modification of which the adult form was evolved.
It is further assumed that the change of the embryo into the adult type follows the
same general course as the development of the remote ancestor into the particular
species under consideration. Speaking broadly, this interpretation is undoubtedly
justifiable. If it were exactly true, it would be necessary only to know the em-
bryology of an animal in order to establish the evolution of the species. Experience,
however, very quickly demonstrates that this procedure is by no means possible,
because the embryo is not a correct or adequate record of the ancestral type. It is
inadequate chiefly for three reasons: first, because the embryo has necessities of its

30 GENERAL CONCEPTIONS.

own, and in the course of evolution embryos acquire special peculiarities by which
they become adapted to the conditions of their life. Such changes in organization
do not correspond to, but on the contrary diverge from, the inherited ancestral
traits, and in so far as they are present they mask or alter those structural fea-
tures of the embryo which represent the ancestral record. Second, because the em-
bryos consist of undifferentiated cells (compare page n). Now, the adult ancestors
representing lower types of organization of course had differentiated tissues, which
enabled them to perform the functions of adult life. One of the first things which
will impress itself upon the student of vertebrate embryology is, that, though he may
find at the proper stage in the embryo the organs of the body clearly developed,
yet, owing to the fact that they consist of relatively undifferentiated cells, they are
incapable, in large part, of performing the functions which they are ultimately to
assume, and the performance of which is the very object of their development.
This change in histological structure brings about a marked unlikeness of the em-
bryo to the assumed ancestral type. Third, the embryo at each stage of its de-
velopment must be regarded as the mechanical cause of the next and of all fol-
lowing stages. It must necessarily, therefore, have in itself peculiarities by which
it is distinguished from all other embryos. It is impossible, accordingly, that all
embryos should be alike. It is only necessary for the student to compare embryos
of various vertebrates one with another to satisfy himself that they have conspicuous
distinctive characteristics. When our knowledge shall have grown sufficintely,
we shall be able to classify vertebrates by their embryos as perfectly as or
perhaps even more perfectly than we can by the consideration of the adult forms.
Every embryo is modified from the very start away from the assumed ancestral
organization, in order that its peculiarities may cause it mechanically to produce the
new form which has been evolved.

In some of the invertebrate animals — as, for instance, among the hydroids and
jellyfishes — the law of recapitulation can be much more easily verified than in the
higher forms which have purely embryonic types of development. From what has
been said, it will be recognized that the likeness of the embryo to the adult lower
form is a general morphological resemblance only, not an exact one, and that there-
fore it is extremely difficult to infer from the embryonic organization what the
ancestral type was. Hitherto all phylogenetic inferences drawn by embryologists
have been largely speculative in character, and, it may be added, have been more
remarkable for their number and variety than for their value.

The resemblance between embryos and lower adult forms has been known for
a century past. It was first adequately asserted in 1811 by J. F. Meckel and
since then has been constantly discussed. More, perhaps, was done to emphasize
it by Louis Agassiz than by anyone else. Von Baer, the creator of modern
scientific embryology, called attention in 1828 to the limitations which must neces-
sarily be put upon Meckel's generalization. It is to be regretted that von Baer's
wise thought on this subject has not been 'more appreciated. He put forth four

ARREST OF DEVELOPMENT. 31

generalizations: first, that which is common to a large group of animals develops
in the embryo earlier than that which is special; second, from the most generalized
stage structures less generalized are developed, and so on until finally the most
special appears; third, the embryo of a given animal form, instead of passing
through the other given forms, separates itself from them more and more; fourth,
therefore, essentially the embryo of the higher forms is never like a lower form,
but only like its embryo. The first to point out the possible phylogenetic signific-
ance of these facts with perfect clearness was Fritz Miiller, in a little book entitled
"Fur Darwin," published in 1864. Ernst Haeckel took up this interpretation and
secured wider attention for it. He termed the law of recapitulation the "biogenetic
law."*

The student will encounter in his practical study many illustrations of the
resemblances which we have been discussing, so that it is unnecessary here to do
more than mention a few for the purpose of illustration. In the embryos of
birds and mammals the pharynx forms a series of lateral pouches which we know
as the gill-pouches, and which develop in the same way as, resemble strikingly,
and are homologous with, the gill-pouches of fishes, which in the fishes give rise
to the so-called gill-clefts. The heart of a young mammalian embryo is a simple
tube with only a single continuous cavity resembling the heart of the lower fishes.
The embryonic kidney or Wolffian body of man resembles, and is homologous with,
the kidney of the frog, but it disappears almost completely before adult life. These
few examples may suffice.

Arrest of Development.

This term is used to designate not the normal, but the abnormal, cessation of
the ontogenetic process. It generally implies the persistence into adult life of an
anatomical condition, normally present in the embryo, which is typically a tem-
porary though essential phase of development. Usually there is no cessation of
the histological differentiation. It is characteristic of these anomalies that they are
more or less definite.

A few illustrations may . render the matter clear. The palate is formed as
two shelves, which grow until they meet in the median line; sometimes they fail
to meet and then the adult has a "cleft palate" the tissues of which, however,
are as fully differentiated as those of the normal palate. In the young embryo a
short blood-vessel (i.e., dorsal part of the left last aortic arch) connects the pulmo-
nary artery with the dorsal aorta (compare page 101), but it later becomes occluded
and finally obliterated. Occasionally, however, it persists as an open vessel, which
is termed the "ductus arteriosus" in the adult. It may grow in size and its walls
become fully differentiated like those of an aorta. The external genitalia of the
male may be arrested in their development, though they continue in such cases

* " Biogenetisches Grundgesetz."

32 GENERAL CONCEPTIONS.

their growth and histogenesis. There results a "pseudo-hermaphrodite" a true male
with external organs of the female type. In such cases the individual usually
presents other female characteristics, such as a wide pelvis and enlarged mammae.

The variations in adult structure due to arrest of development are the most
frequent, important, and significant with which the student of anatomy has to deal.
It is obvious that the possible variations are limited to anatomical conditions, which
actually occur in normal embryos.

CHAPTER II.
THE EARLY DEVELOPMENT OF MAMMALS.

The Spermatozoon.

The spermatozoa of mammals are filaments consisting of a short thick end
called the head, and a very long, delicate thread called the tail. They are of
minute size as compared with the ovum. The head varies greatly
'in shape according to the species. It contains chromatin, hence
it stains darkly with those histological dyes which color nuclei.
The tail consists of three parts: first, the middle piece, which is
next the head, is short and the thickest of the three parts, contains
an axial thread, and probably always has a very fine spiral thread
running round it; second, the main piece, which is the longest
part of the tail; and, third, the end piece, which is not more
than a line, even as seen with very high microscopic powers.

The Human Spermatozoon. — The human spermatozoon is 0.055
mm. long — the head being 0.005 mm., the tail 0.050, and the
middle piece 0.009. It is shown in two views in figure 2. The
head is flattened and pointed. Seen from the flat side, it appears
oval (Fig. 2, A) with the front end generally tapering a little, but
never pointed. The anterior half or two thirds has a brighter
and more transparent part. Seen on edge (Fig. 2, B), the
head has a pointed form with a posterior, thicker, round dark
part. By adjustment of the focus it can be ascertained : that the
sides near the point are depressed. Some writers maintain that
there is a special tip projecting from the head as a cylinder
thread, with a hook at its end. The middle piece, mi, is directly
united with the head by a transverse joint. It is cylindrical and
about as long as, or a little longer than, the head. Its surface is
often granular or rough, and there cling to it a few shreds of
protoplasm. It has a spiral thread, which is easily overlooked
on account of its extreme fineness. The main piece, m, of the
tail is about half as thick as the middle piece. It gradually
tapers and ends abruptly at the beginning of the still finer and very short end
piece, e. The tail probably contains an axial thread, as has been observed in other
3 33

FIG. 2. — HUMAN
SPERMATOZOA.

A, Complete sperma-
tozoon. B, Head
seen from the side.
C, Extremity of
the tail, h, Head.
mi, Middle piece.
m, Main piece, e,
End piece. A 11
highly magnified .
— (After Retzius.)

34 THE EARLY DEVELOPMENT OF MAMMALS.

mammals. The head probably contains a minute body representing a centrosome,
although it has not yet been satisfactorily demonstrated in man.

The spermatozoa, when free in the fluids in which they normally occur, are
capable of active locomotion. This is achieved by means of the tail, which acts as
the swimming organ by vibratory undulations which drive the spermatozoon along,
head foremost. The tail has often been compared to the flagellum which serves
as the locomotive organ for many of the unicellular organisms.

The Fully Grown Ovum Before Maturation.

The structure to be here described is not the true sexual element, but is only
the modified germ-cell which has accomplished its period of growth and is ready
to be transformed into the genuine female sexual element. This transformation is
called the maturation, and is accomplished essentially by the expulsion of the so-
called polar granules. The full-grown mammalian ovum is found in the ovary in
the center of the discus proligerus of the Graafian follicle. It measures usually
from o.io to 0.15 mm. in diameter. It is approximately spherical. In some cases
observers have found a very delicate vitelline membrane covering the protoplasm.
Others have failed to observe this. Outside there is a thick envelope measuring
from 0.02 to 0.03 mm. in diameter and known as the zona pellucida or radiata.
Against the outside of the zona rest the cells of the discus proligerus which consti-
tute the so-called "corona radiata." The nucleus is large, spherical, contains a
distinct nucleolus, and always occupies an eccentric position.* The protoplasm of
the cell is large in amount, granular in appearance, forms a distinct reticulum,
and contains a larger or smaller number of yolk granules which vary considerably in
character, size, and distribution in different mammals. They are usually more or
less concentrated in the central portion of the ovum, leaving the outer portion,
known as the protoplasmic zone, more or less free.

The Human Ovum. — The full-grown human ovum is distinguished among
mammalian ova for the clear development and ready visibility of all its parts, a
peculiarity due chiefly to the small amount of the yolk and the fewness of the
fat granules it contains. Figure 3 represents an ovum from a woman of thirty years.
The specimen was obtained by ovariotomy, examined and drawn in the fresh state,
being in the liquor folliculi. The specimen gave the following measures: The
diameter of the whole ovum, including the zona radiata, 165-170;*; thickness of
zona, 20-34;*; perivitelline fissure, 1.3/1; the clear outer zone of the yolk, 4-6;*;
the protoplasmic zone, 10-21;*; the zone of yolk granules, 82-87;*; nucleus, 25-27;*.

The corona radiata, cor.r, consists of elongated radiating cells with rounded
ends and oval nuclei. The zona pellucida shows a distinct radial striation. This
is probably due to the presence of minute canals running through the zona. The
ovum proper is separated from the zona by a narrow fissure, the perivitelline space,

*The nucleus was formerly termed "germinal vesicle"; the nucleolus, " germinal spot."

OWL AT ION.

35

within which it lies free and loose. Hence when a fresh specimen is examined,
the same side of the ovum, that containing the nucleus and which is the lightest
part, is always found uppermost. In this ovum no vitelline membrane was ob-
served. The body of the ovum may be divided into an inner kernel containing
the yolk granules, and an outer protoplasmal zone, of which the very thin outer-
most layer is clear and therefore more or less differentiated from the broader, deeper
layer, which is granular and constitutes most of the zone, PL The yolk grains
are i ,« or less in diameter. They are highly refringent and of various kinds.
Their characteristics have not yet been
accurately investigated. The nucleus is nearly
spherical and has a conspicuous nucleolus.
In fresh specimens the nucleolus shows
amoeboid movements, even at ordinary
summer temperatures, for several hours after
removal from the ovary. It is .only in
hardened specimens that the reticulum of
the nucleus can be clearly observed.

PL

Nu.

MATURATION.

cor.r, Part of corona radiata. Z, Zona pellucida.
PI, Protoplasm. Y, Yolk. Nu, Nucleus. —
(After W. Nagel.)

Ovulation.

The discharge of the ovum from the
ovary is called ovulation. It results from
structural changes in the Graafian follicle,
and these changes continue after the de- FIG. 3. — FULL-GROWN HUMAN OVUM BEFORE
parture of the ovum, transforming the
Graafian follicle into a so-called corpus
luteum. The exact history of these changes
does not fall within the scope of this work.

The essential steps in the process are the growth of the Graafian follicle and
the thinning of its wall at a point at the surface of the ovary. The thin part
is called the stigma. This breaks through and establishes an opening by which
the ovum, surrounded by the corona radiata, together with the liquor of the follicle,
can escape into the periovarial chamber, whence it makes its way into the Fallo-
pian tube. The growth of tissue in the walls of the collapsed Graafian follicle fills
up the space of the same, constituting a mass which is known as the corpus luteum
on account of its yellow color. The most characteristic elements of this structure
are the large cells which contain the pigment. Each cell has a rounded nucleus
and a large protoplasmic body, which is also more or less rounded in form. The
lutein granules are in these cells. The function of the corpus luteum was long
entirely unknown. Recently the theory has been suggested by Born that these
cells exert an influence upon the uterus by which it is prepared to receive the
ovum. This influence may be suggested to act by means of a chemical substance
.(hormone) produced by the lutein cells and added to the blood, which then affects

36 THE EARLY DEVELOPMENT OF MAMMALS.

the uterus. There are some experimental observations tending to prove the correct-
ness of this theory.

The brilliant color of the corpus luteum is especially characteristic of man, and
has determined the name of the structure. In sheep the pigment is pale brown,
in the cow dark orange, in the mouse brick-red, in the rabbit and pig flesh-color.

The Maturation of the Ovum.

Maturation is the term applied to the series of changes by which the fully
grown egg-cell is transformed into a true female sexual element. Viewed externally
in the living ovum, the process manifests itself chiefly by the separating off of two
small bodies of protoplasm, each of which contains some nuclear material. These
small bodies are generally known by the name of polar globules. They take no
further part in the development, ultimately disintegrate, and are lost. The remain-
ing ovum is capable of impregnation. It is now known that the production of
the polar globules is the result of a special form of cell division, which we term
the "reduction division." When the first polar globule is formed, the egg-cell
divides into one very large cell and a second very small one. When the second
polar globule is formed, the larger of the cells again divides, producing a second
small cell and a new large one. This large one is the true female element.

Maturation as a general process may be described as follows. For figures and
detailed accounts of the process in the mouse, see Chapter V. When an ovum is
about to mature, its nucleus moves nearer that point of the surface which may be
regarded as the center of the so-called animal pole, the region of the ovum, which
contains most of the protoplasm and less of the yolk material. During the migra-
tion of the nucleus, the cell as a whole usually contracts so that a space appears
between it and the zona radiata. Concerning the force that moves the nucleus we
have no definite knowledge. When near the surface, the nucleus as such dis-
appears. Older writers supposed that it was lost altogether, but we now know
that the disappearance of the nucleus is only apparent, not actual, being in reality
a metamorphosis. It is probable that the first step is the discharge of the nuclear
fluid into the surrounding protoplasm, causing the nucleus to become more or less
shriveled. The second step is the dissolution of the membrane of the nucleus so
that the nuclear contents are brought into direct contact and partly mixed
with the protoplasm of the cells. The third step, which in time more or less
accompanies the second, is the gathering of the chromatin of the nucleus into a
definite number of separate granules or chromosomes (tetrads'). These chromosomes
are always conspicuous and are larger than those formed during ordinary cell
division. Their number is also highly characteristic. As is now well known, there
appear during the process of indirect cell division a fairly definite number of chro-
mosomes, a number which is characteristic for each species. In numerous cases
it has been observed that the number of chromosomes in the maturing egg-cell is
exactly one half of that found during the ordinary cell divisions of the* same species..

THE MATURATION OF THE OVUM. 37

The chromatin granules lie at first irregularly. Fourthly, there arises a characteristic
spindle figure such as accompanies mitosis. The chromatin forms an equatorial
plate, each granule being associated with one of the spindle threads. The shape
of the spindle varies, as does also the distribution of the granules of the equatorial
plate. In guinea-pigs the ends of the spindle are pointed and the threads are
straight, the outline of the spindle being like a diamond; in the bat the spindles
are barrel-shaped and the threads are curved. In many cases it is known, and it
may be found to be true of all cases, that the axis of the spindle is at right
angles to the radius of the ovum. The nuclear spindle now changes its position,
becomes first oblique, and then r.adial. One end of the spindle lies close to the
surface of the ovum. The first step is the division proper. The spindle, driven
by an undiscovered power, moves centrifugally until it is partly extruded from the
egg. The projecting end is enclosed in a distinct mass of protoplasm which
gradually increases and soon becomes constricted around its base. The fragments
of chromatin have each divided into two parts (dyads), and one half of each frag-
ment moves toward one end, and the other half toward the other end of the
spindle. The half fragments of each set move together, hence there seem to be
two plates within the spindle. The translation of the groups of chromatin con-
tinues until they reach the end of ^the spindle. The achromatic threads then
break through in the middle, and thus t\ie original nucleus, or at least a part of it,
has been divided. There are now two masses of nuclear substance, one in the
ovum, the other in the polar globule. The result of the whole process is the
formation of two cells extremely unequal in size, and each containing in its nuclear
elements half the number of chromosomes characteristic of the body-cells. The
number of chromosomes has, therefore, been reduced, hence the term reduction
division. It will be noted that the actual reduction in the number of chromosomes
took place when they were first formed in the maturing ovum, while the spindle or
mitotic figure was developing.

The second polar globule is produced a short time after the first. When this
occurs, the nuclear remnants in the ovum do not reconstitute themselves into a
membranate nucleus, as occurs in ordinary cell division, but they change directly
into a second spindle, which lies, as did the first, within the protoplasm of the
animal pole. This second spindle occupies an oblique position, or may even be
parallel with the surface. But it also soon takes up a radial position and pro-
duces a second polar globule in similar manner to the first. The dyads all divide,
and the ovum receives the half number of chromosomes, each of which represents
the fourth part of a tetrad (double chromosome). The second globule is usually
smaller than the first.

It may also happen that the first polar globule may itself divide, so that three
polar globules appear.

The Formation of the Female Pro-nucleus. — The nuclear material which remains
in the main .ovum after the separation of the polar globules -is known as the

38 THE EARLY DEVELOPMENT OF MAMMALS.

female pro-nucleus. The nuclear remnant lies close to the animal pole and in
clear protoplasm. The details of its further history vary according to the species
of animal. Three tendencies are known to affect the pro-nucleus: viz., to move
toward the central position in the ovum, to unite with the male pro-nucleus as soon
as that is formed out of the spermatozoon which enters the ovum, and to assume
the character of a membranate nucleus. As the time of the formation of the male
pro-nucleus is variable, the other tendencies being more constant, the exact history
of the female pro-nucleus may be said to depend principally on the appearance
of the male pro-nucleus. The earlier that event, the less does the female pro-
nucleus move centrifugally and the less does it assume the membranate form.
Even among mammals there is variation.

Time of Maturation. — The time when the polar globules are formed varies
according to the animal, and may be before or after the egg-cell leaves the ovary.
In placental mammals maturation always begins, so far as known, in the ovary,
and is said in some cases to be completed there. But in other cases it is certainly
completed only after ovulation or when the ovum has passed into the Fallopian tube.

Impregnation of the Ovum.

Impregnation is the union of 'the male and female elements to form a single
new cell, capable of initiating by its own division a rapid succession of generations
of descendent cells. The process of union is commonly called the entrance of the
spermatozoon into the ovum. The new cell is called the impregnated or fertilized
ovum. The process of fertilization in the mouse is described and illustrated in
Chapter V.

In all multicellular animals impregnation is effected by three successive steps:
(i) The bringing together of the male and female elements; (2) the entrance of the
spermatozoon into the ovum and the formation of the male pro-nucleus; (3) fusion
of the pro-nuclei to form the segmentation nucleus.

Meeting of the Sexual Elements.— In all amniota the seminal fluid is transferred
from the male to the female passages during coitus, and spermatozoa are thereafter,
in mammals, found in the uterus. In default of actual knowledge it is commonly
believed that the spermatozoa make their way by their own motions into the Fallo-
pian tubes. The ovum, meanwhile (probably, in mammals, while completing its
maturation), travels down the tube. The meeting-point, or site of impregnation,
in placental mammals is about one-third way down from the fimbria to the uterus.
The exact spot is remarkably constant for each species. Nothing is known by
direct observation as to the site of impregnation in man, but there is no jeason to
suppose, as has unfortunately been often done, that the site is either variable or
essentially different from that in other mammals.

The Entrance of the Spermatozoon into the Ovum.— It is probable, in mammals
at least, that only one spermatozoon normally enters the yolk of the ovum and ac-
complishes its fertilization. It has been observed in those animals in which, as in

IMPREGNATION OF THE OVUM. 39

the rabbit, there is formed a more or less considerable space between the yolk and
the zona radiata, that a number of spermatozoa appear in this space, but appar-
ently only one actually fuses with the substance of the ovum. The manner in
which additional spermatozoa are excluded, after the first has entered, is still 'under
discussion. The hypothesis has been suggested that the attractive power of the
ovum is annulled or weakened by the formation of the male pro-nucleus from the
spermatozoon which first enters. With our present knowledge the assumption
appears unavoidable that the ovum exerts a specific attraction upon spermatozoa
of the same animal species. Recent authorities incline to the view that this attrac-
tion is of a chemical nature, for it has been observed that certain chemical sub-
stances may attract very strongly unicellular organisms capable of free locomotion.
The phenomenon is called chemotropism. According to this interpretation, the
attraction of the ovum for the spermatozoon would be termed chemotropic.

At the time of fertilization the ovum in the Fallopian tube is surrounded by
a number of spermatozoa; in the case of the rabbit, perhaps by a hundred, more
or less. They are all, or nearly all, in active motion, for the most part pressing
their heads against the zona radiata. Several of them may make their way through
the zona into the interior. According to Hensen, only those spermatozoa which
enter the zona along radial lines can make their way through. Those which take
oblique courses remain caught in the zona. The female pro-nucleus is already
present, and may be either with or without a membrane, according to the species.
A single spermatozoon makes its way into the yolk proper, passing a short distance
into the interior. It is uncertain whether the whole tail of the spermatozoon enters
the ovum or not. In some of the lower vertebrates and in other animals it ap-
pears to do so. It is probably always the case that at least the main piece of the
tail enters the yolk. The tail, as such, very soon disappears, while the head of
the spermatozoon enlarges, probably by the imbibition of fluid from the surrounding
yolk. The head of the spermatozoon is rich in chromatin, which forms a series
of irregular masses as the head enlarges, producing a network appearance, and is
thus converted into a nucleus-like body, the male pro-nucleus. At the same time
in some animals the growing head surrounds itself by a membrane.

We now have a cell which contains two nucleus-like bodies, one derived from
the head of the spermatozoon and the other from the nucleus of the egg-cell. They
are termed respectively the male and female pro-nucleus. Each pro-nucleus, when
it first appears, is small and gradually enlarges, probably in both cases by the im-
bibition of fluid. The relative size of the two pro-nuclei varies considerably in
different species, and is probably a secondary and relatively unimportant relation.
The proportion between the two probably depends upon the time when the male
pro-nucleus is formed. If the spermatozoon enters early, while the female pro-
nucleus is forming, it may make a pro-nucleus as large as that from the egg-
cell. If, on the other hand, the spermatozoon enters late, the female pro-nucleus
enlarges, acquires a start, and the growing male pro-nucleus is, therefore, smaller.

40 THE EARLY DEVELOPMENT OF MAMMALS.

Concerning the fate of the middle piece of the spermatozoon and its share
in the fertilization in the ovum of mammals, we possess no satisfactory informa-
tion. It has been shown, however, in other animals that this middle piece
produces a centrosome, and the only centrosome which appears in the fertilized
ovum. The theory has been advanced that the ovum, after its maturation, has
no centrosome, that a centrosome is always brought into the ovum by the sper-
matozoon in the manner just indicated. If we regard the centrosome as a perma-
nent cell element, then we must further interpret the addition of the male centro-
some as one of the most important phenomena of fertilization. Whether this
hypothesis is correct or not, we are unable at present to decide.

FIG. 4. — OVUM OF A WORM (SAGITTA) WITH Two FIG.' 5. — OVUM OF A RABBIT, SEVENTEEN HOURS
PRO-NUCLEI. AROUND EACH PRO-NUCLEUS is AFTER COITUS, WITH THE PRO-NUCLEI ABOUT

SHOWN THE ASTER. — (After O. JETertwig.) TO CONJUGATE.

p.g, Polar globules. — (After Rein.)

Astral figures play a conspicuous part in the phenomenon of fertilization in
many animals. Astral figures are produced in the protoplasm of the ovum by its
assuming a special radiating structure. Astral figures may appear around both
the male and female pro-nuclei (Fig. 4). In other cases the astral figure arises
only in association with the head of the spermatozoon or male pro-nucleus. In
mammals, so far as known, no astral figures are developed about either of the
pro-nuclei. There is a clear space in the protoplasm around each nucleus, and
such a clear space has often been noted also when the astral figure is present. It
may possibly be interpreted as a rudimentary aster or center of astral formation.

The two pro-nuclei usually lie at some distance from one another. As soon
as they are formed, or perhaps when they are fully differentiated, they tend to
move toward one another and toward the center of the ovum. Concerning the
path of the male pro-nucleus we possess interesting information from the study of
the ova of the frog and axolotl. In these ova the spermatozoon leaves a trail of
pigment, which consists of two limbs, one passing through the cortical layer of the
ovum nearly perpendicular to the surface, and the other forming an angle with the
first and leading directly to the female pro^-nucleus. The female pro-nucleus tends

IMPREGNATION OF THE OVUM. 41

always to move toward a central position. The force which draws the pro-nuclei
together is unknown. The hypothesis that this force is chemotropic has met with
favor.

The Fusion of the Pro-nuclei. — In the rabbit, as probably in all mammals, both
pro-nuclei lie at first eccentrically, but both move toward each other and toward
the center, meeting when the central position is attained. As they near one an-
other both pro-nuclei perform active amoeboid movements. After they meet they
still continue their amoeboid movements, and travel together to the center of the
ovum (Fig. 5). One of the pro-nuclei assumes a crescent shape and embraces the
other. In the mouse the history is similar. After the two pro-nuclei in this animal
have met, they remain side by side, but they are without membranes. After the con-
junction of the pro-nuclei the chromatin threads become distinct and draw closer
together. Between them appears first a small spot or centrosome with a few radi-
ating lines around it (Fig. 117). From the centrosome arises a spindle of achro-
matic threads (Fig. 118). The chromosomes, both male and female, attach them-
selves to the spindle, and therewith impregnation is completed and mitosis of
the impregnated ovum initiated.

It is now believed that the pro-nuclei never unite to form a distinct mem-
branate nucleus, the so-called segmentation nucleus of earlier writers, but that the
fusion always takes place during the absence of the membranes of the pro-nuclei by
the mingling of their contents. The time of mingling, however, varies as regards
the formation of the chromosomes. It may take place before or after the chromo-
somes are developed. When, as in the mouse, the chromosomes appear as two
distinct groups, it is possible sometimes to determine their number. In the mouse
counting is difficult, but there seems little doubt that each pro-nucleus forms
twelve chromosomes. Hence it results that there are twenty-four chromosomes in the
segmentation spindle. This number, twenty-four, so far as has been determined, is
the number which appears during later stages of segmentation and in all subse-
quent cell divisions of this animal. It is believed to be a general law that the male
and female pro-nuclei each contribute the same number of chromosomes to the seg-
mentation spindle except in those cases where an accessory chromosome is inter-
polated in the development. This number is identical with the number which ap-
pears during the reduction divisions which lead to the maturation of the ovum on
the one hand and the development of the spermatozoon on the other; and,
further, the number is one half the number of chromosomes which appear during
ordinary cell divisions of the species. The most thorough study of the phenomenon
which has yet been made is that by a succession of able investigators upon the
large nematode Ascaris megalocephala. An admirable summary of the process of
fertilization in Ascaris has been given by Oscar Hertwig.*

* " Lehrbuch der Entwicklungsgeschichte," eighth edition, 1906. The large Ascaris is a particularly
favorable object. The student who wishes to pursue the practical study of impregnation further should select
this form.

42

THE EARLY DEVELOPMENT OF MAMMALS.

Segmentation of the Ovum.

Shortly after the entrance of the spermatozoon into the ovum the segmenta-
tion spindle is developed by the union of the pro-nuclei, as described in the pre-
vious section. This spindle leads to a division of the ovum into two cells. These
cells further rapidly divide. As stated on page 10, these early cell divisions are
called the segmentation of the ovum.

The position of the first segmentation spindle is always eccentric, and appears
to be approximately, if not exactly, the same as that of the egg-cell nucleus before
maturation. The axis of the spindle varies greatly in its direction. It is some-
times at right angles to the radius of the ovum, which passes through the polar

FIG. g. — OVUM OF A RABBIT OF TWENTY-FOUR FIG. 7. — OVUM OF A SNAIL (LIMAX CAMPESTRIS)

DURING THE FIRST CLEAVAGE. THE ENVEL-
OPES OF THE OVUM ARE NOT DRAWN IN. X 2OO

diams. — (After E. L. Mark.)

HOURS.

The first cleavage has been completed; the two cells
(segmentation spheres) are appressed; above the
cells lie the polar globules; numerous spermato-
zoa lie in and within the zona pellucida. —
(After Coste.)

globules, but it is more usually oblique to this radius. It was at one time thought
that the plane of division was always at right angles to the radius of the extrusion
of the polar globules, but this view cannot be upheld. After the ovum has divided
into two cells, the polar globules lie in the angle between these two cells (Fig. 6),
because there the globules find room. It is to be noted that the globules accom-
modate themselves to the segmentation spheres, and that the formation of the
spheres is not accommodated to the original position of the globules.

The degree of the eccentricity of the segmentation spindle varies in different
ova, chiefly according to the amount' of yolk; the greater the quantity of yolk in
the ovum, the more marked is the eccentricity.

The actual first cell division (first • cleavage or first segmentation) of a mam-
malian ovum has never been followed by direct observation, the practical diffi-
culties not having hitherto been successfully overcome. Various phases of the di-
vision have, however, been seen, and the internal changes have been studied by
means of sections. It accordingly seems expedient to interpolate the following ac-

SEGMENTATION OF THE OVUM. 43

count of the external appearances of the first segmentation in the living ovum of
the snail, Limax campestris. The eggs of this animal, by their size and in their
mode of segmentation, have a certain resemblance to mammalian ova. The fol-
lowing description is taken from the account by E. L. Mark, published in 1881; it
is nearly in his own words:

In Limax, after impregnation, the region of the segmentation nucleus remains
more clear, but all that can be distinguished is a more or less circular, ill-defined
area, which is less opaque than the surrounding portion of the vitellus. After a
few moments this area grows less distinct. It finally appears elongated. Very soon
this lengthening results in two light spots, which are inconspicuous at first, but which
increase in size and distinctness, and presently become oval. If the outline of the
egg be carefully watched, it is now seen to lengthen gradually in a direction corre-
sponding to the line which joins the spots. As the latter enlarges the lengthening
of the ovum increases, though not very conspicuously. Soon a slight flattening of
the surface appears just under the polar globules; the flattening changes to a de-
pression (Fig. 7), which grows deeper and becomes angular. A little later the fur-
row is seen to have extended around on the sides of the yolk as a shallow de-
pression, reaching something more than halfway toward the vegetative or inferior
pole, and in four or five minutes after its appearance the depression extends com-
pletely around the yolk. This annular constriction now deepens on all sides, but
most rapidly at the animal pole; as it deepens it becomes narrower, almost a fis-
sure. By the further deepening of the constriction on all sides there are formed
two equal masses connected by only a slender thread of protoplasm, situated nearer
the vegetative than the animal pole; the thread soon becomes more attenuated and
finally parts. The first cleavage is now accomplished. Both segments undergo
changes of form; they approach and flatten out against each other, and after a
certain time themselves divide.

The succeeding cleavages of segmentation need to be followed out in greater
detail than yet recorded. In many cases there appear to be three cells in the
next stage, because one of the two primitive segmentation spheres divides sooner
than the other. The more commonly received view is that four cells are produced
next, but it may very well be that there is really a three-cell stage preceding the
four-cell stage of which two figures are presented. The first of these (Fig. 8)
represents the four-cell stage of the ovum of a bat, and the second (Fig. 9) repre-
sents the four-cell stage of the ovum of the Virginian opossum. That of the bat
resembles the picture which we obtained from a number of animals, such as the
rabbit, the guinea-pig, the dog, and others. That of the opossum differs so much
from anything known in other mammals that it may be questioned whether it is
entirely normal. In the mouse the zona is much thinner and assumes an irregular
form, adapting itself to the pressure of the single spheres.

After the four-cell stage, the segmentation proceeds apparently with considerable
irregularity, but we are soon able to see that the cells are grouping themselves

44

THE EARLY DEVELOPMENT OF MAMMALS.

into an uninterrupted external layer and an internal accumulation of cells. The
outer layer is in contact, or nearly in contact, with the zona radiata, and may,
therefore, be termed the subzonal layer (Fig. n, 5.2).* The inner accumulation of
cells is designated as the inner mass, i.m. Figure 10 represents a rabbit ovum of
about seventy hours, according to the observations of van Beneden. He represents
the subzonal layer, EC, as interrupted at one point, where one of the cells of the
inner mass, i.m, is exposed. It is probable, however, that - van Beneden is in
error in regard to this, and that the subzonal layer is really continuous. In the

FIG. 8. — OVUM OF A BAT (VESPERTILIO MURINA)
WITH FOUR SEGMENTATION SPHERES. — (After
van Beneden and Julin.)

X300

FIG. 9. — OVUM OF A VIRGINIAN OPOSSUM, WITH

FOUR SEGMENTS.

.p.g} Polar globules, x, Coagulated material, z, Zona
pellucida. — (After Emil Selenka.)

next stage (Fig. n) we find that the ovum has become larger by the appearance
of a cavity in its interior. This cavity appears between the inner mass, i.m, and
the subzonal layer, but at one side the inner mass remains adherent to, and closely
connected with, the subzonal layer. We now have reached the stage in which the
developing ovum may be designated as the blastodermic vesicle.

As to the interpretation of the parts, it is probable that the subzonal layer
is ectoderm, and that the central cells of the inner mass are also ectodermal and
share in forming the embryonic shield, and finally that the superficial cells of the
inner mass (i.e., those next the cavity of the vesicle) are entodermal. At the stage
we have now reached the blastodermic vesicle has a large part of its walls formed
by the subzonal layer only, so that we call this the stage of the one-layered blasto-
dermic vesicle.

* The subzonal layer is termed trophoblast by A. A. W. Hubrecht, and is held by him to be a special embryonic
structure, developed in order fo establish special relations between the developing ovum and the walls of the uterus
to secure the nutrition of the former. It has seemed best to present a purely objective account of the facts without
entering into a discussion of the very interesting interpretations proposed by Hubrecht.

THE BLASTODERM 1C VESICLE.

45

Arrival in the Uterus. — During the stages described the ovum travels along the
Fallopian tube and reaches the uterus in an early phase of the stage which we
designate as the • blastodermic vesicle. The transit requires about eighty hours in
the mouse, about five days in the opossum, four days in the rabbit, and from eight to
ten days in the dog. The time necessary in man is unknown. It may be sup-
posed to be about one week.

Pro-chorion. — The ovum in many mammals becomes surrounded by a gelatinous
covering, which is secreted by the glands of t the uterus. It may be compared

km.

FIG. 10. — RABBIT'S OVUM or ABOUT SEVENTY

HOURS.

EC, Outer layer, i.m, Inner mass of cells. Z, Zona
pellucida. — (After E. van Beneden.)

S>z.

FIG. ii. — YOUNG BLASTODERMIC VESICLE OF A

MOLE.

i.m, Inner mass of cells. s.z, Outer or subzonal
layer, z, Zona pellucida. — (After W. Heape.)

to the white of the bird's egg. In _the rabbit this envelope becomes enormously
thick about the blastodermic vesicle and in other rodents is voluminous. In the
dog it is less developed, but presents the further peculiarity that the secretion in
the tubular glands may be hardened in connection with the envelope itself, which,
therefore, appears, when the ovum is removed from the uterus, to be studded over
with fine threads resembling villi. The gelatinous envelope has been termed by
Hensen the pro-chorion. The thread-like projections seen in the dog were taken
by Bischoff for true villi, and they have sometimes been referred to as the pro-
chorionic villi. The term pro-chorion has been applied to other structures, as,
for instance, to the subzonal layer of the blastodermic vesicle. The student needs
to be warned against confusing the term pro-chorion in its various applications.

The Blastodermic Vesicle.

The blastodermic vesicle always consists at first of the subzonal layer and an

inner cell mass attached at one point to the subzonal layer, and has a cavity

between the inner mass and the subzonal layer; the vesicle itself is always enclosed

46

THE EARLY DEVELOPMENT OF MAMMALS.

in the zona radiata. The variations offered in different mammals are so great that
a description less general than that given would hardly be applicable, even to the
placental mammals.

The next step in development is the production of a complete second layer
out of the cells of the inner mass. This layer extends completely around the
vesicle and lies close against the subzonal layer, and encloses the main cavity of

the vesicle. The way in which this inner
vesicular layer is developed varies greatly.
In the hedgehog it appears very preco-
ciously, while the blastodermic vesicle is
very small, and afterward it expands rapidly,
while the vesicle as a whole is growing. In
the rabbit and in the mole it is formed
much later, and the one-layered vesicle ex-
pands to a considerable diameter before the
inner mass begins to spread out. The strik-
ing changes through which the inner mass
passes in the mole are illustrated in figure 12.
It forms at first a small globe, A. The
inner mass subsequently flattens out, becom-
ing lens-shaped, thinner, and larger in
area, B. It continues spreading laterally
and separates into three layers. The two
outer layers enter into the formation of the
true ectoderm, C. In the rabbit, and per-
haps in the mole, the outer of the two
FIG. 12.— SECTIONS THROUGH THE INNER MASS OF layers is temporary only in existence. In
BLASTODERMIC VESICLES OF THE MOLE AT some rodents it acquires a very great de-
velopment and leads to the curious phe-
nomenon known as the inversion of the
germ-layers. The innermost of the layers,
Ent, grows at its edges, and its cells spread

out gradually farther and farther under the subzonal layer until they extend com-
pletely around the vesicle and form, by meeting at the opposite pole of the ovum,
a closed, vesicle. Very similar is the process in the rabbit. The cells at the expand-
ing edge of the inner layer are found to spread rapidly, so that during the expansion
they are more or less widely separated from one another. But they continue their
expansion and multiplication until they form a complete inner epithelial layer.

The point where the inner mass and the subzonal layers are connected with
one another marks the site of the future embryonic area.

The blastodermic vesicle grows rapidly in size, partly by the multiplication of
its cells, partly by their becoming flattened out so as to cover a larger surface.

THREE SUCCESSIVE STAGES.

EC, Outer or subzonal layer, z, x, Zona pellucida.
i.m, Inner mass of cells. Ent, Entoderm. —
(After W. Heape.)

THE EMBRYONIC SHIELD.

47

The interior of the vesicle is filled with fluid. As the vesicle grows the fluid in-
creases in amount, and is presumably derived by the ovum from the walls of the
uterus. It is under pressure within the vesicle, as is shown by the manner in which
it spurts out if the vesicle is broken. Nothing exact as to the composition of this
fluid is known, though we may suppose it to resemble more or less the serous fluid
of the adult body. The size and form of the vesicle offer characteristic variations
in mammals. It starts as a more or less nearly spherical body. In the rabbit it
assumes an oval shape, and by the seventh day measures about 4.0 mm., and soon
thereafter becomes attached to the wall of the uterus. In the hedgehog, the guinea-
pig, and the mouse the ovum, while very small and more or less rounded in form,
becomes imbedded in uterine tissue and develops into a special shape in adapta-
tion to its new situation. In the ungulates the vesicle grows enormously, becoming
a very long and slender sac. Thus, for example, in the sheep it may measure
on the fourteenth day not less than 50 cm. in length.

Another respect in which the blastodermic vesicles differ greatly from one an-
other in various mammals is in regard to the early development of the subzonal
layer, or, as we may call it, the ectoderm. In many cases the entire layer under-
goes a precocious development, its cells multiply very rapidly, so that the layer
becomes several cells thick. This thickened layer is known as the trophoderm. In
other placental mammals this thickening is confined to a limited area of the ecto-
derm. For further description see Trophoderm, page 114.

The Embryonic Shield.

Sooner or later in the early history of every blastodermic vesicle, and always

as the first indication of the development of the embryo proper, there appears a

thickening of a small oval area of the outer layer in the region of the inner mass.

FIG. 13. — TRANSVERSE SECTION THROUGH THE EMBRYONIC SHIELD OF THE BLASTODERMIC VESICLE OF A DOG

OF ELEVEN OR FIFTEEN DAYS (PREQISE AGE UNKNOWN). ,

O.L, Outer layer. Ent, Entoderm. X 2oo diams. — (After Bonnet.)

This thickening is known as the embryonic shield. In the fresh specimen it marks
itself by the greater opacity which it causes in the walls of the ovum where it lies.
In those cases where the thickening of the ectoderm to form the trophoderm ex-
tends over the entire blastodermic vesicle, it is very difficult to follow the early
history of the embryonic shield. In other cases, however, where the trophoderm
occupies a special restricted area, the history of the embryonic shield may be more
readily followed. The animals in which it has hitherto been chiefly studied are

48 THE EARLY DEVELOPMENT OF MAMMALS.

the rabbit, dog, cat, and sheep. In all of these the embryonic shield is simply a
thickening of the outer layer (Fig. 13). The embryonic shield is at first small, but
it rapidly expands and assumes a rounded or oval form. There next appears, in
a more or less central position in the shield, a small, darker spot, which marks
what is known as the primitive knot, a peculiarity of which is that it corresponds
to an intimate union of the cells of the inner with those of the outer layer of the
blastodermic vesicle. (Compare Fig. 126, B, page 171.) Soon a linear shadow be-
comes visible extending from the primitive knot toward a point at the periphery
of the embryonic shield (Fig. 14) which represents the embryonic shield of a dog

Oo°

Kn.

O O o

FIG. 14. — SURFACE VIEW OF THE EMBRYONIC SHIELD OF THE BLASTODERMIC VESICLE OF A DOG OF THIRTEEN

TO FIFTEEN DAYS (PRECISE AGE UNKNOWN).

The specimen had been preserved with sublimate and stained with borax-carmin. Sh, Embryonic shield. Kn,
Hensen's knot, p.s, Primitive streak. X 100 diams. — (After Bonnet.)

at about two weeks. The shadow, p.s, from the primitive knot is termed the
primitive streak, and it very soon becomes further characterized by the formation
of a fine groove caused by a depression in the outer layer of cells. This is known
as the primitive groove, and has been observed in all amniote embryos. Its exact
significance has never been satisfactorily ascertained, and its interpretation is still a
matter of scientific discussion. A transverse section through the primitive streak of
a vesicle of a common European mole is shown in figure 15. At about the time the
primitive streak appears the embryonic shield becomes oval in form. In those
animals, such as the carnivora and ungulates, which have a large elongated blasto-
dermic vesicle, we find that the long axis of the embryonic shield is nearly at

GROWTH OF THE EMBRYO AND SEPARATION OF THE YOLK.

49

right angles to the long axis of the vesicle. The size of the shield is about the
same in all mammals which have been heretofore studied.

Growth of the Embryo and Separation of the Yolk.

In all vertebrates the development is strictly of the embryonic type, and
accordingly there is made for the nutrition of the embryo some special provision,

?r

FIG. 15.— TRANSVERSE SECTION THROUGH THE PRIMITIVE STREAK OF AN EMBRYO MOLE.

EC, Ectoderm. En, Entoderm. mes, Mesoderm. p.gr, Primitive groove. Pr, Primflfve streak. — (After W.

Heape.)

which in most cases consists of a stock of yolk material; but in the placental
mammals the provision is made by means of the .placenta for the transfer of
nutriment directly from the mother. In either case the embryo has merely to
assimilate the food already more or less prepared for it. It is perhaps owing to
these provisions that the growth of the vertebrate embryo is extremely rapid. In
the amniota there is a fundamental distinction between the embryo proper and its

bi

FIG. 16. — DIAGRAMS TO ILLUSTRATE THE SEPARATION OF THE EMBRYO FROM THE YOLK.
bl, Blastopore. h, Head of embryo. Ach, Archenteron or entodermal cavity, ec, Ectoderm.

so-called appendages — the yolk-sac, chorion, amnion, and allantois. The append-
ages are all finally sacrificed for the benefit of the embryo, and in mammals, except
for a portion of the allantois retained in the body as the anlage of the bladder,
the four appendages are ultimately cast off altogether and take no part in the
construction of the child after birth. We note, in fact, as we ascend the verte-

4

50

THE EARLY DEVELOPMENT OF MAMMALS.

Op.L.

Ao.
Ph.

Mk.

brate' series, an increasing tendency to give the embryo prominence and to differ-
entiate it more decisively from the embryonic appendages. This becomes so
marked in the higher vertebrates that we speak of the growth of the embryo
T0 epen . almost as a separate thing from

the growth of the appendages.

The embryo is developed from
the axial portion of the embryonic
shield, the position of which is
marked by the primitive streak
(Fig. 14, p.s). In the territory
around the embryo are developed
the first blood-vessels, hence it is
termed the area vasculosa (see
Per.cce. Page 66). About the time that
the blood-vessels begin to appear,
the separation of the embryo from
the shield commences, and the
extra-embryonic portion of the
shield remains as part of the
blastodermic vesicle, or yolk-sac.
This separation is due wholly to
the growth of the embryo.*
The process is illustrated by the
diagrams (Fig. 16), in which for
greater clearness the blastodermic
vesicle is represented filled with
yolk, as it is in the Sauropsida.
Soon after the blood-vessels ap-
FIG. 17.— TRANSVERSE SECTION OF AN EMBRYO CATFISH pear, the head of the embryo has

(AMIURUS); SERIES 25, SECTION 43. grown so much that it not only

Ao, Aorta, bas.g, Basal ganglion of mid-brain. EC, Ectoderm. • Hn +fc f f tVi

epen, Ependymal layer of mid-brain, it, Cavity of mid-brain.

L, Lens. Mk, Meckel's cartilage. N.op, Optic nerve, shield, but projects forward (Fig.
Op. L, Optic lobe. Per.cce, Pericardial ccelom. PA, Pharynx. 16, A, K). Later the caudal end
^Pigment layer of the eye *, Retina. To, Torus. Trab, becomes free in the same
Trabecula cranu. x, Undetermined organ. Yk, Yolk.

X4odiams. (Fig. 1 6, B, C). Cross sections

show a similar expansion of the

embryo laterally (compare the three diagrams, Figs. 29, 45, A, and 45, B). ' Hence,
though the connection between the embryo and the blastodermic vesicle may remain

* The separation of the embryo from the rest of the ovum has long been described as a process of the folding
the germ layers on the under side of the body. The traditional perpetuation of this erroneous description
s regrettable, for the separation of the embryo is really due to the expansion of the embryo, and in no sense to the
constriction of its connection with the yolk.

Ec.—\

ORIGIN OF THE MESODERM. 51

unchanged, or even slightly increase in dimension, yet the growth of the embryo
causes that connection to appear relatively small. A connection of i or 2 mm.
equals at first the entire length of the embryo, but a connection of 4 or 5 mm.
seems small when the embryo is 100 or 200 mm. long.

The relations of the embryo to the yolk in the anamniota are illustrated by
the accompanying figure 17, which represents a transverse section through a young
stage of the catfish (Amiurus). The section passes through the head of the
embryo and shows both eyes and the slender optic nerves, N.op, almost symmet-
rically cut on both sides. The yolk, Yk, is a a large mass heavily laden with yolk-
granules. Between the tissues of the embryo proper and of the yolk-sac there is
a direct continuity. Not only can the ectoderm, EC, be followed around from the
embryo over the yolk-sac, but also a layer of mesoderm. The part of the yolk-sac
which carries the yolk grains is, as above stated, a modification of the entoderm.
There is no amnion.

Origin of the Mesoderm.

The development of the primitive streak and groove is accompanied by the
appearance of the third or middle germ-layer, the mesoderm (Fig. 15, mes). As
shown in the section there figured, the three germ-layers are fused together under-
neath the primitive groove, and are there thicker than elsewhere. As we pass
laterally from the groove, the ectoderm and mesoderm both become thinner and are'
distinctly separated from one another. The entoderm consists of a single thin
layer of cells very closely connected with the mesoderm. The mesoderm occupies
at first only a small area in the immediate neighborhood of the primitive streak.
It grows rapidly, so that its edge extends farther and farther over the blastodermic
vesicle. The mesoderm is to be regarded as the product of the entoderm. Its
exact origin in mammals has not yet been adequately traced. We know, however,
that in birds, reptiles, and elasmobranchs the cells of the inner layer multiply
rapidly, so that the inner layer becomes more than one cell thick. The upper
cells soon split off from the lower and thus form themselves into the middle germ-
layer. The mesoderm therefore is said to be formed by delamination. It seems
probable that in mammals the process is the same.

It may be mentioned that, according to Bonnet, the development of the meso-
derm in the sheep is not quite as above described. It can be first distinguished
at the stage when the primitive knot has appeared, and before the primitive streak
is developed. In the fresh specimen it is seen as a slight turbidity of the vesicular
walls just outside the edge of the shield (Fig. 18), while in the region of the shield
there is no middle layer whatever. By the time the primitive streak has appeared
in the sheep, the formation of the mesoderm has extended under the embryonic
shield, and the relations between the germ-layers then become essentially as above
described.

The cells of the mesoderm are at first quite closely packed, but as the layer

52

THE EARLY DEVELOPMENT OF MAMMALS.

grows they begin to move apart, though remaining connected with one another by
protoplasmic processes. The cells separate least near the primitive streak, but their
separation becomes gradually more and more marked toward the periphery of the
layer, as shown in Fig. 19, which represents a part of the peripheral region of the
mesoderm of a blastodermic vesicle of a rabbit of seven days.

In the details of its expansion the mesoderm varies greatly in different mammals.
In some forms it develops very early and rapidly expands over the entire blasto-

m

FIG. 18.— CENTRAL PORTION OF A SHEEP'S BLASTO- FIG. 19.— BLASTODERMIC VESICLE OF A RABBIT OF

DERMIC VESICLE OF TWELVE TO THIRTEEN SEVEN DAYS. PORTION OF THE MESODERM

-°AYS- OF THE AREA OPACA. — (After Kdlliker.)

Sh, Embryonic shield, kn, Hensen's knot, mes,
Shadow caused by mesoderm developing around
the shield. X 34 diams. — (After Bonnet.)

dermic vesicle, which then becomes three-layered. This seems to be the method of
its growth in man and other primates. In other cases, as in the dog and cat, it
grows more slowly, but ultimately encloses the entire entoderm. In the rabbit,
on the contrary, it never expands more than about three fifths of the way over the
blastodermic vesicle, one part of which, therefore— viz., that opposite the embryo—
never has any mesoderm whatever. This, however, is to be regarded as a special
modification, since we must consider that primitively the mesoderm extended over
the entire vesicle.

The Primitive Axis.

The next stage of development is characterized by the appearance of an accu-
mulation of cells which extends forward from the primitive knot in the axial line.

THE NOTOCHORDAL CANAL. 53

This thickening is termed the primitive axis. German writers commonly designate
it as the "head process" (Kopffortsatz) . The primitive axis may be easily distin-
guished in transverse sections from the primitive streak by the fact that in the for-
mer the thickening occurs in the mesoderm and entoderm, which are closely united,
and it is separated from the outer layer; whereas in the latter the cells of the
thickening are fused with both the entoderm and the ectoderm (compare Fig. 126,
A and C, page 171).

The primitive axis corresponds to the region in which the body proper of the
embryo develops, and represents the beginning of embryonic development in this
restricted sense. It grows quite rapidly in length and width, and as it grows en-
croaches more and more upon the territory of the primitive streak, which is grad-
ually obliterated by merging into the caudal end of the developing embryo, so that
it can no longer be distinguished. The obliteration of the primitive streak is grad-
ual, and there is a series of stages easily observed in amniota in -which we find the
embryonic development in the region of the primitive axis more or less advanced,
while part of the primitive streak still presents to us, more or less clearly, its
original condition.

The Notochordal Canal.

In regard to this canal our knowledge is imperfect. Any account of it which
we can give may need correction. It is a very small canal which runs through the
center of the primitive axis. It ends blindly in front, but opens through the ecto-
derm at its posterior end, at a point corresponding perhaps exactly to the position
of the primitive knot. The first indication of the formation of the canal is an al-
teration in the form of the cells in the center of the primitive axis. These cells
elongate in directions at right angles to the axis. Their nuclei become oval and are
radially placed. The change' begins posteriorly and progresses forward. The radial
cells move apart, so that there arises a longitudinal canal. It may happen that in
mammals, as in birds, the canal is not actually open at its posterior end. If that
should be found to be the case in any instance, it would not alter our interpreta-
tion, for we should then consider that the walls had simply closed togethe r. There
are many instances of tubular structures being temporarily solid in embryonic stages.
Such a condition, for example, has been observed in the oesophagus of elasmo-^
branchs, in the large intestine of birds, and in other cases.

The opening of the notochordal canal is termed the blastopore, and is suppose'd
to be identical with the blastopore of the anamniota.

After the notochordal canal is formed the blastodermic vesicle has, of course,
two cavities: first, the small cavity of the canal; second, the large main cavity of
the vesicle which is surrounded by entoderm. This larger space is designated as
the yolk-cavity. After the canal has acquired a not inconsiderable length its lower
wall develops a series of irregular openings (Fig. 20, nch] on its ventral side, by
which it comes into communication with the large underlying yolk-cavity. These

54

THE EARLY DEVELOPMENT OF MAMMALS.

openings grow until the ventral wall of the notochordal canal is entirely lost. We
then have the two cavities completely fused, making a single cavity bounded by a
continuous layer of cells, the majority of which represents the lining of the yolk-
cavity, but the small minority represents the cells of the notochordal canal. The
continuous layer of cells is known as the permanent entoderm, and the cavity itself,
which is of double origin, is termed the archenteron. At about this time, probably
sometimes earlier, sometimes later, according to the species, the blastopore becomes
permanently closed and the entodermal cavity no longer has an opening to the
exterior. v

i2Si Atnn

FIG. 20. — GERMINAL AREA OF A GUINEA-PIG AT FIG. 21. — LONGITUDINAL SECTION OF THE POSTERIOR

THIRTEEN DAYS AND TWENTY HOURS, SEEN
FROM THE UNDER (ENTODERMAL) SIDE.
o. a, Area opaca. a.p, Area pellucida. nch, Noto-
chordal canal with several irregular openings
through the entoderm. — (After Lieberkilhn.)

END OF A SHEEP EMBRYO OF SIXTEEN DAYS.
Amn, Amnion. a.m, Anal membrane (or plate), pr.s,
Primitive streak. En, Entoderm. Ach, Archenteron,
or entodermal cavity of the embryo. All, Anlage
of allantois. mes, Mesoderm. — (After R. Bonnet.)

In a number of vertebrates it has been demonstrated that the blastopore is
soon divided into two parts: one anterior, which frequently remains open, and
gives rise to the neurenteric canal, and one posterior, which gives rise to the anal
opening. When the spinal cord (medullary canal) is developed it extends so far as
to include the neurenteric canal and exclude the anus. The neurenteric canal is
obliterated during early embryonic life, but so long as it remains open it constitutes
a free communication between the archenteron and the medullary tube (spinal
cord). The anal opening is early closed by a growth of the surrounding cells,
which produces an occluding membrane known as the anal plate (Fig. 21, a.m).
The plate includes a layer of ectodermal and of entodermal cells, but apparently
no mesoderm. It persists for a long time and undergoes a considerable growth,
but ultimately it is perforated to form the permanent anus.

The cells on the dorsal side of the notochordal canal have a different destina-
tion, for they become thickened to make the anlage of the future notochord. It
is to this fact that the canal owes its name.

THE NOTOCHORD.

55

The Notochord.

The notochord (chorda dorsalis) is a rod of peculiar tissue constituting the
primitive axial skeleton of vertebrates. It begins in the embryo immediately
behind the pituitary body and extends to the caudal extremity. It occurs as a
permanent structure in all vertebrates, but undergoes much modification in the
amniota. It appears very early in the course of development, being differentiated
from the median dorsal wall of the notochordal canal, beginning at a time when
the medullary groove (compare page 68) is not fully marked out posteriorly, and is
nowhere closed. The notochordal anlage can be first detected as an axial band
of cells, which at first is not well marked off from the mesoderm of the primitive
axis. The anlage is thicker than the
adjacent entoderm (Fig. 22, nek). The
differentiation of the notochordal cells
begins usually at the anterior end of
the canal and progresses backward. It
appears merely as a specialized part of
the entoderm of the archenteron, but
has a very sharp demarcation. via

FIG. 22. — TRANSVERSE SECTION OF A MOLE EMBRYO
(HEAPE'S STAGE H).

segment. CCR, Coelom. En, Entoderm. nch,

Notochdrd. ao, Aorta, -vt.a, Vitelline artery.

Som, Somatic mesoderm. Spl, Splanchnic meso-
derm.— (After W. Heape.)

The notochordal anlage separates off
and the entoderm proper closes across

under it, SO that the notochordal band am, Amnion. Md, Medullary groove. My, Primitive

lies between the entoderm and the over-
lying ectoderm (floor of the medullary
groove or canal). The two primitive germ-
layers come into actual contact in the

median line, along which, therefore, when the notochord first separates from
the entoderm, there is no middle germ-layer present. This condition exists
in the chick with eight segments described in Chapter V. The separation
of the anlage does not take place at the anterior extremity of the notochord
until somewhat later, so for a considerable period the cephalic end of the noto-
chord remains fused with the entoderm. The separation from the entoderm is
effected in mammals by the entoderm proper shoving itself under the notochord
toward the median line. When the cells from one side meet those of the other
they unite with them and form a continuous sheet of entoderm below the noto-
chordal cells. The process of separation may be followed easily in the develop-
ment of the frog and toad.

After its separation the notochord is a narrow band of cells, which starts
anteriorly from the entoderm (the future lining of the alimentary tract), running
backward to the blastopore. So long as the blastopore or neurenteric canal is
open the notochord terminates in the epithelium lining it. For a certain period
the notochord continues to grow tailward by accretion of cells from the walls of
the blastoporic passage; and after the canal is permanently obliterated, the noto-

56

THE EARLY DEVELOPMENT OF MAMMALS.

chord may still continue to lengthen by acquisitions at its caudal end of additional

cells from the primitive streak.

After it is once formed as a band of cells,
the notochord passes through various changes of
form, but ultimately becomes a cylindrical rod with
tapering extremities. It attains" considerable size in
the embryos of most vertebrates, but in those of
placental mammals it is always small. It is prob-
able that in mammals the notochord, when first
separated from the entoderm, is a broad, flat band,
and that this band subsequently draws together,
diminishing its transverse and increasing its vertical
diameter until it has acquired a 'rounded form.
Finally its outline becomes circular in cross-section.
This series of changes begins near the anterior end
of the notochord and progresses both forward and
backward.

In later stages the mes/)derm again grows
across the median line of the embryo, completely
surrounds the notochord, and forms a special
sheath about it. Still later the mesoderm forms a
broad envelope around the notochord, which we
can soon recognize as the anlage of the chondro style,
out of which the vertebral column and part of the
base of the skull are to be differentiated. Very
soon (Fig. 23) the chondrostylic anlage shows a
series of transverse discs of denser tissue, the
anlages of the intervertebral ligaments, the broader
light spaces between the discs being the anlages of
the vertebrae. In mammals, the notochord assumes
an undulating course, which may be slightly irregular
at first. The typical arrangement is shown in the
figure— the dorsal summit of each flexure is in-
tervertebral, the ventral hollow of each flexure is
vertebral.

FIG.33.-NOTOCHORDANDCHONDROSTYLE ^ UltimatC *** °f the NotOChOrd.

OF A SHEEP EMBRYO OF 14.6 MM. As the vertebral column develops, the notochord

RECONSTRUCTION FROM SAGITTAL slowly disappears in the regions of the vertebra and
SERIES nog. SECTIONS IQO-IQ? ,1

even the space occupied by it is obliterated by the

growth of the body of the vertebra. In the intervertebral discs, on the contrary,
the notochord persists to form the nuclei pulposi of the adult. Each nucleus is

THE ARCHENTERON.

57

enlarged, first, by the withdrawal of the notochordal cells from the vertebrae into the
adjacent intervertebral discs; second, by the growth of the tissue. The cavities
occupied by the nuclei have distinct boundaries and present characteristic forms in
different mammals. The sheath of the notochord is lost, the walls of the cells dis-
appear, the tissue becomes a syncytium (Fig. 24) of granular appearance, and
breaks up into multinucleated reticular masses,
making an irregular network the meshes of which
are filled with a more or less homogeneous sub-
stance resembling mucin, that does not, however,
agree with mucin in its reactions. Tissue of this
character may be easily observed in human embryos
of the third and fourth month. It has been not
infrequently stated that the notochord disappears in
mammals, and that it contributes to the formation
of cartilage. Both statements are now known to
be erroneous. Owing to the persistence of the
nucleus pulposus, the vertebral joint differs funda-
mentally from all other joints in the body of the adult.

FIG. 24. — PIG EMBRYO OF 150 MM.
Notochordal syncytium from nucleus
pulposus. X 800 diams. — (After L.
W. Williams.)

The Archenteron.

The archenteron comprises the entire cavity bounded by the entoderm. At
first it consists chiefly of the cavity of the yolk-sac (Fig. 25), but as it also in-
cludes the embryonic entodermal tract its development in the embryo greatly pre-
dominates as growth continues. As the head of the embryo protrudes, the archenteron
forms a cephalic prolongation, known as the fore-gut (Figs. 25, Vd, and 132, Vd), which
ends blindly in front, but opens behind (caudad) into the general archenteric space,
its opening being termed the fovea cardiaca, fo. Later as the caudal region becomes
protuberant the archenteron sends into it a similar blind prolongation, known as
the hind-gut (Fig. 25, H.g). .As the embryo grows — compare the section on growth,
page 49 — the connection between the embryo and the yolk-sac, which seems so
large in early stages (Fig. 25), increases very little, and therefore becomes relatively
smaller. It never attains more than 3 or 4 mm. The embryo, on the contrary,
grows enormously (Fig. 34), and there is a corresponding enormous lengthening of
the fore-gut and hind-gut. The former is the anlage of the pharynx, oesophagus,
and stomach. The latter is the anlage of the large intestine and most of
the ileum.

During embryonic life the archenteron is divided by the obliteration of the con-
nection between the yolk-sac and the embryonic entoderm. For a time the ori-
ginal point of connection is marked by a small pouch (MeckeVs diverticulum) of the
ileum. The pouch normally disappears, but as an occasional anomaly (arrest of
development) it persists in the adult.

58

THE EARLY DEVELOPMENT OF MAMMALS.

The Oral and Anal Plates.

These two structures resemble one another. Each occupies a small area and is
formed by the intimate union of the entoderm with the ectoderm. When the union
is first formed the two layers are distinct, but they soon fuse, so that no boundary
can be recognized between them. Ultimately both plates break down, their cells

Am.

Cho.

Bs.

- U.A.

Yk.s.

fo.

FIG. 25.— WAX RECONSTRUCTION OF DANDY'S HUMAN EMBRYO WITH SEVEN SEGMENTS BY FREDERICK T. LEWIS.

All, Allantois. Am, Amnion. Bs, Body stalk. Cho, Chorion. fo, Fovea cardiaca. H.g, Hind-gut. Ht, Heart.
Kn, Hensen's knot. Som, Somatopleure enclosing the pericardial cavity. U.A, Umbilical artery. U.V,
Umbilical vein. Vd, Fore-gut, ve, Blood-vessel. Yk.s, Wall of yolk-sac. X 40 diams.

disappearing, and they are replaced by openings, that of the oral plate forming the
opening between the mouth-cavity and the pharynx, that of the anal plate forming
the primitive anal opening. The anal plate, before it breaks down, makes a con-
siderable growth, forming an epithelial mass which plays an important part in the
anatomical modeling of the region. The oral plate disappears very early; the anal
plate much later.

THE DIGESTIVE CANAL.

59

As soon as the head of the embryo has grown so much as to project as an in-
dependent part, we find that the oral plate lies on the under surface of the head, a
little in front of the heart (Fig. 26). The pro-amnion, pro. am, arises from the
somatopleure enclosing the heart, ht, so that, when the oral plate becomes perforate,
the cavity of the entoderm, Ent, will communicate directly with the cavity enclosed
by the pro-amnion, or, in other words, with the permanent amniotic cavity. Figure
72, o.pl, shows the oral plate in a little later stage, shortly after which the plate
ruptures.

A similar anal plate at the
posterior end of the embryo also
lies within the amnion (Fig. 21).
This figure is taken from a sheep
embryo in a very early stage, so
that the anal plate appears to lie
on the dorsal side. By the curl-
ing ventralward or the bending
over of the tail end of the young
embryo the anal plate is gradually
transferred or rolled over on to
the ventral side, where it perma-
nently remains, For the relation
of the anus to the blastopore see
page 54.

pro.anv

FIG. 26. — LONGITUDINAL SECTION OF THE HEAD END OF A

MOLE EMBRYO, STAGE H.

EC, Ectoderm. En, Entoderm. Ent, Anterior end of archenteric
cavity with the oral plate on the cardiac side, fb, Fore-
brain. ht, Heart, m.b, Mid-brain. Mes, Mesoderm. nch,
Notochord. pro.am, Pro-amnion. — (After W. Heape.)

The Digestive Canal.

The digestive canal proper is developed by the growth and modifications of the
fore-gut and hind-gut. The division between the two is a point in the ileum corre-
sponding to the original connection with the yolk-sac, marked in the fetus by
Meckel's diverticulum.

The fore-gut forms the pharynx (and lungs), the oesophagus, stomach, duode-
num, and part of the ileum. It also produces, as appendages to the canal, the
liver and pancreas.

The hind-gut forms most of the ileum and the entire large intestine, together
with the caecum and appendix.

The entoderm persists as the permanent epithelial lining, and produces all the
glands of the digestive tract. It remains a thin layer throughout life. The meso-
derm forms the greater part of the walls, furnishing the connective tissue, the
smooth muscle layers, and the peritoneum, which last consists of the original meso-
thelium and a thin layer of chiefly fibrillar connective tissue.

The general course of the development is shown by figure 27, which represents
outlines of the entodermal canal in three human embryos, uniformly magnified
twelve diameters. An earlier stage is shown in figure 25.

60

THE EARLY DEVELOPMENT OF MAMMALS.

The fore-gut in the 4.2 mm. embryo has lengthened. It communicates freely
with the oral cavity proper, the limit of which is indicated by the hypophysis, Hy,
which is of ectodermal origin. The cephalic portion of the canal has undergone a

FIG. 27.-— OUTLINES OF THE NOTOCHORD AND ENTODERMAL CANAL OF THREE HUMAN EMBRYOS. A, 4.2 MM.

B, 7.0 MM. C, 13. 8 MM.

Al, Allantois. Ch, Notochord. Col, Large intestine. E, Caudal intestine. Ep, Epiglottis. Hy, Hypophysis.
in, Small intestine. La, Larynx. Li, Liver. Li.d, Hepatic duct. Lu, Lung. Md, Mandible. N, Renal
anlage. «?, (Esophagus. P, Dorsal pancreas. St, Stomach. T, Tongue. Th, Thyroid gland. Ur,
Ureter. W, Wolffian duct. Yk.s, Yolk-sac. 1,2,3, nrst> second, and third gill- pouches. Xi2diams. —
(After W. His.)

great widening to form the pharynx, with its characteristic gill-pouches (Fig. 27, A,
i, 2, 3) — compare below. From the caudad end of the pharynx, the anlage, Lu, of
the trachea and lungs has appeared on the ventral side. From the pulmonary an-

THE DIGESTIVE CANAL. 61

lage, Lu, to the hepatic, Li, extends a short tube which comprises the future
oesophagus, stomach, and part of the duodenum. The liver, Li, which arose as
an outgrowth of the entoderm at the fovea cardiaca, has enlarged and become
distinctly an appendage. Between the liver and the yolk-sac, Yk.s, is a short
broad tube, the beginning of part of the small intestine. In the 7.0 mm. embryo,
the fore-gut is much longer, and the differentiation of the oesophagus, oe, stomach,
St, and duodenum, from which the anlage of the dorsal pancreas, P, has developed,
is established. The liver is connected with the duodenum only by the narrow
hepatic duct, Li.d, between which and the yolk -stalk, Yk.s, there is a consider-
able stretch of small intestine. In the 13.8 mm. embryo, the relations have been
greatly altered by the growth and migration of the stomach (Fig. 27, C, Si) which
has descended from its original position into the abdomen, so that it is caudad of
the diaphragm, and lies asymmetrically placed on the left side of the embryo. The
stomach also turns so that its cesophageal end is toward the left, its duodenal
end toward the right, and further revolves so that its left surface faces ventrally.
In the 13.8 mm. embryo, the migration and revolution of the stomach has not ,
been completed. The descent of the stomach involves the elongation of the oeso-
phagus (Fig. 27, C, oe) and the twisting of the duodenum.

The hind-gut has a simpler history. In the 4.2 mm. embryo it has elongated
and terminates blindly in the tail. Its caudal end is somewhat enlarged to form
the cloaca, into which open also the Wolffian ducts and allantois (Fig. 27, A, W
and Al). Between the cloaca and the yolk-sac, Yk.s, extends the cephalad por-
tion of the hind-gut, nearly uniform in diameter. In the 7.0 mm. embryo the
conditions are similar, but the intestinal portion has lengthened and bent ventral-
ward. The insertion of the yolk-stalk, Yk.s, marks the apex of the primitive
intestinal loop. In the 13.8 mm. embryo, the loop has greatly lengthened and
projects into the cavity of the umbilical cord (extra-embryonic ccelom), and a blind
pouch, Coe, has appeared, the anlage of both the caecum and the appendix. It
marks the boundary between the large and small intestines, which as yet differ
very little in diameter.

For some time a portion of the intestine lies in the umbilical cord, and may
form several coils there, but gradually it is withdrawn so as to lie wholly within
the abdomen proper.

The pharynx undergoes many modifications in form, and also produces an im-
portant series of accessory organs, including the thyroid gland, the tonsils, and the
thymus. It comprises the cephalic portion of the fore-gut and originally overlies
the heart (Figs. 25 and 132). The stretch of the fore-gut, which extends from the
pharynx to the fovea cardiaca, remains at first short and narrow, most of the fore-
gut being absorbed in the pharynx, which is produced by the expansion of the
entodermal tube toward both sides of the neck; but the dorso- ventral diameter
remains small. The expansion is greatest a short distance behind the mouth, and
thence diminishes gradually toward the oesophagus, so that the pharynx of the em-

62

THE EARLY DEVELOPMENT OF MAMMALS.

bryo has a rhomboidal form which is complicated, however, by certain irregularities.
These are due to the formation of the gill-pouches, of which there are four distinct

car. I.

Hy.

car.in

pairs in mammals. Some authorities maintain that the ancestors of
had five pairs, and that the pair lost was situated between the present third and
fourth pairs (compare the remarks on "Zimmermann's arch," page 101). Each pouch
is a lateral pocket of the fore-gut, having a tapering form, the apex of which comes
into contact with the ectoderm. At the point of contact, entoderm and ectoderm
fuse to constitute a closing plate, similar to the oral and anal plates. In aquatic
vertebrates the closing plates are lost, and each gill-pouch becomes a true gill-cleft.

The positions of the closing plates
soon after their formation are marked
by an external depression, the ecto-
dermal gill-pouch (Figs. 89 and 94).
The columns of tissue in front of the
first pouch, behind the last, and be-
tween the first and second, the second
and third, and the third and fourth
are known as the five branchial
arches. The first arch is called the
mandibular, the second the hyoid.
In each branchial arch an aortic arch
is developed (see page 99). In mam-
mals each pair of pouches has a
characteristic form in the embryo and
a characteristic differentiation. In the
12.0 mm. pig the first pouch (Fig. 28,
I), has a broad base, tapers toward

apex rises

the second

IV.

Bu.

FIG. 28.— PIG EMBRYO OF 1 2 MM. SERIES 518. OUTLINE the ectoderm, and its
OF THE PHARYNX AS SEEN FROM THE DORSAL SIDE. j ,u j , . ,

FROMAWAXMODELBYA.R.KILGORE. tOWard the dorsal Slde

I, II, in, iv, Gill-pouches. 2, 3, 4, 5, Aortic arches, pouch, II, occupies a more horizontal

Ao, Aorta. Bu, Bursa pharyngis. car. in, internal plane and in form somewhat resembles

arte01!"' ^J^f -^n" T ^^^ the first, with which it is partially

artery. Hy, Hypophysis. Oe, (Esophagus. X 22

diams. merged. The third pouch, III, is

much smaller and is expanded at its

end by a prolongation downward and inward; the prolongation has a somewhat
tubular form and extends far toward the aortic end of the heart; in the dorsal
view of the model it does not show. The fourth pouch, IV, is much smaller than
the others; it resembles the third pouch in having a ventral prolongation, but is
quite variable in form.

The entodermal epithelium of the second to fourth pouches exhibits certain
specializations. One type is illustrated by the tonsil and thymus— the epithelium
is thickened, assumes a reticular structure, and its meshes are invaded by leucocytes.

THE YOLK-SAC.

63

Som

Coe

Another type is illustrated by the epithelial bodies, which are small masses of com-
pact cells resulting from a local epithelial growth, and penetrated by blood-vessels
(sinusoids) — this type includes the parathyroid, nodulus thymicus, and post-
branchial body.

The first gill-pouch becomes the Eustachian tube, the blind distal end being
expanded into the tympanum.

The second pouch is partly obliterated, but its ventral part is converted into
the tonsil.

The third pouch forms an epithelial body, the nodulus thymicus (Fig. 194, Nod)
and its ventral caecal prolongation is converted into the thymus. Its epithelium
is said also to produce a parathyroid.

The fourth pouches give rise to a pair of parathyroids and to the post-branchial
bodies, which develop from the ventral prolongations of the pouches.

The thyroid gland begins as a median evagination of the entoderm on the
ventral side of the pharynx. It starts very early Spl

(human embryo of 3 mm.). The blind end of the
evagination becomes first bilobed, then branching
—the branches are the anlages of the adult fol-
licles. The duct of the gland is soon obliterated,
but its point of origin is often permanently marked
by the foramen caecum at the back of the tongue.

The Yolk-sac.

General Morphology. — The yolk-sac is the con-
tainer of the nutritive yolk destined to be assim-
ilated by the embryo. The principal factor in
its morphological constitution is the entoderm,
which, after the differentiation of the definitive
germ-layers, contains nearly all of the yolk mate-
rial. In the primitive vertebrates, as exemplified
by the marsipobranchs, ganoids, dipnoi, and am-
phibia, we find this yolk material lodged in the C<B> Coeiom. in, Intestinal cavity. Som,

walls Of the primitive digestive tract. It is Situated Somatopleure. Spl, Splanchnopleure.

chiefly on the ventral side of this tract and extends from the point where the heart
is formed toward the tail of the embryo to the point where the allantois is formed.
In other words, it is situated in a region ^corresponding to the territory of the
future abdominal cavity. In the primitive types just referred to, the yolk-bearing
entoderm becomes divided into distinct cells which form a large mass. The con-
dition may be understood from figure 44, which represents a transverse section of
the early stage of an axolotl embryo. The cavity of the entodermal canal (digest-
ive tract) is small. It is bounded on its dorsal side by a single layer of cells
distinctly epithelial in their development, and on the ventral side by a great mass

FIG. 29.— DIAGRAMMATIC SECTION OF
THE YOLK OF A HEN'S EGG AT AN
EARLY STAGE TO SHOW THE RELATION
OF THE PRIMITIVE ENTODERMAL
CAVITY, Ach.

04

THE EARLY DEVELOPMENT OF MAMMALS.

of rounded cells heavily laden with yolk-granules, and containing conspicuously
large nuclei. These large nuclei differ by their size and minute structure very
much from the other nuclei in the embryo. The corresponding nuclei in higher
animals are sometimes called parablast nuclei. Outside of the entoderm comes the
second portion of the yolk-sac, the splanchnic leaf of the mesoderm. If we
imagine the amount of yolk to be gradually increased, so that it would appear
more distinct from the embryo proper, we should then apply to it the term extra-
embryonic. The yolk-sac of the higher forms differs from that of the lower forms
only by its size, as is illustrated by figure 29, which represents a diagrammatic
transverse section of an early stage of the chick, before the formation of the
amnion has begun. The essential relations may be seen by comparing figures 29,

Ent

FIG. 30. — WALL OF THE YOLK-SAC IN THE REGION OF THE AREA OPACA OF A CHICK OF THE SECOND DAY.
Mes, Mesoderm. V,V, Blood-vessels, containing a few young blood-cells. Ent, Entoderm. c, Four distinctly

shown entodermal cells.

44i and 45. As shown in the section (Fig. 29), the yolk-sac, if we may so call it,
is completely enclosed by the somatopleure of the embryo, and in the amniote
embryo the condition is the same. The yolk-sac is surrounded by the somato-
pleure, which, however, in the amniota we call extra-embryonic. The extra-
embryonic somatopleure around the yolk-sac is called in birds the membrana serosa,
and in mammals the chorion.

In amniota we can distinguish in the entoderm of the embryo, or yolk-sac,
three distinct regions. The first of these includes the whole of the entoderm of the
embryo and a certain territory around it. In this region, after the earliest stages
are passed, the entoderm is found to be a very thin layer and to contain very few
yolk-granules, and such few as it contains are small. This portion of the ento-
derm, therefore, seems translucent, an appearance which can easily be noted with
the naked eye, and which has led to the name area pellucida, which has long been
applied to this region. The region all around the area pellucida appears in the
fresh specimen darker, and this is called the area opaca, the second region. The
entoderm in this part consists of columnar cells (Fig. 30, c, and Fig. 31). In the
chick the cells are high cylinder cells of somewhat irregular shape, containing a
loose network of granular protoplasm. The lower ends of the cells are rounded

THE YOLK-SAC. 65

and projecting, and have a well-marked border of dense protoplasm. The nuclei
are variable in size, but for the most .part large, often three or four times greater
in diameter than the neighboring mesodermic nuclei. They usually have one,
sometimes two, conspicuous nucleoli. The nuclei always lie at the upper or basal
ends of the cells, chiefly near one side of the cell. The cells contain yolk-grains
which appear to be undergoing resorption. Toward the area pellucida the cells
are smaller, the network of protoplasm closer, and the yolk-grains are either absent
altogether or, if present, small in size and few in number. The transition to the
thin entoderm of the area pellucida is quite abrupt. In the opposite direction the
area opaca passes gradually, by changing its structure, into the general mass of the
yolk, or area vitellina, the third of the regions of the yolk-sac, so called because it
contains the bulk of the yolk material. The transition of the area opaca into the

FIG. 31. — WALL OF THE YOLK-SAC IN THE REGION OF THE AREA OPACA OF A RABBIT EMBRYO OF THIRTEEN DAYS.
V, Blood-vessels containing young red blood-cells, bl. mes, Mesoderm.

area vitellina is marked by a considerable accumulation of cells which are arising
from the yolk. This accumulation of cells is called the germinal wall. It is the
connecting-link between the epithelium on the dorsal side of the entodermal cavity
and the yolk or area vitellina, which forms the .ventral boundary of the cavity. If
we follow successively the stages, we find that the area pellucida grows at the
expense of the area opaca, and the area opaca at the expense of the area vitellina.
These facts are to be interpreted as phases in the process of the assimilation of
the nutritive yolk. The thin cells of the area pellucida are those in which the
absorption of the yolk has been completed. The larger cells of the opaca are
those in which the assimilation is going on, and it can be easily seen that it is
most advanced in those cells which are nearest the embryo and least advanced in
those cells which are nearest to the germinal wall. In mammals the area pellucida
is well marked and resembles that of birds. The area opaca has well-defined
cylinder cells (Fig. 31) which have rounded ends, but are much smaller than in
birds and contain very little yolk material. Cells of this character extend over also
what we should call the area vitellina, which does not present the special features
which it has in birds, for the reason that the yolk in mammals is so small in
amount and the yolk-sac, therefore, is hollow. Later on the cells pass through
degenerative changes, which need to be more exactly studied. In man the degen-

66

THE EARLY DEVELOPMENT OF MAMMALS.

erative change in the cells of the yolk-sac takes place very early. The mesoderm
of the yolk-sac is at first a thin layer. Very early there appears an angioblast, or
the anlage of the first blood-vessels and blood. In all cases in which the process
has been accurately followed the angioblast makes its first appearance in the region
of the area opaca, where it forms a network of primitive blood-vessels close
against the surface of the yolk. The region occupied by these blood-vessels is
called the area vasculosa. Its boundary in the direction away from the embryo is
everywhere well defined. Gradually the development of blood-vessels progresses
from the region of the area opaca into the region of the area pellucida and extends
into the body of the embryo. We even have the embryo almost completely sur-

raes

FIG. 32. — SECTION OF THE YOLK-SAC OF A YOUNG

HUMAN EMBRYO.
En, Entoderm. mes, Mesoderm. v, Blood-vessels.

—(After Keibel.)

FIG. 33. — HUMAN EMBRYO, 2.15 MM. LONG.
(After W. His.)

rounded by a region of extra-embryonic blood-vessels — the definitive area vasculosa.
Now, it will be remembered that the area opaca is the territory in which the
entodermal cells are actively assimilating the yolk, and we must believe that the
blood-vessels which are thus early developed in close contact with the cells of this
area are destined to take up food material digested by the entodermal cells and
carry it to the embryo. Hence we interpret the early development of the extra-
embryonic vessels as due to physiological necessities.

The mesoderm at first forms a very thin layer over the angioblast. It next
thickens by the multiplication of its cells, and we can then distinguish in it both
the outer mesothelium and the inner mesenchyma. The mesothelium is the per-
manent external cover of the yolk-sac. The mesenchyma grows in between the
primitive blood-vessels, and finally penetrates, at least in part, between the blood-
vessels and the entoderm of the yolk-sac, a condition which is reached very early
in the human embryo (Fig. 32)..

The human yolk-sac is characterized by its small size and by the precocious
expansion of the area vasculosa, so that in the very earliest stage known to us by

THE ORIGIN OF THE NERVOUS SYSTEM.

67

observation blood-vessels are found over the entire sac. At the beginning of the
third week the diameter of the yolk-sac is about equal to the length of the embryo
(Fig. 25). By the end of the third week the sac has become distinctly pear-
shaped, its narrower pointed end being that by which it is connected with the
intestinal canal of the embryo (Figs. 33, 34). The sac continues growing, up to
the end of the fourth week, after which it enlarges very slightly, if at all. Its
diameter is only from 7 to n mm. It is then a pear-shaped vesicle attached by
a long stalk to the intestine, the stalk having been formed by the lengthening of
the neck of the yolk-sac. The cavity of the stalk early becomes obliterated and
the«entoderm in the stalk disappears altogether.

FIG. 34. — HUMAN EMBRYO OF 2.6 MM.— (.4/ter W. His.)

The Origin of the Nervous System.

It will be remembered that the ectoderm of the embryonic shield has at first
a considerable thickness, for it consists of cuboidal or low cylindrical epithelial
cells. The stage which follows next after the appearance of the primitive axis is
characterized by the gradual thinning out of the ectoderm over the peripheral por-
tions of the shield, while in the neighborhood of the axial line the full thickness of
the outer germ-layer is not only retained, but is actually increased. For a time
there is a gradual passage between thicker and thinner parts, but as development
progresses the demarcation rapidly becomes sharper. By these steps the differentia-
tion of the anlage of the central nervous system is accomplished. The thicker cen-
tral portion of the ectoderm constitutes the medullary plate, which begins to appear
shortly after the formation of the primitive streak.. It extends over the primitive
axis, the primitive knot, and the anterior end of the primitive streak (Fig. 35, Md),
and also extends some distance to the right and left of the axial line. It is rounded

68

THE EARLY DEVELOPMENT OF MAMMALS.

A. p. A.o.

md.F

in front, also behind, where, however, it gradually fades out. At the same time
that the medullary plate is being thus differentiated, the central portionbe comes de-
pressed, making the conspicuous furrow, md.F, which begins just in front of the primi-
tive knot and extends nearly to the anterior edge of the medullary plate. This
axial depression is known as the dorsal furrow. Its appearance is shown in cross-
section as illustrated by figure 36, /. The furrow is narrow and deep. Its upper
edge is rounded or curving. By the formation of the furrow the ectoderm of the

medullary plate is brought into actual con-
tact with the anlage of the notochord (Fig.
36, ch), so that the mesoderm can be no
longer in the median line and is conse-
quently divided into right and left parts, as
above mentioned in describing the formation
of the notochord. As the blastopore lies at
or near the primitive knot, it becomes
partly included in the medullary plate. It
may remain open while the medullary plate
is being transformed into the nervous system,'
and in that case may establish a connection
between the cavity of the central nervous
system and that of the entoderm. Such a
communication is termed a neurenteric canal.
Figure 79 represents a wax model recon-
structed from the sections of a human em-
bryo in the stage of the medullary plate.

Md

Kn

pr.s

FIG. 35. — SURFACE VIEW OF THE EMBRYONIC

SHIELD OF A DOG EMBRYO, WITH MEDULLARY It: snOWS clearly the form of the plate, the

PLATE.

deep dorsal groove, the opening of the
A.o, Area opaca. A.p, Area pellucida. Kn, Hen- neurenteric canal, and the remnants of the

sen's knot. Md, Medullary plate. md.F, ... .

Medullary furrow, pr.s, Primitive streak, x .Primitlve groove behind the canal. As the
15 diams. development progresses the medullary plate

extends farther backward and encroaches
upon the territory of the primitive streak until this latter is obliterated.

The Medullary Groove.— Almost or quite as soon as the medullary plate is
formed its lateral portions begin to arise on each side, so that the two halves of
the plate together form a broad open trough known as the medullary groove,
into which, of course, the dorsal groove is merged, so that it no longer can be
recognized (compare Figs. 22 and 147). While the groove is being formed
the medullary plate increases considerably in thickness. The nuclei multiply
rapidly and lie irregularly scattered at various heights. The ectoderm alongside the
medullary plate or groove thins out still further. The development is most rapid
at a point corresponding to the posterior region of the future head. The farther
from this point we go, the less advanced do we find the formation of the groove.

THE STRUCTURE OF THE MEDULLARY CANAL.

69

so that at a certain stage there is a well-marked medullary groove in the cephalic
region, the medullary plate behind that, and the primitive streak at the hind end
of the embryo. But when the streak has disappeared, the medullary groove is
found to extend the entire length of the embryo. Owing to this peculiarity, it is
possible in a single embryo to follow all the principal stages of the formation of
the medullary groove by the examination of a series of transverse sections. Such
a stage is found in the rabbit at nine days, .or in the chick at from thirty to forty
hours of normal incubation (Figs. 129, 130, and 147).

The Medullary Canal. — The medullary
groove gradually deepens, its sides rising
higher and higher and arching more and
more toward one another until the edges
meet and coalesce, thus changing the groove
into a tube — the medullary canal (Figs. 37
and 38, Md}. The closure of the groove
occurs in the cervical region first, and
spreads from there in both directions. As
the closure progresses forward it completes
the canal in the region of the head. It
occurs in such a manner that there is a
very small opening, which is the last point
to close. This opening seems to be a fixed /, Dorsal furrow,
point, occupying always the same relative
position in all vertebrates. It is called the
anterior neuropore. At this time the caudal
end of the medullary groove may be still
open widely, forming the so-called rhom-

boidal sinus, compare figures 129 and 130, and it is the last portion to close. Of
the entire length of the primitive canal, about one half is the anlage of the brain,
while the other half is the anlage of the spinal cord. In the subsequent develop-
ment of the brain the transverse expansion of the canal is most conspicuous, while
in the development of the spinal cord the elongation of the canal predominates.
The dilatation of the brain begins very early.

The medullary canal produces the entire central nervous system. Some of the
cells from its walls migrate out of the wall itself on either side. These cells pro-
duce the ganglia.

The Structure of the Medullary Canal.

When the medullary canal is first formed, it tends to present a rounded out-
line in transverse section. But its lateral walls being thicker than the wall on the
dorsal and ventral sides of the canal, the internal cavity appears somewhat flattened
(Fig. 37). On its ventral side it lies against the notochord. On its dorsal surface

FIG. 36. — CROSS-SECTION OF A HUMAN EMBRYO OF

I .54 MM.

, Ectoderm, ct, Somatic meso-
derm. p, Beginning of the embryonic ccelom.
g, Junction of the extra-embryonic somatic and
splanchnic mesoderm. df, Splanchnic meso-
derm. en, Entoderm. me, Mesoderm. ch,
Notochord. — (After Count Spec.)

70

THE EARLY DEVELOPMENT OF MAMMALS.

ri O
"^ •*-

4 § §<

a J3' B

- u H

o

o .= a

t^ c ;

"£

1 1

QJ e
*j C

^C «

?o;

u

nJ t;

OS ~^C ? O

(d bfl ft,

OT rt O,

a . g 8

c I c S Q
w cs 5

< D

II
£2 3

tfl

^

§'ll

-c S3 o

5/3 ^ o
••i" • !z

tiO *4-t

» ^>^

•0^8

(3 2

S o

•S 51

2 ^
^ "Sj1 >,
w 2 3

•^ * £

* ' J ' I

S « - |

111!

£E 2 &

.^ be X

2 ^1

ts i i ^ >

c^| 2 ||

^ «r J8 C "

o C i C

•< ^ £ -o

THE STRUCTURE OF THE MEDULLARY CANAL.

71

it is in contact with the overlying ectoderm, from which it has, however, completely
separated, and it causes the overlying ectoderm to rise up somewhat. Its sides are
in contact with the mesoderm, which is there developing into the primitive seg-
ments, page 84. The nuclei in the wall of the canal are very numerous, oval in
form, and usually with a single nucleolus. The nuclei are placed in the radial
lines. For some time after the canal has become closed the nuclei multiply very
rapidly by indirect division, but all of the mitotic figures are found close to the
inner surface of the canal, which surface, it will be remembered, corresponds to
the original outer surface of the ectoderm.

Md Seg

Cho,

FIG. 38. — TRANSVERSE SECTION OF A RABBIT EMBRYO OF EIGHT DAYS AND Two HOURS.

Md, Medullary canal. Seg, Primitive segments. Cho, Chorion. Am, Amnion. Som, Somatopleure. C<e,
Coelom. Spl, Splanchnopleure. Ent, Entoderm. Ch, Notochord. Ao, Aorta.

The differentiation of the brain and spinal cord is indicated even during the
stage of the medullary groove. The extreme anterior end of the groove is found
to widen out so as to produce a pair of lateral expansions. As development pro-
gresses and the canal closes, these expansions become more marked and are them-
selves, of course, also closed over, so that when the canal is completed they appear
as lateral diverticula or evaginations of the tube, which are known as the primary
optic vesicles (Figs. 129 and 130). While the vesicles are developing the medullary
tube expands in diameter throughout its cranial or anterior half without any notice-
able change in the general histological structure of its walls. Very soon the expansion
becomes unequal, and the inequalities are such that they produce three dilatations,
which are known as the three primary cerebral vesicles (Fig. 131). The first vesicle
is in the region of the optic outgrowth, the second is just behind this, and the
third is as long as the first and second combined and merges into the spinal cord.
At the time these vesicles become recognizable they occupy about half the entire
length of the rrfedullary tube. Between the first and second vesicles there is a Con-

72

THE EARLY DEVELOPMENT OF MAMMALS.

striction, and one also between the second and the third. The three vesicles are
the anlages, respectively, of the fore-brain, mid-brain, and hind-brain.

In the region of the spinal cord the medullary tube soon becomes somewhat
flattened from side to side, and therefore acquires a characteristic oval configura-
tion as seen in cross-section (Fig. 38). We can now recognize in the cross-sections
four regions: first, the two thick sides; second, in the median dorsal line the thin
portion which we call the deck-plate, and in the median ventral line the thin por-
tion which we call the floor-plate. Later on, each lateral portion becomes sub-
divided into two longitudinal bands, known as the zones of His, and distinguished
from one another as the dorsal and ventral zones (Fig. 116, D.Z, V.Z}. After this
stage there are six longitudinal zones in the embryonic cord. These are, first, the
deck-plate; second and third, the dorsal zones of His; fourth and fifth, the ventral
zones of His; and sixth, the floor-plate. These six zones also appear in the region
of the brain, where, however, .they undergo characteristic modifications. The zones

of His dominate the entire morphology
of the central nervous system, because
a^ t^ie sensory nerves enter the central
nervous system at the lower edge of the
dorsal zone and primarily ramify in the
dorsal zone, and, further, because all
efferent nerve-fibers arise in the ventral
zone and pass out to the body at certain
points on the surface of the ventral zone.

Origin of Nerves.

The essential constituents of a nerve
are the neuraxons, each neuraxon being

FIG. 39.— TRANSVERSE SECTION OF THE DORSAL CORD the prolongation of a nerve cell. In ad-
AND GANGLION OF A CHICK OF NINE DAYS. dition, the neuraxons are usually sur-

V.r, Anterior root. Dr, Posterior root. Gl, Ganglion of roimded by medullary sheaths. The
dorsal root, col, Collateral, with its branches. N,

Medullary neuroblasts with dendrites and axis- y°Ung Cells from whlch the neuraxons
cylinder. (The drawing is from a Golgi prepara- grow Out are called neuroblasts. The
tion. No sheath or neuroglia cells are repre- olfactory nCrve has a Special historv
sented.)— (After Cajal.)

(compare page 76). All the other

nerves are produced by neuroblasts, which are developed from the cells of the
medullary tube.

Shortly after the medullary groove has closed to form a tube, a number of
cells migrate from the medullary wall, just at the junction of the dorsal zone and
the deck-plate, and in sufficient numbers to form a longitudinal band, which is
known as the ganglionic crest, which soon, however, breaks up into separate masses,
the ganglia, which develop symmetrically. Some of the cells of each ganglion
become neuroblasts (Fig. 39, Gl}, each of which forms a centripetal process which

THE SPINAL CORD AND BRAIN. 73

enters the dorsal zone of the medullary tube, and a centrifugal process which passes
downward to join the ventral root. Other ganglionic cells are converted into the
medullary sheaths of the nerve-fibers, and still others migrate along the nerve-
libers and give rise to the peripheral or sympathetic ganglia.

The cells which are retained permanently in the medullary wall differentiate
themselves into two classes: first, the so-called spongioblasts, which become the neu-
roglia of the adult, and second, the neuroblasts (Fig. 39, N}. Each medullary neuro-
blast produces a single neuraxon and several dendrites, although a small number
in certain positions remain always without dendrites. Most medullary neuraxons
are distributed within the spinal cord or brain, but some of those, developed from
cells in the ventral zone only, leave the medullary wall to produce the ventral (Fig.
39, V.r] and lateral nerve-roots. Ventral roots constitute the third, sixth, and
twelfth cephalic nerves, and enter into the composition of all the spinal nerves.
Lateral roots form part of the fifth, seventh, ninth, and tenth nerves, and the
whole of the eleventh cephalic nerve, but probably take no part in the formation of
any true spinal nerve.

The gangiionic crest of the chick is figured and described in Chapter V. The
ganglia, roots, and nerves of the pig embryo are figured and described in Chap-
ter VI.

*

The Spinal Cord and Brain.

The medullary tube, after giving off the cells which form the neural crest,
becomes the definite anlage of the spinal cord and brain. The differentiation of
the brain begins very early, and is marked by an enlargement of the medullary
tube in the region of the future head (compare Fig. 131). As seen there, the
brain takes up nearly half the entire length of the medullary tube. The widening
of the anterior portion of the medullary canal is not uniform, but tripartite. The
brain is divided by two narrower parts into three wider divisions, which are
termed the primary cerebral vesicles, and are named in their order— fore-brain, mid-
brain, and hind-brain. The fore-brain widens very rapidly so ,as to form two lateral
projections, the optic vesicles (Fig. 131, op.V}. The division between the mid-brain
and the hind-brain is at first indistinct, but soon becomes sharply marked off. The
hind-brain then appears about as long as the other two vesicles combined, and
tapers down toward the spinal cord, into which it merges without demarcation.

The walls of the medullary tube acquire throughout certain fundamental char-
acteristics, which are best studied in transverse sections. The side walls become
thickened, but the median, ventral, and median dorsal portions remain thin (Figs.
157, 158, Sp.c): The upper thin part is called the deck-plate; the lower thin part,
the floor-plate. Soon each lateral wall is divided into two longitudinal bands,
designated, respectively, the dorsal and the ventral zone (Fig. 116, D.Z, V.Z}. In
young embryos, as shown in the figure just cited, the two zones are. separated
from one another by an internal notch. Their morphological characteristics depend

74 THE EARLY DEVELOPMENT OF MAMMALS.

upon their relations to the nerve-roots. In the ventral zone are lodged all the
neuroblasts which produce efferent nerve-fibers, which fibers constitute the ventral
nerve-roots in the region of the spinal cord (Fig. 39), and in the region of the
brain and the medulla oblongata produce both lateral and ventral roots. Neuro-
blasts are also differentiated in the dorsal zone, but the neuraxons which they send
out are confined in their distribution to the central nervous' system itself, and never
share in the formation of nerve-roots. The dorsal zone is further characterized by
the fact that the afferent or sensory fibers from the ganglia enter its lower edge
and have their first distribution within the dorsal zone. The stratification of the
thickened portions of the medullary tube begins early. It is initiated by the
appearance of a thin superficial layer, the ectoglia (Fig. «fc2i, Ec.gl), which contains
no nuclei. The thick nucleated portion of the medullary wall changes in appear-
ance as the neuroblasts are differentiated. This occurs in such a way that there
is always a layer of relatively undifferentiated cells next the cavity of the tube.
This is known as the ependyma layer (Fig. 121, Eperi). It persists throughout life,
remaining thin, and never containing nerve-cells. The layer between the ependyma
and the ectoglia is called the gray layer, or cinerea (tin). This layer grows very
rapidly, and the enlargement of the spinal cord and brain depends chiefly upon the
expansion of the cinerea.

The three cerebral vesicles pass through numerous modifications in their form
and cellular structure, yet their primary morphological characters are never obliter-
ated. The brain always has a central cavity, the boundary walls of which con-
stitute the organ. The fore-brain produces from its dorsal zone two lateral hollow
outgrowths, the cerebral hemispheres, the cavities of which constitute the lateral
ventricles of the adult. The median cavity of the fore-brain is the third ventricle;
the communication between the third ventricle and the lateral ventricle is the
foramen of Monro. The cavity of the mid-brain is always small, and its walls are
greatly thickened; it is termed the iter in the adult. The cavity of the hind-brain
is much enlarged and becomes the fourth ventricle. The part of the fore-brain
from which the hemispheres arise lies farthest cephalad, and is termed the telen-
cephalon. It is marked off on the dorsal side by a transverse fold, the velum
transversum, which projects inward. The part of the fore-brain caudad from the
velum is called the diencephalon. In the region of the fourth ventricle we can
observe that the cerebellum and the pons arise from the region adjoining the mid-
brain. This region is called the metencephalon, and the part caudad from it — out
of which the medulla oblongata is differentiated — is called the myelencephalon. The
narrow connection between the mid-brain and the hind-brain is termed the isthmus.
The following table indicates the relations of these embryonic divisions to the adult
parts. In Chapter VI the essential facts of brain development are illustrated and
described.

THE SPINAL CORD AND BRAIN.

75

TABLE OF BRAIN DEVELOPMENT.

Telencephalon

i. Fore-brain

(Lateral and third ventricles)
Kl

2. Mid-brain

(Iter)

Diencephalon

Mesencephalon

Optic vesicles.
Hemispheres.
Olfactory bulb.
Corpus striatum.
Lamina terminalis.
Infundibular gland.

Epiphysis.
Thalamus.
Tuber cinereum.
Pars mammilaris.

Corpora quadrigemina.
Cerebral peduncles.

3. Hind-brain

(Fourth ventricle)

Isthmus.

Cerebellum.

Pons.

Metencephalon

Myelencephalon Medulla oblongata.

R.d.

Can.

The spinal cord develops in an essentially uniform manner except at its caudal
extremity, the development of which is Part. G. B. D.H.

arrested before differentiation sets in.
The undifferentiated extremity becomes
the filum terminale of the adult. Typi-
cally, the three primary layers, ectoglia,
cinerea, and ependyma, are early differ-
entiated. The ectoglia increases in thick-
ness and receives many nerve-fibers,
which run for the most part lengthwise,
and is thus transformed into the white
matter of the adult cord. The cinerea
becomes the gray matter and gradually
assumes the characteristic adult outline in
cross-sections (dorsal and ventral horns).
The ependymal layer remains thin, its
cells contributing to the formation of the
neuroglia frame-work. During the growth
of the cord, the ventral zones enlarge
rapidly and each projects downward,
leaving a notch between them (compare
Fig. 213, Sp.c). The notch is closed on
its dorsal side by the thin floor-plate by which the two ventral zones are con-
nected across. The ventral expansions increase and the notch is thus trans-

Fiss.

V.H.

FIG. 40. — TRANSVERSE SECTION OF SPINAL CORD OF A
HUMAN EMBRYO OF 32 MM.

B, Burdach's column. Can, Central canal. D.H,
Dorsal horn. Fiss, Ventral fissure. G, Goll's
column. Part, Dorsal partition. R.d, Dorsal
nerve root. V.H, Ventral horn.

76 THE EARLY DEVELOPMENT OF MAMMALS.

formed into the ventral fissure, figure 40, Fiss., filled with mesenchyma or, in the
adult, by vascularized connective tissue. Further, the inner surfaces of the dorsal
zone meet and unite, thus obliterating the dorsal portion of the original central
canal, and forming out of the fused ependymal layers the permanent posterior
partition, figure 40, Part. The central canal of the adult, figure 40, Can, corresponds,
therefore, to the ventral part only of the original canal.

Plakodes.

Plakodes are small circumscribed thickenings of the ectoderm, which contribute
to the development of the olfactory, visual, and auditory organs. There are accord-
ingly three pairs of them. They resemble one another very closely in original ap-
pearance and their early history. A plakode, figure 41, Plk, is a rounded area of

-.

EC. Plk. Seg.

Mes. Ent. Endo. Ao.

FIG. 41. — CHICK EMBRYO OF THIRTEEN SEGMENTS. SAGITTAL SERIES 1452, SECTION 52.

Ao, Aorta. EC, Ectoderm. Endo, Endothelium. Ent, Entoderm. Mes, Mesoderm. Plk, Auditory plakode.

Seg, Mesodermic somite. X 100 diams.

syncytial ectoderm of small dimensions, several times as thick as the surround-
ing ectoderm, with which it merges by a rapid transition in the diameter of the
layer. Soon after its appearance it becomes invaginated. By preserving this con-
dition the olfactory plakode forms the olfactory pit. In other cases the invagina-
tion deepens, then closes over, and the vesicle thus formed separates from the over-
lying ectoderm. The vesicle formed by the visual plakode becomes the lens of the
eye. The vesicle formed by the auditory plakode becomes the otocyst. The ulti-
mate development of these organs is considered in the three sections next following.

The Nasal Pits and Olfactory Nerves.

The olfactory plakodes arise as a symmetrical pair of thickenings underneath
the fore-brain. They soon become invaginated, forming two shallow depressions just
in front" of the mouth, as is well shown in figure 165. As development progresses,
the depressions deepen and remain lined throughout by the thickened ectoderm
(Fig. 194, No). The orifices may be temporarily closed by the coalescence of the
epithelium. The olfactory pits acquire a secondary opening into the oral cavity
(Fig. 219). By the expansion of this cavity, the nasal chamber proper of the adult

THE EYE. 77

is produced. The form becomes complicated by the development of the turbinal
folds (Fig. 212) and later by the addition of outgrowths from each nasal pit to
form the accessory sinuses of the adult nasal cavity. The tissue between the two
nasal pits forms the septum (Figs. 220 and 212, Sept}. The epithelium of each
pit sends a special invagination into the nasal septum to make the gland-like struc-
ture known as Jakobson's organ (Fig. 219, Jk.o). It will be seen from the above
that the extent of the nasal epithelium becomes very great, but only a small area
is concerned in the formation of the olfactory organ proper. This area underlies
the olfactory bulb of the fore-brain, and is termed the olfactory epithelium. The
cells of this become elongated and acquire boundaries from one another so as to
make a cylinder epithelium. Some of the cells become the^o-called olfactory cells,
which are more or less isolated and separated from one another by the intervening
supporting cells. The olfactory cells develop on their free ends a few small pro-
jecting hairs, and from their basal ends a single thread-like prolongation, which
becomes a fiber of the olfactory nerve. This fiber penetrates the olfactory bulb,
and there has its terminal arborization, which enters into special relations with the
mitral cells of the bulb. All the fibers of the olfactory nerve arise in this way;
hence the nerve — so far as its development is concerned — is unique in vertebrates.
It differs permanently from all other nerves in that its fibers never acquire any
medullary sheaths, because no cells migrate from the medullary wall or brain into
this nerve as they do into all others.

Later, the complete separation of the nasal and oral cavities is accomplished
by the development of the palate shelves which grow out from the walls of the oral
cavity until they meet in the median line and unite with the lower edge of the

nasal septum, as described and illustrated in Chapter VI.

•

The Eye.

The eye has a complicated history. The optic nerve and retina arise from the
medullary tube (brain). The lens arises from .the visual plakode. The remaining
structures of the eye are of mesodermal origin.

Optic Vesicles.— The optic vesicles arise very early from the extreme cephalic end
of the medullary tube, as two lateral outgrowths (Fig. 131), each of which soon ap-
pears quite as large as the central portion of the medullary tube which produces
it (see Fig. 133). The central portion of the tube, however, grows much more
rapidly than the optic vesicles in order to form the brain. The distal portion of
the optic vesicle expands, and we thus get the condition indicated in figure 154, Op.
The vessel may now be said to be stalked. The stalk is the anlage of the optic
nerve. The larger distal portion of the optic vesicle gives rise to the retina. The
optic vesicle comes in contact with the ectoderm, and over the area of contact the
visual plakode is differentiated. In the next stage the differentiation between the
eyeball and optic nerve is clearly seen (Fig. 154). This is accomplished by modifi-
cations in the distal portion of the optic vesicle. Its outer wall becomes invagi-

78 THE EARI.Y DEVELOPMENT OF MAMMALS.

nated, producing a cup, the wall of which is necessarily double. The layer next the
cavity of the cup increases in thickness and becomes the retina proper. The other
layer remains thin and becomes charged with pigment, and is transformed into the
so-called pigment layer of the retina. The opening of the cup is filled with the
lens (compare also Fig. 17). In accordance with the fact that the optic nerve
and retina are derived from the wall of the medullary tuber we find that their dif-
ferentiation is essentially similar to that of the central nervous system. They
develop neuroglia and nerve-cells with neuraxons. It is only by keeping in mind
these facts that the histogenesis of the adult structure of the retina can be
understood.

Lens. — The visual fllakode, which, it will be remembered, is at first in contact
with the wall of the optic vesicle, is early invaginated (Fig. 153, L). The invagi-
nation closes over, making the lentic vesicle; this rapidly separates from the over-
lying ectoderm, which ultimately becomes the corneal epithelium. The wall of the
vesicle which is nearest the retina and farthest from the epidermis rapidly thickens
and forms the main substance of the lens, at the same time obliterating the cavity
of the vesicle.

Mesoderm. — The mesoderm produces the choroid and sclerotic coats of the
adult eyeball, the connective tissue of the iris and of the cornea, the lining epi-
thelium of the anterior chamber of the eye, and the muscles which move the
eye-ball.

For further details as to the history of the eye see page 331.

The Otocyst.

The otocyst arises by the invagination of the auditory plakode (Fig. 41, Plk)
to form the auditory pit "(Fig. 152, Of), which soon closes, forming an epithelial
vesicle which quickly loses its connection with the overlying ectoderm. The vesicle
or otocyst is the anlage of the membranous labyrinth of the ear. It lies close to
the wall of the myelencephalon (Fig. 191, Of) between the ninth nerve and the
ganglion complex of the seventh-eighth nerves. It becomes pear-shaped, the nar-
row end pointing dorsally. It soon develops a special prolongation (Fig. 42,
D.endo) which extends dorsad near the brain-wall and is known as the ductus
endolymphaticus. The ductus persists throughout life.

The ventral end (Cock) of the otocyst begins to elongate very early, and is
converted gradually into a very long spiral epithelial tube, the scala media cochlea
of the adult. The 'dorsal summit, S.C, of the otocyst is transformed into the
semicircular canals. The middle part of the original vesicle also undergoes remark-
able changes of form to produce the utriculus, which opens into the anterior and
external semicircular canals; the canalis reuniens, which leads to the cochlea; and
the saccidus, a blind pouch on the anterior side of the canalis reuniens.

The entire epithelial labyrinth becomes surrounded by a loose mesenchyma,
which again is surrounded by denser tissue which forms the cartilaginous periotic

THE EARLY HISTORY QF THE MESODERM.

79

capsule. The capsule enters into the formation of the cranium, and when it ossifies
forms also the so-called bony labyrinth.

The auditory ganglion fuses very early with the cephalad wall of the otocyst,
so that in sections no boundary between the ganglion and the epithelium can be
distinguished. The upper part develops into the vestibular, the lower into the coch-
lear ganglion.

R.VIII. D.endo.

S.C.

R.VI1.

Cock.

N.VII.

FIG 42. — PIG EMBRYO OF 12 MM. SERIES 5. RECONSTRUCTION IN WAX OF THE OTOCYST, WITH PART OF THE

BRAIN-WALL AND THE SEVENTH AND EIGHTH NERVES. BY G. C. COE and W. W. BEHLOW. VIEW FROM

THE CAUDAD SIDE.
Br, Brain- wall. Cock, Cochlear anlage. D.endo, Ductus endolymphaticus. N.VII, Seventh nerve. R.

VII, Root of seventh nerve. R. VIII, Root of eighth nerve. S.C, Region from which the semicircular

canals are formed. X 100 diams.

The Early History of the Mesoderm.

Concerning the precise origin and early development of the mesoderm authori-
ties are by no means agreed, and in the interpretations offered there has been more
of hypothesis than of observation. The most accurate observations have so far
been made on the elasmobranchs, lizards, and chick. In these -forms the entoderm
(or segmenting yolk) in the neighborhood of the primitive streak produces cells
which take their place so as to form a layer next to the entoderm. This layer
gradually becomes more and more distinct until it can be definitely recognized as
a separate layer, the mesoderm. It is probable that a similar process goes on in
amphibia and in mammals, so that it is safe to say that the mesoderm probably
arises by this process, which we calh delamination, in all vertebrates. In its first

80

THE EARLY DEVELOPMENT OF MAMMALS.

stage the mesoderm has no distinct boundary against the underlying entoderm. It
is thickest in the neighborhood of the primitive streak and thins out from that in
all directions. It very early comprises two easily recognizable classes of cells. One
of these forms a more or less distinct layer next to the yolk, and so distributes
itself as to form a network of cavities of which these cells become the boundaries,
thus developing the first blood-vessels. The cells which form them constitute the
angioblast. A portion of the angioblastic cells comes to lie in the cavities of these
primitive blood-vessels and is transformed into the first red blood-corpuscles of the
embryo. The second class of cells constitutes the mesoderm proper^ and forms a
more continuous sheet of undifferentiated, somewhat closely compacted cells, ex-
tending out from the primitive streak and lying between the angioblast and the
ectoderm.

The Expansion of the Mesoderm. — After the- mesoderm is once formed as a dis-
tinct layer, it seems to have no longer any connection with the entoderm or ecto-

A

FIG. 43. — THREE DIAGRAMS OF EMBRYONIC AREAS OF CHICKS TO SHOW THE GROWTH OF THE MESODERM.

The mesoderm is indicated by vertical shading, the area opaca by horizontal shading. A.o, Area opaca. A.p,

Area pellucida. mes, Mesoderm. pr, Primitive streak. — (After Duval.)

derm, except in the axial line. Its further expansion is due to the proliferation of
its own cells. During this early expansion the mesoderm assumes in all amniota a
definite and characteristic series of outlines. It is at first pear-shaped (Fig. 43, A),
the anterior end being pointed. It extends a short distance only in front of the
primitive streak and is widest a little distance behind the area pellucida (Ap]. (For
a description of the area pellucida see Chapter V.) The condition in the chick at
about the twentieth hour of incubation is indicated by figure 43, B, drawn on the
same scale as A, and at the close of the first day by figure 43, C. In the last stage
figured it will be noticed that the mesoderm is expanding unequally in front, hav-
ing sent out two lateral wings which leave a median space between them without meso-
derm. These wings continue their growth, and finally meet in front, so that in the
anterior part of the area pellucida there is a small tract without any mesoderm,
although it is completely enclosed by mesoderm. This tract is the pro-amnion. The

THE EARLY HISTORY OF THE MESODERM. 81

actual expanding edge of the mesoderm is quite irregular. The regularity shown in
figure 43 is entirely diagrammatic.

The extent of the growth of the mesoderm over the extra-embryonic region of
the mammalian blastodermic vesicle is very variable. Usually it extends completely
around the vesicle, but in some cases, as in the rabbit, only part way (compare
page 52).

The Origin of the Ccelom. — The next step in the differentiation of the middle
germ-layer is the appearance of two slit-like cavities in it, one on each side.
These cavities do not extend across the median line, for when they appear there is
no mesoderm in the median line of the embryo. The ccelom is the anlage of the
body-cavity, and in part persists in the adult as the pericardial, pleural, and ab-
dominal cavities. Certain parts of its walls share in the production of muscles
and of the excretory organs. The complete history of the ccelom is very complex.
As the coelomatic cavities appear, the cells bounding them take on a distinctly
epithelial character. This limiting layer is termed the mesothelium.

The earliest phases in the development of the ccelom have been exactly fol-
lowed only in a very few instances. In these it has been found that numerous
fissures appear in the mesoderm and unite themselves so as to form a network of
channels which grow, and produce by their fusion the ccelom. The fusion occurs
so that two cavities are developed, one on either side, and parallel with the axis of
the embryo. As the head of the embryo grows the two cavities grow into its
cervical end, following the penetration of the mesoderm, and unite so as to form
below the developing pharynx a single median cavity, the anlage of the future peri-
cardial cavity. In the Sauropsida and in many mammals the pericardial ccelom
merges into two large expansions of the body-cavity which lie just alongside of
the head of the embryo and are known as the amnio-cardiac vesicles (Fig. 131,
A.c.v). (Compare also the account of the splanchnocele, page 87.)

There are very great variations in the development of the ccelom in mammals.
In some cases the coelom grows so as to appear at an early stage in the body of
the embryo (Fig. 37). In other cases it is developed in the entire extra-embryonic
region of the blastodermic vesicle before it is developed in the embryo proper.
This condition has been observed in primates, including man. It results in the
formation of a layer of mesoderm surrounding the yolk-sac, and another layer
underlying the extra-embryonic ectoderm, with a wide ccelomate space between the
two mesodermic layers. This space we call the extra-embryonic ccelom. These
relations are illustrated in figure 45.

As soon as the ccelom has appeared the mesoderm is. divided into two layers, an
outer and an inner. The outer layer is in close contact with the ectoderm. It is
called the somatic mesoderm. The inner layer is in close contact with the entoderm;
it includes the entire angioblast, there . being in early stages no blood-vessels or
blood in the somatic mesoderm. The inner layer is called the splanchnic mesoderm.

82

THE EARLY DEVELOPMENT OF MAMMALS.

/Ent

EC

Somatopleure and Splanchnopleure.

The somatic mesoderm, together with the overlying ectoderm, constitutes the
somatopleure or primitive body-wall. The splanchnic mesoderm, together with the
underlying entoderm, constitutes the Splanchnopleure. The somatopleure and Splanch-
nopleure are, to a large degree, the elementary anatomical parts out of which the
adult structure is produced. Although they each comprise cells belonging to two
germ-layers, they nevertheless develop each almost as a unit, the cells of the two
germ-layers entering into intimate co-operation with one another in the differen-
tiation of organs. In both somatopleure and
Splanchnopleure it is convenient to distinguish two
main regions; namely, the embryonic, which enters
into the constitution of the embryo proper, and
the extra-embryonic, which enters into the forma-
tion of the so-called appendages of the embryo,
that is to say, of parts which exist during em-
bryonic life, but are lost at the time of birth,
and take no share in the permanent body.

In the primitive type of vertebrate develop-
ment there are no embryonic appendages. This
condition is illustrated by figure 44, which is a

transverse section of a young stage of an axolotl.
FIG. 44. — TRANSVERSE SECTION OF AN *

EARLY STAGE OF AN AXOLOTL. This may be readily compared with a blasto-

Ec, Ectoderm, mes, Mesoderm. Md, dermic vesicle of a mammal, if we imagine the

Medullary groove. Ch, Notochord. mags of }k Qr entoderm reduced to a single
Ent, Entoderm. Yk, Yolk. Ach,
• Archenteron or primitive entodermal laJer of cells- We can then easily distinguish the

cavity.— (After Bellond.) ectoderm and the underlying somatic mesoderm,

which together completely enclose the section.

The splanchnic mesoderm lies close against the yolk and is separated from the
somatic by the intervening coelom.

The general homologies of this primitive type of vertebrate embryos with the
type which we find in the amniota may be readily grasped by the aid of the ac-
companying diagrams (Fig. 45), which are based somewhat on the processes as
actually found in the chick. The embryonic structures properly so called are dis-
tinguished by shading. The yolk-sac is large and more or less a separate structure
from the embryo. It is surrounded by a layer of mesoderm represented by a dotted
line. In the direction of the embryo the mesoderm has continued to form part of
the wall of the intestinal canal, In; hence we may say that the Splanchnopleure
forms the wall of the primitive intestinal canal and of the yolk-sac. The yolk-sac
represents a lower portion of the Splanchnopleure. It can be readily seen that we
may compare it with the condition noted in the newt, and have to deal funda-
mentally with a question of relative proportions. The somatopleure, Som, enters into
the formation of the embryo itself, but it also extends beyond. Its disposition be-

SOMATOPLEURE AND SPLANCHNOPLEURE.

83

comes complicated in the amniota by the formation of the amnion itself. We shall
consider here only what is looked upon as the primitive method of the production
of the amnion, and note only that the exact steps of the process are considerably
modified in many mammals, in connection with the early modifications which the
ovum undergoes in order to secure its attachment to the walls of the uterus (see
the section on the trophoderm). The somatopleure forms two folds, one on each side
of the embryo. These folds arch up over the back of the embryo. The inner leaf
or part of each fold is the anlage of the amnion, Am. It consists of a layer of ecto-
derm next to the embryo, and. a layer of mesoderm, represented by the dotted line,

Cho

Cho.

FIG. 45. — GENERALIZED DIAGRAM OF AN AMNIOTE VERTEBRATE EMBRYO.

The first figure shows the, condition before, the second after, the separation of the amnion from the chorion. Am,
Amnion. Cho, Chorion. Cae, Ccelom. In, Intestinal canal. Som, Somatopleure. Spl, Splanchnopleure.
Yolk, Yolk-sac.

turned away from the embryo. The remaining portion of the extra-embryonic so-
matopleure, Cho, extends around both the amnion and yolk-sac, forming a mem-
brane called the chorion, which likewise consists, of course, of ectoderm, which,
however, faces away from the embryo, and of mesoderm (dotted line), which is
turned toward the embryo. As regards the embryo, therefore, the position of the
two germ-layers in the amnion is reversed in the chorion. The two folds continue
to grow until they meet above the back of the embryo and unite. The amnion
(Fig. 45, Am) has thus become a closed membrane surrounding the embryo, and
the chorion, Cho, has become a closed membrane surrounding the amnion, the
embryo, and the yolk-sac.

By the processes indicated we have produced an embryo with its three primary
appendages — the chorion, amnion, and yolk-sac. To these there is to be added a
fourth appendage, the allantois, which also begins its development very early, and
arises as a hollow outgrowth from the under side of the caudal end of the embryo
and expands into the extra-embryonic ccelom or space between the yolk-sac and
the chorion.

84 THE EARLY DEVELOPMENT OF MAMMALS.

The Embryonic Ccelom.

In the body of the embryo proper the ccelom acquires a very complicated dis-
position. It forms, first, a series of small cavities alongside of the medullary tube.
The walls of these cavities are termed the somites. A pair of somites mark out a
primitive segment. It forms, secondly, two large main cavities, which partially unite
in later stages on the ventral side of the embryo, the primitive segments lying more
on the dorsal side. These two large ccelom spaces constitute the splanchnocele, a
term which has reference to the fact that this space surrounds the splanchnic vis-
cera. Finally, it forms a series of so-called head-cavities, of which there are proba-
bly always three on each side of the head. The walls of these head-cavities in
part produce the muscles of the eye. We must now consider the development of
these divisions of the ccelom in the order indicated.

The Primitive Segments. — A segment consists of a pair of cavities symmetrically
placed and bounded by mesothelium. The cavities are portions of the embryonic
coelom. For convenience of description the term somite is applied to one of the
pair of structures which constitute a whole segment. The somites appear very early;
the first pair can be recognized in the chick after twenty to twenty-two hours'
incubation; in the rabbit, at the beginning of the eighth day. In both cases the
medullary groove is still nowhere closed. In amniote embryos, just before the first
segment appears, the mesoderm on either side of the axial line is considerably
thicker than farther away from it. We can, therefore, distinguish two zones, namely,
the thicker segmental zone near the axis, and the thinner, but much wider lateral
or parietal zone (Figs. 129, 131). The first step in the formation of the first seg-
ment is a loosening of the cells in the segmental zone, along a narrow transverse
line. In the chick this occurs about 0.14 mm. in front of the primitive streak, at
a time when only a portion of the medullary groove is formed. Very soon there
appears, close by, a second similar transverse loosening of the cells. The mesoderm
of the segmental zone is thus cleft twice, the mesodermic cells between the two
clefts constituting the first somite, which is somewhat cuboidal in form. The first
segment appears in what later becomes the occipital region. All further segments
are formed successively in a similar manner behind the first, the series growing by
additions caudad. The segments differ somewhat one from another in the details
of their development. The primitive somites, owing to their form and their prox-
imity to the anlage of the central nervous system, were taken by early embryologists
to be the beginnings of the vertebrae, and were, therefore, called the proto-vertebrcE.
This name is still used, although the idea upon which it was based is known to
be erroneous, because the primitive segments form much more than the vertebrae.

The association in time of the development of the medullary groove and primi-
tive segments is important. By the formation of the groove the space between
the ectoderm and entoderm alongside the groove is increased, and it is this space
which gives the mesoderm the opportunity to grow in thickness so as to form the
segmental zone next to the medullary groove.

THE EMBRYONIC CCELOM. 85

In the amniota when the somites are first formed they display usually no actual
cavity, but we must consider that one is morphologically present, since we can
easily observe the line of contact between the opposite walls of the segments. As
observed in transverse sections, the somites are seen to become triangular in outline.
The base of the triangle extends along the side of the medullary canal; the apex
of the triangle lies next to the splanchnocele, and at the point of the triangle the
somatic and splanchnic mesoderm separate widely from one another. Very soon
the apex of the triangle forms a narrow piece (Fig. 46, N), which is known com-
monly as the nephrotome or intermediate cell-mass. While the nephrotome is being
marked off the proximal portion of the segment enlarges, the cells assume a more
distinctly epithelial character (Fig. 46, My), enclosing a considerable space, which,
however, is completely filled by a mass of cells, C, which arise by a proliferation

•«/ «JS«>*S'_<•il3t£lt**S,,•^iQJ, _ SOITl

Spl

FIG. 46. — TRANSVERSE SECTION OF THE MESODERM OF A CHICK EMBRYO WITH ABOUT EIGHTEEN SEGMENTS.
Only the mesoderm of one side has been drawn. The section passes through a recently formed segment. My,

Secondary segment. C, Core of the segment. W.d, Wolffian duct. N, Nephrotome. Cos, Ccelom. Som,

Somatic mesoderm. Spl, Splanchnic mesoderm. X 227 diams.

of the cells from the lower side of the segment. The line around this mass of
cells marking it off from the other wall of the segment indicates the morphological
cavity. In the sheep and the chick it has been observed that the cavities of the
first four segments can be traced through the nephrotome to the splanchnocele.
This represents a primitive condition, one which we find in all the segments of
some of the lower vertebrates. Did we know the development of the amniota
only, we should not have been able to identify the cavity of the somite as mor-
phologically a portion of the crelom. The development in fishes shows conclusively
that it must be so regarded.

The Separation of the Nephrotome. — The nephrotome early loses its connection
on the one side with the enlarged central portion of the somite, and on the other
with the mesodermic walls of the splanchnocele, so that each nephrotome forms
a little mass of cells isolated from, but in definite topographical relation to, the
other parts of the mesoderm. It may be noted that during these early stages one
can always find the anlage of the Wolffian duct on the ectodermal side, and on the
entodermal side the anlage of a blood-vessel. Very soon the nephrotome assumes

86

THE EARLY DEVELOPMENT OF MAMMALS.

a rounded form, and a cavity appears in its interior; it is then often called a
segmental vesicle (Fig. 47, Nephr}. The exact details of the process by which the
nephrotome is separated from the other parts of the middle germ-layer have not
yet been carefully studied. Each nephrotome is the anlage of one of the excretory
tubules of the Wolffian body. It elongates into a tubule, which takes an S-shape,
and extends in the transverse plane of the body. The lateral end of the tubule
unites with, and acquires an opening into, the Wolffian duct. The median end
expands and produces a nephric corpuscle, with the characteristic glomerulus and
capsule.

Am.

Som.

Ao.

Nch.

FIG. 47. — SECTION OF A VERY YOUNG CAT EMBRYO. TRANSVERSE SERIES 413, SECTION 181.
Am, Amnion. A o, Aorta. Md, Medullary tube (spinal cord). My, Outer, My', inner wall of primitive segment
Nch, Notochord. Nephr, Nephrotome (segmental vesicle). Som, Somatopleure. Spl, Splanchnopleure.
Ve, Blood-vessel. W.D, Wolffian duct. X 50 diams.

The portion of the somite which is isolated by the formation of the nephro-
tome lies, of course, next to the medullary canal. The term primitive segment (as
also proto-vertebra) is often applied to this structure as well as to the original
somite before the separation of the nephrotome, but it would be better to refer to
it as the secondary somite* The secondary somite, when first formed, appears more
or less nearly square in surface views, and triangular in cross-sections. As the
medullary canal grows, so does the secondary somite, and it becomes, therefore,
somewhat elongated in its dorso-ventral diameter. After this change in its shape
we can distinguish in transverse sections of an embryo (Fig. 38) the outer wall,
which lies under the ectoderm, and an inner wall, which lies toward the medullary
canal and notochord. In the further history of the somite we can distinguish the
following steps: first, the production of the dermatome (cutis plate) with the ac-
companying transformation of a portion of the cells of the inner wall of the -seg-
ment into the mesenchyma; next, the production of the true muscle-plate;; thirdly,

* This is a new term, here proposed for the first time.

THE EMBRYONIC CCELOM. 87

the breaking-up of the outer wall of the myotome. These portions are sufficiently
described in the practical part, Chapter V.

The Splanchnocele. — The splanchnocele makes its first appearance in the parietal
zone of the mesoderm in the manner above described (Figs. 45 A and B, Coe).
It rapidly increases in size, so that a considerable space separates the somatic from
the splanchnic mesoderm, as shown in figures 160 and 163. When it first ap-
pears, it is a narrow fissure. It rapidly widens, extends toward the axis until it
almost reaches the primitive segments, and also spreads out laterally into the so-
called extra-embryonic region. As above stated, the rate and extent of its extra-
embryonic development vary greatly in different mammals. It develops in birds
earlier and acquires distention first in the future cervical region, where it produces
the amnio-cardiac vesicles (Fig. 136, A.c.v), in the median portion of whose
united cavities the heart is lodged. The splanchnocele of the body proper appears
after the primitive segments, and its expansion takes place at first only in the part
of the mesoderm next to tfre primitive segments. Everywhere as the splanchnocele
develops the mesodermal cells about it assume gradually more and more distinctly
an epithelial character, so that it soon becomes proper to speak of the mesothelium
or boundary epithelial wall of the ccelom.

The splanchnocele is also designated by several other names, and is sometimes
called simply the body-cavity or somatic cavity. Others term it the ventral ccelom.
By English embryologists it is usually called the pleuro-peritoneal space. Its future
subdivisions become early indicated by a transverse ridge of tissue which is known
as the septum transversum. This septum is situated at the posterior end of the
heart, and is developed to allow the great veins to have access to the heart itself.
It is the anlage of the future diaphragm. It separates the ccelom around the heart
from that of the abdomen. It is a product of the splanchnopleure, so that
it arises upon the ventral side of the ccelom. We have, as soon as this septum is
present, the pericardial cavity on its cephalic side, the abdominal cavity on its cau-
dal, and a small pleural cavity on its dorsal side.

The Ccelom of the Head. — No adequate investigation of the early stages of the
mesoderm in the head of amniota has yet been made. We know, however, that
in the lower vertebrates there appear at least three distinct cavities resembling por-
tions of the true ccelom and bounded by epithelial cells, similar to the mesothelium
in character. These cavities are generally regarded as portions of the true ccelom,
and by many writers have been interpreted as true primitive segments. But this
interpretation is not yet beyond doubt. The largest of these is called the mandibu-
lar cavity, because it has a prolongation which extends into the mandible of the
young embryo. In front of it is the first or premandibular cavity, which is much
smaller, and behind it is the third or hyoid cavity, which is intermediate in size
between the first and second (Fig. 48). The head-cavities are best known in the
elasmobranchs. They have also been found clearly developed in reptiles and cer-
tain birds. In mammals no actual cavities have been recorded. There are found

THE EARLY DEVELOPMENT OF MAMMALS.

N.fac.ac. Ot. O. N.gl.ph. N.vag. Dor.com.

Hi.

So. ant

Op. ves.

G.cl. Md.st. Hyo.st.

Prcd.

FIG. 48. — SQUALUS ACANTHIAS, 9.0 MM. SERIES 1495. RECONSTRUCTION TO SHOW THE HEAD-CAVITIES.

BY R. E. SCAMMON.

Ao, Dorsal aorta. Dor. com, Dorsal ganglionic commissure. G.cl, First gill-cleft; two others are open and
marked by similar shading. Ht, Heart. Hyo. st, Stalk of mesoderm connecting the hyoid cavity with
the pericardium. Md. st, Stalk of mesoderm connecting the mandibular cavity with the pericardium.
Nch, Notochord. N. }ac. ac, Facial-acoustic nerve trunk. 2V. gl. ph, Glossopharyngeal nerve. N. vag,
Vagus nerve. TV. ar. tr, Temporary ganglionic mass (Dohrn's "Urtrochlearis"). O, Orifice of the
otocyst. Op.ves, Optic vesicle. Ot, Otocyst. Prcd, Pericardium. So. ant, Anterior head-cavity, a
derivative of the premandibular cavity, and lacking in most animals. So.hyo, Hyoid head-cavity.
So.mand, Mandibular head-cavity. So.prem, Premandibular headcavity X 30 diams.

THE MESENCHYMA.

89

the anlages* of the muscles of the eye, and these are, by hypothesis, homologous
with the cells of the walls of the head-cavities in the lower vertebrates, which cells
produce the muscles of the eye.

The Mesenchyma.

By the term mesenchyma we designate the whole of the mesoderm of the em-
bryo, except the mesothelial lining of the coelom. When fully differentiated his-
tologically, it consists of more or less widely separated cells, connected with one
another by intervening threads of protoplasm, which form a network between the
cells (Fig. 49). The remaining space is filled by a homogeneous structureless ma-
trix or basal substance. It gives rise to a large
number of adult tissues, as shown in the table
on page 19.

In the early development, or histogenesis, of
the mesoderm we can distinguish four stages:
first, that of distinct cells; second, the forma-
tion of the cellular network; third, the forma-
tion of the mesothelium; and, fourth, the differ-
entiation of the mesenchyma. The first stage is
known chiefly through observations on the early
stages of elasmobranchs, reptiles, and birds. In
these types the first cells which are delaminated
from the entoderm to form the anlage of the meso-
derm, are of quite large size and lie between the entoderm, or yolk, and ectoderm,
and are without connection with one another. The number of mesodermic cells in-
creases both by the multiplication of the cells already delaminated and by the addition
of others from the entoderm. Whether this stage occurs in mammals or not, we do
not know at present. In the second stage the primitive cells are found to have ac-
quired connection with one another, the protoplasm of one cell uniting by a process,
or prolongation, with the protoplasm of another cell, and so on until the whole
tissue becomes a network. When the primitive streak has been formed in the
mammalian blastodermic vesicle we find the mesoderm in this condition. The third
stage is brought about by the development of the coelom as above described. The
coelom is bounded by the undifferentiated mesoderm. To produce the fourth stage,
single cells leave the primitive mesodermic layer by migrating out of it on the
side away from the ccelom. The cells left behind are ultimately reduced to a sin-
gle layer, the permanent mesothelium. The emigrant cells constitute the mesen-
chyma and are found to be connected both with one another and with the meso-
thelial cells by protoplasmatic processes, but they do not lie close together, as in
the epithelium, so that there is a considerable, though variable, amount of inter-

* The anlages may be seen in a pig embryo of 10 mm. between the jugular vein and the internal carotid
artery as a group of embryonic cells quite distinct from the surrounding mesenchyma.

J8
FIG. 49. — CHICK EMBRYO OF THE THIRD

DAY.

Mesenchyma from near the otocyst. A, cell
in mitosis.

90 THE EARLY DEVELOPMENT OF MAMMALS.

cellular space. By the migration of the cells and their multiplication, the mesen-
chyma is produced. It fills up all the room between the mesothelium and the two
primary germ-layers so far as it is not occupied by the developing blood-vessels and,
later, by lymph vessels.

Apparently the entire mesothelium may participate in the production of the
mesenchymal cells. Its different regions, however, do not so participate all to an
equal degree, or at the same time. The throwing off of mesenchymal cells may be
observed in certain parts of the embryo in somewhat advanced stages of develop-
ment, and it seems not impossible that the process may be found to occur even
in adult life.

The mesoderm, by the formation of mesenchyma, becomes very early unlike
the other germ-layers. Both ectoderm and entoderm are epithelial membranes.
The mesoderm is partly epithelial, partly mesenchymal, and from the mesenchyma
arise special kinds of tissue which are characteristic of the middle germ-layer, and
never are produced from either the outer or inner germ-layers.

The Origin of the Blood-vessels and Blood.

As stated above (pages 66 and 80), the angioblast and first blood-vessels ap-
pear in the circumscribed region in the mesoderm of the yolk-sac and lie close
against the entodermal cells of the area opaca. The region which they occupy is
termed the area vasculosa. From the area vasculosa the development of blood-
vessels extends, as stated, across the area pellucida into the embryo.* During these
early stages the only blood-vessels are in the splanchnopleure. After their formation
has extended into the body of the embryo, it spreads into the somatopleure also,
which, therefore, acquires its blood-vessels at a later stage. It should be noted,
however, that the development of the blood-vessels begins before the ccelom has
been developed over the area vasculosa. While they are forming, the ccelom
expands; and after it has appeared, the primitive blood-vessels are found always
exclusively in the splanchnic mesoderm.

Definition.— The essential part of a blood-vessel is its endothelial wall. In
early stages all the blood-vessels consist only of endothelium. Arteries and veins
differ but little, if at all, in histological structure during early embryonic stages, and
are distinguished chiefly by the direction of blood-currents passing through them.
Capillary blood-vessels and sinusoids have, as a rule, throughout life merely an en-
dothelial wall. Arteries and veins become strengthened by the development of
special coats around the endothelium which arise by transformations of the mesen-
chymal cells in the immediate neighborhood of the vessels.

The Development in the Chick.— The first indication of the blood-vessels is a ret-
iculate appearance, which can be recognized in the mesoderm in surface views of
the fresh or hardened embryo at the end of the -first day. The reticulate structure
increases rapidly in extent and distinctness during the second day of incubation.

* It has been recorded that in lizards the vascular anlages appear first in the area pellucida.

THE ORIGIN OF THE BLOOD-VESSELS AND BLOOD.

91

It is confined to the region of the mesoderm surrounding the embryo proper, and
which is, therefore, known as the area vasculosa, as above stated (compare Fig. 131).
As soon as there are several primitive segments in the embryo, the network in the
mesoderm shows traces of coloration in irregularly shaped reddish yellow spots,
which are largest and most numerous around the caudal end of the embryo. These
spots are called blood-islands (Fig. 131, Bl.is) because the central cells in them
are transformed into the first blood-corpuscles. The network appearance is due to
the development of the angioblast, which is a set of cells delaminated from the en-
toderm or the yolk, and intervening between the mesoderm proper and the entoderm.
The angioblast at first assumes the form of more or less solid cords. The meshes

FIG. 50. — HUMAN EMBRYO OF 9.4 MM. PROBABLE AGE THIRTY DAYS. X 8 diams.

of the angioblast are partly or wholly filled by mesodermic cells. The ccelom now
appears in the extra-embryonic area, and thereafter the anlages of the blood-vessels
are connected with the splanchnic mesoderm only. The anlages of the blood-
vessel at this stage form a thick network without distinction of stem or branch, ex-
cept that the edge of the area, bounded by a broad band of angioblast, gives rise
to a single large vessel, which is known as the sinus terminalis. The anlages are all
in one layer, none overlying the others, and up to this stage they are all solid.
The terminal sinus becomes connected with the venous system.

The blood-islands are spots where there is a cluster of cells, which remain at-
tached to one another and to the walls of the vessels. The cells develop hemo-
globin in their interior, hence the clusters have a reddish color which renders all
the islands very conspicuous in surface views of fresh specimens. Blood-islands appear
first in the area opaca, but almost immediately after in the pellucida also. They

92

THE EARLY DEVELOPMENT OF MAMMALS.

have at first a rounded or branching form. In the inner part of the pellucida they are
small and stand alone. Toward the periphery they are larger, more closely set,
and more united with one another. Their development is greater around the caudal
end of the embryo.

In the next stage the vascular anlages become hollow, and then may be
called true blood-vessels. When they acquire a lumen, the blood-islands are found
to remain attached usually to the upper side of the vessel like a thickening of its
wall (Fig. 51, bl.is). Very soon after the vessels have become hollow the cells of
the blood-islands break apart and lie free in the cavity of the vessel, thus forming
the first blood-corpuscles. They are characterized by having a rounded nucleus
with a very distinct nucleolus, and a minimal covering of protoplasm only. After

som

FIG. 51. — SECTION OF THE AREA VASCULOSA OF A CHICK EMBRYO OF THE SECOND DAY.

Som, Somatopleure. Spl, Splanchnopleure. EC, Ectoderm. En, Entoderm. bl.is, Blood-isands. V, V, Blood-
vessels. X 227 diams.

the cells have become free the amount of protoplasm in each cell increases. The
cells multiply rapidly by mitotic division. It is believed that all the blood-
corpuscles, both red and white, are descendants of these cells derived from the
blood-islands.

The angioblast continues growing by the development of buds from the vessels
already formed. These buds are rounded or pointed, forming, as it were, spurs.
They often end by meeting one another and uniting. They are usually hollow from
the first, and after they meet one another or an adjacent vessel, the cavities be-
come continuous, and thus the vascular network is extended.

The Development in Mammals. — The origin of the blood-vessels in mammals is
not adequately known. The solid primary anlages appear in the extra-embryonic
area vasculosa and extend later into the embryo. They present well-marked blood-
islands, which make their first appearance in rabbit embryos of the eighth day,
just before the appearance of the first primitive segments. It is characteristic of
most mammals that the entire yolk-sac, probably owing to its small size, becomes,
very early indeed, vascularized throughout.

THE BLOOD-CORPUSCLES. 93

The Growth of the Vessels into the Embryo. — The entrance of the vessels into
the embryo chick begins toward the end of the second day. The buds which form
the extra-embryonic angioblast grow first toward, then into, the embryo. The pene-
trating vessels follow certain prescribed paths. Part of the vessels run along the
posterior edge of the amnio-cardiac vesicles, and enter into connection with the pos-
terior end of the heart, which has meanwhile been progressing, and which — owing
to the early separation of the head end of the embryo from the yolk — is the only
part of the heart which the vessels can reach directly. While the vessels are ap-
proaching the heart their differentiation into various sizes is going on, the smallest
ones to remain as capillaries, the larger ones to become arteries or veins. The
only two veins in the first stage are those above mentioned, which are called the
omphalo-mesaraic. Another set of vessels penetrates along the splanchnopleure of
the body on each side until they attain the small space between the notochord and
somite and the entoderm, where they fuse so as to form a longitudinal vessel, the
anlage of the descending aorta (Fig. 143, Ao.D). It should be noted that this
anlage is primitively double. The aorta appears first in the region toward the
head. It grows forward above the pharynx, bends ventrally just behind the mouth,
dividing as it bends, one branch going around each side of the future pharynx, and
uniting again on the ventral side of the pharynx in the mediari ventral line, in
order to join the anterior end of the tubular heart. The heart begins to beat be-
fore the vessels unite with it. The first blood-cells have already been formed;
hence as soon as union is accomplished the blood circulation starts up, the blood
passing through the aorta to the body, thence by numerous lateral branches to the
area vasculosa, and returning by the two omphalo-mesaraic veins to the heart. It
will thus be seen that almost the entire circulation is extra-embryonic.

The other embryonic blood-vessels are developed by buds from the walls of
the vessels already present in the embryo, in the same general manner as new ves-
sels are formed in the area vasculosa. These buds give rise to the endothelium
only of the embryonic vessels. When a vessel becomes an artery or a vein, the
media and adventitia are added, as above stated, by differentiation of the
surrounding mesenchyma.

During further development many small blood-vessels abort, and often appear
as disconnected bits, closed at both ends and containing corpuscles. Such struc-
tures were at one time supposed to be developing blood-vessels and were accord-
ingly termed " vaso-f ormative cells." The blood-corpuscles in them are of course
not developing, but degenerating .

The Blood-corpuscles.

The first blood-corpuscles are free cells of an indifferent character and capable
of wandering through the walls of the blood-vessels, which in early stages are easily
permeable, as if of a merely gelatinous consistency. The primitive blood-cells, as
they may be called, give rise not only to the permanent blood-corpuscles, both red

94 THE EARLY DEVELOPMENT OF MAMMALS.

and white, but various other cells outside of the vessels, of which two classes are
especially important — the free wandering cells in the mesenchyma or connective
tissue and the giant cells. The latter, however, contribute to the blood, for they
form in the spleen, bone-marrow, and other organs, long processes, like pseudo-
podia, which break up into fragments. These fragments are the blood-plates. It
is probable that all blood-cells and wandering cells are exclusively descendents
of the primitive blood-cells, although some writers maintain that their number
is increased in both the embryo and the adult by transformation of cells of the
mesenchyma.

When the circulation begins, the number of corpuscles is small, but it rapidly
increases by mitotic division of the cells. At the very start, like all cells produced
by segmentation of the ovum, the blood-cells are quite large, but they rapidly de-
crease in size until they reach the "first-stage," in which they appear as small
round cells (in the chick 8.3 to I2.5// in diameter) with a rounded granular reticu-
late nucleus and a minimal amount of protoplasm. In the next stage the amount
of protoplasm increases. We have next to consider separately the cytomorphoses of
the red and white corpuscles.

Red Corpuscles. — By examining the blood of chick embryos of successive ages
we can trace the- differentiation of the red cells. We find that the protoplasm
enlarges for several days, and that during the same time there is a progressive
diminution in the size of the nucleus, which, however, is completed before the area
of protoplasm reaches its ultimate size. The nucleus is at first granular, and its
nucleolus or nucleoli stand out clearly. As the nucleus shrinks, it becomes round
and is colored darkly, and almost uniformly, by the usual nuclear stains. This
change is called pyknosis. The blood-cells of mammals pass through the same
metamorphosis as those of birds. For example, in rabbit embryos of eight days
(Fig. 52, A) the cells have reached the stage with a granular nucleus and well-
developed cell-body. Corpuscles of this kind are characteristic of fishes and
amphibia, and they may, therefore, be designated as the ichthyoid cells. Two days
later the nucleus is already smaller, and by the thirteenth day has shrunk to its
final, dimensions. The cells in this condition are characteristic of the reptiles and
birds, and may be designated, therefore, as sauroid cells. The nucleated stage of
the cells is typical of embryonic life only in mammals. During the fetal period
the nuclei of the red cells gradually disappear and the cells are transformed into
the non-nucleated corpuscles, which occur only in mammals, so that this last may
be designated as the mammalian stage. The nuclei disappear by extrusion from
the cells. Usually they break into fragments, which are then expelled. Some-
times, though rarely, the nucleus goes out intact. The successive stages of the
blood-corpuscles in mammals illustrate the law of recapitulation (page 29). When
the nucleus disappears, the corpuscle becomes smaller. In the human embryo at
one month the red cells are the predominant blood-corpuscles. At two months
they are still the most numerous, although the non-nucleated corpuscles have begun

THE BLOOD-CORPUSCLES.

95

£

t-i

o C

S ""

I*

3 a -a

. * ^

<^ a u
§ *£

O M" *-

© e, d

^ c

s B

s'l
g :

PQ

^J

I

96 THE EARLY DEVELOPMENT OF MAMMALS.

to appear. At three months the non-nucleated corpuscles constitute by far the
majority of all corpuscles in the blood.

Leucocytes. — The primitive blood-cells, being colorless, have been termed leuco-
cytes by some authors, but they are obviously different from the leucocytes of the
adult blood. Some of them become so-called lymphocytes (young leucocytes),
which are distinguishable from the primitive cells by the internal structure of the
nucleus. Others grow in size and follow at least two cytomorphic paths. In one
series the protoplasm develops fine granules, and the nucleus becomes first elongate,
then reniform, and finally beaded. In this form they appear as polymorpho-
nuclear neutrophile leucocytes. In the second series the protoplasm acquires coarser
granules, which are really phagocyted morsels of red blood-cells, and the nucleus
becomes reniform. These cells are termed the eosinophile leucocytes.

The term leucocyte properly embraces all the white corpuscles of the adult
blood, but has been erroneously restricted by some recent authors to the granular
forms. The young stages of the granular leucocytes are sometimes termed myelo-
cytes, because in the adult they occur chiefly in the bone-marrow, which is the
chief sanguifactive tissue in the adult.

It is doubtful whether leucocytes ever develop in the circulating blood. They
appear abundantly after the lymph-glands are formed. The usual explanation is
that some of the wandering primitive blood-cells enter the glands, there multiply,
and in part become leucocytes.

The Origin of the Heart.

The manner in which the head of the embryo becomes free is described on page
49 (compare also Figs. 16 and 132). The origin of the ccelom is described on page
8 1. When the head becomes free, the ccelom is found soon to extend across the
median line. This takes place at the cervical end of the head just where the
tissues of the embryo bend over to join the yolk (Fig. 132, p. 183). This median
ccelom is the beginning of the pericardial cavity. In connection with it the
development of the heart occurs. The formation of this organ is probably initiated
by an ingrowth of the cells of the angioblast, which give rise to the endothelium
of the heart (Fig. 138, Endo). The mesothelium of the dorsal side of the primitive
pericardial coelom produces the muscular walls of the heart (Fig. 129, m.ht). The
early development and primitive relations of this organ can be understood by the
account given in Chapter V of the structure of a chick embryo with eight
segments.

The Germinal Area.

The germinal area is that portion of the amniote ovum (mammalian blastodermic
vesicle) in the center of which the embryo is differentiated. It comprises, therefore,
both the embryo proper and the region immediately surrounding it. In mammals
it corresponds in extent with the embryonic shield (p. 47). In its center we find

THE MAIN VESSELS OF THE AREA VASCULOSA. 97

the anlages of the embryonic structures proper. In its extra-embryonic part we
find the three primitive germ-layers. Underneath the entoderm is the cavity of the
yolk-sac. In the mesoderm we have occurring the development of the ccelora, and
in the splanchnic mesoderm the differentiation of the primitive blood-vessels. These
primitive vessels occupy the sharply denned territory, the edge of which is marked
by the sinus terminalis (Fig. 131, V.t}. The first differentiation in the germinal
area, which can be clearly recognized by the naked eye, is the appearance of the
area pellucida, which is due to the thinning of the entoderm over the central
area. Next ensues the differentiation of the primitive streak (Fig. 14). Further
progress results in the gradual differentiation of the embryo, in the sharp demarca-
tion of the area pellucida, which becomes pear-shaped, and in the appearance of
the blood-vessels and the resulting differentiation of the area vasculosa or opaca.
Figure 131, on page 182, represents the embryonic area of a hen's ovum after about
twenty-seven hours' incubation. The embryo is well advanced in development, for,
although the primitive streak, pr.s, still remains in part and the medullary groove is
still open behind, the brain is already marked out and the head has become partly
free. Alongside the medullary canal lie eight pairs of segments. Around the em-
bryo one easily recognizes the somewhat pear-shaped area pellucida, A.p, and the
darker area opaca, by which it is enclosed. The area vasculosa stands out
conspicuously and is bounded by the already distinguishable sinus terminalis V.t.
Around and underneath is the translucent pro-amnion, pro.am, from which the
mesoderm is altogether absent, and which, therefore, cannot contain any blood-
vessels. Nor are there at this state any vessels in front of the pro-amnion. The
general topographical arrangement is the same in mammals (compare page 179
and Fig. 130).

The Main Vessels of the Area Vasculosa.

Soon after the capillary network of the areas opaca and pellucida has penetrated
the embryo, certain lines of the network begin to widen, and soon distinctly assume
the size and functions of main trunks; some of these unite with the posterior venous
end of the heart (Fig. 59, Ve.ht), which has meanwhile been formed in the em-
bryo, and others become connected with the anterior or aortic end. Even before this
the heart has begun to beat, so that, as soon as all connections are made, the primi-
tive circulation starts up. The arrangement of the vessels is not the same in birds
and mammals. The disposition in birds is indicated by the diagram shown in
figure 53, in which, it should be remembered, the embryo and the capillary net-
work are drawn many times too large in proportion to the area vasculosa. The
area is bounded by a broad circular vessel, the sinus terminalis, S.T, which con-
stitutes a portion of the venous system in birds, for in front of the head of the
embryo the sinus leaves a gap, and is reflected back along the sides of the body of
the embryo to make two large veins, which, after uniting with the other venous
channels coming from various parts of the area vasculosa on each side, enter the

7

98

THE EARLY DEVELOPMENT OF MAMMALS.

embryo as two large trunks, Om.V, known as the omphalo-mesaraic veins; these
two veins unite in a median vessel, the sinus venbsus, S.V, which runs straight
forward and enters the posterior end of the heart. The sinus venosus also receives
the veins from the body of the embryo, namely, the anterior cardinals, Jug, and
posterior cardinals, card. The two cardinals of each side unite, making a short
transverse trunk known as the common cardinal, D.C, which in turn empties into
the sinus venosus. The entire venous current is thus brought to the heart in a
united stream; it passes out through the aorta, the greater part ascends the aortic

P.O.

Ora./L

FIG. 53. — DIAGRAM OF THE CIRCULATION IN A CHICK AT THE END OF THE THIRD DAY, AS SEEN FROM THE UNDER

(ENTODERMAL) SIDE.
The embryo, with the exception of the heart, is dotted; the veins are black. Ao, Aorta. Arc, Aortic arches.

card, Posterior cardinal vein. D.C, Common cardinal vein. Ht, Heart. Jug, Anterior cardinal vein.

Om.A, Omphalo-mesaraic or vitelline artery. Om.V, Omphalo-mesaraic or vitelline vein. S.T, Sinus

terminalis. S.V, Sinus venosus.

arches and passes back through the main aorta, Ao, and divides at the posterior
fork of the aorta, the bulk of the two currents passing out through omphalic" ar-
teries, Om.A, and thence to the capillaries of the area vasculosa and so on to the
venous trunks again. As shown in the figure, which presents the under side of the
area, the left omphalo-mesaraic vein preponderates, and in the latter stages this dif-
ference becomes more marked, until finally the right stem is very inconsiderable in
comparison with the great left vein. The time at which the disparity commences
is extremely variable, as is also the degree of inequality between the two veins.

The following description probably represents what was the primitive condition
of vessels in the mammalian area vasculosa. It applies to an early stage in the
rabbit. An essentially similar arrangement of the vessels exists also at a correspond-

THE AORTIC SYSTEM.

99

ing stage in the dog. The veins are much more symmetrical than in the chick, and
have the same general plan; the sinus terminalis belongs to the venous system, so
that the connection with the arterial circulation, found later, is secondary; the aorta
of the embryo is double, and gives off on each side (segmentally arranged?) trans-
verse branches, one of which develops into the large trunk shown in figure 54; the
network of small vessels forms two layers, of which the upper is connected with
the arteries, the lower with the veins. The change from the earlier condition to
the later has still to be followed.

FIG. 54.— AREA VASCULOSA OF A RABBI?, PRESUMABLY OF ABOUT TWELVE DAYS. — (After Van Beneden and Julin.)

The arrangement of the main vessels in the area vasculosa at a later stage
in the rabbit is quite different. The sinus terminalis forms a complete ring (Fig.
54), and is connected with the arterial system by a single trunk, which corresponds
to the left omphalic artery of the bird. For some time the connection between the
embryonic arteries and the area vasculosa is entirely through capillaries, and the
arterial trunk on the vascular area does not appear in the rabbit for several days.
There are two veins, one arising from each side of the body and passing out on
to the area vasculosa over the back of the embryo; they are the two large upper
vessels in the figure.

The Aortic System.

In early stages the aortic end of the heart terminates under the ventral floor
of the pharynx. The endothelial heart continues as the aorta, which almost at

100

THE EARLY DEVELOPMENT OF MAMMALS.

once branches to the right and left. Each branch forms five vessels, the so-called
aortic arches (Figs. 55, 92, and 93). The first arch is the first formed, the other
four are formed in succession behind it. The arches show a constant relation to
the pharyngeal gill-pouches, there being one pouch between every two arches. The
arches pass dorsalward around the broad pharynx (Fig. 28), and those of each
side become united by a single dorsal longitudinal vessel (Fig. 56, Ao.D}, the
dorsal or descending aorta. The two dorsal aortae pass caudad until they meet
and unite in the median line to form the main aorta.. (Compare Figs. 196, Ao.S;

Ao.D

. IV

II

XEO

Coe.

Ao.M

Car. ex

FIG. 55. — ANTERIOR WALL OF THE PHAR- FIG. 56. — PIG EMBRYO OF 6.0 MM. SERIES 9. AORTIC ARCHES OF

YNX OF A HUMAN EMBRYO OF 3 . 2 MM. LEFT SIDE. FROM A WAX RECONSTRUCTION BY L. M. FERGUSON.

1-5, Gill-arches; the arches are separated I, II, III, TV, V, Aortic arches. Car. ex, External carotid. Ao, Cardiac

from one another by the entodermal aorta. Ao. D, Dorsal aorta. Ao.M Median aorta.

and thecorrespondingectodermal gill-
pouches; the aortic arches are drawn

in dotted lines and arise from the

median cardiac aorta. M, Mouth.

Oe, (Esophagus. Cce, Coelom. X 50

diams.— (After W. His.)

197, Ao, and 198,^0.) In a pig of 7.8 mm. the five aortic arches can be still
traced, but the first arch has begun to disappear, and the condition illustrated in
figure 169 is established. Its ventral portion, /, persists, however, and together with
its own vascular prolongations into the lower Jaw gives rise to the external
carotid (Fig. 101, car. ex). The descending aorta on the dorsal side between the
tops of the first and second arches also persists and is prolonged into the head to
constitute the internal carotid (Fig. 172, car.i). Presently the second arch also
disappears, and both carotids are, as it were, thereby lengthened. This is the
condition which we find in our embryo of 12 mm. (Fig. 172). The third, fourth,
and fifth arches are still present. From the base of the third arch runs forward
the external carotid, and from the summit of the third arch runs forward the

THE AORTIC SYSTEM. 101

internal carotid. The dorsal ends of the third and fourth arches are still con-
nected, but this connection, instead of being a large aortic vessel, as in earlier
stages, has now contracted and almost disappeared, and will soon be lost altogether,
so that in the adult there will be no connection between the dorsal ends of the
third and fourth arches. The fifth arch is still connected with the dorsal end of
the fourth. It gives off the small pulmonary artery to the lungs. On the side
toward the heart the relations of the arches are also changed. The main aortic
vessel which springs from the heart is, in the 12 mm. pig, divided into two vessels
— the pulmonary aorta on the ventral side and the true aorta in a more dorsal
position. The division has 'so taken place that the third and fourth arches are
connected only with the true aorta, while the fifth arch is connected only with the
pulmonary aorta. The part of the fifth arch on the left side between the origin
of the pulmonary artery proper and the main descending aorta offers at this stage
an open communication between the pathways of the pulmonary and of the main
body circulation. This dorsal half of the fifth aortic arch is known as the ductus
arteriosus. It remains throughout the fetal period as an open channel, so that the
blood from the right ventricle flows in part to the lungs, in part into the dorsal
aorta. The lumen of the ductus arteriosus disappears in man soon after birth.
As an anomaly it occasionally persists throughout life, involving serious modifica-
tions of the normal circulation. The dorsal part of the fifth aortic arch of the
right side has a different history, for it aborts early in embryonic life, and there
also occurs an abortion of the entire descending aorta from the end of the fourth
arch on the right side downward to the level of the diaphragm. When this abor-
tion has taken place, the entire aortic stream flows from the heart to the left side
of the embryo. The aortic branches on the right side appear as follows in the
adult: The main stem, from which the five arches originally sprang, is the arteria
innominata, which gives off a stem, the common carotid, from which spring the
two carotids of the right side. Next, a vessel which represents the persistent fourth
right arch, which no longer has any direct communication with the aorta, but at
its end gives off the subclavian and vertebral arteries. The vessel which corre-
sponds to the right fourth arch is usually described as a portion of the stem of the
subclavian artery in the adult. The aortic branches on the left side appeal, as
follows in the adult : first, the common carotid, the stem of the original first to
third arches; second, the subclavian, which includes a part of the fourth arch, but
consists chiefly of what was originally only a branch of the arch. The pulmonary
artery has as its stem a portion of the fifth arch, in man on the left side only,
but the vessel arises as a separate branch from the fifth arch.

There often appear irregular vessels in early stages between the fourth and
fifth arches, and these have been held by some writers to represent an additional
partially aborted aortic arch. If this is the case, then the arch here called the
fifth would be more correctly termed the sixth. The arch supposed to be lost is
sometimes distinguished as Zimmermann 's arch.

102

THE EARLY DEVELOPMENT OF MAMMALS.

The descending and median aortae give off intersegmental vessels, some of
which persist in the adult. The main aorta produces three main branches on its
ventral side, the histories of which are somewhat complicated. They are the
coeliac axis, the superior mesenteric, and the inferior mesenteric.

The Venous System.

The veins do not for the most part, if at all, arise as independent vessels, but
by the transformation of channels in a network of small vessels. Arteries develop
in the same way, but with them the growth of the main stem as such plays a

FIG. 57. — CHICK EMBRYO WITH SEVENTEEN SEGMENTS. DRAWN FROM A SPECIMEN WHICH HAD BEEN AUTO-
INJECTED WITH INDIA-INK. — (After H. M. Evans.)

greater role. The method of development is illustrated by figures 57 and 58, From
the first aortic arch a network of small vessels spreads into the head at the sides
of the fore-brain and mid-brain, gradually occupying an increasing territory- until the
whole head is supplied. The plexus forms a single vascular layer between the brain
and epidermis and is at first without any main channels (Fig. 57). Soon some of
the capillaries enlarge, forming a branching system (Fig. 58), the branches leading
into a main stem which extends from the head to the posterior or venous end of
the heart. This stem, which rapidly increases in size, is the anterior cardinal vein,

THE VENOUS SYSTEM.

103

which is matched by a similar vein on the opposite side of the head. The 'remain-
ing primitive veins and the later secondary veins are all formed in a like manner.
The first veins to be formed are the omphalo-mesaraic, which are evolved from
the vascular network of the area vasculosa, and extend into the body of the em-
bryo, running in the splanchnopleure (Fig. 158, Om.S, Om.D). They approach
one another in the median line (Fig. 59), unite, and are prolonged forward to make
the endothelial heart.

FIG. 58. — CHICK EMBRYO WITH TWENTY-FIVE SEGMENTS. DRAWN FROM A SPECIMEN WHICH HAD BEEX AUTO-
INJECTED WITH INDIA-INK. — (After H. M. Evans.)

There are three other pairs of main primitive veins, all developed entirely within
the body of the embryo: i, the anterior cardinals, which drain the head (com-
pare Fig. 154, card, and Fig. 155, Ve)\ 2, the posterior cardinals, which drain the
body from the tail to the heart and occupy each a characteristic position laterad
from the aorta, and dorsad from the splanchnocele (Fig. 158, card, and Fig. 159,
card, card.s}; the two cardinal veins unite at the level of the venous end of the
heart and form thus on each side a short transverse stem, the common cardinal,
which opens into the heart and was. formerly named the ductus Cuvieri; 3, the

104

THE EARLY DEVELOPMENT OF MAMMALS.

umbilical veins, which are differentiated a little later from the vascular plexus of
the somatopleure. (Compare Figs. 90, 93, and the description of the veins in the
human embryo of the ninth stage, p. 143.)

The pulmonary veins represent a new system of vessels added to those already
mentioned. Their exact origin has not been traced, but about the time the branch-
ing of the entodermal lung begins, they appear and open into the left auricle of
the heart.

The eight pairs of primitive veins pass through complicated metamorphoses
owing principally, first, to the development of new branches to receive the blood
from the organs as they arise; second, to the conversion of small collateral channels

Ao.ht.

Ve.ht.

Om.mes.

FIG. 59. — CHICK OF ABOUT 40 HOURS. VIEW PROM UNDERNEATH OF PART OF THE VASCULAR SYSTEM.

Ao.ht, Aortic limb of endothelial heart. Om.mes, Omphalo-mesaraic vein. PI, Part of the vascular plexus of the

area vasculosa. Ve.ht, Venous end of the endothelial heart. X 25 diams.

into main vessels; and, third, to the obliteration of parts of the original vessels.
The first process is illustrated by the history of the veins of the limbs; the second
by the formation of the lateral vein of the head and of the subcardinal vein of
the Wolffian body; the third, by the disappearance of the right umbilical vein and
of portions of the omphalo-mesaraic veins.

The vena cava inferior is a new pathway formed by utilizing portions of several
originally distinct and separated venous channels. At first the blood from the ab-
dominal viscera must return to the heart chiefly through the posterior cardinal
veins (Fig. 53, card), but the vena cava inferior offers a direct course. Its
development depends primarily upon the union of the cephalad end of the right
Wolffian body with the liver, followed by a vascular fusion of the two organs which
renders it possible for the blood of the right subcardinal vein to pass through the

THE LYMPHATIC SYSTEM. 105

blood-spaces of the liver directly to the heart. This makes a very direct channel, a
more direct one than existed previously when the blood from the subcardinal came to
join that of the cardinal, passing up to the common cardinal and then back to the
heart. The new channel through the liver rapidly enlarges and becomes recogniz-
able as the vena cava inferior. This important venous trunk is a combined vessel,
comprising, first, a part of the hepatic vein; second, a large channel developed
from the sinusoids of the liver; third, the upper part of the right subcardinal vein,
and, fourth, the lower part of the right cardinal.

The Lymphatic System.

The lymph vessels arise in connection with the veins and are probably out-
growths of the vascular endothelium, although some authorities state that they may
begin as sub-endothelial spaces. They may be recognized by their very delicate
but distinct endothelial walls, thus differing from the mesenchymal spaces-, and by
the complete or almost complete absence of blood-corpuscles within them. They
probably end blindly. They appear relatively late, for they do not develop until
after the limb buds are well advanced (pigs of 14 mm.). The first lymph-vessels
develop from the anterior cardinals in the cervical region, and rapidly fuse to
make a pair of very large vesicles, the jugular lymph-sacs (Fig. 60, S.l.j], which
are closely applied to the veins. Each jugular lymph-sac empties into the cardinal
vein near its junction with the subclavian through a valve-like orifice. Whether
this connection is primary or secondary is still uncertain. From each sac, narrow
vessels bud out into the mesenchyma, anastomose with one another, and, by spread-
ing more and more, produce the lymphatic system of the neck and head. Sub-
sequently, the jugular sac is resolved into the deep cervical plexus' of lymphatics.
Similar lymph-sprouts in slightly older embryos produce a. more irregular median
mesenteric sac (Fig. 60, S.l.m) just below the renal anastomosis, R.A; and also
another sac, Cis, dorsal to the first. The mesenteric sac sends branches into the
mesentery to drain the intestine, and at the same time joins the dorsal sac which
enlarges, becoming the cisterna chyli. Still later there are formed in a similar
manner two smaller sacs of dense plexuses, termed the sciatic, for they develop in
connection with the root of the sciatic vein (Fig. 60, Sci). The sciatic sacs are
produced later than the other four. Like the jugular, the four later formed sacs
serve as centers of growth for the lymph-vessels. In addition, other lymphatics
develop from other veins. Especially notable are the sprouts from the azygos
vein, which unite and ultimately give origin to the main lymphatic trunk, the
ductus thoracicus, which joins tailward the cisterna, headward the left jugular sac.

The lymph-glands make their first appearance considerably later than the
vessels (rabbits, 25-30 mm.; human embryos, 30-45 mm.). Each appears be-
tween a vein and a lymph-vessel and is recognizable as mesenchyma crowded
with young leucocytes. The glands are very small at first, but are quite sharply
circumscribed.

106

THE EARLY DEVELOPMENT OF MAMMALS.

FIG. 60. — PIG OF 20 MM. SERIES 59. RECONSTRUCTION OF THE LYMPH-VESSELS AND PART OF THE VENOUS

SYSTEM. BY F. T. LEWIS.

Az, Azygos vein (remnant of the posterior cardinal). Br, Brachial vein. C.C, Common cardinal vein. Ce,
Cephalic vein. Cis, Cisterna chyli. Ex.J, External jugular vein. Fe, Femoral vein. G, Gastric vein.
In.J, Internal jugular vein. R.A, Renal anastomosis between the subcardinal veins. Sci, Sciatic vein. S.l.j,
Jugular lymph-sac. S.l.m, Mesenteric lymph-sac. S.M, Superior mesenteric vein. Th.ep, Thoraco-epigastric
vein. V, Omphalo-mesaraic or vitelline vein. V.C.I, Vena cava inferior. X 8 diams.

THE PANCREAS.

107

The Liver.

When the omphalo-mesaraic veins, the first large veins to appear, are developed,
they are situated in the splanchnopleure and join the heart. They are of such
large size as to cause a projection into the coelom. This projection is the septum
transversum (p. 87). As shown in the diagram (Fig. 16), the entoderm of the
digestive canal of the head of the embryo passes over behind the pericardial cavity
and behind the septum transversum into the yolk-sac. Out of the entoderm cover-
ing the septum transversum on its caudal side, the anlage of the liver is developed
(Fig. 25, fo). This anlage is produced by a rapid proliferation of the entodermal
cells, and they grow toward the space occupied by the omphalo-mesaraic veins
(Fig. 157). An intergrowth of the liver cells and of the endothelium of the veins
takes place. The cavity of the veins becomes subdivided into smaller blood chan-
nels, which we call sinusoids to distinguish them from capillary vessels. The liver
cells arrange themselves in the form of cords which are termed the hepatic cylinders.
Each hepatic cylinder is closely invested by the venous endothelium. The liver
consists at first only of hepatic and endothelial cells and is situated in the septum
transversum.

When the liver becomes larger, it protrudes
from the septum transversum, but does not
separate from it, so that in the adult the liver is
always found attached to the diaphragm, which
is merely the modified septum transversum.

The Pancreas.

The pancreas is a double organ, for it arises
from two distinct anlages: first, the dorsal pan-
creas, which appears as an entodermal evagination
on the dorsal side of the duodenum (Fig. 61,
Panc.d), soon branches, and, continuing to grow,
rapidly develops into the body and tail of the
adult organ. Its connection with the intestine
becomes the dorsal pancreatic duct (ductus San- M, ', 4 Cords of hepatic cells. Panc.d,

-

torini). The ventral pancreas appears a little

later and grows more slowly than the dorsal. It
arises as an Outgrowth (Fig. 6l, Panc.v} from

the entodermal ductus choledochus. It develops
into the head of the pancreas and dorsal pancreatic duct (ductus Wirsungi). The
two anlages expand, meet (Fig. 62), and unite. In the pig, the dorsal anlage is
farther from the stomach than the ventral, but in man it is nearer the stomach.

In the pig, the ventral pancreatic duct is obliterated and the dorsal duct alone
is normally persistent in the adult. In man, on the contrary, the dorsal duct is
normally obliterated and the ventral duct persists. Occasionally both ducts are

Ves-fel*

FIG. 61. — PIG EMBRYO OF 5.5 MM. SERIES
915. WAX RECONSTRUCTION OF THE
STOMACH AND PANCREATIC ANLAGES
BY F. W. THYNG.

Dorsal pancreas. Panc.v, Ventral pan-
creas. St, Stomach. Ves.fel, Gall-
bladder, x, Ventral process of the
dorsal pancreas (situated on the right of
the portal vein). X 55 diams.

108

THE EARLY DEVELOPMENT OF MAMMALS.

found in human adults, and in such cases the dorsal duct has been commonly
termed "accessory."

The cords of pancreatic cells are at first solid, but they for the most part
acquire lumina, thus becoming epithelial gland-tubes. The areas of Langerhans
are small patches of pancreatic cell cords found in the adult without any lumina.

FIG. 62. — PIG EMBRYO OF 20 MM. SERIES 60. WAX RECONSTRUCTION or THE DUODENUM AND PANCREATIC

ANLAGES BY F. W. THYNG.

Div, Duodenal diverticulum. D.chol, Ductus choledochus. D.panc.d, Ductus pancreatis dorsalis. D.pqnc.v,
Ductus pancreatis ventralis. Pane. ace, Pancreas accessor! um (anomaly). Panc.d, Pancreas dorsale. Panc.v,
Pancreas ventrale. St, Stomach, x, Ventral process of the dorsal pancreas, on the right of the portal vein.
X 55 diams.

The Excretory Organs.

No less than three distinct excretory organs are known to occur in vertebrates.

Of these, the first is termed the pronephros, or head kidney, on account of its
position toward the head and in the neighborhood of the heart. It is well developed
and the only excretory organ in certain fishes and in the early larval stages of
amphibia. In elasmobranchs, which occupy in this respect an exceptional position,
and in amniota it exists in a rudimentary form only, except as to its duct, which
plays an important role in the further development. The pronephros consists of

THE EXCRETORY ORGANS.

109

few epithelial tubes which take a somewhat twisting course, but may be said to
run, in general terms, transversely. Each tube begins with a ciliated funnel-shaped
opening (Fig. 63, /) not far from the median line of the embryo, and ends, after
a more or less contorted course, in a longitudinal duct, which, after receiving all
of the tubules, runs toward the posterior end of the embryo and opens into the
extremity of the entodermal or digestive canal. Opposite the funnels, and separate
from the pronephros proper, there is a so-called glomus (Fig. 63, gl), which is a
projection of not inconsiderable size from the mesentery. When fully developed
the glomus contains a rich network of blood-capillaries, so that it somewhat resem-
bles the glomerulus of the kidney. The circulation of the pronephros is sinusoidal.
The second of the excretory organs is termed the mesonephros, Wolffian body,
or fetal kidney. It is the only excretory organ in elasmobranchs. In adult am-

•nch

•Ec

FIG. 63. — FROG (RANA TEMPORARIA) TADPOLE OF 12 .o MM. CROSS-SECTION OF THE PRONEPHRIC REGION.

nch, Notochord. m, Muscles. /, Pronephric funnel, v, Blood-vessel. EC, Ectoderm, t, Pronephric tubule.

gl, Glomus. Lu, Lung. X 90 diams. — (After M. Furbringer.)

phibians it replaces the pronephros, which i§ purely a larval structure. It is pres-
ent in the embryos of all amniota, but undergoes a partial degeneration before
adult life, being itself replaced in adult amniota by the true kidney. The meso-
nephros resembles somewhat the pronephros, especially as found in the ichthyopsida.
It occupies a much larger region of the body than the pronephros. It has no
glomus associated With it, but each tubule contains a glomerulus very similar in
its general structure to the glomerulus of a true kidney. In the ichthyopsida each
tubule begins with a ciliated funnel, and, after making several coils, opens into the
pronephric duct. In the amniota the mesonephros, or, as it is more commonly
called in these animals, the Wolffian body, is essentially an embryonic structure.
Its tubules, however, do not have at any stage the ciliated funnels to be found in
amphibia and fishes, but they have glomeruli and they open into the pronephric

110 THE EARLY DEVELOPMENT OF MAMMALS.

duct, which, on account of its relations to the organs is in this type more com-
monly spoken of as the Wolffian duct. The circulation of the organ is sinusoidal.
Further details are given in the practical part in connection with study of the pig
embryos, pages 252 and 306.

The third of the excretory organs is termed the metamphros, or true kidney.
In the mammalian embryo, after the Wolffian body has acquired a considerable
development, there, appears a small outgrowth of the Wolffian duct at a point near
the junction of the duct with the allantois. It extends dorsad and cephalad (Fig.
172, ki), and may be termed the renal evagination. Its blind end expands to be-
come the pelvis of the kidney, while its stalk remains narrower and is converted
into the ureter, by which the urine is conveyed from the pelvis to the bladder
(allantois). By outgrowths of the epithelium of the pelvis the collecting tubules are
developed. Around the pelvis appears an envelope of special cells, easily recognized
by their darker staining (Fig. 210). These cells are thought to be derived from the
nephrotomes of the segments of the renal region, which have lost their epithelial
arrangement and have migrated to form the envelope. The cells gather themselves
gradually into small clumps, .the number of clumps continuing to increase during a
long period. Each clump assumes an epithelial arrangement and acquires a lumen
and elongates into a tubular form. The tubule elongates, one end joins a collect-
ing tubule, and the lumina of the two structures become continuous. The other
end of the tubule remains closed and is converted into the renal corpuscle. The
tubule now grows rapidly in length and becomes very irregular in its course; the
part which joins the collecting tubule becomes the proximal convoluted tubule; the
part which joins the renal corpuscle becomes the distal convoluted tubule, and the
middle part between these two becomes the loop of Henle.

The Urogenital Ducts.

The genital and excretory organs always develop together and constitute the
urogenital system. The genital glands are always distinct, but the primary excre-
tory or Wolffian duct, after producing the outgrowth to form the renal anlage (p.
309), is transformed into the permanent male duct. The other primary canal is'
termed the Mullerian duct. It is exclusively genital, for with its mate it develops
the uterine tubes, the uterus, and the vagina, but in the male it becomes vestigial.

The Wolffian duct is the excretory canal of the mesonephros (p. 109). It
extends along the ventral surface of the organ; the mesonephric or Wolffian tubules
acquire openings into it, and it itself opens at its caudal end into the base of the
allantois. It serves for a time as a true excretory duct, but loses this function
when the mesonephros degenerates. It meanwhile acquires a secondary connection
with the testis by means of some of the Wolffian tubules in the neigrfborhood of the
genital gland. The tubules in question acquire a connection with the sexual
cords of the testis and, when the cords become seminiferous tubulef, the Wolffian
tubules are ready to conduct the semen to the Wolffian duct (vas defer ens). The

THE ALL AN TO IS. Ill

course of the Wolffian duct is changed during fetal life by the migration of the
testis from its original abdominal position into the scrotum. In the female the
Wolffian duct becomes vestigial.

The Mullerian ducts develop much later. They may be found in the 12 mm.
pig as two short funnels formed by the mesothelium and situated on the ventral
side of the mesonephros near the septum transversum (diaphragm). The funnels
point backward and grow into tubes, which run on the ventral side of the Wolffian
duct and presently connect with and open into the base of the allantois. The
pelvic portions of the two Mullerian ducts approach one another in the median
line and in the female they fuse, making a median epithelial canal, the anlage of
the uterus and vagina. The original entrance to the canal persists as the fimbriate
opening and the stretch of the original canal between the funnel and the uterus
becomes the uterine tube.

The Allantois.

The allantois is a diverticulum of the entodermal canal, and is, therefore, lined
by entodermal epithelium (Fig. -21). It arises on the ventral side of the caudal end
of the embryo in proximity to the anal plate. In its development we can distin-
guish two main types. The first type is illustrated by the sauropsida and the un-
gulates. In them it grows out and rapidly enlarges so as to form a vesicle of
considerable size and connected with the embryo by means of a narrow hollow
stalk. When the allantois develops according to this type, it is spoken of as free,
because it has no connection with the extra-embryonic somatopleure (chorion and
amnion). This form of the allantois may be readily observed in chick embryos,
for by the fourth day it has become a considerable rounded vesicle which lies
in the extra-embryonic ccelom between the yolk-sac and the extra-embryonic soma-
topleure or membrana serosa. During the fifth day it rapidly enlarges, and at the
beginning of the sixth day is nearly or quite as large as the head of the embryo.
In ungulates the growth of the free allantois begins very early and becomes enor-
mous. Its principal expansion is sideways, that is to say, at right angles to the
axis of the embryo, and it becomes a large sac, very much larger, indeed, than the
entire embryo.

The second type of allantois occurs in the placental mammals of unguiculate
series and is not known to occur in 'any species of the ungulate type. In probably
all unguiculates the posterior end of the body has a prolongation which is known
as the body-stalk (Fig. 69, b.s). Into this body-stalk the diverticulum consti-
tuting the allantois extends (Fig. 80, All, and Fig. 25, All). The entoderm of
the allantois is surrounded by mesoderm, which is present in the body-stalk in con-
siderable volume. On the outer surface there extends a layer of ectoderm, so that
the three germ-layers enter into the formation of the body-stalk as they do into the
formation of the embryo. These relations are illustrated by the diagram (Fig. 64).
By means of the body-stalk a connection is established between the embryo and

112

THE EARLY DEVELOPMENT OF MAMMALS.

Cce.

Cho.

Yk.

the extra-embryonic somatopleure or primitive chorion, Cho. Later, when the for-
mation of the amnion is completed, the essential relations are found to be as
illustrated by the diagram (Fig. 64, B). The amnion arises from the distal end
of the body-stalk, but the body-stalk retains its connection with the chorion. When
the allantois becomes free, the connection with the chorion is entirely lost. The
maintenance of that primitive connection in the unguiculates is to be regarded as a
new modification of the relations of the embryonic appendages, evolved only in

the higher animals. The maintenance of
that connection makes possible the modi-
fication in the structure of the chorion,
which is of the greatest morphological
importance. This modification is the
development of the blood-vessels in the
chorion. The anlages of these blood-
vessels are outgrowths of the embryonic
angioblast. They appear so as to form
four vessels which grow through the
length of the body-stalk in the neighbor-
hood of the allantoic diverticulum. Two
Am. of these vessels are veins and two are
Emb arteries. They are termed the umbilical
vessels. The umbilical veins at the
Cce. embryonic end of the body-stalk enter
the somatopleure of the embryo (Fig.
Yk. 186, V. U.S. , V. U.D), through which
they make their way toward the heart
(Fig. 93, Alv). The umbilical arteries
enter the body of the embryo, pass
caudad alongside the allantois (Fig. 210,

A, Before, B, after the formation of the amnion. All, A.um), Curve past the cloaca onto the

Entodermal allantois. Am, Amnion. b.s, Body- dorsal side of the body (Fig. 169, A Mm),
stalk. Cho, Chorion. Cos, Extra-embryonic

coelom. Emb, Anterior end of embryo. Yk, and Jom the Caudal Cnd °f the a°Fta>

Yolk-sac. so that they may be termed the termi-

nal branches of the embryonic aorta.

In early stages they are the largest branches which the aorta has. At the distal
end of the body-stalk the four vessels enter the mesoderm of the chorion, there
branch abundantly, and produce a rich network of blood-vessels throughout the
entire membrane. The unguiculate mammals, therefore, are characterized by this
special feature, the possession of the body-stalk which contains the allantoic diver-
ticulum and gives access for the blood-vessels, and therefore also, of course, for the
blood, to the chorion, which thus becomes vascular. In all other amniota the
chorion is without blood-vessels.

Cho.

FIG. 64. — DIAGRAMS ILLUSTRATING THE RELATIONS OF
THE ALLANTOIS IN UNGUICULATE MAMMALS.

THE ALL AN TO IS. 113

The size of the allantoic cavity in unguiculates varies considerably. In man
it is minimal, forming only a long and very narrow tube (compare Fig. 66, All).
In rodents it expands somewhat, but it never becomes free in the sense that it is
separated from the body-stalk, although it may acquire a partial independence. In
this case it may also become more or less vascular by the development of branches
from the umbilical arteries and veins around the allantois.

In those animals in which the allantois is free, the umbilical arteries and veins
have all their branches in the allantois, there being no body-stalk. The embryo
is without connection with the chorion, and, therefore, these vessels in their rami-
fications are restricted to the allantois.

Relations of the Allantois to the Chorion in Ungulates. — Since the true chorion is
the outermost of the fetal envelopes, it alone can come in contact with the walls
of the uterus. All placental developments, therefore, necessarily depend upon the
chorion. Now, in ungulates, where the chorion is without blood-vessels, there is
no circulatory apparatus to transfer any nutritive material, which may be taken up
by the chorion from the uterus to the embryo, until a second union takes place
between the vascularized allantois and the chorion. The inner surface of the
chorion and the outer surface of the allantois are both mesodermic. The two
mesodermic layers come into contact with one another and unite loosely. The
vessels of the allantoic mesoderm are thus brought into physiological union with
the chorion, but, being allantoic vessels, they are, of course, morphologically differ-
ent from the chorionic vessels of unguiculate mammals. These considerations
demonstrate that the ungulate placenta is allantoic rather than chorionic, and is,
morphologically speaking, essentially different from the true chorionic placenta,
which can be developed only in those animals and embryos which have a perma-
nent body-stalk.

The simple relations of the chorion in the Ungulata to the uterine wall is
illustrated by the accompanying figure 65, which shows a portion of the chorion
of a pig embryo of 15 mm., together with the surface of the uterus to which it was
fitted. The two membranes were accidentally separated in the preparation. The
chorion consists of a layer of cylinder epithelial cells, EC, each of which can be
distinctly made out, and of a layer of mesoderm, Mes, containing only few cells
and blood-vessels, two of which, Ve, are shown in the section; the mesodermic
cells are a little more crowded near the epithelium. Each ectodermal cell is
distinctly marked off from its neighbors by a line. The protoplasm stains some-
what; the nuclei are slightly oval and granular, and are situated near the middle
of the cells. The top of each cell is concave. The uterine epithelium, Ut.Ep,
resembles in the general form of its cells and in the character of its protoplasm
the chorionic ectoderm, but differs from it in that each cell has a convex free end,
and, further, in that the nuclei of the cells are situated near the top of the layer.
When the relations of the two epithelia have not been disturbed, it is readily
observed that the concavity of each chorionic ectodermal cell receives the convex

114

THE EARLY DEVELOPMENT OF MAMMALS.

end of the uterine epithelial cell, so that the two layers are closely fitted together,
cell for cell.

The Bladder.— The allantois extends from the cloaca to the umbilicus, and
beyond the umbilicus into the umbilical cord. It comprises, therefore, an embry-
onic and an extra-embryonic portion. The former is the anlage of the urogenital
sinus, the urethra, and the bladder. The embryonic portion is always united to
the abdominal wall (Fig. 210), the mesenchyma, which surrounds the entodermal
allantois, All, and the umbilical arteries, S.um, being fused in the mid-ventral line

Ut.Ep.

Conn.

EC.

Mes.

FIG. 65. — PIG, 15.0 MM., SERIES 135, SECTION 58, TO SHOW THE RELATIONS OF THE CHORION TO THE UTERUS.
Conn, Connective tissue of the uterus. EC, Chorionic ectoderm. Mes, Choriomc mesoderm. Ut.Ep, Uterine
epithelium. Ve, Chorionic blood-vessel. X 350 diams.

with the mesenchyma of the somatopleure. This connection is retained throughout
life. The opening of the Wolffian ducts into the allantois is established very
early. The ureters at first open into the Wolffian ducts, but they soon migrate
so as to open separately directly into the allantois (bladder) above the ducts.

The Trophoderm.

Trophoderm is the name applied to the special layer of cells developed on the
outer surface of the ectoderm of the mammalian blastodermic vesicle. It has as
yet been observed only in unguiculates. The trophoderm layer may be devel-
oped over the entire surface of the ovum, as in man, or over only a portion
thereof, as in the rabbit and cat. Its principal known function is to destroy the
tissues of the uterus of the mother with which it comes in contact. The destruc-
tion of the tissue is supposed to serve two purposes: First, to supply nutrition to
the embryo. It is from this supposed function that the layer derives its name of

THE UMBILICAL CORD. . 115

trophoderm. Second, to secure the attachment of the ovum to the wall of the
uterus. This preliminary attachment is called the implantation of the ovum. In
some cases the trophoderm is developed very early over the surface of the ovum
(Fig. 74), appearing almost as soon as the stage of the blastodermic vesicle is
reached, and while the vesicle is very small. In such cases the ovum creates a
space for itself by dissolving away the epithelium and connective tissue at a small
spot on the uterine surface, making a cavity in which the ovum lodges. In other
cases the trophoderm is developed later and does not appear over the whole of the
blastodermic vesicle. The area over which it exists in such cases is called the
placental area (compare pages 127 and 179). The trophoderm in these forms
unites very closely indeed with the surface of the uterus (Fig. 37, Tro) and the
uterine tissues undergo degeneration and resorption. We may regard as the first
step toward the production of the placenta proper the disappearance of the tropho-
derm. Our knowledge of its disappearance is incomplete, but it is probable that
it is due to a transformation of the cells of the trophoderm, associated with con-
temporaneous modifications of the chorionic membrane, of which the general result
may be said to be formation of the chorionic villi which constitute the fetal pro-
tion of the placenta. The modified trophoderm cells are supposed to enter into
the formation of the ectodermal covering of these villi.

The Umbilical Cord.

The umbilical cord may be best defined as the tissues connecting the body
proper of the embryo with the amnion. It accordingly includes a portion of the
body-stalk and a certain extent of the body-wall or somatopleure. In early stages
we can hardly speak of an umbilical cord, because the amnion arises close to the
embryo (Fig. 83). As development progresses the body-stalk lengthens out and the
amnion arising from it recedes farther and farther from the embryo, this recession
being assisted by a growth of the somatopleure which leads to the formation of the
umbilical cord proper. By this means a tubular structure is produced, the cavity
of the tube being a prolongation of the coelom of the embryo. During the first
develbpment of the umbilical cord the neck of the yolk-sac becomes constricted and
very much lengthened out, forming the yolk or vitelline stalk. The yolk-stalk
springs within the embryo from the wall of the intestine, runs through the coelom
of the umbilical cord, and makes its exit beyond the amnion, as shown in figure
102. The yolk-sac proper still occupies its original position between the amnion
and chorion. The student should note carefully that the umbilical cord is never
covered by the amnion, for it has unfortunately been often stated that it is so
covered. Ah idea of the relations can be gathered from cross-sections of the cord
(Fig. 66). The ccelom, Cw, is a large cavity and contains the yolk-stalk, Y, with
two blood-vessels, but with its entodermal cavity entirely obliterated. Above the
body-cavity is the duct of the allantois, All, lined by entodermal epithelium, and in

116

.THE EARLY DEVELOPMENT OF MAMMALS.

its neighborhod are two arteries and a single vein. In yet earlier stages there are
two veins. The outer surface of the section is bounded by ectoderm. The further
development of the cord depends upon the growth of the connective tissue and
blood-vessels, the abortion first of the coelom, later of the yolk-stalk, and lastly of
the allantoic duct. Remnants of the allantoic epithelium are, however, often found
in the umbilical cord even at birth. There occurs also a further differentiation of
the connective tissue and of the entoderm.

The umbilical cord is characteristic of mammals. It varies greatly in length.
In the pig it is very short. In man it attains great length and size, becoming at
full term about 55 cm. in length and 12 mm. in thickness. When fully^ developed
the human cord has a whitish color and presents a twisted appearance somewhat

FIG. 66. — SECTIONS OF Two HUMAN UMBILICAL CORDS.

A, From an embryo of 21 mm.; B, from an embryo of sixty-four to sixty-nine days. All, Allantois. Ar, Umbilical
artery. Cce, Coelom. v, Umbilical vein. Y, Yolk-stalk.

like a rope. Its surface is smooth and glistening. The attachment of the cord to
the embryo is known as the umbilicus. Its attachment to the chorion is in the pla-
cental region (chorion frondosum).

The twisting of the cord is well marked externally at the time of birth by the
spiral ridges, within each of which a large blood-vessel runs. The number of spirals
varies from 3 to 32, the turns beginning at the embryo, and, though usually from
left to right, are sometimes from right to left. The twisting begins about the middle
of the second month. Its cause is unknown, but there is no reason to assume that
it is due to revolutions of the embryo. The cord is covered by a layer of epithe-
lium which is continuous at the distal end with the epithelium of the amnion, and
at the proximal end with the epidermis of the embryo. The cord contains typically
no capillaries, and, except in the immediate neighborhood of the embryo, no nerve-
fibers.

THE CH ORION AND AM N ION. 117

The Chorion and Amnion.

These are two membranes which always surround the embryo, the chorion
being the outer, the amnion the inner, membrane of the two. Morphologically, they
are modifications of the extra-embryonic somatopleure. The accompanying diagrams
render this clear. In figure 29, we see that the cavity of the mesoderm, coe, has ex-
tended completely around the yolk. There is a layer of mesoderm, represented by
a dotted line, on the outside of this cavity, which joins with the overlying ecto-
derm, represented in the diagram by a continuous line, to constitute the somato-
pleure, Som. In figure 45, we see the somatopleure folded up on the dorsal side of
the embryo; the leaf of the fold nearest the embryo is the anlage of the amnion,
aw; the rest of the extra-embryonic somatopleure is the anlage of the chorion' In
the second figure of the diagram, the two folds have met above the embryo and
united, thus making a closed inner amniotic and a closed outer chorionic envelope.
The actual appearances of two such stages as in the diagram are illustrated by
figures 38 and 47. By this account we learn that the two envelopes are produced by
a folding of the somatopleure.

When we come to study the development of mammals in detail, we discover
that there are many remarkable variations in the early development of the amnion
of which no general explanation is yet possible; but inasmuch as the folding pro-
cess is the only one in Sauropsida, and also occurs in many mammals of different
classes, it is generally assumed to be the primitive method.

In man the development of the chorion and amnion differs extremely from the
scheme given above. It is described as accurately as our present knowledge per-
mits in Chapter III.

However developed, the fetal envelopes present certain constant characteristics:
both consist of ectoderm and mesoderm, but in the case of the amnion the ecto-
derm is turned toward the embryo, whereas the chorionic ectoderm faces the out-
side. The cavity between the amnion and the embryo becomes filled with the
amniotic fluid, which serves as an important mechanical protection to the develop-
ing embryo. It is through the chorionic ectoderm only that the ovum can come
into actual contact with the walls of the uterus. It is the chorion alone which is
concerned in the formation of the true placenta (compare Chapter VII).

The amnion is a thin, pellucid, non- vascular membrane; the chorion is thicker,
more nearly opaque, and has in man and all nearly related animals a highly de-.
veloped vascular system.

CHAPTER III.
THE HUMAN EMBRYO.

Our knowledge of the early stages of human development is very imperfect.
Upon the fertilization and segmentation of the ovum in man there are no obser-
vations whatever at present. It is not even known exactly how long the ovum
requires for its passage through the Fallopian tube. The earliest stages of which
we have comparatively adequate accounts are those represented by Peters's ovum
(1899) and Herzog's (1909). A number of human embryos in various early stages
after the formation of the medullary canal and up to the stage with four aortic
arches have now been reported and studied, some few of them thoroughly and
carefully.

Calculation of the Age of the Human Embryo. *

The age of the embryo must be reckoned from the date of the fertilization of
the ovum, which presumably occurs in man in the upper third of the Fallopian
tube. It may be that ova become fertilized at various epochs, but fail to continue
their development except when the fertilization occurs at the beginning of a men-
strual period. Ovulation occurs at all periods, but most frequently about the
time of menstruation, which is the expression of structural changes in the uterus
which enable the ovum to implant itself in the uterine wall. Hence only when
fertilization coincides with the beginning of menstruation can conception follow
with the result that the menstrual flow is stopped. Accordingly, the age of the
embryo is usually to be reckoned from the date of the beginning of the first
menstrual period which has lapsed.

Experience, however, shows that sometimes conception occurs without stopping
the menstrual change at the time, but eliminating only the subsequent periods,
and in such cases the age must be estimated from the beginning of the last
menstruation. In the two cases the age of the embryo would differ by a month
(twenty-eight days), and this difference is so great that it obviates errors of
estimate.

Up to the end of the ninth week the form and size of the embryo exhibit a
correlated development, so that generally an embryo at a given stage of develop-
ment in form will agree with its fellows in size; but to this rule there are not in-
frequently exceptions, and sometimes an embryo is found much larger than others

118

THE CLASSIFICATION OF THE EARLY STAGES. 119

at the same stage. Moreover, the variability of embryos is very great, for in
specimens otherwise alike we find this or that organ advanced or retarded in its
development as compared with the embryo as a whole. Nevertheless it is possible
with the information at command to determine with tolerable certainty the age of
an embryo within two days plus or minus, up to the end of the ninth week. For
the course of development during the third month we possess as yet no satisfactory
data, but embryos of full three months are quite frequently obtained, and are very
characteristic in size and configuration (see page 156).

F. P. Mall's formula for calculating the age of human embryos is

A/ioo X length in mm:
The length is measured from the vertex to the breech.

The Classification of the Early Stages.

Any attempt to divide embryos into stages must necessarily establish artificial
groups, for in nature there is no demarcation. Division into stages is for con-
venience, and ought, therefore, to be made by natural and obvious characteristics.
It seems to me that eleven stages may be conveniently discriminated, as follows:

First Stage. — Segmentation of the Ovum: The general process is described on
pages 42 to 45. There are no observations upon this stage in man, or any primate,
except one monkey's ovum in the four-cell s age described by Selenka.
* Second Stage. — Blastodermic Vesicle: The general development of the blasto-
dermic vesicle in mammals is described on page 45. Its development in man is
unknown. During this stage the embryonic shield is differentiated. An ovum
of a monkey in this stage is described on page 127, and one of the very few
known human ova is described on page 128.

Third Stage. — Primitive Streak : Two human ova with a primitive streak be-
fore the formation of the medullary plate have been observed.1 In one of these,
Frassi's embryo, the diameter of the entire ovum was 13 x 5 mm. The diameter
of. the yolk-sac 1.9 x 0.9 mm. The embryonic shield was 1.17 mm. long by 0.6
mm. wide. The primitive groove is shallow and occupies about half the length
of the shield. The anterior end of the groove marks the position of the future
neurenteric canal; its posterior end, the position of the anal plate.

Fourth Stage. — The Medullary Plate: In this stage there are several embryos
known. In all of them the amnion and chorion are already differentiated. There is
a large extra-embryonic coslom. The chorionic vesicle is rounded and somewhat
flattened. In its greatest diameter it measures from 8 to 10 mm. It is beset
with short branching villi which are present over the entire surface. The general
relations are indicated in the accompanying diagram (Fig. 69). The chorion has a
distinct epidermal and mesodermal layer. To its inner surface is attached the
body-stalk wihch unites the embryo and chorion. From it springs the amnion

'The Harvard Embryological Collection has an embryo, Series 825, in fine preservation. It is a little younger
than Frassi's. It is hoped to publish an account of it soon.

120 THE HUMAN EMBRYO.

covering the embryo, which measures only i.o to 1.5 mm., and from the ventral
surface of the embryo arises the yolk-sac, which is of rounded form and about
equal in diameter to the length of the embryo.

Fifth Stage. — The Medullary Groove: The general relations of the embryo and
its appendages are the same as in the previous stage (compare Figs. 82 and 25).
In the cases recorded the chorionic vesicle varied greatly in size. It bore villi
over its entire surface, and the villi were considerably branched. The embryos of
this stage vary in length, but measure about 2.0 mm. The medullary ridges are

FIG. 67. — HUMAN EMBRYO AT THE BEGINNING OF THE THIRD WEEK. — EIGHTH STAGE.
All, Allantois. Am, Amnion. -br, Branchial region. H, Head. Hr, Heart. Yk, Yolk-sac.

very characteristic, rising high above the yolk-sac and enclosing a deep medullary
groove between them. During this stage the formation of the segments is progress-
ing. Thus one of the embryos described had seven segments.

Sixth Stage. — Medullary Tube: In this stage the medullary groove is partly
closed and the heart is clearly differentiated. It must be remembered that the
closure of the medullary groove progresses slowly and is not completed until the
ninth or tenth stage. The embryo measures from 2.2 to 2.5 mm. in length.
The head projects well in front of the yolk. The primitive segments are partly
developed. In one case seven, in another thirteen, were found to have been
formed. The caudal end of the embryo also projects beyond the yolk, but less
than does the head (compare Fig. 83). The auditory imagination is probably not
yet formed. There are no gill-clefts showing externally.

THE CLASSIFICATION OF THE EARLY STAGES. 121

Seventh Stage. — One Gill-cleft Showing Externally: Not known by observation.

Eighth Stage. — Two Gill-clefts Showing Externally: Several embryos in this stage
have been found and some of them accurately studied. They usually have a re-
markable bend in the back (Fig. 67), which imparts to the embryo a very singular
appearance. Nothing similar to this bend or dorsal flexure has been observed in
any other embryos. It has been held by His and others to be a normal condition,
and not the accidental result of a mechanical strain exerted by the yolk-sac. If
the condition is normal, it must exist for only a very brief period, as it is not
encountered in older or younger stages. We may suppose if it is normal that the
change from the concave to the convex position of the embryo, as found in the
next stage, is very abrupt. The head of the embryo (Fig. 67) shows the character-
istic head-bend, and the tail end of the embryo is also bent over ventralward. The
heart is large and very protuberant. It is bent so that we can clearly distinguish
the auricular, ventricular, and aortic limbs. It shows distinctly its inner endothelial
portion and outer mesoderm. The yolk-sac extends from the heart backward to
where the body of the embryo turns to make the dorsal flexure. Between the
heart and the head the oral invagination has been formed, but is still separated by
the oral plate from the entodermic canal. Above the heart on either side is an
open invagination of the ectoderm, the anlage of the so-called otocyst, which in its-
turn is the anlage of the epithelial labyrinth of the adult ear. In one embryo of
this stage there were found twenty-nine primitive segments.

Ninth Stage. — Three Gill-clefts Showing Externally: This is, on the whole, the
best known of the early stages of human development. The embryos described as
belonging to it vary from 2.6 to 4.2 mm. in length. In one of them, in which
the embryo measured 3.2 mm., the chorionic vesicle measured n by 14 mm., and
its supposed age was from twenty to twenty-one days. The general shape of these
embryos is indicated -by figure 89.

The head is bent down and the back is very convex. In figure 89 the tail
is rolled up and turned to the left. Usually, however, the tail turns to the right
and the head is twisted to the left, so that the long axis of the body describes a
large segment of a spiral revolution; the spiral form is marked in embryos a little
older.

Tenth Stage. — Four Gill-clefts Showing Externally: The internal gill-pouches
reach the ectoderm, and for each there arises a corresponding external depression —
that of the fourth arch is often indistinct; hence this stage is more easily recog-
nized by the beginning of the limb-buds. A good embryo near the end of this
stage has been carefully studied by Broman.

Eleventh Stage. — Open Cervical Sinus: The cervical sinus is formed by the invagi-
nation of the ectodermal area of the fourth and fifth and later also of the third
gill-arches. The deep depression thus formed lasts for some time, but closes over
ultimately (embryos of 10 mm.). The eleventh stage comprises a relatively long
period.

122

THE HUMAN EMBRYO.

Hypothetical Development of the Blastodermic Vesicle in Primates.

As there exist no direct observations on the earliest stages of man, we can only
surmise what those stages may be. It is evident that there is a very precocious
development of the mesoderm, of the extra-embryonic ccelom, of the amnion, and
of the trophoderm, because these four features are found very marked in the ear-
liest known stages alike of man, apes, and monkeys. There are certain rodents
and insectivora in which these same peculiarities occur more or less emphasized
in the earliest stages of which we possess knowledge. If we utilize these data as
a basis, we can reconstruct the following hypothetical scheme of the earliest stages
in man.

The accompanying diagrams (Figs. 68 and 69) represent three successive purely
hypothetical stages of the human ovum. They are all conceived to represent longi-

Am.c. '

Ent.

FIG. 68. — Two DIAGRAMS TO ILLUSTRATE THE HYPOTHETICAL EARLY DEVELOPMENT OF PRIMATES.
Am.c, Amniotic cavity. Cae, Ccelom. EC, Ectoderm, in B, bearing the anlages of villi. Ent, Entoderm. Mes',
Somatic mesoderm. Mes.", Splanchnic mesoderm. Tro, Trophoderm.

tudinal sections. In the first stage the ectoderm, EC, forms a moderate sized vesicle
and is already thickened. It should probably be conceived as consisting of an
inner distinctly cellular layer and an outer much thicker trophodermic layer which,
is thickest over what corresponds to the embryonic region. This special thickening
is marked Tro in diagram A. The entoderm, Ent, forms a small vesicle underlying
the thickened portion of the trophoderm. -The mesoderm, Mes, is well advanced in
its development and already contains the large extra-embryonic coelom, Coe, and is
therefore divided into one layer which surrounds the entoderm, and a second layer
which underlies the ectoderm. In other words, the splanchnopleure and somatopleure
are already differentiated. In the next stage (Fig. 68, B) there has been a growth,
the ovum has become larger, the trophoderm has increased in thickness, and in
the mass of thickened ectoderm overlying the yolk-sac there has appeared a cavity
— the future amniotic cavity — which is, of course, entirely surrounded by ectoderm.
The portion of the ectoderm on the under side of this cavity consists of a single
layer of cells which by assuming a cylindrical form constitutes the thickened area-

THE BLAST ODERMIC VESICLE IN PRIMATES. 123

which we can identify as the embryonic shield (compare Fig. 13 and Fig. 68, B).
The solid mass of ectoderm above the amniotic cavity is later to form a part of
the amnion and part of the chorion. • At the posterior end of the embryo there
appears a considerable accumulation of mesoderm (Fig. 69, b.s}, which is the an-
lage of the body-stalk. Into this the entoderm has grown in the form of a cylin-
drical tubular prolongation, the anlage oi the allantois. As a consequence of the
growth of the trophoderm and of the formation of the amniotic cavity, the embryo
or embryonic shield, Emb, together with the yolk-sac, Yk, attached to it, has been
forced down into the interior of the chorionic vesicle. This phenomenon is very
marked in certain rodents and leads to the so-called inversion of the germ-layers.
In the next stage the amnion is formed. This is accomplished by the penetration
of the mesoderm with accompanying extension of the extra-embryonic ccelom into

Ent

' FIG. 69. — DIAGRAM OF AN EARLY STAGE OF A PRIMATE EMBRYO.

All, Allantois. Am, Amnion. b.s, Body-stalk. Cho, Chorion. Emb, Embryo. Ent, Entoderm. In, Ento-
dermal cavity of embryo. Vi, Villi of chorion. Yk, Yolk-sac.

the mass of the ectoderm overlying the amniotic cavity (compare Figs. 68, B, and
69) until the condition shown in figure 69 is brought about. This stage is known
by observation (compare Fig. 80). The amnion, Am, is now completely separated
from the chorion, Cho, which forms a relatively large vesicle and consists of a
thin layer of mesoderm, and a very thick layer of ectoderm, which has an inner
cellular stratum and an outer very much thicker trophodermic stratum. The
trophoderm is now very much altered by the appearance of numerous spaces or
channels in it which develop so that each of these spaces ends blindly toward the
interior of the chorion, buf many of them are open upon the surface of the tropho-
derm. As the ovum at this stage is already embedded in the uterine mucosa, the
'channels in the trophoderm can receive maternal blood, and such is their original

124 THE HUMAN EMBRYO.

function. The embryo and yolk-sac, as compared with the chorionic vesicle, are
very small in size. The body-stalk, b.s, is well developed and contains a well-
marked allantoic anlage, All, formed by the entoderm. The embryo includes as
yet very little, if any, mesoderm. Probably a neurenteric canal exists at this stage.
During the transition of stage B (Fig. 68) to stage C (Fig. 69), the blood-vessels
appear in the mesoderm of the yolk-sac.

Relations of the Embryo to the Uterus: the Two Stages.

The Two Stages. — During the first half or perhaps five months of pregnancy
the decidua reflexa is present. This period is called the first stage, to distinguish
it from the remaining period, or second stage, during which there is no decidua
reflexa. The reflexa during the first stage grows very thin and at the same time
degenerates. It is finally resorbed. The exact date of its disappearance is not
known, but falls somewhere in the fifth month. During the first stage the chorion
laeve is in contact with the decidua reflexa, during the second stage with the
decidua vera. On pages 343 and 345 a typical uterus of each stage is described.

The First Stage. — The study of young human ova and of early stages of
various primates leads us to conceive that the ovum first implants itself in the
mucous membrane of the uterus. The conception "implantation" is the outcome
of very recent researches. The essential idea we have formed of implantation is
that the trophoderm of the ovum corrodes or digests the uterine tissues with which
it comes in contact, and thus produces a cavity in which it is lodged and where
it attaches itself intimately to the maternal tissues. Owing to this process the
ovum is at first partly uncovered, and this condition seems to be permanent in
monkeys. In man and the apes, however, the uterine mucosa grows over the
exposed portion of the ovum, forming a layer of maternal tissue which separates
the ovum from the cavity of the uterus. This layer is the anlage of the decidua
reflexa. As the ovum grows, the decidua reflexa must also expand, and we soon
reach a condition in which the primitive relations of the parts can be easily
followed.

When the uterus becomes pregnant, the mucous membrane of the organ
undergoes changes in structure, and it is then commonly no longer termed the
mucosa, but the decidua or caduca. The decidual membrane is histologically
characterized by, first, modifications in the glands, the epithelium of which in large
part degenerates; second, the transformation of a large number of the connective-
tissue cells into cells of large size, which, on account of their being so extremely
characteristic, are called the decidual cells, and, third, by a growth of its blood-
vessels.

The decidual membrane of the uterus is divided into three regions: first, the
decidua serotina, the area (Fig. 70, s,s) to which the ovum is attached; second, the
decidua vera, comprising all the remaining portions of the mucosa forming part of
the walls of the body of the uterus; third, the decidua reflexa, the arching dome of

RELATIONS OF THE EMBRYO TO THE UTERUS.

125

aternal tissue, r,r, which rises from the walls of the uterus and completely encap-
sules the ovum. The arrangement of the parts is illustrated in figure 70, which
represents a median section of a uterus about five weeks pregnant. The whole
uterus is considerably enlarged. The mucous lining of the uterus is very greatly
thickened. The ovum is attached on the dorsal side of the uterus. This is the
normal position. The diagrams so com-
monly met with which represent the in-
sertion of the ovum at other points should
not be accepted by the student. The reflexa
rises around the ovum, completely covering
it in so as to make a closed bag. The
ovum itself is a sac known as the chorionic
vesicle. The trophoderm has now quite
disappeared, except so far as it persists to
cover the villi. The villi themselves are
shaggy and more or less branched. Their
tips are united either with the surface of
the decidua serotina or with that of the
decidua reflexa. In the interior of the
chorion is lodged the embryo with its yolk-
sac and surrounded by the amnion.

If the walls of the uterus are cut
through and simply reflected, leaving the
bag of the decidua reflexa intact, the ap-
pearances will be found essentially as in
figure 71. The mucosa is enormously hyper-
trophied and contains a great many dilated
irregular blood-sinuses. From the dorsal
side of the organ is suspended a large ', Anterior, & posterior surface, g, Outer limit of

the decidua. s,s, Limits of the decidua serotina.

FIG. 70, — SEMI-DIAGRAMMATIC OUTLINE OF AN
ANTERO-POSTERIOR SECTION OF A HUMAN
UTERUS CONTAINING AN EMBRYO OF ABOUT
FIVE WEEKS.

ch, Chorion, within which is the embryo enclosed
by the amnion, and attached to the chorion by
the umbilical cord; from the cord hangs the
pedunculate yolk-sac. r,r, Decidua reflexa.
c, Cervical canal. — (After Allen Thompson.)

closed bag or sac, the decidua reflexa,
D.ref, nearly filling the cavity of the uterus.
The reflexa presents in the stage figured
the same general appearance as the surface
of the uterus. If the reflexa be open, we
come, of course, upon the villous chorion of the ovum, and find, as above stated,
that only the tips of the villi are united with the surface of the reflexa. In the
fresh state the decidua is reddish gray, spongy or pulpy, soft, and moist. After the
fourth month it acquires, especially in the superficial layers, a duller brownish color,
which subsequently becomes more marked. This coloration is due to the decidual
cells. During the first two or three months the scattered openings of the uterine
glands can still be distinguished over the surface of the serotina and vera. The
surfaces themselves of the vera and reflexa, though somewhat irregular, remain

126

THE HUMAN EMBRYO.

more or less smooth. The inner surface of the reflexa is more irregular and
has protuberant parts united with the tips of the chorionic villi. The surface
of the decidua serotina, on the contrary, becomes very irregular during the
progress of pregnancy, forming little mounds which may become so high as to
resemble columns or so broad as to constitute septa. In later stages the septa
become very well developed, attaining a height of from 5 to 15 mm. They are
irregularly disposed, but subdivide the placenta of later stages into the so-called
cotyledons (compare page 362).

Muse.

OV.

-Ovd.

FIG. 71. — HUMAN UTERUS, ABOUT FORTY DAYS ADVANCED IN PREGNANCY.

Muse, Muscularis. Dv, Decidua vera. D.ref, Decidua reflexa. Ov, Ovary. Ovd, Oviduct (Fallopian tube).
Lig, Round ligament. Vg, Vagina. The uterus has been opened by cutting through the anterior walls
and reflecting the sides. — (After Coste.)

The Second Stage. — The body-stalk becomes converted into the umbilical cord.
This cord runs from the body of the embryo to the chorion (Figs. 70 and 87). It
is always connected with that portion of the chorion which is adjacent to the de-
cidua serotina. It carries the arteries and veins from the body of the embryo to
the chorion. From the end of the umbilical cord the blood-vessels branch out over
the chorion and into the chorionic villi. Thus the chorionic circulation of the em-
bryo centers about the chorionic end of the umbilical cord, and, as this end is in the
part of the chorion overlying the decidua serotina, we have here established from

OVUM OF A MONKEY IN THE SECOND STAGE. 127

the very start an important factor in the further differentiation. From what has
been said it is evident that the portion of the chorion underlying the decidua reflexa
is more remote from the center of the embryonic circulation. In the same way we
find that the decidua reflexa is remote from the blood supply in the uterus, and, as
a matter of fact, we may observe that during the second month of pregnancy the
blood-vessels, both in the decidua reflexa and in the portion of the chorion near it,
begin to disappear and ultimately are completely atrophied. After this atrophy has
been accomplished the circulation of the chorion is restricted to that portion over-
lying the decidua serotina. When the blood-vessels of the chorion under the de-
cidua reflexa abort, the villi also abort, so that this part of the chorion becomes
smooth, and is, therefore, called the chorion lave. Over the serotina the villi con-
tinue to grow, hence the corresponding region of the chorion becomes known as the
chorion frondosum. The chorion frondosum constitutes the fetal portion, the de-
cidua serotina the maternal portion, of the permanent placenta. The maternal blood
circulates in the intervillous spaces, which are bounded by fetal ectoderm. The
fetal blood circulates in the fetal blood-vessels of the chorionic villi. The circu-
latory channels of mother and fetus are always distinct, and no mingling of the
maternal and fetal blood is possible under normal conditions.

>

/ Ovum of a Monkey in the Second Stage.*

This embryo was obtained from a Semnopithecus nasicus in Borneo by Selenka,
who has also described an almost identical stage of S. pruinosus. It rested against
the wall of the uterus and was uncovered, there being no decidua reflexa developed
in monkeys. It measured about 2 mm. in its greatest diameter. Figure 72 repre-
sents a section through the ovum and adjacent tissues of the uterus. The chorionic
.vesicle is very large, but the embryo, Sh, and yolk-sac, Yk, are relatively very
small. The chorion on one side is quite smooth; on the opposite side it has devel-
oped numerous outgrowths, most of which are formed exclusively of the ectoderm,
but a few contain an ingrowth of mesoderm in their interior. The ectoderm on
the side toward the uterus has two layers, an inner cellular layer with relatively
small nuclei, and an outer syncytial or trophodermic layer with larger, nuclei of
variable size. The ovum occupies a depression on the surface of the uterus from
which the uterine tissues have disappeared, with the result of breaking through the
walls of some of the blood-vessels, bl.lac, so that now the maternal blood may es-
cape from these vessels into the spaces left between the irregular outgrowths and
the embryonic chorion. We must assume that the trophoderm of the embryo has
actually dissolved away or digested the tissues of the uterus, thus providing an
attachment for the ovum, securing its embedding in the wall of the uterus, and
establishing an opportunity for the maternal blood to flow into the intervillous spaces.
In later stages of the primates the trophoderm is very much reduced, and therefore

* Compare Classification of Stages, p. IIQ.

128

THE HUMAN EMBRYO.

fulfills its functions in the very earliest stages by establishing these primitive condi-
tions of blood-supply.

A section of the embryo on a larger scale is shown in figure 73. There ap-
pears only the embryonic shield, Sh, which is remarkable for its small area and
great thickness. The yolk-sac is also very small and is lined by a distinct layer of
entoderm, Ent. Above the embryonic shield is the amniotic cavity, which is, of
course, bounded by ectoderm which is continuous with the ectoderm of the embry-
onic shield. The amniotic cavity has a curious extension into the body-stalk, b.s,

Tro.

Cce. Mes. Sh. Yk.

Ant.c.

EC.

Gl. bl.lac. Conn. Gl.

FIG. 72. — BLASTODERMIC VESICLE OF A MONKEY (SEMNOPITHECUS NASICUS) ATTACHED TO THE UTERUS; VERTICAL

SECTION.

Am.c, Amniotic cavity, bl.lac, Blood-lacuna. Cce, Extra-embryonic coelom. Conn, Connective tissue of the
uterus. EC, Ectoderm. Gl, Gl, Uterine glands. Mes, Mesoderm of embryonic chorion. Sh, Embryonic
shield. Tro, Trophoblast. Vi, Mesodermic core of a chorionic villus. Yk, Yolk-sac. — {After E. Selenka.)

by which the embryo is connected with the chorion. The mesoderm is chiefly de-
veloped over the chorion, as shown in figure 72. It is very slightly developed in
the embryo (Fig. 73, mes), but forms a layer over the yolk-sac and over the am-
nion, and forms a considerable mass of tissue to constitute the body-stalk, b.s.

Human Embryo in the Second Stage.

The embryo to be described was investigated by H. Peters. It was found
attached to the dorsal wall of a uterus almost completely embedded in the mucosa,
but it was not wholly covered thereby, so that there was no decidua reflexa yet
present. A blood-clot overlay what would have been otherwise the exposed por-
tion of the ovum. The trophoderm formed an enormously thick layer of very

HUMAN EMBRYO IN THE SECOND STAGE.

129

irregular outline and contained many large spaces filled with maternal blood (Fig.
74). The exact external diameter of the ovum could not, therefore, be determined.
It measured, however, approximately 2.4 mm. by 1.2 mm. The internal diameter
of the chorionic vesicle was about 1.6 by o . 8 mm. The trophoderm is every-
where intimately united with the uterine tissue. The embryo, Sh, is represented
by an embryonic shield consisting of cylinder cells. It is small and lies on the
side of the ovum away from the cavity of the uterus. It rests upon the small
yolk-sac, Yk, and is overlain by the amniotic cavity, Am. c, which is bounded every-
where by ectoderm — on one side, of course, that of the embryonic shield; on the

A.mes.

Cce.

Ent.

FIG. 73. — EMBRYO OF THE PRECEDING FIGURE MORE HIGHLY MAGNIFIED.

Am.c, Amniotic cavity. A.ec, Amniotic ectoderm. A.mes, Amniotic mesoderm. b.s, Body-stalk. Cce, Extra-
embryonic ccelom. Ent, Entoderm. mes', Somatic, mes", splanchnic, mesoderm. Sh, Embryonic shield. —
(After E. Selenka.)

other the thin amniotic ectoderm proper. The mesoderm extends around the ovum,
forming a layer underneath the chorionic ectoderm over the yolk-sac and above the
amnion. At one point, close to the embryo and yolk-sac, it encloses a triangular
space the meaning of which is not known. As indicated in the figure, the meso-
derm was found to have shrunken somewhat, and the appearance of the embryo
and yolk-sac also suggests a somewhat imperfect preservation, histologically speak-
ing, of the tissues. As regards the condition of the uterus, the following points may
be noted. In the neighborhood of the ovum the decidua vera had acquired a
thickness of about 8 mm., while on the opposite or anterior side it was only from
5 to 6 mm. in diameter. Only in the immediate neighborhood of the ovum could
there be seen any differentiation of the mucous membrane into an upper, more
compact layer, and a deeper, looser cavernous layer. The epithelium of the glands
and the tissues of the uterus were well preserved, except in the immediate neighbor-
hood of the ovum. The picture produces the impression that the ovum, in order
to secure a place for itself, has completely destroyed the uterine tissues with
which it has been in contact, thus implanting itself in the maternal tissue. As a

9

130

THE HUMAN EMBRYO.

I 'F
x &

£ "5

* S
w 8

6 5

i-. ^

" x^

I

I i

o "s

H o

h

o ^

w — c "
co pa .g "S

I -.r § fi

" 1

e &

"- -c

Ji o

•g c«

pa

jf

•S 2

£• S

ri
o 4;

-.H 1

.

g > 2

< 'u .2

THE EMBRYO OF A GIBBON IN THE THIRD STAGE. 131

consequence of the destruction of the maternal tissues the walls of some of the
blood-vessels have been broken through, and this has allowed the blood to escape
from those vessels into the lacunae of the trophoderm.

The trophoderm of the ovum offers a very complex picture, owing chiefly to
the changes which it is undergoing. The changes are due apparently to hyper-
trophic degeneration. The layer of the chorionic ectoderm next to the mesoderm
retains more or less evidently a cellular character. The remaining portions tend to
form a syncytium in which the nuclei become enlarged and the cell-boundaries
obliterated, while the protoplasm of the cells also changes in character and becomes
more homogeneous in texture and much denser. The syncytium disappears by re-
sorption, and its disappearance causes the formation of spaces in the trophoderm.
Many different pictures occur in connection with these processes, for in some places
the nuclei tend to gather in groups, in others they disappear; in some instances
strands of degenerative material are left, while nearby some of the trophoderm may
retain its more primitive appearance and be but slightly altered. Finally, it should
be noted that at various points the chorionic mesoderm is growing out into the
trophoderm. Each of these mesodermic outgrowths is to be interpreted as the
anlage of the central portion of a chorionic villus, and out of the neighboring
chorionic ectoderm will be differentiated the ectodermal covering of the villus. It
seems, from a comparison of later stages, that the trophoderm degeneration never
goes so far as to leave any of the chorionic villi without an ectodermal covering.
But this covering varies extremely in its exact character as we find it in later
stages, even in adjacent parts of the same villus, for it may be either a single layer
of cells or a layer of cells covered by a thin coat of syncytium or merely a syncytial
layer (compare page 354). The disappearance of all of the trophoderm, except
so much as remains to share in forming the ectodermal covering 'of the villi, pro-
duces the so-called intervillous spaces of later stages, in which, as above stated,
maternal blood circulates.

An ovum in situ slightly more advanced than Peters's has been described care-
fully by Herzog. The specimen is now in the Harvard Embryological Collection,
Series 1500. It differs from Peters's ovum in having a prolongation of the entoderm
into the body-stalk to make the anlage of the allantois.

The Embryo of a Gibbon in the Third Stage.

The embryo to be described was obtained from a female Hylobaies concolor in
Borneo by Selenka. It is a little more advanced than Frassi's human embryo, men-
tioned on page 119. It still had traces of the primitive streak, at the anterior end
of which was an open neurenteric canal. The medullary plate was partially differen-
tiated from the embryonic shield. It was thoroughly studied by Selenka, and is
one of the best known very early ova of any primate. The entire ovum is repre-
sented in figure 75. The figure was reconstructed from the sections. It shows the
chorionic membrane studded with villi. The diameter of the chorion was about 8.5

132

THE HUMAN EMBRYO.

mm. The number of villi was about one hundred, of which some seventy are
clustered about the region where the embryo was found. The others are scattered
over the surface of the membrane. They are considerably branched. Each one is
.covered by ectoderm which consists of two layers, an inner distinctly cellular, and
an outer one in which the cell-boundaries are indistinct and which is known, there-
fore, as a syncytium and represents the remains of the original trophoderm. Each

Am.

Yk.

Cho

FIG. 75. — EMBRYO OF A GIBBON (HYLOBATES CONCOLOR) IN THE THIRD STAGE.
Am, Amnion. Yk, Yolk-sac. Cho, Chorion. Vi, Villi. — (After E. Selenka.)

villus contains a core of mesodermic tissue. The chorionic membrane is repre-
sented as open in order to show the size and position of the yolk-sac, Yk, and of
the amnion, Am, which encloses the embryo as it rests upon the yolk-sac. The
embryo itself is not shown in the illustration. Both the yolk-sac and the amnion
are, of course, covered by a layer of mesoderm. The entire space between these
two inner structures and the chorion corresponds to the extra-embryonic coelom, the
very precocious and enormous development of which is a special characteristic of

THE EMBRYO OF A GIBBON IN THE THIRD STAGE.

133

primates, including man, and is not at present known to be paralleled by the con-
ditions in the early stages in any other mammals.

A side view of the embryo on a larger scale is represented in figure 76. The
embryo is connected with the chorion by a well-marked body-stalk, b.s, is covered
by the arching amnion, Am, and rests upon the yolk-sac, which in comparison to
the chorionic sac seems very small. The yolk-sac, Yk, already has developed from
it a network of blood-vessels, Ve, which contain blood-corpuscles but have not yet
developed into the embryo itself. The disposition of the vessels is best illustrated by
the section (Fig. 77). The yolk-sac is, of course, lined in its interior by entoderm.
It has formed already a prolongation, All, into the body-stalk. This prolongation
is the anlage of the future allantois. Figure 78 represents a surface view of the

Emb. Ant.

b. s.

Am. EC. F.

Ent:

Yk.

All.

FIG. 76. — EMBRYO OF A GIBBON, SIDE VIEW OF
THE EMBRYO OF FIGURE 75.

Emb, Embryo. Am, Amnion. neu, Neurenteric
canal, b.s, Body-stalk. Yk, Yolk-sac. Ve,
Blood-vessels. All, Allantois. — (After E.
Selenka.)

FIG. 77. — TRANSVERSE SECTION OF THE EMBRYO
OF THE PRECEDING FIGURE.

Am, Amnion. EC, Ectoderm. F, Dorsal furrow.
mes, Mesoderm. Ent, Entoderm. Ve, Blood-
vessel.— (After E. Selenka.)

same embryo, or perhaps one should say, rather, of the embryonic shield. At the
posterior end there is the short primitive streak, the anterior limit of which is
marked by the opening of the neurenteric canal, neu, which passes obliquely down-
ward and forward, as shown also in figure 76. From the end of the neurenteric
canal there extends forward a slight thickening of the entoderm which can be
recognized as the anlage of the notochord, nch. Figure 77 represents a transverse
section through the region of the notochord. It shows the amnion, Am, arching
over the embryo, the thickened ectoderm of the embryonic shield, and the anlage
of the notochord. The mesoderm, mes, of the embryo no longer extends across
the median line and is without any ccelom. At the edge of the embryo the meso-
derm splits and one layer passes over on to the amnion, the other on to the yolk-
sac. In the wall of the yolk-sac, D, one can easily distinguish a layer of the ento-
derm, Ent, and also in the mesodermic portion the young blood-vessels, Ve. Com-
parison with a section of a somewhat older embryo of another gibbon, Hylobates

134

THE HUMAN EMBRYO.

pr.s.

b.s.

FIG. 78. — SURFACE VIEW OF THE
EMBRYONIC AREA OF THE OVUM
SHOWN IN FIGURE 77.

Rafflesi, also described by Selenka, will be found instructive. The relations are
here similar to those shown in the section just described, although the stage is some-
what more advanced, for we see that the amniotic cavity is larger, that the form-
ation of the medullary groove has begun, that the
ccelom is beginning to appear in the embryonic
mesoderm, and that the blood-vessels of the yolk-sac
have increased greatly in size. In this embryo there
were traces of the formation of three segments a little
in front of the neurenteric canal which was still
present and open. This embryo was found to be
attached to the wall of the uterus and to be
enclosed in a decidua reflexa. In later stages the
decidua reflexa of the gibbon unites with the decidua
vera, and is then lost completely by resorption. The
general character of the ovum and its relations to
the uterus justify us in the belief that it is extremely
similar to the human embryo at the same stage.

The Harvard Embryological Collection contains
one very well preserved embryo in the third stage,

b.s, Body-stalk, neu, Neurenteric ca-

nal. n^Notochord. pr.s, Primi- Senes 825- It is a little younger than the gibbon
tive streak. embryo above described. A monograph of this valu-

able specimen is in preparation.

Human Embryo in the Fourth Stage with the Medullary Plate.

The general relations in this stage have been indicated by the diagram (Fig.
69). A more exact idea of the embryonic structures may be gathered from figure
79, a dorsal view of the embryo, from figure 80, which represents a median sec-
tion of the embryo taken from a wax model reconstructed from the sections, and
figure 81, a transverse section through the neuropore. The general disposition of
the parts agrees very closely with the previous stage as described for primates. The
embryo and yolk-sac are very small in comparison with the entire ovum, and they
are connected by means of the body-stalk, b.s, with the chorion, Cho. The body-
stalk contains the entodermal anlage, All, of the allantois. The embryo is covered
by the amnion, Am, which arises in front of the head of the embryo, now becoming
marked off, and runs above the embryo to join the distal end of the body-stalk.
The opening of the yolk-sac, Yk, is about equal to the length of the embryo. The
yolk-sac is, of course, lined by entoderm and has a thick layer of mesoderm sup-
plied already with relatively large blood-vessels containing blood-corpuscles; the
vessels are developed chiefly upon the inferior hemisphere of the yolk-sac. The
embryo measured i . 54 mm. in length. Its dorsal surface is represented in figure
79. This surface is occupied by the very broad medullary plate of thickened ecto-
derm. Toward the middle of its length the medullary plate is somewhat narrower

HUMAN EMBRYO IN THE FOURTH STAGE.

135

than elsewhere. Along its median line runs the deep, narrow, dorsal groove which
at its caudal end widens out and disappears. Just behind it is the opening of the
relatively large neurenteric canal, behind which again follows a remnant of the
primitive groove. A transverse section a little in front of the middle of the em-
bryo is shown in figure 36. The ectoderm, ek, is very much thickened to con-
stitute the medullary plate; the narrow central longitudinal furrow, /, mentioned

Md.gr.

Neu. c.

Cho.

FIG. 79. — RECONSTRUCTION OF A HUMAN EMBRYO i . 54 MM. LONG. THE AMNION HAS BEEN OPENED

TO SHOW THE DORSAL SURFACE OF THE EMBRYO.

Yk, Yolk-sac. Am, Amnion. Md.gr, Medullary groove. Neu.c, Neurenteric canal. Pr.gr, Primitive
groove, b.s, Body-stalk. Cho, Chorion. — (After Count Spec.)

above is very noticeable. Outside of the embryo the ectoderm is reflected on to the
amnion, ct, over the back of the embryo. The entoderm is a thin layer of cells in
the center of which the notochordal band ch can be distinguished. In sections
near the neurenteric canal the notochord is better marked, being there much
thicker than the remaining entoderm, The mesoderm, me, is a distinct layer,
although, as other sections show, it is fused in the median line of the primitive
streak behind the neurenteric canal with both ectoderm and entoderm. Although
the extra-embryonic coelom is fully developed, that of the embryo is present as a
small fissure, p, only. Figure 81 is a section passing through the neurenteric canal,
and shows, therefore, the amnion, am, the thickened medullary plate, e, of the em-
bryo, and the large yolk-sac, d. The yolk-sac is formed, of course, of splanchno-

136

THE HUMAN EMBRYO.

pleure. The thickening of the mesodermic layer in the lower part of the yolk-sac
in order to allow space for the developing blood-vessels, b, b, b, is well shown in the
figure.

Eternod has studied an embryo in this stage. He finds that the heart is
already present underneath the slightly projecting head. From its anterior end
it sends out two aortic branches which run on either side near the notochord, pass
in a gentle curve around the neurenteric canal, come nearer together in the region

.<£'

Ent

All.

Yk.

b.--

FIG. 80. — HUMAN EMBRYO or 1.54 MM. MEDIAN SEC-
TION FROM A WAX MODEL RECONSTRUCTED FROM
SECTIONS.

All, Allantois. Am, Amnion. b.s, Body-stalk. Cho,
Chorion. EC, Ectoderm. Ent, Entoderm. mes,
Mesoderm. Vi, Chorionic villus. Yk, Cavity of
yolk-sac. — (After Count Spec.)

FIG. 81. — HUMAN EMBRYO OF i .54 MM.
Transverse section passing through the neu-
renteric canal and yolk-sac, am, Amnion.
ek, Ectoderm. ct, Amniotic mesoderm.
g, Meeting-point of somatopleure and
splanchnopleure. df, Mesoderm of yolk-
sac, b, b, b, Blood-vessels, en, Entoderm.
n, Neurenteric canal, d, Cavity of yolk-
sac, e, Medullary plate. — (After Count
Spec.)

of the primitive groove, and enter the body-stalk, through which they run parallel
to the allantois and form ramifications in the chorion. He finds also two veins in
the body-stalk which, when they reach the embryo, unite to a single median trunk,
which quickly divides into two vessels which run in the mesoderm of the yolk-sac
near the embryo proper until they reach the venous end of the heart, into which they
open. They each receive a venous branch from the caudal side of the yolk-sac.

Human Embryo in the Fifth Stage with Open Medullary Groove.

Several embryos in this stage have been studied. Two have been studied by
W. His; one he designates as "E" and the other as "SR" (Fig. 82). The
chorionic vesicle of "E" measured 8.5 X 5.5 mm.; of "SR, " 9X8 mm. More

HUMAN EMBRYO IN THE SIXTH STAGE.

137

satisfactory is the embryo described by Dandy, a median section of which is given
in figure 25. The embryo in "E" measured (?) 2.1 mm.; in "SR," 2.2 mm.
(Fig. 82). It will be noticed at once that .the condition is very similar to that
shown in figure 80, but the embryo is somewhat more advanced. The most
important changes in the embryo at this stage are its general growth, so that it
rises above the yolk and has both projecting head and projecting tail. The
medullary groove is very deep and extends the entire length of the embryo. Toward
its caudal end it probably has an open neurenteric canal. The dorsal outline of

Cho

Md.

Am.

Yk.

FIG. 82. — HUMAN EMBRYO WITH OPEN MEDULLARY GROOVE.
Am, Amnion. b.s, Body-stalk. Cho, Chorion. Md, Medullary folds. Yk, Yolk-sac. — (After W. His.)

the embryo is somewhat concave. On the under side of the projecting head,
between it and the anterior limit of the yolk-sac, the anlage o'f the heart has
appeared, and its cavity may be supposed to be in connection with the blood-
vessels of the yolk-sac. The development of segments has begun; Dandy's embryo
had seven. From the under side of the projecting tail end springs the body-stalk,
to the distal end of which the chorion is attached. The chorion is completely
covered by short branching villi. The yolk-sac has still a very broad connection
with the embryo, and contains blood-vessels throughout its entire extent. The
space between it and the chorion, the extra-embryonic coelom, is very large.

Human Embryo in the Sixth Stage with Medullary Canal.

This stage does not include the whole period from the beginning to the com-
pletion of the closure of the medullary groove to form the medullary canal, but
only the first part of this period. The best-known specimen of this stage was
described by Kollmann. It measured 2.2 mm. in length and had the medullary
groove open through the anterior two thirds of its length, but closed along the
caudal third. The embryo had thirteen segments (Fig. 83). The yolk-sac was
attached to the embryo for a distance of 1.5 mm., leaving the head to project

138

THE HUMAN EMBRYO.

0.58 mm. and the tail to project 0.3 mm. The head is already somewhat
enlarged and slightly bent over toward the ventral side. It forms at least one
third of the whole embryo. The dorsal outline of the embryo is concave in the
region where the segments have developed. The caudal end is slightly curved over
and is connected on its under side with the body-stalk, Al, by which the embryo
is attached to the chorion. Between the yolk-sac, Yk.s, and the head, the heart,
Ht, is prominent. By analogy with other vertebrates we assume that the heart-
tube, when it first appears in man, is straight and occupies a longitudinal median

,

-Am

Ht

FIG. 83. — HUMAN EMBRYO OF FROM THIRTEEN TO FOURTEEN DAYS.

Am, Amnion. 5.7, Seventh segment. Md, Medullary groove. Ht, Heart. Yk.s, Yolk-sac. Al, Body-stalk

— (After J. Kollmann.)

position. In this embryo it has already become a relatively large organ and the
tube itself is strongly bent. No anlage of the eye or ear was distinguished. The
amnion was a thin, transparent membrane enveloping the embryo quite closely.
The closeness of the amnion to the embryo was probably accidental (compare Figs.
84 and 85). The chorion was covered externally by branching villi; its diameter,
including the villi, was 18 mm.

Another embryo, the position of which in the series of known stages has long
been a matter of dispute, I feel, after renewed study, must be assigned to a place
very close to Kollmann's embryo just described. The specimen in question was
figured by Coste in his . monumental "Atlas of Embryology."* The embryo was
enclosed in a villous chorion (Fig. 84) and was provided with a large vitelline sac,

* The greatest difficulty comes from Coste's statement as to the magnification of his drawings, according to
which the embryo must have been about 4.4 mm. long, or nearly double the length which we now know to be
normal for embryos in the stage in which this one seems, to be. Other difficulties arise because Coste has given
no further description of this embryo than that which appears in the explanation of his plate. Neither that ex-
planation nor the figures themselves afford any information concerning the dorsal side of the embryo or as to
whether it had a partially open medullary groove or not. Coste's figures indicate that thirteen or fourteen seg-
ments were visible externally. The shape of the head, the size and curvature of the heart, the form of the tail,
and the concavity of the dorsal outline in the segmented region of the embryo all indicate an extremely close resem-
blance to Kollmann's embryo.. As Coste's figures were all made from fresh specimens freehand, we shall prob-
ably commit no error if we assume that the magnification was not correctly given. By making this assumption I
think the difficulties as to placing Coste's embryo vanish.

Coste's private collection was said to be at the College of France, but upon search this specimen could not
be found, so that attempts to ascertain its actual length were without result.

HUMAN EMBRYO IN THE SIXTH STAGE.

139

FIG. 84. — HUMAN OVUM, SAID TO BE FROM FIFTEEN TO EIGHTEEN t>AYS OLD. (Compare footnote, page 138.)
The chorion has been opened and spread out to show the embryo and its adnexa. Al, Body-stalk containing the
allantoic diverticulum. Am, Amnion surrounding the embryo. Vi, Yolk-sac. (Ajter Caste.)

140

THE HUMAN EMBRYO.

-Spl.

Vi, having a very broad connection with the embryo and covered with a network
of vessels, in which was a fluid not yet red. A thick body-stalk, Al, can be seen
running from the under side of the embryo's tail to the chorion; from the anterior
side of the stalk springs the amnion, Am, completely enclosing the embryo. It is

important to notice that in this, as. in still older embryos,
the disposition of the amnion is essentially the same as in
the earliest stages; the line of attachment of the amnion
is down the sides of the allantois and around the embryo
about on a line with the top of the yolk. As regards the
embryo, it is drawn slightly canted on to its left side; its
back is concave; the head end is thickest; behind and
below it can be seen the heart, already a bent tube,
shining through; and on the dorsal side, the light-looking
oesophagus is distinguishable; in the figure a wedge-shaped
shadow intervenes between the straight oesophagus and the
bent heart; the heart causes a conspicuous bulging of the
body between the head and the yolk-sac; the caudal ex-
tremity is thick and rounded and curves upward. Figure
85 is a ventral view of the same embryo after most of the
yolk-sac has been cut off; its walls, Spl (splanchnopleure),
are seen to pass over without any break into those of the
intestinal cavity. In the central line the notochord, s,

can be perceived through the translucent dorsal wall of

FIG. 85. — EMBRYO OF FIGURE ,, . . , . . . a , , , ., , ,

0 „ the intestinal cavity; it is flanked on each side by the

84, SEPARATED FROM THE J '

YOLK-SAC AND VIEWED row of square segments. Behind, we see the large body-

FROM THE UNDER SIDE. stalk, Al, and in front the tubular heart, Ht, with a

Am, Amnion. Hi, Heart. Spl, decided flexure to the right of the embryo; the anterior

Splanchnopleure extending j £ .1 u i -^uj 12 a

beyond the embryo to form end of the heart makes an opposite bend, separating off a
the yolk-sac, s, Noto- limb which becomes the bulbus aorta. The chorion con-
chord with a row of sjsts of ^wo layers, one of which forms the uninterrupted
inner surface of the chorion, while the outer layer alone

*

forms the hollow villi (Figs. 84 and 245); hence, in look-
ing at the inside of the chorion, we seen numerous round

openings which do not penetrate the inner layer. Fortunately, we learn from
Kolliker, who had an opportunity in 1861 to examine the chorion, that the outer
layer was epithelial, with cells of the same character as in the epithelium of older
vascularized villi, and that the inner layer consisted of developing connective tissue,
and carried fine blood-vessels. It thus appears that Coste was the first to observe
the role of the epithelium in the growth of the villi.

Human Embryo in the Seventh Stage with One Gill-cleft Showing Externally.
No human embryo with only one gill-cleft showing externally is known.

side. Al, Body-stalk. (After

Coste.)

HUMAN EMBRYO IN THE EIGHTH STAGE.

141

Human Embryo in the Eighth Stage with Two Gill-clefts Showing Externally.

Several embryos in this stage have been described and some of them studied
anatomically. Those which are best preserved and which we have best reason
to think are normal present a very singular appearance, owing to the deep bend
in the segmented region of the body so as to constitute at the dorsal outline of
the embryo at that point a U-shaped curve (Fig. 86).

This bend is known as the dorsal flexure. Embryos of earlier stages have an
indication of this flexure, as shown in figure 84. Until we have intermediate
stages we cannot be sure that the assumption which seems natural is also correct;
namely, that the deep dorsal flexure of figure 86
is merely an accentuation of the cavity on
the dorsal side of the embryo in earlier stages.
In older embryos the dorsal flexure is normally
absent (compare Fig. 88 and the following
figures). It is possible that the change from
the concave to the convex position is very
abrupt, and it is not improbable that the time
of the occurrence of this change is variable.
The head of the embryo and the tail both
project far beyond the yolk-sac, which, how-
ever, still shows a broad attachment to the
embryo. The right-angled head-bend is well
marked and the region of the fore-brain pro-
jects 'downward so as to leave a depressed area between the head and the heart.
This depression corresponds to the position of the oral cavity. The heart is large,
protuberant, and considerably bent, so that we can distinguish its three primary
limbs. From the under side of the caudal ^nd of the embryo springs the stout
body-stalk by which the embryo is united with the villous chorion. In another
embryo of this stage there were twenty-nine segments present. Above the heart
on the side of the pharyngeal region two external depressions are visible corre-
sponding to the first two gill-clefts. They are elongated in a dorso-ventral direction
and are narrow. This position of the amnion is well shown in figure 86. It arises
from the body-stalk at the side of the embryo along the yolk-sac and cardiac region,
and extends around the embryo, but is not yet fitted closely.

The anatomy of this stage is known to us chiefly through the observations of
His upon two embryos designated by him as Lg. and Sch. i. Lg. measured 2.15
mm.; Sch. i, 2.20 mm. The two embryos resemble one another closely. The
following description applies especially to Lg. The anatomy can be understood
from the accompanying figure 87. The medullary tube extends the entire length of
the embryo and is the principal component of the head. From the region of the
fore-brain has been formed an outgrowth to constitute the optic vesicle, Op. At
the side of the hind-brain and on the dorsal side of the pharynx is situated the

FIG. 86. — HUMAN EMBRYO, 2.15 MM. LONG. —
(After W. His.)

142

THE HUMAN EMBRYO.

OP,

Ao

anlage of the ear, Ot, which at this stage is merely an open invagination of the
ectoderm. The region of the mid-brain is marked by the head-bend, so that the
axis of the fore-brain is approximately at right angles to the axis of the hind-brain.
Another consequence of the head-bend is that the lower process of the head is
brought very close to the pericardial chamber enclosing the heart, Hi. Between
the head and the pericardial sac is situated the oral invagination or future mouth-
cavity, which is still separated from the entodermal
canal by the oral plate, O.pl, which consists merely of
a thin layer of cells belonging to the ectoderm and en-
toderm (compare page 58). The pericardial chamber
is large; in the figure only the endothelial portion of
the heart, Ht, is represented. Around this endothelial
tube is a second and more bulky one from which arises
the muscular wall of the heart. The volume of the
heart is, therefore, much greater than indicated by the
figure, hence the large size of the pericardial chamber.
On the dorsal side of the heart, between it and the
hind-brain, lies the entodermal canal, which is here the
anlage of the pharynx. It has two diverticula or gill-
pouches which are not indicated in the figure. On
the side toward the mouth the endothelial part is
continued beyond the pericardial chamber and gives off

two vessels on each side, the first and second aortic
FIG. 87. — RECONSTRUCTION OF THE .

ANATOMY OF THE EMBRYO arches, which pass around the pharynx and unite again

SHOWN IN FIGURE 86. upon its dorsal side, and then, as the aortae, Ao, de-

Op, Optic vesicle, o.pl, Oral plate. scend along the ventral side of the nervous system,
Ht, Endothelial heart. Li. ,A. ,, ,. ,. , . ,

, . soon uniting m the median line to form the single

Liver. Om, Omphalo-mes-

araic vein. Yk, Yolk-sac, dorsal aorta which runs along nearly to the tail of the
All, Aiiantoic diverticuium embryo, where it forks; and its branches, passing one

formed by the%ntoderm «.„, Qn each sjde Qf ^ jntestinal cana} enter the body-Stalk
Umbilical vein. Ao, Aorta. '

Ot, Otocyst.— (After w. His.) and run to the chorion, where they branch out. Behind

the pharynx the entodermal canal merges into the cavity

of the yolk-sac, Yk, and then beyond the yolk-sac extends again into the tail of
the embryo, forming an expansion there which is known as the cloaca. From
the under side of the cloaca runs out the allantoic diverticuium, All, which extends
as a narrow tube of entoderm through the allantoic stalk to the level of the chorion,
where it ends blindly. The pericardial chamber on its caudal side is bounded by
the septum transversum, in which we find the anlage of the liver, Li, already
present, and through which, on either side, the great vein from the yolk-sac, the
omphalo-mesaraic or vitelline vein, Om, passes to the heart. Of the veins of the
embryo only the umbilical, u.v, is shown in the figure. This vein gathers the
vessels from the chorion, passes through the body-stalk, then runs in the somato-

HUMAN EMBRYO IN THE NINTH STAGE.

143

pleure of the embryo to join the omphalo-mesaraic vein and enter the heart. In
the figure only the general course of the vein is indicated. The fact that it is
situated in the somatopleure could not. well be shown.

Human Embryo in the Ninth Stage with Three Gill-clefts Showing Externally.

Our knowledge of this stage is quite good. The described embryos vary in
length from 2.6 to 4.2 mm. The chorionic vesicles are about 10 mm. in diam-
eter, varying according to the size of the embryo. Figures 88 and 89
represent two embryos of this stage, the latter being the more advanced. The
back of the embryo is normally (or at least usually) convex. The head is bent

FIG. 88. — HUMAN EMBRYO OF 2.6 MM. LENGTH. — (After W. His.)

to one side, usually to the right, and the tail to the other, the whole embryo hav-
ing a slight spiral twist. The embryo has become quite large in proportion to
the yolk-sac. The three gill-clefts are readily seen, the first being the largest, the
third the smallest. The column of tissue between the first cleft and the mouth is
the mandibular process. Between it and the fore-brain lies the shorter rounded
maxillary process. The segments are clearly marked externally along the back
(Fig. 89). The origin of the amnion is shown in figure 89 also. The heart has
grown and something of its more complicated form is indicated in the external
modeling of the embryo. The anlage of the future ear is now a closed vesicle or
otocyst (Fig. 90, of). From the region over the heart almost to the caudal
extremity the segments of the body are distinctly marked externally.

The general anatomy of this stage will be understood by the aid of the accom-
panying figures 90 to 93, which are all reconstructions from sections. The position
of the notochord, Ch, is indicated by a line (Fig. 91). The pharynx is large and

144

THE HUMAN EMBRYO.

wide. It has three lateral outgrowths on each side, i, 2, 3, the gill-pouches. In
front and near the cephalic end of the notochord there is a small median out-
growth, the anlage of the hypophysis, Hy. Toward the neck-bend the pharynx
becomes narrower and passes over into the small entodermal tube, from which we
can detect the outgrowth, Lu, which represents the commencing formation of the
lungs. This narrow tube leads to the space above the yolk-sac, Yk.s. Just

FIG. 89. — HUMAN EMBRYO 4.2 MM.
Yks, Yolk-sac. Am, Amnion. All,
Body-stalk.— (After W, His.)

Car.

FIG. 90. — RECONSTRUCTION or THE
ANATOMY OF THE EMBRYO OF 2.6
MM. IN FIGURE 88.

A, Aortic limb of heart. All, Body-stalk.
Ao, Dorsal aorta. Au, Umbilical
arteries. Car, Posterior cardinal
vein. Jg, Anterior cardinal vein.
Om, Omphalo-mesaraic vein, op,
Optic, vesicle. ot, Otocyst. Vh,
right umbilical vein. — (After W.
His.)

where it passes into the yolk-sac the entoderm has formed the rudiment of the
liver, Li. Figure 55 gives a view of the anterior wall of the pharynx of another
embryo. In front is the large opening of the mouth, M, the oral plate between
the mouth-cavity and the entodermal canal having disappeared. This embryo
being a little older, the traces of the four gill-clefts can already be seen, and there
are four entodermal gill-pouches. The aortic vessels are indicated by dotted lines.
The cardiac aorta reaches the pharynx between the bases of the second and third
gill-arches, and divides into two branches, one on each side. The anterior branch

HUMAN EMBRYO OF THE NINTH STAGE.

145

forks and runs through the first and second arches. The posterior branch forks,
one fork going to the third, and the other, after again forking, supplies the fourth
and fifth branchial arches. This arrangement of the aortic branches is typical.
Between the bases of the first and second arches is a small protuberance which is
the anlage of the tongue and is named by His the tuberculum impar. Studies of
the sections demonstrate that the cavity of the abdominal region (splanchnocele)
has on each side of its dorsal surface a longitudinal ridge, the commencement of
the Wolffian body. The ridge already contains traces of the canals of the Wolffian

FIG. 91. — OUTLINE OF THE ENTODERMAL CANAL OF
A HUMAN EMBRYO OF 4 . 2 MM.

Hy, Hypophysis, i, 2, 3, Lines marking the
position of the pharyngeal gill-pouches. Lu,
Lungs. Li, Liver. Yks, Yolk-sac. Al, Allan-
tois. W, Wolffian duct. Ch, notochord. —
(After W. His.)

Op

-V

FIG. 92. — RECONSTRUCTION OF THE ANATOMY OF A
HUMAN EMBRYO, 3 . 2 MM. LONG, SHOWING THE
ANTERIOR END VIEWED FROM THE VENTRAL SIDE.

Op, Optic vesicle. Ht, Heart. Li, Liver. V, Allan-
toic vein. Au, Auricle of the heart, i, 2, 3, 4,
Aortic arches.

body. Of especial interest is the arrangement of the circulatory apparatus (Figs.
88 and 92). In the first figure the arteries are shaded dark; the heart is an
S-shaped tube which is really double, consisting of an inner endothelial tube con-
tinuous with the arteries and veins at either end of the heart, and an outer meso-
dermic tube which is confined to the heart and is unconnected with the blood-vessels.
The venous end of the heart lies near the yolk-sac. It is convex toward the head.
The arterial end of the heart is convex toward the tail. When viewed from the
ventral side, the venous process .of the heart (Fig. 92, Au} is seen on the left and
the arterial process, Ht, is seen on the right. The heart is continued forward by
the large aorta (Fig. 90, A), which gives off five branches on each side of the neck.
These branches again unite on the dorsal side and run backward to unite with

146

THE HUMAN EMBRYO.

Ot

the fellow-stem, and so form the single median dorsal aorta, Ao, which runs way
back and terminates in two branches, Au, which, curving round, pass out through
the body-stalk and supply the circulation of the chorion. The five branches in
the neck are known as the aortic arches. The column around each branch con-
stitutes the so-called branchial arch. Each branchial
arch is further marked out by the gill-cleft in
front of it and behind it, as shown in figure 90.
The reconstruction of the third embryo in the side
view (Fig. 93) affords further information concern-
ing the disposition of the heart and the large
blood-vessels. The veins, as is there shown, are
(i) the anterior cardinals, J, which are often re-
ferred to as the jugular veins, although they are
not identical with the jugulars of the adult; (2) the
posterior cardinals (compare Fig. 90, Car}; the
posterior and anterior cardinals, coming from the
caudal and cephalic regions, respectively, unite to
form a single transverse stem, the common cardinal,
D.C (the posterior cardinals receive their blood
chiefly from the Wolfiian bodies, and later undergo
complicated metamorphoses); (3) the large umbili-
cal or allantoic veins, Al.v, which pass up from
the chorion through the body-stalk into the somato-
pleure until at the level of the septum trans-
versum, above the liver, Li, they empty into the
common cardinal; (4) the omphalo-mesaraic or
vitelline veins, om, which corne up from the yolk-
sac on either side and meet the common cardi-
nals at the venous end of the heart. This figure
also shows the disposition of the aortic arches and
Art, Allantoic artery. Al.v, Allan- an early stage of the primitive internal carotid

FIG. 93. — RECONSTRUCTION OF THE AN-
ATOMY OF THE HUMAN EMBRYO OF
4 . 2 MM. SHOWN IN FIGURE 89.

Ot, Otocyst. J, Anterior cardinal vein.
car, Carotid artery. 7, First aortic
arch. Au, Auricle. Ven, Ventricle.
Li, Liver, om, Omphalo-mesaraic
vein. Al, Allantoic diverticulum.

toic vein. Am, Origin of the amnion.
D.C, Common cardinal. — (After W.
His.)

artery, car. The muscular, but not the endothelial,
heart is represented in the reconstruction.

Human Embryo in the Tenth 'Stage with Four Gill-clefts Showing Externally.
Few embryos belonging to this stage have been obtained. The one shown in
figure 94 was carefully studied and described by W. His. Its probable age is
twenty-three days. The embryo forms almost a complete circle, the tail being
close to the head. The limb-buds have appeared. . The heart is large and causes
a marked swelling of the body beneath the branchial arches, i, 2, 3, 4, all four of
which show clearly on the surface. The entodermal canal has attained nearly
the condition shown in figure 27, B.

HUMAN EMBRYO IN THE ELEVENTH STAGE.

147

Human Embryo in the Eleventh Stage with the Cervical Sinus in Formation.

The embryo figured (Fig. 95) was described by Mall, and one almost identical
has been studied by H. Piper. Its age is probably twenty-six days. At this
stage the embryo is flexed so as to describe almost a circle, the tail being almost
in contact with the head, yet comparison with figure 94 reveals that the straightening
of the back of the embryo has begun. Although the limbs, A.I and P.I, have
increased in size, they are still only rounded buds. The head, which is bent to
the right, partly conceals the cardiac region. The nasal pit,, Na, is a broad,

IV

al

s.s

FIG. 94. — HUMAN EMBRYO OF ABOUT TWENTY-
THREE DAYS, 4 .o MM. X 15 diams. — (After
W. His, Embryo a.)

al, Anterior limb-bud. B.S, Body-stalk. Op,
Optic vesicle, pi, Posterior limb-bud, iv,
Fourth ventricle, i, Mandibular process.
2, Hyoid arch. 3, 4, Third and fourth gill-
arches.

Cerv.s

A.I.

P.I.

FIG. 95. — HUMAN EMBRYO OF 7 .o MM. X 8 diams.

—(After F. P. Mall.)

A.I, Anterior limb. Cerv.s, Cervical sinus. Li, Liver.
Md, Mandibular process. MX, Maxillary process.
Na, Nasal pit. Op, Eye. P.I, Posterior limb.
Um.c, Umbilical cord. Yk.s, Yolk-stalk.

shallow fossa. The eye, Op, consists of the small optic vesicle and overlying lens.
The maxillary process, MX, is well developed. Behind the mandibular process,
Md, is the first cleft, or anlage of the external auditory meatus. The cervical
sinus, Cerv.s, is in process of development, but on the left side is not so deep as
on the right side, which is figured. The ventral ends of the branchial arches are
continuous with the cardiac region of the body. As shown in the figure, twenty-
four segments are clearly marked externally. The large cardiac region fills out
the space between the anterior 'limb, A.I, and the tip of the head. The ventral sur-
face of the abdomen is prolonged to form the umbilical cord, Um.c, from which
projects the slender yolk-stalk, Yk.s. The position of the liver is indicated by a
distinct protuberance below the foreleg.

148

THE HUMAN EMBRYO.

Human Embryos of the Fourth Week to the Fourth Month.

The following series of illustrations (Figs. 96-113 inclusive) are from specimens
in the Harvard Embryological Collection, all normal or nearly so. To facilitate
comparison figures 97-107 are uniformly magnified five diameters, while figures 108-113
are life size.

Embryos of Four Weeks, 7.5 to 8.0 mm.— They are characterized especially
by the extreme development of the neck-bend. The fourth and fifth branchial
arches are entirely buried in the cervical sinus, and the third arch is turning
in. In other words, the process of invagination of the sinus, though far advanced,

FIG. 96. — HUMAN OVUM WITH EMBRYO OF 9.4 MM.
THE CHORION HAS BEEN PARTLY REMOVED TO
SHOW THE EMBRYO. X 3 diams. — (Minot Collec-
tion, 275.)

FIG. 97. — HUMAN EMBRYO OF 9.6 MM.
SERIES 1001. X 5 diams.

is not completed. The invagination of the ectoderm to form the lens is still open,
though about to close. The back of the embryo is partly straightened. The limb
buds are beginning to expand at their distal ends to make the anlages of the hands
and feet.

Embryos of Twenty-eight to Thirty Days, 8.0 to 10.0 mm. — The form of hu-
man embryos at the end of the first month is very variable, and it has not been
possible hitherto to establish with certainty a typical normal shape. Their length
varies because the head begins to rise with accompanying diminution of the neck-
bend, hence the length may be increased by a change of form without a correspond-
ing growth of the embryo as a whole or advance in structure. Figure 96 illustrates
the proportions of the embryo, yolk-sac, and chorion at this stage. Figure 97 shows
an embryo of 9.6 mm. with the yolk-sac and stalk. In this specimen the oblitera-

EMBRYOS OF THIRTY-ONE TO THIRTY -TWO DAYS.

149

tion of the neck-bend, the growth of the limbs, the narrowness of the opening of
the sinus cervicalis, the elongation of the umbilical cord, and the expansion of the
hind-brain are all evidences of advancing development (compare Fig. 95). From
the distal end of the umbilical cord springs the amnion, beyond which there passes
out from the cord the narrow stalk of the yolk-sac. The cavity in the interior of
the cord is a continuation of the ccelom of the embryo and through it the yolk-
stalk takes its course.

Figure 98 is very instructive, for it represents an embryo which, although 0.2 mm.
shorter than the one shown in figure 97, yet is much more advanced in development,
as is evidenced strikingly by the enlargement of the whole
. head and the elongation of the limbs and the demarcation
of the hand from the rest of the anterior limb. The
orifice of the cervical sinus is narrow. On the ventral
side of the anterior limb, the body shows three rounded
eminences corresponding to the auricle of the heart, the
ventricle of the heart, and .the liver.

Embryos of Thirty-one to Thirty-two Days, 10 to 12
mm. — As typical specimens of this stage we may take two
embryos, one of 10.0 mm. (Figs. 99 and 100), the other
of 11.5 mm. (Fig. 101). Figure 99 shows the embryo,
Series 1000, the chorion and amnion having been opened;
the embryo lies somewhat obliquely on its left side, therefore
figure 100 has been added to give a correct profile compara-
ble with the other figures of the series. As compared with the previous stage (Fig.
97), the back has straightened out somewhat, though the lower end of the body
is still rolled over. The head has risen and increased considerably in size. Be-
tween the end of the region of the hind-brain and the level of the arm the dorsal
outline has become slightly concave. This concavity His designated the "Nacken-
grube." The first gill-cleft, owing to the completed closure of the cervical sinus, is
the only one visible externally.' It is the anlage of the external auditory meatus.
It is separated from the mouth by a prominent mandibular arch. On the cephalic
side of the mouth the maxillary process has become more prominent, but the two
portions of the maxilla do not yet meet in the median line. The primitive seg-
ments are still marked externally. The limbs show indications of their tripartite
division, the fore-limb being more advanced than the hind-limb. The division of
the digits of the hand is just indicated. The abdomen -bulges out, owing to the
growth of the liver. There is a true tail, which is now near its maximum develop-
ment. The umbilical cord has lengthened and shows the commencement of its
spiral twisting. The amnion springs from the end of the cord, leaving only a short
stretch of the body-stalk between the cord proper and the chorion. The amnion
envelops the embryo closely. In embryos slightly older than these the changes in
form above mentioned have progressed further. The body is straighter, the head

FIG. 98. — HUMAN EMBRYO OF
9.4 MM. SERIES 1005.
X 5 diams.

150

THE HUMAN EMBRYO.

is larger, and has risen so as to be about at right angles to the body. The con-
cavity (Nackengrube) below the hind-brain in the outline of the neck is more
marked. The limbs are longer, the fingers more distinct. Where the mandibles
meet in the median line, the separation of lip and chin has begun.

Embryos of Thirty-six Days, 14 mm. — The correlation of age and size for this
stage cannot be recorded as absolute, but we may say that embryos of this length

FIG. 99. — HUMAN EMBRYO OF ro.o MM. WITH THE AMNION, CHORION, AND YOLK-SAC. SERIES 1000. X 5 diams.

(Compare Fig. 100.)

are about five weeks old. The body is now nearly straight (Fig. 102). The
lower limbs project beyond the outline of the body in profile views. The bulging
of the outline at the neck-bend is characteristic of this stage, but in the specimen
figured the protuberance is unusually great. The ventral outline, owing to the large
size of the heart and liver, is very protuberant, and at this stage we find that the
portion of the umbilical cord adjoining the embryo is greatly enlarged, owing to the
distention of its coelom, so that a large cavity is furnished in which there are al-
ways found, as indicated in figure 84, several coils of intestine. This protrusion of
a portion of the intestinal canal, and sometimes even of a small portion of the

EMBRYOS OF THIRTY -EIGHT DAYS.

151

liver, into the extra-embryonic coelom of the umbilical cord is a constant phenom-
enon. It begins at a somewhat earlier stage and continues for a considerable
period. This curious condition has been observed in many different kinds of mam-
mals in the corresponding stage. Later on, the viscera are entirely withdrawn from
the umbilical cord and the cavity itself is wholly obliterated. The umbilical cord
is a hollow prolongation of the body-wall or somatopleure of the embryo, and the
amnion springs from its distal end. The yolk-stalk is very long and narrow. Its
entodermal cavity is obliterated. It is the representative of the original broad con-
nection between the yolk-sac and the entodermal cavity of the embryo, although
it is now only a small appendage of a loop of the intestine. It bears the blood-

FIG. ioo. — HUMAN EMBRYO OF 10.0 MM. SERIES
1000. X 5 diams. (Compare Fig. 99.)

FIG. 101. — HUMAN EMBRYO OF 11.5 MM.

SERIES 1006. X 5 diams.

vessels which run from the embryo and ramify upon the yolk-sac. On the caudal
side of the umbilical cord we find the tissue of the original body-stalk in which
run the allantoic vein and the two allantoic arteries which ramify upon the
chorion.

Embryos of Thirty-eight Days, 16 mm. in a chorionic vesicle of 45 by 40 mm.—
The age of this specimen (Fig. 103) is known by estimate only. This stage repre-
sents the transition of the embryo to the fetus, because after the fortieth day the
form is distinctly human. The head has risen considerably, and the back has
straightened still more, the rectangular neck-bend thus becoming emphasized. The
body has become still more protuberant on the ventral side, and in side views the
arms reach to the outline of the body. In the anterior limb we note the first indi-
cations of the five digits and of the separation of the upper and lower arms. To

152

THE HUMAN EMBRYO.

illustrate the variations in the proportions of embryos and to show a slightly more
advanced stage, figure 104, of a 17.8 mm. embryo is given and also figure 105,
A, B, giving two views of an embryo of 18.1 mm. All three specimens are proba-
bly normal, for it is known that variation is much greater during development
than in the adult, a fact which is to be explained in large part by the temporary
accelerations or retardations of the development of single organs or regions, which
are subsequently compensated for.

Embryos of Forty Days, 19 mm. — The head has risen far toward its definite
position, with the result of a very rapid apparent increase in the total length of

FIG. 102. — HUMAN EMBRYO OF 14.5 MM. SERIES 1003. X 5 diams.

the embryo. The change of position of the head results in bringing the mid-brain
finally directly above the hind-brain, 'the embryo being conceived as having the body
vertical. During the .elevation of the head the concavity (Nackengrube) at the back
of the neck is gradually obliterated. In both head and rump the external modeling,
which in earlier stages indicated more or less the position of the internal organs,
has become blurred, and in the next stage is found to have nearly or quite disap-
peared. The maxillary processes have met and united in the median line. The
anlages of the eyelids have developed. The concha of the ear is indicated. The
arm reaches beyond the heart; the fingers appear as separate outgrowths.

Embryos of Fifty Days, 21 mm. — The author has a fair specimen which came
into his possession with no history whatever, but it agrees very closely with His's

EMBRYOS OF THIRTY-EIGHT DAYS.

153

FIG. 103. — HUMAN EMBRYO OF 16.0 MM.
SERIES 1128. X 5 diams.

FIG. 104. — HUMAN EMBRYO OF 17.8 MM.
SERIES 839. X 5 diams.

A B

FIG. 105. — HUMAN EMBRYO OF 18.1 MM. SERIES 1129. X 5 diams.

154

THE HUMAN EMBRYO.

embryo Ltz, of which he fixes the probable age as just over seven weeks. The
head is nearer its final position than in figure 103, and relatively larger in propor-
tion to the body, In the eye, cornea and conjunctiva are clearly separated; the
face has the fetal form, the nose, mouth, and chin being fully marked off. The
arms are clearly divided into upper and lower segments; the five digits are well
developed; the hands rest over the heart and nearly touch one another. The leg

FIG. 106. — HUMAN EMBRYO OF 22.8 MM. SERIES 871. X 5 diams.

shows the triparite division; the toes are just beginning to be free, but the hind-
limb is much less advanced than the fore-limb. The tail is still a freely projecting
appendage.

Embryos of Fifty-three Days, 22-23 mm- — The specimen (Fig. 106) is probably
quite normal. As compared with the last stage, there are comparatively few changes
of external form; the most noteworthy are perhaps the increased development of the
legs and feet and the commencing disappearance of the free tail. At this time the

EMBRYO OF SIXTY-TWO DAYS.

155

FIG. 107. — HUMAN EMBRYO OF 30 MM. SERIES 913. X 5 diams.

156

THE HUMAN EMBRYO.

protrusion of the coils of the intestine into the ccelom of the umbilical cord is
about at its maximum.

Embryos of Sixty-two Days, 30 mm. — The present specimen (Fig. 107) came
with no data and its age is therefore estimated only. The head is still larger in
proportion to the body than in figure 106. The face shows the two lines which, as
seen in profile, mark the two ridges which run over the cheek, one alongside the
nose to the corner of the mouth, the other from the eye; these ridges are highly
characteristic of the ninth week, and traces of them not rarely persist in the adult
physiognomy. The limbs have grown considerably, the hands being lifted toward
the face; at the elbow there is a considerable bend; the toes are all free, and the
soles of the feet are turned each toward the other. The tail has disappeared as a
free appendage. The external genitalia are considerably developed; the clitoris-penis
projects some distance.

Embryo of Sixty-four Days, 32 mm. — A specimen came with the following his-
tory: "January 4, 1886, last flow began; March 13, 1886, abortion"; between these
two dates are sixty-eight days; but as the flow took place, conception probably
occurred after menstruation, therefore if we deduct four days, making the age
sixty-four days, we shall probably be not far wrong. The head has not yet as-
sumed its final angle with the body. On the other hand, the protuberance of the

abdomen is much reduced, so that the body as a whole
has begun to have a more slender form than in earlier
stages. In this specimen the eyelids have not even begun
to meet; in another they have met, except just in the
center, where is still a loophole.

Embryo of Seventy-five Days, 55 mm. — We figure next
(Fig. 108). a fetus concerning which there are no data.
Comparison with embryos of two and three months leads
us to place it a little under half way between them. The
specimen has essentially the configuration of the young
child; but the head is very large and the body slender; the
position of the limbs is typical; the upper arm is bent
down, the forearm extends toward the chin; -the knee is
bent so as to throw the foot toward the median line; the soles
of the feet are placed obliquely facing one another; the anlages
of the nails can be recognized on both the fingers and toes.

Embryos of the eleventh and twelfth weeks are very rarely obtained. I have
never had a normal one of this period with data to determine the age.

Embryos of Three Months, 78 to 80 mm. In my experience there is no other
age at which abortion of normal embryos occurs so frequently as at three months,
and I possess a number of specimens of this age, which agree very . closely with
one another in size and form. The fetus drawn in figure 109 may be taken to
represent accurately the appearance of the human embryo at three months. The

FIG. 1 08.— HUMAN EMBRYO OF
55 MM. SEVENTY-FIVE
DAYS. NATURAL SIZE.

EMBRYO OF FOUR MONTHS.

157

position of the limbs is typical for this age, but there are slight variations, in that
the hands, one or both, may project more and be higher or lower; usually the right
foot lies in front of the left, but not always. Figure no gives the front view of
the face of the same embryo to show the closed eyelids, the broad triangular nose,
the thick lips, and the pointed chin.

Embryos of Three and One-half Months, 108 to no mm. — I have several speci-
mens which represent this age. Two
of them are figured, one to show the
natural attitude (Fig. in) in utero,
the other (Fig. 112) to show the
natural attitude assumed by the em-
bryo when released from its mem-
branes. The first specimen came to
me with no history, but as it is cer-
tainly a little larger than several other
fetuses of about one hundred and six
days, it is probably a little older.

FIG. 109. — HUMAN EMBRYO OF 78 MM. THREE
MONTHS. NATURAL SIZE.

.Fie. no. — FRONT VIEW or THE FACE OF THE EM-
BRYO SHOWN IN FIGURE 109. NATURAL SIZE.

The head is bent forward (Fig. in); the back is curved; the arms and legs are both
raised toward the face; the right leg is nearly straight, so that the toes are brought
against the forehead, while the left leg is bent at the knee, bringing the left foot
against the right thigh. In this attitude the embryo fills out as perfectly as possi-
ble an oval space, and fits, therefore, the cavity of the uterus. The second speci-
men (Fig. 112) shows the attitude assumed by the embryo when free, and proves
that the position in utero (Fig. in) is a constrained one. This fetus was received
November 30, 1883. The delivery took place on the morning of that day, and the
last menstruation had ceased one hundred and six days previously; the remarkably
fresh condition of the fetus indicated that it had been dead only a very short time,
so that we cannot be far wrong in putting its exact age at one hundred and six days.

Embryo of Four Months, 155 mm.— The fetus shown in figure 113 came in a

158

THE HUMAN EMBRYO.

very fresh condition, January 2, 1887; with the statement: "Conception said to
have taken place September i, 1886; fetus came away January 2, about noon."
The embryo is typical in size and development for four months, except that the

FIG. in. — HUMAN EMBRYO OF 120 MM. (?ONE
HUNDRED AND TEN DAYS.) NATURAL SIZE.

FIG. 112. — HUMAN EMBRYO OF 118 MM. ONE
HUNDRED AND Six DAYS. NATURAL SIZE.

crown is higher than usual, and the antero-posterior diameter of the head some-
what below the average.

The natural attitude in utero is similar to that of figure in; the attitude shown
is that assumed by the fetus when released from the membranes.

EMBRYO OF FOUR MONTHS.

159

FIG. 113. — HUMAN EMBRYO OF 155 MM. ONE HUNDRED AND TWENTY-THREE DAYS. NATURAL SIZE.

CHAPTER IV.

STUDY OF THE SEGMENTATION OF THE OVUM AND OF THE
BLASTODERMIC VESICLE IN MAMMALS.

In selecting material for general laboratory work on the early stages of mam-
mals, we are governed by practical considerations. The white mouse and the
rabbit are both easily kept in the laboratory and their breeding may be accurately
determined. Up to the present time the earliest phases of the development of the
mammalian embryo have been far more thoroughly studied in the white mouse than
in any other mammal. For the next following stages the same remark applies to
the rabbit. Hence these two forms have been chosen for the practical study.

The Maturation, Fertilization, and Segmentation of the Ovum in White Mice.

These animals are selected for the practical study of the earliest stages of
development for two reasons: first, because the processes have been more thoroughly
studied in them than in any other mammals; and, second, because they are easily
kept and breed freely, so that abundant material may be secured with compara-
tively little trouble. Those desiring further information are referred to Sobotta's
and Kirkham's original memoirs.*

Heat occurs twenty-one days after littering, a fact which may be taken
advantage of to secure ova of the desired age. Coitus can take place only during
heat, for it is then only that the vagina is found open; at other times its epithelium
concresces to a solid mass. The spermatozoa do not penetrate into the tube until
some time after the coitus. After the discharge of the semen, the contents of
the large seminal vesicle are ejaculated into the vagina, completely filling it and
hardening into a white plug (bouchon vaginal), as in guinea-pigs. From twenty to
thirty hours later the plug softens and falls out.

The uterine tubes are narrow, much contorted canals. The fimbriate opening
of the tube penetrates the connective tissue about the ovum so that the fimbriae
lie in the periovarial space. There is ciliated epithelium in the proximal region of
the tube only, none in the distal parts or in the uterus itself. During heat the
periovarial space is filled with an abundant clear fluid. This also distends the

* Sobotta, "Die Befruchtung und Furchung des Eies der Maus," Arch. /. mikrosk. Anal., vol. XLV, 15-93,
PI. II-IV (1895).

Kirkham, "The Maturation of the Egg of the White Mouse." Trans. Connecticut Academy, xiu, 65-87,
PI. I-VIII. (Corrects several important errors of the preceding paper.) Also, Biol. Bull., (1910) XVIII, 245.

160

POLAR GLOBULES IN WHITE MICE.

161

proximal part of the tube, forming, as it were, a special sac, with a distended
epithelial lining. At the time of coitus ovulation has generally taken place; the
ovum, still surrounded by the cells of the corona radiata, is found in the fluid of
the distended proximal section of the tube. It is probable that the ova are carried
from the periovarial space not only by the currents created by the cilia of the
fimbriate opening, but also by a sort of pumping action of the tube itself. For
at the beginning of the period of heat we find that the periovarial space contains
much fluid, but later, when the ova are in the tube, this space is empty and the
tube contains fluid. The ovum of the mouse measures only 80^ or less in
diameter, and is therefore the smallest known mammalian ovum. (The ovum of
the cat measures 200^, of the rabbit 161^.) It is surrounded by a very thin
zona pellucida (i6-36//), and contains only a few yolk grains, a portion of which
may be blackened by osmic acid. These ova offer the further special peculiarity
that the first polar globule, which is always formed in the ovary, is lost in 80-90
per cent of the ova, probably by extrusion through the zona pellucida, so that
even after the formation of the second globule, they still often have Only a single
globule within the zona. The second globule is produced only after the ovum has
been transferred to the uterine tube, and then only after a spermatozoon has
entered. The process for formation of the first and second globules is not the
same, although there is a general similarity.

The First Polar Globule. — The first polar globule is formed, as stated, while the
ovum is still in the unruptured Graafian follicle of the ovary. The nucleus moves
toward one side of the ovum and is there transformed into a
mitotic spindle, the axis of which is more or less nearly at
right angles to the radius of the ovum (Fig. 114). The spindle
itself is large, pointed at the ends, with curving achromatic
threads. The chromosomes are probably twelve in number,
but they vary in size and shape, and even in number, which
has been explained as the result of precocious division of some
of them. They gather themselves into an equatorial plate. FlG-
They are elongated, pointed at the ends, with irregular sides,
and are very large. Minute centrioles have been observed at
the end of the spindle, but there are no astral rays extending
from the ends of the spindle into the protoplasm. The chro-
mosomes become somewhat V-shaped. They divide by a trans-
verse separation at the apex of the V. Chromosome halves migrate toward the end
of the spindle. The stages occur probably about twenty-four hours before the
rupture of the follicle. The spindle now assumes a radial position, and one of its
poles lies close to the surface of the ovum, which has meanwhile diminished in size
so that there is a considerable space between the yolk and the zona pellucida.
Division occurs and the first polar globule is formed, and lies in the perivitelline
space. In the mouse it is remarkable, as is also the second, but smaller, polar

114. — OVUM OF
WHITE- MOUSE, WITH
THE FIRST POLAR
SPINDLE IN TANGEN-
TIAL POSITION. X 500
diams. — (After J.
Sobotta.)

162

STUDY OF THE SEGMENTATION OF THE OVUM.

globule, for its large size. It is usually spherical in fresh, oval in preserved
specimens, and measures in the living state from 22-28/4 in diameter. It has a
distinct cell-membrane, a protoplasm which resembles that of the ovum, and may
even contain granules of yolk. Soon after its separation from the ovum its
nucleus becomes well developed and membranate. Except, therefore, that the
number of chromosomes which enter into its formation is half the normal number,
we might say that it differs little from an ordinary cell.

The Second Polar Globule. — After the formation of the first polar globule
ovulation takes place, and during the next changes the ovum is situated in the
ampulla of the Fallopian tubes. In the mouse, unless the ovum is fertilized, it
forms no second polar globule, but instead undergoes autolysis either in the ovary
or in the uterine tube. The nucleus of the ovum does not enter into a condition
of repose, but at once transforms itself, as in other animals, into tl)e second polar
spindle. After the constricting off of the first polar globule, twelve half chromo-
somes (dyads) are left in the ovum. They are drawn into the equator of a new
spindle and ' split longitudinally.

Pg.2

Spz.

FIG. 115. — OVUM OF WHITE MOUSE, DIVIDING TO

PRODUCE THE POLAR GLOBULE.
P.sp.2, Second polar spindle. Spz, Head of

spermatozoon. X 500 diams. (After J .

Sobotta.)

FIG. 116. — OVUM OF WHITE MOUSE, SHOWING THE

METAPHASE OF THE DIVISION PRODUCING THE

FIRST POLAR GLOBULE.
Pg. 2, Second polar globule, pi, Cell-plate. ?,

Female pro-nucleus. X 1500 diams. — (After

J. Sobotta.)

The second polar spindle is smaller than the first. It lies at right angles to
the axis of the ovum and quite close to the surface. It contains twelve thick
achromatic fibers, which do not unite at the poles with one another, but end par-
allel, so that the tip of the spindle is blunted. The chromosomes, when the mem-
brane first disappears, lie irregularly, but shortly after the formation of the spindle
they collect together to form an equatorial plate, somewhat as in the figure.
They are irregular and of uneven size, twelve in number, or possibly the number
may vary somewhat. The chromosomes then divide transversely, and the
halves move rapidly toward the ends of the spindle, which during this change
passes into the radial position (Fig. 115). The twenty-four univalent chromosomes
lengthen into filaments of various sizes, and by their form the second spindle can

FERTILIZATION OF OVUM IN WHITE MICE.

163

be readily identified. The surface of the ovum or the apex of the spindle forms a
protuberance. Division of the achromatic fibers takes place, and there is formed a
well-marked cell-plate (Fig. 116), and presently the polar globule becomes con-
stricted off. The second body is smaller than the first, measuring from 7-1 2^
in diameter, and in the majority of cases is the only one to be found inside the
zona after fertilization. Its twelve chromosomes soon form a resting membranate
nucleus. The cell-plate appears with unusual distinctness. It is at about this
stage that the spermatozoon is found to have entered the ovum (Fig. 117, B) and to
have formed there the male pro-nucleus. During all these stages no centrosome
appears at the poles of the spindle, but centrioles are said to have been observed
at the spindle apices. No astral rays appear in the protoplasm, although in many

Pg.2 Pg.l

FIG. 117. — Two OVA OF WHITE MOUSE. A, WITH TWO POLAR GLOBULES. B, WITH THE SECOND POLAR GLOBULE

ONLY.

Pg. i, First polar globule. Pg. 2, Second polar globule. 6, Male pro-nucleus. 9 Female pro-nucleus.

X 500 diams. — (After J. Sobotta.)

eggs these astral figures are extremely conspicuous. The female pro-nuclear ele-
ments appear at first as a dense cluster of chromatin granules (Fig. 117, J3?), and
fuse apparently into a compact mass, which grows rapidly in size, presumably by
the absorption of fluid from the yolk, and, as it enlarges, acquires a more distinct
outline, and presently shows a network structure in its interior (Fig. 117, A), with
irregular chromatin masses. It continues to grow more and more, and develops
at the same time a series of nucleoli more or less uniform in size. This stage
may be regarded as that of the completed female pro-nucleus.

Fertilization occurs in the ampulla of the uterine tube about 6-10 hours after
the coitus. Unless it occurs the development of the second polar globule does not
take place. It is accomplished, normally, by the penetration of a single spermato-
zoon into the yolk. The tail of the spermatozoon usually enters the egg at least
in part. The head of the spermatozoon can be recognized at first by its shape
(Fig. 116, s). In- position it is typically more or less remote from the polar spindle.
While the second polar globule is forming the head assumes a rounded form, and
becomes the male pro-nucleus (Fig. 117, s). The group of twelve chromosomes left

164

STUDY OF THE SEGMENTATION OF THE OVUM.

in the ovum after the division of the polar spindle becomes the female pro-nucleus
(Fig. 117, 9). Both pro-nuclei now enlarge, the female most, and assume a nearly
spherical form, but have no membrane (Fig. 117, A). They approach one another,
drawing also toward the center of the ovum, until they come to lie side by side,

FIG. 118. — OVUM OF WHITE
MOUSE. BEGINNING OF THE
CONJUGATION OF THE PRO-
NUCLEI. X isoodiams. — (After
Sobotta.)

FIG. 119. — OVUM OF WHITE
MOUSE. CONJUGATION OF THE
PRO-NUCLEI, AND FORMATION OF
THE SEGMENTATION SPINDLE.
X 1500 diams. — (After Sobotta.)

yet separated by a small space. The chromatin of the' two pro-nuclei forms dis-
tinct threads. Next there appears in the space between them a centrosome with a
few radiating lines around it (Fig. 118). From the centrosome arises, just how is
not clear, a spindle of achromatic threads (Fig. 119). The chromatin of each pro-

FIG. 1 20. — OVUM OF WHITE
MOUSE. FIRST SEGMENTATION
SPINDLE WITH THE CHROMO-
SOMES OF THE PRO-NUCLEI STILL
FORMING Two DISTINCT
GROUPS. X 1500 diams. —
(After Sobotta.)

FIG. 121. — OVUM OF WHITE MOUSE. FIRST SEG-
MENTATION SPINDLE WITH EQUATORIAL PLATE
OF CHROMOSOMES. X 1500 diams. — (After
Sobotta.)

nucleus now forms a group of well-defined, elongated, somewhat crooked chromo-
somes. The two groups of chromosomes are quite distinct, and are separated from
one another by the intervening spindle (Fig. 119). The spindle continues to grow,
and the chromosomes of the male pro-nucleus on the one side and the female pro-

FERTILIZATION OF OVUM IN WHITE MICE.

165

nucleus on the other attach themselves to the equatorial region of the spindle (Fig.
120). The spindle continues to grow; the chromosomes become V-shaped and ar-
range themselves as the so-called equatorial plate, in which the chromosomes of
the two pro-nuclei can no longer be distinguished from one another (Fig. 120). At
each end of the spindle is a distinct centrosome with a very faint, small astral
radiation in the neighboring protoplasm. This spindle is the beginning of the divi-
sion of what we may call the segmentation ,,..,,-..... ^i^^^^^^^.^i,^.,. .
nucleus. In the mouse the two pro-nuclei do
not actually fuse into a single nucleus before the
formation of the spindle, which initiates the first
division of the fertilized ovum, so that, strictly
speaking, there is no fusion of the pro-nuclei to
make a segmentation nucleus. There is, never-
theless, a true fusion of the pro-nuclei accom-
plished, although it is somewhat masked by the
early commencement of the first segmentation
spindle, which develops at the same time that the
fusion of the pro-nuclei is being completed.

The chromosomes of the equatorial plate now divide, probably by splitting longi-
tudinally, so that the number of chromosomes is doubled. During the splitting the
chromosomes tend to draw apart from one another. At the same time the spindle,
without changing its length, becomes somewhat narrower. The chromosomes now
move apart from the equator toward the two poles, forming two groups, each group

FIG. 122. — OVUM OF WHITE MOUSE. FIRST
SEGMENTATION SPINDLE.

The chromosomes have divided and have
migrated toward the poles of the spin-
dle, forming two groups. X 1500
diams. — (After Sobotta.)

p.g. Z.

FIG. 123. — OVA OF WHITE MOUSE WITH Two SEGMENTATION SPHERES OR CELLS.

.4, Telophase of the division; the chromosomes are reconstituting the nucleus. B, Membranate nucleus recon-
stituted. I, First cell of segmentation, nu, Nucleus, p.g, Polar globules. Z, Zona pellucida. X 500
diams. — (After Sobotta.)

containing half of the total number of chromosomes (Fig. 122), and at the same
time the whole ovum becomes somewhat elongated in the direction corresponding
with the axis of the spindle. The chromatin granules accumulate at the two poles
of the spindle. The achromatic threads between the poles break through. Then
the actual cleavage of the elongated ovum into two cells becomes marked in the

166 STUDY OF THE SEGMENTATION OF THE OVUM.

protoplasm, and the line of separation of the two cells passes through the equator
of the spindle (Fig. 123, A). The accumulated granules of chromatin then reconsti-
tute the resting membranate nucleus (Fig. 123, B). In brief, the segmentation of
the ovum is a typical indirect or mitotic cell-division. In the mouse the first
cleavage is completed about twenty-six hours after the coitus. The second cleav-
age is not completed until twenty-four hours later. When first formed, the two
segmentation spheres are oval and entirely separated from one another, but subse-
quently they flatten against one another and become appressed, a phenomenon of
which we have no explanation. In most mammals which have been studied there
is more or less space between the segmenting ovum and the zona (see Fig. 6), but
in the mouse this space is reduced to a minimum and the zona is often stretched
into irregular forms during the changes of the ovum.

Method of Obtaining Blastodermic Vesicles from the Rabbit.

The does should be allowed to become pregnant and be isolated until they
have littered; the date of littering should be noted, and thirty days thereafter the
buck be admitted and the exact time of the covering recorded. At the proper
number of days thereafter the animal should be killed and the uterus removed at
once. It may be opened with two pairs of forceps used to split the outer muscular
walls of the organ, beginning the operation at the lower end of the uterus. With a
little care this can be done without rupturing the mucous membrane, which is to
be afterward also opened in a similar manner with the forceps and the blastoder-
mic vesicles exposed. They are small bodies of rounded form and with a brilliant
pearly luster, and are easily observed. During the earlier stages, which occur in
the Fallopian tubes, the ova are very small and difficult to find, but by the time'
the ovum has reached the uterus it has become a blastodermic vesicle measuring
about 0.6 mm. in diameter, and, therefore, easily seen by the naked eye. From
the fourth day after coitus until the beginning of the seventh day the vesicles lie
free in the uterus. Usually early in the seventh day the vesicles, which then
measure about 4.5 by 3.5 mm., begin to attach themselves to the wall of the
uterus, and thereafter are much more difficult to remove. At the beginning of the
fifth day the ova measure about 0.6 to 0.9 mm. in diameter, but vary greatly in
size, and are found more or less near together in the upper portion of the oviduct.
By the end of the sixth day they measure about 4.0 mm. and are distributed
throughout the entire length of the uterus.

The most useful stages are the vesicles from the beginning of the sixth and
seventh days. To preserve the vesicles they must be gently removed from the
uterus, great care being necessary not to injure them, and dropped into Zenker's
or Hermann's fluid. In either of these they may be left for about an hour and
then washed and preserved in the usual manner. Specimens should be examined
in the fresh state, just after they have been preserved, and after they have been
stained, before they are imbedded. For staining, alum cochineal or borax carmine

STUDY OF RABBIT BLASTODERMIC VESICLES IN ALCOHOL. 167

is recommended. Finally, the specimens are to be imbedded in paraffin and cut
in series in the usual manner; sections of from 6 to 8// are desirable. Unfor-
tunately, no method has yet been devised by which these delicate vesicles may be
imbedded without distortion of their form, so that, when the sections are finally
obtained, the blastodermic walls are wrinkled and more or less out of shape. But
fortunately, owing apparently to its greater thickness, the embryonic area usually
escapes distortion and appears in the sections of normal form, or nearly so.

Study of Rabbit Blastodermic Vesicles in Alcohol.

All of the most important points in the structure of the blastodermic vesicles
of the rabbit from the fourth to the seventh day may be fairly well observed by
examining the hardened vesicles in alcohol under the microscope. For such exami-
nations the so-called live-box, such as was formerly much used by microscopists
for the study of living creatures, will be found very convenient. Care must be
taken to have plenty of alcohol around the specimen and not to lower the cover
so much as to exert any pressure upon the vesicle. It is not difficult to place the
vesicles so that any part of their surface may be examined with a No. 7 objective.
In the uncolored specimen the nuclei and even many of the boundaries of the
cells can be clearly made out.

In the following descriptions ages have been chosen at which the important
characteristics can usually be observed. The variation is so great in range during
early stages that the development described below for a given age is often found
in older or younger specimens, and specimens of a given age may exhibit a less
or a more advanced stage of the embryonic formation than is here put down
for that age. In general the correspondence of the stage of development to the
size of the vesicle is more exact than to its age.

Vesicles at Five Days (5 X 24 hours). — At this age the vesicles are always found
in the upper portion of the uterus. Sometimes all of those in one uterus are quite
close together, at other times somewhat scattered and lying singly. The vesicles
are extremely variable in size, for they measure from 0.6 to 0.9 mm. They are
spherical or nearly so, and are surrounded by a thin membrane, which in reality
corresponds to both the zona pellucida and the outer albuminous envelope, which
in the rabbit ovum during segmentation is very thick and conspicuous, but which
is always extremely thin when the stage of the blastodermic vesicle is reached.
Upon the outside of this really double membrane appear a certain number of
small villus-like projections, which are highly refringent. They are probably identi-
cal in character with the villi which have been observed upon the ovum of
the dog (page 45), but are smaller in all of their dimensions. Immediately un-
derneath the external membrane there is a continuous layer of cells belonging
to the ectoderm and extending completely around the ovum. The layer is some-
times designated specifically as the " outer layer" or as the " subzonal layer" It
also extends over the embryonic shield; the portion upon the shield is often termed

168 STUDY OF THE SEGMENTATION OF THE OVUM.

Rauber's layer, it having been first observed by that investigator. The cells of the
outer layer are quite large and their boundaries are easily recognizable in surface
views. Their sides may number four, five, or six, six being perhaps the more
usual number, and are variously disposed, so that the cells differ in shape and size.
During the next two days of development the cells become, if anything, more ir-
regular in outline and somewhat smaller. The boundaries between the cells are
very fine lines; the nuclei are rather large and oval in form, and contain from three
to four or five highly refringent granules. Each nucleus is surrounded by a denser
court of protoplasm in which there are many granules, some of which are highly
refringent. The peripheral portion of the cell is of a loosely reticulate structure
with comparatively wide meshes between the threads of the protoplasm. Occasion-
ally there appear in the protoplasm of these cells narrow, elongated, highly refrin-
gent bodies somewhat resembling bacilli in appearance, and therefore they are
termed the bacilliform bodies. Their nature is unknown; they are more apt to be
found in older vesicles. The outer or subzonal layer can be made out over the
embryonic shield only by very careful observation. In the shield the cells are sev-
eral layers thick. The inner cells are very much smaller in size than the cells of
the outer layer, are more granular, and contain smaller nuclei which take up a
relatively large place in the cell in proportion to its apparent area. Closer observa-
tion, utilizing the fine adjustment of the microscope, will show that there are two
kinds of cells in the inner part: first, those which, like the cells of the subzonal
layer, belong to the ectoderm; and, second, an inner layer of cells, which appar-
ently belongs entirely to the entoderm. In the region of the embryonic shield the
ectoderm is, therefore, made up of two distinct layers of cells. The outer or sub-
zonal (Rauber's layer) disappears during the sixth day of development as a distinct
layer. The cells of the entoderm form a very thin continuous layer on the under
side of the embryonic shield. They may be recognized by the very granular, and
therefore dark,* appearance of their protoplasm, and by the rounded form and
small size of their nuclei. Similar cells may be observed also extending beyond
the limits of the embryonic shield, though not there forming a continuous layer,
except perhaps for a very short distance, but rather lying scattered about in patches
or isolated. As the cuboidal cells of the ectoderm are confined to the region of
the embryonic shield, the cells of the entoderm outside of the shield lie close
against the subzonal layer. Here they may be more easily studied than in the
shield itself. They are very much smaller than the cells of the outer layer and
contain each a nucleus with highly refringent granules, which are now numerous
and smaller than the somewhat similar granules in the overlying nuclei of the ecto-
derm. The farther away we proceed from the edge of the embryonic shield, the
fewer we find the entodermal cells. The extent of their distribution varies greatly,
and apparently more or less in relation to the size of the blastodermic vesicle, since

* As seen by transmitted light

STUDY OF RABBIT BLASTODERMIC VESICLES IN ALCOHOL.

169

in the smallest vesicles of this age we find the cells only a short distance beyond
the edge of the shield, yet in the largest vesicles they have expanded even past the
equator.

Vesicles at Six Days. — At this age the vesicles are found more or less scat-
tered and isolated in position from one another through the upper half of the
uterus. They are nearly spherical and measure from i . o to 1.6 mm. ; their walls
are very transparent and the somewhat more opaque, round or oval embryonic
shield can be readily distinguished with a hand lens (Fig. 124).' Its size varies
with the diameter of the vesicle, being larger in the larger vesicles; but the pro-
portions are not exact, for a vesicle of given diameter may have an embryonic
shield of either larger or smaller dimensions than other vesicles of the same size.
Hence, vesicles of different sizes may have embryonic
shields of similar dimensions. The actual diameter of
the shield is between 0.2 and 0.35 mm. The general
structure of the vesicles is the same as at five days,
but certain differences may be noted. In preserved
specimens the external membrane is very apt to be
wrinkled. The subzonal layer has very much the same
appearance as before, though the cells are somewhat
smaller and it has almost disappeared over the region
of the embryonic shield. The manner of its disap-
pearance has not been definitely settled. There is no FIG. 124. — BLASTODERMIC VESI-
evidence that the cells degenerate or are cast off, hence
one inclines to the hypothesis that the cells of the
subzonal layer become incorporated in the inner layer
of the cuboidal ectodermal cells, for in sections shown at

this stage the ectoderm is one-layered in the region of the shield. The entodermal cells
also have essentially the same appearance as at five days, but they extend considerably
farther around the vesicle, are more numerous, and form a more continuous layer.
Sections show that the subzonal layer outside of the shield is very thin, but its
outer surface is fitted to the inner surface of the zona pellucida. The center of
each cell is somewhat thicker, projecting toward the interior of the vesicle. It is
in this thicker projecting portion that the nucleus is situated. Along the borders of
the cells the layer is of course thinner, and it is under these thinner parts that the
thicker nucleated portions of the entodermal cells are lodged. Hence, in surface
views, the nuclei of the two layers are seen to alternate more or less with one an-
other. This characteristic disposition is not kept^ everywhere, but is subject to
considerable variations. In the very most advanced ova of six days a small spot
sometimes can be observed in the embryonic shield which is noticeable from its
greater opacity. This spot corresponds to Hensen's knot, but it does not usually
show itself distinctly until considerably later.

Vesicles at Seven Days. — Vesicles at this age vary greatly in size, and the stage

CLE OF A RABBIT OF Six DAYS
AND ONE AND ONE-HALF HOURS.
FROM AN ALCOHOLIC SPECIMEN.
X 20 diams.

170 STUDY OF THE SEGMENTATION OF THE OVUM.

of development varies with the size— how exactly, we do not yet know. Prelimi-
narily we may fix on the normal size as being that of vesicles the greatest diam-
eter of which is about 4 mm. Such vesicles are somewhat oval in shape and slightly
flattened on the side bearing the embryonic shield. The membrane enclosing
them is very thin; the albuminoid layer can scarcely be distinguished, but the zona
pellucida is very distinct. The shield (Fig. 125, Sh) is. somewhat elongated and
distinctly pear-shaped. Its long axis is parallel with that of the vesicle. It varies
greatly in its dimensions. Shields i mm. wide and from 1.3 to 1.4 mm. long are
not uncommon. The student will be likely to encounter other dimensions. The
most striking addition is the appearance of a darker area, mes, at the posterior or
pointed end of the shield. This darker area is also somewhat pear-shaped, but
its pointed end is near the center of the shield, its rounded
end a little distance behind the point of the shield. The
darker area owes its formation to the appearance of a new
layer of cells between the ectoderm and entoderm. This
layer consists of loosely connected cells with rounded nuclei
easily distinguishable in surface views from those of the
subzonal layer. The greater part of these cells are certainly
mesodermic, but a portion of them share in the formation
FIG i2<c— BLASTODERMIC °^ *ne primitive streak and notochordal canal, and perhaps
VESICLE OF A RABBIT do not belong to the mesoderm. In the region outside the
AT SEVEN DAYS. embryonic shield the outer layer is easily distinguished; its

Sh, Embryonic shield, mes, 111 i j ^T r j-

/c . ,. cells have marked outlines, but are of smaller dimensions

Mesoderm. (Semi-dia-

grammatic.) x 6 diams. than in earlier stages, their nuclei are large, for the most
part oval, and contain several highly refringent and con-
spicuous granules. The number of granules varies; when there are only two or
three, they are apt to be elongated as if several small granules had united. The
entodermal cells have spread well past the equator of the vesicle and present,
for the most part, a distinctly epithelial arrangement, although at the edge of the
expanding layer the cells are still more or less scattered. The entodermal cells
are easily distinguished by changing the focus of the microscope, when their darker
protoplasm and smaller size, together with their smaller darker nuclei, make them
readily recognizable. The granules in the entodermic nuclei are smaller and more
numerous than in the overlying ectodermal nuclei.

During the next few hours further changes ensue. At the apex of the pear-
shaped mesodermic area there appears a small spot, which is known as Hensen's
knot. At first Hensen's knot consists of a little thickening accompanied by a union
of the cells of the middle layer with those of the overlying ectoderm. Next occurs
the development of the primitive streak, which runs from Hensen's knot backward
toward the apex of the embryonic shield, and very soon thereafter along the line
of the primitive streak there develops the external and shallow primitive groove.
At Hensen's knot the three layers now are found to bet intimately united, so that,

STUDY OF RABBIT BLASTODERMIC VESICLES IN ALCOHOL. 171

though they may everywhere else, when fresh, be separated from one another, the
germ-layers at this point cannot be separated, except by tearing. Finally, in the
next stage there grows out in front of Hensen's knot the so-called head-process, an
axial band of cells in which the notochordal canal develops.

A ..-vvv^^^;vip^

^•>..-i....-^fe''v' :•.!'-;'<.•;.'•'*«• '-<t\

B

^^^

.<?'

v'^vr.;^-^KV:!g;i;:'

'"•" '"£'•&,

FIG. 126. — THREE TRANSVERSE SECTIONS OF A RABBIT EMBRYO OF SEVEN AND ONE-HALF DAYS. SERIES 622,

SECTIONS 247, 260, 381. X 300 diams.

Examination of Cross-sections. — The structure of the embryonic shield is well
shown by cross-sections. Figure 126 represents three sections through the embryonic
shield at seven and one-half days: A, through the head process; B, through Hen-

.
172 STUDY OF THE SEGMENTATION OF THE OVUM.

sen's knot; C, through the primitive groove. The three primitive germ-layers are
easily recognized in each section. They are somewhat separated from one another,
but in life probably lay close together. The upper layer or ectoderm is the thick-
est and consists of low columnar cells; it is characteristic of the embryonic shield,
and at the edge (not included in the figure) of the shield changes to a thin sheet
of cells, which forms the outer layer of the rest of the blastodermic vesicle. The
lowest layer, entoderm, is a thin sheet only one cell thick. The middle layer,
mesoderm, is more irregular, and has begun to thicken, being in places two or
even three cells thick. In the median line the mesoderm enters into special rela-
tions with the other layers. In the middle section, B, which passes through Hen-
sen's knot, it forms a considerable axial thickening, which fuses with the entoderm
below and ectoderm above, and builds with the latter a dome-like projection.
The axial thickening extends forward from Hensen's knot, constituting the so-called
head-process, and in this region, A, the mesoderm is united only with the entoderm.

- Ent.

FIG. 127. — RABBIT EMBRYO OF SEVEN DAYS. TRANSVERSE SERIES 12, SECTION 216, THROUGH THE ANTERIOR

PORTION OF THE EMBRYONIC SHIELD.
EC, Ectoderm. Ent, Entoderm. X 350 diams.

Behind Hensen's knot the thickening extends backward, making the primitive
streak, C, which is characterized by the union of the mesoderm with the ectoderm
only, and by the primitive groove, a shallow median longitudinal depression of the
ectoderm.

The structure of the embryonic shield at seven days is further illustrated by
figures 127 and 128, the former passing across the anterior portion of the shield,
where it is two-layered, and the latter across the posterior portion, in which the
middle layer has appeared. Figure 176 shows the middle portion of a section. It
consists merely of the outer, thicker, ectodermal layer, EC, and the very thin ento-
dermal layer, Ent. Both surfaces of the ectoderm are quite sharply defined. The
nuclei are rather large and show several large, deeply stained nucleoli in each.
The outline of the nucleus is sharp, and, in addition to the larger granules, there
are many smaller ones less deeply stained scattered through the nucleus. The
nuclei vary considerably in size, shape, and position. The protoplasm of the ecto-
dermal cells is lightly stained, and granular in appearance. The boundaries between
adjacent cells are indicated by delicate lines, which extend through the entire thick-
ness of the ectoderm, which is now but a single layer of cells. The original outer

STUDY OF RABBIT BLASTODERM 1C VESICLES IN ALCOHOL.

173

(Rauber's) layer has disappeared. The
entoderm is very thin, but is thickened a
little where each nucleus is lodged. The
nuclei are smaller than those of the ecto-
derm, more darkly stained, and the gran-
ules in them less coarse than those in the
nuclei of the ectoderm. Between the two
layers is a narrow space; whether an
artefact or not is difficult to say. Figure
128 represents a transverse section through
the posterior part of the embryonic
shield where the primitive streak, pr.s, is
just forming. The position of the median
plane is approximately indicated by the
vertical line M. About this plane there
is a considerable accumulation of cells
which merges without boundary into the
superficial cells of the shield. A short
distance from the median line the outer
layer of the shield becomes a distinct epi-
thelium, EC, consisting of a single layer of
cells. The edge of the shield is marked
by a rather abrupt transition to the thin
outer layer of the extra-embryonic region.
On the under side of the section extends
the thin entoderm as a continuous layer,
which is only loosely connected with the
central mass of cells overlying it near the
median plane. Finally, from the median
mass of cells extends laterally the sheet
of mesoderm, Mes, between the outer
and inner germ-layers. The mesodermic
cells are somewhat loosely distributed, and
have round nuclei with distinct chroma-
tin granules and well-marked protoplasmic
bodies, which give off strands by which
the cells are united to one another. The
middle germ-layer is the least compact of
the three.

CHAPTER V.
STUDY OF YOUNG CHICK EMBRYOS.

Method of Obtaining Embryos.

Fertile eggs can usually be obtained from dealers, who can supply them in
quantities as needed, or hens may be kept wifh little trouble especially for the
purpose. In that case the hen herself will be found the best incubator, for the
number of eggs which develop normally under a hen is larger than in an artificial
incubator, and abnormalities of development are less frequent. A good setter will
remain upon the eggs, even though some are removed and replaced by fresh ones,
for about a month. She should be plentifully supplied with water and soft food,
which is best kept at a little distance off, so that she will be obliged to leave the
eggs to feed. A box that is somewhat secluded, and affords some protection,
warmth, and shelter from the light, should be provided. In order to obtain the
most accurate results it is desirable to place the eggs as soon as laid immediately
under the hen. Only by this means can an approximate correlation between the
stage of development and the duration of incubation be secured.

Artificial incubators are now made to work satisfactorily.* The temperature
of an incubator should be maintained at about 38° C. (100.4° F.). It should on
no account be allowed to rise above 40° C. (104° F.), for that destroys a portion
of the eggs and causes the production of many abnormalities in the remainder; and,
if possible, a fall to a lower temperature should be avoided, although the results of
a lower temperature are less disastrous. No incubator should be used which does
not permit a constant supply of fresh air and of moisture. The date should always
be marked on each egg when it is placed in the incubator. If a number of eggs
from a dealer are artificially incubated the same length of time, they are pretty sure
to cover a considerable range of stages, as, of course, 'eggs so supplied are of vary-
ing ages, the exact time of laying not being recorded.

In this work two stages of the chick are especially studied. The first stage
studied is that of a chick with seven segments, which is normal after about twenty-
seven hours' incubation. The second is normally produced after about forty-six
hours' incubation. The embryo should have about twenty-eight segments and three

* The one used at the Harvard Medical School is heated by a kerosene lamp and has a capacity of 100
eggs. It is called the New Method Incubator, and was purchased from M. A. Coffin, Burlington, Mass. In the
market other incubators may be found, doubtless equally good, among them patterns adapted for the use of gas
where that is preferred.

174

METHOD OF OBTAINING EMBRYOS. 175

gill-clefts showing externally. Embryos a little less or a little more developed are
almost equally serviceable.

Removing the Embryo. — Before the egg is opened a basin should be prepared
and filled with normal salt solution warmed to about 40° C. (104° F.). The basin
should be large enough to permit the entire egg to be submerged in it.

Take the egg warm from the incubator or the hen; allow it to rest quietly in
one position for two or three minutes before opening it. This is in order to insure
that the side of the yolk which contains the embryo is turned uppermost. After
an egg is disturbed the yolk will turn and resume its normal position, for which
but a short time is necessary. The egg may now be held in one hand, the shell
cracked, and the pieces of the shell above the yolk be removed with forceps, mak-
ing a hole about an inch in diameter. The inner egg membrane may be removed
with the shell. If any of the white of the egg tends to overflow, it should be
immediately snipped off with a pair of scissors, otherwise it will cause the yolk
to roll over, thus concealing the embryo.

The embryo and germinal area are now to be examined with the naked eye
or, better, with a hand lens. The student will detect very easily the area pellucida,
which lies at right angles to the long axis of the egg, and also see in the middle
of the area a long whitish streak, which marks the anlage of the embryo. Around
the area pellucida can be seen the mottled vascular area which will vary in ap-
pearance according as the development of the blood-vessels and blood-islands is
more or less advanced. The area vasculosa is , a portion of the larger area opaca
which merges at its periphery into the general yolk. In embryos of the second half
of the second day, thirty-six to forty-eight hours, the contraction of the heart can
be readily seen, and usually the outlines of the head of the embryo may be made
out. The germinal area is now to be separated from the rest of the yolk. To
accomplish this, plunge one blade of a sharp pair of scissors into the yolk a little
beyond the edge of the vascular area, and cut rapidly around until a circular inci-
sion has been completed; then take a flat .spatula and plunge it boldly into the
yolk at a depth of perhaps an eighth of an inch underneath the embryo. Next
lift out the embryo together with the yolk and the overlying white of the egg,
steady it a little if necessary on the spatula with a pair of forceps or needle, and
transfer it rapidly to the dish of warm salt solution. With a pair of fine forceps
the edge of the germinal area may be seized, and by gentle motion it may be
separated from the mass of yolk and also from the thin, whitish, overlying mem-
brane of the yolk, and at the same time from so much of the white of the egg
as may have been carried along. As one becomes more practiced in these opera-
tions, it is not difficult to remove the germinal area without taking much yolk
along with it.

The operation may be modified as follows: After the shell is opened the egg
may be tilted so as to allow the white to run off, and as it runs over the edge
it is snipped through with the scissors, and as much of the white removed as is

176 STUDY OF YOUNG CHICK EMBRYOS.

possible in this way. The whole egg is then submerged in the warm salt solution,
an incision around the germinal area made as above, and the embryo floated off
from the yolk.

Preservation of the Embryo. — The next step, after the embryo has been removed
from the yolk and lies in the salt solution, is to put a glass slide in the salt solu-
tion and carefully float the embryo and germinal area upon it, and then remove
them together. The slide is now to be laid flat on the table and the germinal
area spread out carefully upon it. In this operation good results may often be
obtained by allowing a few drops of warm salt solution to fall upon the center of
the germinal area. The currents produced by the falling drops will be sufficient
to spread out the blastoderm in its natural form,* and at the same time to wash
away any superfluous yolk grains that may be adherent to the preparation. At
this stage the preparation should be examined by the student with a low power of
the microscope, as described below. To preserve the specimen, four or five drops
of Zenker's fluid are allowed to fall upon the specimen gently and quietly as it
lies upon the glass slide. The specimen is allowed to stand for about ten minutes
and is then transferred to a dish containing a larger quantity of Zenker's fluid.
The transfer should be made by submerging one end of the slide in the dish and
floating the specimen off. In from two to four hours the hardening of the speci-
mens will be completed. They must then be washed thoroughly by decanting off
the Zenker's fluid and replacing it with water, and this water must itself be replaced
several times during the next twenty-four hours. Further treatment of the specimen
is as described on page 378.

The Making of Serial Sections.— Specimens are best colored with alum cochineal
in toto. They are then imbedded in paraffin and cut into series. The most useful
sections are those which are transverse to the axis of the spinal cord. They should
not exceed 10/1 in thickness.

Embryo Chick with Eight Segments. (About twenty-eight hours' incubation.)

The following description is almost equally applicable to embryos with six or
ten segments.

Examination in the Fresh State. — The embryo when first removed from the
yolk should be placed in a staining-dish with a small quantity of normal salt solu-
tion and examined with a low power of the microscope as a transparent object.
The specimen as a whole has a grayish or brownish gray tint. Most of the germi-
nal area is dark, the transmission of light being stopped by the numerous yolk-
grains contained in the entodermal cells (compare page 64). In the center of the
germinal area the transparent area pellucida is very conspicuous, and has an edge
which is quite sharply defined, more so than after the specimen has been preserved.
It is shaped somewhat like an elongated pear, the broad end of which surrounds

* The student will observe that the fresh blastoderm is verv easily distorted.

EMBRYO WITH EIGHT SEGMENTS.

177

the cephalic end of the embryo (Fig. 129). It should be noted that this figure was
drawn from a hardened, and not from a fresh specimen. The head of the embryo
lies toward the large end of the area pellucida and projects freely above the sur-
face of the germinal area. Underneath the projecting head is a very clear area
with distinct lateral boundaries. It is called the pro-amnion and contains no meso-
derm whatever. Near the head are two characteristic areas, one on each side, easily
recognized by the fact that the surface of the germinal area rises like a dome over

FIG. 129. — CHICK EMBRYO AFTER TWENTY-SEVEN HOURS' INCUBATION, WITH EIGHT PRIMITIVE SEGMENTS.

ALCOHOLIC SPECIMEN, x 15 diams.

each space. The spaces are termed the amnio-cardiac vesicles. They are in reality
local expansions of the ccelom which cause the somatopleure, or upper leaf of the
germinal area, to arch upward on either side of the embryo. By the study of cross-
sections (Fig. 136) the relations may be clearly understood. The posterior limit of
the head is marked by a curving line (to see this sharply the focus must be
lowered) the cavity of which faces the caudal end of the embryo. This line
marks the position of the fovea cardiaca and from it the fore-gut extends into the
head of the embryo. On the cephalic side of the fovea and underneath the fore-
gut the heart will be developed. On the sides of the fovea, running forward

178 STUDY OF YOUNG CHICK EMBRYOS.

toward the median line of the embryo, one can distinguish two darker bands which
represent the beginning of the formation of the blood-vessels, growing in from the
extra-embryonic region to meet in the median line of the embryo and participate
in the formation of the heart. These bands are the anlages of the omphalo-mesa-
riac veins. Behind the fovea appear eight pairs of rather opaque blocks of tissue,
symmetrically placed right and left. These are the primitive segments and are
formed exclusively by the mesoderm. The first pair of blocks lie a short distance
behind the fovea and the last pair a short distance in front of the rhomboidal
sinus (compare below). When new segments are added they are about the same
size as those previously formed, and always arise at the caudal end of the series.
The growth of the embryo in length during these stages depends rather upon the
multiplication of the segments than upon the growth of the single segments. The
principal axial structure is the anlage of the central nervous system, the so-called
medullary groove, already partly converted into a medullary canal; for at this stage
it is closed from the anterior limit of the head to a variable point of the segmented
region of the embryo. For a general account of the origin of the medullary groove
from the ectoderm see page 67. Chicks with eight segments vary extremely as to
the extent of the closure of the groove. The line of .closure can be readily seen.
It is somewhat wavy and irregular in its course, and the closure itself is some-
what irregular, so that we may find one or several points where the closure is not
yet completed although it is complete behind and in front of these points. At the
anterior extremity of the head the closure is always incomplete, there, being an
opening there which persists for some time and is known as the anterior neuropore.
Above the primitive segments, where it is not closed, the medullary groove has its
edges close together, but a short distance behind the last segment the groove widens
abruptly and fades out gradually. This widening is termed the rhamboidal sinus.
The sinus marks the caudal limit of the nervous system and extends so as to em-
brace the cephalic end of the primitive streak. The region of the primitive streak
appears quite dark by transmitted light, owing to the accumulation of cells which
belong chiefly to the mesoderm. This dark appearance extends forward and
merges into a dark band on either side, which runs up to the row of segments.
The dark band is the segmental zone, out of which new segments are differentiated.
On the surface of the primitive streak is a longitudinal furrow, the primitive groove,
which begins just within the rhomboidal sinus and extends backward, often bend-
ing to one side or the other, usually to the left. The groove is shallow in front,
deeper behind, and ends quite abruptly. More careful examination of the area
opaca shows that it already possesses a well-defined area vasculosa, the peripheral
boundary of which is more or less definitely marked. In the fresh specimen only
traces of the formation of the blood-vessels and blood-islands can be made out.

Examination of the Specimen after Hardening. — The specimen, after it has been
hardened, should be examined under the microscope in water or alcohol; and,
again, after it has been stained it should be cleared in oil and further' examined.

EMBRYO WITH EIGHT SEGMENTS.

179

This will enable the student to make out the blood-islands and something of the
blood-vessels in the area vasculosa, and also the shape of the brain which (Fig.
131) has expanded widely just behind the neuropore; the lateral expansions are
the anlages of the optic vesicles (Fig. 133). The remainder of the brain extends
from the optic enlargement to a point a little behind the fovea, fov. It is much
wider than the remaining portion of the medullary canal; it tapers from the optic

FIG. 130. — EMBRYONIC AREA OF A RABBIT WITH EIGHT SEGMENTS, WITH THE PLACENTAL AREA PARTLY TORN OFF.

X 15 diams. (Drawn by T. H. Emerton.)

vesicle and extends backward. One cannot yet distinguish in it positively any
subdivision into mid-brain and hind-brain. On the contrary, its walls are often
somewhat irregularly sinuous and vary considerably from specimen to specimen.

Comparison with a Rabbit Embryo. — In the ovum of the mammalia the ecto-
derm presents a modification known as the trophoderm. In the rabbit this tropho-
derm is developed over a limited region which is called the placental area (Fig.
130, a. pi), by which the embryo is attached to the wall of the uterus. When
the embryo figured was removed, a portion of the placental area remained attached
to the uterus, hence the defect shown in the specimen. The vascular area is

180 STUDY OF YOUNG CHICK EMBRYOS.

nearly circular; its boundary is marked by a well-defined terminal vessel, v.t. The
nearly straight embryo lies in the center and exhibits plainly the medullary canal
and primitive segments. The optic evaginations are already present. The head is
free; on its under side the heart is forming, and beneath it is a relatively large
and conspicuous pro-amnion, pr.a. Blood-vessels are present over the area vascu-
losa, but not yet in the embryo. It will be seen, therefore, that though the pro-
portions differ greatly from those in the chick, the fundamental relations in the
rabbit are the same as in the bird.

Examination of the Specimen after Staining. — After the chick has been stained
in toto it should be cleared in oil of cloves, or other suitable fluid, and further
examined in surface views with low powers of the microscope. For this purpose
it may be placed in a small shallow staining dish. It will be found advantageous
to have also whole chicks with their area? vasculosae permanently mounted in
Damar. Embryos up to about forty-eight hours' incubation are readily prepared in
this way. The germinative area with the embryo is treated like an ordinary section.
Specimens thus mounted must be protected from the pressure of the cover-glass
by putting under two opposite edges strips of paper or, better, of glass to support
the cover. Strips of glass as needed can be cut from broken cover-glasses with
a writing diamond.

Figure 131 represents a chick with eight fully formed segments stained with
alum carmine and viewed as a transparent object.

The distribution of the blood-islands and various details of the structure of the
embryo, which in the fresh specimen are obscure, can be readily observed in the
cleared preparation. The blood-islands, Bl.is, contain crowded young blood-cells
and are conspicuous owing to the intensity with which they are stained. They are
largest and most numerous in the posterior part of the area opaca, and on either
side become gradually smaller and farther apart toward the anterior end and are
absent altogether at the level of the head. A few small islands appear in the area
pellucida around the caudal end of the embryo. The amnio-cardiac vesicles, A.c.v,
are marked by the arching up of the surface of the area pellucida on both sides
of the head. In the embryo, the segments, Som.$, the walls of the medullary tube
(brain, Br, and spinal cord, Sp.c), and the omphalo-mesaraic veins, V.om, are
sharply defined. The first segment is imperfectly formed, and never acquires a
full development; it, together with segments two, three, and four, are called the
occipital segments because they enter into the formation of the occipital region of
the head and never undergo full differentiation like the other segments. In mam-
mals also there are (probably four) occipital segments. The fifth segment of our
chick becomes the first cervical segment of the adult. The medullary tube, Md,
has sharply defined, walls. It is completely closed through the brain region, except
at the anterior neuropore. At its cephalad extremity the tube has expanded later-
ally to form the optic vesicles, Op.V, each of which is the anlage of a retina and
optic nerve. The middle region of the tube, between the optic vesicles, is the first

EMBRYO WITH EIGHT SEGMENTS.

181

cerebral vesicle, fore-brain or prosencephalon. The second cerebral vesicle, mid-brain
or mesencephalon, Br.2, comprises the part of the medullary tube immediately
behind the optic vesicles and includes a little more of the length of the tube
than the vesicles. In older embryos (14-20 segments) it becomes clearly marked
off by constrictions from both the fore- and hind-brain. The third cerebral

G. Pro. am. Np. Op. V. a.

Br.

Br.3

Fov

Sp. c.

Sg. z.

V. t.

Pr. s.

FIG. 131. — CHICK WITH EIGHT FULLY FORMED SEGMENTS, STAINED WITH ALUM COCHINEAL AND MOUNTED IN

DAMAR.

a, Outline of the free portion of the head. A.c.v, Amnio-cardiac vesicle. A.p', A.p", Area pellucida. Bl.is,
Blood-island. Br.2, Second cerebral vesicle. Br.$, Third cerebral vesicle. Fov, Fovea cardiaca. G,
Post-optic ganglionic crest. Md, Medullary groove. Nch, Notochord. Np, Anterior neuropore. Op.V,
Optic vesicle. Ph, Outline of pharynx. Pro., am, Pro-amniotic area. Pr.s, Primitive streak. Sg.z, Segmental
zone or band of mesoderm, from which new segments arise. Som.^, Third somite. Sp.c, Spinal cord. V.om,
Vena omphalo-mesaraica. V.t, Vena terminalis. X 15 diams.

vesicle, hind-brain or rhombencephalon, Br.T,, is as long as the other two and has
a tapering form; it becomes the cerebellum, pons, and. me*dulla oblongata of
the adult. The cavity of the brain is wide, but in the region of the hind-
brain it tapers caudad and so passes over into the narrow cavity of the spinal

182 STUDY OF YOUNG CHICK EMBRYOS.

cord, Sp.c. In this specimen the open medullary groove, Md, begins at the level of
the fourth segment. The outline, a, of the free portion of the head is sharply marked,
as is also the outline, Ph, of the pharynx or fore-gut, which opens at the fovea
cardiaca into the general sub-germinal entodermic cavity. The omphalo-mesaraic
veins, V.om, can be traced peripherally to their junctions with the vascular net-
work of the area pellucida, and if the embryo b,e viewed by its ventral surface, the
two veins can be seen to unite, beneath the fore-gut, with the caudad end of the
heart. The notochord can be seen under the mid- and hind-brain, as a narrow
median band which is slightly irregular, and also very clearly, nch, underneath
the rhomboidal sinus; it fades out at its caudal extremity where it merges into the
undiffer'entiated tissue of the primitive streak. A band of cells, G, can be seen on
either side of the head, extending tailward from the optic vesicle. This band is
usually designated as part of the ganglionic crest, but its origin and fate have
not yet been satisfactorily elucidated.

Longitudinal Section of a Chick. — In order to facilitate the study of the trans-
verse sections of this stage, figure 132 is inserted, which is a nearly median longi-
tudinal section. In consequence of the head end, H, having grown .forward above
the pro-amnion, pro.a, it has become free on all sides, and at the same time the
entodermal cavity has been carried forward with the head, making the so-called
fore-gut of English authors. This fore-gut is the anlage of the pharynx, the oeso-
phagus, and the stomach. Underneath the posterior portion of the fore-gut there
has appeared in the mesoderm'a ccelomic cavity, />,' which serves as the connection
across the median line with the amnio-cardiac vesicles just described in surface
views. We can, therefore, distinguish in the fore-gut the anterior portion from the
posterior portion which overlies the ccelom. This coelom is the anlage of the peri-
cardial cavity* The anterior division of the fore-gut forms the pharynx- proper. It
ends blindly in front. The opening of the fore-gut into the general entodermic
cavity, Ach, is termed the foiea cardiaca, fo. At the posterior end of the embryo
we have a thickened mass of cells constituting the primitive streak, Pr.s. The line
on the under side of .the figure represents the entoderm, and the space underneath
it is a portion of the primitive entodermic cavity.

Study of Transverse Sections.— Attention should be directed, first, to the three
germ-layers, their composition and their rdles in the production of organs; second,
to the exact topographical relations of the various organic anlages, because these
relations are fundamental and determine the anatomical dispositions in the adult.
Before beginning the detailed study of the sections, the student should have a clear
conception of the manner in which the free head of the embryo merges into the
embryonic body and germinative area. Fifteen figures represent as many cross-
sections of an embryo chick with eight fully formed segments, and the ninth seg-
ment beginning. The drawings are uniformly magnified 100 diameters. There are
interpolated figures 132, 138, 139, 147, 149 from other embryos to illustrate certain
details with higher magnifications.

EMBRYO WITH EIGHT SEGMENTS.

183

Section through the Optic Vesicles (Fig. 133). — The section is oval, the head
being flattened dorso-ventrally. Its outer boundary is a layer of cells, EC, consti-
tuting the ectoderm. The inner and outer surfaces of the ectoderm are marked in
the section by distinct lines. With higher powers the ectodermal nuclei are readily
seen; there are no cell boundaries, although the protoplasm is gathered into columns
and strands with clear spaces between. We have in fact to deal rather with a
syncytium than with a layer of cells. On the dorsal side the ectoderm shows a

pro.a

FIG. 132. — LONGITUDINAL SECTION or A YOUNG CHICK EMBRYO.

H, Head. Vd, Anterior portion of digestive canal (Vorderdarm). mes, Mesoderm. fo, Fovea cardiaca. p,
Pericardial coelom. pro.a, Pro-amnion. Ach, Entodermal cavity, in life bounded below by the yolk. Pr.s^
Primitive streak.

thickening, G. If this be followed back in the series of sections it will be found to
be continuous 'both with the ectoderm and with an internal group of cells alongside
the mid-brain (Fig. 134, G). We shall return to the consideration of the group in»
question in connection with the description of the next figure. In the mid-dorsal
line the ectoderm from each side reaches the anterior neuropore, Np, which is still
open, and is reflected inward to form the thicker wall, Md, of the medullary tube,
here widely expanded to form the optic vesicles, Op. The outer ectoderm, EC, and

FIG. 133. — SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 21.
EC, Ectoderm. G, Ganglionic thickening. Md, Wall of medullary tube. Np, Neuropore. Op, Optic vesicle,

X ioo diameters.

inner ectoderm, Md, are everywhere in contact with one another, so that in the
whole section there is but a single germ-layer, the outer. Soon the middle germ-
layer will penetrate between the two sheets of ectoderm, and permanently obliterate
the primitive relations.

Section through the Mid-brain (Fig. 134). — The section of the head is oval, and
bounded everywhere by the ectoderm, EC, or as it may now be called, the epider-
mis. The head is completely free, but underneath lie the layers of the germinal

184

STUDY OF YOUNG CHICK EMBRYOS.

area. Immediately below the head is the pro-amniotic area -(Pro.am) which con-
sists of only two very thin layers of cells, the upper ectoderm, the inner entoderm.
By following through the series of sections it is easy to satisfy oneself that the two
layers of the pro-amnion are directly continuous with the Irke-named layers of the
embryo proper. At a little distance from the head, the lateral limit of the pro-
amnion appears, being marked by the appearance of the mesoderm between the other
two germ-layers. The edge of the mesoderm is sharply defined. The ectoderm has
formed also the thick wall, Md, of the medullary tube, which at this point is
completely closed and has lost its connection with the epidermis. There are no
distinct cell boundaries anywhere in the walls of the medullary tube at this stage.
The nuclei are oval, each with its long axis more or less nearly vertical to the
surface of the tube. Mitotic figures are frequent and occur always near the inner

mes.G.

Md.

Ao.d. Ph. a. Ao.v

FIG. 134. — SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 86.
a, Ventral thickening of ectoderm (part of oral plate). Ao.d, Dorsal aorta. A o.v, Ventral aorta. ,Ec, Ectoderm.
G, Ganglionic crest. Md, Wall of medullary tube, mid-brain, mes, Mesenchyma. Ph, Fore-gut. Pro.am,
Pro-amniotic area. X 100 diams.

or free surface of the medullary wall; in other words, next the cerebral cavity.
The microscopic structure of the tube is similar throughout its whole extent. Under-
neath the brain is the fore-gut, Ph, somewhat crescentic in cross-section, and formed
of a single layer of epithelium, the entoderm, which is thinner on the dorsal, thicker
on the ventral side of the fore-gut, a difference which becomes more marked in
later stages. In the median ventral area the entoderm is somewhat thickened and
adjoins a similar thickening , a, of the underlying ectoderm. The two thickenings
are beginning to unite at present, but are still distinct and easily break apart.
Very soon, however, they fuse into a single lamina, which is known as the oral
plate and in which all trace of the double origin is lost. The ectodermal thicken-
ing, a, is depressed below the level of the ventral surface of the head. By the up-
growth of the tissues around it, the depression is increased, until in later stages it
appears as a deep invagination, lined by ectoderm, and the floor of which is formed
by the oral plate. The invagination is termed the stomodcEum^. and is destined to
form a large part of the mouth. The oral plate soon undergoes autolysis, and by

EMBRYO WITH EIGHT SEGMENTS. 185

its own disappearance creates the oral opening of the fore-gut. Close to the fore-gut
lie four blood-vessels, two above and two below, the dorsal, Ao.d, and ventral,
Ao.v, aortae, respectively. The dorsal vessels are much the larger. If the series
of sections be followed through cephalad the ventral aorta will be found, before
the tip of the fore-gut is reached, to bend dorsalward and join the dorsal aorta of
the same side. If the series Tbe followed through in the caudad direction, it will
be observed that the two ventral aortae draw toward the median line until they
meet and unite in a single trunk, the main aorta, which is continuous with the
heart (Fig. 135, Hf). It is thus learned that the blood leaves the heart at its cepha-
lic end by a single channel, which soon divides; the branches curve upward and
pass to the dorsal side of the pharynx, forming two dorsal channels conducting
the blood-stream backward. The blood-vessels consist each of a very thin layer
of cells, epithelial in character and termed endothelium; the nuclei are flattened
and therefore appear oval in section. All the remainder of the section is occupied
by loosely scattered cells, which are of two sorts: first, those marked mes, which
till the ventral and lateral regions, and constitute the true mesenchyma; the mesen-
chymal cells have nuclei with small amounts of protoplasm around them, and
strands of protoplasm connect the cells together; there are no cell boundaries;
the- t'ssue might be described as an irregular reticulum with nucleated nodes;
second, those cells marked G, which form two lateral groups on the dorsal side
adjoining the mid-brain; these groups have been named the ganglionic crests by
some writers, mesectoderm by others. The cells in question resemble those of the
true mesenchyma, but have more protoplasm around the nuclei and appear
therefore more deeply stained than the mesenchyma proper. If the cells of the
crest be followed dorsally they will be seen to form a narrow band which joins the
ectoderm near the median line, and by following the sections headward, the crests
will be fouHd to merge with ectodermal thickenings (Fig. 133, G). From these
relations it has been inferred that the crest on each side arises from a local pro-
liferation of the ectoderm. The crest is easily seen in surface views of stained
chicks (Fig. 131, G). Two principal views as to the future of the crest cells of the
mid-brain region have been brought forward: first, that they are true ganglionic
anlages, which disappear by autolysis; second, that they are converted into true
mesenchyma.

Section through the Hind-brain (Fig. 135). — The head is no longer free, but
fuses laterally with the layers of the germinal area; hence the ectoderm, EC, instead
of bending over on to the ventral side, bends in the opposite direction— away from the
embryo. The mesoderm stretches across the median line under the embryo. There
is a large space, Coe, in the mesoderm; the space is part of the primitive body-cavity
or ccelom; it extends completely across the embryo and out into the germinal area
on each side. The ccelom is everywhere bounded by a thin epithelial layer, msth,
the mesothelium, which at this stage resembles an endothelium as seen in section;
it forms one part of the mesoderm, the bulky mesenchyma forming the other part.

186

STUDY OF YOUNG CHICK EMBRYOS.

Below the ccelom is another cavity, Pro.am, that of the pro-amnion, lined by ecto-
derm and opening anteriorly. This structure is not further dealt with here, partly
because its history is complicated, partly because it does not occur in the human
embryo. The ventral aortae (Fig. 134, Ao.v) have united into a single blood-chan-
nel, Ht, which we can identify as the blood-channel of the heart; it is called the
endothelial heart. The mesothelium, msth, on the dorsal side of the ccelom forms
a protuberant fold, the mesothelial heart, which surrounds the inner vascular space.
The two heart-walls are some distance apart. The inner heart produces only the
lining endothelium of the adult organ, the mesothelial heart produces all its muscu-
lar and connective-tissue components, and also the pericardium. The difference

mes. Ao.d.

Aid.

Coe. Ent.

nth. Pro.am. msth.

FIG. 135. — SECTION OF CHICK WITH EIGHT SEGMENTS. TRANSVERSE SERIES 86, -SECTION 86.
Ao.d, Dorsal aorta. Coe, Ccelom. EC, Ectoderm. Ent, Entoderm. G. Ganglionic crest. Ht, Aortic end of
the endothelial heart, nch, Notochord. Md, Hind-brain, mes, Mesenchyma. msth, Mesothelium. Ph,
Fore-gut. Pro.am, Pro-amnion. X 100 diams.

in the form of the cross-sections of the hind-brain, Md, mid-brain, and fore-brain
should be noted. Between the brain and fore-gut, and touching both, lies the small
notochord, nch, with a sharp outline. The ganglionic crest, G, of the mid-brain is
still traceable, but occupies a much smaller area, than in figure 134. In the mid-
dorsal line the crest, G, the epidermis, EC, and the medullary tube, Md, are fused
together. In correspondence with the reduction of the crest, the area occupied by
the true mesenchyma, mes, is increased.

Section through the Cephalic (Aortic) End of the Heart (Fig. 136). — The general
topography is similar to that of figure 135. The most striking differences are, that
in the present section the heart is very much larger, is bent to the right of the
embryo (the left of the figure), and has a narrow connection (mesocardium) with
the floor of the pharynx; that the ccelom is much expanded to form the amnio-
cardiac vesicles, A.c.v, one on each side, which are continuous with one another
On the ventral side of the heart; that the lips of the medullary tube are in contact
at c, but have not actually fused; and that there is no pro-amnion, because it does
not extend so far back under the embryo. The following details should be ob-

'EMBRYO WITH EIGHT SEGMENTS.

187

served: The ' epidermis, EC, of the embryo is thickened and fits closely against the
dorsal portion of the hind-brain, with which it is actually fused in the median line.
The ganglionic crest is represented only by a few more lightly stained cells at the
junction of the epidermis with the medullary wall, M d, but is much more developed
in nearby 'Sections, both cephalad and caudad. The fore-gut, Ph, is very wide, the
entoderm on its dorsal side is very thin, but grows thicker toward the lateral
boundaries of the gut, and is thickest in the mid-ventral line, where it forms a
shallow median groove; the nuclei in this groove are all next the external surface
of the entoderm. The mesothelium, msth, is a thin layer, which above the
amnio-cardiac vesicles enters into the formation of the somatopleure or true body

msth. mes. Ao.d.

A.c.v Endo. m.Ht. nch Ph. Spl.

FIG. 136. — SECTION OF A CHICK EMBRYO WITH EIJGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 114.
A.c.v, Amnio-cardiac vesicle. Ao.d, Dorsal aorta, c, Line of Closure of the medullary canal. EC, Ectoderm..
Endo, Endothelial heart. Md, Wall of hind-brain, mes, Mesenchyma. m.Ht, Mesothelial heart, msth,
Mesothelium. nch, Notochord. Ph, Fore-gut. Spl, Splanchnopleure. X IO° diams.

wall. It is not sharply separated from the mesenchyma, mes, as can be very well
seen in the part of the layer underneath the pharynx. When the heart is reached
the mesoderm forms a wide duplicature, m.Ht, }the mesothelial heart, which is a
layer of much greater thickness than the mesothelium proper, and which offers the
important characteristic that it shows no differentiation into mesothelium and
mesenchyma. -Between the mesothelial heart-tube and the endothelial, Endo, there
is a wide space which contains no visible structures, hence we assume that the
two cardiac tubes are kept apart by fluid only. Beneath the ccelom (amnio-cardiac
vesicles, A.c.v) is the Splanchnopleure, Spl, which has two thin layers: the upper
is mesoderm, the lower entoderm. The mesoderm has numerous nuclei, and if
followed out laterally to the area opaca will be found affixed to blood-vessels and
blood-islands, which together constitute the angioblast or anlage of the vascular sys-
tem. It can be observed in most places readily that the angioblast lies beneath the
mesoderm proper and is distinct from it. The entoderm has few nuclei and in
the area pellucida is very thin, but where it passes to the area opaca it gradually
but rapidly thickens, and .is composed of very large columnar cells (compare Fig. 30)

188

STUDY OF YOUNG CHICK EMBRYOS.

with large vacuoles, left by yolk masses which the cells have digested; toward
the periphery vacuoles with partly absorbed yolk may be found.

Section through the Venous End of the Heart (Fig. 137).— The relations differ
but little from those in figure 136 except for the heart and the ganglionic crest.
The heart shows its bend toward the left side (the right in the figure), and in
this bend both the endothelial, Endo, and the mesothelial portions, m.Ht, partici-
pate. The mesocardium, x, is clearly recognizable, and comprises two mesodermic
lamina. By it the heart is suspended throughout its length from the ventral wall
of the fore-gut. The mesocardium soon disappears, and the mesothelial heart there-
upon becomes closed dorsally, and is attached only by its aortic and venous ends
to the neighboring tissues. A strand of cells from the endothelial heart passes

Som. EC.

Ao.D G.

Md.

Acv.

m.Ht.

x Endo.

Ph.

FIG. 137. — SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 130.
Acv, Ammo-cardiac vesicles. Ao.D, Dorsal aorta. EC, Epidermis. Endo, Endothelial heart. G, Ganglionic
crest. Md, Medullary tube (Hind-brain), mes, Mesenchyma. m.Ht, Mesothelial heart. Ph, Fore-gut.
Som, Somatopleure. Spl, Splanchnopleure. x, Mesocardium. X 100 diams.

through the mesocardium and joins the entoderm. The significance of this junc-
tion is not clear. The ganglionic crest, G, is very distinct; it is overlaid by a thin
lamina of the epidermis, and is in texture quite unlike the brain-wall proper, Md.
The cells composing it are considerably individualized and somewhat separated from
one another by clear spaces.

Figure 138 represents a section somewhat more highly magnified through the
heart anlage of a slightly younger embryo. The medullary groove, Md, is not
closed. The ccelom does not yet extend across the median line, but there is only
a thin partition separating the amnio-cardiac vesicles, Am.ves, from one another.
The mesothelial heart, msth, is a relatively thick layer, thrown into irregular folds.
The endothelial heart is represented only by a few scattered angioblastic cells,
Endo, which as yet show no definite order.

The further development of the heart may be understood by the examination
of a somewhat older stage (Fig. 139). As shown in the illustration, the mesothe-
lium has become very protuberant, m.ht, in the median line underneath the fore-
gut, Ph. On either side it rapidly thins out, ,msth. In the protuberant fold we

EMBRYO WITH EIGHT SEGMENTS.

189

can recognize the future muscular heart, as it is sometimes called. The few cells
above described (Fig. 138, Endo) have increased considerably in number and have
joined themselves together in such a manner as to indicate clearly the formation of

FIG. 138. — SECTION OF A CHICK EMBRYO WITH SEVEN SEGMENTS. TRANSVERSE SERIES 510, SECTION 184.

Am.ves, Amnio-cardiac vesicle. EC, Ectoderm. Endo, Cells forming the anlage of the endothelialjheart. Md,

Hind-brain, mes, Mesenchyma. msth, Anlage of mesothelial heart. Ph, Fore-gut.

m.ht

pro. am

FIG. 139. — CHICK EMBRYO, TRANSVERSE SECTION ACROSS THE ANLAGE OF THE HEART IN A STAGE SLIGHTLY MORE

ADVANCED THAN FIGURE 138.

Md, Wall of medullary tube, nch, Notochord. msth, Mesothelium. Ph, Pharynx, pro.am., Tip of pro-amnion.

en.ht, Endothelial heart, m.ht, Muscular heart.

the endothelial heart (Fig. 139, en.ht). At first the cells are irregularly disposed and
have several irregular cavities between them, which soon fuse so as to form two
main cavities running longitudinally. As the two cavities enlarge they meet in the
median line and remain separated at first by a wall of two layers of endothelium.

190

STUDY OF YOUNG CHICK EMBRYOS.

This wall soon • breaks through, and there results a single median tube of endo-
thelium which presently appears to be connected with the mesothelium, m.ht,
by long cell-processes across the wide intervening space. The heart is now a
double tube connected by the mesothelium with the tissues above.

Section through the Wall of the Fovea cardiaca (Fig. 140). — Underneath the whole
of the embryo and the germinative area is the extensive archenteric cavity, bounded
above by the cellular entoderm of the embryo and of the splanchnopleure. The
archenteron includes both the intestinal cavity of the embryo and the cavity of the
yolk-sac, and accordingly its lower floor is the mass of yolk. Into the head of
the embryo runs the closed prolongation of the archenteron, which we have studied
as the fore-gut. The posterior opening of the fore-gut is known as the fovea
cardiaca. The manner in which the entoderm at the fovea bends ventralward to

Sont.

Ao.d. Md.

EC.

Spl. COB. msth. Ent. Ph. V.om.

FIG. 140. — SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 149.

Ao.D, Dorsal aorta. Cos, Coelom. EC, Epidermis. Ent, Entoderm. G, Ganglionic crest. Md, Hind-brain.

msth, Mesoderm of the septum transversum. Ph, Fore-gut. Som, Somatopleure. Spl, Splanchnopleure.

V.om, Omphalo-mesaraic veins. X 100 diams.

pass from the fore-gut on to the splanchnopleure may be readily understood from
figure 139. The present section passes through the nearly vertical wall of entoderm,
Ent, at the fovea. From this wall the anlage of the liver will arise. THe omphalo-.
mesaraic veins, V.om, pass by it to join the caudal or venous end of the heart.
The fore-gut, Ph, is still closed on its ventral side. The veins are in the splanch-
nopleure, and, being cut obliquely, appear somewhat elongated. They" each cause a
1 protuberance of the splanchnopleure toward the ccelom, Coe. The protuberance
is covered by a thick dense layer of mesoderm, Msth, which forms an arch over
the vein, so as to leave a clear space between it and the endothelium of the vein.
The two protuberances constitute the anlage of the septum transversum, which is
itself the anlage of the diaphragm. The region cephalad of the septum is the
cervico-thorax; the region caudad, the future abdomen. If the series of sections be
followed headward, the veins can be traced to their union with the heart in the
median line. If the series be followed tailward, the veins can be traced out into
the area pellucida, where they branch. The hind-brain, Md, is of smaller diameter
than in the previous sections. The epidermis, EC, is closely fitted against the dorsal

EMBRYO WITH EIGHT SEGMENTS.

191

half of the medullary tube and fuses in the mid-dorsal line with the ganglionic
crest, G, which in its turn fuses with the medullary walh The crest, though not
large or conspicuous, can be distinguished readily. The mesenchyma, is clearly
differentiated only above the fore-gut, and on either side just beyond the lateral
boundary of the fore-gut it fuses with the mesotheliurn. >

Som.

Ao.D. Md.

G.

mes. EC.

Spl. Ent. nch. ' a. V.om.

FIG. 141. — SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION -144.
a, Groove corresponding to the prolongation of the lateral portion of the fore-gut. Ao.D, Dorsal aorta. EC,
Epidermis. Ent, Entoderm. G, Ganglionic crest. Md, Wall of Hind-brain, mes, Mesenchyma.
nch, Notochord. Som, Somatopleure. Spl, Splanchnopleure. V.om, Omphalo-mesaraic veins. X 100
diams.

Section, behind the Fovea cardiaca and in- Front of the First Segment (Fig. 141).—
The closed entodermal cavity of the embryo has become open and communicates
freely with tHe yolk-cavity. The omphalo-mesaraic veins occupy a more lateral
position. Otherwise the section differs so little from figure 140, that it does not call
for special description. *

Som.

EC.

Md.

Spl. Cce. Ao.D. nch. Ent. Msth.

. FIG. 142.— SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 162.
Ao.D, Dorsal aorta. Ch, Ccelom. EC, Ectoderm. Ent, Entoderm. G, Anlage of Ganglionic crest, mes,

Mesenchyma of the intersegmental cleft. Msth, Mesodermic lining of the coelom. nch, Notochord. Som,

Somatopleure. Spl, Splanchnopleure. X 100 diams.

Section between the First and Second Segments (Fig. 142). — We are still in the
region of the hind-brain, which extends to the fourth or last occipital segment.
There is no distinction or limit between the embryonic and the vitelline divisions
of the archenteron. The hind-brain, Md, is oval in section; on its dorsal side it
fuses with the ganglionic crest, G, which seems now rather a part of the brain-wall

192

STUDY OF YOUNG CHICK EMBRYOS.

than a separate structure. The mid-ventral wall or floor of the hind-brain is
relatively thin, a feature which marks the transition to the spinal cord, which al-
ways has a thin floor-plate. The notochord, nch, is large and transversely oval in
section. The two dorsal aortae, Ao.D, occupy the same relative positions as in the
previous sections. The ccelom, Coe, is a comparatively narrow fissure, but can be
followed laterally far out into the area op^aca. It is bounded above and below by
mesoderm, Msth, for the most part thin and of a loose texture; but on the lower
side in the embryonic region the mesoderm forms a, broad band, the cells of which
are densely packed. The thick mesodermic band is continuous with that which
forms the covering of the septum trans versum (Fig. 140, Msth). The morphological
significance of the band is undetermined. The mesenchyma, mes, occupies the
space between the hind-brain and ectoderm on the one hand and the aorta and

EC.

Som.m.

Ent.

Cos. Ao.D. nch.

Mes.

seg. Spl.m.

FIG. 143. — SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 180.
Ao.D, Dorsal aorta. Cos, Crelom. EC, Ectoderm. Ent, Entoderm. Md, Hind-brain. Mes, Mesoderm. nch,
Notochord. seg, Mesodermic segment or somite. Som.m, Somatic mesoderm. Spl.m, Splanchnic meso-
derm. X 100 diams. •

ccelom on the other. It consists of loosely scattered cells, connected with one
another by strands of protoplasm. It fuses at the proximal angle of the coelom with
the lining mesoderm thereof. Longitudinal sections demonstrate that the loose
mesenchyma occupies only the narrow space between two segments, and that it
fuses with the denser tissue of both adjacent segments. The intersegmental space
is not a cleft but a partition of loose mesenchyma.

Sections through the Third Segment (Fig. 143). — As the hind-brain ends at the
level of the fourth segment, the present section is near the transition from the
cephalic to the cervical region, segment 5 being the first cervical segment. Accord-
ingly we find that the medullary tube, Md, although not yet closed, has in cross-
section a form resembling that of the cord at this stage. The ectoderm, EC, runs'
from the lips of the medullary groove as a layer which is somewhat thickened over
the embryo, but becomes very thin over the area pellucida. The coelom, Coe, is of
small dimensions, although irregular clefts in the mesoderm of the germinative area
indicate its extension. The somatic mesoderm, Som.m, is a thin layer above the
ccelom; the splanchnic mesoderm is a much thicker layer, Spl.m, below the ccelom.
Both mesodermic layers extend beyond the coelom toward the medullary tube to
form the mesodermic somite, seg, which with its fellow of the opposite side constitutes

EM BR YO WI TH, EIGH T SEGMEN TS. 1 93

a complete segment. The mesoderm does not extend across the median line, being
blocked by the notochord, nch. The somite is bounded mesially by the medullary
tube and notochord, dorsally by the ectoderm, ventrally by the aorta and entoderm.
Its cells are so arranged as to make a dorsal, a mesial, and a ventral wall, and
a core of cells more loosely grouped. Careful study of the segments in various
stages has led to the conclusion that the core belongs to the ventral wall. The
line of contact between the dorsal wall and the core is accordingly the potential
prolongation of the ccelom, and in certain embryos there is a ccelomatic space pres-
ent in the position indicated. The somite has a broader part toward the medullary
wall and a narrower part toward the main coelom. When a segment undergoes its
full development, the narrow part forms a separate structure, the nephrotome.

Section through the Segmental Zones (Fig. 144). — The medullary groove, Md,
is not closed, but is deep and narrow, its dorsal lips nearly in contact with one
another. The embryonic ectoderm, EC, is slightly thickened. The entoderm is

Mes. Seg. z. Md

Ent. Ao. nch. Cce. Ve.

FIG. 144. — SECTION OF A CHICK J^MBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 267.
Ao, Dorsal aorta passing from the embryo to the area vasculosa. Cos, Beginning of coelomatic cavity. EC,
Ectoderm. Ent, Entoderm. Md, Medullary groove. Mes, Mesoderm. nch, Notochord. Seg. z, Seg-
mental zone of the mesoderm. Ve, Blood-vesseJ of the area vasculosa lying below the mesoderm proper.
X 100 diams.

a thin layer which on one side, Ent, shows the beginning of the thickening char-
acteristic of the area opaca. The notochord, nch, is nearly circular in section and
is larger here than nearer the head. The dorsal aortae, Ao, have left their posi-
tion and are passing outward to ramify upon the area pellucida. It is thus evi-
dent that the distribution of the blood from the heart to the area vasculosa takes
place considerably caudad of the veins which collect the blood and return it to the
heart. The blood-vessels, Ao.Ve, lie between the mesoderm proper and the ento-
derm, and constitute with the associated blood-islands the angioblast. The situa-
tion of the angioblast in early stages is typical for all birds and also for mammals.
The mesoderm, Mes, has only irregular spaces, Coe, which by their expansion and
fusion will give rise to the continuous coelom. On either side of the medullary
groove, the mesoderm forms a thickened mass, the segmental zone, Seg.z, which
is markedly constricted where it joins the lateral mesoderm. Out of the constricted
area the nephrotomes are differentiated. The somites arise by transverse cleavage
of the segmental zone, each new somite being formed immediately caudad to the
last-formed somite. The so-called cleavage depends upon a great loosening of the

194

STUDY OF YOUNG CHICK EMBRYOS.

mesenchyma, as shown in figure 142, and not upon the development of an actual
fissure or cell-less space. If the series of sections be followed toward the tail, the
segmental zone will be found to fade out gradually.

Section through the Open Medullary Groove (Fig. 145). — As we have seen in
following the series of sections, the farther tailward we pass the less advanced is
the development. In the present section (Fig. 145), we find that only the notochord,
nch, is separated from its germ -layer. The three germ -layers, the ectoderm, EC,

Aid.

Ales.

Ent. nch. Ve.

FIG. 145. — SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 314.

EC, Ectoderm. Ent, Entoderm. Md, Medullary groove. - Mes, Mesoderm. nch, Notochord. Ve, Blood-
vessels. X loo diams.

mesoderm, Mes, and entoderm, Ent, are merely laminae of undifferentiated cells,
the ectoderm alone showing a modification where it forms the wall of the medul-
lary groove, .Md. The angioblast or layer of blood-vessels, Ve, forms a well-
defined separate anlage, which is clearly distinct from both mesoderm'and entoderm.
The notochord, nch, is of large size, and in part occupies 'a notch on the .under
side of the medullary groove. A little farther caudad the notochord fuses with the
wall of the overlying medullary groove, and still farther on the united structures
fuse also with the entoderm (Fig. 146).

Md

Ales.

Ent. Pr.S.

FIG. 146. — SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. SERIES 642, SECTION 350.
EC, Ectoderm. Ent, Entoderm. Md, Medullary groove. Mes, Mesoderm. Pr.S, Primitive streak. X.ioo

diams.

Section through the Medullary Groove, near its Caudal End (Fig. 146). — The
disposition of the parts is somewhat similar to that in figure 145, but the following
differences are to be noted: The medullary groove, Md, is deeper and more trough-
like in section and its floor merges into the primitive streak, Pr.S, or axial cord of
cells, which merges below with the entoderm and laterally with the mesoderm. The
fusion with all three germ-layers is the essential characteristic of the primitive
streak. The mesoderm, Mes, of the present section is voluminous, and shows in

EMBRYO WITH EIGHT SEGMENTS.

195

he embryo-region no trace of a coelomatic fissure. Over the area pellucida there
is no angioblast between the mesoderm and entoderm, it not having yet penetrated
from the area opaca.

Figure 147 represents, under a considerably higher magnification, three sec-

Seg

EC.

mes

En

FIG. 147. — CHICK EMBRYO WITH SEVEN SEGMENTS. TRANSVERSE SERIES 510, SECTIONS 311, 212, 172.
Three transverse sections across the caudal end of the medullary groove. A, Section through one of the segments.
B, Section posterior to the segments. C, Section just in front of the primitive streak. Md.gr, Medullary
groove, nch, Notochord. EC, Ectoderm, mes, Mesoderm. En, Entoderm. X 230 diams.

196

STUDY OF YOUNG CHICK EMBRYOS.

tions at different levels through the open groove of a slightly younger chick. In the
first, A, the groove is quite deep and the young primitive segment is shown. At
the edge of the groove its thick walls pass over continuously, but quite abruptly,
into the general ectoderm, EC, covering the embryo. Close under the median line
of the medullary groove appears an oval section of the notochord, nch. The ento-
derm, En, is quite thin and somewhat irregular, as is shown in all of the sections.
In B the medullary groove is wide open and quite shallow, the notochord is much
larger and extends from the floor of the medullary , groove to the entoderm and
occupies in part a deep notch in the medullary wall. The notochord prevents the

Md.gr.

FIG. 148. — THREE SECTIONS OF A CHICK EMBRYO WITH EIGHT SEGMENTS. SERIES 642.

A. Section 366, through the cephalic end of the primitive groove. B. Section 400, through the middle of the
primitive groove. C. Section 424, through the caudal end of the primitive groove.

extension of the mesoderm across the median line. In C the medullary groove is
fading out and merging into the beginning of the primitive streak, which forms
a large mass of cells in the median line in which the boundaries between the germ-
layers cannot be determined. Laterally this mass of tissue passes over into per-
fectly distinct germ-layers, of which the middle or mesoderm, mes, is by far the
most voluminous. The walls of the medullary groove are crowded with nuclei
which lie at every possible level, some close to the inner, others close to the outer
surface, and also in every -possible intervening position. The nuclei are much
crowded, there being but little protopjasm. No distinct cell boundaries can be
made out. The nuclei are further remarkable on account of their very conspicu-
ous nucleoli.

EMBRYO WITH TWENTY-FOUR SEGMENTS.

197

Sections through the Primitive Groove (Fig. 148). — In all of these we find merely the
three germ-layers, which are all united in the median line with the axial band of cells,
constituting the primitive streak. The ectoderm has the deep furrow which toward
the head runs into the medullary groove. Caudad, the primitive groove widens
out and is gradually lost. The thinning out of the mesoderm should also be no-
ticed, as the series is followed in the caudad direction. Under a higher power the
character and arrangement of the cells comes out more clearly (Fig. 149).

FIG. 149. — CHICK EMBRYO WITH SEVEN SEGMENTS. TRANSVERSE SECTION ACROSS THE PRIMITIVE GROOVE.
EC, Ectoderm, mes. Mesoderm. Ent, Entoderm. Pr.g, Primitive groove. The large black dots represent

yolk-grains. X 230 diams.

Embryo Chick with about Twenty-four Segments and Three Gill-clefts (about

Forty-six Hours'1 Incubation).

The following description will apply almost equally well to embryos with from
twenty-six to twenty-nine segments.

Examination in toto. — The specimen as a whole, as in the fresh state, has a
grayish tint when viewed by transmitted light. As soon as it is hardened the
opacity of all the tissues is greatly increased. In the center of the germinal area
is the very conspicuous area pellucida, which is somewhat pear-shaped. The por-
tion around the anterior end of the embryo (Fig. 150, A.p) is very wide. In the
center of the area vasculosa appears the embryo, the head end of which is twisted
over so that the left side of the head lies against the yolk. This twisting of the
neck- and head so that they become asymmetrical in position is very characteristic
of birds. Below the .head and somewhat to the right may be seen the tubular
heart, Ht, which, in the fresh specimen, pulsates regularly. Around the area pellu-
cida comes the dark area opaca, in which we readily distinguish the outer boundary
or terminal sinus of the area vasculosa. In this there is already a well-developed
network of blood-vessels through which the blood is circulating, being driven by
the heart. The blood moves out from the embryo by two large vessels, A.vi,
which lie symmetrically, the vitelline or omphalo-mesaraic arteries. These arteries
arise from the dorsal aorta of the embryo and pass out to the area vasculosa,
over which they ramify. -The blood returns to the heart by means of a network
of small vessels, across the posterior part of the area pellucida; the network close
to the embryo fuses into two larger short trunks, one each side. The two trunks

198

STUDY OF YOUNG CHICK EMBRYOS.

are the anlages of the omphalo-mesaraic veins, which gradually grow out and
branch in the extra-embryonic region, enlarging at the same time (compare Fig.
53). The general form of the embryo is indicated by figure i$£>. In the region
of the head we notice the very well-marked head-bend which is established in the
region of the mid-brain, M.b. The medullary tube in the region of the head is

M.b.

A.p.

Am.f.

A.o.

bl.is.

Seg.z. J

Aid.

Pr.g.

FIG. 150. — EMBRYO CHICK WITH ABOUT TWENTY-FOUR SEGMENTS. SURFACE VIEW FROM THE DORSAL SIDE.

A.c.Vf Amnio-cardiac vesicle. Am.f, Posterior edge of the amniotic fold. A.o, Area opaca. A.p, Area pellucida.
A.w, Arteria vitellina (or omphalo-mesaraica). bl.is, Blood-island in the area pellucida. Ht, Heart. M.b,
Mid-brain. Md, Medullary canal or spinal cord. Op, Optic vesicle. Ot, Otocyst. Pr.g, Primitive groove.
Seg, Primitive segment. Seg.z, Segmental zone, i, 2, Gill-arches. X 15 diams.

very much enlarged and is divided into three well-marked primary cerebral vesicles,
which appear distinctly in specimens that have been stained and cleared. The
first of these is quite large, and at its side lies the anlage of the eye, Op, in the
center of which one readily distinguishes the commencement of the lens as a smalt
invagination of the ectoderm with its orifice still open. The second cerebral vesicle
is much smaller .than the first in every dimension. It occupies the region of the
head-bend and is separated from the first vesicle by a constriction, and from the

EMBRYO WITH TWENTY-EIGHT SEGMENTS. 199

thirds vesicle by another constriction. The third vesicle in length more than equals
the first and second combined, and at its widest part is nearly equal in diameter
to the second vesicle. It tapers out toward the caudal end 0f the embryo and
passes over into the much smaller portion of the medullary canal, which represents
the anlage of the spinal cord. At the side of the third vesicle we can see an open
pit, the anlage of the inner ear or otocyst, Ot. On the side of the neck between
the third cerebral vesicle and the heart there are three external depressions which
bound the first and second branchial arches, i, 2, of the embryo. Behind each
arch the depression marks the site of a gill-cleft. The first is the longer, the
second the shorter. Between the projecting head and the first branchial arch the
outline of the embryo makes a depression which marks the position of the develop-
ing, oral cavity. The heart is a large tube, Ht, in a still larger pericardial cavity
(ccelom), the membranous covering of which is somatopleuric. The omphalo-
mesaraic veins join the venous or posterior end of the heart. The heart is very
much bent; its anterior end turns toward the gill-clefts and there gives off the
primitive aortic branches, which finally join ' again so as to form the median dorsal
aorta which sends off the two vitelline arteries, A.m. On either side of the med-
ullary canal can be seen the primitive segments, Seg. The first of these which
is distinct lies close behind the otocyst. At the. posterior end of the embryo addi-
tional segments are still forming, and the precise number of segments varies from
embryo to embryo. The medullary canal, Md, is closed, but beyond its extreme
limit traces of the primitive groove, Pr.g, can still be seen. The network of
blood-vessels over the area vasculosa is very distinct and characteristic. The net-
work, however, does not yet extend into the body of the embryo proper. The
limit of the body of the embryo is suggested by the darker tissue, Seg.z, surround-
ing the spinal cord, Md, on either side. About the hinder end of the embryo, both
iri the pellucida and in the opaca, appear, a number of small spots, the blood-
islands, bl.is, many of which have in the fresh specimen a reddish color. In hard-
ened specimens the opacity of the blood-islands renders them conspicuous, espe-
cially in the area pellucida.

Embryo Chick with Twenty-eight Segments.

The Study of Transverse Sections. — A series of figures from transverse sec-
tions of an embryo of this stage is herewith presented. They have been selected
so as to show the principal typical structures. The position of the sections can
be followed more easily by comparing each transverse section with figure 166,
to determine its place and the organs through which it must pass.

Section through the Right Auditory Imagination (Fig. 151). — Owing to the
curvature of the neck-bend, the section of the head is not symmetrical. It passes
through both the hind-brain, h.b, and the fore-brain, f.b. Underneath the former
appears- a small structure, nch, the notochord, and on one side can be seen the
auditory invagination, O/, which is formed wholly by the locally thickened ectoderm,

200

STUDY OF YOUNG CHICK EMBRYOS.

which is elsewhere quite thin. The ectoderm, EC, covering the dorsal side of the
hind-brain is very thin, but the portion in front of the auditory invagination is
somewhat thicker. The ectoderm of the invagination is very much thickened and
contains numerous somewhat crowded nuclei at all levels. These nuclei are rounded
in form and have one or two very distinct nucleoli. On the posterior side of the
otocyst there is very little mesoderm; on the anterior side, much more. Between the

• Epen.
/f " ^K ^-"'

h.b. __ M Ec

Ot.d

nch.

Ao.

EC

FIG. 151.— SECTION OF CHICK EMBRYO WITH ABOUT
TWENTY-EIGHT SEGMENTS. TRANSVERSE
SERIES 92, SECTION 73.

Ao, Aorta. EC, Ectoderm. f.b, Fore-brain, h.b,
Hind-brain. mes, Mesoderm. nch, Noto-
chord. Ot, Otocyst. Vc, Vein. X 50 diams.

FIG. 152. — SECTION OF CHICK EMBRYO WITH ABOUT
TWENTY-EIGHT SEGMENTS. TRANSVERSE
SERIES 92, SECTION 83.

A o.i, Prolongation toward the fore-brain of the first
aortic arch. Ao.D, Descending aorta, card,
Anterior cardinal vein, cl.pl, Closing plate.
cl.i, First gill-pouch. EC, Ectoderm. Epet. .
Roof of hind- brain. /.&,• Fore-brain, h.b,
Hind-brain, "mes, Mesoderm. nch, Notochord
Op, Optic vesicle. Ot.d, Right otocyst. Ot.s,
Left otocyst. Ph, Pharynx. X 50 diams.

developing otocyst and the notochord there is a blood-vessel, Ve, with merely endo-
thelial walls, a branch of the cardinal vein. Between the hind-brain and fore-
brain near the notochord, the two aortae, Ao, are cut. In their interior there can
usually be seen a certain number of nucleated blood-cells varying somewhat in
size and appearance, but generally having a rounded form with distinct outline and
a well-defined nucleolated nucleus.

Section through the Left Auditory Invagination (Fig. 152). — Owing to the irregu-
lar form of the embryo the sections through the otocyst are not symmetrical. The

EMBRYO WITH TWENTY-EIGHT SEGMENTS.

201

Epen.

card.

sent section shows the opening of the left otocyst, Ot.s, and a closed section
of the right otocyst, Ot.d. At its lower inner edge the outer boundary of the wall
of the otocyst is indistinct, this arppearance being due to the union of the cells of
the acoustic ganglion with the wall of the otocyst. The section also passes through
the first gill-cleft, cl.i, of the right side, and shows very distinctly indeed the
closing plate, cl.pl, which is formed by a
fusion of the ectodermal and entodermal
cells. On the opposite side of the section
the same cleft is imperfectly shown. On
the posterior side of the cleft is the dorsal
aorta, Ao.D, and on the anterior side of
the cleft, extending toward the fore-brain,
f.b, appear the sections of the two pro-
longations of the first arches, Ao.i, toward
the fore-brain. In this specimen each pro-
longation forms a loop, which rejoins its
arch* dorsally. In the region of the fore-
'brain appears a shaving from the edge of
the optic evagination, Op. The anterior
cardinal veins, card, appear just inside of
the otocyst close to the ventral wall of the
hind-brain, h.b.

Section through the Invagination of the
Optic Lens (Fig. 153). — This section also
passes through the hind-brain, h.b, fore-
brain, f.b, and through the openings of FIG. 153. -SECTION OE CHICK EMBRYO WITH ABOUT
J TWENTY-EIGHT SEGMENTS. TRANSVERSE

both invaginations to form the anlages, L, SERIES 92, SECTION 96.

of the lenses Of the eye. These invagina- Ao.D, Descending aorta. Ao.i, First aortic arch:

Mdb.

Ent.

Ret.

f.b.

Ao.2, Second aortic arch, card, Anterior car-
dinal vein. cl.I, First gill-cleft. cl.II, Second
entodermal gill-cleft. EC, Ectoderm. Ent,
Entoderm. Epen, Roof of hind-brain, f.b,
Fore-brain, h.b, Hind-brain. L, Invagination
of lens. Mdb, Mandibular arch, mes, Meso-
defm: .nch, Notochord. Op, Optic vesicle.
Ph, Pharynx. Ret, Retina. X 50 diams.

tions bear a striking resemblance to those

which form the otocysts. The ectoderm,

EC, over the roof of the fore-brain is very

thin and passes abruptly into the thickened

layer which forms the wall of the invagina-

tion. On the ventral side the ectoderm is

somewhat thicker. The wall of the lentic

vesicle is quite thick; its nuclei are numerous, but are situated chiefly on the meso-

dermal side of the layer, so that toward its outer surface the layer is comparatively

free from nuclei. The invagination of the lens rests against the optic vesicle, the

wall of which, Ret, next to the lens is thicker than the posterior or inner wall of the

optic vesicle. The thickened outer portion is the anlage of the retina, the thinner inner

portion is the anlage of the pigment layer covering the retina. The fore-brain,

f.b, has an elongated form with quite thick walls crowded with nuclo' Between

202

STUDY OF YOUNG CHICK EMBRYOS.

Ao.D.

cl.II.

Ao.2.

card.

nch.

cl.II.

Ph.*

Ht.

EC.

it and the hind-brain appears the cavity of the pharynx, Ph, which on the left
side of the embryo shows a prolongation, cl.I, which extends almost to the sur-
face of the embryo. This prolongation is the first gill-pouch. On the dorsal side
of the pharynx appear the two large aortic trunks, Ao.D, and on its ventral side
the two smaller first aortic arches, Ao.i. These are situated in the mandibular

branchial arch, Mdb, which is well marked
externally by a rounded protuberance. The
distal end of the second gill-pouch is shown
on the right side of the section, d, II.

Section through the Optic Stalks (Fig. 154).
—The head of the embryo now appears quite
isolated from the body. It is bounded by a
distinct layer of ectoderm, EC, and contains
the very large fore-brain, f.b, which gives off
on either^ side an optic evagination, Op, the
walls of which are quite thick, about the same
as those of the fore-brain proper. Each optic
evagination is widest toward the side of the
head and is constricted toward the brain,
with which, therefore, it is connected by a
stalk in which we can already recognize the
anlage of the optic nerve. Between the two
optic stalks on the side toward the pharynx

„ the floor of the fore-brain bends downward and

FIG. 154. — SECTION OF CHICK EMBRYO WITH

ABOUT TWENTY-EIGHT SEGMENTS. TRANS- almost joins the superficial ectoderm. All of

VERSE SERIES 92, SECTION 104. the space between the walls of the fore-brain

MTrunkoftheaorta. AO.D, Descending aorta. and the t;c evagination on the. one , hand,

A 0.2, Second aortic arch, card, Anterior . .

cardinal vein. cl.II, Second entodermal and of the Superficial ectoderm of the head

Fore- on the other, is filled with undifferentiated
mesenchyma. In this tissue blood-vessels,
nerves, lymphatics, and muscles will grow, and.
the tissue itself is to produce, the cutis, the
subcutaneous tissue, the skull, the dura mater,

arachnoid membrane, and pia mater. We have in the present undifferentiated
stage of this mesenchyma a most striking contrast with the complicated histo-
logical conditions of the adult. The opposite part of the embryo represents .the
cervical region. At one side -we see a small piece of the heart appearing, Ht, and
higher up is the wide pharynx, Ph, underneath which is a blood-vessel, Ao, the
main aorta. To the left appears another blood-vessel, Ao.2, a portion of the
second aortic arch. The pharynx shows on one side the prolongation of its cavity
which constitutes the second gill-pouch, cl.II. On the dorsal side of the pharynx
'••^ending aortas, Ao.D, that on the right of the figure being joined

f.b.

gill-pouch. EC, Ectoderm, f.b,
brain, h.b, Hind- brain. Ht, Heart, mes,
Mesoderm. My, Muscle plate, nch, Noto-
chord. Op, Optic vesicle. Ph, Pharynx.
X 50 diams.

EMBRYO WITH TWENTY -EIGHT SEGMENTS.

203

by the second aortic arch, near which appears an accumulation of more deeply
colored cells, cl.II, part of the entodermal wall of the second gill-pouch. Between
the pharynx and the hind-brain we have a round section of the small notochord
which appears quite deeply stained, and therefore stands out conspicuously from
the very loose mesenchyma by which it is surrounded. It is not until later stages
that the mesenchymal cells begin to crowd around the notochord to constitute the
anlage of the future vertebral column. At the present stage the differentiation of
the axial skeleton around the notochord has not begun. As. regards the hind-brain,
h.b, we observe that its sides are already considerably thickened, but its dorsal
wall is quite thin and has already expanded considerably, thus initiating the
formation of the thin ependymal roof of the fourth ventricle. On either side of the
hind-brain appears a blood-vessel, card, the anterior cardinal, which by transforma-
tion and migration is to lead to the formation of the jugular veins of the adult.

Section through the Aortic End of the Heart (Fig. 155). — The cervical region
of the head and the tip end of the region of the fore-brain are cut separately.
On the lower side of the pharynx is attached the double heart-tube, of which the
endothelial portion, endo, is in actual contact with the thick entoderm, En, which
forms the floor of the pharynx. The heart-tube shows its bend toward the right
of the embryo. There is a considerable space between the endothelial heart and
the muscular heart, m.ht, and this space is almost wholly free of tissue, except in
the immediate neighborhood of the pharynx itself. Close to the connection of the
heart-tube with the pharyngeal floor there runs off on either side the membrane
of the amnion. Where it starts from the embryo the amnion has considerable
thickness and appears somewhat folded in the section; but as it turns to cover
the embryo it becomes very thin. It consists only of two very delicate layers, meso-
dermic and entodermic, both one cell thick. The two layers lie close together, but
are easily distinguished. On the right-hand side of the embryo the raphe of the
amnion may be observed, raph, and in this section it is constituted by only two
strands of mesoderm which pass over from the amnion on to the chorion, Cho,
or membrana serosa, as it has been called by many embryologists. The arrange-
ment of the envelopes of the head is somewhat more complicated. Underneath the
left side of the section of the cervical portion of the head runs the splanchnopleure,
Spl, in which one can readily distinguish numerous sections of blood-vessels, which,
on the side toward the embryo, are covered by mesoderm, and on the side away
from the embryo are covered by entoderm. If we follow along the splanchnopleure
to a point near the section of the region of the fore-brain, we find that it encounters
a circle of ectoderm, EC, which surrounds that portion of the head. When the
splanchnopleure reaches this ectoderm, its two layers divide or split apart. The
mesoderm bends off toward the right* side of the embryo and forms, together with
a portion of the ectoderm, a part of the true amnion, Am', of the head. The

* The right of the embryo, — the left-hand side of the figure.

204

STUDY OF YOUNG CHICK EMBRYOS.

entoderm, Ent, on the contrary continues in the same direction as before, until it
joins the ectoderm on the left side of the head to form the pro-amnion, Pro.am.
Beyond the head the entoderm and mesoderm again unite and we have a continua-
tion of the splanchnopleure, Spl. Owing to the development of the pro-amnion,
the relations of the fetal envelopes surrounding the head are complicated. The

d.III

FIG. 155. — SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92,

SECTION 114.

Am, Am', Amnion. Ao.D, Descending aorta. Ao.2, Second aortic arch of the left side. Cho, Chorion. cl.II,
Second entodermal gill-pouch. cl.III, Third entodermal gill-pouch. EC, Ectoderm. En, Entoderm of
pharynx, endo, Endothelial heart. Ent, Entoderm of pro-amnion. f.b, Fore-brain. Md, Medulla oblongata.
mes, Mesoderm of amnion. m.ht, Muscular heart, nch, Notochord. Pro.am, Pro-amnion. raph, Raphe of
amnion. Seg, Segment. Spl, Splanchnopleure. Ve, Anterior cardinal vein. X 50 diams.

student may, however, easily satisfy himself that the layer, EC, in figure 157, is
really ectoderm by following it through in the series of sections, for he will then
find that it becomes continuous in other regions, on the one hand, with the ecto-
derm of the true amnion, and, on the other, with the epidermis of the body proper.
In the cervical region we have a transverse section of the lower portion of the

EMBRYO WITH TWENTY-EIGHT SEGMENTS. 205

hind-brain, Md, corresponding to the part of the future medulla oblongata near
its junction to the spinal cord. Underneath it is the section of the notochord, nch,
and on either side sections of a secondary somite, Seg. Just below each somite
is a' cardinal vein, Ve, and below the vein, but nearer to the median line, lies
the dorsal aorta, Ao.D. The pharynx expands on each side; the prolongation on
the left of the embryo is the second gill-pouch, cl.II, that on the right is the
third gill-pouch, cl.III. The pharynx itself is lined by entoderm, En, which is
very thin in the median dorsal line, but immediately below the dorsal aortae it
thickens abruptly and continues as a quite thick layer on to the ventral side. In
the median ventral line it forms a deep groove, and in the walls of this groove we
find that the nuclei are. not distributed through the whole thickness of the ento-
derm, but occupy chiefly its outer or basal portions, so that the portion of the
layer next the cavity of the groove is formed almost wholly of protoplasm. At the
tip of the gill-pouch the entoderm has come into actual contact with the ecto-
derm, and the cells of the two germ-layers have there united, without distin-
guishable boundary being kept between the layers. The fused ectoderm and ent
derm constitute the closing plate of the gill-cleft, and such a plate is formed at the
tip of every gill-pouch. On the left side of the ventral surface of the pharyn/
pears the section of the second aortic arch, A 0.2. Opposite but higher up is
the section of the right third aortic arch. By following along through a few sec-
tions (in the series here studied, from four to six) the junction of these arches
with the endothelial tube of the heart may be observed. The student should verify
this connection and satisfy himself that the endothelium of the blood-vessels is a
continuation of the endothelium of the heart. This fact is of great morphological
and physiological importance. Of the section of the region of the fore-brain little
need be said. The ectoderm has begun to thicken somewhat. • The walls of the
fore-brain, f.b, itself have not begun to show any differentiation into layers. There
is a considerable development of mesenchyma between the brain and the superficial
ectoderm. »

Section through the Venous End of the Heart (Fig. 156). — We have now passed
in our series beyond the level of the head, so that no part of that is included in
the section. The general topography of the part is similar to that of the preced-
ing section (Fig. 155), but there are many important differences of detail. We
are now in the region of the spinal cord, proper, Sp.c, which here offers to us its
characteristic early embryonic form. It is oval in section, -its. walls are thickened
on each side, but are thinned on the dorsal side, where they constitute the deck-
plate, and on the ventral side, where they form the floor-plate; the cavity is narrow
and slit-like. The nofochord ' close under the ventral side of the medullary
tube and below it i^ tbr median dorsal aorta, Ao, a single and very large vessel,
which is formed by fi <• union of .v the two dorsal aortae shown in figure 157,
Ao.D. Immediately below in*, aorta f,\!o,vs the pharynx, Ph, which is nov more
rounded in form and does not e.-iend «ar laterally. Its entodermal lining is mod-

206

STUDY OF YOUNG CHICK EMBRYOS.

erately thick, but it is somewhat thinner near the median dorsal line. On either
side of the pharynx the mesodermal layer, mes, is very thick and stands out con-
spicuously, owing to its dark staining. Above the pharynx it thins out and passes
over on to the somatopleure,', Som, and so on to the amnion, Am. On the ventral
side of jthe pharynx the mesodermal layer passes over into the muscular wall of

m.ht.

FIG. 156. — SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 9?,

SECTION 144.

Am, Am', Amnion. Ao, Aorta. Au, Cardiac auricle. Cho, Chorion. Coe, Ccelom, D.C, Duct of Cuyier.
EC, Ectoderm. Endo, Endothelial heart, mes, Mesoderm. m.ht, Muscular heart. My', Primitive segment.
Ph, Pharynx. Raph, Raphe of amnion. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure.
Ven, Ventricle of heart. X 50 diams.

the heart, m.ht. The heart itself is very large; it has two tubes, the endothelial,
endo, and the muscular, m.ht, which are very distinct. The endothelial cavity is
very large. It is especially expanded immediately underneath the pharynx to form
the auricular end of the heart, which receives the veins. Throughout a large
part of the auricle the cndothelium is closely fitted against the muscular wall.
Farthei ventralward me double heart-tube bends tr the right of the embryo to
form the ventricular limb, Ven, in whiph the epithelial cavity is also enlarged.

EMBRYO WITH TWENTY -EIGHT SEGMENTS. 207

The heart as a whole occupies about one-half the area of the entire section of the
embryo, being of relatively enormous proportions. The cardinal veins, D.C, have
moved down, as compared with the previous section, and -are now found to lie in
the somatopleure, in which there also appear several sections of smaller blood
spaces above the main cardinal vessel. The path of the cardinal through the
somatopleure carries it toward the heart. The vertical part of the vessel, which
effects a union with the heart, is known as the common cardinal. The common
cardinals also deliver the blood from the posterior cardinals to the heart. They are
at somewhat different levels on the two sides of the embryo, that on the right
side being lower and occupying a sort of prominence on the mesothelial side of
the somatopleure. If the cardinal veins are traced along through successive sec-
tions, it will 'be found that they open directly into the auricles of the heart, cross-
ing over the ccelom, Cce. The crossing is accomplished by a growth of the somat-
opleure which unites with ther wall of the heart. The openings of these veins are
at this stage morphologically symmetrical and are entirely distinct from the open-
ings of the omphalo-mesaraic veins, which enter the heart farther tailward. If
sections in the series between the present one and that through the aortic end of
the heart (Fig. 155) be examined, it will be found that the heart in -the middle
part of its course is entirely detached from the pharynx, so that the heart-tube is
suspended by its two ends from the ventral side of the pharynx. . By the crossing
of the cardinal veins the portion of the coelom, Cce, on either side of the pharynx is
shut off from the portion of the coelom around the heart. At the raphe, raph, of
the amnion the ectoderm of the amnion joins that of the chorion, Cho. In the
portion of the somatopleure, Am', which runs from the raphe to the embryo there
area number of spaces of rounded form which appear like so many vesicle?. The
nature of these vesicles is uncertain.*

The secondary somites, My, are very characteristic, and should" be studied
with a higher power. The somite consists of an outer layer and an inner 1,:
"of about equal thickness, and these two layers pass over into one another at the
dorsal and ventral edges of the segment. They are closely pressed against one
another, so that there is no space between them. The outer layer is more deeply
stained than the inner; its nuclei are somewhat less distinct and are rounded in
form. Those of the inner layer are elongated in form, as may be easily observed
by raising and lowering the focus. The outer layer is quite close to the ectoderm,
and the inner layer rests against the large mass of mesenchymal tissue which sur-
rounds the spinal cord, notochord, and aorta.

Section through the Anlage of the Liver (Fig. 157). — In this section the general
topography is similar to that of the last, so that we need describe only the new

* They seem to be bounded on one side by ectoderm, on the other by mi-,oderm; but as »
the significance of tht;se vesicles, I cannot express any opinion. The separaii- < •'^mn.sj?^'
front of, and independently of, the omphalo-mesaraic veins, so far as I
It is a con- ; ological importance.

I

208

STUDY OF YOUNG CHICK EMBRYOS.

structures and relations which appear. A little piece of the ventricular limb of
the heart with its double walls, m.ht,endo, still appears. The section is, strictly
speaking, beyond the venous end of the heart and passes through the sinus venosus
Si. V, which is formed by the union of the omphalo-mesaraic veins entering the
body of the embryo from the splanchnopleure of the yolk-sac, or, in other words,

endo.

Ent.

FIG. 157. — SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92,

SECTION 165.

Am, Amnion. Ao, Aorta, card, Cardinal vein. Cho, Chorion. Cos, Cce', Ccelom. EC, Ectoderm, endo
Endothelial heart. Ent, Entoderm. Li, Liver, mes, Mesoderm. m.ht, Muscular heart. ' msth, Meso-
thelium. My, Primitive segment, nch, Notochord. raph, Raphe of amnion. Si. V, Sinus venosus of
heart. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. Ve, Vein, x, Accumulation of
mesodermic tissue about the omphalo-mesaraic vein. X 50 diams.

from the area vasculosa. ]n the splanchnopleure, Spl, there is a thickening, x,
^derm which .narks the crossing of the veins from the yolk-sac to ' the
The entoderm of the embryo forms a tube, Ent, which
•so-ventral diameter. The entoderm itself is quite
From the ventral side of >he^ entoder-

•

EMBRYO WITH TWENTY -EIGHT SEGMENTS.

209

mal canal spring two small pouches or diverticula, the anlages of the liver. The
left diverticulum is well shown in the figure; the right diverticulum appears a
few sections farther on. It is especially important to note that the entodermal
epithelium of the hepatic diverticulum comes into immediate contact with the endo- .
thelium of the blood spaces. During the later development this relation is preserved,
and there is a complicated intercrescence of the entodermal cells constituting the
liver and of the vascular endothelium.* The intercrescence leads to the forma-

Som.

Spl.

Om.S.

Om. D.

FIG. 158. — SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMKNTS. TRANSVERSE SERIES 92,

SECTION 179.

Am, Amnion. Ao, Aorta, card, Cardinal vein. Cho, Chorion. Cos, Coelom. Ent, Entoderm. In, Intestine.
msth, Mesothelium. My, Muscle plate, nch, Notochord. Om.D, Right omphalo-mesaraic vein. Om.S,
Left omphalo-mesaraic vein. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. X 50 diams.

tion of the sinusoids, which are highly characteristic of the liver and which give
rise to the so-called capillaries of the hepatic lobules of the adult liver. These
"capillaries" are, however, 'always true sinusoids, and morphological!;*' not capil-
laries at all. Owing to the junction of the veins and liver, a portion of the body
cavity, Ccef, at the side of the pharynx is shut off from direct Connection with
the pericardial cavity. The ridge of tissue dividing the two cavitjes from one an-
other is the tjeptum transvcrsum. 11" the ser ns be followed through tail-

* A few sections anterior to this the beginning of the inten i

210

STUDY OF YOUNG CHICK EMBRYOS.

ward, it will be found that at this stage further back the septum transversum is
formed also upon the right side of the body of the embryo. The mesothelium
between the upper division of the ccelom, Cce, and the sides of the entodermal
canal is very much thickened and deeply stained. On either side of the very large
median aorta, Ao, and just above the ccelom, appear the right and left posterior
cardinal veins, card. Concerning the fetal envelopes little need be said, except
to call attention to .the large raphe, raph, of the amnion, which is now a rather
conspicuous ectodermal thickening and seems to. be formed rather at the expense
of. the ectoderm of the amnion than of that of the chorion. Such an ectodermal
raphe is very characteristic of birds; it has in the chick a considerable extent and
therefore appears in many successive sections of the series.

Som. EC. msth. card. My. nch. Sp.c. card.s. b.w. Am. Cho.

In. Ao. Ve. mes. Ent.

FIG. 159. — SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92,

SECTION 220.
.
Am, Amnion. . 1<- JX, jody-wall. card, Right posterior cardinal vein, card.s, Left cardinal vein.

i. EC, Ectoderm. Ent, Entoderm. In, Intestine, mes, Splanchnic mesoderm. msth, Meso-
i. My, Myotome. nch, Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure.
Ve, Vein. X 50 diams.

Section through the Omphalo-mesaraic Veins (Fig. 158). — This section is inter-
mediate in structure between figure 157 and figure 159, here described. We are
now beyond the region of the heart and liver. The cavity of the intestine is open
on the ventral side, so thai the walls of the intestine pass over directly into the
extra-embryonic Splanchnopleure, Spl, in which are lodged the verv^Vide omphalo-
mesaraic veins, Om.D and Om.S, which are entering the body of the embryo
to run forward past the liver anlage (Fig. 157) to join the posterior or venous end
of the heart. It will also be noticed that the amniotic fold does not join its fellow,
and therefore has no raphe, In this condition the amnion is said to be "open."

Section through the Anterior Portion of the Open Intestine (Fig. I5o/. — In this
section the intestinal cavity , 7«, being without a ventral wail, opens directly
the general emi^t- mal cavity under the germinal area and above the yolk-mu^
(compare the diagrams .Figs. 29 and 45). The median plane of the embryo is still

EMBRYO WITH TWENTY -EIGHT SEGMENTS.

211

inclined to the left. The extra-embryonic somatopleure, Som, rises in two high
folds, one on each side of the embryo; the inner portion of each fold, Am, belongs
to the amnion, the outer portion, Cho, to the chorion. The splanchnopleure, Spl,
passes without demarcation into the wall of the intestinal cavity, In. The ento-
derm, Ent, of the extra-embryonic splanchnopleure is very thin, but where it passes
into the embryonic region toward the median line, it thickens a little. The splanch-
nic mesoderm is a thin layer of mesothelium which, of course, bounds the ccelom
everywhere and can be followed continuously over on to the somatopleure. The
splanchnic mesenchyma is loose in texture and surrounds the large blood-vessels.
The splanchnic mesoderm on either side of the intestinal groove appears quite
dark, owing to the condensation of the tissue. Whether this condensation is devel-
oped from the mesothelium or from the mesenchyma it is very difficult to say.

Som.

N. Seg. Sp.c. W.D. EC. Mes.

Cat.

mes' .

FIG. 160. — SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT -SEGMENTS. TRANSVERSE SERIES 92.

SECTION 356.

Cue, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Somatic mesoderm. mes', Splanchnic mesoderm. A',
Xephrotome. nch, Notochord. Seg, Segment. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchno-
pleure. Ve, Bloocl- vessel. W.D, Wolffian duct. X 50 diams.

The somatopleure, Som, where it becomes embryonic, increases greatly in thickness
and forms an arch, b.w, which is the beginning of the formation of the ventral
body-wall of the chick. The form of the arch indicates the commencing closure
of the embryonic somatopleure on the ventral side, by which the body of the em-
bryo will ultimately become shut off from the underlying layers of the blastoderm.
In the median plane of the embryo we find the spinal cord, cut somewhat obliquely,
the notochord, nch, and the very large section of the aorta, Ao. The great
transverse width of the aorta is due to its approaching division toward the caudal
end of the body to form the two branches which run out to the area vasculosa and
are known as the omphalo-mesaraic or vitelline arteries. Before they leave the
body of the embryo each of these arteries gives off a branch which continues in
the body of the embryo not far from the notochord and close to the entoderm.
These branches subsequently become the allantoic arteries. On either side of
the spinal cord lie the secondary somites, My. A short distance from the aorta.

212

STUDY OF YOUNG CHICK EMBRYOS.

on either side appear sections of two rather small blood-vessels, the cardinal veins,
car d. Between the vein on each side and the aorta there is a little accumulation
of denser tissue. If a series of sections is followed through, the Wolffian duct
may be traced into this condensed tissue, and when the duct is differentiated, it
will take the place of this tissue between the aorta and the vein.

Section through the Middle Portion of the Open Intestine (Fig. 160). — Compari-
son of this section with the preceding is instructive as an illustration of the fact
that the differentiation of structures is found less advanced as we proceed toward
the caudal end of the embryo. In the present section the amniotic -folds can
hardly be said to have appeared at all, although the ccelom, Cce, is very wide in-
deed, and there is little differentiation in either the somatopleure, Som, or splanch-
nopleure, Spl, between the embryonic and extra-embryonic regions. The ento-

Som. Cce. Cce.'

nch.

In. Ent.

FIG. 161. — SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 419.
Cce, Ccelom. Cce' , Diverticulum of the ccelom. Ent, Entoderm. In, Intestinal cavity, mes, Mesoderm. nch,

Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure. S.z, Segmental zone. X 50

diams.

derm is a little thicker in the embryo than in the extra-embryonic territory. A
similar difference may be observed in the ectoderm. The embryonic mesoderm in
both somatopleure and splanchnopleure is considerably more developed ' and much
denser than in the extra-embryonic parts. The axial structures of the embryo —
namely, the spinal cord, Sp.c, and notochord, nch — are about the same as further
forward, but the mesoderm is much less advanced than further headward as is
evidenced by the small amount of mesenchyma above the axial structures and
by the slight differentiation of the mesothelium. The condition of the segments
and their relations to the somatic and splanchnic mesoderm are closely similar to
those represented in figure 46. Each somite consists of a larger part, Seg, of
rounded outline, close to the medullary tube, and of a narrower part, the nephro-
tome, N, which connects the inner portion of the somite with the lateral meso-
derm. The secondary somite consists of a distinctly marked wall which extends
around underneath the ectoderm and against the side of the medullary tube, and
of a thick inferior wall which fills up also the center of the somite. Between the
nephrotome and the entoderm are small blood-vessels, Ve.

Section through the Posterior Portion of the Open /; Fig. 161). — This

EMBRYO WITH TWENTY-EIGHT SEGMENTS.

213

section is similar to the last, but we may note especially the following differ-
ences: The spinal cord, Sp.c, shows a comparatively large cavity, which is widest
on the dorsal side, so as to be somewhat triangular in section. In place of the
segments we have only the mass of cells, S.z, which constitutes the segmental zone,
out of which later segments will be differentiated. The segmental zone, S.z, is
of a rather loose texture and merges without boundary into the somewhat denser
mesenchyma of the somatopleure and splanchnopleure of the embryo. The dense
tissue of the somatopleure extends much farther laterally than the corresponding
tissue in the splanchnopleure. The notochord, nch, is very large and fills out the
entire space between the ventral boundary of the spinal cord and the entoderm,
and though the mesoderm comes in contact with the notochord, it does not sur-
round it, the relations here representing an earlier stage of development than any

Som. EC. Cce. cau.i. Sp.c. nch. S.z.

Mes.

Ent.

All.

FIG. 162.— SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 424.

All, Allantois. cau.i, Caudal intestine. Cce, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Mesoderm.
mes' ' , Splanchnic leaf of mesoderm. nch, Notochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanch-
nopleure. S.z, Segmental zone of mesoderm. Ve, Blood-vessel. X 50 diams.

which we find further head ward. The entoderm, Ent, of the embryonic region is
considerably thickened and forms an intestinal channel, In, of very characteristic
form; for the top of this channel is nearly horizontal, while the sides are vertical
and form a distinct angle with the top. In the midst of the mesoderm, on either
side of the intestine, there is a small cavity, Cce', which in two or three sections
further forward is found to unite with the general cavity of the ccelom. The
morphological meaning of this special pocket of the body-cavity is unknown.

From this point onward in the series changes in the appearance of the -sec-
tions take place very rapidly. The two sections next to be described are quite
close in the series to the present one.

'Section through the Caudal Intestine (Fig. 162). — In this section we encounter
the singular fusion of the germ-layers which is characteristic of the caudal extremity
of all vertebrate embryos during early stages. In the median line we see three
distinct cavities. The dorsal of these may be readily identified as the continuation
of the cavity of the spinal cord. The middle and ventral cavities are entodermal;
the upper of the two entodermal cavities, cau.i, represents a prolongation of
the entodermal cavity into the developing tail of the embryo (compare Fig. 16).

214

STUDY OF YOUNG CHICK EMBRYOS.

The lower cavity is the anlage of the allantois, All, which is destined to grow out
during the next few days into a relatively large round vesicle. The tissue on the
ventral side of the spinal^ cord, Sp.c, is connected by a band of cells with the
wall of the caudal intestine, cau.i. If the sections just in front are studied care-
•fully, it can be easily observed that the notochord also passes over without boun-
dary into the same band of cells, which is a mass representing the fusion of the
walls of the medullary canal of the intestine and of the tissue of the notochord.
In this fused tissue we can, with our present means, detect no signs of the corning
differentiation. Just as the walls of the caudal intestine are fused with the tissues
on the dorsal side, so also are they fused on the ventral side' with the tissue of
the allantois. If we follow the tissues laterally, we see that they merge into the
mesoderm proper. From the mesoderm there h^,s been a distinct upgrowth of

Cos. EC. Mes. Sp.c. nch. S.z.

Som.

mes.' Ent.

All.

Ve. msth. Spl.

FIG. 163. — SECTION OF A CHICK EMBRYO WITH TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, SECTION 427.
All, Allantois. Cce, Coelom. EC, Ectoderm. Ent, Entoderm. Mes, Mesoderm. mes', Splanchnic mesoderm.

msth, Mesothelium. nch, }STotochord. Som, Somatopleure. Sp.c, Spinal cord. Spl, Splanchnopleure.

S.z, Segmental zone of mesoderm. Ve,: Blood-vessel. X 50 diams.

tissue of rather loose texture on either side of the medullary canal to form the
segmental zone, 'S.z.

Section through the Allantois behind the Intestine (Fig. 163). — This section is
onjy three in the series beyond that last described, yet it is posterior to the caudal
intestine and shows, therefore, more completely the fusion of the structures in
the axial region. Except for the absence of the caudal intestine, the description
of the last section might apply also to this. The shape of the spinal cord, Sp.c,
is somewhat different, and its merging on the ventral side with the underlying
tissues is more marked. The cavity of the allantois is smaller and almost slit-
like. The other differences do not call for special description.

Horizontal Section (Fig. 164). — The student will find it profitable to make a
series of sections in the horizontal plane, trying to cut them as nearly as possible
parallel with the median plane of the fore-brain and mid-brain.

The accompanying figure 164 is from a section of such a series. It shows
very clearly the general form of the embryo, the curvature of the neck, the sharp
angle of the head-bend, and the almost straight body. In the section represented
the lung stretch of the cavity of the fourth ventricle or hind-brain, Ven. IV. \<

EMBRYO WITH TWENTY-EIGHT SEGMENTS.

215

Ven.IV

nch'

FIG. 164. — HORIZONTAL SECTION OF A CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS.
Am, Am', Am", Amnion. Ao, Aorta. C.ao, Cardiac aorta. Cce, Crelom. D.Ao, Dorsal aorta. Dieii, Dien-
cephalon. Endo, Endothelial heart. H, Cerebral hemisphere. M.b, Mid-brain. Md, Medullary tube.
Mdb, Mandible, m.ht, Muscular heart, nch, nch', nch", Notochord. Op, Optic vesicle. Ph, Pharynx.
Seg, Segment. Seg.z, Segmental zone of mesoderm. Som, Somatopleure. Sp.c, Spinal conl. Yen, Yen-
tricle. Yen. IV, Fourth ventricle or cavity of the hind-brain. X 30 diams.

216 STUDY OF YOUNG CHICK EMBRYOS.

well shown, and it can be readily seen that the hind-brain is nearly equal in
length to the mid- and fore-brains combined. In the floor of the hind-brain ap-
pears a series of curved notches corresponding to the neuromeres. Only a shaving
from the side of the mid-brain, M.b, and two similar shavings from the two parts
of the fore-brain, the diencephalon, Dien, and the cerebral hemispheres, H, appear
in the section. The optic nerve is cut across and appears as a hollow tube.
Underneath the hind-brain a piece of the pharynx, Ph, is cut, and below the
pharynx is the large projecting heart, which is very clearly shown to consist of
an inner or endothelial tube, Endo, and an outer mesothelial tube, m.ht, the anlage
of the muscular portion of the heart. The endothelial tube is cut twice; the
upper portion, Ao, is the aortic trunk, the lower portion, Ven, corresponding to
the ventricle. The heart is, as it were, suspended from the lower wall of the
pharynx. The entoderm of the pharynx is very thin on the dorsal side, and
thicker on the ventral side. Between the head and the pharynx one can see the
projecting mandibular process, Mdb. The small space to the right of this process
in the figure, between it and the head, corresponds to the cavity of the mouth.
Close to the mandibular process, on the side toward the heart, springs the amnion
of the embryo, Am, which passes close around the head of the embryo lying very
near it, and can be followed down to where it rejoins the posterior end of the
embryo, on the left-hand side of the figure. Underneath the posterior part of the
hind-brain can be seen a small piece of the notochord, nch. The notochord
appears twice more in the section, nch' and nch" , in the dorsal region of the em-
bryo. From the end of the hind-brain the cervical region curves to the right.
In it. there is a large cavity, D.Ao, the dorsal aorta. To the left of the dorsal
aorta we begin to get the primitive segments, which are very distinctly marked.
They become gradually wider and wider as we proceed toward the caudal end
of the embryo. There also they are less advanced in their development. A
small bit of the spinal cord appears in section, Md. From the extreme inferior
end of the section a prolongation of the somatopleure can be seen which also
leads off into the formation of the amnion, Am". There appears again a piece,
Sp.c, of the spinal cord and a fragment of the notochord, and on either side of
this a segmental zone, Seg.z, of the mesoderm. On the right there shows a small
portion of the body-cavity, Cos, distinctly bounded on, both sides. Its exterior
boundary is a piece of the true body-wall, Som, of the embryo, and close by it is
another portion of the amnion, Am'. How this is possible may be readily under-
stood by comparison of this figure with figure 161, which represents a transverse
section of a similar embryo in this region.

Histological Differentiation of the Chick Embryo with Three Gill-clefts.

It is important that the student make a thorough examination and study
with a high power of all the cells and tissues of the embryo at this stage so as to
familiarize himself with the embryonic characteristics of the germ-layers. The

HISTOLOGICAL DIFFERENTIATION. 217

cellular homogeneity of the embryo is strikingly evidenced by the nuclei, which
in all parts of the embryo are very similar in size, shape, and structure. They
are all rounded in form, varying between spherical and slightly oval outlines,
which are seldom quite regular. The outline of the nucleus is always well marked,
there being a supefficial layer of nuclear substance, which gives a darker appear-
ance to the edge of the nucleus. In the interior there is a single or sometimes
two, very rarely three, nucleoli, which are quite large and stain deeply. The
strands of substance between the nucleolus and the outer part of the nucleus are
very slight, and the space around the nucleolus, therefore, appears light. The
protoplasm of the cells is never large in amount, so that the cell-body about each
nucleus is not conspicuous, except in the case of the blood-corpuscles, which are,
in this respect, somewhat more advanced than the other cells of the embryo.

The ectoderm offers chiefly variations in its thickness, being very much at-
tenuated in some parts, as, for instance, in the posterior portion of the head, where
the outer ectoderm overlies the hind-brain. Most of the epidermal parts have be-
gun to increase in thickness, and contain nuclei in two or even, three layers.
There are several special thickenings of the epidermal layer, for which the name
of plakodes has been proposed (compare page 76). At the present stage three
pairs of plakodes are seen. The first is the pair of areas which are to be invagi-
nated to form the olfactory pits; the second is the pair which are already invagi-
nated to form the anlages of the lenses of the eyes, and the third pair is also
invaginated to form the otocysts. The portion of the ectoderm which forms the
medullary tube is also very much thickened, except, of course, so far as the floor-
plate and deck-plate have been differentiated. In both the plakodes and in the
thickened portions of the medullary wall the nuclei occupy nearly the whole thick-
ness of the layer, being themselves several layers deep. They are, however, par-
tially absent from that portion of the ectoderm which is near the original external
or free surface. Close to this surface there are, however, a certain number of
nuclei, the large majority of which are in various phases of division, as shown by
the numerous mitotic figures. No mitoses appear, except in tne superficial portion
of the layer. Over the greater part of the amnion the ectoderm is so very thin
as to resemble almost an adult endothelium, but over the chorion or serous mem-
brane it is a little thicker.

The entoderm appears in three distinct forms: first, the large, long, columnar
cells of the area opaca; second, the very thin cells of the area pellucida; and,
third, the somewhat thicker cell-layer in the embryo proper. For an account of
the cells of the area opaca and area pellucida see page 64. The entoderm in the
embryo presents considerable variations in thickness which have been pointed out
in the descriptions of the sections. Where it is thick enough to permit it, the
nuclei are disposed in several layers, and in such places we find that the nuclear
divisions take place only in the superficial portion of the entoderm, the phenome-
non here being similar to that which we have already noted in the ectoderm. The

218 STUDY OF YOUNG CHICK EMBRYOS.

notochord has a sharply defined outline, as if bounded by a distinct membrane.
It contains nuclei which are quite closely placed, but it does not show, at least
in ordinary preparations, any recognizable division into separate cells.

The mesoderm offers several varieties, not so much in the character of the
single cells as in their methods of grouping. We notice, first, that there are parts
of the mesoderm which are quite thick, and in which we cannot perceive any
division into mesothelium and mesenchyma. Such a thick layer of mesoderm may
be observed at either side of the pharynx (Figs. 156, Ph, and 157), or, again,
toward the caudal end of the embryo in both the somatopleure and splanchnopleure,
occupying a larger territory in the former than in the latter (Fig. 163). But for
the most part the mesoderm has progressed beyond this stage and shows clearly
the differentiation of a thin mesothelial layer lining the coelom and the scattered
mesenchymal cells. The mesothelium is quite thin in some parts, almost or quite
as thin as adult endothelium. The mesenchyma consists of cells with small proto-
plasmic bodies connected together by fine threads of protoplasm and with a trans-
parent homogeneous matrix between the cells. It varies greatly in appearance ac-
cording as the cells are more or less closely crowded together, or widely separated
from one another. These differences we designate as varying degrees of condensa-
tion in the mesenchyma. The variations occur in a perfectly definite and constant
manner, though we are far from understanding yet either the cause or the morpho-
logical significance of these variations. The secondary somites vary greatly in
structure, because they are in unlike stages of- differentiation, those toward the tail
being least, and those in the cervical region most, advanced. We can, therefore, in
a single embryo observe several phases of the breaking-up of the inner wall of
the somite to form mesenchyma about the medullary tube and notochord. The
transformation is accomplished by a spreading out and moving asunder of the
cells, and we can also trace a gradual differentiation of the muscle-plate, out of
the inner portion of the somite. The external layer, or so-called cutis-plate,
offers an apparently more ,or less epithelioid structure in all of the somites. The
Wolffian duct is differentiated only through a part of the embryo. It is a small
cord of cells that has as yet no central cavity. The blood-vessels are formed solely
by the endothelium (angioblast). There is nowhere any condensation of the mesen-
chyma about the blood-vessels as yet. There are no capillaries whatever in the
embryo. One of the most important vascular modifications has, however, been
initiated in the anlage of the liver, where we find the vascular endothelium com-
ing into close contact with the entodermal cells of the liver, preparatory to the
later complete differentiation of the hepatic sinusoids. The blood-corpuscles are
round in form with fairly distinct outlines. Their protoplasmic bodies are much
larger than those of any other cells of the embryo at this stage, but their nuclei
resemble in size and structure those of other tissues.

CHAPTER VI.
STUDY OF PIG EMBRYOS.

Method of Obtaining Embryos.

The pig is recommended for embryological study because specimens of the
embryos in sufficiently early stages can be obtained at the larger packing estab-
lishments in considerable numbers and with little trouble or expense. When
this material is not obtainable, rabbit embryos may be substituted, as these ani-
mals are easily kept and breed 'freely (compare page 166). The enormous pre-
cocious development of the chorionic vesicle in pigs produces an enlargement
of the uterus which is usually sufficient, by the time the embryo has attained a
length of 6 mm., to be observable to the untrained eye. It is, therefore, only
necessary to ask the man who removes the viscera from the pigs to lay aside
for examination all of the uteri which appear distended. These should not be
turned about violently, but handled carefully and should be opened immediately.
As soon as the ovum is exposed it will probably be ruptured, and there will
occur a free outflow of opalescent fluid, amniotic and allantoic. With the aid
of .fine forceps and a horn spoon the embryo may be lifted up — and it should
on no account be directly .touched — and transferred to a dish containing Muller's
fluid, in which the specimen should remain for five or ten minutes. It is then
transferred with the help of the horn spoon to Zenker's fluid. Metal instruments
cannot be used on account of the corrosive sublimate in the Zenker solution. In
one or two hours the embryo should be transferred to fresh Zenker solution and
left therein a varying length of time, according to the size of the specimen. In
general it may be said for —

Pigs of 6 to 9 mm 12 hours.

Pigs of 12 mm. ..:... - . 24 hours.

Pigs of 15 mm 36 hours.

Pigs of 20 to 25 mm 48 hours.

It is undesirable to leave any specimen in the Zenker solution more than
forty-eight hours. The Muller's fluid is used for cleaning the specimen. It causes
a granular, non-adherent coagulum to form from the fetal fluids. If the speci-
men is put directly into Zenker's fluid, a fibrous coagulum is formed which often
adheres closely to the embryo so as to obscure its shape. Such a fibrous co-

219

220 STUDY OF PIG EMBRYOS.

agulum cannot be removed without injuring the embryo. After having remained
a .proper length of time in the Zenker solution, specimens are further washed
for twenty-four hours in running water, and then treated with alcohol and iodine
in the usual manner.

The Making of Serial Sections.

Specimens should be colored with alum cochineal in toto, then imbedded in
paraffin and cut into serial sections according to the directions given in Chapter
VIII. It is advantageous to apply a counterstain — orange G is recommended.

Selection of the Planes of Section and the Stages for Practical Study.

It is customary to distinguish three fundamental planes — the transverse, the
sagittal, and the frontal. It is impossible to so define these planes that the defini-
tion shall be exact for all stages. But in general it may be said, reference
being had to the entire embryo, that the transverse plane is one which will be at
right angles to the notochord and medullary tube at the level of the heart; that
the frontal plane will be one. at right angles to this, passing symmetrically through
the limbs of the embryo; and, finally, that the sagittal plane is one parallel to
the median plane of the body. As in younger embryos the form is very asym-
metrical, both the head and caudal end of the embryo being twisted to one side,
the planes which would be true for the body of the embryo in the region of the
heart would not be true elsewhere. For the practical use of the student, there-
fore, in these younger stages it is better to determine the direction of the plane
by the floor of the fourth ventricle, so that by "transverse" will be understood
a plane of section which cuts the head of the embryo symmetrically, no matter
how it may cut the body, and which runs parallel to the floor of the fourth ven-
tricle (medulla oblongata). The frontal plane should be perpendicular to this
and also cut the head of the embryo symmetrically. The sagittal plane in these
cases is also that of the head and not of the body. Such planes are recom-
mended because in the study of the sections more is gained by having the planes
readily understood in the region of the head than in the region of the body. In
later stages, when the body has become straighter, the difference in planes for the
head and the body may be practically left out of consideration, except that for
the heads of older pigs when they are cut alone — as on account of the size
of the body is often desirable — the frontal plane is chosen so as to run at right
angles to the plane of the palate and symmetrically through the embryo. Sec-
tions through the head at right angles to this may be designated as horizontal.*
Students will find that it is very much easier to study transverse and frontal sec-
tions when they are symmetrical. No pains, therefore, should be spared to orient
the embryo properly in the microtome before the sections are cut.

* The system of planes here described is that adopted for the Harvard Embryological Collecti6n, and
has been found convenient in practice.

EXTERNAL FORM OF EMBRYO OF 7.5 MM. 221

Selection of the Stages. — The most profitable stage to study is that of an embryo
of from ii to 13 mm. in length. Each student should have three specimens
of this stage, and it is advantageous that the specimens given each student be
approximately of the same size. The embryos ought to be first studied carefully
as to their external form and then cut into serial sections in the transverse, sagit-
tal, and frontal planes. Of these, the transverse series forms the principal basis
of study, and the other series are to be used principally to clear up the student's
conception of the relation of parts. Embryo pigs of the size specified have the
typical class characteristics of mammalian embryos, and may readily be distin-
guished from the embryos of any other class of vertebrates. The differentiation
of the anlages of all the important organs is accomplished, so that these anlages
can be identified with certainty and their genetic relations to the adult structures
can be clearly grasped by the student. At the same time, although the ana-
tomical differentiation is well advanced, the histological differentiation has made
very little progress, hence the embryos in question are particularly instructive to
beginners. The anatomy of the pig at this stage is, therefore, readily understood
by the student who knows the general anatomy of the adult. Older embryos
are more complicated and yield such long series of sections that the beginner
is apt to be discouraged. Younger embryos, owing to their spiral twisting, are
exceedingly difficult for students" to understand when sectioned. After having
thoroughly mastered the structure of the pig embryo of from 11 to 13 mm., the
student may advantageously extend his study of embryos to other sizes. If, as
is done in this work, the principal study is made with embryos of 12 mm., the
student may proceed to make sections of other stages as follows:

Pig embryo of 6 mm., transverse.

Pig embryo of 9 mm., transverse and sagittal series.

Pig embryo of 17 mm., transverse series.

Pig embryo of 20 mm., transverse and sagittal series.

(Of the head alone, the frontal series.)
Pig embryo of 24 mm., of the head alone, frontal series.

The Study of the External Form.

The student should make a careful and thorough study of the external form
of every embryo, and make, with the aid of the camera lucida, an exact drawing
of every embryo before he cuts it into sections. He will soon learn that such a
drawing is almost indispensable for the interpretation of the sections.

In the following paragraphs, embryos of 7.5, 10, 15, and 20 mm. are figured
and described from specimens which have been hardened in Zenker's fluid and
preserved in alcohol. The description of these stages will be sufficient to enable
the student to understand any of the embryos he is required to study.

Pig Embryo of 7.5 mm. (Fig. 165).— The student maybe helped in the iden-
tification of parts by comparison with figure 166, which has explanatory lettering.

222

STUDY OF PIG EMBRYOS.

The length of the embryo measured in a vertical line as the embryo is placed
in the figure is 7.5 mm., but its greatest length in any direction is 8.0 mm.

The head is somewhat triangular in form, being broadest toward the front (the
left in the figure) and narrowing posteriorly to join the rest of the body. The
upper boundary of the head is a nearly straight line, the extent of which marks
approximately the territory of the hind-brain.

Toward the left the outline forms a rounded curve which marks the territory
of the mid-brain, and then continues obliquely downward in a straighter course un-
til it curves over on to the under side where it forms three notches. The first

notch indicates the position of the mouth, the
second marks the boundary between the first and
second branchial arches, the third the boundary
between the second and third arches. On the tip
of the head, just in front of the mouth, is a shal-
low depression, the anlage of the nasal pit, and
above is the small eye. From the eye to the mouth
runs a shallow furrow, the lachrymal groove. The
first branchial arch is called the mandibular; it is
broad and separated by a furrow from the second.
Between it and the eye lies the maxillary process.
The second branchial arch is termed the hyoid.
The third is smaller and somewhat drawn inward,
while the fourth and fifth have sunken so far as to
produce a deep pit with a triangular outline, which
has been named the cervical sinus.
The body has a long curving dorsal outline terminating in the recurved tail.
Near this outline thirty-seven segments show externally, because each one creates
a protuberance of the ectoderm. The least developed segments are in the tail.
From there toward the head they show a progressive advance in the stage of devel-
opment attained. The two limbs are rounded buds, the anterior being the larger,
and offer no trace of their future articulation. Between them stretches a long
protuberance, which is due to the Wolffian body or mesonephros, the precocious
development of which is characteristic of ungulates. In man at a corresponding
stage the mesonephros is relatively less voluminous. Immediately ventrad from the
fore-limb the two lobes of the liver can be discerned through the translucent body-
walls. Between the liver and the head is the very large heart. The division be-
tween its auricle above and its ventricle below can be seen clearly. The abdominal
region of the body is prolonged outward between the tail and the heart, and so
forms the commencement of the umbilical cord, the end of which is marked by
a thin membrane, the amnion, which has been almost completely removed. In
life the amnion forms a closed sac around the embryo, and is distended by the
amniotic fluid. From the end of the umbilical cord project remnants of the yolk-

FIG. 165. — PIG EMBRYO OF 7 .5 MM.
X 8 diams.

EXTERNAL FORM OF EMBRYO OF 10 MM.

223

sac, and the allantois, both of which pass through the cord to join internal struc-
tures of the embryo.

Pig Embryo of 10 mm. (Fig. 166). — The form of the embryo has undergone
notable changes, as comparison with figure 165 will show. The head is larger, the
expansion in the regions of the mid- and fore-brains being particularly noticeable.
The limb-buds have lengthened, as has also the umbilical cord. The third branchial
arch has disappeared from the surface into the cervical sinus. The head as a
whole lies nearly at right angles with the back, so that the dorsal outline of the

Yen. Md. Au.

C.S.

A.L.

Um

M.L.

P.L

FIG. 166. — PIG EMBRYO OF 10 MM.

A.L, Anterior limb. Au, Auditory, or first gill-cleft. C.S, Cervical sinus. Md, Mandibular process. M.L,
Milk-line. MX, Maxillary process. N, Nasal pit. Op, Eye. P.L, Posterior limb. Seg, Muscular seg-
ment. Um, Umbilical cord. Ven, Floor of fourth ventricle (medulla oblongata). X 8 diams.

head forms a distinct though rounded angle with that of the back. This angle
marks the position of the neck-bend, and also the junction of the brain with the
spinal cord. The very distinct neck-bend is characteristic of the mammalian embryo.
It is less evident in birds and reptiles, absent in amphibians and fishes. Its devel-
opment probably causes the cramping of the ventral cervical region, which leads
to the formation of the cervical sinus, C.S, and to the disappearance from the
surface of the second, third, and fourth gill-clefts. Another consequence of the
neck-bend is the approximation of the nasal regions, N, of the head to the cardiac
region of the body. The cephalic region has a second flexure, the head-bend

224 STUDY OF PIG EMBRYOS.

proper, which occurs at the level of the mid-brain, the nature and significance of
which become clearer when the disposition of the nervous system is studied (compare
Fig. 178). From the mid-brain one axis extends backward through the region of
the hind-brain, Ven, to the neck-bend; the other axis extends vertically downward
to the region of the fore-brain. On the surface of the head we find the nasal pit,
AT", distinctly marked. The eye, Op, shows clearly the outlines of the optic vesicle
and of the lens in the center. It is entirely without lids. The small size of the
eye is a characteristic of the mammalian embryo by which it differs from all saurop-
sidian forms; but, as previously stated, the embryonic eye is slightly larger in certain
other mammals. Below the eye is the maxillary process, MX, which is destined to
form 'the greater part of the upper jaw. The anterior boundary of the maxillary
process is marked, as before, by the lachrymal groove, which runs now from the
angle of the eye, Op, to the nasal pit, N. The mandibular process, Md, out of
which the lower jaw is to be developed, is separated from the maxillary process by
a groove, the boundary between the upper and lower jaws, and is bounded . behind
by a second groove, Au, the anlage of the future meatus auditorius externus.
This groove marks the boundary between the mandibular process and the first, or
hyoid, branchial arch, and is itself the ectodermal member of the first gill-cleft.
The cavity of the hind-brain is very large and is known as the fourth ventricle,
Ven; as it has a very thin roof it can be readily distinguished. The thickened
floor of the fourth ventricle is the anlage of the medulla oblongata. The opening
of the cervical sinus, C.S., is triangular, as before; within it are hidden the third,
fourth, and fifth branchial arches. In slightly older embryos the orifice of the
sinus is further contracted, becoming a small rounded opening which finally closes
over completely. The territory of the mandibular process and cervical sinus corre-
sponds to the pharyngeal region. It is the site of some of the most important,
interesting, and complicated developments by which the embryonic is changed into
the adult anatomy.

The dorsal outline of the body forms a long sweeping curve, ending in the
tail. Comparison with figure 165 shows at once that the straightening out of the
dorsal region is begun, yet at this stage the dorsal side of the embryo is nearly
three times as long as the ventral. The umbilical cord has grown in length, and
is constricted in diameter as it joins the abdomen, yet its connection with the body
occupies practically the entire length of the ventral median line. The position and
number of the segments, Seg, is still shown by the external modeling. Both limbs
are well advanced, the anterior, A.L., more so than the posterior. From the base
of the brain to the base of the hind limb extends the milk-line, M.L, curving so as
to be nearly parallel, to the dorsal outline of the body. Along it the mammary glands
are ultimately developed. Extending across the body are several shadowy lines
shimmering through the translucent body-walls. One marks the position of the
embryonic diaphragm; it extends from the upper edge of the anterior limb ob-
liquely downward toward the edge of the umbilical cord. Another, which extends

EXTERNAL FORM OF EMBRYO OF 15 MM.

225

in a nearly straight line from limb to limb, marks the ventral edge of the large
Wolffian body, or mesonephros, the dorsal limit of which is approximately indicated
by the milk-line, M.L. The outlines of the smaller left dorsal lobe of the liver
are distinct, and map out a pointed area immediately below the fore limb, A.L.
Above the diaphragm lies the protuberant cardiac region, the outlines of which pass
from the umbilical cord below the nasal and pharyngeal regions toward the cer-
vical sinus. The long tapering tail extends alongside the umbilical cord.

FIG. 167. — PIG EMBRYO OF 15 MM. X 8 diams.

Pig Embryo of 15 mm. (Fig. 167). — As compared with figure 166, the present
embryo (Fig. 167) has not only grown in all its dimensions, but has also changed
in form. Unlike the embryo proper, the umbilical cord has grown very little. We
notice at once that the outline of the back is less curved than before, that the ventral
side of the body has acquired a convex outline, and that the head has become
considerably larger, both absolutely and relatively to the body of the embryo, and

226 STUDY OF PIG EMBRYOS.

has, moreover, risen so that the neck-bend is diminished. The limbs are beginning
to show the differentiation of the feet. Examined more carefully, the embryo offers
the following details: the eye, which is characteristically small, has become almond-
shaped, and the circular lens can be seen in the midst of it. In the embryos of
rodents, carnivores, and primates the eye is relatively larger than in the pig. By
the growth of the facial region the development of the snout has been initiated, and
the opening of the nasal pit now appears as the external nares toward the end of
the short snout. The lower jaw is clearly differentiated and the slit of the mouth
is distinct. There has been a great growth of the regions of the fore-brain and
mid-brain, and it is this growth chiefly which has caused the relative expansion of
the head as compared with the rest of the body. The auditory groove now ap-
pears distinctly as the anlage of the external meatus of the ear, behind which a
protuberance can be seen which is the anlage of the external concha of the ear.
The cervical sinus has wholly disappeared. Along the line of the back the primitive
segments are scarcely recognizable in the cervical region, but near the upper
limb they still show distinctly and from there are indicated with increasing
clearness as we pass toward the lower limb. The marks of the segmental divisions
do not extend so far on the dorsal side as in the earlier embryos, but are restricted
to what may be called the segmental ridge. Along the milk-line a series of
small, white, circular spots can be seen. In the specimen figured there were six
of these; their number is variable. They are the anlages of the mammary glands,
and are at this stage merely local thickenings of the ectoderm or epidermis.
There has been a considerable growth of the dorsal region of the body, and
this is perhaps most clearly indicated by the position of the milk-line, which is
much farther away from the median dorsal line than in the 10 mm. pig. Both
limbs are paddle-shaped, and, though still very short, have a broad terminal ex-
pansion, which is the anlage of the foot. The front foot has somewhat the outline
of a truncated pyramid, while the hind foot is more rounded. In the anterior limb
the differentiation into upper and lower divisions is suggested.

Pig Embryo of 20 mm. (Fig. 168). — Comparison of this stage with figure 167
reveals a general progress, but no such striking changes of external form as distin-
guished the embryo of 15 mm. from that of 10 mm. Embryos of this length vary
considerably in their proportions, but the one figured is characteristic of the stage.
The enormous transverse diameter of the body as compared with its length is very
striking, and the very large size of the head in proportion to the body is almost
equally remarkable. In the head the growth of the regions of the fore-brain and
mid-brain has continued, and the divisions between the mid-brain and hind-brain
are marked by concavities in the outline of the head. The eye is both absolutely
and relatively larger. Above it can be distinguished readily the anlages of the great
bristles which develop over the eye, corresponding to the human eyebrow. These
anlages appear as whitish spots, for they are thickenings of the ectoderm. The
snout has increased in length; the external ear has grown longer and has begun to

EXTERNAL FORM OF EMBRYO OF 20 MM.

227

FIG. 168. — PIG EMBRYO OF 20 MM. X 8 diams.

228 STUDY OF PIG EMBRYOS.

assume its permanent pointed form. The limbs have increased considerably in
length, but not yet enough to project beyond the abdomen. In both feet the
differentiation of five toes is clearly indicated. The milk-line, as a line, has almost
completely vanished, but the row of dots, the 'anlages of the mammary glands,
which develop along the milk-line, persists" and will undergo further development
in later stages. The number in the specimen figured is five. As the row of
anlages marks the position of the milk-line, it is readily seen that the line has
migrated ventralward as compared with earlier stages. Comparison of the embryos
of 15 and 20 mm. demonstrates that, during the period comprised between the two
stages, the growth of the dorsal part of the embryo- is far greater than of the
ventral part. Comparison of figures 167 and 168 shows at once that the area
occupied in both figures by the region on the ventral side of the milk-line is
about the same. In the pig of 20 mm. there is no indication of the segmental
structures recognizable in the surface modeling.

Pig Embryo of 7.8 mm. General Anatomy.

Anatomical Reconstructions from the Sections. — Reconstructions are of the greatest
assistance in the study of sections, and much facilitate the identification of all the
parts. Students using this book should, while examining their sections, constantly
refer to the reconstructions. It is unnecessary to give elaborate descriptions of
each of them, since the explanations of the lettering of the figures will suffice for
the identification of all the parts shown. Certain brief explanations as to each of
the figures are, however, desirable. The chief value of reconstructions is to render
clear the topographical distribution of the organs.

The reconstruction* presented in figure 169 shows the general topography of the
embryo, and illustrates chiefly the digestive, vascular, and central nervous systems.
The digestive tract is represented by the outline of its epithelial portion only. In
the actual specimen the walls are thicker, because they include the mesenchyma
immediately surrounding the epithelium. The mouth is an opening between the
head and mandible, Md, and leads directly into the pharynx, Ph. From the dorsal
side of the mouth springs the hypophysis, Hy, which lies close against the wall of
the fore-brain and is destined to form the anterior lobe of the pituitary body.
The pharynx, Ph, is narrow in its dorso-ventral diameter, and is represented in
median section. From its dorsal surface arises the small conical diverticulum,
S.P, situated near the hypophysis. If followed backward, the pharynx is found to
bend tailward and to form two median branches, the more dorsal of which is the
oesophagus, (E. The ventral branch is the trachea which soon bifurcates to form
the main bronchi, of which the right only is shown in the figure, at Bro. The
lateral gill-pouches are not shown in this figure. The oesophagus has lengthened,
and leads to the stomach, St, which has already descended and has so revolved

* Through the kindness of Dr. Thyng, it is possible to use this illustration in advance of its publication
by the author, whose paper on the anatomy of the 7 .8 mm. pig is soon to appear in full.

GENERAL ANATOMY OF EMBRYO OF 7.8 MM.

229

Car. in Md
Isth A. has S.P \ H.B Car. ex Ao.III

Ao.IV

Nch

Ao

A.il

V.om

A.o.m V .port

FIG. 169. — PIG EMBRYO OF 7.8 MM. TRANSVERSE SERIES 1358. RECONSTRUCTION BY FREDERICK W. THYNG.

a, Artery. A .has, Arteria basilaris. A .cand, Caudal artery. A .il, Doifble openings of left common iliac artery
into the aorta. All, Allantois. Ao, Main aorta. Ao.D, Descending aorta. Ao.III, Right third aortic
arch. A o.I V, Right fourth aortic arch. Ao.V, Right fifth aortic arch. A . o.m, Omphalo-mesaraic artery.
A Mm, Umbilical artery. Bro, Bronchus. Car, Carotid loop. Car. ex, External carotid anlage. Car. in,
Internal carotid. Cce.ax, Coeliac axis. Clo, Cloaca. Col, Colon, entodermal part. F.B, Fore-brain.
For.ia, Interatrial foramen. For .iv, Foramen interventriculare. F. ov, Foramen ovale. H.B, Hind-brain.
Hy, Hypophysis. In.caud, Caudal intestine. II, Small intestine, entodermal part. Isth, Isthmus. Ki,
Kidney. Li, Liver. M.B, Mid-brain. Md, Mandible. Nch, Notochord. N.cerv, Cervical nerve. CE, (Esopha-
gus. Pan.d, Dorsal pancreas. Pan.v, Ventral pancreas. (The line does not quite reach the organ,
which lies ventrad from the intestine.) Ph, Pharynx. Som, Somatopleure. S.P, Seessel's pocket. Sp.c,
Spinal cord. St, Stomach. Ve.hep, Hepatic vein. V.om, Omphalo-mesaraic vein. V.port, Portal vein.
V.pul, Pulmonary vein. V.s.m, Superior mesenteric vein. W.D, Wolffian duct. Yk, Yolk-sac. X 14
diams.

230 STUDY OF PIG EMBRYOS.

on its own axis that its dorsal border nearly faces the left. Just below the en-
trance of the oesophagus, the cardiac end of the stomach has a small diverticulum,
which is characteristic for the pig, but does not occur in man. Below the stom-
ach the digestive canal is narrow and forms a long loop, II, which extends t6 the
umbilical opening, where it joins the neck of the yolk-sac, Yk. This portion of
the tract gives rise to the duodenum, jejunum, and most of the ileum. Beyond
the yolk-sac . the intestinal tract is continued as a narrow tube, Col, which leads to
a considerable expansion, the cloaca, Clo, at the base of the tail. From the cloaca
there -extends into the tail a very narrow prolongation of the entodermal canal
known as the tail-gut, or caudal intestine, In.caud. Into the cloaca also open the
Wolffian ducts, W.D, which are the .ducts of the primitive excretory organs of the
embryo. The duct on the left side is represented as cut off close to the cloaca.
More of the right duct is included in order to show the anlage of the true or per-
manent kidney, Ki, which is budding off from it. Returning to the portion of the
intestine (duodenum) next to the stomach, we find clearly displayed the anlage of
the dorsal pancreas, Pan.d. The anlage of the ventral pancreas also appears, Pan.v,
but less clearly. It is an elongated mass of entoderm, lying to the right of the
duodenum and ventral to the portal vein, V.port. It takes its origin from the duct
of the liver. The liver itself, Li, has already acquired a considerable size. On
its ventral surface lies the gall-bladder, which is connected with the liver substance
by .several cords of hepatic cells. In the adult, only one connection between the
gall-bladder and the liver persists.

The central nervous system is represented as* seen in median section, display-
ing the cavity and the inner surfaces of the walls of the cavity. In the region of
the head the tube is already much dilated to make the brain. In the region of
the body it narrows to form the spinal cord, Sp.c, which gradually diminishes in
diameter toward the tail. The brain is clearly subdivided into its three primary
vesicles: the fore-brain, F.B; the mid-brain M.B; and the hind-brain, H.B. Owing
to the head-bend, the mid-brain forms an arch. It is less in diameter than either
mid-brain or hind-brain. The demarcation between the mid-brain and hind-brain,
known as the isthmus, Isth, is strongly marked. The hind-brain, H.B, is longer
than the mid- and fore-brain combined. It diminishes in diameter posteriorly, and
passes without definite demarcation into the spinal cord. Owing to the head-bend,
the fore-brain, F.B, is brought to underlie, as it were, the anterior portion of the
hind-brain, and to overlie the heart. The heart is a very large organ, which is
represented in the figure somewhat to the left of the median line. It consists of a
smaller dorsal or upper chamber — the atrium or auricle, and a lower larger cham-
ber— the ventricle. The auricle shows two openings by which it communicates with
the right auricle. The upper of these, F.ov, is the foramen ovale; the lower is
the so-called interatrial foramen, For.ia. The small pulmonary vein, V.pul, opens
directly into the auricle. The ventricle has a trabecular or sinusoidal structure.
The cavity in the drawing is that of the left side. It communicates with the cavity

GENERAL ANATOMY OF EMBRYO OF 12 MM. 231

of the right ventricle by means of the interventricular foramen, For.iv. Above
the ventricle is seen the main trunk of the aorta, which divides and gives rise to
three aortic arches, Ao.III, Ao.IV, Ao.V. The first and second arches are still
present, but are very slender. Their common stem, Car.ex, forms the future stem
of the external carotid. From the dorsal end of the third arch, Ao.III, there runs
forward the vessel, Car.in, which joins the first and second arches, and itself be-
comes part of the stem of the internal carotid. The aortic arches form on each
side of the descending aorta, Ao.D, which unites with its fellow just above the level
of the stomach to form the main median dorsal aorta, Ao. The aorta gives, off a
series of intersegmental vessels which mount dorsalward between the spinal cord
and the outer surface of the embryo. The main aorta gives off on its ventral side
a vessel, Cce.ax, which becomes the cceliac axis of the adult; and lower down, three
vessels, A.o.m, which anastomose in several places. In their course to the yolk-sac
they cross to the right of the intestine and give off branches to the mesentery. The
two upper roots atrophy later, but the lower persists to form the stem of the
superior mesenteric artery. Just before reaching the cloaca the aorta gives off
vessels, A.il} which run to the hind leg, and develop into the external iliac of
the adult. Just beyond, the aorta gives off a small branch, A.caud, to the tail; and
at the same time bifurcates to form the umbilical arteries, A.um, which take a
sinuous course alongside the allantois, All, and ultimately ramify outside of the
body of the embryo in the walls of the allantois. The allantois itself springs from
the cloaca as a narrow canal which makes a sharp bend and runs to the umbilicus,
gradually expanding. Outside of the embryo, the allantois forms; in the pig — as
in all ungulates— a very large vesicle. In man and the primates, on the contrary,
the allantois is rudimentary. This is one of the most striking differences between
the pig and the human embryo.

Of the veins little is shown in this figure, but the omphalo-mesaraic and portal
veins are included. The former, V.om, arises on' the surface of the yolk-sac,
passes through the umbilicus into the mesentery of the intestine, and extends
parallel with the ileum until it crosses it, a little posterior to the pancreas. It is
there joined by another vein, V.s.m. the superior mesenteric, which is of about
the same caliber. The common trunk formed by the union of these two veins is
the portal vein, V .port, which divides into two branches, extending to the right and
left of the omphalo-mesaraic vessels. The left branch is small. It passes on the
left of the dorsal pancreas and extends to the liver. The right branch passes
between the two pancreatic anlages, then bends to the right and enters the liver,
into the sinusoids of which it discharges its blood-stream.

Pig Embryo of 12.0 mm. General Anatomy.

Dissection of the Viscera (Fig. 170). — The specimen figured measured 12.7
mm. Both limbs and the body-wall on the left side have been removed, displaying
the organs in situ. The umbilical cord has been cut lengthwise so as to display its

232

STUDY OF PIG EMBRYOS.

internal structure. The body proper is divided by the diaphragm, Dia, into an
upper smaller pericardial chamber and a lower larger abdominal chamber. The
diaphragm is a thin membrane, which extends from the level of the base of the
fore leg, F.L, to the ventral wall of the body. The body seems filled chiefly by
three large organs: the heart, Au, Ven, above the diaphragm; and the liver, Li,

Yen

Dia

Urn'
Yk.s

CCBC

COB

Col

II

C:s

Au

- F.L

Li1

Li

W.B

AU

Clo

H.L

Soni

FIG. 170. — PIG OF 12.7 MM. DISSECTION OF THE VISCERA BY RICHARD E. SCAMMOX.

AU, Allantois. Au, Auricle of the heart. Ccec, Caecum. Clo, Cloaca. Coe, Ccelom. Col, Large intestine.
C.s, Cervical sinus, nearly obliterated. Dia, Diaphragm. F. L, Front limb. H. L, Hind limb. //, Small
intestine. Li, Ventral, Li1, dorsal lobe of liver. Som, Cut surface of somatopleure. Um, Wall of the um-
bilical cord. Ven, Ventricle bf the heart. W.B, Wolffian body. Yk.s, Yolk-stalk. X 8 diams.

Li1, and Wolffian body, W.B, below the diaphragm. The heart shows its large au-
ricle, Au, the walls of which are thin and translucent. It entirely conceals the veins,
which enter the heart through the diaphragm, and the aorta, which runs from the
ventricle toward the pharynx. The ventricle, Ven, is much larger than the auricle;
its walls are not translucent; its rounded apex points away from the auricle. The
liver lies close against the diaphragm and shows two lobes: the larger ventral lobe,

234

STUDY OF PIG EMBRYOS.

FIG. 171.

. \

FIG. 172. — PIG EMBRYO OF 12.0 MM. RECONSTRUCTION FROM THE TRANSVERSE SECTIONS, SERIES 5.

For the most part the organs represented are in or near the median plane. The drawing illustrates especially
the .disposition of the alimentary tract, the arterial system, and the heart. A. has, Basilar artery, formed by
the union of the two vertebral arteries, A .v, and joined, under the mid-brain, by the anterior ends of the internal
carotids. A.cau, Caudal artery, the small median prolongation of the dorsal aorta. All, The allantois;
it is joined by the Wolffian duct, and empties into the cloaca, an.pl, The cloacal membrane. Ao, Median
dorsal aorta. Ao.d, Right descending aorta; a small vessel connecting the dorsal ends of the third (carotid)
and fourth aortic arches. Ao.D, Main right descending aorta, passing downward to join its fellow and form
the median aorta. A.s, Subclavian artery. Au, Left auricle of the heart. A.unt, The umbilical artery
which runs in the mesodermic wall of the allantois and joins the caudal end of the dorsal aorta. A .v, Vertebral
artery. "A.vi, The vitelline artery, which becomes the superior mesenteric artery of the adult, c, Artery
known as~lh'e cceliac axis in the adult, car.e, External carotid artery, arising from the base of the third aortic
- arch. car. i, Internal carotid artery. Cbl, Cerebellum, d, Left duct of Cuvier. Dien, Diencephalon. D.V,
Ductus venosus Arantii. ep, Small plug of entodermal epithelium in the rectum; there are several small
irregular passages through the plug. Epgl, Anlage of the epiglottis. /, Interventricular foramen, opening into
the space a, b, c, of Fig. 176. F. M, Foramen of Monro. f.o, Foramen ovale, between the cardiac auricles,
"-bladder. In, Entodermal wall of the intestine, is, One of the series of intersegmental arteries,
^'ind end of the ureter, the anlage of the renal pelvis. Lar, Anlage of the larynx, consisting
°lial plate. Li, Liver. Lu, Entodermal portion of the lung. M.b, Mid-brain- Md.ob,
>7\ A spinal nerve. Neu, Neuromeres of the hind-brain. Oe, (Esophagus, op, Stalk.
•Ik is the anlage of the optic nerve. P. A, Pulmonary artery, pan, Dorsal anlage
art of which becomes the duct; behind the duct is the anlage of the ventral pan-
>wth from the duct of the gall-bladder, p.c, Pericardia! cavity. P.V, Pul-
SV, Stomach; the letters are placed beside the diverticulum which is char-
Mi man. t, Posterior portion of the tongue, an eminence on the floor of
• >:ie ventral ends of the third and fourth gill-arches. T.i, Tuberculum
i of the tongue. Tra, Entodermal trachea. Ug.si, Cloaca. Um.d,
icle, the letters being placed on the septum inferius. V.mes, Superior
.7';, Vitelline vein. W.Dj Wolffian duct, x, Anastomosis between
ins. Yk, Yolk-sac. Yk.s, Entodermal yolk-stalk, connecting the
^-(Draum by F. T.Lewis.)

GENERAL ANATOMY OF EMBRYO OF 12 MM.

235

as.

FIG. 172.

236 STUDY- OF PIG EMBRYOS.

Li, extends to the umbilicus; the smaller dorsal lobe, Li1, abuts against the Wolffian
body. In the fresh specimen the liver is conspicuous by its dark color. The
Wolffian body, W.B, is very large; it extends from the dorsal edge of the dia-
phragm to the level of the hind limb, H.L, or, in other words, to the pelvic end
of the abdomen. The Wolffian tubules can be seen running transversely just
within the surface of the organ and nearly parallel to one another. The large
size of the Wolffian body (fetal kidney or mesonephros) is characteristic of all
known amniote embryos. The umbilical cord projects upward from the ventral
wall of the abdomen; its cut surfaces, Um, are indicated by parallel lines. The
abdominal cavity extends into the cord, forming the umbilical coelom, Cc&. From
underneath the liver and on the right side of the embryo, the small intestine, //,
runs out into the umbilical coelom, makes a sharp loop turn, and passes over into
the large intestine, Col, which runs back to the abdomen on the left side of and
nearly parallel to the ileum, //; it passes under the tip of the liver, then between
the two Wolffian bodies, where it curves in the median plane — though this is not
shown in the figure — and, bending tailward, terminates in the cloaca, Clo. From
the tip of the intestinal loop springs the stalk, Yk.s, of the yolk-sac. The begin-
ning of the large intestine is marked by a small knob, CCKC, the anlage of the
csecum. At this stage the small and large intestines are of about the same di-
ameter. From the cloaca, Clo, a hollow prolongation, All, runs out into the caudad
wall of the umbilical cord; it is the stalk of the allantois.

Anatomical Reconstructions from the Sections. — Six reconstructions of the anatomy
of this stage are figured.* Figures 171, 173, 174, 175, 177, and 179 are based
on the same series which has supplied the transverse section of the 12 mm. pig
figured in the following pages. . The umbilical cord of this embryo (Series 5)
having been damaged, the loop of the intestine in the umbilical cord has been
added to figures 171 and 174 by a reconstruction from another series (No. 518) of
an embryo of the same size.

The following remarks call attention to some of the more important anatomical
relations shown by the reconstructions. The great volume of the central nervous
system as compared with the remaining parts is very striking. Of the other
organs, the three which are most conspicuous by their size are the heart, liver, .
and Wolffian bodies. Another striking peculiarity of the embryo is the great
diameter of the blood-vessels, and especially of the veins, which are of relatively
enormous diameter, being proportionately much larger than in the adult. In
marked contrast with this is the small diameter of the cavity of the trachea and
lungs and of the entire intestinal canal.

Figure 172 represents in the main a median section of the embryo together

* Figures 171, 175, 177, and 179 were made by Dr.F. T. Le\v;s; figure 174 by Mr. P.P. Johnson; figure 173
is from a wax reconstruction by Dr. John L. Bremer. In all cases the reconstructions were made with special
reference to their present use. It gives me much pleasure to acknowledge my obligations to these members of
our laboratory staff.

GENERAL ANATOMY OF EMBRYO OF 12 MM.

237

with the organs of the right side, but with two exceptions, first, the floor of the
pharynx is represented as if cut through considerably to the left of the median
plane; second, the heart is cut to the left of the median plane. The brain and
spinal cord are drawn as if opened to show the modeling of the inner surface of
the medullary tube. The pharynx is so drawn as to indicate something of the
modeling of its floor surface. The opening of the veins into the heart and of the
auricle into the ventricle, and the interventrieular orifices, are shown. Of the
digestive canal only the entoderm is represented, so that the figure displays the
entodermal walls of the oesophagus, stomach, and intestine, and shows the two
pancreatic anlages. Similarly only the entodermal portion of the trachea and

Bu —

M

Oe Tra IV

III

FIG. 173. — PIG, 12 MM. SAGITTAL SERIES 7. WAX RECONSTRUCTION BY J. L. BREMER.

Bu, Pharyngeal bursa. Hy, Hypophysis. M, Mouth-cavity. Oe, (Esophagus. Tra, Trachea. I, II, III, IV,
Gill-pouches, developed as lateral pouches of the pharynx. X 40 diams.

lungs is included, -and the same is true of the caudal end of the Wolffian duct
and of its outgrowth, which forms the anlage of the kidney. The same is further
true of the gall-bladder, of which only the epithelial portion is represented. In
this figure the arterial system is fully displayed. The pulmonary artery and the
aortic trunk are completely separated. A small artery from the pulmonary arch
to the lungs is included, and the figure shows the entire system of branches from
the main aorta.

Figure 175 is in many respects similar to figure 172, and is intended to show
chiefly the disposition of the veins. There are also included in this figure the
Wolffian body and its duct. The pharynx and the heart are supposed to have
been cut through, well to the right of the median plane. This makes it possible
to indicate in the figure the origin of the pulmonary aorta and of the true aorta.
The following are the most important veins: the umbilical, which passes around
the umbilical opening and enf2rs the liver; the portal vein, which receives the

238

STUDY OF PIG EMBRYOS.

Cl.iv

blood from the abdominal viscera, and also delivers it to the liver. In this speci-
men there is quite a wide and free connection within the liver between the portal
and umbilical veins. In other embryos of this size such a connection does not
always exist. The large vena cava inferior is on the right side of the embryo,

and passes through the liver, which thus receives
blood directly from the Wolffian bodies and the
cardinal veins. From the upper side of the liver
the hepatic , vein goes directly to the heart, uniting
with the common cardinals, which receive the
jugular veins from the head and the post-cardinal
veins from the body. The cardinal veins are now
very much changed. In earlier stages they ex-
tended from the common cardinals almost the
entire length of the embryo. Of this great vessel
there now remains connected with the common
cardinal only a comparatively short vessel.

Figure 173 gives a lateral view of the pharynx,
in order to show the four gill-pouches, I, II, III,
IV, as seen from the side. The curvature of the
Pan.d pharynx, and its passage at its posterior end into
the ventral trachea and dorsal oesophagus, are
clearly shown. As regards the gill-pouches, the
first rises upward and terminates in a sharp apex;
the second lies nearly in the same plane as the
portion of the pharynx from which it arises and
has a prolongation toward the third pouch, and
the end of the prolongation bends ventralward;
the third is narrow as it parts from the pharynx,
then bends downward and forward and has a pro-
longation, the anlage of the thymus gland, which
extends toward the root of the aorta; the fourth

Pan.v

Tra

Lu

Oe

Si

Col

FIG. 174. — PIG EMBRYO OF 12.0 MM.

RECONSTRUCTION FROM SERIES 5,

TO SHOW THE ENTODERMAL CANAL,

VIEWED FROM THE VENTRAL SIDE.
( 'IP, Caecum. Cl.iv, Fourth gill-pouch.

Col, Colon. Duod, Duodenum.

Hep, Hepatic duct. 77, Ileum.

La, Larynx. Lu, Lung Oe, (Esoph- also begins with a narrow stalk and has an ex-
panded end, one apex of which extends outward
(to join the cervical sinus), the other inward and
downward, the latter being the anlage of the post-
branchial body. On the dorsal side projects the
pedunculate hypophysis, which is developed, not
from the pharyngeal entoderm, but from the ectoderm of the mouth-cavity proper.
Figure 174 is inserted in order to give a clear idea of the entodermal canal,
as viewed from the ventral side. Only the posterior end of the pharynx is in-
cluded, and the cloaca and the allantois are omitted. The figure represents only
the entoderm without anv of the surrounding .mes'vlerm.

agus. Pan.d, Dorsal pancreas.
Pan.v, Ventra.1 pancreas. St,
Stomach. Tra, Trachea. Tr.Br,
Tracheal bronchus. I'k.s, Yolk-
sac. X 14 diams. — (Drawn by F. P.
Johnson.)

240

STUDY OF PIG EMBRYOS.

.

P'IG. 176. — PIG EMBRYO or 12.0 MM. RECONSTRUCTION FROM THE TRANSVERSE SECTIONS, SERIES 5.

The figure illustrates chiefly the veins of the right side, and shows the right auricle and ventricle of the heart.
A, The right umbilical artery, only a small portion being drawn as it curves around the Wolffian duct, W.D.
a, Tip of aortic septum, which divides the aortic limb of the heart into the pulmonary aorta, P, and main
aorta, Ao; by a growth of the cardiac tissue, a, b, and c of the figure become joined, shutting off the space
around the base of the aorta; this space communicates by the interventricular foramen with the left ventricle,
and serves as the permanent or adult channel of communication between the true aorta, Ao, and the left
ventricle. All, Allantois. Ao, Aortic division of the aortic limb of the heart. Au, Right auricle, b, See
"<;." c, See"a."' card', Superior part of the cardinal' vein (the anlage of the azygos). card", Inferior part of
the posterior cardinal vein, d.i, Opening of the first gill-pouch into the pharynx, the pharynx being indicated
by dotted shading, cl.2, Opening of the second gill-pouch into the pharynx, cl.i,, cl.4, Entodermal portions
of the third and fourth gill-clefts, c.om, Dotted outline of the omental or lesser peritoneal cavity, d, Left
(hut of Cuvier. D.C, Right duct of Cuvier, the main venous trunk entering the heart from the right side.
D.V, Ductus venosus Arantii. F.W, Foramen epiploicum (of Winslow), drawn in black. F-Pp, Foramen,
drawn in black, between the pleural and peritoneal cavities. The foramen is bounded by the lung, liver, and
Wolffian body; the figure shows the pleural side of the opening. If we pass through the foramen as drawn we
reach the abdominal cavity. The outline of the pleural cavity is marked by a dotted line. Gen, Genital
tubercle, represented as somewhat displaced from the median line, which it really occupies. G.R, Genital
ridge. Jug', -/«.?", Jugular or anterior cardinal vein. Li, Liver, l.s, Anlage of the lateral venous sinus.
»i.\; Vein of the inferior maxilla or mandible. P, Pulmonary division of the aortic limb of the heart, p.c,
IVrii urdial cavity. PI, Dotted outline of pleural cavity. Rec, Rectum. Sc, Sub-cardinal vein, which is
derived from the cardinal and on the right side- of the body forms part of the vena cava inferior. Scl, Sub-
c lavian vein, six, Anlage of the superior longitudinal venous sinus, which is formed by the union of th-
veins, /..?., from the sides to make a single vessel between the r(>r,.),r.,] ; ... ' .ier. i'ni.if,

Right umbilical vein v, x Vena

'>, Wo! (Viar.
of I lie n'glil and

GENERAL ANATOMY OF EMBRYO OF 12 MM.

241

sis

cL.i

cl.2.

W.D

cl.5

cU

FIG. 17-0.

1 6

242

STUDY OF PIG EMBRYOS.

243

FIG. 178. — PIG EMBRYO OF 12.0 MM. RECONSTRUCTION FROM THE TRANSVERSE SECTIONS. Si-
To show especially the cephalic nerves, c.i, 0.2, c.$, Cervical nerves, ch.ty, Chorda tympani. curt, <
commissure connecting with the jugular ganglion, j. Dien, Diencephalon. ex, Eternal ^
spinal accessory nerve. F, Froriep's ganglion, which in man completely disappe :
H, Cerebral hemisphere. ;, Jugular ganglion, from which the small avnicular nerve rv>
the glosso-pharyngeus. L, Lens, l.r, Laryngeal branch of the glcsso-pharyngeal nerve,
ficial petrosal nerve, md, Mandibular branch of the trigeminal. Mesi'tt, Mesencephalon. .1
cephalon. mx, Maxillary branch of the trigt-minal. Myelen, Myelencephalon. n, Xodo'-al g:1.^
Nasal pit. Op, Optic cup. oph, Ophthalmic branch of the trigeminal. Ot.
ganglion, ph.r, Pharyngeal branch of the glosso-pharyngeal n^rve. rec. Recurrent laryngeal
s, Superior ganglion, s-l, Semilunar ganglion. Telen, IV'encepha'on. ty, Tympanic nerve
Roof of the fourth ventricle. 3 Oculomotor nerve. 4, Trochli a 5, Trigeminal nerve. 6, A!

nerve. 7, Geniculate ganglion. 8, Acoustic ganglion. 9, Glo so-pharyngeal nerve. 10. \
ir, Spinal accessory nerve. 12, Hypoglossal nerve. Y, 20 diar s. — (Drawn by F. T. Li

244

STUDY OF PIG EMBRYOS.

/.

* V

FIG. 179.

GENERAL ANATOMY OF EMBRYO OF 12 MM.

245

Oe, Tra

FIG.- 180. — PIG EMBRYO OF 12.0 MM. RECONSTRUCTION FROM THE TRANSVERSE SECTIONS, SERIES 5.

The embryo has been drawn as if transparent, to show the form of its pharynx and the relations of the pharyngeal
gill-pouches to the grooves on the outer surface of the embryo. Cbl, Cerebellum. C.S, Cervical sinus. Dien,
Diencephalon. ep.b, Epibranchial body. H, Cerebral hemisphere. Hy, Hypophysis, which arises as an
evagination from the oral cavity. Inf, Infundibular gland, which arises as a median evagination from the
floor of the fore-brain, l.gr, Lachrymal grooVe. m, Maxillary process. M.b, Mid-brain. Mdb, Mandibular
process. Md.ob, Medulla oblongata. na.ex, External nasal opening; the nasal cavity is dotted, na.pl, Nasal
plate separating the nasal pit from the oral cavity; by the rupture of this plate the inlernal nares is formed
later, ni, Internal nares. nod, Nodulus thymicus. Oe, (Esophagus. Op, Eye. Ot, Otocyst. Sp.c, S] . >a!
cord. Th, Median thyroid gland, thym, Anlage of the thymus (part of the third gill-pouch). Tra,
Trachea. Yen. IV, Fourth ventricle, i, Entodermal pouch of the first branchial cleft, the anlage of
the Eustachian tube and tympanum. 2, Entodermal pouch of the second gill-cleft; it actually opens to the
exterior. 3, Entodermal pouch of the third branchial cleft. 4, Entodermal pouch of the fourth gill-cleft,
the lower fork of which is the anlage of the lateral thyroid. X 20 diams. — (Drawn byF. T.Lewis.}

246 STUDY OF PIG EMBRYOS.

Figure 178 shows the disposition of the cephalic and upper cervical nerves
and also the position of the nasal cavity, ' the eye, and the otocyst.

Figure 180 gives an outline of the head and combines an indication of the
external modeling of the gill-arches, with a representation of the shape of the
pharynx.

Pig Embryo of 6 mm. Studied in Sections.

Of this stage three transverse sections are figured in order to give more exact
notions as to the structure of neuromeres, of the pharynx, and of the secondary
segments.

Transverse Section through the Fourth Ventricle (Fig. 181). — The section is taken
through the level of the head, and may be directly compared with figure 189.
The relations are so closely similar that it is unnecessary to describe the present
section (Fig. 181) in detail. The explanation of the figure is sufficient for the
identification of the parts. The otocyst is large and conspicuous, and the arrange-
ment of the nerves is essentially similar to what we find in the older embryos.
The neuromeres, however, are very distinct, especially those upon the left side
of the embryo, N. i, 2, 3, 4. Of these, the third is perhaps the most characteris-
tic. Each neuromere is separated from its fellow by an internal sharp ridge, so
that the inner boundary of each neuromere toward the cavity of the fourth ventricle
is a small arc of a circle. The cells are elongated and are placed radially to the
inner curved surface of the neuromere. A thin but distinct layer of ectoglia is
present. The light line, which marks the boundary between the adjacent neuromeres,
is produced by the comparative absence ol nuclei. As to the number of neuro-
meres our knowledge is still defective; nor have we yet succeeded in making sure
of their exact relation to the nerves of the head, though such a relation evidently
exists. Thus we find, for example, that the facial nerve is always connected with
neuromere 2 of our figure, and the glosso-pharyngeal nerve with neuromere 4.

Transverse Section through the Region of the Branchial Arches (Fig. 182). — The
branchial arches are much more conspicuous at this stage than in later ones, being
separated from one another by deep ectodermal depressions, figure 29, I, II, III,
IV; and, although /// and IV are already being turned in, preparatory to the
formation of the cervical sinus, they are still distinct and their order in the series
is evident. The section (Fig. 182) shows on the dorsal side the spinal cord, in
which we can already recognize the subdivision into dorsal zone, D.Z, and ven-
tral zone, V. Z. To the dorsal zone is appended the dorsal root; from the middle
of the ventral zone comes off the ventral root of a cervical nerve, N. Just between
the dorsal root and the wall of the spinal cord can be seen the section of the ac-
cessory nerve. The secondary somite, My, is sharply defined and has a distinct
growing edge showing at its upper limit in the section. The inner leaf of the
secondary somite is stained more lightly than the neighboring tissue, corresponding
to the modifications which the cells are undergoing preparatory to their change

EMBRYO OF 6 MM. STUDIED IN SECTIONS.

247

into young muscle-fibers. In the 12 mm. pig in this region the cells of the muscle-
plate have already broken apart and no distinct plate can any longer be recognized.
Below the muscle-plate follows the section of the anterior cardinal vein, Card.
Lower down and in the median line we have the section of the pharynx, Ph,

Ven.IV.

Card

A .car-

V en. 111. Md.

FIG. 181. — PIG, 6.0 MM. TRANSVERSE SERIES 9, SECTION 90.

A.car, Carotid artery. Card, Card', Anterior cardinal vein. Md, Medullary wall of the fore-brain. A'.i, X.2
JV.3, N.4, Neuromeres of the hind-brain. A*. 5, Trigeminal ganglion. N.-j, 8, Acustico-facial ganglion.
N.g, Root of the glosso-pharyngeal nerve. Ot, Otocyst. Ven.III, Third ventricle or cavity of the fore-brain.
Ven.IV, Fourth ventricle. X 35 diams.

lined by the epithelial entoderm. The pharynx is surrounded by the very large
aortic vessels, which start from the ventral side of the pharynx, and pass upward
along its sides to join the descending aorta, Ao.d.^, at about the level of the jugu-
lar veins. The vessels shown are the fourth aortic arches. Their symmetry and

248

STUDY OF PIG EMBRYOS.

their relations to the pharynx are beautifully demonstrated in this section. Below
the aorta we find a section of the third internal gill-cleft, cl.III, a narrow, slit-
like cavity lined by entoderm. By following the series of sections, the connection
of this cavity with that of the pharynx can be traced, thus demonstrating that the

Olf.

F.B

D.Z.

Nch.

Ill, IV.

FIG. 182. — PIG, 6.0 MM. TRANSVERSE SERIES 9, SECTION 171.

A».d.4, Descending aorta receiving the right fourth aortic arch. Card, Anterior cardinal vein. cl.III, Third
entode.rmal gill-cleft. D.Z, Dorsal zone of spinal cord. F.B, Fore-brain. Hy, Hyoid branchial arch. Mdb,
Mandibular branchial arch. My, Muscle-plate. N, Nerve. Nch, Notochord. Olf, Olfactory plate.
Ph, Pharynx. V.Z, Ventral zone of spinal cord. I, II, III, IV, First to fourth ectodermal gill-clefts.
X 35 diams.

cleft is an evagination of the pharynx, as are all the gill-clefts. On the left-hand
side of the embryo the junction of the entoderm of the internal pouch with the
ectoderm is shown. The two germ-layers have united to form a typical closing
plate. Above the third gill-cleft, the outline of the embryo shows a deep depres-

EMBRYO OF 6 MM. STUDIED IN SECTIONS.

249

Sp.c.

A .is

Cu.

V.R.

Scler.

sion, ///, IV, which is due to the commencing formation of the cervical sinus.
From the upper end of this depression runs upward the ectodermal fourth cleft, and
from its lower part extend^ downward the ectodermal third cleft. Between the
third and fourth clefts the external surface of the embryo protrudes somewhat. This
protuberance corresponds to the so-called fourth branchial arch. Between the
third external cleft and the second, //, is
a still greater protuberance on the outside
of the embryo. This marks the third
branchial arch. The third aortic arches
are somewhat imperfectly shown, but the
connection of the left third arch with the
central aorta appears. Between the second
and first external clefts we have the second
or hyoid branchial arch, Hy; and, similarly,
between the first or auditory cleft, 7, and
the oral fissure, which separates the head
from the body of the embryo, we have the
very large and protuberant mandibular arch,
Mdb. The head ' of the embryo is com-
pletely separated in this section from the
body. It shows the cavity of the fore-brain,
F.B, bounded by the ectoderm of the med-
ullary wall, and on one side also shows
the thickening of the epidermis, Olf, which
forms the olfactory plate or plakode, which
is to become the lining of the nasal pit.

Transverse Section of the Lower End of
the Embryo (Fig. 183). — Our third section
is very near, the end of the series. Owing
to the curvature of the posterior end of the

body of younger embryos (compare Fig. FIG. 183.— PIG, 6.0 MM. TRANSVERSE SERIES 9,
165; pig, 7.5 mm.), sections taken in the
plane which we call transverse strike the
lumbar region so as to give longitudinal sec-
tions of the spinal cord and primitive seg-
ments. Figure 183, therefore, shows the

cavity of the spinal cord, Sp.c, cut for a very long distance. At the upper and
lower ends of the section, the dorsal side of the spinal cord is cut, and accord-
ingly we see at these levels sections of the ganglia, G, on either side of the
spinal cord. In the middle of our section the ventral portion of the spinal cord
is cut, and here, therefore, the ventral roots, V.R, of the nerves are displayed.
The somites are clearly marked by the external configuration of the embryo, the

SECTION 519.
A. is, Intersegmental artery. Cu, Cutis plate. EC,
Ectoderm. G, Ganglion, muse, Muscle-plate.
Scler, Sclerqtome, auct. Sp.c', Spinal cord.
V.R, Ventral nerve-root. X 50 diams.

250 ' STUDY OF PIG EMBRYOS.

ectoderm, EC, forming an arch over the outside of each segment. Each mesodermic
somite shows three distinct parts: next to the ectoderm the broad, epithelioid
cutis plate, within which comes the spindle-shaped section of the inner portion of
the somite, muse, the anlage of the skeletal muscles; and, third, an expanding
mass of mesenchyma, Scler, which is sometimes termed the sclerotome. This term,
however, is not wholly felicitous, because this mesenchyma forms not only the seg-
ments of the skeleton, but the connective tissue of the whole region about the spi-
nal cord in the dorsal part of the embryo. . The figure shows very clearly that the
ganglia and ventral nerve-roots are arranged in exact conformity to the segments,
and it can be easily observed, by following through the series of sections, that for
each somite there is one ganglion and one ventral root. It also shows that the
ventral roots reach directly to the muscle-plate. The muscle-plate is histologically
partly differentiated, for its cells have already elongated in a direction parallel
with the longitudinal axis of the embryo, and their nuclei also have become much
larger than any other nuclei in the neighboring parts of the embryo, being per-
haps three times as large as the mesenchymal nuclei of the sclerotome. They are
oval in form, contain many fine, and usually one or two somewhat larger granules,
the larger ones being deeply stained; but the nuclei, as a whole, are stained more
lightly than their neighbors. Each somite is very clearly separated from its
neighbors, and between the ends of the adjacent muscle-plates there is a small
clear space entirely free from cells and extending outward to the epidermis. Just
inside of this space in every case is a small blood-vessel, the intersegmental artery,
A. is. The intersegmental arteries are small branches which arise in symmetrical
pairs from the dorsal aorta.

Pig Embryo of 9 mm. Studied in Sections.

Pig embryos of this stage supplement very instructively those of 12 mm. It
will, of course, be advantageous for the student to prepare serial sections himself.
When that is not possible, there should at least be sections prepared for the lab-
oratory which the student may. examine. Four sections are illustrated and described
below. They have been chosen to supplement the descriptions of the sections of
the pig of 12 mm., and they will be found to illustrate certain fundamental mor-
phological relations in the embryo more clearly than older stages.

Sagittal Section to the Right of the Median Plane (Fig. 184). — In the accorri-
panying figure 184 the cephalic end of the embryo is omitted; a portion of the
heart, the entire length of the Wolffian body, and the tail are included. The
dorsal outline of the embryo forms a characteristic curve. A long series of spinal
ganglia, G, is shown arranged in regular succession and following the curvature
of the back. The ganglia are easily recognizable by their dark staining; each of
them is so large as to occupy at least four fifths of the length of the segment to
which it belongs. The boundaries between the adjacent primitive segments are
indicated by the positions of the intersegmental arteries, A. is'. Even when their

EMBRYO OF 9 MM. STUDIED IN SECTIONS.

251

Ao.D. Pul. V.h.c. V.s. Au.

A. is

W.b.

msth.

Cce.

V.msn

FIG. 184. — PIG, 9.0 MM. SAGITTAL SERIES 53, SECTION 213.

A. is, Intersegmental artery. All, Allantois. Ao, Median aorta. Ao.D, Descending aorta. Art.v, Arteria
vitellina. Au, Auricle. Clo, Cloaca. Cue, Ccelom. Ent, Entoderm. G, Ganglion. G.b, Gall-bladder.
Glo, Glomerulus. Li, Liver, msth, Mesothelium. Pul, Lung. Seg, Segment. Som, Somatopleure. Sp.c,
Spinal cord. Um.w', Upper wall of umbilicus. Um.w", Lower wall of umbilicus. Ve, Vein in liver. Yen,
Ventricle of heart. V.h.c, Vena hepatica communis. Vil, Villus. V.msn, Vena sub-cardinalis. V.p, Portal
vein. V.s, Valvula sinistra. W.b', W.b", Wolffian body. X 22 diams.

252

STUDY OF PIG EMBRYOS.

cavities do not show, the position of these vessels is marked by the darker line
of tissue. The origin of one of these intersegmental vessels from the dorsal aorta,
Ao, is indicated in the lower part of the figure. The Wolffian body, W.b', W.b",
extends from the level of the lungs and liver well down toward the pelvic end
of the embryo. Its ventral limit is marked by the body-cavity, Ccc, and it is,
of course, covered by a layer of mesothelium, msth, which here, as everywhere and
at all stages, forms the boundary of the ccelom. In the Wolffian body we dis-
tinguish readily numerous sections' of the epithelial Wolffian tubules, and toward
the ventral side of the organ the characteristic glomeruli, Glo. Between the glo-
meruli and the mesothelium there is a layer of mesenchyma, but between the tubules
there is little tissue, the intertubular spaces being almost entirely occupied by
sinusoids developed from the cardinal vein. The larger sinusoid or venous space,
V.msn, is due to the section of the venous trunk which joins the lower end of
the vena cava inferior, and is known as the sub-cardinal vein. In the upper part
of the figure we encounter a section of the descending aorta, Ao.d, and of the
lungs, Pul, or pulmonary anlage. The latter consists of a ring of entoderm bound-
ing the central cavity and enclosed by a thicker layer of mesenchyma, which, again,
is bounded by a layer of mesothelium. The space or ccelom about the lung is
shown in the figure to be continuous with the ccelom of the abdominal region.
On the ventral side we have the heart partly shown, the ventricle, Ven, being so
cut as to exhibit the trabecular structure of the network of the sinusoidal spaces.
The auricle, Au, is without sinusoids. The great venous trunk, vena hepatica
communis, V.h.c, opens into the auricle, the opening being guarded by two
valves, that on the dorsal side of the opening in the figure, V.s, being the left
valve. The vein receives blood from the liver, Li, and from the Wolffian bodies,
and it persists in the adult as the uppermost part of the vena cava inferior. The
duct-us venosus Arantii, which is so large in the human fetal liver, is less conspicu-
ous in the pig; the ductus is the venous trunk formed by the union of the portal
and umbilical veins within the liver; it joins the vena cava inferior to form the
vena hepatica communis. The liver, Li, consists of liver cells or hepatic cylinders
and numerous sinusoids of many diameters. On the lower side of the liver there
is a considerable accumulation of mesenchyma by which the liver is united on
the one end to the body-wall, Som, to the umbilical wall, Um.w', and to the,
mesentery by which the intestine is suspended from the liver. In this mesen-
chyma is lodged the gall-bladder, which is cut thrice. The reference line G.b
runs to the uppermost of the three sections. The diameter of the gall-bladder
is ^several times that of the entodermal intestine, and its lining epithelium is thicker
than any other epithelium of the embryo at this stage. The section of the bladder
nearest the portal vein, V.p, corresponds to the beginning of the ductus cysticu
Underneath the liver in the section of the mesentery is situated the portal vein,
V.p. From the mesentery extends out the intestine (duodenum). It is a somewhat
cylindrical tube which curves over ventralward and passes out through the opening

• '

EMBRYO OF 9 MM. STUDIED IN SECTIONS. 253

of the umbilicus. It consists of a very small tube of entoderm, Ent, with only a
small internal cavity (compare Fig. 186, Reel.}. The thickness of the intestinal
wall is due chiefly to the considerable development of the mesenchyma. The ex-
ternal covering of the intestine is a layer of mesothelium which becomes the peri-
toneal epithelium of the adult. In the tissue of the organ we distinguish the
narrow vitelline artery, Art.v. The umbilical opening is very wide and is bounded
both above and below by a prolongation, Um.w', Um.w", of the somatopleure
of the embryo. The wall on the upper side is much thicker than on the lower.
The umbilical opening is partly occupied by the duodenum. Appended to
the inferior wall of the umbilicus is the allantois, All, which arises from the
enlarged caudal end (cloaca), Clo, of the intestine. It passes out first inward,
then makes an acute but rounded angle, and extends outward through the um-
bilical opening. It may, therefore, be said to consist of two limbs, one within
the body of the embryo joining the cloaca, and the other passing out through
the umbilical opening. The limb arising from the cloaca is completely united
with the body-wall, and is, of course, upon the side toward the ccelom covered
in by mesothelium. The lining of the allantoic cavity is an epithelium, and is a
portion of the entoderm. Along the second limb of the allantois the mesothelium
on the side toward the cavity of the umbilicus forms a series of clumsy projections,
Vil, the mesothelial ^illi of the allantois. They are smallest toward the embryo
and increase in size distally. With higher power one can see that the mesothelium
of the villi is very thin and the mesenchyma in their interior of quite loose texture.
In later stages the mesothelium grows, the mesenchyma in large part disappears,
and the villi then seem hardly more than small bags of mesothelium with but
little contents, save some coagulum. They continue to enlarge until the embryo
is 17 or 1 8 mm. long, after which they begin to abort. In these older stages the
villi extend far into the abdomen and are packed in between the abdominal
viscera, presenting curious appearances in section. As the tail of the embryo is
bent to one side, it offers us a section of a portion of the spinal cord, Sp.c, and
at its tip a glimpse of three primitive somites, Seg.

Frontal Section through the Mid-brain and Fore-brain (Fig. 185). — Comparison
with figure 165 (pig, 7.5 mm.) will make it clear that in a frontal series
obtain a few sections of the head which include only mid-brain and fore-brain a>id
show no other special cephalic structures. The mid-brain, M.B, is somewhat
rounded in form and passes over into the fore-brain, which is quite long and
which already shows traces of its subdivision into two parts, the diencephalon,
Dien, which lies nearest to the mid-brain, and the prosencephalon, Pros, which
constitutes the terminal portion of the brain and which produces the lateral expan-
sions which are to form the cerebral hemispheres. The expanding prosencephalon
is separated by a constriction from the diencephalon, which in its turn is similarly
separated from the mid-brain. The diencephalon and prosencephalon together rep-
resent the fore-brain. They are subdivisions of the primary first cerebral vesicle.

254

STUDY OF PIG EMBRYOS.

It is important to note that they do not correspond to complete subdivisions, and
have not the same morphological value as the three primary vesicles. The histo-
logical development is much less advanced than in the pig of 12 mm. The ecto-
derm is very thin, consisting for the most part of a single layer, of cells, but here
and there the formation of a second layer is seen to be beginning. The mesoderm
is very simple in character and almost uniform in appearance, but there is a dis-
tinct difference between the mesenchyma around the brain and that underneath the
epidermis, the former having cells farther apart. This is almost the first stage in

the differentiation of the arachnoid zone around the

Md.. - x""*^j«siw ,; brain. The pia mater, however, though quite thin,

is well defined by the condensation of the
mesenchymaF cells and by the somewhat numerous
small blood-vessels in it. The medullary wall is
everywhere quite thick and crowded with nuclei.
In the region of the diencephalon the ectoglia is

Dien. J :. '^i-tliiB ^^&:; ';•'.'• .-0 1 distinctly formed, but elsewhere has hardly begun

its differentiation. On the inside of the medullary
wall, close to the surface, there are everywhere
very numerous mitbtic figures.

Frontal Section through the Umbilical Opening
(Fig. 186). — The illustration is part of the same
section in the series from which figure 185 is taken.
For convenience of comparison the position has
been reversed so as to bring the dorsal side of
the embryo uppermost in figure 186. It results
from this that right and left ' sides of the embryo
FIG. 185.— PIG, 9.0 MM. FRONTAL are reversed in the engraving as compared with
SERIES 54, SECTION 194. the other figures of transverse and frontal sections.

Dien, Diencephalon. M.B, Mid-brain. By examining figure 166 (pig, io mm.) the student

Md, Medullary wall of brain, mes, ..,

,, D T> u i will see that sections in the frontal plane, owing to

Mesenchyma. Pros, Prosencephafon.

"v, Vein, x 22 diams. the curvature of the posterior end- of the body- wall,

furnish transverse sections of the spinal cord of the

pelvic region. Therefore, the section here figured, although part of a frontal series,
is directly comparable to a transverse section of the body. In the upper part of
the figure we have the spinal cord, Sp.c, and on one side of that the ganglion,
G. Owing to the spiral twist of the embryo the section is not symmetrical, so that
the posterior limb, P.L, appears only on one side of the section. Laterad from
the nerve shown in the figure is the large muscle-plate, My, the cells of which are
already beginning to change into muscle-fibers. On the dorsal side of the plate we
find its growing edge, m.pl, where the tissue of the muscle-plate proper bends
over and passes continuously into the external wall of the somite. From this
growing edge the cells are added to the muscle-plate by which it extends upward.

Ve.

Mes.

Pros.

EMBRYO OF 9 MM. STUDIED IN SECTIONS.

255

The similar edge on the ventral side provides for the extension of the muscle-plate
downward. In the median line, below the spinal cord is the small notochord, Nch,
and the large median dorsal aorta, Ao. In the ventral portion of the embryo
appears the large body-cavity into which protrude the Wolffian bodies and the intes-

•tn.pl. G. Sp.c.

V.U.S

• '•'i"ftv^

FIG. 186. — PIG, 9.0 MM. FRONTAL SERIES 54, SECTION 194.

Ao, Aorta, card, Cardinal vein. F, F.ctodermal fold at the border of the limb-bud. G, Ganglion, gen, Genital
ridge. Glo, Glomerulus. In, Small intestine (jejunum), mes, Splanchnic mesoderm (of the intestinal
wall), m.pl, Dorsal growing edge of the muscle-plate. My, Muscle-plate of secondary segment. Nch,
Notochord. P.L, Posterior limb. Reel, Large intestine. Som, Somatopleure. Sp.c, Spinal cord. V.U.D,
Right umbilical vein. V.U.S, Left umbilical vein. W.b, Wolffian body. W.D, Wolffian duct. X 35
diams.

tine. The ccelom also has a downward prolongation into the beginning of the
umbilical cord, and in this prolongation lies the so-called extra-embryonic loop of
the intestine, In. The coelom is bounded everywhere by the layer of mesothelium
represented in the engraving as a continuous line. With a higher power the meso-

256 STUDY OF PIG EMBRYOS.

thelium is seen to consist of a single layer of cells, but varying somewhat in
thickness in different regions. By following the contour of the mesothelium the
student will recognize at once that all of the viscera are, in the anatomical sense,
outside of the ccelom. The Wolffian bodies, W.b, are voluminous organs pro-
jecting from below the aorta on either side of the large intestine, Reel, and extend-
ing far into the abdominal cavity. At the lower ventral edge of the Wolman body
appears the Wolffian duct, W.D, a wide, longitudinal canal into which the Wolffi-
an tubules open. The large size of the duct is characteristic of this stage. In
later stages it is smaller. The tubules are very large, contorted in their course,
and appear, therefore, variously cut. They are formed by a cuboidal epithelium
and are provided with a sinusoidal circulation. The endothelium of the blood
spaces can generally be seen fitting closely against the epithelium of the tubules.
Here and there, however, there is some mesenchyma between the blood spaces and
the walls of the tubules. On the median side of the Wolman body are the
glomeruli, which are of large size, and similar in structure to the glomerulus of
the permanent kidney, though differing from the renal glomeruli in their propor-
tions and in the details of their structure. It is not difficult to make a reconstruc-
tion of the course of a single tubule by following it through a few neighboring
sections. The general course of a tubule is in the transverse plane, but it is
much contorted. Each tubule begins at one of the glomeruli, with which it is
in open communication. . It then bends so as to make a somewhat irregular
S-shaped figure, and finally opens into the Wolffian duct. After leaving the glomeru-
lus it widens somewhat, but before it joins the Wolffian duct it again diminishes in
diameter. The changes in diameter are gradual. The blood spaces or sinusoids
of the Wolffian body are derived from the posterior cardinal veins. The veins and
tubules, wrien the latter first become distinct, lie near together. As development
con tin" ^« both enlarge and encroach upon one another's territory; hence there is
an intimate intercrescence of the blood-vessels and of the tubules, resulting in the
formation of sinusoids. The whole of the Wolffian body might from one point of
view, therefore, be regarded as a modification of the cardinal vein, and morphologi-
cally all of the blood spaces between the tubules belong to that vein. There
remain typically two portions of the cardinal vein which are more or less open and
distinct. The one on the dorsal side of the Wolffian body, card, may be conve-
niently regarded as representing the original cardinal vein. The other, on the ventral
side of the Wolffian body, is at first not a very distinct channel, but gradually
becomes more and more so, and is known by the distinctive name of sub-cardinal
vein. It is a vessel of great morphological importance, since on the right side of
the embryo it acquires a connection with the liver which renders it possible for the
blood of the right sub-cardinal vein to pass through the blood spaces of the liver
directly to the heart. This makes a very direct channel, a more direct one than
existed previously, when the blood from the sub-cardinal came to join that of the
cardinal, passing up to the common cardinal and then back to the heart. The new

EMBRYO OF 9 MM. STUDIED IN SECTIONS. 257

channel through the liver rapidly enlarges and becomes recognizable as the vena
cava inferior. This important venous trunk is a combined vessel, comprising, first,
a part of the sinus venosus of the heart; second, the vena hepatica communis; third,
a large channel developed from the sinusoids of the liver; fourth, the upper part of
the right sub-cardinal vein; and, fifth, the lower part of the right cardinal. The
vena cava inferior has already been developed in the pig embryo of 9 mm. Be-
tween the two Wolffian bodies hangs down the large intestine, Reel, suspended by
its mesentery in the median line. The entodermal portion is a very small circle
of epithelium with an extremely minute lumen, which in the section is scarcely
larger than a single nucleus. The mesentery and intestine are covered by a well-
defined mesothelium and have a considerable amount of mesenchyma, in which
there is no distinct histological differentiation beyond the presence of a number
of small blood-vessels. At this stage the large intestine runs nearly in the median
plane to the pelvic end of the body. In the opposite direction, toward the head,
it bends to the left of the embryo, making a loop which passes over into the end of
the1 ileum. The ileum forms the continuation of the loop and extends into the
ccelom at the bas'e of the umbilical cord. There it bends back and returns toward
the dorsal side of the embryo to pass over into the duodenum and join the stomach.
Owing to the fact that the small intestine extends into the extra-embryonic ccelom
of the umbilical cord, there makes a loop, and returns to the embryonic region,
we get typically a double section of the intestine as shown in the figure, one of
each limb of the loop. The entoderm, In, in these loops forms ^ small ring, which,
however, is much larger than the entodermal ring of the la. ui.c at this

stage. Each loop contains a large amount of mesenchyma, mes. th<
are somewhat crowded, so that the tissue appears dark in the stained* section,
boundary between the body of the embryo and the tissue of i!
marked by the position of the two umbilical veins, that of the lett :
being very much larger than that of the right side, V.U.D. By following do.,
the somatopleure, Som, of the embryo, it will be seen that these veins are lodged
therein, and that the continuation of the somatopleure beyond these veins forms
the substance of the umbilical cord. The limb-bud, P.L, is a large mass of
rather dense mesenchyma, entirely without muscles or nerves and covered by ecto-
derm. At the edge of the limb-bud the ectoderm shows a special thickening, F.
The theory has been advanced that this thickening is homologous with the ecto-
dermal fold -which produces the fin of fishes, or at least that portion of the fin in
which the fin-rays are developed.

Frontal Section through the Second and Third Gill-Clefts
preparation the section hits the posterior wall, Ot, of t:
anterior to the origin of the glosso-pharyngeal nerve. The appe;
section ol the hind-br?in is characteristic for this region of young €n
deck-plate has grown gradually in size and forms a wide n
the ependyrhal roc; !ie fourth ventricle. Owing to this

258 STUDY OF PIG EMBRYOS.

deck-plate, the upper or dorsal limits of the dorsal zones, D.Z, are brought
far apart and the cavity of the hind-brain is thus enlarged. The dorsal zone is
divided by an angle in the interior and by the point of entrance of the nerve-roots
on the exterior from the ventral zone, V.Z. On their dorsal side the dorsal zones
thin out and pass over gradually into the ependyma. The ependyma consists of a
single layer of cells. In the dorsal zone the differentiation of the three primary
layers of the medullary wall has scarcely begun, but in the ventral zones the three
layers are already distinguishable, though not far advanced in their differentiation.

EC. mes. Epen. >.

A. has.

FIG. 187. — PIG, 9.0 MM. FRONTAL SERIES 54, SECTION 459.

/ .has, Basilar artery. Ao.d, Descending aorta of the left side. Ao.^,7 Third aortic arch. Ao.^, Fourth aortic
arch arising from the median ventral aorta. Card, Anterior cardinal vein. cl.II.ex, External portion of the
second gill-cleft. cl.III, Third gill-cleft. D.Z, Dorsal zone of the medulla oblongata. EC, Ectoderm .
Epen, Ependymal roof of the hind-brain. Hy, Hyoid arch, mes, Mesenchyma. Ot, Posterior wall of the
otocyst. P.Ao, Pulmonary aorta. Ph, Pharynx. Som, Somatopleure. V.Z, Ventral zone. X 22 diams.

In the floor-plate there are two layers. Below the medullary tube lies the ba"silar
artery, 14.. has, and below that, not far from the upper wall of the pharynx, lies
the small round notochord in the midst of loose mesenchymal cells, which have
not yet begun to condense themselves about the notochord. The pharynx is a wide
space of rather stnall dorso- ventral diameter, and having a much thinner layer of
v.KK1., ,11 on its dorsal than on its ventral side. Above the pharynx on either
side lies the section of the descending aorta, Ao.d. The reference line to this
vessel crosses a dark mass of cells which belong to the ganglion nodosum of. 4 he
tenth nerve. Below the pharynx the section shows the third aortio arch, AO.T,,
and the fourth aortic arch, Ao.^, just springing off from the median aortic /trunk
above the heart, so that the two fourth arches are connected across the tfiedian

EMBRYO OF 12 MM. STUDIED IN SECTIONS.

'He. Between the third and fourth aortic arches on either side is a small cavity

med by entoderm, cl.III, a diverticulum from the third gill-cleft. Immediately

>elow the otocyst is the anterior cardinal vein, Card. From a point below the

ardinal there extends a prolongation, Hy, which may be taken as a portion of

the hyoid or second branchial arch. It extends downward and consists of a mass

of mescnchyma covered by ectoderm. It encloses a space, cl.II.ex, which may

be regarded as the external portion of the second gill-cleft. In a neighboring

;on (455) the prolongation of the pharynx shown in figure 187 can be traced

farther until it opens into this space, cl.II.ex. The second cleft is open

upon both sides of the embryo, the first and third have closing membranes, the

fourth cleft is not yet so far developed that its entoderm has come in contact with

the epidermis of the embryo. The second cleft probably always becomes open,

differing in this respect from all the others. Why it has this peculiarity we do not

know. The opening does not persist, but the exact history of its closure is at

present unknown. The process, Hy, described as shutting in the external p >rtion

« the second gill-cleft has sometimes been termed the operculum, because it covers

a gill-cleft opening, as does the operculum of a bony fish. In the lower part of

our figure a portion of the somatopleure, Som, is shown where it extends ven-

tralward to form the wall of the pericardial cavity. There is also included in the

drawing a part of the pulmonary aorta, P.Ao.

Pig Embryo of 12 mm. Studied in Sections.

A pig embryo of 12 mm. has been selected as the center of study in this
because its anatomical relations are such that they may be readily grasped by
the student who has already studied 'the 'anatomy of an adult mammal, human
or other. At the same time the development of the organs is so advanced that
their fundamental relations may be observed. From an embryo of this sill
transition to the study of younger embryos is, even for the beginner, comp.i
easy. It is not necessary that the embryo should be of this exact size; ind( cl.
may be somewhat advantageous for the student to have an embryo a millirt
larger, or one, two, or even three millimeters smaller, since the figures and <
tions referring to the 12 mm. stage will enable him to identify all the striH»tttB
to be found and yet call upon him for the exercise of care and judgment in ide
fying, from the data given in the following pages, the various parts in the q
what different stage he may be studying. Of 12 mm. pigs the author has ha<|
his disposal five good series belonging to the Harvard Embryological Collect^

The transverse series is the most important, and should form the bJ|
the study, and accordingly most of the sections figured are from su
Next in importance comes the sagittal series, but it is desirable that e
should have a series in the three standard planes at his disposal for *tn'|;
practical laboratory study each student should be required to make a
accurate camera lucida ^drawings of carefully selected

260

STUDY OF PIG EMBRYOS.

name correctly all the parts shown in each section and to identify the distribution
of the three germ-layers in every case.* A sufficient number of high-power draw-
ings ought to be added to illustrate the character of the various tissues.

284- 340 380

572

Fu;. 188. — RECONSTRUCTION or A PIG EMBRYO OF 12.0 MM. WITH INDICATIONS OF THK PLANES OF -SECTIONS

FIGURED.
an, Cloacal membrane. Ao, Aorta. Au, Auricle. A.um, Umbilical artery, a.v, Vertebral artery, bas, Basilar

artery, c, Anlage of caecum, car, Internal carotid, can. Caudal artery. C.Ex, External carotid artery.

f.b, Fore-brain. G, Spinal ganglion, g, Gall-bladder, h.b. Hind-brain. In, Intestine. Li, Liver, m.b,

Mid-brain. o/>, Optic .vesicle. Ot, Otocyst. pan, Pancreas. Sf>, Spinal cord. St, Stomach. Urn, Umbilical

opening. Yen, Ventricle of heart. Ill, I V, V, Aortic arches.

284 Frontal section, Fig. 203

340 Frontal section, Fig. 204

380 Frontal section, Fig. 205

423 Frontal section, Fig. 206

572 Frontal section, Fig. 207

185 Transverse section, Fig. 189

198 Transverse section, Fig. 191

249 Transverse section, Fig. 192

292 Transverse section, Fig. 193

321 Transverse section, Fig. 194

jx, Transverse section, Fig. 105

470 Transverse section, Fig. 196

513 Transverse section, Fig. 107

f\,j, Transverse section, Fig. 198

*K>r making camera lucida drawings, a i-inch objective will be found convenient.
is recommended. Compare vhe directions for drawing in Chapter VIII.

An Abbe camera

TK.* \S\ ERSE SECTIONS OF EMBRYO OF 12 MM. 261

The accompanying *f.«ure 188 represents the outline of the pig embryo which
was cut into the series of transverse sections from which figures 189 to 198 have
been made. The student can easily identify the parts in the figure by comparison
with that of the pig of 10 mm. (Fig. j66), aided by the accompanying description
of the same. The sections of this embryo are lo/n in thickness, and are 966
in number, not 1200, as the student might expect. The discrepancy is due
to the shrinkage of the embryo when imbedded in paraffin. The shrinkage is
always very great, and in the case of embryos causes a loss of almost 20 per cent
in the length; but as it seems to take place uniformly throughout the embryo, it
causes no distortion, so that the embryo in paraffin is an exact though greatly
reduced copy — so to speak — of the living embryo. It should be remembered that
no correct measurements of the size of organs or cells can be obtained from sec-
tions made by the paraffin method. This limitation upon the use of sections
too often forgotten. The horizontal lines indicate approximately the levels at whic
the sections here7 figured belong. For convenience the direction and position of the
frontal sections represented in figures 203 to 207 are also indicated approximately
on the same outline, although, of course, the frontal series was from another
embryo.

Pig Embryo of 12.0 mm. Study of Transverse Sections.

The figures and descriptions here presented of ten sections have been
selected as illustrating the most important structures, with the exception of the
umbilical opening and of the kidney, which can be better represented in sections
from older or younger stages.

Section through the I * pp>

by the line 185, this sectiQffcJ ^ taken from a level about

and the apex of the otocyst, Ot. It passes, therefore, through the fore-brain,
and the fourth ventricle, Vcn.IV, or cavity of the hind-brain. The section^H
bounded by a thin layer of cpjfiermis, between which and the brain-wall there is
a large amount of mesenc hymal tissue. Alongside the hind-brain lies a series of
important structures imbe \<\£d in the mesenchyma, which are identical upon the

ciH«<i. althoi. Bter somewhat' in the section, as the plane of cutting

was not symm< rse for the head. These structures are in the fol-

lowing order: .V.; rigeminal ganglior. , AT./,8, the acustico-facial ganglion

complex; Ot, the otocyst, an oval vesicle with very distinct epithelial walls. Next
the ninth or glosso-pharyngeal nerve (scarcely appearing in the section on the right
side of the embryo i is shown by the upper ^portion of its ganglion on the '
(the right in tv vi; tb-»

close to the me'*'
posteri< «
thr

262

STUDY OF PIG EMBRYOS.

to the ganglionic commissure which extends above the ^"'gin of the hypoglossal
nerve. On the left the continuity of this commissure is better shown than on the
right, where it offers two parts, one, com, entirely ganglionic, and another, com.II,
which comprises both the ganglionic portion and fibers which share in the forma-

com.II '.

JV.io.

D.E.

N.7,8.

Ven.IV

F.b.

FIG. 189. — PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 185.

com, Ganglionic commissure between hypoglossal and vagus nerves. com.II, Ganglionic commissure with libers
of the root of the spinal accessory nerve. D.E, Ductus endolymphaticus. F.b, fore-brain. Md, Medulla

1 ta. N.f,, Trigominal ganglion. N.-j,&, Acustico-facial ganglion. N.io, Jugular ganglion of the

' >tocvst. Ven.IV, Fourth von*--'--'- ^' ^2 diams.

•TT large, some-
" hind-

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 263

marks in the study of -the topography of the embryonic head. The nerve-cells of
the ganglion are grouped, for the most part, on the side toward the ectoderm,
where they are closely crowded together, making a deeply staining mass. Nearer
the brain-wall the tissue of the ganglion is much less condensed, is somewhat
penetrated by small blood-vessels, and contains a considerable number of nerve-
fibers, which are gathered into small bundles. Toward the brain-wall the bundles
become distinct, and on the right side of the embryo the passage of the nerve-
fibers into the brain can be readily seen. The nerve-fibers -at this stage are
merely neuraxons; that is to say, thread-like prolongations of the bodies of the
nerve-cells (neurones). The fibers are entirely without sheaths. They stain very
lightly, and hence, in the preparation, may be detected by their light appearance.
The nerve-fibers may be conveniently rendered conspicuous by counterstaining the
sections with Lyons blue. The nerve-fibers of the trigeminus, which enter the wall
of the hind-brain, form in part a bundle of fibers, which extends along past the
acustico-facial ganglia within the medullary wall. These fibers represent the com-
mencement of the ascending trigeminal tract of anatomists. The other ganglia
associated with the hind-brain are not well shown in this section. The otocyst
(compare Fig. 42, p. 79) has a very sharply defined epithelial wall and is im-
bedded in loose mesenchymal tissue. On the. right side of the embryo we have
the ductus .endolymphaticus, D.E, the opening of which into the .main cavity of
the otocyst is shown on the left side. The epithelial wall of the ductus is thicker
than that of the greater part of the otocyst proper. The wall, Md,., of the hind
brain exhibits already characteristic differentiations^ for it shows clearly the
primitive laprs; the outer neuroglia layer (ectoglia^is thin, -and appears light in
the sectii- * :; use it takes the stain slightly. It is in this outer neuroglia layer
(ectoglia} that the entire sensory nerve-fibers are p ;.rnriiy distributed, and. there-
fore, it is in a po, rmd l'-> ract
situated. Next to the ectoglia comes the middle layer, in \vbi
the medullary wall are situated, and which is, therefore, termed the neurone < >
gray layer (cinerea), easily recognizable under the microscope by its brighter color,
which is due principally to the fact that the nuclei in this layer, though numerous,
are much less crowded than in the innermost of the three layers, or primitive
ependymal layer, which at this stage is quite thick. Owing to the presence of
nuclei, the gray layer is, of course, stained much more than the ectoglia. The nu-
clei of the brain- wall shoty as yet very little differentiation. There are numerous
mitotic figures which are situated exclusively clpse to the inner surface of the brain-
wall in the fore-brain. The structure «•,*' the for^-brain is similar, but the develo*
ment is less advanced; the differentiation, T)i~4ki neurone layer is only v*
ning, and it has acquired little thickness. In tho'^'Vl^bjain we ><>
along the region between the otocysts, a se*
a scalloped outline to the wall. A ^
nex ^ one of the spac<-

204 STUDY OF PIG EMBRYOS.

a neuromere. The neuromeres correspond in number and position to the neighbor-
ing primitive segments, and are, therefore, to be designated as segmental structures. .
They also bear an evident relation to the development of the nerves, and the J
V accepted hypothesis is that from each neuromere springs a single nerve. The at-
tempts which have been made to verify this hypothesis have met with very serious
difficulties, for the relations are extremely complicated, and until the matter shall
have been much more thoroughly investigated than at present, we must remain
in the dark" as to the precise morphological value of the neuromeres. But, inas-
much as they appear with the greatest constancy in the embryos of all vertebrates,
we cannot help accepting the view that they are really structures of fundamental
importance. At the stage we are studying the neuromeres have already begun to
lose their distinctness, and in slightly older pigs can be traced only with difficulty.
In younger stages their primitive characteristics are better shown (compare page
246). As regards the' blood-vessels in the present section: there are small branches
of the veins, which show outside of the ganglionic commissure, com; parts of the
cardinal vein appear in close proximity to the trigeminal ganglion, and again at
the side of the head. In the median line between the fore-brain and hind-brain,
or nearer to the layer, appears a section of the basilar artery. Near the fore-brain
on either side is the loop of the carotid artery. There are several important points
to be observed in the region between the trigeminal ganglia and the fore-brain. In
order to show these more clearly, a separate illustration (Fig. 190), on a larger
scnle, of this portion of the section is given. The trigeminal ganglion, the wall of
the foio-brain, and the wall of the hind-brain will be at once identified, so that the
correspondence with the general figure is easily followed. Between the trigeminal
ganglion and the fore-brain are four veins, two of . which, Card' and Card'" ', are
larger and are parts of the main cardinal stem passing from the region of the
hind-brain ^to thai- of the, fore-brain, while the ;-, u .. mailer ones, Card", are merely
nches of the same vessel. Close to the section, Card'", of the cardinal nearest
the fore-brain lie the very small sections of the fourth, N.4, and third, N.T,, cere-
bral nerves. The fourth nerve is minute in size and lies just behind the vein.
The third nerve, thougfi somewhat larger, is also very small and lies anterior to
the vein somewhat on its medial side. Both of these nerves, owing to their
small dimensions, are somewhat difficult to observe with the low power. The de-
tailed figure brings out more clear/ other points. It shows very clearly the junc-
tion of the trigeminal. A - , -tico-facial, N.j,8, ganglia with the wall of the
1 brain, and also the div^ 'of that wall into i' three primary layers, the
' "/, the gray layer ..rid .iie inner or ependym layer, ^Epen, and
floor-plate ' •:, /<7r/>/?. Immediately below it is the basilar
'n is the s< I loop of the carotid,
"nd oi thv basilar artery, which
<y symmetric" ' '
spond

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM.

265

designated in the adult as the posterior communicating branch, by which the end
of the carotid proper anastomoses with the basilar artery. At the side of the fore-
brain appears a blood-vessel, Card.L, which might be called the lateral cardinal.
It is a branch of the main cardinal stem and passes over the side of the fore-brain

Ec.gl. AT. 7, 8. Cin. Epen. Raph. Ven.IV.

A.bas.

Pia.

A. car.

Ven.III.

Card.L.

Arach.

FIG. 190. — PORTION OF FIGURE 189 MORE HIGHLY MAGNIFIED.

A.bas, Basilar artery. A. car, Internal carotid artery. Arach, Arachnoid zone. Cin, Neurone layer of medulla.
Cut, Cutis layer. Card', Card", Card'", Anterior cardinal vein. Card.L, Lateral branch of the cardinal vein.
Ec.gl, Ectoglia. Epen, Ependymal layer. G.tri, Trigeminal ganglion. N.-$, Oculo-motor nerve. N.4,
Trochlear nerve. N.$, Sensory root of trigeminus. N."j,8, Acustico-facial ganglion. Pia, Pia mater.
Raph, Raphe of the medulla oblongata. Ven.III, Third ventricle -of the brain. Ven.IV, Fourth ventricle
of the brain. X 50 diams.

toward the median dorsal surface thereof, where it meets the correspondir -
of the opposite side, with \vhich it then unites to form a siagle me('
This vessel ultimately acquires great siz "^^ is known as tht m--x

dinal sinus. Tt is, V>\vn in figure 17?.
in the median 'ine, pc -ist to form

of the vessels
of the ;' '

266 STUDY OF PIG EMBRYOS.

in the embryo are all small branches of the veins when they first appear. Their
great enlargement does not occur until comparatively advanced stages. Finally!
attention should be paid to the following important modifications in the mesenchyma.
Already there has been a rich development of a plexus of fine blood-vessels over
the surface of both the fore- and hind-brain which has been accompanied by a
slight condensation of the mesenchyma between the blood-vessels, thus markirfg a
distinct membrane, in which we can easily recognize the pia mater, Pia. Outside
of the pia mater comes a relatively broad zone, Arach, in which the cells are
widely separated from one another and are connected by slender and long processes,
so that the intercellular spaces are very extensive. This broad zone is the' anlage of
the arachnoid membrane. It is much more differentiated around the ventral portion
of the brain than around the dorsal side. Between the arachnoid zone and the
external epidermis the mesenchyma is somewhat more condensed and the cells are
elongated in form, in part almost spindle-shaped, forming a layer, Cut, which we
may consider the anlage of the cutis, and perhaps, also, of the subcutaneous tissue,
but this is doubtful. Between the arachnoid zone and the cutis zone, so placed
that they cannot be quite said to belong to either one or the other, appear numer-
ous blood-vessels. These form a more or less distinct vascular layer, which ap-
pears with remarkable constancy in all classes of vertebrates, and over a large
part of the body. It may, therefore, be called the panchoroid. It is unquestion-
ably of very great morphological importance, but its history is imperfectly known.

As regards the histological condition of the tissues, the student should make
careful observations. Attention may be directed especially to the following points :
The epidermis at the sides of the section is two-layered and consists of an inner
layer of cuboidal cells, the anlage of the Malpighian layer of the adult, and of
an outer layer of very thin cells, the epitrichium, the nuclei of which are flattened
and appear darkly stained. Toward th^ medLli line, above the hind-brain and
T5eTow T^c * Tore-brain, the epidermis becomes gradually one-layered and much thin-
ner. The mesenchyma exhibits three principal forms of cells: First, those which are
equally branched in all directions, and represent a primitive form of the tissue.
Such may be found in the neighborhood of the basilar artery. Second, the elon-
gated cells of the cutis zone; and, third, the cells of the arachnoid zone above
described. The blood-vessels have very distinct endothelial walls which are very
thin, being thickened only to furnish space lor the nuclei, which, unlike those of
the adult, project not only into the lumen of the vessel, but also against the
surrounding mesenchyma. The red blood-corpuscles are rounded cells, some-
's oval, not infrequently somewhat distorted. Their nuclei are nearly spherical
a number of fine granules. Mitotic figures are quite frequent. A few
i are beginning to char -.- ' v becoming smaller and taking the stain
-onipare page 94). °rvous system the differentiation of the

Vain is mo1- n in the fore-braip but even in the

> Tve-cells and the young neuroglia

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM.

267

cells (neuroblasts and spongioblasts) is not very clear. The nuclei are only just
beginning to acquire distinct nucleoli, such as would be characteristic of later stages.
The nuclei of. the tissues differ markedly from those of the earliest embryonic stages,
but can scarcely be said to have assumed in any of the tissues adult characteristics

N.I i.

FIG. rqr. — Pic tj SERIES 5, SECTION IQS.

Card', Card", Cardinal vein. EC. Ectoderm, l-'.b. ' te-brain. / ••/, ; ''undibular gland. A'. 5, Trigeminal gan-
glion. iV-7,8, Acustico-fachl ganglim. N.< Ganglion- n of the glosso-pharyngeal nerve. .AT.ic "
jug, Jugular ganglion of the vagus i arm. A'.ir, Roo; • ><•' -ory nerve. Md, MedullaWf
gata. Ot, Otocyst. Sir. 'm.iii, . •! vi itriclp. Ven.iv, Fourth ventricle. X -

Section through
section 19 >, and, tii,
bring, out 'three >oinis n

of the Otocyst (Fig.
ions belo;

the

268 STUDY OF PIG EMBRYOS.

spinal accessory nerve, N.n, which arises from the cervical (in the figure upper)
end of the hind-brain and runs forward to join the vagus ganglion, N.io jug,
the jugular ganglion of the adult. 2°, the characteristic relations of the anterior
cardinal vein to the trigeminal ganglion, AT.5. The vein is cut twice, Card' and
Ctf^sC, for it curves around the ganglion, passing on the inside of the ganglion
between it and the wall of the brain. The original vein persists throughout life
in- this position, and enlarges into the cavernous sinus of the adult. Inside or
mesially of the seventh to twelfth nerves the cardinal vein is obliterated, and
is replaced by a new vessel produced by "island formation" outside these nerves,
and designated as the vena capitis lateralis. It is, as it were, interpolated in the
course' of the original vein, and this interpolation is the principal factor in trans-
forming the embryonic anterior- cardinal into the adult jugular vein. In the 12
mm. pig the vena capitis ^ateralis is formed outside the otocyst and of the seventh
and eighth nerves. Later it extends by more island formations outside the ninth
to twelfth nerves also. The jugular, therefore, is to be defined as . the anterior
cardinal vein which, by successive island formations, has migrated to a new posi-
tion outside of the otocyst and cephalic ganglia. 3°, to show the infundibular gland,
Inf, a small evagination from the ventral floor of the fore-brain, F.b. The evagi-
nation is really hollow, but the cavity does not appear in the section figured. It
enters into very close relations with another hollow evagination, which springs from
the dorsal roof of the oral cavity and is known as the hypophysis. The infundib-
ular gland and the. hypophysis become intimately associated with one another in
their further development and give rise to the pituitary body of the adult, the
gland becoming the posterior lobe the hypophysis the anterior lobe of that or-
gan. The- hypophysis may be best studied in sagittal sections (see page 292) .<•
.sent section, figure 191, being at a lower level than figure 189, passes through'
ventral portion of the hind-brain and shows only a narrow part of the. cavity
of the fourth ventricle, Ven.iv. The three lavers in the wall, Md, of the hind-brain
are very distinct. At the anterior end or <-ne hn.A 1 --^.v, ^T-,P~— ~ ^OK, <• light
lines, Str, which are nv^eH KV n'-'-e-nbers. These lines have been identified as
the stria a/>. .,.,.(.#'. They need to be more accurately studied, however, for they
seem r^Oier to be fibers of the lateral root of the facial nerve. Close to the ante-
-sio/'section of the cardinal vein, Card", appear the minute fourth and third nerves,
which, however, are not indicated in the figure. Both lie close to the wall of the
vein on the side away from the trigeminal ganglioi . The fourth nerve lies nearer
' • outside of the embryo, the third nerve nearer the median plane. At about
^e level as this part of the jugular vein, and very close to the wall of the
:* situal i ' -> loop of the internal Chrotic. Lower down, but not ciose
rn, Js the section of the lateral jugular.

"" ''•', // ntiil Optic i---j'igin"i: "\ (Fig. 192). — The
<>r cervical JCgi0n of the spinal cord, on
"" "-'rves. In this and the three

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM.

269

sections next following the complicated pharynx appears in various forms. The
general shape of the pharynx has been described with the aid of a figure of a wax
model of the pharynx made from the same series of sections from which these

D.Z

V.Z

L.R.II.

N.I 2.

L.V.

II

EC.

_ FIG. 192. — PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 249.

Card, Anterior cardinal vein. Car.in, Internal carotid artery. cl.I, First or auditory gill-cleft. D.Z, Dorsal zone
of spinal cord. EC, Ectoderm. H, Anlage of cerebral hemisphere. L, Lens. L~K.II, Lateral rooi of th«-
• iith nerve. L.V, Lateral ventricle. Mx.in, inferior maxillary nerve. NJ\ Facial nerve. X.y.petr*
1'etrosal ganglion of the ninth nerve. A7".io,iT, United vagus and spinal acdpsory nerve. A'. 12, Hypo-i
glossal nerve. Op, Stalk of the optic ^vaginal ion. Op.v, Ophthalmic vein.fPh, Pharynx. Ret. Rttina.
, .^, Ventral zone of spinal cord. X 22 chams.

figures are taken (compare Fig. 173, p. 237). The shaperbi the pharynx and of
its four pairs of latei ^ ..ches at this stage is remarkably' constant, so tha* the s*
dent is not likely to encounter any serious difficulty in /dentif)]
spinal co-,-tl is oval in the section. Its cavity has expinded in thi

270 STUDY OF PIG EMBRYOS.

lateral walls are quite thick, the median ventral wall is thinner, and the median
dorsal wall (deck-plate) is very thin. The three primitive layers of the medullary
tube are very clearly marked out, the ectoglia appearing light, the ependymal layer
appearing dark. The differentiation is much more advanced on the ventral side
of the spinal cord than on the dorsal side, and, indeed, it is only in the ventral
part that the three layers are perfectly differentiated. In the median ventral line
we have the floor-plate, in which we can distinguish only two zones, while in the
deck-plate there is no differentiation of layers whatever. The spinal cord is clearly
divided into a dorsal zone, D.Z, and a ventral zone, V.Z, on each side. The two
dorsal zones are connected across the median line by the thin deck-plate, and the
ventral zones similarly by the thin floor-plate. The lower or ventral limit of the
dorsal zone is marked by the entrance of the dorsal or ganglionic root and by
the fibers, which represent the outgoing lateral roots. In the actual section figured,
the lateral roots, L.R.n, are those which enter into the formation of the eleventh
icrve. The true dorsal root does not appear in the figure. Internally the division
Between the two zones is marked by the lateral angle of the central cavity shown
n the section. In the dorsal zone the differentiation of the three layers has made
slight progress. In the ventral zone, however, the development is far more ad-
vanced. The most characteristic feature of this movement is the growth of the
inerea or neurone layer, which increases in a twofold manner: first, by encroach-
ng upon the inner or ependymal layer; and, second, by the growth of its con-
tituent elements. Examination with a high power shows at once that the cells
tave grown very much. Their nuclei are larger, granular in appearance, rarely
/ith any indication of a distinct nucleolus. Most of the cells are neuroblasts and
ave well-marked protoplasmic bodies, finely granular in texture. They have -many
f them already produced long, slender outgrowths which we can identify as the
buraxons. T" order to study the distribution of the neuraxons and the form of
ae ne1 Tibia ii ;., necessary to apply the Golgi rapid method, by which it can
iiat a portion of the neuraxons is distributed entirely within the
liar iiile another portion passes out to form ventral roots, one of which,

[".12, forming part of the hypoglossal nerve, is shown in the figure. A third
m of the neuraxons, at least in the upper cervical region, as also in the
edulla oblongata, passes' out to form the lateral roots. The positions of the exits
these two bundles of nerve-fibers are constant and characteristic. The ventral
ot always passes- out from the middle of the ventral zone about half-way be-
een the median floor-plate and the dorsal limit of the zone. The lateral root
vays passes out at the u]_^er dorsal limit of the ventral zone and immediately
ow the point of enhance trf" the true dorsal root. Formerly the lateral roots were
distinguished from\the dorsal roots. Following downward in the figure we
V section of the cardinal vein, Card, just inside of which lies the common
T, of the un^ed tenth and eleventh, or vagus and accessorius nerves,
the lower, part of the petrosal ganglion, N.g.petr, of the glosso-

TRANSVERSE SECTIONS OF EMBRl L 12 MM. 271

-

pharyngeal nerve. Lower down and nearer the ectoderm lies the facial nerve,
N.'j, situated in what is called the hyoid arch or mass of tissue intervening be-
tween the first and second gill-clefts. The hyoid arch is further marked by a
bulge in the external^ outline of the section, which leads down into a deep groove
beyond which the outline of the section again rises and arches forward to the eye.
This groove is the external depression of the first gill-cleft and ultimatejy is trans-
formed into the external auditory ifteatus. The position of this groove is well shown
in figure 166, Au, on page 223. Just inside the auditory groove appears the outer
end of the first or auditory internal gill-pouch, cl.I. It is a long, oblique slit,
quite narrow, and is lined by a layer of entoderm. If we follow it along through
several sections, we shall find that higher up its outer end comes in contact with
the ectoderm at the bottom of the auditory groove, and there the two germ-layers,
entoderm -and ectoderm, unite to form a single membrane, the closing plate of the
gill-pouch. Following through the section downward in the series, we can trace
the cleft to its connection with the pharynx, Ph. On the posterior side of the
cleft we find the internal carotid, Car.in. Only the roof of the pharynx, Ph, is
cut, so that it occupies but a small area in the section. On its anterior side it
shows a small knob-like projection toward the floor of the fore-brain. This is
a part of the stalk of the hypophysis: Below the first gill-cleft appears the very
large and conspicuous inferior maxillary nerve, Mx.in, and beneath that the section
of the small ophthalmic vein, Op.v. The fore-brain is quite complicated in shape,
having two lateral expansions, L. V, of its cavity which are destined to form the
lateral ventricles. The walls, H, of the lateral ventricles are the anlages of the
cerebral hemispheres. From the ventral (in the figu ' of the fore-brain

spring on either side the optic stalks, Op. These arc hollow lions of the

brain, which expand at their distal ends to form the retina of
pigment layer. The expanded distal ends constitute each a sort of cup, of^'JliVl
optic stalk is the stem. The cup is two-layered, the space between the two layers be-
ing a prolongation of the central cavity of the brain. The inner of1 the two layers
forms the retina proper and is considerably thickened. The outer layer is quite thin
and is already quite abundantly laden with pigment granules. At .the edge of tb',
cup the pigment layer passes over uninterruptedly into the thick retina layer. Jn
the cavity of the optic cup lies the vesicular lens, L, which arose from an evagma-
tion of the overlying ectoderm. The vesicle is, however, now completely separated
from the layer which produces it. It has at this stage a very largei cavity, and in
cent sections it can be readily seen that the innc: side or that toward the brain
-eady thickening and changing its character so >;rn the main part of

adult lens. The thickening depends chiefly upon the rapid and enyimon
of the epithelial cells of this part of the vesicle, so that they

the so-called fibers of the adult K-ns, (. ndull

Section through -«id Gill-Cleft <. / Oral . c\cl oi

ihis section is such that tn head is cut separately jnd appears in sermon without

STUDY OF PIG EMBRYOS.

EC.

L.V.

N.cerv.i.

Ao.D.

Olf.

P.M.

FIG. 193. PIG, 12.0 MM. TRANSVERSE SERIES 5, SEC-HOX 292.

', Descending aorta. Card, Anterior cardinal vein, car.in, Intern.-)! Carotid artery. cl.II, Second gill-clef'
EC. Ectoderm. H, Anlage of cerebral hemispheres. L.gr, Lachrymal groove. L.V, Lateral ventricle of
brain. W •?.',, \I;mdibular arch. MX, Maxillary process. My, Myotome. nch, Notochord. N.cerv.i,
i- irst 'cervical nerve. N.j, Facial nerve. A". 9, Glosso pharyngeal nerve. Ar.icv Vagus nerve. Ar.n,
Spinal accessory nerve. O.F,.()ral fissure or s[)ace MRwi-en .he head and mandibles. Olf, Olfactory pit.
Pk, Pharynx. P.M, Pia mater. Sp.c, Spinal cord. t< 22 di.-,

f

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 273

connection with the body of the embryo. The space O.F, between the head piece
and body piece, may be designated as% the oral fissure, since it is into this space
that the mouth opens. In general there is considerable resemblance between this and
the section last described, but in the present section the eyes have disappeared
and we get the first indications of the nasal pits, Olf. That on the left side of
the body shows a trace of the cavity of the pit. The posterior part of the pharynx,
Ph, is cut in the section, instead t of the anterior part as in figure 192. The first
gill-cleft does not show, but the second cleft, cl.II, does. It lies posterior to the
first cleft and therefore appears higher up in the figure. The spinal cord, Sp.c,
shows the same general structure as in the previous section. On either side of it
may be seen the small and inconspicuous root of the eleventh or accessory nerve.
It could not be properly represented in the figure. Some distance below the
cord lies the small circular section of the notochord, which differs so slightly .in
staining from the surrounding mesenchyma that it cannot be well made out
without the use of a higher magnifying power. It is enclosed by a distinct mem-
brane which is thick enough to produce a double outline, and contains a consid-
erable number of scattered nuclei, which are, however, nowhere much crowded.
The nuclei are round in form, decidedly larger than those of the surrounding
mesenchyma, granular, and containing each several more conspicuous, darkly
staining granules. There is a very slight gathering of mesenchymal cells about
the notochord, as if to form the anlage of a sheath. Just below the noio<
there is a broad band of somewhat daTker staining, due to a greater conden?
of the mesenchyma in that region, and this condensation represents the beginning
of the formation of the vertebral structures. On either side we find the trans-
formed myotome, My, or anlage of the striated muscular tissue. This tissu
produced from the 'secondary somites of earlier stages. The cells have now sepa-
rated from one another, . have lost their distinctly segmentai grouping, and
begun to elongate into true muscle-fibers. All that can be distinguished
the low power is the somewhat darker appearance of this part of the section,
to the great crow'' at: of the nuclei. Between the muscular anlage and the noto-
chord ' shows a portion of the first cervical nerve, N.cerv.i, and just

; nerve is a small blood-vessel not represented in the figure. Th
a similar blood-vessel symmetrically placed on the opposite side. They ar^ the
small vertebral arteries. The anterior cardinal veins, Card, are large and conkpieu-
ous vessels, but despite their size they have merely endothelial walls and thl;
no condensation of thb^ mesenchymal cells around them, although such a I
densation is to take place later to form the anlages of the muscular and char
ive-tissue coats (media and adventitia) of the adult. On the dorsal side o* :'e
cardinal vein and close to it is a light spot in which can be easily distingu '
\vith the high power, several more or i-=s distinct bundles of nerve-fiber? whk
separated from one another by mesenchyma 1 cells. For this reason it is
\vhat difficul1 to recognize this ner\ viih ti T or to at it

1 8

274- STUDY OF PIG EMBRYOS. .

figure. On the ventral side of the vein there appears a darkly stained mass,
N.io, the nodosal ganglion of the vagus nerve, and outside of this ganglion is
the section of the spinal accessory nerve. Immediately below the nodosal ganglion
we have the internal carotid artery, car.in. A little to the inside of the jugular
is a small vessel, Ao.D, of great morphological importance. The corresponding
vessel appears on the opposite side. Although, very small, this vessel has a dis-
tinct coat of condensed mesenchyma around its endothelium. The two vessels
are the descending aorta, which have almost completely aborted, and in slightly
older specimens will be found to have disappeared altogether. The descending
aortae are the longitudinal trunks by which the dorsal ends of the five aortic
arches of early stages are connected together. The portion shown in this section
is the part of the descending aorta between the tops of the third and fourth aortic
arches. The relations are shown in the reconstruction (Fig. 172). The pharynx,
Ph, is narrow in its dorsal ventral diameter, but wide transversely, and offers the
very characteristic yoke-shaped figure in the section. The distal portions of the
second gill-clefts are shown, and they .appear disconnected from the pharynx,
the connection occurring in sections higher up. Each cleft is somewhat slit-like,
so that its cavity is an oblique fissure and somewhat parallel in position to the
first cleft (Fig. 192). Both the pharynx and the gill-clefts are, of course, lined
throughout by entoderm, which forms a sharply defined layer crowded everywhere
with nuclei, which are of about the same size as those of the surrounding mesen-
chyma. In the pharynx the entoderm is somewhat thinner on the dorsal than
on the ventral side. In the clefts it is thicker than in the pharynx proper, and
especially in the clefts it may be observed that the mitotic figures always occupy
a superficial position. On the dorsal side of the cleft is a very small blood-vessel,
near which, with a higher power, one may see a small nerve, and nearby, but
more dor-salward, a second nerve. Both of these are branches of the glosso-
pharyngeus, and lie behind the cleft. They are, therefore, termed the post-trematic
branches. Below the cleft and somewhat on its median side is a similar third
nerve-branch, the pre-trematic of the glosso-pharyngeus, running in front of the
cleft. The outline of the embryo forms a rounded eminence outsidf^of the second
cleft; it represents in part the hyoid arch. In the midst of .the mesodenii ".$ this
appears a • light area with a few nerve-fibers, the end of the facial nerve, A . ,
The mandibular arch or process, Mdb, is very distinct and prominent. It is
separated from the hyoid arch by a deep external notch, which corresponds to
the external first or auditory cleft. In the interior of trie mandibular process
therct are light spaces differing in their exact distribution on the two sides of 'the
These spaces contain n live-fibers and they represent the inferior maxil-
. We now come to the oral fissure,/0.jF, which separates the body *

he head. In the head portion of the section we have the maxillary process*
•hich is separated in part from the rest of the head by the deep lachrymal
L.gr. On either side there/shows a shaving from the epithelium of tse '

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 27 5

olfactory chamber, Olf. The fore-brain has expanded laterally, L.V, to form the
lateral ventricles, the walls of which, H, are the anlages of the cerebral hemispheres.
On the dorsal side, which is the lower side in the figure, the hemispheres project
somewhat, leaving a median space between them. This median space is filled
with mesenchyma, which may already be regarded as the anlage of the falx. In
the tissue of the falx are two very small blood-vessels, the forward prolongations
of the lateral jugulars, which are to unite to form the median superior longitudinal
sinus. In the previous section these vessels also reappear, but are already united
(Fig. 192). In the median dorsal line the wall of the fore-brain is thin and shows
a characteristic notch. Close to the surface of the fore-brain there is a very dis-
tinctly marked vascular layer, the commencing pia mater, P.M, and with a high
power it can be easily seen that the differentiation of the arachnoid zone has
already begun.

Section through the Third Gill-Cleft and Nasal Pits (Fig. 194). — In this section
the head is clearly separated by a considerable space from the rest of the section.
The transverse diameter of the embryo is here much less than higher or lower,
so that the section as ay-whole seems somewhat narrow. It shows the entire
length of the third gffl^cleft, cl.iii, exhibiting, on one hand, its connection with
the median pharynx, and, on the other hand, its 'dorsal extremity, where its ento-
derm joins the ectoderm. The external outline of the embryo makes a deep de-
pression outside the end of the third cleft. This depression is the cervical sinus
(compare Fig. 166, C.S; pig of 10 mm.). . In the section the cervical sinus dis-
plays a narrow downward prolongation. If followed through in the series of
sections, this prolongation, which is on the inside of the hyoid arch, Hy, will be
found to connect with the second cleft. The spinal cord, Sp.c, presents essen-
tially the same structure as in 116 and 117. Our section passes through
the roots of the second cervical 'frv.2, and shows both the dorsal gan-
glion and the ventral rooi I zone. These two roots join
and form the nerve-trunk, 'ivides, sending one
branch vertically upward into a m;: < cells (the aniage of the
dorsal musculature) and a ventral branch which descfcods a!rpo>
toward the pharynx. Just inside of this ventral branch ''^^H
vertebral artery, Art.v. Between the dorsal summit of the ganglio-
cord there is a minute bundle of nerve-fibers not shown in the fi^.
fibers constitute the commissural trunk of the eleventh nerve. The third gill-
cl.iii, is cut almost symmetrically, and extends from t-he median line to the edge
of the section. It is lined throughout by the entoderm, which at the end of the
cleft on each side has met and fused with the ectoderm to form the epithelial
membrane, the closing plate. The membrane apparently normally remains intact
in mammals. In the ichti: -mbrane bee 1 ! ; --'rxg em-

• ft is OP -he cleft

'he id'.-rgonc a special .on

276

STUDY OF PIG EMBRYOS.

the side of the cleft toward the head. This structure is the anlage of the
nodulus thymicus and is already penetrated by small blood-vessels which are
perhaps not capillaries, but sinusoids. The fate of nodulus is uncertain; it
probably forms the head of the thymus, and not the carotid gland as some

Fourth aorti

FIG. 194. — PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 321.

Fourth aortic arch. Art.v, Vertebral artery. AH, External auditory cleft. Card, Anterior cardinal vein.
cl.iii, Third internal gill-cleft. G.nod, Ganglion nodosum. Hy, Hyoid arch. L.V, Lateral ventricle.
.\'<i. Nasal pit. N.cerv.i, First cervical nerve. N.ceru.2, Second cervical nerve. N.II, Spinal accessory
nerve. IV. 12, Hypoglossal nerve. N.od, Nodulus thymicus. Nv,2, Main trunk of second cervical nerve.
Ol.pl, Olfactory plate. R.ex.n, Ramus exter,nus of the spinal accessory. Sp.c, Spinal cord. Th\r,
Thyroid. Tr, Trachea. X 22 diams.

have suggested. Thfc student should clearly understand that the median region

of the third gill-cleft is really the pharynx proper. From its median

ventral line ' arises the beginning of the trachea, Tr, which shouU, perhaps,

designated as the anlage of the larynx. The entoderm yxtends down

!

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 277

in the median line for a considerable distance, making a figure which, in the
section, is shaped somewhat like an inverted spear-head. In the center of the
section appears a small cavity. Farther down toward the lungs we have only an
epithelial plate with no cavity observable in it (Fig. 195, Tra), the entoderm of
the trachea at this stage forming a solid cord. Ventrad from the trachea, in
the median region and between the two aortic arches, is a small, irregular, deeply
stained mass of cells, Thyr, the anlage of the thyroid gland. These cells are ento-
dermal, the anlage having been developed by a downgrowth of the epithelium of
the floor of the pharynx, although at the present stage the original connection with
the pharynx has been lost. The anlage is now isolated from its parent germ-layer
and is imbedded in mesenchyma. It is solid, for the cavities of the thyroid follicles
are not developed until considerably later. Just above the third gill-cleft may
be seen a large, darkly stained mass, G.nod, the ganglion nodosum of the vagus
nerve. Immediately above it is a section of the anterior cardinal vein, Card.
Between the ganglion and the vein is a bundle of nerve-fibers representing the
twelfth or hypoglossal nerve, which reappears again in the section below the
pharynx, at N.I2. The reason for this double appearance of the hypoglossal
nerve may be seen readily by examination of the reconstruction (Fig. 178). Close
to the ganglion on its outer side is the section of the spinal accessory nerve, N.n.
A little above the jugular vein is the section of the first cervical nerve^ N.cerv.i,
laterad from which is the external branch, R.ex.n, of the spinal accessory nerve.
This branch in the adult innervates the sternocleidomastoid and trapezius muscles.

The lower part of the figure represents the section of the head and shows
the two nasal pits, Na, closed toward the mouth side by the olfactory plate, Ol.pl,
the epithelial membrane somewhat resembling the closing plate of a gill-cleft,
but it is formed by a fusion of the ectoderm on the two sides of the opening
of the nasal pits. When the nasal pits are first formed, they are open throughout
their whole extent. The formation of the • y plate is the first step toward

the separation of the two nasal cavities from i ravity. In later stages this

plate disappears, and its forward portion is replace m:hy ni, so that the

separation of the nasal and oral cavities is permanent posterior portion

of the membrane becomes first very thin, and finally di^ r'-tiier, *'.,. -

establishing a secondary connection between the nose and mo,
chamber. On the dorsal side of the nasal pits (below in Fig. 1(4..
hemispheres are cut separately, their darkly stained walls bounding on <_
the large lateral ventricle, L.V.

Section through the Fourth Gill-Cleft (Fig. 195).— Of the entodermal gill-cleft?
or -pouches the fourth is by far the smallest, and as it appears in sections (rl.l \' :
is inconspicuous. The section figured differs by two striking features from 'those
of the series above described: first, because the head is no longer included; and,
second, because the cardiac structures are beginning to show. On the dorsal side
the spinal cord "is cut at the ievei of U.L ganglion, (7. 3, of the third cervical nerve.

278

STUDY OF PIG EMBRYOS.

The dorsal root of the ganglion joining the spinal cord, Sp.c, is shown on both
sides of the section, and the nerve itself also appears, being best shown on the
left side of the embryo, where a short piece, R.D.-$, of the ramus dorsalis is
included and a much longer piece, ^.^.3, of the ramus ventralis. Just inside of
the nerve at the level of the notochord, Nch, is the cross-section of the vertebral
artery. On the right side of the embryo the section passes through a portion o.f

Sp.c.

FIG. 195. — PIG, I3--.0 MM. TRANSVERSE SERIES 5, SECTION 353.

Ao, Aorta. Ao.4, Fourth aortic a>vh. Au.d, Right auricle. Au.s, Left auricle. Card, Anterior cardinal vein-
Cos, Coelom. d.IV, Four*" gill-pouch. £.3, Ganglion of third cervical nerve, msth, Mesothelium. N .
ctrv.2, Second cervical nerve. Nch, Notochord. N.IO,II, United vagus and spinal accessory nerves. P. A,
Pulmonary artery. PA, Pharynx. R.D. 3, Dorsal ramus of the third cervical nerve. R-V.$, Ventral ramus
•• ' cervical nerve. Som, Somatopleure. Sp.c, Spinal cord. Sym, Sympathetic nerve chain. Tra,
Trachea. Ve, Vein to lower jaw. X 22 diams.

the second cervical nerve, N.cerv.2. The anterior cardinal, Card, is a very large
vessel. Close to its ventral wall appear a few fibers which represent the first cer-
vical rterve, but they are too indistinct to be represented in the figure. They may
easily be found with the higher power. In the median plane is the crescent-shaped
section -of the pharynx, Ph. Between the jugular vein and the pharynx lies the
fourth aortic arch, Ao.^. The right and left arches are at this stage about equal in
size, although the left arch is destined to form the main aortic arch of the body,

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 279

and only a portion of the right arch will persist to form a portion of the stem of
the pulmonary artery. The figure indicates the manner in which these aortic arches
pass up from the heart below on either side of the pharynx. A little above the
aortic arch on either side may be seen a small, round spot, Sym, which is
somewhat conspicuous on account of its deeper staining. It is a section of the
cervical sympathetic. Examination with a higher power shows that it consists of
somewhat crowded cells, some of which have larger nuclei. These are the neuro-
blasts. The mesenchymal cells immediately around the anlage are disposed about it in
somewhat concentric lines. Between the cardinal vein and the aortic arch is situated
the large, conspicuous nerve-trunk, N. 10,1*1, constituted by the united vagus and
spinal accessory nerves. Below this double nerve is a blood-vessel, Ve, a branch
of the cardinal vein. This vessel drains the tongue and facial region of the em-
bryo. It is homologous with the inferior jugular vein of lower vertebrates, and
in mammals gives rise to the lingual and facial veins of the adult, and in some
species forms the external jugular, but the human external jugular is a secondary
anastomosis between the linguo-facial and the junction of the internal jugular and
subclavian veins. The homologies between this vein and those of the adult have
not yet been worked out. Returning now to the pharynx, Ph: on the right side
the prolongation of the pharynx to join the fourth cleft can be clearly followed; on
the left side of the embryo, the right of the figure, the fourth cleft, cl.IV, does
not display its connection with the pharynx, but is a separate, small, epithelial
cavity lined by a cylinder epithelium. Underneath the pharynx appears a vertical
plate, Tra, formed by the entoderm of the trachea. This plate is thinnest in the
middle, somewhat wider toward the top and bottom of the section. It
solid, except for a minute cavity at its dorsal end. This minute ity may be
traced from the opening of the glottis through the series of sections down until it
becomes connected with the comparatively large cavities of the developing bronchi
of the lung. Below the pharyngeal region descends the thick somatopleure, Som,
which encloses the pericardial coelom, Cce, in which the heart is lodged. The inner
surface of the somatopleure is covered by the thin mesothelium, msth. Of the
cardiac structures we note first the section of the main aorta, Ao, and of the
pulmonary aorta, P. A, and finally small sections of the uppermost part of the .two
auricles, Au.d and Au.s. More of the left auricle is included in the section
of the right.

Section through the Anterior Limbs and Heart (Fig. 196). — The section figured is
much lower in the series than the last and was selected in order to illustrate the
anterior limb-buds, the common cardinals, and the heart. The position and shape
of the limb-buds are sufficiently shown in figure 166.' The section demonstrates that
the limb-bud is formed chiefly by a dense mass of mesoderm covered by a thin
layer of ectoderm. The mesoderm consists of very much crowded fells in which it
is very difficult to recognize any distinct differentiations, yet it is probable that
here are mingled both true mesenrhymal cells and cells which originally belonged to

280 STUDY OF PIG EMBRYOS.

the muscle-plates, but which have now broken apart and are developing singly into
muscle-fibers. In certain amphibia the cells from the muscle-plate can be distin-
guished from the mesenchymal cells of the limb, and what we know of the devel-
opment of the muscles in amniota confirms the view that striated muscles and mesen-
chyma are genetically entirely distinct. No skeletal elements whatever have yet arisen
in the limb. We have here a striking illustration of the fact that the skeleton is
very late in its development, and, embryologically speaking, is in no sense the frame-
work upon which the body is built up, but rather a late supplementary develop-
ment. The main morphological features in all parts of the embryo are entirely
fixed by the soft tissues before the skeletal structures arise. Both nerves and blood-
vessels have grown into the limb. The nerves are the ventral branches of the
spinal nerves. Several of these unite together and form the brachial plexus, one
part of which, Br^Plx, is shown in the, section. In the present embryo this nerve-
trunk includes fibers derived from both the sixth and seventh cervical nerves. Just
above the nerve-trunk is the section of the subclavian or axillary vein, which is a
branch from the jugular. The dorsal region of the embryo is relatively larger at
the level of this section than higher up, owing chiefly to the great development of
the mesoderm. The spinal cord, Sp.c, resembles that in figure 195, but is both larger
and more differentiated. On the left side of the embryo the fundamental morpho-
logical characteristics of the spinal nerve are well illustrated in this section. The
dorsal root, D.R, bears the ganglion, G, which joins the dorsal zone of the spinal
cord. The fibers of this root are produced from the cells of the ganglion and grow
from the ganglion into the spinal cord. Other fibers from the same cells grow out
K the opposite direction and form the nerve-trunk or root which descends from the
ganglion in a nearly vertical direction. The ventral root, V. R, arises from the ven-
tral zone, takes an oblique course, and joins the dorsal root a little below the level
of the spinal cord to form a single nerve-trunk, which, however, soon subdivides into
its two primary branches. The first or dorsal branch, R.D, bends at an acute angle
upward and outward. The second or ventral branch, ramus ventralis, continues
dowttWP^d and curves into the limb. Owing to this curvature, in order to follow
its cayrse the nerve must be traced through adjacent sections. If this is done, the
venial ramus will be found to take part in the formation of the brachial plexus.
Some- distance below the spinal cord is the small notochord. Farther down, but
also in the median line, appear two small rings of epithelium. Of these, the smaller
upper one, (E, is the entodermal lining of the oesophagus, and the larger lower one
is the entodermal lining of the trachea. Around each of these rings there has already
occurred a\ slight condensation of the mesenchyma, the first step toward the ulti-
mate differentiation of the submucous and muscular coats of the oesophagus and
trachea. The \entoderm of both the oesophagus and trachea is a moderately thick
layer composed of elongated cells, the nuclei of which are distributed at various
levels, but so as to leave the superficial portion of the layer comparatively free. It
in to.is superficial portion that the mitotic figures always occur. On the ven-

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM.

281

tral side of the trachea and somewhat toward the left, but quite close to it, ap-
pear two small blood-vessels, the pulmonary arteries. They are so small that their
lumina scarcely show in the figure. Two sections nearer the head, the two ves-
sels are found united in a single stem. Opposite the arteries below the trachea

Sp.c.

D.R.

Nch

F.

Som.

FIG. 196. — PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 470.

Ao.S, Left descending aorta. Au.d, Right auricle. Br.Plx, Brachial plexus. C.C.S, -Vena cardinalis communis
sinislra. D.R, Dorsal root of spinal nerve. F, Cardiac fissure. G, Spinal gangfion. L, Anterior limb-bud
Nch, Notochord. Nv, Branch of brachial plexus. (E, (Esophagus. R.D, Ramus dorsalis of spinal nerve.
S.a.c, Septum of the auricular canal. ScLV, Subclavian vein. Som, Somatopleure. Sp.c, Spinal cord.
S.s, Septum superius. Tra, Trachea. Val, Auriculo-ventricular valve. Ven.S, Left ventricle of the heart
V.R, Ventral root of spinal nerve. X 22 diams.

a minute opening, not shown in the figure, marks the tip of th
chamber. To the right and left of the oesophagus appear th
of the two descending aorta-, of which the left, Ao.S, is alread'
the right. Ultimately the greater part of the right •/

iwal

282 STUDY OF PIG EMBRYOS.

of the adult being formed from the left aorta. Lower down in the series the two
descending aortse unite to form the single median dorsal aorta. The common
cardinals, C.C.S, are two enormous venous trunks which deliver the blood to the
heart. They lie symmetrically placed to the right and left of the oesophagus and
trachea. They extend from the level of the descending aortae downward and inward
to the level of the heart. The vein of the left side, C.C.S, is almost symmetrical
with its fellow of the right side, though it has no direct communication with the
heart; but by following down through the series of sections the student can
observe that the left common cardinal connects across with the corresponding vein of
the right side. The right vein opens directly into the right auricle, Au.d, of the
heart. All of the venous blood is collected at this stage by the common cardinals,
except that which comes through the liver. The common cardinals are formed by
the union of the jugular or anterior cardinal vein from the head with the posterior
cardinal vein from the body. The opening of the right vein into the auricle of
the heart is guarded by two small flaps or valves. The lower part of the section
is occupied by the large heart lying in the pericardial chamber. The body-wall,
Som, or somatopleure, which forms the outer covering of this chamber, is quite thin
and without a trace of muscular or skeletal structures. It consists of three distinct
layers — the external ectoderm, the middle mesenchyma, and the internal mesothelium.
The mesothelium is a thin layer of cells which persists throughout life and is
known in the adult as the pericardial epithelium. In the present section it is easy
to follow this layer from the somatopleure past the common cardinals on to the
heart and completely around the outside of the heajrt itself. Everywhere it forms
the covering or boundary of the ccelom of the pericardium. In later stages this
mesothelium will have an especial layer of connective tissue close under it.- The
layer of connective tissue, together with the mesothelium, constitutes the pericardial
membrane of descriptive anatomy. The essential fundamental relations - of this
membrane may, therefore, be easily understood from the present section. From
the study of the adult conditions alone it is extremely difficult for the student to
grasp these relations. The heart is a very large organ. It consists of two auricles
and a ventricle with two limbs. The auricles have thin walls and are separated
from one another by a very thin membrane, the septum superius, S.s. The right
auricle, Au.d, receives upon its dorsal side the opening of the right vein or common
cardinal, the opening being guarded by valves. Of these valves, the one toward
the median line disappears, but the other, toward the right of the embryo, per-
sists to form both the Eustachian and Thebesian valves of the adult. As stated
above; the left common cardinal delivers its blood to the right vein, and so indi-
^eart. The ventricles of the heart are much larger than the auricles,
•icular limb or future left ventricle, Ven.S, is already larger than
external groove, F, which marks the boundary between the
^vn by the section. The trabecular structure of the
'"fords a diagnostic n^ark by which the ventricles.

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 283

if they are cut, may be easily recognized in sections. The development of the
trabeculae corresponds to the formation of blood sinusoids of the heart. The
trabeculae consist of young muscle-cells, and each bundle of cells is closely invested
by the endothelium of the heart. The blood thus circulates freely between the
trabeculae, but remains, as in every blood-channel, separated by the endothelium
from the neighboring tissue. The tissues of the heart are thus enabled to get
their nourishment from the blood circulating through the organ. The sinusoidal
type of circulation which we here encounter appears in the heart of all vertebrate
embryos, and is the permanent form of circulation in the frog. In mammals, on
the other hand, although the sinusoidal circulation is kept throughout life and
the ventricles always have their trabecular structure, yet we find, in addition, the
development of a true capillary circulation to supplement the sinusoidal. This
capillary circulation is supplied by the coronary arteries, and develops compara-
tively late. Between the auricles and the ventricles the heart is narrow. This
constricted region is known as the auricular canal. A broad partition, S.a.c,
divides the cavity of the auricular canal into right and left channels, forming open
vessels between the auricles and ventricles. From the lower edges of these channels
flaps of tissue project into the ventricles. The flaps are the anlages of the atrio-
ventricular valves.

Sections through the Anterior Limbs to Show the Brachial Plexus (Fig. 197).—
Figure 197 was drawn from a single section, except that the nerves in the limbs
represent a reconstruction from several adjacent sections. The limb-bud, A.L,
projects freely from the side of the body, is covered by ectoderm, EC, and filled
with a very dense tissue, the cells of which show no very clear histological dif-
ferentiation. The spinal cord, Sp.c, is fairly well advanced in its development
at this level, and shows a darker, inner layer, Epen, a middle gray layer, cm, and
an outer neuroglia, Ec.gl. The cord is completely surrounded by the developing
pia mater, which is quite thin, but highly vascular. The ganglia are cut almost
symmetrically on the two sides and show their dorsal roots. The descending
trunk from each ganglion is joined by the ventral roots, V.R, which arise from
the ventral zone of the cord in several bundles which unite about the same time
with both one another and the dorsal root to form the main nerve-trunk, N.S,
which enters into the formation of the brachial plexus. Just after the junction
of the two roots the nerve gives off a branch which runs obliquely dorsalward
into the anlage of the dorsal muscles, Muse. This branch is, of course, the dorsal
ramus. The trunk, N, which runs toward the limb is the ventral ramus. Below
the spinal cord is the notochord, Nch, which is completely surrounded by a dense
mass of mesenchymal cells, Vert, the anlage of an intervertebral disc. Triy^
in the section are the two descending aortas, Ao, which are "t
uniting to form the single median dorsal aort"*. On either si<?*
of darker cells from the aorta upward toward the later'
vertebral disc. The dark cells belong to the sympathc'

STUDY OF PIG EMBRYOS.

w

2 I c

— DH eS

3 o *s

6* *.:

.S b 4J

in-S* o> .S

U

o 5 •g

J3 C 'c

^ 5i aj

ao-^ >

z >
•^ —

~

~ -5

5 °

^ i-

o .£

• Ui

CS 1)

M tn

„ O

O P-i

cS en 4J

'0

S

o .2

ff.2^ E

7, u "7i

"M 2 rt

W H £

c ~

C ~ • T3 c

-^ ^ l« TO

u u C •"

W £ - ^

,»/•• ri CS N"

^b 3 C N

^ ^ '^ -"

^N Z CO f~,

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 285

accompanied by nerve-fibers. Below the aorta runs a ring of epithelium, (E,
representing the entoderm of the oesophagus, and farther ventralward a second
layer of epithelium, Tra, the entodermal lining of the trachea. Both of these
rings of epithelium are surrounded by somewhat condensed mesenchyma, the
differentiation of which about the oesophagus is more advanced than about the
trachea. Around the oesophagus next to the epithelium is a thin, looser layer of
mesenchyma, the anlage of the mesodermic portion of the future mucous membrane,
and perhaps also of the submucosa. Outside of this looser mesenchymal envelope
is a second denser layer in which the cells appear elongated, having begun their
differentiation into smooth muscle-cells. To the right and . left of the oesophagus
and at a slightly lower level lie the sections of the vagus nerves, the right nerve
being situated a little higher than the left. To the right and the left of the aorta
appear the very large posterior cardinal veins, card. From the sides of the trachea
project lobes of tissue which represent the anlages of the lungs. These lobes of
tissue are each covered by a layer of mesothelium, and protrude, as it were, into
the ccelom of the pleural cavities, ^leur. Farther to one side the ccelom, Coe,
of the abdominal cavity is also in part shown. It is bounded externally by the
body-wall, Som, of the embryo. Below the trachea in the median line is a small
blood-vessel, a section of the pulmonary vein. As regards the great nerve of the
limb, N.S, it must be remembered that it forms a portion of the brachial plexus
and is joined by other cervical nerves. From the voluminous trunk thus developed
there arise three principal branches; the first, xx, is at the base of the limb, is
small, and runs off dorsally. The other two represent a terminal forking of the
nerve-trunk, one, yy, running to the dorsal side of the limb, the other, zz, to
the ventral side.

Section through the Stomach and Liver (Fig. 198). — We now pass to a r section
well below the heart in order to study the characteristics of the Wolffian body,
stomach, and liver. At this level, as comparison with, figures 194 and 196 will
show, the body of the embryo has its greatest dimensions. The upper edge, Um,
of the umbilical cord appears in this section. The spinal cord with its ganglia
and nerves presents essentially the same features as in figure 196. The notochord,
Nch, forms a small circle in section and is surrounded by the anlage of an inter-
vertebral disc, which appears as an area relatively large, over which the mesen-
chymal cells are more crowded or condensed than elsewhere. At its peripliery the
anlage merges without divisional boundary into the surrounding niesencb' ui.
It is more expanded laterally than ventrally. In the median line belc noto-

chord is the large dorsal aorta, Ao, which is formed by the union of the two
descending aortae shown in figure 196, and which extends h the abdominal

region of the embryo to the pelvic region, where it fork- ;m the two allantoic

arteries, which, passing on either side of the intestine, Lie their course al<«ig

the side of the internal allantois or future blade1 , jiey reach the umbilicus,

where they enter the umbilical cord to su^ * -a -embryonic or placental

286

STUDY OF PIG EMBRYOS.

X

circulation. The aorta is surrounded by mesenchyma, and to this are, so to
speak, appended the large Wolffian bodies, W.B, one on each side. From the
dorsal region of the embryo to the umbilical cord extends the somatopleure or
body-wall, Som, which, like that around the pericardial chamber, consists of an

•Sp.c.

V.U.D.

Um

Om.min.

G.bl.

V.U.S

FIG. 198. — PIG, 12.0 MM. TRANSVERSE SERIES 5, SECTION 633.

Ao, Dorsal aorta. EC, Ectoderm. G, Spinal ganglion. G.bl, Gall-bladder. Gen, Anlage of genital gland.
Li, Liver, mes, Somatic mesenchyma. msth, Somatic mesothelium. ^Y, Spinal nerve. Nch, Notochord.
Om.maj, Omentum ma jus. Om.min, Omentum minus. Som, Somatopleure. Sp.c, Spinal cord. St,
Stomach. Um, Umbilical cord. V '.card, Posterior cardinal vein. V.C.I, Vena cava inferior. Vert, Anlage
of w. '?-;... V.U.D, Right umbilical vein. V.U.S, Left umbilical vein. W.B, Wolffian body. X 22'
diams.

external ectoderm, Ec^ middle mesenchyma, mes, and an internal mesothelium,

'•sth. It is important fur\ the student to understand the arrangement of the germ-

rs in the sonutopleureX The mesothelium is commonly known in the descrip-

:natomy of the :-<Hnlt ,,s the peritoneal epithelium. The peritoneal membrane

TRANSVERSE SECTIONS OF EMBRYO OF 12 MM. 287

V '

consists of this epithelium and of the underlying connective tissue. In sections like
that figured it can be readily followed not only over the inner surface of the body-
wall, but over the surface of the Wolffian body and liver, and upon the left side
of the body also over the surfaces of the greater omentum, stomach, and lesser
omentum. The relations of the abdominal viscera to the peritoneum, which are
so perplexing to the student of adult anatomy, are here shown diagrammatically,
as it were, by the section of the actual embryo. It is evident from such a section
that the abdominal cavity (splanchnocele) is completely bounded by mesothelium,
and that all the abdominal viscera are outside of the cavity. This conception,
which is so important yet so difficult to the student of anatomy, is easily mastered
by the study of embryonic relations. The Wolffian body, W.B, is the fetal or
embryonic kidney, and is also termed the mespnephros (compare page 109). Rela-
tively to other parts, it is much larger in the pig than in man or the rabbit. It
consists of" numerous epithelial tubules very much contorted with blood spaces
between them, of glomeruli which always lie toward the median and inferior side
of the organ, and, finally, of a single longitudinal canal, the Wolffian duct, into
which all of the tubules open. The tubules are formed by the cuboidal epithelium.
The glomeruli resemble in their structure those of the kidney. Each is a bunch
•of blood-vessels covered in by a layer of epithelium which forms one boundary
of the space into which' the glomerulus projects. The opposite side of the space
is also bounded by epithelium, which at the stalk of the glomcrv-'us becomes
continuous with the covering of the glomerulus itself, the whole structure resem-
bling closely that of a Malpighian corpuscle of the true kidne". The space around
each glomerulus is really the beginning of a Wolffian tubule The spaces between
the tubules are almost entirely blood-channels, and are lined by enddtKeliui.
for the most part, is closely fitted against the epithelium of the tul lies. Ov_
ally a small amount of mesenchyma can be found between the tubules, or
the tubules and the nearest endothelium. We have, accordingly, in tilts-,- <>;
a. typical sinusoidal circulation. The blood spaces of the Wolffian body really
belong to the posterior cardinal veins into which the Wolffian tubules in the course
of their development have, as it were, penetrated, although without destroying the
continuity of the vascular endothelium It is by the intercrescencf of the tubules
and of the endothelium that the sinusoidal condition is establish^-
the original channel remains on the dorsal side of the Wolffiar mu.

less free, V '.card. We thus learn that, owing to the developmen of ilu- Wolffian
body, the posterior cardinal veins as such disappear. The Wolffian di,
on the ventral side of the organ, and can easily be traced through as a continuous
tube from section to section. In the figure it may be easily found in the left
mesonephros, it being there the lowermost of the cavities drawn in the organ.
On the median lower surface of the Wolffian body, underneath the glomeruli,
is an accumulation of tissue. Gen, the anlage of the genital gland, which is yet
very slightly advanced. Below the " -n HUM, side of the embryo is a

288

STUDY OF PIG EMBRYOS.

large trunk of the vena cava inferior, V.C.I, on its way past the right dorsal lobe
of the liver. Near the aorta on the left is the mesogastrium, Om.maj, or future
great omentum, by which the stomach is suspended from the median dorsal wall
of the abdomen. The stomach, St, is entirely upon the left side of the body and
is directly connected with the liver by means of the anlage of the lesser omen-
tum, Om.min. The walls of the stomach are constituted by the splanchnopleure,
and, therefore, comprise a layer of thickened entoderm, which bounds the cavity
of the organ, and a relatively thick layer of mesoderm which forms the greater

& f***

© 0G>© e«<%r® <=
- '

Hep

Mes

Ves.ep

Msth

FIG. 199. — SECTION OF GALL-BLADDER OF A 14.0.1™. PIG. FRONTAL SERIES 67, SECTION 171. -
Hep, Hepatic cells. Mes, Mesenchyma. Msth, Mesothelium. Ves.ep, Epithelium of gall-bladder.

t,.ui' JL the wall, and the very thin superficial mesothelium. The entoderm is a
smooth layer of moderate thickness composed of elongated epithelial cells. It
:orms no folds and shows no trace of differentiation into gastric glands. In the
nesenchyma there are some capillary blood-vessels. The mesothelium is thicker
.han over the liver and somatopleure, and contains crowded, more or less nearly
>pherical nuclei. The liver, Li, is by far the largesT organ of the body. It
.ip nearly half the section. It is divided into four main lobes, the two dorsal -and

:ral; two on the right and two on the leu. The reference line. Li, :
ie left dorsal lobe. The liver consists of a complicated network of n

SAGITTAL SECTIONS OF EMBRYO OF 12 MM.

291

'-n

<^
<o

a

^

•^

^"g O u-
"j|

^o"cB |

B S »

M *

w" 'c ^

Q -c

0-li

.2 T3

o- g

oj .0

I e e II

5 * H "8: ^

hs a dj ^>

u

C/! r"
W U

nj

t; ,

O C

< '3

S d -c

"I |

Sl-2

Mfe

os p a

U >H 5

*r^ LL< *^
•B ~ *

a st

'a f

^§5

1 11:

1 s^

s &i

i« *..

3 ^

c O

E BJ

n e a

-c c ^

a, 3 ^

u — D

O 3 *•••<
C X3

.2 3 .-

C ^T1

t; -5

a. £
/j eg

VJ

^X

3 C

-

292 STUDY OF PIG EMBRYOS.

of its various parts to one another. The hind-brain begins at the spinal cord,
Sp.c, and has a very large cavity, the fourth ventricle, Ven.IV. It is separated from
the region of the mid-brain by a constriction which is very marked on the dorsal
side, Isth. The constriction is known as the isthmus. It is always from the dor-
sal side of the isthmus that the fourth nerve takes its origin. It is one of the
fixed landmarks of the brain. The mid-brain, M.B, also has a large cavity, and,
as a whole, forms the great arch which corresponds to the head-bend of the em-
bryo. It passes forward and downward, without any very definite line of demarca-
tion at this stage, into the fore-brain, the cavity of which is larger in diameter
than that of the mid-brain. The fore-brain is partially subdivided into two regions;
the anterior, Pros, is the prosencephalon and gives rise to the lateral outgrowths
which form the cerebral hemispheres. Already the deep depression separates this
part of the fore-brain on its dorsal side from the posterior part, which is termed
the diencephalon. The limits of the diencephalon at this stage are very indistinct;
later its boundary against the mid-brain becomes clearly marked by the differentia-
tion of the epiphysis and posterior commissure. The spinal cord, Sp.c, forms almost
a rightv angle with the axis of the hind-brain. This angle marks and corresponds
to the neck-bend of the embryo. On its dorsal side the hind-brain has a thin epen-
dymal roof, epen, which, however, toward the isthmus thickens considerably to
produce the anlage, Cbl, of the median portion of the cerebellum. On the ventral
side the wall of the hind-brain varies in appearance. Where the section is exactly
median, it displays the raphe or floor-plate of the region. Where it is off the me-
dian plane, it shows instead the thicker, lateral wall of the medulla oblongata.
The walls of the mid-brain on the dorsal side, Q, are almost uniform in thickness
and texture. They are, however, later to be differentiated into the corpora quadri-
gemina. The ventral side of the mid-brain, Ped, is considerably thicker than the
dorsal, and forms a strongly marked arch. It is represented in the adult essen-
tially by a part of the peduncle of the cerebrum. The floor, Dien.fl, of the dien-
cephalon is a thin membrane of which the part nearest to the mid-brain will pro-
duce the mammary bodies, and the part farther from the mid-brain the tuber
cinereum. It has already formed a special outgrowth, Inf, the anlage of the
infundibular gland, which extends put from the brain and arches over the end of the
hypophysis, Hyp. The hypophysis is an outgrowth from the ectodermal lining of
the mouth, Or. Its method of development can be .clearly made out at this stage.
The infundibular gland in older embryos extends farther on the posterior side of
the hypophysis. Meanwhile the hypophysis loses all connection with the epithelium
of the oral cavity, somewhat as does the otocyst with the overlying epidermis
which produces it. The hypophysis proper and the infundibular gland undergo
their further development in intimate association. The result of their differentia-
tion is the pituitary body, which is really a duplex organ. Below the infundibular
gland the wall of the brain shows a thickening, Chi.op, which can be followed
through hi the series laterally until it connects with the optic stalk. This thicken-

SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 293

ing of the brain-wall in later stages furnishes the passage for the fibers of the optic
nerve, and is, therefore, the anlage of the optic chiasma. Between the infundibular
gland and the optic chiasma extends the post-optic lamina, L.p.o. On the opposite
side of the chiasma follows the lamina terminalis, which leads us forward to the
wall of the hemispheres, H. Underneath the hind-brain extends the large basilar
artery, A.bas; at its posterior end, A.bas.p, the basilar artery is joined by the two
vertebral arteries from the fusion of which it is really produced. Underneath the
fore-brain we have the opening of the mouth, Or, from the dorsal side of which
springs the elongated evagination of the hypophysis. The oral cavity runs into the
pharynx, the floor of which is formed in part by the anlage of the tongue, Ton, and
of the epiglottis, Epgl, a rounded eminence . very different in shape at this stage
from the adult epiglottis. The pharynx can be followed along until it passes over
into the oesophagus, (E, which, however, is not well shown, as the section passes
through it away from the true median plane. Between the oesophagus and the
anlage of the epiglottis is a mound of tissue, La, which represents the lateral wall
of the developing larynx. The mound is separated from the anlage of the epiglot-
tis by a deep notch. In the median plane the mound is filled with entoderm
which forms a wide plate through which there is only a narrow opening leading
down into the trachea. Finally, we see from the base of the mandible the somato-
pleure, Som, extending off to form the boundary of the pericardial chamber. The
figure also includes a presentation of the inferior maxillary vein, V. mx.i, and of the
thyroid gland, Thyr, which immediately overlies the main trunk of the ventral
aorta. This aorta gives off on either side of the pharynx three principal branches,
of which the smallest is the base of the carotid and corresponds to the third aortic
. arch. The second and third branches are much larger and correspond to the third
and fourth aortic arches. The pulmonary aorta, P.Ao, is already separated from
the main aorta of the body.

Sagittal Section of the Head through the Principal Ganglia (Fig. 202).— The
section is to one side of the median plane. It exhibits the optic nerve, the tri-
geminal, acustico-facial, petrosal, jugular, and nodosal ganglia; but, on the other
hand, exhibits little of the brain, there being only a shaving from the lateral wall
of the fore-brain, H, and a section of the widest part of the hind-brain which
shows the cavity or lateral recess, R.L, of the fourth ventricle. The auditory vesicle
is cut, Ot. It is formed by a layer of epithelium derived from the ectoderm,
although now not connected with the overlying part of the epidermis by the in-
vagination of which the octocyst is developed. It shows a narrow, upward pro-
longation, the anlage of the ductus endolymphaticus (compare Fig. 42). The
epithelial otocyst lies in a line with the great cephalic ganglia and occupies its
invariable and permanent position behind the acustico-facial ganglion, Ac.F, and in
front of the glosso-pharyngeal, G.petr. The position of the otocyst makes it an
invaluable landmark in the stud rtions of the head. Only the lateral no

of the pharynx. Ph, appears. It forms a \vicl< it slit-like diverticulum.

294

STUDY OF PIG EMBRYOS.

which extend farther laterally the first and second entodermal gill-pouches. In
the figure can be seen a small depression extending downward from the cesophageal
or posterior end of the pharynx. This depression marks the beginning of the second
cleft. Nothing is seen of the third and fourth clefts in this section, as they both
lie nearer the median plane. The pocket or diverticulum of the cervical sinus,

Jug.'" G.jug.

Ph.

Ac.F.

R.L.

EC.

G.petr.

G.nod.

Cerv.S.

N.12.

Cerv.f).

Jug.""

FIG. 202.— PIG, 12.0 MM. No. 7. SAGITTAL SECTION 25.

Ac.F, Acustico-facial ganglion complex. Aur, Auricle of the heart. Cerv.S, Diverticulum of the cervical sinus*
just in front of which shows the anlage of the thymus, which is deeply stained. Cerv. 6, Sixth cervical nerve-
Cce, Ccelom around the heart or pericardial cavity. EC, Ectoderm. G.jug, Ganglion jugulare of the vagus
nerve. G.nod, Ganglion nodosum of the vagus nerve. G.petr, -Ganglion petrosum of the glosso-pharyngeal
nerve. G.tri', Ganglion of the trigeminus nerve. H, Lateral wall of the cerebral hemisphere. Jug' -Jug'"' ,
Jugular vein (Jug', Behind the trigeminus. Jug" , Branch in front of the trigeminus. Jug'" , Main stem
behind the vagus. Jug"", Main stem descending to join the duct of Cuvier). m, An undetermined
structure, probably the anlage of a lingual muscle. Md, Mandible. JV-5, Root of the fifth or trigeminal nerve.
N.op, Optic nerve. N. 12, Twelfth or hypcglossal nerve. Ot, Otocyst. PA. Pharynx. R.L, Recessus lateralis
of the fourth ventricle. Ve, Small branch of the jugular vein. Vent, Ventricle of the heart. X 22 diams.

Cerv.S, lies near the ganglion nodosum, G.nod. From its appearance it might easily
be mistaken for the section of a gill-cleft, but it is in reality lined not by entoderm
but by ectoderm, and its cavity can be easily traced through the series of sections
of the exterior of the embryo where the epithelium lining the sinus becomes con-
tinuous with the epidermis. Cfephalad from the sinus, but close to it, lies a small

SAGITTAL SECTIONS OF EMBRYO OF 12 MM. 295

dark rounded mass, the anlage of the nodulus thymicus (compare Fig. 194, Nod}.
The nodulus anlage is produced by proliferation of the entodermal cells on the
anterior side of the third cleft, and is penetrated by blood-vessels which seem to
be sinusoids, although their history has not been worked out. The great vein of
the head, which for convenience we may term the jugular, — although the applica-
tion of this name to the vein in its present condition is somewhat inexact, — is cut
several times, owing to its irregular course. Its main stem, Jug"" , arises nearly
vertically through the cervical region and is, relatively to the size of the embryo,
of huge diameter. It continues upward, Jug'", along the dorsal side of the vagus
to about half-way between the ganglion nodosum and ganglion jugulare. At that
point the vessel curves inward and forward, and therefore is not encountered
again in this section until, having bent upward again, it shows, Jug', on its way
past the trigeminal ganglion. A branch of the jugular, Jug", is cut just above the
ganglion, and another small and probably not very important branch is shown
at Ve.

The nerves are shown as follows: The optic nerve, N.op, still has its central
cavity, which, nearer the median plane, opens into the third, ventricle of the brain,
and in the section resembles in shape an inverted U. On the side of the nerve
toward the mouth there is a deep notch — the section of the choroid fissure. The
trigeminal ganglion, G.tri, .is very large, and its trilobate form is clearly indicated
by the figure. The lobe to which the reference line, G.tri, runs gives off the ramus
ophthalmicus; the lobe nearest the jugular gives off the ramus maxillaris inferior,
while the middle lobe gives off the ramus maxillaris superior. From the ganglion
the fibers and nerve-cells extend upward to form the root, AT. 5, which joins the
hind-brain at a characteristic point — namely, at the summit of the Varolian bend
and where the hind-brain is widest (compare Figs. 189 and 203). By its great
size and by its topographical association with the lateral apex of the recessus lateralis
of the fourth ventricle, the trigeminal ganglion may always be readily identified
in sections of embryos. The acustico-facial ganglia, Ac.F, may also be readily
determined by their typical position immediately in front of the otocyst, Ot. But
it is quite difficult to identify the four components of this complex structure;
namely, i°, the motor root of the facial nerve; 2°, the facial or geniculate ganglion;
3°, the vestibular ganglion; 4°, the cochlear ganglion. In figure 202 three divisions
are shown. The large, darkly stained division, to which the reference line, Ac.F,
runs and which lies nearest to the otocyst, is the vestibular portion of the acous-
tic ganglion; the small, light area occupying a middle position in the inferior part
of the complex is the motor division of the seventh nerve, or lateral root of the
facial; it can be followed to the brain, which it enters as four bundles of fibers;
its path of entrance is shown better in frontal sections (Fig. 204, t.m). Just in
front of the facial motor root lies a second smaller dark mass, the geniculate gan-
glion of the facial, with an upward prolongation, the sensory root. The ninth or
glosso-pharyngeal nerve is represented by the ganglion petrosum, G.petr, and its

296 STUDY OF PIG EMBRYOS.

ascending sensory root. This nerve may be quickly identified because it is the first
behind the otocyst. The upper ganglion of this nerve, the so-called Ehrenritter 's
ganglion, is represented by an accumulation of cells in the upper part of this
root. As regards the tenth nerve, or vagus, both its ganglia and the fibrous trunk
connecting them are shown. The upper or jugular ganglion, G.jug, is nearly on
a level with the otocyst, while the lower or nodosal ganglion, G.nod, lies near the
cervical sinus. To the nerve-trunk between the two ganglia are adjoined the fibers
of the eleventh or spinal accessory nerve, which does not otherwise appear in this
section. A small piece only of the hypoglossal nerve can be seen, N.I2. The
space occupied by this nerve is blank in the engraving; in the specimen it shows
horizontal fibers.

Pig Embryo of 12.0 mm. Study of Frontal Sections.

The frontal series has special value for the study of the hind-brain and asso-
ciated structures, as the plane of the section is approximately at right angles to
the axis of the hind-brain. It also furnishes instructive pictures of the vena cava
inferior and of the relations of developing vertebrae and nerves.

Portions of three sections illustrating the structure of the hind-brain and asso-
ciated parts are given below. The following remarks on the hind-brain are in-
tended to make clearer the significance of these sections. The wall of the hind-
brain is, of course, produced by the development of the wall of the medullary
tube. Its most striking peculiarity is the enormous expansion of the deck-plate,
which forms the very wide epithelial layer, (Fig. 203, epen), the so-called ependymal
roof of the fourth ventricle. It starts from the upper edge of the dorsal zone,
D.Z, and forms a wide arch which is covered in externally, by a rather thin layer
of mesoderm, mes, and the nearby epidermis, EC, of the embryo. The covering
is so ^slight in development at this stage that in the fresh specimen the roof of
the fourth ventricle, including its coverings, appears as a translucent membrane
through which we can readily distinguish the great cavity of the fourth ventricle
itself. The expanse of the ependymal arch is greatest at the region of the tri-
geminal root. From there backward toward the spinal cord its expanse gradually
diminishes. In correspondence with the growth of the deck-plate the lateral walls
of the medullary tube become bent outward and downward, so that, though they
remain near together on their ventral side, where they are united by the floor-plate
or median raphe (Fig. 205, raph), yet their upper dorsal edges are far apart. In
consequence of this change of their position the original lateral walls appear as
the floor of the hind-brain, and we recognize in' them the anlages of the medulla
oblongata. We distinguish here, as everywhere in the medullary wall, the dorsal
and ventral zones. The ventral zone is intimately 'united with its fellow by the
short median raphe. Between them is a deep fissure (Fig. 204. /) ,which is never
wholly obliterated. The floor-plate undergoes a great development in later stages
and is transformed into the median raphe of the adult medulla. The lateral or

FRONTAL SECTIONS OF EMBRYO OF 12 MM.

297

morphologically dorsal limit of the ventral zone is marked by the exit of the lateral
roots (Fig. 203, L.R). The ventral limit of the dorsal zone is marked by the en-
trance of the sensory or ganglionic fibers (Fig. 203, G.tri; Fig. 204, Fac}. Toward
the dorsal side the dorsal zone gradually thins out and passes over into the
ependyma, epen. The great development of the lateral roots is perhaps the most
important single characteristic of the medulla oblongata. They furnish the principal
motor or efferent nerve-tracts of the brain and form
an important constituent part of four nerves: first,
the trigeminal or fifth; second, the facial or seventh;
third, the glosso-pharyngeal or ninth; and fourth,
the vagus or tenth. There are no lateral roots
known to occur anterior to the medulla oblongata,
unless possibly the fourth nerve, the relations of
which in many respects are peculiar, should turn
out to be a lateral root. In the spinal cord we
find lateral roots in the upper cervical region, and
it is not improbable that they may yet be found
associated with the dorsal roots of spinal nerves

lower f'down. But even in the cervical cord the

•

lateral roots attain but a slight development. The Card
contrast with other portions of the central nervous
system makes the great development of the lateral
roots in the medulla oblongata all the more strik-
ing. The dorsal zone of the hind-brain lags con-
siderably behind the ventral zone in its develop- FIG. 203
ment, and at all stages the ventral zone forms a

PIG, 12.0 MM. FRONTAL
SERIES 6, SECTION 284.

larger proportion of the medulla th n does the Card> Anterior cardinal vein- D-Z' UP~

per portion of the dorsal zone of His.
EC, Ectoderm, epen, Ependymal

roof of the fourth ventricle. G.tri,
Ganglion trigemini. L.R, Lateral
root of the trigeminal nerve, mes,
Mesenchyma. T.S, Tractus soji-
tarius of W. His. X 22 diams.

Section through the Trigeminal Roots (Fig. 203).
—The section passes through the widest part of the
hind-brain, the cavity of which is enormously dis-
tended. It is bounded on the dorsal side only
by the very thin ependymal roof, epen, which

does not form any part of the true nervous structure, although it passes into and
is directly continuous with the dorsal zone, D.Z, which is thus seen to be only a
thickened portion of the wall of the neural tube, just as the ependyma is the
attenuated deck-plate. The trigeminal ganglion, G.tri, is very large and sends
its sensory fibers upward into the dorsal zone to form there a distinct bundle of
nerve-fibers which persists throughout life and is known in the adult as the tri-
geminal tract, T.S. The entering sensory fibers fork; their ascending branches form
the relatively short ascending tract, their descending branches the much longer
descending tract, which gradually grows through the length of the medulla oblon-

298

STUDY OF PIG EMBRYOS.

gata well outside the tractus solitarius, which, however, it joins just before the
spinal cord is reached. The other root of the nerve, L.R, is lateral. It lies below N
the ganglion near the median plane. Its fibers arise from neuroblasts in the ventral
zone and gather together as a distinct bundle which starts near the median line,
takes a curving course through the ventral zone, and makes its exit from the

medullary wall at the dorsal limit of the
zone. It has a striking resemblance to the
root of the facial nerve. We do not yet
know whether such a course of the fibers
is characteristic of all lateral roots or only
of the trigeminal and facial roots. On the
medial side of the trigeminal ganglion is a
large vein, Card, the anterior cardinal vein.
In the median line in the mesenchyma
immediately below the raphe is the section
of the basilar artery, and considerably below
that is the small section of the notochord
which it is very difficult to distinguish with
a low power. Between the notochord and
the cardinal vein is the section of the
carotid artery.

m Section through the Acustico-facial
FIG. 204.— PIG, 12.0 MM. FRONTAL SERIES 6, Ganglion (Fig. 204).— In this section the

SECTION 340.

A. has, Arteria basilaris. D.Z, Dorsal zone of the
medulla oblongata. EC, Ectoderm, epen,
Ependymal roof of the fourth ventricle. /,

thickened ventral wall of the hind-brain
(i. e., the anlage of the medulla oblongata)
is not spread out nearly horizontally, as in

N.I2, Hypoglossal nerve. PA, Pharynx, t.m,
Motor tract of facial nerve. X 22 diams.

Median fissure of the medulla oblongata. Fac, the trigeminal region, but rises obliquely on
Sensory root of the facial nerve. G.gen, Gen- either side from the median line. The

iculate ganglion of the facial nerve. G.vest, • , . j i r, • i r .1 j n

. , , right and left sides of the medulla are

Ganglion vestibuli of the acoustic nerve. Jug, •

Lateral vein, mes, Mesenchyma. Mx.i, In- divided from one another by a deep

ferior maxillary branch of the trigeminal nerve. median fissure, /. In the median line WC

see also the basilar artery, A.bas, and
still lower the wide, slit-like pharynx, Ph,
the outer portion of which ascends obliquely toward the lateral vein, Jug. The
ascending lateral part of the pharynx is a portion of the first gill-pouch or future
Eustachian tube, and is quite clearly marked off from the pharynx proper by
its oblique direction. Of the acustico-facial ganglion complex the section shows
four parts: the ganglion vestibuli, G.vest; the geniculate ganglion, G.gen; the sensory
root, Fac, of the facial nerve arising from the geniculate ganglion and entering
the brain to form there a distinct fiber-tract which is oval in the section and lies
just below the entering vestibular fibers, and is clearly indicated in the drawing;
and, finally, the motor tract, t.m, of the facial nerve. This tract is a very dis-

' FRONTAL SECTIONS OF EMBRYO OF 12 MM.

299

epen

tinctly marked bundle of nerve-fibers which arise from neuroblasts of the ventral
zone, traverse that zone almost horizontally, then bend downward and pass out
from the brain-wall, appearing as the lateral root of the facial nerve. The root
runs first toward and then past the geniculate ganglion. The cardinal vein origi-
nally was inside the ganglia; by island formation it has migrated outside the ganglia,
forming the lateral vein, Jug. In the mandible below the pharynx appear two
nerves. Of these, the upper is the hypo-
glossal, N.I 2, which lies near the angle
formed by the junction of the first gill-cleft
with the pharynx. The lower of the two
nerves, Mx.i, is the inferior maxillary.

Section through the Otocyst (Fig. 205).—
The figure is from a section not far from
the last. The hind-brain has narrowed
considerably; its thickened floor, Md.obl, the
anlage of the medulla oblongata, rises
steeply from the median line. Its ependymal
roof, epen, is less expanded than in figures Fac.m.
204 and 205. It forms a sharp angle in
the * dorsal median line. The median ven-
tral fissure between the two sides of the
medulla is deeper than farther forward.
The pharynx, Ph, is wide and has expanded
laterally into the common beginning of the
first and second gill-pouches. Between the FIG. 205.— PIG, 12.0 MM. FRONTAL SERIES '6,
pharynx and the raphe the basilar artery, SECTION 380.

A.bas, his been CUt transversely. Below ^«, Basilar artery. Coch, Cochlea. Z>.e,Ductus

endolymphaticus. epen, Ependyma. Fac.m,

D.e.

S.c.

Jug.

Coch.

Ph.

Motor division of the facial nerve. Jug, Vena
lateralis capitis. Md.obl, Medulla oblongata.
Ph, Pharynx, raph, Median raphe of the
medulla oblongata. S.c, Anlage of the semi-
circular canals. Ve, Vein. X 22 diams.

it and near the pharynx is the small
notochord, which, however, can b.e clearly
recognized only with the higher power,
and is, therefore, not represented in this or
the preceding figure. The otocyst is a large
epithelial vesicle with three well-marked divisions: First, the common chamber, S.c,
out of which the three semicircular canals are to be differentiated. Second, a
slender canal, D.e., which one easily -identifies as the anlage of the ductus en-
dolymphaticus. It lies between the semicircular canal and the wall of the me-
dulla oblongata. Third, the long, curving, but not spiral cochlea, Coch. The com-
mon chamber formed by the union of these divisions is later subdivided to form
the upper utriculus and lower sacculus. Outside the cochlea lies the cross-section
of the vena 'lateralis capitis, Jug, which appears in the adult as part of the inter-
nal jugular. Just below the lateral vein is the section of the motor portion, Fac.m,
of the facial nerve. The sensory portion of the facial nerve at this stage is

300

STUDY OF PIG EMBRYOS.

much smaller, and runs only a short distance downward from the geniculate gan-
glion and is entirely separate from the motor portion. The morphological constitu-
tion of the facial nerve is still very obscure, and a satisfactory account of its
development is, for the present, impossible.

Section through the Vena Cava Inferior (Fig. 206). — The section displays the
huge vena cava inferior cut through most of its length. For the composition of

Ao.n

Ao.S

P.A.

V.s.c.

'Ao

FIG. 206. — PIG of 12.0 MM. FRONTAL SERIES 6, SECTION 423.

Ao, Main dorsal aorta. Ao.D, Right descending aorta. Ao.S, Left descending aorta. Au.D, Right auricle.
Au.S, Left auricle, c, Vena cava passing through the caval ligament. Nch, Notochord. Om.min, Omentum
minus. P.A, Pulmonary aorta. Sp.e, Spinal cord. St, Stomach. S.V, Sinus venosus. V.C.I, Vena cava
inferior. V.E, Valvula Eustachii. V.S. Valvula sinistra. V.s.c, Vena subcardinalis. W.B, Wolffian body.
X 15 diams.

the vein see page 257. In the section it starts between the Wolffian bodies, W.B, as
a large vessel, V.C.I, formed by the median union of the two subcardinal veins
of the Wolffian bodies. It passes upward, c, through a thin band of tissue, the
caval ligament, to the right of the lesser omentum, Om.min, into the substance of
the liver, Li, through which it takes a slightly sinuous course. Several junctions

FRONTAL SECTIONS OF EMBRYO OF 12 MM. 301

of the hepatic veins with the main vessel are cut in the section. The liver is
attached to the diaphragm. Above the diaphragm the cava is continued, with thin
walls, for a short stretch, S.V, which is the modified sinus venosus of the heacrt,
and which opens directly into the right auricle, Au.D. The opening is guarded
by two valves, the valvula sinistra, V.S, on the left, and the valvula Eustachii,
V.E, on the right, which together prevent the back-flow of the blood !rom~THer
heart into the vein. Above the heart appear the pulmonary aorta, P. A, and the
two descending aortae, Ao.D, Ao.S. The main dorsal aorta, Ao, shows in the
lower part of the section. The stomach, St, lies on .the left side and is closely
attached to the liver by the short and thick anlage of the great omentum, and is
attached to the caval ligament .by the longer band of the lesser omentum, Om.
min. The space bounded by the stomach, the lesser omentum, and the liver is
the lesser peritoneal cavity (bursa omentalis). In the Wolffian body the sub-cardinal
vein, V.s.c, is easily identified, and with a higher power the intertubular sinu-
soids reveal their characteristics clearly, the sinusoidal epithelium being fitted closely
to the surface of the Wolffian tubules. The division of the ccelom by the dia-
phragm into an upper pericardial and a lower abdominal chamber is perfectly demon-
strated by this section. The student should observe that the mesothelium forms
for both chambers the absolutely unbroken boundary of the ccelom.

Section through the Dorsal Vertebra (Fig. 207). — Owing to the curvature of the
embryo the spinal cord is cut twice; once, Sp.c', toward the head end of the
embryo, and again, Sp.c", lower down toward the tail end. Alongside the sections
of the spinal cord appear the large, darkly stained masses of the ganglia, G. The
section also passes through the bases of the anterior limbs, A.L, in one of whi
can see one of the branches, N.br, of the brachial plexus. Between the wo |
of the spinal cord of the section the plane passes on the ventral side of
cord and shows the series of vertebral formations, together with the nc
N',N",- the intersegmental arteries, A.i.s, and the segmental veins, small \essds
which lie close to the intersegmental arteries. The nerves are sections of the
dorsal root below the ganglia. Each nerve has a distinct outline and is partly
penetrated by ingrowing mesenchymal cells which subdivide the nerve into rounded
fiber bundles. In each bundle the nerve-fibers appear as fine dots, which, how-
ever, by the use of the fine adjustment can be followed up and down through
the section, and thus identified as fibers. The single fibers are more or
less isolated from one another, and between them are delicate threads, the nature
of which is not known. Between the adjacent rounded bundles of fibers there
is often a distinct space. The anlages of the intervertebral disc, fv.D, are
formed entirely from condensed mesenchyma, and therefore stand out somewhat
conspicuously in the section owing to their darker staining. Each anlage is bow-
shaped, the concavity of the bow facing toward the tail of the embryo. The end^
of the bow pass behind the nerve-trunk of the segment to which the anlage be-
longs. The anlages extend completely across the median line, and by following

302

STUDY OF PIG EMBRYOS.

Mes.

FIG. 207. — PIG, 12.0 MM. FRONTAL SERIES 6, SECTION 572.

Inlersegmental artery. A.L, Anterior limb, tin, Cinerea of spinal cord. EC, Ectoderm. G, Ganglion.
Iv.D, Intervertebral disc. Mes, Mesoderm. N',N", Nerves. N.br, Nerve-branch of brachial plexus.
Sp.cf, Cephalad portion of spinal cord. -Sp.c". Caudad portion of spinal cord. V.arch, Anlage of arch of
vertebra. X 22 diams.

STUDY OF SECTIONS OF EMBRYO OF 17 MM.

303

through in the series of sections, it may be found that the condensed mesenchyma
surrounds the notochord, which, therefore, passes through the central portion of
each intervertebral anlage. The bodies of the vertebrae at this stage consist merely
of the loose mesenchyma between the intervertebral discs, are entirely without any
distinct limitation, and merge into the surrounding loose mesenchyma. Near the
anterior border of each nerve-trunk, and usually somewhat toward the median
side of it, lie -the intersegmental vessels, which are of small size and vary greatly
in their exact position and number, according as they are more or less branched.
Between the ends of the vertebral bows outside of the nerve-trunks can be seen
with higher power clusters of elongated cells with developing muscle-fibers which
are here still segmentally arranged between the processes of the developing
vertebrae.

Pig Embryo of 17 mm. Study of Sections.

Since the pig of 12 mm. contains the anlages of perhaps every important
part of the body sufficiently advanced in development to be clearly recognized,
we find in the immediate subsequent development that we have to do not so
much with an introduction of new parts as with the differentiation of those which
have already commenced. Embryos of 17 mm. are convenient for the study of
the differentiations referred to. Particularly important for the student to note
are the advances in the development of the vertebrae, of the lungs, of the Wolffian-
bodies and genital glands, and of the kidneys. These points are illustrated in
figures 208 to 210, representing portions of three transverse sections of a 17 mm.
embryo.

Transverse Section through the Lungs (Fig. 208). — The epidermis of the embryo
%as become more distinct owing to its growth in thickness, which is • .ompii. hed
by the increase of the number of layers of cells. The growth is very marked al
the sides of the section about the level of the vertebra. At these points it ca
early seen that upon the outside the epidermis has a very thin layer of flattened
the nuclei of which are themselves also somewhat flattened. This single
jf cells is known as the epitrichium, because the hairs are developed • en-
rneath it. Where the epidermis is thickest, one can observe that the

layers
They
Between
forming the i
developed and
carries the nerves
of more darkly staine
cells proper, the anlage
thelial muscle-cells,

to the mesoderm are closely packed together with round nuclei.
commencing formation of the basal layer of the adult epidermis.
layer and the epitrichium the cells are more loosely placed,
of the mucous layer. The mesenchyma is very much
a large territory in the dorsal region of the embryo. It
blood-vessels and shows at various points accumulations
are of two kinds: first, groups of mesenchymal
the skeleton; and, second, groups of meso-
skeletal muscles. There is little

differentiation otherwise

may note the following changes

304

STUDY OF PIG EMBRYOS.

in it: (i) The anlage of the vertebra, Vert, which is now quite well denned; around
the edge of it the cells have assumed an elongated form and have elongated nuclei;
the elongation is parallel with the surface of the anlage. These cells result from
the commencing differentiation of the perichondrium, which at this stage merges
on the one side into the anlage of the vertebrae, and on the other into the sur-
rounding mesenchyma. The cells of the vertebra have changed into young car-
tilage-cells. They are now distinctly separated from one another by a well-devel-

Vert.

D.R. Ec.gl. Sp.c.

Nch.

Ve'.

Ve".

card.

N.io.

bro.

Lu.

Piece.

EIG? 208. — PIG, 17.0 MM. TRANSVERSE SERIES 51, SECTION 464.

Ao, Aorta, bro, Entodermal bronchus, card, Posterior cardinal vein, cin, Neurone layer (cinerea) of spinal
cord. Cost, Anlage of ribs. D.R, Dorsal root. Ec.gl, Ectoglia. C, Ganglion. Li, Liver. Lu, Lung.
muse, Dorsal musculature. N.io, Vagus nerve. Nch, Notochord. (E, (Esophagus. Pl.cce, Pleural
ccelom. R.D, Ramus dorsalis. R.V, Ramus ventralis. R.sy, Ramus sympathicus. Sp.c, Spinal cord.
Sym, Sympathetic ganglion. Ve', Ve", Branches of the subclavian vein. Vert, Vertebra. X 22 diams.

oped matrix. Each cell occupies a separate space or capsule in the matrix. The
protoplasm of the cell, having changed to a transparent substance and being un-
stained, seems to have disappeared, but the nucleus remains distinct, for it stains
readily, has a sharp outline, and contains a number of dark granules, one or two
of which are conspicuous by their greater size and irregular shape. The nucleus
itself, in most of the cells, is somewhat irregular in outline, as if distorted by

shrinkage. Toward the center of the anlage the cytomorphosis is most advanced.

.

STUDY OF SECTIONS OF EMBRYO OF J7 MM.

more, regularly shaped nuclei. In the center of the vertebra lies the round noto-
chord, Nch, the sheath of which has increased considerably in thickness, and,
being unstained, appears as a clear space between the cells of the notochord
and those of the enclosing vertebra. The nuclei in the notochord are numerous
and somewhat crowded together. (2) The costal processes, Cost, of the vertebra,
which are rod-like and extend quite far down into the somatopleure. The histo-
genetic changes in these processes are similar to those in the vertebra, but less
advanced. They have progressed somewhat more in the proximal than in the
distal portion of the rib. (3) Around the central nervous system the pia mater
has become more distinct, and the arachnoid membrane is indicated by the wide
separation of its cells and the length of the processes connecting them. Its dif-
ferentiation is most easily recognized at the sides of the spinal cord. The outer
limit of the arachnoid is shown by a slight condensation of the mesenchyma which
marks the first step in the differentiation of the dura mater, the anlage of which
is further defined by the elongated form of the mesenchymal cells, by which they
differ from the mesenchymal cells on both sides. (4) There is a distinct layer of
condensed mesenchyma around the aorta, Ao. The layer thus formed consists of
elongated cells, and perhaps corresponds only to the muscular coat of the vessel.
(5) About the oesophagus, (E, the mesenchyma forms two distinct layers. The
inner, next to the epithelium, is of looser texture, and is the anlage of both the
mucous and submucous layers of the adult. The outer layer is denser and con-
sists chiefly of young smooth muscle-cells, which are merely modified mesen-
chymal cells, characterized by the greater development of their protoplasm and
by their elongated form. Traces of the differentiation of the outer layer into
the inner circular muscular coat and the outer longitudinal coat of the adult are
clear iru the section.

spinal cord, Sp.c, has changed its outline as seen in section, being
brc^olest in the ventral zones, which have also begun to expand ventralward
so Ahat the outline of the cord shows on its ventral side a concavity, the first
idication of the ventral fissure. The three layers of the spinal cord are
\Tery distinct. The change in form, however, it can be clearly seen, is due chiefly
to the growth of the gray layer, tin, especially in the ventral zone. The gray
layer in the dorsal zone is still very slightly developed. From the dorsal zone
descends on either side the dorsal nerve-root, D.R, which presently joins
the ganglion, G. The ganglion now occupies a much lower position than in the
earlier stages (compare Fig. 198, G). From the ventral zone springs the ventral
root which unites with the dorsal at the lower tip of the ganglion. From the
nerve-trunk thus formed there is given off almost immediately the dorsal branch,
R.D, which soon ramifies in the midst of a dark mass of tissue, the anlage of the
dorsal musculature, muse. The main nerve-trunk descends ventralward and sends
off at the level of the vertebra a sympathetic branch, R.sy, which runs obliquely
downward and inward toward the aorta, and there terminates in the anlage of the

306 STUDY OF PIG EMBRYOS.

sympathetic chain, Sym, which consists partly of nerve-fibers, partly of ganglion
cells which have migrated along the nerve and taken up their position at its end.
These cells are easily recognized by their very dark staining. Their nuclei are a
little lighter than those of the neighboring mesenchymal cells, but the cells, owing
to -their deep coloration, are conspicuous even when the section is examined only
with the low power. The sympathetic anlage comes in close contact with a por-
tion of the cardinal vein, card, near the aorta. The main nerve-trunk, R.V, con-
tinues obliquely downward and presently forks into an upper and a lower branch.
The cardinal veins, card, lie on either side of the aorta, but they are almost
completely obliterated by the ingrowth of the Wolffian tubules, which subdivide
the vein into numerous smaller channels or sinusoids. The section also shows
two branches, Ve' ', and Ve", of the subclavian vein. The identity of these branches
has not yet been determined. Beneath the aorta, Ao, follows the oesophagus, (E,
the lumen of which is much smaller than that of the aorta. Its epithelium has
the general characteristics of the epithelial entoderm at this stage, being a rather
thick cylinder epithelium. As above mentioned, the differentiation of the mucous
and muscular layers of the oesophagus shows clearly. Below the oesophagus lie
the two large vagus nerves, N.io, and then follow the sections of the two lungs,
Lu. Each lung is a lobe of tissue connected with its fellow across the median
line of the embryo and projecting laterally far into the pleural cavity, Pl.cce.
The lung consists chiefly of a large accumulation of dense mesenchyma in which
the epithelial bronchi, bro, ramify. Every bronchus has a central lumen and its
walls are formed by a moderately thick layer of cylinder entodermal cells. The
surface of each lung is covered by mesothelium, which is shown as a distinct line
in the engraving. The mesothelium can be followed to the root of the lung,
where it is reflected on to the outer wall of the pleural chamber. The pleural
cavity, Pl.cce, is thus everywhere bounded by mesothelium which persists through-
out life, being known in the adult as the pleural epithelium.

Section through the Wolffian Body and Genital Gland (Fig. 209). — The gen-
eral characteristics of the ectoderm, mesenchyma, and nervous system are nearly
the same as in the section last described. On one side the section shows a thick-
ening of the ectoderm, the anlage of a mammary gland, mam (compare page 320).
The branches of the nerves are not so well shown in this section as in the previous
one. The level of our section corresponds to the lower end of the vena cava
inferior, which is marked at this stage by the two large mesonephric veins, V.msn,
which come from the Wolffian bodies and by their union constitute the lower end
of the vena cava. The mesonephric veins are, strictly speaking, portions thereof.
The Wolffian bodies are the most conspicuous structures shown in the section.
They consist chiefly of a great number of tubules, W.t, very much crowded to-
gether. On the median side of the organ appear the large glomeruli, Glo, and- on
their ventral side we have the section of the longitudinal Wolffian duct, W.D.
The tubules of the Wolffian body are formed by a more or less nearly cuboidal

STUDY OF SECTIONS OF EMBRYO OF 17 MM.

307

R.sy.

Ao.

art.

msth.

epithelium, the nuclei of which are decidedly larger than those of the mesenchymal
cells. The nuclei themselves stain deeply, have well-marked outlines, and very
distinct granules in their interior. The protoplasm of the cells also stains some-
what with cochineal, carmine, hematoxylin, etc. There is very little mesenchyma
Nch. ec.gl. Sp.c. G. N. R.V .

R.V

Sym.

Cce.

V.msn.

Gen.

Glo.

W.t.

Som.

W.D.

mst.

In. Li.

FIG. 209.— PIG, 17.0 MM. TRANSVERSE SERIES 51, SECTION 651.

Ao, Dorsal aorta. art, Glomerular artery. Cce, Ccelom. ec.gl, Ectoglia. G, Ganglion. Gen, Genital
gland. Glo, Glomerulus of Wolffian body. In, Intestine. Li, Liver, mam, Mammary anlage1. mst,
Mesentery, msth, Mesothelium. N, Ventral nerve. Nch, Notochord. R.sy, Ramus sympathicus of
nerve. R.V', R.V", Branches of the ventral ramus of the spinal nerve. Som, Somatopleure. Sp.c,
Spinal cord. Sym, Sympathetic ganglion. V.msn, Vena mesonephrica. W.D, Wolffian duct. W.t,
\Yolffian tubule. X 22 diams.

in the organ, but each tubule is closely invested by vascular endothelium; hence the
tubules are separated from one another only by blood spaces, which, morphologi-
cally speaking, are portions of the cavity of the cardinal vein. These blood spaces
are highly characteristic and are typical sinusoids. The intertubular circulation
of the Wolffian body is, so far as known, always sinusoidal. The,, aorta, Ao, is
seen in the figure to give off a small branch, ' art, which runs toward the Wolffian
body. There are numerous such branches, each one of which may be traced to a

308 STUDY OF PIG EMBRYOS.

glomerulus of the mesonephros. Each glomerulus has a capillary circulation,
and the blood on leaving the glomerulus is supposed to be emptied into the venous
sinusoids. More exact investigation of . this point is needed. The mesonephros
is covered by a layer of mesothelium, msth, underneath which is a thin layer of
mesenchyma. The two together constitute the anlage of the peritoneal covering
of the organ. To the median side of the Wolffian body is appended the large
anlage of the genital gland, Gen, which has a constricted connection with the
Wolffian body. Each gland is covered by mesothelium and extends until it comes
in contact with the mesentery, mst. The gland contains two kinds of tissue, one,
the anlage of the medullary, the other of the cortical portion of the gland. The
medullary tissue resembles the neighboring mesenchyma and occupies only a small
territory about the stalk of the organ. The cortical tissue contains cells with much
larger nuclei and clearly developed protoplasmic bodies. It occupies by far the
larger part of the gland. Comparison with figure 198 will show that the genital
anlage at this stage occupies -the same topographical relation to the Wolffian body
as at earlier stages. It differs now from the earlier condition chiefly by its growth
in size and by its advancement in histological differentiation. Below the genital
gland the intestinal canal is cut several times. One portion of the intestine is
seen in the section to be connected by means of the mesentery, mst, with the
median dorsal tissues of the embryo. The intestine is formed by a small tube
of entoderm with a small cavity. The entoderm is a rather thick cylinder epithe-
lium. The greater part in bulk of the walls of the intestine is constituted by
mesenchyma. The external surface is covered by a thin mesothelial layer. The
mesenchyma is beginning to show the differentiation of the external muscular
from the internal mucous coat. There is at this stage no trace whatever of the
development of any folds or glands on the inside of the intestinal canal.

Section through the Kidney (Fig. 210). — This section being much nearer the
caudal end of the embryo, we find, as throughout all the early stages, that the
differentiation of the tissues is less advanced than nearer the head. We have
accordingly, so to speak, an earlier stage in the development of the spinal cord,
Sp.c, -of the nerves, and of the vertebra. In the median line is the large aorta,
Ao, about which the mesenchyma is only slightly condensed. Near the aorta are
the conspicuous anlages of the sympathetic system, Sym, which appear at this level
in a very characteristic hook-shaped pattern. At the dorsal end of the hook the
nerve-fibers are much more numerous than in the ventral portion of the anlage.
The sympathetic cells themselves are extremely conspicuous, owing to^ the depth of
their stain. On either side is situated the anlage of the permanent kidney, Ki.
Each anlage consists of an irregularly branching space bounded by a thick layer
of epithelium, which has somewhat the appearance of the intestinal entoderm at
this stage. If the series of sections be followed through farther toward the tail
of the embryo, the epithelial space will be seen to contract to a relatively small
tube, the ureter, which opens into the Wolffian duct of the same side. The ex-

STUDY OF SECTIONS OF EMBRYO OF 17 MM.

309

panded portion of the cavity shown in our figure corresponds in part to the pelvis
of the adult organ. Its irregular shape is due to the fact that it is forming a
series of outgrowths, which are to give rise to the collecting tubules. Around
the ends of the branches of the renal pelvis is a darker tissue, in which the cells
are very much crowded. It is the material out of which the glomeruli and con-
voluted tubules of the kidney are to be differentiated. By a secondary process
these tubules become united with the branches from the renal pelvis, the branches
forming the collecting tubules only of the adult organ (compare page no). The

Nch

W.D

FIG. 210. — PIG, 17.0 MM. TRANSVERSE SERIES 51, SECTION 759.

All] Allantois. Ao, Aorta. A.um, Umbilical artery, card, Branch of cardinal vein. Cce, Coelom. G, Gan-
glion. Ki, Kidney. N',NV, Nerves. Nch, Notochord. P.L, Posterior limbs. Reel, Large intestine. Sp.c,
Spinal cord. Svm, Sympathetic ganglion. W.b, Wolffian body. W.D, Wolffian duct. X 17 diams.

origin of the renal anlage may easily be followed in earlier stages. It is found that
from the pelvic end of each Wolffian duct there develops a dorsal outgrowth, which
is lined by epithelium. This outgrowth elongates in a headward direction. Its
end expands; the narrow portion is the ureter, the expanded portion the anlage
of the pelvis. The pelvis becomes irregular in shape and forms outgrowths.
Around it appears the condensed tissue just referred to. On the ventral and
lateral sides of the kidneys in our section appear the ends of the Wolffian bodies,
W.b. From the ventral and inner edge of each Wolffian body is a projecting lobe
of tissue in which the Wolffian duct, W.D, is lodged. The walls of the Wolffian
duct are a rather thin, cuboidal epithelium, surrounded by mesenchyma in which
there is no very clear evidence of specialization. Between the Wolffian bodies

310

STUDY OF PIG EMBRYOS.

is suspended the large intestine. It has a small canal formed by entoderm and
very thick mesodermic walls.

Attached to the ventral side of the body- wall of the embryo is the allantois,
All, the cavity of which is quite large, somewhat irregular in shape, and lined
by a cuboidal epithelium, a portion of the entoderm. By following through the
sections it can be seen that the allantois and large intestine join at the cloaca.
The entodermal allantois is surrounded by mesenchyma, which is very much
looser in texture than that of the intestine proper. On either side of the allantois
is a projecting lobe of tissue in which the umbilical artery, A.um, is lodged. The

A.m.

V.vi.

Cce.

Art.

U.V.S. All.

FIG. 211. — PIG, 17.0 MM. FRONTAL SERIES 39, SECTION 64.

A II, Allantois. Art, Umbilical artery. A .vi, Vitelline artery. Cce, Ccelom. EC, Ectoderm. //, Ileum. mes,
Mesenchyma. U.V.S, Left umbilical vein. V.vi, Vitelline vein. X 35 diams.

two arteries pass upward to the umbilicus, then outward to the placenta. Down-
ward they continue to the level of the cloaca, there pass to the dorsal side of
the embryo, and unite with the end of the median dorsal aorta.

Frontal Section of the Umbilical Cord (Fig. 211). — We get in frontaj series
of the embryo sections of the umbilical cord which are more or less nearly trans-
verse. The major part of the area of such sections is occupied by mesenchyma,

STUDY OF SECTIONS OF EMBRYO OF 20 MM. 311

Mes. On the ventral side of the cord is the cavity of the allantois, All, lined by
a thin layer of entoderm, and with no marked condensation of the mesenchyma
around it. A little lower is the large umbilical vein, U.V.S, which appears as a
prolongation of the left umbilical of the body proper, but the part within the cord
is probably the product, of the fusion of the two original veins. A little higher are
the two umbilical arteries, Art, which lie symmetrically as regards the allantois.
All three vessels are strengthened by walls of condensed mesenchyma, which is
much more prominent around the arteries than around the vein. The center of
the cord is occupied by a large irregular space, Cce, a prolongation of the body
cavity. In this umbilical ccelom are lodged the loop of the intestine and the cord
containing the vitelline vein. The intestine is cut twice, the section on the left
passing through the ileum, II, and on the right through the jejunum, which is
much larger than the ileum, having both a larger entodermal portion and a thicker
mesodermal part. The two segments of the intestine are joined together, and in
the part between them are two blood-vessels, one, the inferior, is the vitelline
artery, A.vi, which extends beyond the intestinal loop, to ramify upon the yolk-
sac; the other vessel is the superior mesenteric vein, which does not extend be-
yond the intestine. The mesenchyma of the intestines and of the bit of mesentery
between them consists of very crowded cells, so that the tissue appears darkly
stained. Above the loop lies the cord in which is situated the vitelline vein, V.vi.
The vein is centrally placed; the cord forms a thick wall of very loose mesenchyma
. covered by a thin mesothelial layer. The cord is a very characteristic embryonic
structure; it arises from the mesentery of the duodenum and extends through the
umbilical opening to the yolk-sac. The vein which it contains is thought to
arise by the fusion of the two original vitelline or omphalo-mesaraic veins. Its
union with the superior mesenteric vein to form the portal vein is shown in figure
100. All the surfaces of the ccelom are covered by a distinct mesothelium. The
main tissue of the umbilical cord is a typical loose mesenchyma, Mes. The ecto-
derm, EC, is the direct prolongation of the embryonic epidermis, and consists for
the most part of a single layer of cells, although the formation of a second outer
layer seems to be beginning.

Pig Embryo of 20 mm. Study of Sections.

Nine sections of this stage are figured. In the practical laboratory work em-
bryos a little larger or smaller may serve equally well to illustrate the develop-
mental conditions of this stage.

Transverse Section through the Snout (Fig. 212). — The parts shown are the same
as in figure 219, to the description of which reference is made. The present figure
212 is added to illustrate the development of the palate shelf, Pal. The palate
shelf is a large protuberance on the inner side of the maxillary process. Its inner
edge abuts against the tongue, Ton, its upper edge underlies the maxillo-turbinal
fold, max.tb, and its lower edge forms part of the roof of the oral cavity. Or.

312

STUDY OF PIG EMBRYOS.

At this stage it consists of a large mass of imdifferentiated mesenchyma, covered
by a layer of epithelium. The two palate shelves continue to grow toward one
another until they meet in the median line below the nasal septum, Sept. As they
approach one another the tongue descends. Ultimately the two palate shelves
unite with one another and with the overlying nasal septum. The epithelium of
the two shelves concresces and forms for a time a partition, which marks the point
of union of the two shelves, both with one another and with the nasal septum.
This partition persists for a short time only, for it soon disappears by resorption.

nas.tb.

Sept.

max.tb.

Pal.

Ton.

Or.

Mk.

FIG. 212. — PIG, 20 MM. TRANSVERSE SERIES 59, SECTION 522.

Jk.o, Jakobson's organ, lat, Lateral ethmoid cartilage, max.tb, Maxillo-turbinal fold. Mk, Meckel's cartilage.
nas.tb, Naso-turbinal fold. Or, Oral cavity. Pal, Palate shelf. Sept, Cartilage of nasal septum. Ton,
Tongue. X 22 diams.

The union of the palate shelves separates definitely the nasal and oral cavities
from one another. Their union is gradual, beginning in front and gradually ex-
tending backward. It is a not infrequent anomaly that the palate shelves fail to
unite perfectly. When this occurs, there results the condition known as cleft palate.
Transverse Section through the Lower Part of the Neck (Fig. 213). — The spinal
cord, Sp.c, shows a very great enlargement of the ventral zones, which now pro-
ject downward so as to enclose between them a distinct groove in the median ven-
tral line, which can be identified as the commencing anterior fissure of the cord.
In this groove runs a small, longitudinal blood-vessel, the arteria sulci, which from

STUDY OF SECTIONS OF EMBRYO OF 20 MM.

313

—

~

.3

ui

k. •

—

a

iC

if.

H

-

o

a

>

E

n

~

o'

u

o

M

7^

o"

[>

r^'

-

•^

ug

'.

a

^

I

M

—
E

3
-=

u

a

"~>

3
3

u

'fe

—

X

2.

'3

1

>tt

~?.
—
E

a

^

n

j^

•f.'

E

^-<

^

g

-5:

an

— i

|

-r

U

X) .2

s-g

314 STUDY OF PIG EMBRYOS.

time to time gives off small branches, which enter the substance of the spinal cord.
In the ventral zone the ependymal layer has become quite thin and the middle or
gray layer has acquired great thickness, chiefly owin^ to the growth of the neuro-
blasts, many of which, especially toward the outside of the cord, can now be
readily identified as young nerve-cells. The ectoglia or outer neuroglia layer has
increased in thickness. Many of the processes of the neuroglia cells can be
readily distinguished, running, for the most part, more or less nearly perpendicular
to the surface of the cord. Between the neuroglia fibers are numerous fine dots
which are the cut ends of the nerve-fibers running longitudinally. Although about
these nerve-fibers there are as yet no medullary sheaths developed, it is, never-
theless, proper to speak now of the ectoglia as the external white matter of the
cord. Immediately beneath the entrance of the dorsal root the external outline
of the cord shows a concavity which disappears in later stages. The dorsal
zones are very much smaller than the ventral. The differentiation of their three
primary layers is being completed by the development of a distinct middle layer.
The ectoglia of the dorsal zone resembles that of the ventral zone in structure
and thickness. The spinal ganglia, G, have descended from their original position,
so that they now lie on a level with the lower edge of the spinal cord, and the
nerve-root, by which each ganglion is connected with the dorsal zone of the cord,
has correspondingly elongated. The lower edges of the ganglia come in contact
with the lateral processes of the vertebra. Between the spinal cord and the
vertebra is an area of loose mesenchyma which may be regarded as a portion of
the arachnoid membrane. Close to the upper surface of the vertebra, bounded
dorsally by the tissue just mentioned, are two symmetrically placed blood-vessels.
The intervertebral ligament, Iv.D, is only partially cut. Above it appears the
lighter tissue of the next following vertebra, which is shown better several sections
lower down. The vertebra is distinctly cartilaginous, though not yet fully differ-
entiated, and is surrounded by a distinct fibrous layer, the perichondrium. In
the median line below the vertebra lie 'the oesophagus, ffi, and trachea, Tra, both
tubes lined by entoderm. The cavity of the oesophagus is somewhat crescent-
shaped, that of the trachea triangular. About the oesophagus the mesoderm forms
two layers, an inner lighter layer and an outer muscular layer, the cells of which
are already elongated. The mesenchyma about the trachea is more condensed,
especially on the sides and below, and the condensed tissue is in close contact
with the epithelium. On the dorsal side of the trachea close to the entoderm is
a thin layer of transversely elongated cells. The sympathetic nervous system, Sym,
appears symmetrically placed near the trachea and oesophagus. In section the
sympathetic is round and contains numerous nerve-fibers and characteristic young
sympathetic nerve-cells, by which it is readily recognized. Close to the ventral
side of the sympathetic is the section of the large jugular vein, V.jug, a branch of
which, V.br, lies laterad from the main vessel. This branch receives blood-ves-
sels from the facial region. Between the main jugular and its branch are some

STUDY OF SECTIONS OF EMBRYO OF 20 MM. 315

lymphatic spaces, somewhat irregular in form, and lined by a thin endothelium so
that they present a close resemblance to veins in their structure; if followed up
toward the head the lymphatics are found to unite with the large jugular lymph-
sac (Fig. 60 , s.l.f). Close to the medial wall of the jugular vein is situated the
large trunk of the vagus nerve, N.io. At a little lower level than the vagus
nerves and in the median line lies the anlage of the thyroid gland, which, owing
to its darker staining, is somewhat conspicuous. The cells of the thyroid form
an irregularly shaped branching mass. The spaces between the branches are chiefly
occupied by small endothelial blood-vessels. The arrangement of these cavities
and the relation of their endothelium to the cells of the organ recall the blood
sinusoids of the liver and of the suprarenal capsule. The thyroid cells are com-
pactly arranged without distinct cell-boundaries, but with protoplasm which stains
somewhat and with nuclei of rounded form, distinct outline, and granular appear-
ance, the granules being decidedly more conspicuous than the granules in the nu-
clei of the neighboring mesenchymal cells. Just ventral to each jugular vein is
a small darker body, consisting of closely compacted cells, resembling in appear-
ance those of the thyroid. The body has a very distinct external outline and
is actively growing, for several of its nuclei are in mitosis. The bodies in question
are the parathyroid glands. The rest of the section is mainly occupied by mesen-
chyma and numerous darker masses, muse, the anlages of the various muscles of
the neck and throat. On each side is shown a small piece of the cartilaginous
scapula, Scap. At the lower corner of the section is an indication of the anterior
limb, A.L, and of its vein, Ve".

Section through the Lungs (Fig. 214). — The spinal cord shows very clearly in
the differentiation of the three primary layers of the medullary wall. Its structure
is similar to that shown in figure 208, and need not be again described. The
vertebra, Vert, is now distinctly young cartilage. On its ventral side its boundary
is quite distinct, the formation of the perichondrium having there begun. Laterally
it merges into a dense mesenchyma, by which it is united without demarcation with
the rib, cost', and indirectly with the vertebral arch, V.ar, both of which are
cartilaginous. The cells of the vertebral cartilage occupy rounded cavities, each
of which is marked by a distinct capsule. The matrix between the capsules is
homogeneous, stains slightly, and has acquired a "greater density than in earlier
stages. The cells themselves exhibit traces of their protoplasmic bodies and have
deeply stained nuclei which are quite irregular in shape and very granular. Im-
mediately around the notochord the spaces occupied by the cells are the largest,
the capsules most distinct, and the nuclei most altered. Proceeding toward the
periphery of the cartilage, the cells appear in successively earlier and earlier stages,
until at the very periphery we have normal nuclei and a transition to mesenchyma.
The notochord has contracted, leaving a space between the notochordal cells and the
vertebral cartilage. Immediately below the vertebra are the conspicuous anlages
of the sympathetic system, Sym. They overlie the sections of the posterior cardinal

310

STUDY OF PIG EMBRYOS.

\

u

£

3

E
g

Vi

•"

3

|

o
^

o

(Ll
J

E

^

1

<u

-.

a

h

E

1

j

-^

S"

~

™
•

!

n

"o

U

u

Sc
4

X

u
U

-H'

_5

E

it

ri

H

1
-S'

u

_o

"^

^

0

r

0>

1
D

c

4J

u

s,

E

^

H

. £

*

g

_S

BO

i

2

'C

^

-r

>>

jj

o

5

^_

i

d

.5

^

>,

1

a

en

>

f

,5

nl

^

U

j£>

d

d

W

3

"^

."fr-

en

—

•^-'

H

S

5

W

c

~_^

i/

o

~

d
*•*

i>

•~
u

"z.

_"

w

:=

_

"^

in

H
en

terior

i

i/.

~
_~2

3

1

-

H

o

G

c

B

~

>

-

"H.
_g

a

*<

',

2

•»

n

M

i'

>

H

?
Q

^

0

"->

U

1

. -

t/5

—

^

s

0

"6

§

E
u

^
3
U

^

CS

c3

T^

—

0

c^
u

U

'~

5.

u

-.

5

::

|

^

J*

~^

,.j.

^o

c

^

-

0)

a

5

s

:-

^
tJ

^

—

d

^

M

X

^

£

^

'•r-

B

3

^>

i

(J

-

id

^-'

ri

°f

is

"-"
C

U

o a

-

I.T3

c ?

S >-)

^c3 •

U be
tt . B

STUDY OF SECTIONS OF EMBRYO OF 20 MM. 317

veins, card. These are now quite small vessels, the vena cava inferior having
become the main channel for the return of the blood from the abdominal region to
the heart. The two cardinal veins are not quite symmetrically placed, that on the
left side lying a little lower than that on the right. Between them is situated
the median aorta, Ao, with a relatively thick and well-developed muscular coat,
the deeper staining of which makes it conspicuous even with low powers. The
(esophagus, (E, and trachea, Tra, are not in the median line, but are both dis-
placed toward the right of the embryo.. As compared with earlier stages, both
structures show an advance, first, by the growth of the entoderm, and, second, by
the differentiation of the surrounding mesenchyma. In both oesophagus and trachea
the entoderm is a ring of cylinder epithelium, the tracheal ring being much larger
than the oesophageal. The mesenchyma about the oesophagus forms two distinct
layers, an inner looser layer and an outer denser muscular layer. Around the
trachea the. mesoderm is much condensed. On the dorsal side of the trachea the
cells form next to the epithelium a special layer characterized by the elongated
form of the cells. Between the oesophagus and trachea are situated the vagus nerves,
that of the right side, N.io, occupying a higher position than that on the left,
so that the nerves are not symmetrically placed. The cardinal veins, the aorta,
the oesophagus, the vagus nerve, and the trachea are all imbedded in mesenchyma,
which, together- with these structures, forms the so-called mediastinum by which the
right and left pulmonary cavities, Pl.d, Pl.s, are separated from one another. On
its ventral side the mediastinum joins on to the veins entering the heart. On either
side of the mediastinum at the level of the trachea may be seen the projecting
lung. That on the left side shows clearly the division of the organ into a dorsal
lobe, Lu.d, and a ventral lobe, Lu.v. Each lung consists at this stage chiefly of
mesenchymal tissue and is covered by a layer of mesothelium which forms the
boundary of the pleural ccelom. Within the mesenchyma appear several sections
of the branches of the entodermal bronchi. Each bronchus is lined at this stage
by a rather thick entodermal layer of cylinder cells. The union of the lung with
the mediastinum constitutes the so-called root of the lung. In the root of the lung
is seen the small pulmonary artery, A.pul. The two arteries join a little nearer the
head and on the left side of the embryo to form a single trunk, the main pul-
monary artery. Originally the pulmonary arteries arise symmetrically as branches
from the fifth aortic arches. They soon unite, however, throughout the greater
part of their extent, forming a single vessel. The two arteries shown in our figure
represent the two original symmetrical vessels where they are about to enter the
lungs. On the ventral side of the section various cardiac structures are shown,
but so cut that the picture is not very instructive. It will suffice to refer to the
explanation of the figure for the identification of the parts.

Sections through the Miillerian Ducts (Fig. 215). — The female or Miillerian
ducts are remarkable for their late development. In the 12 mm. pig the
small funnel-shaped in aginations of the mesothelium, which represent the first

318

STUDY OF PIG EMBRYOS.

anlages of the ducts, can just be recognized on the lower mesial surface of the
Wolffian body near the cephalic end of the organ. In the 20 mm. pig the funnels
have lengthened into tubes, which run a short distance caudad, close underneath
the Wolffian duct. Figure 215 A is a section through the right Mullerian funnel,
F. The mesothelium near the funnel is considerably thickened, forming the so-
called tubal band, Ep. The funnel is lodged in a small ridge, Rid, which projects
downward from the mesonephros. The Wolffian duct does not appear in the section
as it does not extend so far head ward. Figure 215 B is a section farther caudad,
to show the oval Mullerian duct, M.D, which immediately underlies the Wolffian
duct, W.D, and causes a small protuberance on the surface of the mesonephros.

V.s.c. Msth

W.t

Rid

W.D

M.D

Rid

FIG. 215. — PIG OF 20 MM. TRANSVERSE SERIES 59, A, SECTION 851; B, SECTION 887.

Ep, Tubal band of thickened mesothelium. F, Funnel-shaped opening of the Mullerian duct. M.D, Mullerian
duct. Msth, Mesothelium. Rid, Ridge containing the Mullerian funnel. V.s.c, Sub-cardinal vein. W.D,
Wolffian duct. W.I, Wolffian tubules. X 150 diams.

A short distance farther on the duct ends in a blind point. It is destined to
continue its backward elongation until it reaches and joins the neck of the allantois.
At just what stage the junction occurs in the pig is undetermined.

Section through the Posterior Limbs (Fig. 216). — Although this section is from
a transverse series, yet, owing to the curvature of the body, it shows the spinal
cord cut very obliquely. The three layers of the cord, the ependymal, epen, the
cinerea or neurone layer, Cin, and the ectoglia are well marked. Something of
the dorsal root, D.R, and of the ganglia, G, of a lumbar nerve are also shown in
the section. The nerves have already joined together to form a very complex
lumbar plexus, sections of portions of which appear at various points. These are
all indicated by the reference letter N in the figure, it being thought not desirable
to attempt an identification of each component of the plexus. The plexus is more
or less symmetrically placed on tho right and left, at about the level of the intes-
tine, Reel. The limbs are large projections extending do\w \vard and containing in

STUDY OF SECTIONS OF EMBRYO OF 20 MM.

Cin. D.R. epen. arach. Sk. EC.

319

V .arc.

A.sul.

Vert. _

Nch.

muse.

Sym.

V.il.

cart".

V.p.

Pen.

FIG. 216. — PIG, 20 MM. TRANSVERSE SERIES 59, SECTION 1253.

arach, Arachnoid membrane. A.sul, Arteria sulci. cart', cart", Cartilaginous anlages of elements of the skeleton
of the limb. Cin, Neurone layer of spinal cord. D.R, Dorsal root. EC, Ectoderm, epen, Ependymal layer
of spinal cord. G, Spinal ganglion, muse, One of the muscular anlages. N, N, N, Nerves of the lumbar
plexus. Nch, Notochord. Pen, Penis. Red, Rectum. Sk, Anlage of the dura mater. Sym, Sympathetic
nerve. T, Tail. Ur, Urethra. V.drc, Vertebral arch. Vert, Vertebra. V.il, Iliac vein. V.p, Border
vein of the limb. X 22 diams.

320 STUDY OF PIG EMBRYOS.

their interior the cartilaginous anlages, cart', cart", of the skeleton of the limb,
and, around these, darker masses of tissue, the developing muscle-fibers. At the
lower edge of each limb is a blood-vessel, V.p, the so-called border or peripheral
vein, which extends completely around the edge of the developing hand and foot.
When the digits are developed, this vein becomes broken up, and out of its divi-
sions are formed the digital vessels. The section also passes through the penis,
Pen, in the center of which is the urethra, Ur. It shows here as a narrow
epithelial band without any cavity, except a very small one at its external
dorsal end. The band is lighter in the center, owing to the fact that the nuclei
are grouped chiefly close to the two surfaces of the band. At the base of the limb
is situated the irregularly shaped section of the iliac vein, V ' .11. In the median
line may be noted the following structures. Immediately underneath the nervous
system is the arteria sulci, A.sul. The vertebra, Vert, and notochord, Nch, resem-
ble corresponding structures in the section described on page 315, except that their
cytomorphosis is slightly less advanced. Below the vertebra lie the paired anlages of
the sympathetic nervous system, Sym, between which is the small median caudal
artery. The intestine, Reel, has its transverse diameter somewhat increased, so that
it appears oval in the section. Around it is beginning the differentiation of the
mucosa and muscularis.

Section through the Mammary Anlage (Fig. 217). — The figure represents a section
through the somatopleure of the embryo in the region of a mammary gland. The
ectoderm, EC, covers the external surface of the somatopleure, as does the meso-
thelium, msth, the inner surface, the space between the two covering layers being
occupied by various mesodermic structures. The ectoderm consists of two or
three layers of cells, the external one of which, Eptr, the epitrichium, is very thin.
To form the mammary anlage, Mam, the ectoderm suddenly thickens and projects
somewhat outward and still more inward into the mesoderm. The epitrichium
passes continuously over the thickening, in the production of which it takes no
share. The inner edge of the ectoderm is marked by a very distinct line or base-
ment membrane, b, against the underlying mesoderm. The cells of the anlage
form two groups, one a band next to the basement membrane, in which the cells
present a somewhat radial arrangement, and the other a central group of cells, many
of which are elongated in a direction somewhat parallel to the surface of the
anlage, so that they form curving lines. The elongated cells in later stages gradu-
ally cornify and fall out, so that the anlage becomes hollow, but its excavation
proceeds very slowly, and in man is not usually completed until after birth. Soon
after the hollowing out of the anlage has begun, it sends out a series of buds from
its inner surface. These buds become elongated, somewhat twisted cords of cells,
and offer at this stage resemblance to embryonic sweat-glands. The outgrowths
subsequently branch and develop central cavities, and are ultimately transformed
into 'the secretory portion of the gland.

Figure 217 also illustrates some important points in regard to the differentia-

STUDY OF SECTIONS OF EMBRYO OF 20 MM.

321

tion of the somatopleure. Parallel to the ectoderm, and some distance from it, is
a layer, Pan, which is marked out by numerous blood-vessels. This is the pan-
choroid layer. There is a slight but unmistakable difference in the mesoderm within
and without this layer, for in the region between the panchoroid and the ectoderm
the cells are somewhat more crowded. They probably represent the anlage of

Mam.

b.

FIG. 217. — PIG, 20. o MM. TRANSVERSE SERIES 59, SECTION 1043.

b, Basement membrane of epidermis, -cost, Costal anlage. Cu, Cutis. EC, Ectoderm. Eptr, Epitrichium,
Mam, Mammary anlage. tnes, Mesenchyma. msth, Mesothelium. Pan, Tunica panchoroidea. per. in.
Peritoneal mesoderm. v e, Blood-vessel. X 250 diams.

the cutis, Cu, and of the cuds only. Within the vascular layer the mesodermic
cells, mes, are not so near to one another, and the processes, by which they are
connected, are more slender. Toward the mesothelium is a broad band of denser
tissue, cost, the rudiment of a rib, the inner boundary of which is further marked
by several blood-vessels, ve. Between the costal anlage and the mesothelium is a
layer of embryonic connective tissue, the cells of which are more crowded toward

322 STUDY OF PIG EMBRYOS.

the mesothelium, so that we may say that two layers of mesenchyma are already
imperfectly differentiated within the rib. The denser layer next the mesothelium
is destined to become still more marked and to transform itself into the connective-
tissue layer of the peritoneum. With the overlying mesothelium it develops into
the peritoneal membrane of descriptive anatomy.

Sagittal Section through the Right Lung and Kidney (Fig. 218). — The lungs
occupy a position in the upper part of the figure and are easily recognized by the
conspicuous- entodermal bronchi, bro, which resemble in microscopic structure the
bronchi of earlier stages. The branches are widely separated from one another by
the voluminous mesenchyma of the organ. . The lung is covered by mesothelium,
msth, and projects into the pleural cavity, Pleu.c, which is lined by a continuation
of the mesothelium of the lung itself. The pleural cavity can be followed down-
ward past the Wolffian body, W.b', and liver, and from there past the genital
gland, Gen, and so on to the lowest part of the abdominal cavity, Ab.cce. The
pleural cavity at this stage is entirely separated from the pericardial, but it is
still directly continuous with the abdominal cavity. On the ventral side (in the
figure, to the right) of the pleural cavity are the great veins, the common cardinal,
C.C, descending from above, and the ductus venosus, Du.v, rising from below. The
pleural cavity is separated from the common cardinal by a lamina of the mesoderm,
x, and from the ductus venosus by a similar but thinner lamina, Y. Both laminae
are bounded on the tdeural side by the mesothelium, and on the venous side by
the endothelium of the vessel. The opening of the veins into the right auricle,
Au.d, does not appear in^this section, though a small bit of the left valve, v.s,
which guards this opening is shown. The Wolffian body is divided into two parts,
an upper, W.b', on a level with the liver, and a lower, W.b", toward the pelvic
end of the abdomen. The lower part is larger than the upper. The two parts are
separated from one another chiefly by the mesonephric vein, V.msn, which is the
principal vessel to take the blood from the Wolffian body. It delivers the blood to
the lower end of the vena cava inferior. The separation of the two parts of the
Wolffian body is, however, further accented by the position of the genital gland,
Gen, and of the kidney, Ki. The structure of the latter organ does not differ much
from that of earlier stages, but the diameter of the tubules has increased, and there
has been an advance in the differentiation of the convoluted tubules and of the
glomeruli. The genital gland (testis) is remarkable for its large size. It is covered
by a layer of mesothelium, underneath which is a rather broad layer of elongated

FIG. 218. — PIG, 20.0 MM. SAGITTAL SERIES 60, SECTION 213.

Ab.cce, Abdominal ccelom. All.vi, Mesothelial villi of the allantois. Au.d, Right auricle, bro, Entodermal
bronchus. C.C, Common cardinal. Cae', Cce", Ccelom. Diaph', Diaph", Diaphragm. Du.v, Ductus
venosus. G.bl, Gall-bladder. Gen, Genital gland. In', In", In'", Intestine. Ki, Kidney. Li, Liver.
Mes, Mesenchyma. msth, Pleural mesothelium. P.cce, Pleural ccelom. Pleu.c, Pleural cavity. Ve. hep,
Ve.hep', Hepatic veins. V.msn, Vena mesonephrica. v.s, Valvula sinistra. W.b', W.b", Wolffian body.
x, Partition separating the pleural cavity from the duct of Cuvier. Y, Partition separating the pleural cavity
from the ductus venosus. X 22 diams.

STUDY 'OF SECTIONS OF EMBRYO OF 20 MM.

323

C.C. v.s, P.cce. Au.d.

Pleu.c

Diaph"

Ki.

W.b

All.vi.

324 STUDY OF PIG EMBRYOS.

mesenchymal cells, the anlage of the tunica albuginea. The interior of the organ
contains a number of contorted epithelioid cords of cells in which there are a cer-
tain number of so-called primitive ova, cells which are distinguished by their larger
size, rounded form, greater transparency, and spherical nuclei. The bands of cells
are known as the sexual cords, and they are separated from one another by loose
mesenchymal tissue. The cords frequently anastomose with one another. They are
the solid anlages of the seminiferous tubules. The question of the origin of these
cords has been much debated, but cannot be considered as yet settled. As to both
the origin and the ultimate fate of the primitive ova in the mammalian testis
we have only incomplete information. The cords remain solid throughout embryonic
life, not acquiring a central cavity until after birth. The liver is a very voluminous
organ permeated everywhere by sinusoidal blood-vessels which offer the greatest
possible variety in size. In the figure only the larger of these blood-vessels have been
drawn in, A large proportion of the smaller sinusoids are crowded with nucleated
red blood-corpuscles, the nuclei of which are small and deeply stained; hence each
cluster of corpuscles stands out as a darker spot in the liver, for the liver cells
themselves stain lightly and have nuclei which, though three or four times the size
of the nuclei of the blood-corpuscles, yet appear relatively pale in the stained
specimen. The blood-corpuscles which form the clusters in the liver differ some-
what from those in the active circulation, for they are smaller, and show less of the
characteristic hemoglobin color. It has been demonstrated that the liver at this
stage furnishes sites for the multiplication of the blood-corpuscles, and the clusters,
which are so conspicuous in the organ, correspond not to blood-corpuscles in active
circulation, but rather to corpuscles which have found a lodging-place in the liver
and are there proliferating. Our knowledge of the blood-forming function of the
embryonic liver is imperfect. Above the liver is the septum transversum or dia-
phragm, Diaph' ',. Diaph" ', which is formed chiefly by mesenchyma. On the lower side
of the liver is another broad accumulation of mesenchyma, Mes, in which is lodged
the gall-bladder, a small section, G.bl, of the entodermal lining of which is included.
The intestine, In', In", In"', is cut several times, because at this stage the intes-
tinal canal forms several coils in the abdominal cavity below the liver and on the
ventral side of the Wolffian bodies. Below the intestines appear the curious meso-
thelial villi, All.vi, of the allantois (compare page 253). At this stage the villi
are little more than large vesicles of mesothelium, which contain in their interior
some coagulum and a very few mesenchymal cells, associated with which are a few
fibers — whether true connective-tissue fibers or not is undetermined. The mesothe-
lium of the villi is a very thin layer of flattened cells.

Frontal Sections of the Head. — The three sections to be described are from
a special series of the head. The plane of the series was made as nearly as possi-
ble transverse and at right angles to the plane of the roof of the mouth. They
illustrate some important points in regard to the development of the facial region
and of the fore-brain. In all of the sections the differentiation of the mesoderm

FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM.

325

around the brain is clearly demonstrated. The pia mater is very distinct. In those
parts of the sections where the brain-wall is cut obliquely, it can be distinguished
only by a somewhat careful observation, as the tissues of the pia mater and of
the brain overlap. All about the brain is the broad zone of the arachnoid (Figs.
220 and 221, arach), easily distinguishable even with a low power by its light colora-
tion. It consists of widely separated cells connected together by very distinct pro-
cesses, and is permeated by a number of small blood-vessels running in various direc-
tions through the layer. Its external boundary is now very distinct, being marked
by a layer, Sk, of somewhat crowded, elongated cells which merge on the side

Sept.

Jk.o. -

Max.

nas.tb.

nax.tb.

MX. sup.

Mdb.

FIG. 219. — PIG, 20.0 MM. FRONTAL SECTION or HEAD. SERIES 40, SECTION 68.

H, Cerebral hemisphere. Jk.o, Jakobson's organ. Max, Maxillary process, max.tb, Maxillo-turbinal fold.
Mdb, Mandible. MX. sup, Superior maxillary nerve, nas.tb, Naso-turbinal fold. Sept, Nasal septum. Sk,
Mesenchymal anlage of the dura mater and skull. X 18 diams.

toward the ectoderm into the general surrounding mesenchyma. Out of this
denser layer (Figs. 219 and 220, Sk) arise both dura mater and the membrane
bones of the skull.

Section through the Anterior Part of the Snout (Fig. 219). — On the dorsal side
appear the two cerebral hemispheres, H, cut separately and each showing the
cavity of its lateral ventricle. On the ventral side the mandible, Mdb, is cut
separately and is separated by the oral fissure from the rest of the section. The
maxillary processes, Max, are large, and each is furnished with an inward prolonga-
tion extending toward the median line. From the oral fissure there extend upward

326

STUDY OF PIG EMBRYOS.

two irregular cavities, the nasal chambers. The two cavities are separated from
one another by a broad mass of tissue, the nasal septum, the ventral edge of which
at this stage forms a portion of the roof of the mouth-cavity. In the center of the
nasal septum is a broad band, Sept, of denser mesenchymal tissue, the anlage of
the cartilaginous septum of the nose. On either side of the nasal septum is the
irregularly shaped nasal cavity, which opens into the mouth between the ven-
tral edge of the nasal septum and the inner edge of the maxillary process. The

arach. ' Sk.

Mdb.

FIG. 220. — PIG, 20.0 MM. FRONTAL SECTION OF HEAD. SERIES 40, SECTION 84.

arach, Arachnoid membrane. H, Cerebral hemispheres. Max.tb, Maxillo-turbinal fold. Mdb, Mandible. MX.
sup, Superior maxillary nerve. Nas.tb, Naso-turbinal fold. N.olf, Olfactory nerve. Sept, Cartilaginous
septum of the nose. Sk, Mesenchymal anlage of the dura mater and skull. Ton, Tongue. X 18 diams.

medial side of each nasal cavity is comparatively regular, but- the external side
shows two prominences, each of which is formed by a mass of mesenchymal tissue
covered by epithelium. The upper of these projections, nas.tb, is the anlage of
the naso-turbinal fold, and the lower, max.tb, the anlage of the maxillo-turbinal
fold. In the nasal septum itself are two oval rings of epithelium, sections of
Jakobson's organs. This organ is an evagination of the epithelial lining of the
nasal cavity, which opens anteriorly and extends backward some distance in the
nasal septum. In the maxillary process may be observed the superior maxillary
nerve, Mx.sup. The number of cells in the nerve has increased, and consequently

FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 327

the division of the nerve-fibers into distinct bundles has become more marked
as compared with the pig embryo of 12 mm.

Section through the Middle of the Snout (Fig. 220). — The relations are very
similar to those described in the previous section, so that it will suffice to note the
three most important differences: first, the absence of Jakobson's organ; second,
the appearance of the tongue, Ton, and third, of the olfactory nerve, N.olf. The
tongue is a protuberance attached to the lower jaw, Mdb. Its connection with
the jaw is rather narrow and corresponds to the frenum. The tongue extends
upward between the maxillary processes until it is almost or quite in contact with
the lower edge of the nasal septum. It is formed by a somewhat dense mass of
tissue in which there is no very evident histological differentiation, and is covered
by a layer of epithelium of moderate thickness and which is probably entirely
derived from the entoderm, for the tongue first appears as a small median pro-
tuberance on the ventral floor of the pharynx, between the first gill-pouches.
The olfactory nerve, N.olf, can be seen joining the lower part of the inner side of
the brain-wall and extending down toward the nasal cavity and branching. Under
the part of the nerve near the brain-wall numerous cells are mingled with the
fibers, and by their crowding render the nerve conspicuous in stained sections.
The fibers of the olfactory nerve differ from all other nerve-fibers known ' in ver-
tebrates. They arise as prolongations of certain of the epithelial cells of the
olfactory region of the nose and grow from these cells into the brain, where they
have their termination in the glomeruli of the olfactory bulb. All other nerve-
fibers arise from nerve-cells either of the central nervous system or of the gan-
glia. Morphologically, therefore, the olfactory nerve takes a unique place, and
is not directly comparable with any other nerve of the brain. The cells which
accumulate in the course of the olfactory nerve do not, so far as known, have any
direct share in the production of the nerve-fibers; nor do they result in the
formation of the medullary sheaths, as they do in other nerves, the olfactory
nerve-fibers remaining naked, as it is termed, throughout life..

Section through the. Fore-brain and Eyes (Fig. 221). — The section passes behind
the nasal cavities, no part of which is shown. The maxillary and mandibular
processes are united and the pharynx, Ph, appears as a closed cavity. On the
dorsal side of the section the fore-brain stands out conspicuously, both from its
dark staining and from being surrounded by the lightly stained broad zone of the
arachnoid, arach. The cavity of the fore-brain has two lateral expansions, L.V,
the lateral ventricles, which extend outward and upward. The walls, H, of the
lateral ventricles are much thinner than the walls of the lower part of the fore-
brain and are the anlages of the cerebral hemispheres. In the median plane the
hemispheres include between themselves a partition, Fx, of mesodermic tissue
which may be designated as the embryonic falx, since within it, though considerably
later, the adult falx will be differentiat d. In the adult, the falx appears as a
prolongation of the dura mater. From the bottom of the falx there extends on

328

STUDY OF PIG EMBRYOS.

each side a fold, Plx, which projects into the cavity of the lateral ventricle. This
fold contains in its interior a prolongation of the mesodermic tissue of the falx,
and it is covered by a continuation of the wall of the hemispheres. The covering
layer of the fold is much thinner than any other portion of the brain-wall shown
in the section, and shows no differentiation into layers. It retains throughout

Fx. ec.gl.

H.

L.V.

Plx.

C.str.

m.rec.sup.
m.retr.b.

hy.gl.

art.

FIG. 221. — PIG, 20 MM. FRONTAL SECTION or HEAD. SERIES 40, SECTION 123.

arach, Arachnoid zone, art, Lingual arteries. C.str, Corpus striatum. ec.gl, Ectoglia. Fx, Falx cerebri. H,
Cerebral hemisphere, hy.gl, Hyoglossal muscle. L, Lens. L.V, Lateral ventricle. Mk, Meckel's car-
tilage, m.rec.sup, Musculus rectus superior, m.retr.b, Musculus retractor bulbi. m.r.lat, Musculus rectus
lateralis (cf. text). Mx.i, Inferior maxillary nerve. ' Ph, Pharynx. Plx, Plexus choroideus lateralis. Ret,
Retina. Sk, Anlage of membranous skull. Ton, Tongue, x, Unidentified structure. X 18 diams.

life an epithelial character and is already to be termed ependyma. The ependyma
of the two folds is connected across the median line, and it forms the median
dorsal boundary of the cavity of the fore-brain. The two folds are the anlages
of the lateral choroid plexus. They are destined to grow much in size and in
complexity of form, but they always remain morphologically what they now are,

FRONTAL SECTIONS OF HEAD, EMBRYO OF 20 MM. 329

vascularized mesenchyma covered by ependyma. The choroid plexus protrudes
into the cavity of the brain in the same way in which the viscera- may be said
to protrude into the abdominal cavity. The cavity of the brain is bounded by
the brain-wall or ependyma, just as the abdominal cavity is bounded by the
peritoneum. The vascular tissue of the choroid plexus is outside of the cavity
of the brain, in the same way that the tissue of the kidney is outside the cavity
of the abdomen. Throughout life the choroid plexus springs, as it does from the
start, from the medial wall of the hemispheres, and it is only at that point that
it can receive its blood-supply. The lateral walls of the hemispheres, H, gradually
thicken as they continue ventralward, and on the ventral side of the brain form
in part the lateral boundary of the medial portion of the brain-cavity, as an especial
thickening of the brain-wall which projects far into the cavity. The thickening,
C.str, is the corpus striatum. Between the summit of the corpus striatum and
the choroid plexus is an open passage through which we may pass from the
median portion of the brain-cavity into the lateral ventricle, L.V. The passage
is the foramen of Munro, which we learn from this section is bounded above
by the choroid plexus, and below by the corpus striatum. On the dorsal and
middle sides of the hemispheres, the ectoglia, ec.gl, is already clearly differentiated.
There is, however, at this stage, no clear indication of the cortex cerebri, al-
though in the slightly older stages it will begin to develop by the accumulation of
neuroblasts immediately beneath the ectoglia. The notochord does not appear
between the brain and the pharynx, the section being too far forward. The noto-
chord stops near the hypophysis. The eyes are not cut quite symmetrically. They
show the lens, L, and retina, Ret, clearly and the left eye of the embryo shows
also the entrance of the optic nerve. On the right side of the embryo, near the
eye, are three areas which are somewhat more darkly stained than the surrounding
mesenchyma. These are the anlages of the muscles of the eye. They have not
yet been studied sufficiently to make their identification certain, but it seems prob-
able that the uppermost of these anlages, m.rec.sup, is the rectus superior, that the
middle one, m.retr.b, is the retractor bulbi, and that the lowest one, m.r.lat, is
the rectus lateralis. Until a reconstruction is . made these identifications can be
recorded as tentative only. The pharynx, Ph, appears as a yoke-shaped slit lined
throughout by entoderm. From its median ventral floor rises the great mass of
the tongue, Ton, over which the dorsal roof of the pharynx forms a closely fitting
arch. A portion of the epithelium of the tongue is loosened from the underlying
tissue, probably owing to defective preservation. Upon the lower side of the
tongue extend downward the anlages of the hyoglossal muscles, hy.gl, between
which are situated the lingual arteries, art. On either side, in the part of the
section corresponding to the mandible, appears Meckel's cartilage, Mk, a some-
what conspicuous and easily identified structure, owing to its dark staining. MeckeVs
cartilage is the primitive skeletal element of the mandibular arch, and is homol-
ogous with the cartilaginous jaw of the lower fishes. It is, for the greater part,

330

STUDY OF PIG EMBRYOS.

a purely embryonic structure, the mandible of the adult being a secondary bone.
By referring to figure 166 (pig, 10 mm.), it can be seen that the mandibular arch
extends upward toward the otocyst and forms the boundary of the first gill-cleft,
the outer division of which becomes the meatus auditorius externus. In other
words, the upper portion of the mandibular arch is in close proximity to the
otocyst and the anlage of the tympanum or middle ear. Meckel's cartilage is a

R

. N

FIG. 222. — RABBIT EMBRYO OF THIRTEEN DAYS; SECTION OF THE EYE.;

EC, Epidermis. L, Lens, mes, Mesenchyma. N, Anlage of optic nerve. P, Pigment layer. R, Retina, tu.v ,

Tunica vasculosa lentis.

rod-like structure extending the entire length of the arch. Its upper end is, there-
fore, close to the future tympanum. While the greater part of Meckel's cartilage
disappears during later development, the upper end persists and takes a direct
share in the formation of the malleus. A little outside of Meckel's cartilage in
our 'section is the inferior maxillary nerve, Mx.i, and still farther lateralward is a
small, darkly stained body, x, which has not yet been identified with certainty.

Pig Embryo of 24 mm. Study of Sections.

Section through the Eye (Fig. 223). — In the pig of 24 mm. the anlages of all

the parts of the adult eye may be said to be present, with the exception of the

pigment layer of the iris, which arises somewhat later by a forward growth of the

STUDY OF SECTIONS OF EMBRYO OF 24 MM. 331

retina and pigment layer. The origin of the retina and lens is illustrated by the
chicken embryo (Figs. 153, 154), and in a more advanced stage by the pig of
12 mm. (Fig. 192). There is added here figure 222, from a section of the eye
of a rabbit embryo of thirteen days, in order to facilitate the comparison between
the 12 mm. stage and the 24 mm. stage of the pig. In figure 222 the ectoderm,
EC, forms an arch over the eye and indicates the commencing formation of the
cornea, the layer of ectoderm being destined to become the external epithelium
of the cornea. Between the lens and the retina there has been an ingrowth of
tissue accompanied by blood-vessels, which forms a more or less distinct covering
over the surface of the lens and constitutes the so-called tunica vasculosa, tu.v. The
space between the retina and lens will increase during the following stages and
will become occupied by a very clear tissue containing a minimal number of cells.
This "clear tissue is the commencement of the vitreous humor. Between the lens
and the overlying ectoderm the mesenchyma has begun to penetrate. This mesen-
chyma will ultimately furnish the connective tissue of the cornea and of the iris.
About the eyeball as yet there is no distinct condensation of tissue such as will
appear later to develop the anlages of the choroid and scleral coats of the eyeball.
In the pig of 24 mm. (Fig. 223) we encounter a marked advance in the differ-
entiation of all parts of the eye. Above and below the eye the anlages of the
eyelids, L.sup, L.inf, have appeared. The anlage is at this stage merely a projecting
fold of the ectoderm filled with mesenchyma and extending a short distance over
the projecting cornea. -The folds will continue to grow until the eyelids meet in
the middle of the eye, covering it completely. The ectoderm of the two lids where
they meet unites. The union of the two lids occurs in all mammals, and in some
cases they do not separate again until after birth, in which case the animals are
said to be "born blind." The ectoderm, EC, of the cornea describes a wide, high
arch, underneath which is a broad band of embryonic connective tissue, corn,
which forms the main thickness of the cornea. Between the connective tissue
of the cornea and the anterior surface of the lens is a clear space, an.ch, which we
can identify as the anterior chamber of the eye, which in the adult is filled only
with the aqueous humor. On the corneal side the anterior chamber is bounded
by a distinct layer of cells, Ep, the internal epithelium of the cornea. This layer
is, however, formed from the mesenchyma, the cells of which develop into the
internal epithelioid covering of the cornea. At the upper and lower edge of the
cornea there is a separate forward growth, Ir, of the connective tissue between
the cornea and the lens. It is the anlage of the connective-tissue layer of the iris.
In later stages it will grow still farther over the lens from all sides, leaving a cen-
tral opening, the pupil, and it will acquire a special pigmented layer on its side
nearest the lens. At the base of the iris anlage is a small blood-vessel, Schl, which
is commonly designated in the adult as the canal of Schlemm. The retina has
increased in thickness and is closely covered by a pigment layer, Pig. The separa-
tion which appears on the inner side of the eyeball between the retina and pig-

332

STUDY OF PIG EMBRYOS.

ment layer in figure 223 is probably artificial, the result of shrinkage during the
preservation of the specimen. At its outer edge the retina, Ret, suddenly thins out
and passes over into the external pigment layer, which is heavily loaded with dark,
yniform, pigment granules, especially crowded together on the side of the layer

L.sup.

N.op.

Vit.

Schl.

tu.v.

Ret.

L.inf.

FIG. 223. — PIG, 24.0 MM. TRANSVERSE SERIES 62, SECTION 428.

an.ch, Anterior chamber of eye. corn, Corneal mesoderm. EC, Ectoderm (epidermis). Ep, Inner epithelium
of cornea. Ir, Mesodermal anlage of iris. L', Outer layer of lens. L", Inner layer of lens. L.inf, Inferior
eyelid. L.sup, Superior eyelid. ^.3, Oculo-motor nerve. N.op, Optic nerve. Pig, Pigment layer. Ret,
Retina. Schl, Canal of Schlemm. tu.v, Tunica vasculosa lentis. Vit, Vitreous humor. X 50 diams.

nearest the retina. In later stages the pigment layer grows forward on the inner
side of the iris, making a fold, so that the iris is covered on the inside by a
double layer of pigmented epithelium, the uvea. The retina resembles closely in
structure the brain-wall in an early stage, for it has on its outer surface a thin

STUDY OF SECTIONS OF EMBRYO OF 24 MM. 333

layer corresponding to the ectoglia, and within a broad, nucleated zone. The
mitotic figures are found only next to the surface of the retina nearest the pig-
ment layer. Since the space between the pigment layer" and the retina corresponds
to the cavity of the brain, it is evident that the position of the mitotic figures
is homologous with their position in the medullary wall elsewhere. The section
of the lens clearly reveals its vesicular structure. The external wall of the lens
vesicle, L', is a comparatively thin epithelial layer which stains quite readily and
therefore stands out clearly in the section. Toward the edges of the lens the outer
layer slightly thickens and then- passes over quite abruptly into the inner layer of
the vesicle, L" ', which is very thick and constitutes by far the greater part of the
bulk of the organ and gives to the lens its characteristic shape. The outer and
inner walls of the lens are in close contact so that there is no actual cavity present.
The epithelial cells of the inner wall have elongated enormously, so much that
they might perhaps already be. termed "fibers." Each cell is supposed to extend
through the entire thickness of the inner wall. The nuclei are placed somewhat
irregularly in the middle portion of the long -cells so that they constitute a more
or less distinct band in the section. Toward the edge of the lens the nuclear band
becomes more distinct, and where the inner wall merges into the outer, the band
becomes narrow and the nuclei are much crowded together. The nuclei of the lens
fibers are oval, being slightly elongated in the same direction as the fibers, and
each nucleus contains usually a distinct nucleolus. Between the lens and the retina
is the vitreous humor, Vit, which has become quite voluminous. It contains a few
mesenchymal cells and a few small blood-vessels, and when examined with a high
power it is, seen to be permeated by a fine network which is probably to be
interpreted as a modification of the protoplasmic threads of the mesenc.hyma. There
are also a very few cells of rounded form and distinct outline, with a single small
granular nucleus, which are probably leucocytes. Against the surface of the lens
there is a delicate homogeneous hyaloid membrane, which can usually be better
seen where by shrinkage it has been loosened from the surface of the lens, as
is apt to occur. Against the hyaloid membrane are a number of small blood-ves-
sels, more numerous than those elsewhere in the vitreous humor, and forming a
fairly distinct vascular membrane around the lens. The membrane, tu.v, is called
the tunica- vasculosa lentis. The blood-vessels of the vitreous humor are chiefly,
possibly at this stage exclusively, branches of the central artery of the retina. The
artery enters the eye through the optic nerve, and sends branches throughout the
vitreous humor. The space originally occupied in the humor by the stem of the
central artery persists, and is called the hyaloid canal. The muscles of the eye
are already differentiated, but their relations cannot be properly understood without
a reconstruction.

Median Sagittal Section (Fig. 224). — The section figured is very nearly median
for the region of the head, but in the body it passes to the left of the median
plane. The area occupied in the section by the neck and head of the embryo is

334

Epen

Ven.IV

Md.ob

Te

Plx.IV.

STUDY OF PIG EMBRYOS.
Cbl. A. M.b.

Ar.hab.

Cce.

W.b

T".

STUDY OF SECTIONS OF EMBRYO OF 24 MM. 335

almost as great as that occupied by the rest of the body. The great size of the
head at this stage is characteristic. Attention is especially directed to the sharp
angle which the medulla oblongata, Md.ob, makes with the spinal cord, Sp.c, and
to the very great bend formed by the floor of the mid-brain, Ar.hab, in conse-
quence of which the floor of the hind-brain above the medulla oblongata and the
floor of the fore-brain are brought quite close together and run in almost parallel
directions. The cavity of the brain is very large. Its walls in the median plane
are, for the most part, thin. From the roof of the diencephalon, Dien, there runs
off a small evagination, Ephys, a shallow pocket or diverticulum of the medullary
wall. It is the anlage of the epiphysis or pineal organ of the adult. It is an
important landmark in the topography of the brain, for its position is always at
the extreme posterior limit of the fore-brain. In the wall of the mid-brain, behind
the epiphysis, for some distance the ectoglia shows considerable thickening and
contains a very large number of nerve-fibers running transversely. They constitute
the posterior commissure, which morphologically belongs to the mid-brain. In later
stages the opening of the epiphysis and the anterior boundary of the posterior
commissure are separated by a narrow band of ependyma. Immediately in front
of the epiphysis-, close to the external surface of the medullary wall, is another
tract of nerve-fibers which is very small and is known as the superior commissure.
The superior commissure develops much later than the posterior, and is much
smaller in size. The two commissures are found in vertebrates of all classes and
are exceedingly constant anatomical features of the brain. Anterior to the epiphy-
sis the dorsal roof of the diencephalon forms a broad arch which descends in
the figure vertically until it ends in a small inward projection, Fix, of the brain-
wall, the anlage of the choroid plexus. Below this point the brain-wall is continued,
forming the lamina lerminalis. It then makes a bend almost at right angles and
runs in a horizontal direction toward the dorsal side of the embryo. This portion
of the brain- wall shows a considerable thickening, the optic chiasma. Behind the
optic chiasma the brain-wall forms a short evagination, the infundibular gland, which
bends over so as • to lie close to the dorsal side of the hypophysis, Hyp. The
hypophysis, which in earlier stages appears clearly as an evagination of the oral

FIG. 224. — PIG, 24.0 MM. SAGITTAL SERIES 63, SECTION 30.

A, Arachnoid space, in this specimen containing extravasated blood. A.Ao, Arch of the main aorta. All.ar,
Allantoic artery. Ao, Dorsal aorta. Ao.D, End of dorsal aorta. Ar.hab, Habenular arch (floor of mid-
brain). A. vert, Vertebral artery joining its mate to form the basilar artery. Bro, Main bronchus of lung.
bro, Branch bronchus within the lung. CW, Cerebellum. Coe, Coelom. Diaph, Diaphragm. Dien, Dience-
phalon. Duo, Duodenum. Epen, Ependymal roof of hind-brain. Ephys, Epiphysis. G, Spinal ganglion.
Hyp, Hypophysis. In, Intestine. Int.v, Anlage of intervertebral ligament. La, Lateral wall of larynx. Li,
Liver. Lu, Lung. M.b, Mid-brain. Md.ob, Medulla oblongata. Nch, Notochord. (E, (Esophagus.
Pen, Penis. Ph, Pharynx. Plx, Choroid plexus of fore-brain. Plx.IV, Choroid plexus of hind-brain.
Sept, Cartilaginous nasal septum. Sp.c, Spinal cord. Sp.ren, Suprarenal capsule. St, Stomach. T',T",
Tail. Te, Testis. Ton, Anterior portion of tongue. Umb, Umbilical cord. Ve, Cardinal vein. Ven,
Ventricle of the heart. Ven. I V, Fourth ventricle, or cavity of the hind-brain. Vert, Vertebra. W.b, Wolman
body. X 8 diams.

336 STUDY OF PIG EMBRYOS.

epithelium (Fig. 201), is now entirely separated from the mouth, and is an epithe-
lial vesicle with an irregular cavity. The epithelium has sent out, especially on
its anterior side, a number of solid outgrowths. The infundibular gland and
hypophysis constitute the pituitary body of the adult. They are surrounded by
loose mesenchymal tissue. The sella turcica, in which the pituitary body of the
adult is lodged, is marked out, because the chondrification, which is to form the
sphenoidal cartilages, has already begun about these structures. The sphenoidal
cartilage is continuous, on the one hand, with that of the nasal septum, Sept, and,
on the other, with that of the vertebral column, Vert. From the opening of the
infundibular gland the brain-wall ascends and joins the habenular arch, where it
suddenly thickens. The arch forms the floor of the mid-brain. The" roof of the
mid-brain, M.b, is quite thin, and forms the large arch in which the differentia-
tion of the anterior and posterior corpora quadrigemina is not yet shown. At its
posterior boundary the wall of the roof of the mid-brain bends inward, marking
the constriction of the so-called isthmus. We now reach the cavity, Ven.IV, or
fourth ventricle, of the hind-brain. This cavity is subdivided into an anterior and
a posterior portion. The boundary is marked on the dorsal side by the inward
projection of the ependymal roof of the ventricle to form the choroid plexus, Plx.
IV, and on the ventral side by the angle formed by the union of the medulla
oblongata, Md.ob, with the vertical peduncles of the brain. The peduncles, con-
tinuing upward, join the habenular arch. In front of the choroid plexus the arch-
ing brain-wall, Cbl, represents the median anlage of the cerebellum. The lateral
portions of the cerebellum are much thicker. Behind the choroid plexus the roof,
Epen, of the fourth ventricle is very thin. The medulla oblongata, Md.ob, is a
thick mass of tissue which passes over abruptly into the spinal cord. The spinal
cord is cut, as a whole, somewhat obliquely. In its upper part, where the reference
line, Sp.c, is placed, the section is almost exactly median, and shows, therefore, the
floor-plate or raphe of the spinal cord. In front of the cord is the vertebral artery,
A. vert, which joins its fellow to form the basilar artery which runs in the median
line the entire length of the hind-brain. The vertebral column is in the cartilagi-
nous stage. It is an absolutely continuous uninterrupted rod of cartilage which
merges at the neck with the cartilaginous skull. The entire continuous cartilaginous
structure is termed the chondrostyle, for the study of which comparison with the
neighboring sections is indispensable. Out of it both the cartilaginous skull and
the vertebrae are differentiated. More or less nearly in the center of the chondro-
style are found the remnants of the notochord, which, however, never extends
anterior to the pituitary body, Hyp. The division of the chondrostyle into separate
vertebrae is indicated by the segmental flexures" of the notochord and by the com-
mencing differentiation of the intervertebral ligaments. The space occupied by the
notochord expands in the region corresponding to the division between each two
vertebrae. The notochord in the intervertebral expansions is expanded and partly
degenerated, forming an enlarged mass of irregular strands of cells, which becomes

STUDY OF SECTIONS OF EMBRYO OF 24 MM. 337

the nucleus pulposus of the adult. From each such mass goes off a narrow exten-
sion of the notochord, through what is to become the body of the vertebra. Some-
times this extension is continuous with the intervertebral portions of the notochord,
but more usually it forms a series of isolated fragments, for the notochord in the
parts corresponding to the bodies of the vertebrae is already in process of resorption.
The diameter of the chondrostyle is nearly uniform in the vertebral region, but
is a little smaller in the part corresponding to each body of a vertebra and a little
wider in the parts corresponding to the intervertebral ligaments. The cartilage of
the body of the vertebra continues past the intervertebral expansion of the noto-
chordal cavity, but the external portion of the chondrostyle opposite each such
expansion exhibits a modification of its cells, for they have become lengthened out
in a direction parallel with the vertebral axis. The tissue thus produced is the
anlage of the intervertebral ligaments. The mouth and pharynx, Ph, form a nar-
row cavity, the floor of which is constituted by the tongue, Ton, the tip of which
has already become free. The surface of the tongue forms a long arch, at the
posterior end of which lies the epiglottis, a projecting fold of tissue which covers
the opening of the trachea. The side of the trachea is marked by the longitudi-
nal fold, La, which separates the trachea proper from the upper end of the ceso-
phagus, (E. At the upper end of the oesophagus there is a small dorsal diverticu-
lum. If the reference line (E were continued a short distance past the oesophagus, it
would lead to the section of the main aorta. A little lower down is the section of
the arch of the aorta, A.Ao. The heart shows chiefly its large ventricle, Ven.
The section is not favorable for an exhibition of its structure or for that of the
lungs, Lu. It does, however, — since in this part of the embryo the section passes to .
one 'side of the median plane, — show the main bronchus, Bro, coming off from the
trachea to the lung, and some of the smaller entodermal bronchial branches, bro,
in the lung itself. The heart and lung are separated from the abdominal cavity
by the diaphragm, Diaph. It is only to the dorsal part of this diaphragm that the
liver, Li, is attached. In earlier stages the liver is connected with the whole of ,
the diaphragm (septum transversum) . We now have a portion of the diaphragm
without connection with the liver. Below the liver is the section of the stomach,
St, the entoderm of which is cut twice. Below the stomach lies the duodenum,
Duo, extending from the dorsal side of the embryo and running toward the um-
bilicus. At the dorsal end of the duodenum is a group or cluster of darkly stained
cells, marking the position of the anlage of the pancreas. Below the duodenum the
loops of the intestine, In, are cut repeatedly. On the dorsal side of these loops is
the section of the genital gland, in this specimen, testis, Te. Dorsalward from
the genital gland is the complicated anlage of the suprarenal capsule, Sp.ren, which
is really a double organ, having one part derived from the sympathetic nervous
system and another from a modification of mesenchymal cells. In a sagittal series
the connection of the anlage with the sympathetic nervous chain of the abdomen
can be readily made out. In the anlage the nerve-fibers and the sympathetic cells

338 STUDY OF PIG EMBRYOS.

are irregularly distributed, although the cells are more or less grouped together.
The sympathetic tissue constitutes the dorsal part of the anlage and gives rise to
the so-called medulla of the adult organ. The ventral portion of the anlage, as
seen in the section, consists of bands or cords of cells separated from one another
by venous sinusoids. The cells are much more closely compacted in this portion
of the anlage than in the sympathetic, and they are further distinguished by hav-
ing nuclei which stain much less deeply. The cords of cells, here seen, develop
into the cortex of the adult organ. The fate of the medulla or .sympathetic portion
of the suprarenal in man is not known. The section passes through the side of
the allantois, and, therefore, shows only one of the lateral arteries, All.ar, but the
allantois still bears a number of degenerating mesothelial villi (compare page 253).
At the pelvic end of the abdomen a small bit of the Wolffian body, W.b, is
displayed.

CHAPTER VII.
STUDY OF THE UTERUS AND THE FETAL APPENDAGES OF MAN.

Histology of the Uterus.

In most mammals the uterus is double. This is the case in the pig, the
rabbit, and the mouse, the three species which furnish material for the practical
study as planned in this book. In these animals each uterus is a long, more or
less cylindrical tube. In primates the double uterus exists only during very early
embryonic stages, after which the two are found united into a single median
uterus. The mammalian uterus is always lined by a mucous membrane, con-
sisting of a superficial epithelium which forms glands, and of a deeper layer of
reticulate connective tissue, in which there are lymph spaces, nerves, and a fairly
abundant blood supply. The mucous membrane is subject to very marked periodic
changes in structure. It is enclosed by the muscular layers of the organ, the
muscle-fibers being of the smooth type. In animals with double uteri the muscle-
fibers form two distinct layers, an inner circular and an outer longitudinal layer.
In the primate 'uterus the disposition of the fibers is far more complicated, and
the two distinct layers cannot be identified. The surface of the uterus, wherever
it is free, is covered by a layer of peritoneum which consists of a layer of flattened
epithelial cells and a thin underlying layer of fibrillar connective tissue.

The human uterus at birth has a mucosa which is about 0.2 mm. thick.
The mucosa is soft, pale gray or reddish gray in color; it consists of a covering
of ciliated epithelium and a connective-tissue layer. It is without glands, the
glands not appearing usually until the third or fourth year, and developing very
slowly up to the age of puberty.

The development of the human uterine glands is accompanied by remarkable
and complex changes of Ahe epithelium.* The adult glands, as shown by figure 225,
are much branched, and the branches occasionally anastomose with one another.
The model, from which the figure is taken, demonstrates that the conception of the
character of the uterine glands, which has hitherto prevailed, is very inadequate.

Menstruation.

The function of menstruation involves great changes in the mucosa of the
body of the uterus. We distinguish three periods: (i) tumefaction of the mucosa,
with accompanying structural changes, taking five days, or, according to Hensen,

* Unpublished investigations by C. A. Hedblom, to whom I am indebted for the privilege of inserting figure 225.

339

340

HUMAN UTERUS AND FETAL APPENDAGES.

ten days; (2) menstruation proper, about four days; (3) restoration of the resting
mucosa, about seven days. The times given are approximate only. The whole

cycle of changes covers about sixteen
days. Since the monthly period is about
four weeks, the period of rest, as thus
calculated, is only about twelve days.

1. Tumefaction. — A few days be-
fore the menstrual flow the mucosa
gradually thickens; the surface becomes
irregular; the openings of the glands lie
in depressions. The connective-tissue
cells are increased in number, and it
is said by some authors in size, but
the increase in size is doubtful; the
number of round cells increases; the
glands expand and become more irreg-
ular in their course; a short time before
hemorrhage begins, the blood-vessels,
especially the capillaries and veins, be-
come greatly distended. We must as-
sume that the connective-tissue cells
proliferate, but we have no satisfactory
observations upon their division. It was
formerly asserted that the menstrual
decidua contains decidual cells, but in
all the specimens the author has studied
there were none present.

2. Menstruation. — When the changes
just described are completed, the decidua
menstrualis is fully formed, and its
partial disintegration begins.. The
process commences with an infiltration

of blood into the subepithelial tissues.
This infiltration has hitherto been com-
monly explained as due to the rupture
FIG. 225.— UTERINE GLAND OF A VIRGIN OF EIGHTEEN of the capillaries; but as no ruptures at
YEARS, WITH A PORTION OF THE SURFACE EPI- this period have been observed, we may

THELIUM. WAX RECONSTRUCTION BY C. A. . ,, •> .,• ,• ,

,. uistly regard this explanation as mad-

HEDBLOM. X 50 diams. J *

missible, and account for the infiltration

per diapedesin. It lasts for a day or two, and is apparently the immediate cause
of a very rapid molecular disintegration of the superficial layers of the mucosa,
which in consequence are lost; the superficial blood-vessels are now exposed, and,

MENSTRUATION. 341

by rupturing, cause the well-known hemorrhagia of menstruation. By the dis-
appearance of its upper portion the mucosa is left without any lining epithelium
and is very much (and abruptly) reduced in thickness. Its surface is formed by
connective tissue and exposed blood-vessels.

3. Restoration of the Mucosa. — At the close of menstruation the mucosa is
2 or 3 mm. thick; the regeneration of the lost layers begins promptly and is com-
pleted in a variable time, probably from five to ten days. The hyperemia rapidly
disappears; the extra vasated blood-corpuscles are partly resorbed, partly cast off;
the spindle-cell network grows upward, while from the cylinder epithelium of the
glands young cells grow and spread up and out so as to produce a new epithelial
covering; new subepithelial capillaries appear. The details of these changes are
imperfectly known; they effect the return of the mucosa to its resting-stage.

Decidua Menstrualis. — Specimens from the first day of menstruation are the
most instructive. They should be preserved in Zenker's fluid; sections may be
made perpendicular to the decidual surface from blocks 10 to 15 mm. cube, and
stained with alum hematoxylin and eosin. The use of Mallory's triple connective-
tissue stain will demonstrate the fibrillar tissue in the decidua and the very large
amount of the same in the muscularis.

The accompanying illustration (Fig. 226) is from a uterus in active menstrua-
tion. The decidual membrane is from i.i to 1.3 mm. thick; its surface is
irregularly tumefied; the gland openings lie for the most part in the depressions.
In the cavity of the uterus there was a small blood-clot. The demarcation
between the decidua and the muscularis is sharp. The upper fourth, d, of the
decidua is broken down and very much disintegrated; its cells stain less readily
than those of the. deep portion of the membrane; the tissue is divided into numer-
ous more or less separate small masses. Some of the blood-vessels are ruptured.
The superficial epithelium, ep, is loosened everywhere; in places fragments of it
have fallen off, and in some parts it is gone altogether; it stains readily with
alum hematoxylin, differing in this respect from the underlying connective tissue.
The deeper layer of the decidua is dense with crowded well-stained cells, which
lie in groups and are probably proliferated connective-tissue cells. They have
small oval or elongated darkly stained nuclei, with very small granular protoplas-
matic bodies. There is no indication of any enlargement of the cells, such as
occurs in the production of true " decidual" cells. There are very few leucocytes.
The glands are enlarged somewhat, and are lined by a normal cylinder epithelium,
which offers no obvious change as compared with that of the glands of the resting
uterus.

• . ••

The Pregnant Uterus: the Two Stages.

When the ovum implants itself in the uterine wall, it becomes covered by a

growth of the mucous membrane or decidua which we know as the decidua

reflexa. For an account of this process see pages 124 to 127, where also proper

342

HUMAN UTERUS AND FETAL APPENDAGES.

cfi t/>

fc 'C
W rt

O ^
en P3

CO bo

J -2

< .s

a .a

S o

\o S

N 3

'a.
W

THE,PREGNANT UTERUS.

343

definitions of the terms decidua reflexa, serotina, and vera are given. -As the
ovum increases in size the decidua reflexa also increases, and gradually becomes
thinner and thinner, until it ultimately disappears. The exact date of its disap-
pearance is not known ; it falls somewhere within the fifth month. Accordingly,
we may divide the period of pregnancy into two phases or stages, each com-
prising about half of the whole period. During the first stage the decidual
reflexa is present (Fig. 227); during the second stage it is absent, so that the
chorion l.aeve comes into direct contact with the decidua vera. In the following
sections a typical uterus of the first and second stages' each is described.

FIG. 227.— HUMAN UTERUS ABOUT ONE MONTH PREGNANT. NATURAL SIZE. (THE UTERUS HAS BEEN OPENED

BY AN INCISION ALONG ITS MEDIAN LlNE, SO AS TO DISPLAY THE SMALL OVAL BAG FORMED BY THE DECIDUA

REFLEXA.)

Human Uterus Three Months Pregnant.

The uterus measures about 3^ inches in transverse diameter, and shows
well-marked venous sinuses on its external surface. It should be opened by a
crucial incision on the anterior side; its walls will -be found about an inch or more
in thickness; it contains a grayish red bag (decidua reflexa), which nearly fills
the cavity of the uterus and encloses the embryo, so that upon opening the womb
we do not encounter the fetus directly. The inner bag has a smooth surface,
but shows a few small pores; it is without blood-vessels and is attached to the
dorsal wall of the uterus. The inner surface of the uterus shows a rich network

344 HUMAN UTERUS AND FETAL APPENDAGES.

of blood-vessels, many of which are large, irregular sinuses. The uterine walls
consist of an outer muscular layer, and an inner decidual layer; the latter takes up
nearly half the thickness of the wall, and is known as the decidua vera. Com-
parison with the seventh month uterus shows that the proportion of the layers
changes, because during gestation the muscular layer increases and the decidual
layer diminishes in thickness. The inner bag, when opened, shows the large
cavity in which .the embryo lies floating in amniotic fluid. The bag is formed by
three very distinct membranes, of which the outermost, the decidua reflexa, is
opaque and the thickest; the two inner ones are thin and transparent; the inner-
most is the delicate amnion; the middle membrane is the chorion, and is quite
distinct from -both the amnion and reflexa; to the latter it is connected by a
number of small branching villi scattered at some distance from one another over
the surface; the villi adhere firmly to the reflexa by their tips. The embryo
(Fig. 109) resembles a child in its general appearance; the length of the head
and rump together is nearly 8 cm., and the head is approximately equal in bulk
to the rump. The umbilical cord is from 5 to 7 mm. in diameter and usually
about 12 cm. long. From its distal end the blood-vessels spread out over the
placental area, and around the edge of the area rises the decidua reflexa, which
does not extend on to the placenta. Floating in the amniotic fluid is a pear-
shaped vesicle, the yolk-sac, which is about 7 mm. in diameter; it has a fine
network of blood-vessels upon its surface, and is connected at its pointed end with
a long, slender pedicle, the yolk-stalk, which runs to the placental end of the
umbilical cord, there enters the cord itself, and runs through its entire length to its
attachment to one of the coils of the intestine of the embryo.* Over the whole
of the placental area the chorion gives off large villous trunks, each of which has
numerous branches, with ramifications of the fetal vessels; the villi fill a space
about i cm. wide between the membrane of the chorion frondosum and the
surface of the uterine decidua serotina, to which the tips of some of the villi
are attached. With care the villi may be separated from the decidua, which is
seen, when it is thus uncovered, to be cavernous;. the caverns are rounded in form
and part of them may be followed, on the one hand, until they connect with the
blood sinuses of the uterus, and, on the other, until they open into the intervillous
spaces, which therefore receive a direct supply of blood from the mother.

The principal difference to be noted between the uterus before and that after
the fifth month in the relations of parts is the presence or absence of the decidua
reflexa as a distinct membrane. Duping the fourth month the reflexa stretches
as the membranes expand, and becomes thinner and thinner until by the end of
the fourth month it is as delicate and transparent as the chorion and lies close
against the decidua vera.

* At this stage a large part of the yolk-stalk within the umbilical cord has degenerated and usually disap-
peared by resorption.

THE PREGNANT UTERUS. 345

Human Uterus Seven Months Pregnant.

If we examine a pregnant uterus at any time during the sixth to ninth month
of gestation, we find essentially the same relations of the parts — the most marked
difference being in the size of the uterus, which increases with the duration of
gestation, to correspond to the growth of the fetus. A description of a uterus
seven months after conception will suffice, therefore, for our present purpose.

Such a uterus is a large, rounded bag with muscular walls, and measures
7 or 8 inches in diameter. Examined externally, it is remarkable especially for the
numerous large sinus-like blood-vessels; its surface is smooth; the texture of the
walls is firm to the touch, but the walls yield to pressure, so that the position of
the child can be felt. As the placenta is situated normally upon the dorsal side,
it is usual to open the uterus by a crucial incision of the ventral wall. The walls
are about one half of an inch thick, sometimes more, sometimes less, and as soon
as they are cut open • we enter at once into the cavity of the uterus containing the
fetus and nearly a pint of serous liquid — the amniotic fluid. The fetus normally
lies on one side, has the head bent forward, the arms crossed over the chest, the
thighs drawn against the abdomen, and the legs crossed (compare Fig. in). It
resembles closely the child at birth, but is smaller; its head is, relatively to the
size of the body, larger; the abdomen is more protuberant, and the limbs propor-
tionately smaller. The inner surface of the uterus is smooth and glistening; if it is
touched with the finger, it is found to be covered by a thin but rather tough mem-
brane, called the amniqn, which is only loosely attached. Examination of the uter-
ine wall, where it has been cut through, shows that its thickness is formed princi-
pally by the muscular layer, which is made up by numerous laminae of fibers,
between which are the large and crowded blood sinuses, similar to those distin-
guishable on the external surface of the uterus. About one fifth or less of the
wall inside the muscularis has a different texture and can be partly peeled off as
two distinct membranes, the innermost of which is the amnion already mentioned,
and the outer is the chorion united with the decidua. The amnion and chorion
are appendages of the embryo; the decidua is the modified mucous membrane of
the. uterus. Let us return to the embryo. From its abdomen there springs a long,
whitish cord, known as the umbilical cord; it is usually .from about one third to
one half an inch in diameter and 40 cm. long, but its dimensions are extremely
variable; it always shows a spiral twist, and contains three large blood-vessels, two
arteries and one vein, all of which can be distinguished through the translucent
tissue. The distal end of the cord is attached to the wall (placenta) of the uterus
usually near the middle of the dorsal side of the organ. It is easily seen that the
blood-vessels of the umbilical cord radiate out from its end over the surface of the
uterus underneath the amnion, branching as they go; they spread, however, only
over a circumscribed area, the placental, where the wall of the uterus is very
much thickened. A vertical section through the placental area -shows that the am-
nion and chorion are widely separated from T;he decidua and muscularis by a

346 HUMAN UTERUS AND FETAL APPENDAGES.

spongy mass soaked with maternal blood. This mass consists of numerous trees of
tissue, which spring with comparatively thick stems from the chorion and branch
again' and again. In these stems and branches are to be found the final ramifica-
tions of the vessels of the umbilical cord; the trees are known as chorionic or
placental mill. Some of their end-twigs are very closely attached to the surface of
the decidua. In the center of the placental area the villi form a mass about three
fourths of an inch thick, but toward the edge of the area the mass gradually thins'
out until at the very edge the chorion and decidua come into immediate contact.
The mass of villi, together with the overlying portions of the chorionic and am-
niotic membranes and the underlying portion of the decidua, constitutes what is
known as the placenta. The decidua of the placental area is called the decidua
serotina; the chorion of the placenta is known as the chorion frondosum. When
birth takes place, the whole placenta is expelled after the delivery of the child; the
placenta of the obstetrician is, therefore, partly of fetal, partly of maternal, origin.

Decidua Vera of the First Stage in Section.

Specimens may be preserved in Zenker's or Tellyesnicky's fluid, or they may
be preserved with less good results in Miiller's or Parker's fluid or in picro-sulphuric
acid. Sections may be made of the, en tire wall in celloidin, or, if it is desired to
get thinner sections, in paraffin, in which case it is advantageous to remove as
much as possible of the muscular coat so as to cut only the decidual membrane.

The following description is based upon a uterus one month pregnant. Figure
228 was obtained from a vertical section of the decidua, by drawing the outlines
of the glands or gland spaces, Gl, and by dotting the entire area occupied
by the connective tissue. The blood-vessels are indicated by double outlines.
The artery, Art, owing to its spiral course, is cut repeatedly. The figure
demonstrates very clearly that the gland cavities are so arranged that the decidua
is divided into an upper compact layer, Comp, and a lower cavernous layer,
Cav, the difference being due to the size and number of the gland cavities. The
amount of epithelium to be observed at this stage varies greatly. It is sometimes
wholly absent from the surface, in other cases absent or present in patches. In the
glands the epithelium has. undergone many modifications. In some parts the original
cylinder epithelium of the glands is well preserved in patches, and such patches
of epithelium are found at every stage- until after delivery. It has been observed
that these patches serve to regenerate the epithelium of the glands, and, by spreading
from the glands on to the surface, to regenerate also the epithelial covering of the
uterine mucosa. But for the most part the glandular epithelium is considerably
altered. We find places in which the cells, though attached to the surrounding
connective tissue, are separated from one another by small fissures. In other places
the cells are a little larger (Fig. 229), each for the most part cleft from its fellow,
and some of them loosened from the wall and lying free in the cavity. Apparently
the cells' which are thus freed become swollen, probably by imbibition, both the

DECIDUA VERA OF FIRST STAGE.

347

Comp.

Cav.

m ,a:

FIG. 228. — VERTICAL SECTION OF A HUMAN UTERUS
(DECIDUA VERA), ONE MONTH PREGNANT.

Comp, Compact layer. Cav, Cavernous layer. D,
Gland-duct. Art, Spiral artery. Cl, Spaces
occupied by epithelial glands. Muse, Muscu-
laris. (For clearness all the glandular epithe-
lium has been omitted from the drawing.)

FIG. 229. — HUMAN UTERUS, ONE MONTH PREG-
NANT. SECTION OF GLAND FROM THE CAVERN-
OUS LAYER, WITH THE EPITHELIUM PARTLY
ADHERENT TO THE WALLS. X 445 diams.

FIG. 230. — HUMAN UTERUS, ONE MONTH PREGNANT. SECTION OF A GLAND FROM THE CAVERNOUS LAYER WITH
THE EPITHELIUM LOOSENED FROM THE WALLS. THE EPITHELIAL CELLS ARE SWOLLEN.

348

HUMAN UTERUS AND FETAL APPENDAGES.

protoplasm and the nuclei becoming enlarged (Fig. 230). The cells lie separately
and almost completely fill the gland cavity. They are no longer cylindrical in
shape, but irregular. Their protoplasm is finely granular and stains rather lightly.
The nuclei are rounded, granular, and with sharp outlines. In somewhat older
stages one finds the cells, replaced by a granular material. The obvious interpreta-
tion of the appearances described is that the glandular epithelium is breaking down
and disintegrating, or, in other words, passing through a special form of degenera-
tion which is highly characteristic. In later stages some of the broken-down

' FIG. 231. — UTERUS ONE MONTH PREGNANT; PORTION OF THE COMPACT LAYER OF THE DECIDUA SEEN IN

VERTICAL SECTION.
Coagl, Coagulum upon the surface. d,df, Decidual cells. X 445 diams.

material forms hyaloid rounded concretions, which, owing to their deep staining, are
somewhat conspicuous. The concretions usually include a number of spherical
vacuolcs.

The formation of decidual cells has already begun' in the upper or compact
layer (Fig. 231). They are modified connective-tissue cells, which have grown in
size and altered their structure. Their bodies stain deeply- with eosin; the nuclei
are round, oval, slightly irregular in shape, coarsely granular, and sharp in outline.
The cells themselves, though irregular and variable in shape, are all more or less
provided with processes running off in various directions. Scattered between the
cells are many sections of the processes. Occasionally it may be seen that two

DECIDUA REFLEX A OF FIRST STAGE.

349

cells are connected. Later on the decidual cells acquire smoother and more rounded
outlines, and appear to lose altogether their connections with one another. In the
cavernous layer there are no decidual cells.

Decidua Reflexa of the First Stage.

The decidua reflexa may be preserved in Zenker's fluid, Parker's fluid, or picro-
sulphuric acid. It should be hardened with the portions of the chorion and cho-
rionic villi adherent to it. It may be im-
bedded in celloidin and the sections stained
with alum hematoxylin and eosin, with
Beale's carmine, or with a so-called fibrin
stain.

As stated above (page 343), the pres-
ence of the decidua reflexa distinguishes
the first stage of pregnancy from the
second, in which the reflexa is absent,
having disappeared by degeneration and
absorption. To observe this process of the
disappearance of the reflexa, membranes
from the second and third months should
be examined.

Section of Decidua Reflexa of Two
Months. — At this time the reflexa starts
from the edge of the placental area as a
membrane of considerable thickness, but it
rapidly thins out, the very thinnest point
being opposite the placenta. Examination
of sections shows that the entire reflexa is
undergoing degeneration which is found to
be more advanced the more remote the
part examined is from the placenta. The
chorion laeve lies very near the reflexa, being
separated only by the villi, which are already
very much altered by degeneration. In the
region halfway between the base and the

apex of the reflexa the tissue (Fig. 232) shows only vague traces of its original
structure. Only here and there can a distinct cell with its nucleus be made out.
Most of the cells have broken down and fused into irregular hyaline masses with-
out organization. Ramifying through the fused detritus appear strands and lines,
which are more darkly stained by both carmine and hematoxylin. On account of
their fibrous appearance, these strands are often spoken of as fibrin, although they
are presumably not the same as the true fibrin from the blood. The fibrin is much

FIG. 232. — SECTION OF HUMAN DECIDUA REFLEXA
AT Two MONTHS.

350 HUMAN UTERUS AND FETAL APPENDAGES.

more developed upon the inner or chorionic side than upon the outer side of the
reflexa. On the inner side it forms a dense network, which fuses with the degen-
erated ectoderm of the chorionic villi wherever the villi are in contact with the de-
cidua. It also ramifies nearly halfway through the decidua, the ramifications being
followed easily, owing to the dark staining of the substance. Over the outside of the
decidua the fibrin forms a much thinner layer or may be only indistinctly formed.

In a decidua reflexa of three months the conditions are essentially the same,
except that the degeneration is further advanced and the membrane thinner.
Traces of cellular structure are still more vague and the fibrin is more developed.
In all parts of the membrane there appear leucocytes which are particularly
numerous and conspicuous in the neighborhood of the placenta. It is natural to
assume that they are concerned in the resorption of the reflexa. There is an inner
thicker layer of fibrin and a thinner outer layer, which is now always present and
distinct. Between these two layers is a stratum in which the remains of the cells
may be seen. Occasionally there is an appearance which suggests surviving de-
cidual cells, and, indeed, in sections taken from parts close to the placenta true
decidual cells may be identified.

The origin of the chorion laeve by the disappearance of its villi is described
on page 367. The sections of the decidua reflexa will enable the student to see
also some of the phases of the degeneration of the villi. They are very much
altered. Their ectoderm undergoes a hypertrophic degeneration and becomes hya-
line tissue, which stains darkly. The degenerated ectoderm of adjacent villi fuses
more or less extensively. The mesoderm of the villi shows a partial loss of its
primitive cellular organization.

Decidua Vera and Chorion Laeve of the Second Stage.

Pieces of the decidua vera of from six to nine months with the chorion and
amnion carefully preserved in situ may be hardened in Miiller's or Tellyesnicky's
fluid. Blocks half an inch or less in size may be imbedded in celloidin, and sec-
tions made perpendicularly to the surface may be stained with alum hematoxylin
and eosin, or with Heidenhain's hematoxylin and orange G, or with picro-carmine.

The decidua reflexa having been resorbed, the chorion (Fig. 233, Cho) has
come into contact with the surface of the uterus, and the chorionic epithelium, c,
is closely adherent to the surface of the decidua, from which the original epithe-
lium has completely disappeared. The amnion is loosely connected with the
chorion by a few strands or threads, which are represented in the figure and the
nature of which is not known. Both the amnion. Am, and the chorion, Cho,
being developed from the original somatopleure (compare page 82), consist of a
mesodermic and an ectodermal layer. The ectoderm of the amnion is a single
layer of cuboidal cells placed on the side of the membrane toward the embryo and
away from the uterus. The ectoderm, c, of the chorion, on the contrary, is next
the uterus. Hence it will be noticed that the mesodermic layers of the amnion

DECIDUA VERA AND CHORION LMVE OF THE SECOND STAGE. 351

and chorion are adjacent. Both membranes are quite thin. The decidua is a
relatively voluminous membrane containing blood-vessels, v, which for the sake of
distinctness have been filled in with black in the drawing. It also contains a
series of elongated spaces, which represent sections of the glands. These spaces,
gl, are present only in the inferior half of the decidua. Owing to their absence
from the superior half, that portion has a more compact structure, and is, there-
fore, designated as the compact layer; the lower portion, being broken up and

c-:,v;K\^v-^^C^!s^^^C^^:-^i^;^^?rt^;»:V

•%:£. ^^'^^^^f^^?^^^^M^^^;

,^^ ^ ^

FIG. 233. — HUMAN UTERUS ABOUT SEVEN MONTHS PREGNANT. VERTICAL SECTION OF THE DECIDUA VERA

WITH THE FETAL MEMBRANES IN SITU.

Am, Amnion. Cho, Chorion. c, Chorionic epithelium. v, Blood-vessel. gl, Glands. muse, Muscularis.

X 40 diams.

made loose in texture by the somewhat numerous gland cavities, is called the
cavernous layer, the caverns, of course, corresponding to the gland spaces. The
gland spaces are now very much stretched out, a condition which results simply
from the general expansion of the uterus during pregnancy. In the gland spaces
appear patches of epithelium still intact, and in the cavities themselves isolated
cells in various phases of degeneration and disintegration, similar to the phases
which may be observed in the decidua vera of one month; but the degeneration is,
on the whole, considerably more advanced than in the early stage. Around

352 HUMAN UTERUS AND FETAL APPENDAGES.

some of the larger blood-vessels there is connective tissue only slightly modified,
and the original structure of the mucous membrane is more or less, but not
perfectly, preserved in the deep portion of the decidua. The majority of the
cells, especially in the compact layer, have grown in size and become transformed
into true decidual cells. In the ectoderm of the chorion, c, the cells lie two or
three deep. They have distinct walls, a very coarsely granular protoplasm, and
nuclei which stain darkly. By these characteristics they are easily distinguished
from the neighboring decidual cells, to which, however, they offer a slight super-
ficial resemblance.*

The Placenta in Situ.

The placenta in its natural position in the uterus follows the curvature of the
uterine walls, hence its free or amniotic surface is slightly concave. Its decidual
surface is strongly convex. It is thickest in the center and thins out gradually
toward its edge. The uterus should be obtained in the freshest possible condition
and be opened by a crucial incision on the ventral side. The embryo should
then be removed, fhe umbilical cord cut through, care being taken to bring as
little pressure as possible on the uterus or the placenta, and the whole organ
placed in the preservative, which should be either Tellyesnicky's or Miiller's fluid.
In view of the large size of the organ, it is very necessary to use large quantities
of the preserving fluid, and this fluid must be changed several times in order to
insure good histological preservation. When the hardening is completed, columns
about one-half inch square may be cut out so as to pass vertically from the inner
to the outer surface of the placenta, preserving the amniotic and chorionic mem-
branes in place. The blocks are to be imbedded in celloidin and ought to
remain at least three days in thin and three days in thick celloidin, so as to insure
a thorough penetration of the imbedding material into the intervillous spaces.
Make the sections so that they pass vertically through the placenta. Stain with
alum hematoxylin and eosin.

Placenta at Seven Months. — A section made according to the method just
described is 'represented in figure 234. The thin amnion, Am, covers the upper
(or inner) surface of the chorionic membrane, Cho. This membrane is. separated
from the decidua, D, by a dense forest of villi, of which innumerable sections
appear. In younger placentas the distance between the chorion and the decidua
is considerably less, and the number of sections of villi is smaller, but the average
size of those sections larger. In the present specimen the distance between the
chorion and the decidua is nearly twice as great as the diameter of the muscular
coat, Me, of the uterus. The ends of some of the villi touch the decidual tissue,
and are imbedded in it. Their imbedded ends are without , covering epithelium,
but their connective tissue is immediately surrounded by hyaline substance which

* It should perhaps be noted that in some comparatively recent text-books the chorionic ectoderm has been
described as the decidua reflexa, an error which is much to be regretted.

THE PLACENTA IN SITU.

353

FIG. 234. — HUMAN PLACENTA IN SITU, ABOUT SEVEN MONTHS. VERTICAL SECTION.

Am, Amnion. Cho, Chorion. Vi, Villous trunk, vi, Sections of villi in the substance of the placenta. D',D",
Decidua serotina. Me, Muscularis. Ve, Uterine artery, opening into the placenta; the fetal blood-vessels are
drawn black; the maternal blood-vessels are white; the chorionic tissue is stippled, except the canalized fibrin,
which is shaded by lines. The remnants of the gland cavities in the decidua are stippled dark. X 6 diams.

23

354

HUM Atf UTERUS AND FETAL APPENDAGES.

is probably degenerated epithelium. The decidua serotina is plainly divided into
an -upper compact, D', and a lower cavernous layer, D". The section figured
passes through an arterial vessel, Ve, which makes an abrupt turn so as to dis-
charge its blood into the intervillous spaces.

The histological structure of all the parts should be carefully studied. (As
regards the structure of the amnion, see page 370.)

The chorion consists of two layers, the outer ectodermic and inner mesodermic.
Over the chorionic membrane proper the ectoderm offers a great variety of appear-
ances. In some places it may be seen to have still its primitive organization, a
single inner layer of distinct cells and an outer syncytial layer, more or less similar

Am.

- J'TTTJ; * »--:rV»' 3'<fcv'-v*~ ^^ v31 C *""'! '" **

__•_-' -*• .--s-^ <3--<:r~*&>* -...,-•»*..,. — — ^-, «*>v. ^-V...^.^--^"""1

"**» " " •** •"" -*M"*:'- - '•'•"' " ' :'-*-;— ••".-•''•J-— ' "•' * "* ~' ---

ZFib-

FIG.- 235. — HUMAN PLACENTAL CHORION AND AMNION OF THE FIFTH MONTH.

Ep, Amniotic epithelium. Am, Amnion. Str, Stroma. Fib, Fibrillar layer. Fbr, Fibrin layer, c, Chorionic
cellular layer of ectoderm. Vi, Chorionic villi. X 71 diams.

to those represented in figure 243. For the most part, however, the chorionic ecto-
derm has been considerably modified from its primitive condition. The inner or
cellular layer exhibits irregular, thickened patches, which present every possible
degree of variation as to their size. A cell patch from a somewhat younger stage
is represented in figure 235 as seen with a low magnification, and another patch
of the age we are studying is represented in figure 236. The patches vary in
appearance; the cells are more distinct in the small patches, less so in the large
patches, in which there are often parts more or less degenerated. The cell-bodies
stain lightly; their nuclei are granular, not very sharply defined, and variable in
size and shape. The cellular layer is always sharply defined against the mesoderm.
Toward the outside the patches offer varying relations. In some cases a part of

THE PLACENTA IN SITU.'

355

a cell patch may form the whole thickness of the ectoderm, as shown in figure 235,
or the whole of a cell patch may do so. More commonly, however, the cellular
patch is covered more or less completely by a special substance, which is termed
canalized fibrin, and which is believed to represent the original outer syncytial
layer in a degenerated condition. The fibrin is a constant, normal, and very
remarkable constituent of the placenta. Its formation seems to begin always in
the outer or syncytial layer of the chorionic ectoderm, but it may also spread into

FIG. 236. — HUMAN CHORION OF SEVEN MONTHS' PLACENTA.
c, Cellular layer, fb, Fibrin layer, ep, Remnants of epithelial layer, mes, Mesoderm. X 445 diams.

the' cellular layer, which then becomes replaced by fibrin, so that this last alone
represents the ectoderm of the chorion. . The fibrin layer consists of a very refrin-
gent substance permeated by numerous channels (Fig. 236, fb). • The substance has
a violent affinity for carmine and hematoxylin, and hence is always deeply colored
in sections stained with either of these dyes. The channels tend to run more or
less parallel to the surface of the chorion, and are connected by numerous
short cross-channels. Some of the channels contain cells or nuclei. The appear-
ances, however, are very variable; the fibrin often sends long outshoots into the

356

HUMAN UTERUS AND FETAL APPENDAGES.

cellular layers. To summarize, we may say that the ectoderm of the chorionic
membrane undergoes patchwise manifold changes. It exists in three general forms:
the nucleated protoplasm or syncytium, the cellular layer, and the canalized fibrin.
A patch of the ectoderm may consist of any one of these modifications or any two,
or of all three. But they have fixed relative positions, for when the syncytium is
present, it always covers the free surface of the chorion; when the cellular layer is
present, it always lies next the mesoderm; and when all three forms are present
over the same part, the fibrin is always the middle stratum.

The mesoderm of the chorion in early stages has a homogeneous matrix, which
about the ninth week begins to change its appearance. In the frondosum, in our
specimen, the matrix has acquired a distinctly fibrous structure. Usually the pro-
duction of fibers is much greater in the immediate neighborhood of the ectoderm,

FIG. 237. — ADENOID TISSUE FROM A VILLUS or A HUMAN PLACENTA or FOUR MONTHS.

/, /, /, Degenerating blood-cells, v, v, Capillary blood-vessels, d, Finer meshwork surrounding a capillary.

X 352 diams.

and this may go so far as to mark out a more or less distinct subectodermal
fibrillar layer (Fig. 235, Fib). There appears to be no mesothelial layer upon the
chorion at this stage, but it seems possible that its presence might be revealed by
the application of proper special methods.

In the mill the ectoderm differs from that of the chorionic membrane in
several respects: (i) The cellular layer after the first month becomes less and less
conspicuous, and after the fourth month is present only in a few isolated patches,
which have been termed the cell-knots. (2) For the most part the villi remain
covered by the syncytial layer, which in many places is thickened. In later stages
these thickenings are small and numerous, constituting the so-called proliferation
islands with many nuclei. Many of the little thickenings appear in sections of

DECIDUA SEROTINA AT SEVEN MONTHS. 357

the villi, and here and there are converted into canalized fibrin. (3) The prolifera-
tion islands are converted into canalized fibrin and at the same time grow and
fuse, forming larger patches, particularly on the larger stems. In this manner are
produced the large areas and columns of fibrin such as appear in our section. (4)
Over the tips of the villi, where they are imbedded in the decidua serotina,
the epithelium apparently degenerates and becomes hyaline tissue, but without
canalization. The mesoderm exists in two principal forms, adenoid tissue and
fibrillar tissue around the blood-vessels. The adenoid tissue (Fig. 237) may be
considered as the proper tissue of the villus. It consists of a network of proto-
plasmic threads, which start from nucleated masses. There are many large meshes,
which are partly occupied by the very large, coarsely granular cells, /, I, which gen-
erally are widely scattered, but sometimes are present in large numbers. These free
cells are extravasated blood-corpuscles, which have increased in size. Probably
they are dead or at least dying and have swollen by imbibition. They undergo
disintegration, their protoplasm, becoming vacuolated; the1 vacuoles increase in size
as the protoplasm is dissolved, until finally the cell-body entirely disappears. About
the capillary blood-vessels, v, the network is more finely spun. Around the larger
blood-vessels the mesoderm has a distinct intercellular substance with a ten-
dency to fibrillar differentiation in quite a wide zone around the blood-vessels. In
this zone the cells become elongated or irregularly fusiform. Around the larger
vessels the cells are grouped in laminae, and apparently are contractile, so that they
must be looked upon as an imperfectly differentiated form of smooth muscular
tissue.

Decidua Serotina at Seven Months.

Specimens may be treated as described for the placenta in situ (page 352).
If, however, the best results are desired, the whole of the uterus should be cut
through and the placenta divided into smaller pieces from i to 2 cm. in diameter,
so as to allow a freer penetration of the preserving fluid. Either Zenker's or
Tellyesnicky's fluid is recommended. In a normal uterus about seven months
pregnant we find the following relations : The serotina is about i . 5 mm. thick,
and contains an enormous number of decidual cells (Fig. 238); the cavernous,
D' ', and compact, D", layers, are very clearly separated; the mucosa is sharply
marked off from the muscularis, although scattered decidual cells have penetrated
between the muscular fibers. The muscularis is about 10 mm. thick and is
characterized by the presence of quite large and' numerous venous thrombi, espe-
cially in the part toward the decidua. The decidua itself contains few blood-vessels.
Upon the surface of the decidua can be distinguished a special layer of denser
decidual tissue, which in many places is interrupted by the ends of the chorionic
villi which have penetrated it, as is well shown in the accompanying figure. The
gland cavities of the spongy layer, D' ', are long and slit-like; they are filled for
the most part with fine granular matter, which stains light blue with hematoxylin;

358

HUMAN UTERUS AND FETAL APPENDAGES.

they also contain a little blood, and sometimes a few decidual cells. There also
occur in them hyaloid concretions — oval bodies several times larger than any of
the decidual cells, and presenting a vacuolated appearance. In uteri over two
months pregnant they are probably invariably present. In many places the glan-
dular epithelium is perfectly distinct; its cells vary greatly in appearance, neighbors
being often, quite dissimilar; nearly all are cuboidal, but some are flattened out;
of the former, a number are small with darkly stained nuclei, but the majority
of the cells are enlarged, with greatly enlarged, hyaline, very refringent nuclei.

VI

D'

©

me

FIG. 238. — THE HUMAN DECIDUA SEROTINA AT SEVEN MONTHS. THE SECTION is TAKEN FROM NEAR THE

MARGIN OF THE PLACENTA.

Vi, Chorionic villi; the intervillous spaces were filled with maternal blood, which is not represented in the figure.
D', Cavernous layer of the decidua. D", Compact layer of the decidua. me Muscularis.

There are also in many of the gland spaces isolated enlarged cells which have
detached themselves from the wall, and in some cases the detached cells nearly
fill the gland cavity, very much as in figure 230.

The decidual cells of the cavernous layer (Fig. 238, D'} are smaller and more
crowded than most of those of the compact layer. The largest cells are scattered
through the compact layer, but are most numerous toward the surface. They
extend around the margin of the placenta and have penetrated the chorion, in
the cellular layer of which they are very numerous; the immigration imparts to
the chorionic layer in question somewhat the appearance of a decidual membrane.
Misled by this peculiarity, some authors have held this layer to be maternal in

THE HUM A N. PLA CEN TA .

.3.-)!)

origin, and accordingly have described it as a "decidua subchorialis" The de-

cidual cells exhibit great variety in their features (Fig. 239). They are nearly

all oval discs, so that their outlines differ according as they are seen lying in the

tissue turned one way or another; they vary greatly in size; the larger they are,

the^more nuclei they contain; the nuclei

are usually more or less elongated; the

contents of the cell granular. Some of

the cells present another type, c; these

are more nearly round, are clear and

transparent; the nucleus is round, stains

lightly, and contains relatively few and

small chromatin granules; such cells are

most numerous about the placental margin.

The Human Placenta.

Specimens of the fresh normal human
placenta may be obtained without diffi-
culty from maternity hospitals. The
placenta should be thoroughly examined

FIG. 239. — DECIDUAL CELLS FROM THE SECTION

REPRESENTED IN FIGURE 238.
in the fresh State by the Student and c> Multinucleate cell; at a seven blood-corpuscles ha\*e

all the points in the description below

verified by him. To make an injected specimen either the starch injection mass
or the colored gelatin mass may be used according as*it is desired to demonstrate
only the coarser or all the branches of the vessels. The" injection should be made
through one of the arteries of the umbilical cord. As there is almost invariably
a cross-anastomosis between the two arteries close to the placenta, it is sufficient
to inject one of them in order to fill the entire system of vessels. The starch mass
may be injected in the cold specimen. If the gelatin mass is used, the specimen
must be submerged in warm water until it is sufficiently heated to keep the gelatin
mass melted during the process of injection. After the gelatin injection is com-
pleted, the placenta may be preserved in 70 per cent alcohol, to every TOO c.c. of
which 2 c.c. of hydrochloric acid have been added. After twenty-four hours replace
the acidulated alcohol by fresh alcohol of 70 per cent, which should be again
changed after another twenty-four hours. Specimens will then keep indefinitely.
Such specimens may be used either for sections of the placenta to be made from
pieces imbedded in celloidin, or for the study of isolated fragments of the villi,
which are pulled out of the placenta by forceps.

The human placenta is a disc of tissue to which the umbilical "cord of the
child is attached by its distal end. As a result of normal labor the amnion
and chorion, by which the fetus in utero is surrounded, are ruptured; the child
is then expelled, but by means of the long umbilical cord remains attached to the
uterus; after an interval the placenta, with which the cord retains its connection,

360 HUMAN UTERUS AND FETAL APPENDAGES.

is loosened from the uterine wall and expelled, together with the fetal envelopes
and portions of the decidual membranes (uterine mucosa) of the mother; the parts
thus thrown off secondarily constitute the so-called after-birth of obstetricians.

The placenta at full term, as thus obtained by natural expulsion, is a moist
mass, containing a great deal of blood, spongy in texture, about 7 inches in
diameter, but very variable in size, being roughly proportionate to the bulk of
the child; usually oval, sometimes round, but not infrequently irregular in shape.
One surface is smooth and covered by a pellucid membrane (the amnion), and
reddish gray in color; to this surface the umbilical cord is attached, and it shows
the arteries and veins branching out irregularly from the cord over the surface of
the placenta (Fig. 240). The opposite surface is rough, lacerated, and usually
covered irregularly with more or less blood, which is often dark and clotted.
When the blood is removed, the surface is seen to be crossed by a system of
grooves which divide the placental tissue into irregular areas, each perhaps an inch
or so in diameter; these areas are called cotyledons. The placenta is about 25 or
30 mm. thick, but thins out rapidly at the edges, and its tissue passes over from
the margin of the placenta.

When in situ, the placenta is fastened to the walls of the uferus by its rough
or cotyledonary surface; its smooth, amniotic surface faces the cavity in which the
fetus lies.

A more detailed examination of the gross appearance of a placenta discharged
at term leads to the following additional observations: The color is a reddish or
purplish gray, varying in tint according to the condition of the blood, and mottled
between the divaricating blood-vessels by patches and networks of pale yellowish
or flesh color. The light pattern is produced by the tissue of the villi shining
through the membrane of the chorion. These appearances are less distinct when
the placenta, as is usually the case, is covered by the thin amnion. The amnion,
however, is very easily detached as far as the insertion of the umbilical cord, to
the end of which it is firmly attached, but it cannot be traced farther because on
the cord itself there is no amnion. The blood-vessels run out in all directions
from the end of the cord; each vessel produces a ridge upon the placental surface,
so that its course is readily followed. The arteries and veins are more easily
distinguished after double injection, as is shown in figure 240.

The two kinds of vessels do not run together; the arteries lie near the surface,
just above the veins; the arteries fork repeatedly, until they are represented only
by small branches and fine vessels; some of the small branches disappear quite
suddenly by dipping down into the deeper-lying tissue in order to pass into the
villi. The veins (Fig. 240) are considerably larger than the arteries; they branch in
a similar manner, but some of the trunks disappear from the surface more abruptly
than is the case with the arteries. There is the greatest possible variability in
the vessels of the placenta; one never sees two placentae with vessels alike.

The insertion of the cord is always eccentric; the degree of eccentricity is

THE HUMAN PLACENTA. 361

variable and is easily seen to be related to the distribution of the vessels. The
insertion may even be entirely outside the placenta, which yet may otherwise
be normally developed. Such insertions are called velamentous. The usual
type is shown in figure 240. The arteries come down together from the cord;
they usually, but not always, anastomose by a short transverse vessel, which
lies about half an inch above the surface of the placenta; it could not be shown

FIG. 240.— HUMAN PLACENTA AT FULL TERM, DOUBLY INJECTED TO SHOW THE SUPERFICIAL DISTRIBUTION OF

THE BLOOD-VESSELS.
The veins are drawn dark and lie deeper than the arteries. One half natural size.

in the figure. Very rarely, if ever, are there any arterial or venous anastomoses
on the surface of the placenta. The arteries there spread out in a manner which
may be described as roughly symmetrical. The veins partially follow the course
of the arteries. When the cord is inserted near the margin the symmetry of the
placental vessels is greater, when the insertion is near the center the symmetry is
less, than in the figure.

362

HUMAN UTERUS AND FETAL APPENDAGES.

The reverse or uterine surface of the placenta is rough and divided into
numerous rounded, oval, or angular portions termed lobes or cotyledons, as stated
above. These vary from half an inch to an inch and a half in diameter. The
whole of this surface consists of a thin, soft, somewhat leathery investment by
the decidual membrane, which dips down in various parts to form the grooves that
separate the cotyledons from each other. This layer is a portion of the decidua
serotina, which, as long as the parts are in situ, constitutes the boundary between

si

FIG. 241. — HUMAN PLACENTA AFTER DELIVERY AT FULL TERM.

A, Vertical section through the margin: D, decidua; vi, aborted villi outside the placenta; Cho, chorion;5&, sinus;
Vi, placental villi; Fib, fibrin. B, Portion of A more highly magnified to show the decidual tissue near b:
v, blood-vessel; d, decidual cell with one nucleus; d', decidual cell with several nuclei.

the placenta and the muscular substance of the uterus, but which at the time of
labor becomes split asunder, so that, while a portion is carried off along with the
placenta and constitutes its external membrane, the rest remains attached to the
inner surface of the uterus. If a placenta is cut through, if is found to consist of
a spongy mass containing a large quantity of blood and bounded by two mem-
branes, each less than a millimeter thick; the upper one is the chorion, covered
by the still thinner amnion, and greatly thickened where the vessels lie in it; the
lower one is the decidual tissue, together with the ends of the villi imbedded in

HISTOLOGY OF THE HUMAN CH ORION. 363

it (cf. especially page 357 and Fig. 238); it represents only a portion of the de-
cidua, the other portion having remained upon the uterine wall. The spongy mass
is found upon examination to consist of an immense number of tufts of fine rods
of tissue, which are irregularly cylindrical in shape. Further examination shows
that they are twigs (Fig. 248) with rounded ends and springing from branchlets
which in their turn arise from branches, and so on until a large main stem is
found, which starts from the chorion. This branching system is richly supplied with
blood from the fetal vessels on the surface of the placenta. The villi are inter-
woven so that the twigs of one branch are interlaced with those of another, and
apparently separate twigs may grow together and their vessels anastomose; but
on this point we are unable to speak positively. The villous twigs next the surface
of the decidua penetrate that tissue a slight distance.

The intervillous spaces are filled, or nearly so, with blood; they form a com-
plex system of channels. The intervillous blood is maternal. Farre says, in his
article in Todd's "Cyclopaedia" (Vol. V, page 716), in reference to the placental
decidua: "Numerous valve-like apertures are observed upon all parts of the surface.
They are the orifices of the veins which have been torn off from the uterus. A
probe passed into any one of these, after taking an oblique direction, enters at
once into the placental substance. Small arteries, about half an inch in length, are
also everywhere observed embedded in this layer. After making several sharp
spiral turns, they likewise suddenly open into the placenta"; and on page 719 he
adds: "These venous orifices occupy three situations. The first and most numerous
are scattered over the inner side of the general layer of decidua which constitutes
the upper boundary of the placenta; the second form openings upon the sides of
the decidual prolongations-- or dissepiments, which separate the lobes [cotyledons]
from each other; while the third lead directly into the interrupted channel in the
margin, termed the circular sinus." The circular sinus (Fig. 241, Si) is merely a
space at the edge of the placenta which is left comparatively free from the villi.
It is not a continuous channel, but is interrupted here and there. Subsequent
writers have gone but little beyond Farre's account, which has been entirely over-
looked by most recent investigators, who, accordingly, have announced as new
discoveries many facts known to Farre. Under these circumstances it is -interesting
to direct renewed attention to Farre's masterly article.

Histology of the Human Chorion.

The chorion may be preserved in Zenker's or Tellyesnicky's fluid or in
Bouin's picro-formalin fluid. Pieces may be stained in toto with alum cochineal
or borax carmine and transverse sections cut in paraffin. The sections may be
advantageously counterstained with eosin or orange G.

For the general history of the chorion see page 82. As it is formed by the
somatopleure, it comprises an outer ectoderm and an inner mesoderm, which
latter comprises mesenchyma and mesothelium.

364 ' HUMAN UTERUS AND FETAL APPENDAGES.

The ectoderm undergoes a very precocious growth producing a very large
number of cells, which form the thick trophodermic layer as described on page 365.
Then follows the stage in which, by degeneration, spaces are produced in the
trophoderm into which the blood of the mother enters and circulates; and at the
same time prolongations of the chorionic mesoderm extend into the trophoderm.
The ectodermal cells arrange themselves as a covering for these mesodermic
outgrowths and so complete a villus. The trophoderm between the developing
villi entirely disappears. The ectoderm, which covers both the villi and the
chorionic membrane proper, consists of two layers, an inner cellular and an outer
syncytial layer. Much of the trophoderm may still remain for awhile around
and beyond the tips of the villi, but it disappears rapidly, probably during the
third week, so that the villi alone are left. The two-layered stage of the ectoderm
is only partially preserved during the later development. Many parts of it become
thinned out so as to contain only one layer of cells, while other parts thicken
and degenerate. These changes may be studied in sections of older placentas
(see Fig. 234).

The mesoderm of the chorion consists at first of mesenchymal cells with a
homogeneous matrix and a layer of mesothelium. In later stages the mesen-
chymal tissue becomes partly fibrillar, and it is doubtful whether the mesothelium
persists or not. During the third week we find the chorion vascular. .. Around
the larger blood-vessels the mesoderm forms a more or less distinct coat in which
the cells are somewhat more crowded together in laminae. After the perivascular
coats have acquired a certain thickness the cells of their inner portions become
more elongated, more regularly spindle-shaped, and more closely packed than
those of the outer layer. The transition from the denser to the looser tissue is
gradual. We are perhaps entitled to call the denser, inner layer the media, and
the outer, looser layer the adventitia, although neither of the layers has by any
means the full histological differentiation characteristic of the like-named layers
of the blood-vessels of the adult. The histogenetic changes in the chorion
frondosum go further than in the chorion Iseve, which may be said to be, as it
were, arrested in its development.

The Chorion with Trophoderm.

When the chorionic vesicle has an internal diameter of from 3 to 6 or 7 mm.,
it will be found to exhibit well-developed trophodermic layers. Such a vesicle
may be hardened in Zenker's fluid or, better, in Flemming's or Hermann's fluid,
as these produce at the same time a differential color (Fig. 242). The chorionic
membrane is quite thin, and consists chiefly of mesoderm, mes, with a covering
of ectoderm, EC, consisting of two layers of cells. The mesoderm extends down
to form the core of the villi shown. These villi are much branched and are also
covered by a layer of ectoderm. At the denser ends of the villi the ectoderm is
very much thickened, forming a great mass of cells, so that the ectoderm con-

THE CH ORION WITH TROPHODERM.

nected with one villus is fused with that of adjacent villi, the whole constituting
a large irregular mass of cells, Tro, the trophoderm. In many places it has
already disappeared, so that there are spaces, lac, in the trophoblastic mass. On
the edges of these spaces the trophoblast is undergoing degeneration, deg, and
where that is occurring it is marked in the figure by the deeper staining of the
degenerated material. Upon examination with a higher power (Fig. 243) it will
be noted that the mesodermic cells are stained much more deeply than the matrix.

ntes.

EC.

lac.

Tro.

FIG. 242.— SECTION OF A VERY YOUNG HUMAN CHORION.

deg, Degenerating ectoderm. EC, Epithelial ectoderm, lac, Lacuna for maternal blood, mes, Mesoderm. Tro,

Trophoderm. Vi, Villi.- X 50 diams.

They hav.e an elongated form and run in various directions, more or less parallel
to the epithelium, EC'. Many of them are cut transversely or obliquely. Aside
from the trophoderm, the ectoderm is everywhere two-layered. The inner layer
is distinctly cellular, the outlines of the cells being very sharply marked and the
nuclei being relatively large. In the. outer layer, which is stained more darkly,
there are no cell boundaries to be recognized, the structure being syncytial. The
nuclei are smaller and more deeply stained than those of the inner layer. • In

366

HUMAN UTERUS AND FETAL APPENDAGES.

the trophoderm we find great masses of cells somewhat similar to those of the
cellular layer upon the chorionic membrane and over the surface of the villi, but
they are larger and more lightly stained. They lie closely packed together; their
nuclei are rounded in form, but vary considerably in size and shape. Many of
them contain one or two distinct spots, which, however, are sometimes absent.
On the edges of the spaces which have been formed, and sometimes apparently
in the interior of the mass of trophoderm, we find bands and lines of degenerative

EC'.

EC".

Tro.

FIG. 243. — PORTION OF THE PRECEDING FIGURE MORE HIGHLY MAGNIFIED.

deg, Degenerating ectoderm. EC', Outer syncytial layer of ectoderm. EC", Inner cellular layer of ectoderm.
mes, Mesoderm of villus. Tro, Trophoblast. X 350 diams.

material in which we can find nuclei, but no distinct cell boundaries. The
substance between the nuclei is more or less uniformly granular in texture and
stains quite deeply. The nuclei of the degenerative material vary extremely in
appearance. In some cases they are small and stain rather deeply, and are then
found to be present in more or less considerable numbers. Occasionally, however,
the nuclei are much larger, and more rarely one sees a nucleus of exceptionally
great diameter.

THE CHORIONIC VILLI. 367

Our knowledge of the human trophoderm being still very imperfect, its full
history is partly a matter of supposition. The appearances described indicate
that the trophoderm undergoes a rapid degeneration, during which the cells fuse,
while their protoplasm becomes a hyaline material. - We must then further suppose
that • the degenerated substance is resorbed and disappears altogether. Finally,
we must assume that the entire trophoderm does not disappear, but that enough
is preserved to form the permanent covering of the villi.

It may be noted that the specimen on which the above description is based
agrees essentially with the specimen described by Siegenbeek van Heukelom, which
is regarded as normal.

The Chorionic Villi.

The villi may be obtained in connebtion with the preparations of the uterus
and placenta. In order to see the youngest stages of the first villi it "is necessary
to have the chorionic membrane of the second or early part of the third week. At
this stage the trophoderm is present and the first villi are appearing (compare page
115). To study the growth and form of the villi, single villi or pieces of villi
should be snipped off from the chorion at various stages. Such pieces may. be
examined as opaque objects in alcohol, or they may be • stained and mounted as
permanent preparations. To obtain injected villi it is best to inject the placenta
through one of the arteries of the umbilical cord, using as the injecting mass
gelatin colored with carmine or Prussian blue. Such injections are very easily
made.

Branching of the Villi. — The formation of a branch is usually initiated by an-
outgrowth of the ectoderm. Branches grow very rapidly; the outgrowth which
forms the branch occurs with every degree of participation of the mesoderm.
The two extremes are, first, the bud consisting wholly of epithelium, which may
become a process with a long, thin pedicle and a thickened free end remaining
sometimes entirely without mesoderm; later the mesoderm penetrates it and
completes the structure. Second, a thick bud with a well-developed cord of con-
nective tis'sue and having a nearly cylindrical form. Between these extremes every
intermediate stage can be found. The tips of the branches are for the most part
free, but some of them come in contact with the surfaces of the decidua and
penetrate it for a short distance. By this means the villi of the embryo are
attached to the decidua of the mother. The villi do not penetrate the glands
of the uterus at any period, as was at one time supposed. The ectoderm on the
tip of the villi, where it is in contact with decidual tissue, undergoes a hyaline
degeneration.

The shape of the villi varies according to the part of the chorion and the age
of the embryo. Over the chorion laeve there is first an arrest of development and
a subsequent slow degeneration of the tissues which lose all recognizable organ-
ization of the protoplasm, and to a large extent of their nuclei also. At the same

368 HUMAN UTERUS AND FETAL APPENDAGES.

time the villi alter in shape (Fig. 244), becoming more and more filamentous. By
the fourth month only a few tapering threads with very few branches remain.
The villi disappear almost completely from the chorion laeve, except near the edge
of the placenta. The villi of the chorion frondosum or placental region, on the
contrary, make an enormous growth. At first they are short, thick-set bodies of
irregular shape, as shown in figure 245. At twelve weeks their form is ex-
tremely characteristic (Figc 246). The main stem gives off numerous branches
at more or less acute angles, and these again other branches, until at last the
terminal twigs are reached. The branches are extremely irregular and variable,
though in general club-shaped and constricted at the base. The branches may
be bigger than the trunk which bears them, or of any less size. In older stages

FIG. 244. — ABORTING VILLUS FROM THE HUMAN FIG. 245. — FRAGMENT OF THE CHORION OF FIGURE

CHORION L^VE OF THE SECOND MONTH. 84, HIGHLY MAGNIFIED.

EC, Ectoderm. Mes, Mesoderm. Vi, Villus formed
wholly by ectoderm.

there is a progressive change. During the fifth month we find the irregularity
of shape, though still very marked, decidedly less exaggerated (Fig. 247). The
branches tend to come off at more nearly right angles. One finds very numerous
free ends, as of course only a small portion of the branches touch the decidual
surface. The branches, too, are less out of proportion to the stems, less constricted
at their bases, -less awkward in form. The gradual changes continue until at full
term, as shown by figure 248, the branches are long, slender, and less closely
set as well as less subdivided than at early stages. They have nodular projec-
tions like branches arrested at the beginning of their development. There are
numerous spots upon the surfaces of the villi. Microscopic examination shows that
these spots are proliferation islands, as we may call them, or little thickenings
of the ectoderm with crowded nuclei. Not all the villi, ^however, have changed to
the slender form, for some still preserve the earlier, clumsier shapes. In sections

THE CHORIONIC VILLI.

369

X 19

FIG. 2^6.— ISOLATED TERMINAL BRANCH OF A
VlLLTJS FROM A HUMAN CHORION OF TWELVE

WEEKS.

FIG. 247. — VILLOUS STEM FROM A HUMAN PLA-
CENTA OF THE FIFTH MONTH. X 9 diams.

Xl9

FIG. 248. — TERMINAL BRANCHES OF A VILLUS FROM A FIG. 249. — PORTION OF AN INJECTED VILLXJS

HUMAN PLACENTA AT FULL TERM. FROM A PLACENTA OF ABOUT FIVE MONTHS.

The little spots indicate proliferation islands of the X 210 diams.
covering epithelium.

24

370

HUMAN UTERUS AND FETAL APPENDAGES.

of placentas of different ages the villi offer characteristic differences; for the younger
the stage, the fewer the total number of branches and the larger their average
size. The older the placenta, the more numerous and smaller are the branches as
they appear in sections (Fig. 234).

Injected Villi. — The arteries and veins of the chorionic membrane enter the
villi. After a short course in the main stalk of a villus, the vessels give rise to
many branchlets, and gradually the character of the circulation changes, until in

the smallest villous twigs there are capillaries only (Fig.
249). The capillaries are remarkable for their large size,
and on this account have been interpreted as arteries and
veins by some of the older writers. Their caliber is
often sufficient for the passage of from two to six
blood-corpuscles abreast. They are very variable in
diameter, and also peculiar, in exhibiting sudden con-
strictions and dilatations. In the short knob-like branches
there is often only a single capillary loop, but as the
branch becomes larger the number of loops increases
and they form anastomoses. In the branches large enough
to admit of it, there are two (or sometimes only one)
longitudinal central vessels, the artery and vein of a
superficial network of capillaries (Fig. 250). The forma-
tion of loops and the large size of the capillaries are not
especially characteristic of the villi, but of the fetal blood-
vessels in general.

The histology of the villi is described in the section
on the placenta in situ, page 356.

FIG. 250. — PORTION OF A
SMALL INJECTED VILLOUS
STEM FROM A PLACENTA OF

The Structure of the Amnion.

The structure of the amnion may be studied in sec-

ABOTJT FIVE MONTHS, x tions, such as will be obtained by the student in con-
nection with the sections of the chicken and pig em-
bryos. These preparations will show the early stages. When the amnion is
first formed, it consists of two layers of cells, both very thin, and with somewhat
widely separated nuclei in each layer. Sometimes the nuclei lie in small groups.
Between the two layers is a distinct space. The layer facing the embryo is a
continuation of the embryonic ectoderm, and is more regular and better defined
than the second or mesodermal layer, which is less regular and sends at in-
tervals protoplasmic processes across the space between the two layers to attach
themselves to the ectoderm.

Human Amnion at Two Months. — A section is shown in figure 251. The
ectoderm, EC, is still very thin, but where the nuclei are placed the layer is a little
thicker. The. mesoderm, on the other hand, has become quite thick, and is

THE STRUCTURE OF THE AM N ION.

371

readily seen to be separated into two parts, a thin mesothelial layer, Mstk, cover-
ing the surface of the amnion toward the chorion, and a mesenchymal layer, Mes,
which makes up the greater part of the membrane. Traces of fibrillar structure
in trie mesenchyma are observable. No blood-vessels, lymphatics, or nerves have"
been found.

EC

-:, Mes

Msth

FIG. 251. — TRANSVERSE SECTION OF A HUMAN AMNION OF Two MONTHS.
EC, Ectoderm. Mes, Mesenchymal mesoderm. Msth, Mesothelium. X 250 diams.

Human Amnion after the Fifth Month. — This should be studied both in sections
and in surface views of the whole membrane, small pieces being mounted with
the ectodermal side up. The preparation may be stained with alum .hematoxylin
and eosin. Sections show that the ectoderm (Fig. 252, ec(] has grown somewhat

FIG. 252. — Two SECTIONS OF THE HUMAN AMNION. FIG. 253: — SURFACE VIEW OF THE HUMAN AMNIOTIC
A, From an embryo of eight months; B, at term. EPITHELIUM OF THE FOURTH MONTH.

ect, Ectoderm, mes, Mesoderm. a, Meso- pi, Protoplasm, pr, Intercellular processes, nu,
thelium. X 340 diams. Nucleus. X 1225 diams.

in thickness. Usually the cells are cuboidal (Fig. 252, A), each with a rounded
top in which is situated the more or less nearly spherical nucleus. Sometimes,
however, the nuclei lie deeper down. Less frequently the epithelium is thin
(Fig. 252, B), and its nuclei, which are transversely elongated, lie farther apart.
As regards the mesoderm, it will be noticed that there is usually, perhaps always,

372

HUMAN UTERUS AND FETAL APPENDAGES.

a layer of nearly homogeneous basal substance or matrix immediately underneath
the ectoderm and remarkable for containing no cells. Sometimes the remaining
portion of the mesoderm is broken up so as to offer a fibrillar structure (Fig. 252,
A), and when that is the case we can no longer make out a distinct mesothelial
layer. At other times the more or less homogeneous matrix can be seen through
the whole thickness of the amnion (Fig. 252, B), and when this is the case the
mesothelium, a, can be readily identified.

In surface views the amniotic ectoderm is seen to consist of more or less
regularly distributed nuclei with cell-bodies connecting with one another by inter-
cellular bridges of protoplasm (Fig. 253). The nuclei, nu, are relatively large,

rounded, and with distinct outlines. They
have a more or less well-marked intranuclear
network with thickened nodes and a small
number of deeply stained granules which
are probably chromatin. Each nucleus is
surrounded by a cell-body, pi, and the ad-
jacent cell-bodies are separated from one
another by clear spaces which are crossed
by threads of material, pr, stretching as
bridges between the neighboring cells. The
protoplasm is vacuolated. The whole picture
thus leads to the view that the epithelium
is a sponge-work of protoplasm somewhat
condensed around each nucleus. As re-
gards the mesoderm, it is very difficult to
obtain clear pictures of the cells, though
the nuclei can be readily observed. They vary greatly in appearance, being some-
times fairly regular and uniform, though always far less so than the nuclei of the
mesenchyma of the embryo proper. In other cases (Fig. 254) the nuclei are exceed-
ingly irregular; some are large with a distinct network, d; others are smaller and
differ but slightly from the normal. Some are very irregular, b, others slightly
irregular, c, and others again strangely elongated and narrow, a. Many other
forms besides those represented in figure 254 may be found. It has been sug-
gested that these varied shapes of the nuclei indicate degenerative changes, and, in
fact, many of the nuclei are actually breaking down, for in some specimens every
stage between a nucleus and scattered granules can be observed, and one may find
nuclei with distinct membranes, without membranes, masses of granular matter stained,
clusters of granules crowded together, and, finally, other clusters more or less scattered.

The Umbilical Cord.

The umbilical cord may be well preserved in Zenker's or Tellyesnicky's fluid.
Transverse sections may be prepared in paraffin and stained with alum hematoxylin

FIG. 254. — NATURAL GROUP or NUCLEI FROM THE
MESODERM OF THE HUMAN AMNION OF THE
FIFTH MONTH. (For lettering see text.)
X 1225 diams.

THE UMBILICAL CORD.

373

and eosin, or with Heidenhain's iron hehiatoxylin and orange G; or, if it is
desired to study the development of the connective-tissue fibrillae, with Mallory's
triple, connective-tissue stain.

FIG. 255. — CROSS-SECTION OF A HUMAN UMBILICAL
CORD AT TERM.

.4, A', Umbilical arteries much contracted. V, Um-
bilical vein. Y, Remnant of allantois. X 12
diams.

FIG. 256. — SECTION OF THE ALLANTOIS FROM A

HUMAN UMBILICAL CORD OF THREE MONTHS.

ent, Allantoic entoderm. mes, Mesoderm. X 340

diams.

FJG. ^.—CONNECTIVE TISSUE FROM THE UMBILICAL CORD OF A HUMAN EMBRYO OF 21 MM. STAINED WITH

ALUM COCHINEAL AND EOSIN.
n, Nucleus, p, Protoplasmic network. X 540 diams.

A general description of the umbilical cord has been given, pages 115 to 116,
and two sections (Fig. 66) are there represented showing the structures which
appear in sections of the umbilical cord. At full term some of these structures

374

HUMAN UTERUS AND FETAL APPENDAGES.

are still present but somewhat* modified (Fig. 255), while others have been partly
or wholly obliterated. As contrasted with the early stages, we find that the
ccelom is entirely obliterated, that the yolk-stalk has usually been completely
resorbed, and that only traces of the allantois can be seen, Y. The blood-vessels
have grown; there are two arteries, A, A', and a single vein, V. Around each of
these is a well-developed muscular coat produced by differentiation of the sur-
rounding mesenchymal cells, which have assumed an elongated form and con-

c.-,. //-• '.0 WVJS'

^ -\v , • m..*"' •

FIG. 258. — CONNECTIVE TISSUE FROM THE UMBILICAL CORD OF A HUMAN EMBRYO OF THREE MONTHS, STAINED

WITH ALUM COCHINEAL AND EOSIN. X 511 diams.

c,c, Cells. /, Fibrillae.

tractile function. It will be remembered that the allantois in man is primitively
a very narrow tubular diverticulum which extends originally nearly to the chorion
(compare Fig. 87). As the umbilical cord lengthens the allantois fails to lengthen
equally. During the second month it increases very little in diameter. After the
second month it appears in sections as a small group of epithelioid cells (Fig. 256)
with distinct walls, irregularly granular contents, and round nuclei; the group may
or may not show a remnant of the original central cavity. Around the cells,
ent, there is a slight condensation of the connective tissue, mes, to form, as it were,
an envelope.

The mesoderm varies in appearance according to the age* of the specimen.

THE STRUCTURE OF THE HUMAN YOLK-SAC.

Its growth and differentiation are rapid. During the second month it consists
merely of numerous cells (Fig. 257) imbedded in a clear substance. The cells
form a complex network of which the filaments and meshes are extremely variable
in size. The nuclei are oval, granular, and do not always have accumulations
of protoplasm about them forming main cell-bodies. (Compare description of
first stage of the mesenchyma, page 89.) By the end of the third month the cells
have assumed nearly their definite form (Fig. 258). Their protoplasm is increased
in amount and forms a large body around each nucleus. The network has become
simpler and coarser, the meshes bigger, and the filaments fewer and thicker. In
the matrix are numerous connective-tissue fibrillae, not yet disposed in bundles.

FIG. 259. — ECTODERM OF AN UMBILICAL CORD OF A HUMAN EMBRYO OF THREE MONTHS.

EC, Ectoderm, mes, Mesoderm. c, Mesenchymal cell, a, Outer layer of ectoderm, b, 'Inner layer of ectoderm.

X 545 diams.

In older cords there is an obvious increase in the number of fibrillae and they
form wavy bundles. In the cord of yet older stages the matrix also contains
mucin which may be stained by alum hematoxylin. In such .cords so stained the
blotch of color appears in the intercellular spaces.

The ectoderm is at first a single layer of cells, as it is also over the body of
the embryo, and as it remains permanently over the amnion. At three months
we find the ectoderm to be two-layered, corresponding to the second stage of the
epidermis of the embryo. In still older stages there is slight increase in the
number of layers of the ectoderm, but it never passes much beyond the stage of
the embryonic epidermis at the fourth month. Figure 259 is from a cord at three
months. The outer layer, a, of ectodermal cells is granular and stains much
more darkly than the inner layer, b, in which also cell bundles are more distinct.

The Structure of the Human Yolk-sac.

The human yolk-sac may be preserved in Zenker's or Tellyesnicky's fluid,
stained in toto with alum .cochineal, imbedded in paraffin, and cut in transverse
sections. Yolk-sacs of the second month are preferable for study.

376

HUMAN UTERUS AND FETAL APPENDAGES.

The general history of the yolk-sac is described on pages 63 and 66. It
becomes a pear-shaped vesicle which in man attains its maximum diameter about
the end of the fourth week. It then measures from 7 to n mm. From its
pointed end runs the long stalk by which it is connected with the intestine. In
very early stages the stalk is hollow and its cavity is lined by entoderm. But
this condition is soon obliterated, the stalk becoming solid and the entoderm dis-
appearing. In this condition we found the yolk-stalk in an embryo of 21 mm.

(Fig. 66, A). The sac itself remains hollow (Fig.
260). It has a lining of entodermal cells, En, and
a thicker layer of mesoderm, mes, containing blood-
vessels, v. The network of the vessels imparts a
characteristic appearance to the external or meso-
dermic surface of the yolk-sac. In the earliest
stages observed the entoderm consisted of a single
layer of cuboidal cells.

Transverse Section of a Yolk-sac of about Two
Months. — The contents of the fresh yolk-sac are
fluid, but coagulate when the organ is hardened.
In the coagulum are found some stained bodies

_ which are supposed to be yolk material. The

OF A VERY YOUNG HUMAN EMBRYO. J

En, Entoderm. mes, Mesoderm. v, entoderm has undergone proliferation and thick-
Blood-vessel.— (After Fr. Keibel.) ening. These cells are more or less irregular and

disposed in two or three layers. Many of the

superficial cells are stained deeply and have small nuclei, while the deeper lying
cells are larger, more lightly stained, and have larger nuclei and more distinct cell
boundaries. The mesoderm consists chiefly of somewhat crowded mesenchymal cells,
the nuclei of which are smaller than the entodermal cells, and a well-marked layer of
mesothelium, which forms the external covering of the yolk-sac. In the mesoderm
appear relatively large blood-vessels, which are usually found filled with blood-cor-
puscles. The blood-vessels have distinct endothelial walls and lie in the part of
the mesoderm toward the. mesothelium, so that they are separated somewhat from
the entoderm and seem often to lie immediately underneath the mesothelium. They
are so large that each vessel causes a protuberance upon the yolk-sac.

FIG. 260. — SECTION OF THE YOLK-SAC

CHAPTER VIII.

METHODS.

Measuring Length of Embryos.

Owing to the many changes during development in the curvature of the. longi-
tudinal axis of the mammalian embryo, it is impracticable to measure that axis
or to employ any one system of measurements to obtain comparable results for
all ages. For this reason the best practice is to measure in all cases the greatest
length of the embryo in its natural attitude along a straight line. The limbs are
not to be included in such measurements. This greatest length in young stages
will not include the head (compare, for example, Fig. 95), but in most stages the
head would be included. Many German authors employ the measurement intro-
duced by His, which he calls the Nackenldnge, which corresponds to the distance
in a straight line from the neck-bend to the caudal bend. As it is impossible to
measure this distance in later stages, it seems best not to use it at all. The length
of an embryo, as given by German authors, is often indicated by the abbreviation
NL., and is, of course, often different from the measures used in this work.

Dissection of Embryos.

Vertebrate embryos may be dissected without serious difficulty. Specimens
hardened in Zenker's fluid are particularly favorable for this purpose. Dissection
should be more extensively practiced than is at present usual in embryological work,
since by it all the viscera, the central nervous system and even the main nerve
trunks may be demonstrated advantageously.

By the following simple method the embryo may be securely attached to
the bottom of the dish in which the dissection is to be made, best in 80 per cent
alcohol. Place the specimen in 95 per cent alcohol for half an hour, then in a
mixture of equal parts of alcohol and ether for fifteen minutes. Put a few drops
of celloidin dissolved in alcohol and ether on the bottom of the dish; put the
specimen in place; allow the celloidin to stand for two or three minutes until a
film is formed and then cover the specimen with 80 per cent alcohol, which will
set the celloidin. After the dissection is finished the specimen may be freed by
dissolving the celloidin with a mixture of alcohol and ether in equal parts.

Methods of Hardening and Preserving.

The three most generally useful methods for preserving embryos are with Zen-
ker's or Tellyesnicky's fluids and 10 per cent formalin. Good results may be had

377

378 METHODS.

with the other reagents. Specimens preserved with Bouin's fluid have the advantage
of staining sharply. To study the medullary sheaths of nerve-fibers, as is necessary
to follow the development of the fiber tracts in later stages, the specimens may
be preserved in Miiller's fluid. Flemming's, Hermann's, and Bouin's fluids are valua-
ble, especially for cytological study, but are applicable only to small pieces.

.1. ZENKER'S FLUID.

Formula: Corrosive sublimate ...» 5 gm.

Potassium bichromate i gm.

Sodium sulphate ' i gm.

Water 100 c.c.

Add 5 c.c. of glacial acetic acid to the fluid immediately before using.

The fluid does not have great penetrating power, but may be used for embryos
of 25 mm. The amount of fluid should be from ten to twenty times the volume
of the specimen, and better results are obtained if the fluid is changed after a few
hours. Chicks of the first and second days are hardened in from two to four hours;
embryos of from 6 to 8 mm. in from eight to ten hours; embryos of 12 mm. in twenty-
four hours; larger embryos in from thirty to forty hours. After the proper interval in
Zenker's fluid the specimens must 'be removed and washed in running water for from
twelve to twenty-four hours. Transfer to 50 per cent alcohol for from one to three
hours, then to 60 per cent, 70 per cent, and 80 per cent. It is indispensable to remove
now the excess of corrosive sublimate by adding sufficient tincture of iodine to
give the alcohol the color of port wine; if the iodine disappears, it must be renewed.
After from one to three days, according to the size of the specimen, transfer
it to fresh 80 per cent alcohol, which must be changed until it no longer extracts
any iodine from the specimen.

2. TELLYESNICKY'S FLUID.

Formula: Bichromate of potassium 3 gm.

Water 100 c.c.

Immediately before using add 5 c.c. glacial acetic acid.

This reagent is employed in the same manner as Zenker's fluid, except that the
treatment with iodine is omitted.

3. FORMALIN.

Formula: Formalin 10 c.c.

Water 7 90 c.c.

Specimens are placed in the fluid, which should be changed in a few hours. On
account of its extreme simplicity this method is used especially for human embryos.
If the fluid is used in large quantity embryos even of 80 mm. may be well pre-
served. They may be kept indefinitely in the solution, and transferred to alcohol
when needed for sectioning. The method has the disadvantage that the transfer
to alcohol, even if made very gradually, always causes a considerable shrinkage.

METHODS OF HARDENING AND PRESERVING. 379

for the 'prevention of which no satisfactory method has been devised. Fortunately,
the shrinkage usually produces no distortion.

4. BOUIN'S FLUID.

Formula: Picric acid, saturated aqueous solution 225 c.c.

Formalin ' 75 c.c.

Glacial acetic acid ; , 15 c.c.

Specimens are kept in the fluid from two to seven days, not longer, according
to their size; transfer to 30 per cent alcohol for one hour, to 50 per cent alcohol
for from one to two hours, to 60 per cent alcohol for twelve hours, and finally to 70
per cent alcohol, which must be changed daily until it no longer shows even a
trace of yellow discoloration by picric acid.

5. MULLER'S FLUID.

Formula: Bichromate of potassium 20 gm.

Sulphate of sodium 10 gm.

Water 1000 c.c.

Miiller's -fluid is a valuable reagent, and for the study of the later stages of the
nervous system indispensable. The objections to its use are that it requires a
long time to act, that it renders the specimens brittle, and makes them somewhat
difficult to stain. It must be used in large quantities and be frequently changed,
and allowed to act on the specimens from three to eight weeks, according to their
size. The appearance of a film or scum indicates that the fluid needs to be changed.

6. PARKER'S FLUID.

Formula:* 70 per cent alcohol .» 100 c.c.

Formaldehyde i c.c.

Very convenient when a simple and expeditious preservative is necessary. The
specimens are placed in the fluid, which ought to be renewed in a few hours.
They may be kept permanently in the fluid, but it is desirable, before using them
for study, to remove the formaldehyde by treating them with fresh 70 per cent
alcohol.

7. FLEMMING'S FLUID.

Formula: i per cent solution of chromic acid . . . . • 50 c.c.

2 per cent solution of osmic acid j2 c.c.

Glacial acetic acid 3 c.c.

This fluid must be used freshly mixed, and must not be kept in the dark. The
specimens must be of small size and as fresh as possible. The amount of fluid
used should be not less than 15 times the volume of the specimen. Speci-
mens are kept in the fluid from twenty-four to forty-eight hours, washed in
running water from four to twenty-four hours, and then transferred to alcohols
of gradually increasing strength. The fluid is useful chiefly for cytological work.

* Differs slightly from the original formula.

380 METHODS.

8. HERMANN'S FLUID.

Formula: i per cent platinum chloride in distilled water 60 c.c.

2 per cent osmic acid in distilled water 8 c.c.

Glacial acetic acid 4 c.c.

Used. in the same manner and with the same precautions as No. 7.

Preservation in Alcohol.

When a specimen is to be preserved with alcohol alone, it should be put
first in 30 or 50 per cent alcohol for an hour or more, then into 60 per
cent for several hours, 70 per cent for from twelve to twenty-four hours, and finally
into 80 per cent, in which it should be kept until required for use. If the
specimen is to be sectioned, it must be placed in 95 per cent alcohol, which must
be renewed at least once, and be allowed to act for twenty-four hours or more,
unless the specimen is very small, when a somewhat shorter time may suffice.

Directions for Imbedding Specimens to be Microtomed.

A. To Imbed in Paraffin:

1. Stain in toto. (See page 382.)

2. Dehydrate in alcohol from three to twenty-four hours.

3. Place in oil of cloves and turpentine (equal parts), one to twenty-four
hours.

4. Place in fresh cloves and turpentine for one to twenty-four hours.

5. Place in soft paraffin at 54° C. for thirty to ninety minutes.

6. Place in hard paraffin at 54° C. for thirty to ninety minutes.

7. Warm a glass plate to about 70° C.; place on it a paper tray or metal
imbedding frame; fill the box with hard paraffin' at 54° C. Warm a spatula and
with it remove the specimen to the tray or frame, and arrange it in a proper posi-
tion. As soon as the paraffin has set, chill it rapidly with cold water, otherwise
the paraffin is likely to crystallize and therefore to cut badly.

B. To Imbed in Celloidin:

1. Dehydrate the mass thoroughly in 95 per cent alcohol, four to twenty-
four hours.

2. Place mass for twenty-four hours in alcohol and ether, equal parts.

3. Place mass in thin syrupy solution of celloidin in equal parts of ether and
alcohol for at least twenty-four hours. (If the specimen contains cavities several
days are necessary to allow the celloidin to penetrate and fill them.)

4. Place mass in thick viscid solution of celloidin in equal parts of ether and
alcohol for twenty-four hours.

5. Set mass on block of vulcanite, compressed fiber, or maple-wood, and as
soon as a film has formed over the surface of the celloidin (two to five minutes)—

6. Immerse in 80 per cent alcohol for twenty-four hours.

7. Cut.

METHODS OF STAINING. 381

Method of Mounting Paraffin Sections.

The student is advised to use for serial sections slides 40 x 76 mm. and
from 1.75 to 2 mm. thick. The thick slides are much better than the thin
ones recommended by dealers. The cover-glasses ought to be 0.17 to 0.18 mm.
thick. A generally convenient size is 35 x 50 mm.

The serial order of the sections should be preserved with the utmost care,
and time spent in arranging the sections in straight rows will be found to be time
saved.

The albumen-glycerin method of fastening the sections to the slide will be
found satisfactory.

Formula: Take the white of one fresh egg, beat slightly until equally fluid, filter it (it will take about
twenty-four hours), and add an equal amount of glycerin. To this fluid add a small piece
of camphor.

i.' Clean the slide thoroughly.

2. Put on a small drop of albumen solution.

3. Spread it out very thin with the finger.

4. Add five or six drops of distilled water, which must flow evenly over the
coating of albumen.

5. Place the sections on slides in regular rows.

6. Warm the slides gently over an alcohol flame, to allow the sections to
flatten. The paraffin must not melt.

7. Drain off the water- as completely as possible, and arrange the sections in
straight rows.

8. Place the slides in oven for twelve to twenty-four hours to evaporate the
water completely.

9. Dissolve off the paraffin in turpentine or xylol.

10. Put in absolute alcohol for three to five minutes.

11. Clear in turpentine or chloroform.

12. Mount in damar varnish. (Canada balsam is undesirable, because it
becomes much discolored with age.)

Methods of Staining.

In embryological work the specimens are usually stained in toto before im-
bedding, either with alum cochineal or with borax carmine, the former being the
more generally useful stain. Staining on the slide is also much used either to
secure a counterstain after the in toto coloration or to secure some special result.
For counterstains eosin, Lyons blue, and orange G are particularly recommended.
A few of the most important special stains are given below.

i. ALUM COCHINEAL.

Formula: Powdered cochineal 6 gm.

Potassic alum 6 gm.

Water .80 c.c.

382 . METHODS.

Boil vigorously for twenty minutes; allow the fluid to settle; decant the clear
fluid; add more water and boil again; filter the entire solution and evaporate the
filtrate to 80 c.c.; add a small piece of thymol or camphor to prevent the growth
of fungi. Alum cochineal is, on the whole, the best reagent for in toto staining,
as it will penetrate quite large objects and color them uniformly throughout, and
gives a good differentiation of the tissues.

For in toto staining place the specimen in water until it sinks; then transfer
it to the cochineal for twenty-four hours, or for large specimens longer; the depth
of the stain will depend upon the strength of the' solution; transfer to clean water
for fifteen to twenty minutes to extract the alum, which otherwise will crystallize
in the tissues when the specimen is placed in alcohol; the object must not be
left too long in water, because it extracts the color also; put in 50 per cent
alcohol for one hour, then successively in 70 per cent, 80 per cent, and 95 per
cent, when the specimen will be ready for imbedding.

2. BORAX CARMINE.

Formula: Best carmine 3 gm.

Borax 2 gm.

Water 50 c.c.

Boil for twenty minutes; allow the solution to cool; 'add water enough to restore
that lost by evaporation, • then add 50 c.c. of 70 per cent alcohol, let the solution
stand twenty-four hours; filter. Borax carmine gives a good nuclear stain and
may be advantageously supplemented by counterstains.

For in toto staining place the specimen in water until it sinks; transfer to the
carmine for twenty-four hours, or longer for large specimens; wash in water for
five minutes; then place it in 70 per cent alcohol, to every 100 c.c. of which 2 c.c.
of hydrochloric acid have been added; after one hour transfer to fresh 70 per
cent alcohol, which must be renewed in an hour or two, and finally transfer to
80 per cent and 95 per cent alcohol, and the specimen will be ready to imbed.

3. ALUM HEMATOXYLIN.

Formula: i. Dissolve 10 grms. hematoxylin in 60 c.c. absolute alcohol.

2. Dissolve 15 grms. ammonia alum in 100 c.c. warm water, and allow the solution to cool.

3. Mix the two solutions and allow the stain to "ripen" in a shallow open dish exposed to

the light for three days.

4. Filter, then add 25 c.c. pure glycerin and 25 c.c. methyl alcohol.

5. After three days filter.

The solution thus prepared will keep indefinitely. It is one of the best nuclear
stains known. Eosin may be used with it as a counterstain giving beautiful
preparations.

i. Place sections from water into a filtered diluted solution of the stain for
from fifteen minutes to twenty-four hours, according to the strength of the staining
solution. (Slow staining gives the best results.)

METHODS OF STAINING. 383

2. Wash thoroughly in water until the section is blue.

3. Dehydrate and mount.

4. COUNTERSTAINS are used either with celloidin sections treated singly, or
with paraffin sections after they have been fastened on the slide. The three here
recommended are alcoholic solutions, and the method of using is the same for all.
If the sections are dried on the slide in an oven for three days, they will adhere
to the glass much more securely during the manipulations of counterstaining.
Lyons blue is less permanent than eosin and orange G.

For staining paraffin sections on the slide it is convenient to have eight jars
or dishes large enough to hold a slide. The slide is transferred from jar to jar
in the order below, being allowed to remain in each jar a few minutes. The
very most scrupulous care is necessary to keep all the fluids clean, and it is indis-
pensable to filter them frequently; the sections on the slide catch and hold the
particles floating in the reagents when they are not clean.

Order of jars: i. Xylol.

2. Xylol.

3. Xylol and absolute alcohol, equal parts.

4. Absolute alcohol.

5. Counterstain.

6. Alcohol of 95 per cent.

7. Absolute alcohol.

8. Xylol.

Eosin Formula: 2 per cent in 95 per cent alcohol.

Orange G. Formula: i per cent in 95 per cent alcohol.
Lyons blue Formula: i per cent in 95 per cent alcohol.

5. HEIDENHAIN'S IRON HEMATOXYLIN.

Formula I: Iron alum 2 gm.

Distilled water 100 c.c.

II: Hematoxylin crystals i gm.

95 per cent alcohol 10 c.c.

Distilled water, to be added after the hematoxylin is dissolved in the

alcohol , 90 c.c.

(If the stain does not work, add 0.5 grm. lithium bicarbonate.)

1. Place sections in the iron solution for from six to ten hours. (Specimens
hardened with Flemming's or Hermann's fluid require longer than specimens from
Zenker's or Tellyesnicky's fluid.)

2. Wash quickly in water.

3. Transfer to the hematoxylin solution for from twtlve to eighteen hours.

4. Wash in tap-water.

5. Decolorize in the iron solution.

6. Wash thoroughly in tap-water.

7. Dehydrate and mount.

384 METHODS.

This stain is useful for cytological work, the study of cell division, etc. The
preparations are often improved by counterstaining with orange G.

6. BEALE'S CARMINE.

Formula: Best carmine i gm.

Ammonia v 3 c.c.

Pure glycerin 96 c.c.

Distilled water 96 c.c.

Alcohol, 95 per cent 24 c.c.

Dissolve in ammonia plus part of the water, add the rest of the water, and
allow the solution to stand in an open dish until the ammonia is nearly all driven
off. Then add the alcohol and glycerin. For use dilute with an equal part of
glycerin. Stain for twenty-four hours in an open dish, which, together with a
second open dish containing acetic acid, is placed under a bell- jar; wash the
sections thoroughly in water and then in very weak hydrochloric acid (i c.c. to
500 c.c. water), and again in water.

Beale's carmine is especially valuable for the study of the central nervous
system and of the placenta.

7. WEIGERT'S COPPER HEMATOXYLIN.

Formula I: Copper solution:

Acetate of copper in saturated aqueous solution.
II: Hematoxylin solution:

Hematoxylin crystals 2 gm.

95 per cent alcohol 20 c.c.

Distilled water 80 c.c.

(If the stain does not work, add 0.5 grm. of lithium bicarbonate.)
Ill: Iron solution:

Ferricyanide of potassium 25 gm.

Borax 20 gm.

Water 2000 c.c.

This stain is indispensable for the study of the nervous system after the
medullary sheaths have begun to develop; the specimens must be preserved in
Miiller's fluid. The method is also valuable for the study of the placenta and
uterus.

1. Place the sections in water.

2. Place the sections in the copper solution for twenty-four hours.

3. Wash quickly in water.

4. Put them in the hematoxylin solution for five to ten minutes. The sec-
tions should turn a deep blue-black.

5. Wash thoroughly in water.

6. Decolorize in the iron solution; the section must be gently moved about
to secure an even decolorization. When part of the section shows a brown color,
it should be removed and examined.

METHODS OF RECONSTRUCTION. 385

7: Wash thoroughly in water to remove the iron solution, no trace of which
can be left without ruining the specimen. (Unless the washing is very thorough,
the sections will gradually fade out after the final mounting.)

8. Dehydrate with alcohol and mount at once in damar.

8. MALLORY'S TRIPLE CONNECTIVE-TISSUE STAIN.

Formula I: Acid fuchsine i . o gin.

Distilled water 1000 . o c.c.

II: Distilled water 100.0 c.c.

Phosphomolybdic acid, to be added before other ingredients i . o gm.

Aniline blue, soluble in water 0.5 gm.

Orange G 2.0 gm.

1. Preserve in corrosive sublimate or Zenker's fluid.

2. Stain the sections in the fuchsine solution one to three minutes.

3. Wash in water very quickly.

4. Place in the phosphomolybdic solution two to twenty minutes.

5. Wash off excess stain in water.

6. Dehydrate in 95 per cent alcohol and mount in damar.

This method gives a perfect differential stain of connective-tissue fibrils, and it
is to be used whenever the fibrils are to be especially studied.

Methods of Reconstruction.

It is often important to obtain definite plastic conceptions of the anatomy
of embryos or parts of embryos too small for dissection. To secure these in the
best form, it is necessary to reconstruct either drawings or models from sections.
The methods employed for these two forms of reconstruction, being different, must
be described separately.

Reconstruction of Drawings from Sections. — To make these reconstructions
satisfactory, it is indispensable to have an accurate outline of the embryo repre-
senting it in the point of view to be used for the reconstruction and enlarged to
the precise scale upon which the reconstruction is to be made. This drawing
must, of course, be made before the embryo is imbedded and sectioned. It is
further necessary to know accurately the plane of the sections and their thickness,
and, finally, the total number of sections in the series must be counted. A con-
venient scale for the reconstruction of the anatomy of mammalian embryos is a
magnification of from 15 to 30 diameters.

Let us suppose that a pig of 12 mm. has been drawn in a side view magnified
20 diameters; that the embryo has been cut into 900 transverse sections and the
approximate plane of the sections is known. It may be more exactly determined
by the study of the sections themselves; for instance, it may be determined
what section is the last to pass through the surface of the head in the region of the
fore-brain and the last to pass through the border of the anterior limb. Then it

386 METHODS.

can be further ascertained through which dorsal segments these two sections pass.
By these data the plane of the two sections can be accurately fixed. Over the
outline of the embryo is now drawn a series of lines which represent the position
of the sections. It is generally sufficient to put in lines which represent only every
second, third, or even fourth section. If at any point where the structure is com-
plicated more details are needed, lines for the additional sections can be interpolated.
In our supposed case, our lines representing every fourth section, there would be
225 parallel lines, and these should be numbered to correspond to the sections
which they represent.

The outlines of the actual sections corresponding to the numbered lines in
the diagram must now be made with the camera lucida. In regard to these great
care is necessary, especially if, as is likely to be the case, the sections are
from embryos imbedded in paraffin, because when an embryo is so imbedded it
always shrinks, and after imbedding is smaller than before. The shrinkage seems
to be uniform throughout and not to disturb the topographical relations even of
the finest structures. Unfortunately the shrinkage is not constant, but varies from
specimen to specimen, hence a camera drawing made from the sections and magni-
fied 20 diameters will not be of the right size to fit in the diagram, and these
drawings, must, therefore, be corrected. This may be done either, as is best, by
making the original camera lucida drawings of the right magnification for direct
use in reconstruction, or they may be made nearly the right magnification and
when they are measured off the necessary correction may be introduced by measur-
ing them with proportional dividers.

From the camera lucida drawings of the single sections the measurements are
taken to fix the position of the parts in the reconstruction.

For a given section the exact position in the reconstruction is fixed by the line
on the outline drawing of the embryo corresponding to the number of the section.
On the drawing of the section the distance of the organ to be reconstructed from
the point in the section corresponding to the outline of the embryo is measured off,
and then marked upon the proper line of the reconstruction diagram. A similar
measurement is then taken from the next section and transferred to the diagram in
the same manner, and so on . with successive sections until a series of dots is
obtained which mark the. outline of the organ. These dots are then connected by
a continuous line, .which will indicate the form and correct position of the organ.
Simple reconstructions may be easily made by these means. When, however, more
complicated reconstructions are attempted, much judgment and skill are necessary
in the selecting of parts which may be successfully represented in a single drawing,
bearing in mind always the point of view which is assumed for the reconstruction,
so that organs may be correctly represented in their relative positions, nearer or
farther from the observer as he looks at the drawing . After the outlines are
completed the shading of the parts must be added, and this often requires a special
degree of skill and a considerable faculty of plastic imagination. As examples of

METHODS OF RECONSTRUCTION. 387

complicated reconstructions, the student is referred to figures 169 and 172, pages
229 and 235.

Oftentimes simpler reconstructions are very helpful in which only a few sections
are combined, as, for example, to show the course and branches of the spinal
nerves in young embryos. In such a case the outline of the middle section of
the series proposed to be combined may be selected to give the outline of the
reconstructed drawing. Camera lucida drawings of this and the neighboring sec-
tions to be included should be made of the desired magnification. The reconstruc-
tion itself may be made upon tracing paper, which is laid successively over the
drawings of the sections and the parts required from each can be added upon the
tracing paper, which will thus combine in a single drawing the parts intended to
be represented. Reconstructions of this kind are easily made by students and are
often very instructive.

Reconstruction with Wax Plates by Barn's Method. — The basis of this method
is to make in wax a magnified reproduction of the single sections, representing in
the wax such portions of the section as it is desired to reproduce in plastic recon-
struction. To this end wax plates must be made which represent a definite magni-
fication of the thickness of a section. For working by this method it is usually
advantageous to employ rather thick sections, say, of 20^. If the magnification
chosen is fifty times, which is practically often convenient, then the wax plates
should be made fifty times 2o/i in thickness, or i mm. The most convenient
plates to work with are those from i to 2 mm. thick. Upon a wax plate of the
requisite thickness a camera lucida drawing is made. This may be done with a
lithographic crayon or with a fine steel point. The drawings must be of exactly
the right magnification; in the illustration chosen, 50 diameters. Next, the wax plate
is put upon a glass or a metal surface where it lies perfectly flat, and with a
sharp thin-bladed knife or scalpel the outlines of the organs which it is intended
to reconstruct are cut out as may be desired. Our bit of wax then represents a
model of the parts selected from the section, and equally magnified in the three
dimensions of space. Wax plates made from successive sections are then piled
up, one on top of the other, in the proper order. If they are rightly superimposed,
an operation which often requires skill and judgment, and always requires the
utmost care, then the pile of plates will correctly represent the form of the parts
included in the reconstruction. To fasten the plates together it is only necessary
to pass a warm metal instrument over the edges of the plates, enough to melt the
wax a little. With proper care this may readily be accomplished without destroy-
ing the surface modeling of the reconstruction.

The simplest method of making wax plates is to have a large tin pan with
vertical sides. This is filled with very hot water, and melted beeswax is poured on
the surface of the water and allowed to cool. Plates of sufficiently exact and even
thickness may be cast in this way, provided the operation is carried out in a quiet
place so that the surface of the water is not disturbed while the wax is hardening.

388 METHODS.

It will be found convenient to have a large plate of iron, not less than one eighth of
an inch in thickness, which may be placed upon supporters. The tin pan should be
set upon this plate and the plate heated by lamps below in order to keep the water
hot enough to allow the wax to spread evenly over the surface of the water. The
water must be freed from air before the wax is poured in, but must not be allowed to
boil after the wax has been added. If bubbles appear in the wax plate, they may be
removed while the wax is still hot by directing the blue flame from a Bunsen burner
down upon them. If the pan is heated directly without the iron plate, it is sure
to warp and become unfit for use. Thin iron plates are also liable to be warped.
To determine the thickness of the plates cast as described we proceed em-
pirically. A weighed quantity of wax is melted and poured into the pan. After
the plate has solidified it is removed by cutting it free from the edges of the pan,
and the thickness of the plate is then measured at various points by micrometer
callipers. From these data it is easy to calculate exactly what thickness of plate
one gram of beeswax represents. To get accurate results it is advisable to cast
several plates of varying thickness and determine the average for one gram in
that way. Having determined what one gram represents in thickness, it becomes
thereafter only necessary to weigh out the proper number of grams in order to
obtain any desired thickness of wax plate. It will be found advantageous to filter
the wax before using it. This may easily be done by a double hot-water filter.
Such a filter may be made of copper. It is desirable to connect it with a Mariotti's
flask to maintain a constant water level.

Directions for Orienting Serial Sections of Embryos. (NOTE: The lower
edge of the ribbon is the one to the left, when the observer has the object between
himself and the knife.)

1. Transverse Series.

Normal thickness: io//.

Dorsal surface to be toward the lower edge of the ribbon.
Series to begin with the head.
In cutting, the left side of the embryo must strike the knife first.

2. Sagittal Series.

Normal thickness: Small embryos, io/*.
Medium " i5//.
Large 20^.

The head of the embryo to be toward the lower edge of the ribbon.
Series to begin with the right side.

In cutting, the ventral side of the embryo must strike the knife first.
^3. Frontal Sections.

Normal thickness: Small embryos, iofi.
Medium " 15;*.
Large 20;*.

MICROTOMES.

389

the

The head of the embryo is to be toward the lower edge of the ribbon.

The series is to begin with the ventral side.

In cutting, the left side of the embryo must strike the knife first.

In mounting leave space for the label at the left-hand end of the slide. Keep

sections in the order cut. Arrange them on the slides in the sequence of

ordinary written lines.

Microtomes.

There are many forms of microtome which may be used with good results
and which will work very satisfactorily for making sections of small objects. The
cutting of larger objects, such as pig embryos of from 15 to 20 mm., and of pieces

FIG. 261. — THE PRECISION MICROTOME.

of the uterus or other organs, is more difficult, and microtomes which work satis-
factorily with small objects often fail to give good even sections of more difficult
objects. For embryological work a microtome ought, therefore, to be selected
which will give perfectly regular sections in long series of any desired thickness
from i/* up to 25/4. It is also desirable for economy of time to have a micro-
tome which works automatically. These considerations lead the author to recom-

390

METHODS.

mend for embryological use especially two forms of microtome made by Messrs.
Bausch & Lomb, of Rochester, N. Y., and designated by them as the "precision"
and " rotary" microtomes.

The precision microtome (Fig. 261) consists, first, of an upper square form
upon which the knife may be clamped in any desired position; second, of two
horizontal ways upon which moves the carriage which bears the object-holder;
and, third, of a micrometer screw with an automatic feeding contrivance on the
under side of the movable carriage. The construction is very solid and great
rigidity of the parts is secured. The microtome may be used for either paraffin
or celloidin cutting. According to the author's experience, this microtome con-
siderably surpasses all other types in the accuracy of the work which may be done

FIG. 262. — THE AUTOMATIC ROTARY MICROTOME.

with it. The rotary microtome was originally made in Germany, and various
patterns have been put upon the market by German, French, English, and Amer-
ican manufacturers. The new pattern recently introduced by Messrs. Bausch &
Lomb embodies a considerable number of improvements, which render the instru-
ment (Fig. 262) very desirable for general laboratory use. It works with accuracy,
is very easy to manipulate, and cuts sections with extreme rapidity. It is adapted
only for paraffin work. For the general use of students, in elementary courses
especially, this microtome is to be preferred to the " precision," as it requires
less care and works more rapidly. A single rotary microtome will be found
sufficient for a class of from twenty to thirty students in embryology.

MICROTOMES. 391

The microtome is an instrument of precision, which implies that it must be
treated with extreme delicacy and kept most scrupulously clean. It will be
found usually, when complaint is made against the microtome, that the complaint
is misdirected, and ought to be not against the machine, but against the owner.
The modern microtome necessarily has several adjustments, every one of which
must be exact and secure. If any one of them is imperfect or insecure, if any
of the movable parts is allowed to become corroded, or gummed up with oil, or
loose, or clogged with dust or dirt of any kind, the microtome will not and cannot
work as an instrument of precision.

The knife used for cutting ought to be regarded as an integral part of the
microtome and as its most delicate and easily injured part. A perfect knife-edge
is the greatest treasure of the microtomist. To sharpen the knife satisfactorily
for fine section cutting is a really serious difficulty. A skillful person, however,
may get a good edge by using the very finest grade of oil-stone. No oil should
be used, but instead a mixture of equal parts of glycerin and water. Before the
knife is honed it must be made as clean as possible. The oil-stone itself also
must be cleaned with equal care, and the mixture of glycerin and water should,
if necessary, be filtered before using to keep it free from dirt. A single particle
of dirt may be the cause of making many microscopic notches in the edge of
a knife. A knife is well sharpened when its edge appears smooth and straight
under a magnifying power of twenty-five diameters. The microtome knife should
be as unlike a razor as possible. It must have a very thick back and be as
heavy and rigid as practicable, so that the actual cutting-edge may be as steady
and inflexible as it can be made. Knives of suitably heavy construction are now
furnished with all the best microtomes.

INDEX.

Abdominal cavity, origin, 87

Abozzo, 9

After-birth, denned, 360

Alcohol, 380

Allantois, chick, 214

general account, no

human, 136, 142, 374

in umbilical cord, 374

origin, 83

in primates, 123

pig, 9.0 mm., 253

17.0 mm., 310, 311

and chorion in ungulates, 113
Alum cochineal, 381 .

hematoxylin, 382
Amiurus, 51

Amnio-cardiac vesicles, chick, 180, 186
Amnion, general account, 117

human, at two months, 370
at three months, 344
after fifth month, 371
at seven months, 345
structure, 370

origin, 83

in primates, 122

raphe, chick, 203, 210
Amniota, 7
Anal plate, 58, 59

origin of, 54
Anamniota, 7
Angioblast, chick, 187

development, 91

early history, 80

first appearance, 66
Anlage, defined,- 9
Annelids, 9
Anterior cavity, 88
Anura, 8

Anus, origin of, 54
Aorta, chick, 191, 192, 200, 202, 205

Aorta, descending, pig, 6.0 mm., 247

12 . o mm., 274, 281
dorsal, pig, 9.0 mm., 255

12. o mm., 282, 283, 285
17.0 mm., 305
first appearance, 93
Aortic arches, chick, 184, 202
general account, 100
human, 142, 144, 145, 146
pig, 6. o mm., 247

12. o mm., 278, 293
Zimmermann's, 100
Aortic system, general account, 99
Appendages, embryonic, absence in lower verte-
brates, 82

Arachnoid, pig, 9.0 mm., 254
12 .o mm., 266
17.0 mm., 305
20. o mm., 325
Archenteron, general account, 57

origin of, 54
Area, germinal, 96

opaca, defined, 64

first appearance, 97
pellucida, defined, 64

first appearance, 97
vasculosa, 50, 66, 91

first appearance, 97
of rabbit, 98
vessels of, 97
vitellina, 65
Arteries, basilaris, pig, 9.0 mm., 258

12 .o mm., 264, 293
carotid, pig, 12 mm., 235, 271, 274

loop, pig, 264
central of retina, 333
intersegmental, pig, 6.0 mm., 249
9.0 mm., 250
12 . o mm., 301
lingual, 329

393

INDEX.

Arteries, posterior communicating branch, 265
pulmonary, pig, 12.0 mm., 281

20. o mm., 317
sulci, 312, 320

umbilical, pig, 17.0 mm., 310, 311
vertebral, pig, 12.0 mm., 273

24 . o mm., 336
vitelline, 311

chick, 211

Ascending trigeminal tract, 263
Astral figures in ovum, 40
Atriozoa, 9
Auricle, pig, 12.0 mm., 282

Bacilliform bodies, 168
Beale's carmine, 384
Biogenetic law, 3 1
Bladder, origin, 114
Blastodermic vesicle, 45

formation of, 44

in primatesl 122

in monkey, second stage, 128

in rabbit, 166

study of, 167-173
Blastopore, 53

division of, 54
Blood, degeneration in chorion, 357

origin, 90

pig, 12 . o mm., 266
20 . o mm., 324
Blood-corpuscles, 93

red, general account, 93
ichthyoid, 94
sauroid, 94
mammalian, 94
Blood-islands, 91

chick, 179
Blood-plates, 94
Blood-vessels, definition, 90

development in chick, 90
in mammals, 92

first appearance, 66

growth into embryo, 93

origin, 90

primitive course, man, 142
Body-cavity of vertebrates, 5
Body-stalk, defined, 1 1 1

origin in primates, 123

vessels of, 112

and umbilical cord, 1 1 5
Borax carmine, 382
Bern's method, 387
Brain, 73

Branchial arches, defined, 62
pig, 6.0 mm., 246, 249
7 . 8 mm., 222
9 . o mm., 258
10. o mm., 224
12 . o mm., 274

Canal, auricular, 283

digestive, 59

hyaloid, 333

neurenteric, origin of, 54, 68

notochordal, 53

of Schlemm, 331
Capsule, periotic, 78
Carnivora, 8
Carotids, origin, 100
Cartilage, first appearance, 304.

Meckel's, 329
Cavity, abdominal, origin, 87

anterior, 88

head, 87

hyoid, 87

mandibular, 87

pericardial, chick, 181, 209
origin, 87

pleural, 87

premandibular, 87
Cells, death of, 15'

germ, 25

lutein, 35

removal of, 16

somatic, 28
Cephalochorda, 9
Cerebellum, pig, 12.0 mm., 292
Cervical sinus, human, 147—149
Cheiroptera, 8
Chick embryo, method of obtaining, i 74

preservation, 176

study of, 174-218

with eight segments, 176

comparison with rabbit, 179
longitudinal section, 180
transverse sections, 181

with twenty-four segments, 197

with twenty-eight segments, differentiation

in, 216

studied in sections, 199-216
Chondrostyle, 56

pig, 24.0 mm., 336
Chorda dorsalis. See Notochord.
Chorion, chick, 203, 211

defined, 64

frondosum, defined, 127

INDEX.

395

Chorion, general account, 1 1 7

human, at three months, 344

degeneration of blood in, 3^7
ectoderm of, 364
histology of, 363
laeve, second stage, 350
mesoderm of, 356, 364
placental, 354
trophoderm of, 365
vijli, 367

laeve, denned, 127

origin, 83

relation to uterus in ungulates, 113
Chromosome, accessory, 28

number in segmentation nucleus, 41
Cinerea, 263, 270

defined, 74
Cisterna chyli, 105
Cochlea, origin, 78
Coelom, defined, 18

double, 8 1

embryonic, 84

of the head, 87

origin, 81

pig, 9.0 mm., 255
12 .o mm., 301

umbilical, 311

ventral, 87

vertebrates, 5
Commissure, ganglionic, 262

posterior, 335

superior, 335
Copper hematoxylin, 384
Cord, umbilical, pig, 17.0 mm., 310

sexual, 322
Corona radiata, 34

Corpora quadrigemina, pig, 12.0 mm., 292,
Corpus luteum, 35

striatum,' 329
Costal processes, 305, 321
Counterstains, 381
Cutis, anlage, 266

pig, 20. o' mm., 321
Cutis-plate, 86
Cytomorphosis, 1 1

Decidua caduca, defined, 124
cavernous layer of, 351
compact layer of, 351
defined, 124

reflexa, at three months, 343
at four months, 344
•atrophy, 127

Decidua reflexa, defined, 124
disappearance, 343
first stage, 349
serotina, at seven months, 345, 357

defined, 124
subchorialis, 359
vera, at three months, 344
defined, 124
first stage, 346
second stage, 350
Decidual cells, development, 348

mature, 359
Deck-plate, 72

pig, 9.0 mm., 257
Degeneration, hypertrophic", 15
Dermatome, 86
Development, arrest of, 91
embryonic, 16
larval, 16
summary of, 10

Diaphragm, pig, 24.0 mm., 337
Diencephalon, origin, 74
Differentiation, 13

two types of, 14
Digestive canal, general account, 59

of vertebrates, 5
Dipnoi, 8

Discus proligerus, 34
Diverticulum, Meckel's, 57
Dorsal furrow, 68

roots, 270, 283
Ducts, Miillerian, no, 317
urogenital, no
Wolffian, no
chick, 212
pig, 12 . o mm., 287
17.0 mm., 309
20. o mm., 318
Ductus arteriosus, 91
defined, 101
Cuvieri, defined, 103
endolymphaticus, 299

origin, 78
thoracicus, 105

venosus Arantii, pig, 9.0 mm., 252
Dyads, 37

Ebauche, 9
Ectoderm, chick, 217

defined, 18
Ectoglia, 263, 314

defined, 74
Elasmobranchs, 8

INDEX.

Embryo, Amiurus, 51

appendages of, 49

arising from embryonic shield, 50

dissection, 377

growth of, 49

human, 118

imbedding, 380

measuring, 377

preservation, 377

separation from yolk, 49
Embryonic shield, 44, 47

rabbit, 170—173
Entoderm, chick, 217

denned, 18

earliest growth, mammals, 46

origin of permanent, 54
Eosin, 383

Ependyma, defined, 74
Ependymal layer, primitive, 263
Epidermis, pig, 12.0 mm., 266

17.0 mm., 303
Epiglottis, 337

pig, 12.0 mm., 293
Epiphysis, 335
Epithelial bodies, 63
Epitrichium, 303, 320
Eustachian tube, origin, 63
Excretory organs, general account, 108
Eye, general account, 76

pig, 12 .o mm., 271
29. o mm., 329
24.0 mm., 331
Eyelids, 331

Facial motor tract, 298
Falx, pig, 12.0 mm., 275

20. o mm., 327
Fertilization of the ovum, 38
Fibrin, in chorion, 355

in decidua reflexa, 349
Filum terminale, 75
Fin compared with limb, 257
Floor-plate, 72
Fluid, Bouin's, 379

Flemming's, 379

Hermann's, 380

Miiller's, 379

Parker's, 379

Tellyesnicky's, 378

Zenker's, 378
Foramen epiploicum, 290

of Monro, 329
defined, 74

Fore-brain, chick, 180, 201

differentiation, 74

origin, 72

pig, 9.0 mm., 253

12 . o mm., 271, 292
Fore-gut, chick, 180, 181, 187

general account, 60

origin, 57
Formalin, 378
Fovea cardiaca, chick, 190

origin, 57
Furrow, dorsal, 68

Gall-bladder, pig, 14.0 mm., 288
Ganglia, auditory, division of, 79
origin, 72
pig, 9. o mm., 250

12. o mm., 261, 283, 301
17.0 mm., 205
20. o mm., 214

Ganglion, acustico-facial, 293, 295, 298
jugular, 293, 296
nodosum, 277, 293
petrosal, 293

trigeminal, 262, 293, 295, 297
Ganglionic crest, 72

chick, 180, 185, 188, 191
Ganoids, 7

Genetic restriction, 14
Germ-cells, general account, 25

of Acanthias, 26

Germ-layers, -general account, 18
specific quality, 19
tissues from, 19
Germinal area, 96

wall, 65

Gibbon, ovum in third stage, 131
Gill-clefts, 62

chick, first, 201
second, 202
third, 205
human, 140—147
pig, 6.0 mm., 248

9.0 mm., 257, 259
12.0 mm., 271, 273, 275, 277
Gill-pouches, origin, 62
Glands, classification, 23
general account, 21
genital, pig, 12.0 mm., 287
17.0 mm., 308
20. o mm., 322
Globules, polar, 37, 161, 162
Gray layer, 263, 270
defined, 74

INDEX.

397

Groove, primitive, 48
Growth, law of unequal, 24
of embryo, 49

Head-bend, 223
Head-process, 53

Heart, chick, 186, 187, 188, 203, 205, 206
general account, 189

origin, 96.

pig, 12 . o mm., 282
Heidenhain's hematoxylin, 382
Hemispheres, cerebral, pig, 12.0 mm., 275
20. o mm., 325, 329

origin, 74

Hensen's knot, 48, 170
Heredity, theory of, 28
Hermaphrodites, 27

pseudo-, 32
Hind-brain, chick, 180, 190, 191, 203

differentiation, 74

origin, 72

pig, 9.0 mm., 258

12. o mm., 263, 292, 296
Hind-gut, general account, 61

origin, 57

Hormone theory of sex, 2 7
Human embryo, 118

age, calculation of, 118

classification of stages, 119

Coste, 138

Dandy, 58, 137

Eternod, 136

Frassi, 119

His, E, 136

Lg, 141

Sch, 141

SR, 136

2.15 mm., 141

2 . 6 mm., 143

3.2 mm., 145

4 . o mm., 147

4 . 2 mm., 144
Kollmann, 137
Mall, 147
Peters, 128
Spec, i. 54. mm., 135
stages, classification of, 119

first, 119

second, 119, 128

third, 119

fourth, 119, 134

fifth, 120, 136

sixth, 1 20, 137

Human embryo, stages, seventh, 121, 140

eighth, 121, 141

ninth, 121, 143

tenth, 121, 146

eleventh, 121, 147

four weeks to four months, 148-159

relations to uterus, 124
Hyoid cavity, 87
Hypophysis, pig, 12.0 mm., 292

24.0 mm., 335
vertebrates, 5

Ichthyopsida, 7
Impregnation of the ovum, 38
Infundibular gland, pig, 12.0 mm., 268, 292

24.0 mm., 335

Inner man in segmentation, 44, 47
Insectivora, 8
Intermediate cell-mass, 85
Intervertebral discs, pig, 12.0 mm., 301

20. o mm., 314
Intestine, caudal, 213

open in chick, 191, 210, 212

pig, 9.0 mm., 252, 255, 257

17.0 mm., 308, 311
Iris, 331

Iron hematoxylin, 382
Isthmus of brain, origin, 74

pig, 12.0 mm., 292
Iter, defined, 74

Jakobson's organ, 326

Kidney, origin of renal anlage, 309

pig, 17.0 mm., 308

20. o mm., 322

Knot, Hensen's, 48, 170

primitive, 48
Kopffortsatz, 53

Lachrymal groove, 222, 224
Lamina terminalis, 335
Larynx, anlage of, 276, 293
Lateral roots, 270, 297
Layer, subzonal, 44
Lens of eye, chick, 201

origin, 78

pig, 12.0 mm., 271
20. o mm., 329
24.0 mm., 333
Lesser peritoneal space, 290, 301

-

398

Leucocytes, general account, 96
Limbs, pig, 9.0 mm., 257
f 2 .o mm., 279
20. o mm., 318
vertebrates, 4
Liver, chick, 207

general account, 107
pig, 9. o mm., 252
12 . o mm., 288
20. o mm., 324
24.0 mm., 337
vertebrates, 5
Lungs, pig, 9.0 mm., 252
12 .o mm., 285
17.0 mm., 306
20. o mm., 317, 322
Lutein, 35
Lymph-glands, 105
Lymph-sacs, 105
Lymphatic spaces, pig, 20.0 mm., 315

system, 105
Lyons blue, 383

Mblleus, 330
Mallory's stain, 385
Mammary anlage, 320

bodies, 292

Mandibular cavity, 87
Marsipobranchs, 7
Marsupials, 8
Mass and surface, 20
Maxillary process, 224

pig, 20. o mm., 325
Maxillo-turbinal fold, '311, 326
Meatus, external auditory, 271
Meckel's cartilage, 329

diverticulum, 57
Mediastinum, 317
Medulla oblongata, pig, 12.0 mm., 296

20. o mm., 336
Medullary canal, differentiation, 7 r

origin, 69

stratification, 74

structure, 69
groove, chick, 178, 180, 194

human, 136

origin, 68
plate, human, 134

origin, 67

Membrana serosa, 64
Menstruation, 339
Mesectoderm, 185
Mesencephalon, chick, 180

INDEX.

Mesenchyma, denned, 18

histogenesis, 89

pig, 12 . o mm., 266
17.0 mm., 303
Mesoderm, chick, 218

defined, 18

early history, 79

origin, 51

somatic, defined, 81

splanchnic, 192
defined, 81

Mesonephros, defined, 109
Mesothelium, origin, 81
Metamerism, 2
Metanephros, origin, no
Metencephalon, origin, 74
Microtomes, 389

knives for, 391
Mid-brain, chick, r8o, 184

differentiation, 74

origin, 72

pig, 9.0 mm., 253

12 . o mm., 292
Milk-line, 225, 226
Monkey, ovum in second stage, 127
Monotremes, 8
Mouth of vertebrates, 5
Mullerian ducts, pig, 20.0 mm., 317
Muscle-plate, 86

pig, 9.0 mm., 254
Muscles, hyoglossal, 329

of eye, 329

origin, 89

Myelencephalon, origin, 74
Myotomes, pig, 12.0 rnm., 273

Nasal pits, 76

pig, 12 . o mm., 277
Naso-turbinal fold, 326
Neck-bend, 223, 292
Necrobiosis, 15
Nephrotome, chick, 193, 212

differentiation, 86

origin, 85
Nerve fibers, pig, 12.0 mm., 263

roots, origin of, 270, 280
Nerves, acoustic, 295, 298 ^r

cervical, 273, 275, 278

facial, pig, 12.0 mm., 271, 295, 298

fourth, pig, 12.0 mm., 264, 268

glosso-pharyngeal, 274, 295

hypoglossal, 277, 296

inferior maxillary, 274

INDEX.

399

Nerves, olfactory, 76, 327

optic, 77, 295

origin, 72

spinal, 280

spinal accessory, 268, 277, 296

superior maxillary, 326

third, pig, 12.0 mm., 264, 268

trigeminal, 295, 297

vagus, 285, 29^6, 317
Nervous system, origin, 67

vertebrates, 4
Neuraxons, 270
Neuroblasts, 72, 73, 270
Neuroglia layer, outer, 263
Neuromeres, pig, 6.0 mm., 246

12 .o mm., 264
Neuropore, anterior, 69, 178
Nodulus thymicus, 27<57~295

origin, 63
Notochord, anlage 'of, 55

growth, 55

pig, 12 .o mm., 273 .
17.0 mm., 305
20 .o mm., 315
24 .o mm., 336

relation to axial mesoderm, 56

ultimate fate, 55

vertebrates, 4
Notochordal canal, 53
Nuclei pulpqsi, 56, 337

(Esophagus, origin, 57

pig, 12. o mm., 280, 285

17.0 mm., 305, 306

20.0 mm., 314, 317

Olfactory nerves, origin, 76

plate, pig, 6.0 mm., 24-9

12 . o mm., 277

Omentum, pig, 12.0 mm., 288, 290
Operculum, pig, 259
Optic chiasma, 292, 335

vesicles, chick, 133, 180, 183, 202

differentiation, 77
Oral plate, 58

chick, 184
human, 142
Orange G, 383
Organsf*constitution of, 20
Otocyst, chick, 199, 200
general account, 78

pig, 12.0 mm., 261, 263, 267, 293, 299
Ova, primitive, 26
Ovulation, 35
mouse, 161

J

Ovum, before maturation, 34
constitution, 12
fertilization, mouse, 163
gibbon, third stage, 131
holoblastic, 10
human, 34

second stage, 128

Peters' s, 128
impregnation, 38
isotropism of, 1 2
maturation, 36
meroblastic, 10
monkey, second stage, 127
mosaic theory of, 12
segmentation, 42

mammals, 160

mouse, 160, 185

Palate, 311

cleft, 91, 312
Panchoroid, 266, 321
Pancreas, general account, 107
pig, 12.0 mm., 290
24.0 mm., 337
Pangenesis, 29
Parablast, 64

Parathyroid glands, 63, 315
Penis, pig, 20.0 mm., 320
Pericardial cavity, chick, 181, 209

origin, 87

Pericardial epithelium, pig, 12.0 mm., 282
Perichondrium, origin of, 304
Peritoneal membrane, pig, 12.0 mm., 286

20. o mm., 322

Peritoneum, pig, 20.0 mm., 322
Pharynx, chick, 184, 205
general account, 61
origin, 57

pig, 12.0 mm., 237, 238, 269, 274
vertebrates, 3

Pia mater, pig, 9.0 mm., 2^4
12.0 mm., 266
17.0 mm., 305
20. o mm., 325

Pig embryo, anatomy, general, 228
7.8 mm. stage, 228
12.0 mm. stage, 231
form, external, 22 1
7 . 5 mm., 221
10 . o mm., 223
15.0 mm., 225
20. o mm., 226
methods of obtaining, 219

400

INDEX.

Pig embryo, sections, diagram of, 260
selection of stages, 221
serial sections, 220
studied in sections, 6.0 mm., 246
9.0 mm., 250
12.0 mm., 259
17.0 mm.i 303
20. o mm., 311
24 . o mm., 330
viscera dissected, 231
Pituitary body, 268, 292, 336
Placenta, allantoic, defined, 113
at seven months, 345, 352
chorionic, defined, 113
cotyledons of, 362
general description, 359
in situ, 352

intervillous spaces, 363
vessels of, 360
Placentalia, 8
Plakodes, chick, 217

general account, 76
Plate, closing of. gill-cleft, 271
Pleural cavity, 322 *

origin, 87

Pleuro-peritoneal space, 87
Plexus, brachial, 280, ,283
lateral choroid, 328
lumbar, 318
Polar globules, 37
mouse, first, 161
second, 162

Post-branchial bodies, 63
Premandibular cavity, 87
Primates, 8
Primitive axis, 52

groove, chick, 178, 197
streak, 50

Pro-amnion, defined, 80
Prochorion, 45
Pronephros, defined, 108
Pronuclei, fusion of, 41

mouse, 164
Pronucleus, female, 37

mouse, 163
male, 39

mouse, 163

Prosencephalon, chick, 180.
Proto vertebrae, defined, 84
Pupil of eye, 331

Rabbit embryo with eight segments, 179
Recapitulation, law of, 29

Reconstructions, by drawings, 385

by wax plates, 387
Reduction division, 37
Regression, 15

Restriction, law of genetic, 14
Rhombencephalon, chick, 180
Rodents, 8

Sauropsida, 7

Sclerotome, pig, 6.0 mm., 250

Sections, orienting, 388

paraffin, mounting of, 381

staining, 381
Segmental vesicle, 86

zone, 178
Segmentation nucleus, 41

of the ovum, 42
in Limax, 43

spindle, 42

Segmented animals, 2
Segments, chick, third, 192

general morphology, 2

occipital, 1 80

primitive, 84
Sella turcica, 336
Sense-organs, 5
Septum, nasal, 312, 326

transversum, chick, 190, 209

relation to ccelom, 87
Sex, 27

cause of, 28

cells, 26
Sexual characteristics, secondary, 27

cords, 322

Shield, embryonic, 44, 47
Sinus, cavernous, 268

cervicalis, 222, 223
human, 147—149
pig, 6. o mm., 249
12 . o mm., 275

lateral, 265

rhomboidal, 69

chick, 178, 1 80

superior longitudinal, 265, 275

terminalis, of chick, 91, 97

venosus, origin, 98
Sinusoids, of heart, 283

of liver, 252, 289, 324

of suprarenals, 338

of Wolffian body, 256, 287, 306, 307

origin in liver, 209
Skull, anlage of, 325
Somatic cavity. See Splanchnocele.

INDEX.

401

Somatopleure, 7, 82
chick,. 187, 211
pig, 12. o mm., 282, 286

20.0 mm., 318
Somites, chick, 192, 207, 212
defined, 84
differentiation, 85
general morphology, 2
origin, 84
secondary, 86.

pig, 6.0 mm., 246, 250
Spermatozoon, 33

entrance into ovum, 38

mouse, 163
Spinal cord, 73

chick, 205
differentiation, 75
pig, 6. o mm.. 246
12 .o mm., 269
17.0 mm., 305
20 . o mm., 312
Splanchnocele, 84, 87

pig, 12.0 mm., 287
Splanchnopleure, 7, 82
chick, 187, 203, 211
pig, 12.0 mm., 288
Spongioblasts, 73
Staining, methods of, 381
Stigma of Graafian follicle, 35
Stomach, origin, 57

pig, 12 .o mm., 288
Stomodaetini, 184
Streak, primitive, 48

rabbit, 170—173
Striae acusticae, 268
Subzonal layer, 44

Suprarenal capsule, pig, 24.0 mm., 337
Surface and mass, 20
Sympathetic system, cervical, 279, 283
pig, 17.0 mm., 305, 308
20 . o mm., 314

Telencephalon, origin, 74

Teleosts, 8

Testis, 322

Tetrads, 36

Thyroid gland, origin, 63

pig, 12.0 mm., 277
20. o mm., 315
Tissue, classification of, 19
Tongue, pig, 12.0 mm., 293
20. o mm., 311, 327
Tonsil, origin, 63
26

Trachea, pig, 12.0 mm., 2 76, 2 79, 280

20. o mm., 314, 317
Trigeminal tract, 263, 297
Trophoblast, (footnote) 44
Trophoderm, degeneration, 366

early stage described, 364

general account, 114

origin, 47
Tubal band, 318
Tuber cinereum, 292
Tunica albuginea, 322

vasculosa lentis, 331, 333
Tunicata, 9

Umbilical cord, amnion from, 115
general account, 115
human, at seven months, 345
ectoderm of, 375
mesoderm of, 374
study of, 372

opening, pig, 9.0 mm., 253
Umbilicus, pig, 9.0 mm., 254
Unguiculates, 8
Ungulates, 8
Urodela, 8
Urogenital ducts, 5

general account, no
ridge, of vertebrates, 5
Uterus, general histology, 339

human, pregnant, two stages of, 124
menstrual changes, 339
pregnant, two stages of, 341
at three months, 343
at seven months, 345
Uvea, 332

Valves, Eustachian, 282, 301
sinistral, 301
Thebesian, 282
Veins, anterior cardinal, origin, 102

pig, 12.0 mm., 268
cardinal, chick, 200, 207, 210

pig, 12.0 mm., 264, 268, 28;
17.0 mm., 306
20. o mm., 317
common, origin of, 98, 103

pig, 12 .o mm., 282
primitive arrangement, 98
iliac, 320

inferior cava, origin, 104, 257
jugular, 279
maxillary, 293

402 INDEX.

Veins, jugular, pig, 12.0 mm., 268 Vesicles, ammo-cardiac, 87

20. o mm., 314 chick, 180, 186

lateral cardinal, 265 cerebral, origin, 71

of the head, 268 optic, origin, 71

lingual, 279 segmental, 86
omphalo-mesaraic, chick, 178, 180, 190, Villi, branching, 367

191, 208, 210 histology of, 356

origin, 93 of allantois, 253, 324

primitive arrangement of, 98, 103 of chorion, 367

ophthalmic, 271 shape of, 367

peripheral (of limb), 320 vessels of, 370
portal, pig, 9.0 mm., 252

12.0 mm., 290 Weigert's hematoxylin, 384

posterior cardinal, origin, 103 Weismannism, 29

pulmonary, 285 Wolffian body, general account, 109

subcardinal, 256 pjg> g 0 mm _ 252) 256

superior longitudinal sinus, 265 120 mm 287

mesenteric, 311 x 7 0 mm | 3o6

umbilical, origin, 104 2O o mm _ 322

pig, 9.0 mm., 257 duct, chick, 212

12.0 mm., 289 tubules, origin, 86
17.0 mm., 311
vitelline, pig, 9.0 mm., 253

1 7 . o mm., 3 1 1 Yolk- Absorption of, 65

Velum transversum, origin, 74 D 'cavity> 53

Vena capitis lateralis, 268, 299 Yolk-sac, angioblast of, 66

cava inferior, 300 entoderm of, 64

development, 257 formed by splanchnopleure, 82

hepatica communis, pig, 9.0 mm., 252 Seneral morphology, 63

Venous system, 102 human' 66

Ventral roots, 270, 283 structure, 375

Ventricle, fourth, pig, 6.0 mm., 246 m umblhcal cord- 374

12.0 mm., 293 mesoderm of, 66
lateral, pig, 12.0 mm., 275

20. o mm., 327 . Zimmermann's arch, 101

of heart, pig, 12.0 mm., 282 Zona pellucida, 34

Vertebrae, pig, 12.0 mm., 303 radiata, 34

17.0 mm., 304 Zones, dorsal, 72

20. o mm., 314, 315 of His, 72

24.0 mm., 337 parietal, defined, 84

Vertebrate type, 2 . segmental, 178, 193, 213

fundamental characteristics of, 3 defined, 84

modifications of, 7 ventral, 72

• 4

THIS BOOK IS DUE ON THE LAST DATE
STAMPED BELOW

AN INITIAL FINE OF 25 CENTS

WILL BE ASSESSED FOR FAILURE TO RETURN
THIS BOOK ON THE DATE DUE. THE PENALTY
WILL INCREASE TO SO CENTS ON THE FOURTH
DAY AND TO $1.OO ON THE SEVENTH DAY
OVERDUE.

QCT 81947

> 1947

0 1991

219SS

SEP 2 9 1955

MQV 2 1 1362

Mo? 9 "=;;••••• -

-rtrrc

LD 21-5rn-l,'39(7053s7)

U.C. BERKELEY LIBRARIES

THE UNIVERSITY OF CALIFORNIA LIBRARY