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UNIVERSITY OF CALIFORNIA
DAVIS

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QUAIN'S ANATOMY

EMBEYOLOGY

QUAIN'S

ELEMENTS OF ANATOMY

EDITORS

EDWARD ALBERT SCHAFER, LL.D, Sc.D., F.R.S.

PROFESSOR OF PHYSIOLOGY AND HISTOLOGY IN THE UNIVERSITY OF EDINBURGH

JOHNSON SYMINGTON, M.D., F.R.S.

PROFESSOR OF ANATOMY IN QUEEN'S COLLEGE, BELFAST

THOMAS HASTIE BRYCE, M.A., M.D.

LECTURER IN ANATOMY, UNIVERSITY OF GLASGOW

IN FOUE VOLUMES

VOL. I.

EMBRYOLOGY
BY T. H. BRYCE

ILLUSTRATED BY MORE THAN 300 ENGRAVINGS
MANY OF WHICH ARE COLOURED

ELEVENTH EDITION

LONGMANS, GREEN, AND CO.

39 PATERNOSTER ROW, LONDON

NEW YORK, BOMBAY, AND CALCUTTA

1908

All rights reserved.

PREFACE

THE present edition of ' Quain's Anatomy ' will appear in four volumes, of which
this one, containing the Embryology, is the first. The remaining volumes will
comprise respectively General and Visceral Anatomy ; the Nervous System and
Sense Organs ; and the Bones, Ligaments, Muscles, and Blood- vessels. Each volume
will be complete in itself and will serve as a text-book for the particular subject
or subjects with which it deals. Thus the first volume is intended to form a com-
plete text-book of Human Embryology, the second a text-book of Histology and
Visceral Anatomy, the third a text- book of Neurology, while the fourth will deal
with the systems which are not included in the second and third volumes.

The work has been completely re- edited and brought up to date. The
volume on Embryology, which was written for the previous edition by Professor
Schafer, has in the present edition been entrusted to Dr. Bryce, who has
re-' written it and has added a large number of illustrations. The advances in our
knowledge of human embryology have necessitated considerable additions to the
text, and this, combined with the profuseness of illustration, has resulted in some
increase in bulk of the volume ; but, in spite of the wealth of detail with which
the subject is treated, this is by no means excessive. There has been some saving
of space by the omission of the bibliographical list which in the former edition
followed each subject. These lists, although at the time useful, are no longer
necessary, since they are to be found in a very complete form in the compendious
' Handbuch der Entwickelungslehre,' by numerous authors, which has lately
been published under the editorship of Professor 0. Hertwig, as well as in
the earlier ' Bibliography of Vertebrate Embryology,' by Professor Minot.

In the matter of illustrations the volume* is indebted to the skill of Mr. A. Kirk-
patrick MaxweU for the original drawings of figs. 116, 118, 119, 121, 122, 129,
130, and 136, as well as for some adaptations of figures from other authors. For
the photographic illustrations of his preparations which are reproduced in
figs. 74, 156, 157, 162, 163, 164, 176, 220, and 246, Dr. Bryce desires to signify his
obligation to Dr. J. H. Teacher. All other original drawings and diagrams, both in
black-and-white and in colour, and most of the adaptations, are from his own pencil.
In addition to the original illustrations, Mr. Gustav Fischer of Jena has supplied
cliches of a number of valuable figures, many of them from the well-known
text-book of Professor Kollmann. Dr. Bryce further begs to acknowledge the
obligation he is under to Professor J. Graham Kerr for reading the proof-sheets of
Section I. The Index to this volume has been prepared by Miss Agnes Picken,
M.A., M.B.Ch.B., Demonstrator of Anatomy in the Department for Women,
University of Glasgow.

CONTENTS

SECTION I

GENERAL EMBRYOLOGY

THE ANIMAL CELL 1

Cytoplasm 1

Centrosome 2

Nucleus 3

Cell -division 4

HISTORY or THE SEX CELLS ... 5

Structure of the Spermatozoon . . 5

Spermatogenesis 6

Structure of Oocyte .... 7

Oogenesis : Formation of Polar Bodies 10

Fertilisation . . . ... 14

Reduction of Chromatin and Nuclear

Changes during Maturation . . 17

Significance of Nuclear Phenomena . 20
Chromosome Theory of Development :

Mendel's Law of Heredity . . . 23

SEGMENTATION OF THE OVUM . . 25

FORMATION OF THE GERMINAL LAYERS 27

of the Entoderm 27

of the Embryonic Ectoderm and

Amnion 29

Entypy of the Germinal Area . . . 30
Formation of the Mesoderm and

Embryonic Axis 32

The Gastrula Theory . . . . 42

PAGE

EARLY CHANGES IN THE BLASTODERM . 48

Neural Canal 48

Notochord. 49

Primitive Segments .... 49

Cleavage of Mesoderm . . . . 51

Separation of Embryo .... 52

Allantois 55

EARLY STAGES IN THE DEVELOPMENT
OF THE MUSCLES, CONNECTIVE

TISSUES, AND BLOOD-VESSELS . 56

The Mesenchyme 58

Blood and "Blood-vessels of Yolk-
sac 59

Early Stages in the Development of
the Heart and Embryonic Blood-
vessels 61

DEVELOPMENT OF FCKTAL MEMBRANES

AND PLACENTA .... 65
Imbedding of Ovum . . . . 65
Changes in the Uterus during Preg-
nancy 67

Fo3tal Membranes 70

Development of Placenta ... 72

The Shed Placenta 77

GENERAL HISTORY OF DEVELOPMENT 80, 92

SECTION II

DEVELOPMENT OF THE ORGANS

PACE

Classification 93

DEVELOPMENT OF THE SKIN AND

CUTANEOUS GLANDS . . . . 93

DEVELOPMENT OF THE NERVOUS SYSTEM 94

Histogenesis of Nerve-tissue . . 94

Origin of Nerve -roots and Peripheral

Nerves 98

MORPHOGENESIS OF SPINAL CORD AND
BRAIN :

Spinal Cord 101

Brain 105

Rhombencephalon (Rhombic Brain) 106

Cerebellum 109

Mesencephalon (Mid-brain) . . . Ill

Prosencephalon (Fore-brain) . . Ill

Cerebral Hemispheres . . . . 115

MORPHOGENESIS OF BRAIN — continued.

Corpus Striatum 119

Hippocampal Formation and Com-
missures 121

Formation of the Fissures . . . 124
DEVELOPMENT OF THE PERIPHERAL

NERVES 125

Spinal Nerves 125

Cerebral Nerves 127

Development of Individual Cerebral

Nerves 130

The Sympathetic 133

DEVELOPMENT OF THE EYE . . . 136

Lens 137

Retina 140

Optic Stalk and Nerve . . . .141

Vitreous Body and Lens Capsule . 141

Vlll

CONTENTS

DEVELOPMENT OF THE EYE — continued.
Protective and Vascular Coats, Iris

and Aqueous Chamber . . .143
Accessory Structures, Eyelids
Lacrymal Glands and Ducts . .144

DEVELOPMENT OF THE EAR . . . 145

Labyrinth 145

Auditory Nerve 148

Accessory Parts of the Organ of Hear-
ing, Middle Ear and External Audi-
tory Meatus 149

DEVELOPMENT OF THE NOSE . . . 151

Palate 154

Olfactory Nerve 155

DEVELOPMENT OF THE ALIMENTARY

CANAL 156

The Mouth 156

Pharynx 158

Tongue 159

(Esophagus, Stomach, and Intestine . 160

Caecum 164

Entodermic Cloaca and Anus . . . 164

DEVELOPMENT OF THE GLANDS OF THE
ALIMENTARY CANAL :

Salivary Glands 165

Lungs, Trachea, and Larynx . . . 166

Thyroid Gland 168

Thymus Gland 169

Parathyroid Glands . . . .172

Liver 173

Pancreas 175

DEVELOPMENT OF UROGENITAL SYSTEM :

Pronephros and Wolffian Duct . . 177

Mesonephros (Wolffian Body) . . 178

Metanephros (Permanent Kidney) . 182
Urinary Bladder . . . .185

Genital Glands 186

Ovary 189

Testicle 191

Fate of Wolffian Ducts . . .192

Miillerian Ducts 193

Prostate Gland 194

PAGE

Descent of Testicle 197

Fate of Entodermic Cloaca . . .201

External Organs, Perineum, and Anus 202

DEVELOPMENT OF SUPRARENAL BODIES 205
DEVELOPMENT OF THE VASCULAR

SYSTEM :

Outward Form of the Heart . . 206
Chambers of Heart and Formation of

Septa 211

The Arteries :

Dorsal Aorta and Aortic Arches . 217

Carotid System 221

Segmental Arteries . . . . 223
Vitelline Arteries . . . .223

Arteries of Limbs 224

The Veins :

Veins of the Liver .... 224

Cardinal Veins 226

Inferior Vena Cava .... 227

Anterior Cardinal (Jugular) Vein . 229

Superior Vena Cava . . . . 232

Veins of Limbs 232

Peculiarities of Foetal Circulation . 233

Changes in the Circulation at Birth . 234

DEVELOPMENT OF LYMPHATIC SYSTEM 235

DEVELOPMENT OF THE SPLEEN . . 236

DEVELOPMENT OF THE BODY- CAVITY :

Pericardium 237

Septum Transversum .... 238

Pleural Cavities 239

Closure of Pleuro-peritoneal Openings 241

Diaphragm 241

Mesentery and Epiploic Sac . . . 244

DEVELOPMENT OF THE MUSCLES . . 246
DEVELOPMENT OF THE SKELETON :

Vertebral Column 250

Ribs and Sternum 252

Skeleton of Limbs . . . .254

Chondro cranium 254

Visceral Skeleton 257

Auditory Ossicles 258

EMBBYOLOGY.

SECTION I.

GENEKAL EMBKYOLOGY.

DEVELOPMENT in the human being, as in all the Metazoa, is initiated by the union
of two specialised cells — the germ-cell or ovum, and the sperm-cell or spermato-
zoon. The ovum, after its union with the spermatozoon, may be conceived as
the central point of a developmental cycle. By its continuous division it gives rise
to the multicellular body, or soma. At a certain stage, in some animals at a very
early stage, of development, there is isolated from the mass of somatic elements
a stirp of cells destined for the reproduction of the species. These are located
in the reproductive glands — the testis in the male and the ovary in the female —
and there undergo an elaborate series of changes which result in the production of
the mature sex-cells, by the union of which a new cycle is again initiated.

From this point of view we may divide the history of development into two
sections :

A. The history of the soma.

B. The history of the sex-cells.

In describing the course of development, it is most convenient to begin at a
point in the cycle at which the reproductive stirp is already laid down, and the
phase has been entered on which leads to the specialisation of the sex-cells.

In order to apprehend clearly the nature and significance of the process of
specialisation of the sex-cells, as well as the general processes of histogenesis of the
somatic elements, it is necessary that the reader should have some knowledge of
the structure of the cell, as the primal element out of which the adult organism is
developed, and the morphological unit of all the tissues and organs of which it is
composed.

THE ANIMAL CELL.

The animal cell is a minute body of microscopic dimensions, consisting
of a speck of living substance of semi-fluid consistence and complex chemical
composition, known as protoplasm. It may or may not possess a limiting
membrane differentiated from the surface-layer of the protoplasm, but always
contains a minute vesicular body within it named the nucleus.

Cytoplasm. — The protoplasm of the cell-body is known as the cytoplasm,
to distinguish it from that of the nucleus, which is termed karyoplasm. A
complete and critical account of the physical characters and composition of the
cytoplasm will be found in the volume of this work devoted to general histology ;
here it will be necessary only to refer to some of the more important points. In
the living condition, it has a homogeneous glassy appearance, with or without
imbedded granules, and a semi-fluid consistency. In some cases it shows,
VOL. i. B

i

2 ANIMAL CELL

even under high powers of the microscope, no signs of any finer structure ; in
other cases, especially after fixation, it exhibits a meshwork or reticular
appearance, which has been variously interpreted as indicating a filar, a spongy,
or an alveolar structure, but in view of the effects produced by most fixing
reagents on colloid solutions, such interpretations must be received with caution.

The granules which are generally present in the cytoplasm may be either
essential constituents of the protoplasm, or included non-protoplasmic bodies of
various kinds. The granules of the former kind (cytomicrosomes) are very minute,
and form apparently an important element in the active protoplasm. They have
been called mitochondria by Benda, and have been shown, as we shall see, to play
an important part in the sex-cells. The non-protoplasmic granules include such
as pigment-granules, yolk-grains, and so on ; but larger inclusions, such as fluid
vacuoles, fat-drops, &c., also occur. All these may be collectively designated
by the terms deutoplasm or paraplasm.

While we generally speak of the tissues as being composed of separate cells, we shall have
occasion in the course of this work to refer to instances of tissues in which the so-called cells

FIG. 2. — POLYMORPHONUCLEAB LEUCOCYTE OF
LEPIDOSIBEN, SHOWING LOBED NUCLEUS,

FIG. 1. — LEUCOCYTE (Lepidosiren paradoxa) ATTRACTION - SPHERE, AND SOME GBA-

SHOWING ATTBACTION-SPHEBE. (T. H.Bryce.) NULES. (T. H. Bryce.)

are joined together by protoplasmic strands. The tissue in such cases is really a multinucleated
protoplasmic network. In other instances, owing to the division of the nuclei without cleavage
of the protoplasm, a multinucleated mass or layer of protoplasm is produced. Any such
multinucleated mass is termed a syncytium.

Centrosome s central particle. — In most cells there is generally to be
demonstrated a point in the cytoplasm, as a rule close to the nucleus, where by
suitable stains a single granule, a double granule, or a group of granules, may be
made visible. These are surrounded by a clear structureless area, round which
the protoplasm may be arranged in a radial fashion. When fully developed, as
in wandering leucocytes (figs. 1 and 2), the whole arrangement is named the
attraction sphere. The substance of the sphere is known as the archoplasm or
centroplasm. The central area, containing the granule or granules, has been
named the centrosome by Boveri. When it contains a single granule, the
particle is called the centriole. In cells in which the granule is the only obvious
feature, it may itself be termed the centrosome. A centrosome is absent in plant-
cells, and it has been proved by Morgan and Wilson that focal points, having
all the characters of centrosomes, may be produced in the protoplasm of the
echinoderm-egg by treatment with chemical reagents.

Nucleus. — The nucleus in the vast majority of cells is a spheroidal or slightly
ovoidal body ; but it may be lobed as in leucocytes (figs. 1 and 2). It has a definite

NUCLEUS

membrane, and is made up of two parts of different chemical and physical
characters, a formed substance with great affinity for certain dyes, and hence called
•chromatin, and a structureless more fluid substance, the achromatin or karyoplasm.
The chromatin is generally in the form of a network, with thickenings at the nodal
points ; but often the nodes are the more prominent feature, and the network

1 8

FIGS. 8 TO 8. — KARYOKINESIS IN BED BLOOD-CORPUSCLES OP LARVAL LEPIDOSIREN. (T. H. Bryce.)

forms only a mesh of fine threads between them. The thickenings are named
karyosomes. The chromatin, which generally takes the form of solid filaments, but
frequently also is seen in the form of granules in a non-staining basis (linin), has a
special affinity for basic dyes, and is hence called basichromatin ; but there are also
granules in the filaments composed of a material which stains with acid dyes, hence

B 2

KAKYOKINESIS

called oxychromatin. The karyoplasm occupies the meshes of the reticulum, and
seems to be of the same nature as the fluid part of the cytoplasm. There are
often, but not always, one or more rounded bodies staining somewhat differently
from basichromatin, the true nucleoli or plasmosomes.

CELL-DIVISION. — While in a few cases it is believed that cells divide
directly, by constriction of the nucleus and then of the cell-body (amitosis), the
almost universal rule is that they divide by a complicated mechanism called indirect
division, mitosis, or karyokinesis. The process is an elaborate device for the exact
partition of the chromatin between the daughter-cells.

Karyokinesis (figs. 3 to 11). — The process of
not present the same picture in every detail

doe&

in

10

indirect cell -division
all classes of cells, varia-
tions occurring accord-
ing to the relative size
of nucleus and cell-
body ; but, apart from
i minor variations, there
is one type of mitosis
which, in certain par-
ticulars, differs essenti-
ally from that seen in
ordinary somatic cells.
This, which is known
as the heterotypical
'•I mitosis, is characteristic
of the sex-cells, and will
be dealt with later ; but,
in order that its cha-
racter and significance
may be more readily
understood, a brief de-
scription of ordinary
mitosis, as it occurs in
somatic cells, will be
here given.

The process is ini-
tiated by the division of
the centrosome (fig. 4).
As the two centro-
somes draw apart in a
direction tangential to
the nucleus, protoplas-
mic radiations become

centred on them, and a spindle system of fibres is drawn out between them (fig. 5).
The network of the nucleus meanwhile takes the form of an apparently continuous
skein (fig. 4), which then arranges itself into loops directed towards the
developing spindle system (fig. 5). The loops then break apart at the opposite
pole of the nucleus, to form a series of V-shaped filaments or chromosomes (fig. 6).
The nuclear membrane meanwhile disappears, the spindle system gradually takes
up a position of equilibrium in the centre of the cell, and the chromosomes arrange
themselves round the equator of the spindle with their apices applied to it (fig. 7).
The chromosomes have in the meantime split longitudinally along their whole
length, and now fche two halves become separated from one another, the apices
of the daughter-V's being drawn towards opposite spindle-poles (fig. 8). The

11

FIGS. 9 TO 11.— KARYOKINESIS IN BED BLOOD-CORPUSCLES
OF LARVAL LspiDOSiHEN (continued). (T. H. Bryce.)

STRUCTUKE OF SPERMATOZOON 5

daughter- V's next get free from one another and pass to the apices of the spindle
where they gather in groups round the poles (fig. 9). They then merge together
again to form the reticulum of each resting daughter-nucleus (figs. 10 and 11).

As the final stages of nuclear division are being completed, the cell-body is
constricted round the equator, and the constriction gradually deepens to divide
the cell into two exactly equal parts. The spindle and polar radiations die away,
the last remains of the system appearing as a strand of fibres gathered into the
narrowing bridge between the two cells. This persists for a time as a bond of
union between them even after the cytoplasm has completely divided (fig. 11).

For convenience of description and reference to the different pictures presented by the
nucleus in different stages of karyokinesis, it has become customary to divide the continuous
process into arbitrary phases. The preparatory stages up to the completed spindle are known as
the prophase ; the stage in which the split rods are being resolved (on the equator of the spindle)
into the daughter- chromosomes as the metaphase ; the stage of separation as the anaphase ; and
the stage of reconstruction as the tdophase.

HISTORY OF THE SEX-CELLS.

STRUCTURE AND DEVELOPMENT OF THE SPERMATOZOON.

Structure of the spermatozoon. — The human spermatozoon (fig. 12) is a
minute body possessed of a head and a long flagellum or tail. The head is conical
when seen in profile, but being compressed in one diameter, it is broadly oval when
seen in face view. At its pointed end it shows a somewhat different staining
reaction from the remainder ; this portion is known as the cap. The base of the
tail shows a thicker section, usually termed the middle piece. It includes two
parts, more distinctly separated from one another in some animals, the neck and
the connecting piece.

The complete length of the spermatozoon is from 52 to 62 /z, the head being
responsible for 4 to 5 //, and the connecting piece for 6 /x, of the total figure.

The pointed process of the head is sometimes called the perforatorium. It is
prolonged in some animals into a hooked projection. The limit of the cap is
marked by a distinct line on the head. The tail has an axial filament which is
prolonged through the connecting piece. In this it is imbedded in a sheath
derived from the cell-protoplasm, which is characterised by the presence of a
remarkable spiral filament. At the junction of the head and neck, and again at
the union of the neck with the connecting piece, there are certain darkly staining
granules derived from the centrosomes of the cell from which the spermatozoon is
developed. The tail filament is related to those which lie at the union of neck
and connecting piece. In some animals the tail has connected with it a very
delicate lateral membranous fold, which ceases a little distance short of the tip
(end piece).

The picture of the spermatozoon is strikingly different from that of a typical cell, and when
seen in active movement produced by the lashing of the motile flagellum, the theory of the
early observers, to which it owes the name of spermatozoon (given to it by von Baer), seems not
unnatural. Kolliker (1841) first showed its true nature, by proving that the head is derived
from the nucleus of the cell from which the spermatozoon is formed. By recent work all the
various parts have been traced back to the constituent elements of the testicular cell.

Spermatogenesis (fig. 13). — The subject of spermatogenesis will be treated
of in the description of the testis ; but some account of the process may here
be given.

The spermatozoa are derived from the cells lining the testicular tubules, which
Are said to multiply by amitosis, with differentiation into elements possessing

SPERMATOGENESIS

distinctive nuclear characters— the spermatogonia. These divide rapidly for a
time, then cease to multiply and give rise to a new generation of cells — the
spermatocytes. In this generation a series of changes in the nucleus is effected which

are of profound significance, and will
be described later. Gradually en-
larging, the spermatocytes divide into
rather smaller elements, the spermato-
cytes of the second order, which in turn
again divide to form the spermatids.
It will thus be seen that from each
spermatogonium, by two successive
divisions, four equivalent spermatids
are formed. Each member of this
group of four becomes, by further
changes, a spermatozoon. The sper-
matids lie near the lumen of the
tubules, and become attached to certain
remarkable elongated striated cells
known as the cells of Sertoli, or foot-
cells. The spermatids have meanwhile
acquired flagella, and remain attached
to the foot- cells in groups, becoming
gradually converted into the young
spermatozoa, which lie in groups with
their tails gathered into the lumen of
the tubule. When mature, the sper-
matozoa are set free in the tubule by
losing their connexion with the foot-
cells, which then shrink back to the wall
of the tubule.

The process of metamorphosis of
spermatid into spermatozoon is one of
much complexity, by which the nucleus
becomes the head, while the protoplasm
is reduced to a rudiment in the middle
piece. The mitochondria of the sper-
matid have been shown by Benda and
Meves to give origin to the spiral fila-
ment. Of the two centrosomes of the
spermatid, one remains as an indepen-
dent body, while the other becomes
related to the nagellum, and the
attraction-sphere (idiosome) undergoes remarkable changes to form the cap.

The accompanying diagrams (fig. 14), founded on drawings and descriptions
by Meves and by Moore and Walker,1 will convey a general impression of the
process of histogenesis leading to the evolution of the spermatozoon.

1 Reports of Thompson-Yates and Johnstone Laboratories, University of Liverpool, No. VII.
Part I. 1906. For earlier papers by Ballowitz, Bardeleben, Lenhossek, Benda, Meves, Moore, Ebner,
Begaud Wilcox, and others, on. mammalian and human spermatogenesis, see Hertwig, Handbuch der
Entwickelungsgeschichte, Literature, i. 431 seq. In the matter of structure see also Betzius, Biolog.
Untersuch., Neue Folge, x. 1902 ; Broman, Anat. Anzeiger. xxi.; Wederhake, ibid, xxvii.

FIG, 12. — HUMAN SPEBMATOZOA. (Broman.)

a and b represent spermatozoa in face (in
different foci), c and d in profile view.

STKUCTUKE OF OVAKIAN OVUM

STRUCTURE AND DEVELOPMENT OF THE OVUM.

Structure of the ovum. — The mature human ovum ready for fertilisation
and outside the Graafian follicle has not yet been observed. It is necessary,
therefore, to begin with a
stage prior to the formation
of the polar bodies, and
homologous with the sperma-
tocyte in the male series —
the stage of the oocyte.

The oocyte (ovarian
ovum) (fig. 15) resembles
that of all other mammals
(with the exception of mono-
tremes) in its minute size.
Immediately before the time
of its discharge from the
Graafian follicle of the ovary
in which it has been formed,
it is a small spherical vesicle
measuring about *22 to

*32 mm. in diameter, and is

FIG. 13. — DIAGRAM OF SPERMATOGENESIS. (T. H. Bryce.)

F, two cells of Sertoli attached to wall of tubule; Spg.,
a spermatogonium ; Spc. I., spermatocyte of the first order ;
Spc. II., spermatocyte of the second order; Spd., spermatids.

just visible as a clear speck
to the naked eye. When it
is examined with the micro-
scope, in a fresh condition in

the liquor folliculi, it is found to be invested by a comparatively thick, clear
covering. This, when the centre of the ovum is exactly focussed, has the

FIG. 14.— DIAGRAM OF THE DEVELOPMENT OF THE SPEBMATID INTO THE SPERMATOZOON. (T. H. Bryce.)

The cell-body is represented in 1 by the outer circle ; its gradual reduction and displacement till it
forms the middle piece with its spiral filament in 9 is shown. The nucleus is represented by the inner
circle ; it forms the main part of the head. The modified attraction- sphere in 1 and 2 shows a central
body and a vesicle (archoplasmic vesicle, Moore and Walker). The central body (shaded) forms the cap ;
the vesicle becomes the tail-sheath (Moore and Walker). The centrosomes are shown as dots; the
growth from one of them of the tail-filament is seen.

appearance in optical section of a clear girdle or zone encircling the ovum
(fig. 15), and was hence named zona pellucida by von Baer (1827). Under a

8

STKUCTUKE OF OVAEIAN OVUM

moderate power of the microscope a faint radial striation can generally be made
out, but this is seen more distinctly in ova which have been fixed by reagents,
and more especially in sections. On this account, and especially since a similar
radially striated membrane forms a characteristic part of the investment of the
ovum in many animals belonging to widely different classes, it is usual, in place
of the name zona pellucida, which has been exclusively used to designate this
investment in mammals, to employ the more general term zona radiata. (Waldeyer,
1870), or to speak of it simply as the striated membrane of the ovum.

FIG. 15. — HUMAN OVUM EXAMINED FRESH IN THE LIQUOR FOLLICULI. (Waldeyer.)

The zona pellucida is seen as a thick clear girdle surrounded by the cells of the corona radiata.
The egg itself shows a central granular deutoplasmic area and a peripheral clear layer, and encloses
the germinal vesicle, in which is seen the germinal spot

The zona radiata of the mammalian ovum is sufficiently tough to prevent the
escape of the contents of the ovum, even when subjected to a considerable
amount of pressure. If, however, the pressure be excessive, the tunic splits,
and the soft contents are extruded (fig. 16). It has, however, to be particularly
noticed that part of the contents remain, at any rate in all but the riper
oocytes, attached to the zona. The striae are believed to be minute pores ; and
it has been shown by Flemming, Heape, Retzius, and Ebner, that they are
occupied by processes of the cells of the corona radiata. This name is given

STKUCTUKE OF OVARIAN O.VUM

FIG. 16. — OVARIAN OVUM OF A MAMMIFEB.
(Allen Thomson.)

a, the entire ovum, viewed under pressure ; the
granular cells have been removed from the outer
surface, the germinal vesicle is seen in the yolk-
substance within ; b, the external coat or zona,
burst by increased pressure, the yolk-protoplasm
and the germinal vesicle having escaped from
within ; c, germinal vesicle more freed from the
yolk-substance. In all of them the macula is
seen.

to an investment of several layers of epithelial cells derived from the
proligerus of the Graafian follicle (fig. 15). In the ripening oocytes these follicular
cells form a syncytial layer immediately applied to the zona. Within the zona a
second membrane of great tenuity has
been described by various authors
(recently by Van der Stricht in the
human ovum), but its existence has
been denied by many, and it is
supposed by some that the processes
of the follicular cells are continued
through the zona into the egg-proto-
plasm. The fact that when the zona
is ruptured in young oocytes the con-
tents do not separate from it is in
favour of such connexion (Ebner).
In young oocytes there does not seem
to be any space round the protoplasm.
The egg-protoplasm is 'filled with
globules and granules of different sizes,
but all possessing a high index of re-
fraction. Compared with that of some
lower mammals, the human ovum is
distinguished by the ill- defined cha-
racters of its specially minute deuto-

plasmic bodies. When examined in the fresh condition the egg (fig. 15) is very
transparent, the deutoplasm being massed towards the centre, while there is a
clear or very finely granular layer round the periphery.

The deutoplasmic bodies in the egg-protoplasm are spoken of collectively as the yolk. There
is great variety in the different orders of animals in respect of the amount of yolk stored in the
egg. The term aleciihal is used to denote an ovum such as that of the mammal, Amphioxus, and
many echinoderms, in which the yolk-particles are absent, or very small in amount and uniformly
distributed, while a heavily yolked egg in which the yolk is accumulated at one end is termed
tdolecithal. There are many different degrees of this condition (amphibians, fishes, reptiles, and
birds). In the very exaggeratedly telolecithal egg of the bird the amount of yolk- substance
is so vastly greater than that of the protoplasm itself thaf it is only in the neighbourhood
of the nucleus (germinal vesicle) itself that the protoplasm can be distinctly recognised.
This point is conveniently distinguished as the animal pole, as opposed to the vegetative pole
marked by accumulation of yolk-material.

The amount and distribution of yolk is the main factor in determining the character of
the egg-cleavage. It is related to the determination of the period when the organism will
assume an independent existence. Thus in the bird the ovum contains necessarily all the
nutriment required by the chick until it is sufficiently developed to emerge from the egg and
obtain food independently. In the frog the yolk is sufficient for the early stages only and the
tadpole is hatched in a very immature condition, while in Amphioxus and many invertebrates
the organism is set free still earlier as a free-swimming ciliated blastula.

In the mammal the conditions are wholly different, and the small amount of nutritive material
in the egg is obviously related to its retention in the uterus, from which it is able directly to derive
its nutriment. It is probable that the alecithal condition of the mammalian egg is a secondary
condition related to the introduction of uterine gestation.

Imbedded in the protoplasm, usually eccentrically, is a large spherical nucleus,
which was termed by its discoverer, Purkinje, the germinal vesicle.2 This, which
is about 30 to 45 ^ in diameter, has all the characters of the nucleus of a

1 Bull, de 1'Acad. Roy. de Medicine de Belgique (4th ser.), xix. 1905.

2 Purkinje discovered the germinal vesicle in the bird's ovum in 1825 ; that of mammals was first
noticed by Coste in 1883.

10 OOGKENESIS

cell. It consists of a nuclear membrane enclosing a clear material or matrix,
imbedded within which may sometimes be seen strands of karyoplasm ; it always
encloses one or more well-marked nucleoli (fig. 15). Frequently there is but one
nucleolus, which is then large and prominent, and has received the name of
germinal spot (macula germinativa, Wagner, 1835).

In the young oocyte (fig. 17) there is a body (idiosome) near the nucleus, corre-
sponding to the attraction- sphere of other cells. It encloses a central granule, and
is itself surrounded by a mass of fine granules (mitochondria). Van der Stricht
identifies 'his body with the yolk-nucleus or body of Balbiani.1 There are also
other cell-inclusions to which various names have been given, but it is doubtful
what significance they possess.

Oog-enesis, — The earlier stages of oogenesis and the development of the
Graafian follicle will be treated of under the head of the ovary. Here we shall

begin with a stage m which the young egg-cell
is already imbedded in the developing gland,
and surrounded by a layer of follicular cells.

It will be recollected that the spermatozoa are derived
from the spermatogonia lining the testicular tubules, and
that the first phase is characterised by marked and
comparatively rapid growth of the spermatocytes. The
process begins at puberty and proceeds continuously
throughout the greater part of the life of the individual.
The young egg- cells or oogonia, on the other hand, have
all become converted into oocytes by the time of birth.
These undergo a very slow process of growth and ripening,
FIG. 17. — YOUNG OOCYTE SUB- anci are discharged singly at periodic intervals during

JCVer - »™*T *hort P-M of -productive activity.

Stricht.)

Showing attraction - sphere, centre- As the oocyte grows the Graafian follicle is

some, and mitochondria. gradually enlarged. The follicular cells, at first

laid down as a single layer (fig. 17), multiply to

form a many-layered investment to the ovum. By the formation of fluid among
them (fig. 18) the cells come to bound a cavity into which, from one part of
the wall, a cellular mass projects surrounding the egg (the discus proligerus).
The cells of the discus proligerus are arranged in an epithelial fashion
round the ovum, and remain in intimate relationship to it during the process
of ripening.

The oocyte is not at first provided with a zona pellucida, and the manner in
which this develops has not yet been quite cleared up. Authorities differ as to
whether it is formed by the follicular cells or is a true egg-membrane secreted
by the ovum.

maturation of the oocyte ; formation of the polar bodies. — By the
term c maturation ' is signified the series of changes which prepare the egg for
fertilisation. During a very prolonged period of growth the deutoplasm becomes
gradually accumulated in the protoplasm. The phenomena observed during this
period have been recently described for the human ovum in great detail by
Van der Stricht.'2 From this it would appear that the mitochondria, which are
regarded as specific cytomicrosomes, are concerned in the elaboration of the yolk.
They first collect round the idiosome, then become scattered, arrange themselves
in chains (chondriomites), solid rods (pseudo-chromosomes), and irregular bodies ;
later they become arranged in double rows to form minute tubules, which extend

1 See also Gurwitsch, Archiv f. mikr. Anat. Ivi. 1900; and Winiwarter, Archives de Biologie, xvii.
1900.

2 Loc. cit.

MATURATION OF OVUM

11

through the protoplasm as a sort of framework. These again disappear as the
deutoplasm collects in the centre of the cell in the form of minute vacuoles with
clear contents, fat-spheres, and minute highly refractile bodies (fig. 19).

When growth and storage of yolk-food are completed, the ovum is matured
by the extrusion of the polar bodies. The maturation stages now to be
described have not been seen in the human ovum, but observations on other
mammals put it beyond question that the process occurs in essentially the same
way as in lower forms. The phases leading up to maturation have been described
by Heape l in the rabbit.

Prior to the beginning of the sexual season in that animal certain of the
Graafian vesicles enlarge, and the growth of those near the surface causes them
to project and form swellings on the surface of the ovary. The wall of the follicle
and the tunic of the ovary is here much attenuated : so much so that in some of
them, when ripe, the structure is sufficiently transparent to allow of the ovum

FIG. 18. — EARLY STAGE OF

The follicular epithelium has become many-layered, and a cavity has appeared among the follicular
cells containing the liquor folliculi. The mass of cells which encloses the ovum is the discus proligerus.

being seen within the vesicle. During procestrum (period preceding ' heat ') the
blood-vessels surrounding the follicle enlarge, and these, running in the thin wall
which projects from the surface of the ovary, give the follicle the bright pink colour
which is characteristic of it. When maturation sets in, the cells of the discus
proligerus begin to withdraw from the ovum, and eventually remain attached to
the zona only by very fine protoplasmic strands. At the same time the ovum
withdraws from the zona, and a narrow perivitelline space appears.

The polar bodies are now thrown off (figs. 20 and 21). They are minute portions
of the egg-substance budded off in quick succession from the same point on the
surface into the perivitelline space. In this they persist for a time, but ultimately
disappear. They were originally termed polar bodies or directive corpuscles, from
the supposition that their presence determines the pole of the egg at which the
first segmentation will take place should the ovum become fertilised. As a matter
of fact, they occupy the pole which becomes afterwards the vegetative pole (Van

1 Proc. Roy. Soc. B. vol. Ixxvi. 1905.

12

MATURATION OF OVUM

. ,

/

der Stricht in the bat),1 and it has now long been known that the process by

which each polar body is formed is in reality a cell-division, in which the

nucleus is equally, but the pro-
toplasm very unequally, divided.
They are thus minute cells,
often called the pole-cells,
and are regarded as abortive
ova. In some cases the first
polar body undergoes division ;
when this occurs, the typical
group of four becomes formed.
Such groups are of frequent
occurrence in the produc-
tion of the gametes, not
only in the animal, but also
in the vegetable kingdom. Only
one of the four is, however,
functional.

The oocyte after the first
division is called the oocyte of
the second order ; and when this
again divides, and the second
polar body is thrown off, the
egg is said to be mature. The
essential difference is that the
large vesicular nucleus of the
oocyte of the first order has been
converted into a smaller nu-
cleus, which retires to the centre

of the egg, there to await the advent of the sperm-nucleus in fertilisation. It is to

the egg at this final stage that, strictly speaking, the term ovum should be applied
During the maturation of the ovum important nuclear phenomena present

themselves, which are essentially similar to those which take place in the

sperm-cells, as was first proved

by Oscar Hertwig in his classical

researches on Ascaris megalocephala,

published in 1890. The full signifi-

cance of these nuclear changes can-

not, however, be made clear until

the process of fertilisation is under-

stood. The treatment of the his-

tory of the nucleus during matu-

ration will therefore be deferred

until that process has been studied.

"

FIG. 19. — HUMAN OVUM AT END OF GBOWTH-PERIOD.
(Van der Stricht.)

Showing distribution of mitochondria in deutoplasmic
zone, with vacuoles, fat-drops, yolk-granules, &c.

FIG. 20.— THE POLAE BODIES AND EGG-NUCLEUS ;

ECHINUS ESCULENTUS. (T. H. Bryce.)

Only a portion of the ovum is represented.

x 1200 diameters.

In the process of spermato-
genesis the centrosome, it will be
recollected, persists and is related to the flagellum, but in the case of the egg
it in many instances seems wholly to disappear during maturation. This has been
made the basis of certain theories of fertilisation which will be alluded to later.

In the above account the case has been described in which maturation is
completed before the egg leaves the ovary. In many animals the spermatozoon

Anat. Anzeiger, Erganzungsheft, xxvii. 1905.

MATURATION OF OVUM

13

enters the egg before the second polar body is extruded. The final phases of
maturation then proceed concurrently with the initial phases of fertilisation.

While there is a general correspondence or homology between the different
cell-generations in spermatogenesis and oogenesis (fig. '22), it must be noted at

3\

i.

*

FlG. 21. — FORMATION OF SECOND POLAR BODY OF THE MOUSE. (Sobotta.)

I. First polar body (a) and its nucleus (6), with second polar spindle.
II. Second polar body, with remains of the spindle and cell-plate, x 1200 diameters

this point that there are two differences between the two series of cells. First,
after the second division is over the egg undergoes no further modification, while
the spermatid becomes converted, by a complicated process of cytohistogenesis,
into the functional spermatozoon ; secondly, all four spermatids become

Spermatogonium

Oogonium

Proliferation
period

Growth
period

Maturation
period

1234 1284

FIG. 22.— SCHEME OF SPERMATOGENESIS AND OOGENESIS. (After Boveri.)

spermatozoa, while only one of the products of division of the original oocyte
becomes a functional ovum.

From the foregoing descriptions it will be clear that in the dimorphism of the sex-cells we
have an instance, of a marked kind, of division of labour. In the most primitive forms among
the protozoa conjugation of cellular individuals occurs, but they are indifferent and equivalent
individuals ; while, on the other hand, in many of the higher Protozoa new conditions of life are
established, the sessile habit, for instance, which necessitates that one of the pair should have a
degree of mobility. Thus we have micro- and macro-gametes. There are instances of colonial

14

FEKTILISATION

forms, in which certain individuals of the colony become macro-, others micro-gametes
(e.g. Volvox), and from this stage the step to the differentiation of dimorphic sex-cells is
theoretically a simple one.

Fertilisation. — The ovum after its expulsion from the Graafian follicle is
received upon the fimbriated end of the Fallopian tube. The fimbrise are covered
by a prolongation of the ciliated lining of the tube, and the action of the cilia
serves to propel the minute ovum into and along the tube towards the uterus.
In this passage it may, if impregnation has occurred, meet with the spermatozoa,
and one of them may penetrate the zona pellucida to fertilise the ovum. It is
possible in some instances for fertilisation to occur on the fimbriated extremity

of the tube, or even in the
Graafian follicle, and this
may result in an extra -
uterine pregnancy.

The details of the pro-
cess of fertilisation have
been observed in a few
mammals, most clearly
in the mouse by Sobotta.
The process can be easily
followed in the transparent
egg of an echinoderm, and
for this reason the pheno-
mena as seen in the com-
mon sea-urchin will first be
briefly described, and then
a comparison will be made
with the facts established
for the egg of the mouse.
This is the more convenient
because the two cases re-
present two types of the
process.

When the spermato-
zoon which is to effect
the fertilisation of the
echinoderm - egg touches
its surface the protoplasm
streams out at the point of
contact, to form what is
known as the entrance cone
(fig. 23, 6). As soon as
the sperm-head is fixed, a membrane is thrown off from the egg. It may
happen, however, that before this is effected several spermatozoa may have
obtained an entrance, and polyspermy results. This is always followed by
abnormal development. The spermatozoon, by the action of the flagellum,
now bores its way into the egg until the whole head and the middle piece
are imbedded in the protoplasm. The flagellum, no longer of service, is
thrown off, and the sperm-head undergoes a rotation through 180° (fig. 23, 6, c),
until the middle piece is directed inwards. Radiations now appear in the
protoplasm centred on the situation of the middle piece, which is no longer
distinguishable, and the sperm-head commences a movement towards the
•centre of the egg. In living eggs the radiations are seen gradually to extend

PIG. 23. — FERTILISATION IN ECHINUS ESCULENTUS ; DRAWN FROM

SECTIONS HIGHLY MAGNIFIED. (T. H. Bryce.)

a, entrance of spermatozoon ; b, commencing rotation of sperm-
head ; c, completed rotation of sperm-head ; commencing sperm-
aster.

FERTILISATION 15

through the whole cell-body, and meanwhile the conical sperm-head assumes
a spheroidal form, and is converted into the sperm-nucleus (fig. 24, a). Several
observers have described the existence of a minute granule in the centre of
the radiation or sperm-aster, and it has been identified as the centrosome of
the spermatozoon. As previously mentioned, it has been supposed that the centro-
some disappears in the ovum during maturation. According to Boveri's theory
of fertilisation, the sperm-centrosome supplies a new divisional centre, and plays
the leading role in initiating cleavage of the egg. Wilson's ' observations, however,
on eggs which undergo parthenogenetic development as the result of treatment by
chemical substances, make it doubtful whether we can interpret the phenomena
in this way. His experiments prove that centrosomes may arise de novo in
the egg-protoplasm, and therefore it is possible that the spermatozoon produces
an effect on the egg-protoplasm such as to produce a centrosome, or physiological
centre of activity, made manifest by the radiations of the aster.

As the result of this protoplasmic activity the sperm-nucleus now changes
its position and moves toward a point not quite in the centre of the egg, while

dti

**$^%y^^ HL,

&*'

FIG. 24. — THREE STAGES IN THE CONJUGATION OF THE SPERM WITH THE GERM-NUCLEUS IN
ECHINUS ; DRAWN FROM SECTIONS, x 1200 D. (T. H. Bryce.

at the same time the germ-nucleus is also drawn to the same point. As the nuclei
approach one another the aster comes into contact with the germ-nucleus, and
the clear area at its centre spreads over the side of that body. The nuclei then
conjugate (fig. 24, 6, c), the aster becomes double, and the radiations die away
during a pause in which the compound nucleus, or, as it is now called, the segmenta-
tion-nucleus, grows in size. The two asters then become related to the poles of the
nucleus, the radiations reappear, and the first nuclear division of the egg is
inaugurated. From the figures it will be clear that the nuclei conjugate while
in the vesicular phase, with the chromatin in the form of a network.

The process in the mouse 2 (fig. 25) is essentially the same, but there are certain
variations. The spermatozoon meets the egg in the second third of the oviduct,
before the second pole-cell has been formed. Entrance is probably effected by the
piercing of the zona, and not through one of the pores in the membrane. The
tail seems to be cast off, and only the head and middle piece enter the egg ; but
in the bat the whole spermatozoon is figured by Van der Stricht as entering (fig. 26).
A rotation of the head occurs, and it then becomes converted into a small vesicular
nucleus, which is at first distinguished from the germ-nucleus by its smaller size;

1 Archiv f. Entwickelungsmechariik, xii. 1901.

- See Sobotta, Arch. f. mikr. Anat. xlv. 1N95. Cf. also Van der Stricht, Anat. Anzeiger, Ergiinz-
ungsheft, 1902, and Rubaschkin, Anat. Hefte, xix. 1905.

16

FEKTILISATION

It expands, however, before conjugation takes place, and the two nuclei, now
of equal size, lie side by side. In each the nuclear network is converted into a
spireme thread, the membrane disappears, and the thread is divided into the
chromosomes. The paternal and maternal chromosomes thus form separate
groups, and these appear to remain distinct as the rods from each nucleus are
gathered into the equatorial plate of the first segmentation-spindle. The mixture
of the chromatin is probably not effected, therefore, until the first segmentation-

FIG. 25 (1-7). — STAGES IN THE FERTILISATION OF THE EGG OF THE MOUSE. (Sobotta.)

1, Entrance of spermatozoon ; 2, rotation of sperm-head ; 3, formation of sperm-nucleus, which
lies to the left ; the germ-nucleus lies to the right ; 4, resolution of nuclei ; 5, vesicular stage of nuclei ;
the smaller is the sperm-nucleus ; 6, enlargement of sperm-nucleus and its approach to germ-nucleus ;
7, first segmentation-spindle, with group of paternal chromosomes to left and of maternal to right.

division has begun ; and it has been proved in Ascaris, which has only four
chromosomes, that the rods are divided, after splitting, between the two first blasto-
meres in such a way that each receives an equal number of paternal
and maternal chromosomes (fig. 27). This is a fact of capital importance,
and plays a prominent part in modern theories of heredity.-

If we analyse the phenomena of fertilisation we must recognise two factors : (1) The initiation
of cell-division, and (2) the union of the nuclei. Though closely bound up with one another,
these two factors are distinct and independent phenomena. Thus it has been demonstrated

THE NUCLEUS IN MATURATION

17

by the brilliant experiments of Boveri, Delage, and others, that portions of sea-urchin eggs broken
by shaking, or cut into fragments (merogoriy), which contain no part of the germ-nucleus,
may, when fertilised by spermatozoa, divide, and ultimately form larvae. The sperm
nucleus is thus sufficient by itself for the segmentation of the egg, a centre-
some being introduced or produced in the protoplasm. Again, it has been shown,
first by Loeb and then by many others, that the eggs of echinoderms and other invertebrates
may be made to segment and form Iarva3 by treatment, in various ways, with certain chemical
substances, by shaking and so forth, without the influence of the spermatozoon. The egg-
nucleus is thus sufficient in itself for segmentation and development of the
egg when, by artificial means, a centrosome is produced in the protoplasm.1

Thus we reach the general conclusion that the union of the nuclei is not the means by which
the developmental process is started, but nevertheless it is the essential factor in fertilisation
— in short, it is the end and aim of the process. The union of the paternal and
maternal chromatin (amphimixis] is the all- important fact, and for this reason, that (without
denying to the protoplasm a certain influence) the chromatin of the nucleus is the material
basis of the hereditary qualities handed on from one generation to another.

Reduction of chromatin. — It is sufficiently obvious that if there is a
fusion of paternal and maternal chromatin in fertilisation at each generation
the amount of chromatin would be doubled,
on the assumption that the mass is con-
stant in all the nuclei of each generation.
The necessary reduction is effected during
the maturation of the sexual cells.

Before entering on a description of the process of
reduction, it is necessary to refer briefly to two related
hypotheses as to the constitution of the nucleus.

It is now practically certain that the number of
chromosomes, is constant in each species, and that out
of a resting nucleus the same number of chromosomes
emerge as entered it at the end of the preceding
division. It is believed by some that the chromo-
somes retain their identity in the resting nucleus, so
that the chromosomes which emerge from it are the
same as those which entered it.- As a corollary to
this theory of the persistent identity of the chromo-
somes, there is a second hypothesis that the paternal
and maternal chromosomes, equally distributed
between the two first blastomeres (fig. 27), have a

separate and persistent identity in all the cells of the soma, and consequently in the spermato-
gonia and oogonia.

Nuclear phenomena during1 development of the sexual cells. — The

history of the sexual cells in respect of the nuclear changes may be divided into
three phases — a pre-reduction, a reduction, and a post-reduction phase.3 The pre-
reduction phase includes all the cell-generations up to that of the spermatogonia and
oogonia (fig. 22). During this period the nuclei behave in all respects like the nuclei
of somatic cells, arid possess the same number of chromosomes. The reduction
phase involves the generations known as spermatocytes and oocytes of the first and
second orders, and is characterised by two divisions differing in their characters from
all other varieties of mitosis, during which the number of chromosomes is
reduced to one-half of the somati c number. The first division is known as

1 Further information as to the observations here referred to will be found in a review of the literature
in the Quarterly Journal of Microscopical Science, xlvi. 1902, by T. H. Bryce.

2 It appears to me that the facts of maturation of the ovum, as will be seen later, form an obstacle to
the present unrestricted acceptance of the theory of the persistent identity of the chromosomes in its
crude form, but that, notwithstanding, it is necessary to assume a segregation in the chromatin mass,
which may, as it were, crystallise at each division into chromosomes of specific characters. — T. H. B.

5 For these phases the terms pre-meiotic, meiotic, and post-meiotic are employed by Moore and
Walker (derived from peiou, to make smaller).

VOL. I. O

FIG. 26. — FERTILISATION ; EGG OF BAT.
(Van der Stricht.)

S.n., sperm-nucleus ; g.n., germ-nucleus ;
P.6., polar bodies.

18

THE NUCLEUS IN MATURATION

the lieter atypical, the second as the homotypical, division. The post-reduction phase
is represented in animal forms by the mature sex-cells, but in certain plants in
which there is alternation of generations it includes all the cells of the sexual genera-
tion. The post-reduction phase comes to an end with the union of the nuclei
of the sex-cells, and it is obvious that as each conjugating nucleus has only one-
half the number of chromosomes possessed by the somatic cells, the somatic

FIG. 27.— DIAGRAM OF FERTILISATION. (After Boveri.)
The number of chromosomes is four : paternal, red ; maternal, blue.

number of chromatin-rods is restored in the segmentation-nucleus
formed by the union of the sperm- with the germ-nucleus.

The phenomena are seen with greater clearness in the sperm-cells than in the
germ-cells, as there are certain modifications of the process in the maturation of
the ovum which will be alluded to later.

The prophase of the first or heterotypical division is very prolonged, and the
chromatin undergoes changes too intricate to be followed in detail here. One

THE NUCLEUS IN MATURATION 19

phase is especially important. During it the chromatin is gathered into a tangled
skein at one side of the nucleus (fig. 28, B). This may be named the synaptic
phase (synapsis of Moore; see p. 22). During this phase a reduction in the
number of chromosomes takes place by a fusion of the somatic
chromosomes in pairs. The manner of the fusion is disputed, some observers
holding that the chromosomes unite end to end ; others that they fuse along their
whole length. The fused rods (now one-half the number characteristic of the
somatic cells) go through elaborate changes, and ultimately form double rods
(fig. 28, a — e), which take various forms (pseudotetrads, rings, &c.), according
to the manner in which the rods are united together. In a very small number
of cases (Ascaris) the prophase figures take the form of four isolated bodies or
tetrads. Such cases fall into a special category, and will be treated separately.

FIG. 28. — SOME STAGES IN SPEBMATOGEXESIS OF MYXINE. (Schreiner.)

a and &, synaptic phase ; c, d, double thread stages ; e, double chromosomes (rings and double rods) ;

/, metaphase of heterotype.

These double-rod prophase figures (' gemini ') are gathered on to the equator of
the first maturation- spindle, and the resulting metaphase figures vary according
to the manner in which they are attached to the threads of the spindle. They
may be attached at their extremities, at their middle points, or nearer one end
(fig. 29). It follows that when the two portions of the double rods are separated
on the spindle, in the first case the chromosome will be a straight rod, in the second
a V, in the third a V with unequal limbs (fig. 29, 6). The result is the same
in each case : the two portions of the double rod are separated, and as these
portions represent not a longitudinally divided chromosome, but the two somatic
chromosomes which have fused in the synaptic phase, whole chromosomes are
separated and distributed on the spindle, instead of the halves of the longitudinally
split chromosome, as in ordinary mitosis. A further complication of the hetero-
typical division is that a cleavage of the rods manifests itself as they pass to the
poles of the spindle, and a double figure again results (figs. 29 c, 30 b). This cleavage
is preparatory to the second or homotypical division, in which the elements are

c2

20

THE NUCLEUS IN MATURATION

separated without further splitting (fig. 30, c). In very many cases there is
no reconstruction of the nucleus between the heterotype and the homotype
division, but in others a partial reconstruction takes place, without however a
loss of the identity of the halves of the double anaphase figures of the heterotype.
The case of true tetrads involves certain difficulties, but the explanation of
their occurrence seems to be that the second, or anaphase cleavage, which is fore-
shadowed in the prophase in many forms, has been completed before the formation
of the first spindle. There has been much diversity of opinion as to the character
of the second cleavage, whether it is a longitudinal splitting or transverse breaking
of the spireme-thread. If it be a longitudinal splitting of the elements of the
' synaptic gemini,' the case falls in with the scheme adopted.

Difficult as the stages are to follow during
maturation of the sperm-cells, they are still further
complicated during the maturation of the egg, by
the fact that an immensely long period of repose
occurs between the early stages of the hetero-
typical prophase and the actual division of the
nucleus, during which the nucleus assumes the
vesicular form characteristic of the ripening
oocyte (fig. 31). As the oocytes are all formed
before birth, the beginning of the prophase must,
in the human subject, be separated by many
years from the actual division. The manner in
which the chromosomes are re-formed out of the
germinal vesicle when maturation sets in, is by
no means cleared up. The chroma tin-threads are,
according to competent observers, completely lost
sight of in the vesicle, and the chromatin gathered
into nucleoli, which again give rise to chromo-
somes. This is a difficulty with which the theory
of persistence of the chromosomes has to contend ;
but in any case double-rod figures appear, which
pass into the metaphase (fig. 32) exactly as in
spermatogenesis. In maturation of the egg in
Echinus the double-rod figures (pseudotetrads) are
attached to the spindle by their extremities. The
figures, though minute,, are simple, and are here
reproduced as an example of the process (fig. 33). l

FlG. 29. — DlAGBAM BEPBESENTING
THE BEHAVIOUR OF THE DOUBLE
CHBOMOSOMES OF THE HETEBO-

TYPICAL DIVISION. (After Gregoire.)
a, Three double chromosomes,
variously arranged ; &, the mode of
attachment to the spindle of the three
types — 1, near the ends; 2, 'opposite
the middle points ; 3, at the ends ;
c, resulting double anaphase figures —
1, tailed V's ; 2, simple V's ; 3, straight

Significance of the nuclear phenomena. — It

is impossible here to deal at length with the history

of opinion on the interpretation and significance of the phenomena described above. In the
last edition of this work two theories were briefly alluded to — the sex theory of Minot
and the hypothesis of Weismann. Minot's tHeory, which applied to the ovum only, postulated
that the cells were hermaphrodite, and supposed that in the extrusion of the polar bodies the male
element was got rid of from the ovum, to be replaced again in fertilisation. This became
untenable when the parallelism of the maturation processes in spermatogenesis and oogenesis
was established, and it was proved that all four elements resulting from the division of the
spermatocytes became functional spermatozoa, while only one of the products of the division of

1 The following works may be consulted on this subject : Ed. B. Wilson, The Cell in Inheritance
and Heredity, 1900 ; Wmiwarter, Arch, de Biol. 1900 ; Korschelt and Heider, Lehrbuch d. vergleich.
Entwickelungsgeschichte d. wirbellosen Tiere, Lieferung ii. 1903. Gregoire. La Cellule, xxii. 1905, gives
a useful summary and bibliography to date of publication. Since then, among other papers, have
appeared Farmer and Moore, Quart. Jour. Micro. Sc. xlviii. ; J. E. Lane-Clay pon, Proc. K. S. 1905;
Janssens, La Cellule, xxii. ; A. and K. E. Schreiner, Arch, de Biol. xxii., and Anat. Anzeiger, xxix. ;
Van der'stricht (bat), Compt. rend, de 1'Assoc. des Anat., 8° Reunion, Bordeaux, 1906 ; Moore and
Walker, The Meiotic Process in Mammalia, Rep. Cancer Research Lab., Univ. Liverpool, 1906.

SIGNIFICANCE OF NUCLEAR CHANGES 21.

the oocyte became a functional ovum. Weismann's theory, very briefly put, postulated a
qualitative reduction of the hereditary substance, brought about by a transverse cleavage
of the chromatin-rods, the halves of which were separated in the second division, the rods or idants
by his hypothesis being organised into lower-grade groups of the ultimate particles of the heredi
tary substance (ids and determinants). During more than a decade of research to which this

6 c

FlG. 30. — FlGUBES ILLUSTRATING BEHAVIOUR OF CHROMOSOMES DURING MATURATION DIVISION.

a, Metaphase of heterotypical division, spermatocyte i. of Batrachoseps (Eisen) ; b, anaphase of
heterotypical division in spermatocyte i. of Salamandra, showing secondary cleavage of chromosomes ;
c, metaphase of homotypical division, spermatocyte 11. of Salamandra (Meves).

hypothesis gave rise, the evidence for and against its actual realisation in fact was pretty equally
divided. The cases in which the two maturation chromosomal cleavages seemed both to be
longitudinal, resulting therefore in an equal distribution of the chromatin-mass to the quaternary
group of gametes, were supported by as cogent evidence as those in which the splitting was
described to be once longitudinal and once transverse, as was necessary for the idea of an
unequal distribution of the supposed hereditary substance. So great was the contradiction

22

CHROMOSOME THEORY OF HEREDITY

among the results of the most eminent of observers that some writers gave expression to the
opinion that the problem was a barren one ; but within the last few years a new light has been
thrown on it by the work of numerous observers. The ultimate result has emerged in the
form of an interesting hypothesis which combines, in a fashion, the theories of Minot and
Weismann.

In 1896 Moore described a phase in which the chromatin of the nucleus is clumped, which,
as already mentioned, he named the synapsis. This proved to be a very general phenomenon
in spermatogenesis, and although missed at first in oogenesis was shown by Winiwarter (1901)
to occur at an extremely early stage in the history of the oocyte. The idea that the double-
rod prophase heterotypical figures might arise from fusion of chromosomes was not quite a
new one. It had been suggested by Korschelt, Wilcox, and Calkins, but it took a new form
when by the observations of Winiwarter, Montgomery, Sutton, Farmer, and Moore, it was

FIG. 31 (a to/). —SOME STAGES IN THE MATUBATION OP THE EGG OF THE BABBIT, x 1700 diameters.

(Winiwarter.)

a, nuclear network converted into delicate looping threads; b, synaptic stage (fine threads);
c, synaptic stage (thick threads) ; d, nucleus now occupied by double filaments ; e, double chromo-
somes ; /, resting vesicular stage of nucleus in which the network is re-established.

shown that in the synaptic phase the chromatin threads or loops fuse in pairs, thus pro-
ducing reduction in the number of chromosomes. Sutton showed in 1902 that there is a
certain order in the fusion, leading to the suggestion that the maternal and paternal
chromosomes— present by hypothesis as persistent individuals in all the somatic nuclei
resulting from the fusion in fertilisation of the male and female germ-nuclei—become fused
in pairs. The double-rod prophase figure therefore represents not a longitudinally split
chromosome, but two chromosomes, one paternal, the other maternal, and the first maturation
division separates the paternal from the maternal.1 It follows that of the four products of
the two maturation divisions, two possess paternal and two maternal rods.

This new idea of reduction of hereditary qualities is different from that of Weismann, but
equally well accounts for a redistribution of hereditary attributes (represented by the chromatin)

1 In some forms it may be that the actual separation takes place in the second division ; the result
is the same.

MENDEL'S LAW

23

during the maturation divisions. The theory is usually known as the chromosome theory
of development and heredity.

It is further of interest that when the process as defined by this theory is analysed, it has sug-
gestive relations to Mendel's ' law of heredity.' This law may be very briefly alluded to here
because of its relation to the cytological data. It may be represented in terms of a single pair
df hereditary qualities thus :

A + B

AB
1

1. A

1
2An

1

i

I I
I B

J 1

I
3. A A

JB i
i

A 2AB B I

in which A and B represent single and distinctive qualities — one being dominant, the other
recessive — of two individuals uniting in a cross, AB. In the cross one quality, A, alone
manifests itself (the dominant] ; the other,
B, is latent (the recessive], so that the off-
spring of the cross appears as pure A. In
the next generation, no new cross in
respect of these qualities being effected,
the offspring appear in the proportion
of three individuals with the A quality
to one with the B quality. The B indi-
viduals now breed pure in all succeeding
generations, and some A individuals do
the same ; but a second set of A's are
really AB'S, and they in the next genera-
tion split up again into pure A's, AB'S
(appearing as A's) and pure B's, and so on.
The theory involves as a corollary
the purity of the gametes in respect of
the qualities, and this purity would be
attained by just such a process as is
assumed by the chromosome theory to
take place in maturation. Thus, if two
AB's cross, the gametes being pure in
respect of the qualities will be either A1
or B1, A2 or B2. Four combinations are
possible between these gametes — viz.

A'A-, A'B2, A-B1, B'B2, giving rise to three classes of individuals, pure A's, mixed AB'S, and pure
B's. The mixed individuals, however, always appear as A's, that quality being dominant and B
recessive, so that there are three A's to one B, as expressed in the ' law.' In higher forms,
all the individual gametes of the four groups are not functional, the three polar cells being
abortive ova, so that the formula requires a slightly different statement. The ovum in respect
of two qualities may be either A or B —

(1) A + A' or B' = AA' or B'A; or

(2) B + A' or B' = BA' or BB'.

In a sufficiently large progeny from a single pair the expectation would still be the same —
viz. one pure A, two mixed AB'S appearing as A's and one pure B. '

1 For some interesting human cases see Batesoii, Brit. Med. Journal, July 14, 1906. The reader
must be referred for further information and for criticism of this theory, as well as for a statement of
other doctrines of heredity, to special treatises on the subject. For the cytological data, see more
especially Boveri, Ergebnisse iiber die Konstitution der chromatischen Kernsubstanz, Jena, 1903 ;
Sutton, ' Chromosomes in Heredity,' Biol. Bull., April 1903. For a statement of Mendel's law, see
Bateson, Mendel's Principles of Heredity, &c. (Cambridge University Press, 1902). The literature is
fully reviewed in Schwalbe's Jahresberichte of recent years. For a criticism from the cytological side,
a paper by R. Fick, Arch. Anat. und Physiol. Anat. Abt. 1905, may be mentioned.

FlG. 32. — FlBST POLAB SPINDLE (METAPHASE OF

HETEBOTYPE), EGG OF MOUSE. (Sobotta.)

24

MATUEATION OF OVUM

FIG, 83. — STAGES OF MATUBATION, EGG or ECHINUS ESCULENTUS. x 1200 diameters. (T. H. Bryce.)

a, Prophase of heterotype : double chromosomes ; &, metaphase of heterotype : separation of
chromosomes I. and their commencing secondary cleavage; c, anaphase of heterotype: completion of
secondary cleavage : extrusion of first polar body ; d, prophase of homotype : chromosomes pass
unchanged into second polar spindle ; e, metaphase of homotype ; /, anaphase of homotype : separation
of chromosomes n. : extrusion of second polar body.

SEGMENTATION OF OVUM

25

HISTORY OF THE SOMA.

FORMATION OF THE BLASTODERM AND EMBRYONIC AXIS.
SEGMENTATION OF THE OVUM.

Immediately after the sperm- and germ-nuclei have conjugated, the egg
divides into two segments or blastomeres (fig. 34). The division of the cell-body
is preceded, as in ordinary cell-division, by the mitotic cleavage of the nucleus,
and, as previously stated, each daughter- nucleus receives an equal complement
of paternal and maternal chromosomes. Each blastomere now divides to form
a group of four segments, which again cleave into eight, and the process of binary
division continues until a mass of small nucleated segments is formed, called the
mulberry mass or morula (fig. 34). This is enclosed by the zona radiata, and
is little, if at all, larger than the single ovum which it replaces within the zona.
The segmentation of the mammalian egg is complete or holoblastic (see p. 27),

z.p.

FIG. 34. — FIRST STAGES OF SEGMENTATION OF A MAMMALIAN OVUM : SEMI-DIAGRAMMATIC.
(Drawn by Allen Thomson after E. Van Beneden's description.)

z.p., zona pellucida ; pgl., polar globules ; a, division into two segments ; Ic, larger and clearer
segment ; sc., smaller, more granular segment ; b, stage of four segments ; c, eight segments ;
d, e, succeeding stages of segmentation showing the more rapid division of the clearer segments
and the enclosure of the darker segments by them.

but is neither quite equal nor quite regular. Some of the cells divide more rapidly
than others, so that groups with an odd number of segments occur, such as 3, 6, 12
or 7, 9, 10 ; and when the morula stage is reached there is a definite grouping of the
segments, the centre of the sphere being occupied by larger, more granular, cells,
surrounded by a layer of smaller, clearer elements. It is not certain at what stage
the distinction between the two categories of cells is established, though some
believe that it is effected at the first cleavage ; but it is clear (even though the
evidence for a definite epiholic process is incomplete) that certain of the cells divide
more rapidly, take a superficial position, and come to cover and enclose the
remainder.

The cleavage of the human ovum has not been observed, and only one stage has so far been
seen in any of the lower Primates (fig. 35). It is a four-cell stage found in an oviduct of
Macacus nemestrinus given to Selenka by Hubrecht. ! The blastomeres are of nearly equal size, two

1 Selenka, Studien iiber Entwickelungsgeschichte der Tiere, Heft x., Wiesbaden; Kreidel, 1903,
p. 831.

26

FOKMATION OF BLASTOCYST

being somewhat oval, two nearly spherical. The observation shows that in the other Primates,
and therefore practically certainly also in man, segmentation takes place in the oviduct, and
after the same fashion as in the lower mammals. It will be observed that there is no zona
radiata represented. In most mammals the zona persists during the earlier phases of develop-
ment, and it is difficult to account for its absence in this and
other cases. It is doubtful whether it is to be ascribed to the
i^^_^^^ preservatives used, or to a normal precocious solution of the

membrane.

Fluid now appears between the peripheral layer
and the central mass, and separates them everywhere
except at one point (fig. 36, b). As the fluid accumu-
lates, the morula is converted into a vesicle (fig. 36, c),
the walls of which are formed of a single layer of small
clear cells, except at the point where the central mass
is attached and projects into the cavity (figs. 36, c, d,

FIG. 35.— SEGMENTING EGG OF and 37). The outer layer takes no part in the building

up of the embryo, but is concerned solely with the
establishment of relations between the ovum and the
uterine mucosa. It has been termed by Hubrecht

the trophoblast. The inner mass provides the material out of which embryo,
yolk-sac, and in man and apes almost certainly also the amnion, are formed,

diameters.

fern

troph.

fcm.

troph.

troph.

•fcm.

FIG. 36. — SECTIONS OF THE OVUM OF THE BABBIT
DUKING THE LATER STAGES OF SEGMENTATION,
SHOWING THE FORMATION OF THE BLASTOCYST.
(E. Van Beneden.)

a, Section showing the enclosure of darker cells,
fcm., by clearer cells, tropli. : the enclosure is
usually complete ; &, more advanced stage in which
fluid is beginning to accumulate between the inner
and outer cells ; c, the fluid has much increased,
so that a large space separates inner from outer
cells except at one part ; d, blastocyst, its wall
formed of a layer of flattened cells (trophoblast),
with a patch of dark granular cells (formative
cell-mass) adhering to it at one part ; z.p., zona
pellucida.

troph.

and may be termed the formative or
embryonic cell-mass. At this stage the
ovum has usually been termed the

blastodermic vesicle, but as the actual blastoderm is not yet formed it is

better to call it the

<;KI;MINAL LAY EMS

27

When the whole ovum is involved in segmentation the egg is termed holoblastic. This variety
of cleavage occurs in all eggs which are either alecithal (see p. 9) or moderately telolecithal, though
in the latter the division is very unequal, as, for instance, in the common frog. In exaggeratedly
telolecithal eggs like those of some fishes, birds, reptiles, and monotremes among mammals, the
cleavage is confined to the animal or protoplasmic pole, and they are then said to be meroblastic.
In all holoblastic eggs, except in the case of the mammal, the whole ovum is utilised for the
formation of the embryo ; while in meroblastic eggs only a small portion forms the embryo,
the remainder becoming the yolk-sac and the egg-membranes. The egg of the placental mammals
is holoblastic, yet the later stages correspond with those of meroblastic ova : hence it is a com-
monly accepted opinion among embryologists that the mammalian ancestry had large yolk-
laden eggs. This opinion is strengthened by the fact that the most primitive mammals — the
Monotremata — have eggs like those of reptiles. We are therefore justified in believing that in the
descent of the mammalia the yolk was lost when the egg came to be retained in the uterus
and established nutritive relations with the maternal tissues, but that it has retained in
some respects its ancestral mode of development. It must be added, however, that, while this
is the view of most embryologists, others (e.g. Hubrecht) maintain the opinion that the
mammalian ovum inherits its mode of development from ancestors with telolecithal holoblastic
eggs, like those of the present-day amphibian forms.

FORMATION OF THE GERMINAL LAYERS.

The youngest known human ovum (fig. 93, p. 65) is already considerably
advanced beyond the stage of the blastocyst. The resemblance between the
early stages in man, apes, and monkeys is very close. In certain particulars they
differ from the early stages of any other mammalian form except Tarsius spectrum.

This creature has been commonly placed among the lemurs, but Hubrecht has shown from
embryological evidence that it is more closely related to the apes and to man, and he has
proposed to limit the order Primates so as to include only man, the apes and monkeys, and
Tarsius. These form embryologically a group by themselves among the mammals, and it is
now possible, thanks to the work of Selenka and of Keibel on the apes, and of Hubrecht on
Tarsius, to combine the data for the lower Primates with the data collected for man first by
His and Graf v. Spee, then by Keibel, Kollman, Peters, Eternod, Mall, Minot, and others, so as
to obtain a fairly complete, if in some points still hypothetical, picture of the early history of
the primate ovum.

The earliest phases have been observed only in Tarsius,1 but from later
resemblances there are cogent reasons for believing that, except in one or two
particulars, these phases may be taken
as representing what actually takes place
in the development of the human ovum.

Formation of the entoderm. —
We shall begin with the stage of the
blastocyst, represented in fig. 37. The
trophoblast forms a continuous layer over
the inner or formative cell-mass, which
projects as a rounded knob into the cavity
of the vesicle. Compared with the blasto-
cyst of the lower Amniota, this mass of
cells is as it were projected (invaginated)
into the interior of the vesicle instead of
being spread out on the surface (fig. 38).

From the inner face of the inner cell-
mass a layer of cells becomes split off,
which is generally called the primitive or
yolk entoderm (lecithophore of Van Beneden)
(fig. 39, A). Contrary to what happens in most lower mammals, this entoderm

1 A. A. W. Hubrecht, ' Furchung und Keimblattbildung bei Tarsius spectrum,' Verhandelingen der
Koninklijke Akademie van Wetenschappen te Amsterdam, viii. 1902.

fern.

"" troph-

FIG. 87. — BLASTOCYST OF TAKSIUS
SPECTRUM. (After Hubrecht.)

fcm., formative cell-mass; troph., trophoblast.

FOKMATION OF ENTODEKM

does not grow out round the wall of the vesicle closely applied to the ectoderm
(see fig. 41), but speedily forms a small closed sac, the entodermic or future
yolk-sac (fig. 39, b and c), separated by a space from the trophoblastic wall

FlG. 38.— DlAGBAM TO SHOW THE DIFFERENCE BETWEEN THE BLASTOCYST (BLASTULA) OF A
PLACENTAL MAMMAL a, AND THAT OF ONE OF THE LOWEB AMNIOTA b. (After Semon.)

In a, the wall of the blastocyst is complete from the first, and the formative cell-mass projects
into its interior ; in b, the wall is completed only at a much later stage by the growth of the
ectoderm over the yolk, and the formative cells spread out on the surface.

of the blastocyst. This peculiarity is clearly secondary, and is due to the
precocious and extensive expansion of the trophoblast shell, while the formative
cell-mass lags behind in development. The entoderm layer, which clings to the

fcm.

rfr

»mb. ect.

FIG. 39. — EARLY STAGES IN THE FORMATION OF THE GERMINAL LAYERS IN TABSIUS SPECTKUM.

(After Hubrecht.)

fcm., formative cell-mass ; ent.^ entoderm ; emb. ect., embryonic ectoderm.

embryonal cell-mass, is composed, as in many lower mammalian forms, of larger
more loosely arranged cells, while the free portion is formed of more flattened
elements.

It is not quite clear how the layer of entoderm cells becomes converted into a sac. There are
three possibilities : 1. The cell-layer grows meridionally in all directions, finally to close in

FORMATION OF EMBRYONIC ECTODERM

29

veutrally. This would correspond to the extension round the wall of the blastocyst that occurs
in lower mammals. 2. The layer bends round in front and grows backwards to close at the
posterior end. 3. The cavity is formed by a splitting among the budded-off entoderm cells so
that they are separated into two lamella?. None of Hubrecht's figures of this stage directly
favour this last possibility, but it has been suggested that the yolk-sac in the human ovum
may be formed out of a solid mass of cells in this way.

Formation of the embryonic ectoderm : bilaminar blastoderm. —

Though the primitive entoderm is already formed, the stage of a bilaminar blastoderm
is not yet reached. The
formative cell-mass is still a
rounded knob, continuous
with, but to be distin-
guished from, the tropho-

phase

place

blast. In the next
a splitting takes

among the cells, so as to
form a cavity (amnio-
embryonic cavity, fig. 40) in
the heart of the knob.
The cells forming the floor
of the cavity arrange them-
selves in a columnar
manner, and form a plate,
which is the embryonic
ectoderm. This plate is
at first necessarily concave
owing to the invagination
of the formative cell-mass.
The fate of the cells form-
ing the roof of the cavity
differs in Tarsius and the
higher Primates. In Tar-
sius, as the plate increases
in size it becomes flattened out ; the primitive invagination (inward projection) is
undone, and the embryonic ectoderm comes to lie free on the surface of the
blastocyst by the disappearance, due to retraction or otherwise, of the cells forming
the roof of the amnio- embryonic cavity. At a considerably later period of
development, when the embryo has been laid down, the cavity is as it were again

FIG. 40. — HYPOTHETICAL STAGE or HUMAN BLASTOCYST

(T. H. Bryce.)

tr., trophoblast ; am., amnio-embryonic cavity ;
ams., amnion-stalk ; ent., entoderm.

FIG. 41.— SECTION OF PART OF THE BLASTODEEMIC VESICLE OF THE RABBIT AT six DAYS.

(From E. Van Beueden.)

a, trophoblast (Rauber's layer) ; b, formative ectoderm ; c, entoderm.

restored by the formation of folds which meet over the embryo and form the
definitive amnion, as will be explained later.

In man and the apes this early stage is as yet unknown ; but the later phases
are most adequately explained by assuming that the roof of the cavity persists,
that the primary invagination is not at this stage undone, and that the formative
ectoderm never comes to lie free on the surface of the blastocyst. The cavity in
the formative cell-mass thus becomes the definitive amniotic cavity, which is

30

ENTYPY OF THE GERMINAL AREA

closed from the first. Its floor becomes the embryonic ectoderm, its roof the
amniotic ectoderm, and this is attached to the trophoblast by a short stalk,
which may be termed the amnion-stalk. Fig. 40, which represents an attempt
to visualise this hypothetical stage of the human ovum, will make these statements
clear ; but for a full understanding of the points at issue, a comprehension of the
phenomenon known as entypy of the germinal area is necessary.

It must be stated here that the view adopted in the text has not the assent of all embryo-
logists. It is clear that the early blastoderm is invaginated in the human ovum, but is the
invagination primary as described above, or is it secondary ? It is held by some that, owing to the
complete imbedding of the ovum in the decidua, the embryonic ectoderm, at first on the surface
of the blastocyst, is very early closed in by precocious amnion folds. As a further result, the
growth of the blastoderm causes it to be inverted into the cavity of the vesicle, and the stage
imagined in fig. 40 would be reached by the fusion of the folds over the embryonic area to form
what has been named the amnion-stalk. Certain observations of Selenka on Hylobates embryos,
and of Mall on abnormal human ova, support this view, which has also been advocated by
Keibel. ' The view adopted in the text is that of Van Beneden, Selenka, Hubrecht, and others,

a

FlG. 42.— DlAGBAMS TO ILLUSTKATE ENTYPY OF THE GEBMINAL AREA. (T. H. BryCG.)

Blastodermic vesicle : a, of rabbit ; b, of mole ; c, of bat ; d, of mouse or rat ; e, of guinea-pig ; /, of
kalong.

Trophoblast represented by continuous black lines or masses ; entoderm by interrupted lines ;
embryonic ectoderm by epithelial cells.

and it differs from the first only in holding that from the circumstances of the complete
imbedding of the ovum the embryo-amnio-genetic ectoderm is invaginated at a still earlier stage,
the blastocyst wall being folded over the formative ectoderm even before it is differentiated
into embryonic ectoderm.

Entypy of the germinal area. — In the rabbit ovum at this stage the
trophoblast is reduced to a thin sheet of flattened cells, against which the cells of the
formative cell-mass spread themselves out at an early stage (fig. 36, d, p. 26).
After the formation of the entoderm, the cells between that layer and the tropho-
blast form a plate of columnar cells, the embryonic ectoderm (fig. 41, a), which is
flat from the first, and directly applied to the covering layer of trophoblast (here

ENTYPY OF THE GERMINAL AREA 31

called Rauber's layer). No amnio-embryonic cavity appears between them. Soon
Rauber's layer disappears, and the embryonic ectoderm lies free on the upper
pole of the blastocyst. In some mammals, such as the mole, pig, and Tupaja, the
germinal area is for a short time distinctly inverted, as in Tarsius ; but the phases
resulting in the opening out of the blastoderm on to the surface are even more
distinctly seen than in that animal. In the mole, for instance (fig. 4-2, 6), the
cavity which hollows out the heart of the formative cell-mass is larger and deeper,
and is roofed in by the trophoblast for a time. The embryonic ectoderm is at
first markedly concave, but this is very soon rectified by the straightening of the
plate ; and the roof of the cavity disappearing, a phase is reached exactly like
that described for the rabbit after the disappearance of Eauber's layer.

In another and considerable series of mammals, the inversion persists rather
longer, and the cavity never opens out on the surface of the blastocyst. but remains
roofed in by the trophoblast layer. This condition was named by Selenka
' entypy of the germinal area.'

FIG. 43. — EMBRYONIC AREA OF MOLE IMMEDIATELY PRIOR TO APPEARANCE or PRIMITIVE STREAK

AND FORMED OF TWO LAYERS ONLY.

FIG. 44. — EMBRYONIC AREA OF MOLE, SHOWING THE PRIMITIVE STREAK AND GROOVE ENDING
POSTERIORLY IN A CRESCENTIC THICKENING.

The area is bilaminar in front, trilaminar in the posterior half.

FlG. 45. — A SOMEWHAT LATER STAGE IN WHICH THE PRIMITIVE STREAK REACHES TWO-THIRDS OF THE
LENGTH OF THE EMBRYONIC AREA, AND ENDS BEHIND IN A KNOB OR THICKENING.

(Figs. 43, 44, and 45 are copied from Heape. They are magnified 49 diameters.)

There are a number of variations in the manner in which the condition manifests itself.

(A) In some of the bats (fig. 42, c) and in the hedgehog the cavity remains roofed in by
trophoblast, and persists as the amniotic cavity, the walls of the definitive amnion being formed
not by the folds as in the other group, but by upgrowth of cells on the inner surface of the
covering trophoblastic layer.

(B) In mice and rats (fig. 42, d) the trophoblast over the formative cell-mass becomes greatly
thickened and invaginated into the interior of the blastocyte, necessarily pushing the mass before
it. A cavity appears in this mass of trophoblast (false amniotic cavity), which becomes continuous
with the primitive amnio-embryonic cavity. The whole blastocyst becoming tubular, the germinal
layers appear reversed, the entoderm being external to the ectoderm. In the further course of
development this persistent inversion of the germinal area is rectified by the tardy straightening
of the blastoderm and opening out of the amniotic cavity.

(C) In the guinea-pig (fig. 42, e) the blastocyst is drawn out into a tubular shape just as in
rats and mice, and the formative cell-mass is inverted in the same fashion into its cavity.
The placental thickening of the trophoblast is not, however, invaginated to the same degree as
the formative cell-mass, so that direct connexion between them is lost. Accordingly, when the
amnio-embryonic cavity is formed, its roof is independent of the trophoblast ; it never opens
into the cavity in the interior of the trophoblast plug (false amniotic cavity), and it becomes
the definitive amnion.

32

FORMATION OF MESODERM

(D) In the kalong (an East Indian bat, Pteropus edulis) (fig. 42, /) the condition is much the
same as in the guinea-pig, but the blastocyst remains rounded ; there is no invagination of the
trophoblast ; no false amnionic cavity ; and there is not much greater inversion of the layers
than occurs in the bats.

In apes and man (fig. 40) some such condition as in Pteropus in all probability exists ; but
there is this difference, that, owing to the great expansion of the trophoblast-shell and to the
tardy formation of the entoderm, there is from the first a space between the trophoblastic wall
of the blastocyst and the entodermic sac.

Formation of the trilaxninar blastoderm : mesoderm and em-
bryonic axis. — The mode of the formation of the middle layer in the Primates
varies in certain important particulars from that generally regarded as typical.
It will be convenient to give first a brief general description of the mode of
formation of the mesoderm in one of the lower mammals — e.g. the mole or rabbit.

The germinal area, at the stage now reached, is a circular disc on the upper
pole of the blastocyst (fig. 43). By unequal growth the disc becomes oval, and at
its smaller end a linear shading appears which is produced by a keel-like thickening

FIG. 46, A AND B. — VIEWS OP THE EMBKYONIC AREA OF THE BABBIT, SHOWING TWO STAGES IN THE

EXTENSION OF THE MESODERM. (Kolliker.)

In A the mesoderm extends on either side of the primitive streak over the posterior part of
embryonic area, and also behind the primitive streak beyond the limits of that area.

In B the mesoderm extends over a circular area which surrounds the embryonic area. The
embryonic area is also trilaminar, except in the middle line in front of the primitive streak.

of the ectoderm, known as the primitive streak (figs. 44 and 45). The first part
of the streak to appear is a knob-like thickening which forms its head (Hensen's
knot). From the primitive streak cells are budded off into the space between
ectoderm and entoderm. They form a loosely arranged layer of branched elements
named the mesoderm. The ectodermic thickening, at first separate from the
entoderm, quickly fuses with it, so that all three layers are continuous in the
primitive streak. By constant proliferation the mesoderm spreads round the
wall of the blastocyst until finally it entirely surrounds it. It will be noticed
that at first the extension is mainly backwards in a continuous sheet behind
the germinal area (fig. 46). This portion of the layer takes no part in the forma-
tion of the embryo, but is concerned in the laying down of the peripheral
mesoderm of the future vascular area. Within the germinal area the sheet is
divided into two lateral wings, separated by the primitive streak from which they
spring (fig. 58, p. 40 ; fig. 66, p. 44 ; and fig. 47). As the mesoderm continues to
spread, the embryo begins to take form in front of the primitive streak, and the
lateral wings of the mesoderm are found extending forwards on each side of a

FORMATION OF MESODERM

33

plate of cells which is the rudiment of the notochord (fig. 66, V. p. 44). In front of
the embryonal axis, in most lower mammals, there is an area named the pro-amnion,
into which the mesoderm does not spread until a later period, and where therefore
the blastoderm is formed merely of ectoderm and entoderm.

p.st

FIG. 47. — DIAGRAM TO ILLUSTRATE THE SPREAD OF THE MESODERM FROM THE PRIMITIVE

STREAK, p.st., IN A TYPICAL LOWER MAMMAL. (T. H. Bryce.)

In a the mesoderm has not yet spread round the entodermic sac, and is undivided ; in b it has
completely surrounded the blastocyst and is divided into somatopleuric and splanchnopleuric layers.
ect, mes, ent, ectoderm, mesoderm, entoderm.

The mesoderm sheet now splits on either side into two lamellae — parietal and
visceral (fig. 47). The parietal layer adheres to the ectoderm, and forms with it the
somatopleure ; the visceral becomes associated with the entoderm, and with it

FIG. 48. — DIAGRAM TO ILLUSTRATE A STAGE IN THE
DEVELOPMENT OF THE MESODERM LATER THAN
IN FIG. 47 b, AND THE FORMATION OF THE

AMNION, IN A TYPICAL LOWER MAMMAL. (T. H.

Bryce.)

">n, am, amnion folds; cce, ccelom; som,
somatopleure ; spl, splanchnopleure ; ent , ento-
derm of yolk-sac, y.s.

FIG. 49. — DIAGRAM TO SHOW A LATER STAGE
IN DEVELOPMENT OF THE AMNION AND
YOLK-SAC THAN IN FIG. 48. (T. H.
Bryce.)

a »i, amnion, now closed ; cce1, intra-embry-
onic ccelom; coe-, extra-embryonic cgelom;
som, somatopleure ; spl, splanchnopleure ; ent,
entoderm of yolk-sac, y.s.

constitutes the splanchnopleure. The splitting first takes place in the embryonic

area, and spreads outwards until in certain forms it completely separates the

blastocyst wall from the entodermic vesicle, now called the yolk-sac. The space

VOL. i. D

34

CONNECTING- STALK

between the layers is named the coelom. As the embryo takes form, it sinks
down, as it were, into the blastocyst, and a fold of the somatopleure comes to
overlap it all round. The edges of the fold ultimately meet over the embryo and
enclose the cavity of the amnion (figs. 48 and 49).

The special feature about the formation of the middle layer in the Primates l is
that the whole extra- embryonic mesoderm is formed, at a relatively early stage,
before the primitive streak has appeared on the germinal disc. The precocious
mesoderm consists in part of cells budded off from the posterior edge of the
germinal area, which spread round the wall of the blastocyst and over the yolk-sac
(figs. 50 to 53). The tissue formed is a loosely arranged layer concerned in the
formation of the lining of the trophoblast and covering of the entodermic
sac, and from the earliest period' it forms a thick stalk of connexion (Haftstiel ;
embryophore) between embryonic area and trophoblast. This stalk becomes
vascularised at an early date, independently of the allantois, and constitutes at this

stage a formation peculiar to the
Primates. In respect of its fate
it may be considered as corre-
sponding to the mesoderm which
extends from the posterior end
of the primitive streak behind

: ,,s the germinal disc in a typical

lower mammal.'2

The actual origin of this early
mesoderm is unknown in man. In
Tarsius it is derived from the ecto-
derm (Hubrecht). In Semnopithecus
nasicus Selenka considers it to be of
entodermic origin, but as it comes
from the same region of the disc
as in Tarsius, it is possible that an
earlier stage would exhibit appear-
ances susceptible of a similar inter-
pretation to that of Hubrecht.

FIG. 50.— SECTION (DIAGRAMMATIC) OF EAKLY EMBRYO OF
TARSIUS SPECTRUM. (After Hubrecht.)

ect, embryonic ectoderm ; T/..S., yolk-sac; c.s., connecting
stalk ; p, thickened trophoblast = ectoplacenta.

The blastocyst ia not completely imbedded in the
uterine mucosa, and only a portion of the trophoblast
therefore takes part in the formation of the placenta. The
' ventral ' mesoderm covers only the posterior surface of
the- yolk-sac.

While in all the Primates a
connecting stalk is a distinctive
feature of the early stages, there are some differences between the conditions in
Tarsius and those in apes and man, determined by the manner in which the ovum is
imbedded, and by the mode of formation of the amnion (cf. figs. 50, 52, and 53). In
Tarsius the ovum is not completely imbedded in the uterine mucous membrane,
and the amnio-embryonic cavity is early opened out ; only a portion of the wall of
the blastocyst is thickened to form the placenta, and the mesoderm passes straight
back from the hinder end of the embryonic plate to it, and covers, at first, the
posterior wall of the yolk-sac only. Owing to the maintenance of the primary inver-
sion in the higher Primates, on the other hand, the mesoderm is, in them, conducted
to the blastocyst- wall by the ' amnion stalk ' (figs. 52 and 53). It surrounds both
yolk-sac and amnion, so that the embryonic rudiment with its two cavities hangs
from the wall of the vesicle completely imbedded in the early mesoderm. The

1 The origin of the mesoderm in the primate blastoderm has been studied in detail only in Tarsius
by Hubrecht. The following account is mainly founded on his description.

• The early mesoderm arising from the hinder border of the germinal area in Tarsius is named
by Hubrecht the ventral mesoblast, because of the theoretical relationship he believes it to bear to
the mesoderm of the ventral lip of the blastopore in the Amphibia. He uses the term ' mesoblast ' in
preference to ' mesoderm,' as he does not place the middle layer complex on the same morphological
plane as the ectoderm and entoderm Riickert (Hertwig's Handbuch, i. Part II.) has, on similar
grounds, adopted the term .ven tral mesoderm to signify the peripheral mesoderm, which appears first, and
springs from the hinder end of the primitive streak in the lower mammals and Amniota generally.

PRIMITIVE STKKAK

35

extra-embryonic ccelom is also occupied by delicate strands of the same tissue
stretching between the yoik-sac and wall of the blastocyst. These constitute what
has been named the magma reticulare.

While this loose mesoderm is developing, the entoderm, forming the roof of the
entodermic sac, becomes in Tarsius thickened into a many-celled plate (fig. 51). This
plate is produced by proliferation from the entoderm (Hubrecht), and is continuous
at the margins of the disc with the yolk-sac mesoderm, in the formation of which it
seems to share. It is therefore a second source of mesoderm- cells, quite independent
of the earlier one. It is named by Hubrecht the protochordal plate, but it will be
here referred to as the primitive entodermic plate, to avoid any theoretical
implication.

In Selenka's figure of a blastoderm of Semnopithecus nasicus (fig. 54) the mesoderm is repre-
sented at this stage as extending forwards into the disc from its hinder end. It would therefore
seem to be derived from the same source as
the mesoderm of the connecting stalk, which,
however, as already said, he refers to the ento-
derm. In Peters' and in Graf v. Spee's early
human ova a similar layer is seen (fig. 55).

From this description it will be gathered
that the ectoderm and entoderm are everywhere
separated in the Primates by a middle layer
before there is any sign of a primitive streak.
Although different from the conventional ac-
count of the origin of the mesoderm, there is no
doubt that the facts are as stated. Their
theoretical significance will be dealt with in a
later paragraph.

Up to the stage now reached, only
that part of the three-layered blastoderm
which we may call the head-plate, because
it will form the extreme head end of the
embryo, is laid down, and we have next
to describe a series of stages by which the
embryonic axis, forming the trunk, is
developed. As in the typical case de-
scribed above, the germinal disc enlarges
(fig. 56), and Hensen's knot (protochordal
knot, Hubrecht) appears as a thickening
of the ectoderm. This thickening extends
inwards and forwards between the two
primary layers (fig. 57), and is continuous
with the thickened entodermal plate in
front. It then becomes fused with the
entoderm on its under aspect. The primi-
tive streak next appears as an extension
backwards of the ectodermic thickening.

that from its sides, as well as from Hensen's knot, wing-like masses of mesoderm
extend laterally between ectoderm and entoderm (fig. 58). They are formed from
cells budded off from the streak, and from this period onwards the new mesoderm
of that part of the blastoderm which lies between the head-plate in front and the
connecting stalk behind, and which gradually increases in length as the embryonal
axis is laid down, may be considered as arising from this source.

Hubrecht describes in Tarsius a tract of middle-layer cells springing from the
•entoderm over a ring-shaped area, continuous in front with the primitive middle-

D2

V >

FIG. 51. — MEDIAN LONGITUDINAL SECTION
THROUGH THE EMBRYONIC PLATE AND YOLK-
SAC OF TAKSIUS AT THE SAME STAGE AS

IN FIG. 50, MORE HIGHLY MAGNIFIED. (After

Hubrecht.)

emb. ect.y embryonic ectoderm ; pp, primitive
entodermic plate ; y.s., yolk-sac ; mes, ventral
mesoderm.

Sections of the primitive streak show

36

PRIMITIVE STREAK

layer cells of the head-plate and passing behind on to the wall of the yolk-sac. The
ring is closed behind when the hind-gut becomes cut off from the yolk-sac. It is
apparently directly related to the development of the blood-vessels. A similar ring

FIG. 52. — HYPOTHETICAL STAGE OF THE HUMAN OVUM IMBEDDED IN THE DECIDUA, SOMEWHAT
YOUNGER THAN PETERS' OVUM, THE TROPHOBLAST IS GREATLY THICKENED, AND LINED WITH
MESODERM, WHICH SURROUNDS ALSO THE EMBRYONIC RUDIMENT, WITH ITS YOLK-SAC AND AMNIO-
EMBRYONIC CAVITY. (T. H. Bryce,)

The embryonic rudiment is proportionally on too large a scale.

was described by the same observer in the shrew, and by Bonnet in the sheep,
but it has not been found in other mammals.

The primitive streak (although this feature is not marked in any of Hubrecht's figures of
Tarsius) becomes deeply indented by a furrow called the primitive groove, and it is to be observed

embryonic ectoderm
\

ectode

mesoderm

r<fer?x-«r--™ — . _ chorion

yolk-sac

amnion
connecting stalk

allanlois

FIG. 53. — MEDIAN LONGITUDINAL SECTION OF AN EARLY HUMAN OVUM 0'4 MM. IN LENGTH.
(After Graf v. Spee, from Kollmann's Entwickelungsgeschichte). x 27 diameters.

that in Graf v. Spec's embryo of 2 mm. (fig. 59), the mesoderm-sheets are represented as
partially subdivided into two lamellse, one connected with the ectoderm, the other with the
entoderm. This feature, also seen in a number of lower forms, has been interpreted by Hertwig
as a vestige of an original development of the mesoderm by ccelomic pouches (see p, 46).

EMBEYONIC AXIS AND NOTOCHOKD-PLATE

37

The growth of the blastoderm at first chiefly affects the region behind Hensen's
knot. The primitive streak elongates and occupies the pointed end of the oval
disc which is now generally called the embryonic shield (fig. 56, b and c).

The next phase is characterised by a shifting of the maximum growth- activity
to the part of the shield in front of Hensen's knot, and the embryonic axis begins
to be laid down from before backwards, either by proliferation of the cells at the
anterior end of the primitive streak, as some think, or by gradual conversion of
the growing streak into the axis, as is indicated by experiments on the developing
blastoderm.1 As the portion of the shield in front of the streak increases in length,
the mass of cells extending from Hensen's knot (protochordal process), already
described, is necessarily lengthened into a column of cells (figs. 60, 61). It
becomes, however, at the same time flattened out into a plate ; and as it is fused

am emliy. ect.

y.s.

FIG. 54.— SECTION OF AN EABLY EMBRYO OF SEMNOPITHECUS NASICUS. (After Selenka.)
am, amnion; emby, ect., embryonic ectoderm; ?/.s., yolk-sac; ent, entoderm ; mes, mesoderm
covering yolk-sac and extending between embryonic ectoderm and entoderm ; con. stk., connecting-
stalk mesoderm.

with the primitive entoderm on its under aspect, the result of this flattening and
opening out is, that it comes to roof-in the cavity under the shield along the
middle line. This plate is named the notochord- plate ,2 and is recognisable in
some cases as a shading in front of the streak in surface views of the blastoderm
(fig. 63). A section of the shield (fig. 64) shows that the plate is continuous on
each side with the general entoderm, and, where plate and entoderm join, also
with wings of mesoderm which spread outwards between ectoderm and entoderm.
In front the plate passes into the thickened primitive entodermic plate, and
behind into the mass of cells at the head end of the primitive streak, out of

1 See Assheton, Proc. Hoy. Soc. 1896, also Anat. Anzeiger, xxvii. ; Kopsch, Verhl. d. fiinfte
internat. zool. Kongress, Berlin, 1901, and Internat. Monatschr. f. Anat. und Phys. xix. ; Peebles,
Archiv f. Entwickelungsmech. vii.

" It has been also termed the archenteric plate (Urdarmplatte, Bonnet), for reasons which will be
given later on.

38

EMBKYONIC AXIS

mes3

FIG. 55.— TKANSVEBSE SECTION THROUGH AN EARLY HUMAN EMBRYO OF (H MM.
(Graf v. Spee ; cf. fig. 53.)

am, aninion ; Dies'1, mesoderm of amnion ; ect, embryonic ectoderm; ent, entoderm ; mes2, meso-
derm of yolk-sac ; mes*, scattered cells between ectoderm and entoderm of germinal disc.

V.,
a

FIG. 56.— SURFACE VIEWS OF GERMINAL DISC OF TARSIUS SPECTRUM AT THREE SUCCESSIVE
STAGES. (After Hubrecht.)

In a the dark spot represents Hensen's knot (protochordal knot, Hubrecht). As the pointed end of
the disc extends backwards, the thickening known as the primitive streak develops on it, c.

NOTOCHOBDAL CANAL

39

which it is continually being added to. The wings of mesoderm springing from
the sides of the notochord-plate are of course continuous with those arising
from the sides of the primitive streak.1 As the axis extends, the streak mesoderm
may be conceived as continually becoming converted into axial mesoderm.

Through this thickened head of the streak a fissure has meanwhile appeared,
which becomes converted into a short canal directed obliquely forwards, and
opening into the cavity of the yolk-sac (figs. 61, 62, 63, ' 65). It .is called the
notochordal or neurenteric canal. In apes (figs. 62 and 63) the canal is considerably
wider than in Tarsius, and it is a very prominent feature in early human embryos
(figs. 65, 72). In some lower mammals — the rabbit, for instance — the canal does
not break through at any stage, although it is well seen in others — for example, in
the mole (fig. 64). In the higher Primates the canal is present at a very early
stage — considerably earlier than in Tarsius ; and it may also be mentioned here
that the primitive streak seems to be comparatively short in all the Primates
(figs. 62, 63, 65). The canal tunnels through the mass of cells in which the

FIG. 57.— MEDIAN LONGITUDINAL SECTION THROUGH A BLASTODERM OF TARSIUS AT A STAGE ABOUT
THE SAME AS REPRESENTED IN FIG. 56, a. (After Hubrecht.)

p.k., Hensen's (protochordal) knot; emb. ect., embryonic ectoderm; T/.S., yolk-sac; pp, primitive
entodermic (protochordal) plate : c.s., ventral mesoderm (connecting stalk).

notochord-plate ends, and it has been given the name ' notochordal canal '
because of its relations to that plate and the notochord which is formed from it.
The term ' neurenteric canal ' signifies properly only the persisting posterior portion
of the notochordal canal, this name being given because it corresponds, when the
neural groove is formed, to the neurenteric canal of lower vertebrates. It has
been already explained that at an earlier stage the protochordal process becomes
fused with the entoderm on its under aspect, and that the column of cells arising
from the process opens out into a plate, and thus comes to roof -in the cavity
under the shield along the middle line. The final result of this process,
therefore, is the same as it would have been had the notochordal
canal tunnelled the whole length of the protochordal column of
cells, and had then broken through into the cavity of the yolk-sac

1 Hubrecht leaves it open whether these lateral sheets are formed in part or in whole from the
protochordal process. Bonnet, in the dog, represents the facts as described in the text. The point is
important theoretically, but not from a mere descriptive point of view.

III.

V.

FIG. 58. — SEBIES OP TRANSVERSE SECTIONS THROUGH THE GERMINAL DISC OF TAHSIUS AT A STAGE

SOMEWHAT EARLIER THAN THE DISC REPRESENTED IN FIG. 56, b. (After Hubrecht.)

Section I. passes through the blastoderm in front of Hensen's (protochordal) knot, and shows only
the ectoderm and primitive entodermal plate. Section II. cuts the head end of the protochordal
process where it is continuous with the primitive entodermic plate. Sections III. and IV. pass through
Hensen's knot, which is seen in V. tapering away into the primitive streak. In III., IV., and V. the
mesoderm-sheets are seen springing from the keel-like thickening of the ectoderm, which in III. and
IV. is observed to be continuous into the entoderm.

pr.st.
FIG. 59. — TRANSVERSE SECTIONS THROUGH THE NEURENTERIC CANAL (NO i.) AND PRIMITIVE

STREAK (NO. n.) OF A HUMAN EMBRYO OF 2 MM. (FIG. 72, P. 49). (After Graf v. Spee.)

e ct, ectoderm ; mes, mesoderm ; ent, entoderm ; pr. st., primitive streak. The mesoderm-sheet

springing from the streak show two lamellae.

SPLITTING OF MESODEIOI

41

by the splitting apart of its floor. Such a process actually takes place
in reptiles, and in an abbreviated fashion in the bats among mammals (figs. 67
and 68, p. 4~>).

Looking back to the complicated series of changes described above, we may divide the
period of the appearance of the mesoderm into two stages. In the first the mesoderm of the

FIG. 60. — MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF TABSIUS SOMEWHAT LATER

THAN THAT SHOWN IN FIG. 57. (After Hubrecht.)

p.A;.,.Hensen's (protochordal) knot continued backwards into the thickening of the primitive streak ;
g.d., germinal disc ; ect, ectoderm of somatopleure ; mes1, mesoderm of ditto ; coe, cceloni ; p.p., primitive
entodermic i protochordal) plate, continuous with _p.^r., protochordal process ; ent, entoderm; all, rudi-
ment of allaiitoic diverticulum ; mes2, mesoderm of splanchnopleure.

connecting stalk (yolk-sac and amnion) and the head-plate is laid down. It is a continuous
and unpaired sheet, and is at no time segmented. In the second phase the primitive streak
appears, and the mesoderm now springs from its sides as bilateral sheets ; the embryonic axis
is differentiated out of the proliferating streak-tissue, the notochordal process and plate take
form, and from the sides of the plate the mesoderm continues to be thrown off as the dorsal
segmented mesoderm of the embryonic body.

per

FIG. 61. — MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF TARSIUS AT A

LATER STAGE THAN THAT SHOWN IN THE LAST FIGURE. (After Hubrecht.)

n.c.. neurenteric canal ; p.p., primitive entodermic (protochordal) plate ;
p-pr., notochordal plate ; am1 anterior, air? posterior amnion fold ; all, allantoic
diverticulum ; per, pericardial ccelom.

At the period now reached the blastoderm has the appearance presented in
fig. 63. Sections demonstrate that the mesoderm everywhere forms a continuous
layer. Within the shield it is still undivided, but outside the shield it spreads as
two lamellae from the edge of the disc, the somatopleuric lamella surrounding the
amnion and lining the wall of the blastocyst, and the splanchnopleuric enveloping
the yolk-sac.

42

GASTKULA THEOKY

The gastrula theory and the mammalian ovum. — It is impossible here to deal
at length with the gastrula theory, because the evidence can only be satisfactorily presented
by extensive comparative treatment. A short statement must therefore suffice.

not.pl.

p.s.

all

FlG. 62. — SUBFACE VIEW OF A
BLASTODEBM OF CEBCOCEBUS

CYNOMOLGUS. (After Selenka.)

n.c., neurenteric canal ; p.s.,
primitive streak.

FIG. 63. — SUBFACE VIEW OF A BLASTODEBM OF
HYLOBATES CONCOLOB. (After Selenka.)

The amnion has been opened to expose the
germinal disc.

am, cut edge of amnion ; y.s., yolk-sac ; not.pl.,
notochord-plate ; n.c., neurenteric canal ;
2>.s., primitive streak ; all, allantoic diverti-
culum in connecting stalk.

It will be observed that in the mammal the two primary layers of the blastoderm, at least
their principal part, are formed by a separation into two strata of the cells of the inner granular
mass, which occupies the interior of the ovum after segmentation. The bilaminar condition

n.f.

ent not.pl. ent

FIG. 64. — TBANSVEBSE SECTION THBOUGH BLASTODEBM OF DOG. (After Bonnet.)
n.p., neural plate ; n.f. neural folds; not.pl., notochordal plate ; ent, entoderm ; mes, mesoderm.

may therefore be said to result from a process of delamination in an originally simple
mass or stratum. But in Amphioxus amongst vertebrates, and in many invertebrates with
holoblastic (alecithal) ova, the bilaminar blastoderm is produced not by delamination, but by

GASTKULA THEORY

43

the iuvagiiiation of one pole of an originally simple hollow sphere — the blastula — the invaginated
portion becoming the primitive entoderm and the remaining part of the wall of the vesicle
forming the primitive ectoderm (fig. 69). This condition, which was discovered by Kowalewsky,
is known as the gastrula stage, and it is regarded by many embryologists, following Haeckel,
as typical of the mode of formation of the bilaminar blastoderm throughout the animal
kingdom. The aperture by which the cavity of the gastrula, whether formed by delamination
or invagination, communicates for a time with the exterior has been termed the blastopore
(Lankester).

There are not wanting observations in mammals pointing to the existence in the bilaminar
blastoderm of a blastoporic aperture. Thus in the mole Heape figures an aperture just before
the primitive streak appears (fig. 70). Similar figures have been published, though for rather
earlier stages, by Selenka in the opossum, by Hubrecht in the shrew and hedgehog, by Keibel in
the rabbit, and by Bonnet in the dog ; while in Tarsius the narrow slit in fig. 57, p. 39, may
have the same significance (Hubrecht). In other mammals, however, no such aperture has

vilii of chorion

notochordal
plate

heart

mesoderm -

^SL chor-ion

-';—• mesoderm

"~ embryophore
"~ primitive streak

allaiitois

— yolk-sac

entoderm

*~ vessels

Fiu. 65. — MEDIAN LONGITUDINAL, SECTION OF THE HUMAN OVUM BEPBESENTED IN FIG. 72, p. 49.
(After Graf v. Spee, from Kollmann.)

The wide and vertical neurenteric canal is seen opening from the amniotic cavity into the yolk-sac.

been seen at this stage, and the opening in all cases has only a transitory existence. It is
doubtful how far it corresponds to the blastopore of a gastrula derived from a holoblastic ovum,
the whole of which is utilised for the formation of the embryo.

The accumulation of yolk in the egg profoundly modifies the process of gastrulation. Thus in
the amphibian holoblastic egg, owing to the character of the segmentation, a considerable proper -
t ion of the yolk-laden entomeres are already within the blastula, and the invagination phenomena
by which the so-called gastrula is completed, represent a second phase of gastrulation concerned
in the formation of a dorsal plate which forms the dorsal wall of the trunk with the notochord
and segmented mesoderm. During the development of the plate the gastrula cavity (archenteron)
is formed by a partial invagination, and the breaking through of the space so produced into the
cavity of the blastula. The archenteron is slit-like, because the gastrula is almost entirely filled
by the entomeres (segmented yolk). In the lower Amniota the animal pole of the telolecithal egg
alone segments. The germinal disc in its early phases represents a small portion of the blastula-
wall, which is not theoretically completed tilt long afterwards, when the blastoderm has grown

44

GASTRULA THEORY

III. -

FIG. 66. — A SERIES OP TRANSVERSE SECTIONS THROUGH THE NEURENTERIC CANAL OF A MOLE

EMBRYO. (Heape.)

The dorsal opening is shown in I., continued into the primitive groove ; the canal passes thence
through the head end of the primitive streak (II.) into the thickened posterior end of the notochord-
plate (III.), along which it extends for some distance (IV.), and eventually opens ventrally in a
median groove (V.).

ec, (me, en, ectoderm, mesoderm, entoderm ; p.gr. (in I. and II.), primitive groove; c, neurenteric
canal ; m.gr. (in III., IV., and V.), medullary groove.

GASTKULA THEORY

45

over the volk. The entoderm is produced by delamination (first stage of gastrulation). It is from
the first entirely within the theoretical bJastula, which is completely filled, as it were, by the ento-
meres and the great mass of unsegmented yolk. Owing to the profoundly altered conditions, the
second phase of gastrulation is further modified and abbreviated, as will be explained presently.

In the Mammalia the egg is holoblastic, but the blastocyst does not represent a blastula
like that of Amphioxus, but is rather like the theoretical blastula of one of the lower Amniota,
empty of yolk and with one portion of its wall invaginated. The entoderm is formed by
delamination, but when the embryonic ectoderm-plate is differentiated a circular blastopore
such as is seen in none of the lower Amniota in some cases breaks through.

Now, analysing the early appearances in the primate blastoderm in terms of the gastrula
theory and in the light of the above interpretations, the posterior edge of the germinal area

FIG. 67. — MEDIAN LONGITUDINAL SECTION OP A BLASTODERM OP VESPERTILIO MUBINUS, SHOWING THE
NOTOCHORDAL CANAL BEFORE ITS OPENING. (Van Beneden.)

n.c., posterior opening of notocho rd- canal ; n'.c',, its anterior opening ; x>.s., primitive streak ;

n.p., notochord-plate.

from which the mesoderm first springs would represent the ventral lip of the blastopore, and
Hensen's knot its dorsal lip. Hensen's knot, however, signifies the commencement of active
changes in the dorsal lip which are concerned in the formation of the embryonic axis, and before
these are initiated a continuous layer of unpaired, and always unsegmented mesoderm is laid
down from the lips of the blastopore, which has no part in the embryo except at its extreme
head end. The formation of the primitive streak, during which the posterior border of the
disc or ventral lip is carried away from Hensen's knot by the growth of the pointed posterior
end of the shield, would signify the drawing out of the gastrula-mouth into a lineal aperture of
which the lips are fused. The second phase of gastrulation is now entered on, during which a
true invagination, but of a modified character, appears to occur. It is associated with the
backward growth of the dorsal lip and the laying down of the dorsal plate (embryonic axis)

n'.c'.

o.n.

FIG. 68.— MEDIAN LONGITUDINAL SECTION OF A BLASTODERM OF VESPERTILIO MUBINUS, SHOWING THE

NOTOCHORDAL CANAL AFTER THE BREAKING THROUGH OF ITS FLOOR. (Van Beneden.)

c.n., neurenteric canal ; c, anterior persisting portion of the notochordal canal ;
other letters as in fig. 67.

in front of it, and the re-establishment of the blastopore in the formation of the notochordal
canal.

In the Reptilia processes are observed which throw a light on the much more modified
phases in mammals, and can again be linked on to the processes seen in those amphibian eggs
which are heavily yolked. On the posterior border of the germinal area a thickening appears
called the primitive plate. On this a slight pocket appears, marking a spot where ectoderm
and entoderm are indistinguishable. The appearances are suggestive of proliferation of cells
to form the entoderm, but there is no true invagination. Later (second phase of gastrulation),
at the anterior end of the primitive plate the pocket deepens to form a true invagination, which,
as the dorsal lip increases in length to form the dorsal plate, becomes extended into a canal.
The floor of the canal fuses with the primitive or yolk entoderm. and then breaks through, so

4(> GASTRULA THEORY

that the cavity under the developing dorsal plate (embryonic axis) is roofed-in by the upper
wall of the canal and opens behind on to the surface of the primitive plate by the mouth of the
original invagination or blastopore. In some Beptilia this invaginated canal is wide, in others
narrow ; but it clearly corresponds to the solid protochordal process of mammals which is
tunnelled in its later stages, and in most forms only at its posterior end, by the notochordal
canal, which opens on the surface as the blastopore. It is a common experience in embryology
to find a developmental process modified, in the sense that a hollow rudiment is replaced by
a solid rudiment which is afterwards hollowed out.

The space under the embryonal axis, formed in the manner described, may be taken, still
following this conception, as representing the dorsal part of the archenteron of Amphioxus

FIG. 69. — FOUR STAGES IN THE DEVELOPMENT OF AMPHIOXUS, ILLUSTRATING THE FORMATION OF THE

GASTRULA. (Hatschek.)

I. Spherical blastoderm ; the cells at the lower pole are larger than the others, and filled with
granules.

II. Invagination of the lower pole producing a cupping of the vesicle.

III. Completion of the invagination ; the blastoderm is now bilaminar, and forms a cup with
narrowed mouth, the blastopore, bl, and a double wall of ectoderm, ec, and entoderm, en.

IV. The ovum is now elongated ; the cavity of the gastrula forms a primitive alimentary canal, the
orifice of which is the blastopore, which is directed dorsally. Extending from this along the dorsal
surface (right in the figure) a shallow groove is seen in optical section : this is the rudiment of the
nervous system.

(fig. 71); and just as in that form, the entoderm of the roof gives origin to the notochord, while
the axial mesoderm arises from its lateral diverticula (ccelomic pouches).

Van Beneden has shown that the floor of the notochordal canal persists for a time in the
bat; but in the Primates, and most other mammals, there is nothing to indicate the true
nature of the developmental phases, the whole series of phenomena being repeated in a still
more abbreviated form. Eternod ' has, however, observed in the early human blastoderm the
occurrence of scattered cells adhering to its under aspect, or to the lip of the neurenteric
canal, which he regards as traces of the floor of the archenteric sac or canal.

1 Anat. Anzeiger, xvi. ; Compt. rend, de 1'Assoc. des Anat., 7e reunion, Geneva, 1905; ibid.
&c Reunion, 1906 ; Bibliograph. Anat. xv.

GASTRULA THEORY

47

The question here arises, What is the fate of the primitive streak ? Is it converted into the
embryonal axis as it increases in length, or is the growth of the embryo effected by a continuous
proliferation from a growth-centre at the head end of the streak ? Experimental data seem to
show that the portion of the blastoderm in front of the streak gives rise to the extreme head end
of the embryo, that the anterior end of the streak forms the rest of the head, while- the remainder
is converted into the trunk, the growth-centre being placed near its posterior end, and forming
ultimately the knob of undifferentiated blastema which gives rise to the tail.

FIG. 70.— LONGITUDINAL SECTION THROUGH THE MIDDLE LINE OF PART OF AN EMBRYONIC AREA IMOLEI

IX WHICH THE PRIMITIVE STREAK HAS BEGUN TO FORM. (Heape.)

The blastoderm is perforated in front of the (short) primitive streak (? blastopore, blp) ; a few
mesoderm-cells are seen anterior to the perforation ; ec, ectoderm ; en, entoderm ; p.s., primitive
streak.

Starting from these premises, and accepting the assumption that the streak represents
the nrastrula-mouth drawn out, the axial increase from before backwards has been interpreted
as signifying the fusion of the lips of the blastopore postulated by the concrescence theories of
His. Minot, and Oscar Hertwig. According to this conception, the bilateral symmetry of the
vertebrate has been brought about by the elongation of the radial gastrula into a cylinder, and
the fusion along the dorsal aspect of the lips of the gastrula-mouth. The fusion takes place from
before backwards, and is manifested by the apparent backward growth of the dorsal lip of the
blastopore, as the embryonal axis is laid down in front of it. This process results in the closing-in

cct

FIG. 71.— SECTIONS ACROSS AN AMPHIOXUS EMBRYO. (Hatschek.)

».f/., neural groove ; n.c., neural canal ; ch, rudiment of notochord ; mes. sow., mesodermic somite. In
I. its cavity is in free communication with the alimentary cavity ; ect, ectoderm ; ent, entoderm ;
at, alimentary cavity. In III. the cavity of the somite has extended on either side of the alimentary
canal and forms a ccelom, or body-cavity (cte).

of the archenteron, the roof of which forms the dorsal aspect or axis of the embryo, with the
notochord and segmented mesoderm.

A considerable tide of opinion has in recent years set in in favour of a somewhat modified
conception of gastrulation. Keibel and Hubrecht ' in 1888 independently worked out the con -
ception of gastrulation in two phases; Hubrecht in 1902 named these two stages kephalogenesis
and notogenesis. The primary gastrula, formed in all Craniota by delamination, has a radial
symmetry and forms the fore-part of the head. The second stage of ontogeny, embracing the

See Hubrecht and Keibel, Quart. Jour. Micro. Sc. xlix. 1905.

48 FORMATION OF NEURAL CANAL

formation of the primitive streak and of the notochord, although involving invagination
phenomena, is not to be reckoned as part of the gastrulation process, but represents a
phylogenetic stage by which a radial ccelenterate form was converted into a proto-vertebrate
by the elongation of the gastrula and the formation of a dorsal plate which became the hoto-
chord. The primitive streak is thus not the gastrula-mouth of ontogeny, but represents the
protostoma of an Actinia-like form, as suggested by Sedgwick and Van Beneden.

Assheton's * theory also involves the acceptance of two ontogenetic phases : a first (which
may be exemplified by the earliest phases in Tarsius) resulting in the formation of the forepart
of the head, and a second represented by the formation of trunk and tail. The idea is that the
lips of the circular blastopore grow actively so as to produce a cylindrical gastrula. The dorsal
lip, however, grows more actively in vertebrates, and produces the back and ultimately the tail
or post-anal part of the axis. The anus represents the blastopore, while the mouth is a new
opening (like the gill-slits) into the alimentary canal. Such a process is greatly modified, of course,
in the Amniota, whether those with mesoblastic eggs or mammals, and the ventral lip of the
gastrula is greatly masked by the presence of the yolk-sac. According to this conception, the
primitive streak is only a phase in the development of the embryonic axis out of the growing
blastema of the blastopore-lip, or secondary ' growth- centre.' •

EARLY CHANGES IN THE BLASTODERM, RESULTING IN THE
FORMATION OF THE EMBRYO.

FORMATION OF THE NEURAL CANAL, NOTOCHORD, AND
MESODERMIC SEGMENTS.

Neural canal. — While the embryonic axis is developing, as described
above, a shallow groove appears on its surface in front of the primitive streak
(fig. 72). This elongates with the axis, and encloses, behind, the anterior end of
the streak with its neurenteric passage. Anteriorly and laterally it is bounded by
a fold of the ectoderm, the groove indeed being produced by the upgrowth of
the limiting folds (figs. 73 and 76). The thickened ectoderm of the groove is called
the neural plate, because the central nervous system is formed from it, and the
bounding folds are termed the neural folds.

By the continued upgrowth of the neural folds (fig. 73) the neural groove is
converted into a deep furrow, and ultimately, by their fusion in the mid-axial line,
into a closed canal (fig. 82, p. 57). The neural plate is then separated from the
surface-ectoderm, and the closed canal becomes isolated as the rudiment of the
cerebrospinal axis. The closure of the canal appears in the human embryo to
begin in the region of the future trunk of the embryo, and proceeds forwards
and backwards. The point where the final closure occurs in front is called the
anterior neuropore. When the neural canal closes posteriorly, the neurenteric canal
comes to lie in its floor, but it is obliterated at an early stage by the fusion of
its lips and soon completely disappears. The anterior end of the neural canal
extends beyond the notochord, and becomes enlarged to form the anterior of
three primary cerebral vesicles round which the brain is formed.

At the point where the lips of the neural folds meet, a mass of ectoderm-cells
forms a thickening known as the neural crest, from which, by a series of changes
afterwards to be described, the nerve-ganglia are formed.

Notochord. — It will be recollected that in last section we considered
the development of a plate of cells which we named the notochord-plate. We

1 Anat. Anzeiger, xxvii. 1905.

2 The literature of the germinal layers in mammals and man up to 1902 will be found fully given in
Hertwig, i. Part I. pp. 81 and 949. For a critical review of the earlier literature, see Born in Merkel
and Bonnet's Ergebnisse d. Anat. u. Entwickelungsgesch. i. 1891 ; and of the later literature, Keibel in
the same publication, x. 1901.

NOTOCHOBD

49

saw that, in the early stages, it lies under the neural groove (fig. 64), is directly
continuous on each side with the primitive entoderm, and at the points where it
joins with that layer also with the lateral sheets of mesoderm. By a process of
differentiation from before backwards, pari passu with the axial growth, the plate
now loses its connexion with the mesoderm-plates, although it continues to pass
directly into the lateral entoderm (fig. 76, III.). It next becomes converted
into a rounded rod of cells, at first continuous with, then detached from, an under-
lying layer of entoderm. The mechanism of this process is probably the doubling
up of the notochordal plate and the fusion of the lips of the groove thus formed in
the mid-axial line, just as in the case of the neural canal. The rod of cells thus
formed is the notochord (figs. 81 and 82). The anterior end of the notochord does
not reach to the anterior end of the embryo, but terminates in a recurved
point against the wall of the hypophysis cerebri (epithelial part of the pituitary
body) in the situation of the future body of the sphenoid bone, and close to the
dorsal attachment of the bucco-pharyngeal membrane '(see Development of the
Mouth). It will be seen, there-
fore, that a portion of the neural
canal is prechordal.

It would seem from the data given
for Tarsius by Hubrecht, and also for
the dog by Bonnet, that the anterior
or head end of the notochord is
formed by differentiation directly out
of the primitive entodermic plate
(fig. 78, p. 54) (protochordal plate,
Hubrecht; Ergcinzungsplatte, Bonnet).

The notochord is essentially an
embryonic structure in mammals,
although it does not completely dis-
appear, for traces of it are to be
found throughout life in the middle of
the intervertebral discs. When fully
developed it is a cylindrical rod com-
posed of clear epithelium-like cells,
enclosed within a special sheath of
homogeneous substance. These cells,
although they may become consider
ably enlarged and vacuolated, undergo
no marked histogenetic change and
take no part in the formation of any
tissue of the adult.

amnion

neural groove

neurenteric canal

primitive streak = —

abdominal stall' , -j.

FIG. 72. — SURFACE VIEW OF EARLY HUMAN EMBRYO, 2 MM.
IN LENGTH. (After Graf v. Spee, from Kollmann's
Entwickelungsgeschichte.) x 80 diameters.

The amnion is opened, and on the blastoderm are seen
the primitive streak, the dorsal opening of the neurenteric
canal, and the neural groove.

Later history of the
mesoderm : formation of
the mesodermic or primi-
tive segments and of the

coelonic — At the time when the neural groove is beginning to appear (figs. 73
and 76) a solid sheet of mesoderm extends outwards from the notochordal
plate between ectoderm and entoderm, to be continuous outside the embryonic
shield with the two layers of the extra-embryonic mesoderm. As the neural folds
rise, the central portions of these sheets expand to occupy the spaces, triangular
in section (fig. 73), which the folds enclose. These longitudinal thickenings
gradually thin off laterally into what is known as the lateral mesoderm (fig. 73).
They give origin to the voluntary muscular tissue of the body, and form what
may be termed the paraxial, as distinguished from the lateral mesoderm. These
paraxial thickenings now become cut up — by the occurrence at regular intervals,

VOL. i. E

50

MESODERMIC SEGMENTS

transversely across the mass, of a process of thinning — into a linear series of
small cubical masses (fig. 74), the mesodermic or primitive segments.1 The
first pair of these segments appears a short distance in front of Hensen's
knot (fig. 75), in what will ultimately become the junction of the head and
trunk of the embryo. They are produced in succession from before backwards,
being gradually added as the embryonal axis increases in length, until the full
number (thirty-five or more for the human embryo) is laid down. It has been
shown in lower forms that the earliest segment to appear is not the most anterior

IV

-

"• '•—.-.-««*••••• •

FIG. 73. — A SEBIES or TBANSVEBSE SECTIONS THBOUGH AN EMBBYO OF THE DOG. (After Bonnet.)
Section I. is the most anterior. In V. the neural plate is spread out nearly flat.

The series shows the uprising of the neural folds to form the neural canal.
ect, ectoderm; ent, entoderm; mes, mesoderm; so, segment ; c, intermediate cell-mass ; l.p., lateral
plate still undivided in I., II., and III. : in IV. and V. split into somatopleuric (sm) and splanchno-
pleuric (sp) lamellae ; p, pericardium ; h, h, rudiments of endothelial heart-tubes. In III., IV.. and V. the
scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vaso-
formative cells which give origin in front, according to Bonnet, to the heart-tubes (h) ; (a) aortae.

of the series, as a number of head-segments develop from behind forwards in front
of that first differentiating. In the human embryo there are probably three such.
The most anterior segment, in higher vertebrates, lies some distance behind the
head end of the notochord in the future occipital region, and there is no trace of
segmentation in front of this point.2 As the segments are being cut out of the

1 Formerly known as ' protovertebrae.' The term 'somite ' is also frequently employed to designate
them.

2 In Petromyzon and Selachians the mesoderm is segmented at least as far forwards as the
notochord extends ; the segments in front of the occipital region undergo retrogressive changes, and
disappear at an early stage.

FORMATION OF INTKA-EMBBYONIC CCELOM

51

paraxial mesoderm, each remains attached to the undivided lateral plate by
a continuous tract called the intermediate mesoderm or intermediate cell-mass
(fig. 73, III. c). According to Felix, this continuous tract is formed by the fusion,
At a very early stage, of the stalks of the segments. As the excretory ducts are
afterwards laid down in this tissue, it corresponds to those portions of the hollow
primitive segments which are named the nephrotomes in the Anamnia.

A cleavage has meanwhile taken place in the lateral mesoderm, dividing it into
a parietal and a visceral plate. The parietal plate is associated with the ectoderm
to form the somatopleure, and the visceral plate with the entoderm to form the

I

FIG. 74.— PHOTOGBAPH OP A CHICKEN EMBRYO, x 20 diameters. (T. H. Bryce.)

The mesodermic segments, eleven in number at this stage, are seen as small cubical masses on each
side of the axis of the embryo. The eleventh is still continuous with the unsegmented axial mesoderm,
which in turn passes behind into the primitive-streak mesoderm. The neural folds have not united,
and they embrace posteriorly the head of the primitive streak. The optic vesicles are prominent
lateral projections from the fore-brain ; the mid-brain vesicle is visible behind the fore-brain, but that of
the hind-brain is hidden by the tubular heart, which receives posteriorly the two vitelline veins from
the vascular area.

splanchnopleure. The space between the layers becomes the intra-em'bryonic
ccelom (body-cavity), and it follows that when the cleavage reaches the borders of
the shield the intra-embryonic will become continuous with the extra-embryonic
coelom, and the relations of the layers will be established which are reached at a
much earlier stage in lower mammals (fig. 77 ; cf. fig. 49, p. 33).

The segments now also show a small cavity in their interior, round which the
cells arrange themselves in an epithelial fashion. The cavity represents a part of
the coelomic cleft, which in lower vertebrates is continuous with the general
ccelom.

E 2

52 SEPARATION OF EMBRYO

The cleavage first makes its appearance at the anterior end of the axis in the
region where the heart-tubes will be formed. Thence it extends backwards, and
at the same time forwards round the head end of the axis, so that the lateral
coelomic spaces are continuous with one another in front, by a pericephalic cleft
which afterwards becomes the pericardium.

The relations of the layers immediately in front of and behind the axis must
finally be referred to. In the axial line the notochord passes in front into the head-
plate. If this be followed forwards (fig. 78), it will be seen that it is continued into
a portion of the blastoderm between the head end of the axis and the pericephalic
ccelom, into which the mesoderm has not extended (or from which it has
disappeared). The ectoderm and entoderm are therefore here in contact, and form
a membrane known as the buccopharyngeal membrane, which later becomes
perforated to form the mouth-opening. The region of the blastoderm between the
buccopharyngeal membrane and the edge of the shield corresponds to the ' pro-

volk-xic

^ amnion

neurenteric canal
_1 allantoic diveriicnlum

FlG. 75. — SUBFACE VIEW OF A BLASTODEEM OF CEBCOPITHECUS CYNOMOLGUS. (After Seleilka.)

The amnion has been opened. The first three segments are visible in front of the neurenteric
canal on each side of the neural groove, which is still open.

amnion ' of lower mammals ; but in the human embryo the ectoderm and entoderm
are, from the first, here separated by mesoderm. This is not split, however, so that
the pericephalic is separated from the extra-embryonic coelom by a bridge of tissue.
Again, at the posterior end of the axis, behind the growing point or tail-knob, the
primitive streak becomes detached from the lateral mesodermic sheets and resolved
into an ectodermal and an entodermal lamella, which together form the cloacal
membrane. This is afterwards perforated to form the urogenital and anal apertures.
Separation of the embryo : history of the yolk-sac and allantois. —
As the embryo increases in length, there is a certain increment also in the breadth
of the embryonic shield ; and although the yolk-sac has much increased in size,
the embryo soon begins to expand in all directions beyond the limits of the mouth
of the sac. A folding-in round the margin of the shield, along the line where
amniotic and embryonic ectoderm meet, consequently takes place. The anterior

SEPARATION OF EMBRYO

53

fold first appears (fig. 79), and as a result of the forward growth of the front end
of the axis a diverticulum of the yolk-sac is formed. This becomes in part the
pharynx, but the fore-gut, as the diverticulum is called, is gradually lengthened by
the deepening of the fold and the coming together of the splanchnopleuric folds,
which are nipped in from each side (fig. 76, I.). In consequence of the formation
of the anterior fold, the buccopharyngeal membrane becomes bent in under the

n.f. n.gr

//Kv. ^ *%,\

III.

FIG. 77. — TRANSVERSE SECTION THROUGH
A HUMAN EMBRYO OF 2'4 MM.
(T. H. Bryce.)

am, amnion ; n.gr., neural groove ;
not, notochord ; f.g., fore-gut ; y.s., yolk-
sac ; a, aorta of right side ; a.v., allantoic
vein of left side : c, ccelom.

Vessels are seen covering the whole
surface of the yolk-sac.

not.pl

FIG. 76. — TRANSVERSE SECTIONS OP THE HUMAN EMBRYO OF 2 MM. REPRESENTED IN FIG. 72.

(After Graf v. Spee.)
In I., which is most anterior, the fore-gut is separated off from the yolk-sac.

n.gr., neural groove ; n.f., neural folds ; ti.pl. (in III.), neural plate ; mes1, intra-embryonic mesoderm
still undivided : the commencing intra-embryonic coelom shows as a space (p) — in I. to the left, and in
II. on both sides ; it becomes the pericardium; am.ect., amniotic ectoderm ; mes-, amniotic mesoderm ;
ent, entoderm of yolk-sac ; mes3, mesoderm of yolk-sac ; not.pl. (in III.), notochordal plate.

head of the embryo and, reversed in position, now forms the still closed anterior
end of the fore-gut. Further, the pericephalic portion of the ccelom, also
reversed in position, comes to lie below the fore-gut, while the bridge of meso-
derm separating it from the extra- embryonic ccplom, and originally at the edge of
the shield, now forms the anterior lip of the primitive umbilical opening, and

54

SEPARATION OF EMBRYO

constitutes what is known as the septum transversum. The folding-in at the
tail end of the embryo takes place rather later, and is complicated by the

presence of the connecting
stalk. In the earliest known
human embryos (fig. 79) there
is a pocket between the
posterior end of the axis and
the upper aspect of the stalk.
As the embryo increases in
length this deepens, the stalk
is displaced forwards, and the

per

primitive streak is bent in to
form, as the anal membrane,
the floor of a diverticulum
named the hind-gut. In front
of the attachment of the stalk
the yolk-sac is further folded
in and the hind-gut is gradu-
ally elongated. Up to this
stage the name connecting
stalk has been applied to the cord of mesoderm uniting the embryonic rudiment
with the chorion. When the tail-fold is produced, it is bent round to the ventral

FIG. 78. — MESIAL LONGITUDINAL SECTION THROUGH THE HEAD

END OF THE GERMINAL DISC OF THE DOG BEFORE THE

FORMATION OF THE HEAD-FOLD. (After Bonnet.)

ect, ectoderm of shield ; not.pl., notochordal plate ; p.p.,
primitive entodermal plate (Ergcinzungsplatte, Bonnet) ; Ip.m.,
buccopharyngeal membrane; per, pericephalic portion of
pericardial coelom.

The notochordal plate (archenteric plate, Bonnet) passes
directly into the primitive entodermal plate (Ergcinzungs-
platte, Bonnet).

villas

ammon

-• core of villus

^jsg^..^ — mesoderm

'connecting stalk
'primitive streak

-yolk-sac

•enloderm

mesoderm —-• —

essels

FIG. 79. — MEDIAN LONGITUDINAL SECTION OF AN EMBRYO OF 2 MM. (see FIG. 72). (Graf v. Spee.)

aspect of the body of the embryo, and may henceforward be appropriately named
the abdominal stalk (Bauchstiel, His).

Between the hind-gut and the fore-gut there is at first a wide opening into the
yolk-sac (fig. 92), which is gradually reduced to a narrow aperture, and the stalk

ALLANTOIS

55

thus formed is drawn out into a long tubular passage, the vitelline duct, which
widens distally into a rounded vesicle called the umbilical vesicle.

Allantois. — In all the Primates the vesicular allantois of lower forms is
represented merely by a narrow tubular passage imbedded in the mesoderm of
the connecting stalk. It appears as a recess of the posterior wall of the yolk-sac
at a very early stage, before the formation of the hind-gut (figs. 53, GO, 61, 79).
This recess is drawn out into a tube as the connecting stalk increases in length.

ect

FIG. 80. — DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF THE RABBIT. THE
SECTIONS SHOW THE MANNER IN WHICH THE PRO-AMNION IS FORMED BY A DIPPING DOWN OF

THE HEAD AND ANTERIOR PART OF THE BODY INTO A DEPRESSION OF THE BLASTODERM, WHICH
AT THIS PART IS FORMED OF ECTODERM AND ENTODERM ONLY. THE DIAGRAMS ALSO ILLUSTRATE
THE MODE OF FORMATION OF THE ALLANTOIS AND OF THE TAIL-FOLD OF THE AMNION IN THIS

ANIMAL. (Van Beneden and Julin.)

ect, ectoderm ; ent, entoderm ; me, mesoderm ; cce, parts of the coelom; cce', pericardial ccelom, the
heart not being represented; pr.a., pro-amnion; pi, seat of formation of the placenta; all, allantois ;
am, amnion.

When the stalk is displaced to the ventral aspect, and the umbilical cord is
formed, the passage persists for a time in the cord, while its in tra- embryonic
portion becomes the urachus.

In lower mammals the entodermic diverticulum varies much in the degree of its development.
In the ungulates and carnivores it forms a large vesicle ; in most rodents (fig. 80) it is less
extensive, being confined to the placental site ; in the guinea-pig it is reduced to a tubular
passage in the body-wall and the stalk is a solid cord of mesoderm ; but in all below Primates
the diverticulum, with its covering layer of mesoderm, projects free into the extra-
embryonic ccelom before it comes into contact with the chorion. In the Primates the
embryonic shield is connected from the first with the chorion by the mesodermic
connecting stalk, and the allantois never projects free into the ccelom. The chorion is thus
vascularised directly and not through the agency of the allantois. This close attachment of the

5G

MYOTOMES AND SCLEROTOMES

embryo to the chorion by the short abdominal stalk is accompanied by a certain retardation
of the development of the hind end of the embryo.

Early stages in the development of the muscles and of the
connective tissue and blood-vessels : mesenchyme. — It will be recollected
that the mesodermic segments were traced to a stage in which each shows a central
lumen round which the cells are arranged in an epithelial fashion (fig. 82). In
some cases the cavity is occupied by branching cells budded off from the ventral
wall. In transverse section each segment is oval in shape, and now the lower part
of the inner and ventral walls becomes resolved into a mass of loosely arranged
cells, wedge-shaped in section, which encroaches on the cavity (sderotome) (figs. 81
and 82). These cells, along with those in the cavity of the segment, divide actively and
wander inwards, to invest the notochord (fig. 83) and ultimately the neural canal,
in a continuous sheet of loose syncytial tissue known as mesenchyme L (fig. 84).
It constitutes the blastema out of which the axial connective and skeletal tissues
are formed.

FIG. 81.— TBANSVEESE SECTION or THE HUMAN EMBRYO OF 2'4 MM. (see FIG. 77), MORE
HIGHLY MAGNIFIED. (T. H. Bryce.)

ent, entoderm of yolk-sac : the lines indicate the points of the splanchnopleuric layers which will
come together to cut off the gut from the cavity of the yolk-sac ; my, outer wall of mesodermic
segment ; me, part of its wall which gives rise to the muscle-plate ; sc, sclerotome ; cce, ccelom. The
other structures as lettered in fig. 77. The amnion, having been torn, is not completed in this
figure.

While the sclerotomes are becoming differentiated, the cavity of the segment
is reduced to a narrow slit bounded by an outer and an inner lamella derived
respectively from the outer and inner wall of the segment (fig. 84). The cells of
the inner lamella elongate, become arranged longitudinally, and are ultimately
(third week) converted into muscle-cells. Hence this lamella is named the
muscle-plate (myotome). It is the rudiment of the voluntary musculature of the
body. The outer lamella of the segment retains its epithelial arrangement for a
time; according to Maurer, it becomes entirely resolved later into a layer of
subcutaneous mesenchyme.

Balfour originally described both inner and outer lamellae as becoming
differentiated into muscle, and in some forms this certainly seems to be the case —
e.g. Lepidosiren (Graham Kerr2) and pig (Bardeen3). As regards the human
embryo, Kollmann (1891) described the outer wall as yielding muscle at least in
part, but Bardeen and Lewis state 4 that the whole outer lamella becomes
muscular tissue.

1 For definition of this term, see p. 58.
5 Johns Hopkins Hospital Keports ix.

2 Eep. Brit. Assoc. 1902.
4 Amer. Journ. of Anat. i.

MYOTOMES AND CCELOM

57

Each primitive segment is thus differentiated into myotome and sderotome, while
its ventral part, concerned in the formation of the excretory ducts, may be termed
the nephrotome. The myotomes retain their segmental disposition, but the sclero-
tomes have really no separate identity, being at once fused into a continuous axial
sheet of mesenchyme. The primitive segments are not, however, the only source
of the embryonic connective tissue. The parietal and visceral plates of
mesoderm become likewise resolved into the mesenchyme of the body-wall and
gut- wall respectively, with the exception of the cells lining the ccelom, which become
the endothelial lining of the body- cavities (mesothdium).

my

w.d.

FIG. 82. — TRANSVERSE SECTION THROUGH THE TRUNK AND HIND-LIMB BUDS OP A RABBIT EMBRYO OF
THE TENTH DAY. THE LINE OF SECTION IS SLIGHTLY OBLIQUE. (T. H. BtyCC.)

n.c., neural canal ; ???.«., mesodermic segment ; sc, ventral wall of segment becoming resolved to form
sclerotome ; /////, outer wall of segment ; w.d., w.rf.,-Wolffian ducts ; on the inner aspect of each a cord of
nephrogenetic tissue; A1, A^, the two primitive aortse cut close to where they separate into the allantoic
arteries, Ai, A2 ; C, coelom ; G, gut.

We have already seen that the mesoderm remains unsegmented in the region
of the head. Moreover, there is here no distinction between paraxial and lateral
mesoderm, and no splitting to form coelomic spaces. The whole unsegmented
mesoderm becomes resolved into a continuous mesenchyme which surrounds the
cerebral vesicles and the head end of the notochord.

In lower vertebrates certain ccelomic cavities appear in the head which are
concerned in the formation of the muscles of the eyes, and of the branchial region.
They correspond to the preoccipital head-segments already alluded -to, and are

58

MESENCHYME

considered by some as equivalent to trunk -segments. They will be referred to
again in a subsequent section.

IKesenchyme. — The term mesenchyme is here employed to denote that part
of the middle layer which is the blastema of the connective tissues. Its use
involves the recognition of two orders of mesoderm. First, mesoderm in the
stricter sense of the term (mesothelium) — i.e. that which we have termed the dorsal or
segmented mesoderm in earlier sections. It is laid down in a coherent layer, in
which the coelom is formed by a process of splitting. It may be considered as
typically developed from ccelomic pouches of the archenteron (see p. 46), although
in general, owing to the accumulation of yolk in the egg and the secondary
modifications resulting therefrom, the rudiment is solid and secondarily excavated.
It gives origin to the skeletal muscles, the endothelial lining of the body- cavities,
the epithelium of the excretory ducts, the germinal epithelium, and the cortical
portion of the suprarenal body. Second, mesenchyme, a syncytial formation

formed of cells budded
off individually from the
epithelial layers, or formed
by resolution of the meso-
derm into a loose mass of
anastomosing cells. From
it are developed the several
. forms of connective tissue,
the unstriped muscular
tissue, perhaps even striped
muscles, and also possibly
the blood and blood-
vessels, though in respect
of the blood there is much
difference of opinion.

The mesenchyme occupies
everywhere the intervals be-
tween the epithelial layers,
and forms a complex on quite
a different morphological plane
from them.

Originally introduced by
0. and R. Her twig,1 the term
* mesenchyme ' has lost its
more strict significance. It is
now known to arise from
several sites, and though in

m.p.

s.p.

FIG. 83. — TRANSVERSE SECTION OP A HUMAN EMBRYO OF THE THIRD

WEEK TO SHOW DIFFERENTIATION OF MESODERMIC SEGMENT.

(Kollmann.)

n.c., neural canal ; ao, aorta ; m.p., muscle-plate ; s.p.t skin-plate ; general a derivative of the
sc, sclerotome. mesoderm, it is said to arise in

some cases directly from the

entoderm. The circular band of ' mesoblast,' for instance, described (after Hubrecht) in the
section on the early formation of the middle layer (footnote, p. 34) as arising from the entoderm
in Tarsius and concerned in the formation of the blood-vessels is a case in point. Further, some
authors (Kastchenko, Goronowitsch, Sewertzoff, Klaatsch, Julia Platt, Lundborg, Koltzoff)
have described tracts of connective tissue arising from the ectoderm. But this has not gone
uncontradicted ; and in general it seems proper to hold that the mesenchyme is essentially and
primarily of mesodermic origin; although, included in the complex, and indistinguishable in
their undifferentiated state from the cells derived from this source, there are possibly other
portions of different parentage. Thus among the cells of mesodermic origin many authors
have described cells which they believe come from the ectoderm or entoderm. Maurer, for

Die Colomtheorie, Jena, 1881 ; and Studien z. Bliittertheorie, 188,1..

BLOOD AND BLOOD-VESSELS OF YOLK-SAC 5£

instance, and others are of opinion that the leucocytes found in the wall of the gut are
derived from the entodermic epithelium ; while, again, there is some reason to believe that
ectoderm-cells may wander from the neural crest, and even from the neural canal along the
primitive ventral nerve-roots, and spread to quite distant parts within the mesoderm.

In connexion with the first appearance of individual cells between the layers, it may here
be mentioned that, according to the observations of Szily,1 the epithelia are all connected together
by protoplasmic threads which are spun out as they draw apart in the course of development.
The spaces between the epithelial layers are not vacant, as they appear to be in sections
treated by ordinary methods, but are occupied by a delicate protoplasmic reticulum, which
forms a basis on which the wandering mesenchyme-cells arrange themselves into a syncytium.

The blood and blood-vessels first appear in the wall of the yolk-sac. In
the lower mammals a vascular area is developed (fig. 85), as in the Sauropsida,

S'ttS?

SSs***. •««£.«

FIG. 84. — TRANSVERSE SECTION THROUGH THE TRUNK OF A RABBIT EMBRYO OF THE

ELEVENTH DAY. (T. H. BryCC.)

•in. p., muscle-plate ; x.p., skin-plate ; sc, sclerotome ; w.d., Wolffian duct ; iv.t., Wolffian tubule. The
rirlge in which the duct and tubule lie is the Wolffian ridge. To the left the section has cut the wall
of a Woffian tubule where it is connected by a cellular strand with the coelomic epithelium. A, aorta ;
C, ccelom ; UV, UV, umbilical veins.

but in the Primates the earliest vessels appear on the under aspect of the sac
(fig. 79) and gradually extend over its upper pole, until the whole sphere is
covered by a vascular network. Further, in Primates there is no terminal sinus.
These are regarded as secondary modifications due to the small size of the
yolk-sac.

The first indication of blood and blood-vessels is the appearance of irregular
projections on the surface of the vesicle due to the formation of the blood-islands
of Pander between the entoderm and mesoderm. The blood-islands are groups of
rounded nucleated corpuscles closely packed together : indeed the cell outlines are

1 Anat. Anzeiger, xxiv. 1903.

60 BLOOD AND BLOOD-VESSELS

not clearly distinguishable. The peripheral layer of cells becomes the endo-
thelium of the vessel- wall, while the central mass is resolved into the primitive
nucleated blood-corpuscles. The islands are united together by cellular processes
which, becoming hollow, produce a continuous network of vessels.

From the third week onwards until the liver is developed, it appears from the descriptions
of Graf v. Spee that the wall of the yolk-sac becomes converted into a tissue resembling liver-
tissue in its simplest form. Saccular dilatations of the entodermic lining of the vesicle are
produced, and from the walls of these dilatations solid masses of cells are budded off. Among

FIG. 85. — VASCULAB ABEA OF THE BABBIT OF ELEVEN DAYS. (v. Beneden and Julin.)

The arteries are represented red, the veins blue ; the capillaries are not shown. The terminal sinus
is seen to be arterial.

the cells are seen ' giant ' elements, derived possibly from the epithelial cells, and within these
are smaller cells closely resembling young nucleated blood -corpuscles.

Once formed, the blood-vessels on the yolk-sac are in direct continuity with vessels which
develop in the connecting stalk, and through them with the vessels of the chorion. In this
xespect the conditions in the Primates again differ from those prevailing in the lower mammals.
Thus Selenka has shown that in Hylobates rafflesi the vessels on the under aspect of the yolk-sac
communicate with the vessels of the chorion by a pair of vessels surrounding the allantoic tube,
before there are any vessels in the embryo itself. This arrangement seems to be present in the
human embryo also, for a similar vascular loop was described by Eternod in his young embryo,
and named by him the sinus ensiforme.

HF.AKT AND EMBRYONIC BLOOD-VESSELS

61

Early stages in the development of heart and embryonic vessels. —

The first embryonic blood-vessels are laid down in the splanchnopleure, and the
earliest channels to appear are two short tubes on each side of the head end
of the embryonic axis. These form the double rudiment of the heart.

not.pl.

ent

FIG. 86.— A. TRANSVERSE SECTION THROUGH THE HEAD OF AN EMBRYO RABBIT OF EIGHT DAYS

AND FOURTEEN HOURS, WITH A PART OF THE PERIPHERAL BLASTODERM. *T8' (Kolliker.)

7?, h, rudiments of the heart ; sr, pharyngeal groove, with notochord-plate.

B. PART OF THE SAME MORE HIGHLY MAGNIFIED. J^. (Kolliker.)

n.p., neural plate (of hind-brain) ; n.g., neural groove ; n.f., neural fold ; ect, ectoderm ; ent, entoderm ;
mes, mesoderm ; p, paraxial mesoderm ; som, somatopleure ; spl, splanchnopleure ; per, pericardia!
coelom ; ah, fold of splanchnopleure which will form wall of heart ; ih, endothelial tube of heart ;
sio, wall of fore-gut; not.pl., notochord-plate.

Various views are held as to the origin of the vessels in the embryo. One is that 'they grow
into the embryo from the vascular area, by a budding of the endothelial walls of the vessels
first laid down there (His) ; another, that the whole closed vascular system is produced by exten-
sion of two primary endothelial sacs, the heart-tubes (Rabl) ; a third, that they arise in sitv.
The third theory is on the whole most consonant with the appearances seen in mammalian

ect

\
not.pl.

cut

spl pei

FIG. 87.— SECTION FROM THE SAME EMBRYO FARTHER FORWARD THAN THAT SHOWN

IN THE PRECEDING FIGURE. (Kolliker.)

Lettering as in fig. 86.

embryos, but varying accounts are given of the actual mode of origin of the channels and the
source of the vaso-formative cells. According to one interpretation, the channels are spaces
in the mesenchyme which become converted into vessels by the transformation of the surrounding
cells into an endothelium ; and these spaces have been looked on hypothetically as the remains

<52 HEAKT AND EMBRYONIC BLOOD-VESSELS

•of the interval between the germ-layers (Ziegler). According to another account, the vessels
appear first as cords or strings of cells which arrange themselves round a lumen, and form the
endothelium of the vessel wall (Riickert). The vessels in terms of both these interpretations
are intercellular spaces ; but according to still other observers, the extension of the channels
is brought about by the formation of spaces within the vaso-formative cells, which are converted
into vessels by being linked up together.

The vaso-formative cells are most generally regarded as mesodermic in origin ; but they
are considered by some as being derived independently from the entoderm, forming thus> an
ontodermic, as distinguished from the mesodermic mesenchyme.

In Tarsius, it will be recollected, a ring-shaped thickening of the entoderm was mentioned as
giving rise to a band of middle-layer cells related to the formation of the vessels. According to
Hubrecht's account, therefore, the vascular mesenchyme arises direct from the entoderm. It
may be noted that the disposition of the ring closely corresponds with the primitive vessels
of the early human embryo as described by Eternod.

In the absence of detail for the early phases of the primate heart, the stages in
the rabbit- embryo may be taken as a type for the development of the parts.

II

I •&

FIG. 88. — TBANSVEESE SECTION THBOUGH THE REGION
OF THE HEART IN A BABBIT EMBBYO OF NINE DAYS,

SHOWING THE COMMENCING FUSION OF THE TWO

TUBES. Y- (Kolliker.)

jt 3-> Ju8ular veins ; ao, aortee ; ph, pharynx ; sow,
somatopleure of body-wall ; bl, bilaminar portion of blasto-
derm forming pro-amnion ; ect, ent, its two layers (ecto-
derm and entoderm) ; p, pericardium ; spl, splanchnopleure ;
afy, outer wall of heart ; ih, endothelial lining of heart ;
e', septum between the tAvo heart-tubes.

FIG. 89.— EMBBYO BABBIT OF EIGHT

DAYS AND EIGHTEEN HOUBS, WITH

NINE PBOTOVEBTEBBJE ; VENTRAL

ASPECT. V- (Kolliker.)

The heart is still a double tube.

At an early period, before the splanchnopleuric folds have begun to fold in to
form the fore-gut, it will be seen (fig. 86) that the pericardial portion of the
ccelom is occupied by a fold of the splanchnic mesoderm. This fold becomes
subsequently closed-in to form the muscular wall of the heart. It encloses a
second tube composed of flattened cells, which becomes the endothelial lining
of the heart. Authorities differ as to the origin of these cells, some deriving them
from the mesoderm, while others trace them direct from the entoderm, either in
the form of an evagination or as a solid cord of cells. When the splanchnopleuric
folds bend in to form the floor of the fore-gut, the two tubes are brought together
(fig. 88) below the pharynx. They at first lie side by side, but soon fuse into
a single median tube by the absorption of the dividing septum. The heart-
tube remains attached to the gut by a mesentery, the mesoeardium poster ius
•(fig. 90).

HEART AND EMBRYONIC BLOOD-VESSELS

63

ect

In the earliest stage described for the human embryo (thirteenth day) the
heart-tubes are still separate from one another. We owe to Eternod ' a description,
arrived at by reconstruction from sections, of the vessels of an embryo of the
thirteenth day, when the channels are still in the course of formation in the mesen-
chyme. The heart-tubes are simply dilated portions of a continuous sinus-like
vessel which surrounds the mouth of the yolk-sac and ends in a common stem in
the abdominal stalk, which is in turn distributed to the chorion (fig. 91). The two
tubes are united for a short distance under the fore-gut into a single vessel, which is
the rudiment of the aortic bulb. From this two vessels sweep back on each side
of the notochord, which is still in the stage of the notochordal plate : these are the
primitive aortas, and the loops between the ventral bulb and the dorsal aorta are
the first or primitive aortic arches.2 Behind, the aortae, sweeping past the neurenteric
canal, bend round the caudal
end of the embryonic axis
into the abdominal stalk, and
pass in this to the chorion.
Where the abdominal stalk
becomes continuous with the
yolk-sac the sinus-like vessel
is joined by a vascular loop
from the back and under side
of the yolk-sac (sinus ensi-
forme), but the vitelline veins
proper, which afterwards con-
vey the blood from the vitel-
line circulation to the heart,
have not yet been laid down.

It would thus seem that
there is a circulation set up
between embryo and chorion
at a very early stage, before
even the yolk circulation is
established. This is to be
correlated with the vestigial
condition of the yolk-sac, and
is another instance of the

remarkable series of variations from the ordinary type which the development of
the primate embryo exhibits.

In the next stage of which we have complete detail, a human embryo of
fifteen days (His' embryo Lg., fig. 92), and for the lower Primates an embryo of
Cercocebus cynomolgus described by Selenka, the yolk circulation is fully estab-
lished, and the course of the vessels has become so modified as to conform to
the condition described for the lower mammals with a large yolk-sac and a vascular
area.

The heart is now a single tube, and shows a subdivision into an auricular,
a ventricular, and a bulbar part. It receives three veins on each side, which join
a transverse vessel placed in the septum transversum, named the sinus venosus.

The three pairs of veins are the vitelline, running in the splanchnopleure from
the yolk-sac ; the allantoic, running in the edges of the somatopleure and

1 Anat. Anzeiger, xv. 1899.

2 It may be mentioned that even at this very early stage, according to Eternod's descriptions, there
are indications of the rudiments of two, perhaps three, connecting vessels on each side representing
future aortic arches.

FIG. 90. — SECTION THROUGH THE REGION OF THE HEART IN
A RABBIT EMBRYO OF TEN DAYS, AFTER THE TWO TUBES

HAVE UNITED INTO A SINGLE MEDIAN ORGAN. (Kb'lliker.)

ao, descending aortee ; ba, bulbus aortas ; ah, its external
wall ; i/c, its endothelial lining ; mp, mesocardium posterius,
uniting the heart to the ventral wall of the pharynx, ph, and
here separating the pleuropericardial ccelom, p, into two
halves, which are, however, united on the ventral side of the
heart ; ent, entoderm of yolk-sac ; df, its mesoderm ; df, meso-
derm of pharynx ; h, mesoderm of somatopleure ; ect, ectoderm.

64

HEART AND EMBRYONIC BLOOD-VESSELS

connected behind with the common stem of the earlier stage in the body-stalk, and
bringing blood from the chorion ; and the ducts of Cuvier, formed by the junction of
two veins from the body of the embryo (anterior and posterior cardinal). The
ducts of Cuvier and allantoic veins effect a junction before they reach the sinus

venosus.

The aortic bulb passes into two ventral vessels, which join with the dorsal
aortse by two arches. The dorsal aortse in turn sweep back on each side of the

all

FIG. 92.— PROFILE . VIEW OF A HUMAN EMBRYO OF
ABOUT FIFTEEN DAYS, WITH THE ALIMENTARY
CANAL IN LONGITUDINAL SECTION. (His.)

p.v., primitive velum ; end, endothelial tube of
heart ; v, yolk-sac ; u.a., umbilical (allantoic) artery ;
u.v., umbilical vein ; all, allantoic diverticulum.

FIG. 91. — DIAGRAM OF THE VASCULAR CHANNELS IN A HUMAN EMBRYO OF THE SECOND WEEK.

(After Eternod.)

The position of the vessel's will be understood if the diagram be compared with the surface view
of the blastoderm at this stage given in fig. 72 (p. 49). The afferent channels (including the
two heart-tubes) are coloured blue; the efferent (aortas) red. m.s., m.s., marginal sinus (primitive
umbilical veins : the anterior dilated portions of the veins are the primitive heart -tubes, h) ;
cs, section of abdominal stalk enclosing all, allantoic diverticulum, a single venous, and two arterial
channels ; n.c,, neurenteric canal. The dotted blue lines indicate the position on the back of the
yolk-sac, and therefore not seen from this view, of the sinus ensiforme.

notochord, giving branches (vitelline arteries) to the yolk-sac ; they terminate by
bending round the tail end of the embryo into the body-stalk, within which they
are carried to the chorion.

The further changes in the heart and vessels will be treated of in the second
part of this volume. It will suffice, at this stage, to have shown that in Primates
the embryonic vessels are connected from the earliest period with the chorion, and
that by the end of the second week the circulation between that membrane and
the embryo is fully established.

IMBEDDING OF OVUM

65

DEVELOPMENT OF THE FCETAL MEMBRANES AND PLACENTA;
IMBEDDING OF THE OVUM.

Having determined the manner in which the principal organs of the body
make their appearance, we must now study in somewhat greater detail than
we have yet done the history of the chorion and amnion, in order to ascertain
how the placenta, or organ which nourishes the foetus, and the membranes which
protect it during its sojourn in the uterus, are developed. It will be necessary,
however, first to describe how the ovum is imbedded in the uterine mucous
membrane, and the changes that take place in that membrane during pregnancy.

':•

FIG. 93. — SECTION THROUGH AN OVUM OF THE FIRST WEEK.
(Reduced from Peters ; from Hertwig's Handbuch.)

The section passes through the embryonic rudiment. Th, thrombus closing an opening on the surface
of the uterine epithelium, U.E. ; D, decidua. The very irregular strands of trophoblast are distinguished
by their darker tint.

Imbedding of the ovum. — The earliest known human ova are already
completely imbedded in the uterine mucous membrane. The site of implantation
in man is normally the posterior wall of the uterus near the fundus.
Peters' and Leopold's ova lay in this position ; their situation was not marked
by any projection of the mucous membrane beyond the general level of the
swollen surface. The blastocyst in Peters' case (fig. 93) was oval in shape.
The wall was relatively thick and already intimately related to the mucous mem-
brane (decidua). Over the ovum there was an area, one millimetre in diameter,
where the uterine epithelium was absent, and here there was a soft thrombus (Th),
which had a narrow stalk occupying the hole in the epithelium, and a broad head
spreading out like a mushroom over the edges of the opening. The uterine glands
in the neighbourhood of the blastocyst took a somewhat concentric course round
VOL. i. F

66

IMBEDDING- OF OVUM

it, as if pushed aside by the growing ovum, and there was absolutely no indication
that any gland-mouths opened into the cavity in the mucous membrane, nor
were there any traces of uterine epithelium lining it. These facts, which have
been recently confirmed in the ovum described by Leopold, clearly indicate that
the ovum is at a very early stage cut off from the general cavity of the uterus,

FIG. 94. — ANTERO-POSTERIOR SECTION OF THE GRAVID
UTERUS AND OVUM OF FIVE WEEKS : DIAGRAM-
MATIC. (Allen Thomson.)

a, anterior, p, posterior uterine wall ; m, muscular
substance ; u, placed in the cavity of the uterus ;
g, the glandular layer of the decidua vera ; r, the
decidua capsularis ; s, decidua basalis ; c, cervix uteri ;
ch, chorion with its villi, which are more highly de-
veloped on the placental side ; e, the embryo enclosed
in the amnion, with the allantoic vessels passing along
a short allantoic stalk into the placenta, and the
umbilical vesicle lying free in the space between
amnion and chorion.

FIG. 95. — DIAGRAMMATIC SECTIONS OF THE UTERINE MUCOUS MEMBRANE, SHOWING THE CHANGES WHICH
THE GLANDS UNDERGO WITH THE SUPERVENTION OF PREGNANCY. (From Kundrat and Engelmann.)

A. Diagram of the glands of the non-pregnant uterus ; m, muscular layer. B. Condition of the
glands at the beginning of pregnancy ; c, compact layer near free surface of decidua ; the glands are
here somewhat enlarged but not very tortuous, and the mucous membrane is rendered compact by
hypertrophy of the interglandular tissue ; sp, spongy layer, containing the middle portion of the
glands greatly enlarged and tortuous, producing a spongy condition in the mucous membrane ;
d, deepest portion of the glands, elongated and tortuous, but not much enlarged.

not, however, as used to be supposed, by an upgrowth round it of the mucous
membrane, but in some different fashion. For a parallel we must look to those
cases among the lower mammals in which the blastocyst remains very small,
and becomes very early surrounded by decidual tissue. Such cases occur among
the Insectivora — e.g. the hedgehog — and also among the Cheiroptera ; but the nearest

DECIDUA 67

analogy is to be found in those mammals in which there is so-called ' inversion of
the germinal layers,' the Muridce (mice and rats) and Cavia (guinea-pig) among
the rodents. In some way the early nesting of the ovum in the decidua is related
to the inversion of the layers, and both phenomena are probably to be correlated
with the very minute size of the blastocyst.

The idea that the imbedding occurs by ' circumvallation ' being given up,
there remain two possibilities — either (a) that the minute ovum is received into
a crypt of the mucous membrane, or (b) that it, in virtue of a biochemical action
of the trophoblastic ectoderm, absorbs the epithelium, and eats its way into the
connective tissue of the mucous membrane. If the first alternative be adopted,
we must conceive the process to take place as it does in the mouse.1 In this
animal the small blastocyst is received into a recess of the uterine cavity. The
epithelium lining this cavity becomes flattened and then disappears, so that
the trophoblast and decidua become closely related. The ectoplacenta blocks
the aperture between the decidual cavity and the lumen of the uterus, and the
cavity later becomes obliterated at the site of implantation, by the disappear-
ance of its epithelium and fusion of the exposed decidual walls. The gland-tubes
disappear as the mucous membrane becomes converted into decidua, and new
capillaries are freely produced, especially in the neighbourhood of the ectoplacenta,
where the maternal part of the placenta is formed.2

If the second alternative be adopted, then we have to conceive the process as
described by Graf v. Spee 3 for the guinea-pig. In that case the ovum reaches
the uterus in the morula or early blastocyst stage, and destroys the epithelium
at the point of contact with it. It becomes imbedded by a process of degeneration
of the connective tissue, which ultimately forms a sort of granulation-tissue capsule
around it.

The evidence afforded by Peters', Leopold's, and other early ova is strongly in
favour of the view that the ovum actually absorbs the uterine tissue before it,
and therefore becomes implanted by the absorptive activities of its ectodermic
covering. In either case, at a very early stage the blastocyst lies surrounded on
all sides by mucous membrane without any trace of an epithelial layer.

Chang-es in the uterus during* pregnancy. — The mucous membrane of
the pregnant uterus is known as the decidua. For convenience of description, the
parts of the mucous membrane immediately enclosing the ovum, and that lining
the general cavity of the uterus, have received distinctive names. Thus the layer
of membrane around the ovum is known as the decidua capsularis (reflexa) ; the part
next the uterine wall where the placenta is afterwards formed is the decidua basalis
(serotina) ; while the membrane lining the cavity of the uterus is termed decidua
vera (fig. 94).

With the subsequent growth and consequent expansion of the ovum the enclosing
decidua capsularis expands also pari passu, encroaching more and more upon
the cavity of the uterus and coming into contact everywhere with the decidua
vera. Eventually it blends entirely with the decidua vera, so that the two layers
are indistinguishable and the cavity of the uterus is obliterated (except at the
cervix uteri).

The ovum is received into the uterus when the mucous membrane is in the
premenstrual phase, and in the earliest pregnancies described (Peters' and Leopold's
cases) the mucosa has all the characters of the menstrual decidua (fig. 90). It is

1 See G. Burckhard, Archiv mikr. Anat. Ivii.

2 Disse (Sitzungsber. Ges. Beford. gesatnmt. Naturwiss., Marburg 1005) has shown that in mice and
rats there occur large giant-cells in the developing decidua, which have a phagocytic action, and
excavate it for the growing ovum. See also Disse, Ergebnisse dor Anat. und Entwick. xv. 1905.

5 Zeitschr. Anat. u. Anthropol. iii.

68 DECIDUA

very soft and markedly oedematous. The glands are enlarged and the blood-
vessels much dilated. There is considerable effusion of blood from ruptured vessels ;
the blood occupying spaces in the loose connective tissue, and even the interior
of gland-tubes, which show desquamation of their epithelium and breaking down
of their walls.

The decidua undergoes further structural changes during the early months of
pregnancy, some of these changes being common to all three parts of the membrane,
whilst others are special to that part (d. basalis) which enters into the construction
of the placenta. The following is a brief account of these changes.

Decidua vera. — With the supervention of pregnancy the mucous membrane
lining the uterus becomes thickened and the tubular glands become both dilated
and greatly elongated. This thickening of the membrane and enlargement of
the glands goes on during the early months of pregnancy until, between the second
and third months, the decidua vera reaches its maximum thickness of more than
a quarter of an inch. Its glands have further undergone so considerable an elonga-
tion that they now no longer pass nearly straight through the membrane, but run
in a tortuous manner from the inner surface to the vascular layer, so that a vertical

vs-

FIG. 96. — SECTION OF UTERINE MUCOUS MEMBRANE DURING MENSTRUATION (Sellheim).

section of the membrane exhibits them cut quite as often obliquely or transversely
as longitudinally. They are also generally dilated, but the dilatation is by far most
marked at the mouths of the glands, which come thus to have a funnel-like shape,
and in the deeper part of the membrane, where the dilatations look in sections like
a series of cavities, lined by cubical or flattened epithelium and separated from
one another by a relatively small amount of interglandular substance. This
gives a spongy appearance to the part in question, and it has been accordingly
termed the stratum spongiosum of the decidua (fig. 95, sp). The deepest part of
the glands — that, namely, which is in contact with, and is imbedded in the superficial
portion of the muscular coat — does not share in this dilatation, and its epithelium
also retains the columnar character. The part of each gland between the funnel-
shaped mouth and the dilatations above described also becomes enlarged, but
not to so great an extent, the hypertrophy of the mucous membrane being here
chiefly confined to the interglandular tissue, which becomes filled with large
epithelium-like cells (decidual cells of Friedlander) and with numerous and large
capillary blood-vessels. This layer of the decidua has been termed the stratum
compactum in contradistinction to the stratum spongiosum external to it
(fig. 95, c).

DECIDUA 69

By the sixth week degenerative changes show themselves. The glandular
epithelium in the stratum spongiosum begins to be shed in places ; and, soon
after, the surface epithelium is thrown off, until in the fourth month all traces
of it have disappeared.

After the fourth month, by which time the great increase in size of the chorionic
vesicle with its contained embryo has brought the decidua capsularis into close
contact with the decidua vera, the latter begins to undergo an atrophic process,
the result to all appearance of the compression and distension to which it is thus
subjected. Its tissue becomes thinner and less vascular, and both the funnel-
shaped mouths of the glands and those parts of the glands which run through
the stratum compactum become gradually obliterated, so that eventually hardly
any trace remains. In the stratum spongiosum the spaces which have resulted
from the dilatation of the gland-tubes lose their lining epithelium, and become
flattened out conformably to the surface, so that they now appear as a layer of
compressed lacunae, separated by thin fibrous trabeculae.

Decidua capsularis (reflexa). — It has already been shown that the decidua
capsularis is not formed, as used to be supposed, by an upgrowth of folds round
the ovum, but is originally that part of the mucous membrane in which the ovum
has excavated a cavity for its lodgment. As the ovum imbeds itself in the stratum
compactum alone, it follows that the decidua capsularis represents only the super-
ficial part of the mucous membrane, and therefore has not a stratum spongiosum
properly so called. Over the ovum there is at first an area in which there is little
or no decidual tissue, the capsule being completed by a fibrinous lamella formed by
the organisation of the blood-clot at the point of entrance. This constitutes
Reickert's scar, which used to be considered as the point of fusion of the folds of the
reflexa. The inner aspect of the capsularis is irregular ; it is not covered with
epithelium, nor do any gland-mouths open into the decidual cavity.

The decidua capsularis resembles at first in every essential respect that portion
of the excavation which lies next the uterine wall and which becomes the decidua
basalis. The inter-relations between the mucous membrane and the villi are at first
similar all round the ovum. As the growing ovum expands, however, the decidua
capsularis and the villi imbedded in it degenerate (fig. 97). A gradual process of
atrophy supervenes until the membrane is reduced to a thin fibrinous or hyaline
lamella in which all traces of glands and vessels have disappeared. By the third
month the capsularis has nearly everywhere come into contact with the decidua vera
so as to obliterate the cavity of the uterus. In the advanced months of pregnancy
it wholly disappears, so that the chorion comes to lie directly against the decidua
vera. The degenerative process at work in the membrane does not seem to be
a fatty degeneration, as long held, but a coagulation necrosis.

Decidua basalis (serotina). — The decidua basalis is that portion of the
mucous membrane which intervenes between the blastocyst and the uterine wall,
opposite the original point of entrance of the ovum (fig. 93). The deeper portions
of the gland-tubes proper to it become much dilated, the final result being the
formation of a spongy layer, with irregular clefts flattened out conformably to the
surface, and from which the epithelium has entirely disappeared. At the same
time all the parts of the glands which are superficial to this layer suffer complete
atrophy, the only portions which remain nearly unaltered being the deepest parts
of the tubes, which are partly imbedded in the muscular coat of the uterus, and
retain their epithelium. After separation of the placenta from the uterine wall
at parturition, the uterine mucous membrane, with its epithelium and glands,
becomes renewed from this deepest portion of the decidua basalis. The blood-
capillaries become much dilated into sinus-like vessels, and the interglandular
tissue becomes crowded with decidual cells, derived, as in the decidua vera, from the

70

AMNION

connective-tissue cells by a process of enlargement, and also by multiplication of the
elements. The decidua is also invaded by foetal tissue, as will be explained later.

Before proceeding to the description of the development of the placenta, it
will be convenient to consider first the later history of the amnion and chorion.

Amnion. — As we have seen in an earlier section, there is good reason for
believing that in man, apes, and monkeys the amnion is closed from the beginning.

FlG. 97.— DlAGBAMMATIC SECTION OF THE PBEGNANT HUMAN UTERUS AT THE SEVENTH OB

EIGHTH WEEK. (Allen Thomson.)

c, c, openings of Fallopian tubes into the cavity of the uterus; c', cervix, filled by a plug of mucus:
the letters c and c1 are placed within the original cavity of the uterus; dv, decidua vera; dr, decidua
capsularis; ds, decidua basalis ; ch, chorion with its villi growing into the decidua capsularis and
d. serotina: in the former the villi are becoming atrophied (chorion Iseve) ; 11, umbilical cord, the dotted
lines indicate blood-vessels within it ; al, allantois ; ?/, yolk-sac (umbilical vesicle) ; y1, its stalk, passing
in the umbilical cord and connected with the intestine of the embryo, i ; am, amnion.

In all the earliest known normal embryos it forms a thin membrane over th<
embryonic shield, consisting of an inner layer of flattened ectoderm-cells and an outer
layer of mesoderm. At first, as will be noticed in a later section, it closely invests
the embryo, but at the beginning of the second month it is distended into a sac oi
considerable dimensions containing an albuminous fluid — the liquor amnii — in whicl

CHORION

71

the embryo floats. During the second month (fig. 97) it comes to fill the cavity
of the chorion, and the extra- embryonic cceiom is obliterated. As the umbilical
cord elongates, the amniou forms a tubular sheath round it, enclosing the vessels,
along with the allantoic and vitelline ducts, which are imbedded in mucous
connective tissue. From the cord it is reflected over the surface of the placenta
on to the chorion, with which it is intimately united to form what are known as
the foetal membranes. Between the amnion and chorion of the placenta lies the
umbilical vesicle.

The flattened ectodermic cells of the amnion become cubical during pregnancy,
and at full time the membrane consists of an epithelial layer, and a lamella of
fibrous tissue. The epithelium has the form of a very regular layer of cubical

FIG. 98. — DIAGRAM OF A HUMAN OVUM AT A (HYPOTHETICAL) STAGE SOMEWHAT YOUNGER THAN
PETERS' OVUM ; IMBEDDED IN THE DECIDUA. (T. H. Bryce.)

Th, blood-clot at point of entrance ; g, g, glands opening on the surface of the mucous membrane ;
d.c., decidua capsularis. The trophoblast partly cellular, partly plasmodial, is seen invading the
decidua, and opening up the dilated capillaries. The extravasated blood occupies the spaces or blood-
lacunse between the strands of trophoblast. The gland-tubules in the decidua take a concentric course
round the ovum.

cells joined by distinct cell-bridges. The liquor amnii varies in amount at different
periods of gestation ; it is relatively most abundant about the fifth or sixth
month. In the later months of pregnancy it contains • urea, which is probably
excreted by the kidneys of the foetus.

Chorion. — We have already seen in an earlier section that the formative cell-
mass, from which entoderm, as well as embryonic and amniotic ectoderm, are
formed, is completely surrounded in the primate ovum by a layer of cells which
has been named the trophoblast. In the youngest known ova the trophoblast
shows a very irregular outer surface (figs. 93 and 98) consisting of cellular strands
separated by spaces containing maternal blood. Round the wall of the vesicle

72

PLACENTA

ttie mesoderm is seen sending out buds which indent the trophoblast. By the
outward growth of these the epithelial strands acquire a mesodermic core, as
will be afterwards more fully explained ; vessels develop in the mesoderm, and the
result is that the whole surface of the chorion becomes occupied by vascular pro-
jections or villi, enclosing foetal vessels and bathed by the maternal blood. At
first equally distributed, the villi in the region of the decidua basalis become larger,
longer, and more branched, while those related to the decidua capsularis remain
relatively smaller and ultimately disappear by atrophy. The part of the chorion
in which the villi persist is known as the chorion frondosum. The remainder is
termed the chorion Iceve (fig. 97). In the third month the chorion frondosum
forms the foetal part of the definitive placenta, while the chorion Iseve, after the
disappearance of the decidua capsularis, comes in contact with the decidua vera,

B.L.

FIG. 99.— PORTION OF THE TROPHOBLAST OF PETERS' OVUM. (After Peters.)

ect, ect, chorionic ectoderm ; mes, mesoderm budding out to form, a villus core ; B.L., B.L., blood-
lacunae lined with a thin layer of syncytium ; tr, tr, cellular layer of trophoblast ; sij, sy, syncytium ;
s?y2, s?/2, mass of syncytium invading the lumen of a maternal blood-capillary, ca, ca ; bz, boundary-zone
between trophoblast and decidua ; c, trophoblast-cell showing alteration of nucleus.

and along with the amnion, as already stated, forms the double membrane lining
the uterine cavity.

Having now sketched the history of the decidua and of the chorion, we are
in a position to study in greater detail the changes which lead to the formation of
the placenta.

Placenta. — We have to distinguish two phases of placentation in the human
subject, a primary and a secondary. In the first phase, the whole chorion is
covered by vascular villi, similarly related to the decidua and the maternal blood,
and the placenta is therefore said to be diffuse. In the second stage, after the
atrophy of the villi of the chorion laeve, the placenta is a discoidal plate formed
from one part only of the chorion (i.e. chorion frondosum), intimately united with
the decidua basalis.

PLACENTA 73

In recent years several early ova l imbedded in the mucous membrane have
been studied by modern histological methods, and the facts elicited have con-
siderably modified the older views as to the structure of the placenta, and the
interrelations of uterine and foetal tissues in it.

In the earliest known stage, the ovum, as already stated, lies imbedded in the
stratum compactum of the mucosa. There is good reason for believing that it has
eaten its way into the mucous membrane by the activities of its trophoblastic
covering. In Peters' ovum (figs. 93 and 98) and the still earlier ovum of Leopold,
the trophoblast is composed of irregular cellular strands attached by their outer ends
to the decidua. In the interspaces between these are lacuna) filled with maternal
blood. The trophoblast is bounded on the foetal side by a fairly regular epithelial
layer (fig. 99, ect), which is lined by mesoderm. It is indented at intervals by the
processes of that layer, which become the cores of the future villi. The blood -lacun-de
reach down to this inner layer. They are everywhere lined by a nucleated proto-
plasmic lamella in which there are no traces of cell-outlines. This is known as the
placental plasmodium or syncytium. The plasmodium where it lines the blood-
lacunae is reduced to a thin endothelium-like layer, but where the decidua and
trophoblast merge it spreads out into masses, which are seen invading the
decidua and pushing their way into the capillaries. The stratum compactum is
beginning to be crowded with decidual cells, the existing capillaries are enormously
enlarged, and there is evidence of the formation of new blood-channels. Immedi-
ately round the ovum there is a zone in which the decidual changes are taking place
more actively ; it contains many large decidual cells, leucocytes and extravasated
red blood-corpuscles, besides multinucleated elements of which it is difficult to
affirm whether they are foetal or maternal derivatives (fig. 99). Some certainly,
probably all, are of trophoblastic origin. In this zone the capillaries are dilated
and sinus-like, and open directly into the blood-lacunae. Masses of syncytium
are seen in the interior of capillaries, and many sections show capillaries which
are lined on the decidual side by endothelium, and on the side of the ovum by
syncytium (fig. 99, sy2). That the vessels are being opened up, and the endothelium
destroyed by the trophoblast, is clearly indicated by these appearances, as well as by
the occurrence of masses of broken-down endothelial cells.

We do not know by direct observation how this stage is reached in the case of
the human ovum, but recent work on the comparative histology of the placenta
leaves no room for reasonable doubt on the main point — viz. that cellular strands,
syncytial masses, and syncytial lining of the blood-lacunae are all equally
derivatives of the chorionic ectoderm.

Origin of the placental syncytium. — The view here adopted that the syncytium is
merely the surface-layer of the chorionic epithelium is now very generally accepted, but there
are some other interpretations of the appearances which may be briefly alluded to.

1. It has been derived by some from the maternal epithelium either of the surface or of
the glands of the mucosa. If the newer views as to the imbedding of the ovum be correct,
such a derivation is very improbable ; but it is finally excluded by the fact that the villi of a
chorionic vesicle, which has become imbedded in the ovary owing to fertilisation of the ovum
having occurred while it was yet in the Graafian follicle, are provided with a well developed
l>lasm<nlial layer. Fig. 100 is a drawing of a villus branch from such a case, and if compared with
fig. 104, p. 76, which is a drawing of a villus from an early uterine pregnancy, it will be seen that
the mesodermic core is covered, in both, by identical layers cellular and syncytial.2

1 See H. Peters, Ueber die Einbettung des menschlichen Eies, &c. Deuticke, Leipzig und Wien,
l.s'.m ; Si«.g,.nbeck van Heukelom, Arch. f. Anat. 1898 ; Marchand, Arch. Gyniikol. 1904 : Anat. Anzeiger
(Erganzungsheft), xxi. : Anat. Hefte, H. 67, xxi. ; Rossi Doria, Arch. Gyniikol. Ixxvi. ; Beneke, Deut.
mod. Wochenschr. Jahrg. xxx. 1904 : Monatschr. f. Geburtsh. u. Gyniikol. xix. ; Graf v. Spee, Verhandl.
deutsch. Ges. Gyniikol. xi. 1905 ; Leopold, Arbeit, a. d. k. Frauenklinik, Dresden, iv. ; H. Happe, Ar>at.
xxxii. ; Keibel, Anat. Anz. (Ergiinzungsheft>, xxx. 1907 ; Frassi, Arch. f. mikr. Anat. Ixx. 1907.

• Ovarian pregnancies are very rare. I am indebted to Dr. Munro Kerr for the opportunity of
examining the sections (prepared by Dr. J. H. Teacher) of an ovary, fixed immediately after removal

74 PLACENTA

2. The outer covering of the villi has been regarded as derived from the decidual tissue,
or from the endothelium of the maternal capillaries. The idea is that there occurs a blending
or interlocking of foetal (chorionic) and maternal (decidual) tissue, so that the villi become clothed
with a layer of decidual tissue (subchorionic membrane, Turner), or that the endothelium of
the dilated capillaries persists as the lining of the blood-spaces (Waldeyer). Such an inter-
pretation is difficult to disprove directly, but it is inconsistent with the newer views regarding
the nature and the activities of the trophoblast. Recent research has provided nearly con-
clusive evidence for the simpler reading of the facts, advanced more especially by Hubrecht and
Van Beneden for lower mammals, and for the human subject by Peters, Leopold, Minot, Webster,
Hart and Gulland, and others, in terms of which the whole placenta, except a thin layer of
decidua on its uterine surface, is derived from the chorion — that, in short, it is a great sponge-
like mass of foetal tissue filled with maternal blood.

If we proceed, then, upon the assumption that the cellular strands (cytdblast,
Van Beneden) and the syncytium (plasmodiblast, Van Beneden) are both derivatives

FIG. 100. — SECTION OF A VILLUS FROM A CASE OF OVARIAN PREGNANCY. (T. H. Bryce.)
sy, syncytium ; L.I., Langhans' layer.

of the chorionic epithelium, the appearances described in Peters' and Leopold's
ova may be explained as follows : At a very early stage, before there is any
differentiation of the embryonic ectoderm, the trophoblast proliferates actively,
and its surface becomes irregular by the outgrowth of epithelial buds. As these
extend into the decidua the maternal tissue is absorbed before them.'2 The
capillaries are opened up by the destruction of their endothelial walls, and the
maternal blood is thus extravasated into the spaces between the strands of the
trophoblast. These spaces now necessarily form a system of intercommunicating
blood-lacunae (fig. 98).

by operation, in which a blastocyst lay imbedded partly in the ovarian stroma and partly in a mass
of fibrin and extravasated blood. The evidence of the sections is conclusive in favour of the foetal
origin of both cellular and syncytial layers.— T. H. B.

1 The trophoblast-cells are supposed to act like phagocytes, and the maternal tissue to serve as
pabulum for the growing ovum. It was on account of the assumed physiological activity of the
chorionic epithelium that Hubrecht gave it the name of ' trophoblast ' — i.e. ' trophic epiblast.' Here,
and elsewhere in this work, the term ' trophoblast ' is used in Hubrecht's original sense, to signify that
part of the ectoderm which does not share in the formation of the embryo, but constitutes the wall of
the blastocyst. Minot has introduced the word troplioderm for the proliferated chorionic ectoderm, or
mantle, which is concerned in the implantation of the egg. The term is not used in this work, because
trophoblast has the priority, and also because in some particulars the processes involved are here
interpreted rather differently than by Minot (Trans. Amer. Gynec. Soc. 1904).

PLACENTA

75

The chorionic epithelium, whether by reason of its active proliferation or other-
wise, becomes, in its superficial lamella, converted into a continuous plasmodial
mass, while the cells in the central portions of the epithelial strands remain isolated
from one another by distinct cell-boundaries. Peters in his original memoir
suggested that the conversion of
the surface-layer of the epithe-
Hum into syncytium was due to
the action of the maternal blood
on it ; but it is more probable
that the differentiation into two
lamellae takes place much earlier
(c/. fig. 40, p. 29), as Van Beneden
has demonstrated for th e placenta
of the bat.

The further changes leading
to the formation of the placenta
will be readily understood by
reference to the diagrams given

Dies,

FIG. 101. — DIAGRAM TO ILLUSTRATE THE FIRST PHASE OF
THE PLACENTA. (After Peters.)

mesoderm ; tr, trophoblast ; b.l., blood-lacuna ;

sy, syncytium ; ca, maternal capillary ; dc, decidua.

in figs. 101 to 103. The tropho-
blast-strands become invaded by
processes from the mesoderm
(fig. 103), and are thus con-
verted into the primary villi.
The villus-stems are at first

simple and attached at their outer ends to the decidua (figs. 101 and 102), but they
soon become drawn out and greatly branched (figs. 102 and 103). As the mesoderm
extends into the epithelial strands, to form the cores of the villi, it is necessarily
covered by both cellular and plasmodial layers. The cellular layer becomes reduced,
as the villi develop, to a single layer of cells, except at their attached ends, and the

core

mcp

FIG. 102. — DIAGRAM TO ILLUSTRATE THE SECOND PHASE OF THE PLACENTA. (After Peters.)

The mesodermic core has now invaded the strands of the trophoblast, and is beginning to branch.
mes, mesoderm; core, core of villus; &y, syncytium; mcp, endothelium of maternal capillary, ca ;
vs, intervillous space ; fib, fibrinous material deposited at junction of trophoblast with decidua.

stems and branches now show the structure seen in fig. 104. In the mesodermic
core capillaries are seen containing nucleated blood- corpuscles from the embryo,
carried thither by the allantoic vessels in the abdominal stalk. The core is
covered by a double membrane, a cellular (Langhans' layer) and a syncytial.

76

PLACENTA

Meanwhile, as the villi become drawn out and branched, the original intercom-
municating blood- lacunas become expanded into the inter villous space, in which
the maternal blood slowly circulates and bathes the villi. The placenta at first

FIG. 103. — DIAGRAM TO ILLUSTRATE THE THIRD PHASE OP THE PLACENTA. (T. H. Bryce.)

The mesodermic processes have further branched, and are now everywhere covered by a single layer
of cells (Langhans' layer) and a lamella of syncytium. At b, where a villus is attached, the cellular layer
retains its primitive arrangement; mes, mesoderm ; ves, ves, vessels going to villi; s?/, syncytium ;
L.L, Langhans' layer; a, cross-section of a villus; dec, decidua; ca, maternal capillary.

extends over the whole chorion, but when the villi of the chorion Iseve degenerate
it becomes confined to the chorion frondosum. Here the villi become continually
more branched, and new villi are formed, until the complicated sponge-work

L.I.

FIG. 104.— SECTION OP A VILLUS FROM AN
OVUM OF THE THIRD WEEK. (T. H. Bryce.)

sy, syncytium; L.I., Langhans' layer.

FIG. 105. — SECTION OF A VILLUS FROM
A PLACENTA AT THE SEVENTH
MONTH. (T. H. Bryce.)

of the discoidal placenta is fully developed. In the later months of pregnancy
the Langhans layer on the villi disappears, and the foetal capillaries are separated
from the maternal blood by the connective tissue of the villus and a thin lamella
of syncytium only (fig. 105).

PLACENTA

77

chcricn.

The changes which occur in the decidua basalis have been already (p. 69)
alluded to. It becomes invaded by masses of syncytium, which penetrate it
even to the muscular layer, and it gradually undergoes a process of degeneration
until it is reduced to a thin layer. This lamella may even be incomplete, for the villi
are sometimes found in direct contact with the muscular tissue. The final term of
the degeneration is the development of a fibrinous layer which serves to mark off
the foetal and maternal tissues. A similar degeneration affects the ends of the
villi, and in the later months this extends to the villi themselves, so that in many
parts the cellular elements have in great part disappeared, as will immediately
be noticed in the description of the full-
time placenta.

The shed placenta. — At full
time the placenta is a discoidal plate
measuring from 16 to 21 cm. in
diameter and 3 to 4 cm. in thickness,
but in shape and dimensions it is
subject to considerable variations. It
is thickest in the centre, and thins
away at the margin where it is con-
tinuous with the chorion and portions
of the decidua.

The surface which has been de-
tached from the uterus shows a number
of irregular areas (cotyledons) sepa-
rated by shallow fissures. The detach-
ment generally takes place through
the remains of the decidua basalis, so
that a thin layer of decidual tissue
covers the ends of the villi. The foetal
surface is covered by the amnion, and
under it are seen the vessels radiating
outwards from the umbilical cord before
they dip into the substance of the
organ. The cord is usually attached
near the centre. It conveys two
arteries to the placenta. They branch
freely but irregularly, and extend out-
wards in the connective tissue of the
chorion to reach the villi in a series
of terrace-like steps, spreading out
horizontally and dipping in vertically
several times. They end. in the villi
in capillary loops (fig. 106), from which
the blood is gathered into veins which, closely following the arteries, finally unite
to form the single umbilical vein round which the arteries coil spirally.

A section across the placenta (fig. 107) shows that the mass of the organ between
the amniotic and chorionic membranes on the foetal side, and the thin covering
of decidua on the uterine side, is made up of immense numbers of arborescent
villi, hanging free in a great space filled with maternal blood. Here and there
the root of a villus is seen springing from the chorion, while the larger stems
and finer branches are seen cut in every direction. A certain number of the
villi are attached to the decidua, but the greater number hang free into the inter-
villous space. Projecting into the placenta from the decidua there are certain

FIG. 106. — DIAGRAM OF THE PLACENTA.
(E. A. Schiifer.)

s, placental sinus ; d.s. decidua basalis ; sp,
spongy layer ; m, muscularis ; a, v, uterine artery
and vein opening into placental sinus.

78

PLACENTA

connective-tissue processes, or septa, to which villi are attached, as well as to the
general decidual surface. These to some extent divide up the placenta into

FIG. 107.— SECTION THKOUGH A NOBMAL PLACENTA OF SEVEN MONTHS IN SITU. (Minot.)

Am, amnion; Cho, chorion ; Vi, root of a villus ; vi, sections of the ramifications of villi in the
intervillous space • the larger blood-vessels within them are represented black ; D, deep layer of the
decidua, showing flattened remnants of enlarged glands in spongy stratum ; Ve, uterine vessel opening
out of placental sinus ; Me, muscular wall of uterus.

PLACENTA

79

loculi. The villi are clothed by a thin layer of continuous protoplasm in which
nuclei are regularly arranged (figs. 105 and 108), and many of them have a
layer of fibrinous material under this syncytium. In the outer part of the
placenta the syncytial layer has in large measure disappeared, to be replaced
by a thick mass of fibrin, and many of the stems have undergone complete
fibrinous degeneration (fig. 108). A dense layer of the same substance also occupies
the outer part of the remnant of the decidua basalis adhering to the placenta.
The blood enters the intervillous space by afferent vessels in the decidua

1 o oo ,»aAtf . o & ooos& oV>0^2 W\ .0. ./ • «•*% I o o o o

oo*oo>— -

o ° ° O

FIG. 108.— SECTION THROUGH A PLACENTA AT FULL TIME.J»V(T. H. Bryce.)

The intervillous space is represented filled with maternal blood. The foetal capillaries were
injected by stripping back the umbilical cord before it was tied, and the corpuscles are represented
solid to distinguish them from the maternal. Notice the layer of syncytium on the villi ; under it, in
many villi, there is a layer of fibrinous material represented by a continuous line; F is a large villus
which has undergone fibrinous degeneration. (From a preparation by Dr. J. H. Teacher.)

connected with small arteries which pursue a spiral course, and are hence
called the ' curling arteries,' while it leaves it by efferent vessels connected with
the veins in the deep part of the remnant of the decidua basalis.1

1 For the literature of the placenta, see Strahl in Hertwig's Handbuch, i. Teil I., II. p. 856 ; also
for some later papers, Kollmann's Handatlas, 1907, appendix, p. 35. See also J. Clarence Webster,
Human Placentation, Chicago, 1901 ; and for comparative data, Arthur Bobiuson (Hunterian
Lectures), Jour, of Anat. and Phys. vol. xxxviii.

80

GENERAL HISTOKY OF DEVELOPMENT

GENERAL, HISTORY OF THE DEVELOPMENT OF
THE HUMAN EMBRYO.

Estimation of ag-e. — In estimating the age of embryos prematurely expelled
from the uterus, we must, in the absence of data as to the early stages, have recourse
to a conventional rule. The rule generally adopted is that formulated by His.
It reckons the duration of pregnancy from the first day of the first omitted
period, the cessation of the menses being the earliest positive sign of
impregnation. '

First month. — The ova of Leopold, of Peters, of Beneke, and one of
those described by Graf v. Spee, must, according to His' rule, be reckoned
to belong to the first few days of pregnancy. Leopold's ovum is probably the
youngest yet discovered, but the relations were somewhat disturbed by intense
congestion of the decidua, and no embryonic rudiment was identified.

mes am

em.pl.

y.s.

FIG. 109.— SECTION OF EMBRYONIC RUDIMENT IN PETERS' OVUM (FIRST WEEK). (After Peters.)

ect, ectoderm of chorion ; mes, mesoderm ; am, amnion ; em.pl., embryonic plate ; y.s., yolk-sac ;
ent, entoderm ; ex.cce, portion of extra-embryonic coelom limited by a strand of the magma reticulare.

We therefore begin with Peters' ovum. In this the cavity of the chorionic
vesicle measures 1-6, '8, and '9 mm. in its three diameters. It is entirely surrounded
by very irregular trophoblast- strands, and the mesoderm is beginning to extend
into these strands to form the primitive villi. The embryonic rudiment (fig. 109)
is still a single layer of ectoderm, to which is attached a small yolk-sac, while it it
covered by a closed amnionic sac. The yolk-sac and amnion are contained withii
and attached to the completely closed chorion, by mesoderm which is alreadj
separated into visceral and parietal layers. The extra-embryonic coelom thi
formed is relatively very large, and is intersected by strands of fibrillae knowi
as the magma reticulare.

The next stage may be illustrated by Spec's embryo v H (fig. 110)
belonging to the beginning of the second week. The chorionic vesicle measures

1 Mall gives a useful formula for estimating the age of human embryos up to 100 mm. in length :
v/lOOxTength in mm. = age in days. In foetuses measuring from 100 to 220 mm. the vertex-breech
millimetre length is approximately equal to the age in days.

FIRST MONTH 81

7 nini. by .V.~> mm., and is entirely covered by villi.1 The embryonic rudiment projects
from the chorion in an oblique direction, and is attached by a broad mesodermic
stalk. The yolk-sac is larger than in the earlier stage, being 1'083 mm. in diameter ;
the embryonic shield is oval, has indications of a primitive streak, and is some-
what concave, the embryonic ectoderm being directly continuous with the flattened
ectoderm of the small closed amnionic sac. The connecting stalk contains a
short diverticulum of the yolk-sac, the rudiment of the allantois.

By the end of the second week the chorionic vesicle measures about 8*5 mm. by
6'5 mm., and is entirely covered by villi in all the specimens described. In Spec's

ectoderm

mesoderm .. ;\<. ——chorion

CUMnOn

yolk-sac

connecting stalk
blood-islands

embryonic plate

/?

rct<n/>'n>i •-- ,/ «SBGLr""lv-~ 'chcrion

mesoderm -

yolk-sac ....... """%£•.

vYv \ ' connecting st-alk

"^••:;cClliiJ>' allantois

B
FIG. 110 A. — EMBRYO OF 0'4 MM. (LETTERED vH) BELONGING TO SECOND WEEK, SHOWING ITS

POSITION AND ATTACHMENT TO INNER ASPECT OF CHORION.

FIG. 110 B. THE SAME IN MESIAL LONGITUDINAL SECTION. (After Graf v. Spee, from Kollmann.)

am, amnion.

embryo Gle (fig. Ill) the embryonic shield measures 1*54 mm. The embryo is
separated by a slight constriction from the yolk-sac, and shows a short primitive
groove, a large neurenteric canal, and a shallow neural groove. The yolk-sac is
covered by projections caused by the development of blood-islands, and distinct
vessels are formed on its lower segment. The connecting stalk has narrowed down
relatively, and, compared with the embryo v H, it would appear that in the laying
down of the embryonic axis in front of the primitive streak a change in the relative
position of parts has taken place, the yolk-sac coming to lie below the shield and

1 The ovum described by Reichert and one described by Mall of about the same age, or rather older,
had merely a circle of villi round the equator, the poles being bare.

VOL. I. G

82

GENERAL HISTORY OF DEVELOPMENT

neural groove — V

r.

neurenteric canal — -ii c

P

I

primitive streak— V

abdominal stalk

FIG. 111. — EMBRYO OF 2 MM. (LETTERED Gle\ ABOUT THIRTEEN
DAYS OLD. (After Graf v. Spee, from Kollmann.)

the connecting stalk at the posterior end. The fore-gut is beginning to be formed,

and the allantoic diverticulum is a distinct tubular passage. The heart, as already

described, is represented by
two lateral vessels which unite
together in front of the neural
groove.

His' embryos E and S R
(fig. 112), estimated to be
about thirteen days, and
Eternod's embryo present the
same characters.

Kollmann's embryo (fig.
113), estimated as fourteen
days old, is farther ad-
vanced. It measures 2'4
mm. in length, and is
separated both in front and
behind from the yolk-sac.
The neural groove is closed
behind, but open in front,
where the cerebral vesicles
form a conspicuous feature.
There are fourteen meso-
dermic segments, and the
heart is now a single coiled
tube. The embryonic axis is
slightly concave.
By the thirteenth day, as illustrated by His' embryo lettered Lg (fig. 114),

the chorionic vesicle has enlarged to 15 mm. by 12 '5 mm., and the villi are

numerous and branched. (In

this particular case they were ^

absent from patches at the

poles.) The amnion closely

surrounds the embryo, which

in most specimens shows a

remarkable dorsiflexion. This

flexure may possibly be a

normal1 though passing fea-
ture, as it is seen also in

some of the lower primate

embryos of the same stage of

development. The yolk-sac

is spherical and about 2 mm.

in diameter. The fore-gut

and hind-gut are formed, but

the cavity of the sac has still a

wide mouth. The neural groove

is closed, and the fore-brain is

bent downward to form the

cranial flexure ; the optic vesicles project from the sides of the fore-brain ; the eai

vesicles are seen as distinct pits above the branchial region, which is marked by two

1 An embryo of 2'5 mm., and estimated as about fifteen days old, studied and modelled by Dr. Petei
Thompson, does not show this dorsiflexion (Jour. Anat. and Phys. xli. April 1907).

amnion

aMom.

yolk-sac —

FIG. 112.— EMBRYO OF 2'2 MM. (LETTERED SB) TWELVE

FIFTEEN DAYS OLD. (His.)

FIRST MONTH

83

slit-like external branchial pouches. The heart is a very prominent structure lying
immediately below the branchial region. About thirty-five pairs of segments can
be counted, and the tail forms a prominent rounded projection, from the under
aspect of which a short thick abdominal stalk connects the embryo with the
chorion, and contains the allantoic diverticulum and allantoic vessels. The

hind-brain

mid-brain

amnion

chorion —

abdom. stalk vilelline vein mouth fore-brain

FIG. 113.— EMBRYO OF 2'4 MM. ABOUT FOURTEEN DAYS OLD. (Kollmann.)

stomodosum is a recess between the under aspect of the fore-brain and pericardial
region, and is separated by the buccopharyngeal membrane from the fore-gut.

Coste's embryo is of about the same age as the one just described.

By the end of the third week a stage is reached represented by His' embryo
lettered Lr (fig. 115). The embryo is almost completely cut off from the
yolk-sac, the mouth of which is narrowed into the yolk-stalk. The embryo
measures in a straight line between its most projecting anterior and posterior
ends 4'2 mm. The head end
is bent on the trunk in the
neck region, forming the cer-
vical flexure. The dorsi-
flexion of the body is no
longer seen, and the tail is
bent in towards the ventral
aspect. The abdominal stalk,
which is now lengthened out
somewhat, passes back to the
chorion on the right side of
the tail.

The olfactory pit is begin-
ning to show on each side of
the fore-brain ; the optic
vesicles are prominent swell-
ings, but the lens pit is not
yet developed ; the auditory

pits are closed, and the branchial region, triangular in shape, shows four branchial
depressions. The ridges bounding the branchial clefts are now named the
maiidibular, the hyoid, and the (three) branchial arches. The mandibular arch*
bounds the stomodceum behind, the hyoid lies between the first and second
branchial clefts, the branchial arches bound the remaining fissures. The hyoid
arch is the largest of the series. The heart, which now lies rather farther back
than before, shows distinct constrictions separating off its several parts. The

o2

— yolk sac

FIG. 114. — EMBRYO OF 2'15 MM. (LETTERED Lg) ABOUT

FIFTEEN DAYS OLD. (His.)

84

GENERAL HISTORY OF DEVELOPMENT

mesodermic segments are distinct, and ventral to them on each side is a longitud
ridge called the Wolffian ridge. This ridge is more prominent at two points,
opposite the posterior end of the heart and allantoic stalk ; the small swellings
thus produced are the earliest signs of the limb-buds.

The general features of a f cetation at the end of the first month are well illustrated
in fig. 116. The chorionic vesicle is laid open. It is still very large relatively
to the contained embryo. The yolk-sac is a rounded vesicle attached to the
embryo by a stout pedicle, and, owing to the flexion of the embryo, the abdominal
stalk and vitelline stalk come to be applied to one another. The embryo is closely
invested by the amnion, and is markedly bent on itself. This bending is best
seen about the twenty-third day, when head and tail touch or even overlap one
another. The greatest diameter of the embryo by the end of the fourth week is
about 7*5 mm., so that, allowing for the flexion, it has obviously much increased
in size. The cervical flexure is very marked.

Fig. 119 represents an enlarged view of an embryo at this stage. On the side
of the head are seen the olfactory pits, above them are the optic vesicles ; the lens-
rudiments have the form of shallow open pits. The auditory vesicles are prominent
rounded swellings, opposite the hyoid arches. The mouth is now a wide cavity

bounded in front (fig. 123}
by a broad field inter-
vening between the olfac-
tory pits called the fronto-
nasal process, behind by
the mandibular arches, and
on each side by lateral
processes named the
maxillary processes, which
project forwards between
the optic vesicles and
mandibular arches. The
mandibular and hyoid
arches are each beginning
to show swellings separ-
ated by constrictions, and
view, being overlapped by the
a recess between the

heart

f>

yolk- stalk

dbdom. stalk

FIG.

115.— EMBRYO OF 42 MM. (LETTERED Lr) EIGHTEEN

TO TWENTY-ONE DAYS OLD. (His.)

and. ves., auditory vesicle ; be, stomodoeum ; fb, fore-brain;
mb, mid-brain ; lib, hind-brain.

the hinder branchial arches are hidden from

anterior arches. This telescoping of the arches produces

branchial region and trunk, known as the precervical sinus.

The mesodermic segments are very prominent objects ; they are thirty-five in
number. In the early part of the fourth week the paired swellings on the Wolffian
ridges become more prominent, and by the end of the week are seen as distinct
buds, the rudiments of the limbs. The heart is relatively very large, and, with the
developing liver behind and slightly above it, forms the prominent rounded swelling
represented in the figure.

It will thus be seen that by the end of the first month all the organs have been
laid down, and the embryo closely resembles any other mammalian embryo at a
corresponding stage — as, for instance, the rabbit embryo at the end of the eleventh
day. During the course of the second month, however, changes take place, which
confer distinctively human features on the embryo.

Second month. — During the first month the chorionic vesicle is, as we have
seen, relatively very large compared with the embryo and its amnion, and the
villi are uniformly distributed. By the end of the second month the distinction
between chorion frondosum and chorion Ia3ve is established, and the amnion has
become greatly enlarged, so as to come in contact with the chorion and obliterate

SECOND MONTH

85

the extra-embryonic coelom. With the enlargement of the amnion the vitelline and
abdominal stalks are bound up together in a tubular prolongation of the membrane
to form the umbilical cord. At the end of the month the cord is about 10 mm.
in length, and still contains at its attachment the primary coils of the small
intestine. The embryo enlarges greatly, but not so rapidly as during the first
month. The back is straightened and the head uplifted, and in consequence the

FIG. 116. — CHORIONIC VESICLE AT THE END OF THE FIRST MONTH, Magnified.
(From a preparation by Dr. J. H. Teacher.)

The vesicle has been opened ; the embryo, closely invested by the amnion, is seen attached by the
abdominal stalk to the chorion.

extreme curvature is to some degree undone. The tissues become condensed and
opaque, so that the internal organs can no longer be made out. The head
remains relatively very large, being at the end of the month nearly as large as the
body of the embryo.

At the end of the fifth week the embryo measures, from neck to breech,
about 12 mm., at the end of the sixth week about 17 mm., at the end of the
seventh week about 20 mm., ajid at the end of the second month about 30 mm.

86

GENERAL HISTOHY OF DEVELOPMENT

(measured from vertex to breech). The chief changes, as far as outward form
is concerned, are the formation of the face and external ear, and the development
of the limbs.

FIG. 117. — CHOBIONIC VESICLE, EMBBYO, AND YOLK-SAC AT THE BEGINNING OF THE SECOND MONTH.
(By permission, from a preparation in the Hunterian Museum, University of Glasgow.)

Face. — In the latter part of the fourth week' the olfactory pits have narrowed
down to form grooves running backwards in the roof of the stomodceum. The

FIG. 118. — CHOBIONIC VESICLE, EMBBYO IN ITS AMNION, AND YOLK-SAC TOVVABDS THE END

OF THE SECOND MONTH.
(By permission, from a preparation in the Hunterian Museum, University of Glasgow.)

margins of these grooves become raised into swellings, which superficially form what
are known as the lateral and mesial nasal processes (processus globulares) (figs. 123,

VUVy

SECOND MONTH

87

124). Between the mesial processes there is a depressed area on the frontonasal
process. In the fifth week an angular projection appears on this area, which

FIG. 119. — EMBRYO OF 7'5 MM. (LETTERED
A) TWENTY-SEVEN TO THIRTY DAYS

OLD. (His.)

FIG. 120.— EMBRYO OF 9'1 MM., THIRTY-ONE TO

THIRTY-FOUR DAYS OLD. (His.)

FIG. 1-21. — EMBRYO OF 15*5 MM. (NECK-
BREECH LENGTH) ABOUT THE BEGINNING
OF THE SIXTH WEEK. (T. H. Bryce.)

This embryo was obtained in a uterus
removed for a large fibroid tumour by Dr.
Oliphant, Glasgow, and fixed in situ with
sublimate solution immediately after the
operation. It may be considered perfectly
normal.

FIG. 122. — EMBRYO OF 30 MM. ABOUT
THE BEGINNING OF THE THIRD
MONTH OR RATHER EARLIER.
(T. H. Bryce.)

(This embryo was fixed in situ.)

afterwards forms the tip of the nose, while the area above it later forms the bridge.
The mesial processes enlarge, and the maxillary processes grow forwards to
meet their outwardly curving ends. The mouth is thus reduced to a narrow slit.

88

GENERAL HISTORY OF DEVELOPMENT

During the sixth week the maxillary processes fuse with the lateral nasal and
mesial nasal processes to form the cheeks, the lateral parts of the upper lip (fig. 125),

FIG. 123.— HEAD OF AN EMBRYO ABOUT
TWENTY-NINE DAYS OLD, FROM

BEFOEE. (His.)

pr.glob., globular extremity of the
mesial nasal process ; mx, maxilla; mn,
mandible; hy, hyoid arch; br', first
branchial arch.

FIG. 124. — HEAD OF AN EMBRYO ABOUT

THIRTY-FOUR DAYS OLD. (His.)

i.m,, placed on the frontal-nasal process and
just above its intermediate depressed part ;
l.n.pr., lateral nasal process ; m.n.pr., mesial nasal
process ; pr.glob., as in the 'previous figure.

FIG. 125. — HEAD OF AN EMBRYO OF ABOUT

SEVEN WEEKS. (His.)

The external nasal processes have united
with the maxillary and globular processes
to shut off the olfactory pit from the orifice
of the mouth.

FIG. 126. — HEAD OF AN EMBRYO
AT THE END OF THE SECOND
MONTH, WITH THE PARTS OF
THE NOSE AND MOUTH BEGIN-
NING TO ASSUME THEIR PER-
MANENT RELATIONSHIPS. (His.)

and the alse of the nose ; while by the end of the second month the mesial nasal
processes fuse with one another to form the mesial part of the upper lip and the

THIRD MONTH

89

columna nasi (fig. 126). The nose is, however, still broad and flat, while the
nostrils remain far apart and look forwards. It is only later that the bridge of
the nose is developed, and the nostrils come to be directed downwards. During
the second month an epithelial plug develops in each nostril, which occludes the
passage for a time. This condition, first described by Kolliker, has recently
been further investigated by Retzius. ! There is no doubt that such a plug exists
in the foatus figured in fig. 122 (see Development of the Nose).

The mandibular processes are at first separated by a flat area, but in the sixth
week they fuse together, and a mesial projection is developed which is the rudi-

FIG. 127. — SKETCHES SHOWING THE DEVELOPMENT OP THE PABTS OF THB EXTERNAL EAR FROM
• PROMINENCES UPON THE MANDIBULAR AND HYOID ARCHES. (His.) Variously magnified.

A, embryo at the end of the first month ; B, embryo of thirty-five days ; C, embryo of thirty-eight
days ; D, embryo at the end of the second month.

1, tuberculum tragicum ; '2, tuberculum anterius helicis ; 3, tuberculum intermedium helicis ;
3c and c, cauda helicis ; 4, tuberculum antihelicis ; 5, tuberculum antitragicum ; 6, tuberculum lobulare :
L, in A, auditory vesicle ; K, lower jaw.

ment of the chin. The lips appear as folds of skin in the sixth week, and by the
end of the eighth week have considerably advanced in development, but the red
margins do not appear till the third month. The eyelids are developed during
the same period as the lips, also as folds of skin. The lacrymal groove between
the eye and stomodceum is covered over by the fusion of the maxillary and lateral
nasal processes. During the second month the branchial arches are telescoped
within the hyoid arch, so that the hyomandibular cleft is alone seen on the
surface, and by the end of the month the sinus prccervicalis is obliterated.

Ear. — Round the hyomandibular cleft small swellings or tubercles appear,
which are named after the different portions of the adult pinna to which they

1 Biolog. Untersuchungen, 1904.

90

GENERAL HISTORY OF DEVELOPMENT

give rise, while the cleft itself becomes the concha and part of the external auditor
meatus. By the end of the month the auricle is so far developed that the adult
parts can be readily recognised. The transformations may be readily understood
from the study of the accompanying series of sketches copied from His, which
show these parts in gradually advancing stages in the human embryo (fig. 127).

decMua

decidua capsularis

FIG 128. — PREGNANT UTERUS AT THE BEGINNING or THE THIRD MONTH.

The uterus has been opened up ; a window has been made in the decidua capsularis, and also in the
amnion, to show the foetus in situ. A large plug of mucus occupies the canal of the cervix. The
bristles indicate the openings of the Fallopian tubes. The cut edge of the decidua capsularis shows
the villi of the chorion laeve. (From a preparation by Dr. J. H. Teacher.)

Limbs. — In the fifth week the limb-buds considerably enlarge, and show a
subdivision into two, then into three, segments (fig. 121). The terminal segment
which forms the hand or foot is broadened out and differentiated into a thicker
basal, and a thinner marginal portion. At the base of the thin marginal segment
the rudiments of the fingers and toes appear as small tubercles, which soon reach
the free margin. During the sixth week the limbs increase in size, the elbow and

LATER .MONTHS ol- PREGNANCY

91

knee form prominent angles, and a certain rotation takes place so that the elbow
oomos to be directed backwards and the knee forwards (fig. 122). The hind-limb
is at first smaller than the fore-limb, and is not so advanced in development. Thus
the fingers project from the free margin of the hand during the sixth week, while

iFiG. 129.— PREGNANT UTERUS OF THE FOURTH MONTH.

The anterior wall of the uterus has been removed to show the fretus in the amnion. (By permission,,
from a preparation in the Hunterian Museum, University of Glasgow.)

the toes do not reach the free border till the seventh week ; the foot is still in the
same plane with the leg. By the end of the eighth week the limbs extend beyond
the body, but the legs are still smaller than the arms. The thumb projects at
an angle different from the other fingers. The ankle-flexure nowr begins to show,
but the limbs are so rotated that the soles of the feet look towards one another.

•92 GENEEAL HISTOEY OF DEVELOPMENT

The tail, which began to disappear in the sixth week, is still distinct at the
of the second month, but reduced to a minute tubercle. The human characl
are all established, and the embryo may now be spoken of as the foetus.

Third month. — Fig. 128 shows the general relations of foetus and uterus at
the beginning of the third month. The decidua capsularis is not yet fused with
the decidua vera. It is still a thickish layer intimately associated with the villi
of the chorion Iseve. The amnion fills the whole chorionic vesicle. The umbilical
cord is a short, stout, and now twisted structure. The foetus by the end of the
month measures about 7 cm. from vertex to coccyx. All its parts have nearly
assumed their relative proportions, though the head is still large. The eyelids
and lips are closed, and the auricle folded. The nails have appeared on fingers
and toes, and the external genital organs are apparent. The vitelline loop of the
intestine is now withdrawn into the body- cavity.

Fourth month (fig. 129). — During the fourth month the foetus increases to
12 cm. or 13 cm. in length from vertex to coccyx. The muscles are now so
far developed as to give rise to movements of the limbs and body.

The skin becomes firmer, and is rose-coloured. Short colourless hairs appear
on the head, and finer downy hairs over the rest of the body. The chin is a more
prominent feature, the arms and legs are of nearly equal length, the umbilicus is
situated close above the pubes, and the sex characters are fully established.

Fifth month. — By the end of the fifth month the foetus measures about
20 cm. from vertex to coccyx, and 25 cm. to 27 cm. if the legs be included in
the measurement. Its weight is now about half a kilogramme. The skin shows
patches of sebaceous matter, and the hair is better developed. The legs are longer
than the arms, and the umbilicus lies farther forwards.

Sixth month. — At the end of the month the length of the foetus from vertex
to heels is 30 cm. to 32 cm. ; it has doubled its weight, which is now about one
kilogramme. The skin is wrinkled and dull red in colour. Sebaceous matter has
increased, especially in the axillae and groins. The hair is darker and stronger,
and the eyebrows and eyelashes appear. The umbilicus is still farther forwards.

Seventh month. — The length of the foetus at the end of the month, from
vertex to heels, is about 35 or 36 cm. ; it weighs about 1J- kilogrammes. Owing
to the deposit of subcutaneous fat, the body has become plumper ; the eyelids
re-open, and the hair is now plentiful on the head. The foetus, if born at this period,
is capable of surviving.

Eighth month. — During the eighth month the foetus increases to 40 to 45 cm.
in length from vertex to heels, but the increase in bulk is more marked ; it now
weighs from 2 to 2J kilogrammes. The skin loses the dull red tint, and becomes
of a bright flesh -colour. Its surface is covered all over with sebaceous matter,
now known as the vernix caseosa : the layer is thickest on the head and in the
axillae and groins.

Ninth month. — At birth the average length of the foetus is about 50 cm.
from vertex to heels, that is, about 20 inches; the average weight is about
3 to 3J kilogrammes, or 6| to 7J pounds. The skin is paler than in the eighth
month, the body is more plump and rounded ; the hair is long and abundant on
the head, but the downy hair (lanugo) on the body has begun to disappear. The
umbilicus now occupies the centre of the body.1

1 For literature other and later than His's Anatomie menschlichen Embryonen, Leipzig, 1885, and
Archiv Anat. u. Phys. Anat. Abt., 1892, see Keibel in Hertwig's Handbuch, I. Teil i., ii. 174 ; and general
literature list, I. Teil i. 83 ; P. Michaelis, Arch. Gyniikol. Ixxviii. ; Retzius, Bi'ol. Untersuchungen,
N.F. xi., Stockholm, 1904. Kollmann (Handatlas Appendix, p. 37) gives a full list of papers. In
addition are the following : Gage, Amer. Jour. Anat. iv. ; Bremner, Amer. Jour. Anat. v. ; Bonnot and
Seevers, Anat. Anzeiger, xxix. ; P. Thompson, Jour. Anat. Phys. xli. ; Ingalls, Arch. mikr. Anat. Ixx.

SECTION II.
DEVELOPMENT OE THE OEGANS OF THE BODY.

Classification. — The organs of the body are generally classified according to
the germinal layer from which their primary or specific elements are derived, but
the middle layer enters into the construction of all organs inasmuch as their
connective-tissue framework, sheathing or covering membranes, and blood-vessels
are derived therefrom. From the histological point of view such a classification
is of importance, because the tissues springing from the several layers show a
certain specific character, both in their history and structural features. This ia
especially true of the epithelia derived from the two primary layers, and it holds
also, though less rigidly, for the derivatives of the dorsal, gastral, or epithelial
mesoderm. The second order of mesoderm — the so-called mesenchyme — isr
however, a blastema of more heterogeneous characters.

With these reservations, the following classification will form the general basis
of the descriptive account of organogeny. From the ectoderm are derived the
epidermis and dermal appendages (hair and nails) ; the epithelium of the sebaceous
and sweat glands as well as of the mammary gland ; of the mouth (in part) ; of the
anal canal (in part) ; of the nasal passages as well as of the glands and cavities
opening thereinto ; the glandular part of the pituitary body ; the enamel of the
teeth ; the whole central and peripheral nervous system ; the epithelium of the
sense-organs ; the sympathetic-chromophil system, and medulla of the suprarenal
body. From the entoderm are derived the epithelium of the alimentary canal
and all the glands connected therewith ; of the Eustachian tube and tympanum ; of
the larynx, trachea, bronchi, and pulmonary alveoli ; of the thyroid and thymus ; of
the urinary bladder and of part of the urethra. From the epithelial mesoderm
are derived the voluntary muscles ; the epithelium of the Wolffian and Miillerian
ducts, and of the excretory tubules both of Wolfnan body and kidney ; the
epithelial lining of the body-cavity ; the cortex of the suprarenal body ; the genital
cords of ovary and testis (and perhaps the germ -cells). From the mesenchyme
are derived the connective tissues, the involuntary muscular tissue, the spleen,
haemolymph and lymphatic glands, the endothelial lining of the heart, blood and
lymph vessels, the red blood-corpuscles, and perhaps the lymph-corpuscles.

DEVELOPMENT OF THE SKIN, CUTANEOUS GLANDS, ETC.

In the section dealing with the formation of the embryo on the blastoderm, it
has already been noted that the ectoderm is early differentiated into a thickened
axial plate, the neural plate, and thinner lateral portions. When the neural canal
has become closed and the embryo is separated from the yolk-sac, the surface
ectoderm forms a continuous layer which gives rise to the epidermis. The
ectoderm at first consists of an epithelial layer of more or less columnar cells ;
but in the stage represented in fig. 81, p. 56, it has taken the form of a thin
lamella of apparently continuous granular protoplasm with a single layer of
nuclei regularly and closely disposed. By the end of the first or beginning of

94 NERVOUS SYSTEM

the second month this layer has resolved itself into two lamellae — a deeper of pol
hedral cells, and a superficial of flattened cells, the rudiment of the stratum corneum
of. the skin. As development proceeds, both of these layers increase in thickness,
and several cellular strata are laid down. By the fifth month the surface cells
begin to be shed, and form with the secretion of the sebaceous glands the cheesy
layer on the skin of the foetus known as the vernix caseosa. Meantime, the
underlying mesenchyme has differentiated into a connective-tissue layer which
becomes the corium or true skin, and projections from this develop into the
vascular papillae. The epithelium of the glands is formed from the deeper layer
of the primitive epidermis. Solid processes grow inwards from this into the
mesenchyme ; in these a lumen is developed later. The sweat-glands open on
to the surface, but the sebaceous glands are formed in connexion with primitive
ingrowths of the epidermis, which give rise to the hairs and their root- sheaths.
The surrounding mesenchyme gives rise to the connective-tissue elements of the
glands, the sheaths, and papillae of the hairs, and also to the muscular tissue of the
arrectores pilce muscles and muscular tissue of the sweat-glands.1

The primitive downy hair of the foetus is known as the lanugo. It is partly
shed in the later months of pregnancy, and is replaced after birth by permanent
hair.

DEVELOPMENT OF THE NERVOUS SYSTEM.2

As has been already described, the whole of the central nervous system takes
origin from the thickened walls of a dorsally situated axial groove, subsequently
converted into a canal, which runs forwards in front of the primitive streak. The
anterior end of this canal becomes enlarged and converted by constrictions into
three vesicles, around which the several parts of the brain are formed. These
enlargements are known as the primary cerebral vesicles. The rest of the neural
canal remains of nearly uniform diameter ; its walls become converted into the
substance of the spinal cord, while the cavity itself becomes eventually the central
canal of the cord. The walls of the neural groove are of course composed of
ectoderm, and it therefore follows that the whole structure of the central nervous
system is laid down in that layer, and consists in the main of more or less modified
ectodermic elements, except where mesodermic tissues subsequently penetrate
into it, conveying blood-vessels into its substance. The same is in all probability
true for all the nerves of the body, cranial and spinal.

Histogenesis of nervous tissue and peripheral nerves. — Before
entering on a description of the phases through which the neural canal passes as
the brain and spinal cord take form, it will be convenient to give a general
account of the changes by which the ectoderm of the wall of the canal is converted
into nervous tissue. The changes are in essence the same in all parts of the
tube, and in all vertebrates.

The neural plate consists at first of a layer of columnar epithelium. The
divisional planes between the cells are rather ill-defined. The outer ends of the
elements are composed of granular protoplasm, the inner ends are finely striated.
The nuclei are in several ranges, placed at different levels ; the innermost are
aJmost without exception in one phase or another of karyokinesis (fig. 130).
Proliferation is thus taking place on the free surface (afterwards the inner surface
of the closed neural canal), and through all successive phases this zone of dividing
nuclei persists until a certain stage of development is reached. It is therefore

1 Some authorities derive the muscular tissue of the sweat-glands from the ectoderm.

2 The literature concerning the morphogenesis of the central nervous system in the Primates will
be found in Ziehen's article in Hertwig's Handbuch II. Part III. p. 386 seq. ; that concerning histo-
genesis, ibid. p. 434 seq. For the literature of the development of the peripheral nervous system
see Neumayer, ibid. p. 621 seq. References to more recent papers mentioned in the text are given in
the footnotes.

NEBVOUS SYSTEM

95

spoken of as the germinal zone. The second stage is characterised by a disap-
pearance of cell-outlines, so that the wall of the canal appears formed of continuous
protoplasmic columns. These then break up (third stage) into a mesh-work

/•mlial <v;/ ///////.<

<^f protopltum yrniinal zonr

'

.«r'~r 4

s clivid ing nuclei

i/i/. mecZ. lam.

FIG. 130. — SECTION OF THE WALL OF THE NEURAL CANAL FROM AN EMBRYO PIG,

JUST AFTER THE CLOSURE OF THE NEURAL GROOVE. (Hardesty).

(terminal zone

£5^ ^7>. ?^7):^^">'7S x^J-r^ """ / ^ r

^^5¥fe^ >£^/^r;E) :SN ^?y •;

. med. lam.

I

jria

^-^^

(I int. med. lam.

\ v'

-V

reticular zone

)>ucl>>(ir zone

germinal zone

FIG. 131. — SECTIONS OF THE WALL OF THE NEURAL CANAL OF THE PIG AT LATER STAGES THAN
THAT SHOWN IN FIG. 130. (Hardesty).

A, from an embryo of 7 mm. ; B, from an embryo of 10 mm. ; g, dividing
nuclei in germinal zone ; r, filaments of myelospongium.

96

NERVOUS SYSTEM

(His '), in which the trabeculse have, in a general way, a radiating disposition
and the oval nuclei, which are distributed throughout the whole thickness of the
wall, have a vertical direction (fig. 131A). On both the inner and outer surfaces th(
trabeculse fuse to form a limiting membrane (membrana limitans internet, and
externa). The nuclei now become densely crowded in the central part of the
thickening wall of the tube, and on the outer aspect a layer appears in which thei
are no nuclei. It is crossed by delicate, radially disposed, protoplasmic strands,
which break up into a system of anastomosing threads. The original epitheliui
has thus been converted into a protoplasmic framework (myelospongium), whicl
has a columnar and radial disposition and shows three layers : (1) An inner germiv
zone ; (2) a middle nuclear mantle zone ; and (3) an outer nuclear-free reticular 01
boundary zone. The columns of the framework are not separate, long-drawn-out
epithelial cells, but the radial trabeculse of a continuous syncytial network, which

are more accentuated than the side
branches (Hardesty2). In the germinal
zone the dividing nuclei are separated
by delicately striated columns of
protoplasm which broaden out and
fuse into the internal .limiting mem-
brane (fig. 131B). This appearance
due to the persistence in this inner
layer of the second stage of the
epithelium, when the cell- outlines
have disappeared and it consists of
columns of protoplasm. The inner
zone is afterwards, when the nuclei
have "ceased to divide, resolved into
the ependymal lining of the canal,
and each ependymal cell is continued,
through the thickness of the wall of
the tube, into a fibre which represents
a radial strand of the myelospongium
(fig. 132). During the transformation
of the neural epithelium into the
myelospongium nuclear proliferation
has been very active, and the crowded
nuclei in the mantle zone (during the
third stage) are obviously the products
of the divisions in the germinal zone.
Among them there can now be de-
tected certain nuclei which appear to
belong to pear-shaped cells with long tapering processes. These are the future
nerve-cells, and were named by His neuroblasts in contradistinction to the cells of
the framework which he termed spongioblasts. It is generally admitted, the exist-
ence of two categories of cells in the early stages being allowed, that both
spongioblasts and neuroblasts spring from the multiplying ectoderm-cells; and
further it is believed that the neuroglia-elements of later stages are also of ectodermic
origin, though mesodermic elements — i.e. common connective-tissue cells — are
necessarily introduced with the blood-vessels which invade the epithelium, The
appearances described have been interpreted in two different senses. According to
the interpretation hitherto almost universally accepted and due to His, each
neuroblast has a separate and isolated protoplasmic body from which a single

FIG. 132. — SECTION OF THE SPINAL CORD TO SHOW
THE ARRANGEMENT OF THE EPENDYMAL AKD
GLIAL FIBRES OF THE DEVELOPING CORD.
(Lenhossek.)

1 Die Entwickelung des menschliclieii Gehirns wabrend der ersten Monate ; Leipzig, 1904.

2 Amer. Jour, of Anat. ii. 1904.

DEVELOPMENT OF NERVE-FUJI,1 K>

97

pr, segment

&

& ^&'

neural canal

FlG. 133.-VENTRAL BOOT OF A SPINAL NEBVE WITH THE SUBBOUNDING 3IESENCHYME IN A
BABBIT-EMBRYO OF THE ELEVENTH DAY. (T. H. Bryce.)

pr.s.

I ' •• 134.-PBIMITIVE SPINAL NEBVE-TBUNK IN AN EMBBYO OF LfiPIDOSIBEN PABADOXA

(Graham Kerr.)

np., wall of neural canal ; n.t., nerve-trunk ; me*, mesenchyme-cell ; pr.s., primitive sheath •

mi/, myotome. The white bodies are yolk-grains.
VOL. I.

98

NERVOUS SYSTEM

protoplasmic process grows to become the axis- cylinder process of the future nerve-
cell, and by further extension a nerve-fibre. The dendritic processes are secondary
outgrowths which, according to the manner of their branching, confer distinctive
characters on the several varieties of nerve-cells. The neuroblasts occupy the
spaces of the myelospongium, and the nerve-fibre processes thread its interstices.
The spongioblasts form supporting elements merely. According to another inter-
pretation, founded on the conception of nerve-tissue put forward by Apathy and
Bethe, and advocated by Held, 1 the neuroblasts are cells of the syncytium in which
neurofibrillae are differentiated. These form a plexus in the cell-body, surround
the nucleus and extend first, as a chief bundle (pear-shaped stage), through one of
the cell-bridges and along a definite path through the syncytial framework to form
the axis-cylinder process, and later into the other cell-bridges to form the dendrites.
While this interpretation differs from the first in so far as that the nerve-fibres are
regarded as neurofibrillar tracts in the substance of the myelospongium, not free

processes threading its meshes,
the two theories agree in respect
that the nerve-fibre is represented
as an active outgrowth from
the nerve-cell. On this point
Held's view differs from that
of Apathy, who conceives the
conducting fibrillae as being laid
down in situ along a protoplasmic
path provided by cells (' nerve-
cells ') united into a syncytium.

Origin of the nerve-
roots. — The motor nerve-root*
appear before the sensory. In
mammalian embryos, when first
detected, the rudiments of the
motor nerves appear as bundles
of extremely delicate proto-
plasmic fibrillae extending among
the mesenchyme- cells from the-
wall of the neural tube, within
which the bundles can be traced

backwards through the commencing reticular framework to neuroblasts in
the mantle zone (fig. 133). Followed outwards, they pass towards the myotome
(fig. 134). In the case of the spinal nerves each motor root is joined by a
contingent of fibrillae from the spinal ganglion, and the mixed nerve so forme
is seen extending into the Wolffian ridge along the mesial aspect of the growing
muscle -rudim ent .

The sensory roots are developed from primitive ganglia which arise from
the neural crest. This is formed of cells derived from the ectoderm at the
angles of the neural folds (fig. 135). From the sides of the crest cells ai
budded off into the space between neural canal, myotome, and surface ectoderm,
and are there collected into paired groups, which are the rudiments of
the spinal and cranial nerve-ganglia. In the trunk these are segmentalb
disposed. It is possible that only a certain proportion of the cells
so utilised, for there is reason to believe that some of them wander farthei
afield. Each ganglion is at first connected with the side of the neural tube by

FIG. 135.— Two STAGES IN THE DEVELOPMENT OF THE

NEURAL CREST IN THE HUMAN EMBRYO. (LenllOSSek.)

1 Anat. Anzeiger, Erganzungsheft, xxix. 1906 ; and Anat. Anzeiger, xxx. 1907.

DEVELOPMENT OF NERVE-ROOTS

99

'

**

conl of ectoderm- cells. Differentiation next occurs in the ganglion exactly as it
does in the neural tube, and bundles of fibres appear among the cells. These
extend through the cellular strand (fig. 136) and enter the wall of the neural tube,
where they form a special bundle in the reticular zone (see p. 101). From the
opposite end of the ganglion, bundles of fibres extend peripherally and join the
fibres of the ventral root
to form the mixed nerve-
trunk. The picture pre-
sented by a preparation
of a developing spinal
ganglion stained by the
Golgi method shows that
the bundles of fibres are
proximal and distal pro-
cesses of bipolar neuro-
blasts, or, as Held be-
lieves, bundles of neuro-
fibrillee extending proxi-
mally and distally from the
ganglion- cells through a
syncytium into which the
primitive ganglion has
been resolved. The mixed
nerve- trunks are at first
composed of rather loosely
arranged fibrils, but soon
they appear as tracts of
compactly arranged bun-
dles of fibres which, traced
outwards, divide up, and
finally have the appearance
of ending freely as single
fibres, on which a terminal
enlargement is shown by
the Golgi method of stain-
ing. This terminal enlarge-
ment is highly character- * :%\n*
istic of developing nerve- if
fibres.

The developing nerve-
paths are studded with
nuclei. It is generally

agreed that these represent the nuclei of the future sheath of Schwann, but
opinion is divided as to their origin.

It is not possible here to give more than the briefest notice of the main hypotheses which
have been advanced regarding the mode of formation and growth of nerve-fibres. They may
be represented in tabular form thus :

I. Outgrowth theory (Bidder, Kupffer, His, Cajal, Kolliker, Lenhossek). — Each nerve-fibre
is the process of a nerve-cell (neuroblast) which by free terminal growth seeks out its proper
end-organ and comes secondarily into relation with it. In the case of the bipolar neuroblast?
there is growth in two directions.

II. Cell-chain theory (Balfour, Marshall, Dohrn, van Wijhe). — Each nerve is the product of
a chain of medullary ectoderm-cells, which extends by proliferation and establishes a secondary
nexus between central and end organ.

H 2

* & a*

-

I

,>;::*

* $\> %

V\

.;>,x..

V

1

• *

FIG. 136.— SPINAL GANGLION AND ANTERIOR NERVE-ROOT IN A

RABBIT- EMBRYO OP THE TWELFTH DAY. (T. H. Bryce.)

100

NERVOUS SYSTEM

III. Syncy tied nucleated network theory (0. Sclmltze). — There is a primary nexus between central
and end organ in the form of a network of anastomosing cells, which becomes differentiated

nto nerve-fibres. Apathy's conception of a plexus of ' nerve '-cells in which neurofibrillae are
deposited to join up ' ganglion '-cell and end-organ is closely akin to this.

IV. Outgrowth with early union theory (Hertwigs). — The end organ is brought very early
nto connexion with the central organ by a protoplasmic process which is differentiated into a

nerve and pulled out as the organs draw apart.

V. Primitive-nerve theory (Baer, Hensen, Sedgwick). — There is a primary nexus between
central and end organ in the form of protoplasmic strands which become fibrillar, and are drawn
out into nerve-fibres as the organs draw apart.

VI. Primitive nexus and outgrowth theory (Held). — There is a primitive protoplasmic nexus
between central and end organ, along which neurofibrillse grow from the nerve-cell outwards.

In the first stage of the nerve the path is
non-nuclear, being furnished by Szily's
inter-epithelial network (see p. 59) ; in the
second stage it is nucleated, being provided
by the cellular (mesenchymic) syncytium.

The sheath of Schwann is variously inter-
preted. According to theories III. and IV.,
the nuclei of the sheath are those of the cells
entering into the formation of the nerve-
fibre, while the remaining hypotheses refer
them to a secondary investment either of
mesenchyme, or of ectoderm cells derived
from the ganglion-crest, or from the neural
tube along the motor root. It has been
stated above that the fibrous stage of the
dorsal roots of the ganglia is preceded by
a cellular stage, and the same is true of
the peripheral branches of certain purely
sensory nerves (A. F. Dixon l and others).
When the fibrous stage is established
these cells envelop the nerve-fibres. It
would appear probable from this that
the sheath of Schwann has an ectodermic
origin in sensory nerves ; and that the same
is true of the motor nerve -fibres is pointed
to by Harrison's experiments in which
the motor nerves were found to develop
without a cellular sheath in tadpoles from
which the neural crest had been removed
by operation (the dorsal roots remaining of
course undeveloped). This derivation of
sheath-cells from the neural-crest ectod<
has suggested that the peripheral extens
of the nerves may be associated with

FIG. 187. — SECTION OF SPINAL CORD OF FOUR-
WEEKS HUMAN EMBRYO. (His.)

• The dorsal roots are continued within the
cord into a small longitudinal bundle which is
the rudiment of the dorsal white column. The
anterior roots are formed by the convergence of
the processes of the neuroblasts. The latter, along
with the elongated cells of the myelospongium,
compose the grey matter. The external layer of
the cord is traversed by radiating fibres which
are the outer ends of the spongioblasts. The
ventral commissure is beginning to appear.

continuous proliferation of indifferent eel

which may become ganglion -cells in the sympathetic, or sheath-cells, in the nerve-tru
(Kohn). On the other hand, the evidence is strong in some cases that the mesenchy
cells furnish a nutritive sheath to the developing nerve (figs. 133, 134), and it see
not impossible that the nuclei in and around the nerve-path may be derived from
sources.

The outgrowth- theory of His has been very generally accepted by anatomisl
and physiologists as the embryological basis of the Neurone-theory, and it hs
obtained its chief support from the Golgi method of staining, as applied moi
especially by Kamon y Cajal. But, as will be obvious from the above stateinei
of the various views which are prevalent on this subject, His' theory is by m
means universally admitted. The work of Held seems to show that, while

1 Trans. Roy. Dublin Soc. vi. (Series IT.), 1896.

SPINAL COED

101

the main principle underlying the theory may be maintained, the divergent
results of different observers in matters of detail cannot yet be brought into line
in any general hypothesis.

MORPHOGENESIS OF THE SPINAL CORD AND BRAIN.

SPINAL CORD. — By the end of the first month the neural canal in the region oi
the trunk shows, instead of the primitive rounded cavity, a narrow cleft-like
lumen, which widens somewhat at its dorsal end (fig. 137). This change is

pr. dors. col.

pr. dors. col.

ventr. col. ventr. horn

ventr. com.

ventr. horn ventr. col.

FIG. 138. — SPINAL CORD OF A HUMAN EMBRYO OP 15'5 MM. (T. H. Bryce.)

c<:nt>: com., ventral commissure ; ventr. col., ventral column : ventr. horn, primitive ventral horn;
l>f. ilnrn. col., primitive dorsal column; m.r. motor, s.r. sensory root.

due to thickening of the lateral walls, which shows the three zones already
described — inner ependymal, middle mantle, and outer reticular ; while the roof
and floor are represented merely by the ependymal lamella. The mantle zone
shows large numbers of neuroblasts whose nerve-fibre paths tend towards the
ventro-lateral aspect and leave the cord as the ventral nerve-roots. By the end
of the fifth week (fig. 138) the lumen shows a dorsal and ventral narrowing and
a mesial expansion, indicating a later demarcation of the lateral walls into dorsal
or alar, and ventral or basal plates. The ependymal roof projects beyond the
general surface as a convex swelling, and on each side of this is a portion of the

102

NERVOUS SYSTEM

reticular zone, which is connected with the dorsal nerve-roots, and is known as
the primitive dorsal column. Ventrally the mantle zone is much expanded, and
forms the rudiment of the ventral horn of the grey crescent on each side ; while
over this the reticular zone is thickened, and projects as the primitive ventral
column. The rudiment of the ventral horn is in the fifth week (fig. 138) sharply
marked off from the rest of the mantle zone by the character of its rounded and less
deeply staining nuclei. The ventral root-fibres can be seen emerging from the
ventro-lateral aspect. Between the horn and the central canal the fibre-paths all tend
towards the ventral commissure. Between the dorsal and ventral expansion is a
recessed area in which the boundary zone is very thin. In succeeding stages this

r f'f"'^:

FIG. 139. — SECTION OF THE SPINAL CORD OF A HUMAN EMBRYO OF 30 MM. (T. H. Bryce.)

is gradually filled up, until the ventral projection reaches the dorsal root, anc
the continuous ventro-lateral column is laid down. The mantle zone of this
region becomes the dorsal horn of the grey crescent. It is at first separal
from its fellow by the now slit-like dorsal portion of the lumen, but it Ia1
becomes isolated as the dorsal septum is laid down.

The anterior or ventral median fissure is at first a shallow recess between the
projecting primitive ventral columns. As these grow in dimensions the lips of
the fissure close in to form a narrow cleft, which encloses a process of pia mater
developed from the mesenchyme surrounding the cord. At the bottom of the
fissure the anterior or ventral commissure is laid down by the extension of nerve-
fibres across the middle line.

SPINAL CORD

103

The so-called posterior or dorsal median fissure appears first in the sixth week
as a distinct infolding of the dorsal plate towards the lumen. At this time the
primitive dorsal columns are still separated by the projection of the plate on
the dorsal aspect of the cord. In the succeeding weeks the upper part of the
primitive canal appears to be obliterated, and only the ventral portion persists
as the definitive central canal of the cord. The apparent closure of the lumen is
associated with great expansion of the primary dorsal columns (future tracts of
Burdach) and the formation of new mesial columns (future tracts of Goll) (fig. 139).
As these columns increase in size, the dorsal horns of the grey crescents are
isolated and clothed on their mesial aspects by white substance. Meanwhile the
walls of the dorsal part of the central canal become apposed, and the ependymal
cells become obliquely arranged, the obliquity of their direction increasing as the
dorsal wall is approached. From this a mesial fibrillar septum extends to the
free surface. These appearances suggest that the canal is not obliterated by a
simple fusion of its walls, but, on the other hand, they cannot be explained wholly
by the growth of the posterior columns and posterior horns with persistence of
the primary canal and extension of the primary infolding of the
dorsal plate. Both factors probably share in the process.

The end part of the central canal is expanded into what
has been termed the terminal ventricle. It forms the dilated
part of the central canal in the conus medullaris.

FIG. 140. — BRAIN AND SPINAL COED EXPOSED FBOM BEHIND IN A FCETUS OF
THBEE MONTHS. (From Kolliker.)

h, the hemispheres ; m, the mesencephalic vesicle or corpora quadrigemina ;
c, the cerebellum ; below this are the medulla oblongata, mo, and fourth
ventricle, with remains of the meinbrana obturatoria. The spinal cord,
s, extends to the lower end of the sacral canal, and shows brachial and crural
enlargements.

In the sixth week the lateral walls show a distinct separation
into alar and basal lamince. With the former 'the afferent nerve-
fibres become connected ; while from the latter the efferent
fibres take origin (His).

The characteristic cylindrical form of the cord is only attained
with the development of the lateral columns. The cervical and
lumbar enlargements are manifest at the end of the third month
(fig. 140).

Up to the fourth month, the cord and the vertebral canal increase in length
pari passu, but the vertebral column then begins to grow more rapidly than the
cord, so that by the time of birth the coccygeal end of the cord is opposite the
third lumbar vertebra, while in the adult its limit is the lower end of the first
lumbar. Along with this relative shifting of the cord and its containing tube
the lower nerve-roots lose their regular rectangular course, and become oblique.
They alone, with the fttum terminate, occupy the lower end of the neural canal,
where they form the cauda equina.

The nerve-fibres of the white columns are at first entirely non-medullated, and
the white substance has a greyish transparent appearance. The medullary sheath
is not formed simultaneously in all parts, but appears at different times in different

1 In the figures published by His of the developing spinal cord, certain other folds of the wall are
represented. Wilson (Jour, of Anat. andPhys. vol. xl.) has recently again drawn attention to these folds.
He suggests that they have morphological significance, and may possibly indicate a triple division of
the neural tube. It is doubtful, however, how far such folds or furrows are to be regarded as normal.
The appearance of the basal plate (see fig. 138) seems to be correlated with the earlier differentiation of
the motor region of the mantle zone. In a well-fixed embryo of 15'5 mm. (obtained at an operation),
and in one of 21 mm. I cannot satisfy myself that there are any irregularities of the wall of th«
such as are seen in His' figures. — T. H. B.

104

NERVOUS SYSTEM

M

NK

FIG. 141. — PROFILE VIEWS OF THE BBAIN OF HUMAN EMBBYOS AT SUCCESSIVE STAGES,
BECONSTBUCTED FBOM SECTIONS. (His.)

A. Brain of an embryo of about fifteen days (the embryo itself is shown in fig. 115) magnifiec
35 diameters.

B. Brain of an embryo about three and a-half weeks old. The optic vesicle has been cut away.

C. Brain of an embryo about seven and a-half weeks old. The optic stalk is cut through.

A, optic vesicle; H, vesicle of cerebral hemisphere; Z, diencephalon ; M, mid-brain; J, isthmi
between mid- and hind-brain ; Hh, cerebellum ; N, rhombic brain ; Gb, otic vesicle ; Bf, fourt
ventricle ; Nk, neck-curvature ; Br, pons-curvature ; Pm, mamillary process ; Tr, infundibuluro
Hp (in B), outline of hypophysis-fold of buccal ectoderm; 111, olfactory lobe. In C the basihxr arter
is represented along its whole course.

BRAIN

105

columns corresponding with the tracts of conduction ; the last of these tracts to
become medullated are the pyramidal tracts.

FIG. 142. — SECTIONS ACBOSS THE BEGIOX OF THE CALAMUS SCRIPTORIUS OF THE BRAIN REPRESENTED

IN FIG. 141, A. (His.)
A, region of the glosso-pharyngeal ganglion. B, region of the auditory facial ganglion.

The membranes are formed from the mesenchyme of the sclerotomes, which
extends over and under the cord and becomes enclosed along with that structure
within the developing vertebral canal. The septa of connective tissue which are

bl

FIG. 148. — SECTIONS ACROSS THE FOURTH VENTRICLE OF A SOMEWHAT OLDER EMBRYO. (His.)

A, section taken through the lower part. B, across the widest part (trigeminus region).

C, through upper part (cerebeilar region).
r, roof of neural canal ; al, alar Jamina ; bl, basal lamina; v, ventral border.

seen penetrating into the substance of the cord from the pia mater grow in from
this mesenchyme, carrying blood-vessels amongst the nervous elements.

FIG. 144. — SECTIONS ACROSS THE LOWER HALF OF THE RHOMBENCEPHALON OF A STILL OLDER EMBRYO,
SHOWING GRADUAL OPENING OUT OF THE NEURAL CANAL AND THE COMMENCING FOLDING OVER OF

THE ALAR LAMINA (at /).

v, ventral border; t, tsenia : ot, otic vesicle; r.L, recessus labyrinthi.

In the succeeding stage (not here represented) the angle at v has almost disappeared, the fold /
{rhombic Up) has extended over the alar lamina, and the two thickened halves are in the same
horizontal plane, covered by a greatly expanded and thinned-out roof.

BRAIN, — The cephalic part of the neural tube closes rather later than the spinal
part. In some mammals (pig ; Keibel) the medullary laminae close only after
organogenesis has proceeded to some extent, and after the cephalic flexure has

106

NERVOUS SYSTEM

begun to be developed. This is probably the case in the human embryo also.
The optic vesicles are formed in the pig as pits on the spread- out neural laminae,
and in the human embryo they are seen as hollow outgrowths from the fore-brain
before the laminae have united. In an embryo at the beginning of the third week
(embryo of 3'1 mm.) the brain-tube shows an anterior and posterior dilatation
connected by a narrower portion which corresponds to a sharp flexure (cephalic
flexure) which has bent the anterior on the posterior segment. The hinder and
larger dilatation is named the hind-brain (rhombencephalon), the anterior the fore-
brain (prosencephalon), while the intermediate portion is termed the mid-brain
(mesencephalon). The terminal wall of the fore-brain as shown in His' model is
not yet completely closed, a foramen being left called the neuropore. From the
fore-brain the optic vesicles project as wide-open diverticula connected now by
narrower portions, the optic stalks.

During the third and fourth weeks, as the primitive brain increases in length
and the embryo becomes more folded, two other bends appear. The first of these
occurs at the junction of the brain and spinal cord, and is due to the head being

bent down on the trunk (fig. 141 A).
It is called the cervical flexure, and
its concavity is necessarily ventral
like that of the primary head bend
(fig. 141s). The second flexure in-
volves the rhombic brain, and is not
determined by the curving of the
embryonal axis. It is produced by
a doubling of the hind-brain on itself.
Its concavity is directed dorsally. and
the point of the bend corresponds to
the position of the future pons ; it
is hence called the pontine flexure
(fig. 141c). While these bends are
forming, the fore-brain, by the increase
of the cephalic flexure, is folded back
below the parts of the tube originally
behind it, so that its ventral aspect
becomes closely applied to the ven-
tral surface of the rhombic brain,

and the space between them is reduced to a narrow cleft filled with the
mesenchymatous investment of the primitive brain. In this tissue the dorsum
sellae is afterwards formed. Owing to this increase in the cephalic flexure the
mid-brain comes to be the most anterior part of the brain-tube, and is for some
time a very prominent feature, although later it loses in prominence by the
greater development of cerebrum and cerebellum. As morphogenesis proceeds,
the walls of the brain- tube, thus sinuously curved, become thickened and expanded
in certain regions to form the several parts of the adult organ, while in other
situations they remain mere ependymal lamellae, which, becoming inflected into the
interior of the primitive vesicles and carrying vascular mesenchyme between their
layers, form the epithelium of the choroid plexuses. Instead of tracing the
development of the brain as a whole from stage to stage, it will be more
convenient to describe separately, through all the phases, the several parts
derived from each primary division of the tube.

Rhombic brain (rhombencephalon). — The hind-brain in its early stages
has the very characteristic feature that the ependymal roof -plate is greatly thinned
and expanded. At first full and convex, this membranous lamella has a triangular

FIG. 145. — MODEL OF THE BRAIN OF A HUMAN
EMBRYO OF 53 MM. (ELEVENTH WEEK) FROM
BEHIND, TO SHOW THE FOURTH VENTRICLE,
RESTIFORM BODIES, AND CEREBELLUM. (His.)

HIND-BRAIN

107

ipe when viewed from the surface. Its apex corresponds to the cervical flexure ;
its base is marked off by a thickened band, which afterwards develops into the
cerebellum. The hind-brain is broadest at the level of this transverse thickened
band ; in front it is connected with the mesencephalon by a thinner portion called
the isthmus (His) ; behind it gradually tapers to its junction with the spinal cord
at the cervical flexure.

In early stages, as shown for the human embryo by Broman and by Peter
Thompson,1 the rhombic brain shows slight constrictions marking off a series of
segments or neuromeres, seven in number.

FIG. 146. — SECTION OF THE HEAD OF A HUMAN EMBRYO OF 30 MM. (BEGINNING OF THIRD MONTH).

Photograph. (T. H. Bryce.)

IV, cavity of rhombencephalon or fourth ventricle ; in the roof of the ventricle the paired cerebellar
plate ; the lateral enlargements are the rhombic lips cut where they pass into the cerebellar plate. N
is placed in the naso-pharynx still continuous between the two palatal folds with the buccal cavity (Af).
The Eustachian tubes are seen opening into the naso-pharynx on each side ; above them in the cranial
base are the auditory labyrinths ; in the floor of the mouth is the tongue. Below the tongue are seen
the distal ends of Meckel's cartilages; above on each side their proximal ends. IAI!U

Pari passu with the development of the pontine flexure the lateral walls of the
hind-brain are opened out, and the epithelial roof becomes greatly expanded
(figs. 142, 143, 144). This seems to be the mechanical effect of the forces causing
the ventral folding ; the behaviour of the tube has been compared to that of a
split indiarubber tube which is bent on itself. The thickened lateral walls are
divided, just as in the spinal portion of the neural canal, by lateral grooves into

1 Jour. Anat. and Phys. xli.

108 NERVOUS SYSTEM

alar and basal laminae. When the tube is opened up these become laid out hori
zontally, and form the mesial and lateral portions of the floor of the fourth ventricle.
The lateral grooves bounding the alar and basal laminae are represented by the
superior and inferior fovea of the adult brain ; the basal lamina becomes the
trigonum hypoglossi and eminentia teres, and the alar lamina the ala cinerea, the
tuberculum acusticum, and locus coeruleus.1 The apex of the fourth ventricle
reaches at first to the cervical bend — i.e. the beginning of the spinal cord. The
closed part of the medulla oblongata is a later formation produced by the neural
laminae closing in again (His), presumably owing to the great development of
the ganglionic masses in the alar laminae which form the clavate and cuneate nuclei.
The obex is the remains of the ependymal roof of this portion of the hind-brain.

At a stage when the alar laminae still stand vertically their dorsal borders
become recurved and form what are known as the rhombic lips (fig. 144B). The
two folds of the lips fuse with one another, and form marginal swellings which
run in front into the cerebellar lamina. They become ultimately the restiform
bodies (third month), and share in the formation of the cerebellum (fig. 145). At
an early stage the afferent fibres of the vagus and glossopharyngeal nerves
which have grown in from the corresponding ganglia form a conspicuous bundle

(funiculus solitarius) on the
^•-,^ „ . .__ ^ .. ^ _,...-'-h"misphere gurface jugt ventral to the

rhombic lip. It represents the
primary dorsal column of
the cord in this region. In

~,emin. teres each rhombic lip a mass of

.,, neuroblasts develops which

tub. Rolando- - - ...•-

corresponds to the dorsal horn
in the cord, and from this
-fun.gradiis mass the cuneate nucleus and

.. I

substance of Kolando are de-

I IF"

veloped. In the mantle zone
of the alar and basal laminae,

FIG. 147.-CEBEBELLUM, MEDULLA OBLONGATA, AND FOUBTH cloge to faQ j^^ agpect, definite
VENTBICLE IN A FCETUS OF THE FIFTH MONTH. (From

Kollmann's Entwickelungsgeschichte.) groups ol neuroblasts constitute

the nuclei of the cerebral

nerves ; but more superficially there are large numbers of scattered neuroblasts,
which persist as the nerve-cells of the formatio reticularis. Again, close to the
middle line on each side, but separated from it by a lamella of the reticular
zone, a large group of neuroblasts forms the rudiment of the corpus dentatum of
the olive.

The reticular formation of the medulla oblongata is produced by a great
expansion of the reticular zone associated with the development of the nerve-
paths connected with the neuroblasts of the rhombic lips and olivary nuclei ;
the mesial raphe, like the ventral commissure of the cord, is partly formed by
these fibres crossing the middle line. As the reticular formation is gradually
added to, the funiculus solitarius becomes deeply buried. Its position in the
adult indicates the original surface of the embryonic medulla oblongata. The
last formations to appear are the pyramids as expansions of the reticular structure
mesial to the olivary nuclei. As they develop, they displace the olives laterally,
while at the same time the ventral median fissure is formed as a recess between
them.

1 According to Wilson (Jour, of Anat. and Phys. voL.xl.), the alar lamina is represented by the area
postrema of Eetzius only, and the subdivision of the floor of the fourth ventricle described in the text
is not the primary one, which is triple, as in some lower forms.

HIND-BRAIN

109

The upper part of the hind-brain, so far as its ventral area is concerned,
undergoes changes essentially similar to those described for the lower part. The
mass of transverse fibres forming the pons is very late in appearing.

FIG. 148. — MODEL OF THE BRAIN OF A HUMAN EMBRYO OF 6'9 MM. (FOUBTH WEEK)

LATERAL ASPECT. (His.)

Cerebellum. — We have already seen that over the rhombic brain the epen-
dymal roof-plate is much thinned and expanded. It is reduced to a simple

islhmu*

FIG. 149. — SAME MODEL AS THAT SHOWN IN FIG. 148; MESIAL ASPECT.

The mouth of the hemisphere vesicle, the future foramen of Monro, is bounded below by the corpus
itriatum and behind by a fold separating the pallium from the lateral wall of the diencephalon
which becomes the thalamus. The niche above the optic recess where the corpus striatum is
continuous with the thalamus is the stalk of the primitive hemisphere-vesicle.

epithelial layer, except along the line of attachment to the rhombic lip, where it
retains some nervous tissue and forms the ligula. Opposite the pontine flexure it
lornis a fold which projects into the cavity of the vesicle, and becomes much

110

NERVOUS SYSTEM

plicated (fig. 151). Between the plaits of the membrane vessels are formed ii
the included mesenchymatous tissue, and thus is produced the choroid plexus of
the fourth ventricle (plica choroidea inferior). If we follow the ependymal lamell
forwards we find it to be continuous with the cerebellar band already mentioned.
Just below this it retains its more primitive characters, and forms the inft
medullary velum. In the region of the isthmus the roof-plate forms similarly th<
superior medullary velum (valve of Vieussens), while laterally it is thickened
produce the superior cerebellar peduncles. The cerebellar band proper becom<
enormously increased in thickness to form the cerebellum. The thickening oj
the roof -plate is at first bilateral, so that two plates are laid down, joined by
thin intermediate band, and separated by a cleft which communicates with the

cavity of the rhombic vesicle (fig. 146).
This cleft is afterwards obliterated
the fusion of its lips, but a part ma};
for some time persist as a small cere-
bellar ventricle (Blake). The central
plate forms the central lobe or vermis ;
the lateral lobes or hemispheres appeal
*./ --^p jater ag rounde(j enlargements of ii

lateral portions. During the earlier
ip* I stages the cerebellum has a smootl

surface, but by the end of the thi]
month it begins to be folded owii
to the increase of its surface area
(fig. 147). Fissures appear which
separate certain definite areas or
primitive lobes from one another.
The first partition involves the lateral
and posterior borders, which are cut
off by lateral fissures (fioccular fissures)
to form the flocculus and paraflocculus.

hemisphere

pineal body —

floccuiar fissures meet on the
vermis, and mark off an area which
becomes the nodule. A deep fissi
(fissura prima of Stroud) also appears
on the vermis which separates the
future culmen and clivus monticuli,
and extends on to the hemispheres.
Later two other fissures mark off the
pyramid from the tuber valvulce (sulcus
prepyramidalis ; fissura secunda, Elliot
Smith) on the one side and the uvula
on the other. These four fissures constitute the main dividing furrows of the
vermis, but during the fourth and fifth months various furrows appear on the
hemispheres which further subdivide its surface. The morphology of the fissures
and lobes of the cerebellum will be considered in the volume of this work
devoted to Neurology, but it may be here stated that the great horizontal fissure
is of late appearance, and is not of morphological importance, being produced
merely by the great growth of the hemispheres in the region of the lobus clivi and
the lobus pyramis, which is so special a feature in the human embryo (Bradley).1

FIG. 150. — MODEL OF THE BEAIN OF A HUMAN
EMBRYO OF 13'6 MM. VIEWED FBOM ABOVE. (His.)

The hemisphere-vesicles are laid open ; in the
left the choroid plexus has been left in position,
in the right it has been removed to show the
corpus striatum. For description, see text.

1 For the literature of the cerebellum, see Hertwig's Handbuch, loc. cit. and Bradley, Journ. of Anat.
and Phys., vol. xxxviii. ; on the mammalian hind-brain, see also the same author, Journ. of Anat. and
Phys. vol. xli.

MID-BRAIN AND FOBE-BRAIN

111

The mid-brain (mesencephalon) is marked off from the isthmus by the
crossing of the trochlear roots, while in front, in early stages, it is separated from
the fore-brain merely by a slight fold in the roof-plate which extends on to the
lateral wall (figs. 148, 149). The cavity is at first relatively large, and owing to
the expansion of the roof it is prolonged backwards over the isthmus (fig. 151). This
diverticulum is ultimately obliterated and the lumen of the vesicle is reduced to
a narrow passage — the aqueductus cerebri (aqueduct of Sylvius). The roof-plate
shows at first a mesial ridge (fig. 150), which in later stages disappears except
at its posterior extremity where it persists as the frcenum of the valve of Vieussens.
It then becomes thickened on each side to form two lateral swellings, subsequently
divided again by a transverse groove into the corpora quadrigemina. The ventral
portion of the vesicle forms the pedunculi (crura) cerebri, of which the tegmentum

cb

FIG. 151. — MEDIAN SECTION THROUGH THE BRAIN OF A TWO
AND A- HALF MONTHS FOETUS. (His.) Magnified 5 diameters.

The mesial surface of the left cerebral hemisphere is seen
in the upper and right-hand part ot the figure; the large
cavity of the third ventricle is bounded above and in front by
a thin lamina ; below is seen the infundibulum and pituitary
body. Filling the upper part of the cavity is the thalamus
opticus ; in front and below this is the slit like foramen of
Monro. Behind the thalamus is seen another slit-like opening
which leads into the still hollow external geniculate body.

olf, olfactory lobe ; p, pituitary body ; e.g., corpora quadri-
gemina; cb, cerebellum ; m o., medulla oblongata.

FIG. 152. — MEDIAN SAGITTAL SECTION
OF THE INFUNDIBULUM AND
PITUITARY DIVERTICULUM IN A
RABBIT-EMBRYO, AFTER THE

OPENING OF THE FAUCES. (From

Mihalkovics )

be, basis cranii with basilar
artery ; if, infundibulum ; tha, floor
of thalamencephalon ; py, pituitary
diverticulum, now closed; p', stalk
of original communication with the
mouth ; pli, pharynx ; ch, notochord
in the spheno-occipital part of the
cranial basis.

is first laid down, while the crusta is, like the other portions of the pyramid tract,
a later formation.

The fore-brain (prosencephalon) undergoes a much more complicated
series of changes, which result in the formation of the thalami, the geniculate bodies,
the pineal body and its peduncles, the optic nerves, chiasma, tracts, and retina,
tuber cinereum and cerebral part of the pituitary body, the corpora mamillaria,
and the cerebral hemispheres.

The remarkable feature in the development of the human brain is the enormous
expansion of the hemispheres till they dominate all the other portions of the brain,
overlapping as they grow the part of the fore-brain from which they spring, then
the mid-brain, and ultimately also the hind-brain.

At a very early stage, as has already been mentioned, the optic vesicles are
developed as hollow diverticula from the fore-brain. The mouths of the diverti-

112

NERVOUS SYSTEM

cilia are gradually closed, and the vesicles remain attached by the optic stalks.
The history of the vesicles will be considered in the chapter on the development of

ri.iencephalon

-*

amillary region ^

optic stalk

infundibulum
FIG. 153. — MODEL OF THE BRAIN OF A HUMAN EMBRYO OF 10'2 MM. (FIFTH WEEK). (His.)

infundibulum

optic recess

FIG. 154. — THE SAME MODEL AS IN FIG. 153 FROM THE MESIAL ASPECT.

The large opening into the hemisphere-vesicle is the foramen of Monro. On the wall of the
cliencephalon the sulcus of Monro is seen separating the thalamus (dark-shaded area) from the
hypothalamus (lighter-shaded area).

of diencephalon

mamillary region

infundinulum

FIG. 155. — MODEL OF THE BRAIN OF A HUMAN EMBRYO OF 13-6 MM.
(BEGINNING OF SIXTH WEEK) FROM THE SIDE. (His.)

FOBE-BEAIN

113

the eye ; but it must here be noticed that between the mouths of the two vesicles
there extends across the floor of the fore-brain a well-marked recess, which is
known as the optic recess (fig. 149). In the posterior fold of this, the optic
commissure is afterwards formed, and it is an important landmark during the
development of the parts.

At first the fore-brain shows the typical regions of the rest of the neural tube — viz. a thin
ependymal roof-plate, a floor-plate of the same constitution, and in the thick lateral walls
a basal and alar lamina separated by a furrow (sulcus of Monro). The thalami are formed
from the so-called alar laminae, as are also the cerebral hemispheres, while the basal laminae
give rise to the hypothalami, the tuber cinereum, infundibulum, and mamillary bodies. It is
<loubtful whether this subdivision has the same morphological value as in the rest of the tube.

op.st. op.it.

FIG. 150. — SECTION OF THE FORE-BRAIN OF A HUMAN EMBRYO AT THE END OF THE FIFTH OR
BEGINNING OF THE SIXTH WEEK. Photograph. (T. H. Bryce.)

h, hemisphere-vesicle ; f.ni., foramen of Monro ; c.s., c.s., corpora striata; op.st., op.st., proximal

ends of optic stalks.

By the thickening of the lateral walls to form the thalami, the cavity of the
vesicle of the fore-brain is reduced to a narrow vertical cleft — the third ventricle
of the adult brain. The thalami ultimately come in contact with one another, and
are joined by a grey lamina known as the intermediate mass or middle commissure.
The ependymal roof at first shows a longitudinal convexity. As seen in His' model
of the brain of a sixth week embryo (fig. 150), the roof broadens out in front where
it reaches on each side the margin of the foramen of Monro, while behind, it
runs into a mesial swelling, which is the rudiment of the pineal body or epiphysis.
Extending forwards from the sides of this, two curved ridges represent the peduncles
of the pineal body. Outside these, again, are two broader bands connected
behind with a semilunar ridge on the roof of the mid-brain; they become the
brachia of the superior corpora quadrigemina. In front of the pineal body, and
between the peduncular ridges, the roof becomes reduced to a simple epithelial layer
which covers later the under aspect of the velum interpositum (tela choroidea of the

VOL. I. I

114

NERVOUS SYSTEM

third ventricle). The pineal body is at first a simple diverticulum of the cavity
of the fore-brain.1 Its ependymal lining becomes thrown into folds, and the cavity
is ultimately obliterated. The stalk remains patent, and forms the pineal recess:
It is at first directed upwards ; but as the hemisphere grows backwards the gland
is thrown over on to the corpora quadrigemina, and the stalk assumes the curvature
seen in the adult brain. The prominent rounded swellings seen on the sides of
the fore-brain (fig. 150) are the rudiments of the lateral geniculate bodies. A section
(fig. 164, p. 120) shows that the external prominences correspond to diverticula
of the cavity of the vesicle. The area on each side, between the geniculate promi-
nence, the superior brachium, and pineal peduncular ridge is the habenular region.
When the hemisphere grows back over the thalamus (see below), the geniculate

h emispJi ere-vesicle

tongue

FIG. 157. — SECTION OF THE CEREBRAL HEMISPHERES FURTHER FORWARDS THAN THE SECTION
GIVEN IN FIG. 156. Photograph. (T. H. Bryce.)

The section should be compared with the model represented in fig. 150. The mesenchyme in the-
great longitudinal fissure between the hemispheres is the primitive falx cerebri ; the choroidal fold is
in process of formation. On the left of the figure he corpus striatum is divided by a furrow into two-
limbs (see floor of right hemisphere in fig. 150)

angle is displaced backwards to its definitive position, and the habenular an
forms the trigonum habenulce and the pulvinar. The grey matter in the habenulai
area becomes the ganglion habenulce, and the two areas are early connected by
commissure across the roof in front of the epiphysis. When the stalk of that bodj
is folded back, the crossing fibres are found in the dorsal lamella of its peduncle
This habenular commissure must not be confused with the posterior commissure
which forms at the junction of mid- and fore-brain below and behind the posterior,
afterwards the ventral, lamella of the pineal stalk. The mesial geniculate bodie

1 In lower vertebrates the pineal diverticulum is bilateral, but only that of the left side devel<
into the pineal gland (Cameron).

CEEKI1KAL HEMISPHERES 115

develop in the same manner as the lateral, showing at first as internal diverticula
and outer prominences on the sides of the fore-brain. In the displacements
resulting from the backward growth of the hemisphere they become pushed back
so as to be seen on the surface of the mid-brain.

The floor of the fore-brain early shows a deep depression behind the optic recess,
which becomes the infundibulum, and gives origin to the cerebral lobe of the
pituitary body. This is at first an open diverticulum (fig. 152), but later it
becomes cut off from the cavity by the obliteration of the lumen of the stalk. The
vesicle thus formed comes into intimate relation with the epithelial portion of the
body, to the posterior aspect of which it is applied, the two becoming bound
together by vascular connective tissue. The epithelial portion is formed as a
diverticulum of buccal ectoderm from the roof of the stomodceum. The diver-
ticulum extends backwards as a flattened cleft (fig. 176, p. 134) which divides
into two horns embracing the infundibulum. Towards the end of the second month

mesencephalon / I cerebral

cerebellum

FIG. 158. — MODEL OF THE BKAIN OF A FCETUS OF 53 MM. (ELEVENTH WEEK). (His.)

The right hemisphere has been cut away to show the mesial surface of the left hemisphere ;
the corpus striatum is seen arching round the stalk of the hemisphere.

the walls become folded and epithelial sprouts grow to form a small mass of
tubules, the lumen of which is afterwards obliterated. The stalk is separated
from the buccal epithelium and becomes absorbed.

The mamillary bodies are developed from a mesial recess of the floor of the
fore-brain between the infundibulum and the anterior basal angle of the floor of
the mid-brain.

Cerebral hemispheres. — The cerebral-hemisphere rudiments first appear as
shallow bays in the fore-part of the alar laminae (fig. 149). In the roof and
anterior wall of the tube the walls of the bays run directly into one another, so that
the two rudiments appear as a single anterior swelling of the fore-brain (fig. 148).

This, with the fore-part of the anterior extremity of the basal part of the fore-brain, is often
termed the telencepkalon, while the remainder is called the diencephalon. Opinions differ
as to whether the hemispheres are to be looked on as separate lateral diverticula, or as two
lobes of a single rudiment.

i2

116

NEKVOUS SYSTEM

mesencephalo n

cerebellum

The hemispheres in an embryo of the fourth week (fig. 148) still form a single
rounded swelling, but the position of the future separation is marked by a longi-
tudinal ridge, which, however, does not reach the lower and anterior portion. Here
an area on each hemisphere is clearly marked off by a slight lateral furrow as the
rudiment of the rhinencephalon. When the hemisphere is looked at from within,
it will be observed that two areas can be distinguished — an upper rounded, the
rudiment of the pallium,1 and a lower triangular, the rudiment of the corpus strmtum.
On the latter are seen three ridges running on to the mesial aspect of the rhin-
encephalic area. The mouth of the vesicle (the future foramen of Monro) has the
following lips, which it is important to distinguish : (1) pallio-thalamic, where
pallial and thalamic walls join one another ; (2) strio-thalamic, where corpus striatum
and thalamus are continuous ; (3) strio-hypothalamic, where corpus striatum

and hypothalamus meet ; and

c. hemisphere opened (4) lamina tcrminalis, where rhin-

encephalon is continuous with
rhinencephalon, and pallium with
pallium.

In the early part of the fifth
week the hemispheres begin to
expand (figs. 153, 154). Their ex-
pansion takes place, like that of
a growing soap-bubble, in every
direction from the stationary mouth
(foramen of Monro). The lamina
terminalis thus comes to lie at the
bottom of a fissure between the two
hemispheres which is occupied by
a lamella of mesenchyme, the
common rudiment of the mesial
pia and the falx cerebri ; the
pallio-thalamic border is at the
same time converted into an
angular arched fold which lies at

the bottom of a cleft between the

The right hemisphere has been opened up to show Posterior part of the expanding

the cavity of the vesicle and the inner aspect of its vesicle and the thalaniUS.

mesial wall. The inward projecting fold round the Th rhinencephalon is now

stalk is the hippocampus; the angular fold m the .

posterior lobe is the calcar avis. marked ore very distinctly from

the pallium by a lateral furrow

(external rninal fissure}. The pallial vesicle soon becomes bean-shaped (fig. 155),
the hilum being represented by a depression above the rhinencephalon. This
depression becomes the fossa of Sylvius, and corresponds to the stalk of the
hemisphere — i.e. the point of union of corpus striatum and thalamus. The
hemisphere -vesicle, anchored as it were to the two ends of the rhinencephalon,
and expanding in all directions, becomes folded round the stalk. Its cavity is
consequently horseshoe-shaped, and the horns of the semilune represent the
future frontal and temporal horns of the lateral ventricle. There is no posterior
horn as yet. It is developed later, when the vesicle has still further expanded
backwards to form the occipital lobe.

1 The terms rhinencephalon &n& pallium are used in a limited and ontogenetic sense. The division
is in a measure arbitrary, as we shall see that a part of the pallium in this sense (limbic lobe) become*
closely related to the rhinencephalon, forming with it the 'rhinencephalon' of Elliot-Smith's definition.
Ontogenetically, however, the above subdivision of the hemisphere-rudiment into rhinencephalon,
pallium, and corpus striatum is not only justified by fact, but necessary in description.

CEREBRAL HEMISPHERES

117

The rhinencephalon at this stage is quite uncovered by the pallium and separated
from it by the external rhinal fissure (fig. 155). It extends back to the extremity of

!/iir. o/facl. med.

cerebellum

it. of Rr

gyr. olfact. lat.

gyr. ambient

gur. semiJunaris

oliue

FIG. 160. — BRAIN OF A FCETUS AT THE BEGINNING OF THE FOURTH MONTH,
FROM BELOW. (From Kollmann.)

The gyrus olfactorius lateralis, gyrus ambiens, and gyrus semilunaris together form tlie

lobus pyriformis.

parietal lobt

olfactory bulb

•gyr. olf. laferalis

</ii/\ scmilunaris
riyr. ambiens

FIG. 161.— BBAIN OF A FOETUS AT THE BEGINNING OF THE FOURTH MONTH. (From Kollmann.)

the temporal horn ; but as the temporal pole grows downwards and forwards, its
posterior extremity becomes covered over, and ultimately reversed in position,

118

NERVOUS SYSTEM

thus giving rise to the uncus. Meanwhile the formation begins to be separated into
its several parts. The anterior extremity becomes cut off from the frontal lobe,
as a hollow stalk which communicates with the tip of the frontal horn of the
cavity. This separated stalk forms the olfactory bulb and tract as well as the
trigone. The remainder of the formation is not so isolated, and is represented by
the anterior perforated spot and the lobus pyriformis (fig. 160). This is a well-
marked swelling during the earlier months, extending from the olfactory tract to the
inner aspect of the temporal lobe, but it becomes greatly reduced, and is ultimately

FIG, 162. — CORONAL SECTION OF THE BRAIN OF A HUMAN EMBRYO OF 30 MM. (BEGINNING OF
THIRD MONTH). Photograph. (T. H. Bryce.)

The section cuts the thalami, cerebral hemispheres, and corpora striata. H, H, posterior lobes of
hemisphere; /, /, frontal lobes. Ill is placed in the habenular region of the third ventricle; in front
of this the cavity is slit-like, and bounded by the bodies of the thalami. Each thalamus terminates in
a blunt-pointed end, the future anterior tubercle ; the anterior portion of the outer surface is oblique,
and separated from the overlapping corpus striatum by a fissure, in which the tsenia semicircularis is
afterwards developed. The infrachoroideal ependymal lamella (see especially on right side of figure)
of the mesial hemisphere-wall is continued over this surface and fused with it. The corpus striatum
being arched, is cut in two places, head (c1, c2) and tail (t) ; between these, the commencing internal
capsule is seen. The head-end of the corpus striatum shows two crura, a mesial (c') and a lateral (c2).
The longitudinal fissure between the frontal lobes is distorted by a large haemorrhage. On the left of
the figure the mesial wall shows a fold, which is the upper end of the fissura prima.

represented only by the uncus and the so-called lateral root of the olfactory tract.
On the mesial aspect of the hemisphere a special rhinencephalic area — trapezoid
plate or field (His), area paraterminalis (Elliot-Smith) is marked off by a distinct
fissure in front (fissura prima), and limited by the lamina terminalis behind
(fig. 164). Its fate will be considered later.

On the mesial wall of the pallium the formation of the choroidal fissure has
already taken place. The vesicle-wall, where it passes over on to the thalamus-
wall, remains ependymal, and is folded into the cavity as a double layer enclosing

CORPUS STRIATfM

119

vascular pial tissue. Thus a plica choroidea is produced (fig. 150), which becomes
so extensive as nearly to fill the interior of the vesicle. The choroidal fissure
begins at the angle where the lamina terminalis and pallio-thalamic border of the
foramen of Monro meet. It is at first short and nearly straight, but gradually
extends round the hemisphere-stalk to the tip of the temporal horn. It is to be
observed that below the fissure there is an ependymal lamella (infrachoroideal
raembrane) which becomes fused with the thalamus (see below).

It will be convenient at this stage to take the development of the corpus
striatum; As we have already seen, it lies in the floor of the hemisphere-
vesicle (figs. 156, 157) ; it is directly continuous, below and behind the primitive

FIG. 1G3. — SECTION THROUGH THE SAME BRAIN AS SHOWN IN FIG. 1G2, SEVERAL SECTIONS

NEARER THE BASE. (T. H. BrVC6.)

The same general description as given under fig. 162 will apply here, but it will be noted that the
fissure between the thalamus and corpus striatum is now interrupted by the large primary ihalamus-
bundle. This marks the stalk of the hemisphere, round which the corpus striatum arches ; as the
stalk enlarges, the fissure (also, of course, arched) is gradually obliterated, and is only represented in
the adult brain by the furrow between caudate nucleus and thalamus, in which the taenia semi-
circularis lies. The hollow on the surface of the hemisphere opposite the stalk is the fossa of Sylvius.

foramen of Monro, with the thalamus, and is connected in front with the
rhinencephalon by three roots. The middle of these fuses with the internal fold
corresponding to the fissura prima (fig. 164) in the floor of the vesicle and cuts off
a pocket of the frontal horn which is continuous with the cavity of the olfactory
stalk. When the lumen of the stalk is obliterated this pocket disappears. The
cleft behind this seems in part obliterated in its basal portion, but persists
above as the space between the caudate nucleus and septum pellucidum in
the adult brain. The body of the corpus striatum grows backwards pari passu
with the development of the temporal horn of the vesicle, and its tail is
thus produced (fig. 150). As the hemisphere-rudiment becomes more and more

120

NERVOUS SYSTEM

arched the corpus striatum becomes highly bowed, surrounding the hemisphere-
stalk and extending from the rhinencephalon in front (future locus perforate
anticus) to the extremity of the temporal horn (fig. 158). As the body is pro-
longed backwards into the tail it overlaps the thalamus, but is necessarily
separated from it by the cleft between thalamus and hemisphere (see fig. 150).
This cleft is obliterated from below, as thalamus and corpus striatum
enlarge, by a fusion of the two bodies, associated with the fusion of th<
ependymal infrachoroidal lamella of the mesial hemisphere-wall with th(

I

FIG. 164. — SECTION THBOUGH THE SAME BRAIN AS IN FIGS. 162 AND 163, MUCH NEARER THE BASE.

(T. H. Bryce.)

The section now cuts the mid-brain M. The diverticula on each side of III, placed in the cavity
the third ventricle, are the rudiments of the geniculate bodies ; H, H, the temporal horns of the hemi-
sphere-vesicles with the tail of the corpus striatum. The notch on the outer surface of each hemisphe
opposite rf is the sulcus marking off the rhinencephalon on the lateral aspect of the brain ; Ilia is
placed in the fore and basal part of the third ventricle. Immediately in front (above in the figure) of
Ilia is the lamina termmalis, here thickened. In front of the lamina terminalis are two triangul
fields (trapezoid areas), separated by a narrow cleft occupied by a delicate prolongation of the primitive
falx. In front of this the falx is much broadened out, and angular projections from it occupy the
fissures primse. The cavity of the hemisphere-vesicle is at this level interrupted by the union of the
projection corresponding to the fissura prima with the lateral crus of the corpus striatum (see
figs. 162, 163). The anterior pocket is the mouth of the cavity in the olfactory stalk.

outer side of the thalamus (fig. 162). The hemisphere-stalk is thus greatly
enlarged, and in the tissue between the thalamus and corpus striatum the
capsule takes form — first, by the formation of the thalamus bundle connecting
that body with the hemisphere (fig. 163) ; and later by the addition of the
pyramid-fibres connected with the crus cerebri.

It may here be pointed out that while the outer reticular zone in the spinal cord becomes the
white covering, this zone remains quite a thin layer on the hemisphere, and the white matter is

HIPPOCAMPAL F010IATK >.\

121

laiil clown between the mantle zone and the ependymal zone by the development of the fibre-paths
conm'fting the thalamus with the hemisphere, and later by the addition of the pyramid-fibres.

The cleft between corpus striatuni and thalamus persists for a time as a groove
in which the stria terminal/is or tcenia semicircularis is formed. During the formation
of the internal capsule the lenticular nucleus and claustrum take shape as isolated
portions of the striate body.

The velum interpositum (tela choroidea ventriculi tertii) arises from the vascular
connective tissue within the longitudinal fissure. This grows in between the
ependymal lamellae of the choroidal fissures to form the choroid plexuses of the
lateral ventricles. It also of course covers the optic thalami, and lies upon
the ependymal roof of the primitive fore-brain, which it inflects into that cavity
to form the choroid plexus of the third ventricle. The ependymal covering of each
lateral choroidal plexus is derived from the mesial wall of the hemisphere, while the

falx

FK;. 1(55. — DIAGRAM OF A TRANSVERSE SECTION OF THE BRAIN TO SHOW THE RELATIONS AND FATE

OF THE MARGIN OF THE MESIAL WALL OF THE HEMISPHERE. (After His.)

Th, thalamus ; Cs, corpus striatum. The mesial wall is infolded, and the sunk grey cortex ends
in a thickened seam, leaving a free edge of white matter. In the temporal horn the parts are labelled
hf, hippocampal fissure ; fa, marginal grey seam ; fi, edge of white substance.

infrachoroidal lamella is prolonged over, and becomes adherent to, the thalamus
(lamina affixa) as far as the stria terminalis, which represents the line of union of
hemisphere- wall and thalamus-wall. In this wray the anterior part of the thalamus
comes to lie in the floor of the lateral ventricle.

The hippocampal formation and the commissures. — If a section
of the mesial wall of the hemisphere be examined in the brain of an
embryo at the beginning of the third month, it will be found that the cortical
ganglionic layer which is forming over the pallial surface ceases before it reaches
the choroidal fissure, in which, as we have seen, the ependymal layer is alone
present. The reticular layer which in the hemisphere develops between
the mantle and the ependymal layers, therefore, comes here to the surface
(tig. 165). From frontal lobe to temporal horn the margin of the pallium thus
consists of a zone in which the cortical grey layer is present, and a zone in which

122 NEKVOUS SYSTEM

it is absent, and this thins away by a margin (tcenia) into the ependymal layer.
This portion of the hemisphere becomes much complicated by the development of
the fornix commissure and the corpus callosum. To make the matter clear, we
shall first imagine it developed without either fornix commissure or corpus
callosum.

The margin of the hemisphere becomes folded early in the third month, to
form a fissure arching from the foramen of Monro to the temporal horn, parallel with
the choroidal fissure. The inflected area is continuous in front with the trapezoid
area (areaparaterminalis). The fissure is a ' complete' one, and has an elevation
within the vesicle corresponding to it. The thickened marginal seam of the grey
matter sunk in the fissure, lies at the lip of the choroidal fissure, and the edge of the
white matter is rolled inwards towards the ventricle, as it thins away into the
ependymal layer covering the choroid plexus (fig. 165); The projection into the
ventricle becomes the hippocampus, the marginal grey seam the fascia dentata,
and the white lip the fimbria. The primitive hippocampal formation thus con-
stituted extends from the front of the foramen of Monro to the temporal horn
arching round parallel with the choroidal fissure. It is continuous in front with
the trapezoid plate (area paraterminalis) and behind with the uncus.

This marginal area of the cerebral cortex is of great morphological importance. In lower
forms, with a largely developed olfactory sense, it forms a prominent part of the brain, while
the rest of the hemisphere is relatively little expanded. In man it remains a diminutive
element, while the remainder of the cortex becomes enormously developed. Elliot-Smith,
seeing that it is in all probability related to the sense of smell, has included it in his
* rhinencephalon,' while to the rest of the pallium he gives the name neopallium. On ontogenetic
grounds it seems better to limit the term * rhinencephalon ' to that very distinct and early
isolated part of the hemisphere-rudiment to which the term has been applied in the foregoing
account. The later formed hippocampal formation is a part of the pallium in the sense defined
in the note on page 116. In respect that it is intimately related to the olfactory apparatus it
might perhaps be termed rhinopallium to distinguish it from the neopallium.

The primitive hippocampal formation becomes profoundly affected by the
development of the commissures, hippocampal and neopallial (corpus callosum).
These are developed in intimate relationship with the lamina terminalis, which, as
we have seen, from the first connects rhinencephalon with rhinencephalon and
pallium with pallium. The lamina terminalis becomes thickened ; but opinion
is divided as to the manner in which this is effected. Some regard the increase
as due to interstitial growth, others believe that the opposed mesial surfaces of the
hemispheres become fused to form the plate in which the commissures appear.
The appearances seen in sections such as are here reproduced (fig. 164) are on the
whole in favour of the latter view. Immediately in front of the ependymal lamina
terminalis the faces of the trapezoid plates have fused to all appearance for a
certain distance. In the band of tissue thus produced the anterior commissure first
appears. It connects the corpora striata and temporal lobes; Both hippocampal
commissure and corpus callosum develop in the upper part of the thickened lamina
terminalis. The hippocampal fibres appear first, and the callosal later. They are
indistinguishable to begin with, but the callosal soon accumulate on the dorsal
aspect of the hippocampal, and form a short plate arching forwards over the
cleft between the two trapezoid plates (areae paraterminales). The cleft open
below is now the cavum septi — the future fifth ventricle — and the two trapezoid
plates will become the two leaves of the septum pellucidum.

Once laid down, the corpus callosum becomes elongated ; there are two
divergent interpretations of the process. According to one view, the growth is
interstitial. The body is said to grow forwards and backwards, and is raised and
separated from the hippocampal commissure. As the original union between the

CEREBRAL COMMISSURES

123

two^is at the posterior end of the corpus callosum, the hippocampal commissure
(or, in other words, the upper part of the lamina terminalis) is stretched and

FIG. 166. — GBAPHIC RECONSTRUCTION OF THE MESIAL, HEMISPHERE -WALL

OF A FOETUS 5'6 MM. LONG. (After His.)

P, stalk of hemisphere ; c. v, anterior and posterior parts of trapezoid area (area paraterminalis) ;
Li., lamina infrachoroidea of mesial hemisphere-wall below, c.f., choroidal fissure; Icm, limbus of
mesial hemisphere-wall below, h.f., hippocampal fissure ; c.c, corpus callosum ; l.t., lamina terminalis ;
., anterior commissure ; f, commencing column (anterior pillar; of fornix ; rh, olfc

rt.r.

olfactory stalk.

c/.

rh

l.t. a.c. un

FIG. 167. — GRAPHIC RECONSTRUCTION OF THE MESIAL HEMISPHERE-WALL OF A FOETUS

OF 8-3 CM. LONG. (After His.)

P, stalk of hemisphere; c.s., cavum septi; b, fimbria, continuous with /, fornix; Icm, limbus;
<'.c., corpus callosum ; h.f.1, callosal fissure ; h.f.'2, hippocampal fissure ; c.f., calcarine fissure ; un, uncus ;
(i.e., anterior commissure ; l.t., lamina terminalis ; o.c., optic commissure ; rh, olfactory stalk ; ves, oiiuline
of cavity of hemisphere.

drawn into a horizontal position forming the psalterium or lyra, while the area
paraterminalis is also drawn out to form the apical portion of the septum pellucidum
(Elliot-Smith and others). According to the other view, the increase in length is

124

NERVOUS SYSTEM

brought about by the free medullary edges of the margins of the hemispheres comii
together, and by the growth of fibres across the bridge so produced (His and others)
This takes place pari passu with the expansion of the pallium, and owing especially
to expansion of the frontal lobes the hippocampal commissure, which remains
small and retains its primitive position, is separated from the fore-part of the
corpus callosum, and the tissue of the trapezoid plates is stretched between them
to form the leaves of the septum pellucidum.

The fate of the hippocampal formation is the same whichever interpretation
be adopted. The corpus callosum is formed within its arc, and causes by its
growth a stretching and atrophy of its forward moiety. The hippocampal fissure
of the temporal horn is represented by the cattosal fissure of the mesial surface.
The grey cortex sunk in the fissure becomes the hippocampus in the temporal horn,

but is reduced to the gyrus cinguli &bovQ
the corpus callosum. The marginal
seam of grey matter becomes the
fascia dentata below, the indusium
above ; while the nervi Lancisii are
fibres arising from this wasted part of
the hippocampal formation, just as the
fornix-fibres spring from the hippo-
campus. The free edge of the white
matter becomes the fimbria, the
posterior pillar (cms), and body of the
fornix. The anterior pillar (column!
fornicis) is formed by a strand of fibres
early formed in the wall of the hemi-
sphere and connected with the thalamus
of its own side (fig. 166). The so-called
peduncle of the corpus callosum is a
strand (fasciculus prcecommissuralis,
Elliot-Smith, or gyrus subcallosus,
Zuckerkandl) formed on the trapezoid
plate (area paraterminalis). It is in-

FlG. 168. — VlEW OF THE INNER SURFACE OF THE
RIGHT HALF OF THE FO3TAL BRAIN OF ABOUT

six MONTHS. (Reichert.)

F, frontal lobe ; P, parietal ; 0, occipital ;
T, temporal ; J, olfactory bulb ; II, optic nerve ;
fp, calloso-marginal fissure; p, p', parts of the
parieto-occipital fissure ; h, calcarine fissure ; g, g,
gyrus fornicatus ; c, c, corpus callosum ; s, septum
pellucidum ; /, placed between the middle commis-
sure and the foramen of Monro ; v, in the upper part
of the third ventricle ; v', in the back part of the
third ventricle ; v", in the lower part of the third
ventricle above the infundibulum ; r, recessus
pinealis; p-v., pons Varolii ; Ce, cerebellum.

timately connected with, indeed is the
anterior termination of, the hippo-
campal formation. When formed it

closes-in the septum pellucidum, and separates it from the other derivatives
of the rhinencephalon below and in front of it.

Formation of the fissures. — We have already seen how the wall of the
vesicle is infolded to form the choroidal and hippocampal fissures. These are, in that
they involve the whole thickness of the wall, complete fissures. Two other such
fissures are produced — the calcarine on the mesial aspect of the hinder part of the
vesicle, and the central portion of the definitive collateral. Corresponding to these
fissures, the calcar avis (fig. 159) and collateral eminence are found as projections
into the ventricle. From among the other fissures the fissure of Sylvius must be
placed in a category by itself. It is formed by the uprising of the edges of the fossa
of Sylvius until with the increasing expansion of the free portion of the pallium they
meet over the grey area on the surface of the hemisphere-stalk. This becomes the
island of Reil, and the meeting lips are named the opercula. The frontal operculum
is last formed (Cunningham), and owing to the development of a triangular field
upon it, the two small anterior limbs of the fissure of Sylvius are produced. The
remaining sulci, which begin to appear in the sixth month, are merely folds
produced during the progress of differentiation of the grey covering of the

SPINAL NERVES

125

hemispheres. They follow a definite pattern determined by differentiation of
areas in the grey matter, which by their own growth, as well as by pressure on
adjoining areas, produce the folding of the cortex. The subject will be fully
treated of in the descriptive account of the brain (see Neurology).

PERIPHERAL NERVOUS SYSTEM.

Spinal nerves. — In the human embryo of the third week the rudiments of the
inal ganglia are connected together by a continuous dorsal band, which extends

VII

VIII a.v. IX X XII XI

fore-bruin

ham itphe

1 T.

1 S.

1 L.

FIG. 1G9. — KECONSTRUCTION OF THE SPINAL AND CEREBRAL NERVES OF A HUMAN EMBRYO
6'9 MM. LONG. (After His.)

V, ventricle; A, auricle of heart; L, liver; a.v., auditory vesicle; Fr.g., Froriep's ganglion.
The Roman numerals indicate the cerebral nerves. 1 C., first cervical ; 1 T,, fir.it thoracic ; 1 L., first
lumbar; 1 S. first sacral nerve.

from the auditory vesicle along the neural tube to its extreme tip (Streeter).1
Though there are no signs as yet of dorsal roots, the ventral roots are present,
and the ventral ends of the ganglia end diffusely among them as they pass out
towards the myotomes (Streeter). The ganglion- crest becomes interrupted in the

1 Amer. Jour, of Anat. iv. 1905.

126

NEKVOUS SYSTEM

fifth week between the ganglia, and the dorsal roots are by that time completed.
Meantime the spinal nerve-roots have been formed by the union of fibres from the
ganglia with the motor root-fibres. During the fourth week, at a time when the
limb-buds are still small and undivided, the segmental nerves begin to be con-
nected by anastomosis (fig. 169). The connecting filaments between the nerve-roots
from the fourth cervical to the first thoracic, arid again from the second lumbar to
the second sacral, constitute the future limb-plexuses (fig. 173). Each segmental

ad

tr.b.

FIG. 170. — TEANSVEKSE SECTION OF A HUMAN EMBBYO AT THE END OF THE FIFTH
OK BEGINNING OF THE SIXTH WEEK. (T. H. BryCC.)

n.c., neural canal; n.a., neural arch; sp.g., spinal ganglion; s.r,, sensory root; m.r., motor root of
spinal nerve; d.b., dorsal branch; v.b., ventral branch; sy, visceral branch of spinal nerve, with
sympathetic ganglion ; st, stomach; sp, spleen; ad, adrenal ; g.g., genital gland ; w.b.t Wolffian body;
p} pancreas ; I, liver ; a, aorta.

nerve early gives off a dorsal branch which passes through the myotome, and a
ventral branch which passes along the inner side of its extended ventral part.
From this a short visceral branch passes to the rudiment of the sympathetic
ganglion (fig. 170). The ventral nerve extends quickly into the neighbourhood of
the Wolffian ridge, and thence much more slowly round the body-wall. All the
limb-nerves as they extend into the growing limb-bud divide into primary dorsal
and ventral branches.

CEREBRAL NERVES

127

Cerebral nerves. — Exclusive of the olfactory and optic, which must be
included in a different category from the others, and will be treated of with the sense-
organs to which they belong, the cerebral nerves divide themselves into a group
of pure motor nerves, the hypoglossus, abducens, trochlearis, and oculo-motorius,
and a group of mixed sensory and motor nerves, the vagus and accessorius
(vagus complex), the glossopharyngeus, the acousticus, facialis, and trigeminus.

The motor roots in both groups spring from the basal lamina of the neural
tube, but the nuclei of origin appear in two series, a mesial and a lateral. The
distinction is well marked as far forwards as the isthmus, but in that portion of the
tube and in the mesencephalon the separation into two distinct ranges disappears
to a considerable extent. The mesial column, which is a continuation upwards of
the ventral nerve column of the cord, gives origin in the region behind the auditory

FIG. 171, A and B. — SECTIONS ACBOSS THE HIND-BRAIN OP A HUMAN EMBRYO 10 MM. LONG. (His.)

In A, the origin of the spinal accessory and hypoglossal nerves is shown, the fibres of both arising
from groups of neuroblasts in the basal lamina of the neural tube. In B, one of the roots of the hypo-
glossal is still seen and in addition the root of the vagus nerve. This is represented as in part arising,
like that of the spinal accessory in A, from a group of neuroblasts in the basal lamina and in parts
continuous with a bundle of longitudinally coursing fibres placed at the periphery of the alar lamina,
and corresponding in situation to the commencing dorsal white columns shown in fig. 137.

vesicle to the hypoglossal nerve (fig. 171). Its roots are in series with those of the
ventral roots of the cervical nerves, and it is to be regarded as representing several
(three or four) segmental — i.e. trunk or spinal — nerves fused into one stem. This
idea is strengthened by the fact that a ganglion-rudiment (Froriep's ganglion),
typical but rudimentary, is occasionally present in connexion with one or more of
the roots. These hypoglossal nerves are connected with the occipital myotomes,
and are occipito- spinal nerves in Fiirbringer's sense — i.e. nerves properly belonging
to a part of the trunk which has become included in the hinder part of the head.
In the pre-otic region the abducens springs from the mesial column (fig. 172), and
its roots are in the same series as those of the hypoglossal. It is regarded therefore
by nearly all observers as the ventral (somatic) root of a segmental nerve. The
trochlearis and oculo-motorius, however, have been variously interpreted, some
observers regarding them as ventral mesial, others as lateral, motor roots ; they

128

NERVOUS SYSTEM

have also been interpreted as being dorsal nerves. Fiirbringer (1902) puts the
trochlear in an intermediate position, but regards it as most resembling a lateral
motor root. His (1904) regarded both it and the oculo-motor as nerves springing
from a region in which the distinction between the columns is less sharp than
elsewhere ; he was not therefore inclined to lay special stress on the point. The
oculo-motor he held to be certainly a mesial (ventral horn) nerve.

The lateral column includes the nuclei of the motor roots of the vagus and
spinal accessory, glossopharyngeal, facial, and trigeminal. The fibres supply the
visceral musculature.

The sensory roots are developed from ganglia which arise from a forward
continuation of the common ganglion-crest, but they show no regular segmental

FIG. 172. — SECTION FROM THE SAME EMBRYO AT THE EXIT OF THE FACIAL NERVE. (His.)
(Several sections have been combined to form this figure.)

VL, fibres of sixth nerve taking origin from group of neuroblasts in basal lamina ; VII.G.g., ganglion
geniculi of the facial; VIII.G.i.c., intracranial ganglion of auditory; VIII.G.v., ganglion vestibuli ;
VIILG.c., ganglion cochleae.

disposition. Moreover, unlike the spinal nerve-ganglia, they come into transient
relation to thickened patches of the surface epithelium (placodes) which are placed
above the gill-arches and have been interpreted in two ways. According to one
view (van Wijhe, Beard, Froriep, and others), the placodes represent branchial
sense-organs which have been lost in phylogeny, and they take no share in the
formation of the cranial nerves. They occur in Selachians in two series, a lateral
and an epibranchial. According to a second opinion (Kupffer, Goronowitsch,
Julia Platt, Koltzoff, and others), these placodes are not rudimentary sense-organs,
but thickenings from which cells are budded off to share in the formation of the
definitive nerve-ganglia.

CBEEBRAL NERVES

129

Thus in Petroinyzon Koltzoff describes the segmental ganglia as originating from a mesial
rudiment derived from the ganglion -crest, and from two peripheral ectodermic rudiments derived
from the lateral and epibranchial placodes. In the region of the head these join to form the
permanent ganglia ; in the region of the trunk the surface placodes do not join with the
central ganglion-rudiments, but remain separate and form the lateral-line organs.

VII
VIII a.v. IX X XI XII

III _

1 Co.

M--vv

•1 L.

PIG. 173. — RECONSTRUCTION OF THE NERVOUS SYSTEM OF A HUMAN EMBRYO OF 10'2 MM. (After His.)

v, ventricle of heart; liv, liver; v.L, vitelline loop of intestine; /, tail; rh, rhinencephalon ;
c./t., cerebral hemisphere ; dien, diencephalon ; mes, mesencephalon ; a.v., auditory vesicle ;
/*'/•.//., Froriep's ganglion ; ph, phrenic nerve. The Roman numerals indicate the nerves. The sixth
cerebral nerve is not labelled, but is seen passing forwards to the eye under the mandibular and
maxillary branches of the fifth nerve.

In embryos of the third week the rhombic brain, as has already been indicated, shows a
subdivision by slight folds into a series of divisions which have been termed neuromeres on
the supposition that they represent a definite segmentation of the neural axis. In the pig
Bradley finds the same number as Thompson in the human embryo — viz. seven — and states
that the cerebellum lies opposite the first and the auditory vesicle opposite the third segment, while
the trigeminal ganglia are related to the second, the acoustico-facial to the fourth, the glosso-
pharyngeal to the sixth, and the vagus to the seventh. Broman describes in addition an eighth
VOL. I. K

130 NEKVOUS SYSTEM

neuromere in front of the cerebellum with which the fourth nerve is connected, and an indist
ninth at the end of the series.

The lateral motor roots have been interpreted as representing in the cerebral nerves
splanchnic efferent fibres which in the cord leave the axis by the ventral root, and are not
therefore separated from the somatic efferent fibres. They may be, on the other hand, represented
by fibres which Lenhossek and Ramon y Cajal have shown to arise in the ventral horn of the
embryonic cord (chick) and to run out in the dorsal roots. The cell column from which the
lateral roots spring may represent the lateral horn column of the cord.

The accessory is (see below) ontogenetically a part of the vagus, wholly therefore a cerebral
nerve. From his studies of the occipital nerves Streeter concludes that ' in all higher vertebrates,
accompanying the conversion of certain gill-muscles into the trapezius and sternomastoid, the
cranial elements (i.e. vagus complex) make a caudal invasion of the spinal cord.' The result of
this invasion, added to the inclusion in the skull of the occipito-spinal segments, is a blurring of
the line of demarcation between the nerves of spinal and those of cranial type, which is distinct
in spite of the inclusion of the occipito-spinal segments in the skull in lower vertebrates.1

Numerous attempts have been made to bring the nerves of the head into a segmental scheme,
and to homologise the cerebral with the spinal nerves. None of these are quite convincing, and
there is still great uncertainty as to what interpretation should be put on the serial characters
of the branchial region.

DEVELOPMENT OF INDIVIDUAL CEKEBEAL NEEVES.

The hypoglossal 2 appears in the third week as a number of rootlets
arranged in three or four segmental groups in series with the cervical motor roots,
and connected with the occipital myotomes. During the fourth week they fuse
into one trunk as they pass towards the floor of the primitive mouth. Owing to
the bend in the neural tube, the hypoglossal and upper cervical nerves are
brought close together ' like the spokes of a wheel ' (Streeter) and grow side by
side into a mass of tissue out of which the tongue and hyoid muscles are developed.
They are bound more or less together by the developing sheaths, and connexions
are established between them. When the muscles take form and draw apart the
nerves are separated out into a plexus in which is foreshadowed the adult
arrangement of the branches supplying this group of muscles.

A ganglion (Froriep's ganglion) is occasionally found in connexion with one of
the roots (figs. 169 and 173).

The vag-us complex includes the vag-us and spinal accessory, which
develop practically as a single structure. The ganglion- crest from which the vagus
ganglion is developed is a continuation forwards of the spinal ganglion-crest as
seen in embryos of the third week. It extends from the first and second cervical
to the auditory vesicle (fig. 175), where it is interrupted, the gap representing the
separation of the vagus from the glossopharyngeal. The motor fibres appear first ;
they form a strand which runs mesial to the crest as low as the third or fourth
cervical segment, and are connected with the lateral-horn region of the neural tube.
This strand is the primary accessory trunk ; in front of its emerging roots and in line
with them are a few scattered bundles at the head of the crest. The ganglion
of the vagus early shows a duplicity — ganglion of the trunk and ganglion of the root.
These separated masses have usually been described as due to a subdivision of
the neural-crest ganglion ; but Streeter supplies evidence which seems to indicate
that the ganglion of the trunk (g. nodosum) may be developed separately. It
related to an ectodermal patch (epibranchial placode) above the gill-arch
The two ganglia are at first separated by a cellular tract, which is afterwards
converted into a fibrous trunk. The ganglion-crest now (by the third week) becomes
broken up into separate clumps by the laying down of fibre-paths. The most
anterior of these form the definitive ganglion of the root ; the remainder (three or

1 Loc. cit., p. 111.

2 The summary of the development of the occipital nerves is mainly founded on Streeter's paper
(loc. cit.}.

ire
; is

:ds

CEREBRAL NERVES

181

FIG. 174. — DIAGRAM SHOWING THE CENTRIPETAL AND CENTRIFUGAL ROOTS OF THE CEREBRAL NERVES

OF THii SAME EMBRYO AS SHOWN IN FIG. 173. (His.)

The places of exit of the nerves are marked by dotted circles or ovals. The efferent nerves (III,
1 1', ui V, VI, VII, part of IX, XI, and XII) are seen to arise within the nerve-centre from groups of
neuroblasts ; the afferent fibres ( V.s, VIII, v and c, most of IX, and X) pass a certain distance
inwards, and for the most part also caudalwards in the nerve-centre, and there end. The ganglion-
rudiments from which they have grown are not shown here. They are represented in the preceding
figure.

Ill

IV

XI

XII

t.a. t.p.
Fiu. 175. — DIAGRAM REPRESENTING THE DISTRIBUTION OF THE CEREBRAL NERVES AND VESSELS OK

THE HEAD IN AN EMBRYO ABOUT THE FIFTH WEEK; FOUNDED ON RECONSTRUCTIONS BY TANDLER,

STREETER, AND MALL. (T. H. Bryce.)

Ill to XII, cerebral nerves ; the sixth is not labelled : it is seen passing forwards below V 3, the
mandibular branch of the fifth. 1 to 7, ganglia of the first seven spinal nerves ; a.v.t auditory vesicle ;
^•a., truncus aortee ; t.p., truncus pulmonalis ; v.a., vertebral artery.

K2

132 NERVOUS SYSTEM

four) are accessory root-ganglia, which gradually diminish in size until they are
found as mere rudiments in the root of the accessory nerve (fig. 173).

The result of the differentiation of the vagus complex is, that the fore-
part or vagus division, becomes predominantly sensory, and the back-part or
accessory division predominantly motor (Streeter). The series of motor roots is
continuous, and both divisions are at first mixed motor and sensory.

The glossopharyng-eal develops in a fashion identical with the vagus, and
in the human embryo it arises independently of it. The petrous ganglion, like the
vagal trunk-ganglion, is associated with an ectodermic thickening (epibranchial
placode) above the third branchial arch. Though in the case of both vagus and
glossopharyngeal the part of the ganglion connected with the placode has possibly
an independent origin, Streeter does not describe any appearances which positively
prove an origin from the placode, as has been described in lower vertebrates. The
main bundle of fibres from the petrous ganglion runs in the third arch forming the
lingual branch, while a second bundle runs into the second arch and becomes the
tympanic branch (fig. 175).

The acoustic nerve arises from a ganglionic mass (acoustico-facial complex)
which lies just in front of the auditory vesicle and is early separated into acoustic
and facial portions. The acoustic portion will be described with the organ of
hearing.

The facial has two roots, a motor and a sensory. The sensory root is derived
from a ganglion distinguished by its larger cells, which is separated off from the
common acoustico-facial ganglion and is named geniculate. It is directly continuous
with an epidermic thickening over the hyo-mandibular cleft (embryo of twenty-
third day: Futamura1).

The acoustico-facial is generally considered a complex nerve connected with the hyoid arch
Giglio-Tos, founding on observations in a seventeenth-day embryo, argues for the independence
of the two nerves in the earliest phases. According to his description, each of the two nerves
has a median proganglion derived from the neural crest and two peripheral proganglia, a lateral
and an epibranchial, with corresponding placodes. The lateral placode of the acoustic nerve is
the auditory epithelium, that of the facial is a separate and distinct thickening ; the epibranchial
proganglia are continuous. The lateral and mesial ganglia are connected by cellular strands
(pronerves), and out of this complex the acoustico-facial ganglion is formed.

The central root-fibres of the geniculate ganglion pass into the neural tube and
end in series with those of the glossopharyngeal (pars intermedia}. The distal
branches appear about the fourth week as strands (A. Francis Dixon) which
become the great superficial petrosal and chorda tympani nerves. They unite
secondarily with the branches of the fifth nerve with which they are connected
in the adult, and are well developed before the peripheral branches of the trunk of
the facial can be recognised. The motor root springs from a group of neuroblasts
in the antero-lateral part of the basal lamina, and at an early stage the fibre-paths
connected therewith show a general mesial direction (His) towards the nucleus of
the sixth nerve, foreshadowing the devious course of the facial root through the
floor of the fourth ventricle. The motor fibres are distributed to the hyoid arch, the
muscles of which they supply ; the sensory branches pass over into the mandibular
arch.

The abducens arises from the mesial column of neuroblasts in line with the
hypoglossal roots. No ganglion has been described in connexion with it. It passes
forward mesial to the trigeminal ganglion to the rudiment of its muscle, the
external rectus of the eye.

The trigeminal nerve has a motor and sensory rudiment. The chief motor
nucleus appears as a group of neuroblasts which forms a keel-like projection of

1 Anatomische Hefte, xxx. 1906.

SYMPATHETIC SYSTEM 183

the wall of the neural tube opposite the ganglion, and immediately to the mesial
side of its entering dorsal root (fig. 174). The group of cells therefore belongs to
the lateral column. The descending root appears later, and is derived from a mass
of neuroblasts lying in close relationship to the oculo-motor and trochlear nuclei
in the floor of the mid-brain. The sensory root arises from a large ganglion, a
derivative (in part at least) of the ganglion-crest (see below). It is situated
opposite the pontine flexure, and in the fourth week has become connected by
a single dorsal root with the neural tube, while peripherally it is already
provided with three primary branches — ophthalmic, maxillary, and mandibular
(figs. 173, 175). The ganglion in an early phase is said by Giglio-Tos to be
connected with the surface ectoderm (placode). The ganglion becomes the
Gasserian ganglion. The various subsidiary ganglia (ciliary, Meckel's, otic, and
sub-maxillary) arise like sympathetic ganglia (see below) in connection with the
several branches of the nerve.

In some lower forms the trigeminal shows traces of a composite character, and it has hence been
suggested that it represents the union of more than one segmental nerve. In a human embryo
of the seventeenth day Giglio-Tos has described a complicated origin for the Gasserian ganglion.
He believes, in the first place, that the rudiment is primarily connected with the mes-
encephalon, and that the nerve becomes displaced backwards. He recognises three neural
' proganglia,' three epibranchial ' proganglia ' connected with surface placodes, and three
' pronerves ' — »'. P. cellular strands between the neural and lateral ganglia. This complex mass fuses
into the single ganglion generally described as the rudiment of the dorsal root.

From the foregoing account it will be seen that ectodermic placodes have been described in
connexion with all the cerebral nerve-ganglia in the human embryo ; it must be left an open
question, however, whether the ganglia which are related to these placodes have not an origin
from the surface ectoderm.

The trochlear is springs from a tract of neuroblasts situated in the isthmus.
They occupy both the mesial and lateral portions of the basal lamina (His, 1904).
The fibres take a dorsal course in the reticular zone to the roof of the isthmus
(future valve of Vieussens), where they cross and, again emerging, pass round the
wall of the mid-brain to their muscle (superior oblique of the eye).

The trochlearis presents the special and puzzling peculiarities — first, that though a ventral
nerve it emerges from the dorsal aspect of the neural tube ; and second, that it crosses with its
fellow to form a dorsal commissure. These facts have not received a satisfying ontogenetic
explanation.1

The oculomotorius springs from a ventral and mesial tract of neuroblasts
in the mesencephalon, and is generally pronounced a mesial or somatic nerve.
The root passes off from the ventral aspect of the neural tube just in the cephalic
bend, and passes backwards in its course to the rudiment of the ocular muscles,
which it supplies.

It has no ganglion-rudiment, but in some forms the nerve-path is beset with nuclei, which have
been regarded as such, and the ciliary ganglion has been sometimes considered as arising by an
aggregation of the outwandering elements on this nerve instead of from the Gasserian ganglion
of the fifth.

DEVELOPMENT OF THE SYMPATHETIC SYSTEM

The problem of the origin of the sympathetic is only a part of the larger one of
the origin of the peripheral nerves. There are two chief views regarding the source
of the ganglion-cells. According to the one (Remak, Kolliker, Paterson), they are
mesodermic ; according to the other (Balfour, His Sr., His Jr., and many others),
they are ectodermic in origin. The first view is based (Paterson) on the inde-
pendent appearance in birds and mammals of an unsegmented strand of mesoderm

1 For a general discussion of the question, see Fiirbringer, ' Morphologische Streitfragen,' Morpho-
logisches Jahrbuch, xxx., 1902.

134

NERVOUS SYSTEM

into which, or through which, the visceral branches of the spinal nerves grow.
The nerve-fibres are said to become connected with the cells, and certain of these
persist to form the ganglia, while those of the intervening portion undergo changes
resulting in the formation of the commissural cords. This view has found little

hemisphere

pituitary
int. car. art. -

geniculate g.
v. cereb. lot. - —

auditory ves.

VII nerve

stapes and —
stapedial art.

IX nerve—

v. cerebralis 1. —
X nerve —

vertebral art

spinal ganglion

- spinal ganglion

FIG. 176. — SECTION OF THE HEAD OF A HUMAN EMBBYO OF 15'5 MM. Photograph. (T. H. Bryce.)

The section will be readily understood if the structures be traced along the line between the two
arrows in fig. 175, p. 131. Ill, cavity of diencephalon.

favour ; and even though the cells may seem to arise in situ, our present knowledge
of the composition of the mesenchyme does not warrant our pronouncing all
elements in it necessarily mesodermic. The second view is presented in several
forms. One account, based more especially on Selachian material, describes the

SYMPATHETIC! SYSTEM 135

ganglia as formed from outgrowths of the spinal nerve-roots (Balfour), the
cells of the outgrowths being identical with those which give origin to the nerve-
trunks, and becoming differentiated 'in situ into ganglion-cells. Another
account regards the sympathetic simply as detached parts of the spina
ganglia, the errant ganglia thus produced remaining attached to the spinal nerve
by the ramus communicans, and becoming secondarily connected together to form
the gangliated chain. His showed that in the human embryo the visceral branches
of the spinal nerves appear before the ganglia. He accordingly modified the
interpretation in the sense that the elements which form the sympathetic ganglia
are not formed nerve-cells, but indifferent cells which, arising in the spinal ganglia,
wander passively or actively along the previously formed nerve-paths to become
aggregated into groups or primitive ganglia, where their transformation into
nerve-cells is completed. A third interpretation goes one step further, and
describes the growth of the sympathetic as a part merely of a general extension
in the developing nerve-paths of indifferent ectoderm cells, which undergo their
differentiation into nerve-cells, sheath-cells, or chromophil- cells only when they
reach their peripheral situation (Kohn [) (see p. 100).

None of these interpretations of the appearances seen are easily capable of
objective proof, but the weight of evidence is decidedly in favour of the purely
ectodermic origin of the sympathetic, and of the discrete spread of indifferent
cells.

The sympathetic first appears in the form of groups of cells closely applied to
the ventral branches of the spinal nerves. Each of these soon becomes a cellular
cord which is the rudiment of the ramus communicans. The ramus communicans
next becomes fibrillar, and the ganglion is produced by proliferation of a
terminal group of cells (fig. 170). The primitive ganglia are secondarily connected
by cellular strands into a continuous cord, which becomes segmented later by the
conversion of the intervening strands into nerve-fibres. There is little doubt that
the whole system of plexuses and ganglia is formed by extension due either to
proliferation or to wandering of the cells from the primary chain. In the neck
the cord is closely related to the vagus, and the branches of the two are bound up
in a common plexus for the supply of the heart and lungs. The superior
cervical ganglion is said to be derived from the ganglion nodosum of the vagus,
and perhaps also the ganglion of the glossopharyngeal (His, Jr.). The abdominal
sympathetic consists at quite early stages of many groups of cells round the
aorta, and many scattered groups which extend into the mesentery, through which
the cells reach the stomach and intestine. The cells form a single layer in the wall
of the stomach, afterwards separated into the two plexuses by the formation of
the muscular coats (His, Jr.).

Chromophil, chromaffin, or phdochrome bodies. — It has within recent years been shown
that, more especially in the region of the abdominal sympathetic, but also along the whole
extent of the sympathetic cord, groups of cells are formed from the primary indifferent
sympathetic cells, which have the special property of staining yellow brown with the
salts of chromic acid. This chromophil system is represented in the adult by the medulla
of the suprarenal body, and perhaps also by the carotid and coccygeal glands. Such chromophi
bodies, first discovered in the human embryo in 1901 by Zuckerkandl, are seen grouped
more especially between the kidneys and suprarenal bodies, extending downwards along the
ureters into the pelvis. They consist of groups of large clear cells with very lightly staining
nuclei (fig. 257, p. 204), and contrast strongly with the groups of densely arranged smaller and
deeply staining cells traversed by nerve-fibres which are the rudiments of the sympathetic ganglia.
It is more especially to the researches of Kohn that the recognition of the system in the human
subject is due. It appears certain that the cells are sympathetic in origin ; they occur not only
in masses, but in scattered groups in the ganglia. The histogenesis is conceived briefly

1 Kohn, Arch. f. mikr. Anat. Ixx. 1907.

136

DEVELOPMENT OF EYE

as follows : The primary sympathetic cells are indifferent (ectodermic) elements which become
differentiated into two families of cells through a stage named in one case the sympaihoblast,
and in the other pMochromoblast (Pol '). which become respectively sympathetic nerve-cells and
chromophil or phaochrome cells. The significance of these researches in connexion with the
adrenal will be alluded to later.

DEVELOPMENT OF THE EYE.2

The eyes begin to develop as a pair of hollow protrusions from the primitive
fore-brain, named the optic vesicles. In some mammals the protrusions appear
before the neural canal is closed in by the fusion of the medullary folds ; in the

optic vesicle j&g

optic vesicle

rii pit *£* £•

- *

1 1 it 1.1 r it -pit

hind-brain

FIG. 177. — TRANSVERSE SECTION OF THE HEAD OF A
HUMAN EMBRYO OF 2'4 MM., SHOWING THE OPTIC
VESICLES AND AUDITORY PITS. (T. H. BryCG.)

FIG. 178. — SIDE VIEW or ANTERIOR

PART OF BRAIN OP A HUMAN
EMBRYO OF THE FOURTH WEEK,
SHOWING THE PRIMARY OPTIC
VESICLE FOLDED AND CUPPED.

(His.)

c.h., cerebral hemisphere (part
of) ; off., olfactory lobe ; opt., optic
cup.

FIG. 179.— SIDE VIEW OF THE SAME PART

OF THE BRAIN IN A STILL MORE AD-
VANCED EMBRYO, THE EYE HAVING
BEEN CUT AWAY. (His.)

opt., cut end of optic stalk, showing
the manner in which it is folded;
i, infundibulum ; olf.p., posterior
of olfactory lobe ; olf.a., anterior part of
the same ; c.h., cerebral hemisphere ;
i.e., tuber cinereum.

pig, for instance, as shown by Keibel, they show as shallow pits on the medullai
folds while these are still spread out flat. The appearances presented by the
very early human embryo drawn in fig. 177, show that this may occui
in the human subject also. The optic vesicle is continuous on its outer side with
the surface ectoderm of the side of the head ; and as this point of attachment does
not move so much during the formation of the cranial flexure as does the attach-
ment to the brain-tube, it follows that the vesicle becomes obliquely placed (His),

1 In Hertwig's Haiidbuch der vergleich. Entwickelungslehre III. Th., for which see references to
recent literature.

-' For literature, see Froriep, Hertwig's Handbuch II. Th. i. and li. p. 261 seq. More recent
references in footnotes.

.

OPTIC CUP AND LENS

187

ith its surface attachment dorsal and caudal, and its central end or stalk ventral
and cranial. The surface ectoderm now becomes thickened and pitted-in so as to
forma cup-shaped depression (fig. 180), which subsequently becomes converted into
a vesicle by the closure of its mouth. This is the rudiment of the lens, and pari
IKLSSU with its formation the optic vesicle becomes doubled up to form the optic
cap. The cavity of the cup is occupied at first by the lens vesicle, but later it
becomes opened out to form the cavity of the eyeball or vitreous chamber, while

aud

FIG. 180.— TRANSVERSE SECTION HEAD OF A RABBIT-EMBKYO OF THE ELEVENTH DAY. (T. H. Bryce.)

fb, fore-brain ; hb, hind-brain ; op.ves., optic vesicles ; lens, lens-plaque ;
aud., auditory vesicles.

the original cavity of the optic vesicle is almost entirely obliterated, appearing
merely as a cleft between the outer and inner walls of the cup.

Development of the lens. — The rudiment of the lens is a disc-shaped ecto-
dermal plaque situated on the side of the head opposite the upper and outer aspect
of the optic vesicle (fig. 178). The plaque is at first closely applied to the outer
wall of the vesicle, but when this begins to be invaginated they draw apart some-
what, remaining connected, however, by protoplasmic strands (fig. 183). This
syncytial connexion is in all probability maintained during the formation and

138

EYE

opening out of the optic cup — a point the significance of which will appear later.
In the few cases in which an open lens-pit has been described in the human
embryo it is figured as a thick-walled, cup-shaped, then flask-shaped depression,
the lips of which come together to form a vesicle which is connected for a time
with the surface ectoderm by an epithelial stalk, but afterwards becomes com-
pletely separated from it. The inner wall of the vesicle at an early stage

FIG. 181. — DEVELOPMENT or THE LENS IN THE BABBIT. (After Rab], from Hertwig's
Handbuch der Entwickelungslehre.)

Nos. 1 to 8 x 130 diameters ; No. 9 x 91 diameters. The stages 1 to 5 embryos from the middle of the
eleventh to the middle of the twelfth day. No. 6 an embryo at the end of the twelfth day.

increases in thickness and encroaches on the cavity, while the outer remains a
thin lamella (fig. 181).

In the rabbit embryo the lens-pit does not coincide with the centre of the lens-plaque ;
therefore in cross-section it is triangular, not hemispherical. The vesicle consequently is also
triangular in section, and it is rather the upper and inner wall which becomes thickened by
the elongation of its cells. The lens in its early stages thus appears in cross-sections of the
embryo to be obliquely placed in the optic cup (fig. 181). It may also be mentioned, though
the significance of the fact is unknown, that in the rabbit, and also in man (Rabl), the vesicle
contains a mass of epithelial cells, which undergo degenerative changes and ultimately disappear.

LENS

139

The thin anterior layer remains throughout life as a simple layer of cubical
cells, and forms the so-called lens-epithelium ; but the cells of the posterior layer
grow forwards into the cavity of the lens-vesicle as the lens-fibres : the central
fibres are the longest and straight (fig. 182), while the rest are slightly curved
with their concavity towards the equator. The fibres become gradually shorter
towards the circumference, where they pass through gradually shortening

FIG. 182. — SECTION OF THE DEVELOPING EYE OF A BABBIT-EMBBYO OF THE THIRTEENTH DAY.

(T. H. Bryce.)

The section passes through the optic stalk, and cuts the groove in which the central artery of the
retina passes into the interior of the optic cup. The cavity of the lens-vesicle is not yet obliterated ;
its anterior wall is formed of a layer of cubical cells • its posterior wall, greatly thickened, is becoming
converted into the lens-fibres. The cavity of the optic cup is almost filled by the lens. On the surface
of the retinal layer of the cup is seen a protoplasmic nuclear-free zone which is the primitive vitreous.
It has shrunk away from the lens, and thereby shows very clearly the vascular layer of mesenchyme on
the posterior aspect of that body. The vessels are seen entering the optic cup through the choroidal
fissure of the optic stalk, and also through the space between the lens and the mouth of the cup.

columnar cells (transitional zone) into continuity with the anterior epithelium.
By the growth of these fibres the cavity of the lens-vesicle becomes obliterated.

In this manner the central part of the lens is developed, and it consists in the
main of fibres which pass in an antero-posterior direction. The remainder of
the lens is formed of fibres which are so disposed as to curve round its margin
and over the ends of the first formed fibres ; they are, moreover, deposited in

140 EYE

successive layers and in three (or more) separate sections, so that their ends abut
against one another in front and behind along tri-radiate (or multi-radiate) lines,
such as may be seen in the macerated lens. These later deposited fibres are all
formed at the equator (at the transitional zone), where cell-multiplication chiefly
takes place, and they grow hence meridionally backwards over the ends of the
already developed antero-posteriorly disposed fibres of the central part of the lens.

Development of the optic cup. — The doubling- in of the optic vesicle is
a gradual process of involution, and from the first the invaginated outer wall is
thicker than the inner (fig. 182). The thinner outer layer of the completed cup
early shows a deposit of pigment, and becomes the hexagonal pigmented
epithelium of the retina, while the thicker inner layer is converted by a
complicated series of changes into the retina.

The optic vesicle at first opens into the cavity of the fore-brain by a wide
aperture. As the vesicle enlarges the lips of this gradually close in, and the stalk
becomes elongated into a hollow cord. The upper wall of this tube is thinner
than the lower. When the optic cup is formed the thin upper wall of the stalk is
continued into the outer layer, while the thick lower wall is continued into the
thick retinal layer of the cup. This is due to the character of the invagination of
the optic vesicle. It is not a simple in-pushing of the outer wall by the growing
lens-vesicle, for the folding is not confined to the part of the wall against which
the lens lies, but also implicates the ventral wall and commencement of the stalk
(fig. 178). A cleft is thus left below the lens which is continued some distance
along the stalk as a fold of its thick lower wall (fig. 179). The cleft and groove
soon become closed in, but before this is effected vessels enter the hollow of the
cup (fig. 182), the fate of which will be discussed later.

The line of closure of the lips of the cleft remains apparent for some time owing to the fact
that when pigment develops in the wall of the cup this so-called choroidal fissure remains
unpigmented for a time. The malformation known as coloboma iridis is attributed to a
persistence of this fissure or unpigmented tract.

Development of the retina. — The thickened inner layer of the optic cup
early shows a distinction into a thicker posterior portion, the pars optica., and a
thinner anterior portion the pars cceca. The line of demarcation becomes marked
by a thickened lip known as the ora serrata. The pars cceca becomes further divided
into the pars ciliaris retina where the inner layer remains as a single lamella of
columnar cells, and the pars iridis where it becomes closely united with the
pigmented outer layer, and spread over the inner surface of the developing iris,
to form the thickly pigmented epithelium known as the uvea.

The pars optica -undergoes histological changes which are, in their essential and primary
features precisely similar to those already described for the general neural epithelium. It is at
first a single layer of high columnar epithelium with closely set nuclei at different levels. The
germinal zone is necessarily on the outer convex side of the lamella, that having been the original
inner surface. As the nuclei multiply a nuclear-free (or nearly free) zone is formed on
the concave aspect, which corresponds to the reticular zone of the general neural epithelium.
Sustentacular or primitive glial elements are laid down and persist as the fibres of Mutter, and an
outer and inner medullary lamina appear. The multiplying nuclei become arranged in zones
separated by narrow reticular bands. This is the expression of the grouping of the neuro-
blasts into radiating cell- complexes, or, interpreted by the syncytial theory, of the arrangement
of the neural syncytium into radiating multinuclear fibrillar paths. The nerve-fibre layer is
formed as elsewhere from the marginal reticular zone. The rods and cones appear first in the
axis of the globe as rounded refractile bodies projecting from the external medullary lamina into
the cleft between the two layers. They are produced progressively from the central point of the
retina to the periphery. Graham Kerr * finds in Lepidosiren paradoxa, in which the cells
are of great size, that in the elements destined to become visual cells a vesicle appears which
contains apparently a fatty substance. As this enlarges the cell bulges the external medullary

1 Quart. Jour. Micro. Sci. xlvi. ; see also Cameron, Jour. Anat. and Phys. xxxix.

RETINA AND OPTIC STALK 141

lamina before it, so that a part projects into the space between the layers of the optic cup. From
this a protoplasmic process is developed, which, elongating, becomes transversely striated and
converted into the cuticular rod.

The optic stalk is, as we have already seen, hollow. Its lower wall is thicker
than its upper and is invaginated by the choroidal fissure at its ocular end. The
fissure soon closes, and the artery, entering the optic cup within the stalk, is
enclosed. The ventricular cavity is obliterated by the middle of the second
month, but the epithelial cells retain for some time longer their radial disposition.
This soon becomes lost, and the stalk becomes converted into a glial network in

^- pp

lens * k\va

FIG. 183. — SECTION OF THE DEVELOPING EYE OF TROUT. (Szily.)

lens, lens-vesicle not yet closed ; ret, inner layer, pp, outer layer of optic cup ; p.v., primitive
vitreous; a, protoplasmic connexions between the cells of the outer and inner walls of the optic cup.

which the nerve-fibres appear at the end of the second month (His). The nerve-
fibres begin in the retina and grow along the stalk towards the brain, extending
into its thickened invaginated lower wall. The point where this is continuous
with the retina at the centre of the optic cup becomes the optic disc. The optic
chiasma is formed by fibres crossing in the posterior boundary of the optic recess,
and the optic tract is a new formation by which the eye is secondarily connected
with the optic thalamus and mid-brain.

Development of the vitreous body and lens capsule.1 — The formed
elements of the vitreous body and the zonule of Zinn are to be regarded as a special

1 The following account is founded on the observations of Toruatola, Rabl, Addario, Van Pee,
Lenhossek, Kolliker, and Szily. My own observations, on which the actual description is based, have
been made on rabbit material. — T. H. B.

142 EYE

development of the syncytial system of nuclear-free protoplasmic threads which
Szily has shown exist between all epithelial formations as they draw apart in the
course of development. In most situations, as has already been indicated,
this system is the basis of the mesenchymatous syncytial network when the
free cells have wandered into it, but in the case of the vitreous it remains
largely cell-free. The lens capsule belongs primarily to the same category, but
in mammals, in which alone a rete vasculosum lentis is developed, mesoderm cells
and blood-vessels enter into its formation.

We have already mentioned the existence of primary protoplasmic connexions
between the lens and the future retinal epithelium (fig. 183). When the lens-pit
closes in, similar connexions are formed between the outer wall of the vesicle and
the surface epithelium. When the retinal layer draws away from the lens, and the

lens ves

FIG. 184. — SECTION OF THE DEVELOPING EYE OF TROUT. (Szily.)

lens, lena; ret, retina; hp, outer layer of optic cup; p-v., primitive vitreous; ves, blood-vessel.
Mesenchyme-cells are seen passing into the space between the surface-ectoderm, the optic cup, and lens.

lens from the surface epithelium, these protoplasmic threads are drawn out into a
mesh- work which fills the optic cup and surrounds the lens-vesicle (fig. 184). The
mesenchyme surrounding the cup does not at first extend beyond its mouth, but in
mammals mesenchyme- cells soon extend round the lens and form a layer on the
back of that body, so that here the primitive syncytial mesh work is replaced by a
lamella of typical mesenchyme. In this vessels appear, and these are supplied
by a vascular loop which extends into the cup through the choroidal fissure. It
ramifies among the vitreous threads, and later becomes the central artery of the
retina and its hyaloid branch. As the optic cup expands this mesenchymatous
lamella clings close to the lens (fig. 182), and the space behind it is seen to be
filled with protoplasmic strands connected with every part of the retinal
epithelium (Eabl, Kolliker, and others). The connexion of the fibrillfp with the
pars optica is lost, but in the ciliary region the attachment persists, and the

VITKEOUS, SCLEROTIC, AND CHOROID

143

<tp

fibrils from the ciliary epithelium form the greater part of the formed portion
of the vitreous, the so-called hyaloid membrane, and zonule of Zinn.

The syncytial meshwork in front of the lens has a different fate. It becomes
invaded by large numbers of mesenchyme-cells, and is converted into a thick
cellular plate between the lens and the surface epithelium. At this stage a delicate
refractile membrane appears round the lens, formed apparently from the mesh-
vrork, and this, together with the mesenchymatous layer behind the lens, and
a delicate lamella of mesenchyme in front of it — separated off at a later stage from
the anterior mass of mesoderm — becomes the lens-capsule.

The membrane thus defined becomes the tunica vascidasa of the later stages of foetal develop-
ment. The portion behind the equator of the lens is supplied by the hyaloid artery, and the
part in front of the equator by the
anterior ciliary arteries. Shortly
before birth the vessels disappear :
the hyaloid artery is obliterated,
but its track in the vitreous per-
sists as the canal of Stilling. The
fore- part of the capsule is named
the pupillary membrane. '

Development of the
protective and vascular
coats ; the iris and
aqueous chamber. — The

optic cup is surrounded by a

specially vascular mass of

mesenchyme, and, as we have

seen, this extends into the

space between the lens and

the surface epithelium. It

here forms a thick cellular

layer, which is the rudiment

in great part of the cornea,

while the surface ectoderm

over it forms the corneal

epithelium. The cornea is at

first identical in structure

with the primitive sclerotic,

with which it is, of course,

continuous. Later it under-

goes histological changes

which cause it to become

transparent. The mesen-

chyme surrounding the optic

cup becomes differentiated

into two layers — an outer

more condensed stratum which forms the sclerotic coat, and an inner, looser stratum,

enclosing many vessels, which becomes the choroid coat. This is thickened

near the margin of the cup to form the ciliary body, and its surface becomes

radially folded over the thickening to give rise to the ciliary processes. On its

outer side the ciliary muscle is laid down.

The aqueous chamber is formed by a separation of the mass of mesenchyme
between lens and surface into two lamellae — a thick anterior layer which becomes

1 For an exhaustive account of the development of the arteries of the mammalian eye, see Fuchs,
Anat. Hefte xxviii. 1905.

FlG. 185. — HOBIZONTAL SECTION THROUGH
EMBBYO-RABBIT OP EIGHTEEN DAYS. ST

AN

o, optic nerve ;

THE EYE OF

1 (Kolliker.)

ve ; p, hexagonal pigment-layer ; r, retina ;
re, ciliary part of the retina ; p', fore-part of the optic cup
(rudiment of the iris-pigment) ; g, vitreous, shrunk away from
the retina, except where the vessels from the arteria centralis
retinae enter it ; i, iris ; ?np, membrana pupillaris ; c, cornea
with epithelium e ; pp,pa, palpebrse ; I, lens ; I', lens-epithelium ;
/, sclerotic ; m, recti muscles. The formation of the aqueous
chamber is just beginning, and is seen as a cleft in front of
the iris on each side.

144 EYE

the cornea, as above described, and a posterior which gives rise to the iris and the
mesenchyme of the pupillary membrane (fig.185). The separation begins just in front
of the ciliary region. Here the posterior lamella is composed of loosely arranged
mesenchyme, applied to the pars iridis of the optic cup. It gives origin to the
stroma of the iris. The cleavage proceeds from the periphery over the front of
the lens, but here only an excessively thin lamella is separated off, in which the
vessels of the pupillary membrane develop. It is at first directly continuous
with the iris (fig. 185), but when the vessels disappear it is dissociated from that
structure, and if any trace of it remains it appears to be incorporated with the
lens-capsule.

Accessory structures. — The eyelids make their appearance as folds
•of integument, subsequently to the formation of the eyeball. About the third
month of foetal life the two folds, one forming the upper and the other the
lower lid, meet and unite by a growth together of the epithelium at the margins
of the folds, so as to cut off the conjunctival sac from the exterior. Shortly
before birth they again become disunited.

A third fold (of the conjunctiva) appears at the inner canthus, and in many
vertebrates develops into a well-marked third eyelid, the membrana nictitans.
In man it remains rudimentary, forming the plica semilunaris.

The glands, hairs, and other structures belonging to the eyelids are developed
in the same way as the corresponding structures in the rest of the integument.

The lacrymal gland is developed in the third month as a number of out-
growths from the deeper layer of the epithelium, at the upper and outer part of
the conjunctival sac. The outgrowths are at first solid, and branch into the
surrounding connective tissue as in the case of racemose glands, subsequently
becoming hollowed out and differentiated into ducts and acini.

The lacrymal canals and ducts were formerly described as being directly
developed by the enclosure of the fissure which separates the lateral nasal
process from the maxillary process (see Development of Face, p. 86), and
which passes in the early embryo from the eye to the upper part of the naso-
buccal cavity (lacrymal fissure). But it has been shown, chiefly by the researches
of Born, that in most animals the canal is at first formed as a thickening of the
rete mucosum of the epidermis, which sinks into the corium along the line of that
fissure. The thickening subsequently becomes separated from the rest of the
epidermis, and hollowed out to form an epithelial tube, which leads from the
conjunctiva into the nasal cavity.

The bifurcation of the duct where it opens on the conjunctiva was formerly believed to be
produced (Ewetsky) by a broadening out of the epithelial cord at the inner canthus, and its
subsequent separation into two parts by an ingrowth of connective tissue in its middle, the
two parts developing into the upper and lower lacrymal canals. It has been shown, however,
(Matys, Fleischer, and Ask ' ), that, although the epithelial cord remains a long time in continuity
with the conjunctiva, both the lacrymal canaliculi and nasal duct are produced by proliferation
of the epithelial blastema in the connective tissue, and come secondarily and simultaneously
into contact with conjunctival and nasal epithelium respectively. The lumen appears first
in the inferior canaliculus, and then at different points in the cord. The upper canal lags
behind the lower in development : it opens at first into the conjunctiva close to the inner
canthus; while the lower canal opens considerably further out along the edge of the lid.
Space is thus left in the lower lid, between the punctum 1 aery male and the inner canthus, for
the development of Meibomian follicles. As the lower canaliculus enlarges, these are compressed,
and therefore atrophy, and the tissue in which they lie becomes the caruncula lacrymalis.

1 Matys, Zeitschr. f . Augenheilk. xiv ; Fleischer, Archiv f. Ophthal. Ixii. ; Ask, Anat. Anzeiger,
xxx. 1907.

EAR

145

DEVELOPMENT OF THE EAK.1

The essential part of the ear — viz. the epithelial lining of the labyrinth — is
developed in much the same way as the crystalline lens, as an invagination of the
external ectoderm, which at first appears as a pit of thickened epithelium (auditory
pit, fig. 177), but is gradually converted by a growing together of the margins of
the pit into a hollow island of ectoderm, the auditory or otic vesicle (fig. 180). This
process occurs somewhat after the formation of the eye has begun, and at quite
a different part of the head — viz. on either side of the hind-brain just over the
upper end of the first post- oral visceral cleft. The vesicle comes at first into close
contact with the hind-brain, except where the ganglionic rudiment of the auditory
nerve projects between them, but it subsequently becomes entirely surrounded
by mesoderm, which separates it from both the neural and external ectoderm.

c.c.

FIG. 186. — STAGES IN THE DEVELOPMENT OP THE MEMBRANOUS LABYRINTH. (W. His, Jr.)

A. Left labyrinth of a human embryo of about four weeks, viewed from the outer side, u, vestibular
part ; c, cochlear part ; r.L, recessus labyrinthi (aqueductus vestibuli).

B. Left labyrinth with parts of the facial and auditory nerves of a human embryo of about four and
a-half weeks, b, surface of the hind-brain ; u, utricular ; s, saccular part of labyrinth; a.u.c., p.s.c.,e.s.c.,
rudimentary folds representing the two vertical and the horizontal semicircular canals ; r.L, upper part
of recessus labyrinthi becoming enlarged into the endolymphic saccule ; c.c., rudiment of cochlea ; n.v.,
vestibular branch of auditory nerve ; g.v., vestibular ganglion (ganglion of Scarpa) ; g.c., cochlear
ganglion ; n.f., facial nerve, with geniculate ganglion, g.g.

C. Left labyrinth of a human embryo of about five weeks, viewed from without and below. Letter-
ing as before. The horizontal canal is still only a fold. The ampullae are beginning to be visible on
the two vertical canals.

The otic vesicle is at first flask- shaped, with the somewhat elongated mouth of
the flask directed externally towards the original point of connexion with the
exterior. In elasmobranch fishes this passage is never closed, but remains
throughout life in the form of a small duct-like tube which passes up through the
cranial wall and opens on the epidermis. In other vertebrates the opening to
the exterior becomes closed, and what remains of the original mouth, or canal of
connexion with the exterior, is visible as a distinct but small process from the

1 For literature see Krause, Hertwig II. Th. i. and ii. p. 133 seq.
in footnotes.
VOL. I.

Reference to more recent papers

146

EAR

upper and inner angle of the vesicle, and is known as the recess of the labyrinth
(fig. 186, r.L). Eventually it develops into a long epithelial tube, which passes

p.s.c.

p.s.c.

FIG. 187. — MODELS OP THE MEMBRANOUS LABYRINTH :
AND B OF AN EMBRYO OF 13 MM.

sacc, saccule; cc, cochlea; r.L, recessus labyrinth!; a.s.c.
canal. The future lateral semicircular canal is represented by
sacc ; vr vestibular, cr cochlear division of auditory nerve.

A OF A HUMAN EMBRYO OF 11 MM.,

(After Streeter.)

superior, p.s.c. posterior semicircular

the fold projecting outwards above

p.s.c.

through the petrous bone, with an expanded end lying within the skull under-
neath the dura mater. This tube and its expanded termination form respectively
the endolymph canal and endolymph saccule (fig. 189).

In the meantime the audi-
tory vesicle becomes elongated
and begins to be irregular.
It shows a larger triangular
swelling in its dorsal part to
which the endolymph canal
is attached, and a smaller
flattened sac which is the
rudiment of the epithelial
canal of the cochlea. At the
junction between the two
moieties a bulging, described
by Denis and named atrium
by Streeter,1 constitutes the
rudiment of the utricle and
saccule. The dorsal or vesti-
bular pouch soon shows a
vertical and a little later a
horizontal fold (fig. 186) ; the
former is the rudiment of the
two vertical (anterior and
posterior) semicircular canals
and their common opening
(crus), the latter the rudiment
of the horizontal (external)
canal. The folds which give rise to the canals are flattened semicircular
hollow protrusions from the wall of the vesicle. The layers of the folds next

1 Amer. Journ. of Anat. vi. 1907.

FIG. 188. — MODEL OF THE MEMBRANOUS LABYRINTH OF A
HUMAN EMBRYO OF 20 MM. (After Streeter.)

u, utricle; s, saccule; cc, cochlea; r.L recessus labyriiithi ;
a.s.c. superior, p.s.c. posterior semicircular canal ; vr ves-
tibular, cr cochlear division of auditory nerve.

INTERNAL EAR

147

come together and coalesce, except near the circumference of the semicircle, which
now forms a tube connected at its ends with the vesicle. Subsequently, by
absorption of the coalesced lamellae, the tube is converted into a free loop.
One of the ends becomes dilated into an ampulla and connected with a branch
of the auditory nerve. In consequence of the manner in which the two
vertical canals arise from the upper vertical fold they are at first in a line with
on«> another, but as they take form they come by differential growth to be

p.s.c.

FIG. 189. — MODEL OP THE MEMBBANOUS LABYBINTH OF A HUMAN EMBRYO OF 80 MM.

(After Streeter.)

u, utricle ; s, saccule ; cc, cochlea ; r.l., recessus labyrinth! (aqueductus vestibuli) ; crus, common
opening of superior and posterior semicircular canals; sin, sinus utriculi lateralis; a.s.c. superior
semicircular canal: ani]).a, its ampulla; p.s.c., posterior semicircular canal: amp.p, its ampulla;
l.s.c. horizontal semicircular canal; vr, vestibular division; cr, cochlear division of auditory nerve';
br, branch from vestibular division of nerve to ampulla of the posterior semicircular canal.

placed at right-angles to one another, the ampullary end of the superior
retaining its original position. While the semicircular canals are forming,
the ventral cochlear portion begins to grow out and become curved on itself,
while the atrium becomes subdivided by a fold into an upper and posterior
chamber connected with the semicircular canals, the utricle, and a ventral and
anterior connected with the cochlea, the saccule. This fold extends into the
attachment of the recess of the labyrinth and separates it longitudinally for a
short distance into two tubes, one of which opens into the utricle and the other

L2

148 EAR

into the saccule, forming the only permanent means of communication between
them. Another fold, or constriction, appears presently, somewhat lower down
and converts the connexion between the saccule and the cochlea into the narrow
duct of Hensen (canalis re-uniens).

In the meantime the cochlea-rudiment at the ventral end of the now labyrinthine
vesicle becomes elongated into a tube, which, as it grows, becomes coiled upon
itself in such a manner as to produce the spiral structure of this part of the auditory
organ (figs. 188, 189). This coiling, however, only occurs in mammals ; in birds,
the cochlea is a short straight blind tube.

All these parts of the labyrinth are, when first formed, simple epithelial tubes
surrounded by and imbedded in embryonic connective tissue. As development
proceeds, and the skull begins to form, a cartilaginous capsule becomes developed
around the several parts of the labyrinth, and this at length becomes ossified.
The cartilaginous capsule does not closely invest the epithelial structures ; they
are immediately surrounded by embryonic connective tissue, which forms an
internal periosteal lining to the capsule and a special covering to the epithelial
tube. These two connective-tissue membranes are everywhere separated from
one another by gelatinous connective tissue, composed of semi-fluid ground sub-
stance and branching corpuscles, except along one border, where they are in
continuity. But in the cochlea the gelatinous tissue is above and below the
epithelial tube, the place of the modiolus being occupied by embryonic tissue
which is not gelatinous, and is connected with that lining the capsule by similar
non-gelatinous tissue separating the turns of the cochlea from one another, and
also running in the position of the future spiral lamina.

The bone, which is formed by ossification of the cartilaginous capsule, is of a spongy nature,
but it becomes coated internally by layers of compact bone deposited by the periosteal
lining. The modiolus and septa of the cochlea, as well as the osseous spiral lamina, are formed
wholly in connective tissue without any preformation in cartilage.

The perilymphatic spaces throughout the whole labyrinth are produced by a gradual vacuo-
lation and disappearance of the gelatinous tissue which surrounds the membranous labyrinth.
In the cochlea this conversion into perilymph begins in the proximal turn of the spiral and
extends hence towards the distal end. It is only with the development of these perilymph- spaces
(scalae) that the cochlear tube, which was previously oval in section, acquires the characteristic
triangular section which we see in the fully formed organ.

The cells which form the wall of the epithelial tube become variously modified in different
parts of" the labyrinth to produce the characteristic structures which there occur — viz. the
hair-cells, the rods of Corti, the sustentacular cells of Deiters, and the epithelium lining the
labyrinth. The membrana tectoria appears as a cuticular deposit over the columnar cells
which are becoming developed into the organ of Corti.

The auditory nerve arises from a ganglionic mass which is early divided into
an acoustic and a facial portion (geniculate ganglion) (fig. 186). The acoustic
ganglion lies on the front edge of the auditory vesicle with its lower end turning on
to its mesial aspect (figs. 186, 187). It consists (in embryos of 7 mm., twenty-sixth
day) of an upper and a lower part. The central root of the ganglion springs from
the upper part, and each division has its own peripheral branches. According to
Streeter's researches, the lower part is not the cochlear ganglion as described by
W. His, Jr. The ganglion spirale develops from the ventral border of the pars inferior,
becomes coiled with the cochlea, isolated from the rest of the common ganglion, and
connected secondarily with the neural tube by a separate nerve-root, the cochlear
root. Thus the pars superior and pars inferior together constitute the vestibular
ganglion. From its upper portion are derived the nerves to the utricle and to
the ampullae of the superior and lateral canals, while from the lower portion come
the nerves to the saccule and ampulla of the posterior semicircular canal (fig. 189).

MIDDLE EAR

149

The primary ganglion is closely applied to the auditory vesicle, and the peripheral
nerves are very short. The secondary ganglia become included in the capsule as
this develops round the labyrinth.

ACCESSORY PARTS OF THE ORGAN OF HEARING : MIDDLE EAR
AND EXTERNAL AUDITORY MEATTJS.

The middle ear and the Eustachian tube are derived from the first
branchial pouch of the pharynx ; the auricular fossa and the external

meatus from the first branchial cleft ; the former are therefore entodermic,
the latter ectodermic derivatives. l

The bottom of the ectodermic cleft, which is shallow above and deeper below,
comes into contact for a time with the entoderm of the corresponding pharyngeal
pouch and its dorsal prolongation. The original depression persists as the fossa
of the auricle (concha and upper auricular fossa), while the different folds of that
structure are produced by a series of elevations which appear on the prominent

concha

incus malleus

cochlea

pharynx

FIG. 190.— RECONSTRUCTION. OF THE TYMPANUM, PRIMITIVE EXTERNAL AUDITORY MEATUS, COCHLEA,

AND OSSICLES OF A HUMAN EMBRYO 24 MM. LONG, FROM THE FRONT. (After Hammar.)

The tympanic cleft (tymp.} is seen extending from the pharynx; at its outer end is a notch
bounded by two recesses of the tympanic cavity, of which only the anterior is seen ; opposite the
notch is the handle of the malleus. Meckel's cartilage (M.c.} and Reichert's cartilage (M.c.) are cut
across at their lower ends : the former is directly continuous with the rudiments of the malleus and incus.
/ir.n.m., primitive external meatus. The spoon-shaped inward part of this is the meatal plate.

lips of the fissure (see Section I. p. 88). The external auditory meatus is in part
produced from an inward tubular prolongation of the lower and deeper part of
the cleft, and in part from a solid epithelial plate which grows obliquely from it
inwards and downwards, below the fissure representing the tympanic cavity
(fig. 190). The cartilaginous portion of the meatus and also a small part of the
roof of the osseous meatus, which have a typical skin-lining, are derived from
the tubular invagination ; the deep portion of the osseous meatus is produced by
the shedding of the central cells of the epithelial plate. The lumen thus pro-
duced lies obliquely like the solid plate, and its upper and inner wall forms ultimately
the ectodermic covering of the tympanic membrane.

The primitive tympanic cavity is derived from the dorsal prolongation of the
first visceral pouch. This and its lateral expansion are at first in contact with
the ectoderm, but the epithelial layers are soon separated again by mesenchyme.
The extremity of the dorsal prolongation is to be recognised at this stage, and through

1 The following account is based on the very^detailed descriptions given by J. August Hammar
( i'psala), Archiv. f. mikr. Anat. lix. 1902.

150

EAK

all later stages, as a pocket named the anterior tympanic recess. From this, ii
a forward direction, a shallow groove extends along the roof of the pharynx, anc
another furrow runs backwards to the dorsal pocket of the second visceral pouch.
This furrow is divided into two portions, a short, sharply descending section and
shallow horizontal posterior prolongation. At the junction of these two a secom
pocket develops which is named the posterior tympanic recess. By the expansioi
of the pouch and the deepening of these furrows the primitive tympanum is lai(
down as a wing-like diverticulum from the pharynx, which extends in a horizonl
and then in a dorsal direction. In shape it is a narrow cleft, the inner wall bein^
rendered salient by the growing cochlea. In the mesenchyme on its outer sid(
the cartilages of Meckel and of Reichert are laid down. The upper end of the forme
passes over the two recesses above named and expands to form the rudiments oi
the malleus and incus. From the malleus a process extends downwards an<
inwards which is the rudiment of its handle (fig. 190). This causes a projection intc

vestibule

M.c.

FIG. 191. — [RECONSTRUCTION OF THE AUDITOBY CAPSULE, THE TYMPANUM, AUDITORY OSSICLES. AND
EXTERNAL AUDITORY MEATUS OF A HUMAN EMBRYO 24'4 MM. LONG, FROM THE FRONT. (After
Hammar.)

M.c . cartilage of Meckel (the proximal end of Meckel's cartilage shows two points, the malleus and
incus respectively) ; B.C., cartilage of Reichert; pr.a.m., the line points to the junction of the outer
portion developed as a pit from the surface and the inner portion developed from the meatal plate.

the outer wall of the tympanum between the anterior and posterior tympanic
recesses. The primitive tympanum is cut off from the pharynx from behind
forwards until it opens only by a short tubal portion. At this stage the cut-off
tubo-tympanic cleft is directed nearly horizontally outwards. Its outer end, with the
two recesses, is obliquely placed, and overlaps the inner end of the meatal plate
above described, a layer of mesenchyme — in which the handle of the malleus lies —
intervening between them. This layer of mesenchyme gives rise to the mem-
brana propria of the drum, and the epithelium to its inner mucous covering. The
handle of the malleus lies between the two tympanic recesses. In later stages
the tubo-tympanic cleft comes to lie more and more antero-posteriorly as the
cranial base pushes forwards, and the short tubal portion becomes elongated,
until the adult position and relationships are attained. During the later months
of pregnancy the lining membrane of the tympanum becomes greatly thickened
and gelatinous, so that the epithelial lamellae are brought together and the lumen

NOSE

151

obliterated. The cavity is again established after birth ; it is believed that
this is due, in part at any rate, to the establishment of respiration. By the
expansion of the cavity in various directions its several recesses are formed, and
the ossicles and the chorda tympani nerve, which, as we have seen, lie at first in
the mesenchyme external to and above the primitive tympanum, come to be
enclosed in folds of the mucous membrane within the fully developed cavity;

DEVELOPMENT OF THE NOSE.1

The olfactory organ appears towards the end pf the third week as an area of
thickened ectoderm on either side of the fore-brain. By the upgrowth of its margins
the area soon becomes depressed below the surface, and the so-called olfactory pit
is produced. The depression is at first pyriform in shape, the smaller end running
towards the stomodoeum (fig. 192). The mouth of the pit next becomes
constricted by the thickening and dra wing-in of its lips ; but at its pointed

Fit. 192. — PBOFILE VIEW or THE HEAD OF A HUMAN

EMBhYO OF NEARLY FOUR WEEKS. (His.)

olf, olfactory depression passing posteriorly into a
deep pit,' the rudiment of Jacobson's organ ; mx,
maxillary process ; tun, raandibular arch ; hy, hyoidean
arch ; brl, br-, first and second branchial arches.

-pr.glol.

FIG. 193. — HEAD OF AN EMBRYO ABOUT
TWENTY-NINE DAYS OLD, FROM

BEFORE. (His.)

pr.glob., globular extremity of the
mesial nasal process. The other
letters as in fig. 192.

•(stomodceal) end the circumference is interrupted, the raised margin ending
mesially and laterally in the mesial and lateral nasal processes (fig. 193). Between
these the pit is continued as a groove or furrow on to the roof of the stomodceum.
We have already seen (p. 88) that the lateral nasal processes form the alse nasi,
and unite with the maxillary processes, which in turn form the cheeks and outer
parts of the upper lip ; also that the mesial nasal processes (processus globulares)
unite with one another to form the central part of the upper lip and philtrum,
and then unite with the maxillary processes to complete the lip. If now the under
aspect of the processes in their first phases be examined, it will be seen that they
extend backwards in the roof of the embryonic mouth, being separated by the
groove already referred to (fig. 194 B). The groove is not, however, an open fissure
communicating with the olfactory pit, but is filled by a raphe of ectoderm produced
by the fusion of the opposing surfaces of the several processes. It is not clear
whether in man the epithelial raphe is a primary formation — an epithelial band
between the mesial and lateral nasal processes (Hochstetter) — -or is secondarily

For literature, see Karl Peter, Hertwig II. Th. i. and ii. p. 78 seq. Later references in footnotes.

152

NOSE

FIG. 194. — A, HEAD OF AN EMBRYO ABOUT THIRTY-FOUR DAYS OLD, FROM BELOW. B, THE ROOF OF

THE PRIMITIVE MOUTH OF THE SAME EMBRYO AFTER REMOVAL OF THE MANDIBLE. (His.)

i.m., placed on the fronto-nasal process, and just above its intermediate depressed part ; l.n.pr., lateral
nasal process; m.n.pr., mesial nasal process; other letters as in fig. 192. The nasal laminae of the
processus globular es and the palatine projections of the maxillary processes are seen in B.

olfactory nerve-fibres

septum

v:.*;^,v--;.^-.c--:.viV;/,at.?t4:.-&i«v->,vr«.v^*

ol. ves.

inf. m.

/ongue

FIG. 195.— SECTION THROUGH THE OLFACTORY VESICLES IN A HUMAN EMBRYO OF 15'5 MM.

(T. H. Bryce.j

ol.ves., olfactory vesicle : on the right the vesicle is still separated from the mouth by the bucco-
nasal membrane, b.n.m. ; on the left this membrane has disappeared, and the section passes through the
primitive choana ; J.o., groove which will become Jacobson's organ ; inf.m., groove which will become
the inferior meatus ; pal., palatal folds.

NOSE

158

produced by the walls of the groove being caught between the growing lateral nasal
and maxillary processes (Lewis). •

The mesial nasal process now becomes united with the lateral nasal and maxillary
processes by the extension of mesenchyme between them, and the epithelial raphe

septal carfil ><</'•

lateral
cartilage

,W.. lateral

'•'•' OJ-'Vs cartilage

plug

plug

FIG. 196. — HORIZONTAL SECTION OP THE NASAL FOSSAE OP A HUMAN EMBRYO OF 30 MM

(T. H. Bryce.)

•J.o., Jacobson's organ ; con., conchoe ; nd, lacrymal ducts ; ndl, their openings into the nasal
fossee ; oss., commencing ossification in maxilla.

is inteiTupted. It persists behind, however, and forms a thin membrane (bucco-
nasal membrane), which breaks through later, so that a passage is established
between the hitherto blind nasal sac and the stomodceum (fig. 195). The two
openings thus produced are the primitive posterior nares (choance). The united

154

NOSE

nasal and maxillary processes in the roof of the stomodoeum constitute the
primitive palate, while the two mesial nasal processes which have meantime fused
together, form a broad primitive septum (intermaxillary process) (fig. 194 B). The
primitive choanse do not correspond in position to the permanent posterior nares,
which are placed much farther back, and are established only in the third month,
when the permanent palate has been developed. The nasal sacs extend backwards
as the face takes shape and the interocular septum is produced, appearing as
narrow clefts in the roof of the primitive mouth. Each is surrounded by
mesenchyme in which a cartilaginous nasal capsule is laid down. On the outer
side — that is in the lateral nasal process — the cartilage takes the form of a
curved plate (fig. 196), connected behind with the trabecular region of the base of
the skull, and ending below in a free margin. The two lateral cartilages join
mesially with a septal cartilage (fig. 196), which has developed in the fronto-
nasal process as a forward projection of the trabecular region of the base of
the skull. The septal, like the lateral cartilage, ends below in a free edge, so
that the capsule is open below. The floor of the fossae is completed by the

FIG. 197. — THE KOOF OF THE MOUTH OF A HUMAN EMBBYO ABOUT TWO AND A-HALF MONTHS OLD,

SHOWING THE DEVELOPMENT OF THE PALATE. (After His.)

p g., processus globularis ; p.g.1, palatal process of process.us globularis ; mx, maxillary process ;
mxl, palatal fold of maxillary process. Close to the angle between this and the palatal process of the
processus globularis, on each side, the primitive choanae.

growth from the lower part of the maxillary processes of the palatal folds
(fig. 197), which unite with one another and then with the lower end of the septum
to form the palate. The palatal folds extend from the line of union of the mesial
nasal and maxillary processes backwards on to the wall of the pharynx. They
are at first below the level of the dorsum of the tongue, and are directed downwards
and inwards (fig. 195). As the tongue sinks between the growing mandibles they
are rotated into a horizontal position and meet in the middle line above the tongue.
The posterior parts of the folds, however, maintain their original direction (Polzl).
We have already seen that the mesial nasal processes, which unite superficially
to form the central part of the upper lip and the philtrum, extend backwards in the
roof of the stomodceum, and there unite to form the intermaxillary process (fig. 197).
This projects farthest back in the middle line, and has two oblique lateral borders,

1 For suggested explanations of this change of position of the palatal folds, see the following
papers : His, W., Abhand. math.-phys. Kl. Sachs. G-es. Wiss. 1901 ; Polzl, Anna, Anat. Hefte, xxvii.
1904 ; Schorr, Anat. Anzeiger, xxx. 1907. See also Gb'ppert, Morph. Jahrb. xxxi. 1903, and Anat. Anzeiger,
xxiii. 1903, on the more general question as to the origin of the secondary palate.

„

ORGAN OF JACOBSON AND OLFACTORY NERVE 155

ith which the palatal folds meet to complete the palate in front. Between them
openings persist for a time — the ducts of Stenson, which correspond to the
permanent passages from the mouth to the nose in lower mammals. Though
obliterated during embryonic life in man, the ducts are represented by strands of
tissue occupying the foramina of the same name in the bony palate.

The hard palate is formed by the extension of bony plates into the membranous
folds. Posteriorly these are absent, and muscular tissue extends into the folds,
giving rise to the soft palate and uvula. The palato-pharyngeal folds repre-
sent the posterior ends of the palatal folds which do not in this region unite
with one another. The turbinate processes appear in the second month, long
before the palate is completed, as projections from the outer wall composed of
a basis of mesenchyme covered by thickened epithelium (fig. 196). In these
projections cartilaginous plates are laid down, connected with the nasal capsule,
which, growing inwards and becoming curved, form the rudiments of the various
conchse. The accessory sinuses are produced by outgrowths from the originally
simple furrows separating the primitive turbinate processes.

The mechanism by which the processes and furrows are formed is variously interpreted.
In the first place, it may be definitely stated that the projections are not due to an inpushing
of the wall by the cartilaginous strands which become the conchae. The folds are present
before cartilage is formed within them (fig. 196). There are two other explanations. The
projections may be either free ingrowing folds of the mucous membrane (Killian, Mihalkovics,
and others) or they may be elevations left by excavations of the furrows in the outer wall
(Legal, Schonemann). According to Schonemann, epithelial ridges grow out in the position of
the future furrows, and these are excavated into epithelial pockets. From this point of view,
the complexities of the nasal fossae are due to the operation of a single process, the early
furrows being produced in the same fashion as the later sinuses. Both factors may be at work
simultaneously (Glas).1

The organ, of Jacobson, though represented by a vestige merely in the adult
human being, is a well-marked structure in the embryo. It appears as a deep
pocket at the stomodoeal end of the olfactory pit, and afterwards, when the mouth
of the pit is closed in, as a pocket on the lateral aspect of the mesial nasal process
(fig. 195). Later, it has the form of a narrow duct, oval in section, running longi-
tudinally in the substance of the septum (fig. 196) and opening anteriorly near the
upper orifice of Stenson's duct. When the septal cartilage becomes formed, a
special curved plate of cartilage is developed which partially encloses the organ
and persists in the adult.

The nostrils are closed for a time by an epithelial plug (fig. 196), the permanent
passages being established by a shedding of the central cells in the epithelial mass.

Olfactory nerve. — What was formerly described by anatomists as the
olfactory nerve is in reality, as we have already seen, a portion of the cerebral
hemisphere cut off to form a hollow stalk, which afterwards (in man) becomes a
solid strand, just as does the optic stalk. The distal end of the olfactory stalk
lies close to the developing olfactory pit, and the two become connected during
the fifth week by nerve-fibres. During the fourth week the lining of the olfactory
pit undergoes histogenetic changes comparable to those seen in the wall of the
neural tube. According to His, a group of cells becomes detached from the
epithelium, and constitutes a ganglion resembling a spinal ganglion. The cells
become bipolar, and their processes, establishing a connexion on the one hand
with the olfactory epithelium, and on the other with the brain, form the olfactory
nerve-fibres. According to Disse (for the bird), the nerve-fibre-producing elements
remain in the epithelium and themselves become the olfactory cells, which are thus
directly connected with the brain by single central processes. This view of the

1 Glas Anat. Hefte, B. xxv. 1904.

156 ALIMENTAKY CANAL

development of the olfactory nerves is in much closer accordance with what is
known of the structure of the olfactory epithelium and of the olfactory bulb in the
adult animal.

The olfactory nerve-fibres are seen at a very early stage running between the
brain and the olfactory pit. They are at first connected (Mihalkovics) with
every part of the epithelium derived from the ectoderm of the pit ; but it is only in
the upper part of the fossae that the permanent connexion by nerve-fibres with
the olfactory lobe persists. In the lower parts of the fossae the epithelium remains
thinner, loses its nerve- connexions, and becomes ciliated, the fossae assuming the
role merely of respiratory passages.

DEVELOPMENT OF THE ALIMENTARY CANAL.1

The early stages in the development of the alimentary canal have already been
described in treating of the formation of the embryo (p. 52 seg.). We resume here
at a phase reached during the third week, in which the primitive tract has assumed
the condition of a tube, formed by the folding-in of the splanchnopleure, and
consisting of an anterior section (fore-gut), a posterior section (hind-gut), and a middle
section (mid-gut), continuous with the cavity of the umbilical vesicle (yolk-sac).
The fore-gut is still closed in front by the buccopharyngeal membrane ; while the
hind-gut is separated from the surface by the primitive cloacal membrane, formed
from the persistent part of the primitive streak. From the hind-gut, further, the
allantoic diverticulum extends as a narrow tube into the body-stalk. We must
now consider the development of the organs derived from the several sections of
the primitive alimentary canal ; but it will be convenient to consider in the first
instance the formation of the buccal cavity.

DEVELOPMENT OF THE MOUTH.

We have already seen (p. 52) that at a very early stage, while the blastoderm
is still a flattened plate, there is an area between the head end of the axis and the
cross portion of the ccelom, in which the ectoderm and entoderm are applied to one
another without any intervention of mesoderm. This has been named the bucco-
pharyngeal membrane. When the head-fold is developed and the fore-gut formed,
the membrane is necessarily bent in below the head end of the embryo. It forms
the floor of a wide and shallow fossa, bounded in front by the down-bent fore-brain,
and behind by the pericardium, and constitutes a septum between this fossa, which
is the stomodoeum, and the fore-gut (fig. 198). This stage is reached by the twelfth
day, but during the third week the depression is converted into an actual chamber
by the forward growth of the fore-brain and by the development of the prominences
which ultimately form the face. These prominences are the fronto-nasal which
overlaps the depression above and in front, the mandibular arches which bound it
behind, and the maxillary processes which close it in on each side. By the end of
the third week (in embryos of 3*2 mm., His), the buccopharyngeal membrane becomes
broken through so as to establish a communication between the stomodoeum and
fore-gut (fig. 199) ; but before this opening is effected, a pocket is developed
immediately in front of the septum, which extends upwards in the angle between
the fore-brain and hind-brain formed by the cephalic flexure. This recess (Kathke's
pocket) comes presently into connexion with a projection from the floor of the
fore-brain, and forms with it the pituitary body (see p. 115). The remains of the

1 For literature of the alimentary tract and its gland, see to date of its publication, Maurer, Hertwig,
Handbuch der Entwickelungslehre II. Th. pp. 241, seq. ; of the respiratory tract ib. p. 105 seq. ; of the
mouth ib. p. 35 ; of the tongue, ib. p. 53.

MOUTH

157

septum seem to persist for a short time, and separate the 'pocket of Rathke, which is
of course ectodermic in origin, from an entodermic pouch called SeesseVs pocket,
developed from the blind anterior end of the fore-gut.1

FIG. 198. — FRONTAL VIEW OF THE UPPER
PART OF A HUMAN EMBRYO OF ABOUT
FIFTEEN DAYS, RECONSTRUCTED

FROM SERIAL SECTIONS. (His.) *-j'.

The pericardium is opened to show
the heart ; between this and the fore-
brain is seen the primitive buccal cavity.

FIG. 200. — FLOOR OF THE PHARYNX OF AN
EMBRYO ABOUT FIFTEEN DAYS OLD, AS

SEEN FROM WITHIN. (His.) 5y).

The first or mandibular pair of arches join in
the middle line ; the second arches are separated
by a rounded prominence (tuberculum impar).
Behind (below) this is the forked prominence
(furcula) bounding a median groove which will
become the laryngeal orifice. In the sections
of each of the first two arches the included
artery is seen. The Roman numerals are
opposite the corresponding arches.

FIG. 199. — SKETCH OF A LONGITUDINAL SEC-
TION THROUGH THE ALIMENTARY CANAL
OF A HUMAN EMBRYO, SOON AFTER THE
DISAPPEARANCE OF THE PRIMITIVE
VELUM. (His.) *T>.

The alimentary canal is shaded through-
out, uk, section of mandibular arch;
BT, hypophysis, behind it the remains of
the pharyngeal septum ; Lg, commencing
lung, the future orifice of the larynx being
opposite K ; Mg, stomach ; £6, liver; Nb, yolk-
stalk ; W, Wolffian duct ; _B, blind portion of
hind-gut ; all, allantois.

The remains of this septum have been termed the primitive velum, but the septum has nothing
whatever to do with the formation of the permanent velum palati, or with the isthmus of the
fauces. The plane of the septum forms in fact an angle with the plane -of the future isthmus

1 Zimmermann (Archiv. f. mikr. Anat. liii.) has described in a human embryo of four weeks two
small vesicles in this region which he regarded as derivations of the pocket of Seessel, and as possible
representatives of the preoral gut (v. Kupfferj of lower forms. See also Nusbaum, Anat. Anzeiger, xii.
and Bonnet, Anat. Hefte, xvi.

158 ALIMENTARY CANAL

faucium, so that the primitive mouth or stomodoeum does not by any means correspond with the
permanent mouth. In fact, the floor of the mouth, including the tongue, is developed behind
the septum, and therefore in connexion with the fore-gut rather than with the stomodoeum ;
whereas the uppermost part of the pharynx, including the choanse, is in front of the septum,
and therefore belongs to the stomodceum.

After the several prominences have united with one another, as described on
p. 86, to form the face, the primitive mouth is converted into a transversely
disposed cleft, and is divided by the development of the palate into an upper
nasal and a lower buccal portion (see p. 154). On the margins of the processus
globulares and maxillary processes where they form the upper, and also on the
mandibular arches where they form the lower border of the mouth, shallow grooves
running parallel to their outer edges appear. These are due to the presence of
ingrowths of the epithelium (Kollman), which are divided by a shedding of the
central cells into two lamellae. The fissures thus produced gradually deepen anc
separate off the lips from the edges of the developing jaws on which the denl
ridges are being developed. The vestibulum oris is produced by an extension oi
the clefts between the cheeks and the alveolar edges of the jaws. The cheel
themselves are formed by a union of the primitive lips as the buccal opening is
gradually constricted.

PHARYNX.

The anterior extremity of the fore-gut becomes dilated to form the pharynx.
Its cavity is greatly flattened dorso-ventrally, but considerably expanded laterally
On the whole, it is funnel-shaped, and is bent on itself owing to the cephalic am
cervical flexures. The cavity soon becomes irregular, due to the development oi
the visceral pouches and certain other evaginations. Four visceral pouches art
present by the fifteenth day (fig. 200), and between them the branchial arches
show as rounded ridges in the interior of the pharynx. All the pouches have ventral
prolongations on to the ventral wall of the pharynx, and all except the fourth have
also dorsal pockets. The ventral prolongation of the first pouch reaches farthest
towards the mid- ventral line, joining the groove round the tuberculum impar
(see below). The remaining pouches do not reach the floor of the pharynx. Each
visceral pouch corresponds to an ectodermic visceral cleft. The external cleft and
internal pouch are at first separated by mesenchyme ; but later this disappears,
and the ectoderm and entoderm come together on the lateral aspect of the pharynx
to form a thin septum between them. It is this septum which is broken through
in gill- breathing animals to form the gill-cleft, but it is probable that the pharynx
does not communicate with the exterior at any stage in mammals.

The branchial arches in the fish develop gill-filaments containing capillary loops connecting
the afferent and efferent branches of the aortic arches. No such filaments are developed in the
higher vertebrates, and it might be expected that, with the loss of gill- respiration the gill-pouches
would also have disappeared. They are retained to provide the cellular rudiments of certain
important organs which arise from the epithelium of the gill-pouches in all vertebrates (Maurer).

Owing to the great expansion of the mandibular and hyoid arches, the funnel-
shape of the pharynx becomes more pronounced as it narrows behind into the
oesophagus, and the hinder pouches take a nearly horizontal direction owing to the
manner in which the hinder arches are telescoped within the hyoid arch (fig. 201).
At first neither the branchial arches nor the visceral pouches reach the mid-ventral
line, so that the floor of the cavity is a flat plate overlying the developing heart.
On this flat surface a depression appears opposite the third and fourth arches, which
is the rudiment of the respiratory passages. This is bounded in front by a transverse
ridge joining the ventral ends of the third arches, and on each side by lateral ridges,

PHARYNX AND TONGUE

159

the whole forming a forked elevation called by His the furcula (fig. 200). In
front of the furcula a rounded swelling is developed in the angular space between
the ventral ends of the first and second arches named the tuberculum impar (His).
Development of the tongue, — It is on the flat floor of the pharynx thus
defined that the tongue takes form. According to His' account, the tuberculum
impar enlarges, projects forward on the oral surface of the mandibular arch, and
forms the body of the organ. It appears, however, from the extensive comparative

FIG. 201.— SIMILAR VIEWS IN OLDER EMBRYOS OP THE SAME PARTS AS IN FIG. 200. (His.) A,

T, tuberculum impar.

; 3,

researches of Kallius, confirmed in the ease of the human embryo by Hammar,1
that the greater part of the body of the tongue is really formed, as Born, indeed,
showed for the pig in 1883, from two lateral swellings which rise from the floor of
the mouth and surround the tuberculum impar. According to Hammar, the tuber-
culum impar forms' in the human tongue only a small portion in front of the
foramen caecum, but Kallius holds that it gives rise to the central part of the
organ. The lateral tongue-ridges are marked off externally by grooves which
deepen, as development proceeds, into
the alveolo -lingual sulci. The root of
the organ is formed from a transverse
ridge which develops between the ventral
ends of the second arches (Born,
Hammar). From this ridge two swellings
grow forwards to embrace, as with the
limbs of a V, the tuberculum impar
(fig. 201). At the apex of the V, and
between its limbs, a deep recess marks
the rudiment of the mesial thyroid. The
line of union between this (copular) part
of the tongue and the body is marked in
the adult by the sulcus terminalis of His,
and the depression is represented by the

foramen ccecum (fig. 202). The root of the tongue is at first continuous with the
ridge which lies between the ends of the third arches and in front of the
primitive glottis. Later a fold develops on this ridge, which is the rudiment of
the epiglottis.

FIG. 202.— SIMILAR VIEW IN A CONSIDERABLY
OLDER EMBRYO, BUT LESS MAGNIFIED.
(His.)

1 See Hammar, Arch. f. mikr. Anat. Ixi. 1£02; C. Rabl, Die Entwickelung des Gesichtes, Heft i.
1(JU2; Kallius, Anat. Anzeiger (Ergiinzungsheft), xxiii. 1903.

160

ALIMENTARY CANAL

In front of the sulcus terminalis the circumvallate papillae begin to show about the beginning
the third month (Graberg). Their position is marked by a pair of ridges diverging in front, but
meeting in the middle line behind. On these ridges circular epithelial thickenings appear whi(
grow inwards and cut, as it were, the papilla out of the mucous membrane. A fissure
appears in the ingrowing wall of epithelium, produced by the shedding of the central cells, whic
becomes the trench round the papilla, while the marginal thickening on the surface into which tl
stratum proprium extends becomes the vallum. The fungiform papillae appear about the begii
ning of the third month and the filiform a trifle later, both as projections of the connects
tissue, over which the epithelium is thickened. The epithelial plaque in the case of the filifori
papillae is afterwards broken up by the irregular thickening of the epithelium over them.

The visceral pouches of the pharynx begin to disappear towards the end of the
first month. The dorsal portions of the first pair become the tubo-tympank

FIG. 203. — PEOFILE SKETCHES OF TWO STAGES IN THE DEVELOPMENT OF THE ALIMENTABY CANAL ix

THE HUMAN EMBRYO. (His.)

Ch., notochord; Sd (in A), median rudiment of thyroid; U.K., section of mandibular arch;
.R.T., hypophysis; Lg., lung ; K., larynx : Mg., stomach; P., pancreas ; Lbg., bile-duct ; Ds., vitelline
duct ; Zg. (in B), tongue ; AIL, allantois ; B., entodermic cloaca ; W., Wolffian duct ; N., hind kidney.

passages (see Development of the Ear), the second in part form the pockets in
which the tonsils develop (Hammar). No traces of the remaining pouches persist
in the adult, but in the embryo their epithelial lining gives origin to important
organs. Thus the third pouches give origin to the thymus, and the fourth to the
lateral portions of the thyroid gland, while both third and fourth supply epithelial
buds which form the parathyroid bodies.

(ESOPHAGUS, STOMACH, AND INTESTINES.

Immediately behind the pharynx, the fore-gut contracts again to form the
O3sophagus, which is very short (fig. 203, A) in the early embryo, owing to the
imperfect development of the neck. Behind the oesophagus the gut widens out into
the dilatation which represents the stomach. This organ, which is at first nearly

INTESTINE

161

straight (fig. 199, A, Mg], soon begins to show the convexity of the greater curvature
on the side next the vertebral column, and the concavity of the lesser curvature
on the opposite border (fig. 203, B, Mg), while the pyloric end becomes tilted away
from the vertebral column, producing the duodenal loop (fig. 203). Finally
the organ becomes turned over on what was previously its right side, which now
becomes the posterior surface ; and the pyloric extremity being also tilted over,
the duodenal loop is thrown over to the right side of the abdomen (fig. 204).
The section of the gut between the stomach and the mouth of the yolk-sac is at

Sth cervical nerve
1st thoracic vert.

pericardium

adrenal

stomach

12th thoracic nerve

Wolffian body

kidney

5th lumbar nerve

I i

sm. intest. caecum

grt. intest, w.d. ur

FIG. 204. — RECONSTRUCTION OF A HUMAN EMBRYO OF 17 MM. (After Mall.)
bl, bladder ; w.d., Wolffian duct ; ur, ureter.

first short and straight. It early shows a forward directed diverticulum which-fis
the rudiment of the liver. It gradually increases in length by the closure of the
mouth of the yolk-sac, until the whole length of the intestine is laid down as a nearly
straight tube which is attached to the posterior wall of the primitive abdominal
cavity by a continuous mesentery (fig. 203, A). As the liver develops and comes
to occupy a large section of the cavity, the intestine, increasing in length, forms a
loop which extends through the wide-open umbilicus (fig. 204). This loop
(vitelline loop) gives attachment at its extremity to the vitelline stalk (fig. 203),

VOL. I.

M

162

ALIMENTAKY CANAL

and consists of a proximal descending and a distal ascending limb. As il
increases in length the two limbs come close together, and the posterk
extremity (afterwards the middle of the transverse colon) is brought into clos
relationship with the end of the duodenal loop (fig. 204). Very early a rotatioi
of the loop commences round its long axis, which brings the distal over th<
proximal limb. On the distal limb an evagination develops which is th<
rudiment of the caecum (fig. 204), and from now onwards it is possible
distinguish the portion of the gut which will become the small, from that whicl

1st thoracic nerve
-2nd thoracic vert.

pericardium

lung

-•adrenal

— 1st lumbar nerve

— stomach

liver

- kidney

~ Woljfianbody

mesentery urogenital sinus
FIG. 205. — KECONSTBUCTION OF A HUMAN EMBBYO OF 24 MM. (After Mall.)

will become the large intestine. The small intestine increases in length more
rapidly than the large intestine, and, while still without the body-wall, it begins I
to be coiled (fig. 205). It shows six bends which presently become six primary
circular coils (Mall). The distal coil ends at the ca3cum, and from this the large
intestine passes straight back in the middle line to the back wall of the abdomen,
where it turns sharply down to end in the rectum (fig. 206). As the result of the
continuance of the axial rotation, the small intestine becomes displaced to the
left, under the large intestine and below the superior mesenteric artery, until it
assumes its definitive position, while the large intestine, also increasing in length,

INTESTINE

163

forms a U-shaped loop surrounding the coils of the small intestine (fig. 206).
The small intestine is next withdrawn into the abdominal cavity, and the
umbilicus is closed. The U-shaped loop of the large intestine is produced by its
proximal portion being carried to the right, so that it comes to be transversely
disposed, passing from right to left. As it lengthens, the caecum descends to
assume its definitive position in the right iliac fossa, and thus we have laid down
an ascending and a transverse colon. The distal portion of the large intestine, at
first in the mesial plane, becomes, on the other hand, displaced to the left, as the

1st thoracic nerve

pericardium

lung

appendix — •—

small intestine

urogenital sinus

x»iall intestine

FIG. 208. — RECONSTRUCTION OF A HUMAN EMBRYO OF 24 MM. SOMEWHAT MORE ADVANCED IN

DEVELOPMENT THAN THAT SHOWN IN FIG. 205. (After Mall.)

•coils of the small intestine increase in number until it ultimately assumes the
position of the adult descending colon. The primary coils of the small intestine
become progressively more complex by the formation of secondary coils, which are
arranged in definite groups so that they can be followed through all the phases
of development (Mall).

The epithelium in the duodenum during the second month increases greatly in thickness by
the active multiplication of the cells (fig. 170, p. 126) until the narrow lumen is nearly or quite
-closed (Tandler).1 As the calibre of the gut increases in the third month the lumen is re-

1 Anat. Anzeiger (Ergiinzangsheft), xviii. 1900.

164

ALIMENTARY CANAL

established. Cases of congenital stenosis of the gut just beyond the pylorus are possibly to be
explained by the persistence of the early occlusion.

The eeecum is at first a uniform conical diverticulum. By the end of the
first month it shows a smaller apical and a larger basal section. The former
becomes the vermiform appendix, the latter the caecum. The caecum is at first
conical (fig. 206), with the appendix passing off from its apex; but later, owing to
the unequal dilatation of its anterior and right walls, the appendix comes to be
attached to its posterior and left aspect.

Formation of the entodermic cloaca and anus. — The hind-gut ends
during the third week in a dilated chamber, which also receives the openings of
the Wolffian ducts (fig. 203 A, B.). This chamber, which is called the cloaca
entodermica, is closed on its ventral aspect by a membrane derived from the
persistent part of the primitive streak. This cloacal membrane consists of
ectoderm and entoderm, which are here in contact with one another without the
intervention of mesoderm, and it forms a septum between the cloaca and a

body-wall

urogenital sinus
rectum

FIG. 207. — PELVIS OF A HUMAN EMBRYO OF 14 MM. (FIVE WEEKS). (After Keibel, from
Kollmann's Entwickelungsgeschichte.)

+ bladder ; * septum uro-rectale.

shallow surface depression (urinogenital fossa). The chamber is then divided by
a septum into a dorsal and a ventral passage, becoming the rectum and urogenital
sinus respectively (figs. 204, 205, 206), and these come to open on the surface by
the absorption of the membrane. The opening into the alimentary canal becomes
the anus. It does not correspond with the posterior extremity of the primitive
hind-gut, for at first there is a postanal cul-de-sac — the tail-gut, which later
becomes reduced and obliterated. The anal opening is completed during the third
month. The process by which it is formed will be dealt with later.

FORMATION OF THE GLANDS OF THE ALIMENTARY CANAL.

Under this head may be included not only those organs which are ordinarily so
termed, but also the lungs, thymus, thyroid, and pituitary body, since the early
development of these organs resembles that of the true secreting glands.

All the organs above enumerated are formed as epithelial involutions, either
solid at first and afterwards becoming hollowed out, or hollow from the first. As

SALIVARY GLANDS

165

these epithelial buds grow into the mesoderm, they may either bifurcate or give off
lateral branches, and in this manner all the ramifications of the ducts of the
compound racemose glands are produced. The blind extremities generally end
eventually in enlarged tubular or saccular dilatations. All the epithelium of the
gland-saccules and ducts is derived from the original epithelial sprout, while the
basement-membranes and connective tissue and blood-vessels of the gland are

FlG. 208. — A, LUNG-RUDIMENTS OF HUMAN EMBRYO OP ABOUT FOUR WEEKS, SHOWING THE BUD-LIKE
ENLARGEMENTS WHICH REPRESENT THE LOBES OF THE FUTURE LUNGS. (His.)

Three buds are seen on the right side, two on the left.
B, LUNGS OF A HUMAN EMBRYO MORE ADVANCED IN DEVELOPMENT. (His.)

derived from the surrounding mesoderm. The salivary glands and most other
glands of the mouth, and part of the pituitary body, which must also be reckoned
as a glandular development, are formed in this way by involution of the buccal
or stomodoeal ectoderm ; while the lungs, liver, pancreas, thyroid, thymus, and all
the small glands of the rest of the alimentary canal are formed of involutions of
the entoderm. The development of the teeth,
which also first make their appearance as
involutions of stomodoeal ectoderm (enamel
germs), will be described after their structure
has been dealt with (in the part of this work
which is devoted to Splanchnology).

Salivary glands. — The submaxillary
gland appears as an epithelial sprout from the
floor of the mouth towards the close of the
fifth week (Sudler),1 and the sublingual rudi-
ment develops on its outer side in the ninth
week (Hammar).'2 The parotid develops in the
lateral wall of the cavity in the angle between
the roof and floor of the primitive mouth.
When the cheeks are formed by the union of
the freed lip -portions of the mouth- opening,
its duct comes to open in the cheek. Accord-
ing to Hammar, Stenson's duct is not formed as an epithelial sprout, as in
the case of the other glands, but appears first at the end of the first month
as a groove in the position specified, which afterwards closes into a canal and
becomes separated from the cheek by the ingrowth of connective tissue. The
duct in the tenth week runs over the masseter to the back of the mandible, where
the epithelial buds are given off which form the secretory tubules of the gland.

FIG. 209. — LUNGS OF A HUMAN EMBRYO

STILL MORE ADVANCED. (His.)

Amer. Journ. of Anat. i. 1902.

Loc. cit.

166 LUNGS

The lung-s. — The lungs begin to develop during the third week from the ventral
part of the pharynx at its junction with the oesophagus (fig. 199, Lg). The lung-
rudiment is at first single and median, and takes the form of an elongated vertical
diverticulum of the fore-gut, communicating freely with that tube, and of course
lined by entoderm. Soon the diverticulum sprouts out at its lower extremity in the
form of two tubes which grow downwards on either side of the heart, into a mass
of mesodermic tissue, which keeps pace in its growth with the lung-rudiment, and
from which the connective tissues of the future lung become ultimately developed.
The extremities of the tubes in question are early seen to be dilated and lobulated
(fig. 208), three lobules being present on the right tube and two on the left. The
division of the lungs into their lobes is thus early indicated.

The further outgrowth of the tabulations produces the rudiments of the principal branches
of the bronchi, one for each future pulmonary lobe. Each of these branches then gradually
progresses in growth, giving off as it proceeds diverticula which form the secondary bronchi,
and these again giving off others until the whole complicated bronchial ramification is
eventually produced. Like the first sprouts from the median diverticulum, all the secondary and
other sprouts are dilated at their termination, and have a lobulated aspect (fig. 208, B ; fig. 220,
p. 174 ; fig. 268, p. 214). This is due to the fact that they are undergoing a further division or
sprouting. This process goes on until the sixth month of intra-uterine life, by which time all
the dilated ends of the growing and sprouting tubes have reached the surface of the lung.
These dilated extremities, which now appear grouped together, and apparently springing several
from a common tube, form the infundibula, but their walls are not at first beset with air-cells.
The formation of these takes place when the bronchial ramification is completed (sixth month,
Kolliker), as small, closely set, pouch-like protrusions of the walls of the infundibula, and of
the terminal bronchial tubes.

There has been much difference of opinion as to whether the branching described above is to be
regarded as monopodial or dichotomous (i.e. whether the bronchial tree is developed by the growth
a chief stem from which secondary side-branches are given off, or by the continuous dichotomous
division of the terminal buds. The truth seems to lie with His, who described for the human
embryo a monopodial division of the primary stems, and a dichotomous division for the secondary
branches. This is the view expressed in the most recent paper on the subject by Flint ' ;
though he found that the secondary branches may also develop by monopodial division for two
or three generations.

The trachea and larynx are formed by a separation from the oesophagus
of the original median diverticulum, from the lower angles of which the bronchial
rudiments have sprung, the separation commencing below and leaving a relatively
small connexion between the two tubes above : this connexion is the rudimentary
glottis. As development advances, both the tracheo-laryngeal and the oesophageal
tubes lengthen, the latter relatively more than the former, so that the lung-
rudiments no longer lie, as was the case at first, in front of and on either side of
the stomach, but extend downwards somewhat short of that organ, separated
from one another by the oesophagus behind and the heart and pericardium in
front. As they thus grow backwards with the lengthening of the trachea, the
lung-rudiments project into the anterior part of the body-cavity or ccelom (dorsal
portion), and receive a covering from its lining membrane, at first only below
and on the external surface, but subsequently on the internal aspect, so as to
separate them from the oesophagus (fig. 220). The portions of the body-cavity
into which the lungs project become shut off from the remainder on the
formation of the diaphragm and pericardium, and form the pleurae.

The rudiment of the larynx appears about the twenty-fifth day, before the
trachea is separated off from the median diverticulum, in the form of two lateral
swellings (Arytdnoidwulste, Kallius), which lie behind the fourth visceral pouches,
and here compress the fore-gut in a sagittal direction. They possibly represent

1 Amer. Journ. of Anat. v. 1906. On this subject see also a recent paper on the comparative
embryology of the lung by Moser, Arch. f. mikr. Anat. Ixiii. 1903.

LARYNX

167

rudimentary fifth branchial arches (Kallius), but they soon lose any apparent
relation to the branchial region by enlarging dorsally and by growing forwards till
they reach almost to the level of the second visceral pouches. They are connected
with the floor of the pharynx by two folds which run into the transverse ridge
intervening between the ventral ends of the third arches. This ridge, and
the lateral folds, form the furcula of His, and, as already mentioned, the
epiglottis appears as a fold on the transverse ridge, while the lateral bands form the
aryteno-epiglottidean folds. On the anterior margin of the arytenoid mass,
external to its pointed end, a swelling, which appears very early, represents the
rudiment of the cartilage of Santorini. At an early stage, after the trachea and
oesophagus have separated, the slit-like cavity between the swellings is for a time
partially obliterated by the cohesion of the opposed epithelial surfaces.

The connective tissue round the now slit-like glottis becomes condensed into
chondroblast. In this precartilaginous stage the rudiments of the arytenoids, the

rvtrnoi<l

\yoid cartilage II.

FIG. 210. — TRANSVERSE SECTION OF THE PHARYNX AND LARYNX OF A HUMAN EMBRYO OF 15'5 MM.

Photograph. (T. H. Bryce.)

c.a., c.a., carotid arteries.

cricoid, and cartilages of the trachea are continuous laterally. The cricoid
chondrifies by two centres, one on each side. These unite ventrally, but the ring
remains open behind for a time, until completed by the extension of chondrification
into the dorsal plate. The tracheal rings develop in the same way, but remain
incomplete. The arytenoids are at first continuous with the cricoid by fibrous
tissue ; the cartilages of Santorini are portions of the arytenoids segmented off.
The epiglottis chondrifies relatively late, and is at first continuous behind with the
cartilages of Wrisberg, which are thus derivatives of the epiglottis (Goppert). The
thyroid is laid down in the form of two lateral plates which are united ventrally
by an intermediate nodule of cartilage (Nicolas). Each plate chondrifies from two
centres, an anterior and a posterior (Kallius), possibly representing separate bars
seen in Echidna, derived from the third and fourth visceral arches (Goppert).

168

THYKOID GLAND

The superior cornu is at first continuous with the great horn of the hyoid on each
side, and the cartilago triticea in the lateral thyro-hyoid ligament of the adult is a
remnant of the cartilaginous connexion.

The thyroid gland is developed partly from a median diverticulum of the
pharyngeal entoderm opposite the ventral ends of the second visceral pouches
(fig. 203, A, Sd), partly from lateral diverticula of the posterior walls of the
fourth visceral pouches. The median diverticulum in most animals early becomes
separated from the pharyngeal entoderm, and is thus converted into an island of
epithelium imbedded in mesenchyme. In the human embryo (fig. 211, A, thr)
it remains for some time in the form of a hollow bifid vesicle, which is connected
with the upper surface of the tongue] by a small duct (ductus thyreoglossus, d) ;
subsequently, however, the vesicle becomes solid, and the duct is obliterated and
disappears, with the exception of a small portion near the orifice, which becomes
converted into the foramen ccecum of Morgagni, f.c.

Occasionally even in the adult a comparatively long duct is found, leading
downwards and backwards from the foramen caecum. This, which has been
termed the ductus lingualis, is the remains of the original thyrolingual duct

fc.

FIG. 211. — SKETCHES SHOWING THE CONDITION or THE THYROID AND THYMUS GLANDS IN A

HUMAN EMBRYO OP ABOUT FIVE WEEKS. (His.)

A, profile sketch from the left side. B, frontal sketch from behind.

t, tongue: fc, foramen caecum; d, ductus thyreoglossus; ep, epiglottis; opposite /, larynx;
tr, trachea; oe, oesophagus; thr, median rudiment of thyroid; thr1, lateral rudiment of thyroid;
thm, developing thymus, seen on the left side of B to be connected with a visceral cleft ; ao (in B),
ascending aorta ; ao', descending aorta ; c, carotid.

connecting the median part of the thyroid Vith the tongue. It may further happen
that the lower part of this connexion also remains in the shape of a tubular
prolongation of the median portion of the thyroid towards the root of the tongue
(ductus thryoideus ; when well developed this portion forms the pyramid). The
so-called accessory thyroid bodies (suprahyoid, prsehyoid glands, &c.) which are
occasionally found near the hyoid bone are also referable to the thyrolingual
duct (His). The mesial rudiment soon becomes enlarged into a lobed mass of
considerable size, which, as the neck elongates, assumes the shape of a horse- shoe
passing across the front of the trachea (figs. 212-214). It becomes converted into
ramifying and anastomosing cell- cylinders, between which vascular connective tissue
is developed. The cell-cylinders subsequently become hollowed out, and finally
are subdivided by growth of the connective tissue into small vesicles, which
gradually become larger from accumulation of colloid in their interior.

The two lateral diverticula, which assist in the formation of the thyroid body,
spring from the fourth visceral pouches (fig. 213). They have at first the
appearance of simple thick- walled saccular glands (fig. 216) partially encircling
the developing larynx. In front of this they come into connexion with the
median rudiment, and eventually blend with it. Like that rudiment, they become

THYMUS

169

entirely separated from the entodermic surface from which they have taken origin,
and are converted into cell- cylinders in which colloid is ultimately formed.

The lateral rudiment of the thyroid apparently corresponds to a structure present in most
vertebrates, and known as the post-branchial body. It is an entodermic pocket which gives
origin to an epithelial body, but this does not yield colloid except in mammals. In Echidna
(Maurer) there is colloid formation, but the body remains distinct from the primitive median
thyroid. Only in placental mammals does it become united with the median rudiment. The
account given above is founded on Bern's original description, but it is right to say that some

<,«•»*.•

rues. thyr.

CM. thym

FIG. 212.— TRANSVERSE SECTION THROUGH THE NECK or A HUMAN EMBRYO OF 15'5 MM., TO SHOW THE

POSITION OF THE THYROID AND THYMUS GLANDS, AND HISTOGENESIS OF WALLS OF O3SOPHAGUS

AND TRACHEA. (T. H. Bryce.)

not, notochord in vertebral body ; sy, sympathetic ganglion ; vg, vagus ; j.v., jugular vein ;
c.a., carotid artery ; thym, thymus ; mes. thyr., mesial thyroid.

hold that, while the diverticulum from the fourth pouch becomes included as a vesicle in the
lateral lobe of the thyroid, it takes little (Simon) or no (Verdun) share in the formation of the
glandular substance.

The thymus in the lower vertebrates appears as a series of buds from the
dorsal pockets of the branchial clefts. The number of buds differs in different
forms, and the number of those which persist and enter into the formation of the
adult gland varies. In birds and mammals it arises from one main rudiment
related to the ventral pocket of the third visceral pouch. To this is added in
birds and some mammals a rudiment from the fourth visceral pouch, but there

170

THYMUS

is an absence of agreement among those who have worked at this difficult subject
as to the presence of this accessory element in man.

parathyroid III.

thymus

mes. thyroid

parathyroid IV.

I at. thyroid

pharynx

-parathyroid III.

t hymns

parathyroid IV.
lat. thyroid

FIG. 213. — KECONSTBUCTION OF BRANCHIAL DERIVATIVES IN A HUMAN EMBRYO OF 14 MM.
(After Tourneux and Verdun.)

c.a. j.v.

parathyroid IV
lat.

raid IV. 1F~*\ fe
. thyroid ~~W

V

mes. thyroid — %
parathyroid III.

lat. thyroid
parathyroid IV.
~" mes. thyroid
-I parathyroid III.

thymus ves.
thymus

u

thymus ves.
thymus

FIG. 214. — KECONSTRUCTION OF THE BRANCHIAL DERIVATIVES IN A HUMAN EMBRYO OF 16 MM.
(After Tourneux and Verdun.)

c.a., carotid artery; j.v., jugular vein; thymus ves., vesicle of thymus.

pyr. c.a. .;'.*>•

j.v. c.a

par. IV
par. III. —

v.c.d.

FIG. 215. — KECONSTRUCTION OF THE THYROID AND THYMUS GLANDS WITH THE VEINS AND ARTERIES.
A, IN A HUMAN EMBRYO OF 26 MM. ; AND B, IN ONE OF 24 MM. (After Tourneux and Verdun.)

In A the thymus lie in front, in B behind the left innominate vein.

thyr., thyroid; thym., thymus; par. IV., parathyroid of fourth pouch; par. III., parathyroid
of third pouch; pyr., pyramidal lobe of thyroid; thyg., thyroglossal duct; c.a., carotid artery;
j.v., jugular vein ; v.c.d., vena cava dextra ; v.c.s., vena cava sinistra ; v.sc.d., vena subclavicularis
dextra ; v.sc.s., vena subclavicularis sinistra.

THYMUS

171

The thymus is thus in its first origin bilateral. A pocket develops from the
third cleft on each side, which extends downwards as a thick- walled tubular
prolongation along the carotid artery. The pocket persists as the thymus vesicle
in the proximal section of each rudiment (fig. 216). From the lower end of the
tube solid epithelial buds are given off, and from these lateral buds again come
off, so that this part of the gland acquires a ramified lobular appearance like an
acinous gland. The acini are, however, solid, and remain so. The two rudiments

par. IV. lot. thyr.

K'«&?»39||5^

•*$m&3&?ri

I
par. III.

FKJ. 216. — SECTION THROUGH THE BRANCHIAL DERIVATIVES IN A HUMAN EMBRYO OF 15'5 MM.

(T. H. Bryce.)

<'.«., carotid artery ; vg, vagus nerve ; j.v., jugular vein ; mes. thyr., mesial thyroid ; lat. thyr., lateral
thyroid ; thym., thymus ; par. IV., parathyroid of the fourth pouch ; par. III., parathyroid of the
third pouch. It appears here as a cellular mass closely related to the thymus rudiment ; in the
adjoining section it becomes free, and forms a rounded cord having exactly the same structure as the
parathyroid of the fourth pouch.

are brought into close contact with one another in front of the trachea (fig. 215),
and unite to form a single-lobed body, which comes to lie in the anterior
mediastinum in close relationship with the pericardium.

The surrounding vascular connective tissue forms a capsule to the gland, extends between
the several buds so as to divide it up into lobules, and also sends processes carrying vessels with
them into the interior of the lobules. Each lobule becomes differentiated into a cortical and
medullary zone, but before this the gland loses its epithelial structure and assumes the appear-

172 PAEATHYKOIDS

ance of a lymphoid organ. The epithelial cells give rise to a reticulum (Hammar ') (figs. 212, 216),
in the meshes of which in the cortical zone lymphoid cells collect in large numbers. The origin
of these lymph-cells has been much disputed, some, following Kolliker, deriving them from the
entodermic epithelial elements (Prenant, Beard, Maurer, Nusbaum, &c.); others, following
Stieda and His, regarding them as invading the rudiment from without (Gulland, Kollmann,
&c.). It has been proved (Hammar, Bryce,2 Stohr3) that leucocytes are already present
before the lymphoid transformation of the gland, so that the thymus cannot be the original
source of the leucocytes as suggested by Beard. Stohr in a recent publication advances the
thesis that the thymus is an epithelial organ throughout, and is not lymphoid, as the characters
of the cells would suggest. The Hassall's corpuscles are very generally, and in all probability
rightly, regarded as cell-nests derived from the epithelial cells, as are also various kinds of
minute cysts, some of them with a ciliated lining, which have been described as occurring
in the adult gland.

The parathyroid bodies develop in intimate relation with the thymus and
lateral thyroid from the entodermic lining of the third and fourth visceral pouches

?-• - - ... ., ___ _ truncus aortic

pericardium

' , post .mesocar din m

' ;">- ..s

''

V/'' / auricular portion of

ant. wall of ^ •' ........ — - heart loop

pericardium T g\ ...-'-'"'"".

- -------- lat. mesocardium

\

. ......... ±--ductofCuvier

septum'
transversum

liver parenchyma .- y.

liver divert ^=

mouth of yoU- _

~" cuelom

- 1 IS-'-— umbilical vein
•^-•.citelltnevein

FIG. 217.— THE LIVER DIVERTICULUM AND PARENCHYME WITH SEPTUM TBANSVEBSUM.
HUMAN EMBRYO OF 3 MM. LONG. (After His, from Kollmann.)

of the pharynx.4 They appear as paired epithelial buds, which soon become
separated from the surface from which they have sprung, and form small isolated
bodies of oval shape (fig. 213). The buds from the third pouches lie at first in front
and to the outer side of the corresponding buds from the fourth pouches, but they
are carried backwards with the thymus, and ultimately lie behind the others
(fig. 214). The epithelial cells of each bud become loosely arranged and form
a syncytial reticulum closely resembling that of the early thymus (fig. 216). Each
body has a distinct capsule, and is quite independent of the other epithelial
derivatives of the branchial pouches. In the early stages the parathyroids are
thus quite unlike the thyroid, but resemble the thymus (fig. 216).

1 Anat. Aiizeiger. xxvii. 1905. In this paper will be found all the most important references to
the Hterature regarding the mammalian thymus. See also Bell, Amer. Journ. of Anat. v. 1906.
- Bryce, Journ. of Anat. and Physiology, xl. 1905.

3 Stohr, Anat. Hefte, xxxi. 1906.

4 The bud from the third pouch is often named the parathymus, while that from the fourth is called
the parathyroid. Here the term parathyroid is used for both bodies in view of their relations to the
thyroid gland in the adult.

LIVER

173

tiver.— The primary rudiment of the liver takes the form of a diverticulum
of the ventral wall of the gut immediately behind the stomach. The diverticulum
first appears as a wide and elongated groove, which becomes closed, and extends

X300

FIG. 218. — SECTION OF THE DEVELOPING LIVER, TO SHOW HOW THE HEPATIC CYLINDERS ENCROACH
ON THE LUMINA OF THE SINUS-LIKE VEINS TO BREAK THEM UP ULTIMATELY INTO CAPILLARY-LIKE

CHANNELS CALLED SINUSOIDS. (Miliot.)

k.c., hepatic cylinders ; ai, sinusoids.

forwards into the substance of the septum transversum (fig. 217). From the fore-
part of the diverticulum cells are rapidly budded off from the epithelium to form
the parenchyma of the gland. The hinder part of the groove does not share in

FIG. 219.— SECTION OP THE DEVELOPING LIVER AT A LATER STAGE THAN IN FIG. 218, TO SHOW THE
MANNER IN WHICH THE HEPATIC CYLINDERS COME TO FORM A TRABECULAR FRAMEWORK OF
PARENCHYMA WITH A NETWORK OF SINUSOIDS IN ITS MESHES. (Minot.)

Ji.c., hepatic cylinders ; si, sinusoids.

this proliferation, and persists as the bile-duct. Before being closed it gives off a
second ventral pouch, which is ultimately converted into the gall-bladder. The
mass of cells budded off from the wall of the tubular diverticulum invades the

174

LIVEE

;;

or

septum transversum, the mesenchyme of which supplies the capsule and co
nective -tissue framework of the organ. As we have already seen, the omphal
mesenteric and allantoic veins pass to the sinus venosus through this septum,
and we now see the growing liver-parenchyma in the form of cellular strands
trabeculae extending round these vessels. The cells of the trabeculae continue to
proliferate very rapidly, and the buds produced invade the lumen of the vessels,
pushing the endothelial walls before them (figs. 218, 219). This process goes on until
the course of the vessels is completely interrupted, and the original channels remain
merely as capillary-like vessels, named by Minot ' sinusoids.' l The ultimate result
is a system of anastomosing epithelial trabeculae, in the meshes of which there is a

FIG. 220. — TBANSVEBSE SECTION OP A HUMAN EMBBYO AT THE END OF THE FIFTH WEEK.
Photograph. (T. H. Bryce.)

s.g., s.g., spinal ganglia, neural cartilages to their outer sides ; s.n., spinal nerve ; not., notochord in
vertebral body ; sy, sympathetic ganglion ; ao, aorta ; Ig, lung ; ce, oesophagus ; m.p.p., membrana
pleuroperitonalis ; H, liver ; B, rib.

second network of sinusoids. The thick trabeculse of early stages are gradually
reduced to columns in which only a single layer of cells bounds the narrow lumen,
which has meanwhile appeared in the solid strands. The slit-like lumen becomes
the bile-capillary. From the second month onwards during the earlier months
of pregnancy the liver is very actively engaged as a blood-forming organ, the
vessels being crowded by young nucleated red blood-corpuscles.

Though the liver-parenchyma is laid down in higher forms as solid anastomosing strands,
in some lower vertebrates (Cyclostomata, Selachians, Amphibians, and certain Reptilia) the

Minot, Procj Boston Soc. Nat. Hist. xxix. N. 10, p. 185.

I '.\NCKE\S

175

trabeculae are hollow from the first, and in Cyclost' raata the arrangement is exactly like a
tubular gland in respect that the branches are separate and not joined into a network.

The tabulated arrangement of the parenchyma is not completed until after
birth. In the younger stages the primitive lobules are larger than those of the
adult, and show an irregular arrangement of the liver trabeculte. In each lobule
there are several anastomosing branches ol the hepatic vein. Later, owing to
extension of branches of the portal vein with a certain amount of connective
tissue into the mass, it is broken up into as many secondary lobules as there are
branches of the hepatic vein. These branches become the central veins of the
lobules ; the trabeculse assume a radial disposition and the intralobular venous
network takes form.

The primary lobing of the liver is determined by the vessels in the septum transversum
along which the cellular strands extend. Accordingly there are four primary groups of liver-
tissue, two right and two left (Brachet). When the septum transversum has become fullv
occupied by liver-substance there is a large ventral mass and an upper lateral expansion on each
side. The primary lobing is largely lost in later stages owing to the formation of secondary
lobes and fissures. The organ begins to be asymmetrical by the eighth week, owing to the
presence of the stomach on the left side and to a greater expansion of liver trabeculse on the

neural canal itolochord aorta dorsal pancreas

liepatic duct

ventral pancreas cystic duct gall-bladder

FIG. 221. — RECONSTRUCTION TO SHOW THE DEVELOPMENT OF THE PANCREAS OF A HUMAN
EMBRYO OF 6'8 MM. (After Piper.)

right side along the vena portjp. The lobus Spigelii appears as a swelling on the inner face of
the right lobe, the surface which forms the outer wall of the epiploic (hepato-enteric) sac
(see Ccelom). It intervenes between the ventral mesentery (gastro-hepatic omentum) and the
vena-caval fold (see Ccelom), and projects into the epiploic sac.

The pancreas is developed from two rudiments, a dorsal and a ventral. The
dorsal pancreas arises as a diverticulum from the duodenum opposite the bile-duct ;
the ventral pancreas springs in the form of two lateral diverticula from the base
of the bile-duct (fig. 221). According to some authorities, these two outgrowths
both give origin to pancreatic tissue, but according to others only the right persists
(Helly).L The dorsal diverticulum becomes the duct of Santorini, the persistent
ventral outgrowth the duct of Wirsung. Each diverticulum gives off lateral offshoots
which form the ducts and alveoli, as in other compound acinous glands (fig. 170,
p. 126, and fig. 222). Owing to the rotation of the duodenum, the two rudiments
are united with one another on the ventral aspect of the portal vein to form a
single body, which extends into the mesoduodenum and mesogastrium. The

1 Helly, Arch. f. mikr. Anat. Ixvii. 1905. Other papers dealing with the mammalian pancreas which
have appeared since Maurer's list (Hertwig, Entwickelungslehre II. Teil. i. and ii., p. 250) are Y«">lki-r,
Arch. f. mikr. Anat. lix. ; Low, Proc. Anat. and Anthro. Soc. University of Aberdeen, 1900-1902 ; Piper,
Arch. f. Anat. 1900 ; Ingalls, Arch. f. mikr. Anat. Ixx.

176

PANCKEAS

opening of the duct of Santorini in the majority of instances becomes obliterated,
and the duct of Wirsung persists. When the rotation of the stomach has
been effected the gland loses its mesial position, and comes to lie across the
back of the abdomen. In man, owing to the fusion of the mesogastrium with
the transverse mesocolon (see Development of the Ccelom), the posterior layer

1 !• -

FIG. 222. — TRANSVERSE SECTION OF A HUMAN EMBRYO OF 80 MM. ABOUT THE BEGINNING OF THE

THIRD MONTH. (T. H. Bryce.)

Below the spinal cord is seen the, body of a vertebra ; on each side of this the kidney ; on the left
(right of figure) below the kidney the adrenal ; on the right only a small portion of the right adrenal is
seen below and external to a large vein. Between the two adrenals and below the aorta (distinguished
from the veins by its thick wall) some irregular groups of cells represent the abdominal sympathetic ;
on the right side (left of figure), just above the large vein, the two rounded clear bodies are chromaffin
bodies. The liver occupies the whole abdominal cavity ; its right and left lobes are separated by the
ventral mesentery (falciform ligament) in which the umbilical vein passes. The stomach is twice cut in
the section ; between it and the adrenal is seen the mesogastrium, with the spleen. The duodenum,
with its much-folded mucosa, is seen on the right ; between it and the stomach the pancreatic tubules
and pancreatic ducts (ducts of Santorini and Wrisberg) ; between stomach, pancreas, and thin portion
of the mesogaster the lesser bag of the peritoneum.

of the mesenteric fold which enclosed the pancreas becomes absorbed, and the gland
comes to he entirely behind the peritoneum.

Owing to the manner in which the alveoli are budded off round the ends of the duct-rudiment,
the terminal portion of duct-epithelium is as it were included in the centre of the alveolus to

ri;<>< JKMTAL SYSTK.M 177

form the centro-acinar cells (Laguesse). Th« cell-islands of Lan^erhans have been gener-
ally iviranlo I as derivatives of the mesenchyme. This view has been a.l\ o. atcd by Hansemann
( I!»oi2), hut it has been shown by Laguesse, Kiister, Pearce, and Helly that they are derived from
tin- gland-epithelium.1 Helly has traced (in the guinea-pig) the Langerhans cells back to a
sta^o in which the pancreas-rudiment is still a solid bud. These special cells are characterised
by dense finely granular protoplasm; they do not share in the formation of the tubules, but
lie loosely arranged in their walls. In the human embryo (Pearce) such cells occur in round or
oval masses and in direct continuity with the epithelial cells. They bud off and form solid
cell- processes which are at first connected with the acini, but later become separated from them
by the ingrowth of mesenchyme. The group thus isolated is vascularised, and by further
histogenetic changes becomes a fully formed cell-islet.

DEVELOPMENT OF THE UEOGENITAL SYSTEM.2

EXCRETORY ORGANS.

The excretory system first appears as a longitudinal duct and a longitudinal
series of epithelial tubules in that part of the mesoderm named the intermediate
cell-mass. This forms a continuous blastema from which every part of the
system takes origin. It consists at first of cells disposed, though somewhat
indistinctly, in two lamellae — outer and inner — which are connected with the
outer and inner layers of the primitive segment internally, and the somatopleuric
and splanchnopleuric layers of the lateral plate externally.

In the Anamnia the corresponding portion of the mesoderm forms in each segment a hollow
stalk derived from the ventral part of the segment, through which the myocoel is
continuous with the splanchnocoel (general body-cavity). In the mammalia, owing to the
condensation of development, the stalks are solid like the segments themselves, and all traces of
segmentation are lost at a very early stage.

In the nephrogenetic blastema the tubules are developed from before back-
wards, and are grouped, as they appear in time and place, into three systems, the
pronephros, mesoncphros, and metanephros.

Pronephros and seg mental ( Woltfian) duct. — The pronephros is developed
as a functional organ in the Anamnia during the larval stage. It is rudi-
mentary in the Amniota, and represented only by vestiges in the Mammalia. When
typically developed it consists of a number of coiled tubules, which, joining one
another at their outer ends, form the segmental duct, while at their inner ends
they open into a chamber, the mesial wall of which is invaginated by a glomerulus —
i.e. a tuft of capillary vessels derived from a branch of the aorta. The chamber
is formed (Brauer in Gymnophiona), by a folding of the mesoderm-layers, from
the ventral part of the hollow primitive segment. It becomes cut off from the
myocoel, but remains connected by a passage with the splanchnocoel (body-
cavity). The tubule is formed as a diverticulum of the chamber by a folding
of the somatic layer of mesoderm, while the peritoneal passage becomes the
nephrostome.

In the human embryo a very rudimentary pronephros is probably to be
recognised in a longitudinal duct, some blind tubules, a peritoneal funnel, and
a vestigial glomerulus, which have been described by several observers,3 in the fifth,
sixth, and seventh segments. It is possible, however, that these vestiges may
represent degenerating mesonephric structures.

1 Hansemann, Verh. deutsch. pathol. Ges. 1901 ; Laguesse, loc. cit. (Hertwig), and Arch. d'Anat.
microsc. v. 1902 ; Pearce, Amer. Jour, of Anat. ii. 1903 ; Ktister, Arch. f. mikr. Anat. Ixiv. 1904 ; Helly,
ibid. Ixvii. 1905.

2 For literature up to 1905, see Felix, Hertwig, Entwickelungslehre, III. Teil i. and ii. p. 852 seq.

" Janosik, Arch. f. mikr. Anat. xxx. ; Tandler, Anat. Hefte, xxviii. ; MacCallum, Amer. Jour, of
Anat. i. ; Gage, ibid. iv. ; Keibel, Anat. Anzeiger (Ergiinzungsheft), xxvii. 1905 ; Ingalls, Arch. f. mikr.
Anat. Ixx. 1907.

VOL. I. N

178

UROGENITAL SYSTEM

The segmental or Wolffian duct appears as a solid cord of cells, which extends
backwards close under the ectoderm external to the nephrogenetic cord as fai
as the cloaca. It soon acquires a lumen (embryo of 3 mm.), and opens into thai
chamber (embryo of 4 -2 mm. ; Keibel).

In the rabbit (Rabl) and marmot (Janosik) the head end of the duct arises by the fusion
of cords of cells which represent rudimentary pronephric tubules, so that the mode of origin
characteristic of the Anamnia is repeated, though in a very abbreviated form. It is not
improbable that the head end of the duct arises in the same fashion in the human embryo.
Graf Spee, Kollmann, Flemming, and others have, however, derived the duct from the ectoderm,
which certainly dips in along the line of the rudiment and lies in contact with it. It is as

neural canal

segment

sderostome

Wolffian duct

FIG. 223. — TRANSVERSE SECTION THROUGH THE TRUNK AND HIND-LIMB BUDS op A RABBIT-EMBRYO OF

THE TENTH DAY. (T. H. Bryce.) ON THE MESIAL ASPECT OF EACH WOLFFIAN DUCT THE NEPHRO-
GENETIC CORD.

yet uncertain whether the ectoderm actually shares in the backward extension of the cellular
cord, or whether the duct grows independently. An ectodermic origin would be explained in
terms of Riickert's theory of the phylogeny of the duct — viz. that it is formed by a union
of the outer ends of the nephridia which^opened primitively on the surface.

IVIesonephros or Wolffian body. — The mesonephros appears first as a ridge
(Wolffian ridge] on each side of the attachment of the primitive mesentery, which
extends from the fifth cervical to the fourth lumbar segment. It is formed by the
enlargement of the intermediate cell-mass, as the tubules and their Malpighian
corpuscles develop in its tissue. As the tubules increase in number the ridges

WOLFFIAN BODY

179

become prominent bodies, which increase in size until the eighth week, after which
they gradually diminish owing to the fact that the tubules undergo degenerative
changes. These degenerative changes commence at an early stage in the head end

%*?;

#~^:.%

uv

*v:;

•'Si

V

FK;. 224. — TRANSVERSE SECTION* THROUGH THE TRUNK OF A RABBIT-EMBRYO OF THE ELEVENTH DAY

(T. H. Bryce.)

me, muscle-plate; cp, skin-plate of segment; sc, ' sclerotome ; w.d., Wolffian duct; w.t,, Wolffian
tubxile. The ridges in which the ducts and tubules lie are the Wolffian ridges. To the left the section
has cut the wall of a Wolffian tubule where it is connected by a cellular cord with the ccelomic
epithelium. A, aorta; (7, coelom ; U.V., U.V., umbilical veins.

primary coils of Wolffian tubuJe
9

\\'»i [ffian duct

Bowman'i

capsule

Fie;. 225.— TRANSVERSE SECTION OF A WOLFFIAN TUBULE IN THE TWENTY-NINTH SEGMENT

OF A RABBIT EMBRYO AT A LATER STAGE THAN THAT REPRESENTED IN FIG. 224. (After

Schreiner.) x 195.

N2

180

UROG-ENITAL SYSTEM

of the Wolffian body, and it is here reduced to a membranous fold which plays an
important part in the formation of the primitive diaphragm, and behind that

kidney

'^TWflHF f

%t&

'*'*>*V.»V<J/f

' *** * :\*t>*$

•A* -**«».*>*,'•>•

w.d.

gen. gl.
FIG. 226. — SECTION OF THE WOLFFIAN BODY OF A HUMAN EMBBYO AT THE END OF THE FIFTH OB

BEGINNING OF THE SIXTH WEEK. (T. H. Bryce.)

gl, gl, glomeruli ; w.d., Wolffian duct ; w.t., w.t., Wolffian tubules ; gen.gl., genital gland.

structure forms the diaphragmatic fold of the Wolffian mesentery. By the fifth
month the tubules have almost entirely disappeared, and in the end only persist as

WOLFFIAN BODY

181

accessory parts of the reproductive apparatus. The Wolffian duct lies external
to the Wolffian body, enclosed in a fold of the peritoneum, which fuses posteriorly
with that of the opposite side to form the genital cord (see below).

mesenchyme

Mitlpi/jhian
body

r/.t.v/v// lnin r

L..^ ... parietal layer
H, of Bowman's
'• '. capsule

genital ridge

FIG. 227.— RECONSTRUCTION OF A WOLFFIAN TUBULE OF A HUMAN EMBRYO OF
10'2 MM. LONG. (From Kollmann.J x 260.

Wolffian

hind-gut

bladder

cloacal membrane

cloaca

outer zone
_ inner zone

—pelvis of
kidney

FIG. 228. — THE PRIMITIVE PELVIS OF THE KIDNEY, FROM A RECONSTRUCTION. (After Schreiner.)

The pelvis is surrounded by nephrogenetic tissue differentiated into an inner epithelial
and outer mesenchymatous zone.

Origin of the tubules. — The intermediate cell-mass becomes differentiated (in the
rabbit-embryo, fig. 223) into an inner part adjoining the segment, which has a mesenchymatous

182

UROG-ENITAL SYSTEM

and an outer part having more of an epithelioid character (nephrogenetic cord, Schreiner). This
outer part forms a cord which becomes separated from the coelomic epithelium, and secondarily
segmented into a series of solid rounded bodies. These soon develop a lumen and form a
series of small vesicles (fig. 224). There are several such in each segment in the rabbit, but it
has been asserted (Kollmann) that the organisation is segmental in the human embryo in the
early stages. In some cases it can be determined that the vesicle is connected with the coelomic
epithelium by a slender string of cells (fig. 224).' From the outer and upper part of the wall a
solid sprout, which is soon hollowed out to form a diverticulum of the vesicle grows against, and
ultimately opens into, the Wolffian duct. This is the rudiment of the tubule proper, the
original vesicle forming the Malpighian capsule. The tubule elongates and becomes S-shaped

(fig. 225). Into the space between
its proximal loop and the upper
originally mesial wall of the now
flattened, spoon - shaped vesicle, a
branch grows in from the adjoining
aorta. From this vessel a knot of
capillaries is formed which lies in the
hollow of the upper wall of the vesicle
and becomes the glomerulus.

The connective- tissue framework
of the Wolffian body is derived from
the mesenchyme of the intermediate
cell mass.

outer zone

pelvis-

Wolffian duct

bladder,

TCetanephros : perma-
nent kidney, — The meta-
nephros arises from the posterior
part of the same nephrogenetic
cord as forms the blastema of
the mesonephros. l The tissue,
however, remains passive during
the time when the tubules are
forming in the Wolffian body,
and becomes related to a diver-
ticulum which grows out from
the dorso-mesial aspect of the
Wolffian duct immediately in
front of its opening into the
cloaca (fig. 228). As the diver-
ticulum increases in length in a
dorsal direction, the nephro-
genetic tissue becomes displaced
and separated from the Wolffian
duct until it comes to lie dorsal
and mesial to it. The diver-
ticulum becomes expanded at its
distal end to form the primitive
pelvis of the kidney, while the stalk represents the future ureter. As the ureter
elongates the kidney gradually changes its position relatively to the Wolffian body,
until eventually it lies above and dorsal to it. The primitive pelvis is surrounded
by the nephrogenetic tissue, which now shows a differentiation into an inner
epithelioid and an outer mesenchymatous zone (fig. 229). The latter is con-
tinuous with the blastema of the mesonephros, but the former is quite separate

FIG. 229. — SECTION OF THE PRIMITIVE PELVIS OF THE

KIDNEY OF A HUMAN EMBKYO OF FIVE WEEKS OLD.

(After Schreiner.)

The inner zone of nephrogenetic tissue is sharply marked
off ; the outer zone passes gradually into the surrounding
mesenchyme.

1 This fact, first demonstrated for the chick by Sedgwick, lias been substantiated for all the
Amniota, including man, more especially by Schreiner, Zeitschr. f. wissen. Zool. Ixxi. 1902. In
recent
though
which the Wolffian body is the anterior end.

-**•"& •"' .tMiij RJJ kj IC.LJ ^jeiLouiii . i. wiootrii,

paper. Janosik (Arch. f. Anat. April 1907) has again thrown doubt on the ontogenetic fact,
he admits the general proposition that the kidney is merely a part of a primitive organ of

KIUNKY 183

from it. The primitive pelvis is at first simple, but soon becomes branched
(fig. 235, p. 187) by the outgrowth from it of bulbous ampullse, each of which

FIG. 280. — SECTION OF A POBTION OF THE COLLECTING TUBE AND TWO TUBULES IN THE

DEVELOPING KIDNEY OF A BABBIT-EMBRYO. (T. H. Bryce.)

On the right the tubule is in the vesicular stage ; on the left the vesicle has elongated, and is
assuming the S shape, but it has not yet joined with the collecting tube.

.. , «i

. -;•;*•• <•> • •. .jg '' -iitprr hf/iii

*& # :_*/*&

collecting tube - —*—?.- ^ -;

: k

:" (.f^fl *"

^ ~ v5v """•• **'•

;: I

-

lower bend

iddle piece

•Bowman's
capsule

FIG. 231. — SECTION OF A DEVELOPING KIDNEY-TUBULE (BABBIT). (T. H. Bryce.)

is covered by a cap of the epithelioid inner zone, which has been broken up into
separate masses during the process of branching. The collecting tubules are
formed by a continuous process of dichotomous budding from the primary

184

UROGENITAL SYSTEM

upper bend

middle piece

branches of the pelvis. The secreting tubules, on the other hand, arise quite
independently from the ' inner zone ' of the nephrogenetic blastema, while the
' outer zone ' provides the connective-tissue framework and capsule of the organ.

Origin of the tubules.1 — At each division of the primary collecting tubules the ' inner
zone ' cap becomes divided into two. The peripheral part of each moiety becomes the new
cap of the new branch ; but the central portion of each becomes separated off as a detached
oval and solid body which lies in the angle between the side branch and the main stem of the
duct. This is the rudiment of the capsule of Bowman, and the further phases in the development
of the tubule correspond exactly to those already described for the mesonephric tubules. A
lumen appears in the solid rudiment (fig. 230), and from the upper extremity of the vesicle
diverticulum passes, which abuts against, and finally opens into the collecting duct. Briefly
stated, the phases in the evolution of the vesicle are as follows. It first becomes comma-shaj
by the thickening of its outer wall. The knee formed deepens into a cleft which separates
the ventral part, the future capsule of Bowman, from the peripheral part, the future tubule.
By a second fold, which appears in the upper part of the inner wall, the tubular portion
becomes S-shaped, the one limb of the S opening from the capsule, the other ending in the

collecting duct (fig. 230). Tl
primitive capsule of Bowman, from
the first hemispherical, become
spoon-shaped by the upgrowth of
its lips. Into the hollow thus pro-
duced the mesenchyme extends,
and in it, either by formation ii
situ, as some think, or by ingrowt
of a bud from a neighbouring
blood-vessel, a glomerulusis formed.
By a further great increase ii
length and coiling of the tubul
round the Malpighian body as a
more or less fixed point (Herring),
the complicated adult tubule take
form. The central limb of
S becomes the first, the periphery
limb the second convoluted tubule
while the bend of the figure, lying
at first in the hollow of the spe
elongates to form the looi
tubule of Henle (figs. 232, 233).
the primary branches of the ampulla* of the primitive pelvis continue to sprout and divide
it necessarily follows that the zone in which tubules are being produced is progressivel
displaced outwards, and thus a zone is laid down in which new tubes are formed during nearly
the whole of foetal life, only ceasing in the eighth month (Herring). As the kidney at an earlj
stage is divided into primary lobules, corresponding to groups of branching primary collectii

1 The account in the text is founded mainly on the descriptions and figures of Herring, Schreiner,
and Carl Hiiber, controlled by personal examination of rabbit-material. The view that the tubule
have a double origin was originally advanced by Kupffer in 1865. It was substantiated by Sedgwi(
and adopted by Balfour. It has recently been confirmed by several observers, more especially the thn
• named; but it should be stated that many authors have preferred the view of Kemak (1855), which w£
maintained by Toldt and accepted by Kolliker, Waldeyer, Minot, and many others, to the effect that the
tubules are in their entirety produced by budding from the original diverticulum. For references to the
literature and for a history of opinion on this subject the reader is referred to Hiiber's paper in the
American Journal of Anatomy, vol. iv. 1905.

Janosik, in the paper referred to in the note to page 182, admits the discontinuous origin of
secreting tubules, but describes the phases of their development differently from any other observer.
The process as he conceives it is one of great complexity and irregularity. The chief points are ar
follows : the primitive capsule and its primitive tubule connecting it with the collecting duct gives of
blind processes, which may become permanent tubules when the capsule is separated from the duct, ai
it very early is. This separation may take place in various ways, so that the primitive capsule remains
connected with the whole or part of its primitive tubule, or be isolated from it. The capsule now
acquires a new connection with the collecting duct at a point nearer the cortex, either directly by some
part of its own tubule, or indirectly through the medium of a different tubule. This may belong eithe
to another capsule, in which case a double tubule results, or may be a tubule which has lost its origins
capsule. Further, the capsules are often double in early stages, due to a union or to a cleaving of th
original rudiment. Henle's tubule cannot be recognised at an early stage as described in the text
apparently any loop may give rise to it.

Henle's
tubule

collecting tube

FIG. 232. — MODEL OF TWO DEVELOPING KTDNEY-TUBULES.
(After Stoerk.)

On the right side the tubule is in the S-shaped stage :
on the left side it is beginning to coil to form the different
parts of the adult tubule.

KIDNKV

185

tubules, the neogenic zone does not form merely a layer on the surface, but extends between the
groups down to the pelvis as a series of septa, the ' primary columns of Berlin' (Haugh). The
primary collecting ducts are at first relatively far apart, but as the division progresses they
come to be set at sharp angles to one another, until at last they form the straight tubules of the
pyramids and medullary rays.

The urinary bladder is developed from the ventral portion of the cloaca
entodermica, which, as we have already seen, becomes divided by a septum into
rectum and urogenital sinus. The sinus and the allantois now form a tubular passage,
on which a dilatation appears implicating the section derived from the cloaca,
and perhaps also part of the allantois. This becomes the bladder, and the allantois
is obliterated to form the urachus. When the division of the cloaca is effected

junctionnl tubule

ggj- 2nd convoluted

tubule

desc. limb of Henle'n-
tubule

afferent )
efferent f

Bowman's
capsule

asc. limb of Herile's tubule

Fiw. 2 :••:•}. -DIAGRAMMATIC SECTION OF A DEVELOPING KIDNEY-TUBULE AND OLOMERULUS.

(T. H. Bryce.)

the Wolffian ducts come to open into the urogenital sinus. The openings are at
first common to the ducts and the ureters, but soon they are caused to open
separately by the lower ends of the Wolffian ducts being taken into the wall of
the sinus. The portion of the sinus intervening between the pairs of openings
then elongates, and the relations are so altered that the ureters open into the
bladder dilatation, and the Wolffian ducts into the sinus proper. This ultimately
forms the prostatic and membranous portions of the urethra in the male, and the
whole urethra and the vestibulum vaginae in the female. The Wolffian ducts at
first open into the urogenital sinus close together, separated by an eminence in
which the fused Miillerian ducts end (fig. 248, p. 196). In the male the eminence
persists as the crista uretkralis, which extends also along that section of the sinus
which becomes elongated as the ureters and Wolffian ducts draw apart. When,

186

UROGENITAL SYSTEM

at a later stage of development, the bladder dilates and the ureters are drawn
apart, the upper part of the crista is expanded into the trigone of the bladder.

DEVELOPMENT OF THE GENITAL GLANDS AND DUCTS

The genital glands develop comparatively late, on the mesial aspects of the
Wolffian bodies (figs. 226, 237). Here the coelomic epithelium becomes thickened
to form what is known as the germinal epithelium (Waldeyer). The cells become

FIG. 234. — SECTION THBOUGH A HUMAN EMBRYO OF 30 MM. ABOUT THE BEGINNING OF THE
THIRD MONTH. (T. H. Bryce.)

On each side of the spinal oord are seen the cartilages of the neural arch ; below the cord the body
of j the primitive vertebra with the notochord passing through it ; between the two are the spinal
ganglia, from which extend downwards the spinal nerves. Below the vertebra the cardinal veins
are united by a large anastomosis ; below this the two common iliac arteries. Between the iliac
arteries the mesentery leaves the posterior abdominal wall ; on each side of this the ureters. External
to the ureters, the Wolffian bodies. On the mesial aspect of each is seen the genital gland connected
by its mesentery; on the outer aspect of each, the Wolffian mesentery containing, to the inner side
the Wolffian, to the outer side the Miillerian duct. Projecting into the abdominal cavity from
below, the allantoic duct attached by a mesentery, in which the two allantoic arteries run. The
intestine is cut in several places, and on each side are seen the lateral lobes of the liver.

columnar and arranged in several layers, while the underlying mesenchyme
becomes somewhat thickened along a projecting ridge, the genital ridge. The
germinal epithelium by proliferation of its cells increases in depth, and among the
proper epithelial elements certain larger and more spherical cells appear (fig. 226,

GENITAL GLANDS

187

cloaca

Wolffian
ducts

-crelom

.~!>''lvi>i of

cloacal
membrane

FIG. 235. — PELVIS; HUMAN EMBBYO OP 11-5 MM. (FOUR AND A-HALF WEEKS).
(After Keibel, from Kollmann's Entwickelungsgeschichte.)

* septum uro-rectale.

umbilical artery

bladder

; ovary

broad
ligament

symphysis >

urogenital sinus, /

^-- '

Mullerian
duct

Wolffian duel

genital papilla — /'

|
urogen. sinus ^JL_

axiom

"" rectum

coccyx

FIG, 236. — PELVIS ; HUMAN EMBBYO OF 25 MM. (EIGHT AND A-HALF TO NINE WEEKS OLD).
(After Keibel, from Kollmann's Entwickelungsgeschichte.)

L, liver ; *septum uro-rectale.

188

GENITAL GLANDS

p. 180). These are the primitive sex-cells, and, as far as any evidence yet
available permits conclusions, they arise from the germinal epithelium in situ.
It is, notwithstanding, a question of great theoretical importance, which must
be left to the future to decide, whether the sex-cells actually do so arise in situ,
or whether they are only segregated here, having been set apart from the somatic
cells at the earliest stages of development. Large sex- cells are already present in
or under the peritoneum at the root of the mesentery in the region of the first
five trunk segments in a human embryo of 4'9 mm. (Ingalls, 1907).

The proliferating epithelium now grows inwards to form strands of cells known
as the genital cords. At first there is a very small amount of connective tissue
between them, but this soon increases in amount, and the cords become

86.

FIG. 237. — SECTION OP THE GERMINAL EPITHELIUM AND ADJACENT STBOMA IN A MALE
CHICK-EMBRYO. (Semon.)

g.ep^ germinal epithelium forming a thickened ridge-like projection ; pr.ov., primitive ova of various
sizes, some in the germinal epithelium and others somewhat beyond the limit of this epithelium ;
st., strands of cells which have grown from the germinal epithelium, and one of which appears connected
with an enlarged primitive ovum.

more clearly defined. The cords consist of epithelial cells, among which are seen
primitive sex-cells, and, it is said, elements which have characters which are
transitional between the two.

According to the researches of Coert and Allen on embryos of the pig and rabbit, the epithelium
over the cranial end of the genital ridge is lower than the true germinal epithelium, but here
again strands are produced by proliferation which are easily distinguishable from the genital
strands by their smaller cells and darker-staining nuclei. These extend inwards and also back-
wards between the gland-rudiment and the Wolffian body, and when later they acquire a lumen
become the rete tubules (see below).

After the gland has reached this stage specialisation begins in the hitherto
indifferent rudiment, and it acquires the distinctive characters of ovary or testis.

OVARY

189

As the glands take form they become separated from the Wolffian body, remaining
attached to it only by a stalk or mesentery — the mesovarium or mesorchium, as
the case may be.

poophoron

FIG. 238. — LONGITUDINAL SECTION THROUGH THE OVABY OF A CAT-EMBBYO CF 9'4 CM. LONG
(SCHEMATIC). (After Coert, from Hertwig's Handbuch der Entwickelungslehre.)

epithelium

*vy^ •>'•**'?'&

medulla

,-€>- ^

FIG. 239. — SECTION OF THE OVARY OF A HUMAN FCETUS OF THE FOURTH MONTH.

The ovary retains longer the primitive characters, in respect that the germinal
epithelium remains many-layered, and retains its proliferative activity. The
future history of the gland is explained by the fact that the mesenchyme grows

190

GENITAL GLANDS

outwards into the germinal epithelium, while the epithelial strands grow inwards,
so that we have an interlocking of epithelial and connective tissues. In the early

epitheliun

oocutes

FIG. 240.— SECTION OF THE OVABY OF A HUMAN FCETUS OF THE SEVENTH MONTH.
(Figs. 239 and 240 from Felix and Biihler, Hert wig's Handbuch der Entwickelungslehre.)

stages, a narrower cortical zone, in which the epithelial strands are closely pressed

together and separated by a very small amount of connective tissue, is distin-
guished from a central medul-
lary zone in which, owing to
the increase in the vascular
stroma, the primary genital cords
are more definitely marked off
from one another (fig. 239) . This
medullary portion of the paren-
chyma is gradually reduced as
the cortical zone increases in
thickness, and the medullary
cords are reduced to a few
epithelial strands, in which ova
are no longer to be seen. The
body of the ovary is formed
from the cortical zone. The
epithelial columns (egg-tubes of
Pfliiger) become separated, then
cut up, by the growth of the
stroma, into cell-islands con-
taining one or more primitive
ova ; these again into smaller
groups or nests of cells, until
ultimately the primitive follicles
are isolated, each containing
a single ovum surrounded by a

layer of follicular cells (fig. 241). From these the Graafian follicles are formed.

This process, by which the stratum germinativum of the ovary is formed, goes

FIG. 241. — SECTION OF THE OVABY OF A NEWLY BOBN
CHILD. (Waldeyer.) Highly magnified.

a, Germinal epithelium dipping in at b, to form an
ovarian tube ; c, c, primordial ova lying in the germ-
epithelium ; d, d, longer tube becoming constricted so as
to form nests of cells ; e, e, larger nests ; /, distinctly
formed follicle with ovum and epithelium ; g, g, blood
vessels.

TESTICLE

191

epithelium

albuginea

on during all the later months of foetal life, but is completed by the time of birth
or shortly after.

According to the important researches of Winiwarter, ' the germinal epithelium comprises two
layers of cells with nuclei of distinctive characters. In the superficial layer, the nuclei stain
deeply as a whole, but the nuclear network is very delicate, and the nuclear membrane is indis-
tinct ; in the deeper layer the nuclei are smaller, but the network is coarser, with distinct
karyosomes, and the nuclear membrane is an obvious feature. There is no true nucleolus in
either variety of nucleus. Winiwarter names them ' noyaux protobroques a and &.' In the epithelial
masses invading the stroma, cells with ' noyaux protobroques b ' are seen in active division, but
there is also a third variety of cell with a large clear nucleus and distinct nucleolus (noyau deuto-
broque). These cells no longer divide, and are the young oocytes. In the nuclear division pre-
ceding the appearance of these cells the prophase of the heterotypical division has been initiated
by the synapsis, in which the chromatin-loops fuse in pairs (see p. 17). The cells with ' noyaux
protobroques b ' become the follicular cells. As these gather round the oocytes they become
arranged in a continuous low columnar epithelium. The cells, however, multiply, and form,
soon, a many-layered investment to the oocyte. The liquor folliculi next gathers among the
cells, and separates a mass surrounding the ovum and attached to the wall of the follicle named
the discus proligerus (cumulus oophorus),
from the cells lining the follicle or stratum
granulosum. Outside this epithelial
layer the connective tissue becomes
condensed round the ovum into the
theca folliculi, which shows two layers,
an external (theca externa) more fibrous,
and an internal (theca interna) more
cellular. The cells in the theca interna
are rounded or polygonal elements of
some size, which exactly resemble the
interstitial cells scattered in the ovarian
stroma. These interstitial cells are
generally regarded as being derived from
the mesenchyme, but Miss Lane- Clay ton 2
has supplied evidence which seems to
show that they may be derived like the
follicular cells from the germinal epithe-
lium. The cells of the corpus luteum 3
are considered by some as arising from
the stratum granulosum, by others from
the cells of the theca interna.

The tubules which have been ob-
served by various authors near the
hilum of the gland, and named the
rete tubules because they are regarded as
being homologous with the tubules of the

rete testis, have been generally regarded as derivatives of Wolffian tubules, but according to the
results of Coert and Allen, referred to above, they arise like the primary genital cords from the
coelomic epithelium.

The testicle is early distinguished from the ovary by the reduction that takes
place in the germinal epithelium, which becomes marked off from the parenchyma
of the gland by a layer of connective tissue, the future tunica albuginea. The
genital cords of the primitive gland become the seminal tubules. These, which
of course are at first solid, consist of smaller cells probably representing the cells
of the germinal epithelium, and larger rounded sex-cells. The tubules acquire
a lumen quite late in their history, and increasing in length become coiled (fig. 242).
The larger rounder cells multiply, and ultimately line the greater part of the wall of

1 Archives de Biok>gie, 1900.

- Journ. of Phys. xxxii. 1905, and Journ. of Obstet. and Gynec. xi. 1907.

3 See ' Ovary ' in volume on Splanchnology.

supporting _ J
cell.

genital cell

FIG. 242. — SECTION OP ONE OF THE GENITAL CORDS
OF THE TESTICLE OF A HUMAN EMBRYO OF
3-5 CM. LONG. (Felix and Biihler.)

192

UKOGEN1TAL SYSTEM

the tubule as spermatogonia. The tubules of the rete testis have long been regarded
as probably derivatives of the Wolffian tubules, but recent work, more especially
of Coert and Allen, points to their origin, as indicated above, from the coelomic
epithelium as epithelial cords which afterwards acquire a lumen. They become
joined with the seminal tubules, and, growing towards the Wolffian body, they
open into the capsules of Bowman of some of the anterior Wolffian tubules.

The interstitial cells of the testis are, like the corresponding cells of the ovary,
generally regarded as connective-tissue elements, but some observers describe
them as arising from the germinal epithelium.

Genital ducts. — The fate of the Wolffian body and Wolffian duct differs
in the sexes. In the male the duct persists as the canal of the epididymis, the
vas deferens, and ejaculatory duct : the seminal vesicle is formed as a diverticulum

FIG 243. — Two FIGURES EXHIBITING A COMPARISON BETWEEN PARTS OP THE GENERATIVE OKGANS IN
THE TWO SEXES. (From Farre, after Kobelt).

A. ADULT OVARY, PAROVARIUM, AND FALLOPIAN TUBE.

a, a, Epoophoron (parovarium) formed from the upper part of the Wolffian body ; b, remains of the
uppermost tubes, sometimes forming hydatids ; c, middle set of tubes : d, some lower atrophied tubes;
e, atrophied remains of the Wolffian duct ; f, the terminal bulb or hydatid ; h, the Fallopian tube,
originally the duct of Mliller ; i, hydatid attached to the extremity ; I, the ovary.

B. THE ADULT TESTIS AND EPIDIDYMIS.

a, a, convoluted tubes in the head of the epididymis developed from the upper part of the Wolffian
body ; b and /, hydatids in the head of the epididymis ; c, coni vasculosi ; d, vasa aberrantia ;
h, remains of the duct of Miiller with i, the hydatid of Morgagni, at its upper end ; I, body of the testis.

from its lower end. Certain of the anterior tubules which have been joined by
the rete tubules remain as the coni vasculosi and vasa efferentia (fig. 243, B).

The organ of Giraldes or paradidymis represents some tubules which have lost their
connexion with the duct, and the vasa aberrantia others which have not been connected with
the rete tubules or have secondarily lost their connexion with them. The head end of the
duct is said to persist as a stalked hydatid, and certain peritoneal tubules which have been
described are supposed to represent the remains of nephrostomes.

In the female (fig. 243, A) the head end of the Wolffian body, which undergoes still
greater reduction than in the male, persists as the rudimentary organ known as the
parovarium. This consists of the head end of the duct and a number of tubules,
which lie in the mesosalpynx (epoophoron), and some remains of tubules in the

THE MULLERIAN DUCTS

193

ligament (paroophoron). The greater part of the duct disappears, but
ints of it are occasionally to be seen in sections across the body or cervix

•'<•>* V* '

glomerulus

••W?

.*&*>&

:^;;su»wjjj

tfoS&h!&!

>»e.

IPNtSPSi

•us8*-"*.

Wolffian duct Miillerian duct

genital ridge

IG. 244.— TRANSVERSE SECTION OF THE HEAD-END OF THE WOLFFIAN BODY OF A HUMAN EMBRYO AT
THE END OF THE FIFTH OR BEGINNING OF THE SIXTH WEEK, SHOWING THE PERITONEAL OPENING
OF THE MULLERIAN DUCT. (T. H. Bryce.)

)f the uterus, or even lying in the vaginal wall. It persists as the duct of Gartner
in some mammals — e.g. the pig.

The Miillerian duct arises on the lateral aspect of the Wolffian body, and
near its anterior end, as a thickening of the coelomic epithelium. This soon shows
a longitudinal depression, which, deepening
at its posterior end, becomes converted into
a funnel-shaped depression. From the
caudal end of this the duct grows backwards
in close relation to the Wolffian duct. In
embryos about the middle of the second
month the two ducts lie in a free fold
projecting from the outer side of the
Wolffian body, the Miillerian duct being
to the outer side (fig. 234). Behind the
Wolffian body this fold, merging with the
Wolffian mesentery, passes on to the lateral
wall of the contracted part of the coelomic
cavity which will form the pelvis (fig. 246).
Here the folds from opposite sides meet in
the middle line; the four ducts are thus
brought close together, and are imbedded

in a mass of tissue called the genital cord (figs. 245, 247). The Miillerian ducts,

as they pass into the genital cord, cross over the Wolffian ducts, and come to lie

close together between them. Within the cord the epithelial tubes fuse, but behind,

VOL. i. o

bladder jM

ureter ._-

urethra

-- Wolffian duct

FIG. 245.— UROGENITAL SINUS, BLADDER,
AND GENITAL DUCTS OF A FEMALE HUMAN
EMBRYO OF 29 MM. (After Keibel.)

194

UKOGENITAL SYSTEM

the lumina remain separate for a time, and the fused walls terminate in an epithelial
thickening which projects into the urogenital sinus between the openings of the
Wolffian ducts (Miillerian eminence) (fig. 248). Here, but at a much later date,
the opening into the sinus is effected. The thickening in which the ducts end is
produced by proliferation of the epithelial cells at their terminal growing points,
the so-called vaginal bulbs. The future history of the ducts differs in the sexes.
In the male they disappear throughout almost their whole length, but the head
end is believed to persist as the Jiydatid of Morgagni, while their fused posterior
ends remain as the prostatic utricle (uterus masculinus).

kid.

FIG. 246.— SECTION THROUGH THE TRUNK OF A HUMAN EMBRYO OF 15'5 MM. AT THE LEVEL
OF THE HIND-LIMBS. (T. H. Bryce.) Photograph.

s.n., spinal nerve ; c.v., c.v., cardinal veins (between 'them the aorta giving off the allantoic arteries) ;
kid, kidney ; ur, ureter ; w.b., Wolffian body ; w.d., Wolffian duct in Wolffian mesentery ; r, rectum ;
bl, bladder ; a.a., a.a., allantoic arteries.

The glandular tissue of the prostate is developed as a series of epithelial
sprouts from the urogenital sinus, which appear in the fourth month, proximal
and distal to the opening of the Wolffian ducts. These acquire a lumen and form
the parenchyma of the gland, the muscular and connective tissue being derived
from the mesenchyme of the genital cord. Similar sprouts form the so-called
Skene's tubules of the female urethra.

In the female the ducts persist, and their upper independent portions become
the Fallopian tubes, while their fused posterior portions form the foundation of
the uterus and vagina. The fusion begins near the lower end (fig. 245), and
proceeds both downwards towards the future orifice and upwards for a certain
length. The extent to which the Miillerian ducts are fused varies in different

THE MULLERIAN DUCTS

195

animals, and certain malformations of the genital tract which occur in the human
subject, involving a greater or lesser degree of duplicity of the tract, are explained
by defective fusion of the ducts. Meanwhile the urogenital sinus shortens, the
vaginal bulbs elongate, and the Miillerian eminence is brought close to the surface.
It follows that when the opening out of the fused solid ends of the ducts takes
place the orifice is superficial, and the urogenital sinus is only represented by the
vestibule of the vagina.

For a considerable period the vagina is solid. Opinions differ as to the origin of this
condition. According to one view (Nagel) it is primary, the vagina being formed from the

FIG. 247. — SECTION THROUGH A HUMAN EMBRYO OF 30 MM. ABOUT THE BEGINNING
OF THE THIRD MONTH. (T. H. Bryce.)

Below the spinal cord is seen the body of a vertebra with the iiotochord, and on each side of the
cord the neural arch ; between them a pair of spinal ganglia. The large cartilages on each side are
the ilia. The peritoneal cavity is bridged over by the genital cord formed by the fusion of the Wolffian
mesenteries ; in the cord are seen the two Miillerian ducts about to fuse, and externally the Wolffian
ducts. Above the genital cord is the rectum ; in the wall of the pelvis, one on either side, the ureters ;
below the cord the bladder and allantoic arteries. On each side of this the peritoneal cavity is interrupted
by a fold, the plica gubernatrix extending from the Wolffian mesentery to the inguinal region,
liotice how at the inguinal attachment of these folds the muscular tissue of the abdominal wall extends
into the gubernaculum.

elongated epithelial mass in which the Miillerian ducts end. According to Berry Hart and
F. Wood- Jones, who adopt a' very similar explanation, two separate epithelial cords are formed,
which fuse to give rise to the solid vagina. According to another view, the condition is
secondary, and is due to the temporary fusion of the epithelial walls. The cavity is produced
or restored by the shedding of the central cells after the fourth month. Opinions also
differ as to the origin of the hymen. According to the most generally accepted view, it
represents the remains of the margins of the opening of the thickened ends of^ the fused
ducts into the urogenital sinus. Berry Hart, and also recently Kempe, have, however, derived
it from the epithelium in which the genital ducts end.

02

196

UKOGENITAL SYSTEM

The epithelial lining of the Miillerian ducts gives rise merely to the epithelium of
the genital tracts. The muscular and connective- tissue elements of the mucous
membrane, and of the wall of the uterus and vagina, are derived from the genital cord,
which undergoes a progressive development as compared with that of the male. The
lower part surrounding the ends of the ducts grows downwards, and apparently
in the process occasions the shortening of the urogenital sinus already referred to.
By the fifth month the muscular wall of the uterus begins to differentiate, and the
mucous membrane to thicken. It is noticeable that during the later months of foetal
life the cervix bears a very large proportion to the body (fig. 249), which is very
short, and it is not till puberty that the body begins to assume its proper relative
proportions.

FIG. 248. — SECTION THROUGH A HUMAN EMBRYO OF 30 MM. ABOUT THE BEGINNING OF THE
THIRD MONTH. (T. H. Bryce.)

Below the spinal cord is seen the body of a sacral vertebra ; on each side the lateral part of the
sacrum and a portion of the ilium ; between the body and the lateral cartilages a pair of spinal nerves.
The section passes through the hip- joints ; between these ventrally the pubic cartilages. Between the
two ischial cartilages the rectum, below this the urogenital sinus with the Wolffian ducts opening into
it ; between the Wolffian ducts the Miillerian eminence, in which is seen the irregular blind end of the
fused Miillerian ducts ; between the pubic cartilages the urethra.

Descent of the ovaries and testes. — In both sexes the genital glands
undergo a displacement from their primitive position in the lumbar region and
come to lie above the brim of the pelvis. From this situation in the later months
of pregnancy the testicles descend into the scrotum, while the ovaries retain their
secondary position until ultimately, with the enlargement of the pelvis, they sink
to the definitive position. The descent of the glands is chiefly effected by the
agency of a fold, in which muscular fibres develop, called the plica gubernatrix
(fig. 247).

The change in position of the different structures is best understood by referring
back to a stage reached about the end of the second month (fig. 251). The Wolffian
body, on each side, is seen to be attached to the posterior abdominal wall by the

DESCENT OF THE TESTICLES

197

Wolffian mesente y, which is continued upwards as a fold to the diaphragm. The
genital gland is fixed to its inner aspect by the genital mesentery, and the two
ducts to its outer side by the urogenital fold (fig. 234). The latter merges
with the Wolffian mesentery behind the Wolffian body to form the urogenital
mesentery, and this in turn unites with its fellow to form the genital cord
(fig. 247). The genital mesentery is continued upwards as the upper, and
downwards as the lower genital fold. The latter joins, at the lower end of the

\

bladder

body rectum

symphysis

cervix

L. maj. L. min. Hymen

FIG. 249.— VERTICAL LONGITUDINAL SECTION OF THE PELVIS : A, OF A FCETUS OF 6 CM. ;

B, OF A FCETUS OF 10 CM. ; C, OF A FCETUS OF THE SEVENTH MONTH. (From -Nagel.)

In A and B : 1, Junction of vagina and uterus ; 2, sinus urogenitalis ; 3, bladder.

Wolffian body, the urogenital fold, while from the same point a fold passes to
the groin, the plica gubernatrix. In the female the adult conditions are easily
deduced from this description. When the Wolffian body disappears, the Wolffian
merges in the urogenital fold to form the mesosalpinx, and the mesovarium
becomes an adjunct to it. The remainder of the urogenital mesentery is the
primitive broad ligament; and when the Fallopian tubes and ovaries sink into
the pelvis, all merge in one wing-like fold of peritoneum. The upper genital

FIG. 250. — DIAGRAMS TO SHOW THE DEVELOPMENT
OF MALE AND FEMALE GENERATIVE ORGANS
FROM A COMMON TYPE. (Allen Thomson.)

A. — DIAGRAM OF THE PRIMITIVE UROGENITAL

ORGANS IN THE EMBRYO PREVIOUS TO
SEXUAL DISTINCTION.

3, ureter; 4, urinary bladder; 5, urachus ;
o£, the genital ridge from which either the ovary
or testicle is formed ; W, left Wolffian body ;
w, w, right and left Wolffian ducts ; m, m, right
and left Miillerian ducts uniting together and
running with the Wolffian ducts in gc, the genital
cord ; ug, sinus urogenitalis ; i, lower part of
the intestine; cl, cloaca; cp, elevation which
becomes clitoris or penis ; Zs, fold of integument
from which the labia majora or scrotum are
formed.

B. — DIAGRAM OF THE FEMALE TYPE

OF SEXUAL ORGANS.

o, the left ovary ; £>o, parovarium
(epoophoron of Waldeyer) ; W, scat-
tered remains of Wolffian tubes near
it (paroophoron of Waldeyer);
d G, remains of the left Wolffian
duct, such as give rise to the duct
of Gartner, represented by dotted
lines; that of the right side is
marked w ; /, the abdominal opening
of the left Fallopian tube ; u, uterus ;
the Fallopian tube of the right side
is marked m; g, round ligament,
corresponding to gubernaculum ;
i, lower part of the intestine ;
va, vagina ; h, situation of the hymen ;
C, gland of Bartholin. (Cowper's
gland), and immediately above it the
urethra; cc, corpus cavernosum
clitoridis ; sc, vascular bulb or corpus
spongiosum ; n, nympha ; I, labium ;
v, vulva.

C.— DIAGRAM OF THE MALE TYPE OF

SEXUAL ORGANS.

/, testicle in the place of its original
formation : e, caput epididymis ; vd, vas
deferens; W, scattered remains of the
Wolffian body, constituting the organ of
Giraldes, or the paradidymis of Waldeyer ;
vh, vas aberrans ; m, Miillerian duct,
the upper part of which remains as the
hydatid of Morgagni, the lower part,
represented by a dotted line descending
to the prostatic vesicle, constitutes the
occasionally existing cornu and tube of
the uterus masculinus ; g, the guberna-
culum ; vs, the vesicula seminalis ;
}jr, the prostate gland ; C, Cowper's gland
of one side ; cp, corpora cavernosa penis
cut short; sp, corpus spongiosum ure-
thrse ; s, scrotum ; t', together with the
dotted lines above, indicates the direction
in which the testicle and epididymis de-
scend from the abdomen into the scrotum.

DESCENT OF THE TESTICLES

199

and upper Wolffian fold become the suspensory ligament of the ovary enclosing
the ovarian vessels. The lower genital fold, and the plica gubernatrix develop
involuntary muscular fibres and become the ovarian and round ligaments respec-
tively. In the male the primitive relations are more departed from, owing to the
descent of the testicle, and are further modified by the disappearance of the
Miillerian ducts. The Wolffian mesentery, the genital mesentery, and the

genital gland

Wolffian
body

allantois •

— diap hragmatic

fold

cranial genital

fold

urogenital fold

*• caudal genital

fold

—plica gubernatrix

FIG. 251. — THE PERITONEAL FOLDS CONNECTED WITH THE WOLFFIAN BODIES AND GENITAL GLANDS
OF A PIG-EMBRYO OF 6'7 CM. LONG. (After Klaatsch.)

urogenital fold are merged in the different portions of the adult mesorchium, but
leave no trace within the abdomen. The plica gubernatrix, which after the atrophy
of the Wolffian body is continuous with the Wolffian and genital mesenteries,
becomes the gubernaculum testis. The testis descends to the iliac region in the
third month, but only enters the internal abdominal ring in the seventh month.
Previously to this a pouch of peritoneum — the processus vaginalis — has descended
into the scrotum along the abdominal ring, pushing before it part of the internal

FIG. 252. — DIAGRAMS TO ILLUSTRATE THE DESCENT OF THE TESTICLE AND THE FORMATION
OF ITS COVERINGS. (O. Hertwig.)

In A the testicle is lying close to the internal abdominal ring. In B it has passed into the sac of
the tunica vaginalis. 1, skin of abdomen ; 1', skin of scrotum ; 2, superficial abdominal fascia ;
2', Cooper's fascia ; 3, muscular and aponeurotic layer of abdominal wall ; 3', cremaster muscle and
spermatic fascia; 4, peritoneum; 4', processus vaginalis; 4", visceral layer of processus vaginalis
covering testicle ; t, testicle ; v.d., vas deferens ; r, internal abdominal ring.

oblique muscle and the aponeurosis of the external oblique, which form respectively
the cremasteric muscle and spermatic fascia (fig. 252). This pouch, after the descent
of the testicle into it, becomes shut off from the abdominal cavity, and forms the
cavity of the tunica vaginalis. The descent of the testicle into the scrotum is
intimately connected with changes in the gubernaculum. The gubernaculum
extends from the integument of the groin, which afterwards forms the scrotum,

200

UKOGENITAL SYSTEM

upwards through the abdominal ring to the lower part of the epididymis. Bound
its attachment to the subcutaneous tissue a thickening is formed which includes
muscular fibres from both transversalis and internal oblique muscles. The
formation of this mass seems already initiated even at so early a stage as that

allantois hind-gut

B

bladder septum hind-gut

tail-gut

bladder hind-gut

hind-gut

genital cloacal
papilla plate

anal depression

genital papilla cloacal plate

FlG. 253. — DlAGBAMS REPRESENTING FOUR STAGES IN THE DEVELOPMENT OF THE CLOACA
ENTODERMICA, THE CLOACAL PLATE, AND GENITAL PAPILLA. (T. H. Bryce.)

figured in fig. 247, p. 195. The muscular tissue forming this inguinal cone
occupies the base of the inguinal pouch, and it is by the outward growth of the
mass that the outpushing of the abdominal wall is produced, and the processus
vaginalis is carried down into the scrotum. When the processus vaginalis is
formed, the gubernaculum lies behind the serous sac. The descent of the testicle

THE EXTERNAL ORGANS 201

is accompanied by a shortening of the gubernacular cord, which thus appears
to draw the organ downwards into the scrotum ; the testicle, following the line
originally taken by the gubernacular cord, passes down along the posterior wall
of the processus vaginalis, which it therefore invaginates from behind.

In many animals the testicles remain throughout life in the abdominal cavity. In others
they only descend into the scrotum during the period of ' heat.' Cases of cryptorchismus, in
which one or both testicles have failed to reach the scrotum, and have remained either in the
inguinal canal or within the abdominal cavity, are not unfrequent in the human subject. In
rare cases the ovaries may also pass through the abdominal ring by a passage corresponding
to the processus vaginalis called the canal of Nuck, and may even be found in the labia majora,
where they resemble in position the testicles within the scrotum.

Fate of the cloaca entodermica : Formation of external genital
organs, perineum, and anus (figs. 253, 254, 255). — The cloaca entodermica,
as we have already seen, is a large chamber connected at its oral end with the
allantois and hind-gut, and closed ventrally by the cloacal membrane (fig. 253, A),
which extends from the umbilicus to the root of the tail. The cloacal chamber,
surrounded by an investment of mesoderm, occupies the whole depth and width
of the hinder part of the body- wall. By the increase in the amount of the investing
mesoderm, dorsal to and on each side of the cloaca, the body-wall is caused to
project between the hind limb buds as an elliptical swelling known as the cloacal
tubercle. For some distance behind the umbilicus the mesoderm reaches the mid-
ventral line, and an even salient surface is produced, which ends below in an angular
projection. This afterwards expands into the genital papilla. Behind the angle the
lateral sheets of mesoderm are separated by the cloacal membrane, and produce
surface folds, which bound a shallow cleft. This extends towards the root of the tail,
but is separated from it by a small projection, also caused by a thickening of the
underlying mesoderm (anal tubercle), and by a slight recess behind it which marks the
posterior limit of the cloacal elevation. Meanwhile important changes are taking
place in the form of the cloacal chamber. Owing to the increase of the mesoderm in
the tongue-like projection between the allantois and hind- gut (fig. 253, A) and also
at the sides of the cloaca, the openings of the allantois and hind-gut are apparently
shifted backwards — in other words, a frontal septum takes shape which separates a
dorsal or rectal from a ventral or urogenital tube, and reduces the cloaca to a
narrow passage between the two (fig. 253, B, C). Whether the process thus sketched
involves an actual division of the chamber by lateral folds, or is merely the expres-
sion of differential growth such that the ventral part of the chamber, with the
Wolfnan ducts attached thereto, expands forwards, while the opening of the gut
is shifted backwards to the caudal part of the dorsal wall, is not yet decided. While
this urorectal septum is forming, the lumen of the ventral part of the chamber is
narrowed to a sagittal cleft, and is encroached on by an epithelial mass which
forms a sagittal plate named the cloacal plate.

The cloacal plate (Kloakalplatte, Keibel ; bouchon cloacal, Tourneux ; Uralplatte, Fleisch-
mann) may be looked on as a thickening of the cloacal membrane in the future urogenital
part of the cloacal fossa. According to Disse (1905) the epithelial cells are entodermic in
origin, the plate being formed by the apposition of the walls of the ventral part of the cloacal
chamber. The same view was advanced earlier by Fleischmann (1902-1904). Tourneux,
who was the first to describe the plate (1889), looked upon it as an ectodermic thickening, and
as representing, when afterwards fissured, the rudiment of an ectodermic cloaca. Otis (1906)
inclines to the same opinion.

The cloacal plate extends as a keel-like thickening into the substance of the
cloacal tubercle, and reaches caudally nearly, but not quite, to the anal tubercle,
Here the cloacal membrane remains as a thinner lamella, which closes in the dorsal

202

UROGENITAL SYSTEM

part of the now narrow cloacal chamber, and lies at the bottom of a shallow
transverse depression (ano-perineal area).

Owing to the increase of the mesoderm round the base of the cloacal elevation,
the lips of the outer depression become raised into the outer genital folds. The
increase is, however, greatest at the anterior or cranial lip, and the salient angle
already alluded to grows in a ventral direction to form the genital papilla. The
cloacal plate is necessarily displaced ventrally and rotated until it lies in the position
seen in fig. 253, D, on the caudal face of the genital papilla, as a mesial ridge which
runs, at the apex of the papilla, into an epithelial horn (seen in fig. 254, B)
produced by a thickening of the surface-ectoderm.

— g.p.
i.g.f.

cut.

lab min. -

lab maj. —

hum."

:. H

. 254.— DEVELOPMENT OP THE EXTEENAL OBGANS. A, MALE EMBRYO OF 23 MM. LONG; B, MALE

EMBBYO OP 29 MM. LONG; C, FEMALE FO3TUS OF 7 MM. LONG (ELEVENTH WEEK); D, FEMALE
F03TUS OF 15 CM. LONG (SIXTEENTH WEEK).

gp, genital papilla ; gls, glans ; clit, clitoris ; o.g.f., outer genital folds ; i.g.f., inner genital folds ;
an, anus; sin, urogenital sinus; lab, min.} labia minora; lab. maj., labia majora ; liyrti, hymen;
cc, coccyx; um.c., umbilical cord.

The cloacal plate at the base of the genital papilla now breaks through, and
the urogenital opening is established. By a loosening of the central cells of the
plate in front of the opening a groove is produced, running on to the caudal face
of the genital papilla, and known as the urethral groove. The edges of this groove,
formed by the salient lateral sheets of mesoderm, form the inner genital folds. The
cloacal chamber has meantime been completely divided, and the mesoderm, which
lies between the dorsal and ventral portions of it, is allowed to reach the surface,
and here forms the perineum. A ring-shaped mesodermic thickening at the same
time fosms round the proctodaeal depression. This at first has the form of two
projections from the anal tubercle, which grow forwards round the anal depression,
and gradually convert it from a transverse into a semilunar, then into a circular,

THE EXTERNAL ORGANS; ANUS 203

depression (Otis). In front the thickening so produced joins the mesoderm forming
the perineal bridge, and forms with it the definitive perineal body separating the
anus from the urogenital opening.

In the female the adult arrangement of parts is readily derived from this neutral
condition described above ; the inner genital folds become the Idbia minora, and
the outer the labia majora, while the genital papilla forms the clitoris. The groove
on the base of the papilla remains open and forms the entrance to the vestibule,
which, as already mentioned, is derived from the much shortened urogenital sinus.
Further, it is owing to the shortening of the sinus that the urethra comes to open
separately on the surface.

In the male the inner genital folds meet to form the bulbous urethra, which is
carried forwards on the cloacal aspect of the genital papilla, first as the solid
epithelial ridge already described, then as a groove produced by the shedding of
the central cells of the solid cord. This groove is closed from behind forwards,
becoming the spongy portion of the urethra. The enlarged end of the papilla becomes
the glans penis. In this the urethra is closed independently, so that the last part
of the tube to be completed is at the junction of glans and body. The external
genital folds meet in the mid- ventral line to form the scrotum. The prepuce in both
sexes is developed by the
ingrowth of a solid ridge of

ectoderm (Berry Hart), which, / ' i . i PJprri*"'*\.' 9lans

by separating into two

lamellae, sets free a cutaneous urethra. ./-—

fold as a cap to the glans.

The anal opening* is
formed later than the uro-

genital; while the urogenital

opening is effected in a

15-8 mm. embryo, the anal

opening is still closed in

one of 29 mm. (Keibel).

From the proctodaeal pit the

ectoderm grows inwards for a FIG. 255.— MALE FCETUS 4£ CM. LONG (TENTH WEEK).

short distance and forms the (From KollmannO

short ectodermic portion of

the anal canal. The remainder of that passage is derived from the terminal, last

closed part, of the cloacal chamber, above which the primitive entodermal tube

of the rectum ends in an ampullated dilatation (fig. 236, p. 187).

It appears from the observations of Tourneux, Retterer, Keibel, Fleischmann and his pupils,
Disse, and others, that the development of the cloacal region is essentially the same in all mammals.
Even in Echidna (Keibel) an entodermic cloaca is completely divided, and the rectum and
urogenital sinus open independently into a secondary ectodermic invagination or ectodermic
cloaca, just as they do on the surface in placental mammals in which there is no external cloaca.

Fleischmann ' believes that the cloaca entodermica, or Urodaum, as he terms it after Gadow,
is not divided by two lateral folds, as first suggested by Rathke. It is, perhaps, after the first
frontal fold separates the rectal opening from the part of the chamber receiving the Wolffian
ducts, not further divided at all, but the stage represented in fig. 235, p. 187, may be reached by
differential growth, the urogenital section being pushed in a ventral and cranial direction, while
the rectum retains its primitive position, and comes to open into a small dorsal chamber
of the urodiium, the pars analis. This becomes separated from the ventral chamber to form
the entodermic portion of the anal canal. Wood Jones * and Keith 3 deny that the cloaca has
any share in the formation of the rectum. The idea involved in Wood- Jones's interpretation,

1 Morphol. Jahrbuch, 1904. The reader will find here (p. 58) a lengthy historical resume of the
literature on this subject.

2 Brit. Med. Jour. 1904, p. 1630. 3 Human Embryology and Morphology (2nd ed.), London, 1904.

204

UKOGENITAL SYSTEM

which was, however, put forward before the appearance of Keibel's paper on Echidna, is
that the urogenital sinus represents the cloaca, and the formation of the upper septum is the

FIG. 256. — SECTION THBOUGH AN EMBRYO OF TALPA EUBOP^A. (After^Soulie.)

zw, bud from coelomic epithelium, the rudiment of suprarenal body (cortical portion) ; v.c.p., posterior
cardinal vein ; glm, glomerulus ; w.d., Wolffian duct ; v, vein.

«!il#J

:jll

&%:#^ "

mtt

FIG. 257. — SECTION THROUGH A CHBOMAFFIN-BODY IN A 44 MM. HUMAN EMBRYO, TO SHOW THE
DIFFERENTIATION OF THE PRIMITIVE INDIFFERENT SYMPATHETIC CELLS. (Kohn.)

p, p, mother chromaffin-cells (Phiiochromoblasts) ; sy, sympathetic cells; b, blood-vessel.

expression of the closure of the mouth of the hind-gut into that chamber, the rectum proper being
a sub-caudal extension of the terminal part of the gut, which becomes secondarily connected
with an ectodermic anal invagination.

SUPRARENAL BODIES

205

SUPRARENAL BODIES (ADRENALS;.

The suprarenal bodies consist of two parts of totally different origin. The
cortex is derived from the mesothelium covering the inner aspect of the fore-part of
the Wolffian body, immediately lateral to the attachment of the mesentery and
in front of the germinal epithelium (fig. 256). It appears as a series of buds which
fuse into a cellular mass imbedded in the mesenchyme of the Wolffian ridge.
The cells become arranged in columns, and the three zones characteristic of the
cortex of the adult gland are early to be made out; The whole body consists

sy sy'

*4;

cap. — •*

if

^Jm

;;;,,

m

§?&$v£r

&&•: '. • t '!>

-* •» * *«v «^f3X

XNJ^&

> VjV/-\»

FIG. 258.— TRANSVERSE SECTION OF^THE SUPRARENAL BODY OF A HUMAN EMBRYO OF 15'5 MM.

(T. H. Bryce.)

sy, sy, the abdominal sympathetic ; sy', sy', groups of cells extending from the sympathetic into the
suprarenal ; cap, capsule of the gland ; a, aorta.

at first of cortical tissue (fig. 258), and in the centre the trabeculae are arranged
in an irregular network with vessels (sinusoid in nature) in the meshes, and
opening into a central venule. The appearance is very like that of a liver
lobule. The medulla is derived from the sympathetic, and is produced by an
ingrowth of cell-groups on the mesial aspect of the gland. In an embryo
at the end of the second month the abdominal sympathetic consists of
numerous groups of cells in the neighbourhood of the aorta. As we have seen
{p. 133), these groups of cells are derived from the ectoderm, and have wandered

206 SUPRARENAL BODIES

from the rudiment of the spinal ganglion, or perhaps from the medullary plate, along
with the ventral nerve-roots. The cellular groups in these earlier phases are not
ganglia, but masses of indifferent or mother- cells which differentiate in two directions,
some becoming ganglion cells, others chromophil or chromaffin cells (i.e. cells having
special affinity for salts of chromic acid and staining yellow therewith). Masses of
these indifferent cells invade the substance of the adrenal, undergo differentiation
into chromaffin elements, and collect in the central part of the body to form the
cell-groups of the medulla. The medulla begins to be marked off from the cortex
in the fourth monfh. The adrenal is rounded in early stages and larger than the
kidney (fig. 170). In the third month it becomes flattened and triangular in section
(fig. 222), but remains relatively large all through foetal life.

The development of the adrenals has been the subject of much discussion. Opinion has
been divided both as to the cortical and medullary parts of the glands. All have agreed
that the cortex arises from the mesoderm, but there have been three main views as to the exact
origin of the cells, some observers tracing them to the general mesenchyme, others to the
epithelium of the excretory ducts, and others to the mesothelium. The latest work of Wiesel
and of Soulie, confirming that of Janosik and Inaba, affords very strong support to the view
that cells are budded off from 'the peritoneal epithelium. ! Recent work, more especially that of
Kohn on the chromaffin elements, has thrown much light on the old problem of the medulla,
and it may be now taken as proved that Balfour's view of the origin of the medulla from the
sympathetic is the correct one. The close association with the sympathetic accounts also for the
presence of ganglion cells under the capsule, at the hilum, and in the medulla of the organ.

DEVELOPMENT OF THE VASCULAK SYSTEM.
THE HEART.

In an earlier section the initial phases in the development of the vascular
system have been described. We there saw that the heart was first laid down as
two tubes which unite together to form a single mesial tube. This lies under the
fore-gut in the pericardium, united to its dorsal wall by a double fold named the
dorsal mesocardium. The mesial tube is divided by constrictions into three
portions — the primitive auricle, ventricle, and aortic bulb. Behind it is connected
with the sinus venosus, a transverse commissural vessel in the septum transversum
which receives three pairs of veins — the vitelline, allantoic, and ducts of Cuvier.
In front it is continued into a ventral stem — the truncus arteriosus — which divides
into two vessels. These, looping round the fore-gut, are continued backwards on
each side of the middle line as the primitive aortse.

The simple heart-tube soon becomes bent on itself. This bending is due to its
increasing in length disproportionately to the pericardial cavity which encloses it.
The dorsal mesocardium disappears between the primitive auricle and the attach-
ment of the bulb to the floor of the fore-gut. These two parts remain in close
relationship during all succeeding phases, but the free loop, becoming enlarged
and displaced backwards, comes to lie behind the original posterior end of the
tube.

DEVELOPMENT OF THE OUTWAKD FOEM OF THE HEART.

In the following account we shall start with the stage reached in the human
embryo by the fifteenth day (fig. 259) . The auricular portion of the heart-tube lies,
still attached by the mesocardium, immediately in front of the septum transversum.
From this point it is directed forwards and to the left. Reaching the anterior limit
of the pericardium, it turns ventrally to join, by a constricted portion named the
auricular canal, the ventral U-shaped ventricular loop. This consists (1) of a

1 The literature of the development of the adrenals is very completely given in the article by Poll
in Hertwig's Handbuch der Entwickelungslehre, p. 603 seq.

THE VASCULAR SYSTKM

207

descending limb which passes from left to right, from the left dorso-anterior to the
right ventro-posterior extremity of the pericardium ; (2) of a transverse portion
directed dorsally ; and (3) of a smaller ascending limb which, passing forwards,

d an m.p.

FIG. 259. — CONDITION OF THE HEART IN THE HUMAN EMBRYO OP ABOUT FIFTEEN DAYS,
RECONSTRUCTED FROM SERIAL SECTIONS. (His.) 4^>.

A, from before, showing external appearance of heart ; B, the same with the muscular substance of

heart removed showing the endothelial tube ; C, from behind.

mn, mandibular arch with maxillary process ; hy, hyoidean arch ; b.a., bulbus aortee ; v, right
ventricle; v', left ventricle; au, auricular part of heart; c.a., canalis auricularis; sr, horn of sinus
venosus with umbilical vein (u.v.), superior vena cava (V.G.S.), and vitelline vein entering it;
d, diaphragm ; ?n.j0., mesocardium posterius ; Z, liver ; b.d., bile-duct.

gradually narrows into the aortic bulb. This, finally inclining inwards, reaches the
mid-dorsal line at its point of attachment to the floor of the fore-gut. The ascending
limb of the loop is separated from the bulb by a slight constriction (the (return

J.au.

b.a.

FIG. 260. — HEART OF A SOMEWHAT MORE ADVANCED HUMAN EMBRYO. (His.) *-£>.
A, from before ; B, from behind.

r.v., right ventricle; l.v., left ventricle; b.a., bulbus aortse; r.au., right auricle; l.au., left auricle;
v.c.s., vena cava superior; u.v., umbilical vein; v.v., vitelline vein ; d, diaphragm.

Halleri), and although it ultimately becomes a portion of the right ventricle, it is
to be regarded at this stage as the proximal segment of the aortic bulb, and a
distinct chamber of the primitive heart (Greil).1 In the following account we

1 Morphol. Jahrbuch. xxxi. 1903. See also Hochstetter, in Hertwig's Handbuch der Entwickelungs-
lehre, III. Teil II

208

VASCULAE SYSTEM

shall speak of the ascending limb as the bulb, and of the remainder of the tube
as the truncus arteriosus. As the heart-tube increases in length, and the peri-
cardium expands in a backward direction, the sinus venosus is drawn into the
cavity out of the septum transversum, necessarily of course carrying with it a
covering of the connective tissue of the septum. The ducts of Cuvier undergo a
similar apparent anterior displacement, and now run in lateral folds, bounding
the aperture between the pericardium and the remainder of the coelom, and named
the lateral mesocardia (fig. 293, p. 239).

As the ventricular loop increases in size, it is more and more displaced in a
backward direction, so that the auricle and its annex, the sinus venosus, which
now consists of a transverse portion and a larger right and a smaller left horn,
come to lie on the dorsal aspect and ultimately in front of the ventricular
segment (fig. 262). The root of the bulb is thus brought into close relationship

right
auricle

left auricle

left ventricle

.FiG. 261.— RECONSTRUCTION OF THE HEART OF A HUMAN EMBRYO OF 6'8 MM. (After Piper.)
The pericardium has been represented as opened to show the ventral aspect of the heart.

with the auricular canal. As a further result of these changes, and consequent
backward expansion of the pericardium, the ducts of Cuvier take a gradually
increasing antero-posterior inclination on their way to reach the sinus venosus.

Meantime the primitive auricle has thrown out ear-like dilatations on each side,
and the descending limb and transverse portion of the ventricular loop have become
dilated to form the primitive common ventricle. The ascending limb remains at
first more tubular ; but it also soon dilates, and, as a result of the increasing distension
of the whole distal part of the loop, the cleft between its two limbs, representing
a centre, itself stationary, round which growth is proceeding, becomes relatively
shorter, until it disappears at the base of the heart, forming there a fold (bulbo-
auricular fold) of the heart-wall, where the wall of the bulb passes directly
into the wall of the auricular canal (fig. 263). The expansion of the loop
is further accompanied by a rotation round the stationary point, of the
dilating ascending limb and truncus arteriosus towards the front (fig. 261).

HEART

209

On the transverse portion of the loop a sulcus now appears, which corresponds
to the developing interventricular septum within the ventricular chamber.

FIG. 262. — VIEW FROM BEHIND OP THE HEART OF A HUMAN EMBRYO OF ABOUT FOUR

WEEKS, MAGNIFIED. (His.)

The two ducts of Cuvier and the inferior cava are seen opening separately into the sinus, which is
ii transversely elongated sac communicating only by a narrow orifice with the right auricle.

The appearance of this sulcus is the expression of a commencing bilateral
•expansion of the common ventricle. We can therefore now name the

FIG. 263. — DIAGRAMS TO ILLUSTRATE HOW IN THE DISTENSION OF THE VENTRICLES THE ' LESSER
CURVATURE' OF THE HEART-TUBE is REDUCED, AND THE PROXIMAL CHAMBER OF THE AORTIC
BULB (B) LOSES ITS MESIAL WALL. (After Keith.)

a.o., aortic bulb (distal part); au, auricle; B, aortic bulb (proximal part); RV, right ventricle;
LV, left ventricle ; P (in b), pulmonary artery.

lateral dilatations the right and left ventricles. It will be observed that the

right ventricle is formed from the right portion of the transverse section of the

VOL. i. p

212

VASCULAK SYSTEM

through the auricular canal. It becomes distended at a very early stage into two
lateral diverticula, which become the auricular appendages (fig. 265).- On the
outer side of the connecting portion between these appendages (the original
tubular auricle) a fold now appears which indicates the position of the future
primary septum. On the inner aspect another fold is also now seen running over
the roof and connected with the venous valves. It is known as the septum
spurium, and corresponds to a muscular fold which stretches these valves in the
reptilian heart (Kose).-

Immediately to the left of this the primary septum extends inwards from the
dorsal wall (fig. 266); This is the septum superius of His, or septum primum of

left ven. valve

septum primum

right ven. valve • ^

right auricle

left auricle

left
ventricle

FIG. 266. — MODEL OF THE HEAET OF A HUMAN EMBBYO OF 6-8 MM. (After Piper.)

. The primitive auricule has been opened up to show the septum primum and the valves guarding
the mouth of the opening of the sinus venosus. The fold passing on to the roof of the auricle and
connected with the venous valves is the septum spurium.

Born. At first it .forms only an incomplete partition, the two sides of the
common auricle communicating below its free thickened lower border by the
ostium primum (Born) (fig. 265). It soon fuses below with the endocardial
thickening in the auricular canal (see below), and the ostium primum is closed, but
a new opening (ostium secundum) appears near its dorsal attachment (fig. 268).
Meantime a second septum (septum secundum, Born) has extended into the
cavity. It is sickle-shaped, and the horns of its free semilunar border fuse with
the ventral attachments of the septum primum. It overlaps the primary
septum and the opening between the two is the foramen ovale. The aperture
is bounded by the free border of the secondary septum, and the free portion of the

HEART.

213

primary septum forms a valvular flap which closes the foramen from the side
of the left auricle. In foetal life blood passes from the right to the left auricle
through the foramen ovale ; but at birth, with the establishment of respiration,
the valvular septum primum unites with the septum secundum and the parti-
tional wall is complete. The annulus ovalis represents the edge of the septum
secundum.

By the end of the first month the left venous valve and the septum secundum
seem to have united with one another to obliterate a space seen in earlier stages
between the left venous valve and the
septum of the auricles. Further the
right venous valve is continuous with
the posterior horn of the septum as it
passes to join the septum primum at
the base of the posterior endocardial
cushion. The sinus venosus thus
comes to lie partly in the septum
secundum as it passes forward to
open into the auricle (Low).1 The
slit-like opening of the sinus venosus
meanwhile opens out, and the cavity
o,f the sinus is completely taken into
that of the auricle, the junction of
the two chambers being indicated
merely by the sulcus terminates of His.
The greater part of the right venous
valve is converted into the Eustachian
valve, but its lower edge becomes the
valve of Thebesius.

The left auricle receives, at a
stage when the primary septum is
appearing, the common stem of the
right and left pulmonary veins con-
veyed to it in the dorsal mesocar-
dium (figs. 266, 267). In later stages
the mouth of the vessel opens up
like the mouth of the sinus venosus,
and its proximal part is taken into the
auricle as its atrium. This process of
expansion proceeds in some animals
until the union of the two pulmonary
veins is reached, so that they come to
open separately into the chamber ; but
in man it is carried still farther, and

PIG. 267. — SECTION THROUGH THE HEART OF A

RABBIT-EMBRYO. (Bom.)

r.s., l.s., right and left horns of sinus receiving
from above the respective ducts of Cuvier ; r.au.,
l.au., right and left auricles ; r.v.t Lv., right and
left parts of the ventricle ; r.v.v., l.v.v., right and
left valves guarding the orifice from the right horn
of the sinus into the right auricle ; au.v.c., one of
the two endocardial cushions which are beginning
to subdivide the common auriculo-ventricular aper-
ture. The dotted line encloses the extent of the
endocardial thickening. The small oval detached
area of endocardial thickening in the right ventricle,
and the swelling opposite it on the septum inferius,
belong to the proximal chamber of the aortic bulb,
and will afterwards unite to separate the conus of the
right from the aortic vestibule of the left ventricle,
s', septum primum growing down between the
auricles and prolonged below by a thickening of
endocardium. Close to this septum in the left
auricle is seen the opening of the pulmonary vein ;
s.m/., inferior septum of the ventricles.

extends to the junction of the two

main tributaries of each vein, in consequence of which the auricle comes to

have four separate pulmonary veins opening into it.

Auricular canal. — By the overlapping of the expanding auricular and
ventricular chambers the auricular canal is telescoped within the cavity of the
ventricle. Its lumen becomes slit-like, and endocardial cushions develop on its
dorsal and ventral walls. These fuse with one another to divide the canal into two
passages, which become the auriculo-ventricular openings. When the auricular

Proc. Anat. and Anth. Soc. Univ. of Aberdeen, 1900-1902.

214

VASCULAE SYSTEM

septum joins the fused cushions these openings lead from the two auricles, but
at first they both communicate with the left side of the ventricular chamber.
Tn consequence, however, of the rotation above described, the canal is moved
towards the right ; and when the septum of the ventricles, presently to be described

FIG. 268. — SECTION OF THE TBUNK OF A HUMAN EMBBYO OF 15'5 MM. (T. H. Bryce.)

The section cuts the pericardium, heart, and lungs. Below the neural canal is the centrum of a
vertebra, which is connected with the neural arch on either side, and with a pair of ribs. To the inside
of the cartilage of the neural arch are seen the spinal ganglia ; outside are the myotomes separated into
dorsal and ventral portions by the dorsal branches of the spinal nerves. Below the vertebra is the aorta
giving off a pair of segmental arteries. On each side of the aorta the cardinal veins, and farther out
the sympathetic cords. Below the aorta the oesophagus and trachea ; on each side the lungs ; the
letters rl and vl are placed in the pleural sacs ; notice how they are separated from the ribs by a thick
lamella of very open connective tissue. The pleural spaces are separated from the pericardium P by
the pleuro-pericardial membranes. The heart is attached to the mesenteric septum by themesocardium.
Attached to this on the left side by a fold is the now much reduced left duct of Cuvier. r.v. right,
Lv. left ventricle ; r.a. right, La. left auricle ; s.p., septum primum ; the arrow marks the ostium
secundum ; r.v.v., right venous valve. Above it is the opening of the sinus venosus, closed on the left
by the left venous valve.

also unites with the fused endocardial cushions, they come to be connected each
with its proper ventricle. The formation of the auriculo-ventricular valves will be
considered in the following paragraph.

Ventricular chamber. — The common ventricular chamber is formed, as
already stated from the descending limb and transverse portion of the ventricular

HEAKT 215

loop, while the ascending limb represents a proximal chamber of the aortic bulb
afterwards taken into the right ventricle. The mesial sulcus seen on the outer
aspect of the heart, representing a fold in its tubular wall, has corresponding to it,
within the cavity, a proje?tion which is the rudiment of the ventricular septum
(septum inferius of His) (fig. 267). As the ventricular dilatations increase in size, this
becomes progressively higher, its upper edge always maintaining the same relative
position to a projection into the cavity from above, which corresponds to the fold
between the two limbs of the primary loop, and which gradually shortens, as the
chambers expand, until it is reduced to a slight crescentic ridge corresponding
to the outer bulbo-auricular fold. By the shortening of the fold separating the
two limbs of the ventricular loop, the ascending limb or bulb loses, as it were,
its mesial wall and its independence as a separate chamber of the heart (fig. 263).
The septum is obliquely placed and is semilunar in shape. The dorsal horn runs
on to the dorsal wall of the auricular canal (fig. 268), while the ventral horn passes
into a fold continuous with the projection into the ventricle just mentioned.
The free edge is inclined towards the right, and the blood is conducted through the
opening left between the chambers into the aortic bulb.

The space between the endothelial lining and the outer wall of the heart- tube
has meantime become occupied by columns and trabeculae springing from the
outer wall; and the endothelial lining, by the
distension of the endothelial tube, becomes
stretched over and around these so that the
greater part of the cavity becomes occupied
by a spongework of muscular columns, which
persist in the adult heart as the columnae
came*.1 The wall of the auricular canal, at FlG. 269._DlAGBAM SHOWINO THE
first solid, also becomes undermined from the DIVISION OF THE LOWEB PART OF

side of the ventricle, and the inner lamella

comes to hang free into the chamber, but (After Gegenbaur and His.)

connected to its walls by muscular trabeculse. A> undivided truncus arteriosus
The septal flaps of the auriculo- ventricular with four endocardial cushions : B, ad-

-, p -i • , r , n T v i vance of the two lateral cushions result-

valves are formed in part from the endocardial ing in the division of the lumen;

Cushions, which are at first Spongy but later be- c> projection of three endocardial
1 1.1 • i a £ cushions in each part ; D, the separation

come membranous, and the marginal flaps from into aorta and ppuim0nary trunks com-
the undermined wall of the auricular canal. The pieted with three cushions in each,
muscular, trabeculse which connect the inner

tube with the ventricular wall become the musculi papillares and chordae tendineae,
the muscles being the basal portions of trabeculae which remain muscular, while
the chords represent strands which have been converted into connective tissue.
The ascending limb of the ventricular loop, at the stage in which it is still more
or less tubular, has a uniformly thick lining. Two endocardial swellings are
formed, one on the dorsal and one on the ventral wall, and then by a process of
undermining, similar to that described for the auricular canal, the chamber is
taken into the right ventricle. The endocardial cushions remain, however, as
projections into the cavity (fig. 267). The fate of these, and the manner in which
final closure of the interventricular opening takes place, will be considered after
the division of^the^bulbjhas been described.

Distal part of aortic bulb. — The truncus arteriosus is subdivided into
two vessels, the^ventral aorta and the pulmonary artery? The division of the
lumen is first effected by two longitudinal endocardial thickenings which fuse with

1 This process is'interpreted rather differently by Lewis (Anat. Anzeiger, xxv. 1904). The spaces
between the trabeculse are considered as equivalent to sinusoids (Minot) produced by the breaking up
of the original endothelial tube by the growing trabeculse.

2 16

VASCULAE SYSTEM

one another. Between these primary thickenings there are two smaller ridges, so
that the cavity is divided into two triangular passages (fig. 269). The main folds
run in a spiral direction from a point on the ventral aorta between the last two
aortic arches to the base of the truncus, and as they lie dorso-ventral in front, and
right and left behind, it follows that when the vessels are separated from one
another they are placed dorso-ventral at their proximal, and right and left at their
distal ends. The actual cleavage of the truncus arteriosus is brought about by two
folds of the connective-tissue wall which correspond to the primary fused endocardial
ridges. These endocardial ridges are prolonged into the part of the right ventricle

uJu ^

VLV.

cat..

FIG. 270. — PROFILE VIEW OF A HUMAN EMBRYO OF ABOUT FIFTEEN DAYS, WITH THE
ALIMENTARY CANAL IN LONGITUDINAL SECTION. (His.)

Two arterial arches are formed at this stage.

FIG. 271. — SIMILAR VIEW OF A SOMEWHAT OLDER EMBRYO, ^SHOWING FIVE ARTERIAL ARCHES.

1, 2, 3, 4, 5, are opposite the respective secondary cerebral vesicles; from the side of the fore-brain
the primary optic vesicle is seen projecting ; ot, otic vesicle, still open in fig. 270 ; p.v., septum between
mouth and pharynx (primitive velum). This has disappeared in fig. 271 ; I, commencing liver in septum
transversum ; v, vitelline stalk; all, allantois enclosed within stalk ; j.v., jugular vein; c.v., cardinal
vein; s.r., sinus venosus within septum transversum; u.a., left umbilical (allantoic) artery ; l.u.v., left
umbilical vein ; end, endothelial tube of heart. The sharp curve of the trunk of the embryo towards
the yolk-sac is normal at this period of development.

formed from the ascending limb of the ventricular loop (proximal chamber of aortic
bulb), and are continuous with the endocardial cushions proper to it. As these
also unite with one another, the aortic septum becomes extended into the right
ventricle to divide its distal portion (aortic bulb) into two passages, pulmonary
and aortic. These become the conus of the right, and the aortic vestibule of
the left ventricle respectively (fig. 263). The semilunar valves are formed by a
hollowing of the endocardial cushion from the distal side at the mouths of the aorta
and pulmonary artery (fig. 269).

The inter ventricular foramen, however, still persists. It is now closed by
a somewhat complicated process. We left the interventricular septum at a

DEVELOPMENT OF ARTERIES 217

stage when its obliquely directed upper free edge bounded a relatively large
interventricular foramen. This becomes constricted by the fusion of the dorsal horn
of the septum with the already fused endocardial cushions of the auricular canal.
This fusion takes place nearer the right than the left edge of the cushions (fig. 268),
so that the septum is placed nearer the right than the left auriculo- ventricular
opening. The two ventricles properly so-called are now completely separated, but
a foramen still connects the left chamber with the distal part of the right ventricle
or proximal chamber of the aortic bulb, meanwhile divided by the fusion of its two
endocardial ridges with one another, and with the aortic septum proper. The
septum of the bulb now unites with the remaining free edge of the interventricular
septum, the pars membranacea septi is formed, and the two sides of the heart are
finally entirely isolated.

DEVELOPMENT OF THE ARTERIES.

The earliest stages in the development of the vessels have been already described
(p. 63). We shall here begin with a stage reached in the human embryo about
the fifteenth day.

Dorsal aorta (fig. 270). — The two dorsal aortse of the early stages come
together in the middle of their course, and fuse into a single mesial vessel. The fusion
proceeds forwards and backwards, but at the head end the primitive arteries remain
separate, one on either side, on the dorsal aspect of the pharynx. Behind, the
vessel divides into the allantoic arteries, which are continued into the body-stalk.
These vessels correspond to branches of the aorta which vascularise the allantois in
lower Amniota, and which appear at a later stage after the yolk-circulation has
been established.

Aortic arches. — By the term aortic arches we understand a series of vascular
loops which are formed in the branchial arches, and join the ventral aortse to the
dorsal aortse round the walls of the pharynx. They appear in succession from
before backwards, one in each branchial arch. On the thirteenth day the primitive
mandibular loop is alone present ; by the fifteenth day a second arch is completed
in the hyoid arch (fig. 270) ; and by the eighteenth day three others have been
added (fig. 271). The first three, along with the ventral and dorsal vessels, by a
series of changes presently to be described, form the carotid system of arteries, the
fourth becomes the systemic arch, and the last the pulmonary arch. Between the
systemic and pulmonary arches a vessel appears at a rather later stage ; this has
been regarded as a rudimentary fifth arch corresponding to the fifth arch of lower
forms. It soon disappears, and takes no share in the formation of any adult
vessels (Zimmermann and Tandler).1 If this be so, the pulmonary arch must be
the sixth of the series.

The observations on the fifth arch in mammalian embryos are not quite in complete accord,
but the variations in development of the vessel, interpreted in this sense, may perhaps be due to
its transitory and vestigial character ; and in view of the conditions prevailing in lower forms
the conclusion that there is a rudimentary fifth arch in mammals also, in front of the
pulmonary, seems justified on the evidence afforded by recent research. -

In the majority of cases the anterior arches have already become incomplete before the
last has been formed, but in the human embryo the series (with the exception of the
rudimentary fifth arch) is complete for a time.

1 Tandler, Morph. Jahrb. xxx.

2 Zimmermann (Verb. Anat. Ges. 1887) first showed the existence of a vascular channel between
the aortic and pulmonary arches in the human embryo. It took the form of a vessel springing from
the systemic arch and joining it again. Tandler (loc. cit.) described a more complete arch (figs. 272,
278) passing from the systemic to the pulmonary arch. For detail as to the lower mammals, see
Zimmermann, Anat. Anzeiger, iv. 1889 ; Lehmann, ibid. xxvi. 1905, and Zool. Jahrb. xxii. ; Locy,
Anat. Anzeiger, xxix. 1906 ; Lewis, ibid, xxviii. 1906. The last named holds it doubtful whether the
irregular channels described as fifth arches are really of that nature.

•218

VASCULAK SYSTEM

At the stage when there are five complete arches (fig. 274) the loops arise in a
radiating fashion from the truncus arteriosus, the two anterior arches being
connected with an ascending, the remaining arches with a descending trunk. As

FIG. 272. — THE AORTIC ARCHES IN A HUMAN EMBRYO OP 5 MM. LONG. (After Tandler.)

The arches are represented in their relations to the visceral pouches of the pharynx. I to VI aortic
arches T.A., truncus arteriosus ; ao, dorsal aorta ; tr, trachea; ce, oesophagus in, island.

the heart is gradually displaced backwards, the truncus arteriosus comes to lie
first opposite the third and then opposite the fourth arch, so that all save the
pulmonary now spring from the ascending limb or, as it may be termed, the

int. car.

tr <x ao.
FIG. 273.— THE AORTIC ARCHES IN A HUMAN EMBRYO OF 9 MM. LONG. (After Tandler.)

I to VI aortic arches; T.A., truncus arteriosus; v.a., ventral aorta; T.P., truncus pulmonalis
p.a., pulmonary artery ; int.car., internal carotid artery. Other lettering as in fig. 272.

ventral aorta. At the stage when the truncus lies opposite the fourth arch the
aortic septum begins to develop in its lumen backwards from the interval
between the systemic and pulmonary arches ; when it has completed the division
of the trunk into the aorta and pulmonary artery the aorta remains connected

ARTERIES

219

with the four anterior pairs of arches and the pulmonary artery with the pulmonary
pair alone.

The first and second arches early become interrupted in their course, but the
dorsal part of the second, and possibly also of the first, persist and take part in the
formation of an embryonic vessel called the stapedial artery (see below). The third
arch remains complete, and is for a time the largest of the series. The dorsal aorta
between the third and fourth arches
next becomes obliterated, and the
carotid system of vessels begins to
take shape. The ventral aorta in
front of the third arch, now discon-
nected from the dorsal aorta, becomes
the external carotid ; the third arch
and the dorsal aorta constitute the
internal carotid ; and the ventral aorta
behind the third arch is the common
carotid. This is at first very short, but
when the heart is displaced back-
wards, and the neck is formed it
becomes much drawn out, and the
internal carotid assumes a directly
ascending course by the straightening
out of the third arch.

The fourth arches persist on both
sides, but the left early assumes larger
proportions. When, somewhat later,
the dorsal aorta on the right side is
obliterated between the fourth arch
and the point of fusion of the two
dorsal aortsR into the descending aorta,
the left fourth arch alone remains in
connexion with that vessel and forms
the aortic arch (in the strict morpho-
logical sense of that term) ; while the
right fourth arch becomes the first part
of the subclavian artery. At an early
stage, when the truncus arteriosus has
been displaced backwards to the level
of the fourth arches, the proximal
segment of the ventral aorta forms a
common stem for the fourth and third
arches. On the right side this becomes
the innominate artery ; but on the" left
side, though the arrangement is at first
symmetrical, the common stem con-
tracts as the fourth arch enlarges,
and is only represented by the portion

of the adult arch between the origins of the innominate and left common
carotid arteries. The pulmonary arch on each side at an early stage gives off
a branch which runs along the developing lung-rudiment of its own side. On
the right side, the portion of the arch distal to this disappears at the same
time as the section of the dorsal aortse with which it was at first connected. On
the left side, however, it persists as the ductus arteriosus connecting the pulmonary

FIG. 274. — PROFILE VIEW OF A HUMAN EMBBYO OF

ABOUT THREE WEEKS, SHOWING ALL THE
CEPHALIC VISCERAL ARCHES AND CLEFTS.

mx, maxillary process ; mn, mandibular arch ;
d.C., duct of Cuvier; j.v., jugular vein; c.v., car-
dinal vein ; v .v.} vitelline vein ; u.v., umbilical vein ;
u.a., umbilical artery ; all, allantois ; pi, placenta!
attachment of allantoic stalk ; olf, olfactory depres-
sion ; ot, otic vesicle.

FIG. 275. — PROFILE VIEW OF A HUMAN EMBRYO OF ABOUT THREE OR FOUR WEEKS, SHOWING THE
PRINCIPAL ARTERIES AND VEINS. (His.)

1, 2, 8, 4, 5, the secondary cerebral vesicles ; hyp, hypophysis ; ot, otic vesicle ; mn, mandibular
arch ; Ig, lung-rudiment ; st, stomach ; W.d., Wolman duct opening into cloaca ; I, II, III, IV, V, the
arterial arches springing from b.a., bulbus arteriosus; p, pulmonary artery; d.C., duct of Cuvier ;
ch, notochord.

FIG. 276. — DIAGRAM TO SHOW THE DESTINATION OF THE ARTERIAL ARCHES IN MAN AND MAMMALS.

(Modified from Rathke.)

The truncus arteriosus and five arterial arches springing from it are represented in outline only,
the permanent vessels in colours — those belonging to the aortic system red, to the pulmonary system
blue. This diagram is retained here in order to present the traditional account of the development
of five aortic arches handed down from Kathke. The recent views are presented in fig. 277.

AETEEIES

221

artery with the aorta. After birth this is obliterated and forms the ligamentum
arteriosum.

The inferior laryngeal nerves at an early stage reach their destination by passing
behind the pulmonary arches. When the heart is displaced backwards and the
systemic and pulmonary arches are carried with it, the points of origin of the nerves
from the vagus trunks are necessarily also drawn backwards, and they assume their
recurrent course. On the left side the primitive relationship to the pulmonary arch
is maintained — i.e. the nerve loops round the obliterated ductus arteriosus ; but
on the right side, on account of the disappearance of this part of the arch, it comes

post, cerebral a.
I

ant. cerebral a.

supra-orbital br.
of stapedial a.

II. div.Vth n.
infra-orbital a.

stapedial a.

III. div. Vtt n
inf. dental a.

int. carotid a.

aortic arch

pulmonary
arch

pulmonary artery

dorsal aorta

FIG. 277. — DIAGRAM TO ILLUSTRATE THE FATE OF THE AORTIC ARCHES AND THE ORIGIN OF THE

MAIN BRANCHES OF THE CAROTID SYSTEM OF ARTERIES. (Founded On Tatldler.)

to loop round the persistent fourth arch — i.e. the first portion of the subclavian
artery.

Carotid system (fig. 277). — The dorsal aortce, now the internal carotids, are
seen at an early stage to be continued from the dorsal roots of the first arches
to the developing brain. Each divides into an anterior and a posterior branch.
The anterior stem ends at first in the mesial nasal process and afterwards in
the septum nasi. This terminal branch is, however, afterwards obliterated, and
the stem ends in the ophthalmic to the developing eye (the future central artery
of the retina), the anterior cerebral, and middle cerebral. The posterior branch

222

VASCULAE SYSTEM

sweeps backwards, gives branches to the brain, afterwards concentrated chiefly
in the posterior cerebral, and joins the cerebral portion of the vertebral (see below)
to form the circle of Willis.

The ventral aortce, now the external carotids, extend forwards into the mandibular
arches. In its course each gives off as secondary branches the superior" thyroid,
lingual, and facial, and then, turning backwards to reach the proximal end^of
Meckel's cartilage, it turns inwards and forwards on its lateral aspect to be joined
to the mandibular branch of the stapedial artery by an anastomosing branch.

The stapedial artery, though only an embryonic vessel in man and some other
mammals, persists in others, such for instance as the rat. It arises in that

XI

XII

t.a. t.p.

FIG. 278. — DIAGRAM REPRESENTING THE NERVES AND ARTERIES OF THE HEAD OP AN EMBRYO or THE

, FIFTH WEEK, FOUNDED ON RECONSTRUCTIONS BY TANDLER, MALL, AND SlREETER. (T. H. Bryce.)

I to XII, cerebral nerves ; 1, 2, 3, three branches of fifth nerve ; the sixth nerve is not labelled — it
is seen passing forwards below 3.

The truncus arteriosus, T.A., is continued forward as the common carotid, which divides into external
and internal carotids. The latter arching forward between the vagus (X) and glossopharyngeal (IX) nerves
passes mesial to the nerve-roots to the brain, and gives off two terminal branches — an anterior which
supplies the posterior middle and anterior cerebral arteries, and a posterior which joins the vertebral,
V.A., to form the primitive circle of Willis. Below the auditory vesicle av., the stapedial artery is seen
passing through the annulus stapedialis and supplying supra-orbital, infra-orbital, and mandibular
branches accompanying the three divisions of the fifth nerve.

animal (Tandler) from the dorsal persisting portion of the second arch, and is
continued forwards as a longitudinal anastomosis between the second and first
arches. In man its origin and course is the same (figs. 277, 278), but the share
taken by the first arch is not certainly known (Tandler). At its origin it passes
through the rudiment of the stapes, and later between the limbs of the ossicle.
The artery gives off two trunks — a superior and an inferior. The superior passes
forward on the mesial aspect of the Gasserian ganglion to the roof of the orbit
(supra-orbital) ; the inferior divides below the Gasserian ganglion into an infra-
orbital branch which runs forwards mesial to the mandibular root of the ganglion,
and a mandibular which accompanies the mandibular nerve (fig. 278). The

ARTERIES 223

mandibular branch becomes joined by an anastomosis with the external carotid;
and later, when the stapedial artery proper becomes obliterated, this anastomosing
vessel becomes the internal maxillary artery ; thus the original branches of the
stapedial become branches of that artery (fig. 277). The descending trunk, which
passes between the roots of the auriculo-temporal nerve, becomes the middle
meningeal and the inferior dental. The infra-orbital branch passes at first mesial
to the mandibular nerve ; but a vascular ring forms, in the generality of cases, on
its outer side, and the deeper vessel becomes obliterated. The occasional orbital
branch of the middle meningeal represents the original forward continuation of
the stapedial artery to the orbit.

Sag-mental vessels ; vertebral and subclavian arteries. — A series of
segmental branches arise from the dorsal aortse, and also from the common aortic
stem. The first of these accompanies the hypoglossal nerve to the side of the brain
(hypoglossal artery], and from this a vessel (cerebral part of vertebral artery) extends
forwards to join the posterior branch of the internal carotid (circle of Willis).
The two vertebrals fuse below the hind-brain to form the basilar artery.
According to de Vriese, the two vessels become connected by a network, and out
of this a new mesial channel is developed. The next seven segmental arteries
and the hypoglossal artery are joined by an anastomosing vessel ; when the
heart and aortic arches are displaced backwards the arteries are obliterated,
but the anastomosing vessel persists as the cervical part of the vertebral
(fig. 278). From the seventh cervical segmental artery the subclavian arises as a
lateral branch and passes into the limb-bud. As the limbs increase in size the
subclavians become larger than their parent vessels ; the vertebral arteries
therefore now appear as branches of the subclavian stems. The upper thoracic
segmental arteries likewise become obliterated, and an anastomosis developed
between them becomes the superior intercostal. The remaining thoracic and
lumbar segmental vessels become the intercostal and lumbar arteries.

Vitelline (omphalo-mesenteric) arteries. — The aorta at an early stage
supplies a number of arteries segmentally arranged to the yolk-sac (Mall ; Tandler x),
but ultimately only one pair persists, The two arteries pass out on each side of
the intestine to the yolk-sac. Later, when the embryo is cut off from the yolk-sac,
only a single stem is found running to the umbilicus in the mesentery of the
vitelline loop. ' Reaching the extremity of the loop, it divides into two branches
which encircle the intestine, uniting again into a single trunk at the attachment
of the yolk-stalk' (Bonnot and Seevers2). This arterial ring suggests that the
two arteries have fused with one another except where they surround the in-
testine, rather than that one of the pair has atrophied, as has usually been
stated. In either case, one side of the ring disappears and a single vitelline
artery is left. When the yolk-circulation has been obliterated, the whole of
the artery distal to the intestine disappears and the remainder persists as
the superior mesenteric artery. The omphalo-mesenteric or superior mesen-
teric artery has thus at first several roots. These are united by a longi-
tudinal anastomosis, which persists as the anterior roots are one by one lost. The
coeliac artery represents one of the primary roots of the omphalo-mesenteric
(Tandler). The remaining visceral arteries arise as secondary branches from the
aorta.

The allantoic arteries run at first, closely applied to the intestine and mesial
to the Wolffian ducts, to the stalks of the allantoic diverticulum. Later another
channel is formed on each side external to the duct (secondary caudal arch
of Young and Robinson), and the mesial vessels disappear. From the ne\f

1 Mall, Journ. of Morph. xii. ; Tandler, Anat. Hefte, xxii. and xxv.
a Bonnot and Seevers, Anat. Anzeiger, xxix. 1906.

224 VASCULAR SYSTEM

vessels the arteries to the posterior extremities and the vessels to the pelvic viscera
arise.

Arteries of the limbs. — The limb -arteries are first laid down in the limb-bud
in the form of a plexus of capillary loops (see fig. 223, p. 178). This plexus arises
probably not from a single vessel, but from several representing a segmental
series. E. Miiller has described a plexus in the developing upper limb related to
the nerves of the brachial plexus which he regards as being suggestive of a primitive
segmental arrangement, but he did not observe a stage in which there was more
than one subclavian artery. ] As the nerves extend into the growing limb-buds, the
capillary loops follow mainly the course of the nerves (De Vriese, E. Miiller2),
and the definitive arteries are formed by the enlargement of certain of these loops.
The main arterial stems are at first central, but later the lateral branches assume
larger proportions, and the primitive arrangement is lost. In the arm the primary
brachial stem is continued to the forearm and hand, as a mesial vessel (in part
the future anterior interosseous) which pierces the carpus and extends to the
dorsal aspect of the hand. This artery soon recedes in importance, and the loop
accompanying the median nerve becomes the main artery of the forearm. This
in turn becomes a secondary stem owing to the enlargement of lateral loops which
become the ulnar and radial arteries. The original artery of the lower limb
accompanies the sciatic nerve, but a new vessel represented by the external iliac
and the femoral appears later. This anastomoses above the knee with the sciatic
artery, and then becomes the main artery of the limb, the original vessel becoming
reduced to the arteria comes nervi ischiatici. The primary stem is continued
to the leg and foot as an interosseous artery, which, like that of the arm, pierces
the tarsus and reaches the dorsum of the foot. It is represented in part by the
peroneal artery. The anterior and posterior tibial arteries are secondary loops enlarged
to form the main arteries.

DEVELOPMENT OF THE PRINCIPAL VEINS.

At the fifteenth day, as we have already seen (p. 63), the ductus venosus receives
three veins on each side, the vitelline from the yolk-sac, the umbilical (allantoic)
from the body-stalk, and the ducts of Cuvier, formed by the union of the anterior
and posterior cardinal veins .

Vitelline veins. — The vitelline or omphalo-mesenteric veins enter the abdo-
men along the vitelline duct and ascend at first along the front of the alimentary
canal, but higher up they come to lie on either side of that tube (duodenum).
Here transverse communications form between the two veins, two in front of, and
one behind the duodenum, so that the gut is encircled by two vascular rings
(figs. 279, 280). Above these venous circles the direct communication with the sinus
becomes lost, the intermediate venous vessel on either side becoming broken up
within the substance of the liver (which has by this time developed around them)
into a vascular network. The vascular network is produced by the breaking up
of the large vessels by the hepatic epithelial cylinders which grow into and cut
up the lumen into a network of capillary-like spaces (sinusoids, Minot).

The vessels which pass from the upper venous ring to the liver sinusoids are
known as vence advehentes : they become the branches of the portal vein ; those
which pass from it into the sinus are the vence revehentes : they become the hepatic
veins.

1 That the limb- arteries are in the first instance segmentally arranged was on theoretical grounds
suggested many years ago by Macalister (Journ. of Anat. and Phys. xx. 1886) and by Mackay
(Memoirs and Memoranda in Anatomy, Cleland, i. 1889). Hans Rabl has recently brought forward
some objective evidence in favour of this hypothesis (Arch. f. mikr. Anat. Ixix. 1907).

a De Vriese, Bertha, Arch, de Biologic, xviii. xxi. ; E. Miiller, Anat. Hefte, xxii. xxvii. ; see also
Gb'ppert, Ergebnisse der Anat. und Entwickelungsgeschichte, xiv. 1905.

DEVELOPMENT OF THE VEINS

225

The lower communication between the vitelline veins takes the form of a
complete longitudinal fusion of the two vessels, at least for some distance. This
fused part receives veins from the intestine and stomach, and becomes the

A/ an/if

right horn
sinus veno.tH:

n

£l

^/ II t 1,'ft /Iff

\ \ '( c~vJ '"'""* ""'

~— s^jr~k_ OS/^) 3^X

right umbilical vein

distal venous rii
right vilt

FIG. 279. — THE VEINS OP THE LIVER OP A HUMAN EMBRYO OF 6'9 MM. (After Ingalls.)

"Hint ve;

left litelline vein

anterior detached portions
of umbilical vein*

rente revehentes

stomach

venai adeehentes
pancreas

bile-duct

obliterated portions
of venous rings

right umbilical iv///

ductus venosus
(Arantii)

lifer

left umbilical vein

vitelline reins

duodenum

FIG. 280. — THE LIVER, AND THE VEINS IN CONNECTION WITH IT, OP A HUMAN EMBRYO, TWENTY-FOUR

OR TWENTY-FIVE DAYS OLD, AS SEEN FROM THE VENTRAL SURFACE. (After His.) (Copied from

Milnes Marshall's Embryology.)

commencement of the portal vein. At a later date the vitelline veins distal to this
point are obliterated, and the new veins in the mesentery become the tributaries
of the portal vein of the adult.

VOL. I. Q

226

VASCULAE SYSTEM

The umbilical veins are for a time double within the abdomen, although
they have fused within the umbilical cord into a single trunk. Diverging from this,
they pass to the sinus on either side in the somatopleure, just where this is becoming
bent round into the amnion (fig. 81, p. 56). After a time, however, it is found that
this direct communication with the sinus is partially interrupted by the development
of a vascular network, and that on the left side a fresh communication has become
established with the upper venous circle of the vitelline veins. The interruption
subsequently becomes complete on both sides (fig. 280), and on the right side the
greater part of the vein becomes atrophied (on both sides the part which originally
opened into the sinus venosus remains evident for a time) (fig. 279). The left vein,
on the other hand, increases in bulk with the development of the placental circulation.
For a short time the whole of its blood, as well as that of the vitelline vein, passes
through the capillaries of the liver. But a branch is soon seen passing from the
upper venous circle direct into the right hepatic vein, near its entrance into the
sinus. This forms the ductus venosus (Arantii) or vena ascendens, and it now carries
most of the blood of the umbilical vein direct to the heart. Subsequently the left

hepatic vein loses its direct communica-
tion with the sinus venosus, and comes
to open into the right hepatic where it is
joined by the ductus venosus. The
channel conveying the blood from the
three vessels is called the common hepatic
vein, and this vein becomes later con-
nected with the vessel which gives rise
to the inferior vena cava (see below).

The lower part of the portal vein is
formed, as we have seen, by the united
vitelline veins. The upper part is formed
as a single trunk out of the double venous
annulus by atrophy of the right half of
the lower ring and the left half of the
upper (fig. 280). The spiral turn around
the duodenum is thereby produced, and
thus it is also that the portal vein at
first appears more directly connected with
the right venae advehentes than with the
left. Most of these embryonic veins are
at first of relatively large size and have an irregular sinus-like character (fig. 279),
which disappears at a later stage of development.

Cardinal veins. — On the thirteenth day, two short transverse venous trunks,
the ducts of Cuvier, open, as has been above stated, one on each side, into the sinus
venosus of the heart. Each is formed by the union of a superior and an inferior
vein, named respectively the anterior and the posterior cardinal.

The posterior cardinal veins are the primitive vessels which return the blood
from the Wolffian bodies and body-walls. They receive segmental veins all along
their course, and in the region of the Wolffian body are largely broken up into
sinus-like spaces (sinusoids). Behind they are continued into vessels which after-
wards become the internal iliacs, and these receive, when the limbs develop, the
sciatic and then the external iliac veins.

At first all the blood from the trunk is returned through the cardinals and
ducts of Cuvier, but by a series of changes the cardinal system of veins is tapped
by the hepatic system, and a new channel is opened up, which becomes the inferior
vena cava. In the region of the Wolffian bodv the cardinals receive a series of

FIG. 281. — UNDER-SURFACE OF THE FCETAL

LIVER, WITH ITS GREAT BLOOD-VESSELS, AT
THE FULL PERIOD.

a, the umbilical vein, lying in the umbilical
fissure, and turning to the right side, at the
transverse fissure (o), to join the vena portse (p) ;
d, the ductus venosus, continuing straight on to
join the vena cava inferior (c) ; some branches
of the umbilical vein pass from a into the
substance of the liver ; g, the gall-bladder, cut.

VEINS

227

small veins from the mesentery, which are united with one another across the front
of the aorta. These unite with one another on each side to form a pair of longitudinal
anastomoses (subcardinal veins, Lewis), running parallel to the cardinal veins and
united with^them at both ends as well as by many small veins along their course
(fig. 282). The cross-connexion between the two subcardinal veins next becomes
limited to one large anastomosis below the superior mesenteric artery (fig. 283). The
right subcardinal now acquires a secondary connexion with the hepatic veins on

t;. cartiina/is

liver
v. caca inferior

FIG. 282. — DEVELOPMENT OF THE INFERIOR VENA CAVA IN THE 'RABBIT-EMBRYO (FIRST STAGE).

(After Lewis.)

the dorsal aspect of the liver, and as the blood comes to take this new short cut
to the heart the portion in front of the anastomosis becomes rapidly enlarged.

According to Hochstetter's account ' (with which that of Lewis here followed otherwise agrees
in every essential respect), this connexion is established by a new channel which arises from the
common hepatic (original right vitelline) vein, and first appears as a small vessel which passes
downwards, through a coalomic fold which is named the caval mesentery along the mesial
surface of the Wolffian body, where it forms the vein called by Lewis the right subcardinal.
A similar vessel appears simultaneously on the left side, and the two are connected by the
anastomosis already mentioned.

The right subcardinal, thus enlarged, becomes a portion of the vena cova, while
the disconnected anterior part of the left subcardinal probably persists as the left
suprarenal vein. The portions of both subcardinals behind the cross-anastomosis
diminish in size and disappear as blood-channels ; the corresponding sections
of the two cardinal veins, which are connected with one another across the middle

1 For references, see Hochstetter's article in Hertwig's Handbuch III. Th. ii. and iii. p. 161 seq.
More recent papers are Lewis (rabbit), Amer. Jour, of Anat. i. ; Dexter (cat), ibid. i. ; Miller (bird),
ihid. ii. ; Bonne (rabbit and sheep), Jour. d'Anat. et 'de la Phys. xxxix. 1904 ; Soulie et Bonne (mole),
ibid. xli. 1906.

Q2

228 VASCULAE SYSTEM

line through the pre-aortic subcardinal anastomosis, on the other hand, enlarge.
As the permanent kidneys assume their definitive position a series of changes takes
place, which need not here be entered on, until ultimately the renal veins are found
opening into the cardinals at the level of the cross-anastomosis, and close to them
the veins from the sex glands, spermatic or ovarian. The cardinals are at first
symmetrical, but the sections behind the anastomosis are now larger than the
sections in front of it.

It will be convenient to trace the fate of these portions separately. A series
of post-aortic anastomoses is formed between the posterior and Wolffian sections
of the veins. One of these (transverse iliac vein) enlarges, and now all the blood
from the pelvis and limbs passes into the right cardinal, which accordingly increases
in size and becomes the posterior part of the vena cava inferior, while the left

v. cardinalis anterior section

v. cava inferior

v. subcardinalis anterior section

^_ __ v. cardinalis

v. subcardinalis 7JL ^ ^f ^^^ posterior section

posterior section

FIG. '283. — DEVELOPMENT OP THE INFEBIOB VENA CAVA IN THE BABBIT-EMBRYO (SECOND STAGE).

(After Lewi?.)

diminishes and ultimately forms the small ascending lumbar vein (fig. 284).
The transverse iliac vein becomes the terminal portion of the left common iliac
of the adult, and the other anastomoses carry the lumbar veins over the vertebral
column to the right cardinal, now the vena cava. The inferior vena cava is thus a
composite vessel formed from (1) the common hepatic vein ; (2) branches of the hepatic
in the liver ; (3) right sub-cardinal; (4) lower part of the right cardinal.1

The upper portions of the posterior cardinals become the azygos veins. The
right remains complete as the vena azygos major ; the left is interrupted by the
development of one or more post-aortic anastomoses. The lower portion becomes

1 In a preliminary note, which has appeared since the above was written, Huntington and McClure
(Amer. Jour, of Anat. vi.) describe the development of the post-cava in the cat rather differently.
According to them, a pair of veins develops dorso-median to the primitive posterior cardinal by longi-
tudinal anastomosis between somatic post-cardinal tributaries. These form the post-renal parts of the
post-cava by a secondary median fusion by means of post-aortic anastomoses, and also become the azygos
veins, replacing the primitive cardinals. Only in its pre-renal section has the primitive cardinal any part
in forming the vena cava inferior.

VEINS

229

the vena azygos minor, opening through an anastomosis into the vena azygos
major. The upper part becomes the hemi-azygos superior.

The anterior cardinal veins return the blood in the first instance from the
developing brain. The primitive vein passes in the early stages mesial to the
auditory vesicle and the roots of the cerebral nerves (fig. 286). Towards the end of
the first month it becomes shifted to their lateral
aspect by the formation of vascular rings round
them, which are joined up on the outer side to
form a new vein (vena capitis lateralis) (fig. 285).
The original vein becomes the internal jugular
behind the last cerebral nerve-roots. In front the
channel where it lies mesial to the Gasserian
ganglion persists as the cavernous sinus ; between
the fifth and twelfth nerves it is apparently
obliterated, but is re-established at a later date
as the inferior petrosal sinus. The vena capitis
lateralis thus extends from the anterior end of
the jugular vein, lateral to the nerve-roots and
auditory vesicle to the fifth nerve. Near its
posterior extremity it is joined by a branch
accompanying the vagus (vena cerebralis posterior)
and at its anterior extremity by a branch emerging
between the fifth and seventh nerves (vena
cerebralis media) (fig. 287). The first collects the
blood from the hind-brain, the second from the
region of the cerebellum. The anterior cardinal
lying mesial to the trigeminal ganglion (cavernous
sinus) receives a vein from the eye (ophthalmic),
and is continued forwards as a vein (vena cerebralis
anterior) which collects the blood from the cerebral
hemisphere rudiment. Its terminal branch encircles
the hemisphere and becomes the superior longitu-
dinal vein. This unites with its fellow of the
opposite side to form the superior longitudinal
sinus, which early shows a dilatation marking its
posterior end, the torcular HerophUi. All the blood
from the fore-part of the brain is at first returned
through the anterior cerebral vein, the persistent
anterior part of the cardinal (cavernous sinus),
and the lateral vein, into the jugular (fig. 287) ;
but as the hemisphere expands the connexion with
the anterior cerebral is lost, and a new anasto-
mosis is formed uniting the longitudinal sinus
with the middle cerebral vein, and then with the
posterior cerebral vein (fig. 288). The vena lateralis
meantime becomes interrupted and finally oblite-
rated, so that all the blood passes through the
anastomosis and posterior cerebral into the
internal jugular. The new channel is the lateral sinus (fig. 289), and the middle
cerebral, which now conveys the blood from the cavernous sinus to the lateral
sinus, is the superior petrosal sinus. Any further Change is only one of position,
due to the backward extension of the hemisphere.

FlG. 284,-^SCHEME OF THE DEVE-
LOPMENT OF THE CHIEF VEINS

OF THE BODY. (G. D. Thane.)

The primitive venous trunks are
indicated by black outlines, and their
names are enclosed within paren-
theses. The definitive veins jare
represented blue.

230

VASCULAR SYSTEM

FIG. 285.— KECONSTBUCTION OF THE HEAD OF A HUMAN EMBBYO OF 9 MM. LONG. (Mall.)

The vena cerebralis lateralis is seen to pass on the outer aspect of the posterior cerebral nerve-roots
and the auditory vesicle 0. The vein on the mesial aspect of V, the ganglion of the fifth nerve, repre-
sents a portion of the primitive jugular. V, Gasserian ganglion; ab, acoustico-facial ganglion;
~0j auditory vesicle; g, glossopharyngeal ; hy, hypoglossal.

AV

SLS

FlG. 286. — DlAGKAM OF THE VEINS OF THE HEAD OF AN EMBBYO OF FOTJB WEEKS OLD.

(After Mall.)

A CV, anterior cardinal vein; VCL, vena capitis lateralis ; SL 8, superior longitudinal sinus;
0V, ophthalmic vein ; V, fifth nerve; AV, auditory vesicle.

VCP

FlG. 287. — DlAGBAM OF THE VEINS OF THE HEAD OF AN EMBBYO OF FIVE WEEKS OLD. (After Mall.)

VJ, jugular^ vein ; VCP, vena cerebralis posterior ; VCM, vena cerebralis media; VAC, vena cerebralis
anterior; TH torcular Herophili ; OF, ophthalmic vein. Other letters as in fig. 286.

VEINS 231

The external jugular is a secondary channel formed by the union of a superficial
facial vein and a vein in the neighbourhood of the ear. It extends backwards
and unites with the primitive or internal jugular, near its junction, with the
subdavian vein from the arm.

FIG. 288. — DIAGRAM OP THE VEINS OP THE HEAD AT THE BEGINNING OP THE THIBD MONTH.
(After Mall.) Lettering as in fig. 287.

A secondary anastomosing vein is formed between the external jugular and the lateral sinus,
which emerges through a foramen in the temporal bone (foramen jugulare spurium). This
occasionally persists in the human adult, and in certain mammals it enlarges and, owing to the
disappearance of the primitive jugular, draws all the blood from the cranial sinuses. Salzer

FIG. 289. — DIAGRAM OF THE VEINS OF THE BRAIN OF AN OLDER FCETUS. (After Mall.)

Lat.Sin., lateral sinus (v. cerebralis posterior) ; Sup.Pet., superior petrosal sinus (v. cerebralia
media) ; SS, Sylvian or middle cerebral vein ; SPS, sphenoparietal sinus ; SB, sinus rectus ;
ILS, inferior longitudinal sinus ; VG, vena galena magna. Other letters as in fig. 287.

(1895) first showed that the view of Luschka, according to which this channel ^represents the
primitive jugular, is untenable ; and Mall, whose account has been here followed, has recently
confirmed Salzer's descriptions in the case of the human embryo.1

1 Salzer, Morph. Jahrb. xiii. ; Mall, Amer. Jour, of Anat. iv. 1905.

232

VASCULAK SYSTEM

The primitive jugular veins are at first symmetrical and join, as already stated,
the posterior cardinals to form the ducts of Cuvier. A communicating branch is,
however, formed between the point of junction of the left jugular and subclavian
veins and the right jugular. This anastomosing vessel is converted into the left
innominate vein. The portion of the right primitive jugular between the transverse
vessel and the right subclavian becomes the right innominate, while the portion
between it and the entrance of the posterior cardinal (vena azygos major), together
with the duct of Cuvier, forms the definitive superior vena cava. On the left side
the portion of the primitive jugular below the anastomosis becomes the superior
intercostal, but the duct of Cuvier becomes obliterated (with the exception of a
portion which in part forms the coronary sinus). Traces of the vessel are to be
recognised even in the adult, in the form of a fibrous strand which runs over the

FIG. 290. — A AND B. DIAGRAMMATIC OUTLINES OF THE VESTIGE OF THE LEFT SUPERIOR CAVA
AND OF A CASE OF ITS PERSISTENCE. (Sketched after Marshall.) 3.

The views are supposed to be from before, the parts of the heart being removed or seen through.

1, 1', internal jugular veins; 2, 2', subclavian veins; 3, right innominate; 3', right or regular
superior cava; 4, left innominate, normal in A, rudimentary in B; 5, in A, the opening of the superior
intercostal vein into the innominate ; 5', vestige of the left superior cava or duct of Cuvier ; 5, 5', in B,
the left vena cava superior abnormally persistent; 6, coronary sinus; 6', coronary veins ; 7, superior
intercostal trunk of the left side (left cardinal vein) ; 8, the principal azygos (right cardinal vein) ;
7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachiaii
valve.

back of the left auricle, and a small vein (oblique vein of Marshall) ; and in front
of the root of the left lung there remains an indication of its former presence in the
form of a small fold of pericardium (vestigial fold of Marshall).

The left duct of Cuvier has been observed persistent as a small vessel in the
adult. Less frequently a right and a left innominate vein open separately into the
right auricle, an arrangement which is also met with in birds and in certain mammals,
and which results from the vessels of the left side being developed similarly to
those of the right, while the cross-branch remains small or absent.

Veins of the limbs. — The veins of the limb -buds form, to begin with, a
vascular loop with two marginal vessels. Of these latter the ulnar and the fibular
are the primary stems. Each extends down the postaxial border of its proper limb

CIRCULATION IN FOETUS 233

and over the dorsal aspect of the future hand or foot to the pre-axial border, there
to become continuous with a smaller and temporary channel, the radial or tibial
vein respectively. These latter veins are replaced by new (secondary) veins which
become the radial and cephalic in the upper arm (opening at first into the external
jugular), and the long saphenous in the lower limb. The primary ulnar vein persists
in the upper arm as the basilic, axillary, and subclavian veins ; the primary
fibular, on the other hand, persists in the leg only, as the short saphenous. The
deep veins which accompany the arteries are later formations.1

PECULIAKITIES OF THE FCETAL ORGANS OF CIRCULATION.

It may be useful here to recapitulate shortly the peculiarities of structure
existing in the advanced stage of the formation of the foetal organs of circulation,
with reference to their influence in determining the course of the blood during
intra-uterine life, and the changes which occur in them upon the establishment
of pulmonary respiration at birth.

The foramen ovale has the form of a free^ oval opening bounded by the
septum secundum, and guarded on the side of the left auricle by a valvular plate
derived from the septum primum, so that the blood can only pass from the right
into the left auricle, not in a contrary direstion.

The Eustachian valve constitutes a crescentic fold of the lining structure of
the heart, which is so situated as to direct the blood entering the auricle by the
inferior cava towards the opening of the foramen ovale.

The ductus arteriosus establishes a communication between the main
pulmonary artery and the aorta, by which the blood from the right ventricle is
carried mainly into the dorsal aorta.

The two large hypogastric or umbilical arteries, prolonged from the iliac
arteries, passing out of the body of the foetus, proceed along the umbilical cord,
to be distributed in the foetal portion of the plicenta. From the placenta the
blood is returned by the umbilical vein, which, after entering the abdomen,
communicates by one branch with the portal vein, and is continued by another,
named ductus venosus, into one of the hepatic veins, through which it joins the
main stem of the vena cava inferior.

Course of the blood in the foetus. — The right auricle of the foetal heart
receives blood from the two venae cavse and the coronary sinus. The blood brought
by the superior cava is simply the venous blood returned from the head and upper
half of the body ; whilst the inferior cava, which is considerably larger than the
superior, conveys not only the blood from the lower half of the body, but also
that which is returned from the placenta and the liver. This latter stream of
blood reaches the vena cava inferior partly by a direct passage — the ductus
venosus — and partly by the hepatic veins, which bring to the vena cava inferior
all the blood circulating through the liver, whether derived from the supply of
placental blood entering that organ by the umbilical vein, or proceeding from
the vena portae or hepatic artery.

The blood of the superior vena cava is believed to pass through the right
auricle into the right ventricle, whence it is propelled into the trunk of the
pulmonary artery. A small part is distributed through the branches of that vessel
to the lungs, and returns by the pulmonary veins to the left auricle ; but, as these
vessels remain small up to the time of birth, by far the larger part passes through
the ductus arteriosus into the descending aorta, and is thence distributed in part
to the lower half of the body and the viscera, and in part along the umbilical arteries
to rhe placenta. From these several organs it is returned by the vena cava inferior,

1 See paper by Lewis, Amer. Jour, of Anat. v. 1905.

234

VASCULAK SYSTEM

s

the vena portse, and the umbilical vein ; and, as already noticed, reaches the right

auricle through the trunk of the inferior cava.

Of the blood entering
the heart by the inferior
vena cava, it is supposed
that only a small part is
mingled with that of the
superior cava, so as to pass
into the right ventricle ; by
far the larger portion is
thought to be directed
by the Eustachian valve
through the foramen ovale
into the left auricle, and
thence, together with the
small quantity of blood
returned from the lungs by
the pulmonary veins, to
pass into the left ventricle,
whence it is sent into the
arch of the aorta, to be
distributed almost entirely
to the head and upper
limbs.

In earlier stages of
development than those
above described, it is cer-
tain that there is little or
no separation of the two
lands of blood, for both the
umbilical veins from the
placenta and the veins from
the yolk-sac and body gene-
rally, pour their blood
together into the sinus
venosus, and the mixed
blood is then forced through
a single somewhat narrowed
orifice (porta vestibuli of
His) into the auricle.

FIG. 291. — DIAGRAMMATIC OUTLINE OF THE ORGANS OF CIRCULA-
TION IN THE FCETUS OF six MONTHS. (Allen Thomson.) -

EA, right auricle of the heart ; E V, right ventricle ; LA, left
auricle; Ev, Eustachian valve; LV, left ventricle; ;L, liver;
K, left kidney ; I, portion of small intestine ; a, arch of the aorta ;
a', its dorsal part ; a", lower end ; vcs, superior vena cava ;
<vci, inferior vena where it joins the right auricle ; vci', its
lower end; s, subclavian vessels ;j, right jugular vein ; c, common
carotid arteries ; four curved dotted arrow lines are carried through
the aortic and pulmonary opening and the auriculo-ventricular
orifices ; da, opposite to the one passing through the pulmonary
artery, marks the place of the ductus arteriosus ; a similar arrow
line is shown passing from the vena cava inferior through the
fossa ovalis of the right auricle, and the foramen ovale into the
left auricle ; hv, the hepatic veins ; 'vp, vena portse ; x to
vci, the ductus venosus ; uv, the umbilical vein ; ua, umbilical
arteries ; nc, umbilical cord cut short ; i, i', iliac v<

CHANGES IN THE CIR-
CULATION AT BIRTH.

The changes which occur
in the organs of circulation
and respiration at birth,
and which ]ead to the estab-
lishment of their perma-
nent condition, are more
immediately determined by
the inflation of the lungs

1 In this diagram the arteries are conventionally coloured red and the veins blue, but these colours
are not intended to indicate the nature of the blood conveyed by the respective vessels.

CHANGES IX CIRCULATION AT BIRTH 235

with air in the first respiration, the accompanying rapid dilatation of the
pulmonary blood-vessels with a greater quantity of blood, and the interruption
to the passage of blood through the pla°ental circulation. These changes are
speedily followed by shrinking and obliteration of the ductus arteriosus, and of
the hypogastric arteries from the iliac trunk to the place of their issue from the
body into the umbilical cord ; by the cessation of the passage of blood through
the foramen ovale, and somewhat later by the closure of that foramen, and by the
obliteration of the umbilical vein as far as its entrance into the liver, and of the
ductus venosus behind that organ.

The process of obliteration of the arteries appears to depend at first mainly on
the contraction of their coats, but this is very soon followed by a considerable
thickening of their substance, reducing rapidly their internal passage to a narrow
tube, and leading in a short time to final closure, even although the vessel may not
present externally any considerable diminution of its diameter. It commences at
birth, and is perceptible after a few respirations have occurred. It makes rapid
progress in the first and second days, and by the third or fourth day the passage
through the umbilical arteries is usually completely interrupted. The ductus
arteriosus is rarely found open after the eighth or tenth day, and by three weeks
it has in almost all instances become completely impervious.

The process of closure in the veins is slower ; but they remain empty of blood
and collapsed, and by the sixth or seventh day are generally closed.

Although blood ceases at once to pass through the foramen ovale from the
moment of birth, or as soon as the left auricle becomes filled with the blood
returning from the lungs, and the pressure within the two auricles tends to be
more equalised during their diastole, yet the actual closure of the foramen is more
tardy than any of the other changes referred to. It is gradually effected by
the union of the valve of the fossa ovalis with the margin of the limbus of
Vieussens on the left side ; but the crescentic margin is generally perceptible
in the left auricle as a) free border beyond the place of union, and not unfre-
quently the union remains incomplete, so that a probe may be passed through
the reduced aperture. In many cases a wider aperture remains for more or less
of the first year of infancy, and in certain instances there is such a failure of the
union of the valve as to allow of the continued passage of venous blood, especially
when the circulation is disturbed by over- exertion, from the right to the left auricle ;
this occurs as the malformation attending the morbus cceruleus.

THE LYMPHATIC SYSTEM.1

Lymph-vessels. — Little that is quite certain is known regarding the develop-
ment of the lymph vascular system. In the lowest vertebrates there is no such
system of vessels distinct from the blood vascular system, and the only channels
comparable to lymph-vessels are spaces in the connective tissue. In mammals
organogenesis is well advanced before there is any sign of walled and valved
lymphatic vessels. Up to that stage there are spaces, in certain situations, which
no doubt contain lymph, and it has been very generally held that the permanent
lymphatics are such spaces round which the connective-tissue cells are arranged to
form the walls of the vessels, Awhile the' communication with the veins is secondarily
acquired (Gulland, Saxer, Sala). On the other hand, Klein described the lymphatics
as developing by the hollowing out of mesenchyme (connective-tissue) cells (vaso-
formative cells), which join with one another to form protoplasmic tubes, the walls of

1 For literature, see Hochstetter, Hertwig III. Th. ii. and iii. p. 165. Also Sabin, Amer. Jour, of
Anat. vols. i. iii. and iv. ; Langer (quoted by Sabin), Sitzungsber. d. k. Akad. d. Wissensck. I. Abth. 1868;
MacCallum, Archiv f. Anat. u. Phys. Anat. Abth. 1902 ; Lewis, Amer. Jour, of Anat. v. 1905 ;
Huntington and McClure, The Anatomical Record, Amer. Jour, of Anat. vi. No. 8, April 1907.

236 LYMPHATIC SYSTEM ; SPLEEN

which are subsequently differentiated around the nuclei to produce the lymphatic
endothelium. Two other views have been taken of the origin of lymphatic
vessels — viz. (1) that of Budge (1880), which derives them from the coelom, but
is not supported by any satisfactory evidence, and has fallen into the background ;
and (2) that of Ranvier, that the lymph-vessels are derived from the veins. Miss
Sabin has supplied the most complete body of evidence in support of this
theory (that the lymphatics arise from the veins). According to her account
(1902-3), the lymphatic system in the pig arises from the venous endothelium at
four points, forming four ducts. These primary ducts dilate to form four
' lymph-hearts' homologous with those of the Amphibia, though not possessed of
muscular walls. From these, as from centres, all the lymphatics grow first along
the veins towards the skin, and second along the aorta and its branches, until
they extend to every part of the body. The process of budding Miss Sabin
supposes to occur as described by Langer and Ranvier — viz. solid buds are
formed which are afterwards hollowed out. In the formation of plexuses the
buds open into neighbouring ducts by absorption of the endothelium, and valves
are formed at the point of junction. The communication between the posterior
lymph-hearts and the veins is lost, but the anterior ducts persist as the right and
left lymphatic ducts of the adult. The primary lymph-glands she describes as
being formed from the four lymph-hearts.

Lewis also derives the lymph- vessels from venous endothelium, not, however,
from four sites, but from several. The openings into the veins he believes to be
secondary, not primary, and holds that the existence of structures comparable
with lymph-hearts has not been demonstrated in mammals. Huntington and
McClure return to the idea that the lymph- vessels are mesenchymatous spaces.
They find that the main lymph- channels are formed along the early veins. They
arise as oval or spindle-shaped spaces outside the intima, in an adventitious
reticular tissue which takes form as the primitively redundant venous channels
shrink. As the intima recedes, following the diminishing column of blood, these
spaces increase in size and number, and, becoming confluent, form large irregular
channels which open secondarily into the veins. They explain the adult
distribution of the larger lymph- vessels by tracing them to embryonic veins,
which are temporary, and subsequently entirely or in great part abandoned.

Lymph-glands. — The lymph-glands arise from a plexus of lymph-vessels
which in section appears as a sinus broken up by connective-tissue trabeculse;
Capillary blood-vessels are formed in the trabeculae connected with the branches of
the artery of the future gland. Round the capillaries free cells (lymphocytes) gather
in the ..connective-tissue spaces, either introduced from the vessels (G-ulland) or
Droduced in situ (Saxer). The lymph-follicles (lymph-cords) are thusjlaid down,
and the central parts of the strands where the Ivmphocvtes are actively multiplying
are the aerm-centres. The original plexus of lymph-vessels forms the} sinus oi the
eland, and is necessarily connected with afferent and efferent channels.1;

The haemolymph-g-lands are apparently developed in a similar fashion, but
the jplexus is venous, not lymphatic.

SPLEEN.-

The spleen appears in the mesogaster as a cellular mass produced by an aggre-
gation ot mesenchyme-cells. It lies close to the dorsal pancreas. The cellular
mass becomes partially detached from the mesentery, but remains connected with
it by a fold (gastro-splenic omentum), through which the vessels] enter. While

1 See further on this subject the account given in the part of this work dealing with Histology.

2 For literature, see Hochstetter, Hertwig III. Th. ii. and iii. pp. 165-66.

BODY-CAVITY 237

still of minute size the organ shows a remarkable notching, of which traces are to
be found in the adult (fig. 170, p. 126 ; fig. 222, p. 176).

The cells are at first closely packed, but spaces containing blood-corpuscles
appear, and the original cellular mass is converted into a trabecular framework.
The spaces become the venous sinuses of the organ. They are from the first
crowded by great numbers of leucocytes of all varieties. The artery is late in
developing. Hound its peripheral branches lymphoid cords are formed, which
become the Malpighian bodies. It is very difficult to determine the origin and fate
of the free cellular elements in the spleen, owing to the minute size and crowded
state of the cells in the higher vertebrates. The question whether they are
produced in situ, or introduced from without, can only be answered indirectly,
and opinion is divided on the point.

In Lepidosiren the cells are of very large dimensions, and from observations on the
developing spleen the present writer v has been able to supply strong evidence in favour of the
view that the original mesenchyme-cells of the rudiment are differentiated into both red and
white blood-corpusdes, thus confirming the work of Laguesse and others on the Selachian
spleen. The spleen sinuses are at first merely spaces in the mesenchyme-mass, which later
become lined by endothelium, derived from the surface cells of the primary cellular trabeculae.
The spleen jn the lower vertebrates is thus an active hoBmapoietic organ. It is probably more
than a mere locus (i.e. a site in which the primitive corpuscles collect and multiply), as there
is evidence to show that blood-cells are actually formed from the indifferent cells of the rudiment.
In most mammals its share in blood-formation is limited to a certain period of foetal life, after
which that function is transferred to the bone-marrow.

DEVELOPMENT OF THE BODY-CAVITY.

The early stages in the development of the ccelom have already been described
on p. 49 seq. It was there explained that the coelomic space withinjthe embryonic
shield had at first the form of a U open behind, the clefts on each side' of the axis
being joined across the front of the shield by the precephalic ccelom. It was
also shown that, while inTthe region of the trunk the space was continuous with the
extra -embryonic ccelom, in the region of the head the future pericardial portion
was separated from it by a lamina of mesoderm. It was further explained that
when the head-fold is foi med the precephalic cleft comes to|lie below, then behind, the
bucco-pharyngeal membrane. It is by the expansion of this space that the definitive
pericardial cavity is developed, and in the following fashion. The pericardial
coalom consists at first of a mesial limb and two horns. The horns lie on either side
of the open pharynx, and their splanchnopleuric mesoderm is doubled in by the
primitive endothelial heart-tubes. At first on the ventral, these subsequently come
to lie on the mesial aspect of the splanchnopleuric folds as they bend in towards
one another. Before the folds meet to close in the floor of the pharynx they have
already become folded-in ventrally, and the mesial cross-portion of the pericardium
has come to lie below them, so that when their union is effected, and the heart-
tubes are brought together, there is no ventral mesentery (Robinson, Volker,
Rouviere).2 By further extension backwards of the mesial cleft the lateral
horns are taken into the cavity, and the pericardial ccelom communicates with
the general ccelom only by two apertures, one on either side of the mesocardium.
When this disappears in part of its extent somewhat later, there is necessarily
only a single aperture extending across the middle line, and it further follows
that the lateral ccelomic spaces must also communicate in this situation with one
another below the free ventral edge of the gut-mesentery.

1 Trans. Eoy. Soc. Ed. xli. 1904.

2 Robinson, Jour, of Anat. and Phys. xxxvii. ; Volker, Bibliogr. Anat. x. ; Rouviere, Jour. d'Anat. et
de la Phys. xl.

238 BODY-CAVITY

Behind the heart the walls of the yolk-sac are nipped in, and come together,
before they are folded in from the front by extension of the mesial pericardial
cleft, so that here a ventral mesentery is produced, as well as a dorsal, and this
continues to be formed until the union of the lateral edges of the shield has
extended beyond the point where somatopleure and splanchnopleure are con-
tinuous with one another. This union primarily extends all round the front edge
of the shield ; when, therefore, the head-fold is formed, and the splanchnopleuric
layers unite with one another, a plate of mesoderm will extend right across
the body of the embryo below the gut and lateral ccelomic spaces, and behind
the pericardium. This is the septum transversum.1 It forms a bridge of tissue
in which the allantoic and Cuverian veins pass inwards and join the vitelline,
to form the sinus venosus. It is at first set nearly transversely, but as the
pericardium extends backwards it becomes rotated, till it lies obliquely in an
antero-posterior and dorso-ventral plane. It forms the dorsal and posterior walls

/.-'• -,-f _._. t nolens aortie

.* .•'';'. post, meaocardium
pericardium

\Y j"\ auricular portion of

ant. wall of , ..--•""" heart loop

pericardium "T" ' ;j\ ... • -

_._ :..-^ lat. mesoca rdi /mi

, ..- „..,,[;[<<( of Cuvier

septum
transversum

liver parenchyma
liver divert

----——umbilical rein
^- — vitelline vein

mouth of yolk-xti*-.,. . •

»sV cnslom

FIG. 292. — THE PEBICABDIAL, PLEURAL, AND ABDOMINAL PORTIONS OF THE CCELOM AND MESOCABDIA
IN A HUMAN EMBRVO OF 3 MM. LONG. (After His, from Kollmann.)

of the pericardium, while in its anterior edge, which bounds the pleuro-pericardial
opening, lie the sinus venosus and the Cuverian veins, which sweep into it
from the somatopleure on each side. Between the dorsal surface of the septum
and the back wall of the body-cavity stretches the mesenteric partition,
containing the gut and its hepatic diverticulum, on each side of which the
vitelline veins extend forwards through its substance to the sinus venosus. The
septum transversum soon becomes greatly thickened by the extension into it
of the epithelial trabeculae of the liver parenchyma. These follow the
walls of the veins, which they surround, imbed, and break up into the sinusoids
already described (p. 174). The primary expansion takes place forwards, but
does not extend to the edge containing the sinus venosus. Later the trabeculee
grow up on each side to form lateral masses of liver-substance which project into

1 The view adopted in the text regarding the formation of the septum transversum is not shared by
embryologists. "- ' " ^ ..........

brought secondarily

all embryologists. Many following Ravn consider that the somatopleure and splanchnopleure
rily into union by the enlargement of the omphalo-mesenteric vein.

SEPTUM TRANSVEKSUM

239

the coelom from below. Between the lateral lobes on the dorsal aspect is a groove
in which the mesenteric partition is inserted. Into this partition the lung-

~x-

\

en r it I' X.

_-• lateral
mesocardiitm

r posterior

mesocar<iii/>ii

pericardium '"

FIG. 293. — FIGUKE OBTAINED BY COMBINING SEVERAL SUCCESSIVE SECTIONS OF A HUMAN
EMBRYO OP 7'5 MM. (FOURTH WEEK). (From Kollmann.)

The arrow indicates the opening of the pleural cavity into the peritoneal.

m.p.p.
r. duct of Cnvier \ w.f. d.p. 1. duct of Cuvier

mitral pillar

lorsal pillar

ai)tero-lat<'>-nl recesx

Woltfan fold _

coeliac artery

— Wolffianfold

FKI. '294. — MODEL OF A HUMAN EMBRYO OF 6'8 MM., SHOWING THE DORSAL WALL OF CCELOM. (After Piper.)

.p'.p., membrana pletiro-peritonalis ; w.f., anterior end of Wolffian fold; d.p., dorsal pillar of
pleuro-peritoneal membrane.

rudiments extend from before backwards, and encroach on the coelomic clefts.
The anterior part of the coelom of each side now becomes the primitive pleural

240

BODY-CAVITY

cavity, the posterior part the peritoneal cavity. The pleural cavities are
at first only short and narrow clefts, bounded behind and on either side
by,:;f certain folds which are concerned in the ultimate separation of the
several 'portions of the coelom from one another, and in the formation of the
diaphragm.

The closure of the pleuro-pericardial opening is chiefly effected by lateral
folds (lateral mesocardia] related to the Cuverian veins (fig. 293). The veins pass

cardinal vein
Wolffianfold J — ~1
dorsal pilla ' — *"

ventral pillar — |
trachea

L hepatic vein

cardinal vein

Wolffianfold
dorsal pillar
cesophagus

ventral pillar
r. hepatic vein

sinus venosus
right auricle

Iruncus arteriosus

canalis auricularis
FIG. 295. — SAME MODEL AS SHOWN IN PIG. 294, IN TKANSVEKSE SECTION.

The figure shows the cranial half of the model. The broad band containing the hepatic veins is the
septum transversum thickened by the liver-trabeculee. The pleuro- peritoneal membrane is seen on
each side stretching between its dorsal and ventral pillars.

in a dorso-ventral direction in the lateral body-walls into the septum transversum,
in which they then take a transverse direction. With the expansion of the body-
wall the dorso-ventral portion of each vein comes to lie in a fold projecting into
the coelom. Owing to the backward displacement of the septum transversum
and its related parts, the veins come to take an increasingly antero-posterior direc-
tion. The lateral folds, and the anterior edge of the septum transversum which
is continuous with them, are necessarily brought from the transverse into a coronal
plane. The terminal portions of the Cuverian veins now run parallel to one

DIAPHRAGM

241

another, and bound the pleuro-pericardial opening. The folds in which they lie
(the original anterior edges of the septum transversum) become expanded owing
to the enlargement of the pleural clefts and pericardial cavity, and their free
margins fuse with the free ventral border of the mesenteric septum so as to shut
off the pericardium from the pleural cavities (fig. 293, p. 239). The septum thus
formed is known as the pleuro-periczrdial membrane.

The closure of the pleuro-peritoneal openings is effected by a very complicated
series of changes, which result in the formation of the diaphragm. The
chief factor in the development of the septum is the extension of the liver
trabeculse into the septum transversum, into certain folds in connexion with it,
and nto the mesenteric septum, until the liver occupies the whole depth of the
body-cavity. The connective-tissue sheet covering it, and derived from the several
parts into which the trabeculas extend, becomes freed from the liver-substance,

pleuro-pericardial openings

•. duct of Cuvier

dorsal pilln r
right lung

liver

mesentery

peritoneal

recexx

floor of
pleural space

cceliac artery in
cut mesentery

FIG. 296. — MODEL OP A HUMAN EMBRYO OF 6'8 MM. (After Piper.)

The coelom is opened, and the dorsal wall removed by cutting through the dorsal mesentery,
lungs and liver are thus exposed from the dorsal aspect.

The

and forms a divisional plane which by a further series of modifications is developed
into the diaphragm.

We have already seen that the liver at first consists of a mass of epithelial
trabeculse occupying the septum transversum, and that the mesenteric septum
is attached to its dorsal aspect. On each side of this the pleural spaces are con-
tinuous with the peritoneal cavity. The openings are in part constricted by folds
named the pleuro-peritoneal membranes (figs. 294-296). These lie at first in a
nearly sagittal plane, with their free edges directed backwards. The dorsal pillar
of each fold is continued on to the dorsal body- wall to be attached to the mesial
side of the Wolffian ridge, and the ventral end is prolonged on the dorsal aspect of
the liver, running behind into the attachment of the mesenteric septum to that
organ. The pleuro-peritoneal membranes are in reality the anterior ends of the
Wolffian ridges, here reduced to membranous folds owing to the atrophy of the
head ends of the Wolffian bodies (Bertelli, Keith, Brachet, Wolfel) (fig. 297).
They separate the anterior end of the pleuro-peritoneal cavity into mesial recesses

VOL. I. R

42

DIAPHRAGM

which become the pleural spaces, and lateral recesses which belong to the peritoneal
cavity. These peritoneal (antero-lateral) recesses of course overlap the pericardium.
Thus, while the anterior edge of the septum transversum internal to the
pleuro-peritoneal membrane separates on each side the pleural space from the
pericardium, the portion lateral to the pleuro-peritoneal fold separates the peritoneal
recess from the pericardium (peritoneo-pericardial membrane) (fig. 297). The
pleuro-peritoneal membranes soon become altered in position owing to the growth
of the body- wall and the extension of the pleural cavities. Their dorsal attach-
ments are shifted laterally until the membrane ultimately assumes an oblique
position, lying in a caudo-dorsal and cranio-ventral direction. At the same
time another fold has formed on the right side of the mesenteric septum, due to
the formation of a ccelomic diverticulum extending inwards and then forwards

p leuro-peritoneal
membrane

sopJiagus

pleural space
lung

pericardiu

ventricle

FlG. 297. -DIAGRAMMATIC SECTION OF THE TRUNK OF AN EMBBYO CALF 15 MM. LONG. (Wolfel.)

into its substance. This cleaves the mesentery into two lamellae, of which the
left follows the gut and the right passes on to the dorsal aspect of the right lobe
of the liver (fig. 296). This light lamella (mesolateral fold of Brachet) has a
free edge bounding the opening (foramen of Winslow) into the diverticulum, and
it has a dorsal and a ventral pillar. The ventral column passes on to the liver to
be continuous with the ventral pillar of the pleuro-peritoneal membrane; the
dorsal is continued on the dorsal wall of the body-cavity in a fold in which the
vena cava inferior runs, and hence is known as the caval mesentery. The pleuro-
peritoneal openings are constricted and ultimately closed by the extension of the
liver trabeculse, first mesially into the caval and mesolateral folds on the right
side, and into the dorsal mesentery round the cardia of the stomach on the left
side, causing a thickening of both lamellae of the mesenteric septum ; second

DIAPHRAGM

243

laterally into the pleuro -peritoneal membranes, causing an uplifting of their
ventral pillars (cf. fig. 295) and drawing-in of the outer and ventral margins of the
openings. The ultimate result is the merging over the surface of the expanding
liver on each side of the mesial layer of the pleuro-peritoneal membrane with
the layer covering the thickened mesenteric septum (the mesolateral fold being
considered part of this septum).

The primitive diaphragm is thus formed first of the septum transversum
(including under that term a part of the pleuro-pericardial membrane) ; second
of the thickened mesenteric septum, and third of the mesial layer of the pleuro-
peritoneal membrane. It consists at first merely of a connective-tissue sheet
covering the liver ; it is at first continuous with the parenchyma of the liver, but
soon becomes separated from it by cleaving into^two layers. The cleavage, how-
ever, does not extend all round the organ, and jthe/esult is that it remains attached
by suspensory bands, which afterwards become the coronary ligaments.

The diaphragm thus consti-
tuted is placed at first very
obliquely, but as it descends to
its definitive position it comes
to lie transversely, and at the
same time increases in circum-
ference by the expansion of the
pleural spaces. These, as we
have already seen, are at first
small, and are placed entirely
dorsal to the pericardium. Owing
to the development of the ribs
the body -wall now becomes
greatly expanded, and a thick
layer of loose tissue develops on
their inner side (fig. 268, p. 214).

This is gradually invaded by the FlG< 298._DlAOBAM OF THE PRIMITIVE DIAPHRAGM TO
pleural sacs, which, pushing into SHOW THE SEVERAL PARTS FROM WHICH IT is BUILT UP.

the body-wall round the peri- (After Broman.)

cardium, gradually come to en- 1, pericardial part derived from septum transyersum;

' J 2 and 3, parts derived from the mesentery : 4, 4, parts

Close that cavity, and extending derived from pleuro-peritoneal membranes : between these

dorsally and the mesenteric portions are the nearly closed
leuro-peritoneal openings ; 5, 5, parts derived from the

fly-walls ; 1, 2, 8, 4, 4, cover the cranial aspect (upper
rfa<

behind into the tissue circum-
scribing the diaphragm form
the costo-diaphragmatic recesses, surface) of the liver.
The pleural sacs thus reach their

final dimensions by excavating the body-wall, and, further, a certain portion of
the circumference of the diaphragm must be referred to tissue really belonging
to the body- wall (fig. 298).

Muscular tissue extends into the diaphragm derived from the transversalis
and the rectus sheets (Keith). The supply of the muscle by the phrenic nerves
shows that part of it is a derivative of cervical myo tomes, which are displaced
backwards as the diaphragm sinks to its permanent level.1

1 The above account of the development of the diaphragm is a mere sketch of a very complicated
process. For further details the reader must be referred to special works on the subject. The earlier
literature will be found fully reviewe<i by Brachet in Merkel and Bonnet, Ergebnisse der Anatomie und
Entwickelungsgeschichte, 1907. References to some more recent papers will be found in Hochstetter's
article in Hertwig's Handbuch, Bd. Ill Th. i. and ii. p. 160. Still more recent papers are Mall, Ball.
of the Johns Hopkins Hospital xii. ; Broman, Anat. Anzeiger Ergiinzungshef t) xxi. ; Bertelli, Arch. Anat.
e Embriol. Ital iv. ; Keith, Jour, of Anat. and Phys. xxxix. ; Wiilfel, Anat. Anzeiger xxx. ; Debeyse,
Bibliograph. Anat. xiv. ; Brachet, Contribution a la signification morphologique du diaphragm dorsal,
Bruxelles 1906.

B2

244

MESENTERY

Development of the mesentery and lesser sac of the peritoneum. —

A. ventral mesentery is not developed behind the umbilicus. The fold so called
in front of it is largely taken up by the liver-parenchyma (fig. 292, p. 238), and
in the adult is represented by the gastro-hepatic omentum and falciform ligament.
The dorsal mesentery extends along the whole length of the gut. It becomes drawn
out into an extensive sheet when the vitelline loop is formed. As the gut
increases in length the neck of the mesentery (which lies within the body while
the rest of the sheet is enclosed in the coelom of the umbilical cord) becomes
narrowed until the end of the vitelline loop (the future mid-point of the transverse
colon) comes into close relationship on the dorsal wall of the abdomen with the end
of the duodenum. This relationship persists through all the later stages, and
when the loop of the colon is formed, a twisting of the neck of the mesentery
necessarily takes place in such fashion that the small gut with its mesentery, i&
carried to the left under the distal (colonic) part of the vitelline loop with its
mesentery (fig. 301). When this rotation is complete, and the colonic loop is

FlG. 299.— DlAGBAM OF THE MESENTEBY, STOMACH,
AND INTESTINE OF A HUMAN EMBBYO OF SIX
WEEKS. (Toldt.)

sf, stomach ; g.c., greater curvature ; I.e., smaller
curvature ; mg, mesogastrium ; spl, spleen ; p, pan-
creas; c, ceecum ; r, rectum; me, mesentery;
ao, aorta ; cl, cceliac axis ; s.mes.a., i.mes.a., supe-
rior and inferior mesenteric arteries.

FlG. 300.— DlAGBAM OF A SECTION ACBOSS THE
ABDOMEN OF A HUMAN EMBBYO OF THE
THIBD MONTH. (Toldt.)

I, I, liver; k, kidneys ; g.o., great omentum ;
g'.o'., omental sac ; s.o., small omentum. The
other letters as in fig. 299.

carried to the right, the meso-colon is necessarily stretched into a transverse plane.
At the same time the portion of the gut distal to the vitelline loop being carried
to the left, as the small intestine develops, the mesentery proper to it assumes a
transverse position and becomes the left half of the transverse mesocolon. The
ascending and the descending colon have at first a free mesentery, but they become
fixed by the disappearance of the posterior layer on each side, and the adult
conditions are realised.

The lesser sac of the peritoneum l is very early roreshadowed by the
formation of the diverticulum in the mesenteric septum mentioned above. Some
observers attribute its formation to the downward growth of the mesolateral fold,
while others regard it as an actual inpushing of the ccelomic space to form a pocket.
The opening into the diverticulum (foramen of Winslow) at first lies between the
rudiment of the right lung and the stomach, but later, by the extension of the
liver-trabeculse into the dorsal pillar of the mesolateral fold and the formation
of the caval lobe of the liver, it comes to lie between the liver and duodenum.

1 A review of the literature of the lesser sac of the peritoneum, by Broman, will be found in
Ergebnisse der Anatomic und Entwickelungsgeschichte, 1905.

LKSSKR SAC OF THE PERITONEUM

245

inesoduodenum

trans, colon

- */>leen

mesocolon
asc. colon

vermiform
appendix

tUelline stalk -

mesogastrium
('Treat omentum)

- duodeno-jejunal
flexure

tlesc. colon
mesocolon

FIG. 301. — DEVELOPMENT OF THE MESENTERY IN THE HUMAN EMBRYO, SEMI-DIAGRAMMATIC.

(From Kollmann.)
The arrow points to the foramen of Winslow.

A B

creas.

(ftlzntestme.

FIG. 802. — DIAGRAMS ILLUSTRATING THE DEVELOPMENT OP THE GREAT OMENTUM. (O. Hertwig.)

A, earlier stage. B, later stage.

st, stomach; s.o., small omentum; s'.o'., omental sac; o', mesogastrium, springing from the posterior
wall of the abdomen, near which in A it encloses the pancreas ; o'1, attachment of mesogastrium to
greater curvature of stomach ; o3, fold of mesogastrium or great omentum growing over coils of small
intestine ; me, mesentery ; m.c., transverse mesocolon ; o4 (in B), dotted line showing the situation of that
lamella of the mesogastrium which at first assisted in enclosing the pancreas, but which has now
disappeared. The next part of this lamella has coalesced with the adjacent lamella of the transverse
mesocolon, and has also disappeared. The coalescence is indicated by the black line.

246 MUSCLES

The cranial extension of the pocket disappears, but when the stomach turns on its
axis the diverticulum enlarges horizontally between liver and stomach, and is
bounded on the left by the displaced mesogaster. The mesogaster is connected,
with the greater curvature — i.e. the original dorsal border of the organ — and is
thus deflected to the left. It now grows down as a double fold, containing an
extension of the lesser sac, to form the greater omentum. This passes down at first
to the left of the colonic loop, but when the definitive positions are assumed it
comes to lie in front of it. An adhesion then takes place between the posterior
layers and the mesocolon, so that the colon comes to be attached to the omentum.
Further, by a disappearance of the posterior layer of the double mesogaster, the
pancreas, which at first lies between its lamellae, comes to lie behind the peritoneum,
just above the line along which the mesogaster is joined to the mesocolon (fig. 302).
The duodenum also loses its mesentery, either by fusion of the visceral and parietal
layers of peritoneum, as some describe it, or by a stripping off of the peritoneum from
its posterior aspect, due to the covering layer not expanding proportionately to the
gut-wall.

DEVELOPMENT OF THE MUSCULAK SYSTEM.1

Muscles of the trunk. — We have already studied the early phases in the
development of the myo tomes (p. 56). In the human embryo during the third
week the muscle-plate is produced from the inner wall of the mesodermic segment ;.
the axial mesenchyme is formed from the sclerotomes ; the mesenchyme of the
somatopleure becomes a thick layer, and the mesenchymatous thickenings on
the Wolffian ridges which constitute the rudiments of the limb-buds are laid down.
During the fourth week the myotomes become greatly lengthened in the trunk and
extend into the somatopleure. Their inner wall has become entirely converted
into muscle-cells, but the outer wall is still epithelial, and the cavity (myoccel)
has become obliterated by the fusion of the two lamellae (cf. fig. 84, p. 59).
During the fifth week the outer wall also becomes muscular (Bardeen and
Lewis ;,* and the myotomes are joined into a dorso-ventral sheet in which
the original segmentation has largely disappeared. This sheet next becomes
subdivided by ingrowing mesenchyme septa into a dorsal and a ventro-lateral
mass. During the sixth week the dorsal begins to be separated from the ventro-
lateral mass ; the dorsal section becomes subdivided into three longitudinal
columns (ilio-costalis, longissimus dorsi, spinalis dor si] ; and the ventral-lateral into
mesial and lateral portions, of which the mesial forms the rectus, and the lateral,
cleaving into three strata, the obliquus externus and internus and transversalis.
It is in the thorax alone that any segmental arrangement is retained, the ventral
projections of the myotomes being separated by the rudiments of the ribs and
giving origin to the intercostal muscles. By the seventh week the premuscular
tissue is all resolved into its definitive divisions (Bardeen and Lewis).

The rectus-muscle rudiments are at first separated by a considerable interval
(Mall), the body-wall between them being formed only by the membrana reunions
or fused somatopleuric layers. The muscle-sheets ultimately grow towards the
middle line, and come into apposition, carrying their nerves with them.

Muscles of the limb?. — In the early stages the limb -buds consist of a mass
of mesenchyme continuous with that of the Wolffian ridges and lateral to the line
of the myotomes (fig. 82, p. 57). As the buds increase in length this becomes
differentiated into a skeletal core and a premuscule sheath. It is not definitely

1 The literature of the development of the muscles will be found collected by Maurer in Hertwig's
Handbuch III. Th. i. p. 76 seq. ; also in Kollmann's Handatlas, Appendix, p. 45 seq.
- Bardeen and Lewis, Amer. Jour, of Anat. i.

MUSCLES

247

known whether in man this premuscular tissue is derived from the myotomes or
arises in situ. In some lower forms there is clear evidence of a growth of
muscle-buds from the myotomes into the limb (fig. 303), while in others it seems
to be formed by a budding-off of cells individually from the myotomes into the
mesenchymatous matrix. While a priori we should expect the striped limb -muscles
to be derived from the myotomes in one way or another, there is no decisive proof

-am.f

FIG. 303.— TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK OF AN

EMBRYO OF SCYLLIUM. (Balfour.)

sp.c, spinal cord; sp.g, ganglion of posterior root; ar, anterior root; dn, dorsal; sp.ii, ventral
branch of spinal nerve ; nip, muscle-plate ; mp', part of muscle-plate already converted into muscle ;
mp.l, part of muscle-plate extending into the limb ; nl, nervus lateralis ; ao, aorta ; ch, notochord ;
sy.g, sympathetic ganglion ; ca.v, cardinal vein ; sd, segmental duct ; st, segmental tube ; du, duode-
num ; hp.d, junction of hepatic duct with it ; pan, rudiment of pancreas connected with another part of
duodenum; umc, opening of umbilical canal (vitelline duct).

that this is the case in higher forms ; some observers therefore conclude that the pre-
muscular tissue is a differentiation in situ (Paterson, Lewis, Bardeen, and others).
Ingalls,1 in a very well-preserved human embryo of 4'9 mm., describes a distinct
budding off of cells from the myotomes into the limb-buds. A stream of cells seems
first to proceed from the outer plate, as was described by Kollmann, but later the

i Arch. f. mikr. Aiiat. Ixx. 1907.

248 HEAD-MUSCLES

inner lamella apparently also contribute cells to the limb-bud. The cleavage of
the premuscule sheath into the rudiments of the adult muscles is completed by
the seventh week (Lewis).1

The premuscule sheath is the rudiment, not only of the muscular tissue proper, but of the
connective-tissue framework, fasciae, and tendons of the muscles. There is no distinction at first
between the cells which will become muscle-cells and those which will give rise to connective-
tissue elements. In quite early stages, according to Bardeen,1 areas are to be made out which
will become muscles, and areas which represent inter muscular spaces. As differentiation
proceeds these spaces become more definite ; the premuscular masses become divided up into
individual muscle-rudiments, and these again into the fasciculi of the individual muscles by the
growth of connective-tissue septa, which are more abundant in embryonic than in fully developed
muscles. The main nerve-paths follow the spaces between the rudiments of the muscle-groups,
and the larger branches of the nerve supplying a muscle-group lie in the septa, between the
members of the group. Differentiation of a muscle-rudiment usually begins at the point of
entrance of the nerve into it. According, further, to Bardeen, ' Metameric segmentation in
the innervation of the limb-muscles is not due to ingrowth into the limb of myotomes, accom-
panied by nerves, but to the fact that a given region in the developing musculature is in the
more direct path of fibres extending into the limb from one or two specific spinal nerves. '

Muscles of the head. — We have already seen that there are three primitive
segments in the occipital region, but that in front of this point there is no trace
of cleavage of the mesoderm. The tongue-muscles supplied by the hypoglossal
(occipito- spinal nerves), and formed in the floor of the primitive mouth, are probably
derived from these occipital myotomes, and come into the same category with the
trunk-musculature. The remaining head-muscles fall into two groups, the muscles
of the eye and the branchial musculature.

The eye-muscles are developed from a cell-complex which appears between
the jugular vein and carotid artery, mesial to the trigeminal ganglion (Eeuter).
This mass is sickle-shaped, and has three limbs — two anterior which embrace the
optic stalk on the inner side, and a posterior. Each limb has its own nerve connected
with it, the upper being associated with the trochlear, the inferior with the oculo-
motor, and the posterior with the abducens. The rudiment moving forward
surrounds the optic stalk, and the two anterior limbs uniting into a ring (Reuter)
the whole complex forms a sort of cup embracing the optic vesicle ; out of the walls
of this the straight and oblique muscles are developed.

The eye-muscles in the mammal are developed in the unsegmented head -mesoderm, but in
the lowest vertebrates (Cyclostomata and Selachia) they are formed in connexion with the
so-called head-cavities, which are supposed to represent primitive segments in the prechordal
part of the head. The first of these gives rise to the muscles supplied by the oculo-motor
nerve, the second to the superior oblique, and the third to the external rectus. The three
limbs of the premuscle-cell-complex in mammals would represent, arguing from their relation
to the three nerves, the three head -cavities of lower forms.

The muscles of the branchial group, which may be termed the visceral
musculature, are derived from the unsegmented mesoderm of the branchial
region. All are supplied by the lateral motor-roots of the cranial nerves.

The masticatory muscles develop in the mandibular arch near the angle which
it forms with the maxillary process (Reuter), appearing as a cell-complex round
the branches of the mandibular division of the fifth nerve. The mass
resembles in shape an inverted Y, the limbs embracing the rudiment of the ramus
of the mandible. The stem forms the temporal, the outer limb the masseter,
and the inner the yterygoids (Reuter for pig).

The muscles supplied by the facial are derivatives of the hyoid arch. The
platysma and all the mimetic muscles wander from this site. There are few develop-

1 Lewis, Amer. Journ. of Anat. i. Regarding the development of the arm-muscles, see also
Griifenberg, Anat. Hefte xxx. ; of the leg-muscles, Bardeen, Amer. Jour, of Anat. vi. 1907.

SEGMENTATION OF HEAD 249

mental data regarding the remaining head-muscles ; but it is probable that the
series supplied by the motor roots of the glossopharyngeal, vagus, and spinal
accessory nerves, which are primarily branchial in their distribution, are to be
looked on as muscles of the branchial arches, corresponding to those originating in
connexion with the branchial sacs of the lower vertebrates. The sternomastoid and
trapezius, though they wander far back in development, may with some reason be
regarded as, in part at least, branchial derivatives.

Segmentation of the head.- — The foregoing paragraphs necessitate here a brief statement
on this obscure and intricate subject.

In the higher Amniota there is certain evidence of segmentation in the occipital region.
Here at an early stage there are three myotomes, and, related to these, three or four occipito-
spinal nerves, which are primarily segmental, but united into a single trunk, the hypoglossal.
In front of this clearly segmented portion of the head there is no trace of segmentation
except in the lower vertebrates. In Selachia the number of ' head-segments ' has been very
variously estimated ; but according to the classical account of van Wijhe there are in all nine,
four metotic (the occipital) and five pre-otic. The pre-otic are much modified, and their claim
to rank as segments has been disputed. They are cavities the walls of which give rise to muscles.
The first three provide the eye-muscles, while the fourth and fifth disappear. In the branchial
arches there is a series of cavities regarded as representing the lateral plates of certain segments.
Their walls give origin to the branchial muscles. The evidence of typical segmentation is more
striking in the Cyclostomata. In Petromyzon (Koltzoff) typical segments occur, the anterior
only being modified in having no skle-plates. They are derived from pouches of the archenteron,
and the lateral plates show cavities, one in each branchial arch. There are two main views
regarding the head-segmentation. The first, maintained by van Wijhe, Miss Platt, Koltzoff,
and others, represents the whole series of segments as belonging to the head proper, and corre-
sponding to trunk-segments. The anterior or pre-otic are rudimentary and greatly modified,
the posterior or metotic are more complete. Their dorsal portions persist and give rise to
myotomes supplied by the occipital nerves (hypoglossal). Their ventral portions give rise to
the muscles of the branchial arches, and their splanchnic and sensory nerves are collected into
the vagus-complex. The second view, maintained by Gegenbaur, Froriep, Fiirbringer, and others,
represents the series of ' head-segments,' as divided into two distinct categories. The metotic
are segments belonging to a part of the trunk which has become included in the head
comparatively recently in phylogeny. The pre-otic are primarily segments of the head proper, and
to them belongs the mesoderm of the branchial arches which have been displaced backwards,
while the occipital segments have been displaced forwards, so that the two regions overlap.
There are thus two genetically distinct parts of the head ; the palingenetic and ccenogenetic of
Gegenbauer, palseocranium and neocranium of Fiirbringer, the prespinal and spinal of Froriep.
The occipital nerves (hypoglossal) belong to the occipital myotomes, and therefore to the
neocranium ; the vagus-complex (including the glossopharyngeal in lower forms as well as the
accessory), belongs to the palseocranium, being formed by^the coalescence at their proximal ends
of the splanchnic fibres of the segmental nerves of the posterior palingenetic segments, of which
only the splanchnic or branchial portions now remain. According to Agar in a recent paper
on the anterior mesoderm in Lepidosiren,1 the preponderance of evidence is in favour of
the Gegenbauer-Fiirbringer view. It seems quite certain that the occipital myotomes really
belong to the trunk, but with regard to the palseocranium the matter is more doubtful.
It is not yet proved that the prechordal ' head-segments ' and branchial segmentation
correspond to the trunk-segmentation, and the ontogenetic facts which have induced Hubrecht
and others to attribute a radial symmetry to the fore-part of the head must not be left out
of sight.

1 Trans. Eoy. Soc. Ed. xlv. Part III. 1907

250

SKELETON

DEVELOPMENT OF THE SKELETON.1

In the following brief account of the development of the skeleton only the
more general features of the early stages will be considered. The details of
ossification and many points which relate to the morphology of the parts will
be dealt with in other parts of this work.

The vertebral column is developed from the mesenchyme which invests
the notochord and neural canal, and is derived from the sclero tomes. This is also
the blastema from which the membranes investing the spinal cord and the
ligaments of the vertebrae are produced. The appearance of the skeletal elements
is preceded by a stage in which a series of paired cellular thickenings is laid down
in the mesenchyme. It has been shown bv v. Ebner and others in the reptilian,
and by 0. Schultze, Weiss, and others in the mammalian embryo, that each sclero-
tome is divided into a cranial and a caudal portion by a narrow transverse cleft

(fig. 304). This appears in
man in the thoracic region
about the end of the third
week (Bardeen).'2 The inter-
vals between the sclero tomes
disappear, and the dividing
cleft is soon obliterated, but
the two portions remain dis-
tinguishable owing to the fact
that the caudal half consists
of more densely cellular tissue.
The mesenchyme immediately
round the notochord loses all
trace of segmental cleavage,
and becomes condensed into
a continuous notochordal
sheath. As Weiss has shown
in the rat, the myotome
extends as a keel-shaped
thickening into the cleft, and
pushes the two portions of
the sclerotome apart, so that

the caudal portions of each sclerotome pair come to lie obliquely, embracing on
their mesial aspect the cranial portions of the succeeding pair. The thickenings
are slowly pushed into the intervals between the myo tomes to which they
properly belong and the succeeding pair, and thus ultimately come to have an
intersegmental position (Bardeen). It must be remembered that these thickenings
are merely areas of condensation in a general blastema ; but it is customary
to speak of them as primitive vertebrae (or scleromeres, Bardeen). Each scleromere
(fig. 305 A) has a pair of dorsal or neural processes (primitive arch), a pair
of ventral or costal processes which extend outwards between the myotomes,
and a mesial plate which is continuous with the condensed mesenchyme of
the notochordal sheath. These plates do not form the future bodies of
the vertebrae ; they occupy rather the position of the future intervertebral
discs. Between the intervertebral plates the notochordal sheath is invested by

1 The literature of the development of the skeleton will be found collected by Braus (skeleton of
limbs), Schauinsel (vertebral column, ribs, and sternum), and Gaup (skull), in Hertwig's Handbuch 111.
Th. ii. and iii. pp. 331 seq., 562 seq., and 855 seq.

2 Amer. Journ. of Anat. iv. 1904.

FIG. 304. — HORIZONTAL LONGITUDINAL SECTION OF THREE

PROTOVERTEBRtf: IN A SNAKE-EMBRYO. (v. Ebner.)

ep, cutaneous ectoderm ; e.m., outer wall of segment ; /, its
margins folded round into i.m., muscle-plate composed of
flattened cells which are becoming elongated into muscular
fibres ; n.c., neural canal, in outline only ; n'.c'., neural
ectoderm forming its walls. Between these and the muscle-
plate is a continuous mass of mesenchyme which has been
derived from the inner parts of the primitive segments, partly
interrupted by the ganglion-rudiments, gl. The original
intervals between the primitive segments here are still indicated
by vessels, v. i.cl., cleft in the mesenchyme (according to
Ebner this is the remains of the original cavity of the
segment).

VERTEBRAE

251

looser mesenchyme, which is derived, like the tissue intervening between the
neural processes, from the anterior portions of the sclero tomes. The plates
are at first relatively thick, but the tissue forming them becomes loosened
posteriorly, and is added to the loose investment of the notochordal sheath,
while in front it is condensed where it adjoins the fissure of Ebner. This
thickened band is the rudiment of the permanent disc, and the looser tissue
intervening between adjoining discs is converted into the body of the
permanent vertebra. It follows from this description that the future body
is contributed to by the anterior portions of a sclerotome pair, and also by
the posterior portions (primitive plates) of the preceding pair, so that a new
segmentation of the skeletal axis is effected which alternates with the primary
myotomic segmentation. The formation of the vertebral bodr is brought about

notoch. disc

neural process

__ intersegmental
artery

i-ostaLprocess

costal process

neural process

inlet-dorsal-,
membrane

notochord peric/iordal septum

FIG. 805. — VIEWS OF MODELS OF BLASTEMAL (MEMBRANOUS) STAGE OF VERTEBRAL COLUMN :
A. FROM AN EMBRYO OF 7 MM., VENTRAL ASPECT, x 33£ diameters. B. FROM AN EMBRYO
OF 9 MM., DORSAL ASPECT, x 25 diameters. C. FROM AN EMBRYO OF 11 MM., LATERAL
ASPECT, x 25 diameters. (After Bardeen.)

as follows : the notochordal sheath becomes prolonged dorso-ventrally into a
kind of septum (fig. 305, C), which extends between the primitive'plates and separates
the loose mesenchyme, alluded to above, into a right and left moiety ; at the same
time the superficial layers of the intervening tissue become condensed into a
continuous lamella uniting the plates, and enclosing the looser tissue on each side
of the septum. This enclosed tissue now becomes converted into cartilage.
There are necessarily at first two chondrogenetic centres, but soon the septum
becomes implicated,|and the notochord is enclosed in a continuous cartilaginous
ring. According to 0. Schultze, the cartilage formation extends also through the
primitive intervertebral plates, so that the column becomes for a time a continuous*
rod of cartilage, and the permanent discs are formed in this secondarily by the
conversion of the hyaline into fibro- cartilage, the only persistent portion of the
primitive membranous plates being the annulus fibrosus of the intervertebra

252

VERTEBRA

discs. Bardeen did not observe this stage in the human embryo. Within the
discs the notochord is enlarged and afterwards converted in each along with the
surrounding tissue into the nucleus pulposus. Within the bodies, on the other
hand, the chorda becomes constricted and ultimately disappears.

The neural arches retain their primitive position, and while the costal processes
are extending between the my o tomes to form the membranous ribs, a transverse
process appears on each side in the angle between it and the neural process,
opposite the attachment of the scleromere to the notochordal sheath. A
chondrogenetic centre appears in each side of the primitive arch, and another in
each costal process. The arch becomes joined to the body, and the cartilaginous
vertebra is completed. The arches, however, remain for a long time open
(figs. 306, and 234, 268, pp. 186, 214). It is not till the fourth month that they are

rib

spinal ganglion

scapula

Y — scapula

humerus

vein thy. \ thy. vein
sternum

FIG. 306.— SECTION OF A HUMAN EMBRYO OF 30 MM. Photograph. (T. H. Bryce.)
s.p., spinal cord; n.c., cartilages of neural arch still separate; thy, thy, thymus.

closed in over the cord to complete the spinal canal. The articular processes are
produced by the growth of cartilaginous rudiments, backwards and forwards,
from the neural processes into the intervening layer of tissue. The primitive
membranous plate at an early stage becomes much thickened ventrally. This
thickened band corresponds to the hypochord rod of lower forms. It is not
a separate structure at any time (rat, Weiss ; man, Bardeen), except in the
case of the atlas, in which, loosened from the body, it persists as the ventral bar
of that bone. It becomes chondrified by two lateral centres (Weiss). The body
of the atlas remains free from the bases of the arches, and is secondarily fused with
the axis as its odontoid process.

Ribs and sternum. — Each vertebra is provided with a rib-process. In the
blastema stage there is no separation between rib and primitive vertebra, and the

KIBS AND STERN I'M

253

same is true, according to some authorities (0. Schultze), for the cartilaginous
stage. The processes remain attached to and become parts of the vertebrae in
the cervical, lumbar, and sacral regions, but in the thoracic region they grow
round the body-wall to form the free ribs. The costo-central joints are
produced by absorption in the matrix between ribs and vertebrae, the surrounding
mesenchyme giving origin to the costo-vertebral ligaments. The rib-process
and growing transverse process are at first united by a continuous blastema.
This is absorbed as anastomoses are established between the segmental
arteries (Bardeen), but between the end of the process and the rib a joint-cavity
is formed and the surrounding mesenchyme gives rise to costo- transverse
ligaments (see fig. 268, p. 214).

The ventral ends of the ribs become continuous with two longitudinal cellular
bands (sternal bands), which are laid down from before backwards. The cellular
strands are derived according to some from the rib -ends, but according to others
they are independent formations ; opposite the first seven pairs of ribs they fuse by
differentiation of the intervening tissue to form the sternum (fig. 306). This
remains cellular for some time after the ribs are converted into cartilage. The

FIG. 307. — MEDIAN SAGITTAL, SECTION OP THE HEAD IN EABLY EMBRYOS OF THE RABBIT.
Magnified. (From Mihalkovics.)

A, from an embryo 5 mm. long. B, From an embryo 6 mm. long.

In A, the faucial opening is still closed ; in B, the septum is perforated at /; c, anterior cerebral
vesicle; me, mesencephalon ; mo, medulla oblongata ; m, medullary epiblast; if (in B), infundibulum ;
sp.e, spheno-ethmoidal ; be, sphenoidal ; and sp.o, spheiio-occipital parts of the basis cranii ; i, fore- gut ;
cJi, notochord ; py, buccal pituitary involution ; am, amnion ; 7i, heart.

process of chondrification begins in the upper lateral angles of the presternum and
in the mesosternum between the ribs (Paterson) ; it proceeds from the margin
inwards, repeating the process by which the chondroblast sternum is formed.
The nietasternum is developed separately, but also from independent lateral
rudiments in all probability.

The usual view adopted regarding the development of the sternum is that of Ruge, who
derives the sternal bands from the ventral ends of the ribs, which unite with one another, as it
were, from before backwards to form the bands. According to Paterson, they are at first quite
separate and independent of the ribs, and are only united with them secondarily. He describes
an early stage in which the primitive sternum is separate from the ribs, but connected with the
rudiments of the shoulder-girdles, and believes, therefore, that ontogenetically the sternum is
an independent structure connected with the limb-girdles. It is not yet clear whether, or
in what degree, the limb-girdles contribute to the formation of the manubrium, and the relations
of the chondroblastic areas to the cartilaginous skeleton are not yet sufficiently elucidated
to warrant a more definite statement than that contained in the text.

254

SKELETON OF EXTREMITIES

Skeleton of extremities. — The skeletal core of the limb-buds is produced
by a condensation of the vascular mesenchyme in the axis of the developing
limbs, and at their bases. The skeletal blastema becomes still further condensed
in the situation of the future bones, and distally into a hand or foot plate.
This stage is named the blastema or prechondral stage. The cellular blastema
is then converted into cartilage by the appearance of centres of chondrification
in the precartilaginous blastema of the several skeletal parts. During this
chondrogenetic stage the limbs acquire the main features of the adult form.
The joints are at first represented by areas of mesenchymatous condensation,
which directly join the cartilages, but by the end of the second month the
joint-cavities have appeared (fig. 248, p. 196) and the surrounding mesenchyme is
condensed and thickened into the capsular ligaments. Intra-articular structures are
derived from the primary blastema. Centres of ossification next appear in the

FIG. 308. — DIAGBAMS OP THE CABTILAGINOUS CRANIUM. (Wiedersheim.)

A, First stage.

Ch, notochord ; Tr, trabeculse cranii ; P.ch, parachordal cartilages ; P, situation of pituitary body ;
N, E, 0, situations of olfactory, visual, and auditory organs.

B, Second stage.

B, basilar cartilage (investing mass of Rathke) ; S, nasal septum and ethmoidal cartilage; Eth,
Eth', prolongations of ethmoidal around olfactory organ, completing the nasal capsule ; Ol, foramina
for passage of olfactory nerve-fibres ; N, E, 0, Ch, Tr, as before.

cartilages, and they closely correspond to the preceding centres of chondrification
(Bardeen.)

Development of the skull. — As in the trunk, so in the head, the noto-
chord is at first the only supporting structure. As we have seen in an earlier
section, it extends forwards to the flexure of the mid-brain, and then returns on
itself to end at the attachment of the buccopharyngeal membrane (fig. 307).
Cartilage begins to be laid down, during the second month, in the mesenchyme
on each side of this part of the notochord, and a basicranial plate is formed enclosing
the chorda and extending from the future foramen magnum to the stalk of the
pituitary body, where it ends in a plate or process which becomes the dorsum settee.

It appears from the work of Jacoby, Levi, Robinson, and others that there are no definite
parachordal nor trabecular cartilages in the human embryo such as occur in lower forms
(fig. 308). Levi describes the appearance of a number of chondroblastic centres, which ultimately
fuse to form the continuous chondrocranium. The basicranial plate is chondrified in two sections,

SKULL

255

an anterior related to the auditory capsule and a posterior or occipital segment. The otic portion
shows no trace even in the membranous stage of any segmentation ; but regarding the occipital
portion, Levi confirms for man the accounts given by Froriep for the calf and recently by Weiss
for the rat. Froriep holds that the occipital region represents the fusion of four rudimentary
vertebrae, corresponding to the three primary roots of the hypoglossal nerve. Of these only the
posterior is at all independent. Its development in the early stages resembles that of the vertebrae,
and it loses its identity only when fused with the parts in front of it. The anterior portion of
the occipital blastema shows faint traces of segmentation, but only in the earliestjphases, by

rist a gulli

lamina cribrosa

ala orbit alis

for. oplicum

'tla temporalis

for. ami. int.
for.jiKjul.

fossa subarciiata

can. nervi

fadalis

udit. caps.

for. endol.

for. liupogloss

foramrn magnum lectum siinoticmn

FIG. 309. — MODEL OF THE CHONDROCRANIUM OF A HUMAN EMBRYO, 8 CM. (From Hertwig's
Handbuch der Entwickelungslehre.)

The membrane-bones are not represented.

the presence of three rudimentary cellular masses (primitive arches) which extend outwards
between the myotomes. After the cartilaginous elements are formed, all trace of segmentation
is lost. The relation of the basicranial plate to the notochord (see below) in mammals makes
the significance of this early segmentation doubtful (Robinson).

The chondrocranium forms only an incomplete case for the brain. The cranial
vault consists of bones which are laid down directly in membrane, and in the
region of the visceral skeleton numerous investing bones are added to complete

256 SKULL

the^adult skull. As these will be fully dealt with in the volume on Osteology, it
will suffice here if a brief account be given of the chondrocranium as a basis for
that description.

The cartilaginous cranium consists of two parts, the neurocranium and the
visceral skeleton. The neurocranium consists of two sections, a parachordal and a
prechordaL''-*; The auditory capsule becomes an integral part of the parachordal, and
the olfactory capsule of the prechordal section. The basicranial plate encloses
the notochord, which ends at the dorsum sellee in a rounded process, its anterior
hooked portion having disappeared. According to Froriep, Robinson, and others, '
the chorda lies on the ventral aspect of the middle portion of the basicranial plate,
being completely invested by cartilage only behind and in the dorsum sellae. The
plate consists, as mentioned above, of an otic and an occipital segment. From the
latter there extends on each side a lateral plate (occipital pillar, Gaup), which in
man is not vertical, as in lower mammals, but is laid out horizontally. This becomes
continuous with the posterior edge of the auditory capsule behind a gap which is
left as the jugular foramen. In each lateral plate which represents the fused occipital
arches is the foramen for the hypoglossal nerve. At first the skull is wide open
behind the hind-brain, being covered merely by the membrana reunions ; but into
this, from the auditory capsule and occipital pillars on each side, a plate of cartilage
extends to close in the foramen magnum and form the supra-occipital. The anterior
part of the basicranial plate becomes continuous with the auditory capsule. This
is an oval mass of cartilage with its long axis directed inwards and forwards, and
divided into an upper and posterior vestibular portion, and a lower and anterior
cochlear portion. Between these is a groove on the upper aspect for the facial nerve,
which in the figure is seen partially bridged over to form a canal. On the mesial
aspect of the capsule is seen the internal auditory meatus, and a deepish fossa
(fossa subarcuata) which extends below the superior semicircular canal. On the
outer aspect a shelf-like process extends which ultimately forms the tegmen tympani.
The prechordal or trabecular portion of the skull stands at first at a considerable
angle from the posterior portion (Hagen). There are, as already mentioned, no
separate trabecular cartilages formed in man. Chondrification occurs in an
already continuous mesial blastema, which is interrupted by the stalk of the
pituitary body. The cartilage extends directly forwards into the mesial nasal
process as the nasal septum. On the upper aspect of the cartilage in front of the
dorsum sellae is a shallow fossa — the future sella turcica, the floor of which is
incomplete for a time where the stalk of the pituitary body passes through it.
A process projects from the cartilage on either side of the fossa ; between this and
the auditory capsule the internal carotid artery enters the skull. To this process
is attached an obliquely placed plate of cartilage (ala temporalis), arising from a
separate centre (Levi), which is the rudiment of the great wing of the sphenoid.
As it extends backwards it surrounds the mandibular division of the trigeminal
nerve and the middle meningeal artery so as to form the foramen ovale and foramen
spinosum. Between it and the orbital wing is a wide gap through which the
maxillary division of the fifth and all the nerves entering the orbit pass. By the
formation of a bridge of cartilage the maxillary division is separated from the
rest of the nerves ; thus the foramen rotundum is formed, while the main fissure
becomes the sphenoidal fissure. In front of the sella turcica a broad bar of
cartilage expands laterally on each side into the orbital or lesser wings (ala orbitalis).
From the posterior borders of each orbital wing a bar is developed which is
attached to the side of the central cartilage in front of the pituitary fossa and
completes the optic foramen. From the anterior border of each orbital wing a

1 See Robinson, Jour, of Anat. and Phys., xxxviii.

VISCERAL SKELETON

257

broad process projects forwards to be attached to the roof of the nasal capsule.
In later stages this is largely reabsorbed. The ethmoidal region of the human
skull is peculiar in respect that the nasal capsules are so rotated that the
apertures of communication between the cranial cavity and the nasal fossae are
placed horizontally. Each aperture is at first single, but is afterwards converted
into the cribriform plate by the formation of cartilaginous bridges. The central
cartilage is continued forwards as the septum nasi, and its upper edge projects
between the olfactory openings as the crista galli. The side walls of the nasal
capsules bend inwards in front to be continuous with the edge of the septum and so
complete the roof of the nasal fossae, but the floor is deficient, and only completed
when the palatal processes have met with the lower free edge of the septum. From
the mesial aspect of the outer wall of the nasal capsule on either side project the
cartilaginous rudiments of the several turbinate processes.

for. optic. ala orbital i
\

palat.

dentale

cart, cricoid.

cart, tfiyreoid.

for. hypogl.

FIG. 310. — THE SAME MODEL AS SHOWN IN FIG. 309, FBOM THE LEFT SIDE.
Certain of the membrane-bones of the right side are represented in yellow.

The visceral skeleton consists of a number of skeletal elements which occupy
the mandibular, the hyoid, and first branchial arches. In the mandibular arch
a bar of cartilage is laid down, known as MeckeTs cartilage. This represents the
primitive mandible ; but it in great part disappears, being displaced by a membrane
bone, the os dentale. A small portion of its ventral end, however, directly ossifies
and forms a part of the lower jaw, and its proximal end is developed into the malleus.
Close to this a separate formation in the blastema in the base of the arch gives
rise to the incus. In the hyoid arch are formed the stapes, the styloid process of
the temporal bone, the stylohyoid ligament, and the lesser cornu of the hyoid bone ;
while in the first branchial arch a short bar of cartilage is deposited which becomes
the greater cornu of that bone. The body of the hyoid bone is an intermediate
formation between the ventral ends of the second and third arches. The develop-
ment of the visceral skeleton is specially interesting as providing the proof
that proximal portions of the first two arches, which in lower vertebrates
VOL. i. s

258

AUDITOEY OSSICLES

form a suspensory apparatus for the mandible, are in higher vertebrates, as
it were, annexed by the organ of hearing to provide an apparatus for sound -
transmission.

Formation of the auditory ossicles. — The development of the auditory
ossicles in the human subject has in recent years been investigated again in great

incus

interhyal
cartilage

handle of malleus
chorda tympani

cartilage of Reichert

FIG. 311. — KECONSTBUCTION OF THE PBOXIMAL ENDS or THE FIRST AND SECOND BRANCHIAL ARCHES

OF A HUMAN EMBRYO OF 16 MM. LONG ; LEFT SIDE, INNER ASPECT. (After Broman.)

V., fifth nerve ; VII., facial nerve.

detail by Broman, and also incidentally by Hammar. The following account sum-
marises the chief points of Broman' s researches. They confirm in the main the
views first enunciated by Reichert in 1837. l In the middle of the second month
the rudiments of the ossicles appear as chondroblast thickenings in a common

head of malleus

tympanic ring

incus

•facial nerve
I— ear-capsule

chorda tympani

handle of malleus

cartilage of Reichert
facial nerve

FIG. 312. — THE SAME MODEL AS SHOWN IN FIG. 811, SEEN FROM THE OUTEB SIDE.

blastema outside the first visceral pouch. This blastema is divided in the
mandibular arch by the trigeminal nerve, and in the hyoid arch by the facial nerve,
into mesial and lateral portions. In the mandibular arch the proximal part of the
lateral mass becomes the rudiment of the incus, while the corresponding part in the
hyoid arch represents the later ohyal cartilage. The distal portions of the lateral

1 Meckel discovered the cartilage which bears his name in the human embryo in 1820. He observed
its continuity with the malleus, but the interpretation of the facts was due to Reichert.

AUDITORY OSSICLES

259

masses, from the first continuous over the first branchial cleft, become separated from
the visceral skeleton, and form the cartilage of the outer ear. The proximal
portion of the mesial thickening in the mandibular arch does not develop farther,
but the distal portion forms the chondroblast of MeckeVs cartilage. Its upper
end early enlarges and becomes the rudiment of the malleus. The proximal part
of the mesial thickening in the hyoid arch becomes thickened round the stapedial
artery, and gives rise to the primitive stapes. This is connected with the incus by
a strand of the common blastema which persists as the rudiment of the long process
of that bone. The distal part of the mesial portion of the hyoid blastema becomes
the hyoid bar. It is at first connected with the stapes by a membranous strand
(pars interhyalis], which soon disappears, so that the stapes is separated from the
rest of the hyoid arch.

In these chondroblast areas cartilage is now developed. Meckel's cartilage
is deposited as a single piece. Its enlarged upper end is the malleus, which has
meantime developed a projection which becomes its handle, and another which
becomes its external process. Between the malleus and incus, which chondrifies

head of malleus

facial nerve —

stapes

cartilage of Meckel

pro. br. of malleus

tympanic ring
handle of malleus
chorda tympani

cartilage of ReicJtert
'acial nerve

FIG. 313. — RECONSTRUCTION OP THE SAME PARTS AS ARE SHOWN IN FIGS. 311, 812 OP AN

EMBRYO OF 55 MM. LONG ; LEFT SIDE, INNER ASPECT. (After Bromail.)

from a separate centre, there is at first a membranous layer which is afterwards
absorbed to give rise to the joint between the bones. In the same manner the
original union between the stapes and the long process of the incus is broken to
form the joint between them. The incus in its chondroblastic stage becomes
attached to the auditory capsule, but is separated from it again when the cartilage
is formed, the original intervening tissue becoming the ligament of the bone.
The stapes is at first free from the ear-capsule, but later becomes attached to it
and ultimately, when in the end of the second month it begins to lose its ring-shape
and assume the adult form, the base takes form in the fenestra ovalis, where the wall
of the capsule is reduced to a layer of perichondrium over it. The stapes and hyoid
cartilage chondrify separately, as does also the laterohyal cartilage mentioned
above. This becomes fixed to the ear-capsule, and by an extension of the cartilage
between it and the hyoid cartilage the latter obtains a direct secondary attachment
to the capsule. The bar thus formed becomes the styloid process. Its basal part
lies in the wall of the tympanum and takes part in the formation of the facial canal.
Its lower part is represented by the styloid ligament and lesser cornu of the hyoid
bone. The ossicles ossify each by a single centre. When the malleus has become

s2

260

AUDITORY OSSICLES

converted into bone the cartilage of Meckel disappears, but meantime on its proximal
end a small membranous ossicle appears, which joins the malleus as its anterior
process.

The tympanic ring is also a membrane bone. It appears in the third month
below and lateral to Meckel's cartilage, and is secondarily attached at a later date
to the petrous and squamous. When this union is effected the ossicles originally
on the outer side of the ear-capsules are included in the tympanic cavity.

Among other recent writers who have worked at this old problem, J. F. Gemmill ' agrees
with Broman in respect of the stapes, but Fuchs,2 working on rabbit-material, has come to con-
clusions shared also by Driiner,3 which revert to those of Parker in his earlier papers (1877), and

m a,

FIG. 314. — CONDITION OF MECKEL'S CABTILAGE AND THE HYOID BAB IN THE HUMAN FCETUS

OF ABOUT EIGHTEEN WEEKS. (Kolliker.)

B is an enlarged sketch by Allen Thomson, showing the relationship of the several parts

better than in A.

z, zygomatic arch ; ma, mastoid process ; mi, portions of the lower jaw left in situ, the rest
having been cut away ; M, Meckel's cartilage of the right side, continued at s, the symphysis, into
that of the left side M', of which only a small part is shown ; T, tympanic ring ; m, malleus ; i, incus ;
s, stapes ; sta, stapedius ; st, styloid process ; p, h, g, stylopharyngeus, stylohyoid, and styloglossus
muscles ; stl, stylohyoid ligament attached to the lesser cornu of the hyoid bone, hy ; th, thyroid
cartilage.

of Gruber (1877) — viz. that the stapes blastema is primarily connected with the auditory capsule
and is only secondarily connected with the hyoid arch. Fuchs and Driiner also believe that the
connection of the malleus and incus with Meckel's cartilage is a secondary one.

The ontogenetic history of the auditory ossicles, as described by Broman, affords additional
evidence that the incus is a separate element, and confirms the view that it represents the
quadrate bone of lower forms ; while the malleus is the upper end of the primitive mandible,
and when ossified is the homologue of the os articulare (Gaup).

1 Brit. Assoc. Report, 1901. 2 ArchivJ. Anat. Suppl. 1905. 5 Anat. Anzeiger xxiv. 1904.

INDEX

ABDOMINAL STALK, 54

Achromatin, 3

Adrenals, development of, 58, 135, 205

medulla of, from chromaffin system, 135,

206

Agar, on anterior mesoderm in Lepidosiren, 249
Age of embryos, estimation of, 80
Alecithal ovum, definition of, 9

type of cleavage in, 27
Alimentary canal, development of, 156

glands of, development of, 164
Allantois, arteries of, 217, 223

blood- supply of, in lower amniota, 217

degrees of development of, in lower
mammals, 53

formation of, 55

fate of, 55, 85, 185

veins of, 206
Allen, on epithelium of genital ridge, 188, 191

on rete tubules, 192
Amitosis, 4
Amnion cavity, origin of, in man, 29

ectoderm, origin of, 30

in guinea-pig, 31

in mice and rats, 31

formation of, in rabbit, 34

stalk, 30

stalk, mesoderm in, in higher Primates,
34

structure of, 70

Amniota, blood-supply of allantois, in lower,
217

higher, head- segmentation in, 249

lower, gastrulation in, 43

peripheral mesoderm in, 34

pronephros rudimentary in, 177
Amphibia, gastrulation in, 43

liver-parenchyma in, 174

lymph-hearts in, 236
Amphimixis, 17
Amphioxus, 42, 46
Anal tubercle, 201

Anamnia, homologue of intermediate cell-
mass in, 177

nephrotomes in, tissue corresponding to,
51

pronephros, a functional organ in larval

stage, 177
Annulus fibrosus of intervertebral discs, 251

ovalis, 213

Anus, formation of, 164, 201
Aorta, bulb of, 63, 207, 215

dorsal, 217, 221

primitive, formation of, 64, 206

ventral, 218

\ Apathy, on formation of neuroblasts, 98
! Apes, entypy of germinal area in, 32

notochordal canal in, 39
Appendix, vermiform, development of, 164
Aqueduct of Sylvius, 111
Archenteron, 47

i Arches, aortic, formation of, 217
fate of, 219

branchial, 83

rudimentary fifth branchial, Kallius,

167

Archoplasm, 2
Area, germinal, entypy of, 30

germinal, in blastocyst, 32

paraterminalis, Elliot-Smith, 118, 122,
123

vascular, origin of early, 32
Arrectores pilae, origin of, 94
Arteries, allantoic, 217, 223

basilar, 223

carotid, 221

centralis retinae, 142

cerebral, 221

ciliary, 143

coeliac, 223

curling, 79

facial, 222

hypoglossal, 223

infra-orbital, 222

innominate, 219

internal maxillary, 223

mandibular, 222

middle meningeal, 223

of the limbs, 224

ophthalmic, 221

segmental, 223

superior intercostal, 223

stapedial, 222

subclavian, 223

superior mesenteric, 223

superior thyroid, 222

vertebral, 222, 223

vitelline, 223

I Arytenoid cartilages, 169
1 Aryteno-epiglottidean folds, 167

262

INDEX

Ascaris megalocephala, nuclear changes in
during maturation, 12

division of chromosomes hi, 16
Assheton, theory of gastrulation of, 48
Atlas, 252
Atrium of right auricle, 211

or rudiment of utricle and saccule,

Streeter, 146
Attraction -sphere, 2

fate of, in spermatid, 6
Auditory pit, 145

vesicle, 145

nerve, 148

ossicles, 257

capsule, 256

Auricular canal, 206, 213
Auriculo-ventricular openings, 213
Azygos veins, 228

BAER, VON, on zona pellucida, 7
spermatozoon, 5
primitive nerve theory, 100

Balbiani, body of, in young oocyte, 10

Balfour, F. M., on outer lamella of myotome, 56
nerves, cell -chain theory of, 99
sympathetic system, origin of, 133, 135
adrenals, 206

Bardeen, on muscle-plate, 56

development of muscles, 246, 248
development of skeleton, 250, 252, 254

Basichromatin, 3

Basicranial plate, formation of, 254

Bat, entypy of germinal area in, 31

formation of notochordal canal in, 41
persistence of floor of notochordal canal

in, 46

differentiation of chorionic epithelium in,
75

Beard, on branchial sense-organs, 128
on thymus, 172

Benda, mitochondria of, 2

Beneden, Van, lecithophore of, 27
on inversion of blastoderm, 30
notochordal canal in bat, 46
phylogeny of primitive streak, 48
origin of placenta in lower mammals, 74
cytoblast and plasmodiblast, 74
differentiation of chorionic epithelium in
bat, 75

Beneke, ovum of, 80

Bertin, primary columns of, 185

Bethe, on neuroblasts, 98

Bidder, outgrowth theory of nerves, 99

Bile-duct, rudiment of, 173

capillaries, formation of, 174

Birds, cochlea in, 148

development of olfactory nerve in, 155

Bladder, gall, rudiment of, 174
urinary, 185, 186

Blastocyst, formation of, 26
in Tarsius spectrum, 27
invagination of cell-mass into, 27
wall, separation of. from yolk-sac, 33
wall, attachment of, to yolk-sac, 35
size of, related to imbedding, 67

Blastoderm, 25
bilaminar, 29
trilaminar, 32
inversion of, theories of, 30
bilaminar, in holoblastic ova, 43

Blastomere, 25
Blastopore, 43

fusion of lips of, 47

as origin of anus, 48
Blastula, formation of, 43

in amphioxus, contrasted with mam-
malian blastocyst, 45
Blood, first appearance of, 59

corpuscles, origin of, 60, 174, 237

islands of Pander, 59

vessels of yolk-sac, 59

vessels of embryo, 61
Bonnet, on origin of mesoderm, 36, 39

blastoporic aperture in dog, 43

notochord in dog, origin of head end of,

49

Bonnot and Seevers, on vitelline artery, 223
Born, on lacrymal canal, 144

development of tongue in pig, 159

development of heart, 212
Boveri, on centrosome, 2

centrosome in fertilisation, 15
Brachet, pleuro-peritoneal membranes of, 241

mesolateral fold of, 242
Brachia of corpora quadrigemina, 113
Bradley, on development of cerebellum, 110

neuromeres in pig, 129
Brain, development of, 105 et seq.

flexures of, 106

blood-supply of primitive, 221

veins of, 229
Branchial arches, clefts, pouches, 83, 149, 158

region, interpretation of serial characters
of, 130

group of muscles, 248

sense-organs of Beard, 128
Brauer, on pronephros in Gymnophiona, 177
Broman, on segments of rhombic brain, 107,
130

development of auditory ossicles, 258, 260
Bronchi, rudiments of, 166
Bryce, on spleen, histogenesis of, 237
Bucco-nasal membrane, 153
Bucco-pharyngeal membrane, 52, 53, 156
Budge, on lymphatic system, 236
Burdach, tract of, 103

C2ECTJM, 164

Calcar avis, 124

Calf, basis cranii in, Froriep, 255

Cameron, on pineal diverticulum in lower

vertebrates, 114

Canals, semicircular, 146, 147, 256
Canalis reunions, or duct of Hensen, 148
Capsule, internal, 120

of lens, 142, 143

auditory, 256

olfactory, 257

of Bowman, rudiment of, 184
Cardinal veins, 226
Carnivora, allantoic vesicle in, 55
Carotid gland, from chromaffin system, 135

system of arteries, 217, 219, 221
Cartilage of Santorini, 167

of Wrisberg, 167
Cartilage triticea, 168
Caruncula lacrymalis, 144
Cauda equina, 103

Caudal arch, secondary, Young and Robinson,
223

INDEX

263

Caudate nucleus, 119
Caval mesentery, 227
Cavum septi, 122
Cell, animal, structure of, 1

process of division, 4
Cells, history of sex, 5 et seq.

vaso-formative, origin of, 61

primitive sex, origin of, 188

interstitial of ovary and testis, 191, 192
Centriole, 2
Centroplasm, 2
Centrosome, 2

in fertilisation, 15
Cephalic flexure, 106, 156
Cerebellum, formation of, 108, 109
Cerebral vesicles, primary, 48

hemispheres, 115 seq.

nerves, development of, 127, 130

arteries, 221

veins, 229
Cervix uteri, 196

Chambers of heart, development of, 211
Cheeks, development of, 158
Cheiroptera, early imbedding of ovum in, 66
Chiasma, optic, 111
Choanse, primitive, 153
Chrondriomites, 10
Chorda dorsalis. See Notochord.
Chorda tympani, 132
Chordae tendinese, 215
Chorion, structure of, 71

frondosum, 72

laeve, 72

vascularisation of, in Primates, 55
Choroid coat, 143

plexuses, origin of epithelium of, 106

plexuses, 110, 121
Choroidal fissure, 118, 140
Chromaffin (phaochrome) cells, 135, 136, 206

bodies, 135
Cliromatin, 3

material basis of hereditary qualities, 17

reduction of, 17
Chromosomes, 4

splitting of, 4

division of, in Ascaris, 16

persistent identity of, 17

reduction of, 17

fusion of, 19

theory of heredity, 22
Ciliary ganglion, 134

body, 143

processes, 143

muscle, 143
Circulation, foetal peculiarities of, 233

course of, in foetus, 233

changes in, at birth, 234
Claustrum, 121

Clavate nucleus, origin of, 108
Clivus monticuli, 110
Clitoris, 203
Cloaca entodermica, formation of, 164

development of bladder from, 185

fate of, 201
Cloacal membrane, 52, 164, 201

tubercle, 201

plate, 201
Cochlea, 147, 256

canal of, 146

ganglion of, 148

Cochlea in birds, 148

modiolus of, 148

spiral lamina of, 148
Coccygeal gland, 135
Coelenterate, conversion of radial, into proto-

vertebrate, 48
Ccelom, definition of, 34

formation of, 49

extra-embryonic tissue in, 35

intra-embryonic, or body-cavity, 51

cavity of primitive segment continuous
with, in lower vertebrates, 51

extra-embryonic, obliteration of, 85
Ccelomic pouches, origin of axial mesoderm
in Reptilia, 46 \

cavities in head, in lower vertebrates,

57, 249
Coert, on epithelium of genital ridge, 188

origin of tubules of rete testis, 192
Colloid, development of, in thyroid, 168
Coloboma iridis, 140
Colon, development of, 163
Columnae carneae, 215

Commissure, midddle, or intermediate mass,
113

posterior, 114

habenular, 114

anterior, 122

hippocampal, 122, 123
Concha, 149
Conchse of nose, 155

Concrescence theories, His, Minot, Hertwig, 47
Coni vasculosi, 192
Conjugation of nuclei, 15
Connective tissue, early stages in develop-
ment of, 56

Conus of right ventricle, 216
Cord, umbilical, formation of, 85

genital, 181, 188, 193

nephrogenetic, 178, 182
Cornea, 143
Corona radiata, 8
Coronary sinus, 211

ligaments of liver, 243
Corpora quadrigemina, 111, 113

mammillaria, 111, 115
Corpus dentatum of olive, 108

caUosum, 122

peduncle of, 124

striatum, rudiment of, 116
development of, 119

luteum, 191
Corpuscles, directive or polar bodies, 11

of Hassal in thymus, origin of, 172
Costal processes, 250, 252
Craniota, formation of gastrula in, 47
Cranium, 254, 256
Cremasteric muscle, 199
Cribriform plate, 256
Cricoid cartilage, 167
Crista galli, 257

urethralis, 185
Crura cerebri, 111
Crus fornicis, 124

of semicircular canals, 146
Crusta, 111
Cryptorchismus, 201
Cumulus oophorus, 191
Cuneate nucleus, origin of, 108
Cunningham, on frontal operculum, 124

264

INDEX

Cuvier, ducts of, 206, 208, 226
fate of, 232
vestiges of, 232

Cyclostomata, liver- parenchyma in, 174
head-segmentation in, 249
development of eye-muscles in, 248
Cytoblast, Van Beneden, 74
Cytomicrosomes, 2

DECIDTTA, structure of, 67, 68, 69

reflexa, or capsularis, 69

serotina, or basalis, 69

basalis, degeneration of, 77

vera, 68

stratum spongiosum of, 68

stratum compactum of, 68
Decidual cells of Friedlander, 68
Deiters, sustentacular cells of, 148
Delage, on artificial parthenogenesis in Echinus,

17

Denis, on rudiment of utricle and saccule, 146
Deutoplasm, 2, 10
Diaphragm, development of, 241, 243

origin of muscular tissue in, 243

nerve -supply of, 243
Diencephalon, 115

Disc, intervertebral, development of, 250
Discus proligerus, 9, 191
Disse, on decidual cells in rats and mice, 67

cloacal plate, 201

development of olfactory nerve in birds,

155

Dixon, A. F., on origin of sheath of Schwann,
100

great superficial petrosal nerve and chorda

tympani, 132

Dog, origin of lateral mesodermic sheets in,
Bonnet, 39

blastoporic aperture in, Bonnet, 43

notochord in, origin of head end of,

Bonnet, 49

Dohrn, cell-chain theory of nerve-fibres, 99
Dorsiflexion in early embryos, 82
Dorsum sellae, origin of, 106, 254
Driiner, on origin of auditory ossicles, 260
Duct, vitelline, formation of, 55

Wolffian, development of, 178

bile, origin of, 173

of Santorini, 175, 176

of Wirsung, 175

Stenson's, development of, 165

Mullerian, 193

ejaculatory, 192

lacrymal, 144
Ducts of Cuvier, 206, 208, 226, 232

genital, development of, 186
Ductus arteriosus, 219, 233, 235

Imgualis, 168

thyreoglossus, 168

thyroideus, 168

venosus (Arantii), 226, 233
Duodenal loop, formation of, 161, 246

lumen, occlusion of, 163

EAR-VESICLES, appearance of, 83

external, 88, 258

development of, 145

middle, 149
Ebner, von, on zona radiata, 8, 9

division of sclerotomes in Reptilia, 250

Echidna, cloaca in, 203

thyroid cartilage in, 167

lateral thyroid in, 169
Echinoderm, fertilisation in, 14
Ectoderm, embryonic, formation of, 29

cells in mesenchyme, 59

organs derived from, 93

origin of early mesoderm from, in Tarsius,

34
Egg-protoplasm, structure of, 9

tubes of Pfliiger, 190
Ejaculatory duct, 192
Elliot-Smith, area paraterminalis of, 118

on rhinencephalon, 122

neopallium, 122

fasciculus prsecommissuralis of, 124
Embryo, head end of, origin of, 47

situation of growth centre in, 47

estimation of age of, 80

general history of development of, 80 et
seq.

separation of, 52, 53
Embryonic ectoderm, 29

axis, development of, 35, 37

cell -mass, 26

vessels, theories as to origin of, 61
Eminence, collateral, 124

Mullerian, 194
Endolymph canal, 146

saccule, 146

Endothelium, lining of heart-tube, 210
Entoderm, primitive, formation of, 27

primitive, formation of, by invagination
in holoblastic ova, 43

early mesoderm from, in Semnopithecus,
34

organs derived from, 93

development of notochord from, in Am phi -

oxus and Reptilia, 46
Entodermic sac, formation of, 28
Entomeres, 43
Entypy of germinal area, 30
Ependymal lining of neural canal, 96
Epibolic process in segmentation, 25
Epididymis, canal of, from Wolffian duct, 192
Epiglottis, 159

Epiphysis cerebri, or pineal body, 113
Epithelial plug in nostril, in second month, 88
Epoophoron, 192
Eternod, sinus ensiformis of, 60

on blood-vessels in embryo of thirteenth
day, 63

characters of thirteenth-day embryo of, 82
Eustachian tube, 149

valve, 213, 233

Excretory organs, development of, 177
Eye, origin of muscles of, in lower vertebrates,
57, 248

development of, 136

muscles of, 248

coats of, 142, 143

anterior chamber of, 143, 144

vitreous and lens-capsule, 142
Eyelid, third, or membrana nictitans, 144
Eyelids, development of, 144

FACE, formation of, 86, 156
Falciform ligament, 244
Fallopian tube, development of, 194
Falx cerebri, rudiment of, 116

INDEX

265

Farmer, on fusion of chromosome in synapsis,

22

Fascia dentata, 122, 124
Fasiculus prsecommissuralis, 124
Fauces, isthmus of, 157
Fertilisation, 14
Fibres of Muller, 140
Filum terminate, 103
Fimbria, 122, 124
Fissura prima, 110, 118

secunda, 110
Fissure, floccular, 110

great horizontal, late appearance of, 110

external rhinal, 116

choroidal, 118, 140

hippocampal, 122, 124

callosal, 124

calcarine, 124

collateral, 124

of Sylvius, 124

sphenoidal, 256

of Ebner, in sclerotomes, 250
Fleischmann, on development of cloaca, 201,

203
Fleming, on zona radiata, 8

origin of Wolffian duct from ectoderm,

178

Flexures of brain, 106
Flint, on division of bronchial stem, 166
Flocculus, 110
Foetus, application of term, 89

history of, 89 et seq.
Fold of Marshall, vestigial, 232

mesolateral, of Brachet, 242
Follicles, primitive, in ovary, 190

development of Graafian, 190
Foramen caecum, of Morgagni, 159, 168

of Winslow, 242, 244

jugular, 256

for hypoglossal nerve, 256

ovale, 212, 213, 233, 256
effects of persistence of, 235

spinosum, 256

rotundum, 256

opticum, 256

of Monro, 113, 116

inter ventricular, 215
closure of, 217

jugulare spurium, 231
Fore-brain, or prosencephalon, 111

alar and basal laminae in, 113
Fore-gut, first formation of; 53
Formatio reticularis, nerve-cells of, 108
Formative cell-mass, 26
Fornix, 124
Fossa subarcuata, 256
Fovea, superior and inferior, 108
Fretum Halleri, 206
Froriep, ganglion of, 127, 130

on branchial sense-organs, 128

theory of head-segmentation, 249

development of basis cranii in calf, 254
Funiculus solitarius, 108
Fiirbringer, on occipito-spinal nerves, 127

origin of trochlear nerve, 128

theory of head-segmentation, 249
Furcula, 159, 167

GALL-BLADDER, rudiment of, 173
Ganglion, acoustico-facial, 132, 148

Ganglion crest, 126]

Froriep's, 127, 130

habenulae, 114

petrous, 132

nodosum, 130

Gasserian, rudiment of, 133

geniculate, 132, 148

ciliary, 133

vestibular, 148

spiral, 148

cochlear, 148

Meckel's, 133

otic, 133

sub-maxillary, 133
Ganglia, spinal, development of, 98, 99

rudiments of, in cerebral nerves, 128

sympathetic, 135

origin of segmental, in Petromyzon, 129
Gartner, duct of, 193
Gastrula theory, 42

applied to Primates, 45
Gastrulation, in amphibian ova, 43

in lower Amniota, 43

stages of, modified by accumulation of
yolk, 45

changes corresponding to, in Primates,
45

two stages of, Keibel and Hubrecht, 47
Gegenbaur, on head- segmentation, 249
Geniculate bodies, 111, 114, 115
Genital organs, external, appearance of, 201

cord, 181, 193, 197

ridge, 186

glands, development of, 186

ducts, development of, 192

ducts, fate of, in the two sexes, 192

mesentery, 199

folds, outer and inner, 202

papiUa, 201, 202
Germ-cells. See Sex-cells.

centres in developing lymph-glands, 236
Germinal layers, formation of, 27

area, entypy of, 30

area, in rabbit, 32

epithelium, 186, 191

spot, 10

vesicle, structure of, 9, 10

zone, development of, in spinal cord, 95
Giglio-Tos, on seventh and eighth nerves, 132

ganglion of trigeminal nerve, 133
Gill-clefts, 83, 158

pouches in higher vertebrates, 158
Giraldes, organ of, 192
Gland, lacrymal, 144

prostate, 194

thymus, 169

thyroid, 168

Glands of alimentary canal, development' of,
164

salivary, 165

sweat, 94

sebaceous, 94

in uterine mucosa, changes in, in preg-
nancy, 68

genital, development of, 186
Glans penis, 203
Globular processes, 87, 151
Glomeruli, development of, in pronephros, 177

in mesonephros, 182

in metanephros, 184

INDEX

Glottis, rudimentary, development of, 166
Goll, rudiments of tracts of, 103
Graafian follicle, origin of, 190
Graham Kerr, on muscle-plate in Lepidosiren,
56

visual cells in Lepidosiren, 140
Greil, on aortic bulb, 207
Groove, primitive, 36

lacrymal, 88

urethral, 202
Gubernaculum testis, 199
Guinea-pig, formation of amnion in, 31

entypy of germinal area in, 31

allantois in, 55

imbedding of ovum in, von Spec, 67
Gymnophiona, pronephros in, 177
Gyrus subcallosus of Zuckerkandl, 124

cinguli, 124

HABENTJLAR REGION, 114

Haeckel, on gastrula stage, 43

Haemolymph-glands, 236

Haftstiel, or connecting stalk, 34

Hair-cells, 148

Hammar, on development of tongue, 159

of tonsils, 160

of tympanum, 149

on thymus, 172
Harrison, on origin of sheath of Schwann,

100
Hart, Berry, on origin of hymen, 195

development of prepuce, 203
Hassall, corpuscles of, origin of, 172
Head-plate in trilaminar blastoderm, 35

end of embryonic axis, origin of, 47

muscles of, 248

cavities in lower forms, 248
Heape, on zona radiata, 8

rudimentary blastopore in the mole, 43

pre- maturation stages in rabbit, 11
Heart, appearance of rudiment of, 61

endothelial lining of, origin of, 62, 206

appearance of constrictions in, 63

development of, 206 el seq.

septa of, 211, 212

valves of, 211, 213, 215, 216

fretum Halleri of, 207

auricular canal of, 206, 213

chordae tendineae of, 215

foramen ovale of, 212, 233

limbus Vieussenii of, 235

muscle of, origin of, 62

column® carneae of, 215

foetal, peculiarities of, 233
Hedgehog, entypy of germinal area in, 31

blastoporic aperture in, Hubrecht, 43
Held on neuroblasts, 98

origin of sensory nerve roots, 99

theory of origin of nerve -fibres, 100
Henle, looped tubule of, 184
Hensen, knot of, 32, 35

primitive nerve theory of, 100

duct of, or canalis reuniens, 148
Hepatic veins, 224

Heredity, chromosome cleavage in, importance
of, 16

Weismann's theory of, 21

Mendel's Law of, 23

Herring, on development of kidney-tubules,
184

Hertwig, on nuclear phenomena in Ascaria
megalocephala, 12

development of mesoderm, 36

concrescence theory of, 47

theory of origin of nerve-fibres, 100

mesenchyme of, 58

Heterotypical mitosis of sex-cells, 4, 18
Hind-brain or rhombencephalon, 106
Hind-gut, formation of, 54

connection of, with Wolffian duct, 164
Hippocampal commissure, 122, 123

formation, fate of, 124
Hippocampus, development of, 121
His, W., Jr., on origin of sympathetic, 135

cochlear ganglion, 148
His, W., Bauchstiel or abdominal stalk of, 54

rule for estimating age of embryos, 80

on embryonic blood-vessels, from vascular
area, 61

concrescence theory of, 47

characters of embryos described by, 82,
83, 84, 86

neuroblasts of, 96

spongioblasts of, 96

on neurone theory, 100

closure of neural lamina?, 108

brain of sixth -week embryo, 113

growth of corpus callosum, 124

direction of motor fibres of facial nerve,
132

development of sympathetic, 133, 135

trapezoid area of, 118

on optic vesicle, development of, 136

appearance of nerve -fibres in optic stalk,
140

development of olfactory nerve, 99,
155

tuber culum impar, 159

furcula of, 159, 167

on accessory thyroid bodies, 168

development of thymus, 172

characters of division of bronchi, 166

isthmus of, 107

septum superius of, 212

sulcus terminalis of, 213

porta vestibuli of, 234
Hochstetter, on inferior vena cava, 227

development of nose, 151
Holoblastic, definition of, 27

segmentation in mammals, 25
Homotypical mitosis of sex-cells, 18, 19
Hubrecht, on trophoblast, 26, 71

phylogeny of mammalian ovum, 27

Tarsius spectrum, 27

formation of entodermic sac, 29

invagination of blastoderm, 30

derivation of early mesoderm in Tarsius.
34, 35, 39

gastrulation, two phases of, 47

blastoporic aperture in mammals, 43

origin of head end of notochord in
Tarsius, 49

vascular mesenchyme, origin of, 62

development of placenta, 74

radial symmetry of fore-part of head,

249

Huntington, on origin of lymph-vessels, 236
Hyaloid membrane, 143

artery, distribution of, 142, 143
Hydatid of Morgagni, 194

INDEX

267

Hylobates, germinal area in, 30

early chorionic vessels in, 60
Hymen, 195
Hyoid arch, muscles of, 248

bone, 257

Hypochordal rod, 252
Hypoglossal nerve, 127, 130, 249, 256
Hypophysis, or ectodermic portion

pituitary body, 115, 256
Hypothalamus, 213

IDIOSOME, of spermatid, 6

of oocyte, 10

Imbedding of ovum, 65, 66, 67
Inaba, on origin of adrenals, 206
Incus, origin of, 257, 258
Indusium, 124
Infundibulum, relation of notochord to, 49

development of, 113, 115

Ingalls, on origin of premuscular tissue in limb-
buds, 247

Inguinal pouch, 200

Insectivora, early imbedding of ovum in, 66
Intermediate cell -mass, formation of, 51

development of excretory organs from,

177

Intervillous space, 76
Intestines, development of, 160 et seq.
Inversion of germ-layers. See Entypy.
Iris, 144

Island of Reil, formation of, 124
Isthmus faucium, 157

of His, 107

JACOBSON'S ORGAN, 155

Jacoby, on basis cranii in human embryo, 254

Janosik, on Wolffian duct in marmot, 178

origin of adrenals, 206

Joint- cavities, development of, in limbs, 254
Jones, F. W., on development of vagina, 195

development of cloaca, 203
Jugular vein, primitive, 229
external, 231

foramen, 256

KATJJUS, on development of tongue in lower

forms, 159
development of larynx in man, 166, 167

Karyokinesis, 4

Karyoplasm, 1, 3

Karyosomes, 3

Keibel, on invagination of blastoderm in

human ovum, 30
blastoporic aperture in rabbit, 43
gastrulation, two phases of, 47
closure of neural tube in pig, 105
development of optic vesicle in pig, 136
Wolffian duct, 178
development of cloacal region, 203
division of cloaca in Echidna, 203

Keith, on aortic bulb, 210

Kephalogenesis, stage of gastrulation, Hu-
brecht, 47

Kidney, permanent or metanephros, 182
primitive pelvis of, 182
development of tubules of, 184
primary lobulation of, 184

Klein, on origin of lymph- vessels, 235

Kohn, on development of nerves, 100

development of sympathetic system, 135

Kohn, chromaffin bodies, 135

development of medulla of adrenals, 206
Kolliker, on structure of spermatozoon, 5
outgrowth theory of nerve-fibres, 99
development of sympathetic system, 133
lens-capsule and vitreous, 142
air-cells of lungs, 166
thymus, 172
Kollmann, on muscle-plate in human embryo,

56
characters of fourteenth-day embryo,

described by, 82
segmental character of Wolffian body in

early human embryo, 182
origin of Wolffian duct from ectoderm,

178

development of mouth, 158
origin of limb-muscles, 247
Koltzoff, on cerebral nerve placodes, 128

origin of segmental ganglia in Petro-

myzon, 129

head-segmentation in Petromyzon, 249
Korschelt, on origin of heterotypical prophase

figures, 22

Kowalewsky on gastrula stage, 43
Kupffer, outgrowth theory of nerve-fibres,

99
on cerebral nerve placodes, 128

LABIA MAJOBA, 203

minora, 203

Labyrinth, recess of the, 145
Lacrymal canals and ducts, 144

groove, 88

gland, 144

Lacrymale, punctum, 144
Laguesse, on cellular elements in Selachian

spleen, 237
Lamina affixa, 121

terminalis, 116, 122, 123

spiral, 148
Laminae of spinal cord, alar and basal, 101,

102
Lancisii, nervi, as vestige of hippocampal

formation, 124

Lane-Clayton, on germinal epithelium, 191
Langer, on development of lymphatics, 236
Langhans' layer, 75, 76
Lankester, on blastopore, 43
Lanugo, 92, 94
Laryngeal nerves, inferior, relation to aortic

arches, 221
Larynx, 166
Lateral sinus, 229
Lecithophore of Van Beneden, 27
Lenhossek, outgrowth theory of nerve-fibres,

99
Lens, rudiment of, from surface ectoderm, 137

epithelium, 139

fibres, 139

vesicle of, 84, 137

transitional zone of, 139

capsule of, 141, 142

Lenticular nucleus, development of, 121
Leopold, ovum of, 65, 75, 80
Lepidosiren, muscle-plate in, 56

visual cells in, 140

fate of cellular elements of spleen in, 237

anterior mesoderm in, Agar, 249
Levi, on development of skull, 254 256

268

INDEX

Lewis, on muscle-plate in human embryo, 56

development of columnse carneae, 215

sub-cardinal veins, 227

origin of lymphatics, 236

origin of premuscular tissue in limb-
buds, 247
Ligament, round, 199

suspensory, of ovary, 197

broad, 197

stylo-hyoid, 259

falciform, 244

coronary, 243

Ligaments, articular, 250, 253, 254
Ligamentum arteriosum, 221
Ligula, 108

Limb-buds, earliest signs of, 84
Limbs, segments of, 89

rotation of, 89

skeleton of, 254

arteries of, 224

veins of, 232

nerves of, primary, ventral and dorsal
branches of, 126

muscles of, 246
Limbus Vieussenii, 235
Linin, 3
Lips, development of, 151, 158

rhombic, formation of, 108
Liquor amnii, 70

folliculi, 191
Liver, rudiment of, 161, 173

parenchyma, development of, 174

veins of, 174, 226

capsule of, 174

activity of, as blood-forming organ in
early months, 174

sinusoids of, 194, 224
Lobules of liver, 175
Lobus pyriformis, 118

Spigelii, 175
Locus coeruleus, 108

Loeb, on parthenogenesis in Echinus, 17
Lung, development of, 166

rudiments of, in body-cavity, 239

division of primitive, into lobes, 166
Luschka, on primitive jugular vein, 231
Lymphatic system, 235
Lymph-cells in thymus, source of, 172

hearts in pig, Sabin, 236

glands, 236

cords, 236
Lyra, origin of, 123

MACACUS NEMESTRINUS, blastomere stage
in, 25

Mackay, on segmental arteries in limbs, 224

Macula germinativa of Wagner, 10

Magma reticularis, 35, 80

Mall, on early human ovum, 81
inversion of germ-layers, 30
intestine, development of, 162
development of body-wall, 246
jugular veins, 231
vitelline arteries, 223

Malleus, origin of, 257

Malpighian corpuscles of spleen, 237
of Wolffian body, 178, 182

Mandible, primitive, 257

Mantle zone, development of, 95

Manubrium sterni, origin of, 253

Marmot, Wolffian duct in, Janosik, 178
Marshall, oblique vein of, 232

vestigial fold of, 232
Maturation of the oocyte, 10, 11

nuclear phenomena during, 17
Maurer, on gill-pouches in higher vertebrates,
158

lateral thyroid in Echidna, 169

on thymus, 172

fate of outer lamella of myotome, 56
McClure, on origin of lymph- vessels, 236
Meatus, external auditory, 149
Meckel's cartilage, 150, 257, 258

ganglion, 133

Medullary nerve-sheath, appearance of, 103
Membrana limitans of spinal cord, 96

nictitans, 144

reunions, 246

tectoria, 148

tympani, 149, 150
Membrane, infrachoroideal, 119

pleuro-pericardial, 241

peritoneo-pericardial, 242

mucous, of uterus and vagina, 196

hyaloid, 143

pupillary, 143

cloacal, 201
Membranes, foetal, 65, 70, 71

of spinal cord, 105, 250
Mendel, Law of Heredity of, 23
Menstrual decidua, 67, 68
Meroblastic segmentation, 27
Merogony in Echinus, 17
Mesenchyme, 56, 58, 59, 62

origin of lymphatics in, 235

of limb-buds, 246

theory, Hertwig, 58
Mesencephalon, or mid-brain, 106, 110

connection with optic stalks, 141

connection of Gasserian ganglion with, 133
Mesentery, caval, 227, 242

of genital gland, 189, 197

primitive intestinal, 161

ventral, 244

dorsal, 244
Mesocardium, dorsal, 62, 206

lateral, 208, 240

ventral, absence of, 237
Mesocolon, 244
Mesoderm, formation of, 32

parietal and visceral layers of, 33

precocious formation of, in Primates, 34

early, origin of, in man, 34

early, origin of, in Tarsius, 34

early, origin of, in Semnopithecus, 34

embryonic rudiment imbedded in, in
higher Primates, 34

as origin of magma reticularis, 35

from entodermal plate, 35

from primitive streak, 35

development of, by coelomic pouches,
Hertwig, 36

from entodermal ring, Hubrecht, 35

in Amphioxus and Reptilia, 46

lateral, history of, 49

paraxial, 49

intermediate, 51

two orders of, 58

anterior, of Lepidosiren, Agar, 249

as origin of sympathetic ganglion- cells, 133

INDEX

269

Mesoduodenum, disappearance of, 246

Mesogaster, 246

Mesolateral fold of Brachet, 242

Mesonephros, 177, 178

Mesorchium, 189, 199

Mesovarium, 189, 197

Mesothelium, or endothelium of body- cavity,

57

Mesosalpinx, 192, 197
Metanephros, 177, 182
Mid-brain, 106, 110
Milhalkovics on olfactory nerves, 156
Minot, sex theory of, 20

concrescence theory of, 47

on placenta, 74

on trophoderm, 74

sinusoids of, 174, 224
Mitochondria, 2
Mitosis in somatic cells, 4

in sexual cells, 17
Modiolus, 148
Mole, inversion of germinal area in, 31

blastoporic aperture in, 43
Monotremes, ovum of, 27
Monro, sulcus of, 113

foramen of, 113, 116
Montgomery on synapsis, 22
Moore, synapsis of, 19, 22
Morbus cceruleus, 235
Morgan and Wilson on artificial production of

centrosomes, 2
Morula or mulberry mass, 25
Mouse, entypy of germinal area in, 31

phagocytic action of decidual cells in, 67
Mouth, 156, 158

non- correspondence of primitive with

permanent, 158

Muller, Eric, on arteries of upper limbs, 224
Miiller, Johannes, fibres of, 140
MiiUerian duct, 193

eminence, 194

Muridse, imbedding of ovum in decidua in, 67
Muscle-plate, 56, 246

Muscles, voluntary, from paraxial mesoderm,
49

early stages in development of, 56

of head, 248

of limbs, 247, 248

of eye, 248

of tongue, 248

of mastication, 248

branchial group, 248

of hyoid arch, 248

platysma, 248

sterno-mastoid, 130, 249

trapezius, 249

cremaster, 199

longissimus dorsi, 246

ilio-costalis, 246

spinalis dorsi, 246

obliquus externus, 246

obliquus internus, 246

transversalis, 246

rectus abdominis, 246
Muscular wall of uterus, 196
Musculature, visceral, cerebral nerve supply

of, 128

Musculi papillares, 215
Myelospongium, 96
Myoccel, 246

Myosepta, 2r>u

Myotomes, differentiation of, from primitive

segments, 56
extension of, into somatopleure, 246

NAGEL, on vagina, development of, 195

Nares, posterior, or choanae, 153

Nasal processes, lateral and mesial, 87, 151, 153

capsule, 154, 257

fossa, development of ciliated epithelium
in, 156

septum, 256
Neocranium, 249
Neopallium, Elliot-Smith, 122
Nephridia, 178

Nephro-genetic blastema, development of
tubules in, 177

cord, 178, 182

Nephrotomes, differentiation of, from primi-
tive segment, 57

in Anamnia, tissue corresponding to, 51
Nephrostome, 177
Nerves, peripheral, histogenesis of, 94

development of, 125

segmental, dorsal and ventral branches of,
126

occipito-spinal, Furbringer, 127

cerebral, development of, 127, 130 seq.

olfactory, 155

great superficial petrosal, 132

chorda tympani, 132

oculo-motorius and trochlearis, 127, 133

trigeminal, 128, 132, 133

abducens, 127, 132

facialis, 128, 132

acousticus, 132

vagus, 128, 130

glosso-pharyngeal, development of, 128,
132

spinal accessory, 128, 130

hypoglossal, 127, 130
Nerve ganglia from neural crest, 48

roots, origin of, 98

fibres, theories of formation of, 99

plexuses for limbs, rudiments, 126
Nervous system, central, origin of, 48

central, development of, 94 et seq.

peripheral, development of, 125
Neural canal, formation of, 48

relation of, to neurenteric canal, 48

crest, formation of, 48, 98

groove, formation of, 48

plate, origin of, 48

folds, 48

arch, development of, 250
Neurenteric canal, 39
Neuroblasts, origin of, 96
Neurocranium, 256
Neuroglia, origin of, 96
Neuromeres, 107, 129
Neurone theory, basis of, 100
Neuropore, anterior, 48
Nicolas, on development of thyroid cartilage,

167

Nose, formation of, 151
Notogenesis, stage of gastrulation, Hubrecht,

47
Notochord, earliest appearance of rudiment of,

32
formation of, 49

270

INDEX

Notochord, traces of, in adult, 49

head end of, relations of anterior primitive
segments to, 50

formation of vertebrae round, 250, 251

disappearance of, in vertebrae, 250

connection with bucco-pharyngeal mem- i
brane, 254

connection with cranium, 254
Notochordal canal, formation of, 39

sheath, development of, 250

plate, 37

canal, opening of, as representing blasto-

pore, 45

Nuck, canal of, 201
Nuclei of origin of cerebral nerves, 127
Nucleolus, true, of animal cell, 4
Nucleus, structure of, 1, 2, 3

pulposus, 251

OBEX, 108

Occipital segment of basi-cranial plate, 256

Occipito-spinal nerves, 127, 130

<Esophagus, 160

Olfactory pit, 83, 151

bulb and tract, origin of, 118

nerve, 155

capsule, 255
Omenta, 236, 244, 246
Oocyte, structure of, 7

maturation of, 10

development of, in germinal epithelium,

191
Oogenesis, 7, 10

synapsis in, 22
Oogonia, 10

Opercula of island of Reil, 124
Opossum, blastoporic aperture in, Selenka, 43
Optic chiasma or commissure, 113, 141

cup, development of, 137, 140

disc, 141

recess, 113

foramen, 256

stalk, formation of, 106

stalk, appearance of nerve-fibres in, 141

tract, 141

vesicles, 82, 106, 136
Ora serrata, 140
Organ of Corti, 148
Organs of the body, classification of, according '

to origin, 93
Os articulare, 260
Os dentale, 257

Ossicles, auditory, formation of, 257
Ostium primum, Born, 212
Ostium secundum, 212
Otis on cloacal region, 201, 202
Ovary, development of, 189

imbedding of blastocyst in stroma of, 73

change of position of, 196
Ovum, alecithal, 9

germinal vesicle of, 9

holoblastic, 25, 27

attachment of, to uterus, 65

site of implantation of human, 65

human, villi in, 75

imbedding of, 65, 66, 67

fertilisation of, 14

maturation of, 10

meroblastic, 27

segmentation of, 25 •

Ovum, zona pellucida of, 7
zona radiata of, 8
mammalian, phylogeny of, 27
development of primitive, 190

Oxychromatin, 3

PAL^OCBANIUM, 249
Palatal folds, 154
Palate, primitive, 154

permanent, formation of, 155
Pallium, 116
Pancreas, development of, 175

ducts of, 176
Papilla, genital, 201
Papillae of tongue, 160
Papillary muscles, 215
Parachordal cartilages, absence of, in human

embryo, 254

Paradidymis, or organ of Giraldes, 192
Paraflocculus, 110
Paraplasm, 2

Parathymus. See Parathyroid.
Parathyroid bodies, 172
Parker, on stapes, 260
Paroophoron, 193
Parovarium, 192
Pars intermedia, of facial nerve, 132

membranacea septi, 217
Paterson, on origin of sympathetic system, 133

limb-muscles, 247

development of sternum, 253
Peduncles, superior cerebellar, 110

of cerebrum, 111

Pelvis of kidney, primitive, 182, 183
Penis, glans, 203

Perforated spot, anterior, 118, 120
Perforatorium of spermatozoon, 5
Pericardium, 52, 206, 237
Perilymph, development of, in inner ear, 148
Perineum, 201, 202
Peritoneum, 242, 244
Peters, on early human ovum, 27

on placenta, 74

ovum of, mesoderm in, 35

imbedding of, in uterine mucous mem-
brane, 65

placental plasmodium in, 73
description of, 80
Petromyzon, segmentation of mesoderm in, 50

head-segmentation in, 249

origin of segmental ganglia in, 129
Pfliiger, egg- tubes of, 190
Phagocytic action of trophoblast-cells, 74
Phaochrome or chromaffin bodies, 135
Phaochromoblast, 136
Pharynx, development of, 53, 158
Philtrum, 151
Pia mater, 105, 116, 250
Pig, muscle-plate in, 56

closure of neural tube in, 105

development of tongue in, 159

development of optic vesicle in, 136

number of neuromeres in, 129

duct of Gartner in, 193

origin of lymphatics in, 236

masticatory muscles in, 248

epithelium of genital ridge in, 188
Pineal body, 111, 113, 114
Pituitary body, cerebral lobe of, 111, 115

glandular portion of, 115/156

INDEX

271

Placenta, 65

formation of, 72 et seq.

description of full-time, 77
Placental syncytium, question of origin of, 73
Placodes, cerebral nerve, relation of ganglia

to, 128, 130, 133
Plasmodiblast, Van Beneden, 74
Plasmodium, placenta!, or syncytium, 73
Plasmosomes, 4
Platt, Julia, on theory of head-segmentation,

249

Pleural cavity, 166, 239, 242
Pleuro-peritoneal cavity, 241
Plexuses of sympathetic, development of, 135
Plica semilunaris, 144

gubernatrix, 196, 197
Polar bodies, formation of, 11, 12
Pole, animal, of ovum, 9

vegetative, of ovum, 9
Pons, formation of, 109
Porta vestibuli of His, 234
Portal vein, 235
Post-branchial body, 169
Pouches, inguinal, 200

visceral, 158
Praehyoid glands, 168
Pregnancy, changes of uterus in, 67

ovarian, plasmodial layer in, 73
Premuscular sheath, in limb-buds, 247
Prepuce, 203
Primates, embryological limits of, 27

time of formation of mesoderm in, 34

gastrula theory applied to, 45

absence of terminal sinus in, 59

earliest blood-vessels on under-aspect of

yolk-sac in, 59
Primitive groove, 36

streak, 32

phylogeny of, 48
fate of, 47

origin of mesoderm from, 35
representing gastrula-mouth, 47
as phase in development of embryonic I
axis, 48

plate, in Reptilia, 45

segments, formation of, 49
Pro-amnion, region in human embryo corre-
sponding to, 52
Process, fronto-nasal, 84, 156

maxillary, 84, 156

lateral nasal, 87, 151

mesial nasal, 87, 151

intermaxillary, 154

turbinate, 155, 257

styloid, 257

odontoid, 252

costal, 250

neural, 250

articular, 252

costal, in cervical, lumbar, and sacral

regions, 252

Processus vaginalis, 199
Proctodceal depression, 202
Procestrum, 11
Pronephros, 177

Prosencephalon or fore-brain, 106, 111
Prostatic vesicle, 194
Protochordal plate (Hubrecht), 35

knot (Hubrecht), 35

process, 37, 46

Protoplasm, 1

Protostoma represented by primitive streak, 48

Protovertebrae. See Primitive Segments.

Psalterium or lyra, 123

Pseudo-chromosomes, in maturation, 10

Pteropus edulis, entypy of germinal area in, 32

Pulmonary arch, 217, 219

Pulvinar, 114

Pupillary membrane, 143

Pur kin je, germinal vesicle of, 9

Pyramid of cerebellum, 110

of thyroid, 168

tracts, 105
Pyramids, appearance of, 108

straight tubules of, in kidney, 185

QUADRATE BONE, homology with incus, 260

RABBIT, maturation of ovum in, 11

germinal area in, 32

formation of mesoderm in, 32

notochordal canal in, 39

blastoporic aperture in, Keibel, 43

development of lens in, 138

Wolffian duct in, 178

tubules of Wolffian body in, 181

epithelium of genital ridge in, 188
Rabl, on origin of vessels, 61

lens- vesicle, 138

vitreous body, 142

Wolffian duct in rabbit, 178
Ramon y Cajal, on posterior roots of spinal

nerves, 130

outgrowth theory of nerves, 99
Ramus communicans, 135
Ranvier, on origin of lymphatics, 236
Rat, entypy of germinal area in, 31

phagocytic action of decidual cells in, 67

persistence of stapedial artery in, 222

primitive vertebrae in, 250

hypochordal rod in, 252

development of basis cranii in, 254
Rathke, diverticulum of, or pocket of, 156

on development of cloaca, 203
Rauber, layer of, 31
Ravn, on formation of septum transversum,

238

Rays, medullary, of kidney, 185
Rectum, 164, 185, 203
Rectus abdominis, 243, 246
Reichert, scar of, 69

disposition of villi in ovum described by,
81

cartilage of, 150

on incus and stapes, 258
Reil, island of, 142

Remak, on origin of sympathetic, 133
Reptilia, notochordal canal in, 41

stages of gastrulation in, 45

blastoporic aperture in, 46

aortic bulb in, Greil, 210

fold in heart analogous to septum
spurium, 212

liver-parenchyma in, 174

division of sclerotomes, v. Ebner, 250
Restiform bodies, 108
Respiratory passages, rudiment of, 158
Rete testis, 192

tubules, 188, 191
* vasculosum lentis, 142

272

INDEX

Retina, development of, 140

pars ciliaris of, 140

rods and cones of, 140

hexagonal pigment-epithelium of, 140

nerve-fibre layer of, 140

central artery of, 142, 221
Retterer, on development of cloaca! region, 203
Retzius, on zona radiata, 8

epithelial plug in nostril, 88
Reuter, on eye-muscles, 248

masticatory muscles in pig, 248
Rhinencephalon, 116,122
Rhinopallium, 122

Rhombencephalon, or hind-brain, 106
Rhombic brain, appearance of segmentation
in, 129

lips, formation of, 108
Ribs, development of, 252
Ridge, Wolffian, appearance of, 84, 178

relation of pleuro-peritoneal mem-
branes to, 241

genital, 186
Robinson, on secondary caudal arch, 223

pericardial coelom, 237
Rodents, allantois in, 55
Rods of Corti, 148
Rolando, origin of substance of, 108
Riickert, on ventral mesoderm, 34

theory of origin of blood-vessels, 62

theory of origin of Wolffian duct, 178
Ruge, on development of sternum, 253

SABIN, on origin of lymphatics, 236
Sac of peritoneum, lesser, 244
Salzer, on primitive jugular vein, 231
Sauropsida, vascular area in, 59
Schonemann, on nasal fossa, 155
Schreiner, on nephrogenetic cord, 182
Schultze, 0., theory of origin of nerves, 100

on cartilaginous stage of vertebral column,
251

division of sclerotomes in mammalia, 250
Schwann, origin of sheath of, 99, 100
Scleromeres or primitive vertebrae (Bardeen),

250

Sclerotic, 143
Sclerotomes, 56, 250
Scrotum, 199, 203
Secreting tubules, origin of, 184
Sedgwick, on phylogeny of primitive streak, 48

theory of primitive nerves, 100
Seessel's pocket, 157
Segmentation nucleus, 15

without fertilisation, 17

of ovum, 25

place of occurrence of, 86

mesodermic, 50

of the head, question of, 249
Segments, mesodermic or primitive, 49, 50

order of appearance of, 50

development of cavity in, 51

preoccipital, cleavage of, in lower verte-
brates, 57

of limbs, appearance of, 89
Selachians, relation of mesodermic segmenta-
tion to notochord in, 50

sympathetic ganglion cells in, Balfour, 135

branchial sense organs in, 128

fate of cellular elements in spleen, 237

liver-parenchyma in, 174

Selachians, development "of eye-muscles in, 248

head-segmentation in, 249
Selenka, on entypy of germinal area, 31

early mesoderm in Semnopithecus, 34, 35

ovum of Macacus nemestrinus, 25

Hylobates embryos, 30

blastoporic aperture in opossum, 43

early blood-vessels in chorion in Hylobates

rafflesi, 60
Seminal tubules, 191

vesicle, 192
Semnopithecus nasicus, early mesoderm in,

34,35
Sensory nerve roots, origin of, 98

roots of cerebral nerves, 128
Septum, aortic, 216, 218

lucidum, 122, 124

spurium, 212

inferius, His, 212

primum, Born, 212

secundum, Born, 212

transversum, 54, 238

sinus venosus in, 206
Sertoli, cells of, in spermatogenesis, 6
Sex, Minot's theory of, 20

establishment of characters of, 90

in genital glands, 189
Sexual cells, history of, 5 et seq.

dimorphism of, 13

development of primitive, 188
Shield, embryonic, 37
Shrew, origin of middle layer in, Hubrecht, 36

blastoporic aperture in, Hubrecht, 43
Simon, on lateral lobe of thyroid, 169
Sinus ensiformis, Eternod, 60

precervical, 84, 88

urogenital, 164, 185

cavernous, 229

lateral, 229

superior petrosal, 229

inferior petrosal, 229

venosus, 206

Sinusoids, Minot, 174, 224
Skeleton, development of, 250
Skin, development of, 93

stratum corneum of, 94
Skull, development of, 254
Sobotta, on fertilisation in mouse, 14
Soma, 1

history of, 25
Somatopleure, in lower mammals, 33

in higher mammals, 51

giving rise to amnion-folds, 34
Soulie, on origin of adrenals, 206
Spee, on early mesoderm in human ovum, 35

changes in wall of yolk-sac, 60

age of ova described by, 80

imbedding of ovum in guinea-pig, 67

origin of Wolffian duct from ectoderm, 178
Sperm -aster, 15
Spermatic fascia, 199
Spermatids, 6
Spermatocyte, 6
Spermatogonium, 6, 192
Spermatozoon, structure of, 5

accession of, to ovum, 9

fertilisation by, 14

changes in, corresponding to maturation,

18
Sperm-nucleus, 15

INDEX

273

Spigelii, lobus, 175
Spinal ganglia, 98, 125

nerves, development of, 125

cord, development of, 101 seq.
fissures of, 102, 103
membranes of, 105, 250
Splanchnoccel, 177
Splanchnopleure, 33, 51
Spleen, development of, 236

venous sinuses in, 237

in Lepidosiren, 237

as haemopoietic organ in lower verte-
brates, 237
Spongioblasts, 96
Stapes, 257, 258
Stenson, ducts of, 155

duct of (parotid), 165
Sternum, development of, 253
Stieda, on origin of lymph-cells in thymus,

172

Stilling, canal of, 143
Stohr on thymus, 172
Stomach, 160, 161
Stomodoeum, 83, 156

connection with pituitary body, 115

connection with nose, 153
Stratum compactum, 68

spongiosum, 68

corneum of skin, 94

germinativum of ovary, 190

granulosum of Graafian follicle, 191
Streeter on occipital nerves, 130

auditory nerve, 148

development of ear, 146
Striae terminalis, 121
Stricht, Van der, on yolk-nucleus in oocyte, 10

polar bodies in bat, 12

fertilisation in bat, 15

on membrane in human ovum, 9
Stylo-hyoid ligament, 257, 259
Styloid process of temporal bone, 257, 259
Subcardinal veins, 227
Sublingual gland, 165
Submaxillary gland, 165
Sulci of brain, principal, 124

auriculo- ventricular, 210
Sulcus of Monro, 113

terminalis of His, 159, 213
Suprarenal bodies. See Adrenals.
Sutton on chromosomes in synapsis, 22
Sylvius, fossa of, 116

fissure of, 124
Symmetry, bilateral, of vertebrates, origin of,

47
Sympathetic system, 133

superior cervical ganglion of, origin of,
135

ganglia, connection with segmental nerves,
126

connection of, with suprarenal bodies,

135, 206

Sympathoblast, 136
Synapsis of Moore in spermatogenesis, 19, 22

in oogenesis, 22
Syncytium, definition of, 2

mesenchymic, 59

placental, 73

development of, in nervous tissue, 96
Szily, interepithelial network of, 59, 142
as origin of nerve-path, 100

TADPOLES, removal of neural crest in, 100
Tasnia semicircularis, 121
Tail-fold, formation of, 54

gut, 164
Tandler, on duodenal epithelium, 163

arterial arches in mammalia, 217

stapedial artery, 222

vitelline arteries, 223

origin of coeliac artery, 223
Tarsius spectrum, blastocyst stage in, 27

inversion of germinal area in, 31

origin of early mesoderm in, Hubrecht,
34

primitive entodermal plate in, 35

notochordal canal in, 39

blastoporic aperture in, Hubrecht, 43

origin of head end of notochord in,
Hubrecht, 49

mesenchyme in, origin of, from entoderm,
58

origin of blood-vessels in, Hubrecht, 62
Tegmentum, 111
Tegmen tympani, 256
Tela choroidea, 110, 113, 121
Telencephalon, 115
Telolecithal ovum, 9

type of cleavage in, 27
Testicle, descent of, 196, 197, 199
Testis, 191, 192
Tetrads, 20
Thalami, optic, 111
Thompson, Peter, on segments in rhombic

brain, 107, 129
Thymus, 160, 169, 172
Thyroid, 159, 160, 168

cartilage, development of, 167
Tongue, development of, 159

muscles of, 248

foramen caecum of, 159

papillae of, 160

sulcus terminalis of, 159
Tonsils, development of, 160
Torcular Herophili, 229
Trachea, 166

Trapezoid plate or field, 118, 122, 124
Trigone of bladder, 186

Trigonum hypoglossi, from basal lamina,
108

habenulae, 114

olfactorium, 118
Triticea cartilago, origin of, 168
Trophoblast or trophic epiblast, 26

in placentation, 67, 74
Truncus arteriosus, 206

cleavage of, 215
Tuber culum impar, 159
Tuber cinereum, 111, 113

valvulae, 110

Tubules in human embryo representing
prone phos, 177

of Wolffian body, origin of, 181

of kidney, origin of, 183, 184
Tunica vasculosa of lens, 143

albuginea, rudiment of, 191

vaginalis, 199

Tupaja, inversion of germinal area in, 31
Turbinate processes, 155
Tympanic membrane, 149
cavity, 149 seq.
ring, 259

T

274

INDEX

UMBILICAL CORD, formation of, 85 . • gj1-^

opening, primitive, formation of anterior
lip of, 53

vesicle, 55
Umbilicus, position of, at early stage, 90

closure of, 163
Uncus, 118, 122

Ungulates, allantoic vesicle in, 55
Urachus, origin of, 55, 185
Ureter, rudiment of, 182
Urethra, 185, 203

Urinary bladder, development of, 185
Urodaum, Fleischmann, 203
Urogenital fossa, 164

system, development of, 177

sinus, 164, 185
Uterus, changes in, in pregnancy, 67, 68, 69

regeneration of mucous membrane of,
after parturition, 69

formation of, 194
Uterus masculinus, 194
Utricle, development of, 147

prostatic, 194
Uvea, 140
Uvula of cerebellum, 110

of soft palate, 155

VAGINA, development of, 195

vestibule, origin of, 185, 194
Valve of Vieussens, 110

Thebesius, 213
Valves, auriculo-ventricular, 211

semilunar, 216

venous, 211

in lymphatic vessels, 236
Vas deferens, 192
Vasa aberrantia, 192

efferentia, 192

Vascular area, development of, in lower
mammals, 59

formation of embryonic blood - vessels
from, theories of, 61

rings round duodenum, 224
Veins, development of, 224

ascending lumbar, 228

azygos, 228

cardinal, 226

cephalic, 233

hepatic, 224

iliac, transverse, 208

iliac, internal, 226

iliac, external, 226

inferior cava, 226, 228

innominate, 232

intercostal, superior, 222

jugular, internal, 229

jugular, external, 231

jugular, primitive, 229

longitudinal, superior, 229

oblique, of Marshall, 232

portal and hepatic, 224, 225

radial, 233

saphenous, long, 233

saphenous, short, 233

sciatic, 226

subcardinal, 227

subclavian, 231

superior cava, 232

suprarenal, left, 228

tibial, 233

Veins, ulnar, 232

umbilical, 226

viteUine, 206, 224
Velum, inferior medullary, 110

superior medullary, 110

interpositum, 113, 121

primitive, 157
Vena ascendens, or ductus venosus, 226

capitis lateralis, 229

cerebralis posterior, 229

cerebralis media, 229

cerebralis anterior, 229
Venae advehentes and revehentes, 224
Ventricle, terminal, of cord, 103

cerebellar, 110

formation of primitive common, 208, 214

primitive common, first appearance of
septum in, 209

lateral, 116

fifth, origin of, 122
Verdun, on thyroid, 169
Vermis of cerebellum, 110
Vernix caseosa, 92
Vertebrae, development of, 250
Vesicle, germinal, 9

umbilical, formation of, 55

blastodermic, 26

lens, development of, 137

auditory or otic, 145

of thymus, 191

seminal, origin of, 192

optic, appearance of, 82

optic, development of, 106, 111, 136
Visceral arches, origin of, 223

musculature of head, 128

pouches, 158
Vitelline loop of intestine, 161, 162

duct, formation of, 55

stalk, 161

veins, 206, 224
Vitreous chamber, rudiment of, 137

body, development of, 141
Vriese, de, on formation of basilar artery, 223

development of arteries of the limbs, 224

WALDEYER, germinal epithelium of, 186

on placental sinuses, 74

zona radiata, 8
Webster, on placenta, 74
Weismann, theory of, 21

Weiss on development of basis cranii in rat,
254

development of primitive vertebras in rat,
250

division of sclerotomes in mammalia, 250
Wijhe, v., cell- chain theory of nerve-fibres, 99

on head-segments in Selachians, 249

placodes as branchial sense-organs, 128

theorv of head-segmentation, 249
Willis, circle of, 222, 223
Wilson, J. T., on folds on wall of neural tube,

103
Wilson, Ed. B., on artificial parthenogenesis,

15
Winiwarter, on germinal epithelium, 191

synapsis in oogenesis, 22
Winslow, foramen of, 242, 244
Wirsung, duct of, 175
Wolffian body, development of, 178

fate of, in male, 192

INDKX '275

Wolffiaii body, atrophic changes of, 241 Yolk nucleus in oocyte, 10

connexion of pleuro- peritoneal membranes phylogeny of, in mammalian ovum, 27

with, 180, 241 sac, 28, 33, 35, 53, 59

Wolffian duct, 177 circulation, 59, 60

fate of, 192 Young and Robinson, secondary caudal arch
Wolffian mesentery, 180, 199 of, 223

Wolffian ridge, 34, 178, 241

Wrisberg, cartilages of, 167 ZIEGLER, on origin of blood-vessels, 62

Zimmermann, on arterial arches, 217

YOLK, accumulation of, gastrulation modified Zona pellucida, 7

by, 43 Zonule of Zinn, 141, 143

amount of, as determining character of Zuckerkandl, gyrus subcallosus of, 124

egg- cleavage, 9 on chromaffin bodies in human embryo,

granules in egg- protoplasm, 9 135

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1908

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Quain, J.

Quain's elements
of anatomy.

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