(navigation image)
Home American Libraries | Canadian Libraries | Universal Library | Community Texts | Project Gutenberg | Children's Library | Biodiversity Heritage Library | Additional Collections
Search: Advanced Search
Anonymous User (login or join us)
Upload
See other formats

Full text of "Quain's Elements of anatomy"

JC-NRLF 




720 725 



111 ,\:\.V:T, B 




LIBRARY 

UNIVERSITY OF CALIFORNIA 
DAVIS 




H 



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- anc i 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 during 1 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-nucleibecome 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 A 1 
or B 1 , A 2 or B 2 . Four combinations are 
possible between these gametes viz. 

A'A-, A'B 2 , A-B 1 , B'B 2 , 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 ; cce 1 , 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 




mes 3 



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 ; mes 2 , 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 ; mes 1 , 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 ; mes 2 , 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 ; am 1 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., 7 e 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 ; mes 1 , 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 ; mes 3 , 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 



F IG . 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 Kerr 2 ) and pig (Bardeen 3 ). 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; A 1 , A^, the two primitive aortse cut close to where they separate into the allantoic 
arteries, A i , A 2 ; 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. * T 8 ' (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-> J u 8 u l ar 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 c 1 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) ; y 1 , 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?y 2 , 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, sy 2 ). 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> ^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"" l v-~ '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 

normal 1 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 now r 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 is r 
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 

^^5fe^ >^/^r;E) :S N ^?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 (Hardesty 2 ). 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"misph e re gurface j ugt ven tral 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 fa Q j^^ agpe ct, 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 j ater ag roun d e( 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 



fl occu i ar fi ssu res 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 (c 1 , c 2 ) 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 (c 2 ). 
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 w r ay 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-- v v 




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: Futamura 1 ). 

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. S T 



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 ; br l , 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:.-&iv->,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 ; nd l , 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 ; 
mx l , 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.) 5 y ) . 

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. 102; 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 



xiall 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; thr 1 , 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&?r i 




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 Stohr 3 ) 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? 

.*&*>& 



:^;;suwjjj 

tfoS&h!&! 



>e. 



IPNtSPSi 




us 8 *-"*. 







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 PJp rri *"'*\.' 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 Kollmann O 

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 



?&$vr 

&&: '. 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> und i v ided 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 aor t a and p p u i m0 nary 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 Seevers 2 ). 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. Miiller 2 ), 
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 plaental 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 r oreshadowed 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 ; o 3 , fold of mesogastrium or great omentum growing over coils of small 
intestine ; me, mesentery ; m.c., transverse mesocolon ; o 4 (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 . gj 1 -^ 

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 



I'KINTKD UY 

si'ornswooDK AND co. LTD., .\ii\v-si KKKT .SO.UARK 

LONDON 



A LIST OF WORKS ON 

MEDICINE, SURGERY AND 

GENERAL SCIENCE 

CONTENTS 

PAGE 

ANATOMY 2 

BACTERIOLOGY 17 

BIOLOGY 12 

CHEMISTRY 19 

HEALTH AND HYGIENE 15 

MEDICINE 2 

MISCELLANEOUS , 9 

MONOGRAPHS ON BIOCHEMISTRY 24 

OPTICS 18 

PHOTOGRAPHY 18 

PHYSIOLOGY 12 

PROCEEDINGS OF THE ROYAL SOCIETY OF MEDICINE ... 11 

SURGERY 2 

TEXT-BOOKS OF PHYSICAL CHEMISTRY 22 

VETERINARY MEDICINE 11 

ZOOLOGY 12 



LONGMANS, GREEN, & CO 

39 PATERNOSTER ROW, LONDON, E.G. 
91 AND 93 FIFTH AVENUE, NEW YORK 

8 HORNBY ROAD, BOMBAY 

303 BOWBAZAR STREET, CALCUTTA 

1909 



MEDICINE, SURGERY, ANATOMY, ETC. 

ASHBY AND WRIGHT. THE DISEASES OF CHILDREN, 

MEDICAL AND SURGICAL. By HENRY ASHBY, M.D. 
Lond., F.R.C.P., late Physician to the Manchester Children's Hospital ; 
and G. A. WRIGHT, B.A., M.B. Oxon., F.R.C.S. Eng., Surgeon to the 
Manchester Royal Infirmary ; Consulting Surgeon to the Manchester Child- 
ren's Hospital. With 15 Plates (I Coloured) and 241 Illustrations in the 
Text. Fifth Edition. Thoroughly Revised, 1905. 8vo, 21s. net. 

BAIN AND EDGECOMBE. THE PHYSIOLOGY AND 
THERAPEUTICS OF THE HARROGATE WATERS, 
BATHS, AND CLIMATE APPLIED TO THE TREAT- 
MENT OF CHRONIC DISEASE. By WILLIAM BAIN, M.D., 
M.R.C.P., and WILFRID EDGECOMBE, M.D. 8vo, 7s. 6d. net. 



BENNETT. WORKS by Sir WILLIAM H. BENNETT, K.C.V.O., 
F.R.C.S., Surgeon to St. George's Hospital. 

RECURRENT EFFUSION INTO THE KNEE-JOINT AFTER 
INJURY, WITH ESPECIAL REFERENCE TO INTERNAL 
DERANGEMENT, COMMONLY CALLED SLIPPED CAR- 
TILAGE : an Analysis of 750 Cases. A Clinical Lecture delivered at 
St. George's Hospital. With 13 Illustrations. 8vo, 3s. 6d. 

CLINICAL LECTURES ON VARICOSE VEINS OF THE 
LOWER EXTREMITIES. With 3 Plates. 8vo, 6s. 

ON VARICOCELE : A PRACTICAL TREATISE. With 4 Tables 
and a Diagram. 8vo, 5s. 

CLINICAL LECTURES ON ABDOMINAL HERNIA: chiefly 
in relation to Treatment, including the Radical Cure. With 12 Diagrams 
in the Text. 8vo, 8s. 6d. 

ON VARIX, ITS CAUSES AND TREATMENT, WITH 
ESPECIAL REFERENCE TO THROMBOSIS. 8vo, 3s. 6d. 

LECTURE ON THE USE OF MASSAGE AND EARLY 
MOVEMENTS IN RECENT FRACTURES AND OTHER 
COMMON SURGICAL INJURIES : SPRAINS AND THEIR 
CONSEQUENCES : RIGIDITY OF THE SPINE, AND 
THE MANAGEMENT OF STIFF JOINTS GENERALLY. 
With 23 Illustrations. 8vo, 6,s. 

THE PRESENT POSITION OF THE TREATMENT OF 
SIMPLE FRACTURES OF THE LIMBS : an Address delivered 
to the British Medical Association. To which is appended a Summary of 
the Opinions and Practice of about 300 Surgeons. 8vo, 2s. 6d. 



MESSES, LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 3 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 



CABOT. A GUIDE TO THE CLINICAL EXAMINATION 
OF THE BLOOD FOB DIAGNOSTIC PUEPOSES. By 

RICHARD C. CABOT, M.D., Physician to Out-Patients, Massachusetts 
General Hospital. With 3 Coloured Plates and 28 Illus. in Text. 8vo, 16s. 



COATS. A MANUAL OF PATHOLOGY. By JOSEPH COATS, 

M.D., late Professor of Pathology in the University of Glasgow. Fifth 
Edition, 1903. Revised throughout and Edited by LEWIS R. SUTHER- 
LAND, M.D., Professor of Pathology, University of St. Andrews. With 
729 Illustrations and 2 Coloured Plates. 8vo, 28s. net. 



CHEYNE AND BURGHARD. A MANUAL OF SUEGICAL 

TEEATMENT. By Sir W. WATSON CHEYNE, Bart., C.B., M.B., 
F.R.C.S., F.R.S., D.Sc., Professor of Clinical Surgery in King's College, 
London ; Surgeon to King's College Hospital, and the Children's Hospital, 
Paddington Green, etc. ; and F. F. BURGHARD, M.D. and M.S. Lond., 
F.R.G.S., Teacher of Practical Surgery in King's College, London ; Surgeon 
to King's College Hospital, and the Children's Hospital, Paddington Green, 
etc. 



PART I. The treatment of General 
Surgical Diseases, including inflam- 
mation, suppuration, ulceration, 
gangrene, wounds and their compli- 
cations, infective diseases and tum- 
ours; the administration of anaesthe- 
tics. With 66 Illustrations. Royal 
8vo, 9s. net. 

PART II. The treatment of the Surgical 
Affections of the Tissues, including 
the skin and subcutaneous tissues, 
the nails, the lymphatic vessels and 
glands, the fasciae, bursee, muscles, 
tendons and tendon-sheaths, nerves, 
arteries and veins ; deformities. 
With 141 Illustrations. Royal 8vo, 
12s. net. 

PART III. The treatment of the Surgical 
Affections of the Bones. Ampu- 
tations. With 100 Illustrations. 
Royal 8vo, 10s. 6d. net. 

PART IV. The treatment of the Surgical 
Affections of the Joints (including 
excisions) and the spine. With 138 
Illustrations. Royal 8vo, 12s. net. 



PART V. The treatment of the Surgical 
Affections of the head, face, jaws, 
lips, larynx and trachea ; and the 
Intrinsic Diseases of the nose, ear 
and larynx, by H. LAMBERT LACK, 
M.D. (Lond.), F.R.C.S., Surgeon to 
the Hospital for Diseases of the 
Throat, Golden Square, and to the 
Throat and Ear Department, the 
Children's Hospital, Paddington 
Green. With 145 Illustrations. 
Royal 8vo, 15s. net. 

PART VI. Section 1. The Surgical 
Affections of the tongue and floor 
of the mouth, the pharynx, neck, 
oesophagus, stomach and intestines. 
With an Appendix on the Examin- 
ation of the Blood in Surgical 
Condition. By W. ESTE EMERY, 
M.D., D.Sc. (Lond.). With 124 
Illustrations. Royal 8vo, 15s. net. 
Section 2. The Surgical Affections 
of the rectum, the liver, pancreas 
and spleen, and genitourinary 
organs, the breast and the thorax. 
With 113 Illustrations. Royal 8vo 
18s. net. 



4 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



MEDICINE, SURGERY, ANATOMY, ETC.- continued. 

COOKE. WORKS by THOMAS COOKE, F.R.C.S. Eng., B.A., B.Sc., 
M.D. Paris, late Senior Assistant Surgeon to the Westminster Hospital. 

TABLETS OF ANATOMY. Being a Synopsis of demonstrations given 
in the Westminster Hospital Medical School. Eleventh Edition in three 
Parts, thoroughly brought up to date, and with over 700 Illustrations from 
all the best sources, British and Foreign. Post 4to. Part I. The Bones, 
7s. Qd. net ; Part II. Limbs, Abdomen, Pelvis, 10s. 6d. net ; Part III. Head 
and Neck, Thorax, Brain, 10s. 6d. net. 

APHOEISMS IN APPLIED ANATOMY AND OPEEATIVE 

SURGERY. Including 100 Typical vivd voce Questions on Surface 
Marking, etc. Crown 8vo, 3s. 6d. 

DAKIN. A HANDBOOK OF MIDWIFERY. By WILLIAM RAD- 

FORD DAKIN, M.D., F.R.C.P., Obstetric Physician and Lecturer on 
Midwifery at St. George's Hospital, Examiner in Midwifery and Diseases of 
Women on the Conjoint Board of the Royal Colleges of Physicians and 
Surgeons in England, etc. With 400 Illustrations. Large crown 8vo, 18s. 

DICKSON. THE BONE-MARROW: a Cytological Study. Forming 
an Introduction to the Normal and Pathological Histology of the Tissue, more 
especially with regard to Blood Formatoin, Blood Destruction, etc. Together 
with a short account of the Reactions and Degenerations of the Tissue in 
Disease. By W. E. CARNEGIE DICKSON, M.D., B.Sc. Edin., F.R.C.P. 
Edin., Lecturer on Pathological Bacteriology and Senior Assistant to the 
Professor of Pathology in the University of Edinburgh ; Assistant Pathologist 
to the Edinburgh Royal Infirmary. With 12 Coloured Plates and 51 Micro- 
Photographs by Richard Muir. Medium 4to, 2 2s. net. 

DICKINSON. WORKS by W. HOWSHIP DICKINSON, M.D. 
Cantab., F.R.C.P., Consulting Physician to St. George's Hospital; Consulting 
Physician to the Hospital for Sick Children, etc. 

ON RENAL AND URINARY AFFECTIONS. Complete in Three 
Parts, 8vo, with 12 Plates, and 122 Woodcuts. 3 4s. Gd. 

THE TONGUE AS AN INDICATION OF DISEASE ; being the 
Lumleian Lectures delivered at the Royal College of Physicians in March, 
1888. 8vo, 7s. 6d. 

OCCASIONAL PAPERS ON MEDICAL SUBJECTS, 1855-1896. 

8vo, 12s. 

FOWLER AND GODLEE. THE DISEASES OF THE LUNGS. 

By JAMES KINGSTON FOWLER, M.A., M.D., F.R.C.P., Physician to 
the Middlesex Hospital and to the Hospital for Consumption and Diseases 
of the Chest, Brompton, etc. ; and RICKMAN JOHN GODLEE, M.S., 
F.R.C.S., Honorary Surgeon-in-Ordinary to His Majesty, Fellow and 
Professor of Clinical Surgery, University College, London, etc. With 
160 Illustrations. 8vo, 25s. 




MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 5 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 



GARROD. THE ESSENTIALS OF MATEEIA MEDICA AND 

THEBAPEUTICS. By Sir ALFRED BARING GARROD, M.D., 
F.R.S., etc. ; late Vice-President of the Royal College of Physicians. Four- 
teenth Edition, Revised and Edited, under the Supervision of the Author, 
by NESTOR TIRARD, M.D. Lond., F.R.C.P., Professor of Materia Medica 
and Therapeutics in King's College, London, etc. Crown 8vo, 12s. 6d. 



GOODSALL AND MILES. DISEASES OF THE ANUS AND 

EECTUM. By D. H. GOODSALL, F.R.C.S., late Senior Surgeon 
Metropolitan Hospital, Senior Surgeon St. Mark's Hospital ; and W. 
ERNEST MILES, F.R.C.S., Assistant Surgeon to the Cancer Hospital, 
Surgeon (out-patients) to the Gordon Hospital, etc. (In Two Parts). 

PART I. Anatomy of the Ano-rectal Region General Diagnosis Abscess 
Ano-rectal Fistula Recto-urethral, Recto-vesical and Recto-vaginal 
Fistulse Sinus over the Sacro-coccygeal Articulation Fissure Haemorr- 
hoids (External and Internal). With 91 Illustrations. 8vo, 7s. 6d. net. 

PART II. Prolapse of the Rectum Invagination of the Rectum Ulceration 
Stricture of the Anus and of the Rectum Malignant Growths of the 
Anus and Rectum Benign Tumours of the Anus and Rectum Foreign 
Bodies in the Rectum Pruritus Ani Syphilis of the Anus and Rectum. 
With 44 Illustrations. 8vo, 6s. net. 



GRAY. ANATOMY, DESCKIPTIVE AND APPLIED. By 

HENRY GRAY, F.R.S., Fellow of the Royal College of Surgeons, late 
Lecturer on Anatomy at St. George's Hospital Medical School. Seven- 
teenth Edition. Edited by ROBERT HOWDEN, M.A., M.B., C.M., 
Professor of Anatomy in the University of Durham. Notes on Applied 
Anatomy, revised by A. J. JEX-BLAKE, M.A., M.B., M.R.C.P., Assistant 
Physician to St. George's Hospital ; and W. FEDDE FEDDEN, M.S., 
F.R.C.S., Assistant Surgeon and Lecturer on Surgical Anatomy, St. 
George's Hospital. With 1,032 Illustrations. Royal 8vo, 32s. net. 



HARE. THE FOOD FACTOR IN DISEASE : Being an investiga- 

tion into the humoral causation, meaning, mechanism and rational treat- 
ment, preventive and curative, of the Paroxysmal Neuroses (migraine, 
asthma, angina pectoris, epilepsy, etc.), bilious attacks, gout, catarrhal 
and other affections, high blood-pressure, circulatory, renal and other 
degenerations. By FRANCIS HARE, M.D., late Consulting Physician to 
the Brisbane General Hospital ; Visiting Physician at the Diamantina 
Hospital for Chronic Diseases, Brisbane ; Inspector-General of Hospitals 
for Queensland. 2 vols. Medium 8vo, 30s. net. 



6 MESSRS. LONGMANS 1 WORKS ON MEDICINE, SURGERY, ETC. 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 



INFLUENCE OF HEEEDITY ON DISEASE (THE), WITH 
SPECIAL EEFERENCE TO TUBERCULOSIS, CANCER, 
AND DISEASES OF THE NERVOUS SYSTEM. A Dis- 
cussion opened by SIR WILLIAM S. CHURCH, Bt., K.C.B., M.D., SIR 
WILLIAM R. GOWERS, M.D., F.R.S. (Diseases of the Nervous System), 
ARTHUR LATHAM, M.D. (Tuberculosis), and E. F. BASHFORD, M.D, 
(Cancer). [From the Proceedings of the Royal Society of Medicine, 1909, 
Vol. II., No. 3.] 4to, 4s. 6d. net. 



LANG. THE METHODICAL EXAMINATION OF THE 

EYE. Being Part I. of a Guide to the Practice of Ophthalmology for 
Students and Practitioners. By WILLIAM LANG, F.R.C.S. Eng., Surgeon 
to the Royal London Ophthalmic Hospital, Moorfields, etc. With 15 
illustrations. Crown 8vo, 3s. 6d. 



LUFF. TEXT -BOOK OF FORENSIC MEDICINE AND 

TOXICOLOGY. By ARTHUR P. LUFF, M.D., B.Sc. Lond., 
Physician in Charge of Out-Patients and Lecturer on Medical Jurisprudence 
and Toxicology in St. Mary's Hospital ; Examiner in Forensic Medicine in 
the University of London ; External Examiner in Forensic Medicine in the 
Victoria University ; Official Analyst to the Home Office. With 13 full- 
page Plates (1 in colours) and 33 Illustrations in the Text. 2 vols., Crown 
8vo, 24s. 



NEWMAN. MOVABLE KIDNEY AND OTHER DISPLACE- 
MENTS AND MALPOSITIONS. By DAVID NEWMAN, M.D., 

F.F.P.S.G., Surgeon to the Glasgow Royal Infirmary. With 25 Illustra- 
tions. 8vo, 5s. net. 



PROBYN-WILLIAMS. A PRACTICAL GUIDE TO THE 
ADMINISTRATION OF ANAESTHETICS. By R. J. 
PROBYN-WILLIAMS, M.D., Anesthetist and Instructor in Anaesthetics 
at the London Hospital ; Lecturer in Anaesthetics at the London Hospital 
Medical College, etc. With 34 Illustrations. Crown 8vo, 4s. 6d. net. 



MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 7 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 

QUAIN. QUAIN'S (Sm KICHARD) DICTIONARY OF MEDI- 
CINE. By Various Writers. Edited by H. MONTAGUE MURRAY, 
M.D., F.R.C.P., Joint Lecturer on Medicine, Charing Cross Medical School, 
and Physician to Charing Cross Hospital, and to the Victoria Hospital for 
Children, Chelsea ; Examiner in Medicine to the University of London. 
Assisted by JOHN HAROLD, M.B., B.Cn., B.A.O., Physician to St. 
John's and St. Elizabeth's Hospital, and Demonstrator of Medicine at 
Charing Cross Medical School, and W. CECIL BOSANQUET, M.A., 
M.D., F.R.C.P., Assistant Physician, Charing Cross Hospital, etc. Third 
and Cheaper Edition, largely Rewritten, and Revised throughout. With 
21 Plates (14 in Colour) and numerous Illustrations in the Text. 8vo, 
21s. net., buckram ; 30s. net., half-morocco. 



QUAIN. QUAIN'S (JONES) ELEMENTS OF ANATOMY. 

The Tenth Edition. Edited by EDWARD ALBERT SCHAFER, F.R.S., 
Professor of Physiology in the University of Edinburgh ; and GEORGE 
DANCER THANE, Professor of Anatomy in University College, London. 
\* The several parts of this work form COMPLETE TEXT-BOOKS OP THEIR 
RESPECTIVE SUBJECTS. They can be obtained separately as follows : 

VOL. I., PART I. EMBRYOLOGY. By E. A. SCHAFER, F.R.S. With 
200 Illustrations. Royal 8vo, 9s. 

VOL. I., PART ii. GENERAL ANATOMY OR HISTOLOGY. 

By E. A. SCHAFER, F.R.S. With 491 Illustrations. Royal 8vo, 12s. 6d. 

VOL. II., PART I. OSTEOLOGY ARTHROLOGY. By G. D. 
THANE. With 224 Illustrations. Royal 8vo, 11s. 

VOL. II., PART II. MYOLOGY ANGEIOLOGY. By G. D. 
THANE. With 199 Illustrations. Royal 8vo, 16s. 

VOL. III., Part I. THE SPINAL CORD AND BRAIN. By E. A. 

SCHAFER, F.R.S. With 139 Illustrations. Royal 8vo, 12s. 6d. 

VOL. III., PART II. THE NERVES. By G. D. THANE. With 102 
Illustrations. Royal 8vo, 9s. 

VOL. III., PART III. THE ORGANS OF THE SENSES. By E. A. 
SCHAFER, F.R.S. With 178 Illustrations. Royal 8vo, 9s. 

VOL. ILL, PART IV. SPLANCHNOLOGY. By E. A. SCHAFER, 
F.R.S., and JOHNSON SYMINGTON, M.D. With 337 Illustrations. 
Royal 8vo, 16s. 

APPENDIX. SUPERFICIAL AND SURGICAL ANATOMY. By 
Professor G. D. THANE and Professor R. J. GODLEE, M.S. With 29 
Illustrations. Royal 8vo, 6s. 6d. 



8 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 



QUAIN. QUAIN'S ELEMENTS OF ANATOMY. The Eleventh 
Edition. Edited by EDWARD ALBERT SCHAFER, 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 ; and THOMAS HASTIE BRYCE, M.A., M.D., Regius Professor 
of Anatomy in the University of Glasgow. 

IN FOUR VOLUMES. Royal 8vo. 

VOL. I. EMBKYOLOGY. By T. H. BRYCE, M.A., M.D. Illustrated 
by more than 300 Engravings, many of which are coloured. 10s. 6d. net. 

VOL. III. NEUBOLOGY. By E. A. SCHAFER and J. SYMINGTON. 
Part I. Containing the General Structure of the Nervous System and 

the Structure of the Brain and Spinal Cord. With 361 Illustrations, 

many of which are coloured. 15s. net. 
Part II. Containing the Descriptive Anatomy of the Peripheral Nerves 

and of the Organs of Special Sense. With numerous Illustrations, 

many of which are coloured. 15s. net. 

%* The other Volumes are in preparation. 

This work has been completely re-edited and brought up to date. The 
volumes will comprise respectively Embryology; 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 His- 
tology and Visceral Anatomy, the third a text-book of Neurology, the 
fourth dealing with the systems which are not included in the second and 
third volumes. 



SCHAFER. THE ESSENTIALS OF HISTOLOGY: Descriptive 

and Practical. For the Use of Students. By E. A. SCHAFER, F.R.S., 
Professor of Physiology in the University of Edinburgh. With 553 Illustra- 
tions some of which are Coloured. Seventh Edition, 1907. 8vo, 10s. Qd. net. 



SMALE AND COLYER. DISEASES AND INJUEIES OF 

THE TEETH, including Pathology and Treatment. By MORTON 
SMALE, M.R.C.S., L.S.A., L.D.S., Dental Surgeon to St. Mary's Hospital, 
Consulting Dental Surgeon, Dental Hospital of London, etc. ; and J. F. 
COLYER, L.R.C.P., M.R.C.S., L.D.S., Dental Surgeon to Charing Cross 
Hospital and to the Dental Hospital of London, Dean of the School, Dental 
Hospital of London. Second Edition Revised and Enlarged by J. F. 
COLYER. With 645 Illustrations. Large Crown 8vo, 21s. net. 



MESSES. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 9 



MEDICINE, SURGERY, ANATOMY, ETC. continued. 



SMITH (H. F.). THE HANDBOOK FOE MIDWIVES. By 

HENRY FLY SMITH, B.A., M.B. Oxon., M.R.C.S. Second Edition. 
With 41 Woodcuts. Crown 8vo, 5s. 



STEVENSON. WOUNDS IN WAK : the Mechanism of their 
Production and their Treatment. By Surgeon-General W. F. STEVENSON, 
C.B. (Army Medical Staff), B.A., M.B., M.Gh. Dublin University; Professor 
of Military Surgery, Royal Army Medical College, London. With 127 
Illustrations. 8vo, 15s. net. 



SYMINGTON. AN ATLAS OF SKIAGEAMS, ILLUSTEATING 
THE DEVELOPMENT OF THE TEETH, with Explanatory 
Text. By JOHNSON SYMINGTON, M.D., F.R.S., Professor of Anatomy, 
Queen's College, Belfast ; and J. C. RANKIN, M.D., Physician in charge 
of the Electrical Department, Royal Victoria Hospital, Belfast. With 12 
Plates. Demy 4to. 10s. 6d. net. 



MISCELLANEOUS. 



ANNUAL CHAEITIES EEGISTEE AND DIGEST: being a Classi- 
fied Register of Charities in or available for the Metropolis, together with 
a Digest of Information respecting the Legal, Voluntary, and other Means 
for the Prevention and Relief of Distress and the Improvement of the 
Condition of the Poor. With an elaborate Index, and an Introduction, 
"How to Help Cases of Distress". By C. S. LOCH, Secretary to the 
Council of the Charity Organisation Society, London. 8vo, 5s. net. 



GASKELL. THE OEIGIN OF VEETEBEATES. By WALTER 

H. GASKELL, M.A., M.D. (Camb.), LL.D. (Edinburgh and McGill Univ., 
Montreal), F.R.S., Fellow of Trinity Hall and University Lecturer in 
Physiology, Cambridge. With 168 Illustrations. 8vo, 21s. net. 



HOBART. THE MEDICAL LANGUAGE OF ST. LUKE. By 

the Rev. WILLIAM KIRK HOBART, LL.D. 8vo, 16s. 



10 MESSRS. LONGMANS 1 WORKS ON MEDICINE, SURGERY, ETC. 



MISCELLANEOUS continued. 

HOPF. THE HUMAN SPECIES : CONSIDEEED FEOM THE 
STANDPOINTS OF COMPAEATIVE ANATOMY, PHYSI- 
OLOGY, PATHOLOGY AND BACTBEIOLOGY. By Dr. 

LUDWIG HOPF. Authorised English Translation. With 7 Plates and 
217 II lustrations in the Text. 8vo, 10s. 6d. net. 

INQUIEY (AN) INTO THE PHENOMENA ATTENDING 
DEATH BY DEOWNING AND THE MEANS OF PEO- 
MOTING EESUSCITATION IN THE APPAEENTLY 

DEOWNED. Report of a Committee appointed by the Royal Medical 
and Chirurgical Society. With 2 Diagrams and 26 Plates. 8vo, 5s. net. 

KING'S COLLEGE HOSPITAL BOOK OF COOKING 

EECIPES : being a Collection of Recipes contributed by Friends of 
the Hospital and Published in aid of the Fund for the Removal of King's 
College Hospital to South London. Crown 8vo, Is. net. 

MOON. THE EELATION OF MEDICINE TO PHILOSOPHY. 

By R. O. MOON, M.A., M.D. (Oxon.), F.R.C.P., Physician to the National 
Hospital for Diseases of the Heart, etc. Crown Svo. 

PAGET. MEMOIES AND LETTEES OF SIE JAMES PAGET, 

Bart., F.R.S., Sergeant-Surgeon to Her late Majesty Queen Victoria. 
Edited by STEPHEN PAGET, F.R.C.S. With Portrait. Svo, 6s. net. 



PETTIGREW. DESIGN IN NATUEE : Illustrated by Spiral and 
other Arrangements in the Inorganic and Organic Kingdoms as exemplified 
in Matter, Force, Life, Growth, Rhythms, etc., especially in Crystals, 
Plants, and Animals. With Examples selected from the Reproductive, 
Alimentary, Respiratory, Circulatory, Nervous, Muscular, Osseous, Loco 
motory, and other Systems of Animals. By J. BELL PETTIGREW, 
M.D., LL.D., F.R.S., F.R.C.P. ; Laureate of the Institute of France ; late 
Chandos Professor of Anatomy and Medicine in the University, St. 
Andrews; Fellow of the Royal Botanical, Medico- Chirurgical, Royal 
Medical, Literary and Philosophical, Harveian and other Societies. 
Illustrated by nearly 2,000 Figures, largely original and from nature. 
In 3 vols. 4to. 63s. net. 



POOLE. COOKEEY FOE THE DIABETIC. By w. H. and 

Mrs. POOLE. With Preface by Dr. PAVY. Fcap. Svo, 2s. 6d. 

RAFFETY. AN INTEODUCTION TO THE SCIENCE OF 

EADIO-ACTIVITY. By CHARLES W, RAFFETY. With 27 
Illustrations. Crown Svo. 4s. 6d. net. 



LONHM.IXS' \\'<>I!K$ ON MEDICINK, KVRGEB.Y, ETC. 11 



THE PROCEEDINGS OF THE ROYAL SOCIETY 
OF MEDICINE. 

The Royal Society of Medicine teas formed in June, 1907, by the amalgamation 

of the following London Medical Societies : 



The Royal Medical and Chirurgical 

Society. 

The Pathological Society. 
The Epidemiological Society. 
The Odontological Society of Great 

Britain. 

The Obstetrical Society. 
The Clinic-al Society. 
The Dermatologies,! Society. 
The British Gynaecological Society. 



The Neurological Society. 

The British Laryngological, Rhinologi- 

cal, and Otological Association. 
The Laryngological Society. 
The Dermatological Society of Great 

Britain and Ireland. 
The Otological Society of the United 

Kingdom. 

The British Electro-therapeutic Society. 
The Therapeutical Society. 



The " Proceedings" of the Royal Society of Medicine c>re published monthly 
from November to July inclusive. The numbers contain the papers of, and the 
discussions read at each of the Sections during the previous month, and are so 
(/r fanned that each Section am, if desired, be detached and bound separately at the 
end of the year. 

The Annual Subscription is 3 3s. net, which may be paid through any 
bookseller. 

The price of each monthly number is 7s. 6d. net. 



VETERINARY MEDICINE, ETC. 

FITZWYGRAM. HOESES AND STABLES. By Lieutenant- 
General Sir F. FITZWYGRAM, Bart. With 56 pages of Illustrations. 
8vo, 3s. net. 

HAYES. TEAINING AND HOESE MANAGEMENT IN INDIA. 

With Hindustanee Vocabulary. By M. HORACE HAYES, F.R.C.V.S. 
(late Captain, " The Buffs "). With Portrait. Crown 8vo, 8s. net. 

STEEL.-UORKS b y JOHN HEWRY STEEL, F.R.C.V.S., F.Z.S., 

A . V. ]). , late Professor of Veterinary Science and Principal of Bmnbay Veterinary 



A TEEATISE ON THE DISEASES OF THE OX ; being a 

Manual of Bovine Pathology. Especially adapted for the use of Veterinary 
Practitioners and Students. With 2 Plates and 117 Woodcuts. 8vo, 15s. 

A TEEATISE ON THE DISEASES OF THE SHEEP ; being 

a Manual of Ovine Pathology for the use of Veterinary Practitioners and 
Students. With Coloured Plate and 99 Woodcuts. 8vo, 12s. 



YOU ATT. WORKS by WILLIAM YOU ATT. 

THE PIOESE. Revised and Enlarged by W. 
With 52 Wood Engravings. 8vo, 7s. 6d. 

THE DOG. Revised and Enlarged. With 33 Wood Engravings. 8vo, 6s. 



THE PIOESE. Revised and Enlarged by W. WATSON, M.R.C.V.S. 
With 52 Wood Engravings. 8vo, 7s. 6d. 



12 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, 






PHYSIOLOGY, BIOLOGY, ZOOLOGY, ETC. 



ASH BY. NOTES ON PHYSIOLOGY FOE THE USE OF 
STUDENTS PEEPAEING FOE EXAMINATION. By 
HENRY ASHBY, M.D. (Lond.), F.R.C.P., late Physician to the General 
Hospital for Sick Children, Manchester ; Lecturer and Examiner in 
Diseases of Children in the Victoria University. With 148 Illustrations 
18mo, 5s. 

BARNETT. THE MAKING OF THE BODY: a Children's Book 

on Anatomy and Physiology. By Mrs. S. A. BARNETT. With 113 
Illustrations. Crown 8vo, Is. 9d. 

BEDDARD. ELEMENTAEY PEACTICAL ZOOLOGY. By 

FRANK E. BEDDARD, M.A. (Oxon.). With 93 Illustrations. Crown 
8vo, 2s. 6d. 

BIDGOOD. A COUESE OF PEACTICAL ELEMENTAEY 

BIOLOGY. By JOHN BIDGOOD, B.Sc., F.L.S. With 226 Illustra- 
tions. Crown 8vo, 4s. 6d. 

BOSE. WORKS by JAGADIS CHUNDER BOSE, M.A. (Cantab.), D.Sc. 
(Lond.), Professor, Presidency College, Calcittta. 



EESPONSE IN THE LIVING AND NON-LIVING. 

117 Illustrations. 8vo, 10s. 6d. 



With 



PLANT EESPONSE AS A, MEANS OF PHYSIOLOGICAL 
INVESTIGATION. With 278 Illustrations. 8vo, 21s. 

COMPAEATIVE ELECTEO-PHYSIOLOGY : A PHYSICO 
PHYSIOLOGICAL STUDY. With 406 Illustrations and Classi- 
fied List of 321 new Experiments. 8vo, 15s. net. 

BRODIE. THE ESSENTIALS OF EXPEEIMENTAL PHY- 

SIOLOGY. For the use of Students. By T. G. BRODIE, M.D., 
Lecturer on Physiology, St. Thomas's Hospital Medical School. With 
2 Plates and 177 Illustrations in the Text. Crown 8vo, 6s. 6d. 



CHAPMAN, THE FOEAMINIFEEA : an Introduction to the Study 
of the Protozoa. By FREDERICK CHAPMAN, A.L.S., F.R.M.S. With 
14 Plates and 42 Illustrations in the Text. 8vo, 9s. net. 



MESSRS. LONGMANS' WORKS OA MEDICINE, SURGERY, ETC. 13 



PHYSIOLOGY, BIOLOGY, ZOOLOGY, ETC. continued. 
FURNEAUX. HUMAN PHYSIOLOGY. By w. PURNEAUX, 

F.R.G.S. With 223 Illustrations. Crown 8vo, 2s. Qd. 

HALLIBURTON. THE ESSENTIALS OF CHEMICAL PHY- 
SIOLOGY. For the Use of Students. By W. D. HALLIBURTON, 
M.D., F.R.S., F.R.C.P., Professor of Physiology in King's College, London. 
With 83 Illustrations. 8vo, 4s. 6d. net. 

HUDSON AND GOSSE. THE EOTIFEEA OE "WHEEL 

ANIMALCULES ". By C. T. HUDSON, LL.D., and P. H. GOSSE, 
F.R.S. With 30 Coloured and 4 Uncoloured Plates. In 6 Parts. 4to, price 
10s. 6d. each; Supplement, 12s. 6d. Complete in Two Volumes, with 
Supplement, 4to, 4 4s. 

%* The Plates in the Supplement contain figures of almost all the Foreign 
Species, as well as of the British Species, that have been discovered since the 
original publication of Vols. I. and II. 

LLOYD. THE TEACHING OF BIOLOGY IN THE SECON- 

DAEY SCHOOL. By FRANCIS E. LLOYD, A.M., and MAURICE 
A. BIGELOW, Ph.D., Professors in Teachers' College, Columbia Univer- 
sity. Crown 8vo, 6s. net. 

MOORE. ELEMENTAEY PHYSIOLOGY AND ANATOMY. 

By BENJAMIN MOORE, D.Sc., Professor of Bio-Chemistry in the 
University of Liverpool. With 125 Illustrations. Crown 8vo, 3s. 6d. 

MACALISTER.-JFO/^S by ALEXANDER MACALISTER, M.D. 

AN INTEODUCTION TO THE SYSTEMATIC ZOOLOGY 
AND MOEPHOLOGY OF VEETEBEATE ANIMALS. 

With 41 Diagrams. 8vo, 10s. 6d. 

ZOOLOGY OF THE INYEETEBEATE ANIMALS, with 77 

Diagrams. Fcp. 8vo, Is. 6d. 

ZOOLOGY OF THE VEETEBEATE ANIMALS, with 59 

Diagrams. Fcp. 8vo, Is. 6d. 

by DANIEL TREMBLY MACDOUGALL, 
TEXT-BOOK OF PLANT PHYSIOLOGY, with 159 niustra- 

tions. 8vo, 7s. Qd. net. 

ELEMENTAEY PLANT PHYSIOLOGY, with 108 Illustrations. 
Crown 8vo, 3s. 



14 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



PHYSIOLOGY, BIOLOGY, ZOOLOGY, ETC. continued. 

MORGAN. ANIMAL BIOLOGY. An Elementary Text-Book. By 
C. LLOYD MORGAN, F.R.S., Principal of University College, Bristol. 
With numerous Illustrations. Crown 8vo, 8s. 6d. 



SCHAFER. DIEECTIONS FOR CLASS WOEK IN PRAC- 
TICAL PHYSIOLOGY : Elementary Physiology of Muscle and Nerve 
and of the Vascular and Nervous Systems. By E. A. SCHAFER, LL.D., 
F.R.S., Professor of Physiology in the University of Edinburgh. With 48 
Diagrams. 8vo, 3s. net. 

THORNTON. WORKS by JOHN THORNTON, M.A. 

HUMAN PHYSIOLOGY. With 284 Illustrations, some of which 
are Coloured. Crown 8vo, 6s. 

ELEMENTARY BIOLOGY, Descriptive and Experimental. With 
numerous Illustrations. Crown 8vo, 3s. 6d. 

ELEMENTARY PRACTICAL PHYSIOLOGY, with 178 illus- 
trations (6 of which are Coloured). Crown 8vo, 3s. 6d. 



WALLER.-WORKS by AUGUSTUS D. WALLER, M.D., F.R.S., 
Hon. LL.D. Edin., Lecturer on Physiology at St. Mary's Hospital Medical 
School, London; late External Examiner at the Victorian University. 

AN INTRODUCTION TO HUMAN PHYSIOLOGY. With 
314 Illustrations. 8vo, 18s. 

LECTURES ON PHYSIOLOGY. FIRST SERiES.-On Animal Elec- 
Iricity. 8vo, 5s. net. 



MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 15 



HEALTH AND HYGIENE, ETC. 



ASH BY, HEALTH IN THE NUESEEY. By HENRY ASHBY, 

M.D., F.R.C.P., Physician to the General Hospital for Sick Children, 
Manchester ; Lecturer and Examiner in Diseases of Children in the 
Victoria University. With 25 Illustrations. Crown 8vo, 3s. net. 

BUCKTON, HEALTH IN THE HOUSE. By Mrs. c. M. 

BUCKTON. With 41 Woodcuts and Diagrams. Crown 8vo, 2s. 

BULL. WORKS by THOMAS BULL, M.D. Thoroughly Revised by 
ROBERT IF. PARKER, M.R.C.S. Eng. 

HINTS TO MOTHEES ON THE MANAGEMENT OF THEIE 
HEALTH DUEING THE PEEIOD OF PEEGNANCY, AND 
HINTS ON NUESING. Fcp. 8vo, sewed, Is. 6d. ; cloth, gilt edges, 
2s. net. 

THE MATEENAL MANAGEMENT OF CHILDEEN IN 
HEALTH AND DISEASE. Fcp. 8vo, sewed, Is. 6d. ; cloth, gilt 
3, 2s. net. 



BUTTERWORTH. MANUAL OF HOUSEHOLD WOEK AND 

MANAGEMENT. By ANNIE BUTTERWORTH. Crown 8vo, 2s. 6d. 

CORFIELD, THE LAWS OF HEALTH. By w. H. CORFIELD, 

M.A., M.D. Fcp. 8vo, Is. 6d. 

CREIGHTON. THE ECONOMICS OF THE HOUSEHOLD. 

Six Lectures given at the London School of Economics during the Winter 
of 1906. By LOUISE CREIGHTON. Crown 8vo, Is. 4d. 

FURNEAUX. ELEMENTAEY PEACTICAL HYGIENE. Sec- 
tion I. By WILLIAM S. FURNEAUX. With 146 Illustrations. Crown 
8vo 2s. 6d. 

JAMES. BALL GAMES AND BEEATHING EXEECISES. 

By ALICE R. JAMES. With Preface by HARRY CAMPBELL, M.D., 
B.S. (London), F.R.C.P. With 17 Illustrations. Crown 8vo, Is. 6d. 



16 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



HEALTH AND HYGIENE, ETC. continued. 

NOTTER AND F \RTH.-WORKSbyJ. LANE NOTTER, M.A., M.D., 
and R. ff. FIRTH, F.R.O.S. 

HYGIENE. With 99 Illustrations. Crown 8vo, 4s. 6d. 

PEACTICAL DOMESTIC HYGIENE. with 84 illustrations. 
Crown 8vo, 2s. 6d. 



POORE. WORKS by GEORGE VIVIAN POORE, M.D., F.R.C.P. 

THE EAETH IN EELATION TO THE PEESERVATION 
AND DESTRUCTION OF CONTAGIA : being the Milroy 

Lectures delivered at the Royal College of Physicians in 1899, together 
with other Papers on Sanitation. 13 Illustrations. Crown 8vo, 5s. 

ESSAYS ON EUEAL HYGIENE. With 12 Illustrations. Crown 
8vo, 6s. 6d. 

THE DWELLING HOUSE. With 36 Illustrations. Crown 8vo, 3s. 6d. 

COLONIAL AND CAMP SANITATION. With 11 Illustrations. 
Crown 8vo, 2s. net. 



PORTER. SCHOOL HYGIENE AND THE LAWS OF 

HEALTH : a Text-Book for Teachers and Students in Training. By 
CHARLES PORTER, M.D., B.Sc., M.R.C.P. (Edin.). With 121 Illustra- 
tions. Crown 8vo, 3s. 6d. 



ROBINSON. THE HEALTH OF OUE CHILDEEN IN THE 

COLONIES : a Book for Mothers. By LILIAN AUSTEN ROBIN- 
SON, M.D. Crown 8vo, 2s. 6d. net. 



TUCKER. NOTES ON THE CAEE OF BABIES AND YOUNG 

CHILDEEN. For the Use of Teachers. By BLANCHE TUCKER, 
Inspectress of Elementary Schools. With an Introduction by DR. HOPE, 
Medical Officer of Health for Liverpool. Grown 8vo, Is. 

WEST, HOW TO NUESE SICK CHILDEEN. By CHARLES 

WEST, M.D., Founder of and late Physician to the Hospital for Sick 
Children, Great Ormond Street, London. With Preface by GEORGE F. 
STILL, M.D., Physician to the Hospital for Sick Children, Great Ormond 
Street, Crown 8vo, Is, net. 



MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 17 



BACTERIOLOGY, ETC. 



CURTIS. THE ESSENTIALS OF PEACTICAL BACTERI- 
OLOGY : an Elementary Laboratory Work for Students and Practitioners. 
By H. J. CURTIS, B.S. and M.D. Lond., F.R.C.S., formerly Surgeon to the 
North-Eastern Hospital for Children ; Assistant Surgeon, Royal Hospital 
for Children and Women, Waterloo Road ; Surgical Registrar and Assistant 
to the Professor of Pathology, University College, London. With 133 
Illustrations. 8vo, 9s. 



ELLIS, OUTLINES OF BACTERIOLOGY (Technical and Agri- 
cultural). By DAVID ELLIS, Ph.D. (Marburg), D.Sc. (London), 
F.R.S.E., Lecturer in Bacteriology and Botany to the Glasgow and 
West of Scotland Technical College, Glasgow. With Illustrations. 8vo, 
7s. 6d. net. 



FRANKLAND. BACTERIA IN DAILY LIFE. By Mrs. PERCY 

FRANKLAND, F.R.M.S. Crown 8vo, 5s. net. 



GOADBY. THE MYCOLOGY OF THE MOUTH: A TEXT- 

BOOK OF ORAL BACTERIA. By KENNETH W. GOADBY, 
L.D.S. Eng., D.P.H. Camb., L.R.C.P., M.R.C.S., Bacteriologist and 
Lecturer on Bacteriology, National Dental Hospital, etc. With 82 
Illustrations. 8vo, 8s. 6d. net. 



KLOCKER. FERMENTATION ORGANISMS. A Laboratory 

Handbook. By ALB. KLOCKER, Assistant in the Carlsberg Laboratory, 
Copenhagen. Translated from the German by G. E. ALLAN, B.Sc., 
Lecturer in the University of Birmingham, and J. H. MILLAR, F.I.C., 
formerly Lecturer in the British School of Malting and Brewing, and 
revised by the Author. With 146 Illustrations. 8vo, 12s. net. 



IS MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



OPTICS, PHOTOGRAPHY, ETC. 



ABNEY, A TEEATISE ON PHOTOGEAPHY. By Sir WILLIAM 

DE WIVELESLIE ABNEY, K.C.B., F.R.S. With 134 Illustrations. 
Crown 8vo, 5s. 



BALY. SPECTEOSCOPY. By E. C. C. BALY, F.I.C., Lecturer on 
Spectroscopy and Assistant Professor of Chemistry, University College, 
London. With 163 Illustrations. Crown 8vo, 10s. 6d. 



DRUDE. THE THEOEY OF OPTICS. By PAUL DRUDE, Pro- 
fessor of Physics at the University of Giessen. Translated from the 
German by C. RIBORG MANN and ROBERT A. MILLIKAN, Professors 
of Physics at the University of Chicago. With 110 Diagrams. 8vo, 15s. net. 



GLAZEBROOK. PHYSICAL OPTICS. By R. T. GLAZEBROOK, 

M.A., F.R.S. With 183 Woodcuts of Apparatus, etc. Crown 8vo, 6s. 



POLLOK. PEACTICAL SPECTEOGEAPHIC ANALYSIS. By 

J. H. POLLOK, D.Sc. Crown 8vo. 



SHEPPARD AND MEES. INVESTIGATION ON THE 
THEOEY OF THE PHOTOGEAPHIC PEOCESS. By S. 
E. SHEPPARD, D.Sc. (Lond.), 1851 Exhibition Scholar of University 
College, London, and C. E. KENNETH MEES, D.Sc. (Lond.). With 65 
Illustrations and Diagrams. Crown 8vo, 6s. Qd. net. 



VANDERPOEL, COLOUE PEOBLEMS : A Practical Manual for 
the Lay Student of Colour. By EMILY NOYES VANDERPOEL. With 
117 Plates in Colour. Square 8vo, 21s. net. 



WRIGHT. OPTICAL PEOJECTION : A Treatise on the Use of the 

Lantern in Exhibition and Scientific Demonstration. By LEWIS 
WRIGHT, Author of " Light : a Course of Experimental Optics ". With 
232 Illustrations. Crown 8vo, 6s. 



MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 19 



CHEMISTRY, ETC. 

ARMITAGE, A HISTOEY OF CHBMISTEY. 

By F. P. ARMITAGE, M.A., F.C.S. Crown 8vo, 6s. 

ARRHENIUS. WORKS by SVANTE ARRHENIUS, Director of the 

Nobel Institute, Stockholm. 

THEOBIES OF CHEMISTEY : being Lectures delivered at the 
University of California, in Berkeley. Edited by T. SLATER PRICE, 
D.Sc., PhiD., F.I.C. 8vo, 5s. 6d. net. 

A TEXT-BOOK OF ELECTEO - CHEMISTEY. Translated 
from the German Edition by JOHN McCRAE, Ph.D. With 58 Illustra- 
tions. 8vo, 9s. 6d. net. 

BUNGE. TEXT-BOOK OF OEGANIC CHEMISTEY FOE 

MEDICAL STUDENTS. By Dr. G. VON BUNGE, Professor of 
Physiology in the University of Basel. Translated by R. H. ADERS 
PLIMMER, D.Sc. 8vo, 6s. net. 

CROOKES. SELECT METHODS IN CHEMICAL ANALYSIS 

(chiefly inorganic). By Sir W. CROOKES, F.R.S. With 68 Illustrations. 
8vo, 21s. net. 

FINDLAY. WORKS by ALEX. FINDLAY, M.A., Ph.D., D.Sc. 

PHYSICAL CHEMISTEY AND ITS APPLICATIONS IN 
MEDICAL AND BIOLOGICAL SCIENCE. Being a Course 
of Seven Lectures delivered in the University of Birmingham. Royal 
8vo, 2s. net. 

PEACTICAL PHYSICAL CHEMISTEY With 92 Illustrations. 

Crown 8vo, 4s. 6d. 

HANSON AND DODGSON. AN INTEEMEDIATE COUESE 
OF LABOEATOEY WOEK IN CHEMISTEY. By EDWARD 
KENNETH HANSON, M.A. (Cant.), F.I.C., Teachers' Diploma (Lond.) ; 
Lecturer in Chemistry, University College, Reading ; Lecturer to the 
Cambridge University Local Lecture Syndicate, and JOHN WALLIS 
DODGSON, B.Sc. (Lond.) ; Director of Evening Classes and Lecturer in 
Chemistry, University College, Reading. With Illustrations. 8vo, 3s. 6d. 

MENDELEEFF. THE PEINCIPLES OF CHEMISTEY. By 

D. MENDELEEFF. Translated from the Russian (Seventh Edition) by 
GEORGE KAMENSKY, A.R.S.M., and Edited by THOMAS H. POPE, 
B.Sc., F.I.C. With 1W Illustrations. 2 vols. 8vo, 32s. net. 

MEYER. OUTLINES OF THEOEETICAL CHEMISTEY. 

By LOTHAR MEYER. Translated by Professors P. PHILLIPS BED- 
SON, D.Sc., and W. CARLETON WILLIAMS, B.Sc. 8vo, 9s. 

MUIR. A COUESE OF PEACTICAL CHEMISTEY. 

By M. M. P. MUIR, M.A. 
Parti. Elementary. Cr. 8vo, 4s. 6d. Part II. Intermediate. Cr. 8vo, 4s. 6d. 



20 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



CHEMISTRY, ETC. continued. 

NEWTH. WORKS by G. S. NEWTH, F.I.C., F.C.S. 

CHEMICAL LECTUEE EXPEEIMENTS. With 230 Illustra- 
tions. Crown 8vo, 6s. 

CHEMICAL ANALYSIS, QUANTITATIVE AND QUALITA- 
TIVE. With 102 Illustrations. Crown 8vo, 6s. 6d. 

SMALLEE CHEMICAL ANALYSIS. Crown 8vo, 2s. 

A TEXT-BOOK OF INOEGANIC CHEMISTEY. With 15 

Illustrations. Crown 8vo, 6s. 6d. 

ELEMENTAEY PEACTICAL CHEMISTEY. with 108 Illustra- 
tions and 254 Experiments. Crown 8vo, 2s. 6d. 



PER KIN. WORKS by F. MOLL WO PERKIN, Ph.D. 

QUALITATIVE CHEMICAL ANALYSIS (OEGANIC AND 

INOEGANIC). With 15 Illustrations and Spectrum Plate. 8vo, 4s. 

PEACTICAL METHODS OF ELECTEO-CHEMISTEY. 8vo, 
6s. net. 

PRICE AND TWISS. A COUESE OF PEACTICAL OEGANIC 

CHEMISTEY. By T. SLATER PRICE, D.Sc., Ph.D., F.I.C., Head 
of the Chemical Department of the Birmingham Municipal Technical 
School, and D. F. TWISS, M.Sc., A.I.C., Lecturer in Chemistry at the 
Birmingham Municipal Technical School. 8vo, 3s. 6d. 

RADCLIFFE AND SINNATT. A SYSTEMATIC COUESE OF 

PEACTICAL OEGANIC CHEMISTEY. By LIONEL GUY RAD- 
CLIFFE, F.C.S. With the assistance of FRANK STURDY SINNATT, 
F.C.S. 8vo, 4s. 6d. 

REYNOLDS. EXPEEIMENTAL CHEMISTEY for Junior students. 

By J. EMERSON REYNOLDS, M.D., F.R.S. Fcap. 8vo, with numerous 
Illustrations. 

PART I. Introductory, Is. 6d. PART III. Metals and Allied Bodies, 3s. 6d. 
PART II. Non-Metals, 2s. 6d. PART IV. Chemistry of Carbon Compounds, 4s. 

SMITH AND HALL, THE TEACHING OF CHEMISTEY 
AND PHYSICS IN THE SECONDAEY SCHOOL. By ALEX- 
ANDER SMITH, B.Sc., Ph.D., Associate Professor of Chemistry in the 
University of Chicago, and EDWIN H. HALL, Ph.D., Professor of Physics 
in Harvard University. With 21 Woodcuts, Bibliographies, and Index. 
Crown 8vo, 6s. net. 






MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 21 



CHEMISTRY, ETC. continued. 
STEWART. KECENT ADVANCES IN OEGANIC CHEM- 

ISTEY. By A. W. STEWART, D.Sc., Lecturer on Stereochemistry 
in University College, London. With an Introduction by J. NORMAN 
COLLIE, Ph.D., LL.D., F.R.S., Professor of Organic Chemistry in 
University College, London. 8vo, 7s. Qd. 

THORPE. A DICTIONARY OF APPLIED CHEMISTEY. 

By Sir T. E. THORPE, C.B., D.Sc. Viet., Ph.D., F.R.S., Principal of 
Government Laboratory, London. Assisted by Eminent Contributors. 
3 vols. 8vo. Vols. I. and II., 2 2s. each ; Vol. III., 3 3s. 

TILDEN. Works by WILLIAM A. TILDEN, D.Sc. London, F.E.S. 
A SHORT HISTORY OF THE PROGRESS OF SCIENTIFIC 

CHEMISTRY IN OUR OWN TIMES. Crown 8vo, 5s. net. 

INTRODUCTION TO THE STUDY OF CHEMICAL PHILO- 
SOPHY. The Principles of Theoretical and Systematic Chemistry. 
With 5 Illustrations. Crown 8vo, 5s. With ANSWERS to Problems. 
Crown 8vo, 5s. 6d. 

PRACTICAL CHEMISTRY. The Principles of Qualitative Analysis. 
Fcp. 8vo, Is. 6d. 

WATTS' DICTIONARY OF CHEMISTRY. Revised and entirely 
Re-written by H. FORSTER MORLEY, M.A., D.Sc., Fellow of, and 
lately Assistant-Professor of Chemistry in, University College, London ; 
and M. M. PATTISON MUIR, M.A., F.R.S.E., Fellow and Prselector 
in Chemistry of Gonville and Caius College, Cambridge. Assisted by 
Eminent Contributors. 4 vols. 8vo, 5 net. 

WESTON. A SCHEME FOR THE DETECTION OF THE 
MORE COMMON CLASSES OF CARBON COMPOUNDS. 

By FRANK E. WESTON, B.Sc., London (First Class Honours), F.C.S., 
Lecturer in Chemistry at the Polytechnic, Regent Street, W. 8vo, 2s. 

WHITELEY. WORKS by R. L.'Whiteley, F.I.C., Principal of the 

Municipal Science School, West Bromwich. 

CHEMICAL CALCULATIONS. With Explanatory Notes, Problems, 
and Answers, specially adapted for use in Colleges and Science Schools. 
With a Preface by Professor F. CLOWES, D.Sc. (Lond.), F.I.C. Crown 
8vo, 2s. 

ORGANIC CHEMISTRY : the Fatty Compounds. With 45 Illustra- 
tions. Crown 8vo, 3s. 6d. 



22 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 

TEXT.BOOKS OF PHYSICAL CHEMISTRY. 

Edited by Sir WILLIAM RAMSAY, K.C.B., F.B.S., D.Sc. 



STOICHIOMETEY. By SYDNEY YOUNG, D.Sc., F.R.S., Professor of 
Chemistry in the University of Dublin ; together with AN INTRODUC- 
TION TO THE STUDY OF PHYSICAL CHEMISTRY, by SIR 
WILLIAM RAMSAY, K.C.B., F.R.S., Editor of the Series. Crown 8vo 
7s. 6d. 



AN INTRODUCTION TO THE STUDY OF PHYSICAL CHEMIS- 
TRY. Being a General Introduction to the Series by SIR 
WILLIAM RAMSAY, K.C.B., F.R.S., D.Sc. Crown 8vo, Is. net. 



CHEMICAL STATICS AND DYNAMICS, INCLUDING THE 
THEOEIES OF CHEMICAL CHANGE, CATALYSIS, AND 
EXPLOSIONS. By J. W. MELLOR, D.Sc. (N.Z.), B.Sc. (Viet.). 
Crown 8vo, 7s. 6d. 



THE PHASE EULE AND ITS APPLICATIONS. By ALEX. 
FINDLAY, M.A., Ph.D., D.Sc., Lecturer and Demonstrator in Chemistry, 
University of Birmingham. With 134 Figures in the Text. Crown 8vo, 5s. 



SPECTEOSCOPY. By E. C. C. BALY, F.I.C., Lecturer on Spectroscopy 
and Assistant Professor of Chemistry, University College, London. With 
163 Illustrations. Crown 8vo, 10s. 6d. 



THEEMOCHEMISTEY. By JULIUS THOMSEN, Emeritus Professor of 
Chemistry in the University of Copenhagen. Translated by KATHARINE 
A. BURKE, B.Sc. (Lond.), Assistant in the Department of Chemistry, 
University College, London. Crown 8vo, 9s. 



MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 23 

TEXT-BOOKS OF PHYSICAL CHEMISTRY, ETC. 

continued. 

ELECTED -CHEMISTEY. PAET I. GENEEAL THEOEY. 
By R. A. LEHFELDT, D.Sc., Professor of Physics at the East London 
Technical College. Including a Chapter on the Relation of Chemical Con- 
stitution to Conductivity, by T. S. MOORE, B.A., B.Sc., Lecturer in 
the University of Birmingham. Crown 8vo, 5s. 



ELECTED - CHEMISTEY. PAET II. APPLICATIONS TO 
ELECTEOLYSIS, PEIMAEY AND SECONDAEY BAT- 
TEEIES, ETC. By N. T. M. WILSMORE, D.Sc. Crown 8vo. 

[In preparation. 



STEEEOCHEMISTEY. By A. W. STEWART, D.Sc., Carnegie Research 
Fellow, Lecturer on Stereochemistry in University College, London. With 
87 Illustrations. Crown 8vo, 10s. 6d. 



THE THEOEY OF VALENCY. By j. NEWTON FRIEND, Ph.D. 
(Wiirz), M.Sc. (Birmingham). Crown 8vo, 5s. 



EELATIONS BETWEEN CHEMICAL CONSTITUTION AND 

PHYSICAL PEOPEETIES. By SAMUEL SMILES, D.Sc. 
Crown 8vo. [In preparation. 



THEEMODYNAMICS. By F. G. DONNAN, M.A., Ph.D. Crown 8vo. 

[In preparation. 



ACTINOCHEMISTEY. By C. E. K. MEES, D.Sc., and S. E. SHEP- 
PARD, D.Sc. Crown 8vo. [In preparation. 



PEACTICAL SPECTEOGEAPHIC ANALYSIS. By J. H. POLLOK, 

D.Sc. Crown Svo. [In preparation. 



24 MESSRS. LONGMANS' WORKS ON MEDICINE, SURGERY, ETC. 



MONOGRAPHS ON BIOCHEMISTRY. 

Edited by R. H. ADERS PLIMMER, D.Sc., and F. GOWLAND HOPKINS, 

D.Sc., F.R.S. 

Royal 8vo. 

In these volumes an attempt is being made to make the subject of Bio- 
chemistry more accessible by issuing a series of monographs upon the various 
chapters of the subject, each independent of and yet dependent upon the others, 
so that from time to time, as new material and the demand therefor necessitate, 
a new edition of each monograph can be issued without reissuing the whole 
series. The expenses of publication and the expense to the purchaser will thus be 
diminished, and by a moderate outlay it will be possible to obtain a full account 
of any particular subject as nearly current as possible. 

THE DEVELOPMENT AND PEESENT POSITION OF BIO- 
LOGICAL CHEMISTBY. By F. GOWLAND HOPKINS, M.A., 
M.B., D.Sc., F.R.S. [In preparation. 

THE NATUEE Ob 1 ENZYME ACTION. By w. M. BAYLISS, 
D.Sc., F.R.S. 3s. net. 

THE CHEMICAL CONSTITUTION OF THE -PROTEINS. 
By R. H. ADERS PLIMMER, D.Sc. In 2 Parts. Part 1, 3s. net; 
Part 2, 2s. 6d. net. 

THE GENEEAL CHAEACTEES OF THE PEOTEINS. By S. B 
SCHRYVER, D.Sc., Ph.D. 2s. 6d. net. 

THE VEGETABLE PEOTEINS. By THOMAS B. OSBORNE, Ph.D. 
3s. 6d. net. 

THE POLYSACCHAEIDES. By ARTHUR R. LING, F.I.C. 

[In preparation. 

GLUCOSE AND THE GLUCOSIDES. By E. FRANKLAND 
ARMSTRONG, D.Sc., Ph.D. [In preparation. 

THE FATS. By J. B. LEATHES, D.Sc. [In preparation. 

COLLOIDS. By W. B. HARDY, M.A., F.R.S. [In preparation. 

H. 2,500 A. U. P. ix/1909. 



THIS BOOK IS DUE ON THE LAST DATE 
STAMPED BELOW 

RENEWED BOOKS ARE SUBJECT TO IMMEDIATE 
RECALL 







LIBRARY, UNIVERSITY OF CALIFORNIA, DAVIS 

Book Slip-50m-8,'66(G5530s4)458 



512483 



Qua in, J. 

Quain's elements of 



Call Number: 



QS4 

Q3 

1908 



9 512483 



Quain, J. 

Quain's elements 
of anatomy. 



HEALTH 
SCIENCES 
LIBRARY 



QS4 

Q3 

1908 

v.l 



LIBRARY 

UNIVERSITY OF CALIFORNIA 
DAVIS