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QUAIN'S

ELEMENTS OF ANATOMY

EDITED BY

EDWARD ALBERT SCHAFER, F.R.S.

PROFESSOR OF PHT^IOLOOY AND HISTOLOOY IN UNIVERSITY COLLEGE, LONDON,

GEORGE DANCER THANE,

PROFESSOR OF ANATOMY IN UNIVERSITY COLLEGE, LONDON'.

IN THREE VOLUMES.

VOL. L— PART I.

EMBRYOLOGY
By PROFESSOR SCHAFER.

ILLUSTRATED BY 200 ENGRAVINGS, MANY OF WHICH ARE COLOURED.

Cdtllj (icKttton.

LONDON
LONGMANS, GREEN, AND CO.

AND NEW YORK: 15 KAST 16th STIlEET
1892

LONDON :
BRADBURY, AGNEW, & CO. LD., PRINTERS, WHITEFRIARS.

CONTENTS OF PAET I.

i.vthodvctiox !
General Considerations
Plan of Organisation
TJie A'ertebrate type .

Segmentation of the Body
Homology .
Symmetry of Form.
Descriptive terms

EMBRYOLOGY.

Structure of Ovarian Ovum ; Matui;a-
Tiox OF Ovum 6, 9

Formation of Polar Globules . . . 9

Fertilization . . . . .11

^Meaning of Polar Globules. . . . 12

Theory of ]\rinot ..... 12

Tlieory of Weissmaim . . . . 14

Recent Literature of the Ovum . . 14
Segmentation of the Ovum ; Forma-
tion OF the Blastoderm . . 16, 17

Gastrula Co.nJition of the Vertebrate
Ovum .....

Views concerning the Gastrulatioii of
Vertebrates ....

Inversion of Blastodermic La3'ers in .some
Mammals ......

Historical view of Blastoderm .

Characters of the Blastodermic Laj'ers

Paiablast theory of His .

Mesenchyme theory of Hertwig .

Kecent Literature of Blastoderm .
Early Changes in Blastoderm

Neural CanaL ....

Notochord

Separation of the Embiyo

Cleavage of Mesoblast

Formation of Body Cavity

Formation of Mesoblastic Somites

Cerebral Vesicles ....

Heart and Vascular System

Recent Literature of Early Changes in
Blastoderm ....
Development of the Fcetal Mem
ERANEK ; Attachment of Ovum to
Uterus

Formation of the Amnion and Chorion

Fonnatiou of the Allantois .

Ciiange.s in tlie Utems and mode of
attachment of Ovum to Uterus .

The Placenta . . - .

Separation of the Decidua at birth, and
regeneration of the Uterine Mucous
Membrane .....

Recent Literature of the Decidua
Devki/^i'mknt of the Nervous System

Of the Spinal Cord

Of the Brain ....

23
23
24
25
26
27
30
30
32
34
36
36
36
37
38

41

42
42
43

46
53

55
55
57
57
61

Further details regarding the develop-
ment of special parts of the Brain . 63
The fifth cerebral vesicle : Bulbar vesicle

or Metencephalon 63

The fourth cerebral vesicle : Cerebellar
vesicle or Epencephalon. . . . 66

The third cerebral vesicle : Mesen-
cephalon : Mid-brain . . . -67
The second cerebral vesicle : Thalamen-
cephalon . . . . . . 67

The first cerebral vesicle : Prosencephalon 68

The Olfactory Lobes 71

Formation of the Fissures andConvolutions 7 1

Development of the Nerves . . 73

Spinal nerves . . . . • • 73

Cranial nerves 75

Optic nerves . . . . • • 79
Olfactory lobe ..... 79
Sympathetic nerves and ganglia . . 81

Recent Literature of the development of
the Nervous System . . . .81
Development of the Eye . . . 83

Of the Retina 86

Of the Lens 86

Capsule of the Lens . . . -87

Vitreous humour 87

Corneal epithelium . . . -87

Sclerotic . . . . . . . 88

Choroid coat ...... 88

Accessory structures 89

Lachrymal gland . . . . -89
Lachiymal canals and ducts . . . 89
Developme.nt of the Ear ; The Laby-
rinth 89

Accessory parts of the Organ of Hearing,
E.xteriial and iliddle Kar . . -93

Dkveloi'me.s'T of the Nosk . • ■ 95
Recent Literature of the development of
the Sense Organs . . . .98

Developm ent of the Ali.mentary Canal 99
Of the Mouth and jjarts in connection

witli it 99

I'harynx ...... lOl

Tongue 102

flisojiliagus, Stomach, and Intestines . 103
The Mesentery ...... 104

The Spleen 108

374765

IV

CONTENTS OF PART I.

Formation of the Anus . . .
Formation of the Glands of the Ali

MENTAET CaNAL

The Lungs , . . . .

The Trachea and Larynx
The Thyroid Body ....
The Thymus . . . .

The Liver

The Pancreas . . . ...

Recent Literature of the development of

the Alimentary Canal and Glands . .
Development of the Ukinary and

Generative Oegans
The Wolffian duct and body
Supra-renal Capsules
The permanent Kidneys ,
The Urinary Bladder
The Miillerian duct .
The Germinal Epithelium
Development of the Ovary.
Of the Testicles .
Descent of the Testicles .
The External Organs
Table of Generative Organs
Recent Literature of the development of

the Urinary and Generative Organs .
Formation of the Vascular System .
Development of the Heart
Peculiarities of the Foetal Heart
Development of the principal Arteries .

page
io8

109
109
no
no
III
112
113

"3

"5
117
120
122
122
122
124
124
125
126
127
130

132

134
134
146
146

page

Destination of the fourth and fifth Arte

rial Arches. ....
Development of the principal Veins,
Peculiai'ities of the fcetal Organs of Circu

lation .....

The Foramen Ovale .
The Eustachian Valve .
The Ductus Arteriosus
The Umbilical Vessels .
Course of the blood in the Foetus
Changes in the Circulation at Birth
The Lymphatic System
Recent Literature of the development uf

the Vascular System ....
Development of the Serous Catities

and of the Muscles and Skeleton
The Serous Cavities ...
Development of the Muscles
Formation of the head Muscles and evi

dences of head segmentation .
Development of the Vertebral Column
Ribs and Sternum ....
The Limbs .....
The Cranium .....
Formation of the visceral skeleton of the

Head ; cartilaginous bars of the visceral

arches ......

Formation of the Auditory Ossicles .
Recent Literature of the development of the

Serous Cavities, Muscles, and Skeleton 169

150
151

15s
155
155
156
156
156
157
157

158

159
159
159

161
162
163
163
165

166
167

ELEMENTS OF ANATOMY.

CORRIGENDA— Vol. I., Past I.

Page 36, line ll//'OHi bottom, should he ^^somatopleural mesoblast" instead of

^^ somatopleurc.^^

Page 36, ,,9 ,, ,, ^^ splanchnopleural incsohlast^' instead of

' ' splanclmopleure. "

Page 117, ,, 4 ,, ,, "(fig. 142, w. d.),'' instead of "(fig. 139, ting.)"

Page 123, middle of page, should be " (second month) " ,, " (fourth month.) "

Page 126, line 3 from, bottom, should be "internal" ,, "intestinal."

Page 135, dcscriidion oj figure 160, should be "in the following figure" instead of

" in the preceding figure."'

Page 141, line ofrom bottom., should be "left end" instead of "right end."

Page 143, description of figure 173, should be "septum superius," "septum inferius,"

instead of "septum superior," "septum inferior."

Page 150, line 11 from top, should be "the whole of" instead of "the proximal

part of."

Page 152, ,, 2 from bottom, shotdd be " sinws \eiios\\s,^' ,, "auricle."

Page 159, ,, 12 ,, ,, ,, " antero-dorsal " ,, "anterio-dorsal."

(For Hochstetter's account of the Development of the inferior vena cava and its
tributaries, consult Vol. II., pp. 544 to 546.)

ti-euts of the minute structure of the component tissues of the body ; the other,
named " Special or Descriptive Anatomy," treats of its several organs, members, and
regions, describing the outward form and internal structure of the parts, their
relative situation and mutual connection, and the successive conditions which they
present in the progress of their formation or development.

To the description of the origin and formation of organs in the embryo, a special
chapter is devoted in this work, under the name Embryology.

The study of anatomy may be viewed in two different aspects ; viz., the physio-
logical and the morphological. In the former, anatomy supplies the materials
relating to structure from which an explanation is sought of the uses or functions
of organs by the physiologist ; and for this purpose the study of histology is of
particular service. In its morphological aspect, anatomy investigates and combines
the facts relating to the structure and relations of organs, from which mav be
deduced general principles as to the construction of the human body or that of

VOL. I. B

IV

CONTENTS OF PART I.

PAGE

Formation of the Anus . . . . io8
Formation of the Glands of the Ali-
mentary Canal . . . .109
The Lungs , . . . , . 109

The Trachea and Larynx . . .110

The Thyroid Body no

The Thymus . . . . . .111

The Liver 112

The Pancreas . . . . ... . 113

Recent Literature of the development of
the Alimentary Canal and Glands . , 113
-Development of thf, TIp.tnatiy and

Destination of the fourth and fifth Arte-
rial Arches. ....

Development of the principal Veins,

Peculiarities of the foetal Organs of Circu
lation .....

The Foramen Ovale .

The Eustachian Valve .

The Ductus Arteriosus

The Umbilical Vessels .

Course of the blood in the Fcetus

Changes in the Circulation at Birth

The Lvmnhatic Svsteni

PAGR

150
151

iSS

155
155
156
156
156
157

ELEMENTS OF ANATOMY.

INTRODUCTION.

Anatomt, in its most extended sense, is tlie science which deals with the
structure of organized bodies. It is divided into departments according to its
subjects ; such as Human Anatomy ; Comparative Anatomy, or the study of the
structure of different animals ; and Vegetable Anatomy, comprehending the
structure of plants.

On examining the structure of an organized body, we find that it is made up of
members or organs, by means of which its functions are executed, such as the root
stem and leaves of a plant, and the heart, brain, stomach and limbs of an animal ;
and farther, that these organs are themselves made up of certain constituent
materials named tissues or textures, such as the cellular, woody, and vascular tissues
of the vegetable, or the osseous, muscular, connective, vascular, nervous, and other
tissues, which form the animal organs.

Most of the tissues occur in more than one organ, and some of them indeed, as
the connective and vascular, in nearly all, so that a multitude of organs, and these
greatly diversified, are constructed out of a small number of constituent tissues ;
and parts of the body, differing widely in form, construction, and uses, may agree in
the nature of their component materials. Again, as the same tissue possesses the
same essential characters in whatever organ or region it is found, it is obvious that
the structure and properties of each tissue may be made the subject of investigation
apart from the organs into whose formation it enters.

The foregoing considerations have led to the subdivision of anatomy into two
branches, the one of which, under the name "General Anatomy," or "Histology,"
treats of the minute structure of the component tissues of the body ; the other,
named " Special or Descriptive Anatomy," treats of its several organs, members, and
regions, describing the outward form and internal structure of the parts, their
relative situation and mutual connection, and the successive conditions which they
present in the progress of their formation or development.

To the description of the origin and formation of organs in the embryo, a special
chapter is devoted in this work, under the name Embryology.

The study of anatomy may be viewed in two different aspects ; viz., the physio-
logical and the morphological. In the former, anatomy sup{)lies the materials
relating to structure from which an explanation is sought of the nscs or functions
of organs by the physiologist ; and for this pur[)Ose the study of histology is of
particular service. In its morphological aspect, anatomy investigates and combines
the facts relating to the structure and relations of organs, from which may be
deduced general principles as to the construction of the human body or that of

VOL. I, B

2 INTRODUCTION.

animals. In the determination of these general principles, or laws of morphology,
it is necessary to combine the knowledge of the anatomy and development of
animals with that of man.

PLAN OF ORGANIZATION.

Vertebrate type. — The general plan of construction of the human body agrees
closely with that which prevails in a certain number of animals, viz., mammals,
birds, reptiles, amphibia, and fishes, and is known as the vertebrate type of organi-
zation. The main feature of that type, and that from which its name is derived,
belongs to the internal skeleton, and consists in the existence of a median longi-
tudinal column, which extends through the whole trunk, and is composed in the fully
developed state of a series of bones termed vertelrce. This vertebral column is formed
in the early embryo around a simple rod-like structure, the primitive skeletal axis,
^'hich is called the notochord, and which in most vertebrate animals disappears to a
greater or less extent in the course of development. The more solid portions of the
vertebrse immediately surrounding the notochord are known as the bodies or centra
(figs. 2 and 3), and constitute a pillar around which the other parts are grouped
with a certain regularity of structure. At one extremity of this pillar is situated the
head, showing in almost all the animals formed upon this type a greater development of
its constituent parts ; and at the other the tail in which an opposite character or that
of diminution prevails ; while on the sides of the main part or trun^, there project,
in relation with some of the vertebral elements, two pairs of symmetrical limbs.

The head and trunk contain the organs or viscera most important to life, such as
the alimentary canal and the great central organs of the vascular and nervous
systems, while the limbs, from which such principal organs are absent, are very
variable and difler widely in the degree of their development among the various
animals formed upon the vertebrate type. In man and the higher animals the trunk
is divisible into neck, chest, abdomen, and pelvis.

The vertebrate form of skeleton is invariably accompanied by a determinate and
conformable disposition of the other most important organs of the body, viz. : —
firstly, the existence on the dorsal aspect of the vertebral axis of an elongated cavity
or canal which contains the brain and spinal cord, or central organs of the nervous
system ; and secondly, the existence on the ventral aspect of the vertebral axis of a
larger cavity, the visceral cavity, body cavity or coelom, in which are contained the
principal viscera connected with nutrition and reproduction, such as the alimentary
canal, the heart and lungs, the great blood-vessels, and the urinary and generative
organs.

The general disposition of the parts of the body and of the more important
viscera in their relation to the vertebral axis are shown in the accompanying
diagrams of the external form and longitudinal and transverse sections of the human
embryo at an early period of its existence.

Segmentation of the body. — The vertebrate type of organisation in the repetition of
similar structural elements in a longitudinal series, has a segmented character, especially in
the axial portion of the body, and this segmentation affects more or less, not merely the
skeletal parts of its structure, but also, to some extent, its other component organs,

A segmented plan of construction is by no means restricted to vertebrate animals, but exists
in several other classes of the animal kingdom, as is most conspicuously seen in the Azthropoda.
such as insects and Crustacea, and in the Annelida or worms. These animals, however, although
showing a serial repetition of parts of like structure, are not considered to belong to the verte-
brate type of organization.

In the human embryo, as in that of all vertebrate animals, the segmentation is most marked
in the muscular system, the nervous and osseous systems becoming tor the most part corres-
pondingly marked off : in the adult the osseous and nervous systems retain in great measui'e
the segmentation which has thus been produced, although in the muscular system it has

PLAX OF OUGANIZATION.

Fig. 1. — Diagram of an early human embryo. (Allen Thomson.)

*, s, indications of the vertebral divisions along the line of the back ; r, it, upper limb ; t, f, lower
limb ; u, umbilical cord. In the cranial part the divisions of the brain are indicated, together with
the eye, and au, the auditory vesicle ; near b, the visceral arches and clefts of the head, forming inter
alia the rudiments of the upper and lower jaws.

Fig. 2.— Semidiagrammjvtic view of a longitudinal section of the embryo rki'resented in
KicuEE 1 ; showing the relations of the principal systems and organs. (Allen Thomson. )

1, 2, 3, 4, 5, primary divisions of the brain in the cranial part of the neural canal ; n, «, spinal
cord in the vertebral part of the canal ; s, spinous process of one of the vertebrae ; ch, chorda dorsalis run-
ning through the axis of the vertebral centra : ch\ the same extending into the base of the cranium : a,
dorsal aorta ; p, phivryngeal cavity ; i, i, alimentary canal : h, ventricular part of the heart, with
which the arterial bulb is .seen joining the aorta by arches ; b, visceral arches of head ; I, liver ; w,
Wolffian body ; v, urinary vesicle or allantois, joining the intestine in the cloaca, c/ ; «, u\ umbilicus.

Fie:. .3.— Transverse .section (diagrammatic) of tiik trunk of the embryo turougu ihi; ui'PER

LIMBS. (Allen Thomson.)
m, iq.inal cord ; n, neural or dorsal arch, including bone, muscle, skin, roots of tlic nerves, &c. j
. chorda dorsalis, surrounded by the vertebral body or centrum ; v, ventral or visceral arc!), or wall
the body ; p, p, body cavity ; /, alimentary canal ; h, heart ; /, I, the rudimentary limbs.

■1.— First dorsal vektebua with the fik.st kib and upper i'art of the sternum, seen from

above. J.

C; centrum ; X, neural cavity ; V, cavity of the chest, visceral cavity.

15 2

4. INTRODUCTION.

become greatly obscured. To the original segments in the embryo the terms protovertehrce,
mesoblastic somites or myotovies have been applied ; those segments or metameres which are
traceable in the adult are often spoken of as vertebral segments. In the limbs, although there
is strong reason for believing that they have originated as outgrowths of certain segments of
the trunk, the repetition of such vertebral elements, and their primitive connection, are
greatly obscured.

Homology. — A certain agreement in structure, situation and connection of
parts or organs constitutes what is called homology, and this term is generally-
employed to indicate the morphological identity of representative parts in different
animals, which may be considered to have its cause in community of origin {homo-
qcny, Lankester), while the anatomical correspondence of parts which are repeated
ill the same animal may be more exactly distinguished as serial homology {homo-
dynamy, Gegenbaur). Thus the arm-bone or humerus of a man is homologous
(homogenetic) with the upper bone of the fore limb of a quadruped, or of the wing
of a bird, while it is at the same time serially homologous (homodynamic) with the
thi"'h bone of man himself, or any other vertebrate animal. It has farther been
found convenient to express by the word analogy that kind of resemblance among
the organs of animals which depends upon similarity of function, and although it
may be accompanied by considerable agreement in structure, yet is not rendered
complete by anatomical relation and connection : for example, the gills of a fish, of
a crab, and of a mussel, serving the same function, are analogous organs, but in no
sense homologous, as all morphological correspondence, or genetic relation, is wanting
between them. Thus also, the upper limb of a man, the fore limb of a quadruped,
the wing of a bird, and the pectoral fin of a fish are homologous but not analogous
structures, the wing of a bat and the wing of a bird are both homologous and
analogous, while the last is analogous to but not homologous with the wing of an
insect.

Symmetry of form. — A remarkable regularity of form pervades the organi-
zation of certain parts of the body, especially the whole of the limbs, the head and
neck, and the framework, at least, and external walls of the trunk of the body.
Thus, if we conceive the body to be divided equally by a plane which passes from its
dorsal to its ventral aspect {median plane), the two halves, in so far as regards the
parts previously mentioned, correspond almost exactly with each other, excepting by
their lateral transposition, — and the human body thus shows in a marked manner
the character of hilateral symmelry. There is, however, a departure from this
symmetrical form in the developed condition of certain of the internal organs, such
as the alimentary canal from the stomach downwards, the heart and first part of the
great blood-vessels, the liver, spleen, and some other viscera.

Bescriptive terms. — In the description of parts so numerous, so various in
form, and so complex in their connections as those composing the human body, there
is difl&culty in finding terms which shall indicate with sufiicient precision their
actual position and their relation to the rest of the organism. This difficulty is
farther increased by the exceptional erect attitude in which the trunk of the human
body is placed as compared with the horizontal position in animals. Hence, a
number of terms have long been in use in human anatomy which are understood in
a technical or restricted sense. For example, the median plane, already referred to,
being that by which the body might be divided into right and left lateral halves,
and the middle or median line being that in which the median plane meets the surface
of the body, the words internal and external are used to denote relative nearness to
and distance from this plane on either side, and may be replaced by mesial and
lateral. The terms sagittal, frontal, and coromd, are also used in indication of
direction within the body : sagittal denoting a dorso-ventral direction in or parallel
to the median plane, frontal or coronal a transverse direction perpendicular to that

DESCEIPTIVE TERMS. 5

plane. The words anterior and posterior, superior and inferior, and several others
indicating position, are employed in human anatomy strictly with reference to the
erect posture of the body. But now that the more extended study of comparative
anatomy and embryonic development is largely applied to the elucidation of the
human structure, it is very desirable that descriptive terms should be sought which
may without ambiguity indicate position and relation in the organism at once in
man and animals. Such terms as dorsal and ventral, neural and visceral, cephalic
and caudal, central and peripheral, proximal and distal, axial and appendicular, pre-
axial and postaxial, are of this kind, and ought, whenever this may be done con-
sistently with sufficient clearness of description, to take the place of those which are
only applicable to the peculiar attitude of the human body, so as to bring the
language of human and comparative anatomy as much as possible into conformity.
In many instances, also, precision may be obtained by reference to certain fixed
relations of parts, such as the verielral and sternal aspects, the radial and ulnar, and
the tibial and fibular borders, the flexor and extensor surfaces of the limbs, and
similarly in other parts of the body.

EMBEYOLOGY/

By E. A. SCHAFER.
FORMATION OF THE BLASTODERM.

STRUCTUEE OF THE OVUM AND CHANGES PEIOR TO SEGMENTATION.

The human body with all its tissues and organs is the product of the development
of a single nucleated cell, the egg-cell, germ-cell, or ovum, which is formed within
the principal reproductive organ of the female or ovary. The commencement of
development is preceded by certain changes in the ovum, which usually occur soon
after its discharge from the ovary, and consist (1) in the emission of certain
constituents of the nucleus which form the so-called polar globules ; (2) in the
accession of the nucleus of a sperm-cell or spermatozoon, which is formed wdthin the
reproductive organ of the male (testicle), and which, blending with the remaining
part of the nucleus of the ovum, appears to take the place of the part which was
discharged in the form of the polar globules.

An account of the structure of the ovum, and of the manner in which the above
changes are efiTected, may therefore appropriately precede the description of the
actual course of development of the ovum.

Structure of the ovarian ovum. — The human ovum resembles that of all other
mammals (with the exception of monotremes) 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 xIt^^ i^^h ('2 mm.)
in diameter, and is just visible as a clear speck to the naked eye. When it is
examined with the microscope, it is found to be invested by a comparatively thick,
clear covering. This, when the centre of the ovum is exactly focussed, has the
appearance in optical section of a clear girdle or zone encircling the ovum (fig. 5),
and was hence named zona pellucida by von Baer (1827). But on more careful
examination with higher magnifying powers, and especially by the examination of
sections, there is not much difficulty in making out the existence of striae passing
radially through the membrane (fig. 6, zf). 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 more convenient,
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, or to
speak of it simply as the striated memlrane of the ovum.

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

1 It is mainly owing to tlie researches of His, published principally in the important monograph
" Anatomie menschlicher Embryonen " (Leipzig, 1880-1885), that our knowledge of the development of
Ihe human embryo is now far more complete than was the case when the last edition of this work was
undertaken, and we are therefore able to keep more closely than was before possible to the human
species in following the course of development of the ovum. Eor the elucidation, however, of many of
the details of development, especially in its earlier stages, it will still be necessary to refer continually
to facts which have been made out only from the study of the embryology of other mammals, as well as
birds, reptiles, fishes, and even invertebrata.

STRUCTURE OF THE OVUM. 7

contents are extruded (fig. 5, b). The striae in the membrane are beheved to
be minute pores, and are supposed, while the ovum is yet within the Graafian
foUicle, to permit the passage of granules of nutrient material into the interior of
the ovum. After the ovum is discharged from the follicle, the spermatozoa may
perhaps find their way into the ovum through these pores. According to Eetzius
the protoplasm of the ovum is united with the follicle-cells by fibres which pass
through the pores of the zona.

Immediately surrounding the zona radiata, as the ovum lies within the mature Graafian
follicle, is a thin stratum of g-ranular substance, probably deposited upon the exterior of the
ovum by the inneiTQOst cells of the discus prolig-erus, which immediately encircle the ovum
within the follicle. When the Graafian follicle bursts and the ovum is set free, this granular
material appears to imbibe water, and, as is specially noticeable in the ovum of the rabbit,
swells up into a clear gelatinous envelope, which has been termed, from a possible homology
with the white of the bird's egg, the albumen. But in the mammal this structure has not
the nutritive importance to the embryo which is possessed by the corresponding formation
in the bii'd, and it disappears during the i^assage of the ovum down the Fallopian tube.

The substance of the ovum within the tunica radiata is known as the vitelJiis
or yolk (fig. 6, vi). It is a soft semi-fluid substance, composed mainly of proto-
plasm, which is filled with globules and granules (yolk-granules) of dilfereut

Fig. :>. — Ovarian ovuji of a mammifer. (Allen Thomson. )

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

Fig. 6. — OVL'M OF THE CAT ; HIGHLY MAGNIFIED. SEMI-DIAGKAMMATIC. (E. A. S.)

zp, zona pellucida, .showing radiated structure ; vi, vitcllus, round which a delicate membrane i.s
Been ; ffv, germinal vesicle ; ys, germinal .spot.

sizes, but all small, and pos.sessing a high index of refraction. Examined in the
fresh condition, the protoplasm between the granules looks jierfectly clear and
Ktructureless, but after treatment with suitable reagents, it may be seen to consist of
!i fine reticulum, which is especially fine and close near the ])erij»hery ui the ovum,
and also around the germinal vesicle, at which places the yolk granules are in less
amount than elsewhere. The substances which occur within an ovum other than
the nucleus and protoplasm, may, as in cells generally, be colle(;tively designated
"dentoplasm " ; they are regarded as furnishing a supply of nutrient matter to the
protojilasm during tlie earliei- stages of development.

Embedded in the protoplasmic vitellus, usually eccentrically, is a large spherical

8 STRUCT QEE OF THE OVAEIAN OVUM.

nucleus, which was termed by its discoverer, Purkinje, the germinal vesicle?- This,
which is about -g^oth inch in diameter, has all the characters of the nucleus of a
cell. It consists of a nuclear membrane enclosing a clear material or matrix,
embedded within which may be seen strands of karyoplasm, enclosing one or more
well-marked nucleoli (fig. <o, gv). Frequently there is but one nucleolus, which is
then large and prominent, and has received the name of germinal spot {macula ger-
minativa, Wagner, 1835).

There is some doubt whether, before fertilization, there is another membrane (vitelline
membrane) enclosing- the vitellus within the zona radiata. The evidence of the presence of
such a membrane is by no means clear, although its existence has been maintained by very
competent observers (v. Beneden, Balfour).

The mammalian ovum (that of monotremes alone excepted) differs from that of other
vertebrates in the relatively small amount of nutritive material (yolk granules, deutoplasm)
which is embedded in its protoplasm. In fishes, amphibia, reptiles, and especially in birds,
the amount of such nutritive material is vastly greater than that of the protoplasm itself, so
that the very existence of the latter is obscured in most parts of the ovum, and it is only in
the immediate neighbourhood of the germinal vesicle that the protoplasm can be distinctly

\....y

Fig. 7. — Diagram of a holoblastic (alecithal) ovum (A) and of a meroblastic (telolecithal)

OVUM (B). (E. A. S.)

Only a small part of the latter is represented. The yolk or food material is represented in both by
clear globules, which in B are seen vastly to preponderate, except in the immediate neighbourhood of
the germinal vesicle.

recognized (fig. 7, B). It is here also that, after fertilization, the more active changes in the
ovum occur, and it is this part alone iu which in the bird and most other oviparous vertebrates
the process of division or segnientation of the yolk and consequent formation of embryonic celb
proceeds. Hence these ova are said to undergo a process of incomplete segmentation, only a
part of the ovum appearing to undergo development, and they are accordingly termed mero-
hlastic to distinguish them from those (like the mammalian ova) in which the yolk or nutri-
tive material is everywhere in relatively small proportion to the protoplasm, the whole of
which undergoes division after fertilization, and participates in the formation of the embryo
(Jiolohlastic ova). This small amount of nutritive material in the mammal is obviously
related to the fact that the mammalian ovum early acquires an attachment to the maternal
system from which it is then able directly to derive its nutriment, whereas the meroblastic
ovum of oviparous vertebrata necessarily contains all the nutriment required by the developing
bird, reptile, or fish, until it is suificiently advanced in development to emerge from the %gg
and obtain food independently. Although, however, the maminalian ovum is holoblastic, it
is none the less clear, from a comparison of the early stages of its development with that of
the bird, that the ancestors of the mammalia must have had ova of the meroblastic type.

Balfour has further conveniently distinguished between those ova in which there is a great
accumulation of nutritive or yolk material at one pole (telolecithal ova, as in the bird,
reptile, and fish amongst vertebrates), those in which the accumulation of yolk is in the middle
of the ovum {crntrolecithal ova, as in arthropods), and those in which it is scattered pretty
equally in small amount throughout the protoplasm without any very marked accumulation

1 Purkinje discovered the germinal vesicle in the bird's ovum in 1825 ; that of mammals was first
noticed by Coste in 1833.

MATUKATION OF THE OVUM. 9

(^aleclthal ova. as in mammals, Amphioxus. echinoderms). It is clear that these conditions
of arrang:ement of the proto- and deuto-plasm within the ovum are the main factors in deter-
mining vai"iations in the process of segmentation.

Maturation of the ovum. Formation of polar globules. — Either before
its escape from the Graafian t'olHcle, or immediately after, the ovum undergoes a
peeuhar change, preparatory to, but nevertheless altogether independent of fertiliza-
tion, which consists of a process of unequal cell-division or germination, and results
in the extrusion from the vitellus of two minute spherical bodies (fig. 8), which have

Fig. 8. — Ovum of the rabbit from the fallopian tube, twelvei

HOURS AFTER IMPREGNATION. (Bischoff. )

On the zona a, spermatozoa are seen, and others in the perivitelline
space ; b, the polar globules.

been termed the j^oJar /jlooules or directive corpuscles,
from a supposition that their presence determines the
pole at which the first segmentation will take place
should the ovum become fertilized. It is, however,
uncertain whether there is any constant relationship of
this kind, but it is none the less clear that the extrusion
of the polar globules is an event of the highest importance for the due development
of the ovum, since until this has happened the ovum appears to be incapable of
complete fertilization and segmentation. ^ What is actually extruded is a small part
of the nucleus of the ovum, or, to speak more precisely, two small parts of its nuclev.s
in succession, probably surrounded by a very thin investment of protoplasm. Prior
to this extrusion, the germinal vesicle aiDproaches the periphery of the vitellus, loses
its distinctness of outline, and after passing through phases which are charac-
teristic of a nucleus which is about to divide, does actually undergo a division into
two, the one part being extruded into a space (perivitelline), which has become
formed in consequence of the shrinking or contraction of the ovum, and the other
part remaining in the vitellus, only, however, to repeat the ^jrocess of division, and
to form a second extruded globule. The remainder of the germinal vesicle, which ir,
now termed the female jjro-nuck us, leaves the periphery of the vitellus for a situation
nearer to the centre, where, if fertilization should supervene, it awaits the advent of
the rnale pro-micleiis, w'hich is formed from a spermatozoon. After the two pro-
nuclei have come together, a new and complete nucleus is formed by their conju-
gation.

The actual formation of polar j^dobules has not hitherto been observed in the human ovum,
ulthou;4-h there is no doubt whatever that it takes place. In the rabbit various stages in the
jtrocesH have been traced by E. v. Beneden and Rein, and it has also been noticed in other
mammals. But the details of the process have been made out most precisely (by Fol, Hertwig,
and others) in the transparent ova of echinoderms, and more recently and minutely (by
E. V. Beneden, Camoy, Boveri, Zacharias. and others) in Ascaris megalocephala, a thread-worm
parasitic in the horse, in which all the changes can be followed in one and the same ovum.
or the various phases fixed Vjy means of reagents in different ova, and these may afterwards bc^
stained and studied with the utmost minuteness. The successive changes in such ova are
represented in figs, y and 10. ■ The polar globules remain visible for a time in the peri-
vitelline fluid, and are even seen, should the ovum become fertilized, during the early stages of
Bfegmentation, but they ultimately disappear and are not known to take any further part in
the subsequent changes which the ovum undergoes.

The fact that throughout the whole animal kingdom the extrusion of polar globules from
the ovum as it becomes mature is almost universal, and that a similar ])rocess has also been
observed to occur in plants indicates the great importance of the plienomenon. The signifi-
cance will be further discussed after the procesH of fertilization of the ovum has been
de»cribe<l.

' The ovum may, however, receive a spermatozoon before tiic completion of tlic formation of polar
globules.

10

FORMATION OF POLAR GLOBULES
B

Fig. 9.

-Formation of the first polar globule in the egg of ascaeis megalocephala.
(v. Gehuchten. )

A. The ovum with the gei-minal vesicle transformed into a spindle of (achi-omatic) fibrils r from the
poles of the spindle other fibrils radiate into the protoplasm. At the equator of the spindle eight
portions of chromatin are visible ; cs, head of a spermatozoon which has previously entered the ovum,
and is becoming transformed into the male pro-nucleus ; m, gelatinous membrane of the ovum.

B. The chromatin particles are seen separated into two sets. The achromatic fibrils are not shown
in this preparation. The ovum is considerably shrunken.

C. Half of the germinal vesicle is extruded into a perivitelline space, and along with a portion of
protoplasm is becoming separated off from the ovum as a polar globule. The extruded half includes
four of the chromatin particles ; the other four remain in the ovum ; m', membrane dividing the polar
globule from the ovum.

B

Fig. 10. — Formation of the second polar globule
in ascaeis megalocephala. (Carnoy.)

A. _ The remainder of the germinal vesicle (after
extrusion of the first globule, g'^) has again become
transfoi-med into a spindle of achromatic fibrils,
with the four remaining chromatin particles at the
equator of the spindle.

B. The spindle, now irregularly Y-shaped, is seen
approaching the surface of the ovum ; g^, first polar
globule ; ns, male pronucleus which has become
formed from a spermatozoon.

C. Extrusion of half of the germinal vesicle
remainder.

cont^i.^twnlf °.l\°^*^' process; the second polar globule, g^ is now separated from the ovum ; ii
T 1 ., ° °V °i"T''*'°i ^^'*''^'f- ^^' °*^'^' *^'° ^'^"^^^ i^ ^l^at is left of the germinal vesicle,
n^ which now forms the female pronucleus ; tis, male pronucleus ; g\ first polar -lobule

FERTILIZATION. 11

Fertilization. — The ovum, ^ffcer its expulsion from the Graafian follicle is
received upon the fimbriated end of the Fallopian tube. The fimbrias 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 have occurred, meet with the spermatozoa, one or
more of which may penetrate the zona pellucida, and fertilize the ovum. It is
possible in some instances for fertilization to occur on the fimbriated extremity of the
tube, or in the body of the uterus, but it is probable that in most cases it happens
in the tube itself.

It is probable that normally only a single spermatozoon enters the vitellus. If it should
happen that two or more enter, normal development does not as a rule occur. Exceptions to
this rule have, however, been recorded.

The changes in the ovum which accompany fertilization have, hke those which
result in the formation of the polar globules, been studied most satisfactorily in the
transparent ova ofechinoderms and in Ascaris. In the former (fig. 11) the spermatozoa

4.

h.

D — m.vv.

r\-T^-£r.

o

0

f'v-

f-Vr.

in.pv.

J-T^-

Fig. 11. — Fertilization of the ovrsi of an echinoderm. (Selenka.)

», spermatozoon ; ra.pr, male pronucleus ; f.pr, female ijronucleus.

1. Accession of u spermatozoon to the ijerijjhery of the vitellus ; 2. Its penetration, and the radial
disposition of the vitelline granules ; .3. Transformation of the head of the spermatozoon into the male
pronucleus ; 4, 5. Blending of the male and female pronuclei.

may be seen to penetrate the gelatinous investment Avhich here takes the place of a
zona pellucida, and the head, of one only as a rule, to imbed itself in the periphery
of the ovum, Avhich becomes slightly protruded at the point of contact. According
to v. Beneden's account, the spermatozoon always enters in Ascaris at a particular
part of the ovum (polar disc), at which part there is an aperture in the vitelline
membrane (micropyle). When once it has passed into the ovum, this aperture
becomes closed, and the head of the spermatozoon rapidly increases in size, and
acquires t?ie appearance of a nucleus which, in contra-distinction to the remains of
the germinal vesicle, or female pro-nucleus, is termed the maU pro-miclcnii. Soon
it leaves the periphery, and pa.sscs towards the centre of the ovum in the direction of
tlie female pro-nucleus. In its passage through the protoplasm it appears to exer-
cise- a peculiar attraction upon the granules in that substance, for these become
arrangr;d in its vicinity in radiating lines. The tail of the spermatozoon
has in the meantime disappeared, whether Ijy being cast off or by l^lending with the
protoplasm of the ovum has not certainly been made out. As the male pi-o-nucleus
approaches the female pro-nucleus, the latter moves somewhat to meet it, and pre-
sently the two pro-nuclei come into contact and together form a new nucleus, com-

12

FERTILIZATION".

plete in all its structure and functions. With the blending of the two pro-nuclei
the act of fertiHzation is completed, and the ovum is now capable of forming
new cells by division. Since the head of the spermatozoon is formed from the
nucleus of a seminal cell, part of which appears to be thrown off prior to the com-
plete maturation of the spermatozoon (Renson, Brown), and the female pro-nucleus is
the nucleus of an egg or germ-ceU, part of which has been removed in the form of the
polar globules, the process of fertiHzation may be described as consisting essentially
of the conjunction of part of the nucleoplasm of a sperm cell with part of the nucleo-
plasm of a germ cell, the result being the production of a complete nucleus endow^ed
with active properties of division and reproduction,

Althoug-h, as has been ali-eady stated, the changes which have just been described are most
clearly to be seen, and have been most completely studied, in the ova of echinoderms and
Ascaris, similar processes have been found to occur in most if not in all animals, and have
even been made out, although not very distinctly, in mammals (in the rabbit by v. Beneden),
There is no doubt, therefore, that the phenomena of fertilization are essentially the same
throughout the whole animal kingdom. As to the exact details of the process there is still
much discrepancy in the accounts given by recent observers. Of all those that given by
V. Beneden of the process of fertilization, and of the subsequent division of the resulting
nucleus in Ascaris, is the most explicit, and appears to negative the idea of a complete fusion
taking place between the elements of the pro-nuclei, at least so far as the chromatin is concerned.
According to this account (v. fig. 12), each of the two pronuclei is seen to possess, previous to their
conjunction, two short chromatin rods (cliromosomes) imbedded in clear nuclear matrix. These
rods undergo various changes, resulting in the formation of a skein within each pro-nucleus
(II., III.), but eventually the skein resolves itself into two V-shaped loops or filaments (IV., V.).
On conjunction the matrix of the two nuclei may appear to blend, although it is doubtful if
they actually fuse together, but the chromatin filaments retain their distinct individuality.
The nucleus which is thus formed by the conjunction, contains, therefore, four similar V-shaped
chromatin filaments, which now split longitudinally (VI., VII.), and after being arranged for
a time at the equator of the now spindle-shaped nucleus (VIII.), four of the resulting filaments
pass towards the one pole, and form eventually the chromatin of the one daughter nucleus,
and four towards the other pole, .£ventually forming the chromatin of the other daughter
nucleus (XL, XII.). It is stated by v. Beneden that of each set of chromatin filaments, or
chromosomes, which thus separate from one another, one half the number is derived from the
male and the other from the female pro-nucleus. If this is the case, and if it should further
be shown that in every subsequent process of division of the resulting cells, the chromatin
filaments of the daughter cells are derived half from male chromatin filaments and half
from female, it necessarily follows that every cell nucleus must be regarded as containirg
both male and female morphological elements.

Meaning of the polar g-lobules. — Theory of Minot. — The question of the hermaphroditism
of cells was first raised by C. S. Minot in connection with the separation of the polar globules.
According to the view advocated by Minot, every cell which results from the division of a
fertilized ovum is hermaphi-odite, for the fertilized ovum is formed by the union of both male

Fig. 12.— Formation and conjdgaiion op the peo-nuclei in ascaris megamcephala.

(E. V. Beneden. )
/, female pronucleus ; m, male pronucleus ; p, one of the polar globules.

I. The second polar globule has just been extruded ; both female and male pronuclei contain two
chromatin particles ; those of the male pronucleus are becoming transformed into a skein.

II. Ihe chromatin m both pronuclei now forms a skein.

Ila. The skein in the pronuclei is more distinct. Two attraction-spheres, each with a central
particle, united by a spindle of achromatic fibres, have made their appearance near the pronuclei. The
male pronucleus has the remains of the body of the spermatozoon adhering to it

ill. ihe pronuclei are enlarged : the skein formation of the chromatin is complete

MEANING OF THE POLAR GLOBULES.

13

Illff

li^..-,^??-

JX.

no.

X.

Ilia. Only the female pronucleus is shown in this Hmn-f Tlio oto,-,, «f +i • •

theiclSaT" ° """■"'""' "" """""■•' "'"' ""■ »PinJl=--*m i, „„v arranged .c„»

IV. Contraction ot the skein and formation of t»-o T-shaped ckroniatin iilamenls in eacli nronuden,

eh,oUali;ll■,s:Ifi:^1,!'r^•:^^.,;fs;°^^^^^

theII!,mrJ'.'l?*''"K'.'*™"f ""•"* °^ ^''' ^""'' ^^'"-^""^'tin loops in the nud.llo of the now clon.'ate.l ovun. •
nn.« ?fT ■ 1. '*r? ^""^'"^ ^ «Pind<e-.shape.l systen. of granules with fil.rils ra.l a<im t-om t J
fwocelL ■'""''' (attracfon-spheres) into the protoplasm ; 'connnencing division of tie tv:^"^:

in.Jt?i '^^''T '^'"f ^"'"'atically the commencing separation of the chromatin filaments of the con
^^^^S'cirS^;t^^,tS '"'^"'' '- ''' -'-ion-spheres. ;f i^'Sr^:

«phe''res'l":,' dl^iS-t ";;! *'' ''""'*" ''^"°"*" ^''^'' "^ *''« ^•'"^••''^' ^'-•^''^'- "f *'- — tion.
.rr fr,u ^"^1 ''i'V'"''**!" filaments are becoming developed into the skeins of the daughter nuclei These
are s t II un.ted by achromatic fiYcs. The protoplasm of the ovum is hecou.ing .livhled.
infn 1 1 V 'f "^''t'^.'- ""'^«' exhibit a chromatin network. Each of the attraction-sidicres las divided
nto two, wh.ch are joined by achromatic fibres, and are connected witli the porird S of t le cd li
the same manner a« the parent sphere shown in Ilia pc.ipnciy ot the cell in

14 RECENT LITERATURE OF THE OVUM.

and female elements, and every one of its descendants must also contain a certain proportion
of eacli. For the sexual conjugration of two cells it is assumed to be necessary that the one
should get rid of the male elements, and retain only the female, and that the other should be
exclusively male. This is effected in the one case, according to Minot, by the extrusion of the
polar globules, which, in this view, represent the male element of the originally hermaphrodite
generative cell, so that when they are extruded this remains wholly female ; in the other case
there is also a separation, and the separated part becomes disintegrated, leaving only the male
portion, or spermatozoon — the separated part in this case represents, therefore, the female
element of the generative cell.

Theory of Weismann. — Minot's theory was adopted by Balfour, who looked upon the
formation of polar cells as having been acquired by the ovum for the express purpose of pre-
venting parthenogenesis. According to this view no polar globules should be formed in parthe-
nogenetic ova, and it was believed by both Minot and Balfour that they would not be found
to occur. It has, however, since been discovered by Weismann and Blochmann that parthe-
nogenetic ova do extrude one polar globule, although the ordinary ova of the same animal
extrude two.

It is clear that this fact renders a modification necessary in the view advocated by Minot
and Balfour. Such modification, or substitute, as it may perhaps more appropriately be
termed, has been furnished by Weismann in his theory of heredity (^Vereriungstheorie). This
theory assumes that every animal and vegetable cell contains two different kinds of living
substance. These are termed by Weismann the nuclear plasma and the nutritive plasma. The
former is endowedi with germinative, directing and hereditary functions, the latter with
assimilation of food and the more purely physical functions (contraction, nerve-conductinn,
secretion, &c.), but these functions are assumed to be carried out under the direction of the
nuclear plasma. The nuclear plasma is further supposed by Weismann to consist of two sub-
stances, viz., a gerininal plasma which is the primitive form, and which alone is endowed with
heredity, and a Mstogenetie plasma which has been derived from the germinal plasma, and which
controls the division, growth, and differentiation of the cell. Fertilization consists in the
bringing to the ovum of a certain amount of germinal plasma from a difiierent individual,
and Weismann assumes that it is necessary for the ovum, prior to fertilization and develop-
ment, to get rid both of its old histogenetic plasma and of so much germinal plasma as may
be brought to it by the spermatozoon, and that it effects this by the extrusion (1) of one
(histogenetic) polar globule, (2) of the other (germinal) globule. If this is what happens, the
primitive or germinal plasma is never wholly eliminated from the ovum, so that it may be
looked upon as transmitting all the accumulated ancestral characters which have been derived
from the vast number of its predecessors. A portion is, however, got rid of in the form of
the second polar globule, and what remains is not necessarily of quite the same constitution in
every case, nor is the portion of germ plasma brought by the spermatozoon necessarily always
similar : these differences in the germinal plasma of the fertilized ovum may account, accord-
ing to Weismann, for the individual differences which occur in the progeny.*

Weismann and Ischikawa have shown that in some animals the segmentation of the ovum
may have advanced through one or two stages before the entry of a spermatozoon. In this
case the spermatozoon (male pro-nucleus) blends with the nucleus of only one of the cells
which have resulted from the segmentation. Probably the sexual cells are the ultimate result
of this conjugiP.*ion.

EECENT LITERATURE,

Beneden, E. v., Recherches sur la maturation de I'osuf et la fecondation. Arch, de biolog., iv.,

1884 ; Fertilization and Segmentation in Ascaris MegalocepTiala, Journal of Microscopic Science, 1888
(Bulletin.de I'academie r. des sciences de Belgique, 1887, t. xiv.) ; Sur la fecondation chez I'ascaride
megalocephale, Anatomisclier Anzeiger. Jahrg., iii., 1888.

Beneden, E. v. et Neyt, A., Nouvelles recherches sur la fecondation et la division mitosique
chez I'ascaride migalocephale (Bullet, de I'acad. royale des sciences de Belgique, 3 ser. t. xiv., 1887).

Blochmann, F., Ueher die Richtungskorper bei Insekteneiern, Morphol. Jabrbuch, Bd. xii., 1887.-
Also in Morph. Jalarb. xv., 1889, and Verhandl. d. naturhist. med. Vereins zu Heidelberg. 1888.

Boveri, T., Zellenstudien, H. 1, Die Bildung der Richstungskor per lei Ascaris meg alocephala und
Ascaris lumbricoides, Jenalsche Zeitschr. f. Naturwiss., Bd. xiv., 1887 ; Zellenstudien. H. 2. Bit
Befrnchtung und Theilung des Eies von Ascaris megalocephala, Jena. Zeitschr., 1888 ; Zellenstudien.
H. 3. JJeher das Verhaken der chromatischen Kernsubstanz bei der Bildung der Richtungskorper u.
bei der Bcfruchtung, Jena. Zeitschr., 1890.

Biitschli, O., Gedanken ilber die morphologische Bedeutung der sogenannten MiohMngskorverchen,
Biolog. Centralbl., Bd. iv., 1884. ^ » a i' >

1 It is difficult to do any justice to Weismann's theory in a short space, and the above is to be taken
as only furnishing a rough sketch of its general outline. For a complete account the reader ia
referred to Weismann's publications upon the subject (see Literature).

KECENT LITERATURE OP THE OVUM. 15

Caldwell, W. H., The Emhryology of Monotremata and Marsupialia, part i., Philosophical
Transactions of the lloval Society of London for the year 1887.

Carnoy, J. B., Les globes polaires de Vascaris clavata. La Cellule, t. iii., 1887 ; La visicult
germinative etles globes polaires chez uuclques nematodes, La Cellule, t. iii., 1887 ; La v6sicule germina-
live ct les globules polaires de I'Ascaris mcgaloccphala, La Cellule, t. ii., 1887 ; -Some Remarks on the
recent Researches of Zacharias and Boveri upon the Fecundation of Ascaris megalocephala. Report ol
the 57th meeting of the British Association for the Advancement of Science at Manchester, 1887.

Cunning-ham, J. T., E. v. Beneden's Researches on the Maturation and Fecundation of the Oimm,
Quart. Journ. of Microsc. Science, Jan., 1885.

Gehuchten, A. v., Nouvelles observations sur la vesicule germinative et les globules polaires de
I' Ascaris megalocephala, Anat. Anz. , No. 25, 1887.

Griitzner, P., Physiologische Untersuchungen iiber die Zeugung, Deutsche medic. Wochenschr. ,
lS8i.

Henking-, H., Ueber Reductionsteilung der Chromosomen in den SamenzeUen von Insecten.
Monthly International Journal of Anatomy and Physiology, vol. vii., 1890.

Herfwig-, Oscar, und Richard, Ueber den Befruchtungs- und Theihmgsvorgang des thierischen
Eies unter dem Einfluss dusserer Agentien, Jena. Zeitschr. f. Naturwiss., Bd. xx., 1887.

Hertwig-, R., Ueber den Einfluss des Chloralhydrats auf die inneren Befruchtungserscheinungen,
Anatom. Anzeiger, 1886.

Kolliker, A. v.. Das Karyoplasma und die Vererbung, eine Kritik der Weismann' schen Theorie
von der Continuii'dt des Keimplasnui, Zeitschr. f. wissensch. Zoologie, Bd. xliv. , 1886.

Kultschitzky, N., Die Befruchtangsvorgdnge bei Ascaris megalocephala. Arch. f. microsc. Anat.,
Bd. xjLxi., 1888 ; Ueber die Eireifung und die Befruchtungsvorgdnge bei Ascaris marginata, Ebendas.,
Bd. xxxii., 1888.

Kupffer, C, Die Befruchtung des Forelleneies, Bayerische Fischerei-Zeitung, 1886.

Minot, Ch. S., Theorie der Gonoblasten, Biol. Centralbl., Bd. ii., 1882.

Mondino, C, e Sala, L., Sur les pMnom^nes de maturation et de ficondation dans les oeufs des
ascarides. Arch. ital. de biol. xii. , 1889.

Nagrel, "W., Das menschliche Ei, Arch. f. micr. Anat., Bd. xxxi., 1888.

Nussbaum, M., Ueber die Verdnderungender Geschlechtsproductebis zur Eifurckung. Ein Beitrag
zur Lehre der Vererbung, Arch. f. mikrosk. Anat., Bd. xxiii., 1884 ; Bildungu. Anzahld. Richtungsk,
bei C'irripedie7i, Zool. Anzeiger, 1889.

Platner, G., Ueber die Befruchtung bei Arion empiricorum, Aixhiv f. mikr. Anat., Bd. xxvii.,
1886 ; Die Bildung d. ersten JRichtungspindel im Ei von Aulastomum gulo. Arch. f. mikrosk. Anat.,
xixiii., 1889.

Rein, G., Beitrdge zur Kenntniss der Reifungserscheinungen und Befruchtungsvorgdnge utti
Saugethierei, Arcliiv f. mikr. Anat., 1883.

Retzius, G., Zur Kenntniss v. Bau d. EierstocTcseies, d:c., Hygiea, Festhand, 1889.

Schultze, O., Ueber Reifung und Befruchtung des Amphibieneies, Anatom. Anz., No. 6, 1886.

Sehlen, D. v., Beitrag zur Frage nach der Mikropyle des Sdugethiereles, Arcliiv. fiir Anat. u.
Physiol., Anat. Abth., 1882.

Selenka, E., Zur Befruchtung des thierischen Eies, Biol. Centralb. V., No. 1, 1885.

Sheldon, L., The Maturation of the Ovum in Peripatus, Quart. Journ. of Mici'oscopical Science,
vol. XXX., 1889.

Tafani, La fecondation et la segmentation etudiees dans les oeufs des rats, Arch, ital. de biologie,
xi., 1SS9.

Thomson, A., Recent Researches on Oogenesis, Quart, Journ. of Micr. Science, vol, xxvi., June,
1886.

Waldeyer, Karyokinesis and its Relation to the Process of Fertilization (Translation), Quarterly
Journal of Microscopical Science, xxx., 1889 (contains an account of recent researches on these subjects).

Weismann, A., Ueber die Vererbung, Jena, 1883 ; Die Continuitdt des Keimplasmas als Gruivi'
lage einer Theorie der Vererbung, Jena, 1885 ; Richtungskorper bei parthenoyenetischen Eiern, Zoolo,,'.
Anz., No. 233, 1886; Ueber die Zald der RichtungskiJrpcr und iiber ihre Bedeutung fiir die Vererbung,
Jena, Fi.scher, 1887 ; Essays upon heredity and kindred biological problems (translations) — Oxford,
Clarendon Press, 1889 ; Ueher die Paracopulation im Daphuidenci, <fcc., 1889.

Weismann, A., u. Ischikawa, C, Ueber die Bildung der Richtungskorper bei thierischen
Eiern, Bericlite der naturf. GeselLsch. zu Freiburg i. Br., Bd. iii., 1887; also Zool. Jahrb., Bd. iv.,
1889-90 ; Ueber partidle Befruchtung, Freiburg i. Br., 1888 ; Biol. Centralbl., Bd. viii., 188S,

Zacharias, O., Xeue Untersuchungen iiber die Copulation der Gesc/dechtsproducte und den
Befruchlungsvorgang bei Ascaris megalocephala, Arch, f, mikrosk. Anat., Bd. xxx., 1887 ; Ueber die
BU/lung der Iliclitungskijrper hcithieriscker Eiern, Biol. Centralb., Bd. viii., 1888 ; Ueber Abweichungtn
vom Typus der Conjugation der GescfUechtskerne, Anat. Anzeiger, iii., 1888.

Zieg-ler, E., Kiinnen erworhene patholorjische Eigenschaften vercrbt werden und wie entstehcn
erbliche Kranklieiten uvl MissbOdungent Beitra^c zur pathoL Aual, u. Physiol., herausgegeben vou
Ziegler und Nauwerk, 1880.

16

SEGMENTATION OF THE OVUM.

EARLY CHANGES IN THE OVUM CONSEQUENT ON PERTILIZATION-SEGMENTATION-
FOBMaTION OF THE BLASTODEBM-THE PBIMITIVE STBEAK AND GROOVE.

Segmentation of the ovum.— Immediately after the completion of the process
of fertilization, the ovum begins to show signs of division into two cells or segments.
The division is preceded by the formation of a spindle-shaped system of achromatic

Fig. 13.

-First stages op segmentation of a mammalian ovum : semi-diagrammatic.
(Drawn by Allen Thomson after E. v. Beneden's description.)

z.p, zona pellucida ; p.gl, polar globules ; a, division into two segments ; ect, larger and clearer
segment • ent, smaller, more granular segment ; h, stage of four segments ; c, eight segments, the ecto-
meres partially enclosing the entomeres ; d. e, succeeding stages of segmentation showing the more rapid
division of the clearer segments and the enclosure of the darker segments by them.

fibres and by changes in the nucleus which are similar to those which take place in
the division of an ordinary cell (v. Histology). According to v. Beneden's observa-
tions in Ascaris these changes occur in each of the two pro-nuclei (fig. 12), and
one-half the number of resulting V-shaped filaments then passes from each to form
each daughter nucleus, which thus contains male and female chromatin elements in
equal amount. Each of the two segments which are thus formed speedily again divides
in the same manner, so that four cells or segments noAV occupy the interior of the ovum.
By a further process of binary division eight cells are formed, then sixteen, thirty-two,
and so on until the originally simple ovum is eventually subdivided into a large
number of small segments, each of which is a nucleated cell, which are aggregated
into a solid spherical mass, not much larger than the original ovum, and known as
the mulberry mass. The cells are not similar throughout, for those at the surface
are clearer and less granular than those which occupy the interior of the mass.
According to v. Beneden's observations in the rabbit and bat, this difference in the
appearance of the cells is traceable even in the first pair of daughter cells, one of
which is larger and clearer than the other, which is darker and more granular (fig.
13, «).! The cells or segments which result from the division of each of these retain
their respective characters, and since the clearer cells divide somewhat more rapidly
than the darker ones, there are for a time at certain stages of the process of segmen-
tation more of the clear cells ; thus, at one stage there are eight clear cells, and only
four darker ones, the latter having not yet undergone division, and later there are

' This statement is denied by Kolliker and other observers, who assert that there is no difference in
the size and appearance of the first segments.

FORMATION OF THE BLASTODERM.

17

sixteen clear cells and only eight darker cells, for the same reason. Further, it is
found that as the segmentation proceeds the clearer cells occupy the superficial part
of the ovum and almost entirely enclose the granular cells which fill the interior

(fig. 1^,«)-

The ovum next undergoes a rapid increase in size owing to the segregation of
fluid between the clear superficial layer of cells and the enclosed granular segments,
which thus become separated from one another except at one part (fig. 14, &). At the
same time the superficial cells multiply, and, becoming flattened out like a pavement
epithelium, form a membrane enclosing the contained fluid. The ovum is now a
thin-walled transparent sac, occupied by fluid and enclosed by two membranes, one

Fig. 14. — Sections of the ovum of the rabbit

DURING THE LATER STAGES OF SEGMENTATION,
SHOWING THE FORMATION OF THE BLASTODER-
MIC VESICLE. (E. V. Beneden. )

a, Section showing the enclosure of darker cells,
ent, by clearer cells, ect ; b, more advanced stage
in which fluid is beginning to accumulate be-
tween the inner and outer cells, the former com-
pletely enclosed ; c, the fluid has much increased,
80 that a large space separates inner from outer
cells except at one part ; d, blastodermic vesicle, its
■wall formed of a layer of flattened cells, with a
patch of dark granular cells adhering to it at one
part ; z.p., zona pellucida.

being the thinned-out zona pellucida and

the other the epithelial membrane just

mentioned. Adherent to one part of the

inner surface of this membrane is the

little mass of dark granular cells which formerly occupied the whole interior of the

mulVjerry mass, and these cells give to the part of the ovum where they occur a

darker appearance, when it is viewed by transmitted light. At this stage of

development the ovum has been termed the blastodermic vesicle (fig. 14, c, d),

although the actual blastoderm is not yet formed.

Formation of the blastoderm. — Soon the granular cells are found to be no
longer accumulated into a small mass but to be spreading out in the form of a
lenticular patch over the inner surface of the vesicle. As this extension jiroceeds
the innermost cells separate off as a distinct layer, the separation starting from
the centre and progressing outwards.

A section through the middle of the ovum now shows three layers (fig. 15) : an
outer, which is the epithelial membrane of the blastodermic vesicle (Rauber's hiyer);
an inner, which may be termed tli(^ priviiliva mlodarm, from the fact that it becomes
the innermost layer of the blastoderm, and an ill-defined middle stratum of somewhat

VOL. I. 0

18

FORMATION OF THE BLASTODERxM.

granular cells, which represents the remainder of the inner granular mass of the
blastodermic vesicle after the separation of the subjacent layer. The three
layers were believed by v. Beneden to represent the three permanent layers of the
blastoderm. But it has been conclusively shown (by Rauber and KoUiker in the
rabbit, and by Lieberkiihn and Heape in the mole) that the middle stratum of this
stage of development is not the permanent middle layer of the blastoderm, for it is

Fig. LS.— Section of part op the blastodermic vesicle op the kabbit at six days. (From

B. van Beneden.)
a, upper layer (Rauber's cells) forming with b, the primitive ectoderm ; c, primitive entoderm.

ect.

Pi^. IG. A SECTION THROUGH PART OF A BILAMINAR BLASTODERM OP THE CAT. (E. A. S. )

ect, primitive ectoderm ; ent, primitive entoderm ; z.p., thinned-out zona pellucida.

before long converted into a layer of columnar cells which becomes closely applied to,
and soon blends with, Rauber's layer, so that the two now form but a single stratum,
which may be denominated the ^nimiiive ectoderm.

Kolliker describes the cells of Rauber's layer as underg-oing a kind of atrophy and gradual
disappearance, taking no part in the formation of the primitive ectoderm. The observa-
tions of Lieberkiihn and Heape, on the other hand, tend to support the view which has been'
given in the text.

Both layers, but especially the primitive ectoderm, are somewhat thickened near
the middle of the ovum over a circular or oval area, which appears slightly darker,
than the rest of the ovum when this is viewed by transmitted light : it is known as the
embri/onic area (fig. 17). The entoderm does not for a long time form a complete invest-
ment to the blastodermic vesicle, for as we have seen it commences to form near the
centre of the ovum, and only gradually grows round within the epithelial investment,
so that it terminates peripherally by a free border. In most mammals which have
been investigated, it has not completely enclosed the ovum when the mesoblast has;
begun to form, but in the cat its growth appears to progress more rapidly, so that,
for a time, the blastodermic vesicle has two complete and distinct epithelial invest-
ments. Whether complete or incomplete, the two layers together constitute what is
known as the lilaminar tlastoderm (fig. 16), the formation of which marks a distinct
stage in the development of all the metazoa.

But the blastoderm does not long remain in the bilaminar condition. In the rabbit
and mole, and probably in most mammals, long before the primitive entoderm has
completely extended itself around the ovum, there occurs a considerable thickening
of the primitive ectoderm at one end — the posterior — of the somewhat oval embryonic
area. This thickening has at first a crescentic form, with the concave edge looking for-
wards, and from the middle of this edge a longitudinal thickening extends for a certain
distance towards the centre of the embryonic area. The thickening is produced by a
proliferation of the cells of the primitive ectoderm, and its consequent downgrowth
towards the primitive entoderm, and it is visible when the ovum is viewed from
above by transmitted Hght, as a streak or shadow which is known as the primitivs

FORMATIOX OF THE BLASTODERM.

19

strealc (figs. 18, 19). Almost as soon as tte primitive streak has become fully formed, it
may be observed to be scored along- its length, except at the anterior end which is that
directed towards the centre of the embryonic area, by a narrow groove — the
primitive groove. The proliferating primitive ectoderm comes into close relationship
below this groove with the primitive entoderm, and the two may be partly or

Fig. 17. — EMBr.YyNic are.v of >iole immediately prior to appearance of primitive streak axd

FORMED OF TWO LAYERS ONLY,

Fig. IS. — Embkyo.mc area of the mole showing the primitive streak and groove ending

POSTERIORLY IN A CRESCENTIC THICKENING.

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

Fig. 19.— A SOMEWHAT LATER STAGE IN WHir'H THE PRIMITIVE STREAK REACHES TWO-THIRDS OP THR
LENGTH OF THE EMBRYONIC AREA, AND ENDS BEHIND IN A KNOB OR THICKENING.

Figs. 17, 18, and 19 are copied from Heape. They are magnified 49 times.

entirely blended, but the union is closest at the anterior end of the primitive streak
where a continuous column of cells unites the primitive ectoderm and entoderm,
so that the two layers are here indistinguishable.

The proliferation of the cells of the primitive streak subsequently proceeds chiefly
at the sides of the primitive groove, and the cells which are produced by this proli-
feration extend themselves laterally between the ectoderm and entoderm to form d

r:^m

Fig. 20. A. and li. — Views op thk embryonic area of the rabbit showing two stages in thu

EXTKXSION OF THK MKSOBLAST. (Kollikor.)

Ill A. the mef(o))Ia»t extends on either f iilc of the primitive streak over the po.stcrior part of the
enihrA-rmic area and also behind the j>rirnitive streak beyond the limits of that area.

In Vj. the roe.-oblaKt extends over a circular area which surrounds tlie eml>ryonic area. The em-
bryonic area i« also trilaminar, except in the middle line in front of the primitive streak.

20

FORMATION OF THE BLASTODERM.

third or intermediate layer. This is mainly derived, as was first pointed out by
Kolliker, from the primitive ectoderm of the groove, but since in this situation the
two primary layers become eventually more or less blended, it is probable that the
primitive entoderm cells also take part in its formation, although this part is in the
mammal evidently a subordinate one. It is further maintained by many embry-

Fig. 21.— Section across the posterior end op the embryonic area of a rabbit at the time
OF the first sign of a primitive streak. (Kolliker.)
ep, epiblast ; ax, its axial part undergoing proliferation (this is shown by the karyokinetic figures,
k) ; me, mesoblasi becoming derived from the proliferating axial epiblast ; hy, hypoblast.

Fig. 22. — LoNarruDiNAL section through the middle line op part op an embrtonio area (mole) in

WHICH the primitive STREAK HAS BEGUN TO FORM. (Heape.)

The blastoderm is perforated in front of the (short) primitive streak (? blastopore, hi])) ; a few meso-
blast cells are seen anterior to the perforation ; ep, epiblast ; hy, hypoblast ; p.sk, primitive streak.

23. — Two SECTIONS ACROSS THE EMBRTONIO AREA OP A BLASTODERM AT THE STAGE SHOWN IN

FIG. 19. (Heape.)

A. Section across the anterior end of the primitive streak and groove.

B. Section across the posterior enlargement of the primitive streak. The epiblast and hypoblast
are seen to be united along the primitive streak, p.sh ; laterally the mesoblast, m., the cells of which
have grown out from the uniting column of axial cells, separates the two primary layers.

■p.gr, primitive groove ; ep, epiblast ; hy, hypoblast ; m, mesoblast.

GASTRULATIOX OF VERTEBRATES.

21

Fig. 24. — Four stages in the development of amphioxus illustrating the formation of the

GASTRULA. (Hatscbek.)

I. Spherical blastodeiin ; 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, hi, and a double wall of epiblast, ep, and hypoblast, hy (or primitive
ectoderm and primitive entoderm).

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.

ologists that cells from the lateral parts of both primary layers are added to the
intermediate layer, and assist in its extension. According to the observations of
Bonnet in the sheep, there is an addition to the middle layer from the peripheral
(thickened) portion of the hypoblast ; this has been long held to be the case with
the blastoderm of the bird, and the cells thus derived (parablastic) have been
considered to have the special function of forming the connective tissues and blood.
Whether, however, this is actually so, most be regarded as at present undecided.
However produced, the appearance of a middle layer causes the originally bilaminar
bla«toderm to be trilaminar, and its three layers have received the names of ectoderm,
mesoderm, and entoderm, or epiblast, mesoblast, and hypoblast.

The K^astrula condition of the vertebrate ovum. — It will bo observed that in the
mammal the two i»rimary lay'jrs of the Ijlastodorm, at lf;a«t their principal part, are formed by
a w;paration into two strata of the cells of the inner ffranular mass which occupies the
interior of the ovum after sefpnentation. The bilaminar condition may therefore Ije .said to
result from a process of dclamination in an originally simple mass or Htratiini. Hut in

22 GASTEULATION OF VERTEBRATES.

Amphioxus among-st Tertebrates,^ and in many invertebrates with holoblastic (alecithal) ova,
the bilaminar blastoderm is produced, not by delamination, but by the invagination of one
pole of an originally simple hollow spherical blastodermic vesicle, the invaginated portion
becoming the primitive entoderm and the remaining part of the wall of the vesicle forming
the primitive ectoderm (fig. 24). This condition, which was discovered by Kowalewsky, is
known as the gastrnla fitage, and it is regarded by most embryologists, following Haeckel, as
typical of the mode of formation of the bilaminar blastoderm throughout the animal kingdom.
The aperture of invagination by which the cavity of the entoderm communicates for a time
with the exterior has been termed the Mastopove (Lankester).

It is not possible in this account of the embryology of the mammal (which must necessarily
be very short) to examine at any length the evidence upon which the opinion rests that a
gastrula stage can be shown to exist at an early stage in the development of the meroblastic
ova of the lower vertebrata. It will be sufficient for the present purpose to state that in
fishes, reptiles, and birds, the ova of all of which are of a markedly meroblastic type, that
part of the ovum in which alone segmentation has occurred, and in which active development
subsequently proceeds, produces a bilaminar blastoderm as in the mammal by the separation ofi^
as a distinct layer of a lower or inner stratum of cells to form the primitive entoderm,
whilst the remaining cells arrange themselves into an upper or outer stratum, the primitive
ectoderm. 2 At one joart of the circular blastoderm which has thus been formed there now
occurs a crescentic thickening of the ectoderm, on the surface of which a pit or depression
becomes formed by an invagination of the ectoderm. This pit extends inwards tmtil it abuts
against a subjacent entodermal thickening, and it may even penetrate the entoderm and
communicate with the cavity below the blastoderm (which afterwards becomes in part con-
verted into the posterior end of the alimentary canal). The invagination in question has been
regarded as a rudimentary blastopore, its time of formation having become shifted to a later
period, and the entoderm having already been formed by delamination altogether independently
of, in place of resulting from, the invagination, as in the typical mode of gastrula formation.

In the mammal a similar invagination of the ectoderm also occurs at the posterior extremity
of the embryonic area, and this invagination has been described by Heape in the mole as
communicating for a time with the cavity of the blastodermic vesicle (fig. 22, II])), which sub-

~ ^^,^_^ Fig. 25.— Surface view of ak embhyonic aiiea of the mole

.-'' ?V IN WHICH THE MEDULLARY GROOVE HAS BEGiUN TO APPEAR

^ ' IN FRONT OF THE PRIMITIVE STREAK. At THE JUNCTION OK

' , THE TWO A SMALL APERTURE IS SEEN : THIS IS THE DORSAL

,' ■■, OPENING OF THE OBLIQUE NEURENTERIC CANAL. (Heaps.)

[ I sequ ently becomes converted in part into the alimentary canal.

i *| In birds and reptiles as well as mammals the invagination in

' % question soon becomes extended forward along the middle line

.J of the blastoderm as a linear groove (primitive groove), Avhich
; indents an ectodermal thickening (primitive streak), and if

/ the posterior invagination represents a blastopore, this

groove must be looked upon as an extension of such blas-
topore, a view which derives support from the fact that
there appears to be a tendency for the primitive groove, at
y least its anterior end. to penetrate to the entoderm, and thus

. _ ' to form here also a canal of communication between the cavity

below the entoderm and the exterior. Such a canal is desig-
nated '• neurenteric," because the anterior end of the primitive streak and groove becomes
eventually enclosed by the neural tube, and the canal then effects a (temporary) communica-
tion between the neural tube and the enteric canal.

Another important point of resemblance between this invagination and the blastopore of
the typical gastrula is the fact that the middle layer of the trilaminar blastoderm begins to
depelope from the margins of the invagination. But in this respect again there is a differ- ^
ence. for whereas in the simplest and most typical forms, such as Sagitta amongst invertebrates,
and Amphioxus amongst vertebrates, the middle layer (mesoblast) originates as a pair of hollow
protrusions of the primitive entoderm (ccelom-invaginations of Hertwig, figs. 28, 29) ; in
mammals and birds it makes its first appearance in the form of solid outgrowths from the
primitive streak.^

Other views concerning- the gastrulation of vertebrates. — Kupffer regards the part

^ Also, according to Hofmann, to some extent in elasmobranch fishes.

2 No layer corresponding with Rauber's layer of the mammal is known to exist in lower vertebrates,
unless that layer is to be regarded as the homologue of the external (corneous) stratum of the epiblast,
v/hich is found at a later stage in fishes and amphibia.

2 Riickert has described an imperfect form of coelom-invagination in elasmobranch fishes, and
Hertwig in amphibia.

HISTOKY OF BLASTODEPaM. 23

wliich has been above alluded to as invaginated ectoderm (primitive groove) as the homologue
of part of the entoderm of more typical forms. If this view be correct, many of the difficulties
in the way of regarding the apertui-e of the invagination as the blastopore, and in explaining
the differences in the mode of origin of the mesoblast are removed ; but, on the other hand,
other difficulties are introduced, and the subject is left by no means clear.

Another view, which was formerly extensively held, regards the blastopore of the meroblastic
vertebrate ovam as bounded by the thickened edge of the bilaminar blastoderm (Haeckel).
According to this view, the cavity of the gastrula is entirely filled up by a mass of
unsegmented or but partially segmented yolk, which also i^rojects for a considerable distance
through the blastopore, forming in fact the great mass of the ovum. The primitive groove
is regarded as a linear prolongation of this thickened edge of the blastoderm towards the
centre of the blastoderm (Balfour), so that the embryo, which developes in front of the
primitive sti-eak. thus comes to have a pseudo-central position in the blastoderm instead
of developing altogether from its margin as in the lower vertebrata and in inverte-
brates. In conformity with this idea, it may be noted that at the thickened rim of the
blastoderm of these meroblastic ova, the two primary layers are continuous with one another
as in the primitive streak. In elasmobranchs an intermediate condition is observed, viz., a
short groove, the margins of which are freely continuous with the margin of the blastoderm.
If, as His and others have described {lidc ■infra), the mesoblast is in part (vascular
and cunnective tissue part) derived from the thickened rim of the blastoderm, this would
furnish another point of resemblance between the primitive streak and that margin.

Ed. V. Beneden has promulgated an entirely different opinion as to the mammalian blasto-
pore from those above described. He regards the condition of the ovum, after the completion
of segmentation and before the formation of a blastodermic vesicle, as representing the
gastrula stage, and looks upon the point where the granular inner mass of cells comes to the
surface between the clear cells which form the outer investment as the blastopore (fig. H, a).
In confoi-mity with this view he considers the layer of clear cells to represent the whole of the
primitive ectoderm, and the granular inner mass the primitive entoderm. But since all the
more recent observations uj^on early mammalian ova agree in affirming the formation of
the three blastodermic layers from the granular inner mass, and that Eauber's layer either
takes no part at all, or only a subordinate part in the formation of the ectoderm of the
embryonic area. v. Beneden's view, in spite of the superficial resemblance of the ovum at
this stage to ceilain gastrula fomis, has not met with general acceptance from embryologists.

Inversion of the blastodermic layers in some mammals. — In the guinea-pig
(Bischoff), rat and mouse (Eraser, Selenka), and in some other rodents, an inversion of the
usual position of the blastodemiic layers is found to occur, the epiblast being innermost,
the hypoblast outermost. The foundation of this inversion is laid early by a process of
invagination and formation of a central cavity in the mass of entomeres, so that when the
blastf,de:-m is difi'erentiated, the innermost cells which are next the (secondary) cavity thus
formed become the epiblast, and the outermo.st the hypoblast, the mesoblast subsequently
fonning between the two by proliferation of epiblast at the primitive groove, as in other
mammals. (For details as to this process of invagination, the student is referred to the papers
by Selenka.)

Historical. — The existence of several lamin£E in the germinal substance of the egg was
first suggested by C. F. Wolff in his celebrated work Tlieoria Gcnerativnis, published in 17.59,
and in his later Memoir On the Development of the Intestine, first published in Nov. Comment.
Acad. Petropol. in 1707 and republished in German by J. F. Meckel in 1812. It is, however, to
the researches of Pander, conducted under the direction of DoUinger of AViirzburg, and pub-
lished in 1817, and those of v. Baer (182G-]8o7), that we owe the first consistent attempt to
connect the development of the several organs and systems of the embryo with the different
con.stituent parts or layers of the blastodei-m. Pander recognised a trilaminar structure of the
blastoderm and distinguished the three layers composing it, in their order from above down-
wards, or from without inwards in the eg'^, as the serous, vascular, and mucous layers.

In \^'>()-:A a further important advance was made in the knowledge of the constitution of
the blastodermic layers, by the discovery by Remak that the greater part of the middle layer
soon after its forma,tion comes to be divided into two laminic, separated by a space which
corresponds to the peiivi.'-ceral cavity {cculom) — a fact which had been partially stated by von
Baer. So marked a division of the middle layer and distinction of the parts which arc
afterwards developed from its two lamina;, has seemed sufficient to some authors to warrant
the recognition of four distinct layers in the blastoderm ; but it will be found on the whole
more convenient to consider the fundamental layers as only tlirce, to which, following the
nomenclature of Foster and Balfour, the designations of cijiblast, mesoblast, and hypoblast are
applied, terms which are synonymous with those of ectoderm, mesoderm, and entoderm,
employed by many authors.

The terms ectoderm and entoderm were first applied t » the two fundamental layers, shown
by Huxley in 1849 to constitute the whole body of ca:lciit crates, and which were correctly
regarded by him as homologous with the two layers of the bilaminar blastoderm, to which w«

24

CHARACTERS OF BLASTODERMIC LAYERS.

have applied the terms primitiTe ectoderm and primitive entoderm. Since the middle layer
is developed from one or both of these primitive layers their permanent representatives are
morphologically different, having- lost the elements which go to form the middle layer, and it
is therefore convenient to accentuate this distinction by the adoption of different terms to
represent the permanent layers.

The generalisation that the formation of a bilaminar blastoderm is typically produced by
the invagination of a hollow spherical unilaminar blastodermic vesicle is due to Haeckel, and
was based largely upon the important researches of Kowalevsky, especially those on Sagitta^
and Amphioxus. The process of delamination which in some animals produces the two
primary layers was originally regarded by Ray Lankester as the typical mode of formation,
but is now generally admitted to be a secondary modification. Finally, it has been shown
(Balfour, Lankester, R. and 0. Hertwig), as is set forth below, that the coelom or body cavity
is typically developed, not by a process of splitting of the mesoblast (although in some
animals this may occur as a secondary modification), but as hollow protrusions from the
primitive alimentary cavity, the cells which bound these protrusions formhig the mesoblast.
Thus from an originally single blastodermic layer by successive processes of invagination
or folding, the three permanent laminEe are ultimately produced.

Such folds may be regarded as formed mechanically by local hypertrophic multiplication
of the cells of the laminee, an increased surface being thus found for the increased number of
cells. In analogous manner the folds which accompany the formation and separation of the
body and the development of the several organs, e.g., the nervous system, alimentary canal,
p,mnion, may also be regarded as resulting mechanically from cell-multiplication. This
mechanical theory of development was first enunciated by Pander, and has of late years been
applied extensively by several embryologists, notably by His {Entwicld. d. Huhnchens, 1868,
find Unserc Korperform, 1874), and Rauber.

Characters of the blastodermic layers. — The three layers of the blastoderm
show from the first distinctive characters (fig. 26). The outer layer, or epiblast, is
epithelial in nature and consists of somewhat irregularly columnar cells closely set side

Fig. 26. — Transverse section through the front end of the primitive streak and
BLASTODERM OF THE CHICK. (From Balfour. )

pr, primitive groove ; m, mesoblast ; ep, epiblast ; Tiy, hypoblast.

by side, forming a single stratum for the most part, except near the middle line, and
becoming thinner and flatter towards the margins of the embryonic area.

The inner layer or hypoblast is also epithelial, but the cells are at first all
flattened, and appear therefore quite thin and linear in sections of the blastoderm.
At a later stage, the hypoblast cells become markedly columnar and enlarged, so
that they considerably exceed the epiblast cells in size.

The middle layer, or mesoblast, which differs, as we have seen, in its mode of
origin, being formed secondarily from one or both of the primary layers, also differs
from them entirely in its appearance and structure. Instead of consisting of cells
closely joined together into a continuous membrane after the manner of an
epithehum, the mesoblast is at first composed of cells which are not thus closely
arranged, but have, on the contrary, a considerable amount of intercellular fluid
between them. They are most irregular in shape, and are often branched and
united with one another, so that much of the mesoblast early resembles an
embryonic connective tissue.

PARABLAST THEORY OF HIS.

25

Before proceeding to describe the commencing development of the embryo it will
be instructive to enumerate the parts which are formed respectively from the three
blastodermic layers. The following is the relation given in tabular form : —

The whole of the nervous sj-stem. including- not only the central organs (brain and
spinal cord), but also the peripheral nerves and sympathetic.

The epithelial structures of the organs of special sense.

The epidermis and its appendages, inchiding the hair and nails.

The epithelium of all the glands opening upon the surface of the skin, including the
i mammary glands, the sweat glands, and the sebaceous glands.

The muscular fibres of the sweat glands.

The epithelium of the mouth (except that covering the tongue and the adjacent
posterior part of the floor of the mouth, which is derived from hypoblast), and that of
g the glands opening into it. The enamel of the teeth.

The epithelium of the nasal passages, of the adjacent upper part of the pharynx,
and of all the cavities and glands opening into the nasal passages.

The urinary and generative organs (except the epithelium of the lu-inary bladder
^ j3 1 and urethra).

^ J I All the voluntary and involuntary muscles of the body (except the muscular fibres
g ■§ / of the sweat glands).

2 « I The whole of the vascular and lymphatic system, including the serous membranes
^ ^ ! and spleen. ^

The skeleton and all the connective tissue structures of the bodv.

© +2

o ^

Tlie epithelium of the alimentary canal from the back of the mouth to the anus,
and that of all the glands which open into this part of the alimentary tube.
The epithelium of the Eustachian tube and tj'mpanum.
The epithelium of the bronchial tubes and air sacs of the lungs.
The epithelium lining the vesicles of the thyroid body.
The ejDithelial nests of the thymus.
The epithelium of the urinary bladder and lu'ethra.

PARABLAST THEORY OP HIS. MESENCHYME THEORY OF HERTWIG.

The observations of His upon the development of the blood and connective tissues
in the bird led him to regard these tissues as originating, not from the mesoblast which
in the chick grows out from the sides of the primitive groove, but from cells which,

laz^

Fig. 27. — Vertical section throlgh tiil blastodfrm of a hen's vaic, taken near the

PERIPHERY. (Strieker. )

E, epibla-st ; //, hypoblast, passing at the periphery into an undiflferentiateil mass of yolk, A,
containing large cells filled witli yolk granules ; M (towards the centre of the blastoderm), mesoblast ;
M 'nearer the periphery), granular cells, apparently derived from A, and lying between the epiblast
and hypoblast.

originating either in the yolk or in the thickened rim of the spreading blastoderm, wander in
centripetally between the primary layers and fill up all the interstices of the centrifugally-
growirig true mesoblast. These in-wandering cells being derived, not like the other cells of
the embryonic area from the more active primarily differentiated central parts of the blasto-
denn. but from the peripheral non-embryonic portion, were collectively named by His
jiuiahldxt. and the tissues (blood and blood-vessels, and all the connective tissues) sujjposed to
be formed from them we-re t<irm(;d jwrtihluntio (all the other tissues of the embryo being
termw], in contra-distinctioii, arrhihlnxlir).

His'rt thcoiy was enunciated as long ago as 1808, although he afterwards introduced
into it certain modifications. For a considerable time it met with little acceptance, but of

26

MESENCHYME THEORY OF HERTWIG.

late years it has obtained, in its modified form as above given, the adherence of many
embryologists, and especially of R. and O. Hertwig, Kupffer, Kollmann, and Waldeyer. R.
and O. Hertwig have given the name of mescncliymc to His's parablast, while retaining the
designation of middle germinal layer or mesoderm for the rest of the mesoblast, from which
it differs (1) in its structure, consisting of loosely arranged Avandering cells, as distinguished
from the epithelium-like lan^e'.I^. of which according to their description the rest of the
mesoderm is composed ; (2) in its derivation, arising as separate cells from the entoderm instead
of in the form of a coherent layer ; and (3) in its further development and destination, giving
origin to the connective tissues and blood-vessels, and perhaps to the plain muscular tissue,
whereas the mesoderm proper gives origin to the skeletal muscles and to the epithelium of the
serous cavities, and of the genital and urinary organs. They describe the true mesoderm as
consisting of two epithelial lamellEe, which form distinct layers of the blastodenn, so that
according to this view the complete blastoderm Avould consist of fLmr layers (epiblast, outer
or somatic mesoblast, inner or splanchnic mesoblast and hypoblast) besides the mesenchyme ;
to which must be added a median strand of cells set aside for the formation of the notochord.
It appears evident from the researches of Kowalevsky in Sagitta and Amphiosus that what
is to be regarded as the typical origin of the mesoblast in Metazoa' takes the form of a pair of
diverticula from the primitive archenteric cavity (fig. 28, la ; fig. 29, I), which hollow diverti-
cula become pinched off from the remainder of that cavity, and. their cavities becoming com-
pressed laterally, are converted into the coslom or body-cavity (serous cavities of vertebrata), the
two walls of this cavity on either side forming respectively the inner and outer mesodermic
layers of R. and 0. Hertwig. In Sagitta, the diverticula occur in the neighbonrhocd of the

Fig. 28. -Formation of mesoblastic somitks m Ajipnioxus, shown in lokgitddinal optical

SECTION. (HatscLek.)

of th;' hvTohl i'lv T '"'^■^° '"''■ ""^^'^ ''}^. ^^«°l^^^st is beginning to form as two longitiulinal folds
soJites ^I S .Tif "'' becoming subchvued from before back by constrictions into separate
somtes. _ I. the same viewed m profile, showing the anterior three somites of one side with their
cavities in free communication with the enteric cavity. The neural canal, « c is continued
posteriorly by a neurenteric canal into the enteric cavity; cj,, epiblast; hy, h^pobllst II c orsa
IJfUAbeTo'st aXio '"'^"'°;, ^""^ ""?*"• ^" moi/nnnfeiJs and 'are'coinpleSety ^epaia'tf In
view a^astm late, St r ^'t^ ' !^°"^^^™'°at^o'i ^^ith the enteric cavity is still seen. Ill, dorsal

run^giitg s*^ miSs of^h: ':^:^z:zr^ ''''-'■ ''- -''^'- -'> -'' ^^°-

SffZ'ed'^v Wif ? '^i%*^f '"I !f * °^ ^^-^S-in of the mesoblast, but in Amphioxus they
befoS l^Sw^i ^ 1 ^^^'^^ °^ *^' '^'^^^ °^ ^^" archenteric cavity, which grow from

aW AmpM^^^^^^ the Z:T^ "T'^'f ""'' 'f ' ^^^'^^^^^ ^^ *^^^ P^^^'^^ I" --^ vertebrates
S i4v eaSv nur^r t'l?*?'"'''*^' ^"' ^™^^ *^*^ ^'^ ^°"^' "^^^ ^^^^^ (although a

<lnirl^7 early and does eventually m any case, occur in them, the coelom bein- thus pro-
duced), nor do they originate so distinctly from the entoderm, but arise rather at the juXn

1 Except the Coslenterata which have only the two primary layers.

RECENT LITEEATUEE OF BLASTODERM.

■n

of this with ectoderm at the margin of the blastopore, and in the hig-her forms, especially
mammals, may even be larg'ely derived from ectoderm. It is nevertheless for many reasons
probable that the origin in a p.iir of hollow diverticula, as above described, is to be looked
upon as the typical one. and that as a solid ovitgrowth, subsequently becoming split or hollow,
as a secondary modification.' It is questionable, however, whether there is so considerable a
difEerence between the external and internal portions of the wall of the diverticulum that

Fig. 29. — Sections across an amphioxds embryo of about the stages snuwx in fio. 28, I. to

111. (Hatschek.)

n.g., neural groove ; n.r., neural canal ; ch, rudiment of notocliovd : mes. som., raesoblastic somite.
In I., its cavity is in free communication with the alimentary cavity ; q:), epiblast ; /(,'/, hypoblast ; al,
alimentary cavity. In III. the cavity of the somite has extended on either side of the aUmentary
canal and forms a ccelom, or body cavity (ca).

these two plates of mesoblast should be regarded each one as of equal morphological importance
with the epi- and hypoblast.

It may also be doubted whether the parablastic cr mesenchyme elements are essentially
different in their origin from the rest of the mesoblast.- In forms which are re-i'arded as
most typical, such as Sagitta and Amphioxus. they are not distinct in origin from that layer.
In the simpler forms amongst the Craniata, as Cyclo.stomata and Amphibia, no origin
distinct from the rest of the mesoldast has been described for these elements, nor has it been
seen in mammals, in which, indeed, it is difficult to conceive an independent source for them.
It is only in the highly modified meroblastic ova that appearances have been noted which have
seemed to justify the ascribing a peripheral origin to the parablastic elements. But the evidence
which has been hitherto adduced in favour of this view cannot be regarded as suilicieut to
justify its unconditional adoption, and it must be regarded as equally open to consideration
whether the derivation from that part of the blastoderm which is most closely connected
with the source of nutriment, viz., the yolk, of tliose elements which are to form the blood
and blood-vessels, and otherwise to minister to the nutrition of the early embryo is not to bo
explained by the modified physiological conditions of these telolecithal ova.

RECENT LITERATURE.

Balfour, F. M., On the structure and hotaolojlcs of the fjcrmbml layers of the anbri/o. Quart.
Journ. Microsc. Sc, vol. xx., 1880. . , , /• w ;• ?

Balfotir, F. M. and F. Deighton, A rcmwed study of the (jcrimnal kujers of tlic cluck.
Quart. Journ. Micro.sc. Scienc, vol. .\xii., 1882. i ^ -ii ,

Beneden, Ed. van, Ilechcrchcs sur Vcmhryoloyic des vmmmifcres. La forvuttiondes feu diets
clu'z h. bnnn. Ardiiv. de biologie, t. i., 1880 ; .S'«/- Evolution, de la lijne primUaw, la frrmatwn
de la votorcnie et da canal cordal chez Ics viamm, feres (lapui et murin). Uullctin de 1 academic
rovalo do Delgiqiie. Ann. v., sdr. iii., t. xii., 188G. , .r- / /• . ,* /„

" Beneden, Ed. van und Ch. Julin, Observations sur la maturation, la JecvuUatMn a la
sfOineiiOUioH de I'w.uf chez les cheirojAeres. Arch, do bid., t. i., 1880.

Bonnet, K., Ueher den Primitivslrei/en und die Chorda der Wiedcrlcauer. Sitzungsher. d.
(ie«dl«chaft f. Morphologie u. PhyBiologie zu Munchen, 1886 ; JJeitrayc zur Emhryoloyie der M udcrkauer.
Arch, fur Anal., 1884, 1889.

' R. and O. Ilcrtwig, "Die Cri;lomtheorie," Jena, 1881.

2 (Jf. IJalfour, "Comparative Embryology," vol. ii. i>i). 290, 297.

28 RECENT LITERATURE OF BLASTODERM.

Boveri, T., Ueber Differenzirung der Zellkerne wdhrend der Furehung des Eies von Ascaris
megalocephala. Anat. Anzeig., 1887.

Butschli, O., Beitrdge zur Gastridatheorie. Morpholog. Jabrbucli, Bd.ix.-, H._ 3, 1884.

Caldwell, W. H., The embryology vf Monotremata and Marsupialia. Philosophical Trans.,
vol. clxxviii., 1888.

Durliain, H., Note on the presence of a neurenteric canal in Eana. Quarterly Journal of
Microscopical Science, N. Ser., vol. 26, 1886.

Duval, M., De la formation du blastoderme dans Vceaf d'oiseau. Annales des sciences
naturelles, 6 ser. t. xviii., No. 1—3, 1884.

Fleisclimaiin, A., Zur EntwicTcelungsgeschichte der RauUhiere. Biolog. Centralbl., Bd. vii.,

1887.

Gasser, Ber ParaUast u. der Keimwall der Vogelkeimscheibe. Sitzungsber. d. naturw. Gesellsch.

zu Marburg, 1883.

Gerlach, L., Ueber die entodermale Entstehung der Chorda. Biol. Centralbl., 1881.

Giacomini, C, Sul canale neurenterico et siol canale anale nelle vesicole blastodermiche di
coniglio. Giorn. della r. accademia di medic, di Torino, No. 4, 5, 1888.

Haddon, A., Note on the blastodermic vesicle, of mammals. Proceedings of the Royal Dublin
Society, N. Ser., vol. iv., 1885.

Haeckel, Bie Gastrceatheorie. Jena Zeitschrift, Bd. viii. ; Nachtrdge zur Gastrceatheorie.
Ibid., Bd. xi. ; Ursprung u. Entwickl. d. thierischen Getoeben. Ibid., Bd. xi.

Hatschek, B., Studien uber EntwicJclung des Amphioxus. Arbeiten a. d. zool. Instit. zu Wien,
Bd, iv. , 1881 ; Ueber die EntwicJclung des Amphioxus. Biolog. Centralbl., Bd. vi., 1887.

Heape, W., The development of the mole {Talpa europcea). The formation of the germinal
layers, a7id early development of the medidlary groove and notochord. Quart. Journ. of Microsc.
Sc, N. S., xci., 1883. Also in Studies from the Morphological Laboratory in the University of
Cambridge, vol. iii.

Hensen, Ueber die Ableitung der Umkehr der Keimbldtter des Meerschweinchevs. Verhand-
liingen des physiologischen Vereins in Kiel, 1881.

Hertwig-, O., Bie hntwiehlung des mittleren Keimblattes der Wirbelthiere. Jena Zeitschrift fiir
Naturw., Bd. xvi., 1882.

Hertwig, O, und R., Bie Colomtheorie. Versuch einer Erkldrung des mittleren Keimblattes.
Jena. 1881 ; Studien zur Bldttevtheorie. Jena, 1883.

His, "W., Ber Kei^nwall des Bilhnereies u. d. Entstehung der parablastischen Zellen. Zeitsch. f.
Anat. u. Physiol., Anat. Abth., 1876 ; Bie Behre vom Bindesubstanzkeim (Parablast), Riickblick nebst
critischer Besprechung einiger neuerer entwicklungsgeschichtlicher Arbeiten. Archiv fiir Anat. u.
Physiol., Anatom. Abtheil., 1882.

Hoffmann, C, K., Bie Bildung des Mesoderms, die Anlage der Chorda dorsalis und die
Entioickelung des Canalis neurentericus bei Vogelembryonen. Amsterdam, 1883.

Hubrecht, A. A. W. , Bie erste Anlage des Hypoblastes bei den Sdugethieren. Eine Erwiderung
an Herrn Prof. Ed. van Beneden. Anatomischer Anzeiger., iii. Jahrg., No. 30, 1888.

Johnson, Alice, On the fate of the blastoptore and the presence of a priiidtive streak in the
Newt [Triton cristatus). Quaiii. Journ. of Microsc. Science, N. S., No. xcvi. , 1884.

Keibel, F., Van Beneden' s Blastoporus und die Rauber'sche Beckschicht. Anatomisch, Anz.,
1887 ; Bie Entwickelungsvorgdnge am hinteren Ende des Meerschweinchenembryos. Arch. f. Anat.
u. Physiol., Anat. Abth., H. 5 u. 6, 1888.

Koller, C, Untersuchungen Uber die Bldtterbildung im HUhnerkeim. Arch. f. mikr. Anat.,
1881 ; Beitrdrie zur Kenntniss des Huhnerkeims im Beginne der Bebrutung. Wiener Sitzungsber.,
Bd. Ixxx., 1881.

Kolliker, Bie Entwicklung der Keimbldtter des Kaninchens. Wiirzburg Festschrift, Leipzig,
1882; Bie embryonalen Keimbldtter u. d. Geivebe. Zeitschr. f. wiss. Zool. xl., 1884; Ueber die
Nichtexistenz eines embryonalen Bindegewebskeims [Parablasts). Sitzungsber. d. phys. medic. Ges.
zu Wiirzburg, 1884,

Kollmann, J., Ber Mesoblast und die Entwickelung der Gewehe bei Wirbelthieren. Biolog.
Centralblatt, Bd. iii., 1 884 ; Ber Randwulst u. d. Ursprung d. StUtzsubstanz. Arch. f. Anat. u.
Physiol., Anat. Abth. 1884.

Kowalevsky, Entwicklung sgeschichte der Sagitta. St. Petersburg Memoirs, xvi., 1871 ;
Entwicklung sgesch. d. Amphioxus, <fcc. Arch. f. mikr. Anat., Bd. xiii., 1877; Ueber die ersten
Entwickelungsiorocesse der Knochenfische. Zeitschr. f. wissenschaft. Zoologie, Bd. xliii. , H. 3, 1886.

Kupffer, C, Bas Ei von Arvicola arvalis und die vermeintliche Umkehr der Keimbldtter an
demselben. Miinchener Sitzungsberichte, H. 5, 1882 ; Bie Gastrulation an den meroblastischen
Eiern der Wirbelthiere und die Bedeutung des Primitivstreifs. Archiv f. Anat. und Physiol., Anat.
Abtheil., 1882 ; Ve. d. Canalis neurentericus der Wirbelthiere. Sitzungsberichte der Gesellsch. f.
Morphol. u. Physiologie za Miinchen. 1887.

Lankester, On the primitive cell-layers of the embryo, &c. Annals and Mag. of Nat. Hist., xi.,
1873 ; Notes on embryology and classification. Quarterly Journ. of Micr. Science, xvii., 1877.

Mitsuktiri, K., and Isliikawa, C. On the formation of the germinal layers in Chelonia.
Journal of the College of Science, Imperial University, Japan, vol. i., 1888.

Morgan, Notes on the fate of the amphibian blastopore, John Hopkins University Circulars, 1889.

Perenyi, I., Bie Entwickl. der Keimbldtter u. d. Chorda in neuer Beleuchtung, Anat. Anzeiger, 4.

Piatt, Studies on the primitive axial segmentation of the chick, Bulletin of the Zool. Museum at
Harvard College, 1889.

Ratal, C, Ueber die Bildung des Mesoderms. Anatomischer Anzeiger, iii. Jahrg., No. 23 — 25, 1888.

EECENT LITERATURE OF BLASTODERM. 29

Rauber, Ueher d. Ursprung des Blutes u. der Bindemhstanzen. Sitzungsb. d. Natuif. Gesellsch
zu Leipzig 1877 ; Die Entwickelung der Gewehe des Sdugethierkorpers und die histolo'iischen Svstemc
Ber. der Naturf. Ges. zu Leipzig, 1883.

Ravn, E., Ueber die mesodermfreie Stelle in der Keimscheibe des Huhncrembryo Archiv f
Anatomie u. Physiologie, Anatom. Abth., 1886.

Bepiachoflf, W., Bemerkungen iiher die Keimbldtter der Wirbelthiere. Zool. Anzeiger, vi., 1883.

_ Komiti, G., Sur Vori/jine du niesoderme et ses rapports avec le vitellus. Arch, ital de biolosie"

1. 11., 1882. ^ '

BoTix, W., Beitrdge zur Entwickehingsmechanik des Embryo. Ueber die hunstUche Hervor-
bnngung halber Embryonen durch eine der beiden ersten Furchungskugeln, dec. Virchow's Archiv
Bd. cxiv., 1888. '

Rtickert, Ueber die Gastrulation der Selachier. Biologisches Centralblatt, Bd. vi.

Ryder, The inversion of the blastodermic layers in Hesperomys. ' American Naturalist
Tol. xxi. '

Schanz, F., Ba-s Schicksal des Blastoporus bei den Amphibicn. Jenaische Zeitschr f Natur-
wissensch., Bd. xiv., 1887.

Selenka. Emil, Keimhlatter und Primitivorgane der Maus. Wiesbaden, 1883 ; Di" Blatterum-
kehrung im Ei der Nagethiere. Wiesbaden, 1884 ; Ueber die Gastrulation der Knocheniische und
Amnioten. Biolog. Centralbl., 1887.

Shipley, A. E., On the formation of the mesoblast, and the persistence of the blastopore in the
lamprey. Proceedings of the Royal Society, 1886 ; On some points in the Development of Petromvzon
fluviatihs. Quarterly Journal of Micros. Science, No. 107, 1887.

Sol&er, Studien z. Entwicklungsgesch. des Coeloms u. des Ceelomepithds der Amphibien Moroh
Jahrb., x. x- f •

Spee, F., Beitrag zur Entwickelungsgeschichte der fruheren Stadien des Meerschweinchens bis zur
yolkndung der Keimblase. Arch. fUr Anat. u. Physiol., Anat. Abth., Heft. i. u. ii., 1883.

Spencer, On the fate of the blastopore in Rana temporaria. Zool. Anzeiger, 1885.

Uskow, N., Die Blutgefdsskeime u. deren Entwickelung bei einem Muhnerembrvo M^m dp
I'acad. de St. Petersb., xxxv., 1888.

Waldeyer, "W., Archiblast und Parablast. Arch. f. mikr. Anat. 1883 ; Die neueren Forschunaen
vm gebiet der Keimblaitlehre. Berliner klinische Wochenschrift, No. 17, 1885.

WolflF, W., Die beiden Keimbldtter und der Mittelkevm. Archiv f. mikroscopische Anatomie
Bd. xiviii., H. 4, 1886. '

Zumstein, J. J., Ueber das mesoderm der Vogelkeimscheibe, Dissert., Bern, 1887.

;3U

EARLY CHANGES IN THE BLASTODEEM.

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

FIRST APPEARANCE OF THE EMBRYO ; FORMATION OF THE NEURAL GROOVE
AND MESOBLASTIC SOMITES.

Neural canal. — The blastoderm in mammals, as we have seen, eventually
completely encloses the cavity of the ovum, and even in the large telolecithal ovum
of the bird, it gradually extends so as to cover a large part of the yolk. But only
a small portion of this membrane takes part in the formation of the body of the
embryo, that portion namely which lies immediately in front of the primitive groove,
and it is here that what may be regarded as the first trace of the embryo makes its
appearance — very soon after the outgrowth of mesoblast from the sides of that

Fig. 30. — Embryonic area, -with outline of part of the vascular area, froji a kabbit's
OVUM OF SEVEN DAYS. '{. (From Kolliker.)

00, vascular area ; ag, embryonic area ; j?r, primitive streak and groove ; rf, medullary groove,

groove— in the form of a shallow furrow (fig. 30, rf), wide behind Avhere it
embraces the anterior end. of the primitive streak, and at first narrow in front, and
bounded on either side and anteriorly by a fold of epiblast ; which folds, in fact,

F]g. 31.— Section across the anterior part op the medullary groove op an early embryo

OF THE GUINEA PIG. (E. A. S.)

cp, folds of epiblast rising up on either side of the middle line, and thus bounding the medullary
groove ; mr/, middle of medullary groove ; hj/, hypoblast, which is in contact with the medullary
epiblast at the middle of the groove, but is elsewhere separated from it by mesoblast, me which has
burrowed forwards between the two primary layers. A cleft is seen in the mesoblast on either side ;
this IS the commencement of the anterior part of the coelom.

I

FOKMATION OF KEUEAL CANAL.

:3I

Ct7?Z

Fig, 32. — Skction.s siiowixfj rt.voks in thk convkrsio.v op the medum-a

NEUKAL CANAI,. FkoM THE TAIL KNI) OF AN EMBHYO OK Till! CAT,

Uy GROOVE

(R A. ».)

/^Z), »»/;, /ty, cpiblast, ine«o)*I,'ist, and liy|)ol)lrist ; vi.r/., medullary groove; n.c. (in IV.), neural
canal ; c/i, notochord ; coi, c<x;lou» ; am, tail fold of the arnuion.

32

THE NOTOCHORD.

produce the groove which is enclosed between them. The groove Avhich is thus
early formed in front of, but not, as was formerly supposed, in continuity with the
primitive groove (Dursy), is no less than the rudiment of the whole central nervous
system, and it is accordingly known as the neural or medullary groove, the folds which
bound it being termed the medullary folds. By the time that the neural groove is
formed, the mesoblast has generally extended forwards from either side of the
primitive streak, burrowing between the epi- and hypoblast, and as the folds become
developed, this mesoblast fills up the space below the epiblast, triangular in section,
which each fold encloses, so that on either side of the neural groove there is now a
longitudinal thickening of mesoblast, entirely separated from its fellow of the
opposite side by the meeting of epi- and hypoblast at the bottom of the neural
groove (fig. 31), and gradually thinning off laterally into what is known as the

nc^.

Fig. 33. — Middle of the section shown in fig. 31,

MAGNIFIED TO SHOW THE lETAILS OP ITS STKUGTURE.

(E. A. S.)

ep, me, hy, m.g., as above ; nch, notocliordal thickening
of median hypoblast.

lateral plate of mesoblast. These two longi-
tudinal thickenings of mesoblast give origin to
most of the muscular and skeletal tissues of the
body ; they form what may be termed the
paraxial as distinguished from the lateral meso-
blast. Somewhat later the medullary folds
become bent over the neural groove, and meet one another in the middle line
(fig. 32). Here they blend together, and the groove becomes converted into a
canal — the neural canal. Of the two layers of epiblast which are formed from
the folds, one is now the roof of the canal, the other is the epiblast of the dorsal
surface of the embryo. The layers are at first in contact with one another, but
subsequently mesoblast passes between them (forming the memlrana reuniens
superior of Eemak). The closure of the neural groove begins in the posterior
cephalic region, and thence extends forwards and backwards.

Notochord. — Eunning along the bottom of the neural groove there may soon
be seen, when the blastoderm is viewed fi.'om above, a linear shading, which appears
to start from the anterior end of the primitive streak and passing forwards becomes
gradually lost towards the anterior end of the neural groove. Transverse sections
across the latter show that this appearance of shading is due to a longitudinal
thickening of the hypoblast along the middle line (fig. 33) ; the central cells of
this layer becoming enlarged and gradually separating themselves oflF to form a rod-
like column, which lies between epi- and hypoblast just below the neural groove
(fig. 32, ch). When so separated, the column is known as the choi'da dorsalis, or
notochord, a structure which, along the middle line of the early embryo, replaces the
mesoblast, and which is at first, as before said, continuous with the united epi- and
hypoblast at the anterior end of the primitive streak. The actual separation of the
notochordal cells from the hypoblast occurs first a little behind the anterior end of
neural groove, and progresses backwards, although the hypoblastic thickening occurs
first at the posterior end of the neural groove and is in fact directly continuous with
the united column of epiblast and hypoblast which forms the anterior end of the
primitive streak. The neurenteric canal passes through the thickened posterior
extremity of the notochord, where this is continuous with the anterior end of the
primitive streak (see fig. 34). It is continued in mammals a short distance along the
notochord as a canal (prolonged forwards into a groove) (fig. 34, 5), which has been

THE NOTOCHOED.

38

III.

Pig. .34.— A 8KIUE.S OF TRANSVERSE SECTIONS THItOUOn THE NEURENTKRIO AND NOTOOHORDAIi CANAL

OK A MOLE EMBRYO. {IIcai)e.)

The embryo was nlightly more advanced than the one reiirosentcd in Fig. 2.'>. The dorsal opening
iKHhown in I., continued into the itriniitivc groove; the canal iiasscs thence through the column of
ceilH which unites the epihiaist ati<l liy|.ol)la.st at tiie front of the primitive streak (11.), into tiie notn-
chordal tliickeniuK of the hyp..hla.st (iU. ), along which it exteud.s for some diHtunce (IV.), and
ev.;ntually opens ventrally (V.) into a median groove, which is formed in the notochordal thiclaiung
(n.ch.c.) .

ep, vie, hy, epihlaBt, mcaoblast, liypohlawt ; jup: (in 1. and 11.), primitive groove ; c, ueureuteno
or noto<;hordal canal ; m.(jr. (in III., IV., and V. ), medidlary groove.

VOL. I. ■'*

34 SEPARATION OF THE EMBRYO.

described under the name of the notochordal canal, and corresponds with the hypo-
blastic invagination, which in Amphioxus (fig. 29, 1., cA) also forms the first stage in
the development of the notochord.

A flattening out and even eventually a duplication of this canal occurs in the chick and in
various mammals at a somewhat later stage than that given in fig. 3i. According to Spee,
its cleft-like cavity may pass laterally into the commencing mesoblastic cleavage (coelom-
invagination ?).

The notochord is essentially an embryonic structure in mammals, although it does
not completely disappear, for traces of it are to be found throughout life in the
centre of the intervertebral discs. When fully developed it is a cylindrical rod
composed of clear epithelium-hke cells, enclosed within a special sheath of homo-
geneous substance. These cells, although they may become considerably enlarged
and vacuolated, undergo no marked histogenetic change and take no part in the
formation of any tissue or organ of the adult.

Separation of the embryo from the blastoderm.— The embryonic rudiment
which thus first makes its appearance in the blastoderm, and which consists

cvtru

coeloin;'

Fig. 35. — Mesial sagittal section through the anterior end of an early sheep embryo

SHOWING the commencing FORMATION OF THE FORE-GUT. (Bomiet. )

ep, epiWast ; Jiy, hypoblast ; /.;/., foregut, formed by folding over of layers ; am, amnion (head fold).
Below the foregut the cephalic coelom is becoming formed as clefts in the mesoblast.

essentially of neural groove, mesoblastic thickenings, and notochord, becomes
at a very early period marked ofP in front by a dipping down of the blastodermic
layers immediately in front of the anterior end of the neural groove, so as to form a
transverse curvilinear sulcus — the anterior limiting sulcus. This is at first wide
and shallow, but soon deepens and narrows, and takes at the same time an oblique
direction, curving downwards and backwards under the front end of the neural
groove. The sulcus is really due to a growth forwards of the anterior end of the
embryonic rudiment, over the part of the blastoderm immediately in front of it, so
that this anterior end now projects as a distinct head (fig. 35).

All the three layers are involved in the forward growth and overfolding which
produces the head, so that a prolongation from the blastodermic cavity, which is of
course lined by hypoblast, becomes included in the head, and the anterior part of
the primitive alimentary canal, ot fore-gut (f.g.) is thereby produced. Formed in this
way its front end is necessarily blind, and for a long while there is no mouth nor any
communication between the fore-gat and the exterior of the embryo. The mouth
becomes formed later by invagination from the exterior.

In the rabbit, and also in the chick, the blastoderm at this time is still bilaminar in and
near the middle line in front of the embryo, for the growth of the mesoblast has not yet

FORMATION OF PRO-AMNION.

35

extended to this part. The head end of the embryo grows forward over thi« bilaminar portion,
and since the embrjo, as it becomes differentiated, tends to sink below the general sui-face of
the blastoderm, the head which now overlies the bilaminar part produces a depression of this
part towards the interior of the vesicle, so that the head of the embryo becomes enclosed by
the bilaminar wall of the depression (fig. 3(5). The enclosing membrane, which is well marked
in the rabbit, has been termed by v. Beneden the prn-aiii/iio)! : by an extension of mesoblast and
of the mesoblastic cleavage between its layers, it afterwards becomes split into somatopleure
and splanchnopleure, and the former becomes continuous with the true amnion (see p. 42).
The stage of pro-amnion. if it exists at all, must disappear very early in the human embryo.

Soon after the appearance of the anterior limiting snlcus, two lateral limiting
sulci are seen running external and parallel to the medullary folds ; these lateral
sulci, as they dip down, mark off the body of the embryo from the rest of the

Fig. 36. — DiAORAJIMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF THE RABBIT. ThB
SECTIONS snow THE MANNER IN WHICH THE PRO-AMNION IS FORMED liY 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 EPIBLAST AND HYPOBLAST ONLY. TlIE DIAGRAMS ALSO ILLUSTRATE
THE MODE OF FORMATION OF THE ALLANTOIS AND OP THE TAILFOLD OF THE AMNION IN THIS

ANIMAL, (v. Beneden and Julin.)

ep, eijiblast ; /(//, hypoljla-st ; me, mesoblast; coe, parts of the ca-lom ; cm', pericardial ccelom, the
heart not being represented ; pr.a., pro-amnion ; pi, seat of formation of tiie placenta ; all, allantois ;
am, amnion.

blastoderm, but they do not for some time progress far in development, the middle part
of the future alimentary tract long remaining in free continuity with the cavity of the
b]a.stodermic vesicle (fig. 15, and fig. 41), d), but becoming gradually more pinc^hed off
from it. That part of the cavity of the original blastodermic vesicle which does not
form a part of the alimentary canal, but remains connected with it by a wide neck
of communication, is known as the yollc-sar. At a later stage, when the body walls
are formed, and the yolk-sac, relatively greatly diminished in size, lies aloogether
ontHide the Iwdy of the foetus, it is merely connected by a long narrow duct, which runs

i» 2

36

CLEAVAGE OF THE MESOBLAST.

along the umbilical cord, with the intestine. This is termed the umbilical duct, and the
yolk-sac itself has received (in mammals) the name oiumUlical vesicle (figs. 50, 51).
Lastly, at the tail end of the embryo a Jdnd-gut is produced. In the human
embryo this appears to be formed by a protrusion from the posterior blind end of the
enteric groove, and after the formation of the allantoic tube, but in most mammals

Fig. 37. — Transverse section op the tail end of an embryo chick of the latter half of

THE SECOND DAT, AT THE PLACE WHERE THK VERTEBRAL SOMITES CEASE. ^f. (From Kollikcr. )

m.f., medullary folds, the neural canal beginning to close ; i).in., paraxial mesoblast ; Z.m., lateral
mesoblast ; ep. , epiblast ; hy, hypoblast ; ao, primitive aorta ; ch, notochord ; ^, ccelomic cleavage of
lateral plate of mesoblast.

and in birds it is produced by a folding over of the tail end of the embryo like that
which occurs at the head end to enclose the fore-gut (figs. 45 to 47). The hind-gut

Fig. 38. — Embryo chick at the end of the second day, seen from
BELOW, 'j^. (From KoUiker. )

Vh, forebrain ; Ab, primary ocular vesicles ; Ch, notocliord ; H, tubular
iieart ; om, vitelline veins ; Vd, entrance to the forepart of the alimentary
canal within the cephalic fold ; in the middle part of the embryo, the
proto vertebral somites are seen (to the number of thirteen pairs) on each side
of the canal of the spinal marrow and notochord.

"^ remains for a considerable time blind, until the anus becomes
formed by invagination from the exterior.

Cleavage of mesoblast. Formation of "body cavity. —

At a very early period, soon indeed after the formation of the
neural groove, two important changes begin in the mesoblast.
One of these is the cleavage of the lateral mesoblast (which is at
first a continuous sheet) into two plates, one of which clings to
the epiblast, and the other to the hypoblast. The cleft is at first
small (fig. 37, Tp), but accumulation of fluid within it soon con-
verts it into a cavity, which gradually spreads until the separation
is very extensive (fig. 39, 'p.])). The layer of mesoblast which
clings to the epiblast eventually forms part of the body-wall,
and is known as the somatopleure ; that which clings to the
hypoblast forms eventually part of the wall of the alimentary
tract, and is known as the splanchnopleure. The cavity between these, which is
formed by enlargement of the original cleft, is the coelom or lody cavity (pleuro-
peritoneal cavity of authors).

Formation of mesoblastic somites.— The other change occurs not in the
lateral but in the paraxial mesoblast, and consists in the occurrence at regular
intervals transversely along the mass, of a process of thinning which produces its
complete separation into distinct segments, so that when the embryo is viewed from
a,bove or below, these segments appear on either side of the neural groove as a
linear series of small quadrangular masses (fig. 38), which were originally termed

'W^,-?'""

FORMATION OF MESOBLASTIC SOMITES.

3?

proiovortcJmc, on the supposition (now known to be ciToneons) tliat they are
the rudiments of the futnre vertebvtB ; they are more appropriately termed the
mesohlastic somites.

M.e.

Fig. 39. — Transverse section through the dorsal region of an emisryo chick of 45 hours.

(From Balfour.)

J, epiblast ; C, hypoblast ; Mc, medullary canal ; Pv, protovertebra or mesoblastic .somite ; Wd,
intermediate cell-mass in which the Wolffian duct is becoming formed ; tio, somatopleure ; <%),
splanchnopleure ; jop, pleuro-peritoneal cavity (coelom) ; op, inner margin of area opaca ; w. thickened
hypoblast of area opaca ; ao, left xjrimitive aorta ; v, blood vessels ; ch, notochord.

In Amphioxus (fig-s. 28, 29), the protovertebrse are formed in common with the body cavity, and
are successively separated off from before backwards from the coelomic fold as hollow cuboid
somites, each of which extends ujjwards around the neural canal and downwards alouji' the
.sides of the alimentary canal, and sub.sequently divides into a dorsal or paraxial part which
forms the protovertebras, and a ventral part which forms the lateral mesoblast. At first the
hollow somites communicate individually with the alimentary cavity, but they become shut
off from this long before the division which has just been mentioned. The ventral segments
run together eventually, to form a continuous serous cavity. In sections of bird or mammalian
embryos (fig. 89), the protovertebraj, althoufjfh on the whole compact masses of meso-
blast, yet often show a tendency to have their central cells loosely arranged, so as to give the
appearance of an irregular cleft in their interior, and sometimes a definite cavity is formed in
them, which may even be continuous with the coelomic cleft in the lateral mesoblast.

Protovertebrfe begin to be marked off in the paraxial mesoblast, a short distance
from the anterior end of the neural groove, in what will eventually become the
cervical region of the embryo. Tliey are produced in succession from before back-
wards, one or two only being at first visible on either side, and others being gradually
added as the embryo grows in length, until a large number may at length be counted,
extending from innnediately behind the cephalic region to the region of the ])rimitive
streak.

Cerebral vesicles. — Meanwhile a change of importance has taken place in
connecti<m with the anterior end of tlie neural groove, which has l)ecome enlarged,
and wjon exhibits a succession of highly characteristic median dilatations, separated
from one another by slight constrictions (fig. 34). These dilatations, at a later
stage, after they have become roofed in, along with the rest of the n(!ural groove,
are known as the cerebral, vehicles. There, is at first a simple enlargement, and
behind this two others form in succession, so that the prmiarij veaiclrn are t liree in

38

CEREBRAL VESICLES.

number, but subsequently the most anterior {fore-lrain) and posterior {/mid-brain)
each forms two vesicles, whereas the middle vesicle {mid-'brain) remains permanently
undivided. Five vesicles are therefore then to be seen, and these give rise eventually

Fig. 40. — Surface tiew op an early embryo of the guinea pig showing the commencement of

THE three primary CEREBRAL VESICLES (1, 2, 3) AS ENLARGEMENTS OF THE MEDULLARY GROOYE.

Semi-diagrammatic.

pr, primitive streak and gi'oove.

Fig. 41.— Rabbit embryo of the 9th day, from the surface. -J. (Kolliker.)

The medullary groove is enlarged anteriorly and the primary optic vesicles are growing ont from
the first cerebral enlargement. On either side of the head, the (double) tubular heart is seen. Eight
pairs of protovertebrifi are formed.

to the five fundamental divisions of the brain, while from the sides of the fore-brain
the rudiments of the optic nerves and retinse grow out as hollow protrusions.

Heart and vascular system. — While this change is progressing in the
neural canal, and the protovertebrse are becoming formed in the paraxial mesoblast,
the first sign of a vascular system is beginning to make its appearance in the
mesoblast on either side of the head in the form of a simple tubular vessel (fig.
41), which becomes developed in the splanchnopleure in this situation. As the
splanchnopleure and its accompanying hypoblast fold round on either side under the
head to form the fore-gut, these two simple tubes necessarily come together in the
middle line, and they then fuse together longitudinally to form a single tube, the
primitive heart (fig. 38) ; this tube runsfor a short distance in the mesoblast immediately
under the fore-gub, and then divides into two branches, which pass laterally, so as to
partially encircle the fore-gut, and thence course backwards along the body of the
embryo on either side of the notochord. These two vessels form the primitive
arteries {primitive aortce), the part of each which encircles the fore-gut as it passes
dorsalwards being known as the ^rs^ aortic arch. On the other hand, the posterior

HEART AND VASCULAR SYSTEM.

39

end of the single tubular heart bifurcates at an obtuse angle to form two large
yenous roots ( primitive veins), which receive the blood from the vascular area on the
yolk-sac when this is developed, and pass it on to the heart. These two primitive
veins become the vitelline veins.

The heart begins to beat very soon after its appearance, even whilst still filled
only with a colourless fluid, and before receiving blood from the vascular area.

Fig. 42. — Vascular area op the rabbit of 10 days. (v. ISeneden and Julin.)
The arteries and arterial capillaries are represented red, the venous capillaries and veins blue.

Afterwards, when receiving and propelling the red blood from that area, and
especially after it has become elongated and bent upon itself, it is one of the most
prominent objects seen on examining the embryo ; projecting as it docs freely into
the yet widely open coelom immediately behind and beneath the cephalic region of
tiie body.

The first vessels to he developed are formed in mesoljlast altogether outside the
body and within a circular area {vascular area), which sun-ounds the developing
embryo for a certain distance. The first appearance of red blood occurs in the form
of isolated rod points {Idood-iHlandH of Pander), wliich are scatteri.'d ab.)ut within
this area, and are especially numerous at its circinuCei-ence, where they fdrm an

40

HEART AND VASCULAR SYSTEM.

almost continuous chain. These red points are small groups of coloured nucleated
blood corpuscles which have been developed within certain of the mesoblast cells in
the manner explained in another portion of this work (see Histology, Development of
blood-corpuscles and blood-vessels). The mesoblast cells in question form the blood-
vessels of the vascular area by becoming united with one another into a capillary
network, which becomes connected mesially with branches of the primitive aortas

Fig. 43. — Vascular area of the rabbit op 11 days. (v. Beneden and Julin. )

The arteries are represented red, tlie veins blue ; the capillaries ai'e not shown.
In both the stages illustrated, the terminal sinus is seen to he arterial.

{vitelline arteries), and peripherally with a circular vessel {terminal sinus), arterial in
mammals but venous in the chick, which forms the circumferential boundary of the
vascular area. From the capillary network of the vascular area the blood is collected
into two vitelline veins, which course backwards and inwards to carry the blood of the
area to the venous roots of the heart. This is the first circulation, or the circulation
of the vascular area. It is also called the vitelhne circulation, because the vascular
area is developed in the mesoblast of the splanchnopleure layer which encloses the
vitellus, and its capillaries are an important means of bringing the food material of
the vitellus to the embryo.

RECENT LITERATURE. 41

RECENT LITERATURE.l

Eeneden, v., and Julin, Recherches sur la formation des annexes fatales r.Tiez les mammifb-es
{Lapin et L'heiropteres). Arcb. de biologie, t. v., 1884.

Carius, Ueher die Entwicklung der Chorda u. der primitiven RachenJiaut bd Mcerschuieinchen
und Kaninchen. Inaug. Diss., Marburg, 1SS8 : Ueber den Kopffortsatz des Kaninchens. Marburg
Sitzungsber, 1887 ; Ueber die Ausbilduny des hinteren K'&rpierendes bei Cavia. Ibid., 1888.

Chiarug-i, Anatomie d'un embryon humain de la longueur de mm. 2'Q en ligne droite. Arch,
ital. de biologie XII.

Ehlers, E., Nebendarm und Chorda dorsalis. Nachrichten d. kgl. Gesellsch. d. Wissensch. zu
Gottingen, 1885.

Fol, H., Description d^un embryon humain de cinq millimetres et six dixiemes. Kecueil
Zool. Suisse, 1884 ; Recherches sur le d^velcppcment des protovei-tebres chez V embryon du poulet
Archives des sciences physiques et naturelles, 1884.

Giacomini, Sul canale neurenterico et sul canale anale, dkc. Giom. de r. accad. di medic d
Torino, 1888.

Keibel, F., Zur Entwicklungsgesch. der Chorda bei Sdugern. Arch. f. Anat. u. Physiol Anat
Abth., 1889.

Kdlliker, v., Ueber die Chordahohle und die Bildung der Chorda beim Kaninchen. Sitzungsber,
der physical. -med. Ges. in Wiirzburg, 1883.

Iiieberkuhn, N., Veber die Chorda bei Sdugethieren. Arch, flir Anat. und Physiol., Anat
Abth., 1884.

Perenyi, J. v., EntwieUung des Amnion, des Wolff 'schen Ganges und der Allantois bei den Repti-
lien. Zool. Anzeiger, No. 274, 1888.

Havn, Ueber die mesodermfreie SteUe in der Keimscheibe des Hiihnerembryo. Arch. f. Anat
1886.

Bomiti, G., Be I'extremile antirieur de la corde dorsale et de son rapport avec lapoche hypophy
taire ou de Rathke chez V embryon du poulet. Archives italiennes de biol., t. VII., 1886.

Shore and Pickering-, The pro-amnion and amnion in the chick. Journal of Anatomy and
Physiology, 1889.

Solg-er, B., Studien zur Entwicklungsgeschichte des Codoms und des Codomepithcls der Amphibien.
Morphol. Jahrbuch X., 1885.

Spee, F. v., Ueler die EntwicUungsvorgange vom Knoten aus in f^dugcthicrkeimscheiben. Anat.
Anzeiger iii., 1888; Beobachtungen an einer menschlichen Keimscheibe mit offener MeduUannne u.
Canali» neurentericus. Arch. f. Anat. u. Physiol., Anat. Abth., 1889.

Strahl, H., Ueber die Entwickelung des Cavalis myelo-entericus und der Allantois der Eidechse.
Archiv fur Anat. und Physiol., Anat. Abtheil., 1881 and 1883.

Works dealing with the general subject of Embryonic Formation and Development.

Balfour, A Treatise on Comparative Embryology. London, 1880.

Foster and Balfour, The Elements of Embryology. Second Edition, 1883. Edited by
Sedgwick & Heape.

Haddon. An introduction to the study of Embryology. London, 1887.

Hertwig', O., Lehrbuch der Entwicklungsgeschichte des Menschen u. d. Wirbclthiere. 2te
Auflage, Jena, 1888.

His, W., Unltrsuchungen ue. d. erste Anlage d. Wirbelthieiieibes. Die erste EntwieUung des
Huhnctiens im Et. Leipzig, 1868 ; Unsere Korperform u. das physiol. Problem ihrer E7istehung,
1871 ; Anatomie mcnschlicher embryonen, 1881 — 1885.

Kblliker, v., Entwicklungsgeschichte des Menschen u. d. hbheren Thiere. 2te Auflage. Leipzig,
1879.

Bauber, FormUldimg u. Formstorung in der EntwieUung von Wirbclthieren. Morph. Jabrb.
vi., 1880.

1 The early changes in the blastodermic layers are dealt with in several of the papers enumcratec^
on pp. 27 to 29.

42

DEVELOPMENT OF THE ECETAL MEMBRANES.

DEVELOPMENT OF THE FCETAL MEMBRANES; ATTACHMENT
OF OVUM TO UTERUS.

Having thus sketched out the manner in which the principal organs of the
body first make their appearance, we may briefly consider the formation of certain
structures which have a purely embryonic existence, and are concerned either with
the nutrition of the foetus and its attachment to the mucous membrane of the
uterus (chorion, allantois, placenta), or serve the purpose of protecting the embryo
against mechanical injuries by suspension in a bag of fluid (amnion).

Pormation of the amnion and chorion. — The amnion, which is only found
in reptiles, birds, and mammals (amniota), is a membranous bag occupied by a clear
albuminous fluid, and covers the whole of the embryo. It is developed from folds of

■false- ccM^i^n/or ohovlOJi^

■vilU' of~
■ihc

Fig. 44. — Diagram of a teansvekse section of a mammalian embryo showing the mode of

FORMATION OP THE AMNION. ThE AMNIOTIC FOLDS HATE NEARLY UNITED IN THE MIDDLE LINE.

Epiblast, blue ; mesoblast, red ; hypoblast and notochord, black.

somatopleure, which are reflected from the head and tail ends and lateral boundaries
of the embryo at an early stage.^ With the sinking of the embryo into the blasto-
dermic vesicle or yolk, these folds grow up over the back (fig. 44) until they meet and
coalesce with one another along the middle line, in such a manner as to form two
distinct membranes, one of which, the inner, is the amnion (true amnion), while the
outer membrane (termed the false amnion) becomes applied to the greatly thinned
remnant of the zona pellucida, and eventually forms a complete external covering
to the ovum and its contents. This external covering of the ovum has been long
known as the chorion — a name which has, however, been applied by some embry-
ologists to other structures.^ It is fixed to the uterus by vilh, which are

1 The head fold is preceded at a yet earlier stage by the bilaminar ^yro-amnion, the formation of
which has been already alluded to (p. 35).

2 The term " chorion " has been applied to various structures by embryologists. Originally used to
denote the external covering of the developing ovum, it was employed successively for the zona pellucida
("primitive chorion"), the epithelial enclosing membrane of the blastodermic vesicle, and finally for
the external amniotic fold or false amnion, when this becomes formed. Lately it has been used to
express the external albuminous envelope of the undeveloped ovum, so that it is probable that
much confusion may arise unless the meaning of the term be in each case clearly defined. It

DEVELOFMEXT OF THE FCErAL MEMBRANES.

4'3

attached to the uterine mucous membrane, and these villi subsequently become
ramified and vascular Avheu the growth of the allantois has brought the umbilical
blood-vessels to the chorion ; but except in the placenta, they at leugth all become
atrophied and disappear.

It will be seen from the manner in which the true and false amnion are formed
by a fold of somatopleure, that these membranes are composed of both epiblast and
mesoblast. In the false amnion the epiblast becomes the outer layer ; in the true
amnion it is the inner layer. The mesoblast of the one is separated from that of
the other by a space occupied by fluid,^ and continuous with the coelom, with which

^fire'-eyn^t

Fig. 45. — Diagram of a longitudinal section of a mammalian embryo, after the completion

OF THE AMNION.

in fact it remains continuous until the body walls of the embiyo have entirely grown
round and coalesced on the ventral surface — the final point of coalescence being the
umbilicus. With this enclosing growth of the body walls the line of reflection of
the amniotic fold is also carried downwards, so that the amnion is eventually
attached around the umbilical cord, by which the foetus appears suspended in the
amniotic fluid (fig. 50).

In the guinea-pig, in which the epiblast is the innermost layer, the amnion is not formed
as a fold, hut results from an extension of the mesoVjlastic cleavage ai'ound the dorsal aspect
of the central cavity ; this cavity thus becomes the cavity of the amnion.

Formation of the allantois. — Both the amnion and the chorion are entirely
extra-embryonic structures, i.e., they are external to the body of the embryo, and,

will he used throughout this article in the seii.sc in which it ha.s hitiierto almost invarialjly been
employe'] in human embryolojfj-, to denote tliat external membrane of the ovum from wliich the
villi (chorionic villi) which grow intf) the uterine mucous mcmi)ninc sj)ring, and this it will be seen
.presently, is that part of the external investment of the blastodermic vesicle, v.hich, when the amnion is
formed, becomes the external amniotic fold or false amnion.

' Except in the later stages of gestation, wiien the amniotic and chorionic mesoblast become loosely
united by jellydike connective tissue.

44 FOKMATION OF THE ALLANTOIS.

although they minister to its protection and nutrition, take no part in the formation
of any of its organs. But the case is different with the structure next to be
described, viz., the allantois, a part of which does in fact eventually become
converted into portions of the urinary and generative systems, although the greater
part is also extra-embryonic, its function being to minister, through its accompanying
blood-vessels, to the nutritive and respiratory exchanges of the foetus.

The j:ime of development of the allantois seems to vary much in mammals, and
there is reason to believe that it is found in the human embryo at a very early
period — indeed the earliest human embryos that have hitherto been described
already possess an allantois. In most animals in which its development has been
studied the allantois has been found to begin as a hollow prolongation of the posterior
end of the primitive alimentary canal (fig. 46). It soon, however, becomes relatively

Fig. 46. — Longitudinal section through the posterior end op an embryo rabbit, showing the

OUTGROWTH OF THE ALLANTOIS. (Kolliker. )

h, epiblast of trunk ; dd, hypoblast ; m, medullary or neural canal ; ch, notocliord ; lid,
commencing hind-gut, which is becoming formed by a folding-over of the tail end of the embryonic
blastoderm, v ; ed, blind end of hind gut ; al, allantois growing out from hind gut into aw, mesoblastic
thickening ; e, epithelium of yolk sac ; df, splanchnopleure ; hp, somato-pleure passing superiorly into
am, tail fold of amnion ; s, is placed within the cavity of the amnion^ and denotes the tail end of the
embryo.

shifted in position so as to come off from the ventral wall of the hind-gut, growing
into the posterior extension of the mesoblastic cleft and eventually into the space
between the false and true amnion, and carrying along with it its mesoblastic covering
(figs. 36, 45). It is therefore composed eventually of two parts, viz., (1) a hypo-
blastic sac which communicates, at first widely but afterwards by a narrowed orifice,
with the hind-gut, and (2) an investment of mesoblast. This last is usually greatly
thickened and very vascular, and is directly supplied with blood by two arteries
(allantoic or umbilical arteries), which appear at first as a direct continuation of
the primitive aortge. As the allantoic vesicle expands into the cavity of the false
amnion, it carries the vascular mesoblast along with it, so that this mesoblast is thus
brought to the inner surface of the chorion, over which its blood-vessels then spread
so as to convert this hitherto non-vascular membrane into one which is richly
supplied with blood-vessels. The chorion has grown in the form of ramified
villi into the substance of the uterine mucous membrane or decidua even
before this advent of the vascular tissue of the allantois, but the chorionic villi
now receive blood-vessels and thus become vascularized, the interchanges between
the foetal and maternal vascular systems, which are afterwards confined to one region
only of the chorion and decidua — that which forms the placenta — occurring in the
first instance over the whole superficies of the ovum.

FORMATION OF THE ALLANTOIS.

45

Like the amnion, an allantois is only formed in the embryo of reptiles, birds, and mammals.*
It varies chiefly in the extent to which its hypoblastic part becomes developed. In reptiles,

Fig. 47. — Longitudinal section at a slightly later stage than that shown in via. 46.

The allantoic protrusion now springs from the ventral wall of the hind gut. Lettering as in fig. 46.
(From Kolliker.)

Fig. 48. — Early human embryo. (From His after Coste.)

The embryo is enclosed within the amnion am, and is attached at its caudal end by the allantoic
stalk, all, to the chorion. The yolk sac, ys, is still distinct from the allantoic stalk.

Pig. 49. — Diagram op LONOiTuniNAL sections thhouoh the human ovum at successive stages

SHOWING the I>n'I'IN« DOWN Of THE EMIiRYO-KUDIMENT INTO THE HLASTODERMIO VESICLE, AND
THE FORMATION OK THE FOREGUT, AMNION, AND STALK OK THE ALLANTOIS BY THE FOLDING OF
THE BLASTODERM. (His.)

Am, head fold of the amnion (pro-amnion in h and c) ; A'6, yolk sac. a and d arc conditions of tlic
embryo which have been seen and described ; b an<l c arc intended to show how the conditions found
in d may be brought about, ami especially liow the stalk of the allantois may be regarded as a direct
continuation of the posterior end of the embryo, which acconling to His docs not lose at any time its
connei;tioii with the chorion or villous external mcmbnine of the ovum. The curved dotted lines in c
indicat<j the formation of the amnion ami false amnion by the upgrowing of lateral folds (which have
not as yet met in the median line).

' For a dis^mssion of the origin and meaning of the amnion and allantois, see lialfour, " Compaiativo
Embryolo-y," II., P- '^^*'>-

46 CHANGES IN THE UTERUS.

in birds, and in some mammals, e.g. ruminants, this portion forms a large sac which occupies
the greater part of the cavity of the false amnion, and is filled by fluid (allantoic fluid) in
which many urinary products can be recognized. (The ducts of the embryonic renal organs open
into the pedicle of the allantois.) But in most mammals the developpient of the hypoblastic
sac is far less extensive, and in some, including the human embryo, the allantois is mainly
represented by a large mesoblastic outgrowth carrying the allantoic (umbilical) vessels to the
chorion. There is also considerable variation in the period at which the allantois begins to
develop. In the human embryo it is certainly formed at a very early period, probably even
before the amnion is completed. In the guinea-pig, also, it appears early, although after the
amnion. In this animal it is first developed as a solid outgrowth of mesoblast which projects
from the line of junction of the hinder end of the amniotic bag with the blastoderm, and
before the formation of a hind gut or of any part of the alimentary tube, a hypoblastic
diverticulum being altogether wanting. In the earliest human ova in which the allantois
has been investigated, it is already a tube of hypoblast which forms a direct prolongation of
the posterior end of the primitive alimentary canal (fig. 49, d), and is enclosed in a short
stalk of mesoblast, by which the posterior end of the embryo is attached to the chorion, and
through which, by the allantoic (umbilical) blood-vessels, the chorionic villi are freely
supplied with blood. From the attachment of this stalk to the placenta (chorionic part),
the hinder extremity of the amnion is reflected. The stalk in question is not the um-
bilical cord, since it does not include the stalk of the yolk sac (vitelline duct), which only
later becomes bound up with it. It is termed by His the abdominal stalk (Bauchstiel),
the term allantois beiug by him restricted to the hypoblastic diverticulum. It is further
considered by His to be probable that the human embryo never becomes completely detached
from the chorion, but that it always retains its attachment to the outer membrane
of the ovum at the hinder end, this abdominal stalk being regarded as a direct prolongation
of the tail end of the embryo (fig. 49). If this should prove to be the case, the
human ovum would form an exception to the usual rule of a complete separation of embryo
from chorion at the formation of the amnion, and subsequent re-attachment by outgrowth of
allantois.

Clianges in the uterus. Mode of attachment of ovum to uterus.^ —

The mucous membrane of the pregnant uterus is known as the decidua. It is
thicker and more pulpy than in the ordinary non-pregnant condition, and the
glands are longer in proportion, but it is otherwise of similar structure except in
the part where the placenta is about to be formed ; here it undergoes important
modifications. The ovum, which has been fertilized and has passed through the
first stages of development in the Fallopian tube, although considerably larger than
the undeveloped ovum, is still an extremely minute object when it reaches the
uterus. Here it speedily becomes imbedded in the soft and thickened mucous
membrane, and this is soon reflected over and completely encloses the ovum, which
thus comes to lie in a cavity within the decidua which is altogether shut off by the
reflected part from the true cavity of the uterus. Different names have been given
to these parts of the uterine mucous membrane which immediately enclose the ovum
to distinguish them from that which Hues the original cavity of the uterus. Thus
the layer of membrane which has grown around the ovum is known as the decidua
reflexa ; the part where the ovum first becomes attached to the uterus and where the
placenta is afterwards formed, is the decidua serotina, while the membrane lining
the true cavity of the uterus, is termed decidua vera (figs. 50, 51).

With the subsequent growth and consequent expansion of the ovum the enclosing
decidua reflexa expands also pari passu, encroaching more and more upon the true
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 indis-
tinguishable and the original cavity of the uterus is obliterated (except at the
cervix uteri).

1 The following account of the formation of the deciduae and of the placenta is confined as much as
possible to what has been observed in the human subject. In other mammals unportant variations in
the mode of attachment of the ovum and in the formation and structure of the placenta are found to
occur.

CHANGES IN THE UTERUS.

,47

Both the decidua vera and the decidua reflexa orio-inally contain tubular and somewhat
tortuous grlands. which were discovered by Sharpey, and were by hina supposed to minister in
the first instance, both to the nutrition and to the attachment of the ovum the latter by
aflfordmp: depressions for the chorionic villi to penetrate into the substance of the decidua It
has however, since been shown that these villi do not directly pass into the giands ' but
rather tend to become attached to the interglandular surface, and indeed at the decidua
serotma. where subsequently the main attachment of the chorionic villi occurs the o-iand-
lumina may become almost entirely obliterated before the villi are here formed But the
gi-catly enlarged glands of the decidua vera very probably furnish a secretion to a^'^i^^t the
nourishment of the ovum previously to the full establishment of the placental circulation

ds

Fig. 50.— DlAOnASIMATIC SKOTIO.N OP THE I'UEONANT HUMAN UTERUS AT THE SEVENTH OR
EIGHTH WEEK. (Alien TliomsoD.

umijiiical cord and connected with the intcHtine of the embryo

The decidua undergoes remarkaLIo sLnu-tiiriil cliiuiocs diirinjr tli(! ciiily monlliH of
pre{,'uancy, some of these changes being coninion to all three partH of tlie membrane,

48

CHANGES IN THE UTERUS,

whilst others are special to that part (d. serotina) which enters into the construction
of the placenta. The following is a brief account of these changes.^

With the supervention of pregnancy the mucous membrane hning the uterus
becomes thickened and the tubular glands become both dilated and greatly elongated.

Fig. 51. — Antero-posterioFv section of the gravid
UTERUS AND OVUM OF FIVE WEEKS. (Semi-dia-
grammatic.) (Allen Thomson.)

a, anterior ; p, posterior uterine wall ; m, mus-
cular substance ; u, placed in the cavity of the uterus ;
[/, the glandular layer of the decidua vera ; r, the
decidua reflexa ; s, decidua serotina ; c, cervix uteri ;
ch, chorion with its villi, which, are more highly deve-
loped 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.

7?S/A

'A

^

0(

7}t'.

Fig. 51*. — 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.

This thickening of the membrane and enlargement of the glands goes on until the
fifth month, so that by this time the decidua vera is nearly half an inch in thickness
and its glands have undergone so considerable an elongation that they now no longer
pass nearly straight through the membrane but run iu a tortuous manner from the

^ For a more complete account of the changes in the uterus, and of the placenta, ths reader is
referred to the list of papers at the end of this section, but especially to the works of Kundrat and
Engelmann, Leopold, and Minot, and for the comparative structure of the placenta to the classical
investigations of Turner.

CHANGES IN THE UTERUS.

49

inner surface to the vascular layer, so that a vertical 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 dratum spongiosum of
the decidua (fig. 51*, B, 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 aljove
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 cehs (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
exteiTial to it (fig. 51*, B, c).

After the fifth month, by which time the great increase in size of the ovum with
its contained embryo has brought the decidua reflexa 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

Fig. 52. — DiAGRAMMATIO SECTION THROUGH THE DECIDUiB

AT THE EDGE OF THE PLACENTA (from Kundrat and
Engelmann).

c, sp, TO, as in fig. 51*, B ; rf v, decidua vera ; d s, decidua
serotina ; d r, decidua reflexa.

stratum spongiosum the spaces which have
resulted from the dilatation of the gland tubes
lose the lining epithelium, and become flattened
out conformably to the surface, so that they now
appear as a layer of compressed lacunae, sepa-
rated by thin fibrous tralxiculse (fig. 52, sp.).

Similar changes occur in the decidua reflexa.
That this is truly a fold of mucous membrane is
evidenced by the fact that gland tubes can ])e
seen to open upon both its surfaces. These gland
tubes early become enlarged, and acquire an

oblique or tortuous course, with dilatations in their deeper parts, i.e., in the middle
of the thickness of the d. reflexa, so as to form here also a sort of spongy tissue.
But the decidua reflexa sooner becomes expanded by the growing ovum into a
relatively thin membrane, and the atrophic changes in the glands occur at an
earlier stage, so that by the time that it has coalesced with the decidua vera hardly
any traces of them can be discerned.

In the decidua serotina (placental decidua) similar changes have been described
in the glands, the final result being the formation of a spongy layer, with ii-regular
clefts flattened out conformably to the surface, and from which the epithelium has
entirely disappeared, accompanied by coinpletf; utro])liy and disappearance of all the
parts of the glands which are superficial to this layei-, the only portions which
remain nearly unaltered being the deepest parts of the tubes, which are partly

VOL. I. ^

50

CHANGES IN THE UTERUS.

Fig. 53.— Section through a nokmal placenta of seven months in situ (Minot).

Am, amnion ; CJio, chorion ; Vi, root of a villus ; vi, sections of the ramifications of villi in the
intervillous spaces, the larger bloodvessels within them are represented black : D, deep layer ot the
decidua, showing flattened remnants of enlarged glands in spongy stratum ; Ve, uterine vein (? artery)
opening out of placental sinus ; 31c, muscular wall of uterus.

CHANGES IN THE UTERUS.

51

imbedded in the mnscnlar coat of the uterus, and retain their epithehum. 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 serotina.

The most important changes of structure occur in the superficial part of the
placental decidua, after the disappearance of the glands. The exact manner in
which these changes take place has not been follow^ed out, but the ultimate result is
the replacement of the whole of this portion of the decidua, with the exception of a

Fig. 54. — Sections illustrating the stuuctuiie of the i'lacenta (Minot).

A, vertical section thioui,'h the margin of a placenta at full term ; D, D, deep layer of decidua ;
Ft, chorionic villi varinnr:ly cut, their bloodvessels injected ; <S/, marginal space of the placenta,
nearly free from villi ; vi, aborted villi beyond the placenta ; Flh, canalized fibrine of Langhans,
prfxluced, according to Minot, by transformation of the superficial layer of the chorionic epiblast.

B, decidual tissue from a placenta at full term ; v, a bloodvessel ; d, d', decidual cells ; the latter
with several nuclei.

comparatively narrow hasal kij/er next to the spongy structure, into a series of inter-
communicating vascular sinuses, which together constitute an innnense flattened
space (intervillous space), bounded internally (toward the uterine wall) by the basal
layer ju.st referred to, and externally by the chorion ; also, according to some
authors, by a thin layer of decidua, the mbchorionie membrane of Turner, which is
described a.s lying immediately under the cliorion of the ovum but so intimately
incorporated with it as to be with difliculty demonstrable as a sepai'ate stratum
except at the edge of the placenta.

From the basal layer, partitions of fil)roiis decidual tissue pass towards the
chorionic surface, and serve to partially sub-divide the lal)ynnth of vascular spaces or
sinuses into a uuriiber of loculi (cotyledons). Each of these loculi is occupied by an

K 2

52

ATTACHMENT OF THE OVUM TO THE DECIDUA.

arborescent tuft of villi continuous with the foetal chorion, and traversed by blood-
vessels which are supplied from the branches of the umbilical arteries. These
blood-vessels form a capillary loop in each villus, and these capillary loops are
separated from the maternal blood in the placental sinuses not only by the capillary
walls and the connective tissue of the viUus, but also by a double layer of flattened
epifchehum-like cells derived either from the chorionic epithelium (Minot),i or from
the decidual tissue, and, perhaps, in part representing an endothelial membrane
belonging to the placental sinuses, which, according to Waldeyer, are lined by endo-
thelium prolonged from that of the uterine vessels.

Some of the chorionic villi are attached (1) by comparatively stout bands of
fibrous tissue to the basal stratum of the decidua, (2) by finer bands of similar
substance both to one another and to the
septal prolongations of the decidua; others
hang freely into the placental sinuses.
These sinuses are supplied directly with
arterial blood from tortuous branches of
the uterine arteries which pass through the

Fig. 55. — Diagram showing the tissues covering

THE VILLI IN THE HUMAN PLACENTA, AND
THEIR RELATION TO THE DECIDUA ACCORDING

TO Turner.

F, foetal tissue ; M, maternal tissue ; d, blood-
vessels in villus; 6! , blood in placental sinus;

c', layer of cells covering villus ; x, basement mem- -^^S- 57. — Diagram of the placenta (E. A. S.).

uSeTeiT-"^!^! ^- f "™i^°i) Po^ti'i^ed from «m, amnion ; c\, chorion ; ., placental sinus ;

uterme vessels; ^s decidua serotma ; t, tissue ds, decidua serotina ; s«, spongv laver • m

op'S ilts'int "''' ' "' "^' "''"'' """'^ """"^^"^ ' «' ^' -terine'^arS f^d vTn openl
opening into sinus. uig ^^^^ placental sinus.

spongy stratum of the decidua serotina and through the basal stratum of the
placenta to open mto the sinuses without the intervention of capillaries. From
the smuses vems, which run very obhquely through the decidua, carry off the blood,
and eventual y pass mto the veins of the muscular wall. The foetal viUi are thus
bathed by slowly flowmg maternal arterial blood, and respiratory and nutritive
exchanges may occur between the two kinds of blood, but there is no actual mixing

^L^^ltl":^^":^"^^^^^ ^ t^'^'f '^' P^r^*^ '' ^'^'^"^'^ by the chorionic epithelium.

.ho^thaJ^tl^n^Sr^^ol^^JiX^^^^^

FORMATION AND STRUCTURE OP THE PLACENTA.

53

of the two fluids, nor is it possible to inject the foetus from the mother through the
placenta, or the mother from the foetus.

The question, whicli has been more than once raised, whether the intervilloiis spaces
normally contain blood, may be now regarded as settled in the affirmative. It has been
usually held that they represent capillaries or veins of the decidua, which have become
dilated and fused together to such an extent as to occupy the whole thickness of the
placental part of that membrane, with atrophy of the intervening- decidual tissue, which
merely remains as a covering to the villi. But since the placental sinuses appear to be
bounded superficially by the chorionic covering of the ovum, and it is in most places not
possible to detect any decidual tissue between them and the chorion, it has been conjectured by
some writers (Kolliker, Langhans) that they have become formed by extravasation of blood into
a space between the ovum and the decidua, into and across which space the chorionic villi
have grown. Although the development of these structures is insufficiently known in Primates,
it has been shown in various mammals (Selenka, Duval, Hubrecht, Masius) that the first
attachment of the blastodermic vesicle to the uterine wall is effected by the external layer of
the epiblast, which sometimes splits off over the embryonic area as a distinct layer, and
which, in some animals (e-0-> hedgehog), becomes greatly thickened, and is connected by
epiblastic villi to the decidua. This external layer of epiblast, for which Hubrecht has pro-
posed the general name of trojjhohJast, causes the absorption of the uterine epithelium both
of the surface and of the glands (where this epithelium has not previously been cast off)
and comes dii'ectly in contact with the enlarging decidual vessels, the endothelium of which
is actively proliferating. Within the thickened trophoblast clefts now make their appear-
ance and are presently found to be occupied by maternal blood, which is derived from the
vessels of the adjacent hypertrophied decidua. This blood flows therefore into spaces in the
trophoblast, which are onlj' bounded by foetal epiblast, and this jJThiiary placental circulation
may be formed before any foetal blood-vessels have reached the chorion. Subsequently, when
the vascular mesoblastic villi become formed they extend into these spaces, pushing before
them the epiblast ; by this layer they remain permanently covered and it also lines the
enlarged spaces into which they have extended.

The placenta is composed of two parts, one foetal, composed of chorion with
its villi ; the other maternal, formed from decidua serotina. In its completely

Fig. 57. — Traxsverse section of a villus from

A PLACENTA OF SEVEN MONTHS (Minot).

Three blood-vessels are seen within tlie villus,
imbedded in a jelly-like connective tissue containing
cells and fibres ; a, a, cell-layer covering villus
(epiblast according to Minot ; according to others
of decidual origin) ; /, a thickened portion of this
cell-layer, which has undergone a fibrinous transfor-
mation (canalised fibrin).

developed condition, it is a circular discoid
mass, weighing about a pound, 7 or 8 inches
in diameter, thickest at the centre (1^ inch),
and thinning away towards the edges,
which are continuous with the compara-
tively thin coalesced deciduae and chorion.
Its inner surface is smooth and concave,
and is closely covered by the amnion as by

a serous membrane ; under this the larger branches of the umbilical vessels course
before dipping into the substance of the placenta. From near the centre of the
organ the umliilical cord passes off to the foetus. Its outer surface is incorporated
with the uterine wall, but when detached from this by tearing through the spongy
tissue of which the deeper part of the decidua is formed (as occurs in parturition at
the expulsion of the fa'tus), the outer surface appears ragged and irregular, in
striking contrast to the smooth amnioii-covered inner surface. Examined under
the microscope, the chorionic tissue (villi) of the placenta is found to be composed
of jelly-like connective tissue, with branched and anastomosing colls (fig. 57) ; in
Bome parts of the larger stems white fibres are seen. What r-cmains of the decidual

M

THE PLACENTA.

tissue has a fibrous appearance, with very imraerous decidual cells, which frequently
obscure the fibres (fig-. 54, b). In and after the fifth month of pregnancy, a number

'^W/J

m

'^1^ ^

Fig. 58. — FfiONT AND SIDE VIEWS OF AN EARLY HUMAN OVUM

FOUR TIMES THE NATURAL SIZE (from Eeichert).

This ovnm is supposed to be of thirteen days after impreg-
nation. The surface bare of villi is that next the wall of the
uterus, showing at e, the opacity produced by the thickened
embryonic disc. The villi covered chiefly the marginal parts of
the surface.

of large multinucleated giant cells are found scat-
tered about in the tissue. They occur most abun-
dantly in the outermost layer of the decidua
serotina, and have been described by Friedlander and by Leopold as growing at a
later stage (eighth or ninth month) into the veins which pass through this layer, so

Fig. 59. — View of the interior of the human gravid uterus at the twenty-fifth day

(from Farre after Coste).
M, uterine wall ; o, ovum with villous chorion ; dv, decidua vera ; ch\ decidua reflexa, divided round
the margin of the ovum, and turned down so as to expose its pitted surface, which has been removed
from the ovum. The right ovary is divided, and shows in section the plicated condition of the early
corpus luteum.

SEPAEATION OF THE UECIDUA AT BIRTH.

as to produce a partial blockage of these veins, preparatory to the detachment of the
placenta from the layer.

The villi do not at first cover the whole sm-face of the ovum, but are deficient at
the embryonic, and perhaps also at the opposite, pole. In the earliest human ovum
which has hitherto been described,
that of Reichert (fig. 58), the villi,
which are quite simple, occur in a
broad zoue around the circumference
of the ovum, leaving the (somewhat
flattened) poles smooth and free
from villi, and on one of these poles
a thickening of the wall of the
vesicle could be detected, which was
probably the embryonic area. But
in all other early human ova which
have been noticed, the chorion,
which is now formed by the false
amnion, is covered with ramified
villi (shaggy chorion), and these are
already vascularized from the allan-
tois, and have grown into the sub-
stance of the decidua reflexa and the
decidua serotina, the formation of
the placenta having already begun.

Separation of the decidua
uterine mucous membrane. — In

60. — Portion of an injected villus from a

PLACENTA OP ABOUT FIVE MONTHS (Minot).

at birth, and regeneration of the

parturition, the pressure of the contracting
muscular walls upon the uterine contents, and especially upon the amniotic fluid,
causes a bulging of the meml3ranes (consisting of the combined deciduas, the
chorion, and the amnion) through the os uteri. When the membranes are ruptured,
the amniotic fluid first escapes, and subsequently the fa'tus is expelled. AVith
fnrthgp contraction of the uterus, the placenta becomes detached from the uterine
wall, separating along the plane of the dilated parts of the glands (stratum spongio-
num of the decidua serotina), and as it is expelled, the separation extends around the
decidua lining the rest of the uterus, which appears in the " after-birth " along with
the chorion and amnion as a thin membranous skirt to the edge of the placenta. The
deepest part of the decidua containing the bases of the uterine glands is everywhere
left in connection with the muscular tissue, and from these basal portions of the
glands, first the whole of the uterine glands, and subsequently the lining epithelium
of the uterus become gradually regenerated.

RECENT LITERATURE.

Allen, W., Omph/ilo-mesenteric remains in mammals. Journ. Anat. and Phj'siol., xvii., 1883.

Beneden, Ed. van, Dc la fixation du hiastocysle d la viuqtieust nterhte rlicz Ic murin {Vesper-
tilio viurinvs). Bullet, de I'Acad. roy. de Belgique, Ser. iii.. T. xv. ; l)e la formation et de la comtita-
lion du placenta chez le murin (Vespcrtilio murinus). Bulletins de I'Acad. roy. de Bclgique, T. xv.,
1888.

Beneden, E. v., et J ulin, lie her rhes sur la forvuition des annexes fo'talcs ihezles mammi/eres.
Arch, de biolo;,'ie, v. 1884.

Bonnet, E,., JJk I'tirinmilch uml ihre Bedeutitng filr die Prucht. Buitrii,'C zur Biologic, 1882 ;
Die Eilidute des I'ferdes. Vcrhandl. d. anat. (iesellHchaft, 1889.

Bumm, Zur Kentitniss der I Iteroplaeentari/efasse. Arcliiv f. Gyniikologic, xxxv., xxxvii. ; Zur
Anat. d. Placenta. Wiirzbiirg Sitzungsb., 1889.

Cadiat, /jull'inUnde. fiaz. med. de PariH, 4 sdr. vi., 1887.

Caldwell, W. H., On the arranf/emcnt of tlie embryonic membranes in marsupial animals,
Quarterly .Jounial of .Microsc. Science, xxiv., 1884.

Colucci, O., J)'i alcuni novi dati di slruttura della placenta umava, 1887.

56 RECENT LITERATURE.

Duval, M., Les placentas discoldes en general, & propos du placenta des rongeurs. Comptea
rendus de la societe de biologie, Ser. viii., T. v,, 1888 ; Le placenta des rongeurs. Journal de I'anat.
et de la physiol., XXV. ,, 7 ^ j •

Ercolani, G. B., Nuove ricercJie di anatorma normale e patologica sulla placenta dei mammx-
feri e della donna. Mem. d. accademia d. scienz. d. Bologna, 1883. (French in Archires italiennes

de biologie, iv.) , . , t> 7,7 • o-x 1 j i -i

Fleischmann, Ueber die erste Anlage der Placenta hei den RauUhieren. bitzb. der physik.
medic. Soc. zu Erlangen, 1886 ; Embryologische Untersuchungen. Heft i. Vntersuchimgen ilber
einheimische Eaubthiere. Wiesbaden, 1888 ; Mittelblatt und Amnion der Katze. Erlangen, 1888.
Frommel, R., Entw. d. Placenta v. Myotus murinus. Wiesbaden, 1888 (Centralbl. f. Gynak.,

1 QQQ\

Hart, B., The meclianism- of the separation of the placenta, dsc. Proc. Roy. Soc. of Edinburgh,
XV., 1889.

Heinz, R., Untersuchungen uber den Ban und die EntwicTcelung der menschhchen Placenta.
Inaug.-Diss. Breslau, 1888.

Heinricius, G., Die EntwicU. der Hunde-Placenta. Sitzung.sb. d. Berlin. Akad., 1889 ; Arch. f.
mikr. Anat. 33, 1889.

Hoffmann, C. K., Ueber das Amnion des zweibldttrigen Keimes. ArcMv f. mikrosk. Anatomic,
xxiii., 1884.

Hofmeier, Zur Anatomie d. Placenta. Wiirzburg Sitzungsb,, 1889.

HubrecM, The placentation of Erinaceus Europceus, with remarks on the phylogeny of the
placenta. Quarterly Journal of Micr. Science, 1889.

Kastschenko, N., Das menschliche Chorionepithel und dessen Rolls hei der Placenta. Arch. f.
Anat. u. Physiol., Anat. Abth., 1885.

Keitoel, P., Zur EntwicUungsgesch. der menschlichen Placenta. Anat. Anzeiger, iv., 1889.

Klein, Ueber die Entstehung der Placenta marglnata. Arch. f. Gynak., xxxvi.

Kundrat und Engrelmann, Untersuchungen iiber die Uteruschleimhaut. Strieker's med.
Jahrb.,1873.

Kuppfer, C, Die Entstehung der Allantois und die Gastrvla der Wirbdthiere. Zoologischer
Anzeiger, ii. , 1879 ; Decidua und Ei des Menschen am Endc des ersten Monats. Miinchener Wochen-
schrift, 1888.

Langhans, Die Losung der mutterlichen Eihdute. Archiv fiir Gynakol., viii., 1876 ; Ueber die
Zellschicht des menschlichen Chorions. Henle's Festgabe, 1882.

Leopold, Studien uber die Uterusschleimhaut wdhrend Menstruation, Schwangerschaft und
Wochenbett. Archiv f. Gynakol., xi., 1877 ; Ueber den Bau der Placenta. Archiv f. Gynakologie,
XXXV., 1889.

Lieljerkiihn, N., Der grilne Saum der Hundeplacenta. Arch. f. Anat. u. Physiol., Anat, Abth.
1889.

Marius, J., De la genese du placenta chez le lapin. Arch, de biologie, ix., 1889.

Masquelin et Swaen, Premieres phases du ddveloppement du placenta maternal chez le lapin.
Arch, de biol., i, 1880.

Minot, Uterus and embryo. Journal of Morphology, Vol. ii., 1889 (includes a tolerably full
Bibliography of the subject).

NitalDuch., R., Beitrage zur Kenntniss der menschlichen Placenta. Inaug.-Diss. Bern, 1888.

Osborn, H. F., Observations upon the foetal membranes of the Opossum, and other Marsupials.
Quarterly Journal of Microsc. Science, xxiii., 1883.

Rolir, K., Die Bezichungen d. miitterl. Gefdsse z. d. intervUlosen Eaumen, dsc. Virchow's Archiv,

Ruge, K., in Schroder, K., Der schwangere u. kreissende Uterus. Bonn, 1886.

Ryder, J. A., The origin of the Amnion. American Naturalist, xx., 1886 ; A theory of the
origin of placental types, and on certain vestigiary structures in the placentae of the mouse, rat, and
field-mouse. American Naturalist, xxi., 1887.

Strahl, H., Die Allantois von Lacerta viridis. Sitzungsb. d. Marburger Gesellsch., 1883; Ueber
den Bau der Placenta; Die Anlagerung des Eies an die Uterusivand. Arch. f. Anat. u. Physiol.,
Anat. Abth., 1889 ; Zur vergleichenden Anat. d. Placenta. Verhandl. d. anat. Gesellschaft, 1889 ;
Placenta von Putorius furo. Anat. Anz. , iv., 1889.

Tafani, A., La circulation dans le placenta de quelques mummifires. Archives italiennes de
biol., viii., 1887.

Turner, "Wm., Lectures on the comparative anatomy of the placenta. Edinburgh (Black), 1876 ;
Smne general observations on the placenta, with especial reference to the theory of evolution. Joum. of
Anat. and Physiol., xii., 1877 ; On the placentation of the Apes, with a comparison of tlie structure of
their placenta with that of the human female. Phil. Trans., Ixix., 1879 ; An additional contribution
to the placentation of the Lemurs. Proceedings of the Royal Society, Vol. xliv., 1888.

Waldeyer, "W., Ueber den Placentarkreislauf des Menschen. Sitzb. d. k. Akad. d. Wissensch.
zu Berlin, vi., 1887 ; Die Placenta von Inuusnemestrinus. Ibid., 1889 ; Menschen- und A ff en-placenta.
Archiv f. mikr. Anat., Bd. 35, 1890,

DEVELOPMENT OF THE SPINAL COED. 57

DEVELOPMENT OF THE NERVOUS SYSTEM.

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 nms forwards in front of the primitive streak, and the anterior
end of which becomes enlarged and converted by constrictions into three successive
vesicles, around which the several parts of the brain are formed, and which are
knoNvn as the primary cerebral vesicles. The remainder of the neural canal is of
nearly uniform diameter, and 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 epiblast, and it therefore
follows that the whole structure of the central nervous system is laid down in
epiblast, and consists in the main of more or less modified epiblastic elements, except
where mesoblastic tissues subsequently penetrate into it, conveying blood-vessels
into its substance. As was shown by Balfour, the same is in all probability true for
all the nerves of the body, cranial and spinal, which either, as with the fibres of the
anterior roots of the spinal nerves, grow directly out from the neural epiblast, or, as
with the fibres of the posterior roots, are formed and grow from masses of epiblast
cells, which are separated oflP at the junction of the neural and general epiblast to
form the gangha, from which the posterior root fibres appear to take origin (His).
An exception must, however, be recorded for the olfactory tracts and bulbs and optic
tracts and nerves, which, although derived from the neural epiblast, yet have a different
mode of origin from all other nerves, both cranial and spinal, since they arise not as
solid outgrowths of that epiblast, but as hollow protrusions from the brain, which
only become solid at a later stage of development. We have then to consider the
manner in which are developed (1) the spinal cord ; (2) the several parts of the
brain ; and (3) the spinal and cranial nerves and their ganglia, as well as the ganglia
and nerves of the so-called sympathetic nervous system.

DEVELOPMENT OP THE SPINAL COED.

Soon after the neural canal is closed (fig. 32, p. 31), it takes the form, along the
greater part of the length of what is afterwards to become spinal cord, of a cleft-
like cavity, with thick sides, and a relatively thin dorsal and ventral boundary (roof
and floor). The parietes of the canal are w4iolly composed of long columnar epithe-
lium cells, whose free borders, which are at first smooth, but later become ciliated,
line the cavity, and whose attached extremities rest upon a homogeneous limiting
membrane which early makes its appearance, bounding the embryonic cord, and
separating it from the surrounding structures. These cells, therefore, extend at
first through the whole thickness of the embryonic cord, and they have the closely-
set, palisade-like character, with the nuclei at different depths, such as it is usual
to find in long columnar epithelium.

After a time, it is found that the ceUs (which have become always longer with
the increasing thickness of the wall of the neural canal) show a tendency to branch
and to unite with the branches of neighbouring cells. In this way a network or
spongework is produced, which extends throughout the greater part of the thickness
of the embiyonic cord ; at the same time the inner parts of the cells which immedi-
ately line the canal retain their palisade-like arrangement, while the external or
attached ends often exhibit a radiating disposition, which gives a characteristic
radial character to the external layer of the reticular stiiicture. The reticulum is

58

DEVELOPMENT OF THE SPINAL COPvD.

termed mijelospoigium, and the cells by which it is formed are spoken of as spongio-
Uasts (His) (fig. 61).

Between the inner ends of the columnar epithelium cells or spongioblasts there
is seen at a comparatively early period (four and five weeks in the human embryo)
a number of rounded cells, with a considerable amount of clear protoplasm, forming

Fig. 61. — Myelospongium from spinal cord of three and a half weeks HtrjiAN EMBRYO (His) 'i?.
Fig. 62. — Inner ends of spongioblasts with germinal cells, g, between them ; from

SPINAL CORD OF HUMAN EMBRYO (His).

Fig. 63. — Inner ends of spongioblasts [Sp] ; a germinal cell {g) and two transitional

CELLS [Tr) FROM SPINAL CORD OF HUMAN EMBRYO (His).

Fig. 64. — Three neuroblasts, each with a nerve-fibre process growing out beyond the

BASEMENT MEMBRANE OF THE EMBRYONIC SPINAL CORD (His).

an interrupted layer in this innermost zone. Their nuclei are mostly in one stage
or another of karyokinesis (fig. 62). They are termed by His the germinal cells, and
according to him they give origin to the cells next to be described.

The third kind of cell {netiroUast) met with in the cord of the early embryo is
one with a relatively large oval nucleus, and little protoplasm, but with a tapering
protoplasmic prolongation directed outwards towards the surface of the cord. These
cells are found in groups, at first only in or near the layer of germinal cells (fig. 63),
but subsequently in the outer layers (fig. 64). The prolongations are the com-
mencements of the nerve-fibres, and they mostly converge either straight or with
an arcuate course towards what will subsequently be the place of exit of the fibres of
the anterior roots.

The. outermost layer of the embryonic cord after the differentiation of the
various kinds of cells above described is free from nuclei, and is composed of the
partly yeticulated, partly radially arranged external or attached extremities of the

DEVELOPMENT OF THE SPINAL CORD.

59

spongioblasts. This may be taken to represent the white matter of the cord at this
stage (all the rest representing grey substance) ; but there are at first no nerve fibres
in it, the only structures which can be at all compared to nerve fil)res being the
prolongations of the neuroblasts, and these lie either as arcuate fibres altogether in
the outer part of the grey substance, or are passing out of the coixi as the beginnings
of the anterior roots from a mass of neuroblasts which forms the rudiment of the
anterior cornu of grey matter (fig. 65). This mass constitutes in the human embryo
of six weeks (fig. 66) the chief portion of each half of the cord. It forms a con-
siderable projection which laterally almost reaches the surface, but ventrally is sepa-
rated from it by a thickening of the external or radial zone, due to the appearance of
longitudinally coursing nerve fibres within it : this is the beginning of the anterior

Fig. 65. — Section of spinal cord of four weeks human embryo (His).

The posterior roots are continued within the cord into a small longitudinal bundle which is the
rudiment of the ]iosterior white column. The anterior roots are formed by the convergence of the
jiroces-ses of the neuroblasts. The latter, along with the elongated cells of the myelospougium compose
the grey matter. The external layer of the cord is traver-ed by radiating fibres which are the outer ends
of the spongioblasts. The anterior commissure is beginning to appear. This figure is much more
magnified than the next one.

Fig. 66. — Transverse section of the cervical part op the si-inal cord of a human
EMBRYO of six WEEKS (from Kiilliker). ^{

c, central canal ; e, its epithelial lining ; at e' (superiorly), the original place of closure of the canal ;
a, the white substance of the anterior columns ; f/, grey substance of aiitero -lateral horn ; p, posterior
column ; ar, anterior roots ; pr, posterior roots.

white column (a). By this time, also, although to a rather less extent, the posterior
white columns have, simultaneously with the "posterior roots, begun to make their
appearance on cither side of the narrow dorsal part of the neural canal (p). There is,
however, only a relatively thin layer of grey matter (neuroblasts) separating the
posterior white columns from the palisade-like lining of the canal, and as yet no
sign of nerve fibres in the sitiuition of the lateral columns, whi(;h are only repre-
sented l)y a thin layer of the radial myelospougiuni. The roof and floor of the
caual are also quite thin and undevelo|)ed.

At this jx^riod there is still no sign of either anterior or posterior (dorsal or
ventral) fissures of the cord. These become formed as the cornua of the grey matter
grow out from the central uni^a, and as the anterior and posterior white columns

60

DEVELOPMENT OF THE SPINAL COED.

increase in extent. The anterior fissure is simply a cleft left between the enlarging
lateral halves of the cord ; the anterior commissure is formed across the bottom of
the cleft, which is thereby separated from the central canal. As for the posterior
fissure, it is uncertain whether it is in part formed from the dorsal portion of the
constricted canal, which has become occupied by an ingrowth of pia mater, and
conyerted into a mere septum of connective tissue, or whether this fissure with its
connective tissue septum becomes formed independently of the central canal, which,
as the fissure extends, gradually atrophies until it is eventually converted into the
rudimentary epithelial tube which is persistent during life.

In the sacral region of birds, the central canal expands into the rhomboidal sinus, and in
the filum terminale of the human cord it remains relatively large. An open enlargement
analogous to the rhomboidal sinus of birds, although relatively smaller, has been described by
Tiedemann in a nine- week human foetus.

The cord is at first oblong oval in section, with an angular depression in each
side which serves to mark off the situation of the future posterior columns and
their corresponding grey matter from the antero-lateral region. These two parts of
the lateral neural epiblast may be distinguished as the dorso-lateral (alar) and the
ventro-lateral (basal) laminse ; with the former, the afferent nerve-fibres become

Fig. 67. — Brain and spinal cord exposed from behind in a fcetus op
THREE MONTHS (from Koliiker).

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

connected, whilst from the latter the efferent fibres take origin
(His). In the human embryo of six weeks, they are well marked
oflF fi'om one another, and their respective connections with the
posterior and anterior nerve-roots are very distinct (fig. 66). In
the upper part of the cord, the lateral nerve-roots (spinal accessory)
also arise from the basal lamina. The characteristic cylindrical
form of the cord is only attained with the development of the
lateral columns. The cervical and lumbar enlargements are mani-
fest at the end of the third month.

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
filum terminale, occupy the lower end of the neural canal, where they form the
eauda 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 diflFerent times in different
parts corresponding with the tracts of conduction ; the last of these tracts to become
medullated are the pyramidal tracts.

The membranes are formed from mesoblast of the protovertebrse, 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 seen
penetrating into the substance of the cord from the pia mater grow in from this
mesoblast, carrying blood-vessels amongst the nervous elements. The neuroglia or

DEVELOPMENT OF THE BRAIN. 61

general sustentacnlar substance of both white and grey matter is probably derived
from the spongioblasts, and is therefore, like the nerve-cells themselves, of epiblastic
origin.

DEVELOPMENT OF THE BRAIN.

TVe have already traced the development of the cephalic part of the neural tube
as far as the formation of the primary cerebral vesicles. These, which are at first
three in number (fig. 40), become subdivided so as to form five in all, which may
be termed in saccession from before back, the first, second, third, fourth, and fifth
secondary vesicles. Of these five parts the first two, which represent the cerebral and
thalamic parts of the future brain (third ventricle), are derived from the first primary
vesicle, and the last two, the cerebellar and bulbar parts (fourth ventricle), from the
third primary vesicle, while the third, middle, or quadrigeminal part, represents the
undivided second primary vesicle (Sylvian aqueduct). These relationships, as well
as the several parts of the brain which are eventually respectively formed in connec-
tion with the vesicles, are shown in the subjoined table.

/ Anterior end of third ventricle, fora-
mina of Monro, lateral ventri-
' First secondary vesicle -| cles, cerebral hemispheres, olfac-
(^l)rosencej)lMlon) tory bulbs and tracts, corpora

I. Anterior primary vesi- J V striata, corpus callosum, fornii..

cle or fore-brain |

Second secondary vesicle ( ^^f. ventricle, optic nerve and
^ ithalaviencejihalon-) '"^^f ^.' °P*^^ thalami, pituitary

\ and pmeal bodies.

II. Middle primary vesicle r Third secondary vesicle i Aqueduct of Sylvius, corpora quad-
or mid-brain C {meseiicephalon) \ rigemina, crura cerebri.

(Fourth secondary vesicle / / Cerebellum. Pons.

(epencephalon) \ Fourth ven- \

It'll
Fifth secondary vesicle ( ( Medulla oblongata.

(metencepkalon)

The first and most striking change which occurs in the primary brain is the
outgi'owth on either side of the first primary vesicle of a hollow protrusion {jwimary
optic vesicle), which becomes developed eventually into optic nerve and retina
(fig. 68). The changes which it undergoes in the formation of these structures will
be considered when the development of the eye is dealt with ; suffice it for the
present to say that the free hollow communication (optic stalk), which at first exists
between the forebrain and optic vesicle, becomes gradually narrowed and at length
obliterated, and that as development proceeds, the connection of the optic stalk
becomes relatively shifted backwards, so that when the anterior part of the fore-brain
is distinct from the posterior part, or thalamencephalon, the optic vesicle is connected
wholly with the latter, a relationship which is maintained permanently, although
partially obscured afterwards by the later connection which is formed between the
optic tract and the mid-brain. Subsequently another pair of hollow outgrowths
sprouts from the fore-brain, and these rapidly extend forwards, laterally and back-
wards ; they form the vesicles of the cerebral hemispheres. From the roof of the fore-
brain (second vesicle) a median hollow protrusion grows upwards and forwards for a
certain distance towards the vortex, and from the floor of the same vesicle another
somewhat similar protrusion passes downwards and backwards towards the roof of
the mouth. The former is the rudiment of the pineal fjland, the latter of the
infundibulum, which becomes involved in the formation of the pituitary body.

The principal parts of the brain appear as thickenings in diilerent parts of the

62

DEVELOPMENT OF THE BRAlK

Vom

walls of the vesicles. Thus the corpora striata are formed in the floor of the hemi-
sphere vesicles, whilst the principal mass of each hemisphere is formed from the
roof and sides (mantle) of those vesicles, and the olfactory lobes are hollow out-
Fig. 68. — FORB-PART OF THE EMBRYO SHOWN IN FIG. 38^

VIEWED FROM THE DORSAL SIDE. '^. (From Kolliker. ).

F, fore-brain ; e, ocular vesicles ; M, mid-brain ; II, hind-
brain ; h, jjart of the heart seen bulging to the right side ;
Vom, omphalo-mesenteric or vitelline veins entering the
heart posteriorly ; Mr, medullary canal, spinal part ;
jp, proto-vertebral somites.

growths from them. The cavities of the hemi-
sphere-vesicles become the lateral ventricles, and
the cavity of the part of the fore-brain (or first
secondary vesicle) from which they spring, forms
the anterior extremity of the third ventricle.
The optic thalamus is formed by a thickening
of the lateral wall of the second vesicle, the
cavity of which comes to be the main part of
the third ventricle ; the corpora quadrigemina
are thickenings in the roof, and the crura cerebri
thickenings of the sides and floor of the third
vesicle, which becomes the aqueduct of Sylvius ;
the cerebellum and pons are respectively thick-
enings of the roof and floor and the crura cere-
belli of the sides of the fourth vesicle (anterior
part of hind-brain), the cavity of which becomes
the anterior (superior) part of the fourth ven-
tricle ; and finally, the medulla oblongata is
developed as a thickening of the wall of the
fifth vesicle, the cavity of which expands from

the central canal of the spinal cord to form the calamus scriptorius of the fourth

ventricle.

On the other hand, certain parts of the walls of the vesicles become thin and

greatly expanded, and even eventually project into the cavities as folds of epithelium

Fig. 69. — Outline of a longitudinal

SECTION THROUGH THE BRAIN OF A

CHICK OF TEN DATS (after Mihalkovics).

h, cerebral hemisphere ; olf, olfactory lobe
and nerve ; st, corpus striatum ; Iv, lateral
ventricle : ac, anterior commissure ; It, lamina
terminalis ; ope, optic commissure ; pit, pitui-
tary gland ; inf, infundibulum ; cai, internal
carotid artery ; v^, third ventricle ; cA*, cho-
roid plexus of third ventricle ; pin, pineal
gland ; hg, corpora bigemina ; mnv, anterior
medullary velum ; below which two last
references are the aqueduct of Sylvius and
crura cerebri; cbl, cerebellum; v*, fourth

ventricle ; ba, basilar artery : ps, pons Varolii ; c7i\ choroid plexus of the fourth ventricle ; obi, medulla

oblongata ; r, roof of fourth ventricle.

covering ramified vascular expansions of pia mater (choroid plexuses). These
vascular expansions occur along the lower border of the mesial surface of each
hemisphere-vesicle (choroid plexuses of lateral ventricles) ; along the roof of the
second vesicle (choroid plexus of third ventricle), and in the roof of the fifth vesicle
(choroid plexus of fourth ventricle).

^.\jA'Aij!-

ba. ps

DEVELOPMENT OF SPECIAL PARTS OF THE BRAIN. 63

"While these changes are going on in its walls the embryonic brain does not
remain straight as at first, with its axis in a line with that of the spinal cord, bnt
undergoes certain flexures (fig. 70), the general result of which is to bend the
anterior end towards the ventral surface. The first of these flexures to make
its appearance is a sharp bend opposite the base of the mid-brain and around the ante-
rior end of the notochord. The result of this flexure, which produces a complete
doubling round of the anteriov part of the brain, is that the mid-brain is for a time
the most prominent part of the encephalon. Later, the growth of the cerebral
vesicles, and of the thalamencephalon, brings these parts again into prominence, and
tends to obscure the flexure, which is, however, never actually obliterated. The second
cerebral flexure, which is also very sharp and well marked, occurs in the region of
the hind-brain (pons Varolii), It is in the opposite direction to the first one, its
concavity being directed towards the dorsum of the embryo, and it produces the
appearance of a deep depression at the part of the brain where it occurs. The third
flexure is a more gradual one. It occurs at the junction of the hind-brain with the
cord, the embryonic medulla oblongata being bent ventralwards from the line of
direction of the medulla spinalis.

The result of these flexures is that the axis of the embryonic brain takes a crook-
shape, passing from the end of the spinal axis at first ventral, then dorsal, and then
again ventral, finally bending sharply backwards towards its termination at the
foramen of Monro.

The second and third flexures become eventually almost entirely obliterated with
the further growth of the brain.

FURTHER DETAILS REOARDINO- THE DEVELOPMENT OF SPECIAL PARTS OF

THE BRAIN.

The fifth cerebral vesicle: bulbar vesicle, or metencephalon. — This
part of the embryonic brain, afterwards to become the medulla oblongata, often
shows at its first appearance — especially in the chick — a series of slight constrictions
(fig. C8), which have by some been taken to indicate a segmentation of the neural tube.
But even where they occur they are quite temporary, and the fifth vesicle soon becomes
a well marked dilatation opening out from the anterior end of the embryonic spinal
cord. Its wall, Hke that of all the other cerebral vesicles, is composed of cells
similar to those of the rest of the neural tube, and the histogenetic changes which
occur to form the nervous tissue are also entirely similar.

Sections across this part of the neural tube are of a compressed oval outline in
the lower pai*t (fig. 71, A, b), but in the upper part, which afterwards becomes the
lower part of the fourth ventricle, the thinning out and lateral expansion of the
dorsal wall of the tube gives to sections of this and the next (fourth) vesicle the
shape of an irregular triangle, or shield, the base of the triangle being directed
towards the dorsum (roof) and the sides bent more or less sharply inwards about
their middle to unite with one another ventrally at the apex of the triangle (figs.
72, 73). This bend serves to mark a division of each side of the tube into
two parts, a dorso-lateral and a ventro-lateral, which correspond, both in their
situation and in their relationship to afferent and effbrent nerves, with the alar
and basal laminae of the embryonic cord (p. 60), with which they are in fact
continuous. The thinning out and lateral expansion of the roof in the region
of the fourth ventricle tends to open up the angle wlii(;h the ventral laminae
form with one another, and to throw the dorsal lamina} more to the side, so that
what were previously the lateral boundaries of the neural tube come to occupy the
80-called floor of the fourth ventricle, and since in this region the roof becomes

NK

Fig. 70. — Profile views of the brain of human embryos at three several staoes,

RECONSTRUCTED FROM SECTIONS (His).

A. Brain of an embryo of about 15 days (the embiyo itself is shown in fig. 117) magnified 35
diameters.

B. Brain of an embryo about three and a half weeks old. The optic vesicle has been ciit 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, first secondary vesicle ; Z, thalamencephalon,
second secondary vesicle ; M, mid-brain ; /, isthmus between mid- and hind-brain ; Hh, fourth
secondary vesicle ; iV, fifth secondary vesicle ; Gh, otic vesicle ; Uf, fourth ventricle ; Nlc, neck
curvature ; Br, pons curvature ; Pm, mammilary process ; Tr, infundibulum ; Hp (in B), outline of
hypophysis-fold of buccal epiblast ; M, olfactory lobe. In C the basilar artery is represented along its
whole course.

DEVELOPMENT OF THE MEDULLA OBLONGATA. 65

reduced to a thin layer of flattened epithelium, the substance of this part of the
medulla oblongata is wholly formed by a thickening of the shifted lateral boundaries
In these, the bend marking the distinction between the ventral and dorsal laminse—
now by change of position mesial and external— continues to be evident, and is in
fact recognizable even in sections of the fully-developed brain.

Of the longitudinal columns of the medulla oblongata the restif orm bodies fir«t become
promment (third month m the human embryo). The Canterior) pyramids are obvious in the fifth
month, and the oUvary tubercle about the sixth. But before any of these, and indeed with

Fig. 71. — Sections across the region op the calamds scriptorius of the brain represented

IN fig. 70, A. (His.)

A, region of the glos.sopharyngeal ganglion,

B, of the auditory-facial ganglion.

Fig. 72. — Sections across the fourth ventricle op a somewhat older bhbrto (Hia.)

A, section tiiken through the lower part.

B, acro8s the widest part (trigeminu.s region),

C, through upf>er part (cerebellar region).

r, TO<j{ of neural canal : al, alar lamina ; W, basal lamina ; r, ventral border.

Fig. 73. — Sf,ction3 across the lowek half op the fourth ve.vtricle op a still olpkr emhrvo,

SHOWIS'J GRADUAL OI'ENINO OUT OK TUK NEURAL CANAL AND THE COMMKNCINO FOLDING OVER OP
THE ALAR LAMINA (at/),

r, ventral border ; t, taenia ; ot, otic vesicle ; rl, recessus labyrinth!.

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

VOL. I. F

CEREBELLAE VESICLE.

the earliest appearance of the nerve roots, tlie white bundles — not yet medullated, how-
ever— which are known as the ascending root of the fifth, and the ascending root of the vagus
and glossopharyngeal (solitary bundle) begin to make their appearance, both being at first on
the surface of the medulla. They gradually, however, become covered in by a folding over of
the dorsal part of the alar lamina, and thus come later to lie imbedded in the substance of
each lateral half of the medulla. This fold is shown in its commencement in fig. 73, A and
B, /. According to His the bundles grow downwards towards the spinal cord from the places
of entrance of the corresponding nerve roots, emerging from the ganglia, as in the case of the
posterior spinal roots ; and after entering the medulla grow gradually along the course of the
future so-called ascending roots, so that the latter are at first visible only in sections taken,
near the places of entrance of the nerve roots into the medulla.

The fourth cerebral vesicle : cerebellar vesicle, or epencephalon. — The

constriction, which is at first obvious between this and the fifth vesicle, does not
long persist, so that the two together form a long boat-shaped cavity which becomes

Fig. 74. — Median section through

THE BKAIN OF A TWO AND A
HALF MONTHS FCETUS. (His.)

Magnified 5 diameters.

The mesial surface of the left
cerebral hemisphere is seen in the
upper and right hand part of 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 geni-
culate body.

olf, olfactory lobe ; p, pituitary
body ; c q., corpora quadrigemina ;
cb, cerebellum ; m. o. , medulla ob-
longata.

the fourth ventricle. As in
that part of this cavity which
has already been described with the fifth vesicle, the roof inferiorly becomes greatly
thinned and expanded. Superiorly the tube becomes gradually more contracted and
the roof thicker, this thickening being the rudiment of the cerelellum and of the
valve of Vieussens (fig. 74). In the meanwhile a considerable thickening of the
lateral boundaries, which, as in the medulla oblongata, have been thrown outwards
by the roof expansion, occurs, and from this the substance of the pons is gradually
formed.

The dorsal and ventral laminse of the lateral walls are still evident in this part of
the embryonic brain. With the former, the sensory fibres of the fifth nerve are
immediately connected ; with the latter, the motor fibres of the fifth and also the
sixth and seventh nerves.

In the human embryo the cerebellum is seen as early as the second month,
forming a thin plate arching over the anterior part of this vesicle (fig. 74). From
this plate, which enlarges only gradually, is formed the middle lobe ; later the lateral
lobes grow out at the sides. The cerebellar surface is at first smooth, but a sub-
division into the subordinate lobes occurs in the fifth month, and the folia appear
about the sixth. In the seventh month all the parts of the organ, except the
amygdalae, are formed.

Of the cerebellar peduncles, the inferior appear in the third month, the middle in
the fourth, and the superior an the fifth. The transverse fibres of the pons develop
pari;passu with the lateral lobes, appearing about the fourth month.

THE SECOND AKD THIRD CEREBRAL VEHICLES.

67

The third cerebral vesicle: mesencephalon: mid-brain. — In this region
no expansion of the vesicle with thinning of the roof occurs, as in the others, but, on
the contrary, the roof undergoes considerable thickening (fig. 71). About the third
month, this thickening becomes separated into two by a median groove. These cor-
respond with the corpora bigemina of lower vertebrates ; it is only in mammals that
they become further subdivided by a transverse furrow. This appears in man about
the fifth month, and the eminences, which are at first large in proportion to the size

Fig. 75. — FCETAL BRAIX OF THE
THIRD MONTH. (His.)

The brain is represented in
profile, but the external '.yall of
the right hemisphere has been
removed to show the interioi- of
the Uiteral ventricle with the cor-
pus striatum curving round the
bend of the fossa of Sylvius. The
curved projections above the cor-
pus striatum are infoklings of the
mesial wall of the hemisphere
vesicle. The lettering as in tig. 74.

of the brain, thus become
the corpora quadrigemiiia.

The fibres of the third
nerve originate in the ven-
tral lamina of this part of
the neural tube : the ieg-
mentum and crusta. become ra.o.
formed as thickenings along
the same lamina : the vesi-
cle itself becomes the aque-
duct of Sijlvius.

In the constriction between the third and fourth vesicles (isthmus of His) the
fourth nerve takes origin in the ventral plate of the neural tube.

The second cerebral vesicle (thalamencephalon). — It is from this part
of the neural tube that the irrbnarii optic vesictcs are developed in the earliest
period, and they are for some time in free communication with its cavity along the
hollow optic stalks. But with the formation of the optic nerves and optic tracts,
the stalks become solid, and are, moreover, connected posteriorly with the mid-brain
by a prolongation backwards of the tracts. The optic thalamus of each side is
formed by a thickening of the lateral wall of the vesicle (figs. 74, 78). The interval
between the thalami forms the cavity of the Vdrd ventricte. Across it the grey
commissure subsequently stretches. Tlie floor becomes prolonged downwards into
the infundibulum, and takes part in the formation of the pituitary lodij (figs, fil), 74).
The roof, on the other hand, becomes like that of the fifth vesicle, thin and expanded,
and remains as a single layer of flat epithelium cells inflected into the ventricle and
sulxsequently occupied by vascular growths of pia mater (choroid plexus of third
ventricle, fig. GIJ, civ-). But at the posterior part of the roof there is a transverse
thickening to form the posterior commissure, and in front of this the roof grows
upwards and forwards, but subscfjuently backwards (in man) as a hollow median
process to form iht pineal (jland (epi])hysis cerebri). The median process soon takes
on a tubular shape (fig. C9, pin), and, after a time, becomes branched, and forms a
numlxjr of tubular follicles lined by ciliated epithelium, and invested by vascular pia
mator. These follicles tend, in man arid mammals, as development proceeds, to
become solid and occupied by calcareous deposit. But in some reptiles the pineal

F 2

68

THE riRST CEREBRAL VESICLE.

tube remains single, and appears as the long stalk — ^partly hollow, and partly solid
— of a rudimentary median or parietal eye, which occupies an aperture in the middle
line of the skull (de Graaf, Baldwin Spencer). The pituitary body (hypophysis
cerebri) is chiefly formed by a diverticulum of the buccal epiblast (diverticulum of
Eathke) which grows upwards towards the base of the second cerebral vesicle, and
dilates into a flask-shaped expansion which is at first simple, but subsequently grows
out to form a small mass of epithelial tubes, the lumen of which becomes eventually
obliterated. Against the posterior wall of this flask-shaped dilatation the infun-
dibulum grows down from the floor of the second vesicle, and its extremity becomes

Fig. 76. — Median sagittal section of the head in eaelt embryoes of the rabbit. Magnified..

(From Mihalkovics.)

A. From an embryo five millimetres long.

B. From an embryo six millimetres long.

In A, the faucial opening is still closed ; in B, the septum is perforated at /; c, anterior cerebral
vesicle ; mc, mesencephalon ; mo, medulla oblongata ; in, medullary epiblast ; if (in B), infundibulum ;
spc, spheno-ethmoidal, ic, sphenoidal, and spo, spheno-occipital parts of the basis cranii ; i, foregut ;
ck, notochord ; py, buccal pituitary involution ; am, amnion : 7i, heart.

Fig. 77. — Median sagittal section of tpe infundibulum and pituitary diverticulum in a

RABBIT EMBRYO, AFTER THE OPENING OP THE FAUCES. (From MihalkovicS. )

ic, basis cranii with basilar artery; if, infundibulum; tha, floor of thalamencephalon ; py, pituitary
diverticulum, now closed ; p', stalk of original communication with the mouth ; ph, pharynx ;.
ch, notochord in the spheno-occipital part of the cranial basis.

intimately connected with the dilatation, but without communicating with its.
cavity, although bound up together by the same vascular connective tissue. In
connection with this extension of the infundibulum, nerve cells and fibres become
formed ; in lower vertebrates they persist and retain their connection with . the
brain. The notochord extends in the basis cranii as far as the pituitary body. Just
before reaching this, it bends ventralwards towards Eathke's diverticulum, and here,
blends with the buccal epiblast (Bonnet).

Dohrii has shown that in Petromyzon the hypophysis developes as a separate median.
diverticulum of the external epiblast which is formed between the nasal pit in front and the
buccal invagination (stomodseum) behind, and grows straight backwards as a canal of some
length towards the point of the notochord, where follicles develope from it and become con-
nected with the infundibulum. Later, its orifice is found to open in common with that of the-
nasal pit.

The first cerebral vesicle : prosencephalon. — This is ifepresented by the
common point at the front of the third ventricle, whence the hemisphere vesicles
diverge through the foramina of Monro, and by these vesicles themselves. The
original vesicle is therefore relatively small, although its lateral outgrowths form by

THE FIRST CEREBRAL VESICLE.

G9

far the largest portion of the brain in higher vertebrates. The corpora striata
appear as thickenings of the floor of the hemisphere vesicles, and outside them the

.^^

r./?7.

Lm.

Fig. 78. — Three sections throooh the fore-brain op a four and a half weeks embryo. (His.)
A. Throut,'h the lower anterior part of the fore-brain ; S, falx : Sf. fold of roof passing below falx

towarfJH the third ventricle ; Bf, fold forming the sulcu.s amnionic ; v.JU, h.Kl., anterior and jioHtcrior

jxirtH of olfactory lobe ; (J», corjjus striatum ; 0. IF, groove continuous with optic .stalk ; J'.s, itms

Bubthalamica ; T.e, tuber cinereura.

li. Kection a littlo further back. Sf in replaced by a lesH promiiioiit but broader fold of the roof,

Ad, which subsequently receives the choroidal vessels, and is therefore tlic choroidal fold ; J/s,

hemisphere vesicle ; 7'h, thalamus ; S.M, sulcus of Monro, below and behind the thalamus.

C. Still further back ; Ad, choroidal fold here projecting into lateral ventricles, but still free from

mesoblaBt and bloodvesaelB ; Ma, mammillary tubercle. The other lettering as before.

70

THE FIRST CEREBEAL VESICLE.

grey and white matter of the island of Keil becomes differentiated. The rest of the
wall of the hemisphere vesicle {inantle of Reichert), although remaining for a time
thinner than the floor, eventually thickens to form the whole of the grey and white
matter of the hemisphere, except along the line where the mesoblast, which is to
form the choroid plexus of the lateral ventricle, passes into the choroidal fold of the
neural epiblast, which becomes thinned out over the invading mesoblast and con-
verted into the epithelial covering of the plexus.

The growth of the hemispheres takes place gradually. They extend at first some-
what forwards and upwards, separated by a thin layer of mesoblast which forms the

f. 7i

Fig. 79. — Transverse sections
through the brain of a
sheep's embryo op 2 "7 cm. in
LENGTH. (From Kolliker.)"

In A tlie section passes througli
the foramina of Monro, in B through
the third and lateral ventricles some-
what further back, st, corpus stria-
tum ; ill, optic thalamus ; t, thu'd
■ventricle ; c, c', rudiment of internal
capsule and corona radiata ; I, lateral
ventricle with choroid plexus, ipl ;
h, hippocampus major ; /, primitive
falx ; a, orbito-sphenoid ; m, i^re-
sphenoid : p, pharynx ; c/i, chiasma ;
o, optic nerve ; in, m, foramina of
Monro ; to, optic tract ; mk, Meckel's
cartilage.

falx, but soon begin to pass
progressively backwards, so
that by the end of the third
month they have covered the
region of the optic thalami, by
the fourth month they have
reached the corpora quadri-
gemina, and by the sixth they
cover not only the corpora
quadrigemina, but also a great
part of the cerebellum, pro-
jecting even beyond this by
the end of the seventh month.
In front, and for some dis-
tance backward over the roof
of the third ventricle, where
the vesicles are not separated
by the falx, their mesial sur-
faces come into contact, and,
during the third month, partly
grow together, but in such a
manner as to leave anteriorly
just in front of the third
ventricle, a triangular area
free in the middle, but completely surrounded at its periphery by the united parts.
Thus is formed the cavity which is known as the Jlfth ventricle or ventricle of the
septimi lucidwn, which at no time has any communication with the vesicles of the
cerebral hemispheres, nor with any other of the cerebral vesicles.

.^ -

THE FIRST CEREBRAL VESICLE.

71

The commissures of the cerebral hemispheres are also formed in the united portion
of the mesial walls of the vesicles ; the anterior is the earliest to appear, thus
coinciding with the early appearance of the corpora striata, which it unites in front.
The anterior part of the fornix, with its pillars, and the corpus albicans (which is
at first single and median) are next formed, followed at a later period by the
posterior pillars, which are seen running backwards on each side into the comu
ammonis as soon as this stracture becomes distinct. The corpus callosum is the
last of the commissures to be formed. Its anterior part appears first, but as the
hemispheres extend backwards the formation of the commissure accompanies the
backward extension.

The olfactory lobes are formed as hollow outgrowths from the lower and
lateral parts of the hemisphere vesicles (figs. 70, 74). In man they soon show a
division into two parts, an anterior and posterior ; of which the latter remains in
close connection with the hemisphere vesicle, while the anterior gi'ows out towards

Fig. 80. — The surface of the fcetal brain at six months. (R. Wagner.)

This figure shows the formation of the principal fissures. A, from above ; 13, from the left side.
F, frontal lobe ; P, parietal ; 0, occipital ; T, temporal ; o, a, a, slight appearance of sulci in the
frontal lobe ; «, Sylvian fissure ; s', its anterior division ; within it, 6', the central lobe ; r, Rolandic
sulcus ; jj, parieto-occipital fissure.

Fig. 81. — View of the inner surface of the

RIGHT HALF OF THE FfETAI, BRAIN OF ABOUT

SIX MONTHS. (Reichei-t.)

F, frontal lobe ; P. parietal ; 0, occipital ;
7*, temporal ; /, olfactory bulb ; //, optic
nerve ; fj>, calloso-raarginal fissure ; p, i/, parts
of the parieto-occipital fissure ; h, calcarine
fissure ; tj, (j, gyrus fomicatus ; c, c, corfjus
callosum ; s, septum lucidum ; /, placed be-
tween the middle commissure and the foramen
of Monro ; r, in the upjjer part of the third
ventricle ; ^•', in the Ijack part of the third
ventricle ; t*", in the lower part of the third
ventricle above the infundibulura ; r, rectssus
pincaiis ; pv, pons Varolii ; 6'e, cerebellum.

the olfactory area of the external epiblast. After the first month these lobes are
relatively small in sizf; and their cavities become gi'adually obliterated, but in some
animals, as in the horse, they are large, and their cavities are permanently in
communication with the anterior cornua of the lateral ventricles. Their further
devolopuir.iit i.s given siib.s<;'|iiently (p. 1'.)).

Formation of the fissures and convolutions. — The enlargement of the
cranium does not always keep pace with the growth in extent of the walls of the

72

THE FPvlST CEREBRAL VESICLE.

hemisphere vesicles, so that it happens that the former are thrown into folds
separated by sulci, and the surface loses its smooth appearance. Such relatively
Tapid growth occurs during the second and third month in the human embryo,
resulting in the production of a number of infoldings of the surface, which are
mostly transverse to the (bent) axis of the brain, although one or two on the mesial
surface run parallel to that axis. These infoldings of the surface, which may be
termed temporary ov primitive sulci, necessarily have a corresponding projection into
the cavity of the thin-w^alled hemisphere vesicle (fig. 75). During the fourth month,
probably owing to a relatively more rapid expansion of the cranium, most of these
primitive sulci become obliterated, and the cerebral surface is again almost smooth.
Three, however, of the primitive sulci remain as permanent fissures of the brain,i
and since the fissure of Sylvius is also now formed, although in a somewhat different
manner, the hemisphere of the human foetus at the beginning of the fifth month is
marked by four well characterised sulci having corresponding projections into the
interior of its cavity. These permanent primitive sulci are the following : —

1. The hippocampal sulcus, corresponding with the projection of the cornu
ammonis (hippocampus) into the lateral ventricle.

%no.p^

Joar. occ.

chic.

Fig. 82.-

-FCETAL BRAIN OF THE BEGINNING OF THE EIGHTH MONTH. (MihalkovicS. ]

A., from above ; B., from the side ; C, mesial surface.
Eo, Rolandic sulcus : Sy, Sylvian fissure ; par.occ, parieto-occipital ; calc, calcarine ; p?-.c, precentral",
'pll, parallel ; int.par, intraparietal ; call. mar, calloso-marginal ; 2i,nc, uucus.

2. The parieto-occipital sulcus, corresponding with the bend of the posterior
cornu of that ventricle,

8. The calcarine sulcus, corresponding with the projection of the calcar avis.
4. The Sylvian fissure, corresponding with the curve of the lateral ventricle.
To these may be reckoned the longitudinal infolding of the mesial wall of the

1 It is, however, uncertain whether the temporary sulci develope into or whether they are replaced
by corresponding permanent sulci. See on this subject a paper by D. J. Cunningham in the Journal of
Anatomy, April, 1890. ^ r r j s

DEVELOPMENT OF THE NERVES. 73

hemisphere just below the hippocampal sulcus, which is caused by the ingrowth of
the choroid plexus of the lateral ventricle.

According to Ecker. the fissure of Sylvius is the first of the primitive sulci to appear. It is
visible before the end of the third month, as a wide, shallow depression, Avhich divides the lower
margrin of the hemisphere into two nearly equal portions, and at the bottom of which is seen
the thickening of the floor of the vesicle from which the corpus striatum and the island of
Reil are developed. This fissure appears to be formed by a cui"ving of the still thin-walled
hemisiAere vesicle over that thickening, around which the vesicle bends ; and its anterior and
posterior parts ultimately meet along the line which marks the posterior limb of the
Sylvian fissure in the developed brain. The anterior limb is produced much later by a further
folding over of that part of the mantle which is in front of the fossa Sylvii. The fissure
remains until nearly the end of foetal life as a widely open depression, at the bottom of
which the island of Reil is readily visible. It closes gradually from behind forwards.

The other sulci are distinguished from the four above-numerated in the fact that
they are depressions of the surface merely, and not infoldings of the whole thickness
of the wall of the hemisphere vesicle. ^ They begin to appear about the end of the
fifth month, the fissure of Rolando being the first to show (figs. 80 and 81).

By the end of the sixth month the precentral and inferior I'rontal sulci, the
intraparietal, the superior occipital, the parallel, the inferior temporal, the calloso-
marginal, and the collateral fissures have become visible, as well as the anterior
limb of the Sylvian fissure.

By the end of the seventh month (see fig. 82) most of the remaining principal
convolutions and fissures have appeared, and those which were previously pi'esent
have increased both in length and depth. They are, however, all comparatively
short and simple. During the eighth month, they continue to increase in length
and depth, and the remaining sulci become gradually developed, but even in the
ninth month there are none of the accessory or secondary furrows which add so much
to the complexity of the developed brain. The last of the principal sulci to make
their appearance are the inferior occipito-temporal.

DEVELOPMENT OF THE NERVES.

Spinal nerves. — At an early period of development, in some cases even before
the closure of the neural gi'oove, in others during or shortly after that event, there

Fig. 83. — Transverse section through the trunk of an embryo

SHARK, TO SHOW THE NEURAL CREST. (Balfour.)

TIC, neural canal ; 2)r, ganglion rudiment running from neural
crest ; x, sub-notochordal rod ; ao, aorta ; sc, j arietal niesoblast ; sp,
visceral me.sobla.st ; mp, muscle plate ; nip', portion of muscle jjlate
converted into muscle ; Vv, portion of proto-vertebra wliicli will give
rise to the vertebra ; al, alimentary canal.

grows out bi-laterally from the angle of junction of the
neural with the general epiblast (fig. 89), and conse-
quently at the dorsal aspect of the neural tube a
continuous ridge or crest of epiblast, which was first de-
scribed by Balfour in elasmobranch fishes : this is termed
the neural crest. At ijitervals along the sides of the
neural crest, corresponding with the middle of each
mesoblastic somite or proto-vertebra, special clavate
enlargements or outgrowths of the neural crest occur

(fig. 8;>, jrr). These grow downwards along the dorso-lateral aspect of the neural canal,
>x;tween the protovertebra and the canal. They remain for u time attached above

■ An exception must, however, be made for tLc collateral fisBurc, which corresponds with the col-
lateral eminence within the veutriclc.

74

DEVELOPMENT OF THE NERVES.

to the dorsal aspect of the neural tube, but that attachment becomes subsequently
lost, and they then form completely isolated portions of epiblast, composed of oval
cells, and lying at the side of the embryonic cord between it and the muscle plates
of the protovertebra? (fig. 84). These are the rudiments of the posterior-root
ganglia. The remainder of the neural crest disappears : at least in most vertebrata.
Some little time after the separation of these gangHon-rudiments, the ventral or
anterior roots of the spinal nerves begin to grow out from the ventro-lateral aspect of
the neural tube. They were originally described by Balfour in elasmobranchs, as
forming bud-like outgrowths from the neural epiblast, the outgrowths being com-
posed of spindle-shaped cells (fig. 84, ar). But according to the recent and extended

Fig. 84. — Section THRorcH the dorsal part of the trunk op a tokpedo embryo. (Balfour.)
pr, g, n, spinal ganglion rudiment ; ar, anterior root ; c7i, notochord ; nc, neural canal ; mp, muscle-
plate.

Fig. 85. — Section of the tentro-lateral angle of the spinal cord of a pristiukus embryo

SHOWING THE OUTGROWTH OF AN ANTERIOR ROOT- RUDIMENT. (His.)

a.r, axis cylinder processes of neuroblasts, forming the anterior root ; g, germinal cells in innermost
part of wall of neural canal.

observations of His in various classes of vertebrates, what actually grow out to form
the anterior roots, are the fibrous prolongations (axis-cylinder processes) of neuro-
blasts (v, aniea, p. 58), which processes converge to the point of exit of the root
and penetrate gradually into the adjoining mesoblast (fig. 85, a. r), where they
come into close contact with the previously formed ganglion rudiments of the pos-
terior roots.i

The fibres of the posterior roots are developed, according to His, as processes
from the oval cells of the ganglion rudiment. These cells are in fact neuroblasts,
and from either end of each cell, which is thus rendered bipolar (fig. 86), a process
becoming eventually the axis-cylinder of a nerve-fibre grows out, one towards the
central organ, the other towards the periphery. The centrally directed processes
soon reach and grow into the embryonic cord at its dorso-lateral aspect, where they
are presently seen in sections occupying an oval area near the periphery of the cord ;

^ The place at which the anterior roots spring from the cord is not opposite to the corresponding
posterior root, but midway between that root and the succeeding one.

CRANIAL NERVES. 75

this area is the beginning of the posterior white column (fig. 87) : the further com-se
and attachments of the ingrowing fibres in the cord are not accurately known, but
they appear to bifurcate and extend both upwards and downwards (Ramon y Cajal).
The peripherally directed fibres grow downwards and join the bundle of fibres of the
anterior root, to form together with them the mixed spinal nerve (fig. 88).

The axis-cylinder processes which are to form the fibres of the anterior roots

Fig. 86.— Bipolar cell from spinal ganglion of a 4 J weeks embryo. (His.) "{»

Fig. 87.— Section of spinal cord of four weeks human embryo. (His.)

The posterior roots are continued within the cord into a small longitudinal bundle whicli is the
rudiment of the po.sterior 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 extenial layer of the cord is traversed by radiating fibres which are the outer ends
of the spongioblasts. The anterior commissure is beginning to appear.

begin to make their appearance about the beginning of the fourth week in the human
embryo (Ilisj. Their growth towards the periphery is slow; even by the end
of the second month they have not reached the tips of the fingers and toes.

Cranial nerves. — The neural crest is continuous along the dorsal aspect of the
cerebral part of the neural tube as far as, and even beyond the mid-bi'ain. As in
the spinal part, clavate enlargements occur here also, but at somewhat ii-rcgular
intervals, and form ganglion rudiments which become separated from the dorsal aspect
of the tube, and acquire a new attachment on the lateral and ventral aspect. Such
ganglion rudiments have been described for the third, fifth, seventh, eighth, ninth,
and tenth nerves.

In the chick the ganglion rudiments belonging to the cranial nerves ajipear as a
thickening of the cephalic epil>last, just where it is folding round into the as yet
unclosed neural canal (fig. 89, vg). This thickening, according to Golowine, is
continuous laterally with a modified portion of the external epiblast (sensory
cpiblastj, and soon becomes subdivided into three ganglionic groups, and these, later,
into separate ganglions. There is a corrcBponding subdivision of the sensory

76

CRAmAL NERVES.

epiblast into rudiments of special sense organs, which become gradually shifted
downwards and outwards away from the ganglion rudiments, and form the so-called

Fig. 88.

-Transveese section through the anterior part op the trunk op an ejibrto of
ScYLLiUM. (Balfour.)

sp.c, spinal cord ; sp.g, ganglion of posterior root ; ar, anterior root ; dn, dorsal ; sp.n, ventral
branch of spinal nerve ; mp, muscle plate ; mp', part of muscle plate already converted into muscle ;
mjy. I, part of muscle plate extending into the limb ; nl, nervus lateralis ; ao. aorta ; ch, notochord ;
sv.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).

"branchial sense-organs" of Beard. They become connected subsequently with
outgrowths from the posterior nerve-rudiments.

These rudimentary sense organs have been described in mammals also (by Froriep). They
appear to represent the special sense organs of the gill clefts of fishes, which were first de-
scribed by Leydig (1850). Beard is of opinion that the nose and ear are also specialised
branchial sense organs, the only ones that are persistent in higher vertebrates. This
differentiation of a special sensory portion of epiblast into rudiments of special sense-
organs, occurs according to Golowine's observations in the chick, not only in the head but also
in the trunk, vi^here they in all probability represent rudiments of " organs of the lateral line,"
such as are seen in fishes.

Various observers have described the ganglion rudiments as actually becoming formed at

CRANIAL NERVES.

77

least in part at the expense of the sensory epiblastic thickening-s. In connection with this
question it is -n-orthy of note that from the sensory thickening which forms the olfactory area
a ganglionic rudiment becomes formed which joins the olfactory lobe of the brain, and o-ives
origin to the olfactory nerve-fibres (His).

Even in the adult, as was shown by Thomsen, traces of pre-existent ganglionic structure
can be found in the root of the third nerve, and similar traces of ganglionic structure liave

-Vg'

Fig. 89. — Traxsvekse section

THROUGH THE POSTERIOR
PART OF THE HEAD OP AN
EMBRYO CHICK OF 30 HOURS.

(From Balfoiu. )

Tib, hind-brain ; vg, vagus
nerve ; ep, epiplast ; ch, noto-
chord ; x, sub-notochordal rod ;
cU, throat ; ht, heart ; pp, body-
cavity ; so, parietal mesoblast ;
sf, visceral mesoblast ; hy, hypo-
blast.

also been described by Gaskell
in the roots of the fourth
nerve, in the motor root of the
fifth, and in the root of the
seventh nerve. If these, as
Gaskell supposes, indicate the
pre-existence of sensory ele-
ments in the roots, it is pro-
bable that these nerves and
ganglia have all been origi-
nally developed like the posterior gangliated roots of the spinal nerves, as outgrowths from
the neural crest. Whether they are joined by outgrowths corresponding to the efferent fibres
of the spinal nerves, or whether they originally contain the elements of the efferent fibres,
and thus resemble the spinal nerves of Amphioxus, in which there are no anterior roots,
but both sensory and motor nerves are contained in the posterior roots, is not at present
known. What is however clear is that the ganglion cells and afferent fibres in the roots of
the third, fourth, motor of fifth and seventh nerves, eventually entirely disappear, the efferent
fibres alone remaining, while in the roots of the sixth, eleventh, and twelfth nerves efferent
fibres only are found, and ganglionic mdiments are not developed at all.

As is shown in another part of this work (Neurology) the nuclei of origin of the efferent
cranial nerves are disposed in two longitudinal series. One of these series comprises the nuclei
of origin of the somatic efferent nerves of Gaskell, which con-espond with the largest fibres
of a typical anterior spinal root, and the series of nuclei is a continuation of the cell-column
of the anterior horn ; the nuclei of this series are those of the hypoglossal, or twelfth, the
sixth, fourth, and third. The other series comprises the nuclei of origin of the splanchnic
efferent nerves of Gaskell, which correspond with the medium-sized and smallest fibres of a
typical anterior spinal root, and the series is a continuation mainly of the cells of the lateral
comu or intermediolateral bract, and partly, perhaps, of the cells of the base of the posterior
horn ; the nuclei of this series are those of the spinal accessory, those of the efferent fibres
of the vagTis and glo*iO-pharyngeal, the facial, and the motor nucleus of the fifth.

Acajrding to the observations of His, the distinction into an anterior or ventral
(somatic) and a lateral (splanchnic) grouj) of efferent fibres is well marked in the
embryo by the fact that the two kinds of efferent fibres take origin from entirely
different parts of the basal lamina of the neural tube, those which correspond witli
the somatic efferent fibres originating from groups of neuroblasts near the middle
line, while the others take their rise near tlie junction of the basal with the alar
lamina (see fig. 1)0). AVith the alar lamina itself the afferent fibres are con-
nect<id, but they do not arise from groups of ceils within it, as do the efferent fibi'cs
within the basal lamina : on the contrary, they effect their connection with the lamina
by growing into it from a ganglion in the manner already described for the posterior
roots of the spinal nerves, and they then appear in most cases to grow downwards
in the direction of the spinal cord. This is stated l)y llis to be the case with the

78

CEANIAL NERVES.

sensory root of the trigeminus, which thus forms the white bundle known as the
ascending root of the fifth, but which in its actual growth is descending ; and also
with the afferent fibres of the glosso-pharyngeal and vagus which grow downwards
in the medulla to form the so-called solitary bundle.^ These are traceable in the
adult as far down as the middle and lower cervical region respectively, but in the
embryo are at first quite short, and limited to near the place of attachment of the

Fig. 90, A and B. — Sections across the hind-brain of a human embryo, 10 mm. long. (His.) to.

In A, the origin of the spinal accessory and hypoglossal nerves is sho'wn, 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 part from
a bundle of longitudinally coursing fibres placed at the periphery of the alar lamina, and corresponding
in situation to the commencing posterior white columns shown in fig. 87.

nerve roots, becoming gradually longer as development proceeds. The solitary bundle
is at first superficial, like the ascending root of the fifth, but it subsequently becomes
covered in by the bending over of the alar plate, and the formation of nervous sub-
stance in this. The ganglion-rudiments from which the ingrowth of these afferent
fibres takes place, become the Gasserian ganglion of the fifth, and the jugular
ganglia of the glosso-pharyngeal and vagus.

The auditory nerve-roots appear also to be formed by an ingrowth from the cells
of its ganglion-rudiment into the alar plate. Subsequently the ganglion-rudiment
becomes subdivided into three parts, one forming an intracranial ganglion, and the
others giving rise to the branches of the nerve to the cochlea and vestibule respectively
(fig. 90, C). The part belonging to the cochlea (ganglion cochlea) forms ultimately
the spiral ganglion ; while the one on the vestibular branch forms the gangiiform
swelling of Scarpa. From a separated part of the ganglion cochleae the nerve to the
posterior semicircular canal passes, as well as that to the macula of the saccule ;
from the vestibular ganglion the nerves to the other ampullae and to the utricle, are
derived. The geniculated ganglion of the facial is derived from the same ganglionic

^ These bundles, therefore, are homologous with the rudiments of the posterior white columns
which are derived from the ingrowing fibres of the spinal ganglia {v. antea, p. 74).

OLFACTORY LOBE.

79

mass as furnishes the ganglia of the auditory nerve, but its cells are early distin-
guishable bv their larger size and clearer appearance. From these cells fibres, which
are probably afferent (His), grow centrally into the hind-brain and peripherally along

m.G.

TMC.v.

MIG.j.

m:.G.c.

Fig. 90, C. — Section fkom the same embryo at the exit of the facial nerve. (His.)
(ucveral sections have been combined to form this figure. )

VL, fibres of sixth nerve taking origin from group of neuroblasts in basal lamina ; Nll.G.g, gang-
lion geniculi of the facial ; YIII.6''.i.c, intracranial ganglion of auditory ; Wll.G.v, ganglion vestibuli ;
VIIL6-'.c, ganglion cochlea;.

the nerve, mingling with its efferent fibres. Some of these afferent fibres may form
the chorda tympani, but there are many more than are found in that nerve.

The optic nerves take origin as hollow outgrowtlis of the brain, which afterwards
become soh'd, while nerve-fibres become developed in their walls. Their mode of
origin will bo further treated of in connection with the development of the eye.

The olfactory lobe, which consists of the olfactory bulb and tract (often
spoken of as the first or olfactory nerve), and the part of the base of the brain
from which the tra^t arises, makes its appearance as a jn-otrusion of the antero-
ventral part of each cerebral hemisphere, extending towards the thickened olfactory
area of the cpiblast (see fig. 70, B, III ; and fig. 74, olf). This primitive olftictory
lobe is early seen to be divided into anterior and posterior pai'ts by a broad sulcus
(fig. 'Jo). Of the two parts, the anterior becomes considerably elongated, and
ultimately forms not only the tract and bulb but also the trigonum ollactorium,
and a small area on the mesial side of this (termed by His BroccCs area) ; whilst
from the jxtsterior, the larger part of the anterior perforated space (mesial to
the lateral olfactory root) and the peduncle of the corpus callosum (gyrus

Fig. 91. — Ckakial nerves of a hujian embryo, 10.2 mm. long. (His.) "f.

The cranial nerves are indicated by Roman, the spinal nerves by Arabic numerals.

c. A, cerebral hemisphere ; th, thalamencephalon ; m.b, mid-brain; Mx, maxillary process; Mn,
mandibular arch ; ffi/, hyoid arch ; the facial nerve is seen to send a branch (chorda tympani) across
the hyoniandibular cleft ; G.g, Gasserian ganglion ; eg, ciliary ganglion ; v, vestibular, and c, cochlear
part of auditory ; g.p, ganglion petrosum of glossopharyngeal : g.j, ganglion jugulare of vagus ; an
anastomosis is seen between these ; g.tr, ganglion trunci of vagus ; F, ganglion described by Froriep as
belonging to the hypoglossal ; r.d, ramus descendens of hypoglossal • ot otic vesicle. The eye is also
represented, and a part of the heart.

Fig. 92. — Diagram showing the centripetal and CENTRiPtTGAL roots of the cranial nerves op

THE SAME EMBRYO. (His. )

The places of exit of the nerves are marked by dotted circles or ovals. The efferent nerves (///.,
IV, mV, VI, VII, part of IX, XI, and XII), are seen to arise v;ithin the nerve centre from groups of
neuroblarsts ; the afferent fibres {V.s, VIII, v and c, most of IX, and X), pass a certain distance in-
wards, and for the most part also caudalwards in the nerve-centre, and there end. The ganglion rudi-
ments from which they have grown are not shown here. They will be found in the preceding figure.

SYMPATHETIC NERVES. 81

subcallosus of Zuckerkandl) are developed. These parts are separated even in the
adult by a sulcus, along which the mesial or internal olfactory root runs.

The olfactory nerve fibres arise, according to His, from neuroblasts which
become formed within the thickened epiblast of the olfactory area (see p. 95).
This epiblast at a certain period of development resembles the neural epiblast, and
whilst some of the cells become spongioblasts, others become pear-shaped, or spindle-
shaped, and their processes grow as nerve fibres towards the olfactory lobe. Not
only, however, do these fibres emerge from the olfactory epiblast, but some of the
ueuroblasts themselves also pass out, and these form a ganglion which lies between
the olfactory lobe and the olfactoiy area. Subsequently this ganglion, the cells
of which are prolonged at either end into nerve fibre processes, becomes attached
to and partially invests the olfactory bulb, with which it ultimately blends, forming
the part whence the olfactory nerve fibres pass to the Schneiderian membrane
(layer of olfactory-glomeruli and nerve fibres), whilst bands of fibres on the other
hand grow centripetally and become the olfactory roots. These are in fact
comparable to the centripetal (so-called " ascending ") roots of the trigeminus
glossopharyngeal, and vagus.

It is not until the third month that the part of the olfactory lobe which forms
the bulb, begins to grow forwards away from the trigonum, and thus to form the
olfactory tract.

The cranial nerves, except the optic and olfactory, and the relations they bear to one
another and to the visceral arches of the head, are shown in fig-. 91 as they occur in the human
embryo of about four weeks. Fig. 92 distinguishes diagrammatically the nerves which o-row
into the nerve centres (centripetal or afferent nerves) from those which grow out from the
centres (centrifugal or efferent nerves), and the extent of growth inwards of the former in the
same embiyo.

The sympathetic neirves and ganglia. — That these are merely outgrowths
of the cerebrospinal nervous system, nearly all recent observations, both morpho-
logical and physiological, clearly show. But even before this fact had come to be
generally recognized, it was known that they are developed in connection with the
spinal nerves (Balfour), and indeed as offshoots from the posterior spinal ganglia
(Schenk and Birdsell, Onodi). They appear for a time as enlargements upon the
main stem of each spinal nerve, but afterwards become connected with this by a
short branch (r. communicans) (fig. 88), and with one another by a longitudinal
commissure. The branch in question contains the splanchnic fibres of the spinal
aerve, and the sympathetic ganglia are its splanchnic or vagrant ganglia (Gaskell).
The splanchnic ganglia of the cranial nerves are probably formed in a similar way,
but their mode of development has not as yet been worked out.

The ciliary ganglion appears to be formed as an outgrowth of the Gasserian ganglion (fig.
91, e.g.) much in the same way as the sympathetic trunk ganglia are formed as offshoots of the
posterior spinal ganglia. In elasmobranchs it is derived from the ophthalmicus profundus
gfanglion, itself an offshoot of the Gasserian (Ewart).

Paterson has recently described the sympathetic chain of ganglia as developing in mammals
Trodents) from a continuous rod of mesoblast lying on either side of the aorta, and as becoming
only secondarily segmented and connected with the cerebro-spinal nerves. But observations
upon earlier embi-yos than were used by Paterson are necessary before the mesoblastic origin
of the rod can be admitted.

RECENT LITERATURE.

Ahlbom, IJiJier die Bedeuturuj der Zirheldruse. Zeitschr. f. wissonscli. Zonlogio. V>i\. xl., 1884.

Barnes, Will., On tlie development of the posterior fissure of the spinal cord and the reduction of
the ceriXrid canal in the pig. Proc. Americ. Acad. Arts and Sciences, 1884.

Beard, J., On the cranial ganrjlia and sef/mental sense organs of fishes. Zoolog. Anzeiger, 1885 ;
The Hystem of branchial sense organs and their associated ganglia in Ichthyopsida. Quarteily Journal
of Micr. Science, 1885 ; The devebiprnent of the p/eripheral nervous system of vertebrates. Quarterly
Journal of .Micr. Science, Oct., 1888.

Bedot, M., liecherchea aur le divcloppement des ncrfs spinaux ehez les Tritons. Ilccueil zoolog.
SaisHC, i., 2, 1884.

VOL. /. O

82 RECENT LITERATURE.

Beer, B., On the development of the Sylvian fissure in the human embryo. Journal of Anatomy
and Physiology, 1890.

Beraneck, E., Recherches sur le diveloppement des nerfs crdniens chez les lizards. Eecueil
zoolog. Suisse, 1884 ; £tude sur les replis medullaires du poulet. Recueil zoolog. Suisse, iv., 1887.

CMarugi, Sullo sviluppo di alcuni nervi cereirali e spinali. Anat. Anzeiger, 1889.

Cunning'h.am, D. J., The complete fissures of the human cerebrum, and their significance in
connection with the growth of the hemisphere and the appearance of the occipital lobe. Journal of
Anatomy, April, 1890.

Dolirn, A., Die Entstehung der Hypophysis bei Petromyzon Planeri. Mittlieil. aus der zoolog.
Station zu Neapel, iv., i., 1883 ; Ueber die erste Anlage und Entwicklung der motorischen Riiclcen-
marksnerven bei den Selachiern, Mittlieil. aus der zoolog. Station zu Neapel, Bd. viii., 1888.

EAwart, J. C, On the development of the ciliai'y or motor oculi ganglion. Proc. Eoy. Soc. xlvii.,
1890.

Proriep, A., Ueber ein Ganglion des Sypoglossus u. Wirbelanlagen in der Occipitalregion,
Arch. f. Anat. u. Physiol., Anat. Abth., 1882 ; Ueber Anlagen von Sinnesorganen am Facialis, Glosso-
pharyngeus und Vagus und iiber die genetische SteU/ung des Vagus zum Hypoglossus und iober die
HerTcunft der Zungenmusculatur, Arch. f. Anat. u. Physiolog., Anat. Abthl., 1885.

Goette, A., Ueber die Entstehung und die Homologien des Hirnanhangs, Zool. Anzeiger, 1883.

Grolowine, E., Sur le developpement du, systhme ganglionnaire chez le poulet, Anat. Anzeiger, 1890

Grraaf, H. W. de, Bijdrage tot de kennis van den bouw en de ontwikkeling der epiphyse bij
Amphibien en Eeptilien. Proefschrift, Leiden, 1886.

His, "W., Ueber das Auftreten der weissen Substanz und der Wurzelfasern am RiickenmarTc
menschlicher Embryonen, Archiv f. Anat. u. Phys., Anat. Abth., 1883 ; Zur Geschichte des mensch-
lichen Riickenmarks und der Nervenwurzeln, Abhandl. d. math. -phys. Kl. d. kgl. Sachs. Gesellsch. d.
Wissensch., Bd. xiii.. No. 6, 1886 ; Die Entwickhing der ersten Nervenbahnen beim menschlichen
Embryo. Uebersichtliche Darstellung, Archiv f. Anat. u. Physiol., Anat. Abth., 1888 ; Zur Geschichte
des Gehirns sowie der ventralen und peripherischen Nervenbahnen beim menschlichen Embryo, Abhandl.
d. math. -phys. Kl. d. kgl. Sachs. Gesellsch. der Wissensch., Bd. xiv., 1888 ; Die Formentwicklung des
menschlichen Vorderhirns, Abhandl. d. konigl. Sachsischen Gesellschaft, Bd. xv., 1889 ; Die Neuroblasten
u. der en Entstehung im embryonalen Mark, Abhandl. d. konigl. Sachs. Gesellschaft, Bd. xv., 1889.

His, W., jun. , Zur Entwicklungsgesch. d. Acustico-facialis-gebietes beim, Menschen, Arch. f. Anat.
u. Physiol., Anat. Abth. 1889.

HofEmann, C K. , Ueber die Metamerie des Nachhirns u. Hinterhirns u. ihre Bcziehung z, d.
segmentalen Kopfnerven bei Reptilienembryonen, Zool. Anzeiger, xii.

Johnson, A., and Sheldon, Lilian, On the development of the cranial nerves of the newt, Pro-
ceed, of the Eoyal Society, vol. xl., 1887.

Kaczander, J., Ueber die Beziehungen des Medullarrohrs zu dem Primitivstreifen, "Wiener
Medicin. Jahrb., 1886.

Kraushaar, R., Entwicklung der Hypophysis und Epiphysis bei Nagethieren, Zeitschr. f.
wissensch. Zoologie, 1884.

Kupffer, Primdre Metamerie der Neuralrohrs der Vertebraten, Munchen. Sitzungsb. , Bd, xv.

McClure, The primitive segmentation of the vertebrate brain, Zool. Anzeiger, xii.

Marshall, A. Milnes, On the early stages of development of the nerves in birds. Journal of Anatomy
and Physiology, 1877 ; The development of the cranial nerves in the chick. Quarterly Journal of Microsc.
Science, 1878 ; On the head cavities and associated nerves of elasmobranchs, Quarterly Journal of
Microsc. Science, 1881.

Mihalkovics, v., Wirbelsaite u. Hirnanhang, Archiv f. mikr. Anatomie, 1875 ; EnticicklungS'
geschichte des Gehirns, Leipzig, 1877.

Onodi, A. D., Ueber die Entwicklung der Spinalganglien und der Nervenwurzeln, Internat,
Monatsschr. f. Anat. u. Histologic, i., 3, 1884 ; Ueber die Entwicklung des sympathischen Nerven-
systems. Arch. f. mikr. Anat., Bd. xxvi., 1885.

Orr, H., Note on the development of Amphibians, chiefly concerning the central nervous system, die.
Quarterly Journal of Micr. Science, xxix., 1889.

Osborn, The origin of the corpus callosum, Morph. Jahrbuch, 1887.

Paterson, A. M., On the fate of the muscle-plate, and the development of the spinal nerves and
limb p)lexuses in birds and mammals, Quarterly Journal of Micr. Science, Aug., 1887 ; The development
of the sympathetic nervous system in mammals. Proc. Roy. Soc, April, 1890.

Rabl, C, Bemerkung iiber die Segmentirung des Hirns, Zoolog. Anz., 1885.

E,abl-Euckard, Gehirn der Knochenfische, Arch. f. Anat. u. Physiol., Anat. Abth. 1882 und
1883.

Robinson, A., On ike developiment of the posterior columns, of the posterior fissure, and of the
central canal of the sp)inal cord. Owens' College Studies, 1890.

R-iiding-er, Ueher die Bildung der Augenblasen, Sitzungsb. der Gesellsch. f. Morphol. zu Munchen,

Spencer, B., On the presence and structure of the pineal eye in Lacertilia, Quarterly Journal of
Micr. Science, 1886.

Strahl, H., und Martin, E., Die Entioicklung des Parietalauges bei Anguis fragilis und Lacerta
vivipara. Arch. f. Anat. u. Phys., Anat. Abth. 1888.

Vignal, "W., Sur le diveloppement des elements de la moelle des mammif&res. Archives de physiol.
1884 ; Recherches sur le developpement de la' substance corticate du cerveau et du cervelet, Archives de
physiologic, 1888.

"Wijhe, J. "W. van, Ueber Somiten und Nerven im Kopfe von Vogel- und Reptilienembryonen,
Zoolog. Anzeiger, 1886. i-J a i if >

Zuckerkandl, E,, Ueber das Riechcentrum, Stuttgart, 1887.

DEVELOPMEXT OF THE EYE.

83

DEVELOPMENT OF THE EYE.

The first development of the eye occurs as a hollow protrusion of the anterior
cerebral vesicle — primary optic vesicle — in the manner akeady mentioned (see
fio-. 68, e, and fig. 93). The vesicle thus formed abuts externally against the
external epiblast of the side of the head (fig. 96) ; and this external epiblast opposite
the most prominent point of the primary optic vesicle, becomes thickened and in-

Fig. 93, A. — Brain of chick op 2nd day,

VIEWED FROM BELOW, TO SHOW THE FORMA-
TION OF THE OPTIC VESICLES BY OUTGKOWTH
OF THE SIDE OF THE FOKE-BRAIN, AND AT
THE SAME TIME BY THE FOLDING OVER OF
THE ENLARGED PART, THE PRODUCTION OF
A GROOVING OR CUPPING OP THE VESICLES.

(His.)

f.hr., m,.lr., h.br., fore-, mid-, and liind-
brain ; opt, optic vesicle ; i, infundibulum.

Fig. 1'3, B. — Brain op human embryo op

THREE WEEKS, SHOWING THE PRIMARY
OPTIC VESICLES AS OUTGROWTHS FROM THE
FOKEBRAIN. (His.)

cl£

ope

Fig. 94. — Side view of anterior

PART OP BRAIN OF MORE ADVANCED
HUMAN EMBRYO, SHOWING THE
PRIMARY OPTIC VESICLE FOLDED
AND CUPPED. (His.)

c.A, cerebral henii.spliere (part of) ;
olf., olfactory lobe ; (jyt., o^jtic cup.

t.'c.

c/r,

■F-

-opt.

Fig. 95. — Side view of the same part of

THE brain in a STILL MORE ADVANCED
EMBRYO, THE EYE HAVING BEEN CUT AWAY.

(His.)

opt., cut end of optic stalk, showing the
manner in which it is folded ; i, infundibu-
lum ; olf. p., posterior part of olfactory lobe ;
olf. a., anterior part of the same ; c./t. , cere-
bral hemisphere ; t.c, tubes cinereum.

vaginated, so as to fonn at first a hollow cup-shaped depression with thickened walls
(figs. 97, 98), and subsequently Ijy the closing in of the epiblast at tlie mouth of the
cup, a hollow island of epithelial cells (fig. 99). This island, which is the rudimentary
lens, lies between, but is entirely distinct from the external epiblast on the one hand,
and the neural ejtiblast of the primary optic vesicle on the other hand. Its forma-
tion is accompanied by a cupping in of the primary optic vesicle (figs. 94, 97, 98),
which is invaginated before it, and this invagination is increased by an ingrowth
of mesoljlast, whifh occurs between the lens and the cujjped optic vesicle, and
which suljsequently forms the vitreous humour. Invaginated in this way the
cavity of the original optic vesicle l^ecomes almost entirely obliterated, and appears
merely a« a cleft between the two layers which form the wall of the so-called " optic
cup." The inner of these two layers is from the first thicker than the outer, and in
it are developed all the parts of the future retina from the membrana limitans interna

S4

DEVELOPMENT OF THE EYE.

to the layer of rods and cones, while from the outer thinner layer the hexagonal
pigmented epithelium of the retina, with its continuation into the uvea, is formed.
The invagination of the primary optic vesicle does not occur only opposite the

Fig. 98. — Pa5,t of a section through the

HEAD OF AN EARLY HUMAN EMBRYO, SHOWING
THE CONNECTION OP THE PRi:\rART OPTIC VESI-
CLES WITH THE FOREBRAIN. (His.)

olf, olfactory area of epiblast ; c.h, part of
forebrain which gives rise to cerebral hemi-
spheres ; ill, thalamencephalon ; 'p.o.v, primary
optic vesicles.

Fig. 97. — Section through the same part of a

MORE ADVANCED EMBRYO, IN WHICH THE LENS IN-
VAGINATION IS FORMED, AND THE PRIMARY OPTIC
VESICLES ARE CUPPED. (His.)

o.c, optic cup ; o.s, optic stalk ; I, lens invagina-
tion ; c.li, cerebral hemisjiheres ; th, thalamencepha-
lon ; i, infundibulum ; olf, olfactory area.

772<S,

0-

C.B.

Fio-.

-Vertical section theough the middle of the developing eye op a chick of the
third day. (e. a, s. )

The section passes longitudinally through the deficiency in the lower part of the optic cup, and
shows the mesoblast extending in between the lens invagination and the pigment layer of the optic cup.

tlial, thalamencephalon; n.ep, neural epiblast; cxp, cutaneous epiblast; o.s., optic stalk ; o, o,
cavity of primary optic vesicle ; Mie, mesoblast ; v, mesoblast passing behind lens to form vitreous ;
I, lens invagination.

Fig. 99. — Section through the eye and optic stalk op a human embryo of five weeks. (His.)

W.C., connection of optic stalk with thalamencephalon ; *Sp, cleft or fold in the stalk, where the
arteria centralis retinee passes in ; P, pigment layer ; E, retina ; L, lens.

DEVELOPMEXT OF THE EYE.

85

place where the lens is becoming involuted, but also belo^y, or ventral, to that place,
so that a section exactly through the middle of the o]itic cup at right angles to the
axis of this part of the head, shows a gap in the boundary of the cup through which
the mesoblast is passing into the space between the lens and the invaginated optic
vesicle (fig. 98, v). This gap or cleft soon becomes closed, but the suture or line of
closure long remains apparent from the fact that wiien pigment begins to be de-
posited in the eye, this so-called chorouM fissure remains for some time unpigmented
(until the sixth week in man).

The ventral invagination is in mammals continued for a considerable distance
into the stalk of the optic vesicle (fig. 95), and the simultaneous inclosure of
mesoblastic tissue leads to the introduction of the central blood-vessels of the retina
within the optic nerve. In birds no such infolding of the stalk occurs.

Tlie lower invagination of the optic cui> serves not only to permit of the passage of meso-
blast behind the lens for the formation of vitreous humour, but also to establish a direct
connection bet\\-een the nerve-fibres which are formed along the course of the optic stalk
(future optic nerve) and the centre of the inner layer of the optic cup (future retina) (0.
Hertwig).

The malformation termed colohoiiia. iridls is attributed to a persistence of the choroidal
cleft, which extends behind the iris along with the retinal pigment or uvea, as far as the
margin of the puijii.

// ^

Fig. 100. — Horizontal section through the eye

OF AN EMBRYO RABBIT OF TWELVE DAYS ANIJ SIX

HOURS. ™. (Kolliker.)

o, optic stalk ; /<', remains of the cavity of the
])riinary optic vesicle ; />, proximal lamella of the
optic cup (pigmentmri nigrum) ; r, distal lamella
(retina) ; I, lens invagination, widely open at ol ;
I', papillar elevation in the bottom of the lens
vesicle ; m, me.sobla.st ; ;i, mesoblast of vitreous ;
i:, a blood-vessel at the anterior border of the ojjtic
cup ; e, cutaneous eiJi blast.

Fig. 101.— Eyeb.\li< of a human embryo op

EuUR \VEEKS cut ACROSS, AND THE ANTERIOR
HALK KEPRliSENTED FROM BEHIND. (Kolliker.) "i**.

fr, the remains of tlie cavity of the primary
optic vesicle ; p, outer layer forming the retinal
pigment ; r, the thickened inner part giving rise
to the columnar and other structures of the retina ;
V, commencing vitreous humour within the optic
cup ; v', the cleft through which a va.-.cular loop,
a, projects from below ; I, the lens with a central
cavity.

The hollow optic stalks are at first ft'eely in communication with the thalam-
cncephalon, or third ventricle. Nerve-fibres grow along tlieir walls, from neuroblasts
which develop in the retinal epibjast, and pa.ss towards the nerve-centre (His), and
the cavities of the stalks become thereby gradually obliterated, the radially sti'iated
epithelial-like arrangement of the wall Iteing, however, long evident. A new con-
nection Ixjcomes subsefjuentiy established between the posterior part of the optic
stalks (optic tracts) and the mesencephalon, whilst the middle parts become united
with one another to form the chiasma.

88

DEVELOPMENT OP THE EYE.

The development of the retina from the inner layer of the optic cup, has not
been fully worked out. In its earlier stages it closely resembles in structure the
wall of the cerebral vesicles, consisting of elongated epithelium-like cells, apparently
arranged in several interlocking layers. Of these cells some become developed into
nerve-fibres and nerve-cells (inner granules and ganglionic layer), others into susten-
tacular tissue, similar to the neuroglia of the central nervous system (molecular
layers, Miillerian fibres), whilst the outermost layer forms the sense- epithelium (W,
Miiller), or , layer of outer granules, which is sharply marked off against the
layer of hexagonal pigment cells by the membrana limitans externa, as is the nerve-
fibre layer from the vitreous humour by the membrana limitans interna. For a long
time there is no trace of the rods and cones. These begin to appear some little time
before birth in man and most animals, but in animals which are born blind, such as
kittens, not until after birth (M. Schultze), in the shape of small protuberances of
the sense-epithelium cells growing beyond the limitans externa, and forming at first
the inner segments of the rods and cones, and subsequently the outer segments also.
The latter as they are developed become imbedded in the inner surface of the
hexagonal pigment cells, which have become developed from the outer layer of the
optic cup.

The anterior third of the optic cup does not undergo the changes above

of.n-

eO

Fig. 102. — Section theough the eye op a babbit embryo, moee advanced in development than
THAT SHOWN IN FIG. 98. (Balfour.)

c, epithelium of cornea; I, lens; me.c, mesoblast growing in to form the .substantia propria of the
cornea; o.n, optic nerve ; rt, retina ; a.c.r, mesoblast for the formation of the vitreous humour, and
the arteria centralis retinae.

described. Its two layers become here developed into the comparatively simple pars
ciliaris retinte, and in front of the ciliary region they extend forwards and inwards
in front of the lens and in close contact with the back of the iris, where they form
the thickly pigmented epithelium, which is known as the uvea and terminates at the
margin of the pupil.

Further development of the lens. — The hollow epiblastic vesicle from
which the lens develops is composed of a thick posterior and a thin anterior layer
which pass into one another at the equator of the lens, and enclose a clear fluid.
In mammals, the vesicle when first formed also contains a small mass of epithelium
cells which have become separated off from the posterior wall (fig. 100), but these

THE VITREOUS HUMOUK. 87

afterwards disappear. 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 :
those in the middle being the longest and straight, while the rest are slightly curved
with their concavity towards the equator, and become gradually shorter towards the
circumference, where they pass through gradually shortening columnar cells {transi-
tional 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 formed, and it consists in the main
of fibres which pass in an antero-posterior direction. The remainder of the lens is
Ibrmed 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 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 chiefly cell-multiplication takes place, and they grow hence
meridionally backwards over the ends of the already developed antero-posteriorly dis-
posed fibres of the central part of the lens.

The capsule of the lens is early visible as a thin homogeneous membrane, the
origin of which is still undetermined. According to some observers (Lieberkiihn,
Arnold, Lowe) it is derived from a thin layer of mesoblast, which passes in between
the lens and the optic cup ; according to others (Kcilliker, Kessler, Balfour), it
appears before any mesoblast has passed in, and they therefore regard it as a
cuticular deposit from the lens cells. In the human embryo, His figures mesoblast
as existing from the first between the lens invagination and the optic cup (v. fig. 97).

In connection with this question it must be remembered that the substantia propria of the
cornea (see below), which is formed of connective tissue, and is therefore mesoblastic in
nature, also at first makes its appearance as a homogeneous deposit before any mesoblast cells
have passed in behind the corneal epithelium.^ Its chemical nature, and its continuity at
the equator with the suspensory ligament and hyaloid membrane, certainly point to the lens
capsule as being a connective tissue, i.e. a mesoblastic structure.

Although the foetal lens like that of the adult is itself non-vascular, it is nevertheless
externally freely supplied with blood-capillaries, which form a vascular tunic completely
surrounding it outside the capsule. These capillaries are supplied by a branch of the arteria
centralis retijiw which passes forwards through the centre of the vitreous humour ; in
front, at the margin of the pupil, they come into continuity with the vessels of the iris. The
most anterior part of this vascular tunic forms a membrane which closes the aperture of the
pupil in the middle periods of f cetal life. In the human eye the whole tunic, together with
the artery which supplies its vessels, becomes atrophied and is lost sight of before birth, but
in some animals the jmpillary meviirane remains apparent for a few days after birth.

The vitreous hninour appears to be formed from the mesoblastic tissue which
haa passed in between the lens and the inner layer of the optic cup by a gradual
formation of a large quantity of ground-substance, whilst the cells of the tissue
almost entirely disappear. The development of the hyaloid membrane has not been
fully traced out, and the same may be said with regard to the zonule of Zinn. They
are probably both formed by part of the same mesoblast as forms the vitreous humour
(Lieberkiihn, Angelucci).

The corneo- sclerotic coat, the choroid coat, and the iris are all derived from the
mesoblast surrounding the optic cup.

The corneal epithelium is a portion of the external epibliist, whicli originally
rests against the front of the lens rudiment. The substantia propria coruese
first appears in the chick as a thin homogeneous layer lying immediately within

' Kesxlcr, however, looks upon this homogeneous dex'osit as being also a cuticular deposit formed by
the epithelial cells.

DEVELOPMENT OF THE EYE.

this epifchelium. Into this homogeneous layer mesoblast cells pass from the margin,
greatly thickening it and producing eventually the regular layers of fibrous tissue,
which are characteristic of the cornea. No cells pass into the most anterior or
into the most posterior stratum, which remain homogeneous (anterior and posterior

Fig. 103. — Horizontal section

THROUGH THE EYE OP AN EMBRYO
RABBIT OF 18 DAYS. ^f. (Kol-

liker. )

0, optic nerve ; p, hexagonal pig-
ment layer ; r, retina ; re, ciliary
part of the retina ; p', forepart of the
optic cup (rudiment of the iris pig-
ment) ; g, vitreous, shrunk away from
the retina, except where the vessels
from the arteria centralis retinae enter
it ; i, iris ; mp, membrana pupillaris ;
c, cornea with epithelium e ; pp, pa,
palpebrse ; I, lens ; I', lens epithe-
lium ; /, sclerotic ; »i, recti muscles.

homogeneous lamellse of Bow-
man). The epithelium of the
posterior homogeneous lamella,
or membrane of Descemet, is
derived from mesoblast cells
which grow in like the cor-
neal corpuscles from the mar-
gin and spread themselves
over the posterior surface of
the cornea, thus separating
this from the iris and anterior
surface of the lens. For a
long while, however, there is no anterior chamber ; this eventually appears as a
cleft-like space between the cornea and the structures immediately behind it.

In mammals, all the above stages of formation have not been described. A
complete layer of mesoblast is early visible lying between the corneal epiblast and
the lens epiblast, and continuous around the margin of the lens with the mesoblast
of the vitreous chamber. In this mesoblast a cleft makes its appearance, separating
it into two parts, one of which adheres to the corneal epiblast, where it forms the
substance of the cornea, the other to the lens capsule forming the pupillary membrane.
This cleft is the rudiment of the anterior chamber. It does not become actually
distended with fluid until a short time before birth (Kolliker).

The sclerotic is formed entirely from mesoblast around the optic cup, probably
continuous with that which forms the cornea, although it. is only later that the
cornea and sclerotic come to be completely amalgamated.

The choroid coat is formed from the mesoblast which is immediately in contact
with the outer layer of the optic cup, and the forward growth of the middle tunic
closely follows that of the margin of the cup. The latter ceases at first at the
margin of the lens, but subsequently grows forwards over the front of the lens as a
thin double layer, which is closely covered externally with a continuation of the
choroidal mesoblast. This is the iris, over the back of which both the layers of the
cup-margin eventually acquire pigment and remain permanently as the uvea. The
ciliary body is formed by a kind of hypertrophy of the optic cup, which developes
radial folds, enclosing thin portions of mesoblastic choroidal tissue, in which, as in
the rest of the choroid, numerous blood-vessels and branched pigment-cells become
formed.

DEVELOPMENT OF THE EAR. 89

Accessory structures. — The eyelids make their appearance gradually as folds
of integumeut, subsequently to the formation of the eyeball (fig. 103). 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. A short time
before birth they again become disunited.

A third fold (of the conjunctiva) appears at the inner canthus, and in many
vertebrates developes into a well-marked third eyelid, the membratia nictitans. In
man it remains rudimentary, forming the plica semilunaris.

The glands, hairs, and other structures belonging to the eyelids, are deve-
loped in the same way as the corresponding structures in the rest of the
integument.

The lachrymal gland is developed in the third month as a number of out-
growths from the deeper layer of the epithehum, at the upper and outer part of the
conjunctival sac. The outgrowths are at first solid, and branch into the surrounding
connective tissue as with other racemose glands, subsequently becoming hollowed
out and differentiated into ducts and acini.

The lachrymal canals and ducts are usually described as being directly
developed by the enclosure of the fissure which separates the lateral nasal process
from the maxillary process (see Development of Nose, p. 95, and figs. Ill, 112),
and which passes in the early embryo from the eye to the upper part of the naso-
buccal cavity (lachrymal 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 is produced, according to
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 lachrymal canals.

DEVELOPMENT OE THE EA"R.

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 epiblast, which at first appears as a pit of thickened epithelium (auditory
pit, fig. 104, A.), but is gradually converted by a growing together of the margins of
Lhe pit into a hollow island of epiblast, the auditory or otic vesicle (fig. 104, B).
This process occurs somewhat after the formation of the eye is laid, 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 meso-
bla.st, which separates it from both the neural and external epiblast.

The hind-brain does not send out a hollow process towards the otic vesicle
con-esponding to the optic processes of the fore-brain, but the auditory nerve
developes from a solid outgrowth of the neural crest in the same way as the
posterior roots of the spinal nerves and parts of many other of the cranial nerves
(see p. 78).

The otic vesicle is at first flask-shaped, with the somewhat elongated mouth of
the flask directed externally towards the original point of connection with the

90

DEVELOPMENT OF THE EAR.

exterior. In elasmobranch fishes this connection is never closed, but remains
throughout hfe 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 connection with
the exterior becomes closed— in the chick during the third day— and what remains

Fig. 104. Sections through region op the hind-brain op human embryos, showing three

STAGES IN the DEVELOPMENT OP THE OTIC VESICLE.

A auditory pits ; B, simple auditory vesicles ; C, auditory vesicles beginning to be fashioned into
parts of the membranous labyrinth.

Fig. 105. — Outline op the right labyrinth op a 3^

WEEKS HUMAN EMBRYO, TO SHOW ITS RELATIONS TO
THE PARTS OP THE AUDITORY NERVE. (W. His, jun.)

a.l, section of hind brain ; g.^', ganglion vestibuli in
contact with the upper part of the labyrinth ; c/.c, ganglion
cochlete in contact with the lower jjart. The fibres of the
corresponding parts of the auditory nerve which have grown
from these ganglia into the hind brain, are seen to cross
one another ; /, facial nerve.

of the original mouth, or canal of connection
with the exterior, is visible as a distinct but
small process from the upper and inner angle
of the vesicle, and is known as the recess of
the lalyrintli (fig. 104 c, r.l). Eventually it
developes into a long epithelial tube, which
passes through the petrous bone, with an expanded end lying within the skull
underneath the dura mater. This tube and its expanded termination form respec-
tively the endolymphatic canal and saccule (fig. 106).

In the meantime the auditory vesicle becomes elongated and begins to be
irregular. Its ventral end projects as a distinct hollow process, at first straight,
but soon becoming curved ; this is the rudiment of the epithelial canal of the cochlea.

DEVELOPMENT OF THE EAR.

91

Other hollow projections appear near the dorsal end of the vesicle ; these form
the two superior semicircular canals ; the horizontal canal appears a little later.

The mode of formation of the canals is some"u-hat peculiar. They first appear as flattened
semicircular hollow protrusions of the wall of the vesicle. Their sides then come together
and coalesce, except near the circumference of the semicircle, which now forms a tube con-
nected at both ends with the vesicle. Subsequently a separation or breach of continuity
occurs over the area of coalescence, so that the rest of the tube is free. One of the ends
becomes dilated into an ampulla and connected with a branch of the auditory nerve.

"Whilst these processes are occurring at the dorsal and ventral ends of che now
elongated vesicle, a fold, or constriction, of the wall is beginning to make its
appearance about the middle, and thus the posterior part which is connected with the
semicircular canals becomes gradually separated (as the utricle) from the anterior
part, which forms the saccule, and is connected with the cochlea. This fold extends
into the beginning of the recess of the labyrinth, and separates it longitudinally fcr a

ccs.a

e.s.c-

p.s.c^.

Fig. 106. — Stages in the development of the membranous labyrinth. (W. His, jun.)

A. Left labyrinth of a human embryo of about four weeks, viewed from the outer side, r, vestibu-
lar part ; c, cochlear part ; r.l, recessus labyrinthi (aqufeductus vestibuU).

B. Left labyrinth with parts of the facial and auditory nerves of a liuman embryo of about 4^
weeks, b.h, surface of the hind brain; «, utricular; s, saccular part of hibyrinth ; a.s.c, 'p.s.c,
e.8 c, rudimentary folds re^jresenting tlie two vertical and tlie horizontal semicircular canals ; r.l, upper
part of recessus labyrinthi becoming enlarged into the endolymjjhatic saccule ; c.c, rudiment of cochlea ;
«.r, vestibular branch of auditory nerve ; g.v, vestibular ganglion (ganglion of Scarpa) ; ff.r, cochlear
ganglion ; n.f, facial nerve, with geniculate ganglion, y.y.

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 ampulliu are beginning to be visible on the
two vertical canals.

short distance into two tubes, one of which opens into the utricle, and the other
into the saccule, forming the only permanent means of communication between
their contents. Another fold, or constriction, appears presently, somewhat lower
down, and converts the connection between the saccule and the cochlea rudiment
into the narrow duct of llensen (rana/i.s ro-vnions).

In the meantime the cochlea-rudiment at the ventral end of the now labyrinthic
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 auditoi-y

92

DEVELOPMENT OF THE EAR.

Fig. 107. — Teansverse and slightly oblique section of the head oe a fcetal sheep, in the
KEGiON OF the HIND BRAIN. (From Foster and Balfour after Boettcher. )

HE, inner surface of the thickened -walls of the hind brain ; EB, recess of the vestibule ; VB,
commencing vertical semicircular canal ; CC, canal of the cochlea ; GC, cochlear ganglion of the right
side ; on the left side. &, the ganglion, and N, the auditory nerve connected with the hind brain.

Fig. 108. — Transverse section of

THE HEAD OF A FCETAL SHEEP OF
FOUR-FIFTHS OF AN INCH IN LENGTH.

(From Foster and Balfour after
Boettcher. )

RV, recessus vestibuli ; VB, vertical
semicircular canal ; CC, cochlear
canal ; Gr, cochlear ganglion ; HB,
horizontal canal.

organ. This coiling, however,
only occurs in mammals ; in
birds, the cochlea is a short
straight blind tube.

All these parts of the laby-
rinth are, when first formed,

? simple epithelial tubes sur-
rounded by and imbedded in
embryonic connective tissue.
As development proceeds, and
the skull begins to form, a
cartilaginous capsule becomes
developed around the se-s'eral
parts of the labyrinth, and
this at length becomes ossified.
The cartilaginous capsule does

are immediately surrounded by

not closely invest the epithehai structures ; they

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

EXTERNAL AND MIDDLE EAR. 93

of semi-fluid sTonnfl substance and branchinji^ 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 fonned 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 vacuolation and disappearance of the gelatinous tissue which sun-ounds 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 (scalse) that the cochlear tube,
which was previously oval in section, acquires the characteristic triangular section
which we see in the fully-formed organ.

The auditory nerve is large and early becomes separated into its two main divisions, vesti-
bular and cochlear. Each division has a large ganglion upon it (fig. 105), which extends
to the anterior wall of the epithelial vesicle, and as the ventral end of the vesicle elongates
and assumes the spiral disposition, the cochlear nerve and ganglion extend along with it and
take the same coiled or spiral form.

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 ai'e becoming developed into the organ of Corti.

ACCESSORY PARTS OF THE ORGAN OF HEARING. EXTERNAL AND MIDDLE EAR.

While the epithelium of the internal ear is formed by an involution of cutaneous
epiblast in the manner which has just been explained, the middle ear with the
Eustachian tube, and the external auditory meatus with the pinna are formed from
the remains of the first visceral cleft, and from the parts of the mandibular and
byoidean arches which immediately bound the cleft. This cleft at an early period
forms an almost complete communication between the pharynx and the exterior,' but
the broad cleft becomes gradually converted into a flattened tube, and this is presently
found to be closed, both by the epiblast and hypoblast, which are from the first
in contact at the bottom of the cleft, and also by an ingrowth of mesoblast, the
rudiment of the membrana tympani being thus formed. There is at first no enlarge-
ment of the flattened tube to represent the tympanic cavity, and the ossicles are
developed not within, but altogether outside the tube, in a mass of gelatinous con-
nective tissue, which is continuous with that forming the embryonic membrana
tympani ; they are formed for the most part by ossification of parts of the carti-
laginous bars, which extend from the otic capsule into the mandibular and hyoidean
visceral arches (see Development of Skeleton). As the tympanic cavity becomes
formed by a gradual enlargement of the blind end of the closed hyomandibular cleft,
the gelatinous tissue retires before it, and as this tissue disappears, the ossicles and
the chorda tympani which were previously entirely enveloped by it, are left projecting
into the tympanic cavity, covered only by thin mucous membrane. The process of
formation of that cavity is not, in fact, completed until after birth, when air becomes
admitted into it through the Eustachian tube.

' Vule footnote on p. 1 02.

94

DEVELOPMENT OF THE EAR.

The embryonic tympanic membrane is at first close to the exterior, the external
meatus being scarcely existent, although the several jDarts of the external ear are very

Fig. 109.— Sketches showing the gradual development op the pabts of the external ear
mTc^niS'^^^^^^^^ ^^^^ ^^^ MANDIBULAR AND HYOiDEAN VISCERAL ARCHES. (His.) Variously

F is an outline sketch showing the several parts of a well-developed adult ear, Ird natural size.
n.rJ\u f°™'^6"°e^ 7 tl^e mandibular arch; 3, prominence between the two\arches, immediately
hvoii.nt.S Pi^longed posteriory into c, behind the hyoidean arch; 4, 5, and 6, prominences on the
hyoiiean arch ; L, m B, otic vesicle (seen also in A) ; K lower jaw

K ,^f *^,«P^'°^^i°«^<=f enumerated 1 forms the tragus; 2, 3, and 3c, the helix; 4, the antihelix :
6, the antitragus ; and 6, the lobule {vide F). >= , , , , , , ^ cuomvn^ ,

DEVELOPMENT OF THE NOSE.

95

early distinguishable as slight protuberances upon the margins of the shallow cleft-
like depression which is all that represents the meatus at this stage (fig. 110, a). Bub
as the body wall becomes thicker, the cleft becomes deepened and more tubular, and
the protuberances upon the mandibular and hyoidean arches become gradually so
transformed and arranged around the external orifice as to be recognizable as the
several parts of the future pinna. The transformations may readily be understood
from the study of the accompanying series of sketches from His, which show these
parts in gradually advancing stages in the human embryo (fig. 109).

DEVELOPMENT OF THE NOSE.

The olfactory organ arises in all vertebrates at an early period of embryonic life
as a depression of external epiblast {olfactory pit) on either side of the fore-brain.
The epiblast in this region becomes thickened, forming an olfadorij area, and a de-

Fig. 110. fROFILE VIEW OF THE HEAD OP A

HUMAN EMBRrO Of NliARLY FOUR WEEKS. (His.)

olf, olfactoiy depression passing posteriorly
into a deep pit, the rudiment of Jacobson's
organ ; rax, maxillary process ; mn, mandibular
arch ; hy, hyoidean arch ; ir', hr-, first and
second branchial arches.

Fig. 111. — Head of an embryo more advanced
IN development than that suown in riG.
110, from before. (His.)

pr.f/loh, globular extremity of the mesial nasal
process. The other letters as in fig. 111.

pression then forms in this area surrounded by a raised margin (figs. !)G, 07, off).
The depression soon appears pyriform, the smaller end extending as a groove towards
the ^omodcEum or buccal invagination (see fig. 110, olf) ; near this end a special
pit is early visible, and becomes developed into Jacobson's organ.

The tliickened boundaries of each olfactory pit and groove are formed by the so-
called mesial and lateral nasal processes (figs. Ill, 112). Tlie mesial nasal processes
are united at their base by a depressed median part of the fronto-nasal process, but
are at first separated below, where they terminate in distinct tubercles, termed by
His the f/lobiilar processes. As development proceeds they extend backwards along
the roof of the emljryonic mouth, forming the nasal lamimn. Eventually the
globular processes coalesce in the middle line to form the intermaxillary process and
the middle part of the lip, while from the depressed siirliuje between them the
lower part of the nasal septum and the pliiltrum are formed, and by a coalescence of
the nasal laminae the rest of the nasal septum is produced. In rodents a notch leads
from the nasal septum through the upper lip to the mouth, and represents an im-
perfect union of the globular jjrocesses.

Above the depr<;S8od surface just referred to, is a triaiigidar part of the fronto-
nasal process which forms an angle with it. This angle eventually becomes the

96

DEVELOPMENT OF THE NOSE.

'§-/"'■

jar. a.l.

Fig. 112. — Head of an embryo still more advanced in development. (Hi.s.)

A, from above ; B, roof of mouth after removal of lower jaw ; ^.73?,, 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 ; the other letters as before. The nasal laminae of the globular processes and the
palatine projections of the maxillary processes are seen in B.

Fig. 113. — Head of an embryo op 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. 114. — Head of an embryo more advanced
IN development, with tee parts of the

NOSE AND MOUTH BEGINNING TO ASSUME THEIR

permanent relationships. (His.)

point of the nose, and the triangular surface above it the bridge ; the alse nasi are
formed by the lateral nasal processes. These processes are less prominent than the
mesial (fig, 112). They curve round the olfactory depressions, and meet the maxillary

DEVELOPMENT OF THE NOSE.

07

processes ; between the two processes (lateral nasal and maxillary) the lachrymal
groove passes from the eye to the nose (figs. Ill, 112). The maxillary processes
also abut in front against the outwardly curing ends of the processus globulares,
which together form as just mentioned, an intermaxillary process, and the three
eventually coalesce to form the upper boundary of the mouth, which is thus shut
off from the anterior orifice of the nasal fossae (fig. 11;^,). But further back the

Fig. 115. — Outline of a transverse vertical section through the nose and upper jaws of a
sheep's embryo with open palate. (From Kolliker.)

The lower jaw and tongue are removed ; m, the mouth ; (/, dental germs ; p, the palate plates
approaching each other in the middle ; /, the nasal fossas ; <; nasal cartilago ; s, septal cartilage ; j, the
two organs of Jacobson with their cartilages internally.

olfactory depressions, which are now developed into cleft-like cavities, and are in-
creasing in complexity by the development of the projections which are to form the
turbinate bones, are still freely in communication with the buccal cavity, and it is
only by the growth of the palatine processes of the maxillte (fig. 115, j;) and their
coalescence in the middle line with one another, and with the lower part of the nasal
septum, that the nasal cavities are cut off from the mouth and from one another,
and now only open posteriorly into the upper part of the pharynx by the posterior
nares {choan(r).

The median or septal part of the external nose, with its columella below, is
formed, as above stated, of the coalesced mesial nasal processes, the alae nasi being
developed from the lateral nasal processes. The septum is at first broad and de-
pressed, so that the nostrils are widely separated from one another (fig. 114), a con-
dition which remains to a certain extent permanent amongst some of the dark races
of mankind.

From the above description it will be seen that the olfactory org-ans are at first altogether
di.stinct from the mouth, that they subsequently pass backward.s, as grooves, deepening into
distinct clefts, along the roof of the moubh and forming in fact the upper part of the embryonic
buccal cavity, and that finally they are again gradually separated from that cavity by the
growth of a horizontal septum to form at fir.st the hard and afterwards the soft palate.

The median union of the palate begins in front about the eighth week and reaches the
back pare and is completed about the tenth week. Imperfect coalescence of these parts pro-
duces the malformations of hare-lip and cleft palate in their various degrees. Usually, how-
ever, in man the coalescence is completed at a comparatively early period of fcetal life,
although a vestige of the original separation may be found in front at the junction of the
maxillary processes with the coalesced globular processes (intermaxillary), as the naso-palatine
canal or incisor foramen, which is occupied by connective tissue, blood-vessels, and a branch
of the fifth nerve. In many mammals, however, an actual communication remains through-
out life between the nostrils and mouth in this situation.

The organ of Jacobson is early visiV)le on either side of the nasal s(i)tiim at its lower part
in the form of a narrow tube, oval in section, running horizontally in the substance of the
Heptum and opening anteriorly near the ujijier orifice of the naso-palatine canal. When the
cartilage of the septum becomes fonned, a sj)ecial curved plate; of cartilage is seen partially
enclosing this organ ; but botli the organ itself and th(! cartilage are less conspicuous in man
than in most mammals. According to (Jc^genbaur the rudiment wlii<;h lias hcen described in

98 RECENT LITERATURE.

the human embryo as the organ of Jacobson is not really that structure, but represents a
special gland which occurs in some lemurs in the lower part of the nasal septum.

The epiblast of the olfactory area early becomes thickened, and resembles in structure the
neural epiblast (His). As in the latter, some of the cells (neuroblasts) become pyriform and
nerve-fibre processes grow out from them, whilst the remainder foi-m long sustentacular
columns, which partially anastomose to form a spongework. The neuroblasts subsequently
pass out towards the olfactory lobe as already described (p. 81).

All the complexities of the nasal fossaj (and they are far more complex and labyrinthic in
many animals than in man) are produced by folds and outgrowths of the original simple
depressions, and the thickened epithelium of these depressions extends over all parts of the
cavities which are thus formed. But it is only in the upper part of the nasal fossae that the
connexion by nerve-fibres with the olfactory lobe becomes established, and it is in this part
only that the true sense-epithelium becomes developed. In the lower, or respiratory part
of the f ossEe the epithelium remains relatively thin and becomes ciliated.

EECENT LITERATURE.

Eye and Nose.

Born, G., Die NasenTiohle u. der TTirdTiennasengang der amnioten Wirhelthiere, Morph. Jahrb.,
1879 u. 1883.

Disse, J., Die AusUldung der Nasenhohk nach der Geburt, Arch. f. Anat. u. Physiol., Anat,
Abth. 1889.

Ewetzky, Th., Zur EntwicTcelungsgeschiclite des Thrdnennasenganges heim Menschen, Archiv f.
Ophthalmol., Bd. xxxiv., 1888.

Gottschau, Zur Entioiokelung der Saugethierlinse, Anat. Anzeiger, 1886.

His, W., AnatomielmenscJdicher Embrgonen, Leipzig, 1880—85 ; Unsere Korperform, dsc, Leip-
zig, 1875 ; Die Formentwickeking des menschlichen Vorderhirns, Abhandl. d. k. Sachsischen Gesell-
schaft, 1889.

Keibel, F., Zur EntwicTcelung des Glash'&i'pers, Arch. f. Anat. u. Phys., Anat. Abth., 1886.

Kessler, Zur Entwickelung des Auges der Wirhelthiere, Leipzig, 1877.

KoUiker, A., Zur Enttvickelung des Auges und Geruchsorganes menschlicher Emiryontn, Fest-
schrift der Schweizer. Universitat Zurich gewidmet, Wiirzburg, 1883.

Koranyi, A., Beitrage zur Entwickelung der Krystallinse hei den Wirhelthieren, Internat. Mo-
natsschr. fiir Anatomie u. Physiologic, 1886.

Leg'al, Die Nasenhohle u. der Thrdnennasengang, Morph. Jahrb., 1883.

Bag-insky, B., Zur Entwickelung der Gehdrsc7in£cke, Verhandl. d. physiol. Gesellschaft zii Berlin,
1885-86.

Beard, J,, On the segmental sense organs of the lateral line and on the morphology of the vertebrate
auditory organ, Zool. Anzeiger, No, 161, 1884.

Boettcher, A., Ueber Entwickelung u. Bau des Gehorlabyrinths, Verhandl. d. kaiserl. Leop.
Carol. Acad., Dresden, Bd. 35.

His, W., jun., Zur Entwickelung sgeschichte des Acustico-faoialis-Gebietes beim Menschen. Arch,
f. Anat. u. Physiol., Anat. Abth., 1889, Supplement Bd.

Hoffmann, C. K., Ueber die Beziehung der ersten Kicmentaschezu der Anlage der Tuba Eustachii
und des Cavum tympani, Archiv f. mikrosk. Anat. xxiii., 1884.

Noorden, v., Die Entwickelung des Labyrinths bei Knochenfischen, Arch. f. Anat. u. Physiol.,
Anat. Abth., 1883.

Riiding-er, N., Zur Entwickelung der hdutigen Bogengdnge der inner en Ohres, Sitzungsber. d.
Akademie d. Wissensch. zu Munchen, Bd. xviii. , 1888.

Tuttle, Alb. H., The relation of the external meatus, tympanum and Eustachian tube to the first
visceral cleft, Proc. American Academy of Arts and Sciences, 1884.

Vassaux, Recherches sur les premieres phases du devcloppement de Vail chez le lapin. Arch,
d'ophthalm., 1888.

DEVELOPMENT OF THK MOUTH.

99

DEVELOPMENT OF THE ALnOlITTAIlY CANAL.

The early development of the primitive alimentary canal has already been briefly
described in treating of the first formation of the embryo (pp. 34, 35), and it was there
explained how the dipping downwards and inwards of the blastodermic layers on
either side of the embryo tends to separate or pinch oflF the part of the blastodermic
vesicle which is immediately underneath the body of the embryo as a distinct tube
(mid-gut) from the remainder of the vesicle, which is now known as the yolk-sac, while
at the same time similar changes occurring in front and behind produce the blind
anterior and posterior extremities of the tube which are known as the fore- and hind-
gut respectively. Although the downfolding in question eventually involves all the
layers of the embryonic blastoderm, the epiblast and the part of the mesoblast which
adheres to it, and together with it forms the splanchnopleure, do not participate in
the process until after the formation of the amnion, so that the alimentary canal for
some time after its formation is enclosed only by the hypoblast and its adherent
mesoblast (splanchnopleure), and projects freely into the wide coelom or space
between the splanchnopleure and somatopleure. The mid-gut also remains for a
time in free communication with the yolk sac, although the communication becomes
gradually narrowed into the vitelline duct. As the somatopleure afterwards grows
down on either side of the alimentary canal, and becomes pinched in around the
vitelline duct and stalk of the allantois, which are thus united into the umbilical
cord, that part of the coelom which is within the body and around the alimentaiy
canal becomes shut off as the pleuroperitoneal cavity from the remainder, which lies
altogether outside the body, and forms the cavity of the false amnion.

Development of the mouth and of the parts iu connection with it. —
The fore-gut terminates blindly at first underneath the head in the region of the

Fig. 116. — Frontal view of the upper part of a human

EMBRYO OF ABOUT FIFTEEN DAYS, RECONSTRUCTED FROM
SERIAL SECTIONS. (His. ) \'

The pericardium is opened to show the heart ; between this
and the fore-brain is seen the primitive buccal cavity. A de-
scription of this figure is given on p. 138.

hind-brain, and the notochord, with the fore- and
mid-brain, curve downwards over the blind extre-
mity, the fore-brain thus causing a rounded pro-
minence in front of and ventral to the extremity of
the alimentary tube (see fig. 45). With the develop-
ment of the heart another prominence becomes
formed on the ventral side of the fore-gut, a little
further back. Between the two prominences, the
one caused h>y the projection of the fore-brain and
the other of the heart, a wide, shallow pit is enclosed
(fig. llfi), at the bottom of which the epiblast

wliich lines it is in contact with the hypoblast of the fore-gut, and the two layers
fuse to form an epithelial membrane, which now forms a septum between the
primitive buccal epiblastic involution or stomodaeum and the fore-gut (fig. 117,
p.v.). This stage is met with in the human eiri])ryo before the twelfth day (Ills),
in the rabbit emliryo at aliout the ninth diiy (Mihalkovics), and in the chick on the
fourth day.

H 2

100

DEVELOPMENT OF THE MOUTH.

The stomodEeum deepens at its upper and anterior part, where it forms a
pocket-like protrusion, which grows a certain distance into the angle formed by the
sharp bend which the hinder part of the fore-brain now makes with the mid-bram.

etioi.

'cdl

VL,X>.

-alZ.

Fig. 117. — PkOFILE view of a human embryo of about 15 bays, with the alimentary CANAl

SHOWN IN LONGITUDINAL SECTION. (His.)

Fig. 118. — Similar view op a somewhat older embryo. (His.)

1, 2, 3, 4, 5, are opposite the respective secondary cerebral vesicles ; from tlie side of the fore-brain
the primary optic vesicle is seen projecting ; ot. otic vesicle ; "p.v., septum between mouth and pharynx
(primitive velum) ; I, commencing liver in septum transversum ; v, vitelline stalk ; all, allantois enclosed
within allantoic stalk ; j v., jugular vein ; c.v., cardinal vein ; s.?'., sinus venosus within septum trans-
versum ; u.a., umbilical (allantoic) artery ; l.u.v-, left umbilical vein. The sharp curve of the trunk of
the embryo towards the yolk-sac is normal. at this stage.

In fig. 117 the otic vesicle is still open, and there are only two aortic arches ; in fig. 118 the otic
vesicle is closed ; there are now five aortic arches. The primitive velum has disippeared.

This pocket (Rathke) is the hypophysis cerebri, or pituitary involution of the buccal
epiblast, and comes presently into connection with the infundibular protrusion of the
neural epiblast, the two together forming the pituitary body (see p. 68). It lies
just al30ve and in front of the pharyngeal septum.

The remains of this septum (when it has become broken througli to allow of a communica-
tion between stomodteum and fore-g-ut), have been termed the primitive vehim, hut 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 faucium, so that the primitive mouth or stomodseum does not by any means
correspond with the permanent mouth. In fact the floor of the mouth, including the tongue,
is developed heMnd the septum, and therefore in connection with the fore-gut rather than with
the stomodffium, whereas the uppermost part of the pharynx, including the choanas, is in front
of the septum, and therefore belongs to the stomodgeum.

THE PHARYNX.

101

The shallow and widely open stomod^um soon deepens and is now seen
to be specially bounded by certain prominences placed above, below, and at the
sides, within and from which the several parts of the face are eventually pro-
duced (figs. Ill, 110). These prominences are the fronlo-ncmil which projects
over the stomodasura, and is formed primarily by the prominence of the fore-
brain, but afterwards acquires a considerable thickening of mesoblast, which ex-
tends into it from the basis cranii ; the mandihular or first visceral arch, which

Fig. 119. — ^Pkofile view of a human embryo of

ABOUT THREE WEEKS, SHOWING ALL THE
CEPHALIC VISCERAL ARCHES AND CLEFTS.

mx, maxillary process ; mn, mandibular arch ;
d.C, duct of Cuvier ; ji\ jugular vein ; c.r, cardinal
vein ; r.r', vitelline vein ; ti.r, umbilical vein ; a. a,
umbilical artery ; idl, allantois ; pi, placental attach-
ment of allantoic stalk ; olf, olfactory depression ;
ot, otic vesicle.

after passing obliquely round the fore-
gut, takes a horizontal direction on its
ventral side, and meets its fellow in the
middle line, the two together forming the
ventral boundary of the stomoda^um : and
the max ill art/ irrocess, which grows from
the base of the mandibular arch, and
projects on either side of the stomodgeum,
filling up the gap between the fronto-
nasal process and the mandibular arch,
and forming the lateral boundaiy.

The separation of the stomodteum into an
upper or olfactory and respiratory part and a
lower permanent buccal cavity, together with
the chan<jes which occur in the fronto-nasal
and maxillary processes to produce the result, has
already been refen-ed to in describing the de-
velopment of the nose (pp. 95 to 97).

Pharynx. — The remainder of the ali-
mentary canal below the mouth is nearly
simple, at first consisting, as before men-
tioned, of a tubular portion in front — the
fore-gut ; a shoi'ter tubular portion behind
— the hind-gut ; and a middle part which
is freely open to the yolk (fig. 117).

The hind-gut remains simple throughout, except that the allantois grows out
from its ventral aspect.^ But in front a diflerentiation soon makes its appear-
ance, the cej)ha]ic part becoming enlarged to form the pharynx, while almost
immediat<.'ly behind this another eiilai-gernent forms the stomacli. 'Wa hypoblast
lining the cavity of the ])harynx gi'ows out on either side successively at four distinct
levels, and to a less extent the epiblast dips in opposite the hypoblastic outgrowths.
In this way eventually four deep clefts Ixjtween the pharynx and the exterior become
formed ; these are known as the cephalic visceral clefts (figs. 110, 111>, in external

' In the liuman embryo the allantois apjiearH to be formed by a direct continuation of the bitcral
foUU, uliich have united to form the main aliinentary tube, while tlio part of tlio tube 'jeliind tho
ailaiitoiH (^bur.sa) a^ipearH as -i blind protrusion (\\\h).

102 DEVELOPMENT OF THE TONGUE.

appearance ; figs. 117, 118, seen from within). i Between them, and also in front of
the first cleft, the pharyngeal wall is greatly thickened so as to exhibit the appear-
ance of curved bars bounding the clefts ; these bars are known as the ce2)lialic
visceral arches, and are five in number, viz. : the first or mandibular^ in front of the
first visceral cleft, between it and the mouth : this is the seat of formation of the
lower jaw ; the second or hijoid arch, between the first and second clefts ; the third
or thyro-liyoid arch, between the second and third clefts, wdiich in fishes and
amphibia develops gill-plates, and is therefore also known as the first bra^ichial
arch ; the fourth between the third and fourth clefts, corresponding with the second
branchial arch of fishes, but small and inconspicuous in man and mammals ; and the
fifth or second branchial still smaller and more inconspicuous, forming the posterior
boundary of the fourth cleft, and hardly recognizable as a distinct bar in man.
After the fourth week, and Avith the increasing flexure of the head, the arches become
somewhat shifted over one another, so that the fourth arch is concealed by the third,
and the third by the second.

The mandibular arches early become united on the ventral aspect ; from the fifth week
their union is complete (fig. Ill, mn) and in man shows eventually no sign of a median groove.
The other arches do not at first reach the middle line (His), the space between their central
ends being occupied by the heart and pericardium ; as these shift backwards a smooth infra-
mandibular surface is left externally.

Development of th.e tongue, — Within the pharynx, the second and third
arches of the two sides are separated by a forked elevation (furcula) with a median

Fjg. 120. — Posterior aspect op the visceral arches op the embryo

SHOWN IN FIGS. 116, 117, AS SEEN FROM THE INTERIOR OF THE
PHARYNX. (His. ) ^f

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.

groove, in front of which is a rounded tubercle {t. impar,
His), which arises in the angular space between the
first and second arches (fig. 120), The groove around
the furcula (simcs arcuatus, His) passes laterally into
the visceral clefts. The second and third arches afterwards unite between the
furcula and tuberculum impar (fig. 121, A). Thus united, the junction forms an
X-shaped mass.

From these conjoined extremities of the second and third arches on either side, the
root of the tongue grows upwards and forwards as two prominences, which diverge
in a V-shaped manner to embrace the anterior or papillary part of the organ which
is developed from the tuberculum impar (fig. 121, B). At the angle of the V is a
deep depression (foramen ccecum) ; this leads into a diverticulum, which forms the
median rudiment of the thyroid body. When the parts of the tongue are united,
there is still for a considerable time a V-shaped groove marking the line of union
(fig. 122), and even in the adult there is often a distinct trace of this groove {sulcus
terminalis, His). Parallel to this, and somewhat in front of it, the papillse vallatse
are developed, and in front of these the other hngual papillse make their appear-
ance (about the end of the second month).

_ 1 According to His, who is confirmed by Bom and by Kolliker. these clefts are not as a rule developed
into complete apertures in birds or mammals ; although the membranes which close them are composed
only of juxtaposed epi- and hypoblast, the mesoblast having disappeared.

DEVELOPMENT OF THE TONGUE.

103

The furcula gives rise to the epiglottis in front (above) the aryepiglottic folds on
either side, and the arytenoid cartilages behind (below) ; the median groove in it leads
to the entrance of the larynx.

Laterally the 2nd arch passes into and forms the palato-giossal arch, and in the visceral
cleft behind this the tonsil develops ; but the ord arch does not form the palato-pharyng-eal
arch ; this is developed from the palatine outgrowths of the maxillary processes.

The visceral arches were first described by Eathke, in 1825. They are often dis-
tinguished as the j)ost-bucciil visceral arches ; certain parts in front of (above) the mouth

Fig. 121.— Similar views of the same parts in older embryos. (His.) A. Y> C- i*

T, tuberculura impar.

being- considered by some morphologists to represent jircr-huccal arches. The visceral clefts,
lying between the arches are, as has been stated, four in number. The first is often known as
the lujomandihilur cleft : it is this one which is concerned -with the formation of the

Fig. 122. — Similar view in a considerably older

EMBRYO, BUT LESS MAGNIFIED. (His.)

Eustachian tube and middle ear as alreadv de-
scribed. The three remaining clefts, which repre-
sent gill-slits of fishes and amphibia, appear, from
the results of recent observations, to be closed in
amniotic vertebrates at all periods of foetal life
(see note on previous page). In some fishes the
branchial arches and clefts are more numerous
than in other vertebrates, and in a few the hyoid
arch also develops a gill.

Through each of the visceral arches an arterial
arch derived from the aortic bulb passes from
front to back reuniting dorsally in front of

the notochord to form the aorta. In branchiate vertebrates, branches of these vessels arc
distributed to the gills. Cartilaginous bars pa.ss, in most vertebrates, from the base of the
skull into each visceral arch, and ossification occurring in or around them, form definite i)aits
of the skeleton as will be afterwards described. In man and mammals these cartilaginous
bars are only found in the first three visceral arches, unless the thyroid cartilage is to be
regardwl as repies(;nting the anterior (ventral) ends of the bar of the 4th arch (Callcnder).
The fourth and fifth visceral arches may be considered as lielonging to the neck rather
than to the head, and the congenital fissures of the neck which sometimes occur as a mal-
formation, and which usually open externally far down in the cervical region, have been
regarded as due to persistence of one or more of the l)ianchial clefts, shifted in position by the
cervical elongation which takes place in later embryonic life.

CEsophagns, stomach, and intestines. — Immediately behind the pliaryn.v, the
fore-gut contnictH again to form the oesophagus, which, in the early embryo, corres-
ponding with the imperfect develoitment of the neck, is very short (figs. 12o, 125, A)

104

(ESOPHAGUS. STOMACH, AND INTESTINES.

and gradually widens oui into the dilatation which represents the stomach. ^ This
organ, which is at first nearly straight (fig.. 125, A, Mg), soon begins to show the con-
Fig. 123. — Sketch of a longitudinal section through

THE ALIMENTARY CANAL OF A HUMAN EMBRYO,
SOON AFTER THE DISAPPEARANCE OF THE PRIMITIVE
VELUM. (His.) to

The alimentary canal is shaded throughout; U.K,
section of mandibular arch ; R. T, hypophysis ; behind
it the remains of the pharyngeal septum ; Lg, com-
mencing lung, the future orifice of the larynx being
opposite K ; Mg, stomach ; Lb, liver ; Nh, yolk stalk ;
W, Wolffian duct ; B, blind portion of hind gut ;
all, allantois.

vexity of the greater curvature on the side next
the vertebral column, and the concavity of the
lesser curvature on the opposite border (fig.
125, B, 3Ig), while the pyloric end becomes
tilted away from the vertebral column, pro-
ducing the duodenal loop (fig. 125, C, D).
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 thus thrown over to the right side of the
abdomen (fig. 120). The small intestine is also
at first quite short and straight, with a wide
aperture to the yolk-sac (fig. 125, A, Nl),
but gradually lengthens as the communication
with the yolk-sac becomes more contracted,
and (besides the loop formed by the tilting of
the pylorus) develops a long Y-shaped loop
opposite the attachment of the vitelline duct
(fig. 125, C, D, and fig. 127).

The loop of intestine to which the vitelline duct
is attached passes, for a time, into the umbilical
cord, close to its attachment, enclosed in a protrusion of the peritoneal cavity (fig. 124). It
occasionally remains in this situation until late in foetal life.

Fig. 124. — Sketch of the human embryo op the

TENTH WEEK, SHOWING THE COIL OF INTESTINE IN

THE UMBILICAL CORD. (Allen Thomson.)

The amnion and villous chorion have been oi^ened and
the embryo drawn aside fi'om them ; v, umbilical vesicle,
connected with the coil of intestine, i, by a small, almost
linear tube. The figure at the side represents the first
part of the umbilical cord magnified ; i, coil of intes-
tine ; vi, vitelline-intestinal duct, alongside of which
are seen omphalo-mesenteric blood-vessels.

The mesentery is developed by a thinning
out and extension of the mesoblastic tissue
which lies between the intestine and the
vertebral column. It forms a continuous membrane along the whole length of the
alimentary canal from the stomach to the rectum, although the part attached to the

» It has been shown (Balfour, Meuron) in most vertebrates— mammals excepted— that at a certain
period of development the lumen of the cesopbagtis becomes for a time completely obliterated at its upper
extremity.

OESOPHAGUS, STOMACH, AND INTESTINES.

A f~ B :»

105

Fig. 125. — Pkofile SKKTciits ok succk.ssivk btaoes in the devkloi'mknt ok thk alimentary canal

IN THE HUMAN EMBKYO. (HIh. )

Ch, notoclionl ; Sd (in 15), median rudiment of tliyroid ; P, iiaiicrefw ; Lby, Ijile duct ; Dfi, vitelline
duct ; Zt/ (in C and D), tongue ; N, pennanent kidrn-y ; rf (in \>), cloaca ; An, anus in course of form-
ation ; .Sf/, sexual i)rominence ; K>t, tail ; C'c, cuucum coii ; 7V, trachea ; K, Liryu.v. Tlie other letter-
in:: an in tig. 12:3.

106

CESOPHAGUS, STOMACH, AND INTESTINES.

stomach is known as mesogastrium and the parts attached to the future colon and
rectum are termed respectively mesocolon and mesorectum. The stomach begins to
assume its characteristic shape while still lying with its longitudinal axis in the

Fig. 126. — Front view of alimentary canal, rather less advanced in

DEVELOPMENT THAN THAT SHOWN IN FIG. 125, D. (His.) ^f

The pharynx and upper part of cesophagus, and termination of the large
intestine are not represented. Lettering as in fig. 125.

median plane of the body ; it is then seen that the mesogastrium
passes to its greater curvature (fig. 127), which, therefore, is
that corresponding to the mesenteric border of the intestine.
And as the pyloric extremity of the stomach and lesser curva-
ture are tilted forwards and upwards, and at the same time
the whole organ turns over on its right side, the mesogastrium
becomes proportionally lengthened to permit of this change of
position, and the right surface of the stomach (now posterior)
rests against the anterior surface of what was previously
the right side of the mesogastrium, the mesogastrium thus
coming to form the posterior boundary of the omental sac (fig.
128). From near its attachment to the stomach a free fold
subsequently grows over the intestines, and becomes the great
omentum.
The gastro-hepatic omentum is formed by the gradual thinning of a mass of
mesoblastic tissue which from the first connects the ventral wall of the stomach with
the anterior wall of the abdomen, and within which the hypoblastic outgrowth

~S.7^&S.CC

ao.

Fig. 127. — Diagram of the mesentery, stomach and intestine of a human embryo of six

WEEKS. (Toldt. )
St, stomach; g.c, greater curvature; l.c, smaller curvature; mg, mesogastrium; spl, spleen;
p, pancreas ; c, csecum ; r, rectum ; me, mesentery ; ao, aorta ; cl, cceliac axis ; s.mes.a, i.mes.a, supe-
rior and inferior mesenteric arteries.

Fig. 128. — Diagram of a section across the abdomen of a human embryo of the third

month. (Toldt. )

I, I, liver ; k, kidneys ; g.o, great omentum ;, r/.o', omental sac ; s.o, small omentum. (The dotted
lino has not been carried quite far enough.) The other letters as in fig. 127.

which forrns the liver becomes developed. The part of this mesoblastic connexion
which lies between the liver and stomach becomes the gastro-hepatic or lesser
omentum, and its free border which was at first directed downwards (caudalwards)
becomes with the descent of the stomach directed anteriorly (ventrally), and eventu-
ally with the turning of that organ laterally it also is directed towards the right, and
thus comes to form the anterior boundary of the entrance into the omental sac.

(ESOPHAGUS, STOMACH, AND INTESTINES.

107

The large intestine is not at first marked ofi" from the small hy any difference in
calibre. Its commencement is distinguishable about the sixth week in the human
eml)ryo by the appearance of the caecum, which gradually grows out (tigs. 125, D, and
127), forming at first a lateral protrusion of uniform calibre, but subsequently re-
maining narrow at its blind extremity to form the vermiform appendix, while the
remainder of the CEecum and the colon increase in size. This protrusion occurs on
the U -shaped loop above described, and a little beyond the attachment of the
vitelline duct.

With the increasing length of the gut it becomes thrown into coils, and the
earliest and most important of these is that by which the limb of the (J'^l^^psd loop

A

diaja^^r,^^

Tnallintestinz.

Fig. 129. — Diagrams illustrating the development of the great omentum. (0. Hertwig.)

A, earlier stage.

li, later stage.

»<, stomach ; .s.o, small ornentiim ; &' .o', omental sac ; o', mcsogastriiim, springing from the
posterior wall of the abdomen, near which in A it enclo.'cs the jjancreas ; o-, attachment of mcsogastrium
to greater curvature of stomach ; o'*, fold of mesogastrium or great omentum growing over coils
of small intestine ; nat, mesentery ; iti.a, transverse mesocolon : o^ (in B), dotted line showing the
situation of that lamella of the mesogastrium which at first assisted in enclosing the ^lancreas but
which Las now disappeared. The next part of this lamella has coalesced with the adjacent lamella of
the transverse mesocolon, and has also disai)ijeared. The coalescence is indicated by the black line.

with which the large intestine is continuous tiu'ns over on to the right side of the
peritoneal cavity, and thus throws the colon in an archlike disposition across the
commencement of the small intestine, and paiallcl with the longitudinal axis of the
stomach (commencement shown in fig. 12G). Within this arch of the large intestine
the coils of the jejunum and ileum become dis[)osed as the intestine lengthens.
Their mesentery spreads out at its intestinal attachment so as to adapt itself to the
increasing length of the gut, while its vertebral attachment, relatively mucli shorter,
loses to a great extent its primitive disposition, and acquires oblique and transverse
lines of attachment ; this is notably the case with the transverse mesocolon.

Althou<rh the mosontery in most parts incrcasoH in Icnf^th and expansion with the further
ffrowth of the intestine, the contrary is the case with the mesentery of the duodenum, and of
the UHcendin^i' and descending' colon. All these jtarts possesH at fiist a complete mesentca-y like
the rest of the intestine, but that of the duodenum disuppi'ars entirely, so that this part of the
int<^Htine becomes fixed to tlie ])osterior wall of the abdomen, and the same i)rocess takes i)lac.e
to a lesser and variable ext<int with the ascendiiif^ and descendintr mesocolon. Since the trans-
verse colon lies across the abdomen immediately below the stomach, it and its mc^sentcry, trauH-
versely diKpoHcd, aluo lie immediately below and ))ehind the mesogastrium (now f( Ided into the
great omentum). The two morabrantH come in fact into close contact, and eventually com-

108 FORMATION OF THE ANUS.

pletely adhere (4th month and onwards) ; and this causes the pancreas to appear to lie alto-
gether behind the peritoneal cavity, in place of being situated between the two layers of the
mesogastrium as is at first the case in the human embryo, and as is frequently found in other
mammals during life.

The free or floating part of the great omentum is formed by an extension of that part of
the omental fold which turns upwards towards the greater curvatui'e of the stomach from
the surface of the transverse colon. The fold is at first clearly double, and in some animals
remains so, but in man its two layers coalesce a little while after bhth, and after a year or
two can no longer be separated. It extends gradually, first over the transverse colon (third
month), later over the coils of the small intestine.

The spleen becomes formed within the substance of the mesogastrium (fig.
127, spl). It is developed wholly from mesoblast, and in close connection with the
pancreas. It appears during the second month in the human embryo, and grows
slowly during fcetal life, the Malpighian corpuscles being the last parts to appear.

Formation of the anus. — The anal invagination of the epiblast, which eventu-
ally by absorption of the septum between it and the hypoblast of the hind-gut opens
into the alimentary tube, is termed the procfodoonm. The junction with the hind-
gut occurs at a little distance from the posterior extremity, so that there is a blindly
terminating post-anal or subcaudal portion of the gut beyond the junction with the
prootodseum ; this, however, shrinks and disappears even before the absorption
of the septum. This part of the hind-gut represents a cloaca, since it receives
through the allantois the ducts of the urinary and genital organs. The sepa-
ration of the permanent anus from the urogenital orifice, which occurs in all
mammals above mouotremes, is the result of a later process of development (see
p. 127).

In mammals the actual amount of proctodseal invagination is very small. The
septum between the hind-gut and the exterior (anal membrane) is throughout formed

CX.frz^

Fig. 130. — Longitudinal section through the posterior end of a sheep's embryo, showing the

ANAL membrane. (Bounet.)

ep, epiblast ; liy. hypoblast ; mes, mesoblast ; h.g, hindgut ; am, amnion ; an, anal membrane ;
f.s, primitive streak ; all, ahantois-rudiment.

by two epithelial layers only, viz., hypoblast and epiblast, which here are in contact
with one another without the intervention of mesoblast (fig. 180, mi). This con-
dition of juxtaposition of the two layers is in fact directly derived ft-om the union
of the two layers which occurs at the primitive streak and groove, and if the latter
be looked upon as representing the blastopore, the anus may'^in a sense be considered
to be formed from a part of that aperture. In some lower vertebrates the anus has
been shown to be directly produced from the blastopore.

THE LUNGS.

109

FOKMATION OF THE GLANDS OF THE ALIMENTARY CANAL.

Under this liead may be included not only those organs which are ordinarily so
termed, but also the lungs, and the thymus and thyroid bodies, since the early
development of these three 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 these
epithelial buds grow into the mesoblast, 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 derived from the
surrounding mesoblast. The salivary glands and most other glands of the mouth,
and part of the hypophysis, which must also be reckoned as a glandular development,
are formed in this way by involution of the buccal or stomodseal epiblast ; 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 hypoblast. The development of
the teeth, which also first make their aiDpearance as involutions of stomodteal epiblast
(enamel germs), will be described after their structure has been dealt with (in the
part of this work which is devoted to Splanchnology).

The lungs. — The lungs begin to develope from the ventral part of the pharynx
at its junction with the oesophagus, in the beginning of the third da;y in the chick,

Fig. 131. — Lung rudiments of human embryo of about 4 weeks, showing the bud-like

ENLARGEMENTS WHICH REPRESENT THE LOBES OF THE FUTURE LUNG.S. (llis. )

Three buds are seen on the right side, two on the left.
Fig. 1.32. — Lungs of a human embryo more advanced in development. (His.)

and in the human embryo at a correspondingly early period (fig. 123, X^). 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 hyjjoblast. Soon the diverticulum sprouts out at its lower extremity
in the form of two tubes which grow downwards on either side Ijchind and on either
side of the heart, into a mass of mesoblastic tissue, which keeps pace in its growth
with the hypoblastic 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 (lig. 131), tbrce lobules being present on
the right tube, and two on the left, the division of the lungs into their lobes being
thus early indicated.

T\i(i further outgrowth of the lobulations produces the rudiments of tin; i)riii(;i(iiil Iji-.anchcs
of the bronchi, one for eucii future imlmonary lobe, and each of these brunches then ■,nadualiy

no

THE THYEOID BODY.

Fig. 133. — Lungs of a human embryo

STILL MORE ADVANCED. (His.)

progi-esses in growth, g-iving off as it proceeds lateral diverticula, wliicli 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. \2»,Lg :

figs. 131, 132, 133. This is due to the fact that they
are undergoing a further division or sprouting. This
process goes on until the sixth month of intrauterine
life, by v^^hich 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 forma-
tion of these takes place when the bronchial ramifica-
tion is comp]eted (sixth month, Kolliker), as small,
closely-set, pouch-like protrusions of the walls of the
infundibula, and of the terminal bronchial tubes.

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 connection between the two
tubes above : this connection 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 somew^hat
short of that organ (fig. 125), 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 mem-
brane, at first only below and on the external surface, but subsequently on the
internal aspect, so as to separate them from the oesophagus. 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 pleurse.

The pulmonary blood-vessels are comparatively late in being developed, the
arteries penetrating into the lung tissue only on the twelfth day in the chick.

The thyroid hody is developed partly as a median diverticulum of the pharyn-
geal hypoblast opposite the ventral ends of the second visceral arches (fig. 125, B, Sd);
partly as a (bilateral) diverticulum of the posterior wall of the fourth visceral cleft.
The median diverticulum in most animals early becomes separated from the
pharyngeal hypoblast, and is thus converted into an island of epithelium imbedded
in mesoblast. In the human embryo, as His has shown (fig. 134, A, thr), it remains
for some time in the form of a hollow bifid vesicle, w^hich 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 ctecum. This, which has been termed the ductus liiigualu,
is the remains of the original thyrolingual duct connecting the median part of the thyroid
with the tongue. It may further happen that the lower part of this connection also remains
in the shape of a tubular prolongation of the median portion of the thyroid towards the root
of the tongue (^chictus tliryoulens ; when well developed this forms the pyramid'). The so-
called accessory thyroid bodies (supra-hyoid, praehyoid glands, &c.) which are occasionally
found near the hyoid bone, are also referable to the thyrolingual duct (His, Anatomie

THE THYMUS.

Ill

menschlicher Embryonen, iii., p. 101, where reference to the literature of these accessory-
thyroids may be found).

The bilateral diverticula, which assist in the formation of the thyroid body, spring
from the fourth visceral cleft (Bora) (fig-. IP.o, ihr). They have at first the appearance
of simple saccular glands partially encircling the developing larynx (fig. l?A, thr').
In front of this they come into connection with the median rudiment, and eventually
blend with it. Like that rudiment, they become entirely separated fi-om the hypo-
blastic surface from which they have taken origin, their cavity disappears, and they

thm

Fig. 134. — Sketches showing the condition of the thyroid and thymus glands in a human

EMBRYO or about FIVE WEEKS. (His.)

A, profile sketch from the left side.

B, fr.intal sketch from behind.

t, tongue ; d, ductus thyreoglossus ; ep, epiglottis ; opposite I, larynx ; tr, trachea ; (e, resophagus ;
thr, median rudiment of thyroid; thr', lateral rudiment of tliyroid ; thm, developing thymus, seen on
the left side of B to be connected with a visceral cleft ; ao (in B), ascending aorta ; no', descending aorta ;
c, carotid.

are converted into ramifying and anastomosing cell-cylinders, between which vascular
connective tissue becomes developed. The cell-cylinders subsequently become hol-
lowed 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.

In most Vertebrates, the lateral and median parts of the thyroid remain distinct ; the
former are the organs known as mpra-per 'rear dial hadh-x in elasmobranchs, and as ticcr.sxnn/
thyroids in other animals. Only in mammals do they become united into one organ as
in man.

The thymns is also developed as a growth of the epithelium (hypoblast) of some
of the visceral clefts ; in birds from the tliird and fourth (fig. ];>;"), Ihijm), in reptiles
from the second, third, and fourth, and in lower Vertebrates from several clefts (de

Fig. 13.5. — Diagram showing the visceral clefts fro.m whioii the

THYMOS AND LATERAL PARTS OP THE THYROID ARE DKVELOI'ED IN

THK CHICK, (de Meuron. )

1, 2, 3, 4, indicate the corresijonding visceral clefts ; tlnjm, rudiments of
thymus ; thr, median rudiment of thyroid ; thr', lateral rudiments of
thyroid.

Menron). In mammals the thymus appears as a (bihiteral)

tubular prolongation l)ackwards of the extremity of tlio

tliird visceral cleft (KolHker), receiving, according to de

Meuron, an acfxission from the hypoblast of the fourth cleft,

as in birds. Tlic tube, which lias a nariow lumen, and

comparatively thick epitJielial lining, is surrounded hy vascuhr connecl i\'(^ tissue,

within which numei'ous lyMi|)hoid cells becfjme accumulated, and grows downwards

along the side and in front of the trachea, where, in nuinunals. it gen(!raliy unites

112 THE LIVER.

with its fellow to form a median organ. Its lower end then gives oflP solid, bud-like
excrescences, and lateral buds come off again from these, so that this pait of the
organ acquires a ramified, lobulated appearance like an acinous gland. The acini are,
however, solid, and remain so, although the upper end of the tube still has a narrow
lumen.

The lymphoid cells next invade the epithelium, growing into every part of the
tubular gland, and converting it into a mass of adenoid tissue. In this process the
epithelium becomes broken up into small isolated portions, some of which remain in
the medullary portion of the lobules as the epithelial nests which are seen in sections
of the fully developed organ, and are known as the concentric corpuscles of Hassall.

The liver. — This organ arises in the form of two diverticula of hypoblast, which
grow from the ventral wall of the duodenum immediately beyond the stomach (figs.
117, 118, Z, 123, LV). They extend into a mass of mesoblastic tissue which connects
the stomach and duodenum with the anterior wall of the abdomen, and which (with
the mesentery, with which it is continuous round the gut) separates the body-cavity
here into a right and left half. In this tissue is the omphalomeseraic or vitelline
vein (and later the umbilical vein) proceeding on either side to the sinus venosus,
and the liver diverticula grow into the mesoblast above and in front of these veins.
Here they ramify, giving off solid buds of cells which grow into columns or cylinders,
and these again give off lateral diverticula of the same kind. So far the development
of the liver resembles that of a compound tubular or acino-tubular gland, except that
the ramifications of the original gland diverticula are from the first solid instead of
hollow. But soon an important diflFerence appears in the fact that the cylinders
unite and anastomose with one another everywhere to form a close network, and
from the cords of this network solid sprouts are again constantly being given off to
form fresh cylinders, thus produciug a yet closer and more intricate network. In
the meantime, capillary blood-vessels are formed in the mesoblastic tissue in which
this formation of cell-cylinders of hypoblast is going on, and these vessels, which
form a network interlocking with that of the anastomosing cell-cylinders, become
connected with branches of the vitelline vein on the one hand {ven(B advehentes),
and on the other with veins (ven(^ revehentes) which pass towards the siuus venosus,
and eventually are found opening as the hepatic veins into the inferior vena cava.

The two original hollow diverticula are the rudiments of the right and left hepatic ducts.
The common bile duct is formed later by a protrusion of that part of the duodenal wall with
which the original diyerticula are connected. This protrusion also eventually receives the
duct of the pancreas, which becomes shifted towards it. As the common bile duct lengthens,
the liver becomes separated from the duodenum, with which it was at first in close connection.
The portal and interlobular bile ducts are formed by the hollowing out of some of the
anastomosing cell-cylinders, so that a lumen is produced within them surrounded by hepatic
cells, which lose their original polyhedral character, and become changed into the columnar
epithelium of the ducts, the anastomoses between the cell-cylinders here disappearing.
The remaining cylinders form the secreting substance of the liver. The biliary canaliculi
appear as minute passages between the cells, and come into continuity with the bile ducts.
With a further development of the connective tissue of the organ, the glandular substance of
the liver, which was at fii-st continuous throughout, becomes separated into lobules, and the
network of cell-cylinders tends with multiplication of their cells to become fused into a con-
tinuous mass within each lobule, the bile canaliculi forming by numerous lateral junctions and
anastomoses a close network of intercellular passages within the lobule.

The grail "bladder and cystic duct are formed by a diverticulum from the common bile
duct, which appears in the second month.

In the elasmobranch fishes, and in amphibia, there is only a single hepatic diverticulum.
The anastomosing cell-cylinders which sprout from this are not solid, but hollow, with a
narrow lumen, and the liver has from the fii-st and retains permanently the character of a
compound gland formed of anastomosing tubules. In reptiles the cylinders also have from
the first a naiTow lumen. In birds and mammals the cylinders are solid, as in man.

As the liver grows, it projects on either side into the pleuroperitoneal cavity. The
mesoblast which unites it to the anterior wall of the abdomen, becomes thinned out to form

THE PANCREAS. 113

the suspensory ligament. That which unites it to the ventral wall of the stomach and duo-
denum also becomes thinned out ; it forms the small or gastro-hepatic omentum. The liver is
at first an exactly symmetrical organ, the right and left lobes being equal in size and
symmetrically placed. After the fourth month the right lobe begins relatively to increase in
size, and at birth the proportion of this to the left lobe is as I'G to 1. The liver also at first
grows very rapidly, so that by the second month it nearly fills the abdomen, and causes a
well marked prominence on the ventral aspect of the embryo. At this time it is calculated to
constitute nearly one half the weight of the body. The proportion, however, gradually
decreases, until at term the relative weight of the liver to the whole body is as 1 to 18. The
further changes which the blood-vessels which pass to the liver undergo will be considered
with the development of the venous system.

The pancreas is developed as a hollow hypoblastic diverticulum from the dorsal
wall of the duodenum opposite the hepatic diverticula, and somewhat later than these
(fig. 12d,B,C,'D,p): This hollow process grows into the mesogastrium or gastro-
duodenal mesentery, which at this time is well developed, and ramifies within this,
producing by its off'-shoots the ducts and alveoli as with other compound acinous
glands. As the duodenal loop becomes formed, and this and the pyloric end of the
stomach are turned over towards the right side, the pancreas loses its median sym-
metrical position, and with the mesentery which encloses it now lies across the back
of the abdomen. This is the condition in which the gland is found in most mammals.
But in man, with the fusion of the mesogastrium (great omentum) to the transverse
mesocolon, the posterior layer of the mesenteric fold which encloses the pancreas
becomes absorbed (Toldt), and the gland becomes fixed across the back of the
abdomen, and is now apparently altogether behind the peritoneum (see fig. 12, 129).

RECENT LITEKATTJRE.

Bemmelen, J. F. van, Entwihkeling en metamorphose der kieuw of viceral-spalten en der aorta-
bogen bij emhryonen van Tropidonotus natrix en LaceHa miiralis, Kin. Akad. v. Wet. Amsterd. Afd.
Natuusk., 1885 ; Die Visceraltaschen u. Aortenbogen bei Reptilien u. Vogeln, Zool. Anzeiger, 1886 ;
Die Halsgegend der Reptilien, Zoo\. Anz., 1887.

Bonnet, K., Ueber die Entwicklung der Allantois und die BUdung des Afters bei den Wieder-
kduern und iiber die Bedeutung der Primitivrinne und des Primitivstreifs bei den Einbryonen der
Sdugethiere, Anat. Anzeiger, 1888.

Bom, G., i'eber die Derivate der embryonalen Schlundbogen und Schlundspalten, Archiv f.
mikr. Anat, Bd. ixii., 1883.

Cadiat, Du developpement desfentes et arcs branchiaux chez I'embryon, Journal de I'anat., &c.,
1883.

Chievitz, J. C, Beitrdge zur Entwicklung sgeschichte der Soeicheldriisen, Arch. f. Anat. u.
Physiol., Anat. Abtheil., 1885.

Demon, F., Diveloppement de la portion sousdiaphragmatique du, tube digestif. LUle, 1884.

Dohrn, A., Die Thyroidea bei Petromyzon, Amphioxus u. Tunicaten, Mitth. aus der zool.
Station z. Neapel, 1886.

Fischelis, Ph., Beitrdge zur Kenntniss der Entwicklungsgeschichte der Gl. thyrcoidea u. Gl.
thymus, Arch. f. mikr. Anat., Bd. xxv., 1885.

His, "W., Ueber den Sinus prcecervicalis und die Thymusanlage, Archiv f. Anat. u. Physiol.,
Anat. Abth., 1886 ; Zur Bildungsf/eschichte der Lungen heim menschlichen Emhryo, Archiv f. Anat.
und Physiol., Anat. Abtheilung, 1887 ; Schlundspalten u. I'hymusanlage {Brief an F. Mall), Arch.
f. Anat. u. Physiol., Anat. Abth., 1889.

Kastschenko, N., Das Schicksal der embryonalen Schlundspalten bei Sdugethieren, Archiv f.
mikrosk. Anat., Bd. xxx., 1887 ; Das SchlundspaUengebiet des Hiihnchcns, Arch. f. Anat. u. Phj-s.
Anat. Abth., 1887.

Liessner, B., Ein Beitrag zur Kenntniss der Kiemenspalten und ihrer Anlagen bei amnioten
Wirbeltkieren, Morpholog. Jahrbuch, Bd. xiii., 1888.

Mall, F. P., Entvncklung der Branchialbogen und Spalten des Hilhnchens, Arch. f. Anat. u.
Physiol., Anat. Abth., 1887 ; The branchiid clefts of the dog, with special reference to the origin of
the thymus gland, Studies from the Biol. Laboratory of John Hopkins University, iv. , 1888.

Ueuron, P. de, Recherches sur le cUveloppement du thymus et de la. (/lande thyroide, Rccueil
zool. Suisse, iii., 1880; Sur le ddveloppement de I'tx soph age, Coinpt. rend., 1886.

Minot, Ch. S., Evolution of the Lungs, Proceed, of the Zoolog. Society of London, 1886.

OstroumofP, A. , Ueber den Blastoporiis u. d. Schwanzdarm bei Eidechsen u. Selachiern, Zool.
^Vnzeig';r, 1889.

Philip, R. "W., Beitrdge zur Lehre iiber die Entwicklung del' Trachea, Mitth. aua d. embryoL
Inst. d. UniverH. Wien, Bd iL, 1883.

114 EECENT LITEEATUKE.

Piersol, Gr. A., Ueber die EnfwicMung der embryonalen Schlundspalten und ihre Dcrivate lei
Sdugethieren, Zeitschr. f. wiss. Zool, Bd. xlvii., 1888.

kabl, C, Zur Bildung.igeschichte des Halses, Prager medic. Woclienschr., 1886 u. 1887.
Retterer, E. , Du diveloppement de la region anale des mammiferes, C. r. de la society de biologie,

1890.

Robinson, A., Olservations on the earlier stages in the development of the lungs of rats and
mice, Journal of Anatomy and Physiology, 1889.

Scbwink, T., Ueber den Zwischenkiefer und seine Nachlarorgane hei Sdugethieren, 1888.

Stieda, Untersuchungen ueher die Entwickl. der Glandula thymus, Glandula thyoideaund Glandula
carotica. Leipzig, 1881.

Swartz, D., Untersuchungen des Schwanzendes hei den Embryonen der Wirhelthiere, Zeitsch. f.
wiss. Zool. xlviii., 1889.

Toldt, C. , JBau u. Wachsthumsverdnderungen der Gekrose des menschlichen BarmJcanales,
Wiener Denkschriften, 1879 ; Die DarmgeJcrose u. Netze im gesetzmdssigen u. im gesetzwidrigen
Zustand. Ibid., 1889.

TJsko-w, N. , Bemerkungen zur Entivicklungsgeschichte der Leber und der Lungen, Archiv f.
mikrosk. Anatomie, Bd. xxii. , 1883.

Wolfler, A., Ueber die Entwickl. u. den Bau der Schildrilse. Berlin, 1880.

DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS. 115

DEVELOPMEISTT OF THE URIWARY AND GENERATIVE ORGANS.

The urinary and generative organs originate in connection with the intermediate
cell-mass, a portion of mesoblast which is seen in sections of the early embryo lying

Fig. 136. — Part of a transverse section op a chick exbrto of 2 days. 6 hours. (Ktilliker.) =5°

uw, ijrotovertebra ; mp, lateral mesoblast ; dfp, splanchnopleuric mesoblast ; kp, soniatopleuric
mesoblast ; p, pleuro-peritoneal cleft (crelom) ; ivcj, Wolffian duct ; wk, part of intermediate cell-mass
from which Wolffian body will become develojied.

between the paraxial mesoblast and the pleuro-peritoneal cleft, and abutting against
the external epiblast (fig. oi), p. )M).

Fig. 137. — Section thkocgh an external glomerulus
OF the pronephros from a chick op about 4 days'
incubation. (Balfour.)

yl, glomerulus ; f)e, peritoneal epithelium ; Wd, Wolffian
duct ; uo, aorta ; 'iiic, mesentery.

vX <>.' r^ as

Some of the cells of this intermediate cell-mass
become differentiated into a longitudinally run-
ning cord, which subsequently acquires a lumen,
and is then known as the Wolffian dud (from its
discoverer, Caspar Friedrich Wolff) (fig.lSC, wg).
Posteriorly the duct opens into the cloaca. The
anterior part of the duct becomes connected with

invaginations of the peritoneal epithelium, between which vascular glomeruli project
fre*jly into the peritoneal cavity (fig. 1;j7). These glomeruli constitute the head
kidney, fore-kidney or p'onepkros} Along its inner side, somewhat further back-

' Hertwig. According to IJalfour and Hedgwick, these glomeruli form tlic anterior part of the
Wolffian body, and the head kidney ia represented by the Miillerian iuvagiuationa referred to later on
(seep. 122 and fig. 145).

I 2

116 DEVELOPMENT OF THE URINARY AND GENERATIVE ORGANS.

wards, a series of transversely coursing tubes becomes developed in the intermediate
cell-mass. These tubes are connected for a time with other involutions of the
peritoneal epithelium (fig. 141), but subsequently lose their connection with that

Fig. 138. — Diagrams of the arrangement of the urinary and genital organs in elasmobranchs,

(Balfour.)

A. — Diagram of the primitive condition of the kidney in an elasmobranch embryo.

pd, segmental duct ; opening at o, into the body cavity and at its other extremity into the cloaca ;
X, line of separation between the Wolffian duct above and the Mtillerian duct below ; st, segmental tubes,
opening at one end into the body cavity and at the other into the segmental duct.

B. — Diagram of the arrangement of the urino-genital organs in an adult female

elasmobranch.

m.d, Mtillerian duct ; ^v.d, "Wolffian duct : s.t, segmental tubes ; five of them are represented with
openings into the body cavity, and five posterioiiy correspond to the metanephros ; oi', the ovary ; d,
ureter.

0. — Diagram of the arrangement of the urino-genital organs in an adult male elasmobranch.

m.d, rudiment of Mtillerian duct ; iv.d, Wolffian duct, serving at vd as vas deferens ; s.t, segmental
tubes, two represented with openings into the body cavity ; d, ureter ; t, testis ; nt, canal at the base
of the testis ; V.E, vas efferentia ; Ic, longitudinal canal of the Wolffian body.

epithelium, and acquiring glomeruli at one part, at another part open into the
Wolffian duct. They form the mid-kidney, Wolffian body or mesonephros, which

THE WOLFFIAN DUCT AND BODY. 117

presently projects as a distinct vascular organ along the dorsal part of the peritoneal
cavity on either side of the mesentery. Subsequently another duct becomes deve-
loped along the outer side of the Wolffian body, along which it runs backwards to
open also into the cloaca : in fi'ont it communicates with the pleuroperitoneal cavity
by one or more funnel-shaped apertures (tig. 143, s). This is the Miillerian dud, so
named after Johannes ^Mi'iller ; m some of the lower vertebrates it arises in common
with the "Wolffian duct. From the lower end of each Wolffian duct a hollow pro-
trusion (tig. 125, C and D, N) grows upwards into a mass of mesoblast continuous
with that of the Wolffian body ; with the branches of this protrusion glomeruli and
convoluted tubes also become connected, and thus the permanent kidney {hind-kidney,
metanephros) is produced. Lastly, the ccelomic epithelium covering the inner side of
the Wolffian body becomes thickened (tig. 143, a), and within it are found larger
cells, from which the generative products in both sexes (ova and spermatozoa) are
eventually derived. This epithelium is accordingly known as the germinal epithelium.
The duct of Miiller becomes in the female the oviduct or Fallopian tube ; in the
male it becomes atrophied. The Wolffian duct in the male becomes the epididymis
and vas deferens ; while the vasa efferentia and tubes of the rete testis are formed
as outgrowths from the Wolffian body ; in the female these parts have no permanent
function.

The head kidney, although permanent and functional in tishes, is only a rudi-
mentary organ in the embryo of higher vertebrates, and soon disappears. The
Wolffian body is well developed in all vertebrates ; in fishes and amphibia it is an
important part of the permanent urinary apparatus, and also serves to cany away
the male sexual products (tig. 138). In higher vertebrates (amniota) it no longer
continues to perform excretory functions, but still supplies the efferent apparatus of
the testis.

The details of the development of these parts may next be considered.

The Wolfa-an duct and body. — The commencement of the Wolffian duct is
seen at a very early period of development (second day in the chick, eighth day in
the rabbit) as a thickening of the intermediate cell-mass in the anterior region oi
the trunk (fifth somite) (fig, 39, Wd). The outgrowth projects towards the epiblast, and

"??'.&-.

Fig. 139. — Transverse section of an embryo chick of thirty-six hodrs. ''" (E. A. S.)

n.c, medullary tube ; p, protovertebra ; cp, epiblast ; me, lateral mesoblast split into splanchnopleuro
anrl Romatopleure ; c<£, pleuro-pentoneal cavity between them ; cce', cavity of protovertebra. continuous
oil the right hide with the lateral mesobla-stic cleavage; W.d., Wolffian duct; W.h., mesoblast of
Wolffian body ; ch, notochord.

developes from before backwards ; a solid cord of mesobltist thus becomes formed, which
gradually becomes detached from the remainder of the intermediate cell-mass, lying
close to the epiblast (tig. 13!), uny). Soon after it is thus formed, a Imnen ajijxjars in
it and extends botli forwards and backwards. The posterior end, which is still solid,
is presently found to be attached to the epiblast, and apparently continues to grow
backwards along and at the expense of the epiblast until it reaches the posterior end

118

THE WOLFFIAN 13UCT AND BODY.

of the body, where it becomes detached from the epiblast, and is connected with and
opens into the hind- gut (cloaca).

I have here followed what has appeared to me the most probable account of the ori<?m of
the duct (Martin, Strahl), but it is right to state that in the opinion of some observers
(Hensen. Spec, Flemming) the formation and growth of the duct in connection with the
epiblast is primary, especially in mammals, and the duct is originally formed by a longitudinal
thickening and involution of the epiblast, which only secondarily becomes connected with the
intermediate cell-mass. Compare also Haddon, Origin of segmental duct, Proc. Eoy. Dublin
Society, Vol. V.

In teleosteans (Rosenburg) and amphibia (G-otte) the Wolffian duct has been described as
developing in the form of a longitudinal groove-like invagination of the somatopleural meso-
blast (Balfour, Comp. Emb., vol. ii., pp. .580, .582), but more recent researches appear to indi-
cate that in these animals also the epiblast may be concerned in its formation.

The Wolffian body developes in the intermediate cell-mass between the Wolffian
duct and the body-cavity as a series of transverse tubes which lie at right angles to
the course of the Wolffian duct, and open into it at regular intervals. The usual

Fig. 140. — Transverse sec-
tion OP THE TRUNK OF A
CAT EMBRYO, SHOWING THE
VESICULAR STAGE OF THE
WOLFFIAN TUBULES.

(E. A. S.)

m.j)-, muscle plate ; ao,
aorta ; in.g., mid-gut ; am,
amnion ; iv, vesicle of Wolffian
body ; w.d.. Wolffian duct ;
cce, coslom.

mode of formation of
these tubes — which are
sometimes termed seff-
men tal tubes — appears
to consist in the accu-
mulation at regular intervals, corresponding with the somites, of rounded masses
of mesoblast on the mesial or ventral side of the Wolffian duct (fig. 142, w.l).),
which masses become afterwards hollowed out so as to form small vesicles, at
first isolated, but afterwards growing towards and opening into the Wolffian
duct (fig. 140).i Corresponding with these vesicles there become formed invagi-
nations of the epithelium of the body-cavity (fig. 141, si), which is thickened
along the inner side of the Wolffian projection, and grows at regular intervals
towards the vesicles. These ingrowths may at first communicate by funnel-shaped
openings, which in some lower vertebrates are lined by ciliated epithelium, with
the body-cavity, but the openings in higher vertebrates become closed again before
communication with the Wolffian duct is established. Finally, the connection
between the Wolffian tubes and the peritoneal epitheHum is completely severed, and
the condition of simple or curved transverse tubes, blind at their inner ends and
opening at their outer ends into the AVolffian duct, is produced (fig. 142, B). After
a time the blind extremities are seen to be enlarged and spoon-shaped, and glomeruli

1 According to v. Wijhe the hollow condition is the primary one in elasmobranchs, and the cavity of
each vesicle represents an intermediate part of the coelom of the segment (meso-ccelom), the dorsal
ccelom being represented by the cavity of the proto-vertebra and the ventral ccelom Ly the pleuroperi-
toneal space. I have myself observed this condition of a hollow intermediate cell-mass communicating
on the one hand with the cavity of the protovertebra and on the other with the cleft of the lateral
mesoblast, in a chick of 36 hours (see fig. 139).

In mammals the Wolffian vesicles are more numerous than the segments.

THE WOLFFIAN DUCT AND BODY.

119

are observed developing in the bowl of the spoon from mesoblast cells, which
presently become entirely enclosed by the end of the tube. Subsequently a second

Fig. 141. — Tkaxsverse section through the trunk of a duck embryo with about twenty-pour

MESOBLASTIC SOMITES. (Balfour.)

am, amnion ; so, somatopleure ; sp, splanchnopleure ; 7cd, Wolffian duct : st, segmental tube with
peritoneal involution ; ca.v, cardinal vein ; m.a, muscle-phitc ; ap.f/, spinal ganglion ; gp.c. spinal cord ;
ck, notochord ; ao, aorta ; hy, hypoblast.

and a third set of tubes become developed in a similar manner, but without perito-
neal invaginations, and also open directly into the Wolffian duct. Lastly, other tubes

Fig. 142. — ^Transverse sections of sheep emuryoes, showincj two stages in the development of
the wolffian body. (Bonnet.)
w.d, Wolffian duct; w.h. Wolffian body; p.v, protovertebra ; cli, noto chord ; n.c, neural canal;
am, amnion ; oo, aorta ; i, intestine; j/.s, yolk-sac.

with glomeruli become formed between, and open into those which arc already
connected with that duct. All these tubes arc short and straight when first deve-
loped, but aftei-wardg lengthen and become converted into convoluted uriuifei-ous
tubes, whioli, like tho.se of the permanent kidneys, begin in a dilated extremity
enclosing a tuft of capillary blood-vessels (glomerulus), which are supplied by
branches of the primitive aortse.

120

THE WOLFFIAN DUCT AND BOD 7.

When completely formed, the Wolffian bodies are seen on opening the abdomen
of the embryo as long prominent vascular organs projecting into the peritoneal
cavity on either side of the intestine, and showing in section numerous Malpighian
corpuscles and uriniferous tubules variously cut (fig. 143).

Soon after having attained its complete condition of development, the Wolffian
body begins to undergo atrophic changes. These proceed much further in the

Fig. 143. — Transverse section op the

WOLFFIAN BODY OF THE CHICK ON THE

FOURTH DAT. (Waldcyer. )

m, mesentery ; L, body wall ; a', thickened
epithelium from which the involution of the
anterior part of the Miillerian duct z, is
taking place ; a, thickened germinal epithe-
lium in which are seen primitive ova, o ;
E, modified mesoblast which will form the
stroma of the ovary ; WK, tubules of Wolffian
body variously cut ; y. Wolffian duct. Two
glomeruli are shown in the Wolffian body.

female sex than in the male, but the
tubules of the organ do not entirely
disappear in either sex. In the
female they form the rudimentary
organ which is known as the far'
ovarium {epooplioro7i of Waldeyer),
while the main tube of that struc-
ture represents a remnant of the
Wolffian duct. But in many animals,
e.g., the sow, the Wolffian duct re-
mains as the duct of Gartner, a
strong, slightly undulated tube, which is traceable, at first free in the broad ligament
of the uterus, and lower down becoming incorporated with the wall of the uterus
and vagina, upon which last it becomes lost. Traces of this tube can sometimes be
seen in sections across the body or cervix of the adult human uterus, and even lying
in the Avail of the vagina.

In the male the Wolffian duct forms the tube of the epididymis, the vas deferens,
and the ejaculatory duct ; the seminal vesicle being formed as a diverticulum from
its lower part. The coni vasculosi and tubuli eflFerentes are in all probability formed
by the persistence of some of the tubules of the Wolffian body. The Malpighian
corpuscles of these tubules have long disappeared, but previous to their disappearance
solid columns of epithelial cells, afterwards becoming tubules, grow from the walls of
those corpascles towards the germinal epithelium (fig. 153), where, in the male, they
become continuous with and enclose cells derived from that epithelium (which
subsequently form the epithelium of the seminiferous tubes), and thus produce the
walls of the seminiferous tubules and the rete testis. In the female sex there is also
a growth of solid cellular columns towards the germinal epithelium, but no connec-
tion becomes established between them, and the columns do not become tubular.
The organ of Giraldes and the vasa aberrantia of Haller are probably the remains
of one or more Wolffian tubules.

Suprarenal capsules. — These organs are intimately connected in their development with
the Wolffian bodies. According- to the observations of Weldon some of the cellular columns
which grow from the Malpighian corpuscles of the upper part of the Wolffian body towards
the germinal epithelium give offsets which pass upwards towards the inferior vena cava, and
there become developed into the cortical substance of the suprarenal capsules. (Mihalkovics,
on the other hand, states that the strands of cells which grow from the upper part of the

SUPRAKENAL CAPSULES.

121

"Wolffian body to take part in the formal ion of the suprarenal capsules have been derived by-
proliferation from the grerminal epithelium.) It had longr been believed that the two parts of
these organs, cortical and medullary, are separate in origrin : the former being derived, as was
thought, from cells which are of mesoblastic origin, the latter being developed in connection
with the sympathetic ganglia. In elasmobranchs and some other lower vertebrates, they

Fig. 144. — ISTERNAL ORGANS OF A FEMALE HUMAN FffiTCS OF 3J INCHES LONG, OR ABOUT 14 WEEKS.

Magnified (from Waldeyer).

0, the ovary full of primordial ova ; e, tubes of the upper part of the "Wolffian body forming the
epoophoron (parovarium of Kobelt) ; W, the lower part of the Wolffian body forming the paroophoron
of His and Waldeyer ; w', the Wolffian duct ; M, the Mtillerian duct ; m', its upper fimbriated opening.

Fig. 145. — Internal genital organs of a male human embryo of 3i inches long (from Waldeyer).

t, body of the testicle with seminal canals formed ; e, epididymis, or upper part of Wolffian body ;
W, Wolffian body, lower part, becoming paradidymis or organ of Giraldes : w' Wolffian duct, becoming
vas deferens ; (/, gubernaculum.

Fig. 146. — Two figures exhibiting a comparison between parts of the generative organs in
THE TWO SEXES (from Farre, after Kobelt).

A. — Adult ovary, parovarium and fallopian tube.

a, n, Epoophoron (parovarium) formed from the upper part of the Wolffian body ; h, remains of the
opperrnoKt tubes, sometimes forming liydatids ; r, middle set of tubes ; d, some lower atrophied tubes ;
e, atrophied remains of the Wolffian duct ; /. the terminal bulb or hydatid ; h, the Fallopian tube,
originally the duct of Miiller ; i, hydatid attached to the extremity ; I, the ovary.

B. — The adult tkstis and epididymis.

a, a, convoluted tubes in the head of the epididymis developed from the upper part of the Wolffian
body : h and /. hydatids in the head of the epididymis ; c, coni vasculosi ; d, vasa aberrantia ; /',
remainii of the duct of Muller with i, the hydatid of Morgagni, at its upper end ; I, body of the testis.

]32 THE MULLERIAN DUCT.

consist throughout life of two separate portions, one median and single, the other, derived
from the sympathetic ganglia, paired ; in birds, reptiles, and mammals these distinct portions
are combined into the two paired organs (Balfour). But in these also, as has been shown by
Mitsukuri for mammals, the medullary or nervous part is at first distinct and outside the
cortical, into which it gradually insinuates itself, retaining, however, its connection with the
neighbouring sympathetic ganglia.

The permanent kidneys arise (1) as protrusions from the posterior end of the
Wolffian ducts (see fig. 125, C and D, N), which grow forwards towards the lower part
of the Wolffian bodies, and form the ureters and the collecting tubules of the kidney ;
(2) from a portion of the intermediate cell-mass situated posterior to the Wolffian
body, and within which convoluted tubes and Malpighian corpuscles, and eventually
the remaining parts of the uriniferous tubules become developed. But before these
changes occur in this intermediate cell-mass, it shifts its position relatively to the
Wolffian body, eventually coming to lie above and behind that organ. The con-
voluted tubes, with their Malpighian corpuscles, appear to be developed independently
of the ureter and collecting tubes, as in the case of the Wolffian tubules and the
Wolffian duct, a communication between them being only subsequently established.

The glomeruli are apparent in the eighth week in the human foetus. In the
third month the papillae are formed, and in the fourth month the loops of Henle are
seen. The tubes are wider in the foetus than in the adult ; the expansion of the
kidney as growth advances must therefore be due mainly to an increase in length of
the tubules, since new tubules and glomeruli do not appear to be formed. The
human kidney is at first lobulated, the lobules corresponding in number to the
Malpighian pyramids, but by the end of the first year after birth, the kidneys have
usually nearly lost their lobulated appearance.

The nrinary bladder is formed by a spindle-shaped dilatation of the stalk of
the allantois (second month). The upper pole of the spindle extends as the urachiis
into the umbilical cord ; it not unfreqnently remains hollow for some length within
the cord (Luschka). The lower pole of the spindle which passes towards the cloaca
becomes the first part of the urethra of the male, and the whole of the urethra
of the female. The rest of the male urethra is formed and enclosed by the folds of
integument which produce the penis (see p. 128). The ureters, which are originally
prolonged from and open into the Wolffian ducts, subsequently become shifted in
position, so as eventually to open into the enlargement of the allantoic stalk, from
which the bladder is formed.

The Miillerian duct. — In lower vertebrates, as was shown by Balfour for
elasmobranchs, this duct takes origin by the splitting off of the ventral part of a
longitudinal segmental or Wolffian duct, the dorsal part remaining as the Wolffian
duct proper, and receiving the segmental and uriniferous tubes, while the ventral part
retains the funnel-shaped orifice, by which the segmental duct communicated ante-
riorly with the body cavity, and comes to open posteriorly into the cloaca by an
orifice distinct from that of the Wolffian duct (fig. 138 and fig. 147). In amniotic
vertebrates, the process of formation of a Miillerian duct is somewhat different.
It arises on the outer side of the already fairly well developed Wolffian body, and
some little distance from the anterior end of that organ, as a thickening of the
peritoneal epithelium (fig. 143, a'), which thickening becomes invaginated towards
the adjacent Wolffian duct, in the form of three successive funnel-shaped depressions
(fig. 148), somewhat similar to those which are connected with the previously formed
Wolffian segmental tubes. The invaginations are connected together by a con-
tinuous epithelial ridge, forming a cord which becomes disconnected from the
peritoneal cavity except at the anterior invagination, and subsequently acquires
a lumen. The short tube which is thus formed, soon begins to grow backward
as a solid rod of cells, which comes in close contact as it proceeds with the Wolffian

THE M0LLEEIAN DUCT.

123

duct (fig. 149). To this duct it presently adheres intimately, and then continues
Fig. 147. — Four sections through the anterior part of the

SEGMENTAL DUCT OP A SCYLLIUM EMBRYO. (Balfour. )

The figure shows how the segmental duct becomes split into tlie
WolfEan duct dorsally and the Mtillerian duct or oviduct ventrally ;
]Vd, Wolffian duct ; od, ]\Iiilleiian duct or oviduct ; sd (in D),
segmental duct.

to grow backwaras for a certain distance as a thicken-
ing of the epithelium of that tube, the thickening
becoming gradually separated off' fi'om before back-
wards, and the lumen passing along it. Further back
it ceases to grow thus in conneetiou with tbe Wolffian
duct, but is prolonged as an independent cellular cord,
which lies in a groove along the side of the W^olffian
duct (Balfour and Sedgwick).

Entering the (jenital cord} the two Miilleriau ducts
lie at first on the mesial side of the corresponding
"Wolffian ducts, but lower down pass behind them ;
they finally come again between these ducts, lying
close together, and, according to Mihalkovics, approach
close to the siuus urogcnitalis, which by this time is
formed out of the ventral part of the cloaca (see p. 128)

without actually opening into it for some time. The j\Iiillerian ducts fuse together
below into a single tube (fourth month) ; the fusion begins not at the lower end,

A

^^mk

wd 1

,B

WK^mmm

^]^^-.cZ

m

^

%^ a~»^^ .. h

C-

D

^1

Fig. 148. — Sections from the chick showing two of the peritoneal invaginations which givk
RISE to the anterior PART OF THE MiJLLERiAN uucT. (Balfour and Sedgwick.)

gr", second invagination ; (jr\ third invagination ; r^, epithelial ridge between them ; Wd^
WolflBan duct. These structures form the pronephros of Balfour and Sedgwick (see note, p. 115).

Fig. 149. — Two SECTIONS from the chick.
showing the junction of the ter-
minal solid portion of the Mul-
lekian duct with thk Wolffian
duct. (Balfour and Sedgwick.)

In A, the terminal portion of the duct
Ib quite diHtinct ; in B it has united
with the wall of the Wolffian duct, md,
Mullerian duct ; Wd, Wolffian duct.

but a short distance away from
this (fig. 100, ?>), and proceeds
both downwards towai'ds the
future orifice and upwards for a

' A name given to the thickened mass of tissue which Kurroundn the Woltlian ducts as they course
together to the cloaca behind tlie stalk of the alJantois (afterwards the base of the bladder).

19A

DEVELOPMENT OF THE OVAEY.

certain length, the amount of this upward extension of the fused ducts varying in
different animals.

The united part of the Miillerian ducts afterwards forms the foundation of the
vagina and uterus in the female, and the prostatic vesicle, or uterus masculinus in

Fig. 150. — Transverse sections of the

GENITAL COED IN A FEMALE CALF EM-
BRYO. Magnified fourteen diameters.
(Kblliker. )

1, near the upper end ; 2 and 3, near the
middle ; 4, at the lower end ; a, anterior,
p, posterior aspect ; to, Miillerian ducts
united or separate ; iv^ Wolffian ducts.

the male ; the upper or fore part of
the Miillerian duct disappears in
the male, in the female it forms the
oviduct (Fallopian tube).

The hydatids of Morgagni are
remnant of part of the Miillerian

believed
duct.

to represent in the ma^e the

In the human embryo of the thii-d month the uterus is bifid, and it is by the upward ex-
tension of the median fusion that the triangular body of the uterus is produced. The bifid
condition corresponds with the bicorned uterus of many animals, and the process of fusion
above described explains the occasional malformation of a partial or comxplete division of the
uterus and vagina into two passages. Up to the fifth month there is no distinction between
vagina and uterus. Then the os uteri begins to be seen, and the cervix uteri subsequently
becomes manifest as a part, which is at fii'st thicker and larger than the rest of the organ.

In some animals the prostatic vesicle of the male is prolonged into cornua and tubes like
the uterus of the female.

The germinal epithelium. — This name was given by Waldeyer to the thick-
ened epithelium lying along the inner side of the Wolffian projection (fig. 143, a).
The cells become at first columnar, and then two, three, or even several layers thick,
while at the same time the mesoblast below them becomes increased in amount, and
thus a marked projection is produced, which in some vertebrates forms a distinct
ridge — the genital ridge. Amongst the cells of the germinal epithelium, some are
seen which are larger and more spherical than the others, these are the primordial
ova (fig. 148, o), and occur in both sexes ; in fact, up to a certain point, the differ-
ence of sex of the embryo is not apparent.

Bevelopment of the ovary. — In the female sex the germinal epithelium soon
becomes much thickened, and begins to grow down into the mesoblastic stroma in the
form of columns of epithelium cells, which enclose amongst them some of the prim-
ordial ova.^ These columns constitute the egg-tuhes of Pfliiger (fig. 152). They
are separated from one another by mesoblast, which grows towards and into the
germinal epithelium simultaneously with the down-growth of the egg-tubes, and
there is thus produced a complete interlocking of strands of connective and epithelial
tissue, which together constitute the ovary. The egg-tubes next become broken up
into rounded groups or " nests " of germinal epithelial cells, each of which may
enclose one or more primordial ova. The primordial ova eventually develope into
ordinary ova, two or more frequently fusing together to form a single ovum (Balfour),
■while from the remaining cells in the "nest " the epithelium of the Graafian foHicle
is eventually produced. In many of the cell nests, primordial ova cannot at first be

^ Mihalkovics states that the cells which are to form the follicular epithelium iirst sink into the
stroma, and that afterwards the primordial ova follow them, and become enclosed by them.

DEVELOPMENT OF THE TESTICLE.

J2-

distinguished, bnt become formed subsequently by an increase in size of one or more
of the cells. The further changes which take place in the Graafian follicle are
described with the structure of the ovary (v. Splanchnology). The remainder of

Fig. 151. — Traxsverse section through the ovary op an embryo shark (scyllium), showing the

GERM-EPITHELIUM FORMING TRIMITIVE OVA. (Balfour.)

At po, the germ-ei/ithelium and primitive ova ; the lightly-shaded part is the ovarian stroma,
covered elsewhere by flattened epithelium.

the germmal epithelium which is left covering the surface after the formation of
the egg-tubes, constitutes the permanent epithelium of the ovary.

Most, if not all, of the pei-manent ova are produced, at least in the human subject, long
before birth. In the human ovary the nests of cells which are to form the Graafian follicles

Fig. 152. — Section op the ovary of a

NEWLY-BORN CHILD. HiGHLY MAG-
NIFIED. (Waldeyer. )

a, Germinal epithelium dipping in at
b, to form an ovarian tube ; c, c, prim-
ordial 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.

are more equally diffused through the
substance of the ovary than in most
animals, in many of which the young
follicles remain forming a stratum
near the surface. In the human
embrj'O of from four months up to
the period of birth, the ovary seems
to be formed of little else than a mass
of younj^ ova, closely surrounded by
flattened cells of the germinal epithe-
lium and constituting thus minute
Graafian follicles ; the amount of

Btromu being at this time relatively small. It has been calculated that the ovaries may
at this stage contain as many as 70,000 primordial ova.

Development of the Testicle. — The germinal o])ithclium does not undergo so
marked an liy|)<:)tropliy in tli'; male as in tlic female. IJiit it hecoiiies thickened,
and enlarged cells, corresponding to the primordial ova in the female, arc found in
it. Further, small strands of the epithelium dip down into the subjacent mcsoblast,

126

DESCENT OF THE TESTICLES.

which grows simultaneously into the epithelium, and eventually cell-nests are sepa-
rated and included in the mesoblastic tissue. Whether these nests are derived from
the division of the primordial ova only, or whether they also include other cells of the
germinal epithelium is not clear. It would appear that from these cell-nests the
epithelium of the seminiferous tubules is developed, although all stages of the
process have not been observed. The cell-nests eventually become connected with
the outgrowths from the WoMan bodies (fig. 153, st), which as ah-eady mentioned,

Fig. 153. —Section of the GERiitNAL epithelium and adjacent stroma in a male chick embryo.

(Semoii. )

g.tp, germinal epithelium forming a tliickened 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 "Wolffian body towards the germinal epithelium, and one of
whicli appears connected with an enlarged primitive ovum.

form the rete testis and the efferent tubes of the testicle. The reproductive gland is
in both sexes at first attached directly to the Wolffian body (fig. 156, A, ot), which
itself is attached by a fold of peritoneum to the back of the abdominal cavity.
This fold becomes the mesovarium or mesorcMum as the case may be. A band also
passes from the Wolffian body upwards to the diaphragm, and another fold contain-
ing involuntary muscular fibres — the plica gubernatrix — runs down towards the
groin from the lower part of the Wolffian body and the duct. This band, as the
Wolffian body becomes atrophied, is found to be attached to the reproductive organ,
constituting the gubernaculum testis in the male, and the round ligament of the ovary
in the female (fig. 156, g).

Descent of the testicles. — The testicles originally lie in the lumbar region of
the abdomen. From this part they become shifted, at first to the intestinal
abdominal ring, opposite which they are found in the sixth month, and which they
enter in the seventh month, then down the inguinal canal into the scrotum, which

THE EXTERNAL ORGANS.

127

they usually enter by the end of the eighth month. But previously to this, a pouch
of peritoneum — i\\Q processus vaginalis — has descended into the scrotum along the
abdominal ring, pushing before it part of the internal oblique muscle and the
aponeurosis of the external oblique, which form respectively the cremasteric muscle

Fig. 154. — Diagrams to illustrate the descent op the testicle and the formation of its

COVERINGS. (0. Hertwig. )

In A the testicle is lying close to the internal abdominal ring. In B it has passed into the s;\c 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 sper-
matic fascia ; 4, peritoneum ; 4', processus vaginalis ; 4" visceral layer of processus vaginalis covering
testicle ; t, testicle ; v.d., vas deferens ; r, internal abdominal ring.

and spermatic fascia (fig. 154). 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, as before mentioned,
from the integument of the groin, which afterwards forms the scrotum, upwards
through the abdominal ring to the lower part of the epididymis. When the pro-
cessus vaginalis is formed, the gubernaculum lies behind the serous sac. The
descent of the testicle is accompanied by a shortening of the gubernacular cord,
which thus appears to draw the organ downwards into the scrotum, and the testicle
following the line originally taken by the gubernacular cord, also 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 durinjj;- 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
infjuinal canal or within the abdominal cavity, are not unfrequent in the human subject.

The ovaries also undergo a considerable change of position, accompanied by a
shortening of the band which corresponds with the gubernaculum testis in the male.
This band, as it passes by the united part of the Mlillerian ducts which are forming
the body of the uterus, becomes attached laterally to that organ, and the descent ol'
the ovary is normally arrested at the side of the uterus. In rare cases, however, the
ovaries pa&s through the abdominal ring by the canal of Nuck, and may even be
found in the labia majora, where they resemble in position the testicles within the
scrotum.

The External Organs. — The external organs are up to a certain time entirely
of tlie same form in Ijoth sexes, and the several organs which afterwards distinguish
the male and female externally have a common origin (see lig. ir.5). A cloaca exists
till after the fifth week, and the genital eminence from which the clitoris or penis is
formed makes its appearance in the course of the fifth or sixth week in fi'ont (jf and
within the orifice of the cloaca. In the course of the seventh and eighth weeks this
orifice is seen to be divided into two parts ; but the exact manner in which the sepa-
ration of the two apertures takes place has not been accurately traced. Tlic process

128

THE EXTERNAL ORGANS.

is connected with the formation of the urogenital cord as an independent structure,
and results in the division of the cloaca into a dorsal or anal and a ventral or
urogenital part {itrogenital sinus). Somewhat later, in the ninth or tenth week, a
transverse integumental band completes the division, which band forms the whole of
the perineum of the female, and the part of the perineal integument in the male
which is situated behind the scrotum.

Of the two apertures the dorsal one or anus is of small size, and is surrounded by
a small circular integumental ridge ; the anterior or urogenital aperture forms a

Fig. 155. — Development of the external sexual

ORGANS IN THE MALE AND FEMALE FROM THE
INDIFFERENT TYPE. (Ecker. )

A, the external sexual organs in an embryo of about
nine weeks, in which external sexual distinction is not
yet established, and the cloaca still exists ; B, the same
in an embryo somewhat more advanced, and in which,
without marked sexual distinction, the anus is now
separated from the urogenital aperture ; C, the same in
an embryo of about ten weeks, showing the female type ;
D, the same in a male embryo somewhat more advanced.
Througbout the figures the following indications are
employed ; pc, sexual eminence (penis or clitoris) ; to
the right of these letters in A, the umbilical cord ;
p, penis ; c, clitoris ; cl, cloaca ; iig, urogenital open-
ing ; a, anus ; Is, cutaneous elevation which becomes
labium or scrotum ; I, labium ; s, scrotum ; co, caudal
or coccygeal elevation.

narrow vertical slit wider behind than before, and running forward as a furrow into
the rudiment of the penis, or clitoris.

The well marked eminence in the integument which forms this rudiment, at first
indifferent in the two sexes, is surrounded by a deep circular fold of the integument
which encompasses its base, and which is the foundation of the mons veneris and
labia majora in the female, and when united by median fusion, of the scrotum in the
male. The lips of the urogenital furrow, which in the female are converted into the
nymphse, and in the male unite as the integument below the penis, are both at first
precisely the same in all embryoes. In the open condition, which continues until
the eleventh or twelfth week, the parts appear alike in both sexes, and resemble
the more advanced female organs. The rudiments of Bartholin's or Coivper's
glands appear at an early period as involutions of epithelium, near the root of the
rudimentary clitoris or penis, on each side of the genito-urinary passage.

In the female, the outer circular fold of integument enlarges at the sides so as to
cover the clitoris as the labia majora. The clitoris itself remains relatively small,
and the groove on its under surface becomes less and less marked, owing to the
opening out, and subsequent extension backwards, of its margins to form the nymi^m.
The vascular bulbs, sunk more deeply in the tissues than in the male organ, remain
distinct and separate, except at one point where they run together in the gians
clitoridis. The liymm begins to appear about the fifth month as a fold of the lining
membrane at the opening of the genital passage into the urogenital sinus. Within
the vestibule, which is the shortened but widened remains of the urogenital sinus,
the urethral orifice is seen, the urethra itself undergoing considerable elongation.

In the male, on the contrary, the fmis continues to enlarge, and the margins of
the groove along its under surface gradually unite from the primitive urethral orifice
behind, as far forwards as the glans, so as to complete the long canal of the male
urethra, which is therefore a prolongation of the urogenital sinus. This is accom-
plished about the fifteenth week. When the union remains incomplete, the abnormal

THE EXTERNAL ORGANS. 129

condition named hypospadias is produced. In the meantime the prepuce is formed,
and, moreover, the lateral cutaneous folds also unite from behind foiTvards, along the
middle line or raphe, and thus complete the scrotum, into which the testicles descend
in the course of the eighth month of foetal life, as before described.

The corpora cavernosa, which are at first separate, become united in their distal
portions in both sexes ; but the corpus spongiosum urethrjE which is also originally
divided in all embryocs, aud in the female remains so in the greater part of its
extent, becomes enlarged in the male in the glans penis, and its two parts become
united mesially both above and below the urethra, so as to enclose the whole of that
tube from the bulb forwards to the glans.

The following Table and Diagrams exhibit the corresponding parts of the
urino-generative organs in the two sexes : —

130

GENITO-UEINAEY OKGANS OF THE TWO SEXES.

■<

Pi
O

O

fi

i

1^

O

Ph

D

p3

M

pi

g

(^

o

o

tH

o

rt

9

f^

Ci o

O 02

rCl

t;

fl

C(l

(t;

pi

.

?«

o
CO

CO

1=1

o

cti

1=1

SB^

T^

-s a

M o P

pi

c3

, p ,

Ti -u tS

4^

1=J

U

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^

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m

t^

P^

P

a

." <!> O
OQ ^ HH

52 1^

■75

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o

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P

P

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P

p

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03

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P

^

o-.

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p

^

p

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iM

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rH

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yy

00

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c3

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p^

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pi P ^

cc P O

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P S

Ph P

rP -P

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o
O

a

co^ 5

"S 2
o Cm DQ

■73

p

c3

O

-P

■XJ

2

rP

Ph

s

P
P

c3

P P

P >► 5i3 o

P

p
o

CO

-73

o

CO

OQ

«*=.

p

DC

Ti

'%

O

P

r^

.+J

P

ci

s

— '

CO

a;i

a>

o
d

o

o

P

C
O

P, o Ph
P P P

P

CO

tu

R o
P O

P

lYxijaaJC)

•SILMIg

lYimaQOHfi

THE EXTERNAL ORGANS.

131

Fig. 156. — Diagrams to show the development

OF MALE AND FEMALE GENERATIVE ORGANS

FROM A common TYPE. (Allen Thomson.)

A. — Diagram of the primitive uro-genital
organs in the embryo previous to sexual
distinction.

3, ureter ; 4, urinary bladder ; 5, uraclius ;
ot, the genital ridire from which either the ovaiy
or testicle is formed ; W, left Wolffian body ;
WAC, right and left ^Yolffian ducts ; m.m, right
and left lliillerian ducts uniting together and
running with the Wolffian ducts in r/c, the genital
cord ; ng, sinus urogenitalis ; i, lower jiart of
the intestine ; cl, cloaca ; ep. elevation which
becomes clitoris or penis ; Is, 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 ; po, 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 Fallo-
pian tube ; u, uterus ; the Fallopian
tube of the right side is marked m ;
X), round ligament, corresponding to
gubernaculum ; i, lower part of the
inte.stine ; va, vagina ; h, situation
of the hymen ; C, gland of Bartholin
(Cowper's gland), and immediately
above it the urethra ; cc, corijus ca-
vernosum clitoridis ; sc, vascular bulb
or corpus spongiosum ; n, nympha ;
/, labium ; v, vulva.

C. — Diagram of the male type or
sexual organs.

t, testicle in the place of its original
formation ; r, caput epididymis ; vd, vas
deferens ; W, scattered remains of the
Wolffian bod}', constituting the organ of
Giraldes, or the paradidymis of Waldeyer;
vh, va-s aberrans ; w, Jliillerian duct,
the ufiper part of wliich remains as the
hydatiil of Morgagiii, the lower part,
represented by a dotted line descending
to the prostatic vesicle, constitutes the
occa-sionaliy existing cornu and tube of
the uterus masculinus ; (j, the guberna-
culum ; VH, the ve.-iicula seminalis ; pr.
the prostate gland ; C, Cowper's gland of
one sid ; ; cp, corpora cavernosa penis
cut short ; up, coq>us spongiosum urc-
thrai ; g, scrotum ; t' , together with the
dotti.-d lines above, indicates the direction
in which the testicle and epididymis de-
jiccod from the abdomen into the scrotum.

K 2

182 REGENT LITERATUEE.

RECENT Lia?EEATITRE.

Ackeren, F. v., Bcitr. sur Fntwiokelungsgeschichte d. weiblichen Sexualorgane des Menschen,
Zeitsclir. f. wiss. Zool. , 1889.

Beard, J., The origin of the segmental duct in Elasmohranchs, Anat. Anzeiger, 1887.

Benda, C, Die Entwickl. des Sdugethierhodens, Verhandl. der anatom. Gresellschaft, 1889.

Bierfreund, M., Ue. die Einmundungsweise der Miiller'schen Gdnge in d. Sinus urogenitalis bei
deni menschlichen Embryo, Zeitsch. f. Geburtshiilfe u. Gynak, xvii.

Bonnet, R., Ueber die ektodermale Entstehung des Wolff ^schen Ganges lei den SdugetTiieren,
Miinchener med. Wochensclir., 1887 ; Emhryologie der Wiederkduer, Arch. f. Anat. u. Physiol.,
Anat. Abth., 1889.

Bramann, F. , Beitrag zur Lehre von dem Descensus testiculorum, die, Arch. f. Anat. u. Physiol,
Anat. Abth., ]884.

Brandt, Ue. d. Zusammenhang der Glandula suprarenalis mit dem Parovarium, die, Biol.
Centralbl. Ix.

Cadiat, 0., M6moire sur Vutirus et les trompes (ddveloppement). Journ. de I'anatomie,
1884 ; Du ddvelopperaent du canal de Vurethre et des organes ginitaux de I'embryon, Jouxn. de
I'anatomie, 1884.

Dolirn, Ueber die Gartner'' schen Kandle heim TFeibe, Arch. f. Gynakologie , xxi., 1883.

Emery, C, Recherches embryologiques sur le rein des mammiferes, Archives italiennes de biologie,
t. iv., 1883.

Flemming-, "W., Die ektoblastische Anlage des Urogenitalsystems heim Kaninchen, Archiv. f.
Anat. u. Physiol., Anat. Abth., 1886.

Gasser, Embryonalreste am mdnnlichen Genitalapparat, Marbnrg. Sitzungsb. , 1882.

Gottscliau, M., Struktur u. embryonale Entwickl. der Nebennieren bei Sdugethieren, Arch. f.
Anat. u. Physiol, Anat. Abth., 1883.

Haddon, Suggestion respecting the epiblastic origin of the segmental duct, Proceed, of the Royal
Dublin Society, Feb. 16, 1887.

HoflEmann, C. K., Zur Entwicklungsgeschichte der Urogenitalorgane bei den Anamnia, Zeitschr.
f. wiss. Zool., Bd. xliv., 1886 ; Zur Entwickl. der Urogenitalorgane bei den Reptilien. Zeitschr. f. wiss.
Zool. xlviii.

Janosik, I., Bemerkungen uber die Entwicklung der Nebenniere, Archiv. f. mikrosk. Anatomie, Bd,
xxii., 1883 ; Histologisch-enibryologische Untersuchungen uber das Urogenital-system, Sitzungsber. d.
Wiener Akad., 1885.

Kallay, A., Die Niere im friihen Stadium des Embryonallebens, Mittb. aus d. embryol. Inst, d,
Univers. Wien, 1885.

Kocks, Ueber die Gartner' schen Gdnge beim Weibe, Arch. f. Gynakologie, xx., 1883.

Kollraann, Ueber die Verbindung zwischen Colom it, Nephridium, Festschrift. Basel, 1882.

Lock-wood, C. B., The development and transition of the testis, normal and abnormal, Journal of
Anat. and Phys. , vols. xxi. and xxii., 1887 and 1888.

Martin, E., Ueber die Anlage der Urniere beim Kaninchen, Archiv. f. Anat. u. Physiol., Anat.
Abth., 1888.

Martin, P., Zur Entwickl. der cavernosen Korper, dec, Deutsche Zeitschr. f. Thiermed., xvi.

Mihalkovics, Gr. v., Untersuchungen uber die Entwicklung des Ham- und Geschlechts-apparates
der Amnioten, Intern. Monatsschr. f. Anat. u. Histol., 1885, 1886.

Mitsukuri, K., The ectoblastic origin of the Wolffian duct in Chelonia, Zool. Anzeiger,
1888.

Nagrel, Ueber den Wolff' schen Korper des menschl. Embryo, Zeitschr. f. Geburtshiilfe u. Gynak.,
1889 ; Ueber die Entwickl. des Urogenitalsystems des Menschen, Arch. f. mikr. Anat., xxxiv., 1889 j
Ue. das Vorkommen von Primordialeiern ausserhalb der KeimdriisenanLage beim Menschen,
Anatomischer Anzeiger, iv.

Perenyi, J. v.. Die ektoblastische Anlage des Urogenitalsystems bei Rana esculenta und Eacerta
viridis, Zoolog. Anzeiger, 1887.

Hanson, G-. , Contributions h 7'embryologie des organes d'excrdtions des oiseaux et des mamm.ife,res.
These. Bruxelles, 1883.

Rotli, Ueber einige Urnierenreste heim Menschen, Festschrift. Basel, 1882.

Eiickert, J., Ueber die Entstehung der Excretionsorgane bei Selachiern, Arch. f. Anat. u, Physiol.,
Anat. Abth., 1888.

Soliniieg-elow, E., Studien ueber die Entwickl. des Sodens u. Nebenhodens, Arch. f. Anat. u.
Physiol., Anat. Abth., 1882.

Sedg\pick, A. , On the early development of the anterior part of the Wolffian duct and body in the
chick, d:c.. Quarterly Journal of Microsc. Science, 1881.

Semon, R., Die indifferente Anlage der Keimdrusen beim Huhnchen und ihre Differenzirung
zum Hoden, Jenaische Zeitschr. f. Naturw., Bd. xxi., 1887.

Spee, F., Ueber directe Betheiligung des Ektodefms an der Bildung der Urnierenanlage des
Meerschweinchens, Archiv f. Anat. u. Physiol., Anat. Abth., 1884; Ueber weitere Befunde zur Ent-
wicklung der Urniere, Mittheil. f. d. Verein Schleswig-Holstein, 1886.

Strahl, H. , Zur Bildung der Kloake des Kaninchenembryo, Archiv f . Anat. u. Phys. , Anat. Abth. ,
1886 ; Ueber den Wolff'schen Gang und die Segmentalbldschen bei Lacerta, Marburger Sitzungsberichte,
1886.

RECENT LITERATURE. 135

Toumeux, P., Sur les premiers developpenients du cloaque, du tulercule ginital, et de Vanuschez
I'embryon de mouton, Journal de I'anatomie, 1S88 ; Sur le developpement du tubercle cjinital chez le
foetus kumain, dc., Joum. de lanatomie, xxv. ; Sur le mode de fonaation du perinie chez Vembryou
du mouton, Compt. rend, de la Societe de Biologic, 1890.

Toumeux, F., et Leg-ay, Ch., Mdmsire sur le developpement de L'utirus et du vagin, Journ. de
ranatomie, IsS-l.

"Weldon, "W. F. R., Note on, the origin of the suprarenal bodies of -vertebrates, Proceed, of tie
Royal Society, vol. 37, 1885 ; On the suprarenal bodies of vertebrata. Quarterly Journal of Alicr.
Science, 1885.

Wieg-er, G., Ueber die Entstehung u. Enticickl. der Bander des iceiblichen Gcnitalapparates, d-c.,
Arcli- f. Anat. u. Physiol., Anat. Abth., 1885.

"Wijhe, J. "W. v., Die Betheiligung des Ektoderms an der Entwickiung dcs Vomierengange.-,
ZooL Anz., 1S86.

134

DEVELOPMENT OF THE HEART.

FORMATION OF THE VASCULAR SYSTEM.

DEVELOPMENT OF THE HEART.

In mammals, the heart appears in the form of two tubes lying in the cephalic
region, one on either side of the embryo. These are seen at a very early period,
prior, in fact, to the separation of any part of the alimentary canal from the yolk
sac, and to the closure of the neural groove. This bilateral condition was first
observed by Hensen in the rabbit ; it has been seen by His in the human
embryo.

The situation and mode of formation of the bi-tubular heart are well illustrated
by the accompanying figures from KoUiker. They exhibit the condition in the

Kg. 157.

-Rabbit embryo of the 9th day, fhom the
SURFACE. -J. (KoUiker.)

The medullary groove is enlarged anteriorly and the primary
optic, vesicles are growing out from the first cerebral enlarge-
ment. On either side of the head, the bilateral tubular heart
is seen. Eight pairs of protovertebrsg are formed.

rabbit embryo of about eight or nine days — the
time when the heart first makes its appearance.
Fig. 157 shows such an embryo in surface view.
The neural groove, as also the sections show, is
widely open, although the rudiments of the cere-
bral enlargements are apparent in it, and also the
enlargements for the primary optic vesicles. There
are eight pairs of proto-vertebrse, the paraxial
mesoblast in front of these and on either side of
the cerebral enlargements being undivided. Out-
side this undivided cephalic mesoblast is a short
tube dipping in front into it, and passing behind
into a venous trunk, the vitelline or omphalo-
meseraic vein of the same side. The tube lies
within and is immediately surrounded by a clear
space, which is continued forwards beyond it on
either side of the fore-brain ; this space is pro-
longed from the mesoblastic cleft or pleuro-peri-
toneal cavity (coelom).
The two short tubes form the double rudiment of the heart. The situation
which they occupy becomes, when the lateral walls fold over to form the foregut,
the ventral wall of the pharynx, and the two tubes are thus brought together in the
middle line underneath the head part of the ahmentary canal. Here they soon
become fused together to form a single median tube, the hinder end of which is still
continuous with the two vitelline veins, while the anterior end bifurcates near the
anterior end of the foregut into two branches which arch dorsalwards on either side
of that tube, and then pass backwards on each side of the notochord as the two
primitive aortse.

These changes in the position of the primitive heart are partly shown in surface
view in figs. 158, 159, but they can only properly be appreciated by the study of
transverse sections. Fig. 160 is a transverse section through the anterior head
region of the embryo shown in fig, 157. This is anterior to the heart region, but
shows the commencing folding over of the splanchnopleure to form the foregut.

DEVELOPMENT OF THE HEART.

135

The mesoblastic cleft (coelom, pK) is somewhat dilated, but is not doubled in, as in the
heart region. The lateral mesoblast ceases a short distance beyond it. Fig. 161 is
a section through the middle of the head region of the same embryo. Here, while

Fig. 158.

-Embryo rabbit of eight days and eighteen hours, with 9 protovertebrj;, mewed

FROM THE VENTRAL ASPECT. "j*. (Kolliker. )

Fig. 109. — Sketches showing more advanced condition of the bitubular heart of the rabbit.

y. (Allen Thomson.)

A, view from below of an embiyo in wliicli the formation of the heart was somewhat more advanced
than in fig. 158. and of which an outline of the heart is repeated in B. C, from another embryo, shows
the two halves of the heart in the commencement of their coalescence, h, the part of the bent tube
which becomes the ventricle ; a. primitive aortic arches and d(scending aortas ; VV, vitelline veins
entering the heart posteriorly The arrows indicate the course of the blood.

the other parts of the section are much the same as in front, the dilatation of the
coelom, which is in fact the rudiment of the future pericardium, is occupied by an

Fig. ICO. — Section from the samk embryo further forward than that shown in the I'Recediug

figure. (Kolliker.)
J), paraxial mesoblast ; rf, medullary groove ; r, ri<lgc bounding groove ; mp, medullary ])late
of hind brain ; h, epiblast ; hj), 8omatoi)ienrc ; dfp, splanchnopieuro ; fh, anterior part of co'b.iii ;
mes, meBf;bla8t beyond the co^lou) ; dd, hypoblast ; dd' , notochordal thickening ; m, lateral wall of tiic
developing pharynx.

invagination, or fold, of the splanchnic racsoblast. This fold becomes subsequently
entirely separated and the aperture or line of invagination closed ; it forms the

136

DEVELOPMENT OF THE HEART.

muscular wall of the heart. It encloses a second tube composed of flattened
epithelium cells ; this so-called endothelial tuU (His) becomes the lining epithelium
of the endocardium.

i7ih

Fig. 161. — A, Transverse section THRorsH the head ov an embryo rabbit of eight days and

FOURTEEN HOURS, WITH A PART OF THE PERIPHERAL BLASTODERM. \^. (KolUker.)

lik, rudiments of the heart ; sr, pharyngeal groove, with notochordal thickening of hypoblast.

B. — Part of THE SAME more highly magnified, ^f: (KoUiker.)

Lettering as in fig 160. In addition : — a7(A, fold of splanchnopleure to fonn wall of heart;
ilih, endothelial tube of heart.

There is some doubt as to the source of this endothelial tube of the heart. In the preceding
edition of this work it was stated that it is " derived from the deeper part of the visceral meso-
blast ; " this statement being apparently founded upon the statements and figures given by
Kolliker. His ascribes it, like the endothelium of the blood-vessels, to an ingrowth, from tLe
vascular area. The appearance of the section shown in fig. 161 B, seems to lend colour to the
belief that the invagination which, has taken place to form the heart is not the splanchnic
mesoblast only, but has included also th.e Lyisoblastic layer of the splanchnopleure ; the notch
which is seen in the hypoblast near sic, appearing to indicate an intemipted connection with
the endocardial tube. Should future investigations show that this is actually the mode of
formation of the tube, the mammalian heart would be developed in essentially the same
Aianner as has been shown by Riickert to occur in Pristiunxs (an Elasmobranch), where this
organ, which, as in all vertebrates below mammals, is formed only after the foregut is com-
pleted, is developed as a median outgrowth or thickening of the ventral wall of the foregut.
A similar mode of formation has also been noticed in Cyclostomata, Ganoids, and Amphibia. In
reptiles and birds the fia.-st appearance of the heart is as a bHaterai tube, but it becomes visible
only after the foregut is formed, and the two tubes lie from the first close together, and from
the surface appea,r as a single median tube.

Sections at a somewhat later period (fig. 162) show the two tubes lying in con-
tact on the ventral side of the now completed foregut. The septum which divides
them at this period has nothing whatever to do with the permanent intra-cardiac
septum, but soon becomes completely absorbed, so that by the fusion of the two
lateral tubes a single median tube is the result (fig. 163). This median tube remains
attached by a suspensory membrane resembling a mesentery (mesocarclium posterius)
(mp, fig. 163) to the ventral wall of the phaiynx, but the mesocardium anterius,
which also at first results from the fusion, disap'^ears, except at the lower end, and
otherwise the tube becomes free, except where the vitelline veins pass to it from the
yolk sac, a lateral attachment to the body wall being here subsequently formed on
each side (mesocardium Merale of Kdlliker). After it is thus formed, the heart is
for a time median in position and symmetrical (fig. 164, A), but afready in the

DEVELOPMENT OF THE HEART.

137

mammal shows distinct indications of division into its several parts ; indeed, these
parts are apparent even while the two tubes are still distinct, as tlie 'accompanying--
sketches of the rabbit's heart clearly show (fig. 150). The heart does not, howevei^

Fig. 162. — Transverse SECTION THROUGH

THE REGION OF THE HEART IN A RABBIT
EMBRYO OF NINE DAYS. SHOWING THE
COMMENCING FUSION OF THE TWO TUBES.

«>. (Kolliker.)

jj, jugular veins ; ao, descending aoi tse ;
pli, pharyn.x ; hp, epiblast of I'ody-wall ;
ih, endothelial lining of the still divided
heart ; ah, outer wall of the heart ; p, peri-
cardial c i-lom ; df, df, visceral mesoblast
(soniatopleure) ; e', prolongation of the
hypoblast of the foregut and the anterior
wall of the pericardial cavity into the
partition between the two halves of the
heart; hi, bilaminar portion of blastoderm
forming pro-amnion ; ixt, ent, its two
layers (epiblast and hypoblast).

long retain its symmetrical position. It soon becomes bent upon itself, so as to
assume the shape of an S, the anterior part of the tube Ijending over to the right and
the posterior to the left (tig. 1G4, B). At the same time the posterior, or sino-auricular

Fig. 163. — Section through the region of the heart in a rabbit embryo of 10 days, afteu

THE TWO tubes HAVE UNITED INTO A SINGLE MEDIAN ORGAN. (Kollikcr.)

ao, descending aortse ; ha, bulbus aortas ; ah, its external wall ; mp, posterior raesocardium, uniting
the heart to the ventral wall of the pharynx, ph, and here separating the pleuropericardial ca-lorn,
p, into two halves, which are, however, united on the ventral side of the heart ; cnt, hypoblast of yolk
sac ; d/, its mesoblast ; df, mesoblast of jjharynx ; eit, epiblast.

Fig. 164. — Outlines of the anterior half of the embryo chick viewed from below, showing the

HEART in its EARLIER STAGES OF FORMATION. (After licmak. ) "['

A, embryo of about 28 to 30 hours ; 15. of about 36 to 40 hours ; a, anterior cerebral vesicle ;
h, proto- vertebral segments ; c, cephalic fold ; ], 1, vitelline or omphalo-mesenteric veins entering the
heart posteriorly ; 2. their union in the posterior part of the heart ; '6, the middle part of the tube
corresjjonding to the ventricle ; 4 (in ii) the arterial bulb.

end of the heart, gradually comes to h'e behind or dorsal to the ventricular part,
which arches transversely from left to right, where it turns sharply upward (towards
the head), and terminates in the bulb. The tube is divided by slight constrictions
into successive portions, viz. : (I) the part formed by the junction of the princii)al
veins, sinus veuosus ; (-2) the auricular part ; (3) the ventricular part ; and (4) the
aortic bulb.

138

DEVELOPMENT OF THE HEAKT.

The sinus venosus may be described as consisting of two lateral enlargements or
horns, and of a transverse part connecting these horns. The veins which it at this
time receives are the umbilical, the vitelline, and the ducts of Cuvier (formed

d. aw. m.j}.

Fig. 165.— Condition op the heart in the human embryo of about fifteen days, reconstructed

FROJI SERIAL SECTIONS. (His.) ''^

A, from before, showing extern.al appearance of heart ; B, the same with the muscular substance of
heart remoYed showing the endothelial tube ; C. from behind.

m«, mandibular arch with maxillary process ; liy, hyoidean arch ; h.a, bulbus aortse ; v, right
ventricle ; v' , left ventricle ; aM, auricular part of heart ; c.a., canalis auricularis ; &.r, horn of sinus
venosus with umbilical vein {xi.v), superior vena cava (t'.c.s), and vitelline vein entering it ;
d, diaphragm ; m p, mesocardium posterius ; I, liver ; h.d, bile duct.

by the junction of the primitive jugular from the head and the cardinal from
the trunk). The three veins are nearly symmetrical on the two sides, and enter the

l.au

Fig. 166. — Heart of a somewhat more advanced human embryo. (His.) \°
A, from before ; B, from behind.

T.v, right ventricle ; l.v, left ventricle ; h.a, bulbus aortfe ; T.au, right auricle ; l.au, left auricle ;
v.c.s, vena cava superior ; u.v, umbilical vein ; v.v, vitelline vein ; d, diaphragm.

corresponding horn of the sinus (jBg. 168). The sinus is at first in free communi-
cation with the common auricular cavity, but the junction presently becomes
narrowed, and the resulting aperture, which eventually acquires a slit-like character.

DEVELOPMENT OF THE HEART.

139

is found to open from the right horn of the sinus into the right part of the common
auricle. The sinus now forms a trausversely disposed sac, lying below and behind
the common auricle, with a larger right and a smaller left horn (the latter being

r.au. l.au.

Fig. 1G7. — Heart of human embryo slightly more advanced than that shown in fig. 166. (His.)

A, interior of auricle and ventricle displayed.

B, endotljelial tulie.

a.c, auricular canal ; a.i, area interposita of His ; m, posterior mesocardium ; r.au, l.au, right and
left auricles ; l.v, left ventricle ; r.v, right ventricle ; h.a, bnlbus aorta:!.

tapered off into the left duet of Cuvier) ; in this condition it has been termed by His
sacciis rciiniens { fig. 1 C9, B, and fig. 171). The umbilical and yitelline veins soon open
into it by a common trunk, which becomes the upper end of the vena cava inferior.

OCO'

v.c.s.

Fig. 168. — Heaut OF rabbit EMUKVo. (Horn, j

A, from before ; B, from behind,

».», sinu.s venoHUH ; l.v, left ventriclo ; r.v, right ventricle ; h, linllnis aortii' ; no' , first aortic arch ;
ao", hecoml aortic arch ; r.au, right auricle ; l.au, left auricle ; wuh r, umliilical vein ; ri.v, vitelline
vein ; v.c.d, vena cava superior.

The slit-like orifice of the sinus in the back of the right auricle is guarded by two
valve-like folds of the endocardium, which project into the cavity of the auricle fright

140

DEVELOPMENT OF THE HEART.

Z.s.c.

vurvb.v.UvvM. cZ.f.

Fig. 169. — Anterior and posterior aspect of the heart op a somewhat older rabbit embryo.

(Born.)

p.a, pulmonary artery ; s.r, s.l, s.tr, right and left horns and transverse part of sinus respectively;
r.s.c, l.s.c, right and left superior cavse ; pe, aperture of pulmonary vein ; Ji.v, hepatic veins; d.v,
ductus venosus ; me, mesocardium posterius.

The other letters as in fig. 168.

Fig. 170. — Section through the heart of a rabbit embryo at the stage shown in fig. 169.

(Born.)
r.s, l.s, right and left horns of sinus receiving from above the respective superior vense cavae ; r.au.
Law, right and left auricles ; r.v, l.v, 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 ; cm.v.c, one of the
two endocardial cushions which are beginning to sub-divide the common auriculo-ventricular aperture.
The dotted line encloses the extent of the endocardial thickening ; s', first septum superior 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.inf, inferior septum of the ventricles.

Fig. 171. — View from behind of the heart of a human embryo of about 4 weeks, magnified. (His.)
_ The two superior cava, right and left, and the inferior cava are seen opening separately into the
sinus which is a transversely elongated sac communicating only by a narrow orifice with the right
auricle.

DEVJ£LOPxMi.NT OF THE HEAKT.

HI

and left venous valves) (fig. 170, r.v.v., l.v.v.). These pass above into a muscular fold
of the auricular wall, which extends over the roof of the auricle heart parallel to the
septum atriorum, and is known as the septum spiirium (fig. 173, B).i It disappears at
length, probably by uniting with the septum atriorum. Subsequently the venous orifice
opens out, and the right horn of the sinus, which is now seen to receive all the great
veins except the left duct of Cuvier, becomes gradually incorporated with the cavity
of the auricle. The transverse part of the sinus and its left horn are continuous
with the left duct of Cuvier (fig. 171), and eventually the transverse part forms the
coronary sinus. From the right venous valve the Eustachian valve is formed, and
the development of the Thebesian valve is also connected with its lower end
(Schmidt). The left venous valve disappears.

The transversely placed ventricular part of the heart receives at first at its lefc
end the orifice of the common auricle, which opens into its posterior wall (fig.

y.cizc.z^^,

S.inJ.

Fig. 172. — Diagram to show the formation of the septum of the ventricles and bulb, and the

MODK OF division OF THE COMMON AURICULO- VENTRICULAR APERTURE. (Born. )

au.v.c (in A and B), auriculo-ventriculai' aperture, partially divided into two by endocardial cushions ;
r.au.v, l.au.v, right and left auriculo-venti-icular apertures which have resulted from the division of the
common aperture ; r.v, l.v, right and left ventricles ; h, bulbus aortse, replaced in C, by p. a and a.o,
pulmonary artery and aorta ; s.b, septum bulbi ; s.inf, septum inferius ventriculorum ; o (in A), orifice
between the two ventricles.

172, A, a.v.c). At its right end it turns sharply upwards into the aortic bulb, into
which it gradually tapers, although there is at a certain point a constriction of the
endothelial tube, where the semilunar valves are subsequently formed {f return Halleri).
Soon the right and left halves of the ventricle are separated externally by a
groove which extends from below, partially encircling the tube (fig. 169). If the
interior of the heart is examined at this stage, it is seen that a muscular septum,
corrcspoudiug internally to this groove, is growing upwards and backwards from
the antero-inferior part of the tube, and is gradually separating it into two parts,
which become the right and left ventricles respectively (fig. 170, s.inf). This
septum {septum inferius of His) is placed obliquely to the long axis of the tube,
and extends eventually nearly to the level of the auriculo-ventricular orifice, which
has by this time become shifted along the posterior wall of the tube, so as to open
into it about its middle instead of at the right end, as was previously the case
(fig. 172, B). The septum of the ventricles remains incomplete for some time,
a communication between the two ventricles being maintained above it. Even-
tually the septum inferius unites with prolongations, (1) from the endocardial
cushions which divide the common auriculo-ventricular orifice into right and left

' Thi8 muscular prolongation may, hh Bom suggests, bo of use in assisting the action of the valves,
and in preventing their being forced backwards into the sinus when the auricle contracts.

142 DEVELOPMENT OF THE HEART.

orifices ; (2) from the endo-cardial aortic septum, which divides the bulb into aorta
and pulmonary artery. Thus the septum of the ventricles is completed by endo-
cardial connective tissue, a fact which is indicated even in the adult heart by the
existence of the thin septum membranaceum which forms the uppermost part of
the inter- ventricular septum.

The common auricle in the meantime becomes shifted relatively upwards over
the back of the ventricles, carrying the sinus along with it, but it still lies behind
rather than over the ventricles, and the aperture of communication passes from
behind forwards, from the left part of the auricle into the corresponding half of the
ventricle. This constricted aperture soon becomes elongated into a short canal,
which is known as the auricular canal. Its orifice into the ventricle is from the
first somewhat flattened, and bounded by two lips, an upper and a lower. As deve-
lopment proceeds, it broadens out towards the median plane of the ventricular tube,
and becomes gradually shifted, first towards, and eventually over the line of constric-
tion which marks off the future right and left ventricles from one another (fig. 172).
The ventricular septum has by this time extended almosj up to the transversely
elongated slit-like orifice, and its lips, still upper and lower in relative position,
become greatly thickened by the formation of cushions of endocardium, which gro^^■
towards one another in the middle of the slit, and presently fuse into a median
thickening which converts the single M-shaped aperture into two triangular
openings, leading one into each ventricle (fig. 172, C). Meanwhile, the septum of
the ventricles growing towards the base abuts against, and at length comes into direct
continuity with the fused endocardial cushions, but this connection ds nearer to the
right than to the left auriculo-ventricular aperture.^ There is still, as above stated,
a small orifice of direct communication between the left and right ventricles above
the free edge of the ventricular septum, and this is not closed until the descent of the
septum of the bulb, and its union with the septum of the ventricles, completes the
interventricular septum.

The above account of the division of the auricular canal is based upon that given
by Born for the rabbit, and in some respects differs from the description which was
given by His from an examination of human embryoes. According to His, the endo-
cardial cushions, which by their union subdivide the auricular canal, are preceded by
and connected with a growth of endocardial tissue, which springs from the posterior
auricular wall, and they together form a septal jDrolongation {septum intermedium),
which projects like a stopper into the auricular canal, and divides the latter into the
two auriculo-ventricular orifices, and also grows down beyond that canal to meet the
uprising ventricular septum (fig. 173). The shortening of this canal is in part effected
by a kind of intussusception which takes place, and which causes its wall to be
folded into the ventricular cavity ; these folds, with probably some thickening of
endocardium, form the bases of the lateral flaps of the auriculo-ventricular valves
(fig. 175). The bases of the mesial or septal flaps are formed by a downward growth
of the edges of the endocardial septum between the two orifices. Both lateral and
mesial flaps become continuous with the spongy muscular substance which at this
time occupies most of the cavity of the ventricles (fig. 175). As development
proceeds, the flaps, which are at first thick and soft, become thin and membranous,
and become free fi'om muscular substance except near their free edges. These
muscular bands become tendinous near their insertion into the valves, and thus form
the chordcb iendinecc ; the parts which are not thus transformed become the iKipillary
muscles.

The septum of the auricles appears at the upper and back part of the auricular

* Hence the riglit auriculo-ventricular orifice lies close to the ventricular septum, but the left orifice
is separated from it by an interval, into which the root of the aorta becomes continued (Born),

DEVELOPMENT OF THE HEAKT.

143

cavity, where its situation is externally marked by a groove. The free edge of this
septum grows forwards and downwards, and the septum (fig. 170, s') gradually
separates the auricular cavity into a right and left half, the separation beiug com-
pleted by the junction of its free edge, which shows a distinct endocardial thickening,

y.cs.

^.a

S.Sp. 5.O.

Z.cc-.

Fit' 173.— Two STAGES in TUE formation of the septum intermedium in Tin: IIKAKT <iF Tin: HUMAN
*"■ "*' EMBRYO. (His.)

In A the septum is represented as growing from a triangular area to tlic left of the sino-auricular
orifice ; in II it has coalesce I with the endocardial cushions, and lies like a stopper in the auricular

^^r\'i I a rii-ht and h-ft auricle ; r.v, l.v, right and left ventricle ; s.r, sinus venosiis ; Eu.v, Eustachian
valve'; n.^p. septum s)-uriuin ; 8.s, septum superior ; s.hif, teptum inferior ; s.i, septum intermedium ;
v.c.e, vena cava superior d extra.

144 DEVELOPMENT OF THE HEAHT.

with the fused cushion-like thickenings which are subdividing the common auricular
orifice. But before the originally free communication between the two auricles is thus
closed, a new aperture makes its appearance above and at the back of this septum,
and gradually enlarges, so that a passage is thus re-established, but in a different
situation. This new orifice is the foramen ovale, it becomes closed by a second
septum, which also starts from the superior auricular wall, a little to the right of the
original attachment of the first septum, and gradually grows forwards and downwards
over the orifice. This second septum becomes the Ivnbus Vieussenii, the first one
forms the so-called valve of the foramen ovale (Born).

According- to His' account of the process in tlie human embryo, the septum atriorum is
formed by an anterior, or lower, and a posterior, or upper, sickle-shaped projection, which between
them enclose the foramen ovale, and form resiDectively the limbus Vieussenii and the valve of
the foramen ovale ; the connective tissue growth which he describes as growing- from the
posterior auricular wall towards the auric ulo- ventricular orifice takes an important part in
the formation of the lower septal projection (septum intermedium). There is reason, how-
ever, to believe that the process, as above described by Born for the rabbit, is materially the
same in all the higher vertebrates, including man, and that the successive growth of both
septa from the upper and posterior auricular wall was not noticed by His on account of the
lack of a series of human embryos sufficiently complete to show all the stages of growth.

Somewhat late in the course of development (after the appearance of the auricular
septum), the pulmonary veins are seen entering the left auricle. Before reaching the
auricle they have united to form a single vessel, and this opens into the auricle near
the septum (fig. 170, p.v.). In some animals, as the rabbit, this represents the
permanent mode of termination of the pulmonary veins, but in man the right and
left veins come to open separately into the auricular cavity, either by division of the
common trunk (His), or by opening out of the common trunk, and its absorption
into the auricle in the same way as the right horn of the venous sinus is absorbed
into the right am'iole (Born). The two resulting vessels may again divide, so that
four pulmonaiy veins ultimately terminate in the left auricle.

The aortic hull) becomes subdivided into two vessels, the ascending aorta and
the pulmonary artery. The division is produced by a septum which arises as two
longitudinal thickenings of the lining membrane (endocardium). These grow from
opposite sides, and gradually meeting, fuse together in the middle of the bulb. The
folds take an oblique course down the bulb, for above they are anterior and posterior,
but below are right and left, hence the resulting vessels after separation are anterior
and posterior below and right and left above. The endocardial thickenings extend
somewhat below the origin of the bulb, and unite with one another and with the
septum of the ventricles, which they complete, and of which they form the mem-
branous part. The ventricular part of the heart is now completely divided into two,
each communicating with the corresponding division of the arterial bulb. There are
at first no semilunar valves, the soft thickened endocardial tissue of the bulb appear-
ing to exercise a sort of valvular action. The valves are formed as three projec-
tions of this tissue at the base of each vessel, at first thick and soft, but subsequently
becoming thinner and membranous. The common aortic trunk has four such
thickenings at the lower end, and the septum of the bulb as it descends is prolonged
into the right and left of these, so that the dumb-bell-shaped orifice is divided into
two triangular apertures, the bulging sides of which are formed by the endocardial
cushions and become developed into the semilunar valves (fig. 174).

The aortic septum begins between the fourth and fifth aortic arches, and is so dis-
posed, that the fourth arch continues the aortic half of the bulb, the fifth the pul-
monary half. After thg completion of the septum, an external groove makes its
appearance along the line of the endocardial thickenings, and deepening gradually,
splits the bulb into two separate vessels.

DEVELOPMENT OF THE HEAKT.

145

Fig. 174. — Diagram showing the

DIVISION OP THE LOWER PART OP
THE BULBUS AORT^, AND THE FOR-
MATION OF THE SEMILUNAR VALVES.

(After (legenbaur and His. )

A, undivided truncus arteriosus
■with four endocardial cushions ; B,
advance of the two lateral cushions
resulting in the division of the lumen ;
C, projection of three eudocardiai
cushions in each part ; D, the separa-
tion into aorta and pulmonary trunks
completed.

Fig. 175. — Sections through the

HEART OF HUMAN EMBRYOES, SHOW-
ING two stages in THE FORMATION
OF THE CJJRDIAC SEPTA AND OF THE
ACRICULO-VENTRICULAR A'ALVES.

(His.)

A, from an embryo of 5 or 6 weeks.
R. V, right auricle ; L. V, left auri-
cle ; S.}:d, right horn of sinus ; S.7-.S,
left horn of sinus ; V.E, Eustachian
valve ; s.int, septum superior and
endocardial cushion (septum interme-
dium, His) ; s.inf, septum inferius
ventriculorum. This septum, as well
as the bulk of the ventricle, is a mus-
cular sponge at this stage. Oe, oeso-
phagus ; Br, bronchus.

B, from a somewhat more advanced
embryo. Ad, An, right and left
auricle ; Ost, auriculo- ventricular
ai)ertures ; S.s. septum superior of
auricles ; S.it, endocardial cushion
(septum intermedium) ; <S'.//, septum
inferius ventriiulorum, now denser
and more muscular ; ^fpp, peri-
cardial attachment.

Distinct muscular tissue is
seen in the cardiac ^all, even
as early as the stage of an
S-shaped tube, although the
heart begins to pulsate regu-
larly long before this. The
muscular layer is separated
from the epithelial lining of
the cavities (endothelial tube
of His) by a layer of clear
gelatinous tissue,bridged across
by fine fibres fembryonic con-
nective tissue;. This layer is
most abundant in the ventri-
cular part and aortic bulb, and
here the endothelial tube is
con.eequently much smaller
than the nm.scular tube. Snb-
8«.'quently, in the ventricle, the
gelatinous tissue is invaded by
mu.scular bauds which grow

146

DEVELOPMENT OF THE PRINCIPAL AETEKIES.

into it from the compact outer layer of muscle, and unite with one another to form
a spongework of muscular trabecule, while the endothelium of the cavity becomes
depressed between and over these trabeculse, and lines all the spaces between them,
which thus communicate with the cavity of the ventricle. The ventricles are therefore
now in the same condition in which they are found permanently in many of the
lower vertebrates (e.g. frog).

Ultimately the compact outer layer of muscle becomes greatly increased in thick-
ness, and the spongework of trabeculse occupies a relatively much smaller portion of
the cavity, being developed in part into the columnse carnege of the adult heart.

Peculiarities of the foetal heart. — Besides the peculiarities of structure,
which have been above described, the foetal heart differs in position, in relative size,
and in the thickness of its several parts, from the organ after birth. Thus it is at
first placed immediately under the head, but subsequently, with the development of
the neck, it gradually assumes a position farther back. In early foetal life it is
much larger in proportion to the size of the body than at a later period, and at birth
it is still proportionally large. The walls of both ventricles are of equal thickness
during foetal life, a peculiarity which is evidently connected with the fact that in
consequence of the communication of the pulmonary artery, through the ductus
arteriosus, with the aorta, the blood pressure which they have to overcome is the
same.

EEVELOPMENO? OF a?HE PRINCIPAL AUTERIES.

From the point of insertion of the aortic bulb into the ventral wall of the foregut,
first one, and then in succession four other arterial arches^ become formed, and pass
on either side, one into each visceral arch. Half encircling this part of the

Fig. 176. — DiAGKAMMATIC OUT-
LINES OF THE HEART AND PRI-
MITIVE VESSELS OF THE EMBRYO
CHICK AS SEEN FROM BELOW AND
ENLARGED. (A. T. )

A, soon after tie first establish-
ment of the circulation ; B, C, at
a somewhat later period ; 1, 1,
the veins returning from the vas-
cular area ; 2, 3, 4, the heart,
now in the form of a notched
tube ; 5, 5 (upper), the two primi-
tive aortic arches ; 5, 5 (lower),
the primitive double aorta ; a, the
single or united aorta ; 5', 5', the
continuation of the double aortae
beyond the origin of the large
omphalo-mesenteric arteries, 6, 6.
The division above 4 is repre-
sented as carried rather too far
down.

alimentary canal, they are continued above it into two descending ox primitive aortm.
These two vessels run down the trunk on either side of the notochord, yielding, as
they descend, lateral offsets to the body walls and to the yolk sac. Finally they give
off, at the lower or posterior extremity, two large vessels, which accompany the
ahantois, and furnish blood to the foetal part of the placenta (umUlical or allantoic
arteries).

The primitive aortee do not long remain double. As was first shown by means
of sections by Allen Thomson, they unite in the middle line, the union beginning
in the dorsal region and extending forwards and backwards ; in the latter direction

DEVELOPME^'T OF THE PRINCIPAL ARTERIES.

147

even 'oeyond the origin of the allantoic arteries, the middle sacral artery being in
fact the extremity of the aorta.

Occasionally the union remains incomplete, a median septum being sometimes found as a
malformation of the descending aorta.

The common iliacs are formed by persistence of the roots of the allantoic arteries ; when
the lower limbs are formed they give off to these the external iliacs.

Since their discovery by Rathke in 1825, the arterial arches have been regarded with
much interest as corresponding with those from which the blood-vessels of the gills in
fishes and amphibia are derived. Along with the (subdi\-ided) aortic bulb they give rise, by
various transformations, to the permanent pulmonary and aortic stems and the principal vessels
which spring from them. Most of what is known regarding the mode of their transfonnation
in different animals is due to the researches of Rathke. In the human embryo the subject
has been recently investigated by His, whose account will be here mainly followed.

Uz-OU.

'cUl '

tcv.

u.cc:

^•^-cOZ.

Fig. 177. — Profile view of a human embryo op about fifteen days, with the alimentary canal in

LONGITUDINAL SECTION. (His.)

Two arterial arches are formed at this stage.

Fig. 178.— 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 uf the fore-brain
the primary optic vesicle is seen projecting ; ot, otic vesicle, still open in 177 ; ^J.y, septum between
mouth and pharynx (primitive velum). This has disappeared in 178 ; /, commencing liver in septum
transversum ; f, vitelline stalk ; all, allantois enclosed within stiilk ; j.v, jugular vein ; c.v, cardinal
vein ; s.r, sinus venosus within septum transversum ; u.a, left umbilical (allantoic) artery; u.v, left
umbilical vein. The sharp curve of the trunk of the embryo towards the yolk-sac is normal at this period
of development.

From the point of insertion of the aortic bulb the arterial arches have a radial
disposition as they pass into their respective visceral arches (fig. 181). Tliey at first
effect, as above stated, a complete communication between the aortic bulb and tiie
descending aorta, but 8ub.sequently in most cases the communication becomes
obliterated, and the completeness of the vaacular arch is thus obscured, the only
arclies which in mammals remain jxirvious through their whole extent up to the

L 2

148

DEVELOPMENT OP THK PRINCIPAL ARTERIES.

time of birfch being the fourth and fifth arches of the left side, which form the arch
of the aorta and the ductus arteriosus respectively. This obliteration begins early
in the first and second arches, so that in many animals by the time the posterior
arches are formed the anterior are partially obliterated ; but in man this is not the
case, all five pairs of arches being present and fully pervious for a certain time (figs.
178, 179, from His).

As development proceeds, the point of insertion of the aortic bulb, which is at
first opposite the first arterial arch, becomes, along with the rest of the heart,

Fig. 179. — Profile view of a human embryo of

ABOUT THREE WEEKS, SHOWING THE CEPHALIC
VISCERAL ARCHES AND CLEFTS AND THEIR RELA-
TIONS TO THE ARTERIAL ARCHES. (His. )

rax, maxillary process ; mn, mandibular arcli ;
d.C, duct of Cuvier; j.v, jugular vein; c.v, cardinal
vein ; v.v, vitelline vein ; %i.v, umbilical vein ; u.a,
umbilical artery ; all, allantois ; jpl, placental at-
tachment of allantoic stalk ; olf, olfactory depres-
sion ; ot, otic vesicle.

relatively shifted backwards, and on eacli
side the arterial arches presently appear to
come off from the bulb in two sets, viz. :
the first and second from an ascending
trunk, afterwards the external carotid, and
the third, fourth, and fifth from a descend-
ing trunk.

, As the point of insertion is still further
shifted, the third arch becomes added to
the ascending trunk, the lower part of
which now forms the common carotid.
Finally, by a continuance of the same
process, the fourth arch on each side comes
to spring from the ascending trunk : that
of the right side forming the innominate
artery, that of the left the arch of the
aorta ; and only the fifth arch, from which
the pulmonary arteries spring, has for a
time a descending direction.

From the dorsal part of the first arch a
branch passes towards the brain — this
becomes the upper part of the internal
carotid. When the first and second arches
become obliterated — a change which next
occurs — this branch remains in continuity
with the third arch by the unobliterated dorsal portions of the first and second
arches (upper extremity of primitive aort«) ; these portions, together with the third
arch, form the lower part of the internal carotid, the posterior communication
between the third and fourth arch becoming obliterated. The branches of the
external carotid are produced from the remains (ventral) of the first and second
arches ; the maxillary and temporal arteries from the first, the lingual and ascending
pharyngeal arteries, and probably also the occipital and auricular, from the anterior
part of the second arch.

The division of the bulb into aortic and pulmonary trunks begins just at the
time when the extremity of the aortic bulb has become shifted backward so as

DEVELOPMENT OF THE PRINCIPAL ARTEPtlES.

149

to be opposite the point of junction between the fourth and fifth pairs of arches, so
that now all the arches above this point become separated oii" in connexion with the
trunk of the aorta (ascending aorta) ; the one below it remaining in connexion sith

Fig. 180. PeOFILE VIEVr' OF a human embryo ok about 3 OR -1 WEEKS, SHOWING THE PRINCIPAL. ARTERIES

AND VEINS. (His. )

1, 2, 3, 4, 5. the secondary cerebral vesicles ; hyp, hypophysis ; ot, otic vesicle ; mn, mandibular
arch ; I'j. lung rudiment ; st, stomach ; Wd, Wolffian duct opening into cloaca ; /, //, HIi IVt V,
the arterial arches springing fronr. b.a, bulbus arteriosus ; ^;, pulmonary artery ; d.C, duct of Cuvier.

the pulmonary trunk. After the separation, the aortic bulb, now a double tube,
becomes still further shifted back, and with it the fourth and fifth arches. Since

Fig. 181. — View from behind of the anterior part
OF the mouth and pharynx of a human embryo
of 3i weeks, showing the arterial arches
radiating from the attachment of the aortic

BULB. (His.)

ao, point of attachment of aortic bulb in the anterior
wall of the pharynx ; mn, hy, br\ hr"^, first four visceral
arches ; /, //, ///, IV, tiie corresponding arterial
arches ; V, fifth arterial arch giving off the pulmonary
arterj', p.

the inferior laryngeal nerve passes under the
latter, this nerve must also become shifted
downwards along with these arches. To
allow for this shifting of the l)ull), the
common carotids become proportionally
lengthened.

From the desrxjnding primitive aortas on either side a series of intcr-segmental
art<ines pa.s8 ; the upperniost of these become united to form the vertebral arteries
(which subsequently unite superiorly in the middle line to jiroduce the basilar), the
lower form intercostal arteries. A branch for the upper extremity comes oil" from

150

DEVELOPMENT OF THE FOURTH AND FIFTH ARCHES.

the commencement of the vertebral ; it subsequently far exceeds the parent vessel
in size, and forms the subclavian, this name being extended to what was originally
the commencement of the vertebral from the descending aorta.

Destination of the fourth and fifth arches.— As was above stated, the
aortic trunk, connected below with the left ventricle, is connected above with the
four superior arches, which spring from its two ascending rami ; of these rami the
right is much the smaller, and its root forms the innominate ; the root of the left,
which is much larger, represents the part of the arch of the aorta between the

Fig. 182 — Diagram to show

THE DESTINATION OF THE
ARTERIAL ARCHES IN MAN

AND MAMMALS. (Modified
from Rathke. )

comffzorz'
ZconfHffx

siwclmjicm.

innomincUe^
arteru

carotid
oirch of

cirteriosjzs

The truncus arteriosus, aud
the five arterial arclies spring-
ing from it are represented in
outline only, the permanent
vessels in colours — those be-
longing to the aortic system
red, to the pulmonary system
blue.

trunk'

CLortcu

innominate and the left
common carotid. On the
right side the proximal
part of the fourth
arch remains, and forms
the common subclavio-
vertebral trunk ; on the
left side the whole arch
persists and forms the
remainder of the arch of
the aorta.

The fifth arch of the
right side only persists
as far as the origin of
the branch to the right
lung, the remainder of
the arch disappears. The fifth arch of the left side persists throughout its whole
length daring foetal life, and joins the fourth arch as the ductus arteriosus. After
birth, the part of the arch beyond the branch to the corresponding lung becomes
impervious on this side also, and is converted into the ligamentum arteriosum.

The upper part of the descending primitive aorta disappears entirely on the
right side ; that of the left side forms the commencement of the permanent
descending aorta.

Rathke described both pulmonary arteries as being- given off from the fifth left arch in
mammals, although, admitting that in birds and reptiles each is formed from the corresponding
arch. In birds the permanent aortic arch is the fourth arch of the right side, and not of the
left side as in mammals, and in reptiles both aortic arches remain pervious.

Many of the abnormalities which are observed in the disposition of these arteries, may be
explained by regarding them as a persistence of embryonic conditions.

The development of the arterial arches of the bird has been recently again examined by
Mackay, whose account differs in important particulars from that of Rathke, and indeed of
nearly all previous investigators. He describes the subclavian artery as arising from the third
arch, not from the fourth (this was also given as its origin by Sabatier), springing from the
ventral part of the arch , and running outwards superficial to the pnenmogastric nerve and j ugular
vein. The third arch and its dorsal upward prolongation, form the common carotid, not tho

DEVELOPMENT OF THE PRINCIPAL VEINS. 151

internal carotid, as stated by Rathke, and the ventral prolongation of the truncus arteriosus
forms, not the external carotid, but a small branch from the subclavian, or innominate
artery, to the front of the trachea. These observations have not been as yet extended to
mammals.

Zimmermann has described, both in the rabbit and in the human embryo, an arterial arch
between the aortic and pulmonary arches. If this is of constant occurrence it must be
reckoned as the iifth arch, and the pulmonary will become the sixth.

The histogenetic changes involved in the development of the blood-vessels are described in
the chapter on Histology.

The first vessels appear, as has been already stated, in the mesoblast of the vascular area ;
the lamina of mesoblast in which they are formed is sometimes distinguished as the vascular
lamina. They are said (bj His and others) to grow inwards from the vascular area, but the
manner in which the principal arteries and veins of the body are first developed is not clear,
beyond the fact that they are at first merely endothelial tubes. The muscular tissue of the
primitive aortfe is derived from the lower part of the proto-vertebra (E. Muller).

DEVELOPMENT OF THE PRINCIPAL VEINS.

In this subject also the description given by His of the condition and changes
of the veins in the human embryo will be followed, although it differs in certain
particulars from that which has usually been received.

In the early embryo, before the development of the allautois, two vilelhm or
omplmio-meseraic veins, right and left, bring back the blood from the vascular area
upon the yolk sac, and unite to form a common trunk, which is continued as the
sinus venosus into the auricular extremity of the rudimentary heart.

At the commencement of the placental or allantoic circulation (fourth week in
man) two umliUcal veins are seen coming from the placenta and opening into the
sinus near the vitelline veins (fig. 168). Into this also opens on either side a
transverse vein, the duct of Guvier or superior vena cava, which is formed by the junc-
tion of the primitive jugHlar vein, bringing blood from the head, and the cardinal
vein, which returns the blood from the Wolffian bodies, the vertebral column and
the body walls (figs. 179, 180). The trunk or sinus into which all these veins pour
their blood is now transversely disposed, immediately below the diaphragm, and
forms the saccus reuniens of Ilis, which has been already alluded to (p. 139).

The vitelline or omphalomeseraic veins enter the abdomen along the vitelline
duct and ascend at first along the front of the alimentary canal, but higher up they
are seen on either side of that tube (duodenum and stomach). Here transverse
communications form between the two veins, two in front of and one behind the
duodenum, so that this is encircled by two vascular rings (figs. 180, 183). Above
these venous circles the direct communication with the sinus becomes lost, the inter-
mediate venous vessel or either side becoming broken up within the substance of the
liver (which has by this time developed around them) into a vascular network, the
middle part of which becomes capillary.

The vessels which pass from the upper venous ring to the capillary network are
known as vena, advehenies, tliey become the branches of the portal vein ; those
which pass from it into the sinus are the vencn, revehentes, they become the hepatic
veins.

The lower communication between the vitelline veins takes the form of a com-
plete longitudinal fusion of the two vessels, at least for some distance. This fused
part receives veins from the intestine and stomach, and becomes the commencement
of the portal vein.

The umbilical veins are for a long time double within the abdomen, although
they have fused within the umbilical cord into a single trunk. They diverge from
this and j)as3 up to the sinus on cither side in the somatopleure, just where this is
becoming bent round into the amnion. After a time;, however, it is found that this
direct communication with the hinu.s is partially interrupted by the development of a

152

DEVELOPMENT OF THE PEINCIPAL VEINS.

vascular network, and that on the left side a fresh communication has become esta-
blished with the upper venous circle of the vitelKne veins. The interruption subse-
quently becomes complete on both sides (fig. 183), and on the right side the greater

Fig. 183. — Venous trunks op a human embryo of

ABOUT THREE-AND-A-HALF WEEKS. (His.)

v.c.d, v.c.s, superior vense cavse, right and left;
v.j, v.c, primitive jugular and cardinal vein;
v.u.d, u it. s, umbilical veins, right and left; v.v!',
v.u", upper detached portions of umbilical veins ;
v.v.om, omphalomeseraic or vitelline veins forming
the vena portse. The permanent veins are coloured
blue.

part of the vein becomes atrophied (on
both sides the part which originally opened
into the sinus reuniens remains evident
for a time). The left vein, on the other
hand, increases in bulk with the develop-
ment 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 or vena
ascendms, and it now carries most of the blood of the umbilical vein direct to the
heart. Subsequently the direct communication of the left hepatic vein with the

Fig. 184. — 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 trans-
verse fissure (o), to join the vena portoe(^) ; rf, the
ductus venosus, continuing straight on to join the
vena cava inferior (c) ; some branches of the umbili-
cal vein pass from a into the substance of the liver ;
y, the gall-bladder, cut.

sinus becomes obliterated, and a new com-
munication becomes established with the
ductus venosus ; and, finally, when, with
the growth of the lower limbs and of the
other abdominal and pelvic organs, the inferior vena cava becomes developed, this
also joins the upper end of the ductus venosus.

The lower part of the portal vein is formed, as we have seen, by the united
vitelhne 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. 183). The spiral turn around the duodenum is thus produced, and thus it is
also that the portal vein at first appears more directly connected with the right vense
advehentes than with the left.

Most of these embryonic veins are at first of relatively large size and have an
irregular sinus-like character, which disappears at a later stage of development.

At the time of commencement of the placental circulation, two short transverse
venous trunks, the duds of Cuvier, open, as has been above stated, one on each side,
into the auricle of the heart. Each is formed by the union of a superior and an
inierior vein, named respectively ihQ primitive jugular and the cardinal.

DEVELOPMENT OF THE PKINCIPAL VEINS.

153

The primitire ji'[/i(Iar vein receives the blood from the cranial cavity by channels
in front of the ear, which are subsequently obliterated : in the greater part of its
extent it becomes the external jugular vein ; and near its lower end it receives small
branches, which grow to be the internal jugular and subclavian veins (fig, 185).
The cardinal rei/is are the primitive vessels which return the blood from the
Wolffian bodies, the vertebral column, and the parietes of the trunk. The inferior
vena cava is a vessel of later development, which opens into the trunk of the
umbilical and vitelline veins, above the veniB hepaticse revehentes. The iliac veins,

Fig. 185. — Diagram of the condition ok

THE GREAT VEINS IN THE EMBRYO, AND
OF THEIR TRANSFORMATION INTO THE

PERMANENT VEINS. (After Kolliker.)

j, jugular veins uniting with the subchi-
vian, s; c.a, cardinal veins becoming the
azygos as in B ; d.c, ducts of Cuvier formed
by the union in B of jugular and cardinal,
and becoming the superior vena cava on the
right side, l)ut disappearing on the left side ;
ca', part of left cardinal vein which disap-
pears ; ci, vena cava inferior ; I, hepatic
veins and ductus venosus ; il, common iliac
veins ; cr, external iliacs ; h, hypogastric
becoming the internal iliacs.

which unite to form the inferior
vena cava, communicate with the
cardinal veins. The inferior extre-
mities of the cardinal veins are per-
sistent as the internal iliac veins.
Above the iliac veins the cardinal
veins are obliterated in a consider-
able part of their course ; the upper
portions then become continuous
with two new vessels, the 'posterior
I'erteh'al veins of Rathke, which
receive the lumbar and intercostal
twigs.

As development proceeds, the direction of the ducts of Cuvier is altered by tne
descent of the heart from the cervical into the thoracic region, and becomes the
same as that of the primitive jugular veins. A communicating branch makes its
appearance, directed transversely from the junction of the left subchivian and
jugular veins, across the middle line to the right jugular ; and further down
in the thoracic region between the posterior vertebral veins, a communicating
branch passes obli(juely across the middle line from right to left. The communi-
cating branch between the primitive juguhir veins is converted into the left brachio-
cephalic or innominate vein. The portion of vessel l)etween the right subclavian
vein and the termination of the communicating branch becomes the right brachio-
cephalic vein. The j)ortion of the primitive jugular vein below the communicating
vein, together with the right duct of Cuviei', forms the vena caA'a supci'ioi-, while
the cardinal vein oj)ening into it is the extremity of the great vena azygos. On the
left side, the portion of the primitive jugular vein placed below the coinnnmicating
branch, and the cardinal and posterior vertebral veins, together with the cross
branch between the two posterior vertebral veins, are converted into the left
superior intercostal and left snpericjr and inferior azygos veins. The variability
in the adult arrangement of these vessels de])ends on the dillerent extent to which
the originally continuous vesBcls are developed or atrophied at one uoint or another.

154 DEVELOPMENT OF THE PRINCIPAL YEINS.

The left duct of Cuvier is obliterated. But even in the adult, traces of this vessel
can always be recognised in the form of a fibrous band, or sometimes a narrow vein,

A B

Fig. 186. — A and B. — Diagrammatic outlines of the vestige op the left superior cava and of
A CASE OF ITS PERSISTENCE. (Sketched after Marshall. ) -J-.

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 ; 6, 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 szygos (right cardinal vein) ;
7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian
valve.

which descends obliquely over the left auricle ; 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

Fig. 187. — View of the f^tal heart and great

VESSELS, FROM THE LEFT SIDE, TO SHOW THE VESTIGE
OF THE LEFT SUPERIOR VENA CAVA IN SITU. (This

figure is planned after one of Marshall's. )

rt, right auricle ; h, left auricle and pulmonary veins ;
c, the conus arteriosus of the right ventricle : d, the left
ventricle ; e, descending aorta ; + , vestigial fold of the
pericardium ; /, arch of the aorta, with a part of the
pericardium remaining above ; g. main pulmonary artery
and ductus arteriosus; g' , left pulmonary artery ; 1, 1',
right and left internal jugular veins ; 2, 2', sabclavian
veins ; 3, 3', right innominate and superior vena cava ;
4, left innominate ; 5, 5', remains of the left superior
cava and duct of Cuvier, passing at + in the vestigial
fold of the pericardium, joining the coronary sinus, 6,
below, and receiving above the superior intercostal vein,
7 ; 7', T, the upper and lower intercostal vein.

of the serous membrane of the pericardium, the vestigial fold of Marshall, to whom
is due the first full elucidation of the nature and relations of the left primitive vena
cava superior.

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

PECULIARITIES OF THE FCETAL ORGANS OF CIRCULATION.

155

right auricle, an arrang-ement which is also met ^Yith 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.

A case is recorded by Gruber in which the left vena azygos opened into the
coronary sinus, and was met by a small vein descending from the union of the
subclavian and jugular. Hei-e, then, the jugular veins had been developed in the
usual manner, while the left vena azygos continued to pour its blood iuto the duct
of Cuvier.

PECUIilAEITIES OF THE FCETAL ORGANS OF CIRCULATION.

It may be useful here to recapitulate shortly the peculiarities of stracture
existing in the advanced stage of the formation of the fcetal organs of circulation.

Fig. 188. — Diagrammatic outline of thk

OKGANS OF CIRCULATION IN THE FffiTUS

OF SIX MONTHS. (Allen Thomson.)

RA, right auricle of the he.art ; RV, 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 ; res, superior vena cava ; rci, in-
ferior 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 arc
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 i)assing from the vena cava iul'ei'ior
through the fossa ovalis of the right auricle,
and the foramen ovale into the left auricle ;
hv, the hepatic veins ; v]), vena portaj ; x to
vci, the ductus venosus ; uv, the umbilical
vein ; ua, umbilical arteries ; uc, umbilical
cord cut short ; i i', iliac vessels.

with reference to their influence in
determining the course of the blood
during intra-uterine life, and the
changes which occur in thera upon
the establishment of pulmonary rc-
cpiration at birth.

The foramen ovale retains tlic
form of a free oval opening in the
septum atriorum up to the fourth
month, but in the course of that
month and the next the growth of
the valvular plate which fills up the
floor of the fossa ovalis, becomes
complete, so that in the last three
and a lialf months the blood can
only pa«8 from the right into the left
auricle, not in a cdiitrary dii'ection.

The Enstachian valve constitutes a cresccntic fold of the liiii
the heart, which is so situated an to direct the blood entering the
inferior cava towards the opening of the foramen ovale.

iig sti
auric

ucture of
le by the

156 COURSE OF THE BLOOD IN THE FffiTUS.

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 placenta. From the placenta the blood
is returned by the umbilical vein, which, after entering the abdomen, communi-
cates 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 fcetus. — The right auricle of the foetal heart
receives blood from the two vena cavge 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 fr-om the supply of placental blood
entering that organ by the umbilical vein, or proceeding from the vena portse 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 the
placenta. From these several organs it is returned by the vena cava inferior, the
vena port», 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.

^ Sabatier was the first to call attention particularly to the action of the Eusta-
chian valve in separating the currents of blood entering the right auricle by the
superior and inferior vense cavse. This separation, as well as that occurring
between the currents passing through the aortic arch and the ductus arteriosus
into the descending aorta, was illustrated experimentally by John Eeid. A striking
confirmation of the extent to which the last mentioned division of the two currents
of the foetal blood may take place, without disturbance of the circulation up to the
time of birth, is afforded by the examples of malformation in which a complete ob-
literation has existed in the aortic trunk immediately before the place of the union
of the ductus arteriosus with the posterior part of the aortic arch.

In earlier stages of development than those above described, it is certain that
there is little or no separation of the two kinds of blood, for both the umbilical
veins from the placenta and the veins from the yolk sac and body generally,
pour their blood together into the sinus venosus, and the mixed blood is then
forced through a single somewhat narrowed orifice (porta vestihuU of His) into the
auricle.

THE LYMPHATIC SYSTEM. 15?

CHANGES IN THE OIKOUIiATION AT BIRTH.

The changes which occur in the organs of circulation and respiration at birth,
and which lead to the establishment of their permanent condition, are more
immediately determined by the inflation of the lungs 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 placental 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 by 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 hver, 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 occiu-red. It makes rapid
progi'ess in the first and second days, and by the third or fourth day the passage
tlirough 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 now referred to. It is gradually eflPected by
the union of the forepart 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 unfrequently
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, as occurs in the
malformation attending the morbus cceruleus.

THE LYMPHATIC SYBTHM.

The derelopment of the lymphatic system has been studied in the chick by Budge. Here
there exist a network of lymphatics a;,'reeing' in their general distribution with the blood-
vessels in the vascular area. This is the first lymphatic circulation ; a second one is formed
later, corresponding with the allantoic circulation. The lymphatics of both systems communi-
cate with the ccelom, but only those of the second circulation communicate with veins. The
lymphatics lie in the vascular area above and close to the blood-vessels, but are separated
near the embryo by a layer of mesoblast continuous with the splanchnoi)leure.

The metho<l of development of lymphatic vessels and lymphatic glands, i.s doalt with in
the chapter on Histology.

158 RECENT LITEllATURE.

BBCENT LITERATXTBE.

Blaschek, A., Untersuchung uber Herz, Pericard, Endocard und PericardialhoMe, Mittheil.
aus dem embryol. Institut der Universitat Wien, 1885.

Born, Gr., EntioicUung des Sdugethier-herzens, Archiv f. mikr. Anat., Bd. 33, 1889.

Budge, A., Untersuchung en uber die Entwichelmig des Lymphsy stems heim Huhner embryo. Arch,
f. Anat. u. Physiol., Anat. Abtheil., 1887.

Fiirstig, J., Untersuchung en uber die EntwicJcelung der primitiven Aorten mit besondcrer
Berilchsichtigung der Beziehungen derselhen zu den Anlagen des Herzens, Schriften herausgegeben von
der Naturf. Gesellsch. bei der Univers. Dorpat, i., 1884.

Greenfield, "W. S., Case of malformation of the heart, d-c, Journal of Anat. and Physiol., 1890.

Hochstetter, F., Beitrdge zur Entwickelungsgeschichte des Venensy stems der Amnioten,^OTp'h.<Aog.
Jahrbuch, Bd. xiii., 1888. _ _ _ _ _ , _

Mackay, J. Yale, The development of the branchial arterial arches in birds, loith special refer-
ence to the origin of the subclavians and carotids. Philosophical Transactions, B, 1888.

Marius, J., Quelques notes sur le diveloppement du cceur chez le poulet, Arch, de biol. , 1889.

Mayer, P., Ueber die Entwickelung des Herzens und der grossen Gefdssstdmme bei den Selachiern,
Mitth. d. zool. Station zu Neapel, Bd. vii., 1887.

Miiller, Erik, Studien uber den Ursprung der GefdssmusJculatur, Archiv. f. Anat. u. Physiol.
Anat. Abth., 1888.

Kabl, C, Ueber die Bildwng des Herzens der Amphibien, Morphol. Jahrbuch, 1886 ; Ueber die
Bildung der Herzanlage, Wiener Medicin. Presse, 1886.

Hose, C, Beitrdge zur Entwiokelungsgeschichte des Herzens, Dissert, Heidelberg, 1888, and Morphol
Jahrb. xv.

Riickert, J., Ueber den Ursprung des Herzendothels, Anat. Anzeiger, 1887 ; Ueber die Entstehung
der endothelialen Anlagen des Herzens und der ersten Gefdssstdmme bei Selachier-Embryonen, Biolog.
Centralb., Bd. viii., 1888.

Schirakewitsch, "W., Ueber die Identitdt der Herzbildung bei den Wirbel- und wirbellosen
Thieren, Zoolog. Anz., 1885.

Sob-wink, F., Ueber die Entwickelung der Herzendothels der Amphibien, Anat. Anzeiger, 1890.

Turstigr, J., Mittheilungen uber die Entwickelung der primitiven Aorten nach Untersuchung en an
Huhnerembryonen, Dissert. Dorpat., 1886.

Van Bemmelen, J. F., Die Viseeraltaschen und Aortenbogen bei Reptilien und Vogdn, Zool.
Anzeiger, 1886.

Zimmermann, W., Ueber ein zwischen Aorten- u. Pvhnonalbogen gelegenen Kiemenarterienhogen
beim Kaninchen, Anat. Anzeiger, 1889.

DEVELOPMENT OF THE MUSCLES. 159

DEVELOPMENT OF THE SEROUS CAVITIES AND OF THE
MUSCLES AND SKELETON.

The serous cavities — peritoneum, pleurte, pericardium — are derived from the
original split or cleavage of the mesoblast, which constitutes the ccelom or general
body cavity (pleuro-peritoneal cavity of older authors). This cleft is formed in the
head as well as in the trunk, and when the heart is formed as a double tube, each
half is enclosed within a portion of that cavity, which later on, when the body walls
bend round and meet to enclose the fore-gut, comes, like the heart itself, to occupy a
position on the ventral aspect of the alimentary tube. The part of the coelom which
thus contains the heart is not for some time entirely distinct, but communicates
dorsally by two comparatively narrow channels with the anterior part of the general
body cavity, here separated into lateral halves, which ultimately become the pleura?,^
by the alimentary canal. Subsequently these communications become obliterated,
and the heart-coelom separated as a distinct cavity (pericardial cavity). Below,
where the great veins enter the heart, they pass into a mass of mesoblastic tissue,
which is connected with the anterior body wall (where it receives the umbilical
and vitelline veins), and also with the lateral wall (where it receives the ducts of
Cuvier), and which, as the heart bends, so that the venous end passes behind the
ventricle and bulb, is carried along with the veins up behind that organ, and thus
forms an obliquely placed thick septum, at first incomplete, but subsequently becoming
entirely closed, which separates the heart within the pericardial part of the body
cavity in front and above from the stomach and alimentary canal within the peri-
toneal part of that cavity behind and below. The thick septum, besides containing
the saccus reuniens and the portions of the great veins (vitelline, umbilical, ducts of
Cuvier) which open into that cavity, also contains the rudiments of a part of the
diaphragm and the mesoblastic part of the liver, into which the hypoblastic part
grows from the adjacent duodenum ; it has been termed by His the transverse septum
(see figs. 177, 178). As development proceeds, the septum becomes gradually diffe-
rentiated into its several parts. The great veins become still further shifted behind
the heart, and the saccus reuniens becomes incorporated with that organ. The
liver, which is at first contained entirely within the septum, becomes split off from
its upper layer, which now forms the thin portion of the diaphragm, while the cavity
of the peritoneum extends from either side, and separates them from one another,
except along the attachment of the broad ligaments.

The diaphragm is completed by a growth of mesoblast which occurs on each
side, and cuts off the anterio -dorsal portions of the body-cavity into which
the lungs are invaginated (recessus pulmonales) from the posterior or peritoneal
part.

The serous membranes are formed by differentiation of the lining mesoblast of
the ccelom.

The formation of the omenta, and the changes which the mesenteric folds of
peritoneum undergo, have been already mentioned in connection with the develop-
ment of the abdominal viscera.

Development of the muscles. — The muscles of the trunk are formed from
the protovortebrae. These are at first, as previously described, separate masses of
mesoblast, the cells of which have at the periphery of the mass a tendency to a
radial disposition (fig. 189, A), whilst toward the centre they are loosely arranged,

' The manner in which the plcunn are invaginated by the growing lungH has already been alluded to
(p. 110).

160

DEVELOPMENT OF THE MUSCLES.

and may even leave a more or less distinct space unoccupied by cells (protovertebral
cavity). This cavity has occasionally been noticed (Lockwood, Bonnet) to be
continuous laterally with the mesoblastic (coeloraic) cleavage (fig. 189, B ; see also

yz.c.

Fig. 189. — Two SECTIONS op a sheep
EMBRYO. (Bonnet. )

A, shows the cavity within the proto-
vertebrte. In B, the protovertebra on
the left side of the section is united with
the lateral mesoblast ; on the right side its
cavityalsois continuous with the coelomic
cleft in that mesoblast. am, amnion ;
n.c, neural canal ; p.v, protovertebra ;
ao, aorta ; p.p, pleuro-peritoneal space
(ccelom).

fig. 139, p. 117), and it is
probably the morphological equi-
valent of the coelom in this part
of the mesoblast. Whether there
be originally a cavity or not in it,
the protovertebra presently be-
comes filled up with cells and
then forms a fairly compact mass
of cells which are mostly irregu-
larly arranged, but externally
(next to the cutaneous epiblast)
become regularly disposed into
an epithelium-like plate of co-
lumnar cells. This is known as
the muscle plate, and when the
inner part of the protovertebra becomes broken up as a distinct mass and joins
with the neighbouring protovertebra to form the membranous vertebral column
(see below), the muscle plates still remain distinct : in them therefore the original

Fig. 190. — Transverse section

OF THE TRUNK OP A CAT EMBRYO,
SHOWING MUSCLE PLATES.

(E. A. S.)

m.p. , muscle plate ; ao, aorta ;
m.g., mid-gut; am, amnion; w,
vesicle of Wolffian body; w.d.,
Wolffian duct.

mesoblastic segmentation
continues to be exhibited.
They do not long remain
as a single epithelium-like
layer, for the extremities
of this layer fold sharply
round and become continu-
ous with a cell-stratum,
which immediately lines the interna, surface of the columnar layer and forms an
imier muscle-plate (fig. 190). It is uncertain whether the cells of this inner muscle-
plate are derived from part of the columnar layer which has folded over, or whether
they spring from other cells of the protovertebra. Soon after their appearance as a
distinct layer of the muscle-plate they begin to elongate in the sagittal (antero-

FORMATION OF THE MUSCLES OF THE HEAD. 161

posterior) direction (ti«r. 11)1, i.)u.), and it may })reseiitly be observed that they are
becoming developed into longitudinal groups or segments of muscle-fibres which
stretch between the original intervals between the protovertebrte. The destination
of the ontej" layer of the muscle-plate has not been traced with certainty. Balfour

Fig. 191. — HiiRIZoNTAL LONGITUMNAL
SECTION OF THREE PROTOVERTE-
BRJi IX A SSAKE EMBRYO.

(v. Ebner.)

<j>, cut-.meou.i epiblast ; e. m, exter-
nal layer of muscle-plate ; /, its mar-
gins folded round into i.m, internal
layer of muscle-plate composed of
flattened cells which are becoming
elongated into muscular fibres ; ii.c,
neural canal, in outline only ; n'.c',
neural epiblast forming its walls.
Between these and the muscle-plate
is a continuous mesoblastio tissue

which has been derived from the inner parts of the protovertebnB. partly inteiTupted by the ganglion
rudiments, gl. The original intervals between the protovertebra; here are etill indicated by vessels, v.
i.cl, cleft in the deeper protovertebral tissue (according to Ebner this is the remains of the original
protovertebral cavity).

described it as also eventually becoming transforaied into muscle-fibres (in elasmo-
branchs), but others have failed to confirm this opinion, and it is by some believed
that it may assist in the formation of the cutis vera.

Although the muscle-plates are originally mainly concerned with the formation
of the muscles which move the central skeletal axis, it is probable that all the
skeletal muscles both of the trunk and limbs are eventually derived from them (see
below, p. 163).

Formation of the muscles of the head and evidences of head segmentation. — •
Although perhaps no part of the cranium actually represents a veitebra. there is nevertheless
abundant evidence of an original segmentation of the head corresponding with the mesoblastic
somites of the trunk. Such segmentation is shown by the existence of the visceral arches,
which in the typical and lea.st modified vertebrates Qe.fj., elasmobranchs) are at least nine
in number, by the successive separation of the part of the body cavity which extends into
the head into separate portions, or head cavities, one corresponding to each visceral arch, the
parletes of which develop into muscles, and which therefore correspond with the muscle-
plates of the protovertebrai of the trunk (this is in fact the typical mode of formation of
mesoblastic somites, r. page 26), and lastly by the mode of development of the cranial
nerves and their relations to the visceral and branchial arches, which correspond in a general
way with the relations of the ventral branches of the spinal nerves to the ribs.

The formation and destination of the head cavities have been investigated of late yeai-s
(chiefly in elasmobranchs, but also in reptiles and birds) by Balfour. Milnes Marshall, and
van Wijhe, and the result of these investigations tends to show that, in all, nine portions of
the original head cavity become separated off on either side, their formation proceeding from
behind forwards. Each somite cavity becomes subsequently divided into a dorsal part corre-
sponding with the protovertebne of the trunk and a ventral part, corresponding with the
pleuroperitoneal cavity, and lying in the middle of the corresponding visceral arch ('). Both
parts give rise by differentiation of their parietes to mu.scles ; the visceral arch portions to
the muscles of the jaw and hyoidean and branchial apparatus ; the dorsal portions, some (first.
second, and third) to the muscles which move the eyes, some (seventh, eighth, and ninth) to
the muscles whicli connect the head with the shoulder girdle, whilst some, viz., the fourth,
fifth, and sixth, are said to disappear. The first head cavity forms the eye muscles, which arc
supplied by the third nerve ; the second, the muscle supplied by the fourth (.superior oblique) ;
and the third, the muscle supplied by the sixth (external rectus). In higher vertebrates, the
formation of the hea<l cavities and their subsequent destination have not been as yet clearly
followed out, although indications of their exist<;nce are not wanting.

' .According to v. Wijhe the int.rmediate )>art of the typical soniito cavity represents a segmental
(arinifeioiis) organ, but this iriteriiiedialc part l.s not seen in the head, altliough it beginb to appear in
t'le immediately Hucvceding »<jmiU:n.

vol-. I. *'

163 DEVELOPMENT OF THE VERTEBRAL COLUMN.

Development of the vertebral column.— The vertebral column is developed
around the notochord, except at the anterior end of that structure, which is imbedded
in the basis cranii. It is formed from protovertebral mesoblast. The outer part of
each protovertebra is transformed into a muscle-plate, and thus the original meso-
blastic segmentation is maintained. The inner parts of the protovertebrge do not,
however, remain distinct, but blend with one another on each side of the neural
canal to form a longitudinal mass, which extends to the side of and subsequently
encloses the notochord, and finally sends dorsal prolongations over the neural canal,
so that this also receives a continuous mesoblastic investment, forming the mmnlrana
reuniens superior of Remak. The investment is only incomplete opposite the points
where the nerve roots are connected with the spinal cord, and in it there is no sign
of any differentiation into vertebrge. It is continuous with a similar investment
within the cranium, which extends in front of the notochord into the fronto-nasal
process. The mass of mesoblast, which thus encloses the notochord and neural
canal, is often spoken of as the membranous vertebral column and cranium, but it
represents much more than the cartilaginous and bony structures of those parts, all
the investing membranes of the cord and brain and the ligaments of the vertebrae
being also derived from it. From it septa pass between the muscle-plates and serve
to give attachment to the developing muscle-fibres.

The first appearance of the permanent vertebrae is in the form of cartilage, which
becomes formed in this mesoblastic investment on either side of the neural canal,
nearly opposite the interval between each two muscle-plates, to form the neural
arch. This part of the vertebra therefore alternates with the original mesoblastic
somites as represented by the muscle-plates.

According to Froriep, the lateral halves of each cartilaginous neural arch become
united below the notochord before the appearance of the rudiments of the cartilagi-
nous bodies, and the latter appear as median accumulations of cartilage, immediately
]W8terior to the hypochordal part of the cartilaginous arch. In most of the vertebrae
this hypochordal part of the arch soon disappears as a distinct structure, but in the
atlas vertebra the primitive condition is maintained.

The serial arrangement of the musculature represents phylogenetically the original
segmentation of the vertebrate body. The segmentation of the vertebral column,
on the other hand, has been arrived at later, and has been carried out in depen-
dence upon the muscular segmentation.^

The cartilage makes its appearance on the fourth day in the chick, on the
eleventh or twelfth day in the rabbit, and in the fourth or fifth week in man
(Kolliker). It is completed by the sixth or seventh week, soon after which ossifica-
tion commences, To form the intervertebral discs, the mesoblast between the
bodies of the vertebrse acquires a fibro-cartilaginous character, while at the same
time the notochord, which gradually elsewhere becomes reduced in size and eventu-
ally disappears, here undergoes enlargement, and its cells form an irregular network
in the central intervertebral pulp. Its remains arc found at all periods of life in
the middle of the discs (Luschka).

In the osseous fishes there is an intervertebral dilatation of the notochord, the growth of
which proceeds to such a considerable extent as to give rise to a mass of soft gelatinous
substance, which occupies the conical hollows of the biconcave vertebral bodies. But in
birds, reptiles, and amphibia, where synovial articulations are developed between the vertebral
bodies, the notochord soon disappears from the intervertebral spaces, although its remains are
seen for some time in the bodies themselves (Gegenbaur).

In mammals, the notochord is constricted within the cartilaginous vertebral body, but
dilated in the intermediate parts of this rod, the whole cord being monilif orm. At a somewhat
later period small dilatations are also to be seen in the epiphysial cartilages (fig. 193).

' A. Froriep, " Zur Ertwickl. der Wirbelsaule, &c,," Arcliiv f, Anatomie 1883 and 1886,

THE LIMBS.

163

Ribs and Sterntini. — The ribs are formed Ity separate cartilao-inons transfor-
mation in extensions of tlie protovertebral niesol)lasb between the muscle-plates.
According to some, they grow out from the cartilaginous vertebra;, but become separate
before ossification begins. Similar deposits are formed in connexion with the other
vertebrae (except the coccygeal in man), but they here become united by ossification
with and form parts of the vertebrae (see Osteology, Vol. II.). At their ventral

■ h

Fig. 192. — Sections of the vertebral column op a human fietus of eight weeks.

(From Kolliker. )

A, transverse longitudinal .section of several vertebrre. 1, ], cliorda dor.«a]is, its remains thicker
opposite the intervertebral discs ; 2, is placed on one of the bodies of the permanent vertebrss ; 3, on
one of the intervertebral discs.

B, transverse horizontal section through a part of one dorsal vertebra. 1, remains of the chorda
dorsalis in the middle of the boily ; 2, arch of the vertebra ; Z, head of a rib.

Fig. 193. — Cac-ittal section op a dorsal intervertebral lkjament op an advanced
sheep's embryo. (Kolliker.)

l.a, l.p, anterior and posterior ligaments ; /. /, intervertebral ligament ; l\ k\ cartilaginous ends of
two vertebral bodies, iv, w' ; c, enlargement of notochord in the liganiciit ; c', c", enlargements in the
cartilaginous ends of the vertebra-.

e.xtremities the first seven (thoracic) cartilaginous ribs become united on cither side
into a longitudinal cartilaginous plate, and this afterwards joins its fcllcw of the
opposite side to form the sternum (manubrium and body). The xiphoid is of later
formation (Parker). This mode of development of the sternum explains many of
the malformations in the shape of fissures of the sternum of different gradation
which have been observed.

The Limbs.— The limbs arise as outgrowths from the lateral part of the
trunk in the thoracic and pelvic regions in the third day in the chick and in the
third and fourth week in the human embryo. They appear as flattened semilunar
thickenings of the parietal mcsoblast covered by epiblast, budding out from a lateral
ridge which is seen in the early embryo near the line of (!l(;avage of the mesoblast
and close to the outer margins of the miiscle-plat(;s, and several of whi('h subw(|iieiitly
send prolongations into each liinbCj; they are therefore connected with several
mesoblastic somites, as is also indicated by tlieir nerv(! supply.

' ThiH i.s the case in elaHrnobi-anchs (hcc (ig. 194, from lialfourj, liut, according to I'aterson,

164

THE LIMBS.

Each limb consists of a part which is sunk in the substance of the lateral ridge,
and in which the thoracic or pelvic girdle becomes developed, and of a free or

m^W/ifi!^^'-,..,

^ 'J-

lif-i

//

Fig. 194. — Transverse section throttgh an anterior part of the trunk of an embryo of

ScTLLiuir. (Balfour, j

sp.c, spinal cord; sjo.j, ganglion of posterior root; ar, anterior root; dn, dorsal ; s/).7i, ventral
branch of spinal nerve ; nip, muscle plate ; mj/, part of muscle plate already converted into muscle ;
vip.l, part of muscle plate extending into the limb ; nl, nervus lateralis ; ao, aorta ; ch, notochord ;
•'^y-ff, sympathetic ganglion ; ca.v, cardinal vein ; sd, segmental duct ; st, segmental tube ; du, duode-
num ; hp.d, junctiou of hepatic duct with it; pan, rudiment of pancreas connected with another part
of duodenum ; umc, opening of umbilical canal (vitelline duct).

projecting part (fig. 195). This is at first quite simple, and represents the distal
segment of the limb (hand or foot). The other two segments (forearm and leg ;
arm and thigh) are successively marked off between it and the girdle by the
development of transverse furrows representing the joints (fifth and sixth week).
At about the same time four notches appear in the flattened distal extremity,
marking off the intervals between the fingers and toes, and the middle segment
(fore-arm or leg) begins to be flexed upon the proximal (arm or thigh), the

the limbs donot receive such prolongations from the muscle-plates in birds and mammals ; the muscles
develop in situ from previously indiSerent mesoblast.

THE CRANIUM.

I Go

concavity looking forwards in tlie upper limb and backwards in the lower limb.
The limbs also come to be folded ventrally against the body of the embryo.

From the manner in which the flattened limb-bud gi'ows out from the lateral
ridge, it is obvious that its surfaces must at first be dorsal and ventral. The
dorsal surface afterwards becomes extensor and the ventral flexor. The anterior
edge is respectively the radial and tibial : the posterior, the ulnar and fibular. As

Fig. 195. — Outlines of thk anterior extremities

OF HUMAN EMBRYOES AT DIFFERENT AGES. (After

His.)

A, at four weeks ; B, at five weeks ; C, at seven weeks ;
J), at nine or ten weeks.

development proceeds, a half rotation occurs in
opposite directions in the two limbs, resulting
in the middle flexure (elbow, knee) being directed
forwards in the upper, backwards in the lower
hmb.

The bones of the limbs are laid down as
cartilages which appear as separate diflerentia-
tions of the more centrally placed mesoblast,

a portion of mesoblast remaining for a time undifferentiated opposite each synovial
articulation. Within this a cleft subsequently appears, and enlarges to form the
synovial cavity, the mesoblast which bounds the cleft developing eventually into
the synovial membrane and capsular ligaments of the joint.

The craninm. — In the head the notochord extends as far forwards as the
raid-brain. Here also it is invested by a continuous mass of mesoblast, which sends
lateral prolongations over the neural canal as in the trunk (membrana reuniens).

J3

:Eth..

Fig. 19C.— Diagrams of the cartilaginous cranium. (Wiederslu-im.)

A, First staj;e.
67/, iiotochonl ; Tr, trabecula; cranii ; P.r.h, parachordal cartilages ; P, situation of iiituitiuy
-V, E, (>, situations of olfactory, visual and auditory organs.

B, Second stage.
li, basilar cartilage (investing; mass of Katlike) ; .S, nasal sci)tuni and ethmoidal cartilage
litW, prolongations of ethmoidal around olfactory organ, comiilctiiig the nasal capsule ; (>l, loi
for passage of olfactory nerve fibres ; N, E, 0, Ch, Tr, as before.

The main diflerence in development between the c-ianiiim and vertebral column
consists in the fact that no separate cartilaginous dcjKJsitH to form vertebme oricur

body ;

: Eth,

amina

166

FORMATION OF THE VISCERAL SKELETON OF THE HEAD.

within the head, nor can any parts be distinguished which strictly represent vertebrse.
The cartilage of the basis cranii makes its appearance in the form of two longitudinal
bars lying on either side of the notochord {parachordal cartilages), and of two other
bars {traheculcB cranvi of Rathke) which embrace the pituitary body, and which
become united together in front and with the parachordals behind to form a con-
tinuous mass, which posteriorly completely invests the notochord (fig. 196). The
cartilaginous basis cranii may therefore be distinguished into the parachordal and
prechordal parts. Of these the first represents the basi-occipital and basi-sphenoid,
the second the presphenoid and ethmoid portions. From the basis cranii continuous
cartilaginous plates grow on either side over the cerebral vesicles to a greater or
less extent in different animals, least in mammals, where only the occipital region
becomes thus roofed in by cartilage. Anteriorly the united trabeculge cranii stretch
forwards into the fi'onto-nasal process, where they form the ethmoid cartilage and
nasal septum, besides enclosing the nasal pits externally (cartilaffmoiis nasal capsule).
From the sides of the presphenoid portion the orbito-sphenoids (lesser wings),
containing the optic foramina, are developed, and from the sides of the basi-sphenoid,

Fig. 197. YiEW FROM BELOW OP THE CARTILAOilNOrS

CRANIUM WITH ITS OSSIFIC CENTRES IN A HUMAN

FCETUS OP ABOUT POUR MONTHS. (After Huxley.)

The cartilage is dotted to distinguisb it from the bone
which is shaded with lines.

b.o, basi-occipital ; a.o, lateral occipitals ; f.ni, foramen
magnum ; o.c, o'.c', bony deposits in the periotic capsule ;
p.s, post-sphenoid ; pr.s, pre-sphenoid ; o.s, orbito-sphe-
noid ; s.n, septum nasi.

the greater wings or alisphenoids. A cartilagi-
nous capsule, connected with the parachordal
portion of the basis cranii, invests the otic vesicle
(periotic capsule). Within this, bony centres are
eventually formed, which unite to form the
petro-mastoid. In the human embryo, chondri-
fication begins in the fourth or fifth week in
the basilar portion of the skull, and is nearly completed by the eighth
week.

Formation of tiie visceral skeleton of the head : cartilaginous bars of
the visceral arches. — -A cartilaginous bar extends from the periotic capsule and
basis cranii, within each of the first three visceral arches, and passes forwards to
meet its fellow in the middle line. The iar of the mandihidar arch is known as
MeclceVs cartilage. It is visible in all sections of the foetal jaw up to the seventh
month. Its proximal end is attached at first to the basis cranii, afterwards to the
periotic capsule; its distal end joins that of its fellow in the middle line of the
lower jaw. Only near this conjoined part does Meckel's cartilage take part in the
formation of the lower jaw bone, the greater part of this bone being developed by
ossification at several places in the connective tissue around the cartilage. In some
animals a short cartilaginous bar is formed in the maxillary process {palato-pterygoid
lar, fig. 198, A,]3pg). Close to it the palatine and pterygoid bones are formed in
membrane, but the bar itself entirely disappears. The second or Tiyoid bar arises
from the skull close behind the attachment of Meckel's cartilage, and passes along
the second arch. It disappears in part, but in part is converted into the styloid
process, stylo-hyoid ligament, and lesser cornu of the hyoid bone. The body of the
hyoid bone (basi-hyal) is an intermediate formation between the second and third
arches. The har of the third arch is known as the ilujro-hyoid. Its lower end forms
the greater cornu of the hyoid bone ; but its attachment to the skull early disap-

FORMATION OF THE AUDITORY OSSICLES.

167

ii,h^ \/,

jaa.ch nc

Fig. 198, A. — Elements of the skull of an embryo pig, |-ixch long, viewed from below :
sEMiDiAGRAMMATic. (Froiu Balfouv aftcv Parker.)

jia.ch, parachordals ; nc, notochord ; au, otic capsule ; pji, pituitary ; tr, trabeculaj ; c.tr, cornu
of the trabecuia ; ^«, prenasal cartilage; e.n, external wares; ol, olfactory region ; 71. jy/, ixilato-
pterygoid bar enclosed in the maxillo-pterygoid process ; mn, mandibular bar ; A//, hyoidean bar ; th.h,
thyrohyoid bar ; 7a, aperture for facial nerve ; 8«, for glossopharyngeal ; 86, for vagus ; 9, for hypoglossal.

Fig. 198, B. — Side view of the mandibular and hyoid arches in an embryo pig of 1^ inch
IN length. (From Balfour, after Parker. )

t'j, tongue ; ml; Meckel's cartilage ; ml, body of malleus ; mh, its manubi'ium or handle ; t.ty,
tegmen tympani ; i, incus ; gt, stapes ; i.hy, inter-hyal ligament ; st.h, stylo-hyal cartilage ; h.h,
hypobyal ; bh basibranchial ; th.h, rudiment of first branchial arch ; 7a, facial nerve.

MeokcZ's

tfzpvoG ^r^i^^ Meckel's coT-Lda^e^

/k 'l. huMwibruM'Ti.

Jii^oi'd, cccrbUcurC'

lC(J,lc

Mcokjels c<xHi'Iage'

stages i/ictnuhfla.m'

Fig. 169. — Prkpj rations hiiowino thrkk staoks ok thk

CAIiTILAOINOUH bars ok 'j'IIE 1«T and 2nd AKUUE8 IN

■riih KMiiKYo OK thk SHEEP. (Salciisky. )

pears, or in some animals may not
be fonnd at all. This bar is in
all cases of less importance than
the first two, from the proximal
parts of which important struc-
tnres connected in lower verte-
brates with the suspensory appa-
ratus of the lower jaw, in the
higher vertebrates Avith the ap-
paratus for sound transmission
to the internal ear are developed.
Formation of the auditory
ossicles. — if the embryonic de-
velopment of the Ijais is studied
in man and other mammals, it is
found that at a certain jieriod of
1(jetal life JMcckel's cartilage is
directly continuous with the car-
tilaginous malleus, above which,
and at first in direct continuity
with it, is the cartilaginous incus
(fig. ]!)!)) ; these two ossicles
therefore appear as the cnlai-ged
and modified proximal end of
Meckel's cartilage. Somewhat
later the incus becomes detached
from the malleus, but the latter
long remains in continuity with
Meckd'H cartilage (fig. '■lO^^).

168

FORMATIOX OF THE AUDITORY OSSICLES.

The most obvious interpretation of these appearances would seem to be that both the
malleus and incus are developed from the cartilaginous bar of the first or mandibular arch.
This view is not, however, universally held. For, as the several illustrations show, the second
or hyoid bar is also connected to the develoi^ing incus, through which it is joined to the periotic
capsule. Hence it has been inferred by some that the incus belongs to the hyoid bar, and not
to the mandibular (A. Fraser). Others have looked upon it as representing a hyomandibular
cartilage, like that which in Sauropsida forms a common suspensory apparatus for both
mandibular and hyoidean apparatus (Huxley). This hyomandibula itself, however, may repre-

Fig. 200. — Condition of Meckel's cak-

TILAGE AND THE HYOID BAR IN THE
HUMAN FtETUS OF ABOUT 18 WEEKS.

(Kbiliker.)

B, is an enlarged sketch by Allen
Thomson, showing the relationship of
the several parts better than in A.

z, zygomatic arch ; ma, mastoid pro-
cess ; mi, portions of the lower jaw left
in situ, the rest having been cut away ;
if, 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 ; /, incus ; s, stapes ; sta, stape-
dius ; st, styloid process ; ^, A, g, stylo-
pharyngeus, stylohyoid, and styloglossus
muscles ; stl, stylohyoid ligament attached
to the lesser cornu of the hyoid bone, hy ;
th, thyi'oid cartilage.

sent an upward prolongation of
Meckel's cartilage, and would then
belong to the 1st visceral arch
(Peters). Lastly, yet another solu-
tion of the question has been offered,
viz.. that both incus and malleus are
hyomandibular (Albrecht, G-adow).

The last mentioned opinion is based
mainly upon considerations of com-
parative anatomy, which can hardly
be left out of account in dealing with
the morphology of these structures.
In lower Vertebrata the suspensory
apparatus of the lower jaw comprises
besides the hyomandibula, common
to it and to the hyoid apparatus, a
large bone, known as the quadrate,
hj means of which, either directly or
with the intercalation of an os arti-
culare, the lower jaw is united with
the basis cranii and periotic capsule.
Reichert looked upon the incus as the
homologue of the quadrate, and the malleus as that of the os articulare ; and the same view
was taken by Gegenbaur. Huxley, on the other hand, came to the conclusion that the
homologue of the quadrate bone of reptiles and birds is to be found in the malleus, and that
the inciis represents a portion of the hyomandibular bar, which, as above stated, is common to
both first and second arches. Various other observers have concluded that the quadrate of
lower vertebrates is represented in mammals by the zygomatic process of the squamosal.
G-adow, however, looks upon it as represented by the tympanic ring of mammals.

The stapes has been variously described as representing : 1 , a part of the hyoid arch
(Reichert) ; 2, a part of the periotic capsule which has become detached (Parker) ; .S, in part
or wholly, the hyomandibula of lower vertebrates (G-egenbaur, Huxley, Albrecht, Gadow) ;
4, hyomandibula and detached periotic cartilage conjoined (Gradenigo) ; 6, as an independent
circular deposit of cartilage around the stapedial artery ' (Salensky, Eraser). It is at any rate
closely connected with the hyoid bar, which forms from above dovsni the tympano-hyal and

^ This artery disappears in man, but is persistent in many mammals.

RECENT LITERATURE, 169

styloid processes, the stylo-hyoid ligament and the lesser cornua of the hyoid bone (cerate -
hyal).

The remaining bones of the visceral skeleton of the head, viz., the maxillary, malar,
palatine, pterygoid, vomer, nasal, and lachrymal, are all formed in membrane. An account of
their development is given in the Osteology (Vol. II.).

RECENT LITERATURE.

Ahlborn, Ueher die Segmentation des Wirbdthierkorpers, Zeitschr. f. wiss. Zool., xl.

Albrecht, Sur la valeur morphologiquc de V articulation mandibulaire, du cartilage de Mcehcl ct
des ossdcts de I'oulc, Bruxelles, 1883 ; Sur la valeur morphologiquc da la trompe d Eustache, iL-c.,
Bruxelles, 1884.

Balfour, On the development of the skeleton of the paired fins of Elasmohranchil, d-c., Proc. Zool.
Soc, ISSl.

Baur, On the quadrate in the Mammalia, Quarterly Journal of Microsc. Science, August, 1887.

Bemmelen, v., Ueher die Herkunft dcr Extremitdten u. Zungenmuscvlatur bei Eidechsen, Anat.
Anzeiger, 1889.

Braun, M., Ueher den Schwanz bei Sdugethieremhryonen, Deutsche Zeitscbr. f. Thiermedicin, ix.,
1883.

Cadiat, Du developpemcnt de la partie ccphalo-thoracique de Vemhryon, de la formation du dia-
phragma, d-c., Journal de I'anatomie, T. xiv. , 1878.

Dohrn, Bemerkungcn U. d. neuesten Versuch einer Losung des Wirhelthierkopf -Problems, Anat.
Anzeiger, 1890. Also numerous papers in M ittheilungen d. zool. Station zu JVeapcl.

Dollo, On the malleus of the Lacertilia and the malar and quadrate bones of Mammalia, Quarterly
Journal of Microsc. Science, 1883.

Ebner, "V. v., Urwirbel und Neugliedcrung der Wirbelsdule, Sitzungsb. der Wiener Akadem.,
1888.

Fraser, On the development of the ossicula auditus in the higher Mammalia, Phil. Trans., 1882.

Froriep, A., Zur Entivicklungsgeschichte der Wirbelsdule, insbesondere des Atlas und Epistro-
pheus und dcr Occipitalregion, Arch. f. Anat. u. Entwicklungsgesch., 1883 and 18S6.

Gado-w, On the modifications of the first and second visceral arches, with especial reference to the
Jiomologies of the auditory ossicles, Philos. Trans., 1888.

Gegenbaur, Die Metamerie des Kopfes und die Wirhelthcorie des Kopfskelets, Morphol. Jahrb. ,
xiii., 1887.

G-radenigro, G., Die emhryonale Anlage des Mittelohrcs : die morphologische Bedeutung der
Gehorknochtlchen, Wiener med. Jahrbiicher, 1887.

His, W., Mitthcilungen zur Embryologie der Sdugethiere u. des Menschcn, Archiv f. Anat. u.
Physiol., Anat. Abth., 1881.

Hofbnann, C. K., Ue. die morphologische Bedeutv/ng des Gehorknochelchens bei d. Reptilien,
Zool. Anzeiger, xii.

Kostlin, Der Ban des knochernen Kopfes in den vier Klassen der Wirbelihkre, Stuttgart,
1884.

liOckwood, C. B., The early development of the pericardium, diaphragm, and great reins,
Proceed, of the Ptoyal Society, 1887.

Noorden, W. v., Beitrag zur Anatomic der knorpeligen Schddelbasis menschlirher Emhryonen,
Arch. f. Anat. u. Phys., Anat. Abth., 1887.

Parker, W. K., Various important papers on the structure and development of the skull in the
Philosophical Transactions of the Royal Society and the Transactions of the Zoological Society.

Rabl, C, Thcorie des Mesoderms, Morphol. Jahrb., xv.

Bavn, E., Ue. die Bildung der Scheidewand zwischcn Brust- und Bauch-hohle in Sdugethier-
emhryonen, Arch. f. Anat. u. Physiol., Anat. Abth., 1889 ; Untersuchvmgen ue. d. Entwickl. des
Diaphragmas, dc. Arch. f. Anat. u. Physiol., Anat. Abth., 1889.

Salensky, Beitrdge zur Entivicklungsgeschichte der knorpeligen Gehorknochelchen bei Saugethicren,
Morphol. Jahrb., vi.

Strahl u. Carius, Beitr. zur Entwickl. des Herzens u. d. Korperhohlen, Arch. f. Anat. u.
Physiol., Anat. Abtli., 1889.

Strazza, G., Zur Lehre Ubcr die Entwickking dcr Kehlkopfmuskcln, Wiener medic. Jahrbiicher,
1888.

XJskow, N., Ueher die Entmckhtng des ZwerchfeUs, des Pericardiums und des Coloms, Archiv
f. mikrosk. Anatomic, Bd. 22, 1883.

Wijhe, J. W. van, Ueber die Kopfsegmcnte und die Phylogenie des Geruchsorganes der Wirhel-
thiere, Zoolog. Anzeiger, 1880 ; Die Kopfregion dcr Craniotcn bcim Amphioxus, Anat. Anzeiger, iv.

INDEX OF PART I.

Abdojiixal stalk, 46
Accessorj' tliyroids, 1 1 1
Alae nasi, 96

Albrecht on auditory ossicles, 168
Alecithal ova (o, not, \^Kvdos, oil-bottle), defi-
nition, 9

invagination of, 22
Alimentary canal, development of, 99
Allantoic arteries. See Umbilical.

fluid, 46

tube, 36
Allantois {a\\as, sausage, uSos, form), formation
< 43

first appearance of, 44

structure of, 44

in what animals fourid. 45

hypoblastic sac of, 40

pedicle of, 46

variation of period of development, 46

connection, with alimentary canal, loi, n.
with bladder, 122
Amnion {aixvlov, a bowl in which victim's blood
was caught, the caul), earliest appear-
ance of in human embryo, 46

false, 42

formation of, 42

in guinea pig, 43, 46

resulting from invagination, 24

true, 42
Amniota, 42
Amphibia, liver in, 112

Wolffian duct in, 118

heart in, 136
Amphioxus (o//<fii, double, o|us, a point), ale-
cithal ova of, 9

invagination of blastoderm of, 22, 24

origin of mesoblast in, 22, 26

mesenchyme in, 27

notochord in, 34

mesoblastic somites in, 37
Araygdahe {afivySd\r], almond), 66
Anal membrane, the, 108

Analogy, avd, according to, \6yos, ratio), defi-
nition of, 4

examples of, 4
Anatomy, definition of, i

departments of, i
Angfdncci on zonule of Zinn, 87
Annelida (anncllus, little ring), 2
Anterior perforated s{jace, 79
Anus, formation of, 108, 128
Aorta [iujpTri, &.fipu>, to lift), primiuve, formation
of, 38

descending primitive, 146

arch of, 148, 150

bulb of, 144, 147, 148
descending ["Tnianent, 150
Aortic arches. See Am kkial Arches.

A(|ueduct of Sylvius, 62, 67

Archenteric cavity, primitive {apxv beginning,

(vnpov, intestine), 26
Archiblastic cells of His, 25
Arteria centralis retina, 85, 87
Arterial (aortic) arches, formation of, 38, 146

division of aortic septum l.y, 144

origin of principal arteries from, 146, 147,
148

destination of fourth and fifth, 150
Arteries {apT-qpla, atpu, to lift), development of,
146

from arterial arclics, 148, 149

from arches in birds, 150

ascending pharyngeal, 148

auricular, 148

basilar, 149

carotid, 148, 150

common iliacs, 147

innominate, 150

intercostal, 149

inter-segmental, 149

lingual, 148

maxillary, 148

middle sacral, 147

occipital, 148

pulmonary, 148, 150

subclavian, 150

temporal, 148

umbilical, 44, 146, 156

vertebral, 149

vitelline, 40
Arthropuda {apOpov, a joint, iravs, a foot), seg-
mental ]>lan of, 2

centrolecithal ova of, 8
Aryepiglottic folds, 103

Arytenoid cartilages {apvraiva, a pitcher), 103
Ascaris inegalocephala, poli\r globules observed
in, 9

fertilisation in, 11, 12

segmentation of, 16
Auditory pit, 89

vesicle, 89

nerve, 78, 89, 93

ossicles, 167, 168
Auricular caiial, 142
Axis, primitive skeletal, 2

Baku, vo.v, zona pellucidaof, 6

on bilaiiiination of middle layer of blas-
toderm, 23
Balfour, F. M., on amnion and allantois, 45, n

blastodermic layers, nonicnolature of, 23

ccelom, typical development of, 24

head cavities, 16 1

limbs, 163 n.

lens-capsule, 87

u

INDEX OF PAUT I.

Balfour, F. M, — contimced.

Mlillerian duct, 122, 123

muscle plate, outer, 161

nerves, epiblastic, origin of, 57
spinal, anterior roots of, 74

neural crest, 73

oesophagus, obliteration of lumen of, 104, n

ova, formation of, 124

parablast, 27, n.

primitive groove, 23

polar globules, theory of, 14

supra-renals, 122

sympathetic system, 81

Wolffian body, 115, n.
duct, 118

vitelline membrane, 8
Bars, cartilaginous, of mandibular arch, 166, 168

palato-pterygoid, 166

hyoid, 166, 168

thyro-hyoid, 166
Bartholin, glands of, 128
Basal layer of placental decidua, 5 1

attachment of villi to, 52
Basis cranii, 166
Bat, placental sinuses in, 52
Bauchstiel (abdominal stalk), 46
Beard on branchial sense-organs, 76
Beneden, v., vitelline membrane, 8

fo]-mation of polar globules, 9

entrance of spermatozoon in Ascaris, 1 1

division of nucleus in Ascaris, 12, 16

blastodermic layers, 18

blastopore, theory of, 23

pro-amnion of, 35
Bile ducts, 112

Birdsell on sympathetic nerves, 81
Bischotf on inversion of blastodermic layers, 23
Bladder, urinary, 122

Blastoderm {^KaarSs, a bud, Se/jfta, skin), for-
mation of, 17, 18

three layers of, 17

gastrula condition of, 21
stage typical, 22

bilaminar, 21

layers, inversion of, 23

historically considered, 23, 24

characters of layers of, 24

table of development from layers of, 25

four layers of, 26

separation of embryo from, 34
Blastodermic vesicle, definition of, 17

growth in the cat of, 18

invagination of, 22, 23, 24

as yolk-sac, 35

first attachment of to uterus, 53
Blastopore {PXa(TT6s, a bud, irSpos, a passage),
discussion of in meroblastic ova, 22, 23

diverticula, near to in Sagitta, 26

connection of anus with, 108
Blochmann on extrusion of one polar globule by

parthenogenetic ova, 14
Blood, origin from parablastic cells, 21, 25

corpuscles, 40
Blood islands of Pander, 39
Blood vessels, formation of, 40
Body, segmentation of, 2
Body-cavity. See Ccelom
Bonnet on middle blastodermic layer, 21

on connection of notochord with buccal epi-
blast, 68
protovertebral cavity, 160
Born on the lachrymal canal, 89

6

Born — continued,

cephalic clefts, 102, n.

thyroid, iii

septum spurium, 141, n.

auricular canal, 142, 144

ventricular septum, 142, n.

foramen ovale, valve of, 144

pulmonary veins, 144
Boveri on polar globules in Ascaris, 9
Bowman, anterior and posterior homogeneous

lamellae of, 88
Brain, development of, 61

flexures of, 63

fissures and convolutions of, 71
Branchial sense-organs of Beard, 76
Bronchi, 109
Brown, H. H., on extrusion of part of nucleus

of seminal cell, 12
Budge on lymphatic system, 157

C^CUM (blind), 107

Calamus scriptorius (writing pen), 6±

Callender on 4th visceral arch, 103

Canalis re-uniens, 91

Capsule of lens, 87

whence derived, 87
Capsule, cartilaginous nasal, 166

periotic, 166
Carnoy on polar globules in Ascaris, 9
Cat, blastodermic vesicle of, 18
Cauda equina, 60

Centra {nivrpov, a point), of vertebrse, 2
Centi'olecithal [Kevrpov, centre, \i\Kvdos, oil-
bottle), ova in arthropods, 8
Cerebellar peduncles, 66
Cerebellum, 62

formation of, 66

amj'gdalfe of, 66
Cerebral vesicles, 37

table of development from, 61

fifth, or bulbar, 63

fourth or cerebellar, 66

third, mid-brain, 67
second, 67
first, 68
Chick, pro-amnion in, 34

rudiments of cranial nerve ganglia in, 75
Choanas {x^avos, a pit for melting metal in), 97,

100
Chorda dorsalis, 32. See NoTOCHOED.
Chorda tympani, 79
Chordae tendineae, 142
Chorion (xopiov, skin), definition, 42 and n.

junction of allantois with, 44, 46
Choroid coat, 87, 88
Choroid plexuses, 62, 67, 70
Choroidal fissure, 85
Cliromatin, filaments of, 12, 16
Chromosomes (xpaJMS colour, aroi>iJ.a, body), 12
Ciliary body, 88
Circulation, fcetal peculiarities of, 155

course of, 156

changes of at birth, 157
Cloaca, 108, 122, 123, 127
Cochlea (koxAiks, a snail with a spiral shell),
canal of, 90

ganglion of, 78

in birds, 92

modiolus of, 93

bone of, 93

spiral lamina of, 93

INDEX OF PART I.

lU

Ccelenterates {koTKos, hollow, evrepov, intestine),

23

Coelora (/coTAoy, hollow), layers of, 2, 23

typical origin from invagination, 24, 26

formation of, 36, 99

connection of mesoblastic somites with, 37,

160
formation of pleurae from, no
of pericardium from, 135 I
serous cavities from, 159
Coelom-invaginations, 22
connection of, with notochordal invagina-
tion, 34
Coloboma iridis {KoKd^oofi^, a part taken away

in mutilation), 85
Columns of cord, anterior white, first rudiment

of, 59

posterior white, 59, 75, 7S, n.

structure of, 60
Commissure, anterior of cord, 60

grey, 67

posterior, 67
Commissures of cerebral hemispheres, 71
Congenital fissures of neck, 103
Connective tissue, origin from parablastic cells,

21, 25
Cornea (horny), substantia propria of, 87

epithelium of, 87
Cornu (horn), anterior, first rudiment of, 59
Cornu Ammonis(from resemblance to the horns

of the statue of Zeus- Amnion), 72
Corpora quadrigemina (four-fold bodies), 62, 67

striata, 62, 69, 73
Corpus albicans (white body), 71
Corpus callosura (the thick body), 71

peduncle of, 79
Corti, rods of, 93

Coste, discovery of germinal vesicle by, 8, n.
Cotyledons of placenta (koti^Atj, anything hollow),

51
Cowper, glands of, 12S
Craniata, 27
Cranium, 165

Crura cerebri {crus, leg), 62
Crusta (crust or rind), 67
Crj'ptorchismus (k/ji/tttos, hidden, opxis, testicle),

127
Cunningham, D. F., on sulci of brain, 72, n.
Cutis vera, formation of from outer muscle plate,

161
Cuvier, ducts of, 138, 139, 141, 151, 152, 153

vestiges of, 154
Cyclostoniata {kiik\os, a circle, aT6(j.a, mouth),
parablastic elements in blastoderm of, 27
heart in, 136
Cystic duct (kuo-tu, the bladder), 112

Decidua {rlccidcre, to fall off), 44
structure of, 46
rcflexa, 46
serotina, 46
vera, 46

function of glands of, 47
thickness of vera, 48
stratum spongiosum of, 49
stratum coinpactum of, 49
atrophy of glands of, 49
placental, 51

giant cells in scrotina, 54
•separation of at birth, 55

Decidual cells of Friedlander, 49
Definition of anatomical terms, 4, 5

mesial,

lateral,

frontal,

sagittal [sagitta, an arrow),

coi'onal (corona, a crown),

dorsal (dorsuvi, a back),

ventral {venter, belly),

neural {fevpov, a cord),

visceral,

cephalic (fcecfaAij, head),

caudal {cauda, tail),

axial,
Deiters, sustentacular cells of, 93
Delamination, process of in blastoderm, 21

non-occurrence in invertebrata, 22

Lankester on, 24
Descemet, membrane of, 88
Descriptive terms, 4
Deutoplasm, 7, 8
Diaphragm {Sid, through, (ppdyfia, a fence,)

159

Directive corpuscles.. 9. See Polar Globulks.
Discus proligerus, albumen deposited on ovum

by, 7
Dohrn on pituitary body, 68
DoUinger, researches of Pander under, 23
Ductus arteriosus, 146, 150, 156

lingualis, no

thyreoglossus, no

thyroideus, no

venosus, ii;2, 156
Duodenal loop, 104
Dursy on continuit}' of piimitive with neural

groove, 32
Duval on epiblastic connection of blastodermic
vesicle to uterine wall, 53

Ear, a specialised branchial sense-organ, 76

development of, 89

accessory parts, 93
Echinoderms vex'""^, urchin, St'p/na, skin), im-i

pregnation in, n, 12
Ecker on the Sylvian fissure, 73
Ectoderm {eKrds, outside, Sepjxa, skin), primitive,
18, 19, 24

invagination of, 22, 23
Egg tubes, 124

Elasmobranclis {■^Aaer/uo, lamina, ffpdyx^a, gills),
ccelom invagination in, 22, n.

primitive groove in, 23

neural crest in, 73

spinal nerves, anterior roots in, 74

origin of ciliary ganglion in, 81

auditory vesicle in, 90

supra-pericardial bodies in, in

liver in, 112

Wolffian vesicles in, 118, n.

su])ra-renal capsules in, 121

Miillerian duct in, 122

heart in, 136

muscle plate, outer, in, 161

visceral arches in, 161

head cavities in, 161

limbs in, 163, n.
Embiyo, human, segmentation whore most

marked, 2
Embryo, separation of, from blastoderm, 34
Embryology {inlipZov, a thing newlv burn, ao-voj,
word), definition, I

IV

INDEX OF PART I.

Embryonic area, 1 8

Endocardium {evSov, within, KapZia, heart), 136

Endolymphatic canal, 90

Engelmann on placenta, 48, n.

Enteric canal (ePTepov, intestine), 22
groove, 36

Entoderm (eVriJy, inside, Sep/xa, skin), primitive,
17, 18, 19, 24
blastopore, relation to, 22, 23
perforation of by primitive groove, 22

Entomeres, mass of {eurSs, within, fiepos, part),

Epencephalon {im, over, eyKecpaXos, brain), 66
Epiblast (iwi, over, ^Kaa-rSs, germ), 21

homology with Rauber's layer, 22, n.

character of cells, 24

formation of medullary folds by, 32

formation of ganglia by, 73, 75

sensory epiblast, 76

formation of lens from, 83
Epiglottis [eni, upon, yAcorris, mouth of the

windpipe), 103
Epiphysis cerebri {inKpvca, to grow upon, cere-
brum, brain), 67
Epoophoron (eVi, upon, oi6v, egg, (pfpoo, to carry)

ofWaldeyer, 120
Eustachian tube, 93

valve, 141, 155
Ewetsky on bifurcation of lachrymal canal, 89
External amniotic fold, 43, n.
Eye, development of, 83

retina, 86

hyaloid membrane of, 87

corneo-sclerotic coat of, ?>'j

choroid coat of, 87, 88

iris of, 87, 88

anterior chamber of, 88
Eyelids, 89

Fallopian tube, 7

fimbrisR of, 11

development of. 117, 124
False amnion, 42, 43, n.
Falx cerebri (a sickle), 70
Fertilization, 11

Filmn terminale (terminal threail), 60
Fissure of Sylvius, 72, 73

of Kolando, 73
Flemming on Wolffian duct, 118
Fol on polar globules in echinoderms, 9
Foramen ceecum of Morgagni (blind opening), 102,

no
Foramen of Monro, 63, 68

ovale, 144

closure of, 155, 157
Fore brain, 38

Fore-gut, first formation of, 34
Fornix (an arch), 71
Foster on nomenclature of the blastodermic

layers, 23
Eraser on inversion of blastodermic laj^ers, 23

auditory ossicles, 168
Fretum Halleri (a strait or channel), 141
Friedlander, decidual cells of, 49

on giant cells in decidua serotiun, 54
Froriep on branchial sense-organs in mammals,
76

vertebral column, 162
Furcula, the (forked prop), 102, 103

Gadow on auditory ossicles, 16S

Gall bladder, 112

Ganglia (ydyyAwv, a swelling), posterior-root, 74

rudiments of, in cranial nerves, 75, 76

Gasserian, 78, 81

jugular, 78

cochleae, 78

spiral, 78

geniculate, 78

sympathetic, 81

ciliary, 81

ophthalmicus profundus of elasraobranchs,
81
Ganoids (ydvos, bright, elSos, form), heart in, 136
Gartner, duct of, 120
Gaskell on cranial ganglia, 77

nuclei of cranial nerves, 77

sympathetic ganglia, 81
Gastrula {yaaryip, belly), 21

views of formation, 22, 23
Gegenbaur on homodynamy, 4

Jacobson's gland, 97

remains of notochord, 162

auditory ossicles, 168
Generative organs, male, 117, 120, 125

external, 127

female, 124

table of, 130
Genital cord, 123

ridge, 124
Germ cell, 12
Germinal cells, 58
Germinal epithelium, 117

connection with Wolffian tubules, 120

formation of ovary from, 124
of testicle from, 125
Germinal spot, 8
Germinal vesicle, structure of, 8
Giant cells of placenta, 54
Gill slits, 103
Giraldds, organ ol, 120
Girdle, thoracic and pelvic, 164
Globular processes of His, 95
Glomeruli (glomus, ball of thread), 119, 122
Golowine on ganglion rudiments, 75

special sense-organ rudiments, 76
Gotte on Wolffian duct, 118
Graaf, de, on pineal eye, 68
Graafian follicle, 6

union of cells of, with ovum. 7

■changes prior to escape of ovum, 9

formation of, 124, 125
Gradenigo on the stapes, 168
Gruber on a case of venous abnormality, 155
Gubernaculum testis (a helm), 126

changes of in descent of testicle, 127
Guinea-pig, formation of amnion in, 43

allantois in, 46
Gyrus subcallosus of Zuckerkandl, 81

Hadden, on origin of segmental duct, 118
Haeckel, on gastrula stage and gastrulisation,

22, 23, 24
Haller, vasa aberrantia of, 120
Hare lip, 97

Hassal, concentric corpuscles of, 112
Head, first appearance of, 34

muscles of, 161

evidences of segmentation of, 161

skeleton of, 166

visceral skeleton of, 166

INDEX OF PART I.

Heape, on blastodermic layers in the mole, i8

rudimentary blastopore in the mole, 22
Heart, formation of, 38, 136
beginning of beat, 39
endothelial tube of, 134
folding and division ot, 137, 141, 142
saccus reunieiis, 139
septum spurium of, 141
valves of, 141, 142
fretum Halleri of, 141
septum inferius of, 14 1
auricular canal of, 142
septum intermedium of, 142
chordfe tendinere of, 142
foramen ovale of, 144
limbus Vieussenii of, 144
muscle of, 145
colurante cnrueae of, 146
foetal, peculiarities of, 146
Hedgehog, placental sinuses in, 52, n.

epiblastic attachment to uterus in, 53
Henle, loops of, 122
Hansen, canalis re-uniens of, 91
on "Wolffian duct, 118
heart, bilateral, 134
Hennaphroditism ('Ep.u^s, ' Acppodtrri, a male and

female god), 12
Hertwig, 0. and E., on polar globules in
echinoderms, 9
cnelom-invaginations of, 22
coelom, typical development of, 24
mesodermic layers of, 26, 27, n.
mesenchyme of, 26
Hind brain, 38
Hind gut, formation of, 36

connection with allantois, 46

with Wolffian duct, iiS
His, "W., abdominal stalk of, 46

on accessory thyroid bodies, no

allantois, loi, n.

arterial arches, 147

ascending roots in medulla, 66

auricular canal, 142

axis cylinders, growth of, 75

blood-vessels, 151

Broca's area, 79

cephalic clefts, 102, n.

endothelial tube of heart, 136

on epiblastic origin of ganglia and po.sterior

roots, 57
germinal cells of, 58
on geniculate ganglion, 79
gloiiular processes of, 95
on heart, bilateral, 134
isthmus of, 67
on laminre of cord, 60
lens capsule, 87

mechanical theory of development, 24
mesobla-st, origin of in part from thickened

rim of blastoderm, 23
nerve fibres, somatic and splanchnic, 77
olfactory epiblast, 98

nerve fibres, origin of, 77, 81
parablast, theory of, 25, 26
on ]iinna, 95
porta ve.stiljuli of, 156
on pulmonary veins, 144
researches of, 6, n.
saccus reunicns of, 139
6ex>tum atriorum, 144
inferius of, 141
between stomodaiumandfore-gut, 99

His — contimted.

septum, transverse of, 159

sinus arcuatus, 102

on spinal nerves, roots of, 74

spongioblasts of, 58

sulcus terminalis, 102

thyroid, no

on tuberculum impar, 102

visceral arches, 102
Histology {iarov, a web ; \6yos, word), defini-
tion, I
Hofmann, gastrulation in elasmobranchs, 22, n.
Holoblastic ova (oAos, whole ; ^\asr/,s, a germ),
definition of, 8

invagination of, 22
Homodynamy {ofi6s, the same ;Suj'a,uis, power), 4
Homogeny {^ij^os, the same ; yevos, descent), 4
Homology (S/xos, the same ; \6yos, word), definition
of, 4

serial homology, 4

examples of, 4
Hubrecht, on epiblastic connection of blasto-
dermic vesicle with uterus, 53
Huxley, on two layers of coelenterates, 23

auditor}' ossicles, 168
Hymen, the (the god of marriage), 128
Hyoid, the (T elSos, the form of the Greek letter

T), 166, 169
Hypoblast {vw6, under ; PAacrrSs, germ), 21

characters of cells of, 24

formation of notochord from, 32

allantois from, 44, 46
Hypophysis cerebri (viro(pvcc,to grow Iroiu below),

68, 100. And see Pituitary Body.
Hypospadias {inr6, under ; cwdu, to tear), 129

Imptiegkation, II
Incisor firamen, 97
Incus (:ui anvil), 167, 168
Infundiliulum, the (a funnel), 61, 67, 68

of lung, no
Intermediate cell mass, the formation of
"Wolffian duct from, 115

of permanent kidney from, 122
Intermaxillary process, 95
Intervertebral disc, 162
Intervillous si)ace, 51

question of blood in, 53
Intestines, 104

large, 106
Invagination in holoblastic ova, 22

aperture of, 22

resemblance to blastopore, 22
Inversion of blastodermic lavers, 23
Iris, 86, 87, 88

Ischikawa on segmentation of ovum before fer-
tilization, 14
Island of P.eil, 70, 73
Isthmus fauciuni, 100
of His, 67

JaCobson's organ, 95, 97

KAnvo KINESIS {i<dpvou, a kernel ; kiVtjitis, move-
ment) of germinal cells, 58

Karyoplasm {napvov, a kernel ; trXdaixa, a mould)
of germinal vesicle, 8

Kessler, on lens cai)sule, 87

substantia propria of cornea, 87, n.

VI

INDEX OF PART I.

Kidney, head or fore, 115, 117

mid, 116

hind or permanent, 117, 122
Kolliker on llanlier's layer, 18

intermediate layer of blastoderm, 20

placenta, 53

lens capsnle, 87

the anterior chamber, 88

cephalic clefts, 102, n.

air cells of lung, 1 10

thymus, 11 1

heart, 134

endothelial tube of heart, 136

mesocardium laterale of, 1 36

cartilage of vertebrae, 162
Kollmann,'parabiastic theory of His, adherence

to, 26
Kowalevslvv, researches on Sagitta and Amphi-

oxus, 24, 26
Knndrat on placenta, 48, n.
Kupffer, gastrulaticn, view of, 22

His' parablastic theory, adherence to, 26

Labia majora (greater lips), 128
Ijabyrinth, the, the recess of, 90

perilymphatic spaces of, 93
Lachrymal canals and ducts, 89, 97
fissure, 89
gland, 89
Laminse of cord, alar, basal (dorso-lateral)

(ventro-lateral), 60, 63, 65, 66, 77
Langhans on placental sinuses, 53
Lankester, on homogeny, 4

blastopore, 22

primary layers of blastoderm produced by
delamination, 24
Larynx, no

Lemurs [lemur, ghost), nasal gland in, 98
Lens, 83, 86

vesicle of, 87

transitional zone of, 87

capsule of, 87
Leopold on placenta, 48, u.

giant cells in decidua serotina, 54
Leydig on special sense organs of gill clefts, 76
Lieberkiihn on blastodermic layers in mole, 18

on lens capsule, 87

zonule of Zinn, 87
Liganientum arteriosum, 150
Limbs, 4, 163

segments of, 164

rotation of, 165

bones of, 165
Limbus (a band) Vieussenii, 144, 157
Literature, recent, of the ovum, 14

blastoderm, 27

embryo, formation of, 41

general subject of embryonic formation and
development, 41

decidua, 55

nervous system, development of, 81

sense organs, 98

alimentary canal and glauds, lungs, 113

urinary and generative organs, 132

blood system, 158

serous cavities, muscles and skeleton, 169
Liver, 106, 112

weight of, 113

formation of veins of, 151, 152
Lockwood, on protovertebral cavity, 160

Lungs, development of, 109
Luschka, on urachus, 122

notochord, remains of, 162
Lymphatic system, 157

Mackay on arterial arches of the bird, 150
Macula genninativa (germinal spot), 8 ■
Malleus (a hammer), 167, 168
Malpighian corpuscles of spleen, 108

ofWolffian body, 120

of permanent kidney, 122
Marshal!, vestigial fold of, 154

Milnes, on head cavities, 161
Martin on Wolffian duct, 118
Masiirs on epiblastic connection of blastodermic

vesicle with uterus, 53
Mechanical theory of development, 24
Meckel, J. F., 23
Meckel's cartilage, 166, 167, 168
Medulla oblongata, 62

formation of, 63
Medullary folds, formation of, 30, 32

groove. See Neural groove.
Membrana limitans of retina, externa, 86

interna, 83, 86

nictitans, 89

reuniens, superior of Remak, 32, 162, 165

tectoria, 93

tympani, 93
Membranes of cord, development of, 60
Meroblastic ova, {fi4pos, a part, jiXamos, a germ),
definition of, 8

gastrular stage in, 22

Haeckel, view concerning blastopore in, 23

mesenchyme in, 27
Mesenchyme {/xeaos, middle, X"i"''S j"ice), 26,

27
Mesencephalon {/xiffos, middle, eyic€(pa\os, brain,

67 ...

connection with optic stalks, 85
Mesentery, the {(jL4a-os, middle, evrepov, intestine),

104, 107
Mesoblast, formation of, 20, 21

origin, varieties of, 22

His' view of origin, 23

character of cells, 24

connection with mesenchyme, 26

paraxial and lateral, 32

cleavage of, 36

formation of vessels in, 39

growth of with allantois, 44
Mesoblastic somites, 37, 159, 162

cavity in, 37, 160
Mesocaidium, anterius, posterius. lateralis, 136
Mesocolon [koXov, great intestine), 106
Mesogastrium (yain-np, belly), 106, 107
Mesonephros (vecppos, kidney), 116
Mesorchium (opx^s, testicle), 126
Mesoi'ectum, 106
Mesovarium, 126

Metameres (^era, following, fj-^pos, a part), 4
Metanephros {f^tTci, behind, ve(pp6s, kidney),

117
Metazoa {fJiira, after, C'^ov, animal), 18, 26
Metencephaloii {fJierd, behind, eyici<pa.\os, brain),

63
Meuron, de, on obliteration of human oesopha-
gus, 104, n.
thymus, in
Micropyle {fxiKp6s, small, itvAt;, opening), ll
Mid brain, 38

INDEX OF PART I.

Vll

Mihalkovics on stomodffium in rabbit, 99

supva-renal capsules, 120

Mtillerian ducts, 123

ovar}-, 124, n,
Minot, polar globules, theory of, 12, 14

on placenta, 48 n.

chorionic epithelium, bounding placental
sinuses, 52, n.
Mitsukuri on supra-renals, 122
Modiolus, the (nave of a wheel), 93
Mole, primitive ectoderm in, i8

blastopore in, 22
Monotremes {fj-ovos, single, rprifia, a hole), 6

amount of yolk of, 8

cloaca in, 108
Morbus cceruleus (the blue disease), 157
Morgagni, hydatids of, 124
Morphology (fj.op<pri, form, K6yos, word), deter-
mination of, 2
Mouth, formation of, 99

non-correspondence of primitive with per-
manent, 100
Mulberry mass, 16

Miiller, E., on muscular tissue of primitive
aorta?, 151
Johannes, Miillcrian duct named after,

, "7
"W., sense epithelium of, 86
Miillerian dm-t, 117, 122, 123, 124, 127

invaginations, 115, u.
Mulleiian fibres, 86
Muscle plate, 160
inuei-, 160
outer, 161
Muscles, 159
Myelosjiongium (^uueAo's-, marrow, (nroyyos,

sponge), 58
Myotomes [fj-iios, a muscle, Ttnuu, I cut), 4

Nasal processes, 95

laminse, 95

septum, 95
Naso-palatine canal, 97
Kerves, fifth ascending, root of, 66, 78

vagus and glosso-pliaryngeal, ascending root
of, 66, 78

fifth sensory fibres of, 66, 78
motor fibres of, 66

sixth, seventh, 66

third, fourth, 67

spinal, formation of, 73

roots of, 74

axis, cylinders of, 75

traces of ganglia in cranial, 77

cranial, 75

auditory, 78, 89, 93

sympathetic, 81

nuclei of origin of cranial, 77

inferior laiyngeal, shifting of, 149
Nerve fibres, origin from neurolila.sts, 58, 59

origin of somatic and splanchnic, 77

ollactory, 81
Nervous system, general view of development, 57
Neural canal, 32

parietes of, structure of, 57

crest, 73, 75

groove, formation and closure of, 30, 32

formation of cerebral vesicles IVoni, 37
Neurenteric canal {vtvpov, nerve, tvTtpov, in-
testine), 22

piercing of notochord by, 32

Neuroblasts [vivpov, nerve, ^Xa(n6s, germ), 58,

59, 74, 98
Neuroglia {vivpov, nerve, 7A.10, glue), origin of,

60
Nose, a specialised branchial sense organ, 76

formation of, 95
Notochord {vHnos, the back, x^P^tj, a string of
g"t), 2

formation of, 32

au embryonic structure, 34

structure of, 34

connection with buccal epiblast, 68

formation of vertebrfe round, 162

connection with cranium, 165
Notochordal canal, 34
Nuck, canal of, 127
Nuclei of origin of cranial nerves, 77
Nucleolus of ovum, 8
Nucleoplasm of germ and sperm cells, 12
Nucleus (a kernel) of ovum, changes in, 6, 9

chromatin filaments of, 12
Nymphae, 128

CESOPHAGUS (o1(Taj = (pfpw, to carry, <payi7u, to

eat), 103, 104, n.
Olfactory {olfacio, to smell) area, 95

bulb, 81

epithelium, 98

glomeruli, 81

pit, 95

lobes, 62, 71

junction of ganglionic rudiment with, 77

formation of, 79
Olivary tubercle, 65
Omental sac, 106
Omentum, great, 106, loS

gastro-hepatic, or small, 106, II3
Omphalomeseraic vein (oiUifaAds, navel, fnaapaioi;

mesentery. Sec Vitelline Vein.
Onodi on sympathetic nerves. Si
Optic chiasma {xicia-f/.a, two lines placed cross-
wise), 85

cup, 83

nerves, origin from fore-brain, 38, 67, 79

stalks, formation of, 85

thalamus, 62, 67

tracts, 67, 85

vesicles, juimary, 61, 67, 83
Organ, definition, i
Organization, plan of, 2
" Organs of the lateral line," 76
Os articulare (joint bone), 168
Otic vesicle {ois, wtihos, ear, of the ear), 89
Ovary, 6, 124, 125

round ligament of, 126

change of position of, 127
Oviparous {ovum, egg, 2'(''^<J, to produce) verto-

brata, 8
Ovum (egg), alecithal, 9

attachment of to uterus, 46

centrolecithul, 8

deutoplasm of, 7

development, changes prior to, 6

gastrula of, 21

germinal vesicle of, 8

holoblastic, 8

human, excejition to rule of sepaiation from
chorion, 46

human, villi in earliest described, 55

VIU

INDEX OF PART I.

0 vum — continu ed.
fertilization of, ii
mammalian, descent of, 8
maturation of, 9
meroblastic, 8
permanent, 125

polar globules, extrusion from, 9
primordial, 124
segmentation of, 16
size of, 6
strife of, 6
structure of, 6
yolk of, 7
zona pellucida or zona radiata of, 6

Palate, the, 97

cleft, 97
Pancreas {■nuv, all, Kpe'as, flesli), 108, 113
Pander, researches on blastoderm, 23

mechanical theory of, 24

blood islands of, 39
Papillary muscles, 142
Parablastic cells (irapd, beside, ^KaarTos, germ),

21, 25, 27
Parachordal cartilages {napd, beside, X"P^'h,

string ; near the notochord), 166
Parker, on formation of xiphoid, 163

stapes, 168
Parovarium {irapd, beside; ovarium, ovary), 120
Pars ciliaris retinae, 86
Parthenogenesis {irapdevos, virgin ; yei/eais,

origin), polar globules in, 14
Parturition, separation of membranes in, 55
Paterson on origin of S3'mpathetic system, 81

limb muscles, 163, n.
Penis, 128, 129

Pericardium ("fp-, about ; KapSia, heart), 135, 159
Peritoneum (irspi, about ; reivw, to stretch), 159
Pcrivitelliue space (vitellus, yolk), 9
Peters on hyomandibula, 168
Petromyzon {-rrerpos, a stone ; /j-vCw, to suck ;

lamprey), pituitary body in, 68
Pfliiger, egg tubes of, 124
Pharyngeal septum, 100
Pharynx, loi

Philtrum {(pi\Tpov, loveliness), 95
Pineal gland {pinea, a fir-cone), 61, 67

as rudimentary e3'e, 68
Pinna, the (a featliei-), 95
Pituitary body (pituita, phlegm), 61, 67

formation of, 68

in Petromyzon, 68

connection with stomodseum 100
Placenta (a cake), 44

formation of, 49, 51

basal layer of, 5 1

subehorionic membrane of, 51

locirli or cotyledons of, 51

structure ot sinuses of, 52

intervillous spaces of, 51, 53

■weight and size of, 53

microscopical structure of, 53, 54
Placental decidua, 51

sinuses, structure of, 52
Plane, median, 4

Plasmse of Weismann, nuclear, nutritive, ger-
minal, histogenetic, 14
Pleurae {irXevpd, a rib), no, 159
Pleuro-peritoneal cavity. See Ccelom

Plica gubernatrix (guiding fold), 126

semilunaris, 89
Polar disc, 1 1

globules, 6

formation of, 9, 12
where observed, 9
of Ascaris megalocephala, 9
of plants, 9
Minot's theory of, 12
Weismann's theory of, 14
Pons (bridge), 62

formation of, 66
Porta vestibuli (gate of the vestibule), 156
Prae-hyoid glands, no
Pregnancy, changes of uterus in, 48
Primary placental circulation, 53
Primitive groove, formation of, 19

as extension of blastopore, 22
as homologue of part of entoderm,

23
Baltour's view of, 23
connection with neural groove,

3°, 32
streak, formation of, 19

an ectodermal thickening in-
dented by blastopore, 22
origin of mesoblast Irom., 22
connection with notochord, 32
velum (velum, a curtain), 100
Pristiurus, heart in, 136
Proamnion, 35, 42, n.
Process, nasal, 95

fronto-nasal, 95, loi
maxillary, 97, loi
globular, 95
tympano-hyal, 168
styloid, 169
Processus vaginalis, 127
Proctodseum [irpwicjos, the anus), 108
Pronephros {-fph, before ; ve^pos, kidney), 115
Pro-nucleus, female, male, 9, 11, 12

chromatin filaments, in, 16
Prosencejihalou (Trpo'y, besides; e7/<:e^aA.os, brain),

68
Prostatic vesicle, 124
Protovertebrss (Trpwros, first ; vertebra, I turn),

37

See Mesoblastic somites.
Protovertebral cavity, 37, 160
Pulmonary blood-vessels, 1 10
Pupillarj'' membrane, ^"j, 88
Purkinje, germinal vesicle of, 8
Pyramidal tracts, the last to be meduUated, 60
Pyramids, anterior, 65

Quadrate, the, it

Pabbit, polar globules observed in, 9
]>ro-amniou in, 35
stomodsenm in, 99
Pamon y Cajal, on posterior roots of spinal

nerves, 75
Pathke, diverticulum of, 68
on hypophysis cerebri, 100
visceral arches, 103
arterial arches, 147, 150
posterior vertebral veins of, 153
traljeculae cranii of, 166
Eauber, layer of, 17, 18, 22, n., 23

on the mechanical theory of development, 24

INDEX OF PAET I.

IX

Eecessus pulmonales (recesses ot the lungs),

159
Reichert on villi in earliest ovum, 55

mantle of, 70

on incus and stapes, 168
Eeid, John, on fretal circulation, 156
Rein on the polar globules in the rabbit, 9
Remak on bilamiuation of middle blastodermic
layer, 23

membraua i-cuniens superior of, 32, 162
Renal organs, embryonic ducts of, opening into

allantoic pedicle, 46
Kenson on extrusion of part of nucleus of seminal

cell, 12
Restiform bodies, 65
Retina (rde, a net), origin from tore-brain, 38

development of, 83, 86

membrana; limitautes of, 83, 86

rods and cones of, 84, 86

hexagonal pigmented epithelium of, 84,
86

central blood-vessels of, 85, 87

ganglionic layer, 86

sense epithelium of, S6
Retzius on the zona radiata, 7
Rhomboidal sinus, 60
Ribs, development of, 163
Rodents, inversion of blastodermic layers in,

Paterson on sympathetic in, 81

nasal septum in, 95
Rosenburg on Wolffian duct, 118
Round ligament, 126

Riickert on ooelom invagination in elasmo-
branchs, 22, n.

heart in Pristiurus, 136

S.VBATiER, on origin of subclavian artery,
150

Eustachian valve, action of, 156
Saccule, 78, 90
Saccus reuniens, 139, 151
ISagitta, origin of mesoblast in, 22

Kowalevsky on, 24, 26

mesenchyme in, 27
Salensky on the stapes, 16S
Sauropsida {cravpa, a lizard ; oi/zir, look), hyo-

mandibular arch in, 168
Scalte {scala, a stair) of cochlea, 93
Scarpa, gangliform swelling of, 78
Schenk, on sympathetic nerves, 81
Schmidt, on Thebesian valve, 141
Schneiderian membrane, 81
Schultze, M., on rods and cones, 86
Sclerotic ((TicX-npos, hard), 88
Scrotum, 127, 128
Sedgwick, on "Wolffian body, ii5,n.

MUllerian duct, 123
Segmental tubes, 118
Segmentation of ovurn, complete, incomplete, 8

main factors of, 9

before fertilization, 14

after fertilization, 16
Selcnka, on inversion of blastodermic layers in
rat and mouse, 23

epiblastic connection of blastodermic vesicle
with uterus, 53
Semicircular canuls, 78, 91
Seminal cell, 12

Septum lucidum (transparent partition), ventricle
of, 70
spurium, 141
inferius, 141
membranaceum, 142
intermedium, 142, 144
transverse, the, 159
Serous cavities, 159
Sexual cells, origin of, 14

conjugation of, Minot's theory of, 14
Sinus arcuatus (arched hollow), 102 "
venosus, 112, 13S, 151
urogenitalis, 123, 128
Skull, formation of, 166

cliondrification of, 166
Somatopleure {ffw/xa, body ; trKiupd, the side),

35
formation of, 36, 99
origin of amnion from, 42
Somites {aSifxa, body), mesoblastic, 4
Spec, on connection of mesoblastic cleavage with
notochordal canal, 34
epiblastic origin of Wolffian duct, 118
Spencer, Baldwin, on ])ineal eye, 68
Sperm cell, 12

Spermatozoon {airepixa, seed ; CQ>ov, animal),
accession of, to ovum, 6, 9
fertilization by, 11
fate of tail of, 1 1
maturation of, 12
Spinal accessory, origin from basal lamina,

60
Spinal cord, development of, 57
outermost layer of, 58
anterior cornu of, first rudiment of, 59
columns of, 59
fissures of, 59
anterior commissure of, 60
dorso-lateral and ventro-lateral laminoB of,

60
filum terminale of, 60
Cauda equina of, 60
growth of, pari passu with vertebral canal,

60
membranes of, 60
Splanchno])leure ((rnAdyxvov, the inwards :
■Khivpa, the side), 35
formation of, 36, 99
Spleen, 108
Spongioblasts [aird'yjus, sponge ; ^\aar6s, germ),

58
origin of neuroglia from, 61
Stapes (a stii'rup), 168
Sternum, 163
Stomach, 104
Stomodaeum {ardi-ia, mouth), 6S, 99

connection with nose, 95
Strahl, on Wolffian duct, 118
Stratum compactum, 49

spongiosum, 49, 55
Subchorionic membrane of furner, 51
Sulci of brain, primitive, 72
principal, 73
Sulcus, anterior liniitihg and latenil liniitinfr,

34, 35
terniinalis, 102
Supra-hyoid glands, no

-pericardial bodies, iii
Suprarenal cayisules, 120
Suspensory ligament, 113
Symmetry of form, 4
bilateral, 4

INDEX OF PART I.

Sympathetic ganglia, 8i

connection of, with supra-renal capsules,

122

Teeth, 109

Tegmentum (a cover), d^

Teleosteans (reAeos, complete, hariov, bone),

Wolffian duct in, 118
Telolecithal ova {reKos, end, and XriKvOos, oil-
bottle), 8, 27, 30
Terminal sinus, 40
Testicle, 117, 125

descent of, 126, 127
Testis. See Testicle.
Textures, general, 1

Thalamencephalon {edXaixos, chamber, ijKecpaKos,
brain), 61, 67

connection with optic stalks, 85
Thebesian valve, 141

Thomsen on ganglionic traces in third nerve, 77
Thomson, Allen, on primitive aortae, 146
Thymus (eiu, to offer as a sacrifice), 1 1 1
Thyroid {OvpeSs, a shield), 102

formation of, no

pyramid of, no
Tiedemann on rhomboidal sinus, 60
Tissues, general, i
Toldt on mesentery of pancreas, 1 1 3
Tongue, 100

formation of, 102

foramen csecum of, 102

papillae of, 102

sulcus terminalis of, 102
Trabeculse cranii [trahs, a beam, diminutive of),

166
Trachea [Tpaxvs, rough), 1 10
Trigonum olfactorium [rpiySivos, triangular), 79
Trophoblast (rpop-fi, nourishment, /SAoo-tJs,

germ), of Hubrecht, 53
Truncus arteriosus, 151
Tuberculum impar (unpaired tubercle), 102
Tuimer on placenta, 48, n.

subchorionic membrane of, 5 1
Type, vertebrate, main features of, 2

Umbilical cord, 36, 43, 104, 122, 151

Umbilical duct, 36

Umbilical vesicle, 36

Umbilicus (the navel), 43

Urachus [oupov, urine ; Ix'^) 'to hold), 122

Ureter (oupeco, to make water), 122

Urethra, 122, 128

Uriniferous tubules, 119

Uterus, mode of attachment of ovum to, 46

changes in pregnancy, 48 — 55

regeneration of after parturition, 55

formation of, 124

bifid stage of, 124

OS and cervix of, 124
Uterus masculinus, 124
Utricle {utriculus, a little womb), 91
Uvea {uva, a cluster of grapes), 84, 85, 86, i.

Vagina (a sheath), 124
Valve of Vieussens, 66
fasa aberrantia of Haller, 120
Vascular area, 39, 151
lamina, 151

Veins, azygos, 153

cardinal, 151, 152, 153

connection of, with heart, 138, 139

iliac, 153

inferior cava, 152, 153

innominate, 153

intercostal, superior, 153

jugular, 153

jugular, primitive, 151, 152, 153

primitive, formation of, 39

portal and hepatic, I51, 152

posterior vertebral, 153

pulmonary, 144

subclavian, 153

superior cava, 151, 153

umbilical, 138, 151, 152, 156

vitelline, 39, 40, 112, 138, 151, 152
Velum palati (curtain of the palate), 100
Vena ascendens, 152

Vense, advehentes and revehentes, 112, 151, 152
Venous annulus of duodenum, 151, 152
Ventricles, lateral, 62

third, 62, 67

fourth, 62, 66

fifth, 70
Vererbungstheorie of AVeismann, 14
Vermiform appendix, 107
Vertebra (vcrto, I turn), 2

permanent, 162
Vertebral column, development, 2, 162

bodies, 2

segments, 4
Vertebrate type, 2

Vesicles of the cerebral hemispheres, 61
Vestibule, 78
Villi {villus, shaggy hair) of chorion, 43

vascularisation of, 44, 52

connection with glands of decidua, 47

zone of, 55
Visceral arches, formation of ossicles of ear from,

.93

first, or mandibular, 10 1, 102

cephalic, 102

second or hj^oid, 102

third or thyro-hyoid, 103

palato-pharyiigeal, 103

post- and pree-buccal, 103

blood supply of, 103

cartilaginous bars of, 103, 166
Visceral clefts, 103

first formation of middle ear from, 93

cephalic, loi

hyomandibular, 103

fourth, formation of thyroid from, no, in
Vitelline arteries, 40

circulation, 40

duct. 46, 104, 151

membrane, 8

veins, 39, 40, 112, 138, 151, 152
Vitellus, structure of, 7. See YoLic.
Vitreous humour (vihntm, glass), 83, 85, 87

Wagner, macula germinativa of, 8
Waldeyer on His' parablastic theory, 26

placental sinuses, 52

epoophoron of, 120

germinal epithelium of, 124
Weismann, theory of, 14
Weldon on supra- renal capsules, 120
Wijhe, v., on Wolffian vesicles, 118, n.

head cavities, 161

typical somite cavity, 161, n.

INDEX OF PART I.

XI

"WolfiF, C. F. , first suggestion of laminre in blasto-
derm by, 23

duct of, 115
Wolffian body, 115, n., 116, 117, 118, 122

atrophic changes of, 120

connection ■with testis, 120, 126

veins of, 151
Wolffian duct, 115, 116, 117, 118

remnant of, in parovarium, 120

as duct of Gartner, 120

male generative organs, 120

permanent kidney, 122

connection with Miillerian duct, 123

Wolffian vesicles, 118, 118, n.

Yolk, structure of, 7
sac, 31;, 99, 104
vitelline circulation round, 40

Zacharias, polar globules in Ascaris, 9
Zimmermann on arterial arches, 151
Zona pellucida (transparent zone), 6, 7

penetration of, by spermatozoon, 1 1
Zonule of Zinn, 87
Zuckerkandl, gyrus subcallosus of, 81

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