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= An Introduction to the Study of Embryology=
[[File:Alfred Cort Haddon.jpg|thumb|alt=Alfred Cort Haddon|Alfred Cort Haddon (1855–1940)]]


By


Alfred C. Haddon, M.A. (Cantab.), M.R.I.A.


Professor Of Zoology In The Royal College Of Science, Dublin.


AN


Philadelphia : P. Blakiston, Son & Co., 1012 Walnut Street. 1887.


INTRODUCTION
{|
| valign=middle|
To the memory of


TO THE
his beloved master and friend,


STUDY OF EMBRYOLOGY.
[[Embryology History - Francis Balfour|'''Francis Maitland Balfour''']]




BY
This Book is dedicated by the Author.
| [[File:Francis Balfour.jpg|alt=Francis Balfour (1851-1882)|thumb|200px|link=Embryology History - Francis Balfour|Francis Balfour (1851-1882)]]
|}


ALFRED C. HADDON, M.A. (Cantab.), M.R.I.A.


PROFESSOR OF ZOOLOGY IN THE ROYAL COLLEGE OF
==Preface==
SCIENCE, DUBLIN.




With gtumnwtf gltetration#.  
Although there are at the present time, in addition to the special accounts in various text-books of Human and Comparative Anatomy, two Students - Manuals in the English language solely devoted to the study of Embryology, it has appeared to me that a relatively small work, giving a general review of the subject, might prove of use to students.


A knowledge of the main facts of Comparative Anatomy and Systematic Zoology has been assumed for the reader, the book being especially designed for Medical Students, or for those who already possess a general acquaintance with the Animal Kingdom.


PHILADELPHIA :
It will be noticed that many of the more difficult problems of Ontology and Phylogeny and special modes of development have either been merely alluded to or entirely ignored - as, for instance, the segmentation of the ovum and the formation of the germinal layers in Insecta and Teleostei. This has been of set purpose, as my main object in writing this book has been to give a brief connected account of the principal organs, omitting or barely mentioning structures and phenomena, which may be regarded as of secondary importance.


P. BLAKISTON, SON & CO.,
The facts of development have been largely supplemented by hypotheses; and an endeavour has been made so to present the latter, that the student could not mistake them for the former.


1012 WALNUT STREET.  
It is inevitable that, in compiling such an introductory textbook as this, many subjects must be treated in a manner similar to that in which they have been dealt with by previous authors ; and therefore I have not hesitated to borrow from them when occasion required.


1887.  
In order to facilitate references, very recent, important, or doubtful observations have been associated in many cases with the investigator -s name. It must be distinctly understood that I do not necessarily personally adopt statements or views which have been incorporated in the book; they are merely put forward for what they are worth.


[ All Rights Reserved.']
The beginner is advised to pay attention only to the large type in the first reading, as purely theoretical subjects or matters of detail are printed in the smaller type. Most of the figures have been so drawn as to admit of their being coloured ; and the student is recommended to tint each germinal layer and the organs derived from it in a uniform manner throughout the book : thus the epiblast and its derivatives might be coloured pink, and the hypoblast tinted blue. A uniform system of colouration will be found to be of great assistance to the memory.


The sources from which the figures have been taken are in all cases acknowledged, and in the cases where no source is given the illustrations are original. Figs. 40, 41, 44, 45, 80, 81, and 178* have appeared previously in the Proceedings of the Eoyal Dublin Society.


The classification adopted will be found in an Appendix. All the genera mentioned in the text have been inserted, in order that their systematic position may be seen at a glance.




TO
A Bibliography has also been appended, which is designed to serve simply as a guide to the more recent literature, and no attempt has been made to render the list exhaustive. It will be noticed that most of the Memoirs cited are of later date than the year 1880. The more important earlier papers are recorded in the late Professor Balfour -s “Treatise of Comparative Embryology.- As any student who seriously studies Embryology must consult that invaluable work, I have considered it superfluous to repeat the Bibliography given by Balfour. The prevalent custom of authors of giving references to the literature of the subject under discussion renders it comparatively easy to discover what has already been written thereon.


IT b c /!!> c m o v p
Finally, I would here express my warmest thanks to my friend Professor G. B. Howes, of the Normal School of Science, South Kensington, for his kindness in reading the proofs and in making many valuable suggestions.
 
OF
 
HIS BELOVED MASTER AND FRIEND,  
 
FRANCIS MAITLAND BALFOUR,
 
This Book
 
IS DEDICATED
 
BY
 
 
THE AUTHOR.  
 
 
 
PREFACE.  
 
 
Although there are at the present time, in addition to the special
accounts in various text-books of Human and Comparative Anatomy, two Students - Manuals in the English language solely devoted
to the study of Embryology, it has appeared to me that a relatively
small work, giving a general review of the subject, might prove
of use to students.
 
A knowledge of the main facts of Comparative Anatomy and
Systematic Zoology has been assumed for the reader, the book
being especially designed for Medical Students, or for those
who already possess a general acquaintance with the Animal
Kingdom.
 
It will be noticed that many of the more difficult problems of
Ontology and Phylogeny and special modes of development have
either been merely alluded to or entirely ignored  - as, for instance,
the segmentation of the ovum and the formation of the germinal
layers in Insecta and Teleostei. This has been of set purpose,
as my main object in writing this book has been to give a brief
connected account of the principal organs, omitting or barely
mentioning structures and phenomena, which may be regarded as
of secondary importance.
 
The facts of development have been largely supplemented by hypotheses; and an endeavour has been made so to present the
latter, that the student could not mistake them for the former.
 
It is inevitable that, in compiling such an introductory textbook as this, many subjects must be treated in a manner similar
to that in which they have been dealt with by previous authors ;
and therefore I have not hesitated to borrow from them when
occasion required.
 
In order to facilitate references, very recent, important, or
doubtful observations have been associated in many cases with
the investigator -s name. It must be distinctly understood that I
do not necessarily personally adopt statements or views which
have been incorporated in the book; they are merely put forward
for what they are worth.
 
The beginner is advised to pay attention only to the large
type in the first reading, as purely theoretical subjects or matters
of detail are printed in the smaller type. Most of the figures
have been so drawn as to admit of their being coloured ; and the
student is recommended to tint each germinal layer and the
organs derived from it in a uniform manner throughout the
book : thus the epiblast and its derivatives might be coloured
pink, and the hypoblast tinted blue. A uniform system of
colouration will be found to be of great assistance to the
memory.
 
The sources from which the figures have been taken are in all
cases acknowledged, and in the cases where no source is given
the illustrations are original. Figs. 40, 41, 44, 45, 80, 81,
and 178* have appeared previously in the Proceedings of the
Eoyal Dublin Society.
 
The classification adopted will be found in an Appendix. All
the genera mentioned in the text have been inserted, in order
that their systematic position may be seen at a glance.
 
 
A Bibliography has also been appended, which is designed to
serve simply as a guide to the more recent literature, and no
attempt has been made to render the list exhaustive. It will be
noticed that most of the Memoirs cited are of later date than the
year 1880. The more important earlier papers are recorded in
the late Professor Balfour -s “Treatise of Comparative Embryology.-
As any student who seriously studies Embryology must consult
that invaluable work, I have considered it superfluous to repeat
the Bibliography given by Balfour. The prevalent custom of
authors of giving references to the literature of the subject under
discussion renders it comparatively easy to discover what has
already been written thereon.
 
Finally, I would here express my warmest thanks to my friend
Professor G. B. Howes, of the Normal School of Science, South
Kensington, for his kindness in reading the proofs and in making
many valuable suggestions.
 
 
AN INTRODUCTION
 
TO
 
THE STUDY OF EMBRYOLOGY.
 
 
CHAPTER I.  MATURATION AND FERTILISATION OF THE OVUM.
 
Introduction.  - Embryology is the term usually applied to the
whole cycle of changes undergone by an animal in passing from
an egg to the adult condition. It is, in other words, the History
of its Development.
 
The name of embryo (or foetus, in mammalian embryology) is
restricted to the unborn young. At birth the young may closely
resemble the parent, or be very dissimilar ; in the latter case, it is
known as a larva, and undergoes a series of changes or a metamorphosis before it attains the adult state.
 
Even closely allied animals may be “ born - at very different
stages in their development ; the higher animals are, however,
generally born at a relatively later stage than those lower in the
animal scale. They are thus better fitted for the struggle for
existence, and expend less energy during their development than
if they had to provide for themselves.
 
In the higher animals the young also have the further advantage
of the watchful care of their parents, a factor which must have
materially influenced the evolution of the race.
 
Embryology may be studied under two aspects. The first, or
Ontogeny, deals solely with the history of the individual, and
traces the development of the animal as a whole, and of its various
organs.
 
The second, or comparative aspect, compares the development
of animals, and taking those phases which are common to all or
 
A
 
 
2
 
 
THE STUDY OF EMBRYOLOGY.
 
 
 
to many, attempts therefrom to deduce or reconstruct the evolution
of the animal kingdom. This study is known as Phytogeny.
 
The chief result of all embryological inquiry has been to demonstrate that the history of the individual recapitulates in its main
features the evolution of the race, and thereby to give positive
evidence in favour of the Theory of Evolution, in the general
acceptance of the term.
 
It is very important to bear in mind that larval forms, as well
as adults, have to adapt themselves to external conditions, and
that they are consequently liable to be variously modified, and,
within limits, to be highly specialised. These modifications often
have no relation to the adult structure, and consequently can
have no phylogenetic significance.
 
Some preliminary knowledge of Zoology and Comparative Anatomy is necessary in order to appreciate fully the phases in the
development of any one animal, and it is, of course, essential in
studying the general principles of Embryology, as constant reference must be made to the structure of different forms. Such a
knowledge will be assumed for the readers of this book.
 
The Animal Kingdom is divided by zoologists into the Protozoa, or unicellular animals, and the Metazoa, or those animals
composed of a number of cells so united together as to form
tissues. As the ' latter alone produce ova or eggs, the science of
Embryology deals solely with the Metazoa. Although there is
considerable variation in the details of the classification of the
Metazoa, zoologists are tolerably well agreed upon the main
divisions, and in this work that classification and terminology
are adopted which are in most general use in English-speaking
countries.
 
Reproduction amongst the Protozoa consists in a direct or indirect method of cell- division, each product of such division
forming a new individual (fig. I, B, c, d). This process may, or
may not, be preceded by a temporary apposition or permanent
fusion of two or more individuals. The conjugating individuals
may either be apparently quite similar (fig. i, e), or may exhibit
certain differences (fig. I, f) ; but conjugation is always effected
between forms which are similarly motile  - that is, ciliated individuals invariably conjugate with ciliated, and amoeboid with
other amoeboid, forms. Even among those Elagellate Infusoria
which pass through a comparatively complicated life-history, an
individual in the flagellate stage never conjugates with another in
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
o
 
O
 
 
the amoeboid condition. Active reproduction of one kind or other
usually occurs after conjugation.
 
Some Protozoa form compound masses, but the individuals
composing the colony are, with rare exceptions (Proterospongia),
similar to one another, and have a practically independent existence.
 
Although asexual reproduction by various modes of budding
and fission is known in nearly all the groups of the Metazoa, the
sexual method is of invariable occurrence. The essential act of
 
 
 
Fig. i. - Reproduction amongst Protozoa. Not drawn to scale.
 
A-C. Fission in an Amoeba. A. The nucleus has divided into two. B. Two
contractile vacuoles have also formed and the protoplasm is dividing. C. The
process is complete. [ After Howes.']
 
D. Fission in Paramsecium bursaria. There are two contractile vacuoles and two
paranuclei, but the nucleus has not yet completely divided.
 
E. Conjugation of Stylonychia mytilus, illustrating also the fragmentation of the
nucleus.
 
F. Conjugation of Vorticella microstoma. Two free-swimming microzooids have
attached themselves to a fixed form. They all possess a curved nucleus and
a contractile vacuole. [D-F after Stein.]
 
c.v. contractile vacuole ; n. nucleus ; nl. paranucleus.
 
 
this form of reproduction consists in the fusion of a flagellate cell
or spermatozoon with an amoeboid cell, the egg or ovum (figs,
io and n).
 
In a very few cases the spermatozoa are either amoeboid, as in
Nematodes, some Arachnids, and Limulus, or often passive and
rayed, as in most Crustacea; but in the great majority of animals
the spermatozoa are flagellate and actively motile (fig. 2).
 
The ovum, under very rare and exceptional conditions, may
develop into a new organism without previous fertilisation by a
spermatozoon ; this phenomenon is known as 'parthenogenesis.
 
 
4
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The ovum and spermatozoon unite to form the fertilised ovum
or oosperm, which then undergoes rapid cell-division ; the cells
thus produced remain in contact with one another, and though at
first usually very similar, certain groups of cells soon take upon
themselves definite characters, and thus initiate the primitive
tissues.
 
Accepting the view that the Metazoa were derived from colonial Protozoa, it follows
that every cell of the primitive Metazoa was capable of forming fresh colonies by
 
 
 
Fig. 2. - Spermatozoa [from various sources]. Not drawn to scale.
 
i. Sponge; 2. Hydroid; 3. Nematode; 4. Crayfish; 5. Snail; 6. Electric
Ray ; 7. Salamander ; 8. Horse ; 9. Man. In many spermatozoa, as in
Nos. 7 and 9, an extremely delicate vibratile band is present.
 
cell-division. Many Metazoa possess the power of asexually producing new forms
by fission or by budding ; but the tissues implicated in this process must be regarded
as being essentially undifferentiated in character.
 
Owing to the advantage derived from physiological differentiation of labour, the
reproductive function came to be chiefly retained by certain cells, the remainder
specialising along other lines. Those cells which pre-eminently retain the reproductive
function are restricted in their position, and the tissue which they constitute (the
germinal tissue) is contained within what is known as a generative organ or gland.
When ripe, the germ-cells become detached, and commence a free existence.
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
5
 
 
After fertilisation, the ovum, or the embryo into which it
develops, is in a few cases retained within the oviduct of the
mother for a longer or a shorter period, and may temporarily
even be intimately, but very rarely structurally, connected with
the walls of the oviduct or uterus, as will subsequently be described.
 
The primitive germ-cells of animals are, practically, precisely
similar to one another (fig. 175), and, when first recognisable as
germ- cells, it is impossible to tell whether they will develop into
ova or sperm-cells. In this connection it is suggestive to find
that both the ovaries and the testes in Sagitta are developed from
a single primitive germ-cell, which makes its appearance at a very
early stage of development. The primitive germ-cells may more
especially be said to correspond to the Protozoon ancestors of the
Metazoa.
 
Before dealing further with the history of the germ-cells, however, it will be advisable to describe briefly their mode of origin.
 
The Ovum.  - The primitive ova usually form part of a definite
epithelium, of which most of the cells, or it may be only a very
small number, develop into ripe ova. The germinal epithelium is
well supplied with nutritive fluid (either blood or the fluid contents of the cavity of the body), which serves for the growth of
the ova. From the nutriment thus provided the ova generally
store up a greater or less amount of reserve food-material, which
is known as “ yolk - or “ food-yolk.-
 
It would be foreign to the purpose of this work to enter into a comparative
account of the development of ova from primitive germinal-cells. As a general rule,
certain of the cells of the germinal epithelium are directly converted into ova. In
Vertebrates, the germinal epithelium is borne upon a distinct germinal ridge ; the
epithelium increases in thickness, and becomes broken up into cords or trabeculae
(ovarian tubes of Pfluger), which, by mutual ingrowth, lie in the stroma or mesoblastic core of the germinal ridge. Isolated masses or nests may also be formed (fig. 3).
 
Balfour has shown that in Elasmobranchs and other forms, in addition to the
foregoing or direct origin of the ova, the protoplasm of the cells forming the nests
fuses into a single mass containing the nuclei of the previously distinct ova.
Various changes are undergone, but eventually a few of the nuclei segregate protoplasm round themselves to form the ova, the remainder having broken down to
pabulum for the permanent ova.
 
Beddard finds that in Protopterus two kinds of ova are developed  - (a.) The ovum
is a mass of granular protoplasm, containing a germinal vesicle limited by a distinct
membrane, inside of which is a peripheral layer of germinal spots. Later the protoplasm becomes vacuolated, and largely differentiates to form yolk-granules. (6.) The
ovum arises from the fusion of a nest of germinal cells lying within a follicle ; not
only is yolk formed within the central mass, but it is also produced within the
columnar cells of. the follicular epithelium. These cells proliferate and migrate into
 
 
6
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the interior of the ovum ; eventually they disappear. The yolk of these ova appears
to be largely derived from the follicular cells.
 
The yolk consists of highly refractive particles, which vary considerably in their
appearance and structure. As a rule, the yolk elements are small vesicles, which
usually contain smaller vesicles and other bodies (fig. 28, b). In Birds the whole of
the yolk at first consists of these white yolk spheres ; but during the development
of the egg, some of the white yolk spheres become modified to form the yellow yolk
(fig. 28, A and c). In the ripe unincubated egg the yellow yolk constitutes the great
mass of the “yolk,- the white yolk being restricted to a peripheral and several concentric layers, and to a central mass which extends in a constricted neck, and again
widens out to form a bed, upon which the blastoderm rests (fig. 28, A, w, y ).
 
 
It not unfrequently happens (many Hydrozoa, Insects, some
Vertebrates, &c.) that certain of the primitive germ-cells feed
upon neighbouring germ-cells, so that the growth of the ovum
 
 
 
Pig. 3.  - Section through a Portion of the
Ovary of a Mammal. Illustrating the mode
of development of the Graafian follicles. [From.
Wiedersheim. ]
 
D. discus proligerus ; Ei. ripe ovum ; G. follicular cells of germinal epithelium ; g. bloodvessels ; K. germinal vesicle (nucleus) and germinal spot (nucleolus) ; KE. germinal epithelium ;
Lf. liquor folliculi ; Mg. membrana- or tunicagranulosa or follicular epithelium ; Mp. zona
pellucida ; PS. ingrowths from the germinal epithelium, ovarian tubes, by means of which some
of the nests retain their connection with the epithelium ; S. cavity which appears within the
Graafian follicle ; So. stroma of ovary ; Tf. theca
folliculi or capsule ; U. primitive ova. When' an
ovum with its surrounding cells has become separated from a nest, it- is known as a Graafian follicle.
 
 
and its store of food-yolk are made at the expense of its fellow
germinal cells. In most Platyhelminths that portion of the primitive germinal epithelium which is destined to provide pabulum
for the ova proper is separated from the ovary as yolk-glands, or
vitellaria , and their products, yolk-cells or yolk-granules, surround
the ova after they have left the ovary, and before they are enclosed
within the egg-capsules. The yolk-cells may be regarded as germinal cells which have lost the power of reproduction, hut retained
that of forming yolk. Either the ovum or the embryo in due
course feeds upon this reserve of food.
 
When many ova are deposited within the same egg-capsule as
in some forms of Prosobranch Gastropods (Buccinum), the more
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
7
 
 
advanced embryos devour those that are imperfectly developed, so
that a very limited number, sometimes only a single individual,
eventually escape from one capsule.
 
The fusion of several germinal cells with one ovum does not
correspond to the multiple conjugation of some Protozoa, as in the
 
 
 
Fiq. 4. - Diagrams of Ova [from, various sources after Geddes ]. Not drawn to scale.
 
a. Diagram of a typical ovum with a delicate egg-membrane, granular protoplasm, nucleus (germinal vesicle), and nucleolus (germinal spot), b. Amoeboid
ovum of Hydra [after Kleinenberg ]. c. Early ovum of a Sea-Urchin (Toxopneustes
variegatus) with pseudopodia-like processes extending into the gelatinous eggmembrane (vitelline membrane) in order to obtain nutriment from without ;
afterwards they become much finer and more regular, causing the vitelline
membrane to have a striated appearauce ; hence it is termed the * • Zona radiata -
 
- the striae are really delicate pores [after Selenka ]. d. Nearly ripe ovum of
Strongylocentrotus lividus with its zona radiata [after Herticig].
 
 
formation of plasmodia ; it is merely the assimilation of several
cells by one ovum, much as an Amoeba feeds upon its prey.
 
An ovum is a small free cell which is characterised in the
resting-stage by possessing a large clear nucleus, the germinal
vesicle, and a well-marked highly refractive nucleolus, the ger
 
Fig. 5. - Ovum of the Cat. Highly magnified ;
semi-diagrammatic. [From Quain, after Schafer.]
 
gs. germinal spot ; gv. germinal vesicle ; vi. vitellus, or protoplasm of ovum filled with yolk granules, round which a delicate membrane was seen ;
zp. zona pellucida ( Zona radiata ) ; only a few
radial pores are drawn.
 
 
 
minal spot ; in many cases several germinal spots occur. Pigs. 4
and 5 illustrate various kinds of ova.
 
The protoplasm usually has, as has just been mentioned, the
power of storing up albuminoid matter as reserve food material by
a differentiation of its own substance in the form of yolk-granules
or spheres. The amount of food-yolk varies greatly ; in some few
 
 
8
 
 
THE STUDY OF EMBRYOLOGY.
 
 
instances none appears to be differentiated ; often only a little is
formed ; more frequently there is a considerable amount ; and in
the eggs of Elasmobranchs and of Sauropsida an enormous quantity
is deposited. The distribution of the yolk within the egg also
varies, being either chiefly concentrated at one pole ( telolecithal ),
or towards the centre {centrolecithal), or evenly distributed throughout ( alecithal ).
 
As the amount of protoplasm in an ovum containing much foodyolk is relatively small, the storing of the yolk-granules within its
substance would naturally cause it to be distended. In those ova
with a very large amount of yolk, the protoplasmic reticulum
scarcely more than serves to keep the yolk-granules together
 
 
Fig. 5*. - Typicai. Cell and Nucleus
of the Intestinal Epithelium of a
Flesh-Maggot (asticot), treated with osmic acid vapour. [From Camay. ]f
 
bn. continuous band of nucleine, contracted to the centre of the nucleus, and
showing numerous twists ; me. membrane of the cell ; mn. membrane of the
nucleus ; pc. protoplasm of the cell,
showing the radiating reticulum and the
enchylema enclosed in its meshes ; pn.
plasma of the nucleus, showing a reticulum and a plasmic enchylema, as distinct
as those of the protoplasm.
 
The structure of an ovum is practically
identical with that of such a tissue-cell
as the above.
 
 
During development, certain cells of the embryo reconvert the
food-yolk into active protoplasm.
 
The germinal vesicle of the unripe ovum, as Carnoy points out,
has the same general structure as the ovum itself; that is, it
consists of an extremely fine protoplasmic reticulum , the meshes of
which are filled with a granular fluid {enchylema). The reticulum
also forms a delicate nuclear membrane. But, in addition to the
above, the nucleus possesses a distinctive substance, which i&
variously termed nucleine , nucleoplasm , or, from its being readily
stained by the action of certain reagents, chromatin. In very
young ovarian ova, the chromatin occurs in the form of a very
long, extremely and irregularly contorted thread or nuclear filament.
 
The nuclear filament is condensed in more mature ova into a
 
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
9
 
 
single spherical mass, the germinal spot, or into a few or a large
number of smaller germinal spots.
 
Ova may be either naked (fig. 4, A and b), or surrounded by
one or more membranes (fig. 4, c, D, and fig. 5). The primary eggmembranes (vitelline membranes) are usually differentiated from
the protoplasm of the ovum itself.
 
In Vertebrates two egg-membranes are usually present, an external delicate vitelline membrane, which is probably formed by the
ovum itself, but in some cases a similar membrane may be secreted
by the epithelium of the ovarian follicle. This membrane is often
termed a chorion. Below the vitelline membrane a thicker membrane, perforated by innumerable fine radial pores, is differentiated
out of the peripheral layer of the ovum. It is known as the. zona
radiata or zona pellucida. The secondary egg-membranes are either
 
 
Fig. 6.  - A Fowl -s Egg after about
Thirty Hours - Incubation. Viewed from
above, the upper portion of the shell
being removed. [ From Kolliker after Von
Baev. J
 
' a. shell ; 6. shell-membrane ; b'. airchamber at broad end of egg between
the two layers of the shell-membrane ;
c. the boundary between the outer and
middle portion of the albumen ; d. the
internal layer of more fluid albumen,
which also extends round the yolk as a
thin sheath ; e. chalaza ; v. vitellus or
yolk ; av. area opaca, or that portion of
the blastoderm which extends over the
yolk ; the heart-shaped central portion,
ao, is the vascular area of the area opaca.
In the centre is the embryo surrounded
by the area pellucida.
 
 
secreted by accessory generative glands, or by the glandular wall of
the oviduct. When a secondary egg-membrane is impregnated with
calcareous deposits, it is known as an egg-shell. The secondary
egg-covering often encloses an albuminous glairy fluid  - the white
of egg  - which serves for the protection and further nutriment of
the embryo (figs. 6, 74, 75). The albumen also is secreted, either
by special glands (most Invertebrates), or by the wall of the
oviduct (Vertebrates).
 
Maturation of the Ovum.  - Before or after fertilisation, certain
changes, which are of considerable interest, take place in the ovum.
The germinal vesicle often becomes amoeboid, and passes to one pole
of the ovum, and the germinal spot disappears (fig. 7, b-d) ; in fact,
both the germinal vesicle and spot disappear as such, and pass into
those karyolitic figures which characterise cell-division (see p. 18).
 
 
 
10
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The resulting nuclear spindle is placed vertically, the peripheral
nuclear star, or “ aster,- being situated in a small protuberance
from the surface of the ovum. This process is segmented off from
the ovum, and a minute cell is formed, containing a portion
of both the protoplasm and the nucleus of the parent-cell
(fig. 7, F and l).
 
 
 
A. Ripe ovum with excentric germinal vesicle and spot ; B-D. Gradual metamorphosis of germinal vesicle and spot, as seen in the living egg, into two
asters ; F. Formation of first polar cells and withdrawal of remaining part of
nuclear spindle within the ovum ; G. Surface view of living ovum in the first
polar cell ; H. Completion of second polar cell ; I. A later stage, showing the
remaining internal half of the spindle in the form of two clear vesicles ; K. Ovum
with two polar cells and radial striae round female pronucleus, as seen in the
living egg. [B, F, H, and I, from picric acid preparations.] L. Expulsion of first
polar cell.
 
 
This phenomenon is repeated, and two cells are budded off from
the ovum ; these are known as the “ polar cells - (or as polar bodies,
polar globules, directive bodies, &c.), from the fact that they are
invariably derived from that pole of the ovum at which the epiblast
or upper-layer cells will be developed; hence, also, this pole is
 
 
 
Fig. 8. - Formation op Polar Cells
in Ovum of Elysia viridis.
 
The upper pole of the ovum becomes
amoeboid during the formation of the
polar cells. The second polar cell is in
process of formation.
 
 
usually termed the upper pole of the ovum (see figs. 12 and 17).
During the production of the polar cells, the ovum, especially at
its upper pole, may exhibit amoeboid movements ; this is well shown
in the ovum of Elysia (fig. 8).
 
Although the polar cells may remain attached to the developing
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
11
 
 
ovum for some time, they take no share in the formation of the
embryo, and are simply to be regarded as superfluous bodies.
 
What remains of the primitive nucleus passes towards the centre
of the ovum, usually in an inactive or resting condition, being
without radial striae. It is known as the female pro-nucleus.
 
The ovum is now in a passive condition, and ready to be
fertilised. The extrusion of the polar cells, though occasionally
taking place after fertilisation (ex. Elysia, fig. 8), is really to be
regarded as the last term in that series of changes which occurs
before impregnation, and to be, in fact, anticipatory of it.
 
Before following the history of the ovum further, it will be
necessary to return to the sperm-cells.
 
The Spermatozoon.  - Although we find considerable variation
in certain details of structure, there is a general similarity in the
appearance of the spermatozoa of animals, a head and vibratile
tail being of almost universal occurrence : the most important
exceptions have already been mentioned (p. 3 and fig. 2).
 
The primitive sperm-cells or mother-cells of the spermatozoa arise from a tissue
corresponding to that which gives origin to the primitive ova (p. 242, fig. 175). The
exact manner in which the spermatozoa are developed varies in different animals,
and has been variously described by numerous investigators. This being the case,
it will be advisable to give simply a sketch of what appear to be the most important
facts in spermatogenesis , as this process is termed.
 
Those cells of the generative epithelium which develop into male sexual cells
undergo cell-division in the ordinary manner, and may give rise to a considerable
number of cells ( spermatoblasts ). Each spermatoblast is converted into a spermatozoon, and, in doing so, gives rise to a small mass of protoplasm, the so-called seminal
granule , or globule, or accessory corpuscle, which appears to have no further function.
Fig. 9, a-h, illustrates this process in the Rat.
 
Instead of becoming distinct, the spermatoblasts or incipient spermatozoa may
remain aggregated together ( spermosphere or sperm-morula), and surround a central
non-nucleated protoplasmic mass (the sperm-Uastophore ), as in the case of the Snail
and Earthworm (fig. 9, o-s).
 
In Elasmobranchs (fig. 9, i-n) the nucleus of the sperm-cell (sometimes called the
spermatocyst) alone divides, forming a number of daughter-nuclei, the remains of the
parent-nucleus still persisting. The protoplasm of the cell differentiates into the
tails of the spermatozoa, while the daughter-nuclei constitute the main portion of
their heads. The ripe spermatozoa are liberated by the rupture of the wall of the
sperm-cell, leaving behind the parent-nucleus and a small remnant of unused protoplasm. This latter is merely an abbreviated variation of the former process, and the
residual nucleus and protoplasm clearly correspond to the accessory corpuscle or to
the sperm-blastophore in the preceding forms.
 
The nucleus of each daughter sperm-cell constitutes the head of a spermatozoon ;
it is surrounded by an extremely delicate film, which is produced from one end into
a fine flagellum, and sometimes also into an almost imperceptible undulating membrane ; these are formed by the protoplasm of the spermatoblast. Every spermatozoon is thus a true morphological cell.
 
Kolliker, however, maintains that the entire mammalian spermatozoon is simply
a free nucleus.
 
 
12
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Fertilisation of the Ovum.  - It is needless to recount the
various ways by which spermatozoa may reach ova ; suffice it to
say, that either within the female or in the surrounding water a
 
 
 
A-H. Isolated sperm-cells of the Rat, showing the development of the spermatozoon, and the gradual transformation of the nucleus into the spermatozoon
head. In G the seminal granule is being cast off. [ After H. H. Brown.]
 
I-M. Sperm-cells of an Blasmobranch. The nucleus of each cell divides into a
large number of daughter-nuclei, each one of which is converted into the rodlike head of a spermatozoon.
 
N. Transverse section of a ripe cell, showing the bundle of spermatozoa and the
passive nucleus. [I-N after Semper .]
 
O-S. Spermatogenesis in the Earthworm : O. young sperm-cell ; P. the same
divided into four ; Q. spermatosphere with the central sperm-blastophore •
R. a later stage ; S. nearly mature spermatozoa. [After Blomfield .]
 
 
spermatozoon comes into contact with an ovum, and either penetrates any membrane which may surround it, or passes through an
aperture (micropyle) left in the egg-membrane.
 
When the spermatozoon is approaching the actual surface of an
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
13
 
 
ovum, a process from the latter sometimes rises up to meet it, and
a fusion is effected (fig. io, a-d, and fig. n, a). The head of the
spermatozoon penetrates the ovum, while the tail, after vibrating
feebly, is absorbed.
 
The head, or rather the nucleus of the spermatozoon, is converted into an aster or star, and is now known as the male pro
 
 
Fxo. io Fertilisation of Ovum of a Star-fish (Asterias glacialis). [ From Geddes after FoL]
 
In A-D the spermatozoa are represented as imbedded within the mucilaginous
coat of the ovum. In A a small prominence is rising from the surface of the
ovum towards the nearest spermatozoon ; in B they have nearly met, and in G
they have met. D. The spermatozoon has penetrated the ovum, and a vitelline
membrane with a crater-like opening has been formed, which prevents the entrance of other spermatozoa. H. ovum showing polar cells and approach of the
male and female pro-nuclei ; the protoplasm is radially striated round the former.
 
E. F. G. later stages in the coalescence of the two nuclei.
 
nucleus. It travels towards the female pro-nucleus, which, it will
be remembered, is situated in the centre of the ripe ovum (fig.
io, h). The female pro-nucleus becomes somewhat amoeboid, and
fusion occurs between the two elements, thus forming a new
nucleus (fig. io, e-g). While this is taking place, the ovum itself
often exhibits amoeboid movements (fig. n, a).
 
 
 
Fig. ii.  - Fertilisation of Ovum of Elysia viridis.
 
A. ovum sending up a protuberance to meet the spermatozoon ; B. approach of
male pro-nucleus to meet the female pro-nucleus ; F.PN. female pro-nucleus ;
 
M.PN. male pro-nucleus; S. spermatozoon.
 
The fertilised ovum is a very different body from the primitive
ovum, as it consists of a portion of the original protoplasm and
nucleus of the latter reinforced by those of another cell, which
is usually derived from a different animal. The new nucleus is
called the segmentation nucleus , and it may be well to adopt Balfour -s name of oosperm for the fertilised ovum.
 
 
14
 
 
THE STUDY OF EMBRYOLOGY.
 
 
There is some doubt whether the male pro-nucleus has the full value of a true
nucleus, and this has led Flemming to define fertilisation as the union of “the chromatin of a male with that of a female nuclear body.- Yan Beneden has recently
shown that the essential act of fertilisation consists in the grouping together (or
probably, more accurately, of the fusion) of the chromatin or germ-plasma of the
nucleus of the spermatozoon with that of the nucleus of the ovum. During the first
division of the oosperm, and in all the succeeding phases of segmentation, each new
cell receives an equal share of the paternal and maternal chromatin. (See chap. ii.
p. 19, and fig. 13.)
 
The fertilisation of an ovum by a spermatozoon is paralleled
by the permanent conjugation Cf such Protozoa as Vorticella and
many Monads. In each case the phenomenon is followed by rapid
cell-division  - the resulting cell-units remaining separate in the
Protozoa, whereas they group themselves together so as to form an
aggregate of a higher series in the Metazoa.
 
00 o o
 
Significance of the Maturation and Fertilisation of the Ovum.  - There have
been numerous speculations concerning the significance of the polar-cells. The
view now generally accepted is that first propounded by Minot, and subsequently
(but independently) proposed by Balfour, which suggests that the polar-cells
represent what may be regarded as the male element of the primitive germinal
cell, the sexes not being supposed to be differentiated in the latter. The ovum
is thus preparing itself for the reception of a vigorous element derived from a
different source. Similarly, the accessory corpuscle, or its equivalent, is regarded as
the female portion of the primitive sperm-cell, the remaining nuclear matter and
protoplasm being used up in the manufacture of unisexual (male) cells. The mature
ovum, being unisexual, is free to conjugate with a male cell. The two are mutually
complemental, and after union constitute a single perfect unit.
 
The relations between the male and female elements may, according to this view,
be thus tabulated
 
Indifferent germinal cells, which eventually specialise into
 
ovum (oospore), | sperm-cell (spermospore).
 
Each by cell- division develops into
 
(oosphere), | (sperm-morula) spermosphere, *
 
which is composed of
A. a passive element,
 
polar-cells, j sperm-blastophore (seminal globules or
 
granules) ;
 
B. an active sexual element,
 
mature ovum, I spermatoblasts, which are directly
 
I converted into spermatozoa.
 
The union of the latter constitutes the fertilised ovum (oosperm).
 
Minot proposed the common term of thelyblast for a mature ovum and for a spermblastophore, and arsenoblasts for the polar-cells and spermatozoa. Sexual reproduction
would thus consist in the union of a thelyblast from one source with an arsenoblast
from another source.
 
 
* In Mammals the sperm-cell gives rise to spermoblasts, each of which gives off a
seminal globule, the remainder differentiating into a spermatozoon.
 
 
MATURATION AND FERTILISATION OF THE OVUM.
 
 
15
 
 
Unfortunately, tlie terms employed in describing the various stages in the development of the generative elements are not used in a synonymous sense by the various
investigators and writers on the subject ; those in the most general use have been
here adopted.
 
A more simple view is that the extrusion of the polar-cells prevents the parthenogenetic development of the egg, merely by eliminating a considerable quantity of
nuclear matter. The researches of Van Beneden on Ascaris have demonstrated in a
quantitive manner the amount of chromatin thus lost ; the precise amount for Ascaris
being three-fourths of that present in the nucleus of the ovarian ovum.
 
The spermatozoon supplies a sufficient amount of new chromatin to enable the
embryo to develop. According to this view, there is no essential distinction between
the chromatin of the male as opposed to that of the female germ-cell.
 
At the end of this work will be found a summary of Weismann -s and Geddes -
conclusions respecting the significance of the maturation and fertilisation of the ovum.
 
The next series of changes undergone by the oosperm is that
known as segmentation. The unicellular oosperm divides, by ordinary cell-division, into a large number of cell-units. The resulting
mass is a multicellular organism, whose “ life - consists of the
sum-total of the activities of its component cells. It is thus an
individual of a higher order than a Protozoon, and one possessing
an infinitely greater capacity for progressive evolution.
 
 
( 16 )
 
 
CHAPTER II.
 
SEGMENTATION AND GASTRULATION.
 
On the cessation of the various phenomena related above, the
oosperm becomes spherical in contour, and its nucleus reappears
as a clear rounded vesicle, enclosing a distinct round nucleolus.
 
This nucleus is properly termed the “ segmentation nucleus as
it differs fundamentally from the original nucleus of the unfertilised
ovarian ovum. The name “ germinal vesicle,- which is commonly
applied to the nucleus of ova, is open to the objection that it
is used indiscriminately for the nucleus both before and after
fertilisation ; it will here be confined to the former condition.
 
A - Invertebrates. - Typical or Alecithal Segmentation. -
In order to gain a clear comprehension of the segmentation of the
oosperm, it will be advisable to take as an example a form in which
the process is not obscured by secondary details. The early stages
of segmentation can be readily studied in the eggs of most Freshwater Molluscs. The fSTudibranch Mollusc Elysia viridis also serves
very well for this purpose ; and the following account refers to the
segmentation, as seen in the living egg, of that form.
 
After a resting-stage, the nucleus divides into two, the nucleolus
having immediately before similarly divided (fig. 12, A-c), and each
new nucleus travels to an opposite pole of the oosperm. Whilst
this is taking place, the nuclei, as such, disappear ; being apparently
replaced by two stars, some of the rays of which meet one another
in the middle line. These polar stars , as they are termed, are
composed of the radially arranged granular protoplasm of the
cell. The polar stars then entirely separate, and the oosperm
usually becomes distinctly amoeboid, especially at its upper pole.
A shallow groove makes its appearance on the surface of the oosperm
midway between the two nuclear foci. The groove rapidly deepens
(fig. 12, D, e), and eventually divides the oosperm into two distinct
spherical halves, which immediately afterwards become appressed
together. The nucleus has by this time reappeared as a clear spo'
 
 
SEGMENTATION AND GASTEULATION.
 
 
17
 
 
in the centre of each polar star ; the rays of the latter disappear
and the chromatin collects to form a new nucleolus (fig. 12, F, g).
Each segmentation sphere now passes through a short resting-stage.
 
The application of staining reagents reveals the nature of the
changes undergone by the nucleus in segmenting oosperms. In all
cases the nucleus is transformed into a spindle-like arrangement of
delicate fibres, termed the “nuclear spindle- round the apices of
which the cell-protoplasm radiates as the above-mentioned polar
stars. The chromatin aggregates at the centre of each fibre, and
divides transversly into two, each moiety travelling along its own
 
 
 
Fig. 12.  - Early Stages of Segmentation of Elysia viridis (drawn from the
living egg).
 
A. oosperm in state of rest after the extrusion of the polar cells ; B. the
nucleolus alone has divided ; C. the nucleus is dividing ; D. the nucleus, as
such, has disappeared, first segmentation furrow appears ; E. later stage ;
F. oosperm divided into two distinct segmentation spheres, the clear nuclear
space in the centre of the aster of granules is growing larger ; G. resting-stage
of appressed two spheres ; H. I. similar stages in the production of four
spheres ; K. formation of eight-celled stage.
 
 
fibre towards the nearest apex of the nuclear spindle (fig. 1 3, d -f).
The fibres between the two receding masses of chromatin thin out,
and eventually disappear. Finally, the nuclear substance segregates
into an ordinary resting nucleus and nucleolus.
 
The Behaviour of the Nucleus in Cell-Division.  - The behaviour of the nucleus
during cell- division has received a great deal of attention within the last few years ;
and as it is a subject of considerable importance, it will be advisable to give a brief
account of the process.
 
The nucleus of a typical tissue-cell consists of a rounded vesicle containing a
nuclear matrix, which is termed “ achromatin- as it is only lightly coloured on the
 
B
 
 
18
 
 
THE STUDY OF EMBRYOLOGY.
 
 
application of staining reagents. The achromatin is permeated by a delicate network or reticulum of a denser substance, the “nucleoplasm- or “chromatin- which
also forms the delicate wall of the vesicle. This network readily stains deeply, and
the intersections of the fibres usually give a dotted appearance to the nucleus.
When the cell is in a resting condition, the chromatin is, as a rule, concentrated
either into several rounded bodies, or more frequently into a single mass, the
nucleolus ; but this is usually, if not always, connected with the wall of the nucleus
by delicate strands of chromatin.
 
During the process of division in such a nucleus as that just described, the con
 
 
A-H. karyokinesis of a tissue-cell. A. nuclear reticulum in its ordinary state.
 
B. preparing for division ; the contour is less defined, and the fibres thicker and
less intricate. C. wreath-stage ; the chromatin is arranged in a complicated
looping round the equator of the achromatin spindle. D. monaster-stage ; the
chromatin now appears as centripetal equatorial V -s, each of which should be represented as double. B. a migration of the half of each chromatin loop towards
opposite poles of the spindle. F. di aster-stage ; the chromatin forms a star
round each pole of a spindle, each aster being connected by strands of achromatin.
 
G. daughter wreath-stage ; the newly formed nuclei are passing through their
retrogressive development, which is completed in the resting-stage, H.
 
d-f. karyokinesis of an egg-cell, showing the smaller amount of chromatin than
in the tissue-cell. The stages d. e. f. correspond to D. E. F. respectively. The
polar star at the end of the spindle is composed of protoplasm granules of the
cell itself, and must not be mistaken for the diaster (F). The coarse lines represent the chromatin, the fine lines the achromatin, and the dotted lines cellgranules [chiefly modified from Flemming]. X-Z. direct nuclear division in the
cells of the embryonic integument of the European Scorpion [ after Blochmann ].
 
tour becomes less defined, owing to the disappearance of its membrane ; the very fine
close network appears looser in texture and coarser in fibre ; and a contorted looped
rosette or wreath of chromatin is eventually formed (fig. 13, A-c). The peripheral
loops fracture, leaving a star-like group of Y-shaped bars of chromatin (aster or
single star), the angles of which point towards the centre. By this time the
achromatin has been transformed into a nuclear spindle, and the chromatin wreath
and single aster lie at right angles to it in its equatorial plane (c, d). Each bent
chromatin bar next divides longitudinally (the division is not shown in the figure),
and the loops, instead of pointing inwards, become directed, some towards one pole
of the long axis of the nucleus, and some towards the other, forming a double star
 
 
 
 
SEGMENTATION AND GASTRULATION.
 
 
19
 
 
or diaster. It is important to remember that each half of every longitudinally split
chromatin bar of the single aster travels towards an opposite pole of the spindle to
form the daughter-stars (fig. 13, e, f). Thus, the chromatin of every new nucleus
is not formed by the simple partition of the parent nuclear network, but by an
actual longitudinal splitting of the chromatin fibre itself, by this means ensuring a
perfectly equable division, while the preliminary breaking up of the network into
bent bars facilitates the process. The daughter-stars thus formed gradually pass
through the reverse process, and each, after becoming a wreath, is transformed into
a fine reticulum enclosing the achromatin, as in the parent nucleus (fig. 13, G, h).
 
When the daughter-nuclei are in the stellar stage, the protoplasm of the cell itself
becomes constricted, and the cell is usually quite divided by the time the wreathstage is attained.
 
This mode of cell-division is known as the “ indirect method ,- and the whole process is termed “ Jcaryokinesis. -
 
The following schema of the phases of indirect cell-division is modified from
Flemming :  -
 
 
Resting Stage.
 
/ Wreath form.
 
|> j Star form.
 
oq ( Transition phase.
 
 
Mother-nucleus.
. 1 . Spira.
 
 
Daughter-nucleus.
 
5. Dispira.
 
 
2. Aster. 1 4. Diaster.
 
-* 3. Metakinesis.
 
 
Usually, in segmenting oosperms and in many vegetable cells, the chromatin is less
abundant, and the achromatin appears to take a larger share in nuclear division
than in tissue-cells. At the stage when the chromatin is equatorially situated (the
“equatorial plate,- which is the equivalent of the wreath and aster stage), the
achromatin forms a well-marked spindle-shaped bundle of fibres, the apices of which
correspond with the centres of the future nuclei. Later, the chromatin separates
into two portions, each of which travels along the achromatin fibres to each apex of
the spindle, the diaster stage. The intervening achromatin threads break across the
middle and are withdrawn.
 
Van Beneden, who has most carefully studied these phenomena in the oosperm of
Ascaris, states that the achromatin spindle is probably always present in the ordinary
tissue-cells though difficult of detection ; but it is readily visible in egg-cells when
they are properly treated with reagents.
 
During karyokinesis, the granules of the protoplasm of the cell often become
radially arranged with regard to the foci of the daughter-nuclei. These alone can
be seen in the living egg (fig. 13, d, e, /), and they should not be mistaken for the
chromatin fibres, which are only visible after suitable treatment.
 
 
The foregoing is a brief summary of the views generally held respecting karyokinesis. Carnoy, however, gives a somewhat different account. As previously
mentioned, he finds all cells to be composed of a fine protoplasmic reticulum enclosing
a fluid enchylema , which contains various substances in solution and particles in
suspension. The nucleus is similarly constituted, but it possesses in addition a
contorted nuclear filament of chromatin. The nuclear reticulum evidently corresponds to the above-mentioned achromatin. According to Carnoy, the convolutions
of the nuclear filament very rarely fuse at their intersections so as to constitute an
actual network. The wall of nucleus is never formed by the chromatin, but solely
at the expense of its reticulum. The latter also forms the nuclear spindle in dividing
cells. The polar stars are formed by the reticulum of the cell.
 
It is probable that there is considerable variation in the method of indirect
nuclear division amongst the Metazoa. Yery rarely, the nucleus simply divides in
half without forming karyolitic figures. This is known as “ direct nuclear division ,-
and has been observed by Blochmann in the embryonic integument of the European
Scorpion, ^and by Ranvier in the division of leucocytes in Axolotl.
 
 
20
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Our present knowledge appears to warrant the following generalisation concerning
the evolution of the nucleus. In some of the simplest of all organisms (Protista)
no nucleus has yet been observed ; probably, however, it will be demonstrated that
nucleoplasm ( i.e ., chromatin) is present in diffused granules, as Gruber has shown is
the case in a few Ciliate Infusoria. A concentration of the chromatin occurs in other
Protozoa, forming either several small nuclei or a single large one. In most Protozoa
the nucleus divides directly, as it may do, though very rarely, amongst the Metazoa.
Physiological differentiation has also acted upon the nucleus of Protozoa, and has
resulted in great variation in structure and behaviour. In some Heliozoa and Ciliata
the nuclear division bears considerable resemblance to the indirect method characteristic of the tissue -cells of Metazoa.
 
In segmenting oosperms, the process of nuclear division is not so complicated as it
is in the tissue-cells of adults - partly owing, perhaps, to a paucity of chromatin ;
 
 
 
Fig. 14. - Early Stages of Segmentation of Elysia viridis (drawn from the living'egg).
 
A. oosperm in state of rest after the extrusion of the polar cells ; B. the nucleolus
alone has divided; C. the nucleus is dividing; D. the nucleus, as such, has disappeared, first segmentation furrow appears ; E. later stage ; F. oosperm divided
into two distinct segmentation spheres  - the clear nuclear space in the centre of
the aster of granules is growing larger ; G. resting-stage of two appressed spheres ;
 
H. I. similar stages in/the production of four spheres; K. formation of eightcelled stage.
 
but it is probable that there is an approach in some cases to the direct method of
nuclear division which is so common amongst the Protozoa ; as, for example, in the
segmenting oosperm of Elysia (fig. 14, b, c), which may be compared with the nuclear
division of the Amoeba (fig. 1, a-c). This irresistibly suggests a retention by certain
segmenting oosperms of the ancestral method of nuclear division.
 
The segmenting typical oosperm was left at a stage in which
two segmentation spheres had been formed.
 
A second series of changes soon takes place, the long axis of
the nuclear spindle lying in the same plane as the first, but at
right angles to it ; four segmentation- spheres are thus formed, all
lying in the same plane (fig. 14, H, 1).
 
 
SEGMENTATION AND GASTRULATION.
 
 
21
 
 
After another short resting-stage, each of the four spheres divides
in a manner essentially identical with the preceding. As the
nuclear spindle assumes a position at right angles to the two previous directions, the third groove is in a horizontal plane, and a
mass of eight cells is produced, four above and four below (fig.
14, k). The segmentation has thus taken place in the three dimensions of space.
 
In the most regular cases of segmentation, the eight spheres are
vertically divided to form sixteen spheres, eight above and eight
below (fig. 15). In the next stage, a furrow is formed on each
side of the first horizontal or equatorial fissure, and these deepen
to produce a mass of thirty-two spheres, consisting of four rows of
eight cells each. A sixty-four-celled stage is next reached (fig.
15); but usually, after this, the regular rhythm is lost, and the
 
 
12 4 s
 
 
 
Fig. 15.  - Segmentation of Oosperm of Frog. [After Eclcer .]
 
 
The numbers above the figures refer to the number of segments at that stage.
 
The dotted lines represent the position of the next furrows or planes of segmentation. The segmentation, though regular, is somewhat unequal owing to the
presence of yolk.
 
order of the segmentation becomes obscure. We thus get the
number of cells in each successive stage as follows :  - A. 1 ; B. 2 ; c. 4 ;
D. 8 ; E. 16; F. 32 ; G. 64 ; n. 00, that is, in geometrical progression.
It must, however, be definitely understood that this is not the
invariable rhythm of segmentation, but only a generalised type
(for example, the Nudibranchs and other Mollusca do not conform
to it).
 
The result of segmentation is the formation of a multicellular
body, usually enclosing a central cavity  - “ Segmentation cavity - or
“ Blastocoel - (fig. 16, a). The body itself is variously termed “ Bias tula - or “ Blastosphere.- Excepting in special cases, the wall of the
Blastula consists of a single layer of cells.
 
Typical Gastrulation.  - An oosperm devoid of food-yolk
(known as alecithal ), or one in which the segmentation is quite
 
 
 
 
22
 
 
THE STUDY OF EMBRYOLOGY.
 
 
regular, has been assumed, but this rarely obtains ; more or less
food-yolk is usually present, and its presence is a disturbing factor
of great importance. Before, however, discussing the effects of
food-yolk upon an oosperm, it will be advisable to continue the
history of the simpler condition ; the ova of Echinodermata being
particularly suitable for this purpose.
 
On the completion of the Blastula stage, a slight depression
occurs at the pole opposite to that where the polar cells are situated. This is often preceded by a flattening of that pole of the
 
 
 
A. blastula ; B. later stage, showing the thickening and flattening of the lower
pole and appearance of, mesoderm ; C. commencement of gastrulation ; D. later
stage ; E. early larval stage with commencing oval invagination ; D. and E. from
living embryos, after Metschnikoff.
 
arc. archenteron ; bl. blastocoel ; bp. blastopore ; ep. epiblast ; hy. hypoblast ;
m. mesoblast (mesamceboids) ; m.s. mesoblast cell secreting a spicule ; st. stomodseum.
 
blastula (fig. 1 6, b), the cells of the flattened region assuming a
more columnar form, the first indication of a histological differentiation. The invagination deepens until a cup-like cavity is
formed (fig. 1 6, d), and eventually there is usually a complete inversion of this pole of the blastula. The growth of the embryo
is so rapid that the size and general form of the body is at first
little altered by this process, but soon the absolute size is increased and the embryo becomes oval in shape.
 
These phenomena result in the formation of a two-layered embryo, which has an orifice at one end, the Blastopore or primitive
 
 
SEGMENTATION AND GASTRULATION.
 
 
23
 
 
mouth, opening into a central sac, the cavity of invagination,
Archenteron , or primitive stomach. The outer layer is the Epiblast (Ectoderm), the inner layer lining the archenteron is the
Hypoblast (Endoderm), and between these layers is a larger or
smaller cavity, which is the more or less reduced segmentation
cavity. Such an embryo is known as a Gastrula (fig. 1 6, c).
 
A modification of ordinary invagination is sometimes met with which is worthy of
special notice :  - In many Nudibranch Mollusca, the blastula is somewhat quadrate
in contour and flattened, being notunlike a book in shape (fig. 17). The gastrula
is formed by a kind of rolling over, combined with a slight amount of invagination.
An elongated blastopore is the result ; this closes over from behind forwards, the
anterior extremity (as indicated by the polar cells) appearing to persist as the mouth.
 
An extreme example of the method of gastrula-formation by the rolling round of
 
 
 
Fig. 17.  - Gastrulation of Fiona nobilis.
 
A. oblong flattened blastula (plakula), two embryos in one egg-shell, the lower
one seen endwise ; B. gastrula in process of formation ; C. gastrula stage  - the
slit-like blastopore (bp.) will be still further reduced from behind forwards.
 
a two-layered flat embryo (the Plakula of Biitschli) is found in the Nematode Worm,
Cucullanus. Intermediate stages are, however, to be found in other forms. It will
be noticed that in the plakula stage one surface of the embryo is epiblastic, while
the other is hypoblastic, and Biitschli compares such an embryo with the problematical organism Trichoplax adhserens [i 7 . E. Schulze ]i
 
The effect of food-yolk upon these changes has now to be considered. Though, as previously mentioned, food-yolk is of only
secondary significance, yet its presence often greatly influences the
manner of segmentation and the early development.
 
Effect of Food- Yolk, Telolecithal Segmentation. - In the formation of a gastrula by simple invagination, the pole of the oosperm
opposite the polar cells ultimately becomes the gastric region of
 
 
24
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the embryo. As might he expected, the yolk, which is merely
stored-up nutritive material, is usually almost entirely confined to
those cells which have a nutritive function, i.e., the hypoblast cells.
Ova in which the yolk is especially concentrated at one pole are
termed “ telolecithal As a matter of fact, it is generally possible
to distinguish between the two layers in the blastula before invagination commences  - the epiblast cells being smaller and more
transparent, while those of the hypoblast are larger, rounder, and
more opaque. This distinction is often to be observed at still
earlier stages ; at the stage of eight segmentation-spheres the four
upper cells may be purely epiblastic, while the four lower may be
primitive or yolk-hypoblast. According to some investigators,
even the first furrow may indicate the first epiblastic sphere ; but
the recent researches of Agassiz and Whitman show this to be
very doubtful (see p. 268).
 
It is not difficult to conceive that the distension of the hypoblast-cells with inert matter would cause them to segment with
difficulty, and this would hinder their invagination, while an increase in the amount of yolk would still further retard the process,
so that a condition might be reached in which it would be impossible for the distended hypoblast cells to be invaginated at all,
and the inertness of the large quantity of yolk would allow of only
a very few hypoblast-cells being formed. Though the epiblast has
been scarcely affected by the increment of yolk in the lower cells,
its behaviour with regard to them is necessarily modified, and since
the hypoblast cannot be invaginated, the epiblast is obliged to
grow round it (fig. 18, a-d).
 
One effect, then, of the addition of food-yolk to the ovum is to
cause the normal method of gas trula- formation by invagination
(“ embold-) to be modified into that of overgrowth (“ epiboU -). The
segmentation cavity is almost obliterated and the blastopore is
greatly reduced, and occasionally may be entirely absent as a distinct orifice (Cephalopoda).
 
In certain Prosobranch Gastropods, with a large quantity of
yolk ( e.g ., Ianthina, Fusus), the oosperm divides into two, and
again into four, large segmentation-spheres (fig. 18, A and a ) ; four
small cells are next segmented off from the upper poles of these
spheres. There are then, at this stage, four small clear epiblast
cells and four large opaque yolk- spheres. The yolk again gives
rise to four small cells (fig. 18, B and 5), and the first four epiblast
cells and the four cells just formed themselves divide, so as to
 
 
SEGMENTATION AND GASTRULATION.
 
 
constitute a group of sixteen cells resting upon four large yolkspheres (fig. 1 8, c). By further cell-division a cap of small
epiblast cells is formed, which gradually extends round the yolkspheres (fig. 1 8, c, D, and e ), leaving a small uncovered area at the
ventral pole, which corresponds to the blastopore of other forms
(fig. 1 8, d and f-bjp). The ventral wall of the archenteron (mesenteron of embryo) appears to be formed, or at least partially, by an
 
 
 
 
Fig. i8.  - Segmentation of Two Prosobranchs; to illustrate the effect of the increase of food-yolk.
 
 
A-D. Ianthina ; the epiblast cells form a cap which gradually grows round the
yolk-cells (primitive hypoblast), a-g. Fusus [ after Boibretzky ] ; a-c. surface
views from above ; d. ventral view ; e-g. sections ; bp. blastopore ; int. commencing intestine ; ms. mesoblast ; oes. stomodseum ; p.k. primitive or larval
kidney ; sh. shell gland; y, y.liy. yolk-cells or yolk hypoblast.
 
ingrowth of cells at the posterior lip of the blastopore ; the dorsal
wall is certainly produced by the formation of cells (hypoblast) by
the yolk-spheres (or primitive hypoblast). The hypoblast cells,
especially those situated in the gastric region, actively assimilate
the yolk. The blastopore, in some species at least, persists as the
mouth, the oesophagus being produced by a further ingrowth of
epiblast at that orifice. The segmentation in Nassa, as described
by Bobretzky, is somewhat different from the above.
 
 
26
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The increase in the amount of food-yolk amongst Invertebrates
culminates in the Cephalopoda, in which group segmentation
results in the formation of a cap of small cells resting upon the
large yolk. This yolk may be regarded as one immense lower-layer
segmentation-sphere distended with food-yolk, or better, perhaps,
as several fused together (primitive hypoblast). The cap of cells is
really the epiblast ; such a cap or layer of cells (one or more cells
deep) resting on the yolk is termed a “ blastoderm - Soon nuclei
make their appearance on the circumference of the yolk at its upper
pole, which nuclei, by aggregating protoplasm round themselves,
form the future hypoblast cells. This apparently anomalous proceeding is merely a masked form of segmentation, the protoplasm
of the lower cells is so enfeebled by the mass of yolk that it cannot
divide; but as segmentation must take place, the nuclei, either
alone or with a minute portion of protoplasm, travel to the
periphery, and there, by the assimilation of the yolk, build up their
cells. A similar phenomenon is common amongst Vertebrates.
 
In two great divisions of the Vermes, the Platyhelminthes and
the Chsetopods, do we find, as in the Mollusca, that the segmentation may be more or less uniform, resulting in a hollow blastula,
which further develops into a typical invaginate gastrula ; or, on
the other hand, sufficient yolk may be present to cause unequal
segmentation, the partial or total obliteration of the blastocoel and
the production of an epibolic gastrula.
 
Amongst the Piatyhelminths, Lineus (fig. 49) and Leptoplana
(fig. 50); and the Earthworm and Ehynchelmis (fig. 53) for the
Oligochseta illustrate these two types of gastrula formation.
 
Syncytial Segmentation. - Sedgwick has very recently shown
that, even later than the gastrula-stage in the development of the
species of Peripatus from the Cape of Good Hope, no definite
cell-walls are present. The embryo is, in fact, a syncytium (fig.
19). What corresponds to segmentation in other forms is here
effected by the multiplication of nuclei, which aggregate round
themselves small portions of the continuous vacuolated protoplasm.
 
The gastrula arises by a process of epibole, and is at first solid.
The archenteron (mesenteron) is simply a large vacuole within a
multinucleated mass of protoplasm.
 
It remains to be seen whether the segmentation-spheres in other
developing ova are in all cases separate cells, or whether there may
not be a direct or indirect protoplasmic continuity between all the
cells of an embryo.
 
 
SEGMENTATION AND GASTRULATION.
 
 
27
 
 
There is a considerable resemblance between such an embryo as
that given in fig. 19, B, and the parenchymula of Obelia, fig. 46, E.
 
Centrolecithal Segmentation.  - The food-yolk is not always
concentrated within the future hypoblast cells, since amongst the
Arthropoda it is usually equally divided between all the segmentation-spheres, the protoplasm of which is mainly peripherally situated.
The passiveness of the yolk, generally, not only prevents any entire
segmentation, but causes a separation to take place between the pro
 
 
A. Blastula with about sixteen epiblast cells. B. Early gastrula stage  - the
central large vacuole is the commencing archenteron. C. Completed gastrula.
 
The whole of the protoplasm is directly continuous and largely vacuolated ; the
hypoblast is at first non-nucleated.
 
toplasm and the yolk ; thus an external continuous single layer of
cells is formed, within which lies a central mass of yolk, more or
less free from protoplasm. Such ova are termed “ centrolecithal.-
 
The details of segmentation vary somewhat in the Crustacea.
In such a comparatively simple case as Callianassa (fig. 20), the
nucleus divides in the ordinary manner, without affecting the yolk,
till sixteen nuclei are produced ; by this time they have travelled
to the periphery of the oosperm, and an external protoplasmic
layer is formed, which, on the further multiplication of the nuclei,
 
 
28
 
 
THE STUDY OF EMBRYOLOGY.
 
 
becomes divided into distinct cells, thus constituting a shell of
cells entirely surrounding a solid core of unsegmented yolk.
 
 
/ 2 â–  3 4
 
 
 
6 e 7 8
 
 
 
Fig. 20.  - Segmentation of Oosperm of Callianassa subterranea. [After Mereschkovjsici.]
 
1-4. The nucleus divides into 2, 4, 8, 16, and the nuclei travel fi'om the centre
towards the surface without affecting the oosperm itself. 5-6. 16-cell stage. 5.
 
The oosperm possesses a broad external protoplasmic layer, which passes into the
central yolk, the former is raised into slight prominences, which correspond to
the underlying nuclei ; in 6 the different cells are segmented off from one another,
but not from the central yolk. 7. Further cell-division has occurred, and the
cells are cut off from the yolk. 8. A single-layered blastoderm of columnar cells
surrounds the yolk.
 
In the Fresh- water Crayfish, however, the greater portion of the
yolk itself segments, forming the so-called “ yolk-pyramids - (figs.
 
 
 
 
Fig. 21.  - Blastula and Gastrula of Fresh-water Crayfish (Astacus fluviatilis).
 
[After Reichenbach and Huxley .]
 
A. Ovum with the blastoderm, bl., not yet separated from the imperfectly
segmented yolk, v. B. Ovum in which the epiblast, ep.b., is completely separated
from the yolk, and the archenteric invagination to form the mid-gut or mesenteron, m.g., has taken place, b.p., blastopore.
 
21 and 22, a). Subsequently the nucleated peripheral portion of
each segment breaks away from the yolk-pyramids (fig. 21, b) ;
 
 
SEGMENTATION AND GASTRULATION.
 
 
29
 
 
and although the latter retain their segmentation for a long time,
they are not to be regarded as having the value of cells, but are
merely masses of non-nucleated yolk, with little, if any, active
protoplasm.
 
Amongst the Crustacea, invagination takes place at one pole of
the blastula and a gastrula is formed, the residual yolk being of
necessity contained w T itkin the segmentation-cavity. The yolk is
gradually absorbed by the hypoblast cells, which emit pseudopodial
processes for that purpose (fig. 22, e, p). Thus the primitively
 
 
 
Fig. 22. - Figures Illustrating the Development of Astacus.
 
[ From T. J. Parker after Reichenbach.]
 
A. Section through part of oosperm during segmentation. B and C. Longitudinal
sections during the gastrula stage. D. Highly magnified view of the anterior lip
of blastopore to show the origin of the primary mesoblast from the wall of the
archenteron. E. Two hypoblast-cells to show the intra-cellular digestion of yolk
spheres. F. Hypoblast-cells giving rise endogenously to the secondary mesoblast.
 
a. Archenteron ; b. blastopore ; c. central yolk mass ; ec. epiblast ; en. hypoblast; n. nuclei; p. pseudopodial process; p.ms. primary mesoblast; s.rns.
secondary mesoblast ; w.y. white yolk ; y. yolk granules ; y.p. yolk pyramids.
 
 
small hypoblast cells become greatly distended with yolk. This
is what occurs in the Crayfish, and with variations is characteristic
of the Crustacea.
 
The peculiar segmentation of most Insects may be regarded as
an extreme modification of the Crustacean type.
 
B. Chordata - Alecithal Segmentation.  - The egg of Amphioxus having very little food-yolk, undergoes entire and regular
segmentation up to the stage of thirty-two spheres. The segmentation then becomes slightly irregular, but results in an almost
t\
 
 
30
 
 
THE STUDY OF EMBRYOLOGY.
 
 
spherical blastula, the cells of the lower pole of which are slightly
larger than the remainder. This pole flattens (fig. 23, a) and
invaginates to form a wide-mouthed gastrula, in which the segmentation cavity is obliterated (fig. 23, c). The blastopore narrows
to a small orifice, and the epiblast becomes ciliated. The gastrula
elongates and its dorsal side becomes flattened. Thus the blastopore comes to have a dorso-terminal position (fig. 57, d). A pair
of “ hinder-pole mesoderm cells - early make their appearance on
the future ventral side of the lip of the blastopore ; their further
history is noticed later (p. 61).
 
Effect of Increase of Food-Yolk. - In the Chordata, as in most
Invertebrates, the yolk is stored up in the lower portion of the
oosperm, and it is consequently contained within the segmentationspheres of that pole  - in other words, within the hypoblast. These
 
 
 
Fig. 23.  - Blastula and Gastrula of Amphioxus. [ From Claus after Hatschelc.]
 
 
A. Blastula with flattened lower pole of larger cells. B. Commencing invagination. C. Gastrulation completed ; the blastopore is still widely open, and one of
the two hinder-pole mesoderm cells is seen at its ventral lip. The cilia of the
epiblast cells are not represented.
 
 
cells usually have a somewhat complicated history, especially when
greatly charged with yolk ; as the primitive hypoblast in that case
is only partially concerned in the formation of the digestive tract
of the future embryo, it is sometimes termed yolk-hypoblast or
lower -layer cells, to distinguish it from the hypoblast of the
adult.
 
The effect of the increase of yolk in the vertebrate oosperm on
segmentation and gastrulation resembles in the main that which
occurs in some Molluscs. The segmentation is unequal, and the
blastocoel is reduced in extent. The epiblast grows round the
enlarged hypoblast, and consequently the gastrulation is asymmetrical. The invagination of the hypoblast is but partial, and tends
to be increasingly reduced. The primitive blastopore of the true
gastrula stage is more and more filled up by yolk- cells (the yolk
 
 
 
SEGMENTATION AND GASTKULATION.
 
 
31
 
 
plug, fig. 24), and it becomes almost if not entirely obliterated as
an actual orifice.
 
The epiblast usually at first consists of a single layer of cells,
and its history is simple.
 
During gastrulation a definite ingrowth of hypoblast occurs at
the dorsal lip of the blastopore. This is most marked in forms
where there is but a small amount of yolk, and least so in ova
with a great deal of yolk. This hypoblast is sometimes spoken of
as invaginated hypoblast  - or, better, axial hypoblast, as it extends
along the median line of the roof of the archenteron. As this
tissue gives rise to the notochord, it is called by Hertwig and
others Chorda- ent oblast (see figs. 59-64, ax. hy .)
 
The sides and floor of the archenteron are bounded by the yolkcells in forms which have a relatively small amount of yolk. In
these ova the yolk cells which immediately bound the archenteron
are usually directly transformed into the definite hypoblast of the
digestive portion of the alimentary canal or mesenteron. These
cells may be called the digestive or gut-hypoblast, or simply
hypoblast; this is the Darm-entoblast of the Germans (figs. 60-65,
hy). These cells are distinctly different in character from the axial
hypoblast. In forms with a great deal of food yolk these cells
have a slightly different origin, as will be shortly described.
 
The remaining yolk-cells may be termed the yolk-hypoblast;
and, like the unsegmented yolk, they simply serve as pabulum for
the developing embryo.
 
In telolecithal ova with a large amount of yolk, only a small
cap of primitive hypoblast-cells is formed ; in this case these are
usually termed lower-layer cells. These lower-layer cells more or
less entirely surround the segmentation cavity, and themselves
rest upon the large unsegmented yolk (figs. 25, 26, 31).
 
The segmentation cavity or blastocoel in all alecithal ova is bounded on the one
hand by the epiblast and on the other by the hypoblast (figs. 16, 19, 23, 24). Even
in such an extreme telolecithal type as the Bird, Duval has shown that the same
condition obtains in a very early stage (fig. 29). Thus the encroachment of the
lower-layer cells round the segmentation cavity in the Elasmobranchs (fig. 26) is a
purely secondary condition of no special import.
 
As will be described in its appropriate place, the primitive
hypoblast also gives rise to the main mass of the mesoblast. It is
convenient to restrict the name of archenteron to the cavity of the
early gastrula stage, and after the formation of the mesoblast to
term the corresponding cavity the mesenteron (that is, the hypo
 
32
 
 
THE STUDY OF EMBRYOLOGY.
 
 
blastic portion of the alimentary canal comprising the pharynx,
oesophagus, stomach, and intestine). The reasons for this will presently appear sufficiently obvious.
 
The effects of the gradual increase of food-yolk in oosperms will
now be illustrated in more detail.
 
In the Lamprey (figs. 60, 6i), and slightly more so in the Newt
(figs. 58, 59), enough yolk is present to cause the cells of the
primitive hypoblast to be larger than those of the epiblast, and
to induce an asymmetrical invagination. The axial hypoblast
 
 
 
Fig. 24. - Blastula and Gastrula Stages of the Frog (Rana temporaria).
[After Gotte .]
 
 
A. early blastula stage ; B. late blastula ; C. commencing gastrula ; D. later
stage; E. completed gastrula stage, longitudinal section to one side of the
median line.
 
al. archenteron (mesenteron) ; hi. blastoccel ; blp. blastopore ; ep. deeper, and
ep'. epidermal layer of epiblast ; Tty. hypoblast ; to. dorsal, and to', ventral mesoblast ; n.p. neural plate of future brain ; y.Tty. yolk hypoblast.
 
is very distinct, and the yolk-cells forming the sides and floor
of the archenteron are transformed into the hypoblast of the
mesenteron.
 
More yolk is present in the Frog -s oosperm, but the first stages
of segmentation are only slightly affected by this increase. The
third furrow, instead of being equatorial, is nearer to the upper or
black pole of the egg (figs. 1 5 and 24) : as this pole is less burdened with yolk than the lower pole, it is only to be expected that
segmentation should be more rapid and complete there. In the
 
 
SEGMENTATION AND GASTKULATION.
 
 
o o
 
oo
 
final blastula stage, the segmentation-cavity is bounded above by
two layers of epiblast, an epidermal and an inner nervous layer,,
the latter eventually becoming three cells thick.
 
The epiblast gradually extends over the surface of the primitive
hypoblast and the uncovered portion (yolk-plug, anus of Rusconi)
is reduced to a small round white spot, entirely surrounded by the
darkly pigmented epiblast (fig. 24, hip). The posterior extremity
of the future embryo is formed by the dorsal lip of the blastopore.
At this point an ingrowth of cells occurs (fig. 24, D. hy), which
constitutes the hypoblastic dorsal wall of the mesenteron. The
ingrowth of the hypoblast continues, and a slit-like archenteron
appears between it and the yolk hypoblast. Meanwhile the
segmentation-cavity has been pushed to one side, and eventually
disappears. The gastrula in the Frog is thus formed partly by
invagination ( emhoU ), partly by overgrowth ( epiboU ).
 
In some forms (e.g., Sturgeon) the primitive hypoblast extends
up the sides of the segmentation-cavity and helps to form its roof.
 
Given more yolk, further complications would arise. Balfour has
drawn an ideal type (fig. 25), intended to illustrate the passage from
the Amphibian to the Elasmobranch gastrula. The segmentationcavity is entirely surrounded by lower-layer cells, and below these
again is the unsegmented yolk penetrated by a protoplasmic
reticulum. This is merely an exaggeration of the tendency to a
separation which occurs in the primitive hypoblast between cells
containing less from those containing more yolk. On reference to
the Frog -s ovum in fig. 24, c, a mass of smaller primitive hypoblast
cells (m) will be seen at the lips of the blastopore, which corresponds to the cap of lower-layer cells of fig. 25, A.
 
Asymmetrical invagination is assumed to occur in this ideal
type, the invaginated hypoblast forming the roof of the archenteron,
while a portion at least of its floor is derived from cells which form
round those scattered nuclei (fig. 25, b, n) which appear below the
archenteron, and which are themselves derived from the nuclei of
the primitive yolk-cells.
 
Telolecithal Segmentation and Gastrulation. - Owing to the
immense amount of yolk in the oosperm of an Elasmobranch, segmentation is only very partial. The protoplasm of the oosperm
mainly segregates to the upper pole, and here also the yolk granulesare of smaller size : this area is termed the germinal disc. A
delicate protoplasmic network extends throughout the whole of the
yolk.
 
c
 
 
34
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Segmentation commences by a groove extending nearly across
the germinal disc ; this is crossed by a second at right angles to it ;
subsequently other grooves appear, and horizontal fissures convert
these into polygonal masses, each of which is provided with a
nucleus, and is, in fact, a segmentation sphere (compare fig. 27).
Eventually a circular cap (blastoderm) of minute cells is formed,
 
 
 
Fig. 25. - Three Diagrammatic Longitudinal Sections through an Ideal
Type of Vertebrate Embryo, Intermediate in the Mode of Formation
of its Layers between Amphibia or Lamprey and Elasmobranchii.
 
[ From Balfour .]
 
al. mid-gut ; ep. epiblast ; hf. head-fold ; Tiy. hypoblast ; m. mesoblast ;
n. nuclei of the yolk ; nc. neural canal ; sg. segmentation-cavity.
 
of which an upper layer is distinctly columnar and constitutes
the epiblast, while the underlying mass of rounded or polygonal
cells is the primitive hypoblast or lower-layer cells. A cavity, the
segmentation-cavity, soon occurs within the latter. Although the
blastoderm is sharply defined from the underlying yolk, the latter
must be regarded as essentially homologous with the lower-layer
cells, the main difference being that the primitive hypoblast
 
 
SEGMENTATION AND GASTRULATION.
 
 
35
 
 
segments into definite cells in that area where there is sufficient
protoplasm, whereas in the greater portion of its mass it is unable
to segment, owing to the preponderance of food-yolk. Nevertheless, the nuclei belonging to the latter divide, and the nuclei thus
produced (figs. 25, B, c, 26, n) may be seen at the upper surface of
the so-called yolk. In process of time these free nuclei form cells,
of which some pass into the blastoderm, and others will constitute
the floor of the mesenteron.
 
At one region the blastoderm projects slightly from the yolk,
forming what is termed the embryonic rim. At this spot the
epiblast, bending round the rim, imperceptibly passes into a columnar layer (hypoblast proper), which is being differentiated from the
lower-layer cells (fig. 26). This differentiation extends anteriorly,
 
 
e.i
 
m
 
%
 
 
Fig. 26.  - Longitudinal Section through the Blastoderm ok an Elasmobranch
during Gastrulation. [Modified, from Balfour.]
 
a. archenteron (mesenteron) ; e.r. embryonic rim ; ep. epiblast ; liy. hypoblast
- the line points to the spot where the invagination occurs at the dorsal rim of
the blastopore ; 1. 1. lower-layer cells or primitive hypoblast ; m. mesoblast ;
k. nuclei of the yolk ; s.c. segmentation cavity.
 
In the corner a nucleus of the yolk is shown very highly magnified, and a portion of the protoplasmic network connected with the nucleus.
 
and a space is left between the developing hypoblast and the
underlying yolk. The embryonic rim is the dorsal lip of the
blastopore ; the anteriorly progressive differentiation of the lowerlayer cells into true hypoblast corresponds with the gastrula invagination of other types, and the cavity between the hypoblast
and the yolk is the archenteron or the future mesenteron. Those
lower-layer cells which do not participate in the hypoblast constitute the mesoblast (see p. 67).
 
The blastopore proper is situated at the posterior end of the
embryo. The blastoderm gradually extends over the yoke in every
direction except immediately behind the embryo, which thus comes
to be situated at the end of a bay or sinus. In process of time
the yolk is entirely surrounded by the blastoderm, the edges of
which unite in a linear manner (primitive streak) behind the
embryo (fig. 35, B, bl).
 
 
 
- J  -
 
 
36
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The segmentation of the Fowl -s egg corresponds sufficiently
closely with that of an Elasmobranch to obviate a description.
Fig. 27 illustrates a superficial view of the segmenting blastoderm,
and figs. 28-31 show sections at various stages of segmentation.
Duval states that the segmentation-cavity appears very early
(fig. 29) ; it is bounded above by a single layer of epiblast-cells,
and at first below by a single layer of primitive hypoblast cells ;
 
 
 
Fig. 27. - Surface Views of Six Stages in the Segmentation of a Fowl -s Oosperm.
[ From K'olliker after Coste. ]
 
 
All the eggs were taken from the lower portion of the oviduct. The shading
outside the germinal disc represents the yolk. Diameter of the germinal disc,
3 mm.
 
1. Earliest stage ; a. the first furrow. 2. Stage of four imperfect cells separated
by furrows. 3. Stage of nine meridian furrows and cross-furrows have also appeared, which divide the disc into nine large peripheral cells and seven small
central cells. 4. A later stage nuclei are to be seen in the central clearer cells ; the
cells are polygonal through mutual pressure. 5. Further stage in segmentation ;
the cells gradually decrease in size towards the centre. 6. Completion of segmentation ; the blastoderm consists of an upper layer of small cubical cells (epiblast)
and a lower-layer mass of larger cells.
 
 
but the latter soon becomes composed of several layers and the
segmentation- cavity is obliterated. The blastoderm of a newlylaid egg (figs. 30 and 31) consists of a definite epiblastic layer and
an inferior irregular mass of rounded cells, the primitive hypoblast
(lower-layer cells), which lies loosely on the yolk. In the upper
surface of the yolk free nuclei occur, which have the same significance as those of the Elasmobranch ovum, i.e., they represent
primitive hypoblast cells whose walls are not limited. After incu
 
 
SEGMENTATION AND GASTRULATION.
 
 
37
 
 
bating for an hour or two the latter mass is differentiated into a
lower stratum of flattened cells, the hypoblast proper, and scattered
mesoblast cells lying between the epiblast and hypoblast. The
 
 
Fig. 28.  - Semi - Diagrammatic
Section through a Fowl -s
Blastoderm corresponding
to No. 3, Fig. 27. [Modified from Kolliker and Balfour.]
 
B. white yolk spheres ; C. isolated yellow yolk-sphere.
 
bl. blastoderm ; w.y. white
yolk, the upper finely granular
layer of which is the seat of cellformation ; y.y. yellow yolk.
 
Fig. A. is cut off just where
the white yolk is expanding to
form the central mass.
 
 
 
imperfect cavity (sub-germinal cavity) between the hypoblast and
yolk corresponds with the archenteron of other forms.
 
Duval has figured a longitudinal section of the blastoderm of a
 
 
Fig. 29.  - Section through the Blastoderm of an Unfertilised newlylaid Fowl -s Ovum in the Blastula Stage. [After DuvaL]
 
ep. epiblast ; n. free nuclei in the yolk,
which aggregate protoplasm round
themselves to form primitive hypoblast
cells; s.c. segmentation-cavity (blastocoel). The white yolk is left blank ; it
rests upon the coarser yellow yolk, represented by dots.
 
 
 
Canary about this stage (fig. 32). The slit between the blastoderm
and the yolk is at the posterior end of the future embryo, and corresponds with the slit-like archenteric invagination of the Lamprey
 
 
 
Fig. 30.  - Semi-Diagrammatic Section through the Germinal Disc of a Fowl during
the Later Stages of Segmentation.
 
 
The central cells are the smallest owing to rapid segmentation ; the large peripheral cells of the epiblast rest directly upon the white yolk ; ep. epiblast ; 1 . 1 .
primitive hypoblast ; n. free nuclei in the yolk ; w.y. white yolk.
 
(fig. 6o) or Frog (fig. 24), or better still, with stage B, or one somewhat earlier, of the diagram of the ideal vertebrate (fig. 25). The
yolk of the Bird is clearly homologous with the yolk-cells of the
Frog ; and the surface of the yolk uncovered by the blastoderm
 
 
38
 
 
THE STUDY OF EMBRYOLOGY.
 
 
of the one corresponds with that area of the yolk-cells not surrounded by the epiblast in the other.
 
It must be remembered that the blastoderm at this stage covers
only a very small extent of the surface of the ovum, and that figures
so greatly enlarged as figs. 30-32 rather tend to give an exaggerated
idea of the relative size of the blastoderm with regard to the
 
 
 
Fig. 31.  - Portion of a Section through an Unincubated Fowl -s Oosperm. [From Klein.}
 
The thin upper layer a. (epiblast) is composed of a single layer of columnar
cells ; at the edge it rests directly upon the white yolk ; b. irregularly disposed
lower-layer cells (primitive hypoblast) ; c. the larger so-called formative cells
resting on the white yolk ; /. archenteron ; the segmentation-cavity is the nearly
obliterated space between the epiblast and hypoblast.
 
 
rest of the oosperm. The blastoderm of this stage is considerably
smaller than the central pale area in fig. 6.
 
In a surface view of the blastoderm of a newly laid Fowl -s egg,
a central transparent nearly circular space (area pellucida) is seen
surrounded by an opaque ring (area opaca). The former appearance is due to the fact that the blastoderm is separated from the
yolk by a shallow space filled with a fiuid, whereas the area opaca
 
 
 
Fig. 32. - Section through the
Blastoderm of an Unfertilised Canary -s Ovum tn the
Gastrula Stage. [After Duval.}
 
Bp. blastopore ; ep. epiblast, below which is the primitive hypoblast or lower-layer cells ; n. free
nuclei, which will form primitive
hypoblast cells.
 
 
rests on the yolk itself. The embryo is developed within the area
pellucida alone (see fig. 6). The area opaca gradually extends
over the whole surface of the ovum enclosing the yolk, its lowerlayer of primitive hypoblast gradually assimilates the enclosed
yolk. That portion of the area opaca immediately surrounding
the area pellucida develops a large number of blood-vessels and
is known as the area vasculosa. Nutritive matter is transmitted
 
 
SEGMENTATION AND GASTEULATION.
 
 
to the blood by the hypoblast of the area opaca, and by it conveyed
to all the regions of the body of the embryo.
 
To anticipate, as the embryo is being formed, an anterior, and
later a posterior, fold in the blastoderm make their appeareance,
which mark the anterior and posterior extremities of the embryo ;
they are known as the head and tail folds. The head-fold travels
backwards and the tail-fold forwards in such a manner as to constrict the embryo from off the yolk. Less marked lateral folds
also appear. Eventually the embryo is quite constricted off the
yolk, so that it is merely connected with the latter (or yolk-sac, as
 
 
Fig. 33. - Surface View of the
Pellucid Area of the Blastoderm of a Fowl of Twenty
Hours. Magnified 24 diameters. [From Kolliker after
His.]
 
Ao. area opaca ; Ap. area pellucida ; Pr. primitive streak ;
vAf. head-fold.
 
 
 
it is now termed) by a narrow stalk. The development of the
embryonic structures known as the amnion and the allantois will
be considered in another section (p. 78).
 
The Primitive Streak.  - The first noticeable sign of incubation
in the blastoderm of the Amniota when viewed from above is the
appearance of an opaque band which extends some distance forwards from the posterior margin of the area pellucida. This is the
primitive streak, and its opacity is due to the presence of a greater
thickness of cells than occurs elsewhere. Shortly after the primitive streak is formed a shallow groove (the primitive groove)
 
 
40
 
 
THE STUDY OF EMBRYOLOGY.
 
 
extends along its whole length. The area pellucida soon becomes
oval in outline, and the primitive streak assumes a more central
position (fig. 33).
 
In a transverse section through the primitive streak on its first
appearance, the blastoderm is seen to consist of an external layer
of columnar epiblast ; inferiorly there is a layer of flattened cells
(hypoblast) which extends from one yolk-wall to the other. Between those two layers is the third germinal layer or mesoblast.
On each side, especially close to the yolk or germinal wall, the
mesoblast cells are loosely heaped up, whereas in the centre they
form a dense mass, which, appearing through the epiblast, gives its
characteristic appearance to the primitive streak. In the region
of the primitive streak a fusion of the epiblast with the axial
 
 
 
Ftg. 34. - Transverse Section through the Anterior End of the Primitive Streak
of a Fowl -s Blastoderm about the Age of Fig. 34. [From Balfour . ]
 
 
Showing the rounded mesoblast cells arising from the primitive streak and the
stellate cells of hypoblastic origin.
 
ep. epiblast ; liy. hypoblast ; m. mesoblast ; pv. primitive groove ; yh. yolk of
germinal wall.
 
mesoblast always occurs (figs. 34, 43), and a complete fusion of all
the layers occurs in a limited area in some forms (fig. 43, c).
 
At a slightly later stage, on the appearance of the primitive
groove, the epiblast and hypoblast have much the same character
as before. The axial or primitive-streak mesoblast has, however,
a greater lateral extension (fig. 34), and is readily distinguishable
from the other mesoblastic cells, which have now assumed a stellate character.
 
Although, for the sake of convenience, an account of the formation of the mesoblast is relegated to another chapter, it is
impossible to avoid referring to this germinal layer in this place,
as its history is so closely connected with that of the primitive
streak.
 
 
SEGMENTATION AND GASTRULATION.
 
 
41
 
 
The changes which have occurred are briefly these. The lowerlayer cells or primitive hypoblast have become differentiated into
an inferior sheet of flattened cells (hypoblast) and an intermediate
tissue of scattered cells (mesoblast). In the mesial line behind
the future embryo, the epiblast by rapid cell-division (proliferation) has given rise to a linear mass of axial mesoblast, which later
widens out into a lateral sheet of cells.
 
Nature of the Primitive Streak.  - Very much has been
written concerning the significance of the primitive streak, but it
 
 
 
Fig. 35.  - Diagrams Illustrating the Position of the Blastopore, and the
Relation of the Embryo to the Yolk in various Meroblastic Vertebrate Oosperms.
 
A. Type of Frog. B. Elasmobranch type. C. Amniotie Vertebrate. [ From Balfour.']
 
bl. primitive streak, caused by concrescence of the lips of the blastoderm
behind the embryo ; mg. medullary or neural groove in the centre of the neural
plate; ne. blastopore; yk. part of the yolk not yet enclosed by the blastoderm.
 
 
is now generally admitted that it represents the fusion of the lips
of the blastoderm, which meet behind the blastopore.
 
The embryo develops subsequently in front of the primitive
streak, the posterior end of the one coinciding with the anterior
end of the other (figs. 100, 101). At the anterior end of the
primitive streak a pit usually occurs, which frequently perforates
the blastoderm, and corresponds to the blastopore. In the Lizard,
Weldon finds that the primitive hypoblast first takes on the
character of the permanent hypoblast at the anterior border of this
 
 
 
 
42
 
 
THE STUDY OF EMBRYOLOGY.
 
 
pit (blastopore), in this respect recalling the development of the
so-called invaginated hypoblast of an Elasmobranch. In the
primitive streak of a Lizard all the three layers are fused together.
 
Fig. 35 graphically illustrates how Balfour assumes the primitive
streak to have originated. Fig. A represents a view of a Frog -s
oosperm at a slightly later stage than Fig. 62 ; the yolk-cells are still
slightly uncovered. An Elasmobranch -s oosperm is shown at R ;
owing to the large increase in the yolk the latter is largely uncovered,
but the blastoderm gradually fuses in the middle line behind the
 
 
 
Fig. 36.  - Portion of the Blastoderm of an Abnormal Fowl -s Ovum of
Eighteen Hours - Incubation. [After Whitman .]
 
a 0. ai-ea opaca, a small portion of whscli lias alone been shaded ; a.p. area
pellucida; p.g. primitive groove; p.p.g. posterior prolongation of primitive
groove; p.s. primitive streak ; m.n. marginal notch.
 
posterior end of the embryo, so that the latter comes to be centrally
situated in the blastoderm. By an abbreviation of this process in
the Sauropsida, the primitive streak itself is developed towards the
centre of the blastoderm (fig. 35, c). This diagram indicates the
area pellucida with the developing embryo surrounded by the
area opaca, and beyond this again is the uncovered yolk. The
edge of the area opaca is often notched immediately opposite to
the posterior end of the primitive streak ; and Whitman has
described an abnormal form of a Fowl -s blastoderm (fig. 36) in
which the primitive streak extended right across the area opaca
 
 
SEGMENTATION AND GASTRULATION.
 
 
43
 
 
to the marginal notch, which is plainly a reversion to a stage
analogous to that figured in fig. 35, b.
 
Since the epiblast becomes continuous with the primitive
hypoblast at the lips of the blastopore of the Frog, it follows that
on the junction of such lips there would be a fusion of the layers :
 
 
Fio. 37.  - Section through the YolkBlastopore of Oosperm of Nightingale. [After Duval.]
ep. epiblast ; hy. hypoblast ; m. mesoblast ; y. yolk, forming a yolk-plug in the
blastopore.
 
 
 
this actually occurs in the Lizard. If a differentiation previously
took place between the mesoblast and permanent hypoblast, the
fusion of the layers would be less evident. It is then not surprising that, in such an abbreviated development of the primitive
streak as we find in the Fowl, the hypoblast is already separated
as a distinct layer (fig. 34). A comparison of a transverse sec
 
 
Fig. 38. - First Stages of Segmentation of a Rabbit -s Oosperm : Semi-Diagrammatic.
[From Quain; drawn by Allen Thomson after E. Van Beneden's description.]
 
a. two-cell stage ; b. four-cell stage ; c. eight-cell stage ; d, e. later stages of
segmentation, showing the more rapid division of the outer -layer cells and the
enclosure of the inner -layer cells ; ect. outer-layer cells ; ent. inner-layer cells ;
pgl. polar cells ; zp. zona pellucida.
 
tion of an uncoalesced primitive streak of a Nightingale (fig. 37)
with the almost completed blastopore of a Frog (fig. 62, c, and
d) will further tend to demonstrate the complete homology of the
two stages. Duval has found traces of a similar condition in some
Fowls - eggs, and the same may also be seen in a transverse section
 
 
44
 
 
THE STUDY OF EMBRYOLOGY.
 
 
of the blastopore of a Lizard. Mitsukuri and Ishikawa have very
recently described a perfectly similar stage in the Turtle (Trionyx).
 
Segmentation of the Mammalian Oosperm - Blastodermic
Vesicle.  - So far as is known, the oosperm of all the higher Mammals (Eutheria) undergo total and, at first, regular segmentation.
In the Eabbit, according to Van Beneden, the first furrow separates
what he terms the epiblast from the hypoblast ; but it will be
better, for the present, to call them, with Heape, the outer and
 
 
 
 
Fig. 39. - Sections through the Oosperm of
the Rabbit during the Later Stages of
Segmentation, showing the Formation of
the Blastodermic Vesicle. [ From Quain,
after E. Van Beneden .]
 
a. the outer-layer cells have entirely surrounded the inner-layer cells, except at one spot,
the “blastopore- of Van Beneden; b. the enclosure is complete, and fluid is beginning to accumulate to form the vesicle ; c. and d. later stages ;
ect. outer layer ; ent. inner layer ; zp. zona pellucida.
 
 
inner layer cells (fig. 38). Each sphere divides into two, and
these into two more spheres.
 
In the eight-celled stage (fig. 38, c) one inner-layer cell is more
centrally situated. Further segmentation results in a cap of
smaller, more transparent outer-layer cells surrounding a solid
mass of granular inner-layer cells (fig. 38, e). Eventually the latter
are entirely surrounded, except at one spot, the so-called “ blastopore - of Van Beneden (fig. 39, a), but this is also, rapidly closed
over.
 
The outer layer next enlarges so as to form what is termed the
 
 
SEGMENTATION AND GASTRULATION.
 
 
45
 
 
blastodermic vesicle, while the inner layer remains attached as an
irregular mass to that pole of the ovum where Van Beneden -s
“ blastopore - was situated. Later the blastodermic vesicle increases in size, and is bounded by a single layer of flattened outerlayer cells, and the inner layer forms a small disc of cells attached
to the upper side of the vesicle (fig. 39, d). In the Bat, however,
the “blastopore - of Yan Beneden is larger, and persists until there
is a considerable cavity in the blastodermic vesicle (fig. 40).
 
 
Fig. 40.  - Section through the Blastodermic Vesicle
of a Bat. [After Van Beneden and Julin.]
 
The outer-layer cells should be represented with
large granules. The inner mass consists of finely granular protoplasm with imbedded nuclei, but it is impossible to distinguish the limits of the cells.
 
 
 
It would appear that this inner mass not only gives rise to a
layer of flattened hypoblast cells by a differentiation of its inferior
surface, but that it also gives rise principally, if not entirely, to
the epiblast of the embryo. As will be immediately shown, the
inner-layer disc corresponds with the early blastoderm of other
Vertebrates, the greater portion of the outer-layer cells forming
the external wall of the blastodermic vesicle ; but they also extend
 
 
Fig. 41.  - Diagrammatic Section of a Mammalian Blastodermic Vesicle, in which the Primitive Invagination of
the Blastoderm is Rectified, and the Covering Cells
have Extended over the Blastoderm.
 
ep. epiblast of future embryo ; ep'. non-embryonic epiblast,
or the epiblast of the area opaca ; liy. primitive hypoblast ;
y.s. yolk-sac.
 
 
 
as a covering layer (Deckenschicht) completely over the blastoderm
proper. An extension of the hypoblast subsequently forms a
second layer underlying the epiblast of the blastodermic vesicle.
 
The oosperm appears at this stage (fig. 41) as a vesicle, of which
the upper half is three-layered, the layers being the covering layer,
the epiblast, and the now differentiated hypoblast (fig. 41, hy),
while the lower half consists for some time of a single layer of
 
 
46
 
 
THE STUDY OF EMBRYOLOGY.
 
 
epiblast. The covering cells, however, soon disappear, either
entering into the formation of the embryonic epiblast or become
attached to the decidua (see p. 92) ; in the latter case they would
not form any portion of the embryo proper.
 
A translucent circular patch next appears at what corresponds
with the upper pole of other oosperms (fig. 42), this embryonic
area soon becomes ovoid and is homologous with the area pellucida
of the Fowl. A primitive streak with its groove makes its appearance at the posterior end of the area. In the Mole, according
to Heape, the blastoderm is perforated immediately in front of
where the primitive streak is commencing to form (fig. 43, a) ;
later this spot is marked by a small down-growth of the epiblast,
which really corresponds with the anterior border of the blastopore.
Somewhat more posteriorly a complete fusion takes place between
the epiblast and incipient mesoblast (fig. 43, b), while at the pos
 
Fig. 42.  - Rabbit -s Oosperm Seven Days after
Impregnation. 3.47 mm. in length. Side
view deprived of its envelopes. Magnified
about 10 diameters. [ From KdlliJcer.]
 
ag. Area pellucida, or embryonic area ; ge. inferior limit of the hypoblast ; below this line the
blastoderm consists solely of a single layer of
epiblast.
 
 
terior end of the streak a complete fusion of all the layers occurs
(fig. 43, c) ; but the three layers are distinct beyond the streak'itself.
 
The similarity of a Mammalian blastoderm at this stage with
that of a Bird, or especially of a Lizard, is very striking, and it led
Balfour to propose the view that the Mammalian ovum originally
possessed a large quantity of yolk, since the blastodermic vesicle
is clearly homologous with the yolk-sac and contains a coagulable
fluid comparable to some extent with the yolk. The primitive
streak is the same structure in both Sauropsids and Mammals,
that is, it represents a vanished blastopore.
 
It has since been proved by Haacke and Caldwell that the previously known but discredited fact was true that the Monotremata
are oviparous, and that the eggs are in all essential points perfectly comparable with those of Reptiles. Thus Balfour -s deduction
from purely embryological data has been verified.
 
 
 
 
SEGMENTATION AND GASTKULATION.
 
 
47
 
 
The primitive possession and the subsequent loss of food-yolk must
be taken into consideration when dealing with the early stages of
the development of the higher Mammalia. It has already been
demonstrated how the presence of a large quantity of yolk is a
 
 
 
Fig. 43.  - Sections through the Blastoderm of a Mole (Talpa). [After Heape.]
 
A. Longitudinal section through the middle line of part of an embryonic area
in which the primitive streak has commenced to form ; the blastoderm is perforated in front of the primitive streak. B. Transverse section through the
middle of a well-developed primitive streak ; the epiblast and mesoblast are
fused, but the hypoblast is distinct ; the mesoblast here extends beyond the
embryonic area. C. Same as B, but through the hind-knob of the primitive
streak. All the layers are fused in the embryonic area, but are distinct beyond.
 
bp. blastopore; ep. epiblast; hy. hypoblast; m. mesoblast; p.sk. primitive
streak.
 
disturbing factor; the subsequent loss of this would necessarily still
further complicate matters.
 
Suggested Explanation of Mammalian Segmentation.  - Tlie following suggestions, previously published by the author, may perhaps tend to elucidate the apparent
anomaly of the process of segmentation in a Mammalian oosperm. A somewhat
similar hypothesis was independently arrived at by Minot.
 
 
Fig. 44.  - Diagrammatic Transverse Section
 
THROUGH THE BLASTODERM AND YOLK. OF
 
the Oosperm of a Hypothetical Primitive Mammal.
 
ep. epiblast of future embryo; ep'. non-embryonic epiblast, which is surrounding the yolk ;
hy. primitive hypoblast ; y. yolk.
 
 
 
The oosperm of a hypothetical primitive mammal (the Monotreme -s oosperm is doubtless very similar to this) in which the yolk is still present is represented in fig. 44.
The blastoderm, which rests upon the yolk, consists of an epiblastic layer and a mass of
lower-layer cells ; the yolk is being surrounded by the non-embryonic epiblast [ep').
 
 
48
 
 
THE STUDY OF EMBRYOLOGY.
 
 
An oosperm in which the yolk is supposed to have been lost is shown in fig. 45, a \
and, owing to its absence, the yolk blastoderm or non-embryonic epiblast has precociously completed the blastodermic vesicle, and the blastoderm has sunk into the
cavity of the now empty yolk-sac. This figure practically corresponds with the
oosperm of the Bat figured above (fig. 40).
 
The inner mass is thus composed from the first of epiblast and primitive hypoblast, and the break in the outer layer (“blastopore- of Yan Beneden) merely
indicates the passage from the yolk blastoderm or area opaca to the embryonie
blastoderm or area pellucida.
 
The increase of yolk during the evolution of a meroblastic from a primitively
lioloblastic oosperm results in a growth of the epiblast over the yolk. This also
occurs in the Monotremata ; but even after the yolk was lost this long-inherited
tendency would persist ; and since the yolk is absent, the completion of the overgrowth would necessarily be very precocious ; so it comes about that in the Rabbit
it is completed in about seventy hours (fig. 39).
 
In fig. 45, b, the epiblast has grown over the embryonic area, forming the covering
cells (Deckenzellen). Lastly, the invagination of the embryonic area is rectified
 
 
 
Fig. 45.  - Diagrammatic Transverse Sections through a Hypothetical Mammal Oosperm.
 
A. Stage corresponding to figs. 40, a, and 41. The yolk of the primitive mammalian oosperm is now lost. B. Later stage, corresponding to fig. 39, c and d.
 
The non-embryonic epiblast has grown over the embryonic area to form the
covering cells.
 
ep. epiblast of embryo ; ep'. epiblast of yolk-sac ; hy. primitive hypoblast ;
y.s. yolk-sac or blastodermic vesicle.
 
(fig. 41), and there is a double -layered oosperm, the covering cells forming the spurious third layer, which misled Yan Beneden into describing the oosperm at this
stage as consisting of the three primitive germinal layers.
 
The completion of gastrulation, which in Vertebrates with meroblastic (telolecithal) oosperms is indicated hy the appearance of the
primitive streak, marks the close of the last stage of development
which is common to all the Metazoa.
 
C. Gastrulation by Immigration and Delamination - All
the above-mentioned cases of gastrula formation may be reduced
to one common type  - invagination. There is, however, another
series of phenomena which equally result in the formation of a
double-layered from a single-layered embryo, which only occurs
amongst the Hydromedusae, and possibly in some Sponges.
 
The development of Obelia (fig. 46), which has been recently
 
 
SEGMENTATION AND GASTRULATION.
 
 
49
 
 
studied by Merejkowsky, will serve as a type. The segmentation
is regular, and results in a large oval blastula, the cells of which
are equal in size and ciliated ; the wall is also stated to be perforated by small pores. The embryo next becomes somewhat
narrowed at the posterior end.* One by one the cells at the extreme hinder end of the embryo become amoeboid and pass into
the segmentation-cavity and wander about, congregating at first
chiefly at the hinder extremity ; eventually the entire segmentationcavity is filled up by a cellular network formed by the fusion of
the pseudopodia of these endoderm cells. Metschnikoff proposes
the name “ parenchymula- for such an embryo, which is formed of
an ectodermal layer and a central solid mass of endodermal cells,
 
 
 
Fig. 46. - Formation of the Planula of Obelia. [After Merejkowsky .]
 
A. Longitudinal section of a blastula with a few scattered endoderm cells,
chiefly at the hind-end. B. Posterior extremity of a slightly earlier stage, showing the proliferation of the terminal cell ; the resulting endoderm cells immigrate into the segmentation-cavii y. C. Surface view of a small area of a blastula
with two pores. D. Section through a pore. E. Planula in which the segmentation-cavity is filled up with branched endoderm cells. F. Two-layered ciliated
planula, with a definite archentric cavity, but no mouth. After a short free life
the planula becomes fixed.
 
but without a mouth. The term “ planula - is usually applied to
this and the succeeding stage. The endoderm now applies itself
to the ectoderm as a definite layer, leaving a central cavity ; the
archenteron and the free-swimming planula is a ciliated elongated two-layered embryo, also destitute of a mouth. After a
short free existence, the planula attaches itself by its anterior end,
the ectoderm secretes a perisarc, a mouth and tentacles appear,
and the hydroid stage commences.
 
In this type the endoderm is formed by immigration, which is
positively stated to occur only at one pole of the blastula.
 
W. K. Brooks describes the planula of the Hydromedusoid
 
* The terms u anterior- and. “posterior- have reference merely to the direction
of progression of the larva.
 
D
 
 
50
 
 
THE STUDY OF EMBEYOLOGY.
 
 
Eutima as transparent and pear-shaped ; he actually witnessed the
inner ends of some of the ectoderm cells splitting off (delaminating) to form the endoderm ; this takes place most rapidly at the
small end, but endoderm cells are formed over the whole inner
surface, and they arrange themselves in a single layer one cell
thick around a central digestive cavity.
 
In the specialised Hydromedusa Geryonia (fig. 47), Eol describes
the formation of the endoderm by delamination from all the
primitive cells of the blastula ; a mouth subsequently opens into
the gastric cavity thus formed.
 
These three types appear to form a series, of which the first can scarcely be
doubted to be the most primitive ; and the formation of the endoderm by delamination may be regarded as derived secondarily from immigration.
 
 
 
Fig. 47. - Sections through Three Stages in the Segmentation of Geryonia. [After Fol . ]
 
A. Stage of thirty-two cells ; each cell is divided into an external, finely granular
layer (indicated in the figure by shading) and an inner layer of clearer protoplasm. B. Later stage, in which the outer portion of the cells has given rise to
a second cell, and the inner portions exhibit a protoplasmic reticulum. C. The
endoderm (hypoblast) has been formed by a delamination of the inner portion
of the cells ; it now encloses the alimentary cavity (archenteron). The outer
cells constitute the ectoderm.
 
In some Hydrozoa segmentation is stated to result in a solid
mass of cells (Morula), the outer layer (ectoderm) of which is next
split off from the internal solid mass. A central cavity appears
in the latter ; the cells bounding it are ultimately arranged as a
single layer of endoderm.
 
Although there is still difference of opinion on the subject, the present evidence
points to the view that immigration is closely allied to invagination, of which, indeed, it may be regarded a special form. Delamination has probably arisen through
precociousness in the formation of the endoderm.
 
D. Segmentation and Gastrulation of Sponges.  - There is so much diversity in
the development of Sponges that it is at present impossible to reduce the variations
to one common type, as can be done in other groups of animals.
 
Segmentation, which is fairly regular, results in the formation of a hollow blastula,
the further development of which varies accordingly as a planula or an ampliiblastula
is formed.
 
The Planula .  - The planula is a solid embryo consisting of an external columnar
 
 
SEGMENTATION AND GASTRULATION.
 
 
51
 
 
flagellate ectoderm and a central gelatinous substance containing amoeboid cells. On
becoming fixed the ectodermal cells are greatly flattened and lose their flagella, and
a central cavity appears lined by a distinct endodermal epithelium, which in their
turn become flagellate. The intermediate tissue persists as the mesoderm.
 
The walls of the central cavity bud off flagellate chambers into the mesoderm, and
all the endoderm, excepting that which lines the chambers, is converted into a platelike epithelium.
 
By perforations in its walls oscula and pores arise, and by various foldings of
different parts the adult stage is reached. The structure of Sponges is, as a rule,
greatly complicated by accelerated and retarded growth combined with concrescence
and imperfect gemmation.
 
The Amphiblastula .  - The amphiblastula is a hollow larva, one hemisphere being
formed of granular amoeboid cells, the other of columnar flagellate cells. The latter
(endoderm) eventually are invaginated within the former (ectoderm).
 
The hitherto free-swimming gastrula becomes attached by its blastopore. A
middle layer (mesoderm) is now developed, apparently from the ectodermal cells
[Metschnikoff], but this requires confirmation. The complications which succeed
differ according to the group to which the embryo belongs.
 
Other methods of embryo-formation have been described, but the two above
mentioned may be taken as fairly representative ; the second appears to be almost
confined to the Calcispongise.
 
In all cases the spicules are of mesodermal origin. Nerve-cells and sense-cells
have quite recently been described in a few forms by Stewart, Yon Lendenfeld, and
Sollas (p. 165), these are stated by Yon Lendenfeld to be of mesodermal origin, as
are also the unicellular glands and the muscle cells.
 
The Porifera form such a distinct and divergent group of the Metazoa that their
development appears to have no direct bearing upon that of other Metazoa.
 
 
( 52 )
 
 
CHAPTEE III.
 
FORMATION OF THE MESOBLAST.
 
As has been previously mentioned, a middle or third germinal
layer early makes its appearance in ova between the epiblast and
the hypoblast, which is known as the mesoblast or mesoderm.
 
Although the mesoblast is probably phylogenetically younger
(that is, arose later in the evolution of the primitive Metazoa) than
the gastrula stage, it not unfrequently, so to speak, is developed
precociously ; and throughout the animal kingdom the mesoblast may often be recognised very early in development. This is
why it has been unavoidable to entirely omit any reference to the
mesoblast when dealing with segmentation and gastrulation. In
reading this chapter, it must be remembered that the formation of
the mesoblast is synchronous with the phenomena previously dealt
with.
 
There has been considerable difficulty in comprehending the
nature of the mesoblast, owing to the fact of its diverse origin in
the embryos of various animals ; but, thanks to numerous recent
researches, it is now possible to arrive at a more definite conclusion.
 
It is necessary to bear in mind that two entirely distinct
structures are included under the single name of mesoblast or
mesoderm; these have been termed ‘‘mesenchyme- [Hertwig] and
“ mesothelium - [Minot]. For the sake of clearness these will be
considered apart.
 
i. Origin of the Mesenchyme.  - In the embryos of a number
of forms, amoeboid cells are budded off during the blastula stage,
either from the epiblast or the hypoblast, or from both layers.
Minot has proposed the term “ mesamoeboids - for such wandering
cells, instead of the more cumbersome titles of “mesenchyme
germs - or “ primitive mesenchyme cells - of Hertwig.
 
Mesenchyme alone is present in Sponges; the mesoderm consisting in this group of mesamoeboids derived in the adult from the
 
 
FORMATION OF THE MESOBLAST.
 
 
53
 
 
endoderm cells, although it is stated to arise from the ectoderm in
the embryo.
 
In the Coelenterata the mesoderm may be represented only by
the structureless lamella, as in Hydra ; or by gelatinous tissue in
which are scattered stellate cells (mesamoeboids) mostly of hypoblastic origin in the Scyphomedusae, and mainly of epiblastic
origin in the Ctenophora.
 
 
 
Fig. 48. - Blastula of Echinus; drawn from the living embryo. [After Metschnikoff.]
 
A. Mesamoeboids arising from the hypoblastic pole of the blastula. B. Later
stage ; the blastula is ciliated.
bl. blastoccel ; m. mesamoeboids.
 
During, or even anterior to, the invagination of the archenteron
in Echinoderms (fig. 16), mesamoeboids are budded off from the
incipient hypoblast (fig. 48). These cells wander throughout the
segmentation-cavity and adhere to all the organs as they are
formed, thus forming a mesoblastic investment.
 
In the Platyhelminth Lineus obscurus, Hubrecht has recently
shown that the mesamoeboids arise during the gastrula stage from
 
 
54
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the epiblast and hypoblast (fig. 49), but mainly from the latter ;
and it is probable that the truly mesoblastic organs are derived
solely from the latter (see p. 165). In Leptoplana, four primitive
mesoblast cells are segmented from the four yolk-hypoblast cells,
and very soon they come to be situated at the lips of the blastopore.
As the epiblast grows over the yolk-hypoblast (fig. 50), the meso
 
 
Fio. 49.  - Origin of Mesenchyme in Lineus. [After Hubrecht.]
 
A. Gastrula stage ; mesamoeboids are seen arising from the hypoblast. B.
Later stage, in which the epiblast is giving origin to mesamoeboids.
a. archenteron ; ep. epiblast ; hy. hypoblast ; m. mesoderm.
 
 
blasts, which now appear as four bands, pass to the upper pole
and obliterate the segmentation-cavity. The large amount of
yolk present in the hypoblast has clearly exerted a disturbing
influence upon the origin of the mesoblast.
 
In the Discophora the mesoblast early forms two bands, which
arise from cells which must be regarded as yolk-hypoblast.
 
 
 
 
 
Fig. 50.  - Gastrulatton and Origin of Mesoblast in Leptoplana Tremellaris. [After
Hallez. ]
 
hi. blastopore ; ep. epiblast, ciliated in C ; hy. yolk cells (primitive hypoblast) ; m. mesoblast.
 
 
Stellate mesoblast cells, which may be considered as mesencliymatous, traverse the space between the epiblast and archenteron
in the free-swimming larvse of some Polychsete Worms (Serpula)
before the true coelom is developed. Similar cells are also to be
found in the pre-oral lobe of embryo Oligochsetes.
 
 
 
 
FORMATION OF THE MESOBLAST.
 
 
55
 
 
In the Mollusca, as a whole, the mesoblast is derived from cells
intermediate in position between the epiblast and the hypoblast
(fig. 1 8), but which may be considered as belonging to the latter
rather than to the former.
 
The presence of mesenchyme in any of the higher Metazoa
must for the present be regarded as an open question.
 
2. Origin of the Mesothelium.  - Paired outgrowths from the
archenteron, which ultimately become constricted off as closed
sacs, make their appearance on the completion of the gastrula
stage in such diverse groups of the Metazoa as the following :  -
All the Echinodermata ; the Chaetognatha (Sagitta) ; Bracliiopoda
(fig. 51); Peripatus (fig. 69); Balanoglossus ; and Amphioxus
 
 
Fig. 51.  - Four Stages in the
Development of Argiope.
[After Koiccilevsky.]
 
A. early gastrula stage; B, C.
illustrating the development of
the archenteric diverticula ; C.
stage after the larva has become
divided into three segments.
 
bl. blastopore ; cce. archenteric
diverticula ; me. mescnteron ; f.
provisional setae.
 
 
 
 
(fig. 56). The cavity of these sacs will form the body-cavity or
coelom of the adult, and the walls constitute such mesothelial
tissues as the peritoneum, mesentery, muscles, and the excretory
and generative organs.
 
Amongst the Echinodermata a pair of such diverticula usually
arise from the blind end of the archenteron; sometimes only a
single vesicle is constricted off, which immediately divides into
two. The former is probably the more primitive mode. These
two sacs enlarge and lie one on each side of the archenteron
(fig. 5 2) ; the left further gives rise to the third vesicle, which by
radial prolongations develops into the ambulacral system of these
animals. The two remaining sacs eventually increase in size, so as
to fill up the whole of the segmentation-cavity. The alimentary
canal thus comes to be surrounded by the two vesicles : when
these meet each other in the middle line, their applied walls
 
 
56
 
 
THE STUDY OF EMBRYOLOGY.
 
 
become more or less absorbed, the remains forming the mesentery
of the adult, and the conjoint cavities constitute the coelom or
body-cavity proper. It will be remembered that there exists a
layer of mesenchyme between the epithelium of the body-cavity
(mesothelium) on the one hand, and the epiblast of the body-wall
and the hypoblast of the alimentary canal on the other.
 
With the exception of not giving rise to an ambulacral system,
and a possible absence of mesenchyme in some, the formation of
the coelom is practically identical in the above-mentioned forms
with that of the Echinoderms.
 
Conn has recently stated that in Serpula, which appears to have
 
 
 
Fig. 52. - Three Larval Stages of the Star-Fish (Asterias).
 
A. Late gastrula stage, with commencing archenteric diverticula. B. Coelomic
pouches constricted off. C. Early larval stage, with stomodaeum not yet opening
into the mesenteron : the left coelom has formed the rudiment of the ambulacral
system.
 
a. anus (persistent blastopore, bp .) ; amb. primitive ambulacral vesicle ;
a.v. anal ring of cilia ; arc. archenteron (mesenteron in C.) ; b.c. right, and b'.c'.
left coelomic sac ; int. intestine ; m. mouth (stomodaeum) ; ms. mesamoeboids ;
p.o. pre-oral ciliated band.
 
a more simple development than most other Chsetopods, the
mesoblast arises at the posterior end of the elongated blastopore.
At first stellate mesenchyme cells are formed which stretch across
the segmentation-cavity, and some of which enclose a small
posterior vesicle (anal vesicle). The remaining mesoblast cells
rapidly give rise to two bands of cells, one on either side of the
alimentary canal, and extending forwards to the mouth: these
‘‘mesodermal bands- segment and become hollow, thus forming
the many-chambered body-cavity, and giving rise to the usual
mesoblastic structures. In one Earthworm (Lumbricus trapezoides) the mesoblast is partly derived from “ mesoblasts - which
are distinguishable before the segmentation-spheres are arranged
 
 
FORMATION OF THE MESOBLAST.
 
 
57
 
 
into distinct layers ; but Kleinenberg inclines to the view that they
are epiblastic in origin. The mesoblasts by cell-division form a
pair of latero-ventral mesoblastic bands, which further develop as
in Serpula. As the development of the Oligochoeta is undoubtedly
abbreviated, the origin of the mesoblast is consequently liable to
be modified.
 
In the Fresh- water Oligochsete Bhvnchelmis (Euaxes), as a considerable amount of yolk is present, the gastrulation is epibolic.
The chief portion of the mesoderm arises very early from two
mesoblasts, which are derived from the primitive hypoblast cells.
The two mesoblastic bands occur at the junction of the epiblast
with the hypoblast (fig. 53).
 
There is considerable uniformity in the accounts of the origin of
the mesoblast amongst the Crustacea. It may be formed by paired
 
 
 
Fig. 53. - Gastrulation and Formation of Mesoblast in Khynchelmis (Euaxes).
[After Kowalevs/ci/.]
 
A. Section through blastula stage of twenty cells. B. Late blastula stage, with
commencing mesoblast. C. Epibolic gastrula with paired mesoblastic bands.
ep. epiblast ; hy. yolk, or primitive hypoblast ; to . mesoblast.
 
 
proliferations from the hypoblast cells of the neck of the archenteron
during gastrulation (fig. 54), or from one or a pair of cells which,
in the blastula stage, occupy a position between the future epiblast
and hypoblast, and which sink into the segmentation-cavity. It is
probable that the latter case is merely a precocious abbreviation of
the former. The presence of mesenchyme in this group is not yet
satisfactorily established, though Beichenbach has described the
development of what he terms “ secondary mesoblast - from the
hypoblast cells of the Crayfish (fig. 54, f) on the completion of the
gastrula stage.
 
The origin of the mesoblast in the Tracheate Arthropoda is
still obscure. In Insects it is partly derived from a ventral groove
of the epiblast, and in Spiders from an analogous solid keel. The
latter is probably a modification of the former process, and Balfour has homologised the mesoblastic groove of Insects with the blastopore of a vanished gastrnla stage. In both groups the mesoblast
 
 
 
A. Section through part of oosperm during segmentation. B and C. Longitudinal sections during the gastrula stage. I). Highly magnified viuw of the
anterior lip of blastopore, to show the origin of the primary mesoblast from the
wall of the archenteron. B. Two hypoblast-cells to show the intra-cellular digestion of yolk-spheres. F. Hypoblast-cells giving rise endogenously to the secondary
mesoblast.
 
a. archenteron ; b. blastopore ; c. central yolk mass ; ec. epiblast ; en. hypoblast ; 7i. nuclei ; p. pseudopodial process; p.ms. primary mesoblast ; s. ms.
secondary mesoblast ; w.y. white yolk ; y. yolk spheres ; y.p. yolk pyramids.
 
 
appears to be added to by cells arising from the yolk-hypoblast.
A pair of mesoblastic bands soon appear, much as in the Chaeto
 
 
Fig. 55.  - Diagrammatic Representation of an
ideal Gastrula Stage of an Insect at the
Time when the Archenteric Diverticula
are Formed. [After 0. and R. Hertwig.]
 
bl. blastopore ; ep. epiblast ; hy. mesenteric
hypoblast ; m. parietal or somatic layer of mesoblast  - between this and the hypoblast is the
visceral or splanchnic layer of mesoblast ; 71 . nerve
cord ; y. yolk-cells or primitive hypoblast.
 
 
pods, which similarly segment, each segment containing a portion
of the coelom.
 
In all the invertebrate groups the mesoblast mainly arises from cells which grow
inwardly from the lip of the blastopore. In closely allied forms the primitive cells
 
 
FORMATION OF THE MESOBLAST.
 
 
59
 
 
vary from an apparently epiblastic to an apparently hypoblastie, or to an intermediate place of origin. The extreme variations may be neglected, as being in all
probability of only secondary significance.
 
Since this was in type Sedgwick has shown that the somites in Peripatus (fig. 69)
do not directly arise as archenteric diverticula, but are separated from a pair of
mesoblastic bands as in Chsetopoda. The somites are at first ventro-lateral in
position, but soon acquire a dorsal extension and divide into two parts. The dorsal
parts come into contact above the enteron, but do not unite with their fellows ;
anteriorly they are early obliterated, but persist posteriorly as the generative glands.
The ventral moieties remain distinct, and consist of a small vesicle situated in the
base of the appendages, leading from which is a small coiled tube (nephridium),
which acquires an external opening. The Hertwigs have interpreted the formation
of the mesoblast in Insects in terms of archenteric diverticula (fig. 55), but the
undoubtedly primitive character of Peripatus renders its development especially
important. Although the cavities of mesoblastic bands and archenteric diverticula
are homologous, their exact relation to one another is somewhat obscure.
 
Whatever views may be held as to the precise position of the Chaetognatha,
Bracliiopoda, and Balanoglossus, the presence of archenteric diverticula in these
 
 
 
Fig. 56.  - Transverse Sections of Embryos of Amphioxus. [After Hatsche/c.]
 
A. Section through the first somite or primitive segment of an embryo in
which the fifth somite is being formed. B. Section through the same region of
an embryo with eight somites. C. Section through the centre of the body of an
embryo with eleven somites.
 
al. mesenteron ; be. coelom ; m. muscle fibres ; n. neural plate and canal ;
nch. notochord.
 
 
forms proves that it occurred in several of the primitive Worms ; so it maybe safely
concluded that the mesoblast (for the most part, at all events) of the Gepliyrea,
Polyzoa, and Nematoda belongs to this category.
 
It will probably be shown that mesothelial mesoblast occurs also in all Mollusca.
It is probable that the pericardium of this group represents the true body-cavity of
other orders ; but even if this is the case, there would be a marked preponderance of
mesenchyme over mesothelium in the mesoblast.
 
There are not sufficient data to come to a definite conclusion concerning the exact
nature of the mesoblast of the Platy helminths.
 
 
Origin of the Mesoblast in the Chordata.  - There appears
to be no valid reason for refusing to accept Bateson -s conclusion
that Balanoglossus is a persistent representative of an early stage
 
 
 
60
 
 
THE STUDY OF EMBRYOLOGY.
 
 
in the evolution of the Chordata from the unsegmented Worms.
He has extended the observation of others that the mesoblast in
this remarkable form is derived from archenteric diverticula in
a manner very similar to that which is characteristic of the
Echinodermata. But the details of mesoderm formation in this
form and in the Tunicata must be passed by.
 
In Amphioxus the formation of the mesoblast is of remarkable
simplicity. The development of this form (p. 29) was traced to
 
 
 
Fig. 57.  - Three Larval Stages of Amphioxus.
 
[From Claus, after Hatschek.]
 
D. Stage with two somites (primitive segments), seen in optical longitudinal section.
 
E. Stage with nine somites, seen from
above, showing the asymmetry of the segments. F. Living larva with mouth and
first gill-slit, seen from the left side; the
second, fourth, and sixth bent lines represent respectively the posterior boundary
of the first, second, and third somite of the
opposite (right) side.
 
Bl. ventral blood-vessel ; Ch. notochord ;
 
D. intestine ; K. gill-slit ; MF. unsegmented mesoderm fold ; N. neural canal ;
 
0. mouth ; Oe. anterior orifice of neural
canal ; Us. somites.
 
 
an elongated gastrula stage with a dorso-posterior blastopore.
Two small pouches soon arise from the archenteron (fig. 56, a)
near the anterior end of the embryo, one on each side of the
median dorsal line. These are followed by others, which are
successively developed from before backwards (fig. 57, D, e). These
extend laterally along the dorsal side of the embryo ; but, as seen
in fig. 57, E and F, they are not placed symmetrically opposite one
another.
 
 
FORMATION OF THE MESOBLAST.
 
 
61
 
 
The archenteric diverticula very shortly become constricted off
from the archenteron, or mesenteron, as it should now be termed
(fig. 56, b), and form a series of closed sacs (mesoblastic somites or
primitive segments). Each somite encloses a distinct cavity or
coelom. The somites gradually extend in a ventral direction,
enclosing the alimentary canal (mesenteron) (fig. 56, c) ; and by
the subsequent fusion of their cavities form the small coelom or
body-cavity of the adult.
 
The outer layer of the somites is known as the somatic or
peripheral mesoblast, the inner layer being termed the splanchnic
or visceral mesoblast. The dorsal moities of the somites lose their
cavities, and become transformed into the great lateral muscle of
the larva and adult ; but the primitive segmentation is permanently
retained.
 
It is readily apparent (fig. 56, a) that the mesoblast is derived
from two regions of the hypoblast. The ventral layer is continuous
with the digestive portion of the hypoblast ; while the dorsal half
is derived from the axial hypoblast. The remainder of this latter
is converted into the notochord (fig. 56, B, nch). The separation of
the somites and the notochord from the archenteron appears to be
due to the dorsal growth and coalescence of the digestive hypoblast
below these structures. The cavity of the archenteron equals that
of the mesenteron + the coeloms of the mesoblastic somites.
 
There would seem to be no mesenchymatous elements in the
mesoblast of Amphioxus, unless the pair of “hinder-pole mesoderm
cells- (fig. 23) are to be regarded as such. They arise from the
hypoblast at the ventral lip of the completed gastrula, and are
stated by Hatschek to give rise solely to the caudal mesoderm.
 
The origin of the mesoblast in the Newt (Triton) is very instructive, as it serves to elucidate the formation of the mesoblast in
Eeptilia and to reduce the latter to the type of Amphioxus. On
the completion of the gastrula stage the mesoblast is only to be
found close to the blastopore (fig. 58). The main portion grows
out as a pair of lateral sheets dorsal to and at each of the blastopore (fig. 58, B, m). The brothers Hertwig at first described
the mesoblast as composed from the commencement of two
distinct layers, the outer growing from the epiblast of the lips
of the blastopore, and the inner from the primitive hypoblast.
Each lateral sheet is, however, at first a solid mass of cells, which
gradually extends forwards and downwards, i.e., anteriorly and
ventrally. According to Scott and Osborn, the lateral meso
 
62
 
 
THE STUDY OF EMBRYOLOGY.
 
 
blast also increases at the expense of the yolk-hypoblast. The
mesoblastic sheets very early split into two layers, an external
somatic and an internal splanchnic. The cavity between the two
layers extends ventralwards, and forms the body-cavity or coelom.
The anterior extension of the paired or dorsal mesoblast appears
 
 
 
Fig. 58.  - Late Gastrula Stage of the Newt (Triton). [After 0. Hertioig.]
 
A. Median vertical longitudinal section. B. Horizontal section through the
same.
 
a. archenteron ; bp. blastopore ; d.l. dorsal lip of blastopore ; ep. epiblast ; hy.
dorsal or axial hypoblast ; l.l. lateral lip of blastopore ; m. dorsal mesoblast ; v l.
ventral lip of blastopore ; v.m. unpaired ventral mesoblast ; y.hy. yolk-hypoblast.
 
to occur at the expense of the hypoblast, in a similar manner to
that described for Amphioxus (fig. 59). Hertwig describes the
dorsal layer as arising from the “ Chorda-entoblast - (axial or
notochordal hypoblast), and the ventral from the “Darm-entoblast - (digestive or gut hypoblast).
 
Fig. 59.  - Transverse Section of the Dorsal
Portion of an Embryo
Newt (Triton). [After
0. Hertwig .]
 
a. mesenteron; ax.hy.
axial hypoblast in process
of forming the notochord ;
b.c. coelom (body -cavity) ;
ep. epiblast ; hy. digestive
hypoblast ; n.p. neural
plate ; so.m. somatic mesoblast ; sp.m. splanchnic
mesoblast.
 
 
In the Newt, and all the higher Chordata, as in Amphioxus,
the axial hypoblast or notochord is in direct contact with the
neural epiblast, consequently the dorsal mesoblast is distinctly
paired. There is a ventral growth of unpaired mesoblast from the
lower lip of the blastopore (fig. 58, A, v, m). This occurs at the
 
 
 
FORMATION OF THE MESOBLAST.
 
 
63
 
 
spot where the epiblast and hypoblast pass into each other, and it
is difficult to say which layer has the larger share in its formation ;
if either, it is perhaps the epiblast.
 
The formation of the mesoblast in the Lamprey is, according to
Calberla, practically identical with that in the Newt; in some
 
 
 
Fig. 6o. - Late Gastrula Stage of Lamprey. [After Scott.]
 
A. Median longitudinal vertical section. B. Section to one side of A.
 
The relation of the hypoblast to the mesoblast is more clearly seen in fig. 61.
a. archenteron (mesenteron) ; bp. blastopore ; ep. epiblast ; hy. hypoblast of
mesenteron, axial hypoblast in A. ; m. paired mesoblast ; m' ventral unpaired
mesoblast ; y.liy. yolk-hypoblast.
 
 
respects it is simpler than in the latter, owing to less food- yolk
being present in the ovum. The position of the paired mesoblast
is clearly shown in figs. 60 and 6 1. The two latero-dorsal sheets
extend from the lip of the blastopore some distance forwards, but
 
 
 
Fig. 6i.  - Transverse Sections through the Upper Portions of Two Embryo Lampreys
(Petromyzon planeri). [After Calberla .]
 
 
A. Same stage as fig. 63. B. Later stage.
 
a. archenteron ; ax.hy. axial (notochordal) hypoblast ; ep. epiblast ; hy. (digestive) hypoblast ; m. mesoblast ; n.k. neural keel ; n.p. neural plate ; y.h. yolkhypoblast.
 
 
they have not yet acquired any lateral or rather ventral extension ;
dorsally they are separated from one another by the axial hypoblast (figs. 60, A, and 6 1, a).
 
According to Scott, a single layer of mesoblast (fig. 60, m')
surrounds the lateral and ventral surface of the yolk-hypoblast
 
 
, N
 
 
64
 
 
THE STUDY OF EMBRYOLOGY.
 
 
from which it is derived ; he also states that the paired mesoblast
grows forward from the blastopore, and that it does not exhibit
any intimate relation with the axial hypoblast.
 
In his recently published paper, Shipley states that in Petromyzon fluviatilis the first formation of the mesoblastic plates
appears to take place by a differentiation of the hypoblastic yolkcells in situ , and not from invaginated cells. The subsequent
downward growth is brought about by the cells proliferating along
the free ventral edge of the mesoblast ; these cells then growing
 
 
 
Fig. 62.  - Origin of Mesoblast in the Frog. [After 0. Hertwig .]
 
 
A. Median longitudinal vertical (sagittal) section through a gastrula with a
wide blastopore. B. Enlarged view of a portion of the same. C. Horizontal
(frontal) section through a stage similar to that of A. D. Lateral lip of a
corresponding stage. E. Horizontal section through a nearly closed blastopore.
 
F. Section through the anterior lip of a closed blastopore.
 
a. archenteron ; ax.liy. axial hypoblast; be. blastocoel ; bl. blastopore; d.l.
dor~al, l.l. lateral, v.l. ventral lip of blastopore; ep. epiblast ; hy. hypoblast;
m. lateral mesoblast ; v.m. ventral mesoblast; y. Ivy. yolk hypoblast.
 
ventralwards push their way between the yolk-cells and the epiblast.
 
In the Frog the mesoblast has a fundamentally similar origin to
that above described, but the invagination of the mesoblast is less
marked. The greater portion of the mesoblast is apparently derived by the metamorphosis of the small cells of the yolk-hypoblast in situ (figs. 24 and 62) ; the result being that there is very
early a sheath of mesoblast, one or more cells thick, below the
epiblast. The mesoblast is only interrupted along the median
dorsal line. The explanation of figs. 24 and 62 sufficiently illus
 
 
 
 
FORMATION OF THE MESOBLAST.
 
 
65
 
 
trate tlie character of the mesoblast of the Frog on its first
appearance.
 
Mitsukuri and Ishikawa have very recently shown that in
the Turtle (Trionyx) the formation of the mesoblast closely recalls
the same process in the Newt. Fig. 63, which represents a transverse section through the hind-portion of the head, demonstrates
the paired mesoblast as arising by proliferation from the hypoblast
at the spot where the digestive hypoblast is contiguous with the
 
 
 
Fig. 63.  - Mesoblast of Trtonyx. [ After Mitsukuri and Ishikawa .]
 
A. Transverse section through the head region before the closure of the neural
groove. B-D. Portions of successive sections of the same embryo.
 
am. amnion; ax.hy. axial hypoblast; Ip. a. epiblastic and hy.a. hypoblastic
layer of amnion ; liy. hypoblast ; m. mesoblast ; nc. neural canal ; ncli. notochord.
 
axial or notochordal hypoblast. In this case, as in so many other
instances, the proliferation may be regarded as a degenerate form
of invagination.
 
Behind the blastopore the mesoblast arises, as in Amphibia, as
an unpaired mass, and in this region there is a fusion of the
three germinal layers, thus forming a primitive streak.
 
The formation of the mesoblast in the Lizard (fig. 64) is intermediate between that which occurs in the Turtle and the Fowl.
The paired mesoblast has much the same origin as that to be
 
E
 
 
66
 
 
THE STUDY OF EMBRYOLOGY.
 
 
shortly described for the Fowl. It arises posteriorly from the
walls of the blastopore as a pair of lateral sheets, which are free
for the greater portion of their extent, but are fused in the median
line of the posterior region of the embryo with the axial hypoblast. Anteriorly the mesoblast is derived from branched cells,
 
 
 
Fig. 64. - Transverse Section through a Portion of the Blastoderm of a Lizard
(Lacerta Muralis). [Ajter Weldon.]
 
The section illustrates the double origin of the mesoblast in the embryonic
region, i.e., in front of the primitive s' reak.
 
a. mesenteron ; ax.hy. axial hypoblast, which is about to develop into the notochord ; ep. epiblast ; hy. hypoblast ; m. mesoblast, partly derived from the axial,
and partly from the permanent.hypoblast ; n.f. neural fold ; n.g. neural groove.
 
which are budded off partly from the axial, and partly from the
lateral hypoblast (fig. 64, m).
 
The origin of the mesoblast has been very carefully studied in
Birds. One portion of the mesoblast arises as a pair of lateral
plates by the proliferation of the epiblast along the line of the
 
 
 
Fig. 65.  - Transverse Section through the Anterior End of the Primitive Streak
of a Fowl -s Blastoderm about the Age of Fig. 34. [ From Balfour.]
 
Showing the rounded mesoblast cells arising from the primitive streak and the
stellate cells of hypoblastic origin.
 
ep. epiblast ; hy. hypoblast ; m. mesoblast ; p.v. primitive groove ; yh. yolk of
germinal wall.
 
primitive streak (fig. 65). Balfour even says that during this
period many sections through the primitive streak give an
impression of the mesoblast being involuted along the lips of a
groove. A second portion of the mesoblast is that which gives
rise to the lateral plates of mesoblast in the head and trunk of the
 
 
FORMATION OF THE MESOBLAST.
 
 
67
 
 
embryo. This is formed of stellate cells, which are at first readily
distinguishable from the rounded cells of the former class ; they
arise from the hypoblast mainly on each side of the median line,
and especially in the region in front of the primitive streak ; in
other words, in the embryonic region. They are continuous behind
with the lateral wings of mesoblast which grow out from the
primitive streak, and on their inner side are also at first continuous
with the cells which form the notochord.
 
The third portion of the mesoblast is derived partly from those
cells of the lower-layer cells which do not form the permanent
hypoblast, and which are scattered between that layer and the
epiblast (figs. 30-34), and partly from the germinal wall, or that
ridge of cells, nuclei, and yolk-granules which in the early stages
 
 
 
Fig. 66.  - Section through the Germinal Ridge of a Fowl -s Blastoderm. [After Kcllmann.]
 
a. archenteron ; ep. epiblast ; hy. hypoblast ; m. mesoblast cells (mesamceboids
or “ Poreuten -) which have been derived from the primitive hypoblast cells of the
germinal ridge ; y. yolk ; y . yolk-spheres ingested by the primitive hypoblast.
 
of incubation forms the marginal boundary of the lower -layer
cells or primitive hypoblast (figs. 65, 66). The large primitive
hypoblast cells of the germinal wall are undoubtedly nutritive
in function, and ingest the underlying yolk. By cell-division
they give origin to amoeboid wandering cells (fig. 66, m), which
are stated by Kollmann to form the primitive vascular system, the
blood, and also the connective tissue. In either case, the cells have
the same morphological value since they are derived from lowerlayer cells before the hypoblast proper is differentiated.
 
While the paired mesoblast referred to above is clearly mesothelial in character, the mesoblast which arises from the lowerlayer cells and the germinal wall appears to be mesenchymatous
in nature.
 
The development of the mesoblast in the Mole (Talpa) (fig. 67)
has been shown by Heape to agree very closely with that de
 
68
 
 
THE STUDY OF EMBRYOLOGY.
 
 
scribed above for Birds. Posteriorly the mesoblast arises where
the epiblast and hypoblast are fused at the primitive streak, and
clearly owes its existence to both. In the region in front of the
primitive streak the mesoblast is proliferated from the hypoblast
as two lateral masses which posteriorly unite with the abovementioned mesoblast. There also appears to be an actual continuity between the developing notochord and the dorsal portion
of the paired mesoblast.
 
There is some diversity of opinion amongst other investigators concerning the
origin of the mesoblast amongst Mammals. It may be concluded that the Mole,
being an Insectivore, would probably not have a very specialised development for
 
 
 
Fig. 67.  - Sections through the Blastoderm of a Mole (Talpa). [After Heape.\
 
A. Longitudinal section through the middle line of part of an embryonic area
in which the primitive streak has commenced to form ; the blastoderm is perforated in front of the primitive streak. B. Transverse section through the
middle of a well-developed primitive streak; the epiblast and mesoblast are
fused, but the hypoblast is distinct; the mesoblast here extends beyond the
embryonic area. C. Same as B, but through the hind-knob of the primitive
streak. All the layers are fused in the embryonic area, but are distinct beyond.
 
bp. blastopore ; ep. epiblast ; hy. hypoblast ; m. mesoblast ; p.s/c. primitive
streak.
 
a Mammal, and, for the present, the above statement may be regarded as holding
good for Mammalia generally.
 
When the embryology of the Prototheria (Ornithodelphia) is investigated, it will
doubtless be found to resemble that of the Lizard in many points, and will demon*
strate that any peculiarities in the development of Mammals is due first to the
presence, and secondly to the subsequent loss, of food-yolk.
 
Although in most Vertebrates the mesothelial mesoblast is at first solid, it very
shortly splits into two layers, a peripheral or somatopleur, and a visceral or splanchnopleur (figs. 59, 71). The pleuro-peritoneal cavity or coelom thus produced is
strictly homologous with the persistent body-cavity of such forms as have hollow
archenteric diverticula.
 
It is evident from the foregoing summary that the derivation of the true bodycavity or coelom from archenteric diverticula occurs in one or more examples of
 
 
FORMATION OF THE MESOBLAST.
 
 
69
 
 
nearly all the main groups of the animal kingdom. In the majority of cases it occurs
in generalised, or, geologically speaking, in ancient types. It may then be safely
concluded that this is the primitive method of the formation of the coelom. This
statement does not preclude the possibility of interstitial spaces or cavities occurring,
as in Platyhelmintlies, Arthropoda and Mollusca ; but these, not being lined by an
epithelium derived from the archenteron, should always be distinguished as pseudoccelous or archiccelous cavities, as opposed to a true body-cavity. It is known that
mesodermal (mesenchymatous) cells bounding a pseudoccel (archiccel), or cavities
derived therefrom, may sometimes become flattened and form an endothelium,
t - There can be no doubt that the lateral sheets of mesoblast of Vertebrates with
telolecithal ova are identical with the mesoblastic somites of Amphioxus, and the
latter again with the archenteric diverticula of many Invertebrates.
 
A very instructive series can be traced from such an alecithal ovum as that of
Amphioxus through the Lamprey, Newt, Frog, Turtle, and Lizard, to the extreme
telolecithal type of the Bird. The lateral proliferation of the hypoblast in the
Lizard and Fowl (figs. 64, 65) to form the mesoblast is possibly a secondary process.
Throughout this series the axial hypoblast takes its share in formation of the paired
mesoblast along with what has been spoken of as the digestive hypoblast. %
 
The primitive-streak mesoblast, as it is termed, is the equivalent of the mesoblast
which arises from the lips of the blastopore ; as, for example, in the Newt. A
reference to the section dealing with telolecithal gastrulation and the nature of the
primitive streak will render further comment needless.
 
Allusion has previously been made to the origin of certain indifferent mesoblast
cells from the primitive hypoblast, which appear to differ in character from the
former, and which have been regarded as being mesenchymatous in nature.
 
Summary.  - The following is a brief resume of the mesoblastic
elements of the Metazoa.
 
Mesamoeboids arise, apparently indiscriminately, from the endoderm (hypoblast) of larval and adult Sponges, and from the same
layer in Coelenterates. The cells which migrate from the ectoderm
into the gelatinous tissue in the latter group are practically epiblastic
mesoderm. In most of the Coelenterates arch enteric diverticula are
found, which never become separated from the alimentary canal.
 
In the Echinoderms, mesamoeboids arise in the blastula stage,
chiefly, if not entirely, from the incipient hypoblast ; and after the
formation of the gastrula, archenteric diverticula arise, which become completely shut off to form the body- cavity of the adult.
 
The mesamoeboids of the Platyhelminths are derived, in some
cases, at all events, from both layers of the gastrula.
 
The exact nature of the mesoblast of Molluscs has not yet been
satisfactorily demonstrated.
 
The Arthropods and their ancestors, the Segmented Worms,
possess an enterocoelous body-cavity, although, in the great
majority of cases, its method of development masks its real
nature. The presence of mesenchymatous mesoblast in these
groups has been questioned.
 
 
70
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Lastly, in Vertebrates the mesothelial mesoblast is extremely
well developed ; according to some investigators, mesenchyme is
also present.
 
It is worthy of note that mesenchyme is certainly phylogenetically older than mesothelinm, and that those requirements which
caused it to first develop may have continually recurred ; so that
whereas all the tissues or organs derived from mesothelium are,
to a certain extent, homologous, those composed of mesenchyme
may not necessarily be so.
 
In this connection it is interesting to note that, according to
Sedgwick, at an early stage in the development of Vertebrates,
most of the connective tissues of the wall of the body and gut are
derived by a process of growth outwards of cells from the epithelium of the body-cavity. The same, he believes, holds good for
the connective tissue and blood-vessels of the Wolffian body.
 
Ccelomic cavities. -Sedgwick has very recently drawn attention
to the history of the cavities enclosed by mesothelial mesoblast.
He finds from his researches on Peripatus that it is probable that
throughout the Arthropoda the cavity of the body and all the
vascular spaces are pseudoccelous. The lumen of the generative
organs is in all cases coelomic, as is also the nephridial apparatus of
Peripatus. The excretory organs of other Arthropoda require
re-investigation.
 
In Mollusca the pericardium, nephridia and possibly the ducts
of the generative organs are coelomic. The vascular system and all
the lacunae in the body are pseudoccelous.
 
In the Chaetopoda and Chordata the cavity of the body is entirely
coelomic, and from its walls are derived the nephridia and the
generative organs. The pseudocoel (archicoel) is only represented
in the adult by the complicated system of vascular channels.
 
 
( 71 )
 
 
CHAPTER IV.
 
GENERAL FORMATION OF THE BODY AND DEVELOPMENT OF THE
EMBRYONIC APPENDAGES.
 
The three germinal layers, the development of which lias now
been traced, constitute the rough material, so to speak, for the
further building up of the embryo. No new formative tissue will
make its appearance, and it now remains to follow the further
history of these layers. Before this can be done in detail, it is
necessary to gain some idea concerning the formation of the
embryo as a whole, and of some of the various secondary structures which are often associated with larval or foetal life.
 
In all those forms possessing a small amount of food-yolk, such
as the Ccelenterata, Echinodermata, most Vermes, a few Mollusca,
Amphioxus, Lamprey, and Amphibia, the embryo lias been carried
to a stage which may roughly be stated to consist of an oval
or rounded body with usually a single layer of epiblast. The
primitive stomach or archenteron is lined with a single layer of
hypoblast, and opens to the exterior by the usually posteriorly situated blastopore. The archenteron is more or less surrounded by
mesoblast, which, as has just been shown, may have a single or a
multiple origin.
 
Ccelenterata  - Radial Symmetry. - The Hydroids may, in
general terms, be said never to advance beyond this stage. In
the fixed forms, which may be regarded as tentaculate gastrulse,
the mesoblast is merely represented by the inconspicuous structureless lamina, the gelatinous tissue of the medusoid forms with
its stellate cells, clearly having relation to their mode of life. The
development of many Hydroids is obscured by abbreviation.
 
The Actinozoa can also be briefly dismissed, but they arrive at a
higher stage of evolution than the Hydrozoa. A further ingrowth
of epiblast takes place at the blastopore, so that a mouth and
oesophagus lined by epiblast are formed. Such an epiblastic ingrowth is known as a stomodseum. The walls of the body are
 
 
72
 
 
THE STUDY OF EMBRYOLOGY.
 
 
further symmetrically and bilaterally infolded. The cavity of the
body (archenteron) is thus divided into a number of diverticula or
pouches, separated by mesenteries, which primarily extend to the
wall of the depending oesophagus. The epiblast (ectoderm) does
not enter into the mesenteries.
 
The Actinozoa have advanced beyond the purely gastrula stage
by acquiring a stomodseum and persistent archenteric diverticula.
The compound and skeleton-producing forms exhibit no real
advance upon this plan. Hseckel maintains that the Scyphomedusse and Actinozoa are offshoots from a primitive branch
(Scvphopolypi) of the Coelenterata.
 
 
 
ig. 68. - Ideal Section through the Long Axis of a Sea-Anemone.
 
The sides of the mouth and oesophagus are supposed to be appressed together,
leaving only the two extremities open, which, in this case, form two channels of
communication with the temporary stomach.
 
b.c. inter-mesenterial chambers or body-cavity: m. edge of mesentery; ces.
oesophagus or stomodteum ; st. temporary stomach, formed by the contiguous
upper digestive edges of the mesenteries ; t, t. axial tentacles in longitudinal
section. The mesoderm is merely represented by the line between the ectoderm
and endoderm.
 
It is a very significant fact that, so far as is known at present, digestion takes
place in the Actinozoa only by means of the enlarged edges of the mesenteries.
When food is introduced into the body, the edges of the mesenteries close round it
and thus form a temporary stomach, which, for the time being, is cut off from the
archenteric diverticula. This “stomach- communicates with the exterior by the
elongated mouth. The latter is often temporarily constricted at the sides, merely
leaving an orifice at each end, which simulates a mouth and anus, as shown in fig. 68.
Wilson and others, appreciating these facts, have speculated upon the possible origin
of the higher Metazoa from such a primitive form.
 
Formation of Body-Cavity.  - In the Echinodermata a distinct
advance in structure is made consequent upon a free as opposed
to a sessile existence. Owing, probably, to the hypoblast actually
lining the wall of the body in the Actinozoa, the gastric pouches
 
 
 
GENERAL FORMATION OF THE BODY, ETC. 73
 
can only be formed by ingrowths of the hypoblast and mesoblast
into the archenteron. There is, however, in the Echinodermata
(fig. 52) a large space, the segmentation-cavity, between the
archenteron and the body-wall (epiblast now being lined by
mesamoeboids). Thus archenteric diverticula can oe directly
formed, and, being developed, can surround the archenteron. This
method of forming a true body-cavity is also characteristic of all
the ccelomatous Metazoa, although it may be greatly modified
and abbreviated. The actual formation of the body-cavity in
representative examples of the Metazoa has already been briefly
described.
 
Metamerism.  - When only a single pair of archenteric diverticula are formed, the animal is, in the true sense of the term,
unsegmented. But usually a considerable number of diverticula
appear, either directly from the archenteron, as in Amphioxus
(fig. 57), or indirectly from the lateral mesoblastic bands, the
abbreviated but usual method (p. 56). These forms are termed
segmented, and the segments may remain more or less distinct
(Chaetopod Worms) or become almost obliterated (Vertebrates).
 
The question of metameric segmentation is too intricate a one to be
here discussed. It must suffice to point out that, while externally
unsegmented, many Platyhelminth Worms have a repetition of
their internal organs, especially in the case of the gastric diverticula and the generative glands. In the Chsetopoda the body is
divided into a large number of mesoblastic somites, and more or
fewer of the organs may be implicated in this metamerism. In
the great majority of Arthropods the segmentation tends to become obscured  - it only affecting the exoskeleton, the appendages,
the muscular system, and the nervous system.
 
The metamerism of the Chordata has many peculiar features, as
several important organs are unaffected by it, and others only
partially so. The neural plate and notochord always appear very
early, and are from the first unsegmented. Whether primitively
segmental or not, the nerves would necessarily acquire a serial
position if the muscles were segmented. The segmentation of the
vertebral column is unquestionably a secondary phenomenon. As
Bateson points out in dealing with the ancestry of the Chordata,
the segmentation of the gill-slits has been acquired within the
group of the Chordata, as nothing resembling them occurs outside
it. The liver is from the first a single structure (e.g., Amphioxus
upwards), and never shows any indication of having a paired
 
 
74
 
 
THE STUDY OF EMBRYOLOGY.
 
 
or multiple origin. Although the mesodermal segmentation of
Amphioxus is so marked (figs. 5 6, 57), the metamerism of the
Chordata is really very partial, and there is insufficient evidence in
support of the view that the Chordata were derived from segmented ancestors ; the converse proposition is perhaps more in
accordance with the facts. Hubrecht has brought forward numerous arguments in favour of his belief that the Nemertean Worms
and the Chordata arose from a common stock. Dohrn and
Semper are the leading advocates of the Annelidan ancestry of
the Chordata.
 
Bilateral Symmetry.  - Metamerism and bilateral symmetry
are the results of the progression of the animal in a determinate
direction, and this also induces the development of paired ambulatory appendages and the specialisation of an anterior region or
head, and conversely of a passively following region or tail.
 
It is as a result of the different impressions made upon them,
and of their response to these stimuli, that the different regions of
the body possess such marked and constant characters.
 
 
When an animal is sessile, external influences may act upon it equally in every
direction, and in response to these the animal acquires a radial symmetry ; but when
an animal progresses in a definite direction, the two sides of the body will be subject
to somewhat different conditions from those affecting the extreme anterior extremity;
On the development of distinct muscles to assist in progression, the stress of the
muscles would probably make the bilateral symmetry more marked. It is also evident that there would accrue a distinct advantage to the organism if the muscles
were symmetrically situated and were of comparatively short length, as by this means
they could act in concert or in opposition and give considerable power of motion to
the animal. This is exactly the condition of the muscular somites (muscle-plates),
which have already been described for numerous embryos.
 
Those Echinodermata which can move in any direction, such as
the Starfish, have a radial symmetry, which almost completely
masks their fundamental bilateral symmetry as exhibited in the
embryo. Almost without exception the remaining Metazoa are
entirely bilaterally symmetrical.
 
A far greater degree of specialisation can be reached in segmented animals, as the serial multiplication of organs gives the
necessary material for concentration, as a consideration of the
anterior segments of the higher Worms and the concentration and
adaption which has taken place in the head and anterior region of
Arthropoda will fully demonstrate.
 
The region in front of the stomodseum usually projects forward
as the pre-oral lobe. This? portions of the body, and that immedi
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
atelv surrounding the mouth, are collectively known as the head.
From its position, the head is the seat of most of the sense-organs,
and of the most specialised portion of the nervous system.
 
A post-anal extension of the body constitutes the tail; this very
rarely exhibits any features of special interest apart from the
mechanical function of propulsion, which it sometimes performs.
 
Paired lobes from the head or sides of the body are usually
developed, which are jointed only in Arthropods. The dorsal processes on the head are usually sensory in function ; when ventral
cephalic appendages are present they are modified to form masticatory organs (jaws). The paired lateral appendages of the body
variously serve for progresssion, prehension, or respiration.
 
It must be taken as granted that the form of any given embryo
is determined by two causes, first by inheritance, and secondly by
the special conditions in which it is placed. It is one of the most
difficult of embryological problems to distinguish between these
two, and to discover whether the larval form has any special
phylogenetic significance.
 
The characteristic larval forms of most groups of animals are
now recognised to be of such great importance that they are
described in most zoological text-books, and therefore need not
be here dealt with.
 
Fate of the Blastopore.  - The fate of the blastopore is so varied
as to have led to very different conceptions concerning its real
nature, since the blastopore may persist as the mouth or the anus,
or as both, or it may form neither.
 
A. Invertebrates.  - Without entering deeply into controversial
questions, it may be regarded as a generally received opinion that
the blastopore was primitively elongated (see fig. 17). In Peripatus
(fig. 69), which is admittedly an unspecialised form, the elongated
blastopore becomes constricted in the middle, thus leaving an orifice
at each end, one of which persists as the mouth and the other as
the anus. Both these orifices communicate with the archenteron,
and as the body elongates the apertures become widely separate,
and form the terminal openings of the alimentary canal. The
ventral or neural aspect of the body thus corresponds with the
surface on which the blastopore occurs, the fused lips of the
blastopore coinciding with the median ventral line.
 
As an ingrowth of epiblast usually occurs round the lips of the
blastopore, the cavities of the mouth and anus are lined with
epiblast. As has been previously mentioned, the oral invagination
 
 
76
 
 
THE STUDY OF EMBKYOLOGY.
 
 
is termed the stomodseum, and the anal is called the proctodseum.
As a rule, the stomodaeum and proctodseum constitute a very small
portion of the alimentary canal as compared with that which is
formed by the archenteron (mesenteron). In Crustacea, however,
the hypoblastic portion of the alimentary canal is, as a rule, relatively very minute (fig. 140).
 
In Nudibranchs the elongated blastopore closes over from behind
forwards, so that only the oral aperture persists (fig. 17). In the
Pulmonate Mollusc Paludina it is the anus which remains unclosed, and in most cases when the blastopore persists it does so
as the anus.
 
 
 
Fig. 69.  - Embryos of Peripatus Capensis. [After Balfour.]
 
A. Surface view of gastrula with, elongated somewhat constricted blastopore.
B. Later embryo in which the sides of the still more elongated blastopore have
grown together ; five mesoblastic somites are present. C. Transverse section
through the blastopore of the last.
 
a. anus (proctodseum) ; b. c. body-cavity (coelom) ; bp. blastopore ; ep. epiblast ;
hy. hypoblast ; m. mouth in B, mesoblastic somite in C ; me. mesenteron.
 
 
Chordata.  - The relation of the mouth and the anus of the
Chordata to the blastopore is a problem which is at the present
time receiving considerable attention.
 
The belief, however, is gaining ground that the neural aspect of
the body in Vertebrates is identical with that of Invertebrates ; in
other words, the terms dorsal and ventral have opposite meanings
as ordinarily applied in these two groups..
 
An ancestral form, of the Chordata may be conceived as having been an elongated
animal with a mouth and anus which were the persistent terminal orifices of the
elongated blastopore. The body was produced in front of the mouth into a pre-oral
lobe, but the anus was situated at the extreme hinder end of the animal. The
segmented body-cavity was derived from archenteric diverticula, as is now the case
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
77
 
 
in Amphioxus. The nervous system was differentiated from the external skin, and,
being derived from a nervous ring round the primitive blastopore, consisted of a
ventral plate mainly situated between the mouth and the anus ; the symmetrical
halves of which it is composed would result from the junction of the lips of the blastopore. In front of the mouth the neural plate was greatly enlarged in connection
with the specialisation of the pre-oral lobe to form the brain, on which the pit-like
eyes were situated (fig. 139).
 
The folding over of the neural plate to form a neural tube greatly diminished the
facility of the communication of the archenteron with the exterior. In the larval
Amphioxus the archenteron for a long time opens into the posterior end of the
neural canal, through what is known as the neurenteric canal (fig. 57), the neural
canal itself opening to the exterior by an anterior pore. But the anterior region of
the archenteron (pharynx) communicated with the exterior by means of the developing gill-slits ; and it is assumed by Dohrn and others that an anterior pair of gillslits gradually became modified to form the vertebrate mouth. Sedgwick, however,
believes that the mouth is homologous all through the Metazoa, and that it always
retains its original position at the anterior end of the true primitive blastopore.
 
Most embryologists consider the anus of Vertebrates to be a new structure, but
Sedgwick regards it as the posterior extremity of the primitive blastopore. In the
Lamprey, and several Amphibia, the blastopore is stated to remain permanently
open, and to persist as the anus. Weldon describes the proctodaeum in the Lizard
as arising within the region of the primitive streak. If the second view be established, it follows that, as in many Invertebrates, the anus of the Chordata assumes
a secondary position on the opposite, abneural, side of the body to its place of origin,
owing to the elongation of the body. This prolongation constitutes the tail of the
Chordata, see figs. 98, 99, which illustrate this for the Frog.
 
It has been further supposed by Cunningham that the neural plate of the primitive Chordata was folded along the median line, so as to form a groove into which
the primitive mouth and anus opened. By this time the anterior region of the
archenteron was perforated by paired slits, forming the characteristic respiratory
pharynx of the group.
 
The primitive mouth opened into the archenteron near the anterior extremity of
the neural plate. The folding over of the latter to form the neural canal would
render the former useless, and a pair of gill-slits are supposed to have assumed its
function. Cunningham suggests that the infundibulum (see p. 1 10) is the remnant
of the primitive mouth, a view which he maintains is supported by the relations of
that diverticulum.
 
The invagination of the neural plate caused the eyes, which appear to have been
simple pit-like depressions of the pre-oral lobe, to develop as outgrowths from the
anterior region of the brain (fig. 139) ; the relative position of the ganglionic to the
retinal layer of the optic vesicle entirely supports this conclusion. An account of
the development of the eye will be given later (p. 157). Other sense-organs were
developed according to the requirements of the animal.
 
The limbs of Vertebrates are now usually considered to be specialisations of a
primitively continuous lateral ridge or fin.
 
Accepting the interpretation given above of the homology of the Vertebrate embryo,
the following fusions of the embryonic layers must be supposed to occur (see fig. 62).
 
1. The fusion of the lips of the primitive blastopore, extending from the primitive
mouth to the primitive anus, a region which roughly corresponds with the neural
plate. Miss Johnson has described a primitive groove and a primitive streak with
the fusion of the layers in this region in the Newt.
 
2. The union of the lips of the blastoderm behind the embryo in telolecithal ova,
forming the “primitive streak- of most authors.
 
3. The junction of the edges of the blastoderm as they unite after extending over
the yolk.
 
 
78
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Free Larvae. - Embryos may commence a free existence in practically any stage of development, though the age at which an embryo
is hatched or born is definite for the species, if not for the group.
 
Those forms which commence their free existence at an early
stage of development possess many larval structures and organs
to enable them to hold their own in the struggle for existence.
During their further life-history they pass through regular stages
of development, which are usually attained by gradual growth ; but
in some cases (e.g., Arthropoda) the changes are hurried over during
moults of the skin. Speaking generally, alecithal ova are soonest
hatched.
 
The acquirement of food-yolk is associated with a prolongation
of pre-natal existence, but the tendency to undergo a metamorphosis still persists. Consequently rudimentary organs occur during
development which receive their explanation in the loss of a free
larval life, and even moultings of the skin may occur.
 
In Vertebrates higher than the Amphibia (Amniota) certain
foetal appendages are developed, which must now be considered.
 
Foetal Membranes of Birds.  - The following account of the
embryonic appendages refers to the Fowl, but doubtless it is equally
applicable to other Birds.
 
Owing to the large amount of yolk present in the ova of Birds
the embryonic area is relatively small. At first the germinal disc
is flat, but soon the anterior extremity of the embryo is limited
by a fold in the area pellucida, which is known as the head-fold,
and, as was described on p. 39 (fig. 72), the embryo is gradually
constricted off from the yolk, which is henceforth known as the
yolk-sac (umbilical vesicle of Mammals).
 
The middle germinal layer (mesoblast) early splits into two layers ;
the outer layer unites with the epiblast to form the somatopleur
or body-wall and the inner unites with the hypoblast and constitutes the somatopleur. The space thus produced, and which is
surrounded by the mesoblast, is the future body-cavity (coelom),
but it is often termed the pleuro-peritoneal cavity, as being the
cavity which encloses the lungs and abdominal viscera ; as will
be subsequently described, the lungs come to be enclosed in a
special portion of the coelom. The splitting of the mesoblast
first occurs in the embryonic area, but as the mesoblast extends
farther and farther round the yolk, it continues to split, as will be
seen in figs. 70-75. Thus when the mesoblast entirely surrounds
the yolk-sac (fig. 72, F and G; 7 5, d), the latter really lies within
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
79
 
 
the body- cavity (pleuro-peritoneal cavity) of the embryo. By this
time the yolk-sac is greatly reduced in size owing to the absorption of the yolk by the hypoblast and blood-vessels of the area
vasculosa, and ultimately it dwindles away.
 
 
Fig. 70. - Transverse Section of an Embryo
Fowl of Three Days -
Incubation. The size
of the embryo is exaggerated. [Prom KolUlcer.]
 
am. amniotic cavity; blh.
extension of the pleuroperitoneal cavity outside
the embryo ; d. vitelline
membrane ; dr. cavity of
the mesenteron ; ect. epiblast ; ent. hypoblast ; g.
yolk ; mes. border of the
splanchnic mesoblast (area
vasculosa) ; r. edge of the
blastoderm, here consisting
only of epiblast and hypoblast ; s. serous or subzonal
membrane.
 
 
S
 
 
 
Amnion.  - About the twentieth hour of incubation of a Fowl -s
egg a semilunar fold of the blastoderm appears in front of the
future anterior extremity of the embryo (fig. 33). This fold, which
 
 
Fig. 71. - -Details of the Edge of the Mesoblast of a Fowl -s
Ovum about the Stage of Fig. 70. [After Duval.]
 
ep. epiblast ; hy.n. free nuclei in the yolk, which will give rise to
the hypoblast of the yolk-sac ; pp. pleuro-peritoneal cavity or coelom ;
so.m. somatic mesoblast; sp.m. splanchnic mesoblast ; y. yolk. J4
 
 
 
is a reduplicature of the somatopleur, is the anterior fold of the
amnion. Somewhat later a second fold makes its appearance
behind the posterior extremity of the embryo ; this unites with the
 
 
80
 
 
THE STUDY OF EMBEYOLOGY,
 
 
anterior fold through the production of lateral folds, and the
embryo lies in a shallow depression bounded by the amniotic fold.
The folds now increase in size (fig. 72, A, D, b), and soon unite in
 
 
 
Fig. 72. - Diagrams to Illustrate the Development oe the Amnion and Allantois.
 
[From Bell, after Foster and Balfour.']
 
In A the embryo ( e ) is being constricted off from the yolk-sac, and the folds of
the amnion are to be seen rising up at either end of the embryo, the anterior fold
(at) being, the larger ; in B the amniotic folds nearly meet, and in C they have
entirely coalesced. In D, which is a litt'e later stage than A, the allantois (al) is
budding out from the intestine ; in E, which is a stage corresponding with C, the
allantois is seen extending round the embryo. In F the yolk-sac (y) is reduced
in size, and in G it is being withdrawn into the body of the embryo. The allantois
in F and G is omitted for the sake of sim plicity.
 
These diagrams only very roughly indicate the relations of the parts. In all
the embryo is represented by horizontal shading, the pleuro-peritoneal cavity is
dotted, and the yolk-sac has concentric lines. The dotted ^line indicates the
vitelline membrane.
 
 
the median line above the embryo ; their walls coalesce, and finally
break down at the points of apposition, so that the enclosed cavity
becomes continuous (figs. 72, c, E, and 79, 2, 3, 4).
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
81
 
 
The closure of the amniotic orifice by the fusion of the folds
takes place from before backwards, till, at the commencement of
the third day, a small opening is left over the tail, which then
closes over.
 
The inner membrane of the amnion (amnion proper) thus forms
a complete sac round the embryo (figs. 70-83), and the enclosed space is the cavity of the amnion containing the liquor
amnii. The outer amniotic membrane (false amnion or serous
membrane) lies immediately below the vitelline membrane.
 
The space between the true and the false amnion, as will be
clearly seen on reference to figs. 70-79, is merely an extension
of the body-cavity or coelom (pleuro-peritoneal cavity). It is
everywhere bounded externally by the somatopleur, and internally by the splanchnopleur, which invests the yolk (fig. 72, B
 
 
Fio. 73 .  - Formation of the
Allantois. Longitudinal section of the posterior
extremity of an embryo
Fowl of the third day.
Osmic acid preparation
strongly contracted by
the reagent. [From KoLlikerJ] Magnified 150.
 
al. rudiment of the allantois ; am. amnion ; cl. cloaca ;
d. posterior border of the intestino-umbilical orifice ; d'
rectum ; dg. splanchnopleur,
where the intestinal wall
passes round the yolk, thus
forming the anterior border
of the tail-fold ; s. posterior
extremity of embryo.
 
 
and e). The body-cavity is thus gradually extending below the
yolk-sac at F (fig. 72), the two sides have met, and have quite
coalesced in G.
 
Allantois.  - During the formation of the folds of the amnion a
sac projects from the splanchnopleur of the hind-gut into the
body-cavity. This is the allantois ; it is lined internally with
hypoblast (figs. 72-75). The allantois grows rapidly, extending
all round the embryo in the space enclosed by the false amnion.
 
The further history of the allantois in Birds has recently been
carefully studied by Duval. He finds that the outer membrane
of the allantois fuses with the serous membrane, or, as it is preferable to call it, the subzonal membrane. (The compound tissue
thus formed consists of an outer epiblastic epithelium, a middle
layer produced by the fusion of the mesoblast of the subzonal
membrane (somatic mesoblast) with that of the allantois (splaneh
 
 
 
82 '
 
 
THE STUDY OF EMBRYOLOGY.
 
 
nic mesoblast), and an inner epithelium, the hypoblastic lining of
the allantois, fig. 74, b).
 
Instead of remaining, as it were, within the confines of the
body-cavity of the embryo, the allantois protrudes beyond the inferior margin of the yolk-sac, of course carrying the subzonal
membrane with it (fig. 75, A, b).
 
The inferior folds of the allantois enclose the albumen and
meet one another below the embryo (fig. 75, c). They next considerably overlap each other, and eventually fuse together (fig. 75, d).
 
 
A B.
 
 
 
Fig. 74.  - A. Diagrammatic Loncitudinal Section through the Egg of a Fowl.
B. Detail of a Portion of the same at a Time when the Allantois
reached the Spot marked x in A. [After Duval . ]
 
al. cavity of allantois ; alb. albumen ; ali. mesenteron ; al.hy. hypoblastic
epithelium of allantois; al.m. mesoblast of allantois; am. cavity of amnion;
6. blood-vessel; emb. embryo; ep. epiblastof outer layer of amnion (serous membrane) ; ep.am. epiblastic epithelium of inner layer of amnion (amnion proper);
m.am. mesoblastic layer of latter; sh. egg-shell; sorn. somatic mesoblast of outer
layer of amnion ; v.m. vitelline membrahe; •}• point where the mesoblastic tissue
of the allantois fuses with that of the serous membrane.
 
 
The remaining albumen of the egg is thus enclosed, in a space
bounded above by the ventral wall of the yolk-sac, and below by
the folds of the allantois. This space is termed by Duval the
placental sac. Simple villi grow out from the epiblast lining the
placental sac to absorb the contained albumen, the nutriment
being conveyed to the embryo by the blood-vessels of both the,
yolk-sac and the allantois.
 
It is interesting to note that at first villi arise from the epiblast
of the inferior pole of the yolk-sac (fig. 75, a, b). Later they are
developed from that portion of the non-embryonic epiblast which
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
83
 
 
is lined by the allantois ; in other words, from a true chorion (see
p. 90).
 
The cavity of the amnion gradually extends all round the em
 
 
Fig. 75. - Diagrams Illustrating the Formation of the Placental Sac in Birds.
 
[After Duval.]
 
A. Section of an egg of a Warbler (“ Fauvette -) corresponding to that of a Fowl
from the eighth to the tenth day. B. Detail of a portion of the above. C. Ventral portion of an egg of the same, corresponding to that of a Fowl about the
fifteenth day. The two allantoic culs-de-sae have come into contact, forming a
placental sac with internal villi. D. Diminished placental sac of the same,
shortly before hatching.
 
al. cavity of allantois, the thick line in B-D indicates its hypoblastic epithelium ; al.e., al.i. outer and inner layers of the allantois ; am. amnion ; ep. epiblast
of serous or sub-zonal membrane,  - the dotted line between the epiblast and the
hypoblast of the allantois indicates diagrammatically the distinction between the
mesoblast of the serous membrane and that of the allantois ; hy. hypoblast surrounding the yolk,  - the folds of the hypoblast enclose biood-vessels which have
been developed from the splanchnic mesoblast ; hy.n. free nuclei which will form
the vitelline hypoblast ; m. unsplit mesoblast ; p.p. extra-embryonic body-cavity
(pleuro-peritoneal cavity); p.s. placental sac ; sh. egg-shell ; sp.m. splanchnic
mesoblast ; v. epiblastic villi of placental sac ; v.m. vitelline membrane ; y. yolk.
 
 
bryo, but for some time leaves a narrow pedicel surrounding the
stalks of the yolk-sac and allantois (fig. 72, G, the latter is omitted
in this fig., and figs. 79, 5 ; 83). This pedicel is known in Mammals
 
 
84
 
 
THE STUDY OF EMBRYOLOGY.
 
 
as the umbilical cord. In Birds it ruptures just before hatching
after the withdrawal of the yolk-sac into the body-cavity of the
embryo. In Mammals it is only severed after birth.
 
Foetal Membranes of Reptiles.  - Our knowledge of the foetal membranes of Reptiles is still very imperfect.
 
The amnion first appears as a hood covering that anterior portion of the embryo
which very early sinks into the yolk-sac. The anterior fold of the amnion consists of
both epiblast and somatic mesoblast, and it gradually extends backwardly in conjunction with lateral folds which arise along the sides of the neural plate. The posterior
fold of the amnion does not appear to be present.
 
The allantois probably resembles that of Birds.
 
Haacke has shown that in the Lizard Trachydosaurus asper the egg-shell is absent
except for a small disc-shaped rudiment which lies between the yolk-sac and the
uterus ; thus the embryo is readily seen through the thin walls of the uterus and the
transparent embryonic membranes. This Lizard is viviparous, and the vascular wall
of the yolk-sac is only separated from the special capillary network of the uterine
vessels, which is concerned in the nutrition of the embryo, by the porous and friable
rudiment of the egg-shell.
 
Foetal Membranes of Mammals.  - The early stages in the development of the embryo in Mammals closely resemble those of
Birds ; but there are a few important differences in the nature of
the foetal membranes. The differences are mainly due in Mammals higher than the Monotremes to the absence of an egg-shell
with its membranes, and of the albumen and yolk. The ovum is
merely protected by the zona radiata (zona pellucida), within
which a delicate membrane has been observed (fig. 5).
 
The hollow yolk-sac or blastodermic vesicle grows rapidly ;
being distended by a contained fluid, the zona becomes very thin
and early disappears. As has previously been mentioned (p. 45),
the germinal area alone of the oosperm possesses the three germinal
layers ; the remaining portion of the upper half of the oosperm is
lined with epiblast and primitive hypoblast, whereas the lower half
of the blastodermic vesicle is composed solely of epiblast (fig. 42).
 
Simple non- vascular villi, which serve to attach the embryo to
the walls of the uterus, usually project from the epiblast of the
blastodermic vesicle (subzonal membrane). In the Babbit they
only occur on that area of the epiblast under which the mesoblast
will not extend (figs. 77, 78), with the exception of a horse-shoe
shaped patch which early makes its appearance in the region of
the future placenta, and with which it shortly becomes identified
(eg. 76.pi).
 
The following account of the development of the amnion is
taken from Van Beneden and Julin -s recent researches on the
development of the Babbit.
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
85
 
 
Pro-amnion.  - The mesoblast (fig. 76, a.v) extends for some
distance from the embryo in every direction, except immediately
around the head ; but the two limbs of mesoblast which bound
this emargination gradually extend round some distance in front
of the head and eventually unite (fig. 76). Thus it comes about
that there is a nearly circular area in front of the head in which
the blastoderm consists of epiblast and hypoblast only.
 
This area early sinks into the cavity of the blastodermic vesicle,
and the anterior extremity of the embryo projects into this depression, which Yan Beneden and Julin term the pro-amnion (figs.
76-78, p.am).
 
Amnion.  - Very slightly later the true amnion is developed,
but only over the posterior end of the embryo (figs. 76, 77). It
 
 
Fig. 76. - Diagrammatic Dorsal View
of an Embryo Rabbit with its Membranes at thf. Stage of Nine Somites. \ Modified from Van Beneden
and Julin.]
 
al. allantois, showing from behind the
tail fold of the embryo ; am. anterior
border of true amnion ; a.v. area vasculosa, the outer border of which indicates
the farthest extension of the mesoblast ;
bl. blastoderm, here consisting only of
epiblast and hypoblast ; o.m.v. omphalomesenteric or vitelline veins ; p.am. proamnion ; pi. non-vascular epiblastic villi
of the future placenta; s.t. sinus terminalis.
 
 
 
rapidly grows forwards until it comes in contact with the raised
anterior rim or fold of the pro-amnion, with which it fuses. The
cavity of the amnion coalesces with the space (extra-embryonic
pleuro-peritoneal cavity) resulting from the splitting of the mesoblast, which now extends in front of the embryo and the proamnion.
 
In process of time the pro-amnion gradually atrophies, and the
true amnion correspondingly advances forwards.
 
It is now generally admitted that the amnion was primitively caused by the embryo sinking into the yolk-sac by its own weight. The protection to the embryo
by the formation round it of what is virtually a water-sac resulted in the precocious
development of the amnion before the embryo in its ontogeny had any appreciable
weight.
 
The pro-amnion probably originated from a similar bearing-down of the heaviest
 
 
86
 
 
THE STUDY OF EMBRYOLOGY.
 
 
(anterior) end of the embryo, when .the blastoderm of that region was still diploblastic (two-layered). The pro-amnion is, in fact, an exaggeration of the head-fold.
 
 
 
Fig. 77.  - Diagrammatic Median Vertical Longitudinal Sections through the Embryo
Rabbit. [ After Van Beneden and Julin .]
 
A. Section through embryo of fig. 76. B. Section through embryo of eleven days.
al. allantois; am. amnion; a.ms. anterior median plate of mesoblast, formed
by the junction of the anterior horns of the area opaca ; a.pl. area placentalis ;
a.v. area vasculosa ; ch. chorion; ece. coelom of embryo; cce f . extra embryonic
portion of the body-cavity ; ep. epiblast ; hy. hypoblast ; m. unsplit mesoblast ;
o.a. orifice of amnion; pi. placenta; pro.a. proamnion; s.t. sinus terminalis ;
v. epiblastic villi of blastodermic vesicle.
 
Van Beneden and Julin affirm that it not only occurs in Rodents, but also in Bats
and the Dog, and that it probably exists for a short period in the Fowl and in Lizards.
 
 
 
Fig. 78.  - Fietal Envelopes of a
Rabbit Embryo. [ From Minot,
 
after Van Beneden and Julin.]
Later stage than fig. 77, B. The amnion has become fused with the blastoderm in front of the embryo, and its
cavity is therefore continuous with the
extra-embryonic portion of the bodycavity in front of the embryo.
 
Al. allantois ; am. amnion ; am'.
portion of the amnion united with the
walls of the allantois ; A.pl, area
placentalis; Av. area vasculosa; Ch.
chorion; Coe. coelom or body-cavity;
coe". extra-embryonic portion of the
body-cavity ; Coel. anterior portion of
the same, produced by the fusion of the
cavity of the amnion with that of the
anterior portion of the area opaca ; Ec.
epiblast ; En. alimentary canal of the
embryo; Ent. hypoblast; PI. placenta;
pro. A. proamnion ; T. sinus terminalis-;
V. villi of blastodermic vesicle ; Y.
cavity of blastodermic vesicle.
 
 
It would appear, therefore, that the pro-amnion is a structure which is comnlon to a
greater or less extent to the Sauropsida and Mammalia.
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
87
 
 
The anterior fold of the true amnion is certainly absent in the
Babbit, and this may prove to be the case for Mammals generally,
now attention has been drawn to the question. At all events, the
posterior fold of the amnion is always well developed.
 
By this time the partially vascular yolk-sac has gradually
diminished in size; and the vascular allantois is greatly increasing
in size and importance, and is functionally replacing the yolk-sac.
 
Allantois.  - The Mammalian allantois has a similar origin to
that in Birds (figs. 77, 79). It extends to a greater or less extent
between the amnion and the serous membrane or subzonal
membrane.
 
The outer membrane of the highly vascular allantois fuses, as
in Birds, with the subzonal membrane, the villi of which become
vascular and usually grow more complex. The compound membrane thus formed is known as the chorion. That portion of the
chorion which enters into immediate connection with the uterus of
the mother constitutes the foetal portion of the placenta.
 
As will be shown later (p. 259), the proximal portion of the stalk
(urachus) of the allantois persists as the urinary bladder, and it is
generally admitted that the urinary bladder (urocyst) of Amphibia
is a homologous organ with that of the Amniota. It is thus a
fair assumption to make that the allantois is merely the precociously
developed urinary bladder.
 
In the lower Yertebrates the egg is usually laid in water, and the larva is, as a rule,
early hatched, respiration being effected by gills situated on the gill-arches.
 
In Alytes and Notodelphis ovipara and some other Anura, large external gills are
developed while the embryo is still within the egg-covering, their function apparently
being to give increased facility for respiration to the unhatched young. A similar
condition also occurs in some Elasmobranchs.
 
Certain Anura, however, have such an abbreviated larval existence that the young
are hatched as small Frogs, and in some of these the external gills atrophy early
(Pipa americana), or are said to be entirely absent (Rhinoderma darwinii, Nototrema
marsupiatum). In Pipa the long tail of the tadpole functions as a respiratory organ
[Peters], and the same holds good for Hylodes. Boulenger finds that the abdomen
of the just-hatched Rana opisthodon is provided with a lateral series of symmetrical
folds, which probably have a respiratory function.
 
The abo vo facts tend to show that some Frogs are losing their ancestral larval
breathing organs, and are utilising other organs for respiratory purposes ; and it is
very significant that this occurs amongst those Frogs which do not deposit their eggs
in water. It is then not difficult to imagine that some primitive Amphibian which
had acquired an increase of food-yolk (as a few recent Anura have done) would find
in the urinary bladder an organ which could be pressed into the service of aerial
respiration.
 
If we may assume that some such Amphibian was the ancestor of the Amniota, we
have a clue to the significance of the total absence of even rudimentary gill-filaments
on the gill arches of even the youngest embryos of the less specialised Amniota in
 
 
88
 
 
THE STUDY OF EMBRYOLOGY.
 
 
/ / *
 
 
 
Fig. 79. - Five Diagrammatic Figures Illustrating the Formation of the Fcetal
Membranes of a Mammal. [From Kolliker. J
 
In 1, 2, 3, 4, the embryo is represented in longitudinal section.
 
1. Oosperm with zona pellucida, blastodermic vesicle, and embryonic area. 2.
Oosperm with commencing formation of umbilical vesicle and amnion. 3. Oosperm
with amnion about to close and commencing allantois. 4. Oosperm with villous
subzonal membrane, larger allantois, and mouth and anus. 5. Oosperm in which
the vascular mesoblast of the allantois has extended round the inner surface of
the subzonal membrane, and united with it to form the chorion ; the cavity of
the allantois is aborted. The yolk-sac (umbilical vesicle) has greatly diminished.
The large amniotic cavity surrounds the umbilical cord. This fig. represents an
early human ovum.
 
a. epiblast of embryo ; a'. epiblast of non-embryonic part of the blastodermic
vesicle ; ah. cavity of the amnion ; al. allantois ; am. amnion ; as. amniotic
sheath round the umbilical cord ; ch. chorion ; ch.z. villi of chorion ; d. zona
pellucida (radiata) ; d'. processes of zona ; dd. embryonic hypoblast ; df. area
vasculosa ; dg. stalk of yolk-sac; ds. yolk-sac (umbilical vesicle); e. embryo ; hh.
pericardial cavity ; i. non-embryonic hypoblast ; kh. cavity of the blastodermic
vesicle, which practically is equivalent to the yolk-sac ; ks. head-fold of amnion ;
m. embryonic, m' non-embryonic, mesoblast ; r. space between chorion and
amnion containing albuminous fluid ; sh. subzonal (serous) membrane ; st. sinus
terminalis ; sz. subzonal villi ; vl. ventral body-wall in the region of the heart.
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
89
 
 
the supposition that the loss of larval gills was a pre-amniote character. This was
rendered possible before the lungs were functional in ontology by the acquisition of
an accessory respiratory organ ; in this case it was the thin-walled vascular urinary
bladder. The adoption of this organ for respiratory purposes causes it to grow
enormously in size, and at the same time to appear earlier. Hence the great
development it now attains.
 
It has just been shown that in Birds the epiblast which underlies the yolk-sac is produced into villi (fig. 75, b, v), which absorb
the nutritive albumen before the allantoic villi are developed.
The same also occurs in the lower Mammalia.
 
In the Virginian Opossum (Didelphys), according to Osborn,
when the allantois is still very small, the yolk-sac is provided
with simple vascular villi (fig. 80, v), which, in addition to serving
to attach the embryo to the uterine wall, are undoubtedly nutritive
in function. In these Mammalia there is no albumen to feed
 
 
Fig. 80.  - Diagram of the Fcetal Membranes of
the Virginian Opossum. \_After Osborn .J
 
Two villi are shown greatly enlarged. The processes of the cells, which have been exaggerated,
doubtless correspond to the pseudopodia described
by Caldwell.
 
al. allantois ; am. amnion ; st. sinus terminalis ;
sz. subzonal membrane ; v. villi on the subzonal
membrane in the region of the yolk-sac ; ys. yolksac. The vascular splanchnopleur (hypoblast and
mesoblast) is indicated by the black line.
 
 
upon, but better nutriment can be directly obtained by osmosis
from the mother.
 
Caldwell has shown that in the Native Bear (Phascolarctos
cinereus) (fig. 81) the inferior non- vascular moiety of the yolk-sac
is, even up to a comparatively late period, surrounded only by
hypoblast and the non-embryonic epiblast (subzonal membrane).
The cells of the latter send out pseudopodia (fig. 81, amb), which
fit in between the cells of the uterine epithelium. Although the
allantois is larger than in the preceding form, and comes into
contact with the subzonal membrane, no villi are formed by it ;
in other words, in the Marsupials the true chorion, if present, is
rudimentary, and, so far as is known, never develops villi. The
previous researches of Owen point to the same conclusion.
 
Prom the nature of the case, no adhesion occurs between the
embryo and the parent in the Prototheria, any more than in
Sauropsida. In the Metatheria a very slight connection does
 
 
 
90
 
 
THE STUDY OF EMBRYOLOGY.
 
 
occur, but in this union the subzonal membrane surrounding the
yolk-sac alone takes part. As the latter was the sole nutritive
organ of the embryos of the earlier Mammals, it would probably
but slowly part with this function.
 
Ryder has suggested that the degeneracy of the yolk in the Mammalian oosperm
-may be due to the development of the so-called uterine milk from the uterine
glands, and it subsequently completely disappeared in consequence of the perfectly
parasitic connection temporarily subsisting between the mother and the embryo. (The
latter supposition was first put forward by Balfour.) At this stage of evolution the
allantois was respiratory,  -as it practically is in the Sauropsida, Monotremes, and
, Marsupials, and the yolk-sac was becoming less nutritive in function.
 
As the allantois is used in Birds to absorb the albumen, so in
the higher Mammals (Eutheria) it develops villi where it is fused
with the subzonal membrane, and forms the chorion.
 
 
 
Fig. 8i.  - Diagram of the Fcetal Membranes of the
Native Bear. \After Caldwell .]
 
al: allantois; am. amnion ; amb. amoeboid processes
of the subzonal epiblast in the non-vascular region of
the yolk-sac ; hy. hypoblast of the non-vascular region
of the yolk-sac; s.t. sinus terminalis; s.z. subzonal
membrane ; y.s. yolk-sac. The black line indicates the
vascular splanchnopleur (hypoblast and mesoblast).
A greatly magnified portion of the ventral wall of the
yolk-sac is also given.
 
 
The term chorion is now limited to those areas of the subzonal
^membrane to which the yolk-sac or the allantois are attached.
Balfour distinguished the former of these as the false and the
latter as the true chorion. In the Babbit (fig. 82) the false chorion
is very large, and the true (or placental) chorion relatively small ;
but in most Mammals the true chorion has a much greater ex^
tension.
 
 
It is possibly owing to the large size of the yolk-sac that the
allantois, forms such a small chorion in the Babbit. There is a remarkably close resemblance between the general disposition and
structure of the foetal membranes in the Babbit (%s. 78, 82) and
some Marsupials (figs. 80, 81). In both, the epiblast (subzonal
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
91
 
 
membrane) of the yolk-sac (blastodermic vesicle) gives rise to non' vascular villi only in the region where the mesoblast has not
extended. The allantois also unites with the subzonal membrane
above the embryo to a small extent ; but in the Rabbit vascular
villi are developed at this spot, which thus form a true placenta.
 
The epiblast of the blastodermic vesicle appears to give rise to
villi in other Mammals, but more precise information is required
on this point.
 
The nature and position of the villi of the chorion vary considerably. The villi fit into depressions of crypts of the uterine
wall, the conjoint structure being known as the placenta.
 
The placenta of the Rodentia, Insectivora, and Chiroptera is
usually dorsally situated and discoidal, as in the Rabbit, and ife
 
 
Fig. 82.  - Diagrammatic Longitudinal
Section of Oosperm of Rabbit at
an Advanced Stage of Pregnancy.
[From Kolliker after B ischoff.]
 
' a. amnion ; al. allantois with its bloodvessels ; c. embryo; ds. yolk-sac; ed,
ed', ed". hypoblastic epithelium of the
.yolk-sac ana its stalk (umbilical vesicle
and cord) ; fd. vascular mesoblastic
membrane of the umbilical cord and
vesicle; pi. placental villi formed by
the allantoig and subzonal membrane;*r. space filled with fluid between the
amnion, the allantois,- and the yolk-sac ;
st. sinus terminalis (marginal vitelline
blood-vessel) ; u. urachus or stalk of the
allantois.
 
 
^co-extensive with the area of contact between the allantois and the
^subzonal membrane. In these forms the yolk-sac is in contact
with the larger portion of the subzonal membrane.
 
In Edentata the placenta may be discoidal (Loricata), or domeshaped (Pilosa), or zonary (Tubulidentata), that is, occupying a
•broad band round the chorion, leaving the ends free from villi, or
diffuse (Squamata).
 
In the Dog the large vascular yolk-sac does not fuse with the
subzonal membrane. The allantois first grows out on the dorsal
side of the embryo, where, coalescing with the subzonal membrane,
it forms an at first discoidal placenta. The villi soon extend, so
as to form, a zonary placenta. The zonary placenta is found in
the Carnivora, llyrax, and Elephas.
 
 
t i -r
 
 
 
92
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The extension of the placenta over the whole of the chorion
results in what is termed a diffused placenta ; this is characteristic
of the Perissodactyla, the Suina, the Tragulina, the Tylopoda, the
Sirenia, the Cetacea, the Lemuroidea.
 
The collection of the villi into groups constitutes what is known
as a cotyledonary placenta. This variety is confined to the Pecora.
In the Giraffe, the placenta is partly diffused and partly cotyledonary. Weldon finds that in the Pour-horned Antelope (Tetraceros) the whole surface of the chorion is thrown into vascular
ridges, exactly as in the Pig, and the cotyledons are very few in
number (twenty to thirty), other Antelopes having sixty or more.
The Bovidse possess a large number of cotyledons, while the
Cervidse have only a very few. In Moschus, however, the placenta
is finely folded, cotyledons being absent.
 
In the Anthropoidea, the villi are at first diffuse, but ultimately
they are restricted to the ventral surface, forming a secondary
discoidal placenta (metadiscoidal).
 
The simplest kind of placenta is one in which the papilla-like
villi of the chorion fit into corresponding depressions in the uterus.
The villi are ranged in irregular ridges in the Pig. In such forms
the chorion can be withdrawn at birth from the placenta; in other
words, the placenta is non-deciduate.
 
The following animals have a non-deciduate placenta :  - Artiodactyla, Perissodactyla, Sirenia, Cetacea, Lemuroidea, and some
Edentata (Squamata). But in some of these the villi are more or
less branched and complicated ; and in many of the Pecora this
interlocking is so close that the parts of the epithelium of the
maternal cotyledons may be carried away at birth.
 
In all the other Eutheria the foetal villi are so intimately connected with the uterine wall, that at birth a greater or less portion
is brought away with the allantois (after-birth). This form of
placenta is known as the deciduate.
 
The uterus merely develops short tubular crypts to surround
the foetal villi in the case of those Mammals with a simple nondeciduate placenta. But in those with a deciduate placenta the
wall of the uterus undergoes varied structural modifications, which
reach their extreme form in the Anthropoidea, where the foetal
villi are immersed in large uterine blood-sinuses.
 
Very shortly after the human ovum has entered the uterus, the
walls of the latter grow round and incapsulate it (fig. 83). The
reflected portion of the uterus is called the decidua reflexa. That
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
93
 
 
portion of the wall to which the embryo is attached is known as
the decidua serotina, the decidua vera being the remaining surface of the uterus. All these structures are cast off in the act of
birth.
 
The decidua reflexa is more or less developed in a few other
Mammals, e.g., Seals, and some Insectivora.
 
 
Inversion of Germinal Layers in Rodents.  - A peculiar inversion of the germinal
layers in the blastoderm of the Guinea-pig was first described by Bischoff, and later
confirmed by Hensen and Schaffer. Four papers were simultaneously published at
the end of the year 1882, in each of which there was practically an identical solution
 
 
 
Fig. 83.  - Diagrammatic Section of Pregnant Homan Uterus, with
Contained Fcetus. {From Huxley after Longet .]
 
al. allantoic stalk (urachus); am. amnion; c. cervix uteri ; ch. chorion; dr.
decidua reflexa ; ds. decidua serotina ; du. decidua vera ; l. Fallopian tube
(oviduct) ; nb. umbilical vesicle or yolk-sac ; 2. foetal villi of the true placenta ;
2'. villi of the non-placental part of the chorion.
 
The portion of the uterine wall to which the embryo is attached is the decidua
serotina ; that portion which grows round the embryo is the decidua reflexa,
while the general wall of the uterus, not related to the embryo, is the decidua
vera.
 
 
of this difficult problem. The forms studied were the Field-vole (Arvicola arvalis)
by Kupffer, the House-mouse (Mus musculus) by Selenka, the Guinea-pig (Cavia
cobaya) by Hensen, and the House-mouse, the Rat (Mus decumanus), and the Guineapig by Fraser. Slightly later, fresh light was thrown on the subject by Spee, and
lastly Heape -s researches on the Mole (Talpa europea) have supplied additional
information.
 
The explanation of the phenomenon is briefly as follows. As has already been
described, the solid mass of inner-layer cells, attached to one pole of the blastodermic
vesicle in the Rabbit (fig. 39, b, c ), flattens out to form the germinal disc (fig. 39, d).
 
In the Mole the primitive inversion of the blastoderm is retained slightly longer,
the embryonic epiblast forming a cup-like depression at one pole of the blastodermic
vesicle ; the secondary cavity thus formed being filled with loose cells of epiblastic
origin. The whole is roofed over by a layer of covering cells, which is continuous
 
 
94
 
 
THE STUDY OF EMBRYOLOGY.
 
 
with the outer wall of the blastodermic vesicle (compare fig. 45, b). Later, in this
Insectivore the blastoderm becomes flattened out, and development proceeds much
as in the Rabbit.
 
In the Field- vole the ovum forms a normal blastodermic vesicle, with a blastoderm
consisting of epiblast and primitive hypoblast (fig. 41). The layer of covering cells
which overlies the embryonic epiblast is the seat of an early and rapid proliferation
(fig. 83V a), thus forming a mass of cells which pushes the blastoderm before it
(fig. 83*, b). The embryo is developed from the centre of the germinal area, the folds
of the amnion arising between the embryo and the covering cells (fig. 83*, c).
 
In the House-mouse and Rat the blastoderm is pushed for a considerable distance
within the blastodermic vesicle by the proliferating epiblast (fig. 83*, e). Subsequently
an elongated cavity appears within the latter, extending along the whole length of
 
 
 
Fig. 83*.  - Diagrams Illustrating the Inversion of the Germinal Layers
in the Blastodermic Vesicle of Rodents.
 
A-C. Field-vole [after Kupffer ]. D. House-mouse [ after SelenJca ]. E-H. Rat
[after Fraser ]. None of the figures are drawn to scale.
 
a. commencement of the folds of the amnion ; all. allantois ; b.v. blood-vessel
of uterine wall ; c.c. covering cells which primitively overlie the blastoderm, and
which serve to connect the future placental- pole of the blastodermic vesicle with
the wall of the uterus ; e.a. embryonic area of blastoderm ; emb. embryo ; e.p. embryonic epiblast ; ep'. non-embryonic epiblast, or epiblast of blastodermic vesicle ;
f.a. false amnion or serous membrane; hy. hypoblast; m. mesoblast ; n.a. neuramniotic cavity (amni otic cavity); s.c. secondary cavity; y.s. cavity of yolk-sac
or blastodermic vesicle. In the Rat (E-H) the wall of the blastodermic vesicle
consists of two layers, epiblast and hypoblast ; only the former is shown in the
diagrams. The notch above the line pointing from m in H indicates the neurenteric canal, and marks the posterior end of the embryo.
 
 
the previously solid plug of epiblast cells. This cavity clearly corresponds to the
hollow simple cup-like invagination of the blastoderm of the Mole and Field-vole.
The germinal disc occupies the bottom of the depression, and the embryo develops
on the upper surface of the secondary cavity (fig. 83*, h, emb) ; thus, to borrow an
illustration, at a certain stage (fig. 83*, c, f) the embryo bears the same relation to
the secondary cavity that the embryo Fowl does to the cavity of the amnion at an
early stage in the formation of the amniotic folds (fig. 72, a, b). Whether it is
rectified (Talpa) or not (Arvicola, Mus), the body of the embryo always lies morphologically outside the blastodermic vesicle.
 
The primitive elongated secondary cavity of the House-mouse and Rat soon becomes
constricted into two vesicles, one of which occupies the fundus of the involution, while
the other lies in its stalk (fig. 83*, f, g). The former has been named by Fraser the
neur-amniotic cavity, as it is from the walls of this vesicle alone that the embryo is
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
95
 
 
formed. This vesicle is merely the isolated extremity of the primitive secondary
cavity ; its wall is composed of an inner layer of epiblast and an outer layer of
primitive hypoblast, and the dorsal surface of the embryo consequently projects into
its central cavity. In other words, it is the cavity of the amnion of more normal
embryos (fig. 79, 3-5, ah).
 
The second vesicle encloses what Fraser terms the false amnion cavity. The
epiblastic epithelium lining it is the exact equivalent of the false amnion or serous
membrane of other Amniotes.
 
During further development these two vesicles become separated by a considerable
space from one another. The mesoblast, which has by this time made its appearance
in the embryonic area, extends into this “interamniotic space,- and the allantois also
penetrates into it as an, at first, solid bud of cells (fig. 83*, H, all). The interamniotic
space into which the mesoblast and allantois immigrate is simply the extra-embryonic
body-cavity (pleuro-peritoneal cavity) of other forms (figs. 72, 77, 78, coe).
 
The last term of the series is found in the Guinea-pig, in which Rodent the neuramniotic cavity, with its embryonic area, appears to be precociously separated from
the upper pole of the blastodermic vesicle, so as to form a vesicle at the opposite pole.
The neur-amniotic vesicle is thus a hollow ball composed of two layers of cells, the
outer layer being the primitive hypoblast and the inner layer the epiblast. There is
a thick ingrowth or plug (“ Trager -) of epiblast cells at the upper pole, as in the
House-mouse (fig. 83*, d).
 
Summary of Evolution of Foetal Membranes.  - Food-yolk is
stored up in the primitive hypoblast of most Vertebrates, sometimes to an enormous extent. In the latter case the embryo is, as
it were, pinched off from the large yolk-sac.
 
During its development the embryo digests and absorbs the
yolk by means of the surrounding hypoblast and the vascular
splanchnopleur. In the case of a few Elasmobranchs the vascular yolk also obtains nutriment directly from the blood-vessels
of the enlarged oviduct (uterus) of the mother, prominences from
the yolk-sac fitting into depressions of the oviduct. In the Teleost
Anableps the vascular yolk-sac is provided with villi, which
absorb nutriment from the fluid secreted by the walls of the
dilated ovarian chamber, within which the embryos are developed
[Wyman].
 
T. J. Parker finds in Mustelus antarcticus that the pregnant oviduct was subdivided
into five to eight compartments, each containing one embryo. The wall of each compartment can be resolved into two *â–  layers : an outer highly vascular membrane
(pseudo-chorion), derived, from the oviduct ; and an inner cuticular non-vascular
layer, secreted by the former. As the enclosed cavity is tense with a fluid giving the
reactions of the amniotic fluid, as generally understood, he proposes to call the latter
membrane the pseudamnion.
 
In Birds, simple villi develop from the yolk-sac for the purpose
of absorbing the albumen.
 
When the ancestors of the Metatheria (Didelphia) and Eutheria
(Monodelphia) were ceasing to deposit their eggs, and were retain
 
96
 
 
THE STUDY OF EMBRYOLOGY.
 
 
in g the by-this-time shell-less ova within the oviduct, the ova
were placed in a most favourable condition for obtaining supplemental nutriment. The vascular yolk-sac would readily become
slightly attached to the wall of the oviduct, as in some Elasmobranchs and Lizards (Trachydosaurus and Cyclodus [Haacke]).
 
The nutriment (blood of the oviduct or uterus, and probably
the secretion of the uterine glands) thus at the disposal of the
ovum was more easily assimilated than the yolk; and it is not
surprising that the yolk-sac gradually lost its yolk, and that the
embryo became entirely dependent upon the maternal bloodvessels. The yolkless yolk-sac of Mammals is known as the
blastodermic vesicle.
 
The blastodermic vesicle was primitively the only means of
connection between the embryo and the parent, as it still is in the
Metatheria, and at first is in the embryos of the Eutheria.
 
By this time the allantois, from being an almost purely respiratory organ, became attached to the serous or subzonal membrane,
and assumed a nutritive function. In Birds (and probably in Reptiles), the allantoic villi also absorb the albumen which lies within
the egg-shell. In the Eutheria, the egg-shell being absent, the
villi enter into direct union with the uterine wall. As the allantois
became more closely attached to the uterus, it gradually usurped
the functions of the yolk-sac, and eventually entirely superseded
it.
 
The allantoic villi are collectively termed the placenta, and
distinct lines of specialisation in the disposition of the villi and
structure of the placenta can be traced in the Eutheria, the
main object to be gained being the increase in the facility for
transfusion between the maternal and foetal fluids. The result is
that in the higher forms the villi become more complex, and instead of being readily withdrawn from the uterine crypts at birth,
they fuse with the uterine wall, and thus form a deciduate, as
opposed to a non-deciduate placenta.
 
The complex foetal membranes of the higher Eutheria are
evidently the result of the gradual differentiation of pre-existing
structures.
 
Amnion of Insects.  - The Insects are characterised by possessing an embryonic protective membrane, which is termed the
amnion. It consists of a reduplicature of the epiblast, which extends over the ventral (neural) aspect of the body and encloses all
the appendages.
 
 
GENERAL FORMATION OF THE BODY, ETC.
 
 
97
 
 
The two amniotic folds unite and fuse in the median ventral
line below the developing embryo, and the two membranes thus
formed separate and constitute a double covering for the embryo,
as in the case of the Amniota.
 
In the Insects, the two folds of the amnion are purely epiblastic
in origin, but they may conveniently receive the same relative
names as those of the Amniota, the outer one being called the
serous membrane, and the membrane next to the embryo is termed
the amnion proper.
 
If they have not previously disappeared, the amniotic membranes are either absorbed or cast off at hatching.
 
 
( 93 )
 
 
CHAPTER Y.
 
ORGANS DERIVED FROM THE EPIBLAST.
 
As the epiblast constitutes the external skin of the embryo, it
naturally has a protective function ; and it gives rise in the adult
to the epidermis, together with those portions of the organs of active
or passive defence which arise from the epidermis. It further
gives origin to numerous glands, and also to the nervous system
and to the sensory portion of the sense-organs. The functions of
this layer may be summed up as protective, secretory, respiratory,
and sensory.
 
Protective Structures  - Invertebrates.  - When the outer skin
(epidermis) of an animal consists of a single layer of cells, it is
usually protected by a more or less continuous and structureless
membrane or cuticle.
 
The cuticle may become cornified, as in most compound
Hydrozoa and some Polyzoa; or calcified, as in the Hydrocorallinae and other Polyzoa; or chitinised, as in the majority of
Arthropoda. In most Crustacea the cuticle is both calcified and
chitinised. Such an indurated cuticle (exoskeleton) may be produced into spines and other weapons of attack or defence.
 
The horny axial skeleton (coenenchyma) of the Antipathidse
(and possibly of the Gorgoniidse) has been shown by Yon Koch to
be the secretion of an invaginated ectodermal epithelium. It has
been recently stated by Klaatsch that in some Hydrozoa (Clytia)
the perisarc is produced by the outer layer of the ectoderm itself
becoming chitinised. According to Yon Koch and Fowler, the
hard parts of the Hexacoralla are also probably secreted by the
ectoderm. The cells which secrete the spicules of Alcyonaria are
also of epiblastic origin.
 
Iu many cases the outer layer (ectosarc) of the protoplasm of
the epiblast cells gives rise to one or numerous delicate contractile
protoplasmic hair-like processes (flagella or cilia), which penetrate
the cuticle, when present, and have a lashing movement. They
serve for the progression of the embryo or adult, or to set up a
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
99
 
 
current in the surrounding medium for the procuring of food,
aeration of the tissues, discharge of waste matter, and other
purposes.
 
The epiblast cells (usually termed ectoderm) may in the Coelenterata develop within themselves, by a modification of their own
protoplasm, sacs containing a long coiled thread,  - the thread -cells
or nematocysts,  - which can be suddenly projected and form
powerful stinging organs. In the Turbellarian worms analogous
short rods often occur.
 
The shells of Brachiopods are secreted by the outer surface of
the delicate pallial membrane, and therefore may be regarded as a
 
 
 
Fig. 84.  - Veliger Larvae of Mollusca.
 
A. Side view of veliger of Purple-snail (Ianthina). B. Longitudinal vertical
optical section of early veliger of the Pond-snail (Lymnseus stagnalis), [After
Hoices .] C. Optical section of primitive kidney of embryo Murex. D. Section of
shell-gland of the same.
 
a. archenteron ; an. ciliated patch in position of future anus; bl. blastocoel
(archicoel) ; blp. blastopore ; c. tuft of cilia above thickened epiblast at the apex
of the head ; ep. epiblast ; /. foot ; hy. hypoblast ; mn. mantle-fold ; ms. mesoblast ; pig. spot of violet pigment ; p.k. primitive kidney ; r. invagination to
form the sac of the radula ; sh. shell; sh.g. shell-gland; st. stomodseum ; v.
velum ; y. yolk-cells, forming the liver in B.
 
special form of cuticle. The shells of adult Mollusca are composed
of three layers, of which the cuticle or epiostracum (“ epidermis -)
and the prismatic layer are secreted by the thickened edge of the
mantle, while the general upper surface of the mantle secretes the
nacreous layer.
 
In all Molluscan embryos an invagination of columnar epiblast
(figs. 84 and 18) takes place on the dorsal side behind the velum.
This is known as the shell-gland, and is of invariable occurrence :
later it flattens out; the surface thus formed, the mantle, secretes
the larval shell. In the Lamellibranchs the axial line of the
shell-area remains uncalcified, and persists as the ligament and
 
 
 
100
 
 
THE STUDY OF EMBRYOLOGY.
 
 
hinge-line of the adult. The primitive shell of Mollusca at first
forms the apex of the permanent one, but it usually disappears
in time.
 
A pit-like depression of the mantle occurs in all embryo Cephalopoda, which may
be termed the shell-sac. This soon atrophies in Octopus, while that of the Squid
and Cuttlefish secretes the 11 £ pen - and “ cuttle-bone - respectively. The shell-sac is
often regarded as the equivalent of the shell-gland of other Mollusca, but Lancaster
has shown that it cannot have the simple significance which it appears to possess :
the student is referred to his paper for a statement of the argument. The conclusion
arrived at is, in brief, that the shell-sac of embryo Cephalopoda is not only equivalent
to the shell-gland of other Mollusca, but in addition corresponds with an upgrowth
of mantle-folds over the original external shell, much in the same manner as the shell
of Aplysia is concealed.
 
In all cases the shells of the Mollusca are entirely epiblastic
in origin, and are consequently, morphologically speaking, always
external, or exoskeletal, structures.
 
 
 
Fig. 85. Sections of Skin of Embryo Birds. [After Jeffries.']
 
A. Section of epidermis of hi hours - Fowl embryo. B. Of 134 hours - Fowl.
C. Of 17 days - Duckling.
 
e. epitrichial layer ; m. mucous layer ; t. transitional cells.
 
 
Chordata.  - In the majority of Chordata embryos the epiblast
consists at first of a single layer (figs. 23, 26, 29-32, 43); but in
the Anura (figs. 24 and 62) two layers are present. Of these two
layers, the lower alone is the active layer, and from it are developed
the glandular and nervous structures. In the Urodela the primitively single layer (figs. 58, 59) early becomes double, the lower
one of which behaving as in the Anura.
 
The epidermis of Amphioxus permanently remains as a single
layer.
 
In all other embryo Vertebrates, the epiblast, from being single,
becomes double layered, owing to the primitive epiblast giving rise
to a layer of flattened epithelial cells, the epitrichial layer (fig. 85).
This may be regarded as the primitive horny or protective layer of
the epidermis. The lower layer is the mucous or Malpighian
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
101
 
 
epithelium, and persists throughout life as the active and regenerative layer of the epidermis. Later the mucous epithelium
gives rise to cells of irregular shape which eventually become
more or less spindle-shaped (transitional cells of Jeffries). The
epitrichial layer is always shed, and the oldest transitional cells,
by a process of drying and consequent shrinkage, become the
horn-cells of the adult.
 
The horny layer is present in all the purely terrestrial Vertebrates ( e.g ., Mammalia, Aves, and Eeptilia), but not in other
 
 
A B
 
 
 
Fig. 86. - Six Stages in the Development of Haik.
 
[From Wiedersheim .]
 
C. derma ; CZ. central zone of hairgerm, which forms the hair-shaft with its
medulla or pith and its sheaths ; Dr. sebaceous gland; F. mesoblastic sheath or
follicle ; HK. hair-knob ; P. commencement of formation of hair-papilla ; P\ the
same at a later stage when it has become
vascular; PZ. peripheral zone of hairgerm, later giving rise to the outer rootsheath ; Sc. stratum comeum of epidermis ;
SM. stratum Malpighii.
 
 
 
forms. In these latter there are parenchymatous cells very
similar to an early stage in the development of the horn-cells.
 
The horn-cells are doubtless an adaptation to, or result of, an aerial life, and consequent drying of the surface of the body. Such protected surfaces as cavities of
the ear and nostrils do not develop horn-cells, although the evanescent embryonic
epitrichial layer is present. It is thus the effete epiblastic cells themselves which
constitute the protective layer.
 
A transverse section of the epidermis of Man, which may be taken as being typical
of Mammalia generally, shows a superficial horny layer ( stratum comeum ), and a
deeper-seated Malpighian layer or rete viucosum. The latter has a basal layer of
columnar cells, from which the whole epidermis is derived, and there is a complete
transition between this layer and the flattened scales, which are thrown off the surface by desquamation. Anatomists usually distinguish several layers in the epider
 
 
102
 
 
THE STUDY OF EMBRYOLOGY.
 
 
mis, but the three layers already referred to, the mucous epithelium, the transitional
cells ( = mucous layer), and the horny layer, are alone of morphological importance.
 
Nails, claws, hoofs, horny beaks, the horny sheaths of the horns
of the Bovidse, are merely local condensations of the horny layer
of the epidermis, while hairs are similar linear extensions.
 
A hair commences as a minute solid ingrowth of the columnar
layer of the epidermis into the derma (fig. 86). A small bulb, the
hair papilla, containing nutritive capillaries, grows up from below
 
 
 
Fig. 87. Development of Feathers. [After Jeffries.]
 
A. Transverse section of a feather papilla near the tip, twenty days - Duckling.
 
B. The same lower down of an eighteen days - Duckling. C. Longitudinal section
of the same as A ; there is a capillary within the pulp full of blood-corpuscles.
 
D. Transverse section of a pin-feather of an embryo Robin.
 
b. primitive barbs ; bl. incipient barbules ; c. capillary ; e. epitrichial layer :
h. horn cells; m. mucous or Malpighian layer; m.b. mucous layer of barb;
p. pulp; p.s. pith of shaft; s. shaft; t. transitional cells; v. vane; * point of
division between the two vanes.'
 
into the hair follicle. The outer cells of this papilla elongate, become cornified, and thus form a hair, which soon forces its way to
the exterior through the follicle.
 
Down-feathers arise from large papillae, which contain a central vascular mesohlastic pulp ; as the papillae grow in length, they tend to sink below the surface,
more especially at the posterior side, thus producing the backward slant of most
feathers ; the depressions are known as feather-follicles. Two thickenings of the
epidermis appear on the upper surface of the papilla and encroach on the pulp, starting from the top, and slowly extending downwards (fig. 87, A, b ). Whilst these first
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
103
 
 
two barbs are growing, the epitrichial layer becomes more compact and the transitional cells horny, thus forming a protective case for the incipient feather. As the
papilla grows, more barb folds appear (fig. 87, b, b). The barbs are formed by the
cells at the angle of the thickenings, as seen in section, while the cells on the sides
arrange themselves in columns (fig. 87, c, bl), which bend slightly towards the tip of
the papilla, and ultimately form the barbules. The walls of the cells of the barbs
and barbules finally become converted into a kind of horn, and the protoplasmic
contents dry up.
 
The contour feathers of adult Birds are developed upon the same plan as the
down-feathers, by a renewed growth of the primitive papilla. Two primary barbfolds appear as before, and are very shortly followed by numerous others. The two
primaries unite to form the two halves of the shaft, and are joined later by those on
the sides. At the side of the papilla, opposite to that where the shaft is formed, is
a slight inversion of the mucous epithelium ; it is here that the separation will
occur which results in the two vanes of the feather. As the barbs are set at an angle
of about forty-five degrees, the portions farthest from the shaft in a transverse section are sections of the tips of lower barbs. In feathers with a hollow shaft, the two
sides bend in and enclose a column of the pulp (fig. 87, d), which subsequently
dries up ; in solid shafted feathers the sides are simply flattened together. When
the feather is matured, the covering falls off and the pulp withers away, and the
barbs separate into the two vanes. Thus it comes about that the upper surface of
the shaft and barbs of a feather (together with the whole of the barbules) is formed
from horn cells or modified transitional cells, whereas the lower surface is composed
of degraded mucous epithelium. The quill is produced by a cornification of the
walls of the lower portion of the papilla.
 
As hairs consist of the horny layer of the epidermis only, it is
evident that they can scarcely he regarded as strictly homologous
with feathers; the latter are never found out of the group of Birds,
and the former are equally peculiar to Mammals.
 
The scutse which occur on the legs of Birds are mere folds of
skin with a horny layer, a mucous epithelium, and a mesodermal
core. They occasionally bear feathers.
 
The scales of Snakes, of Chelonia (tortoise-shell), and also of
some Lizards are purely epidermal structures ; but those of other
Lizards (Anguis, Cyclodus, Scincus) and the scutes of Crocodiles,
and of the Armadillos amongst Mammals, are partly derived from
the epidermis, but chiefly from the corium; in other words, they
are mainly of mesoblastic origin (see p. 193). The scales of the
Manis, like the horn of the Rhinoceros, are formed of hairs agglutinated together.
 
Teeth are not purely epiblastic organs, hut they may be conveniently dealt with here. There is little doubt that teeth were
primitively structures similar to the placoid scales of Elasmobranchs
which have been retained and emphasised in the jaws.
 
A placoid scale arises as an ingrowth from the derma into the
epidermis, the basal columnar cells of the latter are pushed up and
 
 
104
 
 
THE STUDY OF EMBRYOLOGY.
 
 
thus form a kind of sheath to the papilla. The basement membrane,
which is a product of the epidermis, becomes thickened and calcified
at the apex of the papilla, and constitutes an enamel cap, the
papilla becoming converted into dentine, bone, and pulp. The
point of the scale eventually forces its way to the exterior.
 
In the development of a milk (deciduous) tooth a prolongation
from the epidermis arises which passes into the derma (fig. 88) ;
the inferior end becomes dome-shaped, forming the “ enamelorgan - (fig. 89). A papilla of the derma projects into the hollow
of the dome, and soon becomes vascular ; the papilla produces the
dentine and cement of the tooth, while the columnar layer of the
 
 
b
 
 
Fig. 88. Fig. 89.
 
Early Stages in the Development of Milk-Teeth.
 
[From Landois and Stirling.']
 
Fig. 88.  - a. dental ridge; b. commencement of the enamel organic, dentine
germ, first trace of the pulp ; d. first indication of the mesoblastic investment or
tooth-sac.
 
Fig. 89.  - a. dental ridge; 1 upper, 3 lower or secreting layer of the enamel
organ (b), 2 intermediate epiblast cells ; c. dental papilla, with capillary ; d. commencement of dental sac; e. enamel germ of the corresponding permanent
tooth.
 
enamel-organ or germ, which overlies the papilla, is stated to
secrete the enamel layer (fig. 90). The permanent teeth are similarly developed, but the enamel germ arises as a bud (fig. 89, e )
from that of the deciduous tooth. Huxley, and after him Miss
Nunn, asserts that the enamel, like the dentine of teeth and
scales, owes its origin to odontoblasts, and is therefore mesoblastic ;
and that the cuticula dentis is formed by the metamorphosis, either
in whole or in part, of the enamel cells, which have nothing whatever to do directly with the formation of the enamel. However
this may be, the large size and the invariable presence of the
enamel organ prove that it has, or has had, an important function
in the production of teeth.
 
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
105
 
 
In Elasmobranchs all the teeth of each jaw are developed from a common rod of
tissue, which is derived from a ridge-like proliferation of the mesoblast into the epidermis of its jaw. The enamel cap of each tooth is formed in the same manner as
in the placoid scales.
 
Scott finds that the horny teeth of the metamorphosing Lamprey are developed
from the deeper layer of the epiblast which rises in a cap-like manner over a mesoblastic papilla : this appears to be the representative of the enamel organ. A second
tooth is developed vertically below the first. The original papilla and enamel organ
are functional throughout the life of the animal.
 
Horny teeth and a horny sheath to the jaws occur in larval Anura.
 
Epiblastic Glands.  - The epidermis is the seat of origin of
many and varied glands. The simplest cases are where certain cells
become enlarged and secretive, forming unicellular glands. These
 
 
 
*</
 
 
Fig. 90. - Later Stage in the Development op a Tooth. [From Wiedersheim.]
 
Bg. connective tissue which forms the dental sac: D.S. dentine; E.M. epithelium
of mouth; Ma. membrana adamantina (cuticula dentis); O. odontoblasts; SK.
enamel germ ; ZK. tooth germ.
 
alone occur in the Ccelenterata ; in higher forms they often coexist with multicellular glands.
 
Multicellular glands may be simple (figs. 18, 84) or compound
(figs. 91, C-E ; 93, A, m.gl. ; 141, f). The development of such a very
complex gland as a salivary or mammary gland is as follows :  - A
simple solid process from the epidermis sinks into the derma;
branches sprout out from its blind end ; these acquire a central
cavity, elongate, and greatly increase in number, until a muchbranching tubular organ is developed. The ultimate ramifications
in the above-mentioned glands expand into secretory pouches or
alveoli.
 
 
106
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Although there is a solid ingrowth of the epidermis, it is the
Malpighian layer alone which forms the secretory tissue of the
gland; the central epidermal cells eventually disappear. The
solid ingrowth of the incipient gland is clearly a secondary
process.
 
Other glands may always remain simple tubes, or at most
become slightly branched.
 
Thus the most complex type of gland reproduces in its own
development those simpler conditions which it must have passed
through in the course of its evolution, and which are severally the
permanent states of other glands. In the simplest glands all the
cells are secretory, but as complication arises the stem and main
 
 
 
Fig. 91. - Diagrams to Illustrate the Evolution oe
Complex Glands. [ After
 
Huxley. ]
 
A. Section of an ideally simple
skin, showing the mucous and
horny layer of the epidermis (ep),
and a capillary (c) within the
derma or cutis ( d ). B. A simple
gland, with its capillary network.
C, D, E. Glands of increased complexity. The vascular supply is
omitted in these figures. .
 
 
branches lose this function and constitute ducts to convey the fluid
secreted by the terminal portions.
 
All the glands opening on the general surface of the body are of
epibiastic origin ; such are the sweat, scent, anal, poison, adhesive,
byssus, slime, spinning and mammary glands. The salivary glands
of Insects develop as paired invaginations from the ventral plate of
the mouth, behind the stomodaeum, and on the inner side of the
mandibles.
 
According to Klaatsch, the mammary glands develop from a shallow depression,
the glandular area or areolar epithelium, the margin of which is slightly raised.
This condition is permanent in Monotremes (fig. 92). In adult Man, the glandular
area is raised to form the nipple ; the same occurs in the Mouse, but the glands
have a single duct. The nipple of Carnivores, Pigs, Horses, and especially that of
Ruminants, is formed by the upward growth of the raised margin in such a manner
that the glaudular area forms a pit, at the bottom of which the glands open.
 
 
ORGANS DERIVED FROM THE ETCBLAST.
 
 
107
 
 
It seems probable that the mammary glands are greatly enlarged and modified
sebaceous glands, the hairs to which they belong having disappeared in course of
 
 
 
Fig. 92.  - Diagrams of the Arrangements of the Ducts of the Mammary
Glands in Various Mammals. [ From Bell after KLaatsch.]
 
Tlie glandular area of the epidermis is indicated by the thicker line.
 
A. A dult Echidna. B. Human embryo. C. Human adult. D. Adult Mouse.
 
E. Embryo Cow. F. Adult Cow.
 
time (see figs. 86, e and f, Dr; 93, b, h) ; but Rein denies this, and believes that they
are organs sui generis. Gegenbaur has lately shown that the so-called mammary
 
 
 
Fig. 93.  - Development of Mammary Glands of Marsupials.
 
A. Embryo of Phalangista vulpina (9.5 cm.) B. The same of Perameles gunii
(8.6 cm.) Vertical section through rudiment of the mammary depression. [After
Klaatsch.~\
 
d. derma; ep. horny layer of epidermis; h. hair; m. Malpighian layer of
epidermis ; rn.gl. milk-glands ; p. processes of Malpighian layer, indistinguishable
from hair or gland rudiments ; s.gl. sebaceous gland.
 
glands of the Monotremes are phylogenetically distinct from those of other Mammals.
They consist of tubular glands, modified derivatives of the sudoriferous type.
 
 
108
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Those glands derived from the stomodaeum (p. no) and proctodaeum are also epiblastic in origin.
 
In the “Veliger- larval stage of marine Prosobranch Gasteropoda
a group of epiblast cells, on each side of the body behind the
velum enlarge, become vacuolate, and constitute what are generally regarded as provisional renal organs (fig. 84, A, p.k, c). The
red or violet pigment spots occurring in the Veligers of Opisthobranchs and a few other Molluscs may be of a similar nature
(fig. 84, A, pgt).
 
De Meuron describes the primitive renal organs of Helix as
arising from epiblastic invaginations, and not as being mesoblastic
in origin, as are, according to Eabl, the kidneys of the aquatic
Pulmonata. This organ is a tube with a ciliated internal orifice as
in other Pulmonata. Pol had previously described the provisional
excretory organs of the terrestrial Pulmonates as a pair of nonciliated epiblastic pits, with no internal orifice. The permanent
kidney appears to be formed as an epiblastic invagination supplemented by mesoblastic tissue.
 
Muscular Elements.  - The root-like prolongations of the large
ectodermal cells of Coelenterates are contractile, and practically
form an external muscular sheath to the body. The brothers
Hertwig have demonstrated distinct ectodermal muscle-cells in
the Actiniae, and Hubrecht has found similar cells in the Nemertea.
 
The non-striated muscle-fibres which surround some sweat glands
are stated by Eanvier to be derived from the epidermis in Man.
 
Respiratory Organs.-  - Prom the nature of the case, the external skin of the body must always act as a respiratory surface,
except when it is surrounded with an impervious cuticle or exoskeleton ; but certain areas are usually more especially devoted to
the interchange of those gases which constitute what is known as
respiration.
 
Invertebrates.  - The external organs for aquatic respiration
are mostly delicate filaments or plates (branchiae or gills), within
which the blood freely circulates.
 
The manner in which such organs are developed is so selfevident as to need no special comment. In certain cases, as, for
example, in the gills of some Lamellibranchs (e.g. } Anodonta, Dreissena), the primitively simple bent gill-filaments form the perforated
plate-like gills of the adult by concresence between their two
limbs, and by the union of the filaments with each other.
 
It may be here noted that the respiratory plumes of Serpulaceae
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
109
 
 
amongst the Cbtetopoda are supported by (probably mesoblastic)
cartilaginous bars.
 
Hartog and others have shown that anal respiration occurs in
probably all larval Crustacea, and also in some adults. Leaf-like
respiratory organs occur in the rectum of larval Dragon-flies. It
will be shown that in the Arthropoda the rectum is derived from
the proctodseum (see p. 1 1 1).
 
Aerial respiration has supplemented branchial respiration in
some Mollusca and Crustacea by the upper portion of the branchial
chamber becoming vascular and functioning as a lung ( e.g ., Ampullaria, Birgus). In the Pulmonate Mollusca the gills have entirely
disappeared. Lankester has suggested the probable evolution of
the pulmonary sacs of the aerobranchiate Arachnida (Scorpions
and Spiders) from the lamellate gills of their Limulus-like ancestors (Haematobranchiata).
 
The trachese of the tracheate Arthropoda appear to have been
derived from simple diffused cutaneous glands, which have evolved
into delicate branching respiratory tubes.
 
Hubrecht has shown that the ciliated pits which penetrate the posterior brainlobes of the higher Nemertean worms arise solely from the epiblast, and not partially
from the oesophagus, as previous observers had stated. It is probable that these pits
have a sensory as well as a respiratory function.
 
Chordata.  - The epidermis is undoubtedly respiratory in some
Chordata, especially amongst the Amphibia.
 
The characteristic respiratory organs of the Chordata are of
hypoblastic origin (see p. 177).
 
External or epiblastic gills are, however, developed in a few
forms. These are the larval external gills of the Ganoid
Polypterus, the Teleost Cobitis, and the permanent external gills
of the Dipnoid Protopterus, and of some Urodeles. In other
Amphibia they are purely larval organs. The so-called external
gills of embryo Elasmobranchs are merely the extremely long
filaments of the internal gills of the posterior lamellae, only, of each
arch, which protrude beyond the clefts.
 
In certain Teleosts {e.g., Anabas, some Siluroids) accessory respiratory organs,
which are supported by very delicate contorted bony plates, may grow out from the
upper portion of the gill-arches into the branchial chamber (p. 179). They are thus
necessarily invested by the epidermis, and constitute organs for aerial respiration.
In Saccobranchus and a few other Fishes, air is respired by means of membranous
sacs which evaginate from the branchial chamber and push their way along the
lateral muscles of the body. These Teleosts have thus acquired new epiblastic
organs for breathing air direct ; but the problem of aerial respiration has been more
satisfactorily solved by the utilisation of the air-bladder of Fishes and the development of lungs in the ancestral Amniota.
 
 
110
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Stomodaeum.  - The invagination of the epiblast, which in most
animals forms the month of the adult, is known as the stomodseum.
There is reason to believe, as has already been stated (p. 75), that
in the Invertebrates the stomodseum corresponds with the anterior
extremity of the primitive mouth (blastopore). In the Chordata
the same interpretation is held by some embryologists, but Dohrn
and his school believe that the stomodseum is a new formation
possibly corresponding to a pair of fused gill-slits.
 
Whatever theory may be held concerning its nature, the fact
remains that in a large number of animals the mouth arises as an
epiblastic invagination which subsequently unites with the blind
anterior end of the mesenteron (archenteron). The following types
will serve as examples:  - Asterias (fig. 52), Lymnseus (fig. 84, b),
Astacus (fig. 140), and Petromyzon and Bombinator (fig. 94).
 
Usually the stomodseum forms but an insignificant portion of
the alimentary canal; but in the Arthropoda, especially in the
Crustacea (fig. 140, f.g), it is of considerable size. In the latter
group the stomodseum forms the large crop or masticatory
“ stomach,- which in the Decapoda is complicated by the development of the gastric mill and the filtering apparatus. In Insects
it forms the oesophagus, crop, and proventriculus or gizzard, when
such are present. The mouth, oesophagus, and masticatory apparatus of Botifers are also derived from the stomodseum.
 
All the structures and glands developed from the stomodseal
epithelium are necessarily of epiblastic origin, amongst which
may be mentioned the radula sac of Mollusca (fig. 84 B, r ), with
its contained odontophore, the enamel organ of teeth, and various
mucous and salivary glands, but not the salivary glands of
Insects.
 
The Pituitary Body (Hypophysis Cerebri). - The pituitary
body arises in most Vertebrates as a tubular invagination of the
roof of the mouth (stomodseum) approaching the infundibulum
(see p. 127). The upper end becomes swollen, and the stem
gradually atrophies. The enlarged distal portion, which is surrounded by vascular tissue, becomes lobed, and the central lumen
may or may not persist; it ultimately enters into close union with
the infundibulum (figs. 107, hph ; 108, pit, 109, H; 112, hp; n 6,
hph ) ; but it is only in the Mammalia that the two structures fuse
with one another.
 
According to Scott, the pituitary body arises in the Lamprey as an epiblastic invagination between the olfactory epithelium and the stomodseum (fig. 94, a). In the
Frog it appears before the invagination of the stomodseum; but owing to the large
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
Ill
 
 
size of the latter, and the rapid growth of the cerebral hemispheres, the pituitary
body is carried into the mouth (fig. 94, c). In other forms the early development of
the cerebral lobes, probably combined with a later appearance of this now function less organ, causes it to be apparently derived from the roof of the stomodaeum itself.
 
Miss Johnson and Miss Sheldon state that in the Frog and Newt the stomodaeum
is at first a solid ingrowth of the deeper layer of the epiblast ; the lower part of the
ingrowth fuses with the fore-gut, while the upper part projects freely and forms the
pituitary body.
 
In Teleosts, according to Dohrn, the hypophysis arises as a pair of hypoblastic
e vagin ations in front of the mouth, and Hoffmann finds that the earliest rudiment
of the hypophysis is developed in the common Snake from the anterior end of the
archenteron ; the same apparently occurs in the human embryo (fig. 143, H.p).
 
The organ has probably a pre -vertebrate, and possibly a pre-chordate, significance.
 
Proctodaeum.  - The arguments in favour of the stomodaeum
corresponding to the anterior end of the primitive blastopore of
 
 
 
Fig. 94.  - Diagrams to Illustrate the Relation of the Pituitary Invagination
to the Stomodaeum.
 
A. Longitudinal vertical section through the head of an embryo Lamprey just
before hatching \from Scott], B, C. Similar sections through the head of an embryo, and of a young Tadpole, respectively, of a Toad (Bombinator) [ from Scott,
after Gotte],
 
ep. epiblast ; hy. hypoblast ; inf. infundibulum ; ncli. notochord ; olf. olfactory
epithelium ; ph. pharynx ; pt. pituitary invagination ; st. stomodseum ; th. thyroid invagination.
 
Invertebrates apply to the proctodaeum with regard to its posterior
extremity. The blastopore, or a portion of it, however, often persists as the anus, or the anus shortly appears at the spot where it
has closed up.
 
Any invagination of epiblast at the anus constitutes a proctodaeum. In most Invertebrates the proctodaeum is small, but the
long rectum of Crustacea (fig. 140) is derived from this invagination ;
it is also large in other Arthropods.
 
The Malpighian tubules of the Arachnida and Insecta arise as a
single pair of evaginations from the anterior portion of the proctodaeum ; but these usually increase in number.
 
The proctodaeum forms the cloaca of many of the lower Yerte
 
112
 
 
THE STUDY OF EMBRYOLOGY.
 
 
brates, or at all events its outer portion, the anterior section being
formed by the dilated end of the alimentary canal, into which the
urogenital organs open (figs. 73, 143, c). The epiblastic section of
the cloaca is sometimes marked off from the hypoblastic portion
by a small fold.
 
Cloaca of Amniota.  - In a recent paper Gadow states that “ the cloaca of the Amniota consists originally, either permanently or in the embryo only, of three successive
chambers. I. The Proctodceum [Lankester]. The outermost anal chamber of epiblastic origin, with its derivatives : (i.) bursa Fabricii in Birds, (2.) various hedonic
glands in most Amniota, (3.) the copulatory organs, the, at least partly, epiblastic
nature of which is indicated by the frequently developed horny armament of the
glans, by the various sebaceous glands, and by development. II. The Urodceum
[Gadow]. Hypoblastic, this is the middle chamber or primitive cloaca, into which
open the urinogenital ducts and through which pass the faeces. With its differentiations : (i.) urinary bladder, ventral ; (2.) anal sacs in Tortoises, dorsal. III. The
Coprodceum. This is the innermost cloacal chamber.
 
“ The urodaeum is the oldest portion of the whole cloaca, then follows the proctodaeum, and, lastly, the coprodaeum has secondarily assumed cloacal functions.-
 
Nervous System.  - The nervous system and the sensory surfaces
of the sense-organs are, as has been stated, derived from the epiblast.
In scarcely any other section of Embryology is more light thrown
upon the significance of the facts of development by a comparative
study of the adult condition of these structures in the lower
animals. For the sake of convenience the development of the
central nervous system will be first considered, and afterwards that
of the sense-organs.
 
Invertebrates.  - In the majority of Invertebrates the central
nervous system originates from certain areas of the epiblast. The
cells of these areas are usually more or less columnar, and undergo
rapid cell-division (proliferation). The mass of cells thus formed
sinks into the underlying mesoblast, and eventually differentiates
into nerve-cells or ganglion-cells, and into nerve-fibres. Outgrowths from the incipient nerve-centres (ganglia) form nerves
and commissures.
 
Nerve-cells and nerve-fibres occur in all the higher or more
active Coelenterata. They are undoubtedly modified ectodermal
cells which have assumed a deeper position, and in the case of
nerve-fibres have become greatly elongated. As all the ectodermal
cells are connected with one another by means of their basal rootlike processes, the nervous system is from the first connected with
the superficial ectoderm cells on the one hand, and the deeper
seated muscle-cells on the other, that is, of course, when the latter
are present.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
113
 
 
This undifferentiated nervous system is generally diffused over
certain areas, chiefly the oral surface, or it may be restricted to a
circum-oral ring, as in certain Hydroids.
 
Even in adult Starfish the nervous system is scarcely separated
from the epidermis ; and it has recently been shown that in most
Echinoderms a nervous network surrounds the whole animal.
There is, however, a more concentrated nervous tract round the
mouth and along the ambulacral areas in all Echinoderms. The
Carpenters and Marshall have proved the existence of an additional
aboral nervous system in Crinoids.
 
A diffused nervous system lying immediately below the epidermis occurs, according to Hubrecht, in the lower Neinertean
worms, in addition to the lateral cords of the higher forms (see
also p. 165).
 
 
 
Fig. 95.  - Sections to Illustrate the Development of the Nervous System in an
Earthworm (Lumbricus trapezoid.es). [After Kleinenberg.
 
A. Through the head. c.c. cephalic portion of the body-cavity ; c.g. cephalic
ganglion ; ce. oesophagus.
 
B. Through the ventral wall of the trunk, c. body-cavity ; ep. epiblast ; hy.
hypoblast ; m. longitudinal muscles ; n. ventral nerve cord ; so. somatic mesoblast ; sp. splanchnic mesoblast ; v.g. ventral groove ; v.v. ventral blood-vessel.
 
Hatschek describes the nervous system of Criodrilus as first
arising as an anterior ectodermal thickening which extends backwards as a cord on either side of the mouth forming the oesophageal
commissures. The process of thickening continually extends backwards, resulting in the formation of the double ventral nerve-cord.
The nervous system of the Earthworm (Lumbricus), according to
Kleinenberg, develops from the epiblast as two long cords on
each side of a shallow ciliated median ventral groove (fig. 95).
The two cords early unite, and segmental ganglionic enlargements
are soon indicated.- The cephalic ganglion is apparently at first
quite independent of the ventral cords. Hatschek states that the
ventral groove invaginates, and takes part in the formation of the
nerve-cord.
 
In the Mollusca the nervous system is usually developed in the
 
H
 
 
 
114
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ordinary manner by proliferation of the epiblast. This occurs in
two regions. In the early Veliger larva of Gasteropods, or at the
corresponding stage of other Molluscs, a pair of cephalic plates is
formed on the pre-oral lobe within the velum by the rapid celldivision of the locally thickened epiblast. These give rise to the
cephalic ganglia. The pedal ganglia arise from a pair of similar
areas in the foot. Tig. 96, A, shows the proliferating areas which
are giving rise to the cephalic and pedal ganglia in a Prosobranch
Gasteropod (Purpura) ; these are seen in section in fig. 96, B.
 
The nerve-cords of Chitons have been shown by Kowalevsky to arise throughout
their whole length from the epiblast in the region corresponding to that which they
occupy in the adult. They form, in fact, a double nervous ring surrounding the
latero-ventral aspect of the bodjn On recalling the relationships of the mouth and
anus witli the primitive blastopore (p. 76), it will be found that the nervous system
 
 
 
A. Side view of early veliger. B. Transverse section of the same.
c.g. cephalic ganglion; /. foot; m. mesoblast ; mtl. tuantleedgejp.gr. pedal
ganglion ; s. shell ; v. velum ; y. yolk.
 
of these primitive Mollusca constitutes a double circum-oral ring, in other words, a
nervous system comparable with that of many Coelenterates.
 
Kowalevsky also finds that in Dentalium the cephalic ganglia are derived from pitlike invaginations of the cephalic plates. The depressions soon lose their connection
with the external epiblast and later their central cavity disappears. The pedal
ganglia at first arise from an unpaired area ; this divides, and each ganglion increases
at the expense of the epiblast of the foot.
 
Lankester states that in the Cephalopoda, the white body
originates from the epiblast of the head in the same manner and
in the same position as the cephalic ganglia in other Mollusca, but
that the true ganglia are of mesoblastic origin, the white body
becoming an apparently functionless structure.
 
Bobretzky also derives the nerve-ganglia of Cephalopoda and of the Prosobranch
Gasteropod Fusus from the mesoblast. Two explanations suggest themselves.
1. That the earliest stage of these structures has not yet been observed. 2. That
if the observations are correct, it is a secondary phenomenon due to precocious
segregation (see p. 165).
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
115
 
 
The formation of the nervous system in the Lamellibranchiata
is, so far as is known, quite normal.
 
The ventral nerve-cord in the Crayfish arises in the median
ventral line on each side of a central groove ; this thickened epiblast is continued along anteriorly round the stomodaeum, and
passes into the incipient cerebral ganglia, which are formed in the
centre of the pro-cephalic lobes.
 
According to Eeichenbach, the development of the nervous
system is somewhat more complicated, but the above account is
probably substantially correct.
 
The development of the nervous system is very uniform throughout the Arthropoda ; the ventral cord may arise as a single or a
double thickening of the ventral epiblast ; the median groove may
be shallow, deep, or absent : it is stated to sometimes take part in
the formation of the nerve-cord. The cerebral ganglia are apparently always continuous with the ventral cord.
 
The series of ganglia and the commissures connecting them,
which together constitute the central nervous system of Invertebrates, is thus developed directly from the epiblast. These commissures are usually composed of nerve-cells as well as of nervefibres ; in fact, the ganglia are merely local thickenings of the
commissures with a preponderance of the nerve-cells.
 
The nerves proper develop as prolongations from the central
nervous system, and may give rise to other ganglionic enlargements.
 
Nature of the Invertebrate Brain.  - The portion of the central nervous system
situated in front of the mouth (pre-oral) is always associated with the eyes, and constitutes the primitive brain. Lankester has appropriately termed this the archicerebrum. All the nerves which originate from it supply the pre-oral region of the
head.
 
The brain of most, if not of all, Worms is an archi- cerebrum, as is also the preoral nervous system of the Amphineurous Mollusca (Neomenia, Chiton).
 
There is a tendency in the Arthropoda for the anterior appendages with their
ganglia to shift forwards. In this manner a composite brain (syn-cerebrum) is
formed. As the nervous system is composed of two lateral halves, there is no antecedent improbability in the migration forward of the ganglia.
 
All the appendages of the Nauplius larva of Crustacea are post-oral; and Pelseneer
has recently shown that in Apus the ganglia of the first pair of antennae have
migrated to the brain, although their nerves apparently arise from the (esophageal
commissure. The concentration is still greater in other Crustacea ; thus in this
group the brain is always a syn-cerebrum.
 
Balfour has shown that in the Spider the ganglia of the Chelieerae are post-oral,
but they soon become fused with the pre-oral ganglia.
 
The antennae of Insects and Myriapods develop from the pro-cephalic lobes, and
are always innervated by the pre-oral ganglia. Therefore the antennae of these forms
 
 
116
 
 
THE STUDY OF EMBRYOLOGY.
 
 
are probably not homologous with those of the Crustacea, and their brain is an archicerebrum. Hatschek states that the ganglia of the mandibular segment disappears
in the oesophageal commissures, and that the sub-cesophageal ganglion is formed
by the ganglia of the two maxillary segments.
 
An analogous concentration occurs in the brain of the higher Mollusca.
 
There is in the embryos of Arthropoda a pair of ganglia for each segment of the
body, but a fusion of ganglia often occurs in the thoracic region of the body, notably
in the case of the Brachyura and Spiders ; in the former case the concentration occurs
around the sternal artery.
 
Central Nervous System of the Chordata.  - Throughout the
Chordata the central nervous system appears very early, usually as
a more or less well-defined plate of columnar epiblast (neural or
medullary plate) in the median dorsal line of the embryo (figs. 59,
61, 9 7, 100). A central shallow longitudinal groove (neural or
medullary groove) appears in this plate ; it is often widely open at
both ends. The neural plate extends from the dorsal rim of the
blastopore to what will he the anterior extremity of the embryo.
 
 
Fig. 97.  - Embryo of Frog, with Split-like
Blastopore and well-developed Neural Folds. [After 0. Her twig. ~\
 
bl. blastopore ; d.f. dorsal furrow : n. neural
folds.
 
 
The walls of the neural groove bend over, and, fusing in the
median line, convert the groove into a canal, the neural or medullary canal (figs. 63, 64, 102). The enlarged anterior portion of the
neural tube is the incipient brain; the remainder will develop into
the spinal cord. Before closing over the canal becomes ciliated
in Amphioxus and the Fowl.
 
Neurenteric Canal.  - In those Vertebrate embryos which have but little food-yolk,
the blastopore occurs as an opening from the archenteron to the exterior, and the
neural groove arises immediately dorsal and anterior to it ; the neural folds, as a
matter of fact, extend round each side of the blastopore (fig. 97).
 
The supposed relation of the blastopore of such embryos to the primitive blastopore, and the position of the latter with regard to the nervous system, has already
been briefly mentioned (p. 76).
 
When the neural folds unite in the median line to form the neural canal, their
posterior portion which surrounds the blastopore may also close over. ;By this
means the blastopore would be shut off from the exterior by this overgrowth, and
would necessarily open into the posterior extremity of the neural canal. The short
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
117
 
 
tube connecting the cavities of the nervous system and archenteron is known as the
neurenteric canal (fig. 99, ne). The ventral portion of the canal is also termed the
post-anal gut.
 
It was till quite recently supposed that this occurred in the Cyclostomi and
Amphibia ; but in these groups it appears that the blastopore persists as the anus,
consequently what was termed the post-anal gut (solid in the Newt), which was
imagined to extend between the closed-over blastopore and the new anus, is merely a
ventral extension of the neural canal, owing to the growth of the tail taking place
above the blastopore (figs. 98, 99).
 
 
Fig. 98.  - Diagrammatic Longitudinal Section THROUGH THE EMBRYO OF A FROG.
[From Balfour after Gotte.]
 
al. alimentary canal (archenteron) ; m.
mesoblast ; nc. neural canal ; yk. yolk-cells ;
x. point of junction of epiblast and hypoblast
at the dorsal lip of the blastopore. For the
sake of simplicity the epiblast is represented
as if composed of a single row instead of two
layers of cells.
 
 
 
In those forms in which the blastopore, as such, is obsolete, being partially represented by the primitive streak (see p. 41), the neurenteric canal may be lost, but
in many ( e.g ., Lizard, Goose, Duck, Parrot, Mole) it still occurs and occupies the
same relative position. In the Fowl and other Amniota the canal is lost, but
traces of it may occur.
 
The closure of the neural groove takes place from behind forwards in Tunicates and Amnhioxus. but usuallv in Vertebrates it
 
 
Fig. 99.  - Longitudinal Section
 
THROUGH AN ADVANCED EMBRYO of a Toad (Bombinator). [ From Balfour after
Gotte.]
 
an. anus, this should be represented as an opening into the
alimentary canal ; ch. notochord ;
l. liver ; m. mouth (stomodseum) ;
me. neural (medullary) canal ;
ne. neurenteric canal,  - between
this and an is the so-called postanal gut ; pn. pineal gland.
 
 
 
first closes in the region of the neck or hind-brain (fig. 100). The
closure in some cases takes place more rapidly backwards, but in
others the brain is the first to close over (fig. 10 1).
 
It is important to note that in the Tunicates and Amphioxus an anterior pore
(neural pore) persists for some time after the rest of the canal is completed. At this
stage (fig. 57, oe) the cavity of the archenteron can only communicate with the exterior through this pore. For suggestions concerning a possible significance of this
arrangement, the reader is referred to papers by Sedgwick and Van Wijhe.
 
 
118
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In the Teleostei, Lepidosteus, and Lamprey (fig. 61, b), the
central nervous system arises as a solid axis of epiblast cells ;
the epidermal layer may, however, be carried down into this keel
to line the subsequently acquired central lumen ; but Shipley denies
that this occurs in the Lamprey. This variation has only a
secondary significance.
 
 
 
Fig. ioo. - Embryo Fowl, 3 mm. long, of about
twenty-four hours, seen from above, magnified
thirty-nine diameters. [From Kolliker].
 
Mn. union of the medullary folds in the region
of the hind-brain ; Pr. primitive streak ; Pz. parietal zone ; Bf. posterior portion of widely open
neural groove ; Rf'. anterior part of neural groove ;
Rw. neural ridge ; Stz. trunk zone ; vAf. anterior
amniotic fold ; vD. anterior umbilical sinus showing through the blastoderm.
 
His divides the embryonic rudiment into a
central trunk zone, and a pair of lateral or parietal
zones.
 
 
In those forms in which the epiblast is early separable into an
epidermic and nervous or mucous layer (some Ganoids and Anura)
(fig. 24, e), the nervous tract is entirely formed at the expense of
the latter, while the epidermal layer of the medullary plate persists
as the epithelium of the central canal of the nervous system.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
119
 
 
It will be convenient first to trace the further history of the
spinal cord and its nerves, and afterwards that of the brain and
the cranial nerves. The nervous system at this stage consists of a
tube of epiblast several cells thick, with an anterior enlargement
(fig. 98). This is practically the adult condition in Amphioxus,
except that in this form there is no increase in size of the neural
canal anteriorly.
 
 
 
Fig. ioi.  - Embryo Fowl, 4.2 mm. long, of the second day, seen from above, magnified a little
over fifty diameters. [From Kolliker.]
 
Ao. area opaca or vasculosa, bounded by tbe rudiment of the terminal vessel ;
the more external portion of this area has not been shaded, and the blood-vessels
are not represented ; Ap. area pellucida ; Hh. hind-brain ; Mh. mid-brain ; Vh.
fore-brain ; om. rudiments of omphalo-mesenteric veins ; omr. point where the
closure of the neural groove is travelling backwards ; Uw. muscle-plates ; other
lettering as in fig. 100.
 
Spinal Nerves.  - Immediately after the neural tube has become
quite disconnected from the epidermis, paired outgrowths from
the dorsal portion of the nervous wall arise at definite intervals
(fig. 102). These grow ventral-wards, and are the dorsal (afferent,
sensory, or posterior) roots of the spinal nerves. An enlargement,
which is apparent very early, is the rudiment of the ganglion. A
short time after the appearance of the dorsal roots, the ventral
 
 
120
 
 
THE STUDY OF EMBRYOLOGY.
 
 
(efferent, motor, or anterior) roots sprout from the inferior angle of
the spinal cord ; eventually they fuse with the former.
 
In Amphioxus there are large nerves with dorsal roots, and the ventral roots are
epresented by a few loose nerve-fibres which do not unite with the former. The
ventral roots form distinct nerves in the Marsipobranchs, but in Myxine alone are
they united with the dorsal into a common trunk.
 
In a fully developed spinal nerve (fig. 103) a dorsal branch
(ramus dorsalis) passes off to the dorsal region immediately below
the ganglion ; below the latter a branch (ramus intestinalis) passes
to the sympathetic system, and finally the main trunk (ramus
ventralis) divides into its peripheral branches.
 
The dorsal roots of the spinal nerves are generally stated to arise from a median
dorsal ridge of cells, termed by Marshall the “neural crest.- Later, they emerge
more from the sides of the spinal cord ; and, in some forms, all or some of the
 
 
Fig. 102.  - Transverse Section through the Trunk of an
Embryo Dog-Fish (Pristiurus). [ From Baljour.']
 
al. alimentary canal ; ao. aorta ; mp. muscle-plate ; mp'. portion of muscle-plate converted into muscle ; nc. neural canal ;
pr. dorsal root of spinal nerve arising from the neural crest ; sc.
somatic mesoblast ; sp. splanchnic mesoblast ; Vv. portion of the
vertebral plate which will give rise to the vertebral bodies ; x.
subnotochordal rod.
 
 
roots on each side are temporarily connected together by a longitudinal commissure (fig. 104). It is possible that the lateral attachment is not, as some
investigators believe, an entirely new formation, but that it is due to the upward
growth of the dorsal portion of the spinal cord, and the commissures may be each
lateral half of the neural crest.
 
It is, however, conceivable that while the apparent shifting of the attachment of
the dorsal roots may primitively be due to the dorsal growth of the spinal cord
itself, in some cases, at all events, a second connection due to concrescence may have
originated lower down on the sides of the spinal cord.
 
Sympathetic Nervous System - The sympathetic ganglia arise,
according to Balfour, as enlargements of the main branches of the
spinal nerves. Later they are removed from their nerves, but are
still connected by short nerves (fig. 103).
 
Schenck and Birdsell state that in Mammals the main portion of the sympathetic
system arises from the lower portion of the spinal ganglia, and that especially in
the neck the sympathetic cords arise as a continuous ganglionated chain.
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
121
 
 
Histogenesis of the Spinal Cord.  - When the neural canal is completed, its walls
are several cells deep ; the thickness increases, and gradually differentiation occurs.
 
 
 
Fig. 103.  - Transverse Section through the Anterior Part of the Trunk of an
Embryo Dog-Fish (Scyllium). [ From Balfour. J
 
As a matter of fact, the ventral nerve roots do not arise immediately below the
dorsal but half-way between two dorsal roots.
 
ao. aorta; ar. ventral root ; ca.v. cardinal vein ; ch. notochord; du. duodenum ;
dn. ramus dorsalis ; hp.d. point of junction of hepatic duct with duodenum ; mp.
muscle-plate ; mp'. part of muscle-plate already converted into muscles ; mp. 1. part
of muscle-plate which gives rise to the muscles of the limbs ; nl. nervus lateralis ;
pan. pancreas ; sd. segmental duct; sp.c. spinal cord; sp.g. ganglion of dorsal
root ; sp.n. spinal nerve ; st. segmental tube ; sp.g. sympathetic ganglion ;
umc. umbilical canal.
 
The peripheral cells lose their cellular appearance, become much elongated in a
longitudinal direction forming nerve-fibres. The nerve-fibres are at first non
 
Fig. 104.  - Vertical Longitudinal Section through
Part of the Trunk of a Young Scyllium Embryo. [From Balfour.]
 
ar. ventral (anterior) roots of spinal nerves ; com.
commissure uniting the dorsal ends of the dorsal nerveroots ; ge. epithelium lining the body-cavity in the region
of the future germinal epithelium ; pr. ganglia of the
dorsal (posterior) roots ; sd. segmental duct ; st. segmental tubes.
 
 
 
medullated, and occur in greatest profusion in certain definite tracts (white matter),
usually ventral or lateral, but soon extending all round the cord. The remaining
 
 
122
 
 
THE STUDY OF EMBRYOLOGY.
 
 
primitive cells metamorphose into the nerve-cells of the grey matter, with the exception of those cells which line the central canal, and which always retain their
epithelial character.
 
The nerve-cells are at first rounded and apolar. His states that in the human
embryo radial processes arise very early, and that the majority of the cells are at
first bipolar.
 
The central canal retains its primitive slit-like appearance in transverse sections
for a long time, but the exact form of the canal in section varies according to the
region of the body and age of the embryo. Ultimately it becomes reduced by
closure from above downwards to the minute round canal of the adult, which therefore represents the ventral portion of the primitive canal.
 
The ventral (anterior) fissure is produced by lateral downgrowths of the cord,
while the dorsal (posterior) fissure has in the Pig, according to Barnes, the following
origin. After the dorsal (posterior) columns of white matter nearly meet one
another in the median dorsal line, they grow downwards as two horns (Burdach -s
tract) ; in the narrow space between them are wedged two masses of cells (Goll -s tract),
which are either derived from the cord, or more probably are of mixed origin, i.e .,
partly mesoblastic (fig. 105). They are separated below by “horn fibres,- derived
 
 
 
 
Fig. 105.  - Diagrams Illustrating the Formation of the Anterior and
Posterior Fissures of the Lumbar Region of the Sfinal Cord in a
Pig. [After Barnes.]
 
A. From an embryo 43 mm. in length.
 
B. ,, ,, 65 „ ,,
 
C. ,, „ 97 >>
 
af anterior (ventral) fissure ; b. Burdach -s column ; c.c. central canal ; o. Goll -s
column ; g.m. grey matter ; p.h.f. posterior horn fibres ; w.m. white matter.
 
from the degraded epithelial cells of the retreating central canal. The dorsal fissure
is thus produced by ingrowths of the dorsal columns of white matter, and the
atrophy of the tissue lying between them. The downgrowth appears to be independent of the reduction of the canal, as the latter may be reduced to nearly its minimum
length before the former commences (fig. 105, c).
 
Development of the Vertebrate Brain. - The enlarged anterior portion of the neural canal early exhibits definite dilatations ;
of these, three primary brain vesicles are usually recognised, the
fore-, mid-, and hind- vesicles (fig. 106, Vh, Mh, Eh), but these must
not be regarded as having equal morphological value.
 
The middle-brain vesicle is apparently simple in character, but
the last is undoubtedly compound, being formed of several imperfect dilatations, each of which is comparable with the mid-vesicle.
The anterior one of these (fig. 106, Hh) is always well marked, and
dorsally gives rise to the cerebellum.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
123
 
 
A noticeable feature in the embryonic brain is the downward
curvature of its anterior portion. The flexure is slight in those
forms which have small cerebral hemispheres (Cyclostomi, Ganoidei, Teleostei, Amphibia), but well marked in the remaining
groups. The “ cranial flexure,- as it is termed, is apparently rectified as development proceeds, but this is merely due to the increased
 
 
size of the cerebral hemispheres,
thing, becomes more pronounced.
 
 
Fig. 106.  - Dorsal View or Anterior Portion
of Embryo Fowl at the End of the
Second Day, 4.27 mm. long. Magnified 40
diameters. [From Kolliker .]
 
Abl. optic vesicle ; H. heart ; Hh. cerebellar
dilatation of the primitive brain ; Mil. mid-brain ;
Mr. neural canal ; Mr', wall of mid-brain ; Uw.
muscle-plates ; Vh. anterior primary brain vesicle ;
Venn, omphalo-mcsenteric vein.
 
 
The primitive flexure, if anyYh
 
 
 
Before describing the development of the brain, it will be advisable to give a brief
account of the structure of such an unspecialised type of brain as that of the Frog.
 
The posterior region of the Frog -s brain, the medulla oblongata, gradually passes
behind into the spinal cord or myelon. It is triangular in shape, with thick sidewalls and floor, but the roof is very thin, and richly supplied with blood-vessels
forming the choroid plexus. The central canal of the spinal cord expands in the
medulla to form the fourth ventricle.
 
The dorsal anterior wall of this region of the brain is thickened and dorsally produced (fig. 107, cbl), and is known as the cerebellum.
 
The roof of the brain in front of the cerebellum is produced into two thick-walled
hollow vesicles, the optic lobes. The cavity of the region of the brain, into which
the optic lobes open, is the iter a tertio ad quartum ventriculum (or passage between
the third and fourth ventricle), or more shortly, the iter. The anterior end of the
 
 
124
 
 
THE STUDY OF EMBRYOLOGY.
 
 
iter is narrowed ; in the dorsal wall of this neck lies a transverse bundle of nervefibres, the posterior commissure.
 
The cavity of the brain again expands to form the third ventricle ; this brain
region is the thalamencephalon. The anterior portion of its roof is prolonged to form
the pineal gland, and the posterior portion of its floor forms the sac-like infundibulum, to the extremity of which the pituitary body is attached. A fan- shaped
bundle of nerve-fibres passes down the side walls of the thalamencephalon, and
decussating on its ventral wall, forms the optic-chiasma (fig. 107, o.ch). The median
anterior wall of the thalamencephalon is called the lamina terminalis ; about
half-way up is situated the “ anterior commissure - of authors, but this latter is
really composed of a separated upper and lower bundle. Osborn has recently shown
that the upper bundle (which occurs in all Amphibia and Reptiles) is a rudimentary
corpus callosum, as it contains the fibres of the dorso -medial moiety of the hemi
 
A B C D
 
 
 
A. Dorsal view. B. Ventral view. C. Horizontal section. D. Side view [ after
Howes]. E. Longitudinal section [after Osborn].
 
a.c'‘". anterior commissure (pars olfactoria and pars temporalis); chi cerebellum ; cc. corpus callosum ; c.h. cerebral hemisphere ; c.pl. 3 and 4. choroid plexus
of the third and fourth ventricles respectively ; f.m. foramen of Munro ; hph.
hypophysis (pituitary body); inf. infundibulum; iter, aqueduct of Sylvius;
l.tm. lamina terminalis; my. myelon ; op. optic lobe; op.ch. optic chiasma;
op.th. optic thalamus ; s.c. superior commissure ; 1. olfactory nerve ; 11. optic
nerve ; iv. fourth cranial nerve ; 3 and 4. third and fourth ventricles.
 
spheres. The lower bundle (Reptiles, Amphibia, Fishes) represents the anterior
commissure of Mammals (fig. 109, Ca). Two regions are discernible in the lower
bundle, the pars olfactoria and the pars temporalis ; the latter, feebly developed in
the Amphibia, increases with the progressive development of the temporal lobe.
 
The antero-lateral angles of the thalamencephalon are produced into a pair of
elongated lobes, the cerebral hemispheres. They gradually narrow in front, but
again slightly enlarge to form the olfactory lobes ; from their anterior extremities
the olfactory nerves (fig. 107, 1) pass off to the nose. The olfactory lobes are fused
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
125
 
 
together in the middle line. The common cavity, lateral ventricle, of each hemisphere and olfactory lobe communicates with the third ventricle through the foramen
of Munro.
 
A diagram of a section of the brain of an embryo Fowl (fig. 108) may be
advantageously compared with the Frog -s brain. It will be at once noticed that the
 
Fig. 108.  - Diagrammatic Outline
of a Longitudinal Section
THROUGH THE BRAIN OF A FOWL
 
Embryo of Ten Days. [From
Quain after Mihalkovics .]
 
ac. anterior commissure ; amv. anterior medullary velum ; below this are
the aqueduct of Sylvius and the crura
cerebri ; ba. basilar artery ; bg. corpora
bigemina ; cai. internal carotid artery ;
cbl. cerebellum ; ch$, ch i . choroid plexus
of the third and fourth ventricles respectively ; h. cerebral hemisphere ; inf. infundibulum ; It. lamina terminalis ; Iv.
lateral ventricle ; obi. medulla oblongata ; olf. olfactory lobe and nerve ; opc.
optic commissure ; pin. pineal gland ;
pit. pituitary body; ps. pons Varolii ;
r. roof of fourth ventricle; st. corpus
striatum ; v$. third ventricle ; v*. fourth
ventricle.
 
thalamencephalon with the hemispheres and the cerebellum are in this case relatively
much larger, and the optic lobes smaller. This is increasingly the case as development proceeds.
 
A figure of a vertical section through the human brain is given (fig. 109) to
illustrate the disproportionate increase in size of the cerebral hemispheres over the
rest of the brain, and other Mammalian characteristics.
 
 
 
 
Fig. 109. - Longitudinal Section of an Adult Human Brain.
 
[ From Wiedersheim after Reichert .]
 
Aq. aqueduct of Sylvius ; B. corpus callosum ; Ca. anterior commissure ; Cm. middle
commissure ; Col. lamina terminalis ; Cp. posterior commissure ; FM. foramen of
Munro ; G. fornix ; H. pituitary body ; HH. cerebellum ; MH. corpora quadrigemina ;
 
NH. medulla oblongata ; P. pons Yarolii ; R. spinal cord ; Sp. septum lucidum ; T.
infundibulum ; Tch. tela choroidea ; To. optic thalamus ; VH. cerebrum ; Z. pineal
gland ; I. olfactory lobe and nerve ; II. optic nerve.
 
The Posterior Primary Brain Vesicle.  - At first the walls of
the hind- vesicle have a fairly uniform thickness (figs. 159, 160),
hut a noticeable change occurs when the above-mentioned anterior
thickening (cerebellum) increases in size. The side walls of the
posterior multiple division, medulla oblongata, become much
 
 
126
 
 
THE STUDY OF EMBRYOLOGY.
 
 
thickened and grow away from each other dorsally, leaving a very
thin roof which possesses but little nervous tissue (figs. 125, 126).
In transverse sections the medulla at this stage has a very
characteristic triangular outline (figs. 1 12, 126).
 
The side walls and floor of the medulla become greatly thickened, and local enlargements form the olivary bodies and pyramids.
The thin roof of the cavity of the medulla, fourth ventricle, soon
becomes very vascular, and is known as the choroid plexus of
the fourth ventricle.
 
The minor enlargements of this region of the brain alluded to
above disappear very early and leave no trace.
 
The Cerebellum at first appears as a thickened anterior dorsal
border to the medulla ; in many types this undifferentiated condition is practically retained throughout life (Marsipobranchs, some
Ganoids. Dipnoi, Amphibia, and some Eeptiles). In other forms the
roof becomes greatly enlarged ; in Elasmobranchs the cerebellum is
relatively very large, and at an early stage appears to be composed
of two lateral halves. In Birds a central lobe appears and grows
to a very large size ; the walls being much folded, constitute what
is termed an abor- vitae ; there are two small lateral lobes or flocculi.
In the development of the higher Mammals the central lobe
(vermis) is the first to appear, and remains relatively large for
some time, but the lateral lobes (hemispheres) usually eventually
dwarf the former. In connection with this it is interesting to note
that the cerebellum in the Monotremes consists almost entirely
of the median lobe, and that in the Marsupials the lateral lobes
are still small. The cerebellar fissures at first appear on the
vermis and then extend to the hemispheres.
 
The Pons Yarolii, being the ventral commissure connecting the
two hemispheres of the cerebellum, has a proportionate development with them, and appears rather late. In the Monotremes it
is scarcely more developed than in many Sauropsida.
 
The Middle Primary Brain Vesicle. - The mid- vesicle, or, as
it is usually termed, the mid-brain, has a much simpler history
than the other regions of the brain. The cavity always remains
small, and is known as the Aqueductus Sylvii or iter a tertio ad
quartum ventriculum. In most of the lower Vertebrates the roof
is produced into two vesicles, the optic lobes or corpora bigemina
(fig. 107, ojj). In Birds these assume a lateral position, and the
roof of the mid-brain is thin. In Mammals the roof gives rise to
the solid corpora quadrigemina (fig. 109).
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
127
 
 
In an early stage of their development in Mammals the corpora quadrigemina are
said to appear as an indistinct pair of lobes, a phase comparable with the optic lobes
(corpora bigemina) of the lower Vertebrates. But Kolliker states that the anterior
pair are at first separated from one another by a short longitudinal groove and only
partially from the posterior undivided mass. Later the posterior bodies are completed
by a meeting of the lateral grooves and a posterior extension of the median groove.
In the Monotremes the anterior bodies are well marked, the posterior being inconspicuous, and, according to Owen, not separated by a median groove.
 
The floor of the mid- vesicle is greatly thickened, and forms the
crura cerebri. The relative size of this section of the brain is very
much greater in the embryo than in the adult.
 
The Anterior Primary Brain Vesicle.  - The primitively single
cavity of the fore-vesicle is very early produced into a pair of lateral
vesicles, the optic vesicles (figs. 106, 1 10), the further history of
 
 
Fig. no.  - Horizontal Section of the
Brain of a Rabbit of Ten Days. Magnified 40 diameters. [From Kolliker.']
ab. mesoderm ; as. peduncle of optic vesicle
(83 fj. diam.); ch. notochord ; g. thickening of
the epiblast in the region of the future olfactory pits ; i. infundibulum ; m. mid-brain ;
mes. optic vesicle (26 mm. high) ; v. anterior
brain vesicle ; v. veins.
 
 
which is connected with the development of the eye (pp. 1 57-167).
The fore vesicle grows anteriorly, and a small downgrowth from
the roof indicates the distinction between the anterior and posterior
divisions of the fore-brain. The posterior division is the thalamencephalon (figs, m-115); the anterior will give rise to the
cerebral hemispheres and olfactory lobes.
 
The anterior portion of the floor of the thalamencephalon thickens
to form the optic chiasma, while the posterior part is produced
into a blind backwardly directed pouch, the infundibulum (figs,
no, 115, 135).
 
In the lower Vertebrates the infundibulum is usually relatively large, but in the
higher forms it is much reduced. In Teleostei ventral-lateral swellings of the infundibulum constitute the lobi inferiores ; the single tuber cinereum of Mammals
occupies a similar position. The corpus albicans, which is single in the lower
Mammals, but double in Man and the higher Apes, though single when first developed, arises behind the infundibulum.
 
 
 
128
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The pituitary body (figs. 107, hph; 109, h; 112, hp; 116, hph) (hypophysis cerebri)
becomes more or less intimately connected with the fundus of the infundibulum, but
it is in nowise a nervous structure (see p. 100).
 
The walls of the thalamencephalon greatly increase in thickness,
and form the optic thalami (fig. 111). The middle or soft com
 
 
Fig. hi.  - Horizontal Section of Anterior Portion of the
Brain of an Embryo Sheep, 15 mm. long. Magnified 5
diameters. [From Kolliker.]
 
h. cerebral hemispheres ; m. position of the future foramen of
Munro ; 0. recess which, deeper down, passes into the optic nerve ;
t. third ventricle ; t'. central portion of thalamencephalon, in front
is the lamina terminalis ; th. optic thalamus.
 
 
missure of Mammals unites these structures anteriorly across the
cavity of this region of the brain (third ventricle). It is probably homologous with a commissure described by Balfour in Elasmobranchs, and by Osborn in Amphibia (supra-commissura) (fig.
 
 
Fig. 112.  - Horizontal Section of the
Head of an Embryo Sheep, 15 mm.
long. Magnified 50 diameters. [From
Kdlliker.]
 
d. thin roof of fourth ventricle q ; g.
Gasserian ganglion ; gr. nerve-cells in floor
of fourth ventricle; h. cerebral hemisphere ; hp. hypophysis (pituitary body) ;
l. lateral ventricle ; to. position of future
foramen of Munro ; ms. axial portion of
skull ; 0. cavity of optic stalk ; p. nervefibres of pyramid ; s. lamina terminalis ;
t. posterior and deeper portion of third
ventricle ; t'. anterior portion of the same.
 
 
107, s.c.), which crosses the roof of the third ventricle immediately
in front of the pineal gland.
 
The pineal gland, or epiphysis cerebri, develops as a diverticulum from the roof of the third ventricle (figs. 107-109, 116). It
usually becomes a long narrow tube, the lumen of which may persist throughout life, but usually the proximal end atrophies to a
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
129
 
 
thread-like stalk, while the distal portion is enlarged, and becomes
lobular or branched. The enlarged termination may remain outside the cranium (Raja and Anura) or become imbedded within
it (Acantliias and some Lizards), but in most cases it lies beneath
the roof of the skull. In Elasmobranchs and some Urodela the
pineal gland retains its sac-like character (fig. 138*, b).
 
Ahlborn regards the pineal gland as the rudiment of a primitive unpaired eye,
from its position, origin, and mode of development, and compares it with the unpaired eye of Amphioxus and larval Ascidians. This view has since been confirmed
by De Graaf, who has shown that in Anguis the epiphysis has the structui'e of an eye
constructed on the invertebrate plan. Spencer has still more recently extended this
discovery to Hatteria and other Lizards (fig. 138*, c-e). This organ is lodged within
the parietal foramen. A similar foramen is found in the skulls of Labyrinthodonta
and certain extinct Reptilia, and also, as Osborn has pointed out, in the Mesozoic
Mammal Tritylodon (see also p. 162).
 
Behind the pineal gland the optic thalami are further connected
across the roof of the brain in the Elasmobranchii, Amphibia (fig.
107, p.c), Sauropsida, and Mammalia (figs. 109, Cp; 11 6, p.com)
by a transverse commissure, the posterior commissure. This is
always situated at the base of the posterior peduncle of the pineal
gland.
 
In front of the pineal gland the greatly thinned roof of the
third ventricle, velum interpositum, becomes very vascular, and
forms the choroid plexus of the third ventricle or tela choroidea
(figs. 107-109, 1 16, ch.p 3).
 
The cerebral hemispheres usually arise as a pair of lobes from
the roof of the anterior or cerebral portion of the fore-brain, each
containing a cavity, lateral ventricle, which is continuous with that
of the central nervous system (figs. 107, 108, hi, 112).
 
That portion of the fore-brain lying in the median line between
the cerebral hemispheres is the lamina terminalis (figs. 108, 116,
l.t ), and it extends from the roof of the thalamencephalon to the
optic chiasma.
 
The Y-shaped passage connecting the lateral ventricles with the
third ventricle is the primitive foramen of Munro. Though at
first wide (fig. in, m), it is ultimately greatly narrowed (fig.
109, F, m).
 
There is throughout the Vertebrate series considerable diversity
in the size and structure of the cerebral hemispheres. Their
condition in the Amphibia has already been described.
 
The cerebral hemispheres show a marked increase in size in the
Sauropsida, and reach their culminating point in the Birds ; but
 
1
 
 
130
 
 
THE STUDY OF EMBRYOLOGY.
 
 
even here they attain a low stage of evolution as compared with
the hemispheres of the Mammalia.
 
Not only do the cerebral hemispheres in Mammals grow forward,
but they extend backward so as to hide the thalamencephalon
and the mesencephalon in a dorsal view, and even project beyond
the cerebellum in Man (fig. 109) and the higher Apes. The com
Fig. 113.  - Lateral View of the Brain of an
Embryo Calf of 5 cm. [From Balfour
after Mihalkovics.\
 
The outer wall of the hemisphere is removed,
so as to give a view of the interior of the left
lateral ventricle.
 
am. hippocampus major (cornu ammonis) ;
cb. cerebellum ; d. choroid plexus of lateral
ventricle ; fm. foramen of Munro ; hs. cut wall
of cerebral hemisphere ; in. infundibulum ; mb.
mid-brain ; op. optic tract ; ps. pons Varolii,
close to which is the fifth nerve with the
Gasserian ganglion ; st. corpus striatum ; iv. v.
roof of fourth ventricle.
 
plexity of this region of the adult brain is due to local thickening,
reduction, infolding, and fusion.
 
The external walls of the primitively simple cerebral hemispheres become greatly thickened, while the inner walls  - i.e., those
in contact with one another in the median line  - are extremely thin.
 
The mesoblastic sheath surrounding the developing brain grows
downwards as a lamina into the longitudinal fissure between the
 
 
Fig. i 14.  -Brain of a Human Embryo of Six
Months. Natural Size. [From Kolliker.]
 
c. cerebellum ; /. flocculus ; fs. fossa Sylvii ; 0.
y s \ olivary body ; ol. olfactory bulb ; p. pons Varolii.
 
o'l
" ' F
 
S
 
 
hemispheres. From this will be derived the falx cerebri and the
choroid plexus (fig. 1 1 5, / and pi).
 
The floors of the hemispheres become much thickened and
constitute the corpora striata. These protrude so much into the
lateral ventricles as to cause them to assume a curved appearance
in a longitudinal vertical section (fig. 1 1 3, st), thus constituting the
anterior and posterior cornua of the lateral ventricles.
 
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
131
 
 
The position of the corpus striatum is indicated in an external
side view of a cerebral hemisphere by the fossa Sylvii (fig. 1 14,/s),
which demarcates the frontal and temporal lobes.
 
Owing to their backward extension, the corpora striata become
increasingly connected with the optic thalami (fig. 1 1 5, st } th ),
with which they ultimateh fuse so completely that the line of
separation cannot be recognised.
 
The corpora striata are connected together by the anterior
commissure which traverses the anterior wall of the third ven
 
 
Fig. 115.  - Transverse Section of'The Brain of an Embryo Sheep, 2.7 cm. long.
 
Magnified 10 diameters. [From Kolliker.]
 
a. cartilage of orbito-sphenoid ; c. peduncular fibres ; ch. optic chiasma ; /.
median cerebral fissure ; h. cerebral hemispheres, with a convolution upon their
inner wall projecting into the lateral ventricles, l ; m. foramen of Munro ; 0.
optic nerve ; p. pharynx ; pi. lateral plexus ; s. termination of the median fissure
which forms the root of the third ventricle ; sa. body of the anterior sphenoid ;
st. corpus striatum ; t. third ventricle ; th. anterior deep portion of the optic
thalamus.
 
tricle. This is the earliest developed commissure which connects
the cerebral hemispheres, and is found, though of smaller size, in
the Sauropsida and Ichthyopsida. It lies in the substance of the
lamina terminalis (figs. 107- 109, 116 a.c).
 
The inner wall of each hemisphere projects into its lateral
ventricle as two longitudinal ridges extending from the foramen
of Munro to nearly the posterior end of the descending cornua.
The upper one, hippocampus major or cornu ammonis (figs. 1 13, am;
1 1 5, h), is a solid nervous structure, while the lower ridge is very
thin and folded, and by the ingrowth into it of a large number of
 
 
132
 
 
THE STUDY OF EMBRYOLOGY.
 
 
blood-vessels from the falx forms the choroid plexus of the lateral
ventricles (figs. 1 1 3, d ; 1 1 5, pi).
 
The cerebral hemispheres of Mammals unite with one another
in front of and above the lamina terminalis ; the fused internal
walls being very thin, are termed the septum lucidum or septum
pellucidum (figs. 109, Sp; 116, s.l). In Man the two walls
enclose a slit-like cavity, the so-called fifth ventricle. As this
space is really only a portion of the longitudinal fissure between
the hemispheres enclosed by overgrowth, it, morphologically speaking, lies outside the brain, and consequently is not lined by an
epithelium, like the true ventricles.
 
The fornix (fig. 1 1 6, fx) is a band of nerve-fibres which unites
the hemispheres along the inferior border of the septum. In front
it divides into two anterior pillars or columns, each of which,
passing in front of the foramen of Munro and behind the anterior
 
Fig. 116.  - Longitudinal Vertical Section
 
THROUGH THE ANTERIOR PART OF THE
 
Brain of an Embryo Babbit of 4 cm.
[After Mihalkovics.]
 
a.com. anterior commissure ; c.h. cerebral
hemisphere'; c.p. cerebral peduncles ; cal.
corpus callosum; ch.p. 3. chloroid plexus
of the third ventricle ; f.m. foramen of
Munro ; fx. fornix ; hph. hypophysis (pituitary body) ; inf. infundibulum ; iter, acqueductus; l.t. lamina terminalis; to . b. midbrain ; olf. olfactory lobe ; op. ch. optic
chiasma; p.com. posterior commissure ; pin.
pineal gland ; p. V. pons Varolii ; s.l. septum
lucidum ; F.3. third ventricle.
 
commissure, terminates in the corpus albicans (or in each of the
two corpora in Man). Behind, the fornix also divides into two
posterior pillars or crura, each of which eventually passes into the
hippocampus major in the descending cornu of the lateral ventricle
of its side.
 
The characteristic commissure of the Mammalia, the corpus
callosum, arises last of all in the upper portion of the septum
lucidum, and serves to directly connect the two cerebral hemispheres. The curved anterior section (genu) is the first portion to
develop, and this alone occurs in the Monotremata and Marsupials ;
in these groups the anterior commissure is relatively very large.
The corpus callosum keeps pace with the hemispheres as they
increase in size and extend backwards. As was stated on p. 124
a rudiment of the corpus callosum is found in Amphibia and
Beptiles.
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
133
 
 
In the lower Vertebrates the cerebral hemispheres are smooth
throughout life, but in the higher Mammals the surface of the
hemispheres is thrown into a number of folds (convolutions) with
deep grooves, or sulci between them.
 
Kolliker was the first to distinguish two kinds of cerebral convolutions and sulci, which he now terms primitive and secondary.
The former appear early, and all but disappear long before birth.
The sulci are the expression of actual infoldings of the walls of
the hemispheres, and correspond with those local thickenings
which constitute such structures as the corpus striatum, hippocampus major, &c. The sulcus of the first of these (fig. 114,/s)
is the only one which markedly persists throughout life.
 
The secondary convolutions begin to appear about the middle
of foetal life in Man. They affect only the more superficial portion
of the cerebral walls, and probably originate by arrest of growth
in the sulci, accompanied by active growth in the convolutions ;
the arrest of growth may be partly induced by the pressure of the
main blood-vessels of the hemispheres.
 
In many of the lower Mammals the cerebral hemispheres are
smooth, i.e., free, or nearly so, from the secondary convolutions.
The order of the appearance of the convolutions is too special a
subject to be dealt with here ; but, speaking in general terms, the
cerebral convolutions of the brains of certain adult Lemurs and
Monkeys correspond with stages observed in the development of
the human brain.
 
The olfactory lobes (Bhinencephala) usually arise as hollow
prolongations from the antero-ventral end of the cerebral hemispheres (figs. 107, 108, 1 16). According to Marshall, they arise in
Elasmobranchs (fig. 120, ol.v ) and Birds after the appearance of
the olfactory nerves. They are relatively large in the adults of
low forms, and in the embryos of the higher Mammals.
 
In all Mammals the olfactory lobes are at first hollow, the
cavities being prolongations of the lateral ventricles ; in Man the
lobes become solid and quite small (figs. 109, 1; 114, ol). In the
lower Mammals they constitute the anterior extremity of the
brain ; but owing to the forward growth of the cerebral hemispheres in the higher Mammals, they eventually occupy a ventral
position.
 
The envelopes of the brain are entirely of mesoblastic origin.
 
 
134
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Summary of the History of the Mammalian Brain. - The primitive neural
tube dilates to form certain vesicles, all of which have not the same morphological
value. They may be thus tabulated :  -
 
 
Primary Vesicles.
 
Secondary Vesicles.
 
Huxley,
 
Wilder, “Quain.-
 
 
/ Fore -brain*
 
Prosencephalon
 
Prosencephalon
 
Anterior or
Fore- Vesicle
 
< Inter-brain or )
 
Thalamencephalon
 
( Thalamencephalon or
 
 
1  -Tween-brain )
 
( Diencephalon
 
Middle or
Mid- Vesicle
 
j- Mid-brain
 
Mesencephalon
 
Mesencephalon
 
Posterior or
 
(Hind-brain
 
Metencephalon
 
Epencephalon
 
Hind- Vesicle
 
(After-brain
 
Myelencephalon
 
Metencephalon
 
 
The greater portion of the walls of these primitive vesicles become enormously
thickened, thus the anterior portion of the roof of the hind-vesicle (hind-brain)
forms the cerebellum, and the floor and sides develop the olivary bodies, pyramids,
&c., and anteriorly the pons Yarolii.
 
The corpora bigemina (or quadrigemina) are developed from the roof of the middle
vesicle, and the crura cerebri from the floor.
 
In the anterior vesicle, the floor of the thalamencephalon develops the corpus
albicans and optic chiasma, and the walls of the optic thalami. The floor of each
half of the prosencephalon (cerebral hemispheres) develops the corpora striata, and
the inner walls the hippocampus major ; the external walls are greatly thickened.
 
But portions of the primitive vesicles remain thin and develop vascular plexi ;
these are :  - The roofs of the myelencephalon (medulla) and thalamencephalon, and
part of the inner walls of the prosencephalon.
 
The cerebral hemispheres grow backward, and their lateral vesicles are considerably altered in shape and their cavities reduced by the ingrowth of the walls and
floor ; as, for example, the hippocampi and corpora striata.
 
The lateral elements of the brain are co-ordinated by the development of transverse
commissures, of which the following are the- most important :  - Pons Yarolii for the
cerebellum, posterior commissure, anterior portion of the roof of the mesencephalon,
middle commissure across the third ventricle, and the anterior commissure in its
front wall. This, with the fornix at the base of the septum lucidum and the corpus callosum above it, serve to directly connect the cerebral hemispheres with each
other. The decussation of the fibres of the optic chiasma, strictly speaking, come
under this head.
 
The Cranial Nerves.  - The dorsal roots of the cranial nerves,
like those of the spinal nerves, arise from the dorsal portion of the
cerebro-spinal axis. A neural crest, continuous with that of the
spinal cord, is probably always present.
 
Most of the cranial nerves are usually regarded as homologous
with the spinal nerves, and as having a segmental significance, but
 
* Corresponding to the German Yorderhirn, Zwischenhirn, Mittelhirn, Hinterhirn,
and Nachhirn.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
135
 
 
considerably modified, owing to the great changes which have taken
place in the cephalic region.
 
The following is a brief summary of what is known concerning
the development of the cranial nerves. The numeration and
terminology is that which is usually adopted by anatomists.
 
XII. and XI. The Twelfth or Hypoglossal, and the Eleventh
or Spinal Accessory Nerves.  - Neither of these nerves is constant as a cranial nerve throughout the vertebrate series. For the
present they may be dismissed, as they are regarded by some as
belonging to the spinal series (see p. 14 1). Their development is
not well known.
 
X. The Tenth or Vagus Nerve.  - The tenth nerve arises from
the neural ridge in the myelencephalon (medulla) behind the audi
 
 
Fig. 117. - Diagram Illustrating the General Distribution of the Cranial Nerves.
 
[Modified from Beard,.]
 
A-C. The three anterior head-cavities. I.-X. The cranial nerves (ordinary numeration).
 
au. auditory vesicle; bi -by. seven branchial clefts; ci. ciliary ganglion;
h. hyoid cleft; int. intestinal branch of vagus nerve; m. mouth; olf. olfactory
pit ; oph. v. and vn. ophthalmic division of the trigeminal and facial nerves
respectively ; oph. prof, ophthalmicus profundus ; pal. palatine branch of the facial
nerve. The radix longa unites the ciliary with the Gasserian ganglion (v.).
 
tory involution ; it soon develops a large ganglion, beyond which it
is produced as the intestinal branch. Later several anterior roots
arise from the ventral surface of the brain and join the vagus.
This nerve sends a pair of branches to supply the two sides of the
posterior branchial (visceral) clefts (see p. 177). In the Marsipobranchii, and in Notidanus, the last six of the seven branchial clefts
are supplied by thir nerve; in other Vertebrates the number is
less. Thus the tenth nerve is usually regarded as equivalent
to at least six segmental nerves, the single origin of the tenth
nerve being supposed to be of secondary significance. For several
reasons Amphioxus cannot be utilised for comparison, one being
that there is no correspondence between the number of the body
segments and branchial clefts in that form.
 
 
13 G
 
 
THE STUDY OF EMBRYOLOGY.
 
 
IX. The Ninth or Glosso-Pharyngeal Nerve. - The ninth
nerve usually has a common origin with the tenth nerve, but it
very soon becomes distinct, and, like the latter, it acquires numerous
roots. This nerve passes immediately behind the auditory capsule
and expands above the first branchial cleft into a ganglion. From
the latter a thick posterior branch is distributed to the anterior
border of the first branchial arch, and a thinner branch to the
posterior border of the hyoid arch.
 
VIII. The Eighth or Auditory Nerve. - The eighth nerve
(fig. 126, E, viii) arises in such close contiguity with the seventh
that it is usually stated to be a branch of it ; but Beard maintains
that it is a true segmental nerve. It is a short thick nerve with
a large ganglion, and is solely the sensory nerve of the ear.
 
VII. The Seventh or Facial Nerve - The seventh nerve
early develops as an outgrowth from the neural crest on the dorsal
surface of the myelencephalon just in front of the auditory capsule.
At an early stage it acquires a new or secondary attachment to
the side of the brain ; but, unlike any other nerve, cranial or spinal,
the original or primary root is retained as well as the secondary
[Marshall]. The main branch of this nerve passes down the
anterior side of the hyoid arch (p. 178); a smaller branch (praespiracular) forks over the hyomandibular cleft (spiracle) ; in
Mammals it joins the mandibular division of the fifth nerve, and
is known as the chorda tympani. The seventh nerve also gives
rise very early to two anterior branches, the upper (portio facialis
of the ophthalmicus superficialis) passes to the front end of the
head along with the ophthalmic division of the fifth nerve. The
lower or palatine (superficial petrosal of Mammals) runs superficially to the superior maxillary division of the fifth.
 
VI. The Sixth or Abducent Nerve.  - The sixth nerve arises
from the median ventral line of the brain below the seventh nerve,
and never develops ganglion cells. It is an exclusively motor
nerve, which supplies the rectus externus muscle of the eyeball,
and also in some forms the retractor muscle of the bulb of the
eye and the nictitating membrane.
 
V. The Fifth or Trigeminal Nerve.  - The fifth nerve develops
from the neural ridge in front of the seventh nerve. After expanding into a large ganglion (Gasserian ganglion), it arches over the
mouth, the main trunk (mandibular or inferior maxillary) beingdistributed over the lower jawq and the smaller (superior maxillary)
over the upper jaw. The dorsal division of the fifth nerve emerges
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
137
 
 
anteriorly from the Gasserian ganglion, and follows the ophthalmic
division of the seventh nerve to its distribution at the anterior end
of the head ; it is known as the portio profunda or minor of the
ophthalmicus superficialis. A nerve connecting the Gasserian
with the ciliary ganglion is usually termed the ophthalmic division
of the fifth nerve ; it appears not to be a branch of that nerve.
 
IY. The Fourth, Pathetic or Trochlear Nerve. - In its
earliest recognised condition the fourth nerve has the same
position that it occupies in the adult, viz., the dorsal surface of
the extreme hinder border of the mid-brain. It invariably innervates the superior oblique eye-muscle, and in many Vertebrates
sends sensory branches to the conjunctiva and the skin of the
upper eyelid.
 
III. The Third or Oculomotor Nerve. - Marshall thinks it
is probable that the third nerve grows from the neural crest on
the top of the mid-brain ; but as in the adult it arises very near
the mid-ventral line, it must undergo the maximum amount of
change of position. But Beard states that the nerve described by
Marshall is really the radix longa, and believes, though he has no
direct evidence to give, that the oculomotor does not arise from
the neural crest. This nerve is associated with the ciliary or
ophthalmic ganglion, and is distributed to all the muscles of the
eyeball except those supplied by the fourth and sixth nerves, as
well as to the levator palpebrge superioris and the circular muscles
of the iris.
 
II. The Second or Optic Nerve.  - The second nerve is merely
a degenerate portion of the brain itself, being the stalk of the optic
vesicle (p. 160).
 
I. The First or Olfactory Nerve.  - The first nerve arises from
the dorsal part of the sides of the anterior cerebral vesicle before
the cerebral hemispheres have commenced to develop. Owing to
the enormous development of the latter in the higher Vertebrates,
the nerve comes to occupy a ventral position. It is exclusively
distributed to the nasal fossse (figs. 117, 120, 1).
 
Hypotheses concerning the Segmental Value of the Cranial Nerves.  - Recently
both Spencer and Beard have shown that after the (dorsal) roots of the cranial
nerves arise from the neural ridge, they fuse with the epiblast at the level of the
notochord. The epiblast cells at these spots proliferate the masses of cells thus
developed, forming the cranial ganglia ; and at the same time a rudimentary structure is formed, termed by Beard the branchial sense organ, and by Spencer the
sense organs of the lateral line in the head. As these organs at first only appear in
the gill-bearing region of the body, the former term is perhaps the preferable.
 
 
138
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Miss Johnson and Miss Sheldon, who have still more recently investigated the
development of the cranial nerves in the Newt, admit the fusion of the cranial nerves
with the incipient serial sense organs (mucous canals or lateral line organs of the
head). They deny that the ganglion is derived from this fusion, but state that it
takes its origin from the original outgrowth from the neural ridge, as Redot has
also shown for the spinal nerves in Triton.
 
Beard states that the dorsal root of a cranial nerve develops in the following
manner. The nerve grows downwards from the neural ridge below, hut unconnected
with the epiblast. About the level of the notochord it fuses with the epiblast, but
part of the nerve passes on to the lateral muscle-plates of the segment (fig. 118);
this main or posterior branch (post-branchial nerve) of Beard, chiefly innervates the
gill-muscles. Proliferation at the junction of the nerve with the epiblast gives rise to
the ganglion of the dorsal root, and externally to the rudiment of the primitive branchial sense organ of that root. As the ganglion separates from the skin a nervous
tract is left, the so-called dorsal branch (supra-branchial nerve). The anterior or
praebranchial nerve, and probably the pharyngeal branch, are also derived from
this proliferation.
 
Following up this discovery, Beard has attempted a re-enumeration of the seg
 
Fig. i 18. - Diagrammatic Transverse Section THROUGH THE GILL-BEARING REGION
OF AN ELASMOBRANCH OR OTHER ICHTHY
opsid. [After Beard. ]
 
Neural canal not yet closed over. On the
left side the gill muscle-plate is shown, and
on the right tue gill cleft.
 
br.g. branchial ganglion ; br.o. branchial
sense-organ; cl. visceral cleft; d.n. dorsal
(posterior) root of segmental nerve ; h.c.
head-cavity; l.m.p. lateral muscle-plate; n.c.
neural canal ; nch. notochord ; ph. pharynx ;
p.n. post-branchial nerve.
 
 
ments of the head, and a review of the nature of the nerves themselves. The
following is briefly his position.
 
The ganglia known to human anatomists as the olfactory bulbs, ciliary, Gasserian,
geniculate, auditory, petrous, and pneumogastric ganglia, all belong to the same
series, and are associated with primitive sense organs. The table given on p. 140,
when compared with fig. 117, will elucidate their relationships, and but few remarks
will be necessary.
 
On this hypothesis the nerves arising from the cranial neural crest and uniting
with the primitive sense organ of its segment correspond to some extent with the
dorsal branches of the spinal nerves. The nerves and their ganglia are: (1) the
olfactory nerve and ganglion ; (2) the radix longa (or the nerve uniting the ciliary
with the Gasserian ganglion) and the ciliary ganglion ^ (3) the trigeminal nerve
and the Gasserian ganglion ; (4) the facial nerve with its ganglion ; (5) the auditory
nerve and ganglion ; (6) the glosso-pharyngeal nerve and its ganglion ; (7+ ) the vagus
nerve with its segmental branches and their associated ganglia.
 
The association of the dorsal root of the ciliary nerve (radix longa) with the
Gasserian ganglion, instead of its directly arising from the brain, is explained by
Beard as being due to the primitive outgrowths being very close together.
 
 
 
ORGANS DERIVED FROM THE EP1BLAST.
 
 
139
 
 
The oculomotor (III.), trochlear (IV.), and abducent (VI) nerves are regarded as
the anterior roots of the radix longa (ciliary), trigeminal (V.), and facial (VII.) nerves
respectively. They all supply the eye-muscles, the latter being developed from the
first two (? three) head cavities.
 
The fact that there are two anterior branches (ophthalmic and palatine) of the
seventh nerve, is one reason for supposing that there may be a missing head segment
between the third and fourth of the above enumeration. Independently of this,
there are two pre-oral segments ; and counting the auditory as a true segment, there
are nine post-oral in the Fish, with the greatest number of gill-clefts (Notidanus).
This makes a total of at least twelve segments in the Vertebrate head.
 
Little need be added concerning the segmental sense organs, as they usually at
first appear as patches of columnar cells lining a slight depression of the epidermis.
 
Serial Cranial Sense Organs.  - The organs of the lateral line consist of a series of
mucous canals containing groups of sense-cells which are segmentally disposed in
the trunk (see p. 148). The canals are variously distributed in the head, but in the
body they almost invariably extend along the middle line of each side, as far as the
tail. This system of sense organs is only found in Fishes, Urodele Amphibia, and
the larvae of the Anura.
 
In the head the canals are innervated by cranial nerves, the lateral line proper
being supplied by the lateral branch of the vagus.
 
The lateral line itself is developed from a backward growth of the epiblastic
proliferation, which gives rise to the sense organ of the vagus. This ploughs its way
along the superficial epiblast and the indifferent epiblast cells, which are thus thrust
aside are probably lost [Beard] (fig. 103).
 
As in other cases, the nerve of the sense-organ is formed from the deeper layer
of the sensory thickening.
 
The extension of these (primitively branchial) sense organs to the hinder end of
the body is supposed by Beard to be of only secondary significance. Some authors,
however, believe that the connection of the (segmental) organs of the lateral line
with the vagus is itself secondary.
 
Of the primitive segmental sense organs, the first has become retained and modified
as the olfactory organ. In most Ichthyopsida the organs of the lateral line of the
head are still innervated by certain cranial nerves (ciliary, trigeminal, facial, and
glosso-pharyngeal). The auditory organ may possibly be a highly specialised segmental sense organ, its histological structure also lending support to this view.
The posterior organs persist as the organs of the lateral line of the body in the
Ichthyopsida.
 
The presence of primitive branchial sense organs is not confined to the Ichthyopsida. Froriep has discovered rudiments of them for the facial, glosso-pharyngeal,
and vagus segments in Cow and Sheep embryos ; and Beard finds them in the Fowl
for the ciliary and trigeminal, in addition to the above segments. In all cases they
disappear very soon.
 
Thymus Gland.  - The paired serial rudiments of the thymus gland arise in a
manner which is very suggestive of their having possessed a primitive branchial
sensory function. For the sake of convenience the development of this composite
gland will be described in another section (p. 184).
 
The primitive branchial clefts suffer great reduction. The more or less rudimentary hyoidean cleft (spiracle) is lost in the Teleosts. Most Fishes have but five true
branchial clefts. The absolute extinction of the branchial clefts is well exhibited in
the higher adult Urodele Amphibia ; but in these and in all higher animals the
hypoblastic evagination concerned in the hyoid cleft more or less persists as the
Eustachian tube or recess (p. 180).
 
 
Table of Cranial Segments and their Nerves and Sense-Organs.
 
 
140
 
 
THE STUDY OF EMBRYOLOGY.
 
 
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ORGANS DERIVED FROM THE EPIBLAST.
 
 
141
 
 
This table will be found to differ from fig. 117 in having two hyoid segments, and
consequently in accounting for a total of thirteen segments. The first segment
corresponds with the fore-brain vesicle, the second with the mid-vesicle, and the
remainder with the hind region of the brain.
 
Froriep divides the Mammalian head into three regions:  - (1) prepituitary or
trabecular, with the nose and eyes ; (2) pseudo-vertebral, with the trigeminal, facial,
glosso-pharyngeal, and composite vagus nerves, which supply the pharyngeal clefts ;
(3! vertebral, consisting of the occipital bone and hypoglossal nerve. He has found
that in the embryos of Ruminants there are rudiments of three distinct protovertebrae
in front of the first cervical (spinal) nerve and behind the vagus. In front of each
of these rudiments ventral nerve roots arise, which all unite in a single trunk, the
hypoglossus. A single dorsal ganglionated root unites with this composite nerve.
Thus the hypoglossus is a fusion of at least three segmental nerves, and the occipital
region corresponds to as many vertebrae. (This view has been independently arrived
at by M c Murrich on purely anatomical grounds.) It must further be admitted that
the occipital region of the cranium is not identical throughout the vertebrate series.
 
Ahlborn, from his studies on Petromyzon and Anura, has also arrived at the view
that the hinder portion of the skull and the anterior cervical vertebrae may not
respectively be homologous in different Craniates. He has come to the conclusion,
mainly from a consideration of the cephalic mesodermic segments, that there were
primitively nine pairs of spinal nerves in the hind-brain, of which Nerves III., IV.,
and VI. , had only motor roots ; but as neural segmentation (neuro-merism) is
secondary, the spinal-like cerebral nerves of the craniota cannot be compared with
the segmental spinal nerves.
 
An endeavour has been made to give a brief account of some of the views which
are held respecting the significance of the cranial nerves, and of a few of the attempts
which have been made to utilise the nerves in solving the problem of the segmentation of the Vertebrate head. It must, however, be borne in mind that there are
very good reasons for regarding the apparent segmentation of the cephalic region as
an arrangement perfectly distinct from the metamerisation of the trunk.
 
Sense Organs.  - The simplest organs of sense are epiblastic
cells, which, haying a stiff hair-like process, are excited by vibrations in the external medium (fig. 119). These sense-cells are
usually collected into groups or series, and constitute definite
sense organs.
 
Sense organs may be roughly grouped into those which appreciate
vibrations of air or matter, and those which are stimulated by
light.
 
It is usually possible to distinguish between sense organs which
have a tactile, olfactory, gustatory, and auditory function; but in
the lower animals it is probable that other kinds of vibrations may
be appreciable which give rise to sensations of a less distinct, or
even of a different character. These various senses are doubtless
differentiations of a primitive tactile sense ; this is rendered more
probable from the similarity in their development and their fundamental similarity of structure.
 
Tactile Organs.  - Tactile organs are direct modifications of
epidermal cells ; they may either be the simplest of sense-cells, or
 
 
142
 
 
THE STUDY OF EMBRYOLOGY.
 
 
may be more or less differentiated. Numerous kinds of tactile
organs are described in works on comparative anatomy and histology. They may be generally diffused or restricted to certain
prolongations of the body, more especially of the anterior end,
such as tentacles, palpi, and antennae.
 
Olfactory Organs.  - The higher invertebrate Metazoa alone
possess any organs which can be recognised as olfactory. In the
 
 
 
 
Fig. i 19.  - Sense-Cells of Ccelenterates.
 
A. Isolated sense-cells from dorsal nerve-ring in connection with two multipolar ganglion cells (from iEginura myosura). [After Haeckel.']
 
B-E. Isolated elements from the upper nerve-ring of Carmarina hastata. [After
0. and R. Hertwig. ]
 
B. Ordinary small sense-cell. C. Large sense-cell. D. Large ganglion cell. E.
Ordinary ganglion cells and nerve-fibrills.
 
F. Three supporting cells and one sense-cell from tentacle of Anthea cereus.
 
G. Isolated sense-cell from the same. [After 0. and R. Hertwig.]
 
Arthropoda these are minute bristles which are connected with
nerve-fibrils. The olfactory organ of Mollusca (osphradium of
Lankester) consists of a patch of sense-cells which is situated
over each gill.
 
A pit or papilla behind or above each eye is stated to be the
olfactory organ of the Cephalopoda.
 
In Amphioxus a single ciliated pit, situated on the left side at
the anterior end of the neural canal, is usually spoken of as an
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
143
 
 
olfactory organ ; but Hatscliek has shown that it is of hypoblastic
origin (p. 185).
 
An undoubted olfactory organ is present in all higher Chordata.
It first appears as a pair of tracts of columnar epiblast at the
anterior end of the body, immediately in front of the stomodaeum
(fig. 94, A, olf). The sensory epithelium invaginates as two shallow
pits (fig. 1 1 7, 120), which soon deepen. Although the internal
epithelium (Schneiderian membrane) is thrown into folds to
increase the sensory surface, or the surface may be further increased
by the projection of coiled, and sometimes very complicated, cartilages and bones (turbinal bones), yet the sac-like character and
the primitive opening of the nasal pits are always retained.
 
The single nasal sac of the Cyclostomi has probably no phylo
 
 
Fig. 120.  - Sections through Two Stages in the Development of the Olfactory
Organ of an Embryo Dog-Fish (Scyllium). [After A. M. Marshall.']
 
A. Early, B. Later stage.
 
c.h. cerebral hemisphere ; f.b. fore-brain ; olf. olfactory pit ; ol.v. olfactory vesicle
or lobe ; jpn. pineal gland ; sch. Schneiderian folds ; i. olfactory nerve.
 
genetic significance, as in the younger stages there are distinct
evidences of a double nature. In all other Vertebrates the nose
is paired from the first.
 
In Elasmobranchs the orifice of the olfactory pit is ventrally
situated. In the Ganoids and Teleosts a distinct and often wide
bridge of tissue divides the orifice of the nasal sac into an afferent
and an efferent orifice, which always come to be situated on the
dorsal aspect of the snout.
 
A groove extends in many Elasmobranchs from each nasal sac
to the mouth ; the central flap of skin between the grooves is the
nasal valve or fronto-nasal process (fig. 121), The lateral folds of
the fronto-nasal process sometimes fuse with the cephalic integument across the nasal groove, in this way forming two apertures
to the nasal sac.
 
 
 
 
144
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The walls of this groove grow over and coalesce in the middle
in Dipnoi and all higher animals, thus forming a canal which
opens in front by the anterior nares or nostrils, and behind as the
posterior nares. The latter are situated just behind the upper lip
in Dipnoi and Urodela. In Anura and higher forms they lie
somewhat farther back, but they are, in all, morphologically in
 
 
 
Fig. i 2 i. - U nder Surface of Head of Dog-Fish.
 
/. nasal flap, reflected on the left side of the fig. ; g. nasal groove ;
to. mouth ; na. opening of olfactory organ.
 
 
front of the palatine bones. With the formation of the palate,
the mouth cavity becomes subdivided into two, a lower buccal
cavity and an upper nasal passage. The secondary posterior
nares thus established may be carried back, as in the Crocodilia,
Myrmecophaga, and in some Cetacea, even to the extreme hinder
end of the mouth.
 
The development of the nasal passage in the Fowl is briefly as follows. The
edge of the nasal pit develops a thickened border, except towards the mouth, thus
 
 
 
 
 
Fig. i22 . - Ventral Views of the Heads of Embryo Fowls, (i) At the end
of the fourth day of incubation. (2) At the commencement of the fifth
day. [From Kolliker. ]
 
an. outer nasal process ; in. inner nasal process ; V. second visceral arch
(hyoid) ; to. mouth ; n. nasal or olfactory pit ; nf. nasal groove ; o. superior, and
u. inferior, maxillary process of the first (mandibular) visceral arch ; s. cavity of
pharynx ; sp. choroidal fissure of the eye ; st. fronto-nasal process.
 
 
leaving a shallow groove, the nasal groove. The central portion of this groove is
converted into a canal by the lower angle of the fronto-nasal process overlapping,
and ultimately fusing, with the superior maxillary process. The nasal canal thus
formed opens well within the mouth by the posterior nares.
 
The adult condition of the nasal groove in some Elasmobranchs
(fig. 1 21) corresponds with a transient stage (fig. 122) in the embryos of those Vertebrates which have posterior nares.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
145
 
 
Tlie organ of Jacobson is primitively developed as a pair of:
diverticula from the nasal sac. These are at first large, but their
subsequent development is less rapid than that of the olfactory
sacs. Eventually they give rise to comparatively small organs,
which usually open directly into the mouth independently of the
posterior nares.
 
A shallow depression, which extends from the eye to the nasal
pit while the nasal groove is still open, separating the outer nasal
process (as the outer raised border of the nasal pit is termed)
from the superior maxillary process, is known as the lachrymal
groove.
 
The lachrymal duct is formed from a solid cord of epiblast cells
which separates from the floor of the groove. It subsequently becomes hollow, and places the orbit in communication with the
nasal chamber.
 
Gustatory Organs.  - The gustatory organs always retain so
simple a condition that they require no special mention.
 
Auditory Organs.  - The so-called auditory organs of the invertebrate Metazoa are very varied in origin and position, but, except in the case of a few Medusae, they are all epiblastic structures.
 
Some of these organs appear to possess a truly auditor} 7- function. Balfour has
suggested that in some cases their function may be to enable the animals provided
with them to detect the presence of other animals in their neighbourhood, through
the undulatory movements in the water caused by the swimming of the latter. In
the case of the Medusae, however, the vibrations of waves reflected from the shore
and rocks would affect these organs, and may possibly warn the Medusae of danger.
 
Two forms of auditory organ are found amongst the Medusae,
tlie first alone being purely epiblastic, and consisting of an open
sac, which may be converted into a complete cup. These occur
along the base of the velum in the Vesiculate Hydromedusae. Some
of the cells form a concretion (otolith) within their walls, and
others are sense-cells with auditory hairs, which lie close to the
former (fig. 123, A, b).
 
The second form is found in the Trachymedusae and Acraspeda,
and consists of a modified tentacle, the terminal endodermal cells
of which secrete otoliths, but the auditory hairs are solely ectodermal. The whole structure is usually more or less enclosed
within a reduplicature of the ectoderm, sometimes forming a
vesicle which entirely surrounds the auditory tentacle. In all
cases the auditory cells of the Medusae are connected with the
peripheral nerve-ring (fig. 123, u.n.r).
 
K
 
 
146
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Paired otocysts containing several otoliths, rarely one, occur in some Nemerteans,
Nematodes, and a few Annelids. Practically nothing is known of their structure,
and their origin is also unknown; this also applies to the unpaired otocyst of
Planarians.
 
The otocysts of Mollusca develop as epiblastic pits (fig. 124)
close to the proliferating areas which form the pedal ganglia.
Very rarely they are at first solid. The pits are converted into
rounded vesicles, from which a small ciliated canal (ductus Kollikeri)
 
 
 
Fig. 123.  - Auditory Organs of Various Medusae. [After 0. and R. Hertwig.']
 
A. Open auditory pit of Mitrocoma annae. B. Closed auditory sac of iEquorea
forskalea. C. Endodermal otoliths in a modified tentacle of Cunina lativentris.
 
a.h. auditory hairs; c.c. circular canal; ec. ectoderm; en. endoderm ; l.n.r.
lower nerve-ring ; m. muscle-fibres ; m.v. muscle of velum ; ot. otolith; s.c. sensecells ; s.l.v. supporting lamina of velum ; u.n.r. upper nerve-ring.
 
often projects, this being the remnant of the tube which for a time
connects the vesicle with the orifice of the primitive invagination.
At first a single small concretion is secreted by one of the cells of
the vesicle; this may increase in size, and persist as a single otolith ;
in other cases it remains small, and a large number of minute concretions are added (Pteropods, Dentalium, Nautilus, most Gasteropods). Earely the numerous otoliths fuse to form a single large one
 
 
cg pr^o :
 
 
 
Fig. 124. - Two Stages in the Development of the Otocyst in Murex.
 
A. Open pib. B. Closed vesicle, with
small otolith.
 
ep. epiblast ; m. mesoblast.
 
 
(Paludina, Decapods). The interior of the vesicle is clothed with
cilia; but in the specialised otocysts of Heteropods there is a
patch of definite auditory cells (macula acustica), and a similar
ridge (crista acustica) occurs in Decapods. The otocyst often
shifts its position anteriorly, and usually comes to be innervated
from the cephalic ganglion.
 
The Arthropoda never possess otocysts comparable with those
of other Invertebrates. Unicellular hairs, or setse on various parts
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
147
 
 
of the body, especially on the antennae of Crustacea, are generally
regarded as auditory; they are usually lodged within cuticular
depressions.
 
In the Candida! (Shrimps and Prawns) the auditory hairs usually occur on the basal
joints of the antennules and on the tail; auditory pits may occur at both ends of the
body. In the Schizopods a large otolith is present, which is secreted by the walls
of the sac, and is renewed after moult. The auditory sac is situated in the caudal
endopodite. The auditory hairs are restricted in Decapods to the basal joint of the
antennules ; they are usually feathered, and often bent. The otocvst in these forms
may be widely open (Palinurus), but the opening is usually reduced to a narrow fissure
In Hippolyte the sac is completely closed. Only in the Crabs does the otocyst become
 
 
 
Fig. 125.  - Transverse Section through the Auditory Involutions of an
Embryo Fowl of the Second Half of the Second Day. Magnified 84
diameters. [From Kolliker.]
 
a. descending aortse ; am. amnion, with, its two layers ; am', amniotic suture,
situated on the right side and not drawn in its whole extent ; c. root of the
inferior cerebral vein ; dfp. splanchnic mesoblast (fibro-intestinal layer) of the
pharynx, continuous with the external envelope of the heart and forming
an inferior cardiac mesentery; H. heart; hp. somatopleur passing into the
amnion ; ihh. endothelium of the heart ; ph. pharynx ; va. widely open auditory
sacs.
 
at all complicated. The otoliths are entirely foreign particles, and appear to be introduced by the animal itself.
 
A remarkable sense organ, usually stated to be acoustic, is found in certain
Hexapoda, and is situated either on the thorax or at the base of the legs. It consists
essentially of a series of nerve-fibres, each of which passes into a nerve- cell, from
which arises a multicellular elongated structure, usually containing a stiff rod. The
multicellular fibre is usually attached to a tympanum, supported by a chitinous ring.
The whole structure is always situated over an air sac.
 
In Appendicularia there is a single otocyst on the left side of the ganglion, consisting of a spherical sac enclosing a spherical otolith which is supported by delicate
isolated hairs. In other pelagic Tunicates there are two symmetrically placed otocysts ;
their development is not known. In fixed Ascidians an otolith is developed from a
single cell on the dorsal and right side of the brain. This cell projects into the
cavity of the brain, and its free end is pigmented. Eventually the cell becomes
 
 
148
 
 
THE STUDY OF EMBRYOLOGY.
 
 
stalked, and travels round the right side of the brain until it reaches the summit
of a patch of cylindrical sense-cells, the crista acustica. The adult organ thus
consists of a crista acustica on the floor of the anterior region of the brain and projecting into its cavity, upon which is perched an oval otolith, the lower part of
which is clear and refractive, while the upper half is pigmented. This is the only
known example of a cerebral auditory organ.
 
The Organs of the Lateral Line  -In Teleostei the sense
organs of the lateral line appear in segmental patches of simple
sense-cells ; each area is then invaginated to form a short groove,
which partially closes over. The fusion of these channels forms the
canal of the lateral line, but numerous external openings are left.
The lateral line of Elasmobranchs is at first a solid cord of cells,
 
 
 
Fig. 126.- - Early Stages in the Development of the Vertebrate Ear.
 
A-D. Four stages in the development of the labyrinth of a Fowl. [After
Meissner.'] E. Transverse section through the auditory pit of a Fowl -s embryo of
fifty hours. [After Marshall.] F. Transverse section through the head of a foetal
Sheep (16 mm. in length) in the region of the hind-brain. [ After Bottcher.]
a.c.v. anterior cardinal (jugular) vein ; am. amnion ; ao. aortic arch ; c.g. cochlear
ganglion ; d.c. ductus cochlearis; h.b. hind-brain ; nch. notochord ; ph. pharynx ;
r.v. recessus (aqueductus) vestibuli; v. vestibulum ; v.c. vertical semicircular
canal ; vm. auditory nerve.
 
but this is probably an abbreviated process. In Chimaera the lateral
line persists in the adult as an open groove. (See also p. 139.)
 
The Vertebrate Ear.  - The auditory organ of Vertebrates may
possibly prove to be a highly specialised organ of the lateral line
series. The auditory sac first appears as a shallow depression of
the epiblast in the region of the posterior brain vesicle above
the first (hyoid) visceral cleft (figs. 125, 126). It soon becomes a
flask- shaped vesicle which is separated from the skin, although in
some Elasmobranchs the primitive opening to the exterior is retained throughout life.
 
The stalk of invagination persists as the aqueductus vestibuli,
and its blind swollen distal extremity is the saccus endo-lymphaticus or recessus vestibuli (figs. 126 and 127, r.v).
 
The swollen portion of the primary auditory vesicle is modified
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
149
 
 
to form the utriculus and the semicircular canals, while a Ventral
diverticulum gives rise to the cochlea and the sacculus hernisphericus.
 
The rudiments of the anterior and posterior semicircular canals
grow out from the lateral wall of the vesicle as two flattened processes. Their central walls become applied together, obliterating
the cavity, except at the circumference, and eventually the centre
is absorbed, leaving two ring-like canals. The horizontal semicircular canal is developed somewhat later in a similar manner.
 
The Cylostomi possess two imperfect vertical canals, which,
with the utriculus, form a ring-shaped membranous labyrinth. All
other Vertebrates have the three semicircular canals.
 
The body of the primitive vesicle persists as the vestibule or
utriculus.
 
 
Fig. 127. - Transverse Section of Auditory Labyrinth of an Embryo Cow, lines in length.
Magnified 30 diameters. [From Kolliker .]
 
a. boundary of the cavity in the cranial wall containing the epithelial labyrinth (6), which does not
everywhere fill up the cavity ; c. mouth of cochlea ;
c'. lagena of cochlea ; ch. notochord ; rv. recessus
vestibuli ; se. horizontal (external) semicircular canal ;
sh. cranial cavity ; sr. mouth of sacculus hemisphericus (?) ; ss. vertical semicircular canal ; v. vestibulum.
 
 
 
The cochlea of Mammals higher than the Monotremes consists
of a helicoid spiral tube, connected with the utriculus by a narrow
canalis reuniens. It develops as a simple process from the inferior
end of the auditory vesicle. The various stages in its development
in the higher forms are permanently retained in the adults of
various lower animals.
 
The sacculus hemisphericus is a round vesicle which is evaginated
from the base of the cochlea shortly after the appearance of the
horizontal canal. A constriction opposite the mouth of the
aqueductus causes the passage between the utriculus and the
sacculus to diverge slightly up the aqueductus instead of pursuing
a straight course (fig. 128).
 
The simple epiblastic aural invagination becomes in this manner
a complicated labyrinth. The sense-cells are restricted to certain
tracts, and, with the exception of the organ of Corti, they retain a
very simple character. The auditory hairs project into the fluid
(endolymph) contained within the labyrinth. The otoliths or
 
 
150
 
 
THE STUDY OF EMBRYOLOGY.
 
 
otoconia are masses of carbonate of lime secreted by the lining
epithelium.
 
The neighbouring mesoblast enters into relation with the auditory apparatus, the cells immediately surrounding the labyrinth
being converted into a connective tissue investment (the membranous labyrinth). The whole being protected by a cartilaginous, and, in most animals, a subsequently osseous capsule, which
is known as the osseous labyrinth. The latter is undeveloped at
one spot, the fenestra ovalis in Elasmobranchs, Amphibia, and
higher animals. A second foramen occurs in Mammalia, the
fenestra rotunda.
 
Between the membranous and osseous labyrinths imperfect lymph spaces are
found in the Sauropsida ; these are well developed in the Mammalia.
 
In the cochlea of the latter two longitudinal lymph-spaces are formed, the dorsal of
 
 
Fig. 128.  - Diagram of the Auditory Labyrinth : A. of a Fish ;
B. of a Bird ; C. of a Mammal.
[From Bell after Waldeyer .]
 
6. lagena ; c. cochlea ; cr. canalis
reuniens; k. coil (helix) of the
cochlea ; r. recessus vestibuli ;
s. sacculas ; u. utriculus or vestibulum with the three semicircular
canals ; v. csecal sac.
 
 
which (scala vestibuli) communicates with the cavity round the membranous labyrinth, and at the apex of the cochlea is continuous with the ventral space (scala tympani). The latter terminates blindly at the fenestra rotunda. The fluid contained
within these lymph spaces is the perilymph.
 
It must not be forgotten that the cavity (scala media or canalis cochleae) lying
between the two scalae is the sensory portion of the cochlea, and is alone lined by
epiblast. The scalae and the bony labyrinth are protective structures.
 
In most Fish the labyrinth or internal ear is more or less enclosed within the ear
capsule, and is quite cut off from the outer world, the sound vibrations passing
through the skull to the ear. But in some Teleosts the fenestra ovalis or its equivalent is in connection with the air-bladder through the intervention of a chain of
ossicles (e.g., Cyprinoids and Siluroids). (See p. 181.)
 
Howes calls attention to a fenestra in the roof of the chondrocranium of many
Elasmobranchs situated behind the orifice of the aqueductus vestibuli, the covering
of which evidently functions as a tympanic membrane.
 
The hypoblastic diverticulum of the pharynx, which forms the
hyoid cleft of Eishes (see p. 178), may acquire an external opening
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
151
 
 
in some Amphibia which soon closes over. In all higher Vertebrates it persists as a blind recess, the Eustachian tube, dilating
distally into a chamber (tympanic cavity) which partially surrounds the utriculus.
 
The external auditory meatus corresponds to the lower section of the outer or
epiblastic portion of the original hyoid cleft. The meatus is formed principally, if
not entirely, by the growth of the surrounding tissue in such a manner as to leave
a deep tube. A pit (Hunt -s depression), corresponding to the upper section of the
cleft, soon disappears. The external ear, concha or auricle, appears early (in the Pig)
as a small triangular flap arising from the anterior border of the hyoid arch opposite
the meatus ; it corresponds in position with the operculum of Fishes.
 
The tympanum in Mammals is at first a vertical thick wall of tissue separating
the Eustachian tube from the shallow external depression, much as in Amphibia.
By the subsequent extension of the two tubes the tympanum is reduced to a thin
membrane, and is situated in a plane perpendicular (instead of parallel) to the
surface of the head. The outer epithelium of the tympanum is clearly of epiblastic
origin, while the inner epithelium is hypoblastic.
 
There is in Amphibia and Sauropsida a bony rod, the columella
auris, extending from the fenestra ovalis to the tympanum. The
greater portion, according to Parker, is a dismembered section of J
the hyoid arch ; the base (stapes) being a plug of cartilage severed j
from the auditory capsule.
 
A chain of three ossicles, the stapes, incus and malleus, connects
the tympanum with the fenestra ovalis in Mammals ; the first of
these is homologous with the Reptilian stapes, but there has been
a good deal of discussion concerning the nature of the last two
bones. Huxley and Parker -s original view was, that the incus is
the proximal portion of the hyoid arch and the malleus is the
arrested quadrate ; the processus gracilis of the malleus representing the primitive continuation into Meckel -s cartilage. The current
view in Germany is that both the incus and the malleus belong to
the mandibular arch (in which case the former might represent
the quadrate and the latter the articular element of the lower 1
jaw). This homology, which was independently arrived at by
Salensky and Eraser, now receives Parker -s unqualified support.
According to Reichert, the stapes is part of the hyoid arch, but
Salensky and Fraser hold that it arises from a mesoblastic blastema |j
which surrounds the mandibular artery, hence the perforation of
the stapes.
 
Albrecht maintains, however, that the quadrate cannot form part of the chain of
auditory ossicles of Mammalia, and that the zygomatic portion of the squamosal is
the homologue of the quadrate of Sauropsida. Dollo supports this conclusion, and adds
that he has found an element in Lacertilia which he homologises with the malleus of
Mammalia. He slightly modifies Albrecht -s series of homologies in the following
 
 
152
 
 
THE STUDY OF EMBRYOLOGY.
 
 
} manner. The symplectic + hyomandibular of Teleosts or the suspensorium of Fishes
! generally equals the columella of Urodeles and the four ossicles of Anura. These,
again, are equivalent to the malleus + columella of Sauropsida and the malleus +
 
' incus + os lenticulare + stapes of Mammalia.
 
Visual Organs.  - The more or less definite appreciation of those
vibrations of ether which result in the sensation of sight is a
faculty which is readily acquired by the outer cells of the body,
hence what are termed eyes have appeared perfectly independently
in numerous groups of the animal kingdom. Even in the same
order of animals eyes of quite dissimilar morphological value may
occur, as, for example, the eyes in the shells of certain Chitons
[Moseley], on the back of Onchidium [Semper], on the edge of
the mantle, and on the siphon of numerous Lamellibranchs ; but
it is almost certain that the cephalic eyes of the Odontophora,
when present, including even the transient eyes of larval Chitons,
are homologous all through the group.
 
It is probable that the power of distinguishing light from darkness is a primary characteristic of protoplasm ; if this be so, it
would necessarily be readily retained by epiblastic cells, especially
if pigment is present. Semper has suggested that a simple rounded
tubercle covered with a transparent cuticle, or a mere local thickening of the cuticle, would serve to concentrate rays of radiant
energy and would stimulate the adjacent cells ; but eyes appear
to have been derived from the much more elementary condition of
a small patch of pigmented epithelium. From such a simple
beginning almost any kind of eye can be derived without special
difficulty.
 
Eyes of Invertebrates.  - Eyes consisting of but slightly modified epithelial cells covered by a thickened cuticle occur in nearly
all the lower Metazoa. It is characteristic of the eyes of the
Invertebrates that the light falls directly on the sensory (retinal)
cells, their inferior extremities being connected with nerve-fibrils
which transmit the stimulus to the nerve centres. The dorsal eyes
of Onchidium and the pallial eyes of Pecten and Spondylus offer
a remarkable exception to this rule, as in these Molluscs the rays
of light, after passing through the cornea and lens, have to penetrate a layer of nerve-fibres before impinging upon the sense-cells.
Patten has shown that in Pecten this is due to the primitive optic
cup being converted into a vesicle, of which the lower (inner) wall
becomes aborted, the retina being formed of the upper (outer) wall.
The sensory surface of the latter would necessarily be internal to
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
153
 
the cup, and the nerve layer external. The same general arrangement also occurs in the eye of the Chordata.
 
The simplest eyes in the Arthropoda are those of the larvae of
certain Insects ; in these the hypodermis forms a slight depression
(fig. 129), the lowermost cells of which form the retina, and are
connected with the fibres of the optic nerve ; a biconvex thickening of the cuticle forms the lens.
 
Lankester, working on the lines of Grenacher, has suggested the following stages
of evolution as occurring in the Arthropod eye :  -
 
Instead of remaining distinct (non-retinulate), the retinal cells may he aggregated
together to form what is termed a retinula, as in the lateral eyes of Scorpions and
Limulus, and the eyes of Myriapoda.
 
A higher stage of differentiation consists in the division of the retinal cells into an
outer vitreus and an inner retinal layer. These double-layered eyes (diplostichous, as
opposed to the above-mentioned single-layered or monostichous eyes) may either be
composed of separate cells (non-retinulate), as in the dorsal eyes of Spiders and the
simple eyes of adult Insects, or the sensory cells may be grouped into retinulse.
 
The retinulate diplostichous eyes may either be provided with a single lens
 
 
Fig. 129.  - Section of Eye of Larva
of a Water- Beetle (Dytiscus). [From
Bell after Grenacher .]
 
An example of a non-retinulate, monostichous, monomeniscus eye.
 
g-p. optic cup ; h. hypodermis (epidermis) ; l. lens ; 0. optic nerve ; r. retina.
 
 
(monomeniscous), as in the central eyes of Scorpions and Limulus, or the cornea may
become divided into a number of lenses or facets (polymeniseous), as in the compound
eye of Insects and Crustacea.
 
It seems that a non-retinulate eye cannot be polymeniseous, since the segregation of
retinulse is the developmental antecedent of the segregation of the lens. Hence we
may have monostichous polymeniseous eyes (lateral eyes of Limulus) as well as diplostichous polymeniseous eyes, but all non-retinulate eyes are monomeniscus. The compound (poly meniscus) eye is formed, not by the gradual concrescence of a number of
simple eyes, but by the segregation of the elements of a simple eye, which affects first
the retina and then the lens.
 
All these structures are modifications of the epiblast.
 
It is stated that in Astacus the corneal lenses and the crystalline cones are directly
developed from the epiblast of the optic pit which very early makes its appearance on
the procephalic lobes of the embryo ; while the retinulse with their rhabdoms,
together with the optic ganglion and nerve, are developed from the cephalic ganglion.
But, it will be remembered, the latter also arises from a proliferation of the epiblast
of the same area. The pigment is stated to be derived from neighbouring mesoblast
cells, but the visual pigment is probably epiblastic.
 
Patten believes the development of the Decapod eye to be as follows :  - The cephalic
epithelium (hypodermis) gives rise, by cell proliferation, to two layers  - an inner one,
the brain ; and an outer one, the permanent epidermis. That part of the brain arising
from the seat of the future eye gives rise to the optic ganglion, which is never entirely
 
 
 
154
 
 
THE STUDY OF EMBRYOLOGY.
 
 
separated from the seat of its origin. That part of the epidermis from which the
optic ganglion originated again thickens and divides into two layers, an outer
corneal hypodermis and an inner ommateal layer, consisting of retinophorae surrounded by their circles of retinulae (see p. 156).
 
Kingsley has very recently found that in Crangon, the cephalic pits, which Reichenbach formerly believed to be concerned in the development of the cephalic ganglia,
 
 
 
Fig. 130. - Ocellus of Larval Insect. [After Patten.']
 
ax.n. axial nerve; c.c. corneal cuticula; c.liy. corneal
epidermis (hypodermis ) ; rtf. retinophorae. Each retinophora (retinal cell of Grenacher) consists of a group of
four cells round an axial nerve. The cuticular portion or
rod of each retinophora is provided witli a plexus of
nerve-fibrils (not shown in fig.), and projects into the
optic vesicle ; rtn. retinulae or pigmented cells ; v. b.
vitreous body.
 
A section of a retinophora showing the peripheral and
axial nerves is placed by the side of the figure.
 
 
are the rudiments of the eyes. Each optic pit is converted into a vesicle which sinks
below the epidermis. The outer portion of the optic vesicle develops into the retina,
while the inner portion forms the ganglionic layer. Later mesoblastic cells migrate
between the retina and the ganglionic layer ; these subsequently become pigmented.
Nerves grow from the ganglionic layers into the retinal elements. The eyes are only
connected with the cephalic ganglia at about the time of hatching.
 
 
 
A-B. Gasteropod (Murex). C-D. Cephalopod (Loligo). [The latter after LanJcester .]
 
c.g. proliferation to form cephalic ganglion ; m. mesoblast ; op. optic pit ; p.
pigment ; r. retina.
 
According to Patten, the primitive optic pit (fig. 1 29) is converted
into an optic vesicle (fig. 130), the anterior wall of which atrophies,
while the posterior is greatly thickened to form the retina. This
view differs fundamentally from Grenacher -s.
 
The cephalic eyes of the Mollusca arise as a single pit of the
epiblast from the area from which the cephalic ganglia proliferate,
and at the base of the tentacles (fig. 131, a).
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
155
 
 
Fraisse first demonstrated that the eyes of the Limpet (Patella)
never advance beyond this stage of development (fig. 132), and
that Haliotis is intermediate between this larval eye and the eyes
of such Gasteropods as Fissurella (fig. 132, c) or Helix (fig • 133 . B).
 
In the last two forms, as in most other Odontophora, the embryonic pit is converted into a vesicle, the inner wall of which
constitutes the retina. The lens is a cuticular deposit. The
outer wall of the vesicle, together with the overlying epidermis,
form the cornea. The eyes of Chaetopoda and Peripatus are very
similar to this.
 
The stalked eyes of the Nautilus (fig. 133, a) always persist as
 
 
A B C
 
 
 
Fig. 132  - Diagrams Illustrating Three Stages in the Evolution of Eye of
Gasteropods. [A and C. after Fraisse ; B. after Patten.} A. Patella. B.
Haliotis. C. Fissurella.
 
 
In A. the eye persists as a simple optic cup. In B. the lower or retinulate layer
of the cuticle is converted into the retinal rods ; the corneal layer is divided into
a semi-fluid inner portion ( v.b ) and a harder outer portion (i). In C. the optic
cup is converted into a vesicle, and the epidermis is continued under the
cornea.
 
c. cornea; c.c. corneal cuticula; ep. epidermis; l. lens; op.n. optic nerve;
r. retina; r.r. rods of retinophorse ; v.b. vitreous body.
 
a simple optic pit, although considerable differentiation occurs in
the retinal cells.
 
The most complex type of eye occurring amongst the Invertehrata is found
in the Dibran chiate Cephalopoda. In these forms the two stages just mentioned are
passed through, hut a second smaller lens is secreted by the corneal epiblast
immediately in front of the former, and an annular pigmented fold of skin (fig. 133)
which develops round the front of the eyeball functions as an iris. Later a circular
fold surrounds the eye ; it may either grow completely over, or leave a smaller or
larger central aperture. This fold becomes transparent and forms a secondary cornea ;
the space between it and the lens is known as the anterior optic chamber. An eyelid is usually superadded. The secondary cornea passes below into a tough mesoblastic sheath or “ sclerotic,- which is further protected externally by a cartilaginous
capsule. The optic cavity is bounded behind by the several layered retina, and in
front by the lens ; a ciliary body is developed where the retina joins the lens. The
outer wall of the eyeball contained within the anterior optic chamber is sometimes
termed the choroid.
 
 
156
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The complexity of this type of eye is “merely the result of secondary folds of the
external skin (iris, cornea, eyelid), more or less enclosing the typical Molluscan
eye. The white body is a problematical structure which is situated at the side of
the optic ganglion (see p. 114). Although the eye of these Cephalopoda strangely
simulates that of Vertebrates, there is a profound morphological dissimilarity, which
is readily apparent when their development is compared together.
 
 
 
Fig. 133.  - Three Diagrammatic Sections of the Eyes of Mollusca. [After Grenacher.]
 
 
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.
ci. epithelium of ciliary body; co. cornea; e.l. eyelid; ep. epidermis; i.l.r.
inner layer of retina; ir. iris; l. lens; l'. outer segment of lens; n.s.r. nervous
stratum of retina ; op.g. optic ganglion ; op. n. optic nerve ; R. retina.
 
The nature and evolution of eyes of certain Invertebrates has most recently been
studied by Patten ; his views briefly are that the structural element (ommatidium)
of all eyes consists of from two to four colourless cells (retinophorse) surrounded by a
circle of pigmented ones (retinulse). The external cuticle consists of two layers, an
outer structureless one (corneal cuticula), and an inner layer (retinidial cuticula),
 
Fig. 134.  - Diagram Representing the
Transformation of Epidermal,
Cells into Sense- and NerveCells in Mollusca. [After Patten .]
 
a. neuro-epithelial cell with its nervous
prolongation, transformed in c to a bipolar and in d to a multipolar nerve-cell
(g ) ; d. a myo-epithelial cell with its
radiating fibres forming a basal membrane, two hypodermic nerves (n) are
shown, the fibrils of which form a network ( nt . retia terminalia) on the upper
portion of the cell and in the lower layer
of the cornea ; at e the essential portion
(ommatidium) of an invaginate eye is
figured : the central retinophora (rtf) is
composed of two cells, whose nuclei persist, enclosing an axial nerve (ax. n) which
supplies its retinal rod ; the two lateral
pigment cells, retinulse (rtn), have also
retinal rods (rtn), which, however, disappear in more specialised eyes ; their
nerves (n) form a network on the rods ;
c.c. corneal cuticula ; r.c. retinidial cuticula.
 
filled with the retia terminalia or ultimate ramifications of the hypodermic nerves.
The cuticular secretion of each cell forms a rod containing a specialised part of the
retia terminalia (retinidium).
 
In the more specialised ommatidia the rods of the retinulse disappear, leaving the
double (ex. Molluscs, Worms) (fig. 134) or quadruple (crystalline cone of Arthropoda)
(fig. 130) rods of the retinophorse.
 
 
a. 1. c. d. e
 
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
157
 
 
The apposed walls of the retinopliorse disappear to a greater or less extent, so that
the nerve-fibres between the cells come to lie in the centre of the group, and constitute the axial nerve (fig. 134, ax.n)
According to Patten, the epidermis of Molluscs consists mainly of columnar cells,
the inferior expansions of which form the basal membrane. The cuticle secreted by
these cells consists of two layers, an outer corneal layer (fig. 134, c.c) and an inner
retinidial layer ( r.c ). The nerve-fibres of the skin ramify into an extremely delicate
fibrillar network on the upper portion of these cells, and into the lower (retinidial)
layer of their corresponding euticular areas or rods (fig. 134, n.t). An eye is initiated
by the appearance of (red) pigment in one or more of these cells, the red pigment
(ommerythrine) being peculiarly sensitive to light vibrations. An optic element or
ommatidium consists of a group of such pigmented cells (retinulse) round one or
more colourless nervous cells (retinophorse). Although at first all the cells of an
ommatidium are sensitive, the retinophorse persists as the truly sensitive cells, while
the retinulse take on secondary functions. It must be distinctly understood that
Patten alone is responsible for the above conclusions.
 
Lankester draws attention to the fact that “ it is difficult to make out what precisely is the situation and the limit of the pigment in all Arthropod eyes.- Pigment
granules are often very freely developed in the protoplasm of the ordinary hypodermis
(epidermis) cells and of the indifferent cells (both perineural and interneural) of the
ommateum. Should the nerve-end cells be pigmented, the pigment granules are confined to the surface of the cell, leaving the axis transparent.
 
“ The relation of pigment to the optical apparatus cannot be said to be at present
properly understood. It is perfectly certain that in some eyes, and possibly in all,
pigment does not play a primary part in the physiological process set going by light.
Light acts with full effect upon transparent protoplasm, and no pigment is necessary,
converting the energy of light into the energy of heat, in order that the protoplasm
of cells may constitute an apparatus sensitive to light. The function of pigment in
an eye is a secondary one, as we learn from the sight of albino varieties. What precisely the significance of pigment may be in relation to the cells in which the optic
nerve ends, is not yet agreed upon by physiologists.-
 
Eyes of Vertebrates.  - The eyes of the Vertehrata are of a compound nature, part being developed from the brain and part
from the outer skin of the head ; both these elements are therefore of epiblastic origin, and they are protected by mesodermal
structures.
 
The first rudiment of the eye to appear is a pair of diverticula,
which bud out from the sides of the anterior cerebral vesicle
(figs. 106, abl, and no, mes), and which are known as the primary,
optic vesicles. They usually arise as soon as the primitive brain
shows traces of serial dilatations (cerebral vesicles) ; but in some
Mammals, at all events, the optic vesicles are recognisable before
the cerebral neural groove is converted into a canal.
 
The optic vesicles at first have a wide opening into the brain,
but they are soon partially constricted off, and their narrowing
stalks will develop into the optic nerve. The constriction which
separates the optic vesicle from the brain extends from above and
from the front, so that the stalk of the vesicle is situated at the
 
 
158
 
 
THE STUDY OF EMBRYOLOGY.
 
 
base of the brain, and arises from the posterior region (thalamencephalon) of the anterior cerebral vesicle.
 
The external wall of the optic vesicle invaginates until it is
completely inverted (fig. 135), recalling the manner in which a
blastula is typically converted into a gastrula.
 
The epiblast of the head, which lies immediately external to the
optic vesicles, becomes columnar, and invaginates as a rounded
vesicle at the same time that the optic vesicle is introverted. The
sac thus formed is the rudiment of the lens (fig. 1 1 2). As this
becomes constricted off, the outer skin again becomes continuous,
and is eventually transformed into the cornea.
 
 
 
Fig. 135.  - Horizontal Section through the Head of an Embryo Fowl, Illustrating
the Development of the Eye.
 
A. Embryo of fifty-four hours - incubation. [ After Marshall .] The section is
oblique ; on one side it passes through the optic stalk.
 
B. Section of about the same age, thi-ough another plane. C. Later stage.
 
a. a. aortic arches ; a.c.v. anterior cardinal (jugular) vein ; au. auditory vesicle ;
c.h. cerebral hemispheres ; /. 6. fore-brain ; h.b. hind-brain; inf. infundibulum ;
l. lens ; l.t. lamina terminalis ; nch. notochord ; o.c. optic cup ; p. pigment layer
of the retina ; ph. pharynx ; pit. pituitary body ; r. retina; v.c. visceral clefts.
 
The eye at this stage consists of a stalked double-layered cup,
containing a hollow sphere, and bounded externally by the skin
(figs. 1 1 2, 1 35 a). The cavity within which the lens lies is known
as the secondary optic vesicle, or, more correctly, as the optic cup.
The lens does not grow so rapidly as the optic cup, and consequently is soon relatively much smaller, and comes to be
embraced by the rim of the mouth of the cup (figs. 135, c, 136).
 
The various elements of the eye will now be described separately,
but previously certain points concerning the mode of the invagination of the optic vesicle require consideration.
 
The invagination does not occur solely on the outer face of the
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
159
 
 
optic vesicle, but also, in a linear manner, along its ventral line.
The cup thus has a wide mouth, plugged by the rudiment of the
lens, and a ventral slit (choroidal fissure) which opens into the
cavity of the eyeball (fig. 136, ch.f).
 
To again borrow a simile, the orifice of invagination of the optic
cup may be said to resemble a linear blastopore with an anterior
enlargement. The latter persists, but the former ultimately becomes closed.
 
It is at present an open question how far the invagination to
form the optic cup is primitively the result of the pressure of the
lens.
 
 
 
Fig. 136.  - Diagram Illustrating the Position of the Choroidal Fissure.
 
A. Surface view, from the side. B. Skeletal view, the greater portion
of the optic cup being supposed to be cut away.
 
ch.f. choroidal fissure ; l. lens ; o.n. optic nerve ; p. pigment layer ; r. retina.
 
From the first, the inner or anterior layer of the optic cup is
thicker than the outer or posterior, and it becomes increasingly
so. The former is the rudiment of the retina, while the latter
persists as the pigment layer within which the retinal rods are
imbedded (the so-called pigmented epithelium of the choroid)
(figs. 13 7 ,p.ch; 138,3)).
 
The retina soon becomes several cells deep, but it is probable
that for some time, at least, each cell extends throughout its whole
thickness. The histogenesis of the retina is still obscure. It however appears to be unquestionable that the layer of rods and cones
is developed from the epithelial layer of the central nervous system
(fig. 139) ; and that the main portion of the retina, with its nerve
 
160
 
 
THE STUDY OF EMBRYOLOGY.
 
 
fibres and nuclear layers, together with the inner (anterior) layer
of nerve-fibres and nerve-cells, is formed from the more specially
nervous portion of the cerebral epiblast.
 
The optic nerve is, as has already been stated, derived from the
stalk of evagination of the optic vesicle. The reduction of the
cavity of the canal by the thickening of its internal walls takes
place centripetally, i.e., from the eye to the brain.
 
 
 
Fig. 137. -Section of the Eye of a Fowl at the Fourth Day.
 
[From Balfour .]
 
e.p. superficial epiblast of the side of the head ; R. true retina, anterior wall of
the optic cup; p.Ch. pigment epithelium of the choroid, posterior wall of the optic
cup ; b. is placed at the extreme lip of the optic cup, at what will become the margin
of the iris ; l. lens, - the hind-wall, the nuclei of whose elongated cells are shown at n.l,
now forms nearly the whole mass of the lens, the front wall being reduced to a layer
of flattened cells, el ; m. the mesoblast surrounding the optic cup, and about to form
the choroid and sclerotic,  - it is seen to pass forward between the lip of the optic cup
and the superficial epiblast.
 
Filling up a large part of the hollow of the optic cup is seen a hyaline mass, the
rudiment of the hyaloid membrane and of the coagulum of the vitreous humour (»/).
 
In the neighbourhood of the lens it seems to be continuous, as at cl, with the tissue a,
which appears to be the rudiment of the capsule of the lens and suspensory ligament.
 
While the optic stalk is still partially open, fibres appear in its
proximal portion, some of which pass over into the root of the
other, and thus initiate the optic chiasma. The nerve-fibres later
extend along the optic stalk.
 
The optic nerve is at first continuous with both layers of the
optic cup (fig. 136), but in process of time the connection with
the outer or pigment layer is lost.
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
161
 
 
In Mammals the distal portion of the optic stalk is flattened and its cavity
obliterated whilst the optic cup is forming ; and since the stalk itself partakes in
the invagination, the choroidal fissure may be said to extend for some distance along
the nerve. The central blood-vessels of the retina (fig. 138) enter this groove, and
are subsequently surrounded by the overgrowth of the nerve. The retinal circulation
is entirely confined to these vessels and their capillaries. Kolliker suggests that the
invagination of the optic stalk is due to the pressure of the mesoblast, which develops
into the blood-vessels.
 
The retina is unprovided with true retinal blood-vessels in animals lower than the
 
 
 
Fig. 138. - Horizontal Section of the Eye of a Rabbit of Eighteen Bays.
 
Magnified 30 diameters. [ From Kolliker .]
 
ap. orbito-sphenoid (lesser wings of the sphenoid) ; c. cornea, with its anterior
epithelium, e ; f. rudiment of the choroid ; g. vitreous body detached from the retina
by shrinkage, except behind, where the central artery of the retina passes into it ;
i. iris ; l. crystalline lens ; l'. epithelium on the anterior face of the lens ; m, m. rectus
superior, and r. inferior muscles ; mp. membrana pupillaris ; o. optic nerve ; p. outer
pigmented layer of the retina ; p'. anterior border of secondary optic cup, where the
retina proper passes into the pigmented layer ; pa. upper eyelid ; pp. lower eyelid ;
r. retina ; re. pars ciliaris retinae.
 
 
Mammals, but their place is possibly to some extent taken by the vascular structures
which penetrate the cavity of the eyeball through the choroidal fissure. These
are known as th e processus falciformis in Ichthyopsida, and the pecten in Sauropsida.
 
The lens was left as an oval vesicle, with uniformly thick walls.
Very soon the cells of the front wall become thinner and flattened,
while those of the inner wall elongate and entirely obliterate
the cavity of the vesicle (fig. 137). The latter cells early become
 
L
 
 
162
 
 
THE STUDY OF EMBRYOLOGY.
 
 
strap-shaped and acquire their final disposition (fig. 138). At no
time is the wall of the lens more than one cell deep.
 
The lens capsule is a cuticular membrane probably secreted by
the epithelial cells of the lens.
 
The vitreous humour appears to be derived from a fluid transudation from the vascular ingrowth, which enters the retinal chamber through the choroidal fissure. In some cases a few embryonic
mesoblast cells occur.
 
The anterior epithelium of the cornea is formed by the growing
together of the epiblast after the formation of the lens. Its
deeper or proper substance is of mesoblastic origin, and is derived
from an ingrowth of the neighbouring mesoblast. A similar but
shorter inferior fold constitutes the iris. The mesoblast cells of
the incipient cornea occupy a space which lies between the epithelium of the cornea and a flattened epithelium (membrane of
Descemet), which is also of mesoblastic origin.
 
The aqueous humour is a watery fluid which occupies the cavity
between the lens and the cornea.
 
Eyelids are developed as simple folds of the skin ; their inner
surface is lined by a mucous membrane, the conjunctiva, which
also covers part of the sclerotic and the exposed surface of the
cornea. There may be three eyelids, a dorsal, a ventral, and an
anterior, the nictitating membrane, arising from the inner angle of
the eye.
 
The eyelids are rudimentary or absent in Eishes, except in some
Elasmobranchs. All three eyelids are present in most Amphibia
and Sauropsida, but the nictitating membrane is rudimentary in
Mammals.
 
In many Mammals the two eyelids meet together and unite
during a period of embryonic life. A similar condition is permanent throughout life in Snakes and some Lizards, the lachrymal
ducts opening into the space thus formed between the fused lids
and the cornea.
 
Lachrymal glands occur in the Amniota. Their character varies
greatly in the different groups, but they always arise as solid
ingrowths of the conjunctiva.
 
The sclerotic and choroid coats of the eye are protective envelopes developed from the mesoblast.
 
Epiphysial Eye.  - The possession of a rudimentary median eye,
lodged in the parietal foramen and developed from the pineal gland,
in several Lizards has already been alluded to (p. 129). The lens
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
163
 
 
of this eye is a direct derivative of the optic cup, and what light
reaches it impinges directly on the retina without first penetratingthrough the retinal layer of the fibres of the optic nerve (fig.
138*, d). Thus, as in the Pectinidae and Onehidium, certain
Invertebrates have accessory eyes constructed on the vertebrate
plan, so some Lacertilia amongst the Vertebrates possess a typically
invertebrate unpaired eye. A radical distinction between the
pineal eye of Lizards and the eyes of Invertebrates consists in the
fact that the essential constituents (retina and lens) of the former
 
 
 
Fig. 138*. - Diagrams Illustrating the Evolution of the Epiphysis
(Pineal Gland). [After Spencer .]
 
A. Early stage of epiphysis in Bufo cinerea ; this corresponds with the early stage
in larval Tunicates and the probable condition in the ancestral Chordate. B. Early
stage in all higher Chordata ; permanent in Elasmobranchs and Cyclodus. C. Later
stage in Anura and Sauropsida ; permanent in Chameleo. D. Adult stage in certain
living Lacertilia, e.g., Hatteria, Varanus ; probable condition in Labyrinthodonta,
and in ancestors of Reptilia and Aves. Final stage in many Lacertilia, e.g., Calotes,
Seps, Leiodera. F. Anura, adult. G. Aves, adult. H. Mammalia, adult.
 
It will be seen from the above figure that the epiphysial or pineal eye of certain
living Lizards is differentiated from the distal vesicular portion of the pineal gland.
The central section being converted into an optic nerve, the proximal practically
forms an optic lobe. A-D. illustrate the development of the organ to its most
specialised condition. - E-H. indicate various phases of degeneration. The shaded
portion indicates the parietal bone. In D. the anterior portion of the vesicle is modified to form a lens, the posterior wall differentiating into an inner pigmented retinal
layer and an outer layer of nerve-cells.
 
 
are entirely differentiated from a diverticulum of the brain (fig.
138*, c-e), -whereas in the latter they are invariably epidermal
structures.
 
 
Hypothetical Evolution of the Vertebrate Eye.  - The fact that the optic cup is
developed from the anterior brain vesicle is at first sight very anomalous. The
following considerations, however, may tend to throw some light upon it.
 
It will be remembered that an ancestral form of the Chordata was assumed (p. 76)
to possess a nervous system but little differentiated from the epiblast extending
along the primitive oral aspect of the body, and expanding in front of the mouth.
Upon this region a pair of cuplike eyes was supposed to be situated, the eyes having
 
 
 
164
 
 
THE STUDY OF EMBBYOLOGY.
 
 
essentially the same structure as in Patella (fig. 132). This condition is diagrammatically represented in fig. 139, a, b, the latter being a supposed transverse section
through the pre-oral region of a. It will be seen that the eye-pits are connected
with the pre-oral neural epiblast, much in the same manner as the eye-pits of
Mollusca (fig. 1 31) are developed in connection with the proliferations which form
the cephalic ganglia (fig. 96, c.g).
 
The involution of the nervous area to form the neural canal also implicated the
optic pits (fig. 139, c). Since this figure was drawn, Heape has shown that in the
Mole the optic vesicles appear as depressions of the cephalic neural plate even before
the neural groove is established. Heape figures a section which very closely resembles
the diagram given in c, fig. 139. On the closure of the neural tube the pits would
appear as vesicles (optic vesicles) opening into the anterior cephalic enlargement.
 
A local thickening of the overlying lateral epiblast to form a lens might be a
mechanical cause for the invagination of the optic vesicle to form the optic or retinal cup. Every subsequent stage of evolution, being an optical improvement,
could be accounted for once the retinal cup was established.
 
Fig. 139 also illustrates that the visual sense-cells (rods and cones) are derived
from the epithelial layer of the central nervous system, in other words, from the
 
 
 
Fig. 139. - Diagrams Illustrating a Hypothetical Evolution of the
Vertebrate Eye.
 
A. Surface view of head of a hypothetical type. B. Vertical section of same across
the optic pits. C. Invagination of the pre-oral neural plate and optic pits. D. The
process completed. E. Formation of lens and optic cup.
 
ep. epidermal layer of epiblast of head; ep.b. epithelium of brain; l. lens; m.
mouth ; n. nervous layer of epiblast of head ; n.b. nervous layer of brain ; n.p. neural
plate ; n.r. nervous layer of retina ; o.p. optic pit ; op.v. optic vesicle; p.b.v. primary
brain vesicle ; p.o. pre-oral neural plate ; r.r. layer of rods of retina.
 
external epiblastic epithelium ; that is to say, from precisely the same layer which
gives rise to the similar elements in Invertebrates. The deeper or nervous layer of
the epiblast is concerned in the formation of the layer of nerve-fibres and nerve-cells
of the retina.
 
The transparency of the body of the primitive Chordata, assumed by Lankester,
would enable light to reach the optic pits, although the latter were situated within
the brain. But as the animal became more opaque, it may be assumed that the
visual apparatus (optic vesicles) would grow out towards the sides of the head through
which most light would penetrate. The lens is clearly a secondary structure. On
this hypothesis the eye could be functional whilst it was undergoing this unique
metamorphosis.
 
Observations on the Evolution of the Nervous System and Sense Organs.  -
 
The origin of the nervous system and sense organs from the epiblast is one of
the best attested of embryological discoveries, and from the foregoing brief account
they would appear to be universally so derived. The only general statement, however, that can be made is that nerve and sense cells have arisen in response to a
stimulus, or, more correctly, as the result of a stimulus.
 
As a matter of fact, such a stimulus would most readily and frequently act upon
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
165
 
 
the exterior of the body, and therefore upon epiblastic tissue ; hence the almost
universal origin of these structures from that layer; but there are a few exceptions which are of considerable interest.
 
The brothers Hertwig have demonstrated that in addition to the diffused ectodermal nervous system present in the Actinise, there is a distinct layer of nerve fibres
and cells, and in some cases of sense-cells, which can only be derived from the
endoderm. The occurrence of the latter may possibly receive an explanation from
the fact that the mesenteric chambers open widely into the digestive cavity of the
body in these animals. As the wide mouth and oesophagus are so generally open, there
is really considerable facility for stimuli, such as vibrations in the external medium,
to act upon the internal tissues. In both cases, therefore, the differentiation occurs
in tissues directly exposed to the surrounding medium.
 
Quite recently Hubrecht has discovered that the nervous system, i.e., the brain
and lateral nerve cords of the Nemertean Worm Lineus obscurus are derived from the
mesenchyme. Certain of these wandering cells (mesamoeboids) apply themselves to
the interior of the body- wall in definite areas, and there differentiate into the nervous
system of the adult. The mesenchyme has a double origin, being partly derived
from the epiblast and partly from the hypoblast (fig. 49). Although direct proof is
not attainable, it is fair to assume that the nervous system is developed out of the
epiblastic rather than from the hypoblastic mesenchyme. If this be the case, it is
probably another example of “ precocious segregation.-
 
As has been already mentioned (p. 114), Bobretzky states that the nervous system
of the Prosobranch Gasteropod Fusus is derived from the mesoblast, and that the
wandering cells apply themselves to certain areas of the epiblast, as in the case of Lineus,
but in all the other Gasteropods which have been examined, and even in the allied
forms of Purpura (fig. 96) and Murex, the nerve centres have an epiblastic origin.
Bobretzky -s statement must therefore be received with caution. The same applies
to Fol -s account of the origin of the pedal ganglia from the mesoblast of the foot of
Limax, while the cephalic ganglia are developed from the epiblast of the velum.
 
Lastly, the origin of the sense-cells and nerve-cells of Sponges, which have been
described by Stewart, Yon Lendenfeld, and Sollas, is still somewhat uncertain.
They have been stated to be mesodermal (mesenchyme) elements, from the fact that
the ectoderm of Sponges always occurs as a delicate flattened epithelium and never
exhibits any transitional stages into sense-cells, in this respect offering a marked
contrast to that of Coelenterates. Whereas the position and appearance of the nerve
and sense-cells irresistibly suggest a mesodermal origin.
 
One important point should not be lost sight of in these considerations. It is
that protoplasm from its very nature is what has been termed “ irritable,- that is to
say, it responds to stimuli. This irritability is inherent to all cells, and probably is
never lost while the cell lives ; certain cells have this function greatly developed,
while in others it is more or less diminished. It is probable that stimuli may readily
pass from one cell to another in most tissues, as animal cells are usually in close
contiguity when not in actual continuity. In many adult animals, and usually in
embryos, different tissues may be connected together by branched mesoblastic cells
(indifferent connective tissue), which may also be amoeboid. If these latter cells
retain their irritability, there is probably nothing to prevent their transmitting as
well as receiving stimuli. They may thus serve as incipient nerve-fibres ; and it is
further possible that this function may be sufficiently pronounced to cause the formation of a definite nervous tissue which is purely mesoblastic in origin. This
secondary nervous system might be developed in adults as well as in embryos. The
observations of Yon Lendenfeld on Sponges tend to support this hypothesis.
 
From numerous researches on the nervous system of the lower Metazoa, it is not
difficult to trace the stages by which ectodermic (epiblastic) cells are gradually
modified into nerve-cells.
 
 
166
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In the primitive Metazoon most of the external cells of the body were probably
ciliated, and had very similar functions. In process of time certain cells would
gradually acquire a greater degree of sensitiveness, while others would become more
protective in function. If, for instance, a cilium-like prolongation of a cell lost its
power of contractility and became rigid, it would then, as a mechanical necessity,
vibrate in response to the vibrations of the surrounding medium. These induced
vibrations would act as stimuli to the cell and excite a manifestation of irritability,
which might expend itself in various ways. Most sense-cells are constructed on
this plan ; they are, in fact, epidermal cells with a stiff projecting hair or rod-like
process, and are interiorly continuous with other cells.
 
Chatin has recently found that all intermediate stages can be found between the
auditory rods and ciliated cells of the auditory epithelium of the labyrinth in
Batrachia.
 
It is now demonstrated that the cells of the tissues of the Coelenterata are connected with each other by means of very delicate, usually branching, root-like processes, which serve for the contraction and general co-ordination of the parts or
whole of the organism or colony. The sense-cells form no exception, and in some of
them the upper sensory portion appears to be gradually becoming smaller, while the
lower portion, which contains the nucleus, is swollen (fig. 119, c, g). As the nucleus
is mainly the centre of the activity of the cell, it may be assumed that in these cells
general irritability is preponderating over special sensibility, and that it only needs
a slight further specialisation to constitute a cell wholly given over to irritability ;
in other words, a nerve-cell. The same process also occurs in the skin of Molluscs.
In fig. 134, a, b, c, d, diagrammatically represent the gradual transformation of a
sense-cell, a, into a multipolar nerve-cell, g.
 
The nerve-cell retains connection with the neighbouring cells by its root-like
processes, and thus may be united with a sense-cell on the one hand, and with a
glandular or muscular cell on the other. By this double connection the nerve-cell
may receive a stimulus from a sense-cell, and by the excitation of its own irritability
may transmit the stimulus in an intensified form to the distal cell, and the latter
will be stimulated to perform its special function.
 
The foundation of a distinct nervous system will thus be laid, and the multiplication and localisation of sense-cells and nerve-cells has probably been effected to a
large extent independently in the different groups.
 
This suggestion concerning the evolution of the nervous system seems to be
warranted from a consideration of the histology of adult Coelenterates (fig. 119) and
Molluscs (fig. 134) ; but even if it be a correct interpretation of the facts in these
groups, it is possible that in other forms the history may be somewhat different.
For example, nerve-cells may originate by the division of certain epidermal cells into
an outer protective portion and an inner more irritable or nervous moiety, the
latter always retaining connection with the former by means of protoplasmic
threads.
 
In the embryos of the lower Chordata the epiblast primitively consists of a single
layer ; in Amphioxus alone is this condition retained in the adult. In the Urodele
Amphibia the epiblast is single layered till the completion of the gastrula stage; but
in the Anura the epiblast is several layers thick in the blastula stage.
 
In all cases the distinctly nervous elements of the central nervous system and
sense organs is formed entirely from the deeper layer of the epiblast. Thus there is
in the Anura and some other groups, Ganoids and Teleosts, an early separation of the
epiblast into the epithelial and the mucous or nervous layer.
 
Spencer has recently stated that the segmental nerves and ganglia in the Frog
arise in situ by a local persistence of this deeper layer ; thus there is, as he points
out, in Amphibia a primitive nervous sheath to the body, the nervous tracts being
local retentions of this diffused nervous system. Later still Misses Johnson and
 
 
ORGANS DERIVED FROM THE EPIBLAST.
 
 
167
 
 
Sheldon, from their studies on the Newt and Frog, support the generally received
view of the outgrowth of the nerves from the neural ridge.
 
In this connection it is interesting to notice that Bateson has shown that in
Balanoglossus (the lowest known member of the Chordate series) the central nervous
system arises as a delamination of a solid cord of epiblast in the dorsal middle line of
the middle third of the body of the embryo ; this, by invagination of its two ends, is
afterwards extended as a tube in both directions. Other collections of nerve-fibres are
afterwards deposited in various parts of the body, and finally a general network of
nerve-fibres occurs on the under surface of the skin of the body, especially in the
line of the gill-slits. The tail-like processes of the epiblast cells run into the different
superficial nervous tissue, and many fibres pass into the subjacent mesoblastic tissues.
The fibres entering this nerve-substance on its outer side are plainly sensory, or at
all events afferent, and the fibres passing from it on its inner side are presumably
motor, or at least efferent, seeing that they innervate the mesoblast.
 
li It is clear, then (as Bateson points out), that on the separation from the skin of
a cord thus composed the relations of the efferent fibres will not be changed, as they
still remain in contact with the mesoblast. But, on the other hand, if this nervecord be entirely separated from the skin, the supply of outer or afferent fibres is cut
off from it, unless cords of epiblast remain to connect it with the skin. Applying
this reasoning to the particular case of the separation of the dorsal cord, we see that
the afferent fibres are entering it on its dorsal side, and that the efferent fibres are
leaving it on its ventral side. If the nervous system arose in this way, the dorsal
roots were from the first sensory, and did not arise as differentiations of roots of
mixed function, as has often been supposed.-
 
The epithelium lining the cavity of the central nervous system and the sensory
epithelium of the sense organs are derived from, or from what corresponds to, the
external layer of the epiblast. Exceptions occur in the auditory sacs of Ganoids and
Teleosts, which are solely developed from the deeper layer of the epiblast, and in
the optic vesicles of Teleosts, which are formed as solid buds from the solid nervous
keel which will develop into the brain. In this, as in many other respects, the
development of the Teleosts is extremely modified.
 
 
(168 )
 
 
CHAPTER VL
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
In a previous section the archenteron was left as a simple sac
or tube, opening to the exterior anteriorly by the stomodseum, and
posteriorly by the proctodaeum.
 
From what was said concerning the effects of the presence of a
large amount of food-yolk, it will be obvious that there will be a
discrepancy in the relative time of the development of various
hypoblastic structures ; for example, in telolecithal ova the ventral
wall of a considerable portion of the alimentary canal must of
necessity be completed very late.
 
The primitive function of the hypoblast is undoubtedly alimentation, but in the course of evolution it has acquired several other
functions. The digestive organs will now be first considered, and
subsequently other hypoblastic derivatives will be described.
 
Digestive Organs.  - The simple sac-like archenteron of the
gastrula, as has already been described, is produced into pouches
in a large number of animals.
 
When this occurs in Sponges the characteristic hypoblast cells
(choano-flagellate cells) become restricted to the extremities (ciliated
chambers) of the often complicated diverticula. All the exhalent
canals are lined with flattened hypoblast cells.
 
The gastric diverticula of Coelenterates appear to be chiefly concerned with the circulation or distribution of the nutritive fluid, the
actual process of digestion being probably confined to the stomach
of the Hydromedusse, and the edges of the mesenteries in the
Actinozoa (fig. 68).
 
In the Coelomata, or those animals provided with a true body
cavity, these diverticula are cut off from the gastric cavity, and
are henceforth spoken of as mesodermal structures.
 
The gastric diverticula of the Turbellarians, of certain Nemerteans, and of the Leeches, cannot be regarded as coelomic diverticula which have never severed their connection with the
archenteron.
 
 
ORGANS DEBITED FROM THE HYPOBLAST.
 
 
169
 
 
It has been shown (p. 29) that in most centrolecithal ova (e.g.,
Crustacea) some of the hypoblast cells engulf the food-yolk which
lies within the segmentation-cavity (fig. 22). In other ova the
yolk is originally located within the primitive hypoblast. In both
cases it is digested by those cells.
 
The actual conversion of the primitive hypoblast into special
digestive cells has not been fully investigated, but it must be
readily effected, as digestion and assimilation are primary properties
of protoplasm.
 
The hypoblastic portion (mesenteron) of the alimentary canal is
always divisible into definite regions, and, with the exception of
most of the Arthropoda, it forms by far the largest section of the
tract.
 
The various regions of the alimentary canal of different animals
which appear to be similar had received corresponding names
before their development was known, consequently many apparent
morphological anomalies must be expected.
 
Usually among the , Invertebrates the stomodseum is prolonged
as the oesophagus ; the mesenteron includes the stomach and intestine and their associated glands, while the proctodeum is small.
The Arthropoda, as a whole, are an exception to this rule, for in
Insects the mesenteron is that portion of the alimentary canal
lying between the crop or proventriculus, when that is present, and
the point of origin of the Malpighian tubes. The mesenteron may
be a simple tube, or divided into regions, of which the anterior may
possess numerous small caeca (some Beetles) or eight large ones
(Cockroach). In low forms, such as the Myriapoda and Peripatus,
the mesenteron is long and simple.
 
In the lower Crustacea the mesenteron is relatively long. There
are in Amphipods, in addition to the two or four digestive caeca,
which are so commonly present throughout the Crustacea, two
long narrow tubes which open into the extreme hinder end of the
mesenteron. These tubes are undoubtedly excretory, but, as
Spencer has shown, they are hypoblastic and not epiblastic, they
cannot be regarded as homologous with the Malpighian tubules of
the Tracheata (p. 111).
 
The mesenteron of the Decapod Crustacea is restricted to the
usually minute chamber between the so-called pyloric chamber
(fig. 140) and the commencement of the intestine (proctodaeum) ;
it is separated from the former by valves. It is to this that the
term stomach should be restricted. The digestive gland or so
 
170
 
 
THE STUDY OF EMBRYOLOGY.
 
 
called “liver- opens by a wide aperture on each side into the
mesenteron. The latter is the only portion of the alimentary canal
of these animals which is not lined by cuticle.
 
In the Mollusca (figs. 1 8 and 84) only the buccal cavity is lined
by epiblast, the stomach and intestine being archenteric derivatives.
The stomodaeum gives rise to the buccal cavity and its organs
(radula or odontophore, salivary glands), and to the oesophagus.
The proctodaeum is very small. In the Cephalopoda the ink sac
 
 
 
Fig. 140. - Diagrammatic Sections of Embryos of the Cray-Fish (Astacus
Fluviatilis). [From Huxley after Reichenbach.]
 
C. Longitudinal section of an ovum in which the rudiments of the abdomen, of the
hind-gut, and of the fore-gut have appeared. D. Later stage of similar embryo. E.
Longitudinal section of newly-hatched embryo.
 
a. anus ; e. eye ; ep.b. epiblast : f.g. fore-gut (stomodaeum) ; f.g\ its oesophageal, and
f.g 2 . its gastric portion ; h. heart; li.g. hind-gut (proctodaeum) ; m. mouth; m.g. midgut, mesenteron, or archenteron ; v. yolk. The dotted portions in D and E represent
the nervous system.
 
early grows out as a simple diverticulum from the ventral wall of
the hinder end of the intestine.,
 
Invertebrate Digestive Gland or “Liver.- - The large digestive gland associated with the mesenteron in the higher Invertebrates (Molluscs and Arthropods) is usually spoken of as a “liver.-
As a matter of fact, it is now known to be a more universal digestive gland than its name would apply, and that it more closely
corresponds in function with the Vertebrate pancreas, combining,
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
171
 
 
as it does, the function of liver and pancreas, it has been appropriately termed the hepato-pancreas. It is a complex gland which
typically develops from the wall of the mesenteron (fig. 140) in
the usual manner, but, in some forms, the liver appears to be
formed by a metamorphosis of the remnant of the yolk-cells which
remain after the formation of the mesenteron (fig. 84, B, y).
 
Mesenteron of Chordata.  - The hypoblastic portion of the
alimentary canal of the Chordata is divisible into the following
regions: pharynx, oesophagus, stomach, and intestine (figs. 14 1,
 
143).
 
The egg being yolkless in Amphioxus, the archenteron (fig. 57)
is directly converted into the alimentary canal of the adult.
 
The effect on the formation of the mesenteron by the presence
 
 
Fig. 141. - Isolated Alimentary Canal of Embryo Dog of
Twenty-Five Days. Multiplied 5 diameters. [ From Kolliker
after Bischoff .]
 
a. pharyngeal or branchial pouches ; 6. rudiment of laryngeal portion
of the pharynx ; c. lungs ; d. stomach ;•/. liver; g. dorsal wall of the
vitelline sac, with which the intestine still communicates by a large
orifice (the umbilicus) ; h. rectum.
 
The inner white line indicates the hypoblast ; the surrounding dark
border representing the splanchnic or visceral (mesoblastic) sheath of
the alimentary tract. Compare with A, fig. 143.
 
 
A
 
at first of a small, and then of a gradually increasing amount of
food-yolk, has already been described (p. 30). The constriction
off of the digestive tract from the yolk-sac in telolecithal ova takes
a comparatively long time, and not a few Fish are hatched with
the yolk-sac still depending from their bodies. In fig. 141, which
illustrates the isolated alimentary canal of an embryo Dog, viewed
from the ventral surface, it will be seen that all the main organs
have made their appearance while the umbilicus is still widely
open (see also fig. 143). The neck of the yolk-sac gradually narrows to form the vitelline duct, and the first fold of the intestine
(figs. 144, I; 143, c) occurs at the spot where the vitelline duct
joins it. A diverticulum which occasionally occurs in Man in the
lower part of the ileum is the persistent base of the vitelline duct ;
and not unfrequently the proximal portion of the vitelline duct
 
 
 
172
 
 
THE STUDY OF EMBRYOLOGY.
 
 
may persist in Birds as a short tube connected with the small
intestine.
 
Pharynx.  - The pharynx probably extended along a considerable
length of the body in the primitive Chordata, as is still the case
in Amphioxus and Lampreys. The lateral walls were devoted to
respiratory purposes, as will be described subsequently.
 
A deep ciliated groove, the endostyle, extends along the median
ventral line of the pharynx (branchial sac) in Ascidians. The
cilia work from before backwards and thus carry the mucus, which
is secreted by the glandular cells of the endostyle, along with entangled food particles into the oesophagus.
 
The hypopharyngeal ridge of Amphioxus, with its glandular
cells, has a similar function.
 
This region corresponds to the non-respiratory ventral portion
of the pharynx of Balanoglossus.
 
Fig. 142. - Diagrammatic Longitudinal Section through
the Head of a Larval
Lamprey (Pefcromyzon. [From
Claus after Balfour .]
 
Ab. optic vesicle; C. heart; cb.
cerebellum ; c.h. cerebral hemisphere ;
Chd. notochord; H. hypophysial
(thyroid) involution ; inf. infundibulum ; ks. branchial pouches ; m.b.
mid-brain ; md . medulla ; N. nervous
system; O. stomodseum; 01. olfactory
pit ; ot. auditory vesicle, represented
as visible; pn. pineal gland (below
which the optic thalamus is shown) ;
v.cuo. ventral aorta; ve. velum. The
oblique line between the velum and
the first branchial pouch represents
the left of a pair of ciliated grooves
which converge on the median ventral
line to meet the orifice of the thyroid.
 
 
A considerable groove is developed in the front portion of the
floor of the pharynx in the larval Lamprey (fig. 142), and to a
decreasing extent in higher forms.
 
We may therefore conclude that the ventral portion of the primitive pharynx
was concerned in the transmission of food. The special mechanism by which this
was effected afterwards degraded into the median element of the gland known
as the thyroid body (see p. 183). It is possible that this change of function was
correlated with the increase in size of the primitive Chordata and the consequent
ability to eat larger prey. The latter, from their size, would not have the tendency
to escape through the gill-slits, which minute organisms could easily do, and would
further pass into the oesophagus without requiring the assistance of the ventral
groove. The latter, owing to disuse, would naturally degenerate.
 
Throughout the Xchthyopsida the pharynx gradually becomes
greatly shortened, as is also the case in Amphibia and Amniota.
 
(Esophagus.  - The oesophagus calls for no special mention. It
is a simple tube of variable length, which in some forms (Crocodilia
and many Birds) has a ventral saccular dilatation or crop.
 
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
173
 
 
Stomach.  - The oesophagus may pass imperceptibly or abruptly
into the stomach. The stomach is usually a simple dilatation of the
alimentary canal (figs. 141-144). Its exact form varies considerably, but it only becomes at all complicated in a few Mammals
(e.g., Sloths, Cetacea, Buminants, some Marsupials and Bodents).
 
There is an instructive modification in the stomach of Buminants
during growth. In the early foetus the relative size of the compartments and general form of the stomach are almost exactly
those of the adult. After birth, owing to the milk-diet, the
growth of the peptic stomach or abomasus is greatly in excess of
that of the others; but as a herbivorous diet is acquired, the
characteristic form of the adult stomach is re-acquired.
 
To secure increase of secreting surface without proportionate
extent of superficies, crypts or pockets of digestive cells were
developed forming simple glands. In time these became more
complex, as was previously described for epiblastic glands (p. 106),
the cells which actually secrete the digestive fluid being restricted
to the blind extremities or alveoli of the gland.
 
Three types of such glands are found in Mammals ; the simple
tubular crypts of Lieberkuhn in the small intestine. A gland
with a non-glandular duct and a few simple tubules is illustrated
by the peptic and pyloric glands of the stomach, and the glands
of Brunner in the pylorus, while the liver and pancreas represent
the most specialised form of gland.
 
Liver.  - The “ liver - in Amphioxus, alone of all Chordata, retains
its primitive tubular form. It is the earliest hypoblastic gland
to be developed, and it is relatively very large in foetal life. It
appears to be entirely absent in Balanoglossus.
 
In some of the lower Vertebrata (Elasmobranchs and Amphibia)
(fig. 99) the liver arises from a single ventral diverticulum from
the intestine, which soon becomes bilobed. In Birds and Mammals (fig. 1 41) the liver appears to be bilobed from the first.
 
The incipient liver buds out into a local thickening of the splanchnic mesoblast,
which thus becomes penetrated by a number of rod-like prolongations (hepatic cylinders) of the primitive diverticula. As a rule the hepatic cylinders appear to be solid,
but in Elasmobranchs Balfour found that they are hollow, as they are also stated to
be in Amphibia. A system of ducts appears in due course. The hepatic cylinders
have the peculiarity, which is unique among glands, of uniting with one another at
numerous points, thus forming a network within the meshes of which the enveloping
mesoblast develops into blood-vessels.
 
The gall-bladder is simply an enlargement of, or a diverticulum
from, the main duct of the liver. Its presence is very variable ;
 
 
174
 
 
THE STUDY OF EMBKYOLOGY.
 
 
the number and position of the ducts of the liver opening into the
intestine are also inconstant in various animals.
 
Pancreas.  - The pancreas occurs very constantly among the
Vertebrates. It is absent in the Cyclostomi and Perennibranchiate
Amphibia, and rudimentary or absent in many Teleosts. The pancreas may be partially imbedded in the liver in Ganoids, and completely so in Siluroids. It first appears as a tubular outgrowth
from the dorsal wall of the intestine, opposite to, but slightly
behind, the diverticulum, which forms the rudiment of the liver.
According to His, the pancreatic rudiment at first appears in front
 
 
 
Fig. 143. - Four Stages in the Development of the Human Alimentary Canal,
 
AS SEEN FROM THE LEFT SIDE AND ISOLATED. \After His .]
 
all. stalk of allantois; b.p. bursa pelvis; c. caecum; ep. epiglottis; g.e. genital eminence; k. kidney; l. liver; la. larynx ; l.d. duct of liver; Ig. lung; l.j. lower jaw; p.
pancreas ; pr. proctodaeum ; R.p. Rathke -s pouch (hypophysial evagination), behind
it in A and B is Seesfeel -s pouch ; st. stomach ; t. tongue ; tky. median rudiment of
thymus- gland; tr. trachea; u. ureter; umb. umbilical vesicle; v.d. vitelline duct;
W.d. Wolffian duct.
 
 
of the liver in the human embryo, and later shifts its position to
behind that viscus (fig. 143, b-d). Hollow diverticula arise from
the main duct, which continually subdivide. The surrounding
mesoblast develops as usual into blood-vessels and connective
tissue. In some cases two pancreatic diverticula have been observed.
 
Intestine. - The intestine is the post-gastric portion of the
mesenteron. It is always a straight tube in epabryos, and persists
as such in many of the lower Chordata. In other forms it becomes
variously looped, owing to its length exceeding that of the body
; cavity within which it lies.
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
175
 
 
The posterior portion of the intestine in the adult, but not in
the embryo, is usually of markedly greater diameter than the
anterior portion or small intestine ; it is known as the large
intestine.
 
The secreting and absorbing surface of the alimentary canal is
increased in the lowest Vertebrates by the development of a longitudinal fold projecting into the cavity of the intestine, which is
known as the spiral valve.
 
The fold is slightly developed in the Cyclostomi, and reaches its
highest state of development in some Elasmobranchs. It becomes
less marked in the Ganoids, and traces of it may be found in the
intestine of a few Teleosts. In no higher Vertebrate has it been
definitely recognised. A similar fold is found in the intestine of
some Ascidians ; such a fold may be compared with the typhlosole
of certain Invertebrates (ex. Earthworm and Fresh-water Mussel).
 
 
Fig. 144.  -Rough Diagrams Illustrating the
Change in Relative Position undergone by
the Digestive Tract in Mammals. [ From
Landois and Stirling.]
 
6. colon ; o. vitelline duct ; r. rectum ; t. small intestine ; v. stomach.
 
 
 
Concomitantly, according to Wiedersheim, with the disappearance of the spiral valve in Fishes a number of hollow diverticula
(pyloric caeca) make their appearance from the anterior region of
the small intestine (duodenum). These are found in some Ganoids,
in which group their development is not always inversely proportional to that of the spiral valve, and in most Teleosts, but in
no other animals. Their function appears to be, in some forms,
to increase the absorbing surface of the intestine, as a digestive
function may be present or absent [Stirling, Macullum]. In a
few Teleosts they occur side by side with the pancreas.
 
Those animals which possess a spiral valve have, in the main, an
alimentary canal which pursues a straight course through the body
cavity. In other forms (excepting Teleosts) the greater length of
the intestine probably renders a spiral valve superfluous.
 
The relative length of the alimentary canal is largely dependent
 
 
 
176
 
 
THE STUDY OF EMBRYOLOGY.
 
 
upon the nature of the food of the animal. This is well illustrated
in the case of the Frog -s tadpole. When still subsisting upon
its stored-up food-yolk, the alimentary tract retains its primitive
straight course (figs. 98, 99). After the tadpole is hatched it commences to feed upon decaying vegetable matter, and the intestine
grows to a great length, and is coiled up like a watch-spring.
Later on the young Frog takes to an animal diet, and the intestine
is relatively very much shorter, and is only slightly looped.
 
The valvulae conniventes of Man, and similar folds in other
animals, also serve to increase the absorbing surface of the small
intestine. The development of all these structures is too obvious
to require description.
 
In Mammals the end of the large intestine, where it passes into
the small intestine, is usually enlarged to form the csecum. In
Man there is at first no csecum (fig. 143, A-c), then a simple conical
projection appears (fig. d) ; later the csecum lengthens, but the
terminal portion does not keep pace with the growth of the base,
and consequently becomes much narrower in calibre. The basal
portion eventually grows so large that it is commonly called the
csecum, while the true csecum is designated as the vermiform
appendix. Several of the stages in the development of the human
csecum are permanently retained in the adult stage in certain
Mammals. It is not known whether the so-called vermiform
appendix of the Wombat is, as in the higher Primates, a remnant
of an originally elongated apex of the true csecum.
 
In some Armadillos the csecum is distinctly bilobed, and in
Cyclothurus didactylus there are two distinct cseca. In addition
to a capacious true csecum, Hyrax possesses a pair of simple conical
Cseca in the large intestine.
 
In most Birds there are two cseca of variable length at the
commencement of the large intestine.
 
A csecum is usually stated to first appear in Eeptiles, where it
never attains a large size ; but Huxley has described and Howes
has figured a representative of it in the Frog.
 
A simple rectal gland is found in Elasmobranchs.
 
Endodermal Muscles.  - Muscular processes arising from the
endodermal cells have been demonstrated by Jickeli in Hydra;
these run transversely round the body, as opposed to the longitudinal direction of the similar fibres of the ectodermal cells. Endodermal muscular fibres have been demonstrated in the Actiniae
by the brothers Hertwig.
 
ORGANS DERIVED FROM THE HYPOBLAST. 177
 
Respiratory Organs of Invertebrates.  - In but few Invertebrates does the alimentary tract function directly in respiration.
The endoderm lining the general cavity of the body in Actinozoa
is, however, probably largely concerned in respiration, especially in
such forms as Edwardsia, Cerianthus, and Peachia, which live imbedded in the sand.
 
Respiration probably occurs all along the intestine in Proneomenia, and along the rectum in ETeomenia.
 
The anal respiration of many Crustacea is, as has already been
stated (p. 109), really proctodseal.
 
The respiratory trees of most Holothuroidea are probably of
hypoblastic origin. In other Echinoderms the ambulacral system
is partially respiratory.
 
Chordata.  - The anterior portion of the chordate mesenteron is
mainly devoted to respiration ; this may appropriately be termed
the branchial region, or, more shortly, the pharynx.
 
In most Chordata several pairs of wide lateral pouches arise
from the sides of the pharynx and come into close contact with
the external skin. There is apparently a slight invagination of
the latter to meet the former ; an absorption of the applied membranes results in the formation of lateral slits (branchial or visceral
clefts), by means of which the cavity of the pharynx is put into
direct communication with the exterior.
 
Delicate processes of the hypoblastic epithelium covering the
intermediate bars (branchial or visceral arches) constitute the gills
or branchiae. These are richly supplied with blood by the branchial vessels (p. 226). True gills, however, are never developed in
the Amniota at any period of life.
 
Almost invariably the anterior (hyomandibular) visceral cleft
is the first to appear, the remainder appearing in order from before
backwards.
 
 
The worm-like Balanoglossus has pharyngeal gill-slits which arise in the same
manner as those of Vertebrates ; for a long time there is only one pair, hut subsequently they are repeated in pairs, increasing in number with the increase in the
size of the body [Bateson]. The collar at the base of the proboscis grows backward as
an opercular fold to a variable extent in different species of Balanoglossus, but it
never extends beyond three gill-slits. The enclosed cavity is termed the atrial
cavity by Bateson.
 
Van Beneden and Julin have shown that all Ascidians have but a single pair of
visceral clefts, which arise as a pair of pharyngeal pouches met by corresponding
epiblastic depressions. This condition is permanently retained by the interesting
tailed form Appendicularia. In all other Ascidians the gill-clefts fuse together to
form a single chamber (peribranchial cavity or atrium), which almost entirely sur
M
 
 
178
 
 
THE STUDY OF EMBRYOLOGY.
 
 
rounds the pharynx (branchial sac). It is probable that the atrial pore is the persistent opening of the fused gill-slits. The atrium may be formed more especially
from the hypoblastic or the epiblastic portion of these clefts. The numerous and
usually irregular orifices (stigmata) in the pharynx clearly do not correspond with
the gill-slits of higher forms, but are merely secondary perforations. We may say,
with these authors, “ the Tunicata are Chordata with a single pair of branchial clefts,
while the Yertebrata are furnished with several, and the Cephalochorda (Amphioxus)
with a great number.-
 
In Amphioxus also a single pair of gill-slits first makes its
appearance. This is subsequently followed by a large number
(70-100), which slant obliquely from before backward. In the
young form the gill-slits open directly to the exterior, but they
are eventually covered by a pair of dorsal folds of skin which
grow downwards, leaving a space between themselves and the gillslits (the branchial chamber or atrium). The two flaps of skin
meet below the body and fuse throughout their whole extent except at one spot, the branchial or atrial pore. It will be readily
apparent that the branchial chamber of Amphioxus is by no means
homologous with that of Ascidians.
 
The number of gill-clefts never exceeds eight pairs in the
Yertebrata. There are seven in the Cyclostomi and in Hexanchus,
eight in Notidanus (Heptanchus), but six in all other Elasmobranchii ; amongst the Teleostei a further reduction in the number
of clefts occurs, owing to the suppression of the hyoid pair.
 
The first cleft succeeding the mouth is termed the hyomandibular
or hyoid cleft (spiracle), as it lies between the mandibular and
hyoid arches. The second is correspondingly the hyobranchial or
first branchial, and is bounded by the hyoid and the first branchial
arches. The remaining slits are the branchial clefts.
 
Dohrn finds that the pair of ciliated grooves which lie in front
of the gill-pouches in the Lamprey (fig. 142) is developed in the
same manner as the branchial pouches, but an external opening
is never acquired. This supposed lost pair of visceral clefts is
termed by Dohrn spiracular or thyroidean.
 
Primitively all the visceral clefts were undoubtedly respiratory
in function, and in many Eiasmobranchs the mandibular border
of the spiracle bears a rudimentary gill. In Chimaera, some Ganoids,
and many Teleosts, the hyoid border of the second cleft possesses
only a rudimentary gill (opercular pseudobranch), which undergoes
all stages of degeneration amongst the Teleosts, all the anterior
gill-filaments having atrophied. The posterior gills have a tendency to disappear in Teleosts, the greatest reduction occurring in
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
179
 
 
Amphipnous cuchia, in which one branchial arch alone bears
branchial filaments.
 
The gill-clefts in the Cyclostomes and Elasmobranchs are left
quite exposed on the surface of the neck, but in Chimsera, Ganoids,
Teleosts, and Dipnoi they are protected by a fold of skin (operculum), supported by skeletal elements ; the branchiostegal
membrane and its supporting skeleton are derivatives of the
hyoid arch. In some forms the border of the operculum fuses
with the skin of the body, merely leaving a small orifice on each
side leading from the branchial chamber.
 
In Amphibia the hyoid pharyngeal pouch never communicates
with the exterior, but persists as the Eustachian recess. In larval
life four, or rarely three (some Urodela), branchial clefts appear.
The first, second, and third branchial arches develop external gills
which may be covered by epiblast. These usually atrophy, and
internal, probably hypoblastic, gills are developed on each side of
the three branchial clefts. The internal gills are always lost, but
in some Urodeles the external gills are retained throughout life.
Cope has recently stated that the Siren loses and then re-acquires
its external gills. Other Urodeles, which normally lose their gills
when adult, may, however, become oexually mature while still retaining their gills (Axolotl).
 
An opercular fold grows back from each hyoid arch in Anura,
and fusing above and below with the skin of the body, envelops
the gills within a branchial chamber. At first the branchial
chambers open widely to the exterior by an orifice on each side ;
these persist in Dactylethra, according to Huxley. In Bombinator
and certain other forms the openings of the branchial chambers
unite to form a single ventral orifice. In the majority of Anura
(Rana, Bufo), the two branchial chambers communicate by a ventral canal, and the opening of the right chamber is closed up,
leaving a single asymmetrical pore on the left side.
 
External gills are present in some Ichthyopsida, but they have
already been alluded to (p. 109).
 
The external gill filaments of Elasmobranch embryos arise as
simple elongations of the posterior lamellae of each arch, the
anterior not elongating at all. Dohrn finds that yolk is present in
these filaments and in their veins, and also in the posterior branchial vein and the efferent arteries, but never in the heart or in
the branchial artery. It would thus appear that these elongated
filaments serve also to absorb the yolk.
 
 
180
 
 
THE STUDY OF EMBKYOLOGY.
 
 
In none of the Amniota do the visceral clefts bear gills at
any period of life. In all forms there are four pairs of clefts,
the last two being very small in Mammals. The visceral arches
between the clefts are well marked (fig. 145, k", k"'), each possessing a central artery ; hut in Mammals the last cleft is not bounded
by a posterior arch. In Man, at least, none of the visceral clefts
are actually perforated [His], and the fourth and fifth external
visceral furrows are withdrawn into a fold or sinus of the neck
(sinus prsecervicalis), (figs. 146, 147).
 
The visceral clefts close up and entirely disappear, with the exception of the first (hyoid or hyomandibular), which, as has already
been described (151), persists as the Eustachian tube and tympanic
cavity.
 
 
Fig. 145. - Head of Embryo Rabbit of Ten Days.
 
Magnified 12 diameters. [ From Kolliker.]
 
a. eye ; at. atrium or primitive auricle of the heart ;
b. aortic bulb ; k\ k", k" -. first (mandibular), second (hyoid),
third (xst branchial) visceral arch ; m. mouth ; 0. superior
maxillary process, and u. inferior maxillary (mandibular)
process of the right side ; s. mid-brain, which forms the
interior extremity of the body ; v. anterior portion of head
and fore-brain ; v. ventricle of the heart.
 
 
Intestinal Respiration.  - Many Teleosts swallow atmospheric
air, which passes along the alimentary canal and is ejected by the
anus. There can be no doubt that this is a method of supplementary respiration. In these forms the hypoblast of the intestine
is a respiratory tissue. Gage finds that the papillate mucous
membrane of the pharynx of the American fresh-water Turtle,
Aspidonectes spinifer, is distinctly respiratory in function, but this
does not appear to hold good for other forms [Haswell].
 
Air-Bladder.  - A tubular diverticulum grows out from the
dorsal side of the oesophagus or stomach in most Ganoids (fig.
152, A, a.b) and Teleosts. In the Salmon and Carp [Yon Baer] it
arises just in front of the liver, and slightly to the right side. It
grows backwards, and in some cases forwards as well. Excepting
in some Teleosts this structure persists as the air-bladder.
 
It is possible that the primitive diverticulum from the mesenteron, which afterwards developed into the air-bladder, was originally connected with secretion. A
 
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
181
 
 
small sac of unknown function occurs on the dorsal wall of the gullet in some
Elasmobranchs.
 
The gases contained in the alimentary canal, and also, perhaps, air swallowed for
respiratory purposes, would naturally tend to collect in a dorsal diverticulum. A
hydrostatic apparatus would thus be formed, the muscular walls of the sac (airbladder) enabling the quantity of the contained gases to be regulated.
 
In some Fish (Physoclysti) the duct (pneumatic duct) by means of which the airbladder communicates with the alimentary canal becomes closed ; and in others, as
in the Pleuronectidse, the air-bladder may entirely disappear.
 
In the Physoclysti the amount of gas in the air-bladder is regulated by diffusion
through a network of blood-vessels. Under some conditions the fish may respire with
the air secreted in the air-bladder by its own blood-vessels ; but this is a purely
accessory and temporary mode of respiration.
 
The air-bladder in some Ganoids and Teleosts, and notably in the Dipnoids, is
cellular and very vascular, and atmospheric air is in some of them known to be
sucked in through the mouth, so that the air-bladder functions like a true lung.
 
In Gurnards and other Teleosts the air-bladder is used in making grunting sounds.
In many Teleosts the air-bladder functions as an accessory auditory organ, either by
impinging directly on the vestibulum of the internal ear, or by being indirectly
connected with it by means of a chain of ossicles. The auditory function is most
highly developed in the Siluroids, in which group the air-bladder becomes strangely
modified, and may come anteriorly into close contact with the body-wall immediately behind the shoulder-girdle. The body- wall may become extremely thin at this
spot, so as to form a regular tympanum. It is interesting to note that this tympanic
membrane, like the tympanum of the ear, is lined externally by epiblast and internally by hypoblast.
 
In no organ of Vertebrates is there so varied a change of function as there is in
this enteric diverticulum of Fishes.
 
Lungs.  - The lungs are developed from the ventral wall of
the oesophagus immediately behind the pharynx as an elongated
groove, which abruptly terminates posteriorly (fig. 143, A, Ig).
This ventral groove becomes constricted off from the oesophagus,
except at its anterior end (glottis), where it still retains its connection with the pharynx (fig. 143, la). The blind slightly swollen
extremity of the newly formed tube is the rudiment of the lung,
and the duct is the trachea.
 
The lung very early exhibits a bilobed character (figs. 14 1, c;
146, c, T). Some observers state that it is from the first distinctly
paired.
 
In most Amniota the surrounding splanchnic mesoblast becomes
greatly thickened, and the hypoblastic sac-like lungs burrow into
the stroma, dividing and subdividing as they advance. Eventually
an extremely ramified system of tubes is formed in Mammals,
each ultimate branch of which being terminally distended into a
sacculated ampulla (infundibulum).
 
The primitive sac-like character of the lungs (fig. 143, b) is
retained in the Amphibia and most Reptilia, the walls being
merely infolded to give increased respiratory surface.
 
 
182
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In the Chameleons variable branched prolongations of the lungs project freely into
the body cavity. Analogous diverticula appear in the embryos of Birds, and ultimately form the air-sacs. Prolongations from the latter pass into many of the bones
in most Birds, the penetration of these delicate sacs into the bones being due to
bone-absorption consequent on pressure.
 
The cartilaginous rings of the bronchi and trachea and the
cartilages of the larynx are of mesoblastic origin.
 
The air-bladder of the Dipnoids is clearly homologous with the same organ of
other Fishes, but in this remarkable group of animals the air-bladder is distinctly
double ; its walls are greatly infolded (“spongy - or “ cellular -) and very vascular ;
the blood supply is taken directly from the last aortic arch, and not from the caeliac
artery, the blood being returned directly to the heart, and not to the liver, as in other
Fishes ; lastly, the wide pneumatic duct opens on the ventral wall of the throat
(the same also occurs in the Ganoid Polypterus). In all these points the air-bladder
of the Dipnoi resembles the lungs of Amphibia. From these facts it is usually
concluded that lungs are directly derived from the air-bladder of Fishes.
 
Minot, however, has suggested that the lungs have been evolved by the modification of a pair of gill-pouches, which do not break through in the neck, but grow down
into the thorax (figs. 14 1, c, 146, c, l).
 
Albrecht considers it erroneous to homologise dorsal with ventral organs, and
points out the difficulty of the migration of the dorsal air-bladder to a sub -oesophageal
position. In the Gymnodont Teleosts, in addition to the dorsal air-bladder, there
are ventral air-sacs proceeding from the oesophagus, by means of which these fishes
can inflate themselves. These sacs are considered by him as homologous with lungs,
and heterologous to the dorsal air-bladder. The air-bladder of Polypterus would
therefore be the homologue of the lungs. Dorsal diverticula from the oesophagus
opposite the larynx may normally (Pig) or abnormally (Man) be present.
 
 
Tongue.  - Born finds that in the Pig the tongue is developed
from the anterior portion of the ventral floor of the pharynx. The
space between the ventral ends of the first and second visceral
arches is at first depressed; but later a longitudinal ridge grows
up, separated from the arches on each side by a groove. The
anterior portion of this ridge grows forward and becomes the free
part of the tongue. The tongue does not extend back beyond the
second arch, but the posterior portion of the ridge projects between
the third and fourth arches and develops the epiglottis. As Minot
points out, the epithelium covering of the tongue is thus hypoblastic in origin.
 
If the above statement is correct, the taste-buds on the papillae are hypoblastic
sense organs. The gustatory goblet-cells on the tongue of Amphibia possibly have a
similar origin. The goblet-shaped organs in the mouth and pharynx of Fishes may
have a similar function, but those of the mouth appear to be homologous with
similar organs situated in the skin. Macullum has very recently recorded the
occurrence of taste-buds in the oesophagus of the Sturgeon ; he also states that he
has found them in Ampliioxus as far back as the opening of the hepatic caecum.
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
183
 
 
According to His, the tongue in Man has a double origin. From
the anterior region of the ventral space (mesobranchial area),
between the visceral arches of an early embryo, a small round
projection (tuberculum impar) is formed ; behind this are a pair of
folds (furcula), which eventually will form the epiglottis. The
ventral portions of the second and third arches grow towards their
fellows of the opposite side between the tuberculum and the
furcula. The basal growths of the arches form the roots of the
future tongue, and unite together behind the tuberculum impar;
 
 
 
Fig. 146. - Diagrams Illustrating the Visceral Arches and Development op the
Tongue in the Human Embryo. [After His.]
 
Seen from above, the dorsal (posterior) wall of the pharynx being supposed to be
cut away. In B. the branchial blood-vessels are indicated.
 
F. furcula ; l. lungs ; R. roots of the tongue ; s.pc. sinus praecervicalis ; T. tuberculum
impar, body of the tongue in D ; 1-5. visceral arches.
 
the median pit between these structures is the rudiment of the
median lobe of the thyroid body. The tongue is formed by the
fusion of the two roots with the tuberculum.
 
Thyroid Body.  - The generally received view of the significance of the thyroid
body has already been mentioned (p. 172).
 
In the Lamprey (fig. 142) the thyroid body arises as a wide diverticulum from the
floor of the anterior portion of the pharynx. The orifice becomes restricted to a pore
and eventually disappears. During larval life it consists of a median ciliated portion
communicating with a pair of complicated lateral glandular sacs.
 
In some higher forms the thyroid is stated to develop as a tubular diverticulum
or solid down-growth from the anterior region of the pharynx, which later becomes
bilobed. Subsequently it is quite detached from the pharynx, and is produced into
 
 
184
 
 
THE STUDY OF EMBRYOLOGY.
 
 
a number of hollow or solid processes, between which connective tissue septa and
blood-vessels enter.
 
Born reconciles various conflicting observations regarding the origin of the thyroid
body in Mammals by finding that, according to his investigations, the organ has a
double origin. An unpaired portion arises as an invagination from the floor of the
pharynx opposite the front edge of the second visceral cleft. It separates from the
pharyngeal epithelium, expands laterally, and migrates backwards. The other
portion of the thyroid is derived from the paired remnants of the fourth visceral
clefts. These are at first somewhat pear-shaped hollow sacs, but on becoming connected with the central portion they acquire a spongy interior. Pischelis confirms
Born -s statements from his researches on the Pig, Rabbit, and Birds. His finds, in
the human embryos, that the median thyroid rudiment arises as a hollow diverticulum
between the third visceral arches, and that the lateral portions are evaginated from
the posterior end of the pharynx near the glottis (fig. 147, l.thyr). The several parts
become separated from their parent tissues and sink into the deeper portion of the
neck. The duct of invagination of the median portion persists for some time as the
ductus thyreoglossus (fig. 147, d.thyr). The foramen csecum, cornu medium, and the
various bursse which may be present iq the adult are rudiments of this duct.
 
The last investigation on the thyroid body is that of De Meuron, who finds
that the median element is always (Elasmobranch, Frog, Lizard, Fowl, Sheep,
and Man) developed from a median pit in the pharynx at the level of the second
visceral arch. He homologises the supra-pericardial bodies of Elasmobranchs [Van
Bemmelen] and Amphibia [De Meuron] with the accessory or lateral thyroid bodies
of the Amniota (the left alone occurs in Acanthias and Lacerta). The structure of
both resembles that of the median element. These bodies arise as a pair of diverticula
behind the sixth branchial cleft (seventh visceral), which is imperfectly developed in
many Elasmobranchs. It may be concluded that these represent a degenerate pair
of gills as in Heptanchus, in which there are seven branchial clefts, the supra-pericardial bodies are absent. In the higher Fishes and larval Amphibia the lateral
rudiments of the thyroid develop directly from the pharynx behind the last (fifth
branchial cleft. Owing to a further reduction of the clefts, which also disappear
without leaving a trace and a consequent shortening of the pharynx, the lateral
thyroids appear to develop from the fourth branchial cleft ; this is most marked in
Mammals. The similarity in structure of the fully-developed lateral thyroids with
the median element and their close connection in adult Mammals rather tend to
support Dohrn -s hypothesis concerning the primitive condition of the median thyroid,
i.e., that it represents a pair of degraded hyoid clefts.
 
Thymus Gland.  - Maurer finds that in the embryo Trout the thymus takes its
origin from four thickenings of the epithelium of the visceral clefts on each side
of the body, a rudiment being situated in the dorsal angle of each of the four clefts.
A proliferation of the epithelium takes place, and the four rudiments on each side
become fused together. Each lateral thymus gland sinks into the underlying mesoblast and takes on the character of a lymphatic gland.
 
Dohrn states that there is primitively one thymus rudiment for each branchial
cleft in Elasmobranchs, but the fifth disappears in the Sharks. The separation of
these rudiments from the epithelium is due to the shortening of the clefts and the
bending of the visceral arches. The parts thus isolated gave rise to a new organ, the
thymus, which was afterwards transmitted by heredity to higher Vertebrates.
 
In the Pig, according to Born, the thymus arises as a pair of ventral evaginations
from near the inner openings of the third pair of visceral clefts, the outer portions
of which atrophy. The end of the thyroid rests against the pericardium at the spot
where the aorta leaves it. The central cavity disappears, and many branches grow
out from the solid cord, mainly in the direction of the heart.
 
His finds in the human embryo that the primary (epithelial) rudiment of the
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
185
 
 
thymus arises from the epithelium of the inner portion of the fourth, third, and
partly also of the second visceral clefts. These parts become massed together and
separated from the outer skin (fig. 147, thm). He asserts that it is developed from
the epidermis and cannot be regarded as a hypoblastic structure, since in Man none
of the visceral clefts become perforated.
 
It is evident that the function of the gland in the Amniota is secondarily acquired,
and that it is a degraded epithelial organ, "which, from its relation to the gill-clefts
in Fishes, may possibly have been some form of sense organ.
 
De Meuron has also studied the development of the thymus. In Ichthyopsida and
Sauropsida it arises as solid thickenings of the epithelium of the dorsal side of the
branchial clefts. In Fishes it arises from the first four branchial clefts, in the Lizard
from the second, third, and fourth, and in Birds from the third and fourth visceral
 
 
A
 
 
 
Fig. 147. - Development of the Thymus Gland and Thyroid Body in the
Human Embryo. [After His.]
 
 
A. Transverse section through the hinder portion of the head. B. Transverse
section through the larynx of an older embryo. C. Profile reconstruction of the
thyroid and thymus glands, seen from below. D. The same seen from the side.
 
ao.a, ao.d. ascending and descending aorta ; c. carotid artery ; d.thyr. ductus thyreoglossus ; ep. epiglottis ;/.c. foramen caecum ;j.v. jugular vein ; l. larynx; l.thyr. lateral
thyroid rudiment; m.thyr. median thyroid rudiment; ce. oesophagus; p.a. plicae
aryepiglotticse ; ph. pharynx ; r.t. roots of the tongue ; t. tongue ; thm. rudiment of the
thymus gland ; tr. trachea ; ix. glossopharyngeal ; x. vagus ; xi. hypoglossal ; xn.
spinal accessory nerves ; 1-5. visceral arches.
 
 
clefts. In the last three groups the thickening of the third cleft is the largest. In
Anura the second visceral cleft alone develops a thymus. The history of the thymus
is very different in Mammals ; dorsal rudiments are developed, as in Birds, from the
third and fourth visceral clefts, but nearly the whole of the adult organ is derived
from a ventral caecum from the third branchial cleft.
 
Gustatory Organ of Amphioxus.  - The organ usually known as the olfactory organ
of Amphioxus consists of an outer ciliated sac opening to the exterior and also into
an inner sense-organ, which again communicates with the mouth. Hatschek finds
that the whole organ is developed from the left of a pair of archenteric diverticula in
front of the mouth, and that it is therefore of purely hypoblastic origin. It probably
is an organ of taste. Hatschek and Dohrn are inclined to homologise it with the
hypophysis.
 
 
186
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Excretory Organs.  - The only excretory organs which appear
to be of hypoblastic origin are the paired urinary tubes which occur
in Amphipods. They arise from the extreme hind-end of the
mesenteron, there being a distinct break of continuity when the
latter ceases and the hind-gut (rectum) begins [Spencer]. Their
development is unknown.
 
Skeletal Structures.  - Notochord.  - The primitive axial supporting rod or skeleton (notochord), or chorda dorsalis, which is
peculiar to the Chordata, and from which they derive their name,
is of hypoblastic origin.
 
Hemichordata.  - Bateson has shown that in the larval Balanoglossus (B. kowalevskii) the median dorsal wall of the pharynx is
constricted off and grows forward as a short hollow diverticulum
of hypoblast, which afterwards becomes solid except posteriorly,
where its lumen opens throughout life into the pharynx. The cells
scon become vacuolated as in the notochord of higher forms.
 
Urochordata.  - In Ascidians the notochord is developed solely
in the tail, it being derived from the dorsal wall of the caudal
arclienteron.
 
Cephalochordata.  - In Amphioxus the notochord is, as it were,
pinched off from the median dorsal wall of the archenteron (fig.
56 nch). Ultimately its folded appearance and its connection
with the archenteron are lost. It is constricted off from before
backwards.
 
Vertebrata.  - In the lower Vertebrates the notochord is distinctly derived from the dorsal wall of the mesenteron (archenteron). Hertwig -s researches on the development of the Newt
(Triton) show that the dorsal hypoblast (usually referred to in this
book as invaginated hypoblast and the chorda entoblast of Hertwig
and others) not only is distinctly columnar, as opposed to the
rounded ordinary hypoblast cells, but it also lines a distinct
groove (fig. 148). The two sides of the notochordal groove, as it
may be termed, come together and form a solid rod of cells, the
arrangement of which gives no indication of their origin. The
notochordal groove is scarcely apparent in the Frog.
 
Mitsukuri and Ishikawa have demonstrated that the notochord
in the Snapping Turtle (Trionvx japonicus) (fig. 149) is developed
in a manner perfectly comparable with that of the Newt. Indications of a similar origin of the notochord are found in Lizards, and
notably in Mammals.
 
In Birds the axial hypoblast very early becomes converted into
 
 
ORGANS DERIVED FROM THE HYPOBLAST.
 
 
187
 
 
the rudiment of the notochord, and this may occur almost before
the permanent hypoblast can be recognised as such; hence the
supposition of some authors that the notochord was derived from
the mesoblast.
 
 
Fig. 148. - Transverse Section of the Dorsal
Portion oe an Embryo
Newt (Triton). [ 4 /f«‘
O. Hertivig.]
 
a. mesenteron ; ax.hy. axial
hypoblast in process of forming the notochord; b.c. coelom
(body-cavity) ; ep. epiblast ;
hy. digestive hypoblast ; n.p.
neural plate ; so.m. somatic
mesoblast; sp.m. splanchnic
mesoblast.
 
 
 
The rudiment of the notochord consists of a solid rod of cells
lying between the neural tube and the mesenteron. Posteriorly
it is connected with the fusion of the layers which occurs at the
 
 
 
(Fig. 149. - Formation of Notochord in Trionyx. [ After Mitsukuri and Ishikawa.]
 
A. Transverse section through the head region before the closure of the neural groove
B. D. Portions of successive sections of the same embryo. The shading of the epiblast
is purely diagrammatic.
 
am. amnion ; ax.hy. axial hypoblast ; ep.a. epiblastic, and hy.a. hypoblastic layer of
amnion; hy. hypoblast; to. mesoblast; n.c. neural canal: nch. notochord.
 
dorsal lip of the blastopore (fig. 62), or, when there is no distinct
blastoporic passage, as in the Fowl, it passes into the primitive
streak. At a later stage the notochord terminates anteriorly
 
 
188
 
 
THE STUDY OF EMBRYOLOGY.
 
 
behind the infundibulum, its extremity being often recurved.
Posteriorly the notochord terminates at the end of the tail.
 
A definite sheath (elastica limitans interna) is soon formed as a
secretion from the peripheral cells of the notochord. The cells of
the notochord become vacuolated, so that the notochord has a
spongy appearance ; a few nuclei surrounded by a little protoplasm remain attached to some of the meshes of the network (figs.
150, 152, 173, 175, ch).
 
The notochord and its sheath are replaced in most Vertebrates,
leaving only a small rudiment, as will be mentioned in the description of the development of the vertebral column (pp. 196-199).
 
Sub-Notochordal Rod.  - A solid rod of cells is developed from
the dorsal wall of the alimentary canal in Ichthyopsida after the
formation of the notochord (figs. 150, 173, 175, x).
 
This sub-notochordal rod, as it is termed, has about the same
extension as the notochord. Its function or homology is unknown,
but it appears to persist as the sub-vertebral ligament in the
Sturgeon.
 
Significance of the Notochord.  - Few embryological problems are more obscure
than the probable phylogenetic significance of the notochord. The embryological
evidence points to its hvpoblastic origin. We are justified in assuming the primitive, or at all events the archaic, nature of its development in the Amphioxus
(fig. 56) and the Newt (fig. 148). The variations which are met with in other
Vertebrates can be reduced to the type of the Newt, as is proved by the Chelonia
(fig. 149).
 
The development of the urochord in the Ascidians is manifestly a degraded process.
 
The restricted notochord of Balanoglossus develops in an essentially similar manner
to that of Amphioxus, but the central lumen is retained for a much longer period.
It is interesting to note that in some Amniota a transient canal occurs at the
posterior end of the notochord.
 
Upon an examination of the figures given by authors illustrating the development
of the notochord in Balanoglossus, Amphioxus, the Newt, Chelonia, Lizards, and
Mammals, the conclusion seems to be almost inevitable that we must regard the
notochord as a secondary structure. It may be that the ancestor of the Chordata
possessed a longitudinal groove along the neural aspect of its alimentary tract, which
may have had some special secretory (? mucous) function. The extremely early
acquisition of distinctive histological characters may be recalled in this connection.
 
The closure of the notochordal groove in ontology at the time of the constriction
off of the archenteric diverticula from the mesenteron is suggestive of phylogenetic
synchrony.
 
It is not difficult to imagine that a rod of cells, even though containing at first a
small lumen, might form a mechanical support to the body which would prove of considerable value, and, being internal, it would grow with the growth and requirements of the animal.
 
Urinary Bladder.  - The urinary bladder is properly speaking a hypoblastic organ,
but it is more convenient to deal with it at the same time as the uro-genital ducts
(P- 2 59)
 
( 189 )
 
 
CHAPTER VII.
 
ORGANS DERIVED FROM THE MESOBLAST.
 
However it arises, the mesoblast gives rise to the deeper layer
of the skin, i.e., the derma or cutis.; to the whole of the muscular
system in animals higher than the Ccelenterata ; to nearly all the
internal supporting structures of the body ; to the lining membrane
of the body-cavity, peritoneum, in the broadest sense of the term :
to the whole of the vascular system ; to the excretory organs ; and
to the generative glands.
 
Indifferent Mesoblast.  - Under the term indifferent mesoblast
may be classed the general parenchyma of the body of the lowest
Metazoa.
 
In the Porifera, between the two primitive epithelia of the body
irregular amoeboid cells occur in greater or less abundance, imbedded in a jelly-like matrix. Sollas suggests the appropriate
term of archseocytes for such cells. The origin of these mesamoeboids has been described ; they function in various ways, probably
mainly in nutrition, by carrying food-products to various parts of
the organism, and in the transportation of waste matter, in this
respect resembling the leucocytes of higher animals. Many of
the mesamoeboids secrete spicules; some develop into m uscle- cells ;
others constitute germ-cells, and some are stated to act as nervecells.
 
The oval or anastomosing stellate cells in the gelatinous tissue
of Scyphomedusse arise mostly from the hypoblast, and the
muscular stellate cells of Ctenophora from the epiblast, though
some are stated by Chun to be of hypoblastic origin. There may
be connective-tissue cells in the fibrillar lamina of Actinozoa.
 
The mesamoeboids enclosed within the spacious segmentationcavity of larval Echinoderms have many functions to perform ; as
Metschnikoff has shown, they devour degenerate tissues (see p. 274),
and they also secrete the larval skeleton (fig. 16, m.s.).
 
The spongy parenchyma which fills up the space between the
 
 
190
 
 
THE STUDY OF EMBRYOLOGY.
 
 
epiblast of the skin and the hypoblast of the meseateron in
Platyhelminths appears to be of mesenchymatous origin. These
cells are essentially “ indifferent - in character, and Lankester has
shown how that in the Leech this tissue, which he terms skeletotrophic, may insensibly pass into blood-vessels and blood-cells on
the one hand, or into connective tissues generally on the other.
A good deal of the intermediate parenchymatous tissue of Molluscs
might be placed in this category.
 
In higher forms the wandering cells of the body (colourless blood
corpuscles, leucocytes), retain their amoeboid nature, and probably
have diverse functions. The generative or germ-cells may be considered as the least specialised cells in the body.
 
Dermal Mesoblast.  - That mesoblastic tissue which immediately
underlies the embryonic epiblast, and which constitutes the derma
or cutis of the adult, may be termed dermal or peripheral mesoblast.
 
Such, for instance, are those mesamceboids which in Echinoderms
are enclosed between the lining membrane of the body-cavity and
the epiblast. They constitute the main thickness of the body-wall,
and are productive of muscles, ligaments, and the calcareous
spicules, plates, and spines.
 
It would be superfluous to enumerate the various aspects which
the dermal mesoblast assumes.
 
The derma of Vertebrates typically consists of  - (i.) Connective
tissue fibres and elastic fibres. The fibres of the derma in Ichthyopsida are usually arranged in more or less regular vertical and
horizontal bundles, whereas those of the Amniota are irregularly
felted together. (2.) Pigment cells and wandering leucocytes.
(3.) Often a deeper layer of fat cells. (4.) Non-striated muscular
fibres; and, lastly, it is penetrated by blood-vessels and nerves
from the one side, and by glands and hair-bulbs on the other.
 
Muscular System.  - There is considerable uncertainty with regard to the exact origin of the muscular system of many Invertebrates. In some cases it is wholly or partially mesenchymatous
(Echinodermata, Platyhelminths). In the Echinoderms the epithelial cells of the archenteric diverticula are stated by Metschnikoff to possess muscular processes, but it is not known whether
these furnish all the muscular elements of the body-wall. The
external muscle fibres, which cause the movements of the spines of
the Echinoids, are almost certainly not so derived. The muscles
are known to be mesothelial in origin in the Earthworm ; but even
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
191
 
 
in the Chaetopoda and Arthropoda (?) mesenchymatous elements
are stated by some observers to be present, and these may possibly
form muscle-cells.
 
In the Chordata the muscular system is entirely of mesothelial
origin, being derived from the somatic and splanchnic layers.
 
The first muscles to make their appearance in Amphioxus
 
 
Fig. 150.  - Transverse Section trough the Trunk oe an
Elasmobranch Embryo (Pristiurus). [From Balfour.]
 
al. mesenteron ; ao. aorta ; mp. muscle-plate ; uip'. portion of
muscle-plate converted into muscle ; nc. neural canal ; pr. dorsal root
of spinal nerve arising from the neural crest ; sc. somatic mesoblast ;
sp. splanchnic mesoblast; V.v. portion of the vertebral plate which
will give rise to the vertebral bodies ; x. subnotochordal rod.
 
The intermediate cell mass connects the dorsal and ventral mesoblast ; it is seen on the left-hand side of the figure, between the lines
pointing to x and ao.
 
 
 
(fig- 56, c, m) are the longitudinal muscles which lie on each side
of the notochord ; they arise as differentiations of the basal portion
of the splanchnic cells of that region.
 
If fig. 56 is compared with figs. 150 and 175, it will be seen that
the great lateral muscles of Elasmobranchs are developed from
similar splanchnic cells, and the same may be traced in an early
stage in the muscle-plates of the Amniota. In the embryo Bird
 
 
Fig. 151. - Horizontal Section through the Trun
of an Embryo Fowl.
 
The section passes through the notochord and shows the
separation of the cells to form the vertebral bodies from the
muscle-plates.
 
ep. epiblast ; l.m. longitudinal muscles differentiated from
the splanchnic portion of the muscle-plate, m.p ; nch. notochord ; v.r. vertebral rudiment.
 
 
 
the first-formed muscles have a longitudinal direction, and are
divided into segments.
 
A horizontal section through a portion of the body of an embryo
Fowl (fig. 1 51) on the level of the notochord clearly exhibits the
segmented character of the dorsal mesoblast. The section is taken
at a stage when the splanchnopleur has differentiated into an inner
vertebral rudiment (p. 199) and an outer layer of longitudinal
muscles, while the somatopleur is unmodified. A comparison of
 
 
192
 
 
THE STUDY OF EMBRYOLOGY.
 
 
this figure with that of Amphioxus brings out the fact that the
dorsal portion of the body is characterised by a series of mesoblastic pouches, each of which contains an isolated portion of the
body-cavity. This primitive character is masked in most other
forms, but in all the Chordata the great lateral muscles are developed therefrom.
 
Balfour terms each mesoblastic pouch a somite, which is the
equivalent of a protovertebra of many authors, reserving the
name of muscle-plate to the somite after it has given rise to the
vertebral rudiment, as it is then entirely metamorphosed into the
voluntary muscular system.
 
The muscle-plates increase in size and extend into the ventrolateral wall of the embryo. The splanchnopleur is first converted
into muscle-cells, the somatopleur becomes implicated later.
 
The musculature of the limbs early appears as dorsal and ventral bands, which originate from processes from the muscle-plates
(fig. 103, mp.l). These become segmented off from the muscleplates, which then pass into the ventral wall of the body.
 
We may conclude that the primitive continuous lateral fin was put in motion by
muscular processes from each muscle-plate ; and that when the limbs were differentiated from the fin, some, at least, of the segmental muscles were so grouped as to
form the muscles of the limbs.
 
The muscles of the head, including the eye-muscles, arise from
the walls of the cephalic somites (p. 140), in the same manner
as those of the body.
 
The transformation of an epithelial -cell into a muscle-cell occurs by the differentiation of the protoplasm into the contractile fibrils either at one side or peripherally ;
in the former case the original nucleus is lateral, in the latter it is situated in the
centre of each cell.
 
Dermal Skeletal Structures. - Invertebrates. - Mesodermal
exo-skeletal structures scarcely occur amongst the Invertebrates.
The Holothuroidea have thin perforated calcareous plates or
spicules imbedded in their skin; all the other Echinoderms are
characterised by an extensive development of solid calcareous
plates and spines.
 
Chordata.  - The dermal skeletal elements of the Chordata may
be conveniently reduced to one type, namely, to a placoid scale, the
development of which has already been noticed (p. 103). Minute
placoid scales or denticles scattered over the skin constitute the
shagreen of Elasmobranchs. Each denticle has a basal plate
formed of bone.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
193
 
 
The large dermal plates of Ganoids and some Teleosts are by
some regarded as formed by the fusion of the basal plates of numerous denticles, the polished surface of the plate being due to a
deposit of enamel.
 
The thin scales of Amia, most Teleosts, (and Dipnoi) are undoubtedly the somewhat degraded representatives of the bony plates of their Ganoid ancestors. In
many cases the supposed epiblastic portion (enamel) of the scales and dermal plates
atrophies or is undeveloped.
 
The dermal plates, which have a purely mesoblastic origin,
form the group of bones known as membrane bones (see also p.
2 io). To this category belong the parosteal elements of the skull,
and the “ clavicles - of Teleosts.
 
Recent Amphibia are peculiarly deficient in a dermal exo-skeleton. Bony plates
occur in skin of the back in Ceratophrys dorsata and Ephippifer aurantiacus, and
scutes in the Ctecilians.
 
The scutes (often called scales) of Lacertilia and Crocodilia are
formed as ossifications in the derma. The scale-papilla may be
best compared to an extremely flattened feather-papilla, which,
like the latter, is set at an angle within a follicle. The mesoblastic
core ossifies, and the overlying Malpighian layer of the epiblast
possibly in some cases deposits a layer of enamel.
 
Among recent Reptilia the Chelonia have by far the most developed dermal exo-skeleton, which forms a dorsal carapace and
a ventral plastron. Parosteal riblike bones (splints) occur in the
ventral wall of the abdomen of Hatteria and Crocodilia. Similar
ossifications are occasionally present in the intermuscular septa of
Teleosts.
 
The bony plates which occur in the sclerotic in Birds, Reptiles,
and many Pishes belong to this category.
 
No dermal skeletal structures occur in the trunk of Birds, and
but few in Mammals, the most noticeable being the extensive
scutes of the Armadillos.
 
Mesoblastic Endo- Skeletal Structures.  -Invertebrates.  -
 
The supporting or endo-skeletal structures of the Invertebrates
are almost universally of epiblastic origin. The following are the
chief mesoblastic formations.
 
The spicules of Sponges arise from a single mesamoeboid ; when bundles of delicate
spicules (trichites) occur, the whole mass is developed from a single cell.
 
The exact origin of the gelatinous supporting tissue of Coelenterates (mesogloea)
has not been fully made out. The calcareous skeleton of the Hexacoralla and the
calcareous spicules of the Octocoralla are secreted by cells derived from the ectoderm.
 
N
 
 
194
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The horny axial 'skeleton of the Gorgon iidse, the alternate horny and calcareous
axis of the Isidinse, and the calcareous stem of Corallium, may prove to be epiblastic, like the horny axis of the Antipatliidse.
 
In the free-swimming larvae (Plutei, &c.) of the Echinoidea, Ophiuroidea, and
Crinoidea, a calcareous spicular skeleton is secreted by the mesamceboids (fig. 16, m.s).
 
A cartilaginous axis supports the branchial plume of the Serpulae.
 
True cartilage occurs in the Cephalopods and in connection with the odontophore
in Gasteropods ; the former are the only Invertebrates in which the brain is protected by a cartilaginous brain-case.
 
Chordata.  - An endo-skeleton which supports the body and
grows with its growth is one of the principal characteristics of the
Chordata as a whole. It would perhaps be hardly too much to
say that the possession of this and the adaptive axial skeleton was
probably the main factor in the evolution of the group. The endoskeleton of the Chordata includes an axial and appendicular
elements. The former consists primitively of the notochord with
its skeletogenous sheath, and secondarily of the vertebral column
and the cranium.
 
The appendicular skeleton is derived from the primitive supports
of the locomotory organs (fins). These at first were entirely independent of the axial skeleton, but a more or less intimate connection has subsequently been acquired with the latter.
 
Other structures have appeared in the walls of the body which
have all come to be connected with the axial skeleton; for example,
the ribs in the somatopleur of the trunk, the internal branchial
visceral bars in the splanchnopleur of the pharynx, and the labial
cartilages of the face.
 
Vertebral Column.  - The notochord with its sheath persists
as the axial skeleton in Amphioxus, the Cyclostomes, Dipnoi, and
Selachian Ganoids. In all the higher Vertebrates a skeletogenous
sheath is developed round the notochord.
 
Skeletogenous Sheath of Notochord.  - The skeletogenous or
cartilaginous sheath of the notochord is developed from a layer of
mesoblast cells which range themselves round the elastica limitans
interna (fig. 152, b). The layer increases in thickness, and forms
a continuous unsegmented tube of fibrous tissue with flattened
concentrically arranged nuclei. Outside this layer another sheath
is developed, variously known as the elastica limitans externa or
outer sheath of the notochord.
 
This unconstricted condition of the notochord is retained by the
adult Cartilaginous Ganoids and Dipnoi (fig. 153, A). In Chimsera
there are added thin calcareous rings, which bear no relation to
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
195
 
 
the neural arches, and are more numerous. In some Elasmobranchs
true vertebrae are imperfectly developed.
 
In all other forms the notochord is serially constricted by the
development of true vertebral centra, and is eventually partially
or entirely replaced by the mesoblastic vertebral column.
 
Vertebral Arches and Vertebral Bodies. - In Amphioxus the
neural canal is merely protected by a sheath of connective tissue ;
but in the true Vertebrates a series of cartilaginous bars, neural
 
 
 
Fro. 152 . - Notochord of Lepidosteus. [ After Balfour and Parker .]
 
A. Transverse section through the anterior part of the trunk of an embryo about a
month after hatching, showing the connection of the air-bladder with the throat. B.
 
Portion of transverse section through the vertebral column of a larva of 5.5 cm.
 
(through the vertebral region).
 
a.b. air-bladder; c.sh. cuticular sheath of notochord; h.a. haemal arch; i.s. interspinous bone; l.l. ligamentum longitudinale superius; m.e. membrana elastica externa ; n.a. neural arch ; n.a' . dorsal element of same ; n.c. neural canal ; nch. notochord; ces. oesophagus \ pc. pericardium; p.g. pectoral girdle; pr.n. pronephros; sh.
sheath of notochord (elastica limitans interna) ; v. ventricle.
 
The cartilage is dotted ; its bony sheath is left blank in B.
 
arches, are developed, which at first laterally, and then dorsally as
well, protect the neural canal.
 
In the lowest true Vertebrates, the Cyclostomi, the neural
arches are irregularly arranged bars of cartilage which do not meet
over the neural canal.
 
Fishes.  - In other forms the neural arches first appear as a pair
of continuous cellular ridges resting on the skeletogenous sheath
of the notochord. A similar ventral ridge, which is better developed
in the caudal region, is known as the haemal ridge (fig. 1 52).
 
 
196
 
 
THE STUDY OF EMBRYOLOGY.
 
 
The neural ridges become enlarged at each intermuscular
septum in Fishes. These enlargements are converted into cartilage and form the neural arches. The haemal arches develop in
a similar manner ; but it is only in the region of the tail that the
haemal bars unite in the median ventral line to form a true haemal
arch.
 
 
In developing and young, and a few adult Elasmobranchs, in certain young and
adult Ganoids (Sturgeon, Polyodon, Amia), and in Chimaera, intervertebral or intercalary neural arches are developed. Interhaemal arches are developed in some cases.
 
The neural arches always make their appearance between the spinal nerves. The
interneural arches, when present, usually arise between the dorsal and ventral roots
of the nerves.
 
In the adult Scyllium the dorsal root of a spinal nerve passes through the intercalated cartilage, and the ventral root traverses the neural arch immediately in front.
 
 
 
Fig. 153. - Diagram Representing the Various Types of Vertebral Columns
in Longitudinal Section. [ From. Gegenbaur .]
 
A. Primitive type, with no vertebral segmentation. B. Type of Fishes, with
vertebral constrictions of the notoehord. C. Amphibian type, intervertebral constrictions of the notochord by intervertebral rings of the cellular sheath. D. Intervertebral constriction of the notochord as in Sauropsida. E. Vertebral constriction
of the notochord of Mammals, the intervertebral regions of the cartilaginous sheath
being converted into intervertebral ligaments.
 
c. notochord; cs. cuticular sheath of notochord; g. intervertebral articulations;
iv. intervertebral regions; s. cellular or cartilaginous sheath ; v. vertebral regions or
bodies of the vertebrae.
 
 
The skeletogenous sheath of the notochord also undergoes segmentation, and an annular thickening occurs in the vertebral
region (fig. 153, b). This ring becomes converted into hyaline
cartilage and encroaches on the notochord, which becomes considerably constricted at these points, but not in the intervals. In
the intervertebral regions the sheath of the notochord assumes a
fibrous character.
 
From their mode of formation the vertebrae of Fislies are biconcave (amphicoelous).
The gelatinous intervertebral spheres are the degraded remnants of the unconstricted
portions of the notochord. Lepidosteus is the only Fish in which the centra of the
vertebrae directly articulate with one another, the faces of the bodies or centra of the
vertebrae being convex in front and concave behind (opisthocoelous). In this form
the bases of the neural and haemal arches extend into the intervertebral regions*
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
197
 
 
forming cartilaginous rings. Each intervertebral ring becomes divided into two
parts, which will respectively form the anterior face of a given vertebral centrum
and the posterior face of that in front of it. There is thus in this Ganoid a secondary
intervertebral constriction of the notochord ; the latter entirely disappears, except in
the tail.
 
The greater part of the bodies of the vertebrae and of the arches are ossified in
Lepidosteus and Teleosts from the membranous perichondrium.
 
The neural arches rarely unite with their fellows in Fishes, the
neural arch being completed above by accessory cartilages and a
longitudinal elastic band.
 
The various forms of Fishes - tails are described later (p. 203).
 
 
 
Fig. 154. - Longitudinal Section through the Vertebral Column oe Various
Urodeles. [After Wiedersheim.]
 
A. Ranodon sibericus; JB. Amblystoma tigrinum; C. Gyrinophilus porphyriticus
(Vertebrae, i, 2, 3). D. Salamandrina perspicillata.
 
a.h. articular head, and a.s. articular socket of vertebral body ; b. peripheral bony
covering of centrum ; cli. notochord; i.c.s. intervertebral thickening of cartilaginous
sheath; ligt. intervertebral ligament ; m. c. marrow cavity ; t. transverse processes and
ribs; v.c. vertebral cartilage and fat cells; x. vertebral constriction of notochord in
Amblystoma tigrinum without cartilage and fat cells.
 
The cartilage is dotted and the bone is left white.
 
Amphibia.  - The Amphibia present us with an interesting series
of phases in the development of the vertebral column.
 
At first, in IJrodele larvae, as in most Fishes, the notochord is vertebrally constricted,
and the cellular sheath, which is the equivalent of the skeletogenous sheath of Fishes,
is early surrounded by a delicate layer of bone which is formed in the investing connective tissue. This biconcave character of the vertebrae is retained by the Csecilians
and the gilled Urodeles.
 
Later, in the intervertebral regions the sheath becomes greatly thickened, forming
deep cartilaginous rings, which constrict and ultimately obliterate the notochord
(fig. 154, c).
 
 
198
 
 
THE STUDY OF EMBEYOLOGY.
 
 
Finally, an articular cavity is produced by absorption in each intervertebral region,
in such a manner that the convex cartilaginous anterior extremity of one vertebra
articulates with a corresponding concavity in the preceding vertebra. Thus the
caducibranch Urodeles have opisthocoelous vertebrae.
 
Three stages can be distinguished in the development of the
vertebral column of Urodeles  - (i) a connection of the vertebrae by
means of the intervertebrally expanded notochord, as in Fishes
generally ; (2) a union of the centra by means of intervertebral
masses of cartilage; (3) an articular condition. An ossification of
the articular surfaces of the centra of the vertebrae occurs in Lepidosteus, Anura, and most Amniota.
 
It may be noted that the articular facets appear to be the only cartilaginous
portions of the vertebrae of Urodeles, their vertebrae being ossified from membrane
(connective tissue), as in Lepidosteus and Teleosts. In the Anura the vertebrae ossify
from cartilage as in the Amniota. The notochord persists in a cartilaginous form
within the centra of the vertebrae for a long time, and may even be found in adult
Frogs. The articular facets of the vertebral bodies are mostly concave in front and
convex behind (procoelous) in Anura.
 
Haemal arches are present in the tail of Urodeles, as are also
transverse processes which may bear ribs. In the Anura the urostyle is formed by the fusion of the two anterior caudal vertebrae
with the cellular sheath of the notochord.
 
Sauropsida. - -The cellular sheath of the notochord and the neural
arches from the first form a continuous structure.
 
In Hatteria and the Geckos, alone of living Eeptiles, are the
vertebrae biconcave, owing to the vertebral constriction of the
notochord. This condition was common amongst the extinct
forms. All the other Sauropsida agree in the sheath encroaching
on the notochord in the intervertebral regions (fig. 153, d). A
split occurs in the centre of each intervertebral enlargement, as in
Amphibia, which forms the interarticular cavity. In Eeptiles the
articular facets of the centra are usually procoelous ; they are saddleshaped in at least the cervical region of Birds. Intervertebral
discs or menisci occur between the vertebrae of Crocodiles, and,
except in the cervical region, of Birds also.
 
Mammalia.  - The view that Mammals have arisen from some
group of unspecialised Eeptiles receives additional support from
the mode of origin of the vertebrae, as the notochord is from the
first vertebrally constricted. The intervertebral regions become
wholly converted into the fibro-cartilaginous menisci, intervertebral ligaments, in the centre of which the notochord persists
in a degraded form as the nucleus pulposus or gelatinous pulp of
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
199
 
 
the intervertebral disc. Articular surfaces are never developed
between the bodies of the vertebrae, although they occur on the
neural arches. Vertebral epiphyses are peculiar to Mammals ;
they are found amongst Monotremes only in the caudal region, but
are universally present in other Mammals, except the Sirenia.
 
Evolution of the Vertebral Column - It is interesting to
note that at its first appearance the foundation tissue of the
skeletogenous sheath is segmented (fig. 1 5 1, v.r), the segments
corresponding with the muscle-plates ; but this segmentation is
soon lost.
 
The final segmentation of the vertebral column is alternate to that of the muscleplates, so that the centre of each vertebra is opposite to the intermuscular septa.
 
As Balfour says, “The explanation of this character in the segmentation is not
difficult to find. The primary segmentation of the body is that of the muscle-plates,
which were present in the primitive forms in which vertebrae had not appeared. As
soon, however, as the notochordal sheath was required to be strong as well as flexible,
it necessarily became divided into a series of segments.
 
“ The condition under which the lateral muscles can best cause the flexure of the
vertebral column is clearly that each myotome shall be capable of acting on two
vertebrae, and this condition can only be fulfilled when the myotomes are opposite
the intervals between the vertebrae. For this reason, when the vertebrae became
formed, their centres were opposite, not the middle of the myotomes, but the intermuscular septa.-
 
The stages of evolution were thus  - (1) the formation of axial
skeletal mesoblast round the notochord by the segmented muscleplates ; (2) the fusion of these elements to form a flexible continuous sheath round the notochord and nervous axis; (3) the
secondary segmentation of the vertebral column above described.
The last stage consists of two phases  - (a) cartilaginous, ( b ) osseous.
 
Ribs.  - In most Ganoids and Teleosts the ribs arise as the cutoff extremities of the haemal processes; in the caudal region, where
the haemal processes approach one another, the key of the arch is
formed by the fused ribs. The same probably occurs in the
Dipnoi.
 
The differentiation of the ribs is independent of that of the
haemal processes in Elasmobranchs, in which group they arise as
cartilaginous bars in the connective tissue of the intermuscular
septa, eventually they become connected with the haemal processes.
 
The ribs appear to develop in Amphibia and Amniota much in
the same way as in the Elasmobranchs, but in these groups they
are attached to the neural arches or to the transverse processes.
 
Ribs are present in the embryos of all Amniotes throughout the
vertebral column except in the tail. In the Amniota the cervical
 
 
200
 
 
• THE STUDY OF EMBRYOLOGY.
 
 
ribs usually fuse with the transverse processes, but one or more
(rarely all) may remain free. Several ribs unite to form the sternum ; their ventral moities are often incompletely or entirely
unossified, and constitute the sternal ribs. Behind these “ true -
ribs there are usually others, often termed “ false,- which do not
reach the sternum.
 
In all Vertebrates the pelvis is always supported by sacral ribs ;
these may remain distinct, as in Urodeles, or may fuse with the
transverse processes of their sacral vertebrae.
 
The occasional presence of abdominal parosteal splints has
already been noticed (p. 193). They have been erroneously termed
“ abdominal ribs - by some authors.
 
As a matter of fact, but little is really known concerning the
development of ribs, and our knowledge must be increased before
it is possible to satisfactorily determine the homologies of these
structures.
 
Sternum.  - The sternum is derived from a fusion from before
backwards of the ventral extremities of the ribs. The pair of
cartilaginous bars thus formed fuse together to form a central
plate which is later segmented off from the ribs. In Mammals
especially the sternum ossifies from a series of paired centres. It
is doubtful how far the so-called sternum of Amphibia is strictly
homologous with the sternum of the Amniota.
 
Miss Lindsay has come to the conclusion that the sternum of Birds has undergone
an anterior shortening, consequent upon the lengthening of the neck and the shortening of the trunk in the Avian as compared with the Reptilian type, owing to which
the sternum has been severed from the ribs that formed it. The “ manubrium - or
“ rostrum - of the Avian sternum has nothing in common with the manubrium sterni
of Mammals ; it is a secondary outgrowth for the attachment of the sterno-clavicular
ligaments. Miss Lindsay gives the following classification of the parts of the sternum. A. Part common to Sauropsida and Mammalia : Costal sternum arising in two
bands ; connected with sternal ribs in the adult, but often losing its connection with
the ribs which took part in its early formation. B. Part common to Ratitae and
Carinatae, but wanting in early embryos of the former, but never of the latter :
Metasternum. C. Part apparently common to both Ratitse and Carinatae, but really
of different origin : Anterior lateral process ; added to costal sternum in the Ostrich,
formed by atrophy of anterior ribs in the Fowl and Gannet. D. Part absent in
Ratitae, but common to all Carinatae : lceel ; the median ventral outgrowth of B. The
posterior lateral process is common to some Ratitae and to most Carinatae. The accessory processes of metasternum, the rostrum, and the xiphoid ends of posterior processes
are variable in Carinatae.
 
Pectoral Girdle.  - Two distinct elements occur in the pectoral
girdle, the one being the primitive cartilaginous element, the
other consisting of superadded dermal bones (clavicles).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
201
 
 
Without entering upon disputed details, it may be asserted in
general terms that the primitive girdle consisted of a pair of
laterally placed cartilaginous bars, each of which supported a
pectoral fin, and which possibly arose by the fusion or extension
of the basal elements of the fin itself.
 
In most Vertebrates the girdle is developed from such a pair
of plates, which subsequently are segmented into certain pieces.
Taking the articulation of the fore-limb as a starting-point, the
dorsal portion is known as the scapula, and the ventral as the
coracoid element. The latter is usually divisible into an anterior
bar or pre-coracoid, and into a posterior coracoid proper. The
girdle always becomes connected with the sternum.
 
Balfour found that in Elasmobranclis the girdle developed external to the muscleplate.
 
The clavicles first appear in the Ganoids as large dermal scutes which have become applied to the cartilaginous girdle. In the Sturgeon there are three pairs of
these scutes, the dorsal or supraclavicles, which are connected with the otic capsules
of the cranium by the intervention of the post-temporal bones ; the lateral elements
are the clavicles, while the infraclavicles (interclavicles) meet each other in the
median ventral line.
 
In Teleosts the dermal scutes have become subdermal bones ; the interclavicles
are replaced by a single median element, and postclavicles may be added. In* these
fishes the clavicles have, so to speak, usurped the place of the original girdle, so that
while the limb is borne by the scapular and coracoid, the latter are supported by the
enormously developed clavicles.
 
According to Gotte, the interclavicles are segmented oft' from the ventral ends of
the clavicles in Birds, and, extending between the inner edges of the two halves of
the sternum, give rise when the latter unite to the keel (crista sterni). It is most
probable that the keel is a new structure, secondarily acquired in response to the
need of increased surface of attachment for the pectoral muscles. It may ossify from
a single or a pair of centres. The clavicles fuse in the middle line to form the
furculum.
 
In a recent paper Howes homologises the two small coracoid ossifications so constantly present in the Eutheria with the coracoid and epicoracoid of Prototheria
(Monotremes), the former being the “ coracoid epiphysis - and the latter the “ coracoid - of human anatomists. These two elements are readily seen in the young
Rabbit, which is in this respect in an intermediate condition between the Prototherian and the Eutherian type of shoulder girdle. The Mammalian “ clavicles -
may now be definitely regarded as ossifications around pre-existing bars of cartilage
which are at first continuous with the scapulae. The “ clavicles - thus correspond
with the precoracoid of Anura.
 
Gotte has shown that the cartilaginous predecessor of the Mammalian clavicle early
unites with its fellow in the median line ; the tract resulting from this coalescence
eventually segments into five pieces, viz., paired clavicular bars, two small nodules
which represent the “ lateral episterna- of Gegenbaur or the “omosterna- of Parker
and a median episternum. The lateral episterna are stated by him to become
attached to the clavicle or converted into the sterno-clavicular ligament. The middle
piece enters into connection with the omosternum, and either becomes confluent therewith (Mole) or undergoes a retrogressive metamorphosis within its perichondrium
(Lepus). Thus if the lateral bars represent, as unquestionably they appear to do, the
 
 
202
 
 
THE STUDY OF EMBKYOLOGY.
 
 
primary predecessors of the clavicles, this median episternum can only represent that
of the interclavicle. This being so, all the elements of the Prototherian shoulder
girdle are represented in that of the Eutheria.
 
There is so much contradiction in the accounts of the development of the clavicular elements in the Amniota that it is at present
difficult to determine their precise homology.
 
It is possible that the “ clavicles - of the Ganoids and Teleosts
form a series by themselves, and that the “ clavicle - of Amphibia
and Amniota is merely an ossified precoracoid.
 
Pelvic Girdle.  - The pelvic girdle arises as a pair of cartilaginous
bars much in the same way as the pectoral girdle develops.
 
Dorsal to the articulation for the hind-limb is a single element,
the ilium ; but ventrally there are two elements, an anterior pubis
and a posterior ischium. The space between them is known as
the obturator foramen.
 
Locomotory Appendages.  - Throughout the animal kingdom,
when distinct organs for locomotion occur, apart from ciliated
areas, they always develop as folds of the epiblast supported by
an axial layer of mesoblast.
 
The epidermal surface may not be specially modified, but the
mesoblast is differentiated into muscles, often numerous and
complex in their action, which serve to put the appendage or
limb in motion. Nerves are always, and sense-cells usually,
present.
 
The appendages of the Craniata are always productions from
the body-wall, and, being solid, never contain a diverticulum
from the body-cavity; the reverse is the rule in Invertebrates.
The skeletal elements of the appendages are axial in the Craniata,
and, as a rule, external in Invertebrates.
 
Invertebrates.  - The “ arms - of the Starfishes are mere prolongations of the body ;
but owing to the reduction of the body-cavity in them, the arms of the Ophiuroidea
and Crinoidea have a superficial resemblance to mere appendages. In all, the calcareous axial skeleton is of mesoblastic origin.
 
The parapodia of the Chsetopoda are segmentally paired lateral prolongations of
the body-wall, the cavity of which communicates with the body-cavity. The walls
are usually greatly thickened, and normally bear setae, and often scales, cirri, and
gills. The setae are of epiblastic origin ; often a pair are immersed so deeply within
each parapodium that they must serve to give a certain amount of rigidity to the
structure, and thus function as a skeletal element. The parapodia may be rudimentary, and even absent (Earthworm).
 
The appendages of the Arthropoda are jointed tubular paired processes from
the ventro-lateral aspect of each segment. The mainly chitinous exoskeleton is
secreted by the epiblast ; the muscles are entirely internal. The limbs at first develop
as hollow buds, their cavity freely communicating with the body-cavity; but in
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
203
 
 
most cases the limb subsequently becomes solid. In several Arachnoids the alimentary canal sends prolongations into the limbs.
 
The larval velum and the adult locomotory organs of the Mollusca call for no
special mention.
 
Chordata.  - The locomotory appendages of the Chordata fall
into two classes, the median and the paired.
 
Unpaired Limbs.  - A median unpaired fin is characteristic of
all the Ichthyopsida ; in its fullest development it extends along
the dorsal side of the body, commencing behind the head, passing
round the tail, and, running forward along the ventral aspect of
the tail, it terminates just behind the anus. The median ventral
fin, however, extends in front of the anus in the adult Amphioxus
and in embryo Teleosts.
 
Median Fin.  - Usually the median fin is interrupted, above and
below, in front of the end of the tail, so that definite regions are
established which are known as the dorsal, caudal, and anal fins.
The dorsal fin is frequently further subdivided.
 
Its development is very simple, since the fin arises as a lamellar
fold of the epiblast, within which the mesoblast is modified to
form muscles ; and, later, fine supporting rods or fin-rays are developed, which are quite independent of the axial skeleton, although
they may subsequently be closely connected with the neural and
hsemal spines. The fin-rays never occur in the unpaired fin of
Amphioxus or Amphibia.
 
The median fin is found in all larval Amphibians, and it is more
or less developed in those adult Urodeles which retain an aquatic
mode of life. The males of the ISTewt have it largely developed
during the breeding season.
 
A dorsal fin occurs in many Cetacea. Here it is a fold of the
skin which is supported by fibrous and fatty tissue, but without
any skeletal elements. It, of course, has no connection with the
dorsal fin of Fishes, but has been independently acquired.
 
Caudal Fin.  - There can be no doubt that primitively the notochord extended as
a straight tapering rod to the extreme posterior end of the animal, and that the
caudal fin passed symmetrically round it. Such a protocercal or diphycercal tail
is found in Amphioxus, Cyclostomi, Dipnoi, and larval Elasmobranchs, Ganoids,
Teleosts, and Amphibia.
 
The next stage in the development of the tail in Teleosts is characterised by the
greatly increased size of the ventral lobe, resulting in the dorsal flexion of the
notochord. This is the permanent condition in most Elasmobranchs, and is known
as a heterocercal tail.
 
The ventral lobe projects still farther, and the dorsal portion which contains the
notochord dwindles away, merely forming a kind of dorsal border to the permanent
caudal fin, a condition which is characteristic of Ganoids.
 
 
204
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Finally the tail becomes symmetrical externally ; the fin-rays are supported by one
or two greatly developed haemal arches (hypural bones). The now ossified vertebral
column apparently ends abruptly, but a rodlike bone, the urostyle, can usually be
detected, which extends obliquely into the upper part of the base of the fin. The
urostyle is the unsegmented ossified sheath of the upturned posterior extremity of the
notochord. This is usually, but not invariably, the condition which obtains in the
tails of Teleosts. The tail of the ordinary adult Teleost is, strictly speaking, as heterocereal as that of Elasmobranchs or Ganoids ; but having a superficial symmetry,
it is usually termed homocercal.
 
The protocercal nature of the larval tail is retained in Urodele Amphibia, but the
notochord is replaced by the segmented vertebral column.
 
Paired Limbs.  - Paired limbs are developed in all Craniata
higher than the Cyclostomi, except in a few groups in which they
have become lost.
 
Dohrn believes that he has found a rudiment of the pelvic fins of the Lamprey in
the longitudinal folds bordering the anus and rudiments of muscles in the Ammocoete-stage.
 
In the Elasmobranchs, and to a less extent in Birds, the paired
limbs are developed from a larval lateral ridge, which extends
from behind the gill-clefts to the anus.
 
The ridge consists of a fold of epiblast with a core of mesoblast.
It is rapidly produced into an anterior and posterior process ; the
intervening portion (Wolffian ridge) disappears, leaving the fore
and hind pair of limbs.
 
In most animals the lateral ridge is not visible, each pair of
limbs being apparently independent of the other. It is now
generally held that the paired limbs are to be regarded as special
developments of a pair of posteriorly converging lateral fins, which
had essentially the same structure as the median fin.
 
The axial mesoblast of the limbs differentiates into cartilage, and
forms the skeleton of the appendages.
 
Two main types of limb occur in the Craniata : the one found
in Eishes is known as the ichthyopterygium; the other, peculiar to
Amphibia and Amniota, is termed the cheiropterygium.
 
There is much controversy respecting the nature of the ichthyopterygium, based largely on speculation, but with very little positive
embryological evidence ; the subject is therefore quite beside the
scope of this book.
 
The relation of the ichthyopterygium to the cheiropterygium is
also at present obscure ; the structure of the latter is fundamentally
identical in all those animals in which it occurs. The main differences are attributable to modifications in accordance with the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
205
 
 
habits of the animal, to the loss of certain elements, and to the
fusion of parts primarily distinct.
 
Skull.  - The skull is a composite structure, and in order to gain
a clear conception of it as a whole it is necessary to bear in
mind the distinctness of the parts involved. The morphology of
the skull is one of the most intricate of zoological problems,
consequently only the main points can be touched upon here, and
these but lightly.
 
The old view of the segmentation of the skull, which regarded it
as composed of four modified vertebrae, is now entirely abandoned,
thanks to the labours of Huxley, Parker, Gegenbaur, and others.
In that view the radical distinction between membrane bone and
cartilage, with the bones ossified- from it, was entirely overlooked,
and no recourse had been made to embryology.
 
According to the now generally received opinion, without itself
being distinctly segmented, the head corresponds to some dozen or
so of the anterior segments of the body, excluding an unsegmented
portion in front of the mouth, the pre-oral lobe.
 
The skull is essentially composed of an axial brain-box or
cranium, and of three pairs of sense-capsules, and various bars
which surround the mouth and visceral clefts, and which collectively form what are termed the visceral arches. To the primitive
cartilaginous cranium, and the bones which may develop within
it, are usually added a large number of dermal bones. Por the
sake of simplicity, the cranium, the visceral arches, and the dermal
bones will be considered more or less separately.
 
In Amphioxus (Cephalochordata) the notochord extends in
front of the neural tube ; in all the Craniata the notochord terminates anteriorly immediately behind the infundibulum (fig. 94) ; its
extremity being usually bent downwards, being probably acted
upon by the cranial flexure or by the down-growth of the infundibulum.
 
Cranium.  - A layer of mesoblast at first surrounds the brain
and constitutes what is known as the membranous cranium, the
notochord extending along its floor as far as the infundibulum.
 
A continuous tract of cartilage is next developed on each
side of the notochord, hence termed parachordal ; and a separate
pair of bowed rods appears in front, the trabeculae cranii. The
posterior extremities of the trabeculae embrace the apex of the
notochord (fig. 155, a). The curved trabeculae enclose a space
known as the primitive pituitary space; in front they usually fuse
 
 
206
 
 
THE STUDY OF EMBRYOLOGY.
 
 
together below the nasal capsules. A median rod of cartilage,
the prenasal rostrum, is often present between the anterior ends
of the trabeculse.
 
The parachordals early fuse with each other, and entirely enclose
the notochord, with its skeletogenous sheath, to form the basilar
plate.
 
The cartilaginous auditory capsule also unites with the basilar
plate, which forms a ventral support for the posterior half of the
brain. The notochord gradually atrophies, and, as a rule, entirely
disappears.
 
The trabeculse enlarge in size and fuse with the basilar plate ; in
the nasal region a considerable amount of cartilage is formed, and
the pituitary space is reduced (fig. 155, b).
 
 
 
A. Early stage, with the trabeculse and parachordals as simple bars and membranous sense capsules. B. Later stage, in which a fusion of the above elements has
occurred and the cartilaginous nasal and auditory capsules are incorporated in the
cranium. C. Side view of about same stage as B.
 
a.o. antorbital process ; au. auditory capsule ; hr. branchial arches ; c. cornua
trabeculse ; hy. hyoid arch ; mn. Meckel -s cartilage, mandibular arch ; na. nasal
capsule ; n.ar. neural arch ; nch. notochord ; op. optic capsule ; p. ch. parachordal :
pl.pt. palato-pterygoid arch; p.o. post-orbital process ; p.t.s. pituitary space ; q. quadrate ; r. rostrum ; tr. trabeculse.
 
The nasal capsule is supported anteriorly by the outwardly
curved extremities of the trabeculse, the cornua trabeculse; and
posteriorly by a spur of cartilage, the preorbital process. Thus,
like the auditory capsule, the nasal capsule is early engrafted into
the cranium. The optic capsules or eyeballs always remain free.
 
The floor of the cranium being thus laid, the walls are raised by
vertical upgrowths from the sides of the basal cartilage. Between
the auditory capsules the walls usually meet above the brain and
form the posterior cranial roof ; and a solid upgrowth of cartilage
often occurs anteriorly between the nasal capsules.
 
The primitive cartilaginous cranium (chondro-cranium) thus consists of a ventral plate and lateral walls of cartilage, which enclose
the auditory and olfactory capsules, and a posterior roof. The
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
207
 
 
floor is perforated by the pituitary space, through which also the
internal carotid artery at first passes. The cranial nerves pass
through apertures (foramina) left during the extension of the
cartilage.
 
Definite regions can be made out in the cliondro-cranium at this
stage, which may now be enumerated.
 
The posterior roofed extremity of the skull, occipital region,
articulates with the anterior vertebra (except in Cyclostomes,
some Elasmobranchs, Ganoids (except Lepidosteus), and Dipnoids,
in which forms the persistent notochord is continued into the skull,
or the occipital region is fused with more or fewer of the anterior
vertebrae. In front of the occipital is the auditory region, and
between them is the aperture (foramen lacerum posterius) for the
glosso-pharyngeal (ix.) and vagus (x.) nerves (fig. 156).
 
The sphenoidal region extends from the auditory to the nasal
capsule ; an anterior and posterior pair of cartilages usually grow
up from the basi-sphenoidal cartilage, which are respectively known
as the orbito- and ali-sphenoid plates. A large slit-like orifice
(foramen lacerum medius) is left between the auditory capsule and
the ali-sphenoid; in it is lodged the Gasserian ganglion, and through
it emerges the trigeminal (v.) nerve. Between the ali- and orbitosphenoid is a cleft (foramen lacerum anterius or sphenoidal fissure)
through which the optic nerve (11.) and the motor nerves of the
eyeball (in., iv., vi.) pass ; the fourth nerve sometimes passes out
independently above the optic foramen. The cartilage at the base
of the ali-sphenoids (basi-sphenoid) is continuous with that below
the orbito-sphenoids (pre-sphenoid).
 
Between the nasal capsules is the ethmoid region.
 
Such a chondro-cranium as that described above is practically
the permanent condition of the crania of all Fishes, except the
Teleosts and bony Ganoids. In the Ganoids ossification commences in more or fewer of these cartilaginous areas, and, with
some variation, the bones which result from these centres of
ossification occur all through the Vertebrate series.
 
It not unfrequently happens that cartilage extends beyond its
primitive area and encroaches on other regions or surrounds certain
nerves or blood-vessels, or two or more ossifying tracts may fuse
to form a compound bone. On the other hand, portions of the
chondro-cranium may atrophy, or even not be developed at all.
 
Visceral Arches.  - Cartilaginous bars are early developed in
the lateral walls of the pharynx between the visceral clefts. These
 
 
208
 
 
THE STUDY OF EMBRYOLOGY.
 
 
visceral arches, as they are termed, are primitively very similar,
and each consists of a simple bar of cartilage, which later may
become segmented, and usually more or less ossified. The greatest
number occurring in any animal are found in the Cyclostomi and
Notidanus, where there are nine in all : as a rule, there are seven
in the Ichthyopsida and fewer in the Amniota.
 
The first is the mandibular arch, the second is the hyoid, and
the remainder are known as branchials.
 
 
It was formerly thought that the branchial basket-work of the Cyclostomi belonged
t o a different series of cartilages from the visceral arches of Gnathostomatous Crani
 
 
Fig. 156. - The Chondro-Cranium and Visceral Skeleton with the Anterior Part of the Vertebral Column of a Dog-Fish (Scyllium
canicula). Seen from the right side; the labial cartilages are omitted. [ Ajttr
A. M. Marshall .]
 
A. auditory capsule ; B. post-orbital groove ; c. inter-orbital canal ; D. pre-spiracular
(meta-pterygoid) ligament, with the pre-spiracular cartilage ; e. upper jaw (pterygoquadrate arcade) ; f. lower jaw (Meckel -s cartilage) ; G. hyo-mandibular cartilage ;
h. cerato-hyal; 1. pharyngo-branchial ; K. epi-branchial ; L. cerato-branchial ; M.
extra-branchial ; n. vertebral neural arch ; no. olfactory capsule ; 0. centrum of
vertebra ; p. intervertebral neural arch ; R. neural spine ; s. foramen for the ventral
root of a spinal nerve ; t. foramen for the dorsal root of the preceding nerve ; u.
orbital grooves, lodging the ophthalmic branches of the fifth and seventh nerves ; w.
aperture at end of orbital groove through which the above-mentioned branches leave
the orbit ; z. ethmo-palatine (palato-trabecular) ligament. 11. optic foramen ; in.
foramen for third nerve ; iv. foramen for fourth nerve ; V. foramen for the main
branches of the fifth and seventh nerves and for the sixth nerve ; va. foramen for the
ophthalmic branch of the fifth nerve ; vna. foramen for the ophthalmic branch of the
seventh nerve ; ix. foramen for the ninth or glosso-pharyngeal nerve.
 
 
ates, the former being supposed to be developed on the outer wall of the so-called
head-cavities, while the latter arose from their inner wall. The extra-branchial cartilages of Elasmobranchs (fig. 156) were also supposed to be rudiments of the external
series. Dohrn has, however, shown that there is no real distinction between these
elements, and that the branchial skeleton of Lampreys is as truly internal as that of
other Craniates, the main distinction being that in Cyclostomes the visceral arches
are unsegmented. Dohrn also finds that the extra-branchials of Elasmobranchs are
merely the dorsal and ventral cartilaginous branchial rays of their respective arches,
which early shift their position.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
209
 
 
Mandibular Arch.  - From the mandibular arch a bud grows
forward on each side in front of the mouth, and a separation
occurs in the arch at the angle of the mouth, so that an upper and
a lower jaw cartilage result, which articulate together. The upper
jaw arch is termed the palato-quadrate or pterygo-quadrate
arcade; the lower bar forms Meckel -s cartilage. The portion of
the primitive arch above the pteryogoid bud is the metapterygoid,
and possibly constitutes the primitive means of attachment of the
jaw with the cranium.
 
The mandibular arch may posteriorly be supported solely by
the proximal element of the hyoid arch (hyomandibular), or
partially by the latter and partly by its own proximal portion
(metapterygoid ?), or the mandibular arch is directly attached to
the cranium without the intervention of the hyoid arch. The
first mode of attachment, known as hyostylic, occurs in many
Elasmobranchs, and in most Ganoids and Teleosts ; the second or
amphistylic is found in the Notidanidse and Cestracion ; the last,
autostylic, is peculiar to Holocephali, Dipnoi, Amphibia, and
Amniota.
 
The upper jaw arch may anteriorly be quite independent of the
cranium, or attached by a ligament ethmo-palatine or palatotrabecular ligament, or by a cartilaginous bar, the palatine. In
the Holocephali and Dipnoi the whole of the pterygo-quadrate bar
is fused with the base of the cranium.
 
The quadrate, or that region on which the lower jaw articulates,
is usually cut off as a distinct element, and serves, in the Sauropsida, as the support (suspensorium) for the mandible. In Mammals
it is pressed into the service of the internal ear as the incus (p. 1 5 1).
 
In the cartilaginous Fishes, Meckel -s cartilage, or the primitive
cartilage of the lower jaw, is very massive ; but in other forms,
although always present in early life, its place is generally usurped
by membrane bones.
 
The proximal articulating element is segmented off in Mammals,
and now generally regarded as the malleus.
 
Ossifications occur in certain centres of the cartilage, or in the
perichondrium, but the details of these ossifications in this and the
succeeding visceral arches do not fall within the scope of this book.
 
Hyoid Arch.  - The upper portion of the hyoid arch segments
off in Fishes as a distinct cartilage, the hyomandibular, to which
allusion has just been made. The inferior moiety becomes divided
into-several rod-like pieces, which may become ossified.
 
 
0
 
 
210
 
 
THE STUDY OF EMBRYOLOGY.
 
 
From his researches on the development of Fishes, Dohrn finds that the problem of
the original number of visceral clefts and arches is not so simple as is generally imagined. He is satisfied that what is usually regarded as the hyoid arch is certainly
a double structure. He also regards the spiracular cartilage as being the rudiment
of another arch, and he is inclined to believe that both the upper and the lower jaws
are cartilages belonging to distinct arches. According to him, the enumeration of
the visceral arches of the jaw and hyoid regions would be : I. upper jaw; 2. lower
jaw ; 3. spiracular cartilage ; 4. hyomandibular ; 5. hyoid. The clefts between the
mandibular and hyoid arches have become difficult to recognise as such ; the median
thyroid body may perhaps represent the coalesced rudiments of one pair.
 
Branchial Arches.  - The greatest number of branchial arches
obtains in Heptanchus, Notidanus ; where there are seven, in most
Fish there are five (fig. 156); this number may be considerably
reduced in Teleosts. The originally continuous bars become
jointed, and may ossify.
 
Basi-hyoid and basi-branchial cartilages are universally present. A cartilage which
Huxley believes may represent a basi-mandibular element is present in the Cyclostomi.
 
In the adults of the Caducibranchiate Amphibia and Amniota
the post-oral visceral skeleton is greatly reduced, and is represented by the so-called “ hyoid.- In reality this composite structure consists of a flat plate or body, which results from the fusion
of the median pieces of the hyoid and first branchial arch. The
anterior or greater cornua are the persistent hyoid arches, and the
posterior or lesser cornua are the degenerate first branchials. What
appears to be a vestige of the second branchial arch has been
described by Howes for Phocsena, and several branchial elements
enter into the “ hyoid - in most Amphibia.
 
Dermal Bones.  - The ehondro-cranium of Elasmobranchs is
simply covered by the skin of the head. In Ganoids the brain
is further protected by large bony plates, which assume a more
or less regular disposition. Certain of these plates persist in
Teleosts as the dermal bones of the skull, and similar bones with
an analogous distribution are found in higher animals.
 
An irregular median series is sometimes present, but these are
crowded out by a paired series, which form the roof-bones of the
skull.
 
Membrane bones are developed on the side of the face, along
the upper and lower jaw, on the roof of the mouth, and outside
the hyoid arch.
 
Those parosteal bones (ex. parasphenoid, vomers), which are
developed within, or in some cases at the side of the mouth, appear
to be primarily due to the fusion of the basal portion of teeth.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
211
 
 
All the above elements collectively constitute the skull.
 
Body-Cavity.  - The mode of formation of the body-cavity is
necessarily dependent upon the development of the mesoblast, and
has already been incidentally dealt with ; there is, therefore, no
need to repeat the former descriptions or inferences.
 
It is necessary to remember that, with the exception of the lower
Worms and Molluscs, diverticula or pouches (somites) grow out
from the archenteron and become separated from it. The sacs
thus formed increase in size and surround the alimentary canal.
Their outer wall (somatic mesoblast) becomes applied to the
epiblast to form the body-wall (somatopleur) and their inner wall
(splanchnic mesoblast) together with the hypoblast constitutes the
somatopleur ; their cavity is the coelom or true body-cavity.
 
The mesothelium which forms the walls of the somites may
differentiate into various structures, but it nearly always gives rise
to a delicate epithelium (peritoneum or serous membrane) on the
surface facing the body-cavity. The somatic epithelium is known
as the parietal layer, the splanchnic as the visceral layer of the
peritoneum.
 
As the visceral or splanchnic walls of each pair of somites
approach one another they form a double-layered membrane, the
mesentery. In some animals the primitive dorsal and ventral
mesentery may persist, but usually the mesentery is largely absorbed
leaving strands of tissue (mesenteries) which sling the alimentary
canal.
 
In Cyclops, according to Urbanovics, the body-cavity is formed by a fusion of
paired excavations of a mesoblastic band ; the disepiments between only disappear
very late. The dorsal and ventral mesenteries persist ; the dorsal mesentery contains a space which is a remnant of the blastocoel, and plays an important part
in the circulation in the absence of the heart. It is difficult to understand why this
should not be regarded as a true heart ; it may be a rudimentary structure, but the
development is similar to that of a heart (p. 215).
 
The behaviour of the somites in Amphioxus gives us a key to
the original mode of formation of the body-cavity in the ChordatP.
The segmented somites are developed from the dorso-lateral angles
of the archenteron (fig. 56) ; subsequently they extend ventralwards forming the somatic and splanchnic mesoblast, finally the
upper portion loses its central cavity and becomes converted into
the lateral muscles of the body, which always retain their original segmentation. The ventral portions of the somites not only
fuse with their fellows, but form a continuous body-cavity which
extends along the whole length of the body.
 
 
212
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In all Chordata the primitive segmentation of the body is
retained solely by the dorsal moieties of the somites  - primarily
in the muscles, secondarily in the vertebral column, and partially
in the excretory organ. The two former are developed from the
main portion of the dorsal halves of the somites, which eventually
are entirely separated from the ventral halves. Before this is
effected they are connected by what is known as the “ intermediate
cell mass - (figs. 1 50, 174, 178*). This tissue gives rise to the excretory organ (p. 243).
 
At the origin of the mesentery, the peritoneun is columnar
throughout a considerable length of the body-cavity, and constitutes the germinal epithelium (fig. 175, p.o).
 
Mesentery.  - As the alimentary canal of Vertebrates is at first
a simple straight tube, so the mesentery which slings it forms a
simple fold. With the appearance of distinct regions in the alimentary canal, those portions of the mesentery which suspend them
receive corresponding names; thus the mesogastrium, the mesocolon,
and the mesorectum.
 
In Man the stomach is at first an antero-posterior dilation of the mesenteron, as is
permanently the case in most of the lower Vertebrates. The stomach soon turns
over towards the right side, so that the mesogastric border is turned to the left, but
the stomach still retains its longitudinal direction, as in some adult Mammals. The
new left border bulges out to form the greater curvature, and the stomach assumes
by degrees a transverse direction, carrying the mesogastrium with it. As a result of
this rotation of the stomach, a mesogastric sac is formed which is the commencement
of the omentum ; the orifice of the sac is the foramen of Winslow. The omentum
increases in size and extends down to the colon.
 
In Fishes the kidneys remain above the dorsal wall of the bodycavity ; in Amphibia and higher forms they project slightly into
the coelom, being more or less suspended by folds of the peritoneun.
 
The generative glands are suspended within distinct folds of the
peritoneun, which are known as the mesorchium for the testis, and
mesoarium for the ovary.
 
It must be borne in mind that the viscera which are described
as lying within the body-cavity are all, morphologically speaking,
outside it. The body-cavity or coelom is a closed sac lined by a
serous membrane. Various viscera may sink into the contained
cavity, but they always push before them the serous membrane,
which thus forms a fold round them. All structures such! as
blood-vessels or nerves pass to and from , the viscus between the
laminae of the fold of the serous membrane.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
213
 
 
Pericardium.  - The anterior portion of the primitive body-cavity undergoes certain
changes. A horizontal septum is formed, connecting the splanchnopleur with the
somatopleur of each side on a level with the ductus Cuvieri at the spot where they
enter the sinus venosus, and really serving to support these vessels.
 
The transverse septum extends anteriorly and posteriorly ; below lies the heart,
and above is the alimentary canal. As the septum stretches from the body-wall to
what may be termed the dorsal mesocardium (fig. 159), it naturally divides the anterior region of the body-cavity into a ventral pericardium and a pair of dorso-lateral
cavities ; these all communicate anteriorly and posteriorly. By further growth
forwards of the septum, the pericardium is cut off from the anterior dorsal horns of
the body-cavity. The septum extends posteriorly along the under side of the liver
till it reaches the ventral wall of the body, where the liver is attached by its ventral mesentery (falciform ligament) ; but a posterior canal, usually opening into the
general body-cavity by two orifices, persists in Elasmobranchs.
 
The pericardium is thus a specialised portion of the body- cavity, and therefore it
is lined by its serous membrane, which was primitively continuous with that of the
coelom. As its viscus, the heart, depends into the pericardial cavity in the same
manner as the mesenteron depends into the body-cavity, so it is covered by the
reflected or visceral portion of the pericardium, the outer being known as the parietal
portion.
 
In air-breathers the developing lungs project on each side of the throat into the
dorso-lateral extensions of the body-cavity above the pericardium. This condition
is practically retained in Amphibia and most Reptiles. The common body- cavity is
thus often termed in them the pleuro-peritoneal cavity.
 
Diaphragm.  - The diaphragm later makes its appearance by a dorsal extension of
the posterior wall of the pericardium, which cuts off the pleural -cavities from the
abdominal coelom. The diaphragm is at first tendinous ; the muscle grows in later
from the dorsal side, probably from the muscle-plates.
 
Uskow enumerates the following grades of development :  -
 
1. The ventral and dorsal portions of the diaphragm are fully developed ; they
completely divide the coelom, and have muscles. The diaphragm is entirely separated from the pericardium, except two thin lamellae (Rabbit).
 
2. Similar, but a part of the diaphragm remains united with the pericardium (Man).
 
3. Same as 2, but the diaphragm contains no muscles, and its ventral part is completely fused with the pericardium (Fowl).
 
4. Similar to 3, but the dorsal part is not completely developed, remaining in
a primitive condition (Lizard) or in an early stage (Frog).
 
5. Like 4, the diaphragm is not separated from the pericardium, persisting at the
stage of the septum transversum (Myxinoids and Ammocoete).
 
6. The Teleosts form a distinct type ; although, as in the Salmon, there is a certain
separation of the diaphragm from the pericardium, even more than in Birds, yet the
dorsal portion is completely wanting.
 
Pleurae.  - The serous membrane of the pleural cavities is termed the pleura, and,
 
' as in the case of the pericardium, a parietal layer (costal pleura) and a visceral layer
(pulmonary pleura) are present. The mediastinal space is that space which occurs
between the closed pulmonary serous sacs, the mediastinum itself being formed
by the junction of the parietal pleura of each side.
 
In the adult males of the higher Eutheria the primitive coelom is divided into the
following perfectly distinct serous sacs :  - The two pleurae and the pericardium,
which together form the thoracic cavity, the abdominal cavity, and the paired
tunica vaginalis (p. 262).
 
Abdominal Pores.  - A pair of apertures, by means of which
the abdominal cavity is placed in direct communication with the
 
 
214
 
 
THE STUDY OF EMBRYOLOGY.
 
 
exterior, occurs in Cyclostomi, Elasmobranchii, Ganoidei, a few
Teleostei, Dipnoi, Chelonia, and Crocodilia. Occasionally there is
only a single pore.
 
These abdominal pores, as they are termed, usually open into
the cloaca on each side of the urogenital aperture, but they may
occur outside the cloaca, and either in front or behind.
 
 
In Cyclostomes, Scott states they are developed from the hypoblastic section of
the cloaca ; in other forms they arise as epiblastic pits, but the pores in Cyclostomes
may not be homologous with those of other animals. The abdominal pores of most
Teleosts have also been regarded as not homologous with those of other fish (see
p. 258).
 
They serve for the egress of the generative products in Cyclostomes and a few
Teleosts.
 
Abdominal pores are entirely absent in Amphibia and Birds, and have not been
recognised in Mammals. It is, however, possible that the inguinal canals of
Mammals, which have a similar relation to the urogenital orifice, may prove to be
remnants of the abdominal pores of their hypotherian ancestors.
 
 
The branchial or atrial pore of Amphioxus is often erroneously
termed an abdominal pore; its mode of formation (p. 178) proves
these two pores have nothing in common.
 
The Vascular System.  - The vascular system consists of a
closed network of vessels containing a fluid (plasma), within which
float free cells (blood corpuscles). The whole is invariably derived
from the mesoblast.
 
There are yet numerous gaps in our knowledge of the development of blood-vessels in various animals. Two modes of formation
have been described for both Invertebrates and Chordata.
 
Development of Blood-Vessels.  - In the vascular area of the
blastoderm of Amniotes, the mesoblast cells form a protoplasmic
network. Some of the nuclei of these cells rapidly divide and
form masses of nuclei. The protoplasm round each nucleus acquires a red colour (haemoglobin), and, on the deliquescence of the
central portion of the protoplasmic network become liberated as
red-blood corpuscles. The peripheral nuclei form the nuclei of
the walls of the vessels.
 
A similar mode of formation of blood-vessels has been described by Lankester in the adult Leech, and it is probably of
wide occurrence.
 
The process may be summed up as a liquid vacuolation of certain
reticular mesoblastic tissue. Some of the nuclei remain in the
walls of the channels, others (red blood-corpuscles) with free
mesoblastic elements (white blood-corpuscles) are suspended in
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
215
 
 
the fluid (plasma) thus formed, and, on the assumption of contractility by the walls of the main vessels, they are hurried along
in the general circulation.
 
The second mode of vascular development consists in linear
masses of mesoblast cells being formed, the outermost of which
arrange themselves into a tube containing the central free cells
or corpuscles. This occurs in the trunk of Vertebrate embryos,
and is usually described for Invertebrates generally.
 
In the lower Invertebrates the vascular system is either not at
all or very imperfectly developed. The Chsetopod Worms have a
large dorsal (abneural) blood-vessel, which is very contractile and
drives the blood from behind forwards ; some of the lateral
branches are also contractile. The Mollusca and Arthropoda
possess a distinct heart, which in the latter may be considered as a
concentration of the elongated dorsal vessel of the higher worms.
Although many of the blood-vessels in Amphioxus are contrac
Fig. 157. - Diagrams Illustrating the Formation of the a
Heart of (A) Inverte- A
 
BRATES AND (B) CHORDATA.
 
 
The neural aspects are placed the £'/
same way in both diagrams to faci- Of
litate comparisons. U\
 
al. mesenteron ; cor. coelom or vx
body-cavity ; ep. epidermis ; ht. \
cavity of heart ; me. mesocardium ;
m.p. muscle-plate; n. central nervous system ; nch. notochord ; so.
peritoneum (somatic mesoblast) ;
v.m. ventral mesentery (of Invertebrates).
 
tile, no distinct heart is present. In the true Vertebrates a
heart is always present, and the blood-vessels retain their contractibility to a greater or less extent.
 
Formation of the Heart.  - The origin of the heart in many
Invertebrates is still a matter of some uncertainty. From the
recent investigations of Biitschli, Schimkewitsch, and others, it
would appear that the cavity of the heart, at least in certain of
the Annelida and Arthropoda, is a persistent portion of the segmentation-cavity which has been enclosed between the vertical
walls of the archenteric diverticula where they join one another
to form the dorsal (abneural) mesentery (fig. 157, a).
 
Patten, on the other hand, maintains that although in the Cockroach (Blatta)
the heart is formed by the junction of the two folds of mesoblast, the cavity of the
heart is not the space included between the two folds, but is in reality an enclosed
portion of the true body-cavity. The folds of the mesoblast pulsate long before a
special heart is formed, and a circulation occurs through the irregular sinuses of the
body-cavity. Blood corpuscles arise before the formation of the heart by the liberation of indifferent cells, and afterwards from the walls of the heart itself.
 
 
 
216
 
 
THE STUDY OF EMBRYOLOGY.
 
 
In the Spider [Balfour, but not Schimkewitsch] and in some of
the higher Crustacea (Asellus [Dohrn], Astacus and Palsemon
 
 
 
Fig. 158. - Transverse Section through the Head of a Rabbit of Eight Days
Fourteen Hours. [ From Kolliker.]
 
A. Magnified 48 diameters. - h.h. paired rudiment of heart ; sr. cavity of mesenteron.
 
B. Part of A magnified 152 diameters.  - ahh. muscular wall of heart; dd. hypoblast;
dd'. thickening of hypoblast to form the notochord ; dfp. splanchnic mesoblast; h. epiblast ; hp. somatic mesoblast ; ihh. epithelioid layer (endothelium) of heart ; mes. lateral
undivided mesoblast ; mp. neural plate ; ph. pericardial section of body-cavity ; rf. neural
groove ; rw. neural fold ; sp. intermediate cell-mass ; sw. part of the hypoblast which will
form the ventral wall of the pharynx.
 
 
[Bobretzky] ) the heart is said to arise from a solid rod of mesoblast cells, of which the central portion becomes the corpuscles.
This may, however, prove to be only a secondary mode of formation.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
217
 
 
The formation of the heart in Vertebrata appears to be essentially identical with that in Invertebrates, the cavity of the heart
being that space which is left between the median walls of the
lateral halves of the body-cavity as they approach one another
below the throat (fig. 157, b).
 
 
 
Fig. 159. -' Transverse Section through the Cardiac Region of an Embryo Fowl of
Thirty-nine Hours. Magnified 61 diameters. [ From Kolliker .]
 
a. aortic arches; dfp'. somatic mesoblast of throat ; Ent. epiblast of wall of throat;
g. vessels of the internal border of the area opaca ; h. epiblast ; hh. body-cavity of
neck ; hp. dorsal somatic mesoblast ; hzp. muscular wall of heart ; ihh. endothelium
of heart ; m. neural canal ; ph. pharynx ; s. septum formed by the junction of the two
endothelial tubes; uhg. inferior cardiac mesentery (mesocardium) formed by the
meeting of the splanchnopleur below the developing heart ; the corresponding though
widely separated folds between the heart and the pharynx may he termed the dorsal,
and the former the ventral, mesocardium.
 
Shipley has very recently shown that the heart of the embryo Lamprey develops
in the same manner, the endothelium being derived by the splitting of the approaching walls of the splanchnopleur. The blood-corpuscles originate from the free edges
of the lateral plates of mesoblast.
 
 
218
 
 
THE STUDY OE EMBRYOLOGY.
 
 
As Balfour has shown, the heart will from the first appear as
single or double, according to the relative time of its formation.
 
In Elasmobranchs and Amphibia the throat early becomes constricted off from the yolk, and in these groups the heart appears
as a single tube in the ventral (abneural) mesentery (mesocardium) ;
but in those forms (ex. Teleosts, Birds, figs. 159, 160, and Mammals, fig. 158) in which the ventral wall of the throat is formed
later than the first appearance of the heart, the latter necessarily
develops as two tubes.
 
 
 
Fig. 160. - Transverse Section through the Cardiac Region of an Embryo Fowl
of Thirty-nine Hours. Magnified about 95 diameters. [ From Kolliker.]
 
The section passes through the point where the omphalo-mesenteric veins open into
the heart, and therefore behind fig. 159.
 
a. aortic arch ; ch. notochord ; dfp. splanchnic mesoblast ; dfp'. somatic mesoblast
of throat ; ent. epiblast of wall of throat ; h. epiblast ; h'. thickened portion of epiblast
where the auditory sacs will be formed ; hh. body-cavity of neck ; hp. somatic mesoblast; hzp. muscular wall of heart; ihh. endothelium of heart; m. neural canal; ph.
pharynx ; uhg. inferior cardiac mesentery.
 
The Fowl occupies a somewhat intermediate position, since the
extreme anterior end of its heart arises as an almost single tube ;
but it diverges posteriorly, each limb of the A thus formed being
one of the veins which bring the blood back from the yolk
(vitelline veins) (fig. 161).
 
The internal epithelium (endothelium) of the heart is single or
double like the heart itself ; but when the two tubes unite to form
the single heart, the endothelial tubes also coalesce ; but just at first
there is a median septum left (fig. 159, s), indicating where the
two tubes have joined ; this soon breaks through, and a single
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
219
 
 
tube results, the thick walls of which early become very muscular.
 
It follows from what has been stated concerning the mode of
development of the Fowl -s heart, that at an early stage an anterior
section would show the imperfect coalescence of the lateral endothelial tubes, whereas one taken a little farther behind would only
exhibit the approximation of these tubes. Thus in the same
embryo several stages in the development of the heart would be
illustrated. The anterior section would be the most advanced,
and the approaching vitelline veins would represent a much
earlier period (see figs. 159-160).
 
The primitively straight tubular heart of the Chordata undergoes
 
 
Fig. 161. - Ventral View of Embryo Fowl
at the End of the Second Day. 4.27
mm. long; removed from the blastoderm.
[From Kolliker.]
 
Ab. optic vesicle ; Ch. notochord ; H. heart ;
om. omphalo-mesenteric or vitelline vein; Vd.
indicates the backward extension of the headfold : in front of this point the pharynx is
interiorly completed ; while behind, the alimentary tract is still open below to the yolk.
 
 
 
a sigmoid flexure, at first slight (fig. 161, h), but eventually the
S-like flexure is complete (fig. 162, 3). The dorsal limb constitutes the auricular portion (atrium), and the ventral forms the
ventricular part. The dorsal and ventral portions are separated
by a constriction.
 
There is present in Fishes in connection with the atrium a posterior thin-walled sac, the sinus venosus, into which the collecting
veins (ductus Cuvieri) open, and into the single auricle. A pair of
valves guard the orifice leading to the ventricle.
 
The posterior region of the ventricular moiety becomes the
ventricle of the adult, while the anterior portion is divided into a
 
 
220
 
 
THE STUDY OF EMBRYOLOGY.
 
 
posterior conus arteriosus and an anterior bulbus arteriosus. The
conus is long, and provided with several transverse rows of valves,
except in most Teleosts, in which group it is rudimentary or absent.
The non- valvular bulbus leads to the branchial arteries.
 
Among the Dipnoi the blood-vessel bringing blood from the
air-bladder (lungs) to the heart opens into a small second (left)
auricle. The conus and ventricle may also be partially divided
 
 
in two by an imperfect longitudinal septum.
 
 
 
Fig. 162. - Development of the Mammalian Heart. [From Landois and Stirling.']
 
 
1. Heart with slight curvature. 2. Sigmoid flexure of the heart. 3. Formation of
the auricular appendages, and external furrow in the ventricle. 4. Commencing
division of the truncus arteriosus. 5. Dorsal view ; the auricle has been opened to
show the ventricular septum ; the aorta (a) and pulmonary artery open into their
respective ventricles. 6. Diagrammatic view from above of the mode in which venae
cavae open into the auricle. 7. Ventral view of heart of full-time foetus.
 
A. auricular portion of heart; a. aorta; B. ductus arteriosus Botalli; 5 . bulbus
arteriosus; c. carotid; c,c. innominate; Ci. inferior vena cava; Cs. superior vena
cava; L. left ventricle ; o. 0.1. auricular appendages; p. pulmonary artery; R. right
ventricle; s. left subclavian artery; V. ventricular portion of heart; v. auricle and
veins entering the heart ; x. arrow showing the flow of blood from the superior vena
cava through the valve into the right auricle, and y that of the inferior vena cava
through the valve into the left auricle ; 1 and 2. right and left pulmonary arteries.
 
The single auricle of the primitively piscine heart of the
Amphibia is early provided with a pair of lateral appendages,
and an. oblique septum is developed which divides the single
auricle (atrium) into a right and left chamber. The ventricle
remains single ; the conus arteriosus (pylangium) has a longitudinal valve and a row of valves at each end ; there is also a bulbus
arteriosus (synangium), which, however, is rudimentary in the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
221
 
 
Anura. The conus and bulbus are usually collectively known as
the truncus arteriosus. A sinus venosus is also present.
 
The flexure of the developing heart is very marked in the
Amniota. The auricular portion develops lateral appendages ; the
large venous trunk which opens into this region (fig. 162, 3, v) is
composed of the superior and inferior venae cavae. This common
trunk is later absorbed into the enlarging auricle, and thus arises
the separate termination of these vessels (fig. 162, 4, v). The
constriction between the auricular and ventricular divisions of the
heart is known as the canalis auricularis.
 
The heart begins to divide into a right and left half on the third
day in the Fowl and about the fourth week in Man, the division
first occurring in the ventricle. The ventricular septum arises
from the ventral wall and rapidly extends to the dorsal, dividing
 
 
Fig. 163. - Lateral View op
 
Heart of Human Embryo, y.f 3 7
the Right Side being cut
AWAY. [After His.]
 
a. aortic channel ; c.v. coronary
vein ; d. diaphragm ; l. liver ;
p. pulmonary channel ; s.a. septum aorticum in the bulbus
aorta; s.inf. septum inferius ;
s.int. septum intermedium; s.r.
sinus reuniens; s.s. septum superius ; v.c.i. vena cava inferior;
v.c.s.l. vena cava superior (left) ;
v.c.s.r. right superior cava ; v.e.
Eustachian valve.
 
 
the ventricle into two somewhat curved chambers, one more to
the left and above, the other to the right and below. Thus the
large undivided auricle communicates by a right and left auriculoventricular opening with the corresponding ventricle (fig. 162, 5).
 
A fold appears on the ventral wall of the auricle, dividing the
cavity into a right and left chamber. The fold extends only a
short distance, thus forming an incomplete septum (the auricular
septum). The right and left auricles communicate throughout
embryonic life by means of the aperture thus left, the foramen
ovale (fig. 163).
 
The vena cava inferior opens into the right auricle directly
opposite the auricular septum, and its blood has a tendency to
flow through the foramen ovale into the left auricle. The right
vena cava superior joins the vena cava inferior, and its blood also
. passes ,into the left auricle. The left vena cava superior opens
 
 
 
222
 
 
THE STUDY OF EMBRYOLOGY.
 
 
independently into the right auricle, and its blood flows into the
right ventricle.
 
A valve, the Eustachian valve, next develops from the dorsal
wall of the right auricle to the right of the entrance of the vena
cava inferior into the auricle, and between it and the right and
left superior venae cavse. This serves to still further direct the
blood from the vena cava inferior into the left auricle, and at the
same time to retain the blood of the superior venae cavae within
the right side of the heart. In many of the higher Mammals,
including Man, the right vena cava superior disappears during
foetal life.
 
A second fold arises from the dorsal wall in the median line of
the auricles ; this projects freely across, and to the left side of, the
foramen ovale, thus forming a valve which prevents the blood
from flowing back from the left to the right auricle.
 
The left auricle is at first larger than the right. Later the
cavities approximate in size, and the foramen ovale is much
smaller.
 
Lastly, the truncus arteriosus is longitudinally divided in Birds
by a septum, which arises between the fourth and fifth pair of
arches and extends in a somewhat spiral manner to close to the
ventricular orifice. In Mammals the truncus (fig. 162, 4, a.p)
appears to be constricted dorsally and ventrally to form the aorta
and pulmonary artery.
 
Semilunar valves are developed in the short interspace between
the orifice and the free end of the septum of the truncus. The
dorsal and ventral valves first appear, the former as a continuous
ridge, the latter as a pair of small processes. The septum of the
truncus extends between the latter, and, entirely dividing the
' ventricular orifice, fuses with the ventricular septum.
 
By the division of the truncus in Birds, the fifth pair of arches
communicates with the right ventricle, while the third and fourth
pairs communicate with the left ventricle; of these, the former
becomes the pulmonary arteries, and the two latter the carotid
and aortic arches respectively (p. 227). In Mammals, also, the
rio-ht ventricle is continuous with the last aortic arch, the four
anterior arches, or what remains of them, being connected with
the left ventricle.
 
In all Reptiles, except the Crocodiles, the primitively single ventricle is retained.
The ventricular septum was independently acquired by Crocodiles, Birds, and
Mampaals ; thus in these three groups it is what is termed homoplastic, but not
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
223
 
 
homogenetic. An interesting example of the “falsification of the embryological
record- is afforded, as Bell points out, by the development (ontogeny) of the
ventricles, as in those forms in which they become distinct the ventricular septum
develops prior to the auricular septum, whereas in the true phylogeny the reverse
occurred. This is a case of what Haeckel calls cenogeny, and is no doubt dependent
on the requirements of the organism.
 
The complicated series of changes undergone in the evolution of the Vertebrate
heart is apparently mainly the result of the modifications which have occurred in the
respiratory organs. Without going into details, the following facts are worthy of note :
- The respiratory tract of Amphioxus is extremely long, the “ heart - is undifferentiated, and the median ventral vessel (subintestinal vein) is contractile ; in Fishes the
pharynx is much shorter, and is increasingly reduced in the more specialised forms ;
the flexure of the heart may be related to the shortening of the neck ; the assumption of aerial respiration by the air-bladder, and a change in the origin of its afferent,
and in the destination of its efferent blood-vessels ; the necessity for the brain and
sense-organs being supplied with well-oxydised blood.
 
Development of the Vascular System in Vertebrates.  -
 
In the following brief account of the evolution of the vascular
 
 
Fig. 164. - Diagrammatic Outlines
of the Early Arterial System
of a Mammal Vertebrate Embryo. [After Allen Thomson.]
 
A. At a period corresponding to the
36th or 38th hour of incubation. B. Later
stage, with two pairs of aortic arches.
 
h. bulbus arterious of heart ; v. vitelline
arteries; 1-5. the aortic arches; the
dotted lines indicate the position of the
future arches.
 
 
 
system in Vertebrates, the plan is adopted of first describing the
development of the circulatory system in Amniota, and afterwards
that of the Ichthyopsida. A considerable number of minor points
are omitted in order to avoid unduly lengthening the section and
complicating the subject.
 
Very early in the development of the embryo the inner portion
of the area opaca (p. 38) becomes so permeated with a network
of blood-vessels as to receive the name of the area vasculosa.
This net- work soon becomes connected with the embryonic vascular system, but before this is accomplished the heart has already
commenced to beat.
 
The embryonic circulation of Amniotes may be conveniently
divided into four sections  - (1.) The early stages of the embryonic
circulation. (2.) The vitelline circulation. (3.) Later stages of
 
 
224
 
 
THE STUDY OF EMBRYOLOGY.
 
 
foetal circulation. (4.) The allantoic circulation. It is impossible
to describe the first without considering the others ; but it is important to bear in mind the essentially secondary character of the
second and fourth systems.
 
1. Early Stages of Embryonic Circulation. - It is customary
to speak of all those vessels which carry blood away from the
heart as arteries, and those which return the blood as veins. The
arterial system will be. described before the venous.
 
In its earliest stage the arterial system consists of a parallel
pair of arteries, each of which arises from the single bulbus
 
 
Fig. 165. - Diagram of the Embryonic
Vascular System.
 
[From Wiedersheim.\
 
A. atrium: A. A. dorsal aorta; Ab. branchial vessels ; Acd. caudal artery ; All. allantoic (hypogastric) arteries ; Am. vitelline arteries ; B. bulbus arteriosus ; c, c.' external and internal carotids ; D. ductus
Cuvieri (precaval veins) ; E. external iliac
arteries ; HC. posterior cardinal vein ; Ic.
common iliac arteries ; KL. gill clefts ; RA.
right and left roots of the aorta; S, S'.
branchial collecting trunks or veins; Sb.
subclavian artery ; Sb'. subclavian vein ;
Si. sinus venosus; V. ventricle; VC. anterior cardinal vein ; Vra. viteliine veins.
 
 
arteriosus of the heart, and bends round at the anterior end of the
pharynx to its dorsal side. Still remaining distinct, each aorta, as
it is termed, runs backwards along either side of the notochord
below the mesoblastic somites (fig. 176). About half-way down an
artery is given off at right angles by each aorta, which, as it passes
to the yolk-sac (area vasculosa), is called the vitelline artery
(comp. fig. 166, R.Of.A, L.Of.A).
 
Somewhat later the two aortse unite together to form a short dorsal aorta, which lies beneath the notochord. The two aortae soon
 
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
225
 
 
separate and dwindle away in the tail. The vitelline arteries arise
from each trunk behind the median fusion, and are so large that
nearly all the blood passes through them. The arteries which arise
from the heart running forwards, upwards, and backwards are known
as arches. Thus the dorsal aorta is produced by the junction of a
pair of aortic arches. Very shortly afterwards a second and a
third pair (figs. 164-167) are developed behind the primitive pair.
 
The embryonic venous system at this stage consists of an anterior
 
 
AA
 
 
 
Fig. 166.  -Diagram of the Circulation of the Yolk-Sac of the Fowl at the End
of the Third Day of Incubation. [ From Foster and Balfour . ]
 
The veins are marked in outline, and the arteries are black. The whole blastoderm
has been removed from the egg, and is supposed to be viewed from below, hence the
apparent reversal of the sides.
 
A A. the second, third, and fourth aortic arches; the first has become obliterated
in its median portion, but is continued at its proximal end as the external carotid,
and at its distal end as the internal carotid ; AO. dorsal aorta ; D.C. ductus Cuvieri ;
 
H. heart; L.of.A. left vitelline artery; L.of. left vitelline vein; R.Of. right vitelline
vein; R.of.A. right vitelline artery; S.Ca.V. superior (anterior) cardinal vein; S.T.
sinus terminalis ; S. V. sinus venosus.
 
and posterior pair of longitudinal veins (cardinal veins), which run
superficial to the aorta. The anterior (superior) cardinal or jugular
veins unite with the inferior or posterior cardinals to form a
common trunk, ductus Cuvieri (figs. 166, 169), which returns the
blood to the heart. Posteriorly the blood is collected from the
yolk-sac by the vitelline veins (fig. 166, L.Of R.Of), and transmitted to the heart by the median sinus venosus.
 
P
 
 
226
 
 
THE STUDY OF EMBKYOLOGY.
 
 
2. Vitelline Circulation.  - The vascular supply of the yolk-sac may be conveniently
described here. The area vasculosa extends to some distance round the embryo, but
it is at first undeveloped in the median line in front of the embryo ; it is thus somewhat U-shaped. When the vitelline circulation is first established, the blood enters
through the two large vitelline arteries previously mentioned. These arteries divide
and subdivide until they terminate in capillary vessels.
 
The lateral periphery of the area vasculosa is bounded by a blood-vessel, the sinus
terminalis, which also extends round the anterior horns of the area and down their
inner side. The blood from the capillaries flows in three directions : ( i ) most of it is
collected by the large vitelline veins and conveyed straight to the heart ; (2) part
flows forward along the anterior portion of the sinus terminalis, round the anterior
prolongation, and back along the inner margin of the notch, where it enters the root
of the vitelline vein ; and (3) lastly, a small quantity proceeds along the posterior
half of the sinus terminalis, and is lost in small capillaries, but it ultimately returns
by the vitelline veins.
 
When the vitelline circulation is fully developed (fig. 166), the flow of the blood
differs slightly from the condition just described. The sinus terminalis forms a
 
 
 
Fig. 167. - Diagrams of the Aortic Arches of a Mammal.
 
[From Landois and Stirling after Rathke.]
 
1. Arterial trunk with one pair of arches, and an indication where the second and
third pair will develop. 2. Ideal stage of five complete arches ; the four clefts are
shown on the left side. 3. The two anterior pairs of arches have disappeared.
 
4. Transition to the final stage.
 
A. aortic arch; ad. dorsal aorta; ax. subclavian or axillary artery ; Ce. external
carotid ; Ci. internal carotid ; dB. ductus arteriosus Botalli ; P. pulmonary artery ;
 
5. subclavian artery ; ta. truncus arteriosus ; v. vertebral artery.
 
 
complete ring round the area. The distribution of the vitelline arteries and veins
is mainly the same, but there is a slight alteration in the second and third channels
for the return of the blood. Of the anterior recurrent veins the left is always the
larger, and sometimes the right is aborted, so that the blood from the anterior region
of the area vasculosa is returned solely by the left anterior recurrent vein. Of
course in this case a fusion of the anterior limbs of the area vasculosa has occurred
in front of the embryo (fig. 76). On the junction of the lateral halves of the sinus
terminalis behind the embryo, the blood is returned by a single median posterior
recurrent vein into the left vitelline vein (fig. 1 66, L.of).
 
 
3. Later Stages of Foetal Circulation. - Five pairs of aortic
arches make their appearance (figs. 146, 165, 167, 168), hut usually
the first two have atrophied before the last is formed. The arteries
lie towards the inner side of each visceral arch ; there is one for
the mandibular, hyoid, and each of the three branchial arches.
 
The common ventral trunk (ventral aorta) is continued beyond
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
227
 
 
the mandibular arch, as the external carotid and the internal
carotid is a similar anterior extension of each dorsal aortic trunk
(figs. 167, 168, 170). After the disappearance of the first two
aortic arches, the aortic trunk connecting the dorsal end of the
third arch with the fourth disappears, except in Lizards, but a
rudiment, the ductus Botalli, can be traced in some Beptiles. In
this manner the internal and external carotids arise from the ventral
aorta of the third arch (common carotid), as shown on figs. 167, 168.
 
The fourth arch always gives rise, as in Amphibia, to the
dorsal aorta. This pair of arches persists in Beptiles ; but on the
longitudinal division of the truncus arteriosus, the channel leading
from the left side of the ventricle is continuous with the right
 
 
Fig. 168. - Diagram Illustrating
the Transformations of the
Aortic Arches in a Lizard, A ;
a Snake, B ; a Bird, C ; a Mammal, D. Seen from below. [ After
Itathke .]
 
a. internal carotid; b. external
carotid; c. common carotid. A. -d.
ductus Botalli between the third and
fourth arches; e. right aortic arch;
/. subclavian ; g. dorsal aorta ; h. left
aortic arch; i. pulmonary artery;
k. rudiment of the ductus Botalli
between the pulmonary artery and
the aortic arches. B. - d. right aortic
arch ; e. vertebral artery ; /. left aortic
arch ; h. pulmonary artery ; i. ductus
Botalli of the latter. C. - d. origin
of aorta ; e. fourth arch of the right
side (root of dorsal aorta) ; /. right
subclavian ; g. dorsal aorta ; h. left
subclavian (fourth arch of the left
side) ; i. pulmonary artery ; k. and l.
right and left ductus Botalli of the
pulmonary arteries. D. - d. origin of
aorta ; e. fourth arch of the left side
(root of dorsal aorta) ; /. dorsal aorta ;
g. left vertebral artery; h. left subclavian ; i. right subclavian (fourth
arch of the right side) ; k. right vertebral artery ; l. continuation of the
right subclavian; m. pulmonary
artery ; n. ductus Botalli of the latter
(usually termed ductus arteriosus).
 
 
 
 
 
fourth arch (fig. 168, A, e; B, d ), and from it also arise the
carotids (c). The left fourth arch (a, h; B ,/), is connected with
the right side of the ventricle, but it unites with its fellow to form
the dorsal aorta (g).
 
In Birds the right (fourth) aortic arch alone retains its connection with the aorta, the left arch persists as the left subclavian
artery (fig. 168, C, h). The reverse occurs in Mammals (fig. 1 68, d ).
In both there is a single aortic arch, which springs from the left
side of the ventricle.
 
The fifth arch is known as the pulmonary, as it invariably
supplies the lungs ; it arises from the right side of the ventricle.
In all the Sauropsida the right and left arches persist as the right
 
 
228
 
 
THE STUDY OF EMBRYOLOGY.
 
 
and left pulmonary arteries respectively, except in Snakes, in
which the left alone persists (fig. 168, B, h). In Mammals the
left arch disappears, and the right goes to the lungs (fig. 168, D, m).
 
In some forms traces of the communication between the fourth
and fifth arches may remain as the ductus Botalli. A comparison
of' figs. 164-168 will render the development of the adult from the
embryonic condition perfectly comprehensible.
 
The development of the main venous trunks, as it occurs in
Birds, will be briefly described as a standard of comparison with
other Amniota.
 
As the embryo increases in size new veins appear, and an
anterior (superior) vertebral vein, bringing back blood from the
head and neck, and a subclavian from the wing (fig. 169, A, s.c)
open into the anterior cardinal or jugular vein. The two ductus
Cuvieri persist as the superior venae cavae. In the lower Mammals
there are two superior venae cavae, as in Sauropsida, but more
usually an anastomosis (left brachio-cephalic or innominate vein)
is developed between the right and left jugular veins (fig. 172),
and eventually the whole of the blood of the left superior vena
cava is conveyed to the right side. The base of the left superior
vena cava remains as the coronary sinus.
 
The posterior or inferior cardinal veins which pass along the
outer border of the kidneys unite behind with the caudal veins,
and anteriorly they open into the ductus Cuvieri. The intercostal veins begin to be connected with a new longitudinal trunk
(posterior vertebral vein), which is continuous with the anterior
vertebral, and gradually lose their connection with the posterior cardinals. Owing to their diminished function, the anterior portions
of the posterior cardinals disappear ; their posterior moieties become the venae renales advehentes. As a result of this change,
the blood from each side of the wall of the body of the embryo,
instead of entering the heart through the posterior cardinal, is collected by the posterior vertebral, and, together with the anterior
vertebral, passes into the jugular and the ductus Cuvieri (superior
vena cava or precaval) (fig. 169, B, c).
 
The two posterior vertebrals are at first symmetrical, but in
Beptiles, when transverse anastomoses develop between them, the
right becomes the larger. In Mammals (fig. 171) the left posterior
vertebral usually becomes rudimentary, and is known as the
hemiazygos vein ; it is connected by a transverse anastomosis with
the right posterior vertebral or azygos vein.
 
 
ORGANS DERIVED FROM THE MESOBLAST. 22D
 
While these changes have been going on, a new and important
vein, the vena cava inferior, has made its appearance. At first it
is a small vein arising in two roots from the inner border of the
kidneys, and unites with the allantoic vein (to be described shortly)
before it enters the heart. The atrophy of the anterior portion of
the posterior cardinals is doubtless due to the newly developed
vena cava inferior carrying the venous blood of the kidney direct
to the heart. On its way to the heart the vena cava inferior passes
through the liver, from which it receives a few vessels, venae
reheventes (fig. 169, b).
 
Renal Portal System.  - In Reptiles the blood from the caudal
veins and the posterior portion of the posterior cardinal veins
(venae renales advehentes) is broken up into capillaries in the
 
 
 
Fig. 169. - Diagram of Three Stages in the Development of the Venous
Circulation of the Fowl. [ After Balfour .]
 
A. At the commencement of the fifth day. B. During the later days of incubation.
 
C. At the commencement of respiration by means of the lungs.
 
all. allantoic (anterior abdominal) vein; a.v. anterior (superior) vertebral vein; cr.
crural veins; c.v. caudal vein; cy.m. coccygeo-mesenteric vein ; d.C. ductus Cuvieri;
d.v , ductus venosus; h. heart; hy. hypogastric veins; h.v. hepatic vein; i.v. inferior
vertebral vein ; j. jugular vein (superior or anterior cardinal) ; k. kidney ; l. liver ; m.
mesenteric vein; p.c. posterior (inferior) cardinal vein; pul. pulmonary vein; p.v.
portal vein; s.c. subclavian vein; v. vitelline vein ; v.c.i. vena cava inferior ; v.x.r.
right superior vena cava. The ductus venosus passes through the liver in A and B.
 
kidneys, and passes thence to the heart by the vena cava inferior.
This is known as the renal portal system.
 
In Birds and Mammals this does not occur ; the blood from the
tail and hind-limbs passes directly into the vena cava inferior, and
not indirectly through the kidneys. This comes about in Mammals
by the development of the common iliac veins, which collect the
blood from the hind-quarters; the posterior portion of the cardinal
veins enter the common iliac as the hypogastric (fig. 169, 0, hy).
 
Hepatic Portal System.  - As has already been described, the
blood from the yolk-sac is conveyed by the vitelline veins direct
to the heart. A small vein early appears in connection with
 
 
230
 
 
THE STUDY OF EMBRYOLOGY.
 
 
developing mesenteron. This mesenteric vein (fig. 169, B and c, m)
joins the vitelline vein ; their common trunk (ductus venosus or
omphalo-mesenteric trunk) becomes enveloped within the rapidly
growing liver, and sends off branches into that viscus. As these
branches increase in size they convey more and more blood, and
the ductus venosus, which originally passed directly into the heart,
is proportionately diminished, until eventually all the blood from
the yolk-sac and mesentery passes into the hepatic branches, venae
advehentes, and is collected by the vense reheventes and transmitted
to the vena cava inferior. There is nothing remarkable in the
association of the vitelline and mesenteric veins, as it has been
already shown that the yolk-sac is practically merely the hypertrophied ventral wall of the mesenteron, consequent upon the
occurrence of food-yolk. It may be stated in another way by
saying that the vessels from the digestive tract break up in the
liver into capillaries before entering the heart.
 
In Birds and Mammals the right vitelline vein soon disappears.
 
4. Allantoic Circulation.  - There is in Amphibia a vein, anterior
abdominal, which receives blood from the hind-limbs and from
the urocyst (bladder), and passing along the median ventral wall
of the abdomen it enters the liver.
 
There are in Beptiles, as in Anura (p. 234), at first two
anterior abdominal veins developed. These run along the anterior
abdominal wall and enter the ductus Cuvieri; posteriorly they
are connected with the system of the posterior cardinal by the
epigastric veins, and also with the bladder. On account of the
precocious development of the bladder to form the allantois, these
veins are known as allantoic veins. The left disappears, so that a
single allantoic vein enters the heart after having been joined by
the inferior vena cava. Later the two unite nearer the liver, and
finally the anterior abdominal (allantoic) vein joins the portal
system.
 
In Birds the two anterior abdominal veins unite and fall into
the ductus venosus (fig. 169, b) ; the single stem comes to be very
long, owing to the rapid growth of the allantois, and it forms the
allantoic vein. The right anterior abdominal disappears ; the left
bifurcates on reaching the allantois (fig. 169, b, all).
 
The vitelline veins are at first very large (fig. 166), and the
allantoic vein quite small, but their relative size is reversed as the
allantois increases and the yolk-sac diminishes in importance.
The mesenteric vein joins these two, and thus the large portal
 
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
231
 
 
vein is formed. Although the allantoic vein disappears before
hatching, the caudal and posterior pelvic veins are connected with
the portal vein in the adult by the coccvgeo-mesenteric vein (fig.
169, C, cy, m).
 
In Mammals the two primitive anterior abdominal (allantoic)
veins are very early developed, and unite in front with the vitelline
vein. The right allantoic vein (fig. 171, b, u), like the right
vitelline vein (o'), soon disappears. The long common trunk of
 
 
Ci
S
 
 
Fig. 170. - Diagram of the Arrangement of the Principal
Vessels in a Human Foetus.
 
[From Claus after Fcker.]
 
Ao. aortic trunk; Am. amnion;
Aod. dorsal aorta; Az. azygos vein ;
C. anterior cardinal vein ; Cc. common carotid; Ce. external carotid;
Ci. internal carotid ; X>. ductus venosus Arantii ; DC. ductus Cuvieri ;
Dv. vitelline duct ; H. ventricle ; L.
liver ; N. umbilical vesicle (yolk-sac) ;
O. vitelline (omphalomesenteric) ar
tery ; O -, vitelline vein ; P. lung ;
S. subclavian artery; U. allantoic
(umbilical) arteries with their placental ramifications, U"; TJ -. allantoic
vein; V. auricle; V.c. vena cava
inferior; Vp. portal vein ; i, 2, 3, 4, 5.
the arterial arches - the persistent
left aortic arch is not visible.
 
 
O'
 
the (left) allantoic and vitelline veins (ductus venosus) passes
through the liver.
 
In its passage through the liver, according to Kolliker, the
ductus venosus gives off branches near its entrance, and receives
branches from the anterior end of the liver (fig. 171, b). The
main duct, unlike what occurs in the Sauropsida, persists throughout life as the ductus venosus Arantii (fig. .171, D , 1 ).
 
When the placenta is developed, the allantoic circulation becomes extremely important. The vitelline vein, on the other hand,
is greatly reduced, and, with the larger mesenteric vein, it constitutes the portal vein. Later the portal vein (fig. 17 1, D, p)
enters one of the venae advehentes of the allantoic vein (p').
 
The vena cava inferior and the ductus venosus at first unite
together and enter the heart by a common trunk (fig. 17 1, A, ci, l)
Owing to the increased size of the former, the venae reheventes or
hepatic veins open into it, and not into the ductus venosus. The
 
 
 
232
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ductus venosus itself (ductus venosus Arantii) comes to be a small
branch of the vena cava (fig. 171, d).
 
The allantoic vein degenerates at the end of foetal life into the
solid cord known as the round ligament, and all the venous supply
of the liver comes from the portal vein.
 
 
Beddard finds that there is in the adult Echidna an anterior abdominal (allantoic)
vein, which arises from the under surface of the bladder, and passing along the ventral
wall of the body, is distributed to the left half of the liver.
 
 
An anastomosis between the iliac and portal veins is not established in Mammals.
 
The allantoic arteries arise from the dorsal aorta as branches of
 
 
 
 
 
 
Fig. 171 - Diagrams Illustrating the Development oe the Great Veins in Mammals.
[From Quain after Kiilliker.]
 
A. Plan of the principal veins of the human embiro
of about four weeks, or soon after the first formation
of the vessels of the liver and the vena cava inferior.
 
B. Hepatic circulation at a somewhat earlier stage.
 
C. Principal veins of the foetus at the time of the first
establishment of the placental circulation. D. Hepatic
circulation at the same period.
 
az. azygos vein, above p (in C) - the oblique line is the
vein by which the hemiazygos joins the azygos vein ;
ca. posterior cardinal veins; ca'. (inC) the remains of
the left cardinal vein by which the superior intercostal
veins fall into the left innominate vein ; cr. external
iliac or crural veins ; ci. vena cava inferior ; dc. ductus
Cuvieri ; h. hypogastric or internal iliac veins, in the
line of continuation of the primitive cardinal veins;
il. the division of the vena cava inferior into the
common iliac veins ; j. jugular or anterior cardinal
veins ; l. ductus venosus ; V. hepatic veins ; li. (in C) in
dotted lines, the transverse branch of communication
between the jugular vein which forms the left innominate vein; m. mesenteric veins ; o. vitelline or omphalomesenteric vein ; </. right vitelline vein ; p. portal vein ;
p'p'. venae advehentes ; ri. right innominate vein ; s. subclavian vein ; u. allantoic, umbilical or (left) anterior
abdominal vein ; u\ (in B) the temporary right allantoic
vein.
 
 
the common iliac arteries (figs. 165, Ic , 17 1, U). On the disappearance of the allantois they remain as the hypogastric arteries.
 
Circulation in Ichthyopsida.  - Having now described the
development of the circulation in the Amniota, it will be necessary to briefly refer to the circulation in Ichthyopsida.
 
Fig. 165, which represents the embryonic circulation of an
Amniote in a diagrammatic manner, will, with a few alterations,
serve to illustrate the circulation in Fishes. The vitelline arteries
{Am) and the allantoic arteries {All) are not present, and the
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
233
 
 
blood from behind is returned to the heart by the subintestinal
vein, and not by the vitelline veins (Vm).
 
The first or mandibular arterial arch is represented by a small
vessel which arises from the branchial vein of the hyoid arch, and
supplies the rudimentary gill (pseudobranch) of the spiracle. In
other Fish this artery of the first arch disappears.
 
The second or hyoid arterial arch is functional throughout life
in Elasmobranchs ; usually it remains as a small vessel which goes
to the pseudobranch of the hyoid. The artery is said to persist
in Protopterus amongst the Dipnoi.
 
The air-bladder is supplied with arterial blood from the caeliac
artery or direct from the aorta, except in some Ganoids (Polypterus
 
I.
 
 
Fig. 172. - Venous Circulation
in Mammalian Embryo.
[From Landois and Stirling .]
 
I. Early arrangement of veins.
II. Final disposition.
 
Ad. right innominate vein; ,4s.
left innominate vein; Az. azygos
vein ; b. subclavian veins ; Ci. vena
cava inferior ; ci. posterior vertebral
veins; Cs. vena cava inferior; cs.
anterior cardinal vein ; DC. ductus
Cuvieri (superior venae cavge) ; /.
external iliac vein; h. hypogastric
vein ; Hz. hemiazygos vein ; Ie. external jugular vein ; Ji. internal
jugular vein; om. vitelline or omphalo-mesenteric vein ; U. umbilical
or allantoic vein ; V. ventricle ; Vc.
vena cava inferior.
 
 
and Amia) and Dipnoids, where the last branchial arch sends an
artery direct to the air-bladder.
 
In Amphibia the first aortic arch (mandibular) is never developed, and the second (hyoid) arises later than the succeeding
arches ; it never unites dorsally with the latter, and only gives
rise in part to the lingual artery.
 
Of the four branchial aortic arches present in larval Amphibia,
only the second, in the Anura, retains its connection with the
dorsal aorta. The first becomes the carotid arch, and gives rise
to the carotids ; the second forms the systemic arch ; the third is
rudimentary or absent (Anura) in the adult, while the fourth or
pulmonary supplies the lungs. A narrow anastomosis or ductus
 
 
 
234
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Botalli may unite the second, third, and fourth arches in adult
Urodeles.
 
The venous system of Fishes primitively consists of a median
unpaired subintestinal vessel extending from the end of the tail
to the heart ; indeed, the heart may be considered as a specialised
portion of this vessel. Later, cardinal veins are developed, as in
Embryonic Amniotes, but in Fishes they persist as the main
venous trunks. The caudal portion of the subintestinal vessel
acquires a secondary connection with the posterior cardinal veins.
In some cases this, its anastomosis, breaks up into capillaries in
the mesonephros, thus forming a renal portal system.
 
After the appearance of the cardinal veins the main portion of
the subintestinal vein disappears, but a remnant of one of its
branches occurs in some Elasmobranchs as the vein of the spiral
valve, and it also leaves its trace in the hepatic portal system.
 
A branch from the subintestinal goes to the yolk-sac, and the
common trunk is imbedded in the developing liver. Later, vessels
from the alimentary viscera are developed, which break up in the
liver. The hepatic veins convey blood from the liver to the sinus
venosus of the heart.
 
In some Fishes vessels from the anterior abdominal wall enter
into the portal circulation. These may be regarded as the forerunners of the paired anterior abdominal veins.
 
The ductus venosus and the caudal vein may be regarded as the
representatives of the subintestinal vein in Amniota.
 
In Fishes the air-bladder ranks as an ordinary viscus of the
mesenteric series, as its blood enters into the hepatic portal system
before being returned to the heart ; the only exception occurring is
in the Dipnoi, where the pulmonary vein, as it may now be called,
carries the blood direct to the left auricle. The same obtains in
Amphibia.
 
The Amphibia initiate a new departure in the development of
a vena cava inferior, which functionally replaces the larval posterior
cardinal veins. The hepatic veins enter into the vena cava inferior.
On the disappearance of the posterior cardinals the ductus Cuvieri
(superior venae cavae) are connected only with the anterior cardinals
(jugular veins).
 
At first two anterior abdominal veins occur, and open anteriorly
into the sinus venosus, having previously united with a vein from
the truncus arteriosus. An epigastric branch from the iliac vein
and veins from the urocyst or bladder (allantois) join them, after
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
235
 
 
which they unite into a single vessel. The atrophy of the right
vein is said to result in a single anterior abdominal vein. A
secondary connection occurs between the anterior abdominal and
the portal system, which persists in the adult.
 
In other respects the Amphibia are essentially piscine in their
vascular system.
 
Summary of the History of the Aortic Arches.  - As there is still some uncertainty
concerning the* fate of some of the aortic arches in the various groups of Vertebrates,
it may not be superfluous to briefly recapitulate the facts as at present known. In
this summary, as in the foregoing account, the view is adopted which is most generally current, viz., that there is one prehyoid aortic arch, usually termed the mandibular or first aortic arch, the hyoid is the second, while in most Fishes there are four
branchial aortic arches. Dohrn terms the aortic arch immediately in front of the
hyoid the arteria thyreoidea mandibularis, or shortly the thyroid artery (the mandibular of Balfour), which, in Elasmobranchs, after receiving a venous commissure from
the hyoid arch, is called the spiracular artery, as it supplies the spiracle.
 
First aortic arch (mandibular?), present in all embryonic Vertebrata, except the
Amphibia, only persisting in Elasmobranchii, and that imperfectly, as the
spiracular artery.
 
Second aortic arch (hyoid), present in all embryonic Vertebrata, but imperfect in
larval Amphibia. Persistent in Elasmobranchii, usually so in Ganoidei, rudimentary in Teleostei (as artery of pseudobranch), may disappear in some
Dipnoi, and partially persists as the lingual artery in Amphibia and Amniota.
 
Third aortic arch (first branchial), present in all larval forms, and persists as a
complete arch in all Fishes. In adult Amphibia and Amniota it loses its connection with the other arches and. gives rise to the common carotid trunks.
 
Fourth aortic arch (second branchial), retains its connection with the dorsal aorta
throughout the Vertebrate series.
 
Fifth aortic arch (third branchial), persists in all adult Fishes, and to a diminished
degree in adult Urodela (still uniting with the dorsal aorta), is lost during the
metamorphosis of Anura [Boas], and disappears in the Amniota.
 
Sixth aortic arch (fourth branchial), persists throughout the Vertebrate series. In
some Ganoidei (Polypterus, Amia) and Dipnoi also giving a branch to the airbladder, and in adult Amphibia and Amniota supplying the lungs. In adult
Urodela alone is a connection still left with the dorsal aorta, and in Anura
a large cutaneous branch is given off.
 
It is usually stated that the pulmonary artery of Anura and Amniota is the third
branchial aortic arch, and that the fourth disappears. The subject requires reinvestigation, as probably there is a fusion of these two arches, both of them losing their
connection with the dorsal aorta, but the fourth branchial still giving origin to the
pulmonary artery. Boas has shown that this is actually the case in the young Frog,
and in Salamandra the third branchial arch has the appearance of a diminishing
artery. It is, moreover, very improbable that the arterial supply of the lungs should
shift from the last arch to the one in front of it. If this be admitted, the term “ fifth
aortic arch - in the above description of the development of the arterial arches in
Amniota must be understood as implying fifth + sixth aortic arch, making seven
arches in all.
 
Changes Undergone in the Circulation of Foetal Mammals. - The earliest phases
in the circulation have already been described. Later all the venous blood passes
directly into the right auricle. The venous blood from the head and upper portion
of the body is returned by the two venae cavae superiores (innominate veins). In
most Mammals the proximal portion of the left superior vena cava atrophies ; so all
 
 
236
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the blood from the right and left sides of the anterior region of the body comes t6 be
returned by the single (right) superior vena cava.
 
The primitive posterior cardinal veins, and later the posterior vertebrals (azygos
and hemiazygos), convey blood from the latero-dorsal walls of the trunk to the
superior vena cava. The venous blood of the cardiac circulation passes by the
coronary vein into the right auricle.
 
The main portion of the blood from the hinder region of the body is brought back
by the vena cava inferior, which is by this time rapidly rising into importance. The
decreasing blood from the yolk-sac and the gradually increasing mesenteric venous
blood passes by the portal vein into the allantoic vein (here known as the ductus
venosus), which passes straight through the liver and enters the right auricle along
with the vena cava inferior. At its entrance into the liver the ductus venosus gives
rise to a few veins (vense advehentes), and receives again a small number of veins
(venae reheventes) before leaving that viscus. The liver is also supplied with arterial
blood by a branch (hepatic artery) from the dorsal aorta. As the vena cava inferior
increases in size the hepatic veins (venae reheventes) open into it.
 
The blood of the superior vena cava passes ventral and to the right side of the
Eustachian valve, and, together with a small quantity of blood from the inferior
vena cava, passes into the right ventricle, and thence along the pulmonary artery
to the lungs. During foetal life the latter are not distended ; consequently only
a very small quantity of blood is concerned in the pulmonary circulation : this
is returned by the pulmonary veins to the left auricle. The remaining blood passes
through the wide ductus arteriosus (Botalli) (figs. 167, 168) into the dorsal aorta, just
beyond the spot where the carotid and subclavian arteries arise.
 
Only a small portion of the blood returned by the vena cava inferior passes into
the right ventricle ; by far the greater portion is diverted 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, passes into the
left ventricle, then it passes along the ascending arch of the aorta (fourth aortic
arch of the left side), and is mainly distributed to the head and fore-part of the body
by the carotid and subclavian arteries. A small quantity probably passes along with
the blood from the ductus arteriosus down the descending or dorsal aorta.
 
To recapitulate, and omitting minor details :  - The blood from the anterior region
of the body enters the right auricle by the superior vena cava, thence to the right
ventricle and pulmonary artery. A small quantity passes to the lungs and back to
the right auricle (pulmonary circulation) ; the greater portion flows through the
ductus arteriosus to the dorsal aorta, and thence to the posterior region of the body.
This blood is returned by the vena cava inferior to the right auricle, where it is
diverted by the Eustachian valve to the left auricle, and, entering the left ventricle,
passes by the aortic arch to the anterior region of the body.
 
It will be evident from the above that the blood returned by the allantoic veins is
distributed to the anterior region of the body after passing through the liver. Thus
the large developing brain is supplied with the most nutritious and aerated blood
available, while the grosser organs have distributed to them the blood which has
already circulated through the anterior region of the body. A large portion of the
blood from the dorsal aorta passes into the allantoic (placental) circulation, and
becomes partially purified in the placental villi by diffusion of gases with the maternal
blood. In the embryo, as in the adult, it is the right ventricle which pumps the
blood into the respiratory organ ( i.e ., placenta or lungs).
 
During the later portion of intra-uterine existence, the blood returned by the inferior vena cava increasingly mixes with that of the superior vena cava, and a gradual
approach to the adult arrangement is observable.
 
The rapid dilatation of the lungs and the loss of the placenta at birth result in a
considerable modification in the circulation.
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
237
 
 
The vessels of the distended lungs become filled with a large quantity of blood,
which, being returned into the left auricle, equalises the pressure of the blood on
each side of the auricular septum, and no blood passes from one auricle into the other.
The free fold of the foramen ovale gradually becomes fused with the margin of the
foramen, and thus permanently completes the septum. As was previously mentioned,
this valvular fold of the auricular septum was so arranged that, even during foetal
life, blood could only flow from the right into the left auricle. A larger or smaller
portion of the foramen ovale may remain unclosed for a long period, or even throughout life.
 
The ductus arteriosus rapidly diminishes in size, and normally entirely disappears ;
the same fate also befalls the allantoic (umbilical) arteries. The allantoic (umbilical)
vein is obliterated as far as its entrance into the liver, and the ductus venosus disappears within that organ.
 
 
Excretory Organs.  - An excretory organ consists essentially of
a tube or duct which leads from the interior of the animal to the
exterior ; such a tube is termed a nephridium.
 
The internal orifice of a nephridium opens into the archicoel
(Platyhelminths) or coelom (Coelomata) ; in the latter case the
special part of the coelom into which it enters may be more or less
separated from the general body-cavity; thus the nephridium may
open into the pericardium or into a Malpighian body (Vertebrates).
The orifice itself (nephrostome) may be funnel-shaped and richly
ciliated, the cilia working outwards ; or there may be a single long
cilium, which has a screw-like action, lying within the nephrostome
(Platyhelminths, Rotifers).
 
The tube itself may be straight, bent upon itself, or coiled.
Each tube may either open independently to the exterior (Invertebrates), or the nephridia of each side may communicate with a
common duct which opens posteriorly (Vertebrates).
 
A pair of nephridia only may be present (Platyhelminths,
Rotifers, Nematodes, Gephyrea, Polyzoa, Brachiopoda, Mollusca),
or numerous pairs may occur, in which case there may be a single
pair (most Chsetopoda, a few Vertebrata) or several pairs of nephridia for each segment of the body in which they occur.
 
In addition to carrying away nitrogenous waste, the nephridia, or
some of them, may also act as the efferent ducts of the generative
organs (Brachiopods and some Chsetopods, Molluscs, and vasa
efferent] - a of Vertebrates).
 
Invertebrates.  - Our knowledge of the development of the
excretory organs of Invertebrates is in a very unsatisfactory
condition.
 
The excretory system of Platyhelminths and Rotifers consists
in the main of a pair of lateral longitudinal vessels, from which
 
 
238
 
 
THE STUDY OF EMBRYOLOGY.
 
 
numerous fine branches arise which open into the interstices of
the spongy mesenchyme (archicoel), into the blood-vessels in some
Nemerteans (according to Oudemans), or into the “ body-cavity -
of Rotifers. The longitudinal trunks may open anteriorly or
posteriorly either independently or by a common orifice ; in the
latter case the conjoint vessels may expand into a contractile
vesicle. In Nemerteans the nephridial canals communicate with
the exterior by one or numerous ducts, which are always situated
above the nerve-trunks [Oudemans].
 
The only observations on the development of this system are
those of Hubrecht -s for the Nemertean Worm (Lineus obscurus).
He finds that a pair of vesicular outgrowths arise from the hypoblastic oesophagus ; although their further development could not
be traced, he believes they are the rudiments of the nephridia.
 
The paired segmental organs or nephridia of Chsetopods appear
to be developed from the peritoneal epithelium of the body-cavity,
either on the posterior wall of the transverse septa or on the bodywall. The external opening is secondarily acquired.
 
There are several forms of excretory organs amongst Arthropods.
Peripatus possesses segmental organs similar to those of Annelids,
except that, from Balfour -s account, they appear to be devoid of
cilia. The Amphipod Crustacea have hypoblastic intestinal cseca
(pp. 169, 186), while the Insects have epiblastic rectal Malpighian
tubules (p. ill). The excretory organs of the Decapod Crustacea
are the green glands situated in the basal joint of the antennae, the
outer chamber of whic^ appears to be developed as an epiblastic
invagination. The so-called shell-glands of Crustacea may also
be excretory organs.
 
Provisional renal organs are developed in the embryos of most
of the groups of Odontophorous Mollusca. A pair of V-shaped
tubes, with an internal opening into the cavity of the head and
an external orifice on the ventral surface behind the mouth, is
present in the aquatic Pulmonata, and possibly in some other
forms. Rabl and Hatschek ascribe to them a mesoblastic origin,
but Fol states that they arise as epiblastic invaginations. Certain
epiblastic larval excretory organs have already been described
( P . 108).
 
The adult renal organ (organ of Bojanus) has been variously
described to have an epiblastic and a mesoblastic origin. Rabl
states that in Planorbis a mass of mesoblast cells appears near the
end of the intestine, which, becoming vesicular, attaches itself to
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
239
 
 
the epiblast to the left of the anus, and acquires an external
opening. The internal pericardial orifice does not appear to be
acquired till after the formation of the heart.
 
Chordata.  - No distinct urinary organs occur in AscidiaUs, unless the neural gland has this function.
 
Hatschek has recently discovered a true kidney in Amphioxus,
which has the structure and development of a nephridium. It
develops in the larva as a mesodermal ciliated funnel and canal on
the left side only of the mouth in the region of the first somite.
In the adult the nephridium lies in a narrow portion of the bodycavity, near the ventral body of the notochord, overlying the left
carotid (which is a continuation of the left aorta). It appears to
open into the pharynx.
 
The Vertebrate excretory system consists of three parts  - (i.)
Head-kidney or Pronephros; (2.) Wolffian body or Mesonephros;
(3.) Kidney proper or Metanephros. These three portions are
never functional at the same time, and are to be regarded as
differentiations of the primitive kidney which have occurred in
the evolution of the Vertebrates.
 
1. Pronephros.  - The first part of the excretory system to
develop is the duct  - variously termed segmental duct, pronephric
duct, the duct of the primitive kidney, or archinephric duct. The
pronephros, when present, is always connected with the anterior
extremity of this duct.
 
The following is the generally received account of the development of the segmental duct, but the duct has been shown by several
investigators to have an ejoiblastic origin (fig. 178*, s.d). The significance of this will be shortly pointed out (p. 249).
 
I11 the Amphibia the segmental duct appears as a groove
(fig. 173, A, s.d) along the outer angle of the dorsal region of the
body- cavity, which commences just behind the branchial region.
The groove is continuously constricted off from before backwards
so as to form a canal or duct ; except anteriorly, where the constriction only takes place at intervals, leaving two (Urodela), three
(Anura), or four (Ceecilia) openings. The short tubes connecting
these openings or nephrostomata with the segmental duct increase
in length and form the segmental tubes, which correspond in
number with the segments which the pronephros occupies.
 
The duct immediately behind these tubules becomes coiled.
A vascular process from the peritoneum, the glomerulus, projects
on each side of the aorta into a dilated section of the body-cavity,
 
 
240
 
 
THE STUDY OF EMBRYOLOGY.
 
 
which becomes partially cut off from the rest of the coelom (fig!
173, b). The whole of these structures collectively constitutes the
pronephros.
 
The segmental duct eventually opens posteriorly into the cloaca.
 
The pronephros develops in the Teleostei in a similar manner,
except that there is only one anterior opening (nephrostome) ; and
the part of the body-cavity into which it opens, and in which the
glomerulus lies, becomes completely constricted off, so as to form
what is practically an enormous Malpighian body (fig. 173, c).
 
 
 
A. Transverse section through a very young Tadpole of a Toad (Bombinator) at the
middle of the body. [After Giitte .] B. Diagram illustrating the partial isolation of
the glomerulus within a pouch of the body-cavity. C. Transverse section through the
pronephros of a Trout ten days before hatching. [After Balfour .] D. Diagram of the
pronephros of Lepidosteus. [After Balfour and Parker .]
ao. dorsal aorta; b.c. body-cavity; ep. two-layered epiblast ; /. peritoneal funnel;
gl. glomerulus ; m. mesenteron ; rn.p. muscle-plate ; n. neural tube ; ncli. notochord ;
n.s. nephrostome; p.o. opening of pronephric tubule into the isolated portion of the
body-cavity : s.d. segmental duct ; so.rn. somatic, and sp.m. splanchnic, mesoblast ;
t. pronephric tubule ; x. subnotochordal rod ; y.hy. yolk hypoblast.
 
The same arrangement occurs in young larvse of the Ganoid
Lepidosteus (Balfour and Parker), except that a tubular communication with the body-cavity (fig. 173, d) is retained for some time.
 
 
It is usually stated that in some Teleosts the head-kidney (pronephros) is the only
excretory organ of the adult ; in most it occurs together with the Wolffian body
(mesonophros), and in a few it disappears altogether. Balfour, however, has shown
that in certain typical forms (and therefore probably in all) the pronephros, when it
persists, loses its excretory function and degenerates into a lymphatic gland. In
those specialised Teleosts {e.g., Lophius) in which the pronephros only is supposed to
occur, the mesonephros has probably been mistaken for that organ. Weldon suggests
that the head-kidney of Teleosts may be regarded as a suprarenal body.
 
 
The pronephros occurs in all the Ichthyopsida, except the
Elasmobranchii, but only functions during a period intervening
between hatching and the attainment of full maturity ; in other
words, the pronephros is always a larval organ, and never con
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
241
 
 
statutes an active part of the excretory system in the adult state.
It is either absent or imperfectly developed in those types
(Elasmobranchii and Amniota), which undergo the greater part
of their development within the egg or before birth.
 
In the Elasmobranchs the segmental duct arises anteriorly as a
solid ridge of cells from the somatic layer of the intermediate cellmass. Erom this ridge a solid column of cells grows back to the
cloaca without coming into contact with any neighbouring structures. A central cavity or lumen soon appears, and the duct
opens widely into the body-cavity anteriorly.
 
The development of the pronephros has been most carefully
studied in the Fowl. In this form the segmental duct arises as a
solid ridge from the parietal mesoderm, just ventral to the muscleplates. The ridge, which extends to five segments, is constricted
 
 
 
Fig. 174. - Diagrams Illustrating the Development oe the Pronephros
in the Fowl.
 
ao. aorta ; b.c. body-cavity ; ep. epiblast with its epitrichial layer ; hy. hypoblast ;
m.s. mesoblastic somite; n.c. neural canal; nch. notochord; p.t. pronephric tubule;
so. somatic, and sp. splanchnic, mesoblast.
 
off at intervals from the intermediate cell-mass, but remains
attached at certain points. The duct grows backwards as in
Elasmobranchs. The further history of this duct will be described
later. The pronephros extends in the Fowl over the seventh to
the eleventh segments inclusive, the most anterior mesoblastic
somite behind the auditory involution being counted as the first.
 
As the pronephros is the first part of the excretory system to be
developed, and often is the sole excretory organ for a considerable
period, it is usually concluded that it and its duct (the segmental
duct) are the most primitive parts of the vertebrate excretory
system. The mode of its development in the Amphibia may also
be regarded as primitive, especially since Shipley has shown that
the anterior portion of the pronephros of the Lamprey develops in
a similar manner.
 
2. Mesonephros.  - The Wolffian body or mesonephros is largely
 
Q
 
 
242
 
 
THE STUDY OF EMBRYOLOGY.
 
 
developed in all Vertebrates, but it does not persist as an excretory
organ in adult Amniota.
 
The mesonephros consists of a number of serially arranged
primary tubules, segmental or Wolffian tubules, which may be
segmentally arranged (Elasmobranchs, some Amphibia, and at
first in Reptiles), but usually a variable number of tubules are
formed in each segment. Each tubule opens on the one hand into
the segmental duct, and on the other into a Malpighian body.
The latter sometimes (Elasmobranchs and Amphibia) communicates with the body-cavity by a short tube (peritoneal funnel).
In addition to the primary tubules there may be an inconstant
number of dorsally placed secondary, tertiary, &c., tubules, which
correspond with and are developed from the primary tubules.
 
 
Fig. 175. - Transverse Section
 
THROUGH THE TRUNK OF A
 
Young Embryo Elasmobranch
 
(Scyllium). [ From, Balfour .]
 
ao. dorsal aorta ; ch. notochord ; mp.
somatic, and mp'. splanchnic, layer of
muscle-plate; p.o. primitive germinal
cells ; pr. dorsal root of spinal nerve ;
sd. segmental duct ; sp.c. neural canal ;
sp.v. spiral valve of intestine ; v. subintestinal vein : vr. rudiment of vertebral body ; W. white matter of spinal
cord; x. subnotochordal rod.
 
 
These dorsal secondary tubules resemble in their structure the
primary tubules, and usually open into the latter just before they
enter the segmental duct. In the larval Amphibia only, the
secondary and other tubules are known to have peritoneal funnels
arising from their Malpighian bodies. It is worthy of note that
the nephrostomata are connected with the Wolffian tubules in
larval Anura, but that later on they become separated from them,
and open into the renal-portal vein [Wiedersheim].
 
The primary Wolffian tubules are usually stated to be derived
as solid ingrowths from the peritoneum towards the segmental
duct ; but Sedgwick has shown that in Elasmobranchs they have
the following development. It lias previously been stated (p. 212)
that the muscle-plates of Elasmobranchs are dorsal extensions of
 
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
243
 
 
the body-cavity, which become cut off (fig. 1 50, m.p) by the coming
together of the somatic and splanchnic mesoblast. The continuous
lion-segmented band of cells connecting the non-segmented muscleplates with the peritoneal epithelium being known as the intermediate cell-mass. Sedgwick found that the passage connecting
the body-cavity with that of the muscle-plates persists for some
time. Its connection with the ventral dilation of the muscle-plate
cavity is carried 'ventral wards as far as the outer dorsal corner of
the segmental duct, so that it appears as a canal opening into the
body- cavity just internal to the segmental duct, and thence curling
 
 
 
Fig. 176. - Transverse Section through the Trunk of a Duck Embryo with about
Twenty-Four Mesoblastic Somites. [From Balfour.']
 
vm. amnion : ao. aorta ; ca.v. cardinal vein ; ch. notochord ; hy. hypoblast ; m.s.
muscle-plate ; so. somatopleur ; sp. splanchnopleur ; sp.c. spinal cord : sp.g. spinal
ganglion : st. segmental tube ; wd. Wolffian (segmental) duct.
 
round its dorsal wall, opens into the muscle-plate cavity. The
ventral wall of this passage is formed of large columnar cells, the
inner and dorsal wall of much flatter cells.
 
At the next stage of development the passage becomes quite
separated from the muscle-plate cavity, and now lies as a blind
tube (fig. 175, st) opening into the body-cavity internal to the
segmental duct, with which it soon unites and forms a segmental
tubule.
 
Sedgwick has also further shown that in the Fowl, in the region
of the body between the twelfth and fifteenth somites inclusive,
the segmental tubes (Wolffian tubules) have a double origin: (1)
 
 
244
 
 
THE STUDY OF EMBRYOLOGY.
 
 
from outgrowths from the Wolffian or segmental duct; (2) as parts
of the intermediate cell-mass.
 
The intermediate cell-mass is at first continuous with the
peritoneal epithelium in every section, but this connection soon
becomes lost at certain points and maintained at others. At the
points where the continuity is retained, a peritoneal funnel is subsequently formed by the development of a lumen extending from
the body-cavity into the intermediate cell-mass.
 
The tubules have at this stage their characteristic and wellknown S-shape (fig. 176). They consist of the following parts:  -
 
 
 
A - C. A series of successive sections through the thirteenth segment of an embryo
with thirty-one or thirty-two segments, A being the most anterior. In A and B the
tubule is connected with the peritoneal epithelium ; and a lumen has appeared in it,
which is continued behind into the part of the tubule separated from the peritoneal
epithelium, as in C.
 
D - E. Sections through the thirteenth or fourteenth segment of an embryo with
thirty-four or more segments, showing the first appearance of the external and internal glomeruli, D and E correspond to, and are further developments of, B and C.
 
E. Diagrammatic longitudinal vertical section, showing the relations of the further
developed external and internal glomerulus.
 
b.c. body-cavity ; c.v. cardinal vein ; e.gl. external glomerulus ; gl. glomerulus ; i.c.m.
intermediate cell-mass ; i.gl. internal glomerulus ; me. mesentery ; p.f. peritoneal
funnel ; W.d. Wolffian duct.
 
(1) The now hollow Wolffian duct; (2) the outgrowth from it to
the intermediate cell-mass forming the upper limb of the S ; (3)
the intermediate cell-mass with the commencing lumen from
the body-cavity.
 
At a slightly later stage (fig. 177, A-c) there is a distinct lumen
opening into the body- cavity, which is continued behind into the
part of the intermediate cell-mass which has separated from the
peritoneal epithelium (c, i.c.m). This part will in the next stage
(fig. 177, e) become converted into that part of a tubule in which
a Malpighian body is developed, while the anterior part will form
a much wider peritoneal funnel (nephrostome).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
245
 
 
A glomerulus is formed about this time on the anterior wall of
the peritoneal funnel of each segmental tubule. The glomerulus
increases in size, and its lower portion hangs down freely into the
body-cavity (fig. ijy, f, gl). Before the period of the greatest
development of the glomerulus the mouth of the peritoneal funnel
becomes closed, thus dividing the glomerulus into an anterior
lower or external portion and into a posterior upper or internal
portion, the latter persists as the glomerulus of the Malpighian
body of the Wolffian body. The external portion afterwards disappears.
 
Behind the fifteenth segment the segmental tubules develop
entirely from the cells of the intermediate cell-mass. At first the
intermediate. cell-mass is at points distinctly continuous with the
peritoneal epithelium; at others it is less so. It soon breaks away
and occurs as a solid cord of cells, connected at intervals with the
peritoneal epithelium.
 
At the next stage the intermediate cell-mass entirely breaks
away from the peritoneal epithelium, and lies as a cellular blastema
(the Wolffian blastema) just internal to the Wolffian duct. The
Wolffian blastema almost directly breaks up into the structures
constituting the first rudiments of the Wolffian tubules.
 
Posteriorly, from about the twentieth segment, the intermediate
cell-mass has never any connection with the peritoneal epithelium,
and gives rise to the Wolffian blastema quite independently of the
peritoneal epithelium.
 
The cells of the blastema group themselves into tubules, one
end of which forms the Malpighian body, and the other opens into
the Wolffian duct. There appear to be outgrowths from the duct
to meet the tubules.
 
Although the Wolffian blastema extends as far back as the
thirty-fourth segment, it does not break up into Wolffian tubules
behind the thirtieth segment. From the thirty-first to the thirtyfourth segment it undergoes a different fate, and is known as the
kidney blastema.
 
In the anterior region of the mesonephros there appears to be only one primary
tubule (Wolffian tubule) for each segment of the body, but the number increases up
to the twentieth (counting from the auditory involution). All the segments from
the twentieth to the thirtieth inclusive contain five or six primary tubules.
 
The secondary or dorsal tubules are also more numerous behind than in front, the
most anterior segment being about the twenty-first. Some primary tubules, according to Sedgwick, have as many as four secondary tubules ; thus in the twenty-eighth
segment there are twenty secondary tubules (five sets of four).
 
Balfour has shown that the secondary tubules^develop in Elasmobranchs in con
 
246
 
 
THE STUDY OF EMBRYOLOGY.
 
 
nection with the Malpighian bodies of the primary tubules. A process from one
Malpighian body grows forward and unites with the preceding tubule just before it.
enters into the Wolffian duct. The stalk of origin degrades into a fibrous band or is
aborted. The tertiary, &c., tubules probably arise from the same rudiment.
 
The secondary Malpighian body is produced in the Fowl, according to Sedgwick,
by the division of the primary glomerulus into two parts, the upper one forming the
secondary and the lower the primary glomerulus ; and by the simultaneous development of certain folds which separates the dorsal secondary tubule from the ventral
primary tubule.
 
The somewhat later origin of the posterior tubules of the mesonephros in the Fowl, and their development from a blastema, is a
distinct approach towards the mode of origin of the metanephros,
now to be described.
 
It appears to be probable that in the Teleosts and Amphibia
the segmental tubules of the mesonephros develop in situ from a
blastema analogous to that in the posterior region of the Fowl.
The tubules subsequently acquire openings into the Wolffian tube
on the one hand, and into the body-cavity on the other.
 
3. Metanephros.  - The kidney proper or metanephros, as a gland
distinct from the mesonephros, only occurs in Amniota. In the
Fowl it develops from a blastema which is at first perfectly continuous with, and indistinguishable from, that which gives rise to
the posterior portion of the Wolffian body. Although the kidney
blastema arises at a comparatively early stage in development, still
it is not till a much later stage that it shifts its position and begins
to show signs of developing into the segmental tubules. This
retarded development is analogous with the late appearance in
Amphibia of the mesonephros as compared with the pronephros.
 
The first distinct structure to develop is the ureter, which arises
as a dorsal outgrowth from the hinder part of the Wolffian duct.
The ureter grows forward in close connection with the abovementioned blastema, which has by this time broken away from the
mesonephric blastema and assumed a position dorsal to it (fig.
178, c).
 
The metanephric blastema extends in the Fowl from the thirtyfirst to the thirty-fourth segments, and collects round swellings of
the ureter from which kidney tubules grow out. These tubules
burrow into the blastema, and they are increased by segregation of
the blastema cells.
 
The ureter soon loses its connection with the Wolffian duct,
and acquires an independent opening into the cloaca.
 
The primitive continuity of the metanephric with the mesonephric
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
247
 
 
blastema, together with the general similarity of the development
of the renal tubules and the identity of their adult structure, proves
that the metanephros is merely a special portion of the primitive
Wolffian body, which develops late.
 
The acquisition by the posterior portion of the Wolffian body in
some Elasmobranchs and Amphibia of efferent ducts opening into
 
 
 
Fig. 178. - Development oe Metanepheos in the Fowl. {Adapted from Sedgwick.}
 
A. Transverse section through an embryo at the end of the fourth day. B. Longitudinal vertical section through an embryo of about the same age, showing the
absolute continuity of the kidney blastema with the hindermost part of the Wolffian
blastema, in which the development of Wolffian tubules is taking place. C. Transverse section through an embryo at the end of the sixth day.
 
ao. dorsal aorta: b.c. body-cavity ; c.v. cardinal vein; k.b. kidney blastema; k.t.
kidney tubule; M.d. Mullerian duct; mes. mesentery; vch. notochord; pe peritoneum ; T. testis ; u. ureter ; v.c. vertebral centrum ; W.B. Wolffian body ; Wd.
Wolffian duct ; Wt. 1 primary, and Wt . 2 secondary, Wolffian tubule.
 
 
the urogenital sinus or into the extreme posterior end of the
Wolffian duct, is, as Balfour pointed out, a definite step towards
the formation of a metanephros. According to Mikalovics, the
mesonephros remains functional till the second year in Lizards,
and thus is functional at the same time as the metanephors.
 
 
248
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Summary of the Development of the Vertebrate Excretory Organ.  - From his
 
investigations on the development and phytogeny of the vertebrate excretory organs,
Sedgwick has arrived at the following conclusions. For the facts and arguments
upon which they are based, recourse must be had to his papers.
 
The pronephros attains a functional development in all the Ichthyopsida (except
the Elasmobranchii), but usually only during larval life.
 
The segmental duct arises first as a ridge from the parietal peritoneum. This
ridge usually contains a diverticulum from the body-cavity, and is continuously constricted off to form a duct.
 
Except anteriorly, where the constriction only takes place at intervals, leaving the
openings of the pronephros (except in Teleostei, where there is only one opening).
 
These openings correspond in number with the segments which the pronephros
occupies.
 
A vascular structure, called a glomerulus, is formed, projecting on each side of the
aorta into a special dilatation of the anterior part of the body-cavity. (Myxine
forms a peculiar exception to this otherwise universal fact.)
 
This dilated part of the body-cavity may become partially or completely separated
off to form a capsule, into which the glomerulus projects and the anterior end of the
segmental duct opens.
 
The development of the pronephros in the Fowl is essentially identical with the
above, except in the absence of a continuous glomerulus opposite the nephrostomata ;
but that in the Elasmobranch is greatly modified and reduced.
 
In those animals which possess a functional larval pronephros, the mesonephros
develops from a blastema ; this is undoubtedly an abbreviated method. The lateness and consequent modification of the development of the mesonephros in these
Ichthyopsida is due to the fact that the larva already possessed a functional excretory organ, and devoting all its energy in developing those organs which it will
really require as a larva, it leaves over the development of the organs not so required
until later ; and in order that it may not be burdened by useless organs, the cells,
which will give rise to the tubules, are so reduced as hitherto to have escaped
observation. If the phylogenetic order had been adhered to, these cells would have
arisen quite early in embryonic life, and from the parietal mesoblast in the normal
manner. In the Amphibia the mesonephros increases in size and complexity with
the growth of the larva.
 
On the other hand, the mesonephric tubules develop in what is clearly a more
primitive manner in those forms in which the pronephros is functionless. In
Elasmobranch s, and in the anterior region in the Fowl, the tubules are practically
persistent tubular portions of the body-cavity (since the intermediate cell-mass is a
continuation of the coelomic epithelium), which soon acquire an opening into the
segmental duct. The early discontinuity of the tubules with the duct is, however, a
secondary feature. In Birds a segmental glomerulus is developed in connection with
each nephrostome, part of which is converted into the glomerulus of the Malpighian
body. In Elasmobranchs only internal Malpighian bodies are formed.
 
It may fairly be assumed that the Wolffian tubules were primitively segmentally
arranged (as still occurs in the development of Elasmobranchs, Csecilia, and at first
also in the Lizard). A shifting of position has, however, occurred, probably partly
owing to the shortening up of the organ, so that the number of tubules may exceed
that of the segments over which the mesonephros extends. The number of tubules
in a segment usually increases with the growth of the embryo, and at the same time
the organ is complicated by the development of secondary tubules.
 
In Birds the pronephros is continuous with the mesonephros. The discontinuity
in Amphibia is due to the causes mentioned above, but it may not really be so great
as it appears.
 
The segmental tubules of the Ichthyopsidan pronephros open into a special recess
of the body-cavity, into which the elongated glomerulus projects (fig. 173) ; imme
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
249
 
 
diatelv above this lies the muscle-plate. A comparison of figs. 173- 17 7, will demonstrate that the intermediate cell-mass corresponds to this region. It has been shown
that in Birds the peritoneal funnels, the Malpighian body, and a portion at least of
the mesonephric tubules are derived from the intermediate cell-mass. The external
and internal glomeruli of the Avian Wolffian body are developed from the same
region, and all the secondary glomeruli are derived from the internal glomerulus.
The internal glomeruli of Elasmobranclis are clearly homologous with those of Birds.
According to this view, a Malpighian body is to be regarded as an isolated portion of
the body- cavity, comparable with the condition which obtains in the Teleostean
pronephros. Therefore the mesonephros, in all particulars, is merely a continuation
posteriorly of the primitive vertebrate excretory organ, which, for various reasons,
has acquired a more or less independent and modified origin.
 
What applies to the mesonephros also holds good for the metanepliros. The distinction of the latter organ from the former is more apparent than real, as Sedgwick
has fully demonstrated. As a matter of fact, the tubules of the liindermost region of
the mesonephros develop in an almost similar manner to those of the metanepliros.
We have therefore a complete series in the mode of origin of the excretory tubules,
from the primitive condition in the pronephros of Amphibia to the modified method
by the rearrangement of the cells of a blastema, as occurs in the metanepliros of
Amniotes.
 
From the foregoing brief summary it will be seen that the excretory organs of the
adult are usually developed from the walls of the body-cavity in the invertebrate and
vertebrate Coelomata (and therefore the same possibly occurred in the lower Chordata).
It must not, however, be rashly concluded that these organs are necessarily homologous. It is possible that a similar and homologous simple renal organ (arcliinephridium) occurred in the unsegmented vermian ancestors of the Chsetopoda and
Chordata ; but the segmental organs of the one are probably homoplastic, rather
than strictly homologous with the segmental tubules of the other.
 
It is tempting to regard the origin of the Nemertean excretory organ, as described
by Hubrecht, as a degenerate form of the production of a true body-cavity (coelom)
by archenteric diverticula, which, in this case, solely develop into nephridia. If
this be granted, a further step may be taken, and, accepting Rabl -s account of the
development of the Molluscan excretory organ, we may assume that the formative
cells of the mesoblastic vesicle actually arose from the archenteron. Should this
prove to be the case, the Molluscan nephridia would be comparable with those of
the majority of other animals. It is also difficult to believe that the Molluscan
pericardium is not a true coelomic cavity.
 
 
Epiblastic Origin of the Segmental Duct.  - Since the above
account of the development of the vertebrate excretory septem was
in type, a preliminary note by Yon Perenyi has appeared, in which
he confirms and extends the discovery of the epiblastic origin of
the segmental (archinephric) duct. Hensen, Graf Spee, and Flemming have demonstrated that in the Eabbit and Guinea-pig the
primitive nephric duct (probably not the whole excretory system,
as they assume, although without evidence to support them) arises
by delamination from the epiblast at the level of the intermediate
cell-mass, with which it later becomes associated. Afterwards
Van Wijhe found the same held good for Elasmobranchs, and most
recently Yon Perenyi asserts that in the Edible Frog the segmental
 
 
250
 
 
THE STUDY OF EMBRYOLOGY.
 
 
duct develops as a canal-like splitting from the inner (nervous)
cell layer of the epiblast, and quite close to the place of origin of
the developing somites. In the Lizard, also, it appears as a thick
cell-mass separating off from the epiblast.
 
There can now be little doubt that the segmental duct arises from the epiblast.
This discovery will necessarily lead to a modification of our views concerning the
morphology of the vertebrate excretory organs.
 
The segmental tubules (nephridia) appear to be strictly mesoblastic, and the above
account of their development may be taken as probably being fairly accurate. The
origin of these nephridia may have been primitively similar to those (segmental
organs) of the Chsetopod Worms, the main distinction between the two being that
each nephridium of the latter opens directly to the exterior. As has been already
stated, Hatschek has described a single nephridium in Amphioxus in all respects comparable with a vermian nephridium.
 
We have, then, only to assume that a pair of similar vermian nephridia occurred
in each body-segment of the ancestral Vertebrate, and that the nephridia of each
side of the body opened externally into a lateral groove. It would further only be
 
 
 
Fig. 178*. Transverse Section of Embryo Rabbit (4 mm. in length, stage of
sixteen somites). [After Flemming .]
 
The section is taken just in front of the posterior termination of the intestine. The
right side of the figure is the left of the body. There is a small rupture in the left (right
of figure) mesoblastic somite. All the shading is diagrammatic.
 
al. mesenteron (intestine) ; cce. coelom (body-cavity) ; ep. epiblast; hy. hypoblast; i.c.m.
intermediate cell mass; n.c. neural canal; s.d. segmental duct ; som. somatic mesoblasb ;
sp. splanchnic mesoblast.
 
 
necessary for the groove to deepen and next to form a canal (in the same manner that
the neural groove is converted into a canal) to bring about the vertebrate arrangement.
Thus in Vertebrates, as in Invertebrates, the nephridia open by epiblastic pores, but
in the former the area upon which they open is precociously converted into a canal,
which subsequently acquires a secondary opening to the exterior through the cloaca.
 
As we are justified in assuming the persistence of the blastopore as the anus in
early Chordata, the nephric groove, if it were continued behind round to the anus,
would practically open into the extreme hinder end of the mesenteron  - in other
words, into the urodseum. Probably about the same time that the nephric groove
was being converted into the nephric canal (segmental duct), the proctodseum was
being invaginated. The latter would push before it the posterior orifice of the
nephric canal along with the primitive anus (blastopore). On the hypothesis just
sketched out, the nephridia of Vertebrates always open by their original epiblastic
pores, primitively directly to the exterior, secondarily into a canal separated from the
epiblast ; also the archinephros could be equally effectually functional throughout
the whole period of its modification.
 
Urogenital Ducts of Vertebrates - For the sake of simplicity
the ducts of the^Vertebrate renal organs have been referred to as if
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
251
 
 
solely connected with those organs ; as a matter of fact, they become
intimately connected with the generative organs. The modifications
which occur in the glands and ducts of the primitive excretory
system of the Vertebrates may be regarded as being largely due
to their secondary connection with the generative organs.
 
Segmental or Archinephric Duct.  - The development of the
segmental duct has already been described. Tor the sake of
clearness it has been assumed that the segmental duct functions
first as the duct of the pronephros, and secondly as that of the
mesonephros. This is not, however, exactly the case, as in most
cases there is a horizontal division or separation of the duct into
two tubes. A ventral tube is termed the Mullerian duct ; while
the dorsal, from its association with the mesonephros, is known
as the mesonephric or Wolffian duct.
 
Mullerian Duct.  - The two ducts are formed in Elasmobranchs
by the splitting off from before backwards of a nearly solid cord
of cells from the ventral wall of the segmental duct. A very small
portion of the lumen of the segmental duct may perhaps be continued into the Mullerian duct. The latter soon grows in size, and
forms an elongated tube in the female quite distinct from the
Wolffian duct. The longitudinal separation from the segmental
duct occurs in such a manner that the whole of the anterior
extremity, with its peritoneal opening, belongs to the Mullerian
duct, which now forms a complete tube opening posteriorly into
the cloaca and anteriorly into the body-cavity. In these forms
the single, primitively solid, pronephric tubule persists as the peritoneal opening of the Mullerian or oviduct of the adult.
 
The development of the Mullerian duct in Amphibia is very
much the same as in Elasmobranchs. In the Salamander the
Mullerian duct is split off from the segmental duct behind its
anterior extremity, and acquires an independent opening into the
body-cavity slightly behind the pronephros. IJnlike what occurs
in Elasmobranchs, the undivided anterior extremity of the segmental duct with the pronephros retains its connection with the
Wolffian duct.
 
In the Fowl, Balfour and Sedgwick have shown that the anterior
end of the Mullerian duct arises as three grooves connected by an
internal thickening of the peritoneum of that region. The thickening separates as a solid rod of cells, which, before long, acquires a
central lumen. The whole structure now consists of a short tube
opening anteriorly into the body-cavity by three short ductules.
 
 
252
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Posteriorly the Miillerian duct is closely connected with the
segmental duct. The backward growth of the Mullerian duct
takes place at the expense of a thickening of the ventral wall of
the segmental duct. In other words, the avian Mullerian duct is
formed posteriorly by the splitting of the segmental duct, as in
Elasmobranchs and Amphibia. The permanent abdominal opening
of the Mullerian duct (oviduct) corresponds with the anterior of
the three grooves, the two posterior grooves disappearing.
 
Balfour regarded the three peritoneal funnels of the Mullerian duct as the sole
representative of the head-kidney (pronephros) in the Fowl. Sedgwick still adheres
to the earlier published view as to the meaning of the peculiar structures at the
anterior end of the Mullerian duct, hut supposes them to have been derived from
the anterior part of the excretory system after its modification to form the
pronephros.
 
The tubules of the Mullerian duct of the Fowl arise behind the anterior end of
the segmental duct, and therefore more or less posterior to the pronephros. In
Amphibia the single (solid) tubule is situated behind the pronephros. In both cases
we must assume either that the Mullerian tubules are modified and backwardlyshifted pronephric tubules, or that they belong to the region between the pronephros
and the mesonephros proper. In any case, the Mullerian duct is split off from the
segmental duct. Further researches may modify the account given of the development of the pronephros of Elasmobranchs.
 
The Miillerian duct opens at the anterior end of the body-cavity
in the lower Vertebrates; in Elasmobranchs, for instance, the
conjoint orifice of the two ducts is situated on the ventral wall
of the oesophagus just behind the pericardium. In the higher
Vertebrates the Mullerian ducts are situated in the posterior
abdominal region. The hydatid (fig. 181, A, h") which is sometimes
present near the coelom ic orifice of the oviduct is probably a
degraded rudiment of a primitive tubule.
 
In those Ichthyopsida which possess them, in the Sauropsida
and in the Ornithodelphia (Monotremata or Prototheria), the paired
Mullerian ducts (oviducts) open into that portion of the cloaca
which is known as the urogenital sinus (fig. 179, a). Occasionally
only one oviduct maybe developed; in Birds it is usually the right
which atrophies.
 
f The Didelphia (Marsupials or Metatheria) have a modification of their Mullerian
ducts, which is very different from that of other Mammals. It may be here mentioned that three regions are distinguishable in the Mullerian ducts of these and
higher Mammals  - an anterior or distal narrow tube (Fallopian tube or “oviduct-),
which opens into the body-cavity by usually fimbriated lips ; a median swollen uterus,
and a posterior or proximal vagina.
 
In the Didelphia the Mullerian ducts with their three regions are at first perfectly distinct, and practically remain so ; that is, there are two vaginae, uteri and
Fallopian tubes. Later, in the young, the anterior (distal) ends of the vaginae
 
 
ORGANS DEBITED FROM THE MESOBLAST.
 
 
253
 
 
approach one another ; at the point where they touch they form a median sac,
which grows backwardly towards the urogenital sinus. At first this vaginal caecum
is a double tube, corresponding to each vagina ; but the median septum is usually
soon absorbed. At this stage the two uteri open into the anterior extremity of the
vaginal cul-de-sac, into the upper end of which the two vaginae also open. The blind
posterior end of the caecum becomes closely connected with the end of the urogenital
sinus, between the posterior vaginal orifices ; and, as Fletcher has proved, the two
cavities may communicate even in virgin animals, and they certainly do communicate
after the first birth. (Unless very exceptionally, there is, according to Fletcher, no
direct communication in Macropus major between the vaginal caecum and the urogenital sinus, even after young have been produced.)
 
In the Monodelpliia (Eutheria) the Mullerian ducts fuse with
one another to an increasing extent from behind forwards. In
 
 
 
Fig. 179. - Various Forms of Mammalian Uteri.
 
A. Ornifchorhynchus [ after Owen]. B. Didelphys dorsigera [ after Brass]. C. Phalangista vulpina [a 'ter Brass]. D. Double uterus and vagina ; Human anomaly [ after
Farre], E. Lepus cuniculus (Rabbit), uterus duplex [after T. J. Parker]. F. Uterus
bicornis. G. Uterus bipartitus. H. Uterus simplex (Human). [F - H after Wieders heim.]
 
a. anus; cl. cloaca; o.d. oviduct; o.t. os tineas (os uteri); ov. ovary; r. rectum; s.
vaginal septum ; n.b. urinary bladder ; ur. ureter ; ur.o. orifice of same ; u.s. urogenital sinus ; ut. uterus; v. vagina; v.c. vaginal caecum.
 
all there is a single vagina, but in some of the lower forms, e.g .,
Rodents, an imperfect vaginal septum may he present. Usually it
merely divides the orifice (os uteri or os tincse) of one uterus from
that of the other (fig. 179, e). In such forms the uteri are quite
distinct.
 
In other Mammals the uteri come together, and by concrescence
form a common uterus, which also, in some cases, possesses a short
median septum. In these forms there are paired cornua uteri
opening into a single corpus uteri, which communicates with the
vagina by a single os uteri.
 
Eig. 179 illustrates various forms of uterus met with amongst
 
 
254
 
 
THE STUDY OF EMBRYOLOGY.
 
 
the Eutheria. In the most specialised case, as in Man, the uterus
(h, ut) has a pyriform shape, and the Eallopian tubes arise abruptly
from its anterior corners.
 
It is interesting to find that anomalies may occur in the human uterus, which
illustrate the evolution of that organ. Thus a median septum may partially or
wholly extend along the uterus, and the vagina even may be similarly divided (fig.
 
179, D).
 
The Mullerian duct is rudimentary or entirely absent in the
adult male. In the former case it may be represented by a solid
cord for the whole of its length (Dipnoi and some Amphibia, fig.
 
180, A, mg), or only the anterior portion may remain (Elasmobranchs and some Lizards), which degrades into the so-called
hydatid of Morgagni in Man (fig. 181, B, m). The posterior section
is usually stated to persist as the uterus masculinus (figs. 183, III. u,
1 84), present in many Mammals, and especially large in the Eabbit
and the Horse ; but Kolliker now believes this structure to be a
derivative of the Wolffian duct.
 
The oviduct is normally present as a complete duct in the males
of the Dipnoi and of some Ganoids [Ayers, Wiedersheim], and
abnormally in Lizards [Howes].
 
To recapitulate :  - The segmental duct is the duct of the primitive vertebrate
excretory organ. The pronephros was either the sole excretory organ, or it has come
to be the only functional portion of the kidney in free-living larval forms, owing to
the retardation of the posterior region of the primitive excretory organ. At all
events, the segmental duct at first functions as the pronephric duct.
 
One or more of the tubules in the anterior region of the primitive kidney acquired
the office of carrying ova to the exterior. This probably occurred after the full
development of the pronephros, and in the intermediate region between it and the
mesonephros. Possibly at one time the segmental duct conveyed ova to the exterior,
together with secretions from both pronephros and mesonephros.
 
From certain causes the pronephros atrophied or changed its function and became
a lymphatic gland ; the segmental tube then carried ova and mesonephric secretions.
The duct gradually became constricted in such a manner that the ova were conveyed
in a ventral groove, which subsequently was converted into a canal.
 
In some such manner the segmental duct may have differentiated into a ventral
Mullerian duct or oviduct, and into a dorsal mesonephric duct or Wolffian duct.
 
Wolffian Duct or Mesonephric Duct. - In those forms (Ichthyopsida) in which the mesonephros remains functional throughout
life its duct naturally persists, although it also acts as the efferent
duct of the generative gland in the males of the Elasmobranchii,
Lepidosteus, and Amphibia.
 
Branches grow out from the anterior (three or four in Elasmobranchs) segmental or Wolffian tubules (though probably not from
their peritoneal openings  - Balfour) and enter the testis, where
they form a longitudinal canal (fig. 180, a). These branches, vasa
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
255
 
 
efferentia, convey the semen to the Wolffian body after previous
uniting into a longitudinal canal, (the longitudinal canal of the
Wolffian body) (fig. 180). Branches of this so-called testicular
 
 
B
 
 
 
 
Fig. 180 --Diagram of the Urogenital Apparatus of a Male (A) and
Female (B) TJrodele. Founded on Triton tseniatus. [ From Wiedersheim
after J. W. Spengel.]
 
a. collecting tubules of the mesonephros ; GN. anterior sexual portion of kidney (parorchis of the male) ; Ho. testis ; Ig. Wolffian or Ley dig -s duct, urogenital duct in male, A,
and urinary duct in female, B, Ur; mg(Od). Mullerian duct, rudimentary in male, mg';
 
N. posterior non-sexual portion of kidney ; Ot. peritoneal aperture of oviduct ; Ov. ovary ;
 
Ve. vasa efferentia of testis which fall into the longitudinal canal (f) of the Wolffian body ;
this testicular network (ft) is rudimentary in the female, B.
 
network enter certain Malpighian bodies, and the semen is thence
carried by their tubules to the Wolffian duct.
 
The anterior or sexual portion of the Wolffian body in the male
is rudimentary so far as excretory purposes are concerned, and, as
in the male, a functionless rudiment of the Mullerian duct is
present, so a rudimentary testicular network is developed in the
 
 
256
 
 
THE STUDY OF EMBRYOLOGY.
 
 
female Urodeles (fig. 180, b), and the anterior portion of the Wolffian body is also feebly developed.
 
In the Elasmobranchs and Amphibia the collecting tubes of the
non-sexual posterior portion of the Wolffian body unite together
to form one to two primary tubes (ureters) before entering the
posterior extremity of the Wolffian duct. Thus the Wolffian tube
acts as a vas deferens, and the posterior portion of the mesonephros
is practically an incipient metanephros.
 
In the males of the Amniota, tubules grow out from certain
anterior Malpighian bodies of the Wolffian body in the embryo,
and come into connection with the seminal tubuli of the testis.
 
 
 
Fig. i8i. - Generative Organs of Hitman Adult. [After Kobelt.]
 
A. Female. B. Male.
 
The Mullerian duct (31) in the female functions as the oviduct or Fallopian tube ;
from below its fimbriated abdominal opening is seen an hydatid, probably the rudiment of a pronephric tubule ; in the male the blind end of the Mullerian duct forms
the hydatid of Morgagni, m. The Wolffian body persists in three sections - (i.) the
anterior as rudimentary tubes, sometimes forming hydatids, h ; h'. terminal bulb or
hydatid in female ; ( 2 .) the middle set of tubes (c) or coni vasculosi, forming the epididymis of the male and the epoophoron of the female, ep ; ( 3 .) the posterior rudimentary
tubules, paroophoron of female and vasa aberrentia of male. The fold of mesentery
slinging the ovary ( 0 ) is the mesorchium ; t. testis.
 
 
With the exception of two or three, these tubules become detached
from the Wolffian body ; those that remain act as vasa efferentia
(coni vasculosi of Mammals). Several rudimentary outgrowths
from the Malpighian bodies may persist as hydatids or as vasa
aberrentia (fig. 181, b, v.a).
 
The Wolffian duct in the male Amniota is transformed into the
vas deferens ; its anterior portion becomes extremely convoluted,
and forms the canal of the epididymis, the head of the epididymis
being formed from the testicular network, which, as has just been
described, is secondarily developed from the Malpighian bodies.
 
In the female rudimentary structures of a similar nature occur
(figs. 1 8 1, A, h, v.a); the anterior tubules form hydatids, the posterior degenerate into solid cords. These structures are collectively
known as the parovarium (epoophoron or Kosenmiiller -s organ).
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
257
 
 
The Wolffian duct more or less disappears in the female. In
Snakes and several Lizards the posterior portion may remain as a
small functionless canal, and in some Mammals (Pig, Ruminants.
 
 
Fig. 182. - Urogenital Organs of a
Female Human F<etus of 3 \ Inches
Long, or about Fourteen Weeks.
[ From Quain after Waldeyer .]
 
e. tubes of the anterior part of the
Wolffian body, forming the epoophoron
of Waldeyer (parovarium of Kobelt) ; M.
Mullerian duct ; m. its anterior fimbriated
orifice ; 0. ovary full of primordial ova ;
W. posterior part of the Wolffian body,
forming the paroophoron of His and
Waldeyer ; W. Wolffian duct.
 
 
 
Fox, Cat, and some Monkeys) the middle portion may persist as
Gaertner -s duct.
 
The posterior or non-sexual portion of the Wolffian body de
 
 
I. Ideal undifferentiated condition. ~H. reproductive gland lying on the tubules of
the Wolffian body, W : M. Mullerian duct ; S. urogenital sinus. II. Transformations
in the female.  - F. fimbriated orifice, with hydatid (M) of the Fallopian tube, T; O.
ovary ; P. parovarium ; U. uterus. III. Transformations in the male.  - a. vas
aberrans ; E. epididymis with hydatid, h ; u. uterus masculiuus ; V. vas defereus ;
urogenital sinus. 4. Monotrematous, and 5. Eutheriau, stages in development of the
posterior passages,  -a. allantois ; b. bladder ; u. urachus ; d and M. rectum ; k. cloaca ;
 
&. urogenital sinus; m. perineum.
 
grades into the para-epididymis or organ of Girald&s in the male
(fig. 184, w), and into the paroophoron in the female (fig. 185, w).
 
" R
 
 
258
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Generative Ducts of Ganoids and Teleosts.  - There are not at present sufficient
data upon which to satisfactorily determine the homology of the Teleostean oviduct.
Rathke, Balfour, and Huxley have demonstrated that the Teleostei form an extreme
of the Ganoid series, and that the oviduct of the Smelt (Osmerus) is in every way
identical with that of Amia. Some Teleosts, such as the Salmon, have no oviduct,
their ova dehiscing into the body-cavity to pass to the exterior through the abdominal pore. Huxley points out that in the Sturgeons (Sturios) and Lepidosteus the
renal are much wider than the generative ducts, and the communication between
them is effected far in front of the external aperture ; while in Polypterus and Amia
the oviducts are wider than the ureters, and they communicate nearer the external
opening ; in Osmerus the common aperture of the oviducts lies in front of the
opening of the ureter ; and lastly, in the Salmo the abortion of the oviducts, commenced in Osmerus, is completed, and the so-called “abdominal pore- is the homologue of half of the urogenital opening of the Ganoids, and has nothing to do with
the “ abdominal pores - of these fish and of the Selachians. Against this view must
be placed the fact, discovered by Rathke and confirmed by Bridge, that in Mormyrus
oxyrhynchus the ordinary generative ducts coexist with abdominal pores. There is,
unfortunately, no complete account of the development of the Ganoid oviduct ; it is
possible that it represents, in part at least, the Mullerian duct of other forms, but
Balfour has suggested that it is a modified segmental tubule of the mesonephros.
 
There is also great uncertainty concerning the nature of the duct of the testis in
Teleosts. What has been said above for the oviduct also applies largely to the
efferent duct of the testis. Balfour has proved that the anterior portion of the
Wolffian body in Lepidosteus is connected with the testis as in Elasmobranchs, and
thus in that Ganoid the Wolffian duct functions as the vas deferens.
 
Weber, who has very recently investigated the subject, has come to the conclusion
that the genital pore in female Salmonidae is the homologue of that of other Teleostei ;
it communicates with a pair of peritoneal funnels which open widely into the bodycavity. These may in some instances extend forwards close to the ovary (Mallotus,
Osmerus). The peritoneal funnels are incompletely homologous with the oviducts of
those Teleostei with so-called enclosed ovaries, and neither are homologous with the
oviducts of other Vertebrates. In the male Salmonidae the vasa defferentia of the
testes open to the exterior by a pore common to the ureters, precisely as in other
Teleostei. In old Salmonids, in males as well as in females, a pair of true abdominal
pOres occur, a pore being situated on each side of the anus. They are not concerned
in the evacuation of ova; in individual cases one or both may be absent. Weber
considers these abdominal pores as rudimentary structures, perhaps as remnants of
segmental ducts. He homologises them with the abdominal pores of Holocephali,
Elasmobranchii, Ganoidei, and Mormyridse. The so-called abdominal pore of the
Cyclostomi and Mursenidse may be compared with the genital pore of the Salmonidse
and other Teleostei (c/. p. 214).
 
Metanephric Duct.  - The duct of the metanephros or kidney
proper is known as the ureter. At first it opens into the Wolffian
duct, but it early acquires an independent opening into the cloaca
(urogenital sinus).
 
In Sauropsida and Monotremes the ureters open into the urogenital sinus quite independently of the urinary bladder. In the
higher Mammals the ureters open directly into the bladder.
 
Suprarenal Bodies.  - The suprarenal bodies of Vertebrates were shown by Balfour
to have a double origin. The medullary substance is derived from an extension of
 
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
259
 
 
the ganglia of the sympathetic system, while the cortical substance is of mesoblastic
origin.
 
Weldon has lately demonstrated that the cortical substance of the suprarenal bodies
arises as a proliferation of the peritoneum, just internal to the segmental tubules,
throughout the whole extent of the mesonephros. This blastema subsequently
surrounds the outgrowths from the ganglia.
 
In Bdellostoma, as Weldon has shown, the head-kidney has become modified so
as to form an organ functionally analogous to the suprarenals ; while in Teleosteans
a most remarkable series of modifications, affecting every region of the kidney, has
been described by Balfour and Emery.
 
Weldon holds that the same causes which led to the degeneration of the original
renal pronephros (causes among which the specialisation of the pericardium and the
development of the air-bladder and lungs may have played a considerable part),
the same causes which led to the establishment of the mesonephros as the chief seat
of renal secretion, may, and indeed must, have rendered advantageous the suppression
of any glandular organ in the pronephric region ; and thus when, in consequence of
the change of function of the Wolffian duct more and more, the mesonephros became
useless as a kidney, it is easy to understand how some of its component parts underwent in their turn the same change of function as had been undergone by tlie
anterior part of the renal organ at an earlier period of its evolution.
 
 
Urinary Bladder.  - A dilated portion of the Wolffian ducts
which occurs in many Fishes is usually termed a urinary bladder.
In Amphibia a thin-walled vesicle (urocyst) develops from the
ventral wall of the cloacal section of the mesenteron, and is
homologous with the urinary bladder of the Amniota. On referring to the mode of development of the Wolffian duct, it will be
obvious that the piscine “ urinary bladder - is not in any sense of
the term homologous with that of the Amniota.
 
In the Amniota the urinary bladder is a persistent portion of
the stalk of the allantois (p. 81), which becomes converted into a
vesicle. That portion of the stem of the allantois distal to the
bladder which remains within the body-cavity after the formation
of the umbilical cord becomes degraded into a solid cord, and is
known as the urachus (figs. 143, 184, 185).
 
The bladder opens on the ventral wall of the cloaca in Amphibia and in those Sauropsida in which it persists throughout
life (Chelonia and Lacertilia). In these the ureters open independently into the cloaca.
 
In the Monotremes the bladder opens into the anterior end of
the urogenital sinus (fig. 179, A, u.s .), into which the ureters and
generative ducts also debouch. The urogenital sinus or vestibule
may be regarded as the proximal portion of the allantoic stalk.
 
In all higher Mammals the ureters open directly into the
bladder itself, owing to the increase in length of the primitively
short interspace between the orifices of the ureters and generative
 
 
260
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ducts. This narrow lengthened portion of the urogenital sinus
 
The urethra and generative
 
 
Fig. 184. - Diagram of the Mammalian Type of Male Sexual
Organs, [ From Quain .] Compare
with fig. 185.
 
C. Cowper -s gland of one side ; cp. corpora cavernosa penis, cut short ; e. caput
epididymis ; g. gubernaculum ; i. rectum ;
m. hydatid of Morgagni, the persistent
anterior end of the Mullerian duct, the
conjoint posterior ends of which form the
uterus masculinus ; pr. prostate gland ; s.
scrotum ; sp. corpus spongiosum urethrae ;
t. testis (testicle) in the place of its original
formation, the dotted line indicates the
direction in which the testis and epididymis change place in their descent from
the abdomen into the scrotum; vd. vas
deferens ; vh. vas aberrans ; vs. vesicula
seminalis; W. remnants of Wolffian body
(the organ of Giraldes or paradidymis of
Waldeyer), 3, 4, 5, as in fig. 185.
 
 
ducts open into the anterior extremity of the urogenital sinus in
Marsupials and many of the lower Eutheria (compare fig. 179).
 
This condition always persists in the male (fig. 184), as the
urogenital sinus traverses the penis. In the females, however, of
 
 
 
is known as the urethra (fig. 184).
 
 
 
Fig. 185. - Diagram of the Mammalian Type of Female Sexual Organs.
 
[From Quain.]
 
This diagram should be carefully compared with fig. 184, it will be seen that the
dotted lines in one indicate functional organs in the other, and help to demonstrate
the significance of certain rudimentary structures.
 
C. gland of Bartholin (Cowper -s gland) ; c.c. corpus cavernosum clitoridis ; dG.
remains of the left Wolffian duct, which may persist as the duct of Gaertner; /.
abdominal opening of left Fallopian tube ; g. round ligament (corresponding to the
gubernaculum) ; h. hymen ; i. rectum ; l. labium ; m. cut Fallopian tube (oviduct or
Mullerian duct) of the right side ; n. nympha ; 0. left ovary ; po. parovarium : sc. vascular bulb or corpus spongiosum ; u. uterus ; v. vulva ; va. vagina ; W. scattered
remains of Wolffian tubes (paroophoron) ; w. cut end of vanished right Wolffian duct :
3. ureter ; 4. bladder passing below into the urethra ; 5. urachus or remnant of stalk
of allantois.
 
 
the more specialised Eutheria the urogenital sinus becomes much
shortened and flattened out, so that eventually it is merely re
 
ORGANS DERIVED FROM THE MESOBLAST.
 
 
2G1
 
 
presented by the space known as the vestibule of the vulva (fig.
185, 189). In the forms in which this occurs the urinary and
generative ducts come to have independent openings to the exterior. The accompanying diagrams illustrate the changes undergone in the human female foetus.
 
At an early period (figs. 186 and 143, a) the allantois and Mullerian duct communicate with the rectum, but not with the exterior. The proctodaeum is next developed (fig. 187), and forms a
cloaca, into which the urogenital ducts and the rectum open. The
cloaca is then divided into an anterior or ventral part, the urogeni
 
 
Fig. 188. Fig. 189.
 
Diagrams Illustrating the Evolution of the Posterior Passages.
 
[From, Landois and Stirling .]
 
Fig. 186. - Allantois continuous with rectum. Fig. 187.  - Cloaca formed. Fig. 188.  -
 
Early condition in male, before the closure of the folds of the groove on the
posterior side of the penis. Fig. 189.  - Early female condition.
 
a. commencement of proctodseum ; all. allantois ; b. bladder ; c. penis ; CL.
cloaca ; m. Mullerian duct ; R. rectum ; u. urethra ; s. vestibule ; su. urogenital sinus ;
v. vas deferens in fig. 188, vagina in fig. 189.
 
tal sinus, and into a posterior or dorsal portion, the anus (fig. 189),
by a downgrowth of the tissue between the rectum and Mullerian
duct, which forms the perineum. At a latter stage the bladder
forms a rounded vesicle, and the urogenital sinus becomes much
more shallow. Fig. 188 represents a stage in the male corresponding to fig. 190, B, before the urogenital orifice has become
enclosed by the base of the raphe of the penis.
 
Mammalian External Generative Organs.  - The external generative organs of
the Eutheria develop as follows :  - Anteriorly to the cloaca an elevation (genital eminence) appears, and surrounding it in front and on each side is a large cutaneous
fold (fig. 190). The anus is next separated from the urogenital sinus by the formation of the perineum. The genital eminence grows rapidly, forming a cylinder,
which is grooved on its posterior surface ; the two folds of the groove extend back
 
262
 
 
THE STUDY OF EMBRYOLOGY.
 
 
wards, so as to lie between the urogenital orifice and the large folds (fig. 190, b). So
far the development is precisely the same for both sexes.
 
In the female the genital eminence usually remains comparatively small, and is
known as the clitoris ; its groove becomes less marked ; the posterior edges of the
groove persist as the nymph se or labia minora. The anterior portion of the large
cutaneous fold becomes the mons veneris, and the lateral folds greatly increase in
size, so as, in most cases, to enclose the clitoris and constitute the labia majora.
 
In the male the genital eminence increases in size to form the penis. The margins
of the groove close over, so as to convert it into a canal, the posterior ends at the
same time growing over the urogenital orifice, so that the urogenital sinus is directly
continued through the penis. The lateral portion of the large cutaneous fold
unite together behind the penis, and fusing in the middle line, form the scrotum  -
the raph^ indicating the line of junction. In some of the lower Mammals {e.g.,
Rabbit) the scrotal sacs remain distinct. The so-called “ urethra - of the male consists of three distinct regions : (1) the urethra proper, “prostatic portion,- extending from the neck of the bladder to the orifices of the vasa deferentia and the uterus
masculinus ; (2) the urogenital sinus, “membranous portion and (3) the canal of
the penis or “spongy portion,-
 
On each side of the urogenital sinus corresponding to the large fold is a perforation
of the inner wall of the abdomen, which is known as the internal inguinal ring. In
 
 
 
Fig. 190. - Development of the External Sexual Organs in the
Human Male and Female from
the Undifferentiated Condition. [After Ecker.]
 
A. Embryo of about nine weeks, in
which the external sexual distinction is
not yet established and the cloaca still
exists. B. An older embryo, without
marked sexual distinction ; the anus is
now separated from the urogenital aperture. C. Female embryo of about ten
weeks. D. Male embryo somewhat more
advanced.
 
a. anus ; can. tail : c. clitoris ; cl. cloaca ;
l. labium ; Is. undifferentiated sexual
fold ; p. penis ; p.c. undifferentiated sexual eminence ; s. scrotum ; ug.o. urogenital opening.
 
 
the male a sac-like diverticulum of the peritoneum, the processus vaginalis, passes
through the abdominal ring into the scrotum. In some Eutheria the testes always
remain within the abdominal cavity, but in others they temporarily or permanently
pass through the abdominal ring, and into the peritoneal pouch within the scrotum.
Normally, in those animals in which a permanent descent of the testes occurs, the
inguinal rings close, and the testes are enclosed within a serous sac ; when this
does not take place, a portion of the intestine may force its way through the ring
into the scrotum, and thus produce a hernia.
 
 
Generative Organs.  - The sexual cells are usually developed
from a distinct epithelium; the Sponges form an apparent exception, as the sexual cells are derived from the mesenchymatous
mesoderm, which is itself, however, probably solely derived from
the endoderm.
 
Weismann and others have recently shown that, as a rule, the
sexual cells arise from the endoderm of the stolon or stems of the
fixed Hydroids, and subsequently migrate to what are termed the
 
 
ORGANS DERIVED FROM THE AIESOBLAST.
 
 
263
 
 
generative organs. These latter may be situated within fixed
(sporosacs) or detachable (medusoids) lateral buds or gonophores.
The sexual cells mature in their secondary location. The sexual
cells of Hydra are usually stated to be of ectodermic origin, but
the prevalence of the former mode of origin in the marine Hydroids,
combined with the fact of the presence of chlorophyll in the ovum
of Hydra viridis, render it quite possible that a migration occurs
also in this degraded fresli-water form.
 
In all the other Coelenterates the ova and spermatozoa arise
from the hypoblast of the mesenteric pouches or canals.
 
Lang states that in certain Turbellarian Worms (Polyclades)
the sexual cells are developed at the expense of the epithelium
of the gastric diverticula, that is, from the hypoblast.
 
Nothing definite is known concerning the development of the
generative glands of Molluscs.
 
In Sagitta, although it belongs to the Coelomata, a pair of
primitive sexual cells appears as early as the gastrula stage, subsequently each cell develops into the ovary and testis of its side.
 
It is characteristic of most, if not of all the Coelomata, that the
generative organs arise from the epithelium of the body-cavity.
There are no precise accounts of the mode of formation of the
generative organs, or gonads, as they are more concisely termed by
Lankester, amongst the Invertebrates. The structure of such
organs is never complicated, and the dehiscence of free epithelial
cells, as in the case of ova, is not specially remarkable.
 
The maturation of the ovum and its acquisition of food-yolk,
and the difficult problem of spermatogenesis, have already been
alluded to (p. 14).
 
In the Vertebrates the germ-cells are modifications of a special
linear tract (germinal epithelium) of the peritoneum, between the
mesonephros and the insertion of the mesentery (fig. 175, 'p.o).
The germinal epithelium may project more or less into the bodycavity to form a germinal ridge (fig. 178, c).
 
It is now possible to make a general statement and affirm that in the great majority
of cases, at least, the sexual pells arise from the endoderm (hypoblast) in the
Acoelomata ; but in those forms in which the archenteron is produced into radial
pouches, chambers, or canals, they occur on the walls of such diverticula.
 
In the Coelomata the gonads are developed from the coelomic epithelium ; but as
this is derived primitively from archenteric diverticula, the generative epithelium
is practically a homologous tissue throughout the Metazoa.
 
The sexual products may find their way to the exterior by very
different means. In some cases it is by the rupture or destruction
 
 
264
 
 
THE STUDY OF EMBEYOLOGY.
 
 
of the parent ; they may migrate through the parental tissues, or
dehisce into the body-cavity.
 
From the body-cavity they may pass to the exterior through
abdominal pores (Cyclostomi and some Teleosts), or be conveyed
by more or less modified nephridia (Chsetopoda, Gephyrea, Brachiopoda, Mollusca, and some Vertebrates (see p. 2 37).
 
External generative or copulatory organs occur in the higher
members of many groups, to render more certain the fertilisation
of the ovum.
 
 
( 265 )
 
 
CHAPTER VIII.
 
GENERAL CONSIDERATIONS.
 
Complexity of Embryological Phenomena. - -The phenomena
of Embryology are of a very complex nature, owing to abbreviation
or precociousness in the development of certain organs, and in
the occurrence of a series of transformations which have reference
solely to the ancestry of the individual, the latter often bearing no
discernible relation whatever to the adult condition.
 
The irrelevance of these metamorphoses to the adult state is in
some cases emphasised by the fact of their suppression in certain
members of a group, as, for example, amongst the Scyphomedusse.
The genus Pelagia, although closely related to Aurelia, develops
directly from the egg wuthout the intervention of the Scyphistoma
larva; and even Aurelia may abnormally have an abbreviated
development. The characteristic larval forms of the Echinozoon
Echinoderms are omitted in the development of those forms in
which the young are reared in brood-pouches or similar protective
chambers. The following will serve as types :  - Leptychaster kerguelenensis, Ophiacantha vivipara, Hemiaster cavernosus, and
Psolus ephippifer. The direct development of Astacus is an
example of the suppression of metamorphoses amongst the Crustacea, but in this Decapod a good deal of food-yolk is present.
 
The passing through of a free larval existence must be considered
as constituting a drain upon the energy of the organism, and this
loss naturally affects the adult condition. As Sollas points out,
when such a larva “ finally reaches the adult state, it has already
to a considerable extent worn out its machinery and expended its
powers of converting energy. A still more important consequence,
however, would seem to follow from the premature aging due
to a free larval existence, and that is the comparatively early
exhaustion of the powder of undergoing transformational change ;
the adult or comparatively stable state is reached sooner than it
otherwise would be, and the chances of further development are
 
 
266
 
 
THE STUDY OF EMBRYOLOGY.
 
 
correspondingly diminished.- It has been pointed out by several
authors that the individual which is best equipped as an adult
is that which has rapidly passed through its embryonic condition
under circumstances where it has been extraneously nourished
and protected. Again, to quote from Sollas, “ The longer life in
the mature state, acquired by those forms which are saved from
the drudgery of a larval existence, offers increased opportunities for
evolution to the adult animals, so that a progressive development,
starting from higher and higher platforms, is directly favoured.
But not only is a longer existence assured to the adult  - existence
in the embryonic state is shortened ; and perhaps here the influence
of seclusion is most clearly exhibited, for the energy which would
be expended in a free larva in activities other than those involved
in producing structural change is here solely devoted to that end,
and hence the embryonic stages are passed over by secluded forms
with comparative rapidity.-
 
In studying the development of animals, it must always be
remembered that what is known as the “ struggle for existence - is
continually acting upon the larval form as an individual, and that
while the larva has to adapt itself to present conditions and to
supply its own wants, the rudiments, or the formative tissue
(blastema), of future organs may be precociously formed. This is
the main reason for the complications and abbreviations which
occur so frequently in the development of animals. Occasionally
larval forms, so to speak, run wild, and do not develop into their
normal adults, the form known as Leptocephalus amongst Teleosts
affording a good example of this vagary.
 
The real nature of many einbryological phenomena must remain
unknown until the properties of protoplasm are considerably more
elucidated. At present, we can deal only with the results, and not
with the causes of changes in organic matter.
 
In the course of this work attempts have been made to indicate
how certain organs may have been developed from pre-existing
simpler structures in response to definite stimuli or to the requirements of the organism. The further our knowledge extends the
more certain it appears that evolution is mainly the result of a
mechanical necessity, or, as James Hinton put it, “organic forms
are the result of motion in the direction of least resistance.-
 
Suggestions as to the possible significance of observed embryological facts must be held only in the most tentative manner. It
is easy to frame plausible theories respecting the evolution of
 
 
GENERAL CONSIDERATIONS.
 
 
267
 
 
organs or of the animals themselves, but great caution is necessary
in accepting them, and, at best, they should be regarded as merely
working hypotheses.
 
Sketch of a Possible Evolution of the Metazoa.  - The Protozoa combine all the essential activities of life within the limits
of small independent units of protoplasm, and even in these
differentiation may occur to a considerable extent. Those causes
which result in the production of complicated organs in the
Metazoa also act on unicellular forms, but, having less scope, the
result is less evident. The higher organisation of multicellular
animals is solely attributable to the large number of aggregated
units which constitute their body; the forces acting upon all living
beings must be the same.
 
The formation of masses or colonies of cells (aggregates of protoplasmic units) may possibly be primarily due to imperfect
fission. Cell-division itself ( ie ., reproduction) is usually regarded as
being primitively due to excess of growth consequent upon excess
of nutrition ; Geddes, however, suggests a different interpretation
of the origin of cell-division (p. 279). Amongst the Protozoa reproduction results in the formation of distinct and independent
organisms, each one of which is unicellular like its parent. In
only a few forms are individuals aggregated into colonies, and
in these but little co-ordination occurs.
 
More precise histological research is now demonstrating that in
most, if not in all, animal (and vegetable) tissues the component
cells are united together by strands of protoplasm, often of extreme
tenuity. There may thus be a protoplasmic continuity extending
throughout the whole organism, and possibly all the living cells of
an animal are directly or indirectly connected with one another,
except the lymph and blood-corpuscles.
 
The observations of Sedgwick on the syncytial segmentation
of Peripatus (fig. 19) are in this respect very suggestive, and it
may yet be proved that the complete division of an ovum into
distinct segmentation spheres (fig. 12) is apparent rather than
real.
 
It appears that all the cells of adult Coelenterates are connected
together by means of protoplasmic processes, and it might fairly
be assumed that the cells of a segmented ccelenterate ovum and of
the embryo into which it will develop are similarly united ; but
there is at present no definite embryological evidence to support
this conclusion. The cellular network of the parenchymula larva
 
 
268
 
 
THE STUDY OF EMBRYOLOGY.
 
 
of Obelia (fig. 46) is, according to Merejkowsky, a secondary condition due to the fusion of the processes of amoeboid cells.
 
Whether directly continuous or not, all the cells of a Metazoon
are so grouped as to constitute a co-ordinated whole, the life of
the individual being the sum-total of the activities or lives of the
constituent cells. Theoretically each one of these cells possesses
all the attributes of protoplasm, as, most probably, was actually the
case when the ancestral form was passing from the Protozoon to
the Metazoon condition, a stage which is now represented by the
blastula larva. We may assume that each cell then possessed
nutritive, sensory, metabolic, and reproductive functions ; but in
process of time specialisation occurred, and the concurrent limitation of function resulted.
 
In unicellular animals one pole or aspect of the body is usually
concerned in the ingestion of food, and we are justified in assuming
the same for the Protozoon ancestor of the Metazoa.
 
The segmentation of the ovum is stated to occur in two different
ways. Either, according to the generally received account, it may
from the first divide the cell horizontally into a nutritive (vegetative) and sensory (animal) portion ; or, according to Agassiz and
Whitman, the ovum may divide longitudinally (axially), then transversely, and lastly horizontally. In either case a multicellular
mass is formed, of which the upper pole is more especially sensory
(epiblast) and the lower nutritive (hypoblast). Assuming it to
have been flattened, Biitschli has termed this theoretical ancestral
form a Plakula (p. 23).
 
The series of stages from an unicellular form to an organism,
consisting of two sets or layers of cells, presents us with no special
difficulty, and plausible theories have been framed to account for
the formation of a double-layered gastrula from the single-layered
blastula. It is a matter of some importance to note that embryological evidence, as a whole, supports the conclusion that the
future epiblastic (ectodermic) and hypoblastic (endodermic) cells
are already practically differentiated in the blastula stage, and
that the gastrula was evolved as a result of that differentiation.
It is too often assumed that all the cells of the blastula are
identical in every respect.
 
Brief History of Mesoblastic Tissues.  - The conversion of a
diploblastic form to one with three layers (triploblastic) is readily
conceivable. It is possible that the third layer (mesoblast) primitively arose as the result of excessive nutrition of the nutritive
 
 
GENERAL CONSIDERATIONS.
 
 
269
 
 
cells (hypoblast). The inner moieties of these cells separating
themselves as amoeboid cells (mesamoeboids, or archaeocytes of
Sollas), which would then crawl about in the space (segmentation
cavity) between the two layers. Similar cells arise in some
embryos from the epiblast also. These cells would readily assume
the amoeboid condition, as they were not subject to pressure and
had sufficient space for migration.
 
Whether originally specially nutritive or not, the wandering
cells would readily become modified and change their function ;
their contractile power might be emphasised, and thus they might
be converted into simple muscle-cells. By the secretion of mineral
matter they would form skeletogenous cells. By retaining a free
existence others would serve as carriers of matter, or, in other
words, become corpuscles of the nutrient fluid.
 
Most of the internal supporting (endoskeletal) elements, with the
exception of the notochord of the Chordata and the connective
tissues, are, together with the blood-corpuscles and vascular system,
developed from the mesoblast. Lankester has associated these
series of tissues under the common designation of “ skeletotrophic.-
This he regards as a “ natural group of tissues which is divisible
into  - (i.) Skeletal, including fibrous, adenoid, adipose, bony, and
cartilaginous tissues. (2.) Vasifactive, including capillaries and
embryonic blood-vessels. (3.) Haemolymph, including the haema
or haemaglobinous element and lymph, the colourless element of
vascular fluids.-
 
Lankester further points out that “ the mother-cells of all tissues
are either ‘ entoplastic  - or ‘ ectoplastic, - or both  - that is to say,
the metamorphosis of their protoplasm is either essentially one
occurring at the surface of the protoplasmic corpuscle, or one
occurring deeply within its substance, or the two processes may
go on in connection with the same cell.- Thus hyaline cartilage
is essentially ectoplastic, while notochordal tissue results from a
metamorphosis of the cells and is essentially entoplastic. “ Fibrous
tissue generally is ectoplastic, as the protoplasmic corpuscles remain more or less intact whilst surrounded by the fibrous and
lamellar masses to which they have peripherally or laterally given
origin. This is true of ordinary subcutaneous areolar tissue, of
tendon, of mucous tissue (umbilical cord), and of corneal tissue.
At the same time we find in various Invertebrate groups (not in
the Vertebrata) an entoplastic form corresponding chemically aud
functionally to the ectoplastic forms just cited. This is the vesi
 
270
 
 
THE STUDY OF EMBRYOLOGY.
 
 
cular connective tissue so abundant in the Mollusca, in the
Nemertines, and other Invertebrates. The only tissue which in
form represents this among the connective tissues of Vertebrates is
adipose tissue.- The vesicular cells of Mollusca contain glycogen ;
indeed, a glycogenetic function is now known to be widely distributed in various mesoblastic tissues.
 
“Yet further, the tissues of the connective group which are
specially related to the nutrient fluids (such as blood and lymph),
and which form the wall of the coelom or of blood-channels, may
be entoplastic when they give rise, by internal metamorphosis
(liquid vacuolation), to capillary vessels ; or ectoplastic when they
constitute spongy or lacuniferous cell aggregates, the cells separated
by intercellular channels, such as we find in the ‘ pulp * of lymphglands and the spleen, and in the lacunar tissue of Molluscs.-
 
The formation of gastric pouches (archenteric diverticula) appears
to have resulted from the disproportional growth of the hypoblast.
In forms higher than the Ccelenterates these pouches were constricted off from the central cavity and formed a true body-cavity
or coelom. A nutritive fluid might collect by osmosis within the
body-cavity.
 
The progression of the organism in a determinate direction
would ensure a bilaterally symmetrical arrangement of the organs
of the body, and, consequently, of the archenteric diverticula. A
dorsal and ventral mesentery would result from the appression
of the inner walls of the confluent lateral coeloms, while transverse
mesenteries or septa would occur if the coeloms of the segments
remained distinct.
 
The primitive nutritive corpuscles (mesamoeboids) lie within
the blastocoel (or, as Hubrecht proposes to term it, the archicoel),
and consequently outside the archenteric diverticula. When the
coelomic walls were approaching one another, many of the corpuscles
would be enclosed between them ; and if a small space was left
between the walls of the coeloms, a tube would be formed, lying
within the mesentery, containing amoeboid corpuscles. The walls
of the coeloms possess actual or incipient muscle-fibres, and are
therefore contractile. The contractility of the walls of the mesentery would thus result in a longitudinal contractile tube containing
corpuscles, in other words, a vascular system would be initiated.
The development of the heart in both Vertebrates and Chsetopoda
appears to support this hypothesis of its evolution.
 
Hubrecht claims for the blood- vascular system of the ISTeinertiue
 
 
GENERAL CONSIDERATIONS.
 
 
271
 
 
Worm Linens that it arises merely by the “ connective tissue- not
obliterating the archiccel in these places, and that the indifferent
mesoblast is modified in situ into the endothelium and walls of the
vessels. In most other animals the smaller vessels are formed by
the hollowing out of solid cell-rows and cell-groups.
 
It would be rash to hazard a conjecture concerning the evolution
of the excretory organs until we have more precise information
concerning their development in the lower Metazoa. It is not
improbable that there is no genetic connection between the excretory organs (nephridia) of certain groups ; thus it is difficult to see
the homology in such organs as the green gland of Decapod with
the excretory tubes of Amphipod Crustacea, or these again with
the nephridia of Peripatus and the Malpighian tubules of Insects.
The Vertebrate excretory organs appear almost certainly to have
been evolved from some primitive form of nephridium, from
which the nephridia of the Segmented Worms were independently
differentiated.
 
Embryonic Digestion.  - But little is known concerning digestion and assimilation in embryos. The actual processes must be
assumed to be essentially similar to those occurring in adults. The
following general features, which alone can be dealt with here, are
worthy of notice.
 
As was mentioned very early in this work, an oosperm must be
regarded as an amoeboid Protozoon, which multiplies by fission
very rapidly, but which is precluded from obtaining fresh nutriment directly. The energy requisite for this enormous activity is
provided by the breaking down, through digestion, of the highly
nitrogenous food-yolk which is stored up within the body of the
ovum.
 
In many cases the stored-up nutrient material, yolk, is really
derived from neighbouring ovarian cells which the ovum has
swallowed. (This process, which is simply a case of feeding,
must not be confounded with the formation of a plasmodium or
syncytium by the fusion of previously distinct protoplasmic units.)
The ovum has, in fact, gorged itself preparatory to entering upon
a stage of rapid cell-division. The telolecithal and centrolecithal
distribution of the yolk in the ovum and developing embryo has
been already referred to. In the former case the yolk is actually
stored up within the primitive hypoblast cells, that is, within those
very cells whose function is to digest it. In the second case the
yolk is afterwards transferred to those cells.
 
 
272
 
 
THE STUDY OF EMBRYOLOGY.
 
 
i. Hypoblastic Digestion.  - The act of digestion is almost
entirely performed by the hypoblast. From the nature of the
case all Protozoon digestion must be intracellular, that is, must be
effected within the cell itself. It is now proved that the digestion
of the Coelenterates and of some Turbellarian Worms is largely
intracellular, although extracellular digestion also occurs to some
extent. Even in some of the lower Vertebrates the epithelial cells
of the intestine may send out pseudopodia for the purpose of
ingesting fragments of partially digested food. In other words,
the lower Metazoa have not yet broken away from the traditions
of Protozoon digestion. In this respect early embryos of higher
Invertebrates reproduce the ancestral condition ; for we find in the
Crustacea (fig. 22) that the hypoblast of the gastrula stage feeds
upon the yolk by means of pseudopodia, and the digestion is intracellular.
 
Caldwell states that throughout larval life intracellular digestion
occurs in the first stomach of Phoronis, but that this mode of
digestion ceases when the metamorphosis takes place.
 
Kollmann has recently shown that in the meroblastic ova of the
Lizard and Fowl (fig. 66) the primitive cells of the germinal wall,
in the equivalent of the gastrula stage, engulf and digest the yolk
spheres and granules like an Amoeba eating its prey.
 
It is probable that extracellular digestion, as it occurs in the
more specialised Metazoa, does not take place till ‘‘hepatic- or
other secretory cells make their appearance. Most Prosobranch
Molluscs, such as Purpura and Fusus, possess a large quantity of
food-yolk which is stored up within the hypoblast cells (fig. 18),
and the digestion of which is consequently intracellular. It is
well known that during the veliger stage these Molluscs are truly
cannibals and devour their weaker brethren. This new food
passes into the mesenteron (archenteron), and certain of the hypoblast cells acquire a very different appearance from the remainder
and constitute true digestive cells. Food in process of digestion
is seen within the cavity of the mesenteron. As a matter of fact,
the two modes of digestion take place simultaneously until the
yolk is quite absorbed.
 
This view is rendered the more probable from the fact that in
the Ichthyopsida the distinctive complex digestive glands are
either not at all or only slightly developed. Each individual cell
of the mesenteron may be regarded as individually digestive, and
thus in these forms hypoblastic intracellular digestion occurs.
 
 
GENERAL CONSIDERATIONS.
 
 
273
 
 
Temporary pseudopodia, for the seizure of food particles, are very
generally emitted by the cells of the intestinal epithelium in the
lower Vertebrates. Such highly differentiated glands as the peptic
glands and the glands of Lieberkiihn are found, from the Reptilia
upwards, in an increasing degree. Their secretion acts chemically upon the whole or a portion of the food, and digests it within
the cavity of the alimentary canal. The liver has been omitted in
this connection, as it is not, in the true sense of the term, a digestive
gland. As Wiedersheim has pointed out, there is a well-marked
correlation between the folds of the mucous membrane and the
development of intestinal glands. At first, as in the Cyclostomes,
the folds have only a longitudinal direction, but afterwards transverse folds appear and crypts are formed in order to increase the
secretory surface of the alimentary canal.
 
In Mammals the embryo is nourished directly by the blood
of its mother, and the hypoblast of the foetus has never been
functional in digestion ; it consequently requires what Sollas has
termed a gastric education before it can digest the food of the
adult. (This argument does not apply to those Sharks and Lizards
in which there is a slight connection between the yolk-sac of the
embryo and the blood-vessels of the wall of the oviduct, as in
these forms a large amount of food-yolk is always present.) The
secretion of milk by the mother supplies a readily assimilable
pabulum, and the peculiar character of the first-formed milk probably renders the education still more gradual. A somewhat
similar digestive education occurs in some Birds, such as Pigeons,
the Flamingo, and others.
 
2. Epiblastic Digestion.  - The epiblast very rarely appears to
have a digestive function. Metschnikoff, however, has observed
intracellular ingestion by the ectoderm cells of larval Actiniae, and
Kollmann states that the epiblastic cells of the blastoderm of certain
Sauropsida can take up food by means of pseudopodia and digest
it in the intracellular manner. It has been previously noted that
villi develop from the epiblast which underlies the yolk-sac in
Birds (fig. 75, B, v ), and also from the epiblast of the allantoic
folds (c, v) } which absorb the remaining albumen of the egg. In
the lower Mammals (fig. 8o) similar villi occur, which must be the
means of absorbing nutriment from the uterine wall.
 
3. Mesoblastic Digestion.  - The undifferentiated wandering
mesoblast cells may also be concerned in digestion, but their
ingestion of foreign particles may be due in many cases to the
 
s
 
 
274
 
 
THE STUDY OF EMBRYOLOGY.
 
 
mechanical properties of their protoplasm rather than to an actual
selection. Our knowledge concerning the behaviour of these cells
in embryo Invertebrates is almost entirely due to Metschnikoff,
who has proved that the mesamoeboids are of great physiological
importance from their first appearance, and in this respect they
offer a marked contrast to the mesothelial mesoblast.
 
In Echinoderm larvae, for instance, which undergo rapid metamorphoses, the disappearing organs break down into albuminoid
globules, which are devoured and digested by the mesamoeboids,
or phagocytes, as Metschnikoff terms them. The latter also ingest
small foreign particles which may be forced into the segmentationcavity. In many cases the mesamoeboids fuse to form a plasmodium
or giant cell, in order to effect this more readily ; in some cases the
mesamoeboids merely collect round the foreign body in order to
isolate it.
 
The lymph-corpuscles (leucocytes) have been shown by Wiedersheim and by Schafer to perform an important part in digestion in adult Vertebrates. These cells have been proved to force
their way through the mucous membrane into the cavity of the
intestine, and there to devour fat, and probably amyloid particles ;
they then return, and, crawling between the epithelium cells, pass
into the lacteals. Others, again, merely ingest the food particles
which have penetrated through the intestinal epithelium. In all
cases, probably, the leucocytes pass into the lymphatics, where
their contents are discharged by the disintegration of the cells
themselves. The lymphatic fluid or chyle then passes into the
general circulation, carrying with it the digested food which has
been conveyed from the intestine by the leucocytes, and a large
amount of proteid material derived from their dissolved protoplasm
and nuclei.
 
It is still an open question whether digestion may not be performed in Sponges by the ectoderm as well as by the endoderm.
The wandering mesoderm cells are probably concerned in the
conveyance of nutriment and the removal of waste products, in
addition to those functions which are more generally regarded as
typical of that layer.
 
Closely associated with the subject of embryonic digestion is
the part which the foetal membranes of Amniote Vertebrates play
in nutrition. The reader is referred to the section which deals
with these structures (pp. 78-96) for a summary of the evolution
of the foetal membranes of the Amniota. The gradual Requisition
 
 
GENERAL CONSIDERATIONS.
 
 
275
 
 
by the allantois of the whole of the nutrition of the embryo is
especially noteworthy.
 
Embryonic Respiration.  - The function of respiration must of
necessity occur throughout the whole of embryonic and larval life.
As a rule, it is more active in larvae than in adults ; at all events,
the former always speedily succumb to a deficiency in the supply
of oxygen.
 
The true respiratory process, i.e., the assimilation of oxygen and
the excretion of carbon dioxide, occurs in the ultimate tissues ;
it is only the exchange of the latter gas for the former of the external medium which occurs in what are termed respiratory organs.
 
As Dohrn points out, it is the vascular system which is really
respiratory, and the pressure of a blood-vessel against an epithelium
would cause an evagination of that tissue, be it epiblastic or hypoblastic. Of course the whole skin of the body and the alimentary
tract were the primitive respiratory surfaces. The production of
gill-filaments on a given area is the result of the presence of
blood-vessels ; it is the latter after all, and not merely epithelial
prolongations, which constitute gills.
 
It often happens that in embryos and larval forms the delicacy
of the tissues suffices for the interchange of the gases, so that
special respiratory surfaces are not required. When protective
envelopes are present, they are usually very permeable to gases.
 
The proctodaeum serves as a special respiratory organ in certain
larval Arthropoda; as, for instance, in the ISTauplius larvae generally,
and in the aquatic larvae of Dragon-flies.
 
The higher organisation of the embryos of Vertebrates necessitates a large supply of oxygen, and, consequently, special provision
has to be made by the development of larval respiratory organs,
especially in those forms which undergo a secluded development.
These may either be (i) the phylogenetic respiratory organs, which
are utilised in the ontogeny of the individual, or (2) they may
bear no relation either to the ancestral or to the adult respiratory organs. A pair of examples of each of these two cases will
illustrate the general principle.
 
1. Utilisation of Phylogenetic Respiratory Organs in Ontogeny.  - The ordinary hypoblastic gills of Elasmobranchs appear
early in the embryo, but the filaments on the posterior aspect of
the archs are greatly elongated, so as to form a very characteristic
fringe of gills, which have even been regarded as belonging to a
different category from the normal filaments.
 
 
276
 
 
THE STUDY OF EMBRYOLOGY.
 
 
Both external and internal, i.e., epiblastic (?) and hypoblastic
gills occur in the newly hatched tadpoles of Frogs. Whatever may
be the exact significance of the former, the latter certainly are an
example of the utilisation by a larva of the ancestral mode of
respiration, as the respiratory organs of the adult, in this case the
skin and lungs, have no connection whatever with the former.
 
2. Secondarily Acquired Larval Respiratory Organs.  - The
embryonic respiration of the Arnniota affords a good example of
the second proposition. In none are the walls of the visceral
clefts functional as gills at any time, and, as the lungs are only
functional after birth, accessory respiratory organs must be provided.
 
In Sauropsida the area vasculosa of the yolk-sac forms the first
respiratory surface, this function is next shared with the rapidly
developing vascular allantois, and lastly, owing to its enormous
size, the allantois becomes the sole respiratory organ. As has
been mentioned above (p. 259), the allantois is probably the hypertrophied and precociously developed urinary bladder, and we may
assume that the ancestral forms of the Arnniota, like the Amphibia,
had a large membranous vascular urocyst, which was capable of
being early utilised as a respiratory organ. The topography of the
allantoic blood-vessels, and the fact that the proximal portion of
the allantois actually persists as the adult bladder, support this
view. It is well known that egg-shells are very porous to gases.
 
Respiration in the embryos of the Prototherian Mammals is
doubtless perfectly comparable with that in Reptilian embryos,
whereas, in the Eutheria, aerial respiration is impossible owing to
the embryos being included within the uterus. The foetus in utero
has, however, no need for special organs of respiration, as it is
supplied with aerated arterial blood direct from the main arterial
trunks of the parent. The carbonic acid and other waste products of the embryo are carried away by the maternal venous
circulation.
 
Evolution of Nervous System and Sense Organs.  - The epi
blast naturally forms the protective covering of the organism, and
would readily be modified to meet special requirements. From its
position it would be directly subjected to every vibration in the
external medium, and would therefore be continually receiving
numerous stimuli, which would call into play the sensibility of the
protoplasm of the cells. It is, then, no wonder that sense-cells
originated, or that these became grouped together to form sense
 
 
GENERAL CONSIDERATIONS.
 
 
277
 
 
organs, or that a further differentiation occurred which resulted in
the evolution of a highly specialised nervous system.
 
All these obvious facts were sufficiently noticed in the section
on the “ Organs derived from the Epiblast,- and therefore need not
be reiterated here.
 
Continuity of Germ-Producing Tissue. - The germ-producing
tissue is to be regarded as the direct product of the similar tissue
of its parent, that is to say, a portion, however minute, of germinal
substance is transmitted by the parent to its offspring. The
germinal substance in the latter is increased by the ordinary
method of nutrition and growth, but it still has the same essential
character that was transmitted to it. The offspring in its turn
passes on this germinal substance. There is thus a continuity of
germinal matter, which, since it is transferred in an extremely
minute quantity, must have an inconceivably complex structure,
as it possesses the power of transmitting hereditary characters
even of the most trivial nature.
 
It is maintained by some that the nucleus is the essential
element in the germ-cell, whether ovum or spermatozoon, and that
the cell-protoplasm is merely a nutritive basis. The structure of
the ovum has already been stated to be similar in many respects
to that of ordinary undifferentiated tissue-cells (fig. 5*). The
distinguishing feature of the nucleus over the rest of the protoplasm of the cell consists in its possession of chromatin. As the
chromatin always takes a conspicuous part in segmentation, we
are justified in assuming that the chromatin or nuclein is concerned in the reproductive function. Fertilisation appears to
be mainly the fusion of the nuclein elements of a pair of cells
which are liberated from usually two parents. The resulting compound oosperm develops by segmentation and ulterior differentiation into an organism resembling, and at the same time
differing from, each of its parents both in feature and in inherited
tendency.
 
Weismann recently proposed the view that the nucleus of every germ-cell contains
“ germ-plasma ,- or that substance which enables the germ-cell to build up a new
individual ; and “ histogenetic plasma,- or that substance which enables the germ-cell
to accumulate yolk, secrete membranes, or, in short, to develop itself into its characteristic structure as a ripe ovum or spermatozoon.
 
It is the germ-plasma alone that is required for the development of the embryo.
The histogenetic plasma, having performed its function of building up the germ-cell,
is useless, and has to be got rid off ; so it is extruded as the polar-cells, or as the
passive element in the male germ-cells. If the germ-plasma left in the ovum has
sufficient vigour (which would probably depend upon its quantity), there is nothing to
 
 
278
 
 
THE STUDY OF EMBRYOLOGY.
 
 
prevent its further development into a new individual  - that is, nothing to hinder the
occurrence of parthenogenesis. As a matter of fact, however, this is rarely the case,
and it requires the sudden accession of fresh energy in the shape of a spermatozoon to
enable the germ-plasma of the ovum to further develop. In this view there is no
essential distinction between the nucleus of the ovum and that of the spermatozoon ;
the latter, like the former, is merely germ -plasm : the difference being that, as a rule,
the germ-plasm of the male cell has an entirely different series of inherited characters,
which it can transmit to the segmentation nucleus in the same manner as those of the
female cell are transmitted.
 
The essential act of fertilisation, therefore, does not consist in the fusion of elements
which differ in kind, but merely in the sudden accession of a store of energy which
will enable the ovum to segment and build up a new individual. This brings fertilisation to resemble conjugation yet more closely ; and it further explains how it is
that, in those forms in which parthenogenesis is not known to occur, the ovum may
segment, and proceed a short way on its development.
 
This theory also agrees well with certain facts concerning the asexual reproduction
of animals or plants. During segmentation there is formed in the nuclei of the segmentation-cells fresh histogenetic plasma, which is more especially concerned in the
differentiation of the tissues ; but the germ-plasma may be generally diffused, or it
may be early localised within certain segmentation-cells. Sponges, the Hydra, some
Sea- Anemones, may be taken as examples of the former condition, as in these animals
apparently any portion of the body containing ectoderm and endoderm will serve to
produce a new individual ; and in the case of the two first-named, the germ-cells themselves appear to arise indiscriminately from the mesoderm in the former, and from the
ectoderm (?) in the latter. In other Ccelenterata the germ-cells are of hypoblastic
origin. In the second case, where the germ-plasma is localised to a special tissue,
those segmentation-cells which will form the epiblast possess no germ-plasma, and, consequently, they can only build up specialised tissue. On the other hand, in most
cases at all events, the germ-plasma, which at first is restricted to the nuclei of the
hypoblast cells, becomes, as development takes place, still further localised until it is
situated solely in that tissue which has for its especial function the reproduction of
the individual. In other words, it is restricted to the generative gland. Asexual
reproduction in such groups as the Polyzoa and Ascidians, and certain "Worms, is
rendered possible by the retention of germ-plasma within certain undifferentiated
tissue (funicular tissue, stolon, budding zones, &c.), from which the whole or part of
the new individual may be formed ; but it is impossible to reproduce a perfect individual from any fragment containing epiblastic and hypoblastic tissue, as can be
done in the case of Sponges or the Hydra. In this connection it is interesting to find,
as Gruber has shown, that if an Infusorian be artificially divided, each portion will
become a perfect individual. But if the dismembered portion does not possess a fragment of the original nucleus, the animal thus produced lacks the power of reproduction.
It is perfect in every respect, except that it is deprived of the germ -plasma, which
alone possesses the reproductive function.
 
Geddes has recently discussed the theory of growth, reproduction, sex, and heredity
in terms of the metabolism of protoplasm. “ Protoplasm is regarded as an exceedingly
complex and unstable compound, undergoing continual molecular change or metabolism.
On the one hand, more or less simple dead matter or food passes into life by a series
of assimilative ascending changes, with each of which it becomes molecularly more
complex and unstable. On the other hand, the resulting protoplasm is continually
breaking down into more and more simple compounds, and finally into waste products.
The ascending synthetic constructive series of changes are termed anabolic, and the
descending disruptive series TcatabolicP
 
Growth.  - Herbert Spencer first pointed out that in the growth of- similarly shaped
 
 
GENERAL CONSIDERATIONS.
 
 
279
 
 
bodies the mass increases as the cube of the dimensions, the surface only as the
square, and applies this conception to express the occurrence of cell-division. “Thus,-
as Geddes expresses it, “ in the growing cell the nutritive necessities of the increasing mass are ever less adequately supplied by the less rapidly increasing absorbing surface. The early excess of repair over waste secures the growth of the cell,
but the necessarily disproportionate increase of surface implies less opportunity for
nutrition, respiration, and excretion ; and waste thus overtakes, balances, and
threatens to exceed repair. Three alternatives are then possible  - (i) a temporary
equilibrium may be established and growth ceases, or (2) the increase of waste may
bring about dissolution and death, or, still more frequently, (3) the balance of mass
and surface may be restored by the division of the cell.-
 
“ Reproduction  - (a) Asexual.  - Continued surplus of anabolism involves growth ;
this growth is sooner or later checked by the preponderance of katabolism, and the
most frequent alternative is the restoration of the balance by cell-division. Thus
arises discontinuous growth or asexual reproduction. Budding, simple-division, and
spore formation, like continuous cell-division, are simply different forms of the
necessary separation which must occur at the limit of growth if the continuity of life
is to be preserved. Like continuous cell-division, asexual reproduction occurs when
waste or katabolic processes are in the ascendant. But what holds true in the growth
of the individual cell is valid also in regard to the aggregate. There, too, a limit of
growth must eventually be reached, when discontinuous growth in some form becomes
inevitable. The essential difference is simply that at first in the unicellular individual
the disintegration and reintegration entirely exhaust the organism and conclude its
individual existence, while in higher forms the process becomes more and more
localised.-
 
(6.) Sexual Reproduction.  - A comparative study of the methods of reproduction
which occur amongst the lower plants and Protozoa will demonstrate that “the
almost mechanical flowing together of exhausted cells, as illustrated in plasmodia, is
connected through the known surviving cases of multiple conjugation with normal
conjugation ; - the dimorphism which marks the transition from conjugation to
fertilisation, making the latter indispensable, appears very gradually. “The very
gentleness of the gradation leads one to regard the two processes as analogous
responses to the same physiological necessities. The same disturbance of the balance
between anabolism and katabolism which results in the occurrence of asexual reproduction leads in more developed forms to the separation of the dimorphic and
mutually dependent elements of sexual reproduction. As asexual reproduction occurs
at the limit of growth, so a check to the asexual process involves the appearance of the
sexual, which is thus still further associated with katabolic preponderance.- The
following illustration will suffice :  - Under conditions of favourable temperature and
abundant food the parthenogenetic reproduction of female Aphides can be indefinitely
prolonged, while a lowering of the temperature and diminution of the food at once
reintroduce sexual reproduction.
 
“Nature of Sex.  - In attempting to define the distinctive characteristics of male
and female, it is necessary to begin with the sexual elements themselves. The
difference between male and female is there exhibited in its fundamental and most
concentrated expression. It is in the sexual elements, indeed, that the continuity ot
organic life is secured, the vegetative organs being but appendages to the direct
immortal chain of sex- cells. The large quiescent ovum is the result of a continued
surplus of anabolism over katabolism, while the growing preponderance of katabolism
must find its outward expression in increased activity of movement and in diminished
size ; and the natural result is the flagellate sperm-cell.-
 
In multicellular organisms sexual reproduction makes its appearance when nutrition
is checked. “ Some of the cells are seen differentiating at the expense of others,
accumulating capital from their neighbours ; and if their area of exploitation be suffi
 
280
 
 
THE STUDY OF EMBRYOLOGY.
 
 
ciently large, emphatically anabolic cells or ova result ; while if their area is reduced
by the presence of numerous competitors struggling to become germ-cells, the result
is the formation of smaller, more katabolic, and ultimately male cells. In the same
species distinct organisms may, in the same way, become predominantly anabolic or
katabolic, and may be distinguishable as completely female or male organisms.-
 
The numerous facts which have now been accumulated prove that “ such conditions
as deficient or abnormal food, high temperature, deficient light, moisture, and the
like, are obviously such as would tend to induce a preponderance of waste over repair
- a Jcatabolic diathesis ; and we have just seen that these conditions tend to result in
the production of males. Similarly, such factors as abundant and rich nutrition,
abundant light and moisture, must be allowed to be such as favour constructive
processes and make for anabolism ; and we have just seen that these conditions result
in the production of females .-
 
Oogenesis and Spermatogenesis.  - In the maturation of the ovum, the formation
of polar cells seems rightly interpreted as an extrusion of the katabolic or male
elements from the preponderatingly anabolic ovum ; the converse occurs in spermatogenesis.
 
Fertilisation.  - According to this view of Geddes -, li fertilisation is comparable to
mutual digestion, and the reproductive process has arisen from a nutritive want.
The essentially katabolic male cell, getting rid of all accessory nutritive material
contained in the sperm-blastophore, brings to the ovum a supply of characteristic
katastates, which stimulate the latter to division. The profound chemical differences
surmised by some between the male and female elements are intelligible as the outcome of the predominant anabolism and katabolism in the two elements. The union
of the two sets of products restores the normal balance and rhythm of cellular life.-
 
 
APPENDIX
 
 
APPENDIX A.
 
 
System of Classification adopted, with Enumeration of all the
Genera alluded to in the Body of the Book.
 
 
PROTOZOA.
 
1. Protista.
 
2. Rhizopoda.
 
Lobosa (Protoplasta).  - Amoeba.
 
Heliozoa.
 
&c.
 
3. Corticata.
 
Flagellata.  - Monas Proterospongia.
 
Ciliata.  - Paramsecium, Stylonychia, Vorticella.
 
&c.
 
METAZOA.
 
(Trichoplax  - incert sedis.)
 
I. - PORIFERA.
 
1. Calcarea.
 
Caicispongice.
 
2. Non-Oalcarea.
 
Myxospongice.
 
&c.
 
II.  - CCELENTERATA.
 
A. Int^niol.® (Hydrozoa).
 
Hydromedusse.
 
Eleutheroblastea.  - Hydra.
 
Gymnoblastea ( Ocellata ).
 
Galyptoblastea ( Vesiculata).  - Obelia, Clytia, Eutima, Mitrocoma,
 
AEquorea.
 
Trachymedusce.  - Cunina, AEginura, Geryonia, Carmarina.
Hydrocorallince.
 
SipJionopliora.
 
 
282
 
 
APPENDIX.
 
 
B. T^niol^e.
 
1. Scyphomedusae.
 
Discomedusce ( Acraspeda ).  - Aurelia, Pelagia.
 
&c.
 
(Aotinozoa).
 
2. Hexactiniae (Zoantharia).
 
Malacozoa ( Actiniae ).  - Edwardsia, Peachia, Anthea, Cerianthus.
Hexacoralla ( Madreporaria ;).
 
Antipatharia.
 
3. Octactiniae (Alcyonaria).
 
Alcyoniidce.
 
Gorgoniidce (Isidinae).
 
Coralliidce .  - Corallium.
 
&c.
 
C. Ctenophora.
 
III.  - ECHINODERM AT A.
 
1. Pelmatozoa.
 
Crinoidea.
 
2. Echinozoa.
 
Aster oidea.  - Asterias, Lepty chaster.
 
Ophiuroidea .  - Ophiacantha.
 
Echinoidea.  - Echinus, Toxopneustes, Strongylocentrotus,
Hemiaster.
 
Holothur oidea.  - Psolus.
 
IV.  - VERMES.
 
1. Platyhelminthes.
 
Turbellaria.  - Planaria, Leptoplana
N emert ea.  - Lineus.
 
Trematodoa.
 
Cestoda.
 
2. Rotatoria.
 
3. Nemathelminthes.
 
Nematoda.  - Ascaris, Cucullanus.
 
Chaetognatha.  - Sagitta.
 
4. Annelida.
 
Discophora.  - Hirudo.
 
Chaetopoda.
 
Achceta.
 
Polychceta.  - Serpula.
 
Oligochceta.  - Criodrilus, Rhynchelmis (Euaxes),
Lumbricus.
 
5. PoDAXONIA.
 
Gephyrea.
 
Polyzoa.  - Phoronis.
 
Brachiopoda.  - Argiope.
 
 
APPENDIX.
 
 
283
 
 
V. - MOLLUSCA.
 
I. Odontophora.
 
i. Gasteropoda.
 
1. Isopleura ( Amphineura ).
 
Neomenice .  - Neomenia, Proneomenia.
 
Chcetodermce .
 
Polyplacophora .  - Chiton.
 
2. Anisopleura.
 
A. Streptoneura .  - i. Zygobranchia.  - Haliotis, Fissurella,
 
Patella. 2. Azygobranchia.  - (a) Prosobranchia  -
Ianthina, Paludina, Ampullaria, Nassa, Purpura,
Buccinum, Fusus, Murex. (b) Heteropoda.
 
B. Euthyneura .  - 1. Opisthobranchia.  - Aplysia, Elysia,
 
Fiona. 2. Pidmonata .  - Helix, Limax, Onchidium,
Planorbis, Lymnseus.
 
11. Scaphopoda.  - Dentalium.
 
in. Acropoda.
 
1. Pteropoda.
 
2. Cephalopoda.
 
A. Tetrabranchiata.  - N autilus.
 
B. Dibranchiata.  - 1. Decapoda.  - Sepia, Loligo.
 
2. Odopoda .  - Octopus.
 
II. Acephala.
 
Lamellibranchiata.
 
Isomya, Anodonta.
 
Heteromya , Dreissena.
 
Monomya, Pecten, Spondylus.
 
VI. - ARTHROPODA.
 
I. Crustacea.
 
A. Entomostraca.
 
Phyllopoda.
 
Ostracoda.
 
Copepoda.  - Cyclops.
 
Cirripedia.
 
B. Leptostraca.
 
Nebaliidse.
 
C. Malacostraca.
 
Arthrostraca (Hedriophthalmata).
 
Amphipoda.
 
Isopoda.  - Asellus.
 
 
284
 
 
APPENDIX.
 
 
Thoracostraca.
 
Cumacea.
 
Stomopoda.
 
Schizopoda.
 
Decapoda.  - (Carididse), Crangon, Hippolyte, Palsemon
Callianassa ; Astacus ; Palinurus ; Birgus
(Brachyura), Cancer.
 
II. Arachnida.
 
A. Hsematobranchia.
 
Xiphosura.  - Limulus.
 
B. -Slrobranchia.
 
Scorpionida.  - Scorpio.
 
Pedipalpida.
 
Araneida.
 
C. Lipobranchia.
 
Acarinida , &c.
 
III. Protracheata.
 
Peripatida.  - Peripatus.
 
IV. Myriapoda.
 
Y. Insecta (Hexapoda).
 
Thysanura.
 
Orth o ptera.  - Blatta.
 
Pseudoneuroptera.  - (Libellulidse. )
 
Hemiptera ( Rhynchota ).  - Aphis.
 
Neuroptera.
 
Coleoptera.  - Dy tiscus.
 
Diptera.  - Musca.
 
Lepidoptera.
 
Hymenoptera.
 
VII. CHOKDATA.
 
I. Hemichordata.
 
Enteropneusta.  - Balanoglossns.
 
II. Urochordata (Tunicata).
 
1. Perennichordata.  - Appendicularia.
 
2. Caducichordata.
 
III. Cephalochordata (Hypichthyes).
 
Pharyngobranchii .  - Amphioxus .
 
IV. Vertebrata (Craniata).
 
I. Cyclo&tomi (Myzichthyes).
 
Marsipobranchii.
 
(a) Hyperotreta.  - Myxine, Bdellostoma.
 
(5) Hyperoartia.  - Petromyzon.
 
 
APPENDIX.
 
 
285
 
 
II. Gnathostomata.
 
A. ICHTHYOPSIDA.
 
1. Chondkichthyes.
 
Holocephali.  - Chimsera.
 
Elasmobranchii (Selachii).  - (Notidanidse), Notidanus, (Hexanchus and Heptanchus) ; Cestracion, Acanthias,
Scy Ilium; Raja, Torpedo, Pristiurus.
 
2. OSTEICHTHYES.
 
Ganoidei.
 
Selachoidei.  - Acipenser, Polyodon.
 
Teleosteoidei.  - Polypterus, Lepidosteus, Amia.
 
Teleostei.
 
Lophobranchii.
 
Pledogncithi.  - (Gymnodontes.)
 
Physostomi.  - (Mursenidse), Amphipnous, (Leptocephalus) ;
Mormyrus ; (Salmonidse), Mallotus, Osmerus, Salmo,
Trutta ; (Cyprinidse), Cyprinus ; Cobitis ; Anableps ;
(Siluridae), Saccobranchus.
 
Anacanthini.  - (Pleuronectidae.)
 
Acanthopteri.  - Trigla, Anabas, Lophius.
 
3. Hekpetichthyes.
 
Dipnoi.  - Protopterus.
 
4. Amphibia.
 
Urodela.  - (Perennibranckiata), Siren ; (Axolotl) ; (Caducibranchiata), Amblystoma, Triton, Salamandra, Salamandrina, Gyrinophilns, Ranodon.
 
Gymnophiona.  - (Coeciliidse) Ccecilia.
 
Anura (Batrachia).  - Pipa, Dactylethra ; Rana, Alytes, Bombinator, Bufo, Rhinoderma; Notodelphis, Nototrema,
Hylodes.
 
(Amniota.)
 
B. Sauropsida.
 
1. Reptilia.
 
Chelonia.  - Trionyx, Aspidonectes, Testudo.
 
Lacertilia.  - Hatteria, Ckamseleon, Gecko, Calotes, Leiodera,
Scincus, Anguis, Cyclodus, Seps, Lacerta, Trachydosaurus, Yaranus.
 
Ophidia.
 
Crocodilia.
 
2. Aves.
 
Ratitse.
 
Struthiones , &c.
 
 
286
 
 
APPENDIX.
 
 
Carinatse
Chenomorphce.  - A.nser, Anas, Phoenicopterus.
Aledoromorphce.  - Gallus.
 
Columbce.  - Columba.
 
Coracomorphce.  - Pyrrhula, Luscinia, Sylvia.
Psittacomorphce.  - Psittacus.
 
&c.
 
C. Mammalia.
 
1. Prototheria (Ornithodelphia).
 
Monotr emata.  - Ornithorhynchus, Echidna.
 
2. Metatheria (Didelphia).
 
Marsupialia.
 
Glyrina (Rhizophaga).
 
Macropoda ( Poephaga ), Macropus.
 
Scandentia ( Carpophaga ).  - Phascolarctos, Phalangister.
Rapacia.  - Perameles, Didelphys.
 
3. Eutheria (Moxodelphia).
 
Edentata.
 
Pilosa.  - Myrmecophaga, Cy clothurus.
 
Loricata.  - Dasy pus.
 
Squamcita.  - Manis.
 
Tubulidenta.
 
Sirenia.
 
Cetacea.  - Phocsena.
 
Ungulata.
 
Artiodactyla , Suina.  - Sus, Tragulina, Tragulus ; Tylopoda ; Pecora (Cervidse), Moschus ; Camelopardalis ;
Antilope, Tetraceros, Ovis, (Bovidse) Bos.
Perissodadyla.  - Equus, Rhinoceros.
 
Hyracoidea.  - Hyrax.
 
Proboscidea.  - Elephas.
 
Rodentia.
 
Duplicidentata .  - Lepus.
 
Simplicidentcita.  - Arvicola, Mus, Cavia.
 
Chiroptera.
 
Insectivora.  - Talpa.
 
Carnivora.
 
Pinnipedia.  - (Phocidse).
 
Fissipedia.  - Mustelus, Canis, Felis.
 
Primates.
 
Lemuroidea.
 
A nthropoidea.  - Homo.
 
 
APPENDIX.
 
 
287
 
 
APPENDIX B.
 
BIBLIOGRAPHY OF RECENTLY PUBLISHED WORKS
ON EMBRYOLOGY.
 
THE GENERAL SUBJECT.
 
F. M. Balfour.  - A Treatise on Comparative Embryology. Macmillan
 
& Co., London, 1880.
 
C. Claus.  - Lehrbuch der Zoologie. Translated by A. Sedgwick (Elementary Text- Booh of Zoology). Swan, Sonnenschein & Co., London,
1884.
 
P. Geddes.  - “Reproduction,- Encyclopaedia Britannica , 1887.
 
“Sex,- Ibid., 1887.
 
E. Heckel.  - “Studien zur Gastraea Theorie,- Jena, 1877; also Jenaische Zeitschrift , viii., ix. (Abstracted by E. R. Lankester in
Quart. Jour. Micr. Sci, xiv. 1874, and xvi. 1876.)
 
Schopfungsgeschichte. Leipzig. Translated as The History of
 
Creation. London, 1876.
 
G. B. Howes.  - Atlas of Biology (Snail, Fresh-water Mussel, Crayfish,
 
Frog). London, 1886.
 
Klein.  - Elements of Histology. London.
 
E. Rat Lankester.  - “ Notes on Embryology and Classification,- Quart.
Jour. Micr. Sci., xvii. 1877.
 
C. S. Minot.  - “A Sketch of Comparative Embryology,- American
Naturalist , 1880.
 
- “ Comparative Morphology of the Ear,- American Journal of
 
Otology, 1881-82.
 
A. S. Packard, Jun. - Life Histories of Animals, including Man, or
Outlines of Comparative Embryology. Holt & Co., New York,
1876.
 
- Zoology. New York, 1881.
 
E. A. Schafer.'  - “ Some Teachings of Development,- Quart. Jour. Micr.
Sci., xx. 1880.
 
W. J. Sollas.  - “ On the Origin of Fresh-water Faunas : a Study in
Evolution,- Trans. Roy. Dubl. Soc. (n), iii. 1884,
 
TECHNIQUE.
 
M. Foster and F. M. Balfour.  - The Elements of Embryology. London,
1883.
 
A. B. Lee.  - The Microtomisfs Vade-Mecum. London, 1885.
 
 
288
 
 
APPENDIX.
 
 
C. 0 . Whitman.  - Methods of Research in Microscopical Anatomy and
Embryology. Boston, 1885.
 
GENERAL EMBRYOLOGY OF INVERTEBRATES.
 
W. K. Brooks.  - Handbook of Invertebrate Zoology for Laboratories and
Seaside Work (Embryology of Echinoderms, Crab, Fresh-water
Mussel, Squid). Boston, 1882.
 
T. H. Huxley.  - The Anatomy of Invertebr at ed Animals. Churchill, 1877.
(Also other Text-books of Anatomy, Physiology, Zoology, and Biology.)
 
GENERAL EMBRYOLOGY OF VERTEBRATES.
 
M. Foster and F. M. Balfour.  - The Elements of Embryology. London,
1883.
 
O . Hertwig.  - Lehrbuch der Entwicklungsgeschichte des Menschen und
 
der Wirbelthiere. Fischer, Jena, 1886 (1st part).
 
T. H. Huxley.  - The Anatomy of Vertebrated Animals. London, 1871.
A. Kolliker.  - Entwicklungsgeschichte des Menschen und der hdheren
Thiere. Leipzig.
 
Quain's Elements of Anatomy , ii. London.
 
R. Wiedersheim.  - Grundriss der vergleichenden Anatomie der Wirbelthiere. Jena, 1884. Translated by W. N. Parker {Elements of
the Comparative Anatomy of Vertebrates). Macmillan & Co.,
London and New York, 1886.
 
CELLULAR BIOLOGY.
 
W. K. Brooks.  - “Alternation of Periods of Rest with Periods of
Activity in the Segmenting Eggs of Vertebrates,- Studies Biol.
Lab. Johns Hopkins Univer., Baltimore , ii. 1882.
 
J. B. Carnoy.  - La Biologie cellulaire . Van In & Cie, Lierre, 1884.
 
- La Cellule. Lierre. (A new and important journal devoted to
 
cellular biology.)
 
J. T. Cunningham.  - “Review of Recent Researches on Karyokinesis
and Cell Division,- Quart. Jour. Micr. Sci., xxii. 1882.
 
W. Flemming.  - Zellsubstanz, Kern und Zelltheilung. Leipzig, 1882.
 
P. Geddes.  - “Theory of Growth, Reproduction, Sex, and Heredity/'
 
Proc. Roy. Soc. Edinb., 1886 (abstract in Encyc. Brit., article
“Sex-).
 
- “Reproduction,- Encyclopaedia Britannica, 1886.
 
Heitzmann.  - Microscopic Morphology of the Animal Body in Health
and Disease. New York, 1883.
 
Kolliker.  - “ Die Bedeutung der Zellenkerne fur die Vorgange der
Vererbung,- Zeit. fur wiss. Zool. , xlii. 1885.
 
“ Das Karyoplasma und die Vererbung, eine Kritik der Weismann
'schen Theorie von der Kontinuitat des Keimplasma,- Zeit. fur
wiss. Zool., xliv. 1886.
 
 
APPENDIX.
 
 
289
 
 
E. Metschnikoff.  - “Researches on the Intracellular Digestion of Invertebrates,- Quart. Jour. Micr. Set ., xxiv. 1884. (Translation
of “ Untersuchungen iiber die intracellulare Verdauung bei wirbellosen Thieren,- Arbeiten a. d. Zoolog. Instit. Wien , 1883.)
 
“The Ancestral History of the Inflammatory Process,- Ibid.,
 
xxiv., 1884. (Translation of “ Untersuchungen iiber mesodermalen
Phagocyten einiger Wirbelthiere,- Biolog. Centralblatt , iii. (18),
1883.)
 
C. S. Minot.  - “Theorie der Genoblasten,- Biolog. Centralblatt , ii. 1882.
 
W. Preyer.  - Specielle Physiologic des Embryo , Untersuchungen iiber die
Lebensersclieinungen von der Geburt. Leipzig, 1885.
 
A. Rauber.  - “ Neue Grundlegungen zur Kenntniss der Zelle,- Morph.
Jahrb., viii. 1882.
 
J. A. Ryder.  - “The Law of Nuclear Displacement, and its Significance
in Embryology,- Science , i. 1883.
 
A. E. Schafer.  - “ On the Part Played by Amoeboid Cells in Intestinal
Absorption,- Internat. Month. Journ. of Anat. and Hist., ii.
 
Wiedersheim.  - “ Ueber d. mechanische Aufnahme die Nahrungsmittel
in der Darmschleinhaut,- Deutsch. Naturforsch. Vers am. in
Freiburg , 1883.
 
 
OOGENESIS.
 
E. Van Beneden.  - Recherches sur la Maturation de VCEuf, la Fecondation
et la Division cellulaire , 1883.
 
J. T. Cunningham.  - “E. Yan Beneden -s Researches on the Maturation
and Fecundation of the Ovum,- Quart. Jour. Micr. Sci., xxv. 1885.
A. Thomson.  - “Recent Researches on Oogenesis,- Ibid., xxvi. 1886.
(See also La Cellule.)
 
SPERMATOGENESIS.
 
J. E. Blomfield.  - “On the Development of the Spermatozoa.- Part i.,
Lumbricus, Quart. Jour. Micr. Sci., xx. 1880; Part ii., Helix
and Rana , Ibid., xxi. 1881.
 
“Review of Recent Researches on Spermatozoa,- Quart. Jour.
 
Micr. Sci., xxiii. 1883.
 
P. Geddes and A. Thomson. - “ History and Theory of Spermatogenesis,-
Proc. Roy. Soc. Edinb., 1885-86 (Bibliography).
 
0 . S. Jenson.  - “Recherches sur la Spermatog<hiese,- Arch, de Biol., iv.
1883.
 
Yon la Yalette St. George.  - “Ueber d. Genese d. Samenkorper,-
Arcliiv f. Mikr. Anat., xv. 1878.
 
“ Spermatologische Beitrage.- 1. Arch, fiir Mikr. Anat., xxv.
 
1885; 11., iii. Ibid., xxvii. 1886; iv. Ibid., xxviii. 1886.
 
(See also La Cellule.)
 
T
 
 
290
 
 
APPENDIX.
 
 
EARLY STAGES OF DEVELOPMENT AND ORIGIN
OF TISSUES.
 
0 . Butschli.  - “Bemerkungen zur Gastrseatheorie,- Morph. Jahrb ., ix.
1884.
 
H. W. Conn.  - “ Marine Larvae and their Relation to Adults,- Studies
Biol. Lab. Johns Hopkins XJniver ., Baltimore, iii. 1885.
 
W. H. Caldwell.  - “Blastopore, Mesoderm, and Metameric Segmentation,- Quart. Jour. Micr. Sci., vol. xxv. 1885.
 
S. Grosglik.  - “Schizocoel oder Enterocoel,- Zool. Anz., x. 1887.
 
0 . and R. Hertwig.  - Die Ccelomtheorie, Versuch einer Erkldrung des
mittleren Keimblattes. Jena, 1881.
 
0 . Hertwig.  - Die Entwicklung des mittleren Keimblattes der Wirbelthiere.
Jena, 1883.
 
A. Hyatt.  - “Larval Theory of the Origin of Cellular Tissues,- Proc.
Bost. Soc. Nat. Hist., xxiii. 1884.
 
A. Kolliker.  - “ Die embryonalen Keimblatter und die Gewebe,- Zeit.
fur wiss. Zool., xl. 1884.
 
“ J. Kollmann -s Akroblast,- Zeit. fiir wiss. Zool., xli. 1884.
 
J. Kollmann.  - “ Der Randwulst und der Ursprung der Stiitzsubstance,-
Arch. f. Anat. u. Phys., Anat. Abthl., 1884.
 
“ Der Mesoblast und die Entwicklung der Gewebe bei Wirbel
thieren,- Biol. Centralbl., iii. 1884.
 
“ Gemeinsame Entwickelungsbahnen der Wirbelthiere,- Arch.
 
Anat. Phys., Anat. Abtheil., 1885 ; Zeit. fiir wiss. Zool., xli. 1885.
C. Kupffer.  - “Die Gastrulation an den meroblastiscben Eiern der
Wirbelthiere und die Bedeutung des Primitivstreifs,- Arch. Anat.
Phys., Anat. Abtheil., 1882-84.
 
E. Metschnikoff.  - “ Vergleichend-embryologische Studien. (3)Ueber die
Gastrula einiger Metazoen (Echinus, Lineus, Phoronis, Polygordius,
Ascidia, Discoporella),- Zeit. fiir iviss. Zool., xxxvii. 1882.
 
C. S. Minot. - “ Preliminary Notice of Certain Laws of Histological
Differentiation,- Proc. Bost. Soc. Nat. Hist., xx. 1879.
 
“Origin of Mesoderm,- Science, ii. 1883.
 
A. Rauber.  - “Noch ein Blastoporus,- Zool. Anz., vi. 1883.
 
W. Repiachoff.  - “Ueber die Morphologische Bedeutung der jiingsten
Saugethierkeime,- “ Bemerkungen iiber die Keimblatter der
Wirbelthiere,- “Zur Morpbologie des Primitivstreifens,- Zool.,
Anz., vi. 1883.
 
J. A. Ryder.  - “ On the Position of the Yolk-blastopore as Determined
by the Size of the Vitellus,- American Naturalist, 1885.
 
“The Arcbistome Theory,- American Naturalist, 1885.
 
A. Sedgwick.  - “On the Origin of Metameric Segmentation and some
other Morphological Questions,- Quart. Jour. Micr. Sci., xxiv.
1884.
 
 
APPENDIX.
 
 
291
 
 
W. Waldeyer.  - “Archiblast und Parablast,- Arch. f. Milcr. Anat .,
xxii. 1883.
 
W. Wolff.  - “Die beiden Keimblatter und der Mittelkeim,- Arch. f.
Milcr. Anat., xxviii. 1886.
 
 
INCERT SEDIS.
 
F. E. Schulze.  - “Tricoplax adhaerens, n. g. et n. sp.,- Zool. Anz., vi.
1883.
 
 
PORIFERA.
 
A. Goette.  - “Ueber die Entwicklung der Spongillen,- Zool. Anz., vii.
1884; viii. 1885.
 
C. Keller.  - “Sfcudieniiber Organisation und Entwicklung der Chalineen,-
Zeit. fur wiss. Zool., xxxiii. 1879.
 
R. Von Lendenfeld.  - “ On the Systematic Position and Classification
of Sponges - (with a complete List of Publications relating to
Sponges), Proc. Zool. Soc., 1887.
 
“ Synocils, Sinnesorgane der Spongien,- Zool. Anz., x. 1887.
 
W. Marshall.  - “Die Ontogenie von Reniera filigrana 0 . Schmidt,-
Zeit. fur wiss. Zool., xxxvii. 1882.
 
F. E. Schuze.  - “ Untersuchungen fiber den Ban und die Entwicklung
der Spongien, Keunte Mittheilung, die Plakiniden,- Zeit. fur
â– wiss. Zool., xxxiv. 1880.
 
W. J. Sollas.  - “On the Development of Halisarca lobularis ( 0 .
Schmidt),- Quart. Jour. Micr. Sci., xxiv. 1884.
 
“Sponges,- Encyclopaedia Britannica, 1887.
 
 
CCELENTERATA.
 
A. G. Bourne.  - “ Recent Researches upon the Origin of the Sexual Cells
in Hy droids : a Review,- Quart. Jour. Micr. Sci., xxiii. 1883.
W. K. Brooks.  - “ On the Life History of Eutima, and on Radial and
Bilateral Symmetry in Hy droids,- Zool. Anz., vii. 1884.
 
“ The Life History of the Hydromedusse : a Discussion of the
 
Origin of the Medusae and the Significance of Metagenesis,- Mem.
Bost. Soc. N. H., iii. 1886.
 
J. W. Fewkes.  - “ On the Development of Agalma,- Bull. Mus. Corny.
Zool., Harvard, xi. 1885.
 
“ Selections from Embryological Monographs, Acalephs,- Mem.
 
Mus. Comp. Zool., Harvard, ix. 1884.
 
“Bibliography- (for above), Bull. Mus. Comp. Zool., xi. 1884.
 
G. H. Fowler.  - “ The Anatomy of the Madreporaria.- I., Quart. Jour.
 
Micr. Sci., xxv. 1885; II., Ibid., xxvii. 1886.
 
A. Gotte.  - “ Ueber die Entwicklung der Aurelia aurita und Cotylorhiza
borbonica,- Zool. Anz., viii. 1885; [Ann. Mag. Nat. Hist. (5),
xvi. 1885].
 
 
292
 
 
APPENDIX.
 
 
E. ELeckel.  - Metagenesis und Hypo genesis von Aurelia aurita. Ein
Beitrag zur Entwickelungsgeschichte und zur Teratologie der
Medusen. Jena, Fischer, 1881.
 
C. Hartlaub.  - “ Beobachtungen iiber die Entstehung der Sexualproducte
bei Obelia,- Zool. Anz., vii. 1884 ; Zeit. fur wiss. Zool., xli. 1884.
 
A. Korotneff.  - “Zur Kenntnis der Embryologie von Hydra,- Zeit
fiir wiss. Zool., xxxviii. 1883.
 
A. Kowalevskt.  - “ Zur Entwicklungsgeschichte der Lucernaria,- Zool.
Anz., vii. 1884.
 
A. Kowalevsky et Marion.  - “Documents pour l -Histoire embryogenique
des Alcyonaire Ann. du Musee d -Hist. Nat. de Marseille , i.
1882-83.
 
E. L. Mark.  - “ Selections from Embry ological Monographs, Polyps,-
Mem. Mus. Comp. Zool., Harvard, ix. 1884.
 
C. De Merejkowsky. - “Histoire du Developpement de la Meduse Obelia,-
Bull. Soc. Zool. de France , viii. 1883.
 
E. Metschnikoff. - “ Vergleicliend-embryologische Studien,- Zeit. fiir
wiss. Zool.
 
1. “ Entodermbildung bei G-eryoniden,- xxxvi. 1881.
 
2. “ Ueber einiger Stadien der in Carmarina parasitirenden
 
Cunina,- xxxvi. 1881.
 
4. “ Ueber die Gastrulation und Mesodermbildung der Ctenophoren,- xlii. 1885.
 
E. Metschnikoff. - Embryologische Studien an Medusen. Wien, 1886.
 
J. Thallwitz.  - “Ueber die Entwicklung der Mannlichen Keimzellen
bei den Hydroideen,- Jenaische Zeit. Nat., xviii. 1885.
 
A. Weismann.  - Die Entstehung der Sexualzellen bei den Hydromedusen.
Zugleich als Beitrag zur Kenntniss des Baues und der Lebenserscheinungen dieser Gruppe. Jena, 1883. Abstracted by H. N.
Moseley in Nature, xxix. 1883. “Die Entstehung der Sexualzellen bei den Hydromedusen,- Biol. Centralbl., iv. 1884.
 
E. B. Wilson.  - “The Development of Renilla,- Phil. Trans., clxxiv.
1884.
 
 
ECHINODERMATA.
 
A. Agassiz.- - “ Selections from Embryological Mongraphs.- (ii.) “Echinodermata,- Mem. Mus. Comp. Zool., Harvard, 1883. “Bibliography-
to accompany the same, Bull. Mus. Comp. Zool., x. 1882.
 
P. H. Carpenter.  - “ Notes on Echinoderm Morphology, viii. On Some
Points in the Anatomy of Larval Comatulse,- Quart. Jour. Micr.
Sci., xxiv. 1884.
 
H. Ludwig.  - “Entwicklungsgeschichte der Asterina gibbosa, Forbes,-
Zeit. fiir wiss. Zool., xxxvii. 1882.
 
 
APPENDIX.
 
 
293
 
 
E. Metschnikoff.  - “ Vergleichend-embryologische Studien. 3. Ueber
die Gastrula einiger Metazoen (Echinus),- Zeit . fur wiss. Zool. ,
xxxvii. 1882.
 
“ Embry ologische Mittheilungen iiber Echinodermen,- Zool.
 
Anz., vii. 1884.
 
-  - “ Ueber die Bildung der Wanderzellen bei Asterien und Echini
den,- Ibid., xlii. 1885.
 
E. Perrier.  - “ Sur le Develloppement des Comatules,- Comptes Rendus,
xcviii. 1884. (Ann. Mag. Nat. Hist. (5), xiii. 1884.)
 
E. Selenka.  - “ Studien iiber Entwicklungsgeschichte der Thiere. (ii.)
Die Keimblatter der Echinodermen.- Wiesbaden, 1883.
 
“Das Mesenchym der Echiniden,- Zool. Anz., vii. 1884.
 
VERMES.
 
A.  - PLAT YHELMINTHES.
 
P. Hallez.  - Contributions cl VHistoire naturelle des Turbellaries. Thesis
ct la Faculte des Sciences p. le grade d. Docteur es Sci. Nat. Lille,
1879.
 
A. A. W. Hubrecht.  - “Contributions to the Embryology of the
Nemertea,- Quart. Jour. Micr. Sci., xxvi. 1886. (Abstract of
Proeve eener ontwikkelungs geschiedenis van Lineus obscurus,
Barrois. U trecht, 1885.)
 
“The Relation of the Nemertea to the Vertebrata,- Quart. Jour.
 
Micr. Sci., xxvii. 1887.
 
J. Jijima.  - “Ueber die Embryologie von Dendrocoelum lacteum,- Zool.
Anz., vi. 1883.
 
“ Untersuchungen iiber den Bau und die Entwickelungs
geschichte der Siisswasser-Dendrocoelen (Tricladen),- Zeit. fur
wiss. Zool., xl. 1884.
 
A. Lang.  - “Die Polycladen,- Fauna u. Flora d. Golfes v. Neapel, 1884.
(Full Bibliography.)
 
E. Metschnikoff.  - “Die Embryologie von Planaria polychroa,- Zeit.
fur wiss. Zool., xxxviii. 1883.
 
A. C. Oudemans.  - “The Circulatory and Nephridial Apparatus of the
Nemertea,- Quart. Jour. Micr. Sci. Suppl., xxv. 1885.
 
W. Salensky.  - “Zur Entwickelungsgeschichte der Borlasia vivipara,-
Biol. Centralbl., ii. 1883.
 
“ Recherches sur le D6veloppement du Monopora vivipara (Borlasia vivipara, Uljan),- Arch, de Biol., v. 1884.
 
“Bau und Metamorphose des Pilidium,- Zeit. fur wiss. Zool.,
 
xliii. 1886.
 
E. Selenka.  - Zoologische Studien. II. Zur Entwickelungsgeschichte der
Seeplanarien. Ein Beitrag zur Keimbldtterlehre und Descendenztheorie. Leipzig, Engelmann, 1881.
 
 
294
 
 
APPENDIX.
 
 
H. Schauinsland.  - “ Die embryonale Entwickelung der Bothriocephalen, ,, Jenaische Zeit. Nat., xix. 1885.
 
W. Schwarze.  - “Die postembryonale Entwicklung der Trematoden,-
Zeit. fur wiss. Zool ., xli. 1885.
 
B.  - CHiETOPOD A AND DISCOPHORA.
 
J. Beard.  - “On the Life History and Development of the Genus
Myzostoma (F. S. Leuckart),- Mittheil. Zoolog. Stat. Neapel , v.
 
1884.
 
H. W. Conn.  - “Development of Serpula,- Zool. Anz., vii. 1884.
 
R. Yon Drasche.  - Beitrage zur Entwickelung der Polychceten. I. Entwickelung von Potamoceros triquetor , 1884; II. Entwickelung von
Sabellaria spinulosa, Hermione hystrix und eine Phyllocide , 1885.
Wien.
 
“Einige Worte zu der Mittheilung H. W. Conn -s uber die Entwicklung von Serpula,- Zool. Anz., viii. 1885.
 
J. W. Fewkes.  - “ On the Larval Forms of Spirorbis borealis,- American
Nat., xix. 1885.
 
A. Goette.  - Abhandlungen zur Entwickelung sgeschichte d. Thiere. I. Un
tersuchungen zur Entwick. der Wiirmer. Leipzig, Yoss, 1882.
 
B. Hatschek.  - “ Zur Entwicklung des Kopfes von Polygordius,- Arbeit.
 
Zool. Inst. Wien, vi. 1885.
 
“Entwicklung der Trochophora von Eupomatus uncinatus,
 
Philippi (Serpula uncinata),- Ibid. (And other studies on the
development of Annelids in the same journal.)
 
J. Jijima.  - “ On the Origin and Growth of the Eggs and Egg-Strings in
Nephilis, with some Observations on the Spiral Asters,- Quart.
Jour. Micr. Sci., xxii. 1882.
 
N. Kleinenberg.  - “ The Development of the Earthworm Lumbricus
trapezoides, Dug£s,- Quart. Jour. Micr. Sci., xix. 1879.
 
“Die Entstehung des Annelids aus der Larve von Lopado
rhynchus. Nebst Bemerkungen iiber die Entwicklung anderer
Polychaeten, - Zeit. fur wiss. Zool., xliv. 1886.
 
A. Kowalevsky.  - “ Embryologische Studien an Wiirmern und Arthropoden,- Mem. Acad. Petersbourg (vii.), xvi. 1871.
 
E. Ray Lankester.  - “ On the Connective and Yasifactive Tissues of
the Medicinal Leech,- Quart. Jour. Micr. Sci., xx. 1880.
 
J. Nusbaum.  - “ Zur Entwicklungsgeschichte der Hirudineen (Clepsine),-
Zool. Anz., vii. 1884.
 
“ Zur Entwicklungsgeschichte der Geschlechtorgane der Hirudineen (Clepsine),- Ibid., viii. 1885.
 
W. Salensky.  - “Etudes surle Developpement des Annelides. 3. Pileolaria; 4. Aricia foetida; 5. Terebella Meckeli,- Arch, de Biol., iv.
1883; “2me. partie, Developpement de Branchiobdella,- vi. 1885.
 
 
APPENDIX.
 
 
295
 
 
W. T. Sedgwick and E. B. Wilson.  - General Biology {Earthworm).
New York, 1886.
 
C. - NEMATODA, &c.
 
E. Van Beneden.  - “ Recherches sur la Maturation de l -CEuf et la Fecondation (Ascaris megalocepliala),- Arch, de Biol., iv. 1883.
 
E. Van Beneden et C. Julin.  - “La Spermatog6n&se chez l -Ascaris
megalocephala,- Bull. Acad. Sci. Belgique (3), vii. 1883.
 
P. Hallez.  - “Sur le Developpement des Nematodes,- Gomjotes Rendus,
ci., 1885 ; Bull. Sci. Hist. Depart, du Nord , vii. viii. 1885.
 
R. Leuckart.  - “Ueber die Entwieklung der Spliserularia bombi,- Zool.
 
Anz., viii. 1885.
 
V. Linstow.  - “Ueber einen neuen Entwicklungsmodus beiden Nema
toden,- Zeit. fur wiss. Zool., xlii. 1885.
 
0 . Hertwig.  - Die Chcetognatlien, Hire Anatomie, Systematik und Entwicklungsgeschichte. Jena, 1880.
 
D. - ROTATORIA.
 
G. Tessin.  - “ Ueber Eibildung und Entwieklung der Rotatorien,- Zeit.
fiir wiss. Zool., xliv. 1886.
 
0 . Zacharias.  - “Ueber Fortplanzung und Entwickelung von Rotifer
vulgaris/ - Ibid., xli. 1884.
 
E - GEPHYREA.
 
B. Hatschek.  - “Ueber Entwieklung van Sipunculus nudus,- Arb. Zool.
Inst. Wien, v. 1883.
 
POLYZOA.
 
W. H. Caldwell.  - “ Preliminary Note on the Structure, Development,
 
and Affinities of Phoronis/ - Proc. Roy. Soc ., xxxiv. 1882.
 
S. F. Harmer.  - “The Structure and Development of Loxosoma,- Quart.
 
Jour. Micr. Sci., xxv. 1885.
 
“ On the Life History of Pedicellina,- Ibid., xxvii. 1886.
 
E. R. Lankester.  - “Polyzoa,- Encyclopaedia Britannica, ix. 1885.
 
A. Ostrooumoff.  - “Note sur la Metamorphose du Cyphonautes,- Zool
Anz., viii. 1885.
 
W. J. Vigelius.  - “Zur Ontogenie der marinen Bryozoen,- Mittheil.
Zoolog. Stat. Neapel, vi. 1886.
 
BRACHIOPODA.
 
A. Kowalevsky.  - “Observations sur le Developpement des Brachiopodes,-
Arch. d. Zool. Exper. (5), 1. 1883. (Translation by MM.
 
Oehlert and Deniker of the original paper, published at Moscow
in Russian, 1874.)
 
 
296
 
 
APPENDIX.
 
 
E. R. Lankester.  - “ Brachiopoda,- Encyclopaedia Britannica , ix. 1885.
A. E. Shipley.  - “On the Structure and Development of Argiope,-
 
Mittlieil. Zoolog. Stat. Neapel , iv. 1883.
 
MOLLUSCA.
 
F. Blochmann.  - “ Ueber die Entwicklung der Neritina fluviatilis,
 
MiilL,- Zeit. f Ur wiss. Zool., xxxvi. 1881.
 
“ Beitrage zur Kenntnis der Entwicklung der Gasteropoden,-
 
Ibid., xxxviii. 1883.
 
N. Bobretzky.  - “ Studien iiber die embryonale Entwickelung der
Gasteropoden,- Arch. f. Mikr. Anal., xiii. 1879.
 
P. Fraisse.  - “ Ueber Molluskenaugen mit embryonalem Typus,- Zeit.
fur wiss. Zool. , xxxv. 1881.
 
H. Grenacher.  - “ Zur Entwickelungsgescbicbte der Cephalopoden.
Zugleich ein Beitrag zur Morpbologie der hoheren Mollusken,-
Zeit. fur wiss. Zool., xxiv. 1874, p. 419.
 
A. C. Haddon.  - “ Notes on the Development of Mollusca,- Quart.
 
Jour. Micr. Sci, xxii. 1882.
 
B. Hatschek.  - “Ueber Entwicklungsgescbichte von Teredo,- Arb. a. d.
 
Zool. Inst. Wien, iii. 1880.
 
R. Horst.  - “ On the Development of the European Oyster ( Ostrea
 
edulis, L.),- Quart. Jour. Micr. Sci., xxii. 1882.
 
S. Jourdain.  - “Sur le D 4 veloppement du Tube digestif des Limaciens,-
 
Comptes Rendus, xcviii. 1 884.
 
M. A. Kowalevsky.  - “ Etude sur PEmbryogenie du Dentale,- Ann. d.
Mus. d'Hist. Nat. de Marseille, Zool., i. 1882-83.
 
“ Embryongenie du Chiton polii (Philippi), avec quelques Remarques
 
sur le D^veloppement des autres Chitons,- Ann. d. Mus. d'Hist.
Nat. de Marseille, Zool., i. 1883.
 
E. R. Lankester  - “ Observations on the Development of the Cephalopoda,- Quart. Jour. Micr. Sci., xv. 1875.
 
“Mollusca,- Encyclopaedia Britannica (ix.), xvi. 1883.
 
E. L. Mark.  - “Maturation, Fecundation, and Segmentation of Limax
campestris, Binney,- Bull. Mus. Comp. Zool. Harvard, 1881.
(With full bibliography up to 1879.)
 
J. Playfair M‘Murrich.  - “ On the Existence of a Post-Oral Band of
Cilia in Gasteropod Yeligers,- Johns Hopkins Univer. Circ.,
Baltimore, v. 1885.
 
“ A Contribution to the Embryology of the Prosobranch Gastero
pods,- Studies Biol. Lab. Johns Hopkins Univer ., Baltimore ,
iii. 1886.
 
P. De Meuron.  - “Sur les Organes renaux des Embryons d -Helix,-
Comptes Rendus, xcviii. 1884.
 
 
APPENDIX.
 
 
297
 
 
W. Patten.  - “ The Embryology of Patella,- Arbeiten aus d. Zool. Inst,
z. Wien , vi. 1885.
 
“ Eyes of Molluscs and Arthropods,- Mittheil. Zoolog. Stat.
 
Eeapel, vi. 1886.
 
C. Rabl.  - “Beitr. zur Entwicklungsgesch. der Prosobranchier,-$#z?m<7S&er.
Akad. Wien, lxxxvii. 1883.
 
J. A. Ryder.  - “The Metamorphosis and Post-Larval Stages of Development of the Oyster,- Report U. S. Fish. Com. for 1882, 1884.
 
“ A Sketch of the Life History of the Oyster,- Rep. U. S. Geol.
 
Surv. (1882-83), iv. Appendix, ii. 1885.
 
W. Salensky.  - “ Zur Entwickelungsgeschichte von Vermetus,- Biol.
Centralbl., v. 1885.
 
Schmidt.  - “ Beitrag zur Kenntniss der post-embryonalen Entwickelung
der Hajaden,- Arch, fiir Naturg ., li. 1885.
 
M. Ussow.  - Untersuchungen u. d. Entwickelung d. Cephalopoden,-
Arch, de Biol., ii. 1881.
 
H. E. Ziegler.  - “Die Entwicklung von Cyclas cornea, Lam. (Sphaerium
corneum, L.),- Zeit. fiir wiss. Zool., xli. 1885.
 
 
ARTHROPODA.
 
A. - CRUSTACEA.
 
H. Blanc.  - “ D4veloppement de l -CEuf chez la Cuma rathkii,- Rec. Zool.
Suisse, ii. 1885.
 
C. Claus.  - “Neue Beitrage zur Morphologie der Crustaceen,- Arbeit.
Zoolog. Inst. Wien, vi. 1885.
 
H. W. Conn.  - “The Significance of the Larval Skin of Decapods,-
Studies Biol. Lab. Johns Hopkins Univer., Baltimore, iii. 1884.
 
Y. Delage.  - “Evolution de la Sacculine, - , Arch. Zool. Exper. (2), ii.
1884.
 
M. Hartog.  - “ On the Anal Respiration of the Copepoda,- Proc. Manchester Lit. and Phil. Soc., xix. 1881.
 
T. H. Huxley.  - “ The Crayfish,- International Scientific Series, xxviii.
 
1881.
 
C. Ishikawa.  - “ On the Development of a Fresh-water Macrurous Crustacean, Atyephira compressa, De Haan,- Quart. Jour. Micr. Sci .,
xxv. 1885.
 
J. S. Kingsley.  - “ The Development of the Compound Eye of Crangon,-
Zool. Anz., ix. 1886.
 
H. De Lacaze-Duthiers.  - “ Laura gerardiae, Type nouveau de Crustac^
parasite,- Mem. Acad. Sci. Paris, xlii, 1885.
 
 
298
 
 
APPENDIX.
 
 
C. Yon Mereschkowski.  - “Eine neue Art von Blastodermbildung bei
den Decapoden,- Zool. Anz ., v. 1882.
 
N. Nassanow. - “ Zur embryonalen Entwicklung von Balanus,- Zool.
Anz., viii. 1885.
 
T. J. Parker.  - “ An Account of Reichenbach -s Researches on the Early
Development of the Fresh-water Crayfish,- Quart. Jour. Micr.
Sci. , xviii. 1878.
 
P. Pelseneer.  - “ Observations on the Nervous System of Apus,- Quart.
Jour. Micr. Sci ., xxv. 1885.
 
Reichenbach.  - “Die Embryonanlage und erste Entwickelung des Flusskrebses,- Zeit. fiir iviss. Zool., 1877.
 
W. Schimkewitsch.  - “ Einige Bemerkungen fiber die Entwicklungsgeschichte des Flusskrebses,- Zool. Anz., viii. 1885.
 
W. B. Spencer.  - “ The Urinary Organs of the Amphipoda,- Quart. Jour.
Micr. Sci., xxv. 1885.
 
Ulianin B.  - “ Zur Entwickelungsgeschichte der Amphipoden,- Zeit. fiir
wiss. Zool., xxxv. 1881.
 
F. Urbanowicz.  - “ Zur Entwicklungsgeschichte der Cyclopiden,- Zool.
Anz., vii. 1884.
 
B. - ARACHNIDA.
 
J. Barrois.  - “ Le D^veloppement de Chelifer,- Comptes Rendus, xcix.
1884.
 
F. Blochmann.  - “Ueber direkte Kerntheilung in der Embryonalhiille
der Skorpione,- Morph. Jahrb., x. 1885.
 
W. K. Brooks and A. T. Bruce.  - “Abstract of Researches on the
Embryology of Limulus polyphemus,- Johns Hopkins Univer.
Circ., Baltimore, v. 1885.
 
J. S. Kingsley.  - “Notes on the Embryology of Limulus,- Quart. Jour.
Micr. Sci., xxv. 1885.
 
E. R. Lankester and A. G. Bourne.  - “The Minute Structure of the
 
Lateral and Central Eyes of Scorpio and of Limulus,- Ibid., xxiii.
1883.
 
H. L. Osborne.  - “ Metamorphosis of Limulus polyphemus,- Johns
Hopkins Univer. Circ., Baltimo 7 'e , v. 1885.
 
A. S. Packard, Jun.  - “On the Embryology of Limulus polyphemus
in.,- Proc. Amer. Philos. Soc., xxii. 1885. {American Nat.,
xix. 1885.)
 
W. Schimkewitsch.  - “ Zur Entwicklungsgeschichte der Araneen,- Zool.
Anz., vii 1884.
 
C. - PROTRACHEATA.
 
F. M. Balfour.  - “ The Anatomy and Development of Peripatus
 
capensis,- Quart. Jour. Micr. Sci., xxiii. 1883.
 
 
APPENDIX.
 
 
299
 
 
R. S. Bergh.  - “Die Entwicklung des Westindischen Peripatus- Arten,-
Kosmos, 1885.
 
J. Yon Kennel.  - “ Entwicklungsgeschichte von Peripatus,- Zool. Anz .,
vi. 1883.
 
“ Entwickelungsgeschichte von Peripatus Edwardsii, Blanch und
 
Peripatus torquatus sp. n.,- Arbeiten a. d. Zool.-Zoot. Inst. Wurzburg, vii. ; Ibid., viii.
 
A. Sedgwick.  - “The Development of Peripatus capensis,- Quart. Jour.
Micr. Sci., xxv. 1885.
 
“The Development of the Cape Species of Peripatus II.,- Ibid.,
 
xxvi. 1886. “ III.,- Ibid., xxvii. 1887.
 
D. - MYRIOPODA.
 
F. G. Heathcote.  - “ The Early Development of Julus terrestris,- Quart.
Jour. Micr. Sci., xxvi. 1886.
 
H. N. Moseley.  - “ Myriopoda,- Encyclopedia Britannica, xvii. 1884.
 
E.  - INSECTA.
 
A. T. Bruce.  - “ Origin of the Endoderm in Lepidoptera,- Johns Hop
kins Univer. Circ., Baltimore, v. 1885.
 
L. Camerano.  - “ Osservazioni intorno alia neotinia negli insetti,- Bull.
Soc. Entomol. Ital., 1885.
 
B. Grassi.  - “ Studi sugli Artropodi. Intorno alio siruppo delle Api
 
neir uovo,- Atti. Accad. Gioenia Sci. Nat. Catania, (3), xviii.
1885. (An important paper with Bibliography.)
 
R. Hertwig.  - “ Ueber die Anlage der Keimblatter bei den Insecten,-
Jena. Zeits. f. Nat., xiv., Suppl., 1881.
 
A. Korotneff.  - “Die Embryologie der Gryllotalpa,- Zeit. fur wiss. Zool.,
xli. 1885.
 
A. Kowalevsky.  - “Beitrage zur nachembryonalen Entwicklung der
Musciden,- Zool. Anz., viii. 1885.
 
J. Nusbaum.  - “Yorlaufige Mittheilung iiber die Chorda der Arthropoden,- Zool. Anz., vi. 1883.
 
“Bau, Entwicklung und morphologische Bedeutung der Ley
dig -schen Chorda der Lepidopteren,- Ibid., vii. 1884.
 
“The Embryonic Development of the Cockroach. Studies in
 
Comparative Anatomy, iii. The Structure and Life History of
the Cockroach (Periplancta orient alis). An Introduction to the
 
Study of Insects. By L. C. Miall and A. Denny. London, 1886.
J. A. Osborne.  - “ On the Embryology of Botys hyalinalis,- Science
Gossip, xxi. 1885.
 
 
300
 
 
APPENDIX.
 
 
W. Patten.  - “The Development of Phryganids, with a Preliminary
Note on the Development of Blatta germanica,- Quart Jour.
Micr. Set ., xxiv. 1884.
 
H. Yon Wielowiejski.  - “ Zur Kenntnis der Eibildung bei der Feuerwanze,- Zool. Anz ., viii. 1885.
 
E. Witlaczil.  - “ Entwicklungsgeschichte der Aphiden,- Zeit. fur tviss.
Zool., xl. 1884.
 
O. Zacharias.  - “ Neue Untersuchungen iiber die Entwicklung der viviparen Aphiden,- Zool. Anz., vii. 1884.
 
CHORDATA (General).
 
W. Bateson.  - “The Ancestry of the Chordata,- Ibid., xxvi. 1886.
 
A A. W. Hubrecht.  - “On the Ancestral Form of the Chordata,- Quart.
Jour. Micr. Set, xxiii. 1883.
 
A. S. Packard.  - “Aspects of the Body in Vertebrates and Arthropods,-
American Naturalist , 1881.
 
J. A. Ryder.  - “ On the Availability of Embryological Characters in the
Classification of the Chordata,- American Naturalist , 1885.
 
HEMICHORDATA.
 
W. Bateson.  - -“ The Early Stages in the Development of Balanoglossus
(sp. incert.),- Quart. Jour. Micr. Sci., xxiv. 1884.
 
“ The Later Stages in the Development of Balanoglossus
 
kowalevskii, with a Suggestion as to the Affinities of the Enteropneusta,- Ibid., Suppl., xxv. 1885.
 
“ Continued Account of the Later Stages in the Development of
 
Balanoglossus kowalevskii, and of the Morphology of the Enteropneusta,- Ibid., xxvi. 1886.
 
UROCHORDATA.
 
E. Van Beneden et C. Julin.  - “La Segmentation chez les Ascidiens
et ses Rapports avec l -Organisation de la Larve,- Bull. Acad.
Belg ., vii. 1884 (Arch, de Biol., v. 1884).
 
• “ Recherches sur la Morphologie des Tuniciers,- Arch. d. Biol.,
 
v. 1885.
 
“Des Orifices branchiaux externes des Ascidies et la Formation
 
du Cloaque, & c.,- Bull. Ac. Roy. Sci. Belg. (3), viii. 1885.
 
L. Chabry.  - “ La Segmentation des Ascidies simples,- Journ. de CAnat.
Phys., xx. 1885.
 
J. S. Kingsley.  - “ Some Points in the Development of Molgula manhattensis,- Proc. Bost. Soc. Nat. Hist., xxi. 1882.
 
A. Sabatier.  - “ Sur les Cellules du Follicule et les Cellules granuleuses
chez les Tuniciers,- Rec. Zool. Suisse, i. 1884 (Rev. Montp. (3),
iv. 1884).
 
 
APPENDIX.
 
 
301
 
 
A. Sabatier.  - “Sur les CEufs des Ascidiens,- Mem. Ac. Sci. Montpellier ,
 
x. 1885.
 
W. Salenskt.  - “ Neue Untersuchungen iiber die embryonale Entwicklung der Salpen,- Mittheil. Zool. Stat. Neapel , iv. 1882.
 
O . Seeliger.  - “ Die Entwicklungsgeschicbte der socialen Ascidien,-
 
Jenaische Zeit. Nat., xviii. 1884.
 
“Die Entwicklungsgeschichte der socialen Ascidien,- Jenaisclie
 
Zeit. Nat., xviii. 1885.
 
CEPHALOCHORDATA.
 
B. Hatschek.  - “ Studien iiber Entwicklung des Ampliioxus,- Arbeit.
 
Zoolog. Instituts. zu. Wien, iv. 1881.
 
“ Mittheilungen iiber Amphioxus,- Zool. Anz., vii. 1884.
 
VERTEBRATA (General).
 
P. Albrecht.  - “ Sur la Yaleur morphologique de la Trompe d -Eustache
 
et les Derives de l -Arc palatin, de l -Arc mandibulaire et de l'Arc
hyoidien des Vertebres,- Soc. Anat. Path. d. Bruxelles, 1884.
 
“Ueber die morphologische Bedeutung der Pharynxdivertikel,-
 
Centralbl. fur Chirugie, 1885 ( Nature , xxxi. 1885, p. 380).
 
F. Ahlborn.  - “Ueber die Segmentation des Wirbelthierkorpers,- Zeit.
fiir wiss. Zool., xl. 1884.
 
“ Ueber die Bedeutung der Zirbeldriise (Glandula pinealis, &c.),-
 
Zeit. fiir wiss. Zool., xl., 1884.
 
F. M. Balfour.  - “ On the Nature of the Organ in Adult Teleosteans
and Ganoids which is usually regarded as the Head-Kidney or
Pronephros,- Quart. Jour. Micr. Sci., xxii. 1882.
 
J. Beard.  - “The System of Branchial Sense Organs and their Associated Ganglia in Ichthyopsida : a Contribution to the Ancestral
History of Vertebrates,- Ibid., xxvi. 1885.
 
J. F. Van Bemmelen.  - “Die Visceraltaschen und Aortenbogen bei
Reptilien und Vogeln,- Zool. Anz., ix. 1886.
 
0 . Cadiat.  - “Du Developpement des Fontes et Arcs branchiaux chez
l -Embryon,- Journ. Anat. et. Phys., xix. 1883.
 
-  - “Du Developpement du Canal de l -Urethre et des Organes g^nitaux
de l -Embryon,- Journ. Anat. et de la Physiol., xx. 1884, p. 242.
 
“Memoire sur l -Uterus et les Trompes,- Ibid., p. 409.
 
J. H. Chievitz.  - “ Beitrage zur Entwickelungsgeschichte der Speicheldrusen,- Arch. Anat. Phys., Anat. Abthiel., 1885.
 
A Dohrn.  - “ Studien zur Urgeschichte des Wirbelthierskorpers,-
Mittheil. Zoolog. Stat. Neapel.
 
1. “Der Mund der Knochenfische.-
 
11. “ Die Entstehung und Bedeutung der Hypophysisbeiden
Teleostiern,- Ibid., iii. 1884.
 
 
302
 
 
APPENDIX.
 
 
hi. “Die Entstehung der Hypophysis bei Petromyzon Planeri,-
Ibid., iv. 1883.
 
iv. “ Die Entwicklung und Differenzirang der Kiemenbogen
der Selachier.-
 
v. “Zur Entstehung und Differenzirung der Yisceralbogen
bei Petromyzon Planeri.-
 
vi. “Die paarigen und unpaaren Flossen der Selachier,-
 
Ibid., v. 1884.
 
vii. “ Entstehung und Differenzirung des Zungenbein- und
 
Kiefer-Apparates der Selachier.-
 
viii. “ Die Thyreoidea bei Petromyzon, Amphioxus und Tunicaten,- Ibid., vi. 1885.
 
ix. “Die unpaare Elosse in ihrer Bedeutung fiir die Beur
theilung der genealogischen Stellung der Tunicaten
und des Amphioxus, und die Reste der Beckenflosse
bei Petromyzon.-
 
x. “Zur Phylogenese des Wirbelthierauges,- Ibid., vi. 1885.
 
xi. “ Spritzlochkieme der Selachier, Kiemendeckelkieme der
 
Ganoiden Pseudo branchie der Teleostier,- Ibid., vii.
1886.
 
M. L. Dollo.  - “On the Malleus of the Lacertilia and the Malar and
Quadrate Bones of Mammalia,- Quart. Jour. Micr. Sci., xxiii.
1883.
 
C. Emery.  - “TJeber die Beziehungen des Cheiro pterygiums zum Ichthyopterygium,- Zool. Anz., x. 1887.
 
P. Fischelis.  - “ Beitrage zur Kenntniss der Entwickelungsgeschichte
der Gl. Thyreoidea und Gl. Thymus,- Arch. f. mikr. Anat., xxv.
1885.
 
A. Eroriep.  - “Ueber ein Ganglion des Hypoglossus und Wirbelanlagen
in der Occiptalregion,- Arch. f. Anat. u. Pliys ., Anat. Abtheil.,
 
1882.
 
“ Zur Entwickelungsgeschichte der Wirbelsaule, insbesondere
 
des Atlas und Epistropheus und der Occipitalregion, I.,- Ibid.,
 
1883. “ II.,- Ibid., 1886.
 
“ TJeber Anlagen von Sinnesorganen am Facialis, Glossopharyn
geus und Yagus, iiber die genetische Stellung des Yagus zum
Hypoglossus, und iiber die Herkunft der Zungenmusculatur,-
Ibid., 1885.
 
H. Gadow.  - “ Remarks on the Cloaca and on the Copulatory Organs of
the Amniota,- Proc. Roy. Soc., xl. 1886.
 
W. Haacke.  - “TJeber eine neue Art uterinaler Brutpflege bei Wirbelthieren- (H F. E. Gungersen, “ Eine Berichtigung -), Zool. Anz.,
viii. 1885.
 
A. C. Haddon.  - “ Suggestion Respecting the Epiblastic Origin of the
Segmental Duct,- Proc. Roy. Dubl. Soc. (N.S.), v. 1887.
 
 
APPENDIX.
 
 
303
 
 
W. His.  - “Ueber den Sinus pmecervicalis und iiber die Thymusanlage,-
Arch. f. Anat. u. Phys., Anat. Abtheil ., 1886.
 
C. K. Hoffmann.  - “ Ueber des Amnion des Zweiblatterigen Keimes,-
Arch.f. mikr. Anat., xxiii. 1884.
 
“Ueber die Beziehung der ersten Kiementasche zu der Anlage
 
der Tuba Eustachii und des Cavum tympani,- Ibid., xxiii. 1884.
 
“ Zur Entwicklungsgeschichte der Urogenitalorgane bei den
 
Anamnia,- Zeit. fur iciss. Zool., xliv. 1886.
 
J. E. Jeffries.  - “Scales, Feathers, and Hairs,- Proc. Bost. Soc. Nat.
Hist., 1883.
 
J. Kollmann.  - “ Die Doppelnatur des excretorischen Apparates bei den
Cranioten,- Zool. Anz. v. 1882.
 
E. Legal.  - “Die Nasenhohlen und der Thranennasengang der Amnioten
Wirbelthiere,- Morph. Jahrb., viii. 1882.
 
A. M. Marshall.  - “The Morphology of the Vertebrate Olfactory
Organ,- Quart. Jour. Micr. Sci., xix. 1879.
 
The Segmental Value of the Cranial Nerves (Thesis), 1882.
 
P. De Meuron.  - “Recherches sur le Developpement du Thymus et de la
Glande Thyroide,- Recueil Zool. Suisse, iii. 1886.
 
C. S. Minot.  - “Amnion,- Reference Handbook of the Medical Sciences.
 
Boston, 1886.
 
Emily Nunn.  - “On the Development of the Enamel of the Teeth
of Vertebrates,- Proc. Roy. Soc., xxxiv. 1882.
 
D. Onodi.  - “ Ueber die Entwickelung des sympathischen Nervenseptems,
 
1. and 11.,- Arch. f. mikr. Anat., xxvi. 1886.
 
H. F. Osborn.  - “The Origin of the Corpus Callosum: a Contribution
upon the Cerebral Commissures of the Vertebrata,- Morph. Jahrb.
xii. 1886.
 
“ Observations upon the presence of the Corpus Callosum in the
 
Brains of the Amphibians and Reptiles ; - “ Note upon the
Cerebral Commissures in the Lower Vertebrata and a Probable
Fornix Rudiment in the Brain of Tropidonotus,- Zool. Anz., ix.
1886.
 
R. Owen.  - “ On the Homology of the Conario-hypophysial Tract, or the
so-called Pineal and Pituitary Glands,- Journ. Linn. Soc., xvi.
1882.
 
T. J. Parker.  - “Notes from the Otago University Museum, ix. On the
Nomenclature of the Brain and its Cavities,- Nature, xxxv. 1886.
J. Von Perkin yi.  - “Die ektoblastische Anlage des Urogenitalsy stems bei
Rana esculenta und Lacerta viridis (Vorlaufige Mitheilung),-
Zool. Anz., x. 1887.
 
J. A. Ryder. - “An Outline of a Theory of the Development of the
Unpaired Fins of Fishes,- American Naturalist , 1885.
 
â–   - “The Origin of the Amnion,- Ibid., 1886.
 
 
304
 
 
APPENDIX.
 
 
M. Sagemehl.  - Vntersuchungen iiber die Entwichlung der Spinal
nerven, Inaugural Dissertation, Dorpat, 1882.
 
“ Aus welchem Keimblatt entwickeln sich die Spinalnerven der
 
Wirbelthiere,- Sitz. Dorpat. Naturf. Ges. Wien, , 1884.
 
W. Schimkewitsch.  - “ Deber die Identitat der Herzbildung bei den
Wirbel- und wirbellosen Thieren;- “Noch Etwas liber die
Identitat der Herzbildung bei den Metazoen,- Zool. Anz., viii.
 
1885.
 
Schwalbe. - “Das Ganglion Oculomotorii,- Jenaische Zeitschrift , xiii.
1879.
 
W. B. Scott.  - “ On the Development of the Pituitary in Petromyzon, and
the Significance of that Organ in other Types,- Science, ii. 1883.
A. Sedgwick.  - “ On the Original Function of the Canal of the Central
Nervous System of Vertebrata,- Proc. Carnb. Phil. Soc., 1884.
 
H. Strahl.  - “Ueber Entwicklungsvorgange am Kopf und Schwanz von
Reptilien- und Saugethier-embryonen,- Zool. Anz., vii. 1884.
 
W. D -Arcy Thompson.  - “ On the Hind-limb of Ichthyosaurus, and on
the Morphology of the Vertebrate Limbs,- Jour. Anat. Phys., xx.
 
1886.
 
F. Tourneux et Ch. Legay.  - “ M^moire sur le Developpement -de l -Utfous
et du Vagin, envisage principalement chez le Foetus humain,-
Journ. Anat. et de la Physiol., xx. 1884, p. 330.
 
J. Turstig.  - “ Entwickelung des primitiven Aorten,- Sitzb. Dorpat.
Naturf. Ges., vii. 1885.
 
A. H. Tuttle.  - “ The Delation of the External Meatus, Tympanum, and
Eustachian Tube to the First Visceral Cleft,- Proc. American
Acad. Arts. Sci., xix. 1883.
 
N. Uskow.  - “ Ueber die Entwickelung des Zwerchfells, des Pericardiums
 
und des Coeloms,- Arch. f. mihr. Anat., xxii. 1883.
 
“ Bemerkungen zur Entwickelungsgeschichte der Leber und der
 
Lungen,- Ibid., xxii. 1883.
 
M. Weber.  - “Die Abdominalporen der Salmoniden nebst Bemerkungen
liber die Geschlechtsorgane der Eische,- Morph. Jahrb., xii. 1886.
W. E. R. Weldon.  - “ On the Suprarenal Bodies of Vertebrata,- Quart.
Jour. Micr. Sci., xxv. 1885.
 
“ On the Head-Kidney of Bdellostoma, with a Suggestion as to
 
the Origin of the Suprarenal Bodies,- Ibid., xxiv. 1884.
 
J. W. Van Wijhe.  - “Ueber den vorderen Neuroporus und die phylogenetische Function des Canalis neurentericus der Wirbelthiere,-
Zool. Anz., vii. 1884.
 
“ Ueber Somiten und Nerven im Kopfe von Vogel und Reptilien
embryonen,- Ibid., ix. 1886.
 
“Die Betheiligung des Ectoderms an der Entwicklung des
 
Vornierenganges,- Ibid., ix. 1886.
 
 
APPENDIX.
 
 
30$
 
 
CYCLOSTOMI.
 
E. Calberla.  - “ Zur Entwicklung des Medullarrohres und der Cliorda
 
dorsalis der Teleostier und der Petromyzonten,- Morph. Jahrb.,
iii. 1 87 7.
 
A. Dohrn.  - “Die Entstehung der Hypophysis bei Petromyzon Planeri,-
Zool. Anz ., v. 1882.
 
AY. K. Parker.  - “ On the Skeleton of the Marsipobranch Pishes, Part
L, The Myxinoids (Myxine and Bdellostoma) • Part II., Petromyzon,- Phil. Trans., 1883.
 
AY. B. Ranson and D -Arcy AY. Thompson.  - “On the Spinal and
Ausceral Nerves of Cyclostomata,- Zool. Anz., ix. 1886.
 
AAA B. Scott.  - “Beitrage zur Entwicklungsgeschichte der Petromyzonten,- Morph . Jahrb., vii. 1881.
 
“Preliminary Account of the Development of the Lampreys,-
 
Quart. Jour. Micr. Sci., xxi. 1881.
 
A. E. Shipley.  - “On Some Points in the Development of Petromyzon
fiuviatilis Quart. Jour. Micr. Sci., xxvii. 1887.
 
EL ASMOBR AN CHII.
 
J. F. Yan Bemmelen.  - “Ueber vermuthliche rudimentare Kiemenspalten
bei Elasmobrancbiern,- Mittlieil. Zoolog. Stat. Neapel, vi. 1885.
G-. B. Howes.  - “The Presence of a Tympanum in the Genus Raia,-
Jour. Anat. and Phys ., xvii. 1883.
 
A. M. Marshall.  - “ On the Head Cavities and Associated Nerves of
Elasmobranchs,- Quart. Jour. Micr. Sci., xxi. 1881.
 
A. M. Marshall and AY. B. Spencer.  - “ Observations on the Cranial
Nerves of Scyllium,- Ibid., xxi. 1881.
 
T. J. Parker.  - “ On the Gravid Uterus of Mustelus antarcticus,- Trans.
 
New Zealand Inst., xv. 1SS2 (1883).
 
T. AY. Yan AYijhe.  - “Ueber die Mesodermsegmente u. d. Entwicklung
der Nerven des Selachierkopfes,- Konigliche Akad. v. Wiss. zu
Amsterdam, 1882.
 
GANOIDEI.
 
F. M. Balfour and AY. K. Parker.  - “ On the Structure and Develop
ment of Lepidosteus,- Phil. Trans., 1882.
 
J. P. M‘Murrich.  - “The Cranial Muscles of Amia calva (L.), with a
Consideration of the Relations of the Post-Occipital and Hypoglossal Nerves in the various Yertebrate Groups,- Studies Biol.
Lab. Johns Hopkins TJniver., Baltimore, iii. 1885.
 
AY. K. Parker.  - “ On the Structure and Development of the Skull in
Sturgeons (Acipenser ruthenus and A. sturio),- Phil. Tra'is.,
1882.
 
U
 
 
306
 
 
APPENDIX.
 
 
W. K. Parker.  - “On the Development of the Skull in Lepidosteus
osseus,- Ibid., 1882.
 
TELEOSTEI.
 
A. Agassiz and C. 0. Whitman.  - “ On the Development of Some Pelagic
Fish-Eggs,- Proc. Amer. Acad, of Arts and Sci., xx. 1884.
 
G. Brook. - “ On the Origin of the Hypoblast in Pelagic Teleostean Ova,-
Quart. Jour. Micr. Sci., xxv. 1885.
 
J. T. Cunningham.  - “ On the Nature and Significance of the Structure
known as Kupffer -s Yesicle in Teleostean Embryos,- Proc. Roy.
Soc. Edinb., xiii. 1884.
 
“ The Significance of Kupffer -s Yesicle, with Remarks on other
 
Questions of Yertebrate Morphology,- Quart. Jour. Micr. Sci.,
xxv. 1885.
 
“ On the Relations of the Yolk to the Gastrula in Teleosteans
 
and in other Yertebrate Types,- Ibid., xxvi. 1885.
 
C. K. Hoffmann.  - “Zur Ontogenie der Knochenfische,- Konigliche
Alcad. v. Wissen. zu Amsterdam, 1882.
 
“Zur Ontogenie der Knochenfische,- Ar cl civ fur Micr. Anat.,
 
xxiii. 1884.
 
M. Yon Kowalewski.  - “ Ueber die ersten Entwicklungsprocesse der
Knochenfische,- Zeit. fur wiss. Zool ., xliii. 1886.
 
E. Maurer.  - “ Schilddriise und Thymus der Teleostier,- Morjph. Jahrb.,
 
xi. 1885.
 
J. P. M‘Murrich.  - “ On the Osteology and Development of Syngnathus peckianus (Storer),- Ibid., xxiii. 1883.
 
“The Cranial Ribs of Micropterus,- Science, iii. 1884.
 
C. Yon Noorden.  - “Die Entwickelung des Labyrinthes bei Knochenfischen,- Arch. f. Anat. u. Phys ., Anat. Abtheil., 1883.
 
J. A. Ryder.  - “ A Contribution to the Embryography of Osseous Fishes
 
with special Reference to the Development of the Cod (Gadus
morrhua),- Report U. S. Fish. Com. for 1882-84.
 
“ On the Development of Yiviparous Osseous Fishes and of the
 
Atlantic Salmon,- Proc. U. S. Nat. Mus ., 1885.
 
“ On the Development of Osseous Fishes, including Marine and
 
Fresh- Water Forms,- Report TJ. S. Fish. Com. for 1885-86.
 
R. W. Shufeldt.  - “ Osteology of the Large-Mouthed Black Bass,-
Science, iii. 1884.
 
K. F. Wenckebach.  - “Beitrage zur Entwicklungsgeschichte der Knoch
enfische,- Arch. f. mikr. Anat., xxviii. 1886.
 
DIPNOI.
 
F. E. Beddard.  - “ Observations on the Ovarian Ovum of Lepidosiren
 
(Protopterus),- Proc. Zool. Soc., 1886.
 
“ Observations on the Development and Structure of the Ovum
 
in the Dipnoi,- Proc. Zool. Soc., 1886.
 
 
APPENDIX.
 
 
307
 
 
AMPHIBIA.
 
M. Bedot.  - “ Recherches sur le Dbveloppement des Nerfs spinaux chez
les Tritons,- Recueil Zool. Suisse, i. 1884.
 
H. E. Durham.  - “ Note on the Presence of a Neurenteric Canal in
Rana,- Ibid., xxvi. 1886.
 
E. Gasser.  - “ Znr Entwicklung von Alytes ohstetricans,- Sitzunsber. d.
Marburger Naturges., 1882.
 
A. Gotte.  - Die Ent wicJdungsgeschich te der UnJce.
 
T. Iwakawa.  - “The Genesis of the Eggs in Triton,- Quart. Jour. Micr.
Sci., xxii. 1882.
 
Alice Johnson.  - “On the Fate of the Blastopore and the Presence of a
Primitive Streak in the Newt ( Triton cristatus ),- Ibid., xxiv. 1884.
 
Alice Johnson and Lilian Sheldon.  - “Notes on the Development of
the Newt ( Triton cristatus ),- Ibid., xxvi. 1886.
 
W. K. Parker.  - “ On the Structure and Development of the Skull of
the Common Frog ( Rana temporaria, L.),- Phil. Trans., 1871.
 
“On the Structure and Development of the Skull in the Batrachia,
 
part ii.,-' Ibid., 1876 ; part iii., 1881.
 
“On the Structure and Development of the Skull in Urodelous
 
Amphibia, part ii.,- Phil. Trans., 1877.
 
“ On the Structure and Development of the Skull in the Uro
deles,- Trans. Zool. Soc., xi. 1882.
 
C. Rabl.  - “Ueber die Bildung des Herzens der Amphibien,- Morph,
Jahrb., xii. 1886.
 
P. B. and C. E. Sarasin.  - “Ueber die Entwicklungsgeschichte von
Epicrium glutinosum,- Arb. Zool.-Zoot. Inst. Wurzburg, vii. 1885.
 
0 . Schultze.  - “Beitrag zur Entwicklungsgeschichte der Batrachier,-
Arch. f. Mihr. Anat., xxiii. 1884.
 
“ Untersuchungen liber die Reifung und Refruchtung des Am
phibienies, I.,- Zeit. fur iviss. Zool., xiv. 1887.
 
W. B. Scott and H. F. Osborn.  - “ On Some Points in the Early Development of the Common Newt,- Quart. Jour. Micr. Sci., xix.
1879.
 
B. Solger.  - “ Studien zur Entwicklungsgeschichte des Coeloms und des
Coelom epithels der Amphibien,- Morph. Jahrb., x. 1885.
 
W. B. Spencer.  - “ Some Notes on the Early Development of Rana
temporaria,- Quart. Jour., Micr. Sci, Suppl., xxv. 1885.
 
REPTILIA.
 
J. F. Van Bemmelen.  - “Die Halsgegend der Reptilien,- Zool. Anz., x.
1887.
 
W. Haacke.  - “Ueber eine neue Art uterinaler Brutpflege bei Reptilien,-
Zool. Anz., viii. 1885.
 
 
308
 
 
APPENDIX.
 
 
C. K. Hoffmann.  - “ Contribution a l -Histoire du D4veloppement des
Reptiles,- Arch. Neerlandaises d. Sci. exactes et Nat., xvii. 1882.
 
- -  -  - *â–  ‘ Beitrage zur Entwicklungsgeschichte der Reptilien,- Zeit. fur
wiss. Zool ., xl. 1884.
 
“ Weitere Untersuchungen zur Entwicklungsgeschichte der
 
Reptilien,- Morph. Jahrb., xi. 1885.
 
W. Lwoff  - “Beitrage zur Histologie der Haut der Reptilien,- Bull.
Soc. Imp. Nat. Moscou , lix. 1885.
 
K. Mitsukuri and C. Ishikawa  - “On the Formation of the Germinal
Layers in Chelonia,- Quart. Jour. Micr. Sci., xxvii. 1886.
 
W. K. Parker.  - “Report on the Development of the Green Turtle
(Chelone viridis, Schneid.),- The Zoology of the Voyage of HM.S.
Challenger, 1880.
 
“On the Structure of the Skull in the Chameleons,- Trans.
 
Zool. Soc., xi. 1881. [Cf. G. A. Boulenger, Proc. Zool. Soc., 1886.
 
• “ On the Structure and Development of the Skull in the
 
Crocodilia,- Ibid., xi. 1883.
 
C. F. Sarasin.  - “ Reifung fiber Furchung der Reptiliencier,- Arb. aus.
d. Zool.-Zoot. Inst. Wurzburg, vi. 1883.
 
W. B. Spencer.  - “ On the Presence and Structure of the Pineal Eye in
Lacertiiia,- Quart. Jour. Micr. Sci., xxvii. 1886.
 
H. Strahl.  - “ Ueber die Entwickelung des Canalis Myelo-entericus und
der Allantois der Eidechse,- Arch. f. Anat. u. Pliys., Anat. Abtheil.,
1881.
 
“ Beitrage zur Entwickelung von Lacerta agilis,- Ibid., 1882.
 
* “Beitrage zur Entwickelung der Reptilien,- Ibid., 1883.
 
“ Ueber Canalis neurentericus und Allantois bei Lacerta viridis,-
 
Ibid., 1883.
 
“ Ueber Entwicklungsvorgange am Yorderende des Embryo von
 
Lacerta agilis,- Ibid., 1884.
 
■ “Ueber friihe Entwicklungsstadien von Lacerta agilis;- “Die
 
Entwicklungsvorgange am Yorderen Enae von Lacerta agilis und
vivipara,- Zool. Anz., vi. 1883.
 
“Ueber Wachsthumsvorgange am Embryonen von Lacerta
 
agilis,- Abhandlung der Tenckenbergischen naturforschenden
Gesellschaft. Frankfurt, 1884.
 
“Die Dottersackswand und der Parablast cler Eidechse,- Zei t.
 
filr. wiss. Zool., xliv. 1887.
 
W. F. R. Weldon.  - “Note on the Early Development of Lacerta
muralis,- Quart. Jour. Micr. Sci., xxiii. 1883.
 
AVES.
 
E. M. Balfour and F. Deighton.  - “A Renewed Study of the Germinal
Layers of the Chick,- Quart. Jour. Micr. Sci., xxii. 1882.
 
 
.APPENDIX.
 
 
309
 
 
F. M. Balfour and A. Sedgwick.  - “On the Existence of a Head
Kidney in the Embryo Chick, and on Certain Points in the
Development of the Mullerian Duct,- Ibid., xix. 1879.
 
M. Braun.  - “Aus der Entwickelungsgeschichte der Papageien, III.,-
Verhandl. phys.-med. Ges. Wiirz., xv. 1880.
 
“Die Entwickelung des Wellenpapageis (Melopsittacus undulatus ,
 
Sh.), II.,- Arbeit. Zool.-Zoot. Inst. Wurzburg , v. 1881.
 
A. Budge.  - “Untersuchungen iiber die Entwickelung des Lymphsystems
beim Hiihnerembryo,- Arch. f. Anat. u. Phys., Anat. Abtheil.,
1887.
 
M. Duval.  - “Etudes histologiques et morphologiques sur les Annexes
des Embry ons d -Oiseau, - Jour. Anat. et d. Physiol., xx. 1884.
 
“ De la Formation du Blastoderme dans l -CEuf d -Oiseau,- Ann.
 
d. Sci. Nat. (6), Zool., xviii. 1884.
 
E. Gasser.  - “Beitrage zur Ivenntnis der Yogelkeimscheibe,- Arch. f.
 
Anat. u. Phys., Anat. Abtheil., 1882.
 
Alice Johnson.  - “ On the Development of the Pelvic Girdle and Skeleton
of the Hind-Limb in the Chick,- Quart. Jour. Micr. Sci., xxiii.
1883.
 
C. Koller.  - “ Untersuchungen iiber die Blatterbildung im Hiihnerkeim,-
Arch. f. mikr. Anat., xx. 1882.
 
Beatrice Lindsay.  - “ On the Avian Sternum,- Proc. Zool. Soc., 1885.
A. M. Marshall. - “ On the Early Stages of Development of the Nerves
in Birds,- Jour, of Anat. and Phys., xi. 1877.
 
- “The Development of the Cranial Nerves in the Chick,- Quart.
 
Jour. Micr. Sci., xviii. 1878.
 
G. Romiti. - “De l -Extremit 4 ant6rieure de la Corde dorsale et de son
 
Rapport avec la Poche hypophysaire ou de Rathke chez l -Embryon
du Poulet,- Arch. Ital. de Biol., vii. 1886.
 
C. 0 . Whitman.  - “A Rare Form of the Blastoderm of the Chick, and
its Bearings on the Question of the Formation of the Vertebrate
Embryo,- Quart. Jour. Micr. Sci., xxiii. 1883.
 
W. Wolff.  - “Ueberdie Keimblatter des Hiihnes, - Arch.f. mikr. Anat.,
xx. 1882.
 
MAMMALIA.
 
W. K. Parker.  - On Mammalian Descent (The Hunterian Lectures for
1884). London, 1885.
 
A. and B. - PROTOTIIERIA AND METATHERIA.
 
F, E. Beddard.  - “Note on the Presence of an Allantoic (Anterior
 
Abdominal) Vein in Echidna,- Zool. Anz., vii. 1884.
 
W. H. Caldwell.  - “ On the Arrangement of the Embryonic Membranes
in Marsupial Animals,- Quart. Jour. Micr. Sci., xxiv. 1884.
 
 
310
 
 
APPENDIX.
 
 
W. H. Caldwell.  - “ The Embryology of Monotremata and Marsupialia,-
in (Abstract of preliminary paper) Proc. Roy. Soc., 1887.
 
J. J. Fletcher.  - “On the Existence after Parturition of a Direct
Communication between the Median Vaginal Cul-de-sac, so called,
and the Urogenital Canal, in certain Species of Kangaroos,- Proc.
Linn. Soc. N. S. W., vi. 1881.
 
“ On Some Points in the Anatomy of the Urogenital Organs in
 
Females of certain Species of Kangaroos,- Ibid., vi. 1882-83.
 
C. Gegenbaur.  - Zur Kenntniss der Mammarorgane der Monotremen.
Leipzig, Engelmann, 1886.
 
W. Haacke.  - “ Meine Entdeckung des Eierlegens der Echidna,- Zool.
Anz., vii. 1884.
 
W. Haacke.  - “On the Marsupial Ovum, the Mammary Pouch, and the
Male Milk-Glands of Echidna liystrix,- Proc. Roy. Soc., xxxviii.
1885.
 
J. J. Lister and J. J. Fletcher  - “ On the Condition of the Median
Portion of the Vaginal Apparatus in the Macropodidse,- Proc.
Zool. Soc., 1881.
 
H. F. Osborn.  - “Observations upon the Foetal Membranes of the
Opossum and other Marsupials,- 7 &fc?.,'xxiii. 1883.
 
E. B. Poulton. - “ The Structures connected with the Ovarian Ovum of
 
Marsupialia and Monotremata,- Quart. Jour. Micr. Sci., xxiv.
1884.
 
W. B. Spencer.  - “The Eggs of Monotremes,- Nature, xxxi. 1884.
 
EUTHERIA.
 
P. Albrecht.  - “ Note sur le Centre du Proatlas chez un Macacus
arctoides I. Geciffr.,- Bull. Mus. Roy. d -Hist. Nat. Belg., ii. 1883.
 
“ Note sur le Pelvisternum des Edent^s,- Bull. Acad. Roy. de
 
Belg., vi. 1883.
 
■ “ Sur les Elements morphologiques du Manubrium du Sternum
 
chez les Mammiferes,- Livre Jubilaire, Soc. Medecine de Gand,
1884.
 
“Ueber die Chorda dorsalis und 7 Knocherne Wirbelcentren
 
im Knorpeligen Nasenseptum eines erwachsenen Rindes,- Biol.
Centralbl., v. 1885.
 
F. M. Balfour.  - “ On the Evolution of the Placenta, and on the
 
Possibility of Employing the Characters of the Placenta in the
Classification of the Mammalia,- Proc. Zool. Soc., 1881.
 
W. Barnes.  - “ On the Development of the Posterior Fissure of the
Spinal Cord and the Reduction of the Central Canal in the Pig,-
Proc. Amer. Acad, of Arts and Sci., xix. 1883.
 
E. Van Beneden et C. Julin.  - “Recherches sur la Formation des
Annexes foetals chez les Mammiferes (Lapin et Cheiroptkres),-
Arch, de Biol., v. 1884.
 
 
APPENDIX.
 
 
311
 
 
B. Bonnet.  - “ Beitrage zur Embryologie cler AViederkauer, gewonnen am
 
Schafei,- Arch . Anat. Phys ., Anat. Abthiel ., 1884.
 
G. Born.  - “ Ueber die Derivate der embryonalen Schlunsbogen imd
 
Schlundspalten bei Saugethieren,- Arch, mikr. Anat., xxii. 1883.
M. Braun.  - “ Besondere Entwickelungsverhaltnisse am Scliwanzende
von Saugethieren,- Sitzungsber. Dorpater. Naturf. Gesell. , vi.
1884.
 
H. H. Brown.  - “ On Spermatogenesis in the E t,- Quart. Jour. Micr.
 
Set., xxv. 1885.
 
C. Emery.  - “ Ricerche embriologiche sul rene dei Mammiferi,- Atti. d.
 
R. Acc. d. Lined (Rome), (3), xv. 1885.
 
AY. Flemming.  - “ Die ektoblastische Anlage des Urogenitalsystems beim
Kaninchen,- Arch. f. Anat. u. Phys., Anat. Abtheil., 1886, p. 236.
A. Fraser.  - “ On the Development of the Ossicula anditus in the
Higher Mammalia,- Phil. Trans., 1882.
 
“ On the Inversion of the Blastodermic Layers in the Rat and
 
Mouse,- Proc. Roy. Soc., xxxiv. 1883.
 
A. C. Haddon.  - “R ote on the Blastodermic Vesicle of Mammals,-
Proc. Roy. Dubl. Soc. (N.S.), iv. 1885.
 
AY. Heape.  - “T he Development of the Mole ( Talpa europea ), the Formation of the Germinal Layers, and Early Development of the
Medullary Groove and Notochord,- Quart. Jour. Micr. Sci. xxiii.
1883.
 
The Development of the Mole ( Talpa europea), the Ovarian
 
Ovum, and Segmentation of the Ovum,- Ibid., xxvi. 1886.
 
“ The Development of the Mole ( Talpa europea), Stages E to
 
J,- Ibid., xxvii. 1886.
 
Y. Hensen.  - “E in frillies Stadium des im Uterus des Meerschweinchens
festgewachsenen Eies,- Arch. f. Anat. u. Phys., Anat. Abtheil.,
1883.
 
“Bemerkungen betreffend die Mittheilungen von Selenka und
 
Ivupffer iiber die Entwickelung der Mause,- Ibid., 1883.
 
L. Hiltner.  - “U eber die Entwicklung des Nervus opticus der
Saugethiere,- Biol. Centralbl., v. 1885.
 
AY. His.^  - -“Anatomie Menschlicher Embryonen.- “I. Embryonen des
ersten Monats,- 1880; “II. Gestalt- und Grossenentwicklung bis
zum Scliluss des 2 Monats,- 1882; “III. Zur Geschichte der
Organe,- 1885 (with Atlas). Leipzig.
 
G. B. Howes.  - “ The Morphology of the Mammalian Coracoid,- Journ.
 
Anat. and Phys., xxi. (N.S. 1), 1887.
 
H. Klaatsch.  - “ Zur Morphologie der Saugethier-Zitzen,- Morph. Jahrb.,
 
ix. 1883.
 
R. Kraushaar.  - “Entwicklung der Hypophysis und Epiphysis bei
Nagethieren,- Zeit. fur wiss. Zool., xli. 1884.
 
==Appendix==
 
 
C. Kupffer,  - “Das Ei von Arvicola arvalis tmd die vermeintliclie
Umkehr der Keimblatter an demselbem,- Sitz. d. Math.-Phys. Cl.
7 c. b. AJcad. d. Wiss. z. Milnchen , v. 1882.
 
U. Lieberkuhn.  - “ Ueber die Cborda bei Saugethieren,- Arch. f. Anat.
u. Phys ., Anat. Abtheil ., 1882-84.
 
S. Lothringer.  - “ Untersuchungen an der Hypophyse einiger Saugethiere und des Menschen,- Arch. f. miter. Anat., xxviii. 1886.
 
K. Mitsukuri.-  - “On the Development of the Suprarenal Bodies in
Mammalia,- Quart. Jour. Micr. Sci ., xxii. 1882.
 
W. K. Parker.  - “ On the Structure and Development of the Skull in
the Pig (Sus scrota),- Phil. Trans., 1874.
 
“ On the Structure and Development of the Skull in the Mammalia.- Part ii. , “Edentata;- Part iii., “ Insectivora - (with
Bibliography), Ibid.. 1885.
 
G. Pouchet et L. Chabry.  - “ Contribution a TOdontologie des Mammiferes,- Jour. Anat. et de Physiol ., xx. 1884, p. 149.
 
G. Rein.  - “Untersuchungen iiber die embryonale Entwicklungsgeschichte
der Milchdriise I.,- Arch. f. miter . Anat., xx. 1882.
 
R. Rubattel.  - “Recherches sur le Developpement du Cristallin chez
rHomme et quelques Animaux sup^rieurs,- Pec. Zool. Suisse, ii.
1885.
 
W. Salensky.  - “Beitrage zur Entwicklungsgeschichte der Knorpeligen
Gehorknochelchen bei Saugetbieren,- Morph. Jahrb., vi. 1880.
 
E. Selenka.  - Studien iiber Entwicklungsgeschichte der Thiere. I. Keimbldtter und Primitive Organe der Maus. Wiesbaden, 1883.
III. Die Bldtterumteehrung im Ei der Nagethiere. Wiesbaden,
1884.
 
G. E. Spee.  - 11 Beitrag zur Entwickelungsgesehichte der friiheren Stadien
des Meerschweinchens bis zur Vollendung der Keimblase,- Arch,
f. Anat. u. Phys., Anat. Abtheil., 1883.
 
“ Ueber directe Betheiligung des Ektoderms an der Bildung der
 
Urnierenanlage des Meerschweinchens,- Arch. f. Anat. u. Phys.,
Anat. Abtheil., 1884, p. 89 (full Bibliography).
 
K. Strahl.  - “ Zur Bildung der Cloake des Kaninchenembryo,- Arch,
f. Anat. u. Phys., Anat. Abtheil., 1886.
 
 
INDEX
 
 
Abdominal cavity, 213.
 
pores, 213, 214, 258.
 
ling, 262.
 
splints or “ribs,- 193, 200.
 
Abducens, 140.
 
Abneural blood-vessel, 215.
 
mesentery, 215.
 
Abomasus, 173.
 
Acantkias, pineal gland, 1 29; thyroid
body, 184.
 
Achromatin, 17.
 
Acraspeda, auditory organ, 145.
 
Actiniae, ectodermal muscle fibres, ic8 ;
ectodermal and endodermal nervous
system, 165 ; endodermal muscles,
176; reproduction, 278.
 
Actinozoa, stomodaeum, 71 ; arcbenteric
diverticula, 72 ; mesenteries, 72 ; digestion, 72, 168 ; endodermal respiration,
177 ; connective tissue cells, 189 ; ectodermal digestion in larvae, 273. See
“Actiniae,- “ Hexacoralla,- “ Octocoralla. -
 
Adhesive glands, 106.
iEginura, sense cells, 1 42.
iEquorea, auditory organ, 146.
After-birth, 92.
 
After-brain, 134.
 
Air-bladder, 180; homology of, 182;
 
blood supply, 182, 233-235.
 
Air-sacs of birds, 182 ; of gymnodontes,
182.
 
Alcyonaria, spicules, 98.
 
Alecithal ovum, 8 ; segmentation, 16, 17,
20, 21, 29.
 
gastrulation, 21-23, 3°
Alisphenoid cartilage, 207.
 
Allantoic arteries, 224, 231, 232, 237.
 
vein, 229-233, 236, 237.
 
Allantois, 81 ; of birds, 81-84; °f reptiles, 84 ; of mammals, 87-97 5 evolution, 87, 276; circulation in, 230-232,
234
 
Alveoli of glands, 105, 173.
 
Alytes, larval respiration, 87.
 
Amblystoma, vertebral column, 197.
 
Ambulacral system, 55, 56.
 
Amia, scales, 193 ; intercalary arches,
196 ; blood supply of air-bladder, 233,
235 ; oviduct, 258.
 
Ammocoete. See “ Lamprey.- Rudiments of pelvic fins, 204 ; diaphragm,
213.
 
Amnion, 79 ; of fowl, 79 ; amnion proper and false amnion, 8 1 ; of reptiles,
84 ; of the rabbit, 85 ; mammals, 8797, 231 ; insects, 96.
 
Amniota, primitive streak, 39, 41, 42 ;
embryonic appendages, 78 ; urinary
bladder, 78 ; allantoic respiration, 88,
276; cloaca and derivatives, 1 12;
neurenteric canal, 117 ; pharynx, 172 ;
absence of gills, 1 77 ; thyroid body,
184 ; thymus gland, 185 ; derma, 190 ;
muscles, 19 1 ; ribs, 199 ; clavicles, 202 ;
cheiropterygium, 204 ; skull, 207-21 1 ;
vascular system, 223-232, 235-237 ;
pronephros, 241 ; mesonephros, 242 ;
vasa efferentia, 256 ; embryonic respiration, 276. See also “ Reptilia,- “Aves,-
and “ Mammalia.-
 
Amoeba, reproduction, 3 ; mode of feeding, 7, 272 ; nuclear division, 20.
 
Amphibia. See also “ Frog.- Fate of
blastopore, 77, 1 1 7 ; urocyst, 87, 230,
234, 259 ; larval respiratory organs,
87, 276 ; epiblast, 100, 16 6 ; horny
teeth of larvae, 105 ; epiblastic respiration and external gills, 109 ; post-anal
gut, 117; nervous system, 116-118;
cranial flexure, 123 ; corpus callosum,
124, 132; anterior commissure, 124;
cerebellum, 126; supra-commissura,
128; pineal gland, 1 1 7, 129 ; posterior
commissure, 129 ; fenestra ovalis, 150;
columella auris, 151, 152; eyelids,
 
 
314
 
 
INDEX.
 
 
162 ; epiphysial eye, 163 ; relation of
epiblast to nervous system, 166, 167 ;
pharynx, 172 ; liver, 173 ; Eustachian
recess, 179 ; visceral clefts, 179 ; lungs,
181, 182; gustatory cells, 182 ; suprapericardial bodies, 1 84 ; thyroid body,
 
184 ; thymus gland, 185; muscles, 191;
dermal exo-skeleton, 193 ; vertebral
column, 196-198; ribs, 199, 200;
sternum, 200 ; pectoral girdle, 20 1 ;
median fin, 203 ; tail, 203 ; paired
limbs, 204 ; mandibular arch, 209 ;
hyoid, 210; position of kidneys, 212 ;
pleuro-peritoneal cavity, 21 3 ; absence
of abdominal pores, 214; heart, 218,
220, 221 ; aortic arches, 227, 233, 235 ;
venous system, 234, 235 ; anterior
abdominal vein, 230, 234, 235 ; segmental duct, 239, 240, 249, 251 ;
pronephros, 239, 240, 241, 252 ; mesonephros, 242, 246, 248; incipient metanephros, 247, 256 ; Mullerian duct
(oviduct), 251, 252; rudimentary in
male, 254 ; connection between testis
and mesonephros, 254-256; ureters,
256.
 
Amphiblastula, 51.
 
Amphicoelous vertebrae, 196-198.
 
Amphineura, nervous system, 115. See
“ Chiton.-
 
Amphioxus, segmentation of oosperm,
29, 30 ; mesothelium, 55 ; archenteric
diverticula, 59, 60-62 ; mesenchyme,
61 ; mesodermal segmentation, 74 ;
liver, 73, 173; epiblast, 100, 166;
ciliated neural canal, 116; nervous
system, 1 1 7, 119; neural pore, 1 17 ;
nerves, 120; unpaired eye, 1 29; gillslits, 135, 178 ; olfactory organ, 142,
 
185 ; alimentary canal, 1 7 1 ; pharynx,
172; hypopharyngeal ridge, 172;
atrium, 178; atrial pore, 178, 214;
taste-buds, 182 ; gustatory organ, 185 ;
notochord, 186, 188, 194, 205 ; median
fin, 203 ; caudal fin, 203 ; body-cavity,
21 1 ; muscles, 19 1, 21 1; blood-vessels, 215, 223, 239; nephridium, 239,
250.
 
Amphipnous, reduction of gills, 179.
 
Amphipoda, diverticula of mesenteron,
169, 186, 238.
 
Amphistylic skull, 209.
 
Ampullaria, respiration, 109.
 
Anabas, accessory respiratory organs, 109.
 
Anableps, development of embryos in
ovary, 95.
 
 
Anabolism of protoplasm, 278-280.
 
Anal glands, 1 06.
 
Anguis, scales of, 103 ; pineal eye, 129.
Annelids, otocysts of, 146 ; heart, 215 ;
segmental organs, 238. See also
“ Chsetopoda.-
 
Annodonta, gills of, 108.
 
Antelope, placenta, 92.
 
Antennse of insects, 1 1 5 ; of Crustacea,
1 16.
 
Antennules, 147.
 
Anterior abdominal vein, 229-232, 234.
 
(superior) cardinal veins, 224, 225,
 
228, 229, 231-234.
 
commissure, 124, 132, 134.
 
nares, 144.
 
primary brain vesicle, 127, 1 34.
 
(superior) vertebral veins, 228, 229.
 
Anthea, sense cells, 142.
 
Anthropoidea, placenta, 92.
 
Antipathidee, csenenchyma of, 98, 194.
Ant- orbital process, 206.
 
Anura. ^“Amphibia,- “Frog,- and
“ Bombinator.-
 
Anus, 75, hi, 250, 261.
 
Aorta, 121, 220-222, 224-227.
 
Aortic arches, 147, 148, 183, 185, 217,
218, 220-227, 2 3 I » 233, 235.
 
Apes, corpus albicans, 127 ; cerebral
hemispheres, 130.
 
Aphides, parthenogenetic reproduction,
279.
 
Aplysia, shell of, 1 00.
 
Appendicularia, auditory organ, 147 ;
visceral clefts, 177.
 
Aqueduct of Sylvius, 124, 125. See
“ Iter.-
 
Aqueductus vestibuli, 148-150.
 
Aqueous humour, 162.
 
Arachnida, spermatozoa of, 3 ; respiration
of, 109; Malpighian tubules, 111 ; appendages, 203.
 
Arbor-vitse, 126.
 
Archseocytes, 189, 266.
 
Archenteric diverticula, 55 -69, 73, 270.
Archenteron, 23.
 
Archiccel, 69, 70, 237, 238, 270, 271.
Archinephric duct, 239, 249, 25 1.
Archinephridium, 249.
 
Archinephros, 250.
 
Area opaca, 9, 38, 119, 2 17, 223.
 
pellucida, 9, 38, 78.
 
vasculosa, 9, 38, 79, 85, 88, 119,
 
214, 223, 224, 226.
 
Areolar epithelium, 106, 107.
 
Argiope, early development, 55.
 
 
INDEX.
 
 
315
 
 
Armadillo, scutes of, 103, 193; caecum,
176.
 
Arsenoblast, 14.
 
Arteria thyreoidea mandibularis, 235.
 
Arthropoda, segmentation of egg, 27 ;
mesoblast, 57, 69; pseudoccel, 69, 70;
coelomic cavities, 70 ; segmentation of
body, 73 ; head, 74, 75 ; appendages,
75, 202 ; moulting of larvae, 78 ; exoskeleton, 98 ; anal respiration, 109,
275 ; aerial respiration, 109 ; stomodaeum and derivatives, no; proctodaeum, in ; nervous system, 115, 1 16 ;
olfactory hairs, 142 ; auditory hairs,
147 ; otocysts, 146, 147 ; acoustic
 
organ of hexapoda, 147 ; eyes, 153 -
157; mesenteron, 169; “liver,- 170;
muscles, 191 ; heart, 215 ; excretory
organs, 238.
 
Artiodactyla, placenta, 92.
 
Arvicola, inversion of blastoderm, 93.
 
Ascaris, maturation of ovum, 15 ; karyokinesis in, 19.
 
Ascidians. See “ Tunicata.-
 
Asellus, heart, 216.
 
Aspidonectes, pharyngeal respiration,
180.
 
Astacus. See “ Crayfish.-
 
Aster, 10, 18.
 
Asterias. See “ Starfish.-
 
Atrial, pore of amphioxus, 178, 214.
 
Atrium of heart, 219, 220, 224.
 
balanoglossus, 1 77; tunicata, 1 77;
 
amphioxus, 178.
 
Auditory epithelium of batrachia, 166.
 
ganglion, 138.
 
nerve, 136, 1 38-141.
 
organ, 139 ; medusae, 145 ; vermes,
 
146 ; mollusca, 146 ; arthropoda, 146,
 
147 ; tunicata, 147 ; cerebral auditory
organ of tunicata, 148 ; vertebrate,
147-152 ; myxine, 149 ; mesoblastic
labyrinth, 150 ; meatus and concha,
15 1; auditory ossicles, 151.
 
region of cranium, 207.
 
Aurelia, occasional direct development,
265.
 
Auricle of heart, 219, 222.
 
Auricular septum, 221, 222, 237.
 
Autostylic skull, 209.
 
Aves. See “ Birds.-
 
Axial nerve of retinophora, 156.
 
Axillary artery, 226.
 
Azygos vein, 228, 231-233, 236.
 
Axolotl, division of leucocytes in, 19 ;
retention of gills, 1 79.
 
 
Balanoglossus, mesothelium of, 55 ; archenteric diverticula of, 59, 60 ; origin
of nervous system, 167 ; pharynx, 172 ;
absence of liver, 173 ; respiratory
organs, 177; notochord, 186, 188.
 
Barbs and barbules, 102, 103.
 
Basi-hyoid and basi-branchial cartilages,
210.
 
mandibular, 210.
 
sphenoid cartilage, 207.
 
Basilar artery, 125.
 
plate, 206.
 
Bat, blastodermic vesicle of, 45, 48; proamnion, 86.
 
Batrachia. See “Frog,- auditory epithelium, 166.
 
Bdellostoma, head-kidney, 259.
 
Beaks, 102.
 
Beetle, gastric caeca, 169.
 
Biconcave vertebrae, 196-198.
 
Bilateral symmetry, 74, 270.
 
Birds (Aves), yolk, 6; segmentation, 31,
3^> 37, 38 ; mesoblast, 66, 69 ; festal
membranes, 78, 273 ; umbilical cord,
96 ; yolk-sac, 82, 95, 96 ; epidermis of,
101 ; feathers of, 102 ; scutae, 103 ;
bursa Fabricii, 112; cerebellum, 126;
corpora bigemina, 126; cerebral hemispheres, 129; olfactory lobes, 133 ;
auditory labyrinth, 150 ; pineal gland,
163 ; persistence of vitelline duct, 171,
172; crop, 172; liver, 173 ; caeca, 176;
air-sacs from lungs, 182 ; thyroid, 184 ;
thymus, 185; notochord, 186; muscles,
191 ; sclerotic bones, 193; vertebral
column, 198; sternum, 200; pectoral
girdle, 201; diaphragm, 213; absence
of abdominal pores, 214; heart, 218223 ; division of truncus, 222 ; ventricular septum, 222 ; circulation, 225230 ; mesonephros, 248, 249.
 
Birgus, respiration, 109.
 
Bladder, 230, 259 ; veins of, 234.
 
Blastema, 266 ; Wolffian, 245, 246 ; metanephric, 246-249.
 
Blastocoel, 21-37.
 
Blastoderm, 26, 34-48.
 
Blastodermic vesicle, 44-48.
 
Blastosphere, 21.
 
Blastopore, 22 et seq. (“Blastopore- of
Van Beneden, 44, 48) ; fate of, 75, no,
in, 1 16, 250.
 
Blastula, 21-37, 49, 50, 268, 270.
 
Blatta, gastric cseca, 169 ; heart, 215.
 
Blood corpuscles, 214, 2 15-2 17, 267 ; red,
white, or colourless, 190. 214.
 
 
316
 
 
INDEX.
 
 
Blood-vessels, origin of, 67, 70, 214, 215.
 
Body cavity, 61, 68, 72, 78, 21 1, 212.
 
Bombinator, pituitary body, ill ; neural
canal, 1 1 7 ; branchial chamber, 179;
segmental duct, 240.
 
Bovidse, placenta, 92 ; horns of, 102.
 
Brachio-cephalic vein, 228.
 
Brachiopoda, mesothelium of, 55, 59;
shell, 99 ; nephridia, 237 ; generative
ducts, 237, 264.
 
Brachyura, thoracic ganglion, 116 ;
auditory organ, 147.
 
Brauchise. See “Respiratory Organs.-
 
Branchial arches, 178, 183, 185, 206, 208,
210.
 
arteries of, 226, 22 7, 233, 235 ; veins,
 
224.
 
clefts. See “Visceral Clefts.-
 
sense-organ, 1 37-141.
 
Branchiostegal membrane, 1 79.
 
Bronchi, 182.
 
Buccinum, ova of, 6 ; cannibalism of
embryos, 7.
 
Budding zones, 278.
 
Bufo, pineal gland, 163 ; branchial
chamber, 179.
 
Bulbus arteriosus, 220, 221, 223, 224.
 
Bursa Fabricii, 112.
 
pelvis, 174.
 
Byssus glands, 106.
 
Caducibranch amphibia, hyoid, 210.
 
Csecilia, scutes, 193 ; vertebral column,
197 ; pronephros, 239 ; segmental
Wolffian tubules, 248.
 
Caecum, 176.
 
Caeliac artery to air-bladder, 182, 233.
 
Calcispongiae, amphiblastula of, 51.
 
Calf. See “ Cow.-
 
Callianassa, segmentation, 27, 28.
 
Calotes, pineal age, 163.
 
Canalis auricularis, 221.
 
Canary (Pyrrhula), blastoderm of, 37 ;
gastrula of, 32.
 
Caput epididymis, 260.
 
Cardinal veins, 224-232, 234.
 
Carididae, auditory organs, 147.
 
Carinatae, sternum, 200.
 
Car marina, sense cells, 142.
 
Carnivora, placenta, 91 ; nipple of, 106.
 
Carotid artery, internal, 125, 207, 224,
226, 22 7 ; external, 224, 226, 227.
 
Carotids, 220, 224, 226, 227, 233, 236.
 
Carp (Cyprinus), air-bladder, 780.
 
Cartilage, invertebrates, 194 ; chordata,
1 94-2 10.
 
 
Cat (Felis domestica), ovum of, 7 ; Gaertner -s duct, 257.
 
Caudal artery, 224.
 
fin, 203, 204.
 
vein, 228, 229, 231, 234.
 
Cavia, inversion of blastoderm, 93, 95.
 
Cell-division, 17, 267.
 
Centrolecithal ovum, 8, 271 ; segmentation, 27-29.
 
Cenogeny, 223.
 
Cephalochordata, notochord of, 186.
 
Cephalopoda, gastrulation, 24, 26 ; shellsac, 100 ; nervous system, 1 14 ; olfactory pit, 142; eye, 154-156; ink-sac,
1 70; cartilaginous brain-case, 194.
 
Ceratophrys, dermal bones, 1 93.
 
Cerebellum, 122-126, 134.
 
Cerebral hemispheres, 123-125, 127, 128134
Cerianthus, endodermal respiration, 177.
 
Cervidse, placenta, 92.
 
Cestracion, mandibular arch, 209.
 
Cetacea, placenta, 92 ; posterior nares,
144 ; stomach, 173 ; dorsal fin, 203.
 
Chsetopoda, segmentation, 26 ; mesoblast,
54, 56, 69 ; coelom, 58, 69, 70 ; segmentation of body of, 73 ; otocyst,
146 ; muscles, 19 1 ; parapodia 202 ;
setae, 202 ; vascular system, 7°> 21 5 ;
generative ducts, 237, 264 ; nephridia,
70, 237, 238, 249, 250 ; heart, 270.
 
Chaetognatha. See “ Sagitta.-
 
Chameleon, pineal gland, 163; branched
lungs, 182.
 
Cheiropterygium, 204.
 
Chelonia, scales of, 103 ; anal sacs of, 1 12 ;
dermal skeleton, 193 ; abdominal pores
214; urinary bladder, 259. See also
“ Trionyx.-
 
Chimaera, lateral line, 148 ; pseudo-branch
178; operculum, 179; axial skeleton,
195, 196.
 
Chiroptera, placenta, 91.
 
Chiton, nervous system, 114, 115; eyes
in shell, 152; larval eyes, 1 52.
 
Choano -flagellate cells, 168.
 
Chorda-entoblast, 31, 62, 1 86.
 
Chorda tympani, 136.
 
Chordata, segmentation, alecithal, 29, 30 ;
effect of increase of yolk, 30-33 ; telolecithal, 33-39 ; origin of mesoblast,
59 69 ; axial hypoblast, 62; coelomic
cavities, 70 ; metamerism, 73, 74, 212 ;
ancestry, 74 ; fate of blastopore, 7078, no, hi, 250; epiblastic protective
structures, 100; epiblastic respiration,
 
 
INDEX.
 
 
317
 
 
109; stomodaeum, no; central nervous
system, 116 ; olfactory organ, 143 ; ancestral form, 76, 77, 163, 164; origin of
nervous system, 166, 167; mesenteron,
1 7 1 ; pharynx, 172 ; liver, 173 ; intestine
174; branchial region, 177 ; notochord,
186 ; muscular system, 191, 192 ;
 
dermal skeleton, 192 ; endo-skeleton,
194 ; locomotory appendages, 203 ;
paired limbs, 204; body cavity, 21 1 ;
formation of blood, 214 ; heart, 219 ;
urinary organs, 239, 249.
 
Chondro-cranium, 205, 207.
 
Chorion (vitelline membrane), 9 ; (allantoic or true), 87, 90 ; (yolk-sac or
false), 90 ; (pseudo-chorion), 95.
 
Choroid of cephalopoda, 155; of vertebrates, 160-162; pigment layer of,
159-161.
 
Choroidal fissure, 158-162, 144.
 
Choroid plexus of fourth ventricle, 1 23126, 134; of third ventricle, 124, 125,
129-134; of lateral ventricles, 130134 .
 
Chromatin, 8, 18, 277.
 
Chyle, 274.
 
Cilia, 98.
 
Ciliary ganglion, 135, 137-141.
 
nerve, 138, 139.
 
Ciliata, nuclear division in, 20.
 
Ciliated chambers of sponges, 168.
 
Cirri, 202.
 
Clavicles, 200-202.
 
Claws, 102.
 
Clitoris, 260, 262.
 
Cloaca of vertebrates, ill; amniota,
1 12 ; opening of segmental duct, 240,
250 ; of ureter, 246 ; of bladder, 259 ;
ornithorhynchus, 253 ; prototheria,
257; eutheria, 261.
 
Clytia, perisarc, 98.
 
Cobitis, external gills of larva, 109.
 
Coccygeo-mesenteric vein, 229, 231.
 
Cochlea, 149, 150.
 
Cockroach (Blatta), gastric caeca, 169 ;
heart, 215.
 
Coelenterata, mesoderm, 53, 69, 71, 189,
193; radial symmetry, 71; nematocysts, 99 ; unicellular glands, 105 ;
ectodermal muscle-fibres, 108; nervous
system, 112, 1 14 ; sense-cells, 142;
evolution of nervous system, 166 ;
gastric diverticula, 168, 270; skeletal
structures, 98, 193, 194; origin of
sexual cells, 263, 278 ; connection of
cells, 267 ; intracellular digestion, 272
 
 
See “Hydrozoa,- “ Scyphomedusae,-
and “ Actinozoa.-
 
Coelom, 61, 68, 72, 78, 21 1, 212, 213,
2 37, 270.
 
Coelomata, internal opening of nephridia,
237 ; origin of excretory organs, 249 ;
gonads, 263.
 
Coenenchyma, 98, 193, 194.
 
Columella auris, 15 1, 152.
 
Common carotid artery, 227, 231, 235.
Common iliac arteries, 224, 232.
 
veins, 229, 232.
 
Concha, 151.
 
Coni vasculosi, 256.
 
Conjunctiva, nerve supply, 137, 162.
Connective tissue, 67, 70, 189, 190.
Conus arteriosus, 220, 221.
 
Convolutions of cerebral hemispheres,
 
* 33
Coprodaeum, 112.
 
Copulatory organs, 112, 260, 262, 264.
Coracoid, 201.
 
Coracoid epiphysis, 201.
 
Corallium, coenenchyma, 194.
 
Corium, 103.
 
Cornea of mollusca, 155, 156; of vertebrates, 153-164.
 
Corneal cuticula, 156, 1 57.
 
Cornu ammonis, 130, 131.
 
Cornua trabeculae, 206.
 
uteri, 253.
 
Cornu medium, 184.
 
Coronary sinus, 228.
 
vein, 221, 236.
 
Corpus albicans, 127, 132.
 
Corpora bigemina, 125-127, 134; quadrigemina, 1 25-1 27, 134.
 
cavernosa, 260.
 
Corpus callosum of amphibia and reptiles, 124, 132 ; of mammals, 125, 132
134
striatum, 125, 130-134.
 
spongiosum urethrae, 260.
 
uteri, 253.
 
Costal pleura, 2 1 3.
 
Costal sternum,
 
Cow (Bos taurus), nipple of, 107 ; brain
of embryo, 130 ; branchial sense organ,
139; auditory labyrinth, 149.
Cowper -s gland, 260.
 
Crab (Cancer), thoracic ganglion, 116 •
auditory organ, 147.
 
Crangon, eye, 154.
 
Cranial flexure, 123, 205.
 
Cranial sense organs, 139, 205.
 
Cranium, 205-207.
 
 
318
 
 
INDEX.
 
 
Crayfish, spermatozoon of, 4 ; segmentation of, 28, 29 ; mesoblast of, 57,
58 ; stomodgeum and derivatives, 1 10;
nervous system, 1 1 5 ; eyes, 153; alimentary canal, 170; heart, 216; direct
development, 265.
 
Crinoidea, nervous system, 1 13; larval
skeleton, 194 ; arms, 202.
 
Criodrilus, nervous system, 1 1 3.
 
Crista acustica, decapoda (cephalopoda),
146 ; tunicata, 148.
 
sterni, 200, 201.
 
Crocodilia, scutes of, 103, 1 93; posterior
nares, 144 ; crop, 172 ; abdominal
splints, 193, 200 ; vertebral menisci,
198; abdominal pores, 214; ventricular septum, 222.
 
Crop, arthropoda, no, 169; vertebrates,
 
172.
 
Crura cerebri, 125, 127.
 
Crura of fornix, 132.
 
Crural veins, 229, 232.
 
Crustacea, spermatozoa, 3 ; segmentation, 27, 28, 29 ; mesoblast, 57 ; alimentary canal, 76 ; exoskeleton, 98 ;
anal respiration, 109, 1 77 ; aerial respiration, 109 ; stomodgeum and derivatives, Iio; proctodgeum, III; nervous system, 115, 1 16 ; auditory
 
organs, 1 47 ; eyes, 1 5 3, 1 54 ; digestion of yolk, 29, 169 ; mesenteron,
169, 170; digestive cgeca, 169; heart,
216; excretory organs, 238; shell
gland, 238 ; suppression of metamorphoses, 265 ; digestion of yolk, 272.
 
Crypts of Lieberkiihn, 173.
 
Crystalline cone, 156.
 
Ctenophora, origin of middle layer, 53,
189.
 
Cucullanus, gastrulation of, 23.
 
Cunina, auditory organ, 146.
 
Cutaneous artery, 235.
 
Cuticula dentis, 104, 105.
 
Cuticle, 98, 152-156.
 
Cutis, 106, 190.
 
Cuttlefish (Sepia), shell-sac, 100.
 
Cyclodus, viviparity of, 96 ; scales of,
103 ; pineal gland, 163.
 
Cyclops, body cavity, 211.
 
Cyclostomi, fate of blastopore, 1 17 ;
post-anal gut, 117 ; cranial flexure,
123; cerebellum, 126; vagus nerve,
135 ; nasal sac, 1 43 ; auditory labyrinth, 149; absence of pancreas, 172;
spiral valve, 175 ; gill pouches, 178 ;
axial skeleton, 194, 195 ; caudal fin,
 
 
203 ; absence  -of paired limbs, 204 ;
cranium, 207 ; visceral arches, 208 ;
basi - mandibular, 210; diaphragm,
213; abdominal pore, 214, 258, 264;
intestinal folds, 273. See “ Lamprey.-
 
Cyclothurus, cgecum, 176.
 
Cyprinoids, accessory auditory apparatus,
150.
 
Dactylethra, branchial chamber, 179.
 
Darmentoblast, 31, 62.
 
Decapoda (cephalopoda), otolith, 146 ;
 
eye, 155, 156.
 
(crustacea), gastric mill, 1 10 ; auditory organ, 147; eye, 153, 154; mesenteron, 169, 170; “liver,- 170;
green gland, 238.
 
Decidua reflexa, 92.
 
serotina, 93.
 
vera, 93.
 
Deckenschicht, 45, 48.
 
Delamination, 48-50.
 
Dentalium, nervous system, 1 14 ; otolith,
146.
 
Dentine, 104.
 
Derma, 102, 190.
 
Dermal, bones of skull, 210.
 
skeleton, 192, 193, 210.
 
Diaphragm, 213, 221.
 
Diaster, 18, 19.
 
Dibranchiata, eye, 155, 156.
 
Diencephalon, 134.
 
Didelphia. See “ Metatheria.-
 
Didelphys. See “ Opossum.-
 
Digestion, 169 ; in actinozoa, 72 ; embryonic digestion, 27 1 ; hypoblastic
digestion, 272.
 
Diphycercal tail, 203.
 
Diploblastic blastoderm, 86 ; organism,
268.
 
Diplostichous eye, 153.
 
Dipnoi, cerebellum, 126; nostrils, 144 ;
operculum, 179; air-bladder, 181, 182 ;
scales, 193 ; axial skeleton, 194 ; ribs,
199; caudal fin, 203; cranium, 207;
mandibular arch, 209 ; abdominal
pores, 214 ; circulation in air-bladder,
182, 220; left auricle, 220; septum
of conus, 220 ; arterial arches, 233, 23 5 ;
blood supply of air-bladder, 233, 235 ;
Mullerian duct (oviduct) in male, 254.
 
Directive bodies (or cells), 10.
 
Discophora, mesoblast of, 54.
 
Dispira, 19.
 
Dog (Canis familiaris), pro-amnion, 86 ;
placenta, 91 ; alimentary canal, 1 7 1 .
 
 
INDEX.
 
 
319
 
 
Dog-fish, spinal nerves of, 120, 121 ; olfactory organ, 143, 144; muscles, 1 9 1 ;
neural arches, 196; skull, 208.
Dog-whelk. See “ Purpura.-
 
Dorsal aorta, 224-228, 233, 235, 236.
 
blood-vessel, 215,
 
fin, 203.
 
mesentery, 2 1 5.
 
Down feathers, development of, 102.
Dragon-flies (Libellulidae), anal respiration in larvae, 109, 275.
 
Dreissena, gills of, 108.
 
Duck (Anas), skin of embryo, 100 ; feathers of, 102 ; neurenteric canal, 117 ;
mesonephros, 243.
 
Ductus arteriosus Botalli, 220, 226, 227,
236, 237.
 
Botalli, 220, 226-228.
 
Cuvieri, 213, 219, 224, 225, 228
230, 231, 234.
 
Kollikeri, 146.
 
thyreoglossus, 184, 185.
 
venosus, 229-234, 236, 237.
 
arantii, 23 1.
 
Duodenum, 175.
 
Dytiscns, larval eye, 153.
 
Ear, 148-151 ; epitrichial layer in, 101.
 
See “Auditory Organ.-
 
Earthworm (Lumbricus), spermatogenesis
of, 11, 12 ; gastrulation of, 26; mesoblast of, 56; nervous system, 113 ;
typhlosole, 175 ; muscles, 190 ; absence of parapodia, 202.
 
Echidna, mammary gland of, 107 ; anterior abdominal vein, 232.
Echinodermata, segmentation of, 22 ;
mesamoeboids of, 53, 69, 189, 274 ;
mesothelium of, 55, 56, 60 ; ambulacral system, 55, 177 ; formation of
bodv-cavity, 56, 72 ; radial symmetry,
74; nervous system, 1 1 3 ; mesamoeboids
devour degenerate tissues, 189, 274 ;
spicular skeleton, 22, 189 ; dermal,
mesoblast, and calcareous plates, 190 ;
muscles, 190; dermal skeleton, 192 ;
larval skeleton, 194 ; larval forms, 265 ;
phagocytes, 274.
 
Echinus, 53. See “ Sea-Urchin.-
Ectoderm, 23.
 
Ectoplastic tissues, 269.
 
Ectosarc, 98.
 
Edentata, placenta, 91, 92.
 
Edwardsia, endodermal respiration, 177.
Efferent ducts of generative organs, invertebrates, 237 ; chordata, 251-261.
 
 
Egg-shell, 9.
 
Eighth cranial nerve, 136, 138-141.
 
Elasmobranchii, ovum, 5, 8 ; spermatogenesis, II, 12 ; segmentation of egg,
3D 33-35 5 primitive streak, 41, 42 ;
nourishment of embryo in uterus, 95,
96, 273 ; placoid scales, 103, 104, 192 ;
teeth, 105; external gills, 109, 1 79 ;
cerebellum, 126; supra-commissura,
128; pineal gland, 129, 143 ; posterior
commissure, 129; olfactory lobes, 133,
143 ; olfactory pit, 143 ; nasal groove,
142-144 ; lateral line, 148, 121 ; auditory organ, 148, 150; tympanum, 150;
eyelids, 162 ; pineal gland, 163 ; liver,
173; spiral valve, 175, 234; rectal
gland, 176; visceral clefts, 178-179 ;
yolk absorbed by external gills, 179 ;
oesophageal sac, 181 ; thyroid body, 184 ;
supra-pericardial bodies, 184; thymus
gland, 184 ; muscles, 19 1 ; shagreen,
192 ; axial skeleton, 195, 196 ; ribs,
199 ; pectoral girdle, 201 ; caudal fin,
203, 204 ; paired limbs, 204 ; cranium,
207, 208 ; visceral arches, 208-210 ;
connection of pericardium with coelom,
213; abdominal pores, 214; heart,
218; hyoid aortic arches, 233, 235;
vein of spiral valve, 234 ; thyroid and
spiracular artery, 235 ; pronephros,
240, 241, 248 ; segmental duct, 241,
249 ; mesonephros, 242, 243, 248, 249 ;
secondary tubules, 245, 246 ; incipient
metanephros, 247, 256 ; epiblastic
origin of segmental duct, 249 ; segmental duct, 251, 252; Mullerian
duct, 251, 252; oviduct, 251 ; its anterior orifice, 252 ; Mullerian duct in
male, 254 ; connection between testis
and mesonephros, 254-256 ; ureters,
256 ; larval respiration, 275.
 
Elastica limitans externa, 194, 195.
 
interna, 188, 194, 195.
 
Elastic fibres, 190.
 
Electric ray (Torpedo), spermatozoon
of, 4.
 
Elephas, placenta, 91.
 
Eleventh cranial nerve, 135.
 
Elysia, polar cells of, 10, 1 1 ; fertilisation
of ovum, 13 ; segmentation of, 1 6, 1 7 ;
nuclear division, 20.
 
Embole, 24, 33.
 
Embyronic membranes. See “ Foetal
membranes.-
 
Enamel, 104.
 
organ, 104, 1 10.
 
 
320
 
 
INDEX.
 
 
Enchylema, 8, 19.
 
Endoderm, 23.
 
Endolymph, 149.
 
Endoskeletal structures, 269 ; invertebrates, 193, chordata, 194.
 
Endostyle, 172
 
Endothelium, 69 ; of heart, 216-218,271.
 
Entoplastic tissues, 269.
 
Epencephalon, 1 34.
 
Epiblast, 23 ; organs derived from, 98 ;
protective structures, invertebrates,
98 ; chordata, 99 ; teeth, 103 ; glands,
105 ; excretory organs of mollusca,
108, 238 ; muscular elements, 108 ;
respiratory organs, 108 ; stomodseum,
HO; pituitary body, no; Malpighian
tubules, hi, 169, 238; proctodseum,
in ; cloaca of amniota, 112; nervous
system, 112; coelenterata, 112; vermes,
1 13; mollusca, 114 ; arthropoda, 115 ;
nature of invertebrate brain, 115, 116;
chordata, 1 16 ; neurenteric canal, 116,
1 17 ; spinal nerves, 119 ; sympathetic
system, 120 ; spinal cord, 121, 122 ;
vertebrate brain, 122- 134 ; cranial
nerves, 1 34-1 39 ; serial cranial sense
organs and thymus gland, 1 39 ; sense
organs, 141 ; olfactory organs, 142145 ; auditory organs, invertebrates,
145-147 ; chordata, 147-152; lateral
line, 148 ; eyes of invertebrates, 1 5 2157; of vertebrates, 157-162; evolution of nervous system and sense
organs, 164-167 ; segmental duct, 239,
249; external generative organs, 112,
261, 262; digestion, 273.
 
Epibole, 24, 33.
 
Epidermis, 98 ; human, 101.
 
Epididymis, 256, 257, 260.
 
Epigastric vein, 230, 234.
 
Epiglottis, 174, 182, 185.
 
Epicoracoid, 20 1.
 
Epiostracum, 99.
 
Epiphyses of vertebrae, 199.
 
Epiphysis cerebri, 128.
 
Epiphysial eye, 129, 162, 163.
 
Episterna, 201, 202.
 
Epitrichial layer of epiblast, 100, 241.
 
Epoophoron, 256, 257.
 
Equatorial plate, 19.
 
Ethmoid region of cranium, 207.
 
Ethmo-palatine ligament, 209.
 
Euaxes. See “ Rhynchelmis.-
 
Eustachian tube or recess, 139, 151, 179,
180.
 
valve, 221, 222, 236.
 
 
Eutheria, segmentation of, 44 ; allantois
of, 90 ; placenta, 92 ; evolution of
placenta, 96 ; Mullerian ducts, 253 ;
foetal respiration, 276.
 
Eutima, planula of, 50.
 
Exoskeleton of invertebrates, 98-100,
 
192-194.
 
of vertebrates, 103, 192, 193.
 
Excretory organs, epiblastic, of larval
mollusca, 108, 238 ; of adult mollusca,
108, 238 ; Malpighian tubules of insecta, in, [169, 238, 271; segmental
duct, 239, 249 ; hypoblastic, amphipoda, 169, 186, 238, 271 ; mesoblastic,
70 ; invertebrates, 237 ; platyhelminths, 237, 238, 249 ; chsetopoda,
70, 238, 271 ; arthropoda, 70, 238,
27 1 ; mollusca, 70, 238, 239, 249 ;
chordata, 70, 239 ; ascidians, 239 ;
amphioxus, 239 ; vertebrata, 239-250,
271 ; pronephros, 239 ; mesonephros,
241 ; metanephros, 246 ; summary of
development of vertebrate excretory
organ, 248 ; epiblastic origin of segmental ducts, 239, 249 ; urogenital
ducts, 250-259 ; urinary bladder, 87,
 
1 12, 259.
 
External carotid artery, 224-227, 231.
 
gills, 108, 109, 179.
 
iliac artery, 224, 232.
 
Extra-branchial cartilages, 208.
 
Extra-cellular digestion, 272.
 
Eye, 152; evolution of vertebrate eye,
77, 163-164; eyes of mollusca, 152,
I54-IS7; arthropoda, 152-154, 157 ;
vertebrates, 157-167; function of pigment, 157; epiphysial (pineal) eye,
129, 162, 163; “vertebrate- eyes of
invertebrates, 152, 163 ; “ invertebrate -
eyes of vertebrates, 129, 163 ; muscles,
192.
 
Eyelid, nerve supply, 137 ; development,
161, 162; of cephalopoda, 155, 156.
 
Facial nerve, 136, 138-141.
 
Falciform ligament, 213.
 
Fallopian tubes, 252-257.
 
“Falsification of embryological record,-
223.
 
Falx cerebri, 130, 13 1.
 
Feathers, development of down, 102 ; of
pin or contour, 103.
 
Fenestra ovalis, 150.
 
rotunda, 150.
 
Fertilisation of ovum, 12, 277-280; starfish, elysia, 13 ; significance of, 14, 15.
 
 
INDEX.
 
 
321
 
 
Field-vole (Arvicola), inversion of blastoderm, 93, 94.
 
Fifth cranial nerve, 136, 138-141, 207.
 
ventricle, 132.
 
Fin-rays, 203.
 
Fiona, segmentation of, 23.
 
First cranial nerve, 137- 140.
 
Fishes, anterior commissure, 1 24 ; mucous
canals of head, 139 ; auditory labyrinth,
1 50 ; accessory auditory apparatus, 1 50 ;
operculum, 151 ; suspensorium, 152;
eyelids, 162 ; yolk-sac in larvae, 1 7 1 ;
blood supply to air-bladder, 182, 233,
235 ; goblet organs, 182 ; thyroid body,
184; thymus gland, 185; scales, 193 ;
sclerotic bones, 193 ; ichthyopterygium,
204 ; cranium, 207 ; visceral arches,
209, 210 ; position of kidneys, 212 ;
heart, 2 19 ; reduction of pharynx,
139, 178, 223; circulation, 232-235.
 
Fissurella, eye, 155.
 
Flagella, 98.
 
Flagellate infusoria, 2.
 
Flamingo (Phoenicopterus) feedingyoung
 
2 73
Flesh-maggot (Musca?), cell of, 8.
 
Flocculi, 126.
 
Fresh- water mussel (Anodonta), typhlosole, 175.
 
Frog, segmentation of egg, 21, 32, 33 ;
blastopore, 41, 42 ; mesoblast, 64, 69 ;
larval respiration, 87 ; urinary bladder,
87; stomodseum, no, ill; pituitary
body, no, in ; nervous system, 1161 1 8 ; brain, 1 23-1 25 ; pineal gland,
 
1 1 7, 129, 163 ; mucous canals of head
of larvae, 139; neuromerism of head,
14 1 ; posterior nares, 144 ; auditory
ossicles, 152 ; auditory epithelium,
166; origin of nerves, 166, 167; intestine of tadpole, 176; caecum, 176;
branchial chamber, 179 ; thyroid body,
184; thymus gland, 185; notochord,
186 ; vertebral column, 197, 198 ;
pectoral girdle, 201 ; diaphragm, 213 ;
heart, 220 ; aortic arches, 233 ; venous
system, 234, 235 ; anterior abdominal
veins, 230, 234 ; segmental duct, 239,
249 ; pronephros, 239-241 ; mesonephros, 242 ; epiblastic origin of segmental duct, 249.
 
Fronto-nasal process, 143, 144.
 
Foetal membranes, birds, 78 ; reptiles,
84 ; mammals, 84 ; summary of evolution of, 95 ; respiration, 276.
 
Food-yolk. See “Yolk.-
 
 
Foramen caecum, 184, 185.
 
lacerum anterius,  - medius,  - pos
terius, 207.
 
of Munro, 125, 128-134.
 
of Winslow, 212.
 
ovale of heart, 221, 222, 236, 237.
 
Fore-brain, 134.
 
vesicle, 122, 127, 1 34.
 
Fornix, 132.
 
Fossa Sylvii, 130, 131, 133.
 
Four-horned antelope (Tetracerus), placenta, 92.
 
Fourth cranial nerve, 1 37-140, 207.
 
ventricle, 123-126, 134.
 
Fowl (Gallus), egg, 9 ; segmentation of
oosperm, 36-38 ; blastoderm, 39 ;
primitive streak, 40, 43, 66 ; abnormal
blastoderm, 42 ; mesoblast, 66 ; germinal wall or ridge, 67, 272 ; foetal
membranes, 78 ; pro - amnion, 86 ;
ciliated neural canal, 1 1 6 ; neurenteric
canal, 1 1 7 ; nervous system, 118, 1 19;
brain, 123, 125, 134; branchial sense
organs, 139; nasal passage, 144; auditory organ, 147, 148; eye, 158, 160 ;
thyroid body, 184 ; notochord, 187 ;
muscles, 191 ; diaphragm, 213; heart,
2 1 7-2 1 9, 221 ; circulation of yolk-sac,
225, 226 ; pronephros, 241, 248, 252 ;
mesonephros, 243-246, 248 ; metanephros, 246-249 ; Mullerian duct, 251,
252 ; segmental duct, 252 ; oviduct,
252 ; digestion of yolk in germinal
wall, 272.
 
Fox (Canis vulpes), Gsertner -s duct, 257.
 
Funicular tissue, 278.
 
Furcula (of tongue), 183.
 
Furculum, 201.
 
Fusus, segmentation of, 24, 25 ; nervous
system, 114, 165; digestion of yolk,
272.
 
Gartner -s duct, 257, 260.
 
Gall-bladder, 173.
 
Gands of Brunner, 173.
 
Ganoidei, nervous system, 1 18 ; cranial
flexure, 1 23 ; cerebellum, 1 26 ; nasal
pit, 143 ; auditory sac, 167 ; pancreas,
174; spiral valve, 175; pyloric cseca,
 
1 75 J pseudo-branch, 178; operculum,
179; air-bladder, 180, 181, 195; dermal plates, 193, 210 ; axial skeleton,
194-197 ; ribs, 199 ; clavicles, 201,
202 ; caudal fin, 203 ; cranium, 207 ;
mandibular arch, 209 ; dermal bones of
skull, 210; abdominal pores, 214, 258 ;
 
X
 
 
322
 
 
INDEX.
 
 
aortic arches, 235 ; Mullerian duct
(oviduct) in male, 254 ; generative
ducts, 258.
 
Gasserian ganglion, 136, 138-140, 207.
Gasteropoda, otolith, 146 ; eyes, 154, 155 ;
 
cartilage of odontophore, 194.
 
Gastrula, 23, 268.
 
Gastrulation, 16, 21 ; ecliinodermata, 22 ;
nudibranckiata, 23 ; cucullanus, 23 ;
prosobrancks, 24, 25 ; cephalopoda,
 
26 ; vermes, 26 ; peripatus, 26, 27 ;
Crustacea, 29; ampkioxus, 30; lamprey, newt, frog, 32, 33 ; ideal type,
33, 34 ; elasmobranck, 35 ; fowl, 37,
38 ; canary, 3 7, 38 ; nightingale, 43 ;
rabbit, 46, 47 ; obelia, 48, 49 ; eutima,
geryonia, 50 ; kydrozoa, 50 ; sponges,
 
5 °, 5 1
Gecko, vertebral column, 198.
 
Generative organ or gland, 4, 59, 70,
262-264.
 
Geniculate ganglion, 138.
 
Genital eminence, 174, 261, 262.
Gephyrea, mesoblast of, 59 ; nepkridia,
237, 264.
 
Germ cells, 5, 190, 277.
 
Germinal epitkelium, 5, 212, 263.
 
spot, 7, 9.
 
-  - vesicle, 7, 8.
 
wall or ridge, 40, 67, 272.
 
Germ-plasma, 277, 278.
 
Geryonia, delamination of, 50.
 
Gills. See “Respiratory Organs.-
 
slits. See “ Visceral Clefts.-
 
Giraffe (Camelopardalis), placenta, 92.
Gizzard, artkropoda, 1 10.
 
Glands, epiblastic, unicellular, 105 ; multicellular, simple and complex, 106,
107 ; mammary, 106, 107 ; provisional
renal organ and pigment spots of
mollusca, 108; of stomodseum, no;
of proctodseum, 101, 112 ; pineal, 129,
162, 163; tkyrnus, 139, 184, 185;
lackrymal, 162 ; kypoblastic, 169-171 ;
endostyle, 172 ; gastric, liver, 173 ;
pancreas, 174; intestinal, 175, 176.
 
of Bartkolin, 260.
 
of Lieb^rkukn, 273.
 
Glomerulus of pronepkros, 239, 240,
248 ; of mesonephros external, 244,
245 ; internal, 244-246.
Glosso-pharyngeal nerve, 136, 138-141.
Glottis, 1 81.
 
Goblet cells, 182.
 
Gonopkores, 263.
 
Gonads 26
 
 
Goose (Anser), neurenteric canal, 117.
Gorgoniidse, csenenckyma of, 98, 194.
Graafian follicle, 6.
 
Green gland, 238.
 
Growtk, 278, 279.
 
Gubernaculum, 260.
 
Guinea-pig (Cavia), inversion of germinal
layers, 93, 95 ; epiblastic origin of
segmental duct, 249.
 
Gurnards (Trigla), noise made by airbladder, 1 8 1.
 
Gustatory organs, 145-182 ; of amphioxus, 185.
 
Gymnodontes (Teleosts), air-sacs, 182.
Gyrinopkilus (Urodele), vertebral column,
 
197.
 
Haemal arch, 195, 196.
 
Haemoglobin, 214, 269.
 
Haemolymph, 269.
 
Hair, development of, 101, 102, 107.
Haliotis eye, 155.
 
Hatteria, pineal eye, 129, 163 ; abdominal
splints, 193 ; vertebral column, 198.
Head, 74, 75.
 
cavity, 138-140, 208.
 
fold, 39.
 
kidney, 239 ; of Teleosts, 240. See
 
“ Pronepkros.-
 
Heart, invertebrates, 215, 216 ; vertebrata, 21 7-223.
 
Hedonic glands, 112.
 
Helix, spermatozoon of, 4 ; provisional
renal organ of, 108 ; eye, 155, 156.
Hemiazygos vein, 228, 232, 233, 236.
Hemickordata, notockord of, 186.
 
Hepatic artery, 236.
 
cylinders, 173.
 
portal system, 229, 234.
 
veins, 229, 231, 232, 234, 236.
 
Ilepato-pancreas, 171.
 
Heptanckus, visceral clefts, 139, 178;
cranial segments, 140 ; absence of
pericardial bodies, 184; branchial
arches, 210.
 
Hernia, 262.
 
Heterocercal tail, 203.
 
Heteropoda, otocyst, 146.
 
Hexacoralla, csenenchyma, 98, 193.
Hexanchus, visceral clefts, 178.
Hexapoda, acoustic organ, 147.
Hind-brain, 134.
 
brain vesicle, 122, 1 25, 1 34.
 
Hippocampus major, 1 30-1 32, 134.
Hippolyte, auditory sac, 147.
Histogenetic plasma, 277.
 
 
INDEX.
 
 
323
 
 
Holocephali ( see “Chimera-), mandibular arch, 209 ; abdominal pores, 256.
 
Holothuroidea, respiratory trees, 177 ;
dermal skeleton, 192.
 
Homocercal tail, 204.
 
Homoplastic, 222, 249.
 
Hoofs, 102.
 
Horn-cells of epidermis, 101 ; horn fibres
of spinal cord, 120.
 
Horns of bovidae, 102 ; rhinoceros, 103.
 
Horse (Equus), spermatozoon of, 4 ;
nipple of, 106 ; uterus masculiuus,
254
Hunt -s depression, 15 1.
 
Hydatid of oviduct, 251, 256, 257; of
Morgagni, 254, 257, 260 ; of Wolffian
body (mesonephros), 256.
 
Hydra, ovum of, 7 ; structureless lamella
of, 53 > endodermal muscles, 176 ;
origin of sexual cells, 263, 278 ;
asexual reproduction, 278.
 
Hydrocorallinse, csenenchyma, 98.
 
Hydroids, spermatozoon, 4 ; planula, 49 ;
adult condition, 71 ; nervous system,
1 13; origin of sexual cells, 262, 263.
See “ Hydra - and “ Hvdrozoa.-
 
Hydromedusse, gastrulation, 48-50 ; gelatinous tissue, 71 ; digestion, 168.
 
Hydrozoa, primitive germ-cells, 6 ; morula, 50 ; radial symmetry, 71 ; cuticle
or perisarc, 98. See “ Hydroids - and
“ Hydromedusse.-
 
Hylodes, larval respiration, 87.
 
Hymen, 260.
 
Hyobranchial visceral cleft, 178.
 
Hyoid arch, 178, 206, 209, 210 ; artery
of, 226, 227, 233, 234, 235.
 
of amphibia and amuiota, 210.
 
Hyostylic skull, 209.
 
Hyomandibular, 152, 177.
 
cleft, 178 ; cartilage, 208-210.
 
Hypoblast, 23-48, 168 ; organs derived
from the hypoblast, digestive organs
of invertebrates, 168-170; “liver- or
hepato-pancreas, 170, 1 7 1 ; mesenteron
of chordata, 171-176 ; pharynx, 1 72 ;
stomach, 173 ; liver, 173 ; pancreas,
174; intestine, 174-176; endodermal
muscles, 176 ; respiratory organs of
invertebrates, 1 77 ; of chordata, 177180 ; intestinal respiration, 180 ; airbladder, 180, 181 ; lungs, 181, 182 ;
tongue, 182, 183 ; thyroid body, 183,
184 ; thymus gland, 184, 185 ; gustatory organ of amphioxus, 186 ; excretory organs, 186 ; notochord, herni
 
chordata, urochordata, cephalochordata,
186 ; vertebrata, 186- 188 ; sub-notochordal rod, 188 ; urinary bladder,
18S ; digestion, 272, 273.
 
Hypodermis, 153, 154.
 
Hypogastric vein, 229 ; artery, 224, 232.
 
Hypoglossus nerve, 135, 140; multiple
nature of, 14 1.
 
Hypopharyngeal ridge, 172.
 
Hypophysis cerebri, no, III, 185. See
“ Pituitary body.-
 
Hyrax, placenta, 91 ; caeca, 170.
 
Hypural bones, 204.
 
Ianthina, segmentation of, 24, 25 ;
veliger of, 99.
 
Iehthyopsida, anterior commissure, 13 1 ;
innervation of lateral line of head, 139 ;
process falciformis, 161 ; pharynx,
172; thymus, 185; sub-notochordal
rod, 188 ; derma, 190 ; circulation,
232-235 ; pronephros, 240, 248 ; mesonephros, 248, 254 ; Mullerian ducts,
252 ; digestive glands, 272.
 
Ichthyopterygium, 204.
 
Iliac veins, 232, 234.
 
Ilium, 202.
 
Incus, 1 5 1, 152, 209.
 
Inferior (posterior), cardinal veins, 224,
228-230, 232, 234, 236.
 
vertebral vein, 229.
 
Infraclavicles, 201.
 
Infundibulum, 77, no, in, 124, 125,.
127, 128, 132, 134.
 
of lungs, 181.
 
Infusoria, artificial division, 278.
 
Inguinal ring or canal, 214, 262.
 
Ink-sac, 1 70.
 
Immigration (gastrulation), 48-50.
 
Inner-layer cells, 44.
 
Innominate vein, 220, 228, 232, 233, 235.
 
Insects, primitive germ-cells of, 6 ; segmentation of, 29 ; mesoblast of, 57 ;
blastopore of, 58 ; coelom of, 58, 59 ;
amnion, 96 ; salivary glands of, 106,
no; stomodseum and derivatives, no;
proctodseum and Malpighian tubules,
in, 169, 238; auditory organ, 147;
eyes, 153, 154; mesenteron, 169.
 
Insectivora, mesoblast, 68 ; placenta, 91 ;
decidua reflexa, 93.
 
Interamnionic space, 95.
 
Intercalary (interhsemai and interneural)
arches, 196.
 
Interclavicles, 201.
 
Intercostal veins, 228, 232.
 
 
324
 
 
INDEX.
 
 
Intermediate cell-mass, 191, 212, 243
245, 249, 250.
 
Internal carotid artery, 224, 227, 231.
 
Intervertebral discs and ligaments, 198,
199
Intestine, invertebrates, 169, 170 ; chordata, 171-176.
 
Intracellular digestion, 272.
 
Inversion of germinal layers in rodents,
93
Iris, nerve supply, 137, 161, 162 ; of
cephalopoda, 155, 156.
 
Ischium, 202.
 
Isidinae, caenencliyma, 194.
 
Iter (a tertio ad quartum ventriculum) }
123-126, 134.
 
Jugular vein, 228, 229, 232-234.
 
Karyokinesis, 18-20.
 
Katabolism of protoplasm, 278-280.
 
Keel of sternum, 200, 201.
 
Kidney (proper) of vertebrates, 239,
 
246. See “ Metanephros.-
 
Labia majora, 260, 262.
 
minora, 262.
 
Labyrinthodonta, parietal foramen, 129;
probable pineal eye, 163.
 
Laoerta. See “ Lizard.-
 
Lachrymal duct, 145, 162.
 
glands, 162.
 
Lacteals, 274.
 
Lamellibranchiata, shell-gland, 99 ; gills
of, 108 ; nervous system, 1 1 5.
 
Lamina terminalis, 124, 125, 128, 129,
131, 132, 134.
 
Lamprey (petromyzon), segmentation of
egg, 32 ; mesoblast, 63, 64, 69 ; fate of
blastopore, 77 ; teeth, 105 ; stomodseum, no, hi ; pituitary body, no,
III ; development of nervous system,
1 18 ; neuromerism of head, 141 ;
pharynx, 172; gill - pouches, 178;
thyroid body, 183 ; rudiments of pelvic
fins, 204 ; visceral arches, 208 ; heart,
217; pronephros, 241. See “Cyclostomi.-
 
Larvae (free), 78.
 
Larynx, 182, 1 74.
 
Lateral fin, 77, 1 92.
 
line, 139, 148.
 
ventricle, 124, 1 25, 128-134.
 
Leech (Hirudo), gastric diverticula, 168 ;
skeleto-trophic tissue, 190 ; origin of
blood-vessels, 214.
 
 
Leiodera, pineal eye, 163.
 
Lemurs, cerebral convolutions, 1 33.
 
Lemuroidea, placenta, 92.
 
Lens capsule, 162.
 
Lens of eye, invertebrates, 152-157 ;
vertebrates, 158-164.
 
Lepidosteus, development of nervous
system, 118; axial skeleton, 195-198;
cranium, 207 ; pronephros, 240 ; connection between testis and mesonephros, 254, 258 ; oviduct, 258.
 
Leptocephalus, 266.
 
Leptoplana, segmentation of, 26 ; mesamoeboids of, 54.
 
Lepty chaster, direct development, 265.
 
Lepus. See “Rabbit.-
 
Leucocytes, 190, 274 ; digestion by, 274.
 
Levator palpebrae superioris, nerve of,
 
137
Leydig -s duct, 255.
 
Ligamentum longitudinale superius, 195.
 
Limax, eye, 156 ; origin of ganglia, 165.
 
Limbs, musculature, 192 ; unpaired,
203 ; paired, 204.
 
Limpet (Patella), eye, 155.
 
Limulus, spermatozoa of, 3 ; respiration
of, 109 ; lateral and central eyes, 153.
 
Lineus, segmentation of egg, 26 ; mesamoeboids of, 53, 54 ; mesenchymatous origin of nervous system, 165 ;
origin of nephridia, 238 ; vascular
system, 271.
 
Lingual artery, 233, 235.
 
Liquor amnii, 81.
 
Liver, so-called, of invertebrates, 1 70,
171 ; of chordata, 173, 174; blood
supply, 229-237.
 
Lizard (Lacerta, except when otherwise
mentioned), primitive streak, 41-43 ;
blastopore, 44 ; mesoblast, 65, 69 ;
proctodseum, 77 ; embryonic membranes of trachydosaurus, 84 ; connection of yolk-sac with oviduct in
latter and cyclodus, 96} 273 ; proamnion, 86 ; scales of various lizards,
103, r 93 5 neurenteric canal, 1 1 7 ;
pineal gland and pineal eye in various
lizards, 129, 162, 163 ; auditory
 
ossicles (various), 1 5 1 ; closure of eyelids (various), 162 ; thyroid body,
184; thymus gland, 185; notochord,
186, 188 ; diaphragm, 213 ; aortic
arches, 227 ; mesonephros functional
with metanephros, 247 ; Wolffian
tubules, segmental, 248 ; epiblastic origin of segmental duct, 250 ; Mullerian
 
 
INDEX.
 
 
325
 
 
duct (oviduct) in male, 254 ; persistent
Wolffian duct, 257; bladder, 259;
digestion of yolk in germinal wall,
272.
 
Lobi inferiores, 127.
 
Locomotory appendages, 202 ; invertebrates, 202, 203 ; chordata, 203, 204.
 
Loligo, shell-sac, 100 ; eye, 154.
 
Lophius, pronephros, 240.
 
Loricata, placenta, 91.
 
Lower-layer cells, 33, 34, 36.
 
Lumbricus. See “ Earthworm.-
 
Lungs, 78 ; invertebrates, 109 ; vertebrates, 1 81 ; homology, 182; blood
supply, 222, 226-228, 231, 233, 2352 37
Lymnseus, veliger of, 99 ; stomodaeum,
1 10 ; radula sac, 1 10.
 
Lymph, 267, 269, 270 ; corpuscles, 274.
 
Lympatbic gland, 184, 240, 254, 274.
 
Macropus, communication between vaginal caecum and urogenital sinus, 253.
 
Macula acustica, 146.
 
Malleus, 15 1, 152, 209.
 
Mallotus, peritoneal ducts of ovary, 258.
 
Malpighian body, 237, 242, 244, 245, 246,
248, 249, 256 ; pronephros of teleostei,
240.
 
layer of epidermis, 100-101, 106.
 
tubules, hi, 169, 238.
 
Mammalia, ovary of, 6 ; spermatozoa,
II, 14; segmentation blastodermic
vesicle, 44 ; mesoblast, 68 ; umbilical
cord, 84 ; pro-amnion, 86 ; epidermis,
101 ; hair, 101, 103 ; scales, 103 ;
pituitary body, no ; sympathetic nervous system, 1 20 ; anterior commissure, 124; brain, 1 25-1 34; cerebellum,
126; corpora quadrigemina, 126, 127;
posterior commissure, 129; cerebi*al
hemispheres, 130 ; septum lucidum,
132; corpus callosum, 132; convolutions, 133 ; olfactory lobes, 133.
 
Mammals, superficial petrosal nerve, 136 ;
neural segmentation (neuromerism)
of head, 141 ; cochlea, 149 ; fenestra
rotunda, 150; labyrinth, 150; scalse,
150; tympanum, 1 5 1 ; auditory ossicles, 150, 152 ; retinal blood-vessels,
161; nictitating membrane, 162; eyelids closed in some embryos, 162 ;
pineal gland, 163; stomach, 173;
digestive glands, 1 73, 174; intestine,
174-176 ; liver, 173 ; caecum, 176 ;
visceral arches, 180 ; lungs, 181 ;
 
 
thyroid body, 184, 185 ; thymus gland,
184, 185 ; notochord, 186, 188 ; dermal skeleton, 193 ; vertebral column,
196, 198, 199; sternum, 200; pectoral girdle, 20 1 ; mandibular arch, 209 ;
hyoid, 210 ; rotation of stomach, 212 ;
omentum, 212 ; inguinal canals, 214 ;
heart, 218-223 ; division of truncus,
222 ; ventricular septum, 222 ; aortic
arches, 226, 235 ; venous system, 228233 ; circulation in foetus, 235-237 ;
uterus, 253 ; vestigal structures of
Wolffian body in adult, 256-257, 260 ;
ureter, 258.
 
Mammary glands, 105, 106 ; of monotremes, 106, 107 ; evolution of, 107.
 
Man (Homo), spermatozoon, 4 ; foetal
membranes, 88 ; placenta, 92, 93 ;
epidermis, 101 ; mammary gland, 106,
 
107 ; muscle-fibres of sweat glands,
 
108 ; brain, 125 ; corpus albicans, 127 ;
cerebral hemispheres, 130 ; fifth ventricle, 132 ; convolutions of cerebrum,
133 ; olfactory lobes, 1 33 ; persistence
of vitelline duct, 1 7 1 ; pancreas, 174 ;
valvulae conniventes, 176 ; caecum,
176 ; visceral clefts, 180, 183 ; laryngeal diverticula, 182 ; tongue, 183 ;
thyroid body, 184, 185 ; thymus gland,
184, 185 ; rotation of stomach, 21 2 ;
omentum, 212 ; diaphragm, 213 ; heart,
220-222 ; foetal circulation, 231 ; uterus, 92, 93, 253, 254; double uterus
and vagina in anomaly, 253, 254 ;
hydatid of Morgagni, 254, 256 ; uterus
masculinus, 254; generative glands and
ducts, 256, 257, 260.
 
Mandibular arch, 209 ; artery of, 1 5 1 ,
226, 227, 233, 234, 235.
 
artery, 151, 235.
 
Manis, scales of, 103.
 
Mantle of mollusca, 99.
 
Manubrium sterni, 200.
 
Marsipobranchs = Cyclostomi. See “Lamprey- and “Myxine.-
 
Marsupials. See “ Metatheria.-
 
Maturation of ovum, 9 ; significance of,
14 , 15
Meckel -s cartilage, 151, 206, 208, 209.
 
Mediastinum, 213.
 
Medulla oblongata, 123-126, 1 34, 135.
 
Medullary. See “ Neural.-
 
Medusae, mesoblast of, 53, 71, 189 ;
auditory organs, 145.
 
Medusoids, 263.
 
Membrana adamantina, 105.
 
 
INDEX.
 
 
326
 
Membrane, bones of skull, 210.
 
of Descemet, 162.
 
Membranous cranium, 205.
 
portion of urethra, 262.
 
Menisci, 198.
 
Meroblastic oosperms, 48.
 
Mesamoeboids, 52-54, 57, 69, 189, 190,
269, 270, 274.
 
Mesencephalon, 130, 1 34.
 
Mesenchyme, 52-55, 69, 70.
 
Mesenteric vein, 229, 231, 232.
 
Mesenteries of actinozoa, 72.
 
Mesenteron, 25, 31 ; invertebrates, 1691 7 1 ; chordata, 171-176.
 
Mesentery, 21 1, 212, 270.
 
Mesoarium, 212, 256.
 
Mesoblast, 35, 50, 52 ; mesenchymatous
mesoblast, 52; of sponges, 51, 52;
ccelenterates, 53 ; echinoderms, 53 ;
platyhelminths, 53 ; chaetopoda, 54 ;
mollusca, 55 ; vertebrates (?), 67-69 ;
mesothelial mesoblast echinodermata,
55; sagitta, 55, 59 ; brachiopoda, 55,
89 ; peripatus, 55, 59, 76 ; balanoglossus, 55, 59; chaetopoda, 56, 57, 59,
69 ; Crustacea, 57 ; tracheata, 57 ;
amphioxus, 55, 59, 60; triton, 61 ;
lamprey, 63 ; frog, 64 ; trionyx, 65 ;
lizard, 65 ; fowl, 66 ; mole, 67 ; organs
derived from mesoblast, indifferent
mesoblast, 189, 190 ; dermal mesoblast, 191 ; muscular system, 190-192;
dermal skeletal structures invertebrates, 192 ; chordata, 192, 193 ; endoskeletal structures, invertebrates, 193,
194; chordata, 194 ; vertebral column,
1 94-1 99 ; skeletogenous sheath of notochord, 194 ; vertebral arches and
bodies, 195 - 199 ; fishes, 195, 197;
amphibia, 197, 198; sauropsida, mammalia, 198 ; ribs, 199 ; sternum, 200 ;
pectoral girdle, 200-202 ; pelvic girdle,
202 ; locomotory appendages, 202, 203 ;
median fin, 203 ; caudal fin, 203, 204 ;
paired limbs, 204; skull, 205-21 1;
cranium, 205-207 ; visceral arches,
207-210 ; mandibular arch, 209; hyoid
arch, 209, 210 ; branchial arches, 210 ;
dermal bones, 210 ; body cavity, 21 1212 ; mesentery, 212 ; pericardium,
213; diaphragm, 213; pleurae, 213;
abdominal pores, 213, 214; vascular
system, 214-237 ; development of
blood-vessels, 214, 215; formation of
heart, 2 1 5-223 ; vascular system of
vertebrates, 223-237 ; early embryonic
 
 
circulation, 224, 225 ; vitelline circulation, 226 ; foetal circulation, 226-230 ;
allantoic, 230-232 ; circulation in ichthyopsida, 232-235 ; excretory organs,
237 ; invertebrates, 237-239 ; chordata, 239 ; pronephros, 239-241 ; mesonephros, 241-246 ; metanephros, 246247; urogenital ducts, 250-258 ; suprarenal bodies, 258, 259 ; generative
organs, 262-264 ; digestion, 273274.
 
Mesoblastic (mesodermal) bands, 73 ;
chaetopoda, 56, 57 ; arthropoda, 58 ;
peripatus, 59.
 
Mesobranchial area of tongue, 183.
 
Mesocardium, 213, 215, 217, 218.
 
Mesocolon, 212.
 
Mesoderm, 51, 52, 69, 72.
 
Mesogastrium, 212.
 
Mesogloea (of coelenterata), 193.
 
Mesonephric duct, 251, 254.
 
(Wolffian) blastema, 245-249.
 
Mesonephros, 239, 241-246, 248, 249,
254-257 ; elasmobranchii, 242, 243,
245, 248, 249 ; teleostei, 246 ; amphibia, 242, 246, 248; fowl, 243-246, 248,
249 ; rudimentary portion in male
elasmobranchii and amphibia, 255 ;
vestigal structures in adult amniota,
256-257, 260.
 
Mesorchium, 212.
 
Mesorectum, 212.
 
Mesothelium, 52, 55-70, 21 1.
 
Metakinesis, 19.
 
Metamerism, 73, 74, 141.
 
-  - of body, 73.
 
Metanephric blastema, 246.
 
duct, 258.
 
Metanephros, 239, 246-249 ; fowl, 246249 ; incipient in elasmolranchia and
amphibia, 247.
 
Metapterygoid, 209.
 
Metasternum, 200.
 
Metatheria, foetal membranes, 89, 90,
96 ; mammary glands, 107 ; cerebellum, 126 ; corpus callosum, 132 ; stomach, 173 ; Mullerian duct, 252 ;
oviduct, 252, 253 ; vaginal caecum,
253 ; urogenital sinus, 252, 260.
 
Metazoa, 2; reproduction of, 3; kariokinesis, and direct nuclear division of,
19.
 
Metencephalon, 134.
 
Mid-brain, 126, 134.
 
Middle-brain vesicle, 122, 126, 134.
 
Milk, 273.
 
 
INDEX.
 
 
327
 
 
Mitrocoma, auditory organ, 146.
 
Mole (Talpa), blastoderm of, 46, 47 ; mesoblast of, 67, 68 ; inversion of blastoderm, 93; neurenteric canal, 1 1 7 ;
optic vesicles, 164 ; pectoral girdle,
201.
 
Mollusca, segmentation of egg, 21 ; mesoblast, 55, 59, 69, 190 ; pericardium, 59,
70, 249 ; archiccelous cavities, 69, 70 ;
shell-gland, 99 ; provisional renal organs, 108, 238 ; pigmeut spots, 108 ;
respiratory organs, 108, 109 ; radula
sac, 99, no, 170; nervous system,
113-116; osphradiura, 142; otocysts,
146; eyes, 152, 154-157, 163, 164;
evolution of sense-cells, 166 ; alimentary canal, 170; salivary glands, 170;
“liver,- 170, 171 ; velum, 99, 203;
heart, 215 ; nephridia (organs of Bojanus), 70, 108, 237-239, 249 ; generative ducts, 70, 237, 264.
 
Monads, conjugation of, 14.
 
Monaster, 18.
 
Monkeys, cerebral convolutions, 133 ;
Gsertner -s duct, 257.
 
Monodelphia. See “ Eutheria.-
 
Monomeniscous eye, 153.
 
Monostichous eye, 153.
 
Monotremata. See “ Prototheria.-
 
Mons veneris, 262.
 
Mormyrus, generative ducts, 258 ; abdominal pores, 258.
 
Morula, 50.
 
Moschus, placenta, 92.
 
Motoroculi, 137, 140.
 
Mouse (Mus), inversion of blastoderm,
93, 94 ; mammary gland of, 106, 107.
 
Mouth, 75.
 
Mucous canals of head, 138, 1 39.
 
layer of skin, 100, 101.
 
Mullerian duct, 251 ; elasmobranchs,
251, 252; amphibia, 251, 252; fowl,
251, 252; sauropsida, prototheria,
 
252 ; metatheria, 252, 253 ; eutheria,
 
2 53 5 various forms of mammalian
uteri, 253 ; occurrence in male, 254,
255 ; recapitulation of evolution of
Mullerian duct, 254 ; hydatids of, 252.
 
Mursenidse, abdominal pore, 258.
 
Murex, shell - gland of, 99 ; primitive
kidney, 99 ; development of otocyst,
146 ; development of eye, 1 54; nervous
system, 165.
 
Mus, inversion of blastoderm, 93, 94.
 
Muscle-plate, 192.
 
Muscles, 269 ; epiblastic, 108 ; innerva
 
tion of eye-muscles, 136, 137 ; endodermal muscles, 176 ; non-striated,
190; muscular system, invertebrates,
190, 191 ; chordata, 191, 192; histogenesis, 192 ; of eye, 192.
 
Mustelus, uterus, 95.
 
Myelencephalon, 134-136.
 
Myelon. See “Spinal Cord.-
 
Myoepithelial cell, 156.
 
Myotome, 199.
 
Myriapoda, antennae, 1 1 5 ; eyes, 153;
mesenteron, 169.
 
Myrmecophaga, posterior nares, 144.
 
Myxine, nerves of, 120 ; diaphragm, 213 ;
absence of glomerulus to pronephros,
248.
 
Nails, 102.
 
Nasal fossae, 137.
 
Nassa, segmentation of, 52.
 
Native bear (Phascolarctos), foetal membranes of, 89.
 
Nauplius, appendages, 1 1 5 ; proctodaeal
respiration, 275.
 
Nautilus, otolith, 146; eye, 155, 156.
 
Nematocysts, 99.
 
Nematoda, spermatozoa of, 3, 4 ; mesoblastof, 59 ; otocysts, 146 ; nephridia,
23 7
Nemertea, relation to chordata, 74 ; ectodermal muscle-cells, 108 ; ciliated pits
of, 109; nervous system, 1 1 3 ; otocysts,
146 ; gastric diverticula, 168 ; nephridia, 238, 249 ; vesicular connective
tissue, 270.
 
Neomenia, nervous system, 1 15; rectal
respiration, 177.
 
Nephric groove and canal, 250.
Nephridia, 59, 70, 237-250.
 
Nephrostome, 237, 239, 249. 244.
 
Nerves, spinal, no.
 
Nervous system, 112-167, 276; evolution of, in chordata, 77, 167 ; development in invertebrates, 112-116; in
chordata, 1 16-141.
 
Neural arch, 195, 196.
 
canal, 1 16.
 
crest or ridge, 120, 134-138.
 
groove, 1 1 6.
 
plate, 73 ; (unsegmented), 1 1 6-1 18,
 
164, 187.
 
pore, 1 17.
 
Neur-amniotic cavity, 94.
 
Neurenteric canal, 1 1 6.
 
Neuro-epithelial cell, 156.
 
Neuromerism, 141.
 
 
328
 
 
INDEX.
 
 
Newt (Triton), segmentation of, 32 ;
mesoblast of, 61, 62, 69 ; axial hypoblast, primitive streak of, 77 ; epiblast
of, 100; pituitary body, III; postanal gut, 1 17; cranial nerves, 138,
167 ; neural crest, 138 ; lateral line
organs of head, 138 ; notochord, 186188 ; median fin, 203 ; urogenital organs, 255.
 
Nictitating membrane, 162.
 
nerve supply, 136.
 
Nightingale (Luscinia luscinia), blastopore, 43.
 
Ninth cranial nerve, 136, 138- 141.
Non-retinulate eye, 153.
 
Nostril, epitrichial layer in, 101 ; development, 144.
 
Notidamus, vagus, 135 ; gill clefts, 139;
 
visceral arches, 208-210.
 
Notochord (unsegmented, 73), 186 ; of
hemichordata, 186 ; of urochordata,
186 ; of cephalochordata, 186 ; of vertebrata, 186-188 ; significance of, 188;
sheaths of, 194, 195 ; fate of, 195-199.
Notochordal groove, 1 86.
 
tissue, 269.
 
Notodelphis, larval respiration of, 87.
Nototrema, larval respiration of, 87.
Nuclear division, 17 ; indirect (karyokinesis), 1 8, 19, 20 ; direct, 18, 19, 20.
spindle, 17.
 
Nucleine, nucleoplasm. See “ Chromatin.-
 
Nucleus pulposus, 198.
 
Nudibraucliiatae, segmentation of, 21,
23 ; fate of blastopore, 76.
 
Nymphse, 260, 262.
 
Obelia, 2 7 ; segmentation and planula
of, 48, 49, 268.
 
Obturator foramen, 202.
 
Occipital region, 20 7.
 
region of head, 141.
 
Octactiniae, spicules, 193.
 
Octopus, shell-sac of, 100.
 
Oculomotor nerve, 134, 139-141.
Odontoblasts, 104.
 
Odontophora, cephalic eyes, 152, 1 55 ;
renal organs, 238.
 
Odontophore, 1 10; cartilage of, 194.
(Esophagus, gastropods, 25 ; actinozoa,
71; insects, no; invertebrates, 169,
170; chordata, 171-172.
 
Olfactory bulbs, 138. 1
 
lobes, 124, 125, 127, 130, 132, 133,
 
3 M.
 
 
Olfactory nerves, 124, 1 25, 1 33, 135,
 
I 37» 140, 143.
 
organ, 139; invertebrates, 1 42;
 
chordata, 142.
 
Oligochaeta, gastrulation, 26 ; mesoblast*
 
54, 57
Olivary bodies, 126, 1 34.
 
Omentum, 212.
 
Ommateal layer, 154.
 
Ommatidium, 1 55 -1 57.
 
Ommerythrine, 157.
 
Ornosterna, 20 1.
 
Omphalo-mesenteric vitelline veins, 85,
123, 218, 219, 224-226, 229-234 ;
arteries, 223-226, 23 1,
 
Onchidium, dorsal eyes, 152, 163.
 
Ontogeny, 1.
 
Oogenesis, 280.
 
Oosperm, 3, 13, 14, 277.
 
Oosphere, 14.
 
Oospore, 14.
 
Operculum of fishes, 15 1, 1 79 ; of balanoglossus, 177.
 
Ophiacantha, direct development, 265.
 
Ophiuroidea, larval skeleton, 194 ; arms,
202 .
 
Ophthalmic division of the fifth nerve,
 
I35-I37
ganglion, 137.
 
Ophthalmicus superficialis, 135-136.
 
Opisthobranchs, pigment spot in veliger,
108.
 
Opisthoeoelous vertebrae, 196, 198.
 
Opossum (Didelphys), foetal membranes
of, 89 ; oviducts, 253.
 
Optic chiasma, 124, 125, 127, 131, 134,
160.
 
cup, 158, 164.
 
lobes, 123-127, 134.
 
nerve, 137, 157, 160, 161,207; of
 
pineal eye, 163 ; of invertebrates,
153 - 156 .
 
thalamus, 124, 125, 128, 130, 131,
 
134 .
 
vesicles, 123, 127, 157-161, 164.
 
Orbito-sphenoid cartilage, 207.
 
Organ of Bojanus, 238.
 
of Corti, 149.
 
of Giraldes, 257, 260.
 
of Jacobson, 145.
 
Organs of lateral line, 139, 148.
 
Ornithodelphia. See “ Prototheria.-
 
Ornithorhynchus, oviducts, 253.
 
Os lenticulare, 152.
 
Osmerus, oviduct, 258.
 
Osphradium, 142.
 
 
INDEX.
 
 
329
 
 
Os tincae, 253.
 
Os uteri, 253.
 
Otoconia, 150.
 
Otocyst, vermes, 146 ; mollusca, 146 ;
arthropoda, 146, 147 ; tunicata,
 
417.
 
Otolith, medusae, 145 ; vermes, 146 ;
mollusca, 146 ; arthropoda, 147 ; tunicata, 147 ; vertebrates, 149.
Outer-layer cells, 44.
 
Ovarian tubes of Pfliiger, 5.
 
Oviduct, 5, 254 ; uterine compartments
in elasmobranchs, 95 ; reptiles, 84, 96 ;
birds, 252 ; mammals, 96, 252-254.
Ovum, 3, 4, 5, 7, 277-280 ; of vertebrates,
5 ; elasmobranchii, 5, 8 ; protopterus,
5 ; hydra, 7 ; sea-urchin, 7 ; cat, 7 ;
fowl, 9 ; sauropsida, 8 ; structure, 8 ;
egg-membranes, 9 ; maturation, 9-1 1 ;
fertilisation, 12-15.
 
Pal^mon, heart, 216. See “Prawn.-
Palatine, 209.
 
division of facial, 136.
 
Palato - quadrate or palato - pterygoid,
206, 209.
 
trabecular ligament, 209.
 
Palinurus, auditory sac, 147.
 
Pallial membrane, 99.
 
Paludina, fate of blastopore, 76; otolith,
146.
 
Pancreas, 1 74.
 
Parachordal, 205, 206.
 
Paradidymis, 260.
 
Para-epididymis, 257, 260.
 
Paramsecium, reproduction, 3.
 
Parapodia, 202.
 
Parasphenoid, 210.
 
Parenchymula, 27, 49, 267.
 
Parietal layer of peritoneum, 21 1 ; of
pericardium, 2 1 3.
 
Paroophoron, 257, 260.
 
Parorchis, 255.
 
Parovarium, 256, 257, 260.
 
Parrot (Psittacus ?), neurenteric canal,
11 7 .
 
Pars olfactoria of anterior commissure,
 
124.
 
temporalis of anterior commissure,
 
124.
 
Parthenogenesis, 3, 15, 278.
 
Patella, eye, 155, 164.
 
Pathetic nerve, 137.
 
Peachia, endodermal respiration, 177.
Pecora, placenta, 92.
 
Pecten, pallial eyes, 152, 163.
 
 
Pecten of eye, 161.
 
Pectinidae, pallial eyes, 163.
 
Pectoral girdle, 200-202.
 
Pelagia, direct development, 265.
 
Pelvic girdle, 202.
 
Penis, 260-262.
 
Peptic glands, 173.
 
Perameles, mammary gland of, 107.
 
Perennibranchiate amphibia, absence of
pancreas, 174.
 
Peribranchial cavity of tunicata, 177.
 
Pericardium of mollusca, 59, 249 ; of
chordata, 195, 213, 259; connection
with nephridia, 237.
 
Perichondrium, 197, 201, 209.
 
Perilymph, 1 50.
 
Perineum, 257, 261.
 
Peripatus, segmentation and gastrulation,
26, 27, 267 ; mesothelium, 55 ; formation of somites, and origin of nephridia
and generative organs, 59, 70 ; fate of
blastopore, 75, 76 ; mesenteron, 169 ;
segmental organs (nephridia), 59, 70,
238.
 
Perisarc, 98.
 
Perissodactyla, placenta, 92.
 
Peritoneal funnel, 240, 242, 244.
 
Peritoneum, 21 1, 212.
 
Petromyzon. ^“Lamprey.-
 
Petrous ganglion, 138.
 
Phagocytes, 274.
 
Phalangista, mammary glands of, 107 ;
vaginae, 253.
 
Pharynx of chordata, 1 7 1, 172, 177-180.
 
Phascolarctos, foetal membranes of, 89.
 
Phocaena, hyoid, 21c.
 
Phoronis, intracellular digestion in larva,
272.
 
Phylogeny, 2.
 
Physoclisti, air-bladder, 181.
 
Pigeon (Columba), feeds young, 273.
 
Pigment-cells of derma, 190 ; of eyes,
I52-I57
Pigmented layer of choroid, 159-161.
 
Pig (Sus), placenta, 92 ; nipple of, 106 ;
spinal cord of, 120; ear, 1 5 1 ; laryngeal diverticula, 182 ; tongue, 182 ;
thyroid body, 184 ; thymus gland,
184 ; Gaertner -s duct, 257.
 
Pilosa, placenta, 91.
 
Pineal eye, 129, 162, 163.
 
gland, 124, 125, 128, 129, 132, 134,
 
162, 163.
 
Pipa, larval respiration of, 87.
 
Pituitary body, no, III, 124, 125, 128,
132, 134, 185.
 
 
330
 
 
INDEX.
 
 
Pituitary space, 205, 206.
 
Placenta, 84, 87, 91, 96 ; discoidal, 91,
dome-shaped, 91 ; zonary, 91 ; diffuse,
91, 92 ; cotyledonary, 92 ; metadiscoidal, 92 ; of giraffe, tetraceros, moschus, 92 ; non-deciduate, 92 ; deciduate, 92 ; placental circulation, 224,
230-233, 236, 237.
 
Placental sac of birds, 82 ; of warbler,
 
S 3
Placoid scales, 103, 192, 193.
 
Plakula, 23, 268.
 
Planaria, otocyst, 146.
 
Planorbis, organ of Bojanus, 238.
 
Planula, 49-51.
 
Plasma of blood, 214, 2 1 5.
 
Plasmodium, 274, 276.
 
Platyhelminths, generative organs of, 6 ;
segmentation of, 26 ; mesoblast of, 53?
59, 69 ; archiccelous cavities of, 69 ;
repetition of internal organs of, 73 ;
otocysts, 146 ; parenchyma, 189, 190 ;
muscles, 190 ; nephridia, 237.
 
Pleurae, 213.
 
Pleuro-peritoneal cavity, 78-83, 213.
 
Pleuronectidae, absence of air-bladder,
 
181.
 
Pleuroperitoneal cavity, 68, 78.
 
Plicae aryepiglotticae, 185.
 
Plutei, spicules, 1 94.
 
Pneumatic duct, 181.
 
Pneumogastric ganglion, 138.
 
Poison glands, 1 06.
 
Polar cells, 10-15, 277-280.
 
cells (Polar bodies or globules), formation of, in asterias, 10 ; elysia, 10,
11.
 
Polyclades, origin of sexual cells,
263.
 
Polymeniscous eye, 153.
 
Polyodon, axial skeleton, 194, 196.
 
Polypterus, external gills of larva, 109 ;
ventral orifice of pneumatic duct, 182 ;
blood supply of air-bladder, 233, 235 ;
oviduct, 258.
 
Polyzoa, mesoblast, 59 ; cuticle, 98 ;
nephridia, 237 ; asexual reproduction,
278.
 
Pond-snail. See “ Lymnaeus.-
 
Pons Yarolii, 1 25, 126, 132, 134.
 
Poreuten, 67.
 
Porifera, spermatozoon, 4 ; gastrulation,
48, 50, 51 ; planula, 50 ; amphiblastula, 51 ; mesenchyme, 52, 69,
189 ; nervous system, 51, 165, 189 ;
gastric diverticula, 168 ; archaeocytes,
 
 
189; spicules, 51, 189, 193; musclecells, 51, 189 ; germ-cells, 189, 262,
278 ; digestion, 274 ; asexual reproduction, 278.
 
Portal circulation, 234, 235.
 
vein, 229-232, 236
 
Portio facialis, 136.
 
profunda (or minor), 137.
 
Post-branchial nerve, 138.
 
clavicles, 201.
 
Posterior brain vesicle, 125, 134.
 
-  - (inferior) cardinal veins, 224, 228230, 232, 234, 236.
 
commissure, 124, 125, 1 29, 1 32,
 
134 *
 
nares, 144.
 
pelvic vein, 23 1.
 
vertebral vein, 228, 236.
 
Post-temporal bones, 201.
 
Prae branchial nerve, 138.
 
Praespiracular nerve, 136.
 
Prawn (Palaemon), auditory organ, 147 ;
heart, 216.
 
Precaval vein, 224, 228.
 
Precoracoid, 201, 202.
 
Prehyoid aortic arch, 235.
 
Prenasal rostrum, 206.
 
Preoral lobe, 74, 76, 77, 164, 205.
Preorbital process, 206.
 
Prepituitary region of head, 141.
Presphenoid cartilage, 207.
 
Prespiracular ligament, 208.
 
Primates, vermiform appendix, 1 76.
Primitive groove, 39, 40.
 
ova, 5 ; germ-cells, 6.
 
streak, 39 ; nature of, 41.
 
Pristiurus, 120, 191. See “Dog-fish.-
Proamnion of rabbit, 85 ; rodents, bat,
dog, sauropsida, 86.
 
Procephalic lobes, 153.
 
Processus falciformis, 161.
 
'gracilis, 1 51.
 
vaginalis, 262.
 
Proccelous vertebrae, 198.
 
Proctodaeum, 76, ill ; invertebrates,
hi, 169, 170; vertebrates, 112, 250,
261.
 
Proneomenia, intestinal respiration, 177.
Pronephric duct, 239, 254.
 
Pronephros, 239, 254 ; amphibia, 239,
240, 248 ; teleostei, 240 ; lepidosteus,
240 ; elasmobranchii, 240, 241, 248 ;
fowl, 241, 248 ; lamprey, 241 ; degeneration in fishes, 259.
 
Pronucleus, female, 11, 13 ; male, 13.
Prosencephalon, 134.
 
 
INDEX.
 
 
331
 
 
Prosobranch gastropods, ova of, 6 ; segmentation of, 24, 25 ; provisional kidney of, 108 ; pigment spots, 108 ;
nervous system, 1 14; digestion of
yolk, 272.
 
Prostate gland, 260.
 
Prostatic portion of urethra, 262.
 
Protista, absence of nucleus in, 20.
 
Protocercal tail, 203.
 
Protoplasm of ovum, 8 ; of tissue-cells,
8, 19.
 
Protoplasmic continuity of animal cells,
267.
 
Protopterus, ovum of, 5 ; external gills,
109 ; hyoid aortic arch, 233.
 
Protospongia, 3.
 
Prototheria, eggs of, 46-48, 89 ; mammary glands of, 106, 107 ; cerebellum,
126; corpora quadrigemina, 127; pons,
127; corpus callosum, 1 32 ; cochlea,
149 ; vertebral epiphyses, 199 ; pectoral girdle, 201 ; Mullerian ducts
(oviducts), 252, 253 ; urogenital sinus,
252, 253 ; ureter, 258; urinary bladder,
259
Pro to vertebra, 192.
 
Protozoa, 267 ; reproduction of, 2, 267,
279 ; nuclear division, 20 ; digestion,
272.
 
Pro ventriculus of insects, no, 169.
 
Pseudamnion of mustelus, 95.
 
Pseudobranch, 178, 233; artery of, 235.
 
Pseudochorion of mustelus, 95.
 
Pseudoccel, 69, 70.
 
Pseudo-vertebral region of head, 141.
 
Psolus, direct development in P. ephippifer, 265.
 
Pteropoda, otolith, 146.
 
Pterygo- quadrate, 208, 209.
 
Pubes, 202.
 
Pulmonary artery, 220-222, 226, 228,
233, 235-236.
 
plura, 213.
 
sacs (Arachnida), 109.
 
vein, 229, 234, 236.
 
Pulmonata, blastopore, 76 ; primitive
renal organ, 108, 238 ; respiration, 109.
 
Purple snail. See “Ianthina.-
 
Purpura, veligen, and origin of nervous
system, 114, 165; digestion of yolk, 272.
 
Pylangium, 220.
 
Pyloric caeca, 175.
 
glands, 173.
 
Pyramids, 126, 134.
 
Quadrate, 151, 152, 206, 209.
 
 
Rabbit (Lepus), segmentation, blastodermic vesicle, 44, 45, 46 ; foetal
membranes of, 85, 87, 90, 91 ; development of brain, 127, 132 ; eye, 161 ;
visceral arches, 180; thyroid body,
184 ; pectoral girdle, 201 ; diaphragm,
213 ; heart, 216 ; epiblastic origin of
segmental duct, 249, 250 ; vaginal
septum, 253 ; uterus masculinus, 254 ;
scrotal sacs, 262.
 
Radial symmetry, coelenterates, 71 ; ecliinodermata, 74.
 
Radix longa, 135, 137, 140.
 
Radula, 99, no.
 
Raja, pineal gland, 129.
 
Rana. See “ Frog.-
 
Ranodon, vertebral column, 197.
 
Raphd, 261, 262.
 
Rat (Mus decumanus), spermatogenesis,
II, 12 ; inversion of blastoderm, 93, 94.
 
Rathke -s pouch, 174. See “ Hypophysis.-
 
Ratit®, sternum, 200.
 
Rectum, arthropoda, 109, III.
 
Recessus vestibuli, 148.
 
Rectus externus, nerve supply of, 136.
 
Renal organs. See “ Excretory Organs.-
 
portal system, 229, 234.
 
portal vein, connection with Wolffian tubules in anura, 242.
 
Reproduction, 277-280 ; in protozoa, 2,
3 ; in metazoa, 3-5, 277-280 ; asexual,
278, 279.
 
Reptilia, foetal membranes of, 84 ; epidermis of, 101 ; corpus callosum, 124,
132; anterior commissure, 124; cerebellum, 126; auditory ossicles, 15 1,
152; caecum, 176; lungs, 1 81 ; sclerotic bones, 193 ; vertebral column,
198 ; pleuro-peritoneal cavity, 213 ;
ventricle, 222 ; aortic arches, 227 ;
venous system, 228-230 ; mesonephros,
242 ; digestive glands, 273,
 
Respiratory organs, epiblastic, invertebrates, 108 ; anal, in arthropoda, 109,
177 ; chordata, 109 ; accessory, in
teleostei, 109 ; hypoblastic, invertebrates, 177; chordata, pharyngeal, 177
180 ; intestinal, 180 ; air-bladder, 180,
 
181 ; lungs, 181-182.
 
Rete mucosum, 101.
 
Retia terminalia, 157.
 
Reticulum, 8, 19.
 
Retina, invertebrates, 153- 1 57; vertebrates, 158-164 ; histogenesis, 159.
160.
 
Retinophora, 154.
 
 
332
 
 
INDEX.
 
 
Retinidial cuticula, 156, 157.
 
Retinidium, 157.
 
Retinula, 153, 156.
 
Retractor of bvilb of eye, nerve supply
of, 136.
 
Rhabdoms, 1 53 - 1 57.
 
Rhinencephala, 133, 134.
 
Rhinoceros, horn of, 103.
 
Rhinoderma, larval respiration of, 87.
 
Rhynchelmis, gastrulation, 26, 57 ; mesoblast, 57.
 
Ribs, 199; “true- and “false,- 200;
“abdominal,- 200, 193.
 
Robin (Luscinia rubicula), development
of feathers of, 102, 103.
 
Rodentia, pro-amnion of, 86 ; placenta,
91 ; inversion of germinal layers, 93 ;
stomach, 173 ; vaginal septum, 253.
 
Rods and cones of retina, 159, 164.
 
Rosenmiiller -s organ, 256.
 
Rostrum, 200.
 
Rotifera, stomodseum and derivatives,
no; nephridia, 237, 238.
 
Round ligament, 232, 260.
 
Ruminants, nipple of, 106 ; occipital
region of, 141 ; stomach, 1 73; Gaertner -s duct; 257.
 
Saccobranchus, accessory respiratory
organ, 1 09.
 
Sacculus hemisphericus, 1 49, 150.
 
Saccus endolymphaticus, 148.
 
Sagitta, generative organs of, 5 > 263 ;
mesothelium of, 55, 59.
 
Salamander (Salamandra), spermatozoon
of, 4 ; vertebral column, 197 ; Mullerian duct, 251.
 
Salivary glands, 105, Iio; of insects,
106, no; mollusca, 170.
 
Salmon (Salmo), air-bladder, 180; diaphragm, 213 ; abdominal pore, 258.
 
Salmonidae, genital pore, 258.
 
Sauropsida, eggs, 8, 9 ; primitive streak,
42 ; pro-amnion, 86 ; pons Varolii,
126 ; cerebral hemispheres, 129 ; anterior commissure, 131 ; auditory labyrinth, 150 ; auditory ossicles, 1 5 1, 152;
pecten of eye, 16 1 ; eyelids, 162 ;
pineal gland and eye, 163 ; axial
skeleton, 196-198 ; Mullerian ducts,
252 ; ureter, 258 ; digestion by epiblast of blastoderm, 273.
 
Scala media, tympani, vestibuli, 150.
 
Scales, reptiles, 103, 1 93 ; placoid, 103,
192 ; manis, 103 ; of teleosts, 193 ;
chaetopoda, 202.
 
 
Scapula, 201.
 
Scent glands, 106.
 
Schneiderian membrane and folds, 143.
 
Schizopoda, auditory orgau, 147.
 
Scincus, scales of, 103.
 
Sclerotic bones, 193.
 
of cephalopoda, 155, 156 ; of vertebrates, 160-162.
 
Scorpion, direct nuclear division in, 18,
19 ; pulmonary sacs of, 109 ; central
and lateral eyes, 153.
 
Scrotum, 260, 262.
 
Scutee of birds, 103.
 
Scutes of crocodiles, 103, 193 ; armadillos, 103, 193 ; lacertilia, 193 ; caecilia,
 
193
Scy Ilium, 121, 143, 144, 196, 208. See
“Dog-fish.-
 
Scyphistoma, 265.
 
Scyphomedusae, 72 ; origin of middle
layer, 53 ; stellate cells in disc, 189 ;
direct development, 265.
 
Scyphopolypi, 72.
 
Sea-anemone. See “ Actinozoa.-
 
anatomy of, 72. See “Actiniae-
 
and “ Actinozoa.-
 
Seals (Phocidae), decidua reflexa, 93.
 
Sea-urchin, ovum of, 7 ; mesamoeboids
of, 53 ; external muscles, 190 ; larval
skeleton, 194.
 
Sebaceous glands, ioi, 107, 112.
 
Secondary mesoblast of crayfish, 57.
 
Second cranial nerve, 137.
 
Seessel -s pouch, 174.
 
Segmental duct, 239-252, 254 ; epiblastic
origin, 239, 249, 250 ; amphibia, 239,
 
240, 249 ; fishes, 240, 241, 249 ; fowl,
 
241.
 
organs, 238, 250.
 
tubules, 242-250, 254 ; secondary
 
tubules, 242, 245-248 ; testicular network of elasmobranchii and amphibia,
2 55> 256.
 
Segmentation, 16, 267, 268 ; elysia, 16,
17, 20, 21 ; frog, 21 ; mollusca, 21, 24,
25, 26 ; vermes, 26 ; peripatus, 26, 27 ;
Crustacea, 27, 28, 29 ; insects, 29 ;
amphioxus, 29 ; bird, 31 ; lamprey,
newt, frog, 32 ; sturgeon, 33 ; ideal
type* 33 34 ; elasmobranch, 33, 35 ;
fowl, 36; rabbit, 43, 44,45-46; hypothesis of mammalian segmentation, 47.
 
cavity, 21, 2 1 5, 269 ; nucleus, 13.
 
Segmentation of body, metamerism, 73,
74, 141 ; of head, 137-141 ; of vertebral
column, 199.
 
 
INDEX.
 
 
333
 
 
Segmented worms. See “ Chaetopoda.-
Selachians, abdominal pores, 258. See
“ Elasmobranchii.-
 
Semicircular canals, 149.
 
Semilunar valves, 222.
 
Seminal globules or granules, II, 14.
Sense organs, 98, 14 1, 142 ; serial cranial
sense organs, 1 37-140; tactile organs,
 
141 ; olfactory organs, invertebrates,
 
142 ; chordata, 142-145 ; gustatory
organs, 145 ; auditory organs, invertebrates, 145-147; chordata, 147-152;
organs of lateral line, 148 ; visual
organs, invertebrates, 1 52-1 57 ; vertebrates, 157-162 ; epiphysial (pineal)
eye, 162, 163 ; evolution of vertebrate
eye, 163-164 ; evolution of sense organs, 164-167 ; gustatory organ of
amphioxus, 185 ; taste buds, 182.
 
Seps, pineal eye, 163.
 
Septum aorticum, 22 1.
 
inferius, 221.
 
intermedium, 221.
 
lucidum, 125, 132.
 
pellucidum, 125, 132.
 
superius, 22 1.
 
transversum, 2 1 3.
 
Serous membrane of vertebrates, 81-96 ;
 
of insects, 97.
 
sacs, 213, 262.
 
Serpula, mesoblast of, 54, 56 ; gills of,
 
108 ; cartilage in gills, 194.
 
Setse, 146, 202.
 
Seventh cranial nerve, 136, 1 38- 14 1.
Shagreen, 192.
 
Sharks, thymus gland, 184 ; connection
between embryo and parent, 273.
Sheep (Ovis), development of brain, 128,
131 ; branchial sense organ, 139; auditory organ, 148 ; thyroid body, 184.
Shell, brachiopoda, 99 ; m oil u sea, 99.
 
gland, of mollusca, 25, 99, 100 ; of
 
Crustacea, 238.
 
Shrimps, auditory organ, 147.
 
Siluroidea, accessory respiratory organs,
 
109 ; accessory auditory apparatus,
150, 181 ; pancreas, 174.
 
Siren, external gills, 179.
 
Sirenia, placenta, 92 ; absence of vertebral epiphyses, 1 99.
 
Sinus prsecervicalis, 180, 183.
 
• reuniens, 221.
 
terminalis, 85, 86, 88, 91,119, 225,
 
226.
 
Sinus venosus, 219, 221, 224, 225, 234.
Sixth cranial nerve, 136, 139- 140.
 
 
Skeletal tissues, 269.
 
Skeletogenous cells, 269.
 
sheath of notochord, 194.
 
Skeleto-tropliic tissue, 190, 269.
 
Skin invertebrates, 96 ; chordata, 100.
 
Skull, 205-21 1.
 
Slime glands, 106.
 
Sloth (Pilosa), stomach, 173.
 
Smelt (Osmerus), oviduct, 258.
 
Snail, spermatozoon of, 4, 11.
 
Snakes (Ophidia), scales of, 103 ; pituitary body, III; closure of eyelids,
162 ; aortic arches, 227, 228 ; persistent Wolffian duct, 257.
 
Somatopleur, 68, 78, 21 1.
 
Somite, 59, 61, 192, 21 1.
 
Spermatoblast, 11, 14.
 
Spermatocyst, 11.
 
Spermatogenesis, 280 ; in rat, snail,
earthworm, elasmobranchs, 11, 12.
 
Spermatozoa, 1 1 - 15, 277-280; of nematoda, 3, 4 ; arachnida, 3 ; limulus, 3 ;
Crustacea, 3 ; sponge, 4 ; hydroid, 4 ;
crayfish, 4 ; snail, 4, 1 1 ; electric ray,
4 ; salamander, 4; horse, 4 ; man, 4 ;
rat, 11, 12 ; earthworm, 11, 12; elasmobranchs, II, 12.
 
Sperm blastophore, 11, 14.
 
morula, 11, 14.
 
Spermosphere, 11, 14.
 
Spermospore, 14.
 
Sphenoidal fissure, 20 7.
 
region of cranium, 207.
 
Spicules, alcyonaria, 98 ; sponges, 189,
193 ; echinoderms, 1 90, 192.
 
Spider (Arancida), mesoblast, 57 ; pulmonary sacs, 109 ; thoracic ganglia,
1 16 ; eyes, 153 ; heart, 216.
 
Spinal accessory nerve, 135.
 
cord, development of, 116-119;
 
histogenesis of, 12 1, 1 23-1 25.
 
Spinning glands, 106.
 
Spira, 19.
 
Spiracle, 136, 178, 233 ; artery of, 235.
 
Spiracular cartilage, 208, 210.
 
Spiral valve, 175 ; vein of, 234.
 
Splanchnopleur, 68, 78, 21 1.
 
Spondylus, pallial eyes, 152, 163.
 
Sponge. See “ Porifera.-
 
Spongy portion of urethra, 262.
 
Sporosacs, 263.
 
Squamatn, placenta, 91, 92.
 
Squamosal, 1 5 1.
 
Squid. See “ Loligo.-
 
Stapes, 15 1, 152.
 
Starfish, formation of polar cells, 10 ;
 
 
334
 
 
INDEX.
 
 
fertilisation of ovum, 13 ; archenteric
diverticula, 56; stomodaeum, no;
nervous system, 113 ; arms, 202.
Sterno-clavicular ligament, 200, 201.
Sternum, 200-202.
 
Stigmata of tunicata, 178.
 
Stolons, 278 ; of hydroids, 262.
 
Stomach, invertebrates, 169, 1 70; chordata, 1 7 1 - 1 74 ; man, 212.
 
Stomodaeum of actinozoa, 71 ; scyphomedusae, 72, 76, no; chordata, no;
structures derived from, no ; invertebrates, 169, 170.
 
Stratum corneum, 101.
 
Malpighii, 101.
 
Sturgeon (Acipenser), segmentation of
egg, 33 ; taste-buds in oesophagus,
182 ; sub-vertebral ligament, 188 ;
axial skeleton, 194, 196 ; clavicles,
201 ; renal ducts and oviducts, 258.
Sturio. See “ Sturgeon. -
 
Stylonychia conjugation, 3.
 
Subclavian artery, 224, 226, 227, 231,
236 ; vein, 224, 228-229, 232, 233.
Subintestinal vein, 223, 233, 234.
Sub-notochordal rod, 188, 191, 240, 242.
Sub- vertebral ligament, 1 88.
 
Subzonal membrane, 79, 81, 84, 87, 90.
Sudoriferous glands, 107.
 
Suina, placenta, 92.
 
Sulci of cerebral hemispheres, 133.
Superficial petrosal, 136.
 
Superior (anterior) cardinal veins, 224,
225, 228, 229, 231-233, 234.
 
intercostal veins, 232.
 
oblique eye-muscle, nerve of, 137.
 
(anterior) vertebral veins, 228, 229.
 
Supra-branchial nerve, 138.
 
clavicle, 201.
 
commissura, 128.
 
pericardial bodies, 184.
 
Suprarenal bodies, 240, 258-259.
Suspensorium, 152, 209.
 
Sweat glands, 106 ; muscle fibres of, 108.
Sympathetic nervous system, 1 20, 121,259.
Symplectic, 152.
 
Synangium, 220.
 
Syncytial segmentation, 26, 267.
Systemic aortic arch, 233.
 
Tactile organs, 14 1.
 
Tail, 74, 75, 77.
 
Tail-fold, 39.
 
Taste-buds, 182.
 
Teeth, development of, 103 ; milk-teeth,
104 ; horny teeth, 105.
 
Tela choroidea, 1 25, 129.
 
 
Teleostei ( see Preface), accessory respiratory organs, 1 09 ; pituitary body,
III; nervous system, 118 ; cranial
flexure, 123 ; lobi inferiores, 127 ; reduction of branchial clefts, 139, 178 ;
nasal sac, 143 ; accessory auditory apparatus, 150, 181 ; hyomandibular,
152, 178 ; epidermis, 166 ; origin of
auditory sac and optic vesicle, 167 ;
pancreas, 174; spiral valve, 175; pyloric caeca, 175 ; pseudobranch, 178 ;
operculum, 179 ; intestinal respiration, 180 ; air-bladder, 180 ; respiratory functions of air-bladder, 181 ;
scales, 193; parosteal bones, 193, 210;
sclerotic bones, 193 ; vertebral column,
196, 197 ; ribs, 199 ; pectoral girdle,
201, 202 ; caudal fin, 203 ; cranium,
207 ; mandibular arch, 209 ; dermal
bones of skull, 210 ; diaphragm, 213 ;
abdominal pores, 214, 258, 264 ; heart,
218 ; conus arteriosus, 220 ; aortic
arches, 235 ; pronephros, 240, 248 ;
mesonephros, 240 ; generative ducts,
258 ; degeneration of pronephros, 259.
 
Telolecithal ova, 8, 271 ; segmentation,
23, 3 r > 33 5 gastrulation, 24, 33.
 
Temporal lobe, 124.
 
Tenth cranial nerve, 135, 1 38-141.
 
Testicular network, 255, 256.
 
Testis (testicle), 5, 260, 262, 263.
 
Tetraceros, placenta, 92.
 
Thalam encephalon, 1 24-12 5, 1 27- 1 34, 1 58.
 
Thelyblast, 14.
 
Third cranial nerve, 1 37, 139-141.
 
ventricle, 124-125, 128-134.
 
Thoracic cavity, 213.
 
Thorax (insects), 147.
 
Thread-cells, 99.
 
Thyroid, in, 172, 183, 185, 210.
 
Thyroidean visceral cleft, 178, 184 ;
 
artery of, 235.
 
Thymus gland, 139, 184, 185.
 
Toad. See “ Bombinator,- “ Bufo.-
 
Tongue, 182-184.
 
Tortoise (Testudo), anal sacs, 112.
 
-  - shell, 103.
 
Toxopneustes, 7. See “ Sea-Urchin.-
 
Trabeculae cranii, 205, 206.
 
Trabecular region of head, 14 1.
 
Trachea (of lungs), 181, 182, 174.
 
Tracheae, evolution of, 109.
 
Tracheate arthropoda, mesoblast of, 57 ;
respiration of, 1 09 ; Malpighian tubules, 169.
 
Trachydosaurus, embryonic membranes
of, 84, 96.
 
 
INDKX.
 
 
335
 
 
Trachy medusae, auditory organ, 145, 146. 1
Trager, 95.
 
Tragulina, placenta, 92.
 
Transitional cells of epidermis, 101.
Trichites (of sponges), 193.
 
Trichoplax, 23.
 
Trigeminal nerve, 136, 138-141, 207.
Trionyx, blastopore, 44 ; formation of
blastopore, 65, 69 ; notochord, 186188. See “ Chelonia. -
 
Tripoblastic organism, 268.
 
Triton. £ee“Ne\vt.-
 
Tritylodon, parietal foramen, 129.
Trochlear nerve, 137, 139- 141.
 
Trout (Trutta), thymus gland, 184 ; pronephros, 240.
 
Truncus ateriosus, 220-222, 226, 227,234.
Tuber cinereum, 127.
 
Tuberculum impar, 183.
 
Tubulidentata, placenta, 91.
 
Tunica vaginalis, 21 3.
 
Tunicata, mesoderm of, 60; neural canal,
 
1 17 ; neural pore, 1 17 ; auditory organ,
147, 148 ; pineal gland, 163 ; pharynx
(branchial sac), 172; endostyle, 172 ;
fold in intestine, 175 ; asexual reproduction, 278 ; visceral clefts, 177 ;
atrium, 177, 178 ; notochord, 1 86, 1 88.
Turbellaria, epidermal rods, 99 ; gastric
diverticule, 168 ; origin of sexual cells,
263 ; intracellular digestion, 272.
Turbinal bones, 143.
 
Turtle, pharyngeal respiration, 180. See
“ Trionyx.-
 
Twelfth cranial nerve, 135, 140, 141.
Tylopoda, placenta, 92.
 
Tympanic cavity, 1 5 1 .
 
Tympanum of hexapoda, 147 ; elasmobranchs, 150 ; amphibia, mammals,
151 ; teleosts, 181.
 
Typhlosole, 175.
 
Umbilical artery, 231, 237; vein, 233, 237.
 
cord, 84, 259.
 
vesicle, 78, 231.
 
Umbilicus, 171-174.
 
Urachus, 87, 259, 260.
 
Ureter, 246, 247 ; ureters of elasmobranchia and amphia, 256 ; amniota, 258.
Urethra, 260, 261 ; various regions of
“ urethra - of male, 262.
 
Urinary bladder, 87, 112, 188, 259, 260;
homology of, 87, 259 ; of fishes, 259 ;
amphibia, 87, 230, 259 ; of amniota,
259, 260 ; veins, 230, 234.
Urochordata, notochord of, 186.
 
Urocyst, 87, 230, 259, 276.
 
 
Urodoeum, 112, 250.
 
Urodela, epiblast of, 100, 1 66 ; external
gills of larvae, 109 ; pineal gland, 129 ;
mucous canals of head, 138, 139; reduction of visceral clefts, 1 39 ; columella, 152 ; respiratory organs, 1 79 ;
vertebral column, 1 97-198 ; median
fin, 203 ; tail, 204; hyoid, 210 ; aortic
arches, 233-235 ; segmental duct, 239;
urogential apparatus, 255, 256.
Urogenital ducts of vertebrates, 250 ;
segmental duct, 251 ; Mullerian duct,
251 ; oviduct, 251, 252 ; of metatheria, 25 1 ; of eutheria, uterus, 252254; Wolffian duct, 254 ; vas deferens,
256 ; generative ducts of ganoids and
teleosts, 258 ; metanephric ureter, 258 ;
urogenital sinus, 252. 253, 258, 259 ;
urethra, 260.
 
sinus, prototheria, 252, 253 ; meta
tlieria, 253 ; eutheria, 253-262.
Urostyle, 198, 204.
 
Uterine glands (aud milk), 90.
 
Uterus, 5 ; of elasmobranchs, 95 ; of
lizards, 84 ; of mammals, 84, 87, 9294, 96, 252 ; uteri of metatheria, 252,
253 ; of eutheria, 253, 254, 260 ; human
anomaly, 253, 254; bipartitus, 253;
duplex, 253 ; simplex, 253.
 
Uterus masculiuus, 254, 257, 260, 262.
Utriculus, 149.
 
Vagina, 252-254 ; vaginae of metatheria
 
252, 253 ; vaginal caecum, 253; vagina
of eutheria, 253, 260, 261 ; vaginal
septum, 253 ; double human anomaly,
 
253 , 254.
 
Vagus nerve, 135, 138-141, 207.
 
Valves of heart, 219-223 ; of conus
arteriosus, 222.
 
Valvulae conniventes, 176.
 
Vane of feathers, 102, 103.
 
Varanus, pineal eye, 163.
 
Vasa aberrentia, 256, 257, 260.
 
efferentia, 237, 254-257.
 
Vascular system, 70, 214; development
of blood-vessels, 2 14, 270, 27 1 ; formation of heart, invertebrates, 215; vertebrates, 217, 270; development of
vascular system in vertebrates, 223 ;
early stages in embryonic circulation,
224 ; vitelline circulation, 226 ; later
stages of vitelline circulation, 226 ;
venal portal system, 229 ; hepatic
portal system, 229 ; allantoic circulation, 230 ; circulation in ichthyopsida,
232 ; summary of history of aortic
 
 
336
 
 
INDEX.
 
 
arches, 235 ; changes undergone in
circulation of foetal mammals, 235.
 
Yas deferens, 256, 262.
 
Vasifactive tissues, 269.
 
Veliger larva of mollusca, 99, 108, 114.
 
Velum (Mollusca), 99, 203 ; medusae, 145,
146.
 
Vena cava, inferior, 220-222, 229-233,
234, 236 ; superior, 220-222, 228, 229,
 
233-236.
 
Venae advehentes, 230, 231, 236.
 
reheventes, 229, 230, 231, 236.
 
renales advehentes, 228, 229.
 
Ventricular septum, 220-223.
 
Ventricle of heart, 1 95, 219, 224.
 
Vermes, brain of, 1 15 ; eye, 156 ; asexual
reproduction, 278.
 
Vermiform appendix of caecum in primates and wombat, 1 76.
 
Vermis of cerebellum, 126.
 
Vertebral artery, 226, 227.
 
column, 194 ; fishes, 195 ; amphibia,
 
197 ; sauropsida, 198 ; mammalia,
 
198 ; evolution of, 199.
 
region of head, 14 1.
 
rudiment, 191, 192.
 
Vertebrates, germinal epithelium, 5 ;
 
primitive germ cells, 6, 263 ; eggmembranes, 9 ; completion of gastrulation, 48 ; mesoblast, 68-70 ; metamerism, 73 ; fate of blastopore, 76-78 ;
evolution of nervous system, 77 ; foetal
appendages, 78 ; aquatic larvae, 87 ;
yolk sack, 95 ; epiblast, 100-102 ;
pituitary body, 1 10; proctodaeum,
III ; cloaca, 112 ; neurenteric canal,
 
1 16; brain, 122; nasal sacs, 143;
posterior nares, 144; ear, 148-151;
eyes, 157 ; hypothetical evolution of
eyes, 77, 163; liver, 1 73 ; pancreas,
174; intestine, 175 ; gill-clefts, 178;
notochord, 186 ; derma, 190 ; vertebral column, 194, 195 ; sacral ribs,
200; pectoral girdle, 201 ; locomotory
appendages, 202 ; paired limbs, 204 ;
skull, 205 ; mesentery, 212 ; formation
of blood, 215; heart, 217; vascular
system, 223 ; nephridia or excretory
organs, 237, 239, 248, 250 ; mesonephros, 242 ; urogenital ducts, 250,
264 ; generative organs, 263, 246 ;
digestive connective tissue, 270 ; digestion, 272 ; function of leucocytes, 274.
 
 
Vesicula seminalis, 260.
 
Vesicular cells of mollusca, 270.
Vesiculate hydroraedusae, auditory organ
of, 145.
 
Vestibule, 259.
 
Visceral arches, 207.
 
clefts, 73, 77, no, 135, 138, 141,
 
117-180, 183.
 
layer of peritoneum, 2 1 1 ; of pericardium, 213.
 
Visual organs, 152-167.
 
pigment, 153, 157.
 
Vitellaria, 6.
 
Vitelline arteries, 223-226, 231.
 
duct, 17 1 - 1 75, 231; occasional
 
persistence in man, 17 1 ; and birds, 172.
 
membranes, 9, 81.
 
veins, 85, 123, 218, 219, 224-226,
 
229, 234.
 
Vitreous body, 155.
 
humour, 162.
 
Viviparous lizards, 84.
 
Vomer, 210.
 
Vorticella, reproduction of, 3, 14.
 
Vulva, 260.
 
Warbler (Sylvia), embryonic membranes of, 83.
 
Water-beetle, larval eye, 153.
 
White body of cephalopod eye, 114, 156.
Wolffian blastema, 245-249.
 
body, 70, 239. See “Mesonephros.-
 
duct, 243-247, 251, 254-260 ; testicular network, 255 ; rudimentary
portion in adult, 256-257, 260 ; vas
deferens, 256.
 
ridge, 204.
 
tubules. See “Segmental tubules.-
 
Wombat (Phascolomys)j vermiform appendix of caecum, 176.
 
Worms. See “ Vermes,- i c Chaetopoda,-
“ Platyhelminths. -
 
Xiphoid, 200.
 
Yolk, 5, 6 ; of birds, 6, 37 ; digestion
of, 272 ; absorption of yolk by gillfilaments of embryo elasmobranchs, 1 79.
Yolk glands of platyhelminths, 6.
 
P lug, 33.
 
pyramids, 28, 29.
 
sac, 39, 46, 78-96, 231.
 
Zona pellucida (Zona radiata), 7, 9, 43, 84.  


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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.

   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B
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This historic 1887 embryology textbook by Haddon was designed as an introduction to the topic. Currently only the text has been made available online, figures will be added at a later date. My thanks to the Internet Archive for making the original scanned book available.
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

An Introduction to the Study of Embryology

Alfred Cort Haddon
Alfred Cort Haddon (1855–1940)

By

Alfred C. Haddon, M.A. (Cantab.), M.R.I.A.

Professor Of Zoology In The Royal College Of Science, Dublin.


Philadelphia : P. Blakiston, Son & Co., 1012 Walnut Street. 1887.

To the memory of

his beloved master and friend,

Francis Maitland Balfour


This Book is dedicated by the Author.

Francis Balfour (1851-1882)
Francis Balfour (1851-1882)


Preface

Although there are at the present time, in addition to the special accounts in various text-books of Human and Comparative Anatomy, two Students - Manuals in the English language solely devoted to the study of Embryology, it has appeared to me that a relatively small work, giving a general review of the subject, might prove of use to students.

A knowledge of the main facts of Comparative Anatomy and Systematic Zoology has been assumed for the reader, the book being especially designed for Medical Students, or for those who already possess a general acquaintance with the Animal Kingdom.

It will be noticed that many of the more difficult problems of Ontology and Phylogeny and special modes of development have either been merely alluded to or entirely ignored - as, for instance, the segmentation of the ovum and the formation of the germinal layers in Insecta and Teleostei. This has been of set purpose, as my main object in writing this book has been to give a brief connected account of the principal organs, omitting or barely mentioning structures and phenomena, which may be regarded as of secondary importance.

The facts of development have been largely supplemented by hypotheses; and an endeavour has been made so to present the latter, that the student could not mistake them for the former.

It is inevitable that, in compiling such an introductory textbook as this, many subjects must be treated in a manner similar to that in which they have been dealt with by previous authors ; and therefore I have not hesitated to borrow from them when occasion required.

In order to facilitate references, very recent, important, or doubtful observations have been associated in many cases with the investigator -s name. It must be distinctly understood that I do not necessarily personally adopt statements or views which have been incorporated in the book; they are merely put forward for what they are worth.

The beginner is advised to pay attention only to the large type in the first reading, as purely theoretical subjects or matters of detail are printed in the smaller type. Most of the figures have been so drawn as to admit of their being coloured ; and the student is recommended to tint each germinal layer and the organs derived from it in a uniform manner throughout the book : thus the epiblast and its derivatives might be coloured pink, and the hypoblast tinted blue. A uniform system of colouration will be found to be of great assistance to the memory.

The sources from which the figures have been taken are in all cases acknowledged, and in the cases where no source is given the illustrations are original. Figs. 40, 41, 44, 45, 80, 81, and 178* have appeared previously in the Proceedings of the Eoyal Dublin Society.

The classification adopted will be found in an Appendix. All the genera mentioned in the text have been inserted, in order that their systematic position may be seen at a glance.


A Bibliography has also been appended, which is designed to serve simply as a guide to the more recent literature, and no attempt has been made to render the list exhaustive. It will be noticed that most of the Memoirs cited are of later date than the year 1880. The more important earlier papers are recorded in the late Professor Balfour -s “Treatise of Comparative Embryology.- As any student who seriously studies Embryology must consult that invaluable work, I have considered it superfluous to repeat the Bibliography given by Balfour. The prevalent custom of authors of giving references to the literature of the subject under discussion renders it comparatively easy to discover what has already been written thereon.

Finally, I would here express my warmest thanks to my friend Professor G. B. Howes, of the Normal School of Science, South Kensington, for his kindness in reading the proofs and in making many valuable suggestions.

Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2024, April 18) Embryology Book - An Introduction to the Study of Embryology. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Introduction_to_the_Study_of_Embryology

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