The Works of Francis Balfour 3-12: Difference between revisions

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(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."  
(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."  
Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. III. 1876 1877.  
Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. III. 1876 1877.
 
 
 
==CHAPTER XIII. GENERAL CONCLUSIONS==
 
 
 
I. THE MODE OF ORIGIN AND HOMOLOGIES OF THE GERMINAL LAYERS.
 
IT has already been shewn in the earlier chapters of the
work that during the first phases of development the history of
all the Metazoa is the same. They all originate from the coalescence of two cells, the ovum and spermatozoon. The coalesced
product of these cells the fertilized ovum then undergoes a
process known as the segmentation, in the course of which it
becomes divided in typical cases into a number of uniform cells.
An attempt was made from the point of view of evolution to
explain these processes. The ovum and spermatozoon were
regarded as representing phylogenetically two physiologically
differentiated forms of a Protozoon ; their coalescence was equivalent to conjugation : the subsequent segmentation of the
fertilized ovum was the multiplication by division of the organism resulting from the conjugation ; the resulting organisms,
remaining, however, united to form a fresh organism in a higher
state of aggregation.
 
In the systematic section of this work the embryological
history of the Metazoa has been treated. The present chapter
contains a review of the cardinal features of the various histories, together with an attempt to determine how far there are
any points common to the whole of these histories ; and the
phylogenetic interpretation to be given to such points.
 
Some years ago it appeared probable that a definite answer
 
 
 
332 INVAGINATION.
 
 
 
would be given to the questions which must necessarily be
raised in the present chapter ; but the results of the extended
investigations made during the last few years have shewn that
these expectations were premature, and in spite of the numerous
recent valuable contributions to this branch of Embryology,
amongst which special attention may be called to those of
Kowalevsky (No. 277), Lankester (Nos. 278 and 279), and
Haeckel (No. 266), there are few embryologists who would venture to assert that any answers which can be given are more
than tentative gropings towards the truth.
 
In the following pages I aim more at summarising the
facts, and critically examining the different theories which can
be held, than at dogmatically supporting any definite views of
my own.
 
In all the Metazoa, the development of which has been investigated, the first process of differentiation, which follows
upon the segmentation, consists in the cells of the organism
becoming divided into two groups or layers, known respectively
as epiblast and hypoblast.
 
These two layers were first discovered in the young embryos of vertebrated animals by Pander and Von Baer, and have been since known as
the germinal layers, though their cellular nature was not at first recognised. They were shewn, together with a third layer, or mesoblast, which
subsequently appears between them, to bear throughout the Vertebrata
constant relations to the organs which became developed from them. A
very great step was subsequently made by Remak (No. 287), who successfully worked out the problem of vertebrate embryology on the cellular
theory.
 
Rathke in his memoir on the development of Astacus (No. 286) attempted at a very early period to extend the doctrine of the derivation of
the organs from the germinal layers to the Invertebrata. In 1859 Huxley
made an important step towards the explanation of the nature of these
layers by comparing them with the ectoderm and endoderm of the Hydrozoa ; while the brilliant researches of Kowalevsky on the development of
a great variety of invertebrate forms formed the starting point of the current
views on this subject.
 
The differentiation of the epiblast and hypoblast may
commence during the later phases of the segmentation, but
is generally not completed till after its termination. Not
only do the cells of the blastoderm become differentiated
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
333
 
 
 
into two layers, but these
very large number of
 
 
 
two layers, in the case of a
ova with but little food-yolk, con
 
 
(fig. 198) the
require further
 
 
 
 
FIG. 198. DIAGRAM
OF A GASTRULA.
 
(From Gegenbaur.)
 
a. mouth ; b. archenteron ; c. hypoblast ; d. epiblast.
 
 
 
stitute a double-walled sack the gastrula
characters of which are too well known to
description. Following the lines of phylogenetic speculation above indicated, it may
be concluded that the two-layered condition
of the organism represents in a general way
the passage from the protozoon to the metazoon condition. It is probable that we may
safely go further, and assert that the gastrula
reproduces, with more or less fidelity, a stage
in the evolution of the Metazoa, permanent
in the simpler Hydrozoa, during which the
organism was provided with (i) a fully developed digestive cavity (fig. 198 b) lined by
the hypoblast with digestive and assimilative
functions, (2) an oral opening (a), and (3) a
superficial epiblast (d}. These generalisations, which are now widely accepted, are no doubt very valuable,
but they leave unanswered the following important questions :
 
(1) By what steps did the compound Protozoon become
differentiated into a Metazoon ?
 
(2) Are there any grounds for thinking that there is more
than one line along which the Metazoa have become independently evolved from the Protozoa ?
 
(3) To what extent is there a complete homology between
the two primary germinal layers throughout the Metazoa ?
 
Ontogenetically there is a great variety of processes by which
the passage from the segmented ovum to the two-layered or
diploblastic condition is arrived at.
 
These processes may be grouped under the following heads :
1. Invagination. Under this term a considerable number
of closely connected processes are included. When the segmentation results in the formation of a blastosphere, one half of the
blastosphere may be pushed in towards the opposite half, and a
gastrula be thus produced (fig. 199, A and B). This process is
known as embolic invagination. Another process, known as epibolic invagination, consists in epiblast cells growing round and en
 
 
334
 
 
 
INVAGINATION.
 
 
 
closing the hypoblast (fig. 200). This process replaces the former
process when the hypoblast cells are so bulky from being distended
by food-yolk that their invagination is mechanically impossible.
 
 
 
 
FlG. 199. TWO STAGES IN THE DEVELOPMENT OF HOLOTHURIA TUBULOSA,
 
VIEWED IN OPTICAL SECTION. (After Selenka.)
A. Stage at the close of segmentation. B. Gastrula stage.
 
mr. micropyle ; fl. chorion ; s.c. segmentation cavity; bl. blastoderm; ep. epiblast;
hy. hypoblast ; ms. amoeboid cells derived from hypoblast ; a.e. archenteron.
 
There are various peculiar modifications of invagination
which cannot be dealt with in detail.
 
Invagination in one form or other occurs in some or all the
members of the following groups :
 
The Dicyemidae, Calcispongiae (after the amphiblastula stage) and Silicispongiae, Coelenterata, Turbellaria, Nemertea, Rotifera, Mollusca, Polyzoa,
Brachiopoda, Chaetopoda, Discophora, Gephyrea, Chaetognatha, Nematelminthes, Crustacea, Echinodermata, and
Chordata.
 
The gastrula of the Crustacea is peculiar, as is also that
of many of the Chordata (Reptilia, Aves, Mammalia), but
there is every reason to suppose
 
 
 
 
FIG. 200. TRANSVERSE SECTION
THROUGH THE OVUM OF EUAXES
DURING AN EARLY STAGE OF DEVELOPMENT, TO SHEW THE NATURE OF
EPiiiOLic INVAGINATION. (After Kowalevsky. )
 
ep. epiblast ; ms. mesoblastic band ;
hy. hypoblast.
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
335
 
 
 
that the gastrulae of these groups are simply modifications of the
normal type.
 
2. Delamination. Three types of delamination may be
distinguished :
 
a. Delamination where the cells of a solid morula become
divided into a superficial epiblast, and a central solid mass
in which the digestive cavity is subsequently hollowed out
(fig. 201).
 
 
 
 
FlG. 201. TWO STAGES IN THE DEVELOPMENT OF STEPHANOMIA PICTUM,
TO ILLUSTRATE THE FORMATION OF THE LAYERS BY DELAMINATION. (After
 
Metschnikoff.)
 
A. Stage after the delamination; ep. epiblastic invagination to form pneumatocyst.
 
B. Later stage after the formation of the gastric cavity in the solid hypoblast.
po. polypite ; /. tentacle ; pp. pneumatocyst ; ep. epiblast of pneumatocyst ; hy. hypoblast surrounding pneumatocyst.
 
b. Delamination where the segmented ovum has the form
of a blastosphere, the cells of which give rise by budding to
scattered cells in the interior of the vesicle, which, though they
may at first form a solid mass, finally arrange themselves in the
form of a definite layer around a central digestive cavity
(fig. 202).
 
c. Delamination where the segmented ovum has the form
of a blastosphere in the cells of which the protoplasm is differentiated into an inner and an outer part. By a subsequent
 
 
 
336
 
 
 
DELAMINATION.
 
 
 
process the inner parts of the cells become separated from the
outer, and the walls of the blastosphere are so divided into two
distinct layers (fig. 205).
 
Although the third of these processes is usually regarded
as the type of delamination, it does not, so far as I know, occur
in nature, but is most nearly approached in Geryonia (fig. 203).
 
The first type of delamination is found in the Ceratospongiae,
some Silicispongiae (?), and in many Hydrozoa and Actinozoa,
and in Nemertea and Nematelminthes (Gordioidea ?). The
second type occurs in many Porifera \Calcispongi(e (A see t fa),
Myxospongice], and in some Coelenterata, and Brachiopoda
( Thecidium).
 
Delamination and invagination are undoubtedly the two
most frequent modes in which the layers are differentiated, but
 
C
 
 
 
 
FIG. 202. THREE LARVAL STAGES OF EUCOPE POLYSTYLA. (After Kowalevsky.)
A. Blastosphere stage with hypoblast spheres becoming budded off into central
cavity. B. Planula stage with solid hypoblast. C. Planula stage with a gastric
cavity, ep. epiblast ; hy. hypoblast ; al. gastric cavity.
 
there are in addition several others. In the first place the
whole of the Tracheata (with the apparent exception of the
Scorpion) develop, so far as is known, on a plan peculiar to
them, which approaches delamination. This consists in the
appearance of a superficial layer of cells enclosing a central
yolk mass, which corresponds to the hypoblast (figs. 204 and
214). This mode of development might be classed under
delamination, were it not for the fact that the early development
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
337
 
 
 
of many Crustacea is almost the same, but is subsequently
followed by an invagination (fig. 208), which apparently corre
 
 
 
 
FIG. 203. DIAGRAMMATIC FIGURES SHEWING THE DELAMINATION OF THE
 
EMBRYO OF GERYONIA. (After Fol.)
 
A. Stage at the commencement of the delamination ; the dotted lines x shew
 
the course of the next planes of division. B. Stage at the close of the delamination.
 
cs. segmentation cavity ; a. endoplasm ; b. ectoplasm ; ep. epiblast ; hy. hypoblast.
 
spends to the normal invagination of other types. There are
strong grounds for thinking that the tracheate type of forma
 
 
 
FIG. 204. SEGMENTATION AND FORMATION OF THE BLASTODERM IN CHELIFER.
 
(After Metschnikoff. )
 
In A the ovum is divided into a number of separate segments. In B a number of
small cells have appeared (bl) which form a blastoderm enveloping the large yolkspheres. In C the blastoderm has become divided into two layers.
 
B. III. 22
 
 
 
338 ORIGIN OF THE GASTRULA.
 
tion of the epiblast and hypoblast is a secondary modification of
an invaginate type (vide Vol. II. p. 457).
 
The type of some Turbellaria (Stylochopsis ponticus) and that
of Nephelis amongst the Discophora is not capable of being
reduced to the invaginate type.
 
The development of almost all the parasitic groups, i.e. the
Trematoda, the Cestoda, the Acanthocephala, and the Linguatulida, and also of the Tardigrada, Pycnogonida, and other
minor groups, is too imperfectly known to be classed with either
the delaminate or invaginate types.
 
It will, I think, be conceded on all sides that, if any of the
ontogenetic processes by which a gastrula form is reached are
repetitions of the process by which a simple two-layered gastrula
was actually evolved from a compound Protozoon, these processes are most probably of the nature either of invagination or
of delamination.
 
The much disputed questions which have been raised about
the gastrula and planula theories, originally put forward by
Haeckel and Lankester, resolve themselves then into the simple
question, whether any, and if so which, of the ontogenetic
processes by which the gastrula is formed are repetitions of the
phylogenetic origin of the gastrula.
 
It is very difficult to bring forward arguments of a conclusive
kind in favour of either of these processes. The fact that
delaminate and invaginate gastrulse are in several instances
found coexisting in the same group renders it certain that there
are not two independent phyla 'of the Metazoa, derived respectively from an invaginate and a delaminate gastrula 1 .
 
1 It is not difficult to picture a possible derivation of delamination from invagination ; while a comparison of the formation of the inner layers (mesoblast and hypoblast) in Ascetta (amongst the Sponges), and in the Echinodermata, shews a very
simple way in which it is possible to conceive of a passage of delamination into
invagination. In Ascetta the cells, which give rise to the mesoblast and hypoblast, are
budded off from the inner wall of the blastosphere, especially at one point ; while in
Echinodermata (fig. 199) there is a small invaginated sack which gives rise to the
hypoblast, while from the walls of this sack amoeboid cells are budded off which give
rise to a large part of the mesoblast. If we suppose the hypoblast cells budded off
at one point in Ascetta gradually to form an invaginated sack, while the mesoblast
cells continued to be budded off as before, we should pass from the delaminate type of
tta t<> the invaginate type of an Echinoderm.
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 339
 
The four most important cases in which the two processes
coexist are the Porifera, the Coelenterata, the Nemertea, and the
Brachiopoda. In the cases of the Porifera and Ccelenterata,
there do not appear to me to be any means of deciding which of
these processes is derived from the other ; but in the Nemertea
and the Brachiopoda the case is different. In all the types of
Nemertea in which the development is relatively not abbreviated there is an invaginate gastrula, while in the types with a
greatly abbreviated development there is a delaminate gastrula.
It would seem to follow from this that a delaminate gastrula has
here been a secondary result of an abbreviation in the development. In the Brachiopoda, again, the majority of types develop
by a process of invagination, while Thecidium appears to
develop by delamination ; here also the delaminate type would
appear to be secondarily derived from the invaginate.
 
If these considerations are justified, delamination must be in
some instances secondarily derived from invagination ; and this
fact is so far an argument in favour of the more primitive nature
of invagination ; though it by no means follows that in the
invaginate process the steps by which the Metazoa were derived
from the Protozoa are preserved.
 
It does not, therefore, seem possible to decide conclusively in
favour of either of these processes by a comparison of the cases
where they occur in the same groups.
 
The relative frequency of the two processes supplies us with
another possible means for deciding between them ; and there is
no doubt that here again the scale inclines towards invagination.
It must, however, be borne in mind that the frequency of the
process of invagination admits of another possible explanation.
There is a continual tendency for the processes of development
to be abbreviated and simplified, and it is quite possible that the
frequent occurrence of invagination is due to the fact of its
being, in most cases, the simplest means by which the twolayered condition can be reached. But this argument can have
but little weight until it can be shewn in each case that invagination is a simpler process than delamination ; and it is rendered
improbable by the cases already mentioned in which delamination has been secondarily derived from invagination.
 
If it were the case that the blastopore had ih all types the
 
22 2
 
 
 
340
 
 
 
BLASTOPORE.
 
 
 
same relation to the adult mouth, there would be strong grounds
for regarding the invaginate gastrula as an ancestral form ; but
the fact that this is by no means so is an argument of great
weight in favour of some other explanation of the frequency of
invagination.
 
The force of this consideration can best be displayed by a
short summary of the fate of the blastopore in different forms.
 
The fate of the blastopore is so variable that it is difficult
even to classify the cases which have been described.
 
(1) It becomes the permanent mouth in the following forms 1 :
Ccelenterata. Pelagia, Cereanthus.
 
Turbellaria. Leptoplana (?), Thysanozoon.
 
Nemertea. Pilidium, larvae of the type of Desor.
 
Mollusca. In numerous examples of most Molluscan groups, except the
 
Cephalopoda.
 
Chcetopoda. Most Oligochaeta, and probably many Polychseta.
Gephyrea. Phascolosoma, Phoronis.
Nematelminthes. Cucullanus.
 
(2) It closes in the position where the mouth is subsequently formed.
Ccelenterata. Ctenophora (?).
 
Mollusca. In numerous examples of most Molluscan groups, except the
 
Cephalopoda.
Crustacea. Cirripedia (?), some Cladocera (Moina) (?).
 
(3) It becomes the permanent anus.
Mollusca. Paludina.
 
Chatopoda. Serpula and some other types.
Echinodermata.Mmosl universally, except amongst the Crinoidea.
 
(4) It closes in the position where the anus is subsequently formed.
Echinodermata. Crinoidea.
 
(5) It closes in a position which does not correspond or is not known
to correspond 2 either with the future mouth or anus. Porifera Sycandra.
Ccelenterata Chrysaora*, Aurelia*. Nemertea* Some larvae which develop without
a metamorphosis. Rolifera*. Mollusca Cephalopoda. Polyzoa*. Brachiopoda
Argiope, Terebratula, Terebratulina. Ch(Etopoda Euaxes. Discophora Clepsine.
Gephyrea Bonellia*. Chatognatha. Crustacea Decapoda. Chordata.
 
The forms which have been classed together under the last
heading vary considerably in the character of the blastopore.
In some cases the fact of its not coinciding either with the mouth
 
1 The above list is somewhat tentative ; and future investigations will probably
shew that many of the statements at present current about the position of the blastopore are inaccurate.
 
2 The forms in which the position of the blastopore in relation to the mouth or
anus is not known <ire marked with an asterisk.
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 341
 
or anus appears to be due simply to the presence of a large
amount of food-yolk. The cases of the Cephalopoda, of Euaxes,
and perhaps of Clepsine and Bonellia, are to be explained in
this way : in the case of all these forms, except Bonellia, the
blastopore has the form of an elongated slit along the ventral
surface. This type of blastopore is characteristic of the Mollusca
generally, of the Polyzoa, of the Nematelminthes, and very
possibly of the Chaetopoda and Discophora. In the Chaetognatha (fig. 209 B) the blastopore is situated, so far as can be
determined, behind the future anus. In many Decapoda the
blastopore is placed behind, but not far from, the anus. In the
Chordata it is also placed posteriorly to the anus, and,
remarkably enough, remains, in a large number of forms, for
some time in connection with the neural tube by a neurenteric
canal.
 
The great variations in the character of the gastrula, indicated
in the above summary, go far to shew that if the gastrulae, as we
find them in most types, have any ancestral characters, these
characters can only be of the most general kind. This may
best be shewn by the consideration of a few striking instances.
The blastopore in Mollusca has an elongated slit-like form,
extending along the ventral surface from the mouth to the
anus. In Echinodermata it is a narrow pore, remaining as the
anus. In most Chsetopoda it is a pore remaining as the mouth,
but in some as the anus. In Chordata it is a posteriorlyplaced pore, opening into both the archenteron and the neural
canal.
 
It is clearly out of the question to explain all these differences
as having connection with the characters of ancestral forms.
Many of them can only be accounted for as secondary adaptations for the convenience of development.
 
The epibolic gastrula of Mammalia (vide pp. 215 and 291) is
a still more striking case of a secondary embryonic process, and
is not directly derived from the gastrula of the lower Chordata.
It probably originated in connection with the loss of food-yolk
which took place on the establishment of a placental nutrition
for the foetus. The epibolic gastrula of the Scorpion, of Isopods,
and of other Arthropoda, seems also to be a derived gastrula.
These instances of secondary gastrulse are very probably by no
 
 
 
342 BLASTOPORE.
 
 
 
means isolated, and should serve as a warning against laying too
much stress upon the frequency of the occurrence of invagination.
The great influence of the food-yolk upon the early development
might be illustrated by numerous examples, especially amongst
the Chordata (vide Chapter XL).
 
If the descendants of a form with a large amount of food-yolk
in its ova were to produce ova with but little food-yolk, the type
of formation of the germinal layers which would thereby result
would be by no means the same as that of the ancestors of the
forms with much food-yolk, but would probably be something
very different, as in the case of Mammalia. Yet amongst the
countless generations of ancestors of most existing forms, such
oscillations in the amount of the food-yolk must have occurred
in a large number of instances.
 
The whole of the above considerations point towards the
view that the formation of the hypoblast by invagination, as it
occurs in most forms at the present day, can have in many
instances no special phylogenetic significance, and that the
argument from frequency, in favour of invagination as opposed
to delamination, is not of prime importance.
 
A third possible method of deciding between delamination
and invagination is to be found in the consideration as to which
of these processes occurs in the most primitive forms. If there
were any agreement amongst primitive forms as to the type of
their development this argument might have some weight. On
the whole, delamination is, no doubt, characteristic of many
primitive types, but the not infrequent occurrence of invagination
in both the Ccelenterata and the Porifera the two groups which
would on all hands be admitted to be amongst the most
primitive deprives this argument of much of the value it might
otherwise have.
 
To sum up considering the almost indisputable fact that
both the processes above dealt with have in many instances had
a purely secondary origin, no valid arguments can be produced
to shew that either of them reproduces the mode of passage
between the Protozoa and the ancestral two-layered Metazoa.
These conclusions do not, however, throw any doubt upon the
fact that the gastrula, however evolved, was a primitive form of
the Metazoa ; since this conclusion is founded upon the actual
 
 
 
 
 
 
ORIGIN OF TIIH GERMINAL LAYERS.
 
 
 
343
 
 
 
existence of adult gastrula forms independently of their occurrence in development.
 
Though embryology does not at present furnish us with a definite
answer to the question how the Metazoa became developed from the Protozoa, it is nevertheless worth while reviewing some of the processes by
which this can be conceived to have occurred.
 
On purely a priori grounds there is in my opinion more to be said for
invagination than for any other view.
 
On this view we may suppose that the colony of Protozoa in the course
of conversion into Metazoa had the form of a blastosphere ; and that at
one pole of this a depression appeared. The cells lining this depression we
 
 
 
 
F
 
 
 
FIG. 205. DIAGRAM SHEWING THE FORMATION OF A GASTRULA IJY
DELAMINATION. (From Lankester.)
 
Fig. i, ovum; fig. 2, stage in segmentation; fig. 3, commencement of delamination
after the appearance of a central cavity ; fig. 4, delamination completed, mouth forming at M. In figs, i, 2, and 3, EC, is ectoplasm, and En. is endoplasm. In fig. 4,
EC. is epiblast, and En. hypoblast. E. and F. food particles.
 
may suppose to have been amoeboid, and to have carried on the work of
digestion ; while the remaining cells were probably ciliated. The digestion
may be supposed to have been at first carried on in the interior of the cells,
as in the Protozoa; but, as the depression became deeper (in order to
increase the area of nutritive cells and to retain the food) a digestive
secretion probably became poured out from the cells lining it, and the mode
of digestion generally characteristic of the Metazoa was thereby inaugurated.
It may be noted that an intracellular protozoon type of digestion persists in
the Porifera, and appears also to occur in many Ccelenterata, Turbellaria,
 
 
 
344 PASSAGE FROM THE PROTOZOA TO THE METAZOA.
 
&c., though in most of these cases both kinds of digestion probably go on
simultaneously 1 .
 
Another hypothetical mode of passage, which fits in with delamination,
has been put forward by Lankester, and is illustrated by fig. 205. He
supposes that at the blastosphere stage the fluid in the centre of the colony
acquired special digestive properties ; the inner ends of the cells having at
this stage somewhat different properties from the outer, and the food being
still incepted by the surface of the cells (fig. 205, 3). In a later stage of the
process the inner portions of the cells became separated off as the hypoblast ; while the food, though still ingested in the form of solid particles by
the superficial cells, was carried through the protoplasm into the central
digestive cavity. Later (fig. 205, 4), the point where the food entered became
localised, and eventually a mouth became formed at this point.
 
The main objection which can be raised against Lankester's view is that
it presupposes a type of delamination which does not occur in nature except
in Geryonia.
 
Metschnikoff has propounded a third view with reference to delamination. He starts as before with a ciliated blastosphere. He next supposes
the cells from the walls of this to become budded off into the central cavity,
as in Eucope (fig. 202), and to lose their cilia. These cells give rise to
an internal parenchyma, which carries on an intracellular digestion. At a
later stage a central digestive cavity is supposed to be formed. This view
of the passage from the protozoon to the metazoon state, though to my
mind improbable in itself, fits in very well with the ontogeny of the lower
Hydrozoa.
 
Another view has been put forward by myself in the chapter on the
Porifera*, to the effect that the amphiblastula larva of Calcispongias may
be a transitional form between the Protozoa and the Metazoa, composed of
a hemisphere of nutritive amoeboid cells, and a hemisphere of ciliated cells.
The absence of such a larval form in the Ccelenterata and higher Metazoa
is opposed, however, to this larva being regarded as a transitional form,
except for the Porifera.
 
It is obvious that so long as there is complete uncertainty as
to the value to be attached to the early developmental processes,
it is not possible to decide from these processes whether there is
only a single metazoon phylum or whether there may not be two
or more such phyla. At the same time there appear to be strong
 
1 J. Parker, "On the Histology of Hydra fusca," Quart. Journ. Micr. Science,
vol. xx. 1880; and El. Metschnikoff, " Ueb. die intracellulare Verdauung bei Ccelenteraten," Zoologischer Anzeiger, No. 56, vol. in. 1880 and Lankester, " On the
intracellular digestion and endoderm of Limnocodium, " Quart. Journ. Micr. Science,
vol. xxi. 1881.
 
! Vol. n. p. 149.
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 345
 
arguments for regarding the Porifera as a phylum of the
Metazoa derived independently from the Protozoa. This seems
to me to be shewn (i) by the striking larval peculiarities of the
Porifera ; (2) by the early development of the mesoblast in the
Porifera, which stands in strong contrast to the absence of this
layer in the embryos of most Ccelenterata ; and above all, (3) by
the remarkable characters of the system of digestive channels.
A further argument in the same direction is supplied by the fact
that the germinal layers of the Sponges very probably do not
correspond physiologically to the germinal layers of other types.
The embryological evidence is insufficient to decide whether the
amphiblastula larva is, as suggested above, to be regarded as the
larval ancestor of the Porifera.
 
Homologies of the germinal layers. The question as to
how far there is a complete homology between the two primary
germinal layers throughout the Metazoa was the third of the
questions proposed to be discussed here.
 
Since there are some Metazoa with only two germinal layers,
and other Metazoa with three, and since, as is shewn in the
following section, the third layer or mesoblast can only be
regarded as a derivative of one or both the primary layers, it is
clear that a complete homology between the two primary germinal
layers does not exist.
 
That there is a general homology appears on the other hand
hardly open to doubt.
 
The primary layers are usually continuous with each other,
near one or both (when both are present) the openings of the
alimentary tract.
 
As a rule an oral and anal section of the alimentary tract
the stomodaeum and proctodaeum are derived from the
epiblast ; but the limits of both these sections are so variable,
sometimes even in closely allied forms, that it is difficult to avoid
the conclusion that there is a border-land between the epiblast
and hypoblast, which appears by its development to belong in
some forms to the epiblast and in other forms to the hypoblast.
If this is not the case it is necessary to admit that there are
instances in which a very large portion of the alimentary canal
is phylogenetically an epiblastic structure. In some of the
Isopods, for example, the stomodaeum and proctodaeum give
 
 
 
346 ORIGIN OF THE MESOBLAST.
 
rise to almost the whole of the alimentary canal with its appendages, except the liver.
 
The origin of the Mesoblast. A diploblastic condition of
the organism preceded, as we have seen, the triploblastic. The
epiblast during the diploblastic condition was, as appears from
such forms as Hydra, especially the sensory and protective
layer, while the hypoblast was the secretory and assimilating
layer; both layers giving rise to muscular elements. It must
not, however, be supposed that in the early diploblastic ancestors
there was a complete differentiation of function, but there is
reason to think that both the primary layers retained an
indefinite capacity for developing into any form of tissue 1 . The
fact of the triploblastic condition being later than the diploblastic
proves in a conclusive way that the mesoblast is a derivative of
one or both the primary layers. In the Ccelenterata we can
study the actual origin from the two primary layers of various
forms of tissue which in the higher types are derived from the
mesoblast 2 . This fact, as well as general a priori considerations,
conclusively prove that the mesoblast did not at first
originate as a mass of independent cells between the
two primary layers, but that in the first instance it
gradually arose as differentiations of the two layers,
and that its condition in the embryo as an independent layer of undifferentiated cells is a secondary
condition, brought about by the general tendency
 
1 The Hertwigs (No. 270) have for instance shewn that nervous structures are
developed in the hypoblast in the Actinozoa and other Coelenterata.
 
2 There is considerable confusion in the use of the names for the embryonic layers.
In some cases various tissues formed by differentiations of the primary layers have
been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out
the inconvenience of this nomenclature. In the case of the Coelenterata it is difficult
to decide in certain instances (e.g. Sympodium) whether the cells which give rise to a
particular tissue of the adult are to be regarded as forming a mesoblast, i.e. a middle
undifferentiated layer of cells, or whether they arise as already histologically differentiated elements from one of the primary layers. The attempt to distinguish by a
special nomenclature the epiblast and hypoblast after and before the separation of the
mesoblast, which has been made by Allen Thomson (No. 1), appears incapable of
being consistently applied, though it is convenient to distinguish a primary and a
secondary hypoblast. A proposal of the Hertwigs to adopt special names for the
outer and inner limiting membranes of the adult, and for the interposed mass of
organs, appears to me unnecessary.
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 347
 
towards a simplification of development, and a retardation of histological differentiation 1 .
 
The Hertwigs have recently attempted (No. 271) to distinguish two types of differentiation of the mesoblast, viz. (i) a
direct differentiation from the primitive epithelial cells ; (2) a
differentiation from primitively indifferent cells budded off into
the gelatinous matter between the two primary layers.
 
It is quite possible that this distinction may be well founded, but no
conclusive evidence of the occurrence of the second process has yet been
adduced. The Ctenophora are the type upon which special stress is laid,
but the early passage of amoeboid cells into the gelatinous tissue, which
subsequently become muscular, is very probably an embryonic abbreviation ;
and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing
through the gelatinous tissue.
 
The conversion of non-embryonic connective-tissue cells into muscle cells
in the higher types has been described, but very much more evidence is
required before it can be accepted as a common occurrence.
 
In addition to the probably degraded Dicyemida:: and Orthonectidae, the Ccelenterata are the only group in which a true
mesoblast is not always present. In other words, the Ccelenterata are the only group in which there is not found in the
embryo an undifferentiated group of cells from which the
majority of the organs situated between the epidermis and the
alimentary epithelium are developed.
 
The organs invariably derived, in the triploblastic forms,
from the mesoblast, are the vascular and lymphatic systems, the
muscular system, and the greater part of the connective tissue
and the excretory and generative (?) systems. On the other
hand, the nervous systems (with a few possible exceptions) and
organs of sense, the epithelium of most glands, and a few
exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers.
 
The fact of the first-named set of organs being invariably
derived from the mesoblast points to the establishment of the
two following propositions: (i) That with the differenti
1 The causes which give rise to a retardation of histological differentiation will be
dealt with in the second part of this chapter which deals with larval characters and
larval forms.
 
 
 
348 ORIGIN OF THE MESOBLAST.
 
ation of the mesoblast as a distinct layer by the process
already explained, the two primary layers lost for the
most part the capacity they primitively possessed of
giving rise to muscular and connective-tissue differentiations 1 , to the epithelium of the excretory organs,
and to generative cells. (2) That the mesoblast throughout the triploblastic Metazoa, in so far as these forms
have sprung from a common triploblastic ancestor, is
an homologous structure.
 
The second proposition follows from the first. The mesoblast
can only have ceased to be homologous throughout the triploblastica by additions from the two primary layers, and the
existence of such additions is negatived by the first proposition.
 
These two propositions, which hang together, are possibly
only approximately true, since it is quite possible that future
investigations may shew that differentiations of the two primary
layers are not so rare as has been hitherto imagined.
 
Ranvier 2 finds that the muscles of the sweat-glands are developed from
the inner part of the layer of epiblast cells, invaginated to form these
glands.
 
Gotte 3 describes the epiblast cells of the larva of Comatula as being at a
certain stage contractile and compares them with the epithelio-muscular
cells of Hydra. These cells would appear subsequently to be converted
into a simple cuticular structure.
 
It is moreover quite possible that fresh differentiations from
the two primary layers may have arisen after the triploblastic
condition had been established, and by the process of simplification of development and precocious segregation, as Lankester
calls it, have become indistinguishable from the normal mesoblast. In spite of these exceptions it is probable that the major
part of the muscular system of all existing triploblastic forms
has been differentiated from the muscular system of the ancestor
or ancestors (if there is more than one phylum) of the triplo
1 The connective-tissue test of the Tunicata, though derived from the epiblast, is
not really an example of such a differentiation.
 
1 M. L. Ranvier. " Sur la stricture des glandes sudoripares." Comptes Rendus,
Dec. 29, 1879.
 
1 A. Gotte, "Vergleich. Entwick. d. Comatula mediterranea." Archiv f. mikr.
Anat. vol. XI I. p. 597.
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
349
 
 
 
blastica. In the case of other tissues there are a few instances
which might be regarded as examples of an organ primitively
developed in one of the two primary layers having become
secondarily carried into the mesoblast. The notochord has
sometimes been cited as such an organ, but, as indicated in a
previous chapter, it is probable that its hypoblastic origin can
always be demonstrated.
 
A
 
 
 
 
 
FIG. 206. EPIBOLIC GASTRULA OF BONELLIA. (After Spengel.)
 
A. Stage when the four hypoblast cells are nearly enclosed.
 
B. Stage after the formation of the mesoblast has commenced by an infolding of
the lips of the blastopore.
 
ep. epiblast; me. mesoblast; bl. blastopore.
 
The nervous system, although imbedded in mesoblastic
derivates in the adults of all the higher triploblastica, retains
with marvellous constancy its epiblastic origin (though it is
usually separated from the epiblast prior to its histogenic
differentiation) ; yet in the Cephalopoda, and some other
Mollusca, the evidence is in favour of its developing in the
mesoblast. Should future investigations confirm these conclusions, a good example will be afforded of an organ changing the
layer from which it usually develops 1 . The explanation of such
a change would be precisely the same as that already given for
the mesoblast as a whole.
 
The actual mode of origin of various tissues, which in the
true triploblastic forms arise in mesoblast, can be traced in the
 
1 The Hertvvigs hold that there is a distinct part of the nervous system which was
at first differentiated in the mesoblast in many types, amongst others the Mollusca.
The evidence in favour of this view is extremely scanty and the view itself appears to
me highly improbable.
 
 
 
350 ORIGIN OF THE MESOBLAST.
 
Ccelenterata 1 . In this group the epiblast and hypoblast both
give rise to muscular and connective-tissue elements ; and
although the main part of the nervous system is formed in the
epiblast, it seems certain that in some types nerves may be
derived from the hypoblast' 2 . These facts are extremely interest
 
 
 
FlG. 207. TWO TRANSVERSE SECTIONS THROUGH EMBRYOS OF HYDROPHILUS
 
PICEUS. (After Kowalevsky.)
 
A. Section through an embryo at the point where the two germinal folds most
approximate.
 
B. Section through an embryo, in the anterior region where the folds of the
amnion have not united.
 
gg. germinal groove; me. mesoblast; am. amnion; yk. yolk.
 
ing, but it is by no means certain that any conclusions can
be directly drawn from them as to the actual origin of the
mesoblast in the triploblastic forms, till we know from what
diploblastic forms the triploblastica originated. All that they
shew is that any of the constituents of the mesoblast may have
originated from either of the primitive layers.
 
1 The reader is referred for this subject to the valuable memoirs which have been
recently published by the Hertwigs, especially to No. 270. He will find a general
account of the subject written before the appearance of the Hertwigs' memoir in
pp. 180-182 of Volume II. of this treatise.
 
- It would be interesting to know the history of the various nervous structures
found in the walls of the alimentary tract in the higher forms. I have shewn
(Development of Elasmobranch Fishes, p. 172) that the central part of the sympathetic system is derived from the epiblast. It would however be well to work over
the development of Auerbach's plexus.
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
351
 
 
 
For further light as to the origin of the mesoblast, it is
necessary to turn to its actual development.
 
The following summary illustrates the more important
modes in which the mesoblast originates.
B
 
 
 
 
FIG. 208. FIGURES ILLUSTRATING THE DEVELOPMENT OF ASTACUS.
(From Parker; after Reichenbach. )
 
A. Section through part of the ovum during segmentation, n. nuclei ; w.y. white
yolk ; y.p. yolk pyramids ; c. central yolk mass.
 
B. and C. Longitudinal sections of the gastrula stage, a. archenteron ; b. blastopore; ms. mesoblast; ec. epiblast; en. hypoblast, distinguished from epiblast by
shading.
 
D. Highly magnified view of anterior lip of blastopore, to shew the origin of the
primary mesoblast from the wall of the archenteron. /. ms. primary mesoblast ; ec.
epiblast ; en. hypoblast.
 
E. Two hypoblast cells to shew the amoeba-like absorption of yolk spheres.
y. yolk ; n. nucleus ; p. pseudopodial process.
 
F. Hypoblast cells giving rise endogenously to the secondary mesoblast (s.tns.) ;
n. nucleus.
 
I. It grows inwards from the lips of the blastopore as a pair
of bands. In these cases it may originate (a) from cells which
are clearly hypoblastic, (b} from cells which are clearly epiblastic,
(c) from cells which cannot be regarded as belonging to either
layer.
 
Mollusca. Gasteropoda, Cephalopoda, and Lamellibranchiata. In
Gasteropoda and Lamellibranchiata the mesoblast sometimes originates
 
 
 
352
 
 
 
DEVELOPMENT OF THE MESOBLAST.
 
 
 
from a pair of cells at the lips of the blastopore, though very probably some
of the elements subsequently come from the epiblast ; and in Cephalopoda
it begins as a ring of cells round the edge of the blastoderm.
 
Polyzoa Entoprocta. It originates from a pair of cells at the lips of the
blastopore.
 
Chaetopoda. Euaxes. It arises as a ridge of cells at the lips of the
blastopore (fig. 200).
 
Gephyrea. Bonellia. It arises (fig. 206) as an infolding of the epiblastic lips of the blastopore.
 
Nematelminthes. Cucullanus. It grows backwards from the hypoblast
cells at the persistent oral opening of the blastopore.
 
Tracheata. Insecta. It grows inwards from the lips of the germinal
groove (fig. 207), which probably represent the remains of a blastopore.
Part of the mesoblast is probably also derived from the yolk-cells. A similar
though more modified development of the mesoblast occurs in the Araneina
(fig. 214).
 
Crustacea. Decapoda. It partly grows in from the hypoblastic lips of
the blastopore, and is partly derived from the yolk-cells (fig. 208).
 
 
 
 
FIG. tog. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A. and C.
 
after Biitschli, and B. after Kowalevsky.)
The three embryos are represented in the same positions.
 
A. Represents the gastrula stage.
 
B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.
 
C. Represents a later stage, in which the mouth involution (m) has become continuous with the alimentary tract, and the blastopore has become closed.
 
m. mouth; al. alimentary canal; ae. archenteron; bl.p. blastopore; pv. perivisceral cavity; sp. splanchnic mesoblast; so. somatic mesoblast; ge. generative
organs.
 
2. The mesoblast is developed from the walls of hollow
outgrowths of the archenteron, the cavities of which become
the body cavity.
 
 
 
 
 
 
ORIGIN OF THE GERMINAL LAYERS.
 
 
 
353
 
 
 
Brachiopoda. The walls of a pair of outgrowths form the whole of the
mesoblast.
 
Chaetognatha. The mesoblast arises in the same manner as in the
Brachiopoda (fig. 209).
 
Echinodermata. The lining of the peritoneal cavity is developed from
the walls of outgrowths of the archenteron, but the greater part of the mesoblast is derived from the amoeboid cells budded off from the walls of the
archenteron (fig. 210).
 
 
 
 
ME
 
 
 
Mp. Pld.
 
FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA
DOL1OLUM AT THE -END OF THE FOURTH DAY.
 
Vpv. vaso-peritoneal vesicle; ME. mesenteron; Sip., Ptd. blastopore, proctodjeum.
 
Enteropneusta (Balanoglossus). The body cavity is derived from two
pairs of alimentary diverticula, the walls of which give rise to the greater
part of the mesoblast.
 
Chordata. Paired archenteric outgrowths give rise to the whole mesoblast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in
other Chordata is probably secondarily derived from this.
 
3. The cells which will form the mesoblast become marked
out very early, and cannot be regarded as definitely springing
from either of the primary layers.
 
Turbellaria. Leptoplana (fig. 212), Planaria polychroa (?).
 
Chsetopoda. Lumbricus, &c.
 
Discophora.
 
It is very possible that the cases quoted under this head ought more
properly to belong to group i.
 
4. The mesoblast cells are split off from the epiblast.
 
Nemertea. Larva of Desor. The mesoblast is stated to be split off
from the four invaginated discs.
 
B. III. 23
 
 
 
354
 
 
 
DEVELOPMENT OF THE MESOBLAST.
 
 
 
5. The mcsoblast is split off from the hypoblast.
 
Nemertea. Some of the types without a metamorphosis.
 
Mollusca. Scaphopoda. It is derived from the lateral and ventral cells
of the hypoblast.
 
Oephyrea. Phascolosoma.
 
Vertebrata. In most of the Ichthyopsida the mesoblast is derived from
the hypoblast (fig. 213). In some types (i.e. most of the Amniota) the mesoblast might be described as originating at the lips of the blastopore (primitive streak).
 
6. The mesoblast is derived from both germinal layers.
 
Tracheata. Araneina (fig. 214). It is derived partly from cells split off
from the epiblast and partly from the yolk-cells ; but it is probable that the
statement that the mesoblast is derived from both the germinal layers is
only formally accurate ; and that the derivation of part of the mesoblast
from the yolk-cells is not to be interpreted as a derivation from the
hypoblast.
 
Amniota. The derivation of the mesoblast of the Amniota from both
the primary germinal layers is without doubt a secondary process.
 
The conclusions to be drawn from the above summary are by
no means such as might have been anticipated. The analogy
of the Ccelenterata would lead us to expect that the mesoblast
 
 
 
 
FIG. 211.
 
 
 
A.
B.
 
 
 
SECTIONS OF AN AMPH'IOXUS EMBRYO AT TIIRKE STAGES.
 
(After Kowalevsky.)
Section at gastrula stage.
Section of a somewhat older embryo.
C. Section through the anterior part of still older embryo.
np. neural plate; nc. neural canal; mcs. archenteron in A, and mesenteron in B
and C ; ch. notochord ; so. mesoblastic somite.
 
would be derived partly from the epiblast and partly from the
hypoblast. Such, however, is not for the most part the case,
though more complete investigations may shew that there are a
greater number of instances in which the mesoblast has a mixed
origin than might be supposed from the above summary.
 
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 355
 
I have attempted to reduce the types of development of the
mesoblast to six ; but owing to the nature of the case it is not
always easy to distinguish the first of these from the last fourOf the six types the second will on most hands be admitted to
be the most remarkable. The formation of hollow outgrowths
of the archenteron, the cavities of which give rise to the body
cavity, can only be explained on the supposition that the body
cavity of the types in which such outgrowths occur is derived
from diverticula cut off from the alimentary tract. The lining
epithelium of the diverticula the peritoneal epithelium is
clearly part of the primitive hypoblast, and this part of the
mesoblast is clearly hypoblastic in origin.
 
 
 
 
 
FIG. 112. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN
 
THREE STAGES OF DEVELOPMENT. (After Hallez.)
cp. epiblast ; ;;/. mesoblast; hy. yolk-cells (hypoblast); bl. blastopore.
 
In the case of the Chaetognatha (Sagitta), Brachiopoda, and
Amphioxus, the whole of the mesoblast originates from the walls
of the diverticula ; while in the Echinodermata the walls of the
diverticula only give rise to the vaso-peritoneal epithelium, the
remainder of the mesoblast being derived from amoeboid cells
which spring from the walls of the archenteron before the origin
of the vaso-peritoneal outgrowths (figs. 199 and 210).
 
Reserving for the moment the question as to what conclusions can be deduced from the above facts as to the origin of the
mesoblast, it is important to determine how far the facts of
embryology warrant us in supposing that in the whole of the
triploblastic forms the body cavity originated from the alimentary
diverticula. There can be but little doubt that the mode of
origin of the mesoblast in many Vertebrata, as two solid plates
split off from the hypoblast, in which a cavity is secondarily
developed, is an abbreviation of the process observable in
Amphioxus ; but this process approaches in some forms of
 
232
 
 
 
356
 
 
 
ORIGIN OF THE MESOBLAST.
 
 
 
 
 
Vertebrata to the ingrowth of the mesoblast from the lips of the
blastopore.
 
It is, therefore, highly _ en A.
 
probable that the paired ingrowths of the mesoblast from
the lips of the blastopore may
have been in the first instance
derived from a pair of archenteric diverticula. This process of formation of the mesoblast is, as may be seen by
reference to the summary,
the most frequent, including
as it does the Chaetopoda,
 
the Mollusca, the Arthro- FIG. 213. Two SECTIONS OF A YOUNG
101 ELASMOBRANCH EMBRYO, TO SHEW THE
 
pOda, &C. MESOBLAST SPLIT OFF AS TWO LATERAL
 
MASSES FROM THE HYPOBLAST.
 
While there is no difficulty in
the view that the body cavity may
have originated from a pair of enteric diverticula in the case of the
forms where a body cavity is present, there is a considerable difficulty in
holding this view, for forms in which there is no body cavity distinct from
the alimentary diverticula.
 
Of these types the Platyelminthes are the most striking. It is, no doubt,
possible that a body cavity may have existed in the Platyelminthes, and
become lost ; and the case of the Discophora, which in their muscular and
connective tissue systems as well as in the absence of a body cavity resemble
the Platyelminthes, may be cited in favour of this view, in that, being closely
related to the Chaetopoda, they are almost certainly descended from ancestors
with a true body cavity. The usual view of the primitive character of the
 
 
 
nig. medullary groove ; ep. epiblast ; ;;/.
mesoblast ; hy. hypoblast ; n.al. cells formed
around the nuclei of the yolk which have
entered the hypoblast.
 
 
 
1 The wide occurrence of this process was first pointed out by Rabl. He holds,
however, a peculiar modification of the gastrsea theory, for which I must refer the
reader to his paper (No. 284) ; according to this theory the mesoblast has sprung
from a zone of cells of the blastosphere, at the junction between the cells which will
be invaginated and the epiblast cells. In the bilateral blastosphere, from which he
holds that all the higher forms (Bilateralia) have originated, these cells had a
bilateral arrangement, and thus the bilateral origin of the mesoblast is explained.
The origin of the mesoblast from the lips of the blastopore is explained by the
position of its mother-cells in the blastosphere. It need scarcely be said that the
views already put forward as to the probable mode of origin of the mesoblast,
founded on the analogy of the Ccelenterata, are quite incompatible with Rabl's
theories.
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 357
 
Platyelminthes, which has much to support it, is, however, opposed to the
idea that the body cavity has disappeared.
 
If Kowalevsky 1 is right in stating that he has found a form intermediate
between the Ccelenterata and the Platyelminthes, there will be strong grounds
for holding that the Platyelminthes are, like the Ccelenterata, forms the
ancestors of which were not provided with a body cavity.
 
Perhaps the triploblastica are composed of two groups, viz. (i) a more
ancestral group (the Platyelminthes), in which there is no body cavity as dis
 
 
 
FIG. 214. SECTION THROUGH AN EMBRYO OK AGEI.ENA LABYRINTHICA.
The section is represented with the ventral plate upwards. In the ventral plate
is seen a keel-like thickening, which gives rise to the main mass of the mesoblast.
yk. yolk divided into large polygonal cells, in several of which are nuclei.
 
tinct from the alimentary, and (2) a group descended from these, in which
two of the alimentary diverticula have become separated from the alimentary tract to form a body cavity (remaining triploblastica). However this
may be, the above considerations are sufficient to shew how much there is
that is still obscure with reference even to the body cavity.
 
If embryology gives no certain sound as to the questions just
raised with reference to the body cavity, still less is it to be
hoped that the remaining questions with reference to the origin
of the mesoblast can be satisfactorily answered. It is clear, in
the first place, from an inspection of the summary given above,
that the process of development of the mesoblast is, in all the
higher forms, very much abbreviated and modified. Not only is
its differentiation relatively deferred, but it does not in most
cases originate, as it must have done to start with, as a more or
 
1 Zoologischer Anzeiger, No. 52, p. 140. This form has been named by Kowalevsky Cceloplana Metschnikowii. Kowalevsky's description appears, however, to be
quite compatible with the view that this form is a creeping Ctenophor, in no way
related to the Turbellarians.
 
 
 
358 EVOLUTION OK THE MESOBLAST.
 
 
 
less continuous sheet, split off from parts of one or both the
primary layers. It originates in most cases from the hypoblast,
and although the considerations already urged preclude us from
laying very great stress on this mode of origin, yet the derivation of the mesoblast from the walls of archenteric outgrowths
suggests the view that the whole, or at any rate the greater part,
of the mesoblast primitively arose by a process of histogenic
differentiation from the walls of the archenteron or rather from
diverticula of these walls. This view, which was originally put
forward by myself (No. 260), appears at first sight very
improbable, but if the statement of the Hertwigs (No. 270), that
there is a large development of a hypoblastic muscular system
in the Actinozoa, is well founded, it cannot be rejected as
impossible. Lankester (No. 279), on the other hand, has urged
that the mode of origin of the mesoblast in the Echinodermata
is more primitive ; and that the amoeboid cells which here give
rise to the muscular and connective tissues represent cells which
originally arose from the whole inner surface of the epiblast. It
is, however, to be noted that even in the Echinodermata the
amoeboid cells actually arise from the hypoblast, and their mode
of origin may, therefore, be used to support the view that the
main part of the muscular system of higher types is derived
from the primitive hypoblast.
 
The great changes which have taken place in the development of the mesoblast would be more intelligible on this view
than on the view that the major part of the mesoblast primitively
originated from the epiblast. The presence of food-yolk is
much more frequent in the hypoblast than in the epiblast ; and
it is well known that a large number of the changes in early
development are caused by food-yolk. If, therefore, the mesoblast has been derived from the hypoblast, many more changes
might be expected to have been introduced into its early
development than if it had been derived from the epiblast. At
the same time the hypoblastic origin of the mesoblast would
assist in explaining how it has come about that the development
of the nervous system is almost always much less modified than
that of the mesoblast, and that the nervous system is not, as
might, on the grounds of analogy, have been anticipated, as a
rule secondarily developed in the mesoblast.
 
 
 
 
 
 
ORIGIN OF THE GERMINAL LAYERS. 359
 
The Hertwigs have recently suggested in their very interesting memoir
(No. 271) that the Triploblastica are to be divided into two phyla, (i)
the Enteroccela, and (2) the Pseudocoela ; the former group containing the
Chaetopoda, Gephyrea, Brachiopoda, Nematoda, Arthropoda, Echinodermata, Enteropneusta and Chordata ; and the latter the Mollusca, Polyzoa,
the Rotifera, and Platyelminthes.
 
The Enteroccela are forms in which the primitive alimentary diverticula
have given origin to the body cavity, while the major part of the muscular
system has originated from the epithelial walls of these diverticula, part
however being in many cases also derived from the amoeboid cells, called by
them mesenchyme, by the second process of mesoblastic differentiation mentioned on p. 347.
 
In the Pseudoccela the muscular system has become differentiated from
mesenchyme cells ; while the body cavity, where it exists, is merely a split in
the mesenchyme.
 
It is impossible for me to attempt in this place to state fully, or do
justice to, the original and suggestive views contained in this paper. The
general conclusion I cannot however accept. The views of the Hertwigs
depend to a large extent upon the supposition that it is possible to distinguish histologically muscle cells derived from epithelial cells, from those
derived from mesenchyme cells. That in many cases, and strikingly so in
the Chordata, the muscle cells retain clear indications of their primitive
origin from epithelial cells, I freely admit ; but I do not believe either that
its histological character can ever be conclusive as to the non-epithelial
origin of a muscle cell, or that its derivation in the embryo from an indifferent amoeboid cell is any proof that it did not, to start with, originate from an
epithelial cell.
 
I hold, as is clear from the preceding statements, that such immense
secondary modifications have taken place in the development of the mesoblast, that no such definite conclusions can be deduced from its mode
of development as the Hertwigs suppose.
 
In support of the view that the early character of embryonic cells is no
safe index as to their phylogenetic origin, I would point to the few following
facts.
 
(1) In the Porifera and many of the Ccelenterata (Eucope polystyla,
Geryonia, &c.) the hypoblast (endoderm) originates from cells, which according to the Hertwigs' views ought to be classed as mesenchyme.
 
(2) In numerous instances muscles which have, phylogenetically, an
undoubted epithelial origin, are ontogenetically derived from cells which
ought to be classed as mesenchyme. The muscles of the head in all the
higher Vertebrata, in which the head cavities have disappeared, are examples
of this kind ; the muscles of many of the Tracheata, notably the Araneina,
must also be placed in the same category.
 
(3) The Mollusca are considered by the Hertwigs to be typical Pseudocoela. A critical examination of the early development of the mesoblast in
these forms demonstrates however that with reference to the mesoblast they
 
 
 
360 FCETAL AND LARVAL DEVELOPMENT.
 
must be classed in the same group as the Cluetopoda. The mesoblast (Vol.
II. p. 227) clearly originates as two bands of cells which grow inwards from
the blastopore, and in some forms (Paludina, Vol. II. fig. 107) become divided
into a splanchnic and somatic layer, with a body cavity between them. All
these processes are such as are, in other instances, admitted to indicate
Enteroccclous affinities.
 
The subsequent conversion of the mesoblast elements into amoeboid cells,
out of which branched muscles are formed, is in my opinion simply due
to the envelopment of the soft Molluscan body within a hard shell.
 
In addition to these instances I may point out that the distinction between the Pseudocoela and Enteroccela utterly breaks down in the case
of the Discophora, and the Hertwigs have made no serious attempt to
discuss the characters of this group in the light of their theory, and that the
derivation of the Echinoderm muscles from mesenchyme cells is a difficulty
which is very slightly treated.
 
 
 
II. LARVAL FORMS: THEIR NATURE, ORIGIN AND AFFINITIES.
 
Preliminary considerations. In a general way two types
of development may be distinguished, viz. a foetal type and a
larval type. In the foetal type animals undergo the whole or
nearly the whole of their development within the egg or within
the body of the parent, and are hatched in a condition closely
resembling the adult ; and in the larval type they are born at an
earlier stage of development, in a condition differing to a greater
or less extent from the adult, and reach the adult state either
by a series of small steps, or by a more or less considerable
metamorphosis.
 
The satisfactory application of embryological data to morphology depends upon a knowledge of the extent to which the
record of ancestral history has been preserved in development.
Unless secondary changes intervened this record would be complete ; it becomes therefore of the first importance to the
cmbryologist to study the nature and extent of the secondary
changes likely to occur in the fcetal or the larval state.
 
The principles which govern the perpetuation of variations
which occur in either the larval or the fcetal state are the same
as those for the adult condition. Variations favourable to the
survival of the species are equally likely to be perpetuated, at
whatever period of life they occur, prior to the loss of the reproductive powers. The possible nature and extent of the
 
 
 
LARVAL FORMS. 361
 
 
 
secondary changes which may have occurred in the developmental history of forms, which have either a long larval existence,
or which are born in a nearly complete condition, is primarily
determined by the nature of the favourable variations which can
occur in each case.
 
Where the development is a fcetal one, the favourable variations which can most easily occur are (i) abbreviations, (2) an
increase in the amount of food-yolk stored up for the use of the
developing embryo. Abbreviations take place because direct
development is always simpler, and therefore more advantageous;
and, owing to the fact of the foetus not being required to lead an
independent existence till birth, and of its being in the meantime nourished by food-yolk, or directly by the parent, there are
no physiological causes to prevent the characters of any stage of
the development, which are of functional importance during a
free but not during a fcetal existence, from, disappearing from the
developmental history. All organs of locomotion and nutrition
not required by the adult will, for this reason, obviously have a
tendency to disappear or to be reduced in foetal developments;
and a little consideration will shew that the ancestral stages in
the development of the nervous and muscular systems, organs
of sense, and digestive system will be liable to drop out or be
modified, when a simplification can thereby be effected. The
circulatory and excretory systems will not be modified to the
same extent, because both of them are usually functional during
fcetal life.
 
The mechanical effects of food-yolk are very considerable,
and numerous instances of its influence will be found in the
earlier chapters of this work 1 . It mainly affects the early stages
of development, i.e. the form of the gastrula, &c.
 
The favourable variations which may occur in the free larva
are much less limited than those which can occur in the fcetus.
Secondary characters are therefore very numerous in larvae, and
there may even be larvae with secondary characters only, as, for
instance, the larvae of Insects.
 
In spite of the liability of larvae to acquire secondary characters, there is a powerful counterbalancing influence tending
 
1 For numerous instances of this kind, vide Chapter XI. of Vol. in.
 
 
 
362 METAL AND LARVAL DEVELOPMENT.
 
towards the preservation of ancestral characters, in that larvae
are necessarily compelled at all stages of their growth to retain
in a functional state such systems of organs, at any rate, as are
essential for a free and independent existence. It thus comes
about that, in spite of the many causes tending to produce
secondary changes in larvae, there is always a better chance of
larvae repeating, in an unabbreviated form, their ancestral history,
than is the case with embryos, which undergo their development
within the egg.
 
It may be further noted as a fact which favours the relative
retention by larvae of ancestral characters, that a secondary
larval stage is less likely to be repeated in development than an
ancestral stage, because there is always a strong tendency for
the former, which is a secondarily intercalated link in the chain
of development, to drop out by the occurrence of a reversion to
the original type of development.
 
The relative chances of the ancestral history being preserved
in the foetus or the larva may be summed up in the following
way : There is a greater chance of the ancestral history being
lost in forms which develop in the egg ; and of its being masked
in those which are hatched as larvae.
 
The evidence from existing forms undoubtedly confirms the
a priori considerations just urged 1 . This is well shewn by a
study of the development of Echinodermata, Nemertea, Mollusca,
Crustacea, and Tunicata. The free larvae of the four first groups
are more similar amongst themselves than the embryos which
develop directly, and since this similarity cannot be supposed to
be due to the larvae having been modified by living under precisely similar conditions, it must be due to their retaining
common ancestral characters. In the case of the Tunicata the
free larvae retain much more completely than the embryos
certain characters such as the notochord, the cerebrospinal
canal, etc., which are known to be ancestral.
 
1 It has long been known that land and freshwater forms develop without a
metamorphosis much more frequently than marine forms. This is probably to be
explained by the fact that there is not the same possibility of a land or freshwater
species extending itself over a wide area by the agency of free larvre, and there k,
therefore, much less advantage in the existence of such larva:; while the fact of such
larviu being more liable to be preyed upon than eggs, which are either concealed, or
carried about by the parent, might render a larval stage absolutely disadvantageous.
 
 
 
LARVAL FORMS. 363
 
 
 
Types of Larvae. Although there is no reason to suppose
that all larval forms are ancestral, yet it seems reasonable to
anticipate that a certain number of the known types of larvae
would retain the characters of the ancestors of the more important phyla of the animal kingdom.
 
Before examining in detail the claims of various larvae to
such a character, it is necessary to consider somewhat more at
length the kind of variations which are most likely to occur in
larval forms.
 
It is probable a priori that there are two kinds of larvae,
which may be distinguished as primary and secondary larvae.
Primary larvae are more or less modified ancestral forms, which
have continued uninterruptedly to develop as free larvae from
the time when they constituted the adult form of the species.
Secondary larvae are those which have become introduced into
the ontogeny of species, the young of which were originally
hatched with all the characters of the adult; such secondary
larvae may have originated from a diminution of food-yolk in
the egg and a consequently earlier commencement of a free
existence, or from a simple adaptive modification in the just
hatched young. Secondary larval forms may resemble the
primary larval forms in cases where the ancestral characters were
retained by the embryo in its development within the egg; but
in other instances their characters are probably entirely adaptive.
 
Causes tending to produce secondary changes in larv<z. The
modes of action of natural selection on larvae may probably be
divided more or less artificially into two classes.
 
1. The changes in development directly produced by the
existence of a larval stage.
 
2. The adaptive changes in a larva acquired in the ordinary
course of the struggle for existence.
 
The changes which come under the first head consist essentially in a displacement in the order of development of certain
organs. There is always a tendency in development to throw
back the differentiation of the embryonic cells into definite
tissues to as late a date as possible. This takes place in order
to enable the changes of form, which every organ undergoes, in
repeating even in an abbreviated way its phylogenetic history,
to be effected with the least expenditure of energy. Owing to
 
 
 
364 CHANGES IN LARWE.
 
this tendency it comes about that when an organism is hatched
as a larva many of the organs are still in an undifferentiated
state, although the ancestral form which this larva represents
had all its organs fully differentiated. In order, however, that
the larva may be enabled to exist as an independent organism,
certain sets of organs, e.g. the muscular, nervous, and digestive
systems, have to be histologically differentiated. If the period
of foetal life is shortened, an earlier differentiation of certain
organs is a necessary consequence ; and in almost all cases the
existence of a larval stage causes a displacement in order of
development of organs, the complete differentiation of many
organs being retarded relatively to the muscular, nervous, and
digestive systems.
 
The possible changes under the second head appear to be
unlimited. There is, so far as I see, no possible reason why an
indefinite number of organs should not be developed in larvae to
protect them from their enemies, and to enable them to compete with larvae of other species, and so on. The only limit to
such development appears to be the shortness of larval life,
which is not likely to be prolonged, since, ceteris paribus, the
more quickly maturity is reached the better it is for the species.
 
A very superficial examination of marine larvae shews that
there are certain peculiarities common to most of them, and it is
important to determine how far such peculiarities are to be
regarded as adaptive. Almost all marine larvae are provided
with well-developed organs of locomotion, and transparent
bodies. These two features are precisely those which it is most
essential for such larvae to have. Organs of locomotion are
important, in order that larvae may be scattered as widely as
possible, and so disseminate the species ; and transparency is
very important in rendering larvae invisible, and so less liable to
be preyed upon by their numerous enemies 1 .
 
These considerations, coupled with the fact that almost all
free-swimming animals, which have not other special means of
protection, are transparent, seem to shew that the transparency
 
1 The phosphorescence of many larvze is very peculiar. I should have anticipated
that phosphorescence would have rendered them much more liable to be captured by
the forms which feed upon them; and it is difficult to see of what advantage it can be
to them.
 
 
 
LARVAL FORMS. 365
 
 
 
of larvae at all events is adaptive ; and it is probable that organs
of locomotion are in many cases specially developed, and not
ancestral.
 
Various spinous processes on the larvae of Crustacea and
Teleostei are also examples of secondarily acquired protective
organs.
 
These general considerations are sufficient to form a basis for
the discussion of the characters of the known types of larvae.
 
The following table contains a list of the more important of
such larval forms :
 
DICYEMID^. The Infusoriform larva (vol. n. fig. 62).
 
PORIFERA. (a) The Amphiblastula larva (fig. 215), with one-half of the body
ciliated, and the other half without cilia; (b) an oval uniformly ciliated larva, which
may be either solid or have the form of a vesicle.
 
CCELENTERATA. The planula (fig. 216).
 
TURBELLARIA. (a) The eight-lobed larva of Miiller (fig. 222); (b) the larvae of
Gotte and Metschnikoff, with some Pilidium characters.
 
NEMERTEA. The Pilidium (fig. 221).
 
TREMATODA. The Cercaria.
 
ROTIFERA. The Trochosphere-like larvae of Brachionus (fig. 217) and Lacinularia.
 
MOLLUSCA. The Trochosphere larva (fig. 218), and the subsequent Veliger larva
(fig. 219).
 
BRACHIOPODA. The three-lobed larva, with a postoral ring of cilia (fig. 220).
 
POLYZOA. A larval form with a single ciliated ring surrounding the mouth, and
an aboral ciliated ring or disc (fig. 228).
 
CH/ETOPODA. Various larval forms with many characters like those of the
molluscan Trochosphere, frequently with distinct transverse bands of cilia. They are
classified as Atrochoe, Mesotrochse, Telotrochse (fig. 225 A and fig. 226), Polytrochae,
and Monotrochae (fig. 225 B).
 
GEPHYREA NUDA. Larval forms like those of preceding groups. A specially
characteristic larva is that of Echiurus (fig. 227).
 
GEPHYREA TUBICOLA. Actinotrocha (fig. 230), with a postoral ciliated ring of
arms.
 
MYRIAPODA. A functionally hexapodous larval form is common to all the
Chilognatha (vol. n. fig. 174).
 
INSECTA. Various secondary larval forms.
 
CRUSTACEA. The Nauplius (vol. n. fig. 208) and the Zosea (vol. II. fig. 210).
 
ECHINODERMATA. The Auricularia (fig. 223 A), the Bipinnaria (fig. 223 B), and
the Pluteus (fig. 224), and the transversely-ringed larvae of Crinoidea (vol. II. fig. 268).
The three first of which can be reduced to a common type (fig. 231 c).
 
ENTEROPNEUSTA. Tornaria (fig. 229).
 
UROCHORDA (TUNICATA). The tadpole-like larva (vol. in. fig. 8).
 
GANOIDEI. A larva with a disc with adhesive papillae in front of the mouth
(vol. in. fig. 67).
 
ANUROUS AMPHIBIA. The tadpole (vol. in. fig. 80).
 
 
 
366
 
 
 
TYPES OF LARVAE.
 
 
 
Of the larval forms included in the above list a certain
 
 
 
 
en
 
 
 
e.g.
 
 
 
FlG. 21=1. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS.
 
(After Schultze. )
 
A. Amphiblastula stage.
 
B. Stage after the ciliated cells have commenced to be invaginated.
 
c.s. segmentation cavity; ec. granular epiblast cells; en. ciliated hypoblast cells.
 
number are probably without affinities outside the group to
which they belong. This is the case with the larvae of the
 
C
 
 
 
 
FIG. 216. THREE LARVAL STAGES OF EUCOPE POLYSTYI.A. (After Kowalevsky.)
 
A. Blastosphere stage with hypoblast spheres becoming budded into the central
cavity.
 
B. Planula stage with solid hypoblast.
 
C. I'lanula stage with a gastric cavity.
 
</>. epiblast: hy. hypoblast; al. gastric cavity.
 
 
 
LARVAL FORMS.
 
 
 
367
 
 
 
Myriapoda, the Crustacean lame, and with the larval forms of
the Chordata. I shall leave these forms out of consideration.
 
There are, again, some larval forms which may possibly turn
out hereafter to be of importance, but from which, in the present
state of our knowledge, we cannot draw any conclusions. The
infusoriform larva of the Dicyemidse, and the Cercaria of the
Trematodes, are such forms.
 
Excluding these and certain other forms, we have finally left
for consideration the larvae of the Ccelenterata, the Turbellaria,
the Rotifera, the Nemertea, the Mollusca, the Polyzoa, the
Brachiopoda, the Chaetopoda, the Gephyrea, the Echinodermata,
and the Enteropneusta.
 
The larvae of these forms can be divided into two groups.
The one group contains the larva of the Ccelenterata or Planula,
the other group the larvae of all the other forms.
 
The Planula (fig. 216) is characterised by its extreme simplicity. It is a two-layered
organism, with a form varying
from cylindrical to oval, and
usually a radial symmetry. So
long as it remains free it is not
usually provided with a mouth,
and it is as yet uncertain whether
or no the absence of a mouth is
to be regarded as an ancestral
character. The Planula is very
probably the ancestral form of
the Ccelenterata.
 
The larvae of almost all the
other groups, although they may
be subdivided into a series of
very distinct types, yet agree in
the possession of certain common
characters 1 . There is a more or
less dome-shaped dorsal surface,
and a flattened or concave ventral surface, containing the open
1 The larva of the Brachiopoda does not possess most of the characters mentioned
below. It is probably, all the same, a highly differentiated larval form belonging to
this group.
 
 
 
 
Id
 
 
 
ov
 
 
 
FIG. 217. EMBRYO OF BRACHIONUS URCEOLARIS, SHORTLY BEFORE
IT is HATCHED. (After Salensky.)
 
m. mouth ; ms. masticatory apparatus ; me. mesenteron ; an. anus ;
Id. lateral gland ; ov. ovary ; t. tail
(foot) ; tr. trochal disc ; sg. supraoesophageal ganglion.
 
 
 
368 LARWE OF THE TRIPLOBLASTICA.
 
ing of the mouth, and usually extending posteriorly to the
opening of the anus, when such is present.
 
The dorsal dome is continued in front of the mouth to form
a large prceoral lobe.
 
There is usually present at first an uniform covering of cilia ;
but in the later larval stages there are almost always formed
definite bands or rings of long cilia, by which locomotion is
effected. These bands are often produced into arm-like processes.
 
The alimentary canal has, typically, the form of a bent tube
with a ventral concavity, constituted (when an anus is present)
 
 
 
 
FIG. 218. DIAGRAM OF AN EMBRYO OF PLEUROBRANCHIDIUM.
 
(From Lankester.)
 
/. foot; ol. otocyst; m. mouth; v. velum; ng. nerve ganglion; ry. residual yolk
spheres; sAs. shell-gland; i. intestine.
 
of three sections, viz. an oesophagus, a stomach, and a rectum.
The oesophagus and sometimes the rectum are epiblastic in
origin, while the stomach always and the rectum usually are
derived from the hypoblast 1 .
 
To the above characters may be added a glass-like transparency ; and the presence of a widish space possibly filled with
gelatinous tissue, and often traversed by contractile cells,
between the alimentary tract and the body wall.
 
1 There is some uncertainty as to the development of the oesophagus in the
Echinodermata, but recent researches appear to indicate that it is developed from the
hypoblast.
 
 
 
LARVAL FORMS.
 
 
 
369
 
 
 
Considering the very profound differences which exist
between many of these larvae, it may seem that the characters
just enumerated are hardly sufficient to justify my grouping
them together. It is, however, to be borne in mind that my
grounds for doing so depend quite as much upon the fact that
A B
 
 
 
 
FIG. 219. LARVAE OF CEPHALOPHOROUS MOLLUSCA IN THE VELIGER STAGE.
(From Gegenbaur.)
 
A. and B. Earlier and later stage of Gasteropod. C. Pteropod (Cymbulia).
v. velum; c. shell; /. foot; op. operculum ; t. tentacle.
 
they constitute a series without any great breaks in it, as upon
the existence of characters common to
the whole of them. It is also worth
noting that most of the characters which
have been enumerated as common to the
whole of these larvae are not such secondary characters as (in accordance with the
considerations used above) might be expected to arise from the fact of their
being subjected to nearly similar conditions of life. Their transparency is, no
doubt, such a secondary character, and it
is not impossible that the existence of
ciliated bands may be so also ; but it is
quite possible that if, as I suppose, these
larvae reproduce the characters of some
ancestral form, this form may have
existed at a time when all marine
animals were free-swimming, and that it
may, therefore, have been provided with at least one ciliated
band.
 
 
 
 
FIG. 220. LARVA OF
ARGIOPE. (From Gegenbaur ; after Kowalevsky.)
 
m. mantle ; b. setre ;
d. archenteron.
 
 
 
B. III.
 
 
 
2 4
 
 
 
370 THE ECHINODERM GROUP.
 
The detailed consideration of the characters of these larvae,
given below, supports this view.
 
This great class of larvae may, as already stated, be divided
into a series of minor subdivisions. These subdivisions are the
following :
 
1. The Pilidium Group. This group is characterised by
the mouth being situated nearly in the centre of the ventral
surface, and by the absence of an anus. It includes the Pilidium
 
 
 
 
FlG. 221. TWO STAGES IN THE DEVELOPMENT OF PlLIDIUM.
 
(After Metschnikoff.)
 
ae. archenteron; oe. oesophagus; st. stomach; am. amnion; pr.d. prostomial
disc ; pod. metastomial disc ; c.s. cephalic sack (lateral pit).
 
of the Nemertines (fig. 221), and the various larvae of marine
Dendrocoela (fig. 222). At the apex of the praeoral lobe a
thickening of epiblast may be present, from which (fig. 232) a
contractile cord sometimes passes to the oesophagus.
 
2. The Echinoderm Group. This group (figs. 223, 224
and 231 C) is characterised by the presence of a longitudinal
pastoral band of cilia, by the absence of special sense organs in
the praeoral region, and by the development of the body cavity
as an outgrowth of the alimentary tract. The three typical
divisions of the alimentary tract are present, and there is a more
or less developed praeoral lobe. This group only includes the
larvae of the Echinodermata.
 
 
 
LARVAL FORMS. 371
 
 
 
3. The Trochosphere Group. This group (figs. 225, 226)
is characterised by the presence of a praeoral ring of long cilia,
the region in front of which forms a great part of the praeoral
lobe. The mouth opens immediately behind the praeoral ring
of cilia, and there is very often a second ring of short cilia
parallel to the main ring, immediately behind the mouth. The
 
B.
 
 
 
 
 
FIG. 222. A. LARVA OF EURYLEPTA AURICULATA IMMEDIATELY AFTER
HATCHING. VIEWED FROM THE SIDE. (After Hallez.) m. mouth.
 
B. MULLER'S TURBELLARIAN LARVA (PROBABLY THYSANOZOON). VIEWED
FROM THE VENTRAL SURFACE. (After Muller.) The ciliated band is represented by
the black line. m. mouth ; u.l. upper lip.
 
function of the ring of short cilia is nutritive, in that its cilia are
employed in bringing food to the mouth ; while the function of
the main ring is locomotive. A perianal patch or ring of cilia is
often present (fig. 225 A), and in many forms intermediate rings
are developed between the praeoral and perianal rings.
 
The praeoral lobe is usually the seat of a special thickening
of epiblast, which gives rise to the supra-cesophageal ganglion of
the adult. On this lobe optic organs are very often developed
in connection with the supra-oesophageal ganglion, and a contractile band frequently passes from this region to the oesophagus.
 
The alimentary tract is formed of the three typical divisions.
 
The body cavity is not developed directly as an outgrowth
of the alimentary tract, though the process by which it originates
is very probably secondarily modified from a pair of alimentary
outgrowths.
 
24 2
 
 
 
372
 
 
 
TORNARIA.
 
 
 
Paired excretory organs, opening to the exterior and into the
body cavity, are often present (fig. 226 nph}.
 
This type of larva is found in the Rotifera (fig. 217) (in which
it is preserved in the adult state), the Chaetopoda (figs. 225 and
226), the Mollusca (fig. 218), the Gephyrea nuda (fig. 227), and
the Polyzoa (fig. 228)'.
 
 
 
 
FIG. 223. A. THE LARVA OF A HOLOTHUROID.
B. THE LARVA OF AN ASTEROID.
 
m. mouth; si. stomach; a. anus; I.e. primitive longitudinal ciliated band; pr.c.
pneoral ciliated band.
 
4. Tornaria. This larva (fig. 229) is intermediate in most
of its characters between the larvae of the Echinodermata (more
especially the Bipinnaria) and
the Trochosphere. It resembles
Echinoderm larvae in the possession of a longitudinal ciliated
band (divided into a praeoral
and a postoral ring), and in the
derivation of the body cavity
and water-vascular vesicle from
alimentary diverticula ; and it
resembles the Trochosphere in
the presence of sense organs on
the praeoral lobe, in the existence
of a perianal ring of cilia, and in
the possession of a contractile
band passing from the praeoral lobe to the oesophagus.
 
 
 
 
FIG. 224.
LOCENTRUS.
 
m. mouth :
d. stomach ;
 
 
 
A LARVA OF STROXGY(From Agassiz.)
 
a. anus ; o. oesophagus ;
c. intestine ; v ' . and v.
 
 
 
ciliated ridges ; w. water- vascular tube ;
r. calcareous rods.
 
 
 
1 For a discussion as to the structure of the Polyzoon larva, vide Vol. II. p. 305.
 
 
 
LARVAL FORMS. 373
 
 
 
5. Actinotrocha. The remarkable larva of Phoronis (fig.
230), known as Actinotrocha, is characterised by the presence of
(i) a postoral and somewhat longitudinal ciliated ring produced
into tentacles, and (2) a perianal ring. It is provided with a
prseoral lobe, and a terminal or somewhat dorsal anus.
 
6. The larva of the Brachiopoda articulata (fig. 220).
The relationships of the six types of larval forms thus briefly
 
characterised have been the subject of a considerable amount of
controversy, and the following suggestions on their affinities
must be viewed as somewhat speculative. The Pilidium type of
larva is in some important respects less highly differentiated
 
 
 
 
FIG. 225. Two CH^TOPOD LARWE. (From Gegenhaur.)
 
o. mouth ; i. intestine ; a. anus ; v. praeoral ciliated band ; w. perianal ciliated
band.
 
than the larvae of the five other groups. It is, in the first place,
without an anus ; and there are no grounds for supposing that
the anus has become lost by retrogressive changes. If for the
moment it is granted that the Pilidium larva represents more
nearly than the larvae of the other groups the ancestral type of
larva, what characters are we led to assign to the ancestral form
which this larva repeats ?
 
In the first place, this ancestral form, of which fig. 231 A is
an ideal representation, would appear to have had a dome-shaped
body, with a flattened oral surface and a rounded aboral surface.
Its symmetry was radial, and in the centre of the flattened oral
surface was placed the mouth, and round its edge was a ring of
cilia. The passage of a Pilidium-like larva into the vermiform
bilateral Platyelminth form, and therefore it may be presumed
of the ancestral form which this larva repeats, is effected by the
 
 
 
 
374 ORIGIN OF PILIDIUM LARVA.
 
larva becoming more elongated, and by the region between the
mouth and one end of the body becoming the pneoral region,
and by an outgrowth between the mouth and the opposite end
developing into the trunk, an anus
becoming placed at its extremity in
the higher forms.
 
If what has been so far postulated
is correct, it is clear that this primitive
larval form bears a very close resemblance to a simplified free-swimming
Ccelenterate (Medusa), and that the
conversion of such a radiate form into
 
..... , , , , . , , i FlG. 226. POLYGORDIUS
 
the bilateral took place, not by the LARVA- ( After Hatschek.)
elongation of the aboral surface, and ;;/ mouth; ^ ^.^
 
the formation of an anus there, but by phageal ganglion ; nph. nephri, , , . r . 1 i r dion ; me.p. mesoblastic band :
 
the unequal elongation of the oral face, aw< anus f oL stomach .
 
an anterior part, together with the dome
 
above it, forming a praeoral lobe, and a posterior outgrowth the
 
trunk (figs. 226 and 233) ; while the aboral surface became the
 
dorsal surface.
 
This view fits in very well with the anatomical resemblances
between the Coelenterata and the Turbellaria 1 , and shews, if true,
that the ventral and median position of the mouth in many
Turbellaria is the primitive one.
 
The above suggestion as to the mode of passage from the radial into the
bilateral form differs largely from that usually held. Lankester 2 , for
instance, gives the following account of this passage :
 
" It has been recognised by various writers, but notably by Gegenbaur
and Haeckel, that a condition of radiate symmetry must have preceded the
condition of bilateral symmetry in animal evolution. The Diblastula may
be conceived to have been at first absolutely spherical with spherical
symmetry. The establishment of a mouth led necessarily to the establishment of a structural axis passing through the mouth, around which axis the
body was arranged with radial symmetry. This condition is more or less
perfectly maintained by many Ccelenterates, and is reassumed by degrada
1 Vide Vol. II. pp. 179 and 191. In this connection attention may be called
to Cceloplana Mdschnikowii, a form described by Kowalevsky, Zoologischer Anzeiger,
No. 52, p. 140, as being intermediate between the Ctenophora and the Turbellaria.
As already mentioned, there does not appear to me to be sufficient evidence to prove
that this form is not merely a creeping Ctenophor.
 
Qiiart. Journ. of Micr. Science, Vol. XVH. pp. 422-3.
 
 
 
LARVAL FORMS.
 
 
 
375
 
 
 
tion of higher forms (Echinoderms, some Cirrhipedes, some Tunicates).
The next step is the differentiation of an upper and a lower surface in
 
 
 
 
 
FIG. 227. LARVA OF ECHIURUS. (After Salensky.)
;#. mouth ; an. anus ; sg. supra-oesophageal ganglion (?).
 
relation to the horizontal position, with mouth placed anteriorly, assumed by
the organism in locomotion. With the differentiation of a superior and
inferior surface, a right and a left side, complementary one to the other, are
necessarily also differentiated. Thus the organism
becomes bilaterally symmetrical. The Ccelentera
are not wanting in indications of this bilateral
symmetry, but for all other higher groups of animals
it is a fundamental character. Probably the development of a region in front of, and dorsal to the
mouth, forming the Prattomium, was accomplished
pari passu with the development of bilateral symmetry. In the radially symmetrical Ccelentera we
find very commonly a series of lobes of the bodywall or tentacles produced equally with radial symmetry, that is to say all round the mouth, the
mouth terminating the main axis of the body that
is to say, the organism being ' telostomiate.' The
later fundamental form, common to all animals above the Ccelentera, is
attained by shifting what was the main axis of the body so that it may be
described now as the ' enteric ' axis ; whilst the new main axis, that parallel
with the plane of progression, passes through the dorsal region of the body
running obliquely in relation to the enteric axis. Only one lobe or outgrowth
of those radially disposed in the telostomiate organisms now persists. This
lobe lies dorsally to the mouth, and through it runs the new main axis. This
lobe is the Prostomium, and all the organisms which thus develop a new
main axis, oblique to the old main axis, may be called prostomiate."
 
 
 
FIG. 228. DIAGRAM
OF A LARVA OF THE
 
POLYZOA.
 
m. mouth ; an. anus ;
st. stomach; s. ciliated
disc.
 
 
 
376
 
 
 
COMPARISON -BETWEEN TYPES OF LARVAE.
 
 
 
It will be seen from this quotation that the aboral part of the body is supposed to elongate to form the trunk, while the prasoral region is derived from
one of the tentacles.
 
Before proceeding to further considerations as to the origin
of the Bilateralia, suggested by the Pilidium type of larva, it is
necessary to enter into a more detailed comparison between our
larval forms.
 
A very superficial consideration of the characters of these
forms brings to light two important features in which they differ,
viz. :
 
(l) In the presence or absence of sense organs on the prasoral
lobe.
 
 
 
 
FlG. 229. TWO STAGES IN THE DEVELOPMENT OF TORNARIA.
 
(After Metschnikoff.)
 
The black lines represent the ciliated bands.
 
in. mouth; an. anus; br. branchial cleft; ht. heart; c. body cavity between
splanchnic and somatic mesoblast layers ; w. so-called water-vascular vesicle ; v.
circular blood-vessel.
 
(2) In the presence or absence of outgrowths from the
alimentary tract to form the body cavity.
 
The larvae of the Echinodermata and Actinotrocha (?) are
without sense organs on the praeoral lobe, while the other types
 
 
 
LARVAL FORMS.
 
 
 
377
 
 
 
of larvae are provided with them. Alimentary diverticula are
characteristic of the larvae of the Echinodermata and of Tornaria.
 
If the conclusion already arrived at to the effect that the
prototype of the six larval groups was descended from a radiate
ancestor is correct, it appears to follow that the nervous system,
in so far as it was differentiated, had primitively a radiate form ;
and it is also probably true that there were alimentary diverticula
in the form of radial pouches, two of which may have given
origin to the paired diverticula which become the body cavity in
such types as the Echinodermata, Sagitta, etc. If these two
points are granted, the further conclusions seem to follow (i)
that the ganglion and sense organs of
the praeoral lobe were secondary structures, which arose (perhaps as differentiations of an original circular
nerve ring) after the assumption of a
bilateral form; and (2) that the absence
of these organs in the larvae of the
Echinodermata and Actinotrocha (?)
implies that these larvae retain, so
far, more primitive characters than the
Pilidium. The same may be said of
the alimentary diverticula. There are
thus indications that in two important
points the Echinoderm larvae are more
primitive than the Pilidium.
 
The above conclusions with reference to the Pilidium and Echinoderm
larvae involve some not inconsiderable
difficulties, and suggest certain points for further discussion.
 
In the first place it is to be noted that the above speculations
render it probable that the type of nervous system from which
that found in the adults of the Echinodermata, Platyelminthes,
Chsetopoda, Mollusca, etc., is derived, was a circumoral ring,
like that of Medusae, with which radially arranged sense organs
may have been connected ; and that in the Echinodermata this
form of nervous system has been retained, while in the other types
it has been modified. Its anterior part may have given rise to
supra-cesophageal ganglia and organs of vision ; these being
 
 
 
 
FIG. 230. ACTINOTROCHA.
(After Metschnikoff.)
 
/. mouth ; an. anus.
 
 
 
378
 
 
 
PRIMITIVE TYPE OF NERVOUS SYSTEM.
 
 
 
developed on the assumption of a bilaterally symmetrical form,
and the consequent necessity arising for the sense organs to
be situated at the anterior end of the body. If this view is
correct, the question presents itself as to how far the posterior
part of the nervous system of the Bilateralia can be regarded as
derived from the primitive radiate ring.
 
 
 
 
FIG. 231. THREE DIAGRAMS REPRESENTING THE IDEAL EVOLUTION OF VARIOUS
 
LARVAL FORMS.
 
A. Ideal ancestral larval form.
 
B. Larval form from which the Trochosphere larva may have been derived.
 
C. Larval form from which the typical Echinoderm larva may have been
derived.
 
m. mouth ; an. anus ; st. stomach ; s.g. supra-cesophageal ganglion.
The black lines represent the ciliated bands.
 
A circumoral nerve-ring, if longitudinally extended, might
give rise to a pair of nerve-cords united in front and behind
exactly such a nervous system, in fact, as is present in many
Nemertines 1 (the Enopla and Pelagonemertes), in Peripatus 2 ,
and in primitive molluscan types (Chiton, Fissurella, etc.).
From the lateral parts of this ring it would be easy to derive the
ventral cord of the Chaetopoda and Arthropoda. It is especially
deserving of notice in connection with the nervous system of the
 
1 Vute Hubrecht, "Zur Anat. und Phys. d. Nerven-System. d. Nemertinen," Kbn.
Akad. Wiss., Amsterdam ; and " Researches on the Nervous System of Nemertines,"
Quart. Journ. of Micr. Science, 1880.
 
* Vide F. M. Balfour, " On some points in the Anat. of Peripatus capensis," Quart.
Jourt:. of Micr. Science, Vol. xix. 1879.
 
 
 
LARVAL FORMS. 379
 
 
 
above-mentioned Nemertines and Peripatus, that the commissure
connecting the two nerve-cords behind is placed on the dorsal
side of the intestine. As is at once obvious, by referring to the
diagram (fig. 231 B), this is the position this commissure ought,
undoubtedly, to occupy if derived from part of a nerve-ring which
originally followed more or less closely the ciliated edge of the
body of the supposed radiate ancestor.
 
The fact of this arrangement of the nervous system being
found in so primitive a type as the Nemertines tends to establish
the views for which I am arguing ; the absence or imperfect
development of the two longitudinal cords in Turbellarians may
very probably be due to the posterior part of the nerve-ring
having atrophied in this group.
 
It is by no means certain that this arrangement of the nervous
system in some Mollusca and in Peripatus is primitive, though it
may be so.
 
In the larvae of the Turbellaria the development of sense organs in the
praeoral region is very clear (fig. 222 B) ; but this is by no means so obvious
in the case of the true Pilidium. There is in Pilidium (fig. 232 A) a thickening of epiblast at the summit of the dorsal dome, which might seem, from
the analogy of Mitraria, etc. (fig. 233), to correspond to the thickening of the
praaoral lobe, which gives rise to the supra-cesophageal ganglion ; but, as a
matter of fact, this part of the larva does not apparently enter into the
formation of the young Nemertine (fig. 232). The peculiar metamorphosis,
which takes place in the development of the Nemertine out of the Pilidium 1 ,
may, perhaps, eventually supply an explanation of this fact ; but at present
it remains as a still unsolved difficulty.
 
The position of the flagellum in Pilidium, and of the supra-cesophageal
ganglion in Mitraria, suggests a different view of the origin of the supraoesophageal ganglion from that adopted above. The position of the ganglion
in Mitraria corresponds closely with that of the auditory organ in Ctenophora ; and it is not impossible that the two structures may have had
a common origin. If this view is correct, we must suppose that the apex of
the aboral lobe has become the centre of the praeoral field of the Pilidium
and Trochosphere larval forms 2 a view which fits in very well with their
structure (figs. 226 and 233). The whole of the questions concerning the
nervous system are still very obscure, and until further facts are brought to
light no definite conclusions can be arrived at.
 
1 Vide Vol. ii. p. 204.
 
2 The independent development of the supra-cesophageal ganglion and ventral
nerve-cord in Chaetopoda (vide Kleinenberg, Development of Lumbricus trapezoides)
agrees very satisfactorily with this view.
 
 
 
380 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA.
 
 
 
The absence of sense organs on the praeoral lobe of larval
Echinodermata, coupled with the structure of the nervous system
of the adult, points to the conclusion that the adult Echinoder
 
 
 
FlG. 232. A. PlLIDIUM WITH AN ADVANCED NEMERTINE WORM. B. RlPE
EMBRYO OF NEMERTES IN THE POSITION IT OCCUPIES IN PlLIDIUM. (Both after
Biitschli.)
 
ft. oesophagus ; st. stomach ; i. intestine ; fr. proboscis ; lp. lateral pit (cephalic
sack) ; a. amnion ; n. nervous system.
 
mata have retained, and not, as is now usually held, secondarily
acquired, their radial symmetry; and if this is admitted it follows
that the obvious bilateral symmetry of Echinoderm larvae is a
secondary character.
 
The bilateral symmetry of many Ccelenterate larvae (the
larva of ,/Eginopsis, of many Acraspeda, of Actinia, &c.), coupled
with the fact that a bilateral symmetry is obviously advanta
 
 
LARVAL FORMS. 381
 
 
 
geous to a free-swimming form, is sufficient to shew that this
supposition is by no means extravagant ; while the presence of
only two alimentary diverticula in Echinoderm larvae is quite in
accord with the presence of a single pair of perigastric chambers
in the early larva of Actinia, though it must be admitted that
the derivation of the water-vascular system from the left
diverticulum is not easy to understand on this view.
 
A difficulty in the above speculation is presented by the fact
of the anus of the Echinodermata being the permanent blastopore,
and arising prior to the mouth. If this fact has any special
significance, it becomes difficult to regard the larva of Echinoderms and that of the other types as in any way related ; but if
the views already urged, in a previous section on the germinal
layers, as to the unimportance of the blastopore, are admitted,
the fact of the anus coinciding with the blastopore ceases to be
a difficulty. As may be seen, by referring to fig. 231 C, the
anus is placed on the dorsal side of the ciliated band. This
position for the anus adapts itself to the view that the Echinoderm larva had originally a radial symmetry, with the anus
placed at the aboral apex, and that, with the elongation of the
larva on the attainment of a bilateral symmetry, the aboral apex
became shifted to the present position of the anus.
 
It may be noticed that the obscure points connected with the absence of
a body cavity in most adult Platyelminthes, which have already been dealt
with in the section of this chapter devoted to the germinal layers, present
themselves again here ; and that it is necessary to assume either that alimentary diverticula, like those in the Echinodermata, were primitively
present in the Platyelminthes, but have now disappeared from the ontogeny
of this group, or that the alimentary diverticula have not become separated
from the alimentary tract.
 
So far the conclusion has been reached that the archetype of
the six types of larvae had a radiate form, and that amongst
existing larvae it is most nearly approached in general shape
and in the form of the alimentary canal by the Pilidium group,
and in certain other particulars by the Echinoderm larvae.
 
The edge of the oral disc of the larval archetype was probably
armed with a ciliated ring, from which the ciliated ring of the
Pilidium type and of the Echinodermata was most likely derived.
The ciliated ring of the Pilidium varies greatly in its characters,
 
 
 
382 PRIMITIVE RADIAL SYMMETRY OF ECHINODERMATA.
 
 
 
and has not always the form of a complete ring. In Pilidium
proper (fig. 232 A) it is a simple ring surrounding the edge of
the oral disc. In Muller's larva of Thysanozoon (fig. 222 B) it is
 
 
 
 
FlG. 233. TWO STAGES IN THE DEVELOPMENT OF MlTRARIA. (After Metschnikoff.)
m. mouth; an. anus; sg. supra-cesophageal ganglion; br. and b. provisional
bristles ; pr.b. prasoral ciliated band.
 
inclined at an axis to the oral disc, and might be called praeoral, but
such a term cannot be properly used in the absence of an anus.
 
 
 
 
FIG. 234. CYPHONAUTES (LARVA OF MEMBRANIPORA). (After Hatschek.)
 
m. mouth ; a '. anus ; f.g. foot gland ; x. problematical body (probably a bud).
 
The aboral apex is turned downwards.
 
 
 
LARVAL FORMS. 383
 
 
 
The Echinoderm ring is oblique to the axis of the body, and,
owing to the fact of its passing ventrally in front of the anus,
must be called postoral.
 
The next point to be considered is that of the affinities of the
other larval types to these two types.
 
The most important of all the larval types is the Trochosphere,
and this type is undoubtedly more closely related to the Pilidium
than to the Echinoderm larva. Mitraria amongst the Chaetopods
(fig. 233) has, indeed, nearly the form of a Pilidium, and mainly
differs from a Pilidium in the possession of an anus and of
provisional bristles ; the same may be said of Cyphonautes (fig.
234) amongst the Polyzoa.
 
The existence of these two forms appears to shew that the
praeoral ciliated ring of the Trochosphere may very probably be
derived directly from the circumoral ciliated ring of the Pilidium;
the other ciliated rings or patches of the Trochosphere having a
secondary origin.
 
The larva of the Brachiopoda (fig. 220), in spite of its peculiar
characters, is, in all probability, more closely related to the
Chaetopod Trochosphere than to any other larval type. The
most conspicuous point of agreement between them is, however,
the possession in common of provisional setae.
 
Echinoderm larvae differ from the Trochosphere, not only in
the points already alluded to, but in the character of the ciliated
band. The Echinoderm band is longitudinal and postoral. As
just stated, there is reason to think that the praeoral band of
the Trochosphere and the postoral band of the Echinoderm
larva are both derived from a ciliated ring surrounding the oral
disc of the prototype of these larvae (vide fig. 231). In the case
of the Echinodermata the anus must have been formed on the
dorsal side of this ring, and in the case of the Trochosphere on
the ventral side ; and so the difference in position between the
two rings was brought about. Another view with reference to
these rings has been put forward by Gegenbaur and Lankester,
to the effect that the praeoral ring of the Trochosphere is derived
from the breaking up of the single band of most Echinoderm
larvae into the two bands found in Bipinnaria (vide fig. 223) and
the atrophy of the posterior band. There is no doubt a good
deal to be said for this origin of the praeoral ring, and it is
 
 
 
384 PHYLOGENETIC CONCLUSIONS.
 
strengthened by the case of Tornaria ; but the view adopted
above appears to me more probable.
 
Actinotrocha (fig. 230) undoubtedly resembles more closely
Echinoderm larvae than the Trochosphere. Its ciliated ring has
Echinoderm characters, and the growth along the line of the
ciliated ring of a series of arms is very similar to what takes
place in many Echinoderms. It also agrees with the Echinoderm
larvae in the absence of sense organs on the praeoral lobe.
 
Tornaria (fig. 229) cannot be definitely united either with
the Trochosphere or with the Echinoderm larval type. It has
important characters in common with both of these groups, and
the mixture of these characters renders it a very striking and
well-defined larval form.
 
Phylogenetic conclusions. The phylogenetic conclusions
which follow from the above views remain to be dealt with.
The fact that all the larvae of the groups above the Ccelenterata
can be reduced to a common type seems to indicate that all the
higher groups are descended from a single stem.
 
Considering that the larvae of comparatively few groups have
persisted, no conclusions as to affinities can be drawn from the
absence of a larva in any group; and the presence in two groups
of a common larval form may be taken as proving a common
descent, but does not necessarily shew any close affinity.
 
There is every reason to believe that the types with a
Trochosphere larva, viz. the Rotifera, the Mollusca, the Chaetopoda, the Gephyrea, and the Polyzoa, are descended from a
common ancestral form ; and it is also fairly certain there was a
remote ancestor common to these forms and to the Platyelminthes.
A general affinity of the Brachiopoda with the Chaetopoda is
more than probable. All these types, together with various
other types which are nearly related to them, but have not
preserved an early larval form, are descended from a bilateral
ancestor. The Echinodermata, on the other hand, are probably
directly descended from a radial ancestor, and have more or less
completely retained their radial symmetry. How far Actinotrocha 1 is related to the Echinoderm larvae cannot be settled.
Its characters may possibly be secondary, like those of the
 
1 It is quite possible that Phoronis is in no way related to the other Gephyrea.
 
 
 
LARVAL FORMS. 385
 
 
 
mesotrochal larvae of Chaetopods, or they may be due to its
having branched off very early from the stock common to the
whole of the forms above the Ccelenterata. The position of
Tornaria is still more obscure. It is difficult, in the face of the
peculiar water-vascular vesicle with a dorsal pore, to avoid the
conclusion that it has some affinities with the Echinoderm larvae.
Such affinities would seem, on the lines of speculation adopted
in this section, to prove that its affinities to the Trochosphere,
striking as they appear to be, are secondary and adaptive. From
this conclusion, if justified, it would follow that the Echinodermata
and Enteropneusta have a remote ancestor in common, but not
that the two groups are in any other way related.
 
General conclusions and summary. Starting from the
demonstrated fact that the larval forms of a number of widely
separated types above the Ccelenterata have certain characters
in common, it has \&&\ provisionally assumed that the characters
have been inherited from a common ancestor ; and an attempt
has been made to determine (i) the characters of the prototype
of all these larvae, and (2) the mutual relations of the larval
forms in question. This attempt started with certain more or
less plausible suggestions, the truth of which can only be tested
by the coherence of the results which follow from them, and
their capacity to explain all the facts.
 
The results arrived at may be summarised as follows :
 
1. The larval forms above the Ccelenterata may be divided
into six groups enumerated on pages 370 to 373.
 
2. The prototype of all these groups was an organism
something like a Medusa, with a radial symmetry. The mouth
was placed in the centre of a flattened ventral surface. The
aboral surface was dome-shaped. Round the edge of the oral
surface was a ciliated ring, and probably a nervous ring provided
with sense organs. The alimentary canal was prolonged into
two or more diverticula, and there was no anus.
 
3. The bilaterally symmetrical types were derived from
this larval form by the larva becoming oval, and the region in
front of the mouth forming a praeoral lobe, and that behind the
mouth growing out to form the trunk. The aboral dome became
the dorsal surface.
 
On the establishment of a bilateral symmetry the anterior
 
15. in. 25
 
 
 
386 GENERAL CONCLUSIONS.
 
part of the nervous ring gave rise (?) to the supra-cesophageal
ganglia, and the optic organs connected with them ; while the
posterior part of the nerve-ring formed (?) the ventral nerve-cords.
The body cavity was developed from two of the primitive
alimentary diverticula.
 
The usual view that radiate forms have become bilateral by
the elongation of the aboral dome into the trunk is probably
erroneous.
 
4. Pilidium is the larval form which most nearly reproduces
the characters of the larval prototype in the course of its
conversion into a bilateral form.
 
5. The Trochosphere is a completely differentiated bilateral
form, in which an anus has become developed. The praeoral
ciliated ring of the Trochosphere is probably directly derived
from the ciliated ring of Pilidium, which is itself the original ring
of the prototype of all these larval forms.
 
6. Echinoderm larvae, in the absence of a nerve-ganglion or
special organs of sense on the prseoral lobe, and in the presence
of alimentary diverticula, which give rise to the body cavity,
retain some characters of the prototype larva which have been
lost in Pilidium. The ciliated ring of Echinoderm larvae is
probably derived directly from that of the prototype by the
formation of an anus on the dorsal side of the ring. The anus
was very probably originally situated at the aboral apex.
 
Adult Echinoderms have probably retained the radial symmetry of the forms from which they are descended, their nervous
ring being directly derived from the circular nervous ring of their
ancestors. They have not, as is usually supposed, secondarily
acquired their radial symmetry. The bilateral symmetry of the
larva is, on this view, secondary, like that of so many Coelenterate
larvae.
 
7. The points of similarity between Tornaria and (i) the
Trochosphere and (2) the Echinoderm larvae are probably
adaptive in the one case or the other ; and, while there is no
difficulty in believing that those to the Trochosphere are
adaptive, the presence of a water- vascular vesicle with a dorsal
pore renders probable a real affinity with Echinoderm larvae.
 
8. It is not possible in the present state of our knowledge
to decide how far the resemblances between Actinotrocha and
Echinoderm larvae are adaptive or primary.
 
 
 
LARVAL FORMS. 387
 
 
 
BIBLIOGRAPHY.
 
(257) Allen Thomson. British Association Address, 1877.
 
(258) A. Agassiz. " Embryology of the Ctenophorae." Mem. Amer. Acad. of
Arts and Sciences, Vol. X. 1874.
 
(259) K. E. von Baer. Ueb. Entivicklungsgeschichte d. Thiere. Konigsberg,
18281837.
 
(260) F. M. Balfour. "A Comparison of the Early Stages in the Development
of Vertebrates." Quart. Joum. of Micr. Set., Vol. XV. 1875.
 
(261) C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastraa-tlieorie. Wien,
1874.
 
'(262) C. Glaus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.
 
(263) A. Dohrn. Der Ursprung d. Wirbelthiere u. d. Princip des Functionsivechsels. Leipzig, 1875.
 
(264) C. Gegenbaur. Grttndriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also Translation. Elements of Comparative Anatomy. Macmillan & Co.
1878.
 
(265) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1874.
 
(266) E. Haeckel. Studien z. Gastraa-theorie, Jena, 1877; and also jtenaisc/ic
Zeitschrift, Vols. vin. and IX. 1874-5.
 
(267) E. Haeckel. Schopfungsgeschichte. Leipzig. Vide also Translation,
The History of Creation. King & Co., London, 1878.
 
(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Anthropogeny. Kegan Paul & Co., London, 1878.
 
(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden."
Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.
 
(270) O. and R. Hertwig. "Die Actinien." Jenaische Zeitschrift, Vols. xm.
and xiv. 1879.
 
(271) O. and R. Hertwig. Die Ccelomtheorie. Jena, 1881'.
 
(272) O. Hertwig. Die Chatognathen. Jena, 1880.
 
(273) R. Hertwig. Ueb. d. Bau d. Ctenophoren. Jena, 1880.
 
(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill,
1877.
 
(274*) T. H. Huxley. "On the Classification of the Animal Kingdom."
Quart. J. of Micr. Science, Vol. xv. 1875.
 
(275) N. Kleinenberg. Hydra, eine anatomisch-cntwickhingsgeschichtiiche Untersuchung. Leipzig, 1872.
 
(276) A. Kolliker. Entwicklungsgeschichte d. Menschen it, d. hoh. Thiere.
Leipzig, 1879.
 
(277) A. Kowale vsky. " Embryologische Studien an Wiirmern u. Arthropoden."
Mem. Acad. Petersbourg, Series vil. Vol. xvi. 1871.
 
(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the
Basis of the Genealogical Classification of Animals." Ann. and Mag. of Nat. Hist.
1873
1 This important memoir only came into my hands after this chapter was already
in type.
 
25 2
 
 
 
388 BIBLIOGRAPHY.
 
 
 
(279) E. R. Lankester. "Notes on Embryology and Classification." Quart.
Jonrn. of Micr. Set., Vol. XVII. 1877.
 
(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme."
Zeit.f. wiss. Zool., Vol. xxiv. 1874.
 
(281) E. Metschnikoff. " Spongiologische Stuclien." Zeit.f, wiss. Zool.,
Vol. xxxn. 1879.
 
(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
of Comparative Embryology. Holt and Co., New York, 1876.
 
(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol.
x. 1876.
 
(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschnecke (Planorbis)." Morph.
Jahrbuch, Vol. v. 1879.
 
(285) H. Rathke. Abhandlungen 2. Bildung und Entwicklungsgesch. d. Menschen
. d. Thiere. Leipzig, 1833.
 
(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig,
1829.
 
(287) R. Remak. Untersuch. iib. d. Entwick. d. Wirbelthiere. Berlin, 1855.
 
(288) Salensky. " Bemerkungen iib. Haeckels Gastrsea-theorie." Archiv f.
Na turgesch ich te, 1874.
 
(289) E. Schafer. "Some Teachings of Development." Quart. Jonnt. of Micr.
Science, Vol. xx. 1880.
 
(290) C. Semper. "Die Verwandtschaftbeziehungen d. gegliederten Thiere.
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. III. 1876-7.
 
 
 
PART II.
 
ORGANOGENY.
 
 
 
PART II.
ORGANOGENV.
 
INTRODUCTION.
 
OUR knowledge of the development of the organs in most of
the Invertebrate groups is so meagre that it would not be profitable to attempt to treat systematically the organogeny of the
whole animal kingdom.
 
For this reason the plan adopted in this section of the work
has been to treat somewhat fully the organogeny of the Chordata, which is comparatively well known ; and merely to indicate
a few salient facts with reference to the organogeny of other
groups. In the case of the nervous system, and of some other
organs which especially lend themselves to this treatment, such
as the organs of special sense and the excretory system, a wider
view of the subject has been taken ; and certain general principles underlying the development of other organs have also been
noticed.
 
The classification of the organs is a matter of some difficulty.
Considering the character of this treatise it seemed desirable to
arrange the organs according to the layers from which they are
developed. The compound nature of many organs, e.g. the eye
and ear, renders it, however, impossible to carry out consistently
such a mode of treatment. I have accordingly adopted a rough
classification of the organs according to the layers, dropping the
principle where convenient, as, for instance, in the case of the
stomodaeum and proctodseum.
 
The organs which may be regarded as mainly derived from
 
 
 
392 INTRODUCTION.
 
 
 
the epiblast are (i) the skin; (2) the nervous system; (3) the
organs of special sense.
 
Those from the mesoblast are (i) the general connective
tissue and skeleton ; (2) the vascular system and body cavity ;
(3) the muscular system ; (4) the urinogenital system.
 
Those from the hypoblast are the alimentary tract and its
derivates ; with which the stomodaeum and proctodaeum and
their respective derivates are also dealt with.
 
BIBLIOGRAPHY.
 
General works dealing with the development of the organs of the
 
Chordata.
 
(291) K. E. von Baer. Ueber Entwicklungsgeschichte d. Thiere. Konigsberg,
18281837.
 
(292) F. M. Balfour. A Monograph on tlic development of Elasmobrancli Fishes.
London, 1878.
 
(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Sdtigethiere ti. d. Menschen.
Leipzig, 1842.
 
(294) C. Gegenbaur. Gnindriss d. vergleichenden Anatomic. Leipzig, 1878.
Vide also English translation, Elements of Comp. Anatomy. London, 1878.
 
(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I.
London, 1874.
 
(296) Alex. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
 
(297) W. His. Untersitch. iib. d. erste Anlage d. Wirbelthierleibcs . Leipzig,
1868.
 
(298) A. Ko Hiker. Entwicklungsgeschichte d. Menschen u. der hoheren Thiere.
Leipzig, 1879.
 
(299) H. Rathke. Abhandlungen it. Bildung mid Entwicklungsgeschichle d.
Menschen it. d. Thiere. Leipzig, 1838.
 
(300) H. Rathke. Entwicklungs. d. Natter. Kbnigsberg, 1839.
 
(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.
 
(302) R. Remak. Untersuchnngen iib. d. Entwicklung d. Wirbelthiere. Berlin,
18501855.
 
(303) S. L. Schenk. Lehrbuch d. vei'gleich. Embryologie d. Wirbeltliicre.
Wien, 1874.
 
 
 
.
 
CHAPTER XIV.
THE EPIDERMIS AND ITS DERIVATIVES.
 
 
 
IN many of the Ccelenterata the outermost layer of the blastoderm is converted as a whole into the skin or ectoderm.
The cells composing it become no doubt in part differentiated
into muscular elements and in part into nervous elements, &c. ;
but still it may remain through life as a simple external
membrane. This membrane contains in itself indefinite potentialities for developing into various organs, and in all the true
Triploblastica these potentialities are more or less realized.
The embryonic epiblast ceases in fact, in the higher forms, to
become converted as a whole into the epidermis, but first gives
rise to parts of the nervous system, organs of special sense, and
other parts.
 
After the formation of these parts the remnant of the
epiblast gives rise to the epidermis, and often unites more or
less intimately with a subjacent layer of mesoblast, known as
the dermis, to form with it the skin.
 
Various differentiations may arise in the epidermis forming
protective or skeletal structures, terminal sense organs, or
glands. The structure of the epidermis itself varies greatly, and
for Vertebrates its general modifications have been already
sufficiently dealt with in chapter XII. Of its special differentiations those of a protective or skeletal nature and those of a
glandular nature may be considered in this place.
 
Protective epidermal structures. These structures constitute a general cuticle or an exoskeleton of scales, hairs,
feathers, nails, hoofs, &c. They may be entirely formed from
 
 
 
394 TH E EXOSKELETON.
 
 
 
the epidermis either as (i) a cuticular deposit, or as (2) a
chitinization, a cornification, or calcification of its constituent
cells. These two processes run into each other, and are in many
cases not easily distinguished. The protective structures of the
epidermis may be divided into two groups according as they are
formed on the outer or the inner side of the epidermis. Dermal
skeletal structures are in many cases added to them. Amongst
the Invertebrata the most widely distributed type of exoskeleton
is a cuticle formed on the outer surface of the epidermis, which
reaches its highest development in the Arthropoda. In the same
class with this cuticle must be placed the molluscan and brachiopod shells, which are developed as cuticular plates on special
regions of the epidermis. They differ, however, from the more
usual form of cuticle in their slighter adhesion to the subjacent
epidermis, and in their more complicated structure. The test of
Ascidians is an abnormal form of exoskeleton belonging to this
type. It is originally formed (Hertvvig and Semper) as a
cuticle on the surface of the epidermis ; but subsequently
epidermic cells migrate into it, and it then constitutes a tissue
similar to connective tissue, but differing from ordinary epidermic
cuticles in that the cells which deposit it do so over their whole
surface, instead of one surface, as is usually the case with
epithelial cells.
 
In the Vertebrata the two types of exoskeleton mentioned
above are both found, but that developed on the inner surface of
the epidermis is always associated with a dermal skeleton, and
that on the outer side frequently so. The type of exoskeleton
developed on the inner side of the general epidermis is confined
to the Pisces, where it appears as the scales; but a primitive
form of these structures persists as the teeth in the Amphibia
and Amniota. The type developed on the outer side of the
epidermis is almost entirely 1 confined to the Amphibia and Amniota, where it appears as scales, feathers, hairs, claws, nails, &c.
For the histological details as to the formation of these various
organs I must refer the reader to treatises on histology, confining
my attention here to the general embryological processes which
take place in their development.
 
1 The horny teeth of the Cyclostomala are structures belonging to this group.
 
 
 
THE EPIDERMIS AND ITS DERIVATIVES.
 
 
 
395
 
 
 
The most primitive form of the first type of dermal structures
is that of the placoid scales of Elasmobranchii 1 . These consist,
when fully formed, of a plate bearing a spinous projection.
They are constituted of an outer enamel layer on the projecting
part, developed as a cuticular deposit of the epidermis (epiblast),
and an underlying basis of dentine (the lower part of which may
be osseous) with a vascular pulp in its axis. The development
(fig. 235) is as follows (Hertwig, No. 306). A papilla of the
dermis makes its appearance, the outer layer of which gradually
calcifies to form the dentine and osseous tissue. This papilla is
covered by the columnar mucous layer of the epidermis (e), from
which it is separated by a basement membrane, itself a product
of the epidermis. This membrane gradually thickens and calcifies, and so gives rise to the enamel cap (o). The spinous point
gradually forces its way through the epidermis, so as to project
freely at the surface.
 
The scales of other forms of fishes are to be derived from those of
Elasmobranchii. The great dermal plates of many fishes have been formed
by the concrescence of groups of such scales. The dentine in many cases
partially or completely atrophies, leaving the major part of the scale formed
of osseous tissue ; such plates often become parts of the internal skeleton.
 
 
 
 
d
 
 
 
5\
 
 
 
 
FIG. 235. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYONIC SHARK,
TO SHEW A DEVELOPING PLACOID SCALE. (From Gegenbaur ; after O. Hertwig.)
 
E. epidermis ; C. layers of dermis ; d. uppermost layer of dermis ; p. papilla of
dermis ; e. mucous layer of epidermis ; o. enamel layer.
 
1 For the most important contributions on this subject from which the facts and
views here expressed are largely derived, vide O. Hertwig, Nos. 306 808.
 
 
 
396 THK KXOSKELETON.
 
 
 
The teeth, as will be more particularly described in the section on the
alimentary tract, are formed by a modification of the same process as the
placoid scales, in which a ridge of the epithelium grows inwards to meet
a connective tissue papilla, so that the development of the teeth takes place
entirely below the superficial layer of epidermis.
 
In most Teleostei the enamel and dentine layers have disappeared, and
the scales are entirely formed of a peculiar calcified tissue developed in the
dermis.
 
The cuticle covering the scales of Reptiles is the simplest
type of protective structure formed on the outer surface of the
epidermis. The scales consist of papillae of the dermis and
epidermis ; and are covered by a thickened portion of a twolayered cuticle, formed over the whole surface of the body
from a cornification of the superficial part of the epidermis.
Dermal osseous plates may be formed in connection with these
scales, but are never of course united with the superficial
cuticle.
 
Feathers are probably special modifications of such scales. They arise
rom an induration of the epidermis of papillae containing a vascular core.
The provisional down, usually present at the time of hatching, is formed by
the cornification of longitudinal ridges of the mucous layer of the epidermis
of the papillee ; each cornified ridge giving rise to a barb of the feather. The
horny layer of the epidermis forms a provisional sheath for the developing
feather below. When the barbs are fully formed this sheath is thrown- off,
the vascular core dries up, and the barbs become free except at their base.
 
Without entering into the somewhat complicated details of the formation
of the permanent feathers, it may be mentioned that the calamus or quill is
formed by a cornification in the form of a tube of both layers of the epidermis
at the base of the papilla. The quill is open at both ends, and to it is
attached the vexillum or plume of the feather. In a typical feather this
is formed at the apex of the papilla from ridge-like thickenings of the mucous
layer of the epidermis, arranged in the form of a longitudinal axis, continuous with the cornified mucous layer of the quill, and from lateral ridges.
These subsequently become converted into the axis and barbs of the plume.
The external epidermic layer becomes converted into a provisional horny
sheath for the true feather beneath.
 
On the completion of the plume of the feather the external sheath is
thrown off, leaving it quite free, and the vascular core belonging to it shrivels
up. The papilla in which the feather is formed becomes at a very early
period secondarily enveloped in a pit or follicle which gradually deepens as
the development of the feather is continued.
 
Hairs (Kolliker, No. 298) are formed in solid processes of
the mucous layer of the epidermis, which project into the
 
 
 
THE p;PIDERMIS AND ITS DERIVATIVES. 397
 
subjacent dermis. The hair itself arises from a cornification of
the cells of the axis of one of the above processes ; and is
invested by a sheath similarly formed from the more superficial
epidermic cells. A small papilla of the dermis grows into the
inner end of the epidermic process when the hair is first formed.
The first trace of the hair appears close to this papilla, but soon
increases in length, and when the end of the hair projects from
the surface, the original solid process of the epidermis becomes
converted into an open pit, the lumen of which is filled by the
root of the hair. Hairs differ in their mode of formation from
scales in a manner analogous to that in which the teeth differ
from ordinary placoid scales ; i.e. they are formed in inwardly
directed projections of the epidermis instead of upon free
papillae at the surface.
 
Nails (Kolliker, No. 298) are developed on special regions of the epidermis, known as the primitive nail beds. They are formed by the cornification
of a layer of cells which makes its appearance between the horny and
mucous layers of the epidermis. The distal border of the nail soon becomes
free, and the further growth is effected by additions to the under side and
attached extremity of the nail.
 
Although the nail at first arises in the interior of the epidermis, yet its
position on the outer side of the mucous layer clearly indicates with which
group of epidermic structures it should be classified.
 
Dermal skeletal structures. We have seen that in the
Chordata skeletal structures, which were primitively formed of
both an epidermic and dermic element, may lose the former
element and be entirely developed in the dermis. Amongst the
Invertebrata there are certain dermal skeletal structures which
are evolved wholly independently of the epidermis. The most
important of these structures are the skeletal plates of the
Echinodermata.
 
Glands. The secretory part of the various glandular structures belonging to the skin is invariably formed from the
epidermis. In Mammalia it appears that these glands are
always formed as solid ingrowths of the mucous layer (Kolliker,
No. 298). The ends of these ingrowths dilate to form the true
glandular part of the organs, while the stalks connecting the
glandular portions with the surface form the ducts. In the case
of the sweat-glands the lumen of the duct becomes first
established. Its formation is inaugurated by the appearance of
 
 
 
398 THE EXOSKELETON.
 
 
 
the cuticle, and appears first at the inner end of the duct and
thence extends outwards (Ranvier, No. 311). In the sebaceous
glands the first secretion is formed by a fatty modification of the
whole of the central cells of the gland.
 
The muscular layer of the secreting part of the sweat-glands
is formed, according to Ranvier (No. 311), from a modification
of the deeper layer of the epidermic cells.
 
The Mammary Glands arise in essentially the same manner as the other glands of the skin 1 . The glands of each side
are formed as a solid bud of the mucous layer of the epidermis.
From this bud processes sprout out, each of which gives rise to
one of the numerous glands of which the whole organ is formed.
Two very distinct types in the relation of the ducts of the
glands to the nipple are found (Gegenbaur, No. 313).
 
BIBLIOGRAPHY OF EPIDERMIS.
General.
 
(304) T. H. Huxley. " Tegumentary organs." Tocld's Cyclopaedia of Anat.
and Physiol.
 
(305) P. Z. Unna. " Histol. u. Entwick. d. Oberhaut." Archiv f. mikr. Anat.
Vol. xv. 1876. FzV&also Kolliker (No. 298).
 
Scales of tJic Pisces.
 
(306) O. Her twig. " Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zahne d. Selachier." Jenaische Zeitschrift, Vol. vin. 1874.
 
(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrln<ch,
Vol. n. 1876. (Siluroiden u. Acipenseridre.)
 
(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Alorph. Jahrbuch, Vol. v. 1879.
 
FeatJiers.
 
(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.
 
(310) Th. Studer. "Beitrage z. Entwick. d. Feder." Zeit. f. wiss. Zool., Vol.
xxx. 1878.
 
Sweat-glands.
 
(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes
A'f/iiiits, Dec. 29, 1879.
 
1 For a very different view on this subject vide Creighton (No. 312).
 
 
 
BIBLIOGRAPHY OF EPIDERMIS. 399
 
 
 
Mammary glands.
 
(312) C. Creighton. "On the development of the Mamma and the Mammary
function." Jour, of Anat. and Phys. , Vol. XI. 1877.
 
(313) C. Gegenbaur. " Bemerkungen iib. d. Milchdriisen-Papillen d. Saugethiere." Jenaische Zeit., Vol. vn. 1873.
 
(314) M. Huss. "Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wiederkauern." Jenaische Zeit., Vol. vil. 1873.
 
(315) C. Langer. " Ueber d. Ban u. d. Entwicklung d. Milchdriisen." Denk.
d. k. Akad. Wiss. Wien, Vol. III. 1851.
 
 
 
CHAPTER XV.
 
 
 
NERVOUS SYSTEM.
 
 
 
Origin of the Nervous System.
 
ONE of the most important recent embryological discoveries
is the fact that the central nervous system, in all the Metazoa in
which it is fully established, is (with a few doubtful exceptions)
derived from the primitive epiblast 1 . As we have already seen
that the epiblast represents to a large extent the primitive
epidermis, the fact of the nervous system being derived from the
epiblast implies that the functions of the central nervous system,
which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by
step removed from the surface, and has finally become in the
higher types a well-defined organ imbedded in the subdermal
tissues.
 
Before considering in detail the comparative development of
the nervous system, it will be convenient shortly to review the
present state of our knowledge on the general process of its
evolution.
 
This process may be studied either embryologically, or by a
comparison of the various stages in its evolution preserved in
living forms. Both the methods have led to important results.
 
1 Whether there is any part of it in many types not so derived requires further
investigation, now that it has been shewn by the Hertwigs that part of the system
develops from the endoderm in some Coelenterata. O. Hertwig holds that part of it
has a mesoblastic origin in Sagitta, but his observations on this point appear to me
very inconclusive. It would be very advantageous to investigate the origin of
. \ucrl >ach's plexus in Mammalia.
 
 
 
NERVOUS SYSTEM. 401
 
 
 
The embryological evidence shews that the ganglion-cells of
the central part of the nervous system are originally derived
from the simple undifferentiated epithelial cells of the surface of
the body, while the central nervous system itself has arisen from
the concentration of such cells in special tracts. In the Chordata at any rate the nerves arise as outgrowths of the central
organ.
 
Another important fact shewn by embryology is that the
central nervous system, and percipient portions of the organs of
special sense, especially of optic organs, are often formed from
the same part of the primitive epidermis. Thus the retina of
the Vertebrate eye is formed from the two lateral lobes of the
primitive fore-brain.
 
The same is true for the compound eyes of some Crustacea.
The supracesophageal ganglia of these animals are formed in the
embryo from two thickened patches of the epiblast of the procephalic lobes. These thickened patches become gradually
detached from the surface, remaining covered by a layer of
epidermis. They then constitute the supraoesophageal ganglia ;
but they form not only the ganglia, but also the retinulae of the
eye the parts in fact which correspond to the rods and cones in
our own retina. The accessory parts of these organs of special
sense, viz. the crystalline lens of the Vertebrate eye, and the
corneal lenses and crystalline cones of the Crustacean eye, are
independently formed from the epiblast after the separation of
the part which becomes the central nervous system.
 
In the Acraspedote Medusae the rudimentary central nervous
system has the form of isolated rings, composed of sense-cells
prolonged into nervous fibres, surrounding the stalks of tentaclelike organs, at the ends of which are placed the sense-organs.
 
This close connection between certain organs of special sense
and ganglia is probably to be explained by supposing that the
two sets of structures actually originated part passu.
 
We may picture the process as being somewhat as. follows :
 
It is probable that in simple ancestral organisms the whole body was
sensitive to light, but that with the appearance of pigment-cells in certain
parts of the body, the sensitiveness to light became localised to the areas
where the pigment-cells were present. Since, however, it was necessary
that stimuli received by such organs should be communicated to other parts
 
B. III. 26
 
 
 
402 EVOLUTION OF THE NERVOUS SYSTEM.
 
of the body, some of the epidermic cells in the neighbourhood of the
pigment-spots, which were at first only sensitive in the same manner as
other cells of the epidermis, became gradually differentiated into special
nerve-cells. As to the details of this differentiation embryology does not as
yet throw any great light ; but from the study of comparative anatomy there
are grounds for thinking that it was somewhat as follows: Cells placed on
the surface sent protoplasmic processes of a nervous nature inwards, which
came into connection with nervous processes from similar cells placed
in other parts of the body. The cells with such processes then became
removed from the surface, forming a deeper layer of the epidermis below
the sensitive cells of the organ of vision. With the latter cells they remained
connected by protoplasmic filaments, and thus they came to form a thickening of the epidermis underneath the organ of vision, the cells of which
received their stimuli from those of the organ of vision, and transmitted the
stimuli so received to other parts of the body. Such a thickening would
obviously be the rudiment of a central nervous system, and is in fact very
similar to the rudimentary ganglia of the Acraspeda mentioned above. It
is easy to see by what steps it might become larger and more important,
and might gradually travel inwards, remaining connected with the senseorgan at the surface by protoplasmic filaments, which would then constitute
nerves. The rudimentary eye would at first merely consist of cells sensitive
to light, and of ganglion-cells connected with them ; while at a later period
optical structures, constituting a lens capable of throwing an image of
external objects upon it, would be developed, and so convert the whole
structure into a true organ of vision. It has thus come about that, in the
development of the individual, the retina is often first formed in connection
with the central nervous system, while the lenses of the eye are independently evolved from the epidermis at a later period.
 
A series of forms of the Ccelenterata and Platyelminthes
affords us examples of various stages in the differentiation of a
central nervous system 1 .
 
In sea-anemones (Hertwigs, No. 321) there are, for instance, no organs
of special sense, and no definite central nervous system. There are, however,
scattered throughout the skin, and also throughout the lining of the digestive
tract, a number of specially modified epithelial cells, which are no doubt
delicate organs of sense. They are provided at their free extremity with a
long hair, and are prolonged on their inner side into fine processes which
penetrate into the deeper part of the epithelial layer of the skin or digestive
wall. They eventually join a fine network of protoplasmic fibres which forms
a special layer immediately within the epithelium. The fibres of this network are no doubt essentially nervous. In addition to fibres there are,
 
1 Our knowledge on this subject is especially due to the brothers Hertwig (Nos.
320 and 321), Eimer (No. 318), Claus (No. 317), Schafer (No. 326), and Hubrecht
(No. 323).
 
 
 
NERVOUS SYSTEM.
 
 
 
403
 
 
 
 
FIG. 236. NEUROEPITHELIALSENSECELLS OFAURELIA
 
AURITA. (From
 
Lankester ; after
Schafer.)
 
 
 
moreover, present in the network cells of the same character as the multipolar
ganglion-cells in the nervous system of Vertebrates, and some of these cells
are characterised by sending a process into the superjacent epithelium.
Such cells are obviously intermediate between neuroepithelial cells and ganglion-cells ; and it is probable
that the nerve-cells are, in fact, sense-cells which have
travelled inwards and lost their epithelial character.
 
In the Craspedote Medusae (Hertwigs, No. 320)
the differentiation of the nervous system is carried
somewhat further. There is here a definite double
ring, placed at the insertion of the velum, and usually
connected with sense-organs. The two parts of the
ring belong respectively to the epithelial layers on
the upper and lower surfaces of the velum, and are not
separated from these layers ; they are formed of fine
nerve-fibres and ganglion-cells. The epithelium above
the nerve rings contains sense-cells (fig. 237) with a
stiff hair at their free extremity, and a nervous prolongation at the opposite end, which joins the nervefibres of the ring. Between such cells and true ganglioncells an intermediate type of cell has been found (fig.
237 B) which sends a process upwards amongst the
epithelial cells, but does not reach the surface. Such cells, as the Hertwigs
have pointed out, are clearly sense-cells partially transformed into ganglioncells.
 
A still higher type of nervous system has been met with amongst some
primitive Nemertines (Hubrecht, No. 323), consisting of a pair of large
cephalic ganglia, and two well-developed lateral ganglionic cords placed
close beneath the epidermis. These cords, instead of giving off definite
nerves, as in animals with a fully differentiated nervous system, are connected with a continuous subdermal nervous plexus.
 
The features of the embryology and the anatomy of the
nervous system, to which attention has just been called, point to
the following general conclusions as to the evolution of the
nervous system.
 
(1) The nervous system of the higher Metazoa appears to
have been evolved in the course of a long series of generations
from a differentiation of some of the superficial epithelial cells of
the body, though it is possible that some parts of the system
may have been formed by a differentiation of the alimentary
epithelium.
 
(2) An early feature in the differentiation consisted in the
growth of a series of delicate processes of the inner ends of
 
26 2
 
 
 
404
 
 
 
EVOLUTION OF THE NERVOUS SYSTEM.
 
 
 
certain epithelial cells, which became at the'same time especially
differentiated as sense-cells (figs. 236 and 237).
 
 
 
 
FIG. 237. ISOLATED CELLS BELONGING TO THE UPPER NERVE-RING OF CARMARINA
HASTATA. (After O. and R. Hertwig.)
 
A. Neuro-epithelial sense-cell, c. sense-hair.
 
B. Transitional cell between a neuro-epithelial cell and a ganglion -cell.
 
(3) These processes gave rise to a subepithelial nervous
plexus, in which ganglion-cells, formed from sense-cells which
travelled inwards and lost their epithelial character (fig. 237 B),
soon formed an important part.
 
(4) Local differentiations of the nervous network, which was
no doubt distributed over the whole body, took place partly in
the formation of organs of special sense, and partly in other
ways, and such differentiations gave rise to a central nervous
system. The central nervous system was at first continuous
with the epidermis, but became separated from it and travelled
inwards.
 
(5) Nerves, such as we find them in the higher types,
originated from special differentiations of the nervous network,
radiating from the parts of the central nervous system.
 
The following points amongst others are still very obscure :
 
(1) The steps by which the protoplasmic processes from the primitive
epidermic cells became united together so as to form a network of nervefibres, placing the various parts of the body in nervous communication.
 
(2) The process by which nerves became connected with muscles, so
that a stimulus received by a nerve-cell could be communicated to and
cause a contraction in a muscle.
 
It is probable, as stated in the above summary, that the nervous net
 
 
NERVOUS SYSTEM. 405
 
 
 
 
work took its origin from processes of the sense-cells. The processes of the
different cells probably first met and then fused together, and, becoming
more arborescent, finally gave rise to a complicated network.
 
The primitive relations between the
nervous network and the muscular system
are matters of pure speculation. The
primitive muscular cells consist of epithelial cells with muscular processes (fig. 238),
but the branches of the nervous network
have not been traced into connection with FIG. 238. MYO-EPITHELIAL
 
the muscles in any Ccelenterata except CELLS OF HYDRA. (From Gegenthe Ctenophora. In the higher types a baur 5 after Kleinenberg.)
continuity between nerves and muscles ' contractile fibres; processes
 
in the form of motorial end plates has
 
been widely observed. Even in the case of the Ccelenterata it is quite
clear from Romanes' experiments that stimuli received by the nerves are
capable of being transmitted to the muscles, and that there must therefore
be some connection between nerves and muscles. How did this connection
originate?
 
Epithelial cells with muscular processes (fig. 238) were discovered by
Kleinenberg (No. 324) in Hydra before epithelial cells with nervous processes were known, and Kleinenberg pointed out that Hydra shewed the
possibility of nervous and muscular tissues existing without a central nervous
system, and suggested that the epithelial part of the myo-epithelial cells was
a sense-organ, and that the connecting part between this and the contractile
processes was a rudimentary nerve. He further supposed that in the subsequent evolution of these elements the epithelial part of the cell became a
ganglion-cell, while the part connecting this with the muscular tail became
prolonged so as to form a true nerve. The discovery of neuro-epithelial
cells existing side by side with myo-epithelial cells demonstrates that this
theory must in part be abandoned, and that some other explanation must be
given of the continuity between nerves and muscles. The hypothetical
explanation which most obviously suggests itself is that of fusion.
 
It seems quite possible that many of the epithelial cells of the epidermis
and walls of the alimentary tract were originally provided with processes,
the protoplasm of which, like that of the Protozoa, carried on the functions
of nerves and muscles at the same time, and that these processes united
amongst themselves into a network. Such cells would be very similar to
Kleinenberg's neuro-muscular cells. By a subsequent differentiation some
of the cells forming this network may have become specially contractile, the
epithelial parts of the cells ceasing to have a nervous function, and other
cells may have lost their contractility and become solely nervous. In this way
we should get neuro-epithelial cells and myo-epithelial cells both differentiated from the primitive network, and the connection between the two would
also be explained. This hypothesis fits in moreover very well with the
condition of the neuro-muscular system as we find it in the Coelenterata.
 
 
 
406 INVERTEBRATA.
 
 
 
BIBLIOGRAPHY.
Origin of the Nervous System,
 
(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the
British Association." 1880.
 
(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen,
Discomedusen." Denk. d. math.-naturwiss. Classe d. k. Akad. Wiss. Wien, Vol.
xxxvin. 1877.
 
(318) Th. Eimer. Zoologische Studien a, Capri. I. Ueber Beroe ovatus, Ein
Beitrag 2. Anat. d. Rippenquallen. Leipzig, 1873.
 
(319) V. Hen sen. " Zur Entwicklung d. Nervensystems. " Virchmifs Archiv,
Vol. xxx. 1864.
 
(320) O. and R. Hertwig. Das Nerveiisystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.
 
(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriicksichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xin. 1879.
 
(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift,
Vol. xiv. 1880.
 
(323) A. W. Hubrecht. "The Peripheral Nervous System in Palaeo- and
Schizonemertini, one of the layers of the body- wall." Quart. J. of After. Science,
Vol. xx. 1880.
 
(324) N. Kleinenberg. Hydra, eine anatomisch-entwicklungsgeschichtliche Untersuchung. Leipzig, 1872.
 
(325) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Acad. Petersbourg, Series VII., Vol. XVI. 1871.
 
(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita."
Phil. Trans. 1878.
 
Nervous system of the Invertebrata. Our knowledge of
the development of the central nervous system is still very
imperfect in the case of many Invertebrate groups. In the
Echinodermata and some of the Ghaetopoda it is never detached
from the epidermis, and in such cases its origin is clear without
embryological evidence.
 
In the majority of groups the central nervous system may be
reduced to the type of a pair of cephalic ganglia, continued posteriorly into two cords provided with nerve-cells, which may
coalesce ventrally or be' more or less widely separated, and be
unsegmented or segmented. Various additional visceral ganglia
may be added, and in different instances parts of the system
may be much reduced, or peculiarly modified. The nervous
system of the Platyelminthes (when present), of the Rotifera,
Brachiopoda, Polyzoa (?), the Mollusca, the Chaetopoda, the
 
 
 
NERVOUS SYSTEM. 407
 
 
 
Discophora, the Gephyrea, the Tracheata, and the Crustacea,
the various small Arthropodan phyla (Pcecilopoda, Pycnognida,
Tardigrada, &c.), the Chaetognatha (?), and the Myzostomea,
probably belongs to this type.
 
The nervous system of the Echinodermata cannot be reduced
to this form ; nor in the present state of our knowledge can that
of the Nematelminthes or Enteropneusta.
 
It is only in the case of members of the former set of groups
that any adequate observations have yet been made on the
development of the nervous system, and even in the case of
these groups observations which have any claim to completeness
are confined to certain members of the Chaetopoda, the Arthropoda and the Mollusca. An account of imperfect observations
on other forms, where such have been made, will be found in the
systematic part of this work.
 
Chaetopoda. We are indebted to Kleinenberg (No. 329) for
the most detailed account which we have
of the development of the central nervous
system in the Chaetopoda.
 
The supracesophageal ganglion with
the cesophageal commissure developes independently of the ventral cord. It arises
as an unpaired thickening of the epiblast, p IG- 239 . SECTION
close to the dorsal side of the oesophagus THROUGH THE HEAD OF
 
A 'YOUNG EMBRYO OF
 
at the front end of the head (fig. 239), LUMBRICUS TRAPEZOIDES.
which becomes separated from the epi- < After Kleinenber s-)
 
, e.g. cephalic ganglion ;
 
blast, and extends obliquely backwards CCi cephalic portion of the
and downwards in a somewhat arched body cavity ;*. oesophagus.
form ; its lower extremities being somewhat swollen. The
inner portion of this curved rudiment becomes converted into
commissural nerve-fibres, while the cells of the outer and upper
portion assume the characters of ganglion-cells. The commissural fibres are continued downwards to meet the ventral
chord, but their junction with the latter structure is not effected
till late in embryonic life.
 
The ventral cord is formed by the coalescence of a pair of
linear cords, the development of which takes place from before
backwards, so that when their anterior part is well developed their
posterior part is hardly differentiated. These cords arise, one on
 
 
 
 
408
 
 
 
CH^TOPODA.
 
 
 
 
FIG. 240. SECTION THROUGH
PART OF THE VENTRAL WALL OF THK
TRUNK OF AN EMBRYO OF LUMBRIcus TRAPEZOIDES. (After Kleinenberg.)
 
m. longitudinal muscles ; so. somatic mesoblast ; sp. splanchnic mesoblast; hy. hypoblast; Vg- ventral
nerve-cord; w. ventral vessel.
 
 
 
each side of a ventral ciliated furrow, first as a single row of epiblast cells, and subsequently as several rows (fig. 240, Vg). While
still united to the external epiblast, they extend themselves below the cells lining the ventral
furrow, and unite into a single
nervous band, which however
exhibits its double origin by its
bilobed section. Before the two
cords unite, the groove between
them becomes somewhat deep,
but subsequently shallows out
and disappears. The nervous
band, before separating from the
epiblast, exhibits, in correspondence with the mesoblastic segments, alternate swellings and
constrictions. The former become the ganglia, and the latter the
connecting trunks.
 
As soon as the cord becomes free from the epiblast, it
becomes surrounded by a sheath, formed of somatic mesoblast.
In each of the ganglionic enlargements there next appears on
the dorsal surface a pair of areas of punctiform material, the
substance of which soon differentiates itself into .nerve-fibres.
These areas, by uniting from side to side, give rise to the
transverse commissures, and also by a linear coalescence to the
longitudinal commissures of the cord. The cellular parts of the
band surrounding them become converted into a ganglionic
covering of the cord.
 
In each ganglion the cells of this ganglionic investment
penetrate as a median septum into the cord. A fissure is next
formed, dividing this septum into two ; it is subsequently
continued for the whole length of the cord.
 
Arthropoda. In the Tracheata and the Crustacea the
development of the ventral cord is in the main similar to that in
the Chaetopods, while that of the supracesophageal ganglia is as
a rule somewhat more complicated. No such clear evidence of
an independent development of these two parts, as in the case
of the Chaetopods, has as yet been produced.
 
The most primitive type of nervous system amongst the
 
 
 
NERVOUS SYSTEM. 409
 
 
 
Tracheata is that of Peripatus, where it consists of large supraoesophageal ganglia, continuous with a pair of widely separated
but large ventral cords united posteriorly above the anus. These
cords have an investment of ganglion- cells for their whole length,
and are imperfectly divided into ganglia corresponding in
number with the feet.
 
The ventral cords are formed as two separate epiblastic
ridges (fig. 241, v.n], continued in front into a pair of thickenings
 
 
 
 
FIG. 241. SECTION THROUGH THE TRUNK OF AN EMBRYO OF PERIPATUS.
The embryo from which the section is taken was somewhat younger than that of
fig. 242.
 
sp.m. splanchnic mesoblast ; s.m. somatic mesoblast ; me. median section of body
cavity ; Ic. lateral section of body cavity ; -v. 11. ventral nerve cord ; me. mesenteron.
 
of the procephalic lobes, which are at first independent of each
other, and from which a large part of the supracesophageal
ganglia takes its origin. . After the latter have become separated
from the epiblast an invagination of the epiblast covering them
grows into each lobe (fig. 242), and becoming constricted from
the superficial epiblast, which remains as the epidermis, forms a
not unimportant part of the permanent supracesophageal ganglia.
 
In the Arachnida the mode of development of the nervous
system is essentially the same, and the reader will find a
detailed account of it for Spiders in Vol. II. pp. 447 451. The
ventral cords are here formed as independent and at first widely
separated strands (fig. 243, vii), which for a long time remain far
apart ; they are subsequently divided into ganglia and become
united by transverse commissures.
 
The supracesophageal ganglia are formed as two independent
 
 
 
4io
 
 
 
ARTHROPODA.
 
 
 
thickenings of the procephalic lobes (fig. 244), which eventually
separate from the superficial skin. There is formed however in
 
 
 
 
FIG. 242. HEAD OF AN EMBRYO PERIPATUS. (From Moseley.)
The figure shews the jaws (mandibles), and close to them epiblastic involutions,
which grow into the supracesophageal ganglia. The antennas, oral cavity, and oral
papillae are also shewn.
 
each of them a semicircular groove (fig. 244, gr) lined by the
superficial epiblast, which becomes detached from the skin, and
is involuted to form part of the ganglia.
 
A similar mode of formation of both the ventral cords and
the supraoesophageal ganglia obtains in Insects (fig. 245). The
 
 
 
 
FIG. 243. TRANSVERSE SECTION THROUGH THE VENTRAL PLATE OF AGELENA
 
LABYRINTHICA.
 
The ventral cords have begun to be formed as thickenings of the epiblast, and the
limbs are established.
 
me.s. mesoblastic somite; vn. ventral nerve-cord; yk. yolk.
 
ventral cords are however much less widely separated than in
Spiders, and early unite in the median line. In the supraoesophageal ganglia the invaginated epiblast has in Lepidoptera
(Hatschek) the form of a pit on the dorsal border of the
antennae.
 
 
 
NERVOUS SYSTEM.
 
 
 
Hatschek states that there takes place an invagination of a median part
of the skin between the two ventral cords, for the details of which I must
refer the reader to Vol. II. p. 410. He has made more or less similar
statements for the earthworm, but his observations in both instances are
open to serious doubt.
 
 
 
ce.s
 
 
 
 
FIG. 244. SECTION THROUGH THE PROCEPHALIC LOBES OF AN EMBRYO OF
 
AGELENA LABYRINTHICA.
 
st. stomadaeum; gr. section through semi-circular groove in procephalic lobe;
ce.s. cephalic section of body cavity.
 
Full details as to the development of the nervous system in
the Crustacea are still wanting ; a fairly complete account of
 
 
 
nie.s
 
 
 
 
 
FlG. 245. TWO TRANSVERSE SECTIONS THROUGH THE EMBRYO OF HYDROPHILUS.
 
(After Kowalevsky.)
 
A. Transverse section through an embryo in the region of one of the stigmata.
 
B. Transverse section through an older embryo.
 
vn. ventral nerve-cord ; am. amnion and serous membrane ; me. mesoblast ; me.s.
somatic mesoblast; hy. hypoblast (?) ; yk. yolk-cells (true hypoblast); st. stigma of
trachea.
 
 
 
412 GEPHYREA.
 
 
 
what is known on the subject is given in Vol. n. pp. 521 2. It
appears that the ventral cord may either arise as an unpaired
thickening of the epiblast (Isopoda), marked however by a
shallow median furrow, or from two cords which eventually
coalesce 1 . It is not certain how far the supracesophageal
ganglia are usually in the first instance continuous with the
ventral cord. In Astacus, the early stages of which have been
elaborately investigated by Reichenbach (No. 331), they are
stated to be so ; the supracesophageal ganglia are moreover described by this author as having a somewhat complicated origin.
Five elements enter into their composition. There is first
formed a pair of pits on the procephalic lobes, which become
very deep during the Nauplius stage, and are continuous with a
pair of epiblastic ridges which pass round the mouth, and join
the ventral cords just described. The walls of the pits are
believed to form a part of the embryonic ganglia which gives
rise to the retina as well as to the optic ganglia. The ridges
form the remainder of the ganglia and the cesophageal commissures ; while the fifth element is supplied by a median
invagination in front of the mouth, which appears at a much
later date than the other parts.
 
In the Isopoda supracesophageal ganglia are stated to arise
as thickenings of the procephalic lobes, which become eventually
detached from the epidermis.
 
The ventral cord is at first unsegmented, but soon becomes
partially divided by a series of constrictions into a number of
ganglia, corresponding with the segments. The development of
the commissural and ganglionic portions takes place much as in
the Chaetopoda.
 
The Gephyrea approach closely the types so far dealt with, but the
ventral cord in the Inermia is formed as an unpaired thickening of the
epiblast. In Echiurus, as has been shewn by Hatschek in an interesting
paper on the larva of this species, published since the appearance of the first
volume, there is a pair of ventral cords 2 . In correspondence with a general
segmentation of the body, which is subsequently lost, these cords become
 
1 Reichenbach (No. 331) holds that the walls of the groove between the two
strands of the ventral cords become invaginated and assist in the formation of the
ventral cord.
 
8 " Ueber Entwicklungsgeschichte d. Echiurus." Arbeit, a. d. zool. Instit. Wien
Vol. ill. 1880.
 
 
 
NERVOUS SYSTEM.
 
 
 
4'3
 
 
 
segmented. The two cords unite in the median line, and Hatschek, in
accordance with his general view on this subject, states that their junction is
effected by means of a median cord of invaginated epiblast. The segmentation of the cords subsequently becomes lost. The supracesophageal ganglia
arise as an unpaired median thickening of the procephalic lobe. No traces
of segmentation in the ventral cord have been observed by Spengel in
Bonellia, and the supracesophageal ganglion is formed in this genus as an
unpaired band.
 
 
 
In all the groups above considered the nervous system
clearly presents the same type of development with various
modifications.
 
It is formed of two parts, viz. (i) the supracesophageal
ganglia, and (2) the ventral cord.
 
In the simpler forms, Chaetopoda and Gephyrea, the supracesophageal ganglia are usually stated to be formed as an
unpaired thickening at the apex of the praeoral lobe, which in
most cases becomes subsequently bilobed.
 
In the Arthropoda the unpaired praeoral lobe of the Chaetopoda is replaced by the so-called procephalic lobes, which are
themselves bilobed ; and the supracesophageal ganglia are
formed of two independent halves ; further complications in
development are also generally found.
 
There is not as yet sufficient evidence to decide whether the
supracesophageal ganglia were primitively developed continuously with, or independently of, the ventral cords.
 
The ventral cord appears in the embryo as two independent
unsegmented strands, although in a few cases (some Crustacea
and Gephyrea) these cords, by an abbreviation in development,
arise as an unpaired median thickening of the epiblast.
 
The form of nervous system of the Chaetopoda, Arthropoda,
and Gephyrea is clearly therefore to be derived, as was first
pointed out by Gegenbaur, from a more or less similar type to
that now found in the Nemertines ; and as suggested in the
chapter on larval forms (vide p. 378) may perhaps be derived
from the elongation of a circular ring, of which the anterior end
has become developed into the supracesophageal ganglia, the
lateral parts into the two lateral strands, while the posterior
part persists in some forms in the junction of the ventral cords
above the anus (Enopla and Peripatus).
 
 
 
414 MOLLUSCA.
 
 
 
Mollusca. While study of the anatomy of the nervous system of the
Mollusca, especially of certain primitive genera (Chiton, Haliotis, Fissurella,
&c.) leaves little doubt that it is formed on the same type as that of the
groups just spoken of, the development, so far as our imperfect knowledge
enables us to make definite statements on the subject, is somewhat abnormal 1 .
 
In the Gasteropoda and Pteropoda the supracesophageal ganglia appear
most probably to be developed either as paired thickenings of the epiblast
of the velar area, or as invaginated pits of the velar area, which become
detached from the surface, and then become solid (Hyaleacea and Limax).
In either case the supracesophageal ganglia appear to be developed quite
independently of the pedal ganglia. The latter, as might be anticipated, are
earlier in their development and more constant than the various visceral
ganglia ; and, if the views above expressed are correct, are homologous
with the ventral cord of the Chaetopods and Arthropods. Their actual
development is very imperfectly known.
 
The most precise statements on the subject, viz. those of Bobretzky and
Fol, would lead us to suppose that they arise in the mesoblast, but it seems
more probable that they are formed as thickenings of the sides of the foot.
 
In the Cephalopods all the ganglia are stated to be differentiated in the
mesoblast (Lankester, Bobretzky).
 
Hatschek 2 has recently given a detailed description of the development
of the supracesophageal and pedal ganglia of Teredo. He finds that the
former ganglia arise as an unpaired thickening of the epiblast in the centre
of the velar area, and the latter as an unpaired thickening of the epiblast
of the ventral side of the body between the mouth and the anus. The two
ganglia would thus seem to be disconnected with each other in their
development.
 
(327) F. M. Balfour. "Notes on the development of the Araneina." Quart.
J. of Micr. Science, Vol. XX. 1880.
 
(328) B. Hatschek. " Beitr. z. Entwicklung d. Lepidopteren." Jenaische
Zeitschrift, Vol. xi. 1877.
 
(329) N. Kleinenberg. "The development of the Earthworm, Lumbricus
Trapezoides." Quart. J. of Micr. Science, Vol. XIX. 1879.
 
(330) A. Kowalevsky. " Embryologische Studien an Wiirmem u. Arthropoden." Mem. Acad. Petersbourg, Series vin., Vol. XVI. 1871.
 
(331) H. Reichenbach. " Die Embryonalanlage u. erste Entwick. d. Flusskrebses." Zeit.f. wiss. Zool., Vol. xxix. 1877.
 
1 Vide Vol. ii., pp. 273, 274.
 
2 " Ueber Entwicklungsgeschichte von Teredo." Arbeit, a. d. zool. Instit, IVieit,
Vol. in. 1880.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 415
 
 
 
THE CENTRAL NERVOUS SYSTEM OF THE VERTEBRATA 1 .
 
The formation of the cerebro-spinal axis of the Chordata
from the medullary plate has already been treated at length
(pp. 301 304). Before entering into the consideration of the
morphological value of the various parts of this cord, it will be
convenient to describe the more important features of its
ontogeny. For this purpose the two parts into which the
nervous axis becomes at an early period divided, viz. the spinal
cord and the brain, may be dealt with separately.
 
The Spinal Cord, shortly after the closure of the medullary
canal, has, in all the true Vertebrata, the form of an oval tube ;
the walls of which are of a fairly uniform thickness, and are
composed of several rows of elongated cells. This cord, as
development proceeds, usually becomes vertically prolonged in
transverse section, and the central canal which it contains also
becomes vertically elongated. The variations in shape of the
spinal canal are very great at different periods and in different
parts of the body, and an attempt to chronicle them would
appear, in the present state of our knowledge, to be quite
valueless' 2 . Fig. 117, in which the spinal cord of the chick
of the third day is shewn in transverse section, illustrates the
character of the cord at the stage just described. Up to this
time the walls of the spinal canal have exhibited an uniform
structure. A series of changes now however takes place, which
results in the differentiation (i) of the epithelium of the central
canal, (2) of the grey matter of the cord, and (3) of the external
coating of white matter.
 
The relative time at which each of these parts becomes
developed is not constant in the different forms.
 
The white matter is apparently the result of a differentiation
of the outermost parts of the superficial cells of the cord into
 
1 For the development of the central nervous system in Amphioxus and the
Tunicata the reader is referred to the chapters dealing with those two groups.
 
2 Lowe (No. 341) holds that at an early stage of development three regions can
always be distinguished in any section of the central canal, viz. (i) a ventral narrow
slit, (2) a median enlargement, and (3) a dorsal slit. Such a form can no doubt often
be observed, but my own observations do not lead me to attach any special importance
to it.
 
 
 
41 6 SPINAL CORD.
 
 
 
longitudinal nerve-fibres, which remain for a long period without
a medullary sheath. These fibres appear in transverse sections
as small dots. The white matter forms a transparent investment
of the grey matter and would seem to contain neither nuclei nor
cells 1 . The white matter may from the first form only two
masses, one on each side, forming a layer on the ventral and
lateral parts of the spinal cord but not extending to the dorsal
surface (Elasmobranchii, fig. 185, W) ; or it may form four
patches, viz. an anterior and a posterior white column on each
side, which lie on a level with the origin of the anterior and
 
 
 
 
c
 
 
 
FIG. 246. SECTION THROUGH THE SPINAL CORD OF A SEVEN DAYS' CHICK.
 
pew. dorsal white column ; lew. lateral white column ; acw. ventral white column ;
c. dorsal tissue filling up the part where the dorsal fissure will be formed ; pc. dorsal
grey cornu ; ac. anterior grey cornu; ep. epithelial cells; age. anterior commissure;
pf. dorsal part of spinal canal ; spc. ventral part of spinal canal ; af. anterior fissure.
 
posterior nerve-roots (the Fowl, Human embryo, etc.). In
whichever of these forms the white matter appears, it is always,
at first, a layer of extreme tenuity, which rapidly increases
 
1 This holds true at first for Elasmobranchii, but at a later stage there are present
numerous nerve-cells in the white matter, so that the distinction between the white
and grey matter becomes much less marked than in higher types; in this respect Elasmobranchii present an approximation to Amphioxus.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 417
 
in thickness in the subsequent stages, and extends so as
gradually to cover the whole cord (fig. 246).
 
The anterior white commissure is formed very shortly after
the first appearance of the white matter. The grey matter and
the central epithelium are formed by a differentiation of the
main mass of the spinal cord. The outer cells lose their
epithelial-like arrangement, and, becoming prolonged into fibres,
give rise to the grey matter, while the innermost cells retain
their primitive arrangement, and constitute the epithelium of the
canal. The process of formation of the grey matter would
appear to proceed from without inwards, so that some of the
cells, which have, on the formation of the grey matter, an
epithelial-like arrangement, subsequently become converted into
true nerve-cells.
 
As has already been mentioned, the central epithelium of
the nervous system probably corresponds with the so-called
epidermic layer of the epiblast.
 
The grey matter soon becomes prolonged dorsally and
ventrally into the posterior and anterior horns. Its fibres may
especially be traced in two directions: (i) round the anterior
end of the spinal canal, immediately outside its epithelium and
so to the grey matter on the opposite side, forming in this way
an anterior grey commissure, through which a decussation of
the fibres from the opposite sides is effected : (2) dorsalwards
along the outside of the lateral walls of the canal.
 
There is at this period no trace of the ventral or dorsal
fissure, and the shape of the central canal is not very different to
what it was at an earlier period. This condition of the spinal
cord is especially instructive, as it is very nearly that which is
permanent in Amphioxus.
 
The next event of importance is the formation of the ventral
or anterior fissure. This owes its origin to a downgrowth of the
anterior horns of the cord on each side of the middle line. The
two downgrowths enclose between them a somewhat linear
space the anterior fissure which increases in depth in the
succeeding stages (fig. 246, af}.
 
The dorsal or posterior fissure is formed at a later period
than the anterior, and accompanies the atrophy of the dorsal
section of the embryonically large canal of the spinal cord.
B. III. 2 7
 
 
 
41 8 SPINAL CORD.
 
 
 
The exact mode of its formation appears to me to be still
involved in some obscurity.
 
In the Elements of Embryology the development of the posterior fissure
was described in the following way :
 
" On the seventh day the most important event is the formation of the
posterior fissure,
 
" This is brought about by the absorption of the roof of the posterior of
the two parts into which the neural canal has become divided.
 
"Between the posterior horns of the cord, the epithelium forming the
roof of the, so to speak, posterior canal is along the middle line covered
neither by grey nor by white matter, and on the seventh day is partially
absorbed, thus transforming the canal into a wedge-shaped fissure, whose
mouth however is seen in section to be partially closed by a triangular
clump of elongated cells (fig. 246, c]. Below this mass of cells the fissure
is open. It is separated from the 'true spinal canal' by a very narrow space
along which the side walls have coalesced. In the lumbar and sacral regions
the two still communicate.
 
"We thus find, as was first pointed out by Lockhart Clarke, that the
anterior and posterior fissures of the spinal cord are, morphologically speaking, entirely different. The anterior fissure is merely the space left between
two lateral downward growths of the cord, while the posterior fissure is part
of the original neural canal separated from the rest of the cavity (which goes
to form the true spinal canal) by a median coalescence of the side walls."
 
I confess that I have some doubts as to the complete accuracy of the
above statement.
 
Kolliker gives a full account of the gradual atrophy of the central canal ;
but I do not fully understand his statements with reference to the formation
of the posterior fissure, which in fact appears to be only incidentally
mentioned. It would seem from his account that a shallow and somewhat
wide dorsal fissure is formed to start with, in the human embryo, by two
projections of the posterior white horns. On the atrophy of the central
canal this furrow becomes narrowed, but Kolliker does not definitely state
how it becomes deepened so as to give rise to the permanent dorsal fissure.
 
It seems to me probable, though further investigations on
the point are still required, that the dorsal fissure is a direct
result of the atrophy of the dorsal part of the central canal
of the spinal cord.
 
The walls of the canal coalesce dorsally, and the coalescence
gradually extends ventralwards, so as finally to reduce the
central canal to a minute tube, formed of the ventral part of the
original canal. The epithelial wall formed by the coalesced
walls on the dorsal side of the canal is gradually absorbed.
 
The epithelium of the central canal, at the period when its
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 419
 
atrophy commences, is not covered dorsally either by grey
or white matter, so that, with the gradual reduction of the
dorsal part of the canal, and the absorption of the epithelial wall
formed by the fusion of its two sides, a fissure between the two
halves of the spinal cord becomes formed. This fissure is the
posterior or dorsal fissure. In the process of its formation the
white matter of the dorsal horns becomes prolonged so as to
line its walls ; and shortly after its formation the dorsal grey
commissure makes its appearance, which is not improbably
derived from part of the epithelium of the original central
canal.
 
Development of the Brain.
 
The brain is formed from the anterior portion of the medullary plate. When the medullary plate first becomes differentiated it is not possible to distinguish between the region of
the brain and that of the spinal cord. The brain region is
however usually very early indicated by a widening of the
medullary plate, but does not become sharply marked off from
the region of the spinal cord. In many Ichthyopsida (Elasmobranchii (fig. 28, C) and Amphibia (fig. 77, A)) the anterior
dilatation gives to the medullary plate, before its sides meet to
form a canal, a spatula-like form ; which is either not present or
less marked in Reptilia, Aves and Mammalia.
 
The length of the brain as compared to the spinal cord is
always very great in the embryo, and in the earliest developmental periods the disproportion in the size of the brain is
specially marked, owing to the full number of the somites of
the trunk not having been formed. In Elasmobranchii the
brain is about one-third of the whole length of the embryo at
the stage immediately following the closure of the medullary
canal.
 
The first differentiation of the brain into distinct parts is
a very early occurrence, and may take place before (Mammalia)
or during the closure of the medullary folds. The brain first
becomes divided into two successive lobes or vesicles by a
single transverse constriction, and subsequently the posterior of
these again 'becomes divided into two, so that three lobes
 
272
 
 
 
42O
 
 
 
THE BRAIN.
 
 
 
are formed known as the fore- the mid- and the hind-brain ; of
these the hind-brain is usually the longest. In some instances a
bilobed stage can hardly be recognised. This primitive division
of the brain is shewn in many of the figures already given.
The reader may perhaps best refer to fig. 108. On the closure
of the medullary groove the lumen of the medullary canal
is continued uninterruptedly through the brain, but dilates
considerably in each of the cerebral vesicles.
 
The anterior lobe of the brain becomes converted into the
cerebral hemispheres, the thalamencephalon, the primary optic
vesicles, and the parts connected with them. The middle lobe
becomes the optic lobes (corpora bigemina or corpora quadrigemina in Mammalia) and the crura cerebri ; while the posterior
lobe becomes converted into the cerebellum and medulla
oblongata.
 
Before describing in detail the changes by which the primary
vesicles of the brain become converted into the above parts, it will
be convenient to say a few words
about the general development of
the brain.
 
The most striking peculiarity
with reference to the general development of the brain is a curvature
which appears in its axis, known as
the cranial flexure. The flexure
takes place through the mid-brain ;
and causes the fore-brain to be
gradually bent downwards so that
the axis of its floor forms, first, a
right angle with that of the hinder
part of the brain, and subsequently,
as a rule, an acute angle.
 
During these changes the brain,
in most Amniota at any rate, becomes in the first instance
retort-shaped, the cerebral vesicle forming the swollen part of
the retort, but subsequently the retort-shape is lost owing to the
great development of the vesicle of the mid-brain, which forms
the termination of the long axis of the embryo. Figs. 29, 76,
 
 
 
 
FIG. 247. LONGITUDINAL
SECTION THROUGH THE BRAIN
OF A YOUNG PRISTIURUS EMBRYO.
 
cer. commencement of the
cerebral hemisphere ; pn. pineal
gland ; In. infundibulum ; pt. ingrowth from mouth to form the
pituitary body ; mb. mid-brain ;
cb. cerebellum ; ch. notochord. ;
al. alimentary tract ; laa. artery
of mandibular arch.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
421
 
 
 
mb
 
 
 
pn.
 
 
 
 
and 1 1 8, are representative figures of embryos of various
vertebrate forms at a period when the mid-brain forms the
termination of the long axis of the body.
 
It is generally stated that the
cranial flexure is at its maximum
at the stage represented in these
figures, and there can be no
doubt that viewed from the exterior the cranial flexure ceases
to be so marked a feature, and
finally disappears as the embryo
gradually grows older ; but
though the mid-brain ceases to
form the termination of the long
axis of the embryo, the flexure
of the brain becomes in many
forms absolutely more marked ;
while in other forms, though
stated to diminish, it does not
entirely vanish.
 
The general nature of the
changes which take place will
perhaps best be understood by a
comparison of figs. 247 and 248
representing longitudinal sections at two stages through the brain of an embryo Elasmobranch. The actual cranial flexure, i.e. flexure of the floor of
the brain, is obviously greater in the older of the two brains,
though viewed from the exterior the axis of this brain appears
to be quite straight. In the younger stage, fig. 247, the midbrain (mb) forms the end of the long axis of the body, while in
the older one the cerebral hemispheres (cer) have grown very
greatly, especially forwards and dorsalwards. They have thus
come to lie in front of the mid-brain, and to form the end of the
long axis of the body, and have at the same time compressed
the originally large thalamencephalon against the mid-brain.
The same general features may be seen in fig. 250 representing
a longitudinal section of the brain of an embryo fowl, and fig. 255
representing a longitudinal section of the brain of a Mammal.
 
 
 
FIG. 248. LONGITUDINAL SECTION
THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF
DEVELOPMENT.
 
cer. cerebral hemisphere ; pn. pineal gland ; op.th. optic thalamus, connected with its fellow by a commissure
(the middle commissure). In front of
it is seen a fold of the roof of the forebrain, which is connected with the choroid plexus of the third ventricle ; op.
optic chiasma ; //. pituitary body ; in.
infundibulum ; cb. cerebellum ; ati.v.
passage leading from the auditory vesicle to the exterior ; mel. medulla oblongata ; c.in. internal carotid artery.
 
 
 
422 HISTOGENESIS OF THE BRAIN.
 
The infundibulum or perhaps rather the point of origin
of the optic nerves is to be regarded as the anterior termination
of the axis of the base of the brain.
 
The cranial flexure is least marked in Cyclostomata (fig. 253), Teleostei,
Ganoidei, and Amphibia, while it is very pronounced in Elasmobranchii,
Reptilia, Aves, and Mammalia. In Teleostei, and still more in Cyclostomata,
it permanently remains slight, owing to the small development of the
cerebral hemispheres.
 
In addition to the cranial flexures, two other flexures make their
appearance in the base of the brain. A posterior at the junction of the
brain and spinal cord, and an anterior at the boundary between the
cerebellum and medulla oblongata, just at the point where the pons Varolii
is formed in Mammalia. The anterior of these is the most marked and
constant ; it is shewn in fig. 250. It arises considerably later than the main
cranial flexure, and since it is turned the opposite way it assists to a considerable extent in causing the apparent straightening of the cranial axis.
 
Histogenetic changes 1 . The walls of the brain are at first
very thin and, like those of the spinal cord, are formed of a
number of ranges of spindle-shaped cells. The processes of
each of these cells are stated to be continued through the whole
thickness of the wall. In the floor of the hind- and mid-brain a
superficial layer of delicate nerve-fibres is formed at an early
period. This layer appears in the first instance on the floor and
sides of the hind-brain, and very slightly, if at all, later on the
floor and the sides of the mid-brain. The cells internal to the
nerve-fibres become differentiated into an innermost epithelial
layer lining the cavities of the ventricles, and an outer layer of
grey matter.
 
The similarity of the primitive arrangement and histological
character of the parts of the brain behind the cerebral hemispheres to that of the spinal cord is very conclusively shewn by
the examination of any good series of sections. In both brain
and spinal cord the white matter forms a cap on the ventral and
lateral parts considerably before it extends to the dorsal surface.
In the medulla the white matter does not eventually extend to
the roof owing to the peculiar degeneration which that part
undergoes.
 
1 It is not within the scope of this work to give an account of the histogenesis of
the brain; in the statement in the text only a few points, of some morphological
importance, are touched on.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 423
 
In the case of the fore-brain the earliest histological changes,
except possibly in Mammals, take place on the same general
plan as those of the remainder of the central nervous system 1 ;
but though the general plan is the same, yet the early histological distinction between the fore-brain, and the mid- and hindbrain is more marked than the distinction between the latter
and the spinal cord.
 
On the floor and sides of the thalamencephalon, and apparently the whole of the hemispheres of the lower types, there is
formed, somewhat later than in the remainder of the brain, a
very delicate layer of white matter. The inner part of the wall,
which still remains comparatively thin, is not at first clearly
divided into an epithelial and nervous layer. This distinction
soon however becomes more or less apparent, though it is not so
marked as in most other parts of the brain ; and it appears that
in the subsequent growth the greater part of the original
epithelial layer becomes converted into nervous tissue.
 
In Mammals the same plan of differentiation would seem to
be followed, though somewhat less obviously than in the lower
types. The walls of the hemispheres become first divided
(Kolliker) into a superficial thinner layer of rounded elements,
and a deeper and thicker epithelial layer, and between these the
fibres of the crura cerebri soon interpose themselves. At a
slightly later period a thin superficial layer of white matter,
homologous with that of the remainder of the brain, becomes
established.
 
The inner layer, together with the fibres from the crura
cerebri, gives rise to the major part of the white matter of the
hemispheres and to the epithelium lining the lateral ventricles.
 
The outer layer of rounded cells becomes divided into
(i) a superficial part with comparatively few cells, which,
together with its coating of white matter, forms the cortical
part of the grey matter, and (2) a deeper layer with numerous
cells which forms the main mass of the grey matter of the
hemispheres.
 
The development of the several parts of the brain will now
be described.
 
1 I have worked out these changes in Elasmobranchii, Amphibia (Salamandra)
and Aves.
 
 
 
424
 
 
 
THE HIND-BRAIN.
 
 
 
The hind-brain. The hind-brain is at first an elongated,
funnel-shaped tube, the walls of which are of a nearly uniform
thickness, though the roof and floor are somewhat thinner than
the sides. It forms a direct continuation of the spinal cord, into
which it passes without any sharp line of demarcation. The
ventricle it contains is known as the fourth ventricle.
 
The sides become in the chick marked by a series of transverse constrictions, dividing it into lobes, which are somewhat indefinite in number.
The first of these remains permanent, and its roof gives rise to the cerebellum.
It is uncertain whether the other constrictions have any morphological
significance. More or less similar constrictions are present in Teleostei.
In Elasmobranchii the medulla presents on its inner face at a late period a
series of lobes corresponding with the roots of the vagus and glossopharyngeal
nerves, and it is possible that the earlier constrictions may potentially
correspond to so many nerve-roots.
 
Throughout the Vertebrata an anterior lobe of the hindbrain becomes very early marked off, so that the primitive
hind-brain becomes divided into two regions which may be
 
 
 
 
cc
 
 
 
AOA
 
 
 
FIG. 249. SECTION THROUGH THE HIND -BRAIN OF A CHICK AT THE END
OF THE THIRD DAY OF INCUBATION.
 
IV. Fourth ventricle. The section shews the very thin roof and thicker sides of
the ventricle. Ch. Notochord ; CV. Anterior cardinal vein; CC. Involuted auditory
vesicle ; CC points to the end which will form the cochlear canal ; RL. Recessus
labyrinth! (remains of passage connecting the vesicle with the exterior) ; hy. Hypoblast
lining the alimentary canal; AO., AOA. Aorta, and aortic arch.
 
 
 
NERVOUS SYSTEM OF THE VERTEBKATA. 425
 
conveniently spoken of as the cerebellum (figs. 247 and 248, cb)
and medulla oblongata. The floor of these regions is quite
continuous and is also prolonged without any break into the
floor of the mid-brain.
 
The posterior section of the hind-brain, which forms the
medulla, undergoes changes of a somewhat complicated character. In the first place its roof becomes in front very much
extended and thinned out. At the raphe, where the two lateral
halves of the brain originally united, a separation, as it were,
takes place, and the two sides of the brain become pushed apart,
remaining united by only a very thin layer of nervous matter,
consisting of a single row of flattened cells (fig. 249). As a
result of this peculiar growth in the brain, the roots of the nerves
of the two sides, which were originally in contact at the dorsal
summit of the brain, become carried away from one another, and
appear to arise at the sides of the brain.
 
The thin roof of the fourth ventricle is triangular, or, in
Mammalia, somewhat rhomboidal in shape. The apex of the
triangle is directed backwards.
 
At a later period the blood-vessels of the pia mater form a
rich plexus over the anterior part of the thin roof of the medulla,
which becomes at the same time somewhat folded. The whole
structure is known as the tela vasculosa, or choroid plexus
of the fourth ventricle (fig. 250, chd 4). The floor of the
whole hind-brain becomes thickened, and there very soon
appears on its outer surface a layer of non-medullated nervefibres, similar to those which first appear on the spinal cord.
They are continuous with a similar layer of fibres on the floor
of the mid-brain, where they constitute the crura cerebri. On
the ventral floor of the medulla is a shallow continuation of the
anterior fissure of the spinal cord.
 
In Elasmobranchii and many Teleostei the restiform tracts are well
developed, and are anteriorly continued into the cerebellum, of which they
form the peduncles. Near their junction with the cerebellum they form
prominent bodies, which are regarded by Miklucho-Maclay as representing
the true cerebellum of Elasmobranchii.
 
In Elasmobranchii a dorsal pair of ridges projects into the cavity of the
fourth ventricle, corresponding apparently with the fasciculi teretes of the
Mammalia.
 
In Mammalia there develop, subsequently to the longitudinal fibres
 
 
 
426 THE HIND-BRAIN.
 
 
 
already spoken of, first the olivary bodies of the ventral side of the medulla,
and at a still later period the pyramids. The fasciculi teretes in the cavity of
the fourth ventricle are developed shortly before the pyramids.
 
When the hind-brain becomes divided into two regions the
roof of the anterior part does not become thinned out like that
of the posterior, but on the contrary, becomes somewhat thickened and forms a band-like structure roofing over the anterior
part of the fourth ventricle (fig. 247 and fig. 253, cb).
 
This is a rudiment of the cerebellum, and in all Craniate Vertebrates it at first presents this simple structure and insignificant size. In Cyclostomata, Amphibia and many Reptilia
this condition is permanent. In Elasmobranchii, on the other
hand, the cerebellum assumes in the course of development a
greater and greater prominence (fig. 248, cb), and eventually
overlaps both the optic lobes in front and the medulla behind.
In the later embryonic stages it exhibits in surface-views the
appearance of a median constriction, and the portion of the
ventricle contained in it is prolonged into two lateral outgrowths.
 
Miklucho-Maclay, from his observations on the brains of adult Elasmobranchii, was led to regard what is here called the cerebellum as identical
with the mid-brain, and the true mid-brain as part of the thalamencephalon.
Miklucho-Maclay was no doubt misled by the large size of the cerebellum,
but, as we have seen, this body does not begin to be conspicuous till late in
embryonic life.
 
The mid-brain and thalamencephalon (according to the ordinary interpretations) have in the embryo of Elasmobranchs exactly the same relations
as in the embryos of other Vertebrates ; so that the embryological evidence
appears to me to be conclusive against Miklucho-Maclay's view.
 
In Birds the cerebellum attains a very considerable development (fig. 250, cbl\ consisting of a folded central lobe with an
arbor vitae, into which the fourth ventricle is prolonged. There
are two small lateral lobes, apparently equivalent to the flocculi.
Anteriorly the cerebellum is connected with the roof of the midbrain by a delicate membrane, the velum medullas anterius,
or valve of Vieussens (fig. 250, vtna). The pons Varolii of
Mammalia is represented by a small number of transverse
fibres on the floor of the hind-brain immediately below the
cerebellum.
 
In Mammalia the cerebellum attains a still greater develop
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 427
 
ment The median lobe or vermiform process is first developed.
In the higher Mammalia the lateral parts forming the hemi
fXJ^ cmfl l,. n
 
vnut
 
cU
 
 
 
 
ats inS fo s
 
FIG. 250. LONGITUDINAL SECTION THROUGH THE BRAIN OF A CHICK OF TEN
 
DAYS. (After Mihalkovics.)
 
Jims, cerebral hemispheres; alf. olfactory lobe; alf^. olfactory nerve; ggt. corpus
striatum ; oma. anterior commissure; chd-$. choroid plexus of the third ventricle;
pin. pineal gland; cmp. posterior commissure; trm. lamina terminalis; chm. optic
chiasma; inf. infundibulum; hph. pituitary body; bgm. commissure of Sylvius (roof
of iter a tertio ad quartum ventriculum) ; vma. velum medullae anterius (valve of
Vieussens); cbl. cerebellum; chd 4. choroid plexus of the fourth ventricle; obt 4. roof
of fourth ventricle ; obi. medulla oblongata ; pns. commissural part of medulla ; inv.
sheath of brain ; bis. basilar artery ; crts. internal carotid.
 
spheres of the cerebellum become formed as swellings at the
sides at a considerably later period, and are hardly developed in
the Monotremata and Marsupialia.
 
The cerebellum is connected with the roof of the mid-brain in front and
with the choroid plexus of the fourth ventricle behind by delicate membranous
structures, known as the velum medullae anterius (valve of Vieussens) and
the velum medullae posterius.
 
The pons Varolii is formed on the ventral side of the floor of the
cerebellar region as a bundle of transverse fibres at about the same time as
the olivary bodies.
 
The mid-brain. The changes undergone by the mid-brain
are simpler than those of any other part of the brain. We
have already seen that the rnid-brain, on the appearance of the
cranial flexure, forms an impaired vesicle with a vaulted roof and
curved floor, at the front end of the long axis of the body (fig.
1 1 8, MB}. It is at this period in most Vertebrates relatively
much larger than in the adult ; and it is only in the Teleostei
that it more or less retains in the adult its embryonic proportions.
 
 
 
428 THE FORE-BRAIN.
 
 
 
The cavity of the mid-brain, greatly reduced in size in the
higher forms, is known as the iter a tertio ad quartum ventriculum, or aqueductus Sylvii.
 
The roof of the mid-brain is sharply constricted off from the
divisions of the brain in front of and behind it, but these
constrictions do not extend to the floor.
 
In some Vertebrates the region of the mid-brain is stated to
undergo hardly any further development. In the Axolotl it
remains according to Stieda 1 as a simple tube with nearly uniformly thick walls. In the majority of forms it undergoes, however, a more complicated development.
 
In Elasmobranchs the sides become thickened to form the optic lobes,
which are soon separated by a median longitudinal groove. The floor
becomes thickened to form the crura cerebri. The primitive simple median
cavity becomes imperfectly divided into a median portion below, and two
lateral diverticula in the optic lobes.
 
In Teleostei the changes, resulting in the formation of (i) a pair of
longitudinal ridges projecting from the roof into the cavity of the iter,
constituting the fornix of Gottsche, and (2) of the two swellings on the floor,
forming the tori semicirculares, are more complicated, but have not been
satisfactorily worked out. In Bombinator and the Anura generally the
changes are of the same nature as those in Elasmobranchii, except that the
prolongations of the ventricle into the optic lobes are still further constricted
off from the median portion, which forms the true iter.
 
In Reptilia and Aves the development of the mid-brain takes place on
the same type as in Elasmobranchii and the Anura. In Birds the optic
lobes are pushed very much aside, and the roof of the iter is greatly thinned
out. In Mammalia the sides of the mid-brain give rise to two pairs of
prominences the corpora quadrigemina instead of the two optic lobes of
other Vertebrata. The prominences, which do not contain prolongations of
the iter, become first visible on the appearance of an oblique transverse furrow,
while the anterior pair alone are separated by a longitudinal furrow. In the
later stages of development the longitudinal furrow is continued so as to
bisect the posterior pair.
 
The floor, which is bounded posteriorly by the pons Varolii, becomes the
crura cerebri. The corpora geniculata interna also belong to this division
of the brain.
 
Fore-brain. In its earliest condition the fore-brain forms a
single vesicle without a trace of separate divisions, but very
early it buds off the optic vesicles, whose history is described
with that of the eye.
 
1 " Ueb. d. Bau d. centralen Nervensystem d. Axolotl." Zdt.f. wlss. Zool., Vol.
xxv. 1875.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
429
 
 
 
The optic vesicles become gradually constricted off from the
fore-brain in a direction obliquely backwards and downwards.
They remain, however, attached to it at the anterior extremity
of the base of the fore-brain (fig. 251, op.v.). While the above
changes are taking place in the optic vesicles the anterior part
 
 
 
opy
 
 
 
 
 
FIG. 251. SECTION THROUGH THE
FRONT PART OF THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER
IMPREGNATION.
 
al. alimentary tract ; fb. thalamencephalon ; /. lens of eye ; op.v. optic vesicle. The
mesoblast is not represented.
 
 
 
FIG. 252. LONGITUDINAL
SECTION THROUGH THE BRAIN
OF A YOUNG PRISTIURUS EMBRYO.
 
cer. commencement of cerebral
hemisphere; pn. pineal gland; In.
infundibulum ; pt. ingrowth of
mouth to form the pituitary body ;
mb. mid-brain ; cb. cerebellum ; ch.
notochord; al. alimentary tract;
laa. artery of mandibular arch.
 
 
 
of the fore-brain becomes prolonged, and at the same time
somewhat dilated. At first there is no sharp boundary between
the primitive fore-brain and its anterior prolongation, but there
shortly appears a constriction which passes from above obliquely
forwards and downwards. This constriction is shallow at first,
but soon becomes much deeper, leaving however the cavities of
the two divisions of the fore-brain united ventrally by a somewhat wide canal (fig. 252).
 
Of these two divisions the posterior becomes the thalamencephalon, while the anterior and larger division (cer) forms the
rudiment of the cerebral hemispheres and olfactory lobes. For
a considerable period this rudiment remains perfectly simple, and
exhibits no signs, either externally or internally, of a longitudinal
constriction dividing it into two lobes.
 
From the above description it may be concluded that the
 
 
 
430 THE THALAMENCEPHALON.
 
rudiment of the cerebral hemispheres is contained in the
original fore-brain. In spite however of their great importance
in all the Craniata, it is probable that the hemispheres were
either not present as distinct structures, or only imperfectly
separated from the thalamencephalon, in the primitive vertebrate
stock.
 
The thalamencephalon. The thalamencephalon varies so
slightly in structure throughout the Vertebrate series that a
general description will suffice for all the types.
 
It forms at first a simple vesicle, the walls of which are of
a nearly uniform thickness and formed of the usual spindleshaped cells.
 
 
 
md.
 
 
 
 
FIG. 253. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.
 
The larva had been hatched three days, and was 4*8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues.
 
c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ;
mb. mid-brain ; cb. cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.
 
The cavity it contains is known as the third ventricle. Anteriorly it opens widely into the cerebral rudiment, and posteriorly
into the ventricle of the mid-brain. The opening into the
cerebral rudiment becomes the foramen of Munro.
 
For convenience of description I shall divide it into three
regions, viz. (i) the floor, (2) the sides, and (3) the roof.
 
The floor becomes divided into two parts, an anterior part,
giving origin to the optic nerves, in which is formed the optic
chiasma ; and a posterior part, which becomes produced into an
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
431
 
 
 
at first inconspicuous prominence the rudiment of the infundibulum (fig. 252, In}. This comes in contact with an involution
from the mouth, which gives rise to the pituitary body (fig. 252,
//), the development of which will be dealt with separately.
 
In the later stages of development the infundibulum becomes
gradually prolonged, and forms an elongated diverticulum of
the third ventricle, the apex of which is in contact with the
pituitary body (figs. 252, 254, in, and figs. 250 and 255, inf}.
 
Along the sides of the infundibulum run the commissural
fibres connecting the floor of the mid-brain with the cerebrum.
 
In its later stages the infundibular region presents considerable variations
in the different vertebrate types. In Fishes it generally remains very large,
and permanently forms a marked diverticulum of the floor of the thalamencephalon. In Elasmobranchii the distal end becomes divided into three
lobes a median and two lateral. The lateral lobes appear to become the
sacci vasculosi of the adult.
 
In Teleostei peculiar bodies known as the lobi inferiores (hypoaria) make
their appearance at the sides of the a2r ,. y
 
infundibulum. They appear to correspond in position with the tuber cinereum of Mammalia 1 . In Birds, Reptiles, and Amphibia the lower part of
the embryonic infundibulum becomes
atrophied and reduced to a mere fingerlike process the processus infundibuli.
 
In Mammalia the posterior part of
the primitive infundibulum becomes the
corpus albicans, which is double in
Man and the higher Apes ; the ventral
part of the posterior wall forms the
tuber cinereum. Laterally, at the junction of the optic thalami and infundibulum, there are placed the fibres of
the crura cerebri, which are probably
derived from the walls of the infundibulum. A special process grows out
from the base of the infundibulum,
which undergoes peculiar changes, and
becomes intimately united with the
pituitary body ; in which connection it
will be more fully described.
 
 
 
rncl
 
 
 
 
c.in
'Pt
 
 
 
FIG. 254. LONGITUDINAL SECTION
THROUGH THE BRAIN OF SCYLLIUM
CANICULA AT AN ADVANCED STAGE OF
DEVELOPMENT.
 
cer. cerebral hemisphere ; pn. pineal gland ; op. th. optic thalamus, connected with its fellow by a commissure
(the middle commissure). In front of
it is seen a fold of the roof of the forebrain, which is the choroid plexus of
the third ventricle ; op. optic chiasma ;
ft. pituitary body ; in. infundibulum ;
cb. cerebellum ; au.v. passage leading
from the auditory vesicle to the exterior ; mel. medulla oblongata ; c . in.
internal carotid artery.
 
 
 
1 For the relations of these bodies, vide L. Stieda, "Stud. lib. d. centrale Nervensystem d. Knochenfische." Zeit. f. wiss. Zool. Vol. xvni. 1868.
 
 
 
432 THE PINEAL GLAND.
 
 
 
The sides of the thalamencephalon become very early
thickened to form the optic thalami, which constitute the most
important section of the thalamencephalon. They are separated,
in Mammalia at all events, on their inner aspect from the
infundibular region by a somewhat S-shaped groove, known as
the sulcus of Munro, which ends in the foramen of Munro. They
also become in Mammalia secondarily united by a transverse
commissure, the grey or middle commissure, which passes
across the cavity of the third ventricle. This commissure is
probably homologous with, and derived from, a commissural
band in the roof of the thalamencephalon, placed immediately
in front of the pineal gland which is well developed in Elasmobranchii (fig. 254).
 
The roof undergoes more complicated changes. It becomes
divided, on the appearance of the pineal gland as a small
papilliform outgrowth (the development of which is dealt with
separately), into two regions a longer anterior in front of the
pineal gland and a shorter posterior. The anterior region
becomes at an early period excessively thin, and at a later
period, when the roof of the thalamencephalon is shortened by
the approach of the cerebral hemispheres to the mid-brain, it
becomes (vide figs. 250 and 255, chd 3, and 254) considerably
folded, while at the same time a vascular plexus is formed in
the pia mater above it. On the accomplishment of these
changes it is known as the tela choroidea of the third ventricle.
 
In the roof of the third ventricle behind the pineal gland
there appear in Elasmobranchii, the Sauropsida and Mammalia
transverse commissural fibres, forming a structure known as the
posterior commissure, which connects together the two optic
thalami.
 
The most remarkable organ in the roof of the thalamencephalon is the pineal gland, which is developed in most Vertebrates as a simple papilliform outgrowth of the roof, and is at
first composed of cells similar to those of the other parts of the
central nervous system (figs. 250, 252, 254 and 255, pn or pin}.
In the lower Vertebrata it is directed forwards, but in Mammalia,
and to some extent in Aves, it is directed backwards.
 
In Amphibia it is described by Gotte (No. 296) as being a product of the
point where the roof of the brain remains latest attached to the external skin.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 433
 
The figure which Gotte gives to prove this does not appear to me fully to
bear out his conclusion ; which if true is very important. Although I
directed my attention specially to this point, I could find no indication in
Elasmobranchii of a process similar to that described by Gotte, and his
observations have not as yet been confirmed for other Vertebrates. Gotte
compares the pineal gland to the long-persis.ting pore which leads into the
cavity of the brain in the embryo of Amphioxus, and we might add the
Ascidians, and, should his facts be confirmed, the conclusion he draws from
them would appear to be well founded.
 
The later stages in the development of the pineal gland in
different Vertebrates have not in all cases been fully worked
out 1 .
 
In Elasmobranchii the pineal gland becomes in time very
long, and extends far forwards over the roof of the cerebral
 
 
 
 
FIG. -255. LONGITUDINAL VERTICAL SECTION THROUGH THE ANTERIOR PART
OF THE BRAIN OF AN EMBRYO RABBIT OF FOUR CENTIMETRES. (After
Mihalkovics.)
 
The section passes through the median line so that the cerebral hemispheres are
not cut ; their position is however indicated in outline.
 
spt. septum lucidum formed by the coalescence of the inner walls of part of the
cerebral hemispheres; cna. anterior commissure; frx. vertical pillars of the fornix;
cal, genu of corpus callosum; trm. lamina terminalis; hms. cerebral hemispheres;
olf. olfactory lobes; acl. artery of corpus callosum; fmr. position of foramen of
Munro; chdi,. choroid plexus of third ventricle ; pin. pineal gland; cmp. posterior
commissure; bgm. lamina uniting the lobes of the mid-brain; chm, optic chiasma ;
hph. pituitary body; inf. infundibulum ; pns. pons Varolii; pde. cerebral peduncles;
agd. iter.
 
1 For a full account of this subject vide Ehlers (No. 337).
B. Ill, 28
 
 
 
434 THE PINEAL GLAND.
 
hemispheres (fig. 254/w). Its distal extremity dilates somewhat,
and in the adult the whole organ forms (Ehlers, No. 337) an
elongated tube, enlarged at its free extremity, and opening at
its base into the brain. The enlarged extremity may either be
lodged in a cavity in the cartilage of the cranium (Acanthias), or
be placed outside the cranium (Raja).
 
In Petromyzon its form is very different. It arises (fig.
2 53 P n ) as a sack-like diverticulum of the thalamencephalon
extending at first both backwardsand forwards. In the Ammoccete the walls of this sack are deeply infolded.
 
The embryonic form of the pineal gland in Amphibia is very
much like that which remains permanent in Elasmobranchii ;
the stalk connecting the enlarged terminal portion with the
brain soon however becomes solid and very thin except at its
proximal extremity. The enlarged portion also becomes solid,
and is placed in the adult externally to the skull, where it forms
a mass originally described by Stieda as the cerebral gland.
 
In Birds the primitive outgrowth to form the pineal gland
becomes, according to Mihalkovics, deeply indented by vascular
connective tissue ingrowths, so that it assumes a dendritic
structure (fig. 250 pin).
 
The proximal extremity attached to the roof of the thalamencephalon forms a special section, known as the infra -pineal
process. The central lumen of the free part of the gland finally
atrophies, but the branches still remain hollow. The infra-pineal
process becomes reduced to a narrow stalk, connecting the
branched portion of the body with the brain. The branched
terminal portion and the stalk obviously correspond with the
vesicle and distal part of the stalk of the types already described.
In Mammalia the development of the pineal gland is, according
to Mihalkovics, generally similar to that of Birds. The original
outgrowth becomes branched, but the follicles or lobes to which
the branching gives rise eventually become solid (fig. 255 pin).
An infra-pineal process is developed comparatively late, and is
not sharply separated from the roof of the brain.
 
No satisfactory suggestions have yet been offered as to the
nature of the pineal gland, unless the view of Gotte be regarded
as such. It appears to possess in all forms an epithelial structure,
but, except at the base of the stalk (infra-pineal process) in
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 435
 
Mammalia, in the wall of which there are nerve-fibres, no
nervous structures are present in it in the adult state.
 
The pituitary body. Although the pituitary body is not
properly a nervous structure, yet from its intimate connection
with the brain it will be convenient to describe its development
here. The pituitary body is in fact an organ derived from the
epiblast of the stomodaeum. This fact has been demonstrated
for Mammalia, Aves, Amphibia and Elasmobranchii, and may
be accepted as holding good for all the Craniata 1 . The epiblast
in the angle formed by the cranial flexure becomes involuted to
form the cavity of the mouth. This cavity is bordered on its
posterior surface by the front wall of the alimentary tract, and
on its anterior by the base of the fore-brain. Its uppermost end
does not at first become markedly constricted off from the
remainder, but is nevertheless the rudiment of the pituitary
body.
 
Fig. 256 represents a transverse section through the head of
an Elasmobranch embryo, in which, owing to the cranial flexure,
the fore part of the head is cut longitudinally and horizontally,
and the section passes through both the fore-brain (fb) and the
hind-brain. Close to the base of the fore-brain are seen the
mouth (in), and the pituitary involution from this (pf). In
contact with the pituitary involution is the blind anterior
termination of the throat (/) which a little way back opens to
the exterior by the first visceral cleft (l. v.c.}. This figure alone
suffices to demonstrate the correctness of the above account of
the pituitary body; but its truth is still further confirmed by
fig. 252; in which the mouth involution (pt) is in contact with,
but still separated from, the front end of the alimentary tract.
Very shortly after the septum between the mouth and throat
becomes pierced, and the two are placed in communication, the
pituitary involution becomes very partially constricted off from
the mouth involution, though still in direct communication with
it. In later stages the pituitary involution becomes longer and
 
1 Scott states that in the larva of Petromyzon the pituitary body is derived from
the walls of the nasal pit; Quart, jf. of Micr. Science^ Vol. xxi. p. 750. I have not
myself completely followed its development in Petromyzon, but I have observed a
slight diverticulum of the stomodaeum which I believe gives origin to it. Fuller
details are in any case required before we can admit so great a divergence from the
normal development as is indicated by Scott's statements.
 
283
 
 
 
436
 
 
 
PITUITARY BODY.
 
 
 
Ky
 
 
is dilated terminally ; while the passage connecting it with the
mouth becomes narrower and narrower, and is finally reduced to
a solid cord, which in its turn disappears.
 
Before the connection between the pituitary vesicle and the
mouth is obliterated the cartilaginous cranium becomes developed,
and it may then be seen that the infundibulum projects through
the pituitary space to come into close juxtaposition with the
pituitary body.
 
After the pituitary vesicle has lost its connection with the
mouth it lies just in front of the infundibulum (figs. 250 and
255 hph and fig. 254 pf) ; and soon becomes surrounded by
vascular mesoblast, which grows in
and divides it into a number of
branching tubes. In many forms
the cavity of the vesicle completely
disappears, and the branches become
for the most part solid [Cyclostomata
and some Mammalia (the rabbit),
Elasmobranchii, Teleostei and Amphibia]. In Reptilia, Aves and most
Mammalia the lumen of the organ is
more or less retained (W. Miiller, No.
344).
 
Although in the majority of the
Vertebrata there is a close connection
between the pituitary body and the
infundibulum, there is no actual fusion
between the two. In Mammalia the
case is different. The part of the infundibulum which lies at the hinder
end of the pituitary body is at first a
simple finger-like process of the brain
(fig. 255 inf), but its end becomes
swollen, and the lumen in this part
becomes obliterated. Its cells, originally similar to those of the other
parts of the nervous system and even
(Kolliker) containing differentiated nerve-fibres, partly atrophy,
and partly assume an indifferent form, while at the same time
 
 
 
 
FIG. 256. TRANSVERSE SECTION THROUGH THE FRONT PART
OF THE HEAD OF A YOUNG PR1STIURUS EMBRYO.
 
The section, owing to the cranial flexure, cuts both the fore- and
the hind-brain. It shews the premandibular and mandibular head
cavities \pp and 2//, etc. The
section is moreover somewhat
oblique from side to side.
 
fb. fore-brain; /. lens of eye;
m. mouth ;pt. upper end of mouth,
forming pituitary involution ; lao.
mandibular aortic arch ; \pp. and
ipp. first and second head cavities ;
\vc. first visceral cleft; V. fifth
nerve ; aim. auditory nerve ; VII.
seventh nerve; aa. roots of dorsal
aorta ; acv. anterior cardinal vein ;
ch. notochord.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 437
 
there grow in amongst them numerous vascular and connectivetissue elements. The process of the infundibulum thus metamorphosed becomes inseparably connected with the true pituitary
body, of which it is usually described as the posterior lobe. The
part of the infundibulum which undergoes this change is very
probably homologous with the saccus vasculosus of Fishes.
 
The true nature of the pituitary body has not yet been made out. It is
clearly a rudimentary organ in existing craniate Vertebrates, and its
development indicates that when functional it was probably a sense organ
opening into the mouth, its blind end reaching to the base of the brain. No
similar organ has as yet been found in Amphioxus, but it seems possible
perhaps to identify it with the peculiar ciliated sack placed at the opening
of the pharynx in the Tunicata, the development of which was described at
p. 1 8. If the suggestion is correct, the division of the body into lobes in
existing Vertebrata must be regarded as a step towards a retrogressive
metamorphosis.
 
Another possible view is to regard the pituitary body as a glandular
structure which originally opened into the mouth in the lower Chordata, but
which has in all existing forms ceased to be functional. The intimate relation
of the organ to the brain appears to me opposed to this view of its nature,
while on the other hand its permanent structure is more easily explained on
this view than on that previously stated. In the Ascidians a glandular
organ has been described by Lacaze Duthiers^n juxtaposition to the ciliated
sack, and it is possible that this organ as well as the ciliated sack may be
related to the pituitary body. In view of this possibility further investigations
ought to be carried out in order to determine whether the whole pituitary
body is derived from the oral involution, or whether there may not be a
nervous part and a glandular part of the organ.
 
The Cerebral Hemispheres. It will be convenient to treat
separately the development of the cerebral hemispheres proper,
and that of the olfactory lobes.
 
Although the cerebral hemispheres vary more than any other
part of the brain, they are nevertheless developed from the
unpaired cerebral rudiment in a nearly similar manner throughout the series of Vertebrata.
 
In the cerebral rudiment two parts may be distinguished, viz.
the floor and the roof. The former gives rise to the ganglia at
the base of the hemispheres corpora striata, etc. the latter to
the hemispheres proper.
 
1 " Les Ascidies simples des Cotes de France." Archives de Biologie exper. et
generate, Vol. III. 1874, p. 329.
 
 
 
43
 
 
 
THE CEREBRAL HEMISPHERES.
 
 
 
The two lobes
 
 
 
 
cc
 
 
 
The first change which takes place consists in the roof
growing out into two lobes, between which a shallow median
constriction makes its appearance (fig. 257).
thus formed are the rudiments of the two hemispheres. The cavity of each
of them opens by a widish
aperture into the vestibule
at the base of the cerebral
rudiment, which again opens
directly into the cavity of
the third ventricle (3 v).
The Y-shaped aperture thus
formed, which leads from
the cerebral hemispheres
into the third ventricle, is
the foramen of Munro. The
cavity (lv) in each of the
rudimentary hemispheres is
a lateral ventricle. The part of the cerebrum which lies between
the two hemispheres, and passes forwards from the roof of the
third ventricle round the end of the brain to the optic chiasma,
is the rudiment of the lamina terminalis (figs. 257 It and 255 trm}.
Up to this point the development of the cerebrum is similar in
all Vertebrata, but in some forms it practically does not proceed
much further.
 
In Elasmobranchii, although the cerebrum reaches a considerable size (fig. 254 cer\ and grows some way backwards over
the thalamencephalon, yet it is not in many forms divided into
two distinct lobes, but its paired nature is only marked by
a shallow constriction on the surface. The lamina terminalis in
the later stages of development grows backwards as a thick
median septum which completely separates the two lateral
ventricles 1 (fig. 263).
 
There are, it may be mentioned, considerable variations in
 
 
 
op.t/t
 
 
 
FIG. 257. DIAGRAMMATIC LONGITUDINAL HORIZONTAL SECTION THROUGH THE
FORE-BRAIN.
 
j>.v. third ventricle ; lv. lateral ventricle ;
//. lamina terminalis ; ce, cerebral hemisphere ; op.th. optic thalamus.
 
 
 
1 A comparison of the mode of development of this septum with that of the septum
lucidum with its contained commissures in Mammalia clearly shews that the two
structures are not homologous, and that Miklucho-Maclay is in error in attempting to
treat them as being so.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 439
 
the structure of the cerebrum in Elasmobranchii into which it is
not however within the scope of this work to enter.
 
In the Teleostei the vesicles of the cerebral hemispheres
appear at first to have a wide lumen, but it subsequently
becomes almost or quite obliterated, and the cerebral rudiment
forms a small bilobed nearly solid body. In Petromyzon (fig.
253 c/i) the cerebral rudiment is at first an unpaired anterior
vesicle, which subsequently becomes bilobed in the normal
manner. The walls of the hemispheres become much thickened,
but the lateral ventricles persist
 
In all the higher Vertebrates the division of the cerebral
rudiment into two distinct hemispheres is quite complete, and
with the deepening of the furrow between the two hemispheres
the lamina terminalis is carried backwards till it forms a thin
layer bounding the third ventricle anteriorly, while the lateral
ventricles open directly into the third ventricle.
 
In Amphibians the two hemispheres become united together
immediately in front of the lamina terminalis by commissural
fibres, forming the anterior commissure. They also send out
anteriorly two solid prolongations, usually spoken of as the
olfactory lobes, which subsequently fuse together.
 
In all Reptilia and Aves there is formed an anterior commissure, and in the higher members of the group, especially Aves
(fig. 250), the hemispheres may obtain a considerable development. Their outer walls are much thickened, while their inner
walls become very thin ; and a well-developed ganglionic
mass, equivalent to the corpus striatum, is formed at their
base.
 
The cerebral hemispheres undergo in Mammalia the most
complicated development. The primitive unpaired cerebral
rudiment becomes, as in lower Vertebrates, bilobed, and at the
same time divided by the ingrowth of a septum of connective
tissue into two distinct hemispheres (figs. 260 and 26 \f and
258 I). From this septum is formed the falx cerebri and
other parts.
 
The hemispheres contain at first very large cavities, communicating by a wide foramen of Munro with the third ventricle
(fig. 260). They grow rapidly in size, and extend, especially
backwards, and gradually cover the thalamencephalon and the
 
 
 
440
 
 
 
THE CEREBRAL HEMISPHERES.
 
 
 
mid-brain (fig. 258 I,/). The foramen of Munro becomes very
much narrowed and reduced to a mere slit.
 
The walls are originally , ^
 
nearly uniformly thick, but
the floor becomes thickened
on each side, and gives rise
to the corpus striatum (figs.
260 and 261 st). The corpus
striatum projects upwards
into each lateral ventricle,
giving to it a somewhat
semilunar form, the two
horns of which constitute
the permanent anterior and
descending cornua of the
lateral ventricles (fig. 262 st).
 
 
 
 
 
FIG. 258. BRAIN OF A THREE MONTHS'
HUMAN EMBRYO: NATURAL SIZE. (From
Kolliker.)
 
i. From above with the dorsal part of
hemispheres and mid-brain removed ; i.
From below, f. anterior part of cut wall of
the hemisphere ; f ' . cornu ammonis ; f/io.
optic thalamus ; cst. corpus striatum ; to.
optic tract ; cm. corpora mammillaria ; /.
pons Varolii.
 
 
 
With the further growth of the hemisphere the corpus
 
 
 
CftZ
 
 
 
Ams
 
 
 
 
spt.
 
 
 
FIG. 259. TRANSVERSE SECTION THROUGH THE BRAIN OF A RABBIT OF FIVE
CENTIMETRES. (After Mihalkovics.)
 
The section passes through nearly the posterior border of the septum lucidum,
immediately in front of the foramen of Munro.
 
hms. cerebral hemispheres ; cal. corpus callosum ; amm. cornu ammonis (hippocampus major) ; cms. superior commissure of the cornua ammonis ; spt. septum
lucidum ; frx i. vertical fibres of the fornix; ana. anterior commissure ; trm. lamina
terminalis; str. corpus striatum; Iff. nucleus lenticularis of corpus striatum; vtr i.
lateral ventricle; vtr 3. third ventricle; ipl. slit between cerebral hemispheres.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 44!
 
striatum loses its primitive relations to the descending cornu.
The reduction in size of the foramen of Munro above mentioned
is, to a large extent, caused by the growth of the corpora striata.
The corpora striata are united at their posterior border with
the optic thalami. In the later stages of development the area
of contact between these two pairs of ganglia increases to an
immense extent (fig. 261), and the boundary between them
becomes somewhat obscure, so that the sharp distinction which
exists in the embryo between the thalamencephalon and cerebral
hemispheres becomes lost. This change is usually (Mihalkovics,
 
 
 
 
FIG. 260. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO
 
OF 27 CM. IN LENGTH. (From Kolliker.)
The section passes through the level of the foramen of Munro.
st. corpus striatum ; m. foramen of Munro ; t. third ventricle ; pi. choroid plexus
of lateral ventricle; f. falx cerebri; th. anterior part of optic thalamus; ch. optic
chiasma; o. optic nerve; c. fibres of the cerebral peduncles; h. cornu ammonis;
/. pharynx; sa. pre-sphenoid bone; a. orbito-sphenoid bone; s. points to part of the
roof of the brain at the junction between the roof of the third ventricle and the lamina
terminalis ; /. lateral ventricle.
 
Kolliker) attributed to a fusion between the corpora striata and
optic thalami, but it has recently been attributed by Schwalbe
(No. 349), with more probability, to a growth of the original
surface of contact, and an accompanying change in the relations
of the parts.
 
 
 
442 THE CEREBRAL HEMISPHERES.
 
The outer wall of the hemispheres gradually thickens, while
the inner wall becomes thinner. In the latter, two curved folds,
projecting towards the interior of the lateral ventricle, become
formed. These folds extend from the foramen of Munro along
nearly the whole of what afterwards becomes the descending
cornu of the lateral ventricle.
 
The upper fold becomes the hippocampus major (cornu
ammonis) (figs. 259 amm, 260 and 261 /i, and 262 am). When
 
 
 
 
P'IG. 261. TRANSVERSE SECTION THROUGH THE BRAIN OF A SHEEP'S EMBRYO
OF 27 CM. IN LENGTH. (From Kolliker.)
 
The section is taken a short distance behind the section represented in fig. 260, and
passes through the posterior part of the hemispheres and the third ventricle.
 
st. corpus striatum ; th. optic thalamus; to. optic tract; t. third ventricle; d. roof
of third ventricle; c. fibres of cerebral peduncles; c' '. divergence of these fibres into
the walls of the hemispheres ; e. lateral ventricle with choroid plexus //; h. cornu
ammonis; f. primitive falx; am. alisphenoid; a. orbito-sphenoid ; sa. presphenoid; /.
pharynx; mk. Meckel's cartilage.
 
the rudiment of the descending cornu has become transformed
into a simple process of the lateral ventricle the hippocampus
major forms a prominence upon its floor.
 
The wall of the lower fold becomes very thin, and a vascular
plexus, derived from the connective-tissue septum between the
hemispheres, and similar to that of the roof of the third ventricle,
 
 
 
NERVOUS SYSTEM OF THE VKRTEBRATA. 443
 
is formed outside it. It constitutes a fold projecting far into the
cavity of the lateral ventricle, and together with the vascular
connective tissue in it gives rise to the choroid plexus of the
lateral ventricle (figs. 260 and 261 //).
 
It is clear from the above description that a marginal fissure
leading into the cavity of the lateral ventricle does not exist in
the sense often implied in works on human anatomy, in that the
epithelium covering the choroid plexus, which forms the true
wall of the brain, is a continuous membrane. The epit/ielium of
the choroid plexus of the lateral ventricle is quite independent
of that of the choroid plexus of the third ventricle, though at the
foramen of Munro the roof of the third ventricle is of course continuous with the inner wall of the lateral ventricle (fig. 260 s).
The vascular elements of the two plexuses form however a continuous structure.
 
The most characteristic parts of the Mammalian cerebrum
are the commissures connecting the two hemispheres. These
commissures are (i) the anterior commissure, (2) the fornix, and
(3) the corpus callosum, the two latter being peculiar to Mammalia.
 
By the fusion of the inner walls of the hemispheres in front
of the lamina terminalis a solid septum is formed, known as the
septum lucidum, continuous behind with the lamina terminalis,
and below with the corpora striata (figs. 255 and 259 spt). It is
by a series of differentiations within this septum that the above
commissures originate. In Man there is a closed cavity left in
the septum known as the fifth ventricle, which has however no
communication with the true ventricles of the brain.
 
In the septum lucidum there become first formed, below, the
transverse fibres of the anterior commissure (fig. 255 and fig.
259 cma), and in the upper part the vertical fibres of the fornix
(fig. 255 and fig. 259 frx 2). The vertical fibres meet above
the foramen of Munro, and thence diverge backwards, as the
posterior pillars, to lose themselves in the cornu ammonis (fig.
259 amm}. Ventrally they are continued, as the descending or
anterior pillars of the fornix, into the corpus albicans, and thence
into the optic thalami.
 
The corpus callosum is not formed till after the anterior
commissure and fornix. It arises in the upper part of the region
 
 
 
444
 
 
 
THE OLFACTORY LOBES.
 
 
 
 
(septum lucidum) formed by the fusion of the lateral walls of the
hemispheres (figs. 255 and 259 cal), and at first only its curved
anterior portion the genu
or rostrum is developed. ^
This portion is alone found
in Monotremes and Marsupials. The posteriorportion,
which is present in all the
Monodelphia, is gradually
formed as the hemispheres
are prolonged further backwards.
 
Primitively the Mammalian cerebrum, like that
of the lower Vertebrata, is
quite smooth. In many of
the Mammalia, Monotremata, Insectivora, etc., this
condition is nearly retained
through life, while in the
majority of Mammalia a
more or less complicated system of fissures is developed on the
surface. The most important, and first formed, of these is
the Sylvian fissure. It arises at the time when the hemispheres, owing to their growth in front of and behind the
corpora striata, have assumed a somewhat bean-shaped form.
At the root of the hemispheres the hilus of the bean there
is formed a shallow depression, which constitutes the first trace
of the Sylvian fissure. The part of the brain lying in this fissure
is known as the island of Reil.
 
The olfactory lobes. The olfactory lobes, or rhinencephala,
are secondary outgrowths of the cerebral hemispheres, and contain prolongations of the lateral ventricles, but may however be
solid in the adult state. According to Marshall they develop in
Birds and Elasmobranchs and presumably other forms later
than the olfactory nerves, so that the olfactory region of the
hemispheres is indicated before the appearance of the olfactory
lobes.
 
In most Vertebrates the olfactory lobes arise at a fairly early
 
 
 
FIG. 262. LATERAL VIEW or THE BRAIN
OF A CALF EMBRYO OF 5 CM. (After Mihalkovics.)
 
The outer wall of the hemisphere is removed, so as to give a view of the interior of
the left lateral ventricle.
 
hs. cut wall of hemisphere ; st. corpus
striatum; am. hippocampus major (cornu ammonis) ; d. choroid plexus of lateral ventricle ;
fm. foramen of Munro; op. optic tract; in.
infundibulum ; mb. mid-brain ; cb. cerebellum ;
IV. V. roof of fourth ventricle ; ps. pons Varolii, close to which is the fifth nerve with
Gasserian ganglion.
 
 
 
NERVOUS SYSTEM OF THE VKRTEBRATA.
 
 
 
445
 
 
 
stage of development from the under and anterior part of the
hemispheres (fig. .250 olf}. In Elasmobranchs they arise, not
 
 
 
 
FIG. 263. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)
 
ch. cerebral hemispheres ; ol.v. olfactory vesicle ; olf. olfactory pit ; Sch. Schneiderian folds ; I. olfactory nerve. The reference line has been accidentally taken through
the nerve to the brain ; pn. anterior prolongation of pineal gland.
 
from the base, but from the lateral parts of the brain (fig. 263),
and become subsequently divided into a bulbous portion and a
stalk. They vary considerably in their structure in the adult.
 
In Amphibia the solid anterior prolongations of the cerebral
hemispheres already spoken of are usually regarded as the
olfactory lobes, but according to Gotte, whose view appears to
me well founded, small papillae, situated at the base of these
prolongations, from which olfactory nerves spring, and which
contain a process of the lateral ventricle, should properly be
regarded as the olfactory lobes. These papillse arise prior to the
solid anterior prolongations of the hemispheres.
 
In Birds the olfactory lobes are small. In the chick they
arise (Marshall) on the seventh day of incubation.
 
 
 
General conclusions as to the Central Nervous System.
 
It has been shewn above that both the brain and spinal cord
are primitively composed of a uniform wall of epithelial cells,
and that the first differentiation results in the formation of an
external layer of white matter, a middle layer of grey matter
(ganglion cells), and an inner epithelial layer. This primitive
 
 
 
446 GENERAL CONCLUSIONS.
 
histological arrangement, which in many parts of the brain at
any rate, is only to be observed in the early developmental
stages, has a simple phylogenetic explanation.
 
As has been already explained in an earlier part of this chapter
the central nervous system was originally a differentiated part of
the superficial epidermis.
 
This differentiation (as may be concluded from the character
of the nervous system in the Ccelenterata and Echinodermata)
consisted in the conversion of the inner ends of the epithelial
cells into nerve-fibres ; that is to say, that the first differentiation
resulted in the formation of a layer of white matter on the inner
side of the epidermis. The next stage was the separation of a
deeper layer of the epidermis as a layer of ganglion cells from
the superficial epithelial layer, i.e. the formation of a middle
layer of ganglion cells and an outer epithelial layer. Thus,
phylogenetically, the same three layers as those which first make
their appearan-ce in the ontog'eny of the vertebrate nervous system
became successively differentiated, and in both cases they are
clearly placed in the same positions, because the central canal of
the vertebrate nervous system, as formed by an involution, is at
the true outer surface, and the external part of the cord is at the
true inner surface.
 
It is probable that a very sharp distinction between the white
and grey matter is a feature acquired in the higher Vertebrata,
since in Amphioxus there is no such sharp separation ; though
the nerve-fibres are mainly situated externally and the nerve-cells
internally.
 
As already stated in Chapter Xll. the primitive division of
the nervous axis was probably not into brain and spinal cord,
but into (i) a fore-brain, representing the ganglion of the praeoral lobe, and (2) the posterior part of the nervous axis, consisting of the mid- and hind-brains and the spinal cord. This view
of the division of the central nervous system fits in fairly satisfactorily with the facts of development. The fore-brain is, histologically, more distinct from the posterior part of the nervous
system than the posterior parts are from each other ; the front
end of the notochord forms the boundary between these two parts
of the central nervous system (vide fig. 253), ending as it does at
the front termination of the floor of the mid-brain, and finally,
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 447
 
the nerves of the fore-brain have a different character to those of
the mid- and hind-brain.
 
This primitive division of the central nervous system is lost
in all the true Vertebrata, and in its place there is a secondary
division corresponding with the secondary vertebrate head
into a brain and spinal cord. The brain, as it is established in
these forms, is again divided into a fore-brain, a mid-brain and a
hind-brain. The fore-brain is, as we have already seen, the
original ganglion of the praeoral lobe. The mid-brain appears
to be the lobe, or ganglion, of the third pair of nerves (first pair
of segmental nerves), while the hind-brain is a more complex
structure, each section of which (perhaps indicated by the constrictions which often appear at an early stage of development)
giving rise to a pair of segmental nerves is, roughly speaking,
homologous with the whole mid-brain.
 
The type of differentiation of each of the primitively simple
vesicles forming the fore-, the mid- and the hind-brains is very
uniform throughout the Vertebrate series, but it is highly instructive to notice the great variations in the relative importance of
the parts of the brain in the different types. This is especially
striking in the case of the fore-brain, where the cerebral hemispheres, which on embryological grounds we may conclude to
have been hardly differentiated as distinct parts of the fore-brain
in the most primitive types now extinct, gradually become more
and more prominent, till in the highest Mammalia they constitute
a more important section of the brain than the whole of the
remaining parts put together.
 
The little that is known with reference to the significance of
the more or less corresponding outgrowths of the floor and roof
of the thalamencephalon, constituting the infundibulunv and
pineal gland, has already been mentioned in connection with the
development of these parts.
 
 
 
(332) C. J. Cams. Vcrsnch einer Darstellnng d. Nervensy stems, etc. Leipzig,
 
1814.
 
(333) J. L. Clark. " Researches on the development of the spinal cord in Man,
Mammalia and Birds." Phil. Trans., 1862. .
 
 
 
448 BIBLIOGRAPHY.
 
 
 
(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. "
Centralblatt f. d, med. Wissenschaften, 1868. Nr. 8.
 
(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen and der
hoheren Wirbelthiere. Tiibingen, 1869.
 
(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen
der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v.
Ecker und Lindenschmidt. Vol. ill. 1868.
 
(337) E. Ehlers. "Die Epiphyse am Gehirn d. Plagiostomen." Zeit. f. wiss.
Zool. Vol. xxx., suppl. 1878.
 
(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des
Menschen. Auf Grund cntwicklungsgeschichtlicher Untersucfumgen. Leipzig, 1876.
 
(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoitfs Archiv,
Bd. xxx. 1864.
 
(340) L. Lowe. "Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Saugethiere u. d. Menschen." Berlin, 1880.
 
(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nervensystems d. Wirbelthiere." Mittheil. a. d. embryo!. Instit. Wien, Vol. II. 1880.
 
(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ."
Quart. J. of Micr. Science, Vol. XIX. 1879.
 
(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.
 
(344) W. Mil Her. " Ueber Entwicklung und Bau der Hypophysis und des
Processus infundibuli cerebri. " yenaische Zeitschrift. Bd. VI. 1871.
 
(345) H. Rahl-Riickhard. "Die gegenseitigen Verhaltnisse d. Chorda,
Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d.
einzelnen Theile d. Fischgehirns." Morphol. Jahrbttch, Vol. vi. 1880.
 
(348) H. Rathke. " Ueber die Entstehung der glandula pituitaria." Mutter's
Archiv f. Anat. und Physiol., Bd. V. 1838.
 
(347) C. B. Reichert. Der Bau des mcnschlichen Gehirns. Leipzig, 1859 u 1861.
 
(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns."
Zeitschrift f. wiss. Zoologie, 1862. Bd. xi.
 
(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns. " Sitz. d.
Jenaischcn Gesell.f. Med. u. Naturwiss. Jan. 23, 1880.
 
(350) F'ried. Tiedemann. Anatomic und Bildtmgsgeschichte des Gehirns im
Foetus des Menschen. Niirnberg, 1816.
 
 
 
THE DEVELOPMENT OF THE CRANIAL AND SPINAL NERVES 1 .
 
All the nerves are outgrowths of the central nervous system,
but the differences in development between the cranial and
spinal nerves are sufficiently great to make it convenient to
treat them separately.
 
1 Remak derived the posterior ganglia from the tissue of the mesoblastic somites,
and following in Remak's steps most authors believed the peripheral nervous system
to have a mesoblastic origin. This view, which had however been rejected on
theoretical grounds by Hensen and others, was finally attacked on the ground of
observation by His (No. 297). His (No. 352, p. 458) found that in the Fowl " the
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 449
 
Spinal nerves. The posterior roots of the spinal nerves, as
well as certain of the cranial nerves, arise in the same manner,
and from the same structure, and are formed considerably before
the anterior roots. Elasmobranch fishes may be taken as the
type to illustrate the mode of formation of the spinal nerves.
 
The whole of the nerves in question arise as outgrowths of a
median ridge of cells, which makes its appearance on the dorsal
side of the spinal cord (fig. 264 A, pr). This ridge has been
called by Marshall the neural crest. At each point, where a
pair of nerves will be formed, two pear-shaped outgrowths
project from it, one on each side ; and apply themselves closely
to the walls of the spinal cord (fig. 264 B, pr). These outgrowths are the rudiments of the posterior nerves. While still
remaining attached to the dorsal summit of the neural cord they
grow to a considerable size (fig. 264 B, pr).
 
The attachment to the dorsal summit is not permanent, but
 
spinal ganglia of the head and trunk arose from a small band of matter which is
placed between the medullary plate and epiblast, and the material of which he called
the 'intermediate cord'." He further states that: "Before the closure of the
medullary tube this band forms a special groove the 'intermediate groove' placed
close to the border of the medullary plate. As the closure of the medullary plate
into a tube is completed, the earlier intermediate groove becomes a compact cord.
In the head of the embryo a longitudinal ridge arises in this way, which separates the
suture of the brain from that of the epiblast. In the parts of the neck and in the
remaining region of the neck the intermediate cord does not lie over the line of
junction of the medullary tube, but laterally from this and forms a ridge, triangular
in section, with a slight indrawing." This intermediate ridge gives rise to four
ganglia in the head, viz. the g. trigemini, g. acousticum, g. glossopharyngei, and
g. vagi, and in the trunk to the spinal ganglia. In both cases it unites first with the
spinal cord.
 
I have given in the above account, as far as possible, a literal translation of His'
own words, because the reader will thus be enabled fairly to appreciate his meaning.
 
Subsequently to His' memoir (No. 297) I gave an account of some researches of
my own on this subject (No. 351), stating the whole of the nerves to be formed as
cellular outgrowths of the spinal cord. I failed fully to appreciate that some of the
stages I spoke of had been already accurately described by His, though interpreted by
him very differently. Marshall, and afterwards Kolliker, arrived at results in the main
similar to my own, and Hensen, independently of and nearly simultaneously with
myself, published briefly some observations on the nerves of Mammals in harmony
with my results.
 
His has since worked over the subject again (No. 352), and has reaffirmed as a
result of his work his original statements. I cannot, however, accept his interpretations on the subject, and must refer the reader who is anxious to study them more
fully, to His' own paper.
 
B. III. 29
 
 
 
450
 
 
 
SPINAL NERVES.
 
 
 
 
FIG. -264 A. TRANSVERSE SECTION THROUGH A PRISTIURUS EMBRYO SHEWING THE PROLIFERATION
OF CELLS TO FORM THE NEURAL
CREST.
 
pr. neural crest ; nc. neural canal ;
ch. notochord ; ao. aorta.
 
 
 
 
FIG. 2646. TRANSVERSE SECTION THROUGH THE TRUNK OK
AN EMBRYO SLIGHTLY OLDER
THAN FIG. 28 E.
 
nc. neural canal ; pr. posterior
root of spinal nerve ; x. subnotochordal rod ; ao. aorta ; sc . somatic mesoblast ; sp. splanchnic
mesoblast ; mp. muscle-plate ;
mp'. portion of muscle-plate converted into muscle ; Vv. portion
of the vertebral plate which will
give rise to the vertebr.il bodies ;
al. alimentary tract.
 
 
 
before describing the further fate
of the nerve-rudiments it is necessary to say a few words as to
the neural crest. At the period
when the nerves have begun to
shift their attachment to the
spinal cord, there makes its appearance, in Elashiobranchii, a
longitudinal commissure connecting the dorsal ends of all
the spinal nerves (figs. 265, 266
com}, as well as those of the
vagus and glosso-pharyngeal
nerves. This commissure has
as yet only been found in a complete form in Elasmobranchii ;
 
 
 
 
FIG. 265. VERTICAL LONGITUDINAL
SECTION THROUGH PART OF THETRUNK
OF A YOUNG SCYLLIUM EMBRYO.
 
com. commissure uniting the dorsal
ends of the posterior nerve-roots ; pr.
ganglia of posterior roots; ar. anterior
roots; st. segmental tubes; sd. segmental
duct; g.c. epithelium lining the body
cavity in the region of the future germinal
ridge.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
451
 
 
 
but it is nevertheless to be regarded as a very important morphological structure.
 
 
 
 
FIG. 266. SPINAL NERVES OF SCYLLIUM IN LONGITUDINAL SECTION TO SHEW
THE COMMISSURE CONNECTING THEM.
 
A. Section through a series of nerves.
 
B. Highly magnified view of the dorsal part of a single nerve, and of the
commissure connected with it.
 
com. commissure; sp.g. ganglion of posterior root; ar. anterior root.
 
It is probable, though the point has not yet been definitely
made out, that this commissure is derived from the neural crest,
which appears therefore to separate into two cords, one connected
with each set of dorsal roots.
 
 
 
7' r
 
 
 
 
FIG. 267. SECTION THROUGH THE DORSAL PART OF THE TRUNK OF A
 
TORPEDO EMBRYO.
 
pr. posterior root of spinal nerve ; g . spinal ganglion ; n. nerve ; ar. anterior root
of spinal nerve; ch. notochord; nc. neural canal; mp. muscle-plate.
 
29 2
 
 
 
452 SPINAL NERVES.
 
 
 
Returning to the original attachment of the nerve-rudiments
to the medullary wall, it has been already stated that this
attachment is not permanent. It becomes, in fact, at about the
time of the appearance of the above commissure, either extremely
delicate or absolutely interrupted.
 
The nerve-rudiment now becomes divided into three parts
(figs. 267 and 268), (i) a proximal rounded portion, to which is
attached the longitudinal commissure (pr) \ (2) an enlarged
portion, forming the rudiment of a ganglion (g and sp g}\ (3) a
distal portion, forming the commencement of the nerve (#).
The proximal portion may very soon be observed to be united
with the side of the spinal cord at a very considerable distance
from its original point of attachment. Moreover the proximal
portion of the nerve is attached, not by its extremity, but by its
side, to the spinal cord (fig. 268 x\ The dorsal extremities of
the posterior roots are therefore free.
 
This attachment of the posterior nerve-root to the spinal cord is, on
account of its small size, very difficult to observe. In favourable specimens
there may however be seen a distinct cellular prominence from the spinal
cord, which becomes continuous with a small prominence on the lateral
border of the nerve root near its proximal extremity. The proximal extremity of the nerve is composed of cells, which, by their small size and
circular form, are easily distinguished from those which form the succeeding
or ganglionic portion of the nerve. This part has a swollen configuration,
and is composed of large elongated cells with oval nuclei. The remainder
of the rudiment forms the commencement of the true nerve. This also is, at
first, composed of elongated cells 1 .
 
1 The cellular structure of embryonic nerves is a point on which I should have
anticipated that a difference of opinion was impossible, had it not been for the fact
that His and Kolliker, following Remak and other older embryologists, absolutely
deny the fact. I feel quite sure that no one studying the development of the nerves in
Elasmobranchii with well-preserved specimens could for a moment be doubtful on
this point, and I can only explain His' denial on the supposition that his specimens
were utterly unsuited to the investigation of the nerves. I do not propose in this
work entering into the histogenesis of nerves, but may say that for the earlier stages
of their growth, at any rate, my observations have led me in many respects to the
same results as Gotte (Entwick. d. Unke, pp. 482 483), except that I hold that
adequate proof is supplied by my investigations to demonstrate that the nerves are
for their whole length originally formed as outgrowths of the central nervous system.
As the nerve-fibres become differentiated from the primitive spindle-shaped cells, the
nuclei become relatively more sparse, and this fact has probably misled Kolliker.
Lowe, while admitting the existence of nuclei in the nerves, states that they belong to
mesoblastic cells which have wandered into the nerves. This is a purely gratuitous
assumption, not supported by observation of the development.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
453
 
 
 
It is extremely difficult to decide whether the permanent attachment of
the posterior nerve-roots to the spinal cord is entirely a new formation, or
merely due to the shifting of the original point of attachment. I am inclined
to adopt the former view, which is also held by Marshall and His, but may
refer to fig. 269, shewing the roots after they have become attached to the
side, as distinct evidence in favour of the view that the attachment simply
becomes shifted, a process which might perhaps be explained by a growth of
the dorsal part of the spinal cord. The change of position in the case of
some of the cranial nerves is, however, so great that I do not think that it is
possible to account for it without admitting the formation of a new attachment.
 
The anterior roots of the spinal nerves appear somewhat
later than the posterior roots, but while the latter are still quite
small. Each of them (fig. 269 ar) arises as a small but distinct
conical outgrowth from a ventral corner of the spinal cord,
before the latter has acquired its covering of white matter.
From the very first the rudiments of the anterior roots have a
somewhat fibrous appearance and an indistinct form of peripheral
 
 
 
 
FIG. 268. SECTION THROUGH THE DORSAL REGION OF A PRISTIURUS EMBRYO.
pr. posterior root; sp.g. spinal ganglion; n. nerve; x. attachment of ganglion to
spinal cord ; nc. neural canal ; mp. muscle-plate ; ch. notochord ; i. investment of
spinal cord.
 
termination, while the protoplasm of which they are composed
becomes attenuated towards its end. They differ from the
posterior roots in never shifting their point of attachment to the
spinal cord, in not being united with each other by a commissure,
and in never developing a ganglion.
 
 
 
454
 
 
 
SPINAL NERVES.
 
 
 
The anterior roots grow rapidly, and soon form elongated
cords of spindle-shaped cells with wide attachments to the spinal
cord (fig. 267). At first they pass obliquely and nearly horizontally outwards, but, before reaching the muscle-plates, they
take a bend downwards.
 
One feature of some interest with reference to the anterior
roots is the fact that they arise not vertically below, but
alternately with the posterior roots : a condition which persists
in the adult. They are at first quite separate from the posterior
roots ; but about the stage represented in fig. 267 a junction is
effected between each posterior root and the corresponding
anterior root. The anterior root joins the posterior at some
little distance below its ganglion (figs. 265 and 266).
 
Although I have made some efforts to
determine the eventual fate of the commissure uniting the dorsal roots, I have not
hitherto met with success. It grows thinner
and thinner, becoming at the same time
composed of fibrous protoplasm with imbedded nuclei, and finally ceases to be recognisable. I can only conclude that it
gradually atrophies, and ultimately vanishes.
 
After the junction of the posterior and
anterior roots the compound nerve extends
downwards, and may easily be traced for
a considerable distance. A special dorsal
branch is given off from the ganglion on
the posterior root (fig. 275 dn\ According
to Lowe the fibres of the anterior and posterior roots can easily be distinguished in
the higher types by their structure and
behaviour towards colouring reagents, and
can be separately traced in the compound
 
 
 
 
FIG. 269. TRANSVERSE SKI TION THROUGH THE DORSAL REGION OF A YOUNG TORPEDO EMBRYO TO SHEW THE ORIGIN OF
THE ANTERIOR AND POSTERIOR
ROOTS OF THE SPINAL NERVES.
 
pr. posterior root of spinal
nerve ; ar. anterior root of spinal
nerve; mp. muscle-plate; ch. notochord; vr. mesoblast cells which
will form the vertebral bodies.
 
nerve.
 
 
 
So far as has been made out, the development of the spinal
nerves of other Vertebrates agrees in the main with that in
Elasmobranchii, but no dorsal commissure has yet been discovered,
except in the case of the first two or three spinal nerves of the
Chick.
 
In the Chick (Marshall, No. 353) the posterior roots, during their early
stages, closely resemble those in Elasmobranchii, though their relatively
smaller size makes them difficult to observe. They at first extend more or
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
455
 
 
 
less horizontally outwards above the muscle-plates (as a few of the nerves
also do to some extent in Elasmobranchii), but subsequently lie close to the
sides of the neural canal. They are shewn in this position in fig. 116 sp.g.
There does not appear to be a continuous crest connecting the roots of the
posterior nerves. The later stages of the development are precisely like
those in Elasmobranchii.
 
The anterior roots have not been so satisfactorily investigated as the
posterior, but they grow out, possibly by several roots for each nerve, from
the ventral corners of the spinal cord, and subsequently become attached
to the posterior nerves.
 
I have observed the development of the posterior roots in Lepidosteus, in
which they appear as projections from the dorsal angles of the spinal cord,
extending laterally outwards and, at first, having their extremities placed
dorsally to the muscle-plates.
 
The cranial nerves 1 . The earliest stages in the development of the cranial nerves have been most satisfactorily studied,
especially by Marshall (No. 354), in the Chick, while the later
stages have been more fully worked out in Elasmobranchii,
where, moreover, they present a very primitive arrangement.
 
 
 
hi,
 
 
 
 
fy
 
 
 
FIG. 270.
 
 
 
TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE
 
HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.
 
hb. hind-brain; vg. vagus nerve; cp. epiblast; ch. notochord; x. thickening of
hypoblast (possibly a rudiment of the subnotochordal rod) ; al. throat ; ht. heart ;
//. body cavity ; so. somatic mesoblast ; sf. splanchnic mesoblast ; hy. hypoblast.
 
1 The optic nerves are for obvious reasons dealt with in connection with the
development of the eye.
 
 
 
456 CRANIAL NERVES.
 
 
 
In the Chick certain of the cranial nerves arise before the
complete closure of the neural groove. These nerves are formed
as paired outgrowths of a continuous band composed of two
laminae, connecting the dorsal end of the incompletely closed
medullary canal with the external epiblast. This mode of
development will best be understood by an examination of fig.
270, where the two roots of the vagus nerve (vg) are shewn
growing out from the neural band. Shortly after this stage the
neural band, becoming separated from the epiblast, constitutes
a crest attached to the roof of the brain, while its two laminae
become fused. The relation of the cranial nerves to the brain
then becomes exactly the same as that of the posterior roots of
the spinal nerves to the spinal cord.
 
It does not appear possible to decide whether the mode of development
of the cranial nerves in the Chick, or that of the posterior roots of the spinal
nerves, is the more primitive. The difference in development between the
two sets of nerves probably depends upon the relative time of the closure of
the neural canal. The neural crest clearly belongs to the brain, from the
fact of its remaining connected with the latter when the medullary tube
separates from the external epiblast.
 
It is not known whether the cranial nerves originate before the closure of
the neural canal in other forms besides the Chick.
 
The neural crest of the brain is continuous with that of the
spinal cord, and on its separation from the central nervous axis
forms on each side a commissure, uniting the posterior cranial
nerves with the spinal nerves, and continuous with the commissure connecting together the latter nerves.
 
Anteriorly, the neural crest extends as far as the roof of the
mid-brain 1 . The pairs of nerves which undoubtedly grow out
from it are the third pair (Marshall), the fifth, the seventh and
auditory (as a single root), the glossopharyngeal, and the various
elements of the vagus (as separate roots in Elasmobranchii, but
as a single root in Aves). Marshall holds that the olfactory
 
1 Marshall holds that the neural crest extends in front of the region of the optic
vesicle. I have been unable completely to satisfy myself of the correctness of this
statement. In my specimens the epiblast along the line of infolding of this part of
the roof of the brain is much thickened, but what Marshall represents as a pair of outgrowths from it like those of a true nerve (No. 354, PI. n. fig. 6) appears to me in my
specimens to be part of the external epiblast ; and I believe that they remain connected
with the external epiblast on the complete separation of the brain from it.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 457
 
nerve probably also originates from this crest. It will however
be convenient to deal separately with this nerve, after treating of
the other nerves which undoubtedly arise from the neural crest.
 
The cranial nerves just enumerated present in their further
development many points of similarity ; and the glossopharyngeal nerve, as it develops in Elasmobranchii, may perhaps be
taken as typical. This nerve is connected by a commissure with
those behind, but this fact may for the moment be left out of
consideration. Springing at first from the dorsal line of the
hind-brain immediately behind the level of the auditory capsule,
it apparently loses this primitive attachment and acquires a
secondary attachment about half-way down the side of the
hind-brain. The primitive undifferentiated rudiment soon becomes divided, exactly like a true posterior root of a spinal
nerve, into a root, a ganglion and a nerve. The main branch of
the nerve passes ventralwards, and supplies the^ first branchial
arch (fig. 271 gl}. Shortly afterwards it sends forwards a
smaller branch, which passes to the hyoid arch in front ; so that
the nerve forks over the hyobranchial cleft. A typical cranial
nerve appears therefore, except as concerns its relations to the
clefts, to develop precisely like the posterior root of the spinal
nerve.
 
Most of the cranial nerves of the above group, in correlation
with the highly differentiated character of the head, acquire
secondary differentiations, and render necessary a brief description of what is known with reference to their individual development.
 
The Glossopharyngeal and Vagus Nerves. Behind the ear
there are formed, in Scyllium, a series of five nerves which pass down to
respectively the first, second, third, fourth and fifth branchial arches.
 
For each arch there is thus one nerve, whose course lies close to the
posterior margin of the preceding cleft ; a second anterior branch, forking
over the cleft and passing to the arch in front, being developed later. These
nerves are connected with the brain by roots at first attached to the dorsal
summit, but eventually situated about half-way down the sides. The
foremost of them is the glossopharyngeal. The next four are, as has been
shewn by Gegenbaur 1 , equivalent to four independent nerves, but form
together a compound nerve, which we may briefly call the vagus.
 
1 "Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. VI.
1871.
 
 
 
CRANIAL NERVES.
 
 
 
This compound nerve together with the glossopharyngeal soon attains a
very complicated structure, and presents several remarkable features. There
are present five branches (fig. 271 B), viz. the glossopharyngeal (gl) and
four branches of the vagus, the latter probably arising by a considerably
greater number of strands from the brain 1 . All the strands from the
brain are united together by a thin commissure (fig. 271 B, vg) } continuous
with the commissure of the posterior roots of the spinal nerves, and from
this commissure the five branches are continued obliquely ventralwards and
backwards, and each of them dilates into a ganglionic swelling. They all
become again united together by a second thick commissure, which is
continued backwards as the intestinal branch of the vagus nerve. The
nerves, however, are continued ventralwards each to its respective arch.
 
 
 
A6
 
 
 
t'A
 
 
 
 
FlG. 271. VIEWS OF THE HEAD OF El.ASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.
 
A. Pristiurus embryo of the same stage as fig. 28 F.
 
B. Somewhat older Scyllium embryo.
 
///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl.
glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland ; mb. midbrain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op.
eye; au.V. auditory vesicle; m. mesohlast at base of brain; t/i. notochord; /it. heart;
Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head.
 
 
 
1 " Ueber d. Kopfnerven von Hexanchus," etc., Jenaische Zeitschrift, Vol. vi. i S; i .
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 459
 
From the lower commissure springs the lateral nerve, at a point whose
relations to the branches of the vagus I have not certainly determined.
 
With reference to the dorsal commissure, which is almost certainly
derived from the original neural crest, it is to be noted that there is a
longish stretch of it between the last branch of the vagus and the first
spinal nerve, which is probably the remains of a part of the commissure
which connected the posterior branches of the vagus, at a stage in the
evolution of the Vertebrata, when the posterior visceral clefts were still
present. These branches of the vagus are probably partially preserved in
the ramifications of the intestinal stem of the vagus (Gegenbaur). The
origin of the ventral commissure, continued as the intestinal branch of the
vagus, has not been embryologically worked out.
 
The lateral nerve may very probably be a dorsal sensory branch of
the vagus, whose extension into the posterior part of the trunk has been
due to the gradual backward elongation of the lateral line 1 , causing
the nerve supplying it to elongate at the same time (vide Section on lateral
line).
 
In the Chick the common rudiment for the vagus and glossopharyngeal
nerves (Marshall), which has already been spoken of, subsequently divides
into two parts, an anterior forming the glossopharyngeal nerve, and a
posterior forming the vagus nerve.
 
The seventh and auditory nerves. As shewn by Marshall's
and my own observations 'there is a common rudiment for the seventh and
auditory nerves. This rudiment divides almost at once into two branches.
The anterior of these pursues a straight course to the hyoid arch (fig.
271 A, VII.} and forms the rudiment of the facial nerve ; the second of the
two (fig. 271 A, au.ti), which is the rudiment of the auditory nerve, develops
a ganglionic enlargement and, turning backwards, closely hugs the ventral
wall of the auditory involution (fig. 272).
 
The seventh or facial nerve soon becomes more complicated. It early
develops, like the glossopharyngeal and vagus nerves, a branch, which
forks over the cleft in front (spiracle), and supplies the mandibular arch
(fig. 27 1 B). This branch forms the praespiracular nerve of the adult, and
is homologous with the chorda tympani of Mammalia. Besides however
giving rise to this typical branch it gives origin, at a very early period,
to two other rather remarkable branches ; one of these, arising from its
dorsal anterior border, passes forwards to the front part of the head, immediately dorsal to the ophthalmic branch of the fifth to be described
directly. This nerve is the portio major or superficialis of the nerve usually
known as the ramus ophthalmicus superficialis in the adult 2 .
 
1 The peculiar distribution of branches of the fifth and seventh nerves to the
lateral line, which is not uncommon, is to be explained in the same manner.
 
2 The two branches of the ramus ophthalmicus superficialis were spoken of as the
ram. opth. superficialis and ram. opth. profundus in my Monograph on Elasmobranch
Fishes. The nomenclature in the text is Schwalbe's, which is probably more correct
than mine.
 
 
 
460 CRANIAL NERVES.
 
 
 
The other branch of the seventh is the palatine branch superficial
petrosal of Mammalia the course of which has been more fully investigated
by Marshall than by myself. He has shewn that it arises "just below the
root of the ophthalmic branch," and " runs downwards and forwards, lying
parallel and immediately superficial to the maxillary branch of the fifth
nerve." This branch of the seventh nerve appears to bear the same sort of
relation to the superior maxillary branch of the fifth nerve, that the
ophthalmic branch of the seventh does to the ophthalmic branch of the fifth.
 
Both the root of the seventh and its main branches are gangliated.
 
The auditory nerve is probably to be regarded as a specially differentiated part of a dorsal branch of the seventh, while the ophthalmic branch
may not improbably be a dorsal branch comparable to a dorsal branch of
one of the spinal nerves.
 
The fifth nerve. Shortly after its development the root of the fifth
nerve shifts so as to be attached about half-way down the side of the brain.
A large ganglion becomes developed close to the root, which forms the
rudiment of the Gasserian ganglion. The main branch of the nerve grows
into the mandibular arch (fig. 271 A, V), maintaining towards it similar
relations to those of the posterior nerves to their respective arches.
 
Two other branches very soon become developed, which were not
properly distinguished in my original account. The dorsal one takes a
course parallel to the ophthalmic branch of the seventh nerve, and forms,
according to the nomenclature already adopted, the portio profunda of the
ophthalmicus superficialis of the adult.
 
The second nerve (fig. 271 A) passes forwards, above the mandibular
head cavity, and is directed straight towards the eye, near which it meets
and unites with the third nerve, where the ciliary ganglion is developed
(Marshall). This branch is usually called the ophthalmic branch of the
fifth nerve, but Marshall rightly prefers to call it the communicating branch
between the fifth and third nerves 1 .
 
Later than these two branches there is developed a third branch, passing
to the front of the mouth, and forming the superior maxillary branch of the
adult (fig. 271 B).
 
Of the branches of the fifth nerve the main mandibular branch is
obviously comparable to the main branch of the posterior nerves. The
superficial ophthalmic branch is clearly equivalent to the ophthalmic branch
of the seventh. The superior maxillary is usually held to be equivalent to
that branch of the posterior nerves which forms the anterior limb of the fork
over a cleft. The similarity between the course of this nerve and that
of the palatine branch of the seventh, resembling as it does the similar
course of the ophthalmic branches of the two nerves, suggests that it may
perhaps really be the homologue of the palatine branch of the seventh, there
 
1 Marshall thinks that this nerve may be the remains of the commissure originally
connecting the roots of the third and fifth nerves. This suggestion can only be tested
by further observations.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 461
 
being no homologue of the typical anterior branch of the other cranial
nerves.
 
The third nerve. Our knowledge of the development of the third
nerve is entirely due to Marshall. He has shewn that in the Chick there is
developed from the neural crest, on the roof of the mid-brain, an outgrowth
on each side, very similar to the rudiment of the posterior nerves. This
outgrowth, the presence of which I can confirm, he believes to be the third
nerve, but although he is probably right in this view, it must be borne
in mind that there is no direct evidence on the point, the fate of the
outgrowth in question not having been satisfactorily followed.
 
At a very considerably later period a nerve may be found springing
from the floor of the mid-brain, which is undoubtedly the third nerve, and
which Marshall supposes to be the above rudiment, which has shifted its
position. It is shewn in Scyllium in fig. 271 B, ///. A few intermediate
stages between this and the earliest condition of the nerve have been
imperfectly traced by Marshall.
 
The nerve at the stage represented in fig. 271 B arises from a ganglionic
root, and " runs as a long slender stem almost horizontally backwards, then
turns slightly outwards to reach the interval between the dorsal ends of the
first and second head cavities, where it expands into a small ganglion."
This ganglion, as first suggested by Schwalbe (No. 359), and subsequently
proved embryologically by Marshall, is the ciliary ganglion. From the
ciliary ganglion two branches arise ; one branch continuing the main stem
of the nerve, and obviously homologous with the main branch of the other
nerves, and the other passing directly forwards " along the top of the first
head cavity, then along the inner side of the eye, and finally terminating at
the anterior extremity of the head, just dorsal of the olfactory pit."
 
The partial separation, in many forms, of the ciliary ganglion from the
stem of the third nerve has led to the erroneous view (disproved by the
researches of Marshall and Schwalbe) that the ciliary ganglion belongs to
the fifth nerve. The connecting branch of the fifth nerve often becomes
directly continuous with the anterior branch of the third nerve, and the two
together probably constitute the nerve known as the ramus ophthalmicus
profundus (Marshall). Further embryological investigations will be required
to shew whether this nerve is homologous with the nasal branch of the fifth
nerve in Mammalia.
 
Relations of the nerves to the head-cavities. The cranial
nerves, whose development has just been given, bear certain very definite
relations to the mesoblastic structures in the head, of the nature of somites,
which are known as the head-cavities. Each cranial nerve is typically
placed immediately behind the head-cavity of its somite. Thus the main
branch of the fifth nerve lies in contact with the posterior wall of the
mandibular cavity, as shewn in section in fig. 272 V. ipp and in surface view
in fig. 271 ; the main branch of the seventh nerve occupies a similar position
in relation to the hyoid cavity ; and, as Marshall has recently shewn, the
main branch of the third nerve adjoins the posterior border of the front
 
 
 
462
 
 
 
CRANIAL NERVES.
 
 
 
cavity, described by me as the premandibular cavity. Owing to the early conversion of the walls of the posterior headcavities into muscles, their relations to the
nerves are not quite so clear as in the
case of the anterior cavities, though, as
far as is known, they are precisely the
same.
 
Anterior nerve-roots in the brain.
 
During my investigations on the development of the cranial nerves I was
unable to find any roots comparable with
the anterior roots of the spinal nerves,
and propounded an hypothesis (suggested
by the absence of anterior spinal roots
in Amphioxus 1 ) that the head and trunk
had become differentiated from each other
at a stage when mixed motor and sensory
posterior roots were the only roots present, and I supposed the cranial and
spinal nerves to have been independently
evolved from a common ground form,
the resulting types of nerves being so
different that no roots strictly comparable
with the anterior roots of spinal nerves
were to be found in the cranial nerves.
 
The views put forward by me on this
subject, though accepted by Schwalbe
 
 
 
Vll
 
 
 
 
FIG. 272. TRANSVERSE SECTION
THROUGH THE FRONT PART OF THE
HEAD OF A YOUNG PRISTIURUS
 
EMBRYO.
 
The section, owing to the cranial
flexure, cuts both the fore- and the
hind-brain. It shews the pramandibular and mandibular head-cavities
\pp and ipp, etc.
 
fb. fore-brain; /. lens of eye; m.
mouth ; pt. upper end of mouth,
forming pituitary involution; \ao.
mandibular aortic arch; ipp. and
ipp. first and second head-cavities ;
ivc. first visceral cleft ; V. fifth
nerve ; aun. ganglion of auditory
nerve ; VII. seventh nerve ; aa. dorsal aorta ; acv. anterior cardinal
vein ; ch. notochord.
 
 
 
(No. 357), have in other quarters not
met with much favour. Wiedersheim holds that it is impossible to believe
that the cranial nerves are simpler than the spinal nerves. Such simplicity,
which is clearly not found, I have never asserted to exist ; I have only
stated that the cranial nerves, in acquiring the complicated character they
have in the adult, do not develop anterior roots comparable with those
of the spinal nerves. Marshall also strongly objects to my views, and has
made some observations for the purpose of testing them, leading to some
very interesting results, which I proceed to state, and I will then explain my
opinion concerning them.
 
The most important observation of Marshall on this subject concerns
the sixth nerve. In both the Chick and Scy Ilium he has detected a nerve
(the first development of which has unfortunately not been made out) arising
by a series of roots from the base of the hind-brain. By tracing this nerve
to the external rectus muscle of the eye he has satisfactorily identified
 
 
 
1 Schneider holds that anterior roots are present in Amphioxus, but I have been
unable to satisfy myself of their presence.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 463
 
it as the sixth nerve. " Neither in the nerve nor in its roots are there any
ganglion cells." This nerve he finds to be placed vertically below the roots
of the seventh nerve ; and it is not visible till much later than the cranial
nerves above described.
 
In addition to this nerve Marshall has found, both in the third nerve
and in the fifth nerve, a series of non-gangliated roots, which arise in a
manner not yet satisfactorily elucidated, considerably later than, and in front
of, the main roots. These roots join the gangliated roots on the proximal
side of the ganglion or in the ganglion 1 ; and Marshall believes them to be
homologous with the anterior roots of spinal nerves, while he holds the
sixth nerve to be an anterior root of the seventh nerve.
 
In addition to these nerves Marshall holds certain ventral roots, which
occur in Elasmobranchs close to the boundary of the spinal cord and
medulla, and which probably form the hypoglossal nerve of higher types, to
be anterior roots of the vagus. It is very difficult to prove anything
definitely about these nerves, but, for reasons stated in my work on
Elasmobranch Fishes, I am inclined to regard them as anterior roots of one
or more spinal nerves.
 
Before attempting to decide how far Marshall's views about the so-called
anterior roots of the seventh, the fifth and the third nerves are well founded
it will conduce to clearness to state the characters and relations of the two
roots of spinal nerves.
 
The posterior root is (i) always purely sensory ; (2) it always develops a
ganglion. The anterior root is (i) always purely motor ; (2) it always joins
the posterior root below the ganglion, except in Petromyzon (though not in
Myxine) where the two roots are stated to be independent.
 
How far do Marshall's anterior and posterior roots of the cranial nerves
exhibit these respective peculiarities ?
 
With reference to the sixth and seventh nerves he states " we must
regard the sixth nerve as having the same relation to the seventh that the
anterior root of a spinal nerve has to the posterior root." On this I would
remark (i) that the posterior root of this nerve is a mixed sensory and
motor nerve and therefore differs in a very fundamental point from that of
a spinal nerve ; (2) the sixth nerve though resembling the anterior root of
a spinal nerve in being motor and without a ganglion, differs from the
nearly universal arrangement of spinal nerves in not uniting with the
seventh.
 
With reference to the fifth nerve it is to be observed that it is by no
means certain that the whole of the motor fibres are supplied by the socalled anterior roots, and that these roots differ again in the most marked
manner from the anterior roots of spinal nerves in joining the main root of
the nerve above (nearer the brain), and not as in a spinal nerve below the
 
1 These non-gangliated roots of the fifth nerve are not to be confounded with the
motor root of the fifth nerve in higher types. They appear to form the anterior root
of the adult which gives origin to the ramus ophthalmicus.
 
 
 
464 CRANIAL NERVES.
 
 
 
ganglion. The gangliated root of the third nerve is purely motor 1 , and its
so-called anterior roots again differ from the anterior roots of spinal nerves,
in the same manner as those of the fifth nerve.
 
With reference to the glossopharyngeal and vagus nerves I would
merely remark that no anterior root has even been suggested for the
glossopharyngeal nerve and that the posterior roots of both these nerves
contain a mixture of sensory and motor fibres.
 
In view of these facts, my original hypothesis appears to me to be
confirmed by Marshall's observations.
 
The fact of all the posterior roots of the above cranial nerves (except
the third which may be purely motor) being mixed motor and sensory roots
appears to me to demonstrate that the starting-point of their differentiation
was a mixed nerve with a single dorsal root ; and that they did not therefore
become differentiated from nerves built on the same type as the spinal
nerves with dorsal sensory and ventral motor roots. The presence of such
non-gangliated roots as those of the third and fifth nerves is not a difficulty
to this view. Considering that the cranial nerves are more highly differentiated than the spinal nerves, and have more complicated functions to
perform, it would be surprising if there had not been developed nonganglionated roots analogous to, but not of course homologous with, the
anterior roots of the spinal nerves 2 .
 
As to the sixth nerve further embryological investigations are requisite
before its true position in the series can be determined ; but it appears to
me very probable that it is a product of the differentiation of the seventh
nerve.
 
The fourth nerve. No embryological investigations have been
made with reference to the fourth nerve. It is possible that it is a segmental
nerve comparable with the third nerve, and that the only remnant still left
of the segment to which it belongs is the superior oblique muscle of the eye.
If this is the case there must have been two praemandibular segments, viz.
that belonging to the third nerve, and that belonging to the fourth nerve.
Against this view of the fourth nerve is the fact, urged with great force by
Marshall, that the superior oblique muscle is in front of the other eye
muscles, and that the fourth nerve therefore crosses the third nerve to
reach its destination.
 
The Olfactory nerve. It was shewn in my monograph on Elasmobranch Fishes that the olfactory nerve grew out from the brain in the
 
1 If Marshall's view about the ramus ophthalmicus profundus (p. 461) is correct,
the third must still be, as it no doubt was primitively, a mixed motor and sensory
nerve.
 
2 In the higher types, as is well known, the fifth nerve has its roots formed on the
same type as a spinal nerve. The fact that this is not the case in the lower types,
either in the embryo or the adult, is a clear indication, to my mind, that the mammalian arrangement of the roots of the fifth nerve has been secondarily acquired, a
fact which is a most striking confirmation of my views as to the differences between
the cranial and spinal nerves.
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA. 465
 
same manner as other nerves ; and Marshall (No. 355), to whom we are
indebted for the greater part of our knowledge on the development of this
nerve, has proved that it arises prior to the differentiation of the olfactory
lobes.
 
The earliest stages in the development of the nerve have not been
made out. Marshall, as already stated, finds that in the Chick the neural
crest is continued in front of the optic vesicles, and holds that this fact is
strong a priori evidence in favour of the nerve growing out from it. As
mentioned above, note on p. 456, I cannot without further evidence accept
Marshall's statements on this point. In any case Marshall has not yet been
 
 
 
 
FIG. 273. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF ScvLLiUM. (Modified from figures by Marshall and myself.)
 
c.h. cerebral hemispheres; ol.v. olfactory vesicle ; olf. olfactory pit; Sch. Schneiderian folds; /. olfactory nerve. The reference line has been accidentally taken
through the nerve to the brain; pn. pineal gland.
 
able again to find an olfactory nerve till long after the disappearance of the
neural crest. The olfactory nerve at the next stage observed forms an outgrowth of fusiform cells springing on either side from near the summit of
the fore-brain ; and at fifty hours it ends close to a slight thickening of the
epiblast forming the first rudiment of the olfactory pit, with the walls of
which it soon becomes united.
 
The growth of the cerebral hemispheres causes its point of insertion in
the brain to be relatively shifted ; and on the development of the olfactory
lobes (vide pp. 444, 445) it arises from them (fig. 273). In Elasmobranchs
there is a large development of ganglion cells near its root. From Marshall's
figures these appear also to be present in the Chick, but they do not seem to
have been found in other forms. In both Teleostei and Amphibia the
olfactory nerves are at first extremely short.
 
Marshall holds that the olfactory nerve is a segmental nerve equivalent
to the third, fifth, seventh etc. nerves. It has been already stated that in my
opinion the origin of the olfactory nerves from the fore-brain, which I hold
to be the ganglion of the prseoral lobe, negatives this view. The mere fact
 
B. HI- 30
 
 
 
466 SYMPATHETIC NERVOUS SYSTEM.
 
of these nerves originating as an outgrowth from the central nervous
system is no argument in favour of Marshall's view of their nature ; and
even if Marshall's opinion that they arise from the neural crest should turn
out to be well founded, this fact would not prove their segmental nature,
because their origin from this crest would, as indicated in the next
paragraph, merely seem to imply that they primitively arose from the
lateral borders of the nerve-plate from which the cerebro-spinal tube has
been formed.
 
Situation of the dorsal roots of the cranial and spinal
nerves. The probable explanation of the origin of nerves from the neural
crest has already been briefly given (p. 316). It is that the neural crest
represents the original lateral borders of the nervous plate, and that, in the
mechanical folding of the nervous plate to form the cerebro-spinal canal, its
two lateral borders have become approximated in the median dorsal line to
form the neural crest. The subsequent shifting of the nerves I am unable
to explain, and the meaning of the transient longitudinal commissure
connecting the nerves is also unknown. The folding of the neural plate
must have extended to the region of the origin of the olfactory nerves, so
that, as just stated, there would be no special probability of the olfactory
nerves belonging to the same category as the other dorsal nerves from the
fact of their springing from the neural crest.
 
 
 
BIBLIOGRAPHY OF THE PERIPHERAL NERVOUS SYSTEM.
 
(351) F. M. Balfour. "On the development of the spinal nerves in Elasmobranch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A monograph on the development of Elasmobranch Fishes. London, 1878, pp. 191 216.
 
(352) W. His. " Ueb. d. Anfange d. peripherischen Nervensystems." Archiv
f. Anat. it. Physiol., 1879.
 
(353) A. M. Marshall. " On the early stages of development of the nerves in
Birds." Journal of Anat. and P/iys.,No\. xi. 1877.
 
(354) A. M. Marshall. "The development of the cranial nerves in the Chick."
Quart, y. of Micr. Science, Vol. xvm. 1878.
 
(355) A. M> Marshall. "The morphology of the vertebrate olfactory organ."
Quart. J. of Micr. Science, Vol. xix. 1879.
 
(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.
 
(357) C. Schwalbe. "Das Ganglion oculomotorii." Jenaische Zeitschrift,
Vol. xili. 1879.
 
Sympathetic nervous system.
 
The discovery that the spinal and cranial nerves together
with their ganglia were formed from the epiblast was shortly
afterwards extended to the sympathetic nervous system, which
has now been shewn to arise in connection with the spinal and
 
 
 
NERVOUS SYSTEM OF THE VERTEBRATA.
 
 
 
467
 
 
 
cranial nerves. The earliest observations on this subject were
those contained in my Monograph on Elasmobranck Fishes
(P- T 73)> while Schenk and Birdsell (No. 361) have since
arrived at the same result for Aves and Mammalia.
 
In my account of the development of these ganglia, it is
stated that they were first met with as small masses situated at
the ends of short branches of the spinal nerves (fig. 275 sy.g).
More recent investigations have shewn me that the sympathetic
ganglia are at first simply swellings on the main branches of the
spinal nerves some way below the ganglia. Their situation
may be understood from fig. 274, sy.g,
which belongs however to a somewhat
later stage. Subsequently the sympathetic ganglia become removed from the
main stem of their respective nerves,
remaining however connected with those
stems by a short branch (fig. 275, sy.g).
I have been unable to find a longitudinal
commissure connecting them in their
early stages; and I presume that they
are at first independent, and become subsequently united into a continuous cord
on each side.
 
The observations of Schenk and
Birdsell on the Mammalia seem to indicate that the main parts of the sympathetic system arise in continuity with
the posterior spinal ganglia : they also shew that in the neck
and other parts the sympathetic cords arise as a continuous
ganglionic chain. The observations on the topographical
features of the development of the sympathetic system in
higher types are however as yet very imperfect.
 
The later history of the sympathetic ganglia is intimately
bound up with that of the so-called supra-renal bodies, which
are dealt with in another chapter.
 
 
 
 
FIG. 274. LONGITUDINAL VERTICAL SECTION
THROUGH PART OF THE
BODY WALL OF AN ELASMOBRANCH EMBRYO SHEWING
PARTOFTWOSPINAL NERVES
AND THESYMPATHETICGANGLIA BELONGING TO THEM.
 
ar. anterior root ; pr. posterior root ; sy.g. sympathetic
ganglion ; tnp. part of muscle-plate.
 
 
 
302
 
 
 
468
 
 
 
SYMPATHETIC NERVOUS SYSTEM.
 
 
 
 
time.
 
 
 
FIG. 275. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.
 
The section is diagrammatic in the fact that the anterior nerve-roots have been
inserted for their whole length ; whereas they join the spinal cord half-way between
two posterior roots.
 
sp.c. spinal cord; sp.g. ganglion of posterior root; ar, anterior root; d.n. dorsally
directed nerve springing from posterior root; mp. muscle plate; mp'. part of muscle
plate already converted into muscles ; mp. /. part of muscle plate which gives rise to
the muscles of the limbs; /. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric)
duct; st. segmental tube; dn. duodenum; pan. pancreas; hp.d. point of junction of
hepatic duct with duodenum; nmc. umbilical canal.
 
 
 
NERVOUS SYSTEM OF THE VEKTEBRATA. 469
 
 
 
BIBLIOGRAPHY OF THE SYMPATHETIC NERVOUS SYSTEM.
 
(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes.
London, 1878, p. 173.
 
(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung
d. Ganglien d. Sympatheticus. " Mittheil. a. d. embryologischen histit. Wien, Heft
in. 1879.
 
 
 
CHAPTER XVI.
ORGANS OF VISION.
 
IN the lowest forms of animal life the whole surface is sensitive
to light, and organs of vision have no doubt arisen in the first
instance from limited areas becoming especially sensitive to light
in conjunction with a deposit of pigment. Lens-like structures,
formed either as a thickening of the cuticle, or as a mass of cells,
were subsequently formed ; but their function was not, in the first
instance, to throw an image of external objects on the perceptive
part of the eye, but to concentrate the light on it. From such a
simple form of visual organ it is easy to pass by a series of steps
to an eye capable of true vision.
 
There are but few groups of the Metazoa which are not provided with optic organs of greater or less complexity.
 
In a large number of instances these organs are placed on the
anterior part of the head, and are innervated from the anterior
ganglia. It is possible that many of the eyes so situated may
be modifications of a common prototype. In other instances
organs of vision are situated in different regions of the body, and
it is clear that such eyes have been independently evolved in each
instance.
 
The percipient elements of the eye would invariably appear
to be cells, one end of each of which is continuous with a
nerve, while the other terminates in a cuticular structure, or
indurated part of the cell forming what is known as the rod or
cone.
 
The presence of such percipient elements in various eyes is
therefore no proof of genetic relationship between these eyes*
but merely of similarity of function.
 
Embryological data as to the development of the eye do not
 
 
 
ORGANS OF VISION.
 
 
 
471
 
 
 
exist except in the case of the Arthropoda, Mollusca and Chordata. From such data as there are, combined with study of the
adult structure of the eye, it can be shewn that two types of
development are found. In one of these the percipient elements
are formed from the central nervous system, in the other from
the epidermis. The former may be called cerebral eyes. It is
probable however that this distinction is not, in all cases at
any rate, so fundamental as might be supposed ; but that in
both instances the eye may have taken its origin from the
epidermis. In the eyes in which the retina is continuous with
the central nervous system, these two organs were probably
evolved simultaneously as differentiations of the epidermis, and
continue to develop together in the ontogenetic growth of the eye.
 
Some of the eyes in which the retina is formed from the epidermis have also probably arisen simultaneously with part of the
central nervous system, while in other instances they have arisen
as later formations subsequently to the complete establishment
of a central nervous system.
 
Coelenterata. The actual evolution of the eye is best
shewn in the Hydrozoa. The simplest types
are those found in Oceania and Lizzia 1 . In
"Lizzia. the eye is placed at the base of a
tentacle and consists of (fig. 276) a lens (/)
and a percipient bulb (oc). The lens is a
simple thickening of the cuticle, while the
percipient part of the eye is formed of three
kinds of elements: (i) pigment cells; (2)
sense cells, forming the true retinal elements,
and consisting of a central swelling with the
nucleus, a peripheral process representing a
hardly differentiated rod, and a central process continuous with (3) ganglion cells at
the base of the eye. In this eye there is
present a commencing differentiation of a
ganglion as well as of a retina.
 
The eye of Oceania is simpler than that of Lizzia
in the absence of a lens. Claus has shewn that in
 
 
 
 
oc.
 
 
 
(From Lankester; after
Hertwig.)
 
/. lens; oc. perceptive part of eye.
 
 
 
1 O. and R. Hertwig. Das Nei~uen system #. Sinnesorgane d. Medtisen.
1878.
 
 
 
Leipzig,
 
 
 
472
 
 
 
MOLLUSCA.
 
 
 
Charybdea amongst the Acraspeda a more highly differentiated eye is
present, with a lens formed of cells like the vertebrate eye.
 
Mollusca. In a large number of the odontophorous Mollusca
eyes, innervated by the supracesophageal ganglia, are present
on the dorsal side of the head. These eyes exhibit very various
degrees of complexity, but are shewn both by their structure and
development to be modifications of a common prototype.
 
The simplest type of eye is that found in the Nautilus, and
although the possibility of this eye being degenerated must be
borne in mind, it is at the same time very interesting to note
(Hensen) that it retains permanently the early embryonic structure of the eyes of the other groups.
 
It has (fig. 277 A) the form of a vesicle, with a small opening
in the outer wall, placing the cavity of the vesicle in free communication with the exterior. The cells lining the posterior face
of the vesicle form a retina (7?); and are continuous with the
fibres of the optic nerve (N.op). We have no knowledge of the
development of this eye.
 
In the Gasteropods the eye (fig. 277 B) has the form of a
closed vesicle: the cells lining the inner side form the retina,
while the outer wall of the vesicle constitutes the cornea. A
 
 
 
 
N.op
 
 
 
G.op
 
 
 
FIG. 277. THREE DIAGRAMMATIC SECTIONS OF THE EYES OF MOLLUSCA.
 
(After Grenacher.)
A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod.
 
Pal. eyelid; Co. cornea; Co.ep. epithelium of ciliary body ; Ir. iris; Int, Int" 1 ...
Int*. different parts of the integument; /. lens; I 1 , outer segment of lens; R. retina;
N.op. optic nerve; G.op. optic ganglion; x. inner layer of retina; N.S. nervous
stratum of retina.
 
 
 
ORGANS OF VISION. 473
 
 
 
cuticular lens is placed in the cavity, on the side adjoining the
cornea. This eye originates from the ectoderm, within the velar
area, and close to the supra-cesophageal ganglia, usually at the
base of the tentacles. According to Rabl (Vol. II. No. 268) it is
formed as an invagination, the opening of which soon closes ;
while according to Bobretzky (Vol. II. No. 242) and Fol it arises
as a thickening of the epiblast, which becoming detached takes
the form of a vesicle. It is quite possible that both types of
development may occur, the second being no doubt abbreviated.
The vesicle, however formed, soon acquires a covering of pigment,
except for a small area of its outer wall, where the lens becomes
formed as a small body projecting into the lumen of the vesicle.
The lens seems to commence as a cuticular deposit, and to grow
by the addition of concentric layers. The inner wall of the vesicle
gives rise to the retina.
 
The most highly differentiated molluscan eye is that of the
Dibranchiate Cephalopoda, which is in fact more highly organized
than any other invertebrate eye.
 
A brief description of its adult structure l will perhaps render more clear
my account of the development. The most important features of the eye
are shewn in fig. 277 C. The outermost layer of the optic bulb forms a kind
of capsule, which may be called the sclerotic. Posteriorly the sclerotic abuts
on the cartilaginous orbit, which encloses the optic ganglion (G. op~) ; and in
front it becomes transparent and forms the cornea Co, which may be either
completely closed, or (as represented in the diagram) perforated by a larger
or smaller opening. Behind the cornea is a chamber known as the anterior
optic chamber. This chamber is continued back on each side round a
great part of the circumference of the eye, and separates the sclerotic from a
layer internal to it.
 
In the anterior optic chamber there are placed (i) the anterior part of
the lens (7 1 ) and (2) the folds of the iris (Ir). The whole chamber, except
the part formed by the lens, is lined by the epidermis (InP and Infi}.
Bounding the inner side of the anterior optic chamber is a layer which is
called the choroid (Int 1 } which is continued anteriorly into the fold of the
iris (Ir). The most superficial layer of the choroid is the epithelium already
mentioned, next comes a layer of obliquely placed plates known as the
argentea externa, then a layer of muscles, and finally the argentea interna.
The argentea interna abuts on a cartilaginous capsule, which completely
invests the inner part of the eye.
 
The lens is a nearly spherical body composed of concentric lamellae of a
structureless material. It is formed of a small outer (7 1 ) and large inner
 
1 Vide Hensen, Zeit. f. wiss. Zool. Bd. XV.
 
 
 
474 CEPHALOPODA.
 
 
 
(/) segment, the two being separated by a thin membrane. It is supported
by a peculiar projection of the wall of the optic cup, known as the ciliary
body (Co.ep), inserted at the base of the iris, and mainly formed of a
continuation of the retina. This body is however muscular, and presents a
series of folds on its outer and inner surfaces, which are especially developed
on the latter.
 
The membrane dividing the lens into two parts is continuous with the
ciliary body. Within the lens is the inner optic chamber, bounded in front
by the lens and the ciliary body, and behind by the retina.
 
The retina is formed of two main divisions, an anterior division adjoining
the inner optic chamber, and a posterior division (N.S) adjoining the
cartilage of the choroid. The two layers are separated by a membrane.
Passing from within outwards the following layers in the retina may be
distinguished :
 
(1) Homogeneous membrane. | Anterior division of
 
(2) Layer of rods. retina
 
(3) Layer of granules imbedded in pigment. J
 
(4) Cellular layer.
 
 
 
(5) Connective tissue layer.
 
 
 
Posterior layer of retina.
 
 
 
(6) Layer of nerve-fibres.
 
At the side of the optic ganglion is a peculiar body, known as the white
body (not shewn in the figure), which has the histological characters of
glandular tissue.
 
The first satisfactory account of the development of the eye
is due to Lankester (No. 365). The more important features in
it were also independently worked out by Grenacher (No. 363),
and are beautifully illustrated in Bobretzky's paper (No. 362).
The eye first appears as an oval pit of the epiblast, the edge of
which is formed by a projecting rim (fig. 278 A). The epiblast
 
A
 
 
 
 
FlG. 278. TWO SECTIONS THROUGH THE DEVELOPING EYE OF A CEI'HALUl'OD
 
TO SHEW THE FORMATION OF THE OPTIC CUP. (After Lankester.)
 
layer lining the floor of the pit soon becomes considerably thickened. By the growth inwards of the rim the mouth of the pit
 
 
 
ORGANS OF VISION.
 
 
 
475
 
 
 
is gradually narrowed (fig. 278 B), resembling at this stage the
eye of Nautilus, and finally closed. There is thus formed a
flattened sack, lined by epiblast, which may be called the primary
optic vesicle. Its cavity eventually forms the inner optic chamber.
The anterior wall of the sack is lined by a much less columnar
layer than the posterior, the former giving rise to the epithelium
on the inner side of the ciliary processes, the latter to the retina.
The cavity of the sack rapidly enlarges, and assumes a
spherical form. At the same time a layer of mesoblast grows
in between the walls of the sack and the external epiblast.
 
 
 
 
FIG. 279. TRANSVERSE SECTION THROUGH THE HEAD OF AN ADVANCED
EMBRYO OF LoLlGO. (After Bobretzky.)
 
gls. salivary gland; g.vs. visceral ganglion; gc. cerebral ganglion; g.op. optic ganglion; adk. optic cartilage; ak. and_y. lateral cartilage or (?) white body; rt. retina;
gm. limiting membrane of retina ; vk, ciliary region of eye ; cc. iris ; ac. auditory sack
(the epithelium lining the auditory sacks is not represented) ; vc. vena cava ; ff. folds
of funnel ; x, epithelium of funnel.
 
Two new structures soon arise nearly simultaneously (fig. 279),
which become in the adult eye the iris (cc) and the posterior
segment of the lens. The iris is formed as a circular fold of the
skin in front of the optic vesicle. It consists both of epiblast
and mesoblast, and gives rise to a pit lined by epiblast. The
posterior segment of the lens arises as a structureless rod-like
body, which is shewn in fig. 279 depending from the inner side
 
 
 
476 CEPHALOPODA.
 
 
 
of the anterior wall of the optic vesicle. Its exact mode of origin
is somewhat obscure. The following is Lankester's account of
it 1 : "It is formed entirely within the primitive optic chamber,
and at first depends as a short cylindrical rod from the middle
point of the anterior wall of that chamber, that is to say, from
the point at which the chamber finally closed up. It grows subsequently by the deposition of concentric layers of a horny material
round this cone. No cells appear to be immediately concerned
in effecting the deposition, and it must be looked upon as an
organic concretion, formed from the liquid contained in the
primitive optic chamber."
 
The lens would thus appear to be a cuticular structure. It
gradually assumes a nearly spherical form ; and is then composed
of concentrically arranged layers (fig. 280, /if).
 
While the lens is being formed, the ciliary epithelium of the
optic vesicle becomes divided into two layers, an outer layer of
large cells and an inner of small cells. Both layers are at first
continuous across the anterior wall of the optic chamber in front
of the lens, but soon become confined to the sides (fig. 280 A,
cc and gz). The inner layer is stated by Lankester to give rise
to the muscles present in the adult. The mesoblast cells also
disappear from the region in front of the lens, and the outer
epithelium is converted into a kind of cuticular membrane. By
these changes the original layers of cells in front of the lens
become reduced to mere membranes, a change which appears
to be preparatory to the appearance of the anterior segment of
the lens. The formation of the latter has not been fully followed
out by any investigator except Bobretzky. His figures would
seem to indicate that it is formed as a cuticular deposit in
front of the membrane already spoken of (fig. 280 B, vl). The
two segments of the lens appear at any rate to be separated by
a membrane continuous with the ciliary region of the optic
vesicle.
 
Grenacher believes that the front part of the lens is formed in a pocketlike depression of the epiblastic layer covering the outer side of the optic
cup ; and Lankester thinks that the lens " pushes its way through the median
anterior area of the primitive optic chamber, and projects into the second or
anterior optic chamber where the iridian folds lie closely upon it."
 
1 "Devel. of Cephalopoda." Q. J. Micro. Scien. 1875, p. 44.
 
 
 
ORGANS OF VISION.
 
 
 
477
 
 
 
While the lens is attaining its complete development there
appears a fresh fold round the circumference of the eye, which
gradually grows inwards so as to form a chamber outside the
parts already present. This chamber is the anterior optic
chamber of the adult. In most Cephalopods (fig. 277 C) the
edges of the fold do not quite meet, but leave a larger or smaller
aperture leading into the chamber containing the iris, outer
segment of the lens, etc. In some forms however they meet
and coalesce, and so shut off this chamber from communication
with the exterior. The edge of the fold constitutes the cornea
while the remainder of it gives rise to the sclerotic.
 
The retina is at first a thick layer of numerous rows of oval
 
 
 
 
<>
 
 
 
 
FIG. 280. SECTIONS THROUGH THE DEVELOPING EYE OF LOLIGO
 
AT TWO STAGES. (After Bobretzky.)
 
///. inner segment of lens ; vl. outer segment of lens ; a and a. epithelium lining
the anterior optic chamber; gz. large epiblast cells of ciliary body; cc. small epiblast cells of ciliary body ; ms . layer of mesoblast between the two epiblastic layers
of the ciliary body; of. and if. fold of iris; rt. retina; rt". inner layer of retina;
st. rods ; aq. cartilage of the choroid.
 
 
 
478 ONCHIDIUM.
 
 
 
cells (fig. 279). When the inner segment of the lens is far
advanced towards its complete formation pigment becomes
deposited in the anterior part of the retina, and a layer of rods
grows out from the surface turned towards the cavity of the
optic vesicle (fig. 280 A, st). At a slightly later stage the retina
becomes divided into two layers (Bobretzky), a thicker anterior
layer, and a thinner posterior layer (fig. 280, rt and rf}. The
former is composed of two strata, (i) the rods and (2) a stratum
with numerous rows of nuclei which becomes in the adult the
granular layer with its pigment. The posterior layer gives rise
to the cellular part of the posterior division of the retina, while
layers of connective tissue around it give rise to the connective
tissue of this portion of the retina (layer 6 in the scheme on
p. 474). The nervous layer is derived from the optic ganglion
which attaches itself to the inner side of the connective tissue layer.
 
The greater part of the choroid is formed from the mesoblast
adjoining the retina, but the epithelium covering its outer wall
is of epiblastic origin.
 
It is difficult to decide from development whether the Molluscan eyes, so far dealt with, originated in the first instance part
passu with the supra-cesophageal ganglia or independently at a
later period. On purely a priori ground I should be inclined to
adopt the former alternative.
 
In addition to the above eyes there occur amongst Mollusca highly
complicated eyes, of a very different kind, in two widely separated groups,
viz. certain species of a genus of slug (Onchidium), and certain Lamellibranchiata. These eyes, though they have no doubt been evolved independently of each other, present certain remarkable points of agreement. In
both of them the rods of the retina are turned away from the surface, and
the nerve-fibres are placed, as in the Vertebrate eye, on the side of the retina
which faces outwards.
 
The peculiar eyes of Onchidium, investigated by Semper 1 , are scattered
on the dorsal surface, there being normal eyes in the usual situation on the
head. The eyes on the dorsal surface are formed of a cornea, a lens
composed of i 7 cells, and a retina surrounded by pigment ; which is
perforated in the centre by an optic nerve, the retinal elements being in the
inverted position above mentioned.
 
The development of these eyes has been somewhat imperfectly studied
in the adult, in which they continue to be formed anew. They arise by a
 
1 Ueber Sehorgane von Typus d. Wirbdthieraugen, etc., Wiesbaden, 1877, anf l
Archiv f. mikr. Anat. Vol. xiv. pp. 118 122.
 
 
 
ORGANS OF VISION. 479
 
 
 
differentiation of the epidermis at the end of a papilla. At first a few
glandular cells appear in the epidermis in the situation where an eye is
about to be formed. Then, by a further process of growth, an irregular mass
of epidermic cells becomes developed, which pushes the glandular cells to
one side, and constitutes the rudiment of the eye. This mass, becoming
surrounded by pigment, unites with the optic nerve, and its cells then differentiate themselves, in situ, into the various elements of the eye. No
explanation is offered by Semper of the inverted position of the rods, nor is
any suggested by his account of the development. As pointed out by
Semper these eyes are no doubt modifications of the sensory epithelium of
the papillce.
 
The eyes of Pecten and Spondylus 1 are placed on short stalks at the
edge of the mantle, and are probably modifications of the tentacular
processes of the mantle edge. They are provided with a cornea, a cellular
lens, a vitreous chamber, and a retina. The retinal elements are inverted,
and the optic nerve passes in at the side, but occupies, in reference to its
ramifications, the same relative situation as the optic nerve in the Vertebrate
eye. The development has unfortunately not yet been studied.
 
Our knowledge of the structure or still more of the development of the
organ of vision of the Platyelminthes, Rotifera, and Echinodermata is too
scanty to be of any general interest.
 
Chaetopoda. Amongst the Chaetopoda the cephalic eyes of Alciope
(fig. 281) have been adequately investigated as to their anatomy by Greeff.
These are provided with a large cuticular lens (/), separated from the retina
by a wide cavity containing the vitreous humour. The retina is formed of a
single row of cells, with rods at their free extremities, continuous at their
opposite ends with nerve-fibres. The development of this eye has not been
worked out. Eyes not situated on the head are found in Polyophthalmus,
and have probably been evolved from the more indifferent type of senseorgan found by Eisig in the allied Capitellidas.
 
Chaetognatha 2 . The paired cephalic eyes of Sagitta are spherical
bodies imbedded in the epidermis. They are formed of a central mass of
pigment with three lenses partially imbedded in it. The outer covering of
the eye is the retina, which is mainly composed of rod-bearing cells ; the
rods being placed in contact with the outer surface of each of the lenses. In
the presence of three lenses the eye of Sagitta approaches in some respects
the eye of the Arthropoda.
 
Arthropodan eye. A satisfactory elucidation of the phylogeny of Arthropodan eyes has not yet been given.
 
All the types of eyes found in the group (with exception of
 
1 Vide Hensen (No. 364) and S. J. Hickson, "The Eye of Pecten," Quart. J. of
Micr. Science, Vol. xx. 1 880.
 
2 O. Hertwig. " Die Chaetognathen." Jenaischc Zcitschrift, Vol. XTV. 1880.
 
 
 
480 ARTHROPODA.
 
 
 
that of Peripatus) 1 present marked features of similarity, but I
am inclined to view this similarity as due rather to the character
of the exoskeleton modifying in a more or less similar way all
the forms of visual organs, than to the descent of all these eyes
from a common prototype. In none of these eyes is there
present a chamber filled with fluid between the lens and the
 
 
 
 
FIG. 281. EYE OF AN ALCIOPID (NEOPHANTA CELOX). (From Gegenbaur;
 
after Greef.)
 
i. cuticle; c. continuation of cuticle in front of eye; /. lens; h. vitreous humour;
o. optic nerve; o. expansion of the optic nerve; b. layer of rods; /. pigment layer.
 
retina, but the space in question is filled with cells. This
character sharply distinguishes them from such eyes as those of
Alciope (fig. 281). The types of eyes which are found in the
Arthropoda are briefly the following :
 
(i) Simple eyes. In all simple eyes the corneal lens is
formed by a thickening of the cuticle. Such eyes are confined
to the Tracheata.
 
There are three types of simple eyes, (a) A type in which
the retinal cells are placed immediately behind the lens, found
 
1 The eye of Peripatus is similar neither to the eye of the Arthropoda, nor to that
of the Choetopoda, but resembles much more closely the Molluscan eye. The hypodermis and cuticle form together a highly convex cornea, within which is a large optic
chamber, the posterior wall of which is formed by the retina. The optic chamber
would appear to contain a structureless lens, but it is possible that what I regard as a
lens may, on fuller investigation, turn out to be only a coagulum.
 
 
 
ORGANS OF VISION.
 
 
 
481
 
 
 
 
(Lowne) in the larvae of some Diptera (Eristalis), and also in
some Chilognatha.
 
(b] A type of simple eye found in some Chilopoda, and in
some Insect larvae (Dytiscus, etc.) (fig. 282), the parts of which
are entirely derived from the epidermis. There is present a
lens (/) formed as a thickening of the cuticle, a so-called vitreous
humour (gl] formed of modified hypodermis cells, and a retina
(r) derived from the same source.
 
The outer ends of the retinal cells
terminate in rods, and their inner
ends are continuous with nervefibres.
 
(c) A type of simple eye found
in the Arachnida, and apparently
some Chilopoda, and forming the
simple eyes of most Insects, which
differs from type (a) in the cells
of the retina forming a distinct
layer beneath the hypodermis ; the
latter only obviously giving rise to
the vitreous humour.
 
The development of the simple eyes has not yet been
studied.
 
The simple eyes so far described are always placed on the
head, and are usually rather numerous.
 
(2) Compound eyes. Compound eyes are almost always
present in the Crustacea, and are usually found in adult Insects.
In both groups they are paired, though in the Crustacea a median
much simplified compound eye may either take the place of the
paired eyes in the Nauplius larva and lower forms, or be present
together with them during a period in the development of higher
forms.
 
The typical compound eye is formed (fig. 283) of a series of
corneal lenses (c) developed from the cuticle; below which
are placed bodies known as the crystalline cones, one to each
corneal lens ; and below the crystalline cones are placed bodies
known as the retinulae (r) constituting the percipient elements
of the eye, each of them being formed of an axial rod, the
rhabdom, and a number of cells surrounding it.
 
B. in. 3 1
 
 
 
FIG. 282. SECTION THROUGH
THE SIMPLE EYE OF A YOUNG DYTISCUS LARVA. (From Gegenbaur ; after
Grenadier.)
 
/. corneal lens ; g. vitreous hu
mour ; r. retina ; o. optic nerve ; h.
hypodermis.
 
 
 
482
 
 
 
ARTHROPODA.
 
 
 
The crystalline cones are formed from the coalescence of cuticular
deposits in several cells, the nuclei of which usually remain as Semper's
nuclei. These cells are probably simple hypodermis cells, but in some
forms, e.g. Phronima, there may be a continuous layer of hypodermis cells
between them and the cuticle. In various Insect eyes the cells which
usually give rise to a crystalline cone may remain distinct, and such eyes
have been called by Grenacher aconouseyes, while eyes with incompletely
formed crystalline cones are called by him pseudoconouseyes.
 
The rhabdom of the retinulae is, like the crystalline cone, developed by
the coalescence of a series of parts, which are primitively separate rods
placed each in its own cell : this condition of the retinulas is permanently
retained in the eyes of the Tipulidae.
 
The development of the compound eye has so far only been
satisfactorily studied in some Crustacea by Bobretzky (No. 367) ;
by whom it has been worked out in Palaemon and Astacus, but
more fully in the latter, to which the following account refers :
 
The eye of Astacus takes its
origin from two distinct parts, (i)
the external epidermis of the procephalic lobes which will be spoken
of as the epidermic layer of the
eye, (2) a portion of the supracesophageal ganglia, which will be
spoken of as the neural layer of
the eye. The mesoblast is moreover the source of some of the
pigment between the two above
layers. The epidermic layer gives
rise to the corneal lenses, the
crystalline cones, and the pigment
around the latter. The neural
layer on the other hand seems to
give rise to the retinulae with their rhabdoms, and to the optic
ganglion.
 
After the separation of the supra-cesophageal ganglia from the superficial
epiblast, the cells of the epidermis in the region of the future eye become
columnar, and so form the above-mentioned epidermic layer of the eye.
This layer soon becomes two or three cells deep. At the same time the
most superficial part of the adjoining supra- oesophageal ganglion becomes
partially constricted off from the remainder as the neural layer of the eye,
but is separated by a small space from the thickened patch of epidermis.
 
 
 
 
FIG. 283. DIAGRAMMATIC REPRESENTATIONS OF PARTS OF A COMPOUND ARTHROPOD EYE. (From
Gegenbaur.)
 
A. Section through the eye.
 
B. Corneal facets.
 
C. Two segments of the eye.
 
c. corneal (cuticular) lenses ; r.
retinulae with rhabdoms ; n. optic
nerve ; g. ganglionic swelling of optic
nerve.
 
 
 
ORGANS OF VISION. 483
 
 
 
Into this space some mesoblast cells penetrate at a slightly later period.
Both the epidermic and neural layers next become divided into two strata.
The outer stratum of the epidermic layer gives rise to the crystalline cones
and Semper's nuclei ; each crystalline cone being formed from four coalesced
rods, developed as cuticular differentiations of four cells, the nuclei of which
may be seen in the embryo on its outer side. The lower ends of the cones
pass through the inner stratum of the epidermic disc, the cells of which
become pigmented, and constitute the pigment cells surrounding the lower
part of the crystalline cones in the adult. The outer end of each of the
crystalline cones is surrounded by four cells, believed by Bobretzky to be
identical with Semper's nuclei 1 . These cells give rise in a later stage (not
worked out in Astacus) to the cuticular corneal lenses.
 
Of the two strata of the neural layer the outer is several cells deep, while
the inner is formed of elongated rod-like cells. Unfortunately however the
fate of the two neural layers has not been worked out, though there can be
but little doubt that the retinuke originate from the outer layer.
 
The mesoblast which grows in between the neural and epidermic layers
becomes a pigment layer, and probably also forms the perforated membrane
between the crystalline cones and the retinulas.
 
The above observations of Bobretzky would appear to
indicate that the paired compound eyes of Crustacea belong to
the type of cerebral eyes. How far this is also the case with the
compound eyes of Insects is uncertain, in that it is quite possible
that the latter eyes may have had an independent origin.
 
The relation between the paired and median eye of the
Crustacea is also uncertain.
 
In the genus Euphausia amongst the Schizopods there is present a series
of eyes placed on the sides of some of the thoracic legs and on the sides of
the abdomen. The structure of these eyes, though not as yet satisfactorily
made out, would appear to be very different from that of other Arthropodan
visual organs.
 
The Eye of the Vertebrata. In view of the various
structures which unite to form it, the eye is undoubtedly the
most complicated organ of the Vertebrata ; and though its
mode of development is fairly constant throughout the group,
it will be convenient shortly to describe what may be regarded
as its typical development, and then to proceed to a comparative
view of the origin of its various parts, and to enter into greater
detail with reference to some of them. At the end of the section
 
1 There would appear to be some confusion as to the nomenclature of these parts
in Bobretzky's account,
 
31
 
 
 
4 8 4
 
 
 
PRIMARY OPTIC VESICLE.
 
 
 
 
there is an account of the accessory structures connected with
the eye.
 
The formation of the eye commences with the appearance of
a pair of hollow outgrowths from the anterior cerebral vesicle or
thalamencephalon, which arise in many instances, even before
the closure of the medullary canal. These outgrowths, known
as the optic vesicles, at first open freely into the cavity of the
anterior cerebral vesicle. From this they soon however become
partially constricted, and form vesicles (fig. 284, a], united to the
base of the brain by comparatively narrow hollow stalks, the
rudiments of the optic nerves.
The constriction to which the
stalk or optic nerve is due takes
place obliquely downwards and
backwards, so that the optic
nerves open into the base of the
front part of the thalamencephalon
(fig. 284, ff).
 
After the establishment of the
optic nerves, there take place (i)
the formation of the lens, and (2)
the formation of the optic cup
from the walls of the primary optic vesicle.
 
The external or superficial epiblast which covers, and is in
most forms in immediate contact with, the most projecting
portion of the optic vesicle, becomes thickened. This thickened
portion is then driven inwards in the form of a shallow open
pit with thick walls (fig. 285 A, o), carrying before it the front
wall (r) of the optic vesicle. To such an extent does this
involution of the superficial epiblast take place, that the front
wall of the optic vesicle is pushed close up to the hind wall, and
the cavity of the vesicle becomes almost obliterated (fig. 285 B).
 
The bulb of the optic vesicle is thus converted into a cup
with double walls, containing in its cavity the portion of
involuted epiblast. This cup, in order to distinguish its cavity
from that of the original optic vesicle, is generally called the
secondary optic vesicle. We may, for the sake of brevity, speak
of it as the optic cup; in reality it never is a vesicle, since it
 
 
 
FIG. 284. SECTION THROUGH
THE HEAD OF AN EMBRYO TELEOSTEAN, TO SHEW THE FORMATION OF
 
THE OPTIC VESICLES, ETC. (From
 
Gegenbaur; after Schenk.)
 
c. fore-brain ; a. optic vesicle ; b.
stalk of optic vesicle ; d. epidermis.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA.
 
 
 
485
 
 
 
 
 
always remains widely open in front. Of its double walls
the inner or anterior (fig. 285 .
 
B, r) is formed from the front
portion, the outer or posterior
(fig. 285 B, u] from the hind portion of the wall of the primary
optic vesicle. The inner or anterior (r), which very speedily becomes thicker than the other, is
converted into the retina : in
the outer or posterior (), which
remains thin, pigment is eventually deposited, and it ultimately
becomes the tesselated pigmentlayer of the choroid.
 
By the closure of its mouth
the pit of the involuted epiblast
becomes a completely closed sac
with thick walls and a small
central cavity (fig. 285 B, /). At
the same time it breaks away
from the external epiblast, which
forms a continuous layer in front of it, all traces of the original
opening being lost. There is thus left lying in the cup of the
secondary optic vesicle, an isolated elliptical mass of epiblast.
This is the rudiment of the lens. The small cavity within it
speedily becomes still less by the thickening of the walls,
especially of the hinder one.
 
At its first appearance the lens is in immediate contact with
the anterior wall of the secondary optic vesicle (fig. 285 B). In
a short time however, the lens is seen to lie in the mouth of the
cup (fig. 288 D), a space (vh] (which is occupied by the vitreous
humour) making its appearance between the lens and anterior
wall of the vesicle.
 
In order to understand how this space is developed, the
position of the optic vesicle and the relations of its stalk must
be borne in mind.
 
The vesicle lies at the side of the head, and its stalk is
directed downwards, inwards and backwards. The stalk in fact
 
 
 
FIG. 285. DIAGRAMMATIC SECTIONS ILLUSTRATING THE FORMATION
OF THE EYE. (After Remak.)
 
In A the thin superficial epiblast h
is seen to be thickened at x, in front of
the optic vesicle, and involuted so as
to form a pit o, the mouth of which has
already begun to close in. Accompanying this involution, which forms the
rudiment of the lens, the optic vesicle
is doubled in, its front portion r being
pushed against the back portion u, and
the original cavity of the vesicle thus
reduced in size. The stalk of the vesicle
is shewn as still broad.
 
In B the optic vesicle is still further
doubled in so as to form a cup with a
posterior wall u and an anterior wall r.
In the hollow of this cup lies the lens /,
now completely detached from the
superficial epiblast xh.
 
 
 
486
 
 
 
CHOROID FISSURE.
 
 
 
 
slants away from the vesicle. Hence, when the involution of
the lens takes place, the direction in which the front wall of the
vesicle is pushed in is not in a line with the axis of the stalk,
as for simplicity's sake has been represented in the diagram
(fig. 285), but forms an obtuse angle with that axis, after the
manner of fig. 286, where / represents the cavity of the stalk
leading away from the almost obliterated cavity of the primary
vesicle.
 
Fig. 286 represents the early stage at which the lens fills the
whole cup of the secondary vesicle. The subsequent condition
is brought about through the rapid
growth of the walls of the cup. This
growth however does not take place
equally in all parts of the cup. The
walls of the cup rise up all round except
that point of the circumference of the
cup which adjoins the stalk. While
elsewhere the walls increase rapidly
in height, carrying so to speak the lens
with them, at this spot, which in the
natural position of the eye is on its
under surface, there is no growth : the
wall is here imperfect, and a gap is left.
Through this gap, which afterwards
receives the name of the choroidal
fissure, a way is open from the mesoblastic tissue surrounding the optic
vesicle and stalk into the interior of the
cavity of the cup.
 
From the manner of its formation the gap or fissure is
evidently in a line with the axis of the optic stalk, and in order
to be seen must be looked for on the under surface of the optic
vesicle. In this position it is readily recognised in the embryo
seen as a transparent object (fig. 1 18, chs).
 
Bearing in mind these relations of the gap to the optic stalk,
the reader will understand how sections of the optic vesicle at
this stage present very different appearances according to the
plane in which the sections are taken.
 
When the head is viewed from underneath as a transparent
 
 
 
FIG. 286. DIAGRAMMATIC
SECTION OF THE EYE AND
THE OPTIC NERVE AT AN
EARLY STAGE. (From Lieberkiihn.)
 
To shew the lens / occupying the whole hollow of
the optic cup, the inclination
of the stalk s to the optic
cup, and the continuity of the
cavity of the stalk s' with that
of the primary vesicle c ; r.
anterior, u. posterior wall of
the optic cup.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA.
 
 
 
487
 
 
 
 
object the eye presents very much the appearance represented in
the diagram (fig. 287).
 
A section of such an eye taken along the line y, perpendicular
to the plane of the paper, would give a figure corresponding
to that of fig. 288 D. The lens,
the cavity and double walls of the
secondary vesicle, the remains of the
primary cavity, would all be represented (the superficial epiblast of
the head would also be shewn) ;
but there would be nothing seen of
either the stalk or the fissure. If
on the other hand the section were
taken in a plane parallel to the
plane of the paper, at some distance
above the level of the stalk, some
such figure would be obtained as
that shewn in fig. 288 E. Here the
fissure f is obvious, and the communication of the cavity vh of the
secondary vesicle with the outside
of the eye evident ; the section of
course would not go through the
superficial epiblast. Lastly, a section, taken perpendicular to the
plane of the paper along the line z,
i.e. through the fissure itself, would
present the appearances of fig. 288 F,
where the wall of the vesicle is
entirely wanting in the region of
the fissure marked by the position
of the letter f. The external epiblast has been omitted in this figure.
 
With reference to the above description, taken with very slight alterations
from the Elements of Embryology, Pt. I., two points require to be noticed.
Firstly it is extremely doubtful whether the invagination of the secondary
optic vesicle is to be viewed as an actual mechanical result of the ingrowth
of the lens. Secondly it seems probable that the choroid fissure is not
simply due to an inequality in the growth of the walls of the secondary optic
cup, but is partly due to a doubling up of the primary vesicle from the side
 
 
 
FIG. 287. DIAGRAMMATIC REPRESENTATION OF THE EYE OF
THE CHICK OF ABOUT THE THIRD
DAY AS SEEN WHEN THE HEAD IS
VIEWED FROM UNDERNEATH AS A
TRANSPARENT OBJECT.
 
/. the lens ; /'. the cavity of the
lens, lying in the hollow of the
optic cup ; r. the anterior, u. the
posterior wall of the optic cup ; c.
the cavity of the primary optic
vesicle, now nearly obliterated. By
inadvertence u has been drawn in
some places thicker than r, it
should have been thinner throughout, s. the stalk of the optic cup
with s' its cavity, at a lower level
than the cup itself and therefore
out of focus; the dotted line indicates the continuity of the cavity
of the stalk with that of the primary
vesicle.
 
The line z z, through which the
section shewn in fig. 288 F is supposed to be taken, passes through
the choroidal fissure.
 
 
 
488 SECONDARY OPTIC CUP.
 
along the line of the fissure, at the same time that the lens is being thrust in
in front. In Mammalia, the doubling up involves the optic stalk, which
becomes flattened (whereby its original cavity is obliterated) and then folded
in on itself, so as to embrace a new central cavity continuous with the cavity
of the vitreous humour. And in other forms a partial phenomenon of the
same kind is usually observable, as is more particularly described in the
sequel.
 
Before describing the development of the cornea, aqueous
humour, etc. we may consider the further .growth of the parts,
whose first development has just been described, commencing
with the optic cup.
 
During the above changes the mesoblast surrounding the
optic cup assumes the character of a distinct investment, whereby
the outline of the eye-ball is definitely formed. The internal
portions of this investment, nearest to the retina, become the
choroid (i.e. the chorio-capillaris, and the lamina fusca; the
pigment epithelium, as we have seen, being derived from the
epiblastic optic cup), and pigment is subsequently deposited in
it. The remaining external portion of the investment forms the
sclerotic.
 
The complete differentiation of these two coats of the eye
does not however take place till a late period.
 
The cavity of the original optic vesicle was left as a nearly
obliterated space between the two walls of the optic cup. By
the end of the third day the obliteration is complete, and the two
walls are in immediate contact.
 
The inner or anterior wall is, from the first, thicker than the
outer or posterior ; and over the greater part of the cup this contrast increases with the growth of the eye, the anterior wall
becoming markedly thicker and undergoing changes of which we
shall have to speak directly (fig. 289).
 
In the front portion however, along, so to speak, the lip of
the cup, anterior to a line which afterwards becomes the ora
serrata, both layers cease to take part in the increased thickening,
accompanied by peculiar histological changes, which the rest of
the cup is undergoing. Thus a hind portion or true retina is
marked off from a front portion.
 
The front portion, accompanied by the mesoblast which
immediately overlies it, is behind the lens thrown into folds, the
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 489
 
ciliary ridges ; while further forward it bends in between the
lens and the cornea to form the iris. The original wide opening
of the optic cup is thus narrowed to a smaller orifice, the pupil ;
and the lens, which before lay in the open mouth of the cup, is
now inclosed in its cavity. While in the hind portion of the
cup or retina proper no deposit of black pigment takes place in
 
D
 
E
 
 
 
 
 
FIG. 288.
 
D. Diagrammatic section taken perpendicular to the plane of the paper, along
the linejjy, fig. 287. The stalk is not seen, the section falling quite out of its region.
vh. hollow of optic cup filled with vitreous humour ; other letters as in fig. 285 B.
(After Remak.)
 
E. Section taken parallel to the plane of the paper through fig. 287, so far behind
the front surface of the eye as to shave off a small portion of the posterior surface of
the lens /, but not so far behind as to be carried at all through the stalk. Letters as
before ; f. the choroidal fissure.
 
F. Section along the line zz, perpendicular to the plane of the paper, to shew the
choroidal fissure/, and the continuity of the cavity of the optic stalk with that of the
primary optic vesicle. Had this section been taken a little to one side of the line zs,
the wall of the optic cup would have extended up to the lens below as well as above.
Letters as before. The external epiblast is omitted in this section.
 
the layer formed out of the inner or anterior wall of the vesicle ;
in the front portion forming the region of the iris, pigment is
largely deposited throughout both layers, though first of all in
the outer one, so that eventually this portion seems to become
nothing more than a forward prolongation of the pigment epithelium of the choroid.
 
Thus, while the hind moiety of the optic cup becomes the
retina proper, including the choroid-pigment in which the rods
and cones are imbedded, the front moiety is converted into the
ciliary portion of the retina, covering the ciliary processes, and
into the uvea of the iris ; the bodies of the ciliary processes and
the substance of the iris, their vessels, muscles, connective tissue
and ramified pigment, being derived from the mesoblastic choroid.
The margin of the pupil marks the extreme lip of the optic
 
 
 
490
 
 
 
THE RETINA.
 
 
 
vesicle, where the outer or posterior wall turns round to join the
inner or anterior.
 
The ciliary muscle and the ligamentum pectinatum are both
derived from the mesoblast between the cornea and the iris.
 
The Retina. At first the two walls of the optic cup do not
greatly differ in thickness. On the third day the outer or posterior
becomes much thinner than the inner or anterior, and by the
middle of the fourth day is reduced to a single layer of flattened
 
 
 
c.t
 
 
 
p.Ch
 
 
 
 
FIG. 289. SECTION OF THE EYE OF CHICK AT THE FOURTH DAY.
 
e.p. superficial epiblast of the side of the head ; /?. 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. /. the lens. The hind wall, the nuclei of whose elongated cells are shewn at /,
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, y.
In the neighbourhood of the lens it seems to be continuous as at d with the tissue a,
which appears to be the rudiment of the capsule of the lens and suspensory ligament.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 491
 
cells (fig. 289, p.C/i). At about the 8oth hour its cells commence
to receive a deposit of pigment, and eventually form the so-called
pigmentary epithelium of the choroid ; from them no part of the
true retina (or no other part of the retina, if the pigment-layer in
question be supposed to belong more truly to the retina than to
the choroid) is derived.
 
On the fourth day, the inner (anterior) wall of the optic cup
(fig. 289, R) has a perfectly uniform structure, being composed of
elongated somewhat spindle-shaped cells, with distinct nuclei.
On its external (posterior) surface a distinct cuticular membrane,
the membrana limitans externa, early appears.
 
As the wall increases in thickness, its cells multiply rapidly,
so that it soon becomes several cells thick : each cell being
however probably continued through the whole thickness of the
layer. The wall at this stage corresponds closely in its structure
with the brain, of which it may properly be looked upon as part.
According to the usual view, which is not however fully supported by the development, the retina becomes divided in the
subsequent growth into (i) an outer part, corresponding morphologically to the epithelial lining of the cerebro-spinal canal,
composed of what may be called the visual cells of the eye, i.e.
the cells forming the outer granular (nuclear) layer and the rods
and cones attached to them ; and (2) an inner portion consisting
of the inner granular (nuclear) layer, the inner molecular layer,
the ganglionic layer and the layer of nerve-fibres corresponding
morphologically to the walls of the brain. According to Lowe,
however, only the outer limbs of the rods and cones, which he
holds to be metamorphosed cells, correspond to the epithelial
layer of the brain.
 
The actual development of the retina is not thoroughly understood.
According to the usual statements (Kolliker, No. 298, p. 693) the layer of
ganglion cells and the inner molecular layer are first differentiated,
while the remaining cells give rise to the rest of the retina proper, and
are bounded externally by the membrana limitans externa. On the inner
side of the ganglionic layer the stratum of nerve-fibres is also very early
established. The rods and cones are formed as prolongations (Kolliker,
Babuchin), or cuticularizations (Schultze, W. Miiller) of the cells which
eventually form the outer granular layer. The layer of cells external to
the molecular layer is not divided till comparatively late into the inner
and outer granular (nuclear) layers, and the interposed outer molecular
layer.
 
 
 
492 THE OPTIC NERVE.
 
 
 
Lowe's account of the development of the retina in the Rabbit is in many
points different from the above. He finds that three stages in the differentiation of the layers of the retina may be distinguished.
 
In the first stage, in an embryo of four or five millimetres, the following
layers are present, commencing at the outer side, adjoining the external wall
of the secondary optic cup.
 
(1) A membrane, which does not however, as usually believed,
become the membrana limitans externa.
 
(2) A layer of clear elements, derived from metamorphosed cells,
constituting the outer limbs of the rods and cones.
 
(3) A layer of dark rounded elements.
 
(4) An indistinctly striated layer, the future layer of nerve-fibres.
The third of these layers gives rise to all the eventual strata of the
 
retina proper, except the outer limbs of the rods and cones.
 
In the next stage, when the embryo has reached a length of 2 cm., this
layer becomes divided into three strata : viz. an outer and inner layer of
dark elements and a middle one of clearer elements. The two inner of these
layers become respectively the inner molecular layer and the layer of ganglion cells, while the outer layer gives rise to the parts of the retina external
to the inner molecular layer.
 
In the newly born animal the outer darker layer of the previous stage
has become considerably subdivided. Its outermost part forms a stratum
of darkly coloured elements, which develop into the inner limbs of the rods
and cones. It is bounded internally by a membrane the true membrana
elastica externa. The part of the layer within this is soon divided into the
outer and inner granular layers, separated from each other by the delicate
outer molecular layer. Thus, shortly after birth, all the layers of the retina
are established in the Rabbit. It is important to notice that, according to
Lowe's views, the outer and inner limbs of the rods and cones are metamorphosed cells. The outer limbs at first form a continuous layer, in which
separate elements cannot be recognised.
 
At a very early period there appears a membrane on the side of the
retina adjoining the vitreous humour. This membrane is the hyaloid membrane. The investigations of Kessler and myself lead to the conclusion that
it may be formed at a time when there is no trace of mesoblastic structures
in the cavity of the vitreous humour, and that it is therefore necessarily
developed as a cuticular deposit of the cells of the optic cup. Lieberkiihn,
Arnold, Lowe, and other authors regard it however as a mesoblastic
product ; and Kolliker believes that a primitive membrane is developed
from the cells of the optic cup, and that a true hyaloid membrane is
developed much later as a product of the mesoblast.
 
For fuller information on this subject the reader is referred to the
authors quoted above.
 
The optic nerve. The optic nerves are derived, as we have
said, from the at first hollow stalks of the optic vesicles. Their
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 493
 
cavities gradually become obliterated by a thickening of the
walls, the obliteration proceeding from the retinal end inwards
towards the brain. While the proximal ends of the optic stalks
are still hollow the rudiments of the optic chiasma are formed
from fibres at the roots of the stalks, the fibres of the one stalk
growing over into the attachment of the other. The decussation
of the fibres would appear to be complete. The fibres arise in
the remainder of the nerves somewhat later. At first the optic
nerve is equally continuous with both walls of the optic cup ; as
must of necessity be the case, since the interval which primarily
exists between the two walls is continuous with the cavity of the
stalk. When the cavity within the optic nerve vanishes, and
the fibres of the optic nerve appear, all connection is ruptured
between the outer wall of the optic cup and the optic nerve, and
the optic nerve simply perforates the outer wall, and becomes
continuous with the inner one.
 
There does not appear to me any ground for doubting (as has
been done by His and Kolliker) that the fibres of the optic nerve
are derived from a differentiation of the epithelial cells of which
the nerve is at first formed.
 
Choroid Fissure. With reference to the choroid fissure we
may state that its behaviour varies somewhat in the different
types. It becomes for the greater part of its extent closed,
though its proximal end is always perforated by the optic nerve,
and in many forms by a mesoblastic process also.
 
The lens when first formed is an oval vesicle with a small
central cavity, the front and hind walls being of nearly equal
thickness, and each consisting of a single layer of elongated
columnar cells. In the subsequent stages the mode of growth
of the hind wall is of precisely an opposite character to that of
the front wall. The hind wall becomes much thicker, and tends
to obliterate the central cavity by becoming convex on its front
surface. At the same time its cells, still remaining as a single
layer, become elongated and fibre-like. The front wall on the
contrary becomes thinner and thinner and its cells flattened.
 
These modes of growth continue until, as shewn in fig. 289,
the hind wall / is in absolute contact with the front wall el, and
the cavity thus becomes entirely obliterated. The cells of the
hind wall have by this time become veritable fibres, which, when
 
 
 
494 THE VITREOUS HUMOUR.
 
seen in section, appear to be arranged nearly parallel to the optic
axis, their nuclei nl being seen in a row along their middle. The
front wall, somewhat thickened at either side where it becomes
continuous with the hind wall, is now a single layer of flattened
cells separating the hind wall of the lens, or as we may now say
the lens itself, from the front limb of the lens-capsule ; of the
latter it becomes the epithelium.
 
The subsequent changes undergone consist chiefly in the continued elongation and multiplication of the lens-fibres, with the
partial disappearance of their nuclei.
 
During their multiplication they become arranged in the
manner characteristic of the adult lens of the various forms. The
lens-capsule, as was originally stated by Kolliker, appears to be
formed as a cuticular membrane deposited by the epithelial cells
of the lens.
 
The views of Lieberkiihn, Arnold, Lowe and others, according to
which the lens-capsule is a mesoblastic structure, do not appear to be well
founded. The contrary view, held by Kolliker, Kessler, etc., is supported
mainly by the fact that at the time when the lens-capsule first appears
there are no mesoblast cells to give rise to it. It should however be stated
that W. Miiller has actually found cellular elements in what he believes to
be the lens-capsule of the Ammoccete lens. Considering the degraded
character of the Ammoccete eye, evidence derived from its structure must
be accepted with caution.
 
The vitreous humour. The vitreous humour is derived
(except in Cyclostomata) from a vascular ingrowth, which differs
considerably in different types, through the choroid slit. Its
real nature is very much disputed. According to Kessler's view,
it is of the nature of a fluid transudation, but the occasional
presence in it of ordinary embryonic mesoblast cells, in addition
to more numerous blood-corpuscles, gives it a claim to be regarded
as intercellular substance. The number of cells in it is however
at best extremely small and in many cases there is no trace of
them. In Mammals there appear to be some mesoblast cells invaginated with the lens, which are not improbably employed in
the formation of the vessels of the so-called membrana capsulopupillaris. In the Ammoccete the vitreous humour originates
from a distinct mesoblastic ingrowth, though the cells which give
rise to it subsequently disappear.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 495
 
 
 
The development of the zonula of Zinn in Mammalia, which ought to
throw some light on the nature of the vitreous humour, has not been fully
investigated. According to Lieberkiihn (No. 373, p. 43), this structure
appears in half-grown embryos of the sheep and calf.
 
He says "At the point where the ciliary processes and the ciliary
part of the retina are entirely removed, one sees in the meridian bundles
of fine fibres, which correspond to the valleys between the ciliary processes and fill them ; also between these bundles there extend, as a thin
layer, similar finely striated masses, and these would have been on the
top of the ciliary processes." He further states that these fibres may be
traced to the anterior and posterior limb of the lens-capsule, and that
amongst them are numerous cells. Kolliker confirms Lieberkiihn's statements. There can be little doubt that the fibres of the zonula are of the
nature of connective tissue : they are stated to be elastic. By Lowe they
are believed to be developed out of the substance of the vitreous humour,
but this does not appear to me to follow from the observations hitherto
made. It seems quite possible that they arise from mesoblast cells which
have grown into the cavity of the vitreous humour, solely in connection
with their production.
 
The integral parts of the eye in front of the lens are the
cornea, the aqueous humour, and the iris. The development
of the latter has already been described, and there remain to be
dealt with the cornea, and the cavity containing the aqueous
humour.
 
The cornea. The cornea is formed by the coalescence of
two structures, viz. the epithelium of the cornea and the cornea
proper. The former is directly derived from the external epiblast,
which covers the eye after the invagination of the lens. The
latter is formed in a somewhat remarkable manner, first clearly
made out by Kessler.
 
When the lens is completely separated from the epidermis
its outer wall is directly in contact with the external epiblast
(future corneal epithelium). At its edge there is a small ringshaped space bounded by the outer skin, the lens and the edge
of the optic cup. In the chick, which we may take as typical,
there appears at about the time when the cavity of the lens is
completely obliterated a structureless layer external to the above
ring-like space and immediately adjoining the inner face of the
epiblast. This layer, which forms the commencement of the
cornea proper, at first only forms a ring at the border of the
lens, thickest at its outer edge, and gradually thinning off to
 
 
 
496 THE CORNEA.
 
 
 
nothing towards the centre. It soon however becomes broader,
and finally forms a continuous stratum of considerable thickness,
interposed between the external skin and the lens. As soon as
this stratum has reached a certain thickness, a layer of flattened
cells grows in along its inner side from the mesoblast surrounding the optic cup (fig. 290, dm). This layer is the epithelioid
layer of the membrane of Descemet. After it 1 has become
 
 
 
 
FIG. 290. SECTION THROUGH THE EYE OF A FOWL ON THE EIGHTH DAY
OF DEVELOPMENT, TO SHEW THE IRIS AND CORNEA IN THE PROCESS OF
FORMATION. (After Kessler.)
 
ep. epiblastic epithelium of cornea; cc. corneal corpuscles growing into the structureless matrix of the cornea; dm. Descemet's membrane; ir. iris; cb. mesoblast of
the iris (this reference letter points a little too high).
 
The space between the layers dm. and ep. is filled with the structureless matrix of
the cornea.
 
completely established, the mesoblast around the edge of the
cornea becomes divided into two strata ; an inner one (fig. 290,
cb) destined to form the mesoblastic tissue of the iris already
described, and an outer one (fig. 290, cc] adjoining the epidermis.
The outer stratum gives rise to the corneal corpuscles, which are
the only constituents of the cornea not yet developed. The
corneal corpuscles make their way through the structureless
corneal layer, and divide it into two strata, one adjoining the
epiblast, and the other adjoining the inner epithelium. The two
strata become gradually thinner as the corpuscles invade a larger
and larger portion of their substance, and finally the outermost
portion of them alone remains as the membrana elastica anterior
and posterior (Descemet's membrane) of the cornea. The corneal
 
1 It appears to me possible that Lieberkiihn may be right in stating that the
epithelium of Descemet's membrane grows in between the lens and the epiblast before
the formation of the cornea proper, and that Kessler's account, given above, may on
this point require correction. From the structure of the eye in the Ammocoete it
seems probable that Descemet's membrane is continuous with the choroid.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 497
 
corpuscles, which have grown in from the sides, thus form a layer
which becomes continually thicker, and gives rise to the main
substance of the cornea. Whether the increase in the thickness
of the layer is due to the immigration of fresh corpuscles, or to
the division of those already there, is not clear. After the
cellular elements have made their way into the cornea, the latter
becomes continuous at its edge with the mesoblast which forms
the sclerotic.
 
The derivation of the original structureless layer of the cornea is still
uncertain. Kessler derives it from the epiblast, but it appears to me more
probable that Kolliker is right in regarding it as derived from the mesoblast. The grounds for this view are, (i) the fact of its growth inwards
from the border of the mesoblast round the edge of the eye, (2) the peculiar
relations between it and the corneal corpuscles at a later period. This
view would receive still further support if a layer of mesoblast between
the lens and the epiblast were really present as believed by Lieberkiihn.
It must however be admitted that the objections to Kessler's view of its
epiblastic nature are rather a priori than founded on definite observation.
 
The observations of Kessler, which have been mainly followed in the
above account, are strongly opposed by Lieberkiihn (No. 374) and Arnold
(No. 370), and are not entirely accepted by Kolliker. It is especially on
the development of these parts in Mammalia (to be spoken of in the sequel)
that the above authors found their objections. I have had through Kessler's
kindness an opportunity of looking through some of his beautiful preparations, and have no hesitation in generally accepting his conclusions, though
as mentioned above I cannot agree with all his interpretations.
 
The aqueous humour. The cavity for the aqueous humour
has its origin in the ring-shaped space round the front of the
lens, which, as already mentioned, is bounded by the external
skin, the edge of the optic cup, and the lens. By the formation
of the cornea this space is shut off from the external skin, and on
the appearance of the epithelioid layer of Descemet's membrane
a continuous cavity is developed between the cornea and the
lens. This cavity enlarges and receives its final form on the
full development of the iris.
 
Comparative view of the development of the Vertebrate Eye.
 
The organ of vision, when not secondarily aborted, contains in all
Vertebrata the essential parts above described. The most interesting cases
of partial degeneration are those of Myxine and the Ammoccete. The
development of such aborted eyes has as yet been studied only in the
 
B. III. 3 2
 
 
 
498
 
 
 
THE AMMOCCETE EYE.
 
 
 
Ammocoete 1 , in which it resembles in most important features that of other
Vertebrata.
 
Eye of Ammoccetes. The optic vesicle arises as an outgrowth of
the fore-brain, but the secondary optic cup is remarkable in the young larva
for its small size (fig. 291, opv). The thicker outer wall gives rise to the
retina, and the thinner inner wall to the choroid pigment. The lens is formed
as an invagination of the single-layered epidermis (fig. 291, /). As development proceeds the parts of the eye gradually enlarge, and the mesoblast
around the hinder and dorsal part of the optic cup becomes pigmented.
There is at first no cavity for the vitreous humour, but eventually the
growth of the optic cup gives rise to a space, into which a cellular process
of mesoblast grows at a slight notch in the ventral edge of the optic cup
(W. Muller, No. 377). This notch is the only rudiment of the choroid
fissure of other types. The mesoblastic process
is probably the homologue of the processus
falciformis and pecten, and appears to give rise
to the vitreous humour ; for a long time it
retains its connection with the surrounding
mesoblast. Its cells eventually disappear, and
it never contains any vascular structures.
 
The lens for a long time remains as an oval
vesicle with a central cavity. In a later stage,
when the Ammoccete is fully developed, the
secondary optic cup forms a deep pit (fig. 292, r) ;
in the mouth of which is placed the lens (/).
The two walls of the retina have now the normal
vertebrate structure, though the pigment is as
yet imperfectly present in the choroid layer.
The lens has the embryonic forms of higher
types (cf. fig. 289), consisting of an inner thicker
segment, the true lens, and an outer layer forming the epithelium of the lens capsule. The
edge of the optic cup, which forms the rudiment
of the epiblast of the iris, is imperfectly separated
from the remainder of the optic cup ; and a
mesoblastic element of the iris, distinct from
Descemet's membrane (dm\ can hardly be spoken of.
 
There is no cavity for the aqueous humour in front of the lens ; and
there is no cornea as distinct from the epidermis and subepidermic tissues.
The elements in front of the lens are (i) the epidermis (ep} ; (2) the dermis
(dc) ; (3) the subdermal connective tissue (sdc) which passes without any
sharp line of demarcation into the dermis ; (4) a thick membrane, continuous with the mesoblastic part of the choroid, which appears to represent
Descemet's membrane. The subdermal connective tissue is continued as an
 
 
 
 
FIG. 291. HORIZONTAL
 
SECTION THROUGH THE
HEAD OF A JUST HATCHED
LARVA OF PETROMYZON
SHEWING THE DEVELOPMENT OF THE LENS OF THE
 
EYE.
 
th.c. thalamencephalon ;
op.v. optic vesicle ; /. lens of
eye ; h.c. head cavity.
 
 
 
The most detailed account is that of W. Muller (No. 377).
 
 
 
ORGANS OF VISION OF THE VERTEBRATA.
 
 
 
499
 
 
 
investment round the whole eye ; and there is no differentiated sclerotic and
only an imperfect choroid.
 
In a still later stage a distinct mesoblastic element for the iris is formed.
When the Ammoccete is becoming a Lamprey, the eye approaches the
surface ; an anterior chamber is established ; and the eye differs from that
of the higher types mainly in the fact that the cornea is hardly distinguished
from the remainder of the skin, and that a sclerotic is very imperfectly
represented.
 
Optic vesicles. The development of the primitive optic vesicles, so
far as is known, is very constant throughout the Vertebrata. In Teleostei
and Lepidosteus alone is there an important deviation from the ordinary
type, dependent however upon the mode of formation of the medullary keel,
the optic vesicles arising while the medullary keel is still solid, and being at
first also solid. They subsequently acquire a lumen and undergo the
ordinary changes.
 
The lens. In the majority of groups, viz. Elasmobranchii, Reptilia,
Aves, and Mammalia, the lens is formed by an open invagination of the
epiblast, but in Amphibia, Teleostei and Lepidosteus, where the nervous
 
 
 
S.d.c
 
 
 
 
FIG. 292. EYE OF AN AMMOCCETES LYING BENEATH THE SKIN.
 
ep. epidermis; d.c. dermal connective tissue continuous with the sub-dermal
connective tissue (s.d.c}, which is also shaded. There is no definite boundary to this
tissue where it surrounds the eye.
 
m. muscles; dm. membrane of Descemet ; /.lens; v.h. vitreous humour ; r. retina;
rp. retinal pigment.
 
layer of the skin is early established, this layer alone takes part in the
formation of the lens (fig. 293, /). The lens is however formed even in
these types as a hollow body by an invagination ; but its opening remains
permanently shut off from communication with the exterior by the epidermic
 
322
 
 
 
500 THE CORNEA.
 
 
 
layer of the epiblast. Gotte describes the lens as formed by a solid
thickening of the nervous layer in Bombinator. This is probably a mistake.
 
The cornea. The mode of formation of the cornea already described
appears to be characteristic of most Vertebrata except the Ammocoete. It
has been found by Kessler in Aves, Reptilia and Amphibia, and probably
also occurs in Pisces. In Mammals it is not however so easy to establish.
There are at first no mesoblast cells between the lens and the epiblast (fig.
295) but in many Mammals (vide Kessler, No. 372, pp. 91 94) a layer of
rounded mesoblast cells, which forms Descemet's membrane, grows in
between the two, at a time when it is not easy to recognise a corneal
lamina, as distinct from a simple coagulum.
 
After the formation of this layer the mesoblast cells grow into the
corneal lamina from the sides, and becoming flattened arrange themselves
in rows between the laminae of the cornea. The cornea continues to
increase in thickness by the addition of laminae on the side adjoining the
epiblast.
 
We have already seen that in the Lamprey the cornea is nothing else
but the slightly modified and more transparent epidermis and dermis.
 
The optic nerve and the choroid fissure. It will be convenient to consider together the above structures, and with them the
vascular and other processes which pass into the cavity of the optic cup
through the choroid fissure. These parts present on the whole a greater
amount of variation than any other parts of the eye.
 
I commence with the Fowl which is both a very convenient general type
for comparison, and also that in which these structures have been most
fully worked out.
 
During the third day of incubation there passes in through the choroid
slit a vascular loop, which no doubt supplies the transuded material for
the growth of the vitreous humour. Up to the fifth day this vascular loop is
the only structure passing through the choroid slit. On this day however a
new structure appears, which remains permanently through life, and is
known as the pec ten. It consists of a lamellar process of the mesoblast
cells round the eye, passing through the choroid slit near the optic nerve,
and enveloping part of the afferent branch of the vascular loop above
mentioned. The proximal part of the free edge of the pecten is somewhat
swollen, and sections through this part have a club-shaped form. On the
sixth day the choroid slit becomes rapidly closed, so that at the end of the
sixth day it is reduced to a mere seam. There are however two parts of
this seam where the edges of the optic cup have not coalesced. The
proximal of these adjoins the optic nerve, and permits the passage of the
pecten and at a later period of the optic nerve ; and the second or distal one
is placed near the ciliary edge of the slit, and is traversed by the efferent
branch of the above-mentioned vascular loop. This vessel soon atrophies,
and with it the distal opening in the choroid slit completely vanishes. In
some varieties of domestic Fowl (Lieberkiihn) the opening however persists.
The seam which marks the original site of the choroid slit is at first
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 501
 
conspicuous by the absence of pigment, and at a later period by the deep
colour of its pigment. Finally, a little after the ninth day, no trace of it is
to be seen.
 
Up to the eighth day the pecten remains as a simple lamina ; by the
tenth or twelfth day it begins to be folded or rather puckered, and by the
seventeenth or eighteenth day it is richly pigmented and the puckerings
 
 
 
 
FIG. 293. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOS
TEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION.
al. alimentary tract ; fb. thalamencephalon ; /. lens of eye ; op.v. optic vesicle.
The mesoblast is not represented.
 
have become nearly as numerous as in the adult, there being in all seventeen
or eighteen. The pecten is almost entirely composed of vascular coils,
which are supported by a sparse pigmented connective tissue ; and in the
adult the pecten is still extremely vascular. The original artery which
became enveloped at the formation of the pecten continues, when the latter
becomes vascular, to supply it with blood. The vein is practically a fresh
development after the atrophy of the distal portion of the primitive vascular
loop of the vitreous humour.
 
There are no true retinal blood-vessels.
 
In the formation of the optic cup the extreme peripheral part of the optic
nerve, which is in immediate proximity with the artery of the pecten,
becomes folded. The permanent opening in the choroid fissure for the
pecten is intimately related to the entrance of the optic nerve into the
eyeball ; the fibres of the optic nerve passing in at the inner border of the
pecten, coursing along its sides to its outer border, and radiating from it as
from a centre to all parts of the retina.
 
In the Lizard the choroid slit closes considerably earlier than in the
Fowl. The vascular loop in the vitreous humour is however more developed.
The pecten long remains without vessels, and does not in fact become at all
 
 
 
502 THE CHOROID FISSURE.
 
vascular till after the very late disappearance of the distal part of the
vascular loop of the vitreous humour.
 
The arrangement of the ingrowth through the choroid slit in Elasmobranchii (Scyllium) has been partially worked out, and so far as is at present
known the agreement between the Avian and Elasmobranch type is fairly
close.
 
At the time when the cavity between the lens and the secondary optic
cup is just commencing to be formed, a process of mesoblast accompanied
by a vascular loop passes into the vitreous humour, through the choroid slit,
close to the optic nerve. The vessel in this process is no doubt equivalent
to the vascular loop in the Avian eye, but I have not made out that it projects beyond the mesoblastic process accompanying it. As the cavity of the
vitreous humour enlarges and the choroid slit elongates, the process through
it takes the form of a lamina with a somewhat swollen border, and projects
for some distance into the cavity of the vitreous humour.
 
At a later stage, after the outer layer of the optic cup has become pigmented, the distal part of the choroid slit adjoining the border of the lens
closes up ; but along the line where it was present the walls of the optic cup
remain very thin and are thrown into three folds, two lateral and one
median, projecting into the cavity of the vitreous humour. The median
fold is in contact with the lens, and the vascular mesoblast surrounding the
eye projects into the space between the two laminae of which it is formed.
In passing from the region of the lens to that of the optic nerve the lateral
folds of the optic cup disappear, and the median fold forms a considerable
projection into the cavity of the vitreous humour. It consists of a core of
mesoblast covered by a delicate layer derived from both strata of the optic
cup. Still nearer the optic nerve the choroid slit is no longer closed, and
the mesoblast, which in the neighbourhood of the lens only extended into the
folds of the wall of the optic cup, now projects freely into the cavity of the
vitreous humour, and forms the lamina already described. It is not very
vascular, but close to the optic nerve there passes into it a considerable
artery.
 
In the young animal the choroid slit is no longer perforated by a mesoblastic lamina. At its inner end it remains open to allow of the passage of
the optic nerve. The line of the slit can easily be traced along the lower
side of the retina ; and close to the lens the retinal wall continues, as in the
embryo, to be raised into a projecting fold. Traces of these structures are
visible even in the fully grown examples of Scyllium.
 
As has been pointed out by Bergmeister the mesoblastic lamina projecting into the vitreous humour resembles the pecten at an early stage of
development, and is without doubt homologous with it. The artery which
supplies it is certainly equivalent to the artery of the pecten.
 
There can be no doubt that the mesoblastic lamina projecting into the
vitreous humour is equivalent to the processus falciformis of Teleostei, and
it seems probable that the whole of it, including the free part as well as that
covered by epiblast, ought to be spoken of under this title. The optic nerve
 
 
 
ORGANS OF VISION OF THE VERTEJ5RATA.
 
 
 
503
 
 
 
 
in Elasmobranchii is not included in the folding to which the secondary
optic vesicle owes its origin, and would seem to perforate the walls of the
optic cup only at the distal end of the processus falciformis.
 
In Teleostei there is at first a vascular loop like that in Birds, passing
through the choroid fissure. This has been noticed by Kessler in the Pike,
and by Schenk in the Trout. At a later period a mesoblastic ingrowth with
a blood-vessel makes its way in many forms into the cavity of the vitreous
humour, accompanied by two folds in the walls of the free edges of the
choroid fissure (fig. 294). These structures, which constitute the processus
falciformis, clearly resemble very closely the
mesoblastic process and folds of the optic cup
in Elasmobranchii. The processus falciformis
comes in contact with, and perhaps becomes
attached to the wall of the lens ; and persists
through life.
 
In Triton there is no vascular ingrowth
through the choroid fissure, but a few mesoblastic cells pass in which represent the vascular
ingrowth of other types. The optic nerve perforates the proximal extremity of the original
choroid slit.
 
The absence of an embryonic blood-vessel
does not however hold good for all Amphibia,
as there is present in the embryo Alytes (Lieberkiihn) an artery, which breaks up into a capillary
system on the retinal border of the vitreous
humour.
 
In the Ammoccete the choroid slit is merely represented by a slight
notch on the ventral edge of the optic cup, and the mesoblastic process
which passes through the choroid slit in most types is represented by a
large cellular process, from which the vitreous humour would appear to be
derived.
 
Mammalia differ from all the types already described in the immense
fcetal development of the blood-vessels of the vitreous humour. There are
however some points in connection with the development of these vessels
which are still uncertain. The most important of these points concerns
the presence of a prolongation of the mesoblast around the eye into the
cavity of the vitreous humour. It is maintained by Lieberkiihn, Arnold,
Kolliker, etc., that in the invagination of the lens a thin layer of mesoblast
is carried before it ; and is thus transported into the cavity of the vitreous
humour. This is denied by Kessler, but the layer is so clearly figured by
the above embryologists, that the existence of it in some Mammalia (the
Rabbit, etc.) must I think be accepted.
 
In the folding in of the optic vesicle, which accompanies the formation
of the lens, the optic nerve becomes included, and on the development of
the cavity of the vitreous humour an artery, running in the fold of the optic
 
 
 
FIG. 294. HORIZONTAL
SECTION THROUGH THE EYE
OF A TELEOSTEAN EMBRYO.
(From Gegenbaur ; after
Schenk.)
 
s. choroid fissure, with
two folds forming part of the
processus falciformis ; a. choroid layer of optic cup ; b.
retinal layer of optic cup ; c.
cavity of vitreous humour ; d.
lens.
 
 
 
504
 
 
 
THE CIIOROID FISSURE.
 
 
 
nerve, passes through the choroid slit into the cavity of the vitreous humour
(fig. 295, acr). The sides of the optic nerve subsequently bend over, and
completely envelope this artery, which at a later period gives off branches to
the retina, and becomes known as the arteria centralis retinas. It is
homologous with the arterial limb of the vascular loop projecting into the
vitreous humour in Birds, Lizards, Teleostei, etc.
 
Before becoming enveloped in the optic nerve this artery is continued
through the vitreous humour (fig. 295), and when it comes in close proximity
 
 
 
a. c.
 
 
 
 
,m, e o
 
 
 
FIG. 295. SECTION THROUGH THE EYE OF A RABBIT EMBRYO OF
 
ABOUT TWELVE DAYS.
 
c. epithelium of cornea ; /. lens ; mec. mesoblast growing in from the side to form
the cornea: rt. retina ; a.c.r. arteria centralis retinae; of.n. optic nerve.
 
The figure shews (i) the absence at this stage of mesoblast between the lens and
the epiblast : the interval between the two has however been made too great ; (2) the
arteria centralis retinae forming the vascular capsule of the lens and continuous with
vascular structures round the edges of the optic cup.
 
to the lens it divides into a number of radiating branches, which pass round
the edge of the lens, and form a vascular sheath which is prolonged so as to
cover the anterior wall of the lens. In front of the lens they anastomose
with vessels, coming from the iris, many of which are venous (fig. 295) and
the whole of the blood from the arteria centralis is carried away by these
veins. The vascular sheath surrounding the lens receives the name of the
membrana capsulo-pupillaris. The posterior part of it appears (Kessler,
No. 372) to be formed of vessels without the addition of any other structures
and is either formed simply by branches of the arteria centralis, or out of
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 505
 
the mesoblast cells involuted with the lens. The anterior part of the
vascular sheath is however inclosed in a very delicate membrane, the
membrana pupillaris, continuous at the sides with the epithelium of
Descemet's membrane. On the formation of the iris this membrane lies
superficially to it, and forms a kind of continuation of the mesoblast of the
iris over the front of the lens.
 
The origin of this membrane is much disputed. By Kessler, whose
statements have been in the main followed, it is believed to appear
comparatively late as an ingrowth of the stroma of the iris ; while Kolliker
believes it to be derived from a mesoblastic ingrowth between the front wall
of the lens and the epiblast. According to Kolliker this ingrowth subsequently becomes split into two laminae, one of which forms the cornea, and
the other the anterior part of the vascular sheath of the lens with its membrana pupillaris. Between the two appears the aqueous humour.
 
The membrana capsulo-pupillaris is simply a provisional embryonic
structure, subserving the nutrition of the lens. The time of its disappearance varies somewhat for the different Mammalia in which this point has
been investigated. In the human embryo it lasts from the second to the
seventh month and sometimes longer. As a rule it is completely absorbed
at the time of birth. The absorption of the anterior part commences in the
centre and proceeds outwards.
 
In addition to the vessels of the vascular capsule round the lens, there
arise from the arteria centralis retinas, just after its exit from the optic nerve,
in many forms (Dog, Cat, Calf, Sheep, Rabbit, Man) provisional vascular
branches which extend themselves in the posterior part of the vitreous
humour. Near the ciliary end of the vitreous humour they anastomose with
the vessels of the membrana capsulo-pupillaris.
 
In Mammals the choroid slit closes very early, and is not perforated
by any structure homologous with the pecten. The only part of the slit
which remains open is that perforated by the optic nerve ; and in the centre
of the latter is situated the arteria centralis retinas as explained above.
From this artery there grow out the vessels to supply the retina, which
have however nothing to do with the provisional vessels of the vitreous
humour just described (Kessler). On the atrophy of the provisional
vessels the whole of the blood of the arteria centralis passes into the
retina.
 
It is interesting to notice (Kessler, No. 372, p. 78) that there seems to be
a blood-vessel supplying the vitreous humour in the embryos of nearly all
vertebrate types, which is homologous throughout the Vertebrata. This
vessel often exhibits a persisting and a provisional part. The latter in
Mammalia is the membrana capsulo-pupillaris and other vessels of the
vitreous humour ; in Birds and Lizards it is the part of the original vascular
loop, not included in the pecten, and in Osseous Fishes that part (?)
not involved in the processus falciformis. The permanent part is formed by
the retinal vessels of Mammalia, by the vessels of the pecten in Birds and
Lizards, and by those of the processus falciformis in Fishes.
 
 
 
506 THE IRIS.
 
The Iris and Ciliary processes. The walls of the edge of the
optic cup become very much thinner than those of the true retinal part. In
many Vertebrates (Mammalia, Aves, Reptilia, Elasmobranchii, etc.) the
thinner part, together with the mesoblast covering it, becomes divided into
two regions, viz. that of the iris, and that of the ciliary processes. In the
Newt and Lamprey this differentiation does not take place, but the part in
question simply becomes the iris.
 
 
 
Accessory Organs connected wit/i t/te Eye.
 
Eyelids. The most important accessory structures connected with
the eye are the eyelids. They are developed as simple folds of the integument with a mesoblastic prolongation between their two laminas. They
may be three in number, viz. an upper and lower, and a lateral one the
nictitating membrane springing from the inner or anterior border of the
eye. Their inner face is lined by a prolongation of conjunctiva, which is
the modified epiblast covering the cornea and part of the sclerotic.
 
In Teleostei and Ganoidei eyelids are either not present or at most
very rudimentary. In Elasmobranchii they are better developed, and the
nictitating membrane is frequently present. The latter is also usually found
in Amphibia. In the Sauropsida all three eyelids are usually present, but in
Mammalia the nictitating membrane is rudimentary.
 
In many Mammalia the two eyelids meet together during a period of
embryonic life, and unite in front of the eye. A similar arrangement
is permanent through life in Ophidia and some Lacertilia ; and there is a
chamber formed between the coalesced eyelids and the surface of the cornea,
into which the lacrymal ducts open.
 
Lacrymal glands. Lacrymal glands are found in the Sauropsida
and Mammalia. They arise (Remak, Kdlliker) as solid ingrowths of the
conjunctival epithelium. They appear in the chick on the eighth day.
 
Lacrymal duct. The lacrymal duct first appears in Amphibia, and
is present in all the higher Vertebrates. Its mode of development in the
Amphibia, Lacertilia and Aves has recently been very thoroughly worked
out by Born (Nos. 380 and 381).
 
In Amphibia he finds that the lacrymal duct arises as a solid ridge of
the mucous layer of the epidermis, continued from the external opening
of the nasal cavity backwards towards the eye. It usually appears at
about the time when the nasal capsule is beginning to be chondrified. As
this ridge is gradually prolonged backwards towards the eye its anterior
end becomes separated from the epidermis, and grows inwards in the
mesoblast to become continuous with the posterior part of the nasal sack.
The posterior end which joins the eye becomes divided into the two
collecting branches of the adult. Finally the whole structure becomes
separated from the skin except at the external opening, and develops a
lumen.
 
 
 
ORGANS OF VISION OF THE VERTEBRATA. 507
 
In Lacertilia the lacrymal duct arises very much in the same manner as
in Amphibia, though its subsequent growth is somewhat different. It
appears as an internal ridge of the epithelium, at the junction of the superior
maxillary process and the fold which gives rise to the lower eyelid. A solid
process of this ridge makes its way through the mesoblast on the upper
border of the maxillary process till it meets the wall of the nasal cavity, with
the epithelium of which it becomes continuous. At a subsequent stage
a second solid growth from the upper part of the epithelial ridge makes its
way through the lower eyelid, and unites with the inner epithelium of the
eyelid ; and at a still later date a third growth from the lower part of the
structure forms a second junction with the epithelium of the eyelid. The
two latter outgrowths form the two upper branches of the duct. The
ridge now loses its connection with the external skin, and, becoming
hollow, forms the lacrymal duct. It opens at two points on the inner
surface of the eyelid, and terminates at its opposite extremity by opening
into the nasal cavity. It is remarkable, as pointed out by Born, that the
original epithelial ridge gives rise directly to a comparatively small part of
the whole duct.
 
In the Fowl the lacrymal duct is formed as a solid ridge of the epidermis,
extending along the line of the so-called lacrymal groove from the eye to the
nasal pit (fig. 120). At the end of the sixth day it begins to be separated
from the epidermis, remaining however united with it on the inner side of
the lower eyelid. After its separation from the epidermis it forms a solid
cord, the lower end of which unites with the wall of the nasal cavity. The
cord so formed gives rise to the whole of the duct proper and to the lower
branch of the collecting tube. The upper branch of the collecting tube is
formed as an outgrowth from this cord. A lumen begins to be formed on
the twelfth day of incubation, and first appears at the nasal end. It arises
by the formation of a space between the cells of the cord, and not by
an absorption of the central cells.
 
In Mammalia Kolliker states that he has been unable to observe
anything similar to that described by Born in the Sauropsida and Amphibia,
and holds to the old view, originally put forward by Coste, that the duct is
formed by the closure of a groove leading from the eye to the nose between
the outer nasal process and the superior maxillary process. The upper
extremity of the duct dilates to form a sack, from which two branches pass
off to open on the lacrymal papillae. In view of Born's discoveries Kolliker's
statements must be received with some caution.
 
 
 
The Eye of tJte Tunicata.
 
The unpaired eye of the larva of simple Ascidians is situated
somewhat to the right side of the posterior part of the dorsal
wall of the anterior cephalic vesicle (fig. 296, O\ It consists of
a refractive portion, turned towards the cavity of the vesicle of
 
 
 
508 THE EYE OF THE TUNICATA.
 
the brain, and a retinal portion forming part of the wall of the
brain. The refractive parts consist of a convex-concave meniscus in front, and a spherical lens behind, adjoining the concave
side of the meniscus. The posterior part of this lens is im
 
 
 
FIG. 296. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer. )
Only the anterior part of the tail is represented.
 
IV'. anterior swelling of neural tube; N. anterior swelling of spinal portion of
neural tube ; n. hinder part of neural tube ; ch. notochord ; K. branchial region of
alimentary tract; d. oesophageal and gastric region of alimentary tract; 0. eye;
a. otolith ; o. mouth ; s. papilla for attachment.
 
bedded in a layer of pigment The retina is formed of columnar
cells, with their inner ends imbedded in the pigment which
encloses the posterior part of the lens. The retinal part of the
eye arises in the first instance as a prominence of the wall of
the cerebral vesicle : its cells become very columnar and pigmented at their inner extremities (fig. 8, V, a). The lens is
developed at a later period, after the larva has become hatched,
but the mode of its formation has not been made out.
 
General considerations on the Eye of the Chordata.
 
There can be but little doubt that the eye of the Tunicata belongs to the
same phylum as that of the true Vertebrata, different as the two eyes are.
The same may also be said with reference to the degenerate and very
rudimentary eye of Amphioxus.
 
The peculiarity of the eye of all the Chordata consists in the retina being
developed from part of the wall of the brain. How is this remarkable feature
of the eye of the Chordata to be explained ?
 
Lankester, interpreting the eye in the light of the Tunicata, has made
the interesting suggestion 1 "that the original Vertebrate must have been a
transparent animal, and had an eye or pair of eyes inside the brain, like that
of the Ascidian Tadpole."
 
1 Degeneration, London, 1880, p. 49.
 
 
 
ORGANS OF VISION. 509
 
 
 
This explanation may possibly be correct, but another explanation appears
to me possible, and I am inclined to think that the vertebrate eyes have not
been derived from eyes like those of Ascidians, but that the latter is a
degenerate form of vertebrate eye.
 
The fact of the retina being derived from the fore-brain may perhaps be
explained in the same way as has already been attempted in the case of the
retina of the Crustacea ; i.e. by supposing that the eye was evolved simultaneously with the fore part of the brain.
 
The peculiar processes which occur in the formation of the optic vesicle
are more difficult to elucidate ; and I can only suggest that the development
of a primary optic vesicle, and its conversion into an optic cup, is due to the
retinal part of the eye having been involved in the infolding which gave rise
to the canal of the central nervous system. The position of the rods and
cones on the posterior side of the retina is satisfactorily explained by this
hypothesis, because, as may be easily seen from figure 285, the posterior face
of the retina is the original external surface of the epidermis, which is
infolded in the formation of the brain ; so that the rods and cones are, as
might be anticipated, situated on what is morphologically the external surface
of the epiblast of the retina.
 
The difficulty of this view arises in attempting to make out how the eye
can have continued to be employed during the gradual change of position
which the retina must have undergone in being infolded with the brain in
the manner suggested. If however the successive steps in this infolding
were sufficiently small, it seems to me not impossible that the eye might have
continued to be used throughout the whole period of change, and a transparency of the tissues, such as Lankester suggests, may have assisted in
rendering this possible.
 
The difficulty of the eye continuing to be in use when undergoing
striking changes in form is also involved in Lankester's view, in that if, as I
suppose, he starts from the eye of the Ascidian Tadpole with its lenses
turned towards the cavity of the brain ; it is necessary for him to admit that
a fresh lens and other optical parts of the eye became developed on the
opposite side of the eye to the original lens ; and it is difficult to understand
such a change, unless we can believe that the refractive media on the two
sides were in operation simultaneously. It may be noted that the same
difficulty is involved in supposing, as I have done, that the eye of the
Ascidian Tadpole was developed from that of a Vertebrate. I should
however be inclined to suggest that the eye had in this case ceased for a
period to be employed ; and that it has been re-developed again in some of
the larval forms. Its characters in the Tunicata are by no means constant.
 
Accessory eyes in the Vertebrata.
 
In addition to the paired eyes of the Vertebrata certain organs are
found in the skin of a few Teleostei living in very deep water, which, though
clearly not organs of true vision, yet present characters which indicate that
 
 
 
510 ACCESSORY EYES IN THE VERTEBRATA.
 
they may be used in the perception of light. The most important of such
organs are those found in Chauliodus, Stomias, etc., the significance of which
was first pointed out by Leuckart, while the details of their structure have
been recently worked out by Leydig 1 and Ussow. They are distributed not
only in the skin, but are also present in the mouth and respiratory cavity, a
fact which appears to indicate that their main function must be something
else than the perception of light. It has been suggested that they have the
function of producing phosphorescence.
 
Another organ, probably of the same nature, is found on the head of
Scopelus.
 
The organs in Chauliodus are spherical or nearly spherical bodies
invested in a special tunic. The larger of them, which alone can have any
relation to vision, are covered with pigment except on their outer surface.
The interior is filled with two masses, named by Leuckart the lens and
vitreous humour. According to Leydig each of them is cellular and receives
a nerve, the ultimate destination of which has not however been made out.
According to Ussow the anterior mass is structureless, but serves to support
a lens, placed in the centre of the eye, and formed of a series of crystalline
cones prolonged into fibres, which in the posterior part of the eye diverge
and terminate by uniting with the processes of multipolar cells, placed near
the pigmented sheath. These cells, together with the fibres of the crystalline
cones which pass to them, are held by Ussow to constitute a retina.
 
Eye of the Mollusca.
 
(362) N. Bobretzky. " Observations on the development of the Cephalopoda "
(Russian). Nachrichten d. kaiserlichen Gesell.d. Freundcder Natunviss. Anthropolog.
Ethnogr. bei d. Universitiit Moskau.
 
(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
f. wiss. Zool., Bd. xxiv. 1874.
 
(364) V. Hensen. " Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss.
Zool., Vol. xv. 1865.
 
(365) E. R. Lankester. " Observations on the development of the Cephalopoda." Quart, y. of Micr. Science, Vol. xv. 1875.
 
(366) C. Semper. Ueber Sehorgane von Typus d. Wirbelthieratigen. Wiesbaden,
1877
Eye of the Arthropoda.
 
(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.
 
(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Palinurus nnd Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.
 
1 F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani." Archiv f. Anal,
und Phys., 1879. M. Ussow. " Ueb. d. Bau d. augenahnlichen Flicken einiger
Knochenfische." Bui. d. la Soc. d. Naturalistes de Moscon, Vol. i.iv. 1879. Vide
for general description and further literature, Giinther, The Study of Fish>-s t Edinburgh,
1880.
 
 
 
ORGANS OF VISION. 51 1
 
 
 
(369) E. Claparede. " Morphologic d. zusammengesetzten Auges bei den Arthropoden." Zeit. f. wiss. Zool., Bd. x. 1860.
 
(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden.
Gottingen, 1879.
 
Vertebrate Eye.
 
(371) J.Arnold. Beitrage zur Entwicklungsgeschichte des Auges. Heidelberg,
1874.
 
(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wilrzburger natiinuissenschaftliche Zeitschrift, Bd. 8.
 
(373) L. Kessler. Zur Entwicklung d. Attges d. Wirbelthiere. Leipzig, 1877.
 
(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.
 
(375) N. Lieberkiihn. "Beitrage z. Anat. d. embryonalen Auges." Archiv
f. Anat. imd Phys., 1879.
 
(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese
der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.
 
(377) V. Mihalkowics. " Untersuchungen iiber den Kamm des Vogelauges."
Archiv f. mikr. Anat., Vol. ix. 1873.
 
(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wirbelthiere." Festgabe Carl Ltidwig. Leipzig, 1874.
 
(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische."
Wiener Sitzungsberichte, Bd. LV. 1867.
 
Accessory organs of the Vertebrate Eye.
 
(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien.''
Morphologisches Jahrbuch, Bd. II. 1876.
 
(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. v. 1879.
 
Eye of the Tunicata.
 
(382) A. Kowalevsky. "Weitere Studien lib. d. Entwicklung d. einfachen
Ascidien." Archiv f. mikr. Anat., Vol. vil. 1871.
 
(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f.
mikr. Anat., Vol. vii. 1872.
 
 
 
CHAPTER XVII.
 
 
 
AUDITORY ORGAN, OLFACTORY ORGAN AND SENSE
ORGANS OF THE LATERAL LINE.
 
 
 
Auditory Organs.
 
A GREAT variety of organs, very widely distributed amongst
aquatic forms, and also found, though less universally, in land
forms, are usually classed together as auditory organs.
 
In the case of all aquatic forms, or of forms which have
directly inherited their auditory organs from aquatic forms,
these organs are built upon a common type ; although in the
majority of instances the auditory organs of the several groups
have no genetic relations. All the organs have their origin in
specialized portions of the epidermis. Some of the cells of a
special region become provided at their free extremities with
peculiar hairs, known as auditory hairs; while in other cells
concretions, known as otoliths, are formed, which appear often
to be sufficiently free to be acted upon by vibrations of the
surrounding medium, and to be so placed as to be able in their
turn to transmit their vibrations to the cells with auditory hairs 1 .
The auditory regions of the epidermis are usually shut off from
the surface in special sacks.
 
The actual function of these organs is no doubt correctly
described, in the majority of instances, as being auditory; but it
appears to me very possible 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
 
1 The function of the otoliths is not always clear. There is evidence to shew that
they sometimes act as dampers.
 
 
 
AUDITORY ORGANS. 513
 
 
 
unclulatory movements in the water, caused by the swimming of
the latter.
 
Auditory organs with the above characters, sometimes freely
open to the external medium, but more often closed, are found
in various Ccelenterata, Vermes and Crustacea, and universally
or all but universally in the Mollusca and Vertebrata.
 
In many terrestrial Insects a different type of auditory organ
has been met with, consisting of a portion of the integument
modified to form a tympanum or drum, and supported at its
edge by a chitinous ring. The vibrations set up in the membranous tympanum stimulate terminal nerve organs at the ends
of chitinous processes, placed in a cavity bounded externally by
the tympanic membrane.
 
The tympanum of Amphibia and Amniota is an accessory
organ added, in terrestrial Vertebrata, to an organ of hearing
primitively adapted to an aquatic mode of life ; and it is interesting to notice the presence of a more or less similar membrane
in the two great groups of terrestrial forms, i.e. terrestrial Vertebrata and Insecta.
 
Nothing is known with reference to the mode of development or evolution of the tympanic type of auditory organ found
in Insects, and, except in the case of Vertebrates, but little is
known with reference to the development of what may be called
the vesicular type of auditory organ found in aquatic forms.
Some very interesting facts with reference to the evolution of
such organs have however been brought to light by the brothers
Hertwig in their investigations on the Ccelenterata; and I
propose to commence my account of the development of the
auditory organs in the animal kingdom by a short statement of
the results of their researches.
 
Ccelenterata. Three distinct types of auditory organ have
been recognised in the Medusae ; two of them resulting from
the differentiation of a tentacle-like organ, and one from ectoderm cells on the under surface of the velum. We may commence with the latter as the simplest. It is found in the
Medusae known as the Vesiculata. The least differentiated
form of this organ, so far discovered, is present in Mitrotrocha,
Tiaropsis and other genera. It has the form of an open pit ;
and a series of such organs are situated along the attached edge
 
B. in, 33
 
 
 
514 AUDITORY ORGANS OF THE CCELENTERATA.
 
of the velum with their apertures directed downwards. The
majority of the cells lining the outer, i.e. peripheral side of the
 
 
 
 
FIG. 297. AUDITORY VESICLE OF PHIALIDIUM AFTER TREATMENT WITH
DILUTE OSMIC ACID. (From Lankester; after O. and R. Hertwig.)
 
d l . epithelium of the upper surface of the velum; d 2 . epithelium of the under
surface of the velum ; r. circular canal at the edge of the velum ; nr l . upper nervering ; h. auditory cells ; hh. auditory hairs ; np. nervous cushion formed of a
prolongation of the lower nerve-ring. Close to the nerve-ring is seen a cell, shewn as
black, containing an otolith.
 
pit, contain an otolith, while a row of the cells on the inner, i.e.
central side, are modified as auditory cells. The auditory cells
are somewhat strap-shaped, their inner ends being continuous
with the fibres of the lower nerve-ring, and their free ends being
provided with bent auditory hairs, which lie in contact with the
convex surfaces of the cells containing the otoliths.
 
By the conversion of such open pits into closed sacks a more
complicated type of auditory organ, which is present in many of
the Vesiculata, viz. ^Equorea, Octorchis, Phialidium, &c., is
produced. A closed vesicle of this type is shewn in fig. 297.
Such organs form projections on the upper surface of the velum.
They are covered by a layer of the epithelium (d 1 } of the upper
surface of the velum, but the lining of the vesicle (d*} is derived
from what was originally part of the epithelium of the lower
surface of the velum, homologous with that lining the open pits
in the type already described. The general arrangement of the
cells lining such vesicles is the same as that of the cells lining
the open pits.
 
A second type of auditory organ, found in the Trachymeclusa,", appears in its simplest condition as a modified tentacle.
 
 
 
AUDITORY ORGANS. 515
 
 
 
It is formed of a basal portion, covered by auditory cells with
long stiff auditory hairs, supporting at its apex a club-shaped
body, attached to it by a delicate stalk. An endodermal axis is
continued through the whole structure, and in one or more of
the endoderm cells of the club-shaped body otoliths are always
present. The tails of the auditory cells are directly continued
into the upper nerve-ring.
 
In more complicated forms of this organ the tentacle becomes
enclosed in a kind of cup, by a wall-like upgrowth of the
 
 
 
 
FIG. 298. AUDITORY ORGAN OF RHOPALONEMA. (From Lankester; after O.
and R. Hertwig.)
 
The organ consists of a modified tentacle (hk) with auditory cells and concretions, partially enclosed in a cup.
 
surrounding parts (fig. 298) ; and in some forms, e.g. Geryonia,
by the closure of the cup, the whole structure takes the form of
a completely closed vesicle, in the cavity of which the original
tentacle forms an otolith-bearing projection.
 
The auditory organs found in the Acraspedote Medusae
approach in many respects to the type of organ found in
the Trachymedusse. They consist of tentacular organs placed
in grooves on the under surface of the disc. They have a
swollen extremity, and are provided with an endodermal axis
for half the length of which there is a diverticulum of the gastrovascular canal system. The terminal portion of the endoderm
is solid, and contains calcareous concretions. The ectodermal
cells at the base of these organs have the form of auditory cells.
 
Mollusca. Auditory vesicles are found in almost all Mollusca on the ventral side of the body in close juxtaposition to
the pedal ganglia. Except possibly in some Cephalopods, these
 
332
 
 
 
516 AUDITORY ORGANS OF THE VERTEBRATA.
 
vesicles are closed. They are provided with free otoliths,
supported by the cilia of the walls of the sack, but in addition
some of the cells of the sack are provided with stiff auditory
hairs.
 
In many forms these sacks have been observed to originate
by an invagination of the epiblast of the foot (Pahtdina, Nassa,
Heteropoda, Limax, Clio, Cephalopoda and Lamellibranchiata).
In other instances (some Pteropods, Lymnaeus, &c.) they appear,
by a secondary modification in the development, to originate by
a differentiation of a solid mass of epiblast.
 
According to Fol the otocysts in Gasteropods are formed by
cells of the wall of the auditory sacks ; and the same appears to
hold good for Cephalopoda (Grenacher) 1 shewing that free otoliths
have in these instances originated from otoliths originally placed
in cells.
 
Crustacea. In the decapodous Crustacea organs, which have been
experimentally proved to be true organs of hearing, are usually present on
the basal joint of the anterior antennae. They may have (Hensen, No. 384)
the form either of closed or of open sacks, lined by an invagination of the
epidermis. They are provided with chitinous auditory hairs and free otoliths.
In the case of the open sacks the otoliths appear to be simply stones transported into the interior of the sacks, but in the closed sacks the otoliths,
though free, are no doubt developed within the sacks.
 
The Schizopods, which, as mentioned in the last chapter, are remarkable
as containing a genus (Euphausia) with abnormally situated eyes, distinguish
themselves again with reference to their auditory organs, in that another
genus (Mysis) is characterized by the presence of a pair of auditory sacks in
the inner plates of the tail. These sacks have curved auditory hairs supporting an otolith at their extremity.
 
The development of the auditory organs in the Crustacea has not been
investigated.
 
The Vertebrata. The Cephalochorda are without organs
of hearing, and the auditory organ of the Urochorda is constructed
on a special type of its own. The primitive auditory organs of
the true Vertebrata have the same fundamental characters as
those of the majority of aquatic invertebrate forms. They consist
of a vesicle, formed by the invagination of a patch of epiblast,
and usually shut off from the exterior, but occasionally (Elasmo
1 For the somewhat complicated details as to the development of the auditory
sacks of Cephalopods I must refer the reader to Vol. II., pp. 278, 279, and to
Grenacher (Vol. i., No. 280).
 
 
 
AUDITORY ORGANS.
 
 
 
517
 
 
 
branchii) remaining open. The walls of this vesicle are always
much complicated and otoliths of various forms are present in its
cavity. To this vesicle accessory structures, derived from the
walls of the hyomandibular cleft, are added in the majority of
terrestrial Vertebrata.
 
The development of the true auditory vesicle will be considered
separately from that of the accessory structures derived from the
hyomandibular cleft.
 
In all Vertebrata the development of the auditory vesicle
commences with the formation of a thickened patch of epiblast,
at the side of the hind-brain, on the
level of the second visceral cleft.
 
t.v.v
 
This patch soon becomes invaginated
in the form of a pit (fig. 299, aup), to
the inner side of which the ganglion
of the auditory nerve (ami), which as
shewn in a previous chapter is primitively a branch of the seventh nerve,
closely applies itself.
 
In those Vertebrata (viz. Teleostei, Lepidosteus and Amphibia) in which the epiblast is early divided into a nervous and
epidermic stratum, the auditory pit arises
as an invagination of the nervous stratum
only, and the mouth of the auditory pit is
always closed -(fig. 300) by the epidermic
stratum of the skin. Since the opening of
the pit is retained through life in Elasmobranchii the closed form of pit in the above
forms is clearly secondary.
 
In Teleostei the auditory pit arises as a
solid invagination of the epiblast.
 
 
 
 
T/t,
 
 
 
FIG. 299. SECTION THROUGH
THE HEAD OF AN ELASMOBRANCH
EMBRYO, AT THE LEVEL OF THE
AUDITORY INVOLUTION.
 
aup. auditory pit; aun. ganglion of auditory nerve ; iv.v. roof
of fourth ventricle; a.c.v. anterior
cardinal vein; aa. aorta; I.aa.
aortic trunk of mandibular arch ;
pp. head cavity of mandibular
arch ; Ivc. alimentary pouch which
will form the first visceral cleft;
77?. rudiment of thyroid body.
 
 
 
The mouth of the auditory vesicle gradually narrows, and in most
 
forms soon becomes closed, though in Elasmobranchii it remains
permanently open. In any case the vesicle is gradually removed
from the surface, remaining connected with it by an elongated
duct, either opening on the dorsal aspect of the head (Elasmobranchii), or ending blindly close beneath the skin.
 
In all Vertebrata the auditory vesicle undergoes further
 
 
 
5 i8
 
 
 
AUDITORY ORGANS OF THE VERTEBRATA.
 
 
 
changes of a complicated kind. In the Cyclostomata these
changes are less complicated than in other forms, though whether
this is due to degeneration, or to the retention of a primitive
 
 
 
 
FIG. 300. SECTION THROUGH THE HEAD OK A LEPIUOSTEUS EMBRYO ON
 
THE SIXTH DAY AFTER IMPREGNATION.
au.v. auditory vesicle ; au.n. auditory nerve ; ch. notochord ; hy. hypoblast.
 
state of the auditory organ, is not known. In the Lamprey the
auditory vesicle is formed in the usual way by an invagination
 
 
 
cv
 
 
 
 
cc
 
 
 
AOA
 
FIG. 301. SECTION THROUGH THE HIND-BRAIN OK A CHICK AT THE END
OF THE THIRD DAY OF INCUBATION.
 
IV. fourth ventricle. The section shews the very thin roof and thicker sides of
the ventricle. Ch. notochord ; C V. anterior cardinal vein; CC. involuted auditory
vesicle (CC points to the end which will form the cochlear canal) ; RL. recessus
labyrinthi (remains of passage connecting the vesicle with the exterior) ; hy. hypoblast
lining the alimentary canal; AO., AO.A. aorta, and aortic arch.
 
 
 
AUDITORY ORGANS. 519
 
 
 
of the epiblast, which soon becomes vesicular, and for a considerable period retains a simple character. As pointed out by Max
Schultze, a number of otoliths appears in the vesicle during
larval life, and, although such otoliths are stated by J. Miiller to
be absent both in the full-grown Ammoccete and in the adult,
they have since been found by Ketel (No. 387). The formation
of the two semicircular canals has not been investigated.
 
In all the higher Vertebrates the changes of the auditory
sacks are more complicated. The ventral end of the sack is
produced into a short process (fig. 301, CC}\ while at the dorsal
end there is the canal-like prolongation of the lumen of the sack
(RL}, derived from the duct which primitively opened to the
exterior, and which in most cases persists as a blind diverticulum
of the auditory sack, known as the recessus labyrinthi or
aqueductus vestibuli. The parts thus indicated give rise to
the whole of the membranous labyrinth of the ear. The main
body of the vesicle becomes the utriculus and semicircular canals,
while the ventral process forms the sacculus hemisphericus and
cochlear canal.
 
The growth of these parts has been most fully studied in
Mammalia, where they reach their greatest complexity, and it
will be convenient to describe their development in this group,
pointing out how they present, during some of the stages in their
growth, a form permanently retained in lower types.
 
The auditory vesicle in Mammalia is at first nearly spherical,
and is imbedded in the mesoblast at the side of the hind-brain.
It soon becomes triangular in section, with the apex of the triangle pointing inwards and downwards. This apex gradually
elongates to form the rudiment of the cochlear canal and sacculus
hemisphericus (fig. 302, CC). At the same time the recessus
labyrinthi (R.L) becomes distinctly marked, and the outer wall
of the main body of the vesicle grows out into two protuberances,
which form the rudiments of the vertical semicircular canals
( V.B}. In the lower forms (fig. 305) the cochlear process of the
vestibule hardly reaches a higher stage of development than that
found at this stage in Mammalia.
 
The parts of the auditory labyrinth thus established soon
increase in distinctness (fig. 303) ; the cochlear canal (CC}
becomes longer and curved ; its inner and concave surface being
 
 
 
520
 
 
 
AUDITORY ORGANS OF THE MAMMALIA.
 
 
 
lined by a thick layer of columnar epiblast. The recessus labyrinthi also increases in length, and just below the point where
the bulgings to form the vertical semicircular canals are situated,
there is formed a fresh protuberance for the horizontal semi
 
 
V.B
 
 
 
 
FIG. 302. TRANSVERSE SECTION OF THE HEAD OF A FCETAL SHEEP (16 MM. IN
LENGTH) IN THE REGION OF THE HIND-BRAIN. (After Bottcher.)
 
HB. the hind -brain.
 
The section is somewhat oblique, hence while on the right side the connections of
the recessus vestibuli R.L., and of the commencing vertical semicircular canal V.B.,
and of the ductus cochlearis CC., with the cavity of the primary otic vesicle are seen :
on the left side, only the extreme end of the ductus cochlearis CC, and of the semicircular canal V.B. are shewn.
 
Lying close to the inner side of the otic vesicle is seen the cochlear ganglion GC ;
on the left side the auditory nerve G and its connection N with the hind-brain are also
shewn.
 
Below the otic vesicle on either side lies the jugular vein.
 
circular canal. At the same time the central parts of the walls
of the flat bulgings of the vertical canals grow together, obliterating this part of the lumen, but leaving a canal round the
periphery ; and, on the absorption of their central parts, each of
the original simple bulgings of the wall of the vesicle becomes
converted into a true semicircular canal, opening at its two
extremities into the auditory vesicle. The vertical canals are
first established and then the horizontal canal.
 
 
 
AUDITORY ORGANS.
 
 
 
521
 
 
 
Shortly after the formation of the rudiment of the horizontal
semicircular canal a slight protuberance becomes apparent on the
 
 
 
 
FIG. 303. SECTION OF THE HEAD OF A FCETAL SHEEP 20 MM. IN LENGTH.
 
(After Bottcher.)
 
R. V. recessus labyrinthi ; V.B. vertical semicircular canal ; H.B. horizontal semicircular canal; C.C. cochlear canal ; G. cochlear ganglion.
 
inner commencement of the cochlear canal. A constriction arises
on each side of the protuberance, converting it into a prominent
hemispherical projection, the sacculus hemisphericus (fig. 304,
S.R\
 
The constrictions are so deep that the sacculus is only connected with the cochlear canal on the one hand, and with the
general cavity of the auditory vesicle on the other, by, in each
case, a narrow though short canal.
 
The former of these canals (fig. 304, b) is known as the canalis
reuniens. At this stage we may call the remaining cavity of the
original otic vesicle, into which all the above parts open, the utriculus.
 
Soon after the formation of the sacculus hemisphericus, the
 
 
 
522 AUDITORY ORGANS OF THE MAMMALIA.
 
cochlear canal and the semicircular canals become invested with
cartilage. The recessus labyrinthi remains however still enclosed
in undifferentiated mesoblast
 
Between the cartilage and the parts which it surrounds there
remains a certain amount of indifferent connective tissue, which
is more abundant around the cochlear canal than around the
semicircular canals.
 
As soon as they have acquired a distinct connective-tissue
coat, the semicircular canals begin to be dilated at one of their
terminations to form the ampullae. At about the same time a
constriction appears opposite the mouth of the recessus labyrinthi,
which causes its opening to be divided into two branches one
towards the utriculus and the other towards the sacculus hemisphericus ; and the relations of the parts become so altered that
communication between the sacculus and utriculus can only take
place through the mouth of the recessus labyrinthi (fig. 305).
 
When the cochlear canal has come to consist of two and a
half coils, the thickened epithelium which lines the lower surface
of the canal forms a double ridge from which the organ of Corti
is subsequently developed. Above the ridge there appears a
delicate cuticular membrane, the membrane of Corti or membrana tectoria.
 
The epithelial walls of the utricle, the recessus labyrinthi, the
semicircular canals, and the cochlear canal constitute together the
highly complicated product of the original auditory vesicle. The
whole structure forms a closed cavity, the various parts of which
are in free communication. In the adult the fluid present in this
cavity is known as the endolymph.
 
In the mesoblast lying between these parts and the cartilage,
which at this period envelopes them, lymphatic spaces become
established, which are partially developed in the Sauropsida, but
become in Mammals very important structures.
 
They consist in Mammals partly of a space surrounding the
utricle and semicircular canals, and partly of two very definite
channels, which largely embrace between them the cochlear canal.
The latter channels form the scala vestibuli on the upper side
of the cochlear canal and the scala tympani on the lower. The
scala vestibuli is in free communication with the lymphatic cavity
surrounding the vestibule, and opens at the apex of the cochlea
 
 
 
AUDITORY ORGANS.
 
 
 
523
 
 
 
into the scala tympani. The latter ends blindly at the fenestra
rotunda.
 
The fluid contained in the two scalae, and in the remaining
lymphatic cavities of the auditory labyrinth, is known as perilymph.
 
The cavities just spoken of are formed by an absorption of
 
 
 
Ch.
 
 
 
JUB
 
 
 
C.C
 
 
 
FIG. 304. SECTION THROUGH THE INTERNAL EAR OF AN EMBRYONIC SHEEP
28 MM. IN LENGTH. (After Bottcher.)
 
D.M. dura mater; R. V. recessus labyrinthi ; H.V.B. posterior vertical semicircular canal ; U. utriculus ; H.B. horizontal semicircular canal; b. canalis reuniens ;
a. constriction by means of which the sacculus hemisphericus S.R. is formed ; f.
narrowed opening between sacculus hemisphericus and utriculus ; C. C. cochlea ;
C.C. lumen of cochlea; K.K. cartilaginous capsule of cochlea; K.B. basilar plate;
Ch. notochord.
 
 
 
524 ORGAN OF CORTI.
 
 
 
parts of the embryonic mucous tissue between the perichondrium
and the walls of the membranous labyrinth.
 
The scala vestibuli is formed before the scala tympani, and
both scalae begin to be developed at the basal end of the cochlea :
the cavity of each is continually being carried forwards towards
the apex of the cochlear canal by a progressive absorption of the
mesoblast. At first both scalae are somewhat narrow, but they
soon increase in size and distinctness.
 
The cochlear canal, which is often known as the scala media
of the cochlea, becomes compressed on the formation of the
scalae so as to be triangular in section, with the base of the triangle
outwards. This base is only separated from the surrounding
cartilage by a narrow strip of firm mesoblast, which becomes the
stria vascularis, etc. At the angle opposite the base the canal
is joined to the cartilage by a narrow isthmus of firm material,
which contains nerves and vessels. This isthmus subsequently
forms the lamina spiralis, separating the scala vestibuli from
the scala tympani.
 
The scala vestibuli lies on the upper border of the cochlear
canal, and is separated from it by a very thin layer of mesoblast,
bordered on the cochlear aspect by flat epiblast cells. This membrane is called the membrane ofReissner. The scala tympani
is separated from the cochlear canal by a thicker sheet of mesoblast, called the basilar membrane, which supports the organ
of Corti and the epithelium adjoining it. The upper extremity
of the cochlear canal ends in a blind extremity called the cupola,
to which the two scalae do not for some time extend. This
condition is permanent in Birds, where the cupola is represented
by a structure known as the lagena (fig. 305, II. L}. Subsequently the two scalae join at the extremity of the cochlear canal ;
the point of the cupola still however remains in contact with the
bone, which has now replaced the cartilage, but at a still later
period the scala vestibuli, growing further round, separates the
cupola from the adjoining osseous tissue.
 
The ossification around the internal ear is at first confined to the
cartilage, but afterwards extends into the thick periosteum between the
cartilage and the internal ear, and thus eventually makes its way into the
lamina spiralis, etc.
 
The organ of Corti. In Mammalia there is formed from the
 
 
 
AUDITORY ORGANS.
 
 
 
525
 
 
 
epithelium of the cochlear canal a very remarkable organ known as the organ
of Corti, the development of which is of sufficient importance to merit a
brief description. A short account of this organ in the adult state may
facilitate the understanding of its development.
 
The cochlear canal is bounded by three walls, the outer one being the
osseous wall of the cochlea. The membrane of Reissner bounds it towards
 
 
 
 
U
 
 
 
FIG. 305. DIAGRAMS OF THE MEMBRANOUS LABYRINTH. (From Gegenbaur.)
 
I. Fish. II. Bird. III. Mammal.
 
U. utriculus ; S. sacculus ; US. utriculus and sacculus ; Cr. canalis reuniens ;
R. recessus labyrinthi ; UC. commencement of cochlea ; C. cochlear canal ; L. lagena ;
PC. cupola at apex of cochlear canal; V. csecal sack of the vestibulum of the cochlear
canal.
 
the scala vestibuli, and the basilar membrane towards the scala tympani.
This membrane stretches from the margin of the lamina spiralis to the
ligamentum spirale ; the latter being merely an expanded portion of the
connective tissue lining the osseous cochlea.
 
The lamina spiralis is produced into two lips, called respectively the
labium tympanicum and labium vestibulare ; it is to the former and
longer of these that the basilar membrane is attached. At the margin of the
junction of the labium tympanicum with the basilar membrane the former is
perforated for the passage of the nervous fibres, and this region is called the
habenula perforata.
 
The labium vestibulare, so called from its position, is shorter than the
labium tympanicum and is raised above into numerous blunt teeth. Partly
springing out from the labium vestibulare, and passing from near the inner
attachment of the membrane of Reissner towards the outer wall of the
cochlea, is an elastic membrane, the membrana tectoria. Resting on the
basilar membrane is the organ of Corti.
 
Considering for the moment that a transverse section of the cochlear
 
 
 
$26 ORGAN OF CORTI.
 
 
 
canal only one cell deep is being dealt with, the organ of Corti will be found
to consist of a central part composed of two peculiarly shaped rods widely
separated below, but in contact above. These are the rods or fibres of
Corti. On their outer side, i.e. on the side towards the osseous wall of the
canal, is a reticulate membrane which passes from the inner rod of Corti
towards the osseous wall of the canal. With their upper extremities fixed in
that membrane, and their lower resting on the basilar membrane are three
(four in man) cells with auditory hairs known as the outer 'hair cells,'
which alternate with three other cells known as Deiters' cells. Between
these and the outer attachment of the basilar membrane is a series of cells
gradually diminishing in height in passing outwards. On the inner side of
the rods of Corti is one hair cell, and then a number of peculiarly modified
cells which fill up the space between the two lips of the lamina spiralis.
 
It will not be necessary to say much in reference to the development of
the labium tympanicum and the labium vestibulare.
 
The labium vestibulare is formed by a growth of the connective tissue
which fuses with and passes up between the epithelial cells. The epithelial
cells which line its upper (vestibular) border become modified, and remain
as its teeth.
 
The labium tympanicum is formed by the coalescence of the connective
tissue layer separating the scala tympani from the cochlear canal with part
of the connective tissue of the lamina spiralis. At first these two layers are
separate, and the nerve fibres to the organ of Corti pass between them.
Subsequently however they coalesce, and the region where they are
penetrated by the nervous fibres becomes the habenula perforata.
 
The organ of Corti itself is derived from the epiblast cells lining the
cochlear canal, and consists in the first instance of two epithelial ridges or
projections. The larger of them forms the cells on the inner side of the
organ of Corti, and the smaller the rods of Corti together with the inner and
outer hair cells and Deiters' cells.
 
At first both these ridges are composed of simple elongated epithelial
cells one row deep. The smaller ridge is the first to shew any change. The
cells adjoining the larger ridge acquire auditory hairs at their free extremities,
and form the row of inner hair cells ; the next row of cells acquires a broad
attachment to the basilar membrane, and gives origin to the inner and outer
rods of Corti.
 
Outside the latter come several rows of cells adhering together so as to
form a compact mass which is quadrilateral in section. This mass is
composed of three upper cells with nuclei at the same level, which form the
outer hair cells, each of them ending above in auditory hairs, and three
lower cells which form the cells of Deiters. Beyond this the cells gradually
pass into ordinary cubical epithelial cells.
 
As just mentioned, the cells of the second row, resting with their broad
bases on the basilar membrane, give rise to the rods of Corti. The breadth
of the bases of these cells rapidly increases, and important changes take
place in the structure of the cells themselves.
 
 
 
AUDITORY ORGANS. 527
 
The nucleus of each cell divides ; so that there come to be two nuclei or
sometimes three which lie close together near the base of the cell. Outside
the nuclei on each side a fibrous cuticular band appears. The two bands
pass from the base of the cell to its apex, and there meet though widely
separated below. The remaining contents of the cell, between the two
fibrous bands, become granular, and are soon to a great extent absorbed ;
leaving at first a round, and then a triangular space between the two fibres.
The two nuclei, surrounded by a small amount of granular matter, come to
lie, each at one of the angles between the fibrous bands and the basilar
membrane.
 
The two fibrous bands become, by changes which need not be described
in detail, converted into the rods of Corti each of their upper ends growing
outwards into the processes which the adult rods possess.
 
Each pair of rods of Corti is thus (Bottcher) to be considered as the
product of one cell ; and the nuclei embedded in the granular mass between
them are merely the remains of the two nuclei formed by the division of the
original nucleus of that cell 1 . The larger ridge is for the most part not
permanent, and from being the most conspicuous part of the organ of Corti
comes to be far less important than the smaller ridge. Its cells undergo a
partial degeneration ; so that the epithelium in the hollow between the two
lips of the lamina spiralis, which is derived from the larger ridge, comes to
be composed of a single row of short and broad cells. In the immediate
neighbourhood however of the inner hair cell, one or two of the cells derived
from the larger ridge are very much elongated.
 
The membrana reticularis is a cuticular structure derived from the parts
to which it is attached. .
 
Accessory structures connected with the organ of hearing- in
Terrestrial Vertebrata.
 
In all the Amphibia, Sauropsida and Mammalia, except the
Urodela and a few Anura and Reptilia, the first visceral or hyomandibular cleft enters into intimate relations with the organs
of hearing, and from it and the adjoining parts are formed the
tympanic cavity, the Eustachian tube, the tympanic membrane
and the meatus auditorius externus. The tympanic membrane
serves to receive from the air the sound vibrations, which are
communicated to fluids contained in the true auditory labyrinth
by one ossicle or by a chain of auditory ossicles.
 
The addition to the organ of hearing of a tympanic membrane
to receive aerial sound vibrations is an interesting case of the
 
1 It is not clear from Bottcher's description how it comes about that the inner rods
of Corti are more numerous than the outer.
 
 
 
528 THE TYMPANIC CAVITY.
 
adaptation of a structure, originally required for hearing in
water, to serve for hearing in air ; and as already pointed out,
the similarity of this membrane to the tympanic membrane of
some Insects is also striking.
 
There is much that is obscure with reference to' the actual
development of the above parts of the ear, which has moreover
only been carefully studied in Birds and Mammals.
 
The Eustachian tube and tympanic cavity seem to be derived
from the inner part of the first visceral or hyomandibular cleft,
the external opening of which becomes soon obliterated. Kolliker holds that the tympanic cavity is simply a dorsally and
posteriorly directed outgrowth of the median part of the inner
section of this cleft; while Moldenhauer (No. 392) holds, if I
understand him rightly, that it is formed as an outgrowth of a
cavity called by him the sulcus tubo-tympanicus, derived from
the inner aperture of the first visceral cleft together with the
groove of the pharynx into which it opens ; and Moldenhauer is
of opinion that the greater part of the original cleft atrophies.
 
The meatus auditorius externus is formed at the region of a
shallow depression where the closure of the first visceral cleft
takes place. It is in part formed by the tissue surrounding this
depression growing up in the form of a wall, and Moldenhauer
believes that this is the whole process. Kolliker states however
that the blind end of the meatus becomes actually pushed in
towards the tympanic cavity.
 
The tympanic membrane is derived from the tissue which
separates the meatus auditorius externus from the tympanic
cavity. This tissue is obviously constituted of an hypoblastic
epithelium on its inner aspect, an epiblastic epithelium on its
outer aspect, and a layer of mesoblast between them, and these
three layers give rise to the three layers of which this membrane
is formed in the adult. During the greater part of fcetal life it
is relatively very thick, and presents a structure bearing but
little resemblance to that in the adult.
 
A proliferation of the connective tissue-cells in the vicinity of
the tympanic cavity causes in Mammalia the complete or nearly
complete obliteration of the cavity during fcetal life.
 
The tympanic cavity is bounded on its inner aspect by the
osseous investment of the internal ear, but at one point, known
 
 
 
AUDITORY ORGANS. 529
 
 
 
as the fenestra ovalis, the bone is deficient in the Amphibia,
Sauropsida and Mammalia, and its place is taken by a membrane ; while in Mammalia and Sauropsida a second opening,
the fenestra rotunda, is also present.
 
These two fenestrae appear early, but whether they are
formed by an absorption of the cartilage, or by the nonchondrification of a small area, is not certainly known. The upper of
the two, or fenestra ovalis, contains the base of a bone, known
in the Sauropsida and Amphibia as the columella. The main
part of the columella is formed of a stalk which is held by
Parker to be derived from part of the skeleton of the visceral
arches, but its nature is discussed in connection with the skeleton,
while the base, forming the stapes, appears to be derived from
the wall of the periotic cartilage.
 
In all Amphibia and Sauropsida with a tympanic cavity, the
stalk of the columella extends to the tympanic membrane ; its
outer end becoming imbedded in this membrane, and serving to
transmit the vibrations of the membrane to the fluid in the
internal ear. In Mammalia there is a stapes not directly
attached to the tympanic membrane by a stalk, and two additional auditory ossicles, derived from parts of the skeleton of the
visceral arches, are placed between the stapes and the tympanic
membrane. These ossicles are known as the malleus and incus,
and the chain of the three ossicles replaces physiologically the
single ossicle of the lower forms.
 
These ossicles are at first imbedded in the connective tissue
in the neighbourhood of the tympanic cavity, but on the full
development of this cavity, become apparently placed within
it ; though really enveloped in the mucous membrane lining it.
 
The fenestra ovalis is in immediate contiguity with the walls
of the utricle, while the fenestra rotunda adjoins the scala
tympani.
 
Hunt (No. 391) holds, from his investigations on the embryology of
the pig, that " the Eustachian tube is an involution of the pharyngeal
mucous membrane ;" and that "the meatus is an involution of the integument " while " the drum is formed by the Eustachian tube overlapping the
extremity of the meatus." Urbantschitsch also holds that the first visceral
cleft has nothing to do with the formation of the tympanic cavity and
Eustachian tube, and that these parts are derived from lateral outgrowths
of the oral cavity.
 
B. III. 34
 
 
 
530 THE TYMPANIC CAVITY.
 
The evolution of the accessory parts of the ear would be very difficult
to explain on Darwinian principles if the views of Hunt and Urbantschitsch
were correct ; and the accepted doctrine, originally proposed by Huschke
(No. 389), according to which these structures have originated by a ' change
of function' of the parts of the first visceral cleft, may fairly be held till
more conclusive evidence has been brought against it than has yet been
done.
 
Tunicata. The auditory organ of the Tunicata (fig. 306) is
placed on the under surface of the anterior vesicle of the brain.
 
 
 
 
FIG. 306. LARVA OF ASCIDIA MENTULA. (From Gegenbaur ; after Kupffer.)
 
Only the anterior part of the tail is represented.
 
N'. anterior swelling of neural tube ; IV. anterior swelling of spinal portion of
neural tube; n. hinder part of neural tube; ch. notochord ; A", branchial region
of alimentary tract ; d. cesophageal and gastric region of alimentary tract ; O. eye ;
a, otolith ; o. mouth ; s. papilla for attachment.
 
It consists of two parts (i) a prominence of the cells of the floor
of the brain forming a crista acustica, and (2) an otolith projecting into the cavity of the brain, and attached to the crista by
delicate hairs.
 
The crista acustica is formed of very delicate cylindrical
cells, and in its most projecting part is placed a vesicle with
clear contents. The otolith is an oval body with its dorsal half
pigmented, and its ventral half clear and highly refractive. It
is balanced on the highest point of the crista.
 
The crista acustica would seem to be developed from the
cells of the lower part of the front vesicle of the brain. The
otolith however is developed from a single cell on the dorsal and
right side of the brain. This cell commences to project into the
cavity of the brain and its free end becomes pigmented. It
gradually grows inwards till it forms a spherical prominence in
the cavity of the brain, to the wall of which it is attached by a
 
 
 
AUDITORY ORGANS. 531
 
 
 
stalk. At the same time it travels round the right side of the
vesicle of the brain (in a way not fully explained) till it reaches
the summit of the crista, which has become in the meantime
established.
 
The auditory organ of the simple Ascidians can hardly be
brought into relation with that of the other Chordata, and has
most probably been evolved within the Tunicate phylum.
 
BIBLIOGRAPHY.
 
Invertebrata.
 
(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeit.f. wiss.
ZooL, Vol. xm. 1863.
 
(385) O. and R. Hertwig. Das Nervensystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.
 
Vertebrata.
 
(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d.
kaiserl. Leop. Carol. Akad. d. Wissenschaft., Vol. xxxv.
 
(387) C. H asse. Die vergleich. Morphologic u. Histologied. hiiutigen Gehororgane
d. Wirbelthiere. Leipzig, 1873.
 
(388) V. Hensen. "Zur Morphologic d. Schnecke." Zeit. f. wiss. ZooL, Vol.
XIII. 1863.
 
(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim
bebriiteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. vi.
 
(390) Reissner. De Auris internes formatione. Inaug. Diss. Dorpat, 1851.
 
Accessory parts of Vertebrate Ear.
 
(391) David Hunt. "A comparative sketch of the development of the ear and
eye in the Pig. " Transactions of the International Otological Congress, \ 876.
 
(392) W. Moldenhaueir. "Zur Entwick. d. mittleren u. ausseren Ohres."
Morphol. Jahrbuch) Vol. III. 1877.
 
(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trommelfelles." Mittheil. a. d. embryol. Instit. Wien, Heft i. 1877.
 
Olfactory organ.
 
Amongst the Invertebrata numerous sense organs have been
described under the title of olfactory organs. In aquatic animals
they often have the form of ciliated pits or grooves, while in the
Insects and Crustacea delicate hairs and other structures present
on the antennae are usually believed to be organs of smell. Our
knowledge of all these organs is however so vague that it
 
342
 
 
 
532
 
 
 
OLFACTORY PIT.
 
 
 
would not be profitable to deal with them more fully in this
place. Amongst the Chordata there are usually well developed
olfactory organs.
 
Amongst the Urochorda (Tunicata) it is still uncertain what
organs (if any) deserve this appellation. The organ on the
dorsal side of the opening of the respiratory pharynx may very
possibly have an olfactory function, but it is certainly not homologous with the olfactory pits of the true Vertebrata, and as
mentioned above (pp. 436 and 437), may perhaps be homologous
with the pituitary body.
 
In the Cephalochorda (Amphioxus) there is a shallow ciliated
pit, discovered by Kolliker, which is situated on the left side of
the head, and is closely connected with a special process of the
 
 
 
 
FlG. 307. VIEWS OF THE HEAD OF ELASMOBRANCH EMBRYOS AT TWO STAGES
AS TRANSPARENT OBJECTS.
 
A. Pristiurus embryo of the same stage as fig. 28 F.
 
B. Somewhat older Scyllium embryo.
 
///. third nerve ; V. fifth nerve ; VII. seventh nerve ; au.n. auditory nerve ; gl.
glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. midbrain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op.
eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; kt. heart;
Vc. visceral clefts; eg. external gills; //. sections of body cavity in the head.
 
 
 
OLFACTORY ORGANS. 533
 
front end of the brain. It is most probably the homologue of
the olfactory pits of the true Vertebrata.
 
In the true Vertebrata the olfactory organ has usually the
form of a pair of pits, though in the Cyclostomata the organ is
unpaired.
 
In all the Vertebrata with two olfactory pits these organs
are formed from a pair of thickened patches of the epiblast, on
the under side of the fore-brain, immediately in front of the
mouth (fig. 307, ol). Each thickened patch of epiblast soon
becomes involuted as a pit (fig. 308, N), the lining cells of
which become the olfactory or Schneiderian epithelium. The
surface of this epithelium is usually much increased by various
foldings, which in the Elasmobranchii arise very early, and are
bilaterally symmetrical, diverging on each side like the barbs of
a feather from the median line. They subsequently become
very pronounced (fig. 309), serving greatly to increase the
surface of the olfactory epithelium. At a very early stage the
olfactory nerve attaches itself to the olfactory epithelium.
 
In Petromyzon the olfactory organ arises as an unpaired thickening of
the epiblast, which in the just hatched larva forms a shallow pit, on the
ventral side of the head, immediately in front of the mouth. This pit
rapidly deepens, and soon extends itself backwards nearly as far as the
infundibulum (fig. 310, 0!}. By the development of the upper lip the opening
of the olfactory pit is gradually carried to the dorsal surface of the head, and
becomes at the same time narrowed and ciliated (fig. 47, ol). The whole
organ forms an elongated sack, and in later stages becomes nearly divided
by a median fold into two halves.
 
It is probable that the unpaired condition of the olfactory organ in the
Lamprey has arisen from the fusion of two pits into one ; there is however
no evidence of this in the early development ; but the division of the sack
into two halves by a median fold may be regarded as an indication of such a
paired character in the later stages.
 
In Myxine the olfactory organ communicates with the mouth through
the palate, but the meaning of this communication, which does not appear
to be of the same nature as the communication between the olfactory pits
and the mouth by the posterior nares in the higher types, is not known.
 
The opening of the olfactory pit does not retain its embryonic characters. In Elasmobranchii and Chimaera it becomes
enclosed by a wall of integument, often deficient on the side of
the mouth, so that there is formed a groove leading from the
nasal pit towards the angle of the mouth. This groove is
 
 
 
534
 
 
 
EXTERNAL AND INTERNAL NARES.
 
 
 
MB.
 
 
 
:u
 
 
 
 
usually constricted in the middle, and the original single
opening of the nasal sack thus becomes nearly divided into two.
In Teleostei and Ganoids the division of the nasal opening into
two parts becomes complete, but the ventral opening is generally
carried off some distance from the mouth, and placed, by the
growth of the snout, on the upper surface of the head (figs. 54
and 68). In all these instances it is
 
/ tftM
 
probable that the dorsal opening of
the nasal sack is homologous with
the external nares, and the ventral
opening with the posterior nares of
higher types. Thus the posterior
nares would in fact seem to be represented in all Fishes by a ventral
part of the opening of the original
nasal pit which either adjoins the
border of the mouth (many Elasmobranchii) or is quite separate from
the mouth (Teleostei and Ganoidei).
In the Dipnoi, Amphibia and all the
higher types the oral region becomes
extended so as to enclose the posterior nares, and then each nasal pit
acquires two openings ; viz. one outside the mouth, the external nares,
and one within the mouth, the internal or posterior nares. In the
Dipnoi the two nasal openings are very similar to those in
Ganoidei and Teleostei, but both are placed on the under surface
of the head, the inner one being within the mouth, and the
external one is so close to the outer border of the upper lip that
it also has been considered by some anatomists to lie within the
mouth.
 
In all the higher types the nasal pits have originally only a
single opening, and the ontogenetic process by which the
posterior nasal opening is formed has been studied in the
Amniota and Amphibia. Amongst the Amniota we may take
the Chick as representing the process in a very simple form. The
general history of the process was first made out by Kolliker.
 
 
 
FIG. 308. SIDE VIEW OF THE
HEAD OF AN EMBRYO CHICK OF
THE THIRD DAY AS AN OPAQUE
OBJECT. (Chromic acid preparation.)
 
C.H. cerebral hemispheres ;
F.B. vesicle of third ventricle;
M.B. mid-brain; Cb. cerebellum;
H.B. medulla oblongata; N. nasal pit ; ot. auditory vesicle in the
stage of a pit with the opening not
yet closed up; op. optic vesicle,
with /. lens and ch.f. choroidal
fissure.
 
i F. The first visceral fold ;
above it is seen the superior maxillary process.
 
2, 3, 4 F. Second, third and
fourth visceral folds, with the
visceral clefts between them.
 
 
 
OLFACTORY ORGANS.
 
 
 
535
 
 
 
The opening of the nasal pit becomes surrounded by a ridge
except on its oral side. The deficiency of this ridge on the side
of the mouth gives rise to a kind of shallow groove leading from
 
 
 
 
FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN
EMBRYO OF SCYLLIUM. (Modified from figures by Marshall and myself.)
 
c.h. cerebral hemispheres; oLv. olfactory vesicle; olf, olfactory pit ; Seh. Schneiderian folds ; /. olfactory nerve. The reference line has been accidentally taken
through the nerve to the brain.
 
the nasal pit to the mouth. The ridge enveloping the opening
of the nasal pit next becomes prolonged along the sides of this
groove, especially on its inner one; and at the same time the
superior maxillary process grows forwards so as to bound the lower
 
 
 
ma
 
 
 
 
FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.
 
The larva had been hatched three days, and was 4-8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues.
 
c.h. cerebral hemisphere; th. optic thalamus; in. infundibulum ; pn. pineal gland;
mb. mid-brain; cb. cerebellum; md. medulla oblongata; au.v. auditory vesicle; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v. ao. ventral aorta ; ht. ventricle of heart ; ch. notochord.
 
 
 
536 EXTERNAL AND INTERNAL NARES.
 
 
 
part of its outer side. The inner and outer ridges, together
with the superior maxillary process, enclose a deep groove, connecting the original opening of the nasal pit with the mouth.
The process just described is illustrated by fig. 311 A, and it
may be seen that the ridge on the inner side of the groove
forms the edge of the fronto-nasal process (k).
 
On the sixth day (Born, 394) the sides of this groove unite
together in the middle, and convert it into a canal open at both
ends the ventral openings of the canals of the two sides being
placed just within the border of the mouth, and forming the
posterior nares ; while the external openings form the anterior
nares. The upper part of the canal, together with the original
 
 
 
 
 
FIG. 311. HEAD OF A CHICK FROM BELOW ON THE SIXTH AND SEVENTH DAYS
OF INCUBATION. (From Huxley.)
 
/". cerebral vesicles ; a. eye, in which the remains of the choroid slit can still be
seen in A ; g. nasal pits ; k. fronto-nasal process ; /. superior maxillary process ;
i. inferior maxillary process or first visceral arch; 2. second visceral arch; x. first
visceral cleft.
 
In A the cavity of the mouth is seen enclosed by the fronto-nasal process, the
superior maxillary processes and the first pair of visceral arches. At the back of it is
seen the opening leading into the throat. The nasal grooves leading from the nasal
pits to the mouth are already closed over.
 
In B the external opening of the mouth has become much constricted, but it is
still enclosed by the fronto-nasal process and superior maxillary processes above, and
by the inferior maxillary processes (first pair of visceral arches) below.
 
The superior maxillary processes have united with the fronto nasal process, along
nearly the whole length of the latter.
 
nasal pit, is alone lined by olfactory epithelium ; the remaining
epithelium of the nasal cavity being indifferent epiblastic epi
 
 
OLFACTORY ORGANS.
 
 
 
537
 
 
 
thelium. Further changes subsequently take place in connection
with the posterior nares, but these are described in the section
dealing with the mouth.
 
In Mammalia the general formation of the anterior and
posterior nares is the same as in Birds ; but, as shewn by Dursy
and Kolliker, an outgrowth from the inner side of the canal
between the two openings arises at an early period ; and
becoming separate from the posterior nares and provided with a
special opening into the mouth, forms the organ of Jacobson.
The general relations of this organ when fully formed are shewn
in fig. 312.
 
In Lacertilia the formation of the posterior nares differs in some
particulars from that in Birds (Born). A groove is formed leading from
the primitive nasal pit to the mouth, bordered on its inner side by the
swollen edge of the fronto-nasal process, and on its outer by an outernasal process ; while the superior maxillary process does not assist in
bounding it. On the inner side of the narrowest part of this groove
there is formed a large lateral diverticulum, which is lined by a continuation of the Schneiderian epithelium, and forms the rudiment of
Jacobson's organ. The nasal groove continues to grow in length, but
soon becomes converted into a canal by the junction of the outer-nasal
process with the fronto-nasal process. This canal is open at both ends :
at its dorsal end is placed the original opening
of the nasal pit, and its ventral opening is
situated within the cavity of the mouth. The
latter forms the primitive posterior nares. The
superior maxillary process soon grows inwards
on the under side of the posterior part of the
nasal passage, and assists in forming its under
wall. This ingrowth of the superior maxillary
process is the rudiment of the hard palate.
 
On the conversion of the nasal groove into
a closed passage, the opening of Jacobson's
organ into the groove becomes concealed ; and
at a later period Jacobson's organ becomes
completely shut off from the nasal cavity, and
opens into the mouth at the front end of an
elongated groove leading back to the posterior
nares.
 
In Amphibia the posterior nares are formed
in a manner very different from that of the
Amniota. At an early stage a shallow groove
is formed leading from the nasal pit to the mouth ; but this groove instead
 
 
 
 
J
 
 
 
FIG. 312. SECTION THROUGH
 
THE NASAL CAVITY AND JA
COBSON'S ORGAN. (From
Gegenbaur.)
 
sn. septum nasi ; en. nasal
cavity ; y. Jacobson's organ ;
d, edge of upper jaw.
 
 
 
538 ORGANS OF THE LATERAL LINE.
 
of forming the posterior nares soon vanishes, and by the growth of the front
of the head the nasal pits are carried farther away from the mouth.
 
The actual posterior nares are formed by a perforation in the palate,
opening into the blind end of the original nasal pit.
 
Considering that the various stages in the formation of the posterior nares
of the Amniota are so many repetitions of the adult states of lower forms, it
may probably be assumed that the mode of formation of the posterior nares
in Amphibia is secondary, as compared with that in the Amniota.
 
A diverticulum of the front part of the nasal cavity of the Anura is
probably to be regarded as a rudimentary form of Jacobson's organ.
 
BIBLIOGRAPHY.
 
(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten
Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.
 
(395) A. Kollicker. " Ueber die Jacobson'schen Organe des Menschen."
Festschrift f. Rienecker, 1877.
 
(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ."
Quart. Journ. of Micr. Science, Vol. xix., 1879.
 
Sense organs of the lateral line.
 
Although I do not propose dealing with the general development of
various sense organs of the skin, there is one set of organs, viz. that of the
lateral line, which, both from its wide extension amongst the Ichthyopsida
and from the similarity of some of its parts to certain organs found amongst
the Chastopoda 1 , has a great morphological importance.
 
The organs of the lateral line consist as a rule of canals, partly situated
in the head, and partly in the trunk. These canals open at intervals on
the surface, and their walls contain a series of nerve-endings. The
branches of the canal in the head are innervated for the most part by
the fifth pair, and those of the trunk by the nervus lateralis of the vagus
nerve. There is typically but a single canal in the trunk, the openings
and nerve-endings of which are segmentally arranged.
 
Two types of development of these organs have been found. One of
these is characteristic of Teleostei ; the other of Elasmobranchii.
 
In just hatched Teleostei, Schulze (No. 402) found that instead of the
normal canals there was present a series of sense bulbs, projecting freely
on the surface and partly composed of cells with stiff hairs. In most
 
1 The organs which resemble those of the lateral line are the remarkable sense
organs found by Eisig in the Capitellidse (Mittheil. a. d. ZooL Station zu Neapel,
Vol. I.) ; but I am not inclined to think that there is a true homology between these
organs and the lateral line of Vertebrata. It seems to me probable that the
segmentally arranged optic organs of Polyophthalmus are a special modification of the
more indifferent sense organs of the Capitellidse. The close affinity of these two
types of Chsetopods is favourable to this view.
 
 
 
SENSE ORGANS. 539
 
 
 
cases each bulb is enclosed in a delicate tube open at its free extremity ;
while the bulbs correspond in number with the myotomes. In some
Teleostei (Gobius, Esox, etc.) such sense organs persist through life ; in
most forms however each organ becomes covered by a pair of lobes of the
adjacent tissue, one formed above and the other below it. The two lobes
of each pair then unite and form a tube open at both ends. The linear
series of tubes so formed is the commencement of the adult canal ; while
the primitive sense bulbs form the sensory organs of the tubes. The
adjacent tubes partially unite into a continuous canal, but at their points
of apposition pores are left, which place the canal in communication with
the exterior.
 
Besides these parts, I have found that there is present in the just hatched
Salmon a linear streak of modified epidermis on the level of the lateral
nerve, and from the analogy of the process described below for Elasmobranchii it appears to me probable that these streaks play some part in the
formation of the canal of the lateral line.
 
In Elasmobranchii (Scyllium) the lateral line is formed as a linear
thickening of the mucous layer of the epidermis. This thickening is at
first very short, but gradually grows backwards, its hinder end forming a
kind of enlarged growing point. The lateral nerve is formed shortly after
the lateral line, and by the time that the lateral line has reached the level
of the anus the lateral nerve has grown back for about two-thirds of that
distance. The lateral nerve would seem to be formed as a branch of the
vagus, but is at first half enclosed in the modified cells of the lateral line
(fig. 275, nl) 1 , though it soon assumes a deeper position.
 
A permanent stage, more or less corresponding to the stage just described
in Elasmobranchii, is retained in Chimasra, and Echinorhinus spinosus,
where the lateral line has the form of an open groove (Solger, No. 404).
 
The epidermic thickening, which forms the lateral line, is converted
into a canal, not as in Teleostei by the folding over of the sides, but by
the formation of a cavity between the mucous and epidermic layers of the
epiblast, and the subsequent enclosure of this cavity by the modified cells
of the mucous layer of the epiblast which constitute the lateral line.
The cavity first appears at the hind end of the organ, and thence extends
forwards.
 
After its conversion into a canal the lateral line gradually recedes from
the surface ; remaining however connected with the epidermis at a series
of points corresponding with the segments, and at these points perforations
are eventually formed to constitute the segmental apertures of the system.
 
The manner in which the lumen of the canal is formed in Elasmobranchs bears the same relation to the ordinary process of conversion
of a groove into a canal that the formation of the auditory involution
 
1 Gotte and Semper both hold that the lateral nerve, instead of growing in a
centrifugal manner like other nerves, is directly derived from the epiblast of the
lateral line. For the reasons which prevent me accepting this view I must refer the
reader to my Monograph on Elasmobranch Fishes, pp. 141 146.
 
 
 
540 ORGANS OF THE LATERAL LINE.
 
in Amphibia does to the same process in Birds. In both Elasmobranchii
and Amphibia the mucous layer of the epiblast behaves exactly as does
the whole epiblast in the other types, but is shut off from the surface by
the passive epidermic layer of the epiblast.
 
The mucous canals of the head and the ampullae are formed from the
mucous layer of the epidermis in a manner very similar to the lateral line ;
but the nerves to them arise as simple branches of the fifth and seventh
nerves, which unite with them at a series of points, but do not follow
their course like the lateral nerve.
 
It is clear that the canal of the lateral line is secondary, as compared
with the open groove of Chimaera or the segmentally arranged sense bulbs
of young Teleostei ; and it is also clear that the phylogenetic mode of
formation of the canal consisted in the closure of a primitively open groove.
The abbreviation of this process in Elasmobranchii was probably acquired
after the appearance of food-yolk in the egg, and the consequent disappearance of a free larval stage.
 
While the above points are fairly obvious it does not seem easy to
decide a priori whether a continuous sense groove or isolated sense bulbs
were the primitive structures from which the canals of the lateral line
took their origin. It is equally easy to picture the evolution of the canal of
the lateral line either from (i) a continuous unsegmented sense line, certain
points of which became segmentally differentiated into special sense bulbs,
while the whole subsequently formed a groove and then a canal ; or from
(2) a series of isolated sense bulbs, for each of which a protective groove
was developed ; and from the linear fusion of which a continuous canal
became formed.
 
From the presence however of a linear streak of modified epidermis
in larval Teleostei, as well as in Elasmobranchii, it appears to me more
probable that a linear sense streak was the primitive structure from which
all the modifications of the lateral line took their origin, and that the
segmentally arranged sense bulbs of Teleostei are secondary differentiations
of this primitive structure.
 
The, at first sight remarkable, distribution of the vagus nerve to the
lateral line is probably to be explained in connection with the evolution
of this organ. As is indicated both by its innervation from the vagus,
as also from the region where it first becomes developed, the lateral line
was probably originally restricted to the anterior part of the body. As it
became prolonged backwards it naturally carried with it the vagus nerve,
and thus a sensory branch of this nerve has come to innervate a region
which is far beyond the limits of its original distribution.
 
BIBLIOGRAPHY.
 
(397) F. M. Balfour. A Monograph onthe development of Elasnwbranch Fishes,
pp. 141 146. London, 1878.
 
(398) H. Eisig. "Die Segmentalorgane cl. Capitelliden." Mitthcil. a. d. zool.
Station zu Neapel> Vol. I. 1879.
 
 
 
BIBLIOGRAPHY. 541
 
 
 
(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
 
(400) Fr. Leydig. Lehrbuch d. Histologie des Memchen u. d. Thiere. Hamm.
 
1857
(401) Fr. Leydig. Neue Beitrdge z. anat. Kenntniss d. Hautdecke u. Hautsinnesorgane d. Fische. Halle, 1879.
 
(402) F. E. Schulze. " Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und
Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.
 
(403) C. Semper. "Das Urogenitalsy stem d. Selachier." Arbeit, a. d. zoo!.zoot. Instil. Wiirzburg, Vol. II.
 
(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische."
Archiv f. mikr. Anat., Vol. xvn. and xvm. 1879 an< * !88o.
 
 
 
CHAPTER XVIII.
 
 
 
THE NOTOCHORD, THE VERTEBRAL COLUMN, THE
RIBS AND THE STERNUM.
 
 
 
INTRODUCTION.
 
AMONGST the products of that part of the mesoblast which
constitutes the connective tissue of the body special prominence
must be given to the skeleton of the Vertebrata, from its importance in relation to numerous phylogenetic and morphological
problems.
 
The development of the skeleton is however so large a
subject that it cannot be satisfactorily dealt with except in a
special treatise devoted to it ; and the following description must
be regarded as a mere sketch, from which detail has been as far
as possible excluded.
 
In the lowest Chordata the sole structure present, which
deserves to be called a skeleton, is the notochord. Although
the notochord often persists as an important organ in the true
Vertebrata, yet there are always added to it various skeletal
structures developed in the mesoblast. Before entering into a
systematic description of these, it will be convenient to say a
few words as to the general characters of the skeleton.
 
Two elements, distinct both in their genesis and structure,
are to be recognized in the skeleton. The one, forming the true
primitive internal skeleton or endoskeleton, is imbedded within
the muscles and is originally formed in cartilage. In many
instances it retains a cartilaginous consistency through life, but
in the majority of cases it becomes gradually ossified, and
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN. 543
 
converted into true bone. Bones so formed are known as
cartilage bones.
 
The other element is originally formed by the fusion of the
ossified bases of the dermal placoid scales already described in
Chapter xiv., or by the fusion of the ossified bases of teeth
situated in the mucous membrane of the mouth. In both
instances the plates of bone so formed may lose the teeth or
spines with which they were in the first instance covered, either
by absorption in the individual, or phylogenetically by their
gradually ceasing to be developed. The plates of bone, which
originated by the above process, become in higher types directly
developed in the connective tissue beneath the skin ; and
gradually acquire a deeper situation, and are finally so intimately interlocked with parts of the true internal skeleton, that
the two sets of elements can only be distinguished by the fact
of the one set ossifying in cartilage and the other in membrane.
 
It seems probable that in the Reptilia, and possibly the
extinct Amphibia, dermal bones have originated in the skin
without the intervention of superjacent spinous structures.
 
In cases where a membra nebone, as the dermal ossifications are usually called, overlies a part of the cartilage, it may
set up ossification in the latter, and the cartilage bone and membrane bone may become so intimately fused as to be quite inseparable. It seems probable that in cases of this kind the
compound bone may in the course of further evolution entirely
lose either its cartilaginous element or its membranous element ;
so that cases occasionally occur where the development of a
bone ceases to be an absolutely safe guide to its evolution.
 
As to the processes which take place in the ossification of
cartilage there is still much to be made out. Two processes are
often distinguished, viz. (i) a process known as ectostosis, in
which the ossification takes place in the perichondrium, and
either simply surrounds or gradually replaces the cartilage, and
(2) a process known as endostosis, where the ossification actually
takes place between the cartilage cells. It seems probable
however (Gegenbaur, Vrolik) that there is no sharp line to be
drawn between these two processes ; but that the ossification
almost always starts from the perichondrium. In the higher
types, as a rule, the vessels of the perichondrium extend into
 
 
 
544 MEMBRANE BONES AND CARTILAGE BONES.
 
the cartilage, and the ossification takes place around these
vessels within the cartilage; but in the lower types (Pisces, Amphibia) ossification is often entirely confined to the perichondrium ; and the cartilage is simply absorbed.
 
The regions where ossification first sets in are known as
centres of ossification; and from these centres the ossification
spreads outwards. There may be one or more centres for a
bone.
 
The actual causes which in the first instance gave rise to
particular centres of ossification, or to the ossification of particular parts of the cartilage, are but little understood ; nor have
we as yet any satisfactory criterion for determining the value to
be attached to the number and position of centres of ossification.
In some instances such centres appear to have an important
morphological significance, and in other instances they would
seem to be determined by the size of the cartilage about to be
ossified.
 
There is no doubt that the membrane bones and cartilage bones can
as a rule be easily distinguished by their mode of development ; but it is
by no means certain that this is always the case. It is necessarily very
difficult to establish the homology between bones, which develop in one
type from membrane and in another type from cartilage ; but there are
without doubt certain instances in. which the homology between two bones
would be unhesitatingly admitted were it not for the difference in their
development. The most difficult cases of this kind are connected with the
shoulder-girdle.
 
The possible sources of confusion in the development of bones are
obviously two. (i) A cartilage bone by origin may directly ossify in membrane, without the previous development of cartilage, and (2) a membrane
bone may in the first instance be formed in cartilage.
 
The occurrence of the first of these is much more easy to admit than
that of the second ; and there can be little doubt that it sometimes takes
place. In a large number of cases it would moreover cause no serious
difficulty to the morphologist.
 
BIBLIOGRAPHY of the origin of the Skeleton.
 
(405) C. Gegenbaur. " Ueb. primare u. secundare Knochenbildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium." Jcnaischc Zcitschrifl, Vol. III. 1867.
 
(406) O. Hertwig. " Ueber Ban u. Entwicklung d. Placoidschuppen u. d.
Ziihne d. Selachicr." Jenaische Zeitsckrift, Vol. viu. 1874.
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
 
 
545
 
 
 
(407) O. Hertwig. " Ueb. cl. Zahnsystem d. Amphibien u. seine Bedeutung
f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supplementheft, 1874.
 
(408) O. Hertwig. " Ueber d. Hautskelet cl. Fische." Morphol. Jahrbmh,
Vol. II. 1876. (Siluroiden u. Acipenseriden.)
 
(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbnch, Vol. v. 1879.
 
(410) A. Kolliker. " Allgemeine Betrachtungen iib. die Entstehung d. knochernen Schadels d. Wirbelthiere. " Berichte r. d. kijnigl. zoot. Anstalt z. Wiirzburg,
1849.
 
(411) Fr. Leydig. " Histologische Bemerkungen lib. d. Polypterus bichir."
Zeit.f. wiss. Zool., Vol. v. 1858.
 
(412) H. Miiller. " Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc." Zeit. f. wiss. ZooL, Vol. ix. 1859.
 
(413) Williamson. "On the structure and development of the Scales and
Bones of Fishes." Phil. Trans., 1851.
 
(414) Vrolik. " Studien lib. d. Verknocherung u. die Knochen d. Schadels d.
Teleostier. " Niederliindisches Archiv f. Zoologie, Vol. I.
 
 
 
NotocJtord and Vertebral column.
 
The primitive axial skeleton of the Chordata consists of the
notochord and its sheath. It persists as such in the adult in
Amphioxus, and constitutes, in embryos of all Vertebrata, for a
considerable period of their early embryonic life, the sole representative of the axial skeleton.
 
The Notochord. The early formation of the notochord
has already been described in
detail (pp. 292 300). It is
developed, in most if not all
cases, as an axial differentiation of the hypoblast,
and forms at first a solid
cord of cells, without a
sheath, placed between the
nervous system and the dorsal wall of the alimentary
tract, and extending from
the base of the front of the
 
 
 
 
mid-brain to the end of the
tail. The section in the
region of the brain will be
dealt with by itself. That
H. HI.
 
 
 
FIG. 313. HORIZONTAL SECTION THROUGH
THE TRUNK OF AN EMBRYO OF SCYLLIUM
CONSIDERABLY YOUNGER THAN F IN FIG. 28.
 
The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.
 
ch. notochord ; ep. epiblast ; Vr. rudiment
of vertebral body; ;;//. muscle-plate; mp'.
portion of muscle-plate already differentiated
into longitudinal muscles.
 
35
 
 
 
546
 
 
 
NOTOCHORD.
 
 
 
in the trunk forms the basis round which the vertebral column
is moulded.
 
The early histological changes in the cells of the notochord
are approximately the same in all the Craniata. There is
formed by the superficial cells of the notochord a delicate sheath,
which soon thickens, and becomes a welldefined structure. Vacuoles (one or more to
each cell) are formed in the cells of the
notochord, which enlarge till the whole notochord becomes almost entirely formed of
large vacuoles separated by membranous
septa which form a complete sponge-like
reticulum 'fig. 313). In the Ichthyopsida
most of the protoplasm with the nuclei is
carried to the periphery, where it forms a
special nucleated layer sometimes divided
into definite epithelial-like cells (fig. 314),
while in the meshes of the reticulum a few
nuclei surrounded by a little protoplasm still
remain. In the Amniotic Vertebrata, probably owing to the early atrophy of the
notochord, the distribution of the nuclei in
the spaces of the mesh-work remains fairly
uniform.
 
 
 
 
FIG. 314. SECTION
THROUGH THE SPINAL
COLUMN OF A YOUNG
SALMON. (From Gegenbaur.)
 
cs. sheath of notochord ; k. neural arch ;
k'. haemal arch; m.
spinal cord; a. dorsal
aorta ; z'. cardinal
veins.
 
 
 
In the early stages of development the spaces in the notochordal spongework, each containing a nucleus and protoplasm, probably represent cells.
In the types in which the notochord persists in the adult the mesh-work
becomes highly complicated, and then forms a peculiar reticulum filled with
gelatinous material, the spaces in which do not indicate the outlines of
definite cells (figs. 315 and 318).
 
Around the sheath of the notochord there is formed in the
Cyclostomata, Ganoidei, Elasmobranchii and Teleostei an elastic
membrane usually known as the membrana elastica externa.
 
In most Vertebrates the notochord and its sheath either
atrophy completely or become a relatively unimportant part of
the axial skeleton; but in the Cyclostomata (fig. 315) and in the
Selachioidean Ganoids (Acipenser, etc.) they persist as the
sole representative of the true vertebral axis. The sheath becomes
very much thickened; and on the membrana elastica covering
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
 
 
547
 
 
 
Ch
 
 
 
it the vertebral arches directly rest. In Klasmobranchii the
sheath of the notochord undergoes a more complicated series of
changes, which result first of all in the formation of a definite
unsegmented cartilaginous tube 1
round the notochord, and subsequently (in most forms) in the
formation of true vertebral bodies.
Between the membrana elastica
externa and the sheath of the
notochord a layer of cells becomes
interposed (fig. 316, n}, which lie
in a matrix not sharply separated
from the sheath of the notochord.
The cells which form this layer
appear to be derived from a special
investment of the notochord, and
to have penetrated through the
membrana elastica externa to
reach their final situation. The
layer with these cells soon increases 7/> cardmal vems in thickness, and forms a continuous unsegmented tube of
fibrous tissue with flattened concentrically arranged nuclei (fig.
317, Vb}. Externally is placed c f,
the membrana elastica externa
(met}, while within is the cuticular
sheath of the notochord. This
tube is the cartilaginous tube
spoken of above and is known as
the cartilaginous sheath of
the notochord.
 
 
 
 
FIG. 315. SECTION THROUGH
THE VERTEBRAL COLUMN OF AMMOCCETES. (From Gegenbaur.)
 
Ch. notochord ; c s. notochordal
sheath ; m. spinal cord ; a. aorta ;
 
 
 
^ \
 
 
 
FIG. 316. LONGITUDINAL SECTION THROUGH A SMALL PART OF
THE NOTOCHORD AND ADJOINING
PARTS OF A SCYLLIUM EMBRYO, AT
THE TIME OF THE FIRST FORMATION OF THE CARTILAGINOUS
SHEATH.
 
ch. notochord; sc. sheath of notochord; n. nuclei of cartilaginous
sheath; me.e. membrana elastica
externa.
 
 
 
The exact origin of the cartilaginous
tube just described is a question of fundamental importance with reference to
the origin of the vertebral column and
the homologies of its constituent parts ;
but is by no means easy to settle. In the account of the subject in my
memoir on Elasmobranch Fishes I held with Gegenbaur that it arose from
 
1 This tube consists of a peculiar form of fibrous tissue rather than true cartilage,
though part of it subsequently becomes hyaline cartilage.
 
352
 
 
 
548
 
 
 
SHEATH OF THE NOTOCHORD.
 
 
 
a layer of cells outside the sheath of the notochord, on the exterior of which
the membrana elastica externa was subsequently formed. To this view
Gotte (No. 419) also gave his adhesion. Schneider has since (No. 429)
stated that this is not the case, but that, as described above, the membrana
elastica externa is formed before the layer of cartilage. I have since worked
over this subject again, and am on the whole inclined to adopt Schneider's
correction.
 
It follows from the above description that the cartilaginous
tube in question is an essential part of the sheath of the notochord, and that it is to some extent homologous with the notochordal sheath of the Sturgeon and the Lamprey, and not an
entirely new formation.
 
This sheath forms the basis of the centra of the future
vertebrae. In a few adult forms, i.e. Chimaera and the Dipnoi, it
 
 
 
 
FIG. 317. TRANSVERSE SECTION THROUGH THE VENTRAL PART OF THE
NOTOCHORD AND ADJOINING STRUCTURES OF AN ADVANCED SCYLLIUM EMBRYO
AT THE ROOT OF THE TAIL.
 
Vb. cartilaginous sheath of the notochord ; ha. hasmal arch ; vp. process to which
the rib is articulated ; mcl. membrana elastica externa ; ch. notochord ; ao. aorta ;
. caudal vein.
 
 
 
retains its primitive condition, except that in Chimaera there
are present delicate ossified rings more numerous than the
arches ; while in the Notidani, Laemargi and Echinorhini the
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN. 549
 
indications of vertebrae are imperfectly marked out. The further
history of this sheath in the forms in which true vertebrae are
formed can only be dealt with in connection with the formation
of the vertebral arches.
 
In Teleostei there is present, as in Elasmobranchii, an elastica externa,
and an inner notochordal sheath. The elastica externa contains, according
to Gotte, cells. These cells, if present, are however very difficult to make
out, but in any case the so-called elastica externa appears to correspond with
the cartilaginous sheath of Elasmobranchii together with its enveloping
elastica, since ossification, when it sets in, occurs in this layer. The sheath
within becomes unusually thick.
 
In the Amphibia and in the Amniota no membrane is
present which can be identified with the membrana elastica
externa of the Elasmobranchii, Teleostei, etc. In Amphibia
(Gotte) there is formed round the notochord a cellular sheath,
which has very much the relations of the cartilaginous tube
around the notochord of Elasmobranchii, and is developed in
the same way from the perichordal connective tissue cells. It
is only necessary to suppose that the rnembrana elastica externa
has ceased to be developed (which in view of its extreme delicacy
and unimportant function in Elasmobranchii is not difficult to
do) and this cellular sheath would then obviously be homologous
with the cartilaginous tube in question. In the Amniota an
external sheath of the notochord cannot be traced as a distinct
structure, but the connective tissue surrounding the notochord
and spinal cord is simply differentiated into the vertebral bodies
and vertebral arches.
 
Vertebral arches and Vertebral bodies.
 
Cyclostomata. The Cyclostomata are the most primitive
forms in which true vertebral arches are present. Their ontogeny
in this group has not been satisfactorily worked out. It is
however noticeable in connection with them that they form for
the most part isolated pieces of cartilage, the segmental
arrangement of which is only imperfect.
 
Elasmobranchii. In the Elasmobranchii the cells forming
the vertebral arches are derived from the splanchnic layer of the
mesoblastic somites. They have at first the same segmentation
 
 
 
55O NEURAL AND H^MAL ARCHES.
 
as the somites (fig. 313, Vr), but this segmentation is soon lost,
and there is formed round the notochord a continuous sheath of
embryonic connective tissue cells, which gives rise to the arches
of the vertebrae, the tissue forming the dura mater, the perichondrium, and the general investing connective tissue.
 
The changes which next follow result in what has been
known since Remak as the secondary segmentation of the
vertebral column. This segmentation, which occurs in all
Vertebrata with true vertebrae, is essentially the segmentation
of the continuous investment of the notochord and spinal cord
into vertebral bodies and vertebral arches. It does not however
follow the lines of the segmentation of the muscle-plates, but is
so effected that the centres of the vertebral bodies are opposite
the septa between the muscle-plates.
 
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 inter-muscular septa.
 
These considerations fully explain the characters of the secondary
segmentation of the vertebral column. On the other hand the primary
segmentation (fig. 313) of the vertebral rudiments is clearly a remnant
of a condition when no vertebral bodies were present ; and has no greater
morphological significance than the fact that the cells of the vertebrae
were derived from the segmented muscle-plates, and then became fused
into a continuous sheath around the notochord and nervous axis ; till
finally they became in still higher forms differentiated into vertebrae and
their arches.
 
During the stage represented in fig. 28 g, and somewhat
before the cartilaginous sheath of the notochord is formed, there
appear four special concentrations of the mesoblastic tissue
adjoining the notochord, two of them dorsal (neural) and two of
them ventral (haemal). They are not segmented, and form four
ridges, seated on the sides of the notochord. They are united
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
 
 
551
 
 
 
with each other by a delicate layer of tissue, and constitute the
substance in which the neural and haemal arches subsequently
become differentiated.
 
At about the time when the first traces of the cartilaginous
sheath of the notochord arise, differentiations take place in the
neural and haemal ridges. In the
neural ridge two sets of arches are
formed for each myotome, one
resting on the cartilaginous sheath
of the notochord in the region
which will afterwards form the centrum of a vertebra, and constituting
a true neural arch ; and a second
separate from the cartilaginous
sheath, forming an intercalated
piece 1 . Both of them soon become
hyaline cartilage.
 
There is a considerable portion
of the original tissue of the neural
ridge, especially in the immediate
neighbourhood of the notochord,
which is not employed in the formation of the neural arches. This
tissue has a fibrous character and
becomes converted into the perichondrium and other parts.
 
The haemal arches are formed
from the haemal ridge in precisely
the same way as the neural arches,
but interhsemal intercalated pieces
are often present. In the region
of the tail the haemal arches are
continued into ventral processes
which meet below, enclosing the aorta and caudal veins.
 
1 The presence of intercalated pieces in the neural arch system of Elasmobranchii,
Chimaera, etc. is probably not the indication of an highly differentiated type of
neural arch, but of a transitional type between an imperfect investment of the spinal
cord by isolated cartilaginous bars, and a complete system of neural arches like that
in the higher Vertebrata.
 
 
 
 
FIG. 318. SECTION THROUGH
THE VERTEBRAL COLUMN OF AN
ADVANCED EMBRYO OF SCYLLIUM
IN THE REGION OF THE TAIL.
 
na. neural arch ; ha. haemal
arch ; ch. notochord ; sh. inner
sheath of notochord ; ne. membrana
elastica externa.
 
 
 
552 NEURAL AND H^iMAL ARCIIKS.
 
Since primitively the postanal gut was placed between the
aorta and the caudal vein, the haemal arches potentially invest
a caudal section of the body cavity. In the trunk region they
do not meet ventrally, but give support to the ribs. The
structures just described are shewn in section in fig. 318, in
which the neural (110) and haemal (ha) arches are shewn resting
upon the cartilaginous sheath of the notochord.
 
While these changes are being effected in the arches the
cartilaginous sheath of the notochord undergoes important differentiations. In the vertebral regions opposite the origin of the
neural and haemal arches (fig. 318) its outer part becomes
hyaline cartilage, while the inner parts adjoining the notochord
undergo a somewhat different development, the notochord in this
part becomes at the same time somewhat constricted. In the
intervertebral regions the cartilaginous sheath of the notochord
becomes more definitely fibrous, while the notochord is in no
way constricted. A diagrammatic longitudinal section through
the vertebral column, while these changes are being effected, is
shewn in fig. 320 B.
 
These processes are soon carried further. The notochord
within the vertebral body becomes gradually constricted, especially in the median plane, till it is here reduced to a fibrous
band, which gradually enlarges in either direction till it reaches
its maximum thickness in the median plane of the intervertebral
region. The hyaline cartilage of the vertebral region forms a
vertebral body in which calcification may to some extent take
place. The cartilage of the base of the arches gradually spreads
over it, and on the absorption of the membrana elastica externa,
which usually takes place long before the adult state is reached,
the arch tissue becomes indistinguishably fused with that of the
vertebral bodies, so that the latter are compound structures,
partly formed of the primitive cartilaginous sheath, and partly
of the tissue of the bases of the neural and haemal arches.
Owing to the beaded structure of the notochord the vertebral bodies take of necessity a biconcave hourglass-shaped
form.
 
The intervertebral regions of the primitive sheath of the notochord form fibrous intervertebral ligaments enclosing the unconstricted intervertebral sections of the notochord.
 
 
 
NOTOCHORD AND VERTKBKAL COLUMN. 553
 
A peculiar fact may here be noticed with reference to the formation
of the vertebral bodies in the tail of Scyllium, Raja, and possibly other
forms, viz. that there are double as many -vertebral bodies as there are
myotomes and spinal nerves. This is not due to a secondary segmentation
of the vertebras but, as I have satisfied myself by a study of the development, takes place when the vertebral bodies first become differentiated.
The possibility of such a relation of parts is probably to be explained by
the fact that the segmentation of the vertebral column arose subsequently
to that of the nerves and myotomes.
 
Ganoidei. In Acipenser and other cartilaginous Ganoids
the haemal and neural arches are formed as in Elasmobranchii,
and rest upon the outer sheath of the notochord. Since however
the sheath of the notochord is never differentiated into distinct
vertebrae, this primitive condition is retained through life.
 
Teleostei. In Teleostei the formation of the vertebral arches and
bodies takes place in a manner, which can be reduced, except in certain
minor points, to the same type as that of Elasmobranchii.
 
There are early formed (fig. 314 k and k] neural and haemal arches
resting upon the outer sheath of the notochord. The latter structure,
which, as mentioned on p. 549, corresponds to the cartilaginous sheath
of the notochord of Elasmobranchii, soon becomes divided into vertebral
and intervertebral regions. In the former ossification directly sets in
without the sheath acquiring the character of hyaline cartilage (Gotte, 419).
The latter forms the fibrous intervertebral ligaments. The notochord
exhibits vertebral constrictions.
 
The ossified outer sheath of the notochord forms but a small part of
the permanent vertebrae. The remainder is derived partly from an ossification of the connective tissue surrounding the sheath, and partly from
the bases of the arches, which do not spread round the primitive vertebral
bodies as in Elasmobranchii. The ossifications in the tissue surrounding
the sheath usually (fig. 319) take the form of a cross, while the bases of
the arches (k and k'} remain as four cartilaginous radii between the limbs
of the osseous cross. In some instances the bases of the arches also become
ossified, and are then with difficulty distinguishable from the other parts
of the secondary vertebral body. The parts of the arches outside the
vertebral bodies are for the most part ossified (fig. 319). In correlation
with the vertebral constrictions of the notochord the vertebral bodies are
biconcave.
 
Amphibia. Of the forms of Amphibia so far studied
embryologically the Salamandridae present the most primitive
type of formation of the vertebral column.
 
It has already been stated that in Amphibia there is present
 
 
 
554
 
 
 
VERTEBRAL COLUMN OF AMPHIBIA.
 
 
 
around the notochord a cellular sheath, equivalent to the
cartilaginous sheath of Elasmobranchii. In the tissue on the
dorsal side of this sheath a series of cartilaginous processes
becomes formed. These processes are the commencing neural
arches ; and they rest on the cellular sheath of the notochord
opposite the middle of the vertebral regions.
 
A superficial osseous layer becomes very early formed in
each vertebral region of the cellular
sheath ; while in each of the intervertebral regions, which are considerably shorter than the vertebral,
there is developed a ring-like cartilaginous thickening of the sheath,
which projects inwards so as to
constrict the notochord. At a
period before this thickening has
attained considerable dimensions
the notochord becomes sufficiently
 
constricted in the centre of each FlG - 319- VERTICAL SECTIONTHROUGH THE MIDDLE OF A VER
vertebral region to give a biconcave TEBRA OF Esox LUCIUS (PIKE).
form to the vertebrae for a very < From Gegenbaur.)
short period of fcetal life.
 
 
 
 
The stage with biconcave vertebrae is
retained through life in the Perennibranchiata and Gymnophiona.
 
 
 
ch. notochord ; cs. notochordal
sheath; /. and K. cartilaginous
tissue of the neural and haemal
arches ; h. osseous hremal process ;
n. spinal canal.
 
 
 
The chief peculiarity which distinguishes the later history of
their vertebral column from that of fishes consists in the
immense development of the intervertebral thickenings just
mentioned, which increase to such an extent as to reduce the
notochord, where it passes through them, to a mere band ; while
the cartilage of which they are composed becomes differentiated
into two regions, one belonging to the vertebra in front, the
other to that behind, the hinder one being convex, and the
anterior concave. The two parts are not however absolutely
separated from each other.
 
By these changes each vertebra comes to be composed of (i) a
thin osseous somewhat hourglass-shaped cylinder with a dilated
portion of the notochord in its centre, and (2 and 3) of two
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
 
 
555
 
 
 
halves of two intervertebral cartilages, viz. an anterior convex
half and a posterior concave half. The vertebrae thus come to
be opisthoccelous. A longitudinal section through the vertebral
column at this stage is diagrammatically shewn in fig. 320 C.
 
To the centre of each of these vertebrae the neural arches,
the origin of which was described above, become in the
meantime firmly attached ; and grow obliquely upwards and
A B CUE
 
 
 
 
FIG. 320. DIAGRAM REPRESENTING THE MODE OF DEVELOPMENT OK THE
 
VERTEBRA IN THE DIFFERENT TYPES. (From Gegenbaur.)
 
A. Ideal type in which distinct vertebrae are not established.
 
B. Type of Pisces with vertebral constrictions of the notochord.
 
C. Amphibian type, with intervertebral constrictions of the notochord by the
intervertebral parts of the cellular sheath.
 
D. Intervertebral constriction of the notochord as effected in Reptilia and Aves.
 
E. Vertebral constriction of the notochord as effected in Mammalia, the intervertebral parts of the cartilaginous sheath being converted into intervertebral ligaments.
 
c . notochord ; cs. cuticular sheath of notochord ; s. cartilaginous sheath ; v. vertebral regions ; iv. intervertebral regions ; g, intervertebral joints.
 
backwards, so as to meet and unite above the spinal cord. The
transverse processes of the vertebrae would seem (Pick) to be
developed independently of the arches, though they very soon
fuse with them. According to Gotte the transverse processes
are double in the trunk, there being two pairs, one vertically
above the other for each vertebra. The pair on each side
eventually fuse together.
 
In the tail haemal arches are formed, which are similar in
their mode of development to the neural arches.
 
The unconstricted portion of the notochord, which persists in
each vertebra, becomes in part converted into cartilage.
 
 
 
556 VKRTKIJRAL COLUMN OF THE AMNIOTA.
 
Anura. In the Anura the process of formation of the vertebral
column is essentially the same as that in the Salamandridte. Two types may
however be observed. One of these occurs in the majority of the Anura,
and mainly differs from that in Salamandra in (i) the earlier fusion of the
arches with the cellular sheath of the notochord ; (2) the more rapid growth
of the intervertebral thickenings of the cellular sheath, which results in the
early and complete obliteration of the intervertebral parts of the notochord ;
(3) the complete division of these intervertebral thickenings into anterior
and posterior portions, which unite with and form the articular surfaces of
two contiguous vertebras. The vertebrae are moreover proccelous instead
of being opisthoccelous.
 
The unconstricted vertebral sections of the notochord always persist till
the ossification of the vertebras has taken place. In some forms they
remain through life (Rana), while in other cases they eventually either
wholly or partially disappear.
 
The second type of vertebral development is found in Bombinator,
Pseudis, Pipa, and Pelobates. In these genera the formation of the
vertebra takes place almost entirely on the dorsal side of the notochord ;
so that the latter forms a band on the ventral side of the vertebral column.
In other respects the history of the vertebral column is the same in the two
cases ; the vertebral unconstricted parts of the notochord appear however
to become in part converted into cartilage. The type of formation of
the vertebral column in these genera has been distinguished as epichordal
in contradistinction to the more normal or perichordal type.
 
Amniota. In the Amniota all trace of a distinction between
a cellular notochord sheath and an arch tissue is lost, and the
two are developed together as a continuous whole forming an
unsegmented tube round the notochord, with a neural ridge
which does not at first nearly invest the neural cord. This tube
becomes differentiated, in the manner already described for other
types, into (i) vertebral regions with true arches, and (2) intervertebral regions.
 
Reptilia. In Reptilia (Gegenbaur, No. 416) a cartilaginous
tube is formed round the notochord, which is continuous with
the cartilaginous neural arches. The latter are placed in the
vertebral regions, and in these regions ossification very early sets
in, while the notochord remains relatively unconstricted. In the
intervertebral regions the cartilage becomes thickened, as in
Amphibia, and gradually constricts the notochord. The cartilage in each of the intervertebral regions soon becomes divided
into two parts which form the articular faces of two contiguous
vertebrae.
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN. 557
 
The general character of the vertebral column on the completion of these changes is shewn in fig. 320 D. The later changes
are relatively unimportant. The constricted intervertebral
sections of the notochord rapidly disappear, while the vertebral
sections become partially converted into cartilage, and only
cease to be distinguishable at a considerably later period.
 
The ossification extends from the bodies of the vertebrae into
the arches and into the articular surfaces, so that the whole
vertebrae eventually become ossified.
 
The Ascalabotae (Geckos) present an exceptional type of vertebral
column which has many of the characters of a developmental stage in
other Lizards. The body of the vertebra is formed of a slightly hourglassshaped osseous tube, united with adjoining vertebras by a short intervertebral cartilage. There is a persistent and continuous notochord which,
owing to the small development of the intervertebral cartilages, is narrower
in the vertebral than in the intervertebral regions.
 
Aves. In Birds the cellular tube formed round the notochord
is far thicker than in the Reptilia. It is continuous in the
regions of the future vertebrae with neural arches, which do not
at first nearly enclose the spinal cord.
 
On about the fifth day, in the case of the chick, it becomes
differentiated into vertebral regions opposite the attachments of
the neural arches, and intervertebral regions between them ; the
two sets of regions being only distinguished by their histological
characters. Very shortly afterwards each intervertebral region
becomes segmented into two parts, which respectively attach
themselves to the contiguous vertebral regions. A part of each
intervertebral region, immediately adjoining the notochord, does
not however undergo this division, and afterwards gives rise to
the ligamentum suspensorium.
 
The notochord during these changes at first remains
indifferent, but subsequently, on about the seventh day in the
chick, a slight constriction of each vertebral region takes place ;
so that the vertebrae have temporarily, as they have also in
Amphibia, a biconcave form which repeats the permanent
condition of most fishes. By the ninth and tenth days, however,
this condition has completely disappeared, and in all the intervertebral portions the notochord has become distinctly constricted, and at the same time in each vertebral portion there
 
 
 
558
 
 
 
VERTEBRAL COLUMN OF MAMMALIA.
 
 
 
have also appeared two constrictions of the notochord giving
rise to a central and to two terminal enlargements.
 
On the twelfth day the ossification of the cartilaginous centra
commences.
 
The first vertebra to ossify is the second or third cervical,
and the ossification gradually extends to those behind. It does
not commence in the arches till somewhat later than in the
bodies. For each arch there are two centres of ossification, one
on each side.
 
The notochord persists for the greater part of foetal life and
even into post-fcetal life. The larger vertebral portions are
often the first completely to vanish. They would seem in many
cases at any rate (Gegenbaur) to be converted into cartilage,
and so form an integral part of the permanent vertebrae.
Rudiments of the intervertebral
portions of the notochord may
long be detected in the ligamenta
suspensoria.
 
Schwarck (No. 420) states that in both
the intervertebral and the vertebral regions,
though less conspicuously in the former,
the cartilage is divided into two layers, an
inner and an outer. He holds that the
inner layer corresponds to the cartilaginous
notochordal sheath of the lower types,
and the outer to the arch tissue. Ossification (Gegenbaur) of the centra appears
in a special inner layer of cartilage, which
is probably the same as the inner layer of
the earlier stage, though this point has
not been definitely established.
 
 
 
FIG. 321. LONGITUDINAL SECTION THROUGH THE VERTEBRAL
COLUMN OF AN EIGHT WEEKS'
HUMAN EMBRYO IN THE THORACIC
REGION. (From Kolliker.)
 
v. cartilaginous vertebral body ;
//. intervertebral ligament; ch, notochord.
 
 
 
Mammalia. The early development of the perichordal
cartilaginous tube and rudimentary neural arches is almost the
same in Mammals as in Birds. The differentiation into vertebral
and intervertebral regions is the same in both groups ; but
instead of becoming divided as in Reptilia and Birds into two
segments attached to two adjoining vertebrae, the intervertebral
regions become in Mammals wholly converted into the intervertebral ligaments (fig. 322 /*'). There are three centres of ossifications for each vertebra, two in the arch and one in the centrum.
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
 
 
559
 
 
 
The fate of the notochord is in important respects different
from that in Birds. It is first constricted in the centre of the
vertebra (figs. 320 E and 321) and disappears there shortly after
the ossification ; while in the intervertebral regions it remains
relatively unconstricted (figs. 320 E, 321 and 322 c] and after
 
 
 
 
FlG. 3-22. LONGITUDINAL SECTION THROUGH THE INTERVERTEBRAL LIGAMENT
AND ADJACENT PARTS OF TWO VERTEBRA FROM THE THORACIC REGION OF AN
ADVANCED EMBRYO OF A SHEEP. (From Kolliker.)
 
la. ligamentum longitudinale anterius ; lp. ligamentum long, posterius ; li. ligamentum intervertebrale ; k, k'. epiphysis of vertebra ; w. and iv' '. anterior and
posterior vertebrae ; c. intervertebral dilatation of notochord ; c'. and c'. vertebral
dilatation of notochord.
 
undergoing certain histological changes remains through life as
part of the nucleus pulposus in the axis of the invertebral ligaments 1 .
There is also a slight swelling of the notochord near the two
extremities of each vertebra (fig. 322 c' and c"}. In the persistent vertebral constriction of the notochord Mammals retain
a more primitive and piscine mode of formation of the vertebral
column than the majority either of the Reptilia or Amphibia.
 
1 This view was first put forward by Lushka, and his surmises have been confirmed by Kolliker and other embryologists. Leboucq (No. 424) however holds that_
the cells of the notochord in the intervertebral regions fuse with those of the
adjoining tissue ; and Dursy and others deny that the nucleus pulposus is derived
from the notochord.
 
 
 
560 BIBLIOGRAPHY.
 
 
 
BIBLIOGRAPHY of Notochord and Vertebral column.
 
(415) Cartier. "Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeitschrift furwiss. ZooL, Bd. xxv. Suppl. 1875.
 
(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wirbelsaule der Amphibien nnd Reptilien. Leipzig, 1862.
 
(417) C. Gegenbaur. " Ueber die Entwickelung der Wirbelsaule des Lepidosteus mil vergleichend anatomischen Bemerkungen." Jenaische Zeitschrift, Bd. in.
1863.
 
(418) C. Gegenbaur. " Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische
Zeitschrift, Vol. v. 1870.
 
(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d.
Wirbelthicre. " II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol.
xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879
(Teleostier).
 
(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der
Wirbelsaule u. s. w." Hasse, Anatomische Studien, 1872.
 
(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d.
Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.
 
(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung
der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical,
medic in. Gesellschaft in Wiirzburg, Bd. X.
 
(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier
insbesondere iiber die Wirbel der Lamnoidei." Abhandlungen der senkenbergischen
naturforschenden Gesellschaft in Frankfurt, Bd. v.
 
(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale
chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.
 
(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische nnd
Reptilien. Berlin, 1853.
 
(426) Aug. Miiller. " Beobachtungen zur vergleichenden Anatomic der Wirbelsaule." Miiller's Archiv. 1853.
 
(427) J. Miiller. " Vergleichende Anatomic der Myxinoiden u. der Cyklostomen mil durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungen der
koniglichen Akademie der Wissenschaften zu Berlin. 1834.
 
(428) W. Miiller. "Beobachtungen des pathologischen Instituts zu Jena, I.
Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.
 
(429) A. Schneider. Beitrage c. vergleich. Anat. u. Entwick. d. Wirbelthicre.
Berlin, 1879.
 
 
 
Ribs and Sternum.
 
Ribs. Embryological evidence on the development of the
ribs, though somewhat inadequate, indicates that they arise as
cartilaginous bars in the connective tissue of the intermuscular
septa, and that they are placed, in Elasmobranchii and
 
 
 
RIBS. 561
 
Amphibia, on the level of division between the dorso-lateral and
ventro-lateral divisions of the muscle-plates. This does not
appear to hold true for either Ganoidei or Teleostei. In
Teleostei they are entirely below the muscles along the lines of
the intermuscular septa, and this is partially true for Ganoidei,
though not wholly so in Lepidosteus. They may be attached
either to the haemal (Pisces) or neural (Amphibia and Amniota)
arches. The connective tissue from which they are formed is
continuous with the processes of the vertebrae to which they are
attached ; but the conversion of the tissue into cartilage takes
place more or less independently of that of the arches, although
in many cases the cartilage of the two becomes continuous, the
separation of the ribs being then effected by a subsequent
process of segmentation (Pick, No. 431). It is possible that the
ribs of Pisces may not be homologous with those of Amphibia
and the Amniota, but till the reverse can be proved it is more
convenient to assume that the ribs are homologous structures
throughout the vertebrate series.
 
In Elasmobranchii the ribs are relatively of less importance in the
adult than in the embryo. By a careful examination of their early development, I have satisfied myself that the differentiation of the ribs is independent of that of the haemal processes to which they are attached, although
the differentiation proceeds in such a manner that, when both are converted
into cartilage, they are quite continuous. Subsequently the ribs become
segmented off from the haemal processes. At the junction of the tail and
trunk, where the haemal processes commence to be ventrally prolonged,
eventually to unite in the region of the tail below the caudal vein, the ribs
are attached to short processes which spring from the sides of the haemal
arches (fig. 317). The ventral haemal arches of these" fishes are therefore
clearly in no part formed by the ribs.
 
In Ganoidei and Teleostei there is very great difficulty in determining
the homologies of the ribs.
 
In the cartilaginous Ganoidei there are well developed rib-like structures, which might be regarded as homologous with Elasmobranch ribs,
and indeed probably are so ; but at the same time their relations are in
some respects very different from those of Elasmobranch ribs in the caudal
region. In Ganoids the ribs, in approaching the tail, become shorter and then
fuse with the ends of the haemal processes, and finally in the caudal region
form together with the haemal arches a closed haemal canal which superficially resembles that in Elasmobranchii.
 
In Lepidosteus and Amia, especially the former, the same phenomenon
is still more marked ; and in Lepidosteus it is easy, in passing backwards,
 
B. III. 36
 
 
 
562 STERNUM.
 
 
 
to trace the ribs bending ventral-wards, and uniting ventrally in the caudal
region to form, with the haemal processes, a complete haemal canal.
 
It might have been anticipated that the Teleostean Ganoids would
resemble the Teleostei, but, from an examination of adult Teleostei, it
would seem to be clear that the relations of the parts are the same as in
Elasmobranchii, i.e. that the ribs have no share in forming the haemal
canal in the tail. Aug. Miiller and Gotte have however brought embryological evidence (though not of a conclusive character), to shew that in the
embryo the ribs really fuse with the haemal processes in the tail, and so
assist, as in the Ganoids, in forming the haemal canal. Gotte moreover holds
that the ribs in Elasmobranchii are not homologous with those of Teleostei
and Ganoids ; but that the haemal arches in the tail are homologous in the
three groups.
 
Without necessarily following Gotte in these views it is worth pointing
out that the undoubtedly close affinity between the bony Ganoids and the
Teleostei is in favour of the view on the haemal arches of Teleostei at
which he has arrived on embryological grounds.
 
In Amphibia the formation of the ribs from the connective tissue of the
intermuscular septa, their secondary attachment to the transverse processes
of the neural arches, and their subsequent separation was first clearly
established by Pick (No. 431), whose statements have since been confirmed
by Hasse, Born, &c., and in part by Gotte, who holds however that, though
converted into cartilage independently of the transverse processes, they
are formed in membrane as outgrowths of these processes.
 
In the Amniota the ribs are also independently established (Hasse and
Born), though they subsequently become united to the transverse processes
and to the bodies of the vertebrae, or to the transverse processes only.
This junction is however stated by the majority of authorities, never to
be effected by the fusion of the cartilage of the two parts, but always by
fibrous tissue ; though Hoffmann (No. 435) takes a different view on this
subject, holding that the ribs are at first continuous with the intervertebral
regions of the primitive cartilaginous tube surrounding the notochord.
 
Sternum. In dealing with the development of the sternum
it will be convenient to leave out of consideration the interclavicle or episternum which is, properly speaking, only part of
the shoulder-girdle and to confine my statements to the sternum
proper.
 
This structure is found in all the Amniota except the
Ophidia, Chelonia, and some of the Amphisbaenae.
 
From the older researches of Rathke, and from the newer
ones of Gotte, etc., it appears that the sternum is always formed
from the fusion of the ventral extremities of a certain number of
ribs. The extremities of the ribs unite with each other from
 
 
 
STERNUM. 563
 
 
 
before backwards, and thus give rise to two cartilaginous bands.
These bands become segmented off from the ribs with which
they are at first continuous, and subsequently fuse in the median
ventral line to form an unpaired sternum. The Mammalian
presternum (manubrium sterni) and xiphosternum have the
same origin as the main body of the sternum (Ruge, No. 438).
 
In the Amphibia there is no structure which admits from its
mode of development of a complete comparison with the
sternum of the Amniota ; and it must for this reason be
considered doubtful whether the median structure placed behind
the coracoids in the Anura, which is usually known as the
sternum, is really homologous with the sternum of the
Amniota 1 .
 
The remaining Ichthyopsida are undoubtedly not provided
with a sternum.
 
BIBLIOGRAPHY of Ribs and Sternum.
 
(430) C. Glaus. " Beitrage z. vergleich. Osteol. d. Vertebraten. I. Rippen u.
unteres Bogensystem. " Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.
 
(431) A. E. Fick. " Zur Entwicklungsgeschichte . d. Rippen und Querfortsatze." Archivf. Anat. und Physiol. 1879.
 
(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil
vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.
 
(433) A. Gotte. " Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergiirtel." Archivf. mikr. Anat., Vol. xiv. 1877.
 
(434) C. Hasse u. G. Born. " Bemerkungen lib. d. Morphologic d. Rippen."
Zoologischer A nzeiger, 1879.
 
(4S5) C. K. Hoffmann. "Beitrage z. vergl. Anat. d. Wirbelthiere." Niederland. Archiv Zool., Vol. IV. 1878.
 
(436) W. K. Parker. " A monograph on the structure and development of the
shoulder-girdle and sternum." Ray Soc. 1867.
 
(437) H. Rathke. Ueb. d. Bau n. d. Entrmcklung d. Brustbeins d. Saurier.
 
i853
(438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen.
 
Morphol. Jahrbuch., Vol. VI. 1880.
 
1 The so-called sternum of the Amphibia develops in proximity with certain
rudimentary abdominal ribs, and Ruge has with some force urged (against Gotte)
that it may be for this reason a rudimentary structure of the same nature as the
sternum of the higher types.
 
 
 
362
 
 
 
CHAPTER XIX.
THE SKULL.
 
THREE distinct sets of elements may enter into the composition of the skull. These are (i) the cranium proper, composed
of true endoskeletal elements originally formed in cartilage, to
which are usually added exoskeletal osseous elements, formed
in the manner already described p. 542, and known in the
higher types as membrane bones. (2) The visceral arches formed
primitively as cartilaginous bars, but in the higher types largely
supplemented or even replaced by exoskeletal elements. (3) The
labial cartilages.
 
These parts present themselves in the most various forms,
and their study constitutes one of the most important departments of vertebrate morphology, and one which has always
been a favourite subject of study with anatomists. At the end
of the last century and during the first half of the present
century the morphology of the skull was handled from the point
of view of the adult anatomy by Goethe, Oken, Cuvier, Owen,
and many other anatomists, while Duges and, nearer to our own
time, Rathke, laid the foundation of an embryological study of
its morphology. A new era in the study of the skull was
inaugurated by Huxley in his Croonian lecture in 1858, and in
his lectures on Comparative Anatomy subsequently delivered
before the Royal College of Surgeons. In these lectures
Huxley disproved the then widely accepted view that the skull
was composed of four vertebrae ; and laid the foundation of a
more satisfactory method of dealing with the homologies of its
constituent parts. Since then the knowledge of the development
of the skull has made great progress. In this country a number
 
 
 
THE SKULL.
 
 
 
565
 
 
 
of very interesting memoirs have been published on the subject
by Parker, which together constitute a most striking contribution
to our knowledge of the ontogeny of the skull in a series of
types ; and in Germany Gegenbaur's monograph on the cephalic
skeleton of Elasmobranchii has greatly promoted a scientific
appreciation of the nature of the skull.
 
In the present chapter only the most important features in
the development of the skull will be touched on.
 
It will be convenient to describe, in the first instance, the
development of the cartilaginous elements of the skull.
 
The Cranium. The brain is at first enveloped in a continuous layer of mesoblast known as the membranous cranium,
into the base of which the anterior part of the notochord is
prolonged for some distance.
The primitive cartilaginous
cranium is formed by a differentiation within the membranous cranium, and is always
composed of the following parts
 
(fig- 323) :
 
(1) A pair of cartilaginous
plates on each side of the
cephalic section of the notochord, known as the parachordals (pa. ck}. These plates together with the notochord (nc)
enclosed between them form a
floor for the hind- and midbrain. The continuous plate,
formed by them and the notochord, is known as the basil ar
plate.
 
(2) A pair of bars forming
the floor for the fore-brain,
 
 
 
iff
 
 
 
 
Cl?
 
 
 
pa.ch.
 
 
 
CIA
 
 
 
FIG. 323. HEAD OK EMBRYO DOGFISH, SECOND STAGE ; BASAL VIEW OF
CRANIUM FROM ABOVE, THE CONTENTS
HAVING BEEN REMOVED. (From
 
Parker.)
 
ol. olfactory sacs ; an. auditory capsule;
nc. notochord; py. pituitary body ; pa.ch.
parachordal cartilage ; tr. trabecula ; inf.
infundibulum ; C.ir. cornua trabeculse ;
pn. prenasal element ; sp. spiracular cleft ;
br. external branchiae; Cl. 2, 4. visceral
clefts.
 
 
 
known as the trabeculae (tr).
These bars are continued forward from the parachordals. They
meet behind and embrace the front end of the notochord ; and
after separating for some distance bend in again in such a way
 
 
 
566
 
 
 
THE PARACHORDALS AND NOTOCHORD.
 
 
 
as to enclose a space the pituitary space. In front of this
space they remain in contact and generally unite. They extend
forwards into the nasal region (pn}.
 
(3) The cartilaginous capsules of the sense organs. Of these
the auditory (ait) and olfactory capsules (ol} unite more or less
intimately with the cranial walls ; while the optic capsules,
forming the usually cartilaginous sclerotics, remain distinct.
 
The parachordals and notochord. The first of these sets
of elements, viz. the parachordals and notochord, forming
together the basilar plate, is always an unsegmented continuation of the axial tissue of the vertebral column. It forms the
floor for that section of the brain which belongs to the primitive
postoral part of the head (vide p. 314), and its extension is
roughly that of the basioccipital of the adult skull. Its mode of
development is almost identical with that of the vertebral
column, except that the notochord, even in many forms where it
persists in the vertebral column, disappears in the basilar plate ;
though in a certain number of cases remnants of it are found in
the adult state.
 
 
 
It will be convenient to say a few words
notochord in the head. It always extends
along the floor of the mid- and hind-brains,
but ends immediately behind the infundibulum. The limits of its anterior extension
are clearly shewn in fig. 43. The front end
of the notochord often becomes more or
less ventrally flexed in correspondence with
the cranial flexure ; its anterior end being
in some instances (Elasmobranchii) almost
bent backwards (fig. 324).
 
Kolliker has shewn that in the Rabbit 1 ,
and I believe that a more or less similar
phenomenon may also be observed in Birds,
the anterior end of the notochord is united
to the hypoblast of the throat in immediate
contiguity with the opening of the pituitary
body ; but it is not clear whether this is to
be looked upon as the remnant of a primitive
attachment of the notochord to the hypoblast, or as a secondary attachment.
 
 
 
here with reference to the
nib
 
 
 
 
FIG. 324. LONGITUDINAL
SECTION THROUGH THE BRAIN OF
A YOUNG PRISTIURUS EMBRYO.
 
cer. commencement of the cerebral hemisphere; pn. pineal gland ;
///.infundibulum ; //.ingrowth from
mouth to form the pituitary body ;
nib. mid-brain ; cb. cerebellum ; ch,
notochord; al. alimentary tract;
laa. artery of mandibular arch.
 
 
 
" Embryologische Mittheilungen." Festschrift d. Nattirfor. G^//., Halle, 1879.
 
 
 
THE SKULL.
 
 
 
567
 
 
 
Before the parachordals are formed the anterior end of the notochord has
usually undergone a partial atrophy ; and its front end often becomes
somewhat dorsally flexed. Within the basilar plate it often exhibits two
or more dilatations, which have been regarded by Parker and Kolliker as
indicative of a segmentation of this plate ; but they hardly appear to
me to be capable of this interpretation.
 
In Elasmobranchs where, as shewn above, a very primitive
type of development of the vertebral column is retained, we find
that the basilar plate is at first formed of (i) the notochord
invested by its cartilaginous sheath, and (2) of lateral masses of
cartilage, the parachordals, homologous with the arch tissue of the
vertebral column. This development probably indicates that the
basilar plate contains in itself the same elements as
those from which the neural arches and the centra of
the vertebral column are formed ; but that it never passes
beyond the unsegmented stage at first characteristic of
the vertebral column. The hinder end of each parachordal
forms a condyle articulating with the first vertebra ; so that in the
cartilaginous skull there are always two occipital condyles. The
basilar plate always grows up behind (fig. 326, so], and gives rise
to a complete cartilaginous ring enveloping the medulla oblongata, in the same manner that the neural arches envelope the
spinal cord. This ring forms an occipital cartilaginous ring ; in
front of it the basilar plate becomes laterally continuous with the
periotic cartilaginous capsules, and the occipital ring above
usually spreads forward to form a roof for the part of the brain
between these capsules. In the higher Vertebrates the periotic
cartilages may be developed continuously with the basilar plate
 
 
 
The trabeculae. The trabeculae, so far as their mere anatomical relations are concerned, play the same part in forming the
floor for the front cerebral vesicle as the parachordals for the
mid- and hind-brains. They differ however from the parachordals in one important feature, viz. that, except at their
hinder end (fig. 323), they do not embrace between them the
notochord.
 
The notochord constitutes, as we have seen, the primitive
axial skeleton of the body, and its absence in the greater part of
the region of the trabeculae would probably seem to indicate, as
 
 
 
568
 
 
 
THE TRABECUL^i.
 
 
 
pointed out by Gegenbaur, that these parts, in spite of their
similarity to the parachordals, have not the same morphological
significance.
 
 
 
C V 1
 
 
 
 
FlG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL/E
OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After Parker.)
 
In order to shew this, the whole of the upper portion of the head has been sliced
away. The cartilaginous portions of the skull are marked with the dark horizontal
shading.
 
cv i. cerebral vesicle (sliced oft") ; e. eye ; nc. notochord ; iv. investing mass ;
9. foramen for the exit of the ninth nerve ; d. cochlea ; hsc. horizontal semicircular
canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior
end of the investing mass ; pts. pituitary space ; tr. trabeculse. The reference line tr.
has been accidentally made to end a little short of the cartilage.
 
The nature of the trabeculae has been much disputed by morphologists.
The view that they cannot be regarded as the anterior section of the
vertebral axis is supported by the consideration that the forward limit of the
primitive skeletal axis, as marked by the notochord, coincides exactly with
the distinction we have found it necessary to recognise, on entirely independent grounds, between the fore-brain, and the remainder of the nervous axis.
But while this distinction between the parachordals and the trabeculas must .
I think be admitted, I see no reason against supposing that the trabecuke
may be plates developed to support the floor of the fore-brain, for the same
physiological reasons that the parachordals have become formed at the sides
of the notochord to support the floor of the hind-brain. By some anatomists
the trabeculse have been held to be a pair of branchial bars ; but this view
has now been generally given up. They have also been regarded as equivalent to a complete pair of neural arches enveloping the front end of the brain.
The primitive extension of the base of the fore-brain through the pituitary
 
 
 
THE SKULL.
 
 
 
569
 
 
 
space is an argument, not without force, which has been appealed to in
support of this view.
 
In the majority of the lower forms the trabeculys arise quite
independently of the parachordals, though the two sets of
elements soon unite ; while in Birds (fig. 325) and Mammals the
parachordals and trabeculae are formed as a continuous whole.
The junction between the trabeculae and parachordals becomes
marked by a cartilaginous ridge known as the posterior clinoid.
 
The trabeculae are usually somewhat lyre-shaped, meeting in
front and behind, and leaving a large pituitary space between
their middle parts (figs. 323 and 325). Into this space there
 
 
 
so
 
 
 
 
f
 
 
 
bbr
 
 
 
cbr
 
 
 
FJG. 326. SIDE VIEW OF THE CARTILAGINOUS CRANIUM OF A FOWL ON THK
 
SEVENTH DAY OF INCUBATION. (After Parker.)
 
pn. prenasal cartilage ; aln. alinasal cartilage ; ale. aliethmoid ; immediately below
this is the aliseptal cartilage, eth. ethmoid ; pp. pars plana ; ps. presphenoid or
interorbital ; pa. palatine ; pg. pterygoid ; z. optic nerve ; as. alisphenoid ; q.
quadrate ; st. stapes ; fr. fenestra rotunda ; hso. horizontal semicircular canal ;
psc. posterior vertical semicircular canal : both the anterior and the posterior semicircular canals are seen shining through the cartilage, so. supraoccipital ; eo. exoccipital ; oc. occipital condyle ; nc. notochord ; mk. Meckel's cartilage ; ch. ceratohyal ; bh. basi-hyal ; cbr. and ebr. cerato-branchial ; bbr. basibranchial.
 
primitively projects the whole base of the fore-brain, but the
space itself gradually becomes narrowed, till it usually contains
only the pituitary body. The carotid arteries always pass through
it in the embryo ; but in the higher forms it ceases to be
perforated in the adult. The trabeculae soon unite together both
in front and behind and form a complete plate underneath the
fore-brain, and extending into the nasal region 1 . A special
 
1 In Man (Kolliker) the trabeculce form from the first a continuous plate in front
of the pituitary space, and the latter very early acquires a cartilaginous floor.
 
 
 
5/0 THE TKAHECUL/E.
 
 
 
vertical growth of this plate in the region of the orbit forms the
interorbital plate of Teleostei, Lacertilia and Aves (fig. 326, ps),
on the upper surface of which the front part of the brain rests.
The trabecular floor of the brain does not long remain simple.
Its sides grow vertically upwards, forming a lateral wall for the
brain, in which in the higher types two regions may be distinguished, viz. an alisphenoidal region (fig. 326, as) behind,
growing out from what is known as the basisphenoidal region
of the primitive trabeculae, and an orbitosphenoidal region in
front growing out from the presphenoidal region of the trabecula,\ These plates form at first a continuous lateral wall of
the cranium. At the front end of the brain they are continued
inwards, and more or less completely separate the true cranial
cavity from the nasal region in front. The region of the cartilage
forming the anterior boundary of the cranial cavity is known as
the lateral ethmoid region, and it is always perforated for the
passage of the olfactory nerves.
 
The cartilaginous walls which grow up from the trabecular
floor of the cranium generally extend upwards so as to form a
roof, though almost always an imperfect roof, for the cranial
cavity. In the higher types, in Mammals more especially, this
roof can hardly be said to be formed at all. The region of the
trabeculae in front of the brain is the ethmoid region. The basal
part of this region forms an internasal plate, from which an
internasal septum may grow up (fig. 326). To its sides the
olfactory capsules are attached, and there are usually lateral
outgrowths in front forming the trabecular cornua, while from
the posterior part of the ethmoidal plate, forming the anterior
boundary of the cranial cavity, there often grows out a prefrontal
or lateral ethmoidal process.
 
These and other processes growing out from the trabeculse have
occasionally been regarded as rudimentary praeoral branchial arches. I
have already stated it as my view that the existence of branchial arches
in this region is highly improbable, and I may add that the development
of these structures as outgrowths of the skull is in itself to my mind a nearly
conclusive argument against their being branchial arches, in that true
branchial arches hardly ever or perhaps never arise in this way.
 
The sense capsules. The most important of these is the
auditory capsule, which, as we have seen, fuses intimately with
 
 
 
THE SKULL.
 
 
 
the lateral walls of the skull. In front there is usually a cleft
separating it from the alisphenoid region of the skull, through
which the third division of the fifth nerve passes out. This cleft
becomes narrowed to a small foramen (fig. 327, V). The
sclerotic cartilage is always free, but profoundly modifies the
region of the cranium near which it is placed. The nasal investment forms in Elasmobranchs (fig. 327, No) a capsule open
 
 
 
 
FIG. 327. SKULL OF ADULT DOGFISH, SIDE VIEW. (From Parker.)
O. C, occipital condyle ; Au. periotic capsule; Pt.O. pterotic ridge ; Sp. 0. sphenotic process ; S. Or. supraorbital ridge ; Na. nasal capsule ; P.N. prenasal cartilage;
77. optic foramen ; V. trigeminal foramen ; PL TV., Qu. pterygo-quadrate arcade ;
M.Pt. metapterygoid ligament (including a small cartilage) ; Pl.Tr, ethmo-palatine
or palato-trabecular ligament ; Mck. lower jaw ; Sp. spiracle; H.M. hyomandibular;
C.Hy, ceratohyal ; m.h.l. mandibulo-hyoid ligament; Ph.Br. pharyngobranchial ;
E.Br. epibranchial ; C.br. ceratobranchial ; H.Br. hypobranchial ; B.Br. basibranchial ; Ex.Br. extrabranchial ; l\ 2 , 3 , 4 , 5 . labial cartilages ; the dotted lines
within Mck. indicate the basihyal.
 
below, and continuous with the ethmoid region of the trabeculse.
In most types however it becomes more closely united with the
ethmoid region and the accessory parts belonging to it.
 
The cartilaginous cranium, the development of which has
been thus briefly traced, persists in the adult without even the
addition of membrane bones in the Cyclostomata, Elasmobranchii
(fig. 327) and Holocephali. In the Selachioid Ganoids it is also
found in the adult, but is covered over by membrane bones. In
all other types it is invariably present in the embryo, but becomes
in the adult more or less replaced by osseous tissue.
 
 
 
572 THE BRANCHIAL BARS.
 
 
 
Branchial skeleton.
 
The most primitive type of branchial skeleton in any existing
form would appear to be that of the Petromyzonidae, which is
developed in a superficial subdermal tissue, and consists of a
series of bars united by transverse pieces, so as to form a basketwork. It is known as an extra-branchial system, and an early
stage of its development in the Lamprey is shewn in fig. 47. In
the higher forms this system is replaced by a series of bars,
known as the branchial bars, so situated as to afford support to
the successive branchial pouches. Outside these bars there may
be present in some primitive forms (Elasmobranchii) cartilaginous
elements, which are supposed to be remnants of the extrabranchial system (fig. 327, Ex.Br] ; while a series of membrane
bones is also usually added to them, which will be dealt with in a
separate section. The branchial bars are developed as simple
cartilaginous rods in the deeper parts of the mesoblast which
constitutes the primitive branchial arches.
 
The position of the branchial bars in relation to the somatopleure and
splanchnopleure can be determined from their relation to the so-called head
cavities. These cavities atrophy before the formation of the cartilaginous
branchial bars, but it will be observed (fig. 328), that the artery of each
arch (aa) is placed on the inner side of the head cavity (//). The cartilaginous bar arises at a later period on the inner side of the artery, and
therefore on the inner side of the section of the body cavity primitively
present in the arches.
 
An anterior arch, known as the mandibular arch, placed in
front of the hyo-mandibular cleft, and a second arch, known as
the hyoid arch, placed in front of the hyo-branchial cleft, are
developed in all types. The succeeding arches are known as
the true branchial arches, and are only fully developed in the
Ichthyopsida.
 
In some Sharks (Notidani) seven branchial arches may be
present (not including the hyoid and mandibular). In other
Ichthyopsida five are usually present, in the embryo at any rate,
while in the Amniota there are usually two or three post-hyoid
membranous arches, in the interior of which a cartilaginous bar
is usually formed. The general form of these bars at an early
 
 
 
THE SKULL.
 
 
 
573
 
 
 
 
 
 
 
Fir
 
 
HORIZONTAL
 
 
 
stage of development is shewn in the
dog-fish (Scyllium) in fig. 329.
 
The simple condition of these bars
in the embryo renders it highly probable that forms existed at one time
with a simple branchial skeleton of
this kind : at the present day however
 
J SECTION THROUGH THE PEN
such forms no longer exist. The first ULTIMATE VISCERAL ARCH
arch has in all cases changed its F RUS AN EMHRYO <
function and has become converted ^ epiblast; vc. pouch of
 
into a supporting skeleton for the hypoblast which will form the
, , ,11 -1 1 ., i i i. walls of a visceral cleft ; pp.
 
mouth ; the hyoid arch, though retain- segme nt of body-cavity in vis
ing in Some forms its branchial func- ceral arch ;aa. aortic arch.
 
tion, has in most acquired additional functions and has undergone in consequence various peculiar modifications. The true
branchial arches retain their branchial functions in Pisces and
some Amphibia, but are secondarily modified and largely
aborted in the abranchiate forms. Since the changes undergone
 
c.a
 
 
 
 
Bnl
 
 
 
ffm
 
 
 
LrJt
 
 
 
Sn.f
 
 
 
FIG. 329. HEAD OF EMBRYO DOGFISH, n LINES LONG. (From Parker.)
TV. trabecula ; Pl.Pt. pterygo-quadrate ; M.Pt. metapterygoid region; Mn.
mandibular cartilage ; Hy. hyoid arch; Br. i. first branchial arch; Sp. mandilmlohyoid cleft; C/ 1 . hyo-branchial cleft; Lch. groove below the eye; Net. olfactory
rudiment; E. eyeball; An. auditory mass; C i, 2, 3. cerebral vesicles; Hm.
hemispheres; f.n.p. nasofrontal process.
 
by the true branchial bars are far less complicated than those of
the hyoid and mandibular bars it will be convenient to treat of
them in the first instance.
 
These bars are, as already mentioned, most numerous in
certain very primitive forms (seven in Notidanus), while as we
ascend the series there is a gradual tendency for the posterior of
them to disappear. This tendency is the result of a gradual
atrophy of the posterior branchial pouches, which commenced at
 
 
 
574
 
 
 
THE BRANCHIAL BARS.
 
 
 
a stage in the evolution of the Chordata long prior to the
appearance of cartilaginous or osseous branchial bars, and
reaches its climax in the Amniota.
 
In a fully developed branchial bar the primitively simple rod
of cartilage becomes divided into a series of segments, usually
four, articulated so as to be more or less mobile : and either
remaining cartilaginous or becoming partially or wholly ossified.
Each bar (fig. 327) forms a somewhat curved structure, embracing
the pharynx. The dorsal and somewhat horizontally placed
segment is known as the pharyngobranchial (Ph.Br), the next
two as the epibranchial (E.Br) and ceratobranchial (C.Br), and
the ventral segment as the hypobranchial (H.Br). There is
also typically present a basal unpaired segment, uniting the bars
of the two sides, known as the basibranchial (B.Br). The arches
often bear cartilaginous rays which support the gill lamellae.
 
In Teleostei dental plates are usually developed as an
exoskeletal covering on parts of the branchial arches.
 
In the Amphibia four or three branchial arches are present in
the embryo. These parts are more or less completely retained
in the Perennibranchiata and Caducibranchiata, but in the
Myctodera and Anura they become largely reduced, and
entirely connected with the hyoid.
 
In the Anura they never reach any considerable development,
and are soon reduced to a
plate (fig. 330) the coalesced
basihyal and basibranchial
plate the posterior processes
of which represent the remnants of the branchial arches.
 
 
 
According to Parker the posterior process of this plate in the
adult is a remnant of the fourth
branchial bar ; the next one is
the third branchial bar, while the
anterior lamina behind the hyoid
is stated by him (though this is
somewhat doubtful) to be a remnant of the first two bars.
 
In the Amniota, the branchial arches become still more
 
 
 
 
Pmx
 
 
 
FIG. 330. YOUNG FROG, WITH TAIL
JUST ABSORBED ; SIDE VIEW OF SKULL.
(From Parker.)
 
An. auditory capsule; in front of it is
the cranial side wall ; A.N. external nostril ;
St. stapes; Mck. Meckelian cartilage; B.Hy.
basihyobranchial plate; St.Hy. stylohyal
or ceratohyal; Br.i. first branchial arch.
 
Bones: E-0. exoccipital; Pr.O. prootic ; Pa. parietal ; Fr. frontal ; Na. nasal ;
Pmx. premaxillary ; MX. maxillary; Pt.
pterygoid; Sq. squamosal; Qn-J't. quadra tojugal; Art. articular; D. dentary.
 
 
 
THE SKULL. 575
 
 
 
degenerated, in correlation with the total disappearance of a
branchial respiration at all periods of life. Their remnants
become more or less important parts of the hyoid bone, and are
solely employed in support of the tongue. Their basal portions
are best preserved, forming parts of the body of the hyoid. The
posterior (thyroid) cornua of the hyoid are remnants of the true
arches. Of these there are two in the Chelonia and Lacertilia,
and one in the Aves and Mammalia. In Aves the cornu formed
from the first branchial arch (fig. 331, cbr) is always larger than
that of the true hyoid arch (cJi).
 
Mandibular and Hyoid arches. The adaptations of both
the mandibular and hyoid bars, to functions entirely distinct from
 
 
 
 
FlG. 331. VIEW FROM BELOW OF THE BRANCHIAL SKELETON OF THE SKULL
 
OF A FOWL ON THE FOURTH DAY OF INCUBATION. (After Parker.)
cv i. cerebral vesicles ; e. eye ; fn. frontonasal process; n. nasal pit; tr. trabeculre ;
pts. pituitary space ; mr. superior maxillary process ; pg. pterygoid ; pa. palatine ;
q. quadrate; mk. Meckel's cartilage; ch. cerato-hyal ; bh. basihyal ; cbr. ceratobranchial ; ebr. proximal portion of the cartilage in the third visceral (first branchial)
arch; bbr. basibranchial ; i. first visceral cleft; 2. second visceral cleft; 3. third
visceral arch.
 
those which they primitively served, are most remarkable ; and
the adaptations of the two bars are in many cases so intimately
bound together, that it is not possible to treat them separately.
 
The most important change of function is undoubtedly that
of the mandibular arch, which becomes entirely converted into a
skeleton for the jaws. It may be noted as a peculiarity of the
 
 
 
576 MAND1BULAR AND HYOID BARS.
 
mandibular arch that it is never provided with an unpaired basal
element.
 
The simplest forms of metamorphosis are those undergone
by Elasmobranchii, of which the Dog-fish (Scyllium) and Skate
(Raja) have been studied (Parker, No. 456). In some of these
forms, e.g. the Skate, part of the mandibular bar is still related to
the hyo-mandibular cleft (the spiracle).
 
Elasmobranchii. In Scyllium the hyoid and mandibular
arches are at first very similar to those which follow. Soon
however each of them sends an anteriorly directed dorsal process
(fig. 329). The regions which may be distinguished owing to the
growth of these processes have received names from ossifications
in them which are found in other types. The anterior process of
the mandibular arch is known as the pterygo-quadrate bar
(Pl.Pt) ; the dorsal end of the primitive bar from which it starts
(M.Pt] is known as the metapterygoid process; while the
ventral end of the bar forms the Meckelian cartilage. The
upper end of the hyoid arch is known as the hyomandibular.
 
In a somewhat later stage changes take place which cause
these parts practically to assume the adult form (fig. 327). The
mandibular arch becomes segmented at its bend into (i) a
pterygo-quadrate bar (Pl.Pf) which grows forwards in front of
the mouth, and forms an upper jaw, and (2) a Meckelian cartilage
(Mck} which is placed behind the mouth, and forms a lower jaw.
The two jaws are articulated together, and the cartilages of the
two sides composing them meet each other distally.
 
At the articulation of the Meckelian cartilage with the quadrate part of the pterygo-quadrate is situated a ligament (M.Pf),
which takes the place of the metapterygoid process of the
previous stage, and passes up on the anterior side of the spiracle,
to be attached to the cranium in the front part of the auditory
region. This ligament, which is supplemented by a second
ligament, the ethmopalatine ligament, passing from the
pterygo-quadrate bar to the antorbital region of the skull, is not
the most important support of the jaw. The main support is, on
the contrary, given by the hyoid arch ; the hyomandibular
segment of which (H.M) as well as the adjoining segment (ceratohyoid C.Hy) are firmly attached by ligament to the mandibular
 
 
 
THE SKULL.
 
 
 
577
 
 
 
arch. The hyomandibular is articulated with the cranium
beneath the pterotic ridge (Pt.O),
 
In the type just described, the hyoid and mandibular arches
undergo less modification than in almost any other case. The
hyoid arch has altered its form, but retains its respiratory function. It has however acquired the secondary function of supporting the mandibular arch. The mandibular arch is divided
into two elements, which form respectively the upper and lower
jaws. It is not directly articulated with the skull, and its mode
of support by the hyoid arch has been called by Huxley (No.
445) hyostylic.
 
The development of the hyoid and mandibular arches in the
Skate is characterised by a few important features (fig. 333). The
anterior element of the hyoid
 
arch, which forms the hyo- \ ^ Sp
 
mandibular (H.M], becomes
entirely separate from the
posterior part of the arch, and
only serves to support the
jaws. The posterior part of
the arch (Hy} carries on the
respiratory functions of the
hyoid, and is closely connected with the first branchial
arch. The upper or metapterygoidelementof the mandibular arch (M.Pt} has a
considerable development,
 
 
 
 
FIG. 333. HEAD OF EMBRYO SKATE, i\
IN. LONG. (From Parker.)
 
Tr. trabecula ; Pl.Pt. pterygo- quadrate
bar ; Mn. mandibular bar ; M.Pt. metapterygoid cartilage ; H.M. hyomandibular ; Hy. remainder of hyoid arch ; Br. \.
first branchial arch ; Sp. mandibulo-hyoid
cleft or spiracle ; Pn. pineal gland ; Au. au
ditory vesicle ; C. i, C. 2, and C. 3. vesicles
of the brain.
 
 
 
and, becoming separated from
the remainder of the arch, forms a mass of cartilage with one or
two branchial rays, in the front wall of the spiracle, and constitutes a section of the mandibular arch still retaining traces
of its primitive function in supporting the wall of a branchial
pouch.
 
Although the development of other Elasmobranch types is
not known, it is necessary to call attention to the mode of
support of the mandibular arch in certain forms, notably Notidanus, Hexanchus and Cestracion, where the pterygo-quadrate
region of the mandibular arch is directly articulated to the
B. in. 37
 
 
 
578 MANDIKULAR AND HYOID BARS.
 
cranium between the optic and trigeminal foramina. In the
two former genera the metapterygoid region of the arch is moreover continuous with the pterygo-quadrate, and articulates with
the post-orbital process of the auditory region of the skull. In
spite of these attachments the mandibular arch continues to be
partially supported by the hyomandibular. The skulls in which
the mandibular arch has this double form of support have been
called by Huxley amphistylic.
 
Considering the in many respects primitive characters of the
forms with amphistylic skulls it seems not improbable that they
 
 
 
SOr
 
 
 
l } a.ch.
 
 
 
 
Gtly
 
 
 
FIG. 334. CRANIAL SKELETON OF A SALMON FRY, SECOND WEEK AFTER
HATCHING; MEMBRANE BONES, EYEBALLS, AND NASAL SACS REMOVED. (From
Parker.)
 
T.Cr. tegrnen cranii; 'S. Or. supraorbital band; Fo. superior fontanelle; Au.
auditory capsule ; Pa.ch. parachordal cartilage; Ch. notochord; 7>. trabecula; above
the trabecula, the interorbital septum is seen, passing into the cranial wall above and
reaching the supraorbital band; //. optic foramen; V. trigeminal foramen; /', I".
labial cartilages ; PI. Ft. palatopterygoid bar ; M. Pt. metapterygoid tract ; Qu. quadrate region; Mck. Meckelian cartilage; H.M. hyomandibular cartilage; Sy.
symplectic tract; I.Hy. interhyal; C.Hy. ceratohyal; II. fly. hypohyal; G.ffy.
glossohyal; Br.\. first branchial arch.
 
preserve the original mode of support of the mandibular arch ;
from which differentiations in two directions have taken place, viz.
differentiations in the direction of a complete support of the
mandibular arch by the hyoid, which is characteristic of most
Elasmobranchii and, as will be shewn below, of Ganoidei and
Tclcostei ; and differentiations towards a direct articulation or
attachment of the mandibular arch to the cranium, without the
 
 
 
THE SKULL. 579
 
 
 
intervention of the hyoid. The latter mode of attachment is
called by Huxley autostylic. It is found in Holocephala,
Dipnoi, Amphibia and the Amniota.
 
Teleostei. In addition to that of Elasmobranchii, the skull
of the Salmon is the only hyostylic skull in which, by the admirable investigation of Parker (No. 451), the ontogeny of the hyoid
and mandibular bars has been satisfactorily worked out. Apart
from the presence of a series of membrane bones, the development of these bars agrees on the whole with the types already
described.
 
The hyoid arch, though largely ossified, undergoes a process
of development very similar to that in Raja. It is formed as a
simple cartilaginous bar, which soon becomes segmented longi
ft 1.3 Sp.
 
 
 
 
FIG. 335. YOUNG SALMON OF THE FIRST SUMMER, AKOUT 2 INCHES LONG;
 
SIDE VIEW OF SKULL, EXCLUDING BRANCHIAL ARCHES. (From Parker.)
 
The palato-mandibular and hyoid tracts are detached from their proper situations,
 
a line indicating the position where the hyomandibular is articulated beneath the
 
pterotic ridge.
 
oL olfactory fossa; c.tr. trabecular cornu; /*. /''. upper labial cartilages ; p.s.
 
presphenoid tract ; t.cr. tegmen cranii ; s.o.b. supraorbital band; fo. superior fonta
nelle; n.c. notochord; b.o. basilar cartilage; //'. trabecula; p.c. condyle for palatine
 
cartilage; 5. trigeminal foramen ; fa. facial foramen; 8. foramen for glossopharyngeal
 
and vagus nerves; mk. Meckelian cartilage; op.c. opercular condyle.
 
Bones: e.o. exoccipital; s.o. supraoccipital; e.p. epiotic; pt.o. pterotic; sp.o.
 
sphenotic ; op. opisthotic; pro. prootic; I'.s. basisphenoid ; al.s. alisphenoid; o.s.
 
orbitosphenoid ; I.e. ectethmoid or lateral ethmoid ; pa. palatine ; pg. pterygoid ;
 
m.pg. mesopterygoid ; mt.pg. metapterygoid ; qu. quadrate; ar. articular; h.m.
 
hyomandibular; sy. symplectic ; i.h. interhyal ; ep.h. epiceratohyal ; c.h. ceratohyal ;
 
h.h. hypohyal; g.h. glosso- or basihyal.
 
372
 
 
 
580 MANDIBULAR AND HYOID BARS.
 
tudinally into an anterior and a posterior part (fig. 334). The
former constitutes the hyomandibular (H.M], while the latter,
becoming more and more separated from the hyomandibular,
constitutes the hyoid arch proper ; owing to the disappearance
of the hyobranchial cleft, it loses its primitive function, and
serves on the one hand to support the operculum covering the
gills, and on the other to support the tongue. It becomes
segmented into a series of parts which are ossified (fig. 335) as
the epiceratohyal (ep./t) above, then a large ceratohyal (c/t),
followed by a hypohyal (JiJi), while the median ventral element
forms the basi- or glossohyal (gJi).
 
The hyomandibular itself is articulated with the skull below
the pterotic process (fig. 334, H.M}. Its upper element ossifies
as the hyomandibular (fig. 335, fun.}, while its lower part (fig.
334, Sy), which is firmly connected with the mandibular arch,
ossifies as the symplectic (fig. 335, sy). A connecting element
between the two parts of the hyoid bar forms an interhyal (i/i).
 
There are more important differences in the development of
the mandibular arch in Elasmobranchii and the Salmon than in
that of the hyoid arch, in that, instead of the whole arcade of
the upper jaw being formed from the mandibular arch, a fresh
element, in the form of an independently developed bar of
cartilage, completes the upper arcade in front ; but even with
this bar the two halves of the upper branch of the arch do not
meet anteriorly, but are separated by the ends of the trabeculae.
 
The anterior bar of the upper arcade is known as the
palatine ; but it appears to me as yet uncertain how far it is to
be regarded as an element, primitively belonging to the upper
arcade of the mandibular arch, which has become secondarily
independent in its development ; or as an entirely distinct
structure which has no counterpart in the Elasmobranch upper
jaw. The latter view is adopted by Parker and Bridge, and a
cartilage attached to the hinder wall of the nasal capsule of
many Elasmobranchii is identified by them with the palatine rod
of the Teleostei.
 
The arch itself is at first very similar to the succeeding
arches ; its dorsal extremity soon however becomes broadened,
and provided with an anteriorly directed process. This part (fig.
334, M.Pt and Qii] is then segmented from the lower region,
 
 
 
THE SKULL. 581
 
 
 
and forms what may be called the pterygo-quadrate cartilage,
though not completely homologous with the similarly named
cartilage in Elasmobranchs ; while the lower region forms the
Meckelian cartilage (Mck], which has already grown inwards, so
as to meet its fellow ventrally below the mouth. The whole
arch becomes at the same time widely separated from the axial
parts of the skull.
 
Nearly simultaneously with the first differentiation of the
mandibular arch, a bar of cartilage the palatine bar already
spoken of is formed on each side, below the eye, in front of the
mouth. The dilated anterior extremity of this bar soon comes
in contact with an anterior process of the trabeculse, known as
the ethmopalatine process.
 
In a later stage the pterygoid end of the pterygo-quadrate
cartilage unites with the distal end of the palatine bar (fig. 334,
Pl.Pt], and there is then formed a continuous cartilaginous
arcade for the upper jaw, which is strikingly similar to the
cartilaginous upper jaw of Elasmobranchii.
 
A large dorsal process of the primitive pterygo-quadrate now
forms a large metapterygoid tract (M.Pt] ; while the whole arch
becomes firmly bound to the hyomandibular (H.M}.
 
In the later stages the parts formed in cartilage become
ossified (fig. 335). The palatine is first ossified, the pterygoid
region of the pterygo-quadrate is next ossified as a dorsal
mesopterygoid (m.pg] and a ventral pterygoid proper (pg).
The quadrate region, articulating with the Meckelian cartilage,
becomes ossified as a distinct quadrate (qu\ while the dorsal
region becomes also ossified as a metapterygoid (int.pg).
 
In the Meckelian cartilage a superficial ossification of the
ventral edge and inner surface forms an articulare (ar) ; but the
greater part of the cartilage persists through life.
 
Some of the above ossifications, at any rate those of the palatine and
pterygoid, seem to be started by dental osseous plates adjoining the cartilage. They will be spoken of further in the section dealing with the membrane bones.
 
Amphibia. The development of the autostylic piscine skulls
has unfortunately not yet been studied ; and the most primitive
autostylic types whose development we are acquainted with are
 
 
 
582 MANDIBULAR AND HYOID BARS.
 
those of the Amphibia ; on which a large amount of light has
been shed by the researches of Huxley and Parker.
 
The modifications of the hyoid arch are comparatively simple
and uniform. It forms a rod of cartilage, which soon articulates
in front with the quadrate element of the mandibular arch, and
is subsequently attached by ligaments both to the quadrate and
to the cranium. In those Amphibia in which external gills and
gill clefts are lost, it fuses with the basal element of the hyoid
(fig. 330), which, together with the basal portions of the following
arches, forms a continuous cartilaginous plate. On the completion of these changes the paired parts of the hyoid arch have
the form of two elongated rods, known as the anterior cornua of
the hyoid, which attach the basihyal plate to the cranium behind
the auditory capsule.
 
It is still uncertain whether there is any distinct element corresponding
to the hyomandibular of fishes.
 
Parker holds that the columella auris of the Anura is the homologue
of the hyomandibular. The columella develops comparatively late and
independently of the remainder of the hyoid arch, but the similarity
between its relations to the nerves and those of the hyomandibular is
put forward by Parker as an argument in favour of his view. The early
ligamentous connection between the quadrate and the upper end of the
primitive hyoid is however an argument in favour of regarding the upper
end of the primitive hyoid as the hyomandibular element, not separated
from the remainder of the arch.
 
The history of the mandibular arch is more complicated than
that of the hyoid. The part of it which corresponds with the
upper jaw of Elasmobranchii exhibits most striking variations in
development ; so striking indeed as to suggest that the secondary
modifications it has undergone are sufficiently considerable to
render great caution necessary in drawing morphological conclusions from the processes which are in some instances observable. A more satisfactory judgment on this point will be .
possible after the publication of a memoir with which Parker is
now engaged on the skulls of the different Anura.
 
The membrane bones applying themselves to the sides of the
mandibular arch are relatively far more important than in the
lower types. This is especially the case with the upper jaw
where the maxillary and premaxillary bones functionally replace
the primitive cartilaginous jaw ; while membranous pterygoids
 
 
 
THE SKULL.
 
 
 
583
 
 
 
and palatines apply themselves to, and largely take the place of,
the cartilaginous palatine and pterygoid bars.
 
Two types worked out by Parker, viz. the Axolotl and the
common Frog, may be selected to illustrate the development of
the mandibular arch.
 
In the Axolotl, which may be taken as the type for the
Urodela, the mandibular arch is constituted at a very early
stage of (i) an enlarged dorsal element, corresponding with the
pterygo-quadrate of the lower types, but usually known as the
quadrate ; and (2) a ventral or Meckelian element. The Meckelian bar very early acquires its investing bones, while the dorsal
part of the quadrate becomes divided into two characteristic
 
 
 
 
FIG. 336. YOUNG AXOLOTL, i\ INCHES LONG ; UNDER VIEW OF SKULL,
 
DISSECTED, THE LOWER JAW AND GILL ARCHES HAVING BEEN REMOVED.
 
(From Parker.)
 
nc. notochord ; oc.c. occipital condyle; f.o. fenestra ovalis; si. stapes; tr. trabecular cartilage; i.n. internal nares; c.tr. cornu trabeculse; pd. pedicle of quadrate;
(/. quadrate; pg. outline of pterygoid cartilage; 5'. orbito-nasal nerve; 7. facial nerve.
 
BonCS I pa.s. parasphenoid ; e.o. exoccipital ; v. vomer; px. premaxillary ; mx.
maxillary; pa. palatine; pg. pterygoid.
 
processes, viz. an anterior dorsal process which grows towards
and soon permanently fuses with the trabecular crest, and a
posterior process known as the otic process, which applies itself
to the outer side of the auditory region. The anterior of these
processes, as pointed out by Huxley, is probably homologous
with the anterior process of the pterygo-quadrate bar in Notidanus, which articulates with the trabecular region of the
cranium, while the otic process is homologous with the meta
 
 
584 MANDIBULAR AND HYOID BARS.
 
pterygoid process. Hardly any trace is present of an anterior
process to form a pterygoid bar, but dentigerous plates forming
a dermal palato-pterygoid bar have already appeared.
 
At a somewhat later stage a fresh process, called by Huxley
the pedicle, grows out from the quadrate, and articulates with
the ventral side of the auditory region (fig. 336, pd). Shortly
afterwards a rod of cartilage grows forward from the quadrate
under the membranous pterygoid (pg), which corresponds with
the cartilaginous pterygoid bar of other types (fig. 336), and an
independent palatine bar, arising even before the pterygoid
process, is formed immediately dorsal to the dentigerous palatine
plate (pa\ and is attached to the trabecula. These two bars
eventually meet, but never become firmly united to the more
important membrane bones placed superficially to them.
 
The mandibular arch in the
Frog stands, so far as development is concerned, in striking
contrast to the mandibular
arch of the Axolotl, in spite of
the obvious similarity in the
arrangement of the adult parts
in the two types. FlG . 33? . EMBRYO FROG, JUST BE
In the earliest stage it FORE HATCHING ; SIDE VIEW OF HEAD,
 
WITH SKIN REMOVED. (From Parker.)
forms a simple bar in the ,, lf , , - . , .. ,
 
Na. olfactory sack; E. involution for
 
membranous mandibular arch, eyeball; Ati. auditory sack; 7>. trabe11 i , .1 cula; Mn. mandibular : Hy. hyoid ; Br.I.
 
parallel to and very similar to first branchial arch . ' th / gili.buds are
 
 
 
 
are
 
the hyoid bar behind (figf 337, seen on the first two branchial arches; /.
M \ T u, * u labial cartilages.
 
Mn). In the next stage ob
served, that is to say in Tadpoles of four, five, to six lines long,
an astonishing transformation has taken place. The mandibular
arch (fig. 338) is turned directly forwards parallel to the
trabecula, to which it is attached in front (p.pg) and behind
(pd}. The proximal part of the arch thus forms a subocular
bar, and the space between it and the trabecula a subocular
fenestra. In front of the anterior attachment it is continued
forwards for a short distance, and to the free end of this projecting part is articulated a small Meckelian cartilage directed
upwards (mk}. The Meckelian cartilage is at this stage placed
in front of the nasal sacks, in the lower lip of the suctorial
 
 
 
THE SKULL.
 
 
 
585
 
 
 
mouth. The greater part of the arch, parallel with the trabeculae,
is equivalent to what has been called in the Axolotl the
 
 
 
mJr
 
 
 
 
FIG. 338. TADPOLE OF COMMON TOAD, ONE-THIRD OF AN INCH LONG ;
CRANIAL AND MANDIBULAR CARTILAGES SEEN FROM ABOVE ; THE PARACHORDAL
 
CARTILAGES ARE NOT YET DEFINITE. (From Parker.)
 
nc. notochord; ms. muscular segments; au. auditory capsule; py. region of
pituitary body; tr. trabecula; c.tr, cornu trabeculae ; p-pg. palatopterygoid bar ; pd.
pedicle; q. quadrate condyle; mk. Meckelian piece of mandibular arch; s.o.f.
subocular fenestra ; u.l. upper labial cartilage. The dotted circle within the quadrate
region indicates the position of the internal nostril.
 
quadrate, while its anterior attachment to the trabeculae is the
rudiment of the palato-pterygoid cartilage. The posterior
attachment is known as the pedicle.
 
The condition of the mandibular arch during this and the next stage
(fig. 339) is very perplexing. Its structure appears adapted in some way to
support the suctorial mouth of the Tadpole.
 
Reasons have been offered in a previous part of this volume for supposing that the suctorial mouth of the Tadpole is probably not simply a
structure secondarily acquired by this larva, but is an organ inherited from
an ancestor provided through life with a suctorial mouth.
 
The question thus arises, is the peculiar modification of the mandibular
arch of the Tadpole an inherited or an acquired feature ?
 
If the first alternative is accepted we should have to admit that the
mandibular arch became first of all modified in connection with the
suctorial mouth, before it was converted into the jaws of the Gnathostomata ; and that the peculiar history of this arch in the Tadpole is a
more or less true record of its phylogenetic development. In favour of this
 
 
 
586
 
 
 
MANDIBULAR AND HYOID BARS.
 
 
 
view is the striking similarity which Huxley has pointed out between
the oral skeleton of the Lamprey and that of the Tadpole ; and certain
peculiarities of the mandibular arch of Chimaera and the Dipnoi can perhaps
best be explained on the supposition that the oral skeleton of these forms
has arisen in a manner somewhat similar to that in the Frog ; though with
reference to this point further developmental data are much required.
 
On the other hand the above suppositions would necessitate our
admitting that a great abbreviation has occurred in the development of
the mandibular arch of the otherwise more primitive Urodela ; and that
the simple mode of growth of the jaws in Elasmobranchii, from the
primitive mandibular arch, is phylogenetically a much abbreviated and
modified process, instead of being, as usually supposed, a true record of
ancestral history.
 
If the view is accepted that the characters of the mandibular arch of
the Tadpole are secondary, it will be necessary to admit that the adaptation
of the mandibular arch to the suctorial mouth took place after the suctorial
mouth had come to be merely a larval organ.
 
In view of our imperfect knowledge of the development of most Piscine
skulls I would refrain from expressing a decided opinion in favour of
either of these alternatives.
 
 
 
or.p
 
 
 
eth
 
 
 
 
FIG. 339. TADPOLE WITH TAIL BEGINNING TO SHRINK; SIDE VIEW OF SKULL
 
WITHOUT THE BRANCHIAL ARCHES. (From Parker.)
 
n.c. notochord; au. auditory capsule; between it and eth. the low cranial side wall
is seen; eth. ethmoidal region; st. stapes; 5. trigeminal foramen; 2. optic foramen;
ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu
trabeculae; ./. upper labial, in outline; su. suspensorium (quadrate); pd. its pedicle;
ot.fr. its otic process; or.p. its orbitar process; t.m. temporal muscle, indicated by
dotted lines passing beneath the orbitar process; pa.pg. palatopterygoid bar; ;;//.
Meckelian cartilage; /./. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The
upper outline of the head is shewn by dotted lines.
 
As the tail of the Tadpole gradually disappears, and the
metamorphosis into the Frog becomes accomplished, the
mandibular arch undergoes important changes (fig. 339): the
 
 
 
THE SKULL.
 
 
 
palato-pterygoid attachment (pa.pg) of the quadrate subocular
bar becomes gradually elongated ; and, as it is so, the front end
of the subocular bar (su) rotates outwards and backwards, and
soon forms a very considerable angle with the trabeculae. The
Meckelian cartilage (ink) at its free end becomes at the same
time considerably elongated. These processes of growth continue till (fig. 330) the palato-pterygoid bar (Pf) forms a subocular bar, and is considerably longer than the original subocular region of the quadrate ; while the Meckelian cartilage
(Mck] has assumed its permanent position on the hinder border
of the no longer suctorial mouth, and has grown forwards so as
nearly to meet its fellow in the median line.
 
The metapterygoid region of the quadrate gives rise to a
posterior and dorsal process (fig. 339, ot.pr), the end of which is
constricted off as the tympanic annulus (fig. 340, a.f) ; while
 
 
 
pmx
 
 
 
 
FIG. 340. YOUNG FROG, NEAR END OF FIRST SUMMER ; UPPER VIEW OF
SKULL, WITH LEFT MANDIBLE REMOVED, AND THE RIGHT EXTENDED OUTWARDS. (From Parker.)
 
b.o. basioccipital tract; s.o. supraoccipital tract; fo. frontal fontanelle; e.n,
external nostril; internal to it, internasal plate; a.t. tympanic annulus.
 
Bones : e.o. exoccipital; pr.o. prootic, partly overlapped by/, parietal; f. frontal ;
eth. rudiment of sphenethmoid ; na. nasal ; pmx. premaxillary ; mx. maxillary; /-.
pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; s<j. squamosal; ar. articular; d. dentary; m.mk. mento-Meckelian.
 
the proximal part of the process remains as the otic (metapterygoid) process, articulating with the auditory cartilage.
 
The pedicle (pd} retains its original attachment to the skull.
 
 
 
588 MAND1BULAR AND IIYOID BARS.
 
The palato-pterygoid soon becomes segmented into a transversely placed palatine, and a longitudinally placed pterygoid
(fig. 340). With the exception of a few ossifications, which present no features of special interest, the parts of the mandibular
arch have now reached their final condition, which is not very
different from that in the Axolotl.
 
Sauropsida. In the Sauropsida the modifications of the
hyoid and mandibular arches are fairly uniform.
 
The lower part of the hyoid arch, including the basihyoid,
unites with the remnants of the arches behind to form the hyoid
bone, to which it contributes the anterior cornu and anterior part
of the body.
 
The columella is believed by Huxley and Parker to represent,
as in the Anura, the independently developed dorsal (hyomandibular) element of the hyoid, together with the stapes with which
it has become united 1 .
 
The membranous mandibular arch gives off in the embryos
of all the Sauropsida an obvious bud to form the superior
maxillary process, and the formation of this bud appears to
represent the growth forwards of the pterygoid process in Elasmobranchii, which is indeed accompanied by the formation of a
similar bud ; but the skeletal rod, which appears in the axis
of this bud, is as a rule independent of that in the true arch
(fig- SS 1 ./^. PS}- The former is the pterygo-palatine bar; the
latter the Meckelian and quadrate cartilages.
 
The pterygo-palatine bar is usually if not always ossified
directly, without the intervention of cartilage.
 
Born has recently shewn that Parker was mistaken in supposing that
the palato-pterygoid bone is cartilaginous in Birds. In the Turtle a short
cartilaginous pterygoid process of the quadrate would seem to be present
(Parker, No. 458).
 
The quadrate and Meckelian cartilages are either from the
first separate, or very early become so.
 
1 The strongest evidence in favour of Huxley's and Parker's view of the nature of
the columella is the fusion in the adult Sphenodon of the upper end of the hyoid with
the columella (vide Huxley, No. 445). From an examination of a specimen in the
Cambridge museum I do not feel satisfied that the fusion is not secondary, but have
not been able to examine the junction of the hyoid and columella in section. For a
different view to that of Huxley vide Peters, "Ueb. d. Gehorknochelchen u. ihr
Verhaltniss zu. Zungenbeinbogen b. Sphenodon." Berlin MoHOtsbtnekU, 1874.
 
 
 
THE SKULL.
 
 
 
589
 
 
 
The quadrate cartilage ossifies as the quadrate bone, and
supplies the permanent articulation for the lower jaw. Its upper
end exhibits a tendency to divide into two processes, corresponding with the pedicle and otic processes of the Amphibia.
The Meckelian cartilage becomes soon covered by investing
bones, and its proximal end ossifies as the articulare. The
remainder of the cartilage usually disappears.
 
Mammalia. The most extraordinary metamorphosis of the
hyoid and mandibular arches occurs in the Mammalia, and has
been in part known since the publication of the memoir of
Reichert (No. 461).
 
Both the hyoid and mandibular arches develop at first more
completely than in any of the other types above Fishes; and are
 
 
 
 
pn.ch nc
 
 
 
FIG. 341. EMBRYO PIG, TWO-THIRDS OF AN INCH LONG ; ELEMENTS OF THE
 
SKULL SEEN SOMEWHAT DIAGRAMMATICALLY FROM BELOW. (From Parker.)
pa.ch. parachordal cartilage; nc. notochorcl; au. auditory capsule; py. pituitary
body; tr. trabeculse; c.lr. trabecular cornu; pn. prenasal cartilage; e.n. external
nasal opening; ol. nasal capsule; p-pg- palatopterygoid tract enclosed in the
maxillopalatine process; mn. mandibular arch ; hy. hyoid arch; th.h. first branchial
arch; ja. facial nerve; 8a. glossopharyngeal ; 86. vagus; 9. hypoglossal.
 
articulated to each other above, while the pterygo-palatine bar
is quite distinct. The main features of the subsequent development are undisputed, with the exception of that of the upper end
of the hyoid, which is still controverted. The following is Parker's
(No. 452) account for the Pig, which confirms in the main the
view originally put forward by Huxley (No. 445).
 
The mandibular and hyoid arches are at first very similar
 
 
 
5QO MANDIBULAR AND HYOID BARS.
 
(fig. 341 mn and hy), their dorsal ends being somewhat incurved,
and articulating together.
 
In a somewhat later stage (fig. 342) the upper end of the
mandibular bar (mb\ without becoming segmented from the
ventral part, becomes distinctly swollen, and clearly corresponds
to the quadrate region of other types. The ventral part of the
bar constitutes the Meckelian cartilage (mk).
 
The hyoid arch has in the meantime become segmented into
two parts, an upper part (z), which eventually becomes one of
 
 
 
 
FIG. 342. EMBRYO PIG, AN INCH AND A THIRD LONG; SIDE VIEW OF
MANDIBULAR AND HYOID ARCHES. THE MAIN HYOID ARCH IS SEEN AS DISPLACED BACKWARDS AFTER SEGMENTATION FROM THE INCUS. (From Parker.)
 
tg. tongue; ink. Meckelian cartilage; ml. body of malleus; mb. manubrium or
handle of the malleus; t.ty. tegmen tympani; i. incus; st. stapes; i.hy, interhyal
ligament; st.h. stylohyal cartilage; h.h. hypohyal ; ^.//.basibranchial; th.h. rudiment
of first branchial arch; -ja. facial nerve.
 
the small bones of the ear the incus and a lower part which
remains permanently as the anterior cornu of the hyoid (st./i).
The two parts continue to be connected by a ligament.
 
The incus is articulated with the quadrate end of the mandibular arch, and its rounded head comes in contact with the
stapes (fig. 342, st) which is segmented from the fenestra ovalis.
The main arch of the hyoid becomes divided into a hypohyal
(h.h) below and a stylohyal (st. h] above, and also becomes articulated with the basal element of the arch behind (b/i).
 
In the course of further development the Meckelian part of
the mandibular arch becomes enveloped in a superficial ossification forming the dentary. Its upper end, adjoining the quadrate
region, becomes calcified and then absorbed, and its lower, with
the exception of the extreme point, is ossified and subsequently
incorporated in the dentary.
 
The quadrate region remains relatively stationary in growth
 
 
 
TIIK SKULL. 591
 
 
 
as compared with the adjacent parts of the skull, and finally
ossifies to form the malleus bone of the ear. The processus
gracilis of the malleus is the primitive continuation into Meckel's
cartilage.
 
The malleus and incus are at first embedded in the connective tissue adjoining the tympanic cavity (hyomandibular cleft,
vide p. 528) ; and externally to them a bone known as the
tympanic bone becomes developed so that they become placed
between the tympanic bone and the periotic capsule. In late
fcetal life they become transported completely within the tympanic cavity, though covered by a reflection of the tympanic
mucous membrane.
 
The dorsal end of the part of the hyoid separated from the
incus becomes ossified as the tympano-hyal, and is anchylosed
with the adjacent parts of the periotic capsule. The middle part
of the bar just outside the skull forms the stylo-hyal (styloid
process in Man) which is attached by ligament to the anterior
cornu of the hyoid (cerato-hyal).
 
While the account of the formation of the malleus, incus, and stapes
just given is that usually accepted in this country, a somewhat different
view of the development of these parts has as a rule been adopted in
Germany. Reichert (No. 461) held that both the malleus and the incus
were derived from the mandibular bar ; and this view has been confirmed
by Giinther, Kolliker and other observers, and has recently been adopted
by Salensky (No. 462) after a careful research especially directed towards
this point. Reichert also held that the stapes was derived from the hyoid
bar ; but, though his observations on this point have been very widely
accepted, they have not met with such universal recognition as his views
on the origin of the malleus and incus. Salensky has recently arrived
at a view, which is in accord with that of Parker, in so far as the independence of the stapes of both the hyoid and mandibular arches is concerned.
Salensky however holds that it is formed from a mass of mesoblast
surrounding the artery of the mandibular arch, and that the form of the
stapes is due to its perforation by the mandibular artery. A product of
this artery permanently perforates the stapes in a few Mammalia, though
in the majority it atrophies.
 
In view of the different accounts of the origin of the incus the exact
nature of this bone must still be considered as an open question, but
should Reichert's view be confirmed the identification of the incus with
the columella of the Amphibia and Sauropsida must be abandoned.
 
 
 
592 MEMBRANE BONES.
 
 
 
Membrane bones and ossifications of the cranium.
 
The membrane bones of the skull may be divided into two
classes, viz. (i) those derived from dermal osseous plates, which
as explained above (p. 542) are primitively formed by the coalescence of the osseous plates of scales ; and (2) those formed by
the coalescence of the osseous plates of teeth lining the oral
cavity. Some of the bones sheathing the edge of the mouth
have been formed partly by the one process and partly by the
other.
 
In the Fishes there are found all grades of transition between
simple dermal scutes, and true subdermal osseous plates forming
an integral part of the internal skeleton. Dermal scutes are best
represented in Acipenser and some Siluroid Fishes.
 
Where the membrane bones still retain the character of dermal
plates, those on the dorsal surface of the cranium are usually
arranged in a series of longitudinal rows, continuing in the region
of the head the rows of dermal scutes of the trunk ; while the
remaining cranial scutes are connected with the visceral arches.
The dermal bones on the dorsal surface of the head are very
different in number, size, and arrangement in different types of
Fishes ; but owing to their linear disposition it is usually possible to
find a certain number both of the paired and unpaired bones
which have a similar situation in the different forms. These
usually receive the same names, but both from general considerations as to their origin, as well as from a comparison of different
species, it appears to me probable that there is no real homology
between these bones in different species, but only a kind of general
correspondence 1 .
 
It is not in fact till we get to the types above the Fishes that
we can find a series of homologous dorsal membrane bones
covering the roof of the skull. In these types three paired sets
of such bones are usually present, viz, from behind forwards the
parietals, frontals and nasals, the latter bounding the posterior
surface of the external nasal opening. Even in the higher
 
1 For some interesting remarks on the arrangement of these bones in Fishes, vide
Bridge, "On the Osteology of Polyodon folium." Phil. Trans., 1878.
 
 
 
THE SKULL. 593
 
 
 
types these bones are liable to vary very greatly from the usual
arrangement.
 
Besides these bones there is usually present in the higher
forms a lacrymal bone on the anterior margin of the orbit
derived from one of a series of periorbital membrane bones
frequently found in Fishes. Various supraorbital and postorbital
bones, etc. are also frequently found in Lacertilia, etc. which are
not impossibly phylogenetically independent of the membrane
bones inherited from Fishes; and may have been evolved as
bony scutes in the subdermal tissue of the papillae of the sauropsidan scales.
 
The visceral arches of Fishes, especially of the Teleostei, are
usually provided with a series of membrane bones. In the true
branchial arches these take the form of dentigerous plates ; but
no such plates are found in the Amphibia or Amniota.
 
The opercular flap attached to the hyoid arch is usually
supported by a series of membrane bones, which attain their
highest development in the Teleostei. One of these bones, the
praeopercular, is very constant and is primitively attached
along the outer edge of the hyomandibular. It seems to be
retained in Amphibia as a membrane bone, overlapping the
attachment of the quadrate and known as the squamosal ;
though it is not impossible that this bone may be derived from a
superficial membrane bone, widely distributed in Teleostei and
Ganoids, which is known as the supra-temporal. In Dipnoi
the bone which appears to be clearly homologous with the
squamosal would seem from its position to belong to the series of
dorsal plates, and therefore to be the supra-temporal ; but it is
regarded by Huxley (No. 446) as the praeopercular 1 .
 
In the Amniota the squamosal forms an integral part, of the
osseous roof of the skull ; but in the Sauropsida it continues, as
in Amphibia, to be closely related to the quadrate.
 
A larger series of persistent membrane bones are related to
the mandibular, and its palato-quadrate process.
 
Overlying the palato-quadrate process are two rows of bones,
 
1 It is not impossible that the solution of the difficulty about the praeopercular is
to be found by supposing that the praeopercular as it exists in Teleostei is derived
from a dorsal dermal plate, and that in the Dipnoi this plate retains more nearly than
in Teleostei its primitive position.
 
B. III. 3 8
 
 
 
594 MEMBRANE BONES.
 
 
 
one row lying at the edge of the mouth, on the outer side of the
pterygo-palatine process, and the other set on the roof of the
mouth superficial to the pterygo-palatine process.
 
The outer row is formed of the praemaxilla, maxilla, jugal,
and very often quadrato-jugal. Of these bones the maxilla
and prsemaxilla, as is more especially demonstrated by their
ontogeny in the Urodela, are partly derived from dentigerous
plates and partly from membrane plates outside the mouth;
while the jugal, and quadrato-jugal when present, are entirely
extra-oral. In the Amphibia and Amniota the praemaxillae and
maxillae are the most important bones in the facial region, and
are quite independent of any cartilaginous substratum.
 
The second row of bones is clearly constituted in the Dipnoi
and Amphibia by the vomer in front, then the palatine, and
finally the pterygoid behind. Of these bones the vomer is
never related to a cartilaginous tract below, while the palatines
and pterygoids usually are so. The position and growth of the
three bones in many Urodela (Axolotl) are especially striking
(Hertwig. No. 442). In the Axolotl they form a continuous
series, the vomer and palatine being covered by teeth, but the
pterygoid being without teeth. The vomer and palatine originate from the united osseous plates of the bases of the teeth,
while the pterygoid is in the first instance continuous with the
palatine.
 
In Teleostei, Amia, etc., there are dentigerous plates forming
a palatine and pterygoid, which in position, at any rate, closely
correspond with the similarly named bones in Amphibia ; and
there is also a dentigerous vomer which may fairly be considered
as equivalent to that in Amphibia.
 
In the Amniota the three bones found in Amphibia are always
present, but with a few exceptions amongst the Lacertilia and
Ophidia, are no longer dentigerous. The cartilaginous bars,
which in the lower types are placed below the palatine and
pterygoid membrane bones, are usually imperfectly or not at all
developed.
 
On Meckel's cartilage important membrane bones are almost
always grafted. On the outside and distal part of the cartilage a
dentary is usually developed, which may envelope and replace
the cartilage to a larger or smaller extent. Its oral edge
 
 
 
THE SKULL. 595
 
 
 
is usually dentigerous. The splenial membrane bone is the
most important bone on the inner side of Meckel's cartilage, but
other elements known as the coronoid and angular may also be
added. In Mammalia the dentary is the only element present
(vide p. 590).
 
On the roof of the mouth a median bone, the parasphenoid,
is very widely present in the Amphibia and Fishes, except the
Elasmobranchii and Cyclostomata, and has no doubt the same
phylogenetic origin as the vomer and membranous palatines and
pterygoids.
 
It is less important in the Sauropsida, and becomes indistinguishably fused with the sphenoid in the adult, while in
Mammalia it is no longer found.
 
Ossification of the Cartilaginous Cranium. In certain
Fishes the cartilaginous cranium remains quite unossified, while
completely enveloped in dermal bones. Such for instance is its
condition in the Selachioid Ganoids. In most instances, however,
the investment of the cartilaginous cranium by membrane bones
is accompanied by a more or less complete ossification of the
cartilage itself.
 
In the Dipnoi this occurs to the smallest extent, the only
ossifications occurring in the lateral parts of the occipital region,
and forming the exoccipitals.
 
In Teleostei and bony Ganoids, a considerably greater number
of ossifications occur in the cartilage.
 
In the region of the occipital cartilaginous ring there appears
a basioccipital and supraoccipital and two exoccipitals.
The basioccipital is the only bone on the floor of the skull
ossifying that part into which the notochord is primitively continued 1 .
 
In the region of the periotic cartilage a large number of
bones may appear. In front there is the prootic, which often
meets the exoccipital behind ; behind there is above and in close
connection with the supraoccipital the epiotic, and below in
close connection with the exoccipital the opisthotic. On the
dorsal side of the cartilage there is a projecting ridge composed
mainly of a bone known as the pterotic, sometimes erroneously
 
1 The notochord appears also to enter into the posterior part of the region which
ossifies as the basisphenoid.
 
383
 
 
 
59 6 OSSIFICATIONS OF THE CARTILAGINOUS CRANIUM.
 
called the squamosal, and continued in front by the sphenotic.
The pterotic, or the cartilaginous region corresponding to it,
always supplies the articular surface for the hyomandibular.
 
In the floor of the skull, in the region of the pituitary body,
there is formed a basisphenoid; while in the lateral parts of the
wall of this part of the cranium, there is a bone known as the
alisphenoid.
 
In front, parts of the lateral walls of the cranium ossify as the
orbitosphenoids.
 
In view of the very imperfect ossification of the cartilaginous
cranium of the Dipnoi, and of the fact that there is certainly no
direct genetic connection between the Teleostei on the one hand,
and the Amphibia and Amniota on the other, it is very difficult
to believe that most of the ossifications of the cranium in the
Amphibia and Amniota have more than a general correspondence
with those in the Teleostei.
 
In the Amphibia the ossifications in the cartilage are comparatively few. In the occipital region there is a lateral ossification
on each side of the exoccipital. the basioccipital region being
unossified, and the supraoccipital at the utmost indurated by a
calcareous deposit.
 
The periotic capsule is ossified by a prootic centre, which
meets the exoccipital behind.
 
The front part of the cartilaginous cranium is ossified by a
complete ring of bone the sphenethmoid bone which embraces
part of the ethmoid region, and of the orbitosphenoid and
presphenoid regions.
 
In the Amphibia the cartilaginous cranium, with its centres
of ossification, is easily separable from the membranous investing
bones.
 
In the Amniota the cartilaginous cranium, whose development
in the embryo has already been described, becomes in the adult
much more largely ossified, and the bones which replace the
primitive cartilage unite with the membrane bones to form a
continuous bony cranium.
 
The centres of ossification become again much more numerous.
In the occipital segment analogous centres to those of Teleostei
are again found ; and it is probable that the exoccipitals are
homologous throughout the series, the supraoccipital and basioc
 
 
THE SKULL. 597
 
 
 
cipital bones of the higher types being merely identical in position
with the similarly named bones in Fishes.
 
In the periotic there are usually three centres of ossification,
first recognised by Huxley. These are the prootic, the epiotic
and opisthotic, the situations of which have already been defined.
Of these the prootic is the most constant.
 
In Reptiles, the prootic and opisthotic frequently remain
distinct even in the adult.
 
In Birds, the epiotic and opisthotic are early united with the
supra- and exoccipital ; and at a later period the prootic is also
indistinguishably fused with the adjacent parts.
 
In Mammals the three ossifications fuse into a continuous
whole the periotic bone which may be partially united with
the adjacent parts.
 
In the pituitary region of the base of the cranium a pair of
osseous centres or in the higher types a single centre (Parker 1 )
gives rise to the basisphenoid bone, and in front of this another
basal or pair of basal ossifications forms the presphenoid, while
laterally to these two centres there are formed centres of
ossification in the alisphenoid and orbitosphenoid regions, which
may be extremely reduced in various Sauropsida, leaving the
side walls of the skull almost entirely formed of membrane or
cartilage.
 
In the ethmoid region there may arise a median ossification
forming the mesethmoid and lateral ossifications forming the
lateral ethmoids or prefrontals ; which may assist in forming the
front wall of the brain-case, or be situated quite externally to the
brain-case and be only related to the olfactory capsules.
 
The labial cartilages. In most Fishes a series of skeletal structures,
known as the labial cartilages, are developed at the front and sides of the
mouth, and in connection with the olfactory capsules ; and these cartilages
still persist in connection with the olfactory capsules, though in a reduced
form, in the higher types. They are more developed in the Cyclostomata
than in any other Vertebrate type.
 
The meaning of these cartilages is very obscure ; but, from their being in
part employed to support the lips and horny teeth of the Cyclostomata and
the Tadpole, I should be inclined to regard them as remnants of a primitive skeleton supporting the suctorial mouth, with which, on the grounds
already stated (p. 317), I believe the ancestors of the present Vertebrata
to have been provided.
 
1 According to Kblliker there are two centres in Man in both the basisphenoid
and presphenoid.
 
 
 
598 BIBLIOGRAPHY.
 
 
 
BIBLIOGRAPHY.
 
(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a
leur differents ages." Paris, Mem. savans etrang. 1835, and An. Set. A 7 af. Vol. I.
1834.
 
(440) C. Gegenbaur. Untersuchwigen z. vergleich. Anat. d. Wirbelthiere, III.
Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.
 
(441) Giinther. Beob. iib. die Entwick. d. Gehororgans. Leipzig, 1842.
 
(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f.
d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr. Anat., Vol. xi. 1874, suppl.
 
(443) T.H.Huxley. " On the theory of the vertebrate skull." Proc. Royal
Soc., Vol. ix. 1858.
 
(444) T. H. Huxley. The Elements of Comparative Anatomy. London, 1869.
 
(445) T.H.Huxley. "On the Malleus and Incus." Proc. Zool. Soc., 1869.
 
(446) T.H.Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.
 
(447) T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."
Journ. of Anat. and Phys., Vol. X. 1876.
 
(448) T.H.Huxley. The Anatomy of Vertebrated Animals. London, 1871.
 
(449) W. K. Parker. "On the structure and development of the skull of the
Common Fowl (Callus Domesticus)." Phil. Trans., 1869.
 
(450) W. K. Parker. "On the structure and development of the skull of the
Common Frog (Rana temporaria)." Phil. Trans., 1871.
 
(451) W. K. Parker. "On the structure and development of the skull in the
Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.
 
(452) W. K. Parker. "On the structure and development of the skull in the
Pig (Sus scrofa). " Phil. Trans., 1874.
 
(453) W. K. Parker. "On the structure and development of the skull in the
Batrachia." Part n. Phil. Trans., 1876.
 
(454) W. K. Parker. "On the structure and development of the skull in the
Urodelous Amphibia." Part in. Phil. Trans., 1877.
 
(455) W. K. Parker. "On the structure and development of the skull in the
Common Snake (Tropidonotus natrix)." Phil. Trans., 1878.
 
(456) W. K. Parker. " On the structure and development of the skull in Sharks
and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.
 
(457) W. K. Parker. "On the structure and development of the skull in the
Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans.,
1879.
 
(458) W. K. Parker. "The development of the Green Turtle." The Zoology
of the Voyage of H. M.S. Challenger. Vol. I. pt. V.
 
(459) W. K. Parker. "The structure and development of the skull in the
Batrachia." Pt. in. Phil. Trans., 1880.
 
(460) W. K. Parker and G. T. Belt any. The Morphology of the Skull.
London, 1877.
 
(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1839.
 
(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Miiller's
Archiv, 1837.
 
(462) W. Saleusky. "Beitragez. Entwick. d. knorpeligen Gehorknochelchen."
Morphol. Jahrbuch, Vol. VI. 1880.
 
Vide also Kolliker (No. 298), especially for the human and mammalian skull;
Gotte (No. 296).
 
 
 
CHAPTER XX.
 
 
 
THE PECTORAL AND PELVIC GIRDLES AND THE
SKELETON OF THE LIMBS.
 
 
 
TJie Pectoral girdle.
 
Pisces. Amongst Fishes the pectoral girdle presents itself
in its simplest form in Elasmobranchii, where it consists of a
bent band of cartilage on each side of the body, of somewhat
variable form, meeting and generally uniting with its fellow
ventrally. Its anterior border is in close proximity with the
last visceral arch, and a transverse ridge on its outer and
posterior border, forming the articular surface for the skeleton
of the limb, divides it into a dorsal part, which may be called
the scapula, and a ventral part which may be called the
coracoid.
 
In all the remaining groups of Fishes there is added to the
cartilaginous band, which may wholly or partially ossify, an
osseous support composed of a series of membrane bones.
 
In the types with such membrane bones the cartilaginous
parts do not continue to meet ventrally, except in the Dipnoi
where there is a ventral piece of cartilage, distinct from that
bearing the articulation of the limb. The cartilage is moreover
produced into two ventral processes, an anterior and a posterior,
below the articulation of the limb ; which may be called, in
accordance with Gegenbaur's nomenclature, the praecoracoid
and coracoid. Of these the praecoracoid is far the most
 
 
 
600 THE PECTORAL GIRDLE.
 
prominent, and in the majority of cases the coracoid can hardly
be recognised. The coracoid process is however well developed
in the Selachioid Ganoids, and the Siluroid Teleostei. In
Teleostei the scapular region often ossifies in two parts, the
smaller of which is named by Parker praecoracoid, though it is
quite distinct from Gegenbaur's praecoracoid. The membrane
bones, as they present themselves in their most primitive state
in Acipenser and the Siluroids, are dermal scutes embracing the
anterior edge of the cartilaginous girdle. In Acipenser there
are three scutes on each side. A dorsal scute known as the
supra-clavicle, connected above with the skull by the posttemporal ; a middle piece or clavicle, and a ventral or infraclavicle (inter-clavicle), which meets its fellow below.
 
In most Fishes the primitive dermal scutes have become
subdermal membrane bones, and the infra-clavicle is usually not
distinct, but the two clavicles form the most important part of
the membranous elements of the girdle. Additional membrane bones (post-clavicles) are often present behind the main
row.
 
The development of these parts in Fishes has been but little
studied.
 
In Scyllium, amongst the Elasmobranchii, I find that each
half of the pectoral girdle develops as a vertical bar of cartilage
at the front border of the rudimentary fin, and externally to the
muscle-plates.
 
Before the tissue forming the pectoral girdle has acquired
the character of true cartilage, the bars of the two sides meet
ventrally by a differentiation in situ of the mesoblastic cells, so
that, when the girdle is converted into cartilage, it forms an
undivided arc, girthing the ventral side of the body. There is
developed in continuity with the posterior border of this arc on
the level of the fin a horizontal bar of cartilage, which is
continued backwards along the insertion of the fin, and, as will
be shewn in the sequel, becomes the metapterygium of the adult
(figs. 344, bp and 348, mp). With this bar the remaining skeletal
elements of the fin are also continuous.
 
The foramina of the pectoral girdle are not in the first
instance formed by absorption, but by the non-development of
the cartilage in the region of pre-existing nerves and vessels.
 
 
 
THE PECTORAL GIRDLE. 6oi
 
The development of these parts in Teleostei has been recently investigated
by 'Swirski (No. 472) who finds in the Pike (Esox) that the cartilaginous
pectoral girdle is at first continuous with the skeleton of the fin. It forms
a rod with a dorsal scapular and ventral coracoid process. An independent
mass of cartilage gives rise to a prascoracoid, which unites with the main
mass, forming a triradiate bar like that of Acipenser or the Siluroids.
The coracoid process becomes in the course of development gradually
reduced.
 
'Swirski concludes that the so-called praecoracoid bar is to some extent
a secondary element, and that the coracoid bar corresponds to the whole of
the ventral part of the girdle of Elasmobranchii, but his investigations do
not appear to me to be as complete as is desirable.
 
Amphibia and Amniota. The pectoral girdle contains a
more or less constant series of elements throughout the
Amphibia and Amniota ; and the differences in structure
between the shoulder girdle of these groups and that of Fishes
are so great that it is only possible to make certain general
statements respecting the homologies of the parts in the two
sets of types.
 
The generally accepted view, founded on the researches of
Parker, Huxley, and Gegenbaur, is to the effect that there is a
primitively cartilaginous coraco-scapular plate, homologous with
that in Fishes, and that the membrane bones in Fishes are
represented by the clavicle and inter-clavicle in the Sauropsida
and Mammalia, which are however usually admitted to be
absent in Amphibia. These views have recently been challenged
by Gotte (No. 466) and Hoffmann (No. 467), on the ground of
a series of careful embryological observations ; and until the
whole subject has been worked over by other observers it does
not seem possible to decide satisfactorily between the conflicting
views. It is on all hands admitted that the scapulo-coracoid
elements of the shoulder girdle are formed as a pair of cartilaginous plates, one on each side of the body. The dorsal half
of each plate becomes the scapula, which may subsequently
become divided into a supra-scapula and scapula proper ; while
the ventral half forms the coracoid, which is not always separated
from the scapula, and is usually divided into a coracoid proper,
a praecoracoid, and an epicoracoid. By the conversion of parts
of the primitive cartilaginous plates into membranous tissue
various fenestrae may be formed in the cartilage, and the bars
 
 
 
602 THE NATURE OF THE CLAVICLE.
 
bounding these fenestrae both in the scapula and coracoid
regions have received special names ; the anterior bar of the
coracoid region, forming the praecoracoid, being especially
important. At the boundary between the scapula and the
coracoid, on the hinder border of the plate, is placed the glenoid
articular cavity to carry the head of the humerus.
 
The grounds of difference between Gotte and Hoffmann and
other anatomists concern especially the clavicle and inter-clavicle.
The clavicle is usually regarded as a membrane bone which may
become to some extent cartilaginous. By. the above anatomists,
and by Rathke also, it is held to be at first united with the
coraco-scapular plate, of which it forms the anterior limb, free
ventrally, but united dorsally with the main part of the plate ;
and Gotte and Hoffmann hold that it is essentially a cartilage
bone, which however in the majority of the Reptilia ossifies
directly without passing through the condition of cartilage.
 
The interclavicle (episternum) is held by Gotte to be
developed from a paired formation at the free ventral ends of
the clavicles, but he holds views which are in many respects
original as to its homologies in Mammalia and Amphibia. Even
if Gotte's facts are admitted, it does not appear to me necessarily
to follow that his deductions are correct. The most important
of these is to the effect that the dermal clavicle of Pisces has no
homologue in the higher types. Granting that the clavicle in
these groups is in its first stage continuous with the coracoscapular plate, and that it may become in some forms cartilaginous before ossifying, yet it seems to me all the same quite
possible that it is genetically derived from the clavicle of Pisces,
but that it has to a great extent lost even in development its
primitive characters, though these characters are still partially
indicated in the fact that it usually ossifies very early and
partially at least as a membrane bone 1 .
 
In treating the development of the pectoral girdle systematically it will be
convenient to begin with the Amniota, which may be considered to fix the
nomenclature of the elements of the shoulder girdle.
 
1 The fact of the clavicle going out of its way, so to speak, to become cartilaginous
before being ossified, may perhaps be explained by supposing that its close connection
with the other parts of the shoulder girdle has caused, by a kind of infection, a change
in its histological characters.
 
 
 
II IK PECTORAL GIRDLE.
 
 
 
603
 
 
 
Lacertilia. The shoulder girdle is formed as two membranous plates,
from the dorsal part of the anterior border of each of which a bar projects
(Rathke, Gotte), which is free at its ventral end. This bar, which is usually
(Gegenbaur, Parker) held to be independent of the remaining part of the
shoulder girdle, gives rise to the clavicle and interclavicle. The scapulocoracoid plate soon becomes cartilaginous, while at the same time the clavicular bar ossifies directly from the membranous state. The ventral ends
of the two clavicular bars enlarge to form two longitudinally placed plates,
which unite together and ossify as the interclavicle.
 
Parker gives a very different account of the interclavicle in Anguis. He
states that it is formed of two pairs of bones 'strapped on to the antero-inferior part of the prassternum,' which subsequently unite into one.
 
Chelonia. The shoulder girdle of the Chelonia is formed (Rathke) of
a triradiate cartilage on each side, with one dorsal and two ventral limbs.
It is admitted on all hands that the dorsal limb is the scapular element,
and the posterior ventral limb the coracoid ; but, while the anterior ventral
limb is usually held to be the praecoracoid, Gotte and Hoffmann maintain
that, in spite of its being formed of cartilage, it is homologous with the
anterior bar of the primitive shoulder-plates of Lacertilia, and therefore the
homologue of the clavicle.
 
Parker and Huxley (doubtfully) hold that the three anterior elements of
the ventral plastron (entoplastron and epiplastra) are homologous with the
interclavicle and clavicles, but considering that these plates appear to belong
to a secondary system of dermal ossifications peculiar to the Chelonia, this
homology does not appear to me probable.
 
Aves. There are very great differences of view as to the development
of the pectoral arch of Aves.
 
About the presence in typical forms of the coraco-scapular plate and two
independent clavicular bars all authors are agreed. With reference to the
clavicle and interclavicle Parker (No. 468) finds that the scapular end of the
clavicle attaches itself to and ossifies a mass of cartilage, which he regards
as the mesoscapula, while the interclavicle is formed of a mass of tissue between the ends of the clavicles where they meet ventrally, which becomes
the dilated plate at their junction.
 
Gegenbaur holds that the two primitive clavicular bars are simply clavicles, without any element of the scapula ; and states that the clavicles are
not entirely ossified from membrane, but that a delicate band of cartilage
precedes the osseous bars. He finds no interclavicle.
 
Gotte and Rathke both state that the clavicle is at first continuous with
the coraco-scapular plate, but becomes early separated, and ossifies entirely
as a membrane bone. Gotte further states that the interclavicles are formed
as outgrowths of the median ends of the clavicles, which extend themselves
at an early period of development along the inner edges of the two halves of
the sternum. They soon separate from the clavicles, which subsequently
meet to form the furculum ; while the interclavicular rudiments give rise, on
the junction of the two halves of the sternum, to its keel, and to the ligament
 
 
 
604 THK PECTORAL GIRDLE.
 
connecting the furculum with the sternum. The observations of Gotte,
which tend to shew the keel of the sternum is really an interclavicle, appear
to me of great importance.
 
A prascoracoid, partially separated from the coracoid by a space, is present in Struthio. It is formed by a fenestration of a primitively continuous
cartilaginous coracoid plate (Hoffmann). In Dromaeus and Casuarius clavicles are present (fused with the scapula in the adult Dromaeus), though
absent in other Ratitae (Parker, etc.).
 
Mammalia. The coracoid element of the coraco-scapular plate is
much reduced in Mammalia, forming at most a simple process (except in the
Ornithodelphia) which ossifies however separately 1 .
 
With reference to the clavicles the same divergencies of opinion met with
in other types are found here also.
 
The clavicle is stated by Rathke to be at first continuous with the coracoscapular plate. It is however soon separated, and ossifies very early, in the
human embryo before any other bone. Gegenbaur however shewed that
the human clavicle is provided with a central axis of cartilage, and this observation has been confirmed by Kolliker, and extended to other Mammalia by
Gotte. The mode of ossification is nevertheless in many respects intermediate between that of a true cartilage bone and a membrane bone. The
ends of the clavicles remain for some time, or even permanently, cartilaginous, and have been interpreted by Parker, it appears to me on hardly
sufficient grounds, as parts of the mesoscapula and praecoracoid. Parker's
so-called mesoscapula may ossify separately. The homologies of the episternum are much disputed. Gotte, who has worked out the development of the
parts more fully than any other anatomist, finds that paired interclavicular
elements grow out backwards from the ventral ends of the clavicles, and
uniting together form a somewhat T-shaped interclavicle overlying the front
end of the sternum. This condition is permanent in the Ornithodelphia,
except that the anterior part of the sternum undergoes atrophy. But in the
higher forms the interclavicle becomes almost at once divided into three
parts, of which the two lateral remain distinct, while the median element
fuses with the subjacent part of the sternum and constitutes with it the presternum (manubrium sterni). If Gotte' s facts are to be trusted, and they
have been to a large extent confirmed by Hoffmann, his homologies appear to
be satisfactorily established. As mentioned on p. 563 Ruge (No. 438) holds
that Gotte is mistaken as to the origin of the presternum.
 
Gegenbaur admits the lateral elements as parts of the interclavicle, while
Parker holds that they are not parts of an interclavicle but are homologous
with the omosternum of the Frog, which is however held by Gotte to be a
true interclavicle.
 
1 This process, known as the coracoid process, is held by Sabatier to be the
pnecoracoid ; while this author also holds that the upper third of the glenoid cavity,
which ossifies by a special nucleus, is the true coracoid. The absence of a praecoracoid in the Ornithodelphia is to my mind a serious difficulty in the way of
Sabatier's view.
 
 
 
THE PECTORAL GIRDLE. 605
 
Amphibia. In Amphibia the two halves of the shoulder girdle are
each formed as a continuous plate, the ventral or coracoid part of which is
forked, and is composed of a larger posterior and a smaller anterior bar-like
process, united dorsally. In the Urodela the two remain permanently free
at their ventral ends, but in the Anura they become united, and the space
between them then forms a fenestra. The anterior process is usually (Gegenbaur, Parker) regarded as the praecoracoid, but Gotte has pointed out that
in its mode of development it strongly resembles the clavicle of the higher
forms, and behaves quite differently to the so-called praecoracoid of Lizards.
It is however to be noticed that it differs from the clavicle in the fact that it
is never segmented off from the coraco-scapular plate, a condition which has
its only parallel in the equally doubtful case of the Chelonia. Parker holds
that there is no clavicle present in the Amphibia, while Gegenbaur maintains
that an ossification which appears in many of the Anura (though not in the
Urodela) in the perichondrium on the anterior border of the cartilaginous
bar above mentioned is the representative of the clavicle. Gotte's observations on the ossification of this bone throw doubt upon this view of Gegenbaur ; while the fact that the cartilaginous bar may be completely enclosed
by the bone in question renders Gegenbaur's view, that there is present both
a clavicle and prsecoracoid, highly improbable.
 
No interclavicle is present in Urodela, but in this group and in a number
of the Anura, a process grows out from the end of each of the bars (praecoracoids) which Gotte holds to be the clavicles. The two processes unite
in the median line, and give rise in front to the anterior unpaired element of
the shoulder girdle (omosternum of Parker). They sometimes overlap the
epicoracoids behind, and fusing with them bind them together in the median
line. Parker who has described the paired origin of the so-called omosternum,
holds that it is not homologous with the interclavicle, but compares it with
his omosternum in Mammals.
 
 
 
BIBLIOGRAPHY.
 
(463) Bruch. " Ueber die Entwicklung der Clavicula und die Farbe des
Blutes. " Zeit.f. wiss. Zool., \\. 1853.
 
(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a
leurs differens ages." Memoires des savants etrang. Academic royale des sciences de
Finstitut de France^ Vol. vi. 1835.
 
(465) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomie der Wirbelthiere, 2 Heft. Schultergiirtel der Wirbelthiere. Bmstflosse der Fische. Leipzig,
1865.
 
(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel." Archivf. mikr, Anat. Vol. xiv. 1877.
 
(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederlandisches Archivf. ZooL,Vol.v. 1879.
 
(468) W. K. Parker. "A Monograph on the Structure and Development of the
Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.
 
 
 
606 PELVIC GIRDLE.
 
 
 
(469) H. Rathke. Ueber die Entwicklung der Schildkrbten. Braunschweig,
1848.
 
(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der
Saurier, 1853.
 
(471) A. Sabatier. Comparaison des ceinfures et des membres antMeurs et posttrtturs d. la Serie d. Vertttrh. Montpellier, 1880.
 
(472) Georg 'Swirski. Untersuch. iib. d. Entwick. d. Schultergiirtels n. d.
Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.
 
 
 
Pelvic girdle.
 
Pisces. The pelvic girdle of Fishes is formed of a cartilaginous band, to the outer and posterior side of which the basal
element of the pelvic fin is usually articulated. This articulation
divides it into a dorsal iliac, and ventral pubic section. The iliac
section never articulates with the vertebral column.
 
In Elasmobranchii the two girdles unite ventrally, but the
iliac section is only slightly developed. In Chimaera there is a
well developed iliac process, but the pubic parts of the girdle
are only united by connective tissue.
 
In the cartilaginous Ganoids the pelvic girdle is hardly to be
separated from the skeleton of the fin. It is not united with its
fellow, and is represented by a plate with slightly developed
pubic and iliac processes.
 
In the Dipnoi there is a simple median cartilage, articulated
with the limb, but not provided with an iliac process. In bony
Ganoids and Teleostei there is on each side a bone meeting its
fellow in the ventral line, which is usually held to be the rudiment of the pelvic girdle ; while Davidoff attempts to shew that
it is the basal element of the fin, and that, except in Polypterus,
a true pelvic girdle is absent in these types.
 
From my own observations I find that the mode of development of the pelvic girdle in Scyllium is very similar to that of
the pectoral girdle. There is a bar on each side, continuous on
its posterior border with the basal element of the fin (figs. 345
and 347). This bar meets and unites with its fellow ventrally
before becoming converted into true cartilage, and though the
iliac process (il) is never very considerable, yet it is better developed in the embryo than in the adult, and is at first directed
nearly horizontally forwards.
 
Amphibia and Amniota. The primitive cartilaginous pelvic
 
 
 
PELVIC GIRDLE. 607
 
 
 
girdle of the higher types exhibits the same division as that of
Pisces into a dorsal and a ventral section, which meet to form
the articular cavity for the femur, known as the acetabulum.
The dorsal section is always single, and is attached by means
of rudimentary ribs to the sacral region of the vertebral column,
and sometimes to vertebrae of the adjoining lumbar or caudal
regions. It always ossifies as the ilium.
 
The ventral section is usually formed of two more or less
separated parts, an anterior which ossifies as the pubis, and a
posterior which ossifies as the ischium. The space between them
is known as the obturator foramen. In the Amphibia the two
parts are not separated, and resemble in this respect the pelvic
girdle of Fishes. They generally meet the corresponding elements
of the opposite side ventrally, and form a symphysis with them.
The symphysis pubis, and symphysis ischii may be continuous
(Mammalia, Amphibia).
 
The observations on the development of the pelvic girdle in
the Amphibia and Amniota are nearly as scanty as on those of
Fishes.
 
Amphibia. In the Amphibia (Bunge, No. 473) the two halves of
the pelvic girdle are formed as independent masses of cartilage, which
subsequently unite in the ventral line.
 
In the Urodelous Amphibia (Triton) each mass is a simple plate of
cartilage divided into a dorsal and ventral section by the acetabulum.
The ventral parts, which are not divided into two regions, unite in a
symphysis comparatively late.
 
The dorsal section ossifies as the ilium. The ventral usually contains
a single ossification in its posterior part which forms the ischium ; while
the anterior part, which may be considered as representing the pubis,
usually remains cartilaginous ; though Huxley (No. 475) states that it has
a separate centre of ossification in Salamander, which however does not
appear to be always present (Bunge). There is a small obturator foramen
between the ischium and pubis, which gives passage to the obturator nerve.
It is formed by the part of the tissue where the nerve is placed not becoming converted into cartilage.
 
There is a peculiar cartilage in the ventral median line in front of the
pubis, which is developed independently of and much later than the true
parts of the pelvic girdle. It may be called the praepubic cartilage.
 
Reptilia. In Lacertilia the pelvic girdle is formed as a somewhat
triradiate mass of cartilage on each side, with a dorsal (iliac) process, and two
ventral (pubic and ischiad) processes. The acetabulum is placed on the
outer side at the junction of the three processes, each of which may be
 
 
 
6o8 PECTORAL AND PELVIC GIRDLES.
 
considered to have a share in forming it. The distal ends of the pubis
and ischium are close together when first formed, but subsequently separate.
Each of them unites at a late stage with the corresponding process of the
opposite side in a ventral symphysis. A centre of ossification appears in
each of the three processes of the primitive cartilage.
 
Aves. In Birds the parts of the pelvic girdle no longer develop as a
continuous cartilage (Bunge). Either the pubis may be distinct, or, as in the
Uuck, all the elements. The ilium early exhibits a short anterior process,
but the pubis and ischium are at first placed with their long axes at right
angles to that of the ilium, but gradually become rotated so as to lie parallel with it, their distal ends pointing backwards, and not uniting ventrally
excepting in one or two Struthious forms.
 
Mammalia. In Mammalia the pelvic girdle is formed in cartilage
as in the lower forms, but in Man at any rate the pubic part of the cartilage is formed independently of the remainder (Rosenberg). There are
the usual three centres of ossification, which unite eventually into a single
bone the innominate bone. The pubis and ischium of each side unite with
each other ventrally, so as completely to enclose the obturator foramen.
 
Huxley holds that the so-called marsupial bones of Monotremes and
Marsupials, which as shewn by Gegenbaur (No. 474) are performed in cartilage, are homologous with the praepubis of the Urodela ; but considering
the great gap between the Urodela and Mammalia this homology can only
be regarded as tentative. He further holds that the anterior prolongations
of the cartilaginous ventral ends of the pubis of Crocodilia are also structures of the same nature.
 
 
 
BIBLIOGRAPHY.
 
(473) A. Bunge. Untersuch. z, Entwick. d. Beckengiirtels d. Amphibien,
Reptilien u. Vogel, Inaug. Diss. Dorpat, 1880.
 
(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne
d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.
 
(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
Proc. of Roy. Soc., Vol. xxvm. 1879.
 
(476) A. Sabatier. Comparaison des ceintures et des membres anterieurs et
posterieurs dans la Serie d. Vertebrcs. Montpellier, 1880.
 
Comparison of Pectoral and Pelvic girdles.
 
Throughout the Vertebrata a more or less complete serial
homology may be observed between the pectoral and pelvic
girdles.
 
In the cartilaginous Fishes each girdle consists of a continuous
band, a dorsal and ventral part being indicated by the articulation
of the fin ; the former being relatively undeveloped in the pelvic
 
 
 
LIMBS. 609
 
girdle, while in the pectoral it may articulate with the vertebral
column. In the case of the pectoral girdle secondary membrane
bones become added to the primitive cartilage in most Fishes,
which are not developed in the case of the pelvic girdle.
 
In the Amphibia and Amniota the ventral section of each
girdle becomes divided into an anterior and a posterior part, the
former constituting the praecoracoid and pubis, and the latter the
coracoid and ischium ; these parts are however very imperfectly
differentiated in the pelvic girdle of the Urodela. The ventral
portions of the pelvic girdle usually unite below in a symphysis.
They also meet each other ventrally in the case of the pectoral
girdle in Amphibia, but in most other types are separated by
the sternum, which has no homologue in the pelvic region, unless
the praepubic cartilage is to be regarded as such. The dorsal or
scapular section of the pectoral girdle remains free ; but that of
the pelvic girdle acquires a firm articulation with the vertebral
column.
 
If the clavicle of the higher types is derived from the membrane bones of the pectoral girdle of Fishes, it has no homologue
in the pelvic girdle ; but if, as Gotte and Hoffmann suppose, it is
a part of the primitive cartilaginous girdle, the ordinary view as
to the serial homologies of the ventral sections of the two girdles
in the higher types will need to be reconsidered.
 
Limbs.
 
It will be convenient to describe in this place not only the
development of the skeleton of the limbs but also that of the
limbs themselves. The limbs of Fishes are moreover so different
from those of the Amphibia and Amniota that the development
of the two types of limb may advantageously be treated separately.
 
In Fishes the first rudiments of the limbs appear as slight
longitudinal ridge-like thickenings of the epiblast, which closely
resemble the first rudiments of the unpaired fins.
 
These ridges are two in number on each side, an anterior
immediately behind the last visceral fold, and a posterior on the
level of the cloaca. In most Fishes they are in no way connected, but in some Elasmobranch embryos, more especially in
Torpedo, they are connected together at their first development
B. in. 39
 
 
 
6io
 
 
 
PAIRED FINS OF ELASMOBRANCHII.
 
 
 
by a line of columnar epiblast cells 1 . This connecting line of
columnar epiblast is a very transitory structure, and after its
disappearance the rudimentary fins become more prominent,
consisting (fig. 343, &) of a projecting ridge both of epiblast and
mesoblast, at the outer edge of which is a fold of epiblast only,
which soon reaches considerable dimensions. At a later stage
the mesoblast penetrates into this fold and the fin becomes a
simple ridge of mesoblast, covered
by epiblast. The pectoral fins
are usually considerably ahead
of the pelvic fins in development.
 
For the remaining history it
is necessary to confine ourselves
to Scylliurn as the only type
which has been adequately
studied.
 
The direction of the original
ridge which connects the two fins
of each side is nearly though not
quite longitudinal, sloping somewhat obliquely downwards. It
thus comes about that the attachment of each pair of limbs is
somewhat on a slant, and that
the pelvic pair nearly meet each
other in the median ventral line
a little way behind the anus.
 
The elongated ridge, forming
the rudiment of each fin, gradually projects more and more, and
so becomes broader in proportion to its length, but at the same
time its actual attachment to the side of the body becomes
shortened from behind forwards, so that what was originally the
attached border becomes in part converted into the posterior
border. This process is much more completely carried out in
the case of the pectoral fins than in that of the pelvic, and the
changes of form undergone by the pectoral fin in its development may be gathered from figs. 344 and 348.
 
 
 
 
FIG. 343. SECTION THROUGH
THE VENTRAL PART OF THE TRUNK
OF A YOUNG EMBRYO OF SCYLLIUM AT
THE LEVEL OF THE UMBILICAL CORD.
 
b. pectoral fin ; ao. dorsal aorta ;
cav. cardinal vein ; ua. vitelline artery ; u.v, vitelline vein ; al. duodenum ; /. liver ; sd. opening of segmented duct into the body cavity ;
mp. muscle plate ; ;. umbilical
canal.
 
 
 
1 I''. M. I'alfour. Monograph on Elasmobranfh l-'hhes, pp. 1012.
 
 
 
 
LIMBS. 6ll
 
Before proceeding to the development of the skeleton of
the fin it may be pointed out that the connection of the two
rudimentary fins by a continuous epithelial line suggests the
hypothesis that they are the remnants of two continuous lateral
fins 1 .
 
Shortly after the view that the paired fins were remnants of
continuous lateral fins had been put forward in my memoir on
Elasmobranch Fishes, two very interesting papers were published
by Thacker (No. 489) and Mivart (No. 484) advocating this
view on the entirely independent grounds of the adult structure
of the skeleton of the paired fins in comparison with that of the
unpaired fins 2 .
 
The development of the skeleton has unfortunately not been
as yet very fully studied. I have however made some investigations on this subject on Scyllium, and 'Swirski has also made
some on the Pike.
 
In Scyllium the development of both the pectoral and pelvic
fins is very similar.
 
In both fins the skeleton in its earliest stage consists of a bar
springing from the posterior side of the pectoral or pelvic girdle,
and running backwards parallel to the long axis of the body.
The outer side of this bar is continued into a plate which
 
1 Both Maclise arid Humphry {Journal of Anat. and Pkys., Vol. v.) had
previously suggested that the paired fins were related to the unpaired fins.
 
2 Davidoff in a Memoir (No. 477) which forms an important contribution to our
knowledge of the structure of the pelvic fins has attempted from his observations to
deduce certain arguments against the lateral fin theory of the limbs. His main
argument is based on the fact that a variable but often considerable number of the
spinal nerves in front of the pelvic fin are united, by a longitudinal commissure, with
the true plexus of the nerves supplying the fin. From this he concludes that the pelvic
fin has shifted its position, and that it may once therefore have been situated close
behind the visceral arches. If this is the strongest argument which can be brought
against the theory advocated in the text, there is I trust a considerable chance of its
being generally accepted. For even granting that Davidoff's deduction from the
character of the pelvic plexus is correct, there is, so far as I see, no reason in the
nature of the lateral fin theory why the pelvic fins should not have shifted, and on the
other hand the longitudinal cord connecting some of the spinal nerves in front of the
pelvic fin may have another explanation. It might for instance be a remnant of the
time when the pelvic fin had a more elongated form than at present, and accordingly
extended further forwards.
 
In any case our knowledge of the nature and origin of nervous plexuses is far too
imperfect to found upon their character such conclusions as those of Davidoff.
 
392
 
 
 
612
 
 
 
PAIRED FINS OF ELASMOBRANCHII.
 
 
 
extends into the fin, and which becomes very early segmented
into a series of parallel rays at right angles to the longitudinal
bar.
 
In other words, the primitive skeleton of both the fins
consists of a longitudinal bar running along the base of the fin,
 
 
 
 
FIG. 344. PECTORAL FIN OF A YOUNG EMBRYO OF SCYLLIUM IN LONGITUDINAL AND HORIZONTAL SECTION.
 
The skeleton of the fin was still in the condition of embryonic cartilage.
b.p. basipterygium (eventual metapterygium) ; fr. fin rays; p.g. pectoral girdle in
transverse section; /. foramen in pectoral girdle; pc. wall of peritoneal cavity.
 
and giving off at right angles series of rays which pass into the
fin. The longitudinal bar, which may be called the basipterygium, is moreover continuous in front with the pectoral or
pelvic girdle as the case may be.
 
The primitive skeleton of the pectoral fin is shewn in
longitudinal section in fig. 344, and that of the pelvic fin at a
slightly later stage in fig. 345.
 
A transverse section shewing the basipterygium (inpi) of the
pectoral fin, and the plate passing from it into the fin, is shewn
in fig. 346.
 
Before proceeding to describe the later history of the two
fins it may be well to point out that their embryonic structure
completely supports the view which has been arrived at from
the consideration of the soft parts of the fin.
 
My observations shew that the embryonic skeleton of the
paired fin consists of a series of parallel rays similar to those
of the unpaired fins. These rays support the soft part of the fin
which has the form of a longitudinal ridge, and are continuous
at their base with a longitudinal bar, which may very probably
 
 
 
LIMBS.
 
 
 
613
 
 
 
be due to secondary development. As pointed out by Mivart, a
longitudinal bar is also occasionally formed to support the
cartilaginous rays of unpaired
fins. The longitudinal bar of
the paired fins is believed by
both Thacker and Mivart to
be due to the coalescence of
the bases of primitively independent rays, of which they
believe the fin to have been
originally composed. This
view is probable enough in
itself, but there is no trace
 
 
 
 
FIG. 345. PELVIC FIN OF A VERY
YOUNG FEMALE EMBRYO OF SCYLLIUM
STELLARE.
 
bb. basipterygium ; pu. pubic process
of pelvic girdle ; il. iliac process of pelvic
girdle.
 
 
 
in the embryo of the bar in question being formed by the
coalesceace of rays, though the fact of its being perfectly
continuous with the bases of the rays is somewhat in favour
of this view 1 .
 
A point may be noticed here which may perhaps appear to be a
difficulty, viz. that to a considerable extent in the pectoral, and to some
extent in the pelvic fin the embryonic cartilage from which the fin-rays
are developed is at first a continuous lamina, which subsequently segments
into rays. I am however inclined to regard this merely as a result of the
mode of conversion of the indifferent mesoblast into cartilage ; and in any
case no conclusion adverse to the above view can be drawn from it, since
I find that the rays of the unpaired fin are similarly segmented from a
continuous lamina. In all cases the segmentation of the rays is to a large
extent completed before the tissue in question is sufficiently differentiated
to be called cartilage by an histologist.
 
Thacker and Mivart both hold that the pectoral and pelvic
girdles have been evolved by ventral and dorsal growths of the
anterior end of the longitudinal bar supporting the fin-rays.
 
There is, so far as I see, no theoretical objection to be taken
to this view, and the fact of the pectoral and pelvic girdles
originating continuously, and long remaining united with the
 
1 Thacker more especially founds his view on the adult form of the pelvic fins in
the cartilaginous Ganoids ; Polyodon, in which the part which constitutes the basal
plate in other forms is divided into separate segments, being mainly relied on. It is
possible that the segmentation of this plate, as maintained by Gegenbaur and Davidoff,
is secondary, but Thacker's view that the segmentation is a primitive character seems
to me, in the absence of definite evidence to the reverse, the more natural one.
 
 
 
614
 
 
 
THE PELVIC FIN.
 
 
 
longitudinal bars of their respective fins is in favour of rather
than against this view. The same may be said of the fact that
the first part of each girdle to be formed is that in the neighbourhood of the longitudinal bar (basipterygium) of the fin, the
dorsal and ventral prolongations being subsequent growths.
 
The later development of the skeleton of the two fins is more
conveniently treated separately.
 
The pelvic fin. The changes in the pelvic fin are comparatively slight. The fin remains through life as a nearly horizontal
lateral projection of the body, and the longitudinal bar the
 
 
 
 
FIG. 346. TRANSVERSE SECTION THROUGH THE PECTORAL FIN OF A YOUNG
 
EMBRYO OK SCYLLIUM STELLARE.
mpt. basipterygial bar (metapterygium) ; fr. fin ray; m. muscles; hf. horny fibres.
 
basipterygium at its base always remains as such. It is for a
considerable period attached to the pelvic girdle, but eventually
becomes segmented from it. Of the fin rays the anterior
remains directly articulated with the pelvic girdle on the separation of the basipterygium (fig. 347), and the remaining rays
finally become segmented from the basipterygium, though they
remain articulated with it. They also become to some extent
transversely segmented. The posterior end of the basipterygial
bar also becomes segmented off as the terminal ray.
 
The pelvic fin thus retains in all essential points its primitive
arrangement.
 
 
 
LIMBS.
 
 
 
6l 5
 
 
 
The pectoral fin. The earliest stage of the pectoral fin
 
 
 
 
There
 
 
 
FIG. 347. PELVIC FIN OF A YOUNG MALE EMBRYO OF SCYLLIUM STELLARE.
 
bp. basipterygium ; m.o. process of basipterygium continued into clasper; il. iliac
process of pectoral girdle ; pit. pubis.
 
differs from that of the pelvic fin only in minor points,
is the same longitudinal
or basipterygial bar to
which the fin-rays are
attached, whose position
at the base of the fin is
clearly seen in the transverse section (fig. 346,
mpf). In front the bar is
continuous with the pectoral girdle (figs. 344 and
 
348).
 
The changes which
take place in the course of
the further development
are however very much
more considerable in the
case of the pectoral than
in that of the pelvic fin. "' 3+8. F^OJJL ,,, v.
 
By the process spoken m p t me tapterygium (basipterygium of earlier
 
stage); me.p. rudiment of future pro- and mesopterygium ; sc. cut surface of scapular process ;
cr. coracoid process;/;', foramen;/, horny fibres.
 
 
 
 
of above, by which the
attachment of the pec
 
 
6l6 THE PECTORAL FIN.
 
toral fin to the body wall becomes shortened from behind
forwards, the basipterygial bar is gradually rotated outwards,
its anterior end remaining attached to the pectoral girdle.
In this way this bar comes to form the posterior border of the
skeleton of the fin (figs. 348 and 349, mp], constituting what
Gegenbaur called the metapterygium, and eventually becomes
segmented off from the pectoral girdle, simply articulating
with its hinder edge.
 
The plate of cartilage, which is continued outwards from the
basipterygium, or as we may now call it, the metapterygium,
into the fin, is not nearly so completely divided up into fin-rays
as in the case of the pelvic fin, and this is especially the case
with the basal part of the plate. This basal part becomes
in fact at first only divided into two parts (fig. 348) a small
anterior part at the front end (me.p), and a larger posterior along
the base of the remainder of the fin. The anterior part directly
joins the pectoral girdle at its base, resembling in this respect
the anterior fin-ray of the pelvic girdle. It constitutes the
rudiment of the mesopterygium and propterygium of Gegenbaur.
It bears four fin-rays at its extremity, the anterior not being
well marked. The remaining fin-rays are borne by the edge of
the plate continuous with the metapterygium.
 
The further changes in the cartilages of the limb are not
important, and are easily understood by reference to fig. 349
representing the limb of a nearly full-grown embryo. The
front end of the anterior basal cartilage becomes segmented
off as a propterygium, bearing a single fin-ray, leaving the
remainder of the cartilage as a mesopterygium. The remainder
of the now considerably segmented fin-rays are borne by the
metapterygium.
 
The mode of development of the pectoral fin demonstrates
that, as supposed by Mivart, the metapterygium is the homologue of the basal cartilage of the pelvic fin.
 
From the mode of development of the fins of Scyllium conclusions
may be drawn adverse to the views recently put forward on the structure of the fin by Gegenbaur and Huxley, both of whom consider the
primitive type of fin to be most nearly retained in Ceratodus, and to
consist of a central multisegmented axis with numerous rays. Gegenbaur
derives the Elasmobranch pectoral fin from a form which he calls the
archipterygium, nearly like that of Ceratodus, with a median axis and two
 
 
 
LIMBS.
 
 
 
6I 7
 
 
 
rows of rays ; but holds that in addition to the rays attached to the median
axis, which are alone found in Ceratodus, there were other rays directly
articulated to the shoulder-girdle. He considers that in the Elasmobranch
fin the majority of the lateral rays on the posterior (median or inner
according to his view of the position of the limb) side have become
aborted, and that the central axis is represented by the metapterygium ;
while the pro- and mesopterygium and their rays are, he believes, derived
from those rays of the archipterygium which originally articulated directly
with the shoulder-girdle.
 
Gegenbaur's view appears to me to be absolutely negatived by the facts
of development of the pectoral fin in Scyllium ; not so much because the
pectoral fin in this form is necessarily to be regarded as primitive, but
because what Gegenbaur holds to be the primitive axis of the biserial fin
is demonstrated to be really the base, and it is only in the adult that it is
conceivable that a second set of lateral rays could have existed on the
posterior side of the metapterygium. If Gegenbaur's view were correct
we should expect to find in the embryo, if anywhere, traces of the second
set of lateral rays ; but the fact is that, as may easily be seen by an inspection of figs. 344 and 346, such a second set of lateral rays could not possibly have existed in a type .
of fin like that found in the
embryo 1 . With this view of
Gegenbaur's it appears to
me that the theory held by
this anatomist to the effect
that the limbs are modified
gill arches also falls ; in
that his method of deriving
the limbs from gill arches
ceases to be admissible,
while it is not easy to see
how a limb, formed on the
type of the embryonic limb
of Elasmobranchs, could be
derived from a visceral arch
with its branchial rays 2 .
 
Gegenbaur's older view
 
 
 
 
FIG. 349. SKELETON OF THE PECTORAL FIN
AND PART OF PECTORAL GIRDLE OF A NEARLY
RIPE EMBRYO OF SCYLLIUM STELLARE.
 
m.p. metapterygium ; me.p. mesopterygium ;
//. propterygium ; cr. coracoid process.
 
 
 
1 If, which I very much doubt, Gegenbaur is right in regarding certain rays found
in some Elasmobranch pectoral fins as rudiments of a second set of rays on the
posterior side of the metapterygium, these rays will have to be regarded as structures
in the act of being evolved, and not as persisting traces of a biserial fin.
 
2 Some arguments in favour of Gegenbaur's theory adduced by Wiedersheim as
a result of his researches on Protopterus are interesting. The attachment which he
describes between the external gills and the pectoral girdle is no doubt remarkable,
but I would suggest that the observations we have on the vascular supply of these
gills demonstrate that this attachment is secondary.
 
 
 
6l8 THE CHEIKOPTERYGIUM.
 
that the Elasmobranch fin retains a primitive uniserial type appears to me
to be nearer the truth than his more recent view on this subject ; though I
hold that the fundamental point established by the development of these
parts in Scyllium is that the posterior border of the adult Elasmobranch fin
is the primitive base line, i.e. the line of attachment of the fin to the side of
the body.
 
Huxley holds that the mesopterygium is the proximal piece of the axial
skeleton of the limb of Ceratodus, and derives the Elasmobranch fin from
that of Ceratodus by the shortening of its axis and the coalescence of some
of its elements. The secondary character of the mesopterygium, and its
total absence in the embryo Scyllium, appears to me as conclusive against
Huxley's view, as the character of the embryonic fin is against that of
Gegenbaur ; and I should be much more inclined to hold that the fin of
Ceratodus has been derived from a fin like that of the Elasmobranchii by
a series of steps similar to those which Huxley supposes to have led to the
establishment of the Elasmobranch fin, but in exactly the reverse order.
 
With reference to the development of the pectoral fin in the Teleostei
there are some observations of 'Swirski (No. 488) which unfortunately do
not throw very much light upon the nature of the limb.
 
'Swirski finds that in the Pike the skeleton of the limb is formed of a
plate of cartilage, continuous with the pectoral girdle ; which soon becomes
divided into a proximal and a distal portion. The former is subsequently
segmented into five basal rays, and the latter into twelve parts, the number
of which subsequently becomes reduced.
 
These investigations might be regarded as tending to shew that the
basipterygium of Elasmobranchii is not represented in Teleostei, owing to
the fin rays not having united into a continuous basal bar, but the observations are not sufficiently complete to admit of this conclusion being
founded upon them with any certainty.
 
Tlie ckeiropterygium.
 
Observations on the early development of the pentadactyloid
limbs of the higher Vertebrata are comparatively scanty.
 
The limbs arise as simple outgrowths of the sides of the
body, formed both of epiblast and mesoblast. In the Amniota,
at all events, they are processes of a special longitudinal ridge
known as the Wolffian ridge. In the Amniota they also bear
at their extremity a thickened cap of epiblast, which may be
compared with the epiblastic fold at the apex of the Elasmobranch fin.
 
Both limbs have at first a precisely similar position, both
being directed backwards and being parallel to the surface of
the body.
 
 
 
I 111: CHEIROPTERYGIUM.
 
 
 
619
 
 
 
In the Urodela (Gotte) the ulnar and fibular sides are
primitively dorsal, and the radial and tibial ventral : in Mammalia however Kolliker states that the radial and tibial edges
are from the first anterior.
 
The exact changes of position undergone by the limbs in the
course of development are not fully understood. To suit a
terrestrial mode of life the flexures of the two limbs become
gradually more and more opposite, till in Mammalia the corresponding joints of the two limbs are turned in completely
opposite directions.
 
Within the mesoblast of the limbs a continuous blastema
becomes formed, which constitutes the first trace of the skeleton
of the limb. The corresponding elements of the two limbs,
viz. the humerus and femur, radius and tibia, ulna and fibula,
carpal and tarsal bones, metacarpals and metatarsals, and
digits, become differentiated within this, by the conversion
of definite regions into cartilage, which may either be completely
distinct or be at first united. These cartilaginous elements
subsequently ossify.
 
The later development of the parts, more especially of the carpus and
tarsus, has been made the subject of considerable study ; and important
results have been thereby obtained as to the homology of the various
carpal and tarsal bones throughout the Vertebrata ; but this subject is too
special to be treated of here. The early development, including the succession of the growth of the different parts, and the extent of continuity
primitively obtaining between them, has on the other hand been but little
investigated ; recently however the development of the limbs in the Urodela has been worked out in this way by two anatomists, Gotte (No. 482)
and Strasser (No. 487), and their results, though not on all points in complete harmony, are of considerable interest, more especially in their bearing
on the derivation of the pentadactyloid limb from the piscine fin. Till
however further investigations of the same nature have been made upon
other types, the conclusions to be drawn from Gotte and Strasser's observations must be regarded as somewhat provisional, the actual interpretation
of various ontological processes being very uncertain.
 
The forms investigated are Triton and Salamandra. We may remind
the reader that the hand of the Urodela has four digits, and the foot five,
the fifth digit being absent in the hand 1 . In Triton the proximal row of
carpal bones consists (using Gegenbaur's nomenclature) of (i) a radiale, and
(2 and 3) an intermedium and ulnare, partially united. The distal row is
formed of four carpals, of which the first often does not support the first
1 This seems to me clearly to follow from Gotte and Strasser's observations.
 
 
 
620 THE GHE1ROPTERYGIUM.
 
metacarpal ; while the second articulates with both the first and second
metacarpals. In the foot the proximal row of tarsals consists of a tibiale,
an intermedium and a fibulare. The distal row is formed of four tarsals, the
first, like that in the hand, often not articulating with the first metatarsal,
the second supporting the first and second metatarsals ; and the fourth the
fourth and fifth metatarsals.
 
The mode of development of the hand and foot is almost the same. The
most remarkable feature of development is the order of succession of the
digits. The two anterior (radial or tibial) are formed in the first instance,
and then the third, fourth and fifth in succession.
 
As to the actual development of the skeleton Strasser, whose observations
were made by means of sections, has arrived at the following results.
 
The humerus with the radius and ulna, and the corresponding parts in
the hind limb, are the first parts to be differentiated in the continuous plate
of tissue from which the skeleton of the limb is formed. Somewhat later a
cartilaginous centre appears at the base of the first and second fingers
(which have already appeared as prominences at the end of the limb) in the
situation of the permanent second carpal of the distal row of carpals ; and
the process of chondrification spreads from this centre into the fingers and
into the remainder of the carpus. In this way a continuous carpal plate
of cartilage is established, which is on the one hand continuous with the
cartilage of the two metacarpals, and on the other with the radius and ulna.
 
In the cartilage of the carpus two special columns may be noticed, the
one on the radial side, most advanced in development, being continuous with
the radius ; the other less developed column on the side of the ulna being
continuous both with the ulna and with the radius. The ulna and radius are
not united with the humerus.
 
In the further growth the third and fourth digits, and in the foot the fifth
digit also, gradually sprout out in succession from the ulnar side of the
continuous carpal plate. The carpal plate itself becomes segmented from the
radius and ulna, and divided up into the carpal bones.
 
The original radial column is divided into three elements, a proximal the
radiale, a middle element the first carpal, and a distal the second carpal
already spoken of. The first carpal is thus situated between the basal cartilage of the second digit and the radiale, and would therefore appear
to be the representative of a primitive middle row of carpal
bones, of which the centrale is also another representative.
 
The centrale and intermedium are the middle and proximal products of
the segmentation of the ulnar column of the primitive carpus, the distal
second carpal being common both to this column and to the radial column.
 
The ulnar or fibular side of the carpus or tarsus becomes divided into a
proximal element the ulnare or fibulare the ulnare remaining partially
united with the intermedium. There are also formed from this plate two
carpals to articulate with digits 3 and 4 ; while in the foot the corresponding
elements articulate respectively with the third digit, and with the fourth and
fifth digits.
 
 
 
THE CIIF.IROPTERYGIUM. 621
 
Gotte, whose observations were made in a somewhat different method to
those of Strasser, is at variance with him on several points. He finds that
the primitive skeleton of the limb consists of a basal portion, the humerus,
continued into a radial and an ulnar ray, which are respectively prolonged
into the two first digits. The two rays next coalesce at the base of the
fingers to form the carpus, and thus the division of the limb into the brachium,
antebrachium and manus is effected.
 
The ulna, which is primitively prolonged into the second digit, is
subsequently separated from it and is prolonged into the third ; from the side
of the part of the carpus connecting the ulna with the third digit the fourth
digit is eventually budded out, and in the foot the fourth and fifth digits arise
from the corresponding region. Each of the three columns connected
respectively with the first, second, and third digits becomes divided into three
successive carpal bones, so that Gotte holds the skeleton of the hand or foot
to be formed of a proximal, a middle, and a distal row of carpal bones each
containing potentially three elements. The proximal row is formed of the
radiale, intermedium and ulnare ; the middle row of carpal i, the centrale
and carpal 4, and the distal of carpal 2 (consisting according to Gotte of two
coalesced elements) and carpal 3.
 
The derivation of the cheiropterygium from the ichthyoptcrygium. All
anatomists are agreed that the limbs of the higher Vertebrata are derived
from those of Fishes, but the gulf between the two types of limbs is so great
that there is room for a very great diversity of opinion as to the mode of
evolution of the cheiropterygium. The most important speculations on the
subject are those of Gegenbaur and Huxley.
 
Gegenbaur holds that the cheiropterygium is derived from a uniserial
piscine limb, and that it consists of a primitive stem, to which a series of
lateral rays are attached on one (the radial) side ; while Huxley holds that the
cheiropterygium is derived from a biserial piscine limb by the "lengthening of the axial skeleton, accompanied by the removal of its distal
elements further away from the shoulder-girdle and by a diminution in the
number of the rays."
 
Neither of these theories is founded upon ontology, and the only ontological evidence we have which bears on this question is that above recorded
with reference to the development of the Urodele limb.
 
Without holding that this evidence can be considered as in any way
conclusive, its tendency would appear to me to be in favour of regarding the
cheiropterygium as derived from a uniserial type of fin. The humerus or
femur would appear to be the basipterygial bars (metapterygium), which
have become directed outwards instead of retaining their original position
parallel to the length of the body at the base of the fin. The anterior
(proximal) fin-rays and the pro- and mesopterygium must be supposed to
have become aborted, while the radius or ulna, and tibia or fibula are two
posterior fin-rays (probably each representing several coalesced rays like the
pro- and mesopterygium) which support at their distal extremities more
numerous fin-rays consisting of the rows of carpal and tarsal bones.
 
 
 
622 THE CHEIROPTERYGIUM.
 
This view of the cheiropterygium corresponds in some respects with that
put forward by Gotte as a result of his investigations on the development of
the Urodele limbs, though in other respects it is very different. A difficulty
of this view is the fact that it involves our supposing that the radial edge of
the limb corresponds with the metapterygial edge of the piscine fin. The
difficulties of this position have been clearly pointed out by Huxley, but the
fact that in the primitive position of the Urodele limbs the radius is ventral
and the ulna dorsal shews that this difficulty is not insuperable, in that it is
easy to conceive the radial border of the fin to have become rotated from its
primitive Elasmobranch position into the vertical position it occupies in the
embryos of the Urodela, and then to have been further rotated from this
position into that which it occupies in the adult Urodela and in all higher
forms.
 
BIBLIOGRAPHY of the Limbs.
 
(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen
d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.
 
(478) C. Gegenbaur. Untersuckungen z. vergleich. Anat. d. Wirbelthiere.
Leipzig, 1864 5. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d.
Fische.
 
(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im
Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeitsckrift, Vol. V. 1870.
 
(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol.
vii. 1873.
 
(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere."
Morphologisches Jahrbuch, Vol. II. 1876.
 
(482) A. Gotte. Ueb. Entivick. u. Regeneration d. Gliedmaassenskelets d. Molche.
Leipzig, 1879.
 
(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the
classification of Fishes." Proc. Zool. Soc. 1876.
 
(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological
Trans., Vol. x.
 
(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen
d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthieren." Zeil.f. iviss. Zool.,
Vol. xxin. 1873.
 
(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi
d. Menschen. " Morphologisches Jahrbuch, Vol. I. 1875.
 
(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u.
Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.
 
(488) G. 'S wirski. Untersitch. iib. d. Entwick. d. Schultergitrtels u. d. Skelcls d.
Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.
 
(489) J. K. Thacker. "Median and paired fins. A contribution to the history
of the Vertebrate limbs." Trans, of the Connecticut Acad., Vol. ill. 1877.
 
(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut
Acad., Vol. iv. 1877.
 
 
 
CHAPTER XXI.
 
 
 
THE BODY CAVITY, THE VASCULAR SYSTEM, AND THE
VASCULAR GLANDS.
 
 
 
The Body cavity.
 
IN the Ccelenterata no body cavity as distinct from the
alimentary cavity is present ; but in the remaining Invertebrata
the body cavity may (i) take the form of a wide space separating
the wall of the gut from the body wall, or (2) may be present in
a more or less reduced form as a number of serous spaces, or
(3) only be represented by irregular channels between the
muscular and connective-tissue cells filling up the interior of the
body. The body cavity, in whatever form it presents itself, is
probably filled with fluid, and the fluid in it may contain special
cellular elements. A well developed body cavity may coexist
with an independent system of serous spaces, as in the Vertebrata and the Echinodermata ; the perihaemal section of the
body cavity of the latter probably representing the system of
serous spaces.
 
In several of the types with a well developed body cavity it
has been established that this cavity originates in the embryo
from a pair of alimentary diverticula, and the cavities resulting
from the formation of these diverticula may remain distinct, the
adjacent walls of the two cavities fusing to form a dorsal and a
ventral mesentery.
 
It is fairly certain that some groups, e.g. the Tracheata, with
imperfectly developed body cavities are descended from ancestors
which were provided with well developed body cavities, but how
far this is universally the case cannot as yet be definitely
decided, and for additional information on this subject the
 
 
 
624 CIIORDATA.
 
 
 
reader is referred to pp. 355 360 and to the literature there
referred to.
 
In the Chaetopoda and the Tracheata the body cavity arises
as a series of paired compartments in the somites of mesoblast
(fig. 350) which have at first a very restricted extension on the
ventral side of the body, but eventually extend dorsalwards and
vcntralwards till each cavity is a half circle investing the
alimentary tract ; on the dorsal side the walls separating the two
 
 
 
 
FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA
LABYRINTHICA.
 
The section is taken slightly to one side of the middle line so as to shew the relation of the mesoblastic somites to the limbs. In the interior are seen the yolk
segments and their nuclei.
 
i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument.
 
half cavities usually remain as the dorsal mesentery, while
ventrally they are in most instances absorbed. The transverse
walls, separating the successive compartments of the body
cavity, generally become more or less perforated.
 
Chordata. In the Chordata the primitive body cavity is
cither directly formed from a pair of alimentary diverticula
(Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic
plates of the two sides of the body (fig. 20).
 
As already explained (pp. 294 300) the walls of the dorsal
sections of the primitive body cavity soon become separated
from those of the ventral, and becoming segmented constitute
the muscle plates, while the cavity within them becomes
 
 
 
I
 
 
 
THE BODY CAVITY.
 
 
 
625
 
 
 
the
 
 
 
obliterated : they are dealt with in a separate chapter. The
ventral part of the primitive cavity alone constitutes the
permanent body cavity.
 
The primitive body cavity in the lower Vertebrata is at first
continued forwards into the region of the head, but on the
formation of the visceral clefts the cephalic section of the body
cavity becomes divided into a series of separate compartments.
Subsequently these sections of the body cavity become obliterated ; and, since their walls give rise to muscles, they may
probably be looked upon as equivalent to the dorsal sections of
the body cavity in the trunk, and will be treated of in connection
with the muscular system.
 
As a result of its mode of origin the body cavity in
trunk is at first divided into two
lateral halves ; and part of the mesoblast lining it soon becomes distinguished as a special layer of epithelium, known as the peritoneal epithelium, of which the part bounding the
outer wall forms the somatic layer,
and that bounding the inner wall the
splanchnic layer. Between the two
splanchnic layers is placed the gut.
On the ventral side, in the region of
the permanent gut, the two halves
of the body cavity soon coalesce,
the septum between them becoming
absorbed, and the splanchnic layers
of epithelium of the two sides uniting
at the ventral side of the gut, and
the somatic layers at the median
ventral line of the body wall (fig.
 
 
 
 
In the lower Vertebrata the body
cavity is originally present even in
the post-anal region of the trunk, but
usually atrophies early, frequently
before the two halves coalesce.
 
On the dorsal side of the gut the
B. III.
 
 
 
FIG. 351. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 
28 F.
 
sp.c. spinal canal ; W. white
matter of spinal cord ; pr. posterior nerve-roots ; cA. notochord ;
x. sub-notochordal rod ; ao. aorta ;
nip. muscle-plate ; nip 1 , inner layer
of muscle-plate already converted
into muscles; Vr. rudiment of
vertebral body ; si. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve ; v. subintestinal vein ;
p.o. primitive generative cells.
 
40
 
 
 
626 ABDOMINAL PORES.
 
 
 
two halves of the body cavity never coalesce, but eventually the
splanchnic layers of epithelium of the two sides, together with a
thin layer of interposed mesoblast, form a delicate membrane,
known as the mesentery, which suspends the gut from the dorsal
wall of the body (figs. 119 and 351). On the dorsal side the
epithelium lining of the body cavity is usually more columnar
than elsewhere (fig. 351), and its cells partly form a covering for
the generative organs, and partly give rise to the primitive
germinal cells. This part of the epithelium is often known as
the germinal epithelium.
 
Over the greater part of the body cavity the lining epithelium becomes in the adult intimately united with a layer of the
subjacent connective tissue, and constitutes with it a special
lining membrane for the body cavity, known as the peritoneal
membrane.
 
Abdominal pores. In the Cyclostomata, the majority of the Elasmobranchii, the Ganoidei, a few Teleostei, the Dipnoi, and some Sauropsida
(Chelonia and Crocodilia) the body cavity is in communication with the
exterior by a pair of pores, known as abdominal pores, the external
openings of which are usually situated in the cloaca 1 .
 
The ontogeny of these pores has as yet been but very slightly investigated.
In the Lamprey they are formed as apertures leading from the body cavity
into the excretory section of the primitive cloaca. This section would
appear from Scott's (No. 87) observations to be derived from part of the
hypoblastic cloacal section of the alimentary tract.
 
In all other cases they are formed in a region which appears to belong
to the epiblastic region of the cloaca ; and from my observations on Elasmobranchs it may be certainly concluded that they are formed there
in this group. They may appear as perforations (i) at the apices of
papilliform prolongations of the body cavity, or (2) at the ends of cloacal
pits directed from the exterior towards the body cavity, or (3) as simple
slit-like openings.
 
Considering the difference in development between the abdominal pores
of most types, and those of the Cyclostomata, it is open to doubt whether
these two types of pores are strictly homologous.
 
In the Cyclostomata they serve for the passage outwards of the generative products, and they also have this function in some of the few Teleostei
in which they are found ; and Gegenbaur and Bridge hold that the primitive
mode of exit of the generative products, prior to the development of the
Miillerian ducts, was probably by means of these pores. I have elsewhere
 
1 For a full account of these structures the reader is referred to T. W. Bridge,
"Pori Abdominales of Vertebrata. " Journal of Anat. and Physiol. , Vol. XIV., 1879.
 
 
 
THE BODY CAVITY.
 
 
 
627
 
 
 
 
suggested that the abdominal pores are perhaps remnants of the openings
of segmental tubes ; there does not however appear to be any definite
evidence in favour of this view, and it is more probable that they may have
arisen as simple perforations of the body wall.
 
Pericardial cavity, pleural cavities, and diaphragm.
 
In all Vertebrata the heart is at first
placed in the body cavity (fig. 353 A),
but the part of the body cavity containing it afterwards becomes separated as
a distinct cavity known as the pericardial cavity. In Elasmobranchii, Acipenser, etc. a passage is however left
between the pericardial cavity and the
body cavity ; and in the Lamprey a
separation between the two cavities does
not occur during the Ammoccete stage.
In Elasmobranchii the pericardial
cavity becomes established as a distinct
space in front of the body cavity in the
following way. When the two ductus
Cuvieri, leading transversely from the
sinus venosus to the cardinal veins, become developed, a horizontal septum,
shewn on the right side in fig. 352, is
formed to support them, stretching
across from the splanchnic to the somatic side of the body cavity, and
dividing the body cavity (fig. 352) in
this part into (i) a dorsal section formed
of a right and left division constituting
the true body cavity (pp), and (2) a
ventral part the pericardial cavity (pc).
The septum is at first of a very small
longitudinal extent, so that both in
front and behind it (fig. 352 on the left
side) the dorsal and ventral sections of the body cavity are in
free communication. The septum soon however becomes prolonged, and ceasing to be quite horizontal, is directed obliquely
upwards and forwards till it meets the dorsal wall of the body
 
40 2
 
 
 
-ht
 
 
 
FIG. 352. SECTION
THROUGH THE TRUNK OF A
SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN 28 F.
 
The figure shews the separation of the body cavity from
the pericardial cavity by a
horizontal septum in which
runs the ductus Cuvieri ; on
the left side is seen the narrow
passage which remains connecting the two cavities.
 
sp.c. spinal canal ; w. white
matter of spinal cord ; pr.
commissure connecting the
posterior nerve-roots ; ch. notochord ; x. sub-notochordal
rod ; ao. aorta ; sv. sinus venosus ; cav. cardinal vein ; ht.
heart ; pp. body cavity ; pc.
pericardial cavity ; as. solid
oesophagus ; /. liver ; nip. muscle-plate.
 
 
 
628 THE PERICARDIAL CAVITY.
 
Anteriorly all communication is thus early shut off between the
body cavity and the pericardial cavity, but the two cavities still
open freely into each other behind.
 
The front part of the body cavity, lying dorsal to the pericardial cavity, becomes gradually narrowed, and is wholly
obliterated long before the close of embryonic life, so that in
adult Elasmobranch Fishes there is no section of the body cavity
dorsal to the pericardial cavity. The septum dividing the body
cavity from the pericardial cavity is prolonged backwards, till it
meets the ventral wall of the body at the point where the liver
is attached by its ventral mesentery (falciform ligament). In
this way the pericardial cavity becomes completely shut off from
the body cavity, except, it would seem, for the narrow communications found in the adult. The origin of these communications
has not however been satisfactorily worked out.
 
The septum between the pericardial cavity and the body
cavity is attached on its dorsal aspect to the liver. It is at first
nearly horizontal, but gradually assumes a more vertical position,
and then, owing to the obliteration of the primitive anterior
part of the body cavity, appears to mark the front boundary of
the body cavity. The above description of the mode of formation of the pericardial cavity, and the explanation of its relations
to the body cavity, probably holds true for Fishes generally.
 
In the higher types the earlier changes are precisely the
same as those in Elasmobranch Fishes. The heart is at first
placed within the body cavity attached to the ventral wall of
the gut by a mesocardium (fig. 353 A). A horizontal septum is
then formed, in which the ductus Cuvieri are placed, dividing
the body cavity for a short distance into a dorsal (/./) and
ventral (p.c) section (fig. 353 B). In Birds and Mammals, and
probably also in Reptilia, the ventral and dorsal parts of the
body cavity are at first in free communication both in front of
and behind this septum. This is shewn for the Chick in
fig- 353 A an d B, which are sections of the same chick, A being
a little in front of B. The septum is soon continued forwards
so as completely to separate the ventral pericardial and the
dorsal body cavity in front, the pericardial cavity extending at
this period considerably further forwards than the body cavity.
 
Since the horizontal septum, by its mode of origin, is
 
 
 
THE BODY CAVITY.
 
 
 
629
 
 
 
necessarily attached to the ventral side of the gut, the dorsal
part of the primitive body space is divided into two halves by a
median vertical septum formed of the gut and its mesentery
(fig- 353 B). Posteriorly the horizontal septum grows in a
slightly ventral direction along the under surface of the liver
(fig- 354)j till it meets the abdominal wall of the body at the
insertion of the falciform ligament, and thus completely shuts
off the pericardial cavity from the body cavity. The horizontal
septum forms, as is obvious from the above description, the
dorsal wall of the pericardial cavity 1 .
 
A. B.
 
 
 
 
 
FIG. 353. TRANSVERSE SECTIONS THROUGH A CHICK EMBRYO WITH TWENTYONE MESOBLASTIC SOMITES TO SHEW THE FORMATION OF THE PERICARDIAI,
CAVITY, A. BEING THE ANTERIOR SECTION.
 
p.p. body cavity; p.c. pericardial cavity; al. alimentary cavity ; au. auricle; v. ventricle; s.v. sinus venosus; d.c. ductus Cuvieri ; ao. aorta; nip. muscle-plate; me.
medullary cord.
 
With the complete separation of the pericardial cavity from
the body cavity, the first period in the development of these
parts is completed, and the relations of the body cavity to the
 
1 Kolliker's account of this septum, which he calls the mesocardium laterale (No.
298, p. 295), would seem to imply that in Mammals it is completed posteriorly even
before the formation of the liver. I doubt whether this takes place quite so early as
he implies, but have not yet determined its exact period by my own observations.
 
 
 
630
 
 
 
THE PERICARDIAL CAVITY.
 
 
 
pericardial cavity become precisely those found in the embryos
of Elasmobranchii. The later changes are however very different. Whereas in Fishes the right and left sections of the body
cavity dorsal to the pericardial cavity soon atrophy, in the
higher types, in correlation with the relatively backward situation of the heart, they rapidly become larger, and receive the
lungs which soon sprout out from the throat.
 
The diverticula which form the lungs grow out into the
splanchnic mesoblast, in front of
the body cavity ; but as they
grow, they extend into the two
anterior compartments of the body
cavity, each attached by its mesentery to the mesentery of the
gut (fig. 354, lg). They soon moreover extend beyond the region of
the pericardium into the undivided
body cavity behind. This holds
not only for the embryos of the
Amphibia and Sauropsida, but
also for those of Mammalia.
 
To understand the further
 
rrianfrps in rhp nerirardial ravitv FlG> 354- SECTION THROUGH
 
pencaraiai cavity THECARDIACREGION OF AN EMBRYO
 
it is necessary to bear in mind its OF LACERTA MURALIS OF 9 MM. TO
 
, ,. ,, ,. . . SHEW THE MODE OF FORMATION OF
 
relations to the adjoining parts. THE PERICARDIAL CAVITY.
 
 
 
 
'-/it
 
 
 
It lies at this period completely
ventral to the two anterior pro
 
 
ht. heart ; pc. pericardial cavity ;
al. alimentary tract; lg. lung; /.
liver ; pp. body cavity ; md. open
longations of the body Cavity COn- end of Mullerian duct ; wd. Wolffian
. . duct; vc. vena cava inferior; ao.
 
taming the lungs (fig. 354). Its aorta; ch. notochord; me. medullary
 
dorsal wall is attached to the gut, cord>
 
and is continuous with the mesentery of the gut passing to the
dorsal abdominal wall, forming the posterior mediastinum of
human anatomy.
 
The changes which next ensue consist essentially in the
enlargement of the sections of the body cavity dorsal to the
pericardial cavity. This enlargement takes place partly by the
elongation of the posterior mediastinum, but still more by the
two divisions of the body cavity which contain the lungs
extending themselves ventrally round the outside of the peri
 
 
THE BODY CAVITY.
 
 
 
631
 
 
 
cardial cavity. This process is illustrated by fig. 355, taken
from an embryo Rabbit. The two dorsal sections of the body
cavity (pl.p] finally extend so as completely to envelope the
pericardial cavity (pc\ remaining however separated from each
other below by a lamina extending from the ventral wall of the
pericardial cavity to the body wall, which forms the anterior
mediastinum of human anatomy.
 
By these changes the pericardial cavity is converted into a
closed bag, completely surrounded at its sides by the two lateral
halves of the body cavity, which were primitively placed
 
 
 
SJ3. C.
 
 
 
 
FIG. 355. SECTION THROUGH AN ADVANCED EMBRYO OF A RABBIT TO SHEW
HOW THE PERICARDIAL CAVITY BECOMES SURROUNDED BY THE PLEURAL
CAVITIES.
 
ht. heart; pc. pericardial cavity; //./ pleural cavity; Ig. lung; al. alimentary
tract; ao. dorsal aorta; ch. notochord; rp. rib; st. sternum; sp.c. spinal cord.
 
dorsally to it. These two sections of the body cavity, which in
Amphibia and Sauropsida remain in free communication with
the undivided peritoneal cavity behind, may, from the fact of
their containing the lungs, be called the pleural cavities.
 
In Mammalia a further change takes place, in that, by the
formation of a vertical partition across the body cavity, known
as the diaphragm, the pleural cavities, containing the lungs,
 
 
 
632 THE VASCULAR SYSTEM.
 
become isolated from the remainder of the body or peritoneal
cavity. As shewn by their development the so-called pleurae or
pleural sacks are simply the peritoneal linings of the anterior
divisions of the body cavity, shut off from the remainder of
the body cavity by the diaphragm.
 
The exact mode of formation of the diaphragm is not fully
made out ; the account of it recently given by Cadiat (No. 491)
not being in my opinion completely satisfactory.
 
BIBLIOGRAPHY.
 
(491) M. Cadiat. "Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de
1'cesophage." Journal de F Anatomic et de la Physiologic, Vol. xiv. 1878.
 
 
 
Vascular System.
 
The actual observations bearing on the origin of the vascular
system, using the term to include the lymphatic system, are
very scanty. It seems probable, mainly it must be admitted on
d priori grounds, that vascular and lymphatic systems have
originated from the conversion of indefinite spaces, primitively
situated in the general connective tissue, into definite channels.
It is quite certain that vascular systems have arisen independently in many types ; a very striking case of the kind being
the development in certain parasitic Copepoda of a closed
system of vessels with a red non-corpusculated blood (E. van
Beneden, Heider), not found in any other Crustacea. Parts of
vascular systems appear to have arisen in some cases by a
canalization of cells.
 
The blood systems may either be closed or communicate
with the body cavity. In cases where the primitive body cavity
is atrophied or partially broken up into separate compartments
(Insecta, Mollusca, Discophora, etc.) a free communication
between the vascular system and the body cavity is usually
present ; but in these cases the communication is no doubt
secondary. On the whole it would seem probable that the
vascular system has in most instances arisen independently of
the body cavity, at least in types where the body cavity is
 
 
 
THE VASCULAR SYSTEM. 633
 
present in a well-developed condition. As pointed out by the
Hertwigs, a vascular system is always absent where there is not
a considerable development of connective tissue.
 
As to the ontogeny of the vascular channels there is still much to be
made out both in Vertebrates and Invertebrates.
 
The smaller channels often rise by a canalization of cells. This process
has been satisfactorily studied by Lankester in the Leech 1 , and may easily
be observed in the blastoderm of the Chick or in the epiploon of a newlyborn Rabbit (Schafer, Ranvier). In either case the vessels arise from a network of cells, the superficial protoplasm and part of the nuclei giving rise
to the walls, and the blood-corpuscles being derived either from nucleated
masses set free within the vessels (the Chick) or from blood-corpuscles
directly differentiated in the axes of the cells (Mammals).
 
Larger vessels would seem to be formed from solid cords of cells, the
central cells becoming converted into the corpuscles, and the peripheral cells
constituting the walls. This mode of formation has been observed by
myself in the case of the Spider's heart, and by other observers in other
Invertebrata. In the Vertebrata a more or less similar mode of formation
appears to hold good for the larger vessels, but further investigations are
still required on this subject. Gotte finds that in the Frog the larger vessels
are formed as longitudinal spaces, and that the walls are derived from the
indifferent cells bounding these spaces, which become flattened and united
into a continuous layer.
 
The early formation of vessels in the Vertebrata takes place in the
splanchnic mesoblast ; but this appears due to the fact that the circulation
is at first mainly confined to the vitelline region, which is covered by
splanchnic mesoblast.
 
The Heart.
 
The heart is essentially formed as a tubular cavity in the
splanchnic mesoblast, on the ventral side of the throat, immediately behind the region of the visceral clefts. The walls of this
cavity are formed of two layers, an outer thicker layer, which has
at first only the form of a half tube, being incomplete on its
dorsal side; and an inner lamina formed of delicate flattened
cells. The latter is the epithelioid lining of the heart, and the
cavity it contains the true cavity of the heart. The outer layer
gives rise to the muscular wall and peritoneal covering of the
heart. Though at first it has only the form of a half tube (fig.
 
1 "Connective and vasifactive tissues of the Leech." Quart. J. of Micr. Science,
Vol. XX. 1880.
 
 
 
634
 
 
 
THE HEART.
 
 
 
356), it soon becomes folded in on the dorsal side so as to form
for the heart a complete muscular wall. Its two sides, after thus
meeting to complete the tube of
the heart, remain at first continuous
with the splanchnic mesoblast surrounding the throat, and form a provisional mesentery the mesocardium which attaches the heart to
the ventral wall of the throat. The
superficial stratum of the wall of
the heart differentiates itself as the
peritoneal covering. The inner epithelioid tube takes its origin at the
time when the general cavity of the
heart is being formed by the separation of the splanchnicmesoblastfrom
the hypoblast. During this process
(fig. 357) a layer of mesoblast remains close to the hypoblast, but connected with the main mass
 
 
 
 
FIG. 356. SECTION THROUGH
THE DEVELOPING HEART OF AN
EMBRYO OF AN ELASMOBRANCH
(Pristiurus).
 
al. alimentary tract ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ht. heart.
 
 
 
 
FIG. 357. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE
HEAD OF AN EMBRYO CHICK OF THIRTY HOURS.
 
hb. hind-brain; vg. vagus nerve; ep. epiblast; ch. notochorcl ; x. thickening of
hypoblast (possibly a rudiment of the sub-notochordal rod) ; al. throat; ht. heart;
//. body cavity; so. somatic mesoblast; sf. splanchnic mesoblast; Ay. hypoblast.
 
 
 
THE VASCULAR SYSTEM.
 
 
 
635
 
 
 
of the mesoblast by protoplasmic processes. A second layer
next becomes split from the splanchnic mesoblast, connected
with the first layer by the above-mentioned protoplasmic
processes. These two layers form together the epithelioid lining
of the heart ; between them is the cavity of the heart, which soon
loses the protoplasmic trabeculae which at first traverse it. The
cavity of the heart may thus be described as being formed by a
hollowing out of the splanchnic mesoblast, and resembles in its
mode of origin that of other large vascular trunks.
 
The above description applies only to the development of
the heart in those types in which it is formed at a period after
the throat has become a closed tube (Elasmobranchii, Amphibia,
Cyclostomata, Ganoids (?)). In a number of other cases, in
which the heart is formed before the conversion of the throat
into a closed tube, of which the most notable is that of Mammals
(Hensen, Gotte, Kolliker), the heart arises as two independent
 
A.
 
 
 
 
B.
 
 
 
mes fir
 
 
 
 
FIG. 358. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE
 
SAME AGE AS FIG. 144 B. (From Kolliker.)
B is a more highly magnified representation of part of A.
 
rf. medullary groove; mp. medullary plate; riv. medullary fold; h. epiblast ;
dd. hypoblast; dd' . notochordal thickening of hypoblast; sp. undivided mesoblast;
^.somatic mesoblast; dfp. splanchnic mesoblast; ph. pericardial section of body
cavity; ahh. muscular wall of heart; ihh. epithelioid layer of heart; vies, lateral
undivided mesoblast ; sw. part of the hypoblast which will form the ventral wall of
the pharynx.
 
 
 
636
 
 
 
THE HEART.
 
 
 
tubes (fig. 358), which eventually coalesce into an unpaired
structure.
 
In Mammals the two tubes out of which the heart is formed appear at
the sides of the cephalic plates, opposite the region of the mid- and hindbrain (fig. 358). They arise at a time when the lateral folds which form
the ventral wall of the throat are only just becoming visible. Each half of
the heart originates in the same way as the whole heart in Elasmobranchii,
etc. ; and the layer of the splanchnic mesoblast, which forms the muscular
wall for each part (ahh), has at first the form of a half tube open below to
the hypoblast.
 
On the formation of the lateral folds of the splanchnic walls, the two
halves of the heart become carried inwards and downwards, and eventually
 
 
 
 
FlG. 359. TWO DIAGRAMMATIC SECTIONS THROUGH THE REGION OF THE
HIND-BRAIN OF AN EMBRYO CHICK OF ABOUT 36 HOURS ILLUSTRATING THE
FORMATION OF THE HEART.
 
fib. hind-brain ; nc. notochord ; E. epiblast ; so. somatopleure ; sp. splanchnopleure ; d. alimentary tract ; hy. hypoblast ; hs. heart ; of. vitelline veins.
 
 
 
THE VASCULAR SYSTEM.
 
 
 
637
 
 
 
meet on the ventral side of the throat. For a short time they here remain
distinct, but soon coalesce into a single tube.
 
In Birds, as in Mammals, the heart makes its appearance as two tubes,
but arises at a period when the formation of the throat is very much more
advanced than in the case of Mammals. The heart arises immediately
behind the point up to which the ventral wall of the throat is established
and thus has at first a A -shaped form. At the apex of the A , which forms
the anterior end of the heart, the two halves are in contact (fig. 357),
though they have not coalesced; while behind they diverge to be continued
as the vitelline veins. As the folding in of the throat is continued backwards the two limbs of the heart are brought together and soon coalesce
from before backwards into a single structure. Fig. 359 A and B shews the
heart during this process. The two halves have coalesced anteriorly (A)
but are still widely separated behind (B). In Teleostei the heart is formed
as in Birds and Mammals by the coalescence of two tubes, and it arises
before the formation of the throat.
 
The fact that the heart arises in so many instances as a
double tube might lead to the supposition that the ancestral
Vertebrate had two tubes in the place of the present unpaired
heart.
 
The following considerations appear to me to prove that this
conclusion cannot be accepted. If the folding in of the splanchnopleure to form the throat were deferred relatively to the
formation of the heart, it is clear that a modification in the
development of the heart would occur, in that the two halves of
the heart would necessarily be formed widely apart, and only
eventually united on the folding in of the wall of the throat. It
is therefore possible to explain the double formation of the heart
without having recourse to the above hypothesis of an ancestral
Vertebrate with two hearts. If the explanation just suggested
is the true one the heart should only be formed as two tubes
when it arises prior to the formation of the throat, and as a single
tube when formed after the formation of the throat. Since this
is invariably found to be so, it may be safely concluded that the
formation of the heart as two cavities is a secondary mode of
development, which has been brought about by variations in the
period of the closing in of the wall of the throat.
 
The heart arises continuously with the sinus venosus, which in
the Amniotic Vertebrata is directly continued into the vitelline
veins. Though at first it ends blindly in front, it is very soon
connected with the foremost aortic arches.
 
 
 
638 THE HEART.
 
 
 
The simple tubular heart, connected as above described, grows
more rapidly than the chamber in which it is contained, and is
soon doubled upon itself, acquiring in this way an S-shaped
curvature, the posterior portion being placed dorsally, and the
anterior ventrally. A constriction soon appears between the
dorsal and ventral portions.
 
The dorsal section becomes partially divided off behind from
the sinus venosus, and constitutes the relatively thin-walled
auricular section of the heart; while the ventral portion, after
becoming distinct anteriorly from a portion continued forwards
from it to the origin of the branchial arteries, which may be called
the truncus arteriosus, acquires very thick spongy muscular
walls, and becomes the ventricular division of the heart.
 
The further changes in the heart are but slight in the case of the Pisces.
A pair of simple membranous valves becomes established at the auriculoventricular orifice, and further changes take place in the truncus arteriosus.
This part becomes divided in Elasmobranchii, Ganoidei, and Dipnoi into a
posterior section, called the conus arteriosus, provided with a series of
transverse rows of valves, and an anterior section, called the bulb us
arteriosus, not provided with valves, and leading into the branchial
arteries. In most Teleostei (except Butirinus and a few other forms) the
conus arteriosus is all but obliterated, and the anterior row of its valves
alone preserved ; and the bulbus is very much enlarged 1 .
 
In the Dipnoi important changes in the heart are effected, as compared
with other Fishes, by the development of true lungs. Both the auricular
and ventricular chamber may be imperfectly divided into two, and in the
conus a partial longitudinal septum is developed in connection with a
longitudinal row of valves 2 .
 
In Amphibia the heart is in many respects similar to that of the Dipnoi.
Its curvature is rather that of a screw than of a simple S. The truncus
arteriosus lies to the left, and is continued into the ventricle which lies
ventrally and more to the right, and this again into the dorsally placed
auricular section.
 
After the heart has reached the piscine stage, the auricular section
(Bombinator) becomes prolonged into a right and left auricular appendage^
A septum next grows from the roof of the auricular portion of the heart
 
1 Vide Gegenbaur, "Zur vergleich. Anat. d. Herzens." Jenaische Zeit., Vol. n.
1866, and for recent important observations, J. E. V. Boas, "Ueb. Herz u. Arterienbogenbei Ceratodenu. Protopterus," and " Ueber d. Conus arter. b. Butirinus, etc.,"
Morphol. Jahrb., Vol. VI. 1880.
 
2 Boas holds that the longitudinal septum is formed by the coalescence of a row of
longitudinal valves, but this is opposed to Lankester's statements, "On the hearts of
Ceratodus, Protopterus and Chimaera, etc. Zool. Trans. Vol. x. 1879.
 
 
 
THE VASCULAR SYSTEM. 639
 
 
 
obliquely backwards and towards the left, and divides it in two chambers ;
the right one of which remains continuous with the sinus venosus, while
the left one is completely shut off from the sinus, though it soon enters
into communication with the newly established pulmonary veins. The
truncus arteriosus 1 is divided into a posterior conus arteriosus (pylangium)
and an anterior bulbus (synangium). The former is provided with a
proximal row of valves at its ventricular end, and a distal row at its anterior
end near the bulbus. It is also provided with a longitudinal septum, which
is no doubt homologous with the septum in the conus arteriosus of the
Dipnoi. The bulbus is well developed in many Urodela, but hardly exists
in the Anura.
 
In the Amniota further changes take place in the heart,
resulting in the abortion of the distal rows of valves of the conus
arteriosus 2 , and in the splitting up of the whole truncus arteriosus
into three vessels in Reptilia, and two in Birds and Mammals,
each opening into the ventricular section of the heart, and
provided with a special set of valves at its commencement. In
Birds and Mammals the ventricle becomes moreover completely
divided into two chambers, each communicating with one of the
divisions of the primitive truncus, known in the higher types
as the systemic and pulmonary aortae. The character of the
development of the heart in the Amniota will be best understood
from a description of what takes place in the Chick.
 
In Birds the originally straight heart (fig. 109) soon becomes doubled up
upon itself. The ventricular portion becomes placed on the ventral and
right side, while the auricular section is dorsal and to the left. The two
parts are separated from each other by a slight constriction known as the
canalis auricularis. Anteriorly the ventricular cavity is continued into the
truncus, and the venous or auricular portion of the heart is similarly connected behind with the sinus venosus. The auricular appendages grow out
from the auricle at a very early period. The general appearance of the
heart, as seen from the ventral side on the fourth day, is shewn in fig. 360.
Although the external divisions of the heart are well marked even before
this stage, it is not till the end of the third day that the internal partitions
become apparent ; and, contrary to what might have been anticipated from
the evolution of these parts in the lower types, the ventricular septum is the
first to be established.
 
1 For a good description of the adult heart vide Huxley, Article "Amphibia," in
the Encyclopedia Britannic a.
 
2 It is just possible that the reverse may be true, vide note on p. 640. If however,
as is most probable, the statement in the text is correct, the valves at the mouth of
the ventricle in Teleostei are not homologous with those of the Amniota ; the former
being the distal rov/ of the valves of the conus, the latter the proximal.
 
 
 
 
640 THE HEART OF AVES.
 
It commences on the third day as a crescentic ridge or fold springing
from the convex or ventral side of the rounded ventricular portion of the
heart, and on the fourth day grows rapidly across the ventricular cavity
towards the concave or dorsal side. It thus forms an incomplete longitudinal partition, extending from the canalis auricularis to the commencement
of the truncus arteriosus, and dividing the twisted ventricular tube into
two somewhat curved canals, one more
to the left and above, the other to
 
the right and below. These commu- A ^) ) CA
 
nicate with each other, above the free
edge of the partition, along its whole
length.
 
Externally the ventricular portion
as yet shews no division into two parts.
 
By the fifth day the venous end of
the heart, though still lying somewhat
to the left and above, is placed as far FIG. 360. HEART OF A CHICK ON
 
forwards as the arterial end, the whole THE FOURTH DAY OF INCUBATION
 
VIEWED FROM THE VENTRAL SURFACE.
 
organ appearing to be drawn together.
 
The ventricular septum is complete. L ?.- lef t a , uricular appendage; C.A.
 
, e .. , . , , canahs auricularis ; v. ventricle ; b. trun
The apex of the ventricles becomes cus arteriosus.
 
more and more pointed. In the auricular portion a small longitudinal fold appears as the rudiment of the
auricular septum, while in the canalis auricularis, which is now at its greatest
length, there is also to be seen a commencement of the valvular structures
tending to separate the cavity of the auricles from those of the ventricles.
 
About the io6th hour, a septum begins to make its appearance in the
truncus arteriosus in the form of a longitudinal fold, which according to
Tonge (No. 495) starts at the end of the truncus furthest removed from the
heart. It takes origin from the wall of the truncus between the fourth and
fifth pairs of arches, and grows downwards in such a manner as to divide the
truncus into two channels, one of which leads from the heart to the third and
fourth pairs of arches, and the other to the fifth pair. Its course downwards
is not straight but spiral, and thus the two channels into which it divides
the truncus arteriosus wind spirally the one round the other.
 
At the time when the septum is first formed, the opening of the truncus
arteriosus into the ventricles is narrow or slit-like, apparently in order to
prevent the flow of the blood back into the heart. Soon after the appearance
of the septum, however, semilunar valves (Tonge, No. 495) are developed
from the wall of that portion of the truncus which lies between the free edge
of the septum and the cavity of the ventricles 1 .
 
1 If Tonge is correct in his statement that the semilunar valves develop at some
distance from the mouth of the ventricle, it would seem possible that the portion of
the truncus between them and the ventricle ought to be regarded as the embryonic
conus arteriosus, and that the distal row of valves of the conus (and not the proximal
as suggested above, p. 639) has been preserved in the higher types.
 
 
 
THE VASCULAR SYSTEM.
 
 
 
641
 
 
 
The ventral and the dorsal pairs of valves are the first to appear : the
former as two small solid prominences separated from each other by a
narrow groove ; the latter as a single ridge, in the centre of which is a
prominence indicating the point where the ridge will subsequently become
divided into two. The outer valves appear opposite each other, at a
considerably later period.
 
As the septum grows downwards towards the heart, it finally reaches
the position of these valves. One of its edges then passes between the two
ventral valves, and the other unites with the prominence on the dorsal
valve-ridge. At the same time the growth of all the parts causes the valves
to appear to approach the heart, and thus to be placed quite at the top
of the ventricular cavities. The free edge of the septum of the truncus now
 
A. B.
 
 
 
 
 
FlG. 361. TWO VIEWS OF THE HEART OF A CHICK UPON THE FIFTH DAY
 
OF INCUBATION.
 
A. from the ventral, B. from the dorsal side.
 
La. left auricular appendage; r.a. right auricular appendage ; r.v. right ventricle;
l.v. left ventricle; b. truncus arteriosus.
 
fuses with the ventricular septum, and thus the division of the truncus into
two separate channels, each provided with three valves, and each communicating with a separate side of the heart, is complete ; the position of
the valves not being very different from that in the adult heart.
 
That division of the truncus which opens into the fifth pair of arches is
the one which communicates with the right ventricle, while that which
opens into the third and fourth pairs communicates with the left ventricle.
The former becomes the pulmonary artery, the latter the commencement of
the systemic aorta.
 
The external constriction actually dividing the truncus into two vessels
does not begin to appear till the septum has extended some way back
towards the heart.
 
The semilunar valves become pocketed at a period considerably later
than their first formation (from the H7th to the,i65th hour) in the order of
their appearance.
 
At the end of the sixth day, and even on the fifth day (figs. 361 and 362),
the appearance of the heart itself, without reference to the vessels which
come from it, is not very dissimilar from that of the adult. The original
 
 
 
B. III.
 
 
 
4 1
 
 
 
642
 
 
 
THE HEART OF MAMMALIA.
 
 
 
r.a
 
 
 
 
l.v
 
 
 
FIG. 362. HEART OF A
CHICK UPON THE SIXTH DAY
OF INCUBATION, FROM THE
VENTRAL SURFACE.
 
La. left auricular appendage ;
r,a. right auricular appendage ;
r.v. right ventricle ; l.v. left ventricle ; b. truncus arteriosus.
 
 
 
protuberance to the right now forms the apex of the ventricles, and the
two auricular appendages are placed at the anterior extremity of the heart.
The most noticeable difference (in the ventral
view) is the still externally undivided condition of the truncus arteriosus.
 
The subsequent changes which the heart
undergoes are concerned more with its internal structure than with its external shape.
Indeed, during the next three days, viz. the
eighth, ninth, and tenth, the external form of
the heart remains nearly unaltered.
 
In the auricular portion, however, the
septum which commenced on the fifth day
becomes now more conspicuous. It is placed
vertically, and arises from the ventral wall ;
commencing at the canalis auricularis and
proceeding towards the opening into the
sinus venosus.
 
This latter structure gradually becomes
reduced so as to become a special appendage
of the right auricle. The inferior vena cava
 
enters the sinus obliquely from the right, so that its blood has a tendency to
flow towards the left auricle of the heart, which is at this time the larger of
the two.
 
The valves between the ventricles and auricles are now well developed,
and it is about this time that the division of the truncus arteriosus into the
aorta and pulmonary artery becomes visible from the exterior.
 
By the eleventh to the thirteenth day the right auricle has become as
large as the left, and the auricular septum much more complete, though
there is still a small opening, the foramen ovale, by which the two cavities
communicate with each other.
 
The most important feature in which the development of the Reptilian
heart differs from that of Birds is the division of the truncus into three
vessels, instead of two. The three vessels remain bound up in a common
sheath, and appear externally as a single trunk. The vessel not represented
in Birds is that which is continued into the left aortic arch.
 
In Mammals the early stages in the development of the heart present no
important points of difference from those of Aves. The septa in the truncus,
in the ventricular, and in the auricular cavities are formed, so far as
is known, in the same way and at the same relative periods in both groups.
In the embryo Man, the Rabbit, and other Mammals the division of
the ventricles is made apparent externally by a deep cleft, which, though
evanescent in these forms, is permanent in the Dugong.
 
The attachment of the auriculo-ventricular valves to the wall of the
ventricle, and the similar attachment of the left auriculo-ventricular valves
in Birds, have been especially studied by Gegenbaur and Bernays (No. 492),
 
 
 
ARTERIAL SYSTEM. 643
 
 
 
and deserve to be noticed. In the primitive state the ventricular walls
have throughout a spongy character ; and the auriculo-ventricular valves are
simple membranous projections like the auriculo-ventricular valves of Fishes.
Soon however the spongy muscular tissue of both the ventricular and
auricular walls, which at first pass uninterruptedly the one into the other,
grows into the bases of the valves, which thus become in the main muscular
projections of the walls of the heart. As the wall of the ventricle thickens,
the muscular trabeculas, connected at one end with the valves, remain at the
other end united with the ventricular wall, and form special bands passing
between the two. The valves on the other hand lose their muscular
attachment to the auricular walls. This is the condition permanent in
Ornithorhynchus. In higher Mammalia the ends of the muscular bands
inserted into the valves become fibrous, from the development of intermuscular connective tissue, and the atrophy of the muscular elements.
The fibrous parts now form the chordae tendinea?, and the muscular the
musculi papillares.
 
The sinus venosus in Mammals becomes completely merged into the
right auricle, and the systemic division of the truncus arteriosus is apparently not homologous with that in Birds.
 
In the embryos of all the Craniata the heart is situated very
far forwards in the region of the head. This position is retained
in Pisces. In Amphibia the heart is moved further back, while
in all the Amniota it gradually shifts its position first of all into
the region of the neck and finally passes completely within the
thoracic cavity. The steps in the change of position may be
gathered from figs. 109, in, and 118.
 
BIBLIOGRAPHY of the Heart.
 
(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen."
Morphol. Jahrbuch,^o\. II. 1876.
 
(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f.
mikr. Anat., Vol. xiv.
 
(494) A. Thomson. "On the development of the vascular system of the foetus
of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.
 
(495) M. Tonge. "Observations on the development of the semilunar valves
of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX.
1869.
 
Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296),
and Balfour (292).
 
Arterial System.
 
In the embryos of Vertebrata the arterial system consists of
a forward continuation of the truncus arteriosus, on the ventral
 
41 2
 
 
 
644
 
 
 
ARTERIES OF PISCES.
 
 
 
side of the throat (figs. 363, abr, and 364, a), which, with a few
exceptions to be noticed below, divides into as many branches on
each side as there are visceral arches. These branches, after
traversing the visceral arches, unite on the dorsal side of the
throat into a common trunk on each side. This trunk (figs. 363
and 364) after giving off one (or more) vessels to the head (c and
c] turns backv/ards, and bends in towards the middle line, close
to its fellow, immediately below the notochord (figs. 21 and 116)
and runs backwards in this situation towards the end of the tail.
The two parallel trunks below the notochord fuse very early into
a single trunk, the dorsal aorta (figs. 363, ad, and 364, a"}.
 
 
 
 
ttbr v "a,
 
FIG. 363. DIAGRAMMATIC VIEW OF THE HEAD OF AN EMBRYO TELEOSTEAN,
WITH THE PRIMITIVE VASCULAR TRUNKS. (From Gegenbaur.)
 
a. auricle ; v. ventricle ; abr. branchial artery ; c'. carotid ; ad. dorsal aorta ;
s. branchial clefts; sv. sinus venosus; dc. ductus Cuvieri; n. nasal pit
 
There is given off from each collecting trunk from the visceral
arches, or from the commencement of the dorsal aorta, a subclavian
artery to each of the anterior limbs ; from near the anterior end
of the dorsal aorta a vitelline artery (or before the dorsal aortae
have united a pair of arteries fig. 125, R of A and L of A) to the
yolk-sack, which subsequently becomes the main visceral artery 1 ;
and from the dorsal aorta opposite the hind limbs one (or two)
arteries on each side the iliac arteries to the hind limbs ; from
these arteries the allantoic arteries are given off in the higher
types, which remain as the hypogastric arteries after the
disappearance of the allantois.
 
The primitive arrangement of the arterial trunks is with a
few modifications retained in Fishes. With the development of
the gills the vessels to the arches become divided into two parts
connected by a capillary system in the gill folds, viz. into the
 
1 In Mammalia the superior inesenteric artery arises from the vitelline artery,
which may probably be regarded as a primitive crclinco-mescnteric artery.
 
 
 
ARTERIAL SYSTEM.
 
 
 
branchial arteries bringing the blood to the gills from the truncus
arteriosus, and the branchial veins transporting it to the dorsal
aorta. The branchial vessels to those arches which do not bear
gills, either wholly or partially atrophy; thus in Elasmobranchii
the mandibular trunk, which is fully developed in the embryo
(fig. 193, \av}, atrophies, except for a small remnant bringing
blood to the rudimentary gill of the spiracle from the branchial
vein of the hyoid arch. In Ganoids the mandibular artery
atrophies, but the hyoid is usually preserved. In Teleostei both
mandibular 1 and hyoid arteries are absent in the adult, except
that there is usually left a rudiment of the hyoid, supplying the
pseudobranch, which is similar to the rudiment of the mandibular
artery in Elasmobranchii. In Dipnoi the mandibular artery
atrophies, but the hyoid is sometimes preserved (Protopterus),
and sometimes lost.
 
In Fishes provided with a well developed air-bladder this
organ receives arteries, which arise sometimes from the dorsal
aorta, sometimes from the caeliac arteries, and sometimes from
the dorsal section of the last (fourth) branchial trunk. The
latter origin is found in Polypterus and Amia, and seems to have
been inherited by the Dipnoi where the air-bladder forms a true
lung.
 
The pulmonary artery of all the air-breathing Vertebrata is derived from the pulmonary artery of the
Dipnoi.
 
In all the types above Fishes considerable changes are
effected in the primitive arrangement of the arteries in the
visceral arches.
 
In Amphibia the piscine condition is most nearly retained 2 .
The mandibular artery is never developed, and the hyoid artery
is imperfect, being only connected with the cephalic vessels and
never directly joining the dorsal aorta. It is moreover developed
later than the arteries of the true branchial arches behind. The
subclavian arteries spring from the common trunks which unite
to form the dorsal aorta.
 
In the Urodela there are developed, in addition to the hyoid,
 
1 The mandibular artery is stated by Gotte never to be developed in Teleostei, but
is distinctly figured in Lereboullet (No. 71).
 
2 In my account of the Amphibia, Gotte (No. 296) has been followed.
 
 
 
646 ARTERIES OF THE AMNIOTA.
 
four branchial arteries. The three foremost of these at first
supply gills, and in the Perennibranchiate forms continue to do
so through life. The fourth does not supply a gill, and very
early gives off, as in the Dipnoi, a pulmonary branch.
 
The hyoid artery soon sends forward a lingual artery from its
ventral end, and is at first continued to the carotid which grows
forward from the dorsal part of the first branchial vessel.
 
In the Caducibranchiata, where the gills atrophy, the following
changes take place. The remnant of the hyoid is continued
entirely into the lingual artery. The first branchial is mainly
continued into the carotid and other cephalic branches, but a
narrow remnant of the trunk, which originally connected it with
the dorsal aorta, remains, forming what is known as a ductus
Botalli. A rete mirabile on its course is the remnant of the
original gill.
 
The second and third branchial arches are continued as
simple trunks into the dorsal aorta, and the blood from the fourth
arch mainly passes to the lungs, but a narrow ductus Botalli still
connects this arch with the dorsal aorta.
 
In the Anura the same number of arches is present in the
embryo as in the Urodela, all four branchial arteries supplying
branchiae, but the arrangement of the two posterior trunks is
different from that in the Urodela. The third arch becomes at a
very early period continued into a pulmonary vessel, a relativelynarrow branch connecting it with the second arch. The fourth
arch joins the pulmonary branch of the third. At the metamorphosis the hyoid artery loses its connection with the carotid, and
the only part of it which persists is the root of the lingual artery.
The first branchial artery ceases to join the dorsal aorta, and
forms the root of the carotid : the so-called carotid gland placed
on its course is the remnant of the gill supplied by it before the
metamorphosis.
 
The second artery forms a root of the dorsal aorta. The
third, as in all the Amniota, now supplies the lungs, and also
sends off a cutaneous branch. The fourth disappears. The
connection of the pulmonary artery with both the third and
fourth branchial arches in the embryo appears to me clearly to
indicate that this artery was primitively derived from the fonrtli
arc/i as in the Urodela, and that its permanent connection
 
 
 
ARTERIAL SYSTEM.
 
 
 
647
 
 
 
with the third arch in the Anura and in all the Amniota is
secondary.
 
In the Amniota the metamorphosis of the arteries is in all
cases very similar. Five arches, viz. the mandibular, hyoid, and
three branchial arches are always developed (fig. 364), but, owing
to the absence of branchiae,
never function as branchial arteries. Of these the main parts of
the first two, connecting the truncus arteriosus with the collecting
trunk into which the arterial
arches fall, always disappear, usually before the complete development of the arteries in the posterior arches.
 
The anterior part of the collecting trunk into which these
vessels fall is not obliterated
when they disappear, but is on
the contrary continued forwards
as a vessel supplying the brain,
homologous with that found in
Fishes. It constitutes the internal
carotid. Similarly the anterior
part of the trunk from which the mandibular and hyoid arteries
sprang is continued forwards as a small vessel 1 , which at first
passes to the oral region and constitutes in Reptiles the lingual
artery, homologous with the lingual artery of the Amphibia ; but
in Birds and Mammals becomes more important, and is then
known as the external carotid (fig. 125). By these changes the
roots of the external and internal carotids spring respectively
from the ventral and dorsal ends of the primitive third artery,
i.e. the artery of the first branchial arch (fig. 365, c and c'} ; and
thus this arterial arch persists in all types as the common carotid,
 
 
 
 
FIG. 364. DIAGRAM OF THE ARRANGEMENT OF THE ARTERIAL
ARCHES IN AN EMBRYO OF ONE OF THE
 
AMNIOTA. (From Gegenbaur ; after
RATHKE.)
 
a. ventral aorta; a", dorsal aorta;
' 2 > 3> 4> 5- arterial arches ; c. carotid
artery.
 
 
 
1 His (No. 232) describes in Man two ventral continuations of the truncus arteriosus, one derived from the mandibular artery, forming the external maxillary artery,
and one from the hyoid artery, forming the lingual artery. The vessel from which
they spring is the external carotid. These observations of His will very probably be
found to hold true for other types.
 
 
 
6 4 8
 
 
 
ARTERIAL ARCHES OF THE AMNIOTA.
 
 
 
and the basal part of the internal carotid. The trunk connecting
the third arterial arch with the system of the dorsal aorta persists
in some Reptiles (Lacertilia, fig. 366 A) as a ductus Botalli, but
is lost in the remaining Reptiles and in Birds and Mammals (fig.
366 B, C, D). It disappears earliest in Mammals (fig. 365 C),
later in Birds (fig. 365 B), and still later in the majority of
Reptiles.
 
The fourth arch always continues to give rise, as in the Anura,
to the system of the dorsal aorta.
 
In all Reptiles it persists on both sides (fig. 366 A and B),
but with the division of the truncus arteriosus into three vessels
 
 
 
 
ad
 
 
 
FIG. 365. DEVELOPMENT OF THE GREAT ARTERIAL TRUNKS IN THE EMBRYOS
OF A. A LIZARD ; B. THE COMMON FOWL; C. THE PIG. (From Gegenbaur; after
Rathke.)
 
The first two arches have disappeared in all three. In A and B the last three are
still complete, but in C the last two are alone complete.
 
/. pulmonary artery springing from the fifth arch, but still connected with the
system of the dorsal aorta by a ductus Botalli; c. external carotid; <'. internal
carotid; ad. dorsal aorta; a. auricle; v. ventricle; n. nasal pit; m, rudiment of
fore-limb.
 
one of these, i.e. that opening furthest to the left side of the
ventricle (e and d), is continuous with the right fourth arch, and
also with the common carotid arteries (c) ; while a second
springing from the right side of the ventricle is continuous with
the left fourth arch (Ji and f). The right and left divisions of the
fourth arch meet however on the dorsal side of the oesophagus to
give origin to the dorsal aorta (g).
 
In Birds (fig. 366 C) the left fourth arch (h) loses its connection with the dorsal aorta, though the ventral part remains as
 
 
 
ARTERIAL SYSTEM.
 
 
 
649
 
 
 
the root of the left subclavian. The truncus arteriosus is moreover only divided into two parts, one of which is continuous
with all the systemic arteries. Thus it comes about that in
Birds the right fourth arch (e) alone gives rise to the dorsal
aorta.
 
In Mammals (fig. 366 D) the truncus arteriosus is only
divided into two, but the left fourth arch (>), instead of the right,
is that continuous with the dorsal aorta, and the right fourth
arch (/) is only continued into the right vertebral and right
subclavian arteries.
 
The fifth arch always gives origin to the pulmonary artery
(fig. 365, /) and is continuous with one of the divisions of the
truncus arteriosus. In Lizards (fig. 366 A, i), Chelonians and
Birds (fig. 366 C, i] and probably in Crocodilia, the right and
left pulmonary arteries spring respectively from the right and
left fifth arches, and during the greater part of embryonic life
the parts of the fifth arches between the origins of the pulmonary
arteries and the system of the dorsal aorta are preserved as
ductus Botalli. These ductus Botalli persist for life in the
Chelonia. In Ophidia (fig. 366 B, Ji) and Mammalia (fig.
366 D, m) only one of the fifth arches gives origin to the two
pulmonary arteries, viz. that on the right side in Ophidia, and
the left in Mammalia.
 
The ductus Botalli of the fifth arch (known in Man as the
ductus arteriosus) of the side on which the pulmonary arteries
are formed, may remain (e.g. in Man) as a solid cord connecting
the common stem of the pulmonary aorta with the systemic
aorta.
 
The main history of the arterial arches in the Amniota has
been sufficiently dealt with, and the diagram, fig. 366, copied
from Rathke, shews at a glance the character of the metamorphosis these arches undergo in the different types. It merely
remains for me to say a few words about the subclavian and
vertebral arteries.
 
The subclavian arteries in Fishes usually spring from the
trunks connecting the branchial veins with the dorsal aorta.
This origin, which is also found in Amphibia, is typically found
in the embryos of the Amniota. In the Lizards this origin
persists through life, but both subclavians spring from the right
 
 
 
650
 
 
 
ARTERIAL ARCHES OF THE AMNIOTA.
 
 
 
side. In most other types the origin of the subclavians is
carried upwards, so that they usually spring from a trunk
common to them and the carotids (arteria anonyma) (Birds and
some Mammals); or the left one, as in Man and some other
Mammals, arises from the systemic aorta just beyond the
carotids. Various further modifications in the origin of the
subclavians of the same general nature are found in Mammalia,
A 13
 
 
 
 
 
FIG. 366. DIAGRAMS ILLUSTRATING THE METAMORPHOSIS OF THE ARTERIAL
 
ARCHES IN A LlZARD A, A SNAKE B, A BlRD C AND A MAMMAL D. (From
Mivart ; after Rathke.)
 
A. a. internal carotid; b. external carotid ; c. common carotid; d. ductus Botalli
between the third and fourth arches ; e. right aortic trunk ; /. subclavian ; g. dorsal
aorta; h. left aortic trunk; i. pulmonary artery; k. rudiment of ductus Botalli
between the pulmonary artery and the system of the dorsal aorta.
 
B. a. internal carotid; b. external carotid; c. common carotid; d. right aortic
trunk; e. vertebral artery;/, left aortic trunk of dorsal aorta; h. pulmonary artery ;
i. ductus Botalli of pulmonary artery.
 
C. a. internal carotid ; b. external carotid ; c. common carotid ; d. systemic
aorta; e. fourth arch of right side (root of dorsal aorta);/, right subclavian; g. dorsal
aorta; h, left subclavian (fourth arch of left side); i. pulmonary artery; k. and /.
right and left ductus Botalli of pulmonary arteries.
 
D. a. internal carotid; b. external carotid; c. common carotid; d. systemic aorta;
c. fourth arch of left side (root of dorsal aorta);/ dorsal aorta; g. left vertebral
artery; h. left subclavian artery; i. right subclavian (fourth arch of right side); k.
right vertebral; /. continuation of right subclavian; in. pulmonary artery; n. ductus
Botalli of pulmonary artery.
 
 
 
THE VENOUS SYSTEM.
 
 
 
6 5 I
 
 
 
but they need not be specified in detail. The vertebral arteries
usually arise in close connection with the subclavians, but in
Birds they arise from the common carotids.
 
BIBLIOGRAPHY of the Arterial System.
 
(496) H. Rathke. " Ueb. d. Entwick. d. Arterien vv. bei d. Saugethiere von
d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.
 
(-197) H. Rathke. " Untersuchungen lib. d. Aortenwurzeln d. Saurier."
Denkschriften d. k. Akad. Wien, Vol. XIII. 1857.
 
Vide also His (No. 232) and general works on Vertebrate Embryology.
 
TJie Venous System,.
 
The venous system, as it is found in the embryos of Fishes,
consists in its earliest condition of a single large trunk, which
traverses the splanchnic mesoblast investing the part of the
alimentary tract behind the heart. This trunk is directly continuous in front with the heart, and underlies the alimentary
canal through both its praeanal and postanal sections. It is
shewn in section in fig. 367, v, and may be called the subintestinal vein. This vein has been found in the embryos of
Teleostei, Ganoidei, Elasmobranchii and Cyclostomata, and runs
parallel to the dorsal aorta above, into which it is sometimes
continued behind (Teleostei, Ganoidei, etc.).
 
In Elasmobranch embryos the subintestinal vein terminates,
as may be gathered from sections (fig. 368, v.cau), shortly before
the end of the tail. The same series of sections also shews that
at the cloaca, where the gut enlarges and comes in contact with
the skin, this vein bifurcates, the two branches uniting into a
single vein both in front of and behind the cloaca.
 
In most Fishes the anterior part of this vein atrophies, the
caudal section alone remaining, but the anterior section of it
persists in the fold of the intestine in Petromyzon, and also
remains in the spiral valve of some Elasmobranchii. In
Amphioxus, moreover, it forms, as in the embryos of higher
types, the main venous trunk, though even here it is usually
broken up into two or three parallel vessels.
 
It no doubt represents one of the primitive longitudinal trunks of the
vermiform ancestors of the Chordata. The heart and the branchial artery
constitute a specially modified anterior continuation of this vein. The
 
 
 
652
 
 
 
THE SUBINTESTINAL VEIN.
 
 
 
-p.o
 
 
 
rp.r.
 
 
 
dilated portal sinus of Myxine is probably also part of it ; and if this is
really rhythmically contractile 1 the fact would be interesting as shewing that
this quality, which is now localised in the heart, was once probably common
to the subintestinal vessel for its whole length.
 
On the development of the cardinal veins (to be described
below) considerable changes are
effected in the subintestinal vein.
Its postanal section, which is known
in the adult as the caudal vein,
unites with the cardinal veins. On
this junction being effected retrogressive changes take place in the
praeanal section of the original subintestinal vessel. It breaks up in
front into a number of smaller
vessels, the most important of which
is a special vein, which lies in the
fold of the spiral valve, and which is
more conspicuous in some Elasmobranchii than in Scyllium, in which
the development of the vessel has
been mainly studied. The lesser of
the two branches connecting it
round the cloaca with the caudal
vein first vanishes, and then the
larger ; and the two posterior cardinals are left as the sole forward
continuations of the caudal vein.
The latter then becomes prolonged
forwards, so that the two cardinals
open into it some little distance in
front of the hind end of the kidneys.
By these changes, and by the disappearance of the postanal section of the gut, the caudal vein is
made to appear as a supraintestinal and not, as it really is, a
subintestinal vessel.
 
From the subintestinal vein there is given off a branch which
supplies the yolk-sack. This leaves the subintestinal vein close
 
1 J. Miiller holds that this sack is not rhythmically contractile.
 
 
 
 
FIG. 367. SECTION THROUGH
THE TRUNK OF A SCYLLIUM
EMBRYO SLIGHTLY YOUNGER
 
THAN 28 F.
 
sp.c. spinal canal; W. white
matter of spinal cord ; pr. posterior nerve-roots; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle plate; ;;//'. inner layer
of muscle-plate already converted
into muscles; Vr. rudiment of
vertebral body; st. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve; v. subintestinal vein ;
p.o. primitive generative cells.
 
 
 
THE VENOUS SYSTEM.
 
 
 
653
 
 
 
to the liver. The liver, on its development, embraces the
subintestinal vein, which then breaks up into a capillary system
in the liver, the main part of its blood coming at this period
from the yolk-sack.
 
The portal system is thus established from the subintestinal
vein ; but is eventually joined by the various visceral, and sometimes by the genital, veins as they become successively developed.
 
The blood from the liver is brought back to the sinus venosus by veins known as the hepatic veins, which, like the hepatic
capillary system, are derivatives of the subintestinal vessel.
 
There join the portal system in Myxinoids and many
Teleostei a number of veins from the anterior abdominal walls,
representing a commencement of the anterior abdominal or
epigastric vein of higher types 1 .
 
In the higher Vertebrates the original subintestinal vessel never attains a
full development, even in the embryo. It is represented by (i) the ductus
 
 
 
 
FIG. 368. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL
OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.
 
A. is the posterior section.
 
nc. neural canal; al. post-anal gut; alv. caudal vesicle of post-anal gut; x.
subnotochordal rod; mp. muscle-plate; c/i. notochord; cl.al. cloaca; ao. aorta;
v.cait. caudal vein.
 
1 Stannius, Vergleich. Anat., p. 251.
 
 
 
654
 
 
 
THE CARDINAL VEINS.
 
 
 
venosus, which, like the true subintestinal vein, gives origin (in the Amniota)
to the vitelline veins to the yolk-sack, and (2) by the caudal vein. Whether
the partial atrophy of the subintestinal vessel was primitively caused by the
development of the cardinal veins, or for some other reason, it is at any rate
a fact that in all existing Fishes the cardinal veins form the main venous
channels of the trunk.
 
Their later development than the subintestinal vessel as well as their
absence in Amphioxus, probably indicate that they became evolved, at any
rate in their present form, within the Vertebrate phylum.
 
The embryonic condition of the venous system, with a single
large subintestinal vein is, as has been stated, always modified
by the development of a paired system of vessels, known as the
cardinal veins, which bring to the heart the greater part of the
blood from the trunk.
 
The cardinal veins appear in Fishes as four paired longitudinal trunks (figs. 363 and 369), two anterior (/) and two
posterior (c). They unite into two transverse trunks on either
side, known as the ductus Cuvieri (dc), which fall into the sinus
venosus, passing from the body wall to the sinus by a lateral
mesentery of the heart already spoken of (p. 627, fig. 352). The
anterior pair, known as the anterior cardinal or jugular veins,
bring to the heart the blood from the head and neck. They
are placed one on each side above the level
of the branchial arches (fig. 299, a.cv). The
posterior cardinal veins lie immediately dorsal to the mesonephros (Wolfifian body), and
are mainly supplied by the blood from this
organ and from the walls of the body (fig.
275, c.a.v). In many forms (Cyclostomata,
Elasmobranchii and many Teleostei) they
unite posteriorly with the caudal veins in
the manner already described, and in a large
number of instances the connecting branch
between the two systems, in its passage through
the mesonephros, breaks up into a capillary
network, and so gives rise to a renal portal
system.
 
The vein from the anterior pair of fins
(subclavian) usually unites with the anterior
jugular vein.
 
 
 
 
j
 
 
 
FIG. 369. DIAGRAM OF THE PAIRED VENOUS SYSTEM
 
OF A FISH. (From
Gegenbaur. )
 
j. jugular vein
(anterior cardinal
vein) ; c. posterior
cardinal vein; //. hepatic veins ; sv. sinus
venosus ; dc. ductus
Cuvieri.
 
 
 
THE VENOUS SYSTEM. 655
 
The venous system of the Amphibia and Amniota always
differs from that of Fishes in the presence of a new vessel, the
vena cava inferior, which replaces the posterior cardinal veins;
the latter only being present, in their piscine form, during
embryonic life. It further differs from that of all Fishes, except
the Dipnoi, in the presence of pulmonary veins bringing back
the blood directly from the lungs.
 
In the embryos of all the higher forms the general characters
of the venous system are at first the same as in Fishes, but with
the development of the vena cava inferior the front sections of
the posterior cardinal veins atrophy, and the ductus Cuvieri,
remaining solely connected with the anterior cardinals and their
derivatives, constitute the superior venae cavae. The inferior
cava receives the hepatic veins.
 
Apart from the non-development of the subintestinal vein
the visceral section of the venous system is very similar to that
in Fishes.
 
The further changes in the venous system must be dealt
with separately for each group.
 
Amphibia. In Amphibia (Gotte, No. 296) the anterior and posterior
cardinal veins arise as in Pisces. From the former the internal jugular vein
arises as a branch ; the external jugular constituting the main stem. The
subclavian with its large cutaneous branch also springs from the system of
the anterior cardinal. The common trunk formed by the junction of these
three veins falls into the ductus Cuvieri.
 
The posterior cardinal veins occupy the same position as in Pisces, and
unite behind with the caudal veins, which Gotte has shewn to be originally
situated below the post-anal gut. The iliac veins unite with the posterior
cardinal veins, where the latter fall into the caudal vein. The original
piscine condition of the veins is not long retained. It is first of all disturbed
by the development of the anterior part of the important unpaired venous
trunk which forms in the adult the vena cava inferior. This is developed
independently, but unites behind with the right posterior cardinal. From
this point backwards the two cardinal veins coalesce for some distance, to
give rise to the posterior section of the vena cava inferior, situated between
the kidneys 1 . The anterior sections of the cardinal veins subsequently
atrophy. The posterior part of the cardinal veins, from their junction with
the vena cava inferior to the caudal veins, forms a rhomboidal figure. The
iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitu
1 This statement of Gotte's is opposed to that of Rathke for the Amniota, and
cannot be considered as completely established.
 
 
 
656 VEINS OF THE SNAKE.
 
dinal vessel on the outer border of the kidneys, which receives transverse
vertebral veins and transmits their blood to the kidneys, thus forming a
renal portal system. The anterior limbs of the rhomboid formed by the
cardinal veins soon atrophy, so that the blood from the hind limbs can only
pass to the inferior vena cava through the renal portal system. The
posterior parts of the two cardinal veins (uniting in the Urodela directly
with the unpaired caudal vein) still persist. The iliac veins also become
directly connected with a new vein, the anterior abdominal vein, which
has meanwhile become developed. Thus the iliac veins become united
with the system of the vena cava inferior through the vena renalis advehens
on the outer border of the kidney, and with the anterior abdominal veins by
the epigastric veins.
 
The visceral venous system begins with the development of two vitelline
veins, which at first join the sinus venosus directly. They soon become
enveloped in the liver, where they break up into a capillary system, which
is also joined by the other veins from the viscera. The hepatic system has
in fact the same relations as in Fishes. Into this system the anterior
abdominal vein also pours itself in the adult. This vein is originally
formed of two vessels, which at first fall directly into the sinus venosus,
uniting close to their opening into the sinus with a vein from the truncus
arteriosus. They become prolonged backwards, and after receiving the
epigastric veins above mentioned from the iliac veins, and also veins from
the allantoic bladder, unite behind into a single vessel. Anteriorly the
right vein atrophies and the left continues forward the unpaired posterior
section.
 
A secondary connection becomes established between the anterior abdominal vein and the portal system ; so that the blood originally transported
by the former vein to the heart becomes diverted so as to fall into the liver.
A remnant of the primitive connection is still retained in the adult in the
form of a small vein, the so-called vena bulbi posterior, which brings the
blood from the walls of the truncus arteriosus directly into the anterior
abdominal vein.
 
The pulmonary veins grow directly from the heart to the lungs.
 
For our knowledge of the development of the venous system of the
Amniota we are mainly indebted to Rathke.
 
Reptilia. As an example of the Reptilia the Snake may be selected,
its venous system having been fully worked out by Rathke in his important
memoir on its development (No. 300).
 
The anterior (external jugular) and posterior cardinal veins are formed in
the embryo as in all other types (fig. 370, vj and vc] ; and the anterior
cardinal, after giving rise to the anterior vertebral and to the cephalic veins,
persists with but slight modifications in the adult ; while the two ductus
Cuvieri constitute the superior venos cavas.
 
The two posterior cardinals unite behind with the caudal veins. They
are placed in the usual situation on the dorsal and outer border of the
kidneys.
 
 
 
THE VENOUS SYSTEM.
 
 
 
657
 
 
 
 
U
FIG. 370. ANTERIOR
PORTION OF THE VENOUS
SYSTEM OF AN EMBRYONIC
SNAKE. (From Gegenbaur;
after Rathke.)
 
vc. posterior cardinal
vein; vj. jugular vein; DC.
ductus Cuvieri ; vu. allantoic vein ; v. ventricle ; ba.
truncus arteriosus ; a. visceral clefts ; /. auditory
vesicle.
 
 
 
With the development of the vena cava inferior, to be described below,
the blood from the kidneys becomes mainly
transported by this vessel to the heart ; and the
section of the posterior cardinals opening into
the ductus Cuvieri gradually atrophies, their
posterior parts remaining however on the outer
border of the kidneys as the vena? renales
advehentes 1 .
 
While the front part of the posterior cardinal
veins is undergoing atrophy, the intercostal veins,
which originally poured their blood into the
posterior cardinal veins, become also connected
with two longitudinal veins the posterior vertebral veins which are homologous with the
azygos and hemiazygos veins of Man ; and bear
the same relation to the anterior vertebral veins
that the anterior and posterior cardinals do to
each other.
 
These veins are at first connected by trans
verse anastomoses with the posterior cardinals,
but, on the disappearance of the front part of the
latter, the whole of the blood from the intercostal veins falls into the
posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into
the jugular vein.
 
The posterior vertebral veins are at first symmetrical, but after becoming
connected by transverse anastomoses, the right becomes the more important
of the two.
 
The vena cava inferior, though considerably later in its development
than the cardinals, arises fairly early. It constitutes in front an unpaired
trunk, at first very small, opening into the right allantoic vein, close to the
heart. Posteriorly it is continuous with two veins placed on the inner
border of the kidneys 2 .
 
The vena cava inferior passes through the dorsal part of the liver, and in
doing so receives the hepatic veins.
 
The portal system is at first constituted by the vitelline vein, which is
directly continuous with the venous end of the heart, and at first receives
the two ductus Cuvieri, but at a later period unites with the left ductus.
 
1 Rathke's account of the vena renalis advehens is thus entirely opposed to that
which Gotte gives for the Frog, but my own observations on the Lizard incline me to
accept Rathke's statements, for the Amniota at any rate.
 
2 The vena cava inferior does not according to Rathke's account unite behind with
the posterior cardinal veins, as it is stated by Gotte to do in the Anura. Gb'tte
questions the accuracy of Rathke's statements on this head, but my own observations
are entirely in favour of Rathke's observations, and lend no support whatever to
Gotte's views.
 
 
 
B. III.
 
 
 
658 VEINS OF THE CHICK.
 
It soon receives a mesenteric vein bringing the blood from the viscera,
which is small at first but rapidly increases in importance.
 
The common trunk of the vitelline and mesenteric veins, which may be
called the portal vein, becomes early enveloped by the liver, and gives off
branches to this organ, the blood from which passes by the hepatic veins
to the vena cava inferior. As the branches in the liver become more
important, less and less blood is directly transported to the heart, and finally
the part of the original vitelline vein in front of the liver is absorbed, and the
whole of the blood from the portal system passes from the liver into the
vena cava inferior.
 
The last section of the venous system to be dealt with is that of the
anterior abdominal vein. There are originally, as in the Anura, two veins
belonging to this system, which owing to the precocious development of the
bladder to form the allantois, constitute the allantoic veins (fig. 370, vu}.
 
These veins, running along the anterior abdominal wall, are formed
somewhat later than the vitelline vein, and fall into the two ductus Cuvieri.
They unite with two epigastric veins (homologous with those in the Anura),
which connect them with the system of the posterior cardinal veins. The
left of the two eventually atrophies, so that there is formed an unpaired
allantoic vein. This vein at first receives the vena cava inferior close to the
heart, but eventually the junction of the two takes place in the region of the
liver, and finally the anterior abdominal vein (as it comes to be after the
atrophy of the allantois) joins the portal system and breaks up into capillaries
in the liver 1 .
 
In Lizards the iliac veins join the posterior cardinals, and so pour part of
their blood into the kidneys ; they also become connected by the epigastric
veins with the system of the anterior abdominal or allantoic vein. The
subclavian veins join the system of the superior venae cavas.
 
The venous system of Birds and Mammals differs in two important
points from that of Reptilia and Amphibia. Firstly the anterior abdominal
vein is only a foetal vessel, forming during foetal life the allantoic vein ;
and secondly a direct connection is established between the vena cava
inferior and the veins of the hind limbs and posterior parts of the cardinal
veins, so that there is no renal portal system.
 
Aves. The Chick may be taken to illustrate the development of the
venous system in Birds.
 
On the third day, nearly the whole of the venous blood from the body
of the embryo is carried back to the heart by two main venous trunks,
the anterior (fig. 125, S.Ca.V) and posterior (V.Ca) cardinal veins, joining on
each side to form the short transverse ductus Cuvieri (DC), both of which
unite with the sinus venosus close to the heart. As the head and neck
continue to enlarge, and the wings become developed, the single anterior
 
1 The junction between the portal system and the anterior abdominal vein is
apparently denied by Rathke (No. 300, p. 173), hut this must he an error on
his part.
 
 
 
THE VENOUS SYSTEM.
 
 
 
659
 
 
 
 
V.C.L
 
 
 
cardinal or jugular vein (fig. 371, /), of each side, is joined by two new
veins : the vertebral vein, bringing back blood from the head and neck, and
the subclavian vein from the wing (W\
 
On the third day the posterior cardinal veins are the only veins which
return the blood from the hinder part of the body of the embryo.
 
About the fourth or fifth day, however, the vena cava inferior (fig. 371,
V.C.L) makes its appearance. This, starting
from the sinus venosus not far from the heart,
is on the fifth day a short trunk running backward in the middle line below the aorta, and
speedily losing itself in the tissues of the
Wolffian bodies. When the true kidneys are
formed it also receives blood from them, and
thenceforward enlarging rapidly becomes the
channel by which the greater part of the blood
from the hinder part of the body finds its way
to the heart. In proportion as the vena cava
inferior increases in size, the posterior cardinal
veins diminish.
 
The blood originally coming to them from
the posterior part of the spinal cord and trunk
is transported into two posterior vertebral veins,
similar to those in Reptilia, which are however
placed dorsally to the heads of the ribs, and
join the anterior vertebral veins. With their
appearance the anterior parts of the posterior
cardinals disappear. The blood from the hind
limbs becomes transported directly through the
kidney into the vena cava inferior, without
forming a renal portal system 1 .
 
On the third day the course of the vessels from the yolk-sack is very
simple. The two vitelline veins, of which the right is already the smaller,
form the ductus venosus, from which, as it passes through the liver on its
way to the heart, are given off the two sets of vena advehentes and vena
revehentes (fig. 371).
 
With the appearance of the allantois on the fourth day, a new feature is
introduced. From the ductus venosus there is given off a vein which
quickly divides into two branches. These, running along the ventral walls
of the body from which they receive some amount of blood, pass to the
allantois. They are the allantoic veins (fig. 371, U] homologous with the
anterior abdominal vein of the lower types. They unite in front to form a
single vein, which becomes, by reason of the rapid growth of the allantois,
very long. The right branch soon diminishes in size and finally disappears.
Meanwhile the left on reaching the allantois bifurcates ; and, its two
 
 
 
FIG. 371. DIAGRAM OF
THE VENOUS CIRCULATION
IN THE CHICK AT THE COMMENCEMENT OF THE FIFTH
 
DAY.
 
H. heart ; d. c. ductus Cuvieri. Into the ductus Cuvieri
of each side fall/, the jugular
vein, W. the vein from the
wing, and c. the inferior cardinal vein ; S. V. sinus venosus ;
Of. vitelline vein ; U. allantoic vein, which at this stage
gives off branches to the bodywalls ; V.C.l. inferior vena
cava ; /. liver.
 
 
 
The mode in which this is effected requires further investigation.
 
42 2
 
 
 
66o
 
 
 
VEINS OF THE CHICK.
 
 
 
 
branches becoming large and conspicuous, there still appear to be two
main allantoic veins. At its first appearance the allantoic vein seems to be
but a small branch of the vitelline, but as the allantois grows rapidly,
and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.
 
On the third day the blood returning from the walls of the intestine is
insignificant in amount. As however the
intestine becomes more and more developed, it acquires a distinct venous system,
and its blood is returned by veins which
form a trunk, the mesenteric vein (fig. 372,
M") falling into the vitelline vein at its
junction with the allantoic vein.
 
These three great veins, in fact, form a
large common trunk, which enters at once
into the liver, and which we may now call
the portal vein (fig. 372, P. V}. This, at its
entrance into the liver, partly breaks up
into the vena advehentes, and partly continues as the ductus venosus (D.V}
straight through the liver, emerging from
which it joins the vena cava inferior. Before
the establishment of the vena cava inferior,
the venas revehentes, carrying back the
blood which circulates through the hepatic
capillaries, join the ductus venosus close to
its exit from the liver. By the time however that the vena cava has become a large
and important vessel it is found that the
venae revehentes, or as we may now call
them the hepatic veins, have shifted their
embouchment, and now fall directly into
that vein, the ductus venosus making a separate junction rather higher up (fig. 372).
 
This state of things continues with but slight changes till near the end
of incubation, when the chick begins to breathe the air in the air-chamber
of the shell, and respiration is no longer carried on by the allantois. Blood
then ceases to flow along the allantoic vessels ; they become obliterated.
The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere
branch of the portal vein. The ductus venosus becomes obliterated ; and
hence the whole of the blood coming through the portal vein flows into the
substance of the liver, and so by the hepatic veins into the vena cava.
 
Although the allantoic (anterior abdominal) vein is obliterated in the
adult, there is nevertheless established an anastomosis between the portal
system and the veins bringing the blood from the limbs to the vena cava
 
 
 
FIG. 372. DIAGRAM OF THE
VENOUS CIRCULATION IN THE
CHICK DURING THE LATER DAYS
OF INCUBATION.
 
H. heart ; V.S.R. right vena
cava superior; V.S.L. left vena cava
superior. The two venas cavrc
superiores are the original 'ductus
Cuvieri,' they open into the sinus
venosus. J. jugular vein; Su.V.
anterior vertebral vein ; In. V. inferior vertebral vein ; W. subclavian; V.C.I, vena cava inferior;
D. V. ductus venosus ; P. V. portal
vein ; M. mesenteric vein bringing
blood from the intestines into the
portal vein ; O.f. vitelline vein ; U.
allantoic vein. The three last mentioned veins unite together to form
the portal vein ; /. liver.
 
 
 
THE VENOUS SYSTEM.
 
 
 
66l
 
 
 
inferior, in that the caudal vein and posterior pelvic veins open into a
vessel, known as the coccygeo-mesenteric vein, which joins the portal
vein ; while at the same time the posterior pelvic veins are connected with
the common iliac veins by a vessel which unites with them close to their
junction with the coccygeo-mesenteric vein.
 
Mammalia. In Mammals the same venous trunks are developed in
the embryo as in other types (fig. 373 A). The anterior cardinals or
external jugulars form the primitive veins of the anterior part of the body,
and the internal jugulars and anterior vertebrals are subsequently formed.
The subclavians (fig. 373 A, j), developed on the formation of the anterior
limbs, also pour their blood into these primitive trunks. In the lower
Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc.,
the two ductus Cuvieri remain as the two superior venae cavae, but more
usually an anastomosis arises between the right and left innominate veins,
and eventually the whole of the blood of the left superior cava is carried to
the right side, and there is left only a single superior cava (fig. 373 B and C).
 
 
 
 
 
F IG - 373- DIAGRAM OF THE DEVELOPMENT OF THE PAIRED VENOUS SYSTEM OF
 
MAMMALS (MAN). (From Gegenbaur.)
 
j. jugular vein ; cs. vena cava superior; s. subclavian veins; c. posterior cardinal
vein ; v. vertebral vein ; az. azygos vein ; cor. coronary vein.
 
A. Stage in which the cardinal veins have already disappeared. Their position
is indicated by dotted lines.
 
B. Later stage when the blood from the left jugular vein is carried into the right
to form the single vena cava superior ; a remnant of the left superior cava being however still left.
 
C. Stage after the left vertebral vein has disappeared; the right vertebral
remaining as the azygos vein. The coronary vein remains as the last remnant of the
left superior vena cava.
 
A small rudiment of the left superior cava remains however as the sinus
coronartus and receives the coronary vein from the heart (figs. 373 C,
cor and 374, cs).
 
The posterior cardinal veins form at first the only veins receiving the
 
 
 
662
 
 
 
THE VEINS OF MAMMALIA.
 
 
 
blood from the posterior part of the trunk and kidneys ; and on the
development of the hind limbs receive the blood from them also.
 
As in the types already described
an unpaired vena cava inferior becomes
eventually developed, and gradually
carries off a larger and larger portion
of the blood originally returned by the
posterior cardinals. It unites with the
common stem of the allantoic and
vitelline veins in front of the liver.
 
At a later period a pair of trunks
is established bringing the blood from
the posterior part of the cardinal veins
and the crural veins directly into the
vena cava inferior (fig. 374, il}. These
vessels, whose development has not
been adequately investigated, form the
common iliac veins, while the posterior
ends of the cardinal veins which join
them become the hypogastric veins (fig.
374, hy). Owing to the development of
the common iliac veins there is no renal
portal system like that of the Reptilia
and Amphibia.
 
Posterior vertebral veins, similar to
those of Reptilia and Birds, are established in connection with the intercostal
and lumbar veins, and unite anteriorly
with the front part of the posterior
 
 
 
 
FIG. 374. DIAGRAM OF THE CHIEF
 
VENOUS TRUNKS OF MAN. (From
Gegenbaur.)
 
cs. vena cava superior ; s. subclavian vein ; ji. internal jugular ; je.
external jugular ; az. azygos vein ; ha.
hemiazygos vein ; c. clotted line shewing previous position of cardinal veins ;
ci. vena cava inferior ; r. renal veins ;
il. iliac ; hy. hypogastric veins ; h.
hepatic veins.
 
The dotted lines shew the position
of embryonic vessels aborted in the
adult.
 
 
 
cardinal veins (fig. 373 A) 1 .
 
On the formation of the posterior vertebral veins, and as the inferior
vena cava becomes more important, the middle part of the posterior cardinals becomes completely aborted (fig. 374, f), the anterior and posterior
parts still persisting, the former as the continuations of the posterior
vertebrals into the anterior vena cava (az\ the latter as the hypogastric veins
(Ay).
 
Though in a few Mammalia both the posterior vertebrals persist, a
transverse connection is usually established between them, and the one (the
right) becoming the more important constitutes the azygos vein (fig. 374, az),
the persisting part of the left forming the hemiazygos vein (ha}.
 
The remainder of the venous system is formed in the embryo of the
vitelline and allantoic veins, the former being eventually joined by the
mesenteric vein so as to constitute the portal vein.
 
1 Rathke, as mentioned above, holds that in the Snake the front part of the
posterior cardinals completely aborts. Further investigations are required to shew
whether there really is a difference between Mammalia and Reptilia in this matter.
 
 
 
 
 
 
THE VENOUS SYSTEM. 663
 
The vitelline vein is the first part of this system established, and divides
near the heart into two veins bringing back the blood from the yolk-sack
(umbilical vesicle). The right vein soon however aborts.
 
The allantoic (anterior abdominal) veins are originally paired. They
are developed very early, and at first course along the still widely open
somatic walls of the body, and fall into the single vitelline trunk in front.
The right allantoic vein disappears before long, and the common trunk
formed by the junction of the vitelline and allantoic veins becomes considerably elongated. This trunk is soon enveloped by the liver.
 
The succeeding changes have been somewhat differently described by
Kolliker and Rathke. According to the former the common trunk of the
allantoic and vitelline veins in its passage through the liver gives off
branches to the liver, and also receives branches from this organ near its
anterior exit. The main trunk is however never completely aborted, as in
the embryos of other types, but remains as the ductus venosus Arantii.
 
With the development of the placenta the allantoic vein becomes the
main source of the ductus venosus, and the vitelline or portal vein, as it may
perhaps be now conveniently called, ceases to join it directly, but falls into
one of its branches in the liver.
 
The vena cava inferior joins the continuation of the ductus venosus in
front of the liver, and, as it becomes more important, it receives directly
the hepatic veins which originally brought back blood into the ductus
venosus. The ductus venosus becomes moreover merely a small branch of
the vena cava.
 
At the close of foetal life the allantoic vein becomes obliterated up to its
place of entrance into the liver ; the ductus venosus becomes a solid cord
the so-called round ligament and the whole of the venous blood is brought
to the liver by the portal vein 1 .
 
Owing to the allantoic (anterior abdominal) vein having merely a fcetal
existence an anastomosis between the iliac veins and the portal system by
means of the anterior abdominal vein is not established.
 
 
 
BIBLIOGRAPHY of the Venous System.
 
(498) J. Marshall. "On the development of the great anterior veins." Phil.
Trans., 1859.
 
(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Saugethieren." MeckeVs Archiv, 1830.
 
(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbelthiere." Bericht. Jib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.
 
Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke
(Nos. 299, 300, and 301).
 
1 According to Rathke the original trunk connecting the allantoic vein directly
with the heart through the liver is aborted, and the ductus venosus Arantii is a
secondary connection established in the latter part of foetal life.
 
 
 
664 LYMPHATIC SYSTEM.
 
 
 
Lymphatic System.
 
The lymphatic system arises from spaces in the general parenchyma of
the body, independent in their origin of the true body cavity, though communicating both with this cavity and with the vascular system.
 
In all the true Vertebrata certain parts of the system form definite trunks
communicating with the venous system ; and in the higher types the walls of
the main lymphatic trunks become quite distinct.
 
But little is known with reference to the ontogeny of the lymphatic vessels,
but they originate late in larval life, and have at first the form of simple
intercellular spaces.
 
The lymphatic glands appear to originate from lymphatic plexuses, the
cells of which produce lymph corpuscles. It is only in Birds and Mammals,
and especially in the latter, that the lymphatic glands form definite structures.
 
The Spleen. The spleen, from its structure, must be classed with the
lymphatic glands, though it has definite relations to the vascular system.
It is developed in the mesoblast of the mesogastrium, usually about the
same time and in close connection with the pancreas.
 
According to Miiller and Peremeschko the mass of mesoblast which
forms the spleen becomes early separated by a groove on the one side from
the pancreas and on the other from the mesentery. Some of its cells
become elongated, and send out processes which uniting with like processes
from other cells form the trabecular system. From the remainder of the
tissue are derived the cells of the spleen pulp, which frequently contain more
than one nucleus. Especial accumulations of these cells take place at a
later period to form the so-called Malpighian corpuscles of the spleen.
 
BIBLIOGRAPHY of Spleen.
 
(501) W. Miiller. "The Spleen." Strieker's Histology.
 
(502) Peremeschko. " Ueb. d. Entwick. d. Milz." Sitz. d. Wuti. Akad.
Wiss., Vol. LVI. 1867.
 
Suprarenal ^bodies.
 
In Elasmobranch Fishes two distinct sets of structures are found, both of
which have been called suprarenal bodies. As shewn in the sequel both of
these structures probably unite in the higher types to form the suprarenal
bodies.
 
One of them consists of a series of paired bodies, situated on the
branches of the dorsal aorta, segmentally arranged, and forming a chain
extending from close behind the heart to the hinder end of the body cavity.
Each body is formed of a series of lobes, and exhibits a well-marked
distinction into a cortical layer of columnar cells, and a medullary substance
formed of irregular polygonal cells. As first shewn by Leydig, they are
 
 
 
SUPRARENAL BODIES. 665
 
closely connected with the sympathetic ganglia, and usually contain numerous
ganglion cells distributed amongst the proper cells of the body.
 
The second body consists of an unpaired column of cells placed between
the dorsal aorta and unpaired caudal vein, and bounded on each side by the
posterior parts of the kidney. I propose to call it the interrenal body.
In front it overlaps the paired suprarenal bodies, but does not unite with
them. It is formed of a series of well-marked lobules, etc. In the fresh
state Leydig (No. 506) finds that "fat molecules form the chief mass of the
body, and one finds freely imbedded in them clear vesicular nuclei." As
may easily be made out from hardened specimens it is invested by a tunica
propria, which gives off septa dividing it into well-marked areas filled with
polygonal cells. These cells constitute the true parenchyma of the body.
By the ordinary methods of hardening, the oil globules, with which they are
filled in the fresh state, completely disappear.
 
The paired suprarenal bodies (Balfour, No. 292, pp. 242 244) are developed from the sympathetic ganglia. These ganglia, shewn in an early
stage in fig. 380, sy.g, become gradually divided into a ganglionic part and a
glandular part. The former constitutes the sympathetic ganglia of the adult ;
the latter the true paired suprarenal bodies. The interrenal body is however
developed (Balfour, No. 292, pp. 245 247) from indifferent mesoblast cells
between the two kidneys, in the same situation as in the adult.
 
The development of the suprarenal bodies in the Amniota has been most
fully studied by Braun (No. 503) in the Reptilia.
 
In Lacertilia they consist of a pair of elongated yellowish bodies, placed
between the vena renalis revehens and the generative glands.
 
They are formed of two constituents, viz. (i) masses of brown cells placed
on the dorsal side of the organ, which stain deeply with chromic acid, like
certain of the cells of the suprarenals of Mammalia, and (2) irregular cords,
in part provided with a lumen, filled with fat-like globules l , amongst which
are nuclei. On treatment with chromic acid the fat globules disappear, and
the cords break up into bodies resembling columnar cells.
 
The dorsal masses of brown cells are developed from the sympathetic
ganglia in the same way as the paired suprarenal bodies of the Elasmobranchii, while the cords filled with fat-like globules are formed of indifferent
mesoblast cells as a thickening in the lateral walls of the inferior vena cava,
and the cardinal veins continuous with it. The observations of Brunn (No.
504) on the Chick, and Kolliker (No. 298, pp. 953955) n the Mammal,
add but little to those of Braun. They shew that the greater part of the
gland (the cortical substance) in these two types is derived from the mesoblast,
and that the glands are closely connected with sympathetic ganglia ; while
Kolliker also states that the posterior part of the organ is unpaired in the
embryo rabbit of 1 6 or 17 days.
 
The structure and development of what I have called the interrenal body
 
1 These globules are not formed of a true fatty substance, and this is also probably
true for the similar globules of the interrenal bodies of Elasmobranchii.
 
 
 
666 SUPRARENAL BODIES.
 
in Elasmobranchii so closely correspond with that of the mesoblastic part of
the suprarenal bodies of the Reptilia, that I have very little hesitation in
regarding them as homologous 1 ; while the paired bodies in Elasmobranchii,
derived from the sympathetic ganglia, clearly correspond with the part of the
suprarenals of Reptilia having a similar origin ; although the anterior parts
of the paired suprarenal bodies of Fishes have clearly become aborted in the
higher types.
 
In Elasmobranch Fishes we thus have (i) a series of paired
bodies, derived from the sympathetic ganglia, and (2) an unpaired body of mesoblastic origin. In the Amniota these bodies
unite to form the compound suprarenal bodies, the two constituents of which remain, however, distinct in their development.
The mesoblastic constituent appears to form the cortical part of
the adult suprarenal body, and the nervous constituent the
medullary part.
 
BIBLIOGRAPHY of the Suprarenal bodies,
 
(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilien. " Arbeit,
a. d. zool.-zoot. Institut Wurzlttrg, Vol. V. 1879.
 
(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick.
d. Nebennieren." Archiv f. mikr. Anat., Vol. VIII. 1872.
 
(505) Fr. Leydig. Untersiich. iib. Fische u. fieptilten. Berlin, 1853.
 
(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.
 
Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.
 
1 The fact of the organ being unpaired in Elasmobranchii and paired in the
Amniota is of no importance, as is shewn by the fact that part of the organ is unpaired
in the Rabbit.
 
 
 
CHAPTER XXII.
 
 
 
THE MUSCULAR SYSTEM.
 
 
 
 
IN all the Ccelenterata, except the Ctenophora, the contractile elements of the body wall consist of filiform processes of
ectodermal or entodermal epithelial cells (figs. 375 and 376 B).
The elements provided with these processes, which were first
discovered by Kleinenberg, are known as myo-epithelial
cells. Their contractile parts may either be striated (fig. 376)
or non-striated (fig. 375). In some
instances the epithelial part of the
cell may nearly abort, its nucleus
alone remaining (fig. 376 A) ; and
in this way a layer of muscles lying
completely below the surface may
be established.
 
There is embryological evidence
of the derivation of the voluntary
muscular system of a large number of types from myo-epithelial
cells of this kind. The more important of these groups are the
Chaetopoda, the Gephyrea, the Chaetognatha, the Nematoda, and
the Vertebrata 1 .
 
While there is clear evidence that the muscular system of a
large number of types is composed of cells which had their
origin in myo-epithelial cells, the mode of evolution of the
 
1 If recent statements of Metschnikoff are to be trusted, the Echinodermata must
be added to these groups. The amoeboid cells stated in the first volume of this
treatise to form the muscles in this group, on the authority of Selenka, give rise,
according to Metschnikoff, only to the cutis, while the same naturalist states the
epithelial cells of the vasoperitoneal vesicles are provided with muscular tails.
 
 
 
FIG. 375. MYO-EPITHELIAL
CELLS OF HYDRA. (From Gegenbaur ; after Kleinenberg.)
 
m. contractile fibres.
 
 
 
668 THE MUSCULAR FIBRES.
 
muscular system of other types is still very obscure. The
muscles may arise in the embryo from amoeboid or indifferent
cells, and the Hertwigs 1 hold that in many of these instances the
muscles have also phylogenetically taken their origin from
indifferent connective-tissue cells. The subject is however beset
with very serious difficulties, and to discuss it here would carry
me too far into the region of pure histology.
 
The voluntary muscular system of the CJiordata.
 
The muscular fibres. The muscular elements of the
Chordata undoubtedly belong to the myo-epithelial type. The
embryonic muscle-cells are at first simple epithelial cells, but
 
 
 
 
FIG. 376. MUSCLE-CELLS OF LIZZIA KOLLIKERI. (From Lankester ; after
O. and R. Hertwig.)
 
A. Muscle-cell from the circular fibres of the subumbrella.
 
B. Myo-epithelial cells from the base of a tentacle.
 
soon become spindle-shaped : part of their protoplasm becomes
differentiated into longitudinally placed striated muscular fibrils,
while part, enclosing the nucleus, remains indifferent, and constitutes the epithelial element of the cells. The muscular
fibrils are either placed at one side of the epithelial part of the
cell, or in other instances (the Lamprey, the Newt, the Sturgeon,
the Rabbit) surround it. The latter arrangement is shewn for
the Sturgeon in fig. 57.
 
The number of the fibrils of each cell gradually increases,
and the protoplasm diminishes, so that eventually only the
nucleus, or nuclei resulting from its division, are left. The
products of each cell probably give rise, in conjunction with a
further division of the nucleus, to a primitive bundle, which,
 
1 O. and R. Hertwig, Die Calomthcorie. Jena, 1881.
 
 
 
THE MUSCULAR SYSTEM.
 
 
 
669
 
 
 
t>r
 
 
 
 
except in Amphioxus, Petromyzon, etc., is surrounded by a
special investment of sarcolemma.
 
The voluntary muscular system. For the purposes of
description the muscular system of the Vertebrata may conveniently be divided into two sections, viz. that of the head and
that of the trunk. The main part, if
not the whole, of the muscular system
of the trunk is derived from certain
structures, known as the muscle-plates,
which take their origin from part of
the primitive mesoblastic somites.
 
It has already been stated (pp.
292 ^296) that the mesoblastic somites
are derived from the dorsal segmented
part of the primitive mesoblastic plates.
Since the history of these bodies is
presented in its simplest form in Elasmobranchii it will be convenient to
commence with this group. Each
somite is composed of two layers a
somatic and a splanchnic both formed
of a single row of columnar cells.
Between these two layers is a cavity,
which is at first directly continuous
with the general body cavity, of which
indeed it merely forms a specialised
part (fig. 377). Before long the cavity
becomes however completely constricted off from the permanent body cavity.
 
Very early (fig. 377) the inner or splanchnic wall of the
somites loses its simple constitution, owing to the middle part of
it undergoing peculiar changes. The meaning of the changes is
at once shewn by longitudinal horizontal sections, which prove
(% 378) that the cells in this situation (mp') have become
extended in a longitudinal direction, and, in fact, form typical
spindle-shaped embryonic muscle-cells, each with a large
nucleus. Every muscle-cell extends for the whole length of a
somite. The inner layer of each somite, immediately within
the muscle-band just described, begins to proliferate, and produce
 
 
 
FIG. 377. TRANSVERSE
SECTION THROUGH THETRUNK
OF AN EMBRYO SLIGHTLY
OLDER THAN FIG. 28 E.
 
nc. neural canal ; pr. posterior root of spinal nerve ; x.
subnotochordal rod ; ao. aorta ;
sc. somatic mesoblast ; sf>.
splanchnic mesoblast ; mp.
muscle-plate ; mp', portion of
muscle-plate converted into
muscle ; Vr. portion of the
vertebral plate which will give
rise to the vertebral bodies ; al.
alimentary tract.
 
 
 
THE MUSCLE-PLATES.
 
 
 
a mass of cells, placed between the muscles and the notochord
( Vr\ These cells form the commencing vertebral bodies, and
have at first (fig. 378) the same segmentation as the somites
from which they sprang.
 
After the separation of the vertebral bodies from the somites
the remaining parts of the somites may be called muscle-plates ;
since they become directly converted into the whole voluntary
muscular system of the trunk (fig. 379, mp}.
 
According to the statements of Bambeke and Go'tte, the Amphibians
present some noticeable peculiarities in the development of their muscular
system, in that such distinct muscle-plates as those of other vertebrate types
are not developed. Each side-plate of mesoblast is divided into a somatic
and a splanchnic layer, continuous throughout the vertebral and parietal
portions of the plate. The vertebral portions (somites) of the plates soon
become separated from the parietal, and form independent masses of cells
constituted of two layers, which were originally continuous with the
somatic and splanchnic layers of the parietal plates (fig. 79). The outer or
somatic layer of the vertebral plates is formed of a single row of cells, but
the inner or splanchnic layer is made up of a kernel of cells on the side of
the somatic layer and an inner layer. The kernel of the splanchnic layer
and the outer or somatic layer together correspond to a muscle- plate of other
Vertebrata, and exhibit a similar segmentation.
 
Osseous Fishes are stated to agree with Amphibians in the development
of their somites and muscular
system 1 , but further observations
on this point are required.
 
In Birds the horizontal splitting of the mesoblast extends at
first to the dorsal summit of the
mesoblastic plates, but after the
isolation of the somites the split
between the somatic and splanchnic layers becomes to a large extent obliterated, though in the anterior somites it appears in part
to persist. The somites on the
second day, as seen in a transverse section (fig. 115, P.?'.), are
somewhat quadrilateral in form
but broader than they are deep.
 
Each at that time consists of
a somewhat thick cortex of radi
 
 
 
FlG. 378. HORIZONTALSECTION THROUGH
THE TRUNK OF AN EMBRYO OF SCYLL1UM
CONSIDERABLY YOUNGER THAN 28 F.
 
 
 
The section is taken at the level of the
notochord, and shews the separation of the
cells to form the vertebral bodies from the
muscle-plates.
 
ch. notochord ; ep. epiblast ; Vr, rudiment
of vertebral body ; mp. muscle- plate ; mp' .
portion of muscle-plate already differentiated
into longitudinal muscles.
 
 
 
1 Ehrlich, " Ueber den peripher. Theil d. Urwirbel." Archiv f. mikr. Anal.,
Vol. XI.
 
 
 
THE MUSCULAR SYSTEM. 671
 
ating rather granular columnar cells, enclosing a small kernel of spherical
cells. They are not, as may be seen in the above figure, completely
separated from the ventral (or lateral as they are at this period) parts of the
mesoblastic plate, and the dorsal and outer layer of the cortex of the
somites is continuous with the somatic layer of mesoblast, the remainder of
the cortex, with the central kernel, being continuous with the splanchnic
layer. Towards the end of the second and beginning of the third day the
upper and outer layer of the cortex, together probably with some of the
central cells of the kernel, becomes separated off as a muscle-plate (fig. 1 16).
The muscle-plate when formed (fig. 117) is found to consist of two layers,
an inner and an outer, which enclose between them an almost obliterated
central cavity ; and no sooner is the muscle-plate formed than the middle
portion of the inner layer becomes converted into longitudinal muscles.
The avian muscle-plates have, in fact, precisely the same constitution as
those of Elasmobranchii. The central space is clearly a remnant of the
vertebral portion of the body cavity, which, though it wholly or partially
disappears in a previous stage, reappears again on the formation of the
muscle-plate.
 
The remainder of the somite, after the formation of the muscle-plate,
is of very considerable bulk ; the cells of the cortex belonging to it lose
their distinctive characters, and the major part of it becomes the vertebral
rudiment.
 
In Mammalia the history appears to be generally the same as in Elasmobranchii. The split which gives rise to the body cavity is continued to
the dorsal summit of the mesoblastic plates, and the dorsal portions of the
plates with their contained cavities become divided into somites, and are
then separated off from the ventral. The later development of the somites
has not been worked out with the requisite care, but it would seem that they
form somewhat cubical bodies in which all trace of the primitive slit is lost.
The further development resembles that in Birds.
 
The first changes of the mesoblastic somites and the formation of the muscle-plates do not, according to existing statements,
take place on quite the same type throughout the Vertebrata,
yet the comparison which has been instituted between Elasmobranchs and other Vertebrates appears to prove that there are
important common features in their development, which may be
regarded as primitive, and as having been inherited from the
ancestors of Vertebrates. These features are (i) the extension
of the body cavity into the vertebral plates, and subsequent
enclosure of this cavity between the two layers of the muscleplates ; (2) the primitive division of the vertebral plate into an
outer (somatic) and an inner (splanchnic) layer, and the formation
of a large part of the voluntary muscular system out of the inner
 
 
 
THE MUSCLE-PLATES.
 
 
 
sp.c
 
 
 
layer, which in all cases is converted into muscles earlier than
the outer layer.
 
The conversion of the muscle-plates into muscles. It
 
will be convenient to commence this subject with a description
of the changes which take place in
such a simple type as that of the
Elasmobranchii.
 
At the time when the muscleplates have become independent
structures they form flat two-layered
oblong bodies enclosing a slit-like
central cavity (fig. 379, mp). The
outer or somatic wall is formed of
simple epithelial -like cells. The
inner or splanchnic wall has however a somewhat complicated structure. It is composed dorsally and
ventrally of a columnar epithelium,
but in its middle portion of the
muscle-cells previously spoken of.
Between these and the central cavity
of the plates the epithelium forming
the remainder of the layer commences to insert itself; so that between the first-formed muscle and
the cavity of the muscle-plate there
appears a thin layer of cells, not
however continuous throughout.
 
When first formed the muscleplates, as viewed from the exterior,
have nearly straight edges ; soon
however they become bent in the middle, so that the edges have
an obtusely angular form, the apex of the angle being directed
forwards. They are so arranged that the anterior edge of the
one plate fits into the posterior edge of the one in front. In the
lines of junction between the plates layers of connective-tissue
cells appear, which form the commencements of the intermuscular
septa.
 
The growth of the plates is very rapid, and their upper ends
 
 
 
 
FIG. 379. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 
28 F.
 
sp.c. spinal canal ; W. white
matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle-plate; mp' . inner layer
of muscle-plate already converted
into muscles ; Vr. rudiment of
vertebral body ; si. segmental
tube ; sd. segmental duct ; sp.v.
spiral valve ; z/. subintestinal vein ;
P.O. primitive generative cells.
 
 
 
THE MUSCULAR SYSTEM. 673
 
soon extend to the summit of the neural canal, and their lower
ones nearly meet in the median ventral line. The original band
of muscles, whose growth at first is very slow, now increases
with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, mp'). It extends upwards and
downwards by the continuous conversion of fresh cells of the
splanchnic layer into muscle-cells. At the same time it grows
rapidly in thickness by the addition of fresh spindle-shaped
muscle-cells from the somatic layer as well as by the division of
the already existing cells.
 
Thus both layers of the muscle-plate are concerned in forming
the great longitudinal lateral muscles, though the splanchnic layer
is converted into muscles very much sooner than the somatic 1 .
 
Each muscle-plate is at first a continuous structure, extending
from the dorsal to the ventral surface, but after a time it becomes
divided by a layer of connective tissue, which becomes developed
nearly on a level with the lateral line, into a dorso-lateral and
a ventro-lateral section. The ends of the muscle-plates
continue for a long time to be formed of undifferentiated
columnar cells. The complicated outlines of the inter-muscular
septa become gradually established during the later stages of
development, causing the well-known appearances of the muscles
in transverse sections, which require no special notice here.
 
The muscles of the limbs. The limb muscles are formed
in Elasmobranchii, coincidently with the cartilaginous skeleton,
as two bands of longitudinal fibres on the dorsal and ventral
surfaces of the limbs (fig. 346). The cells, from which these
muscles originate, are derived from the muscle-plates. When
the ends of the muscle-plates reach the level of the limbs they
bend outwards and enter the tissue of the limbs (fig. 380).
Small portions of several muscle-plates (m.pl) come in this way
to be situated within the limbs, and are very soon segmented
off from the remainder of the muscle-plates. The portions of
the muscle-plates thus introduced soon lose their original dis
1 The brothers Hertwig have recently maintained that only the inner layer of the
muscle-plates is converted into muscles. In the Elasmobranchs it is easy to demonstrate the incorrectness of this view, and in Acipenser (vide fig. 57, mp) the two layers
of the muscle-plate retain their original relations after the cells of both of them have
become converted into muscles.
 
B. in. 43
 
 
 
674
 
 
 
THE MUSCLE-PLATES.
 
 
 
3,-n,
 
 
 
 
FIG. 380. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF THE TRUNK
OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDER THAN FIG. 29 B.
 
The section is diagrammatic in so far that the anterior nerve-roots have been
inserted for the whole length ; whereas they join the spinal cord half-way between
two posterior roots.
 
sp.c. spinal cord; sp.g. ganglion of posterior root; ar. anterior root; dn. dorsally
directed nerve springing from posterior root; nip. muscle-plate; mp'. part of muscleplate already converted into muscles; vi.pl. part of muscle-plate which gives rise to
the muscles of the limbs; /. nervus lateralis; ao. aorta; ch. notochord; sy.g. sympathetic ganglion; ca.v. cardinal vein; sp.n. spinal nerve; sd. segmental (archinephric)
duct; st. segmental tube; du. duodenum; pan. pancreas; hp.d. point of junction of
hepatic duct with duodenum ; umc. umbilical canal.
 
 
 
THE MUSCULAR SYSTEM. 675
 
tinctness. There can however be but little doubt that they
supply the tissue for the muscles of the limbs. The muscleplates themselves, after giving off buds to the limbs, grow
downwards, and soon cease to shew any trace of having given
off these buds.
 
In addition to the longitudinal muscles of the trunk just described,
which are generally characteristic of Fishes, there is found in Amphioxus a
peculiar transverse abdominal muscle, extending from the mouth to the
abdominal pore, the origin of which has not been made out.
 
It has already been shewn that in all the higher Vertebrata
muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with
reference to their muscular system, a fish- like stage. The
middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very
early period, and the outer layer for a long time remains formed
of indifferent cells. That these muscle-plates give rise to the
main muscular system of the trunk, at any rate to the episkeletal
muscles of Huxley, is practically certain, but the details of the
process have not been made out.
 
In the Perennibranchiata the fish-like arrangement of muscles is retained through life in the tail and in the dorso-lateral parts of the trunk.
In the tail of the Amniotic Vertebrata the primitive arrangement is also
more or less retained, and the same holds good for the dorso-lateral trunk
muscles of the Lacertilia. In the other Amniota and the Anura the
dorso-lateral muscles have become divided up into a series of separate
muscles, which are arranged in two main layers. It is probable that the
intercostal muscles belong to the same group as the dorso-lateral muscles.
 
The abdominal muscles of the trunk, even in the lowest Amphibia,
exhibit a division into several layers. The recti abdominis are the least
altered part of this system, and usually retain indications of the primitive
inter-muscular septa, which in many Amphibia and Lacertilia are also
to some extent preserved in the other abdominal muscles.
 
In the Amniotic Vertebrates there is formed underneath the vertebral
column and the transverse processes a system of muscles, forming part
of the hyposkeletal system of Huxley, and called by Gegenbaur the subvertebral muscles. The development of this system has not been worked
out, but on the whole I am inclined to believe that it is derived from
the muscle-plates. Kolliker, Huxley and other embryologists believe
however that these muscles are independent of the muscle-plates in their
origin.
 
432
 
 
 
676 THE HEAD-CAVITIES.
 
 
 
Whether the muscle of the diaphragm is to be placed in the same
category as the hyposkeletal muscles has not been made out.
 
It is probable that the cutaneous muscles of the trunk are derived
from the cells given off from the muscle-plates. Kolliker however believes
that they have an independent origin.
 
The limb-muscles, both extrinsic and intrinsic, as may be concluded
from their development in Elasmobranchii, are derived from the muscleplates. Kleinenberg found in Lacertilia a growth of the muscle-plates
into the limbs, and in Amphibia Gotte finds that the outer layer of the
muscle-plates gives rise to the muscles of the limbs.
 
In the higher Vertebrata on the other hand the entrance of the muscleplates into the limbs has not been made out (Kolliker). It seems therefore
probable that by an embryological modification, of which instances are so
frequent, the cells which give rise to the muscles of the limbs in the higher
Vertebrata can no longer be traced into a direct connection with the muscleplates.
 
TJte Somites and muscular system of the head.
 
The extension of the somites to the anterior end of the body
in Amphioxus clearly proves that somites, similar to those of
the trunk, were originally present in a region, which in the
higher Vertebrata has become differentiated into the head. In
the adult condition no true Vertebrate exhibits indications of
such somites, but in the embryos of several of the lower Vertebrata structures have been found, which are probably equivalent
to the somites of the trunk : they have been frequently alluded
to in the previous chapters of this volume. These structures
have been most fully worked out in Elasmobranchii.
 
The mesoblast in Elasmobranch embryos becomes first split
into somatic and splanchnic layers in the region of the head ;
and between these layers there are formed two cavities, one on
each side, which end in front opposite the blind anterior extremity of the alimentary canal ; and are continuous behind
with the general body-cavity (fig. 20 A, vp}. I propose calling
them the head-cavities. The cavities of the two sides have
no communication with each other.
 
Coincidently with the formation of an outgrowth from the
throat to form the first visceral cleft, the head-cavity on each
side becomes divided into a section in front of the cleft and a
section behind the cleft ; and at a later period it becomes, owing
to the formation of a second cleft, divided into three sections :
 
 
 
THE MUSCULAR SYSTEM.
 
 
 
677
 
 
 
vn~.
 
 
 
 
(i) a section in front of the first or hyomandibular cleft; (2) a
section in the hyoid arch between the hyomandibular cleft and
the hyobranchial or first branchial cleft ; (3) a section behind
the first branchial cleft.
 
The front section of the head-cavity grows forward, and soon
becomes divided, without the intervention of a visceral cleft, into
an anterior and posterior division.
The anterior lies close to the eye,
and in front of the commencing
mouth involution. The posterior
part lies completely within the mandibular arch.
 
As the rudiments of the successive visceral clefts are formed, the
posterior part of the head-cavity becomes divided into successive sections, there being one section for
each arch. Thus the whole headcavity becomes on each side divided
into (i) a premandibular section ; (2)
a mandibular section (vide fig. 29 A,
PP] > (3) a hyoid section ; (4) sections
in each of the branchial arches.
 
The first of these divisions forms
a space of a considerable size, with
epithelial walls of somewhat short
columnar cells (fig. 381, ipp}. It is
situated close to the eye, and presents a rounded or sometimes a
triangular figure in section. The
two halves of the cavity are prolonged ventralwards, and meet below
the base of the fore-brain. The
connection between them appears to last for a considerable time.
These two cavities are the only parts of the body-cavity within
the head which unite ventrally. The section of the head-cavity
just described is so similar to the remaining sections that it
must be considered as serially homologous with them.
 
The next division of the head-cavity, which from its position
 
 
 
FIG. 381. TRANSVERSE SECTION THROUGH THE FRONT PART
OF THE HEAD OF A YOUNG PRISTIURUS EMBRYO.
 
The section, owing to the cranial flexure, cuts both the foreand the hind-brain. It shews the
premandibular and mandibular
head-cavities ipp and ipp, etc.
The section is moreover somewhat
oblique from side to side.
 
fb. fore-brain ; /. lens of eye ;
m. mouth ; pt. upper end of mouth,
forming pituitary involution; lao.
mandibular aortic arch; ipp. and
ipp. first and second head-cavities;
\vc. first visceral cleft; V. fifth
nerve ; aim. auditory nerve ; VII.
seventh nerve ; aa. dorsal aorta ;
acv. anterior cardinal vein ; ch,
notochord.
 
 
 
678 THE HEAD-CAVITIES.
 
may be called the mandibular cavity, presents a spatulate shape,
being dilated dorsally, and produced ventrally into a long thin
process parallel to the hyomandibular gill-cleft (fig. 20, pp}.
Like the previous space it is lined by a short columnar epithelium.
 
The mandibular aortic arch is situated close to its inner side
(fig. 381, 2pp). After becoming separated from the lower part
(Marshall), the upper part of the cavity atrophies about the time
of the appearance of the external gills. Its lower part also
becomes much narrowed, but its walls of columnar cells persist.
The outer or somatic wall becomes very thin indeed, the
splanchnic wall, on the other hand, thickens and forms a layer
of several rows of elongated cells. In each of the remaining
arches there is a segment of the original body-cavity fundamentally similar to that in the mandibular arch (fig. 382). A dorsal
dilated portion appears, however, to be present in the third or
hyoid section alone (fig. 20), and even
there disappears very soon, after being
segmented off from the lower part
(Marshall). The cavities in the posterior parts of the head become much
reduced like those in its anterior part,
though at rather a later period. FlG . 382 . HORIZONTAL
 
It has been shewn that the divi- SECTION THROUGH THE PENULTIMATE VISCERAL ARCH OF
 
sions of the body-cavity in the head, AN EMBRYO OF PRISTIURUS.
with the exception of the anterior, e p. epiblast; vc. pouch of
early become atrophied, not so how- hypoblast which will form the
 
walls of a visceral cleit ; //.
CVer their walls. The cells forming segment of body-cavity in vis
the walls both of the dorsal and ven- ceral arch ; aa ' aortic arch '
tral sections of these cavities become elongated, and finally
become converted into muscles. Their exact history has not
been followed in its details, but they almost unquestionably
become the musculus contrictor superficialis and musculus interbranchialis 1 ; and probably also musculus levator mandibuli and
other muscles of the front part of the head.
 
The anterior cavity close to the eye remains unaltered much
longer than the remaining cavities.
 
1 Vide Vetter, " Die Kiemen und Kiefermusculatur d. Fische." Jenaische Zcltschrift, Vol. vn.
 
 
 
 
THE MUSCULAR SYSTEM.
 
 
 
679
 
 
 
Its further history is very interesting. In my original account
of this cavity (No. 292, p. 208) I stated my belief that its walls
gave rise to the eye-muscles, and the history of this process has
been to some extent worked out by Marshall in his important
memoir (No. 509).
 
Marshall finds that the ventral portion of this cavity, where
its two halves meet, becomes separated from the remainder.
The eventual fate of this part has not however been followed.
Each dorsal section acquires a cup-like form, investing the
posterior and inner surface of the eye. The cells of its outer
wall subsequently give rise to three sets of muscles. The middle
of these, partly also derived from the inner walls of the cup,
becomes the rectus internus of the eye, the dorsal set forms the
rectus superior, and the ventral the rectus inferior. The obliquus
inferior appears also to be in part developed from the walls of
this cavity.
 
Marshall brings evidence to shew that the rectus externus (as
might be anticipated from its nerve supply) has no connection
with the walls of the premandibular head-cavity, and finds that
it arises close to the position originally occupied by the second
and third cavities. Marshall has not satisfactorily made out the
mode of development of the obliquus superior.
 
The walls of the cavities, whose history has just been recorded, have definite relations with the cranial nerves, an account
of which has already been given at p. 461.
 
Head-cavities, in the main similar to those of Elasmobranchii, have been found in the embryo of Petromyzon (fig. 45,
/ic\ the Newt (Osborn and Scott), and various Reptilia (Parker).
 
BIBLIOGRAPHY.
 
(507) G.M.Humphry. " Muscles in Vertebrate Animals." Journ. of Anat.
and Phys., Vol. vi. 1872.
 
(508) J. Miiller. " Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie
u. Myologie." Akad. Wiss., Berlin, 1834.
 
(509) A. M. Marshall. "On the head cavities and associated nerves of
Elasmobranchs." Quart. J. of Micr. Science, Vol. xxi. 1881.
 
(510) A. Schneider. " Anat. u. Entwick. d. Muskelsystems d. Wirbelthierc."
Silz. d. Oberhessischen Gesellschaft, 1873.
 
(511) A. Schneider. Beitrdge z. vergleich. Anat. . Entwick. d. Wirbelthiere.
Berlin, 1879.
 
Vide 2^0 Gotte (No. 296), Kolliker (N o. 298), Balfour (No. 292), Huxley, etc.
 
 
 
CHAPTER XXIII.
 
 
 
EXCRETORY ORGANS.
 
 
 
EXCRETORY organs consist of coiled or branched and often
ciliated tubes, with an excretory pore opening on the outer surface
of the body, and as a rule an internal ciliated orifice placed in the
body-cavity. In forms provided with a true vascular system,
there is a special development of capillaries around the glandular
part of the excretory organs. In many instances the glandular
cells of the organs are filled with concretions of uric acid or some
similar product of nitrogenous waste.
 
There is a very great morphological and physiological similarity between almost all the forms of excretory organ found in
the animal kingdom, but although there is not a little to be said
for holding all these organs to be derived from some common
prototype, the attempt to establish definite homologies between
them is beset with very great difficulties.
 
Platyelminthes. Throughout the whole of the Platyelminthes these organs are constructed on a well-defined type, and
in the Rotifera excretory organs of a similar form to those of the
Platyelminthes are also present.
 
These organs (Fraipont, No. 513) are more or less distinctly
paired, and consist of a system of wide canals, often united into a
network, which open on the one hand into a pair of large tubes
leading to the exterior, and on the other into fine canals which
terminate by ciliated openings, either in spaces between the
connective-tissue cells (Platyelminthes), or in the body-cavity
(Rotifera). The fine canals open directly into the larger ones,
without first uniting into canals of an intermediate size.
 
 
 
EXCRETORY ORGANS.
 
 
 
68 1
 
 
 
The two large tubes open to the exterior, either by means of
a median posteriorly placed contractile vesicle, or by a pair of
vesicles, which have a ventral and anterior position. The former
type is characteristic of the majority of the Trematoda, Cestoda.
and Rotifera, and the latter of the Nemertea and some Trematoda.
In the Turbellaria the position of the external openings of the
system is variable, and in a few Cestoda (Wagner) there are
lateral openings on each of the successive proglottides, in addition
to the terminal openings. The mode of development of these
organs is unfortunately not known.
 
Mollusca. In the Mollusca there are usually present two
independent pairs of excretory organs one found in a certain
number of forms during early larval life only 1 , and the other
always present in the adult.
 
The larval excretory organ has been found in the pulmonate
Gasteropoda (Gegenbaur, Fol 2 , Rabl), in Teredo (Hatschek), and
possibly also in Paludina. It is placed in the anterior region of
the body, and opens ventrally on each side, a short way behind
the velum. It is purely a larval organ, disappearing before the
close of the veliger stage. In the aquatic Pulmonata, where it is
best developed, it consists on each side of a V-shaped tube, with
a dorsally-placed apex, containing an enlargement of the lumen.
There is a ciliated cephalic limb, lined by cells with concretions,
and terminating by an internal opening near the eye, and a nonciliated pedal limb opening to the exterior 3 .
 
Two irreconcilable views are held as to the development of
this system. Rabl (Vol. II. No. 268) and Hatschek hold that it
is developed in the mesoblast ; and Rabl states that in Planorbis
it is formed from the anterior mesoblast cells of the mesoblastic
bands. A special mesoblast cell on each side elongates into two
processes, the commencing limbs of the future organ. A lumen
is developed in this cell, which is continued into each limb, while
 
1 I leave out of consideration an external renal organ found in many marine
Gasteropod larvte, vide Vol. II. p. 280.
 
2 H. Fol, "Etudes sur le devel. d. Mollusques. " Mem. Hi. Archiv d. Zool.
exfJr. et gener., Vol. VIII.
 
3 The careful observations of Fol seem to me nearly conclusive in favour of this
limb having an external opening, and the statement to the reverse effect on p. 280 of
Vol. ii. of this treatise, made on the authority of Rabl and Biitschli, must probably be
corrected.
 
 
 
682 POLYZOA.
 
the continuations of the two limbs are formed by perforated
mesoblast cells.
 
According to Fol these organs originate in aquatic Pulmonata
as a pair of invaginations of the epiblast, slightly behind the
mouth. Each invagination grows in a dorsal direction, and after
a time suddenly bends on itself, and grows ventralwards and
forwards. It thus acquires its V-shaped form.
 
In the terrestrial Pulmonata the provisional excretory organs
are, according to Fol, formed as epiblastic invaginations, in the
same way as those in the aquatic Pulmonata, but have the form
of simple non-ciliated sacks, without internal openings.
 
The permanent renal organ of the Mollusca consists typically
of a pair of tubes, although in the majority of the Gasteropoda
one of the two tubes is not developed. It is placed considerably
behind the provisional renal organ.
 
Each tube, in its most typical form, opens by a ciliated funnel
into the pericardial cavity, and has its external opening at the
side of the foot. The pericardial funnel leads into a glandular
section of the organ, the lining cells of which are filled with
concretions. This section is followed by a ciliated section, from
which a narrow duct leads to the exterior.
 
As to the development of this organ the same divergence of
opinion exists as in the case of the provisional renal organ.
 
Rabl's careful observations on Planorbis (Vol. II. No. 268) tend
to shew that it is developed from a mass of mesoblast cells, near
the end of the intestine. The mass becomes hollow, and,
attaching itself to the epiblast on the left side of the anus,
acquires an opening to the exterior. Its internal opening is not
established till after the formation of the heart. Fol gives an
equally precise account, but states that the first rudiment of the
organ arises as a solid mass of epiblast cells. Lankester finds
that this organ is developed as a paired invagination of the.
epiblast in Pisidium, and Bobretzky also derives it from the
epiblast in marine Prosobranchiata. In Cephalopoda on the
other hand Bobretzky's observations (I conclude this from his
figures) indicate that the excretory sacks of the renal organs are
derived from the mesoblast.
 
Polyzoa. Simple excretory organs, consisting of a pair of
ciliated canals, opening between the mouth and the anus, have
 
 
 
EXCRETORY ORGAN>.
 
 
 
68 3
 
 
 
been found by Hatschek and Joliet in the Entoproctous Polyzoa,
and are developed, according to Hatschek, by whom they were
first found in the larva, from the mesoblast
 
Brachiopoda. One or rarely two (Rhynchonella) pairs of
canals, with both peritoneal and external openings, are found in
the Brachiopoda. They undoubtedly serve as genital ducts, but
from their structure are clearly of the same nature as the
excretory organs of the Chaetopoda described below. Their
development has not been worked out.
 
Chaetopoda. Two forms of excretory organ have been met
with in the Chaetopoda. The one form is universally or nearly
universally present in the adult, and typically consists of a pair
of coiled tubes repeated in every segment. Each tube has an
internal opening, placed as a rule in the segment in front of that
in which the greater part of the organ and the external opening
are situated.
 
There are great variations in the structure of these organs,
which cannot be dealt with here. It may be noted however that
the internal opening may be absent, and that there may be
several internal openings for each organ (Polynoe). In the
Capitellidae moreover several pairs of excretory tubes have been
shewn by Eisig (No. 512) to be present in each of the posterior
segments.
 
The second form of excretory organ has as yet only been
found in the larva of Polygordius, and will be more conveniently
dealt with in connection with the development of the excretory
system of this form.
 
There is still considerable doubt as to the mode of formation
of the excretory tubes of the Chaetopoda. Kowalevsky (No. 277),
from his observations on the Oligochasta, holds that they develop
as outgrowths of the epithelial layer covering the posterior side
of the dissepiments, and secondarily become connected with the
epidermis.
 
Hatschek finds that in Criodrilus they arise from a continuous
linear thickening of the somatic mesoblast, immediately beneath
the epidermis, and dorsal to the ventral band of longitudinal
muscles. They break up into S-shaped cords, the anterior end
of each of which is situated in front of a dissepiment, and is
formed at first of a single large cell, while the posterior part is
 
 
 
684 CHvETOPODA.
 
 
 
continued into the segment behind. The cords are covered by
a peritoneal lining, which still envelopes them, when in the
succeeding stage they are carried into the body-cavity. They
subsequently become hollow, and their hinder ends acquire
openings to the exterior. The formation of their internal
openings has not been followed.
 
Kleinenberg is inclined to believe that the excretory tubes
take their origin from the epiblast, but states that he has not
satisfactorily worked out their development.
 
The observations of Risig (No. 512) on the Capitellidae
support Kowalevsky's view that the excretory tubes originate
from the lining of the peritoneal cavity.
 
Hatschek (No. 514) has given a very interesting account of
the development of the excretory system in Polygordius.
 
The excretory system begins to be formed, while the larva is
still in the trochospere stage (fig. 383, npli), and consists of a
provisional excretory organ, which is placed in front of the future
segmented part of the body, and occupies a position very
similar to that of the provisional excretory organ found in some Molluscan
larvae (vide p. 68 1).
 
Hatschek, with some shew of reason, holds that the provisional excretory organs of Polygordius are homologous with those of the Mollusca.
 
In its earliest stage the provisional
excretory organ of Polygordius consists of a pair of simple ciliated tubes, FIG. 383. POLYOORDIUS
 
, . , r 11-1 LARVA. (After Hatschek.)
 
each with an anterior funnel-like open- m _ moulh . ^ supraKBSO .
 
ing situated in the midst of the meSO- phageal ganglion ; nph. nephri11 11 . , dion ; ine.p. mesoblastic band;
 
blast cells, and a posterior external an _ anus 5 oL stomach .
opening. The latter is placed immediately in front of what afterwards becomes the segmented region
of the embryo. While the larva is still unsegmented, a second
internal opening is formed for each tube (fig. 383, np/i) and the
two openings so formed may eventually become divided into
five (fig. 384 A), all communicating by a single pore with the
exterior.
 
When the posterior region of the embryo becomes segmented,
 
 
 
 
EXCRETORY ORGANS.
 
 
 
685
 
 
 
paired excretory organs are formed in each of the posterior
segments, but the account of their development, as given by
Hatschek, is so remarkable that I do not think it can be
definitely accepted without further confirmation.
 
From the point of junction of the two main branches of the
larval kidney there grows backwards (fig. 384 B), to the hind
end of the first segment, a very delicate tube, only indicated by
its ciliated lumen, its walls not being differentiated. Near the
front end of this tube a funnel, leading into the larval body
cavity of the head, is formed, and subsequently the posterior end
of the tube acquires an external opening, and the tube distinct
walls. The communication with the provisional excretory organ
is then lost, and thus the excretory tube of the first segment is
established.
 
The excretory tubes in the second and succeeding segments
are formed in the same way as in the first, i.e. by the continuation of the lumen of the hind end of the excretory tube from
the preceding segment, and the subsequent separation of this
part as a separate tube.
 
The tube may be continued with a sinuous course through
 
 
 
 
 
A
A
 
A
+
 
A.
 
 
 
Y
 
Y
Y
Y
Y
 
 
 
J)
 
 
 
FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY
SYSTEM OF POLYGORDIUS. (After Hatschek.)
 
several segments without a distinct wall. The external and
internal openings of the permanent excretory tubes are thus
secondarily acquired. The internal openings communicate with
the permanent body-cavity. The development of the perma
 
 
686 GEPHYREA.
 
 
 
nent excretory tubes is diagrammatically represented in fig.
384 C and D.
 
The provisional excretory organ atrophies during larval life.
 
If Hatschek's account of the development of the excretory system of
Polygordius is correct, it is clear that important secondary modifications
must have taken place in it, because his description implies that there sprouts
from the anterior excretory organ, while it has its own external opening, a
posterior duct, which does not communicate either with the exterior or with
the body-cavity! Such a duct could have no function. It is intelligible
either (i) that the anterior excretory organ should lead into a longitudinal
duct, opening posteriorly ; that then a series of secondary openings into the
body-cavity should attach themselves to this, that for each internal opening
an external should subsequently arise, and the whole break up into separate
tubes ; or (2) that behind an anterior provisional excretory organ a series of
secondary independent segmental tubes should be formed. But from Hatschek's account neither of these modes of evolution can be deduced.
 
Gephyrea. The Gephyrea may have three forms of excretory organs, two of which are found in the adult, and one,
similar in position and sometimes also in structure, to the
provisional excretory organ of Polygordius, has so far only been
found in the larvae of Echiurus and Bonellia.
 
In all the Gephyrea the so-called 'brown tubes' are
apparently homologous with the segmented excretory tubes of
Chaetopods. Their main function appears to be the transportation of the generative products to the exterior. There is but a
single highly modified tube in Bonellia, forming the oviduct and
uterus ; a pair of tubes in the Gephyrea inermia, and two or
three pairs in most Gephyrea armata, except Bonellia. Their
development has not been studied.
 
In the Gephyrea armata there is always present a pair of
posteriorly placed excretory organs, opening in the adult into
the anal extremity of the alimentary tract, and provided with
numerous ciliated peritoneal funnels. These organs were stated
by Spengel to arise in Bonellia as outgrowths of the gut ; but in
Echinrus Hatschek (No. 515) finds that they are developed from
the somatic mesoblast of the terminal part of the trunk. They
soon become hollow, and after attaching themselves to the
epiblast on each side of the anus, acquire external openings.
They are not at first provided with peritoneal funnels, but these
parts of the organs become developed from a ring of cells at
 
 
 
EXCRETORY ORGANS.
 
 
 
687
 
 
 
their inner extremities ; and there is at first but a single funnel
for each vesicle. The mode of increase of the funnels has not
been observed, nor has it been made out how the organs themselves become attached to the hind-gut.
 
The provisional excretory organ of Echiurus is developed at
an early larval stage, and is functional during the whole of
larval life. It at first forms a ciliated tube on each side, placed
in front of that part of the larva which becomes the trunk of the
adult. It opens to the exterior by a fine pore on the ventral
side, immediately in front of one of the mesoblastic bands, and
appears to be formed of perforated cells. It terminates internally in a slight swelling, which represents the normal internal
ciliated funnel. The primitively simple excretory organ becomes
eventually highly complex by the formation of numerous
branches, each ending in a slightly swollen extremity. These
branches, in the later larval stages, actually form a network, and
the inner end of each main branch divides into a bunch of fine
tubes. The whole organ resembles in many respects the excretory organ of the Platyelminthes.
 
In the larva of Bonellia Spengel has described a pair of
provisional excretory tubes, opening near the anterior end of
the body, which are probably homologous with the provisional
excretory organs of Echiurus (vide Vol. II., fig. 162 C, se).
 
Discophora. As in many of the types already spoken of,
permanent and provisional excretory organs may be present in
the Discophora. The former are usually segmentally arranged,
and resemble in many respects the excretory tubes of the
Chaetopoda. They may either be provided with a peritoneal
funnel (Nephelis, Clepsine) or have no internal opening
(Hirudo).
 
Bourne 1 has shewn that the cells surrounding the main duct
in the medicinal Leech are perforated by a very remarkable
network of ductules, and the structure of these organs in the
Leech is so peculiar that it is permissible to state with due reserve
their homology with the excretory organs of the Chaetopoda.
 
The excretory tubes of Clepsine are held by Whitman to be
developed in the mesoblast.
 
1 "On the Structure of the Nephridia of the Medicinal Leech." Quart. J. of
Micr. Science, Vol. XX. 1880.
 
 
 
688 ARTHROPODA.
 
 
 
There are found in the embryos of Nephelis and Hirudo
certain remarkable provisional excretory organs the origin and
history of which are not yet fully made out. In Nephelis they
appear as one (according to Robin), or (according to Biitschli)
as two successive pairs of convoluted tubes on the dorsal side of
the embryo, which are stated by the latter author to develop
from the scattered mesoblast cells underneath the skin. At
their fullest development they extend, according to Robin, from
close to the head to near the ventral sucker. Each of them is
U-shaped, with the open end of the U forwards, each limb of the
U being formed by two tubes united in front. No external
opening has been clearly made out. Fiirbringer is inclined from
his own researches to believe that they open laterally. They
contain a clear fluid.
 
In Hirudo, Leuckart has described three similar pairs of
organs, the structure of which he has fully elucidated. They
are situated in the posterior part of the body, and each of them
commences with an enlargement, from which a convoluted tube
is continued for some distance backwards; the tube then turns
forwards again, and after bending again upon itself opens to the
exterior. The anterior part is broken up into a kind of
labyrinthic network.
 
The provisional excretory organs of the Leeches cannot be
identified with the anterior provisional organs of Polygordius
and Echiurus.
 
Arthropoda. Amongst the Arthropoda Peripatus is the
only form with excretory organs of the type of the segmental
excretory organs of the Chsetopoda 1 .
 
These organs are placed at the bases of the feet, in the
lateral divisions of the body-cavity, shut off from the main
median division of the body-cavity by longitudinal septa of
transverse muscles.
 
Each fully developed organ consists of three parts :
 
(i) A dilated vesicle opening externally at the base of a
foot. (2) A coiled glandular tube connected with this, and
subdivided again into several minor divisions. (3) A short
terminal portion opening at one extremity into the coiled tube
 
1 Vide F. M. Balfour, " On some points in the Anatomy of Peripatus Capensis."
Quart. J, of Micr. Science, Vol. XIX. 1879.
 
 
 
EXCRETORY ORGANS. 689
 
 
 
and at the other, as I believe, into the body cavity. This
section becomes very conspicuous, in stained preparations, by
the intensity with which the nuclei of its walls absorb the
colouring matter.
 
In the majority of the Tracheata the excretory organs have
the form of the so-called Malpighian tubes, which always (vide
Vol. II.) originate as a pair of outgrowths of the epiblastic
proctodaeum. From their mode of development they admit of
comparison with the anal vesicles of the Gephyrea, though in
the present state of our knowledge this comparison must be
regarded as somewhat hypothetical.
 
The antennary and shell-glands of the Crustacea, and
possibly also the so-called dorsal organ of various Crustacean
larvae appear to be excretory, and the two former have been
regarded by Claus and Grobben as belonging to the same
system as the segmental excretory tubes of the Chaetopoda.
 
Nematoda. Paired excretory tubes, running for the whole
length of the body in the so-called lateral line, and opening in
front by a common ventral pore, are present in the Nematoda.
They do not appear to communicate with the body cavity, and
their development has not been studied.
 
Very little is known with reference either to the structure or
development of excretory organs in the Echinodermata and the
other Invertebrate types of which no mention has been so far
made in this Chapter.
 
Excretory organs and generative ducts of the Craniata.
 
Although it would be convenient to separate, if possible, the
history of the excretory organs from that of the generative
ducts, yet these parts are so closely related in the Vertebrata, in
some cases the same duct having at once a generative and a
urinary function, that it is not possible to do so.
 
The excretory organs of the Vertebrata consist of three
distinct glandular bodies and of their ducts. These are (i) a
small glandular body, usually with one or more ciliated funnels
opening into the body cavity, near the opening of which there
projects into the body cavity a vascular glomerulus. It is
situated very far forwards, and is usually known as the head
44
 
 
 
690 ELASMOBRANCHII.
 
 
 
kidney, though it may perhaps be more suitably called, adopting
Lankester's nomenclature, the pronepliros. Its duct, which forms
the basis for the generative and urinary ducts, will be called the
segmented duct.
 
(2) The Wolffian body, which may be also called the
mesonepJiros. It consists of a series of, at first, segmentally
(with a few exceptions) arranged glandular canals (segmental
tubes) primitively opening at one extremity by funnel-shaped
apertures into the body cavity, and at the other into the
segmental duct. This duct becomes in many forms divided
longitudinally into two parts, one of which then remains
attached to the segmental tubes and forms the Wolffian or
mesonepJiric duct, while the other is known as the Milllerian
dnct.
 
(3) The kidney proper or metanephros. This organ is only
found in a completely differentiated form in the amniotic Vertebrata. Its duct is an outgrowth from the Wolrfian duct.
 
The above parts do not coexist in full activity in any living
adult member of the Vertebrata, though all of them are found
together in certain embryos. They are so intimately connected
that they cannot be satisfactorily dealt with separately.
 
Elasmobranchii. The excretory system of the Elasmobranchii is by no means the most primitive known, but at the
same time it forms a convenient starting point for studying the
modifications of the system in other groups. The most remarkable peculiarity it presents is the absence of a pronephros.
The development of the Elasmobranch excretory system has
been mainly studied by Semper and myself.
 
The first trace of the system makes its appearance as a knob
of mesoblast, springing from the intermediate cell-mass near the
level of the hind end of the heart (fig. 385 K,pd). This knob is
the rudiment of the abdominal opening of the segmental duct,
and from it there grows backwards to the level of the anus a
solid column of cells, which constitutes the rudiment of the
segmental duct itself (fig. 385 B, pd). The knob projects
towards the epiblast, and the column connected with it lies
between the mesoblast and epiblast. The knob and column do
not long remain solid, but the former acquires an opening into
the body cavity (fig. 421, sd) continuous with a lumen, which
 
 
 
EXCRETORY ORGANS.
 
 
 
691
 
 
 
makes its appearance in the column (fig. 386, sd). The knob
forms the only structure which can be regarded as a rudiment of
the pronephros.
 
 
 
spn
 
 
 
spn
 
 
 
 
FlG. 385. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAL
 
CLEFTS.
 
The sections illustrate the development of the segmental duct (pd) or primitive
duct of the pronephros. In A (the anterior of the two sections) this appears as a
solid knob (pd) projecting towards the epiblast. In B is seen a section of the column
which has grown backwards from the knob in A.
 
spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord; X. subnotochordal rod; mp. muscle-plate; mp' . specially developed portion of muscle-plate;
ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchnopleure ; //.
body cavity; ep. epiblast; al. alimentary canal.
 
While the lumen is gradually being formed, the segmental
tubes of the mesonephros become established. They appear to
arise as differentiations of the parts of the primitive lateral plates
of mesoblast, placed between the dorsal end of the body cavity
and the muscle-plate (fig. 386, st) 1 , which are usually known as
the intermediate cell-masses.
 
The lumen of the segmental tubes, though at first very small,
soon becomes of a considerable size. It appears to be established
in the position of the section of the body cavity in the intermediate cell-mass, which at first unites the part of the body
cavity in the muscle-plates with the permanent body cavity.
The lumen of each tube opens at its lower end into the dorsal
part of the body cavity (fig. 386, st}, and each tube curls obliquely
 
1 In my original account of the development I held these tubes to be invaginations
of the peritoneal epithelium. Sedgwick (No. 549) was led to doubt the accuracy of
my original statement from his investigations on the chick ; and from a re-examination of my specimens he arrived at the results stated above, and which I am now
myself inclined to adopt.
 
442
 
 
 
692
 
 
 
ELASMOBRANCHII.
 
 
 
sp.c
 
 
 
 
backwards round the inner and dorsal side of the segmental
duct, near which it at first ends blindly.
 
One segmental tube makes its
appearance for each somite (fig. 265),
commencing with that immediately
behind the abdominal opening of the
segmental duct, the last tube being
situated a few segments behind the
anus. Soon after their formation
the blind ends of the segmental tubes
come in contact with, and open into
the segmental duct, and each of them
becomes divided into four parts.
These are (i) a section carrying the
peritoneal opening, known as the
peritoneal funnel, (2) a dilated vesicle
into which this opens, (3) a coiled
tubulus proceeding from (2), and
terminating in (4) a wider portion
opening into the segmental duct. At
the same time, or shortly before this,
each segmental duct unites with and
opens into one of the horns of the
cloaca, and also retires from its
primitive position between the epiblast and mesoblast, and assumes a
position close to the epithelium lining
the body cavity (fig. 380, sd}. The
general features of the excretory
organs at this period are diagrammatically represented in the
woodcut (fig. 387). In this fig. pd is the segmental duct and
o its abdominal opening; s.t points to the segmental tubes,
the finer details of whose structure are not represented in the
diagram. The mesonephros thus forms at this period an elongated gland composed of a series of isolated coiled tubes, one
extremity of each of which opens into the body cavity, and the
other into the segmental duct, which forms the only duct of the
system, and communicates at its front end with the body cavity,
and behind with the cloaca.
 
 
 
FIG. 386. SECTION THROUGH
THE TRUNK OF A SCYLLIUM EMBRYO SLIGHTLY YOUNGER THAN
 
28 F.
 
sp.c. spinal canal; W. white
matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
nip, muscle-plate ; nip', inner layer
of muscle-plate already converted
into muscles ; Vr, rudiment of
vertebral body ; st. segmental
tube; sd. segmental duct; sp.v.
spiral valve ; v. subintestinal vein ;
p.o. primitive generative cells.
 
 
 
EXCRETORY ORGANS. 693
 
 
 
The next important change concerns the segmental duct,
which becomes longitudinally split into two complete ducts in
the female, and one complete duct and parts of a second duct in
the male. The manner in which this takes place is diagrammatically represented in fig. 387 by the clear line x, and in
transverse section in figs. 388 and 389. The resulting ducts are
(i) the Wolffian duct or mesonephric duct (wd\ dorsally, which
remains continuous with the excretory tubules of the mesonephros, and ventrally (2) the oviduct or Miillerian duct in the
female, and the rudiments of this duct in the male. In the
 
 
 
 
 
FIG. 387. DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN
 
ELASMOBRANCH EMBRYO.
 
pd. segmental duct. It opens at o into the body cavity and at its other extremity
into the cloaca; x. line along which the division appears which separates the segmental
duct into the Wolffian duct above and the Miillerian duct below; s.t. segmental
tubes. They open at one end into the body cavity, and at the other into the segmental duct.
 
female the formation of these ducts takes place (fig. 389) by a
nearly solid rod of cells being gradually split off from the
ventral side of all but the foremost part of the original segmental
duct. This nearly solid cord is the Miillerian duct (pd}. A
very small portion of the lumen of the original segmental duct
is perhaps continued into it, but in any case it very soon acquires
a wide lumen (fig. 389 A). The anterior part of the segmental
duct is not divided, but remains continuous with the Mullerian
duct, of which its anterior pore forms the permanent peritoneal
opening 1 (fig. 387). The remainder of the segmental duct (after
the loss of its anterior section, and the part split off from its
ventral side) forms the Wolffian duct. The process of formation
of these ducts in the male differs from that in the female chiefly
 
1 Five or six segmental tubes belong to the region of the undivided anterior part
of the segmental duct, which forms the front end of the Mullerian duct ; but they appear to atrophy very early, without acquiring a definite attachment to the segmental
duct.
 
 
 
694
 
 
 
ELASMOBRANCHIL
 
 
 
in the fact of the anterior undivided part of the segmental duct,
which forms the front end of the Miillerian duct, being shorter,
 
 
 
 
trd/
 
 
 
 
FIG. 389. FOUR SECTIONS
THROUGH THE ANTERIOR
I'ART OF THE SEGMENTAL
DUCT OF A FEMALE EMBRYO
OF SCYLLIUM CANICULA.
 
The figure shews how the
segmental duct becomes split
into the Wolffian or mesonephric duct above, and Miillerian duct or oviduct below.
 
wd. Wolffian or mesonephric duct; od. Miillerian
duct or oviduct ; sd. segmental duct.
 
 
 
FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A
 
SCYLLIUM EMBRYO ILLUSTRATING THE
FORMATION OF THE WOLFFIAN AND MlJLLERIAN DUCTS BY THE LONGITUDINAL
SPLITTING OF THE SEGMENTAL DUCT.
 
me. medullary canal; mp. muscle-plate;
ch. notochord; ao. aorta; cav. cardinal
vein; st. segmental tube. On the left side
the section passes through the opening of
a segmental tube into the body cavity. On
the right this opening is represented by
dotted lines, and the opening of the segmental tube into the Wolffian duct has
been cut through; iv.d. Wolffian duct;
m.d. Miillerian duct. The section is taken
through the point where the segmental
duct and Wolffian duct have just become
separate; gr. the germinal ridge with the
thickened germinal epithelium ; /. liver ;
i. intestine with spiral valve.
 
and in the column of cells with which it is continuous being
from the first incomplete.
 
The segmental tubes of the mesonephros undergo further
important changes. The vesicle at the termination of each peritoneal funnel sends a bud forwards towards the preceding
tubulus, which joins the fourth section of it close to the opening
 
 
 
EXCRETORY ORGANS.
 
 
 
695
 
 
 
 
into the Wolffian duct (fig. 390, px). The remainder of the
vesicle becomes converted
into a Malpighian body (mg}.
 
By the first of these changes 10^-4 M @W>f
a tube is established connecting each pair of segments
of the mesonephros, and
though this tube is in part
aborted (or only represented
by a fibrous band) in the
anterior part of the excretory
organs in the adult, and most
probably in the hinder part,
yet it seems almost certain
that the secondary and tertiary Malpighian bodies of
the majority of segments are
developed from its persisting
blind end. Each of these
 
 
 
FIG. 390. LONGITUDINAL VERTICAL
SECTION THROUGH PART OF THE MESONEPHROS OF AN EMBRYO OF SCYLLIUM.
 
The figure contains two examples of the
budding of the vesicle of a segmental tube
(which forms a Malpighian body in its own
segment) to unite with the tubulus in the
preceding segment close to its opening into
the Wolffian (mesonephric) duct.
 
ge. epithelium of body-cavity; st. peritoneal funnel of segmental tube with its
peritoneal opening; mg. Malpighian body;
px. bud from Malphigian body uniting with
preceding segment.
 
 
 
secondary and tertiary Malpighian bodies is connected with a
convoluted tubulus (fig. 391, a.mg), which is also developed from
the tube connecting each pair of segmental tubes, and therefore
falls into the primary tubulus close to its junction with the
 
 
 
st.c
 
 
 
 
w.d
 
 
 
FIG. 391. THREE SEGMENTS OF THE ANTERIOR PART OF THE MESONEPHROS OF A
NEARLY RIPE EMBRYO OF SCYLLIUM CANICULA AS A TRANSPARENT OBJECT.
The figure shews a fibrous band passing from the primary to the secondary Malpighian bodies in two segments, which is the remains of the outgrowth from the
primary Malpighian body.
 
sf.o. peritoneal funnel; p. ing. primary Malpighian body; a.mg. accessory Malpighian body; w.d. mesonephric (Wolffian) duct.
 
 
 
696 ELASMOBRANCI1II.
 
 
 
segmental duct. Owing to the formation of the accessory tubuli
the segments of the mesonephros acquire a compound character.
 
The third section of each tubulus becomes by continuous
growth, especially in the hinder segments, very bulky and
convoluted.
 
The general character of a slightly developed segment of
the mesonephros at its full growth may be gathered from fig.
391. It commences with (i) a peritoneal opening, somewhat
oval in form (st.d) and leading directly into (2) a narrow tube,
the segmental tube, which takes a more or less oblique course
backwards, and, passing superficially to the Wolffian duct (w.d},
opens into (3) a Malpighian body (p.mg) at the anterior extremity of an isolated coil of glandular tubuli. This coil forms
the third section of each segment, and starts from the Malpighian body. It consists of a considerable number of rather
definite convolutions, and after uniting with tubuli from one,
two, or more (according to the size of the segment) accessory
Malpighian bodies (a.mg) smaller than the one into which the
segmental tube falls, eventually opens by (4) a narrowish
collecting tube into the Wolffian duct at the posterior end of
the segment. Each segment is probably completely isolated
from the adjoining segments, and never has more than one
peritoneal funnel and one communication with the Wolffian duct.
 
Up to this time there has been no distinction between the
anterior and posterior tubuli of the mesonephros, which alike
open into the Wolffian duct. The collecting tubes of a considerable number of the hindermost tubuli (ten or eleven in
Scyllium canicula), either in some species elongate, overlap,
while at the same time their openings travel backward so that
they eventually open by apertures (not usually so numerous as
the separate tubes), on nearly the same level, into the hindermost section of the Wolffian duct in the female, or into the
urinogenital cloaca, formed by the coalesced terminal parts of
the Wolffian ducts, in the male; or in other species become
modified, by a peculiar process of splitting from the Wolnian
duct, so as to pour their secretion into a single duct on each
side, which opens in a position corresponding with the numerous
ducts of the other species (fig. 392). In both cases the modified
posterior kidney-segments are probably equivalent to the per
 
 
EXCRETORY ORGANS. 697
 
 
 
manent kidney or metanephros of the amniotic Vertebrates, and
for this reason the numerous collecting tubes or single collecting
tube, as the case may be, will be spoken of as ureters. The
anterior tubuli of the primitive excretory organ retain their early
relation to the Wolffian duct, and form the permanent Wolffian
body or mesonephros.
 
The originally separate terminal extremities of the Wolffian
ducts always coalesce, and form a urinal cloaca, opening by a
single aperture, situated at the extremity of the median papilla
behind the anus. Some of the peritoneal openings of the segmental tubes in Scyllium, or in other cases all the openings,
become obliterated.
 
In the male the anterior segmental tubes undergo remarkable modifications, and become connected with the testes.
Branches appear to grow from the first three or four or more of
them (though probably not from their peritoneal openings),
which pass to the base of the testis, and there uniting into a
longitudinal canal, form a network, and receive the secretion of
the testicular ampullae (fig. 393, nf). These ducts, the vasa
efferent ia, carry the semen to the Wolffian body, but before
opening into the tubuli of this body they unite into a canal
known as the longitudinal canal of the Wolffian body (l.c\ from
which pass off ducts equal in number to the vasa efferentia,
each of which normally ends in a Malpighian corpuscle. From
the Malpighian corpuscles so connected there spring the convoluted tubuli, forming the generative segments of the Wolffian
body, along which the semen is conveyed to the Wolffian duct
(v.d). The Wolffian duct itself becomes much contorted and
acts as vas deferens.
 
Figs. 392 and 393 are diagrammatic representations of the
chief constituents of the adult urinogenital organs in the two
sexes. In the adult female (fig. 392), there are present the
following parts :
 
(1) The oviduct or Mullerian duct (m.d) split off from the
segmental duct of the kidneys. Each oviduct opens at its
anterior extremity into the body cavity, and behind the two
oviducts have independent communications with the general
cloaca.
 
(2) The mesonephric ducts (w.d), the other product of the
 
 
 
698
 
 
 
ELASMOBRANCHII.
 
 
 
segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anterior persisting
segment of the mesonephros on each side, and unite behind to
 
 
 
 
FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
 
IN AN ADULT FEMALE ELASMOBRANCH.
 
m.d. Miillerian duct; w.d. Wolffian duct; s.t. segmental tubes; five of them are
represented with openings into the body cavity, the posterior segmental tubes form
the mesonephros ; ov. ovary.
 
open by a common papilla into the cloaca. The mesonephric
duct receives the secretion of the anterior tubuli of the primitive
mesonephros.
 
(3) The ureter which carries off the secretion of the kidney
proper or metanephros. It is represented in my diagram in its
most rare and differentiated condition as a single duct connected
with the posterior segmental tubes.
 
(4) The segmental tubes (.$-./) some of which retain their
 
 
 
-S.t:
 
 
 
 
FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS
 
IN AN ADULT MALE ELASMOBRANCH.
 
m.d. rudiment of Miillerian duct; w.d. Wolffian duct, marked vd in front and
serving as vas deferens; s.t. segmental tubes; two of them are represented with openings into the body cavity; d. ureter; /. testis; nt. canal at the base of the testis;
VE, vasa efferentia; Ic. longitudinal canal of the Wolffian body.
 
 
 
EXCRETORY ORGANS. 699
 
 
 
original openings into the body cavity, and others are without
them. They are divided into two groups, an anterior forming
the mesonephros or Wolffian body, which pours its secretion
into the Wolffian duct ; and a posterior group forming a gland
which is probably equivalent to the kidney proper of amniotic
Craniata, and is connected with the ureter.
 
In the male the following parts are present (fig. 393):
 
(1) The Mlillerian duct (m.d], consisting of a small rudiment attached to the liver, representing the foremost end of the
oviduct of the female.
 
(2) The mesonephric duct (w.d] which precisely corresponds
to the mesonephric duct of the female, but, in addition to
serving as the duct of the Wolffian body, also acts as a vas
deferens (vd}. In the adult male its foremost part has a very
tortuous course.
 
(3) The ureter (d\ which has the same fundamental constitution as in the female.
 
(4) The segmental tubes (s.t). The posterior tubes have
the same arrangement in both sexes, but in the male modifications take place in connection with the anterior tubes to fit them
to act as transporters of the semen.
 
Connected with the anterior tubes there are present (i) the
vasa efferentia (VE], united on the one hand with (2) the
central canal in the base of the testis (/), and on the other with
the longitudinal canal of the Wolffian body (/<?). From the
latter are seen passing off the successive tubuli of the anterior
segments of the Wolffian body, in connection with which Malpighian bodies are typically present, though not represented in
my diagram.
 
Apart from the absence of the pronephros the points which
deserve notice in the Elasmobranch excretory system are (i)
The splitting of the segmental duct into Wolffian (mesonephric)
and Mullerian ducts. (2) The connection of the former with
the mesonephros, and of the latter with the abdominal opening
of the segmental duct which represents the pronephros of other
types. (3) The fact that the Mullerian duct serves as oviduct,
and the Wolffian duct as vas deferens. (4) The differentiation
of a posterior section of the mesonephros into a special gland
foreshadowing the metanephros of the Amniota.
 
 
 
/OO CYCLOSTOMATA.
 
 
 
Cyclostomata. The development of the excretory system
amongst the Cyclostomata has only been studied in Petromyzon
(Miiller, Furbringer, and Scott).
 
The first part of the system developed is the segmental duct.
It appears in the embryo of about 14 days (Scott) as a solid
cord of cells, differentiated from the somatic mesoblast near the
dorsal end of the body cavity. This cord is at first placed
immediately below the epiblast, and grows backwards by a
continuous process of differentiation of fresh mesoblast cells. It
soon acquires a lumen, and joins the cloacal section of the
alimentary tract before the close of foetal life. Before this
communication is established, the front end of the duct sends a
process towards the body cavity, the blind end of which acquires
a ciliated opening into the latter. A series of about four or five
successively formed outgrowths from the duct, one behind the
other, give rise to as many ciliated funnels opening into the body
cavity, and each communicating by a more or less elongated
tube with the segmental duct. These funnels, which have a
metameric arrangement, constitute the pronephros, the whole
of which is situated in the pericardial region of the body
cavity.
 
On the inner side of the peritoneal openings of each pronephros there is formed a vascular glomerulus, projecting into
the body cavity, and covered by peritoneal epithelium. For a
considerable period the pronephros constitutes the sole functional part of the excretory system.
 
A mesonephros is formed (Furbringer) relatively late in
larval life, as a segmentally arranged series of solid cords,
derived from the peritoneal epithelium. These cords constitute
the rudiments of the segmental tubes. They are present for a
considerable portion of the body cavity, extending backwards
from a point shortly behind the pronephros. They soon separate
from the peritoneal epithelium, become hollowed out into canals,
and join the segmental duct. At their blind extremity (that
originally connected with the peritoneal epithelium) a Malpighian
body is formed.
 
The pronephros is only a provisional excretory organ, the
atrophy of which commences during larval life, and is nearly
completed when the Ammoccete has reached 180 mm. in length.
 
 
 
EXCRETORY ORGANS. 70 1
 
Further changes take place in connection with the excretory
system on the conversion of the Ammoccete into the adult.
 
The segmental ducts in the adult fall into a common urinogenital cloaca, which opens on a papilla behind the anus. This
cloaca also communicates by two apertures (abdominal pores)
with the body cavity. The generative products are carried into
the cloaca by these pores ; so that their transportation outwards
is not performed by any part of the primitive urinary system.
The urinogenital cloaca is formed by the separation of the portion
of the primitive cloaca containing the openings of the segmental
ducts from that connected with the alimentary tract.
 
The mesonephros of the Ammoccete undergoes at the metamorphosis complete atrophy, and is physiologically replaced by
a posterior series of segmental tubes, opening into the hindermost portion of the segmental duct (Schneider).
 
In Myxine the excretory system consists (i) of a highly developed pronephros with a bunch of ciliated peritoneal funnels opening into the pericardial section of the body cavity. The coiled and branched tubes of which
the pronephros is composed open on the ventral side of the anterior portion
of the segmental duct, which in old individuals is cut off from the posterior
section of the duct. On the dorsal side of the portion of the segmental duct
belonging to the pronephros there are present a small number of diverticula,
terminating in glomeruli : they are probably to be regarded as anterior
segmental tubes. (2) Of a mesonephros, which commences a considerable
distance behind the pronephros, and is formed of straight extremely simple
segmental tubes opening into the segmental duct (fig. 385).
 
The excretory system of Myxine clearly retains the characters of the
system as it exists in the larva of Petromyzon.
 
Teleostei. In most Teleostei the pronephros and mesonephros coexist through life, and their products are carried off by
a duct, the nature of which is somewhat doubtful, but which is
probably homologous with the mesonephric duct of other types.
 
The system commences in the embryo (Rosenberg, Oellacher,
Gotte, Furbringer) with the formation of a groove-like fold of the
somatic layer of peritoneal epithelium, which becomes gradually
constricted into a canal; the process of constriction commencing
in the middle and extending in both directions. The canal does
not however close anteriorly, but remains open to the body
cavity, thus giving rise to a funnel equivalent to the pronephric
funnels of Petromyzon and Myxine. On the inner side of this
 
 
 
702
 
 
 
TELEOSTEI.
 
 
 
funnel there is formed a glomerulus, projecting into the body
 
cavity ; and at the same time that
 
this is being formed the anterior end
 
of the canal becomes elongated and
 
convoluted. The above structures
 
constitute a pronephros, while the
 
posterior part of the primitive canal
 
forms the segmental duct.
 
The portion of the body cavity
with the glomerulus and peritoneal
funnel of the pronephros (fig. 395,
po) soon becomes completely isolated from the remainder, so as to
form a closed cavity (gl). The
development of the mesonephros
does not take place till long after
that of the pronephros. The segmental tubes which form it are
stated by Fiirbringer to arise from
solid ingrowths of peritoneal epithelium, developed successively from
before backwards, but Sedgwick
informs me that they arise as differentiations of the mesoblastic cells
near the peritoneal epithelium. They
soon become hollow, and unite with
the segmental duct. Malpighian
bodies are developed on their median
portions. They grow very greatly
in length, and become much convoluted, but the details of this
process have not been followed out.
 
The foremost segmental tubes are situated close behind the
pronephros, while the hindermost are in many cases developed
in the post-anal continuations of the body cavity. The pronephros appears to form the swollen cephalic portion of the kidney
of the adult, and the mesonephros the remainder ; the so-called
caudal portion, where present, being derived (?) from the postanal segmental tubes.
 
In some cases the cephalic portion of the kidneys is absent
 
 
 
 
FIG. 394. PORTIONS OF THE
MESONEPHROS OF MYXINE. (From
Gegenbaur; after J. Miiller.)
 
a. segmental duct ; b. segmental tube; c. glomerulus ; d. afferent,
e. efferent artery.
 
B represents a portion of A
highly magnified.
 
 
 
EXCRETORY ORGANS. 703
 
 
 
in the adult, which probably implies the atrophy of the pronephros ; in other instances the cephalic portion of the kidneys is
the only part developed. Its relation to the embryonic proncphros requires however further elucidation.
 
In the adult the ducts in the lower part of the kidneys lie as
a rule on their outer borders, and almost invariably open into a
 
 
 
 
pr
 
 
 
FIG. 395. SECTION THROUGH THE PRONEPHROS OF A TROUT AND ADJACENT
PARTS TEN DAYS BEFORE HATCHING.
 
pr.n. pronephros ; po. opening of pronephros into the isolated portion of the body
cavity containing the glomerulus ; gl. glomerulus ; ao. aorta ; ch. notochord ; x.
subnotochordal rod ; al. alimentary tract.
 
urinary bladder, which usually opens in its turn on the urinogenital papilla immediately behind the genital pore, but in a few
instances there is a common urinogenital pore.
 
In most Osseous Fish there are true generative ducts continuous with the investment of the generative organs. It
appears to me most probable, from the analogy of Lepidostcus,
to be described in the next section, that these ducts are split off
from the primitive segmental duct, and correspond with the
Miillerian ducts of Elasmobranchii, etc. ; though on this point
we have at present no positive embryological evidence (vide
general considerations at the end of the Chapter). In the
female Salmon and the male and female Eel the generative
products are carried to the exterior by abdominal pores. It is
possible that this may represent a primitive condition, though it
 
 
 
704
 
 
 
GANOIDEI.
 
 
 
is more probably a case of degeneration, as is indicated by the
presence of ducts in the male Salmon and in forms nearly allied
to the Salmonidae.
 
The coexistence of abdominal pores and generative ducts in
Mormyrus appears to me to demonstrate that the generative
ducts in Teleostei cannot be derived from the coalescence of the
investment of the generative organs with the abdominal pores.
 
Ganoidei. The true excretory gland of the adult Ganoidei
resembles on the whole that of Teleostei, consisting of an
elongated band on each side the mesonephros an anterior
dilatation of which probably represents the pronephros.
 
There is in both sexes a Mullerian duct, provided, except
in Lepidosteus, with an abdominal funnel, which is however
situated relatively very far back in the abdominal cavity. The
Mullerian ducts appear to serve as generative canals in both sexes.
In Lepidosteus they are continuous with the investment of the
generative glands, and thus a relation between the generative ducts
and glands, very similar to that in Teleostei, is brought about.
 
Posteriorly the Mullerian ducts and the ducts of the mesonephros remain united. The common duct so formed on each
side is clearly the primitive segmental duct. It receives the
secretion of a certain number of the posterior mesonephric
tubules, and usually unites with its fellow to form a kind of
bladder, opening by a single
pore into the cloaca, behind
the anus. The duct which
receives the secretion of the
anterior mesonephric tubules
is the true mesonephric or
Wolffian duct.
 
The development of the
excretory system, which has
been partially worked out in
Acipenscr and Lepidosteus 1 ,
is on the whole very similar
to that in the Teleostei. The
first portion of the system to
 
 
 
 
FIG. 396. SECTION THROUGH THE
TRUNK OF A LEPIDOSTEUS EMBRYO ON
THE SIXTH DAY AFTER IMPREGNATION.
 
me. medullary cord ; ms. mesoblast ; sg.
segmental duct ; ch. notochord ; .r. subnotochordal rod; hy. hypoblast.
 
 
 
1 Acipenser has been investigated by Fiirbringer, Salensky, Sedgwick, and also
by myself, and Lepidosteus by W. N. Parker and myself.
 
 
 
EXCRETORY ORGANS.
 
 
 
705
 
 
 
be formed is the segmental duct. In Lepidosteus this duct is
formed as a groove-like invagination of the somatic peritoneal
epithelium, precisely as in Teleostei, and shortly afterwards
forms a duct lying between the mesoblast and the epiblast
(fig. 396, sg}. In Acipenser (Salensky) however it is formed as
 
 
 
 
FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER
 
EMBRYO. (After Salensky.)
 
Rf. medullary groove ; Alp. medullary plate ; Wg. segmental duct ; Ch. notochord ; En. hypoblast ; Sgp. mesoblastic somite ; Sp. parietal part of mesoblastic
plate.
 
a solid ridge of the somatic mesoblast, as in Petromyzon and
Elasmobranchii (fig. 397, Wg).
 
In both forms the ducts unite behind with the cloaca, and a
pronephros of the Teleostean type appears to be developed.
This gland is provided with but one 1 peritoneal opening, which
together with the glomerulus belonging to it becomes encapsuled
in a special section of the body cavity. The opening of the
pronephros of Acipenser into this cavity is shewn in fig. ^<^>,pr.n.
At this early stage of Acipenser (larva of 5 mm.) I could find
no glomerulus.
 
The mesonephros is formed some distance behind, and some
time after the pronephros, both in Acipenser and Lepidosteus,
so that in the larvae of both these genera the pronephros is for
a considerable period the only excretory organ. In Lepidosteus
especially the development of the mesonephros occurs very
late.
 
The development of the mesonephros has not been worked
out in Lepidosteus, but in Acipenser the anterior segmental
tubes become first established as (I believe) solid cords of cells,
attached at one extremity to the peritoneal epithelium on each
 
1 I have not fully proved this point, but have never found more than one
opening.
 
 
 
B. III.
 
 
 
45
 
 
 
GANOIDEI.
 
 
 
side of the insertion of the mesentery, and extending upwards
and outwards round the segmental duct 1 . The posterior segmental tubes arise later than the anterior, and (as far as can be
determined from the sections in my possession) they are formed
independently of the peritoneal epithelium, on the dorsal side
of the segmental duct.
 
In later stages (larvae of 7 10 mm.) the anterior segmental
tubes gradually lose their attachment to the peritoneal epithelium. The extremity near the peritoneal epithelium forms a
Malpighian body, and the other end unites with the segmental
duct. At a still later stage wide peritoneal funnels are es
 
 
sjy.c
 
 
 
mjo
 
 
 
pr.n
 
 
 
 
FIG. 398. TRANSVERSE SECTION THROUGH THE REGION OF THE STOMACH OF A
 
LARVA OF ACIPENSER 5 MM. IN LENGTH.
 
st. epithelium of stomach ; yk. yolk ; ch. notochord, below which is a subnotochordal rod; pr.n. pronephros ; ao. aorta; mf. muscle-plate formed of large cells,
the outer parts of which are differentiated into contractile fibres ; sp.c. spinal cord ;
b.c. body cavity.
 
tablished, for at any rate a considerable number of the tubes,
leading from the body cavity to the Malpighian bodies. These
 
1 Whether the segmental tubes are formed as ingrowths of the peritoneal
epithelium, or in situ, could not be determined.
 
 
 
EXCRETORY ORGANS. 707
 
funnels have been noticed by Furbringer, Salensky and myself,
but their mode of development has not, so far as I know, been
made out. The funnels appear to be no longer present in the
adult. The development of the Mullerian ducts has not been
worked out.
 
Dipnoi. The excretory system of the Dipnoi is only known in the
adult, but though in some respects intermediate in character between that of
the Ganoidei and Amphibia, it resembles that of the Ganoidei in the
important feature of the Mullerian ducts serving as genital ducts in both
sexes.
 
Amphibia. In Amphibia (Gotte, Furbringer) the development of the excretory system commences, as in Teleostei, by
the formation of the segmental duct from a groove formed by a
fold of the somatic layer of the peritoneal epithelium, near the
dorsal border of the body cavity (fig. 399, u). The anterior end
of the groove is placed immediately behind the branchial
region. Its posterior part soon becomes converted into a canal
by a constriction which commences a short way from the front
end of the groove, and thence extends backwards. This canal
at first ends blindly close to the cloaca, into which however it
soon opens.
 
The anterior open part of the groove in front of the constriction (fig. 399, n] becomes differentiated into a longitudinal
duct, which remains in open communication with the body
cavity by two (many Urodela) three (many Anura) or four
(Cceciliidae) canals. This constitutes the dorsal part of the
pronephros. The ventral part of the gland is formed from the
section of the duct immediately behind the longitudinal canal.
This part grows in length, and, assuming an S-shaped curvature,
becomes placed on the ventral side of the first formed part of
the pronephros. By continuous growth in a limited space the
convolutions of the canal of the pronephros become more numerous, and the complexity of the gland is further increased by the
outgrowth of blindly ending diverticula.
 
At the root of the mesentery, opposite the peritoneal openings
of the pronephros, a longitudinal fold, lined by peritoneal epithelium, and attached by a narrow band of tissue, makes its
appearance. It soon becomes highly vascular, and constitutes a
glomerulus homologous with that in Petromyzon and Teleostei.
 
452
 
 
 
AMPHIBIA.
 
 
 
a*'
 
 
 
The section of the body cavity which contains the openings
of the pronephros and the glomerulus,
becomes dilated, and then temporarily
shut off from the remainder. At a
later period it forms a special though
not completely isolated compartment.
For a long time the pronephros and
its duct form the only excretory organs
of larval Amphibia. Eventually however the formation of the mesonephros
commences, and is followed by the
atrophy of the pronephros. The mesonephros is composed, as in other
types, of a series of segmental tubes,
but these, except in Cceciliidae, no
longer correspond in number with the
myotomes, but are in all instances
more numerous. Moreover, in the
posterior part of the mesonephros in
the Urodeles, and through the whole
length of the gland in other types,
secondary and tertiary segmental tubes
are formed in addition to the primary
tubes.
 
 
 
 
FIG. 399. TRANSVERSE SECTION THROUGH A VERY YOUNG
TADPOLE OF BOMBINATOR AT
THE LEVEL OF THE ANTERIOR
END OF THE YOLK-SACK. (After
 
Gotte.)
 
a. fold of epiblast continuous
with the dorsal fin; is", neural
cord; m. lateral muscle; as 1 .
outer layer of muscle-plate; s.
lateral plate of mesoblast ; b.
mesentery ; u. open end of the
segmental duct, which forms the
pronephros ; f. alimentary tract ;
f. ventral diverticulum which
becomes the liver; e. junction of
yolk cells and hypoblast cells ;
d. yolk cells.
 
 
 
The development of the mesonephros
commences in Salamandra (Fiirbringer) with
the formation of a series of solid cords, which
in the anterior myotomes spring from the
peritoneal epithelium on the inner side of the
segmental duct, but posteriorly arise independently of this epithelium in the adjoining
mesoblast. Sedgwick informs me that in the
 
Frog the segmental tubes are throughout developed in the mesoblast, independently of the peritoneal epithelium. These cords next become detached
from the peritoneal epithelium (in so far as they are primitively united to it),
and after first assuming a vesicular form, grow out into coiled tubes, with a
median limb the blind end of which assists in forming a Malpighian body,
and a lateral limb which comes in contact with and opens into the segmental
duct, and an intermediate portion connecting the two. At the junction of
the median with the intermediate portion, and therefore at the neck of the
Malpighian body, a canal grows out in a ventral direction, which meets the
 
 
 
EXCRETORY ORGANS. 709
 
peritoneal epithelium, and then develops a funnel-shaped opening into the
body cavity, which subsequently becomes ciliated. In this way the peritoneal
funnels which are present in the adult are established.
 
The median and lateral sections of the segmental tubes become highly
convoluted, and the separate tubes soon come into such close proximity that
their primitive distinctness is lost.
 
The first fully developed segmental tube is formed in Salamandra maculata in about the sixth myotome behind the pronephros. But in the region
between the two structures rudimentary segmental tubes are developed.
 
The number of primary segmental tubes in the separate myotomes of
Salamandra is as follows :
 
In the 6th myotome (i.e. the first with a true
 
segmental tube) 12 segmental tubes
 
yth roth myotome 23
 
IIth ... 34
 
I2th 3 4 or 4 5
 
I3th y> 45
 
1 3th i6th 56
 
It thus appears that the segmental tubes are not only more numerous than
the myotomes, but that the number in each myotome increases from before
backwards. In the case of Salamandra there are formed in the region of
the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out
of independent solid cords, which arise in the mesoblast dorsally to the tubes
already established.
 
The secondary segmental tubes appear to develop out of these cords
exactly in the same way as the primary ones, except that they do not join the
segmental duct directly, but unite with the primary segmental tubes shortly
before the junction of the latter with the segmental duct. In this way compound segmental tubes are established with a common collecting tube, but
with numerous Malpighian bodies and ciliated peritoneal openings. The
difference in the mode of origin of these compound tubes and of those in
Elasmobranchii is very striking.
 
The later stages in the development of the segmental tubes have not been
studied in the other Amphibian types.
 
In Cceciliidas the earliest stages are not known, but the tubes present in
the adult (Spengel) a truly segmental arrangement, and in the young each of
them is single, and provided with only a single peritoneal funnel. In the
adult however many of the segmental organs become compound, and may
have as many as twenty funnels, etc. Both simple and compound segmental
tubes occur in all parts of the mesonephros, and are arranged in no definite
order.
 
In the Anura (Spengel) all the segmental tubes are compound, and an
enormous number of peritoneal funnels are present on the ventral surface,
but it has not yet been definitely determined into what part of the segmental
tubes they open.
 
 
 
710 AMPHIBIA.
 
 
 
Before dealing with the further changes of the Wolffian body
it is necessary to return to the segmental duct, which, at the
time when the pronephros is undergoing atrophy, becomes split
into a dorsal Wolffian and ventral Mullerian duct. The process
in Salamandra (Fiirbringer) has much the same character as in
Elasmobranchii, the Mullerian duct being formed by the gradual
separation, from before backwards, of a solid row of cells from
the ventral side of the segmental duct, the remainder of the duct
constituting the Wolffian duct. During the formation of the
Mullerian duct its anterior part becomes hollow, and attaching
itself in front to the peritoneal epithelium acquires an opening
into the body cavity. The process of hollowing is continued
backwards pari passu with the splitting of the segmental duct.
In the female the process is continued till the Mullerian duct
opens, close to the Wolffian duct, into the cloaca. In the male
the duct usually ends blindly. It is important to notice that
the abdominal opening of the Mullerian duct in the Amphibia
(Salamandra) is a formation independent of the pronephros, and
placed slightly behind it ; and that the undivided anterior part
of the segmental duct (with the pronephros) is not, as in Elasmobranchii, united with the Mullerian duct, but remains connected
with the Wolffian duct.
 
The development of the Mullerian duct has not been satisfactorily
studied in other forms besides Salamandra. In Cceciliidae its abdominal
opening is on a level with the anterior end of the Wolffian body. In other
forms it is usually placed very far forwards, close to the root of the lungs
(except in Proteus and Batrachoseps, where it is placed somewhat further
back), and some distance in front of the Wolffian body.
 
The Mullerian duct is always well developed in the female, and serves as
oviduct. In the male it does not (except possibly in Alytes) assist in the
transportation of the genital products, and is always more or less rudimentary, and in Anura may be completely absent.
 
After the formation of the Mullerian duct, the Wolffian duct
remains as the excretory channel for the Wolffian body, and, till
the atrophy of the pronephros, for this gland also. Its anterior
section, in front of the Wolffian body, undergoes a more or less
complete atrophy.
 
The further changes of the excretory system concern (i) the
junction in the male of the anterior part of the Wolffian body
with the testis ; (2) certain changes in the collecting tubes of the
 
 
 
EXCRETORY ORGANS.
 
 
 
711
 
 
 
posterior part of the mesonephros. The first of these processes
results in the division of the Wolffian body into a sexual and a
non-sexual part, and in Salamandra and other Urodeles the
division corresponds with the distribution of the simple and
compound segmental tubes.
 
Since the development of the canals connecting the testes with
the sexual part of the Wolffian body has not been in all points
satisfactorily elucidated, it will be convenient to commence with a
description of the adult arrangement of the parts (fig. 400 B). In
most instances a non-segmental system of canals the vasa effcrentia (ve) coming from the testis, fall into a canal known as the
longitudinal canal of the Wolffian body, from which there pass off
transverse canals, which fall into, and are equal in number to, the
primary Malpighian bodies of the sexual part of the gland. The
spermatozoa, brought to the Malpighian bodies, are thence transported along the segmental tubes to the Wolffian duct, and so to
the exterior. The system of canals connecting the testis with
the Malpighian bodies is known as the testicular network. The
number of segmental tubes connected with the testis varies
very greatly. In Siredon there are as many as from 30 32
(Spengel).
 
The longitudinal canal of the Wolffian body is in rare instances
(Spelerpes, etc.) absent, where the sexual part of the Wolffian body is
slightly developed. In the Urodela the testes are united with the anterior
part of the Wolffian body. In the Cceciliidas the junction takes place in an
homologous part of the Wolffian body, but, owing to the development of the
anterior segmental tubes, which are rudimentary in the Urodela, it is
situated some way behind the front end. Amongst the Anura the connection
of the testis with the tubules of the Wolffian body is subject to considerable
variations. In Bufo cinereus the normal Urodele type is preserved, and in
Bombinator the same arrangement is found in a rudimentary condition, in
that there are transverse trunks from the longitudinal canal of the Wolffian
body, which end blindly, while the semen is carried into the Wolffian
duct by canals in front of the Wolffian body. In Alytes and Discoglossus
the semen is carried away by a similar direct continuation of the longitudinal canal in front of the Wolffian body, but there are no rudimentary transverse canals passing into the Wolffian body, as in Bombinator. In Rana the transverse ducts which pass off from the longitudinal
canal of the Wolffian body, after dilating to form (?) rudimentary Malpighian
bodies, enter directly into the collecting tubes near their opening into the
Wolffian duct.
 
 
 
712 AMPHIBIA.
 
 
 
In most Urodeles the peritoneal openings connected with the primary
generative Malpighian bodies atrophy, but in Spelerpes they persist. In
the Cceciliidie they also remain in the adult state.
 
With reference to the development of these parts little is
known except that the testicular network grows out from the
primary Malpighian bodies, and becomes united with the testis.
Embryological evidence, as well as the fact of the persistence of
the peritoneal funnels of the generative region in the adults
of some forms, proves that the testicular network is not developed
from the peritoneal funnels.
 
Rudiments of the testicular network are found in the female Cceciliidae
and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and
of transverse canals passing from this to the Malpighian bodies, together
with some branches passing into the mesovarium.
 
Amongst the Urodela the collecting tubes of the hinder non-sexual part
of the Wolffian body, which probably represents a rudimentary metanephros,
undergo in the male sex a change similar to that which they usually undergo
in Elasmobranchii. Their points of junction with the Wolffian duct are
carried back to the hindermost end of the duct (fig. 400 B), and the collecting
tubes themselves unite together into one or more short ducts (ureters) before
joining the Wolffian duct.
 
In Batrachoseps only the first collecting tube becomes split off in
this way ; and it forms a single elongated ureter which receives all the
collecting tubes of the posterior segmental tubes. In the female and in
the male of Proteus, Menobranchus, and Siren the collecting tubes retain
their primitive transverse course and open laterally into the Wolffian duct.
In rare cases (Ellipsoglossus, Spengel} the ureters open directly into the
cloaca.
 
The urinary bladder of the Amphibia is an outgrowth of the
ventral wall of the cloacal section of the alimentary tract, and is
homologous with the allantois of the amniotic Vertebrata.
 
The subjoined diagram (fig. 400) of the urogenital system of
Triton illustrates the more important points of the preceding
description.
 
In the female (A) the following parts are present :
 
(1) The Mullerian duct or oviduct (od) derived from the
splitting of the segmental duct.
 
(2) The Wolffian duct (sug) constituting the portion of the
segmental duct left after the formation of the Mullerian duct.
 
(3) The mesonephros (r), divided into an anterior sexual part
 
 
 
EXCRETORY ORGANS.
 
 
 
7'3
 
 
 
connected with a rudimentary testicular network, and a posterior
part. The collecting tubes from both
parts fall transversely into the Wolffian duct.
 
(4) The ovary (ov).
 
(5) The rudimentary testicular
network.
 
In the male (B) the following
parts are present :
 
(1) The functionless though fairly
developed Miillerian duct (;).
 
(2) The Wolffian duct (sug).
 
(3) The mesonephros (r) divided
into a true sexual part, through the
segmental tubes of which the semen
passes, and a non-sexual part. The
collecting tubes of the latter do not
enter the Wolffian duct directly, but
bend obliquely backwards and only
fall into it close to its cloacal aperture, after uniting to form one or two
primary tubes (ureters).
 
(4) The testicular network (ve)
consisting of (i) transverse ducts
from the testes, falling into (2) the
longitudinal canal of the Wolffian
body, from which (3) transverse canals are again given off to the Malpighian bodies.
 
Amniota. The amniotic Vertebrata agree, so far as is known, very
closely amongst themselves in the
formation of the urinogenital system.
 
The most characteristic feature of the system is the full
development of a metanephros, which constitutes the functional
kidney on the atrophy of the mesonephros or Wolffian body,
which is a purely embryonic organ. The first part of the
system to develop is a duct, which is usually spoken of as the
Wolffian duct, but which is really the homologue of the seg
 
 
 
FIG. 400. DIAGRAM OF THE
URINOGENITAL SYSTEM OF TRITON. (From Gegenbaur ; after
Spengel.)
 
A. Female. B. Male.
r. mesonephros, on the surface
of which numerous peritoneal funnels are visible ; sug. mesonephric
or Wolffian duct; od. oviduct
(Miillerian duct); in. Miillerian
duct of male ; ve. vasa efferentia of
testis ; t. testis ; ov. ovary ; up.
urinogenital pore.
 
 
 
714 AMNIOTA.
 
 
 
mental duct. It apparently develops in all the Amniota nearly
on the Elasmobranch type, as a solid rod, primarily derived
from the somatic mesoblast of the intermediate cell mass (fig.
401 W.d}\
 
The first trace of it is visible in an embryo Chick with eight
somites, as a ridge projecting from the intermediate cell mass towards the epiblast in the region of the seventh somite. In the
course of further development it continues to constitute such a
ridge as far as the eleventh somite (Sedgwick), but from this
point it grows backwards in the space between the epiblast and
mesoblast In an embryo with fourteen somites a small lumen
has appeared in its middle part and in front it is connected with
rudimentary Wolffian tubules, which develop in continuity with
it (Sedgwick). In the succeeding stages the lumen of the duct
gradually extends backwards and forwards, and the duct itself
also passes inwards relatively to the epiblast (fig. 402). Its hindend elongates till it comes into connection with, and opens into,
the cloacal section of the hind-gut' 2 .
 
It might have been anticipated that, as in the lower types,
the anterior end of the segmental duct would either open into
the body cavity, or come into connection with a pronephros.
Neither of these occurrences takes place, though in some types
(the Fowl) a structure, which is probably the rudiment of a
pronephros, is developed ; it does not however appear till a later
stage, and is then unconnected with the segmental duct. The
next part of the system to appear is the mesonephros or
Wolffian body.
 
This is formed in all Amniota as a series of segmental tubes,
which in Lacertilia (Braun) correspond with the myotomes, but
in Birds and Mammalia are more numerous.
 
In Reptilia (Braun, No. 542), the mesonephric tubes develop as segmentally-arranged masses on the inner side of the Wolffian duct, and
appear to be at first united with the peritoneal epithelium. Each mass soon
becomes an oval vesicle, probably opening for a very short period into the
 
1 Dansky and Kostenitsch (No. 543) describe the Wolffian duct in the Chick as
developing from a groove opening to the peritoneal cavity, which subsequently
becomes constricted into a duct. I have never met with specimens such as those
figured by these authors.
 
2 The foremost extremity of the segmental duct presents, according to Gasser,
curious irregularities and an anterior completely isolated portion is often present.
 
 
 
EXCRETORY ORGANS.
 
 
 
715
 
 
 
peritoneal cavity by a peritoneal funnel. The vesicles become very early
detached from the peritoneal epithelium, and lateral outgrowths from them
give rise to the main parts of the segmental tubes, which soon unite with the
segmental duct.
 
In Birds the development of the segmental tubes is more complicated 1 .
 
The tubules of the Wolffian body are derived from the intermediate cell
mass, shewn in fig. 401, between the upper end of the body cavity and the
 
 
 
g.o.
 
 
 
 
FIG. 401. TRANSVERSE SECTION THROUGH THE DORSAL REGION OF AN
 
EMBRYO CHICK OF 45 HOURS.
 
M.c. medullary canal ; P.v. mesoblastic somite ; W.d. Wolffian duct which is in
contact with the intermediate cell mass ; So. somatopleure ; S.p. splanchnopleure ;
p.p. pleuroperitoneal cavity ; ch. notochord ; op. boundary of area opaca; v. bloodvessel.
 
muscle-plate. In the Chick the mode of development of this mass into the
segmental tubules is different in the regions in front of and behind about the
sixteenth segment. In front of about the sixteenth segment the intermediate
cell mass becomes detached from the peritoneal epithelium at certain points,
remaining attached to it at other points, there being several such to each
segment. The parts of the intermediate cell mass attached to the peritoneal
epithelium become converted into S-shaped cords (fig. 402, st] which soon
unite with the segmental duct (wd}. Into the commencement of each
of these cords the lumen of the body cavity is for a short distance
prolonged, so that this part constitutes a rudimentary peritoneal funnel.
 
1 Correct figures of the early stages of these structures were first given by
Kolliker, but the correct interpretation of them and the first satisfactory account of
the development of the excretory organs of Birds was given by Sedgwick (No. 549).
 
 
 
716
 
 
 
AMNIOTA.
 
 
 
In the Duck the attachment of the intermediate cell mass to the peritoneal
epithelium is prolonged further back than in the Chick.
 
In the foremost segmental tubes, which never reach a very complete
development, the peritoneal funnels widen considerably, while at the same
time they acquire a distinct lumen. The section of the tube adjoining
the wide peritoneal funnel becomes partially invaginated by the formation of
a glomerulus, and this glomerulus soon grows to such an extent as to project
through the peritoneal funnel, the neck of which it completely fills, into the
body cavity (fig. 403, gl). There is thus formed a series of free peritoneal
glomeruli belonging to the anterior Wolfnan tubuli 1 . These tubuli become
however early aborted.
 
In the case of the remaining tubules developed from the S-shaped cords
the attachment to the peritoneal epithelium is very soon lost. The cords
acquire a lumen, and open into the segmental duct. Their blind extremities
constitute the rudiments of Malpighian bodies.
 
 
 
am
 
 
 
 
FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO WITH
 
ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.
 
am. amnion ; so. somatopleure ; sp. splanchnopleure ; ivd. Wolffian duct ; st. segmental tube; ca.v. cardinal vein; m.s. muscle-plate; sp.g. spinal ganglion; sp.c.
spinal cord ; ch. notochord ; ao. aorta ; hy. hypoblast.
 
1 These external glomeruli were originally mistaken by me (No. 539) for the
glomeralus of the pronephros, from their resemblance to the glomerulus of the
Amphibian pronephros. Their true meaning was made out by Sedgwick (No.
550).
 
 
 
EXCRETORY ORGANS.
 
 
 
717
 
 
 
In the posterior part of the Wolffian body of the Chick the intermediate
cell mass becomes very early detached from the peritoneal epithelium, and
at a considerably later period breaks up into oval vesicles similar to those of
the Reptilia, which form the rudiments of the segmental tubes.
 
Secondary and tertiary segmental tubules are formed in the Chick, on the
dorsal side of the primary tubules,
as direct differentiations of the mesoblast. They open independently into
the Wolffian duct.
 
In Mammalia the segmental tubules (Egli) are formed as solid masses
in the same situation as in Birds and
Reptiles. It is not known whether
they are united with the peritoneal
epithelium. They soon become oval
vesicles, which develop into complete
tubules in the manner already indicated.
 
 
 
 
After the establishment of
the Wolffian body there is formed
in both sexes in all the Amniota
a duct, which in the female
becomes the oviduct, but which
is functionless and disappears
more or less completely in the
male. This duct, in spite of certain peculiarities in its development, is without doubt homologous with the Mullerian duct of
 
 
 
FIG. 403. SECTION THROUGH THE
EXTERNAL GLOMERULUS OF ONE OF
THE ANTERIOR SEGMENTAL TUBES OF
AN EMBRYO CHICK OF ABOUT IOO H.
 
gl. glomerulus ; ge. peritoneal epithelium ; Wd. Wolffian duct ; ao.
aorta ; me. mesentery. The segmental
tube, and the connection between the
external and internal parts of the glomerulus are not shewn in this figure.
 
 
 
 
FIG. 404. SECTIONS SHEWING TWO OF THE PERITONEAL INVAGINATIONS WHICH
GIVE RISE TO THE ANTERIOR PART OF THE MULLERIAN DUCT (PRONEPHROS).
(After Balfour and Sedgwick. )
 
A is the nth section of the series.
B i 5th
 
C i8th ,, ,,
 
gri. second groove ; gr$. third groove ; ri. second ridge ; wit. Wolffian duct.
 
 
 
7 i8
 
 
 
AMNIOTA.
 
 
 
the Ichthyopsida. In connection with its anterior extremity
certain structures have been found in the Fowl, which are
probably, on grounds to be hereafter stated, homologous with
the pronephros (Balfour and Sedgwick).
 
The pronephros, as I shall call it, consists of a slightly
convoluted longitudinal canal with three or more peritoneal
openings. In the earliest condition, it consists of three successive
open involutions of the peritoneal epithelium, connected together
by more or less well-defined ridge-like thickenings of the
epithelium. It takes its origin from the layer of thickened
peritoneal epithelium situated near the dorsal angle of the body
cavity, and is situated some considerable distance behind the
front end of the Wolfifian duct.
 
In a slightly later stage the ridges connecting the grooves
become partially constricted off from the peritoneal epithelium,
 
 
 
 
FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND
GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160 times.
m. mesentery; Z. somatopleure ; a', portion of the germinal epithelium from
which the involution (2) to form the pronephros (anterior part of Miillerian duct) takes
place; a. thickened portion of the germinal epithelium in which the primitive
germinal cells C and o are lying ; E. modified mesoblast which will form the stroma
of the ovary ; WK. Wolffian body ; y. Wolffian duct.
 
 
 
EXCRETORY ORGANS. 719
 
and develop a lumen. The condition of the structure at this
stage is illustrated by fig. 404, representing three transverse
sections through two grooves, and through the ridge connecting
them.
 
The pronephros may in fact now be described as a slightly
convoluted duct, opening into the body cavity by three groovelike apertures, and continuous behind with the rudiment of the
true Miillerian duct.
 
The stage just described is that of the fullest development
of the pronephros. In it, as in all the previous stages, there
appear to be only three main openings into the body cavity ; but
in some sections there are indications of the possible presence of
one or two additional rudimentary grooves.
 
In an embryo not very much older than the one last
described the pronephros atrophies as such, its two posterior
openings vanishing, and its anterior opening remaining as the
permanent opening of the Miillerian duct.
 
The pronephros is an extremely transitory structure, and its
development and atrophy are completed between the QOth and
i2Oth hours of incubation.
 
The position of the pronephros in relation to the Wolffian
body is shewn in fig. 405, which probably passes through a
region between two of the peritoneal openings. As long as the
pronephros persists, the Mullerian duct consists merely of a very
 
 
 
 
FlG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID
PORTION OF THE MtJLLERIAN DUCT WITH THE WOLFFIAN DUCT. (After Balfour
 
and Sedgwick.)
 
In A the terminal portion of the duct is quite distinct ; in B it has united with the
walls of the Wolffian duct.
 
md. Mullerian duct ; Wd. Wolffian duct.
 
 
 
72O AMNIOTA.
 
 
 
small rudiment, continuous with the hindermost of the three
peritoneal openings, and its solid extremity appears to unite
with the walls of the Wolffian duct.
 
After the atrophy of the pronephros, the Miillerian duct
commences to grow rapidly, and for the first part of its course it
appears to be split off as a solid rod from the outer or ventral
wall of the Wolffian duct (fig. 406). Into this rod the lumen,
present in its front part, subsequently extends. Its mode of
development in front is thus precisely similar to that of the
Miillerian duct in Elasmobranchii and Amphibia.
 
This mode of development only occurs however in the
anterior part of the duct. In the posterior part of its course its
growing point lies in a bay formed by the outer walls of the
Wolffian duct, but does not become definitely attached to that
duct. It seems however possible that, although not actually
split off from the walls of the Wolrfian duct, it may grow backwards from cells derived from that duct.
 
The Miillerian duct finally reaches the cloaca though it does
not in the female for a long time open into it, and in the male
never does so.
 
The mode of growth of the Miillerian duct in the posterior part of its
course will best be understood from the following description quoted from
the paper by Sedgwick and myself.
 
"A few sections before its termination the Miillerian duct appears as a
well-defined oval duct lying in contact with the wall of the Wolffian duct on
the one hand and the germinal epithelium on the other. Gradually, however,
as we pass backwards, the Miillerian duct dilates ; the external wall of the
Wolffian duct adjoining it becomes greatly thickened and pushed in in its
middle part, so as almost to touch the opposite wall of the duct, and so form
a bay in which the Miillerian duct lies. As soon as the Miillerian duct has
come to lie in this bay its walls lose their previous distinctness of outline,
and the cells composing them assume a curious vacuolated appearance. No
well-defined line of separation can any longer be traced between the walls of
the Wolffian duct and those of the Miillerian, but between the two is a
narrow clear space traversed by an irregular network of fibres, in some of
the meshes of which nuclei are present.
 
The Miillerian duct may be traced in this condition for a considerable
number of sections, the peculiar features above described becoming more
and more marked as its termination is approached. It continues to dilate
and attains a maximum size in the section or so before it disappears. A
lumen may be observed in it up to its very end, but is usually irregular in
outline and frequently traversed by strands of protoplasm. The Miillerian
 
 
 
EXCRETORY ORGANS. 721
 
duct finally terminates quite suddenly, and in the section immediately
behind its termination the Wolffian duct assumes its normal appearance,
and the part of its outer wall on the level of the Miillerian duct conies into
contact with the germinal epithelium."
 
Before describing the development of the Mullerian duct in other
Amniotic types it will be well to say a few words as to the identifications
above adopted. The identification of the duct, usually called the Wolffian
duct, with the segmental duct (exclusive of the pronephros) appears to be
morphologically justified for the following reasons : (i) that it gives rise to
part of the Mullerian duct as well as to the duct of the Wolffian body ;
behaving in this respect precisely as does the segmental duct of Elasmobranchii and Amphibia. (2) That it serves as the duct for the Wolffian
body, before the Mullerian duct originates from it. (3) That it develops in a
manner strikingly similar to that of the segmental duct of various lower
forms.
 
With reference to the pronephros it is obvious that the organ identified
as such is in many respects similar to the pronephros of the Amphibia.
Both consist of a somewhat convoluted longitudinal canal, with a certain
number of peritoneal openings ;
 
The main difficulties in the homology are :
 
(1) the fact that the pronephros in the Bird is not united with the
segmental duct ;
 
(2) the fact that it is situated behind the front end of the Wolffian body.
It is to be remembered in connection with the first of these difficulties
 
that in the formation of the Mullerian duct in Elasmobranchii the anterior
undivided extremity of the primitive segmental duct, with the peritoneal
opening, which probably represents the pronephros, is attached to the
Mullerian duct, and not to the Wolffian duct ; though in Amphibia the
reverse is the case. To explain the discontinuity of the pronephros with the
segmental duct it is only necessary to suppose that the segmental duct and
pronephros, which in the Ichthyopsida develop as a single formation,
develop in the Bird as two independent structures a far from extravagant
supposition, considering that the pronephros in the Bird is undoubtedly
quite functionless.
 
With reference to the posterior position of the pronephros it is only
necessary to remark that a change in position might easily take place after
the acquirement of an independent development, and that the shifting is
probably correlated with a shifting of the abdominal opening of the
Mullerian duct.
 
The pronephros has only been observed in Birds, and is very
possibly not developed in other Amniota. The Mullerian duct
is also usually stated to develop as a groove of the peritoneal
epithelium, shewn in the Lizard in fig. 354, md., which is continued backward as a primitively solid rod in the space between
B. ill. 46
 
 
 
722
 
 
 
AM N IOTA.
 
 
 
the Wolffian duct and peritoneal epithelium, without becoming
attached to the Wolffian duct.
 
On the formation of the Miillerian duct, the duct of the
mesonephros becomes the true mesonephric or Wolffian duct.
 
After these changes have taken place a new organ of great
importance makes its appearance. This organ is the permanent
kidney, or metanephros.
 
Metanephros. The mode of development of the metanephros has as yet only been satisfactorily elucidated in the Chick
(Sedgwick, No. 549). The ureter and the collecting tubes of
the kidney are developed from a dorsal outgrowth of the hinder
part of the Wolffian duct. The outgrowth from the Wolffian
duct grows forwards, and extends along the outer side of a mass
of mesoblastic tissue which lies mainly behind, but somewhat
overlaps the dorsal aspect of the Wolffian body.
 
This mass of mesoblastic cells may be called the metanephric blastema. Sedgwick, of the accuracy of whose
account I have satisfied myself, has shewn that in the Chick it is
derived from the intermediate cell mass of the region of about
the thirty-first to the thirty-fourth somite. It is at first continuous with, and indistinguishable in structure from, the portion
of the intermediate cell mass of the region immediately in front
of it, which breaks up into Wolffian tubules. The metanephric
blastema remains however quite passive during the formation of
the Wolffian tubules in the adjoining blastema ; and on the
formation of the ureter breaks off from the Wolffian body in
front, and, growing forwards and dorsalwards, places itself on
the inner side of the ureter in the position just described.
 
In the subsequent development of the kidney collecting tubes
grow out from the ureter, and become continuous with masses of
cells of the metanephric blastema, which then differentiate themselves into the kidney tubules.
 
The process just described appears to me to prove that the
kidney of the A mniota is a specially differentiated posterior section
of the primitive mesonephros.
 
According to the view of Remak and Kolliker the outgrowths from the
ureter give rise to the whole of the tubuli uriniferi and the capsules of the
Malpighian bodies, the mesoblast around them forming blood-vessels, etc.
On the other hand some observers (Kupffer, Bornhaupt, Braun) maintain, in
 
 
 
EXCRETORY ORGANS. 723
 
 
 
accordance with the account given above, that the outgrowths of the ureter
form only the collecting tubes, and that the secreting tubuli, etc. are formed
in situ in the adjacent mesoblast.
 
Braun (No. 542) has arrived at the conclusion that in the Lacertilia the
tissue, out of which the tubuli of the metanephros are formed, is derived
from irregular solid ingrowths of the peritoneal epithelium, in a region
behind the Wolffian body, but in a position corresponding to that in which
the segmental tubes take their origin. These ingrowths, after separating
from the peritoneal epithelium, unite together to form a cord into which the
ureter sends the lateral outgrowths already described. These outgrowths
unite with secreting tubuli and Malpighian bodies, formed in situ. In
Lacertilia the blastema of the kidney extends into a postanal region.
Braun's account of the origin of the metanephric blastema does not appear
to me to be satisfactorily demonstrated.
 
The ureter does not long remain attached to the Wolffian
duct, but its opening is gradually carried back, till (in the Chick
between the 6th and 8th day) it opens independently into the
cloaca.
 
Of the further changes in the excretory system the most important is the atrophy of the greater part of the Wolffian body,
and the conversion of the Wolffian duct in the male sex into the
vas deferens, as in Amphibia and the Elasmobranchii.
 
The mode of connection of the testis with the Wolffian duct
is very remarkable, but may be derived from the primitive
arrangement characteristic of Elasmobranchii and Amphibia.
 
In the structures connecting the testis with the Wolffian body
two parts have to be distinguished, (i) that equivalent to the
testicular network of the lower types, (2) that derived from the
segmental tubes. The former is probably to be found in peculiar
outgrowths from the Malpighian bodies at the base of the testes.
 
These were first discovered by Braun in Reptilia, and consist
in this group of a series of outgrowths from the primary (?)
Malpighian bodies along the base of the testis : they unite to
form an interrupted cord in the substance of the testis, from
which the testicular tubuli (with the exception of the seminiferous cells) are subsequently differentiated. These outgrowths,
with the exception of the first two or three, become detached
from the Malpighian bodies. Outgrowths similar to those in
the male are found in the female, but subsequently atrophy.
 
Outgrowths homologous with those found by Braun have
 
46 2
 
 
 
724 AMNIOTA.
 
 
 
been detected by myself (No. 555) in Mammals. It is not
certain to what parts of the testicular tubuli they give rise, but
they probably form at any rate the vasa recta and rete vasculosum.
 
In Mammals they also occur in the female, and give rise to
cords of tissue in the ovary, which may persist through life.
 
The comparison of the tubuli, formed out of these structures,
with the Elasmobranch and Amphibian testicular network is
justified in that both originate as outgrowths from the primary
Malpighian bodies, and thence extend into the testis, and come
into connection with the true seminiferous stroma.
 
As in the lower types the semen is transported from the
testicular network to the Wolffian duct by parts of the glandular
tubes of the Wolffian body. In the case of Reptilia the anterior
two or three segmental tubes in the region of the testis probably
have this function. In the case of Mammalia the vasa efferentia,
i.e. the coni vasculosi, appear, according to the usually accepted
view, to be of this nature, though Banks and other investigators
believe that they are independently developed structures. Further
investigations on this point are required. In Birds a connection
between the Wolffian body and the testis appears to be established as in the other types. The Wolffian duct itself becomes,
in the males of all Amniota, the vas deferens and the convoluted
canal of the epididymis the latter structure (except the head)
being entirely derived from the Wolffian duct.
 
In the female the Wolffian duct atrophies more or less
completely.
 
In Snakes (Braun) the posterior part remains as a functionless canal,
commencing at the ovary, and opening into the cloaca. In the Gecko
(Braun) it remains as a small canal joining the ureter ; in Blindworms a
considerable part of the canal is left, and in Lacerta (Braun) only interrupted
portions.
 
In Mammalia the middle part of the duct, known as Gaertner's canal,
persists in the females of some monkeys, of the pig and of many ruminants.
 
The Wolffian body atrophies nearly completely in both
sexes ; though, as described above, part of it opposite the testis
persists as the head of the epididymis. The posterior part of
the gland from the level of the testis may be called the sexual
part of the gland, the anterior part forming the non-sexual part.
 
 
 
EXCRETORY ORGANS. 725
 
The latter, i.e. the anterior part, is first absorbed ; and in some
Reptilia the posterior part, extending from the region of the genital
glands to the permanent kidney, persists till into the second year.
 
Various remnants of the Wolffian body are found in the adults of both
sexes in different types. The most constant of them is perhaps the part in
the female equivalent to the head of the epididymis and to parts also of the
coiled tube of the epididymis, which may be called, with Waldeyer, the
epoophoron 1 . This is found in Reptiles, Birds and Mammals ; though in a
very rudimentary form in the first-named group. Remnants of the anterior
non-sexual part of the Wolffian bodies have been called by Waldeyer
parepididymis in the male, and paroophoron in the female. Such remnants
are not (Braun) found in Reptilia, but are stated to be found in both male
and female Birds, as a small organ consisting of blindly ending tubes with
yellow pigment. In some male Mammals (including Man) a parepididymis
is found on the upper side of the testis. It is usually known as the organ of
Giraldes.
 
The Mlillerian duct forms, as has been stated, the oviduct in
the female. The two ducts originally open independently into
the cloaca, but in the Mammalia a subsequent modification of
this arrangement occurs, which is dealt with in a separate
section. In Birds the right oviduct atrophies, a vestige being
sometimes left. In the male the Miillerian ducts atrophy more
or less completely.
 
In most Reptiles and in Birds the atrophy of the Miillerian ducts is
complete in the male, but in Lacerta and Anguis a rudiment of the anterior
part has been detected by Leydig as a convoluted canal. In the Rabbit
(Kolliker) 2 and probably other Mammals the whole of the ducts probably
disappears, but in some Mammals, e.g. Man, the lower fused ends of the
Miillerian ducts give rise to a pocket opening into the urethra, known as the
uterus masculinus ; and in other cases, e.g. the Beaver and the Ass, the
rudiments are more considerable, and may be continued into horns homologous with the horns of the uterus (Weber).
 
The hydatid of Morgani in the male is supposed (Waldeyer) to represent
the abdominal opening of the Fallopian tube in the female, and therefore to
be a remnant of the Miillerian duct.
 
Changes in the lower parts of the urinogenital ducts in the Amniota.
 
The genital cord. In the Monodelphia the lower part of
the Wolffian ducts becomes enveloped in both sexes in a special
 
1 This is also called parovarium (His), and Rosenmiiller's organ.
 
2 Weber (No. 553) states that a uterus masculinus is present in the Rabbit, but
his account is by no means satisfactory, and its presence is distinctly denied by
Kolliker.
 
 
 
726
 
 
 
AMNIOTA.
 
 
 
cord of tissue, known as -the genital cord (fig. 407, gc), within the
lower part of which the MUllerian ducts are also enclosed. In
the male the MUllerian ducts in this cord atrophy, except at
their distal end where they unite to form the uterus masculinus.
The Wolffian ducts, after becoming the vasa deferentia, remain
for some time enclosed in the common cord, but afterwards
separate from each other. The seminal vesicles are outgrowths
of the vasa deferentia.
 
In the female the Wolffian ducts within the genital cord
atrophy, though rudiments of them are for a long time visible or
even permanently persistent. The lower parts of the MUllerian
ducts unite to form the vagina and body of the uterus. The
junction commences in the middle and extends forwards and
backwards ; the stage with a median junction being retained
permanently in Marsupials.
 
The urinogenital sinus and external generative organs.
In all the Amniota, there open at first into the common cloaca
the alimentary canal dorsally, the allantois ventrally, and the
Wolffian and MUllerian ducts and ureters laterally. In Reptilia
and Aves the embryonic condition is retained. In both groups
the allantois serves as an embryonic urinary bladder, but while
it atrophies in Aves, its stalk dilates to form a permanent
urinary bladder in Reptilia. In Mammalia the dorsal part of
the cloaca with the alimentary tract becomes first of all partially
constricted off from the ventral, which then forms a urinogenital
sinus (fig. 407, ug). In the course of development the urinogenital sinus becomes, in all Mammalia but the Ornithodelphia,
completely separated from the intestinal cloaca, and the two
parts obtain separate external openings. The ureters (fig. 407,
3) open higher up than the other ducts into the stalk of the
allantois which dilates to form the bladder (4). The stalk
connecting the bladder with the ventral wall of the body constitutes the urachus, and loses its lumen before the close of
embryonic life. The part of the stalk of the allantois below the
openings of the ureters narrows to form the urethra, which opens
together with the Wolffian and MUllerian ducts into the urinogenital cloaca.
 
In front of the urinogenital cloaca there is formed a genital
prominence (fig. 407, cp), with a groove continued from the
 
 
 
EXCRETORY ORGANS. 727
 
urinogenital opening ; and on each side a genital fold (&). In
the male the sides of the groove on the prominence coalesce
together, embracing between them the opening of the urinogenital cloaca ; and the prominence itself gives rise to the penis,
 
 
 
 
FIG. 407. DIAGRAM OF THE URINOGENITAL ORGANS OF A MAMMAL AT AN
EARLY STAGE. (After Allen Thomson ; from Quain's Anatomy.)
 
The parts are seen chiefly in profile, but the Miillerian and Wolffian ducts are
seen from the front.
 
3. ureter; 4. urinary bladder ; 5. urachus; of. genital ridge (ovary or testis) ; W.
left Wolffian body ; x. part at apex from which coni vasculosi are afterwards
developed ; w. Wolffian duct ; m. Miillerian duct ; gc. genital cord consisting of
Wolffian and Mullerian ducts bound up in a common sheath ; i. rectum ; ug. urinogenital sinus ; cp. elevation which becomes the clitoris or penis ; Is. ridge from
which the labia majora or scrotum are developed.
 
along which the common urinogenital passage is continued.
The two genital folds unite from behind forwards to form the
scrotum.
 
In the female the groove on the genital prominence gradually
disappears, and the prominence remains as the clitoris, which is
therefore the homologue of the penis : the two genital folds form
the labia majora. The urethra and vagina open independently
into the common urinogenital sinus.
 
 
 
728 GENERAL CONCLUSIONS.
 
General conclusions and Summary.
 
Pronephros. Sedgwick has pointed out that the pronephros
is always present in types with a larval development, and either
absent or imperfectly developed in those types which undergo
the greater part of their development within the egg. Thus it
is practically absent in the embryos of Elasmobranchii and the
Amniota, but present in the larvae of all other forms.
 
This coincidence, on the principles already laid down in a
previous chapter on larval forms, affords a strong presumption
that the pronephros is an ancestral organ ; and, coupled with
the fact that it is the first part of the excretory system to be
developed, and often the sole excretory organ for a considerable
period, points to the conclusion that the pronephros and its duct
the segmental duct are the most primitive parts of the
Vertebrate excretory system. This conclusion coincides with
that arrived at by Gegenbaur and Fiirbringer.
 
The duct of the pronephros is always developed prior to the
gland, and there are two types according to which its development may take place. It may either be formed by the closing
in of a continuous groove of the somatic peritoneal epithelium
(Amphibia, Teleostei, Lepidosteus), or as a solid knob or rod of
cells derived from the somatic mesoblast, which grows backwards
between the epiblast and the mesoblast (Petromyzon, Elasmobranchii, and the Amniota).
 
It is quite certain that the second of these processes is not a
true record of the evolution of 'the duct, and though it is more
possible that the process observable in Amphibia and the
Teleostei may afford some indications of the manner in which
the duct was established, this cannot be regarded as by any
means certain.
 
The mode of development of the pronephros itself is apparently partly dependent on that of its duct. In Petromyzon,
where the duct does not at first communicate with the body
cavity, the pronephros is formed as a series of outgrowths from
the duct, which meet the peritoneal epithelium and open into
the body cavity ; but in other instances it is derived from the
anterior open end of the groove which gives rise to the segmental
duct. The open end of this groove may either remain single
 
 
 
EXCRETORY ORGANS. 729
 
(Teleostci, Ganoidei) or be divided into two, three or more
apertures (Amphibia). The main part of the gland in either
case is formed by convolutions of the tube connected with the
peritoneal funnel or funnels. The peritoneal funnels of the
pronephros appear to be segmentally arranged.
 
The pronephros is distinguished from the mesonephros by
developmental as well as structural features. The most important of the former is the fact that the glandular tubules of
which it is formed are always outgrowths of the segmental duct ;
while in the mesonephros they are always or almost always 1
formed independently of the duct.
 
The chief structural peculiarity of the pronephros is the
absence from it of Malpighian bodies with the same relations as
those in the meso- and metanephros; unless the structures found
in Myxine are to be regarded as such. Functionally the place
of such Malpighian bodies is taken by the vascular peritoneal
ridge spoken of in the previous pages as the glomerulus.
 
That this body is really related functionally to the pronephros appears to
be indicated (i) by its constant occurrence with the pronephros and its
position opposite the peritoneal openings of this body ; (2) by its atrophy at
the same time as the pronephros ; (3) by its enclosure together with the
pronephridian stoma in a special compartment of the body-cavity in
Teleostei and Ganoids, and its partial enclosure in such a compartment in
Amphibia.
 
The pronephros atrophies more or less completely in most
types, though it probably persists for life in the Teleostei and
Ganoids, and in some members of the former group it perhaps
forms the sole adult organ of excretion.
 
The cause of its atrophy may perhaps be related to the fact that it is
situated in the pericardial region of the body-cavity, the dorsal part of which
is aborted on the formation of a closed pericardium ; and its preservation in
Teleostei and Ganoids may on this view be due to the fact that in these types
its peritoneal funnel and its glomerulus are early isolated in a special cavity.
 
Mesonephros. The mesonephros is in all instances composed of a series of tubules (segmental tubes) which are
developed independently of the segmental duct. Each tubule is
 
1 According t.o Sedgwick some of the anterior segmental tubes of Aves form an
exception to the general rule that there is no outgrowth from the segmental or
metanephric duct to meet the segmental tubes.
 
 
 
730 GENERAL CONCLUSIONS.
 
typically formed of (i) a peritoneal funnel opening into (2) a
Malpighian body, from which there proceeds (3) a coiled glandular tube, finally opening by (4) a collecting tube into the
segmental duct, which constitutes the primitive duct for the
mesonephros as well as for the pronephros.
 
The development of the mesonephridian tubules is subject to
considerable variations.
 
(1) They may be formed as differentiations of the intermediate cell mass, and be from the first provided with a lumen,
opening into the body-cavity, and directly derived from the
section of the body-cavity present in the intermediate cell
mass; the peritoneal funnels often persisting for life (Elasmobranchii).
 
(2) They may be formed as solid cords either attached to
or independent of the peritoneal epithelium, which after first
becoming independent of the peritoneal epithelium subsequently
send downwards a process, which unites with it and forms a
peritoneal funnel, which may or may not persist (Acipenser,
Amphibia).
 
(3) They may be formed as in the last case, but acquire no
secondary connection with the peritoneal epithelium (Teleostei,
Amniota). In connection with the original attachment to the
peritoneal epithelium, a true peritoneal funnel may however be
developed (Aves, Lacertilia).
 
Physiological considerations appear to shew that of these
three methods of development the first is the most primitive.
The development of the tubes as solid cords can hardly be
primary.
 
A question which has to be answered in reference to the segmental tubes
is that of the homology of the secondarily developed peritoneal openings of
Amphibia, with the primary openings of the Elasmobranchii. It is on the
one hand difficult to understand why, if the openings are homologous in the
two types, the original peritoneal attachment should be obliterated in
Amphibia, only to be shortly afterwards reacquired. On the other hand
it is still more difficult to understand what physiological gain there could be,
on the assumption of the non-homology of the openings, in the replacement
of the primary opening by a secondary opening exactly similar to it.
Considering the great variations in development which occur in undoubtedly
homologous parts I incline to the view that the openings in the two types
are homologous.
 
 
 
EXCRETORY ORGANS.
 
 
 
731
 
 
 
In the majority of the lower Vertebrata the mesonephric
tubes have at first a segmental arrangement, and this is no
doubt the primitive condition. The coexistence of two, three, or
more of them in a single segment in Amphibia, Aves and
Mammalia has recently been shewn, by an interesting discovery
of Eisig, to have a parallel amongst Chaetopods, in the coexistence of several segmental organs in a single segment in
some of the Capitellidae.
 
In connection with the segmental features of the mesonephros it is perhaps worth recalling the fact that in Elasmobranchii as well as other types there are traces of segmental
tubes in some of the postanal segments. In the case of all the
segmental tubes a Malpighian body becomes established close
to the extremity of the tube adjoining the peritoneal opening, or
in an homologous position in tubes without such an opening.
The opposite extremity of the tube always becomes attached to
the segmental duct.
 
In many of the segments of the mesonephros, especially in
the hinder ones, secondary and tertiary tubes become developed
in certain types, which join the collecting canals of the primary
tubes, and are provided, like the primary tubes, with Malpighian
bodies at their blind extremities.
 
There can it appears to me be little or no doubt that the
secondary tubes in the different types are homodynamous if not
homologous. Under these circumstances it is surprising to find
in what different ways they take their origin. In Elasmobranchii a bud sprouts out from the Malpighian body of one
segment, and joins the collecting tube of the preceding segment,
and subsequently, becoming detached from the Malpighian body
from which it sprouted, forms a fresh secondary Malpighian
body at its blind extremity. Thus the secondary tubes of one
segment are formed as buds from the segment behind. In
Amphibia (Salamandra) and Aves the secondary tubes develop
independently in the mesoblast. These great differences in
development are important in reference to the homology of
the metanephros or permanent kidney, which is discussed
below.
 
Before leaving the mesonephros it may be worth while putting forward
some hypothetical suggestions as to its origin and relation to the pro
 
 
732 GENERAL CONCLUSIONS.
 
nephros, leaving however the difficult questions as to the homology of the
segmental tubes with the segmental organs of Chastopods for subsequent
discussion.
 
It is a peculiarity in the development of the segmental tubes that they at
first end blindly, though they subsequently grow till they meet the segmental
duct with which they unite directly, without the latter sending out any
offshoot to meet them 1 . It is difficult to believe that peritoneal infundibula
ending blindly and unprovided with some external orifice can have had an
excretory function, and we are therefore rather driven to suppose that the
peritoneal infundibula which become the segmental tubes were either from
the first provided each with an orifice opening to the exterior, or were united
with the segmental duct. If they were from the first provided with external
openings we may suppose that they became secondarily attached to the duct
of the pronephros (segmental duct), and then lost their external openings, no
trace of these structures being left, even in the ontogeny of the system.
It would appear to me more probable that the pronephros, with its duct
opening into the cloaca, was the only excretory organ of the unsegmented
ancestors of the Chordata, and that, on the elongation of the trunk and its
subsequent segmentation, a series of metameric segmental tubes became
evolved opening into the segmental duct, each tube being in a sort of way
serially homologous with the primitive pronephros. With the segmentation
of the trunk the latter structure itself may have acquired the more or less
definite metameric arrangement of its parts.
 
Another possible view is that the segmental tubes may be modified
derivatives of posterior lateral branches of the pronephros, which may at
first have extended for the whole length of the body-cavity. If there is any
truth in this hypothesis it is necessary to suppose that, when the unsegmented ancestor of the Chordata became segmented, the posterior
branches of the primitive excretory organ became segmentally arranged,
and that, in accordance with the change thus gradually introduced in them,
the time of their development became deferred, so as to accord to a certain
extent with the time of formation of the segments to which they belonged.
The change in their mode of development which would be thereby introduced is certainly not greater than that which has taken place in the case of
segmental tubes, which, having originally developed on the Elasmobranch
type, have come to develop as they do in the posterior part of the mesonephros of Salamandra, Birds, etc.
 
Genital ducts. So far the origin and development of the
excretory organs have been considered without reference to the
modifications introduced by the excretory passages coming to
serve as generative ducts. Such an unmodified state of the
 
1 As mentioned in the note on p. 729 Sedgwick maintains that the anterior
segmental tubes of the Chick form an exception to this general statement.
 
 
 
EXCRETORY ORGANS. 733
 
 
 
excretory organs is perhaps found permanently in Cyclostomata 1 and transitorily in the embryos of most forms.
 
At first the generative products seem to have been discharged
freely into the body-cavity, and transported to the exterior by
the abdominal pores (vide p. 626).
 
The secondary relations of the excretory ducts to the
generative organs seem to have been introduced by an opening
connected with the pronephridian extremity of the segmental
duct having acquired the function of admitting the generative
products into it, and of carrying them outwards ; so that
primitively the segmental duct must have served as efferent duct
both for the generative products and the pronepJiric secretion (just
as the Wolffian duct still does for the testicular products and
secretion of the Wolffian body in Elasmobranchii and Amphibia).
 
The opening by which the generative products entered the
segmental duct can hardly have been specially developed for
this purpose, but must almost certainly have been one of the
peritoneal openings of the pronephros. As a consequence (by a
process of natural selection) of the segmental duct having both a
generative and a urinary function, a further differentiation took
place, by which that duct became split into two a ventral
Mullerian duct and a dorsal Wolffian duct.
 
The Mullerian duct was probably continuous with one or
more of the abdominal openings of the pronephros which served
as generative pores. At first the segmental duct was probably
split longitudinally into two equal portions, and this mode
of splitting is exceptionally retained in some Elasmobranchii ;
but the generative function of the Mullerian duct gradually
impressed itself more and more upon the embryonic development, so that, in the course of time, the Mullerian duct
developed less and less at the expense of the Wolffian duct.
This process appears partly to have taken place in Elasmobranchii, and still more in Amphibia, the Amphibia offering in
this respect a less primitive condition than the Elasmobranchii ;
while in Aves it has been carried even further, and it seems
possible that in some Amniota the Mullerian and segmental
 
1 It is by no means certain that the transportation outwards of the genital products
by the abdominal pores in the Cyclostomata may not be the result of degeneration.
 
 
 
734 GENERAL CONCLUSIONS.
 
ducts may actually develop independently, as they do exceptionally in individual specimens of Salamandra (Fiirbringer). The
abdominal opening no doubt also became specialised. At first it
is quite possible that more than one pronephric abdominal
funnel may have served for the entrance of the generative
products ; this function being, no doubt, eventually restricted to
one of them.
 
Three different types of development of the abdominal
opening of the Mullerian duct have been observed.
 
In Amphibia (Salamandra) the permanent opening of the
Mullerian duct is formed independently, some way behind the
pronephros.
 
In Elasmobranchii the original opening of the segmental
duct forms the permanent opening of the Mullerian duct, and no
true pronephros appears to be formed.
 
In Birds the anterior of the three openings of the rudimentary
pronephros remains as the permanent opening of the Mullerian
duct.
 
These three modes of development very probably represent
specialisations of the primitive state along three different lines.
In Amphibia the specialisation of the opening appears to have
gone so far that it no longer has any relation to the pronephros.
It was probably originally one of the posterior openings of this
gland.
 
In Elasmobranchii, on the other hand, the functional opening
is formed at a period when we should expect the pronephros to
develop. This state is very possibly the result of a differentiation by which the pronephros gradually ceased to become
developed, but one of its peritoneal openings remained as the
abdominal aperture of the Mullerian duct. Aves, finally, appear
to have become differentiated along a third line ; since in their
ancestors the anterior (?) pore of the head-kidney appears to
have become specialised as the permanent opening of the
Mullerian duct.
 
The Mullerian duct is usually formed in a more or less complete manner in both sexes. In Ganoids, where the separation
between it and the Wolffian duct is not completed to the cloaca,
and in the Dipnoi, it probably serves to carry off the generative
products of both sexes. In other cases however only the female
 
 
 
EXCRETORY ORGANS.
 
 
 
735
 
 
 
products pass out by it, and the partial or complete formation
of the Mullerian duct in the male in these cases needs to be
explained. This may be done either by supposing the Ganoid
arrangement to have been the primitive one in the ancestors of
the other forms, or, by supposing characters acquired primitively
by the female to have become inherited by both sexes.
 
It is a question whether the nature of the generative ducts of
Teleostei can be explained by comparison with those of Ganoids.
The fact that the Mullerian ducts of the Teleostean Ganoid
Lepidosteus attach themselves to the generative organs, and thus
acquire a resemblance to the generative ducts of Teleostei,
affords a powerful argument in favour of the view that the
generative ducts of both sexes in the Teleostei are modified
Mullerian ducts. Embryology can however alone definitely
settle this question.
 
In the Elasmobranchii, Amphibia, and Amniota the male
products are carried off by the Wolffian duct, and they are
transported to this duct, not by open peritoneal funnels of the
mesonephros, but by a network of ducts which sprout either
from a certain number of the Malpighian bodies opposite the
testis (Amphibia, Amniota), or from the stalks connecting the
Malpighian bodies with the open funnels (Elasmobranchii).
After traversing this network the semen passes (except in
certain Anura) through a variable number of the segmental
tubes directly to the Wolffian duct. The extent of the connection of the testis with the Wolffian body is subject to great
variations, but it is usually more or less in the anterior region.
Rudiments of the testicular network have in many cases become
inherited by the female.
 
The origin of the connection between the testis and Wolffian body is still
very obscure. It would be easy to understand how the testicular products,
after falling into the body-cavity, might be taken up by the open extremities
of some of the peritoneal funnels, and how such open funnels might have
groove-like prolongations along the mesorchium, which might eventually be
converted into ducts. Ontogeny does not however altogether favour this
view of the origin of the testicular network. It seems to me nevertheless the
most probable view which has yet been put forward.
 
The mode of transportation of the semen by means of the mesonephric
tubules is so peculiar as to render it highly improbable that it was twice
acquired, it becomes therefore necessary to suppose that the Amphibia and
 
 
 
736 GENERAL CONCLUSIONS.
 
Amniota inherited this mode of transportation of the semen from the same
ancestors as the Elasmobranchii. It is remarkable therefore that in the
Ganoidei and Dipnoi this arrangement is not found.
 
Either (i) the arrangement (found in the Ganoidei and Dipnoi) of the
Miillerian duct serving for both sexes is the primitive arrangement, and the
Elasmobranch is secondary, or (2) the Ganoid arrangement is a secondary
condition, which has originated at a stage in the evolution of the Vertebrata
when some of the segmental tubes had begun to serve as the efferent ducts
of the testis, and has resulted in consequence of a degeneration of the latter
structures. Although the second alternative is the more easy to reconcile
with the affinities of the Ganoid and Elasmobranch types, as indicated by
the other features of their organization, I am still inclined to accept the
former ; and consider that the incomplete splitting of the segmental duct in
Ganoidei is a strong argument in favour of this view.
 
Metanephros. With the employment of the Wolffian duct
to transport the semen there seems to be correlated (i) a
tendency of the posterior segmental tubes to have a duct of
their own, in which the seminal and urinary fluids cannot become
mixed, and (2) a tendency on the part of the anterior segmental
tubes to lose their excretory function. The posterior segmental
tubes, when connected in this way with a more or less specialised
duct, have been regarded in the preceding pages as constituting
a metanephros.
 
This differentiation is hardly marked in the Anura, but is
well developed in the Urodela and in the Elasmobranchii ; and
in the latter group has become inherited by both sexes. In the
Amniota it culminates, according to the view independently
arrived at by Semper and myself, (i) in the formation of a
completely distinct metanephros in both sexes, formed however,
as shewn by Sedgwick, from the same blastema as the Wolffian
body, and (2) in the atrophy in the adult of the whole Wolffian
body, except the part uniting the testis and the Wolffian duct.
 
The homology between the posterior metanephridian section of the
Wolffian body, in Elasmobranchii and Urodela, and the kidney of the
Amniota, is only in my opinion a general one, i.e. in both cases a common
cause, viz. the Wolffian duct acting as vas deferens, has resulted in a more
or less similar differentiation of parts.
 
Fiirbringer has urged against Semper's and my view that no satisfactory proof of it has yet been offered. This proof has however, since
Fiirbringer wrote his paper, been supplied by Sedgwick's observations.
The development of the kidney in the Amniota is no doubt a direct as
opposed to a phylogenetic development ; and the substitution of a direct for
 
 
 
EXCRETORY ORGANS. 737
 
 
 
a phylogenetic development has most probably been rendered possible by
the fact that the anterior part of the mesonephros continued all the while
to be unaffected and to remain as the main excretory organ during foetal
life.
 
The most serious difficulty urged by Fiirbringer against the homology is
the fact that the ureter of the metanephros develops on a type of its own,
which is quite distinct from the mode of development of the ureters of the
metanephros of the Ichthyopsidan forms. It is however quite possible, though
far from certain, that the ureter of Amniota may be a special formation
confined to that group, and this fact would in no wise militate against the
homology I have been attempting to establish.
 
Comparison of the Excretory organs of the Chordata and
Invertebrata.
 
The structural characters and development of the various forms of
excretory organs described in the preceding pages do not appear to me to
be sufficiently distinctive to render it possible to establish homologies
between these organs on a satisfactory basis, except in closely related
groups.
 
The excretory organs of the Platyelminthes are in many respects similar
to the provisional excretory organ of the trochosphere of Polygordius
and the Gephyrea on the one hand, and to the Vertebrate pronephros
on the other ; and the Platyelminth excretory organ with an anterior
opening might be regarded as having given origin to the trochosphere organ,
while that with a posterior opening may have done so for the Vertebrate
pronephros 1 .
 
Hatschek has compared the provisional trochosphere excretory organ of
Polygordius to the Vertebrate pronephros, and the posterior Chastopod
segmental tubes to the mesonephric tubes ; the latter homology having
been already suggested independently by both Semper and myself. With
reference to the comparison of the pronephros with the provisional excretory
organ of Polygordius there are two serious difficulties :
 
(1) The pronephric (segmental) duct opens directly into the cloaca,
while the duct of the provisional trochosphere excretory organ opens anteriorly, and directly to the exterior.
 
(2) The pronephros is situated within the segmented region of the
trunk, and has a more or less distinct metameric arrangement of its parts ;
while the provisional trochosphere organ is placed in front of the segmented
region of the trunk, and is in no way segmented.
 
The comparison of the mesonephric tubules with the segmental excretory organs of the Chaetopoda, though not impossible, cannot be satisfactorily admitted till some light has been thrown upon the loss of the supposed
external openings of the tubes, and the origin of their secondary connection
with the segmental duct.
 
1 This suggestion has I believe been made by Fiirbringer.
B. III. 47
 
 
 
738 BIBLIOGRAPHY.
 
 
 
Confining our attention to the Invertebrata it appears to me fairly clear
that Hatschek is justified in holding the provisional trochosphere excretory
organs of Polygordius, Echiurus and the Mollusca to be homologous. The
atrophy of all these larval organs may perhaps be due to the presence of a
well-developed trunk region in the adult (absent in the larva), in which
excretory organs, probably serially homologous with those present in the
anterior part of the larva, became developed. The excretory organs in the
trunk were probably more conveniently situated than those in the head,
and the atrophy of the latter in the adult state was therefore brought about,
while the trunk organs became sufficiently enlarged to serve as the sole
excretory organs.
 
BIBLIOGRAPHY OF THE EXCRETORY ORGANS.
Invertebrata.
 
(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool.
Stat. z. Neapel, Vol. I. 1879.
 
(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Trematodes et d.
Cesto'ides." Archives de Biologic, Vol. I. 1880.
 
(514) B. Hatschek. "Studien lib. Entwick. d. Anneliden." Arbeit, a. d.
zool. Instit. Wien, Vol. I. 1878.
 
(515) B. Hatschek. "Ueber Entwick. von Echiurus," etc. Arbeit, a. d.
zool. Instit. Wien, Vol. in. 1880.
 
EXCRETORY ORGANS OF VERTEBRATA.
General.
 
(516) F. M. Balfour. "On the origin and history of the urinogenital organs of
Vertebrates." yournal of Anat. and Phys., Vol. X. 1876.
 
(517) Max. Furbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.
 
(518) H. Meek el. Zur Morphol. d. Hani- u. Geschlechtnverkz.d. Wirbelthiere,
etc. Halle, 1848.
 
(519) Joh. Miiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.
 
(520) H. Rathke. " Beobachtungen u. Betrachtungen u. d. Entwicklung d.
Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in
Dantzig, Bd. I. 1825.
 
(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. iibrigen Wirbelthiere." Arb. a. d. zool.-zoot. Instit. Wurzburg, Vol. II.
1875
(522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.
 
 
 
1 The papers of Furbringer, Semper and Waldeyer contain full references to the
literature of the Vertebrate excretory organs.
 
 
 
BIBLIOGRAPHY. 739
 
 
 
ElasmobrancJdi.
 
(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anat.,
Vol. XI. 1875.
 
Vide also Semper (No. 521) and Balfour (No. 292).
 
Cyclostomata.
 
(524) J. Miiller. " Untersuchungen ii. d. Eingeweide d. Fische." Abh. d. k.
Ak. Wiss. Berlin, 1845.
 
(525) W. Miiller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa."
Jenaische Zeitschrift, Vol. VII. 1873.
 
(526) W. Miiller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschri/t, Vol. IX. 1875.
 
(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.
 
(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol.
Jahrbuch, Vol. vn. 1881.
 
Teleostei.
 
(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d.
k. k. Akad. Wiss. Wien, Vol. n. 1850.
 
(530) A. Rosenberg. Untersuchungen iib. die Entivicklung d. Teleostierniere.
Dorpat, 1867.
 
Vide also Oellacher (No. 72).
 
Amphibia.
 
(531) F. H. Bidder. Vergleichend-anatomische u. histologische Untersitchungen
ii. die mdnnlichen Geschleehts- und Harnwerkzeuge d. nackten Amphibien. Dorpat,
1846.
 
(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des
Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.
 
(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.
 
(534) F. Leydig. Anatomie d. Amphibien u. Reptilien. Berlin, 1853.
 
(535) F. Leydig. Lehrbuch d. Hisiologie. Hamm, 1857.
 
(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien."
Sitz. d. naturfor. Gesellsch. Leipzig, 1875.
 
(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d.
zool.- zoot. Instil. Wiirzburg. Vol. III. 1876.
 
(538) VonWittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit.
f. wiss. Zool., Vol. IV.
 
Vide also Gotte (No. 296).
 
Amniota.
 
(539) F. M. Balfour and A. Sedgwick. "On the existence of a head -kidney
in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. xix. 1878.
 
(540 ) Banks. On the Wolffian bodies of the fatus and their remains in the adult.
Edinburgh, 1864.
 
472
 
 
 
74O BIBLIOGRAPHY.
 
 
 
(541) Th. Bornhaupt. Untersuchungen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.
 
(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. iv. 1877.
 
(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblatter u. d.
WolfFschen Ganges im Hiihnerei." Mini. Acad. Imp. Petersbourg, vn. Series, Vol.
xxvil. 1880.
 
(544) Th. Egli. Beitrage zur Anat. und Entwick. d. Geschlechtsorgane. Inaug.
Diss. Zurich, 1876.
 
(545) E. Gasser. Beitrage zur Entwicklungsgeschichte d. Allantois, der
Milllcr'schen Gange u. des Afters. Frankfurt, 1874.
 
(546) E. Gasser. "Beob. iib. d. Entstehung d. Wolff schen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.
 
(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Hiihnerembryonen." Sitz. d. GeseU. zur Befdrderung d. gesam. Naturwiss. Marburg, 1879.
 
(548) C. Kupffer. " Untersuchting iiber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.
 
(549) A. Sedgwick. "Development of the kidney in its relation to the
Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. xx. 1880.
 
(550) A. Sedgwick. "On the development of the structure known as the
glomerulus of the head-kidney in the Chick." Quart. J. of Micros. Science, Vol. xx.
1880.
 
(551) A. Sedgwick. "Early development of the Wolffian duct and anterior
Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory
system." Quart. J. of Micros. Science, Vol. xxi. 1881.
 
(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. xiv. 1879.
 
(553) E. H. Weber. Zusdtze z. Lehre von Baue u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.
 
Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297),
Kolliker (No. 298).
 
 
 
CHAPTER XXIV.
GENERATIVE ORGANS AND GENITAL DUCTS.
 
GENERATIVE ORGANS.
 
THE structure and growth of the ovum and spermatozoon
were given in the first chapter of this work, but their derivation
from the germinal layers was not touched on, and it is this
subject with which we are here concerned. If there are any
structures whose identity throughout the Metazoa is not open
to doubt these structures are the ovum and spermatozoon ;
and the constancy of their relations to the germinal layers
would seem to be a crucial test as to whether the latter have
the morphological importance usually attributed to them.
 
The very fragmentary state of our knowledge of the origin of
the generative cells has however prevented this test being so far
very generally applied.
 
Porifera. In the Porifera the researches of Schulze have
clearly demonstrated that both the ova and the spermatozoa
take their origin from indifferent cells of the general parenchyma, which may be called mesoblastic. The primitive germinal cells of the two sexes are not distinguishable ; but a
germinal cell by enlarging and becoming spherical gives rise
to an ovum ; and by subdivision forms a sperm-morula, from
the constituent cells of which the spermatozoa are directly
developed.
 
Ccelenterata. The greatest confusion prevails as to the
germinal layer from which the male and female products are
derived in the Ccelenterata 1 .
 
1 E. van Beneden (No. 556) was the first to discover a different origin for the
generative products of the two sexes in Hydractinia, and his observations have led to
numerous subsequent researches on the subject. For a summary of the observations
on the Hydroids vide Weismann (No. 560).
 
 
 
742 CCELENTERATA.
 
 
 
The following apparent modes of origin of these products
have been observed.
 
(1) The generative products of both sexes originate in the
ectoderm (epiblast) : Hydra, Cordylophora, Tubularia, all (?) free
Gonophores of Hydromedusae, the Siphonophora, and probably
the Ctenophora.
 
(2) The generative products of both sexes originate in the
entoderm (hypoblast) : Plumularia and Sertularella, amongst
the Hydroids, and the. whole of the Acraspeda and Actinozoa.
 
(3) The male cells are formed in the ectoderm, and the
female in the entoderm : Gonothyraea, Campanularia, Hydractinia, Clava.
 
In view of the somewhat surprising results to which the
researches on the origin of the genital products amongst the
Ccelenterata have led, it would seem to be necessary either to
hold that there is no definite homology between the germinal
layers in the different forms of Ccelenterata, or to offer some
satisfactory explanation of the behaviour of the genital products, which would not involve the acceptance of the first
alternative.
 
Though it can hardly be said that such an explanation has
yet been offered, some observations of Kleinenberg (No. 557)
undoubtedly point to such an explanation being possible.
 
Kleinenberg has shewn that in Eudendrium the ova migrate
freely from the ectoderm into the endoderm, and vice versa ; but
he has given strong grounds for thinking that they originate in
the ectoderm. He has further shewn that the migration in this
type is by no means an isolated phenomenon.
 
Since it is usually only possible to recognise generative
elements after they have advanced considerably in development,
the mere position of a generative cell, when first observed, can
afford, after what Kleinenberg has shewn, no absolute proof
of its origin. Thus it is quite possible that there is really
only one type of origin for the generative cells in the Ccelenterata.
 
Kleinenberg has given reasons for thinking that the migration of the ova
into the entoderm may have a nutritive object. If this be so, and there are
numerous facts which shew that the position of generative cells is often
largely influenced by their nutritive requirements, it seems not impossible
 
 
 
GENERATIVE ORGANS. 743
 
that the endodermal position of the generative organs in the Actinozoa and
acraspedote Medusre may have arisen by a continuously earlier migration of
the generative cells from the ectoderm into the endoderm ; and that the
migration may now take place at so early a period of the development, that
we should be justified in formally holding the generative products to be
endodermal in origin.
 
\Ve might perhaps, on this view, formulate the origin of the generative
products in the Ccelenterata in the following way :
 
Both ova and spermatozoa primitively originated in the ectoderm, but in
order to secure a more complete nutrition the cells which give rise to them
exhibit in certain groups a tendency to migrate into the endoderm. This
migration, which may concern the generative cells of one or of both the
sexes, takes place in some cases after the generative cells have become
recognisable as such, and very probably in other cases at so early a period
that it is impossible to distinguish the generative cells from indifferent
embryonic cells.
 
Very little is known with reference to the origin of the
generative cells in the triploblastic Invertebrata.
 
Chaetopoda and Gephyrea. In the Chaetopoda and
Gephyrea, the germinal cells are always developed in the adult
from the epithelial lining of the body cavity ; so that their origin
from the mesoblast seems fairly established.
 
If we are justified in holding the body cavity of these forms
to be a derivative of the primitive archenteron (vide pp. 356 and
357) the generative cells may fairly be held to originate from a
layer which corresponds to the endoderm of the Ccelenterata 1 .
 
Chaetognatha. In Sagitta the history of the generative
cells, which was first worked out by Kowalevsky and Biitschli,
has been recently treated with great detail by O. Hertwig 2 .
 
The generative cells appear during the gastrula stage, as two
large cells with conspicuous nuclei, which are placed in the
hypoblast lining the archenteron, at the pole opposite the
blastopore. These cells soon divide, and at the same time pass
out of the hypoblast, and enter the archenteric cavity (fig. 408
- A, ge). The division into four cells, which is not satisfactorily
represented ifl my diagram, takes place in such a way that two
 
1 The Hertwigs (No. 271) state that in their opinion the generative cells arise
from the lining of the body cavity in all the forms whose body cavity is a product of
the archenteron. We do not know anything of the embryonic development of the
generative organs in the Echinodermata, but the adult position of the generative
organs in this group is very unfavourable to the Hertwigs' view.
 
2 O. Hertwig, Die Chcetognathen. Jena, 1880
 
 
744
 
 
 
CH^ETOGNATHA.
 
 
 
cells are placed nearer the median line, and two externally. The
two inner cells form the eventual testes, and the outer the
 
 
 
 
FIG. 408. THREK STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after
 
Biitschli, and B after Kowalevsky.)
The three embryos are represented in the same positions.
 
A. Represents the gastrula stage.
 
B. Represents a succeeding stage, in which the primitive archenteron is commencing to be divided into three.
 
C. Represents a later stage, in which the mouth involution (in) has become continuous with the alimentary tract, and the blastopore is closed.
 
///. mouth ; al. alimentary canal ; ac. archenteron ; bl.p. blastopore ; pv. perivisceral cavity ; sp, splanchnic mesoblast ; so. somatic mesoblast ; ge. generative
organs.
 
ovaries, one half of each primitive cell thus forming an ovary, and
the other a testis.
 
 
 
 
FIG. 409. Two VIEWS OF A LATE EMBRYO OF SAGITTA. A, from the dorsal
 
surface. B, from the side. (After Biitschli.)
 
m. mouth ; al. alimentary canal ; v.g. ventral ganglion (thickening of epiblast) ;
<.'/. epiblast ; c.pv. cephalic section of body cavity ; so. somatopleure ; sp. splanchnopleure ; ge. generative organs.
 
 
 
GENERATIVE ORGANS.
 
 
 
745
 
 
 
When the archenteric cavity is divided into a median
alimentary tract, and two lateral sections forming the body
cavity, the generative organs are placed in the common vestibule
into which both the body cavity and alimentary cavity at first
open (fig. 408).
 
The generative organs long retain their character as simple
cells. Eventually (fig. 409) the two ovaries travel forwards, and
apply themselves to the body walls, while the two testes also
become separated by a backward prolongation of the median
alimentary tract.
 
On the formation of the transverse septum dividing the tail
from the body, the ovarian cells lie immediately in front of this
septum, and the testicular cells in the region behind it.
 
Polyzoa. In Pedicellina amongst the entoproctous Polyzoa
Hatschek finds that the generative organs originate from a pair
of specially large mesoblast cells, situated in the space between
the stomach and the floor of the vestibule. The two cells
undergo changes, which have an obvious resemblance to those of
the generative cells of the Chsetognatha. They become surrounded by an investment of mesoblast cells, and divide so as to
form two masses. Each of these masses at a later period
separates into an anterior and a posterior part. The former
becomes the ovary, the latter the testis.
 
Nematoda. In the Nematoda the generative organs are
derived from the division of a single cell which would appear to
be mesoblastic 1 .
 
Insecta. The generative cells have been observed at a very
early embryonic stage in several insect forms (Vol. II. p. 404), but
the observations so far recorded with reference to them do not
enable us to determine with certainty from which of the germinal
layers they are derived.
 
Crustacea. In Moina, one of the Cladocera, Grobben 2 has
shewn that the generative organs are derived from a single cell,
which becomes differentiated during the segmentation. This
cell, which is in close contiguity with the cells from which both
the mesoblast and hypoblast originate, subsequently divides ;
 
1 Fide Vol. n. p. 374; also Gotte, Zool. Anzeiger, No. 80, p. 189.
 
2 C. Grobben. "Die Entwick. d. Moina rectirostris." Arbeit, a. d. zool. Instil.
Wien. Vol. II. 1879.
 
 
 
746
 
 
 
CHORDATA.
 
 
 
sp.c
 
 
 
but at the gastrula stage, and after the mesoblast has become
formed, the cells it gives rise to are enclosed in the epiblast, and
do not migrate inwards till a later stage. The products of the
division of the generative cell subsequently divide into two
masses. It is not possible to assign the generative cell of Moina
to a definite germinal layer. Grobben, however, thinks that it
originates from the division of a cell, the remainder of which
gives rise to the hypoblast.
 
Chordata. In the Vertebrata, the primitive generative cells
(often known as primitive ova) are early distinguishable, being
imbedded amongst the cells of two linear streaks of peritoneal
epithelium, placed on the dorsal side of the body cavity, one on
each side of the mesentery (figs. 405
C and 4io,/0). They appear to be
derived from the epithelial cells
amongst which they lie ; and are
characterized by containing a large
granular nucleus, surrounded by a
considerable body of protoplasm.
The peritoneal epithelium in which
they are placed is known as the
germinal epithelium.
 
It is at first impossible to distinguish the germinal cells which will
become ova from those which will
become spermatozoa.
 
The former however remain within the peritoneal epithelium (fig. 41 1),
and become converted into ova in a
manner more particularly described
in Vol. II. pp. 54 59.
 
The history of the primitive
germinal cells in the male has not
been so adequately worked out as in
the female.
 
The fullest history of them is
that given by Semper (No. 559) for
the Elasmobranchii, the general accuracy of which I can fully support ;
 
 
 
 
FIG. 410. SECTION THROUGH
THE TRUNK OF A SCYLLIUM
EMBRYO SLIGHTLY YOUNGER
 
THAN 28 F.
 
sp.c. spinal cord ; W. white
matter of spinal cord ; pr. posterior nerve-roots ; ch. notochord ;
x. sub-notochordal rod ; ao. aorta ;
mp. muscle-plate ; mp'. inner layer
of- muscle-plate already converted
into muscles ; Vr. rudiment of
vertebral body ; st. segmental
tube; sd. segmental duct; sp.v.
spiral valve ; v. subintestinal vein ;
i>.o. primitive generative cells.
 
 
 
GENERATIVE ORGANS.
 
 
 
747
 
 
 
though with reference to certain stages in the history further
researches are still required 1 .
 
In Elasmobranchii the male germinal cells, instead of remaining in the germinal epithelium, migrate into the adjacent stroma,
accompanied I believe by some of the indifferent epithelial cells.
Here they increase in number, and give rise to masses of variable
form, composed partly of true germinal cells, and partly of
smaller cells with deeply staining nuclei, which are, I believe,
derived from the germinal epithelium.
 
 
 
 
FIG. 411. TRANSVERSE SECTION THROUGH THE OVARY OF A YOUNG EMBRYO
OK SCYLLIUM CANICULA, TO SHEW THE PRIMITIVE GERMINAL CELLS (po) LYING
IN THE GERMINAL EPITHELIUM ON THE OUTER SIDE OF THE OVARIAN RIDGE.
 
These masses next break up into ampullae, mainly formed of
germinal cells, and each provided with a central lumen ; and
these ampullae attach themselves to tubes derived from the
smaller cells, which are in their turn continuous with the
testicular network. The spermatozoa are developed from the
cells forming the walls of the primitive ampulla;; but the
process of their formation does not concern us in this chapter.
 
In the Reptilia Braun has traced the passage of the primitive
germinal cells into the testicular tubes, and I am able to confirm
his observations on this point : he has not however traced their
further history.
 
1 Balbiani (No. 554) has also recently dealt with this subject, but I cannot bring
my own observations into accord with his as to the structure of the Elasmobranch
testis.
 
 
 
MODE OF EXIT OF GENITAL PRODUCTS.
 
 
 
In Mammalia the evidence of the origin of the spermatospores from the germinal epithelium is not quite complete, but
there can be but little doubt of its occurrence 1 .
 
In Amphioxus Langerhans has shewn that the ova and
spermatozoa are derived from similar germinal cells, which may
be compared to the germinal epithelium of the Vertebrata.
These cells are however segmentally arranged as separate
masses (vide Vol. II. p. 54).
 
BIBLIOGRAPHY.
 
(554) G. Balbiani. Lemons s. la generation des Vcrlebrcs. Paris, 1879.
 
(555) F. M. Balfour. "On the structure and development of the Vertebrate
ovary." Quart, J. of Micr. Science, Vol. xvm.
 
(556) E. van Beneden. "De la distinction originelle dutecticule et clel'ovaire,
etc." Bull. Ac. roy. belgique, Vol. xxxvil. 1874.
 
(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrium." Zcit.
f. -wiss. Zool., Vol. xxxv. 1881.
 
(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche." Arbeit, a. d. zool.zoot. Inslit. Wilrzburg, Vol. I. 1874.
 
(559) C. Semper. "Das Urogenilalsystem d. Plagiostomen, etc." Arbeit, a.
d. zooL-zoot. Ins tit. Witrzbiirg, Vol. II. 1875.
 
(560) A. Weismann. "Zur Frage nach dem Ursprung d. Geschlechtszellen bei
den Hydroiden." Zool. Anzeiger, No. 55, 1880.
 
Fitffcalso O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.
 
GENITAL DUCTS.
 
The development and evolution of the generative ducts is as
yet very incompletely worked out, but even in the light of our
present knowledge a comparative review of this subject brings to
light features of considerable interest, and displays a fruitful
field for future research.
 
In the Ccelenterata there are no generative ducts.
 
In the Hydromedusae and Siphonophora the generative
products are liberated by being dehisced directly into the
surrounding medium ; while in the Acraspeda, the Actinozoa
and the Ctenophora, they are dehisced into parts of the gastrovascular system, and carried to the exterior through the mouth.
 
The arrangement in the latter forms indicates the origin of
 
1 An entirely different view of the origin of the sperm cells has been adopted by
Balbiani, for which the reader is referred to his Memoir (No. 554).
 
 
 
GENITAL DUCTS.
 
 
 
749
 
 
 
the methods of transportation of the genital products to the
exterior in many of the higher types.
 
It has been already pointed out that the body cavity in a
very large number of forms is probably derived from parts of a
gastrovascular system like that of the Actinozoa.
 
When the part of the gastrovascular system into which the
generative products were dehisced became, on giving rise to the
body cavity, shut off from the exterior, it would be essential that
some mode of transportation outwards of the generative products
should be constituted.
 
In some instances simple pores (probably already existing at
the time of the establishment of a closed body cavity) become
the generative ducts. Such seems probably to have been the
case in the Chaetognatha (Sagitta) and in the primitive
Chordata.
 
In the latter forms the generative products are sometimes dehisced into
the peritoneal cavity, and thence transported by the abdominal pores to the
exterior (Cyclostomata and some Teleostei, vide p. 626). In Amphioxus
they pass by dehiscence into the atrial cavity, and thence through the gill
slits and by the mouth, or by the abdominal pore (?) to the exterior. The
arrangement in Amphioxus and the Teleostei is probably secondary, as
possibly also is that in the Cyclostomata ; so that the primitive mode of
exit of the generative products in the Chordata is still uncertain. It is
highly improbable that the generative ducts of the Tunicata are primitive
structures.
 
A better established and more frequent mode of exit of the
generative products when dehisced into the body cavity is by
means of the excretory organs. The generative products pass
from the body cavity into the open peritoneal funnels of such
organs, and thence through their ducts to the exterior. This
mode of exit of the generative products is characteristic of the
Chaetopoda, the Gephyrea, the Brachiopoda and the Vertebrata,
and probably also of the Mollusca. It is moreover quite possible
that it occurs in the Polyzoa, some of the Arthropoda, the
Platyelminthes and some other types.
 
The simple segmental excretory organs of the Polychaeta,
the Gephyrea and the Brachiopoda serve as generative canals,
and in many instances they exhibit no modification, or but a
very slight one, in connection with their secondary generative
 
 
 
750 DERIVATION FROM EXCRETORY ORGANS.
 
function ; while in other instances, e.g. Bonellia, such modification is very considerable.
 
The generative ducts of the Oligochaeta are probably derived from
excretory organs. In the Terricola ordinary excretory organs are present in
the generative segments in addition to the generative ducts, while in the
Limicola generative ducts alone are present in the adult, but before their
development excretory organs of the usual type are found, which undergo
atrophy on the appearance of the generative ducts (Vedjovsky).
 
From the analogy of the splitting of the segmental duct of the Vertebrata
into the Miillerian and Wolffian ducts, as a result of a combined generative
and excretory function (vide p. 728), it seems probable that in the generative segments of the Oligochasta the excretory organs had at first both an
excretory and a generative function, and that, as a secondary result of this
double function, each of them has become split into two parts, a generative
and an excretory. The generative part has undergone in all forms great
modifications. The excretory parts remain unmodified in the Earthworms
(Terricola), but completely abort on the development of the generative ducts
in the Limicola. An explanation may probably be given of the peculiar
arrangements of the generative ducts in Saccocirrus amongst the Polychaeta (vide Marion and Bobretzky), analogous to that just offered for the
Oligochaeta.
 
The very interesting modifications produced in the excretory
organs of the Vertebrata by their serving as generative ducts
were fully described in the last chapter ; and with reference to
this part of our subject it is only necessary to call attention to
the case of Lepidosteus and the Teleostei.
 
In Lepidosteus the Mullerian duct appears to have become
attached to the generative organs, so that the generative
products, instead of falling directly into the body cavity and
thence entering the open end of a peritoneal funnel of the
excretory organs, pass directly into the Mullerian duct without
entering the body cavity. In most Teleostei the modification is
more complete, in that the generative ducts in the adult have no
obvious connection with the excretory organs.
 
The transportation of the male products to the exterior in all
the higher Vertebrata, without passing into the body cavity, is
in principle similar to the arrangement in Lepidosteus.
 
The above instances of the peritoneal funnels of an excretory
organ becoming continuous with the generative glands, render it
highly probable that there may be similar instances amongst the
In vertebrata.
 
 
 
GENITAL DUCTS.
 
 
 
751
 
 
 
As has been already pointed out by Gegenbaur there are
many features in the structure of the genital ducts in the more
primitive Mollusca, which point to their having been derived
from the excretory organs. In several Lamellibranchiata 1
(Spondylus, Lima, Pecten) the generative ducts open into the
excretory organs (organ of Bojanus), so that the generative
products have to pass through the excretory organ on their way
to the exterior. In other Lamellibranchiata the genital and
excretory organs open on a common papilla, and in the remaining types they are placed close together.
 
In the Cephalopoda again the peculiar relations of the
generative organs to their ducts point to the latter having
primitively had a different, probably an excretory, function.
The glands are not continuous with the ducts, but are placed in
special capsules from which the ducts proceed. The genital
products are dehisced into these capsules and thence pass into
the ducts.
 
In the Gasteropoda the genital gland is directly continuous
with its duct, and the latter, especially in the Pulmonata and
Opisthobranchiata, assumes such a complicated form that its
origin from the excretory organ would hardly have been
suspected. The fact however that its opening is placed near
that of the excretory organ points to its being homologous with
the generative ducts of the more primitive types.
 
In the Discophora, where the generative ducts are continuous
with the glands, the structure both of the generative glands and
ducts points to the latter having originated from excretory
organs.
 
It seems, as already mentioned, very possible that there are
other types in which the generative ducts are derived from the
excretory organs. In the Arthropoda for instance the generative
ducts, where provided with anteriorly placed openings, as in the
Crustacea, Arachnida and the Chilognathous Myriapoda, the
Pcecilopoda, etc., may possibly be of this nature, but the data
for deciding this point are so scanty that it is not at present
possible to do more than frame conjectures.
 
The ontogeny of the generative ducts of the Nematoda and
 
1 For a summary of the facts on this subject vide Bronn, Klassen u. Ordnungen d.
Thierreichs, Vol. in. p. 404.
 
 
 
752 DERIVATION FROM EXCRETORY ORGANS.
 
the Insecta appears to point to their having originated independently of the excretory organs.
 
In the Nematoda the generative organs of both sexes
originate from a single cell (Schneider, Vol. I. No. 390).
 
This cell elongates and its nuclei multiply. After assuming
a somewhat columnar form, it divides into (i) a superficial
investing layer, and (2) an axial portion.
 
In the female the superficial layer is only developed distinctly
in the median part of the column. In the course of the further
development the two ends of the column become the blind ends
of the ovary, and the axial tissue they contain forms the
germinal tissue of nucleated protoplasm. The superficial layer
gives rise to the epithelium of the uterus and oviduct. The
germinal tissue, which is originally continuous, is interrupted in
the middle part (where the superficial layer gives rise to the
uterus and oviduct), and is confined to the two blind extremities
of the tube.
 
In the male the superficial layer, which gives rise tc the
epithelium of the vas deferens, is only formed at the hinder ond
of the original column. In other respects the development takes
place as in the female.
 
In the Insecta again the evidence, though somewhat conflicting,
indicates that the generative ducts arise very much as in Nematodes, from the same primitive mass as the generative organs. In
both of these types it would seem probable that the generative
organs were primitively placed in the body cavity, and attached
to the epidermis, through a pore in which their products passed
out ; and that, acquiring a tubular form, the peripheral part of
the gland gave rise to a duct, the remainder constituting the true
generative gland. It is quite possible that the generative ducts
of such forms as the Platyelminthes may have had a similar
origin to those in Insecta and Nematoda, but from the analogy
of the Mollusca there is nearly as much to be said for regarding
them as modified excretory organs.
 
In the Echinodermata nothing is unfortunately known as to
the ontogeny of the generative organs and ducts. The structure
of these organs in the adult would however seem to indicate that
the most primitive type of echinoderm generative organ consists
of a blind sack, projecting into the body cavity, and opening by
 
 
 
GENITAL DUCTS. 753
 
 
 
a pore to the exterior. The sack is lined by an epithelium,
continuous with the epidermis, the cells of which give rise to the
ova or spermatozoa. The duct of these organs is obviously
hardly differentiated from the gland ; and the whole structure
might easily be derived from the type of generative organ
characteristic of the Hydromedusae, where the generative cells
are developed from special areas of the ectoderm, and, when ripe,
pass directly into the surrounding medium.
 
If this suggestion is correct we may suppose that the generative ducts of the Echinodermata have a different origin to those
of the majority of 1 the remaining triploblastica.
 
Their ducts have been evolved in forms in which the
generative products continued to be liberated directly to the
exterior, as in the Hydromedusae ; while those of other types
have been evolved in forms in which the generative products
were first transported, as in the Actinozoa, into the gastrovascular
canals 2 .
 
1 It would be interesting to have further information about Balanoglossus.
 
2 These views fit in very well with those already put forward in Chapter xm. on
the affinities of the Echinodermata.
 
 
 
B. III.
 
 
 
48
 
 
 
CHAPTER XXV.
 
THE ALIMENTARY CANAL AND ITS APPENDAGES, IN
THE CHORDATA.
 
THE alimentary canal in the Chordata is always formed of
three sections, analogous to those so universally present in the
Invertebrata. These sections are (i) the mesenteron lined by
hypoblast ; (2) the stomodaeum or mouth lined by epiblast, and
(3) the proctodaeum or anal section lined like the stomodaeum by
epiblast.
 
Mesenteron.
 
The early development of the epithelial wall of the mesenteron
has already been described (Chapter XI.). It forms at first a
simple hypoblastic tube extending from near the front end of the
body, where it terminates blindly, to the hinder extremity where
it is united with the neural tube by the neurenteric canal (fig.
420, ne). It often remains for a long time widely open in the
middle towards the yolk-sack.
 
It has already been shewn that from the dorsal wall of the
mesenteron the notochord is separated off nearly at the same
time as the lateral plates of mesoblast (pp. 292 300).
 
The subnotochordal rod. At a period slightly subsequent
to the formation of the notochord, and before any important
differentiations in the mesenteron have become apparent, a
remarkable rod-like body, which was first discovered by Gotte,
becomes split off from the dorsal wall of the alimentary tract in
all the Ichthyopsida. This body, which has a purely provisional
existence, is known as the subnotochordal rod.
 
 
 
MESENTERON.
 
 
 
755
 
 
 
It develops in Elasmobranch embryos in two sections, one situated in
the head, and the other in the trunk.
 
The section in the trunk is the first to appear. The wall of the
alimentary canal becomes thickened along the median dorsal line (fig. 412,
r), or else produced into a ridge into which there penetrates a narrow
prolongation of the lumen of the alimentary canal. In either case the cells
at the extreme summit become gradually constricted off as a rod, which lies
immediately dorsal to the alimentary tract, and ventral to the notochord
(fig. 413, *).
 
 
 
 
 
FIG. 412. TRANSVERSE SECTION
THROUGH THE TAIL REGION OF A
PRISTIURUS EMBRYO OF THE SAME
AGE AS FIG. 28 E.
 
df. dorsal fin ; sp.c. spinal cord ;
//. body cavity ; sp. splanchnic layer
of mesoblast ; so. somatic layer of
mesoblast; mp'. portion of splanchnic
mesoblast commencing to be differentiated into muscles ; ch. notochord ; x.
subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract ; al. alimentary tract.
 
 
 
FIG. 413. TRANSVERSE SECTION THROUGH THE TRUNK OF AN
EMBRYO SLIGHTLY OLDER THAN
FIG. 28 E.
 
nc. neural canal ; pr. posterior
root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic
mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp'. portion of muscle-plate converted into
muscle ; Vv. portion of the vertebral
plate which will give rise to the vertebral bodies ; al. alimentary tract.
 
 
 
In the hindermost part of the body its mode of formation differs somewhat from that above described. In this part the alimentary wall is' very
thick, and undergoes no special growth prior to the formation of the subnotochordal rod ; on the contrary, a small linear portion of the wall becomes
scooped out along the median dorsal line, and eventually separates from the
remainder as the rod in question. In the trunk the splitting off of the rod
takes place from before backwards, so that the anterior part of it is formed
before the posterior.
 
The section of the subnotochordal rod in the head would appear to
develop in the same way as that in the trunk, and the splitting off from the
throat proceeds from before backwards.
 
482
 
 
 
756 MESENTERY.
 
 
 
On the formation of the dorsal aorta, the subnotochordal rod becomes
separated from the wall of the gut and the aorta interposed between the two
(fig. 367, *).
 
When the subnotochordal rod attains its fullest development it terminates
anteriorly some way in front of the auditory vesicle, though a little behind
the end of the notochord ; posteriorly it extends very nearly to the extremity
of the tail and is almost co-extensive with the postanal section of the
alimentary tract, though it does not reach quite so far back as the caudal
vesicle (fig. 424, b x). Very shortly after it has attained its maximum size it
begins to atrophy in front. We may therefore conclude that its atrophy,
like its development, takes place from before backwards. During the later
embryonic stages not a trace of it is to be seen. It has also been met with
in Acipenser, Lepidosteus, the Teleostei, Petromyzon, and the Amphibia, in
all of which it appears to develop in fundamentally the same way as in
Elasmobranchii. In Acipenser it appears to persist in the adult as the
subvertebral ligament (Bridge, Salensky). It has not yet been found in a
fully developed form in any amniotic Vertebrate, though a thickening of the
hypoblast, which may perhaps be a rudiment of it, has been found by
Marshall and myself in the Chick (fig. 1 10, x).
 
Eisig has instituted an interesting comparison between it and an organ
which he has found in a family of Chaetopods, the Capitellidas. In these
forms there is a tube underlying the alimentary tract for nearly its whole
length, and opening into it in front, and probably behind. A remnant of
such a tube might easily form a rudiment like the subnotochordal rod of the
Ichthyopsida, and as Eisig points out the prolongation into the latter during
its formation of the lumen of the alimentary tract distinctly favours such a
view of its original nature. We can however hardly suppose that there is
any direct genetic connection between Eisig's organ in the Capitellidas and
the subnotochordal rod of the Chordata.
 
 
 
Splanchnic mesoblast and mesentery- The mesentcron
consists at first of a simple hypoblastic tube, which however
becomes enveloped by a layer of splanchnic mesoblast. This
layer, which is not at first continued over the dorsal side of the
mesenteron, gradually grows in, and interposes itself between the
hypoblast of the mesenteron, and the organs above. At the same
time it becomes differentiated into two layers, viz. an outer
cpithelioid layer which gives rise to part of the peritoneal
epithelium, and an inner layer of undifferentiated cells which in
time becomes converted into the connective tissue and muscular
walls of the mesenteron. The connective tissue layers become
first formed, while of the muscular layers the circular is the first
to make its appearance.
 
 
 
ALIMENTARY CANAL. 757
 
Coincidently with their differentiation the connective tissuestratum of the peritoneum becomes established.
 
The Mesentery. Prior to the splanchnic mesoblast growing
round the alimentary tube above, the attachment of the latter
structure to the dorsal wall of the body is very wide. On the
completion of this investment the layer of mesoblast suspending
the alimentary tract becomes thinner, and at the same time the
alimentary canal appears to be drawn downwards and away from
the vertebral column.
 
In what may be regarded as the thoracic division of the general
pleuroperitoneal space, along that part of the alimentary canal
which will form the oesophagus, this withdrawal is very slight, but
it is very marked in the abdominal region. In the latter the at
first straight digestive canal comes to be suspended from the body
above by a narrow flattened band of mesoblastic tissue. This
flattened band is the mesentery, shewn commencing in fig. 117,
and much more advanced in fig. 1 19, M. It is covered on either
side by a layer of flat cells, which form part of the general
peritoneal epithelioid lining, while its interior is composed of
indifferent tissue.
 
The primitive simplicity in the arrangement of the mesentery
is usually afterwards replaced by a more complicated disposition,
owing to the subsequent elongation and consequent convolution
of the intestine and stomach.
 
The layer of peritoneal epithelium on the ventral side of the
stomach is continued over the liver, and after embracing the liver,
becomes attached to the ventral abdominal wall (fig. 380). Thus
in the region of the liver the body cavity is divided into two
halves by a membrane, the two sides of which are covered by the
peritoneal epithelium, and which encloses the stomach dorsally
and the liver ventrally. The part of the membrane between the
stomach and liver is narrow, and constitutes a kind of mesentery
suspending the liver from the stomach : it is known to human
anatomists as the lesser omentum.
 
The part of the membrane connecting the liver with the
anterior abdominal wall constitutes the fa lei form or suspensory ligament of the liver. It arises by a secondary fusion, and
is not a remnant of a primitive ventral mesentery (vide pp. 624
and 625).
 
 
 
758 MESENTERY.
 
 
 
The mesentery of the stomach, or mesogastrium, enlarges in
Mammalia to form a peculiar sack known as the greater
omentum.
 
The mesenteron exhibits very early a trifold division. An
anterior portion, extending as far as the stomach, becomes
separated off as the respiratory division. On the formation
of the anal invagination the portion of the mesenteron behind
the anus becomes marked off as the postanal division, and
between the postanal section and the respiratory division is a
middle portion forming an intestinal and cloacal division.
 
The respiratory division of the mesenteron.
 
This section of the alimentary canal is distinguished by the
fact that its walls send out a series of paired diverticula, which
meet the skin, and after a perforation has been effected at the
regions of contact, form the branchial or visceral clefts.
 
In Amphioxus the respiratory region extends close up to the
opening of the hepatic diverticulum, and therefore to a position
corresponding with the commencement of the intestine in higher
types. In the craniate Vertebrata the number of visceral clefts
has become reduced, but from the extension of the visceral clefts
in Amphioxus, combined with the fact that in the higher Vertebrata the vagus nerve, which is essentially the nerve of the
branchial pouches, supplies in addition the walls of the oesophagus
and stomach, it may reasonably be concluded, as has been pointed
out by Gegenbaur, that the true respiratory region primitively
included the region which in the higher types forms the
oesophagus and stomach.
 
In Ascidians the respiratory sack is homologous with the
respiratory tract of Amphioxus.
 
The details of the development of the branchial clefts in the
different groups of Vertebrata have already been described in
the systematic part of this work.
 
In all the Ichthyopsida the walls of a certain number of
clefts become folded ; and in the mesoblast within these folds a
rich capillary network, receiving its blood from the branchial
arteries, becomes established. These folds constitute the true
internal gills.
 
 
 
ALIMENTARY CANAL.
 
 
 
759
 
 
 
In addition to internal gills external branchial processes covered
by epiblast are placed on certain of the visceral arches in the
larva of Polypterus, Protopterus and many Amphibia. The
external gills have probably no genetic connection with the
internal gills.
 
The so-called external gills of the embryos of Elasmobranchii
are merely internal gills prolonged outwards through the gill
clefts.
 
The posterior part of the primitive respiratory division of the
mesenteron becomes, in all the higher Vertebrata, the oesophagus
and stomach. With reference to the development of these parts
the only point worth especially noting is the fact that in
Elasmobranchii and Teleostei their lumen, though present in
very young embryos, becomes at a later stage completely filled
up, and thus the alimentary tract in the regions of the
oesophagus and stomach becomes a solid cord of cells (fig. 23
A, ces)\ as already suggested (p. 61) it seems not impossible that
this feature may be connected with the fact that the cesophageal
region of the throat was at one time perforated by gill clefts.
 
In addition to the gills two important organs, viz. the
thyroid body and the lungs, take their origin from the respiratory region of the alimentary tract.
 
Thyroid body. In the Ascidians the origin of a groovelike diverticulum of the ventral wall of the branchial sack,
bounded by two lateral folds, and known as the endostyle or
hypopharyngeal groove, has already been described (p. 18).
This groove remains permanently open to the pharyngeal sack,
 
 
 
 
FIG. 414. DIAGRAMMATIC VERTICAL SECTION OF A JUST-HATCHED LARVA
 
OF PETROMYZON. (From Gegenbaur ; after Calberla.)
 
o. mouth ; 6. olfactory pit ; v. septum between stomodteum and mesenteron ;
h. thyroid involution ; n. spinal cord ; ch. notochord; c. heart ; a. auditory vesicle.
 
 
 
760
 
 
 
THE THYROID BODY.
 
 
 
 
and would seem to serve as a glandular organ secreting mucus.
As was first pointed out by W. Miiller there is present in
Amphioxus a very similar and probably homologous organ,
known as the hypopharyngeal groove.
 
In the higher Vertebrata this organ never retains its primitive condition in the adult state. In the larva of Petromyzon
there is, however, present a ventral groove-like diverticulum of
the throat, extending from about the second to the fourth
visceral cleft. This organ is shewn in longitudinal section in
fig. 414, h, and in transverse section in fig. 415, and has been
identified by W. Muller (Nos. 565 and 566) with the hypopharyngeal groove of Amphioxus and Ascidians. It does
not, however, long retain its
primitive condition, but its opening becomes gradually reduced
to a pore, placed between the
third and fourth of the permanent clefts (fig. 416, tli). This
opening is retained throughout
the Ammoccete condition, but
the organ becomes highly complicated, with paired anterior
and posterior horns and a
median spiral portion. In the adult the connection with the
pharynx is obliterated, and the organ is partly absorbed and
partly divided up into a series of glandular follicles, and eventually forms the thyroid body.
 
From the consideration of the above facts W. Muller was led
to the conclusion tJiat the tJiyroid body of the Craniata was
derived from the endostyle or Jiypopharyngeal groove. In all the
higher Vertebrata the thyroid body arises as a diverticulum of
the ventral wall of the throat in the region either of the mandibular or hyoid arches (fig. 417, Tk}, which after being segmented
off becomes divided up into follicles.
 
In Elasmobranch embryos it appears fairly early as a diverticulum from
the ventral surface of the throat in the region of the niandibular arc/i,
extending from the border of the mouth to the point where the ventral aorta
divides into the two aortic branches of the mandibular arch (fig. 417, Th}.
 
 
 
FIG. 415. DIAGRAMMATIC TRANSVERSE SECTIONS THROUGH THE BRANCHIAL REGION OF YOUNG LARV.K OF
PETROMYZON. (From Gegenbaur ; after
Calberla.)
 
d. branchial region of throat.
 
 
 
ALIMENTARY CANAL.
 
 
 
761
 
 
 
Somewhat later it becomes in Scyllium and Torpedo solid, though still
retaining its attachment to the wall of the oesophagus. It continues to grow
in length, and becomes divided up into a number of solid branched lobules
separated by connective tissue septa. Eventually its connection with the
throat becomes lost, and the lobules develop a lumen. In Acanthias the
lumen of the gland is retained (W. Miiller) till after its detachment from the
 
 
 
-- "
 
 
Pti
 
 
 
 
FIG. 416. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A
LARVA OF PETROMYZON.
 
The larva had been hatched three days, and was 4 '8 mm. in length. The optic
and auditory vesicles are supposed to be seen through the tissues. The letter tv
pointing to the base of the velum is where Scott believes the hyomandibular cleft to
be situated.
 
c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum ; pn. pineal gland ;
mb. mid-brain ; cb, cerebellum ; md. medulla oblongata ; au.v. auditory vesicle ; op.
optic vesicle; ol. olfactory pit; m. mouth; br.c. branchial pouches; th. thyroid
involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord.
 
throat. It preserves its embryonic position through life. In Amphibia it
originates, as in Elasmobranchii, from the region of the mandibular arch ;
but when first visible it forms a double epithelial wall connecting the throat
with the nervous layer of the epidermis. It subsequently becomes detached
from the epidermis, and then has the usual form of a diverticulum from the
throat. In most Amphibians it becomes divided into two lobes, and so
forms a paired body. The peculiar connection between the thyroid diverticulum and the epidermis in Amphibia has been noted by Gotte in
Bombinator, and by Scott and Osborn in Triton. It is not very easy to see
what meaning this connection can have.
 
In the Fowl (W. Miiller) the thyroid body arises at the end of the second
or beginning of the third day as an outgrowth from the hypoblast of the
throat, opposite the point of origin of the anterior arterial arch. This
outgrowth becomes by the fourth day a solid mass of cells, and by the fifth
ceases to be connected with the epithelium of the throat, becoming at the
same time bilobed. By the seventh day it has travelled somewhat backwards, and the two lobes have completely separated from each other. By
 
 
 
762
 
 
 
THE THYROID BODY.
 
 
 
the ninth day the whole is invested by a
capsule of connective tissue, which sends
in septa dividing it into a number of lobes
or solid masses of cells, and by the sixteenth day it is a paired body composed of
a number of hollow branched follicles, each
with a ' membrana propria,' and separated
from each other by septa of connective
tissue. It finally travels back to the point
of origin of the carotids.
 
Amongst Mammalia the thyroid arises
in the Rabbit (Kolliker) and Man (His) as
a hollow diverticulum of the throat at the
bifurcation of the foremost pair of aortic
arches. It soon however becomes solid,
and is eventually detached from the throat
and comes to lie on the ventral side of the
larynx or windpipe. The changes it undergoes are in the main similar to those in the
lower Vertebrata. It becomes partially
constricted into two lobes, which remain
however united by an isthmus 1 . The fact
that the thyroid sometimes arises in the
region of the first and sometimes in that of
the second cleft is probably to be explained
 
 
 
 
Tli
 
 
 
FIG. 417. SECTION THROUGH
THE HEAD OF AN ELASMOBRANCH
EMBRYO, AT THE LEVEL OF THE
AUDITORY INVOLUTION.
 
Th. rudiment of thyroid body ;
aup. auditory pit ; aim. ganglion
of auditory nerve ; iv. v. roof of
fourth ventricle ; a.c.v. anterior
cardinal vein ; aa. aorta ; f.aa
aortic trunk of mandibular arch ;
//. head cavity of mandibular
arch ; Ivc. alimentary pouch which
will form the first visceral cleft.
 
 
 
by its rudimentary character.
 
The Thymus gland. The thymus gland may conveniently be
dealt with here, although its origin is nearly as obscure as its function. It
has usually been held to be connected with the lymphatic system. Kolliker
was the first to shew that this view was probably erroneous, and he
attempted to prove that it was derived in the Rabbit from the walls of one
of the visceral clefts, mainly on the ground of its presenting in the embryo
an epithelial character.
 
1 Wolfler (No. 571) states that in the Pig and Calf the thyroid body is formed as a
pair of epithelial vesicles, which are developed as outgrowths of the walls of the first
pair of visceral clefts. He attempts to explain the contradictory observations of other
embryologists by supposing that they have mistaken the ventral ends of visceral
pouches for an unpaired outgrowth of the throat. Stieda (No. 569) also states that in
the Pig and Sheep the thyroid arises as a paired body from the epithelium of a pair
of visceral clefts, at a much later period than would appear from the observations of
His and Kolliker. In view of the comparative development of this organ it is
difficult to accept either Wolfler's or Stieda's account. Wolfler's attempt to explain
the supposed errors of his predecessors is certainly not capable of being applied in
the case of Elasmobranch Fishes, or of Petromyzon ; and I am inclined to think that
the method of investigation by transverse sections, which has been usually employed,
is less liable to error than that by longitudinal sections which he has adopted.
 
 
 
ALIMENTARY CANAL. 763
 
 
 
Stieda (No. 569) has recently verified Kolliker's statements. He finds
that in the Pig and the Sheep the thymus arises as a paired outgrowth from
the epithelial remnants of a pair of visceral clefts. Its two lobes may at first
be either hollow (Sheep) or solid (Pig), but eventually become solid, and
unite in the median line. Stieda and His hold that in the adult gland, the
so-called corpuscles of Hassall are the remnants of the embryonic epithelial
part of the gland, and that the lymphatic part of it is of mesoblastic origin ;
but Kolliker believes the lymphatic cells to be direct products of the
embryonic epithelial cells.
 
The posterior visceral clefts in the course of their atrophy give rise to
various more or less conspicuous bodies of a pseudo-glandular nature, which
have been chiefly studied by Remak 1 .
 
Swimming bladder and lungs. A swimming bladder is
present in all Ganoids and in the vast majority of Teleostei.
Its development however is only imperfectly known.
 
In the Salmon and Carp it arises, as was first shewn by Von
Baer, as an outgrowth of the alimentary tract, shortly in front of
the liver. In these forms it is at first placed on the dorsal side
and slightly to the right, and grows backwards on the dorsal
side of the gut, between the two folds of the mesentery.
 
The absence of a pneumatic duct in the Physoclisti would
appear to be due to a post-larval atrophy.
 
In Lepidosteus the air-bladder appears to arise, as in the
Teleostei, as an invagination of the dorsal wall of the oesophagus.
 
In advanced embryos of Galeus, Mustelus and Acanthias, MikluchoMaclay detected a small diverticulum opening on the dorsal side of the
oesophagus, which he regards as a rudiment of a swimming bladder. This
interpretation must however be regarded as somewhat doubtful.
 
The lungs. The lungs originate in a nearly identical way in
all the Vertebrate forms in which their development has been
observed. They are essentially buds or processes of the ventral
wall of the primitive oesophagus.
 
At a point immediately behind the region of the visceral
clefts the cavity of the alimentary canal becomes compressed
laterally, and at the same time constricted in the middle, so that
its transverse section (fig. 418 i) is somewhat hourglass-shaped,
and shews an upper or dorsal chamber d, joining on to a lower
or ventral chamber / by a short narrow neck.
 
1 For details on these organs vide Kolliker, Entwicklungsgeschichte, p. 88 1.
 
 
 
764
 
 
 
THE LUNGS.
 
 
 
 
The hinder end of the lower tube enlarges (fig. 418 2), and
then becomes partially divided into two lobes (fig. 418 3). All
these parts at first freely communicate, but the two lobes,
partly by their own growth,
and partly by a process of constriction, soon become isolated
posteriorly; while in front they
open into the lower chamber
of the oesophagus (fig. 422).
 
By a continuation forwards
of the process of constriction
the lower chamber of the oesophagus, carrying with it the
two lobes above mentioned,
becomes gradually transformed
into an independent tube,
opening in front by a narrow
slit-like aperture into the oesophagus. The single tube in
front is the rudiment of the
trachea and larynx, while the
two diverticula behind become
(fig. 419, Ig) the bronchial tubes
and lungs.
 
While the above changes
are taking place in the hypoblastic walls of the alimentary
tract, the splanchnic mesoblast
surrounding these structures
becomes very much thickened ; but otherwise bears no marks of
the internal changes which are going on, so that the above
formation of the lungs and trachea cannot be seen from the
surface. As the paired diverticula of the lungs grow backwards,
the mesoblast around them takes however the form of two lobes,
into which they gradually bore their way.
 
There do not seem to be any essential differences in the mode of
formation of the above structures in the types so far observed, viz. Amphibia,
Aves and Mammalia. Writers differ as to whether the lungs first arise as
 
 
 
FlG. 418. FOUR DIAGRAMS ILLUSTRATING THE FORMATION OF THE LUNGS.
 
(After Gotte.)
 
a. mesoblast; b. hypoblast; d. cavity
of digestive canal ; /. cavity of the pulmonary diverticulum.
 
In (i) the digestive canal has commenced to be constricted into an upper
and lower canal ; the former the true
alimentary canal, the latter the pulmonary tube; the two tubes communicate
with each other in the centre.
 
In (2) the lower (pulmonary) tube has
become expanded.
 
In (3) the expanded portion of the
tube has become constricted into two
tubes, still communicating with each other
and with the digestive canal.
 
In (4) these are completely separated
from each other and from the digestive
canal, and the mesoblast has also begun
to exhibit externally changes corresponding to the internal changes which have
been going on.
 
 
 
ALIMENTARY CANAL.
 
 
 
765
 
 
 
re
 
 
 
paired diverticula, or as a single diverticulum ; and as to whether the
rudiments of the lungs are established
before those of the trachea. If the above
account is correct it would appear that
any of these positions might be maintained. Phylogenetically interpreted the
ontogeny of the lungs appears however
to imply that this organ was first an
unpaired structure and has become
secondarily paired, and that the trachea
was relatively late in appearing.
 
The further development of the
lungs is at first, in the higher types
at any rate, essentially similar to
that of a racemose gland. From
each primitive diverticulum numerous branches are given off
In Aves and Mammalia (fig. 355)
they are mainly confined to the
dorsal and lateral parts. These
branches penetrate into the surrounding mesoblast and continue
to give rise to secondary and
tertiary branches. In the meso
 
 
 
At
 
 
 
FIG. 419. SECTION THROUGH
THE CARDIAC REGION OF AN EMBRYO
OF LACERTA MURALIS OF 9 MM. TO
SHEW THE MODE OF FORMATION OF
THE PERICARDIAL CAVITY.
 
ht. heart ; pc . pericardial cavity ;
al. alimentary tract; Ig. lung; /.
liver; pp. body cavity; md. open
end of Mullerian duct; wd. Wolffian
duct ; vc. vena cava inferior ; ao.
aorta; ch. notochord; me, medullary
cord.
 
 
 
blast around them numerous capillaries make their appearance, and the further growth of the
bronchial tubes is supposed by Boll to be due to the mutual
interaction of the hitherto passive mesoblast and of the hypoblast.
 
The further changes in the lungs vary somewhat in the different forms.
 
The air sacks are the most characteristic structures of the avian lung.
They are essentially the dilated ends of the primitive diverticula or of their
main branches.
 
In Mammalia (Kolliker, No. 298) the ends of the bronchial tubes become
dilated into vesicles, which may be called the primary air-cells. At first,
owing to their development at the ends of the bronchial branches, these are
confined to the surface of the lungs. At a later period the primary air-cells
divide each into two or three parts, and give rise to secondary air-cells, while
at the same time the smallest bronchial tubes, which continue all the while
to divide, give rise at all points to fresh air-cells. Finally the bronchial
tubes cease to become more branched, and the air-cells belonging to each
minute lobe come in their further growth to open into a common chamber.
 
 
 
766 THE CLOACA.
 
 
 
Before the lungs assume their function the embryonic air-cells undergo a
considerable dilatation.
 
The trachea and larynx. The development of the trachea and larynx
does not require any detailed description. The larynx is formed as a simple
dilatation of the trachea. The cartilaginous structures of the larynx are of
the same nature as those of the trachea.
 
It follows from the above account that the whole pulmonary
structure is the result of the growth by budding of a system of
branched hypoblastic tubes in the midst of a mass of mesoblastic
tissue, the hypoblastic elements giving rise to the epithelium of
the tubes, and the mesoblast providing the elastic, muscular,
cartilaginous, vascular, and other connective tissues of the
tracheal and bronchial walls.
 
There can be no doubt that the lungs and air-bladder are
homologous structures, and the very interesting memoir of Eisig
on the air-bladder of the Chaetopoda 1 shews it to be highly
probable that they are the divergent modifications of a primitive
organ, which served as a reservoir for gas secreted in the
alimentary tract, the gas in question being probably employed
for respiration when, for any reason, ordinary respiration by the
gills was insufficient.
 
Such an organ might easily become either purely respiratory,
receiving its air from the exterior, and so form a true lung ; or
mainly hydrostatic, forming an air-bladder, as in Ganoidei and
Teleostei.
 
It is probable that in the Elasmobranchii the air-bladder has
become aborted, and the organ discovered by Micklucho-Maclay
may perhaps be a last remnant of it.
 
The middle division of the mesenteron. The middle
division of the mesenteron, forming the intestinal and cloacal
region, is primitively a straight tube, the intestinal region of
which in most Vertebrate embryos is open below to the yolksack.
 
Cloaca. In the Elasmobranchii, the embryos of which
probably retain a very primitive condition of the mesenteron,
this region is not at first sharply separated from the postanal
section behind. Opposite the point where the anus will even
1 H. Eisig, " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei
Anneliden." Mittheil. a. d. zool. Station z. Neafel, Vol. II. 1881.
 
 
 
ALIMENTARY CANAL.
 
 
 
767
 
 
 
tually appear a dilatation of the mesenteron arises, which comes
in contact with the external skin (fig. 28 E, an}. This dilatation
becomes the hypoblastic section of the cloaca. It communicates
behind with the postanal gut (fig. 424 D), and in front with the
intestine ; and may be defined as the dilated portion of the alimentary tract which receives the genital and urinary ducts and opens
externally by the proctodczum.
 
In Acipenser and Amphibia the cloacal region is indicated
as a ventral diverticulum of the mesenteron even before the
closure of the blastopore. It is shewn in the Amphibia at an
early stage in fig. 73, and at a later period, when in contact with
the skin at the point where the anal invagination is about to
appear, in fig. 420.
 
 
 
 
FIG. 420. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF
 
BOMBINATOR. (After Gotte.)
 
m. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; ch.
notochord ; pn. pineal gland.
 
In the Sauropsida and Mammalia the cloaca appears as a
dilatation of the mesenteron, which receives the opening of the
allantois almost as soon as the posterior part of the mesenteron
is established.
 
The eventual changes which it undergoes have been already
dealt with in connection with the urinogenital organs.
 
Intestine. The region in front of the cloaca forms the
intestine. In certain Vertebrata it nearly retains its primitive
character as a straight tube ; and in these types its anterior
part is characterised by the presence of a peculiar fold, which in
a highly specialised condition is known as the spiral valve.
This structure appears in its simplest form in Ammocoetes. It
 
 
 
768 THE INTESTINE.
 
 
 
there consists of a fold in the wall of the intestine, giving to the
lumen of this canal a semilunar form in section, and taking a
half spiral.
 
In Elasmobranchii a similar fold to that in Ammoccetes first
makes its appearance in the embryo. This fold is from the
first not quite straight, but winds in a long spiral round the
intestine. In the course of development it becomes converted
into a strong ridge projecting into the lumen of the intestine
(fig. 388, /). The spiral it makes becomes much closer, and it
thus acquires the form of the adult spiral valve. A spiral valve
is also found in Chimaera and Ganoids. No rudiment of such
an organ is found in the Teleostei, the Amphibia, or the higher
Vertebrata.
 
The presence of this peculiar organ appears to be a very
primitive Vertebrate character. The intestine of Ascidians
exhibits exactly the same peculiarity as that of Ammoccetes,
and we may probably conclude from embryology that the
ancestral Chordata were provided with a straight intestine
having a fold projecting into its lumen, to increase the area of
the intestinal epithelium.
 
In all forms in which there is not a spiral valve, with the
exception of a few Teleostei, the intestine becomes considerably
longer than the cavity which contains it, and therefore necessarily more or less convoluted.
 
The posterior part usually becomes considerably enlarged to
form the rectum or in Mammalia the large intestine.
 
In Elasmobranchii there is a peculiar gland opening into the
dorsal side of the rectum, and in many other forms there is a
caecum at the commencement of the rectum or of the large
intestine.
 
In Teleostei, the Sturgeon and Lepidosteus there opens into
the front end of the intestine a number of caecal pouches known
as the pancreatic caeca. In the adult Sturgeon these pouches
unite to form a compact gland, but in the embryo they arise as
a series of isolated outgrowths of the duodenum.
 
Connected with the anterior portion of the middle region of
the alimentary canal, which may be called the duodenum, are
two very important and constant glandular organs, the liver and
the pancreas.
 
 
 
ALIMENTARY CANAL.
 
 
 
769
 
 
 
ITlf
 
 
 
 
The liver. The liver is the earliest formed and largest
glandular organ in the embryo.
 
It appears in its simplest
form in Amphioxus as a single
unbranched diverticulum of the
alimentary tract, immediately
behind the respiratory region,
which is directed forwards and
placed on the left side of the
body.
 
In all true Vertebrata the
gland has a much more complicated structure. It arises as a
ventral outgrowth of the duodenum (fig. 420, /). This outgrowth may be at first single,
and then grow out into two
lobes, as in Elasmobranchii (fig.
421) and Amphibia, or have from
the first the form of two somewhat unequal diverticula, as in
Birds (fig. 422), or again as in
the Rabbit (Kolliker) one diverticulum may be first formed, and a second one appear
somewhat later. The hepatic diverticula, whatever may be
their primitive form, grow into a special thickening of the
splanchnic mesoblast.
 
From the primitive diverticula there are soon given off a
number of hollow buds (fig. 421) which rapidly increase in
length and number, and form the so-called hepatic cylinders.
They soon anastomose and unite together, and so constitute an
irregular network. Coincidently with the formation of the
hepatic network the united vitelline and visceral vein or veins
(u.v\ in their passage through the liver, give off numerous
branches, and gradually break up into a plexus of channels
which form a secondary network amongst the hepatic cylinders.
In Amphibia these channels are stated by Gotte to be lacunar,
but in Elasmobranchii, and probably Vertebrata generally, they
arc from the first provided with distinct though delicate walls.
B. in. 49
 
 
 
FIG. 421. SECTION THROUGH THE
VENTRAL PART OF THE TRUNK OF A
YOUNG EMBRYO OF SCYLLIUM AT THE
LEVEL OF THE UMBILICAL CORD.
 
b. pectoral fin ; ao. dorsal aorta ;
cav. cardinal vein; ua. vitelline artery ; nv. vitelline vein united with
subintestinal vein ; al. duodenum ;
/. liver ; sd. opening of segmental
duct into the body-cavity ; mp. muscle-plate ; urn. umbilical canal.
 
 
 
770
 
 
 
THE LIVER.
 
 
 
It is still doubtful whether the hepatic cylinders are as a rule hollow or
solid. In Elasmobranchii they are at first provided with a large lumen,
which though it becomes gradually smaller never entirely vanishes. The
same seems to hold good for Amphibia and some Mammalia. In Aves
the lumen of the cylinders is even from the first much more difficult
to see, and the cylinders are stated by Remak to be solid, and he has
been followed in this matter by Kolliker. In the Rabbit also Kolliker finds
the cylinders to be solid.
 
The embryonic hepatic network gives rise to the parenchyma
of the adult liver, with which in
its general arrangement it closely
agrees. The blood-channels are
at first very large, and have a
very irregular arrangement ; and
it is not till comparatively late
that the hepatic lobules with their
characteristic vascular structures
become established.
 
The biliary ducts are formed
either from some of the primitive hepatic cylinders, or, as
would seem to be the case in
Elasmobranchii and Birds (fig.
422), from the larger diverticula of the two primitive outgrowths.
 
The gall-bladder is so inconstant, and the arrangement of
the ducts opening into the intestine so variable, that no general statements can be made about
them. In Elasmobranchii the primitive median diverticulum
(fig. 421) gives rise to the ductus choledochus. Its anterior end
dilates to form a gall-bladder.
 
In the Rabbit a ductus choledochus is formed by a diverticulum from the intestine at the point of insertion of the two
primitive lobes. The gall-bladder arises as a diverticulum of
the right primitive lobe.
 
The liver is relatively very large during embryonic life and
has, no doubt, important functions in connection with the circulation.
 
 
 
 
r
 
 
 
FIG. 422. DIAGRAM OF THE DIGESTIVE TRACT OF A CHICK UPON THE
FOURTH DAY. (After Gotte.)
 
The black line indicates the hypoblast. The shaded part around it is
the splanchnic mesoblast.
 
Ig. lung ; st. stomach ; p. pancreas ;
/. liver.
 
 
 
ALIMENTARY CANAL.
 
 
 
771
 
 
 
The pancreas. So far as is known the development of the
pancreas takes place on a very constant type throughout the
series of craniate Vertebrata, though absent in some of the
Teleostean fishes and Cyclostomata, and very much reduced in
most Teleostei and in Petromyzon.
 
It arises nearly at the same time as the liver in the form of a
hollow outgrowth from the dorsal side of the intestine nearly
opposite but slightly behind the hepatic outgrowth (fig. 422, /).
It soon assumes, in Elasmobranchii and Mammalia, somewhat
the form of an inverted funnel, and from the expanded dorsal
part of the funnel there grow out numerous hollow diverticula
into the passive splanchnic mesoblast.
 
As the ductules grow longer and become branched, vascular
processes grow in between them, and the whole forms a compact
glandular body in the mesentery on the dorsal side of the
alimentary tract. The funnel-shaped receptacle loses its origi nal form, and elongating, assumes the character of a duct.
 
From the above mode of development it is clear that the
glandular cells of the pancreas are derived from the hypoblast.
 
Into the origin of the varying arrangements of the pancreatic
ducts it is not possible to enter in detail. In some cases,
e.g. the Rabbit (Kolliker), the two lobes and ducts arise from a
division of the primitive gland and duct. In other cases, e.g. the
Bird, a second diverticulum springs from the alimentary tract.
In a large number of instances the primitive condition with a
single duct is retained.
 
Postanal section of the mesenteron. In the embryos of
all the Chordata there is a section of the mesenteron placed
behind the anus. This section invariably atrophies at a comparatively early period of embryonic life ; but it is much better
developed in the lower forms than in the higher. At its
posterior extremity it is primitively continuous with the neural
tube (fig. 420), as was first shewn by Kowalevsky.
 
The canal connecting the neural and alimentary canals has
already been described as the neurenteric canal, and represents
the remains of the blastopore.
 
In the Tunicata the section of the mesenteron, which in all probability
corresponds to the postanal gut of the Vertebrata, is that immediately
 
492
 
 
 
 
772 POSTANAL SECTION OF THE MESENTERON.
 
following the dilated portion which gives rise to the branchial cavity
 
and permanent intestine. It has already
 
been shewn that from the dorsal and
 
lateral portions of this section of the
 
primitive alimentary tract the notochord
 
and muscles of the Ascidian tadpole are
 
derived. The remaining part of its walls
 
forms a solid cord of cells (fig. 423, al'},
 
which either atrophies, or, according to
 
Kowalevsky, gives rise to blood-vessels.
 
In Amphioxus the postanal gut, FIG. 423. TRANSVERSE OPTICAL
 
.hough distinctly developed, is no, very %
long, and atrophies at a comparatively (After Kowalevsky.)
early period. The sect i on ; s f rom an embryo of
 
In Elasmobranchii this section of the the same age as fig. 8 iv.
 
alimentary tract is very well developed, ch - notochord ; nc neural 1 canal ;
 
. , , me. mesoblast ; of. hypoblast of
and persists for a considerable period of ta ji <
 
embryonic life. The following is a
history of its development in the genus Scyllium.
 
Shortly after the stage when the anus has become marked out by the
alimentary tract sending down a papilliform process towards the skin, the
postanal gut begins to develop a terminal dilatation or vesicle, connected
with the remainder of the canal by a narrower stalk.
 
The walls both of the vesicle and stalk are formed of a fairly columnar
epithelium. The vesicle communicates in front by a narrow passage with
the neural canal, and behind is continued into two horns corresponding
with the two caudal swellings previously spoken of (p. 55). Where the
canal is continued into these two horns, its walls lose their distinctness of
outline, and become continuous with the adjacent mesoblast.
 
In the succeeding stages, as the tail grows longer and longer, the postanal section of the alimentary tract grows with it, without however undergoing alteration in any of its essential characters. At the period of the
maximum development, it has a length of about -J of that of the whole
alimentary tract.
 
Its features at a stage shortly before the external gills have become
prominent are illustrated by a series of transverse sections through the
tail (fig. 424). The four sections have been selected for illustration out of a
fairly-complete series of about one hundred and twenty.
 
Posteriorly (A) there is present a terminal vesicle (alv) '25 mm. in
diameter, which communicates dorsally by a narrow opening with the
neural canal (nc) ; to this is attached a stalk in the form of a tube, also
lined by columnar epithelium, and extending through about thirty sections
(B al}. Its average diameter is about '084 mm., and its walls are very thick.
Overlying its front end is the subnotochordal rod (x), but this does not
extend as far back as the terminal vesicle.
 
The thick-walled stalk of the vesicle is connected with the cloacal section
 
 
 
ALIMENTARY CANAL.
 
 
 
773
 
 
 
of the alimentary tract by a very narrow thin-walled tube (C of). This for
the most part has a fairly uniform calibre, and a diameter of not more than
035 mm. Its walls are formed of flattened epithelial cells. At a point not
far from the cloaca it becomes smaller, and its diameter falls to -03 mm. In
 
 
 
 
cl.al
 
 
 
FIG. 424. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL
OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F.
 
A. is the posterior section.
 
nc . neural canal ; al. postanal gut ; alv. caudal vesicle of postanal gut ; x.
subnotochordal rod; mp. muscle-plate; ch. notochord; cl.al. cloaca; ao. aorta;
v.cau, caudal vein.
 
front of this point it rapidly dilates again, and, after becoming fairly wide,
opens on the dorsal side of the cloacal section of the alimentary canal just
behind the anus (D al}.
 
Very shortly after the stage to which the above figures belong, at a
point a little behind the anus, where the postanal section of the canal
was thinnest in the previous stage, it becomes solid, and a rupture here
occurs in it at a slightly later period.
 
The atrophy of this part of the alimentary tract having once commenced
proceeds rapidly. The posterior part first becomes reduced to a small
rudiment near the end of the tail. There is no longer a terminal vesicle,
nor a neurenteric canal. The portion of the postanal section of the
alimentary tract, just behind the cloaca, is for a short time represented
by a small rudiment of the dilated part which at an earlier period opened
into the cloaca.
 
In Teleostei the vesicle at the end of the tail, discovered by Kupffer,
 
 
 
774 THE STOMOD/EUM.
 
 
 
(fig- 34> hyv) is probably the equivalent of the vesicle at the end of the
postanal gut in Elasmobranchii.
 
In Petromyzon and in Amphibia there is a well-developed postanal
gut connected with a neurenteric canal which gradually atrophies. It is
shewh in the embryo of Bombinator in fig. 420.
 
Amongst the amniotic Vertebrata the postanal gut is less developed
than in the Ichthyopsida. A neurenteric canal is present for a short period
 
 
 
 
FIG. 425. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.
 
ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy. hypoblast ; p.a.g, postanal gut ; pr. remains of primitive streak folded in on the ventral
side ; al. allantois ; me. splanchnic mesoblast ; an. point where anus will be formed ;
p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.
 
in various Birds (Gasser, etc.) and in the Lizard, but disappears very early.
There is however, as has been pointed out by Kolliker, a well-marked
postanal gut continued as a narrow tube from behind the cloaca into
the tail both in the Bird (fig. 425, p.a.g.} and Mammals (the Rabbit), but
especially in the latter. It atrophies early as in lower forms.
 
The morphological significance of the postanal gut and of the neurenteric canal has already been spoken of in Chapter xii., p. 323.
 
 
 
The anterior section of the permanent alimentary tract is
formed by an invagination of epiblast, constituting a more or
less considerable pit, with its inner wall in contact with the
blind anterior extremity of the alimentary tract.
 
In Ascidians this pit is placed on the dorsal surface (fig. 9, o),
and becomes the permanent oral cavity of these forms. In the
larva of Amphioxus it is stated to be formed unsymmetrically
 
 
 
THE STOMOD/EUM.
 
 
 
775
 
 
 
 
(vide p. 5), but further observations on its development are
required.
 
In the true Vertebrata it is always formed on the ventral
surface of the head, immediately behind the level of the forebrain (fig. 426), and is deeper in Petromyzon (fig. 416, ;) than
in any other known form.
 
From the primary buccal cavity or stomodaeum there grows
out the pituitary pit (fig. 426, pt\ the
development of which has already
been described (p. 435).
 
The wall separating the stomodaeum from the mesenteron always
becomes perforated, usually at an
early stage of development, and
though in Petromyzon the boundary
between the two cavities remains
indicated by the velum, yet in the
higher Vertebrata all trace of this
boundary is lost, and the original
limits of the primitive buccal cavity
become obliterated ; while a secondary buccal cavity, partly lined by
hypoblast and partly by epiblast,
becomes established.
 
This cavity, apart from the organs which belong to it,
presents important variations in structure. In most Pisces it
retains a fairly simple character, but in the Dipnoi its outer
boundary becomes extended so as to enclose the ventral opening of the nasal sack, which thenceforward constitutes the
posterior nares.
 
In Amphibia and Amniota the posterior nares also open well
within the boundary of the buccal cavity.
 
In the Amniota further important changes take place.
 
In the first place a plate grows inwards from each of the
superior maxillary processes (fig. 427, /), and the two plates,
meeting in the middle line, form a horizontal septum dividing
the front part of the primitive buccal cavity into a dorsal
respiratory section (), containing the opening of the posterior
nares, and a ventral cavity, forming the permanent mouth. The
 
 
 
FIG. 426. LONGITUDINAL
SECTION THROUGH THE BRAIN OF
A YOUNG PRISTIURUS EMBRYO.
 
r.unpaired rudimentofthecerebral hemispheres \pn. pineal gland ;
/w.infundibulum ; //.ingrowth from
mouth to form the pituitary body ;
mb. mid-brain ; cb. cerebellum ; ch.
notochord; al. alimentary tract;
Zaa. artery of mandibular arch.
 
 
 
THE TEETH.
 
 
 
 
two divisions thus formed open into a common cavity behind.
The horizontal septum, on the development within it of an
osseous plate, constitutes the hard palate.
 
An internasal septum (fig. 427, e) may more or less completely divide the dorsal cavity into two canals, continuous
respectively with the two nasal cavities.
 
In Mammalia a posterior prolongation of the palate, in which
an osseous plate is not formed, constitutes the soft palate.
 
The second change in the Amniota, which also takes place in
some Amphibia, is caused by the section of the mesenteron into
which the branchial pouches open,
becoming, on the atrophy of these
structures, converted into the posterior part of the buccal cavity.
 
The organs derived from the
buccal cavity are the tongue, the
various salivary glands, and the
teeth ; but the latter alone will engage our attention here.
 
The teeth. The teeth are to be
regarded as a special product of the
oral mucous membrane. It has been
shewn by Gegenbaur and Hertwig
that in their mode of development
they essentially resemble the placoid
scales of Elasmobranchii, and that the latter structures extend
in Elasmobranchii for a certain distance into the cavity of the
mouth.
 
As pointed out by Gegenbaur, the teeth are therefore to be
regarded as more or less specialised placoid scales, whose
presence in the mouth is to be explained by the fact that the
latter structure is lined by an invagination of the epidermis.
The most important developmental point of difference between
teeth and placoid scales consists in the fact, that in the case
of the former there is a special ingrowth of epiblast to
meet a connective tissue papilla which is not found in the
latter.
 
 
 
FIG. 427. DIAGRAM SHEWING THE DIVISION OF THE PRIMITIVE BUCCAL CAVITY INTO THE
RESPIRATORY SECTION ABOVE
AND THE TRUE MOUTH BELOW.
(From Gegenbaur.)
 
p. palatine plate of superior
maxillary process; m. permanent
mouth ; n. posterior part of nasal
passage; e. internasal septum.
 
 
 
Although the teeth are to be regarded as primitively epiblastic structures, they are nevertheless found in Teleostei and Ganoidei on the hyoid
 
 
 
THE STOMOD/KUM.
 
 
 
777
 
 
 
and branchial arches ; and very possibly the teeth on some other parts of
the mouth are developed in a true hypoblastic region.
 
The teeth are formed from two distinct organs, viz. an epithelial cap and
a connective tissue papilla.
 
The general mode of development, as has been more especially shewn
by the extended researches of Tomes, is practically the same for all Vertebrata, and it will be convenient to describe it as it takes place in Mammalia.
 
Along the line where the teeth are about to develop, there is formed
an epithelial ridge projecting into the subjacent connective tissue, and
derived from the innermost columnar layer of the oral epithelium. At the
points where a tooth is about to be formed this ridge undergoes special
changes. It becomes in the first place somewhat thickened by the development of a number of rounded cells in its interior ; so that it becomes
constituted of (i) an external layer of columnar cells, and (2) a central core
of rounded cells ; both of an epithelial nature. In the second place the
organ gradually assumes a dome-shaped form (fig. 428, e), and covers over a
papilla of the subepithelial connective tissue (p] which has in the meantime
been developed.
 
From the above epithelial structure, which may be called the enamel
organ, and from the papilla it covers, which
maybe spoken of as the dental papilla,
the whole tooth is developed. After these
parts have become established there is formed
round the rudiment of each tooth a special
connective tissue capsule ; known as the
dental capsule.
 
Before the dental capsule has become
definitely formed the enamel organ and the
dental papilla undergo important changes.
The rounded epithelial cells forming the core
of the enamel organ undergo a peculiar transformation into a tissue closely resembling
ordinary embryonic connective tissue, while
at the same time the epithelium adjoining
the dental papilla and covering the inner
surface of the enamel organ, acquires a somewhat different structure to the epithelium
on the outer side of the organ. Its cells
become very markedly columnar, and form
a very regular cylindrical epithelium. This
layer alone is concerned in forming the
enamel. The cells of the outer epithelial
layer of the enamel organ become somewhat
flattened, and the surface of the layer is raised into a series of short papilla?
which project into the highly vascular tissue of the dental sheath. Between
 
 
 
 
FIG. 428. DIAGRAM SHEWING THE DEVELOPMENT OF THE
TEETH. (From Gegenbaur.)
 
p. dental papilla ; e. enamel
organ.
 
 
 
778 THE PROCTOD/EUM.
 
the epithelium of the enamel organ and the adjoining connective tissue
there is everywhere present a delicate membrane known as the membrana
praeformativa.
 
The dental papilla is formed of a highly vascular core and a non-vascular
superficial layer adjoining the inner epithelium of the enamel organ. The
cells of the superficial layer are arranged so as almost to resemble an
epithelium.
 
The first formation of the hard structures of the tooth commences at
the apex of the dental papilla. A calcification of the outermost layer of
the papilla sets in, and results in the formation of a thin layer of dentine.
Nearly simultaneously a thin layer of enamel is deposited over this,
from the inner epithelial layer of the enamel organ (fig. 428). Both
enamel and dentine continue to be deposited till the crown of the tooth has
reached its final form, and in the course of this process the enamel
organ is reduced to a thin layer, and the whole of the outer layer of the
dental papilla is transformed into dentine while the inner portion remains
as the pulp.
 
The root of the tooth is formed later than the crown, but the enamel
organ is not prolonged over this part, so that it is only formed of dentine.
 
By the formation of the root the crown of the tooth becomes pushed
outwards, and breaking through its sack projects freely on the surface.
 
The part of the sack which surrounds the root of the tooth gives rise
to the cement, and becomes itself converted into the periosteum of the
dental alveolus.
 
The general development of the enamel organs and dental papillae is
shewn in the diagram (fig. 428). From the epithelial ridge three enamel
organs are represented as being developed. Such an arrangement may
occur when teeth are successively replaced. The lowest and youngest
enamel organ (e) has assumed a cap-like form enveloping a dental papilla,
but no calcification has yet taken place.
 
In the next stage a cap of dentine has become formed, while in the
still older tooth this has become covered by a layer of enamel. As may be
gathered from this diagram, the primitive epithelial ridge from which the
enamel organ is formed is not necessarily absorbed on the formation of a
tooth, but is capable of giving rise to fresh enamel organs. When the
enamel organ has reached a certain stage of development, its connection
with the epithelial ridge is ruptured (fig. 428).
 
The arrangement represented in fig. 428, in which successive enamel
organs are formed from the same epithelial ridge, is found in most Vertebrata except the Teleostei. In the Teleostei, however (Tomes), a fresh
enamel organ grows inwards from the epithelium for each successively
formed tooth.
 
The Proctodceuni.
 
In all Vertebrata the cloacal section of the alimentary tract
which receives the urinogenital ducts is placed in communication
 
 
 
THE PROCTOD/EUM.
 
 
 
779
 
 
 
with the exterior by means of an epiblastic invagination, constituting a proctodseum.
 
This invagination is not usually very deep, and in most
instances the boundary wall between it and the hypoblastic
cloaca is not perforated till considerably after the perforation of the
stomodseum ; in Petromyzon, however, its perforation is effected
before the mouth and pharynx are placed in communication.
 
The mode of formation of the proctodaeum, which is in
general extremely simple, is illustrated by fig. 420 an.
 
In most forms the original boundary between the cpiblast of
the proctodaeum and the hypoblast of the primitive cloaca
becomes obliterated after the two have become placed in free
communication.
 
 
 
 
FIG. 429. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH THE POSTERIOR
END OF AN EMBRYO BlRD AT THE TIME OF THE FORMATION OF THE ALLANTOIS.
 
ep. epiblast ; Sp.c. spinal canal ; ch. notochord ; n.e. neurenteric canal ; hy, hypoblast ; p.a.g. postanal gut ; pr. remains of primitive streak folded in on the ventral
side ; al. allantois ; me. mesoblast ; an. point where anus will be formed ; p.c. perivisceral cavity ; am. amnion ; so. somatopleure ; sp. splanchnopleure.
 
In Birds the formation of the proctodseum is somewhat more complicated than in other types, owing to the outgrowth from it of the bursa
Fabricii.
 
The proctodseum first appears when the folding off of the tail end of
the embryo commences (fig. 429, an} and is placed near the front (originally
the apparent hind) end of the primitive streak. Its position marks out the
front border of the postanal section of the gut.
 
The bursa Fabricii first appears on the seventh day (in the chick), as a
dorsal outgrowth of the proctodaeum. The actual perforation of the septum between the proctodeeum and the cloacal section of the alimentary tract
is not effected till about the fifteenth day of fcetal life, and the approxi
 
 
780 BIBLIOGRAPHY.
 
 
 
mation of the epithelial layers of the two organs, preparatory to their
absorption, is partly effected by the tunneling of the mesoblastic tissue
between them by numerous spaces.
 
The hypoblastic section of the cloaca of birds, which receives the openings of the urinogenital ducts, is permanently marked off by a fold from
the epiblastic section or true proctodaeum, with which the bursa Fabricii
communicates.
 
BIBLIOGRAPHY.
Alimentary Canal and its appendages.
 
(561) B. Afanassiew. "Ueber Bau u. Entwicklung d. Thymus d. Saugeth."
Archivf. mikr. Anat. Bd. xiv. 1877.
 
(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.
 
(563) E. Gasser. "Die Entstehung d. Cloakenoffnung bei Hiihnerembryonen."
Archivf. Anat. u. Physiol., Anat. Abth. 1880.
 
(564) A. Gotte. Beilrdge zur Entivicklungsgeschichle d. Darmkanah im
Hiihnchen. 1867.
 
(565) W. Millie r. "Ueber die Entwickelung der Schilddriise." Jenaische
Zeitschrift, Vol. vi. 1871.
 
(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaische Zeitschrift, Vol. VII. 1872.
 
(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische Untcrsuchungen. 1872.
 
(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d.
Huhns." Zeit.f. wiss. Zool. 1866.
 
(569) L. Stieda. Untersuch. iib. d. Entwick. d. Glandula Thymus, Glandula
thyroidea,u. Glandula car otica. Leipzig, 1881.
 
(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad.
Petrop. 1766.
 
(571) H. Wolfler. Ueb. d. Entwick. u. d. Bau d. Schilddriise. Berlin, 1880.
Vide also Kolliker (298), Gotte (296), His (232 and 297), Foster and Balfour (295),
 
Balfour (292), Remak (302), Schenk (303), etc.
 
Teeth.
 
(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of
Micros. Science, Vol. in. 1855.
 
(573) R. Owen. Odontography . London, 1840 1845.
 
(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative.
London, 1876.
 
(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros.
Science, Vol. xvi. 1876.
 
(576) W. Waldeyer. " Structure and development of teeth." Strieker's Histology. 1870.
 
Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.
 
 
 
INDEX TO VOLUME III.
 
 
 
Abdominal muscles, 675
 
Abdominal pore, 626, 749
 
Acipenser, development of, 102; affinities
of, 1 1 8 ; comparison of gastrula of, 279 ;
pericardial cavity of, 627
 
Actinotrocha, 373
 
Air-bladder of Teleostei, 77; Lepidosteus,
117; blood supply of, 645 ; general account of, 763 ; homologies of, 766
 
Alciope, eye of, 480
 
Alisphenoid region of skull, 569
 
Alimentary canal and appendages, development of, 754
 
Alimentary tract ofAscidia, 18; Molgula,
22; Pyrosoma, 24; Salpa, 31 ; Elasmobranchii, 52; Teleostei, 75; Petromyzon, 93, 97; Acipenser, no; Amphibia, 129, 136; Chick, 167; respiratory
region of, 754; temporary closure of
oesophageal region of, 759
 
Allantois, development of in Chick, 191,
198; blood-vessels of in Chick, 193;
Lacerta, 205, 209; early development of
in Rabbit, 229, of Guinea-pig, 264;
origin of, 309. See also ' Placenta ' and
'Bladder''
 
Alternation of generations in Ascidians,
origin of, 35 ; in Botryllus, 35 ; Pyrosoma, 36; Salpa, 36; Doliolum, 36
 
Alytes, branchial chamber of, 136; yolksack of, 139; branchiae, 141 ; Miillerian
duct of, 710
 
Amblystoma, ovum of, 120; larva of, 142,
 
H3
 
Amia, ribs of, 561
 
Ammocoetes, 95; metamorphosis of, 97;
 
eye of, 498
Amnion, early development of in Chick,
 
185; later history of in Chick, 196;
 
Lacerta, 204, 210; Rabbit, 229; origin
 
of, 3.07. 39
 
Amphibia, development of, 120; viviparous, 121; gastrula of, 277; suctorial
mouth of, 317; cerebellum of, 426; infundibulum of, 431; pineal gland of,
433; cerebrum of, 439; olfactory lobes
of, 444; nares of, 553; notochord and
its sheath, 548; vertebral column of,
554; ribs of, 561 ; branchial arches of,
574; mandibular and hyoid arches of,
582 ; columella of, 582 ; pectoral girdle
of, 605; pelvic girdle of, 607; limbs of,
619; heart of, 638; arterial system of,
f>45 ; venous system of, 655 ; excretory
 
 
 
system of, 707 ; vasa efierentia of, 711;
liver of, 769; postanal gut of, 774;
stomodaeum of, 778
 
Amphiblastula larva of Porifera, 344
 
Amphioxus, development of, i ; gastrula
of, 275 ; formation of mesoblast of, 292 ;
development of notochord of, 293; head
of, 314; spinal nerves of, 461; olfactory organ of, 462 ; venous system
of, 651; transverse abdominal muscle
f> 673; generative cells of, 748; liver
of, 769; postanal gut of, 772; stomodaeum of, 777
 
Amphistylic skulls, 578
 
Angular bone, 594
 
Anterior abdominal vein, 653
 
Anura, development of, 121; epiblast of,
125; mesoblast of, 128; notochord of,
128; hypoblast of, 129; general growth
of embryo of, 131; larva of, 134; vertebral column of, 556 ; mandibular arch
of, 584
 
Anus of Amphioxus, 7 ; Ascidia, 18; Pyrosoma, 28 ; Salpa, 31 ; Elasmobranchii,
57; Amphibia, 130, 132; Chick, 167;
primitive, 324
 
Appendicularia, development of, 34
 
Aqueductus vestibuli, 519
 
Aqueous humour, 497
 
Arachnida, nervous system of, 409; eye
of, 481
 
Area, embryonic, of Rabbit, 218; epiblast
 
of, 219; origin of embryo from, 228
 
area opaca of Chick, 150; epiblast,
 
hypoblast, and mesoblast of, 159
area pellucida of Chick, 150; of Lacerta, 202
 
area vasculosa of Chick, 194; mesoblast of, 1 60; of Lizard, 209; Rabbit,
228, 229
 
Arteria centralis retinas, 503
 
Arterial system of Petromyzon, 97; constitution of in embryo, 643 ; of Fishes,
644; of Amphibia, 645; of Amniota, 647
 
Arthropoda, head of, 313 ; nervous system
of, 409 ; eye of, 480 ; excretory organs
of, 688
 
Articular bone of Teleostei, 581 ; of Sauropsida, 588
 
Ascidia, development of, 9
 
Ascidians. See 'Tunicata'
 
Ascidiozooids, 25
 
Atrial cavity of Amphioxus, 7; Ascidia,
18; Pyrosoma, 24
 
 
 
7 82
 
 
 
INDEX.
 
 
 
Atrial pore of Amphioxus, 7; Ascidia, 20;
Pyrosoma, 28 ; Salpa, 32
 
Auditory capsules, ossifications in, 595,
59.6
 
Auditory involution of Elasmobranchii,
57; Teleostei, 73; Petromyzon, 89,
92; Acipenser, 106; Lepidosteus, 114;
Amphibia, 127; Chick, 170
 
Auditory nerve, development of, 459
 
Auditory organs, of Ascidia, 15; of Salpa,
31; of Ammocoetes, 98; Ganoidei, 108,
114; of Amphibia, 127; of Aves, 170;
general development of, 512; of aquatic
forms, 512; of land forms, 513; of
Ccelenterata, 513; of Mollusca, 515;
of Crustacea, 516; of Vertebrata, 517;
of Cyclostomata, 89, 92, 518; of Teleostei, Lepidosteus and Amphibia,
518; of Mammalia, 519; accessory
structures of, 527; ofTunicata, 528
 
Auriculo-ventricular valves, 642
 
Autostylic skulls, 579
 
Aves, development of, 145; cerebellum
of, 426; midbrain of, 427; infundibulum of, 431; pineal gland of, 434;
pituitary body of, 436; cerebrum of,
439 ; olfactory lobes of, 444 ; spinal
nerves of, 449 ; cranial nerves of, 455 ;
vagus of, 458; glossopharyngeal of,
458; vertebral column of, 557; ossification of vertebral column of, 558;
branchial arches of, 572, 573; pectoral
girdle of, 603; pelvic girdle of, 608;
heart of, 637 ; arterial system of, 647 ;
venous system of, 658; muscle-plates
of, 670; excretory organs of, 714; mesonephros of, 715; pronephros of, 718;
Miillerian duct of, 718, 720; nature of
pronephros of, 721 ; connection of Miillerian duct with Wolffian in, 720 ;
kidney of, 722; lungs of, 764; liver of,
769; postanal gut of, 774
 
Axolotl, 142, 143; ovum of, 120; midbrain of, 427; mandibular arch of, 583
 
Basilar membrane, 524
 
Basilar plate, 565
 
Basipterygium, 612
 
Basisphenoid region of skull, 569
 
Bilateral symmetry, origin of, 373-376
 
Bile duct, 770
 
Bladder, Amphibia, 131 ; of Amniota, 726
 
Blastodermic vesicle, of Rabbit, first development of, 217; of 7th day, 222;
Guinea-pig, 263; meaning of, 291
 
Blastoderm of Pyrosoma, 24; Elasmobranchii, 41; Chick, 150; Lacerta 202
 
Blastopore, of Amphioxus, 2; of Ascidia,
II ; Elasmobranchii, 42, 54, 62 ; Petromyzon, 87; Acipenser, 104 ; Amphibia,
125, 130; Chick, 153; Rabbit, 216;
true Mammalian, 226; comparative
history of closure of, 284, 288; summary of fate of, 340; relation of to
primitive anus, 324
 
 
 
Blood-vessels, development of, 633
 
Body cavity, of Ascidia, 2 1 ; Molgula, 2 1 ;
Salpa, 31; Elasmobranchii, 47 ; of Teleostei, 75 ; Petromyzon, 94 ; Chick,
169; development of in Chordata, 325;
views on origin of, 356 360, 377; of
Invertebrata, 623; of Chordata, 624;
of head, 676
 
Bombinator, branchial chamber of, 136;
vertebral column of, 556
 
Bonellia, excretory organs of, 687
 
Bones, origin of cartilage bones, 542 ;
origin of membrane bones, 543; development of, 543; homologies of membrane bones, 542 ; homologies of cartilage bones, 545
 
Brachiopoda, excretory organs of, 683 ;
generative ducts of, 749
 
Brain, of Ascidia, IT, 15; Elasmobranchii, 56, 59, 60; Teleostei, 77; Petromyzon, 89, 92 ; Acipenser, 105 ; Lepidosteus, 113; early development of in
Chick, 170; flexure of in Chick, 175;
later development of in Chick, 176;
Rabbit, 229, general account of development of, 419; flexureof, 420; histogeny of, 422
 
Branchial arches, prseoral, 570; disappearance of posterior, 573; dental plates
of in Teleostei, 574; relation of to
head cavities, 571 ; see ' Visceral arches'
 
Branchial chamber of Amphibia, 136
 
Branchial clefts, of Amphioxus, 7 ; of
Ascidia, 18, 20; Molgula, 23; Salpa,
32; of Elasmobranchii, 57, 59 01;
Teleostei, 77; Petromyzon, 91, 96;
Acipenser, 105; Lepidosteus, 114, 116;
Amphibia, 132, 133; Chick, 178;
Rabbit, 231; praeoral, 312, 318; of
Invertebrata, 326; origin of, 326
 
Branchial rays, 574
 
Branchial skeleton, development of, 572,
592; of Petromyzon, 96, 312, 571; of
Ichthyopsida, 572; dental plates of in
Teleostei, 574; relation of to head
cavities, 572
 
Branchiae, external of Elasmobranchii, 6r,
62; of Teleostei, 77; Acipenser, 107;
Amphibia, 127, 133, 135
 
Brood-pouch, of Salpa, 29 ; Teleostei, 68,
Amphibia, 12 1
 
Brown tubes of Gephyrea, 686
 
Bulbus arteriosus, of Pishes, 638 ; Amphibia, 639
 
Bursa Fabricii, 167, 779
 
Canalis auricularis, 639
Canalis reuniens, 521
Capitellidre, excretory organs of, 683
Carcharias, placenta of, 66
Cardinal vein, 652
Carnivora, placenta of, 250
Carpus, development of, 620
Cartilage bones of skull, 595 ; homologies
of, 595
 
 
 
INDEX.
 
 
 
783
 
 
 
Cat, placenta of, 250
 
Caudal swellings of Elasmobranchii, 46,
 
55; Teleostei, 72; Chick, 162, 170
Cephalic plate of Elasmobranchii, 55
Cephalochorda, development of, i
Cephalopoda, eyes of, 473 477
Cerebellum, Petromyzon, 93; Chick, 176;
 
general account of development of, 424,
 
425
 
Cerebrum of Petromyzon, 93, 97; Chick,
175 ; general development of, 429, 438;
transverse fissure of, 443
Cestoda, excretory organs of, 68 1
Cetacea, placenta, 255
Chtetognatha, nervous system of, 349;
eye of, 479 ; generative organs of, 743 ;
generative ducts of, 749
Chcetopoda, head of, 313; eyes of, 479;
excretory organs of, 683; generative
organs of, 743 ; generative ducts of, 749
Charybdnea, eye of, 472
Cheiroptera, placenta of, 244
Cheiropterygium, 618; relation of to ich
thyopterygium, 621
 
Chelonia, development of, 210; pectoral
girdle of, 603 ; arterial system of, 649
Chick, development of, 145 ; general
growth of embryo of, 1 70 ; rotation of
embryo of, 173; fcetal membranes of,
185; epiblast of, 150, 166; optic nerve
and choroid fissure of, 500
 
Chilognatha, eye of, 481
 
Chilopoda, eye of, 481
 
Chimasra, lateral line of, 539 ; vertebral
column of, 548; nares of, 533
 
Chiromantis, oviposition of, 121
 
Chorda tympani, development of, 460
 
Chordata, ancestor of, 311; branchial
system of, 312; evidence from Ammocuetes, 312; head of, 312; mouth of,
318; table of phylogeny of, 327
 
Chorion, 237; villi of, 237, 257
 
Choroid coat, Ammoccetes, 99; general
account of, 487
 
Choroid fissure, of Vertebrate eye, 486,
493 ; of Ammocoetes, 498 ; comparative
development of, 500; of Chick, 501;
of Lizards, 501 ; of Elasmobranchii,
502 ; of Teleostei, 503 ; Amphibia, 503 ;
Mammals, 503, 504
 
Choroid gland, 320
 
Choroid pigment, 489
 
Choroid plexus, of fourth ventricle, 425 ;
of third ventricle, 432 ; of lateral ventricle, 442
 
Ciliated sack of Ascidia, 18; Pyrosoma,
26; Salpa, 31
 
Ciliary ganglion, 461
 
Ciliary muscle, 490
 
Ciliary processes, 488; comparative development of, 506
 
Clavicle, 600
 
Clitoris, development of, 727
 
Clinoid ridge, 569
 
Cloaca, 766
 
 
 
Coccygeo-mesenteric vein, 66 1
 
Cochlear canal, 519
 
Coecilia, development of, 143; pronephros
of, 707; mesonephros of, 709; Mill
lerian duct of, 710
 
Coelenterata, larvae of, 367 ; eyes of, 47 1 ;
auditory organs of, 513; generative
organs of, 741
 
Columella auris, 529; of Amphibia, 582 ;
of Sauropsida, 588
 
Commissures, of spinal cord, 417; of
brain, 431, 432, 439, 443
 
Coni vasculosi, 724
 
Conus arteriosus, of Fishes, 638; of Amphibia, 638
 
Coracoid bone, 599
 
Cornea, of Ammocretes, 99 ; general development of, 495 ; corpuscles of, 496 ;
comparative development of, 499; of
Mammals, 499
 
Coronoid bone, 595
 
Corpora geniculata interna, 428
 
Corpora quadrigemina, 428
 
Corpora striata, development of, 437
 
Corpus callosum, development of, 443
 
Corti, organ of, 522; structure of, 525;
fibres of, 525 ; development of, 526
 
Cranial flexure, of Elasmobranchii, 58,
60; of Teleostei, 77; Petromyzon, 93,
94; of Amphibia, 131, 132; Chick,
174; Rabbit, 231; characters of, 321;
significance of, 322
 
Cranial nerves, development of, 455;
relation of to head cavities, 461 ; anterior roots of, 462 464; view on
position of roots of, 466
 
Crocodilia, arterial system of, 649
 
Crura cerebri, 429
 
Crustacea, nervous system of, 41 1 ; eye of,
481; auditory organs of, 515; generative cells of, 745 ; generative ducts of,
 
75
 
Cupola, 524
 
Cutaneous muscles, 676
 
Cyathozooid, 25
 
Cyclostomata, auditory organs of, 517;
olfactory organ of, 532; notochord and
vertebral column of, 546, 549; abdominal pores of, 626 ; segmental duct of,
700 ; pronephros of, 700 ; mesonephros
of, 700 ; generative ducts of, 733, 749 ;
venous system of, 651 ; excretory organs
of, 700
 
Cystignathus, oviposition of, 122
 
Dactylethra, branchial chamber of, 136;
 
branchise of, 136; tadpole of, 140
Decidua reflexa, of Rat, 242 ; of Insecti
vora, 243; of Man, 245
Deiter's cells, 526
Dental papilla, 777
Dental capsule, 777
Dentary bone, 595
Dentine, 780
Descemet's membrane, 496
 
 
 
784
 
 
 
INDEX.
 
 
 
Diaphragm, 631 ; muscle of, 676
 
Dipnoi, nares of, 534; vertebral column
of, 548; membrane bones of skull of,
592 ; heart of, 638 ; arterial system of,
645 ; excretory system of, 707 ; stomodseum of, 777
 
Diptera, eye of, 481
 
Discophora, excretory organs of, 687
 
Dog, placenta of, 248
 
Dohni, on relations of Cyclostomata, 84 ;
on ancestor of Chordata, 311, 319
 
Doliolum, development of, 28
 
Ductus arteriosus, 649
 
Ductus Botalli, 648
 
Ductus Cuvieri, 654
 
Ductus venosus Arantii, 663
 
Dugong, heart of, 642
 
Dysticus, eye of, 481
 
Ear, see ' Auditory organ '
 
Echinodermata, secondary symmetry of
larva of, 380; excretory organs of, 689 ;
generative ducts of, 752
 
Echinorhinus, lateral line of, 539; vertebral column of, 548
 
Echiurus, excretory organs of, 686
 
Ectostosis, 543
 
Edentata, placenta of, 248, 250, 256
 
Eel, generative ducts of, 703
 
Egg-shell of Elasmobranchii, 40 ; Chick,
146
 
Elasmobranchii, development of, 40; viviparous, 40; general features of development of, 55 ; gastrulaof, 281 ; development of mesoblast of, 294 ; notochord of, 294 ; meaning of formation of
mesoblast of, 295; restiform tracts of,
425 ; optic lobes of, 427 ; cerebellum of,
425 ; pineal gland of, 432 ; pituitary
body of, 435 ; cerebrum of, 438 ; olfactory lobes of, 444 ; spinal nerves, 449 ;
cranial nerves of, 457; sympathetic
nervous system of, 466; nares of, 533;
lateral line of, 539; vertebral column of,
549 ; ribs of, 560 ; parachordals of, 567 ;
mandibular and hyoid arches of, 576 ;
pectoral girdle of, 600 ; pelvic girdle of,
607; limbs of, 609; pericardial cavity
of, 627; arterial system of, 644 ; venous
system of, 65 1 ; muscle-plates of, 668 ;
excretory organs of, 690 ; constitution
of excretory organs in adult of, 697;
spermatozoa of, 747 ; swimming-bladder of, 763 ; intestines of, 767 ; liver of,
769; postanal gut of, 772
 
Elrcoblast of Pyrosoma, 28; Salpa, 30
 
Elephant, placenta of, 249
 
Embolic formation of gastrula, 333
 
Enamel organ, 777
 
Endolymph of ear, 522
 
Endostosis, 543
 
Endostyle of Ascidia, 18, 759; Pyrosoma,
25; Salpa, 32
 
Epiblast, of Elasmobranchii, 47 ; Teleostei, 71, 75; Petromyzon, 86; Lcpid
 
 
osteus, 112; Amphibia, 122, 125;
Chick, 149, 166; Lacerta, 203; Rabbit,
216, 219; origin of in Rabbit, 221 ;
comparative account of development
of, 300
 
Epibolic formation of gastrula, 334
 
Epichordal formation of vertebral column,
556
 
Epicrium glutinosum, 143
 
Epidermis, in Ccelenterata, 393; protective structures of, 394
 
Epididymis, 724
 
Epigastric vein, 653
 
Episkeletal muscles, 676
 
Episternum, 602
 
Epoophoron, 725
 
Ethmoid bone, 597
 
Ethmoid region of skull, 570
 
Ethmopalatine ligament of Elasmobranchs, 576
 
Euphausia, eye of, 483
 
Eustachian tube, of Amphibia, 135;
Chick, 1 80; Rabbit, 232; general
development of, 528
 
Excretory organs, general constitution of,
680; of Platyelminthes, 680; of Mollusca, 681; of Polyzoa, 682; of Brachiopoda, 683 ; of Choetopoda, 683 ; of
Gephyrea, 686 ; of Discophora, 687 ; of
Arthropoda, 688; of Nematoda, 689;
of Echinodermata, 689 ; constitution of
in Craniata, 689; of Elasmobranchii,
690; constitution of in adult Elasmobranch, 697; of Petromyzon, 700; of
Myxine, 701 ; of Teleostei, 701 ; of
Ganoidei, 704; of Dipnoi, 707; of
Amphibia, 707; of Amniota, 713;
comparison of Vertebrate and Invertebrate, 737
 
Excretory system, of Elasmobranchii, 49 ;
Teleostei, 78; Petromyzon, 95, 98;
Acipenser, 99; Amphibia, 133
 
Exoccipital bone, 595
 
Exoskeleton, dermal, 393 395 ; epidermal, 393396
 
External generative organs, 726
 
Extra-branchial skeleton, 572
 
Eye, of Ascidia, 16; Salpa, 31; Elasmobranchii, 56, 57, 58; Teleostei, 73;
Petromyzon, 92, 98; Aves, i/o; Rabbit, 229; general development of, 470;
evolution of, 470, 471; simple, 480;
compound, 481 ; aconous, 482; pseudoconous, 482 ; of Invertebrata, 471; of
Vertebrata, 483 ; comparative development of Vertebrate, 497 ; of Ammoccetes, 497 ; of Tunicata, 507 ; of Chordata, general views on, 508 ; accessory
eyes of Fishes, 509; muscles of, 677
 
Eyelids, development of, 506
 
Falciform ligament, 757
 
Falx cerebri, 439
 
Fasciculi terctes, of Elasmobranchii. 426
 
Feathers, development of, 396
 
 
 
INDEX.
 
 
 
785
 
 
 
Fenestra rotunda and ovalis, 529
 
Fertilization, of Amphioxus, 2 ; of Urochorda, 9; Salpa, 29; Elasmobranchii,
46; of Teleostei, 68; Petromyzon, 84 ;
Amphibia, 120; Chick, 145 ; Reptilia,
202 ; meaning of, 331
 
Fifth nerve, development of, 460
 
Fifth ventricle, 443
 
Fins, of Elasmobranchii, 62 ; Teleostei,
78; Petromyzon, 94, 95; Acipenser,
109; Lepidosteus, 118; relation of
paired to unpaired, 611, 612 ; development of pelvic, 614; development of
pectoral, 615; views on nature of paired
fins, 616
 
Fissures of spinal cord, 417
 
Foetal development, 360 ; secondary variations in, 361
 
Foot, 618
 
Foramen of Munro, 430, 438
 
Foramen ovale, 642
 
Forebrain, of Elasmobranchii, 55, 59, 60;
Petromyzon, 93 ; general development
of, 428
 
Formative cells, of Chick, 154
 
Fornix, development of, 443
 
Fornix of Gottsche, 428
 
Fourth nerve, 464
 
Frontals, 592
 
Fronto-nasal process of Chick, 179
 
Gaertner's canals, 724
 
Gall-bladder, 770
 
Ganoidei, development of, 102; relations
of, 118; nares of, 534; notochord of,
546 ; vertebral column of, 546, 553 ;
ribs of, 561 ; pelvic girdle of, 606; arterial system of, 645 ; excretory organs
of, 704; generative ducts of, 734
 
Gastropoda, eye of, 472
 
Gastrula, of Amphioxus, 2; of Ascidia, lo;
Elasmobranchii, 43, 44 ; Petromyzon,
86; Acipenser, 103; Amphibia, 123;
comparative development of, in Invertebrata, 275 ; comparison of Mammalian, 291 ; phylogenetic meaning of, 333 ;
ontogeny of (general), 333 ; phylogeny
of, 338 343 ; secondary types of, 34!
 
Geckos, vertebral column of, 557
 
Generative cells, development of, 74! ;
origin of in Ccelenterata, 741 ; of Invertebrata, 743 ; of Vertebrata, 746
 
Generative ducts, of Teleostei, 704, 735 ;
of Ganoids, 704; of Cyclostomata, 733;
origin of, 733 ; of Lepidosteus, 735,
750 ; development and evolution of,
748 ; of Ccelenterata, 748 ; of Sagitta,
749 ; of Tunicata, 749 ; Cheetopoda,
Gephyrea, etc., 749; of Mollusca, 751;
of Discophora, 751 ; of Echinodermata,
 
75*
 
Generative system of Elasmobranchii, 51
Gephyrea, nervous system of, 412; excretory organs of, 686 ; generative cells of,
743 ; generative ducts of, 749
 
B. III.
 
 
 
Germinal disc, of Elasmobranchii, 40;
Teleostei, 68 ; Chick, 147
 
Germinal epithelium, 746
 
Germinal layers, summary of organs <lrrived from, in Vertebrata, 304 ; historical account of views of, 332 ; homologies of in the Metazoa, 345
 
Germinal wall of Chick, 152, 159; structure and changes of, 160
 
Geryonia, auditory organ of, 5 r 5
 
Gill of Salpa, 31
 
Giraldes, organ of, 725
 
Glands, epidermic, development of, 397
 
Glomerulus, external, of Chick, 716
 
Glossopharyngeal nerve, development of,
 
45 6 > 457
Grey matter of spinal cord, 417; of brain,
 
423
Growth in length of Vertebrate embryo,
 
306
Guinea-pig, primitive streak of, 223;
 
notochord of, 226 ; placenta of, 242 ;
 
development of, 262
Gymnophiona, see ' Ccecilia '
 
Habenula perforata, 525
 
Hairs, development of, 396
 
Halichrerus, placenta of, 250
 
Hand, 619
 
Head, comparative account of, 313; segmentation of, 314
 
Head cavities, of Elasmobranchii, 50 ;
Petromyzon, 90, 96; Amphibia, 127;
general development of, 676
 
Head-fold of Chick, 157, 167
 
Head kidney, see ' Pronephros '
 
Heart, of Pyrosoma, 25; Elasmobranchii,
50, 58 ; Petromyzon, 94, 97 ; Acipenser, 106; Chick, 170 ; first appearance
of in Rabbit, 230; general development
of, 633 ; of Fishes, 635, 637 ; of Mammalia, 638; of Birds, 637, 639; meaning of development of, 637 ; of Amphibia, 638 ; of Amniota, 639 ; change of
position of, 643
 
Hind-brain, Elasmobranchii, 55, 59, 60 ;
Petromyzon, 93 ; general account of,
424
 
Hippocampus major, development of, 442
 
Hirudo, development of blood-vessels of,
633 ; excretory organs of, 688
 
Horse, placenta of, 253
 
Hyaloid membrane, 492
 
Hylodes, oviposition of, 1 21 ; metamorphosis of, -1 37
 
Hyobranchial cleft, 572
 
Hyoid arch, of Chick, 179; general account of, 572, 575 ; modifications of,
e !73> 577 > f Elasmobranchii, 576; of
Teleostei, 577 ; of Amphibia, 582 ;
of Sauropsida, 588; of Mammalia,
 
589
 
Hyomandibular bar of Elasmobranchii,
576, 577 ; of Teleostei, 579 ; of Amphibia, 582
 
50
 
 
 
;86
 
 
 
INDEX.
 
 
 
Hyomandibular cleft, of Fetromyzon, 91 ;
Chick, 179 ; general account of, 572
 
Hyostylic skulls, 582
 
Hypoblast of Elasmobranchii, 5! ; Teleostei, 71, 75; Petromyzon, 86; Acipenser, 104; Lepidosteus, 113; Amphibia,
122, 129; Chick, 151, 167 ; Lacerta,
203; Rabbit, 215, 216, 219 ; origin of
in Rabbit, 220
 
Hyposkeletal muscles, 675
 
Ilyrax, placenta of, 249
 
Incus, 529, 590
 
Infraclavicle, 600
 
Infundibulum of Petromyzon, 92 ; Chick,
175 ; general development of, 430
 
Insectivora, placenta of, 243
 
Insects, nervous system of, 410 ; eye of,
481; generative organs of, 745; generative ducts of, 751
 
Intercalated pieces of vertebral column,
 
55 1
 
Interclavicle, homologies of, 602
 
Intermediate cell-mass of Chick, 183
 
Intermuscular septa, 672
 
Interorbital septum, 570
 
Interrenal bodies, 665
 
Iris, 489 ; comparative development of,
 
506
 
Iris of Ammoccetes, 98
Island of Reil, 444
 
Jacobson's organ, 537
Jugal bone, 594
 
Kidney, see ' Metanephros '
 
Labia majora, development of, 727
 
Labial cartilages, 597
 
Labium tympanicum, 525 ; vestibulare,
 
5 2 5
 
Lacertilia, general development of, 202 ;
nares of, 537 ; pectoral girdle of, 603 ;
pelvic girdle of, 607 ; arterial system
of, 649
 
Lacrymal bone, 593
 
Lacrymal duct, 506
 
Lacrymal glands, 506
 
Lremargus, vertebral column of, 548
 
Lagena, 524
 
Lamina spiralis, 524
 
Lamina terminalis, 438
 
Larva of Amphioxus, 2 ; of Ascidia, 1 5
it ; Teleostei, 81 ; Petromyzon, 89, 95;
Lepidosteus, 117, 318; Amphibia, 134,
142; types of, in the Invertebrata, 363
 
Larvre, nature, origin, and affinities of,
360 386; secondary variations of less
likely to be retained, 362 ; ancestral
history more fully recorded in, 362 ;
secondary variations in development of,
363 ; ontogenetic record of secondary
variations in, 361; of freshwater and
land animals, 362; types of, 36.2; phosphorescence of, 364; of Coelenterata,
 
 
 
367 ; table of, 365 ; of Invertebrata,
367 et seq.
 
Larynx, 766
 
Lateral line sense organs, 538 ; comparison of, with invertebrate, 538 ; development of, in Teleostei, 538 ; development of, in Elasmobranchii, 539
 
Lateral ventricle, 438 ; anterior cornu of,
440 ; descending cornu of, 440 ; choroicl
plexus of, 443
 
Layers, formation of, in Elasmobrancliii,
41, 56 ; Teleostei, 71 ; Petromyzon,
85 ; Acipenser, 103 ; Lepidosteus, 1 1 1 ;
Amphibia, 121; Chick, 150, 152;
Lacerta, 202; Rabbit, 215 227; comparison of Mammalia with lower forms,
226, 289; comparison of formation of
in Vertebrata, 275; origin and homologies of, in the Metazoa, 331
 
Leech, see ' Hirudo '
 
Lemuridre, placenta, 256
 
Lens, of Elasmobranchii, 57, 58 ; Petromyzon, 94, 99; Acipenser, 106 ;
Lepidosteus, 115 ; Amphibia, 127 ;
Chick, 177 ; of Vertebrate eyes, 485 ;
general account of, 493 ; capsule of, 493 ;
comparative development of, 499 ; of
Amphibia, Teleostei, Lepidosteus, 499
 
Lepidosteus, development of, 1 1 1 ; larva
of, 117; relations of, 119; spinal nerves
of, 455; ribs of, 561 ; generative ducts
of, 704, 735 ; swimming-bladder of,
 
763
 
Ligamentum pectinatum, 490
 
Ligamentum suspensorium, 557, 558
 
Ligamentum vesicse medium, 239
 
Limbs, of Elasmobranchii, 59 ; Teleostei,
80 ; first appearance of in Chick,
184 ; Rabbit, 232 ; muscles of, 673 ; of
Fishes, 609; relation of, to unpaired fins
of Fishes, 611, 612; of Amphibia, 61 8
 
Liver of Teleostei, 78 ; Petromyzon, 95,
96; Acipenser, no; Amphibia 130;
general account of, 769
 
Lizard, development of, 202; general
growth of embryo of, 208 ; Mullerian
duct of, 721
 
Lizzia, eye of, 471
 
Lobi inferiores, 431
 
Lungs of Amphibia, 137 ; development
of, 763 ; homology of, 766
 
Lymphatic system, 664
 
Malleus, 529, 591 ; views on, 591
Malpighian bodies, development of accessory in Elasmobranchs, 695
Mammalia, development of, 214; comparison of gastrula of, 291 ; cerebellum
of, 427 ; infundibulum of, 431 ; pineal
gland of, 434; pituitary body of, 436;
cerebrum of, 439 ; spinal nerves of, 449 ;
sympathetic of, 466; vertebral column
of, 558; branchial arches of, 573, 574;
mandibular and hyoid arches of, 589 ;
pectoral girdle of, 604; pelvic girdle of,
 
 
 
INDEX.
 
 
 
787
 
 
 
608 ; heart of, 636 ; arterial system of,
647; venous system of, 661 ; muscleplates of, 671 ; mesonephros of, 714;
testicular network of, 724 ; urinogenital
sinus of, 727 ; spermatozoa of, 747 ;
lungs of, 765 ; intestines of, 768 ; liver
of> 769; postanal gut of, 774; stomodseum of, 775
 
Mammary gland, development of, 398
Man, placenta of, 244 ; general account of
development of, 265 ; characters of embryo of, 270
 
Mandibular arch of Elasmobranchii, 62,
576; Petromyzon, 91 ; Acipenser, 106,
116; Chick, 179; general account of,
 
572, 575; modification of to form jaws,
 
573, 575; of Teleostei, 580; of Amphibia, 582; Sauropsida, 588; Mammalia, 589
 
Mandibular bar, evolution of, 311, 321
 
Manis, placenta of, 256
 
Marsupial bones, 608
 
Marsupialia, foetal membranes of, 240 ; cerebellum of, 426 ; corpus callosum of,
' 443 ; uterus of, 726
 
Maxilla, 594
 
Meatus auditorius externus, of Chick, 181;
development of, 527
 
Meckelian cartilage, of Elasmobranchii,
576; of Teleostei, 581 ; of Amphibia,
584, 585; of Sauropsida, 588 ; of Mammalia, 590
 
Mediastinum anterior and posterior, 630
 
Medulla oblongata, of Chick, 176 ; general development of, 425
 
Medullary plate of Amphioxus, 4, 5 ; of
Ascidia, n; Elasmobranchii, 44, 47,
55; Teleostei, 72; Petromyzon, 88;
Acipenser, 104; Lepidosteus, 1 1 1 ; Amphibia, 126, 127, 131; Chick, 159;
Lacerta, 204; Rabbit, 223, 227, 228;
primitive bilobed character of, 303, 317
 
Medusae, auditory organs of, 513
 
Membrana capsulo-pupillaris, 494, 504,
 
507
 
Membrana elastica externa, 546
 
Membrana limitans of retina, 491
 
Membrana tectoria, 522, 525
 
Membrane bones, of Amphibia, 582 ; of
Sauropsida, 588; of Mammalia, 590;
of mandibular arch, 593 ; of pectoral
girdle, 599, 602 ; origin of, 592 ; homologies of, 593
 
Membranous labyrinth, development of
in Man, 519
 
Menobranchus, branchial arches of, 142
 
Mesenteron of Elasmobranchii, 43 ; Teleostei, 75 ; Petromyzon, 85 ; Acipenser,
104; Amphibia, 123, 124, 129; Chick,
167; general account of, 754
 
Mesentery, 626, 756
 
Mesoblast, of Amphioxus, 6 ; Ascidia,
17, 20; Pyrosoma, 24; Salpa, 30;
Elasmobranchii, 44, 47; Teleostei, 75;
Petromyzon, 86; Acipenser, 105; Lepi
 
 
dosteus, 113; Amphibia, 125, 128, 129;
of Chick, 154, 167; double origin of in
Chick, 154, 158, 159; origin of from
lips of blastopore in Chick, 158; of
area vasculosa of Chick, iOo; Lacerta,
203; origin of in Rabbit, 218, 223; of
area vasculosa in Rabbit, 227; comparative account of formation of, 292 ;
discussion of development of in Vertebrata, 297 ; meaning of development
of in Amniota, 298; phylogenetic origin
of, 346 ; summary of ontogeny of, 349
352 ; views on ontogeny of, 352 360
 
Mesoblastic somites, of Amphioxus, 6 ;
Elasmobranchii, 48, 55 ; Petromyzon,
88 ; Acipenser, 105 ; Lepidosteus,
114; Amphibia, 129, 131; Chick,
161, 1 80; Rabbit, 228; development
of in Chordata, 325; meaning of development of, 331; of head, 676
 
Mesogastrium, 758
 
Mesonephros, of Teleostei, 78, 702; Petromyzon, 95, 98, 700; Acipenser, 1 10,
705; Amphibia, 134, 708; Chick, 184,
714; general account of, 690 ; development of in Elasmobranchs, 691 ; of
Cyclostomata, 700 ; Ganoidei, 705 ;
sexual and non-sexual part of in Amphibia, 710; of Amniota, 713, 724;
summary and general conclusions as
to, 729; relation of to pronephros, 731
 
Mesopterygium, 616
 
Metagenesis of Ascidians, 34
 
Metamorphosis of Amphibia, 137, 140
 
Metanephros, 690; development of in
Elasmobranchii, 697; of Amphibia,
712; of Amniota, 713; of Chick, 722;
of Lacertilia, 723; phylogeny of, 736
 
Metapterygium, 616
 
Metapterygoid, of Elasmobranchii, 576;
of Teleostei, 581
 
Metazoa, evolution of, 339, 342 ; ancestral
form of, 333, 345
 
Mid-brain, of Elasmobranchii, 55, 58,
59; Petromyzon, 92; general account
of development of, 427
 
Moina, generative organs of, 745
 
Molgula, development of, 22
 
Mollusca, nervous system of, 414 ; eyes of,
472; auditory organs of, 515; excretory organs of, 68 1
 
Monotremata, foetal membranes of, 240 ;
cerebellum of, 426; corpus callosum
of, 443 ; cerebrum of, 443 ; urinogenital sinus of, 726
 
Mormyrus, generative ducts of, 704
 
Mouth, of Amphioxus, 7; of Ascidia, 18;
Pyrosoma, 27; Salpa, 31; Elasmobranchii, 57, 60, 61, 62; Petromyzon,
92, 94, 95, 99; Acipenser, 107; Lepidosteus, 118; Amphibia, 129, 132,
"134; Rabbit, 231 ; origin of, 317
 
Mouth, suctorial, of Petromyzon, 99;
Acipenser, 107; Lepidosteus, 116, 317;
Amphibia, 133, 141, 317
 
 
 
;88
 
 
 
INDEX.
 
 
 
Mullerian duct, 690; of Elasmobranchs,
693 ; of Ganoids, 704 ; of Amphibia,
710; of Aves, 717,720; opening of into cloaca, 727; origin of, 733; summary of development of, 733; relation
of to pronephros, 733
 
Muscle-plates, of Amphioxus, 6; Elasmobranchii, 49, 668 ; Teleostei, 670 ;
Petromyzon, 94; Chick, 183, 670; general development of, 669 ; of Amphibia,
670; Aves, 670; of Mammalia, 671;
origin of muscles from, 672
 
Muscles, of Ascidia, II, 17; development
of from muscle-plates, 672; of limbs,
673 ; of head, 676 ; of branchial arches,
678; of eye, 678
 
Muscular fibres, epithelial origin of, 667
 
Muscular system, development of, 667;
of Chordata, 668
 
Mustelus, placenta of, 66
 
Myoepithelial cells, 667
 
Mysis, auditory organ of, 517
 
Myxine, ovum of, loo; olfactory organ
of, 533 ; portal sinus of, 652 ; excretory
system of, 701
 
Nails, development of, 397
 
Nares, of Acipenser, 108; of Ichthyopsida, 534; development of in Chick,
535; development of in Lacertilia, 537;
development of in Amphibia, 537
 
Nasal bones, 592
 
Nasal pits, Acipenser, 108; Chick, 176;
general development of, 531
 
Nematoda, excretory organs of, 689 ;
generative organs of, 745 ; generative
ducts of, 752
 
Nemertines, nervous system of, 311 ; excretory organs of, 68 1
 
Nerve cord, origin of ventral, 378
 
Nerves, spinal, 449 ; cranial, 455 466
 
Nervous system, central, general account
of development of in Vertebrata, 415 ;
conclusions as to, 445; sympathetic,
466
 
Nervous system, of Amphioxus, 4; Ascidia, 15, 16; Molgula, 22; Pyrosoma,
24, 25; Salpa, 30, 31; Elasmobranchii,
44; Teleostei, 77 ; Petromyzon, 89, 93;
Acipenser, 105; Amphibia, 126; comparative account of formation of central,
301; of Sagitta, 349; origin of in
Ccelenterata, 349; of pneoral lobe,
377, 380; evolution of, 400405; development of in Invertebrates, 406;
of Arthropoda, 408; of Gephyrea, 412;
Mollusca, 414
 
Neural canal, of Ascidia, 10; Teleostei,
72; Petromyzon, 88; Acipenser, 105;
Lepidosteus, 114; Amphibia, 126, 131 ;
Chick, 1 66, 171 ; Lacerta, 208; closure
of in Frog and Amphioxus, 279; closure
of in Elasmobranchii, 284; phylogcuctic origin of, 316
 
Neural crest, 449, 456, 457
 
 
 
Neurenteric canal, of Amphioxus, 4, 5 ;
Ascidia, lo; Elasmobranchii, 54; Petromyzon, 88 ; Acipenser, 105 ; Lepidosteus, 113; Aves, 162; Lacerta, 203,
206; general account of, 323; meaning
of, 3 2 3
 
Newt, ovum of, 120; development of,
I2 55 general growth of, 141
 
Notidanus, vertebral column of, 548;
branchial arches of, 572
 
Notochord of Amphioxus, 6; Ascidia,
II, 17; Elasmobranchii, 51; Teleostei,
74; Petromyzon, 86, 94; Acipenser,
104; Lepidosteus, 113; Amphibia, 128,
129; Chick, 157; canal of, in Chick,
163; Lacerta, 204, 205; Guinea-pig,
226; comparative account of formation
of, 292, 325; sheath of, 545; later
histological changes in, 546; cartilaginous sheath of, 547; in head, 566;
absence of in region of trabeculas, 567
 
Notodelphys, brood-pouch of, 121 ; branchiae of, 140
 
Nototrema, brood-pouch of, 121
 
Nucleus pulposus, 559
 
Oceania, eye of, 471
 
Occipital bone, 595
 
CEsophagus, solid, of Elasmobranchii,
61, 759; of Teleostei, 78
 
Olfactory capsules, 571
 
Olfactory lobes, development of, 444
 
Olfactory nerves, Ammoccetes, 99; general development of, 464
 
Olfactory organ, of aquatic forms, 531;
Insects and Crustacea, 531; of Tunicata, 532 ; of Amphioxus, 532 ; of
Vertebrata, 533; Petromyzon, 533;
of Myxine, 533
 
Olfactory sacks, of Elasmobranchii, 60;
Teleostei, 73; Petromyzon, 92, 97;
Acipenser, 106, 108; Lepidosteus, 116;
Chick, 176
 
Oligochreta, excretory organs of, 683
 
Olivary bodies, 426
 
Omentum, lesser and greater, 757
 
Onchidium, eye of, 473
 
Opercular bones, 593
 
Operculum, of Teleostei, 77; Acipenser,
107; Lepidosteus, 117, 118; Amphibia,
 
r 3.5.
 
Ophidia, development of, 210; arterial
system of, 649 ; venous system of, 656
 
Optic chiasma, 430, 493
 
Optic cup, retinal part of, 488 ; ciliary
portion of, 489
 
Optic lobes, 428
 
Optic nerve, development of, 492 ; comparative development of, 500
 
Optic thalami, development of, 431
 
Optic vesicle, of Elasmobranchii, 57 59;
Teleostei, 74, 499 ; Petromyzon, 89, 92 ;
Acipenser, 106; Lepidosteus, 115;
Chick, 170; Rabbit, 229; general development of, 429 ; formation of secon
 
 
INDKX.
 
 
 
7*9
 
 
 
dary, 487 ; obliteration of cavity of, 488 ;
comparative development of, 499; of
Lepidosteus and Teleostei, 499. See
also ' Eye '
 
Ora serrata, 488
 
Orbitosphenoid region of skull, 570
 
Organs, classification of, 391 ; derivation
of from germinal layers, 392
 
Orycteropus, placenta of, 249
 
Otic process of Axolotl, 583; of Frog,
585 et seq.
 
Otoliths, 512
 
Oviposition, of Amphioxus, i ; Elasmobranchii, 40; Teleostei, 68; Petromyzon, 84; Amphibia, 121; Reptilia, 202
 
Ovum, of Amphioxus, i; Pyrosoma, 23;
Elasmobranchii, 40; Teleostei, 68;
Petromyzon, 83 ; Myxine, loo; Acipenser, 102; Lepidosteus, in; Amphibia,
120; Chick, 146; Reptilia, 202 ; Mammalia, 214; of Porifera, 741; migration of in Ccelenterata, 742; Vertebrata, 746
 
Palatine bone, of Teleostei, 580; origin
of, 594
 
Pancreas, Acipenser, no; general development of, 770
 
Pancreatic caeca, of Teleostei, etc. 768
 
Papillae, oral, of Acipenser, 108; Lepidosteus, n6
 
Parachordals, 565, 566
 
Parasphenoid bone, 594
 
Parepididymis, 725
 
Parietal bones, 592
 
Paroophorori, 725
 
Parovarium, 725
 
Pectoral girdle, 599 ; of Elasmobranchs,
600; of Teleostei, 600; of Amphibia
and Amniota, 60 1 ; comparison of with
pelvic, 608
 
Pecten, eye of, 479
 
Pecten, of Ammoccetes, 498; of Chick,
501 ; Lizard, 501 ; Elasmobranchs, 501
 
Pedicle, of Axolotl, 484 ; of Frog, 485
 
Pelobates, branchial apertures of, 136;
vertebral column of, 556
 
Pelodytes, branchial chamber of, 135
 
Pelvic girdle, 606; of Fishes, 606; Amphibia and Amniota, 607 ; of Lacertilia, 607 ; of Mammalia, 608 ; comparison with pectoral, 608
 
Penis, development of, 727
 
Peribranchial cavity, of Amphioxus, 7;
of Ascidia, 18; Pyrosoma, 24
 
Pericardial cavity, of Pyrosoma, 26 ; Elasmobranchii, 49 ; Petromyzon, 94; general account of, 626; of Fishes, 627 ; of
Amphibia, Sauropsida and Mammalia,
628
 
Perichordal formation of vertebral column,
5^6
 
Perilymph of ear, 523
Periotic capsules, ossifications in, 595,
596
 
 
 
Peripatus, nervous system of, 409 ; eye of
480 ; excretory organs of, 688
 
Peritoneal membrane, 626
 
Petromyzon, development of, 83; affinities of, 83, 84; general development
of, 87; hatching of, 89; comparison of
gastrula of, 280; branchial skeleton of,
312, 572; cerebellum of, 425; pineal
gland of, 434 ; pituitary body of, 436 ;
cerebrum of, 439; auditory organ of,
517; olfactory organ of, 533; comparison of oral skeleton of with Tadpole,
586; pericardial cavity of, 627; abdominal pores of, 626 ; venous system of,
651 ; excretory organs of, 700; segmental duct of, 700; pronephros of, 700;
mesonephros of, 700 ; thyroid body of,
760; postanalgut of, 774; stomodx-um
 
of, 775
 
Phosphorescence of larvae, 364
 
Phylogeny, of the Chordata, 327; of the
Metazoa, 384
 
Pig, placenta of, 251; mandibular and
hyoid arches of, 589
 
Pineal gland, of Petromyzon, 93 ; Chick,
175; general development of, 432;
nature of, 432, 434
 
Pipa, brood-pouch of, 121 ; metamorphosis of, 139; yolk-sack of, 140; vertebral
column of, 556
 
Pituitary body, of Rabbit, 231 ; general
development of, 435 ; meaning of, 436 ;
Placenta, of Salpa, 29; Elasmobranchii, 66; of Mammalia, 232; villi of,
235 ; deciduate and non-deciduate, 239;
comparative account of, 239 259 ; characters of primitive type of, 240; zonary, 248; non-deciduate, 250; histology of, 257; evolution of, 259
 
Placoid scales, 395
 
Planorbis, excretory organs of, 68 1
 
Planula, structure of, 367
 
Pleural cavities, 631
 
Pleuronectidae, development of, 80
 
Pneumatoccela, characters of, 327
 
Polygordius, excretory organs of, 684
 
Polyophthalmus, eye of, 479
 
Polypedates, brood-pouch of, 121
 
Polyzoa, excretory organs of, 682 ; generative cells of, 745 ; generative ducts
 
of, 751
 
Pons Varolii, 426, 427
 
Pori abdominales, Ammoccetes, 99
 
Porifera, ancestral form of, 345 ; development of generative cells of, 74!
 
Portal vein, 653
 
Postanal gut of Elasmobranchii, 58, 59,
60; Teleostei, 75; Chick, 169; general account of, 323, 772
 
Prsemaxilla, 594
 
Praeopercular bone, 593
 
Prrcoral lobe, ganglion of, 377, 380
 
Prefrontals, 597
 
Presphenoid region of skull, 570
 
Primitive groove of Chick, 1 55
 
 
 
790
 
 
 
INDEX.
 
 
 
Primitive streak, of Chick, 152, 161;
meaning of, 153; origin of mesoblast
form in Chick, 154; continuity of
hypoblast with epiblast at anterior end
of, in Chick, 156; comparison of with
blastopore, 165 ; fate of, in Chick, 165 ;
of Lacerta, 203; of Rabbit, 221; of
Guinea-pig, 223 ; fusion of layers at, in
Rabbit, 224; comparison of with blastopore of lower forms, 226, 287 ; of
Mammalia, 290
 
Processus falciformis of Ammoccetes, 498 ;
of Elasmobranch, 502 ; of Teleostei , 503
Proctodseum, 778
 
Pronephros, of Teleostei, 78, 701 ; Petromyzon, 95, 99, 700; Acipenser, 106,
no; Amphibia, 134, 707; general account of, 689 ; of Cyclostomata, 700 ;
of Myxine, 701 ; Ganoidei, 705 ; of
Amniota, 714; of Chick, 718; summary of and general conclusions as to,
728; relation of, to mesonephros, 731 ;
cause of atrophy of, 729
Prootic, 596, 597
Propterygium, 616
Proteus, branchial arches of, 142
Protochordata, characters of, 327
Protoganoidei, characters of, 328
Protognathostomata, characters of, 328
Protopentadactyloidei, characters of, 329
Protovertebrata, characters of, 328
Pseudis, Tadpole of, 139; vertebral
 
column of, 556
 
Pseud ophryne, yolk-sack of, 140; Tadpole of, 140
Pterygoid bone, of Teleostei, 581; origin
 
of, 597
 
Pterygoquadrate bar, of Elasmobranchii,
576; of Teleostei, 581; Axolotl, 584;
F r g, 584; ofSauropsida, 588; of Mammalia, 589
 
Pulmonary artery, origin of, 645 ; of
Amphibia, 645 ; of Amniota, 649
 
Pulmonary vein, 655
 
Pupil, 489
 
Pyrosoma, development of, 23
 
Quadrate bone of Teleostei, 581 ; of
Axolotl, 584; Frog, 585; Sauropsida,
588
 
Quadratojugal bone, 594
 
Rabbit, development of, 214; general
growth of embryo of, 227 ; placenta of,
248
 
Radiate symmetry, passage from to bilateral symmetry, 373 376
 
Raja, caudal vertebras of, 553
 
Rat, placenta of, 242
 
Recessus labyrinthi, 519
 
Reissner's membrane, 524
 
Reptilia, development of, 202; viviparous,
202; cerebellum of, 426; infundibulum
of, 431; pituitary body of, 436; cerebrum of, 439; vertebral column of,
 
 
 
556; arterial system of, 648; venous
system of, 656; mesonephros of, 713;
testicular network of, 723; spermatozoa
of, 747
 
Restiform tracts of Elasmobranchii and
Teleostei, 425
 
Retina, histogenesis of, 490
 
Retinulse, 482
 
Rhabdom, 482
 
Rhinoderma, brood-pouch of, 121; metamorphosis of, 1 39
 
Ribs, development of, 560
 
Roseniniiller's organ, 725
 
Rotifera, excretory organs of, 680
 
Round ligament of liver, 663
 
Ruminantia, placenta of, 253
 
Sacci vasculosi, 437
 
Sacculus hemisphericus, 519; of Mammals, 519, 520
 
Sagitta. See ' Chaetognatha'
 
Salpa, sexual development of, 29; asexual
development of, 33
 
Salamandra, larva of, 142; vertebral
column of, 553; limbs of, 619; mesonephros of, 708; Miillerian duct of,
710
 
Salmonidse, hypoblast of, 71; generative
ducts of, 704
 
Sauropsida, gastrula of, 286; meaning of
primitive streak of, 288; blastopore of,
289 ; mandibular and hyoid arches of,
588 ; pectoral girdle of, 60 1
 
Scala, vestibuli, 522; tympani, 523;
media, 522
 
Scales, general development of, 396 ; development of placoid scales, 395
 
Scapula, 599
 
Sclerotic, 488
 
Scrotum, development of, 727
 
Scyllium, caudal vertebrse of, 553; mandibular and hyoid arches of, 578; pectoral girdle of, 600; limbs of, 610; pelvic fin of, 614; pectoral fin of, 615
 
Segmental duct, 690 ; development of in
Elasmobranchs, 690; of Cyclostomata,
700; of Teleostei, 701; of Ganoidei,
704, 705 ; of Amphibia, 707 ; of Amniota, 713
 
Segmental organs, 682
 
Segmental tubes, 690 ; development of in
Elasmobranchs, 691 ; rudimentary anterior in Elasmobranchs, 693 ; development of secondary, 731
 
Segmentation cavity, of Elasmobranchii,
42 44; Teleostei, 69, 85, 86; Amphibia, 122, 125
 
Segmentation, meaning of, 331
 
Segmentation of ovum, in Amphioxus, 2 ;
Ascidia, 9 ; Molgula, 22 ; Pyrosoma,
23; Salpa, 30; Elasmobranchii, 40;
Telostei, 69; Petromyzon, 84; Acipenser, IOT, Lcpidosteus, in; Amphibia, 122, 124; Newt, 125; Chick,
146; Lizard, 202: Rabbit, 214
 
 
 
INDEX.
 
 
 
791
 
 
 
Semicircular canals, 519
 
Sense organs, comparative account of
development of, 304
 
Septum lucidum, 443
 
Serous membrane, Lacerta, 209; of Rabbit, 237
 
Seventh nerve, development of, 459
 
Shell-gland of Crustacea, 689
 
Shield, embryonic, of Chick, 151 ; of
Lacerta, 202
 
SimiadiK, placenta of, 247
 
Sinus rhomboidalis, of Chick, 162
 
Sinus venosus, 637
 
Sirenia, placenta of, 255
 
Sixth nerve, 463
 
Skate, mandibular and hyoid arches of,
 
577
 
Skeleton, elements of found in Vertebrata, 542
 
Skull, general development of, 564 ; historical account of, 564 ; development of
cartilaginous, 566; cartilaginous walls
of, 570; composition of primitive cartilaginous cranium, 565
 
Somatopleure, of Chick, 170
 
Spelerpes, branchial arches of, 142
 
Spermatozoa, of Porifera, 741; of Vertebrata, 746
 
Sphenoid bone, 595
 
Sphenodon, hyoid arch of, 588
 
Spinal cord, general account of, 415;
white matter of, 415; central canal of,
417, 418; commissures of, 417; grey
matter of, 417; fissures of, 418
 
Spinal nerves, posterior roots of, 449;
anterior roots of, 453
 
Spiracle, of Elasmobranchii, 62 ; Acipenser, 105; Amphibia, 136
 
Spiral valve. See 'Valve'
 
Spleen, 664
 
Splenial bone, 595
 
Squamosal bone, 593
 
Stapes, 529; of Mammal, 590
 
Sternum, development of, 562
 
Stolon of Doliolum, 29 ; Salpa, 33
 
Stomodaeum, 774
 
Stria vascularis, 524
 
Styloid process, 591
 
Sub-intestinal vein, 65 1 ; meaning of,
 
651
 
Syngnathus, brood-pouch of, 68
Subnotochordal rod, of Elasmobranchii,
 
54; Petromyzon, 94; Acipenser, no;
 
Lepidosteus, 115; general account of,
 
754; comparison of with siphon of
 
Chsetopods, 756
 
Subzonal membrane, 237; villi of, 236
Sulcus of Munro, 432
Supraclavicle, 600
Suprarenal bodies, 664
Supra-temporal bone, 593
Swimming bladder, see Air bladder
Sylvian aqueduct, 428
Sylvian fissure, 444
Sympathetic ganglia, development of, 467
 
 
 
Tadpole, 134, 139, 140; phylogenetic
meaning of, 137; metamorphosis of,
137; m can ing of suctorial mouth of, 585
 
Tail of Teleostei, 80; Acipenser, 109;
Lepidosteus, 109; Amphibia, 132
 
Tarsus, development of, 620
 
Teeth, horny provisional, of Amphibia,
136; general development of, 776;
origin of, 777
 
Teleostei, development of, 68; viviparous, 68; comparison of formation of
layers in, 286; restiform tracts of, 425 ;
mid-brain of, 425 ; infundibulum of,
431 ; cerebrum of, 439; nares of, 534;
lateral line of, 538; notochord and
membrana elastica of, 549 ; vertebral
column of, 553; ribs of, 561; hyoid
and mandibular arches of, 579; pectoral girdle of, 601 : pelvic girdle of,
606; limbs of, 618; heart of, 637;
arterial system of, 645; muscle-plates
of, 670; excretory organs of, 701 ; generative ducts of, 704, 735, 749; swimming bladder of, 763 ; postanal gut of,
 
Teredo, nervous system of, 414
 
Test of Ascidia, 14; Salpa, 31
 
Testicular network, of Elasmobranchs,
697 ; of Amphibia, 712 ; Reptilia, 723 ;
of Mammals, 724
 
Testis of Vertebrata, 746
 
Testis, connection of with Wolffian body,
in Elasmobranchii, 697; in Amphibia,
710; in Amniota, 723; origin of, 735
 
Thalamencephalon of Chick, 175; general development of, 430
 
Third nerve, development of, 461
 
Thymus gland, 762
 
Thyroid gland, Petromyzon, 92 ; general
account of, 759; nature of, 760; development of in Vertebrata, 761
 
Tooth. See 1 Teeth'
 
Tori semicirculares, 428
 
Tornaria, 372
 
Trabeculas, 565, 567; nature of, 568
 
Trachea, 766
 
Trematoda, excretory organs of, 68 1
 
Triton alpestris, sexual larva of, 143
 
Triton, development of limbs of, 619}
urinogenital organs of, 7 12
 
Truncus arteriosus, 638; of Amphibia,
638; of Birds, 639
 
Turiicata, development of mesoblast of,
293; test of, 394; eye of, 507; auditory organ of, 530; olfactory organ of,
532; generative duct of, 749 ; intestine
of, 767; postanal gut of, 771; stomodseum of, 775
 
Turbellaria, excretory organs of, 68 1
 
Tympanic annulus of *'rog, 587
 
Tympanic cavity, of Amphibia, 135;
Chick, 1 80; Rabbit, 232; general development of, 528; of Mammals, 591
 
Tympanic membrane, of Chick, 180;
general development of, 528
 
 
 
792
 
 
 
INDEX.
 
 
 
Tympanohyal, 591
 
Umbilical canal of Elasmobranchii, 54,
 
57, 58, 59
 
Umbilical cord, 238; vessels of, 239
 
Ungulata, placenta of, 250
 
Urachus, 239, 726
 
Ureters, of Elasmobranchii, 696; development of, 723
 
Urethra, 727
 
Urinary bladder of Amphibia, "Jii; of
Amniota, 726
 
Urinogenital organs, see Excretory organs
 
Urinogenital sinus of Petromyzon, 700;
of Sauropsida, 726; of Mammalia,
727
 
Urochorda, development of, 9
 
Uterus, development of, 726; of Marsupials, 726
 
Uterus masculinus, 726
 
Utriculus, 519
 
Uvea of iris, 489
 
Vagus nerve, development of, 456, 457;
intestinal branch of, 458; branch of to
lateral line, 459
 
Valve, spiral, of Petromyzon, 97; Acipenser, no; general account of, 767
 
Valves, semilunar, 641; auriculo-ventricular, 642
 
Vasa efferentia, of Elasmobranchs, 697 ;
of Amphibia, 711; general origin of,
724
 
Vascular system, of Amphioxus, 8; Petromyzon, 97; Lepidosteus, 116; general
development of, 632
 
Vas deferens, of Elasmobranchii, 697 ;
of Amniota, 723
 
Vein, sub-intestinal of Petromyzon, 97 ;
Acipenser, no; Lepidosteus, 116
 
Velum of Petromyzon, 9 1
 
Vena cava inferior, development of, 655
 
Venous system of Petromyzon, 97; general development of, 651; of Fishes,
651 ; of Amphibia and Amniota, 655 ;
of Reptilia, 656; of Ophidia, 656; of
Aves, 658; of Mammalia, 661
 
Ventricle, fourth, of Chick, 176; history
of, 424
 
Ventricle, lateral, 438, 440; fifth, 443
 
Ventricle, third, of Chick, 175
 
Vertebral bodies, of Chick, 183
 
Vertebral column, development of, 545,
549; epichordal and perichordal development of in Amphibia, 556
 
Vespertilionidse, early development of,
217
 
Vieussens, valve of, 426
 
Villi, placental, of zona radiata, 235 ;
subzonal membrane, 235; chorion, 237;
 
 
 
Man, 246; comparative account of,
2 575 of young human ovum, 265, 269
 
Visceral arches, Amphioxus, 7 ; Elasmobranchii, 57 60; Teleostei, 77; Acipenser, 1 06; Lepidosteus, 116; Amphibia, 133; Chick, 177; Rabbit,
231; prseoral, 570; relation of to head
cavities, 572; disappearance of posterior, 573; dental plates of in Teleostei, 574
 
Visual organs, evolution of, 470
 
Vitelline arteries of Chick, 195
 
Vitelline veins of Chick, 195
 
Vitreous humour, of Ammoccetes, 98 ;
general development of, 494; blood*
vessels of in Mammals, 503 ; mesoblastic ingrowth in Mammals, 503
 
Vomer, 594
 
White matter, of spinal cord, 415; of
brain, 423
 
Wolffian body, see ' Mesonephros '
 
Wolffian duct, first appearance of in
Chick, 183; general account of, 690;
of Elasmobranchs, 693 ; of Ganoids,
704; of Amphibia, 710; of Amniota,
713; atrophy of in Amniota, 724
 
Wolffian ridge, 185
 
Yolk blastopore, of Elasmobranchii, 64
 
Yolk, folding off of embryo from, in
Elasmobranchii, 55; in Teleostei, 76;
Acipenser, 106; Chick, 168, 170
 
Yolk nuclei, of Elasmobranchii, 41, 53;
Teleostei, 69, 75
 
Yolk, of Elasmobranchii, 40; Teleostei,
68; Petromyzon, 96; Acipenser, 109;
Amphibia, 122, 129; Chick, 146; influence of on formation of layers, 278;
influence of on early development,
 
341, 342
 
Yolk-sack, Amphibia, 131, 140, 141; enclosure of, 123
 
.Yolk-sack, development of in Rabbit,
227; of Mammalia reduced, 227; circulation of in Rabbit, 233 ; enclosure
of in Sauropsida, 289
 
Yolk-sack, enclosure of, Petromyzon, 86
 
Yolk-sack, Lepidosteus, 118
 
Yolk-sack of Chick, enclosure of, 160;
stalk of, 174; general account of, 193;
circulation of, 195 ; later history of, 198
 
Yolk-sack of Elasmobranchii, enclosure
of, 62, 283; circulation of, 64
 
Yolk-sack of Lacerta, 209 ; circulation of,
209
 
Yolk-sack, Teleostei, 75, 81; enclosure
of, 75 ; circulation of, 81
 
Zona radiata, villi of, 237
Zonula of Zinn, 495
 
 
 
BIBLIOGRAPHY.
 
 
 
CEPHALOPODA.
 
(1) A. Kowalevsky. " Entwicklungsgeschichte des Amphioxus lanceolatus."
Mem. Acad. Imper. des Sciences de St Pttersbourg, Series vn. Tom. XI. 1867.
 
(2) A. Kowalevsky. "Weitere Studien iiber die Entwicklungsgeschichte des
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(3) Leuckart u. Pagenstecher. " Untersuchungen tiber niedere Seethiere."
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(4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit.
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(5) A. M. Marshall. "On the mode of Ovi position of Amphioxus." your,
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UROCHORDA.
 
(6) P. J. van Beneden. " Recherches s. 1'Embryogenie, 1'Anat. et la Physiol.
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(7) W. K. Brooks. "On the development of Salpa." Bull, of the Museum of
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(8) H. Fol. Eludes surles Appendiculaires du detroit de Mcssine . Geneve et
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(9) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien."
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(10) C. Gegenbaur. " Ueber den Entwicklungscyclus von Doliolum nebst
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(11) A. Giard. "Etudes critiques des travaux d'embryogenie relatifs a la
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(12) A. Giard. " Recherches sur les Synascidies. " Archiv Zool. exper., Vol. I.
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(13) O. Hertwig. "Untersuchungen lib. d. Bau u. d. Entwicklung des Cellulose-Mantels d. Tunicaten." Jenaische Zeitschrift, Bd. vn. 1873.
 
(14) Th. H. Huxley. " Remarks upon Appendicularia and Doliolum. " Phil.
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(15) Th. H.Huxley. " Observations on the anatomy and physiology of Salpa
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(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." Linnean
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(17) Keferstein u. Ehlers. Zoologische Beitrage, 1861. Doliolum.
 
(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien." Mem.
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(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." Nachrichtcn d.
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(21) A. Kowalevsky. "Ueber Knospung d. Ascidien." Archiv f. mikr. Anat.,
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(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." Archiv f.
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B. Hi. a
 
 
 
BIBLIOGRAPHY.
 
 
 
(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." Mailer's Archiv,
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(25) A. Krohn. "Ueber die Fortpfianzungsverhaltnisse d. Botrylliden. " Archiv
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(36) W. Salensky. "Ueber die Entwicklung d. Hoden u. iiber den Generationswechsel d. Salpen." Zeit.f. wiss. Zool., Bd. xxx. Suppl. 1878.
 
(37) C. Semper. " Ueber die Entstehung d. geschichteten Cellulose-Epidermis
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(38) Fr. Todaro. Sopra lo sviluppo e F anatomia delle Salpc. Roma, 1875.
 
(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." Realc
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ELASMOBRANCHII.
 
(40) F. M. Balfour. " A preliminary account of the development of the Elasmobranch Fishes." Quart. J. of Micr. Science, Vol. xiv. 1876.
 
(41) F. M. Balfour. "A Monograph on the development of Elasmob ranch
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(42) Z. Gerbe. " Recherches sur la segmentation de la cicatrule et la formation
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(43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit. fur Anat. u.
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(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus
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(45) R. Leuckart. "Ueber die allmahlige Bildung d. Korpergestalt bei d.
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(46) Fr. Ley dig. Rochen u. Hate. Leipzig, 1852.
 
(47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajae." Kongl.
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(48) Joh. M tiller. Clatter Haie des Aristoteles und iiber die Verschiedenheitcn
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(49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter."
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(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv
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(51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische. "
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BIBLIOGRAPHY.
 
 
 
Ill
 
 
 
(52) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirlwllosen. Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.
 
(53) C. Semper. " Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a. d.
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(54) Wyman. " Observations on the Development of Raja batis." Memoirs of
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TELEOSTEI.
 
(55) Al. Agassiz. " On the young Stages of some Osseous Fishes. I. Development of the Tail." Proceedings of the American Academy of Arts and Sciences,
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(56) Al. Agassiz. "II. Development of the Flounders." Proceedings of the
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(57) K. E. v. Baer. Untersuchungen ilber die Entwicklungsgeschichte der Fische.
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(58) Ch. van Bambeke. "Premiers effets de la fecondation sur les cufs de
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Ch. van Bambeke. " Recherches sur 1'Embryologie des Poissons
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LXXIV.
 
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E. v. Beneden. "A contribution to the history of the Embryonic development of the Teleosteans." Quart. J. of Micr. Sci., Vol. xvm. 1878.
 
(61) E. Calberla. " Zur Entwicklung des Medullarrohres u. d. Chorda
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(62) A. Gotte. "Beitrage zur Entwicklungsgeschichte der Wirbelthiere."
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(63) A. Gotte. " Ueber d. Entwicklung d. Central-Nervensystems der Teleostier." Archiv f. mikr. Anat., Vol. xv. 1878.
 
(64) A. Gotte. " Entwick. d. Teleostierkeime." Zoologischer Anzeiger, No. 3.
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(65) W. His. " Untersuchungen Uber die Entwicklung von Knochenfischen, etc."
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(66) W. His. "Untersuchungen Uber die Bildung des Knochenfischembryo
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(67) E. Klein. "Observations on the early Development of the Common
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(70) M. Lereboullet. "Recherches sur le developpement du brochet de la
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(72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschen u. Thiere. Leipzig,
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a 2
 
 
 
BIBLIOGRAPHY.
 
 
 
CYCLOSTOMATA.
 
(77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri."
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(78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda
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(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtimg am Ei d.
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(82) W. Muller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen. '
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(83) Ph. Owsjannikoff. "Die Entwick. von d. Flussneunaugen. " ^ Vorlauf.
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(84) Ph. Owsjannikoff. On the development of Petromyzon fiuviatihs
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GANOIDEI.
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(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung. "
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BIBLIOGRAPHY,
 
 
 
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vi BIBLIOGRAPHY.
 
 
 
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(139) C. H. Pander. Beitrage z. Entwick. d. Hiinchens im Eie. Wiirzburg,
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(140) A. Rauber. " Ueber die Etnbryonalanlage des Hiihnchens." Centralblatt
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(141) A. Rauber. Ueber die Stellung des Hiihnchens im Entwickhingsplan.
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(142) A. Rauber. " Primitivrinne und Urmund. Beitrage zur Entwicklungsgeschichte des Hiihnchens." Morphol. Jahrbuch, B. II. 1876.
 
(143) A. Rauber. Primitivslreifen und Neurula der Wirbelthiere in normalcr
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(144) R. Remak. Untersuch. iib. d. Entwicklung d. Wirbelthiere. Berlin,
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(145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen
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(146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat.,
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(147) S. L. Schenk. Lehrbuch d. vergleich. Embryol. d. Wirbelthiere. Wien,
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(148) S. Strieker. " Mittheil. iib. d. selbststiindigen Bewegungen embryonaler
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(149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sitzungsber., Vol. LIV. 1866.
 
(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug.
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(151) W. Waldeyer. "Ueber die Keimblatter und den Primitivstreifen bei
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(152) C. F. Wolff. Theoria generationis. Halse, 1759.
 
(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriitcten Hiinchen.
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REPTILIA.
 
(154) C. Kupffer and Benecke. Die erste Entwicklung am Ei d. Keptilien.
Konigsberg, 1878.
 
 
 
BIBLIOGRAPHY, vii
 
 
 
(155) C. Kupffer. "Die Entstehung d. Allantois u. <1. Gastrula d. Wirbclthiere." Zoologischer Anzeiger, Vol. II. 1879, pp. 520, 593, 612.
 
Lacertilia.
 
(156) F. M. Balfour. " On the early Development of the Lacertilia, together
with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.
 
(157) Emmert u. Hochstetter. " Untersuchung lib. d. Entwick. d. Eidechsen
in ihren Eiern." Reil's Archiv, Vol. X. 1811.
 
(158) M. Lereboullet. "Developpement de la Truite, du Lc/ard et du
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(159) W. K. Parker. "Structure and Devel. of the Skull in Lacertilia.''
Phil. Trans., Vol. 170, p. 2. 1879.
 
(160) H. Strahl. " Ueb. d. Canalis myeloentericus d. Eidechse." Schrift. d.
Gesell. z. Be/or, d. gesam. Naturwiss. Marburg. July 23, 1880.
 
Ophidia.
 
(161) H. Dutrochet. " Recherches s. 1. en veloppes du foetus." Mem. d. Soc.
Mcd. if Emulation, Paris, Vol. vm. 1.816.
 
(162) W. K. Parker. "On the skull of the common Snake." Phil. Trans. ,
Vol. 169, Part II. 1878.
 
(163) H. Rathke. EntTvick. d. Natter. Konigsberg, 1839.
 
Chelonia.
 
(164) L. Agassiz. Contributions to the Natural History of the United Slates,
Vol. u. 1857. Embryology of the Turtle.
 
(165) W. K. Parker. "On the development of the skull and nerves in the
green Turtle." Proc. of the Roy. Soc., Vol. xxvin. 1879. Vide also Nature,
April 14, 1879, and Challenger Reports, Vol. I. 1880.
 
(166) H. Rathke. Ueb. d. Entwicklung d. Schildkroten. Braunschweig, 1848.
 
Crocodilia.
 
(167) H. Rathke. Ueber die Entwicklung d. Krokodile. Braunschweig, 1866.
 
MAMMALIA.
 
(168) K. E. von Baer. Ueb. Entwicklungsgcschichte d. Jhiere. Konigsberg,
 
(169) Barry. "Researches on Embryology." First Series. Philosophical
Transactions, 1838, Part II. Second Series, Ibid. 1839, Part II. Third Series, Ibid.
1840.
 
(170) Ed. van Beneden. La maturation de Foeuf, la fecondation et les premieres
phases du developpement embryonaire d. Mammiferes. Bruxelles, 1875.
 
(171) Ed. van Beneden. " Recherches sur 1'embryologie des Mammiferes.
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(172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc.
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(173) Th. L. W. Bischoff. Entivicklungsgeschichte d. Siiugethiere 11. des
Menschcn. Leipzig, 1842.
 
(174) Th. L. W. Bischoff. Entivicklungsgeschichte des Kanmcheneies. Braunschweig, 1842.
 
(175) Th. L. W. Bischoff. Entwicklungsgeschuhte des Hundeeies.
 
schweig, 1845.
 
(176) Th. L. W. Bischoff. Entivicklungsgesclnchte des Meerschivcinchens.
 
Giessen. 1852.
 
 
 
viii BIBLIOGRAPHY.
 
 
 
(177) Th. L. W. Bischoff. Entivicklungsgeschichte des Rehcs. Giesscn, 1854.
 
(178) Th. L. W. Bischoff. " Neue Beobachtungen z. Entwicklungsgesch. des
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(179) Th. L. W. Bischoff. Historisch-kritische B enter kungen z. d. naicstcn
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(180) M. Coste. Embryogenie comparee. Paris, 1837.
 
(181) E. Haeckel. Anthropogenie, Entwicklungsgeschichte des Menschen.
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(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens
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(183) A. Kolliker. Entivicklungsgeschichte d. Menschen u. d. hb'hcren Thiere.
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(184) A. Kolliker. "Die Entwick. d. Keimblatter des Kaninchens." Zoologist her Anseiger, Nos. 61, 62, Vol. in. 1880.
 
(185) N. Lieberkiihn. Ueber d. Keimblatter d. Siiugethiere. Doctor- Jubelfeier
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(186) N. Lieberkiihn. "Z. Lehre von d. Keimblattern d. Saugethiere." Sitz.
d. Gesell. z. Beford. d. gesam. Natunviss. Marburg, No. 3. 1880.
 
(187) Rauber. "Die erste Entwicklung d. Kaninchens." Sitzungsber. d.
naturfor. Gesell. z. Leipzig. 1875.
 
(188) C. B. Reichert. " Entwicklung des Meerschweinchens." Abh. der.
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(189) E. A. S chafer. " Description of a Mammalian ovum in an early condition of development." Proc. Roy. Soc., No. 168. 1876.
 
(190) E. A. Schafer. "A contribution to the history of development of the
guinea-pig." Journal of Anal, and Phys. , Vol. x. and xi. 1876 and 1877.
 
Fcetal Membranes and Placenta of Mammalia.
 
(191) John Anderson. Anatomical and Zoological Researches in Western
Yunnan. London, 1878.
 
(192) K. E. von Baer. Untersuchungen iiber die Gef&ssverbindung zwischen
Mutter und Fruc/tf, 1828.
 
(193) C. G. Cams. Tabulae anatomiam comparali-vam illustrantes. 1831,
1840.
 
(194) H. C. Chapman. "The placenta and generative apparatus of the
Elephant." Journ. Acad. Nat. Sc., Philadelphia. Vol. viii. 1880.
 
(195) C. Creighton. " On the formation of the placenta in the guinea-pig."
Journal of Anat. and Phys., Vol. XII. 1878.
 
(196) Ecker. Icones Physiologicae. 1852-1859.
 
(197) G. B. Ercolani. 7'he utricular glands of the uterus, etc., translated from
the Italian under the direction of H. O. Marcy. Boston, 1880. Contains translations
of memoirs published in the Mem. deW Accad. d. Scienze d. Bologna, and additional
matter written specially for the translation.
 
(198) G. B. Ercolani. Nuove ricerche sulla placenta nei pesci cartilaginosi e
nei mammiferi. Bologna, 1 880.
 
(199) Eschricht. De organis quae respirationi et mttritioni fcetus Mammaliutn
inservinnt. Hafniae, 1837.
 
(200) A. H. Gar rod and W. Turner. "The gravid uterus and placenta of
Hyomoschus aquaticus." Proc. Zool. Soc., London, 1878.
 
(201) P. Hart ing. Het ei en de placenta van Halicore Dugong. Inaug. diss.
Utrecht. " On the ovum and placenta of the Dugong." Abstract by Prof. Turner.
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(202) Th. H. Huxley. The Elements of Comparative Anatomy. London,
1864.
 
(203) A. Kolliker. " Ueber die Placenta der Gattung Tragulus." Verh. der
Wiirzb. phys.-med. Gesellschaft, Bd. x.
 
(204) C. D. Meigs. "On the reproduction of the Opossum (Didelphis Virginiana)." Amer. Phil. Soc. Trans., Vol. x. 1853.
 
(205) H.Milne-Edwards. " Sur la Classification Naturelle." Ann. Sciences
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BIBLIOGRAPHY.
 
 
 
IX
 
 
 
(206) Alf. Milne-Edwards. "Kecherches sur la famille dcs Chcvrutains.' 1
Ann. dcs Sciences Nat., Series V., Vol. II. 1864.
 
(207) Alf. Milne-Edwards. " Observations sur quelqucs points <le I'Kmbryologie des Lemuriens, etc." Ann. Sci. Nat., Ser. V., Vol. xv. 1872.
 
(208) Alf. Milne-Edwards. " Sur la conformation du placenta chcz le Tainandua." Ann. des Sci. Nat., xv. 1872.
 
(209) Alf. Milne-Edwards. " Kecherches s. 1. enveloppes fcetales du Tatou a
neuf bandes." Ann. Sci. Nat., Ser. vi., Vol. vill. 1878.
 
(210) R. Owen. "On the generation of Marsupial animals, with a description
of the impregnated uterus of the Kangaroo." Phil. Trans., 1834.
 
(211) R. Owen. "Description of the membranes of the uterine foetus of the
Kangaroo." Mag. Nat. Hist., Vol. I. 1837.
 
(212) R. Owen. "On the existence of an Allantois in a foetal Kangaroo
(Macropus major)." Zool. Soc. Proc., v. 1837.
 
(213) R. Owen. "Description of the foetal membranes and placenta of the
Elephant." Phil. Trans., 1857.
 
(214) R.Owen. On the Anatomy of Vertebrates, Vol. III. London, 1868.
 
(215) G. Rolleston. " Placental structure of the Tenrec, etc." Transactions
of the Zoological Society, Vol. V. 1866.
 
(216) W. Turner. "Observations on the structure of the human placenta."
Journal of Anat. and Phys., Vol. vn. 1868.
 
(217) W. Turner. "On the placentation of the Cetacea." Trans. Roy. Soc.
Edinb,, Vol. xxvi. 1872.
 
(218) W. Turner. "On the placentation of Sloths (Cholcepus Hoffrnanni)."
Trans, of R. Society of Edinburgh, Vol. xxvn. 1875.
 
(219) W. Turner. "On the placentation of Seals (Halichcerus gryphus)."
Trans, of R. Society of Edinburgh, Vol. xxvii. 1875.
 
(220) W.Turner. "On the placentation of the Cape Ant-eater (Orycteropus
capensis)." Journal of Anat. and Phys., Vol. X. 1876.
 
(221) W. Turner. Lectures on the Anatomy of the Placenta. First Series.
Edinburgh, 1876.
 
(222) W. Turner. "Some general observations on the placenta, with special
reference to the theory of Evolution." Journal of Anat. and Phys., Vol. XI. 1877.
 
(223) W.Turner. " On the placentation of the Lemurs." Phil. Trans., Vol.
166, p. 2. 1877.
 
(224) W.Turner. " On the placentation of Apes." Phil. Trans., 1878.
 
(225) W. Turner. "The cotyledonary and diffused placenta of the Mexican
deer (Cervus Americanus). " Journal of Anat. and Phys., Vol. xm. 1879.
 
 
 
Human Embryo.
 
(226) Fried. Ahlfeld. " Beschreibung eines sehr kleinen menschlichen Eies."
Archiv f. Gynaekologie, Bd. xm. 1878.
 
(227) Herm. Beigel und Ludwig Loewe. "Beschreibung eines menschlichen
Eichens aus der zweiten bis dritten Woche der Schwangerschaft." Archiv f. Gynaekologie, Bd. xn. 1877.
 
(228) K. Breus. " Ueber ein menschliches Ei aus der zweiten Woche der
Graviditat." Wiener medicinische Wochenschrift, 1877.
 
(229) M. Coste. Histoire generale et particuliere du developpement des corps organises, 1847-59.
 
(230) A. Ecker. Icones Physiologicae. Leipzig, 1851-1859.
 
(231) V. Hensen. " Beitrag z. Morphologic d. Korperform u. d. Gehirns d.
menschlichen Embryos." Archiv f. Anat. u. Phys., 1877.
 
(232) W. His. Anatomie menschlicher Etnbryonen, Part I. Embryonen d.
ersten Monats. Leipzig, 1880.
 
(233) J. Kollmann. " Die menschlichen Eier von 6 MM. Grosse." Archiv f.
 
 
 
Anat. und Phys., 1879.
 
(234) W. Krause.
Phys., 1875.
 
(235) W. Krause.
/. wiss. Zool., Vol. xxxv.
 
 
 
Ueber d. Allantois d. Menschen." Archiv f. Anat. und
 
 
 
' Ueber zwei friihzeitige menschliche Embryonen."
1880.
 
 
 
Zeit.
 
 
 
X BIBLIOGRAPHY.
 
 
 
(236) L. Loewe. "Im Sachen cler Eihaute jiingster menschlicher Eicr. "
Archiv fiir Gynaekologie, Bd. xiv. 1879.
 
(237) C. B. Reichert. " Beschreibung einer friihzeitigen menschlichcn Frucht
im blaschenformigen Bildungszustande (sackformiger Keim von Baer) nebst vergleichenden Untersuchungen iiber die blaschenformigen Friichte der Saugethiere und des
Menschen. " Abhandlungcn der konigl. Akad. d, Wiss, zu Berlin, 1873.
 
(238) Allen Thomson. "Contributions to the history of the structure of the
human ovum and embryo before the third week after conception ; with a description
of some early ova." Edinburgh Med. Siirg.Journal, Vol. LI I. 1839.
 
COMPARISON OF THE FORMATION OF THE GERMINAL LAYERS
IN THE VERTEBRATA.
 
(239) F. M. Balfour. "A comparison of the early stages in the development
of Vertebrates." Quart. J. of Micr. Science, Vol. xv. 1875.
 
(240) F. M. Balfour. "A monograph on the development of Elasmobranch
Fishes." London, 1878.
 
(241) F. M. Balfour. " On the early development of the Lacertilia together
with some observations, etc." Quart. J. of Micr. Science, Vol. xix. 1879.
 
(242) A. Gotte. Die Entwicklungsgeschichte d. Unke. Leipzig, 1875.
 
(243) W. His. "Ueb. d. Bildung d. Haifischembryonen." Zeit. f. Anal. it.
Entwick., Vol. II. 1877. Cf. also His' papers on Teleostei, Nos. 65 and 66.
 
(244) A. Kowalevsky. " Entwick. d. Amphioxus lanceolatus." Mem. Acad.
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(245) A. Kowalevsky. " Weitere Studien lib. d. Entwick. d. Amphioxus lanceolatus." Archiv f. mikr. Anal., Vol. XIII. 1877.
 
(246) C. Kupffer. "Die Entstehung d. Allantois u. d. Gastrula d. Wirbelthiere." Zool. Anzeiger, Vol. II. 1879, PP- 5 2 ' 593' 61?.
 
(247) R. Remak. Untersuchungen iib. d. Entiuicklung d. Wirbelthiere, 1850
1858.
 
(248) A. Rauber. Primitivstreifen ti. Neurula d. Wirbelthiere, Leipzig,
1877.
 
PHYLOGENY OF THE CHORDATA.
 
(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
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(250) A. Dohrn. Der Ursprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.
 
(251) E. Haeckel. Schb'pfungsgeschichte. Leipzig. Vide also Translation.
The History of Creation. King and Co. , London. 1876.
 
(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Antliropogeny. Kegan Paul and Co., London, 1878.
 
(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus."
Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. xi. 1867, and Archiv f. ?nikr.
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(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien."
Archiv f. mikr. Anat., Vol. VII. 1871.
 
(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. u. 1875.
 
(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere."
Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876 1877.
 
GENERAL WORKS ON EMBRYOLOGY.
 
(257) Allen Thomson. British Association Address, 1877.
 
(258) A. Agassiz. "Embryology of the Ctenophoroe." Mem. Amcr. Acad. of
Arts and Sciences, Vol. X. 1874.
 
(259) K. E. von Baer. Ueb. Entivicklnngsgeschichle d. Thiere. Konigsberg,
18281837.
 
 
 
BIBLIOGRAPHY.
 
 
 
XI
 
 
 
(260) F. M. Balfour. "A Comparison of the Early Stages in the Development
of Vertebrates." Qttart. Journ. of Micr. Set., Vol. XV. 1875.
 
 
 
(261)
1874.
 
 
 
C. Glaus. Die Typenlehre u. E. HaeckeFs sg. Gastnca-theorie. Wieii,
 
 
 
(262) C. Claus. Grundziige d. Zoologie. Marburg und Leipzig, 1879.
 
(263) A. Dohrn. Der Ursprung d. Wirbdlhiere u. d. Princip des Functionswechsds. Leipzig, 1875.
 
(264) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878.
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(265) A. Gotte. Ent^vicklungsgeschichte d. Unke. Leipzig, 1874.
 
(266) E. Haeckel. Studien z. Gastrcca-theorie, Jena, 1877; anc ' a ' so Jenaische
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(267) E. Haeckel. Schdpfungsgeschichte. Leipzig. Vide also Translation,
The History of Creation. King & Co., London, 1878.
 
(268) E. Haeckel. Anthropogenic. Leipzig. Vide also Translation, Atithropogeny. Kegan Paul & Co., London, 1878.
 
(269) B. Hatschek. "Studien lib. Entwicklungsgeschichte d. Anneliden."
Arbeit, a. d. zool. Instit. d. Univer. Wien. 1878.
 
(270) O. and R. Hertwig. " Die Actinien." Jenaische Zeitschrift, Vols. xiil.
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(271) O. and R. Hertwig. Die Cctlomtheorie. Jena, 1881.
 
(272) O. Hertwig. Die Chatognathen. Jena, 1880.
 
(273) R. Hertwig. Ueb. d. Ban d. Ctenophoren. Jena, 1880.
 
(274) T. H. Huxley. The Anatomy of Invertebrated Animals. Churchill,
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(274*) T. H. Huxley. "On the Classification of the Animal Kingdom."
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(275) N. Kleinenberg. Hydra, eine anatomisch-entivicklungsgeschichte Untersnchung. Leipzig, 1872.
 
(276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hbh. Thiere.
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(277) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden."
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(278) E. R. Lankester. "On the Germinal Layers of the Embryo as the
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1873
(279) E. R. Lankester. " Notes on Embryology and Classification." Quart.
 
Jotirn. of Alter. Set., Vol. xvn. 1877.
 
(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme."
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(281) E. Metschnikoff. " Spongiologische Studien." Zeit. f. wiss. Zool.,
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(282) A. S. P. Packard. Life Histories of Animals, including Man, or Outlines
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(283) C. Rabl. " Ueb. d. Entwick. d. Malermuschel. " Jenaische Zeitsch., Vol.
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(284) C. Rabl. "Ueb. d. Entwicklung. d. Tellerschneke (Planorbis)." Morph.
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(285) H. Rathke. Abhandhmgen z. Bildung und Enlwicklungsgesch.d. Menschen
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(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig,
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(287) R. Remak. Untersuch. ilb. d. Entwick. d. Wirbelthiere. Berlin, 1855.
 
(288) Salensky. " Bemerkungen lib. Haeckels Gastrsea-theorie." Archiv /.
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(289) E. Schafer. "Some Teachings of Development." Quart. Jotint. of Micr.
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(290) C. Semper. " Die Verwandtschaftbeziehungen d. gegliederten Thiere."
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Xll BIBLIOGRAPHY.
 
 
 
GENERAL WORKS DEALING WITH THE DEVELOPMENT OF
THE ORGANS OF THE CHORDATA.
 
(291) K. E. von Baer. Ueber Enlwicklungsgeschichte d. Thiere. Konigsberg,
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(292) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes.
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(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Siiugdhiere u. d. Menschen.
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(294) C. Gegenbaur. Grundriss d. vergleichenden Anatomic. Leipzig, 1878.
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(295) M. Foster and F. M. Balfour. The Elements of Embryology. Part I.
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(296) Alex. Gotte. Entwickhmgsgeschichte d. Unke. Leipzig, 1875.
 
(297) W. His. Untersuch. ilb. d. erste Anlage d. Wirbelthierleibes. Leipzig,
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(298) A. K 6 Hiker. Entwickhmgsgeschichte d. Menschen u. der hoheren Thiere.
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(299) H. Rathke. Abhandlungen u. Bildung und Entwickhingsgeschichle d.
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(300) H. Rathke. Entwicklungs. d. Natter. Konigsberg, 1839.
 
(301) H. Rathke. Entwicklungs. d. Wirbelthiere. Leipzig, 1861.
 
(302) R. Remak. Untersuchungen iib. d. Entwicklung d. Wirbelthiere. Berlin,
18501855.
 
(303) S. L. Schenk. Lehrbuch d. vergleich. Embryologie d. Wirbelthiere.
Wien, 1874.
 
. EPIDERMIS AND ITS DERIVATIVES.
General.
 
(304) T. H. Huxley. " Tegumentary organs." Todd's Cyclopedia of Anat.
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(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv /. mikr. Anat.
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Scales of the Pisces.
 
(306) O. Hertwig. "Ueber Bau u. Entwicklung d. Placoidschuppen u. d.
Zahne d. Selachier." Jenaische Zeitschrift, Vol. vill. 1874.
 
(307) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrbuch,
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(308) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. Polypterus)." Morph. Jahrbuch, Vol. v. 1879.
 
Feathers.
 
(309) Th. Studer. Die Entwick. d. Federn. Inaug. Diss. Bern, 1873.
 
(310) Th. Studer. " Beitrage z. Entwick. d. Feder." Zeit.f. wiss. Zool., Vol.
xxx. 1878.
 
Sweat-glands.
 
(311) M. S. Ranvier. " Sur la structure des glandes sudoripares." Comptes
Rendus, Dec. 29, 1879.
 
 
 
BIBLIOGRAPHY. xiii
 
 
 
Mammary glands.
 
(312) C. Creighton. "On the development of the Mamma and the Mammary
function." Jour, of Anat. and Phys. , Vol. xi. 1877.
 
(313) C. Gegenbaur. " Bemerkungen lib. d. Milchdriisen-Papillen d. Saugethiere." Jenaische Zeit.. Vol. VII. 1873.
 
(314) M. Huss. " Beitr. z. Entwick. d. Milchdriisen b. Menschen u. b. Wiederkauern." Jenaische Zeit., Vol. vil. 1873.
 
(315) C. Langer. " Ueber d. Bau u. d. Entwicklung d. Milchdriisen." Denk.
d. k. Akad. Wiss. Wien, Vol. in. 1851.
 
THE NERVOUS SYSTEM.
Evolution of the Nervous System.
 
(316) F. M. Balfour. " Address to the Department of Anat. and Physiol. of the
British Association." 1880.
 
(317) C. Claus. "Studien lib. Polypen u. Quallen d. Adria. I. Acalephen,
Discomedusen." Denk. d. math.-natiirwiss. Classe d. k. Akad. Wiss. Wien, Vol.
xxxvin. 1877.
 
(318) Th. Eimer. Zoologische Studien a. Capri. I. Ueber Beroe ovatus. Ein
Beitrag z. Anat. d. Rippenquallen. Leipzig, 1873.
 
(319) V. Hensen. " Zur Entwicklung d. Nervensystems. " Virchow's Archiv,
Vol. xxx. 1864.
 
(320) O. and R. Hertwig. Das Nerven system u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.
 
(321) O. and R. Hertwig. "Die Actinien anat. u. histol. mit besond. Beriicksichtigung d. Nervenmuskelsystem untersucht." Jenaische Zeit., Vol. xiii. 1879.
 
(322) R. Hertwig. "Ueb. d. Bau d. Ctenophoren." Jenaische Zeitschrift,
Vol. xiv. 1880.
 
(323) A. W. Hubrecht. "The Peripheral Nervous System in Palseo- and
Schizonemertini, one of the layers of the body-wall." Quart, y. of Micr. Science,
Vol. xx. 1880.
 
(324) N. Kleinenberg. Hydra, eine anatomisch-entwickhmgsgeschichthche Untersuchung. Leipzig, 1872.
 
(325) A. Kowalevsky. " Embryologische Studien an Wtirmern u. Arthropoden." Mem. Acad. Petersboiirg, Series vil., Vol. XVI. 1871.
 
(326) E. A. Schafer. "Observations on the nervous system of Aurelia aurita."
Phil. Trans. 1878.
 
Nervous System of the Invertebrata.
 
(327) F. M. Balfour. "Notes on the development of the Araneina." Quart.
J. of Micr. Science, Vol. xx. 1880.
 
(328) B. Hatschek. "Beitr. z. Entwicklung d. Lepidopteren.' Jenaische
Zeitschrift, Vol. XI. 1877.
 
(329) N. Kleinenberg. "The development of the Earthworm, Lumbncus
Trapezoides." Quart. J. of Micr. Science, Vol. xix. 1879.
 
(330) A. Kowalevsky. "Embryologische Studien an Wiirmern u. Arthropoden." Mem. Acad. Petersbourg, Series vin., Vol. xvi. 1871.
 
(331) H. Reichenbach. "Die Embryonalanlage u. erste Entwick. d. Flusskrebses." Zeit.f. wiss. Zool, Vol. xxix. 1877.
 
Central Nervous System of the Vertebrata.
 
(332) C. J. Carus. Versuch einer Darstellung d. Nervensystems, etc. Leipzig,
 
(333) J. L. Clark. " Researches on the development of the spinal cord in Man,
Mammalia and Birds." Phil. Trans., 1862.
 
 
 
xiv BIBLIOGRAPHY.
 
 
 
(334) E. Dursy. " Beitrage zur Entwicklungsgeschichte des Hirnanhanges. "
Centralblatt f. d. med. \Vissenschaften, 1 868. Nr. 8.
 
(335) E. Dursy. Zur Entwicklungsgeschichte des Kopfes des Menschen und der
hb'heren Wirbelthiere. Tiibingen, 1869.
 
(336) A. Ecker. "Zur Entwicklungsgeschichte der Furchen und Windungen
der Grosshirn-Hemispharen im Foetus des Menschen." Archiv f. Anthropologie, v.
Ecker und Lindenschmidt. Vol. ill. 1868.
 
(337) E. Ehlers. " Die Epiphyse am Gehirn d. Plagiostomen." Zeit.f.wiss.
Zool. Vol. xxx., suppl. 1878.
 
(338) P. Flechsig. Die Leitungsbahnen im Gehirn und Riickenmark des
Menschen. Auf Grtind entwicklungsgeschichtlicher Untersuchungen. Leipzig, 1876.
 
(339) V. Hensen. "Zur Entwicklung des Nervensystems." Virchoisfs Archiv,
Bd. xxx. 1864.
 
(340) L. Lowe. " Beitrage z. Anat. u. z. Entwick. d. Nervensystems d. Saugethiere u. d. Menschen." Berlin, 1880.
 
(341) L. Lowe. " Beitrage z. vergleich. Morphogenesis d. centralen Nervensystems d. Wirbelthiere." Mitthcil. a. d. embryol. Instit. Wien, Vol. u. 1880.
 
(342) A. M. Marshall. "The Morphology of the Vertebrate Olfactory organ."
Quart. J. of Micr. Science, Vol. xix. 1879.
 
(343) V. v. Mihalkovics. Entwicklungsgeschichte d. Gehirns. Leipzig, 1877.
 
(344) W. Miiller. " Ueber Entwicklung und Bau der Hypophysis und des
Processus infundibuli cerebri. " Jenaische Zeitschrift. Bd. vi. 1871.
 
(345) H. Rahl- Ruck hard. "Die gegenseitigen Verhaltnisse d. Chorda,
Hypophysis etc. bei Haifischembryonen, nebst Bemerkungen lib. d. Deutung d.
einzelnen Theile d. Fischgehirns." Morphol. Jahrbuch, Vol. vi. 1880.
 
(346) H. Rathke. " Ueber die Entstehung der glandula pituitaria. " Mullens
Archiv f. Anat. und Physiol. , Bd. V. 1838.
 
(347) C. B. Reich ert. Der Bau des menschlichen Gehirns. Leipzig, 1859 u 1861.
 
(348) F. Schmidt. "Beitrage zur Entwicklungsgeschichte des Gehirns."
Zcitschrift f. wiss. Zoologie, 1862. Bd. xi.
 
(349) G. Schwalbe. "Beitrag z. Entwick. d. Zwischenhirns." Sitz. d.
Jenaischen Gesell.f. Med. u. Natttnviss. Jan. 23, 1880.
 
(350) Fried. Tiedemann. Anatomie und Bildungsgeschichte des Gehirns im
Foetus des Menschen. Niirnberg, 1816.
 
Peripheral Nervous System of the Vertebrata.
 
(351) F. M. Balfour. "On the development of the spinal nerves in Elasmobranch Fishes." Philosophical Transactions, Vol. CLXVI. 1876; vide also, A monograph on the development of Elasmobranch Fishes. London, 1878, pp. 191216.
 
(352) W. His. " Ueb. d. Anfiinge d. peripherischen Nervensystems." Archiv
f. Anat. u. Physiol., 1879.
 
(353) A. M. Marshall. " On the early stages of development of the nerves in
Birds." Jottrnal of Anat. and Fkys.,Vo\. XI. 1877.
 
(354) A. M. Marshall. "The development of the cranial nerves in the Chick."
Quart, y. of Micr. Science, Vol. xvni. 1878.
 
(355) A. M. Marshall. "The morphology of the vertebrate olfactory organ."
Quart, y. of Micr. Science, Vol. xix. 1879.
 
(356) A. M. Marshall. " On the head-cavities and associated nerves in Elasmobranchs." Quart, y. of Micr. Science, Vol. xxi. 1881.
 
(357) C. Schwalbe. "Das Ganglion oculomotorii. " Jenaische Zeitschrift,
Vol. xni. 1879.
 
Sympathetic Nervous System.
 
(360) F. M. Balfour. Monograph on the development of Elasmobranch Fishes.
London, 1878, p. 173.
 
(361) S. L. Schenk and W. R. Birdsell. "Ueb. d. Lehre vond. Entwicklung
d. Ganglien d. Sympatheticus." Mittheil. a. d. cmbryologischen Instit. Wien. Heft
III. 1879.
 
 
 
BIBLIOGRAPHY. XV
 
 
 
THE EYE.
 
Eye of the Mollusca.
 
(362) N. Bobretzky. " Observations on the development of the Cephalopoda "
(Russian). Nachrichtcn d. kaiserlichen Gesell. d. Frennde der Natuna iss. Anthropolog.
Ethnogr. bei d. Universitdt Moskau.
 
(363) H. Grenacher. " Zur Entwicklungsgeschichte d. Cephalopoden." Zeit.
f. wiss. Zool., Bd. xxiv. 1874.
 
(364) V. Hensen. "Ueber d. Auge einiger Cephalopoden." Zeit. f. wiss.
Zool., Vol. xv. 1865.
 
(365) E. R. Lankester. " Observations on the development of the Cephalopoda." Quart. J. of Micr. Science, Vol. xv. 1875.
 
(366) C. Semper. Ueber Sehorganevon Typus d. Wirbelthicraugen. Wiesbaden,
1877.
 
Eye of the Arthropoda.
 
(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.
 
(368) A. Dohrn. " Untersuchungen lib. Bau u. Entwicklung d. Arthropoden.
Palinurus und Scyllarus. " Zeit. f. wiss. Zool., Bd. xx. 1870, p. 264 et seq.
 
(369) E. Claparede. "Morphologic d. zusammengesetzten Auges bei den Arthropoden." Zeit. f. wiss. Zool., Bd. X. 1860.
 
(370) H. Grenacher. Untersuchungen iib. d. Sehorgane d. Arthropoden.
Gottingen, 1879.
 
The Vertebrate Eye.
 
(371) J.Arnold. Beitrage zur Entwicklungsgeschichle des A uges. Heidelberg,
1874.
 
(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wiirzliurger naturwissenschaftliche Zeitschrift, Bd. 8.
 
(373) L. Kessler. Zur Ent^vicklung d. Auges d. Wirbclthiere. Leipzig, 1877.
 
(374) N. Lieberkiihn. Ueber das Auge des Wirbelthierembryo. Cassel, 1872.
 
(375) N. Lieberkiihn. " Beitrage z. Anat. d. embryonalen Auges." Archiv
f. Anat. und Phys., 1879.
 
(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese
der Retina." Archiv f. mikr. Anat., Vol. xv. 1878.
 
(377) V. Mihalkowics. "Untersuchungen iiber den Kamm des Vogelauges."
Archiv f. mikr. Anat., Vol. IX. 1873.
 
(378) W. Miiller. " Ueber die Stammesentwickelung des Sehorgans der Wirbelthiere." Festgabe Carl Ludwig. Leipzig, 1874.
 
(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische."
Wiener Sitzungsberichte, Bd. LV. 1867.
 
Accessory organs of the Vertebrate Eye.
 
(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien."
Morphologisches Jahrbuch, Bd. II. 1876.
 
(381) G. Born. " Die Nasenhohlen u. d. Thranennasengang d. amnioten Wirbelthiere. I. Lacertilia. II. Aves." Morphologisches Jahrbuch, Bd. V. 1879.
 
Eye of the T2tnicata,
 
(382) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen
Ascidien." Archiv f. mikr. Anat., Vol. VII. 1871.
 
(383) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f.
mikr. Anat., Vol. VII. 1872.
 
 
 
xvi BIBLIOGRAPHY.
 
 
 
AUDITORY ORGANS.
Auditory organs of tlie Invertebrata.
 
(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeil.f. wiss.
Zool., Vol. xui. 1863.
 
(385) O. and R. Her twig. Das Nervensystem u. d. Sinnesorgane d. Medusen.
Leipzig, 1878.
 
Auditory organs of the Vertebrata.
 
(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d.
kaiserl. Leap. Carol. Akad. d. Wissenschaft., Vol. xxxv.
 
(387) C. Hasse. Dievergleich. Morphologieu. Histologied. hciutigen Gehororgane
d. Wirbelthiere. Leipzig, 1873.
 
(388) V. Hensen. "Zur Morphologie d. Schnecke." Zeit. f, wiss. ZooI.,Vo\.
 
XIII. 1863.
 
(389) E. Huschke. "Ueb. d. erste Bildungsgeschichte d. Auges u. Ohres beim
bebrliteten Kiichlein." Isis von Oken, 1831, and Meckel's Archiv, Vol. VI.
 
(390) Reissner. De Auris internee formatione. Inaug. Diss. Dorpat, 1851.
 
Accessory parts of Vertebrate Ear.
 
(391) David Hunt. "A comparative sketch of the development of the ear and
eye in the Pig. " Transactions of the International Otological Congress, 1 876.
 
(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres."
Morphol. Jahrbiich, Vol. ill. 1877.
 
(393) V. Urbantschitsch. " Ueb. d. erste Anlage d. Mittelohres u. d. Trommelfelles." Mittheil. a. d. embryol. Instit. Wien, Heft I. 1877.
 
OLFACTORY ORGAN.
 
(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten
Wirbelthiere." Parts I. and II. Morphologisches Jahrbuch, Bd. V., 1879.
 
(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen."
Festschrift f. Rienecker, 1877.
 
(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ."
Quart. Journ. of Micr. Science, Vol. xix., 1879.
 
SENSE-ORGANS OF THE LATERAL LINE.
 
(397) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
pp. 141 146. London, 1878.
 
(398) H. Eisig. "Die Segmentalorgane d. Capitelliden." Mitlhcil. a. d. zool.
Station zu Neapel, Vol. I. 1879.
 
(399) A. Gotte. Entwicklungsgeschichte d. Unke. Leipzig, 1875.
 
(400) Fr. Ley dig. Lehrbuch d. Histologie des Menschen u. d. Thiere. Hamm.
 
T857
(401) Fr. Ley dig. Nene Beitrdge z. anat. Kenntniss d. Haiitdecke u. IJautsinnesorgane d. Fische. Halle, 1879.
 
(402) F. E. Schulze. "Ueb. d. Sinnesorgane d. Seitenlinie bei Fischen und
Amphibien." Archiv f. mikr. Anat., Vol. vi. 1870.
 
(403) C. Semper. "Das Urogenitalsystem d. Selachier." Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. II.
 
(404) B. Solger. "Neue Untersuchungen zur Anat. d. Seitenorgane d. Fische."
Archiv f. mikr. Anat., Vol. xvil. and xvni. 1879 and 1880.
 
ORIGIN OF THE SKELETON.
 
(405) C. Gegenbaur. "Ueb. primare u. secundare Knochenliildung mit besonderer Beziehung auf d. Lehre von dem Primordialcranium." Jciiaischc Zeitschrifl, Vol. in. 1867.
 
 
 
BIBLIOGRAPHY. xvii
 
 
 
(406) O. Hertwig. "Ueber Bau u. Entwicklung cl. Placoidschuppcn u. d.
Ziihne d. Selachicr." Jetiaische Zeitschrift, Vol. vm. 1874.
 
(407) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bcdeutung
f. d. Genese d. Skelets d. Mundhohle." Archiv f. mikr. Anat., Vol. xi. Supplementheft, 1874.
 
(408) O. Hertwig. " Ueber d. Hautskelet d. Fische." Morphol. Jahrlmch,
Vol. u. 1876. (Siluroiden u. Acipenseriden.)
 
(409) O. Hertwig. "Ueber d. Hautskelet d. Fische (Lepidosteus u. I'olypterus)." Morph. Jahrbnch, Vol. v. 1879.
 
(410) A. Kolliker. "AllgemeineBetrachtungenub. die Entstehungd. knocliernen Schadels d. Wirbelthiere. " Berichle v. d. konigl. zoot. Anstalt z. \Viirzlwrg,
1849.
 
(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bichir."
Zeit.f. wiss. Zool., Vol. V. 1858.
 
(412) H. Muller. "Ueber d. Entwick. d. Knochensubstanz nebst Bemerkungen, etc." Zeit. f. wiss. Zool., Vol. IX. 1859.
 
(413) Williamson. "On the structure and development of the Scales and
Bones of Fishes." Phil. Trans., 1851.
 
(414) Vrolik. " Studien iib. d. Verknocherung u. die Knochen d. Schadels d.
Teleostier." Niederldndisches Archiv f. Zoologie, Vol. i.
 
 
 
NOTOCHORD AND VERTEBRAL COLUMN.
 
(415) Cartier. " Beitrage zur Entwicklungsgeschichte der Wirbelsaule." Zeitschrift fur wiss. Zool., Bd. xxv. Suppl. 1875.
 
(416) C. Gegenbaur. Untersuchungen zur vergleichenden Anatomic der Wirbelsaule der Amphibien und Reptilien. Leipzig, 1862.
 
(417) C. Gegenbaur. "Ueber die Entwickelung der Wirbelsaule des Lepidosteus mit vergleichend anatomischen Bemerkungen." Jenaisckc Zeitschrift, Bd. ill.
1863.
 
(418) C. Gegenbaur. "Ueb. d. Skeletgewebe d. Cyclostomen." Jenaische
Zeitschrift, Vol. v. 1870.
 
(419) Al. Gotte. "Beitrage zur vergleich. Morphol. des Skeletsystems d.
Wirbelthiere." II. "Die Wirbelsaule u. ihre Anhange." Archiv f. mikr. Anat., Vol.
xv. 1878 (Cyclostomen, Ganoiden, Plagiostomen, Chimaera), and Vol. xvi. 1879
(Teleostier).
 
(420) Hasse und Schwarck. "Studien zur vergleichenden Anatomic der
Wirbelsaule u. s. w." Hasse, Anatomische Studiett, 1872.
 
(421) C. Hasse. Das natiirliche System d. Elasmobranchier auf Grundlage d.
Bau. u. d. Entwick. ihrer Wirbelsaule. Jena, 1879.
 
(422) A. Kolliker. " Ueber die Beziehungen der Chorda dorsalis zur Bildung
der Wirbel der Selachier und einiger anderen Fische." Verhandlungen der physical,
medicin. Gesellschaft in Wiirzburg, Bd. X.
 
(423) A. Kolliker. " Weitere Beobachtungen iiber die Wirbel der Selachier
insbesondere iiber die Wirbel der Lamnoidei." Abhandhmgen der senkenbergischen
naturforschenden Gesellschaft in Frankfurt, Bd. V.
 
(424) H. Leboucq. " Recherches s. 1. mode de disparition de la corde dorsale
chez les vertebres superieurs." Archives de Biologie, Vol. I. 1 880.
 
(425) Fr. Leydig. Anatomisch-histologische Untersuchungen iiber Fische und
Reptilien. Berlin, 1853.
 
(426) Aug. Muller. "Beobachtungen zur vergleichenden Anatomic der Wirbelsaule." Miiller's Archiv. 1853.
 
(427) J. Muller. " Vergleichende Anatomic der Myxinoiden u. der Cyklostomen mit durchbohrtem Gaumen, I. Osteologie und Myologie." Abhandlungcn der
koniglichen Akademie der Wissenschaften zu Berlin. 1834.
 
(428) W. Muller. "Beobachtungen des pathologischen Instituts zu Jena, I.
Ueber den Bau der Chorda dorsalis." Jenaische Zeitschrift, Bd. VI. 1871.
 
(429) A. Schneider. Beitrage z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.
 
B. III. *
 
 
 
xviii BIBLIOGRAPHY.
 
 
 
RIBS AND STERNUM.
 
(430) C. Claus. " Beitrage z. vergleich. Osteol. d. Vertcbraten. I. Rippen u.
unteres Bogensystem." Sitz. d. kaiserl. Akad. Wiss. Wien, Vol. LXXIV. 1876.
 
(431) A. E. Fick. "Zur Entwicklungsgeschichte d. Rippen und Querfortsritze." Archiv f. Anat. und Physiol. 1879.
 
(432) C. Gegenbaur. "Zur Entwick. d. Wirbelsaule des Lepidosteus mil
vergleich. anat. Bemerk." Jenaische Zeit., Vol. III. 1867.
 
(433) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere Brustbein u. Schultergiirtel." Archiv f. mikr. Anat., Vol. xiv. 1877.
 
(434) C. Hasse u. G. Born. " Bcmerkungen lib. d. Morphologic d. Rippen."
Zoologischer Anzeiger, 1879.
 
(435) C.K.Hoffmann. " Beitrage z. vergl. Anat. d. Wirbelthiere." Niederliind. Archiv Zool., Vol. iv. 1878.
 
(436) W. K. Parker. " A monograph on the structure and development of the
shoulder-girdle and sternum." Ray Soc. 1867.
 
(437) H. Rathke. Ueb. d. Ban u. d. Enlivicklung d. Brustbeins d. Sanricr.
 
1853
(438) G. Ruge. " Untersuch. lib. Entwick. am Brustbeine d. Menschen."
Morphol. Jahrlmch., Vol. VI. 1880.
 
THE SKULL.
 
(439) A. Duges. "Recherches sur 1'Osteologie et la myologie des Batraciens a
leur differents ages." Paris, Mem. savans tirang. 1835, and An. Sci. Nat. Vol. I.
1834.
 
(440) C. Gegenbaur. UntersucJmngen z. vergleich. Anat. d. Wirbelthiere, III.
Heft. Das Kopfskelet d. Selachier. Leipzig, 1872.
 
(441) Giinther. Beob. iib. die Entwick. d. Gehbrorgans. Leipzig, 1842.
 
(442) O. Hertwig. " Ueb. d. Zahnsystem d. Amphibien u. seine Bedeutung f.
d. Genese d. Skelets d. Mundhohle. " Archiv f. mikr, Anat., Vol. xi. 1874, suppl.
 
(443) T. H. Huxley. "On the theory of the vertebrate skull." Proc. Royal
Soc., Vol. ix. 1858.
 
f444) T.H.Huxley. The Elements of Comparative Anatomy . London, 1869.
 
 
 
(445
(446
(447
 
 
 
T. H. Huxley. "On the Malleus and Incus." Proc. Zool. Soc.,
 
T. H. Huxley. "On Ceratodus Forsteri." Proc. Zool. Soc., 1876.
 
T. H. Huxley. " The nature of the craniofacial apparatus of Petromyzon."
 
 
 
Journ. of Anat. and Phys., Vol. X. 1876.
 
(448) T. H. Huxley. The Anatomy of Vertebrated Animals. London, 1871.
 
(449) W. K. Parker. "On the structure and development of the skull of the
Common Fowl (Gallus Domesticus). " Phil. Trans., 1869.
 
(450) W. K. Parker. "On the structure and development of the skull of the
Common Frog (Rana temporaria)." Phil. Trans., 1871.
 
(451) W. K. Parker. "On the structure and development of the skull in the
Salmon (Salmo salar)." Bakerian Lecture, Phil. Trans., 1873.
 
(452) W. K. Parker. "On the structure and development of the skull in the
Pig (Susscrofa)." Phil. Trans., 1874.
 
(453) W. K. Parker. "On the structure and development of the skull in the
Batrachia." Part II. Phil. Trans., 1876.
 
(454) W. K. Parker. "On the structure and development of the skull in the
Urodelous Amphibia." Part in. Phil. Trans., 1877.
 
(455) W. K. Parker. "On the structure and development of the skull in the
Common Snake (Tropidonotus natrix)." Phil. Trans. , 1878.
 
(456) W. K. Parker. "On the structure and development of the skull in Sharks
and Skates." Trans. Zoolog. Soc., 1878. Vol. x. pt. iv.
 
(1.17) W. K. Parker. "On the structure and development of the skull in the
Lacertilia." Pt. I. Lacerta agilis, L. viridis and Zootoca vivipara. Phil. Trans.,
1879.
 
 
 
BIBLIOGRAPHY,
 
 
 
(458) W. K. Parker. "The development of the Green Turtle." The Zoolo-v
of the Voyage of H.M.S. Challenger. Vol. I. pt. v.
 
(459) W. K. Parker. "The structure and development of the skull in the
Batrachia." 1't. in. Phil. Trans., 1880.
 
(460) W. K. Parker and G. T. Bettany. The Morphology of the Skull.
London, 1877.
 
(460*) H. Rathke. Entwick. d. Natter. Konigsberg, 1830.
 
(461) C. B. Reichert. " Ueber die Visceralbogen d. Wirbelthiere." Mailer's
Archiv, 1837.
 
(462) W. Salensky. " Beitrage z. Entwick. d. knorpeligen Gehorknochelchen."
Morphol. Jahrbuch, Vol. VI. 1880.
 
Vide also Kolliker (No. 298), especially for the human and mammalian skull;
Gotte (No. 296).
 
THE PECTORAL GIRDLE.
 
(463) Bruch. "Ueber die Entwicklung der Clavicula und die Farbe des
Blutes." Zeit.f. wiss. Zool., IV. 1853.
 
(464) A. Duges. " Recherches sur 1'osteologie et la myologie des Batraciens a
leurs differents ages." Memoires des savants etrang. Academic royale des sciences de
Finstitut de France, Vol. VI. 1835.
 
(465) C. Gegenbaur. Unterstichungen zur vergleichenden Anatomic der Wirbelthiere, i Heft. Schultergilrtel der Wirbelthiere. Brustflosse der Fische. Leipzig,
1865.
 
(466) A. Gotte. "Beitrage z. vergleich. Morphol. d. Skeletsystems d. Wirbelthiere : Brustbien u. Schultergiirtel. " Archiv f. mikr. Anat. Vol. XIV. 1877.
 
(467) C. K. Hoffmann. "Beitrage z. vergleichenden Anatomic d. Wirbelthiere." Niederldndisches Archiv f. Zool. , Vol. V. 1879.
 
(468) W. K. Parker. " A Monograph on the Structure and Development of the
Shoulder-girdle and Sternum in the Vertebrata." Ray Society, 1868.
 
(469) H. Rathke. Ueber die Entwicklung der Schildkroten. Braunschweig,
1848.
 
(470) H. Rathke. Ueber den Bau und die Entwicklung des Brustbeins der
Satirier, 1853.
 
(471) A. Sab a tier. Comparaison des ceintures et des menibres anteneurs et posterieurs d. la Serie d. Vertebrcs. Montpellier, 1880.
 
(472) Georg 'Swirski. Untersuch. lib. d. Entwick. d. Schultergiirtels u. d.
Skelets d. Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.
 
THE PELVIC GIRDLE.
 
(473) A. Bunge. Untersuch. z. Entwick. d. Beckengilrtels d. Amphibien,
Reptilien u. Vdgel. Inaug. Diss. Dorpat, 1880.
 
(474) C. Gegenbaur. " Ueber d. Ausschluss des Schambeins von d. Pfanne
d. Hiiftgelenkes." Morph. Jahrbuch, Vol. II. 1876.
 
(475) Th. H. Huxley. "The characters of the Pelvis in Mammalia, etc."
Proc. of Roy. Soc., Vol. xxvin. 1879.
 
(476) A. S aba tier. Comparaison des ceintures et des membres anterieurs ct
postb-ieurs dans la Serie d. Vertebres. Montpellier, 1880.
 
SKELETON OF THE LIMBS.
 
(477) M. v. Davidoff. "Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen
d. Fische I." Morphol. Jahrbuch, Vol. v. 1879.
 
(478) C. Gegenbaur. Untersuchungen z. vergleich. Anat. d. Wirbelthiere.
Leipzig, 18645. Erstes Heft. Carpus u. Tarsus. Zweites Heft. Brustflosse d.
Fische.
 
(479) C. Gegenbaur. "Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im
Allgemeinen u. d. Hintergliedmaassen d. Selachier insbesondere." Jenaische Zeilschrift, Vol. V. 1870.
 
 
 
XX BIBLIOGRAPHY.
 
 
 
(480) C. Gegenbaur. " Ueb. d. Archipterygium." Jenaische Zeitschrift, Vol.
vn. 1873.
 
(481) C. Gegenbaur. "Zur Morphologic d. Gliedmaassen d. Wirbelthiere."
Morphologisches Jahrbuch, Vol. II. 1876.
 
(482) A. Gotte. Ueb. Entwick. u. Regeneration d. Gliedmaassenskelets d. Molche.
Leipzig, 1879.
 
(483) T. H. Huxley. "On Ceratodus Forsteri, with some observations on the
classification of Fishes." Proc. Zool. Soc. 1876.
 
(484) St George Mivart. "On the Fins of Elasmobranchii." Zoological
Trans., Vol. x.
 
(485) A. Rosenberg. "Ueb. d. Entwick. d. Extremitaten-Skelets bei einigen
d. Reduction ihrer Gliedmaassen charakterisirten Wirbelthiere." Zeit.f. wiss. Zool.,
Vol. xxin. 1873.
 
(486) E. Rosenberg. "Ueb. d. Entwick. d. Wirbelsaule u. d. centrale carpi
d. Menschen." Morphologisches Jahrbuch, Vol. I. 1875.
 
(487) H. Strasser. "Z. Entwick. d. Extremitatenknorpel bei Salamandern u.
Tritonen." Morphologisches Jahrbuch, Vol. V. 1879.
 
(488) G. 'S wirski. Unterstich. iib. d. Entwick. d. Schnltergiirtels u. d. Skelets d.
Brustflosse d. Hechts. Inaug. Diss. Dorpat, 1880.
 
(489) J. K. Thacker. "Median and paired fins. A contribution to the history
of the Vertebrate limbs." Trans, oftke Connecticut Acad., Vol. III. 1877.
 
(490) J. K. Thacker. "Ventral fins of Ganoids." Trans, of the Connecticut
Acad., Vol. IV. 1877.
 
PLEURAL AND PERICARDIAL CAVITIES.
 
(491) M. Cadiat. " Du developpement de la partie cephalothoracique de 1'embryon, de la formation du diaphragme, des pleures, du pericarde, du pharynx et de
1'cesophage." Journal de FAnatomie et de la Physiologic, Vol. xiv. 1878.
 
VASCULAR SYSTEM.
The Heart.
 
(492) A. C. Bernays. " Entwicklungsgeschichte d. Atrioventricularklappen."
Morphol. Jahrbuch, Vol. 11. 1876.
 
(493) E. Gasser. " Ueber d. Entstehung d. Herzens beim Hiihn." Archiv f.
mikr. Anat., Vol. xiv.
 
(494) A. Thomson. "On the development of the vascular system of the foetus
of Vertebrated Animals." Edinb. New Phil. Journal, Vol. ix. 1830 and 1831.
 
(495) M. Tonge. "Observations on the development of the semilunar valves
of the aorta and pulmonary artery of the heart of the Chick." Phil. Trans. CLIX.
1869.
 
Vide also Von Baer (291), Rathke (300), Hensen (182), Kolliker (298), Gotte (296),
and Balfour (292).
 
The Arterial System.
 
(496) H. Rathke. "Ueb. d. Entwick. d. Arterien w. bei d. Saugethiere von
d. Bogen d. Aorta ausgehen." Miiller's Archiv, 1843.
 
(41)7) PI. Rathke. " Untersuchungen iib. d. Aortenwurzeln d. Saurier."
Denkschriften d. k. Akad. Wien, Vol. xiil. 1857.
 
Vide also His (No. 232) and general works on Vertebrate Embryology.
 
The Venous System.
 
(498) J.Marshall. "On the development of the great anterior veins." Phil.
Trans., 1859.
 
 
 
BIHLIOGRAI'IIY. XXJ
 
 
 
(499) H. Rathke. " Ueb. d. Bildung d. Pfortader u. d. Lebervenen b. Sauge
thieren." Meckel 's Archiv, 1830.
 
(500) H. Rathke. "Ueb. d. Bau u. d. Entwick. d. Venensystems d. Wirbclthiere." Bericht. iib. d. natttrh. Seminar, d. Univ. Konigsberg, 1838.
 
Vide also Von Baer (No. 291), Gotte (No. 296), Kolliker (No. 298), and Rathke
(Nos. 299, 300, and 301).
 
THE SPLEEN.
 
(501) W. Miiller. "The Spleen." Strieker's Histology.
 
(502) Peremeschko. "Ueb. d. Entwick. d. Milz." Silz. d. Wien. Akad.
Wiss., Vol. LVI. 1867.
 
THE SUPRARENAL BODIES.
 
(503) M. Braun. "Bau u. Entwick. d. Nebennieren bei Reptilian." Arbeit,
a. d. zool.-zoot. Institut Wilrzburg, Vol. v. 1879.
 
(504) A. v. Brunn. "Ein Beitrag z. Kenntniss d. feinern Baues u. d. Entwick.
d. Nebennieren." Archiv f. mikr. Anat., Vol. vni. 1872.
 
(505) Fr. Leydig. Untersuch. ilb. Fische u. Reptilien. Berlin, 1853.
 
(506) Fr. Leydig. Rochen u. Haie. Leipzig, 1852.
 
Vide also F. M. Balfour (No. 292), Kolliker (No. 298), Remak (No. 302), etc.
 
THE MUSCULAR SYSTEM OF THE VERTEBRATA.
 
(507) G.M.Humphry. " Muscles in Vertebrate Animals." J our n. of Anat.
and Phys., Vol. vi. 1872.
 
(508) J. Miiller. "Vergleichende Anatomic d. Myxinoiden. Part I. Osteologie
u. Myologie." Akad. Wiss., Berlin, 1834.
 
(509) A. M. Marshall. "On the head cavities and associated nerves of
Elasmobranchs." Quart. J. of Micr. Science, Vol. XXI. 1881.
 
(510) A. Schneider. "Anat. u. Entwick. d. Muskelsystems d. Wirbelthiere."
Sitz. d. Oberhessischen Gesellschaft, 1873.
 
(511) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.
 
Vide also Gotte (No. 296), Kolliker (No. 298), Balfour (No. 292), Huxley, etc.
 
EXCRETORY ORGANS.
 
INVER TEBRA TA .
 
(512) H. Eisig. " Die Segmentalorgane d. Capitelliden." Mitth. a. d. zool.
Slat. z. Neapel, Vol. I. 1879.
 
(513) J. Fraipont. " Recherches s. 1'appareil excreteur des Irematc
Cestoides." Archives de Biologie, Vol. I. 1880.
 
(514) B. Hatschek. "Studien iib. Entwick. d. Annehden. Arbeit, a. d.
zool. Instil. Wien, Vol. I. 1878. .
 
(515) B. Hatschek. "Ueber Entwick. von Echmrus, etc. Arbeit, a.
 
zool. Instit. Wien, Vol. ill. 1880.
 
VERTEBRATA.
 
General.
 
(516) F. M. Balfour. "On the origin and history of the urinogenital organs of
Vertebrates." Journal of Anat. and Phys., Vol. X. 1876.
 
 
 
XXJi BIBLIOGRAPHY.
 
 
 
(517) Max. Fiirbringer 1 . "Zur vergleichenden Anat. u. Entwick. d. Excretionsorgane d. Vertebraten." Morphol. Jahrbuch, Vol. IV. 1878.
 
(518) H. Meckel. Zur Morphol. d. Harn- u. Geschlechtswerkz.d. Wirbelthiere,
etc. Halle, 1848.
 
(519) Job. Mtiller. Bildungsgeschichte d. Genitalien, etc. Diisseldorf, 1830.
 
(520) H. Ratbke. "Beobachtungen u. Betrachtungen ii. d. Entwicklung d.
Geschlechtswerkzeuge bei den Wirbelthieren." N. Schriften d. naturf. Gesell. in
Dantzig, Bd. I. 1825.
 
(521) C. Semper 1 . "Das Urogenitalsystem d. Plagiostomen u. seine Bedeutung f. d. ubrigen Wirbelthiere." Arb. a. d. zool.-zoot. Insiit. Wiirzburg, Vol. u.
 
1875
(522) W. Waldeyer 1 . Eierstock u. Ei. Leipzig, 1870.
 
ElasmobrancJdi.
 
(523) A. Schultz. "Zur Entwick. d. Selachiereies." Archiv f. mikr. Anal.,
Vol. xi. 1875.
 
Vide also Semper (No. 521) and Balfour (No. 292).
 
Cyclostomata.
 
(524) J. M uller. " Untersuchungen ii. d. Eingeweide d. Fische. " Abh. d. k.
Ak. Wiss. Berlin, 1845.
 
(525) W. Muller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa."
Jenaische Zeitschrift, Vol. VII. 1873.
 
(526) W. Muller. "Ueber d. Urogenitalsystem d. Amphioxus u. d. Cyclostomen." Jenaische Zeitschrift, Vol. ix. 1875.
 
(527) A. Schneider. Beitrdge z. vergleich. Anat. u. Entwick. d. Wirbelthiere.
Berlin, 1879.
 
(528) W. B. Scott. "Beitrage z. Entwick. d. Petromyzonten." Morphol.
Jahrbuch, Vol. vn. 1881.
 
Teleostei.
 
(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." Denkschr. d.
k. k. Akad. Wiss. Wien, Vol. II. 1850.
 
(530) A. Rosenberg. Untersuchungen iib. die Enlwicklung d. Teleostierniere.
Dorpat, 1867.
 
Vide also Oellacher (No. 72).
 
Amphibia.
 
(531) F. H. Bidder. Vergleichend-anatomische u. histologisclie Untcrsiiclniii^cn
ii. die mdnnlichcn Geschlec/its- tmd Harmverkzeuge d. nackten Amphibien. Dorpat,
1846.
 
(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des
Reptiles," etc. Mem. Acad. Sciences. Paris. Vol. xi. 1851, pp. 17 95.
 
(533) M. Fiirbringer. Zur Entwicklung d. Amphibienniere. Heidelberg, 1877.
 
(534) F. Ley dig. Analomie d. Amphibien u. Keptilien. Berlin, 1853.
 
(535) F. Leydig. Lehrbuch d. Histologie. Hamm, 1857.
 
(536) F. Meyer. "Anat. d. Urogenitalsystems d. Selachier u. Amphibien."
Sitz. d. naturfor. Gesellsch. Leipzig, 1875.
 
(537) J. W. Spengel. "Das Urogenitalsystem d. Amphibien." Arb. a. d.
zool.- zoot. Instil. Wiirzburg. Vol. in. 1876.
 
(538) Von Wittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien." Zeit.
f. wiss. Zool., Vol. iv.
 
Vide also Gotte (No. 296).
 
1 The papers of Fiirbringer, Semper and Waldeyer contain full references to the
literature of the Vertebrate excretory organs.
 
 
 
BIBLIOGRAPHY. xxiii
 
 
 
Amniota.
 
(539) F. M. Balfour and A. Sedgwick. "On the existence of ahead-kidney
in the embryo Chick," etc. Quart. J. of Micr. Science, Vol. XIX. 1878.
 
(540) Banks. On the Wolffian bodies of the foetus and their remains in the adult.
Edinburgh, 1864.
 
(541) Th. Bornhaupt. UntersucJnmgen iib. die Entwicklung d. Urogenitalsystems beim Hiihnchen. Inaug. Diss. Riga, 1867.
 
(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien."
Arbeiten a. d. zool.-zoot. Instit. Wiirzburg. Vol. IV. 1877.
 
(543) J. Dansky u. J. Kostenitsch. " Ueb. d. Entwick. d. Keimblatter u. d.
Wolffschen Ganges im Htihnerei." Me"m. Acad. Imp. Petersbourg, vn. Series, Vol.
xxvn. 1880.
 
(544) Th. Egli. Beitrdge zur Anat. tmd Entiuick. d. Geschlechtsorgane. Inaug.
Diss. Zurich, 1876.
 
(545) E. Gasser. Beitrdge zur Entwickhmgsgeschichte d. Allantois, der
MiUler' schen Giinge u. des Afters. Frankfurt, 1874.
 
(546) E. Gasser. " Beob. iib. d. Entstehung d. WolfFschen Ganges bei Embryonen von Hiihnern u. Gansen." Arch, fiir mikr. Anat., Vol. xiv. 1877.
 
(547) E. Gasser. "Beitrage z. Entwicklung d. Urogenitalsystems d. Htihnerembryonen." Sitz. d. Cesell. zur Beforderung d. gesam. Naturwiss. Marburg, 1879.
 
(548) C. Kupffer. " Untersuchung liber die Entwicklung des Harn- und Geschlechtssystems." Archiv fiir mikr. Anat., Vol. II. 1866.
 
(549) A. Sedgwick. "Development of the kidney in its relation to the
Wolffian body in the Chick." Quart. J. of Micros. Science, Vol. XX. 1880.
 
(550) A. Sedgwick. "On the development of the structure known as the
glomerulus of the head -kidney in the Chick." Quart. J. of Micros. Science, Vol. XX.
1880.
 
(551) A. Sedgwick. "Early development of the Wolffian duct and anterior
Wolffian tubules in the Chick ; with some remarks on the vertebrate excretory
system." Quart. J. of Micros. Science, Vol. xxi. 1881.
 
(552) M. Watson. "The homology of the sexual organs, illustrated by comparative anatomy and pathology." Journal of Anat. and Phys., Vol. XIV. 1879.
 
(553) E. H. Weber. Zusdtze z, Lehre von Bane u. d. Verrichtungen d. Geschlechtsorgane. Leipzig, 1846.
 
Vide also Remak (No. 302), Foster and Balfour (No. 295), His (No. 297),
Kolliker (No. 298).
 
GENERATIVE ORGANS.
 
(554) G. Balbiani. Lemons s. la generation des Vertebres. Paris, 1879.
 
(555) F. M. Balfour. "On the structure and development of the Vertebrate
ovary." Quart. J. of Micr. Science, Vol. XVIII.
 
(556) E. van Beneden. "De la distinction originelledutecticuleet del'ovaire,
etc." Bull. Ac. roy. belgique, Vol. xxxvn. 1874.
 
(557) N. Kleinenberg. "Ueb. d. Entstehung d. Eier b. Eudendrhim." Zeit.
f. wiss. Zool., Vol. xxxv. 1 88 r.
 
(558) H. Ludwig. "Ueb. d. Eibildung im Theirreiche. " Arbeit, a. d. zool.zoot. Instit. Wiirzburg, Vol. I. 1874.
 
(559) C. Semper. "Das Urogenitalsystem d. Plagiostomen, etc." Arbeit, a.
d. zool.-zoot. Instit. Wiirzburg, Vol. II. 1875.
 
(560) A. Weismann. "Zur Frage nach clem Ursprung d. Geschlechtszellen bei
den Hydroiden." Zool. Anzeiger, No. 55, 1880.
 
Vide also O. and R. Hertwig (No. 271), Kolliker (No. 298), etc.
 
ALIMENTARY CANAL AND ITS APPENDAGES.
 
(561) B. Afanassiew. " Ueber Bau u. Entwicklung d. Thymus d. Saugeth."
Archiv f. mikr. Anat. Bd. XIV. 1877.
 
 
 
XXIV BIBLIOGRAPHY.
 
 
 
(562) Fr. Boll. Das Princip d. Wachsthums. Berlin, 1876.
 
(563) E. Gasser. "Die Entstehung d. Cloakenoffhung hei Hiihneremhryonen."
Archiv f. Anat. u. Physiol., Anat. Abth. 1880.
 
(564) A. Gotte. Beitrage zur Entwicklungsgeschichte 'd. Darmkanah im
Hithnchcn. 1867.
 
(565) W. Miiller. " Ueber die Entwickelung der Schilddriise." ycnaische
Zeitschrift, Vol. vi. 1871.
 
(566) W. Miiller. "Die Hypobranchialrinne d. Tunicaten." Jenaischc Zeitschrift, Vol. VII. 1872.
 
(567) S. L. Schenk. "Die Bauchspeicheldriise d. Embryo." Anatomischphysiologische UntersucJnmgcn. 1872.
 
(568) E. Selenka. " Beitrag zur Entwicklungsgeschichte d. Luftsacke d.
Huhns." Zeit.f. wiss. Zool. 1866.
 
(569) L. Stieda. Untersuch. lib. d. Entivick. d. Glandula Thymus, Glandula
thyroidea, u. Glandula carotica. Leipzig, 1881.
 
(570) C. Fr. Wolff. " De formatione intestinorum." Nov. Comment. Akad.
Petrop. 1766.
 
(571) A. Wblfler. Ueb. d. Entwick. it. d. Ban d. Schilddriise. Berlin, 1880.
Vide also Kolliker (298), Qotte (296), His (232 and 297), Foster and Balfour (2!)5),
 
Balfour (292), Remak (302), Schenk (303), etc.
 
Teeth.
 
(572) T. H. Huxley. "On the enamel and dentine of teeth." Quart. J. of
Micros. Science, Vol. III. 1855.
 
(573) R. Owen. Odontography. London, 1840 1845.
 
(574) Ch. S. Tomes. Manual of dental anatomy, human and comparative.
London, 1876.
 
(575) Ch. S. Tomes. " On the development of teeth." Quart. J. of Micros.
Science, Vol. xvi. 1876.
 
(576) W. Waldeyer. " Structure and development of teeth." Strieker 's Histology. 1870.
 
Vide also Kolliker (298), Gegenbaur (294), Hertwig (306), etc.
 
 
 
 
 
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Cephalochorda | Urochorda | Elasmobranchii | Teleostei | Cyclostomata | Ganoidei | Amphibia | Aves | Reptilia | Mammalia | Comparison of the Formation of Germinal Layers and Early Stages in Vertebrate Development | Ancestral form of the Chordata | General Conclusions | Epidermis and Derivatives | The Nervous System | Organs of Vision | Auditory, Olfactory, and Lateral Line Sense Organs | Notochord, Vertebral Column, Ribs, and Sternum | The Skull | Pectoral and Pelvic Girdles and Limb Skeleton | Body Cavity, Vascular System and Glands | The Muscular System | Excretory Organs | Generative Organs and Genital Ducts | The Alimentary Canal and Appendages in Chordata
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This historic 1885 book edited by Foster and Sedgwick is the third of Francis Balfour's collected works published in four editions. Francis (Frank) Maitland Balfour, known as F. M. Balfour, (November 10, 1851 - July 19, 1882) was a British biologist who co-authored embryology textbooks.



Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.

Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol. III. A Treatise on Comparative Embryology 2 (1885)

CHAPTER XII. OBSERVATIONS ON THE ANCESTRAL FORM OF THE CHORDATA

THE present section of this work would not be complete without some attempt to reconstruct, from the materials recorded in the previous chapters, and from those supplied by comparative anatomy, the characters of the ancestors of the Chordata ; and to trace as far as possible from what invertebrate stock this ancestor was derived.

The second of these questions has been recently dealt with in a very suggestive manner by both Dohrn (No. 250) and Semper (Nos. 255 and 256), but it is still so obscure that I shall refrain from any detailed discussion of it.

While differing very widely in many points both Dohrn and Semper have arrived at the view, already tentatively put forward by earlier anatomists, that the nearest allies of the Chordata are to be sought for amongst the Chaetopoda, and that the dorsal surface of the Chordata with the spinal cord corresponds morphologically with the ventral surface of the Chaetopods with the ventral ganglion chain. In discussing this subject some time ago x I suggested that we must look for the ancestors of the Chordata, not in allies of the present Chaetopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chaetopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally, instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to its existence is well founded, appears now to have perished. The recent researches of Hubrecht on the anatomy of the Nemertines a have, however, added somewhat to the probability of my views, in that they shew that in some existing Nemertines the nerve-cords approach each other very closely in the dorsal line.

With reference to the characters of the ancestor of the Chordata the following pages contain a few tentative suggestions rather than an attempt to deal with the whole subject ; while the

1 Monograph on the development of Elasmobranch Fishes, pp. 170 173.

2 Hubrecht, "Zur Anat. u. Phys. d. Nervensystems der Nemertinen. " Kon. Akad. Wiss. Amsterdam; and "Researches on the Nervous System of Nemertines." Quart. Journ. of Micr. Science, 1880.


312 THE PR^iORAL LOBE.

origin of certain of the organs is dealt with in a more special manner in the chapters on organogeny which form the second part of this work.

Before entering upon the more special subject of this chapter, it will be convenient to clear the ground by insisting on a few morphological conclusions to be drawn from the study of Amphioxus, a form which, although probably in some respects degenerate, is nevertheless capable of furnishing on certain points very valuable evidence.

(1) In the first place it is clear from Amphioxus that the ancestors of the Chordata were segmented, and that their mesoblast was divided into myotomes which extended even into the region in front of the mouth. The mesoblast of the greater part of what is called the head in the Vertebrata proper was therefore segmented like that of the trunk.

(2) The only internal skeleton present was the unsegmented notochord a fact which demonstrates that the skeleton is of comparatively little importance for the solution of a large number of fundamental questions, as for example the point which has been mooted recently as to whether gill-clefts existed at one time in front of the present mouth ; and for this reason : that from the evidence of Amphioxus and the lower Vertebrata 1 it is clear that such clefts, if they ever existed, had atrophied

1 The greater part of the branchial skeleton of Petromyzon appears clearly to belong to an extra-branchial system much more superficially situated than the true branchial bars of the higher forms. At the same time-there is no doubt that certain parts of the skeleton of the adult Lamprey have, as pointed out by Huxley, striking points of resemblance to parts of a true mandibular and hyoid arches. Further embryological evidence is required on the subject, but the statements on this head on p. 84 ought to be qualified.

Should Huxley's views on this subject be finally proved correct, it is probable that, taking into consideration the resemblance of these skeletal parts in the Tadpole to those in the Lamprey, the cartilaginous mandibular bar, before being in any way modified to form true jaws, became secondarily adapted to support a suctorial mouth, and that it subsequently became converted into the true jaws. Thus the evolution of this bar in the Frog would be a true repetition of the ancestral history, while its ontogeny in Elasmobranchii and other types would be much abbreviated. For a fuller statement on this point I must refer the reader to the chapter on the skull.

It is difficult to believe that the posterior branchial bars could have coexisted with such a highly developed branchial skeleton as that in Petromyzon, so that the absence of the posterior branchial bars in Petromyzon receives by far its most plausible explanation on the supposition that Petromyzon is descended from a vertebrate stock in which true branchial bars had not been evolved.


ON THE ANCESTRAL FORM OF THE CHORDATA. 313

completely before the formation of cartilaginous branchial bars ; so that any skeletal structures in front of the mouth, which have been interpreted by morphologists as branchial bars, can never have acted in supporting the walls of branchial clefts.

(3) The region which, in the Vertebrata, forms the oesophagus and stomach, was, in the ancestors of the Chordata, perforated by gill-clefts. This fact, which has been clearly pointed out by Gegenbaur, is demonstrated by the arrangement of the gill-clefts in Amphioxus, and by the distribution of the vagus nerve in the Vertebrata 1 . On the other hand the insertion of the liver, which was probably a very primitive organ, appears to indicate with approximate certainty the posterior limit of the branchial clefts.

With these few preliminary observations we may pass to the main subject of this section. A fundamental question which presents itself on the threshold of our enquiries is the differentiation of the head.

In the Chaetopoda the head is formed of a praeoral lobe and of the oral segment ; while in Arthropods a somewhat variable number of segments are added behind to this primitive head, and form with it what may be called a secondary compound head. It is fairly clear that the section of the trunk, which, in Amphioxus, is perforated by the visceral clefts, has become the head in the Vertebrates proper, so that the latter forms are provided with a secondary head like that of Arthropods. There remain however difficult questions (i) as to the elements of which this head is composed, and (2) as to the extent of its differentiation in the ancestors of the Chordata.

In Arthropods and Chaetopods there is a very distinct element in the head known as the procephalic lobe in the case of Arthropods, and the praeoral lobe in that of Chaetopods ; and this lobe is especially characterized by the fact that the supracesophageal ganglia and optic organs are formed as differentia 1 The extension forwards in the vertebrata of an uninterrupted body-cavity into the region previously occupied by visceral clefts presents no difficulty. In Amphioxus the true body cavity extends forwards, more or less divided by the branchial clefts, for the whole length of the branchial region, and in embryos of the lower Vertebrata there is a section of the body cavity the so-called head-cavities between each pair of pouches. On the disappearance of the pouches all these parts would naturally coalesce into a continuous whole.


314 THE PR/KORAL LOBE.

tions of part of the epiblast covering it. Is such an element to be recognized in the head of the Chordata ? From a superficial examination of Amphioxus the answer would undoubtedly be no ; but then it has to be borne in mind that Amphioxus, in correlation with its habit of burying itself in sand, is especially degenerate in the development of its sense-organs ; so that it is not difficult to believe that its praeoral lobe may have become so reduced as not to be recognizable. In the true Vertebrata there is a portion of the head which has undoubtedly many features of the praeoral lobe in the types already alluded to, viz. the part containing the cerebral hemispheres and the thalamencephalon. If there is any part of the brain homologous with the supracesophageal ganglia of the Invertebrates, and it is difficult to believe there is not such a part, it must be part of, or contain, the fore-brain. The fore-brain resembles the supraoesophageal ganglia in being intimately connected in its development with the optic organs, and in supplying with nerves only organs of sense. Its connection with the olfactory organs is an argument in the same direction. Even in Amphioxus there is a small bulb at the end of the nervous tube supplying what is very probably the homologue of the olfactory organ of the Vertebrata ; and it is quite possible that this bulb is the reduced rudiment of what forms the fore-brain in the Vertebrata.

The evidence at our disposal appears to me to indicate that the third nerve belongs to the cranio-spinal series of segmental nerves, while the optic and olfactory nerves appear to me equally clearly not to belong to this series 1 . The mid-brain, as giving origin to the third nerve, would appear not to have been part of the ganglion of the prseoral lobe.

These considerations indicate with fair probability that the part of the head containing the fore-brain is the equivalent of the praeoral lobe of many Invertebrate forms ; and the primitive position of the Vertebrate mouth on the ventral side of the head affords a distinct support for this view. It must however be admitted that this part of the head is not sharply separated in development from that behind ; and, though the fore-brain is

1 Marshall, in his valuable paper on the development of the olfactory organ, takes a very different view of this subject. For a discussion of this view I must refer the reader to the chapter on the nervous system.


ON THE ANCESTRAL FORM OF THE CHORDATA. 315

usually differentiated very early as a distinct lobe of the primitive nervous tube, yet that such differentiation is hardly more marked than in the other parts of the brain. The termination of the notochord immediately behind the fore-brain is, however, an argument in favour of the morphological distinctness of the latter structure.

The evidence at our disposal appears to indicate that the posterior part of the head was not differentiated from the trunk in lower Chordata ; but that, as the Chordata rose in the scale of development, more and more centralizing work became thrown on the anterior part of the nervous cord, and part passu this part became differentiated into the mid- and hind-brain. An analogy for such a differentiation is supplied in the compound subcesophageal ganglion of many Arthropods ; and, as will be shewn in the chapter on the nervous system, there is strong embryological evidence that the mid- and hind-brains had primitively the same structure as the spinal cord. The head appears however to have suffered in the course of its differentiation a great concentration in its posterior part, which becomes progressively more marked, even within the limits of the surviving Vertebrata. This concentration is especially shewn in the structure of the vagus nerve, which, as first pointed out by Gegenbaur, bears evidence of having been originally composed of a great series of nerves, each supplying a visceral cleft. Rudiments of the posterior nerves still remain as the branches to the oesophagus and stomach 1 .

The atrophy of the posterior visceral clefts seems to have taken place simultaneously with the concentration of the neural part of the head ; but the former process did not proceed so rapidly as the latter, so that the visceral region of the head is longer in the lower Vertebrata than the neural region, and is dorsally overlapped by the anterior part of the spinal cord and the anterior muscle-plates (vide fig. 47).

On the above view the posterior part of the head must have been originally composed of a series of somites like those of the

1 The lateral branch of the vagus nerve probably became differentiated in connection with the lateral line, which seems to have been first formed in the head, and subsequently to have extended into the trunk (vide section on Lateral Line).


3'6


Till. MEDULLARY CANAL.


trunk, but in existing Vertebrata all trace of these, except in so far as they are indicated by the visceral clefts, has vanished in the adult. The cranial nerves however, especially in the embryo, still indicate the number of anterior somites ; and an embryonic segmentation of the mesoblast has also been found in many lower forms in the region of the head, giving rise to a series of cavities known as head-cavities, enclosed by mesoblastic walls which afterwards break up into muscles. These cavities correspond with the nerves, and it appears that there is a praemandibular cavity corresponding with the third nerve (fig. 193, \pp) and a mandibular cavity (2pp) and a cavity in each of the succeeding visceral arches. The fifth nerve, the seventh nerve, the glossopharyngeal nerve, and the successive elements of the vagus nerve correspond with the posterior head-cavities.

The medullary canal. The general history of the medullary plate seems to point to the conclusion that the central canal of the nervous system has been formed by a groove having appeared in the ancestor of the Chordata along the median dorsal line, which caused the sides of the nervous plate, which was placed immediately below the skin, or may perhaps at that stage not have been distinctly differentiated from the skin, to be bent upwards ; and that this groove subsequently became converted into a canal. This view is not only supported by the actual development of the central canal of the nervous system (the types of Teleostei, Lepidosteus and Petromyzon being undoubtedly secondary), but also (i) by the presence of cilia in the epithelium lining the canal, probably inherited from cilia coating the external skin, and (2) by



FIG. 193. TRANSVERSE SECTION THROUGH THE FRONT PART OF THE HEAD OF A YOUNG PRISTIURUS

EMBRYO.

The section, owing to the cranial flexure, cuts both the fore- and the hind-brain. It shews the prsemandibular and mandibular head-cavities ipp and ipp, etc.

fb. fore-brain; /. lens of eye; /. mouth ; pt. upper end of mouth, forming pituitary involution; iao.. mandibular aortic arch; ipp. and ipp. first and second head-cavities ; ivc. first visceral cleft ; V. fifth nerve ; aun. ganglion of auditory nerve ; VII. seventh nerve ; aa, dorsal aorta ; acv. anterior cardinal vein; ^..notochord.


ON THE ANCESTRAL FORM OF THE CHORDATA.


317


the posterior roots arising from the extreme dorsal line (fig. 194), a position which can most easily be explained on the supposition that the two sides of the plate, from which the nerves originally proceeded have been folded up so as to meet each other in the median dorsal line 1 .

The medullary plate, before becoming folded to form the medullary groove, is (except in Amphibia) without any indication of being composed of two halves. In both the embryo and adult the walls of the tube have however a structure which points to their having arisen from the coalescence of two lateral, and most probably at one time independent, cords ; and as already indicated this is the view I am myself inclined to adopt ; vide pp. 303 and

304 The origin and nature of the mouth. The most obvious point connected with the development of the mouth is the fact that in all vertebrate embryos it is placed ventrally, at some little distance from the front end of the body. This feature is retained in the adult stage in Elasmobranchii, the Myxinoids, and some Ganoids, but is lost in other vertebrate forms. A mouth, situated as is the embryonic vertebrate mouth, is very ill adapted for biting ; and though it acquires in this position a distinctly biting character in the Elasmobranchii, yet it is almost certain that it had not such a character in the ancestral Chordata, and that its terminal position in higher types indicates a step in advance of the Elasmobranchii.

On the structure of the primitive mouth there appears to me



al


FIG. 194. TRANSVERSE SECTION THROUGH THE TRUNK OF AN EMBRYO SLIGHTLY OLDER THAN FIG. 28 E.

nc. neural canal ; pr. posterior root of spinal nerve ; x. subnotochordal rod ; ao. aorta ; sc. somatic mesoblast ; sp. splanchnic mesoblast ; mp. muscle-plate ; mp'. portion of muscle-plate converted into muscle ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; al. alimentary tract.


1 Vidf for further details the chapter on the nervous system.


318 PRIMITIVE SUCTORIAL MOUTH.

to be some interesting embryological evidence, to which attention has already been called in the preceding chapters. In a large number of the larvae or embryos of the lower Vertebrates the mouth has a more or less distinctly suctorial character, and is connected with suctorial organs which may be placed either in front of or behind it. The more important instances of this kind are (i) the Tadpoles of the Anura, with their posteriorly placed suctorial disc, (2) Lepidosteus larva (fig. 195) with its anteriorly placed suctorial disc, (3) the adhesive papillae of the larvae of the Tunicata. To these may be added the suctorial mouth of the Myxinoid fishes 1 .

All these considerations point to the conclusion that in the ancestral Chordata the mouth had a more or less definitely suctorial character 2 , and was placed on the ventral surface immediately behind the praeoral lobe; and that this mouth has become in the higher types gradually modified for biting purposes, and has been carried to the front end of the head.

The mouth in Elasmobranchii and other Vertebrates is originally a wide somewhat rhomboidal cavity (fig. 28 G) ; on the development of the mandibular and its maxillary (pterygoquadrate) process the opening of the mouth becomes narrowed to a slit. The wide condition of the mouth may not improbably be interpreted as a remnant of the suctorial state. The fact that no more definite remnants of the suctorial mouth are found in so primitive a group as the Elasmobranchii is probably to be explained by the fact that the members of this group undergo an abbreviated development within the egg.


1 The existing Myxinoid P'ishes are no doubt degenerate types, as was first clearly pointed out by Dohrn ; but at the same time (although Dohrn does not share this view) it appears to me almost certain that they are the remnants of a large and very primitive group, which have very likely been preserved owing to their parasitic or semiparasitic habits ; much in the same way as many of the Insectivora have been preserved owing to their subterranean habits. I am acquainted with no evidence, embryological or otherwise, that they are degraded gnathostomatous forms, and the group probably disappeared as a whole from its incapacity to compete successfully with Vertebrata in which true jaws had become developed.

3 I do not conceive that the existence of suctorial structures necessarily implies parasitic habits. They might be used for various purposes, especially by predaceous forms not provided with jaws.


ON THE ANCESTRAL FORM OF THE CHORDATA. 319

While the embryological data appear to me to point to the existence of a primitive suctorial mouth, very different conclusions have been put forward by other embryologists, more especially by Dohrn, which are sufficiently striking and suggestive to merit a further discussion.

As mentioned above, both Dohrn and Semper hold that the Vertebrata are descended from Chastopod-like forms, in which the ventral surface has become the dorsal. In consequence of this view Dohrn has arrived at the following conclusions : (i) that primitively the alimentary canal perforated the nervous system in the region of the original cesophageal nerve-ring ; (2) that there was therefore an original dorsal mouth (the present ventral mouth of the Cheetopoda) ; and (3) that the present mouth was secondary and derived from two visceral clefts which have ventrally coalesced.

A full discussion of these views 1 is not within the scope of this work ; but, while recognizing that there is much to be said in favour of the interchange of the dorsal and ventral surfaces, I am still inclined to hold that the difficulties involved in this view are so great that it must, provisionally at least, be rejected; and that there are therefore no reasons against supposing



-sd


op

FIG. 195. VENTRAL VIEW OF THE HEAD OF A LEPIDOSTEUS EMBRYO SHORTLY

BEFORE HATCHING, TO SHEW THE LARGE SUCTORIAL DISC.

m. mouth ; op. eye ; sd. suctorial disc.

the present vertebrate mouth to be the primitive mouth. There is no embryological evidence in favour of the view adopted by Dohrn that the present mouth was formed by the coalescence of two clefts.

If it is once admitted that the present mouth is the primitive mouth, and is more or less nearly in its original situation, very strong evidence will be required to shew that any structures originally situated in front of it are the remnants of visceral clefts ; and if it should be proved that such remnants of visceral clefts were present, the views so far arrived at in this section would, I think, have to be to a large extent reconsidered.

The nasal pits have been supposed by Dohrn to be remnants of visceral

1 For a partial discussion of this subject I would refer the reader to my Monograph on Elasmobranch Fishes, pp. 165 172.


320 FORMATION OF THE JAWS.

clefts, and this view has been maintained in a very able manner by Marshall. The arguments of Marshall do not, however, appear to me to have any great weight unless it is previously granted that there is an antecedent probability in favour of the presence of a pair of gill-clefts in the position of the nasal pits ; and even then the development of the nasal pits as epiblastic involutions, instead of hypoblastic outgrowths, is a serious difficulty which has not in my opinion been successfully met. A further argument of Marshall from the supposed segmental nature of the olfactory nerve has already been spoken of.

While most of the structures supposed to be remains of gill-clefts in front of the mouth do not appear to me to be of this nature, there is one organ which stands in a more doubtful category. This organ is the so-called choroid gland. The similarity of this organ to the pseudo-branch of the mandibular or hyoid arch was pointed out to me by Dohrn, and the suggestion was made by him that it is the remnant of a praemandibular gill which has been retained owing to its functional connection with the eye 1 . Admitting this explanation to be true (which however is by no means certain) are we necessarily compelled to hold that the choroid gland is the remnant of a gill-cleft originally situated in front of the mouth ? I believe not. It is easy to conceive that there may originally have been a praemandibular cleft behind the suctorial mouth, but that this cleft gradually atrophied (for the same reasons that the mandibular cleft shews a tendency to atrophy in existing fishes, &c.), the rudiment of the gill (choroid gland) alone remaining to mark its situation. After the disappearance of this cleft the suctorial mouth may have become relatively shifted backwards. In the meantime the branchial bars became developed, and as the mouth was changed into a biting one, the

1 The probability of the choroid gland having the meaning attributed to it by Dohrn is strengthened by the existence of a praemandibular segment as evidenced by the presence of a pnemandibular head-cavity, the walls of which as shewn by Marshall and myself give rise to the majority of the eye-muscles and of a nerve (the third nerve, cf. Marshall) corresponding to it; so that these parts together with the choroid gland may be rudiments belonging to the same segment. On the other hand the absence of the choroid gland in Ganoidei and Elasmobranchii, where a mandibular pseudo-branch is present, coupled with the absence of a mandibular pseudo-branch in Teleostei where alone a choroid gland is present, renders the above view about the choroid gland somewhat doubtful. A thorough investigation of the ontogeny of the choroid gland might throw further light on this interesting question, but I think it not impossible that the" choroid gland may be nothing else but the modified mandibtddr pseudo-branch, a view which fits in very well with the relations of the vessels of the Elasmobranch mandibular pseudo-branch to the choroid. For the relations and structure of the choroid gland vide F. Miiller, Vergl. Anal. Myxinoiden, Part in. p. 82.

It is possible that the fourth nerve and the superior oblique muscle of the eye which it supplies may be the last remaining remnants of a second praemandibular segment originally situated between the segment of the third nerve and that of the fifth nerve (mandibular segment).



ON THE ANCESTRAL FORM OF THE CHORDATA. 321


bar (the mandibular arch) supporting the then first cleft became gradually modified and converted into a supporting apparatus for the mouth, and finally formed the skeleton of the jaws. In the hyostylic Vertebrata the hyoid arch also became modified in connection with the formation of the jaws.

The conclusions arrived at may be summed up as follows : The relations which exist in all jaw-bearing Vertebrates between the mandibular arch and the oral aperture are secondary, and arose paripassu with the evolution of the jaws 1 .

The cranial flexure and the form of the head in vertebrate embryos. All embryologists who have studied the embryos of the various vertebrate groups have been struck with the remarkable similarity


Vgr


aur


vir



FIG. 196. THE HEADS OF ELASMOBRANCH EMBRYOS AT TWO STAGES VIEWED

AS TRANSPARENT OBJECTS.

A. Pristiurus embryo of the same stage as fig. 28 F. B. Somewhat older Scyllium embryo.

///. third nerve; V. fifth nerve; VII. seventh nerve; au.n. auditory nerve; gl. glossopharyngeal nerve; Vg. vagus nerve; fb. fore-brain; pn. pineal gland; mb. midbrain; hb. hind-brain; iv.v. fourth ventricle; cb. cerebellum; ol. olfactory pit; op. eye; au.V. auditory vesicle; m. mesoblast at base of brain; ch. notochord; ht. heart; Vc. visceral clefts ; eg. external gills ; //. sections of body cavity in the head.

1 I do not mean to exclude the possibility of the mandibular arch having supported a suctorial mouth before it became converted into a pair of jaws.

B. III. 21


322 POST-ANAL GUT.


which exists between them, more especially as concerns the form of the head. This similarity is closest between the members of the Amniota, but there is also a very marked resemblance between the Amniota and the Elasmobranchii. The peculiarity in question, which is characteristically shewn in fig. 196, consists in the cerebral hemispheres and thalamencephalon being ventrally flexed to such an extent that the mid- brain forms the termination of the long axis of the body. At a later period in development the cerebral hemispheres come to be placed at the front end of the head ; but the original nick or bend of the floor of the brain is never got rid of.

It is obvious that in dealing with the light thrown by embryology on the ancestral form of the Chordata the significance of this peculiar character of the head of many vertebrate embryos must be discussed. Is the constancy of this character to be explained by supposing that at one period vertebrate ancestors had a head with the same features as the embryonic head of existing Vertebrata ?

This is the most obvious explanation, but it does not at the same time appear to me satisfactory. In the first place the mouth is so situated at the time of the maximum cranial flexure that it could hardly have been functional ; so that it is almost impossible to believe that an animal with a head such as that of these embryos can have existed.

Then again, this type of embryonic head is especially characteristic of the Amniota, all of which are developed in the egg. It is not generally so marked in the Ichthyopsida. In Amphibia, Teleostei, Ganoidae and Petromyzontidae, the head never completely acquires the peculiar characteristic form of the head of the Amniota, and all these forms are hatched at a relatively much earlier phase of development, so that they are leading a free existence at a stage when the embryos of the Amniota are not yet hatched. The only Ichthyopsidan type with a head like that of the Amniota is the Elasmobranchii, and the Elasmobranchii are the only Ichthyopsida which undergo the major part of their development within the egg.

These considerations appear to shew that the peculiar characters of the embryonic head above alluded to are in some way connected with an embryonic as opposed to a larval development ; and for reasons which are explained in the section on larval forms, it is probable that a larval development is a more faithful record of ancestral history than an embryonic development. The flexure at the base of the brain appears however to be a typical vertebrate character, but this flexure never led to a conformation of the head in the adult state similar to that of the embryos of the Amniota. The form of the head in these embryos is probably to be explained by supposing that some advantage is gained by a relatively early development of the brain, which appears to be its proximate cause ; and since these embryos had not to lead a free existence (for which such a form of the head would have been unsuited) there was nothing to interfere with the action of natural selection in bringing about this form of head during fcetal life.

Post-anal gut and neurenteric canal. One of the most


ON THE ANCESTRAL FORM OF THE CHORDATA. 323

remarkable structures in the trunk is the post-anal gut (fig. 197). Its structure is fully dealt with in the chapter on the alimentary tract, but attention may here be called to the light which it appears to throw on the characters of the ancestor of the Chordata.

In face of the facts which are known with reference to the post-anal section of the alimentary tract, it can hardly be doubted that this portion of the alimentary tract must have been at one time functional. This seems to me to be shewn (i) by the constancy and persistence of this obviously now functionless rudiment, (2) by its greater development in the lower than in the higher forms, (3) by its relation to the formation of the notochord and subnotochordal rod.

If the above position be admitted, it is not permissible to shirk the conclusions which seem necessarily to follow, however great the difficulties may be which are involved in their accept


FIG. 197. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF BOM BINATOR. (After Gotte.)

m. mouth; an. anus; /. liver; ne. neurenteric canal; me. medullary canal; ch. notochord; pn. pineal gland.

ance. These conclusions have in part already been dealt with by Dohrn in his suggestive tract (No. 250). In the first place the alimentary canal must primitively have been continued to the end of the tail ; and if so, it is hardly credible that the existing anus can have been the original one. Although, therefore, it is far from easy, on the physiological principles involved in the Darwinian theory, to understand the formation of a new anus 1 ; it is nevertheless necessary to believe that the present

1 Dohrn (No. 250, p. 25) gives an explanation of the origin of the new anus which does not appear to me quite satisfactory.

21 2


324


POST-ANAL GUT.


vertebrate anus is a formation acquired within the group of the Chordata, and not inherited from some older group. This involves a series of further consequences. The opening of the urinogenital ducts into the cloaca must also be secondary, and it is probable that the segmental tubes were primitively continued along the whole post-anal region of the vertebrate tail, opening into the body cavity which embryology proves to have been originally present there. They are in fact continued in many existing forms for some distance behind the present anus. If the present anus is secondary, there must have been a primitive anus, which was probably situated behind the post-anal vesicle ; and therefore in the region of the neurenteric canal. The neurenteric canal is, however, the remnant of the blastopore (vide p. 277). It follows, therefore, that tJie vertebrate blastopore is probably almost, if not exactly identical in position with the primitive aims. This consideration may assist in explaining the remarkable phenomenon of the existence of the neurenteric canal. The attempt has already been made to shew that the central canal of the nervous system is really a groove converted into a tube and lined by the external epidermis. This tube (as may be concluded from embryological considerations) was probably at first open posteriorly, and no doubt terminated at the primitive anus. On the closure of the primitive anal opening, the terminal portions of the post-anal gut and the neural tube, may conceivably have been so placed that both of them opened into a common cavity, which previously had communication with the exterior by the anus. Such an arrangement would necessarily result in the formation of a neurenteric canal. It seems not impossible that a dilated vesicle, often present at the end of the post-anal gut (vide fig. 28*, p. 58), may have been the common cavity into which both neural and alimentary tubes opened 1 .

1 As pointed out in Vol. II. p. 255, there is a striking similarity between the history of the neurenteric canal in Vertebrates, and the history of the blastopore and ventral groove as described by Kowalevsky in the larva of Chiton. Mr A. Sedgwick has pointed out to me that the ciliated ventral groove in Protoneomenia, which contains the anus, is probably the homologue of the groove found in the larva of Chiton, and not, as usually supposed, simply the foot. Were this groove to be converted into a canal, on the sides of which were placed the nervous cords, there would be formed a precisely similar neurenteric canal to that in Vertebrata, though I do not mean to suggest that there is any homology between the two (vide Hubrecht, Zool. Anzeigcr, 1880, p. 589).


ON THE ANCESTRAL FORM OF THE CHORDATA. 325

Till further light is thrown by fresh discoveries upon the primitive condition of the posterior continuation of the vertebrate alimentary tract, it is perhaps fruitless to attempt to work out more in detail the 'above speculation.

Body cavity and mesoblastic somites. The Chordata, or at least the most primitive existing members of the group, are characterized by the fact that the body cavity arises as a pair of outgrowths of the archenteric cavity. This feature 1 in the development is a nearly certain indication that the Chordata are a very primitive stock. The most remarkable point with reference to the development of the two outgrowths is, however, the fact that the dorsal part of each outgrowth becomes separated from the ventral. Its walls become segmented and form the mesoblastic somites, which eventually, on the obliteration of their cavity, give rise to the muscle-plates and to the tissue surrounding the notochord. It is not easy to understand the full significance of the processes concerned in the formation of the mesoblastic somites (vide p. 296). The mesoblastic somites have no doubt a striking resemblance to the mesoblastic somites of the Chsetopods, and most probably the segmentation of the mesoblast in the two groups is a phenomenon of the same nature ; but the difference in origin between the two types of mesoblastic somites is so striking, and the development of the muscular system from them is so dissimilar in the two groups, as to render a direct descent of the Chordata from the Chsetopoda very improbable. The ventral parts of the original outgrowth give rise to the permanent body cavity, which appears originally to have been divided into two parts by a dorsal and a ventral mesentery.

The notochord. The most characteristic organ of the Chordata is without doubt the notochord. The ontogenetic development of this organ probably indicates that it arose as a differentiation of the dorsal wall of the archenteron ; at the same time it is not perhaps safe to lay too much stress upon its mode of development. Embryological and anatomical evidence demonstrate, however, in the clearest manner that the early Chordata were provided with this organ as their sole axial skeleton ;

1 Vide the chapter on the Germinal Layers.


326 GILL-CLEFTS.


and no invertebrate group can fairly be regarded as genetically related to the Chordata till it can be shewn to possess some organ either derived from a notochord, or capable of having become developed into a notochord. No such organ has as yet been recognized in any invertebrate group 1 .

Gill-clefts. The gill-clefts, which are essentially pouches of the throat opening externally, constitute extremely characteristic organs of the Chordata, and have always been taken into consideration in any comparison between the Chordata and the Invertebrata.

Amongst the Invertebrata organs of undoubtedly the same nature are, so far as I know, only found in Balanoglossus, where they were discovered by Kowalevsky. The resemblance in this case is very striking ; but although it is quite possible that the gill-clefts in Balanoglossus are genetically connected with those of the Chordata, yet the organization of Balanoglossus is as a whole so different from that of the Chordata that no comparison can be instituted between the two groups in the present state of our knowledge.

Other organs of the Invertebrata have some resemblance to the gill-clefts. The lateral pits of the Nemertines, which appear to grow out as a pair of oesophageal diverticula, which are eventually placed in communication with the exterior by a pair of ciliated canals (vide Vol. II. pp. 200 and 202), are such organs.

Semper (No. 256) has made the interesting discovery that in the budding of Nais and Chaetogaster two lateral masses of cells, in each of which a lumen may be formed, unite with the oral invagination and primitive alimentary canal to form the permanent cephalic gut. The lateral masses of cells are regarded by him as branchial passages homologous in some way with those in the Chordata. The somewhat scanty observations on this subject which he has recorded do not appear to me to lend much support to this interpretation.

It is probable that the part of the alimentary tract in which gill- clefts are present was originally a simple unperforated tube provided with highly vascular walls ; and that respiration was carried on in it by the alternate introduction and expulsion of sea water. A more or less similar mode of respiration has been recently shewn by Eisig 2 to take place in the fore part

1 In the Chaetopods various organs have been interpreted as rudiments of a notochord, but none of these interpretations will bear examination.

2 " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden." Mittheil. a. d. zoo!. Station zu Neapel, Vol. n. 1881.



ON THE ANCESTRAL FORM OF THE CHORDATA. 327

of the alimentary tract of many Chastopods. This part of the alimentary tract was probably provided with paired cascal pouches with their blind ends in contiguity with the skin.

Perforations placing these pouches in communication with the exterior must be supposed to have been formed ; and the existence of openings into the alimentary tract at the end of the tentacles of many Actinias and of the hepatic diverticula of some nudibranchiate Molluscs (Eolis, &C. 1 ) shews that such perforations may easily be made. On the formation of such perforations the water taken in at the mouth would pass out by them ; and the respiration would be localized in the walls of the pouches leading to them, and thus the typical mode of respiration of the Chordata would be established.

Phylogeny of the Chordata. It may be convenient to shew in a definite way the bearing of the above speculations on the phylogeny of the Chordata. For this purpose, I have drawn up the subjoined table, which exhibits what I believe to be the relationships of the existing groups of the Chordata. Such a table cannot of course be constructed from embryological data alone, and it does not fall within the scope of this work to defend its parts in detail.

MAMMALIA SAUROPSIDA

L- T J

. PROTO-AMNIOTA AMPHIBIA


TELEOSTEI PROTO-PENTADACTYLOIDEI

I


GANOIDEI


-DIPNOI


PROTO-GANOIDEI

HOLOCEPHALI -ELASMOBRANCHII


PROTO-GNATHOSTOMATA


Cyclostomata PROTO-VERTEBRATA

Cephafochorda PROTOCHORDATA Uroc/iorda

In the above table the names printed in large capitals are hypothetical groups. The other groups are all in existence at the present day, hut those printed in Italics are probably degenerate.

The ancestral forms of the Chordata, which may be called the Protochordata, must be supposed to have had (i) a

1 The openings of the hepatic diverticula through the sacks lined with thread cells are described by Hancock and Embleton, Ann. and Mag. of Nat. History, Vol. xv. 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzeiger, No. 23) and apparently attributes their discovery to himself.


328 PHYLOGENY OF THE CHORDATA.

notochord as their sole axial skeleton, (2) a ventral mouth, surrounded by suctorial structures, and (3) very numerous gill-slits. Two degenerate offshoots of this stock still persist in Amphioxus (Cephalochorda), and the Ascidians (Urochorda).

The direct descendants of the ancestral Chordata, were probably a group which may be called the Proto-vertebrata, of which there is no persisting representative. In this group, imperfect neural arches were probably present ; and a ventral suctorial mouth without a mandible and maxillae was still persistent. The branchial clefts had, however, become reduced in number, and were provided with gill-folds ; and a secondary head (vide p. 313), with brain and organs of sense like those of the higher Vertebrata, had become formed.

The Cyclostomata are probably a degenerate offshoot of this group.

With the development of the branchial bars, and the conversion of the mandibular bar into the skeleton of the jaws, we come to the Proto-gnathostomata. The nearest living representatives of this group are the Elasmobranchii, which still retain in the adult state the ventrally placed mouth. Owing to the development of food-yolk in the Elasmobranch ovum the early stages of development are to some extent abbreviated, and almost all trace of a stage with a suctorial mouth has become lost.

We next come to an hypothetical group which we may call the Proto-ganoidei. Bridge, in his memoir on Polyodon 1 , which contains some very interesting speculations on the affinities of the Ganoids, has called this group the Pneumatoccela, from the fact that we find for the first time a full development of the air-bladder, though it is possible that a rudiment of this organ, in the form of a pouch opening on the dorsal side of the stomachic extremity of the oesophagus, was present in the earlier type.

Existing Ganoids are descendants of the Proto-ganoidei. Some of them at all events retain in larval life the suctorial mouth of the Proto-vertebrata ; and the mode of formation of their germinal layers, resembling as it does that in the Lamprey

1 Phil. Trans. 1878. Part II.


ON THE ANCESTRAL FORM OF THE CHORDATA. 329

and the Amphibia, probably indicates that they are not descended from forms with a large food-yolk like that of Elasmobranchii, and that the latter group is therefore a lateral offshoot from the main line of descent.

Of the two groups into which the Ganoidei may be divided it is clear that certain members of the one (Teleostoidei), viz. Lepidosteus and Amia, shew approximations to the Teleostei, which no doubt originated from the Ganoids ; while the other (Selachoidei or Sturiones) is more nearly related to the Dipnoi. Polypterus has also marked affinities in this direction, e.g. the external gills of the larva (vide p. 1 18).

The Teleostei, which have in common a meroblastic segmentation, had probably a Ganoid ancestor, the ova of which were provided with a large amount of food-yolk. In most existing Teleostei, the ovum has become again reduced in size, but the meroblastic segmentation has been preserved. It is quite possible that Amia may also be a descendant of the Ganoid ancestor of the Teleostei ; but Lepidosteus, as shewn by its complete segmentation, is clearly not so.

The Dipnoi as well as all the higher Vertebrata are descendants of the Proto-ganoidei.

The character of the limbs of higher Vertebrata indicates that there was an ancestral group, which may be called the Proto-pentadactyloidei, in which the pentadactyle limb became established ; and that to this group the common ancestor of the Amphibia and Amniota belonged.

It is possible that the Plesiosauri and Ichthyosauri of Mesozoic times may have been more nearly related to this group than either to the Amniota or the Amphibia. The Proto-pentadactyloidei were probably much more closely related to the Amphibia than to the Amniota. They certainly must have been capable of living in water as well as on land, and had of course persistent branchial clefts. It is also fairly certain that they were not provided with large-yolked ova, otherwise the mode of formation of the layers in Amphibia could not be easily explained.

The Mammalia and Sauropsida are probably independent offshoots from a common stem which may be called the Protoamniota.


330 BIBLIOGRAPHY.


BIBLIOGRAPHY.

(249) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes, London, 1878.

(250) A. Dohrn. Der (Jrsprung d. Wirbelthiere und d. Princip. d. Functionswechsel. Leipzig, 1875.

(251) E. Haeckel. Sch'dpfungsgeschichte. Leipzig. Vide also Translation. The History of Creation. King and Co. , London. 1876.

(252) E. Haeckel. Anthropogenie. Leipzig. Vide also Translation. Anthropogeny. Kegan Paul and Co., London, 1878.

(253) A. Kowalevsky. " Entwicklungsgeschichte d. Amphioxus lanceolatus." Mem. Acad. d. Scien. St Petersbourg, Ser. VII. Tom. XI. 1867, and Archivf. mikr. Anat., Vol. xin. 1877.

(254) A. Kowalevsky. "Weitere Stud. lib. d. Entwick. d. einfachen Ascidien." Archivf. mikr. Anat., Vol. VII. 1871.

(255) C. Semper. "Die Stammesverwandschaft d. Wirbelthiere u. Wirbellosen." Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. II. 1875.

(256) C. Semper. "Die Verwandschaftbeziehungen d. gegliederten Thiere." Arbeit, a. d. zool.-zoot. Instit. Wurzburg, Vol. III. 1876 1877.

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