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==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
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GENERAL WORKS DEALING WITH THE DEVELOPMENT OF
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(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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BIBLIOGRAPHY. XV
THE EYE.
Eye of the Mollusca.
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Eye of the Arthropoda.
(367) N. Bobretzky. Development of Astacus and Palaemon. Kiew, 1873.
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The Vertebrate Eye.
(371) J.Arnold. Beitrage zur Entwicklungsgeschichle des A uges. Heidelberg,
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(372) Babuchin. "Beitrage zur Entwicklungsgeschichte des Auges." Wiirzliurger naturwissenschaftliche Zeitschrift, Bd. 8.
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(376) L. Lowe. "Beitrage zur Anatomic des Auges" and "Die Histogenese
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(377) V. Mihalkowics. "Untersuchungen iiber den Kamm des Vogelauges."
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(379) S. L. Schenk. "Zur Entwickelungsgeschichte des Auges der Fische."
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Accessory organs of the Vertebrate Eye.
(380) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. Amphibien."
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(382) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen
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xvi BIBLIOGRAPHY.
AUDITORY ORGANS.
Auditory organs of tlie Invertebrata.
(384) V. Hensen. "Studien lib. d. Gehororgan d. Decapoden." Zeil.f. wiss.
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(386) A. Boettcher. "Bau u. Entwicklung d. Schnecke." Denkschriften d.
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(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
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(392) W. Moldenhauer. "Zur Entwick. d. mittleren u. ausseren Ohres."
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(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
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(395) A. Kolliker. " Ueber die Jacobson'schen Organe des Menschen."
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SENSE-ORGANS OF THE LATERAL LINE.
(397) F. M. Balfour. A Monograph on the development of Elasmobranch Fishes,
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(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.
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(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,
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(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,
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(411) Fr. Leydig. " Histologische Bemerkungen iib. d. Polypterus bichir."
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(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."
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(543) J. Dansky u. J. Kostenitsch. " Ueb. d. Entwick. d. Keimblatter u. d.
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(544) Th. Egli. Beitrdge zur Anat. tmd Entiuick. d. Geschlechtsorgane. Inaug.
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(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 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 Amphioxus lanceolatus." Archiv f. mikr. Anat., Vol. xui. 1877.

(3) Leuckart u. Pagenstecher. " Untersuchungen tiber niedere Seethiere." Mutter's Archiv, 1858.

(4) Max Schultze. " Beobachtung junger Exemplare von Amphioxus." Zeit. f. wiss. Zool., Bd. in. 1851.

(5) A. M. Marshall. "On the mode of Ovi position of Amphioxus." your, of Anat. and Phys., Vol. x. 1876.

UROCHORDA.

(6) P. J. van Beneden. " Recherches s. 1'Embryogenie, 1'Anat. et la Physiol. des Ascidies simples." Mem. Acad. Roy. de Belgique, Tom. xx.

(7) W. K. Brooks. "On the development of Salpa." Bull, of the Museum of Comp. Anat. at Harvard College, Cambridge, Mass.

(8) H. Fol. Eludes surles Appendiculaires du detroit de Mcssine . Geneve et Bale, 1872.

(9) Ganin. "Neue Thatsachen a. d. Entwicklungsgeschichte d. Ascidien." Zeit.f. wiss. Zool., Vol. XX. 1870.

(10) C. Gegenbaur. " Ueber den Entwicklungscyclus von Doliolum nebst Bemerkungen iiber die Larven dieser Thiere." Zeit.f. wiss. Zool., Bd. vu. 1856.

(11) A. Giard. "Etudes critiques des travaux d'embryogenie relatifs a la parente des Vertebres et des Tuniciers." Archiv Zool. experiment., Vol. I. 1872.

(12) A. Giard. " Recherches sur les Synascidies. " Archiv Zool. exper., Vol. I. 1872.

(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. Trans., 1851.

(15) Th. H.Huxley. " Observations on the anatomy and physiology of Salpa and Pyrosoma." Phil. Trans., 1851.

(16) Th. H. Huxley. "Anatomy and development of Pyrosoma." Linnean Trans., 1860, Vol. XXIII.

(17) Keferstein u. Ehlers. Zoologische Beitrage, 1861. Doliolum.

(18) A. Kowalevsky. "Entwicklungsgeschichte d. einfachen Ascidien." Mem. Acad. Pctersbourg, VII. serie, T. x. 1866.

(19) A. Kowalevsky. "Beitrag z. Entwick. d. Tunicaten." Nachrichtcn d. konigl. Gesell.zu Gottingen. 1868.

(20) A. Kowalevsky. "Weitere Studien iib. d. Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vn. 1871.

(21) A. Kowalevsky. "Ueber Knospung d. Ascidien." Archiv f. mikr. Anat., Vol. X. 1874.

(22) A. Kowalevsky. "Ueber die Entwicklungsgeschichte d. Pyrosoma." Archiv f. mikr. Anat., Vol. xi. 1875.

(23) A. Krohn. "Ueber die Gattung Doliolum u. ihre Arten." Archiv f. Natnrgeschichte, Bd. xvm. 1852.

B. Hi. a


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(24) A. Krohn. "Ueber die Entwicklung d. Ascidien." Mailer's Archiv, 1852.

(25) A. Krohn. "Ueber die Fortpfianzungsverhaltnisse d. Botrylliden. " Archiv f. Naturgeschichte, Vol. xxxv. 1869.

(26) A. Krohn. "Ueber die fruheste Bildung d. Botryllenstocke." Archiv f. Naturgeschichte, Vol. xxxv. 1869.

(27) C. Kupffer. " Die Stammverwandschaft zwischen Ascidien u. Wirbelthieren." Archiv f, mikr. Anat., Vol. vi. 1870.

(28) C. Kupffer. "Zur Entwicklung d. einfachen Ascidien." Archiv f. mikr. Anat., Vol. vm. 187-2.

(29) H. Lacaze Duthiers. "Recherches sur 1'organisation et 1'Embryogenie des Ascidies (Molgula tubulosa)." Comptes rendus, May 30, 1870, p. 1154.

(30) H. Lacaze Duthiers. "Les Ascidies simples des Cotes de France" (Development of Molgula). Archiv Zool. exper., Vol. ill. 1874.

(31) R. Leuckart. "Salpa u. Verwandte." Zoologischc Untcrsuchungen, Heft u.

(32) E. Metschnikoff. " Observations sur le developpement de quelques animaux (Botryllus and Simple Ascidians)." Still, d. fAcad. Petersbottrg, Vol. xm. 1869.

(33) H. Milne-Edwards. "Observations s. 1. Ascidies composees des cotes de la Manche." Memoir es d. V Instittit, T. xvm. 1842.

(34) W. Salensky. "Ueber d.embryonaleEntwicklungsgeschichtederSalpen." Zeit.f. wiss. Zool., B. xxvn. 1877.

(35) W. Salensky. "Ueber die Knospung d. Salpen." Morphol. Jahrbuch, Bd. in. 1877.

(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 d. Ascidien." Arbeit, a. d. zool.-zoot. Instit. Wiirzburg, Vol. ri. 1875.

(38) Fr. Todaro. Sopra lo sviluppo e F anatomia delle Salpc. Roma, 1875.

(39) Fr. Todaro. "Sui primi fenomeni dello sviluppo delle Salpe." Realc Accadcmia dci Lincei, Vol. iv. 1880.


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 Fishes." London, 1878. Reprinted from the Journal of Anat. and Fhysiol. for 1876, 1877, and 1878.

(42) Z. Gerbe. " Recherches sur la segmentation de la cicatrule et la formation des prodnits adventifs de Pceuf des Plagiostomes et particulierement des Rates." Vide also Journal de FAnatomie et de la Physiologic, 1872.

(43) W. His. " Ueb. d. Bildung v. Haifischenembryonen." Zeit. fur Anat. u. Entwick., Vol. 11. 1877.

(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus Irevis. " (Russian.) Transactions of the Kiew Society of Naturalists, Vol. I. 1870.

(45) R. Leuckart. "Ueber die allmahlige Bildung d. Korpergestalt bei d. Rochen." Zeit. f. wiss. Zool., Bd. II., p. 258.

(46) Fr. Ley dig. Rochen u. Hate. Leipzig, 1852.

(47) A. W. Malm. " Bidrag till kannedom om utvecklingen af Rajae." Kongl. vetenskaps akademiens fo'rhandlingar. Stockholm, 1876.

(48) Joh. M tiller. Clatter Haie des Aristoteles und iiber die Verschiedenheitcn unler den Haifachen und Rochen in der Entivicklung des Eies. Berlin, 1840.

(49) S. L. Schenk. " Die Eier von Raja quadrimaculata innerhalb der Eileiter." Sitz. der k. Akad. Wien, Vol. LXXIII. 1873.

(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies. " Archiv fiir micro. Anat., Vol. XI. 1875.

(51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische. " Archiv fiir micro. Anat., Vol. xni. 1877.


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. zool.-zoot. Instit. Wiirzburg, Vol. n. 1875.

(54) Wyman. " Observations on the Development of Raja batis." Memoirs of the American Academy of Arts and Sciences, Vol. ix. 1864.


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, Vol. xin. Presented Oct. n, 1877.

(56) Al. Agassiz. "II. Development of the Flounders." Proceedings of the American Acad. of Arts arid Sciences, Vol. xiv. Presented June, 1878.

(57) K. E. v. Baer. Untersuchungen ilber die Entwicklungsgeschichte der Fische. Leipzig, 1835.

(58) Ch. van Bambeke. "Premiers effets de la fecondation sur les cufs de Poissons: sur 1'origine et la signification du feuillet muqueux on glandulaire chez les Poissons Osseux." Comptes Rendus des Seances de f Academic des Sciences, Tome

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LXXIV.

(59)

Osseux. ' Vol. XL.

(60)


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 dorsalis d. Teleostier u. d. Petromyzonten." Morphologisches Jahrbuch, Vol. III. 1877.

(62) A. Gotte. "Beitrage zur Entwicklungsgeschichte der Wirbelthiere." Archiv f. mikr. Anat., Vol. IX. 1873.

(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. 1878.

(65) W. His. " Untersuchungen Uber die Entwicklung von Knochenfischen, etc." Zeit.f. Anat. it. Entwicklungsgeschichte, Vol. I. 1876.

(66) W. His. "Untersuchungen Uber die Bildung des Knochenfischembryo (Salmen)." Archiv f. Anat. u. Physiol., 1878.

(67) E. Klein. "Observations on the early Development of the Common 'Trout." Quart. J. of Micr. Science, Vol. XVI. 1876.

~^* (68) C. Kupffer. " Beobachtungen Uber die Entwicklung der Knochenfische." Archiv f. mikr. Anat., Bd. iv. 1868.

(69) C. Kupffer. Ueber Laichen u. Entwicklung des Ostsee-Herings. Berlin, 1878.

(70) M. Lereboullet. "Recherches sur le developpement du brochet de la perche et de 1'ecrevisse." Annales des Sciences Nat., Vol. I., Series iv. 1854.

(71) M. Lereboullet. " Recherches d'Embryologie comparee sur le developpement de la Truite." An. Sci. Nat., quatrieme serie, Vol. XVI. 1861.

(72) T. Oellacher. " Beitrage zur Entwicklungsgeschichte der Knochenfische nach Beobachtungen am Bachforellenei." Zeit. f. wiss. Zool., Vol. xxn., 1872, and' Vol. xxni., 1873.

(72*) H. Rathke. Abh. z. Bildung u. Entwick. d. Menschen u. Thiere. Leipzig, 1832-3. Part n. Blennius.

(73) Reineck. " Ueber die Schichtung des Forellenkeims." Archiv f. mikr. Anat., Bd. V. 1869.

(74) S. Strieker. "Untersuchungen Uber die Entwicklung der Bachforelle." Sitzungsberichte der Wiener k. Akad. d. Wiss., 1865. Vol. LI. Abth. 2.

(75) Carl Vogt. " Embryologie des Salmones." Histoire Naturelle des Poissons de F Europe Centrale. L. Agassiz. 1842.

(76) C.Weil. " Beitrage zur Kenntniss der Knochenfische." Silzungsbcr. doWiener kais. Akad. der Wiss., Bd. i.xvi. 1872.

a 2


BIBLIOGRAPHY.


CYCLOSTOMATA.

(77) E. Calberla. " Der Befruchtungsvorgang beim Petromyzon Planeri." Zeit.f. iviss. Zool., Vol. xxx. 1877.

(78) E. Calberla. "Ueb. d. Entwicklung d. Medullarrohres u. d. Chorda clorsalis d. Teleostier u. d. Petromyzonten." Morpholog. Jahrbuch, Vol. in. 1877.

(79) C. Kupffer u. B. Benecke. Der Vorgang d. Befruchtimg am Ei d. Neunaugen. Konigsberg, 1878.

(80) Aug. Muller. " Ueber die Entwicklung d. Neunaugen." Miiller s Archiv, 1856.

(81) Aug. Muller. Beobachtungen iib. d. Befruchtungserscheinungen im Ei d. Neunaugen. Konigsberg, 1864.

(82) W. Muller. "Das Urogenitalsystem d. Amphioxus u. d. Cyclostomen. ' Jcnaische Zeitschrift, Vol. IX. 1875.

(83) Ph. Owsjannikoff. "Die Entwick. von d. Flussneunaugen. " ^ Vorlauf. Mittheilung. Melanges Biologiqttcs tires du Bulletin de VAcad. Imp. St Pttersbourg, Vol. vn. 1870.

(84) Ph. Owsjannikoff. On the development of Petromyzon fiuviatihs (Russian).

(85) Anton Schneider. Beitrdge z. vergleich. Anat. a. Entwick. d. Wirbelthiere. Quarto. Berlin, 1879.

(86) M. S. Schultze. "Die Entwickl. v. Petromyzon Planeri." Gekronte Preisschrift. Haarlem, 1856.

(87) W. B. Scott. " Vorlaufige Mittheilung iib. d. Entwicklungsgeschichte d. Petromyzonten." Zoologischer Anzeiger, Nos. 63 and 64. ill. Jahrg. 1880.

GANOIDEI. A cipenseridce.

(88) Knock. "Die Beschr. d. Reise z. Wolga Behufs d. Sterlettbefruchtung. " Bull. Soc. Nat. Moscow, 1871.

(89) A. Kowalevsky, Ph. Owsjannikoff, and N. Wagner. "Die Entwick. d. Store." Vorlauf. Mittheilung. Melanges Biologizes tires du Bulletin d. VAcad. Imp. St Petersbowg, Vol. VII. 1870.

(90) W. Salensky. "Development of the Sterlet (Acipenser ruthenus)." 2 Parts. Proceedings of the Society of Naturalists in the imperial University of Kasan. 1878 and 9 (Russian). Part I., abstracted in Hoffmann and Schwalbe's Jahresbcricht for 1878.

(91) W. Salensky. " Zur Embryologie d. Ganoiden (Acipenser)." Zoologischer Anzeiger, Vol. I., Nos. n, 12, 13.

Lepidosteidce.

(92) Al. Agassiz. "The development of Lepidosteus." Proc. Amer. Acad. of Arts and Sciences, Vol. xm. 1878.

AMPHIBIA.

(93) Ch. van Bambeke. " Recherches sur le developpement du Pelobate brun." Mc/noires coitronncs, etc. de I 1 Acad. roy. de Belgique, 1868.

(94) Ch. van Bambeke. "Recherches sur 1'embryologie des Batraciens." /!iill,-tin dc V Acad. roy. de Belgique, 1875.

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AVES.

(117) K. E. vonBaer. " Ueb. Entwickhmgsgeschichte d. Thiere." Konigsberg, 18281837.

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(122) J. Disse. "Die Entstehung d. Blutes u. d. ersten Gefasse im Hiihnerei.' Archiv f. mikr. Anat., Vol. xvi. 1879.

(123) Fr. Durante. "Sulla struttura della macula germinativa delle uova di Gallina." Ricerche nel Laboratorio di Anatomia della R. Universita di Roma.

(124) E. Dursy. Der Primitivstreif des Hiihnchens. 1867.

(125) M. Duval. "Etude sur la ligne primitive de 1'embryon de Poulet. Annales des Sciences Naturelles, Vol. vn. 1879.

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(131) W. His. Unsere Kbrperform tmd das physiol. Problem ihrer Entstehung. Leipzig, 1875.

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(133) W. His. " Neue Untersuchungen iib. die Bildung des Hiihnerembryo I." Archiv f. Anat. u. Phys. 1877.

(134) E. Klein. "Das mittlere Keimblatt in seiner Bezieh. z. Entwick. d. ers. Blutgefiisse und Blutkorp. im Hiihnerembryo." Sitzungsber. Wien. Akad., Vol. LXIII. 1871.

(135) A. K6 Hiker. Entwicklungsgeschichte d. Menschen u. d. hbheren 7'hii'rc. Leipzig, 1879.

(136) C. Kupffer. " Die Entsteh. d. Allantois u. d. Gastrula d. Wirbelth." Zoolog. Anzeiger, Vol. II. 1879, PP- 5 2O > 593> 612.

(137) C. Kupffer and B. Benecke. " Photogramme z. Ontogenie d. Vogel." Nov. Act. d. k. Leop.-Carol.-Deutschen Akad. d. Naturforscher, Vol. XLI. 1879.

(138) J. Oellacher. "Untersuchungen tiber die Furchung u. Blatterbildung im Hiihnerei." Strieker's Studien. 1870.

(139) C. H. Pander. Beitrage z. Entwick. d. Hiinchens im Eie. Wiirzburg, 1817.

(140) A. Rauber. " Ueber die Etnbryonalanlage des Hiihnchens." Centralblatt fur d. medic. Wissenschaften. 1874 75.

(141) A. Rauber. Ueber die Stellung des Hiihnchens im Entwickhingsplan. 1876.

(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 und pathologischer Beziehung. 1877.

(144) R. Remak. Untersuch. iib. d. Entwicklung d. Wirbelthiere. Berlin, 185055.

(145) S. L. Schenk. "Beitrage z. Lehre v. d. Organanlage im motorischen Keimblatt. Sitz. Wien. Akad., Vol. LVII. 1860.

(146) S. L. Schenk. " Beitrage z. Lehre v. Amnion." Archiv f. mikr. Anat., Vol. vii. 1871.

(147) S. L. Schenk. Lehrbuch d. vergleich. Embryol. d. Wirbelthiere. Wien, 1874.

(148) S. Strieker. " Mittheil. iib. d. selbststiindigen Bewegungen embryonaler Zellen." Sitz. Wien. Akad., Vol. XLIX. 1864.

(149) S. Strieker. "Beitrage zur Kenntniss des Hiihnereies." Wiener Sitzungsber., Vol. LIV. 1866.

(150) H. Virchow. Ueber d. Epithel d. Dottersackes im Hiihnerei. Inaug. Diss. Berlin, 1875.

(151) W. Waldeyer. "Ueber die Keimblatter und den Primitivstreifen bei der Entwicklung des Hiihnerembryo." Zeitschrift fiir ratioudle Medicin. 1869.

(152) C. F. Wolff. Theoria generationis. Halse, 1759.

(153) C. F. Wolff. Ueb. d. Bildung d. Darmcanals im bebriitcten Hiinchen. Halle, 1812.

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 Limnee. II. Embryologie du Lezard." An. Sci. Nat., Ser. iv., Vol. xxvn. 1862.

(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. Archives de Biologic, Vol. I. 1880.

(172) Ed. v. Beneden and Ch. Julin. "Observations sur la maturation etc. de 1'oeuf chez les Cheiropteres." Archives de Biologie, Vol. I. 1880.

(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 Meerschweinchens." Abh. d. bayr. Akad., Cl. n. Vol. X. 1866.

(179) Th. L. W. Bischoff. Historisch-kritische B enter kungen z. d. naicstcn Alittheilungen iil>. d. erste Entwick. d. Siitigethiereier. Miinchen, 1877.

(180) M. Coste. Embryogenie comparee. Paris, 1837.

(181) E. Haeckel. Anthropogenie, Entwicklungsgeschichte des Menschen. Lci])zig, 1874.

(182) V. Hensen. "Beobachtungen lib. d. Befrucht. u. Entwick. d. Kaninchens u. Meerschweinchens." Zeit.f. Anat. u. Entwick., Vol. I. 1876.

(183) A. Kolliker. Entivicklungsgeschichte d. Menschen u. d. hb'hcren Thiere. Leipzig, 1879.

(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 d. Herrn H. Nasse. Marburg, 1879.

(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. Berl. Akad. 1862.

(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. Journal of Anat. and Phys., Vol. xin.

(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 Nat., Ser. 3, Vol. I. 1844.


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. des Sciences St Petersbourg, Ser. vii. Tom. XI. 1867.

(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, London, 1878.

(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. Anat., Vol. XIII. 1877.

(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. Vide also Translation. Elements of Comparative Anatomy. Macmillan Co. 1878.

(265) A. Gotte. Ent^vicklungsgeschichte d. Unke. Leipzig, 1874.

(266) E. Haeckel. Studien z. Gastrcca-theorie, Jena, 1877; anc ' a ' so Jenaische Zeitschrift, Vols. vm. and IX. 1874-5.

(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. and XIV. 1879.

(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, 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-entivicklungsgeschichte Untersnchung. Leipzig, 1872.

(276) A. Kolliker. Entwicklungsgeschichte d. Menschen u. d. hbh. Thiere. Leipzig, 1879.

(277) A. Kowalevsky. " Embryologische Studien an Wurmern u. Arthropoden." Mem. Acad. Petersbourg, Series vii. 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 (279) E. R. Lankester. " Notes on Embryology and Classification." Quart.

Jotirn. of Alter. Set., Vol. xvn. 1877.

(280) E. Metschnikoff. "Zur Entwicklungsgeschichte d. Kalkschwamme." Zeit. f. wiss. Zool., Vol. xxiv. 1874.

(281) E. Metschnikoff. " Spongiologische Studien." 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. Tellerschneke (Planorbis)." Morph. Jahrbuch, Vol. v. 1879.

(285) H. Rathke. Abhandhmgen z. Bildung und Enlwicklungsgesch.d. Menschen u. d. Thiere. Leipzig, 1833.

(286) H. Rathke. Ueber die Bildung u. Entwicklungs. d. Flusskrebses. Leipzig, 1829.

(287) R. Remak. Untersuch. ilb. d. Entwick. d. Wirbelthiere. Berlin, 1855.

(288) Salensky. " Bemerkungen lib. Haeckels Gastrsea-theorie." Archiv /. Naturgeschichte, 1874.

(289) E. Schafer. "Some Teachings of Development." Quart. Jotint. of Micr. Science, Vol. xx. 1880.

(290) C. Semper. " Die Verwandtschaftbeziehungen d. gegliederten Thiere." Arbeiten a. d. zool.-zoot. Instit. Wiirzburg, Vol. in. 1876-7.


Xll BIBLIOGRAPHY.


GENERAL WORKS DEALING WITH THE DEVELOPMENT OF THE ORGANS OF THE CHORDATA.

(291) K. E. von Baer. Ueber Enlwicklungsgeschichte d. Thiere. Konigsberg, ! 828 1837.

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

(293) Th. C. W. Bischoff. Entwicklungsgesch. d. Siiugdhiere u. d. Menschen. Leipzig, 1842.

(294) C. Gegenbaur. Grundriss 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. Entwickhmgsgeschichte d. Unke. Leipzig, 1875.

(297) W. His. Untersuch. ilb. d. erste Anlage d. Wirbelthierleibes. Leipzig, 1868.

(298) A. K 6 Hiker. Entwickhmgsgeschichte d. Menschen u. der hoheren Thiere. Leipzig, 1879.

(299) H. Rathke. Abhandlungen u. Bildung und Entwickhingsgeschichle d. Menschen u. d. Thiere. Leipzig, 1838.

(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. and Physiol.

(305) P. Z. Unna. "Histol. u. Entwick. d. Oberhaut." Archiv /. mikr. Anat. Vol. XV. 1876. Pft&also Kolliker (No. 298).

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, Vol. u. 1876. (Siluroiden u. Acipenseridae.)

(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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