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 Müllerian 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 Müllerian duct.
(3) The mesonephros (_r_), divided into an anterior sexual part
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 Müllerian duct (_m_).
(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 (1) 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.
[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 (Müllerian duct); _m._ Müllerian duct of male; _ve._ vasa
efferentia of testis; _t._ testis; _ov._ ovary; _up._ urinogenital
pore.]
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 segmental 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_)[257].
[257] 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.
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
hind-end elongates till it comes into connection with, and opens into,
the cloacal section of the hindgut[258].
[258] The foremost extremity of the segmental duct presents,
according to Gasser, curious irregularities and an anterior
completely isolated portion is often present.
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 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[259].
[259] Correct figures of the early stages of these structures
were first given by Kölliker, 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).
[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._
blood-vessel.]
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 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. 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 Wolffian tubuli[260]. These tubuli become however
early aborted.
[260] These external glomeruli were originally mistaken by me
(No. 539) for the glomerulus of the pronephros, from their
resemblance to the glomerulus of the Amphibian pronephros. Their
true meaning was made out by Sedgwick (No. 550).
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.
[FIG. 402. TRANSVERSE SECTION THROUGH THE TRUNK OF A DUCK EMBRYO
WITH ABOUT TWENTY-FOUR MESOBLASTIC SOMITES.
_am._ amnion; _so._ somatopleure; _sp._ splanchnopleure; _wd._
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.]
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.
[FIG. 403. SECTION THROUGH THE EXTERNAL GLOMERULUS OF ONE OF THE
ANTERIOR SEGMENTAL TUBES OF AN EMBRYO CHICK OF ABOUT 100 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 MÜLLERIAN DUCT
(PRONEPHROS). (After Balfour and Sedgwick.)
A is the 11th section of the series.
B " 15th " "
C " 18th " "
_gr2._ second groove; _gr3._ third groove; _r2._ second ridge; _wd._
Wolffian duct.]
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 Müllerian
duct of 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 Wolffian duct.
[FIG. 405. SECTION OF THE WOLFFIAN BODY DEVELOPING PRONEPHROS AND
GENITAL GLAND OF THE FOURTH DAY. (After Waldeyer.) Magnified 160
times.
_m._ mesentery; _L._ somatopleure; _a´._ portion of the germinal
epithelium from which the involution (_z_) to form the pronephros
(anterior part of Müllerian 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.]
In a slightly later stage the ridges connecting the grooves become
partially constricted off from the peritoneal epithelium, 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 groove-like apertures, and
continuous behind with the rudiment of the true Müllerian 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
Müllerian duct.
The pronephros is an extremely transitory structure, and its
development and atrophy are completed between the 90th and 120th hours
of incubation.
[FIG. 406. TWO SECTIONS SHEWING THE JUNCTION OF THE TERMINAL SOLID
PORTION OF THE MÜLLERIAN 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._ Müllerian duct; _Wd._ Wolffian duct.]
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
Müllerian duct consists merely of a very 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 Müllerian 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 Müllerian 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 Wolffian
duct, it may grow backwards from cells derived from that duct.
The Müllerian 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 Müllerian 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 Müllerian 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 Müllerian 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
Müllerian duct lies. As soon as the Müllerian 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 Müllerian, 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 Müllerian 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 Müllerian 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 Müllerian duct comes into
contact with the germinal epithelium."
Before describing the development of the Müllerian 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: (1) that it gives rise to part of the Müllerian 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
Müllerian 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 Müllerian 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 Müllerian 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 Müllerian duct.
The pronephros has only been observed in Birds, and is very possibly
not developed in other Amniota. The Müllerian 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 the Wolffian duct and
peritoneal epithelium, without becoming attached to the Wolffian duct.
On the formation of the Müllerian 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 Amniota is a specially differentiated posterior section of the
primitive mesonephros_.
According to the view of Remak and Kölliker 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 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, (1) 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 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. 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[261]. 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.
[261] This is also called parovarium (His), and Rosenmüller's
organ.
The Müllerian 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
Müllerian ducts atrophy more or less completely.
In most Reptiles and in Birds the atrophy of the Müllerian 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 (Kölliker)[262] and probably other Mammals the whole of the
ducts probably disappears, but in some Mammals, _e.g._ Man, the lower
fused ends of the Müllerian 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).
[262] 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 Kölliker.
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 Müllerian 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 cord of tissue,
known as the genital cord (fig. 407, _gc_), within the lower part of
which the Müllerian ducts are also enclosed. In the male the Müllerian
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 Müllerian 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
Müllerian 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
Müllerian 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
urinogenital opening; and on each side a genital fold (_ls_). 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, along which the common
urinogenital passage is continued. The two genital folds unite from
behind forwards to form the scrotum.
[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 Müllerian and
Wolffian ducts are seen from the front.
3. ureter; 4. urinary bladder; 5. urachus; _ot._ genital ridge
(ovary or testis); _W._ left Wolffian body; _x._ part at apex from
which coni vasculosi are afterwards developed; _w._ Wolffian duct;
_m._ Müllerian duct; _gc._ genital cord consisting of Wolffian and
Müllerian ducts bound up in a common sheath; _i._ rectum; _ug._
urinogenital sinus; _cp._ elevation which becomes the clitoris or
penis; _ls._ ridge from which the labia majora or scrotum are
developed.]
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.
_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 larvæ 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 Fürbringer.
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 (Teleostei, 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[263] formed independently of the duct.
[263] According to 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.
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 (1) 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 typically formed of (1) 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.
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 Chætopods,
in the coexistence of several segmental organs in a single segment in
some of the Capitellidæ.
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
pronephros, leaving however the difficult questions as to the homology
of the segmental tubes with the segmental organs of Chætopods 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[264]. 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.
[264] 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.
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 excretory organs is perhaps
found permanently in Cyclostomata[265] and transitorily in the embryos
of most forms.
[265] 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.
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
pronephric 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 Müllerian duct and a dorsal Wolffian duct.
The Müllerian 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 Müllerian
duct gradually impressed itself more and more upon the embryonic
development, so that, in the course of time, the Müllerian 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
Müllerian and segmental ducts may actually develop independently, as
they do exceptionally in individual specimens of Salamandra
(Fürbringer). 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
Müllerian duct have been observed.
In Amphibia (Salamandra) the permanent opening of the Müllerian duct
is formed independently, some way behind the pronephros.
In Elasmobranchii the original opening of the segmental duct forms the
permanent opening of the Müllerian 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 Müllerian 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
Müllerian 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 Müllerian duct.
The Müllerian 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 products pass out by it, and the
partial or complete formation of the Müllerian 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 Müllerian 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 Müllerian 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 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 (1) the arrangement (found in the Ganoidei and Dipnoi) of the
Müllerian 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 (1) 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, (1) 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.
Fürbringer has urged against Semper's and my view that no satisfactory
proof of it has yet been offered. This proof has however, since
Fürbringer 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 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 Fürbringer 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[266].
[266] This suggestion has I believe been made by Fürbringer.
Hatschek has compared the provisional trochosphere excretory organ of
Polygordius to the Vertebrate pronephros, and the posterior Chætopod
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 Chætopoda, 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.
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. l'appareil excréteur des Trematodes
et d. Cestoïdes." _Archives de Biologie_, Vol. I. 1880.
(514) B. Hatschek. "Studien üb. 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. III. 1880.
EXCRETORY ORGANS OF 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.
(517) Max. Fürbringer[267]. "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) Joh. Müller. _Bildungsgeschichte d. Genitalien_, etc.
Düsseldorf, 1830.
(520) H. Rathke. "Beobachtungen u. Betrachtungen ü. d. Entwicklung d.
Geschlechtswerkzeuge bei den Wirbelthieren." _N. Schriften d. naturf.
Gesell. in Dantzig_, Bd. I. 1825.
(521) C. Semper[267]. "Das Urogenitalsystem d. Plagiostomen u. seine
Bedeutung f. d. übrigen Wirbelthiere." _Arb. a. d. zool.-zoot.
Instit._ Würzburg, Vol. II. 1875.
(522) W. Waldeyer[267]. _Eierstock u. Ei._ Leipzig, 1870.
_Elasmobranchii._
(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. Müller. "Untersuchungen ü. d. Eingeweide d. Fische." _Abh. d.
k. Ak. Wiss._ Berlin, 1845.
(525) W. Müller. "Ueber d. Persistenz d. Urniere b. Myxine glutinosa."
_Jenaische Zeitschrift_, Vol. VII. 1873.
(526) W. Müller. "Ueber d. Urogenitalsystem d. Amphioxus u. d.
Cyclostomen." _Jenaische Zeitschrift_, Vol. IX. 1875.
(527) A. Schneider. _Beiträge z. vergleich. Anat. u. Entwick. d.
Wirbelthiere._ Berlin, 1879.
(528) W. B. Scott. "Beiträge z. Entwick. d. Petromyzonten." _Morphol.
Jahrbuch_, Vol. VII. 1881.
_Teleostei._
(529) J. Hyrtl. "Das uropoetische System d. Knochenfische." _Denkschr.
d. k. k. Akad. Wiss. Wien_, Vol. II. 1850.
(530) A. Rosenberg. _Untersuchungen üb. die Entwicklung d.
Teleostierniere._ Dorpat, 1867.
_Vide_ also Oellacher (No. 72).
_Amphibia._
(531) F. H. Bidder. _Vergleichend-anatomische u. histologische
Untersuchungen ü. die männlichen Geschlechts- und Harnwerkzeuge d.
nackten Amphibien._ Dorpat, 1846.
(532) C. L. Duvernoy. "Fragments s. les Organes genito-urinaires des
Reptiles," etc. _Mém. Acad. Sciences._ Paris. Vol. XI. 1851, pp.
17-95.
(533) M. Fürbringer. _Zur Entwicklung d. Amphibienniere._ Heidelberg,
1877.
(534) F. Leydig. _Anatomie d. Amphibien u. Reptilien._ 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. Instit. Würzburg._ Vol. III. 1876.
(538) Von Wittich. "Harn- u. Geschlechtswerkzeuge d. Amphibien."
_Zeit. f. wiss. Zool._, Vol. IV. _Vide_ also Götte (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 foetus and their remains
in the adult._ Edinburgh, 1864.
(541) Th. Bornhaupt. _Untersuchungen üb. die Entwicklung d.
Urogenitalsystems beim Hühnchen._ Inaug. Diss. Riga, 1867.
(542) Max Braun. "Das Urogenitalsystem d. einheimischen Reptilien."
_Arbeiten a. d. zool.-zoot. Instit. Würzburg._ Vol. IV. 1877.
(543) J. Dansky u. J. Kostenitsch. "Ueb. d. Entwick. d. Keimblätter u.
d. Wolff'schen Ganges im Hühnerei." _Mém. Acad. Imp. Pétersbourg_,
VII. Series, Vol. XXVII. 1880.
(544) Th. Egli. _Beiträge zur Anat. und Entwick. d.
Geschlechtsorgane._ Inaug. Diss. Zürich, 1876.
(545) E. Gasser. _Beiträge zur Entwicklungsgeschichte d. Allantois,
der Müller'schen Gänge u. des Afters._ Frankfurt, 1874.
(546) E. Gasser. "Beob. üb. d. Entstehung d. Wolff'schen Ganges bei
Embryonen von Hühnern u. Gänsen." _Arch. für mikr. Anat._, Vol. XIV.
1877.
(547) E. Gasser. "Beiträge z. Entwicklung d. Urogenitalsystems d.
Hühnerembryonen." _Sitz. d. Gesell. zur Beförderung d. gesam.
Naturwiss._ Marburg, 1879.
(548) C. Kupffer. "Untersuchung über die Entwicklung des Harn- und
Geschlechtssystems." _Archiv für 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. _Zusätze z. Lehre von Baue u. d. Verrichtungen d.
Geschlechtsorgane._ Leipzig, 1846.
_Vide_ also Remak (No. 302), Foster and Balfour (No. 295), His (No.
297), Kölliker (No. 298).
[267] The papers of Fürbringer, Semper and Waldeyer contain full
references to the literature of the Vertebrate excretory organs.