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.
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[247], and the other always present in
the adult.
[247] I leave out of consideration an external renal organ found
in many marine Gasteropod larvæ, _vide_ Vol. II. p. 280.
The larval excretory organ has been found in the pulmonate Gasteropoda
(Gegenbaur, Fol[248], 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 non-ciliated pedal limb opening to the
exterior[249].
[248] H. Fol, "Etudes sur le dével. d. Mollusques." Mem. III.
_Archiv d. Zool. expér. et génér._, Vol. VIII.
[249] 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 Bütschli, must
probably be corrected.
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 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 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 Chætopoda
described below. Their development has not been worked out.
Chætopoda. Two forms of excretory organ have been met with in the
Chætopoda. 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 Capitellidæ 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 Chætopoda. Kowalevsky (No. 277), from his
observations on the Oligochæta, 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 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 Eisig (No. 512) on the Capitellidæ 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.
[FIG. 383. POLYGORDIUS LARVA. (After Hatschek.)
_m._ mouth; _sg._ supraoesophageal ganglion; _nph._ nephridion;
_me.p._ mesoblastic band; _an._ anus; _ol._ stomach.]
The excretory system begins to be formed, while the larva is still in
the trochospere stage (fig. 383, _nph_), 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 larvæ (_vide_ p.
681).
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, each with an anterior
funnel-like opening situated in the midst of the mesoblast cells, and
a posterior external 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, _nph_) 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, 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.
[FIG. 384. DIAGRAM ILLUSTRATING THE DEVELOPMENT OF THE EXCRETORY
SYSTEM OF POLYGORDIUS. (After Hatschek.)]
The tube may be continued with a sinuous course through 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 permanent 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 (1) 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 larvæ of Echiurus and
Bonellia.
In all the Gephyrea the so-called 'brown tubes' are apparently
homologous with the segmented excretory tubes of Chætopods. 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 _Echiurus_ 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 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 hindgut.
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 Chætopoda. They may either be provided with
a peritoneal funnel (Nephelis, Clepsine) or have no internal opening
(Hirudo).
Bourne[250] 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 Chætopoda.
[250] "On the Structure of the Nephridia of the Medicinal Leech."
_Quart. J. of Micr. Science_, Vol. XX. 1880.
The excretory tubes of Clepsine are held by Whitman to be developed in
the mesoblast.
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 Bütschli) 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. Fürbringer 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
Chætopoda[251].
[251] _Vide_ F. M. Balfour, "On some points in the Anatomy of
Peripatus Capensis." _Quart. J. of Micr. Science_, Vol. XIX.
1879.
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:
(1) 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 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 proctodæum. 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 larvæ 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 Chætopoda.
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 (1) 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-kidney, though it may perhaps be more
suitably called, adopting Lankester's nomenclature, the _pronephros_.
Its duct, which forms the basis for the generative and urinary ducts,
will be called the _segmental duct_.
(2) The Wolffian body, which may be also called the _mesonephros_. 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 mesonephric
duct_, while the other is known as the _Müllerian duct_.
(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 Wolffian 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 A, _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 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.
[FIG. 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; _mc._ 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;
_pp._ 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_)[252], which are usually known as the
intermediate cell-masses.
[252] 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.
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 backwards round the inner
and dorsal side of the segmental duct, near which it at first ends
blindly.
[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._ subnotochordal 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; _p.o._ primitive generative cells.]
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 (1) 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. 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 Müllerian duct below; _s.t._ segmental
tubes. They open at one end into the body cavity, and at the other
into the segmental duct.]
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 (1) the Wolffian duct or mesonephric duct
(_wd_), dorsally, which remains continuous with the excretory tubules
of the mesonephros, and ventrally (2) the oviduct or Müllerian duct in
the female, and the rudiments of this duct in the male. In the 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 Müllerian duct (_od_). 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 Müllerian duct, of which its anterior pore forms
the permanent peritoneal opening[253] (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 in the fact of the anterior undivided part of the
segmental duct, which forms the front end of the Müllerian duct, being
shorter, and in the column of cells with which it is continuous being
from the first incomplete.
[253] 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 Müllerian duct; but they appear to atrophy very
early, without acquiring a definite attachment to the segmental
duct.
[FIG. 388. DIAGRAMMATIC REPRESENTATION OF A TRANSVERSE SECTION OF A
SCYLLIUM EMBRYO ILLUSTRATING THE FORMATION OF THE WOLFFIAN AND
MÜLLERIAN DUCTS BY THE LONGITUDINAL SPLITTING OF THE SEGMENTAL DUCT.
_mc._ 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; _w.d._ Wolffian duct; _m.d._ Müllerian 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; _l._ liver; _i._ intestine
with spiral valve.]
[FIG. 389. FOUR SECTIONS THROUGH THE ANTERIOR PART 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 Müllerian duct or oviduct
below.
_wd._ Wolffian or mesonephric duct; _od._ Müllerian duct or oviduct;
_sd._ segmental duct.]
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 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 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 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 segmental duct. Owing to the
formation of the accessory tubuli the segments of the mesonephros
acquire a compound character.
[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 Malpighian body uniting with preceding segment.]
[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.
_st.o._ peritoneal funnel; _p.mg._ primary Malpighian body; _a.mg._
accessory Malpighian body; _w.d._ mesonephric (Wolffian) duct.]
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 (1) a peritoneal opening, somewhat oval in form
(_st.o_) 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 Wolffian 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 permanent 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 ampullæ (fig. 393, _nt_).
These ducts, the vasa efferentia, 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 Müllerian 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 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 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 (_s.t_) some of which retain their 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.
[FIG. 392. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT FEMALE ELASMOBRANCH.
_m.d._ Müllerian 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.]
[FIG. 393. DIAGRAM OF THE ARRANGEMENT OF THE URINOGENITAL ORGANS IN
AN ADULT MALE ELASMOBRANCH.
_m.d._ rudiment of Müllerian 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; _t._ testis; _nt._ canal at the base of the testis; _VE._
vasa efferentia; _lc._ longitudinal canal of the Wolffian body.]
In the male the following parts are present (fig. 393):
(1) The Müllerian 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 (1) the vasa
efferentia (_VE_), united on the one hand with (2) the central canal
in the base of the testis (_nt_), and on the other with the
longitudinal canal of the Wolffian body (_lc_). 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 (1) The splitting of
the segmental duct into Wolffian (mesonephric) and Müllerian 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
Müllerian 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.
Cyclostomata. The development of the excretory system amongst the
Cyclostomata has only been studied in Petromyzon (Müller, Fürbringer,
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 (Fürbringer) 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
Ammocoete has reached 180 mm. in length. Further changes take place in
connection with the excretory system on the conversion of the
Ammocoete 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 Ammocoete 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 (1) 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, Götte,
Fürbringer) 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 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.
[FIG. 394. PORTIONS OF THE MESONEPHROS OF MYXINE. (From Gegenbaur;
after J. Müller.)
_a._ segmental duct; _b._ segmental tube; _c._ glomerulus; _d._
afferent, _e._ efferent artery.
B represents a portion of A highly magnified.]
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 Fürbringer 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 postanal
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 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 pronephros requires however
further elucidation.
[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.]
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 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 Lepidosteus, to be described in the next
section, that these ducts are split off from the primitive segmental
duct, and correspond with the Müllerian 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 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 Salmonidæ.
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 Müllerian duct, provided, except in
Lepidosteus, with an abdominal funnel, which is however situated
relatively very far back in the abdominal cavity. _The Müllerian 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_.
[FIG. 396. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE
SIXTH DAY AFTER IMPREGNATION.
_mc._ medullary cord; _ms._ mesoblast; _sg._ segmental duct; _ch._
notochord; _x._ subnotochordal rod; _hy._ hypoblast.]
Posteriorly the Müllerian 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 Acipenser and Lepidosteus[254], is on the whole very
similar to that in the Teleostei. The first portion of the system to
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 a solid ridge of the somatic
mesoblast, as in Petromyzon and Elasmobranchii (fig. 397, _Wg_).
[254] Acipenser has been investigated by Fürbringer, Salensky,
Sedgwick, and also by myself, and Lepidosteus by W. N. Parker and
myself.
[FIG. 397. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN
ACIPENSER EMBRYO. (After Salensky.)
_Rf._ medullary groove; _Mp._ medullary plate; _Wg._ segmental duct;
_Ch._ notochord; _En._ hypoblast; _Sgp._ mesoblastic somite; _Sp._
parietal part of mesoblastic plate.]
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[255] 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. 398, _pr.n._ At this early stage of Acipenser
(larva of 5 mm.) I could find no glomerulus.
[255] I have not fully proved this point, but have never found
more than one opening.
The mesonephros is formed some distance behind, and some time after
the pronephros, both in Acipenser and Lepidosteus, so that in the
larvæ 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 side of the insertion
of the mesentery, and extending upwards and outwards round the
segmental duct[256]. 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.
[256] Whether the segmental tubes are formed as ingrowths of the
peritoneal epithelium, or _in situ_, could not be determined.
[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;
_mp._ muscle-plate formed of large cells, the outer parts of which
are differentiated into contractile fibres; _sp.c._ spinal cord;
_b.c._ body cavity.]
In later stages (larvæ 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 established, for at any rate a
considerable number of the tubes, leading from the body cavity to the
Malpighian bodies. These funnels have been noticed by Fürbringer,
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 Müllerian 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 Müllerian ducts serving as genital ducts in
both sexes.
Amphibia. In Amphibia (Götte, Fürbringer) 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, _u_) becomes differentiated into a longitudinal duct, which
remains in open communication with the body cavity by two (many
Urodela) three (many Anura) or four (Coeciliidæ) 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.
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 Coeciliidæ, 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
Götte.)
_a._ fold of epiblast continuous with the dorsal fin; _is^x._ neural
cord; _m._ lateral muscle; _as^x._ 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
(Fürbringer) 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 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) 1-2 segmental tubes
" " 7th-10th myotome 2-3 " "
" " 11th " 3-4 " "
" " 12th " 3-4 or 4-5 " "
" " 13th " 4-5 " "
" " 13th-16th " 5-6 " "
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 Coeciliidæ 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.
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 Müllerian duct. The process in Salamandra (Fürbringer) has
much the same character as in Elasmobranchii, the Müllerian 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 Müllerian 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 Müllerian 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 Müllerian 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
Müllerian duct, but remains connected with the Wolffian duct.
The development of the Müllerian duct has not been satisfactorily
studied in other forms besides Salamandra. In Coeciliidæ 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 Müllerian 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 Müllerian 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 (1) 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 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 efferentia
(_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 Coeciliidæ 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.
In most Urodeles the peritoneal openings connected with the primary
generative Malpighian bodies atrophy, but in Spelerpes they persist.
In the Coeciliidæ 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 Coeciliidæ
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.