Illustration: Figure 400Fig. 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.
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 segmentalduct. 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. 401W.d)[257].
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].
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 theperitoneal 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].
Illustration: Figure 401Fig. 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.
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 infig. 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.
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.
Illustration: Figure 402Fig. 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.
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.
Illustration: Figure 403Fig. 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. 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.
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.
Illustration: Figure 404Fig. 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 is the 15th section of the series.C is the 18th section of the series.gr2.second groove;gr3.third groove;r2.second ridge;wd.Wolffian duct.
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 is the 15th section of the series.C is the 18th section of the series.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 ofthe 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.
Illustration: Figure 405Fig. 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 cellsCandoare lying;E.modified mesoblast which will form the stroma of the ovary;WK.Wolffian body;y.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 cellsCandoare 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 byfig. 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.
Illustration: Figure 406Fig. 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.
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 infig. 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 verysmall 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 Müllerianduct 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 infig. 354,md., which is continued backward as a primitively solid rod in the space betweenthe 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 themetanephric 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 provethat 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, inaccordance with the account given above, that the outgrowths of the ureter form only the collecting tubes, and that the secreting tubuli, etc. are formedin situin 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, formedin 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 havebeen 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.
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).
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 specialcord 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 theurinogenital 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.
Illustration: Figure 407Fig. 407. Diagram of the urinogenital organs of a Mammal at an early stage.(After Allen Thomson; from Quain’sAnatomy.)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.
Fig. 407. Diagram of the urinogenital organs of a Mammal at an early stage.(After Allen Thomson; from Quain’sAnatomy.)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.
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 istypically 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 theprimitiveduct 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.
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 theexcretory organs is perhaps found permanently in Cyclostomata[265]and transitorily in the embryos of most forms.
At first the generative products seem to have been discharged freely into the body-cavity, and transported to the exterior by the abdominal pores (videp.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 segmentalducts 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 femaleproducts 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 GanoidLepidosteusattach 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,notby open peritoneal funnels of the mesonephros,butby 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 andAmniota 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 fora 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 fœtal 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 organwith an anterior openingmight be regarded as having given origin to the trochosphere organ, while thatwith a posterior openingmay have done so for the Vertebrate pronephros[266].
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 situatedwithinthe segmented region of the trunk, and has a more or less distinct metameric arrangement of its parts; while the provisional trochosphere organ is placedin frontof 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.
Videalso 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.
Videalso 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.
Videalso Götte (No.296).
Amniota.
(539)F. M. BalfourandA. 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 fœtus 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. Danskyu.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.Videalso Remak (No.302), Foster and Balfour (No.295), His (No.297), Kölliker (No.298).