Illustration: Figure 388Fig. 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. 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.
Illustration: Figure 389Fig. 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.
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 openinginto 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 thesegmental duct. Owing to the formation of the accessory tubuli the segments of the mesonephros acquire a compound character.
Illustration: Figure 390Fig. 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. 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.
Illustration: Figure 391Fig. 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.
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 fromfig. 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 permanentkidney 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, thevasa efferentia, carry the semen to the Wolffian body, but before opening into the tubuli of this body they unite into a canal known as thelongitudinal 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.392and393are 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 productof the segmental ducts of the kidneys. They end in front by becoming continuous with the tubulus of the anteriorpersistingsegment 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 theiroriginal 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.
Illustration: Figure 392Fig. 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. 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.
Illustration: Figure 393Fig. 393. Diagram of the arrangement of the urinogenital organs in an adult male Elasmobranch.m.d.rudiment of Müllerian duct;w.d.Wolffian duct, markedvdin 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.
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, markedvdin 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 fœtal 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 Ammocœte has reached 180mm.in length.Further changes take place in connection with the excretory system on the conversion of the Ammocœte 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 Ammocœte 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 thisfunnel 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.
Illustration: Figure 394Fig. 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.
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 cellsnearthe 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 absentin 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.
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.
Illustration: Figure 395Fig. 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.
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 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 (videgeneral 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 itis 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.
Illustration: Figure 396Fig. 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.
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 tobe 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).
Illustration: Figure 397Fig. 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.
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 infig. 398,pr.n.At this early stage of Acipenser (larva of 5mm.) I could find no glomerulus.
The mesonephros is formed some distance behind, and some time after the pronephros, both in Acipenser and Lepidosteus, so that in the 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 eachside 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.
Illustration: Figure 398Fig. 398. Transverse section through the region of the stomach of a larva of Acipenser 5mm.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.
Fig. 398. Transverse section through the region of the stomach of a larva of Acipenser 5mm.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 7-10mm.) 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. Thesefunnels 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 (Cœciliidæ) 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 Cœciliidæ, 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.
Illustration: Figure 399Fig. 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;isx.neural cord;m.lateral muscle;asx. 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.
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;isx.neural cord;m.lateral muscle;asx. 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 theperitoneal 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
In the 7th-10th myotome: 2-3 segmental tubes
In the 11th myotome: 3-4 segmental tubes
In the 12th myotome: 3-4 4-5 segmental tubes
In the 13th myotome: 4-5 segmental tubes
In the 13th-16th myotome: 5-6 segmental tubes
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 Cœciliidæ 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 backwardspari passuwith 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 Cœciliidæ 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 theposterior 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. 400B). 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 Cœciliidæ 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 Cœciliidæ 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 Cœciliidæ and in the females of many Urodela (Salamandra, Triton). These rudiments may in their fullest development consist of a longitudinal canal and of transverse canals passing from this to the Malpighian bodies, together with some branches passing into the mesovarium.
Amongst the Urodela the collecting tubes of the hinder non-sexual part of the Wolffian body, which probably represents a rudimentary metanephros, undergo in the male sex a change similar to that which they usually undergo in Elasmobranchii. Their points of junction with the Wolffian duct are carried back to the hindermost end of the duct (fig. 400B), and the collecting tubes themselves unite together into one or more short ducts (ureters) before joining the Wolffian duct.
In Batrachoseps only the first collecting tube becomes split off in this way; and it forms a single elongated ureter which receives all the collecting tubes of the posterior segmental tubes. In the female and in the male of Proteus, Menobranchus, and Siren the collecting tubes retain their primitive transverse course and open laterally into the Wolffian duct. In rare cases (Ellipsoglossus,Spengel) the ureters open directly into the cloaca.
The urinary bladder of the Amphibia is an outgrowth of the ventral wall of the cloacal section of the alimentary tract, and is homologous with the allantois of the amniotic Vertebrata.
The subjoined diagram (fig. 400) of the urogenital system of Triton illustrates the more important points of the preceding description.
In the female (A) the following parts are present:(1) The Müllerian duct or oviduct (od) derived from the splitting of the segmental duct.(2) The Wolffian duct (sug) constituting the portion of the segmental duct left after the formation of the Müllerian duct.(3) The mesonephros (r), divided into an anterior sexual partconnected with a rudimentary testicular network, and a posterior part. The collecting tubes from both parts fall transversely into the Wolffian duct.(4) The ovary (ov).(5) The rudimentary testicular network.
In the male (B) the following parts are present:(1) The functionless though fairly developed Müllerian duct (m).(2) The Wolffian duct (sug).(3) The mesonephros (r) divided into a true sexual part, through the segmental tubes of which the semen passes, and a non-sexual part. The collecting tubes of the latter do not enter the Wolffian duct directly, but bend obliquely backwards and only fall into it close to its cloacal aperture, after uniting to form one or two primary tubes (ureters).(4) The testicular network (ve) consisting of (1) transverse ducts from the testes, falling into (2) the longitudinal canal of the Wolffian body, from which (3) transverse canals are again given off to the Malpighian bodies.