Paper - The development of the urogenital system in Marsupialia, with special reference to Trichosurus vulpecula 2
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Fraser EA. The development of the urogenital system in Marsupialia, with special reference to Trichosurus vulpecula. Part II. (1919) J. Anat. 53: 97-129. PMID 17103868
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- 1 The development of the urogenital system in Marsupialia, with special reference to Trichosurus vulpecula. Part II
- 1.1 Introduction
- 1.2 Genital Ridge and Development of Rete
- 1.3 Degeneration of Mesonephros and Formation of Urogenital Connection
- 1.4 Epoophoron and Paroophoron
- 1.5 Epididymis and Paradidymis
- 1.6 The Urogenital Connection in Other Marsupials
- 1.7 Diprotodontia
- 1.8 Discussion
- 1.9 Accessory Glands
- 1.10 List of References
- 1.11 Description of Plates
The development of the urogenital system in Marsupialia, with special reference to Trichosurus vulpecula. Part II
By Elizabeth A. Fraser, 1).Sc. (Lond.)
From the Embryological Laboratory, Department of Zoology, University of London, University College
Genital Ridge and Development of Rete
Degeneration of Mesonephros and Formation of Urogenital Connection
Urogenital Connection in Other Marsupials
List of References
Description of Plates
If HE following paper is the continuation of the study of the development of the urogenital system in mars11pials, originally undertaken at the suggestion of Prof. J. P. Hill, to whom I am much indebted for constant advice. As in Part I, Trichosurus forms the basis for this investigation. The development of the rcte and the urogenital connection was found to show considerable variation in different genera of both Monodelphian and Didelphian forms. These variations have been considered in some detail and form an interesting comparison with the conditions which have been observed in other Vertebrates. The development of the accessory glands relates exclusively to Trichosurus. For the photomicrograph (text—fig. 12) of a transverse section through the cloacal glands, I have to thank Mr F. Pittock of the Zoological Department, University College.
Genital Ridge and Development of Rete
It is not easy to determine exactly at what stage the primordium of the genital organ first makes its appearance in Trichosurus. In an embryo of 7 -51mm. (Stage IV), however, a very low ridge can be seen, forming a slight projection into the eoelom, along the mc(lial side of the urogenital fold. The ridge is filled with denser cells, which at the cranial end, between the 7th and 8th spinal ganglia, pass slightly dorsally, and eventually run into the cells forming the primordium of the suprarenal body, the latter at this level being situated just above the groove on the medial side of the urogenital fold. The posterior end of the ridge appears to reach almost as far as the 9th spinal ganglion.
In succeeding stages the genital primordium increases in length and becomes more prominent. By Stage VIII (G.L. 7 -25 mm.) it appears as a definite, though still low ridge (text-fig. 1, A, g.7'.). It has now apparently become disconnected from the suprarenal organ, and gradually increases in thickness as it extends posteriorly (text-fig. 1, B, g.7'.), its hinder portion forming a well marked elevation projecting out into the coclom (text-fig. 1, C and D, g.r.). The ridge is filled with a mass of darkly staining cells, which, on the inner side, lie close to the medial walls of the Malpighian corpuscles of the mesonephros.
Text-fig. l. A, I)’, C’, I). ’1'r2'Clm.s-uru.s' z:u.l[m,'ulu,. ($.11. 7-25 mm. (XII ‘(.12). 'l‘ra11s\*ei-se sections through the urogenital fold, showing the genital ridge (g.r.) at different levels. A low anterior end of ridge; 1) well marked posterior end; B and 0 intermediate regions. (l0l'/. dotrsal aorta; gl. glomerulus; I. lung; p.c. posterior cardinal vein"; .9. suprarenal body; sf. stomach; W.l). \Volf’fian duct.
Text-fig. 2 shows the genital ridge (gm) in an older embryo (G.L. 85 mm., Stage IX). The thicker posterior portion has a much indented margin, and the tapering cranial end runs forwards almost as far as the ostium abdominale tubae (o.a.), but a definite connection with the latter does not seem to be established.
In embryos with a length of 11 mm. (Stage XII), the anterior portion of the ridge is more definitely marked out. As it extends forwards, it‘ gradually passes, from the dorso-medial side of the urogenital fold across the ventral surface of the mesentery, and on to the coelomic epithelium covering the suprarenal body. In this position it runs forwards as a small projecting ridge, triangular in section (text-fig. 3, A, gm), which in front of the mesonephros passes into the epithelium lining the shallow medial bay of the ostium abdominale (cf. Part I, fig. 19, B, 7.).
Text-fig. 2. Trichosurus vulpecula. G.L. 8-5 mm. (’97). Diagram of a longitudinal section of the mesonephros, showing the genital ridge (g.r.) lying along the ventromedial side of the urogenital fold. gl. glomeruli; o.a. ostium abdominale.
The posterior part of the ridge (text-fig. 3, C, g.r.)'is becoming very conspicuous, and by Stage XIV (G.L. 13 mm.) it is already differentiating into the future gonad. The differentiation is, however, very slight, although primitive sex cells are now distinguishable for the first time. The primordium of the gland is also becoming more divided off from the mesonephros, although a mesogenitale is not yet developed.
Just before birth (G.L. 14 mm.) the cells within that part of the ridge situated in front of the primordium of the definitive gonad have become concentrated to form a core of deeply staining cells, which extends from the ostium abdominale all along the ridge into the hilum of the genital organ. This core is the primordium of the rete, and that part of the ridge, within which it lies, may now be termed the rete ridge. In early stages, the rete ridge is simply the reduced anterior continuation of the genital ridge; it is identical with the latter and cannot be distinguished from it, the one gradually passing into the other.
Text-fig. 3.- Trickosums vulpécula. G.L. 11’-5 mm. (VII ’Ol). Transverse sections through the mesonephros showing the genital ridge (g.r.) at different levels. A near the anterior end of the mesonephros, where the genital ridge forms a. ‘small projection close to the suprarenal body (3.); B further posteriorly; 0 thicker posterior portion of ridge. aort. dorsal aorta; gl. glomeruli; s. suprarenal body; W.D. Wolffian duct.
In pouch young with a head length of 7-5 mm., these two regions are more definitely marked out. The rete, which is now composed of irregular cords and strands of cells, extends from the ostium abdominale, between the 19th and 20th spinal ganglia, to just behind the 21st spinal ganglion, where it runs into the gonad. The latter is now connected with the Wolfiian body by a short thick mesogenitale, whilst a bundle of small arteries and veins, related to the dorsal aorta and the Vena cava respectively, passes down along the medial border of the urogenital fold, through the mesogenitale into the genital gland. This vascular bundle, together with a branch from the sympathetic system, forms a conspicuous formation in later stages, running through the mesogenitale on the posterior side of the rete.
In pouch young of this age (H .L. 6-75 to 8 mm.), and again in later stages, definite ingrowths, in the form of hollow invaginations (Pl. VII, fig. 3) and solid cords, are to be seen extending inwards from the coelomic epithelium of the rete ridge, to merge at their inner ends with the cell strands of the rete itself. It must be noted that in late embryos, before the formation of the rete core, the ridge is indented (G.L. 11 mm., Pl. V II, fig. 1) by more or less distinct grooves extending in from the coelomic epithelium (G.L. 14 mm., Pl. VII, fig. 2), but it was not possible to determine whether these ingrowths are actually connected with the development of the rete, as are the more definite invaginations present in early pouch young. Shortly before birth, comparable ingrowths from the coelomic epithelium of the gonad were first observed passing inwards into the substance of the latter.
Having traced the early development of the genital ridge and its differentiation into gonad and rete, the further development of the latter and the establishment of the urogenital connection must be considered in conjunction with the degeneration of the mesonephros.
Degeneration of Mesonephros and Formation of Urogenital Connection
In Part I of this paper the development of the mesonephros was followed from comparatively early embryos up to pouch young with a head length of 7 ~5 mm., at which time it was still a fully developed organ. In a male foetus with a head length of 8 mm. (G.L. 18 mm.), we find that the mesonephros has entered on a new and final phase of its history. Degenerative changes first occur at the anterior end. At this end, on the left side, is a small atrophied tubule, quite isolated, and behind it, the next few tubules are narrow and darkly staining. They have a uniform appearance throughout their length, and are already surrounded by much connective tissue. They run dorsally into the Wolffian duct, b11t the connection with their Malpighian corpuscles, which are small and in process of degeneration, was not very distinct in this foetus. A short distance further back (about -096 mm.) normal secretory tubules make their appearance, and continue throughout the remainder of the mesonephros, which is well developed and apparently functional. On the right side, the degenerate portion is slightly shorter. The rete ridge lies along the medial side of the Wolffian body, gradually increasing in size as it approaches the testis. Within the ridge, the primordium of the rete is composed of irregular cords and strands of darkly staining cells, which, in this foetus, a male, are in close connection with the seminiferous tubules, whilst in a female of nearly the same age, they run into the central mass of the ovary. At the anterior end of the mesonephros, some of the rete cords bend slightly inwards to terminate close to the small anterior legenerating Malpighian corpuscles, and further posteriorly, down to about the 15th glomerulus, strands of cells pass from the rete up to the walls of Bowman’s capsules. Although these strands do not appear to be definitely attached to the walls of the capsules in this foetus, such a connection is already present in a female with _a head length of 7-5 mm. (G.L. 14-75), opposite the ]3th glomerulus.
In a pouch young with a head length of 8-5 mm. (G.L. 20 mm.), similar (legenerative changes are again seen in the mesonephros. At the anterior end of the urogenital fold are a few isolated tubular fragments, and behind these, we find a small number of anastomosing canals with darkly staining walls, each surrounded by several circular layers of connective tissue. The former secretory portions of the tubules, adjoining the Malpighian corpuscles, have now quite lost their characteristic structure. The first transformed secretory tubules of this stage appear to have already lost their glomeruli. A short distance posteriorly, unchanged secretory tubules appear, and connected with them are well formed glomeruli. On the right side, the mesonephros is fully developed almost up to the extreme anterior end, and it has altogether a length of 1-53 mm. The posterior tubules on each side are large, with marked Malpighian corpuscles, the latter having approximately the same diameter as those in pouch young with a head length of 7-5 mm. (cf. Part I). The rete has the same relations as in the last stage (H .L. 8 mm.).
In pouch young with a head length of 9 mm., both 3‘ and $2, the involution of the mesonephros is more marked, the anterior portion of the gland having shrunk considerably, whilst the rete has also undergone a corresponding shortening. Text-fig. 4, A and B, shows the reduction in length which occurs between a foetus with a head length of 7-5 mm. (A) and one of 9 mm. (B). At its extreme anterior end, the rete lies close to the walls of the degenerating Malpighian corpuscles (Pl. VII, fig. 4, r.c.), whilst more posteriorly, it is further removed from these, and sprouts extend out from it to unite with their walls.
At 11 and 12 mm. head length, presexual a.nd postsexual regions, as described by Von Winiwarter and Sainmont (’08), are definitely marked o11t in the mesonephros, ,but the transition between the parts is a gradual one, the intermediate or sexual region lying opposite the reproductive gland. The entire organ is now bent round, so that the presexual region, destined to give rise to the epididymis in the male and to the epoophoron in the female, lies dorso-medially, a11d partly anterior to the postsexual portion, and dorsolaterally to the testis or ovary. The anterior end of the Wolffian duct is wide and curves round medially; from it arise rows of narrow darkly staining tubules, which run down ventrally and are surrounded by layers of connective tissue. Some are of greater diameter and have larger lumina than others, but all have thin walls, and have quite lost the characteristics of secretory tubules. The glomeruli opposite the anterior end of the rete are very small, but they increase in size posteriorly, and are quite large at the beginning of the sexual region. The rete runs through the genital mcsentcry and its darkly staining twisted strands, i11 some of which the cells are beginning to take on a radial arrangement, are united with the walls of the Malpighian corpuscles. This union appears usually to take place before the actual degeneration of the glomeruli, as it is especially well seen in the larger unchanged corpuscles, which lie in the postero-lateral portion of the sexual region (Pl. VIII, fig. 5, 8p7'.). Whether the walls of the Malpighian corpuscles give rise to definite outgrowths, which extend towards and unite with the rete cords, was not definitely determined, but from the appearances presented in this stage I have reason to believe that such outgrowths are developed (Pl. V III, fig. 5, spr. gl.). In any case, however, they must only be short, as the main connection is evidently supplied by sprouts from the rete cords themselves. In the sexual region the tubules are found to be in all stages of degeneration. The postsexual region is still well developed and functional; it lies ventro-laterally to the reproductive gland, its anterior end appearing in the same transverse sections as the presexual portion, and, being cut longitudinally, six or seven glomeruli appear in one section.
Text-fig. 4. Trz7«:ho.mru.e 7:ulpec.ula. Diagrams of two longitudinal sections of the Inesonephros, A 7-5 mm. ll.L._, B 9 mm. H.L., showing the shortening of the anterior end of the mesonephros and of the rete (r.c.) between the two stages. Both drawn to the same scale. d.gl. anterior degenerating glomeruli; gl. glomeruli; ]|I.D. Miillerian duct; W.D. Wolfiian duct.
In pouch young with a head length of 12-5 111111. ($2), tl1e presexual portion has widened dorso-ventrally, owing apparently to increase of connective tissue. In the presexual region, some of the Malpighian eorpuscles have become narrow, and present a drawn-out appearance (text-fig. 5), whilst a few seem to have completely disappeared, so that the strands of the rete are directly connected with the narrow tubules, and by them with the \VolFfian
Text-fig. 5. Tr'icho.s-urus vulpecnlrz. H .L. 12-5 mm. Transverse section through the mesonephros and ovary, showing the mass of rete strands (r.) which extend from the walls of the duct.. At the posterior end of the sexual region there is much connective tissue, and degeneration of the secretory tubules is proceeding actively. The same changes as have been described in the presexual region are now occurring at the anterior end of the postsexual portion, where the glomeruli are small, but further posteriorly the gland evidently still functions, and the glomeruli are large. On the left side, the disposition of the different regions is like that of the last stage, but on the right the bending of the mesonephros is not so marked, the postsexual portion lying posterior to the presexual.
Malpighian eorpuseles (gl.p7'.) of the presexual region of the mesonephros, through the mesovarium (m.) into the ovary (022). The glomeruli are seen to be in progressive stages of degeneration. gl.pt. glomeruli of postscxual region of mesonephros: iM.D. Miillerian duct; W. D. Wolffian duet.
Epoophoron and Paroophoron
In the female in later stages, from a head length of 15 mm. up to a head length of 24 mm., a gradual degeneration of the presexual portion or epoophoro11 occurs. The connective tissue becomes abundant, and the tubules, although retaining their connection with the rete, become detached from the Wolffian duct, which degenerates. The tubules gradually decrease in number, and degenerating glomcruli are present. In the oldest foetus (H .L. 38 mm.), the epoophoron consists of a group of small, mostly hollow tubes, some of which run into the rete. The rete, which becomes distinctly tubular in the foetus with a head length of 15 mm., remains well marked, much more so than in the male of the same age; the cords become thick and darkly staining, with well developed lumina, and, as in earlier stages, they extend into the centre of the ovary. Later, cavities and spaces appear to develop in it.
The paroophoron, which is derived from the postsexual portion of the mesonephros, undergoes a similar degeneration to the epoophoron, but the process is carried still further. 111 the foetus with a head length of 24 mm., it consists of a few narrow tubes and several degenerating Malpighian corpuscles, and into it runs a large nerve from the sympathetic system. finally, in older stages, only a few tubular remnants remain.
Epididymis and Paradidymis
In a male with a head length of 15 mm., the epididymis, which like the epoophoron represents the presexual part of the mesonephros, is quite a distinct structure, and consists of a collection of darkly staining tubules lying dorso-laterally to the testis, and running into the Wolffian duct. They are connected with the rete strands, which are now becoming tubular, and which extend into the mesorchium to unite with the seminiferous tubules. In the postsexual portion, destined to become the paradidymis, degeneration of the tubules is seen throughout its extent. They are narrow and resemble those of the epididymis, but are not so darkly staining as the latter. Though fairly large glomcruli are present a.t the posterior end, the gland is obviously no longer functional.
In a male with a head length of 20 mm. the epididymis has begun to descend with the testis, and lies with the latter a short distance anterior to the scrotal sac in the saceus vaginalis 011 the ventro-lateral side of the body. It is composed of many fine tubules connected with the Wolffian duct, and is surrounded by investing layers of connective tissue. The tubules dilate slightly where they pass into the rete, at the point where the former Malpighian corpuscles were situated. The rete is poorly developed, being less marked than in the foetus of 15 mm. H.L., or in the female with a head length of 24 mm., an(l is composed of fine tubes or strands of cells. In a pouch young of 26 mm. H .L., the epididymis is a conspicuous mass of coiled tubules with darkly staining walls, lying on the dorsal side of the testis; the tubules run into the Wolﬂian duct. The latter is now coiled on leaving the epididymis to form the Vas deferens. The rete consists of a small bundle of Very fine tubules, which pass from the canals of the epididymis through the mesorchium to be connected with the seminiferous tubules at the base of the testis.
In the adult, the epididymis (text-fig. 6) is divided into a caput (01).), which overlies the dorsal side of the testis, and a cauda (cd.), the two portions being united by mea.ns of an elongated narrow part running along the medial border of the gonad. The eauda projects beyond the testis as a bulbous mass, and is connected with the scrotal sac by a short gubernaculum. A short narrow rete (72) runs from the genital gland into the caput of the epididymis.
Text-fig. 6. Trickosurus vulpecula. Adult. .4. Dissection of the scrotal sac, showing the testis and the epididymis of the left side. Anterior View. B. Posterior View of testis and epididymis. cp. caput epididymis; cd. cauda epididymis; o.v.d. opening of vas deferens into urethra; r. rete testis; sc. scrotum; t. testis; v.d. vas deferens; zap. Vascular plexus.
The degeneration of the paradidymis resembles that of the paroophoron. It consists, in the male with a head length of 26 mm., of a few tubules disconnected from the \Volfi°1an duct and surrounded by much connective tissue. It lies ventrally and posteriorly to the epididymis, and is united with the latter by a broad mesentcry, whilst more posteriorly it is joined to the Very narrow mesorchium.
In early stages of Trichosurus, the primordium of the gonad and rete first appears as a low ridge, which projects into the coelom on the medial side of the urogenital fold. The ridge is filled with slightly denser cells, which are not sharply marked off from the adjacent mesenchyme. Anteriorly, these cells are at first continuous with the primordium of the suprarenal body, but, in the next few stages, the cranial end of the ridge is less dist.inct. and difficult to follow; in older embryos (G.L. 11 mm.), however, up to the involution of the mesonephros, the epithelium of the ridge can be traced forwards into continuity with that of the ostium abdominale, and the connection with the suprarenal organ is lost. The ridge gradually increases in thickness towards the posterior end, and, shortly before birth, it becomes distinctly differentiated into two portions, an anterior one, forming the rete ridge, an(l a posterior, representing the primordium of the gonad. Within the rete ridge lies a deeply staining core of cells, the rete core, which extends from the ostium abdominale into the genital gland. For some time before birth, before the formation of the rete core, groove-like invaginations of the coelomic epitheli1.un are to be seen in the region of the rete» ridge, but it is only in pouch young with a head length of 6-7 5 mm. and in following stages, that actual cords of cells are to be observed connecting the coelomic epithelium with the rete core. The latter comes to consist of irregular cords and strands of cells which, in pouch young with a head length of 15 mm., assume a tubular structure, constituting the definitive rete canals.
The involution of the mesonephros sets in in pouch young with a head length of 8 mm. At this time, the cranial end of the rete primordium lies close against the walls of the anterior degenerating Malpighian corpuscles, but more posteriorly it is further removed from the latter, and by means of sprouts from the rete cords definite connections with the walls of Bowman’s capsules are now established. Short outgrowths also seem to be developed from the walls of the latter, but these take a very insignificant part in the formation of the urogenital connection. The changes at the cranial end of the mesonephros are at first precisely similar in both sexes. One or two of the most anterior tubules probably disappear completely. In the following seventeen or eighteen, the secretory portions of the tubules lose their characteristic structure, and take on the appearance of collecting tubules, whilst the Malpighian corpuseles gradually undergo transformation, their glomeruli disappear, their walls thicken, and, after the connection with the rete is established, they become reduced to short uniformly narrow tubes, in continuity with, and of the same diameter the altered secretory portions of the tubules. During this transformation, the connective tissue round the tubules increases in amount, and the whole anterior end of the mesonephros undergoes considerable shrinkage. The degenerating corpuseles are drawn closer together, and the rete, being attached to the latter, becomes correspondingly shortened in antcro-posterior extent. This anterior or presexual portion of the mesonephros is destined to develop into the epididymis in the male and the epoophoron in the female. In the male, the tubules retain their connection with the V/Volffian duct which becomes the vas deferens. They come to consist of a mass of coiled and darkly staining canals, the canals of the epididymis, which communicate with the seminiferous tubules by a bundle of fine rete tubes or vasa efferentia running through the mesorchium.
The urogenital connection in Triehosurus thus involves a large number of rete canals, and 17 or 18 mesonephric tubules, the latter giving rise to the epididymis (text-fig. 8, A).
In the female with a head length of 15 mm., and in later stages, the presexual portion or epoophoron gradually deteriorates, the degenerating tubules become detached from the Wolffian duct and eventually atrophy. The rete, on the other hand, appears to remain well marked, even in later stages, where it forms a conspicuous mass of narrow canals, which stretch from the centre of the ovary, right through the mesovarium, terminating blindly at both ends.
The degeneration of the posterior portion of the mesonephros begins in pouch young with a head length of about 11 mm. The secretory portions of the tubules break down and the glomeruli degenerate. The atrophy, which begins in the sexual portion of the gland, gradually spreads to the postsexual region, until finally only vestiges of tubules and glomeruli remain. These form the paradidymis in the male and paroophoron in the female. The former completely disappears, whilst the latter, in older females, consists of only a few tubular remnants.
The Urogenital Connection in Other Marsupials
The only detailed observations on the rete and urogenital connection of marsupials are those of van den Broek. In a paper on the female urogenital organs, this author (’05) describes and figures a larger or smaller number of branching and anastomosing rete canals in the hilum of the ovary of an adult Petrogale penicillata. These canals were isolated and had no connection with any other structures, and from their appearance van den Broek suggests that they may possibly be a new formation. It may be noted that, in 1901, Berry Hart (’()1) pointed out the occurrence of a canalicular system at the hilum of the ovary in the rat kangaroo. This was continuous with the epoophoron and was therefore considered as representing the paroophoron.
In the male pouch young of Dasyurus (D.C.L. 53 mm.), van den Broek (’07) has shown that the seminiferous tubules unite into a single duct, which runs through the mesorchium, then coils to form the epididymis, and finally passes back as the vas deferens. In a female with a length of 40 mm. (D.C.L.), the cranial end of the Wolﬂlan duct is described as passing through the mesovarium into the hilum of the ovary. I11 older females (D.C.L. 53 and 63 mm.), he found that the anterior end of this duct was simply a blind canal lying near the hilum. From these observations van (le11 Brock comes to the conclusion that the cranial end of the Wolﬂ’1an duct in both sexes grows through the mescntery into the base of the genital gland, forming the pars‘ conjunctiva in the male and remaining in the female of later stages as an isolated duct. He states that the mesonephros atrophies completely, and that iii the male the canals of the epididymis or pars conglomerata arise by sprouting, or more probably by division of tlie primitively single \VollIia11 duct. According to him, no rete is developed. The conditions in Didelphys point, he says, to a similar conclusion. In male pouch young of Didelphys spec. (D.C.L. 102 and 130 mm.), and also in a full-grown D. vi7'gim'ana, a single canal passes from the tubules of the testis through the mcsorchium, and divides into the many canals of the epididymis. In a later communication van den Broek (’10, a and I1) states that the same conditions are present in the Phalangeridae and Macropodinae, and that here also there is at first only a single duct from which the canals of the cpididymis later arise. This author tliereforc supposes that the urogenital connection in marsupials has a widely different origin from that of other 111am1nals.
From our observations on Triehosurus, it is evident that the conditions described by van den Broek do not hold for all marsupials. Van de11 Brock, however, had little or no opportunity of examining embryos or early pouch young, and his conclusions are based on the examination of comparatively late stages. From the unique collection of early stages in the possession of Prof. J. P. Hill, I have been able to investigate a complete series of early pouch young of Dasyurus, also early Didelphys, besides several stages of other forms. It is of some interest to compare these observations with those of Vail den Broek, and with the conditions already described in T richosurus.
Da.s*yur'u.s' viveiwlnus. In very early‘ pouch young of Dasyurus (H.L. 3 to 4 mm., Stages C and D) the rete ridge has the same relations as in embryos of Trichosurus. It passes back from the medial side of the ostium abdominalc as a low ridge, which increases in size as it continues posteriorly into the gonad. Within the ridge is a concentration of cells, but as yet no definite cord has developed. At the next stage (H .L. 4-5 to 5-5 mm., Stages E and F), from the posterior wall of the ostium abdominalc a short tubular groove extends inwards and continues back as a solid and distinct cord of cells, the rete cord, which runs on the inner side of the rete ridge, close to the walls of the Malpighian corpuscles. Near the gonad, the cord becomes thicker, and finally, after passing through the mesentery, runs for a short distance into the gland, where it either ends blindly or is intimately connected with the central cells of the latter. The Mijllerian duct at this stage extends back as far as the posterior end of the mesonephros.
In pouch young with a head length of 6-5 to 8 mm. (Stages G and H), the rete cord in both sexes is becoming transformed into a definite tubular duct. This rete duct runs out from the reproductive gland through the mesentery and along the rete ridge, where it passes very close against the Walls of the Malpighian corpuscles of the mesonephros. Further cranially, the duct moves slightly dorsally, o11t of the rete ridge, and, at the anterior end of the excretory gland, it turns laterally and becomes connected with the cranial end of the WolFfian duct by two, or sometimes possibly three, narrow tubules; these most probably represent the two most anterior mesonephric tubules, which, with their glomeruli, have already undergone transformation. Apart from the union of these two tubules with the rete duct, there is as yet no differentiation at the anterior end of the mesonephros, and the rete ridge itself passes as before into the posterior side of the ostium abdominale.
At the next stage (H.L. 10 and 10-5 mm., Stage I), there is marked atrophy at the cranial end of the mesonephros. Much connective tissue is present, the glomeruli are undergoing degeneration, Whilst the collecting tubules are breaking down and are apparently no longer connected with the \Volffian duct. At the posterior end, the mesonephros is still well developed,
Text-fig. 7. I)a.s3/urus m'vcr2°inu.s. H .L. 12 mm. (About J). 22. Diagram of a transverse section of the ovary and anterior end of the Wolﬂ‘ian duct. The single rete duct (r.d.) passes out from the centre of the ovary (nu) and is united with the Wolffian duct ( W.D.) by two tubes, a larger (a) and a narrower (b), which probably represent the most anterior tubules of the mesonephros. M. I). Miillerian duct.
as are the Malpighian corpusclcs. Inthe male. the seminiferous tubules are for the most part tubular. They unite towards the base of the testis into a solid cord of cells, which soon becomes hollow as it passes through the mesorchium. The rete duct thus formed, shortly after leaving the mesorchium and behind its connection1 with the WolHian duct, buds off many small tubules, some of them mere short euls-de-sac. These tubules continue into the degenerate mass at the anterior end of the excretory organ, and may perhaps represent the sprouts which formerly connected the rete with the Malpighian corpuscles, now completely atrophied. The appearances at this stage point to an abortive attempt 011 the part of the rete to become attached to the mesonephric tubules, s11ch as oce11r in the formation of the epididymis in Trichosurus.
- 1 It was impossible to determine in this foetus whether this involved one or two tubules. The Development of tlte U erogenvltctl System in the M arsupialia 111
In a male with a head length of 11-5 mm. (Stage J), the mesonephros is quite degenerate, even at the posterior end. The cranial portion is simply a mass of connective tissue with remnants of tubules, but more posteriorly remains of secretory tubules and degenerating glomeruli can be recognised. The condition of the rete duct is similar to that of the last stage, but in a slightly older female (H .L. 12 mm.) it is connected with the Wolffian duet by two distinct tubules (text-fig. 7), whilst.» in one with a head length of 12-5 111111. the anterior end of the \VolHian duct has disappeared, the rete duct simply becoming dismembered at its cranial end.
In females with a head length of 18 and 22-5 mm. (Stages K and L) the rete duet arises from the ovary, and passes through the mesovarium as a conspicuous tube with a large cavity and darkly staining epithelial walls. It becomes reduced in size a11d disappears soon after leaving the mesovarium.
In a male with a head length of 18 mm., the testes have begun their descent, and lie close to the ventral wall of the abdomen 011 each side, just cranially to the scrotal sac. The seminiferous tubules, towards the base of the testis, unite into a single duet, which, passing through the mesorchium, becomes much narrower in diameter, and runs forwards and slightly dorsally to become continuous with the vas deferens. From the conditions in the female shown in text-fig. 7, it is clear that either the two anterior tubules of the mesonephros there present, coalesce to form one tube, or that one of them disappears. In an older male (H .L. 225 mm., Stage L), the rete duet, after leaving the mesorchium, continues as a single tube, which, coiling round in a connective tissue basis, forms the primordium of the epididymis, and runs into the vas deferens. In a still older stage (H.L. 3-6 cm., Stage N), a distinct epididymis is present. It is a tubular structure, lying in connective tissue, and is divided into a caput and a smaller cauda, the two united by a narrow intermediate portion. Certainly in the cauda, and most probably also in the eaput, the gland is composed of a single, much co11vol11ted tube, developed by coiling from the single duct. How much of this duct corresponds to the mesoncphric tubule originally connecting the rete and \/Vollfian ducts, and how much is derived from the anterior end of the Wolffian duct alone, it is impossible to ascertain. Whether in late stages there is any branching of the originally single duct into many tubules, as suggested by van den Broek, I have not been able to determine.
The urogenital connection in Dasyurus is thus. composed of a single rete canal and one or two mesoncphric tubules (text-fig. 8, D).
Didelphys. The pouch young of Didelphys consisted of males of D. (mrita with a head length of 8, 9, 10-5, 12, 18 and 20 mm., and two females with a head length of 9-5 and 18 mm. respectively. Observations were also made on a male of 111 etac/zli7'u.s opossum of 13 mm. H .L. 1 12 lﬂzfzwbel/L A. Iflmser
In the earliest stage (H.L. 8111111.) there is only a slight intlieation of dege11eratio11 at the a11terior end of the n1eso11ephros. The rete ridge is well marked. It arises from the medial bay of the ostium abdominale, a11d extends back, increasing greatly i11 size, along the medial side of the urogenital fold. The anterior region is filled with 111asses of darkly staining cells, which, 1nore posteriorly, are connected i11to a single cord. This cord becomes a d11ct as it passes into the mesorchium, a11d finally unites witl1 the seminiferous tubules. At a .few places along the ridge, solid cell cords extend from the coelo111ic
Text-fig. 8. Diagrams illustrating the condition of the rete and mode of origin of the epididymis
in different marsupials. A. Triehosurus. B. Didelphys. C. Perameles. D. Dasyurus. m.t. transformed mesonephrie tubules destined to form epididymis; r. rete; 5.1. seminiferous tubules; W .D. Woli°fian duct.
epithelium i11to the rete core, i11 the same manner as described and figured for Triehosurus. These ingrowths are still more marked in the next foetus (H .L. 9 a11d 9-5 mm.), a11d are also present along the mesogenitale. At this stage the rete ridge is Very prominent, and within it the rete forms a cord of darkly staining cells. The secretory portions of the most anterior tubules of the mesonephros are no longer recognisable, and, on the right side in the male, the first five glomeruli lie close together, and from the ve11tral walls of their capsules a thick solid cord of cells stretches ventrally a11d posteriorly i11to the rete cord. The latter is also becoming attached to the more posterior
Malpighian corpuscles. On the left side, the most anterior degenerate glomerulus appears to be isolated, whilst behind it three are united with the rete as are the first five on the right.
In the next male (H .L. 10-5 mm.), the connection of the rete, which is now becoming a definite duct, with the five anterior transformed tubules of the mesonephros is well seen, especially on the right side (Pl. VIII, fig. 6). The first tubule (t. 1) now consists of a narrow duet running into the Wolffian duct dorsally, and united on the ventral side with the rete; its glomerulus ha.s entirely disappeared. In the second tubule, the. position of the former glomerulus (gl. 2) is marked by a slight swelling of the tube and a thickening of its walls at this point. The other glomeruli (gl. 3, 4, and 5) are seen to be in progressive stages of degeneration. In this foetus, at the base of the right testis, the coelomic epithelium is connected with the rete duct by strands of cells. At an older stage (H .L. 12-5 mm.), eleven transformed mesonephric tubules (possibly twelve on the right side) are united with the rete duct ventrally, and run up dorsally into the \Volffian duct, the portion connecting them with the latter having thinner, more lightly staining walls. Definite Malpighian corpuscles are only recognisable in connection with the hindermost of these tubules. This region is destined _to form the epididymis; behind it, on both sides, there is a zone of degenerate glomeruli and tubules, surrounded by much connective tissue. This zone gradually merges into a posterior region, which still possesses large Malpighian corpuscles and distinct secretory tubules.
In the two oldest males (H .L. 18 and 20 mm.), the epididymis is well developed. It lies dorso-laterally to the testis in a large cavity close to the ventral side of the abdomen, just cranially to the scrotal sac, and consists of many canals, which join together into a single rete duct with darkly staining walls. This duct, which is somewhat smaller than in Dasyurus, Stage K, ru11s ventro-medially through the mesorchium, to be connected with the seminiferous tubules. No paradidymis appeared to be present. I11 the female of the same age (H .L. 18 mm.), a similar duet passes from the centre of the ovary through the mesovarium, but its opposite end was not traced, owing to this portion being absent from the sections. Most probably the duct is a blind t11be as in the female of other forms.
In Metachirus (H .L. 13 mm.), the conditions are much as in Didelphys. On the left side, only three tubes appeared to pass from the rete duct into the epididymis, but, on the right, it was difficult to count the exact number.
The urogenital connection in Didelphys is therefore made up of a single 1‘ete canal, which becomes connected with 11 or 12 mesonephric tubules, the latter developing i11to the epididymis (text-fig. 8, B).
Perameles. In an embryo of Perameles with a length of 6-8 mm., the cranial portion of the excretory organ, which begins on a level with the 4th spinal ganglion, consists of many rudimentary tubules, the majority of which have a solid nephrostomial connection with the coelomic epithelium. Behind the level of the 6tl1 spinal ganglion, tl1e tubules are larger, and definite primordia of Malpighian corpuscles are present. Further posteriorly, they become curved, and much resemble the developing mesonephric tubules in the Triehosurus embryo of 4-5 mm. G.L. (Stage II) (cf. Part I of this paper). The wall of that portion of each of these posterior tubules, which is destined to develop into a Malpighia11 corpuscle, is connected with the coelomic epithelium on the ventro-medial side of the urogenital fold by what must be regarded a rudimentary nephrostomial canal. These structures (Pl. IX, figs. 7a, 1), and 8 and P1. VIII, fig. 7, nph.) are present all along the excretory organ, right down to the posterior end, where additional tubules are still forming. The majority of them take the form of a thick column of denser cells, whose margins are not definitely limited from the surrounding mesenchyme (Pl. IX, fig. 8, ‘nph.). Some of the columns are narrower and not so well developed, whilst a few are longer and partly tubular (Pl. VIII, fig. 7, v.nph.), and in some cases a slight groove in the coelom is present opposite the point of contact with the coelomic epithelium. It is noteworthy, also, that in some instances they tend to run into each other.
The suprarenal primordium stretches from the level of the 5th to behind the level of the 12th spinal ganglion. Its cranial end is poorly developed, but further back it is Well marked, and is seen as a continuous proliferation of cells from the coelomic epithelium, which passes 11p dorsally between the aorta and the mesonephros. Along its posterior region it is connected with the rudimentary mesonephric nephrostomes at this level by bands of cells, which extend out dorso-medially from each nephrostome to blend with the cells of the primordium (Pl. IX, fig. 7 . a, b and c). These outgrowths from the nephrostomes apparently contribute towards the formation of the cortical substance of the suprarenal body.
The genital ridge is not yet developed.
In the next embryo of Perameles (G.L. 7 mm.) and in later stages, the genital ridge is present as a low elevation filled with denser cells, extending along the ventro-medial side of the urogenital fold, and quite similar to that in the early stages of Trichosurus and other forms. The cells within the anterior portion of the ridge, which eventually form the rete core, lie in exactly the same place a.s the degenerate nephrostomial canals present in the embryo of 6-8 mm., and they approach close to the Walls of the Malpighian eorpuscles. In the 6-8 mm. embryo, the cells of those nephrostomial canals, which are not definitely separated from the surrounding mesenchyme (Pl. IX, fig. 8, «nph.), have a similar appearance to the cells in the genital ridge of the next stage (G.L. 7 mm.). The evidence points to the conclusion that the rete core is derived from the nephrostomial canals and nephrostomes.
In a female with a head length of 12-5 mm., the rete is becoming transformed into a single hollow tube, which is connected by a few solid cords with Malpighian corpuscles in the anterior part of the mesonephros. The cranial end of the tube could 11ot be definitely followed, but it eventually runs into the anterior end of the Wolffian duet. In an older female (H.L. 16 mm.) there is a definite rete duct, the cranial end of which was again rather indistinct. On the left side, it appears to be connected with the Wolffian duct by several tubules, which no longer possess glomeruli; the condition on the right is apparently somewhat similar. The cranial end of the mesonephros is now very degenerate, but in the posterior portion glomeruli are still present. In a still older stage (H .L. 37 mm. 9), the rete duct runs out from the centre of the ovary through the mesovarium. It then becomes narrower and passes into the epoophoron, now devoid of tubules and almost completely atrophied, and, coiling round, it terminates in a degenerating cord, which evidently represents the last vestige of the anterior end of the Wolffian duct. In older females, the rete duct does not reach beyond the mesovarium.
In a male with a head length of 19 mm., the epididymis is composed of a small number of short tubules, about four or five, forming a network lying in connective tissue on the dorsal side of the testis. From the conditions in the younger female (H .L. 12-5 mm.), it is very possible that these canals represent a similar number of modified mesonephric tubules.
The urogenital connection in Perameles would thus seem to involve a single rete canal and a small number (presumably 4 or 5) of mesonephric tubules from which the epididymis arises (text-fig. 8, C.).
Phas-c0la7‘ct0s. Observations on a series of embryos and pouch young of Phascolarctos (G.L. 7-5, 9, 11, 12-25, 13-5, 16-5 mm., H.L. 9 and 9-5 mm.§ show that the development of the rete is apparently identical with that of Trichosurus. A similar rete ridge is present in early stages, and in later pouch young (II.L. 9-5 mm.), all of which are female, the rete is composed of a mass of cords, some of which are tubular. They pass from the centre of the ovary through the mesovarium, and are united by solid cords of cells with the walls of the Malpighian corpuscles. Further anteriorly, the rete forms a single cord, which runs forwards close up to the medial side of Bowman’s capsules, and, as in other forms, it is seen to be connected with the coelomic epithelium of the ridge.
In the oldest stage of Phascolomys (G.L. 17-5 mm. 5‘) which was investigated, the rete ridge was well defined, but a rete cord had not yet developed.
Several female Macropods (J1. tlzetides, H .L. 22 mm., Al. ruficollvfs, H.L. 23 and 32 mm., and Petrogale pencillata, H .L. 7-5 to 8 mm.) were also examined. In all these there was a well marked broad" rete, consisting of many small tubules, some larger than others, extending out from the central mass of the ovary through the mesovarium to run into the epoophoron. The latter was made up of narrow canals, representing the remains of the anterior end of the mesonephros. The condition in the oldest female recalls the description and figures given by van den Broek (’05) of the rete ovarii of an adult Petrogale.
8-2 116 Elizczybctia A. Iflraser
Diprotodontia. The number of stages of ge11era older than Triehosurus, available for study, unfortunately does not form a complete series, but the results so far obtained seem to show that in this suborder the conditions described for Triehosurus generally hold good. The rete core becomes attached to a considerable number of Malpighian corpuscles (17 or 18 i11 Triehosurus) at the anterior end of the mesonephros. The rete eventually divides into a large number of fine canals, which run out from the gonad through the mesogenitale into the epididymis or epoophoron. The epididymis arises from the anterior mesonephrie tubules, by transformation of their secretory portions and of their Malpighian corpuscles, as described in Triehosurus (teXt—fig. 8, A).
Polyprotodontia. In all the Polyprotodonts so far investigated, the rete core, instead of breaking up into many fine canals, develops into a single large duct.
In Didelphys, as in Triehosurus, the anterior end of the rete core lies close against the walls of the most anterior Malpighian corpuscles, whilst, more posteriorly, a connection with the latter is established by means of sprouts from the rete itself. During this process, the rete core develops into a single rete duct, which stretches from the anterior end of the mesonephros through the mesogenitale into the gonad. The epididymis arises from a fairly large number of anterior mesonephrie tubules (possibly 11 or 12), which become transformed into uniformly narrow canals as in the Diprotodontia (text-fig. 8, B).
In Perameles, a single rete duct is again developed, but apparently only a very limited number (possibly 4 or 5) of the anterior tubules of the mesonephros take a share in the formation of the epididymis, the rest of the excretory organ completely degenerating (text-fig. 8, C).
In Dasyurus, there appears to be a temporary attachment between the rete duct and a number of Malpighian corpuscles at the anterior end of the mesonephros. Eventually, however, the duct only remains united by a single canal, probably the first mesonephrie tubule, or the first and second fused together, with the \Volffian duct (text-fig. 8, D). The mesonephros completely dcgcnerates i11to a mass of connective tissue, in which the single duct undergoes a complicated coiling to form the epididymis. Van den Broek (’O6) has already put forward the suggestion that the tube’ passing from the Wolffian duct into the genital gland of Dasyurus and Sminthopsis might possibly represent the most cranial tubule of the mesonephros, which grows into the gonad, though he thought this suggestion unlikely. This author did 11ot observe any transformation of mesonephrie tubules into a connecting duct, and he was not in a position to make any investigations on the development of the rete, such as has been described in this paper.
There is much difference of opinion as regards the origin of the rete cords’ in the Amniota. Many of the earlier workers, such as Braun (’77), Weldon (’85), and Hoffmann (’89) in reptiles, Hoffmann (’86) in Arnphibia, Schmiegelow (’82) and Semon (’87) in the chick, Mihalkovics (’85) in the cat, dog, and rabbit, and more recently Sainmont (’O6 and ’08) in the cat, and von Winiwarter (’O8, ’00 and ’10) in the rabbit and man, consider that the rete develops entirely from outgrowths of the walls of a variable number of Malpighian corpuscles at the anterior end of the mesonephros. According to this view, the rete does not appear until shortly before the involution of the mesonephros.
A second opinion, held by Janosik (’85), and later by Coert (’98) in his investigations on the rabbit and cat, and Allen (’O4) in his studies on the pig and also on the rabbit, ascribes the origin of the rete cords to invaginations of the peritoneal epithelium. Coert (’98)1 thinks that the rete tubules develop from a mass of cells lying within the genital ridge, the so-called “Reteblasteem,” which is derived from cells of the coelomic epithelium, which slowly migrate inwards. Allen (’04) describes the primordium of the rete in the pig as tubular invaginations of the peritoneum, which are formed at the same time and in the same manner as the sex cords (both medullary cords and seminiferous tubules). The union with the Malpighian corpuscles is therefore a secondary one.
A few of the more recent authors, namely Bouin (’0O) in the frog, Wichmann (’l2) in the sheep and dog, and firket (’14) and Swift (’l5) in birds, contend that the urogenital connection is derived from; the mesenchyme lying between the peritoneal epithelium and the Malpighian corpuscles. Here again, the rete is regarded as becoming only secondarily connected with the uriniferous tubules. Wichmann and firket consider that the urogenital cords are homologous with pronephric nephrostomes, which have lost their connection with the coelomic epithelium. In Chrysemys Allen (’05) states that the rete arises from a series of “funnel-cords” which take their origin from peritoneal funnels of the Malpighian corpuscles. In this last paper, the divergent views on the development of the rete are well shown in a series of diagrams.
In the early stages of Trichosurus I was not able to trace the exact origin of the cells which come to lie within the rete ridge, but appearances in this genus at‘ first suggested that a gradual infiltration of cells from the coelomic epithelium was taking place during this time, such as is said to occur in the rabbit according to Coert (’98).' With the exception of Perameles, the conditions in the early stages of other marsupials much resembled Trichosurus. The youngest embryo (G’.L. 6-8 mm.) of Perameles is of special interest. In this stage, immediately behind what may be regarded as the region of the pronephros, the portions of the tubules destined to become the Malpighian corpuscles are connected with the eoelomic epithelium by a series of rudimentary nephrostomial canals, which are found as far back as the posterior end of the excretory organ, where tubules are still forming. The majority of these nephrostomial canals have the form of rather thick, cellular columns, of somewhat indefinite outline, owing to the fact that their margins are not sharply marked off from the surrounding mesenchyme. Moreover, adjacent canals tend to run into each other. In a slightly older embryo (G.L. 7 mm.), mesonephrie nephrostomes are no longer recognisable, but between the Malpighian corpuscles and the eoelomic epithelium we find a continuous mass of denser cells lying in a low ridge, which projects into the coelom along the ventro-medial border of the urogenital fold. This ridge is the primordium of the genital ridge, and is exactly similar to that present in early stages of Trichosurus and other forms. The comparison of these two embryos of Perameles leads to the concl11sion that in this genus, and probably also in Trichosurus and other marsupials, the cells within the genital ridge are derived from the nephrostomial canals of the mesonephros, which lose their connection both with the eoelomic epithelium and the Malpighian corpuscles, and unite into a continuous mass of cells.
- 1 My knowledge of Coert’s paper was obtained from the references in the papers of von Wini- Warter (’10), Allen (’04) and Wichmann (’I2). I have had no opportunity of reading the original.
The exact determination of the origin of the cells in the genital ridge in early stages in other forms is evidently a matter of some difficulty. This is noted in the chick by firket (’14), who says (p. 218): “ Il n’est pas facile dc distinguer, lors les premiers stades d11 developpement des glandes génitales, ce qui est tissue mesenchymatique de ce qui est epithelial.” Coert (’98) experienced the same diffieulty iii the rabbit, whilst Allen (’04) finds the peritoneal invaginations much clearer in the pig than in the rabbit. Wiehmann (’12), also, refers to the fact that the elements are more easy to distinguish in some animals than in others. In the sheep, dog, and pig, he describes bridges of cells stretching from Bowman’s capsules to the eoelomic epithelium. These bridges much resemble the rudimentary mesonephrie nephrostomes present in Perameles. According to this author, the cell bridges, lying near the cranial part of the genital primordium, early run into each other, and thus form a continuous blastema, which is indistinctly separated from the surrounding mesenchyme. Out of this blastema the rete later differentiates. This description of the origin of the rete blastema coincides very closely with the conditions described in Perameles. Wichmann, however, considers that in the forms investigated by him, the cell bridges are derived from the primary nephrostome canals (“Erganzungskanalchen”) of the pronephros. This is certainly not the case for Perameles, nor for other marsupials, where the anterior end of the rete ridge always lies behind the region of the pronephros. In coming to this conclusion, Wichmann necessarily assumes that pro- and mesonephrie elements are present in the same segments of the body, but, as already stated in Part I of this paper, there is no evidence of such overlapping in marsupials.
Allen (’O5), as already noted, also ascribes the origin of the rete in Chryscmys to the peritoneal funnels of the Malpighian corpuseles, which become T detached from the latter and develop into “funnel-cords.” In later stages, these unite with short evaginations from Bown1an’s capsules, an(l thus establish the urogenital connection. Such short outgrowths from the walls of the Malpighian corpusclcs were also observed in Trichosurus, but, as in Chrysemys, they are of small importance, and certainly take a very insignificant part in the formation of the rete. Again, in Elasmobranchs, according to Borcea (’O5), the Vasa efferential take their origin from a variable number of nephrostomial canals.
In Stages V and VI of Trichosurus, the anterior end of the genital ridge is continuous with the hinder end of the suprarenal primordium. This connection has been observed and variously interpreted by different. authors. Soulié (’03), who has made extensive studies on the development of the suprarenal bodies in higher vertebrates, considers that the apparently perfect fusion between the two organs is a result of their common origin from the coelomic epithelium, and is thus of only secondary significance. The suprarenal primordium in Trichosurus is certainly well marked before the genital ridge first definitely appears. In the embryo of Perameles with a length of 6-8 mm. the nephrostomes towards the cranial end of the mesonephros are connected by bands of cells with the hinder end of the suprarenal primordium (Pl. IX, fig. 7 c, .v). These structures appear to be almost exactly similar to the “funnel-cords” of Chrysemys; according to Allen, the “funnel-cords” extend dorso-medially from the peritoneal funnels, their proximal ends forming the major part of the rete, whilst their distal ends fuse with the primordium of the suprarenal body, and, in his view, contribute the greater part of the cortical substance of this organ, only a small portion being furnished by peritoneal ingrowths. The connection between the nephrostomes and the suprarenal organ in Perameles is of short duration, and, although they apparently take some part in the development of the latter, the extensive proliferations from the coelomic epithelium, directly medial to the nephrostomes, evidently form the main source from which the suprarenal cortex arises. ‘The nephrostomes opposite the hinder portion of the suprarenal primordium seem to disappear completely, together with their mesonephric canals, only those behind this region developing into the rete. In Trichosurus, the connection between the anterior end of the genital ridge and the suprarenal organ very soon disappears, and a short time after the ostium abdominale is developed, the cranial end of the ridge runs directly into the epithelium of the shallow medial bay of the ostium. The meaning of this relation is obscure, but there is no direct evidence that the ostium abdominale has anything to do with pronephric nephrostomes and canals (cf. Part I of this paper), as supposed by Wichmann (’12, ’14), when he discusses the presence of connections in the pig and in man, during the beginning of the foetal period and sometimes later, between the epithelium of Mi'1ller’s duct and the tubes of the rete ovarii and epoophoron.
- 1 It must be noted that, in Elasmobranchs, according to Borcea, the Vasa eflerentia only represent tl1e transverse tubules, which lead from the collecting canal of the testis to the longitudinal canal of the epididymis, the connections between the latter and the Malpighian corpuseles being termed “canalicules intermédiaires.” This distinction is not possible in the Mammalia, and the term rete or vasa eflerentia, adopted in this paper, refers to the whole length of the canal or canals, which unite the seminiferous tubules with the Malpighian corpuscles. 120 Elizabeth A. Fraser
The rete ridge in Perameles, in embryos older than 6-8 mm., and in early stages of other marsupials, is the direct continuation of the genital ridge and is apparently identical with the latter, exactly as described by Allen (’04) in the pig. The two are indistinguishable, and the differentiation into gonad and rete does not occur until shortly before birth. At this time and in later stages, after the rete core is established, definite cord-like invaginations are seen uniting the coelomic epithelium of the ridge with the rete core. They have the same appearance as the sex cords, which are now developing in the gonad. Allen (’04) considers that the rete cords of the pig are formed from tubular invaginations of the peritoneum, which arise at the sa.me time and in the same manner as the sex cords. When comparing the results of his investigations on ('.hrysemys and mammals, he finds it difficult to be certain of the nature of the rete in the latter, and suggests (p. 93) that either “the sex-cords have disappeared leaving only the funnel-cords,” or, more probably, “the sex-cords which presumably existed in this region have taken 011 the function and character of funnel-cords.” I have had much the same difficulty in interpreting the significance of the definite ingrowths of the coelomic epithelium, which occur in later stages of Trichosurus, Didelphys and other forms, but am inclined to agree with Allen that they represent sex cords. It might be supposed that they are homologous with the sex cords of the definitive gonad, a relic of the time when the latter extended further cranially. The fact that they come into close relation with the rete core suggests that they may take a part in the later development of the rete, but whether this is so or not, I have not been able to determine. It is not impossible that i11 some of the higher mammals, such as the pig, the sex cords, as suggested by Allen, may alone form the urogenital connection, completely replacing the nephrostomes.
The transformation of the rete‘ core into a single duct in the Polyprotodontia recalls the longitudinal canal, which is developed in many of the lower Vertebrates. In most Elasmobranchs, the nephrostomial canals which form the vasa efferentia, terminate in rudimentary vesicles, which unite to form the so-called longitudinal canal of the epididymis (Borcea, ’05), the latter entering into relation with the Malpighian corpuscles of the mesonephros by means of a number of intermediate canals corresponding to the number of nephrostomes. A similar longitudinal canal (“ N ierenrandkanal ”) is present in Amphibia and also in some reptiles, such as the viper. In Amphibia, according to Hoffmann (’86), the vasa efferentia are formed by means of sprouts, which grow out from the Malpighian bodies through the mesogenitale into the gonad. Before the sex is differentiated, other sprouts arise from the part of the vasa efferentia lying nearest to the kidney, and these unite into a canal (“ Langskanal ”) running along.the medial edge of the latter. Semon (’92) considers that this system of cross and longitudinal canals in Amphibia takes its origin from the segmental strands, which represent the inner portions of the primitive connection between the nephrostomes and the coelom. Again, in the viper (Braun, ’77), the longitudinal canal is said to be produced by the 11nion of outgrowths, which spring from the Malpighian bodies of that part of the mesonephros situated near to the gonad. In many reptiles, the longitudinal canal is incomplete, as in the turtle (Allen), or is in the form of a long often broken cell strand, as in the lizard and slowworm (Braun). The single rete duct, which, in certain early marsupial pouch young, resembles the longitudinal canal in reptiles and Elasmobranchs, becomes shortened in later stages, ‘owing to the shrinkage of the anterior end of the mesonephros and to the form of the testis. It finally comes to consist of a short tube, leading from the seminiferous tubules through the mesorchium into the epididymis.
The involution of the mesonephros to form the epididymis and paradidymis in the male, and the epoophoron and paroophoron in the female, follows in many particulars the course of events in the cat as determined by Von \Viniwarter and Sainmont (’08). The transformation of the mesonephric tubules and of the Malpighian corpuscles appears to take place in the same manner as observed by these authors. In the higher Mammalia, a large number of these tubules apparently take part in the formation of the epididymis. In the Marsupialia, however, the number varies considerably in different genera, being 17 or 18 in Trichosurus, 11 or 12 in Didelphys, 3 or 4 in Perameles, and only one (or possibly two) in Dasyurus. A similar variation is found in the Reptilia, and again in Elasmobranchs, Amongst the latter, the epididymisl in Squatina is formed from sixof the anterior mesonephric canals, which unite with a corresponding number of Vasa efferentia, in Mustelus and Galeus there are three to five, whilst in the rays there is only one vas efferens, which becomes connected with the \Volﬂian duct by means of the first incompletely formed mesonephric tubule. The exact mode of origin of the epididymis in reptiles needs further investigation. In this order, it is usual for a number of the connections between the Vasa efferentia and the Malpighian corpuscles to be absorbed during early life. In the lizard and in snakes (Braun, ’ 7, Hoffmann, ’89) many such connections are developed, but they all degenerate with the exception of two or three, which form the ducts leading from the testis into the epididymis in the adult. In adult Chelonians, although the number of canals uniting the testis with the epididymis is a larger one than in lizards or snakes, here also there is a considerable reduction during the process of development. For example, in a young Em.y.s~ lutaria, measuring 2-5 cm., there are, according to Von Moller (’99),
- 1 The epididymis in Elasmobranchs, as defined by Borcea (’05), is that part of Leydig‘s gland which is in connection with the Vasa efferentia.
about 30 rete cords, whilst in the adult this number is reduced to 15. Again in Chrysemys (Allen, ’05), the rete cords amount. to 16 in an embryo of 8 mm., whereas in the fullgrown turtle only 9 were counted. It will be remembered that in Dasyurus (Stages I and J) there is also an abortive attempt on the part of the rete to become attached to several of the tubules at the anterior end of the mesonephros. In snakes, the epididymis is much less developed than in other reptiles, and simply consists of a coiled portion of the vas deferens, which communicates by two or three canals with the testis. This condition presents a striking parallel with the mode of development of the epididymis in Dasyurus. It is of interest to note that the canals of the epididymis in Echidna (Disselhorst, ’97) are also few in number, and separated by much connective tissue, but how these canals arise has not yet been investigated.
Cowper's Glands. Well developed Cowper’s glands are present in the male Trichosurus. They are also developed in the female, but always remain small and probably undergo reduction in older pouch young
The primordia of the glands first appear in a male foetus with a length of 16 mm. (H.L. 6-5 mm.) in the form of two slight solid out
growths from the ventro-lateral .-j '6'-},.—":'.. .'r " ., ,2--. _ , -.3 ..'. . ;:-3; '_;°.':s". ;. wall of the urogenital slnus (text- -:’.‘,:‘;‘ -,°~,-.‘«:::g‘.' "' J"-’.-33 -."l'-.°i'. '.:.'.7n'.-°.'.i fig. 9, c.g.), just at the point where 1 -'.j;‘.» j.-if ‘ v G
the free margin of the cloacal septum reaches the latter. In pouch young with a head length of 7-5 mm., both male and female, these outgrowths have developed into definite short buds, extending out from the sinus wall (Part I, text-fig. 14, c.g.). They grow rapidly and very soon (H.L.
..‘ I , ’ _“"
Text-fig. 9. G’.L. 15mm. H.L.6-75 mm. Transverse 8'5 Stretch up dorsally on section, showing the primordia of Cowper’s glands
each side Of the sinus, ventro- (c.g.), arising from the ventro-lateral wall of the
laterally to the recturn. In SuC_ urogenital sinus. The free margin of the cloacal septum (cl.s.) meets the urogenital sinus in the
Cccdlng Stages’ the prlmordla’ sections immediatelyfollowing. cl.s. dorsal margin 0“ 111311 3111019 and Sohda beC0n1C of cloacal se tum, formin ventral wall of uro«:9 P g
h0]]0wed out and branched, and genital. sinus; ect.cl. ectodermal cloaca; u.g.s. the resulting glands in the male urogemtal Smus‘ (SL 6-5-5‘)
soon attain a larger size than those of the female. In Part I of this paper, text-fig. 25 (l.c.g., r.c.g.) shows a transverse section of Cowper’s glands in a male with a head length of 11 mm., whilst the point of origin of the glands from the urogenital sinus in older stages is shown in Part I, text-figs. 15, 18, 19, 21 and 23 (c.g.). With the general growth of the sinus wa.ll, the glands gradually come to open on the dorsal side of the latter (text-fig. 1()).
In the male with a head length of 26 mm., each gland arises from the sinus by two short hollow ducts, which lie close together. Running up dorsally, they spread out into numerous glandular lobes, which reach up on each side of the rectum. A third duct is now present on each side, arising from the sinus posterior to and quite separate from the first two, but though hollow, it is only a short sprout. In older males, these ducts apparently tend to unite into one common opening, and their number appears to vary slightly. For example, in a foetus with a head length of 3-1 cm. we find the glands leading into the sinus by one dorsal duct, which branches almost directly into two, and by one smaller ventral duct, on each side. Again, in a slightly older specimen (H .L. 3-5 c1n.), there are four narrow ducts, one of which is very small; the largest appears to be independent, but the other three join together almost immediately into a common tube. In the oldest male foetus (H .L. 4-1 cm.), three ducts are again found, lying close together, two of which (text-fig. 10, d ]. cl 2) unite almost at once on leaving the sinus. At this stage, as T0xt_fig_ 10_ H_L_.1cm_ Transverse may be seen from the text-figure, the uro- section of the urogenital sinus at genital sinus at the level of Cowper’s glands, t’h°’1°Ve1°f'5h" °Pe“ingS of C°WP°1"S
. g ‘ . glands (d 1, d 2, d 3). (S1. 9-1-5.) has a three-cornered shape In cross section, the dorso-lateral walls, into which the glands open, being convex towards the cavity of the sinus. The usual number of ducts appears to be three, and ‘these most probably correspond to the three pairs of Cowper’s glands generally present in Marsupials (van den Broek, ’10 a). In most cases, van den Broek (’10 a) finds that one pair is very small, whilst the other two lie so close together that they are diﬂicult to separate from each other. Possibly in Trichosurus, the two anterior ducts originate by secondary division from the primitively single outgrowth, which first appears in pouch young with a head length of 6-5 mm.
In an adult male, dissected by Mr K. S. Tan, there were two pairs of Cowper’s glands, one pair being considerably larger than the other.
In a female with a head length of 38 mm., the glands are not nearly so large, being shorter and less branched than in the male of 26mm. H.L. They spring from an apparently solid duct on each side, which divides i11t0 several small hollow lobes. These stretch up on each side of the urogenital sinus, b11t do not extend above the latter, not reaching nearly so far dorsally as in the male. In the oldest female (H .L. 46 cm.), they ha.ve not increased , in size and a.re perhaps even less well developed.
From our account of the origin of the eloacal septum from the coalesced walls of the entodermal cloaca, it is evident that Cowper’s glands, arising as they do from the ventro-lateral walls of the sinus just where the free margin of the septum runs into the wall of the latter, must be of entodermal origin. This is easily realised by a glance at text-figs. 14 and 15 (c.g.) in Part I of this paper. From the situation of the glands in older pouch young, van den Broek (’07 and ’10 a and b) supposes that they develop from the oral wall of the entodermal cloaca, and regards them as marking the limit between ectoderm and entoderm. Their apparent position between ectoderm and entoderm is due to the fact that, in older stages, the epithelium of the urogenital sinus, from the opening of the latter as far forwards as Cowper’s glands, secondarily takes on an epidermal appearance, as pointed out in Part I of this paper (section on Cloacal). In any case, however, the question of the origin of Cowper’s glands is closely bound 11p with the mode of development of the eloacal septum, _whieh has received various interpretations by different authors (Part I, p. 69).
Urethral Glands. “Urethral glands are developed in the male, but they were 11ot observed in the female. The primordia of the glands were first seen in a male pouch young with a head length of 20 mm. (G.L. 4 em.). At this stage, they appear as short, solid, bud-like outgrowths, which arise from all round the wall of the urogenital sinus, from the entrance of the Wolffian ducts to about half way along the length of the sinus, gradually becoming less marked towards their posterior limit. In a male with a head length of 26 mm. they are better developed, being largest near the entrance of the Wolffian duct, where they branch out into many short solid lobes. They decrease in size posteriorly, and no primordia are apparent beyond the anterior two-thirds of the sinus. These observations on the urethral glands agree with those of van den Broek (’07, ’10 a), who also finds the primordia appearing late in development, i.e. Dasyurus, D.C.L. 53 mm.
In the adult they form a conspicuous glandular swelling round the urethra, the so-called “prostate” gland. The swelling is thickest near the neck of the bladder, and gradually tapers towards the posterior end, which lies some distance in front of the openings of Cowper’s glands.
Cloacal Glands. Cloacal or so-called rectal glands are well developed in both sexes. There is one large dorsal pair of glands and a variable number of smaller subsidiary pairs.
They first appear in the newly born foetus (G.L. 14-5 to 15 1nm.), in the form of a slight outbulging -of the lateral wall of the ectodermal cloaca on each side, close to the entrance of the rectum into the latter. They therefore develop at a slightly earlier stage than do Cowper’s glands. In pouch young with head lengths of 6-5 to 7-5 mm. (G.L. 15 to 17-5 mm.) they are more conspicuous, now forming short solid outgrowths (text-fig. 11, l.r.g., 7*.7".g.). During the next stages, the primordium increases.in length and develops into a short hollow duct, the distal solid end of which is enlarged, and is surrounded by a condensed mass of mcsenchynie. The duct grows gradually longer, and, in pouch young with head lengths of 1]. to 12 mm., extends up dorso-laterally, swelling out at its distal end into a large, almost solid, bulbous enlargement, situated dorso-laterally to the posterior end of the rect111n. The distal enlargement, which represents the primordium of the gland proper, consists at first of a small ce11tral area of lighter cells with faintly staining nuclei, surrounded by a wider darker‘ layer of epithelial cells. Very soo11 (H.L. 12-5 mm.) a central
Cavity appears in which lie Text-fig 11 GL 15mm HL 675 mm Transverse clumps of degenerating, pale section, showing the origin of the cloaeal glands (r.r.g.,
staining cells, Whilst the outer l.r.g.) from the lateral wall of the ectodermal cloaca (ect.cl.). u.g.s. urogenital sinus. (S1. 6-5-11.)
investing layer of epithelium becomes much thinner. The duct, immediately after leaving the gland, branches into several short hollow tubes, which, at this stage, extend round the ventral half of the gland. Meanwhile, from the lateral walls of the cloaca on each side, three more solid outgrowths have arisen close to the primary duct, two lying ventrally, a11d one dorsally to the latter. These are eventually destined to develop into smaller subsidiary glands, which are situated ventrally and postcriorly to the large pair. Their first development is very similar to that of the latter, b11t their central cavity always remains small, their distal ends early dividing up into many small lobes. They are already well marked iii a foetus witli a head length of 26 mm., where we find three subsidiary glands on the left side an(l two on the right; in the most ventral one of each side a central cavity is beginning to form, whilst the others are as yet quite solid. In a much older male (H .L. 4'1 cm.) there are still only two pairs of Very small subsidiary glands. They are deeply lobed, possess a well marked cavity, a11d are surrounded by a ring of narrow tubules. Their ducts communicate with the cloaca, one just dorsally, the other shortly ventrally to the duct of the main cloacal gland.
The oldest foetus, in which the cloaeal glands were examined, was a female with a head length of 4-6 cm. Here, we find a large dorsal anterior pair of glands (text-fig. 12, l.7'.g., 1'.7'.g.), lying dorso-laterally to the rectum on each side. Each has an average diameter of about 2-5 mm. The verylarge central cavity is encircled by an inner wall of a few layers of pale staining cells, around which is a very thin outer investment of small darkly staining cells. The edge of the gland is indented to form small lobes, and around it is a well marked circular layer of striate muscle (c.m.). The duct (d.) is a long narrow tube, which extends back on each side of the rectum to ope11 finally into the lateral side of the cloaca. From the duct, 11ear its e11trance into the gland, arise numerous hollow tubules (h.t.), which run all round the gland, but are especially abundant on the ventral side. Ventral and posterior to the large cloacal gland are four small glands, lying close together 011 each side. Of these, the two most anterior (s.r.g.l.) are the largest, but, nevertheless, they are
Text-fig. 12. H.L. 4-6 cm. E2 Microphotograph of a transverse section, showing the posterior portion of the large dorsal cloacal glands (r.r.g., l.«r.g.), and the most anterior pair /s.r.g. 1) of subsidiary glands. The anterior end of the second pair of subsidiary glands (snag. 2) is also seen, just beginning to appear dorsally to the first pair. c.m. circular layer of muscles; d. duct of large cloacal gland; k.t. narrow tubules surrounding large gland; h.t.s. narrow tubules round subsidiary glands; r. rectum; u.g.s. urogenital sinus. (S1. 8-3-7. x 10.)
very considerably smaller than the main gland. They are surrounded, like the latter, by hollow tubules (I2.t.s.) and by a circular layer of muscle fibres (c.m.); they are lobulated, and contain a well marked cavity. The most ventral of the subsidiary glands on the right side is very small and poorly developed. Their ducts, which are long and narrow, open two dorsally, and two ventrally to the duct of the large gland.
In the adult male, dissected by Mr K. S. Tan, there were two pairs of arge cloacal glands, the more dorsal anterior pair communicating with the cloaca by a long narrow duct on each side, whilst the ducts of the more ventral and slightly larger pair were very short.
According to van den Broek (’O4), there are usually in adult marsupials only two pairs of cloacal glands, of which the more ventral one is often much better developed than the more dorsal. In 1836, Martin (’36) mentions the occurrence of four rectal glands in Trichosurus vulpina, and later, in 1882, Cunningham (’82) figures and describes in Tr2'cho.s'urus maculata three pairs of glands, two near the a11us,_and one pair situated more orally at the side of the rectum, each gland having an independent duct. In the vulpine phalanger, according to this author, the same number of glands were present, but they were smaller in size. Very possibly the number of glands may vary even in the same species. “Whether the four subsidiary pairs in the foetus with a head length of 4-6 cm. always remain small, one or two pairs being quite rudimentary, or whether some of them develop later into definite cloacal glands, I have not determined.
Other accessory glands. In the later stages of pouch young, surrounding the dorsal a11d to a less extent the Ventral side of the cloaca, there are small tubules, which are exactly similar to those round the cloacal glands above described. They are united with the wall of the cloaca by many fine ducts, and evidently arise independently as outgrowths from the cloacal wall. It may be noted that, of these tubules, the more lateral ones appear to be in connection with those around the most dorsal of the subsidiary cloacal glands on each side. These tubular structures apparently correspond to the branching tubular glands (“verzweigte tubulose Drijsen”) described by van den Broek (’07, p. 380) in the neighbourhood of the cloaca, their exact significance being doubtful.
List of References
Development of Rete, Degeneration of Mesonephros.
(’04) ALLEN, B. M. “The Embryonic Development of the Ovary and Testis of the Mammals.” Amer. Jou,rn.. Anat. VOl. III. 1904.
(’05) ———- “The Embryonic Development of the Rete-cords and Sex-cords of Chrysemys.” Amer. J ourn. A nat. vol. V. 1905-6.
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_’77) BRAUN, ‘VI. “Das Urogenitalsystem der cinheimischen Reptilien.” Arb. zool.-zoot. Inst. Wilrzburg. Bd XL. 1877-8.
’05) VAN DEN BROEK, A. J. P. “Untersuchungen iibcr die weiblichen Geschlechtsorgane der Beuteltiere.” .Petrus Camper. D1. III. 1905.
’06) “On the relation of the genital ducts to the genital gland in marsupials.” Kon. A/cad. o. W etenschappen. Amsterdam, 1906. (’07) “ Beitr-age zur Kenntniss der Entwiekelung des Urogenitalappa-rates bei Beuteltieren.’ ’ Petrus Camper. D1. IV. 1907. (’lO a) “Untersuehungen iiber den Bau der mannlichen Geschlechtsorgane der Beuteltierc.” Jllorph. Jakrb. Bd XLI. 1910. (10 b) “Entwicklung und Bau des Urogenital-Apparates der Beutler und dessen Ver hitltnis zu diesen Organen andrer Sanger und niederer Wirbeltiere.” Morph. Jahrb. Bd XLI. 1910. 128 V98) (’97)
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(’04) C32) (’36)
Elizabeth A. Fraser
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Proc. Zool. Soc. 1836.
gl. g1omerulus;_ r.c. rete core; r.r. rete ridge; v.c. Vena caVa; W.D. Wolffian duct; L. lateral; M. medial.
“Die indifierente Anlage der Keimdriisen beim Hiihnchen und ihre Difieren- , J oumal of Anatomy, Vol. LI I I , Parts II and II I fiG. 1 fiG. 2
‘ rounding mesenchyme. (S1. 8-9-11 and 12.
Description of Plates
1. Trichosurus vulpecula, Stage XII, G'.L. 11~5 mm. (VII ’O1). Transverse section of the genital ridge, showing an indentation (ind.) in the ridge. (S1. 16-3-3. x 200.)
2. Trichosurus vulpecula, Stage XV a, G'.L. 14 mm. (XI ’O1). Transverse section of the genital ridge, towards the anterior region, showing a well marked ‘groove (gr.) extending in from the coelomic epithelium. (S1. 20-5-5. x 200.)
3. Trickosurus mtlpecula, G.L. 18 mm., H.L. 6°75 mm. Transverse section of the genital ridge, showing a hollow invagination (in.)‘ from the coelomic epithelium into the rete strands.‘ (S1. 23-5-10. x400.)
4. Trichosurus vulpecula, H.L. 9-10 mm. Transverse section of the anterior end of the rete ridge (r.r.). The anterior end of the rete (r.c.) is seen to lie close against the walls of
the Malpighian corpuscles (gI.). (S1. 2-3-4. x 200.)
. 5. Trichosurus vulpecula, H .L. 11 mm. 9 Transverse section through the mesovarium,
showing the sprouts (spr.) from the rete cords (r.c.), which extend out to unite with the walls of the Malpighian corpuscles (gl.) of the mesonephros. The latter have not as yet undergone degeneration. spr.gl. a short outgrowth which has probably arisen from the wall of the Malpighian corpuscle itself to meet the rete sprout. (S1. 1 1-3-8 and 7. x 200 and reduced by i.) 6. Didelphys aurita, H .L. 10-5 mm. 3‘ Transverse section (composite) through the anterior end of the lmesonephros. The rete (r.c.), which is still a solid cord at its cranial end, is seen to be connected with the M. corpuscles of tubules 2 to 5 of the mesonephros. In the second
tubule the position of the former glomerulus (gl. 2) is marked by a swelling of the tube, whilst .
the binder glomeruli (gl. 3, 4, and 5) are in progressive stages of degeneration. The most anterior tubule (t. 1) is shown united with the Wolffian duct (W.D.); its glomerulus has completely disappeared. The tubule is connected with the rete cord just in front of these sections. ilI.D. Miillerian duct. (S1. 9-3--1 to 3. x 130 and reduced by 1.)
. 7. Pemmeles nasula. 67.13‘. 68 mm. (l3—VII-O5). Three consecutive transverse sections
through the urogenital fold, right side, immediately in front of the 13th spinal ganglion, showing a developing mesonephric tubule. The wall of that portion of the tubule destined to become the Malpighian corpuscle (p.M.c.) is connected with the coelomic epithelium by a rudimentary nephrostome (a and b, nph.), which is united by a band of cells (c, x.) with the binder end of the suprarenal primordium (sr.), a. anterior, c. posterior. (S1. 8-8-13 to 15. x 200.)
. 8., Idem. Transverse section through the urogenital fold and a mesonephric tubule of the left side, on a level with the 14th" spinal ganglion. That portion of the tubule destined to become the Malpighian corpuscle ( p. M .c.) is connected with the coelomic epithelium by a thick mass of cells, the rudimentary nephrostome (nph.), incompletely separated from the surx 200.)
Cite this page: Hill, M.A. (2020, January 28) Embryology Paper - The development of the urogenital system in Marsupialia, with special reference to Trichosurus vulpecula 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_development_of_the_urogenital_system_in_Marsupialia,_with_special_reference_to_Trichosurus_vulpecula_2
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