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The Development of the Urogenital System in the Marsupialia, with Special Reference to Trichosurus Vulpecula Part I

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The Development of the Urogenital System in the Marsupialia, with Special Reference to Trichosurus Vulpecula Part I THE DEVELOPMENT OF THE UROGENITAL SYSTEM IN THE MARSUPIALIA, WITH SPECIAL REFERENCE TO TRICHOSURUS VULPECULA PART I By GWYNNETH BUCHANAN, D.Sc. (MELB.) AND ELIZABETH A. FRASER, D.Sc. (LOND.) From the Embryological Laboratory, Department of Zoology, University of London, University College EDITED WITH AN INTRODUCTORY NOTE BY PROF. J. P. HILL, D.Sc., F.R.S. CONTENTS PAGE Introductory Note .35 Pronephros and Mesonephros 36 Discussion. 42 Metanephros and Ureter . .46 Discussion ..... 50 Cloaca . ... 52 Discussion .....68 Bladder . 70 Wolffian Duct. 71 Mullerian Duet . 74 Summary and Discussion . .82 Suprarenal Body . 87 Conclusions. 88 List of References . .91 Description of Plates . .93 INTRODUCTORY NOTE THE present memoir is the result of the independent investigations of the two authors whose names appear in the title. The research was originally undertaken at my suggestion by Miss Buchanan during her tenure of a travelling scholarship of the University of Melbourne in 1914, and was in- tended to afford as complete an account of the morphogenesis of the marsupial urogenital system as the available material would permit. Miss Buchanan devoted her attention more particularly to the earlier stages of development, concerning which practically nothing was known, and embodied her results in a thesis which, along with other contributions, was accepted for the degree of D.Sc. by the University of Melbourrne. This thesis has not been published and is incorporated in the present memoir. Owing to the ;3-2 36 G. Buchanan and E. A. Fraser limited time at her disposal, Miss Buchanan was unable to deal inl any detail with the later stages. As these seemed worthy of detailed study, notwith- standing the extensive investigations of van den Broek, I invited Miss Fraser to continue the research. In order to do so, Dr Fraser found it necessary to make an independent study of the earlier stages, and with the observations, models, and drawings of Miss Buchanan before her, she has been able. to supplement the latter's account of these stages, whilst, with much additional material at her disposal, she has provided an extended account of the later development. The work has been carried out under my direction, and wherever the two observers have arrived at a different interpretation, I have exercised my editorial functions, and the view expressed is that which seemed to me most in accordance with the evidence. The authors desire to express their thanks to Miss E. A. Steele for the beautiful drawings of the models represented in figs. 23, 24 and 25. PRONEPHROS AND MESONEPHROS Our three earliest embryos of Trichosurus vulpecula (Stages I, a, b, and c) measure 5 mm. in greatest length (G.L.). The anterior end of the Wolffian duct in (a) first appears on the right side on a level with the third spinal ganglion, but, after running for *05 mm., it is absent for *08 mm., again reappearing in the sections between the third and fourth spinal ganglia at the same time as the duct of the left side. In (c) we find the anterior end of the duet opposite the fourth spinal ganglion on both sides, but on the right, after only three sections (.08mm.), it is again discontinuous for -07 mm. From this region down to between the fifth and sixth spinal ganglia are seven or eight degenerate excretory tubules on each side. In (a), the first four of these are the most rudimentary; they are smaller than the remainder and more or less solid, whilst the hinder ones are partly tubular. One or two are isolated-for example, the first on the right, which lies medially to the anterior end of the first portion of the Wolffian duct-but the majority extend out from the dorso-medial side of the duct, and are not connected with the coelomic epithelium (plate I, fig. 1, p.t.). On the left, however, two, and on the right, one, are united with the latter by a solid cellular cord. In (c) the conditions are similar, although the excretory remnants are still more degenerate, seven being exceedingly small, and all solid. As in (a), there are no definite ciliated funnels, but on the left side two of the cords are connected with the coelomic epithelium by a solid strand of cells. There is no metamerism. Plate I, fig. 2 shows a longitudinal section through the anterior end of the left Wolffian duct in (b). In all probability these anterior degenerate tubules are vestiges of the pronephros. From the level of the interspace between the fifth and sixth spinal ganglia backwards, there follow about fifty more tubules, the first of which, especially Development of the Urogenital System in the Marsupialia 37 in (c), is distinguished from the more anterior remnants by its larger size and tubular structure; behind these, down to the level of the twenty-fourth spinal ganglion, are still others in process of formation. All the tubules are united with the dorso-medial or medial side of the Wolffian duct, and do not appear to be connected with the coelomic epithelium. The majority communicate with the cavity of the duct, but some, especially at the anterior end, are only joined with the latter by a solid cord. The largest tubules are simple and almost straight, and have a narrower portion, the future collecting duct, uniting them with the Wolffian duct; glomeruli are not yet developed. In (b), at the anterior end of the left side, two of the collecting ducts have each two tubules running into them. In this same embryo, there appear to be four or five tubules to a segment at the posterior end, but further anteriorly there are apparently only three, or rarely two. As is known, the number of mesonephric tubules to a segment presents great variation in different mammals. In an embryo of the bat, Rhinolophus hipposideros, with twenty- one somites, Van der Stricht (34) describes two to a somite towards the posterior end of the body, and three further anteriorly. In the human embryo, the mesonephric tubules are also dysmetameric from the first, and four may occur to one segment (Felix (14)). Neither is there any strict metamerism in the early mesonephros of the Spermophilus embryo (Janosik (21)), nor in the rabbit, according to Schreiner(31), where three or four tubule primordia are usually found to a segment. Stage II (a, P '98 and b. y '99) measures only 4*5 mm. G.L., but is dis- tinctly older. In embryo (b). the first excretory tubule is solid, and arises from the dorso-medial side ofan isolated portion oftheWolffian duct; it curves round medially and ventrally to meet the coelomic epithelium, at which place there is a definite nephrostome (plate I, fig. 4, c.f.). Altogether in (b), there are about seven rudimentary tubules at the cranial end of the excretory organ, the hinder ones being on the whole less well developed than the more anterior. They all arise from the dorso-medial side of the duct, and most of them are connected with the coelomic epithelium; the majority are small, and all are solid, with the exception of the fourth on the right side, which is especially well developed, and consists of a hollow proximal portion, arising from the dorsal side of the Wolffian duct, followed by a narrow solid piece running medially, and a wider solid distal section extending up dorsally between the aorta and the cardinal vein. In (a) much the same conditions prevail, but in this embryo, the degenerate remnants are not so easily recognised, and are less sharply marked off from the more definite posterior tubules. Almost all the cords are tubular, and several, most noticeably on the left side, are miade up of a medial tubular portion, united, on the one hand, by a narrow solid cord with the Wolffian duct, and, on the other, by a narrow or wider solid connection with the coelomic epithelium, where, in a few cases, a rudinieiitary nephrostome may be seen (plate I, fig. 3, c.f.). From the sixth spinal ganglion backwards occur a further series of tubules, 88 G. Buchanan and E. A. Fraser which are much larger than the preceding ones. In (a) for example, on the left side, they number approximately fifty-seven. The majority are hollow, have roughly an S-shaped form, but four or five at the anterior end are more simple. At about the thirty-seventh tubule, the coiling becomes very slight, and the thirty-eighth to the forty-seventh tubules are joined to the Wolffian duct by a solid or only partially hollow cord of cells. The forty-eighth is isolated, being altogether separated from the duct. On the right side, the first two or three tubules, starting from the sixth spinal ganglion, exactly resemble those occurring anterior to this ganglion; they are small, and appar- ently united by solid epithelial cords with the coelomic epithelium. Posterior to these, are four longer but simple tubules, and behind again, we find about fifty-one more, of which the thirty-sixth to the forty-seventh have only a solid connection with the Wolffian duct, and the last eight or nine are quite isolated. Stage III (a '97). which measures 7 mm., shows a further degeneration at the anterior end of the excretory organ, the first tubule occurring on a level with the fifth spinal ganglion, close behind the cranial end of the Wolffian duct. The latter is again discontinuous on both sides. The first one or two rudimentary epithelial cords are solid, whilst the following three are tubular, and all, except the fourth on the right, which is very small, arise from the Wolffian duct, and have more or less degenerate nephrostomes.
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