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To Trichosurusvulpecula THE DEVELOPMENT OF THE UROGENITAL SYSTEM IN THE MARSUPIALIA, WITH SPECIAL REFERENCE TO TRICHOSURUS VULPECULA PART I I By ELIZABETH A. FRASER, D.Sc. (LOND.) From the Embryological Laboratory, Department of Zoology, University of London, University College CONTENTS PAGE Introduction . 97 Genital Ridge and Development of Rete . 97 Degeneration of Mesonephros and Formation of Urogenital Connection 101 Summary . 106 Uropenital Connection in Other Marsupials . 108 Summary . 116 Discussion . 117 Accessory Glands . 122 Cowper's Glands . 122 Urethral Glands . 124 Cloacal Glands . 121 List of References. 127 Description of Plates . 129 INTRODUCTION liTHE following paper is the continuation of the study of the development of the urogenital system in marsupials, originally undertaken at the suggestion of Prof. J. P. Hill, to whom I am much indebted for constant advice. As in Part I, Trichosirus forms the basis for this investigation. The development of the rete 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 1I1)DGE AND DEVELOPMENT OF IRETE 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 7-5 mm. (Staoc IV), however, a vecry low ridge can be seen, forming a slight projectiom iflto tile cocloll, along the mc(lial side of the urogenital fold. The ridge is filled with dcnscr cells, which at the cranial end, between the 7th Anlatomly l1lI1 7 98 Elizabeth A. Fraser and 8th spinal ganglia, pass slightly dorsally, and eventually run into the cells forming the primordiurn of the siiprarenal 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 siieceeding stages the genital primordium increases in length and becomes more prominent. By Stage VIII (G.L. 7-25 mm.) it appears as a A D P1C. sort. gi. *~~ort. -wo.w.D. Iext-fig. 1. A, IL, C, 1). Trichosurp ul;)(4)crulo. G.L. 725 omm. (XI '02). Trans-erse sections through the urogenital fold, showing the genital ridge (q.r.) at different levels. A low anterior end of ride; D) well marked posterior end; B andl C intermediate regions. aort. (1 sal aorta; yi. glornerulhs; 1. lung; p,.c. posterior car(Iinal vein; s. suprarenal body; st. stomach; W.D. Woiffian duct. definite, though still low ridge (text-fig. 1, A, g.r.). It has now apparently become disconnected from the suiprarcnal organ, and gradually increases iii thickness as it extends posteriorly (text-fig. 1, B, g.r.), 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 Alalpighian corpuscles of the mesonephros. Text-fig. 2 shows the genital ridge (g.r.) in an older embryo (G.L. 8-5 mm., Stage IX). The thicker posterior portion has a much indented margin, and The Development of the Urogenital System in the Marsupialia 99 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, g.r.), which in front of the mesonephros passes into the epithelium lining the shallow medial bay of the ostium ab- dominale (cf. Part I, fig. 19, B. r.). 0-a. V 00 Ao&',0 oO*oO oO° D . 0 Text-fig. 2. Trichosuru.3 vulpecula. G.L. 8-5 mm. ('97). Diagram of a longitudinal section of the mesonephros, showing the genital ridge (g.r.) lying along the ventro-medial side of the urogenital fold. gO. glomeruli; o.a. ostium abdominale. The posterior part of the ridge (text-fig. 3, C, g.r.) 'is becoming very con- spicuous, 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 mesogenitaJe 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 7-2 100 Elizabeth A. Fraser identical with the latter and cannot be distinguished from it, the one gradually passing into the other. aort"- -W.D. I.. aort.- ... Text-fig. 3. Tricho8urs8 vulpecula. G.L. 11-5 mm. (VII '01). 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 (8.); B further posteriorly; C thicker posterior portion of ridge. aort. dorsal aorta; gl. glomeruli; 8. 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 The Development of the Urogenital System in the Marsupialia 101 and 20th spinal ganglia, to just behind the 21st spinal ganglion, where it runs into the gonad. The latter is now connected with the Wolifian 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 (P1. 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., P1. I'II, fig. 1) by more or less distinct grooves extending in from the coelomic epithelium (G.L. 14 mm., PI. 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 dif- ferentiation 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 FORAIA'ION 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 meso- nephros 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, but 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 secretary 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.
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