"URINOGENITAL ORGANS 373 



which in other Mammals become more or less fused with one 

 another proximally, remain distinct throughout, and open into the 

 urinogenital canal anteriorly to the ureters and bladder. 



In the higher Mammals the oviducts become distinctly differ- 

 entiated into three portions, — a Fallopian tiibe, a litems, aod a 

 vagina. The vagina opens to the exterior (Figs. 273, 297, 298), 

 while the Fallopian tube communicates with the abdominal cavity 

 by a ciliated funnel-shaped aperture. 



In Marsupials the fusion of the two oviducts is much less 

 marked than in the higher Mammals, and in order to trace 

 the gradual differentiation of these parts, their condition in Opos- 

 sums (Didelphidae) must now be described in greater detail. 



A dilated portion' of each oviduct (Fig. 297, a), giving rise to a 

 uterus, is plainly distinguishable from the rest, and its narrowed 

 posterior end comes into close contact with its fellow in the middle 

 line. At this point (f) each uterus communicates with the vagina 

 by a distinct os uteri. The vagina curves sharply outwards, and, 

 then backwards, opening close to its fellow into the elongated urino- 

 genital canal. The ureters, as in all other Marsupials in which the 

 vaginae have a similar arrangement, pass between the curved 

 portions of the vaginae to the bladder. 



From the condition of the female generative organs in Didel- 

 phys, that seen in other Marsupials can be easily explained. In 

 Phalangista vulpina and Phascolomys wombat (Fig. 297, B and c) 

 the anterior ends of the knee-shaped bends of the vaginse lie 

 closer together, and begin to extend backwards towards the urino- 

 genital canal, the septum between them disappearing at the 

 same time. A vaginal ccecum is thus formed : this may 

 become more elongated, and finally extend backwards so as to 

 meet the anterior wall of the urinogenital canal, into which it 

 may open by the formation of a so-called third vagina. The 

 anus and urinogenital apertures are surrounded by a common 

 sphincter. 



In the placental Mammals the posterior portions of the Miiller- 

 ian ducts become fused together to form an unpaired vagina, and a 

 definite cloaca exists only in the embryo, although even in the 

 adult in certain cases {e.g., amongst Rodents), the anus and urino- 

 genital aperture may be enclosed by a common fold of the integu- 

 ment as in Marsupials, and a median septum may be present in 

 the vagina distally, indicating its primary double nature. The 

 uterine portions of the oviducts may also fuse with one another to 

 a greater or less extent, and thus the most various forms of uteri 

 result (loterus duplex, hicornis, hipartitiis, and siviplex), as shown in 

 Fig. 298, A to D. The Primates possess a single uterus, and in this 

 case the primitively paired condition of the MuUerian ducts is seen 

 only in the Fallopian tubes. The latter vary much in form, and 

 their abdominal apertures are usually provided with fringe-like 

 appendages (fimbriae). The ureters, unlike those of Marsupials, 



