322 COMPARATIVE ANATOMY. 



the portion of the oviduct lying more posteriorly, or vagina (Vg\ 

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

 and, passing backwards, opens close to its fellow into the elongated 

 urinogenital sinus (Sug). 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 (B). 



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. 252, 

 B and C) the anterior ends of the knee-shaped bends of the vagina 

 (comp. Fig. 252, A, f) come to lie closer and closer together, and 

 begin to extend backwards towards the urinogenital sinus, the 

 septum between them disappearing at the same time. A vaginal 

 caecum is thus formed (Fig. 252, B, C, VgB\ and this may become 

 more elongated, and finally extend backwards so as to meet the 

 upper (anterior) wall of the urinogenital sinus, into which it may 

 open by the formation of a so-called third vagina. This is 

 known to occur in seven species of Halmaturus, two of Petrogale 

 and Osphranter respectively, and in Onychogalea fraenata. 



In all other Mammals the posterior portions of the Miillerian 

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

 cloaca exists only in the embryo (comp. p. 236). A fusion may 

 also take place more anteriorly, and, according to its extent, the 

 most various forms of uteri result (uterus duplex, bicornis, 

 bipartitus, and simplex), as is shown in Fig. 253, A to D. The 

 Primates possess a simple uterus 1 (Fig. 253, B), and in this case 

 the primitively paired condition of the Miillerian ducts is seen only 

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

 abdominal apertures are usually provided with fringe-like append- 

 ages (fimbriae). The ureters, unlike those of Marsupials, always 

 pass to the outer sides of the genital passage, the vagina being 

 single. 



The ovaries are usually small, and rounded or oval in shape, 

 their surface being either smooth, irregular, or furrowed. The 

 point at which the nerves and vessels enter is not covered by 

 peritoneum, and is called the hilum. 



The reader is referred to p. 300 and Fig. 254 for further details 

 as to the more minute histological structure of the ovary and the 

 formation of the ova. 



Remains of the mesonephros, known as the parovarium, are 

 present in the neighbourhood of the ovary, oviduct, and uterus. 

 These usually consist of small caecal tubes, forming a network, 

 which are connected together by a collecting duct. In cases 

 where the Wolrfian duct persists in the female, it passes from 

 the parovarium to the urinogenital sinus, and is spoken of as 

 Gartner's duct (Fig. 236, A, Gg\ as already mentioned oh p. 301. 



1 The abnormalities which sometimes occur in the human uterus and vagina can 

 be often explained as atavisms. 



