480 COMPARATIVE ANATOMY 



genital canal, the septum between them disappearing at the 

 same time. A iagina.1 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 in some cases eventually open by the formation of a so-called 

 third vagina, as, e.g. , in certain species of Macropus and Halmaturus. 

 The anus and urinogenital apertures are surrounded by a common 

 sphincter. 



In nearly all the placental Mammals (Monodelphia) the 

 posterior portions of the Miillerian ducts become fused to form an 

 unpaired vagina, and a definite cloaca exists only in the embryo ; 

 but even in the adult, the anus and urinogenital aperture 

 may in certain cases (e.g. amongst Rodents) be enclosed by a 

 common fold of the integument as in Marsupials, and a median 

 septum is sometimes present in the vagina distally, indicating its 

 primarily 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 (Fig. 361, A to D), viz. 

 uterus duplex, with two ora uterorum (most Rodents), uterus 

 bipartitus and uterus licornis, double in part only and with one os 

 uteri (Carnivores, most Ungulates), and uterus simplex (Primates). 

 In the last-mentioned form the primitively paired condition of the 

 Miillerian ducts is seen only in the Fallopian tubes, which vary 

 much in form. The ureters, unlike those of Marsupials, always 

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

 single. 



The urinogenital canal may, as in Marsupials, be of consider- 

 able length (e.g. amongst Rodents), and a fold of the mucous 

 membrane (hymen) 1 is often present where the vagina opens into 

 it. On the ventral wall of the urinogenital canal, the clitoris 

 (p. 487) is situated. In both male and female the space 

 between the urinogenital aperture and the anus is known as the 

 perinceum. 



The ovaries of monodelphous Mammals 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. 

 Various differences are seen as regards the relations of the peri- 

 toneum to the ovary, and from a simple investment on the ventral 

 side, the organ hardly sinking into the peritoneum at all (e.g. 

 Rabbit, Cat), all stages occur up to its complete investment so 

 as to form an ovarial sac. 



Remains of the mesonephros, known as the parovarium, are 

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

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

 which are connected together by a collecting duct. In cases 



1 A similar fold, closing the apertures of the oviducts in the immature 

 condition, is present in Elasmobranchs. 



