Glass 6. Mammalia. 



491 



genital sinus, which contains also the aperture of the urinary 

 bladder, whilst those of the ureters lie on the dorsal wall of 

 the bladder.* In the placental Mammals there is no cloaca 

 in the adult ; the urinogenital sinus (vestibule) and the 

 rectum open independently, either close togethei- or separated 

 by a considerable interval. The posterior ends of the oviducts 

 are almost always united for a greater or less extent : either the 

 vagina simply ; or the posterior portions, or even the whole length 

 of the uteri also : in the first case there are thus two entirely 

 separate uteri {uterus duplex), as in Rabbits ; in the second, there 



Pig. 400. The Miillerian ducts and urinogemtal sinus of various Mammalia. A of 

 a Marsupial, B uterus duplex, C bicornuate, D simplex, b opening of the 

 bladder into the front end of the ui'inogenital sinus, o external opening of the sinus, 

 t fallopian tube, tr fiumel, u uterus, v vagina. — Orig. 



is a bicornuate uterus (m. hicornis), as in the Horse; in the last, a 

 simple uterus [u. simplex), as in Man. The ureters and the 

 bladder are arranged as in the Marsupials. 



At the junction of the vagina and the vestibule there is in many MammaKa a 

 thin membranous septum (hymen), perforated by a small pore, and broken 

 down at the first coitus. The vestibule varies very much in length ; in some 

 forms it is very long (e.(/., in Hares), in others, very short, hardly apparent (e.g., in 

 Man). Very frequently, a clitoris, a rudimentary organ homologous with the 

 penis of the male, is present in the female ; it is usually a papilla, rarely elongate, 

 and lies on the ventral side of the external apertui'e of the urinogenital sinus. 

 Further, there may be rudiments of the mesonephros (epoophoron, for instance, in 

 man) and of its duct (Gartner's duct in the Ruminants). 



Male Genitalia. In all Mammalian embryos the testest lie 

 close to the dorsal wall of the abdomen, just as in Reptiles and Birds, 



* In some Marsupials the two oviducts are separate throughout their length, in 

 others, the vaginae are united for a certain distance, but open separately into the ixr'ino- 

 genital sinus. 



t In many Mammalia, as in Aves (see p. 4i9), the testis diminishes in size after the 

 breeding season (Eoebuct, Hedgehog, etc). 



