294 MAMMALIA. 



(when a vagina is present), we may distinguish : the uterus duplex^ 

 with more or less complete external separation and double os 

 uteri (Rodents, Marsupials) ; the uterus bipartitus, with single 

 os uteri, but almost complete internal partition (Rodents); the 

 uterus bicornis (fig. 678 >), in which the upper parts, or horns of the 

 uterus are separate ( Vnyulata, Carnivora, Cetacea, Insectivora) ; and 

 finally the uterus simplex (fig. 678 c) with single cavity and very 

 muscular walls (Primates). 



The vestibule, with its glands of Duvernoy (Bartholin), which 

 correspond to the Cowperian glands of the male, is separated from the 

 vagina by a constriction, and sometimes also by a fold of the mucous 

 membrane, called the hymen. The external generative organs consist 

 of the labia majora and labia minora, at the sides of the sexual 

 opening, and of the clitoris. The labia majora are two external 

 folds of skin, and are equivalent to the two halves of the scrotum ; 

 the labia minora are two smaller internal folds, and are not always 

 present. The clitoris possesses erectile tissue and a glans, and is the 

 equivalent of the penis. The clitoris may sometimes (as in Ateles) reach 

 a considerable size, and be perforated by the urethra (Rodents, Moles, 

 Lemurs). In such cases of perforated clitoris, there is, of course, no 

 common urogenital sinus. Morphologically, the female genitalia 

 represent an earlier stage of development of the male organs, which, 

 in the cases of the so-called hermaphrodite formation, may in con- 

 sequence of arrest of development preserve a more or less female 

 structure. As a rule the two sexes are easily distinguished by the 

 different form of the external generative organs. Frequently there 

 is a marked dimorphism in the whole external appearance; the 

 male being larger, having a different hairy covering, being pos- 

 sessed of a louder voice, and provided with stronger teeth or special 

 weapons (horns). On the other hand, the milk glands, which are 

 situate in the inguinal region, on the abdomen, and on the thorax, 

 and which almost always project into teats or nipples, are rudi- 

 mentary in the male sex. 



The breeding time (rut) is usually in the spring, rarely towards 

 the end of summer (Ruminants), or even in the winter (Sus, Carni- 

 vora}. An important phenomenon, which accompanies the rut in 

 the female, and is independent of copulation, is the passage of one 

 or more ova from the Graaffian follicles of the ovary into the 

 oviduct. The ova of the Mammalia were first discovered by 0. E. 

 von Baer. They are extraordinarily small (^ to T ^ line in diameter) 

 and are surrounded by a strongly refractile membrane (zona 



