URINOGENITAL ORGANS 375 



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

 it. On the ventral (anterior) wall of the urinogenital canal, the 

 clitoris (Pp. 380-384) is situated. In both male and female the 

 space between the urinogenital aperture and the anus is known as 

 the perinccum. 



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. 



Remains of the mesoiiephros, known as the parovarium, are present in 

 the neighbourhood of the ovary, oviduct, and uterus. These usually consist 

 of small crecal tubes, forming a network, which are connected together by 

 a collecting duct. In cases where the Wolffian duct persists in the female, 

 it passes from the parovarium to the urinogenital sinus and is spoken of as 

 Gartner's duct (Fig. 273, H). 



A fold of 'the skin of the abdomen forming a pouch or mar-, 

 supium is present in Echidna and to a greater or less extent 

 in Marsupials (p. 28) : this serves to protect the egg (Echidna) 

 and young, which in the latter are born in a very unripe con- 

 dition, thus rendering possible a longer connection between the 

 mother and embryo during lactation. The aperture of the marsu- 

 pial pouch opens anteriorly or posteriorly, according to the mode of 

 life of the animal, and is provided with a muscle capable of clos- 

 ing it. In Marsupial embryos the margins of the lips become 

 partially fused secondarily and temporarily to form a suctorial 

 mouth, by means of which the young, many of the organs of which 

 are still in a " larval " condition, become attached to the teats 

 (compare p. 288). 



In male Mammals, the testes arise in the same relative position 

 as the ovaries of the female. The ovary, however, never moves in 

 the course of development further backwards than the pelvis; but 

 the testis may pass out of the abdomen through an inguinal 

 canal into a purse-like outgrowth of the integument in the inguinal 

 region called the scrotal sac, which is lined by a continuation of the 

 peritoneum, the tunica vaginalis. The two scrotal sacs (which are 

 represented amongst female Primates by the so-called labia majora 

 of the vulva) may remain separate, or unite to form a scrotum : in 

 Marsupials this is situated in front of, and in placental Mammals 

 behind the penis (p. 382). If the inguinal canals remain widely 

 open, they may be withdrawn periodically into the abdomen (as e.y 

 in Rodentia and Insectivora, in which they only descend at sexual 

 maturity) ; this is effected by means of the cremaster muscle, a con- 

 tinuation of the fibres of the internal oblique and transversalis, or of 

 the latter only, and corresponding t-o the " compressor mammne " of 

 Marsupials. When the inguinal canals become reduced, the testes 

 remain throughout life in the scrotum. In many Mammals, how- 



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

 tion, is present in Elasmobranchs. 



