COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 295 



7. A urinary bladder is generally present as an evagination of the ventral wall of the 

 cloaca. It receives the ducts of the kidneys in mammals only. It opens into the cloaca in all 

 forms except adult placental mammals. 



8. The ovaries and testes are paired bodies developed from the ventral surface of the 

 mesomere and projecting into the coelom. 



9. The ovaries are always located within the peritoneal or pleuroperitoneal cavity in all 

 vertebrates. The ducts of the ovaries are the Miillerian ducts or oviducts. They are supposed 

 to be derived from the pronephric duct by splitting, but arise in this way only in elasmobranchs. 



10. The oviducts never have any direct connection with the ovaries. They open into 

 the coelom near the ovaries by an ostium which is believed to represent one or more nephro- 

 stomes. 



11. The oviducts enter the cloaca separately except in the placental mammals. In these 

 latter the oviducts are more or less united and differentiated into regions. This union proceeds 

 from their posterior ends anteriorly, forming in the lowest placental mammals first a common 

 vagina, in higher forms a partially fused uterus with separate horns, and in the primates a 

 single uterus produced by the fusion of two originally separate uteri. The upper portions of 

 the oviducts form narrow uterine tubes which always remain separate. 



12. The testes are located internally except in the higher mammals, where they descend 

 temporarily or permanently into pouches of the body wall located externally in the inguinal 

 region. 



13. The ducts of the testes in all vertebrates (except cyclostomes and teleostomes) are the 

 Wolfiian or mesonephric ducts, also called the vasa deferentia. In the males of groups where 

 the mesonephros is functional the Wolffian ducts have both genital and excretory functions. 

 In groups where a metanephros is present the Wolfiian ducts have only genital functions. 



14. The Wolffian duct, vas deferens, is always directly connected to the testis by the 

 intervention of a portion of the mesonephros. This is differentiated into the vasa efferentia 

 and the epididymis in part. The remainder of the epididymis is part of the Wolffian duct. 

 In male amniotes a portion of the mesonephros thus always persists as part of the male system; 

 in female amniotes the mesonephros takes no part in the functional female apparatus. 



15. A cloaca which receives the intestine and urogenital ducts is present in most verte- 

 brates (cyclostomes, teleostomes and placental mammals excepted). In placental mammals 

 the cloaca splits into a dorsal portion which receives the intestine and opens to the exterior 

 by the anus, and a ventral portion the urogenital canal or sinus which receives the blad- 

 der and the urogenital ducts. The bladder in mammals develops a stalk, the urethra, which 

 leads to the exterior. The ureters shift so as to open directly into the bladder. In all male 

 placental mammals the vasa deferentia (Wolfiian ducts) join the urethra to form a urogenital 

 canal; in most female placental mammals the vagina similarly unites with the urethra form- 

 ing a urogenital canal ; but in primates the urethra and vagina open separately to the exterior, 

 a urogenital canal being absent. The male urogenital canal is generally provided in mammals 

 with various glands, whose secretion is believed to be of importance for the sperm; its terminal 

 portion traverses the penis or organ of copulation and forms the glans of the penis. The 

 penis is formed by the union of two spongy bodies, the cavernous bodies. It begins in reptiles, 

 where the cavernous bodies are separate. 



