262 STRUCTURE OF THE VERTEBRATES 



connection with the testis is the epididymis, and the Wolffian 

 ducts are the vasa deferentia {ims deferens is the singular). 



All amniotes have internal fertilization, and a number of in- 

 tromittent structures have developed. In the turtles, crocodilia, 

 a few birds, and all mammals the copulatory organ is the penis 

 which is homologous throughout these groups. The testes remain 

 in the primitive position in the reptiles, birds, monotremes and 

 a few placentals; the descent of the testes is described in the 

 following section. 



C. Reproductive Structures of the Mammals 



The above description is a generalized discussion of the 

 amniote system and applies particularly to the reptiles, birds 

 and monotremes. The last, although mammals, are typically 

 reptilian in their reproductive organs, both sexes retaining the 

 cloaca, and the female having oviducts with separate openings 

 on either side of the urinary papilla. In the marsupials and 

 placentals the cloaca disappears and other genital structures 

 make their appearance. 



Female. With the division of the cloaca and the separation of 

 the digestive and urinogenital tracts, the female mammals de- 

 velop labia, or lips, guarding the external orifice of the urino- 

 genital openings. These, particularly in the higher mammals, 

 are double, consisting of outer and inner labia. Dorsally, within 

 the labia, is the opening of the vagina which develops largely as 

 a fusion of the posterior ends of the Miillerian ducts. Ventral to 

 this is the single urethral opening; and at the ventral union 

 of the inner labia is a small clitoris which is homologous with 

 the penis of the male. 



The greatest modification occurs in the evolution of the 

 uterus, which retains the minute ova during development of the 

 embryos. The union of the oviducts is very incomplete in the 

 marsupials, with the result that the vagina is paired or separated 

 by a tissue septum, which causes each half to persist as an 

 almost independent unit. At the anterior end of each hemi- 

 vagina there is a marked constriction with a cervical opening 

 into the uterus. Sperms deposited in the vaginae during coitus 



