UROGENITAL SYSTEM. 



335 



and lizards the gonad of one side is in advance of the other, while in forms with 

 large eggs there is a marked tendency for one ovary to degenerate (right in birds) 

 the other alone being functional. 



The oviducts, which are Miillerian ducts, are modified in accordance with the 

 peculiarites of the eggs. The upper portion is usually much coiled and glandular, 

 this part of the tube secreting the white, while parts farther toward the external 

 opening form the shell membrane and the shell. The walls are also somewhat 

 muscular, the muscles acting like constrictors to force the eggs along. The 



FIG. 335. Model of cloacal region of human embryo, 6.5 mm. long, after Keibel 

 a, allantois; c, cloaca; cm, cloacal membrane; k, outgrowth to form kidney and ureter; 

 r, rectum; u, where bladder will develop; uud, Wolffian duct. 



mesonephros and the Wolffian duct are largely degenerate in the female, being 

 represented by rudiments between the oviduct and the vertebral column, best 

 developed in turtles and snakes. 



The testes (figs. 313, 328) are short, round or oval in outline, and in birds one 

 is usually the larger, though both increase in size at the breeding season. The 

 Wolffian duct is solely reproductive (vas deferens), and its anterior, greatly coiled 

 end, together with the vasa efferentia form the epididymis. Traces of the Miillerian 

 duct persist in the male sauropsida. There are several accessory reproductive 

 glands in the reptiles but little is known of their function. 



MAMMALS. In considering the urogenital structures of the mam- 

 mals the following parts are to be kept in mind: They are composed of 



