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 
Fic. 335.—Model of cloacal region of human embryo, 6.5 mm. long, after Keibel 
uw, allantois; c, cloaca; cm, cloacal membrane; k, outgrowth to form kidney and uréter; 
r, rectum; “, where bladder will develop; wd, 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 
