THE UROGENITAL SYSTEM 609 



nephrostome as in the pronephros. The nephrostomal region now forms 

 another connection at its inner end with the sinus of the posterior cardi- 

 nal vein, in which sinus the mesonephric tubules lie surrounded by 

 venous blood. 



Many, as high as two hundred, outer tubules and nephrostomes may 

 be formed from the three units described, and possibly some may be 

 formed also by independent evaginations from the peritoneum, or even 

 by splitting of those previously formed. 



The urinary bladder is a median ventral outgrowth from the wall 

 of the cloaca nearly opposite the openings of the ureters. 



THE REPRODUCTIVE SYSTEM 



The mesonephric duct becomes divided somewhat obliquely into 

 two portions in front of the mesonephros, the more anterior portion now 

 being the Miillerian duct, while the posterior portion forms the Wolffian 

 duct. The Miillerian duct connects with the peritoneal epithelium an- 

 teriorly and empties into the cloaca posteriorly. In the male frog, this 

 duct persists as a mere longitudinal streak on the outer side of the kid- 

 ney, and extends some distance in front of it. In the female frog, this 

 Miillerian duct becomes the oviduct (Fig. 350). 



The Wolffian duct functions as the ureter in both sexes, but in the 

 male, the posterior end of it becomes dilated into a glandular enlarge- 

 ment called the seminal vesicle. 



Already at the six millimeter stage, as the mesentery is being formed 

 by the coming together of the somites from both sides just under the 

 dorsal aorta, a small group of entodermal cells is pinched off from the 

 yolk, which, after completely separating from the yolk, divide longi- 

 tudinally, each half moving laterally. These longitudinal halves are the 

 genital ridges (Fig. 349, B). They lie close to the mesenteric attach- 

 ment, and just beneath the cardinal veins. 



The genital ridges become quite conspicuous in a short time by 

 germ-cell proliferations as well as by the peritoneal cells which cover 

 them, and the mesenchymal cells from the body wall which migrate to 

 this region. 



The mesenchymal cells form the stroma of the ridge ; the peritoneal 

 cells form a thin superficial covering at first, while later they also form 

 the suspending folds (mesorchia of the testes, and mesovaria of the 

 ovaries) of the "fonads. The germ-cells now begin to proliferate and 

 form the nests of cells (Fig. 254), which are to develop into gonads and 

 gametes as already described early in the embryology of the chick. 



The anterior portion of the genital ridge becomes the fat body 

 shortly before metamorphosis, while the posterior portion connects with 

 the mesonephric duct. 



In this posterior region several outgrowths from the Malpighian 

 bodies known as sexual cords can be seen. These become tubular, and 



