396 



THE DEVELOPMENT OF THE CHICK 



tion. The sexual cords now begin to branch and anastomose, 

 and soon form a reticulum with mesenchyme in the meshes. About 

 the thirteenth day the primordial germ-cells, which have been 

 inactive, begin to divide, and a rapid increase in numbers ensues. 





Fig. 228. — Portion of a transverse section through the right testis of a 

 20 day chick embrj'o. The section shows a seminiferous cord in which a 

 lumen is beginning to develop. Note the position and polarization of the 

 spermatogonia (after Swift). 

 Int. c, interstitial cells. L., beginning of lumen. M. C, Mitochondrial 

 granules within a spermatogonium, p. c, supporting cells, derivatives of 

 peritoneal cells of the sexual cords, s. c, seminiferous cord, sp., spermato- 

 gonia, str., stroma. 



The sexual cords are solid up to about the twentieth day of incu- 

 bation; a lumen then begins to appear and they become trans- 

 formed into tubules (Fig. 228). The primordial germ-cells form 

 the spermatogonia, and the peritoneal cells form the supporting 

 cells of the seminiferous tubules (Swift). 



After the sixth day the germinal epithelium of the testis 

 rapidly retrogresses and becomes reduced to a thin peritoneal 



