362 OUTLINES OF CHORDATE DEVELOPMENT 



During the fifth day strands of cells appear, extending between 

 the substance of the gonad and the mesonephric tubules of the 

 region. These are the rudiments of the sexual cords (Fig. 142) . 

 While not definitely demonstrated as yet, it seems probable 

 that the sexual cords are outgrowths of the mesonephric tubules 

 (Malpighian bodies) which extend into the gonad. About the 

 end of the first week of development the sexes are distinguishable 



FIG. 142. Section through the gonad of a chick, the middle of the fifth day, 

 showing the sex cords reaching the germinal epithelium. After Semon. g, 

 Germinal epithelium; m, epithelium of the mesentery (peritoneum); o, primitive 

 ova; s, sex cords; t, connective- tissue stroma. 



through the enlargement of the sexual cords in the male, and the 

 greater thickness of the germinal epithelium in the female. 



Testis. During the second week the primordial germ cells 

 appear all through the stroma, and even migrate into the sexual 

 cords, which continue to increase in size and number until they 

 form altogether a considerable bulk. While the greater part of 

 the mesonephros degenerates, a vestige remains as the para- 

 didymis, that part with which the sexual cords connect. The 

 tubules of this region become the vasa efferentia (epididymis) 

 through the formation, about the end of the third week, 



