394 THE DEVELOPMENT OF THE CHICK 



The sexual cords appear within the gonad on the fifth day; 

 they are soHd cords of epithehal cells that fill up the interior 

 of the gonad and cause it to protrude from the surface of the 

 Wolffian body (Fig. 226); the cords extend from the germinal 

 epithelium, with which they may be in contact, towards the hilum 

 of the gonad (represented at this time by the broad surface 

 opposed to the Wolffian body), and into the Wolffian body where 

 they enter into close connection with the renal corpuscles. In 

 the Wolffian body and intermediate zone they are very irregular 

 in their course and connected by numerous anastomoses, corre- 

 sponding to the rete region of the future testis. Strands of these 

 cells pass dorsally, and, according to some authors, form the 

 cortical cords of the suprarenal capsules (Fig. 226). 



The following views of the origin of the sexual cords in birds 

 have been held: (1) That they arise as outgrowths of the capsules 

 of renal corpuscles (Hoffmann, Semon) and the neck of the 

 Wolffian tubules also (Semon); (2) that they are ingrowths of 

 the germinal epithelium (Janosik); (3) that they differentiate 

 from the stroma (Prenant). The subject is a somewhat difficult 

 and complicated one, but the view that the sexual cords arise 

 as outgrowths of the capsules of renal corpuscles appears to be 

 the best substantiated, and brings the birds into line, in this 

 respect, with the reptiles and amphibia. Hoffmann's observa- 

 tion that the sexual cords lie at first on the lateral side of the 

 blood-vessels intervening between the germinal epithelium and 

 the Wolffian body, and that the cells of the sexual cords are 

 directly continuous with those of the capsules, should be con- 

 clusive. If the cords arose from the germinal epithelium and 

 grew secondarily through the stroma into the Wolffian body, 

 there should be a stage when they occur exclusively median to 

 the blood-vessels intervening between the germinal epithelium 

 and the Wolffian body; but such does not appear to be the case. 

 The relation of the sexual cords to renal corpuscles, germinal 

 epithelium, and suprarenal capsules in Limosa aegocephala is well 

 shown in Fig. 226. 



Sexual Differentiation. The period of morphological indiffer- 

 ence of the gonad is relatively long and the actual sexual differ- 

 entiation appears slowly. It manifests itself (1) in differences in 

 the behavior of the germinal epithelium; (2) of the sexual cords; 

 (3) larger size of the left ovary and ultimate disappearance of the 



