394 THE DEVELOPMENT OF THE CHICK 



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

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

 epithelium 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, corresponding 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 rete 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, Firket). The subject is a somewhat 

 difficult and complicated one, but the view that the rete cords 

 arise as outgrowths of the capsules of renal corpuscles brings the 

 birds into line, in this respect, with the reptiles and amphibia. 

 Hoffmann's observation that the rete 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 cords 

 are directly continuous with those of the capsules, should be 

 conclusive. 



The sexual cords arise as proliferations of the germinal epi- 

 thelium which appear as buds projecting into the stroma (Fig. 

 227). They are definitely limited in time of origin between the 

 middle of the fifth and sixth days of incubation (Swift). They 

 carry with them numerous primordial germ-cells from the germinal 

 epithelium. About the end of the sixth day all free themselves 

 from the germinal epithelium, and a layer of stroma begins to 

 separate them sharply from the latter. They are destined to 

 form the seminiferous tubules in the male, and the so-called 

 medullary cords in the female. 



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; 



