SEX-CORDS AND SPERMATOGONIA IN CHICK 387 



cells of the germiiuil epithelium and stroma have none of the 

 vitellus and inconspicuous spheres. 



In fact the primordial germ-cells do not originate in the ger- 

 minal epithelium as was believed by many of the older investiga- 

 tors, but at a distance from the site of the gonad. They arise 

 in the germ wall entoderm (Swift, '14) and after an extended 

 migration through the blood vessels (Swift '14 and von Beren- 

 berg-Gossler, '14) they reach the splanchnopleure entering into 

 the formation of the mesentery, where they leave the vessels 

 and then migrate in an amoeboid manner into the forming 

 germinal epithelium. 



During all their migration period and up to this time they 

 divide very infrequently. That they do divide is proven by the 

 increase of numbers from stage to stage (Firket, '14, and Swift, 

 '14 and '15) and by the fact that they are frequently seen in 

 small groups, which are probably the result of several successive 

 divisions. 



As was indicated in the short historical review of this article, 

 it is still an open question as to the role played by the primor- 

 dial germ-cells in the formation of the definitive sex-cells in the. 

 male. 



DIFFERENTIATION OF SEX AND ORIGIN OF THE SEXUAL CORDS 



The period of embryonic development in the chick from 5| 

 to 6^ days is of extreme interest and importance, for during that 

 period the sexual cords appear and at its termination it is pos- 

 sible to tell the sex of the individual. 



The sexual cords, which begin to appear at about the 132d 

 hour of development, are the true sexual cords in the male or 

 seminiferous cords, and in the female the medullary cords or 

 cords of first proliferation. As the names imply, there is only 

 one series of cords in the male while in the female there are two 

 — the last series being known as the cortical cords or cords of 

 second proliferation (Mihalkowics, '85, Hoffmann, '92, Firket, 

 '14 and Swift, '15). 



The cords of first proliferation are first noticed as buds or 

 protrusions of the germinal epithelium into the underlying 



