SEX-CORDS AND SPERMATOGONIA IN CHICK 379 



the case might be. This idea is of interest only because of its 

 novelty and historic interest. 



We now come to two important papers, which will always 

 stand out as landmarks, and which, together with those of Wal- 

 deyer, Niissbaum and Mihalkowics, will always be studied in 

 connection with any work on the origin of the sexual cells. I 

 refer to the work of Semon ('87) and Hoffman ('92). 



Semon studied the indifferent gonad and the testis of the 

 chick. Like Balfour ('78), Braun ('77), and Nussbaum ('80), he 

 believed the sex cords to be outgrowths of the capsules of the 

 Wolffian bodies. He saw, as Waldeyer did, two kinds of cells 

 in the germinal epithelium, the columnar epithelial coelomic 

 cells, and the large clear primordial germ-cells. He described 

 the various stages which the columnar cells passed through in 

 becoming the primordial germ-cells. In fact, his opinion' in re- 

 gard to the primordial germ-cells is exactly that of Waldeyer. 

 He found that the sexual cords, growing out from the Malphigian 

 bodies, reached the germinal epithelium about the sixth day and 

 that the primordial germ-cells passed into them. The cords 

 became separated from the epithelium by a connective tissue 

 layer, differentiated from the stroma, called the albuginea. 

 The cords anastomosed freely with one another, became tubu- 

 lar and in this way developed the tubuli contorti seminiferi. 

 The cavity in the cords began to appear during the third week 

 of incubation and at hatching had reached a great size. These 

 tubes contained two kinds of cells, like the germinal epithelium, 

 the small peritoneal cells and the large clear cells which resem- 

 bled and were descendants of the primordial germ-cells. In re- 

 gard to the products of these two varieties of cells it is better 

 to quote Semon: ''Es kann natiirlich keinen Zweifel unter- 

 liegen, das die kleinen Zellen der Segmentalstrange die soge- 

 nannten Stiitzzellen der Samenkanalchen, die Ureier aber die 

 grossen, runden Hodenzellen reprasentiernen." 



Semon and Nussbaum ('80) were the first to show a conti- 

 nuity between the primordial germ-cells and the male sexual cells 

 of the adult. It must be remembered, however, that Semon 

 obtained his ureier from the cells of the germinal epithelium by 



