ORIGIN OF DEFINITIVE SEX-CELLS 443 



epithelium, producing a first series of buds, sent into the under- 

 Ijdng stroma sexual cords which contained some of the large 

 primordial germ-cells. After a short period of rest, the germinal 

 epithelium became active again and sent down into the sexual 

 cords cells which he characterized as primordial eggs. He called 

 the cells surrounding these primordial eggs follicular cells. This 

 author differed from Schmiegelow in believing that the primordial 

 eggs of the sexual cords as well as those seen in the germinal 

 epithelium were of peritoneal origin. 



Prenant ('89) in a very complete series of chick embryos stud- 

 ied carefully the evolution of the gonad and particularly the 

 origin of the primordial eggs — the so-called primordial germ- 

 cells. In the three-day chick, he found on the medial surface 

 of the Wolffian body lateral to the root of the mesentery an 

 elongated ridge covered with peritoneal epithelium. This en- 

 largement, which he called the genital ridge, was covered at 

 three and one-half days with the elongated cells of the germinal 

 epithelium. In this epithelium and the mesenchyme beneath, 

 which forms the main mass of the genital ridge, he discovered 

 the large primordial eggs. Since he did not see any mitotic 

 figures and since these large cells increased in number, he was 

 drawn to the conclusion that they were produced by a process of 

 differentiation from the cells of the germinal epithelium. The 

 author did not hazard any opinion as to the relation of the 

 primordial germ-cells to the definitive ova. 



Just as the work of Waldeyer is the starting point of one epoch 

 in connection with research done on sex-cell formation, so the 

 work of C. K. Hoffmann ('93j should initiate another. 



Up to this time the true value of primordial germ-cells or pri- 

 mordial eggs had not been appreciated. True, a great many 

 investigators, some of whom are mentioned in the preceding 

 pages, had seen and described them, but not one had found them 

 in the embryo previous to the formation of the germinal epithe- 

 lium. Even the theory as to their origin in the gonad, faulty as 

 it was, remained in controversy. Waldeyer, Balfour, von Mihal- 

 cowics and Semon believed that their sole seat of origin was the 

 germinal epithelium; Schmiegelow and Prenant, although not 



