STUDIES ON GERM CELLS ' 449 



In this crustacean there are at one cleavage stage four primordial 

 germ cells which resemble the quartette of cells described by 

 Hertwig in Sagitta, two of which were proved by him to be oogonia 

 and the other two spermatogonia. Pedaschenko supposes that 

 the four primordial germ cells form two pairs, a pair lying 0:1 

 either side of the median line. Each pair becomes a single cell 

 by fusion and the probable disintegration of one nucleus. One 

 cell of each pair is believed by Pedaschenko to be male, the other 

 female ; and the sex of the cell whose nucleus does not degenerate 

 determines the sex of the resulting individual, since a persisting 

 female cell would form an ovary and a persisting male cell a 

 testis. Thus is a potentially hermaphroditic organism changed 

 to a dioecious organism. 



4. THE KEIMBAHN IN VERTEBRATES 



Animals from all classes of vertebrates have been employed 

 for determining the origin of the germ cells, but in no case have 

 these cells been traced back to cleavage stages. Early authors, 

 and even many writers at the present time, believed in the germi- 

 nal epithelium theory of Waldeyer (70). This investigator 

 first distinguished germ cells in the epithelium covering the 

 genital ridge and thought that they evolved from these epithelial 

 cells. The gonotome theory of Riickert ('88) and Van Wijhe 

 ('89) holds that germ cells arise in the embryo from a certain 

 part of the mesoblastic segments called by the latter the 'gono- 

 tome;' from here they are carried into the peritoneum. There 

 can be no doubt from the most recent investigations that the 

 germinal epithelium and gonotome theories are incorrect, and 

 that the germ cells of vertebrates are formed at a much earlier 

 period, giving good basis for the idea that these cells arise from 

 cleavage cells as has been abundantly proved for many inverte- 

 brates. Some of those who have advocated such an early origin 

 of germ cells are Nussbaum ('80) in the trout and frog, Eigen- 

 mann ('92, '96a, '96b) in Cymatogaster, Wheeler ('99) in the 

 lamprey. Beard ('00, '02) in Raja and Pristiurus, Nussbaum ('01) 

 in the chick. Woods ('02) in Squalus, Allen ('06, '07, '09) in Chry- 



