204 GERM-CELL CYCLE IN ANIMALS 



is initiated. The Nebenkern disappears in the 

 oocyte soon after the yolk begins to form. The 

 chromatin threads in the spermatocytes break down 

 and lose their affinity for dyes, but later reappear. 

 In the oocyte, on the contrary, the chromatin threads 

 persist. Demoll concludes from these observations 

 that the Nebenkern always determines the character 

 of the germ cells, which, up to its formation, may be 

 called indifferent germ cells. He further concludes, 

 that, since in dioecious animals sex is determined by 

 the accessory chromosomes, in Helix the sexual 

 specificity of the Nebenkern must be determined 

 by the accessory chromosomes. Such chromosomes 

 were described by Demoll (1912a) in a previous 

 contribution. 



A similar idea has been expressed by von Voss 

 (1914) regarding the differentiation of indifferent 

 germ cells in a flat-worm, Mesostoma ehrenhergi. 

 In the embryo of this hermaphrodite the germ gland 

 is a syncytium containing both the nuclei of future 

 oogonia and future spermatogonia. The cytoplasm 

 is apparently homogeneous throughout. The forma- 

 tion of the oogonia from indifferent germ cells begins 

 with the appearance of a "germinal-vesicle stage"; 

 this is followed by an increase in the amount of 

 cytoplasm surrounding them. Since the cytoplasm 

 appears to be similar in all parts of the syncytium, 

 differentiation must be initiated by the nucleus, 

 and the suggestion is made that perhaps the accessory 

 chromosome may be responsible for the separation 

 of the germ cells into oogonia and spermatogonia. 



