198 GERM-CELL CYCLE IN ANIMALS 



eral. The centrally situated cells now increase in 

 size; but their nuclei retain the original condition; 

 that is, the chromatin is present in irregular clumps. 

 These are nurse cells. After the nurse cells have 

 formed, certain of the peripheral cells increase in 

 volume and pass through an indifferent progermina- 

 tive stage (Fig. 57, E). Then they transform into 

 female progerminative cells, as shown in Fig. 57, F, G. 

 The chromatin clumps break up and become oriented 

 near the nuclear membrane, where they form a layer 

 of more or less rounded bodies bearing chromatic 

 filaments. In the meantime, both nucleus and cyto- 

 plasm increase in amount, especially the cytoplasm. 

 This (Fig. 57, G) represents an oocyte, which does not 

 divide before maturation. 



Ancel concludes from these observations that there 

 are three successive periods of cellular differentiation 

 in the hermaphroditic gland of Helix: (1) the ap- 

 pearance of spermatogonia, (2) nurse cells, and 

 (3) oocytes. Both spermatogonia and oocytes pass 

 through the indifferent progerminative-cell stage, but 

 the nurse cells do not ; there are therefore two sorts 

 of differentiation of the indifferent epithelial cells. 

 Regarding the cy to-sexual determination, the follow- 

 ing hypothesis is advanced : A progerminative in- 

 different cell becomes a male or female element 

 according to its environment at the time of its trans- 

 formation ; if it appears before the nurse cells are 

 formed it becomes a spermatogonium ; if nurse cells 

 are already present it grows into an oocyte. The 

 discovery of certain individuals containing only male 



