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 odcyte, 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 Heliz: (1) the ap- 
pearance of spermatogonia, (2) nurse cells, and 
(3) obcytes. Both spermatogonia and odcytes 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 cyto-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 odcyte. The 
discovery of certain individuals containing only male 
