196 GERM-CELL CYCLE IN ANIMALS 



by him the "besondere Korper," may be unequally 

 distributed between these cells, and that the one 

 which procures the larger portion is the progenitor 

 of the spermatozoa, the other of the ova. The evi- 

 dence for this view is, however, insufficient. 



In Helix both eggs and spermatozoa originate in 

 every acinus of the ovo-testis ; it is therefore an ex- 

 cellent species for the study of the differentiation of 

 the sex cells. According to Ancel (1903) the anlage 

 of the hermaphroditic gland of Helix pomatia appears 

 a few hours before the larva hatches ; it consists of a 

 group of cells situated in the midst of the mesoderm, 

 from which germ layer it seems to originate. It 

 soon loses its rounded form and becomes elongate; 

 then a lumen appears within it, thus changing it into 

 a vesicle whose wall consists of a single layer of cells 

 a true germinal epithelium. Secondary, tertiary, 

 etc., vesicles bud off from the single original vesicle, 

 forming the acini of the fully developed gland. Cel- 

 lular differentiation takes place by the transformation 

 of the germinal epithelial cells into male, nurse, and 

 female elements. An indifferent epithelial cell is 

 shown in Fig. 57, A ; the chromatin granules are con- 

 densed to form irregular clumps. Some of these 

 indifferent epithelial cells increase in size and give 

 rise to indifferent progerminative cells ; the chroma- 

 tin clumps fuse, forming more or less spherical masses 

 (Fig. 57, E). From cells of this sort originate both 

 the oogonia and spermatogonia. The progermina- 

 tive male cell passes through the stages shown in 

 Fig. 57, B-D ; part of the chromatin of the progermi- 



