116 GERM-CELL CYCLE IN ANIMALS 



behave differently from those at other points, since 

 they do not remain to form part of the blastoderm 

 but continue to migrate until they have become 

 entirely separated from the rest of the egg. During 

 this process each of the sixteen nuclei that act in this 

 way becomes surrounded by a halo of granules — 

 part of the pole-disc. Then cell walls appear and 

 sixteen primordial germ cells result. These form a 

 group at the posterior end, each member of which 

 divides twice, thus producing sixty-four germ cells 

 in all. During these divisions, which are mitotic, 

 the pole-disc granules appear to be equally distrib- 

 uted between the daughter cells (Fig. 37, B). 

 A rest period then occurs, as far as cellular multipli- 

 cation is concerned, during which a ventral plate, 

 which later grows into the germ band, develops on 

 the ventral surface of the egg. As in Miastor the 

 germ-band pushes around on the dorsal surface 

 and the group of sixty-four germ cells is carried 

 along with it. In the meantime the germ cells 

 begin to migrate from the amniotic cavity in which 

 they lie through a sort of canal at the bottom of a 

 groove in the germ-band and thus make their way 

 inside of the embryo (Fig. 37, F). That the germ 

 cells actually migrate and are not simply forced 

 about by the surrounding tissues seems certain since 

 they are ameboid in shape and pseudopodia extend 

 out in the direction of their movement (Fig. 37, F) . 



After penetrating into the embryo the germ cells 

 become separated into two groups. It was difficult 

 to count the number in each group, but many 



