NEUROPTERA AND COLEOPTERA 



289 



inner layer which has the effect of crowding the yolk into an eccentric 

 position (Fig. 220). The definitive number of nuclei in the yolk syncitium 

 is four; subsequent division affects the blastomeres only (Figs. 221, 223). 

 At first the cells in the interior are rather loosely arranged (Fig. 221) ; but 

 as development continues, they become deeper and more or less wedge 



y:^ 



^^. 





Fig. 219. — Stylops. Eight-cell stage. 



Fig. 220. — Stylops gwynanae. 

 64-cell stage. 



shaped. The cells in the interior squeeze in between the peripheral cells 

 (Figs. 222, 223) without multiplying in number, and the yolk is pushed 

 into a peripheral position. The shift in the position of the yolk from the 

 morphological ventral position of the germ band to the dorsal position is 



y ^- 



'\j> 





,-^ 



^^j 





Fig. 221.— Stylops praecocis. 128- 

 cell stage. 



Fig. 222. — Stylops praecocis. 

 Section through yolk (y). 



characteristic for the Strepsiptera. An inversion of cell polarity thus 

 occurs. The interior is filled with a granular mass which appears to be a 

 secretion from the peripheral cells. The embryonic rudiment now elon- 

 gates more or less in one dimension accompanied by a lateral flattening, 

 the cavity becoming reniform (Fig. 224). The layer adjacent to the yolk 



