HYMENOPTERA 319 



to the side of the fusion nucleus. The fusion nucleus now undergoes 

 mitotic division (Fig. 268) which is soon followed by a division of the 

 adjacent cytoplasm to form two blastomeres (Fig. 269), the upper undi- 

 vided part of the egg constituting the pole region (trophamnion) with its 

 polar nucleus (Fig. 269, nu. p). The oosome without change becomes 

 associated with one of the daughter nuclei (Fig. 268) and is thus included 

 in the blastomere formed about that nucleus (Fig. 269, os). In the second 

 division, or four-cell stage, the oosome again accompanies one of the 

 blastomeres, usually one of the upper ones. With the third division, 

 producing the eight-cell stage, the oosome also divides so that two cells 

 now contain oosome substance (Fig. 270, Us. os). Meanwhile the polar 

 region gradually grows around the lower, or embryonic, region (Fig. 270) 

 with the polar nucleus beginning actively to divide {nu. p). In the fourth 

 division all the embryonic cells divide except the two that contain the 

 oosome substance. The result is the production of a 14- instead of the 

 typical 16-celled stage. Before this time the oosome has broken down, 

 and its material is spread around the nucleus throughout the cytoplasm of 

 each of the two daughter cells (Fig. 270, os). It is clear that the oosome 

 substance retards division of the cells containing it. In the fifth division 

 all the blastomeres divide, thus producing 28 cells, four of them containing 

 oosome substance. The polar region now has formed as a definite and 

 complete membrane, the trophamnion (ajnt), around the blastomeres 

 (Fig. 271). After this all synchrony in division is lost. The polar nuclei 

 in the trophamnion in part migrate toward the posterior end of the 

 morula-hke embryonic region, the trophamnion itself becoming more 

 envelope-hke (Fig. 272). By the time the blastomeres number more than 

 200 (Fig. 273), there may be some spindle-shaped (csp) and some poly- 

 hedral-shaped cells (c) in the morula-like mass. In this stage the 

 trophamnion is of uniform thickness (Fig. 273, amt) about the embryonic 

 mass, and its nuclei are fairly evenly distributed. 



In Litomastix truncatellus Silvestri (1937) recognizes the oosome sub- 

 stance as the germ-line determinant that is distributed to the few embry- 

 onal cells destined to become sex cells. Leiby (1929) reached the same 

 conclusion with reference to the oosome substance in Copidosoma gelechiac. 

 Patterson (1921), however, was unable to follow the history of the cells 

 containing the oosome substance beyond the 70-cell stage. He believes 

 that this substance has ceased to function as a germ-line determinant in 

 the polyembryonic egg owing to the increase in complexity of develop- 

 ment. The first steps leading to the organization of the polygerm may be 

 observed as early as the 220- to 225-celled stage (Fig. 273). Some of the 

 cells assume a spindle shape (csp) and pushing in between groups of 

 embryonic cells and fusing with other spindle cells form nucleated mem- 

 branes (Fig. 274, csp) around the clumps of embryonic cells. Thus 



