148 GERM-CELL CYCLE IN ANIMALS 



whereas fertilized eggs give rise to females. First 

 and second polar bodies are formed and the first 

 divides, thus making three in all. The events of 

 early cleavage are the same whether the nucleus 

 consists of the female pronucleus only or of the 

 female and male pronuclei fused. Unlike the eggs 

 of monembryonic species, the cleavage nuclei here be- 

 come separated from one another by cell walls and 

 the " nucleolo" from the very beginning is segregated 

 at each division in a single cleavage cell (Fig. 45, D) . 

 This cell divides more slowly than the others ; the 

 "nucleolo" gradually becomes vacuolated, breaks 

 down, and finally is evenly scattered throughout 

 the entire cytoplasm. Just before the sixteen-cell 

 stage is reached the cell containing the disintegrated 

 "nucleolo" divides and the two daughter cells are 

 provided with equal amounts of its substance (Fig. 

 45, F). Silvestri was only able to trace the cells 

 containing the remains of the "nucleolo" until 

 four of these were present. Nevertheless, he con- 

 cludes that these and these alone give rise to the 

 germ cells. This conclusion seems well founded when 

 the history of this "nucleolo" is compared with 

 that of similar bodies (keimbahn-determinants) 

 in the eggs of certain other animals. 



Two regions develop in the eggs of these polyem- 

 bryonic HYMENOPTERA : (1) an anterior or polar re- 

 gion containing the polar bodies, and (2) the posterior 

 embryonic region. The latter again becomes differen- 

 tiated into two regions : (1) an anterior "massa germi- 

 nigera, " which gives rise to normal larvae, and (2) a 



