GERM CELLS IN THE ARTIIROPODA 115 



and a sperm nucleus combined (g.n). Frequently 

 the two polar bodies have not yet been produced 

 when the egg is laid and thus many stages may be 

 encountered in the newly laid eggs. Polyspermy is 

 a normal condition in insects and several sperma- 

 tozoa are often observed among the yolk globules. 

 The keimhautblastem is not homogeneous through- 

 out, for at the posterior end there is embedded in it a 

 disc-shaped mass of darkly staining granules which I 

 have called the pole-disc (g.c.d.) and which resembles 

 the pole-plasm of Miastor, the "Keimwulst" or 

 " Keimbahnplasma " of Chironomus and the " Dotter- 

 platte" of Calliphora, 



The cleavage nucleus divides by mitosis; the 

 daughter nuclei separate slightly, and divide; and 

 this process is continued until nuclei, each surrounded 

 by a small mass of cytoplasm, are scattered more or 

 less regularly throughout the egg. Then a division 

 of the nuclei into tw^o groups occurs; those of one 

 group migrate to the periphery, fuse with the periph- 

 eral layer of cytoplasm, and are cut off by cell walls, 

 thus forming the blastoderm; whereas the other 

 nuclei, the vitellophags, remain behind among the 

 yolk globules which it is their function to dissolve. 

 The blastoderm consists of a single layer of cells, 

 except at the posterior end where its formation has 

 been interrupted by the process resulting in the 

 establishment of the primordial germ cells. 



The primordial germ cells are formed in the fol- 

 lowing manner. The cleavage nuclei at the posterior 

 end of the egg that encounter the pole-disc granules 



