GERM CELLS IN THE ARTHROPODA 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-dise (g.c.d.) and which resembles 
the pole-plasm of Jiastor, the “ Keimwulst”’ or 
“ Keimbahnplasma” of Chironomus and the “‘ Dotter- 
platte” of Callcphora. 
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 two 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 
