224 GERM-CELL CYCLE IN ANIMALS 
tion process does not occur in many species, it is 
probably not a universal phenomenon, and conse- 
quently cannot be of fundamentalimportance. Most 
of the evidence, on the other hand, points toward 
the conclusion that all of the cleavage nuclei are 
qualitatively alike, and that the cytoplasm is the 
controlling factor. 
b. CyTopLasMic oR EXTRACELLULAR NUTRITIVE 
Supstances. It was pointed out on a preceding 
page (p. 101) that one of the characteristics used to 
distinguish primordial germ cells from other embry- 
onic cells is the presence within them of yolk material. 
In many vertebrates the yolk globules persist in the 
primordial germ cells until a comparatively late 
stage, and indeed are often so numerous as to practi- 
cally conceal the nuclei of these cells. A large num- 
ber of the keimbahn-determinants that have been 
described are supposed to consist of nutritive sub- 
stances. Some of the earliest investigators were 
aware of the yolk content of the primordial germ 
cells. For example, in Chironcmus Weismann (1863) 
found four oval nuclei lying in the ‘‘ Keimhautblas- 
tem” at the posterior end of the egg, each of which 
is associated with one or two yolk granules; these 
are the ‘“Polzellen.” In another Dipteron, Simula 
sp., Metchnikoff (1866) records four or five pole- 
cells which possess fine yolk granules in their cell 
substance. The same author (1866) also states that 
when the pseudovum in the pzedogenetic larva of 
Miastor contains twelve to fifteen nuclei, one of 
these, together with the dark yolk-mass in which it 
