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 fundamental importance. 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 

 Substances. 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 Chironomus 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 psedogenetic larva of 

 Miastor contains twelve to fifteen nuclei, one of 

 these, together with the dark yolk-mass in which it 



