142 GERM-CELL CYCLE IN ANIMALS 



(3) The nuclei of the vitellophags resemble the 

 early cleavage nuclei ; they are midway between 

 the other two kinds in size, and their chromatin is in 

 a more diffuse condition. 



Whether these three kinds of nuclei were all 

 potentially alike before their differentiation is an 

 important question. Visibly they are all similar 

 until they become localized in definite regions of 

 the egg, and associated with particular cytoplasmic 

 elements. One cannot help but conclude that they 

 were all potentially alike and that their differentia- 

 tion was brought about through the influence of 

 the cytoplasm in which they happened to become 

 embedded. The writer has shown (Hegner, 1911a) 

 that if the posterior end of a freshly laid egg of 

 Leptinotarsa decemlineata is killed with a hot needle, 

 thus preventing the pole-disc granules and surround- 

 ing cytoplasm from taking part in development, no 

 primordial germ cells will be produced. A large 

 series of similar experiments have also proved that 

 at the time of deposition, " The areas of the peripheral 

 layer of cytoplasm (Fig. 36 khbl.} are already set 

 aside for the production of particular parts of the 

 embryo, and if the areas are killed, the parts of the 

 embryo to which they were destined to give rise 

 will not appear. Likewise, areas of the blastoderm 

 are destined to produce certain particular parts of 

 the embryo" (Hegner, 1911a, p. 251). What 

 becomes of the nuclei that are prevented from enter- 

 ing the injured region of the egg ? No evidence 

 has been discovered to indicate that they disinte- 



