1 84 



H. L. WIEMAN. 



ages take place approximately in the center of the egg. As 

 cleavage proceeds there occurs a separation of the nuclei into two 

 groups, one of which moves outward into the peripheral cyto- 

 plasm to form the nuclei of the " Keimhautblastem," while the 



a ^ b c d e / 



Fig. 4. Six stages in the division of a spherule of the nutritive stream in the proc- 

 ess of yolk formation. X I)200. 



other (the vitellophags) remains scattered through the yolk. Ac- 

 cording to Hegner a definite number of nuclei reaching the poste- 

 rior part of the egg do not remain in the peripheral cytoplasm, 

 but collect about them a number of granules from the pole disc 

 and continue their migration until they are entirely separated from 

 the blastoderm. These nuclei with their accumulations of cyto- 

 plasm constitute the primordial germ cells. 



Fig. 6 represents a section taken through the region of the pole 

 disc shortly after the pole cells have reached the periphery. It 

 will be noticed that the granules of the pole disc are much finer 

 than in the earlier stage shown in Fig. 5. The granules in the 



Fig. 5. Longitudinal section of the posterior end of a mature ovum. X i>ooo 

 />.d., pole disc; k., Keimhaut ; y., yolk. 



cells are distributed on a reticulum and stain intensely with the 

 basic dye, but no more so than the granules that are found in the 

 cytoplasm of the blastema cells. In one case the granules are 

 derived from the pole disc, while in the other they come direct 

 from the food stream. The original source in both cases is the 

 material secreted by the nurse cells. 



