STUDIES ON GERM CELLS 469 



that chrbmatin particles are in some cases cast out of the nuclei 

 in the oocytes of certain animals and continue to exist as such in 

 the cytoplasm for a considerable period. It is also possible that, 

 as Buchner ('10b) maintains, the Keimbahn-determinants may 

 be in reality 'Keimbahnchromidien.' 



This view was suggested by the writer in 1909 (p. 274) to ac- 

 count for the origin of the pole-disc granules in the eggs of Chry- 

 somelid beetles. It was thought that here, as in the Hymenop- 

 tera (Blochmann, '86; et al.), chromatin granules might be cast 

 out of the nuclei of the oocytes, and that these granules might 

 gather at the posterior end to form the pole-disc. It was also 

 suggested that chromatin granules from the nurse-cell nuclei 

 might make their way into the oocyte and later become the 

 granules of the pole-disc. It should not be forgotten, moreover, 

 that these granules stain like chromatin. Finally, mention 

 should be made of the 'anello cromatico' of Giardina ('01) 

 which is associated with the differentiation of the oocytes in 

 Dytiscus (p. 414). 



Conclusion. Certain Keimbahn-determinants consist of nucleo- 

 lar material which is derived from the germinal vesicle and 

 which persists until the primordial germ cells are established. 

 In some cases the Keimbahn cells are characterized by the 

 possession of the complete amount of chromatin, in contrast to 

 the somatic cells which lose a part of this substance. Since, 

 however, the chromatin-diminution process does not occur in 

 many species, it is not a universal phenomenon, and consequently 

 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. 



6. From cytoplasmic or extracellular nutritive substances : Yolk 

 and nurse cells. It was pointed out on page 450 that one of the 

 characteristics used to distinguish primordial germ cells from other 

 embryonic ceUs is the presence of yolk material within them. In 

 many vertebrates the yolk globules persist in the primordial germ 

 cells until a comparatively late stage, and indeed are often so nu- 

 merous as practically to conceal the nuclei of these cells. A large 



