STUDIES ON GERM CELLS 471 



from which the germ cells of these animals eventually arise. The 

 hypothesis that the nucleoli consist of food substance also argues 

 in favor of the idea that the Keimbahn-determinants are nutri- 

 tive. 



The importance of these nutritive substances to the primordial 

 germ cells can be stated with some degree of certainty. Accord- 

 ing to some authorities the primordial germ cells remain in the 

 primitive condition and do not undergo differentiation at the 

 same time, or at least at the same rate, as do the other embryonic 

 cells. On this account their yolk contents are not at first util- 

 ized, since their metabolic activities are so slight. This is more 

 especially true of the vertebrates, in which it has been suggested 

 (Hegner, '09a, p. 276), that the yolk contents of the germ cells 

 are transformed into the energy of motion during the characteris- 

 tic migration of these cells into the germinal epithelium. Why 

 these nutritive substances are segregated in the primordial germ 

 cells is more difficult to answer. 



Finally, it is interesting to note that the differentiation of the 

 indifferent germ cells of HeUx arbustorum into spermatogonia 

 or oogonia has been found to depend upon nutrition (Buresch, 

 '11). "Ob aber eine indifferente Geschlechtszelle sich in mann- 

 licher oder weiblicher Richtung weiter entwickeln wird, das 

 konnen wir schon sehr friih sagen, namlich nach der Lage diesen 

 Zelle naher oder weiter von einen Nahrzelle" (p. 327). 



Yolk nucleus. There are many bodies in the cytoplasm of 

 growing oocytes which have been called yolk nuclei and which 

 may be responsible for the origin of the Keimbahn-determinants. 

 Some of these bodies have already been considered, but the term 

 'yolk nucleus' has been applied to so many different cytoplasmic 

 inclusions (Munson, '12) that no attempt will be made here to 

 describe them nor to trace their history. 



Mitocho7idria. The condition of the chondriosomes in the 

 primordial germ cells of certain vertebrates (Rubaschkin, '10, 

 '12; Tschaschkin, '10) and the theories proposed regarding the 

 role of these bodies in heredity make it necessary to refer to 

 them briefly here. A review of the literature on mitochondria, 

 chondriosomes, plastosomes, etc., would be superfluous since 



JOURNAL OF MOKPHOLOGT, VOL. 25, NO. 3 



