226 GERM-CELL CYCLE IN ANIMALS 



other embryonic cells. On this account their yolk 

 contents are not at first utilized, since their meta- 

 bolic activities are so slight. This is more especially 

 true of the vertebrates in which, it has been sug- 

 gested (Hegner, 1909a, p. 276), the yolk contents 

 of the germ cells are transformed into the energy of 

 motion during the characteristic 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 Helix arbustorum into sper- 

 matogonia or oogonia has been found to depend 

 upon nutrition (Buresch, 1911).^ 



Yolk Nucleus. There are many bodies in the 

 cytoplasm of growing oocytes that have been called 

 yolk nuclei and that 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, 1912) 

 that no attempt will be made here to describe them 

 nor to trace their history. 



Mitochondria. The condition of the chondrio- 

 somes in the primordial germ cells of certain verte- 

 brates (Rubaschkin, 1910, 1912; Tschaschkin, 1910; 

 Swift, 1914) and the theories that have been pro- 



* "Ob aber eine indifferente Geschlechtszelle sich in mannlicher oder 

 weiblicher Richtung weiter entwickeln wird, das konnen wir schon sehr 

 frlih sagen, namlich nach der Lage dieser Zelle naher oder weiter von 

 einer Nahrzelle " (p. 327). 



