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 odgonia has been found to depend 
upon nutrition (Buresch, 1911).1 
Yotx Nucteus. There are many bodies in the 
cytoplasm of growing odcytes 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. 
Mitocuonpria. 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- 
1“Qb aber eine indifferente Geschlechtszelle sich in mannlicher oder 
weiblicher Richtung weiter entwickeln wird, das kénnen wir schon sehr 
friih sagen, nimlich nach der Lage dieser Zelle niher oder weiter von 
einer Nihrzelle ” (p. 327). 
