242 GERM-CELL CYCLE IN ANIMALS 



of chromatin escape from the nucleus and are dif- 

 ferentially distributed to the daughter cells is evidence 

 that nuclear material may play some important role 

 in the progressive changes of cleavage cells. 



It has been shown that in many animals the germ 

 cells do not multiply for a considerable period 

 during the early developmental stages. This period 

 coincides also with that during which the keimbahn- 

 determinants, as a rule, disappear. For example, 

 the germ cells of chrysomelid beetles multiply until 

 there are about sixty -four present, at which time they 

 constitute a group at the posterior end of the egg and 

 the embryo has just started to form ; no further 

 increase in number occurs until the larval stage is 

 reached and the definitive germ glands are established. 

 As soon, however, as the embryo has reached a 

 certain developmental stage, the germ cells migrate 

 into it, and it looks very much as though they remain 

 quiescent until the somatic cells are *'able to protect, 

 nourish, and transport" them. 



The number of primordial germ cells during the 

 **period of rest" is perhaps most definitely known in 

 Miastor, where, as one group of eight and later as two 

 groups of four each, they are present throughout a 

 large part of embryonic development. 



In vertebrates also a long period exists during 

 which division of the primordial germ cells does not 

 take place (Fig. 6) and at least in several species 

 certain cell contents (the mitochondria) remain in an 

 indifferent condition (Rubaschkin, 1910; Tschasch- 

 kin, 1910 ; Fig. 31, B). These facts all indicate that 



