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 réle 
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 
Muastor, 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 
