30 GERM-CELL CYCLE IN ANIMALS 
determinate type being more fully organized than 
those of the indeterminate type. 
The period when the primordial germ cells are es- 
tablished is probably due in part to the state of 
organization of the egg when development begins, 
and it is not strange, therefore, that the primordial 
germ cell may be completely segregated in certain 
eggs as early as the four-cell stage; whereas in 
others germ cells have not been discovered until a 
late larval condition has been reached. An ever 
increasing number of species of animals is being 
added to those in which an early segregation of the 
germ cells has already been recorded. Neverthe- 
less, there are certain zodlogists who still question 
the general occurrence of an early segregation of 
the germ cells, but more careful investigations will 
probably establish the fact of early segregation in 
species in which this has not yet been demonstrated. 
2. Earty MULTIPLICATION OF THE PRIMORDIAL 
Germ Crtits. The number of germ cells present 
at the time of their first appearance in the embryo 
varies in different species. There may be one, as 
in the majority of cases, for example the fly, 
Miuastor (Fig. 17), the nematode, Ascaris (Fig. 51), 
the crustacean, Cyclops (Fig. 48), and the arrow 
worm, Sagitta (Fig. 54) ; or a number, as in chrysome- 
lid beetles (Fig. 36), certain parasitic HYMENOPTERA 
(Fig. 44), and vertebrates (Fig. 6). As a rule the 
primordial germ cell or cells increase in number by 
mitosis soon after they are segregated, and then 
cease to divide for a considerable interval. For 
