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 zoologists 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. EARLY MULTIPLICATION OF THE PRIMORDIAL 

 GERM CELLS. 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, 

 Miastor (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 



