THE GAMETES IN PLANTS AND ANIMALS 



329 



means that the germ cells are probably determined in essentially 

 the same way as other parts of the organism. In various other 

 insects also a germ path has been described. In certain hymenop- 

 tera Hegner ('14^) finds that the granules of the polar cytoplasmic 

 region are derived from the disinte- 

 grated nucleus of a nutritive cell taken 

 up by the egg during its growth, an 

 origin very similar to that which 

 Buchner described in the case of Sagitta. 



In all these cases among the inverte- 

 brates the factors determining what 

 shall become germ cells and what 

 somatic structures apparently exist in 

 the cytoplasm and not in the nuclei. 

 Moreover, the cytoplasmic regions 

 which determine the germ cells are not 

 directly related to the cytoplasm of pre- 

 existing germ cells, but very evidently 

 are simply regions where certain special 

 metabolic conditions exist. Cells 

 arising from these regions become germ 

 cells, just as those arising from other 

 regions become one part or another of 

 the body. It is of interest to note that 

 very generally the germ cells arise from 

 regions of the egg with a relatively low 

 metabolic rate. They very commonly 

 divide more slowly than other cells. 

 In fact, it seems possible that this low 

 metabolic rate, rather than any specific 

 character, determines that they shall 

 not take part in the early development 

 of the body, because other cells react 



more rapidly than they do. They are, so to speak, left behind 

 and only later become an active functional part of the organism. 



Among the vertebrates comparatively early segregation of the 

 primitive germ cells is apparently of wide occurrence in fishes, 



FIG. 145. Early cleavage of 

 Chironomus, a gnat: the spindle 

 at the lower end of the egg 

 represents the primitive germ 

 cell; the cytoplasm about this 

 spindle separates with it from 

 the remainder of the egg and 

 divides into two cells, each of 

 which divides farther. From 

 Hasper, 'u. 



