296 GERM-CELL CYCLE IN ANIMALS 



One of the distinguishing features of many primor- 

 dial germ cells is the presence within their cytoplasm 

 of certain stainable bodies to which I have applied 

 the term *'keimbahn-determinants." Although, as 

 pointed out in Chapter VIII, these inclusions do not 

 appear to consist of the same sort of material in 

 the eggs of different species and hence their signif- 

 icance is problematical, still they seem to be asso- 

 ciated with that particular part of the egg sub- 

 stance which becomes the cytoplasm of the primor- 

 dial germ cells. For this reason, if for no other, 

 the keimbahn-determinants are of the greatest 

 value, since they enable us to determine the position 

 of this germ-cell substance during the stages before 

 the primordial germ cells are established. It is 

 therefore possible to trace the germ-cell substance 

 in such cases as Sagitta (Fig. 54), where there is no 

 morphological continuity of the germ cells. What 

 relation the keimbahn-determinants have to the germ- 

 plasm is not yet definitely known. 



There have, of course, been many objections to 

 the germ-plasm theory. The history of the germ 

 cells in the Coelenterata, upon which Weismann 

 (1882) based a large part of his argument, is consid- 

 ered by Hargitt (see p. 95) to be directly opposed 

 to the hypothesis. According to some zoologists 

 there is no essential difference between the repro- 

 ductive cells and the various sorts of somatic cells ; 

 they have all arisen as the result of division of labor, 

 and the germ cells have been differentiated for pur- 

 poses of heredity just as the muscle cells have been 



