THE GERM-PLASM THEORY 297 



differentiated for causing motion and the nerve cells 

 for receiving and conducting stimuli. That the germ 

 cells remain in a primitive condition during a large 

 part of the embryonic period is accounted for by 

 the fact that they become functional at a compara- 

 tively late stage in ontogeny (Eigenmann, 1896). 

 Asexual reproduction by means of fission or budding 

 has seemed to some to invalidate the theory of ger- 

 minal continuity, but as Montgomery (1906, p. 82) 

 has pointed out, "Perhaps in all cases products of 

 asexual generation contain germ cells. If this were 

 so, it might then be the case that the incapacity 

 of any part of the body of an animal to reproduce 

 asexually, or even to regenerate, would be due to 

 the absence of germ cells in it — but this is merely 

 a suggestion." The probability that the regenerat- 

 ing pieces of coelenterates and the artificial Plas- 

 modia formed by dissociated sponge cells contain 

 germ cells has already been noted (p. 79), but there 

 are cases of the regeneration of sex organs that are 

 not so easily explained. For example, Janda (1912) 

 has found that if the anterior part of the hermaph- 

 roditic annelid, Criodrilus lacuum, is removed, a 

 new anterior end will regenerate containing both 

 ovaries and testes, although not always in their 

 normal positions. 



The study of the germ cells in the cestode Moniezia 

 expansa convinced Child (1906) that germ cells may 

 develop from tissue cells. In this species the germ cells 

 are derived from the parenchymal syncytium, which 

 has undergone a considerable degree of cytoplasmic 



