138 Animal Life and Intelligence. 



the mother ovum. But the reproductive cells are set apart ; 

 they take no share in the differentiation, but remain 

 virtually unchanged, and continue unaltered the proto- 

 plasmic tradition of the original ovum."* In such a case, 

 then, observation flatly negatives the view that the germinal 

 cells are "constituted" by gemmules collected from the 

 body-cells, though, of course (on a modified pangenetic 

 hypothesis), they might be the recipients of such gemmules. 



It is only in a minority of cases, however, that the direct 

 continuity of germinal cells as such is actually demonstrable. 

 In the higher vertebrates, for instance, the future repro- 

 ductive cells can first be recognized only after differentiation 

 of some of the body-cells and the tissues they constitute 

 is relatively advanced. While in cases of alternation of 

 generations, " an entire asexual generation, or more than 

 one, may intervene between one ovum and another." In 

 all such cases the continuity of the chain of recognizably 

 germinal cells cannot be actually demonstrated. 



The impracticability of actually demonstrating a con- 

 tinuity of germinal cells in the majority of cases has 

 induced Professor Weismann to abandon the view that 

 there is a continuity of germinal cells, and to substitute 

 for it the view that there is a continuity of germ-plasm 

 (keimplasma) . "A continuity of gerrn-ce^s," he says,t 

 "does not now take place, except in very rare instances; 

 but this fact does not prevent us from adopting a theory of 

 the continuity of the geim-plasm, in favour of which much 

 weighty evidence can be brought forward." It might, 

 however, be suggested that, although a continuity of 

 germ- cells cannot be demonstrated, such continuity may, 

 nevertheless, obtain, the future germinal cells remaining 

 undifferentiated, while the cells around them are under- 

 going differentiation. The comparatively slight differentia- 

 tion of the body-cells in hydroids renders such a view by 

 no means improbable. But Professor Weismann does not 

 regard such an idea as admissible, at all events, in certain 



* Geddes and Thomson, " The Evolution of Sex," p. 92. 



t Weismann, " Essays on Heredity," English translation, p. 173. 



