262 RECENT CYTOLOGY 



constituents of both parents. And it is an essential 

 point in Weismann's theory that he regards a given 

 germ-cell as containing a considerable number of ids 

 derived from its ancestors, all near ancestors being 

 thus represented. 



But Mendel's experiments and others of the same 

 kind show, in the case of a great number of different 

 characters, that although every essential character is 

 represented in every germ-cell, yet each Mendelian 

 character is represented by a paternal or a maternal 

 determinant only, and not by both. Thus, not 

 only are all immediate ancestors not represented 

 in the germ-cells in respect of any particular 

 character, but only one of the parents is so repre- 

 sented to the complete exclusion, so far as we can 

 tell, of the other parent. In fact, we are led to 

 believe that the germ-cells contain one set of de- 

 terminants only a single id whilst the somatic cells 

 contain two ids only. The Mendelian theory is thus 

 seen to be considerably simpler than the germ plasm 

 theory, which it replaces. At the same time it must 

 not be forgotten that many of the conceptions used in 

 the Mendelian expression of the facts are borrowed 

 from Weismann's theory, and that but for Weismann's 

 work it would have been impossible for us to have got 

 so far in the co-ordination of the facts derived from 

 experiment and microscopic observation respectively. 



The preceding sketch may serve to show how Mendel's 

 observations have been found to throw light upon many 

 of the facts of cytology the meaning of which was pre- 

 viously obscure ; and how it affords at the same time 



