338 



THE EVOLUTION THEORY 



depend must be a minimal quantity of substance. The difference is 

 especially great in animals, even in those species whose eggs may be 

 called small, for instance, those of sea-urchins or of mammals ; even in 

 these the mass of spermatozoon is scarcely a thousandth part, often 

 scarcely a hundred-thousandth part of the mass of the ovum. And 

 yet the inheritance from the father and from the mother is equally 

 great. Now as we know that vital powers have always a material 

 basis, a minute quantity, such as is contained, for instance, in the 

 spermatozoon of Man, must have implicitly in it all the hereditary 



Fig. 69. Ovum of Sea-urchin (Taxopneustes lividus), 

 after E. B. Wilson, zk, cell-substance, k, nucleus 

 (so-called germinal vesicle), n, nucleolus (so-called 

 germinal spot). Below there is a spermatozoon of 

 the same animal (sp), magnified in the same pro- 

 portion, about 750 times. 



-h 



.-■ax 



■ e 



Fig. 68. Diagram of a 

 spermatozoon. After E. B. 

 Wilson, sp, apex. n. nucleus, 

 c, centrosphere. m, middle 

 portion, ax, axial filament. 

 e, terminal filament. 



tendencies of the father; and the conclusion is inevitable that in 

 the ovum there can only be an equally minimal quantity of substance 

 which is the bearer of the hereditary powers, for if there were a larger 

 quantity of hereditary substance in the ovum its power of transmission 

 would also be greater 1 . 



If we inquire as to the part of the spermatozoon which bears 

 this hereditary substance, we may exclude both the tail-thread and 

 the middle piece (Fig. 68), the former because it obviously fulfils 



1 The improbable assumption that the hereditary substance of the father may be in 

 quality altogether different from that or the mother, and so may have the same power 

 of transmission, and yet take up much less room, I leave out of the question altogether. 



