REPRODUCTION. 66l 



body in the development of gametes is the following. He 

 considers that the nucleus of the developing gamete contains 

 two kinds of nucleoplasm, namely, reproductive substance 

 and histogenic (or somatic) substance, and that in the ex- 

 trusion of the polar body the histogenic substance is elimi- 

 nated. Agreeing with Strasburger as to the physiological 

 equivalence of male and female gametes, he believes the 

 significance of the sexual process to be this, that the sudden 

 increase in the bulk of the nucleus determines the division of 

 the cell. Vegetative reproduction takes place in plants be- 

 cause the reproductive substance is widely disseminated 

 throughout the somatic cells. 



Weismann's theory is framed specially with the object of 

 explaining the phenomena of heredity. He denies that acquired 

 characters can be transmitted by sexual reproduction. Hence 

 he cannot accept either the theory of Pangenesis, which 

 assumes the presence in the reproductive cells of gemmules 

 from all parts of the organism, or the view of Naegeli and 

 Strasburger, that a somatic cell may return to the embryonic 

 condition, in other words, that a conversion of somatic sub- 

 stance into reproductive substance may take place. Under 

 these circumstances the assumption of the continuity of the 

 reproductive substance becomes a logical necessity. His 

 theory of variation is also based upon the negation of the 

 inheritance of acquired characters by sexual reproduction. 

 Acquired characters, he asserts, can only be transmitted by 

 asexual reproduction. He therefore refers the origin of 

 acquired characters to the ancestral unicellular plants which 

 multiplied only asexually. These unicellular plants, after 

 multiplying asexually through countless generations, acquired 

 a great variety of characters in response to changes in the 

 external conditions, and then, when sexual reproduction was 

 evolved, the ancestral characters of the two parents were com- 

 bined in the sexually produced individual. The whole pro- 

 cess of evolution since the first appearance of sexual repro- 

 duction is then simply the expression of the repeated re- 

 combination in various ways of the characters acquired by 

 the asexual unicellular progenitors of the race. 



