WEISMANN'S THEORY 147 



contain all the determinants of the plant, for it will 

 produce branches bearing various organs, among 

 others, flowers with sexual cells. A whole begonia 

 will grow from a fragment of leaf planted in moist 

 sand. 



Similar phenomena can be observed in the animal 

 world, such as cases of regeneration. Cut off one of 

 the salamander's legs and a new member, normal in 

 shape and in structure, will develop from the stump ; 

 segments of certain worms will grow a new head with 

 all the organs appertaining to it. To account for 

 these facts Weismann admits the presence, in the cells 

 in question, of two or more categories of determinants, 

 some of which are reserve determinants, inactive under 

 ordinary conditions of life but waking up under the 

 influence of a certain stimulus. Ovum and spermato- 

 zoon contain determinants of both sexes, those of one 

 sex only becoming active. 



This applies to secondary sexual characters as well. 

 When a son inherits his maternal grandfather's black 

 beard, the necessary determinants must have existed 

 in the mother in a latent condition. Likewise, the 

 varied forms observable in social insects must be due 

 to the presence in the ovum of several kinds of deter- 

 minants. A specific stimulus makes certain deter- 

 minants active to the exclusion of the others. Each 

 cell contains, like the initial germ cell, the whole com- 

 plex of determinants, but at every stage of onto- 



