WEISMANN'S THEORY OF HEREDITY 169 



There is no other means of communication between the soma 

 and the germ cells, and hence somatogenic characters, which 

 are acquired in the life-time of the individual body as the direct 

 result of the action of the environment (including use and disuse 

 of organs), cannot be transmitted to the germ cells and therefore 

 cannot be inherited. The only characters which can be inherited 

 are blastogenic characters, which arise by modification of the 

 germ plasm in the germ cells themselves. 



This denial of the transmission of so-called acquired charac- 

 ters constitutes the most important difference between the 

 theories of heredity propounded by Weismann and Darwin. 

 Both these theories postulate the existence of ultra-microscopical 

 material particles, determinants or gemmules, but Weismann's 

 theory allows of no transference of such particles from soma to 

 germ cell, only from germ cell to germ cell and from germ cell to 

 soma. There is supposed to be no mechanism for the trans- 

 mission of somatogenic characters to the next generation. Accord- 

 ing to the older view the germ cells give rise to the soma and the 

 soma to the germ cells alternately. According to the newer one 

 the germ cells give rise to the soma and at the same time to the 

 next generation of germ cells, while the soma gives rise to nothing 

 but itself and ultimately perishes. 



i The contrast between the two views is clearly expressed in the 

 accompanying diagram (Fig. 76), in which, for the sake of sim- 

 plicity, the complication introduced by the process of conjugation 

 of the germ cells has been ignored. 



The inheritance of somatogenic characters being denied, Weis- 

 mann is obliged to seek the origin of variations from some 

 source other than the action of the environment and use and- 

 disuse. We shall return to this point presently. In the meantime 

 we must point out that Weismann's theory harmonizes very well 

 with the phenomena of mitosis, and especially with the remarkable 

 modifications of those phenomena which accompany the matura- 

 tion of the germ cells. 



The entire process of mitosis serves to emphasize the import- 

 ance of the chromatin substance of the nucleus. It is evidently 

 of the utmost consequence that this substance should be accur- 

 ately apportioned between the daughter cells. We accordingly 

 find the elaborate mechanism of centrosomes and nuclear spindle, 

 and a splitting of each individual chromosome, which takes place 

 longitudinally when the chromosomes themselves happen to bo 



