ONTOGENESIS 193 



Weismannism. In the course of time the pen- 

 dulum again swung back toward the preformation 

 standpoint. The increase in knowledge of the data 

 of heredity, and a more exact understanding of the 

 cellular phenomena of zygosis and ontogeny, forced 

 speculative biologists to refer back the structures 

 that develop in morphogenesis to some sort of pre- 

 existing structure in the gametes. Under the hand 

 of Weismann (1834- ) this became an elaborate 

 architecture of determining particles of ultra-micro- 

 scopic size, each of which is the causal agent in bring- 

 ing about the development of some part of the organ- 

 ism. Weismann laid much emphasis on the concept 

 of the continuity of the germ-plasm, in contrast to 

 the soma, a matter already discussed. Development 

 thus becomes a mere sorting out of determinants, 

 and the organism is a sort of mosaic. One obvious 

 corollary of such an hypothesis is the fact that 

 nothing that may befall the soma after development 

 begins can have any influence in modifying the 

 result of development in a succeeding generation, 

 since each generation develops in strict accordance 

 with the determinants in the germ-cells. Most ex- 

 periments seem to prove that this is so. On the 

 other hand, the facts of regeneration and regulation, 

 just cited, are a strong argument against such an 

 inflexible mosaic development. For this and other 

 reasons, the elaborate and complicated architecture 

 which Weismann postulated to exist in the germ- 

 cells is not considered by modern biologists really to 

 exist. Nevertheless, for many reasons, particularly 



