298 GERM-CELL CYCLE IN ANIMALS 



differentiation and therefore consists of real tissue 

 cells. Those parenchymal cells that encounter 

 certain conditions become germ cells. Later (1906) 

 the same author gave an account of the development 

 of spermatogonia in the same animal from the dif- 

 ferentiated muscle cells. These studies, together with 

 the results from experiments on regeneration, have led 

 Child (1912) to the belief ''that this germ-plasm 

 hypothesis and the subsidiary hypotheses which 

 have grown up about it are not only unnecessary 

 and constitute an impediment to biological thought, 

 which has retarded its progress in recent years to a 

 very appreciable extent, but furthermore, that they 

 are not in full accord with observed facts and can 

 be maintained only so long as we ignore the facts." 

 He further maintains that if protoplasm is a physico- 

 chemical substance it is capable of changing its con- 

 stitution in any direction according to the conditions 

 imposed upon it, and that therefore the continuous 

 existence of a germ-plasm with a given specific 

 constitution is vmnecessary. 



The evidence in favor of the germ-plasm theory 

 is so strong that the arguments thus far advanced 

 against it have had but little influence. If, then, we 

 accept germinal continuity as a fact and consider 

 the germ-plasm to be a substance that is not con- 

 taminated by the body in which it lies, but remains 

 inviolate generation after generation, we should next 

 inquire as to the nature of this substance. The 

 generally accepted idea is that the chromatin of the 

 nucleus represents the physical basis of heredity. In 



