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 musclecells. 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 unnecessary. 
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 
