280 THE FACTORIAL HYPOTHESIS 
disintegration of the germ plasm when the body cells 
are produced in order to account for the localization 
of characters; the other view, following the experi- 
mental results and microscopical observations, as- 
sumes, so far as the chromosomal materials are con- 
cerned, that all of the hereditary factors are present 
in every cell in the body. This view is essentially 
that proposed by DeVries in his book on Intracellular 
Pangenesis. The cause of the differentiation of the 
cells of the embryo is not explained on the factorial 
hypothesis of heredity. On the factorial hypothesis 
the factors are conceived as chemical materials in the 
egg, which, like all chemical bodies, have definite 
composition. The characters of the organism are 
far removed, in all likelihood, from these materials. 
Between the two lies the whole world of embryonic 
development in which many and varied reactions 
take place before the end result, the character, emerges. 
Obviously, however, if every cell in the body of one 
individual has one complex, and every cell in the 
body of another individual has another complex that 
differs from the former by one difference, we can treat 
the two systems as two complexes quite irrespective 
of what development does so long as development is 
orderly. 
It is sometimes said that our theories of heredity 
must remain superficial until we know something 
of the reactions that transform the egg into the adult. 
There can be no question of the paramount impor- 
tance of finding out what takes place during develop- 
ment. The efforts of all students of experimental 
