THE FACTORIAL HYPOTHESIS 277 
WEISMANN’S PRA&FORMATION HYPOTHESIS AND THE 
FactToriaL THEORY 
Weismann’s theory of development postulates par- 
ticles in the germ plasm that are sorted out in proper 
sequence to appropriate parts of the body as the 
embryonic cells divide. What determines the order 
of the sorting out of the factors was not explained. 
Weismann’s speculation differed from other preeform- 
ation theories mainly in that he made use of the 
chromosomal mechanism not only to carry the here- 
ditary materials, but also to bring about the sorting 
out of the materials in order to reach their final desti- 
nation in the body. His theory as applied to 
embryonic development failed, both because the facts 
concerning the behavior of the chromosomes during 
segmentation of the egg gave no support to his as- 
sumption of sorting out of the materials of the chromo- 
somes, and also because the data from experimental 
embryology and regeneration indicated very clearly 
that no such sorting process takes place. On the 
other hand, Weismann’s ideas of heredity concern- 
ing.the segregation in the reduction divisions of the 
-egg and sperm of inherited materials present in the 
chromosomes, furnish the basis of our present at- 
tempt to explain heredity in terms of the cell. 
In common with Weismann’s theory, the factorial 
theory of heredity rests on the assumption that the 
germ plasm contains a host of elements, that are in- 
dependent of each other in the sense that one allelo- 
morph may be substituted for another one without 
