THE FACTORIAL HYPOTHESIS 223 



WEISMANN'S PR^EFORMATION HYPOTHESIS AND THE 

 FACTORIAL 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 prseform- 

 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 



