DIFFERENTIATION, HEREDITY, SEX 265 



is of definite, specific, organismal structure. This cytoplasmic 

 structure of the germ is regarded as continuous from one 

 generation of ova to the next, through the germ within the 

 organism, and it thus serves as the physical basis of heredity. 



As more or less opposed to this conception we have a group 

 of hypotheses which may collectively be termed the hypothesis 

 of " nuclear analysis." Here the nucleus alone is regarded as 

 that part of the germ or zygote which bears a specific relation to 

 later differentiations, and within the nucleus, the chromosomes 

 are the elements chiefly concerned. Chromosomes are supposed 

 to possess a structural predetermination that is promorpho- 

 logical; they are unlike and individually behave specifically 

 in determining the characteristics of developmental reactions. 

 This hypothesis, in its various forms, is associated chiefly with 

 the names of Nageli, Roux, Weismann, DeVries, and Oscar 

 Hertwig. It will be recognized as also preformational in its 

 essentials; specific configurations of chromatin represent 

 potentially, corresponding embryonic and adult traits, which 

 become actual by a truly epigenetic series of developmental 

 reactions. 



It has been pointed out frequently that this hypothesis really 

 transfers the idea of germinal localization from the cytoplasm 

 to the nucleus; the structure of the cytoplasm is regarded as 

 real, but as secondary and dependent upon the primary 

 structure of the nuclear elements. These nuclear organs, the 

 chromosomes, are thought to maintain a specific physiological 

 continuity from one generation of ova to the next and thus 

 to constitute a real physical basis of heredity. 



These two general hypotheses have in common the idea of a 

 fixed promorphological structure within the germ which becomes 

 expressed epigenetically. They differ as regards the particular 

 part of the germ whose promorphology is to be regarded as 

 primary, and yet it is quite possible that both hypotheses 

 contain elements of truth. It remains now for us to review 

 some of the more significant facts of development bearing 

 directly upon these ideas in order to determine, perhaps which, 

 perhaps how much of each, is justified. In doing this we shall 



