THE LIFE CYCLE 47 



years has shown that different degrees of differentiation exist in 

 different reproductive cells, but has not afforded any real support 

 to the view that the morphological characters of the adult are 

 represented in some way by distinct entities in the germ. 1 But 

 even if we admit that organic differentiation is, at least to a large 

 extent, an epigenetic process, the real problem still remains. The 

 orderly and definite character of the process, the variety of struc- 

 tural features, and their apparent adaptation to the function which 

 they are to perform, all combine to render the problem one of the 

 greatest interest and significance. 



At present, however, it must suffice to call attention only to 

 certain aspects of the problem. In the first place, in so far as 

 differentiation is really a progressive or epigenetic process, it must 

 depend on changes of some sort in the dynamic processes in different 

 regions of the developing organism. We know that differentiation 

 in its specific features is to a large extent independent of external 

 conditions; therefore the internal conditions must determine these 

 changes. And this brings us to the important question: How 

 can such localized differences in the dynamic processes arise in the 

 developing organism ? The corpuscular theories have accustomed 

 us to regard different morphological parts of the organism as 

 qualitatively different, and it is evident that in many cases they 

 are, but it does not necessarily follow that the qualitative differences 

 are primary, or that all differentiations are qualitative. It is a well- 

 known fact that quantitative differences in the conditions existing 

 in a chemical reaction may result in qualitatively different products, 

 and this is demonstrated for many reactions which occur in the 

 metabolic complex. It cannot then be doubted that qualitative 

 differences may result from quantitative differences in the processes 

 occurring in the organism. We also know that many morpho- 

 logical differences are differences of size, shape, or quantity of some 



1 In view of the present vogue of the factorial hypothesis among investigators 

 in the field of genetics, and particularly of certain attempts to apply it to the chromo- 

 somes, such a statement may appear to many as at least unwarranted, if not incorrect. 

 The factorial hypothesis, however, does not necessarily involve the assumption of 

 factors as distinct entities in the germ, and the attempts to connect particular factors 

 with particular chromosomes or parts of chromosomes are not at present, properly 

 speaking, scientific hypotheses. 



