258 FOUNDATIONS OF BIOLOGY 



larly organized cells, then each of the cells is not equipotent 

 and the mosaic type of development results; but if the initial 

 differentiation is delayed until later, or is relatively slight so 

 that the cells of the early stages are all essentially similar, 

 then during this period each cell is totipotent the whole 

 forms an equipotential system as exhibited by the early 

 stages of the Sea Urchin. Thus we may bring under one 

 viewpoint the apparently paradoxical behavior of the two 

 classes of eggs, for it turns out to be reducible to the common 

 factor, differentiation. In one case this has progressed 

 further than in the other during the early embryonic stages. 

 In both cases, therefore, development is epigenetic in its 

 obvious features. (Fig. 134.) 



However, since cytoplasmic differentiation is a fact 

 whether it appears early or late, we have merely pushed the 

 solution of the problem further back and the question be- 

 comes: Is there a primary differentiation and, if so, where? 

 It is not possible to present here the specific evidence on 

 this point, but the reader's knowledge of the nucleus, and 

 particularly its definite chromosomal architecture, will lead 

 him to anticipate that modern research tends more and more 

 to emphasize - the chromosome as representing a material 

 configuration a packet of chemicals, may we say which 

 is transmitted, in a way, 'preformed' from generation to 

 generation and determines the cytoplasmic characteristics of 

 the cells. As to how the specific physical basis of inheritance, 

 constituting the chromosomes, is related to cytoplasmic 

 organization and to characters which arise later, we can offer 

 no satisfactory explanation or even guess. We must be 

 content with a discussion, in the next chapter, of some of the 

 salient facts of heredity and their definite association with 

 certain chromosome arrangements. 



But in so far as the nucleus possesses an organization 



