278 ANIMAL BIOLOGY 



antithesis, epigenesis. But an explanation is not far to seek. The 

 difference apparently depends, as already suggested, upon the time 

 when chemical differentiation of the egg cytoplasm occurs and the 

 products are localized in special regions. If this occurs before or 

 at fertilization, so that the early divisions give rise to dissimilarly 

 organized cells, then each of the cells is not totipotent and the 

 mosaic type of development results ; but if the initial differentiation 

 and localization 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 equi- 

 potential system — as exhibited by the early stages of the Sea 

 Urchin. Thus we may bring under one viewpoint the apparently 

 contradictory behavior of the two classes of eggs, for it turns out 

 to be reducible to a common factor : the time of differentiation and 

 localization of the products. In one case this has progressed fur- 

 ther than in the other during the early embryonic stages. In both 

 cases, therefore, development is epigenetic in its obvious features. 



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 becomes : 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 architec- 

 ture, will lead him to anticipate that modern research tends more 

 and more to emphasize the gene as representing a material con- 

 figuration — apparently it is a protein molecule — which is trans- 

 mitted, 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, the genes constitut- 

 ing the chromosomes, is related to cytoplasmic organization and 

 to characters which arise later, we can offer no satisfactory expla- 

 nation or even guess. We must be content with a discussion, in 

 the next chapter, of some of the facts of heredity which show that 

 certain chromosomes are causally related to the inheritance of 

 certain characters. 



But in so far as the nucleus possesses an organization which is 

 definitely related to differentiations of the cytoplasm, organ- 

 forming substances, or characters of embryo and adult, we may 

 look upon the chromatin to this extent as representing a sort of 

 primary preformation which is realized by a process of building 



