ORIGIN OF THE INDIVIDUAL 



259 



which is definitely related to differentiations of the cyto- 

 plasm, 'organ forming substances,' or characters of embryo 

 and adult, we may look upon the chromatin to this extent as 



D E 



FIQ. 134. Diagram of zones of cytoplasmic differentiation and their distribution 

 at the first division of the egg. A, immature egg, assumed to have no definite segrega- 

 tion of cytoplasmic stuffs; B, mature egg, with cytoplasmic zones established; C, first 

 division of egg; D and E, two types of two-cell stages; D, Dentalium or Cynthia type, 

 with one cytoplasmic zone entirely distributed to one of the cells, and therefore each of 

 the two cells, if separated, gives rise to an abnormal larva; E, Echinoderm or Amphioxus 

 type, with equal distribution of the zones to both cells, and therefore, if separated, each 

 of the two cells gives rise to a normal larva. (After Wilson.) 



representing a sort of primary preformation which is real- 

 ized by a process of building up epigenesis as one char- 

 acter after another becomes established in the development 

 of the individual. This is the guise in which the old problem 

 of preformation versus epigenesis faces the biologist to-day. 



