DEVELOPMENT 



279 



up — epigenesis — as one character after another becomes estab- 

 lished in the development of the individual. This is the guise in 

 which the old problem of preformation versus epigenesis faces the 

 biologist to-day. 



So the early embryologists were right when, studying the egg 

 of the Frog or Hen, they maintained that development is develop- 



B 



D E 



Fig. 178B. — Diagram to illustrate how the character of the first division 

 of an egg may influence the distribution of the products of cytoplasmic differ- 

 entiation and therefore the potentialities of the resulting cells. A, immature 

 egg, assumed to have no definite segregation 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, 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, 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. 

 (From Wilson.) 



ment and not merely the unfolding of an organism already fash- 

 ioned in more or less definite adult form. But it took two centuries 

 of research to reveal the fact that, below and beyond its super- 

 ficial aspects, there is a germ of truth in the principle of preforma- 



