ONTOGENY 23 



ment. Indeed it seems now to have been merely a restate- 

 ment of the fact that development occurs, leaving this fact to 

 be explained through the operation of some supernatural or 

 miraculous process, for the spontaneous generation of the 

 embryo within the egg was at first definitely assumed. 



Thus we have almost from the beginning of embryological 

 study, two opposing explanations of the visible phenomena 

 of development, preformation explaining development by de- 

 nying it, epigenesis explaining development by reaffirming it. 

 Since this early conflict of opinions, the crudity of which we un- 

 derstand when we think of the means then at hand for observing 

 such minute objects as are many eggs and embryos, there has 

 been constant opposition of morphological and physiological 

 interpretations of development. The modern understanding 

 of preformation is better termed predelineation, or better still, 

 predetermination, less crude, less complete and particular than 

 preformation. What is preformed or predetermined in the 

 germ in some way represents the embryo without being at 

 all like it. The idea of epigenesis, too, is to-day less complete; 

 a certain structural organization is admittedly present in the 

 germ as a heritage from previous generations, and real develop- 

 ment occurs as a physiological process directed by this rudi- 

 mentary structure already present. The history of these opin- 

 ions indicates that neither conception is exclusively true, but 

 that development must involve both predetermination and 

 epigenesis; and the present endeavor is to find out not which, 

 but to what extent each, is true. 



The present understanding of development seems to be an 

 extremely refined predetermination strongly tinged with epi- 

 genesis, using these words in their modern sense. A more 

 extended statement of this modern view and the facts upon 

 which it is based is reserved for a later chapter (Chapter VII). 

 Briefly stated, we believe that while the embryo, not to say 

 adult, is by no means preformed nor even fully predelineated 

 in the germ, yet there is a certain degree of protoplasmic 

 structure or regional differentiation in the germ cells. This is 

 spoken of now as the organization of the germ, and it may be 



