HEREDITY 345 



Preformation and Epigenesis. Having sketched the outUnes 

 of development and traced the chief principles of inheritance, we 

 are now in position to appreciate the two opposite interpretations of 

 development. Begin by facing the problem in a simple statement. 

 Before us is an egg; after a rapid series of changes, before us is an 

 individual consisting of millions of cells, highly organized and 

 intricately constructed. Is this highly organized adult present in 

 miniature in the tgg? Or is development a progressive process, as 

 a chain of technical processes in a factory convert in an orderly 

 fashion crude materials into an end-product? The first interpreta- 

 tion is known as preformation and the second as epigenesis. 



These two views both have long histories and both have colored 

 and still influence biological theory. A moment's thought will con- 

 vince one that only these two interpretations are possible; either 

 development is an unfolding of the egg potencies, or it is a series of 

 reactions that are initiated by fertilization, which build up the adult 

 from outside materials. In their earliest forms both concepts partook 

 of the ridiculous. Early preformationists claimed to have observed a 

 miniature human in a sperm cell. One early worker calculated the 

 possible number of miniature human bodies in the ovary of the 

 Hebrew mother Eve. The earliest form of the doctrine of epigenesis 

 was primarily vitalistic, holding that an outside supernatural power 

 governs the formation of the adult from the tgg. 



Preformation in modern Biology takes the form of the self- 

 evident fact that the protoplasm of a species is peculiar, that only the 

 adult of a particular species can arise from the species ovum. It 

 recognizes the importance of the fact that the egg is organized and 

 that the organization of the adult is accomplished by reason of this 

 basic organization. In the hands of some biologists the gene theory 

 of inheritance becomes primarily a preformationist doctrine; it is 

 held that the genes are in some way actual representatives of adult 

 structures. 



Epigenesis in its modern form regards development as a serial 



