192 GENERAL BIOLOGY 



boxes, each one inclosing a series of increasingly 

 smaller ones. Indeed the theory was called the 

 " encasement " theory. 



Epigenesis. Wolff, the father of modem em- 

 bryology, investigated the developing chick in the 

 shell, discovering, among other things, that the heart 

 actually comes into existence after development 

 begins, and was forced to conclude that there is no 

 evidence whatever of the preexistence of the chick 

 in the germ of the egg. In his Theoria Generationis 

 (1759) he advanced the hypothesis of epigenesis, 

 according to which the development of the germ 

 involves the coming into existence of new structures 

 with each generation. It was thus in direct con- 

 flict with the preformation concept held by the 

 majority of eighteenth-century naturalists and 

 philosophers. As for the means of this epigenetic 

 development, he conceived of a specific internal 

 energy or force (vis essentialis) that permeates living 

 matter. Development (in the hen's egg) is not 

 brought about by the heat of incubation, but by 

 the operation of this somewhat mystic internal 

 force. Wolff's results and speculations were a long 

 time in gaining acceptance, but the gradual im- 

 provement in microscopes, and in technique, 

 made it impossible to accept the naive preforma- 

 tion of the earlier school, and the biological 

 world became persuaded to the epigenetic way of 

 thinking, without, however, accepting the "vis 

 essentialis." 



