44 THE EVOLUTION OF MAN. 



phases of its evolution an internal structure and an external 

 form totally different from those of the developed organism. 

 In none of these phases do we find any pre-formed parts ; 

 nowhere any encasement. In these days we can scarcely 

 continue to call this Theory of Epigenesis a theory, for we 

 have been thoroughly convinced of its correctness in fact, 

 and we are able to demonstrate it in any moment under the 

 microscope. Nor, during the last decade, has any doubt of 

 the truth of Epigenesis been expressed. 



Wolff supplied detailed proof of his Theory of Epigenesis 

 in his scholarly treatise "On the Formation of the Intestinal 

 Canal (1768)." In its complete condition the intestinal 

 canal of the Chick is a very complex, long tube, to which 

 the lungs, the liver, the salivary, and many smaller glands 

 are attached. Wolff showed that there is no trace of this 

 complex tube, with all its various parts, in the embryo 

 Chick during the first period of incubation, but that in its 

 place there is a flat, leaf-shaped body ; and that the whole 

 embryo-body in the earliest period is also of a flat, oblong, 

 leaf-like form. Considering the difficulty of accurately ex- 

 amining conditions so extremely minute and delicate as the 

 first leaf-shaped beginnings of the body of the bird with the 

 indifferent microscopes of the last century, we cannot but 

 admire the rare talent for observation possessed by Wolff', 

 who actually proved the most important facts known in 

 this the darkest portion of Embryology. From this very 

 difficult investigation he even drew the correct conclusion 

 that the entire embryonic body of all higher animals, as 

 well as of birds, is for a while a flat, thin, leaf-shaped 

 plate, which at first appears simple, but subsequently 

 as if composed of several layers. The lowest of all these 



