Manchester Memoirs, Vol. I. {igo6), No. 3. 89 



the embryo and between the parts and the whole ; he has 

 to discover, in a word, what are the external and what 

 the internal factors which govern the process of differentia- 

 tion. Differentiation sets in with the separation of those 

 elementary embryonic organs which we are accustomed 

 to speak of as the layers of the germ ; it is in a precise 

 physiological study of these organs that we must look for 

 the clue to one of the greatest of biological problems, the 

 problem of the epigenetic evolution of the complexity of 

 the adult from the apparent simplicity of the fertilized 

 ovum. 



