402 FOUNDATIONS OF BIOLOGY 



and to reach a level of analysis deep enough to suggest how 

 "the whole future organism is potentially and materially im- 

 plicit in the fertilized egg cell" and thus that "the preforma- 

 tionist doctrine had a well-concealed kernel of truth within 

 its thick husk of error." 



The next great advance came in the accurate and compre- 

 hensive studies of the Russian, VON BAER (1792-1876), pub- 



Fio. 207. Karl Ernst von Baer. 



lished in the thirties of the last century. Taking his material 

 from all the chief groups of higher animals, von Baer founded 

 COMPARATIVE EMBRYOLOGY. Among his achievements may be 

 mentioned: the clear discrimination of the chief developmental 

 stages, such as cleavage of the egg, germ layer formation, tis- 

 sue and organ differentiation; the insistence on the importance 

 of the facts of development for classification; and the dis- 

 covery of the egg of Mammals. His observations on the 



