Embryology. 123 



that while his predecessors had restricted their attention 

 almost exclusively to the readily available chick, he has 

 the credit of founding- comparative embryology. As 

 Bergh says, Von Baer broadened embryology as Cuvier 

 had broadened anatomy, by making it comparative. 

 He thus paved the way for Johannes Miiller and his 

 famous school, and there is a fairly continuous filiation 

 from Von Baer to Balfour. 



It was Von Baer, also, who first showed the import- 

 ance of embryology as an aid to classification, and 

 although his actual achievements in this connection are 

 hardly acceptable nowadays, he has the credit of first 

 suggestion. Even those who are now very cautious as 

 to the use of "the embryological criterion of homo- 

 logy ", will allow that without it the problems of rela- 

 tionship would be much more obscure than they are. 



It was Von Baer who first clearly discriminated the 

 great events in a life-history : (a) The primary processes 

 of egg-cleavage, and the establishment of the germinal 

 layers; (b) the gradual differentiation of the tissues 

 (histogenesis) ; and (c) the blocking-out of the organs 

 (organogenesis), and the shape-taking of the entire 

 organism (morphogenesis). 



But Von Baer is, perhaps, best remembered on 

 account of his formulation of certain laws of develop- 

 ment, which are discussed later on. What is often 

 called "Von Baer's law", is the generalization that the 

 individual development recapitulates the racial history, 

 but it is by no means correct to father this hazardous 

 conclusion on Von Baer. On the contrary, it was one 

 of his endeavours to show that this generalization, care- 

 lessly credited to him, was far from correct. 



The broadening out of embryological inquiry, which 

 began with Von Baer, was continued in the work of 

 Ratke, Kolliker, Lovdn, Sars, Johannes Miiller, Kowa- 

 levsky, Metschnikoff, and many others, until it became 

 possible for Francis Balfour to gather up a thousand 

 scattered papers into an ordered whole in his epoch- 

 making work on comparative embryology (1880-1881). 



Early in the century the poet Chamisso, who accom- 

 panied Kotzebue on his circumnavigation of the globe, 



