the basis of our knowledge of growth and 

 development of animals is obtained from the 

 fortunate opportunity of continued studies . 

 What we know of the development of other 

 animals is for most classes, especially the 

 mammals, to which the physical aspects of 

 man also belong , so isolated, that it can 

 only be understood by comparison with the 

 development of birds. (II, If, p. 16 (p. 8 )) 



And although Baer, as a result, anticipated that the 

 features of similarity of birds and other vertebrates, as 

 well as their specific peculiarities, might be inherent 

 only to avian development, he later did not pay attention 

 to his own warning. Describing the development of the 

 egg in the ovary, the structure of the fertilized egg, 

 the formulation of embryonic organs and membranes and so 

 on in reptiles, birds, and mammals, Baer strove not to 

 keep finding complete parallels. Sometimes he did not 

 see those new formations characteristic of the higher 

 classes of vertebrates. Note that it is impossible to 

 forget about Baer's great service. He was the first to 

 show with complete distinctness the fruitful comparative 

 method in embryology, although he himself used it only 

 within the class of vertebrates and did not see the possi- 

 bility of comparing regularity of development in animals 

 belonging to different morphological types. 



In order that his listeners could establish an objec- 

 tive judgment, and in order that they would not draw general 

 conclusions for their own dogmatism, Baer suggested the 

 following. He intended to begin by stating the history of 

 bird development, then to compare the most essential from 

 the history of development of other classes of animals in 

 order to get to the most important problem — the elucidation 

 of reproduction and development. 



The last division of the book, treating general questions 

 of embryology, was never written, and because the second 

 volume of UBER ENTWICKLUNGSGESCHICHTE was printed 

 without the author's participation, the missing part remained 

 only a reference in the introduction. 



371 



