114 KARL ERNST VON BAER 



der Thicrc. Beobachtnng iuid Reflexion (Konigsberg, 1828). 

 The second volume followed in 1837, but dates really from 

 1834, and was published in an incomplete form. This second 

 volume is intended as an introduction to embryology for the 

 use of doctors and science students. In it von Baer describes 

 in full detail the development of many vertebrate types 

 chick, tortoise, snake, lizard, frog, fish, several mammals and 

 man, basing his remarks largely upon his personal observa- 

 tions, but taking account also of all contemporary work. A 

 separate account of the development of a fish {Cyprinus blicca) 

 appeared in I835. 1 



We shall concentrate attention on the first volume. This 

 volume contains the first full and adequate account of the 

 development of the chick, followed by a masterly discussion 

 of the laws of development in general. 



When we consider that von Baer worked chiefly with a 

 simple microscope and dissecting needles, the minuteness 

 and accuracy of his observations are astonishing. He 

 described the main facts respecting the development of all the 

 principal organs, and if, through lack of the proper means of 

 observation, he erred in detail, he made up for it by his 

 masterly understanding and profound analysis of the essential 

 nature of development. His account of the development of 

 the chick is a model of what a scientific memoir ought 

 to be ; the series of " Scholia " which follow contain the 

 deductions he made from the data, and, in so far as they are 

 direct generalisations from experience, they are valid for 

 all time. 



The first Scholion is directed against the theory of 

 preformation, and succeeds in refuting it on the ground of 

 simple observation. The theme of the second Scholion is 

 that the essential nature (die ]] 7 esenheit} of the animal 

 determines its differentiation, that no stage of development 

 is solely determined by the antecedent stage, but that 

 throughout all stages the M'csoiheit or idea of the 

 definitive whole exercises guidance. This guidance is shown 

 most clearly in the regulatory processes of the germ, whereby 

 the large individual variations commonly presented by the 



' Untersuchungen it. die I'.nt-.'ickelun%sgcsclri<.htc tier FiscJic ; Leipzig 

 1835. 



