124 KARL ERNST VON BAER 



plastic parts with their asymmetrical arrangement and 

 rounded shape belong to the massive type. 



These types of von Baer inevitably recall the " Embranche- 

 ments" of Cuvier, with which they more or less coincide. It 

 seems that von Baer arrived at his types (from the study of 

 adult structure) independently of Cuvier, though the priority 

 of publication rests with Cuvier. 1 



Now it is clear that the development of the individual, 

 which is essentially an Aitsbildniig, a differentiation, is 

 directly comparable with the grade-differentiation of forms 

 within the type. And just as the type rules all its varied 

 modifications, so does the development of the individual 

 take place always within the bounds imposed by type. This 

 is von Baer's chief contribution to the theory of embryonic 

 relationships the law that "the type of organisation 

 determines the manner of development " (p. xxii.). Develop- 

 ment is not merely from the general to the special there arc 

 at least four distinct " general " types, from which the special 

 is developed. The type is fixed in the very earliest stages of 

 development the embryo of a Vertebrate is from the very 

 beginning a Vertebrate (p. 220), and it shows at no time any 

 agreement in total organisation with any In vertebrate. The 

 types are independent of one another ; differentiation and 

 development follow a different course in each of them. Not 

 but what some analogies can be found between the "very 

 earliest stages of embryos of different type. Thus vertebrate 

 and annulate embryos agree in certain points at the time of 

 the formation of the primitive streak. And in the earliest 

 stage of all, the egg-stage, there is probably agreement 

 between all the types. In eggs with yolk, whether vertebrate 

 or annulate, there is always a separation into an animal and 

 a plastic layer. It seems, too, as if a hollow sphere were a 

 constant stage in the development of all animals (pp. 224, 258). 

 Apart from these analogies, development takes an entirely 

 independent course in each of the four main types, and no 

 embryo of one of the higher types repeats in its development 

 the peculiar organisation of any adult of the lower types. 



1 Cuvier, in 1812, Ann. MHS. if' /fist. Nat., xix. ; von llacr in 1816, 

 Nova Acta Acdd. l\>it. ('/ft: Sec Entwickelungsgcsckichte dcr Thiere, 

 i., p. vii., f.n. 



