every form could be considered a type -change from. a more 

 general form; even the last is a modification of the basic 

 type. Hence in individual representatives of a type, some 

 organs are more developed, and in others, other organs. 

 Baer's rejection of the linear succession of developing 

 individual organs led him to deny the possibility of reverse 

 development. In some undefined form he stated, at the end 

 of this corollary, an idea about the fact that development 

 is always progressive and in the animal world leads to the 

 foremost developed system, the cephalic brain. 



The extensive fourth corollary, entitled "The division 

 of animals according to their method of development," is 

 devoted first, to the developmental difference between plants 

 and animals, and secondly, to the differences of individual 

 groups of animals based on processes of development. Such 

 differences correspond to the animal type. Thus, for the 

 vertebrates it is characteristic, according to Baer, to have 

 a double symmetrical development, which he himself studied 

 in detail. Typical for vertebrate development is the 

 formation of two tubes, which are closed at the spinal and 

 abdominal ends and divided at the longitudinal axis by the 

 spinal or vertebral cord. To the elongated animal the 

 symmetrical type of development is inherent, which leads 

 to the formation of one symmetrical tube closed along the 

 abdominal side to the spinal side. It is right to compare 

 symmetrical development with double symmetrical development, 

 but not to deduce one type from the other. 



The preliminary data, obtained by Baer from his embryo- 

 logical study of bivalved molluscs and snails, led him to 

 conclude that this development proceeds according to the 

 principle of transference of the developing parts into 

 spirals; therefore the molluscs have a waved form of 

 development. Baer had little data concerning development 

 of the peripheral or radiate type. 



From comparing the methods of development of animals 

 of the four main types, Baer concluded that "each main type 

 follows a peculiar plan of development." The connection 

 between the character or plan of development he designated 

 by the following aphorism: "The plan of development is 

 nothing other than the emerging type, and the type the result 



360 



