DOCTRINE OF TYPES 123 



one would not expect the vertebrae to appear before the 

 embryo had passed through many Invertebrate stages. But 

 experience shows the direct contrary, for in the chick the 

 rudiments of the vertebral axis appear sooner than any other 

 part. 



The theory of parallelism or recapitulation then is not 

 borne out by the facts, and clearly cannot be the law which 

 we are seeking. But what then is the true relation between 

 the variety of development and the variety of adult structure ? 

 Before answering this question we must review the varied 

 forms of adult organisation and consider in what relations 

 they stand to one another. In particular we must enquire 

 whether they belong to one type or to many. One point is 

 here cardinal we must distinguish between the type of 

 organisation and the grade of differentiation. By "type" 

 von Baer means the structural plan of the organism. 

 " I call the type the spatial relationship of the organic 

 elements and organs " (p. 208). Each type of organisa- 

 tion characterises one of the big groups of animals ; the 

 lesser groups represent " grade " modifications of the 

 type. " The product of the degree of differentiation 

 and the type gives the several great groups of animals 

 which are called classes" (p. 208). Ansbildiuig (differen- 

 tiation) takes place in one or other of several directions, 

 in adaptation, for instance, to life in the water or to life in 

 the air. 



There are, von Baer considers, four main types (i) the 

 peripheral or radiate type, (2) the longitudinal type, (3) the 

 massive or molluscan type, (4) the vertebrate type. The 

 radiate type is shown by discoid infusoria, by medusae, by 

 starfish and their allies. The longitudinal type characterises 

 such genera as Vibrio, Filaria, Gordins, and all the annulate 

 animals. Mollusca, rotifers, polyzoa, and such infusoria as 

 are not included in types (i) and (2) belong to the massive 

 type, in which the body and its parts form rounded masses. 

 The longitudinal type is predominantly "animal," the 

 massive type predominantly "plastic" (vegetative). The 

 vertebrate type has both the " animal " and the " plastic " 

 organs highly developed. In the symmetrical arrangement 

 of the animal parts it resembles the longitudinal type ; its 



