the main types are combined in one middle type, or in which 

 one half of the body represents one type, and the other 

 half represents another." But Baer did not review these 

 intermediate forms in the present work, in order to set off 

 the peculiarities of the four main types ("Haupt-Typen") : 

 the peripheral or radiate, the articulated or elongated, 

 the massive or mollusc, and the vertebrate. 



Baer studied the types of organization for analysis of 

 individual development (219 4a) . Embryonic development 

 involves an increase of the "degrees of development, or 

 histological and morphological differentiation. It is 

 possible to show that between the embryos of higher animals 

 and the constant forms of the lower animals there exists a 

 certain relationship. This, however, does not represent 

 evidence that "each embryo of the higher animal form 

 gradually passes through the form of the lower animals," 

 according to Baer. On the contrary, "the type of each 

 animal from the beginning is fixed in the embryo and governs 

 all development." 



All work on chick development Baer considered "only a 

 long comment to this confirmation." Before anything else 

 the spinal column is separated, then the spinal and abdominal 

 plates, and also the spinal cord. All these rudiments appear 

 very early, and after their appearance "there could not be 

 any speech about the correspondence of the embryo to any 

 invertebrate animal." On the contrary, the first recognized 

 are the properties characteristic of the vertebrate animal. 

 This is true not only for the bird embryo but also for all 

 classes of vertebrates, from which Baer concluded that "the 

 embryo of the vertebrate animals is from the first a verte- 

 brate." Because no adult vertebrate animals exist which 

 could be characterized with no little histological and 

 morphological differentiation as the young chicken embryo, 

 Baer concluded that "the vertebrate embryo in its development 

 does not pass through the stages ... of adult animal form." 



In the formation of individuals, it is possible to 

 establish a general regularity characteristic of the type. 

 Such a rule is represented by the greater early similarity 

 of embryos of different vertebrates and the approach of a 

 later moment of original development. The peculiarities, 



353 



