236 K. E.VON BAER. PHILOSOPHICAL FRAGMENTS. 



the brain, parts which must therefore necessarily be absent in 

 other types. I would here render perfectly obvious how it is 

 the scheme of development which produces the principal cha- 

 racter of the animal. If we suppose that, in any articulate 

 animal which is in the earliest stage of its development, a part 

 of the germ should become raised up on both sides, and so 

 enclose a portion of the external surface, the enclosed part 

 would be an animal central portion. The internal organs 

 would all have the same relation to it as in the Vertebrata, ex- 

 cepting the plastic nerves, which by the influence of the ani- 

 mal nervous system appear to be approximated to this last in 

 the Vertebrata. 



In relation to the external world, however, all the internal 

 organs would be inverted, since the central part itself would 

 lie downwards. If we were to invert the animal, all the outer 

 parts, the extremities and the organs of sense, would be dis- 

 placed ; and supposing that the extensor and flexor sides had 

 not undergone inversion by the addition of the new central part, 

 these also. 



Hence we conclude, that, by the origin of a central part for the 

 animal body, the position of the plastic organs, and their relation 

 to the nearest animal layer, have indeed remained unchanged ; 

 but their relation to the external world, and all which represents 

 this relation in the body, has become inverted. In the former 

 case, where the course of development is simply symmetrical, 

 the central line from which it proceeds becomes the flexor side 

 of the animal ; with a doubly symmetrical development, the side 

 from which it proceeds becomes the extensor side. Towards the 

 flexor side, the extremities and the feet are developed. By this 

 it shows itself to be that which is most essentially turned towards 

 the planet. Towards the extensor side, that turned from the 

 ground, the organs of the senses are developed. 



I commenced this Corollary with the remark, that animals 

 ought to be divided according to their mode of development, 

 and I have shown sufficiently at length that the principal types 

 have their own form of development. I may be permitted to 

 point out here, in a few words, that the safest guide for further 

 division would be found in the history of development, if we 

 were acquainted with it with sufficient exactness, in the different 



