K. E, VON BAER. PHILOSOPHICAL FRAGMENTS. 185 



haps resemble a Vibrio or a Thread-worm ; but never a Medusa 

 or an Ascidian. In the Vertebrata, indeed, it has been attempted 

 to demonstrate an agreement with Medusae in the distribution 

 of the vessels in the germinal membrane ! Here the only thing 

 overlooked was the body of the embryo. In this the essential 

 character of the type is the first to arise; the formation of the 

 vertebrate animal begins with the rudiment of the vertebral 

 column, proceeding from which the dorsal folds pass up- 

 wards and the visceral plates downwards, so as to lay the founda- 

 tions of the upper and lower halves of the body, which charac- 

 terize this form. 



Our view explains also, how the external forms of animal 

 bodies are connected with their vital manifestations, and with the 

 arrangement of the internal parts. The separate organic ele- 

 ments develope themselves into a higher state of perfection by 

 differentiation of the animal substance. Their form and con- 

 nexion, however, depend upon the type which predominates in 

 the animal, or in a principal part of it. Hence from the form of 

 the exterior one can conclude the arrangement of the interior. 

 If we examine, for instance, an Octopus, we see the molluscous 

 structure only in the sac-like part of the body, while in the head 

 the motor organs are arranged in a completely radiate manner 

 around a central point. In the middle is the aperture of the 

 mouth, and it corresponds completely with the type of the ra- 

 diate animal, that the nerves and vessels, before they pass into 

 the separate arms, are combined into a circle in the median disc. 

 We know again that the Cephalopoda most frequently swim with 

 their heads directed downwards ; the radiate portion of the body 

 therefore seems to hold and to move itself in accordance with 

 the mode of locomotion prevalent in its type, overcoming the 

 tendency of the molluscous body. Even in the higher animals 

 we find that when single parts take on the external form of the 

 radiate type, we have a corresponding order of the nerves and 

 vessels. Thus it is with the iris of the Vertebrata. 



Furthermore, our conception explains how, according to the 

 nature of the different types, a given organ arises, and becomes 

 developed earlier in one than in another ; how it is again that 

 the perfection of a single organ does not imply a general corre- 

 spondence in development. It is one of the characters of our 



