PRINCIPAL SYSTEMS OF ZOOLOGY 255 



A. The embryo arises from a primitive part. (Evolutio ex una parte.) 



1°. It grows in two directions, with bilateral symmetry. (Evolutio bigemina.) 



a. The dorsal plates close up. Vertebrata. 



b. The dorsal plates remain open and are transformed into limbs. Articulata. 

 2°. It grows uniformly in every direction. (Evolutio radiata.) And 



a. Incloses the embryonal vesicle entirely. 



a. This takes place very early. Gasteropoda and Acephala. 

 ^. This takes place late. (Temporary vitelline sac.) LLmax. 



b. Contracts above the embryonal vesicle. (Genuine vitelline sac.) Cephalopoda. 



B. The whole body of the embryo arises simultaneously. (Evolutio ex omnibus partibus.) 



1°. It grows in the direction of its transverse axis, 



a. With its hind body. Radiata. (Echinoderms.) 



b. With the fore body, and 



a- The hind body does not grow. Acalephs. 

 /3. The hind body grows longitudinally. Polypi. 

 2°. It grows in the direction of its longitudinal axis. Worms. 



I have already shown how unnatural a zoological system must be 

 which is based upon a distinction between total or partial segmen- 

 tation of the yolk. No more can a diagram of the development of 

 animals which adopts this difference as fundamental be true to na- 

 ture, even though it is based upon real facts. We ought never to 

 single out isolated features by which animals may be united or 

 separated, as most anatomists do; our aim should rather be to ascer- 

 tain their general relations, as Cuvier and K. E. von Baer have so 

 beautifully shown. I think also that the homology of the limbs of 

 Articidata and the dorsal plates of Vertebrata is more than question- 

 able. The distinction introduced between Polyps and Acalephs and 

 these and the other Radiates is not any better founded. It seems also 

 quite inappropriate to call the development of Mollusks evolutio 

 radiata, especially after Baer had designated under that same name 

 the mode of formation of the branch of Radiates, for which it is 

 far better adapted. 



The classification of Vogt^*^ presents several new features, one of 

 which is particularly objectionable. I mean the separation of the 

 Cephalopoda from the other Mollusks, as a distinct primary division 

 of the animal kingdom. Having adopted the fundamental distinction 

 introduced by Kolliker between the animals in which the embryo 

 is developed from the whole yolk, and those in which it arises from 

 a distinct part of it, Vogt was no doubt led to this step in conse- 

 quence of his interesting investigations upon Actason, in which he 



^ Zoologische Brieve; see above, n. 3. 



