364 ESSAY ON CLASSIFICATION. 



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



a. Incloses the embryonal vesicle entirely. 



. This takes place very early. GASTEROPODA and ACEPHALA. 

 /8. This takes place late. (Temporary vitelliue sac.) LIMAX. 



b. Contracts above the embryonal vesicle. (Genuine vitelline 



sac.) CEPHALOPODA. 



B. The whole body of the embryo arises simultaneously. (Eoolutio ex om- 

 nibus partibus.) 

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



a. With its hind body. RADIATA. (Echiuoderms.) 



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. WOKMS. 



I have already shown how unnatural a zoological sys- 

 tem must be which is based upon a distinction between 

 total or partial segmentation of the yolk. 1 No more can 

 a diagram of the development of animals which adopts 

 this difference as fundamental be true to nature, 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 ascertain their general relations, as 

 Cuvier and K. E. von Baer have so beautifully shown. 2 

 I think, also, that the homology of the limbs of Articulata 

 and the dorsal plates of Vertebrata is more than ques- 

 tionable, The distinction, introduced between Polyps and 

 Acalephs and these and the other Radiates, is not at all 

 better founded. It seems also quite inappropriate to call 



1 Chap. III., Sect. 1, p. 288. die gegenseitige Verwandtschaft der 



1 The principles of classification Thiere zu erlangen, die verschiedenen 



advocated by Baer are so clearly ex- Organisationstypen von den verschi- 



pressed by him that I cannot resist edenen Stufen der Ausbildung stets 



the temptation of quoting some pas- imterscheiclen. Das man diesen Un- 



sages from the paper already men- terschied gewohnlich nicht ira Auge 



tioiied above (p. 351), especially now, behalten hat, scheint uns zu den son- 



whenlfeel called upon to oppose the derbarsten Zusammenstellungen ge- 



views of one of his most distinguished fiihrt zu haben." Beitrag'e, etc. 



colleagues. " Vor alien Dingen muss Acta Nova, vol. 13, p. 739. 

 man, uni cine richtige Eiusicht in 



