CiiAP. III. EMBRYOLOGICAL SYSTEMS. 229 



DIAGRAM OF TIIK DEVELOPMENT OF ANIMALS BY KOLLIKER. 



Koi.LiKKH, (A.,) in liis Eiitwickelungsgeschichte dcr Cephalopoilen, Zurirli, 181J, 1 voL 4to., \i. 17.3, 

 has subinittcil llic folluwinf; diagram of tin; development of tlie animal kingdom. 



A. Tlie embryo arises from a priniilivc part. (Evolutio ex una parte.) 



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



a. The dorsal jjlates close up. Vertebrata. 



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

 2°. It gi'ows uniformly in every direction. (Evolutio radiata.) And 



a. Incloses the embryonal vesicle entirely. 



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

 ^. This takes place late. (Temporary vitelline sac.) L i m a x . 



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



A. The wliole body of the embryo arises simultaneously. (Evolutio ex omnibus partibus.) 

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



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



b. With the fore body, and 



n. The hind body does not grow. Acalephs. 

 ^. The hind body grows longitudinally. Polypi. 

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



I have alre.ad}' sllO^VIl how unnatural a zoological system must be which is 

 based upon a distinction between total or partial segmentation of the yolk.^ No 

 more can a diagram of the development of animals, which adopts this difference 

 as fimdamental, 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 sep- 

 arated, 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." I think also, 

 that the homology of the limbs of Articulata and the dorsal plates of Vertebrata 

 is more than questionable. 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. 



' C'liap. III., Seel. 1, [i. 171. gegenseitige Verwandtschaft dcr Tliiere zu erlangen, 



^ The i)rinciples of classification advocated by die ve rsch i c d e n e ii Orga n i sa t i on s t y p en 



Baer are so clearly exjjresscd by him, that I cannot von den v e rsch i e d e n e n Stufen der Aus- 



resist the temptation of <pioting some passages from b i 1 d u n g stets unterschciden. Dass man diesen 



the paper already mentioned above, p. 224, especially Unterschied gewohnlich nicht im Auge bchalten hat, 



now, when I feel callid upon to ojijiose the views of seheint uns zu <lfn sonderbarsten Zusammenstel- 



one of his most distinguished colleagues. " Vor alien lungen gefuhrt zu haben." Beitriige, etc.. Acta 



Dingen muss man, um cine richtige Einsicht in die Nova, vol. 13, p. 739. 



