EMBRYOLOGICAL SYSTEMS. 367 



segmentation of the yolk of the Mammalia could justify 

 the separation of the latter from the other Vertebrates. 

 Had the distinction made by Vogt between theCephalopods 

 and the other Mollusks the value he assigns to it, Limax 

 should also be separated from the other Gasteropods. 

 The assertion, that the Protozoa produce no eggs, deserves 

 no special consideration after what has already been said in 

 the preceding sections respecting the animals themselves. 

 As to the transfer of the Ctenophora to the type of Mol- 

 lusks, it can in no way be maintained. 



Before closing this sketch of the systems of Zoology, 

 I cannot forego the opportunity of adding one general 

 remark. When we remember how completely independent 

 the investigations of K. E. von Baer were from those of 

 Cuvier, how different the points of view were from which 

 they treated their subject, the one considering chiefly the 

 mode of development of animals, while the other looked 

 mainly to their structure; when we further consider how 

 closely the general results at which they have arrived 

 agree throughout, it is impossible not to be deeply im- 

 pressed with confidence in the opinion they both advocate, 

 that the animal kingdom, exhibits four primary divisions, 

 the representatives of which are organized upon four dif- 

 ferent plans of structure, and grow up according to four 

 different modes of development. This confidence is fur- 

 ther increased when we perceive that the new primary 

 groups which have been since proposed are neither cha- 

 racterized by such different plans, nor developed accord- 

 ing to such different modes of development, but exhibit 

 simply minor differences. It is, indeed, a, very unfortu- 

 nate tendency, which prevails now almost universally 

 among naturalists with reference to all kinds of groups, 

 of whatever value they may be, from the branches down 



