EMBEYOLOGICAL SYSTEMS. 357 



its medial axis, the backbone, there arise upward, arches 

 which close in an upper crest; and downward, arches 

 which unite, more or less, in a lower crest. Corresponding 

 to this we see four rows of nervous threads alono; the 



O 



spinal marrow, which itself contains four strings, and a 

 quadripartite gray mass. The muscles of the trunk form 

 also four principal masses, which are particularly distinct 

 in the Fishes. The animal system is therefore doubly 

 symmetrical in its arrangement. It might easily be sho\vn 

 how the vegetative systems of the body correspond to the 

 type of Mollusks, though influenced by the animal system. 



From the illustrations accompanying this discussion of 

 the great types or branches of the animal kingdom, and 

 still more from the paper published by K. E. von Baer in 

 the " Nova Acta," 1 it is evident that he perceived more 

 clearly and earlier than any other naturalist the true rela- 

 tions of the lowest animals to their respective branches. 

 He includes neither Bryozoa nor Intestinal Worms among 

 Eadiata, as Cuvier, and after him so many modern writers, 

 did, but correctly refers the former to the Mollusks, and 

 the latter to the Articulates. 



Comparing these four types with the embryonic deve- 

 lopment, von Baer shows that there is only a general 

 similarity between the lower animals and the embryonic 

 stages of the higher ones, arising mainly from the absence 

 of differentiation in the body, and not from a typical 

 resemblance. The embryo does not pass from one type 

 to the other; on the contrary, the type of each animal is 



1 Beitrjige zur Kenntniss der nie- animals. These " Beitriige" and the 



dern Thiere, Nova Acta Academic papers in which Cuvier characterized 



Naturae Curiosuni, vol. 13, Part 2, for the first time the four great types 



1827, containing seven papers, upon of the animal kingdom, are amon- 



Aspidogaster, Distoma, and others, the most important contributions to 



Cercaria, Nitzschia, Polystoma, Pla- general Zoology ever published. 

 iiaria, and the general affinities of all 



