HISTORY OF ZOOLOGY. 47 



it is laid down likewise as a part of the skin, and only 

 gradually through infolding and cutting off from this does 

 it come to lie internally. One can demonstrate in case of 

 each vertebrate this change of position by cross-sections 

 through the dorsal region of embryos of different ages 



(Fig- 9)- 



The Skeletal System. The skeleton of vertebrates is 



a further example. In the lowest vertebrates, amphioxns 

 and the cyclostoincs, the vertebrae are lacking, and in their 

 place we find a cylindrical cord of tissue, the chorda 

 dorsalis (notochord). In the fishes and anipJdbia usually 

 the notochord persists ; but it is partially compressed and 

 narrowed by the vertebrae, which in the lower forms con- 

 sist of cartilage, and in the higher of bone or a combination 

 of bone and cartilage. Mature birds and mammals finally 

 have a completely ossified vertebral column ; their em- 

 bryos, on the other hand have in the early stages only the 

 notochord (amphioxus stage) ; later this notochord becomes 

 narrowed by the vertebra; (fish-amphibia stage) and finally 

 entirely replaced; the vertebral column is in the beginning 

 cartilaginous, only later becoming ossified. Comparative 

 Anatomy and Embryology thus give the same developmental 

 stage of the axial skeleton: (i) notochord, (2) notochord 

 and vertebral column, the latter at first formed of carti- 

 lage, then of bone. 



We have here spoken of a parallelism between the 

 facts of Comparative Anatomy and those of Embryology. 

 But as a matter of fact we should expect a threefold paral- 

 lelism ; for according to the descent theory the system- 

 atic arrangement of animals is conditioned by a third 

 factor the historical development of the animal world, or 

 Phylogeny. The boundary stones of phylogenesis, the 

 fossils, must now, so we should expect, in the successive 

 geological strata give the same progressive series as the 

 stages of forms found by Comparative Anatomy and Em- 

 bryology. In point of fact we know instances of such 

 threefold parallelisms. Comparative Anatomy teaches that 

 the lowest developed form of a fish's tail is the diphy- 



