124 COMPARATIVE ANATOMY 



manner of origin in the embryo. But Amphioxus has very little that can 

 be regarded as corresponding to the mesenchyme of vertebrates. Unques- 

 tionably cartilage and bone are phylogenetically new skeletal materials 

 as compared to the notochord. Consistently with this assumed fact, 

 the mesenchyme, which is the embryonic source of cartilage and bone, is 

 the last of the germinal building materials to be differentiated in the 

 embryo. 



The principle of "recapitulation" was formulated by Ernst Haeckel 

 (1874) in the phrase "Ontogeny is a brief repetition of Phylogeny." 

 Nineteenth century biology was perhaps inclined to over-work the idea. 

 Nevertheless many facts of vertebrate development justify, or even 

 compel, the inference that there is some direct relation between the 

 phylogenetic and the ontogenetic order. 



Early embryos of reptiles, birds and mammals develop briefly transi- 

 tory pharyngeal clefts (or pouches). These clefts are not accompanied 

 by the formation of functional gill structures such as occur in connexion 

 with the pharyngeal clefts of adult fishes. But they are closely similar 

 to those embryonic pharyngeal clefts which in fishes come to be associated 

 with vascular filaments and the other structures which, altogether, con- 

 stitute functional gills. 



The notochord is developed in the early embryo of all vertebrates. 

 In shark-like fishes (Elasmobranchii) wholly cartilaginous vertebrae are 

 developed around the notochord of which traces persist in the adult. 

 In reptiles, birds and mammals the embryonic notochord becomes sur- 

 rounded by cartilaginous vertebral structures. In later development the 

 cartilage is replaced by bone. The adult vertebrae consist of solid bone. 

 Ordinarily no traces of notochord remain. Nowhere else in the body is 

 bone preceded by any such structure or tissue as notochord. Many 

 bones other than vertebrae are produced directly from mesenchyme with- 

 out the intervention of cartilage. It seems clear, then, that in general 

 the formation of bone is not inherently dependent on preexisting cartilage 

 or notochord. 



Elasmobranch fishes have an all-cartilaginous skull. The skull of 

 adult amniotes is nearly all bone. But the amniote embryo develops 

 first a cartilaginous skull whose general plan resembles that of an elasmo- 

 branch. In later development the cartilage is almost wholly replaced by 

 bone and numerous dermal bones are developed superficially to the 

 cartilage. The-geologkxecord shows that elasmobranchs existedjong 

 >^re any^rtebrartgswith bo,By^eleton^^c^e upon th^cene^.^"''^ 



In the phylogenetic history of the axial skeleton of vertebrates, 

 notochord is the oldest structure and bone is most recent. In the develop- 

 ment of the bony vertebral column of a modern animal, notochord, 

 cartilage and bone succeed one another in phylogenetic order. In all 



