VEETEBE/E OF VEETEBBATA. 



-129 



derived from tlie skeletogenous layer, is developed around the 

 primitive sheath, and on this the cartilaginous and superficially 

 ossified arches are set. 



The axial skeleton of the Selachii is much more highly developed. 

 The rudiments of the superior and inferior cartilaginous arches 

 appear around the notochord; these grow around it and so form 

 cartilaginous circular centra. That part of the cartilage which 

 encloses the chord is marked off from the peripheral part, which 

 is continued into the arches, and the former represents, just as in the 

 Dipnoi, a kind of cartilaginous sheath (skeletogenous chordal sheath), 

 which is deposited on the cuticular sheath. 



The vertebral column of the Selachii varies greatly in structure 

 according to the mode of growth of tho notochord and its skeleto- 

 genous sheath. 



B 



I) 



cs~ 



Fig. 222. Diagram of the changes produced in the notochord by the skeletogenous 

 layer (longitudinal sections), c Chorda, cs Chordal sheath, s Skeletogenous layer. 

 v Bodie3 of the vertebra). iv Intervertebral portion. g Intervertebral joint. 

 A The chordal tube, when all its parts are equally well developed (Fishes). B Interver- 

 tebral growth of the chorda. Formation of amphicoelous vertebras (Fishes). In- 

 tervertebral constriction of the chorda by cartilage ; while the rest of the chorda is 

 retained in the vertebrae (Amphibia). D Intervertebral constriction of the chorda 

 (Reptilia, Aves). E Vertebral constriction of the chorda, where part of tho interver- 

 tebral portion is retained (Mammalia). 



The cartilage sometimes forms a cylindrical tube, in which the 

 vertebras are merely represented by the arches and circular parts of 

 the skeletogenous sheath. The notochord is sometimes developed 

 between the vertebras (Fig. 222, B), and retains its earlier size at tho 

 points where the vertebra (y) and arches were first laid down around 

 it. This arrangement gives rise to biconcave (amphicoelous) vertebras 

 (B), the depressions in which are filled up by the intervertebral 

 chord. This is the way in which the vertebras of nearly all other 

 fishes are formed. 



§ 329. 



In the Gano'i'dei the vertebral column, when simplest in organisa- 

 tion, resembles that of the Selachii. Just as in the Selachii and 



