VERTEBRAL COLUMN. 123 



variations which would, in any other group, be regarded 

 as of great morphological importance. It varies in the 

 extent and uniformity of its segmentation, in the arrangement 

 and number of the cartilaginous pieces which enclose the spinal 

 canal, and their mode of attachment to the vertebral central 

 and in the extent of the calcification of the cartilage. It con- 

 sists typically of a series of amphicoelous vertebrae, through 

 the centra and intervertebral ligaments of which runs the per- 

 sistent but reduced notochord. The neural arches of each 

 vertebra always consist of more than one piece of cartilage on 

 each side ; and the haemal arches extend outwards in the trunk 

 region where they carry short ribs, and downwards, meeting 

 each other ventrally below the caudal artery and vein, in the 

 caudal region. The vertebral column is 

 formed of hyaline and fibro-cartilage, 

 which tissues pass into one another 

 quite gradually. 



A^ tough ; fibroua membrane, containing 

 cells and surrounded by the elastica externa, J & 



is formed (see p. 58) at a comparatively j 



early stage, round the notochord and is L j/,^ 



called the chordal sheath (Fig. 33). This in FIG. 67. Three pos- 



most Plagiostomes becomes differentiated terior trunk verte- 



into alternately short fibrous and longer (aftL HasJeTom^g! 



cartilaginous portions, i.e. segmented (Fig. enbaur). n neural arch, 



66). The fibrous portions become the inter- ^^SS^d^aS- 



vertebral ligaments, while the cartilaginous calated piece with 



portions form the bodies or centra of the ST^ haemafa^h' 



vertebrae ; so that the vertebrae are described 



as being chordo-centrotis. The centra are, however, frequently reinforced 

 by the arch tissue in the manner described on p. 59. 



The cartilaginous arches (neural and haemal), which appear in the neural 

 and haemal ridges of the skeletogenous tissue, may, as stated above, spread 

 out round the notochord outside the elastica externa (which may persist or 

 disappear) and unite with each other and so reinforce the vertebral centra; 

 or they may remain separate from one another. In the latter case the 

 neural and haemal arches are separate from the centra ; in the former 

 they are continuous with the centra. Kolliker states that in some cases 

 the centra are reinforced by calcified fibro-cartilage which proceeds from 

 the perichondrium of the centra between the insertion of the neural and 

 haemal arches (Carchariidae, etc.). In this case we get the cruciform 

 figure in section described below. 



In Chlamydoselachus, Echinorhinus, Hexanchus and Heptan- 

 chus, the cartilaginous notochordal sheath is not definitely seg- 

 mented : it consists of continuous cartilage, though in the centre 



