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 spuial 

 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, j T""*! X^^* 



which tissues pass into one another I 



quite gradually. '■ ' 



^ 



A tough fibroua luembrane, containing u-' 

 c ells and siirrounded by the elastica externa, | 

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

 early stage, round the notochord and is I ■' .' . 

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

 most Plagiostomes becomes differentiated terior trunk verte- 

 ,, " , . T , 12X. J 1 ^rae of Centrophorus 

 into alternately short nbrous and longer (after Hasse from Geg- 

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

 ^n\ m, z?i ^ u 4.^ ■\ with foramen for an- 

 bb). ihe fibrous portions become the inter- terior root ; in inter- 

 vertebral ligaments, while the cartilaginous calated piece with 

 ,. ^ ii 1 1- ; £ i-i foramen for posterior 

 portions form the bodies or centra or the jq^^. f^ haemal arch. 



vertebrae ; so that the vertebrae are described 



as being chordo-centrous. 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 roimd 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 Hejytan- 

 chus, the cartilaginous notochordal sheath is not definitely seg- 

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



