THE SKATE. 33 



11. The intervertebral substance, a gelatinous tissue 

 filling up the biconvex spaces between contiguous vertebrae, 

 and containing the remains of the embryonic notochord. 



V. Examine the caudal vertebras in the same way : 

 make out the following points : 



12. The centrum of each vertebra (Figs, n and 14, c} 

 gives off, on each side, a downwardly-directed haemal 

 process (h.p\ which is seen, by examining the transition 

 region between trunk and tail (Fig. n), to be homologous 

 with the transverse process of a trunk vertebra : the haemal 

 processes, uniting below, form the haemal arch, which 



FIG. 13. Raja nasuta. Longitudinal vertical section of a trunk 

 vertebra (nat. size). 



c, centrum: i.n., inter-neural plate: n.fi, neural process: n.s, 

 neural spine. 



transmits the caudal artery and vein ( 157 and 158): a 

 median process, the haemal spine (h.s), is given off at 

 the junction of the haemal processes of most of the 

 vertebrae, or may occur as a distinct cartilage (see Fig. 1 1). 



13. The absence of inter-neural plates and of ribs. 



14. The gradual reduction in size and simplification in 

 structure of the vertebrae, seen in passing from the anterior 

 to the posterior end of the caudal region. 



15. The skeleton of the two dorsal fins, attached to 

 the posterior caudal vertebrae : each consists of several 

 radial cartilages or fin rays, attached to two basal 

 cartilages, these latter being attached by fibrous tissue to 



D 



