NOTOCHORD AND VERTEBRAL COLUMN. 



455 



somewhat constricted. In the interverteb 

 sheath of the notochord becomes more 

 notochord is in no way constricted. 

 A diagrammatic longitudinal section 

 through the vertebral column, while 

 these changes are being effected, is 

 shewn in fig. 320 B. 



These processes are soon carried 

 further. The notochord within the 

 vertebral body becomes gradually con- 

 stricted, especially in the median plane, 

 till it is here reduced to a fibrous band, 

 which gradually enlarges in either di- 

 rection till it reaches its maximum 

 thickness in the median plane of the 

 intervertebral region. The hyaline 

 cartilage of the vertebral region forms 

 a vertebral body in which calcification 

 may to some extent take place. The 

 cartilage of the base of the arches 

 gradually spreads over it, and on the 

 absorption of the membraua elastica 

 externa which usually takes place long 

 before the adult state is reached, the 

 arch tissue becomes indistinguishably 

 fused with that of the vertebral bodies, 

 so that the latter are compound struc- 

 tures, partly formed of the primitive 

 cartilaginous sheath, and partly of the 

 tissue of the bases of the neural and 

 haemal arches. Owing to the beaded 

 structure of the notochord the vertebral 

 bodies take of necessity a, biconcave 

 hourglass-shaped form. 



The intervertebral regions of the 

 primitive sheath of the notochord firm 

 fibrous intervertebral ligaments enclos- 

 ing the unconstricted intervertebral 

 ssctions of the notochord. 



rid regions the cartilaginous 

 definitely fibrous, while the 



na. 



ha 



FIG. 318. SECTION THROUGH 

 THE VEKTEBHAL COLUMN OF AN AD- 

 VANCE!) EJIUKYO OF SCYLLIUM IN 

 TH3 REGION OF THE TAIL 



i:u. neural arch ; Jni. haemal 

 a'*ch ; c/i. notocbord ; xh. inner 

 s'leath of notocLord ; iu j . membraua 

 elastica externa. 



A peculiar fact may here be noticed with reference to the formation 

 of the vertebral bodies in the tail of Scyllium, Raja, and possibly other 

 forms, viz. that there we double as many vertebral bodies us there are 

 mijoto iites and spinal nerves. This is not due to a secondary segmentation 

 of the vertebrae but, as I have satisfied myself by a study of the develop- 

 ment, takes place when the vertebral bodies first become differentiated. 

 The possibility of such a relation of parts is probably to bo explained by 

 the fact that the segmentation of the vertebral column arose subsequently 

 to that of the nerves and mvotomes. 



