NO TOG HO RD AND VERTEBRAL COLUMN. 



455 



somewhat constricted. In the intervertebral regions the cartilaginous 



sheath of the notochord becomes more definitely fibrous, while the 



notochord is in no way constricted. 



A diagrammatic longitudinal section 



through the vertebral column, while 



these clianges 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 

 cartilasre of the base of the arches 

 gradually spreads over it, and on the 

 absorption of the membrana elastica 

 externa^ which usually takes place long 

 before the adult state is reached, the 

 arch tissue becomes indistinguishably 

 fused with that of the veitebral 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 

 hiiemal 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 f )rm 

 fibrous intervertebral ligaments enclos- 

 ing the unconstricted intervertebral 

 sections of the notochord. 



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

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

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

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

 of the vertebras 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 be explained by 

 the fact that the .segmentation of the vertebral column arose subsequently 

 to tliat of the nerves and mvotoiues. 



Fig. 318. Hection through 

 the vektebral column of an ad- 

 vanced embkyo of scyllium in 

 the region of the tail. 



va. neural arch ; ha. bffimal 

 arch ; ch. notochord ; sh. inner 

 s'leath of notocLord ; ne. membrana 

 elastica externa. 



