552 NEURAL AND H/EMAL ARCHES. 



Since primitively the postanal gut was placed between the 

 aorta and the caudal vein, the haemal arches potentially invest 

 a caudal section of the body cavity. In the trunk region they 

 do not meet ventrally, but give support to the ribs. The 

 structures just described are shewn in section in fig. 318, in 

 which the neural (no) and haemal (ha) arches are shewn resting 

 upon the cartilaginous sheath of the notochord. 



While these changes are being effected in the arches the 

 cartilaginous sheath of the notochord undergoes important differ- 

 entiations. In the vertebral regions opposite the origin of the 

 neural and haemal arches (fig. 318) its outer part becomes 

 hyaline cartilage, while the inner parts adjoining the notochord 

 undergo a somewhat different development, the notochord in this 

 part becomes at the same time 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 changes are being effected, is 

 shewn in fig. 320 B. 



These processes are soon carried further. The notochord 

 within the vertebral body becomes gradually constricted, espe- 

 cially in the median plane, till it is here reduced to a fibrous 

 band, which gradually enlarges in either direction 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 membrana 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 structures, 

 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 verte- 

 bral bodies take of necessity a biconcave hourglass-shaped 

 form. 



The intervertebral regions of the primitive sheath of the noto- 

 chord form fibrous intervertebral ligaments enclosing the uncon- 

 stricted intervertebral sections of the notochord. 



