THE SKELETON 415 



the thoracic region of a five-day chick. The notochord is 

 sUghtly constricted intervertebrally, and the position of the 

 intersegmental artery, of the myotomes and nerves, shows that 

 each vertebral segment is made up of two components repre- 

 senting succeeding sclerotomes. In the region of the neural 

 arches (Fig. 234) the line of union of cranial and caudal vertebral 

 components is indicated by a slight external indentation at the 

 place of union, and by the arrangement of the nuclei on each 

 side of the plane of union. 



Fig. 233. — Frontal section through the notochord and pri- 



mordia of two vertebrae of a 5-day chick; thoracic region. 



Note intervertebral constrictions of the notochord. The 



anterior end of the section is above. 



N., Spinal nerve. Symp., Part of sympathetic cord. v. C, 

 Region of pleurocentrum, in which the formation of cartilage 

 has begun. Other abbreviations as in Fig. 232. 



The parts of the vertebrae formed in the membranous stage 

 are as follows: (1) The vertebral body is formed by tissue of 

 both vertebral components that grows around the perichordal 

 sheath; (2) a membranous process (neural arch) extends from 

 the vertebral body dorsally at the sides of the neural canal; but 

 the right and left arches do not yet unite dorsall}^; (3) a lateral 

 or costal process extends out laterally and caudally (Fig. 233) 

 from the vertebral body between the successive myotomes. 



The union of the right and left cephalic vertebral components 



