THE SKELETON 415 
the thoracic region of a five-day chick. The notochord is 
slightly constricted intervertebrally, and the position of the 
intersegmental artery, of the myotomes and nerves, shows that 
each vertebra! 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. 
coud Sel. = 2s: 
SA Fs 
ESS 
nt oe 
er ee) 
Ree) 
x 
Pest 
pal 
v 
bore @ paeee 
HOE CITRATE pate 
A 
a 
= 
ole ; & 
2 57, O_O Pate Oia Odie o Be 2882 
Fic. 233. — Frontal section through the notochord and _pri- 
mordia of two vertebre 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. 282. 
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 dorsally; (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 
