418 THE DEVELOPMENT OF THE CHICK 
the sclerotomes to form the definite vertebrae; (2) union of the 
cephalic vertebral arches beneath the notochord to form the 
subnotochordal bar; (3) origin of the membranous vertebral 
bodies and of the neural arch and costal processes. 
Chondrification, or development of cartilage, sets in from the 
following centers in each vertebra: (1) the cephalic neural arches 
and subnotochordal bar, forming a horseshoe-shaped cartilage 
at the cephalic end of each vertebra; (2) and (3) right and left 
centers in the body of each vertebra behind the subnotochordal 
bar, which soon fuse around the notochord; (the subnotochordal 
bar probably corresponds to the hypocentrum, and the lateral 
centers (2 and 3) to the pleurocentra of paleontologists); (4) and 
(5) centers in each costal process (Figs. 235 and 236). These 
centers are at first separated by membrane, but except in the 
case of the costal processes, which form the ribs, the cartilage 
centers flow together. The neural arches end in membrane 
which gradually extends dersally around the upper part of the 
neural tube, finally uniting above with the corresponding arches 
of the other side to form the membrana reuniens. The chondri- 
fication follows the extension of the membrane. During this 
time the transverse processes of the neural arch and the zygo- 
pophyses are likewise formed as extensions of the membrane. 
The distinction that some authors make between a primary 
vertebral body formed by chondrification within the perichordal 
sheath, and a secondary vertebral body formed by the basal 
ends of the arches surrounding the primary, is not a clear one 
in the case of the chick. 
On the seventh and eighth days the process of chondrifica- 
tion extends into all parts of the vertebra; the entire vertcbra 
is, in fact, laid down in cartilage on the eighth day, although the 
neural spine is somewhat membranous. Fig. 237 shows the 
right side of four trunk vertebrie of an eight-day chick, prepared 
according to the methylene blue method of Van Wijhe. The 
Fic. 236. — Frontal seetion of the vertebral column and neighboring strue- 
tures of a 6-day chick. Upper thoracic region. Note separate centers 
of chondrifiecation of the neural arch, centrum, and costal processes. An- 
terior end of section above. 
B. n. A.. Base of neural arch. br. N. 1, 2, 3, First, second, and third 
brachial nerves. Cp. R., Capitulum of rib. iv. D., Intervertebral dise. 
Mu., Muscles. N. A., Neural arch. T. R., Tubereulum of rib. VY. C., Cen- 
trum cf vertebra. Other abbreviations as before. 
