418 THE DEVELOPMENT OF THE CHICK 



the sclerotomes to form the definitive vertebrae; (2) union of the 

 cephahc vertebral components 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 palaeontologists) ; (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 dorsally around the upper part of the 

 neural tube, finally uniting above with the corresponding arches 

 of the other side to form the memhrana 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 vertebra 

 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 vertebrae of an eight-day chick, prepared 

 according to the methylene blue method of Van Wijhe. The 



Fig. 236. — Frontal section of the vertebral column and neighboring struc- 

 tures of a 6-day chick. Upper thoracic region. Note separate centers 

 of chondrification 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 disc. 

 Mu., Muscles. N. A., Neural arch. T. R., Tuberculum of rib. V. C, Cen- 

 trum of vertebra. Other abbreviations as before. 



