420 



THE DEVELOPMENT OF THE CHICK 



notochord runs continuously through the centra of the four 

 vertebrae shown. It is constricted intra vertebrally and expanded 

 intervertebrally, so that the vertebral bodies are amphiccelous. 

 The intervertebral discs are not shown. A pre- and postzygapo- 

 physis is formed on each arch. It is by no means certain that the 

 parts separated by the clear streak shown in the figure extending 

 through centra and arches correspond to the sclerotomal com- 

 ponents of the primitive vertebrae, though this was the in- 

 terpretation of Schauinsland as shown in the figure; further 

 study seems necessary to determine the exact relations of the 

 primitive sclerotomal components to the parts of the definitive 

 vertebra. The successive vertebrae have persistent membranous 



Fig. 237. — The right side of four bisected vertebrae of the trunk 

 of an 8-day chick. (After Schauinsland.) 

 caud. V. A., Caudal division of vertebral arch. ceph. v. A., 

 Cephalic division of vertebral arch. N'ch., Notochord. 



connections in the regions of the neural spines, zygapophyses 

 and centra. These are shown in Figs. 238 and 239 (cf. also 

 Fig. 150) ; they are continuous with the perichondrium and all 

 are derived from unchondrified parts of the original membran- 

 ous vertebrae. 



Atlas and Axis (epistropheus). The first and second verte- 

 brae agree with the others in the membranous stage. But, when 

 chondrification sets in, the hypochordal bar of the first vertebra does 

 not fuse with the body, but remains separate and forms its floor 

 (Figs. 238 and 239). The body of the first vertebra chondrifies 

 separately and is attached by membrane to the anterior end of 

 the body of the second vertebra, representing in fact the odon- 

 toid process of the latter. It has later a separate center of ossi- 

 fication, but fuses subsequently with the body of the second 

 vertebra, forming the odondoid process (Fig. 240). 



