420 



THE DEVELOPMENT OF THE CHICK 



notochord runs continuously through the centra of the four 

 vertebra shown. It is constricted intravertebrally and expanded 

 intervertebrally, so that the vertebral bodies are amphicoelous. 

 The intervertebral discs are not shown. Each vertebral centrum 

 consists of united cephalic and caudal parts; the arches arising 

 from the centra are likewise double, but the caudal component 

 diminishes posteriorly and is incomplete in the last vertebra 

 shown. A pre- and postzygapophysis is formed on each arch. 



As it is possible to follow the sclerotomal components of the 

 primitive vertebrae up to this stage continuously, there can be 

 no reasonable doubt that they correspond to the divisions shown 

 by the staining in the cartilaginous vertebrae of Fig. 237. The 



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. 



successive vertebrae have persistent membranous 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 con- 

 tinuous with the perichondrium and all are derived from unchon- 

 drified parts of the original membranous 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). 



