. The development of the chick; an introduction to embryology . 420 THE DEVELOPMENT OF THE CHICK notochord runs continuously through the centra of the fourvertebrae shown. It is constricted intravertebrally and expandedintervertebrally, so that the vertebral bodies are intervertebral discs are not shown. A pre- and postzygapo-physis is formed on each arch. It is by no means certain that theparts separated by the clear streak shown in the figure extendingthrough centra and arches correspond to the sclerotomal com-ponents of the primitive vertebrae, though this was the i
. The development of the chick; an introduction to embryology . 420 THE DEVELOPMENT OF THE CHICK notochord runs continuously through the centra of the fourvertebrae shown. It is constricted intravertebrally and expandedintervertebrally, so that the vertebral bodies are intervertebral discs are not shown. A pre- and postzygapo-physis is formed on each arch. It is by no means certain that theparts separated by the clear streak shown in the figure extendingthrough 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; furtherstudy seems necessary to determine the exact relations of theprimitive sclerotomal components to the parts of the definitivevertebra. The successive vertebrae have persistent membranous. Fig. 237. — The right side of four bisected vertebrse 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. Nch., Xotochord. connections in the regions of the neural spines, zygapophysesand centra. These are shown in Figs. 238 and 239 (cf. alsoFig. 150); they are continuous with the perichondrium and allare 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, whenchondrification sets in, the hypochordal bar of the first vertebra doesnot fuse with the body, but remains separate and forms its floor(Figs. 238 and 239). The body of the first vertebra chondrifiesseparately and is attached by membrane to the anterior end ofthe body of the second vertebra, representing in fact the odon-toid process of the latter. It has later a separate center of ossi-fication, b
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