. The development of the chick; an introduction to embryology . first and second verte-bne agree with the others in the membranous stage. But, whenchondrification sets in, the hvj^ochordal l)ai of the first vertel)ra doesnot fuse with the body, but Icmaius separate and forms its floor(Figs. 238 and 23!)). The l)ody of the first vertebra chondrifiesseparately and is attached by membrane to the anterior end ofthe body of the second vertebra, rc])resenting in fact the odon-toid process of the latter. It has later a separate center of ossi-fication, but fuses subsefpuMitly with tlie body of the se
. The development of the chick; an introduction to embryology . first and second verte-bne agree with the others in the membranous stage. But, whenchondrification sets in, the hvj^ochordal l)ai of the first vertel)ra doesnot fuse with the body, but Icmaius separate and forms its floor(Figs. 238 and 23!)). The l)ody of the first vertebra chondrifiesseparately and is attached by membrane to the anterior end ofthe body of the second vertebra, rc])resenting in fact the odon-toid process of the latter. It has later a separate center of ossi-fication, but fuses subsefpuMitly with tlie body of the secondvertebra, forming the odondoid i)iocess (Fig. 240). THE SKELETON 421 Formation of Vertebral Articulations. In the course of develoj)-nient the intervertebral discs differentiate into a peripheral inter-vertebral ligament and a central suspensory ligament which at firstcontains remains of the notochord. There is a synovial cavity])etween the intervertebral and suspensory ligaments. This dif-ferentiation takes place by a process of loosening and resorption. Fig. 238. — Median sagittal section of the basis cranii and first three vertebral centra of an 8-day chick. B. C, Basi-cranial cartihxge. iv. D. 1, 2, 3, 4, First, second, third, and fourth intervertebral discs. N. T., Floor of neviral tube. s. n. b. 1, 2, First and second subnotochordal liars. V. C. 1, 2, 3, First, second, and third pleurocentra. of cells just external to the perichordal sheath (). The in-tervertebral ligament takes the form of paired, fibrous menisci, oi-, inother words, the intervertebral ligaments are incomplete aroundthe l)odies of the vertebra dorsally and ventrally (Schwarck).Ossification is well advanced in the clavicles, long bones, 422 THE DEVELOPMENT OF THE CHICK and in(Miil)i-ane bones of the skull before it begins in the takes ])hiee in antero-posterior order,so that a series of stagesmay be followed in a single embryo (ef. Fig. 242). There arethree main centers for each
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