3IO THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



From the caudal portions, horizontal tissue masses now grow toward the median 

 line and enclose the notochord, thus establishing the body of each vertebra. 

 Similarly, dorsal extensions form the vertebral arch, and ventro-lateral outgrowths, 

 the costal processes. The looser tissue of the cranial halves also grows mesad and 

 fills in the intervals between successive denser regions. 



The denser caudal half of each sclerotomic mass presently unites with the less 

 dense cranial half of the sclerotome next caudad to form the anlages of the 

 definitive vertebra (Fig. 313 5). Tissue bordering the cranial and caudal portion 

 of the original sclerotome gives rise to the intervertebral discs. Since a vertebra is 

 formed from parts of two adjacent sclerotomes, it is evident that the interseg- 

 mental artery must now pass over the body of a vertebra and the myotomes and 

 vertebrffi alternate in position. 



Following this blastemal stage centers of chondrification appear, two centers 

 in the vertebral body, one in each half of the vertebral arch, and one in each costal 

 process. These centers enlarge and fuse to form a cartilaginous vertebra; the 

 union of the costal processes, which will give rise to ribs, with the body is, how- 

 ever, temporary, an articulation forming later. Transverse and articular processes 

 grow out from the vertebral arch, and the rib cartilages, having in the meantime 

 formed tubercles, articulate with the transverse processes somewhat later. The 

 various hgaments of the vertebral column arise from mesenchyme surrounding 

 the vertebrae. 



Finally, at the end of the eighth week, the stage of ossification sets in. A 

 single center appears in the body, one in each half of the arch, and one near the 

 angle of each rib (Fig. 296 A). The replacement of cartilage to form a sohd mass 

 is not completed until several years after birth. At about tlae seventeenth year 

 secondary centers appear in the cartilage still covering the cranial and caudal ends 

 of the vertebral body and form the disc-Hke, bony epiphysis. These unite with 

 the vertebra proper to constitute a single mass at about the twentieth j^ear. 



While the foregoing account holds for vertebrae in general, a few deviations 

 occur. When the atlas is formed a body differentiates as well, but it is appropri- 

 ated by the body of the epistropheus (axis), thereby forming the tooth-hke dens 

 of the latter. The sacral and coccygeal vertebrae represent reduced types. At 

 about the twenty-fifth year the sacral vertebrae unite to form a single bony mass,, 

 and a similar fusion occurs between the rudimentary coccygeal vertebras. 



The ribs, originating as ventro-lateral outgrowths from the vertebral bodies, 

 reach their highest development in the thoracic region. In the cervical regioa 

 they are short; their tips fuse with the transverse processes and their heads with 



