3io 



THE MORPHOGENESIS OF THE SKELETON AND MUSCLES 



lages of the definitive lerlebra (Fig. 313 B}. Mesenchymal tissue, filling 

 the new intervertebral fissure thus formed, gives rise to the interv,rtebral 

 discs. Since a vertebra is formed from parts of two adjacent sclerotomes, 

 it is evident that the intersegmental artery must now pass over the body 

 of a vertebra, and the myotomes and vertebrae 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, however, temporary, an articulation forming 



Myotome. 



Ectoderm 



>Sderotome 



^Intersegmental 



artery 

 ...Notochord 



Myotome 



Ectoderm 



Notochord 



A nlage of vertebra 

 Intervertebral fissure 

 -Intersegmental artery 



A B 



FIG. 313. Frontal sections through the mesodermal segments of the left side of human 

 embryos. A, at about 4 mm., showing the differentiation of the sclerotomes into less dense 

 and denser regions; B, at about 5 mm., illustrating the union of the halves of successive sclero- 

 tomes to form the anlages of the vertebrae. 



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 

 ligaments of the vertebral column arise from mesenchyme surrounding the 

 vertebras. 



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^). The replacement of cartilage to 

 form a solid mass is not completed until several years after birth. At 

 about the seventeenth year, secondary centers appear in the cartilage still 

 covering the cranial and caudal ends of the vertebral body and form che 

 disc-like, bony epiphyses. These unite with the vertebra proper to con- 

 stitute a single mass at about the twentieth year. 



While the foregoing account holds for vertebras in general, a few devia- 

 tions occur. When the atlas is formed, a body differentiates as well, but 

 it is appropriated by the body of the epistropheus (axis), thereby forming 

 the tooth-like dens of the latter. The sacral and coccygeal vertebrae 

 represent reduced types. At ,about the twenty-fifth year the sacral ver- 

 tebrae unite to form a single bony mass, and a similar fusion occurs between 

 the rudimentary coccygeal vertebrae. 



