2IO 



COMPARATIVE ANATOMY 



between the mesoderm and the notochord. Later, by a continuation of 

 the same migration, the neural tube becomes completely surrounded by 

 mesenchyma. 



Before cartilage is secreted in the mesenchyma, the sclerotome median 

 to each myotome becomes differentiated into a denser posterior portion 

 and a less dense anterior half. As the definitive vertebrae are formed, 

 the posterior half of each vertebral anlage fuses with the anterior half 

 of the following one. By this process the definitive vertebrae come to lie 

 intersegmentally, alternating with the myotomes. The result is obvi- 

 ously adaptive, since only by this arrangement could each myotome 

 become connected with two vertebrae and with two successive ribs. 



EPIDERMIS 



DERMATOME 



MYOTOME 



NOTOCHORD 



The chondrification of each vertebra starts in six centres, two in each 

 centrum, two in each neural arch, and two in the costal processes. During 

 the third month ossification of the vertebrae begins; but it is not com- 

 pleted until several years after birth. At about the seventeenth year, 

 bony epiphyses arise as separate ossifications of the anterior and posterior 

 surfaces of the centra. In the twenty-first year, these fuse with the 

 centra and elongation of the vertebral column ceases. Of the original 

 cartilaginous matrix only the intervertebral regions persist. The fusion 

 of the sacral vertebrae is deferred until the twenty-fifth year. 



The Ribs. Man has twelve pairs of ribs, which form a basket sur- 

 rounding the thoracic cavity. Each rib is a curved flat bone ending 

 ventrally in a costal cartilage. By means of these costal cartilages the 



