598 



Comparative Morphology of Chordates 



Fig. 462. Armadillo sacrum. (A) Lateral view. (B) Dorsal view. 



These rather thick disks or ligaments intervene between adjacent 

 centra, serving as shock-absorbers and imparting the necessary flexi- 

 bility to the column. Other than in mammals, such disks are well 

 developed only in crocodilians and birds. 



In the development of mammalian bones there is a unique 

 feature, best exemplified in vertebrae and the elongated bones of the 

 legs. The embryonic cartilaginous centrum of a vertebra ossifies in 

 three parts, forming a thin anterior plate, a similar posterior plate, and 

 a thicker middle part. Between the middle bony mass and the adjacent 

 thin bony plates (epiphyses), a thin layer of cartilage persists without 

 ossifying until the growth of the vertebral column is completed. In- 

 crease in the length of the centrum is accomplished by progressive 

 deposition of new bone on the anterior and posterior faces of the middle 

 part of the centrum, each of the faces being adjacent to a layer of 

 cartilage. When growth is completed, the epiphyses fuse solidly with 

 the middle mass of the centrum. Similarly, at an early stage in the 

 development of one of the elongated bones of an appendage (Figs. 

 463, 464), a bony epiphysis is formed in each end of the cartilaginous 

 precursor of the bone, and the intermediate region of the cartilage 

 ossifies separately to form the shaft (or diaphysis) of the bone, but a 



