ON OSSIFICATION 



257 



and membrane (Figs. 109, no), but, to us it seems to 

 manifest itself in two degrees, viz. cartilaginous (see Fig. 

 107), and fully osseous (see Fig. 1 10), and that sometimes 

 it remains at the cartilaginous stage, as in the cartilages of 

 the ribs, the joints and air passages, where the property 

 of greater elasticity is retained, with the conferred skeletal 

 stiffening. Moreover, as age advances, a process of 



FIG. 109. PARIETAL BONE OF AN EMBRYO SHEEP. Size of the embryo, 

 2^ inches. (Sharpey. ) 



The small upper figure represents the bone of the natural size, the larger figure is 

 magnified about 12 diameters. The curved line a, b, marks the height to which 

 the subjacent cartilaginous lamella extended. A few insulated particles of bone 

 are seen near the circumference, an appearance which is quite common at this 

 stage. 



stiffening is undergone by these cartilaginous structures, 

 and frequently there are laid down, nodules, or more 

 developed structures, in various localities by preference, 

 which undergo a series of structural changes, due to the 

 deposition in their matrix of earthy material, which 

 ultimately may lead to the development of a supplementary 

 skeleton, or detached osseous structures, the growth of 

 which is usually manifested along what we might call, the 



