DEVELOPMENT OF BONES. 273 



(#) Development of Connective-tissue Bones. 



In those instances in which bones are developed in con- 

 nective tissue certain bundles of connective tissue become cal- 

 cined and form the ground substance of the bone. The con- 

 nective-tissue cells arrange themselves in a layer on the surface 

 of these bundles, and, becoming more rich in protoplasm, are 

 converted into osteoblasts (Fig. 199). There is thus formed a 

 bony plate by the addition of bone on the surface and at the 

 borders of the calcified mass. This increases in thickness by 



Bone cells 



FIG. 199. 



Osteoblasts 





V V 



Blood-vessel Connective tissue Bone 



From a transverse section of the parietal bone of a human embryo. X 220. 



the deposition of new bone on the two surfaces. The older 

 bone between these two layers becomes a spongy bone substance 

 (diploe). In this kind of bone production the osteoclasts are 

 particularly active, for the bones that are so formed are con- 

 stantly undergoing changes in form and relations. These are 

 mainly the lateral bones of the skull, the facial bones, and the 

 upper parts of the occipital bone. 



B. Cartilages. 



The cartilages are covered with a perichondrium, with the 

 exception of those covering the joint-surfaces and those joining 

 together bones. In fully developed cartilages we find no blood- 

 vessels. These, as well as the nerves, exist only in the peri- 

 chondrium (see Cartilage tissue). 



18 



