DEVELOPMENT OF BONE 69 



After a time the cartilage in one or both ends of the long bones begins 

 to ossify independently, and the epiphyses are formed. These are not 

 joined to the shaft until the growth of the bone is completed. Growth 

 takes place in length by an expansion of the cartilage (intermediate car- 

 tilage) which intervenes between the shaft and the epiphyses, and by the 

 gradual extension of the ossification into it ; in width entirely by the 

 deposition of fresh bony layers under the periosteum. In the terminal 

 phalanges of the digits the ossification starts, not from the middle of 

 the cartilage, but from its distal extremity. 



For the regeneration of portions of bone which have been removed 

 by disease or operation it is essential that the periosteum be left. 



if 



FIG. 68. PAKT OF THE GROWING EDGF. OF THE DEVELOPING PARIETAL BONE 

 OF A F<ETAL CAT, ] INCH LONG. 



tp, bony spicules, with some of the osteoblasts embedded in them, producing the lacunae; of, 

 osteogenic fibres prolonging the spicules, with osteoblasts (ost) between them and applied 

 to thein. 



Intramembranous ossification. In this variety of ossification, the 

 bone is not preceded by cartilage at all, and therefore no eiidochon- 

 dral bone is formed, but the calcification occurs in a sort of embryonic 

 fibrous tissue which contains numerous osteoblasts and blood-vessels 

 (fig. 68). The fibres of this tissue (osteogenic fibres), which, like those 

 of fibrous tissue, are collected into small bundles, become enclosed in 

 a calcareous matrix ; and as the fibres grow, the calcification extends 

 further and further, so that bony spicules are formed, which, as they 

 become thickened, run together to form reticular layers, leaving spaces 

 filled with osteoblasts around the blood-vessels. The osteogenic 



