174 TEXT-BOOK OF EMBRYOLOGY. 



the primary marrow cavity. The osteoblasts arrange themselves in rows along 

 the septa of calcined cartilage and a thin layer or lamella of calcium salts is 

 deposited between them and the cartilage. Successive lamellae are deposited 

 in the same manner and some of the osteoblasts become enclosed to form bone 

 cells (Fig. 156). The cartilage in the center gradually disappears. This 

 region where bone formation is going on is known as an ossification center (Fig. 

 153) and the irregular anastomosing trabeculae of bone with the enclosed marrow 

 spaces constitute primary spongy bone. 



From this time on, ossification gradually progresses toward each end of the 

 cartilage, and at the same time a special modification of the cartilage precedes 

 it. Nearest the ossification center the cartilage cell spaces become enlarged and 



Cartilage cell spaces 

 (primary marrow space) 



Disintegrating 

 cartilage cells 



Cartilage cell 

 spaces 



Blood vessel 



m Trabecula 



of cartilage 



L 



Osteogenetic tissue in 

 primary marrow space 



FIG. 155.- From same section as Fig. 153; showing osteogenetic tissue pushing into the cartilage 

 and breaking it up into trabeculae. (Intracartilaginous ossification.) 



arranged in rows and contain cartilage cells in various stages of disintegration. 

 Some of the septa break down, leaving larger, irregular spaces; the remaining 

 septa become calcified (Fig. 153). Passing away from the center of ossifica- 

 tion, there is less enlargement of the cell spaces and they have a tendency to be 

 arranged in rows transverse to the long axis of the cartilage; there is also a lesser 

 degree of calcification. The region of modified cartilage at each end of the 

 ossification center passes over gradually into ordinary hyalin cartilage and is 

 known as the calcification zone. It always precedes the formation of bone as 

 the latter process moves toward the end of the cartilage (Fig. 153). 



Along with the type of ossification just described subperiosteal ossification 

 also occurs (Fig. 153). Beneath the periosteum (perichondrium) is a layer of 

 connective tissue the cells of which are transformed into osteoblasts. Thev 



