206 



THE ORGANS 



vessels. In this way the septa between the cartilage cell spaces are broken 

 down, the cartilage cells disappear, and a central cavity is formed — the primary 

 marrow cavity. From the region of the primary marrow cavity blood-vessels 

 and osteogenetic tissue push in both directions toward the ends of the cartilage 

 which is to be replaced by bone. These break down the transverse septa be- 

 tween the cell spaces, while many of the longitudinal septa at first remain to 

 form the walls of long anastomosing channels, the primary marrow spaces (Fig. 

 125). As in intramembranous bone, these contain blood-vessels, embryonal 



Fig. 124. 



Fig. 12.S. 



Fig. 124. — Intracartilaginous Bone Development. X3S0. Showing osteogenetic 

 tissue pushing its way into the cartilage (periosteal bud) at the ossification centre. 

 a, Periosteum; b, cartilage cell spaces; c, periosteal bud; d, blood-vessel; e, cartilage 

 cells;/, cartilage matrix. 



Fig. 125. — Intracartilaginous Bone Development. Samespecimen asFig. 123 (X3S0), 

 showing osteogenetic tissue pushing its way into the cartilage and breaking it up into 

 trabeculse; also formation of primary marrow spaces and disintegration of cartilage 

 cells, a Disintegrating cartilage cells; b, cartilage trabecula; c, osteogenetic tissue in 

 primary marrow space; d, blood-vessels; e, cell spaces;/, cartilage cells. 



marrow, and osteoblasts, all of which are derived from the osteogenetic 

 tissue brought in from the periosteum by the periosteal buds. The osteo- 

 blasts ne.\t arrange themselves in a single layer along the remains of the 

 calcified cartilage, where they proceed to deposit a thin layer of bone between 

 themselves and the cartilage (Fig. 126). As this increases in thickness some of 

 the osteoblasts are enclosed within the newly formed bone to become bone 

 cells, while the remains of the cartilage diminishes in amount and finally dis- 

 appears. The calcification centre has now become the ossification centre, and 



