THE CONNECTIVE TISSUES. 



the osteoblasts depositing new bone upon the walls of these cylin- 

 drical cavities, layer upon layer, until only a small central channel 

 the Haversian cahal remains as the representative of the large 

 Haversian space. The outer boundary of the Haversian system, 

 therefore, corresponds to the limits of the Haversian space, while 

 the remains of the primary bone-trabeculae constitute the older 

 interstitial lamellae of the adult tissue. 



Osseous tissue, wherever developed, is formed through the 

 agency of the osteoblasts, the deriva- 

 tives and descendants of the special- 

 ized mesoblastic cells of the embryo; 

 whether in endochondral or periosteal 

 formation, the bone-producing elements 



FIG. 64. 



FIG. 65. 





Developing bone both periosteal and endochondral : 

 f, outer fibrous, o, inner osteogenetic layer of perios- 

 teum ; /, trabeculae of periosteal bone covered by the 



Developing bone longitudinal section 

 of embryonal phalanx : e, the primary 

 cartilage of the extremities of the bone ; 



osteoblasts ; e, endochondral bone ; m, primary marrow- a, zone of enlarged and vertically-dis- 

 cavities. posed cartilage-lacunae ; c, zone of calcifi- 



cation ; /, trabeculae of calcified cartrage 

 covered with new bone ; nt, marrow- 

 cavity ; 6, periosteal bone formed directly 

 beneath the overlying periosteum,/. 



arrange themselves over the surfaces of the cartilage-trabeculae or the 

 periosteal fibres respectively, and soon are surrounded by osseous 



