Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 123 



this tube gradually spreads towards either end of the bone until 

 it becomes a delicate but compact osseous sheath. It is pro- 

 duced by an osteogenetic layer (with osteoblast cells), which is 

 developed on the inner face of the membrane. This layer of 

 compact bone, with its well-defined Haversian systems, gradually 

 increases by exogenous growth beneath the periosteum. And 

 as, with the general growth of the bone in thickness, the marrow 

 cavity widens all the spongy bone is here absorbed, and in the 



\^^yi-"j^^^^-~^^ :.^^'-. J.-*^i , 



FIG. 39. DIAGIIAM OF BONE FORMATION. 



Longitudinal section through head of bone. 



1. Cartilage. 2. Columnar cartilage. 3. Vascularized cartilage. 4. Spongy 

 calcined cartilage. 5. Region of deposition of bone lamellae. 6. Reabsorp- 

 tion of bone to form medullary cavity. 7. Compact periosteal bone. 



completed femur the central shaft is entirely composed of com- 

 pact periosteal bone. At the ends of the growing bone, however, 

 endochondral bone is still in process of formation, so that the 



