THE STRUCTURE OF THE ELEMENTARY TISSUES. 



67 



thins out toward each end of the shaft. The embryonic spongy bone is 

 absorbed, through the agency of osteoclasts, its trabeculae becoming 

 gradually thinned and its meshes enlarging, and finally coalescing into 

 one great cavity the medullary cavity of the shaft. 



Stage 5. Absorption of the Inner Layers of the Periosteal 

 Bone. The absorption of the endochondral spongy bone is now com- 

 plete, and the medullary cavity is bounded by periosteal bone: the inner 

 layers of this periosteal bone are next absorbed, and the medullary cavity 

 is thereby enlarged, while the deposition of bone beneath the periosteum 



Fig. 71. Tranverse section of femur of a human embryo about eleven weeks old. , Rudimen- 

 tary Haversian canal in cross-section; 6, in longitudinal section; c, osteoblasts; d, newly formed 

 osseous substance of a lighter color; e, that of greater age; /, lacunae with their cells; g, a cell still 

 united to an osteoblast. (Frey.) 



continues as before. The first-formed periosteal bone is spongy in 

 character. 



Stage 6. Formation of Compact Bone. The transformation of 

 spongy periosteal bone into compact bone is effected in a manner exactly 

 similar to that which has been described in connection with ossification 

 in membrane (p. 61). The irregularities in the walls of the areolae in 

 the spongy bone are absorbed, while the osteoblasts which line them are 

 developed in concentric layers, each layer in turn becoming ossified till 

 the comparatively large space in the centre is reduced to a well-formed 

 Haversian canal (fig. 71). When once formed, bony tissue grows to 

 some extent interstitially, as is evidenced by the fact that the lacuna are 

 rather further apart in full-formed than in young bone. 



