50 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



The embryonic spongy bone, formed as above described, is simply a tem- 

 porary tissue occupying the place of the fetal rod of cartilage; the preceding 

 stages show the successive changes at the center of the shaft. Periosteal 

 bone is at the same time deposited in successive layers beneath the perios- 

 teum at the circumference of the shaft, exactly as described in the section 

 on ossification in membrane, and thus a casing of periosteal bone is formed 

 around the embryonic endochondral spongy bone. The embryonic spongy 

 bone is absorbed, through the agency of the osteoclasts, until the trabeculae 

 are replaced by one great cavity, the medullary cavity of the shaft. 



FIG. 59. Transverse Section of Femur of a Human Embryo about Eleven Weeks Old. 

 a, Rudimentary Haversian canal in cross-section; b, 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.) 



Stage of 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 mem- 

 brane, page 46. The irregularities in the walls of the spongy periosteal 

 bone are absorbed by the osteoclasts, while the osteoblasts which line them 

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

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

 Haversian canal, figure 59. When once formed, bony tissue grows to some 

 extent inter stitially, as is evidenced by the fact that the lacunae are rather 

 further apart in full-formed than in young bone. 



