124 THE TISSUES. 



deeply) inclosed in osseous tissue, making up the greater portion 

 of the section and surrounded by the intramembranous bone. 



In figure 91, more highly magnified, the relations of endochon- 

 dral to intramembranous bone and the details of their mode of 

 development are shown ; also the structure of the periosteum. 



As was stated in the previous section, soon after the formation 

 of the endochondral bone, this is again absorbed ; the process of 

 endochondral bone-formation and absorption extending from the 

 center of ossification toward the ends of the diaphysis. Before the 

 absorption of the endochondral bone, the intramembranous bone 

 has attained an appreciable thickness and surrounds the marrow 

 cavity formed on the absorption of the endochondral bone. Before, 



Fig. 9- Cross-section of developing bone from leg of human embryo, showing endo- 

 chondral and intramembranous bone-development. 



however, the marrow cavity can attain its full dimensions, much of 

 the intramembranous bone must also undergo absorption. While 

 intramembranous bone is being developed from the periosteum and 

 thus added to the outer surface of that already formed, osteoclasts 

 are constantly engaged in its removal from the inner surface of the 

 intramembranous bone. The marrow cavity is thus enlarged, the 

 process continuing until the shaft attains its full size. 



The compact bone of the shaft is developed from the primary 

 spongy intramembranous bone after the following manner : The 

 primary marrow spaces are enlarged by an absorption, through the 

 agency of osteoclasts, of many of the smaller trabeculae of osse- 



