BONE 87 



bony sponge-like structure enclosing vascular mesenchyma, the primary 

 marrow. The spicules of the cancellous bone contain numerous bone cells 

 the representatives of original osteoblasts which have become enmeshed 

 in their own product of osseous matrix and are covered with an epithe- 

 lioid membrane of a single or double layer of osteoblasts, which contribute 



FIG. 96. INTRAMEMBRANOUS BONE FORMATION IN THE LOWER JAW OF A SHEEP 



FETUS. 



a, bone; b, primary marrow cavity; c, osteoblasts; d, growing point of the primitive 

 bone, beyond which primary marrow is developing in the connective tissue of the 

 mesoblast. X 300. (After Bohm and von Davidoff.) 



to the further growth of the bony trabeculae. The marrow includes 

 besides osteoblasts and the specific marrow cells- somewhat less numerous 

 than in the primary marrow of endochondral bone numerous osteoclasts 

 under whose absorptive agency, assisted by the productive activity of 

 the osteoblasts, the inner conformation of the growing bone continually 

 alters its details. Peripheral osteoblasts, arising from the inner layer 

 of the periosteum, produce the more compact external plates of the bone. 



