58 ELEMENTS OF HISTOLOGY. [Chap. vn. 



cartilage at the expense of which the new bone is 

 formed, by the marrow (blood-vessels and marrow- 

 cells or osteoblasts) of the spongy substance in con- 

 tact with the cartilage. 



72. Following the development of a tubular bone 

 after the above-mentioned stage 4, we find that the 

 spongy bone once formed is not a permanent structure, 



Fig. 35. From a Transverse Section through the Tibia of Foetal Kitten. 



a, Fibrous layer of the periosteum ; 6, osteogenetic layer of the periosteum ; 

 c, periosteal hone: d, calcified cartilage not covered yet by hone; below this 

 layer the trabeculoa of calcined cartilage covered with plates of hone- 

 shaded darkly in the figure ; e, boundary between periosteal and endochondral 

 bone. (Atlas.) 



