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HANDBOOK OF PHYSIOLOGY. 



angles to the plane of calcification, while the process of calcification 

 extends into the hyaline matrix between them (figs. 67 and 68). 



Stage 4. Substitution of Periosteal Bone for the Primary 

 Embryonic Spongy Bone. The embryonic spongy bone, formed as 

 above described, is simply a temporary tissue occupying the place of the 

 fcetal rod of cartilage, once representing the femur; and the stages 1, 



NV. 



Fig. 70. Transverse section through the tibia of a foetal kitten, semi-diagrammatic. X 60. 

 P, Periosteum. O, Osteogenetic layer of the periosteum showing the osteoblasts arranged side by 

 side, represented as pear-shaped black clots on the surface of the newly-formed bone. B, The peri- 

 osteal bone deposited in successive layers beneath the periosteum and ensheathing E, the spongy 

 endochondral bone; represented as more deeply shaded. Within the trabeculae of endochondral 

 spongy bone are seen the remains of the calcified cartilage trabeculee represented as dark wavy 

 lines. C, The medulla, with V, V, veins. In the lower half of the figure the eudochondral spongy 

 bone has been completely absorbed. (Klein and Noble Smith.) 



2, and 3 show the successive changes which occur at the centre of the 

 shaft. Periosteal bone is at the same time deposited in successive layers 

 beneath the periosteum, i.e., 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: this casing is thickest at the centre, where it is first formed, and 



