54: HANDBOOK OF PHYSIOLOGY. 



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 

 foetal rod of cartilage, once representing the femur; and the stages 1, 2, 

 and 3 show the sucessive changes which, occur at the centre of the shaft. 

 Periosteal bone is now deposited in successive layers beneath the perios- 

 teum, i. e., at the circumference of the shaft, exactly as described in the 

 section on " ossification in membrane," and thus a casing of periosteal 



B. 



FIG. 63. Transverse section through the tibia of a foetal kitten, semi-diagrammatic. X 60. P. 

 PeriosteiAn. O, osteogenetic layer of the periosteum showing the osteoblasts arranged side by 

 side, represented as pear-shaped black dots on the surface of the newly formed bone. B, the perios- 

 teal bone deposited in successive layers beneath the periosteum and ensheathin ? E, the spongy en- 

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

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

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

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



bone is formed around the embryonic endochondral spongy bone: this 

 casing is thickest at the centre, where it is first formed, and thins out 

 towards each end of the shaft. The embryonic spongy bone is absorbed, 

 its trabeculae becoming gradually thinned and its meshes enlarging, and 



