48 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



beginning at the centers oj ossification, and spreading chiefly up and down 

 the shaft. 



Stage of Substitution of Embryonic Spongy Bone for Calcified Cartilage. 

 The cells of the primary marrow arrange themselves as a continuous epi- 

 thelium-like layer on the calcified trabecula? and deposit a layer of bone, 



\v. 



FIG. 58. Transverse Section through the Tibia of a Fetal Kitten, semidiagrammatic. X 60. 

 P, Periosteum. 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 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 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.) 



and ensheath them. The encased trabeculae are gradually absorbed by 

 the osteoclasts of Kolliker. 



These stages are precisely similar to what goes on in the growing shaft 

 of a bone which is increasing in length by the advance of the process of ossifi- 

 cation into the intermediary cartilage between the diaphysis and epiphysis. 

 In this case the cartilage cells become flattened and, multiplying by division, 



