4 8 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



gradually becomes transformed into calcified trabeculse, figure 57. The en- 

 larging cartilage cells become more transparent, and finally disintegrate, the 

 spaces occupied by them forming the primordial marrow cavities. During 

 this stage the perichondrium has become the periosteum, and is beginning 

 to deposit bone on the outside of the cartilage. 



FIG. 57. Developing Bone of Femur of the Rabbit. X 350. a, Cartilage cells; 

 b, cartilage cells enlarged in the region of calcifying matrix; c, d, trabecuhc of calcifying 

 cartilage covered with e, osteoblasts; /, osteocla'sts eroding the trabecuke; g, h, disap- 

 pearing cartilage cells. The osteoblasts are seen to be depositing layers of bony sub- 

 stance. Loops of blood vessels extend to the limit of the region in which the bone is 

 forming. (Schafer, from Klein.) 



Stage of Vascular ization of the Cartilage. Processes from the osteo- 

 genetic layer of the periosteum containing blood vessels break through the 

 bone into the primordial marrow cavities and form the primary marrow, 

 beginning at the centers of 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- 



