DEVELOPMENT OF BONE. 



65 



(fig. 73), and fine granules of calcareous matter are deposited in the 

 matrix. Simultaneously with this the osteoblasts underneath the 

 periosteum deposit a layer or layers of fibrous lamellae upon the surface 

 of the cartilage, and these lamellae also become calcified (fig. 73, im). 

 As they are formed, some of the osteoblasts (o) are included between 

 them and become bone-corpuscles. 



FIG. 73. SECTION OF PHALANGEAL BONE OF HUMAN FCETUS, AT THE TIME OF 

 COMMENCING OSSIFICATION. (From a preparation by F. A. Dixey.)(Magnified 

 about 75 diameters.) 



The cartilage-cells in the centre are enlarged and separated from one another by dark-looking 

 calcified matrix ; im, layer of bone deposited underneath the periosteum ; o, layer of 

 osteoblasts by which this layer has been formed. Some of the osteoblasts are already em- 

 bedded in the new bone as lacunae. The cartilage-cells are becoming enlarged and 

 flattened and arranged in rows above and below the calcified centre. At the ends of the 

 cartilage the cells are small and the groups are irregularly arranged ; the fibrous perios- 

 teum is not sharply marked off from the cartilage. 



In the second stage some of the subperiosteal tissue eats its way 'through 

 the newly formed layer of bone and into the centre of the calcified car- 



