DEVELOPMENT OF BONE 



55 



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

 of the cartilage, and these lamellae also become calcified (fig. 64, im). 

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

 them and become bone-corpuscles. 



FIG. 64. SECTION OF PHALANGEAL BONE OF HUMAN FCETUS, AT THE TIME OF 

 COMMENCING OSSIFICATION. (Magnified about 75 diameters.) 



The cartilage-cells iu the centre are enlarged and separated from one another by dark-looking 1 

 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 iu the new bone as lacunas. 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 periosteum 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 cal- 

 cified cartilage (fig. 65, ir). This is freely absorbed before it, so that 

 large spaces are produced which are filled with osteoblasts and contain 



