DEVELOPMENT OF BONES. '21 \ 



arranged in rows, or cell columns, between which there is a 

 fibrous ground substance. The individual cells of the columns 

 are separated by thin septa. Still nearer the medullary cavity 

 the cell lacunas are large and flattened against one another. 

 The septa of ground substance become thinner, and finally 

 vanish, and the lacunae in many places coalesce to form larger 

 cavities. 



The ground substance is impregnated with calcium salts 

 and becomes opaque. The spaces in the cartilage open into 

 the marrow cavity, which in consequence seems to have many 

 irregular cavities leading from it. Blood-vessels grow in from 

 the marrow cavity together with marrow and osteoblasts, which 

 on the inner surface of the increased medullary cavity begin 

 the formation of a bony layer. The osteoblasts gradually 

 become surrounded by ground substance which is converted 

 into bone, the osteoblasts themselves becoming bone cells. 



In consequence of the activity of the osteoblasts the whole 

 medullary cavity is lined with a thin layer of bone (Figs. 195 

 and 196), and of the original solid mass of cartilage there 

 remain only irregular pieces covered with bone. The cartilage 

 is thus converted into a spongy bone. As already mentioned, 

 the perichondral ossification goes on at the same time at the 

 surface of the cartilage (Figs. 195 and 196). This is due to 

 the activity of osteoblasts lying between the cartilage and 

 perichondrium, and in this way bone is laid down in layers 

 on the outside of the cartilage. By this so-called apposition 

 the bone increases in thickness. 



The vessels at the surface become enclosed in the develop- 

 ing bone in cavities which form afterward the Haversian canals. 

 The osteoblasts contained in these form concentrically lying 

 lamella in the ground substance of the bone. The epiphyses 

 of the long bones become ossified later than the diaphyses ; but 

 the process in both cases takes place by an endochondral and a 

 perichondral ossification. Areas of calcification are formed, 

 into which blood-vessels grow from the surface of the cartilage 

 or from the diaphysis. A medullary cavity is formed and the 

 ossified borders of the diaphysis and epiphysis approach one 



