BONE. 



49 



Second Stage or Stage of Irruption. In this stage the blood- 

 vessels and osteoblasts of the periosteum form processes which 

 absorb portions of the bone recently made by intramembranous 

 ossification, and of the walls of the primary areolae, thus pro- 

 ducing larger spaces or cavities, sec- 

 ondary areolce or medullary spaces; 

 these contain osteoblasts and blood- 

 vessels, which constitute embryonic 

 marrow. Authorities differ as to the 

 ultimate fate of the cartilage-cells; 

 some think they become osteoblasts, 

 while others teach that they are ab- 

 sorbed. 



Third Stage. The osteoblasts of the 

 embryonic matrix, increased in num- 

 ber by division, form a layer of bone 

 on the surfaces of the walls of the 

 secondary areolse. On this bony wall 

 another layer of osteoblasts forms a 

 second layer of bone, and thus the 

 process continues until only a small 

 canal remains, the Haversian canal. 

 The layers of bone, produced in the 

 manner described, are the lamellae; 

 while such of the osteoblasts as remain 

 between the lamellae become the bone- 

 corpuscles. No satisfactory explanation 

 has been given of the method of pro- 

 duction of the canaliculi. During this 

 stage the process of ossification which 

 began in the center of the bone extends 

 toward the extremities, and thus the en- 

 tire shaft becomes ossified. Histologists 

 describe the multinucleated cells (sim- 

 iliar to the myeloplaxes of the marrow) 

 which are concerned in the absorption 

 of the calcified matrix and bone under 

 the name osteoclasts, reserving the term 

 osteoblasts for the cells which form the 

 bone. 



The shaft of the bone and its ex- 

 tremities remain separated for a period 

 of time which varies in different bones, 

 and increase in length takes place by a growth of cartilage between 

 the shaft and its epiphyses. This intermediate cartilage later ossi- 

 fies, and the union of shaft and extremities is complete. Cartilagi- 

 nous at first like the shaft, the epiphyses undergo ossification in 



FIG. 39. Longitudinal sec- 

 tion through area of ossifica- 

 tion from long bone of human 

 embryo (Huber). 



