176 



BONE AND BONE MAKKOW 



INTRACARTILAGINOUS OSSIFICATION. This process be- 

 gins with the formation of plates of hyaline cartilage whose shape 

 corresponds more or less closely with that of the future bone. 

 This type of fetal cartilage differs from the hyaline cartilage of 

 the adult only in the irregular form and distribution of its carti- 

 lage cells. 



Each plate of fetal cartilage is enveloped by a layer of pre- 

 fibrous tissue, the fetal perichondrium. The outer portion of the 

 fibrocellular layer is destined to become the periosteum of the 

 future bone ; its innermost portion contains many small round 

 cells, which, from their intimate relation to bone production, are 

 known as osteoblasts. The inner portion of the perichondrium 

 forms the osteogenic layer of the future periosteum. 



C 



FIG. 166. THE PRIMARY CHANGES IN INTRACARTILAGINOUS BONE FORMATION. 

 A, metatarsus ; B and C, phalanges, of human embryo. In A, the earliest enlarge- 

 ment of cartilage cells at the center of ossification is shown. B and C are successively 

 later stages. The bones are cut in longitudinal section. Carmin hematoxylin stain. 

 x 27. (After Toldt.) 



Ossification of the cartilage begins at one or more points which 

 are called centers of ossification. In the long bones, in which the 

 process of bone formation can be most readily traced, there are 

 usually three such centers, one near the middle of the cartilagi- 

 nous plate, from which the diaphysis is formed, and one epiphysial 

 center at each extremity. The centers for the epiphyses make 

 their appearance much later than that for the shaft of the bone. 



