86 CONNECTIVE TISSUE CAETILAGE BONE 



3. Absorption of the cartilage matrix between the cells of the rows 



and finally also of the cells themselves. Calcification of per- 

 sistent remnants of cartilage matrix between the rows of 

 cells. 



4. Eruption of the subperiosteal osteogenic tissue, invasion to center 



of cartilage plate, and the formation of primary marrow cavi- 

 ties at the center of ossification. 



5. Gradual extension of the above processes followed by a deposit 



of primary bone by the osteoblasts upon the calcified cartilage 

 spicules. Coincident osteoblastic deposit of perichondrial bone 

 beneath and within the perichondrium of the cartilage plate. 



6. Absorption of portions of the primary bone by the osteoclasts to 



form the large central marrow cavity or medulla. The ab- 

 sorption involves both the endochondral and the perichondrial 

 bone and is accompanied by a further deposit of new bone 

 at the periphery. In the perichondrial bone cylindrical axial 

 channels are formed, in which the deposit of new bone pro- 

 duces the Haversian systems of the compact bony tissue. 



INTRAMEMBRANOUS OSSIFICATION. This is the simpler and more 

 direct method of bone formation. In principle it is identical with peri- 

 chondrial ossification. Endochondral bone development differs from it 

 only in respect of the additional processes involved in the removal and 

 replacement of the hyaline cartilage. 



Membrane bones, including the flat bones of the face and the vault of 

 the cranium, arise directly in the mesenchyma. The first indication of 

 ossification is the enlargement and rounding up of a group (or groups) 

 of mesenchyme cells, and their association in the form of an irregular 

 membrane. Among the cells appear bundles of delicate collagenous 

 fibrils, the osteogenic fibers (Sharpey), radiating beyond the limits of 

 the cell group. The cells of this initial ossific group begin to function 

 as osteoblasts and deposit osseous matrix among the fiber bundles. 

 This original osseous trabecula marks approximately the center of the 

 future bone. The surrounding loose mesenchyma has meanwhile become 

 increasingly vascular. Vaguely outlining the peripheral limits of the 

 definitive bone appears a relatively thick layer of denser, more cellular 

 mesenchyma, the cells in general maintaining a fusiform shape. This 

 represents the primitive periosteum of the forming bone. The bone takes 

 shape internally by the appearance of numerous trabecula?, which arise 

 in the manner described for the initial spicules and then unite into a 



