INTKAMEMBRANOUS OSSIFICATION 181 



replaced by more compact bony tissue, as occurs in the wall of the 

 epiphysis. In its central portions the tissue retains its spongy 

 arrangement and but few Haversian systems are formed. It is 

 thus that the cancellous bone of this part, as also of the ends of the 

 diaphysis, is formed. 



At the point where the expanding centers of ossification of the 

 shaft and epiphysis are about to meet, a line of unossified carti- 

 lage, the epipJiysial line, persists until growth of the bone is com- 

 plete. It is by growth of this cartilaginous disk, with continued 

 formation of cartilage on its surface, that the bone increases its 

 length. 



The following is a resume of the various stages of enchondral 

 ossification : 



1. Formation of the fetal cartilages. 



2. Enlargement of the cartilage cells with a rearrangement 

 into radiating cell rows at the center of ossification. 



3. Absorption of the cartilage matrix and finally also of the 

 cartilage cells. Appearance of calcified remnants of the cartilage 

 matrix. 



4. Eruption of the subperiosteal osteogenetic tissue and the 

 formation of primary marrow cavities at the center of ossification. 



5. Gradual extension of the above processes followed by a de- 

 posit of primary bone by the osteoblasts upon the calcified carti- 

 lage. Coincident osteoblastic deposit of periosteal bone beneath 

 the perichondrium of the cartilage plate. 



6. Absorption of portions of the primary bone by the osteo- 

 clasts to form the large central marrow cavity or medulla. The 

 absorption involves both the enchondral and the periosteal bone 

 and is accompanied by a further deposit of new bone at the periph- 

 ery. In the periosteal bone cylindrical axial channels are formed, 

 in which the deposit of new bone produces the Haversian systems 

 of the compact bony tissue. 



INTRAMEMBRANOUS OSSIFICATION. In this type of bone 

 formation, ossification occurs directly within the preconnective 

 tissue of the mesoblast without the preliminary formation of car- 

 t ilaire. The earliest evidence of ossification consists in an enlarge- 

 ment of the mesenchymal cells which arrange themselves in the 

 form of a membrane at the site of the future bone. Certain of 

 these cells produce the periosteum ; others increase greatly in size, 

 acquire a considerable cytoplasmic body, and assume the functions 

 of the osteoblasts. The osteoblasts which are thus formed, not 



