BONE 



83 



droclasts, the morphological marks of identification of which are not yet 

 known. According to some investigators (e.g., Retterer, 1900) the car- 

 tilage cells do not disintegrate but pass into the marrow cavity where 

 they become osteoblasts. 



Primary Bone. The osteoblasts which thus gain access to the pri- 

 mary marrow cavities, now arrange themselves. along the surface of the 

 remnants of calcined cartilage and 

 begin the deposit at their proximal 

 surface of the fibrous tissue and 

 calcareous salts which compose the 

 primary bone. The osseous matrix 

 is commonly assumed to be the 

 product of a transformation of the 

 exoplasm of the osteoblasts. Many 

 of the osteoblasts apparently be- 

 come entangled in this newly 

 formed tissue and form the bone 

 cells. The fetal cartilage is thus 

 transformed into a spongy mass of 

 primary osseous tissue whose spic- 

 ules are formed by a core of calci- 

 fied cartilage upon which are de- 

 posited successive layers of bony 

 tissue with their included lacuna? 

 and bone cells. In sections stained 

 with hematoxylin and eosin, the 

 central strand of calcified cartilage 

 is colored blue, the primary bone, 

 red. 



Axial sections of long bones at 

 this stage of ossification show all 

 the above changes in regular suc- 

 cession from the fetal hyaline car- 

 tilage at the extremities to the primary bone with its marrow cavities in 

 the center. The process of ossification steadily progresses toward the ends 

 of the bone, the line of enlarged cartilage cells constantly advancing 

 farther and farther from the original center of ossification. 



Absorption of the Newly Formed Bone. It is at this stage, however, 

 that the giant cell osteoclasts become most active and the absorption of 

 the newly formed bone progresses rapidly. The osteoclasts collect along 



FIG. 94. TRABECULA OF PRIMARY 

 ENCHONDRAL BONE, SHOWING A 

 CENTRAL DEEP-STAINING CORE OF 

 CALCIFIED CARTILAGE AND A PER- 

 IPHERAL LAYER OF OSTEOBLASTS. 



Osteoblasts have become incorpora- 

 ted within the bone as bone cells. From 

 the finger of a human fetus. 



