INTRACARTILAGINOUS OSSIFICATION 179 



cavities in the center. The process of ossification steadily pro- 

 gresses at the periphery, the line of enlarged cartilage cells con- 

 stantly advancing farther and farther from the original center of 

 ossification. 



It is at this stage, however, that the giant cell osteoclasts be- 

 come most active and the absorption of the newly formed bone pro- 

 gresses rapidly. The osteoclasts collect along the surface of the 

 spicules of primary bone in considerable numbers and appear to 

 sink into little recesses which they form within the bony tissue. 

 The little bays which are thus formed in the primary bone are the 

 lacunae of Howslip. The continued absorption soon breaks down 

 and removes the trabeculae and partitions of spongy bone and 

 forms a central medullary cavity of constantly increasing size. 



Coincident with these changes within the cartilage the osteo- 

 genic tissue which forms the inner layer of the periosteum produces 

 successive layers of bony tissue upon the surface of the fetal carti- 

 lage. This process of periosteal ossification proceeds in a manner 

 similar to that by which enchondral bone is formed. Osteoblasts 

 arrange themselves upon the surface of the cartilage and deposit 

 successive layers of bony tissue, between which many of these cells 

 are included as bone corpuscles. At irregular intervals the osteo- 

 clasts collect and the primary periosteal bone is absorbed. Into 

 these cavities buds of vascular osteogenic tissue push their way to 

 form canals of considerable length. Upon the surface of the 

 canals which are thus hollowed out of the periosteal bone, the 

 osteoblasts deposit successive concentric layers of bony tissue and 

 the Haversian systems make their appearance. Finally, upon the 

 surface of the periosteal bone successive layers of newly formed 

 bony tissue compose the outer circumferential lamella, while upon 

 the wall of the- medullary cavity a similar endosteal layer of bone- 

 forming cells deposits the inner circumferential lamella. 



With the formation of the periosteal bone the lateral expansion 

 of the organ by enchondral bone formation necessarily ceases. 

 Henceforth increase in diameter of the bone is only produced by 

 continued absorption of the compact bony wall and the formation 

 of new bone beneath the periosteum by frequent repetitions of the 

 processes of periosteal ossification as already described. The rem- 

 nants of those Haversian and circumferential lamellae which are 

 only partially absorbed in this process form the interstitial lamellcB 

 of the mature bone. 



During the processes of enchondral and periosteal ossification 



