84 



CONNECTIVE TISSUE CARTILAGE BONE 



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 lacuna of Hoivship. 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. 



Pericliondrial Ossification. Coincident with these changes within the 

 cartilage the osteogenic tissue which forms the inner layer of the peri- 



chondrium produces succes- 

 sive layers of bony tissue 

 upon the surface of the fetal 

 cartilage. This process of 

 pericliondrial (periosteal) 

 ossification proceeds in a 

 manner similar to that by 

 which bone is formed in 

 membrane which is not 

 closely applied to cartilage. 

 Pericliondrial bone for- 

 mation is essentially of the 

 intramembranous type. In 

 essence there is no valid dis- 

 tinction between endochon- 

 dral, pericliondrial and 

 membrane bone develop- 

 ment, since each involves 

 calcification of a fibrillar 

 matrix by agency of the 

 same cell, the osteoblast. At 

 irregular intervals the osteo- 

 clasts collect and the pri- 

 mary pericliondrial 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 pericliondrial bone, the Haver- 

 sian spaces, 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 external circumferential lamella', while upon the 

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



FIG. 95. TRABECULA OF PRIMARY BONE FROM 

 THE FINGER OF A HUMAN FETUS. 



Three giant cells (osteoclasts) are shown at 

 the right, two resting in Howship's lacunae. 



