BONE 63 



formed. During the resorption giant niultinncleated cells called 

 osteoclasts appear along the ^amellse undergoing resorption. To 

 them has been attributed a ])art in the process of dissolution 

 without other evidence than their presence in such a location. 

 The flat bones grow until they meet adjacent plates, at which 

 time a fusing of their interlacing edges follows and growth ceases. 



Endochondral Ossification. In this type of bone formation a 

 hyaline cartilage forerunner roughly outlines the bone which is to 

 replace it and after its remo\al the resultant bone formation is 

 fimdamentall>' the same as in intramembranous ossification. In 

 the center of the cartilaginous piece ossification activity is preceded 

 by enlargement of the cartilage cells and their arrangement into rows 

 accompanied by a calcification of the hyaline matrix separating them. 

 Two things happen simultaneously. From the inner cellular region 

 of the perichondrium, bud-like vascular tufts burrow into the 

 cartilage and develop a primary marrow tissue. Within the center 

 of the cartilage a number of cartilage cells and some of the matrix 

 disintegrate and are resorbed, and into the spaces thus formed the 

 tufts of primary marrow tissue extend. These invaginations of 

 tissue carry in osteoblasts and other elements of bone-marrow, 

 which continue to extend into the progressively forming cavities. 

 At first, the osteoblasts deposit bony matrix on calcified cartilage 

 spicules not resorbed in the first dissolution process and then at 

 many other points form spicules of bone in the same manner as in 

 intramembranous bone formation. (Fig. 35.) The dissolution of 

 cartilage progresses toward either end of the cartilage piece and 

 marrow tissue deposits bony spicules until the major portion of the 

 cartilage region has been replaced by spongy bone. Associated 

 with the destruction of the cartilage are chondroclasts, large multi- 

 nucleated cells. 



With the initiation of ossification within the cartilaginous piece, 

 there also occurs a change along the periphery where the inner cells 

 of the perichondrium become osteogenic, and this outer fibrous 

 sheath surrounding the developing bone becomes the periosteum. 

 In the same manner as in the plates of membranous bone, osteo- 

 blasts from the periosteum then deposit jieripheral or periosteal 

 lamella? about the spongy bone de\eloping internally. 



In this type of bone there is also a resorption of the early bony 

 matrix in the course of growth and moulding of the mature bone. 

 During these disintegration processes, tubular channels become 

 hollowed out and the osteogenic tissue projecting into them deposits 



