82 



CONNECTIVE TISSUE CAETILAGE BONE 



car. c. 



Primordial Mar- 

 row Cavities. The 

 absorption of the 

 cartilage matrix re- 

 sults in the forma- 

 tion of broad spaces 

 into which osteo- 

 genic buds of prim- 

 itive marrow tissue 

 push their way from 

 the perichondrium. 

 Thus the primordial 

 marrow cavities are 

 formed. The fetal 

 marrow which now 

 occupies these cav- 

 ities is derived from 

 the osteogenic layer 

 of the primitive peri- 

 osteum. The oste- 

 ogenous tissue of this 

 layer, containing os- 

 teoblasts, osteoclasts, 

 and developing 

 blood-vessels,, grows 

 into the cartilage in 

 the form of budlike 

 cords which are pre- 

 ceded by absorption 

 of the adjacent car- 

 tilage matrix. This 

 so-called 'eruptive 

 tissue ' promptly 

 reaches the center of 

 ossification and bur- 

 rows its way into the 

 enlarged cartilage 

 lacunae whose cells are now replaced by primary osteogenic marrow. The 

 destruction of cartilage is initiated and maintained by agency of the 

 osteogenic tissue, presumably through specific cells, the so-called chon- 



FIG. 93. RECONSTRUCTION OF CARTILAGE INTO BONE. 



car. c., cartilage cells in successive stages of degenera- 

 tion; ost, osteoblasts; gi. c., giant cells (osteoclasts) ; 6, 

 young bone; bl. c., blood cells. (From Dahlgren and 

 Kepner.) 



