DEVELOPMENT OF BONE. 



67 



rows becoming calcified, and then the calcified cartilage becoming 

 excavated from behind by the osteoblastic tissue so as to form new 

 medullary spaces (fig. 77). The walls of these are at first formed only 

 by remains of the calcified cartilage-matrix (fig. 77, c), but they soon 

 become thickened by lamellae of fibrous bone (b) which are deposited 

 by the osteoblasts, and between which bone-corpuscles become included, 



FIG. 75. LONGITUDINAL SECTION THROUGH PART OF A PHALANX OF A six MONTHS 

 HUMAN EMBRYO. (Kolliker.) 



The calcified cartilage is completely absorbed almost to the limit of advancing calcification. 

 The darker substance on either side is periosteal bone. The embryonic marrow has shrunk 

 somewhat away from it. 



as in the case of the subperiosteal bone. The latter advances paripassu 

 with the endochondral calcification, but beyond this the uncalcified 

 cartilage grows both in length and breadth, so that the ossification is 

 always advancing into larger portions of cartilage ; hence the endo- 

 chondral bone as it forms assumes the shape of an hour-glass, the 

 cylindrical shape of the whole bone being maintained by additions of 

 periosteal bone to the outside (see fig. 76). The absorption of the 

 calcified cartilage-matrix appears to be effected, as is the case with 



