DEVELOPMENT OF BONE. 45 



the intervening matrix, in which a deposit of lime salts now takes place, as / 

 shown by opacity and by the grittiness when a knife is carried through such 

 ossific centres. On acquiring their maximum size, the cartilage-cells soon 

 give indications of impaired vitality in their shrinking cytoplasm and degen- 

 erating nuclei. The enlarged spaces or lacunae enclosing these cells are 

 known as the primary areoltz. 



Coincidently with, or indeed preceding, these changes within the car- , 

 tilage, a thin peripheral layer of bone has been formed beneath the young 

 periosteum, thickest around the middle of the shaft and fading away towards 

 the ends. x Bud-like processes of the osteogenetic layer grow inward from 

 the periosteum and invade the embryonal cartilage, by absorption of 

 the cartilage-matrix gaining the centre of ossification and there effecting 

 destruction of the less resistant cells and the intervening matrix. In conse- 

 quence of this invasion by the periosteal tissue, a space, the primary 

 marrow-cavity, now occupies the centre of ossification and contains the 

 direct continuation of the osteogenetic layer. 'This tissue, the primary ' 

 marrow, which has thus gained the interior of the cartilage, contributes the 

 cells upon which a double role devolves to remove the embryonal cartilage 

 and to produce bone. 



The cartilage-matrix closing the enlarged cell-spaces on the side 

 towards the primary marrow-cavity progressively suffers absorption, whereby 

 the spaces are opened, converted into secondary areolte, and brought into 

 direct communication with the expanding medullary cavity. The car- 

 tilage-cells escape from their former homes into the marrow-cavity and 

 undergo disintegration, taking no part in the direct production of the 

 bone-tissue. 



Beyond the immediate limits of the primary marrow-cavity, the car- 

 tilage cells in their turn undergo the increase in size and the impairment of 

 vitality described; in addition they often exhibit a conspicuous rearrange- 

 ment, forming columnar groups separated by intervening tracts of calcified 

 matrix (Fig. 57). This characteristic belt, the zone of calcification, sur- , 

 rounds the medullary cavity and marks the area in which the destruction of 

 cartilage is progressing with greatest energy. In consequence of the dispo- 

 sition of the cartilage elements as columnar groups separated by intervening 

 tracts of calcified matrix, a less and a more resistant portion of the cartilage 

 are offered to the attacks of the marrow-tissue by the cell- and the matrix- 

 columns respectively. As the result of this difference, the cells and the 

 immediately surrounding partitions first succumb, while the intercolumnar 

 tracts of calcified matrix remain for a time as irregular indented tapering 

 processes, deeply tinted in sections stained with hematoxylin, which extend 

 beyond the last row of cartilage-cells into the medullary cavity. x These 

 trabeculse of calcified cartilage-matrix serve as supports for the marrow-cells 

 engaged in producing the true bone, these elements, the ostcoblasts, becom- 

 ing arranged along the trabeculae upon which, through the influence of the 

 cells, the immature bone-tissue is deposited. 



Simultaneously with the destructive phase attending the absorption of 

 the embryonal cartilage by the chondroclasts, the constructive process of 

 bone-formation is instituted by the osteoblasts. The osteoblasts rest on the 

 irregular trabeculae of calcified cartilage and bring about the deposit of a 

 layer of bone-matrix upon the surface of the trabeculae, which thus becomes 

 encased within a shell of immature bone. After the latter has attained a 

 thickness at least equal to that of the osteoblasts, some of the latter are grad- 

 ually surrounded by the osseous matrix, until, finally, they lie isolated within 



