DEVELOPMENT OF BONE. 



95 



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Embryonal cartilage 



Cartilage-cells be- 

 coming enlarged 

 and regrouped 



nuclei. The enlarged spaces enclosing these cells are sometimes designated as the 

 prhnary areolce. 



Coincidently with these intracartilaginous changes, a thin peripheral layer of 

 bone has been formed beneath the young periosteum ; from the latter bud-like 

 processes of the osteogenetic layer grow inward from the periphery and invade 

 the embryonal cartilage, by absorption of the cartilage-matrix gaining the centre 

 of ossification and there effecting a destruction of the less resistant cells and inter- 

 vening matrix. In consequence of the penetration of the periosteal processes and 

 the accompanying absorption of the cartilage, a space, the primary marrow-cavity, 

 now occupies the centre of ossification and contains the direct continuation of the 

 osteogenetic layer. This tissue, t\\& primary viarrozu, which has thus gained access 

 to the interior of the cartilage, contributes the cellular elements upon which a double 

 r61e devolves, — to produce osseous tissue and to remove the embryonal cartilage. 



The cartilage-matrix closing the 

 enlarged cell-spaces next the pri- Fig. 125. 



mary marrow-cavity suffers absorp- 

 tion, whereby the cartilage-cells are 

 liberated and the opened spaces are 

 converted into the secondary areolce, 

 and directly communicate with the 

 growing medullary cavity. After 

 the establishment of this communi- 

 cation, the cartilage- cells escape from 

 their former homes and undergo dis- 

 integration, taking no part in the 

 direct production of the osseous tissue. 



Beyond the immediate limits of 

 the primary marrow- cavity the car- 

 tilage-cells, in tuna, repeat the pre- 

 paratory stages of increased size and 

 impaired vitality already described, 

 but in addition they often exhibit a 

 conspicuous rearrangement, where- 

 by they form columnar groups sepa- 

 rated by intervening tracts of calci- 

 fied matrix (Figs. 126, 129). This 

 characteristic belt, or zo7ie of calci- 

 ficatio7i, surrounds the medullary 

 cavity and marks the area in which 

 the destruction of the cartilage ele- 

 ments is progressing with greatest 

 energy. In consequence of the 

 columnar grouping of the enlarged 

 cartilage-cells and the intervening 

 septa of calcified matrix, an arrange- 

 ment particularly well marked in the ends of the diaphysis of the long bones, a less 

 and a more resistant portion of the cartilage are offered to the attacks of the marrow- 

 tissue by the cell- and matrix-columns respectively ; as a result of this difference, 

 the cells and the immediately surrounding partitions are first absorbed, while the 

 intervening trabeculae of calcified cartilage- matrix remain for a time as irregular and 

 indented processes, often deeply tinted in sections stained with haematoxylin, which 

 extend beyond the last cartilage-cells into the medullary cavity. These trabeculae 

 of calcified cartilage-matrix serve as supports for the marrow-cells assigned to pro- 

 duce the true bone, since these elements, the osteoblasts, become arranged along 

 these trabeculae, upon which, through the influence of the cells, the osseous tissue is 

 formed. 



Simultaneously with the destructive phase attending the absorption of the car- 

 tilage, the constructive process is instituted by the osteoblasts by which the bone- 

 tissue is formed. These specialized connective tissue elements, resting upon the 



Enlarged cartilage- 

 cells at centre of 

 ossification 



Periosteum 



Section of tarsal bone of foetal sheep, showing centre of ossifi- 

 cation. X 50. 



