STRUCTURE OF THE ELEMENTARY TISSUES. 



51 



"branched anastomosing channels formed by the enlargement and coales- 

 cence of the spaces in which the cartilage-cells lie, and containing loops 

 of blood-vessels (Fig. 52) and spheroidal-cells which will become osteo- 

 blasts. 



Stage 2. Calcification of Cartilaginous Matrix. Lime-salts 

 are next deposited in the form of fine granules in the hyaline matrix of 

 the cartilage, which thus becomes gradually transformed into a number 

 of calcified trabeculae (Fig. 54, B ), forming alveolar spaces (primary areolce) 

 containing cartilage cells. By the absorption of some of the trabeculaa 

 larger spaces arise, which contain cartilage-cells for a very short time 

 only, their places being taken by the so-called osteogenetic layer of the 

 perichondrium (before referred to in Stage 1) which constitutes the pri- 

 mary marrow. The cartilage-cells, gradually enlarging, become more 

 transparent and finally undergo disintegration. 



Stage 3. Substitution of Embryonic Spongy Bone for Car- 

 tilage. The cells of the primary marrow arrange themselves as a con- 

 tinuous layer like epithelium on the 

 calcified trabeculas and deposit a layer 

 of bone, which ensheathes the calcified 

 trabeculae: these calcified trabeculae, 

 encased in their sheaths of young bone, 

 become gradually absorbed, so that 

 finally we have trabeculae composed en- 

 tirely of spongy bone, all trace of the 

 original calcified cartilage having dis- 

 appeared. It is probable that the large 

 multinucleated giant-cells termed "os- 

 teoclasts" by Kolliker, which are de- 

 rived from the osteoblasts by the mul- 

 tiplication of their nuclei, are the 

 agents by which the absorption of cal- 

 cified cartilage, and subsequently of 

 embryonic spongy bone, is carried on 

 (Fig. 55, G). At any rate they are 

 almost always found wherever absorp- 

 tion is in progress. 



Stages 2 and 3 are precisely similar to what goes on in the growing 

 shaft of a bone which is increasing in length by the advance of the pro- 

 cess of ossification into the intermediary cartilage between the diaphysis 

 and epiphysis. In this case the cartilage-cells become flattened and, 

 multiplying by division, are grouped into regular columns at right angles 

 to the plane of calcification, while the process of calcification extends 

 into the hyaline matrix between them (Figs. 52 and 53). 



Stage 4. Substitution of Periosteal Bone for the Primary 



FIG. 55. A small isolated mass of bone next 

 the periosteum of the lower jaw of human 

 foetus, a, osteogenetic layer of periosteum. 

 G, multinuclear giant cells, the one on the left 

 acting here probably like an osteoclast. Above 

 c, the osteoblasts are seen to become sur- 

 rounded by an osseous matrix. (Klein and 

 Noble Smith.) 



