THE STRUCTURE OF THE ELEMENTARY TISSUES. 



65 



up and down the shaft, etc. Thus the substance of the cartilage, which 

 previous!}' contained no vessels, is traversed by a number of branched anas- 

 tomosing channels formed by the enlargement and coalescence of the 

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

 vessels (fig. 6G) and spheroidal cells which will become osteoblasts. By 

 further absorption of some of the trabecnlas larger spaces are devel- 

 oped, which contain cartilage-cells for a very short time only, their 

 places being taken by the so-called osteogenetic layer of the periosteum 

 which constitutes the primary marrow. 



Stage 3. Substitution of Embryonic Spongy Bone for Carti- 

 lage. The cells of the primary marrow arrange themselves as a contin- 

 uous layer like epithelium on the calcified trabecula3 and deposit a layer 



FIG. 69. A small isolated mass of bone nex* 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 surrounded by an 

 osseous matrix. (Kleiu and Noble Smith.) 



of bone, and ensheath them: the calcified trabecula?, encased in the 

 sheaths of 7oung bone, become gradually absorbed, so that finally we 

 have trabecula3 composed entirely of spongy bone, all trace of the orig- 

 inal calcified cartilage having disappeared. It is probable that the large 

 multinucleated giant-cells termed osteoclasts by Kolliker, which are de- 

 rived from the osteoblasts by the multiplication of their nuclei, are the 

 agents by which the absorption of calcified cartilage, and subsequently 

 of embryonic spongy bone, is carried on (fig. 69, g). At any rate, they 

 are almost always found wherever absorption is in progress. 



These stages are precisely similar to what goes on in the growing 

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

 process of ossification into the intermediary cartilage between the dia- 

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

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



