DEVELOPMENT OF BONE. 143 



consist of numerous cells, elongated in accordance with the 

 long axis of the trabeculse. These cells appear strongly granular 

 in their thicker median portion, and contain a round nucleus. 

 They resemble the osteoblasts found elsewhere, but are elon- 

 gated to a still greater extent in one of their axes. Between 

 these cells with their interlacing processes, fibres extend, 

 either singly or in small bundles, in association with which 

 the processes of these cells run, so that the several trabeculse 

 present collectively the appearance of connective tissue at 

 a certain stage of its development. At a period immediately 

 succeeding the formation of this fibro-cellular material con- 

 stituting the rudiment of the secondary bones, we may 



Fig. 16. 



Fig. 16. Bone trabeculae with osteoblastic layer, from the parietal 

 bone of a human embryo, at the fifth month. 



trace in the most beautiful manner, in preparations which 

 have been teased out, how the whole deposit calcifies and 

 acquires the character of osseous tissue. Each newly formed 

 trabecula of bone is invested on its surface with a layer of 

 osteoblasts, and in proportion to the increasing thickness of 

 the trabeculse the investing layers of osteoblasts assume the 

 character of an epithelial layer (fig 16), as it presents itself in 

 the primary medullary spaces of the long bones, or in the 

 material in which the Haversian canals are about to form. The 



