DEVELOPMENT OF BONE. 7 



cellular substance. By one other repetition of the same process, each 

 cell producing four or five, or six young cells, a cluster is formed, con- 

 taining from thirty to fifty cells. These clusters lie in immediate 

 relation with the punctum ossificationis ; they are oval in figure (about 

 gg-g- in length by -5^ in breadth), and placed in the direction of the 

 longitudinal axis of the bone. The cells composing the cluster lie 

 transversely with regard to its axis. In the first instance they are 

 closely compressed, but by degrees are parted by a thin layer of inter- 

 cellular substance, and each cluster is separated from neighbouring 



Fig. 4." 



clusters by a broader layer (-g^Vo- of inch) of intercellular substance. 

 Such are the changes which occur in cartilage preparatory to the for- 

 mation of bone. 



Ossification is accomplished by the formation of very fine and deli- 

 cate fibres within the intercellular substance : this process commences at 

 the punctum ossificationis and extends from that point through every 

 part of the bone, in a longitudinal direction in long, and in a radiated 

 manner in flat bones. Starting from the punctum ossificationis, the 

 fibres embrace each cluster of cells, and then send branches between 

 the individual cells of each group. In this manner the network, 

 characteristic of bone, is formed, while the cells by their conjunction 



* The same cartilage at the seat of ossification ; the clusters of cells are 

 arranged in columns ; the intercellular spaces between the columns being 

 5^-53 of an inch in breadth. To the right of the figure osseous fibres are 

 seen occupying the intercellular spaces, at first bounding the clusters laterally, 

 then splitting them longitudinally and encircling each separate cell. The 

 greater opacity of the right hand border is due to a threefold cause, the in- 

 crease of osseous fibres, the opacity of the contents of the cells, and the multi- 

 plication of oil globules. In the lower part of the figure some attempt has been 

 made to show the texture of the cells. 



