HIGHER RIGID SUPPORTING TISSUES "J\ 



of changes, beginning when it has arrived within a certain distance 

 of them. Also when it has passed it does not leave fully formed 

 bone behind it. Only when it has passed a certain distance does the 

 tissue finally appear completed. The process thus occupies a zone of 

 some width. This movement is called the line or wave of ossification. 

 At the point where it begins a blood vessel loop enters the future bone, 

 bringing with it the various bone-making cells of the perichondrium 

 and a blood supply for circulation. 



As the line of ossification approaches within a certain distance of 

 any point of the cartilage, the cartilage cells occupying that point begin 

 to change in form as well as position. The change in position is most 

 marked (see Fig. 72). Instead of lying scattered at random with 

 only a somewhat regular proximity of the daughter cells of common 

 ancestry, the cartilage cells arrange themselves in more or less regular 

 rows placed at right angles to the line of advancing ossification. 



How the cells move in order to place themselves thus is not known. 

 As the spaces that they occupy in the matrix become wider and narrower 

 as well as larger than before, and as the increasing size of this space shows 

 that the cartilage cells eat away the matrix, it is probable that they move 

 by dissolving the matrix before them. Once in line, the dissolution of 

 matrix goes rapidly on, and, as the cell spaces are nearer to those next 

 them in the line than to those in the neighboring lines, it follows that 

 the spaces in each line break through their barriers and form channels 

 with irregular strands of matrix lying between them. Sometimes two 

 or more channels will unite to become one larger one. The cartilage 

 cells, meanwhile, slowly swell up, lose their staining power, and by 

 the time the spaces are broken into, have entirely disintegrated. 



When the process first began at the center, a "bud" of the perichon- 

 dral membrane, a connective tissue covering the cartilage, was evagi- 

 nated into the space left by the first dissolution of matrix, carrying with 

 it a loop of blood vessel and its own inner layer of modified cells, that 

 have the power of secreting various salts of lime and depositing them on 

 whatever surface they may rest against. 



A layer of these cells is pushed up into each of the channels and 

 against the surface of its walls, the remnants of the cartilage matrix. 

 Here they begin the deposition of lime salts, the materials of which are 

 supplied them by the loop of blood vessel that grows into each channel. 

 These vessels follow closely behind the perichondral cells which, in their 

 new position and function, are called the osteoblasts or bone-making cells. 



The osteoblasts lay down the lime salts, first in the remnant of car- 

 tilage matrix and then in successive internal layers. Between each neigh- 

 boring pair of layers some of the osteoblasts are left in small chambers 

 of the bone called lacuna. In this position they are called bone cells. 



