DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 17 



that they have atrophied from disuse or perhaps from the encroachment of the 

 vigorous young fibroblasts. It must be borne in mind that these degenerate forms 

 are occasionally found in areas of active proteolysis, associated with densely 

 stained cells, and that they give evidence of some phagocytic activity. They do 

 not appear all to be old cells, for some of them are quite small; young cells, however, 

 as well as mature ones, could, theoretically, fall victims to overwork, poisoning, 

 disuse atrophy, or fibroblastic overgrowth. 



The subsequent history, then, of an area such as that shown in figure 7 of the 

 third day is as follows : while the fibroblasts go on developing scar tissue the macro- 

 phages complete their work of clearing away the products of protoplasmic solution. 

 This being done, they gradually disintegrate in situ. Certain it is that these cells 

 lose their power of motility and literally die in their tracks. Becoming reduced 

 to mere cell skeletons, they collapse, break up, perhaps under the influence of the 

 enzymes of the neutrophilic leucocytes, and doubtless pass off in the tissue fluids. 

 It is quite possible that any solid fragments that remain fall a prey to the neigh- 

 boring phagocytes. This fate seems to overtake most of the macrophages. 



Polymorphonuclear leucocytes were not infrequently found in this specimen, 

 in areas where damaged tissue and macrophages were present. They were not 

 vitally stained. 



The skull sections of SI 1-2 present no additional points of importance. The 

 picture, though similar, is much less striking than in the long bones, the macro- 

 phagic tissue being comparatively slight in amount, although increased over normal. 



The callus of the long bone of 811-2 is still more extensive on the sixth day, 

 and consists, as before, of a rather delicate network of trabeculae inclosing spaces. 

 The spaces of the interior, especially near the old bone, show enlargement, so that 

 there has been some tissue destruction here. The trabeculse are not much thickened. 



Under the low-power there may be seen, in the cleared uncounterstained callus, 

 many more trypanophil cells than in earlier stages, and they are a little more dis- 

 tinctly stained, especially in the interior of the callus. As before, the dyestuff is 

 less obvious in the younger outlying tissue. The trabecula? are more dense and 

 show a fibrous structure. 



Under the high-power the peripheral spaces, smaller and of more recent origin, 

 are filled with cells and contain small capillaries. Farther back the somewhat 

 older spaces are larger and in them is noted a loose, plexiform aggregation of cells 

 (fig. 12). As development proceeds, it is evident that the cells of the spaces separate 

 to form a large-meshed reticulum, while the capillaries become larger. Later still, 

 the capillaries coalesce to form sinuses (fig. 12 BS). While this is going on, either 

 growth or breakdown of the trabeculse may take place. If the former, the walls 

 appear lined with osteoblasts; if the latter, these cells are absent and the walls of 

 the trabeculae are roughened. Both processes may be going on in the walls of the 

 same space. As has been noted, the reticulum cells are often phagocytic, as shown 

 by their trypanophihc reaction (fig. 12 M), and it is apparently in association with 

 the process of disintegration of areas of callus that these cells develop their gor- 



