IBROUS dysplasia of bone, according to F Lichtenstein and Jaffe,e arises from undiCferentiated mesenchymal cells that devclop into fibroblasts, osteoblasts, cartilage cells, and osteoclasts as a congenital anomaly similar to a hamar toma. Schlumberger found a direct metaplasia OE the fibrous connective tissue of this lesion into newly formed, partly calcified, bony trabeculae. Since many of the monostotic lesions of fibrous dysplasia of bone were antedated by trauma, Schlumberger12 suggested that they were reparative responses to in jury rather than hamartomas. T h e reparative tissue was made up chiefly of' fibroblasts and only occasionally did these fibroblasts apply themselves to the periphery of metaplastic bone as osteoblasts. It was Pritchard'ss impression that the lesion was composed chiefly of fibrous tissue and that those osteoblasts that appeared along the margins of bony trabeculae played a part in smoothing off the irregular margins of the metaplastic bone. Nearly a11 observers have considered the major tissue component of fibrous dysplasia of bone, other than the marginal osteoblasts or bone, to be fibrous tissue. Schlumberger could not find cartilage cells in the bone lesions of fibrous dysplasia, except i n association with pathological fractures, and suggested that they were not an integral part 01 the lesion. T h e osteoclasts have been noted by all observers. A varied interpretation has been given to their genesis, number, and role in the lesions of fibrous dysplasia of bone. Lichtenstein and Jaffe," as already noted, suggested that they were derivatives of undifferentiated mesenchyinal cells.