The rôle played by the various components of bone in the healing of fractures has been the subject of many researches since the work of Duhamel1 in 1741. It is, of course, one of great practical importance as well as of theoretical interest. Because of the volume of the literature and the variety of opinions held, a classification of the theories of bone repair is quoted from Bancroft,2 who has summarized this subject as follows: 1. The periosteal theory presupposes that the periosteum and endosteum are definite organs for bone formation and repair, and that the bone cells arise from them and from no other source. 2. The osteoblastic theory may be divided into two subtitles. Type A. This assumes that in bone repair following injury bone cells are liberated from their lacunae and that they reproduce and form new bone. Type B. This assumes that following injury