During recent years the science of bone transplantation has been occupying the attention of orthopedic surgeons to a steadily increasing degree. While the earlier operations were pure experiments attended by success or failure, depending on whether the surgeon chanced to employ the principles that are essential to success, the mass of evidence now at hand as the result of experimentation on animals has clearly outlined these principles and placed the clinical employment of the procedure on a sound scientific basis. This is most fortunate at the present moment when thousands of soldiers are returning to our hospitals with injuries to the bones that urgently call for radical operative treatment before the men can be fitted to return to useful civilian life. CHANGES IN TRANSPLANTED BONE The experiments on which we have based the principles that have guided our operative work have been Fig. 2.—High power photomicrograph of one of the