1. A surgical procedure is described which permits selective removal of the medulla froms the diaphysis of the cat femur and the study of the role of marrow elements in bone regenerations and repair.

2. In this experimental model, endosteal-callus formation, in the presence of marrow, precedes and exceeds periosteal-callus formation in the healing of a partial osteotomy without displacement. Cartilage was formed infrequently in the reactive bone callus and only after severing of the fibrous portions of the periosteum.

3. After removal of the medulla, residual narrow elements in the distal and proximal metaphyses and in the surviving endosteum of the Haversian and Volkmann canals appeared to initiate the repair processes which led to complete reconstitution of the medulla. The sequence of events during bone repair repeated the phases of embryonic membranous-bone development. The restorations of the medulla and its contributions to bone healing emphasize the importance of its role osteogenesis.

4. The disruption of the nutrient vessels during removal of the diaphyseal medulla was followed by reorganizations of the greater portions of the cortex of the diaphysis associated with marked though transient periosteal new-bone formation. The response elicited resembled that seen in hypertrophic pulmonary osteoarthropathy. As healing progressed the periosteal new bone was partly resorbed and partly reintegrated into the remodeled cortex of the diaphysis.

5. The experimental offers opportunities for the study of bone inductions and for the testing of substances which might enhance or delay bone repair.