Three experiments: cardiac ligature, subcutaneous implantation of glass diaphragm and regenerated calcaneus tendon transplantation, produce new bone with marrow. The mechanism proceeds in two steps: 1) after trauma or local irritation, mesenchymal fibroblasts enter in division; this young population remains fibrous indefinitely; 2) those young reactive cells, submitted to local oxygen deficiency, build new substances called "stress proteins". Under anoxia, a potent stressor, transplanted new calcaneus tendon ossifies and marrow develops in its core. No immigrant cells participate in this ossicle as it is rejected in a foreign host. Ectopic ossification is an active phenomenon, young fibroblast population building its own inductor, quite different from passive osteogenesis in which inductive message is produced outside the responsive cell. Fracture repair proceeds by an osteogenic reaction from both periosteum and marrow. A third factor is delivered by the cortex: osteogenin or BMP inductor. A fourth osteogenic factor is to be considered: "autoinduction by local stress anoxia".