The present experiments represent the first portion of a study of the relationships between the host and bone transplants in rabbits and dogs. Fresh autogenous iliac grafts, as well as autogenous iliac bone with attached one-week-old callus, were transplanted to fresh defects and to defects previously created at various time intervals in the ulna (delayed bone transplantation).

Delaying transplantation until the hematoma had been largely replace by a vascular undifferentiated tissue caused the cellular elements arising from the host bone and periosteum to make more rapid contact with the transplant and to invade the transplant more quickly. Introduction of a bone transplant into a fresh or four-day-old bed, on the other hand, resulted in necrosis of all the transplanted elements, since the graft remained dormant in the blood clot and organizing hematoma until later, when contact was made with the invading cellular and osteogenic tissues. When the transplants were placed in older defects that were filled with denser cellular elements, these transplants were often isolated from the invading new bone by the formation of a barrier of connective tissue. They then became encapsulated and were removed by osteoclasis.

Proper timing of the introduction of the graft into a delayed bed appeared to be important for the development of optimum host-transplant relationships. The presence of osteoid seams along the trabecular surfaces of grafts placed in seven-day-old delayed beds in rabbits and ten-day-old delayed beds in dogs before contact of the grafts was made with the bone invading from the host, appeared to represent surviving and proliferating osteogenic elements of the transplant. The formation of new bone in the vicinity of transplanted fragments isolated within the graft bed suggested either osteogenic activity of surviving transplanted endosteal osteoblasts that had migrated into the favorable environment of the delayed bed or an induction phenomenon.