When the thymus is autoplastically transplanted into the marrow cavity of the tibia of a guinea pig, the transplant degenerates, regenerates, and finally atrophies. The degeneration and regeneration of the thymus when so transplanted are similar to the homologous changes occurring when it is autoplastically transplanted into the abdominal wall. For comparative study the material used in previously published work on transplantation into the abdominal wall was reviewed. A difference in the final result of transplantation of thymus in these two situations is observed. In the abdominal wall the thymus regenerates and persists apparently indefinitely. Abdominal wall thymus transplants more than a year old have been observed. In the bone marrow cavity, the regeneration, which is slower and less abundant, is followed by an atrophy of the gland, and finally its complete disappearance. This atrophy is, we believe, the result of the encasement of the thymus in a bony capsule. That the genetic character of the transplanted tissue has much to do with the success of the transplantation in the marrow, is evident from the fact that some other tissues regenerate without encapsulation, even though they may finally disappear.

The blood clot about the thymus which fills the rest of the curetted cavity, is quickly vascularized and replaced by regenerated cellular bone marrow and newly formed bony trabeculæ.

The difference in the fate of the thymus when transplanted in the abdominal wall and in the marrow, is probably due to difference in the reciprocal relations of the thymus to the tissue in those situations. From experience gained in the transplantation of other tissues into the marrow cavity, we have learned that encapsulation of the transplant does not occur with all tissues. The reaction of the marrow to transplanted thymus is similar to its reaction to any tissue foreign to it.

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