Splenectomy in the dog causes, as a rule, a transformation of the fatty marrow of the long bones to a richly cellular red marrow.

During the early periods, one to three months, the change in the marrow is slight and either focal or peripheral; after six to twenty months the replacement of fat by marrow cells is complete or nearly so. Exceptions were, however, seen in four animals representing the 8th, 9th, 10th, and 22d months, respectively. The evidence at hand does not support the theory that this hyperplasia is compensatory either to the anemia caused by splenectomy or to an increased hemolysis in the lymph nodes. It is possible that it may be a concomitant of the activity of the bone marrow in taking over, in the absence of the spleen, the function of storing and elaborating the iron of old blood pigment for future utilization by new red cells, but our studies do not fully support this view.

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