In long continued severe anemia due to bleeding in dogs we may observe all degrees of red marrow spread. The maximal red marrow spread takes in the total marrow area in ribs, vertebrae and long bones. The minimal red marrow spread may involve but 10 to 20 per cent of this maximal area. All gradations between these extremes are observed in this series.

The extent of the red marrow spread is not dependent upon the length of the anemia period nor is it related to the capacity of the dog to produce new hemoglobin and red cells on standard diets.

Evidence points to the liver as concerned with this hemoglobin production and liver function may set the limits for hemoglobin production on these standard diets. This in turn may determine the needful red marrow spread.

During favorable diet periods there is a storage of hemoglobin or hemoglobin precursors which come out later in the control periods as finished red cells. This reserve of hemoglobin producing material is not stored as mature red cells in the marrow or elsewhere but as intermediates stored probably in the liver and red marrow but also perhaps in the kidney and spleen.

Red marrow very rarely gives evidence of exhaustion even after many years of continuous severe anemia (Dog 18-114). This would seem to consist of a shrinkage of the red marrow area rather than a degenerative change of the red marrow cells.

The spleen shows some evidence of erythrogenesis in these dogs. Megakaryocytes in some instances are conspicuous (Fig. 6) and nests of nucleated red cells are found in the spleen pulp but it is unlikely that the spleen contributes any large amount of red cells to the anemic circulation in these experiments.

There is no evidence that erythrogenesis occurs in the liver or lymphatic tissue in these dogs.

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