1. A method has been presented for the quantitative enumeration of the bone marrow cells by adjusting the differential count for the degree of cellularity, which is derived from imprint preparations of femoral marrow.

2. This method has been employed in a study of bone marrow changes at various intervals following 700 r total body x-irradiation in the rat. The findings have been correlated with the peripheral blood counts.

3. On the basis of numerical changes and morphologic observations, the mechanism of the effects of irradiation on the various cell series of bone marrow is evaluated. The erythroid cells show rapid decreases in number due to their marked destruction. The myeloid cells chiefly undergo accelerated maturation resulting in increased segmented forms in both blood and bone marrow. The decreases in megakaryocytes, mast cells and lymphocytes appear to be related to their life spans in the absence of further production. The reticulum cells and plasma cells show neither an absolute increase nor decrease.

4. Formation of new cells, except possibly for plasma cells, from the reticulum cells is completely inhibited for the first nine days following total body irradiation of rats with 700 r. Regeneration is first apparent at twelve days after x-ray as evidenced by areas of erythropoiesis, and to a lesser extent, myelopoiesis. Megakaryocytes reappear in small numbers at fifteen days. Although this regeneration is apparent in only 50 per cent of the animals studied at twelve and fifteen days following irradiation, all these animals showed evidence of transition of reticulum cells to mast cells.

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