During the past decade the problem of the anemias has been approached by new paths. Interest centers not so much in the morphology as in the physiology of the blood, the way the hematopoietic system performs its function. The function of the blood forming organs is to preserve the hemoglobin, and the red and white cells at a certain level. This stability is maintained by a balance between two opposing forces; the continuous destruction of blood cells is compensated by an equivalent regeneration.
There is no single criterion of blood regeneration. When the percentage of hemoglobin, and the number of red and white cells fall within normal limits, and there are no abnormal cells in the blood stream, blood regeneration may be considered normal. The presence in the circulation of immature cells reflects the degree of strain under which the bone marrow is working. Nucleated forms, normoblasts and megaloblasts, indicate