Within recent years much has been learned of the physiology of the blood-making organs from the study of the circulating red blood cells. By determinations of the number of erythrocytes and their size, form and relative content of hemoglobin in the presence of anemia it is often possible to recognize a specific defect in their development and in some instances to supply the deficient factor. Those anemias characterized by small red blood cells and a low color index are generally attributed to a lack of iron, whereas large erythrocytes, carrying more than the usual complement of hemoglobin, are considered to denote anemia of the pernicious group dependent on interference with the maturation of the erythrocytes and due to the lack of a substance found in liver.1Although deficiencies other than iron and the factors which make up the liver principle have been suggested as possible causes of impaired blood