Normal young albino mice were treated with a single intraperitoneal injection of urethane, and the effect of the drug on the mitotic activity of the bone marrow was studied. A notable increase of the mitotic index was observed from ten to forty-four hours after the administration of the drug. At the same time the percentage of prophases was decreased, while the percentage of metaphases was increased. Abnormal mitoses were frequent. Mitotic figures which might be interpreted as reversions from the meta- and anaphase stages to a resting nucleus were seen. Promyelocytes with highly lobated nuclei of bizarre shape appeared.

No changes were found in the myeloid-erythroid ratio, or in the maturation rate of the white blood cells.

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