A standard method is described wliich gives reproducible blood volume values for rats and their various organs and tissues.

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Healing skin wounds were studied in a series of parabiotic rats . The femurs of one parabiont of each pair were shielded whilst both animals were given 800 r from a Co60 source . The animals were wounded 3 days after irradiation . Each animal with partially shielded marrow was then given tritiated thymidine intraperitoneally daily while the cross-circulation was arrested by clamping . After the thymidine-3 H had cleared the blood, the clamp was released. Animals were sacrificed, and wounds were prepared for radioautography 1, 2, and 6 days after wounding. In the wounds of the shielded animals thymidine-3H was observed in epidermis, endothelium, leukocytes, fibroblasts, and mast cells . Only neutrophilic leukocytes, monocytes, and lymphocytes were labeled, as determined by light and electron microscope radioautography, in the wounds of each nonshielded parabiont . None of the many fibroblasts present were found to contain label in the wounds of the nonshielded parabionts through the 6 day period . These observations provide further evidence that wound fibroblasts do not arise from hematogenous precursors and, therefore, must arise from adjacent connective tissue cells .

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