INCORPORATION OF LABELED PHENYLALANINE 993 CONCLUSIONS This study investigated the effect of fetal irradiation on fetal growth and fetal incorporation of labeled phenylalanine into the acidinsoluble fraction of the fetus, both in vivo and in a fetal cell-free system. The following observations were made relative to the 24-hr period following fetal irradiation: 1. The increase in fetal weight observed in the irradiated fetuses was significantly less than that observed in unirradiated fetuses. 2. The in vivo incorporation of labeled phenylalanine into the acidinsoluble fraction of the fetus shows a small but significant decrease following irradiation. 3. The incorporation of labeled phenylalanine into the acid-insoluble fraction of a fetal cell-free system was reduced following irradiation to a greater extent than that observed in the in vivo studies. REFERENCES 1. M. M. P. Bar and T. Boullé, Ulcerations Profondes et Troubles Trophiques Graves de la Paroil Abdominale Produits par les Rayon X Chez une Femme Enceinte: Heuseuse Influence des Rayons Rouges, Bull. Soc. Obstet. Gynec. (Paris), 4: 251 (1901). 2. L. B. Russell, The Effects of Radiation on Mammalian Prenatal Development, in Radiation Biology, Vol. 1, pp. 861-918, A. Hollaender, Ed., McGraw-~ Hill Book Company, Inc., New York, 1954. 3. R. Rugh, Vertebrate Radioblology (Embryology), Annu. Rev. Nucl. Sci.,9: 493-522 (1959). 4. C. D. Van Cleave, Irradiation and the Nervous System, Rowman and Littlefield, Inc., New York, 1963. 5. S. P. Hicks and C. J. D'Amato, Effects of Ionizing Radiations on Mammalian Development, in Advances in Teratology, Vol. 1, pp. 196-250, D. H. Woollam, Ed., Logos Press, London, 1966. 6. L. Jacobsen, Low Dose X-Irradiation and Teratogenesis, Acta Pathol. Microbio, Scand, Suppl.,193: Supp. No. 1 (1968). 7. L. B. Russell and W. L. Russell, An Analysis of the Changing Radiation Response of the Developing Mouse Embryo, J. Cell, Physiol., 43: Suppl. No. 1: 103-149 (1954). . 8. S. P. Hicks, C. J. D'Amato, and D. L. Joftes, The Nature of the Radiosensitive Cells in the Nervous System Studied with Tritiated Thymidine, in Effects of lonizing Radiation on the Nervous System, Symposium Proceedings, Vienna, 1961, pp. 199-205, International Atomic Energy Agency, Vienna, 1962 (STI/PUB/46), 9. R. Rugh, L. Duhamel, C. Somogyi, A. Chandler, W. R. Cooper, R. Smith, and G. Stanford, Sequelae of the LD/50 X-ray Exposure of the Preimplantation Embryo: Days 0.0 to 5.0, Biol, Bull, 131: 145-154 (1966). 10. T. T. Job, G. J. Leibold, and H. A. Fitzmaurice, Blological Effects of Roentgen Rays. The Determination of Critical Periods in Mammalian Development with X-rays, Amer, J. Anat., 56: 97 (1935). 11. S. P. Hicks, C. J. D'Amato, and M. J. Lane, The Development of the Mam* malian Nervous System. I. Malformations of the Brain, Especially the Cerebral Cortex, Induced in Rats by Radiation. Il. Some Mechanismsof the Malformations of the Cortex, J. Comp. Neurol., 113: 435-470 (1959).