In the formation of the fetal rat cerebral cortex, undifferentiated cells proliferate in the primordial ependemyl layer on day 13 of gestation and begin to migrate through the intermediate zone on day 14 to form the cortical plate. The aims of this study were to evaluate whole-body gamma radiation (50, 100, 150, 200 R) and hypothermia-hypercapnia exposure on the 13th day of gestation on the formation of the cortical hemispheres on day 19 of gestation. Fetal brains were removed and subjected to qualitative and morphometric assessment of the various exposure effects on cellular and tissue derangements in the cerebral hemispheres and the cross-sectional area ($\mu {\rm m}^{2}\times 10^{4}$) of the cortical plate. Dose-dependent cellular and tissue derangements were observed in the cortical wall and in the mean cross-sectional area of the cortical plate. Significantly lesser effects on the cerebral hemispheres and cortical plate were observed after two fractionated exposures, with an intervening 6-hr interval, than with single equivalent doses. Hypothermia-hypercapnia alone had no observable effects. Hypothermia-hypercapnia interposed in the 6-hr interval between fractionated exposures resulted in greater damage, comparable to single rather than lesser fractionated dose effects. The single-fractionated dose differences and hypothermia-hypercapnia reversal of fractionated dose decreases were interpreted as an inhibition of repair or recovery in the formation of the fetal rat cortical plate.

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