Decreased mortality has been observed among embryos of rainbow trout as a result of exposure of the sperm to 25 and 50 rad of gamma rays, whereas higher doses of 200 and 400 rad result in increased mortality. The "beneficial" effect of the lower doses is more apparent during the early and intermediate stages of embryo development and the harmful effect of the higher doses is expressed mainly during the intermediate and later stages. Both effects appear to be due to hereditary changes since they each persist over many cell generations. The findings are of theoretical interest in relation to the shape of the lower end of the dose-effect curve for genetic changes, and of practical importance bearing on estimates of radiation-induced prenatal mortality in man.

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