Differentiation as a Determinant of the Reaction of Rat Embryos to X-Irradiation.

X-irradiation of rat embryos on the 8th day of gestation had a more limited effect on subsequent development than did similar treatment on the 9th and 10th days. The only residual effect after doses of 121/2 to 100r on the 8th day was retardation of growth. Exposure to comparable doses on the 9th and 10th days has been shown previously to result in the development of malformations and in an increase in mortality, as well as in retardation of growth. This difference in reactivity appears to be dependent upon whether cellular differentiation within the embryos has begun at the time of irradiation.