The Lethal Effects of X Rays on the Chick Embryo: the Mode and Time of Death and their Bearing on the Determination of the LD₅₀

There are four time intervals following irradiation during which the four-day chick embryo is especially liable to die. Death at each of these times is associated with a characteristic mechanism.

Using 250 kV X rays at 155 r/min, the LD₅₀ for a four-day embryo was found to be 887 r for death within six hours after irradiation, and 717 r for death within ten days after irradiation. A 9 per cent increase in effectiveness was found when the dose-rate was raised from 36 to 151 r/min. Sensitivity to radiation was found to increase by approximately 10 per cent from the third to the fifth day of incubation. It was not possible to show any differences between radiations of H.V.L. from 0·48 to 4·10 mm Cu.