Chick embryos were irradiated on day 4 and day 8 of incubation, and the germ cell population of the gonads was estimated with an analysis of their mitotic stages after intervals of 24 hours and 5 days. Irradiation is followed by an appreciable fall in the number of germ cells. The extent of this fall is independent of the age of the embryo at the time of treatment, and also of the interval between exposure and observation. The percentage of dividing cells increased within 24 hours after irradiation. Even after an interval of 4 days this increase in mitotic cells was persistent. The correlation between the radiosensitivity and the mitotic activity of germ cells is discussed, and it is suggested that the sensitivity of germ cells in division is independent of the influence of sex and age.

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