It has long been known that the ovary is particularly sensitive to radiation, and that it also shows the effect of selective action in a marked degree. For these reasons the ovary of the rabbit has been one of the tissues chosen for studying the differential action of gamma radiation. As frequent use will be made of the two terms “differential” and “selective” action, it may be as well to distinguish them. Selective action: When a beam of radiation passes through a collection of cells some varieties are more affected by the irradiation than others, quite irrespective of the variation in the intensity of the irradiation as it goes from one point to another or from plane to plane. The ovary is a good example. For years it has been known that the ovary can be put temporarily, or even permanently, out of action with a dose of X rays which will cause no appreciable damage either to the skin or to the tissues lying between the skin and the ovary. Moreover, the ovary further shows selective action in that the ce...