218 ATOMIC MEDICINE poorly understood. The degree of injury of various organs is the function wernt P20 Zul cr of the amount of ionization produced within the organ and it is apparent, without extensive discussion, that clinical pictures will vary with the relative degree of injury to various organs and the importance of the organ in the over-all economy of the animal. For example, epilation may be very prominent, if conditions are such that the skin absorbs more radiation than deeper structures. This may occur if a combination of penetrating and slightly penetrating radiations are simultancously received. The protoplasmic injury itself, wherever the cells may be located, is probably produced by a combination of enzyme inhibition, alteration in cellular permeability, mitotic inhibition and chromosome damage. Concomitant with or shortly after the diffuse cellular injurv, ‘toxic’ symptoms (nausea, vomiting, perhaps diarrhea, malaise, headache, anorexia, etc.) develop. These “toxic symptoms” subside within 24 to 48 hours, and during a variable latent period, depending on the dosage, only the hematological signs are easily found. With higher doses of radiation following which there is a very short survival time, there is no latent period, no subsidence of the initial toxic reaction. The vomiting continues, diarrhea develops, prostration occurs and is followed by death within a few days. In this high dose range, in excess of the minimal amount that will kill 100 per cent of the animals, the death is well correlated symptomatically and histologically with severe gastrointestinal injury which is not reversed during the survival period. At lower doses, complete histological recovery takes place within 4 to 4 days (Brecher and Cronkite; Tullis). During the latent period, a series of cellular and histological phenomena are occurring. In the opinion of the authors, the main reason for recurrence of symptoms and ultimate death in the lethal range is the failure of adequate regeneration of hemopoiesis. There may be an adrenal component or other as yet unknownfactors contributing to the lethality but all evidence to date points towards the importance of hemopoiesis and its regeneration in survival from potentially lethal radiation injury. At any rate, the latent period is terminated with the recrudescence of symptoms and signs. The latter are well known, and in the mechanism of development can be fairly well explained as follows. « The panleukopenia is severe and progressive. In the granulopenic state, necrotizing, nonpurulent reactions develop atthe site of infections. Hemorthagic manifestations appear and mayprogress. The hemorrhage is most likely due to a combination of manyfactors, however, thrombocytopenia with a coneomitant poor clot retraction, poor hemostasis, impaired prothrombin utihzation, and the prolonged clotting time, probably can adequately explain all the phenomena. It is not necessary to introduce the concept of release of anticoagulants by irradiation in order to explain the prolongation of the whole-blood clotting time.