It has been estimated that 92 per cent of the total radiation emitted by radium in equilibrium with its subsequent products is given off in the form of α-rays. This, however, cannot be utilized when the source is enclosed in an ordinary container, because the α-rays are absorbed completely by even a small thickness of glass. About 3.2 per cent of the total radiation is emitted in the form of ß-rays, and 4.8 per cent as gamma radiation. The effects produced on the radiated mice of these experiments were due mainly to the ß-rays, which are easily absorbed by tissue. The γ-rays, being only slightly absorbed by organic matter, probably contributed very little to the observed effects.
It is interesting to correlate the different effects produced by the same dose of radiation. The mice which received a dose of 1.9 millicurie hours showed no local effects on the skin or hair. Neither females nor males were sterilized, and the time at which they opened their eyes or reached sexual maturity was not affected, as far as we could tell. The only difference noted between the radiated animals and the controls was in the body weight. This dose accelerated the growth of the young mice, that is, while initially of the same weight, soon after irradiation they became distinctly bigger than the controls, but finally the animals of each group had substantially the same average weight. That this variation in body weight should be accidental is unlikely, since it was observed also in the animals treated by a slightly larger dose (2.4 millicurie hours). The number of animals (seven) which showed this effect is too small to prove conclusively the accelerating effect of small doses of radiation on the body growth of mice. But considering that similar results have been. obtained by radiating plants and beetles, it is reasonable that the observed increase in weight might be attributed, at least in part, to the effects of radiation. Since this paper was first written Russ, Chambers, and Scott have shown that small doses of x-rays accelerate the body growth of rats. In view of this additional evidence there can be little doubt that the increase in weight observed in our experiments was due to the radiation.
A dose of 2.4 millicurie hours applied over the backs of the animals produced no local skin effects, but it accelerated the growth of the mice as in the previous case. In addition it caused permanent sterilization of all the females. A similar result was obtained with 4.9 millicurie hours, except that the effect on the rate of growth was uncertain. A dose of 6.8 millicurie hours produced a definite but mild skin erythema and retarded the development of lanugo hair. But since in this instance the emanation was applied over the heads of the animals, the dose reaching the ovaries was not sufficient to cause sterilization, as already explained. No other definite effect was noted.
In connection with the sterilization of the females it should be noted that a dose of radiation which produced no visible skin changes was sufficient to cause permanent sterility. On account of the greater distance of the ovaries from the source of radiation as compared with that of the skin directly below the tube, and the depth of tissue which the rays had to traverse to reach the ovaries, the amount of radiation acting on the latter was much smaller than the amount falling on the skin. The radiation emitted by the emanation tube is reduced to about 50 per cent of its initial value after traversing 1 mm. of tissue. Still, while the skin was not visibly affected, the mice were sterilized. This shows that the ovaries are influenced very easily by radiation of this type. We can estimate the amount of radiation reaching the ovaries which is sufficient to cause sterility to be less than 25 per cent of the amount necessary to produce visible skin changes in the mice. It should be noted also that whenever sterility of the female mice was induced, it was permanent. Furthermore, those mice which were not rendered sterile by radiation were, as far as the experiments enable us to say, as prolific as the controls. Remembering that a dose of 1.9 millicurie hours had no apparent effect on the ovaries, while a slightly larger dose, 2.4 millicurie hours, caused permanent sterility, it might be concluded that it is not possible to produce temporary sterility by radiation. We know, however, that temporary sterility can be produced, at least when the animals are radiated at a later stage in their development. The mice in our experiments were radiated for the first time soon after birth, and it is not improbable that under these conditions temporary sterility cannot be obtained.
Large sublethal doses produced severe skin burns, retarded the body growth of the animals, but failed to sterilize the males. About one-third of the total skin area of the mice showed marked effects from the radiation. The animals were very sick for a time, and their growth was temporarily stunted. But nevertheless they recovered and finally became apparently normal except for the narrow hairless strip of skin which had been closest to the emanation tube. Only the females were rendered permanently sterile. The males did not show even temporary sterility when the doses of radiation were close to the lethal dose. While the testes of mammals are known to be very easily affected by radiation, still they are more resistant than the ovaries. In addition, in these experiments they were at a greater distance from the source of radiation than the ovaries, and they were better protected by the thicker layer of tissue in the path of the rays. The fact that no sublethal dose in these experiments sterilized the males shows that under the conditions of irradiation adopted the amount of radiation reaching the testes was not sufficient to affect them noticeably. If the source of radiation had been applied closer to the reproductive organs of the males, they would have been sterilized by millicurie hour doses much smaller than the lethal dose.
Some of the radiated animals were killed with ether, and macroscopic and microscopic examinations of the reproductive organs were made. The ovaries of the sterile females were generally atrophied and colored yellow. The normal histological structure was altered. The characteristic findings were the destruction of the Graafian follicles, with absence of ovum cells. The testes and the epididymis of the radiated mice of the present experiment appeared macroscopically and histologically normal, with the presence of abundant spermatozoa. Owing to the method adopted for the irradiation of the mice, the testes were too far from the source of radiation, and too well protected by the intervening tissue to be definitely affected by the rays.