The Action of X-Rays on Nucleated and Non-Nucleated Egg Fragments

This report deals with the question of whether the changes produced in cells by x-rays are due to their action on the nucleus or some other part of the cell. It might be expected that irradiation effects which involve the nucleus, such as mutations, chromosome abnormalities, changes in the mitotic figure, and the like,1 are due to modifications in the nucleus produced directly by the radiation. Since, however, cytoplasm, cell membranes, and surrounding media are by necessity irradiated at the same time in practically all cases, the nuclear changes may result indirectly from changes produced in these. Only a few attempts have been made to determine which part of the cell is most important from the standpoint of vulnerability to radiation. Among the more significant of these are the classic experiments of G. and O. Hertwig (1, 2), who demonstrated the seeming paradox that slightly irradiated sperm causes more disturbance in the development of certain eggs than severely irradiated sperm. The explanation lies in the fact that both lightly and heavily irradiated sperm are capable of participating in fertilization and thus initiating development, but only the lightly irradiated ones are capable of entering into the nuclear activities of the zygotes. Thus, having undergone some injury, the lightly irradiated sperm cause more disturbance in development than the heavily irradiated ones, since the latter take no part in the nuclear functions and accordingly exert no influence on them. These experiments were repeated by Dalcq (3) and similar studies were made by Packard (4) on Chaetopterus eggs, with similar results. Likewise, we have shown that a certain radiobiological change–cleavage delay in Arbacia eggs–can be produced by irradiating sperm alone (5). Since the sperm in such cases is composed almost exclusively of nuclear material, these experiments lead to the presumption that the important irradiation effects are produced in the nucleus. Vintemberger (6) irradiated frogs' eggs through small holes in a lead plate and obtained important changes only when the nucleus was in the “line of fire.”