- 17 - radiation, the effects are classified as somatic or genetic. Somatic cells are those concerned with the maintenance and integrity of the individual and include cells in the bone marrow, blood, liver or nervous system; genetic cells are the reproductive cells of the gonads, which are responsible for the transfer of genetic information from generation to generation. Damage to somatic and genetic cells differs in that somatic effects are limited to the irradiated individual, whereas genetic effects may be passed on to the progeny of the irradiated individual. The biological effects of radiation vary considerably between individuals of the same species but the aifference between widely separated groups of organisms is even greater. In general the more complex the organism, the more vulnerable it is. The lethal dose, fifty per cent at thirty days (LDsq), is about 400 roentgens for man but is two to three times greater for fish. Values for other organisms are given in Figure 1 which is prepared from data reported by Donaldson and Foster (1957). Owing to the great variety of circumstances that existed in the experiments from which these data were obtained, the values in Figure 1 represent only orders of magnitude of effects.