Exposure of mature sperm or ova to x-rays before fertilization results in abnormalities of development. When such abnormalities reappear in succeeding generations, it is plain that the cytologic changes induced occurred in the hereditary elements. In certain plant and animal forms where special cytologic and breeding tests can be performed, it has been shown that some of the abnormalities can be correlated directly with chromosome aberrations (deletions, inversions, and translocations) and also with gene mutations. In the fruit fly, Drosophila, such developmental abnormalities may be a change in the shape of wings, the number of bristles in a particular region, the number of pigmented eye facets, etc.; in plants, they may be modifications in leaf or petal structure, change in color pattern, and the like. In higher forms, including man, characteristics such as complexion, stature, mental capacity, etc., are known to be hereditary and controlled completely or in part by chromosomes. Experiments have demonstrated that various hereditary patterns in higher forms (rats and mice) can likewise be modified by exposure of mature germ cells to radiation—presumably through alteration of the chromatin material. When such chromosome changes lead to modification of vital organs in development and death results prematurely, they are called “lethal mutations.” Actually, chromosome alterations have been observed in lower forms which lead to death before the organisms are old enough to be used in breeding tests but, because of the similarity of the chromosome changes involved, they are presumed to be lethal mutations in the same sense. All chromosome modifications, which presumably can be shown to be inheritable if breeding tests could be carried out, are regarded as genetic. Irradiation of sea urchin sperm or ova before fertilization often leads to multipolar cleavage at the time of the first mitotic division of the zygote. After such changes, several subsequent divisions may occur in the cell progeny, but usually death of the new organism occurs very soon. In other embryos of the same samples, abnormalities do not show until later in development. Depending on which cell group is involved, the abnormality may result in death or various degrees of monster formation. In Drosophila, irradiation of mature sperm in the male and of mature ova in the female leads to similar changes after fertilization. Treatment of frog gametes likewise leads to the most bizarre development. In some embryos, excessive proliferation takes place with almost no differentiation; in others, differentiation occurs but it is extremely abnormal. In mammals, exposure of male or female mice or rats before conception has also led to abnormalities. When the developmental changes take place early in the higher forms and are of the lethal variety, usually resorption occurs and the only effect seen is smaller litters. When changes occur late, abnormalities may be seen in the offspring. In these types of injury, it is safe to presume that many— perhaps indeed a large portion—of the changes induced resulted from chromosome modifications. From our general understanding of heredity it is known that some mutations are inherited as dominants and others as recessives. Thus, recessive abnormalities may be masked by the dominant and not be revealed for many generations, or at least until two recessive mutant genes come together in the same offspring.