The present writer X-rayed eggs and female moths of the silkworm and obtained the following results. (1) Irradiation of X-ray s induces lethal effect on embryos in the eggs. With increase of dosage, the uniformity of developmental processes of the embryos become disturbed, and the percentage of unhatched eggs increases proportionally. (2) Lethal effect takes place usually at the end of the embryonic stage, but a few eggs of each treated series hatch out, even when exposed to a considerably large quantity of X-rays. (3) The effect of X-rays was slightly stronger in the younger eggs than in the older. (4) If the percentage of unhatched eggs in a irradiated series, which is corrected on the basis of that of the control as 0.00 per cent, is represented by x (per cent) and dosage of X-rays at that time by r (r unit), the relation between these two variables may be shown by the following formula: (snip)