The lethal embryonic effects of hypoploid and hyperploid chromosomes has been studied utilizing translocation stocks. One stock, XXY, with two translocations and a second stock, found in the author''s laboratory, with a reciprocal translocation between chromosome II and III. The lethal effect of a deficiency of the X chromosome occurs in the initial stages of development, those hypoploid for a single arm of chromosome II or III at an intermediate stage, while those hyperploid for identical chromosome arms are lethal in the final stages of embryonic development. In such stocks genetic mutants and those with a small deficiency show similar manifestations to those with hyperploid chromosomes.