It was realized in the very early days o f Mendelism that there is some connection between sex determination and Mendelian heredity. Even before the rediscovery o f Mendel’s2 work, Bateson in 1894 pointed out that sex is an example o f discontinuous variation, and included it in the body o f facts which, presented as Materials for the Study of Varia­ tion, nearly brought him independently to the idea of discontinuous hereditary units. Correns3 was the first to give an actual demonstration that the connection was a real one. He showed that in Bryonia the male is heterozygous for sex factors, and produces two classes o f pollen, while the female corresponds to the homozygous recessive. This was soon followed by the discovery o f unequal sex chromosomes by McClung.