The application of the facts of criss-cross inheritance to commercial poultry breeding is widespread, and every year witnesses the hatching of thousands of chickens, the plumage-colour character of whose mothers was genetically silver, whilst that of their sires was genetically gold. Dominant silver and its allelomorph, recessive gold, form a pair of sexlinked characters, the genes corresponding to them being placed in the X-chromosome. Such evidence as exists points directly to the conclusion that in the fowl the male is the homogametic sex (XX), the female the heterogametic (XY or XO). The mating silver hen, (SX)Y, and gold cock, (sX) (sX), gives, in more or less equal numbers, gold daughters, (sX)Y, and silver sons, (SX) (sX), and, since these colours can readily be distinguished in the newly hatched chick, it is possible to identify males and females amongst day-olds by reference to their down colour. The diagnostic value of a sex-linked character depends upon the precision of the distributive mechanism of the sex-chromosomes.