The phenotypic sex of an individual mammal is determined by the sex of its gonads, i.e. testes or ovaries. This in turn is determined by the presence or absence of a small region of the Y chromosome, located near the X-Y pairing region in man and on the short arm of the Y chromosome in the mouse. The testis-determining region of the Y appears to exert its primary effect by directing the supporting-cell lineage of the gonad to differentiate as Sertoli cells, acting at least in part cell-autonomously. The phenotypic sex of a germ cell, i.e. whether it undergoes spermatogenesis or oogenesis, is determined at least in the mouse by whether or not it enters meiotic prophase before birth. This depends not on its own sex chromosome constitution, but on its cellular environment. A germ cell in or near normal testis cords (made up mainly of Sertoli cells) is inhibited from entering meiosis until after birth; one that escapes this inhibition will develop into an oocyte even if it is in a male animal and is itself XY in chromosome constitution.