Whether germ cells succeed in making eggs or sperm depends both on their genetic constitution and on the tissue environment in which they develop. The decision as to whether it is oogenesis or spermatogenesis on which they initially embark depends only on their environment, however, and not at all on their own chromosomes. The foetal testis of the mouse produces an inhibitor of meiosis: germ cells that are exposed to it develop as prospermatogonia. Germ cells in the foetal ovary enter meiosis and develop as oocytes: this may represent the default pathway for germ cell sexual differentiation, or there may exist a meiosis-inducing substance. Experimental evidence suggests that any such substance must be present ubiquitously, not just in the ovary. The stage of foetal development at which meiosis is initiated may be programmed in the germ cell lineage.