Fig. 6. A model for the antagonistic role of meiotic germ cells on the testis pathway. In normal XX gonads, germ cells follow an intrinsic clock and enter meiosis by 13.5 dpc. In normal XY gonads, expression of Sry at 10.5-12.5 dpc initiates cord formation prior to entry of germ cells into meiosis. Cord formation blocks germ cell entry into meiosis. Molecules on the surface of germ cells or short-range diffusible factors from germ cells may be permissive for testis cord formation only until germ cells commit to meiosis when their surface properties may change. A temporal mismatch experimentally created by combining meiotic germ cells with 12.5 dpc XY somatic cells led to failure of testis cord formation.