for SRY, the Y-chromosomal gene responsible for sex determination in mammals. The urogenital ridge is made up of the gonadal anlage and the mesonephros. The mesonephros is involved in the development of the testis, but its role is in differentiation, and not determination. This is indicated by the absence of SRY expression in the mesonephros. SRY expression is expressed exclusively in the developing gonad, lacking a presence in any other tissue in embryos or adults. See also Development of the gonads References External links Embryology of urogenital system