In both male and female embryos, primordial undifferentiated gonads form on the medial wall of the urogenital ridges. Two pair of ducts initially develop in the gonad, the mesonephric duct (Wolffian duct), and the paramesonephric (Mullerian) duct. The primordial undifferentiated germ cells then migrate from the yolk sac to the urogenital ridge during the fourth to eighth week of gestation, where they are required for development of the gonads (testes and ovaries). In the absence of a testicular differentiation factor from the Y chromosome, which directs the production of Mullerian inhibiting substance (MIS) by four to six weeks gestation in males, the germ cells differentiate into primitive oogonia that begin mitosis at about six weeks. The first meiotic division is initiated at about 15 weeks, signaling the transformation of oogonia to oocytes. This meiotic division is then arrested at the first...