The first step in gonadal development takes place when the primordial germ cells migrate from the allantois (portion of the placenta) to the genital ridge, the structure that will become the undifferentiated gonad. The gonadal ridges are protuberances within the coelomic cavity of the developing fetus. The structures of the primitive gonad that are not composed of primordial germs cells (i.e. somatic gonadal cells) are derived from local mesenchymal cells, coelomic epithelium and cells derived from mesonephric tubules. Ultimately Sex determination of the undifferentiated gonad is dependent on the presence of the sex determining region Y (SRY) gene found on the short arm of the Y chromosome. Therefore without the Y chromosome and SRY gene, differentiation to a female gonad will take place.