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A variety of factors contributes to the survival of preovulatory follicles. It is clear that the integrity of the germinal disc region is required for follicle viability given that its destruction results in the rapid onset of atresia in the affected follicle (Yoshimura and Bahr 1995; Yao et al. 1998). The critical supportive factors produced by the oocyte have not been unequivocally identified, but likely include one or more growth factor(s), including those from the EGF family (Yao and Bahr 2001). Continued support by circulating gonadotropins is a second factor, as hypophysectomy leads to the loss of all follicles within the hierarchy. Conversely, in vivo treatment of chicken or quail hens with FSH or equine chorionic gonadotropin (eCG) decreases the rate of atresia and increases the overall number of prehierarchal and preovulatory follicles.