At doses above 5.0 Gy, germ cell number was significantly reduced (Fig. 6, bottom): at 5.0 Gy, the average germ cell number in day 5 embryos was 101. 4 6 23.6, at 7.0 Gy, PGC number was 70.2627.1, at 7.5 Gy, PGC number was 13.36 4.1. Control day 5 embryos contained 25086235 PGCs.