Embryos were irradiated (γ-IR) or not (control) before the MBT (st.6) with either 20 kV (A), 30 kV (B), 40 kV (C), 50 kV (D) or 60 kV (E) of energy for the indicated times, collected at st.8 (MBT) and 4, 6 and 8 h after the MBT, and frozen. Samples equivalent to ten embryos were tested for caspases 3/7 activity using a specific colorimetric substrate as described in the “Materials and Methods” section and normalized as described in the legend of Fig. 2. Points indicate the average of ten embryos at each time stage. Results similar to those presented here were observed in three independent experiments. F. Morphology of Xenopus embryos not irradiated (control) or irradiated with 20, 30, 40, 50 kV for 24, 30, 25, 18, 15 min, respectively, and collected 8 h after the MBT. Scale bar, 250 µm.