F) Percentage of oocytes harbouring one or more single sister chromatids when analysed by chromosome spreads stained with CREST and Hoechst. Where indicated, oocytes were treated with Reversine from GVBD onwards, and treatment was renewed every 8 hours. Early CSF corresponds to 8 hours after GVBD (Reversine treated oocytes undergo anaphase I on average 4-5 hours after GVBD), and late CSF corresponds to 36 hours after GVBD (>29 hours in CSF arrest). PSCS: precocious sister chromatid segregation. 