After a two-step loading with equilibration solution containing 7.5% (v/v) ethylene glycol and 7.5% (v/v) dimethyl sulfoxide, and vitrification solution containing 15% (v/v) ethylene glycol, 15% (v/v) dimethyl sulfoxide and 0.5 mol/L sucrose, embryos were loaded with a narrow glass capillary onto the Cryotop in a volume of < 0.1 µL . After loading, almost all the solution was removed to leave only a thin layer covering the embryos, and the sample was quickly immersed into liquid nitrogen (LN). Subsequently, the plastic cap was pulled over the film part of the Cryotop, and the sample was stored under LN. At warming, the protective cap was removed from the Cryotop while it was still submerged in LN and the Cryotop was immersed directly into a 37˚C medium containing sucrose. The embryos were then sequentially incubated in diluents solution before further in vitro culture for transfer.