Egg freezing is mainly performed by vitrification freezing, which has the highest success rate, usually over 95%. Before freezing eggs in first generation IVF techniques, the mound cells must be removed and the freezing process can cause sclerosis of the zona pellucida, which must be done by second generation fertilization method, i.e. microfertilization, to form embryos before transfer. Fresh eggs are usually fertilized by in vitro fertilization, or IVF. Fresh eggs with soft zona pellucida can be fertilized by the first generation in vitro fertilization method. If the zona pellucida is sclerotic but the cytoplasm is unchanged, sperm must be injected into the egg with the help of a microinjection needle and micro manipulation to form an embryo.