Vitrification is an egg freezing process in which water molecules in an embryo are removed and replaced with a cryoprotectant. The embryos are then plunged directly into liquid nitrogen. This sudden & drastic freezing creates a glass transition known as glass state. This reduces the formation of ice crystals as in slow cryofreezing. Once frozen embryos are contained within labeled sterile straws, which are immersed in liquid nitrogen. These can be safely stored like this for years.