Not all embryos are suitable for freezing so only good quality embryos are chosen to freeze. Embryos can be frozen at different stages of their development – when they are just a single cell, at the two to eight cell stage or later in their development. The embryos are put in a special solution containing substances (cryoprotectants), which help to draw water out from the embryo and provide protection in the cells. This protects them from damage caused by ice crystals forming. They are then frozen, either by cooling them slowly or fast freezing (vitrification) and stored in tanks of liquid nitrogen until you are ready to use them.