Phial of human sperm being removed from a flask of liquid nitrogen at the in vitro fertilisation unit, Cromwell Hospital, London. The sperm is labelled and frozen in liquid nitrogen immediately after ejaculation, where it is stored until required. Prior to being mixed with 2 or 3 ova (eggs) removed from the potential mother, the sperms are removed and allowed to attain ambient temperature: the mixing process is performed under ultra-clean conditions (temperature & pH are also critical), & examined under an optical microscope to see if fertilisation has occurred.