Technician working under sterile conditions in the in vitro fertilisation unit at the Cromwell Hospital, London. Ova (eggs) removed from a potential mother are being pipetted into petri dishes (2-3 in each dish) prior to being mixed with sperm (stored in a frozen state using liquid nitrogen) & examined microscopically for possible fertilisation. The procedure is carried out in a special cabinet, providing a dust-free area with a laminar air-flow, to minimise contamination risk. Temperature & pH are other important criteria: an acid-base indicator is added to the medium to monitor pH.