Petri dishes containing unfertilised human ova (eggs), prepared prior to being mixed with human sperm at the in vitro fertilisation unit at the Cromwell Hospital, London. The ova are removed from the potential mother and pipetted into the dishes (2-3 in each dish): the pink colour of the medium in which they are suspended is due to an acid-base indicator (pH & temperature are critical factors). Cleanliness is also vital; samples are prepared & mixed in an ultra- clean, dust-free cabinet (with a laminar air-flow throughout), to minimise risk of contamination.