At the time of ovulation, the unfertilized egg is a tensioned spring waiting to be sprung.

Unlike sperm cells (and unlike fertilized eggs), recently ovulated eggs -- and eggs obtained from follicles just before ovulation in the ordinary course of IVF -- are tense with electro-physiological actions waiting to happen. When penetrated by a sperm, there's an electrical signal that spreads quickly through the egg to do several things:

- packets of mucus are released by the egg, producing a mucus coat that stops more sperm getting too close;

- the second division of meiosis is completed and the second polar body expelled; and

- the egg begins taking apart the sperm head that will change it into the male pronucleus, ready to merge with the female pronucleus (the process that ends in syngamy-see Chapter 3-by which 46 chromosomes are formed by the 23 from the sperm cell and the 23 from the egg cell).

This vulnerable electric state of the egg -- and the structures called spindles that hold the egg's chromosomes in the right place for completing meiosis and for fertilization -- are vulnerable to a fall in temperature. For example, 10 minutes for an egg at ordinary lab temperature (say, 21°C or 70°F) instead of body temperature (37ºC or 98.4ºF) is enough to produce irrecoverable damage to eggs in IVF programs.

A Melbourne embryologist, Deborah Gook, has had success with quickly freezing and thawing eggs to preserve these spindles (see 'vitrification' in the box, Nature's antifreeze), but many scientists believe the spindles are too delicate -- and the consequences of disrupting the spindles and producing chromosomal aneuploidy too serious -- for this procedure to be relied on and to become widely used.