Traditionally, a candle was used to scan eggs, and the process became known as candling. Obviously, complications could arise when eggs were held up to hot candles, so thanks to recent developments in technology, brighter and cooler light sources are available which do not carry the risk of harming the development of the embryos. By shining a  Candling Torch through the shell, you can often make out signs of progress as and when they take place.

• After seven days, a living embryo will reveal a complicated system of blood vessels which appear to be growing around a dark blob of matter, as shown in the egg on the top left of the diagram above. This blob is the embryo itself, and the network of blood vessels will develop as the days go by. A conspicuous air space at the fat end of the egg will also be clearly visible, and this space will expand as the process continues.

• Any problems early in the incubation process could well kill the vulnerable embryo, and this will become clear during candling. Early death is often recognised by the presence of a narrow smear of blood inside the egg shell, which will develop in time into an unbroken loop of rotten blood, as shown in top and bottom center of the diagram above. The loop may swill back and forth as you move the egg, and this is a fairly certain sign that the embryo is dead.

• It may be that the egg is infertile, in which case you will see no sign of development whatsoever. If you see no progress, don't despair. Put the egg back into the incubator and scan it again in a few days. All embryos develop differently, and it could be that a second scan will reveal obvious signs of progress.

• If you have bought eggs in from a distant breeder, it may be that the internal makeup of the egg has been damaged in transit. This is particularly common in eggs which have been sent through the postal system. While there may be no visible cracks or damage to the shell, the air pocket at the blunt end of the egg is liable to break loose and swill around inside the shell. If this has happened, it's very unlikely that you will get a successful hatch. Sometimes, the air sack is damaged but remains intact. These eggs may start to develop, but they very rarely reach full term and even if they do, the chick will probably have a great deal of trouble hatching.