Candling is an important skill to learn that is used to determine fertility rate and overall health of the embryo. It is a good idea to wait until around a week to candle eggs and do so minimally, so that the embryos are not placed under stress.
- Once you have a suitable light source (preferably bright and with a small diameter), you may hold the egg above the light source. The egg should be positioned so that the larger end is directly against the light.
What to look for:
If the embryo is healthy, there should be an array of blood vessels throughout the egg (pictured below). The embryo may be moving and you may also assess if it is growing steadily or not.
If there is a blood ring (pictured below), this will most likely mean that the embryo has died and the blood vessels have contracted. A yolker is an egg that has not been fertilised and will look relatively clear and the same as when you first put the eggs in the incubator.
Image courtesy of: http://www.notcot.com/archives/2012/05/from-egg-to-chick.php