Candling eggs is a method of shining a bright light on a fertile egg to see what’s going on inside. The resulting shadow on the shell will silhouette a network of spider-like blood vessels and other details of the embryo inside. This technique is widely used in commercial egg production to check for signs of fertility, defects and freshness. It can also be an excellent way to teach children about the process of incubation without breaking open a real egg.
The most important time to candle is between the second and fourth day of incubation when the chick is starting to develop its form inside the egg. At this stage, the chick’s air cell begins to dip downwards, making it easy to spot.
Once this dip takes place, it is generally not a good idea to candle the egg again until around the eighth day when the chick is getting ready to hatch. This is because moving the egg around at this point can cause it to lose its dipped position.
At this point, the chick has almost fully developed, and it is preparing to move into a hatching position. As a result, the movement that began on day nine will begin to slow down and will eventually stop completely as the egg prepares to burst open. This change is often clearly visible when the egg is candled, as the blood vessels start to disappear. You can also see a small blob beginning to form on the surface of the egg.