Candling eggs is a very exciting time. Candling a week after incubation allows you to find out if an egg is fertile and allows you to remove clear eggs. Candling again after another week allows you to remove any ‘dead in shell’ embryos, those that haven’t developed further and usually only show a blood ring inside.
A dark room and a bright torch is all you really need, even an old toilet-roll cut to fit the shape of the egg and held up towards the sun will work well.
With a freshly laid egg, you will see an almost clear egg. There is a feint shadow which is the egg yolk. This is called a ‘Clear Egg’ and is how the egg will remain during the first week of incubation if it is not fertile.
As the embryo develops, over the next few days, a very small darker patch will develop on the ‘shadow’ of the yolk. This is the germainal disc and takes an expert eye to spot this.
As time goes on, blood vessels begin to develop and become thicker and more visible and the embryo grows in size, until day 8 when the eye and the heart have developed and the embryo is starting to take the shape of a chick. If you are lucky, you may see the little peeps heart beating!
The air sack is easy to see. This grows as the egg develops and contains the first few breaths of air the chick will take when it pokes it little beak through on day 21.
The yolk sack itself I have shown in red (why? Because yellow didn’t show up too well and the yolk sack is so full of red blood vessels, it’s now pretty much all red).
The blood vessels emerge like spiders legs into the alubumen (egg white), the thicker of these can be seen when candling your eggs.