|Muscovy duck egg at 9 days incubation|
I love incubating and hatching eggs. It is so amazing to watch the babies grow in their shells. A lot of people do not know that you can see into the eggs.
The easiest ones to see into are white duck eggs because they are large and the shells are white. You set them over a strong light and as you can see in the photo you can see what is going on in there. Colored eggs are much harder, especially dark brown ones, because they obstruct the light more.
My friend Karen asked me to incubate some of her Muscovy duck eggs as her hens weren't setting the nest yet and there were a lot of eggs, maybe too many for them to cover easily when they do decide to sit on them and keep them warm. Muscovy ducks lay very large eggs which take an average of 35 days to incubate. In contrast chicken eggs only take 21 days.
In the photo the darker circle in the center is the embryo, with it's spiderweb of veins. Look at the center of that circle and see the small darker spot, which is the eye and brain beginning to form. This egg has been in the incubator for 9 days.
A lot of things can happen to an egg that will prevent it from hatching. First of course it must be fertile, but not every fertile egg will begin to grow a chick and not every egg that begins to grow will complete it's growth and hatch. Some issues are genetic, just something wrong with the chick itself and some are environmental, like getting too hot or too cold. If the humidity is too high or too low, that can also cause problems. Not even mamma hens have a perfect hatch every time.