Candling eggs is always fun to do. At the same, time it can be stressful. You don't want to accidently throw out a fertile developing egg, or leave in a bad one that can potentially explode. So, far I have been luck on the exploding eggs, but have heard horror stories.
Here are two examples of what I am considering tossers (BAD eggs). You can see both have the bloodline at the bottom before the airspace. One, had developed more than the other, however since there is a bloodline, I am choosing to discard them both.
Here are two examples of good developing eggs. You can see there is either veins, or dark area (chick) developing, and no bloodlines.
We have only had to throw out 6 out of 33 eggs in the incubator. Not too shabby. They will be checked again, at lockdown (day 18).
This batch has 3 blue copper Martan, Cockoo Lavender Ophington and Blue Laced Red Whyandotte.