By day 10 the chick is well formed. The basic body structure is relatively recognisable, the spinal chord and brain are established and the heartbeat is strong enough to be recognised with a specialist chicken egg heart rate monitor (yes, there really is such a thing and of course, I have one! see sources).
As the eggs get further into incubation the shell become more porous, allowing the blood system to take in enough oxygen to keep the embryo alive.
For that reason it's very important that, if you candle, you make sure everything, particularly your hands, is spotlessly clean.
Bacteria entering the eggs is disastrous for hatch levels. In a warm, moist environment like an incubator bacteria multiply very quickly and eggs will become bad, potentially exploding and releasing bacteria into all the other eggs in the incubator.
Always keep a bottle of a good quality hand sanitiser on your incubating table. (This is an affiliate link, which means that if you click and buy a product, I earn a small commission).
Certainly if the eggs have a white or other light coloured shell, you can see the contents easily, and there is no obvious development, the chances are it will not develop now.
As long as it's not smelling or oozing liquid, though, it's fine to leave it for a couple more days if you really can't bear to discard it.
Below is an egg I candled at day 7 and saw no development whatever. Note the 'X' mark at the top which is my shorthand for 'no signs of anything'.
But I candled again at day 12 (this pic) and there was clearly an embryo in there. See the dark spot which is the chick's eye, and the developing blood supply?
This egg developed successfully, and hatched into a Light Sussex chick – the larger of the two in the photo below. (The other is one of my Lemon Millefleur Sablepoot chicks).
Personally, if I have an egg which I'm unsure about at day 10, I mark it and candle again at day 14. If there is still no sign then – discard.
The exception to this is dark-coloured shells like the Marans, which are virtually impossible to see inside, even with a strong candling light. These I keep until the end of incubation, as long as they are showing no signs of deteriorating.
These links will help whether you want to return to the very start of this day-by-day course (button link #1), or if you need to refresh your memory about yesterday (day 9 button link). Otherwise, use the third button to move forward to tomorrow, day 11 of incubation.
A lot of "facts" you'll find on the internet are often people's individual views, based on inaccurate information repeated from poor quality sources.
The information I provide in this article and others is based on both my own experience of incubating and hatching chicken eggs every year for over 13 years, but on evidenced facts from scientific, peer-reviewed research and books from highly respected and experienced poultry keepers such as Gail Damerow.
Some of the trusted sources I have used in this article are these.
Avitronics: Heart Rates. Pub. Avian ID, 2020.
Damerow, Gail: Hatching and Brooding Your Own Chicks. Pub. Storey, 2013. See my review, here.
Hall, C., et al: A new candling procedure for thick and opaque eggs and its application to avian conservation management. Pub. Journal of Zoobiology, 2022.
Hamburger, V and Hamilton, H L: A series of normal stages in the development of the chick embryo. Pub. Journal of Morphology, 1951.
Pescatore, T, and Jacob, J.: Development of the Chick. Pub. University of Kentucky, College of Agriculture, 2019.
Swann, G., and Brake, J.: Effect of Incubation Dry-Bulb and Wet-Bulb Temperatures on Time of Hatch and Chick Weight at Hatch. Pub. Journal of Poultry Science, 1990.
Tona et al: Chicken Incubation Conditions: Role in Embryo Development, Physiology and Adaptation to the Post-Hatch Environment. Pub. Frontiers in Physiology, 2022.