The first 3 days of incubation and the last 3 days of incubation incur the most deaths...the first 3 days due to failure of the embryo to get a good start, the last 3 due to the chick failing to transfer to an air environment or due to bacterial infection or internal abnormalities.
I have cracked infertile and fertile eggs open after being set for a number of days, and while mushier looking, the yolk was still in tact. I have cracked others open and got nothing but slosh...I think it is the amount of deterioration due to bacteria (?) after a failed attempt to grow from the blastodisk into an embryo...which I think a started but failed embryo may cause bacteria to be higher and thus break down the yolk structure. Or that egg simply had a more porous shell that was more susceptible to bacteria (and hence the embryo never took hold) again breaking down the yolk structure.
But I am guessing.
Interesting question, and I might do some reading to see if I can find the answer.
Now I have a question back. I just discarded an egg at day 23 that did not hatch. We had left it for several more days as it was fully dark at one end with a clear air cell (ie looking like a fully formed chick), and had been MOVING the day before! I opened it up expecting to find a fully formed chick that had died without hatching, but only found yellow yolk mush. Okay, seriously. My daughter and I both felt and saw the egg move the day before (but not the chick inside the egg upon candling). I've opened plenty of unhatched dead chicks and they were fully formed, and while they decompose fairly quickly, they don't turn to complete yellow mush without a sign in one day.
So...WHAT MADE THAT EGG MOVE???!!!!!
I don't think it was gases because I've opened those exploding eggs up before too, and this one didn't pop upon opening.
Lady of McCamley