So, when people say that hens are born with as many eggs as they will ever have, that's not entirely accurate. They are actually, effectively, born with the building blocks to produce only a certain number of eggs.
It's a bit complicated and you'll have to forgive me if I'm missing details as it's been quite a while, but to my memory the way it works is like this. Within the developing embryo of a female animal are what's called primordial germ cells. These germ cells are multiplying through mitosis, the process of cells splitting into identical duplicates of themselves, up until a certain point during embryonic development. I don't remember exactly when that is, I think around 2 weeks along in the development of a chicken embryo?
Anyway, at that point, those germ cells stop multiplying via mitosis and switch over to meiosis. Meiosis, of course, is the process by which a cell splits into non-identical cells, producing gametes, in this case true ova or egg cells. When finished with meiosis, egg cells will each have a set of un-paired chromosomes that can be paired with the set of un-paired chromosomes within a sperm cell should fertilization occur. However, these germ cells do not complete meiosis at this point; they freeze during the first phase of meiosis and instead become what are called oocytes, cells that are capable of becoming an egg cell in the future. The oocytes do not complete the meiotic process until after the pullet has hatched, grown, and is approaching point of lay. The influx of hormones associated with that age triggers oocytes one by one to 'un-freeze' and finish meiosis, only developing into true egg cells at that point.
In other words, what a pullet has within her as an embryo and young chick are not truly egg cells. At that point they're still packets of paired chromosomal DNA, including a ZW chromosome pair, and temperature differences during incubation would not change anything about that. It's only after the oocytes are triggered to finish meiosis when the pullet is approaching point of lay and those chromosome pairs have been split that the egg cell is truly formed, and during the process that egg cell has kept either a Z chromosome or W chromosome to determine whether the chick it forms will be male or female.
If anything I've said here is not making sense, feel free to ask for clarification! I never know if I'm wording things well or making things even more confusing
Congratulations, Fluffy! 
