I agree, it's probably the beginning stages of parthenogenesis. I could not find many records of chicken embryos developing beyond the very early stages. There were several studies done in the late 1950s to mid 1960s... one study from 1966 https://academic.oup.com/jhered/article-abstract/57/1/23/835787?redirectedFrom=PDFSince all three of your birds are clearly female, you've had them for 18 months and they've not been with a rooster for at least that long, and they have no way to get out and have a date with a rooster, then if the egg is really fertile, it would have been parthenogenesis.
Now, that's not really common, but reptiles can do it, turkeys can do it, and some chickens can too - namely the dark cornish breed is known for being able to do this. So, even though there is no way your hens mated with a rooster, there IS a slight chance that the eggs are actually fertile.
Might be fun if you could get an incubator and set some and see what happensHowever, with parthenogenesis, most eggs will not make it to hatch and do die before hatching.
"found nucleated cells in 15-18 percent of the blastodiscs of infertile Barred Plymouth Rock and White Leghorn chicken eggs. The process was of extremely brief duration, and development did not resume when eggs were incubated."
I didn't read the whole thing, but most likely that's what happened in the OP's egg.
The egg is considered infertile, because it was never fertilized.
Chicken eggs developing through parthenogenesis are almost never viable, meaning the embryo does not survive to adulthood. (there was one reference about the Cornish Cross, but not sure that any embryos successfully developed or hatched.) Most embryos stop growing early on, like the OP's egg, regardless of whether they are incubated.
