I think I might take a print of that thingy to my biology class tonight.
After researching how the ovaries actually work (the ovary makes the yolk, and the rest of it gets added on as it travels down the tube) and since I think you said she had stopped laying for a while, it could be that it did sorta cook as it traveled too slowly down the tube, and the normally thin layers piled up on top of it, cooked. It's not a fetus, though. Chickens don't work that way. The Cornell Atlas of Avian diseases shows all sorts of gross pics of tumored ovaries. She could easily have passed a tumored egg yolk.
I'm just throwin' out ideas. I have no real evidence to support my hypothesis here.
Ask me again in ten years when I'm a vet. LOL!