Weird Egg From clutch

So I had an 8 egg clutch and one egg didn’t hatch. We opened it up and the fetus was underdeveloped and there was a huge mass inside the egg with the fetus and it doesn’t look like yolk almost like an undeveloped twin? Taking it to my vet for dissection what do you guys think?

The mass you are referring to is just the yolk sac. It will change color and sometimes become hard if the baby is no longer viable.