I'm so sorry for you, how devastating. ounds like it's umbilicus alright, where the yolk sac goes in. Two of mine had a little entrail looking bit of pinky hanging off them when they hatched, but the yolk was inside them. It leaked alittle browny/orange, so I imagine it was indeed yolk. Mine just hatched too quick, one didn't make it, the other was touch and go but is now zooming around with the other two I kept. Actually I kept five, but I had to cull one at a week old and another died a couple of days later, it just wasn't thriving.