I hatched chicks for the first time last week. I had a little chick who had the sac like yours...it was huge! after doing some research I found that it was definately the yolk sac. I was so scared cause everything i read on here said that she would die if she didnt absorb it, so I waited and instead of absorbing it ruptured!!! I thought for sure she would die....there was blood everywhere and she was still laying on her side barely moving after 2 days! well I called a vet and he instructed me to tie the tissue off with a piece of thread as close to the navel as possible and then cut the tissue away under the thread. To my surprise within a few hours she was standing and the little nub has healed.
I got lucky cuz if she wouldnt have started to eat and drink she wouldn't have made it! she is still doing great and is now my favorit chick!
