When a chick is sitting upright with its legs sticking out straight forward it is usually a mushy belly caused by it having absorbed all of the egg without expelling any waste yet. With large eggs this can be common. The chick is unbalanced and can't get its weight over its legs. If it is laying on its side, light weight and very weak, that can be caused by late hatching and has used too much energy trying to get out of the egg. In either case, taping it to a cardboard can correct the problem if done asap.
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