It is most likely just reproductive tissue from the hen. Once in a while they get cysty ovaries like women do if one ruptures the tissue ends up in an egg. If you find them on a regular basis it may be time to cull the hen or take her to a vet if she is a beloved pet. Eggs take a week to nine day to develop far enough to see a mass in the egg. So very unlikely to be an embryo unless the egg was being sat on.