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Here is some info for you. It's a little technical, but you might find some of it useful: https://www.backyardchickens.com/threads/yolk-sac-problems-graphic-pictures.1052862/

Very sorry for your loss. To me it looks like it failed to absorb is yolk, so the cause of death was probably related to that. Can you cut open the yolk sac and take pictures of it? The mass was probably the gizzard.