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.Tried orally giving him a drop... evidently it was too late. Within a matter of minutes he passed away. Hate to do it, but I think I’m going to have to find out was that mass was..