What is inside dinosaur eggs?

I'm curious to know, what exactly lies within the confines of dinosaur eggs? Are they simply hollow shells, or do they contain some sort of developing embryo? And if so, what stage of development would these embryos be in when the egg is first discovered? Moreover, what clues can we gather from the contents of dinosaur eggs to understand more about their biology and reproduction? I'm fascinated by the mysteries that lie within these ancient artifacts, and I'm eager to learn more.