The great majority of the development the chicken fetus is a water living organism just like all other vertebrates, but unlike the embryo of mammals which is provided with energy from the mother the avian embryo have all energy it needs with it inside the egg. The only thing the egg needs to develop and hatch is suitable temperatures, the right humidity and a surrounding which contains the right combination of gases.

This makes the avian embryo a perfect model for research on how environmental variations affect the embryo/fetus. This line of research would be more or less impossible to perform on mammals.