Final answer:
The part of an amniotic egg that provides water and protein to the developing embryo is the yolk sac, which is essential for the early nourishment of the embryo.
Step-by-step explanation:
The part of an amniotic egg that provides water and protein to the embryo is called the yolk sac. The yolk sac is a membranous sac attached to the embryo, playing a crucial role early in development by providing nutrients. It contains the egg yolk, which is a source of nutrients for the developing embryo.
In addition, blood vessels in the yolk sac transport yolk nutrients to the circulatory system of the embryo. While the albumen acts as the embryo's water supply and assists in gas exchange as well as cushioning, the yolk sac is primarily responsible for the nourishment in the early stages of embryo development.