The yolk sac is an essential structure in the early development of embryos in many vertebrates, including mammals, birds, and reptiles. It provides nutrients to the developing embryo before the formation of the placenta. In species like chickens, the yolk sac contains the yolk, which is rich in proteins and fats, supporting growth during the initial stages of development. In mammals, the yolk sac is smaller and plays a different role. It helps in the formation of blood cells and contributes to the development of the umbilical cord. As the embryo grows, the yolk sac gradually diminishes in size and is eventually absorbed or incorporated into the developing body.