The yolk sac is an essential structure in the early development of embryos in many animals, including mammals, birds, and reptiles. It provides nutrients to the developing embryo before the formation of the placenta or other nutritional structures. The yolk sac contains yolk, which is a rich source of proteins and lipids, supporting growth during the initial stages of development. In mammals, the yolk sac is also involved in the formation of blood cells and the early circulatory system. While it is prominent in the embryos of egg-laying species like birds, it becomes less significant in placental mammals, where the placenta takes over the role of nutrient supply as the embryo grows.