What does the yolk sac provide for the embryo?

The yolk sac is an essential structure in embryonic development, particularly in vertebrates, providing crucial nutrients necessary for the growth and development of the embryo. During the early stages of development, the yolk sac contains a rich supply of yolk, which serves as a food reserve for the developing embryo. This is particularly vital during the initial phases when the embryo is not yet fully dependent on the placenta or other structures for nutrient supply.

In many species, the yolk sac also plays a role in the formation of blood cells and contributes to the early circulatory system. As development progresses, the nutrients stored in the yolk are utilized by the embryo to support its growth. Thus, the primary function of the yolk sac is nutrient provision, enabling the embryo to develop properly until it can obtain resources through other means.

Understanding the role of the yolk sac highlights its importance in embryonic nutrition and development, emphasizing why the correct answer focuses on its provision of nutrients.