Final answer:
The size at which the yolk sac becomes visible is during the early stages of embryonic development. It provides nourishment to the embryo and supports primitive blood circulation.
Step-by-step explanation:
Early in the development of the embryo, the yolk sac is visible. It gives the embryo nutrients and is made up of hypoblast cells. Although the yolk sac is present at the start of development, its size eventually decreases as the placenta takes over as the embryo's primary source of nutrition. It provides nutrients and supports the developing embryo's primitive blood circulation. It also serves as the source of blood and germ cells.