Which of the following structures is formed during the early development stages of an embryo and is essential for nutrient exchange?

The placenta is a vital structure formed during early embryonic development and plays a crucial role in nutrient exchange between the mother and the developing fetus. It facilitates the transfer of oxygen, nutrients, and waste products, which are essential for the growth and development of the embryo. The placenta also serves as an endocrine organ, producing hormones necessary for maintaining pregnancy.

In contrast, while the yolk sac, chorion, and allantois play significant roles in early development, their functions are different from that of the placenta. The yolk sac primarily provides early nourishment in species that rely on yolk material, but its role diminishes as the placenta forms. The chorion forms part of the outer layer of the placenta and aids in gas exchange, but it does not handle nutrient exchange independently. The allantois is involved in waste storage and also contributes to the formation of the umbilical cord, but it is not the main structure responsible for nutrient exchange. Thus, the placenta is uniquely positioned to support the developing embryo through direct maternal-fetal nutrient transfer.