What provides nourishment to the embryo before the placenta forms?

The yolk sac is the structure responsible for providing nourishment to the embryo before the placenta forms. During the early stages of development, it serves as a vital source of nutrients, facilitating the embryo's growth. The yolk sac contains yolk, which is rich in proteins and other nutrients that support the developing embryo until the placenta is functional.

As development progresses, the placenta takes over the role of nutrient and gas exchange, but initially, the yolk sac plays a crucial role in early embryonic sustenance. This is significant because it allows the embryo to develop in the crucial early days following fertilization when the connection to the maternal blood supply has not yet been established.

The other options do not serve the same purpose: amniotic fluid primarily provides cushioning and protection for the embryo, the umbilical cord serves as a conduit for nutrients and wastes once the placenta is formed, and the endometrial lining, while essential for implantation, does not directly nourish the embryo in its earliest stages.