What is the name of the fluid-filled sac that surrounds and cushions the embryo?

The fluid-filled sac that surrounds and cushions the embryo is known as the amnion. This membrane is critical during development as it provides a protective environment for the embryo, allowing it to grow without physical stress and allowing for movement. The fluid within the amnion, known as amniotic fluid, serves several essential roles, including maintaining a stable temperature, providing a cushion against external pressure, and facilitating the development of the musculoskeletal system by allowing the embryo to move freely.

The chorion, while also associated with embryonic development, primarily functions in gas exchange and nutrient transfer, establishing a vital connection with the placenta but does not directly cushion the embryo. The placenta is responsible for nutrient and waste exchange between the mother and the developing embryo but does not act as a physical buffer. The yolk sac, present in some early embryonic development stages, provides nutritional support during the initial stages but is not a protective sac like the amnion. Thus, the amnion is specifically designed for the protection and cushioning of the embryo, highlighting its importance in embryonic development.