Final answer:
The function of the amniotic sac is to cushion the fetus during development, providing a protective environment that also enables free movement, muscular development, and temperature regulation.
Step-by-step explanation:
The amniotic sac is a crucial structure that surrounds the fetus during development. It is filled with amniotic fluid, which performs several vital functions, including cushioning the fetus. This protective cushioning helps safeguard the fetus by absorbing and distributing external pressures and impacts that may occur to the mother's abdomen. Additionally, the amniotic sac allows for unrestricted fetal movement which is essential for muscular and skeletal development, and it maintains a constant temperature to protect the fetus from heat loss.
The placenta and umbilical cord facilitate the transfer of oxygen, nutrients, and the removal of carbon dioxide and metabolic wastes, but they are not associated with cushioning which is the exclusive role of the amniotic sac and fluid.