Final answer:
The protective sac around the developing embryo or fetus is the amnion, which forms the amniotic sac to cushion and protect the embryo.
Step-by-step explanation:
The protective sac around the developing embryo or fetus is the amnion. This membrane forms a fluid-filled sac known as the amniotic sac which surrounds and protects the embryo. The placenta is a separate structure that develops between the embryo and mother, allowing for the transfer of substances, and should not be confused with the amnion. The umbilical cord connects the developing fetus to the placenta, facilitating the supply of nutrients and oxygen. Finally, the chorion is another membrane that surrounds the amnion and contributes to the formation of the placenta.
It's important to note the distinct roles and structures involved in embryonic development to understand the different components contributing to the protection and nourishment of the developing fetus.