The fluid-filled bag of thin tissue that develops around the embryo is called the amnion. The amnion is a key component of the embryonic sac, which provides a protected environment for the developing embryo and fetus during pregnancy. It is formed from the inner cell mass of the blastocyst, which is the earliest stage of embryonic development. The amnion grows and expands as the embryo develops, and it eventually surrounds the entire embryo, forming a fluid-filled cavity called the amniotic cavity. This fluid-filled environment provides a cushioned environment for the developing fetus, protecting it from mechanical stress and injury. The amniotic fluid also plays an important role in regulating the temperature and pH of the embryonic environment, as well as providing a source of nutrients and oxygen to the developing fetus.