What is the amniotic sac?

The amniotic sac is indeed a protective membrane surrounding the fetus. This sac plays a vital role during pregnancy, as it contains amniotic fluid, which cushions and supports the developing baby. This fluid-filled structure helps to maintain a stable temperature, provides protection against external forces, and allows for fetal movement, which is important for musculoskeletal development.

The amniotic sac develops from the cells of the blastocyst shortly after fertilization and is composed of two membranes: the amnion and the chorion. These membranes create a sealed environment that is crucial for the fetus's growth and development throughout gestation.

The other options refer to related structures but do not accurately define the amniotic sac. For example, a fluid-filled structure following the umbilical cord does not describe the amniotic sac; rather, it refers to the placenta's role in nutrient transfer. The outer layer of the placenta is distinct from the amniotic sac, which is aimed specifically at protecting the fetus. Lastly, the internal lining of the uterus, known as the endometrium, is also separate from the amniotic sac's function and structure.