The decidua capsularis that initially covered the chorion thins out with the enlargement of the amniotic cavity, and then degenerates. The chorion is getting progressively closer to the uterine wall (decidua parietalis). The amnion also extends, and directly borders the chorion. The chorionic cavity has thus practically disappeared together with the umbilical vesicle. The fetus now occupies a single chamber, the amniotic cavity, which contains the amniotic fluid. This cavity is surrounded by several layers: amnion, chorion and decidua parietalis of the uterine wall. The part of the decidua lying beneath the placenta is called decidua basalis.