The human embryo and extraembryonic tissues
The embryo is located within the amniotic cavity and is connected to the placenta via the umbilical cord. The definitive yolk sac remains a relatively small structure during human development and both the amnion and yolk sac are suspended within the extraembryonic coelom, contained within the chorioallantoic placenta. The placenta is divided into the chorion frondosum and the chorion laeve portions. The chorion frondosum originates in the polar trophoblast and is rich in chorionic villi which extend into the maternal tissues. The chorion laeve originates in the mural trophoblast and has little or no chorionic villi, giving it a smoother appearance.