In a young fetus the zonary placenta (1) almost completly encloses each fetus and the chorioallantoic membrane (2) covers a small portion of the anterior and posterior ends. The fetus floats in a fluid filled space enclosed by the the amniotic membrane (3).
As the fetus matures the membranes expland. In this image the amniotic membrane is covering the head of the
fetus, but has been removed from the posterior end. The arrow is pointing to the umbilical cord that connects the
fetus to the placenta. The fetus has been removed from the chorioallantoic membrane, which can be seen attached
to the posterior margin of the placenta.