The extra-embryonic mesoblast originally forms a wide connection between the amnion and the trophoblast. This connection relatively narrows as it further develops and forms the connecting stalk. The connecting stalk progressively becomes shifted more caudally.
The embryonic anlage now contains three chambers: the amniotic cavity (dorsally located), the secondary umbilical vesicle (ventrally located) and the chorionic cavity, which surrounds the amniotic cavity, embryonic disc and secondary umbilical vesicle.