There are different theories to explain the development from primary to secondary umbilical vesicle. The theory of disintegration and restitution is the most plausible. According to this theory, the spaces between trophoblast on the one hand, and both embryonic disc and primary umbilical vesicle on the other hand, coalesce to form a unique cavity called the extra-embryonic coelom. At S6, the primary umbilical vesicle collapses. The free ends of the opened umbilical vesicle come together again to form a smaller, secondary umbilical vesicle. The separated distal part of the umbilical vesicle remains in the extra-embryonic coelom for a while as exocoelomic cyst. Initially, the extra-embryonic coelom also contains remnants of the extra-embryonic reticulum.
As described for stage 5c, the newly-formed cavity between the embryonic primordium and trophoblast is lined with extra-embryonic mesoderm (or mesoblast). The two possible sources for this mesoderm are the caudal pole of the epiblast of the embryonic disc (caudal proliferation zone, early S6) or the parietal hypoblast. The extra-embryonic mesoderm covers the amnion and lines the inner wall of the trophoblast as parietal mesoderm. It covers the umbilical vesicle as visceral mesoderm. The mesoderm-lined wall of the cavity consists of the syncytiotrophoblast, cytotrophoblast and extra-embryonic mesoderm. It is now known as the chorion, and the cavity is known as the chorionic cavity (extra-embryonic coelom).