Gradually these spaces join with each other until there is only a single fluid-filled cavity, the chorionic cavity between the yolk sac and the chorion. Extraembryonic mesoderm also spreads out to surround the amnion also and to separate it from the cytotrophoblast. As the trophoblast grows, its mesodermal connection with the amnion, embryonic disk and yolk sac becomes restricted to a narrow connecting stalk . Eventually this connecting stalk becomes the umbilical cord and conveys the embryonic blood vessels that connect the embryo with its placenta. In that case their chorions are also partly fused.
Monozygotic twins can implant separately or at only one site.
When the zygote splits in two during early cleavage, two distinct blastocysts can form and then implant at different sites.