Embryonic Development of the Chorion:
The chorion is a bilayered membrane formed by 7.5 dpc, consisting of an outer layer of extraembryonic ectoderm (trophectoderm) and an inner epiblast-derived extraembryonic mesoderm. The chorion bilayer fuses at 8.5 dpc with the allantois and together they form the chorioallantoic placenta.
The chorionic bilayer lines the chorionic cavity which is one of three cavities in the embryo: chorion, amnion
and yolk sac
. The chorionic cavity is formed as the extraembryonic mesoderm splits into two layers, and the embryo (with the dorsal amnion and ventral yolk sac) is separated from the chorion, the outermost layer. The embryo remains enclosed in the chorionic cavity with the connecting stalk serving as the sole anchor to the outer chorion (placenta
) by 13 dpc.