Explanation

Placenta is an organic connection between the foetus and uterine wall for physiological exchange between foetus and mothers blood. The placenta develops at the point of implantation. Extraembryonic membrane, i.e., amnion, allantois, chorion and yolk sac are formed from trophoblast (the ring of cells surrounding the inner cell mass in a developing pro-embryo). Chorion is formed of ectoderm externally and mesoderm inside. Along with allantois, it participates in formation of placenta.