When and where do chorionic villi appear in humans? State their function.

Explanation

The finger like projections called chorionic villi appear from trophoblast after implantation and get surrounded and interdigitated with the uterine tissue jointly, to form a structural and functional unit between the developing embryo (foetus) and maternal body called placenta. The placenta facilitates the supply of `O_2` and nutrients to the embryo and removal of `CO_2` and excretory waste material from the embryo.