When and where do chorionic villi appear in humans?

Chorionic villi are finger-like projections that appear on the trophoblast after implantation. They are surrounded by the uterine tissue and maternal blood. Function: The structural and functional unit formed between the developing embryo and maternal body when chorionic villi and the uterine tissue become interdigitated with each other is called the placenta. It nourishes the developing embryo.