Explain the process of formation of placenta in a human female after the implantation of the blastocyst in the endometrium of the uterus.

After implantation of blastocyst finger-like projections appear on the trophoblast, called chorionic villi, which are surrounded by the uterine tissue, and maternal blood, the chorionic villi and uterine tissue become interdigitated with each other, and form a structural and functional unit between developing embryo or foetus and maternal tissue or body called placenta.