Implantation is a process in which the blastocyst becomes embedded in the endometrium of the uterus. After implantation, the events which lead to the formation of placenta are as follows :
(1) After implantation, finger-like projections appear on the trophoblast and are called chorionic villi.
(2) Chorionic villi are surrounded by the uterine tissue and maternal blood.
(3) The chorionic villi and uterine tissue become interdigitated with each other and jointly form placenta which is a structural and functional unit between developing embryo (foetus) and maternal body.
