A placenta between developing embryo/foetus and the maternal body is formed by the interdigitation of the 

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 (endometrium) become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.