Finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood after implantation.
Functions of Chorionic villi:

1. The chorionic villi and uterine tissue become inter-digitated with each other and together to form a structural and functional unit between developing embryo (foetus) and maternal body called placenta.

2. This facilitates the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo.

Explanation:

Fig. The human foetus within the uterus