Assertion: Finger-like projections appear on the trophoblast called chorionic villi afterimplantation.Reason: Chorionic villi are surrounded by the uterine tissue and maternal blood

The assertion and the reason are both correct and they are related.

Step 1: Understanding the Assertion The assertion is about the development of the embryo after implantation. The trophoblast is the outer layer of the embryo that attaches to the uterine wall during implantation. After implantation, finger-like projections called chorionic villi appear on the trophoblast.

Step 2: Understanding the Reason The reason given is that the chorionic villi are surrounded by uterine tissue and maternal blood. This is also correct. The chorionic villi invade the uterine wall and establish contact with the mother's blood supply.

Step 3: Connecting the Assertion and Reason The assertion and the reason are related because the formation of the chorionic villi (assertion) is a crucial step in establishing the connection between the embryo and the mother's blood supply (reason). The chorionic villi allow for the exchange of nutrients and waste products between the mother and the embryo.