Figure 43.5 A Human Blastocyst at Implantation Adhesion molecules and proteolytic enzymes secreted by trophoblast cells allow the blastocyst to burrow into the endometrium. Once the blastocyst is implanted in the wall of the uterus, the trophoblast cells send out numerous projections—the chorionic villi—which increase the embryo’s area of contact with the mother’s bloodstream. The inner cell mass divides into two embryonic tissues, the hypoblast and the epiblast. The epiblast splits to form the amniotic cavity.