Blastocyst embryo. Conceptual computer artwork of an embryo at the blastocyst stage. This ball of cells forms a few days after the fertilization of an egg. There are two types of cell (blastomere) here: stem cells (yellow) are pluripotent, capable of giving rise to most tissues. Trophoblast cells (green) embed the blastocyst into the wall of the uterus and also develop into a membrane (chorion) that encloses the embryo.