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A blastocyst is an early embryo that is less than a week old, usually forming five days after fertilization from the morula in the uterus and implanting into the uterine wall six days after fertilization. Blastocysts are composed of two distinct cell lineages: the inner cell mass (ICM) that will become the embryo and the trophoblast that will become the placenta. The trophoblast surrounds the ICM and the inner, fluid-filled cavity called the blastocoel.