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Illustration of a human blastocyst implanted in the lining (endometrium) of the womb (uterus). The blastocyst is a ball of embryonic cells that arises from a fertilised egg. The outer trophoblast layer (light pink) of the blastocyst will form the placenta and the inner cell mass will form the embryo (KEYSTONE/SCIENCE PHOTO LIBRARY/NICOLLE R. FULLER)