Embryo development, artwork. The sequence moves clockwise from lower left. In the first stage, the spermatozoon penetrates the ovum. Their nuclei unite and the cells grow rapidly. At approximately day two, the cells form a solid cluster called a morula. As the number of cells increases, the mass descends down the uterine tube, forming a blastocyst, a hollow sphere of cells. In the last stage shown, the blastocyst implants itself in the uterine wall, approximately five days after fertilization.
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