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then four, then eight and then 16 cells (typically by day four after fertilization), it becomes a ball of cells called a morula. During these cellular divisions, the zygote remains the same size, but the number of cells increase. The morula enters the uterus after three or four days--during which a cavity, called the blastocoel, is formed to produce the blastocyst. Once the blastocyst is formed, it undergoes implantation

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