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Development of a human embryo from fertilization to implantation. Compaction of the human embryo occurs on day 4 when it is at the 10-cell stage. The embryo “hatches” from the zona pellucida upon reaching the uterus. During its migration to the uterus, the zona prevents the embryo from prematurely adhering to the oviduct rather than traveling to the uterus. (After Tuchmann-Duplessis et al. 1972.)