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The early stages of embryogenesis are the point at which embryonic stem cell lines are derived. The fertilized egg (day 1) undergoes cell division to form a 2-cell embryo, followed by 4-cell, etc. until a ball of cells is formed by the fourth day. The ball becomes hollow, forming the blastocyst. This is the stage at which pluripotent embryonic stem cell lines are generated. Following the blastocyst stage, the tissues of the embryo start to form and the cells become multipotent.