Embryo undergoes mitotic divisions throughout its development. Since the number of cells increases with each mitotic division, it is important all of them to persist in normal cell division. That way the embryo development will end successfully when a baby is born. But, if all cells of the embryo fail to go through S phase, the DNA will not replicate. As mentioned, DNA replication occurs in the S phase of mitosis. A cell cycle will be interrupted and the mitotic divisions will stop. Consequently, the embryo will die.