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After fertilization, the development of a multicellular organism proceeds by a process called cleavage, a series of mitotic divisions whereby the enormous volume of egg cytoplasm is divided into numerous smaller, nucleated cells. These cleavage-stage cells are called blastomeres.The transition from fertilization to cleavage is caused by the activation of mitosis promoting factor (MPF). MPF was first discovered as the major factor responsible for the resumption of meiotic cell divisions in the ovulated frog egg.
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