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First the sperm enters the egg and the Fertilization Membrane (slow block) elevates off the surface of the egg. 1st cleavage is meridional (N-S), 2nd cleavage is also meridional, but at right angles to the first cleavage plane. 3rd cleavage is equatorial. 1st and 2nd cleavages produce equal sized blastomeres, but the 3rd cleavage produces two size classes of cells. The 4th cleavge results in three cell size classes, the Mesomeres, the large Macromeres and the tiny Micromeres . |