Early cleavage divisions in most embryos are also rapid. One reason for this is that cleaving cells have a modified cell cycle, in which the two gap phases, G1 and G2 are completely omitted, and the cells cycle rapidly between M and S phases. In addition, because the major cytoskeletal systems of the cells are largely devoted to mitosis (microtubules) and cytokinesis (microfilaments) during rapid cleavage division, there is typically an antagonism between periods of rapid cell division and cell movement. Thus as the embryo approaches gastrulation, there is a tremendous slowing in the rate of division (note the slope of the curve at the right).
