Rates of Division

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 ( microtubule) 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 below right).


Increase in cell number in the frog embryo vs. time