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
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
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