Early cleavage divisions in most embryos are
**reductive**, i.e., they divide the original contents
of the oocyte/egg without a concomitant increase in the
total cellular volume of the embryo. This means that (a)
the average diameter of a cell decreases as cleavage
continues, and (b) there is a huge increase in surface area
relative to cellular volume. You can perform a little "back
of the envelope" calculation to see how the surface area of
an array of block-shaped cells increases in a situation
where the divisions are completely reductive, starting with
an embryo that is the diameter of a sea urchin zygote
(about 100 micrometers for common species). After the third
cleavage, the surface area has roughly doubled! While this
calculation is a bit biased, it nevertheless points up the
challenge faced by an embryo undergoing reductive
cleavages. For the numerate, the calculation is pretty much
the same for spherically shaped cells, as one finds in
echinoderm embryos. Click on the button to change the
number of cells...