## Cleavage:

# Surface Area Increase

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.

At right, 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. 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 the uncompacted mouse
embryo, for example.