  A key process in the life of any multicellular organism is its development
from a single egg into a full grown adult. The first step in this process often
consists of forming a tissue layer out of randomly placed cells on the surface
of the egg. We present a model for generating such a tissue, and find that the
resulting cellular pattern corresponds to the Voronoi tessellation of the
nuclei of the cells. Experimentally, we obtain the same result in both fruit
flies and flour beetles, with a distribution of cell shapes that matches that
of the model, without any adjustable parameters. Finally, we show that this
pattern is broken when the cells do not all grow at the same rate.
