The blastoderm prepattern.

During the first 3h of development the Drosophila blastoderm is subdivided into segments along the anterio-posterior axis. This is achieved by three classes of genes: gap, pair-rule and segment polarity. The gap and pair-rule genes interpret cues provided by the localized maternal determinants. These transient intermediate patterns then establish the persistent segment polarity gene expression which defines segments throughout development. In so doing they transform maternal gradients which vary gradually over a distance of 40 or so cells into a precise pattern where individual cells are distinguished from their immediate neighbors.