Even before a fly egg first divides, the structural changes that culminate in an adult fly begin. The body's two major axes emerge first, followed, in the embryo, by the appearance of compartments that will become the mouth, legs, and other organs of the eventual maggot.
Those structural milestones, and later ones in the life cycles of flies and other organisms, are controlled by signaling molecules called morphogens. Since the 1970s biologists have identified numerous morphogens in their favorite fly, Drosophila melanogaster. The morphogens' often whimsical names, like wingless and hedgehog, describe deformities that befall the fly when the corresponding genes mutate.
Although their underlying biochemistry is complex, the ability of morphogens to pattern tissue and trigger growth appears to be a straightforward consequence of the response they evoke in cells and of their spatial distribution and transport properties. What's harder to understand is how—or whether—morphogens determine a growing...
