Over the last 20 years, the essential mechanisms of development have become clearer, mainly because modern molecular genetics has transformed traditional embryology. The longest dispute in embryology has been resolved: we now know that the adult is not preformed in the fertilized egg and that animals arise by step-by-step elaboration from simple beginnings, that is, by epigenesis. Much of this knowledge has come from ingenious experiments on invertebrates such as Drosophila and the nematode, Caenorhabditis elegans. Nevertheless, because so much of the genetic machinery is shared, many of the principles uncovered may apply, with variations, to all multicellular animals.

Here, we describe a simple and beautiful mechanism that is used to build pattern in the development of flies. Like the Central Dogma in molecular biology, the heart of the matter is not so much the individual molecules involved, but more the flow of information and the logic of the system they participate in. With this proviso, it becomes reasonable to ask whether pattern formation in other animals uses all or some of the same logical steps; and does it use similar molecules? Although this review is mainly about flies, we do approach these questions, briefly, with respect to vertebrates. Our aim is not to provide a specialized review of the fly work—this has already been done well (for example, see