In The Art of Genes: How Organisms Make Themselves, Enrico Coen uses artistic metaphors to describe embryonic development, and he succeeds brilliantly. Coen, an internationally recognized plant geneticist working at the John Innes Centre in Norwich, England, asks two questions. First, how is it possible that organisms can "make themselves," in the sense that there is no microscopic homunculus curled up in the head of a spermatozoon? And, second, what precisely are the roles of genes in this process?

The central metaphor used to facilitate his explanation is that of art - particularly the use of color in painting. The overarching thesis of The Art of Genes is that embryonic development in a complex, multicellular organism is much more like the creative act of painting a picture than that of manufacturing an object from a plan or blueprint, or making a precise copy from an original object. Francis Crick entitled his most recent book The Astonishing Hypothesis: The Scientific Search for the Soul, referring to the fact that the immaterial mind is a product of the material brain. Equally astonishing, it might be argued, is Coen's proposal - widely accepted, if not widely articulated - that there is no plan for development; or, to be more precise, what plan there is cannot be separated from its execution. Yeats's immortal question, How can we tell the dancer from the dance? is an exact analogy.

Let's stay with Coen's analogy of the embryo as a painting, however. When a painter begins a painting, she may have only a vague idea of the eventual outcome. A brush stroke is made. A new color is overlaid. Something begins to take shape. The lighting in the room changes subtly and, in response to the appearance of the emerging picture as mediated by the environment of the studio, the artist's conception of the painting changes, so that there is now a slightly - or perhaps radically - altered vision of what the painting will be. New colors and forms are added. The painting changes again. The artist responds and so on, until it can only be said that the process of making the painting is one in which each moment was informed by the previous moment through interactions between the mind of the painter, the picture taking shape on the canvas, and the overall environment. In my view, there has been no better analogy or metaphor put forward to describe the process of embryonic development, which proceeds by the gradual elaboration of a single cell into a multicellular organism, each step guided by changes in gene expression. Gene expression is, in turn, informed by signals from neighboring cells, and from sources outside of the embryo itself.

How is Coen's painting analogy employed to illustrate specific events in embryogenesis? For the most part, Coen explores the development of the fruit fly Drosophila melanogaster, although he also cites examples of his own favorite organism, the snapdragon, Antirrhinum majus. In the Drosophila embryo, Coen explains, there are a series of "hidden colors," which is his term for gene products that act as transcription factors, turning other genes on or off. This pattern of hidden colors, produced by differential gene expression, breaks the symmetry of an early embryo. Just as spectators at a football stadium can spell out messages with squares of colored cardboard when each responds to instructions that are hidden from an overhead viewer, the asymmetries of form in embryos are themselves defined by hidden colors. These different "colors" distinguish anterior from posterior, dorsal from ventral, proximal from distal, and left from right.

Coen goes on to describe the central role that gene regulatory elements play in establishing these asymmetries. Regions flanking each gene contain sites that can be bound by one or more hidden colors themselves, so that genes encoding particular colors are themselves responsive to the presence of other colors. Cells may also produce different "scents" or signaling molecules, which can instruct a neighboring cell to produce a different set of colors (transcription factors), resulting in the subsequent production of a different "scent," which in turn instructs other neighboring cells to change their colors and, ultimately, their fate. As with a painting, cells constantly respond to signals being received from other cells or from the external environment, guided by the regulatory elements flanking the genes encoding their hidden colors, which define the responses available to a particular cell.

It would take a lot of space (a whole book, in fact) to describe the many clever ways that Coen exploits this metaphor to explain the roles of genes in embryonic development, but two more will have to suffice. Where do the hidden colors come from? Their expression is activated by other hidden colors, whose expression is in turn activated by other colors, until you get back to the egg, where even before fertilization there are initial deposits of messenger RNA and proteins. In other words, "the canvas is never blank."

Finally, how do we know about these hidden colors and the ways in which they define regions of the developing embryo? Of course, we use specific nucleic acid probes that hybridize specifically with the messenger RNA that we want to detect. Coen uses the analogy that bees see in the ultraviolet range, and thus their view of a flower is shaped by that fact, and not merely by the light in the visible range to which we are accustomed. We could get an approximation of what it is to see like a bee, however, if we had an ultraviolet camera, which Coen likens to the nucleic acid probe that enables us to see patterns of gene expression that would otherwise be invisible to us. He poses this lovely analogy to describe the task facing all molecular biologists: How do we make out patterns at the level of molecules given the limits of our senses?

As a book reviewer, I have two quibbles. Coen suggests that his analogy succeeds because human creativity is itself a product of development and thus it would make sense that each could be understood in terms of the other. While this is an interesting idea, it is a bit vague and is not presented with the degree of insight and rigor that is found in the rest of the book. And finally, in a book in which color is such a central theme, why are all of the illustrations in black and white?

This book is written for a general audience, but it can be enjoyed by developmental biologists on the cutting edge of the field. It may be that The Art of Genes will find its most lasting value in providing a sort of antidote to an overemphasis on genes as controlling factors in development. Before long, the complete sequence of the human genome will be announced, and one hopes that this single achievement will be seen for what it is - the end of the beginning, not the end in itself. We will have the paint, in all of its many shades, but to understand how the picture is painted will take at least as much effort. Thanks to Enrico Coen and his timely and beautiful book, we now have an accurate and fruitful way to think about the task ahead.