Starting from a single egg cell, embryos first exhibit a series of geometrically quite precise cell divisions corresponding, I suspect, to a simple neighbor-independent substitution system. When the embryo consists of a definite number of cells—from tens to tens of thousands depending on species—the phenomenon of gastrulation occurs, and the hollow sphere of cells that has been produced folds in on itself so as to begin to form more tubular structures. In organisms with a total of just a few thousand cells, the final position and type of every cell seems to be determined directly by the genetic program of the organism; most likely what happens is that each cell division leads to some modification in genetic material, perhaps through rules like those in a multiway system. Beyond a few thousand cells, however, individual cells seem to be less relevant, and instead what appears to happen is that chemicals such as retinoic acid (a derivative of vitamin A) produced by particular cells diffuse to affect all cells in a region a tenth of a millimeter or so across. Probably as a result of chemical concentration gradients, different so-called homeobox genes are then activated in different parts of the region. Each of these genes—out of a total of 38 in humans—yields proteins which then in turn switch on or off large banks of genes, allowing different forms of behavior for cells in different places.