The central problem of developmental biology is to understand how a relatively simple and homogeneous cellular mass can differentiate into a relatively complex and heterogeneous organism closely resembling its progenitor(s) in relevant respects. This is not a new problem. It has been with us since Aristotle, at least. However, it is only recently that we have established a handle on how possibly to solve it. I am not convinced that we have yet grasped the right handle, though.

A decade ago, an advertisement for The Encylopedia of the Mouse Genome appeared in a biotechnology serial. The tagline read: ‘The Complete Mouse (some assembly required)’ (cited in Gilbert and Faber 1996: 136). The parenthetical clause refers, of course, to development. As those of us who have purchased ready-to-assemble furniture know all too well, this is indeed an onerous requirement, for the assembly process may very well have the greatest impact on final outcome! What is true of ready-to-assemble furniture is also true, I contend, of organisms believed to be ‘ready-to-assemble’ from DNA and assorted other material.