“All the information for embryonic development is contained within the fertilized egg. So how is this information interpreted to give rise to an embryo? Does the DNA contain a full description of the organism to which it will give rise; is it a blueprint for the organism? The answer is no. Instead, the fertilized egg contains a program of instructions for making the organism—a generative program—that determines where and when different proteins are synthesized and thus controls how cells behave. A descriptive program such as a blueprint or a plan describes an object in some detail, whereas a generative program describes how to make an object. For the same object, the programs are very different. Consider origami, the art of paper folding. By folding a piece of paper in various directions it is quite easy to make a paper hat or a bird from a single sheet. To describe in any detail the final form simply by marking regions on the flat piece of paper is really very difficult, and not of much help in explaining how to achieve it. Much more useful and easier to formulate are instructions on how to fold the paper. The reason for this is that simple instructions about folding have complex spatial consequences. In development, gene action similarly sets in motion a sequence of events that can bring about profound changes in the embryo. One can thus think of the genetic information in the fertilized egg as equivalent to the folding instructions in origami; both contain a generative program for making a particular structure.”