On the left, patterns of gene expression in a developing Drosophila embryo. On the right, a diagram of the regulatory interactions between the genes which produce this patterninng. Arrows indicate positive regulatory interactions, a line ending in a flat line indicates negative feedback. Sean B. Carroll, Jennifer K. Grenier, and Scott D. Weatherbee (2001) From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, Blackwell Publishing:Cambridge, MA, p. 59, fig. 3.5