Fig. 3
From: The Intelligent Egg, and How It Got That Way: from Genes to Genius in a Few Easy Lessons
How signaling brings about change in cells. Top arbitrary combinations of signals are produced in some parts of a body or tissue; second row they diffuse across the tissue; third row where they are detected by cells expressing the appropriate (signal-specific) detectors; bottom row the detection of a particular combination of signals causes the cells to change their form or behavior. For more, see (Weiss and Buchanan 2009; Weiss et al. 2011)