| Many patterns of cell and tissue organization are specified during development by gradients of morphogens, substances that assign different cell fates at different concentrations. Gradients form by morphogen transport from a localized site, but whether this occurs by simple diffusion or by more elaborate mechanisms is unclear. We attempt to resolve this controversy by mathematical modeling, analysis and computations in addition to comparing and examining recent experimental data in ways that appropriately capture the complexity of the systems. We find that diffusive mechanisms of morphogen transport are much more plausible- and nondiffusive mechanisms much less plausible-than has generally been argued. |