Scripting a place in time.

Evidence in many experimental systems supports the idea that non-uniform distributions of morphogen proteins encode positional information in developing tissues. There is also strong evidence that morphogen dispersal is mediated by cytonemes and that morphogen proteins transfer from producing to receiving cells at morphogenetic synapses that form at sites of cytoneme contacts. This essay considers some implications of this mechanism and its relevance to various contexts including large single cells such as the pre-cellular Drosophila embryo and the ciliate Stentor.