morphogen
A morphogen is a signaling molecule that spreads out from a source and forms a concentration gradient — high near the source, fading with distance. Cells read how much morphogen they sense and respond differently at different levels, like a thermostat: above one threshold do this, below it do that. From a smooth gradient, sharp regions of distinct cell types emerge.
Because a single graded signal can specify several cell fates at once depending on local concentration, morphogens are an economical way for an embryo to turn position into identity. Many are diffusible proteins secreted by an organizing group of cells; the receiving cells contain machinery that converts continuous morphogen levels into discrete on/off decisions about which genes to express.
The concept is powerful but idealized. In real embryos, gradients are shaped by how fast the molecule is made, moves, and is degraded, and cells often refine their first reading over time. Still, the morphogen-gradient idea explains how an initially uniform field of cells can become a precisely patterned tissue.
In the developing limb, a morphogen gradient across the bud tells cells whether to become a thumb (low signal) or a little finger (high signal), so the digits form in their correct order.
A gradient turns smooth differences in concentration into discrete structures.
The fly protein Bicoid is a textbook morphogen: its gradient along the egg sets up head-to-tail position before any segments form, with high levels specifying head structures.