Axis specification and gradients
The development of multicellular organisms requires a tight coordination of developmental decisions. One of the most fundamental strategies of orchestrating such decisions is the use of morphogen gradients. Morphogens provide a means by which cells are given information about their position within a patterning field.

A morphogen is a diffusible signal produced in one part of the body, which can determine cell fates according to its local concentration. The concentration of the morphogen changes over distance, the highest concentration being at its source. In order to translate this gradient into a spatial pattern, cells need to be able to sense different concentrations of the morphogen. Cells that sense a high concentration of the morphogen, and are thus close to the morphogen source elicit a response different to those further away from the source, where the morphogen concentration is low.

Examples of morphogens in the development of Drosophila are manifold. The early patterning of the Drosophila egg as well as those of the adult structures is based on the principle of morphogen gradients in several respects.

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