Color gradient filament is fun stuff to play with. It lets you make 3D prints that slowly fade from one color to another along the Z-axis. [David Gozzard] wanted to do some printing with this effect, ...

Decapentaplegic (DPP) exerts its long-range influence on wing patterning by acting 'directly' at a distance as a gradient morphogen, rather than 'indirectly' as a short-range inducer of other signals.