Early experiments on eyespot morphogenesis used cautery on the butterfly wing eyespot foci to demonstrate that a long range signaling mechanism or morphogen gradient controlled the formation of eyespots in both space and time. The findings cannot be explained by a simple source/diffusion model, but could be explained by either a source/threshold model, in which the focus creates the morphogen, or by the sink model, in which the focus generates a gradient by removing a morphogen which was created elsewhere. Several genes involved in eyespot formation have been identified that can fit into these models, but only two of them have been functionally tested. These genes are the transcription factor Distalless (Dll) and the ligand (a signalling substance that binds a cell surface receptor) Hedgehog (Hh).