"The mechanisms regulating the vertebrate limb mesenchyme of chickens, have been extensively discussed in literature. There are currently two models. The first regards that progressive distalization of limb pattern is based on an autonomous clocklike mechanism, inherent to the undifferentiated mesenchymal cells. The second belives that instructive cues from surrounding tissues are responsible for specifying the PD segments (adapted from [Cooper et al. 2011](https://www.ncbi.nlm.nih.gov/pubmed/21617075))."
This pathway visualises the different steps in limb development, with corresponding literature.