Tetrapod limb development: early theory

This is a simplified version of the model proposed in 1979 by Newman and Frisch. It explains the development of the "big bone, two parallel bones, lots of little bones" pattern discussed in Your Inner Fish. This is the pattern that we see in the land vertebrates (and marine mammals). This model invokes a Turing mechanism that allows the limb to self-pattern with only a small number of dynamic molecules.
Nowadays, we have more complex explanations for limb development. For a summary see my book chapter on the subject. I will update this chapter over the next few months as new research has been published. bit.ly/3g8rVKJ