yea the hox genes are essential for the the specification of the location but the actual growing of the limbs are determined by a series of protein and other factors. proximal limb contains fgf(fibroblast growth factors) wnt(look up wnt pathway) and high levels of retonic acid where distal limbs shows high fgf's high wnt and little to none RA. the sonic the hedgehog gene talked about in the other answer has to to do with the anteriorizing(up)/posteriozing(down) of the autopod(digits, carpals). as to dorsal ventral wnt would be the culprit

How does an embryo know where to grow limbs etc
Are there any interesting, important or (for the non physicist) astonishing examples where the recoil principle (as special case of conservation of linear momentum) is applied beside rockets and guns?

I am teaching physics in high school and was just wondering if there are any more interesting examples than the two textbook standard examples above. 

I am not interested in examples which are not in the domain of macroscopic, classical physics like Mößbauer-effect or something like that.
