The innervation of dorsoventrally reversed chick wings: evidence that motor axons do not actively seek out their appropriate targets.

In normal chick embryos the extensor (dorsal) muscles are innervated by motoneurones lying laterally in the motor horn, while flexor muscles are supplied by more medially placed motoneurones. After reversal of the dorsoventral axis of the forelimb prior to innervation in most cases the opposite pattern is found, the extensors innervated by medial and flexors by lateral motor neurones. In a minority of cases the normal innervation pattern is obtained. Three hypotheses are discussed, two involving specific target affinity between motor axon and target and one involving passive deployment of axons to targets. We conclude that our results favour the latter hypothesis but that we cannot exclude a short-range specific signal.