Evidence for a positional memory in the development of the chick wing bud

Grafts of quail zones of polarizing activity (ZPA), treated with 10000 rad γ-radiation, tend to remain at the base of a limb. Their signalling ability is not passed on to more distal tissue, but the limb goes on to produce a reduplication. This suggests that the effect of a ZPA can be remembered in its absence, and explains why a normal limb can develop if its ZPA is removed.