Positional signalling by Hensen's node when grafted to the chick limb bud.

Hensen's node from stage 4 to stage 10 shows polarizing activity when grafted to the anterior margin of the chick limb bud. It can specify additional digits though its action is somewhat attenuated when compared with the effect of a grafted polarizing region. At stage 10 the activity disappears from the node and is found both posterior to the node and in the future wing region of the flank. The ability of Hensen's node to generate a positional signal suggests that the signal in the limb and early embryo may be similar. The results support the view of the polarizing region as a discrete signalling region.