(II) A new A/P juxtaposition can generate a supernumerary legAs mentioned in connection with segmentation, a repetition of at least three cell states is required to obtain a periodic pattern with an internal polarity. That means, one pair of A/P compartments is separated by at least one state called S ('Separation') that is neither A nor P. Since an A/P confrontation is decisive for the generation of a leg, the removal of the S region would lead to a new P/A confrontation. A corresponding experiment has been done by Horst Bohn . In cockroaches he observed the formation of a supernumerary leg after removal of a part of a metathoracic segment that would never form a segment:
(III) Vertebrate limbs: shift of tissue along the AP axis can lead to polarity reversal.Harrison was convinced that the polarity of the embryonic AP axis imposes the polarity of the limb. Therefore, it was most puzzling for him [7 ] that transplantations of small pieces of tissue can either lead to symmetric limbs or to limbs with a reversed polarity. The figure below illustrates that the model provides a coherent explanation for the observations of Harrison [6,7] and Slack . Top: normal situation. By cooperation a symmetrical morphogen distribution results. Only the A region can respond. The single exponential gradient in the A region (red curve) determines the sequence of the digits (high = little finger). Centre: A P-region is transplanted in front of an A region. A symmetrical P-A-P pattern, a symmetrical positional information (red curve) and thus a symmetrical limb with posterior digits at each end is formed. Bottom: The reverse experiment: A cells are transplanted to a more posterior position. Although the pattern is again symmetric (A-P-A), two exponential gradients (red curves) and thus two limbs result. The posterior arm has a reversed polarity. This observation was very puzzling for Harrison since he assumed that the overall polarity of the body axis orient the AP axis of the limb. Even more striking, since the operation does not change the dorsoventral pattern, if the operation is performed at the right side, the supernumerary limb will have the symmetry of a left limb.