ABSTRACT
An Alternative Explanation An entirely alternative for this RA phenomenology, as those authors acknowledged, is that RA induces a new, ectopic 12 A on the anterior side of the limb bud. A greater concentration of RA would induce more 12 A ceHs as we already know, more 'l:PA cells <~greater degree of duplication iln this situation RA thus becomes a serendipitously discovered inducing molecule and the new ZPA then the morphogen, about the nature of which we still have no idea. In of this it was shown early on (33) that the anterior tissue adjacent to the RA implant can be grafted to the anterior side of another limb bud where it too will induce duplications. However, there are two explanations for this phenomenon: either a new ZP A has indeed been induced or there is enough RA in the adjacent tissue which is carried over in the graft to be responsible for the induction of extra digits.
