Determination of anteroposterior polarity in the axolotl forelimb by an interaction between limb and flank rudiments.

1. It is shown that the mesoderm in the prospective forelimb-bud of the axolotl embryo is thickened and divided into somatic and splanchnic layers, while that of the flank is thinner and divided. The first sign of the limb-bud itself appears at stage 38. 2. A whole, a half or a third of a limb rudiment can develop into a normal or reduplicated limb when transplanted to the flank. 3. An anterior half of a limb rudiment fails to develop when transplanted to the head but will do so if accompanied by a small piece of flank tissue. 4. Small pieces of tissue from a wide area of the flank will cause reduplication of the forelimb if grafted to the anterior margin of the rudiment. It is shown that the whole of the reduplication is formed from host tissue and has the morphological character of the host. 5. Reduplications have posterior structures arranged symmetrically on both sides of midline. Both muscles and cartilages are duplicated. 6. It is suggested that the same interaction between prospective flank and limb is responsible for the capacity for growth on the head, the induction of reduplications and the formation of the anteroposterior pattern of the limb in normal development. 7. A simple rule is proposed which explains the occurrence of reduplications in classical work on the amphibian limb.