Does vitamin A act on pattern formation via the epidermis or the mesoderm?

To determine whether the effect of vitamin A on pattern formation in the regenerating axolotl limb was due to a change in the blastemal mesoderm or the epidermis, tissue grafts were exchanged between normal and retinoic acid‐treated animals. The results were conclusive. The combination of treated epidermis and untreated mesoderm resulted in normal regenerates. The combination of treated mesoderm and untreated epidermis produced regenerates that were serially duplicated in the proximodistal axis just as the controls were. We may, therefore, conclude that vitamin A acts on the mesodermal cells of the blastema in causing proximodistal duplications.