Induction in Metamorphosis
Some of the morphogenetic changes throughout metamorphosis are found fo be quite independent of hormone action. For instance, generally during metamorphosis, the skin of the tail goes through degeneration under hormonal effect. It is observed that when the skin alone is removed from tail and transplanted onto the tail of other metamorphosing larva, it does not regress despite the existence of the hormone. But if the skin is grafted to the trunk with its underlying muscle in a developing tadpole, then it regresses.
So the hormone affects only the muscle directly that induces regression or progression of the skin depending upon the induction it receives from its underlying muscle of tail or trunk respectively. Likely tympanum is another instance of the induction process and is independent of direct hormone, action as its formation is induced by the tympanic cartilage.