Cellular metamorphosis was a technique used by the inhabitants of Antos IV which allowed them to heal injuries at an accelerated pace. It could also be used to change one's outward appearance.
In the 23rd century, the Antos IV natives taught this technique to Captain Garth of Izar, who had sustained life-threatening injuries. Although it was successful in healing Garth, it also caused him to become psychologically unstable, necessitating his placement at the Elba II asylum.
While there, he managed to use cellular metamorphosis to alter his physical form and take on the appearances of other individuals. Using this power, he was able to take over the asylum and capture Captain Kirk and Mr. Spock, who had beamed down to the colony to deliver new medicine. Garth alternately used this technique to impersonate both Spock and Kirk in an attempt to take over the USS Enterprise, but Spock was eventually able to determine his true identity and stun him with a phaser. (TOS: "Whom Gods Destroy")