Cellular metamorphosis was a discipline or technique that enabled practitioner to manipulate their body and rearrange their cell structure, allowing them to imitate that of a person or object of around the same size and mass, to the extent that they became shapeshifters (ST reference: Star Trek Maps, TOS episode: "Whom Gods Destroy").
At some point during the mid-23rd century, this practice was taught to Starfleet Captain Garth of Izar in order for him to recover from serious wounds that he had suffered during the performance of his duty. Whilst the use of the technique allowed him to repair his physical injuries, this came at the cost of severe mental disturbances that drove him insane (TOS episode: "Whom Gods Destroy").
By the 2280s, the Orion Victory League had captured Trisha Fox, daughter of United Federation of Planets Ambassador Robert Fox, and taught her the cellular metamorphosis technique, whereupon she was given the task of assassinating her father. She used this skill to board the USS Enterprise and masquerade as various individuals during her assignment (TOS comic: "Who Is... Enigma?").