Chameleon Arch

The Chameleon Arch was Time Lord technology which could modify the biology of an individual, essentially changing their species. Connected to a TARDIS' databank via a cable, the arch resembled a headset incorporating a biodata module usually resembling a fob watch. The module was placed on the front on the Time Lord's forehead. The watch stored the Time Lord's essence, and, when opened, would return it to the Time Lord.

The arch gave the Time Lord a new personality and memories, provided by a TARDIS. To ensure that the biodata module would not be noticed, it was incorporated with a perception filter.

Using the arch re-wrote a Time Lord's cellular biology and was therefore painful. The Tenth Doctor screamed in agony when undergoing transformation into a human. It was capable of altering them both with or without modifying their outer appearance. (TV: Human Nature / The Family of Blood; AUDIO: The Heavenly Paradigm)

Function
Some traits of the user's true Time Lord self still remained after the use of the Chameleon Arch. For example, John Smith was able to use a simple cricket ball to deliberately cause a complex chain of events to save a mother and her baby from being crushed by a falling piano. (TV: Human Nature) heard drums beating in his head all his life, a burden the Master also faced. (TV: Utopia, The Sound of Drums, The End of Time)

Words like "TARDIS" and "regeneration" and other words of Time Lord lore stirred up residual memories in Professor Yana, (TV: Utopia) while John Smith kept a dream journal in which he chronicled memories of the Doctor's adventures. (TV: Human Nature / The Family of Blood)

Though he had become a human, Quarren Maguire was still time sensitive, and unlike his wife, he sensed, if only briefly, when the timelines changed as the Last Great Time War caught up with them. (AUDIO: One Life)

The Eighth Doctor claimed that destroying a Chameleon Arch while the user was not a Time Lord would render their Time Lord personality and biology irretrievable. (AUDIO: One Life) The Time Lord essence stored in a Chameleon Arch could observe and communicate thoughts to their human self, (TV: Utopia, AUDIO: The Heavenly Paradigm) or to others with psychic abilities, (TV: Human Nature) through direct physical contact. (TV: Human Nature, Utopia)

While Quarren indicated that he could not return to his human personality, (AUDIO: One Life) the Tenth Doctor later told Joan Redfern that he could change back, but actively chose not to. (TV: The Family of Blood)

Notable uses
There was an account suggesting that the Seventh Doctor, prior to his regeneration into his eighth incarnation, made use of a broken Arch to create the fiction that he was half-human to use against the Master. (COMIC: The Forgotten, TV: Doctor Who)

The Time Lord who would become Quarren Maguire was a powerful psychic with the ability to manipulate reality, but he used the Chameleon Arch to escape the Time War as he didn't want to fight. Quarren hid his true Time Lord self in a locket that he shared with his wife, Rupa Maguire, and created a history for the pair of them. After the Daleks attempted to track him down, his identity was exposed by Ollistra while he was attempting to escape with various other human refugees in the Doctor's TARDIS. Determined to save the Eighth Doctor, Quarren opened the Arch and restored himself to his original state, rewriting history to save the TARDIS and send most of the human refugees to safety and the other Time Lords to Gallifrey. Quarren then erased himself from history, as he could not go back to the good man he was. (AUDIO: One Life)

used the Arch to turn into a human infant to escape the Last Great Time War, after witnessing the Daleks take over the Cruciform. (AUDIO: The Heavenly Paradigm) Calling himself Professor Yana, he remained in human form for long enough to grow into adulthood and even old age, until he opened the biodata module and remembered who he was. Due to the effects of the module's perception filter, Yana did not notice anything unusual about his fob watch until it was pointed out by Martha Jones. (TV: Utopia)

The Ninth Doctor gave a biodata module to Plex. (COMIC: The Promise)

The Tenth Doctor used a Chameleon Arch to change himself into the human John Smith to elude the Family of Blood. He left Martha Jones in charge of protecting John Smith, and instructed her to use the fob watch biodata module in case of danger. (TV: Human Nature)

The Tenth Doctor used a representation of his Chameleon Arch to recover his memories, after his mind was taken over by Es'Cartrss of the Tactire. (COMIC: The Forgotten)

The Tenth Doctor also briefly considered that Jackson Lake might be a future incarnation of himself under the influence of an Arch. Lake had what turned out to be just an ordinary fob watch. The initials on the back of the fob watch revealed his true identity to both himself and to the Doctor. (TV: The Next Doctor)

Known Chameleon Arches
During the Last Great Time War, the extremely psychic Time Lord who would adopt the name Quarren Maguire used a Chameleon Arch to make himself human, fearing his abilities would be used for the destruction of more than just the Daleks. His biodata module took the form of a pendant. (AUDIO: One Life)

Narvin gave a Chameleon Arch for providing the Time Lords with Swenyo. (AUDIO: Sins of the Father)

Following the disappearance of Gallifrey, (TV: The Day of the Doctor) there were only two known Chameleon Arches in the universe. One belonged to the Doctor inside his TARDIS, though he gave the biodata module fob watch to Timothy Latimer. (TV: Human Nature / The Family of Blood) In light of its later appearance in the Eleventh Doctor's drawing room (GAME: TARDIS), he seemed to have either recovered it or had duplicates, as the Ninth Doctor gave a modified one to Plex that broke down by the time the Twelfth Doctor retrieved it. (COMIC: The Promise) The other was the Master's, before he left the end of the universe for the 21st century. (TV: Human Nature, Utopia)

Behind the scenes

 * Interestingly, a fob watch strongly resembling a biodata module is one of the objects seen in the Doctor's Gladstone bag in the 1996 Doctor Who TV movie, as he retrieves his sonic screwdriver to seal the Master's remains.
 * Also in the TV movie, among the Doctor's belongings examined by Chang Lee was a normal fob watch.