Regeneration




 * This article is about the Time Lord physiological process. For other uses of Regeneration, see Regeneration (disambiguation)

Regeneration was the process by which Time Lords (and a few other life-forms) could renew themselves by complete physical change, either at will or because of near-fatal illness or injury.

Appearance
During a regeneration, the body often shines with milky white light also known as bio-energy (DW: Logopolis, Time and the Rani, Doctor Who) or with a violent-seeming great discharge and torrent of golden bio-energy. (DW: The Parting of the Ways, Utopia, The Stolen Earth, The End of Time) In other cases, there was no visible energy discharge. (DW: Planet of the Spiders, Destiny of the Daleks)

Just prior to his ninth regeneration, the Doctor warned his companion to keep away from him (during a partial regeneration, Jack did this as well). This would seem to imply that the energy released during the process posed some level of danger to Humans, and possibly that them being too close may pose a danger to the Time Lord. (DW: The Parting of the Ways, The Stolen Earth) However at least one regeneration occurred with other individuals seen in close proximity (although this was assisted by the transient Watcher, who may have helped to contain the regenerative energy). (DW: Logopolis)

The damage done to the TARDIS during the Tenth Doctor's regeneration into the Eleventh may be an example of the potential dangers the regeneration energy can pose to those who stand too close.
 * However it is unclear if the overall damage done was the result of the Doctor regenerating in a position that allowed the energy to flow onto the console of the TARDIS, the regeneration being uncommonly violent, or was simply for dramatic effect.

Physical and mental change
During regeneration, the body of a Time Lord was typically reconfigured into a new form, though this wasn't always the case. (DW: The Doctor's Daughter) In the case of the Doctor, the new form has often been physically younger and healthier (in human terms) than the Time Lord's previous incarnation, though this hasn't always been the case (see, for example, the contrast between his fifth and sixth incarnations). Other Time Lords have regenerated into both younger (Romana) and older (Borusa) bodies. The Time Lord might gain or lose height and body mass. After each regeneration there is often a marked change in personality. During the process of regeneration there are the genetic equivalent of 'bit errors' in the DNA of the regenerated cells. This is what causes the appearance of the Time Lord to change; because even the cells of the brain regenerate as well, their brain chemistry and organisation will change. Although the aspects of their personality caused by "nurture" will not change, the "nature" contribution to their personality will change. (BFA: The Sirens of Time) Time Lords of the Oldblood Houses, born with just one heart, grew a second heart upon regeneration. This included the Doctor, who in his first incarnation had only one heart. (MA: The Man in the Velvet Mask)
 * ''When one of the Doctor's hearts stopped he wonders aloud how humans can cope with just one, suggesting he has never experienced a single heartbeat. (DW: The Shakespeare Code) However as he was in his tenth incarnation at the time, it may have been so long since he last had the experience he may have forgotten what it was like.

The Eleventh Doctor feared that he had become a woman, confirming that it is a possibility when it comes to regeneration (or that the mania sometimes experienced during regeneration caused him to jump to this conclusion hastily). (DW: The End of Time) This was justified by the fact that regeneration is very random and, as the Ninth Doctor said in DW:The Parting of the Ways, 'You never know what you're going to end up with'. He also stated prior to that comment that he could even have grown another head due to regeneration, though he may have been joking as he also suggested that an unlikely "no head" outcome was possible. All known naturally occurring Time Lord regenerations have remained humanoid. Nonetheless, the Doctor's eleventh incarnation made it a priority - even amidst serious damage to his TARDIS - to conduct a physical inventory to make sure he still had two legs and sufficient fingers, suggesting some physical alteration is possible.
 * Although some accounts (DW: The Sound of Drums) suggest that not all Time Lords have similar characteristics to what humans call Caucasian (white), it has yet to be confirmed whether regeneration has the ability to change to colour of a Time Lord's skin or other racial characteristics.
 * The Doctor's ability to regrow a severed limb soon after regeneration (DW: The Christmas Invasion) suggests the possibility that had the eleventh incarnation, for example, discovered a deficiency - a missing leg, for example - he might have been able to rectify the situation.



After-effects
During the first few hours of the regeneration, the Time Lord will often suffer from confusion, erratic behaviour and memory loss. The Doctor, in particular, though not so much other Time Lords, has exhibited mood swings and confusion. It took some time for the regeneration fifth incarnation to remember his own identity. (DW: Castrovalva) On one occasion, he attempted to strangle Peri to death before re-asserting control of himself (DW: The Twin Dilemma) and on another he almost crashed the TARDIS (DW: Children in Need Special) A Zero Room can help with this, as it removed all outside distractions. (DW: Castrovalva)

If they are knocked unconscious, the whole process might be started all over again. (DWN: The Power of the Daleks) After a while, the Time Lord's body will have settled down, though they can regrow limbs within the first 15 hours of the regeneration due to having enough residual energy. (DW: The Christmas Invasion) After his first regeneration, the Doctor implied that his TARDIS helped the process along. (DW: The Power of the Daleks) Some Time Lords, however, may regenerate with little or no overt complications, for example, Romana. (DW: Destiny of the Daleks) Even after the physical transformation, changes might occur, as the Doctor believed could happen. (DW: Robot) The Doctor's hair, for instance, went from longer to shorter to longer in the space of a few days. (MA: Cold Fusion)

Limitations
Though Time Lords could regenerate after severe injuries — one Time Lord even beginning to regenerate a complete body after being decapitated (EDA: The Shadows of Avalon), the process only being cut short when a Time Lord is stabbed through both hearts before it can finish — regeneration does not seem to be guaranteed. The Doctor, for example, was at one point convinced that he was going to die at the hands of the Gelth (DW: The Unquiet Dead), although it is possible he meant the particular incarnation, and there have been numerous occasions in his lifetimes where survival, including regeneration, was not assured. Maxil implied that a fatal blast from a staser (an energy weapon used by the Chancellory Guard on Gallifrey) could prevent regeneration. (DW: Arc of Infinity) Stabbing or shooting a Time Lord through both hearts at the same time (EDA: The Shadows of Avalon, PDA: World Game), or drowning, if it happened quickly enough (DW: Turn Left) could also end a Time Lord's life regardless of how many regenerations they have left. Burning out the hearts simultaneously would have the same effect as stabbing or shooting them simultaneously (DW:Silence in the Library/Forest of the Dead). A Time Lord can also willingly reject regeneration, as demonstrated by the Master (DW:Last of the Time Lords).

As well, the interference of some medications, such as anaesthesia, may disrupt or destroy the regenerative process. (DW: Doctor Who)

The Tenth Doctor also states that regeneration is not possible if a Time Lord dies before the process begins, alluding to the alternate time line where he was killed in the flood that destroyed the Racnoss in DW The Runaway Bride. This fact is supported by almost all of the Doctor's regenerations save for the Seventh to the Eighth, during which the Doctor was declared dead in an operating room. It is possible that in this instance the anesthesia slowed the Doctor's metabolism and the ensuing regeneration process to the point where he appeared dead to human medical science. In all other cases the Doctor's regeneration occurred while he was still alive.

The regenerative cycle
The Time Lords had a limited regenerative cycle of twelve regenerations, after which they had no more incarnations and would suffer permanent death. (DW: The Deadly Assassin, Doctor Who) Time Lords can will themselves to die by regenerating when they have no more regenerations left to use, as Azmael chose to do. (DW: The Twin Dilemma) Other life forms (for example, the Minyans) could have hundreds. Rassilon apparently had physical reasons to impose this restriction. (BFA: Zagreus)

As with most such "rules" there were occasionally exceptions to the twelve regeneration limit. The High Council, offered the Master a new regenerative cycle if he complied with rescuing the various incarnation of the Doctor from the Death Zone. (DW: The Five Doctors) Although he did not receive this award on that occasion, during the Last Great Time War the Master was given the ability to regenerate once more (DW: Utopia).

The crew of the Minyans' ship the P7E, however, had had about a thousand regenerations, by which time, however, they wearied of life. (DW: Underworld) Mawdryn and his followers, who had stolen the Time Lords' regeneration technology, also had an apparently limitless number of incarnations, though they had no control over when it would happen and what form, often grotesque, they would change into. Consequently, they also longed for death, making their mutations of a kind of de facto punishment by the Time Lords for stealing their technology. (DW: Mawdryn Undead)

Attitude toward regeneration
As noted above, regeneration is not guaranteed, as the Doctor on numerous occasions believed he was in danger of actually dying. Even with regeneration a possibility, the Doctor has come to consider such a change as being nonetheless a "death". In recollecting the events surrounding the Master's attempt at stealing the Eye of Harmony, the Doctor referred to his incarnations as "lives". (DW: Doctor Who: The Movie) Iris Wildthyme once confided in Samantha Jones, saying that regeneration was treated on Gallifrey the same way sex is on Earth. (EDA: The Scarlet Empress) Though unspecified in the original series, after the 2005 revival of the series the Doctor's attitude towards regeneration has changed - just prior to the end of his ninth incarnation, the Doctor bade farewell to his companion, ('I'm not gonna see you again. Not like this.') even though he was not actually dying. (DW: The Parting of the Ways). Most recently, the Doctor's tenth incarnation was concerned regarding a prediction made regarding his own impending regeneration, saying "Even if I change, it feels like dying. Everything I am dies. Some new man goes sauntering away...and I'm dead." (DW: The Waters of Mars; The End of Time). He seemed incredibly happy when the prophecy seemed to be wrong and he turned out alive after the Time Lords' defeat, until he heard the sound heralding his "death". The Tenth Doctor made the impression that regeneration was simply death for one incarnation of the Doctor while a new man was born with the same memories - his final words confirmed this, in a way, as he said 'I don't wanna go'.

Control over regeneration
Generally, regeneration is initiated when a Time Lord has been too badly injured to survive. However in some cases Time Lords have been known to exercise control over the process. Romana appeared to regenerate on a whim, (DW: Destiny of the Daleks) while Azmael initiated a thirteenth regeneration in order to end his life (DW: The Twin Dilemma). The process can also be stopped with some difficulty (DWN: The Power of the Daleks), as the Master did in order to spite the Doctor (DW: Last of the Time Lords). This is not always an option, however, as the Doctor notes fearfully that while his companion can die only once, he may have to repeatedly regenerate and live out all of his life times when the TARDIS stalls in the middle of space. (DW: Vengeance on Varos)

The degree of control that Time Lords have over their end appearance is unclear. The Master was able to make his next regeneration as young as the Doctor was at the time, though since most regenerations lead to younger bodies this may have been a coincidence. (DW: Utopia) Romana seemed adept enough at the process to custom design her new appearances, trying several bodies before finally deciding on a copy of Princess Astra. The Doctor criticised Romana for taking on the form of another person, suggesting such things were not unheard of. (DW: Destiny of the Daleks) In contrast to Romana, the Doctor did not appear to have much control over his post regeneration appearance; after his fourth regeneration he commented "that's the problem with regeneration, you never quite know what you're going to get" (DW: Castrovalva), and would restate this immediately prior to his ninth regeneration (DW: The Parting of the Ways) (Although the violent nature of his regenerations as opposed to Romana apparently triggering her regeneration on purpose might have something to do with this difference).

While skilled Time Lords can choose their new form with a voluntarily induced regeneration, the process has the potential to go horribly wrong and leave the Time Lord in a severely misshapen body. The problem is then often exacerbated by the Time Lord trying to solve the problem by immediately starting another regeneration instead of obtaining medical assistance, which only amplifies the defects in the regeneration. The end result of several too close regenerations, whether self-initiated or not, is inevitably a mutated monstrosity that can only be put out of its misery by complete disintegration. (DWN: The Twin Dilemma)

Some Time Lords were capable of momentarily regenerating, or partially regenerating. Though this could use up a lot of regenerative energy, it could give the Time Lord a new set of genes, allowing them to fool genetic sensors. The Doctor used this method to on the planet Purgatory in order to fool the genetic scanner used by the Landsknechte. (NA: Original Sin)

The Time Lords were apparently capable of controlling the regeneration of individual Time Lords, either forcing a regeneration, influencing the new appearance (DW: The War Games) or removing later regenerations (DW: The Ultimate Foe).

It appears Time Lords are capable of resisting regeneration, effectively committing suicide. The Master did this after being shot as an alternative to becoming the Doctor's eternal prisoner. (DW: Last of the Time Lords) The Master is later resurrected by a cult of his followers, but despite a debilitated condition and not completly stable body, he doesn't regenerate. It is possible he was unable to due to his life-force problems. (DW: The End of Time) Similarly, the Doctor appears to resist the process in DW: The End of Time. After exposure to a lethal dose of radiation, he notices the process begining, but resists the transformation long enough to visit his former companions throughout time and space. The complexity of the journey would seem to indicate that the Doctor resisted regenerating completely for at least several hours, and it may have resulted in the unusually violent nature of the regeneration, although it's also possible the nature of the radiation exposure was such that lethal effect did not immediately set in.

Time Lords seemingly developed a means for artificial life forms to undergo regeneration as well; the robot dog K-9 Mark I was fitted with a regeneration circuit which allowed him to upgrade to a significantly more advanced model following his self-destruction. (K9TV: Regeneration)

Regnerative difficulty
While most regenerations seemed to cause moments of mental instability (with some degree of temporary amnesia often noted), some offered particularly profound instances of physical peril. The Doctor feared that his fourth regeneration "was failing" when he found himself reverting into previous personas, and could only be righted with the use of the Zero Room of the TARDIS. (DW: Castrovalva) Later, the Doctor claimed that anesthesia had "nearly destroyed the regenerative process" during his seventh regeneration, as a possible explanation for the particularly severe amnesia he suffered afterwards. (DW: Doctor Who) Still later, one of the the Doctor's hearts stopped beating for a prolonged period of time, and he began to exhale regenerative energy when his ninth regeneration went "a bit wrong", after which he slipped into a coma-like state for most of a day. (DW: Children in Need Special, The Christmas Invasion)

For other species, a metamorphic symbiosis regenerator was said to be able to provide help when regerations failed. (DW: Mawdryn Undead)

Meta-Crisis
A Time Lord can prevent death and regeneration by focusing the regenerative energies into a severed appendage, like the Doctor's hand. His hand siphoned off the excess energy that would have changed his appearance while the Doctor used just enough to heal himself from the injury sustained by the Dalek gun. This resulted in the appendage storing enough energy to actually grow a nearly identical Time Lord after it come in contact with Donna Noble. As a consequence, the Doctor appeared to regenerate and heal but did not change. (DW: Journey's End)


 * It is unknown if this used up one of the Doctor's regenerations.

Cross-species transformations
Though they did not undergo normal regenerations, as noted elsewhere, Mawdryn underwent many mutations of physical form. (DW: Mawdryn Undead) I.M. Foreman, a Gallifreyan (but not a Time Lord) absorbed the DNA around him and underwent indescribable changes as a result of mutations, transcending sex, species and even physical existence itself. (EDA: Interference - Book One, Interference - Book Two) Romana, prior to her regeneration in to her second incarnation, appeared to have taken on a Near-Human blue-skinned form. (DW: Destiny of the Daleks)


 * One account states that the TARDIS itself, rather than Romana, adopted this shape. (ST: The Lying Old Witch in the Wardrobe)

Aborted Regeneration
Occasionally, a regeneration will fail and the regeneration will abort. Though the Time Lord will have regenerated, they are severely deformed. Though Time Lord technology can treat this, in some occasions the damage will be too severe to fix.

After being shot by the War Lords, the War Chief was barely able to survive. While being taken back to the War Lords' planet, his body attempted to regenerate. Due to the massive injuries and the lack of medical care, this regeneration aborted, which meant he had two conjoined individual bodies, poorly fused together. (NA: Timewyrm: Exodus)

Scientific explanation
The exact mechanism that makes regeneration possible has not been stated. Varying explanations may or may not be compatible with each other.


 * One explanation was that Cardinal Rassilon had been investigating a method of regenerating decaying and diseased tissue via a series of permanently carried self-replicating biogenic molecules. The cells of a Gallifreyan body could be repaired, restored and re-organised. This would result in a wholly new physical form. The brain cells would similarly be rearranged, though to a lesser degree, thus ensuring the new incarnation will replicate the memories and personality of the former incarnation. Rassilon intended for this mechanism only to be used upon the Gallifreyan elite. He also inputted a parameter of 12 regenerative cycles to avoid decaying biogenic molecules. (BFA: Zagreus)


 * Another theory attributes regeneration to a "nanomolecular virus" that rebuilds the body much like the "self-replicating biogenic molecules". (REF: The Gallifrey Chronicles)


 * A third theory is that Time Lords have triple-helix DNA: the third strand was added by Rassilon to make regeneration possible. (MA: The Crystal Bucephalus)


 * One partial explanation of the process links it to the release of massive amounts of a hormone known as lindos at moments of extreme trauma, with the hormone triggering the regeneration itself. Recently-regenerated Time Lords can be identified by the raised levels of lindos in their system. (DWN: The Twin Dilemma, BFA: Unregenerate!)

The Doctor

 * First to second incarnation: Apparently succumbed to old age and/or fatigue. (DW: The Tenth Planet)


 * Second to third incarnation: A forced regeneration and exile to Earth by the Time Lords. (DW: The War Games)


 * Third to fourth incarnation: Radiation poisoning from the Great One's cave of crystals on Metebelis III. Regeneration did not not occur immediately, as the Doctor said he was "lost in the time vortex" for an indeterminate length of time. (DW: Planet of the Spiders, ST: "Ancient Whispers")


 * Fourth to fifth incarnation: Fell from the Pharos Project radio telescope. (DW: Logopolis)


 * Fifth to sixth incarnation: Spectrox toxaemia exposure. (DW: The Caves of Androzani)


 * Sixth to seventh incarnation: Injured as the Rani attacked the Doctor's TARDIS (DW: Time and the Rani); seemingly because he hit his head (NA: Head Games), however there were underlying issues of radiation poisoning. (PDA: Spiral Scratch)


 * Seventh to eighth incarnation: Died on the operating table while undergoing exploratory heart surgery by Dr. Grace Holloway. (DW: Doctor Who) The Doctor claimed that the anesthesia kept him "dead too long", and he did not regenerate immediately upon the death of his predecessor.


 * Eighth to ninth incarnation: Unknown; implied to have occurred with relation to the Last Great Time War. (Prior to Rose)


 * Ninth to tenth incarnation: Cellular degeneration caused by absorbing the energies of the time vortex from Rose Tyler, which she in turn had absorbed from the heart of the TARDIS. (DW: The Parting of the Ways)
 * During tenth incarnation: Partial regeneration after being hit by Dalek fire, (DW: The Stolen Earth); aborted by redirecting the energy to his severed hand; indirectly led to creation of both DoctorDonna and clone. (DW: Journey's End) It is not known at present whether this incident counts against the Doctor's allotment of regenerations.
 * Tenth to eleventh incarnation: The Doctor was massively irradiated in the process of saving Wilfred Mott. As with the Third and Seventh Doctors, regeneration did not immediately set in, and this Doctor was able to spend an indeterminate amount of time revisiting past companions before finally succumbing. On this occasion, he regenerated so explosively it caused massive damage to the TARDIS and set it on fire. (DW: The End of Time)

On most of the above occasions, the Doctor has regenerated in the presence of other individuals (usually companions). Known exceptions include his seventh and tenth regenerations which occurred when the Doctor was alone; it is not known for certain whether his second or eighth regenerations occurred in the presence of others.

Of the Doctor's recorded regenerations to date, six have occurred with the Doctor lying in a prone position, while two (as well as the partial regeneration) occurred with him standing upright. At least four of the regenerations (first, third, sixth and eighth) occurred with the Doctor apparently unconscious, while he appeared to remain conscious during his fourth, fifth, ninth and tenth regenerations. His first, fifth, sixth, ninth and tenth regenerations (plus the partial) occurred within the TARDIS; his third, fourth, and seventh occurred outside the ship. (The exact nature of his second and eighth regenerations remains unknown.)

Alternate Doctors

 * Ninth to tenth incarnation:Hit by Dalek fire when they tried to exterminate The Master. (DW: The Curse of Fatal Death)


 * Tenth to eleventh incarnation:Injured while trying to fix the Dalek's Zectronic beam. (DW: The Curse of Fatal Death)


 * Eleventh to twelfth incarnation:Injured while trying to fix the Dalek's Zectronic beam. (DW: The Curse of Fatal Death)


 * Twelfth to thirteenth incarnation:Hit by excess zectronic Energy, zectronic Energy was known to able to destroy the Regenerative process of a Time Lord but nevertheless the Regeneration was successful. (DW: The Curse of Fatal Death)


 * The Doctor (Unbound 3):Shot by Ruth,she shot him again when he regenerated and so on. Full Fathom Five

The Master

 * Used up most of his regenerations escaping from the Darkheart. (MA: The Dark Path)


 * Used up his regenerations, up until his last, recovering from bomb blast set by Susan Foreman. (EDA: Legacy of the Daleks)
 * These accounts contradict each other somewhat. Possibly, not all were used up in the two instances.


 * Utilized the power of the Source to steal the body of Tremas. (DW: hThe Keeper of Traken)
 * It is not known for certain if the incarnation of the Master later "executed" by the Daleks is this same individual.


 * Stole the body of the human Bruce after being vaporized by the Daleks. (DW: Doctor Who)
 * Neither of these new bodies were, strictly speaking, regenerations.


 * Regenerated to survive wound from gun fired by Chantho. (DW: Utopia)
 * The Master had been resurrected to fight the Last Great Time War at this point, and probably had a new regenerative cycle bestowed upon him (per offer made in DW: The Five Doctors).

Romana

 * Regenerated into several forms before settling on a body duplicating that of Princess Astra. (DW: Destiny of the Daleks)
 * One account suggests that she did so to prevent her possession by the evil entity Pandora (BFG: Lies), another that the Key to Time had secretly harmed her (ST: The Lying Old Witch in the Wardrobe). The Doctor, soon after one regeneration, was able to regrow a severed hand (DW: The Christmas Invasion), offering a possible explanation as to how Romana was able to initially choose several different appearances -- including twice choosing that of Astra.


 * Regenerated into an incarnation more suited to fighting the War with the Enemy. (EDA: The Shadows of Avalon)


 * Romana may have regenerated a third time as a result of the War. (PDA: Tomb of Valdemar)

Rassilon
Rassilon was said to have achieved a cycle of perpetual regeneration, becoming immortal. It was for this secret that Lord President Borusa sent four of the Doctor's incarnations and their companions into the Death Zone, where Rassilon lay in eternal sleep in the Dark Tower. (DW: The Five Doctors).

Rassilon again became president in the Last Great Time War; it would seem that he was awakened from his eternal sleep and regenerated to renew himself for the war. It is possible that Rassilon granted every participating Time Lord a new regenerative cycle to avoid losses in the war and resurrected others, presumably with full regeneration cycles like the Master.

This also gives future producers of the show the ability to go beyond Thirteen Doctors and go on to at least Twenty One (if were counting from the Ninth Doctor onwards).

Borusa
Borusa regenerated at least 3 times, having at least 4 lives. (DW: The Deadly Assassin, DW: The Invasion of Time, DW: Arc of Infinity, DW: The Five Doctors)

Azmael

 * Azmael used all his 12 regenerations.(DW: The Twin Dilemma)


 * In his Thirteenth and Final body he regenerated past his limit, killing him and Mestor who possessed Azmaels body after his own was destroyed. (DW: The Twin Dilemma)

Jenny
Regenerated several hours after being shot by Cobb, curiously her appearance did not change.

However it is possible that she entered a healing coma, was restored by residual energy from The Source, or was healed by her body's residual cellular energy (although this is reliant on her 'birth' somehow generating extra cellular energy like a regeneration would). (DW: The Doctor's Daughter)

K'anpo Rimpoche
Attacked by Eight Legs and regenerated by merging with his assistant Cho Je. (DW: Planet of the Spiders)

Iris Wildthyme
Regenerated following a lengthy period of illness following the consumption of a live Kaled mutant. (EDA: The Scarlet Empress)

Rallon
Initiated all twelve of his regenerations to stop the Celestial Toymaker. (PDA: Divided Loyalties)

The Brigadier
Regenerated in 2012, which renewed his youth. (NA: Happy Endings)

Chris Cwej

 * Regenerated by force to survive radiation poisoning (BNA: Tears of the Oracle)

Behind the Scenes

 * Regeneration was first introduced when the First Doctor (William Hartnell) changed into the Second Doctor (Patrick Troughton) in the closing moments of The Tenth Planet. Tradition has it that Hartnell himself thought up the idea of the Doctor being able to change into a new body as a means of keeping the series going after his departure.


 * In The Power of the Daleks, the next story, the Doctor said that he had been "renewed", and also said that the change was partly due to the TARDIS, partly to himself, without elaborating. Originally, the Production team meant for the Doctor to have used the TARDIS to reverse time and "rejuvenate" himself, though they may have abandoned this explanation by the time Patrick Troughton took over the role.


 * The change in the Doctor's appearance was meant to occur several stories earlier, during The Celestial Toymaker, with the Toymaker capriciously having changed the Doctor's appearance out of spite. (The Doctor is invisible and unable to speak for most of the story anyway.) The reason for the change of plan is unknown. Either it was decided to retain Hartnell in the role for a few more stories, or the actor was accidentally issued with a new contract by mistake, making it impossible to recast the Doctor at that moment in time. (A plot device similar to this would occur in The Mind Robber to cover Frazer Hines' temporary replacement by his cousin, Hamish Wilson, after Hines contracted an illness.)


 * The changeover was not actually referred to as "regeneration" until the end of Planet of the Spiders, when the Third Doctor (Jon Pertwee) changed into the Fourth (Tom Baker). Prior to this, the Doctor was simply described as having "changed his appearance."


 * Fans and the general public have long speculated as to whether the Doctor could change sex as a result of a regeneration. The Time Lord I.M. Foreman having changed sex as a result of regeneration implies that this can happen. Female versions of the Doctor appeared in the non-canonical The Curse of Fatal Death and in the Doctor Who Unbound story Exile. It may have gained further support as the 11th Doctor at first thought he had regenerated into a female.


 * In the original 1963-89 series, plus the 1996 TV movie, each regeneration was treated differently on screen, using a variety of special effects ranging from simple cross-fades to a CGI "morph" in the 1996 film. With the return of Doctor Who in 2005, producer Russell T Davies decided regenerations would take a consistent form, with the Time Lord in question standing upright, arms outstretched, and blinding energy flying out from the head, arms, and legs. So far, the four regenerations depicted on screen (three featuring the Doctor in The Parting of the Ways, The End of Time and The Stolen Earth; one featuring the Master in Utopia) have taken on this form. And all four have also occurred within the TARDIS. It remains to be seen if any future on-screen regenerations will retain the same format.


 * After his resurrection on Earth the Master didn't regenerate. It is unclear if he was merely unwilling to do so until his body totally failed or if he could not because his resurrection was sabotaged.