<P> In some cases, future potential incarnations can achieve independent, though temporary, existence . In Planet of the Spiders, a Time Lord, K'anpo Rinpoche, creates a corporeal projection of a future incarnation which has such an existence under the name Cho Je until he regenerates into that incarnation . The Valeyard, an "amalgamation of the darker sides of (the Doctor's) nature, somewhere between (his) twelfth and final incarnation", appears in The Trial of a Time Lord (1986) opposite the Sixth Doctor; the Valeyard is promised the remainder of the Doctor's regenerations . Another example is "The Watcher", who repeatedly appears to the Fourth Doctor in Logopolis (1981), and ultimately merges with him as part of his regeneration into his fifth incarnation . </P> <P> The Time Lords' ability to change species during regeneration is referred to in the television movie by the Eighth Doctor in relation to the Master . Mavic Chen states that the First Doctor's resemblance to an Earth creature is "only a disguise" in The Daleks' Master Plan (1965). In Destiny of the Daleks (1979), the Fourth Doctor's Time Lady companion Romana demonstrates an apparent ability to "try on" different bodies from a number of different species during her regeneration, before settling on a final, humanoid form which physically resembles Princess Astra of Atrios . </P> <P> Steven Moffat has said that reference will be made to Peter Capaldi's previous appearances in the Whoniverse during his tenure as the Twelfth Doctor: "The face is not set from birth . It's not like he was always going to be one day Peter Capaldi . We know that's the case because in The War Games he has a choice of faces . So we know it's not set, so where does he get those faces from? They can't just be randomly generated because they've got lines . They've aged . When he turns into Peter he'll actually have lines on his face . So where did that face come from?" In "Deep Breath" (2014), the Doctor and several other characters speculate about where his new face came from, given that it has lines from frowning . The Doctor considers aloud that it is a face he recognises from somewhere, but due to post-regenerative trauma, he cannot place it . In "The Girl Who Died" (2015), he concludes that he subconsciously chose the face as a reminder of his decision to save the life of Caecilius (also played by Capaldi) in "The Fires of Pompeii" (2008) and to remind him to save people even with possible historical consequences by doing so . </P> <P> The fact that the Master is inhabiting a non-Gallifreyan body when he is offered a new cycle of regenerations (see above) implies that it is possible to grant them to a non-Gallifreyan, albeit one inhabited by a Time Lord mind . In addition, River Song is shown to have the ability to regenerate due to altered DNA that has similarities to Time Lord DNA, a side effect of having been conceived on board the TARDIS as it travelled through the space - time vortex . Non-Gallifreyans are also seen to regenerate in Underworld (1978) and Mawdryn Undead (1983), but with adverse side effects . In Mawdryn Undead, these appear to be the result of mishandling stolen technology, but in Underworld they are implied to be the inevitable result of limited technology that reinvigorates, rather than transforms, the subject's appearance (in this case, the Minyans, with whom the Time Lords shared much of their technology), thereby regenerating' the body, not the soul' . </P>

Why can doctor who only regenerate 12 times