Also, I wonder if this means the Doc can still infinitely regenerate? He might have just been wrong on Trenzalore when he thought he'd run out, and maybe the Time Lords just sent him a special effect which didn't actually do anything
Did they say his/her (whichever at the time) mind was taken from the body, and put into Hartnell's body with a regeneration limit? I know some of the "connective tissue" was missing, and I also had to nip to the loo at one point, so yeah, do we know how the Master knows it's the Doc, and if she's still the same regenerated body as the baby?