Danniesen wrote: With the Master, you yourself can imagine what happens between MacQueen and Jacobi. However you get the impression that MacQueen's Master regenerated into childform, as Derek Jacobi's Master said "I was a... naked child found on the coast of the Silver Devastation. Abandoned, with only this [the fob watch]".
Jacobi's Master believed himself to be Professor Yana at the time, so that probably isn't what actually happened to Macqueen's Master.