The Master was somewhat alarmed by the Doctor, but he didn't let it show. Of course, his two hears, hammering away would give it away. But instead of letting it show, he smiled. A creeping laughter slowly escaped between his teeth and raised in tone and volume quickly.
The laughter ebbed away and he cocked his head to the side. "Tell that to yourself the next time you regenerate" he replied smoothly. "Or better yet, I'm sure he's around somewhere! You can tell him yourself!" he said brightly, grinning like a Cheshire cat. "You do realise I can take exactly what I need from you, Doctor.... That life force, that energy!" he said, his eyes lighting up. For a brief moment, his form changed, as if he had been shocked by electricity. The skeleton beneath became visible for just a moment and then.... it stopped.