Perhaps it was seeing Amy with babe in arms. Perhaps it was the sight of that old cot that made the Doctor remember her. Maybe it was something else, but seeing those words, intertwining circles, concentric, reminded him of her.
And because he is alone - something he doesn't want to think about right now - he tries to find her old room. He wanders through the corridors, recognising doorways. He thinks the TARDIS is being kind to him, because this time it is on the fifth corridor. He stops just short of the doorway and takes a deep breath. He reaches out with one hand and gently traces the circles, vast curving lines and almost-not-there dots. He whispers her name to himself. And then he opens the door.
It's just like he remembers. It always is, as though it were an exhibition in a museum, but without a single trace of dust. Her clothes are bundled around the bottom of her wardrobe. Her bed is almost made but the sheets are crumpled and the pillow is sideways, as if she straightened it in a hurry.
It's like she never left, but he knows that she's gone. It was so long ago, but she's gone. And for a second, he's tempted to try and find her, to see if she's still around, somewhere on Earth. But he knows he most. He made her leave for a reason, and he isn't one to go back on these things.
He remembers his life, so long ago, centuries ago, when he was but a child, and yet held in his arms his beautiful granddaughter.
How far he has come since then.
He retreats, as though he is tresspassing on something private. But he lingers at the door again, tracing his finger across the circular pattern, a single word in Gallifreyan. The one language that the TARDIS has never needed to translate for him. He smiles, sadly.
Arkytior. A beautiful name for a beautiful girl.
He shakes his head as he remembers it. How she had scoffed when she realised its English equivalent and opted to choose a different name instead. Susan.
Just as well, because to him, there would only be one Rose.
He chuckles, and then closes the door and makes his way back to the control room. After all, he has another baby to look after now. Little Melody Pond. Oh, he knew she would grow up to be special. The greatest kind of woman he could think of.
The Doctor pulls a lever and drops the TARDIS into the time vortex.
Time to save River.