You know, maybe it's just me, but the picture just doesn't look as good with that spot filled in. The the composition works better with a central empty point to balance around.
Escher may not be a mathematician, but this mathematician's no artist.
Not a formally trained mathematician, but I think this story is a stunning example of just how much innate mathematical ability the human brain contains. Also, it's noteworthy that the ability to describe complex mathematical concepts without being able to write them down formally seems to be a common thread in a certain kind of genius (e.g. Einstein).
Escher may not be a mathematician, but this mathematician's no artist.