.... For now, and first, a quick look at the reference that some identify the disciple/apostle to Jesus left with the "upward pointing finger as John the Baptist".
Please review Post #79830 - 06/10/05, Topic: "How The Apostle John Was Usually Portrayed", (URL at the end of the Post).
In that example, the figure is not identified as John the Baptist but as Thomas.
John The Baptist was long dead by the time Jesus and his disciple/apostles met for this final time. Accordingly, Leonardo did not paint John The Baptist into the scene.....
Of course John had been killed by this time. I never thought that this was John The Baptist. The point I was making is that the pointing finger has been equated with John The Baptist. In Raphael's 'The School of Athens', it looks like Leonardo, himself, who is pointing. http://www.newbanner.com/AboutPic/SOA.html
It would appear that this gesture had real meaning for Leonardo.
Gestures always mean something.
What was the reason for Thomas's pointing finger?
Since Leonardo's John the Baptist figures seem to be connected to the gesture, I wonder if it is meant to symbolise him in the painting.