This DaVinci painting plays heavily into "The DaVinci Code".  It has some very odd symbology within the image.  Start with the arrangement of the characters: (from left to right) Jesus(?), Mary, John the Baptist(?), and (probably) Uriel. 

Jesus is shown on one knee with his hands in a posture of supplication as if he is praying, but to whom? 

Mary has her right hand on Jesus' back (that's why we assume that figure to be Jesus), and her left hand is poised over John's head, but look at the hand.  Is she blessing John or is her hand more clawed in a threatening gesture? 

John's right hand is upraised in a gesture of blessing.  John blessing Jesus?  Shouldn't it be the other way around? 

Uriel, if that is who is pictured, is looking not at the viewer, but somewhere off to the viewer's left (we would call it 'zoning') and is pointing at Jesus.

This is the original Madonna Of The Rocks which was delivered to the abbey which had commissioned it from DaVinci.  When the monks saw it they rejected the work immediately and categorically.  They didn't care for the implications of the imagery and symbolism DaVinci had painted for them.  They sent it back with instructions to try again. 

Here is what DaVinci came back with. 

The Madonna in London's National Gallery is a very different picture.  The figures are (L-R) John, Mary, Jesus, and Uriel.  We know it's John because he now has the elongated cross which is John's symbol.  John has his hand on Mary's knee; there is not a second hand clasping it in "a posture of supplication".  Mary's hand poised now over Jesus' head is much less threatening.  Uriel is no longer pointing at anyone or anything; (s)he is merely an observer. 

This one passed muster and the abbey paid DaVinci the commission price.  DaVinci kept the original.