The contrast is in the textures, the curved against the flat etc. the idea of the corresponding shape of the egg, and the Knot in the wood serves to empasize this (blather blather!).
Explanations can be really tough - this is an old favorite shot, but I find it very hard to put into words the ambiguities and similarities - maybe that's why I thought it worked . . . . . .
Pepe or Flick - please could you describe it for me!