Easter egg hunt

The BBC has an article by Kelly Grovier about a hidden egg in a Bosch painting:

Did you know that the world’s first Easter egg was created circa 1490? No, not the kind that children hunt in gardens. (That tradition is ancient, dating back to early Christians in Mesopotamia.) I mean the type of hidden messages, or inside jokes, that computer programmers conceal inside their products. The American electronics company Atari first referred to these secret communiqués as 'Easter eggs' 37 years ago, after it was revealed that one of its designers tucked his name into one of its games for players to discover.
The role of covertly planted 'Easter eggs', from which deep truths hatch is nothing new in the history of art. Once spotted, these camouflaged messages are as obvious as a rainbow beach ball in an empty white room.

Do you know where to look? What if I were to give you a great garden, the most famous garden in the history of art, could you find the Easter egg that’s concealed in plain sight: the secret symbol that centers the eye and ties the work together? Lose yourself, if you dare, in The Garden of Earthly Delights (circa 1490-1510), the mesmerizing Renaissance triptych (photo, above) by the early Netherlandis painter Hieronymus Bosch, whose death five hundred years ago, in 1516, will be commemorated this August.
The precise meaning of the masterpiece’s three intensely intricate panels, which appear to take the viewer’s eyes from Eden (in the leftmost panel) to a realm of sensual abandon (in the centre) to Hell (on the right), is the stuff of endless art-historical debate. Amid the carnal romp and riotous revelry of the central panel, Bosch has preserved purity teetering perilously above a surging sea of temptation and sin.
To find the one un-cracked egg in the garden of earthly delights, one need not look behind a single leaf of grass, or pry open a single man-crushing mollusc, or peer into a single strange translucent bulb of Bosch’s bizarre vision. To find it, one’s eyes need merely draw an ‘X’ from the four corners of the work and an egg marks the spot, smack before us at the dead centrt of the painting. Suddenly, the tempestuous vision collapses into a mystical vanishing point. Through the timeless symbol of the unhatched egg, Bosch offers us a way out of his troubled work: the hope of a birth that’s evermore about to be.

Rico says great art is often mysterious...