When the Kundalini energy reaches the 7th subtle centre in the body, at the crown of the head, the yogi realises the single Self, and knows that individual selves are an illusion. This chakra is the Sahasrara - the Lotus of a Thousand Petals. The medieval Italian poet Dante saw it in mystic vision as the Sempiternal Rose, "its petals rising in more than a thousand tiers are the thrones of the blessed."
The Kundalini emerges through a subtle opening known as the Brahmarandhra - the Aperture of Brahma. Sanskrit 'randhra' = a slit, split, opening, aperture, hole, chasm, fissure, cavity. (probably related to the English word 'rend' - to split)
This subtle opening also manifests on the physical level - in infants the bones forming the skull are not completely fused and there is an opening called the fontanelle - a word suggestive of the emergence of the fountain-like, upward-springing Kundalini from the top of the head.