Egyptian Magic in Egyptian Magic by Florence Farr.

“In the same way the AB (will) or Red Vessel of the Heart is represented in the Book of the Dead as containing an egg, and a concave germ: when this concave germ is developed by cultivation the real life and full development of the Ego could begin: this is to say the KA could progress in its celestial evolution, just as the body could progress in its terrestrial evolution.” [via]