What is the Yggdrasil as noted in the book, Thou Art That?

The Yggdrasil, or "World Tree", can be found in the Icelandic Edda (specifically the Hāvamāl, versus 139-140 and 142). The Yggdrasil is All-Father Othin's means of acquiring the Wisdom of the Runes. Othin hangs himself on it for nine days, with an eagle perched above and a worm, Nithhogg gnawing at its roots, and a squirrel, Ratatosk, running up and down communicating messages. Four deer graze around the Yggdrasil. Joseph Campbell points out remarkable similarities to a Mayan cross and to the Cross of Jesus Christ.