Is the strange reproductive process in The Twelve Kingdoms based on a myth or legend? In The Twelve Kingdoms, babies are born from Ranka, or giant eggs that grow on trees.  Is this based on an myth or legend?  Or is it just a way the author invented to explore what happens to a society where the women do not have to give birth?  The idea of babies growing in eggs on trees seems like something I've heard of before somewhere else, but I can't recall where.
I touch on this in my answer to your other question. Eastern and Buddhist myths often use the cosmic egg to explain creation. In Chinese mythology, it is Pangu who hatches from the egg. In Hindu mythology, it is Brahma. In Buddhism, Buddha himself is depicted as hatching from the egg as seen below.



The myth of the World Tree is another that pervades religion.