Final answer:
In the Prose Edda, a six-headed being grew from Ymir's legs, contributing to the ancestry of the Jotuns or giants in Norse mythology.
Step-by-step explanation:
In the Prose Edda, a collection of Old Norse poems and tales, it is described that from Ymir's legs grew a six-headed being. This being is considered to be among the ancestry of the race of Jotuns, or giants, in Norse mythology. The giants were depicted as the primeval beings who were born from the body parts of Ymir, the primordial giant.