The Homunculi mark is what we call Ouroboros. It is an ancient symbol with many interpretations to it and it's generally drawn as a Dragon or a Serpent who devours its own tail. In addition to that symbol, the Homunculi's ouroboros also carries a 6 edged star (also know as the David's star) inside the circle formed by the Dragon. In alchemy, the Ouroboros appeared in many illustrations representing the cicle of the alchemical opus.