Jung believed these visions to be a sort of Alchemical allegory, with the tormented homunculi personifying transmutations—burning or boiling themselves to become something else. The central image of the visions are the Sacrificial Act, which each Homunculus endures. In alchemy the dyophysite nature is constantly emphasized, two principles balancing one another, active and passive, masculine and feminine, which constitute the eternal cycle of birth and death. This is also illustrated in the figure of the uroboros, the dragon that bites its own tail (and which appears earliest in the Chrysopoeia). Self-devouring is the same as self-destruction, but the unison of the dragon's tail and mouth was also thought of as self-fertilization. Hence the text of "Tractatus Avicennae" mentions "the dragon slays itself, weds itself, impregnates itself." In the visions, circular thinking appears in the sacrificial priest's identity with his victim and in the idea that the homunculus into whom Ion is changed devours himself—he spews fourth his own flesh and rends himself with his own teeth. The homunculus therefore stands for the uroboros, which devours itself and gives birth to self. Since the homonculus represents the transformation of Ion, it follows that Ion, the uroboros, and the sacrificer are essentially the same.