From Arthur Clutton-Brock & Percy Bysshe Shelley's "Shelley, the Man and the Poet" (1999), now online at Internet Archive: "It was the result of a conversation about ghosts and horrors after which Shelley suddenly ran shrieking out of the room. He said that he was looking at Mary and thought of a woman he had heard of who had ejres instead of nipples to her breasts. This took possession of his mind and horrified him. When he was recovered Byron proposed that each of the party should write a ghost story. Shelley kept pressing Mary to do this, and at last one night she hit upon the idea of Frankenstein and his Monster."