A tiny skull containing a malformed brain has been found in a Japanese teenager's ovary.
The 16-year-old was undergoing an operation to have her appendix removed when surgeons found a 10cm-wide "monster" of a tumour.
Inside it they found matted hair and a 3cm-wide " brainstem-like structure", according to a report in medical journal Neuropathology.
"A mature cystic teratoma of the ovary… arose in a 16-year-old girl", the journal states.
Specifically the brain was a tiny cerebellum, which sits near the back and controls the body's muscles. It was surrounded by a "skull-like, bony shell".
The word "teratoma" comes from the Greek "teratos", or "monster".
The misplaced parts weren't part of a parasitic twin, as occasionally happens - they were made by the girl's own body. It's believed teratomas are caused by immature egg cell going rogue and producing the wrong kind of tissue.
New Scientist reports while it's not uncommon to find the odd brain cell in teratomas removed from ovaries, it's "extremely unusual" they'd form something close to resembling an actual brain.
The 16-year-old's surgery was successful and she has since recovered.