A teratoma is an encapsulated tumor with tissue or organ components resembling normal derivatives of all three germ layers. There are rare occasions when not all three germ layers are identifiable. The tissues of a teratoma, although normal in themselves, may be quite different from surrounding tissues, and may be highly disparate; teratomas have been reported to contain hair, teeth, bone and very rarely more complex organs such as eye, torso, and hands, feet, or other limbs.
Thanks for the Science Part, Wikipedia...
And now for the teratoma itself:
Credit: The AntiCraft.com