Fetus in fetu: a fetiform teratoma.

Examination of a retroperitoneal fetus in fetu, diagnosed preoperatively, revealed previously unreported histologic findings in the vascular pedicle and membranous capsule that indicated that these structures are not "umbilical cord" or "amnion." These findings include nervous innervation of both structures and a well-defined lamina elastica interna and vaso vasorum in the artery of the vascular pedicle. Thus, strong support is provided for the theory that many examples of fetus in fetu are remarkably complex, well differentiated, highly organized teratomas. Additional arguments that favor the identity of fetus in fetu and teratoma are presented.