Gangopadhyay et al. Journal of Medical Case Reports 2010, 4:96 http://www.jmedicalcasereports.com/content/4/1/96 Page 3 of 7 Figure 2 Abdominal computed tomography of the fetus with a large encapsulated peritoneal cavity mass and mature vertebral skeleton. elements of the three germinative layers. Skin, a verteb- ral column, germinative buds of limbs, central nervous tissue (encephalus and coroidal plexus), a stomach, small and large bowels, pancreas, adrenal glands, kid- neys, upper and lower airways, cardiac striated muscles, and lymphoid tissue-like spleen were found. The histo- pathological study of the specimen supported the con- clusion that the previously imaged calcifications could be assumed to be the skull and bony constituents of the vertebral axis, some parts of the skull, and bony consti- tuents of the rudimentary limbs. Our patient recovered well after the surgery and was discharged. To rule out any recurrence he was followed up through clinical examination, plain abdominal X-ray examination, abdominal ultrasound, and serum alpha- fetoprotein (AFP). We were unable to detect any recur- rence of his previous symptoms one year after the operation. Discussion The term “fetus in fetu” was first used by Johann Frie- drich Meckel during the late 18th century [4]. Subse- quently, Willis described it as a rare condition where a malformed parasitic twin resides in the body of its host, usually in the host’s abdominal cavity [5]. The condition represents an aberration of monozygotic diamniotic twinning where the unequal division of the totipotent inner cell mass of the developing blastocyst leads to the inclusion of a smaller cell mass within a maturing sib- ling embryo. This rare pathology occurs only once every 500,000 births [6]. Fewer than 100 cases worldwide have been reported [7]. The literature rarely describes multiple or twin FIF. The majority of cases of FIF occur during infancy, with the oldest reported case being that of a 47-year-old man [1]. Thakral et al. reported that FIF occurs equally among the male and female populations [8]. In 70% of reported cases, the chief presenting com- plaint is an abdominal mass [9]. The mass is predomi- nantly retroperitoneal in 80% of cases [5], while reported uncommon sites are the oral cavity [4], the sacrococcygeal region [10] and the scrotum [7]. The presence of a vertebral column in the FIF is an important feature that distinguishes it from a teratoma. The clear identification of a verterbal column shows that fetal development of the included twin had advanced at least beyond the primitive streak stage (12 to 15 days of gestation) to a notochord, which is the precursor of the vertebral column [1-3,8]. FIF generally occurs singly. Multiple masses have been found in only a few instances. Our patient exemplifies the occurrence of FIF as a partially developed twin fetus [11]. The mass we found in our patient was enveloped by a sac that contained a second mass, which was suggestive of a twin FIF. There are instances where no symptoms at all occur. In some cases, however, symptoms present as an