A hypothetical mechanism for conjoined twinning postulated by Spencer ([2003] 
Developmental Malformations and Clinical Implications, Baltimore: Johns Hopkins 
University Press, p 1-476) suggests that, after separation, monovular twins fuse 
in one of eight predictable homologous sites. The tripus fetal specimen under 
study embodies characteristics of three types therefore preventing it from 
classification into a simple variant of any one of the eight twin types 
described by Spencer. The aim of this study was to reveal internal structural 
anomalies of the fetal specimen by using magnetic resonance imaging and 
computerized tomography. Dorsally appended to the primary twin is a secondary 
head mass (brain tissue and ocular globe) and two spinal columns converging at 
T4/T5, suggesting rachipagus twinning. The ventral orientation of the secondary 
twin's (right lateral) lower limb suggests parapagus twinning. The caudal 
divergence of the spinal columns and the presence of a secondary hemipelvis, 
separate from the primary pelvis, suggest cephalopagus twinning. Measurements of 
the long bones indicate a gestational age of ∼20-23 weeks. Secondary 
malformations of the primary fetal body include anencephaly, cleft palate, renal 
agenesis, decreased left ventricular outflow, and a prematurely terminating 
descending aorta. This study demonstrates the possibility of using current 
imaging techniques to study very old, formalin-preserved human material for 
documentation and scientific discussion without destroying the specimen, thus 
keeping it intact for posterity.