Abstract
Multiple major malformations, characterized as conjoined twins (cephalothoracopagus monosymmetros), occurred as an incidental finding in a single conceptus from a CD-1 mouse dam. The dam had 14 other normal siblings, and the cephalothoracopagus was regarded as a case of incomplete twinning of “spontaneous” origin. Similar cephalothoracopagus twins occur rarely in various species, including man, and are thought to arise from a monozygotic origin. The cephalothoracopagus twin had essentially a single head with some duplicated structures and a duplicated torso which was fused near the umbilical cord. The conceptus had a common umbilical cord emerging from a single placenta and entering the site of ventral attachment. Several components of the axial and appendicular skeleton were duplicated, but the twin had a normal complement of viscera with the exception of a duplicated heart. The initiation of the cephalothoracogagus anomaly is thought to occur between the two-cell stage and gastrulation and would thus antedate the period of drug administration (gestation days 6-15) in mouse teratology studies. Evaluation of the temporal events of embryogenesis is thus critical in vitiatina a potential relationship of the developmental abnormality to administration of a drug.