title: Parapagus conjoined twin Holstein calf.
abstract: Conjoined twins with a doubling of all cranial structures, having two vertebral columns along their entire body length yet with a single pelvis, have not been described in the literature. The cause of conjoined twinning is incompletely understood; however, two main theories (fission and fusion) have been proposed and disputed.
  
  
  
  A dicephalus, tetrabrachius (parapagus) conjoined twin Holstein heifer calf was studied.
  
  
  
  Two normal heads were present on two necks. The twins were fused in the thoracic region. There were four forelimbs and an abnormal orientation of ribs. Two separate vertebral columns along the length of the animal ended with two tails. There was a single pelvis, and only two hindlimbs were present. The musculature of the medial forelimbs was complete but abnormally positioned. Some medial structures, caudal to the thorax, failed to develop in these twins. There were two hearts, each one supplying one half of the body. The cranial vasculature was doubled and normal. Caudally there were two aortas, each supplying the respective half of the twin. The right caudal vena cava drained all caudal parts of the body while the left caudal vena cava drained only the liver. There were two sets of lungs. Each twin had a separate esophagus that entered a separate stomach. The right abomasum (fourth stomach chamber) was herniated through the diaphragm into the thoracic cavity. The two duodenums from each stomach fused distal to the pyloric sphincters. Caudal to this point of fusion, all structures of the digestive and urogenital systems were single. The calf had a single anus and vulva.
  
  
  
  The anatomical findings in this twin suggest a fission event followed by fusion of parallel embryonic axes.
authors.affiliation: Department of Biomedical Sciences, University of Guelph, Ontario, Canada.
authors.forename: D M
authors.initials: DM
authors.lastname: Vanderzon
authors.forename: G D
authors.initials: GD
authors.lastname: Partlow
authors.forename: K R
authors.initials: KR
authors.lastname: Fisher
authors.forename: W G
authors.initials: WG
authors.lastname: Halina
mesh_terms: D000015:Abnormalities, Multiple / Q000473:pathology*; D000818:Animals; D001842:Bone and Bones / Q000002:abnormalities; D018376:Cardiovascular Abnormalities / Q000473:pathology; D002417:Cattle / Q000002:abnormalities*; D004065:Digestive System Abnormalities / Q000473:pathology; D005552:Forelimb / Q000002:abnormalities*; D006257:Head / Q000002:abnormalities; D018482:Muscle, Skeletal / Q000002:abnormalities; D015619:Respiratory System Abnormalities / Q000473:pathology; D014428:Twins, Conjoined / Q000473:pathology*; D014564:Urogenital Abnormalities / Q000473:pathology