TERATOGENESIS. 597 



one of the twins is rudimentary, the thorax and head being absent, but the 

 extremities present in part (ischiopagus parasiticus) . Ischiopagi seldom 

 survive owing to atresia of the anus. 



In case the attachment extends along the entire trunk of each twin 

 ischiothoracopagus the two sacra are usually fused to form a single sacrum. 

 The thoraces of the twins are joined by means of a common sternum. The 

 upper extremities may all be present (tetrabrachius), or there may be three 

 (tribrachius), or only two (dibrachius) and a very rudimentary third. The 

 lower extremities are subject to the same variations as the upper. The ex- 

 ternal genitalia and anus are single. The alimentary tube is double as far as 

 the lower end. The thoracic viscera are partly double. Monsters of this 

 type may live for years. 



Middle Union. In the case of middle union a ventral or ventro-lateral 

 attachment extends from the umbilicus for a variable distance toward the head. 

 In most cases the umbilicus itself is single. The union may occur in the 

 region of the xiphoid process xiphopagus or it may involve the entire region 

 of the sternum sternopagus or thoracopagus. In the case of xiphopagus, 

 a bridge joining the twins extends from the common umbilicus to the 

 xiphoid processes. The latter are usually united across the bridge. The 

 thoracic cavities are separate. The two livers may be connected by a bridge of 

 hepatic tissue, in which case the two peritoneal cavities are in communication, 

 or the livers may remain separate, in which case the peritoneal cavities do not 

 communicate. The two alimentary tubes may or may not communicate in the 

 region of the stomach. Xiphopagi may live for many years, as instanced by 

 the "Siamese Twins." 



In the case of sternopagus the union extends upward from the common um- 

 bilicus, so that the two sterna are fused into a single bone. There is a com- 

 mon thoracic cavity, separated from the abdominal cavity by a single diaphragm. 

 One or two hearts may be present. The middle portions of the two alimentary 

 tubes form a single tract. The two livers are fused into a common mass. The 

 genitalia are distinct and separate. The extremities may be normal, or in case 

 of a ventro-lateral union, the approximated upper extremities may be fused. 

 Such monsters are usually born dead; if born alive, they survive but a short 

 time owing to defective development of the heart. 



As other varieties of thoracopagus the following may be mentioned : Thora- 

 copagus parasiticus, in which one twin is much arrested in development, a head 

 and heart being present, and attached to the thoracic region of the stronger 

 twin. Gastrothoracopagus dipygus (dipygus parasiticus), in which extremities 

 and trunk are present at least in part and are attached to the lower part of the 

 thorax or to the epigastrium of the other twin. The head is not present. Such 

 twins may live for years, as instanced by Laloo. Cephalothoracopagus diproso- 



