DEVELOPMENTAL HAZARDS OF HUMAN TWINS 155 



In cases of marked difference between the two com- 

 ponents the larger more nearly normal individual has 

 come to be called the autosite, and the smaller more or 

 less abnormal individual, the parasite. This terminology- 

 has probably certain false implications. It implies either 

 that one of the individuals is in some sense an origi- 

 nally superior or primary one and that the other is a sort 

 of secondary outgrowth of the first produced by budding, 

 or that the parasite has come to attach itself secondarily 

 by fusion to the body of the primary individual. Both 

 of these alternative implications are, T believe, obviously 

 ill founded, as will be appreciated when it becomes 

 known that the same vascular anastomoses exist between 

 conjoined twins as prevail for separate one-egg twins. 



CONJOINED TWINS WITH SEPARATE HEARTS AND 

 SEPARATE UMBILICAL CORDS 



The majority of conjoined twins are joined only by 

 the body wall and have most of the viscera separate. 

 There are therefore the same opportunities for one 

 component to injure the other through interference with 

 its blood supply as if they were not united at all. Schatz 

 cites a large number of instances of acardia, mediacardia, 

 microcardia, and macrocardia in conjoined twins, and 

 makes scarcely any distinction on the grounds of sep- 

 arateness or union of twins. He seems to take it for 

 granted that the two types are simply two expressions 

 of the same phenomenon. Without going into any 

 great detail then, it would appear that the developmental 

 hazards of conjoined twins are the same as those of 

 separate twins and we may expect to find the same kinds 

 of inhibited individuals. In cases where complete 



