DEVELOPMENTAL HAZARDS OF HUMAN TWINS 149 



additional circulation induces heart hypertrophy in 

 the stronger twin, and this may have serious conse- 

 quences after birth when the extra burden has been 

 removed. 



Whether one twin injures the other or prolongs the 

 life of the other we find that the most striking physio- 

 logical and anatomical conditions are those that concern 

 the heart. The heart dies and atrophies completely, it 

 becomes weak and small in size or becomes overly strong 

 and too large in size. We shall now consider the various 

 heart anomalies in twins. 



HEARTLESS TWINS (aCARDU) 



The general term acardia is used to designate a con- 

 dition common in twins, especially one-egg twins, charac- 

 terized by atrophy of the heart. Schatz distinguishes 

 between complete lack of heart (holoacardia) and partial 

 lack of heart (hemiacardia) . In a twin exhibiting holo- 

 acardia the circulation is carried on entirely by means of 

 the heart of the other twin. In a twin with hemiacardia 

 the circulation is carried on partly by the foreign heart 

 and partly by its own heart. So long as a twin's own 

 heart continues to function to any extent, or even if the 

 foreign heart is only locally effective, as would be the 

 case if the direction of the blood stream were leversed 

 in an umbilical artery, the condition would be diagnosed 

 not as holoacardia but as hemiacardia. 



There may be as many as twenty-eight different 

 situations depending upon which of the four types of 

 placenta, A,B,C,D (see p. 142), are concerned and which 

 one of the following seven types of vascular irregularity 

 is present in a particular case. 



