CHAPTER XII 



SYMMETRY REVERSAL AND MIRRORTMAGING IN 

 TWINS AND DOUBLE MONSTERS 



SYMMETRY REVERSAL IN HUMAN TWINS 



Ever since double monsters began to attract the atten- 

 tion of biologists the occurrence of reversed symmetry 

 has been a matter of marked interest. The earlier obser- 

 vations are confined to instances of reversal of the asym- 

 metry of certain unpaired organs such as stomach, heart, 

 aortic arch, vena cava. The famihar condition in which 

 the greater curvature of the stomach is to the left, the 

 apex of the heart to the left, aortic arch to the left, and 

 vena cava to the right is by the older writers called 

 situs solitus — the situation normal for single individuals. 

 The unusual condition in which the greater curvature 

 of the stomach is to the right, apex of heart to the right, 

 aorta to the right, and vena cava to the left was at first 

 called situs viscerum transversus. The older authors also 

 use for this phenomenon the terms situs rarior, solito 

 inversus, and situs inversus viscerum. Among English- 

 speaking writers it has come to be referred to as symmetry 

 reversal or mirror-imaging. Rare instances appeared in 

 which a single-born individual exhibited symmetry 

 reversal, and it has commonly been suggested that such 

 individuals are probably surviving right-hand com- 

 ponents of twin pairs in which the left-hand component 

 was either formerly a parasite, afterward completely 

 resorbed, or else had died at an early stage. 



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