SYMMETRY REVERSAL AND MIRROR-IMAGING 165 



In human conjoined twins, especially in those known 

 as anadid3ani, in which there are two separate heads 

 while the bodies are united in the thoracic and abdominal 

 regions, symmetry reversal is quite common. According 

 to Spaeth (i860) there are all degrees of situs inversus 

 viscerum ranging from cases of complete reversal to cases 

 in which there is only a trace of it. Many cases of slight 

 symmetry reversal are overlooked, but it is these marginal 

 cases, which, according to Schatz (1887), involve the 

 greatest danger for the twins possessing them. They are 

 nearly always associated with some irregularity in the 

 circulatory system and such irregularities are almost 

 certain to have serious consequences when the blood 

 supplies of the two individuals come to be united by 

 placental anastomoses. Not only are there numerous 

 instances of situs inversus in conjoined twins but, accord- 

 ing to Spaeth and Schatz, small deviations from the 

 normal situs are often found in separate one-egg twins. 

 Such deviations do not so much concern the larger 

 more obvious organs, such as stomach, heart, aorta, but 

 have to do especially with less conspicuous regions of 

 the circulatory system. 



Schatz in studying separate human one-egg twins, 

 especially the acardii dealt with in chapter x, noted 

 many instances of incomplete situs inversus viscerum. 

 In one case the only reversal present had to do with the 

 chief vessels of the heart; the aorta was on the right, 

 the vena cava on the left. In another case the aorta 

 was to the left of the vertebral column yet to the right 

 of the vena cava. Another case was noted in which 

 there was practically bilateral symmetry of certain 

 normally asymmetrical structures. Even these lesser 



