132 E VOL UTION AND DISEASE. 



turned to the right, the liver is situated in the left 

 side of the abdomen, thus changing places with the 

 spleen. This transposition also affects the main 

 arterial and venous trunks. Mr. Morrant Baker 

 suggested to me that if double embryos and, in some 

 instances, twins are produced by dichotomy of a single 

 ovum, the viscera of one or other embryo should be 

 transposed one should be as it were the reflection of 

 the other vehit in speculo. 



There is much to support this view. In many 

 examples of duplex human children such a trans- 

 position has been actually found ; in following up the 

 inquiry it turns out that the hearts in these monsters 

 are often situated in the median plane of the body, 

 and not infrequently a composite heart dominates the 

 circulation of the two individuals. The question 

 naturally suggests itself, Are all cases of transposition 

 of viscera associated with twin conceptions ? This 

 must stand as a question until we know more of the 

 history of such cases, and it needs careful investigation. 



It was formerly believed, and some still maintain the 

 view, that duplex monsters may arise by the adhesion 

 of two embryos originally distinct. This view has little 

 foundation. Without attempting to seriously discuss 

 the difficulties surrounding this " impaction theory," its 

 supporters will have to explain the relation of such 

 foetuses to each other, and why they are united by 

 homologous parts, such as head to head, back to back, 

 pelvis to pelvis, &c. Should the second embryo be 

 represented merely as a teratoma, it usually resembles 

 the part to which it is attached. By the impaction 



