132 EVOLUTION 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 velut 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 
