200 THE PHYSIOLOGY OF TWINNING 



The situation is greatly complicated, however, when one 

 of the components of the twinned limb undergoes a 

 secondary reduplication. The mirror plane is then on 

 the ulnar or superior side of the twinned limbs. It 

 could not be otherwise without doing away with the 

 mirror-imaging between the first pair of twin com- 

 ponents, for they could not all three have the mirror 

 plane on the radial or inferior side. This secondary 

 reduplication undoubtedly greatly complicates matters 

 and renders the analysis of symmetry reversal extremely 

 difficult. It is easy to state the rule according to which 

 mirror-imaging works out, just as both Bateson and 

 Harrison have done, but it is not nearly so easy to ac- 

 count on physiological grounds for what is so readily 

 formulated. 



It is clear that the complicating factor is the second 

 step in limb-doubling. If we could find a material in 

 which limb-doubling was simpler we could perhaps 

 obtain a less complicated situation that would admit 

 of more ready analysis. It is fortunate that we have 

 just such a series of instances of simple limb-doubHng 

 in human hands and feet. 



DOUBLE HANDS AND FEET IN MAN 



Bateson (1894) has described and figured a number 

 of significant instances of hand and foot anomaly which 

 seem to me to help us to bring the phenomenon of 

 limb-doubling into line with bilateral twinning. These 

 will be listed in a logical series : 



I . The minimal case of mirror-imaging or break-down 

 of dorso-ventral asymmetry of the left hand is one in 

 which the thumb is essentially the same as the little 



