196 THE PHYSIOLOGY OF TWINNING 



This reversal of symmetry reminds one of the sym- 

 metry reversal seen in twins and double monsters. It 

 will be remembered that in those cases there was a 

 delicate equilibrium between the internal factors and 

 the external factors. Sometimes the internal factors, 

 the inherent tendency for a twinned right-hand com- 

 ponent to assume the characteristic left-handed asym- 

 metry of the species, prevails; sometimes the external 

 factor, the close proximity of the left-hand component 

 and a tendency to integrate with it, causes the right- 

 hand component to act as though it were merely a 

 right-hand mirror-image duplicate of the left-hand 

 component, and we have symmetry reversal. 



Our interest in Harrison's work is, for the moment, 

 limited to the phenomenon of limb-doubling. Double 

 and triple limbs arise frequently from the transplanted 

 limb-buds. These are of all grades of completeness and 

 occur under a great variety of different experimental 

 conditions. They occur most commonly, however, when 

 the buds are transplanted farthest away from their nor- 

 mal positions. In most cases of double limbs Harrison 

 is able to distinguish an original (primary) and a second- 

 ary limb which he conceives of as arising as a lateral 

 outgrowth from the primary. 



This symmetry relation is evidently an extremely 

 fundamental phenomenon and in many respects reminds 

 one of bilateral S3anmetry and of symmetry reversal in 

 con j oined twins . W hile in conj oined twins there are many 

 exceptions to mirror-imaging, due to a regulation on the 

 part of the partially separated structures back to the 

 specific asymmetry, there are in the case of these double 

 limbs very few exceptions to the mirror-image rule. These 



