TWINNING (DUPLICITY) IN LIMBS 203 



hand falls between the thumb and the index finger and 

 that the thumb is the reduced equivalent, on the radial 

 side of the limb, of the four fingers on the ulnar side of 

 the limb, we could not have a double hand with more 

 than eight digits. No instances of more than eight 

 digits have come to our attention, and whenever there 

 are eight digits they occur in two mirror-image sets. 



CASES OF SYMMETRY REVERSAL 



In the case of double-headed twins, one component, 

 always the one on the right (the individual belonging 

 to the inferior side) may become sufiiciently independent 

 to resume the specific asymmetry {situs solitus). Some- 

 what equivalent changes may occur in the case of double 

 hands and feet. An interesting double right hand with 

 eight fingers is described and figured by Giraldes, in 

 which the outer digit of the ulnar or superior side is 

 decidedly thumblike while that on the radial side is 

 distinctly a little finger. This is like the complete 

 reversal of asymmetry seen in the sea urchin plutei 

 described by Oshima (192 1). Another similar case is 

 noted by Athol Johnson, but this time in a foot instead 

 of a hand. It is a case of a left foot with nine toes, 

 one only partially subdivided. The four digits on 

 the little-toe side (homologous with the little-finger 

 side of the hand) had four normal digits, none of which 

 is a great toe. The five toes of the great-toe side had 

 the largest digit, much like a great toe, on the outside. 

 The third and fourth digits of this side are only partially 

 divided. This looks like another case of symmetry 

 reversal on the part of the component of the weaker 

 side back to the specific asymmetry of the single limb. 



