i6o 



THE BIOLOGY OF TWINS 



of as a lack of correspondence with the rest." Yet 

 no well-defined idea of the significance of these reversals 

 seems to have occurred to Wilder. Bateson, however, 

 sees in these pecuUar phenomena evidence that twins 



derived from a single 



X 



y 



zygote have been parts 

 of a single system of 

 symmetry. This is 

 evidently the key to 

 the significance of 

 symmetry reversals, as 

 was brought out in the 

 discussion of symmetry 

 reversals of armadillo 

 quadruplets. 



As Wilder has 

 pointed out so clearly 

 in his latest paper 

 ("Palm and Sole 

 Studies "0: 



The. bands of the 

 armadillo carapace, with 

 their variation and the 

 friction ridges of human 

 pakns and soles, are par- 

 tially or whoUy homolo- 

 gous structures, so that 

 their use in determining the degree of s imil arity of twinned indi- 

 viduals is equally warrantable in both cases, while the results 

 may well be compared. Both deal with epidermic structures, 

 the probable homologues of reptilian scales, placed in rows; in 

 both are observed the similar phenomena of the forking and con- 



FiG. S4. — Prints (from Wilder) of 

 the tips of the first three fingers of 

 the left hand of a pair of "identical" 

 twins. Note the reversed symmetry 

 of the index-finger prints. This is 

 a good case of mirror-imaging, so 

 characteristic of monozygotic twins. 



'Biological Bulletin, XXX, Nos. 2 and 3 (1916). 



