VARIATION AND HEREDITY IN TWINS 159 



for ordinary brothers and sisters, but those of the two 

 boys are extraordinarily similar. It may be decided that 

 the two boys are monozygotic and that the girl was 

 derived from a separate zygote. 



The most significant features of the finger prints 

 of duplicate twins are: 



1. In one pair of twins there was an almost complete 

 mirror-imaging of the two palm patterns of the two 

 individuals. The left hand of x corresponds to the right 

 h-and of y, and vice versa. This case corresponds to the 

 rather rare mirror-imaging of "opposites" seen in 

 armadillo quadruplets, and goes far to prove that 

 polyembryony actually occurs in man. 



2. In three other sets symmetry reversal occurs 

 to a limited extent, but, curiously enough, always in 

 connection with the pattern of the index fingers. In 

 two cases the finger-prints of the left index' finger of x 

 and of y show a reversed s^Tnmetry so that one of them 

 mirrors the condition in the left index finger of the 

 other twin. In the third case the symmetry reversal 

 is in the right index finger. These cases are quite 

 homologous to the half-band reversals noted for arma- 

 dillo quadruplets. In human twdns these reversals are 

 fairly numerous considering the small number of twins 

 examined. Fig. 54 shows the finger-prints of one pair 

 of duplicate twins with the reversed patterns in the two 

 upper prints. 



"These reversals of index patterns," says Wilder, 

 "seem to occur with too great frequency to be disposed 



' An interesting parallel exists between the conditions seen in man 

 and in the armadillo. In man mirror-imaging is usually confmcd to the 

 index finger, while in the armadillo it is largely confined to the first 

 band of armor. 



