SYMMETRY REVERSAL AND MIRROR-IMAGING 189 



In separate one-egg human twins mirror-imaging 

 seems to persist mostly in certain integumentary struc- 

 tures such as friction-ridge patterns on index fingers, occa- 

 sional reversals in direction of whirl in the crown of the 

 head hair. Yet there are not a few instances in which 

 one twin is right handed, the other left handed. As in 

 the case of fishes, however, the normal condition in com- 

 pletely separate twins is a complete regulation in both 

 individuals of the specific asymmetry of all structures. 

 In the armadillos, especially in the case of twins derived 

 from a secondary blastoderm of one side, the incidence 

 of mirror-imaging is more frequent, there being nearly 

 as many instances in which some asymmetrical integu- 

 mentary peculiarity is found on opposite sides of a 

 pair of twins as on the same side. Here again the 

 equilibrium at the time of separation of the twin pri- 

 mordium must be extremely delicate and some very 

 minor factor may decide whether the two individuals 

 shall both show unilateral asymmetry of the same side 

 or whether one shall be the mirror-image of the other. 

 In these cases we cannot speak of symmetry reversal 

 because we do not know with regard to any sporadic 

 as3anmetry of the scute pattern what is the specific 

 condition or the situs soliius. All we can say is that 

 both individuals are mirror-images of each other. In 

 the next chapters are discussed further cases of mirror- 

 imaging that must be taken into account in reaching 

 any final judgment as to the causes and significance of 

 symmetry reversals. 



