2OO H. H. NEWMAN. 



monsters are also frequently characterized by mirror-image sym- 

 metry, due to the fact that the two halves are not physio- 

 logically isolated and are therefore parts of a single system of 

 symmetry. 



Occasional vestiges of mirror-imaging have been described by 

 Wilder for the finger-print patterns of duplicate human twins, as 

 when the pattern of the right index finger of twin A is the mirror- 

 image of the left index finger of twin B. These symmetry re- 

 versals in duplicate twins have been emphasized as evidences of 

 the monovic origin of such individuals and the reversals them- 

 selves have been interpreted as the last trace of an early sym- 

 metry formerly common to the two individuals but subsequently 

 largely outgrown. 



Now in the armadillo there are many definite evidences of a 

 system of symmetry common to all of the quadruplets, upon 

 which has been superimposed a secondary symmetry system be- 

 tween twins. This in turn is more or less completely obliterated 

 later by a tertiary symmetry between the antimeric halves of the 

 single individuals. In some sets evident traces of the primary 

 system of symmetry persist as mirror-image relations between 

 individuals of opposite pairs, but it is more usual to find no trace 







of the primary system. The secondary mirror-imaging between 

 pairs is far more commonly in evidence, but is frequently ob- 

 literated by the tertiary mirror-imaging between antimeric 

 halves of the same individual, which latter is the prevailing sym- 

 metry system. An analysis of this intricate interplay of three 

 grades of symmetry systems is by no means a simple task, but 

 in the foregoing descriptions of individual sets we have indicated 

 our interpretation of the various mirror-image and symmetry 

 reversal phenomena that form the basis of such an analysis. In 

 general, mirror-imaging between individuals of opposite pairs is 

 interpreted as an evidence of the early system of symmetry 

 present in the embryonic vesicle before polyembryonic budding 

 began. When the primary buds are formed they are the product 

 of the antimeric halves of the undivided embryo and therefore 

 should have mirror-image relations, but a partial physiological 

 isolation of the two buds permits a certain degree of reorganiza- 

 tion or regulation in the symmetry relations, that tends partially 



