VARIATION AND HEREDITY IN TWINS 153 



cate interplay of three grades of successively operating 

 symmetry systems, the later tending to obliterate the 

 effect of the earlier, but not always successfully. In 

 general, mirror-imaging between opposites is evidence 

 of a residuum of a primary bilateral symmetry that 

 held sway in the blastocyst before polyembryonic 

 fission began. When the primary outgrowths are 

 formed, they are the product of the antimeric halves 

 of the first embryo and should therefore show mirror- 

 image relations. But a partial physiological isolation 

 of the two halves permits a certain reorganization or 

 regulation of new symmetry relations, which tends more 

 or less completely to destroy the original symmetry, 

 yet often leaving a trace of the latter. Similarly, when 

 the secondary outgrowths arise between the primary 

 ones a certain residuum of the primary symmetry may 

 be carried over that frequently manifests itself in 

 mirror-imaging between twins derived from one half 

 of the original embryo. Finally, when each secondary 

 outgrowth organizes its own bilateral s)Tnmetry, it 

 tends to lose, partially at least, the earlier symmetry 

 relations, and to establish its own mirror-imaging of 

 right and left sides. In some cases traces of all 

 three symmetry systems appear in a single set of 

 fetuses, but it is common to find only two systems 

 interacting. 



Mirror-imaging between individuals limited to integu- 

 mentary structures. — When examples of symmetry re- 

 versal were first discovered to occur so frequently in 

 the armor characters of the armadillo, it seemed probable 

 that similar reversals would be found in the visceral 

 arrangements. One would expect to find occasionally 



