SYMMETRY REVERSAL AND MIRROR-IMAGING 175 



noted by the writer in the case of armadillo quadruplets 

 in which both twins showed the same unilateral asym- 

 metry in their scute peculiarities. It was about equally 

 common, however, for them to show mirror-image sym- 

 metry. Here we have a somewhat parallel case and 

 the explanation is the same: that there is in some cases 

 a greater degree of isolation between the twin components 

 than in others, so that sometimes the two individuals 

 act as though entirely independent, and in others as 

 though they still retained some residuum of the earlier 

 interrelationship characteristic of antimeric halves of a 

 single individual, a relation that expresses itself most 

 obviously in mirror-imaging. 



In concluding this discussion of mirror-imaging in 

 fish monsters I "would like to reiterate what appears to 

 me a very fundamental principle: that mirror-imaging 

 is normal for twins derived through separation of the 

 antimeric halves of a single embryonic axis. Such indi- 

 viduals rarely, possibly never, become entirely separate. 

 There is no reason to expect situs inversus in separate 

 twins that have originated from two complete embryonic 

 axes on a single blastoderm. 



EXPERIMENTAL PRODUCTION OF SITUS INVERSUS 

 VISCERUM IN TRITON 



A recent very significant paper by Spemann and 

 Falkenburg (1919) has thrown much light on the problem 

 of reversed symmetry. The object of this investigation 

 was to discover what would be the symmetry relations 

 in twins artificially produced by severing the blastula 

 or gastrula stage down the sagittal plane. This opera- 

 tion effectually isolates the right- and left-hand primordia 



