SYMMETRY REVERSAL AND MIRROR-IMAGING 177 



It would be very remarkable if the left-hand pieces 

 developed into larvae with right-hand situs, for the 

 species shows typically only left-hand asymmetry, as 

 in man and the fishes. There is, however, some cause 

 to emphasize the frequency of situs inversus in the 

 right-hand pieces. Even those right-hand twins that 

 were not classed as definite cases of situs inversus 

 showed what we may legitimately, I think, consider as 

 cases of partial situs inversus, since they were scarcely 

 at all asymmetrical (i.e., lacked the normal left-hand 

 asymmetry) or had only a slight degree of left-hand 

 asymmetry. On the whole, then, we may say that, as a 

 rule, the right-hand pieces show more or less situs inversus 

 or the asymmetry which we would expect to find in an 

 individual derived from the right-hand primordium of a 

 blastoderm that had already established its axis of 

 symmetry. 



Spemann, in an elaborate discussion of the causes 

 of situs inversus in these experimental twins, seems 

 inclined to refer the normal specific (left-hand) asym- 

 metry back to certain asymmetric relations of a molec- 

 ular sort in the egg. He presents as an analogy to 

 the foregoing experiments certain facts brought out by 

 Przibram in connection with crystals. Certain asym- 

 metrical crystals, after one side has been injured by 

 cutting, rebuild themselves so as to be the mirror-image 

 dupHcates of the typical crystal. It is suggested by 

 Przibram that this reversal of asymmetry is due to 

 reversal of the micros true ture of the crystals, possibly 

 involving actual molecular changes. One finds such an 

 analogy extremely attractive and almost unescapable 

 whenever an attempt is made at an ultimate analysis 



