SYMMETRY REVERSAL AND MIRROR-IMAGING 173 



embryos derived from two embryonic shields, each of 

 which has its own right- and left-hand sides, we see no 

 reason in the world why there should be a reversal of 

 the viscera. Each is from the beginning a separate 



Fig. 63. — A ventrolateral view of a trout double monster (anadidy- 

 mus) showing situs inversus viscerum. The component on the reader's 

 right is the true left-hand component and has the typical left-hand 

 asymmetry of the species. The component on the reader's left (the 

 right-hand component) has the reversed asymmetry of the viscera. 

 Letters have the same significance as in Fig. 62. (From Swett.) 



individual except that it happens^ to be on the same 

 yolk sac as its twin partner. It would also be impossible 

 to say which of such twins is the right-hand and which 

 the left-hand component. Therefore right-handed or 

 reversed asymmetry would be quite inappropriate in 



