SYMMETRY REVERSAL AND MIRROR-IMAGING 187 



structures, the hydrocoele and its derivatives, start to 

 grow first on the left. In both of these groups we 

 beheve that the earher onset of rapid growth in certain 

 left-hand primordia inhibits more or less completely the 

 growth of equivalent structures on the right. The 

 phenomenon of growth inhibition in these and other 

 allied cases is probably bioelectric in character. If so, the 

 region of rapid growth at any level of the primary axis 

 is positive to regions of less active growth and there is a 

 one-way bioelectric current, which furnishes the medium 

 of control of one part over another. R. S. Lillie has 

 demonstrated similar relations in connection with metals 

 in solutions of electrolytes. One positive or anodic 

 region seems to have an inhibiting efTect over a given 

 distance so that other similarly charged regions cannot 

 arise near the original anodic region. Whatever be the 

 ultimate physiology of the inhibition exercised over one 

 growing region by another more actively growing region, 

 the actual fact of inhibition is beyond question. 



If now we in some way break down the co-operation 

 of the two half-primordia destined to form the bilateral 

 halves of a single individual, the two halves become more 

 or less completely independent, and twinning results. 

 If the twins are separated down the whole axis, or if 

 they are separated as far back as the posterior end of 

 the body cavity, the two severed halves will each undergo 

 complete regulation, each forming a whole organism 

 with the symmetries and asymmetries characteristic of 

 the species. Thus completely divided human twins, 

 armadillo twins, and fish twins rarely if ever show situs 

 inversus viscerum, but always possess the left-hand 

 asymmetry characteristic of the species. 



