70 . THE PHYSIOLOGY OF TWINNING ' 



of bilateral primordia through suppression of the active 

 focusing of equivalent bilateral anlage upon a single 

 median line. 



Cases of more or less complete isolation may occur. 

 The minimal cases are probably those involving minor 

 degrees of discoordination and consequent asymmetry 

 of the two sides, as in fish embryos in which one 

 side grows and develops less well than the other. It 

 seems likely that the condition known as hemihyper- 

 trophy in man (chapter xi) is to be viewed also as a 

 case of minimal twinning. The maximal cases are those 

 in which the two components are entirely separate except 

 for a common anus or a common median ureter. It may 

 also be true that some completely separate fish twins 

 arise by simply going a short step farther than the last- 

 named condition. Unless this is true it would be difficult 

 to account for the occasional cases of symmetry reversal 

 in one of a pair of completely separate twins. It is my 

 opinion, however, that the great majority of separate 

 twins come from separate embryonic axes and that all 

 true conjoined twins arise by the dichotomy or fission of 

 a single embryonic axis. 



THE EXPERIMENTAL PRODUCTION OF 

 TWINS IN FISHES 



Gemmill expresses the opinion "that the occurrence 

 of double monstrosity (twinning) is due in the main not 

 to environmental factors, but to conditions which are 

 inherent in the germ cell." In another connection he 

 says: "The likelihood cannot be excluded that external 

 factors sometimes induce the production of double 

 monstrosities in the developing eggs of fishes." 



