i68 



THE PHYSIOLOGY OF TWINNING 



d) If we include minor degrees of symmetry reversal 

 we find it to be a much more frequently occurring 

 phenomenon than we had formerly supposed. Complete 

 situs solitus in right-hand components of conjoined 

 twins is, in fact, relatively rare. 



e) The existence of 

 mirror-image S3mimetry 

 in the components of 

 double monsters argues 

 against the theory that 

 they have arisen through 

 the fusion of separate 

 embryonic axes and for 

 the theory of origin 

 through dichotomy or 

 fission of the bilateral 

 halves of a single em- 

 bryonic axis. 



SYMMETRY REVERSAL IN 

 FISH TWINS 



Fig. 57. — A horizontal section 

 through the head region of a partially 

 double trout embryo. The embryo is 

 double as far back as the anterior end 

 of the mid-brain. Note the perfect 

 mirror-imaging of the twinned struc- 

 tures. (After Gemmill.) 



Until very recently 

 no attention seems to 

 have been paid to sym- 

 metry reversal in fish 

 twins. Gemmill (191 2), 

 although he figures in 

 great detail numerous conditions that are essentially 

 symmetry reversals, never calls attention to the con- 

 dition as such. One cannot help but note the beauti- 

 fully perfect mirror-imaging displayed in his horizontal 

 sections through the heads of partially double-headed 



