170 



THE PHYSIOLOGY OF TWINNING 



expect a continuance of the mirror-imaging. Gemmill 

 does not give us any further data on this point, but two 

 recent studies on inverse symmetry in double-monster 

 trout have supplied this deficiency. 



Morrill (1919) has worked on the internal anatomy 

 of some of Stockard's collection of double trout. He 



Figs. 59-61. — Three diagrams showing symmetry reversal in the 

 hearts and principal vessels of trout double monsters. Fig. 59 shows 

 the normal single heart. Fig. 60 shows an incompletely twinned heart. 

 Fig. 61 shows completely divided twinned hearts, the right component of 

 which shows situs inversus viscerum. (After Gemmill.) 



found several cases of complete symmetry reversal and 

 also several cases that were incomplete or doubtful. 

 Never was there any symmetry reversal in the indi- 

 viduals in which body and tail were both separate, but 

 only in cases in which heads were entirely separate and 

 the viscera separate at least back to the pelvic region. 



