1 78 THE PHYSIOLOGY OF TWINNING 



of organic asymmetry. As yet, however, the theory far 

 outstrips the facts. 



Using this idea as a working hypothesis we might 

 readily conceive of a condition in which, because of the 

 fundamental molecular asymmetry characteristic of a 

 group of animals, one side of the body (the left in verte- 

 brates) is the superior side in a physiological sense. 

 This side normally grows more rapidly so as to produce 

 curvatures or unilateral hypertrophies of certain median 

 organs; or else certain other organs, ordinarily paired, 

 grow only on the left side. The right side is the inferior 

 side and, in the presence of the left or superior side, is kept 

 in some sort of subordination. If some agency lowers 

 the dominance of the superior side the inferior side might 

 become independent and might develop its own sym- 

 metry relations without hindrance from the superior side. 



Let us examine the facts of situs inversus in Triton 

 twins in the light of this theory. These isolated right 

 halves will, during the period of regeneration, have a 

 superior side of their own, the outer or right side, and 

 this side will more or less completely dominate the 

 regenerating left side. Thus the molecular asymmetry 

 is doubtless established in a reversed direction. In 

 some right-hand pieces, however, there may be sufficient 

 of the left-hand material present to set the molecular 

 symmetry more or less completely to the left, thus pro- 

 ducing the typical situs of the species. This theory of a 

 superior and an inferior side in vertebrates is in accord 

 with many different facts. We know, for example, that 

 man is typically right handed because the motor centers 

 of the right-hand musculature lie chiefly on the left side 

 of the brain. The abnormal condition of hemihypertro- 



