i86 THE PHYSIOLOGY OF TWINNING 



halves as mirror-images of each other, just as the two 

 halves of a symmetrical crystal are mirror-images. 

 If, in our hypothetical perfectly bilaterally symmetrical 

 organism, the two halves were to be physically or 

 physiologically isolated, we would expect exactly equiva- 

 lent twins, for each half-primordium would regenerate 

 in such a way as to reproduce the exact bilateral S3an- 

 metry present in the original individual of which they 

 are parts. 



As a matter of fact, however, a certain amount of 

 unilateral asymmetry appears to be characteristic of 

 most organisms. In some organisms such asymmetry 

 is very pronounced, as in echinoderms and in gastropod 

 molluscs, while in other organisms it is much less pro- 

 nounced, as in vertebrates and in many arthropods. 

 Whether the unilateral asymmetry affects many organs 

 or a few, whether the extent of the asymmetry be great 

 or little, the basis of the asymmetry seems to be one 

 involving a physiological superiority of one side or the 

 other. In last analysis the difference between the two 

 sides may be reduced to terms of rate of fundamental 

 vital activity, probably measurable in terms of rate of 

 oxidation. The result is that one side develops rather 

 more rapidly than the other, especially in connection 

 with certain structures that arise near the median axis 

 or mirror plane. Thus, in vertebrates, the left side of 

 the stomach, of the heart, and of other median structures 

 grows more rapidly than the right; and certain other 

 structures, such as swim-bladder in fishes and left 

 aorta in mammals, appear only on the left and not on 

 the right. Similarly, in echinoderms the left side of the 

 larva develops more rapidly than the right, and certain 



