TWINNING (DUPLICITY) IN LIMBS 197 



exceptions are, however, significant, since they represent 

 the equivalent of the failures to show mirror-image 

 symmetry in twins and are doubtless due to similar 

 causes. Bateson notes two clear-cut cases of doubled 

 limbs in beetles in which the double appendage branching 

 from the single normal appendage has no S3anmetry 

 relation to the single appendage, but the twin parts 

 are quite distinctly symmetrical between themselves. 

 This is really a breach of the first rule of Bateson, that 

 *'the long axis of the normal appendage and the two 

 extra appendages are in the same plane." Various other 

 authors have cited occasional exceptional cases, but it 

 must not be forgotten that the symmetry rules of Bateson 

 hold with respect to a very high percentage of all cases. 



The analysis of symmetry relations in reduplicated 

 limbs of Amblystoma, as given us by Harrison, has been 

 rendered vastly more difficult than need be owing to 

 the complexity of experimental procedures. Had the 

 author been interested primarily in the study of double 

 limbs and their symmetry relations he could have 

 eliminated a great many complicating factors, such as 

 the changing of grafts from right to left side or the 

 turning of grafts upside down. Under the circumstances, 

 it appears to me remarkable that so definite a result 

 was obtained, and we need hardly despair about the 

 possibility of clearing up this problem if efforts should 

 be directed definitely to that end. Certain facts may be 

 gleaned from a survey of Harrison's work that aid us 

 in our present analysis: (a) The broad axis of the limb 

 is at right angles to the sagittal plane of the body. 

 (b) The ulnar border of the limb is dorsal and the radial 

 border ventral, i.e., the little-finger side of the hand is 



