194 THE PHYSIOLOGY OF TWINNING 



an anomalous leg of a beetle. On the basis of a large 

 number of such instances Bateson is able to set down 

 certain rules of symmetry : 



When extra appendages, arising from a normal appendage, 

 are thoroughly relaxed and extended, the following rules will be 

 found to hold good with certain exceptions to be hereafter specified: 



I. The long axis of the normal appendage and the two extra 

 appendages are in one plane: of the two extra appendages one is 

 therefore nearer to the axis of the normal appendage and the other 

 remoter from it. 



II. The nearer of the two appendages is in structure and position 

 formed as the image of the normal appendage in a plane mirror 

 placed between the normal appendage and the nearer one, at right 

 angles to the plane of the three axes; and the remoter appendage is 

 the image of the nearer in a plane mirror similarly placed between the 

 two extra appendages. 



The symmetry between the nearer double member 

 and the normal appendage may be called primary 

 symmetry and that between the two twinned members, 

 secondary symmetry. The terms major and minor sym- 

 metry are sometimes used quite synonomously with these. 



In a book of the present scope it would neither be 

 desirable nor feasible to enter into an exhaustive survey 

 of the elaborate literature on duplicities and symmetries 

 in limbs. Hence we shall confine ourselves to a few 

 selected phases of the subject that appear to be especially 

 helpful in our attempt to analyze the phenomena of 

 organic symmetry and symmetry reversal. 



EXPERIMENTAL PRODUCTION OF DOUBLE 

 LIMBS IN AMBLYSTOMA LARVAE 



Harrison (1920) has succeeded in producing a large 

 number of double and triple limbs in the larvae of 

 the common salamander Amblystoma by transplanting 



