TWINNING (DUPLICITY) IN LIMBS 



199 



This is, I believe, just what happens when a limb-bud 

 undergoes simple twinning. In such a twinned limb 

 the mirror plane, as in other cases of symmetry reversal, 

 is adjacent to the inferior or radial side of each com- 



A.DU 



MP2(U) 



Fig. 67. — Diagram showing mode of limb reduplication (twinning) 

 in Amblystoma. Pi?, primary limb; P.DU, posterior reduplicating 

 member; A.DU, anterior reduplicating member; MPAR), primary 

 (radial) mirror plane; MPiiU), secondary (ulnar) mirror plane; 1-4, 

 first to fourth digits, respectively. (After Harrison.) 



ponent of the double-monster limb and the superior side 

 is away from the mirror plane in ^ach instance. Such a 

 double limb as this is represented in Figure 67, and should 

 be compared first with a mirror-imaged double monster, 

 such as that of the trout (Fig. 63) and then with the 

 completely double hand shown on page 202 (Fig. 71). 



