TWINNING (DUPLICITY) IN LIMBS 205 



piece of evidence is of an entirely different sort and arises 

 out of data furnished by Detwiler (1920). Using the 

 methods of Harrison, previously referred to, this author 

 transplanted limb-buds of Amblystoma from their 

 normal position to various levels more posterior. Many 

 of these transplanted limb-buds underwent doubling 

 and showed mirror-image symmetry. According to 

 Detwiler, ^' there occurred a gradual increase in the 

 number of reduplications as the limbs became trans- 

 planted farther and farther away from the normal 

 situation." If we take this statement in connection with 

 another series of facts, it becomes significant; for the 

 farther away from the normal position the graft is placed 

 the slower it is to develop. Thus we have the greatest 

 frequency in limb-doubling when the developmental rate 

 of the limb-bud is most retarded; and the causal basis of 

 limb-doubling is seen to be in all probability the same 

 as in other more typical forms of twinning. 



