PRODUCTION OF TWINS AND DOUBLE-MONSTERS 351 



Reduplication of limbs a special case of twinning 



In case only certain parts of individuals are reduplicated, it 

 is to be borne in mind that many organs or systems possess 

 separate axes or gradients of their own. The limbs of verte- 

 brates, for example, have their own gradients, with the apical 

 point at the distal end and the basal point at the proximal or 

 attached end. It is not uncommon for limbs to show redupli- 

 cation, especially under experimental conditions. A very sug- 

 gestive series of experiments involving the reduplication of 

 limbs has recently been published by Detwiler ('20). He trans- 

 planted the anterior limb buds of Amblystoma embryos varying 

 distances back from their normal position. These transplants 

 differentiated in their abnormal positions with varying degrees 

 of success, depending upon their distance from the normal locus. 

 Many limbs developed as mirror-image duplicate appendages. 

 According to the author, "there occurred a gradual increase 

 in the number of reduplications as the limbs became trans- 

 planted farther and farther away from the normal situation." 

 It is also clearly shown in the author's description that the rate 

 of development becomes progressively slower as the redupli- 

 cations becomes more frequent ; at least one may infer this from 

 his statements as to the time of appearance of the first reflexes, 

 which appear in about fourteen days in the normal limb; about 

 two days later when the limb is placed one or two somites back; 

 from two to ten days later than normal when placed three somites 

 back; from fifteen to twenty-six days later normal when placed 

 four somites back ; from seventeen to twenty-two or more days 

 later when placed five somites back. It would appear, then, 

 that reduplication or twinning of limbs is associated with re- 

 tarded development. What causes retardation to be more 

 pronounced the farther from the normal position the transplant 

 is placed is another problem. 



The writer is confident that other cases of reduphcation in 

 development will be found to be associated with retarded devel- 

 opment and that the theory of twinning herewith proposed will 

 be found to have a very far-reaching application to all phenomena 

 of twinning in the broad sense of this term. 



THE JOURNAL OP EXPERIMENTAL ZOOLOGY, VOL. 33, NO. 2 



