CHAPTER XIII 

 DOUBLE TAILS IN VERTEBRATES 



Among bilateral animals of all sorts it is very com- 

 mon to find, instead of the single tails or limbs character- 

 istic of the species, double tails or limbs. Such double 

 structures are unsually known as duplicities^ but are 

 really cases of local twinning, as I shall attempt to show. 



Double tails have been very frequently described 

 in connection with experiments on regeneration of lost 

 tails. Not uncommonly one finds in lizards that the 

 regenerated end of the tail has grown out more or less 

 completely doubled. Similar results have been reported 

 in connection with regenerated tails of various Amphibia. 



Some years ago when the writer (191 5^) was en- 

 gaged in an extensive series of hybridization experi- 

 ments upon the bony fishes it was noted that in some 

 crosses, a considerable percentage of the hybrid larvae 

 had double tails. This was especially true in the cross 

 Tauiogolabrus adspersus $ X Stenotomus chrysops S (Gun- 

 ner $ X butterfish s). This cross furnishes a very 

 extensive assortment of monstrosities among which 

 there were both double-headed and double- tailed indi- 

 viduals. The double-tailed ones were, however, very 

 numerous. Occasional double-tailed individuals were 

 found in the various other crosses, especially in the 

 cross between the eggs of the butterfish and the sperm 

 of Fundulus heteroclitus, where one perfectly symmetrical 

 double tail was found. In all of these experiments the 

 double-tailed condition was found associated with various 



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