88 Supernumerary Digit in Heterodactylous Fowls 



digit appeared on the right side in two cases. Two more examples 

 occurred in a mating between a heterodactylous cock and five-toed hens. 



The tendency for the fifth digit to appear on the left side is evidently 

 strong in birds which are heterozygous as regards number of digits. 



Besides these hetesrodactylous birds a number of the symmetrical 

 five-toed hybrids also showed a fuller development of the fifth digit on 

 the left side. In some cases this left-sided extra digit was made up of 

 a larger number of phalanges, and in some these phalanges were better 

 developed. In two birds rudiments of six toes appeared, in one case on 

 the left and in one on the right side. 



This over-development in excess of the number of digits normally 

 possessed by a five-toed breed is important and suggests that the fifth 

 toe in the five-toed race is not transmitted to hybrid offspring as a 

 stable unit character, but rather that the factor transmitted when a five- 

 toed is crossed with a four-toed breed is a tendency to extra serial 

 segmentation of the rudiments forming the digits and not a tendency 

 to develop a fixed number of (five) digits. 



This serial segmentation may affect the whole digit or the phalanges 

 only, it may be complete or incomplete and in the latter case a fused 

 metatarsal bone or phalanx may be common to two digits. 



Although the numbers in which the reverse cross was tested are 

 small they suggest that heterodactylism with the extra digit on the left 

 side is more frequent when the female parent belongs to the five-toed 

 breed. 



This left-sided incidence of the extra toe in asymmetrical birds is 

 confirmed by other workers. Thus Messrs Bateson and Punnett have 

 kindly furnished me with the following records of extra toed chicks bred 

 by them. In 89 asymmetric birds from different matings the extra digit 

 appeared on the left side in 72 cases and on the right side in 17. In 49 

 birds the extra digit although present on both sides was unequally 

 developed, the more fully developed toe occurred on the left side in 34 

 cases and on the right side in 15. 



Since this paper was written Prof Punnett has also called my 

 attention to articles by D. Barfurth in the Archivfur Entwicklungsmech. 

 der Organismen, XXXI and xxxill. In a summary of different matings 

 between normal and hyper-dactyl ous fowls carried out between 1908 

 and 1910 Barfurth gives a total of 1031 individuals of which 556 were 

 four-toed and 475 hyper-dactylous. Of the hyper-dactylous birds 410 

 were symmetrical and 65 asymmetrical. Of the 6o asymmetrical birds 

 the extra digit was present on the left side in 35 and on the right side 



