90 Supernumerary Digit in Heterodactylous Fowls 



obtained in the distribution of certain abnormal double tail feathers 

 which occur in these Fantail hybrids. Morgan has drawn attention to 

 the occurrence of these double tail feathers in cross-bred pigeons but 

 does not mention any inequality of distribution. 



In the above cross these abnormal tail feathers appeared in 12 cases 

 in the F^ offspring, 5 being on the left and 7 on the right side. In the 

 F^ generation however out of 11 examples 8 occurred on the left side 

 and only 3 on the right. 



In a total of 38 abnormal tail feathers from this and other matings 

 21 occurred on the left side and 17 on the right. 



Evidently the condition of unstable equilibrium in the number and 

 distribution of tail feathers in hybrid pigeons, the result of crossing a 

 fish-tailed with a fan-tailed breed occurs on both sides of the body, but 

 the numbers suggest a slight excess of meristic variation away from the 

 normal 12 feather arrangement on the left as compared with the right 

 side of the body. 



Taking the facts concerning heterodactylism in fowls and tail-feather 

 pattern in hybrid pigeons together we find that (in birds at any rate) 

 there is a tendency during the early cleavage of the ovum which separates 

 the organism into a right and left half, for the factor or factors which 

 control meristic variation to pass to the left rather than to the right 

 half of the developing zygote. 



In this article it is not intended to discuss the question whether 

 any association exists between the left-sided incidence of asymmetrical 

 characters and sex. The atrophy of the right ovary in normal female 

 birds is important in this connection, so also is the fact that in a certain 

 number of female heterodactylous fowls the extra digit on the left side 

 is associated with the development of a spur on the same side. 

 Heinrich Poll {SB. Ges. Naturf. Fr. Berlin, 1909) states that in true 

 hermaphrodite birds the male secondary sex character tends to appear 

 on the right side, that is the side on which the primary sex gland is 

 male in these abnormal birds. There are difficulties in accepting the 

 hormonic theory as a full and complete explanation of the occurrence 

 and distribution of secondary sex characters in birds. The case of the 

 hermaphrodite pheasant with hemilateral asymmetry of secondary sex 

 characters recorded in the Journ. Gen., Feb. 1914, together with other 

 examples recorded by other observers suggests that (in birds at any 

 rate) in addition to the influence exercised by sex hormones on the 

 development of secondary sex characters other factors are concerned in 

 their origin and distribution. One other such factor may be a capacity 



