ON CERTAIN FACTOKS CONCERNED IN THE 

 PRODUCTION OF EYE COLOUR IN BIRDS. 



By C. J. BOND. 



Introduction. 



The work of Hurst, Nettleship, and others in this country, and 

 Davenport in America, has been concerned chiefly with eye colour in 

 mammals, more especially in the human subject. The facts which Hurst 

 established as to the genetic importance of the presence or absence of 

 pigmerit on the anterior surface of the iris in man, seem to be applicable 

 in the main also to the avian iris. I am unable however to find that 

 much work has been done on eye colour in birds from the genetic stand- 

 point, and the observations here recorded must be regarded as an attempt 

 to carry the analysis of this problem a stage further. 



I showed in 1912 (see Nature, Sept. 19, 1912) that in birds, not only 

 the ciliary muscle, but the dilator and constrictor muscles of the iris are 

 composed of fibres of the striated or voluntary kind, and that the move- 

 ments of the pupil in birds are apparently subject to voluntary control. 

 This we should expect if the iris muscles in birds are innervated by 

 meduUated nerve fibres from the cerebro-spinal system. This difference 

 in the histological structure of the intrinsic muscles of the eye-ball in 

 birds and other vertebrates (with the possible exception of the Reptilia) 

 has an important bearing on the evolutionary methods by which the 

 pigmentation of the Iris has been brought about in these different 

 orders. 



Histological. 



The ''Bull" Eye in Birds. 



By this term I mean the eye which owes its black or dark colour to 

 the absence of pigment on the anterior surface of the iris. The delicacy 

 and translucency of the iris tissues allow the posterior uveal pigment to 

 shine through, and this gives rise to an appearance of blackness. The 

 Bull eye in birds thus comes under Hurst's definition of the simplex 



