70 Eye Colour in Birds 



eye. In the Bull eye the chick character is in fact retained during adult 

 life in the bird just as, in the blue eye in the human subject, the simplex 

 character common to most infants (of white as opposed to negro descent) 

 persists into adult life. 



The eye of the white fantail pigeon provides a good example of the 

 " Bull " or simplex avian eye. There is an entire absence of black, 

 brown, or yellow anterior pigment while the connective tissue cells and 

 the striated muscle fibres are sufficiently delicate in structure to allow 

 the posterior hexagonal brown or black pigment to show through, and 

 thus produce the black effect. 



Careful examination of the "Bull" eye in the living bird with a lens 

 in a good light will show a red appearance, like a ruddy glow, covering 

 the peripheral zone of the iris. This appearance is due to a rich plexus 

 of capillary blood vessels with thin walls in this situation, and is not due 

 to the presence of any pigment cells. 



It is important to remember that the majority of black or very dark 

 brown irides in birds owe their dark colour to the presence of interior 

 iris pigment, and are not " Bull " eyes at all. Even among pigeons the 

 "Bull" or simplex eye occurs chiefly in wholly white birds, which are 

 genetically speaking recessive whites, though not of course albinos. 

 Bateson and others have shown that in fowls white feather colour is of 

 two kinds. Dominant white as in the White Leghorn, and Recessive white 

 as in the white Rose-comb Bantam, the Silky fowl, and an extracted 

 white obtained by crossing certain white with coloured strains. It is 

 interesting to note that while (as we should expect) the colour of the 

 iris in the Dominant White Leghorn is red or yellow (gravel eye) the 

 iris colour in the Recessive white Rose-comb Bantam is not "Bull" but 

 red or yellow. 



In the Silky fowl also the black eye is not a "Bull" eye but owes its 

 dark colour to black anterior iris pigment. There are however special 

 points about the black eye of the Silky fowl to which I shall draw atten- 

 tion later. 



In the Stock Dove (Columha oenas) a superficial observation of the 

 eye of the adult bird would record the eye as "Bull" or simplex, i.e. 

 deficient in anterior pigment. Microscopic examination of sections of 

 the iris shows however that the black colour is due to the presence of 

 numbers of branching cells packed with dark brown or black pigment 

 granules. These cells are not only present on the surface of the iris 

 but also among the muscle-fibres and deeper tissues in which there are 

 also cells containing yellow pigment. Thus the black eye of the Stock 



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