76 Eye Colour in Birds 



pigment cells on the front of the iris, and these latter cells are found at 

 a deeper level just above or anterior to the uveal pigment cells. The 

 appearances in the Herring Gull suggest a downward migration of the 

 black branched cells and a surface migration of the yellow cells. This 

 view that the colour change is the result of cell migration receives some 

 confirmation in the case of young Rooks and other members of the 

 Corvidae. The anterior surface of the iris in the nestling Rook (Corvus 

 frugilegxis) is dotted with a number of branching cells loaded with in- 

 tensely black pigment granules. These cells are most numerous at the 

 scleral margin of the iris and are directly continuous with a plexus of 

 similar cells which lines the sclerotic on its choroidal surface. As the 

 nestling grows these cells become more numerous and spread over the 

 anterior surface of the iris, as if migrating towards the pupil from the 

 scleral margin. 



The " Black " Eye in Fowls. 

 . If the Black Orpington (black eye) male be crossed with the Bufif 

 Orpington (gravel eye) female the ^i chickens all show some black 

 anterior iris pigment on a yellowish background. In the pullets the 

 brownish black colour persists in adult life while in the cockerels the 

 black gives place to the yellow or gravel eye. Although in this cross 

 the yellow eye seems to be a sex limited character the sharpness of the 

 limitation is not absolute, for a few of the cockerels show in adult life a 

 surface layer of brownish pigment on a yellow background. In the Malay 

 (daw eye) female crossed with the Black Orpington (black eye) male the 

 cockerels also develop a yellow eye when sexually mature while that of 

 the pullets remains black or dark brown, and in this cross the character 

 seems strictly sex limited. In both crosses the deeper yellow pigmenta- 

 tion is due to two factors as in the yellow eye in other breeds of fowls. 

 These factors are (a) the presence of connective tissue cells containing 

 yellow pigment granules and (h) the deposition of yellow pigment 

 granules in the striated muscle fibres of the iris. 



The " Triplex" Eye or the Eye containing two kinds of anterior Iris 

 pigment. 



Mention has already been made of Hurst's classification of the Duplex 

 eye as one in which anterior iris pigment of some kind either yellow, 

 brown, or black is present. In connection Avith this point I wish to 

 draw attention to a group of eyes in Fowls of which the Silky, the Croad 

 Langshan, the Houdan and some other breeds provide examples, which 

 might be called Triplex eyes. There are, however, some objections to 



