C. J. Bond 75 



gives F^ with black plumage (with some red in the males), and all have 

 more or less black anterior iris pigment due to the presence of these 

 characteristic branching cells. As the ^i chicks develop however inter- 

 esting changes take place. Among the limited number of birds reared 

 up to the present the pullets retain the black eye in adult life, while in 

 the cockerels the iris gradually assumes a yellow colour and at the age 

 of 9 months or earlier, if the birds are sexually mature, the black becomes 

 a giavel or orange eye. Thus the black iris of the hen and the orange 

 of the cock in this cross are sex limited characters as in the case of other 

 gallinaceous birds. In the Golden Pheasant (Chri/solophus pictus) the 

 iris is brown or brownish black in the female and bright yellow in the 

 fully developed male, and to a much less marked degi'ee the same is true 

 of the Mongolian Pheasant (Phasianus mongolicus). An interesting 

 problem arises as to the way in which this change in iris colour is 

 brought about in the adult male. The transition is one from an 

 ^pistatic to a hypostatic character, that is from a higher to a lower 

 grade of pigmentation. The chicks of both sexes of this Malay cross 

 have brown black irides due to the presence of these anterior pig- 

 ment-containing cells and they retain this character while sexually 

 immature. The change to the yellow colour in the developing cockerels 

 occurs in patches on the surface of the iris and seems to be due to the 

 removal of the cells containing brown or black pigment and of the substi- 

 tution in their place of cells containing yellow pigment granules together 

 with (in the case of the half-bred Malay fowl) the deposition of yellow 

 pigment granules in the striated muscle cells of the iris. The question 

 arises as to the disappearance of these brown pigment cells. Is it the 

 result of atrophy and absorption or of migration to deeper parts of the 

 iris ? There are reasons for thinking that both factors are concerned 

 in the process. 



As the iris assumes the yellow^ colour the cells which contain the 

 black or brown pigment coincidently lose their dendritic processes and 

 tend to become rounder in outline. In the Herring Gull {Larus argen- 

 tatus) the yellow iris is not fully developed in the adult male until the 

 fourth year. A careful comparison of sections of the iris in the nestling, 

 the young male, the young female, and adults of the two sexes of the 

 Herring Gull shows that the black colour of the nestling's eye is due to 

 the presence of a plexus of branched cells containing brown or black 

 pigment on the anterior surface of the iris. The female retains more or 

 less of this dull brown colour during adult'life, but in the adult male a 

 layer of cells containing yellow pigment replaces the layer of black 



