72 Eye Colour in Birds 



must not lose sight of the fact however that the substitution of striated 

 muscle- fibres in the avian iris for the plain muscle cells of the mam- 

 malian iris introduces a different histological factor in these different 

 orders. In the mammalia there is every grade of blue eye from the deep 

 violet or almost black shade of the newly born child or kitten to the steel 

 blue eye of the human adult, or the bluish white iris of the blue Dutch 

 rabbit, or the china white iris of the " wall" eyed pig. 



The paleness of the blue colour or greyness of the iris depends on 

 the granular condition or opacity of the tissue cells and the degree to 

 which the posterior pigment is prevented from shining through. Thus, 

 an iris composed of translucent cells gives a bull or black eye, while an 

 iris of which the cells are slightly opaque is blue, and an iris composed 

 of wholly opaque granular cells is china white in colour. All these irides 

 are free from anterior pigment. The different reaction to incident light 

 is due to structural and not pigmentary differences, and to the presence 

 in the cells of these colourless granules. 



Genetically the pearl eye in pigeons acts as a recessive to the red or 

 gravel eye. The daw eye in fowls is also a recessive to the amber eye, 

 and to the black eye when the blackness is due to the presence of 

 anterior pigment. 



The Yellow or "Gravel" Eye. 



In pigeons the yellow eye is due to a net-work of branching cells 

 crowded with small spherical yellow granules. These cells lie on the 

 anterior surface of the iris : they cover the capillary blood vessels and 

 surround the striated muscle fibres which in the pigeon are themselves 

 free from pigment. The difference between the grey-white or pearl eye 

 and the yellow or gravel eye in pigeons is essentially a structural differ- 

 ence. In the pearl eye the granules which fill these connective tissue 

 cells are chalky white to reflected and opaque to transmitted light, 

 whereas in the yellow eye the granules appear yellow both when viewed 

 by reflected and transmitted light. If however the cells which contain 

 them are closely aggregated and prevent the passage of light, then the 

 granules appear black. 



Thus the yellow eye represents the first grade in iris pigmentation. 

 In fowls on the other hand the gravel or yellow eye presents a 

 different problem. In some breeds the orange colour is due to the 

 presence of large numbers of branching connective tissue cells containing 

 yellow or yellow-brown pigment granules, which lie around the capil- 

 laries and muscle-fibres. In other breeds, of which the Dorking and 



