C. J. Bond 71 



Dove is an anteriorly pigmented eye and differs from the normal type of 

 eye colour in pigeons which is white, yellow, orange or red, producing 

 the so-called pearl, gravel or ruby eye. 



In the Rock Pigeon (Columba livia), which is regarded as the ances- 

 tral form, the iris colour is yellow or orange. 



The Pearl Eye. 



Next in grade of pigment to the " Bull " eye comes the so-called 

 pearl eye in pigeons, and the " Daw " eye as seen in the Malay Fowl. 

 Anterior iris pigment is absent in the pearl eye, but the iris tissues are 

 not translucent as in the "Bull " eye. They are crowded with granules 

 which are themselves colourless but prevent the passage of transmitted 

 light, and when seen by reflected light give a grey white appearance to 

 the surface of the iris. 



Microscopic examination of the pearl iris shows that this opacity to 

 transmitted light is due to the presence of these colourless, but more or 

 less opaque, granules, which appear as clear refractile spherules of 

 moderate size. When these are densely aggregated the interference with 

 the passage of light may be sufficient to produce an opaque or black 

 effect. The cells which contain these granules are scattered around the 

 capillaries and amongst the muscle-fibres, and they do not seem to have 

 such definite outlines as the cells which contain the ordinary yellow or 

 brown pigment. It is indeed an open question whether these refractile 

 bodies should be classified as pigment granules, since under dark ground 

 illumination they appear to be of a dull white colour. 



Among Fowls the eye which most nearly corresponds to the pearl eye 

 of pigeons is the so-called " Daw " eye of the Malay Fowl. 



In the "Daw" eye the iris is free from anterior yellow or brown pig- 

 ment, but the connective tissue cells and the muscle fibres are loaded 

 with granules which produce a tissue opacity and prevent the uveal pig- 

 ment from shining through the iris, thus giving a grey appearance to 

 the iris on reflected light. The pearl eye, and perhaps the daw eye, may 

 in fact be regarded as representing the first or lowest grade of iris 

 pigmentation, a condition of cell-opacity without cell-pigment, inter- 

 mediate between the bull eye with its translucent tissue-cells and absence 

 of pigment-granules on the one hand, and the fully pigmented eye with 

 its connective tissue cells loaded with yellow, brown or black pigment 

 granules on the other. 



The pearl eye in pigeons seems to correspond in pigmentary grada- 

 tion with the china-white or "Wall" eye in dogs, horses and pigs. We 



