C. J. Bond 81 



Chemical. 



I regret that I am unable to bring forward any detailed account of 

 the difference between the black, brown, yellow, ruby, and pearl pigment 

 granules from the chemical point of view. There is one definite and 

 constant character which serves to distinguish the yellow from the black 

 granules and that is their different behaviour to reducing agents like 

 formalin. The yellow pigment granules lose their colour after immersion 

 in 5 °/^ formalin for two or three weeks. A coalescence of the small 

 granules into larger globules of yellow lipoid-like material takes place, 

 and these larger globules may eventually merge into larger masses. In 

 some specimens needle crystals have formed inside these yellow oil-like 

 globules (see Herring Gull). The black pigment gi-anules which fill 

 the branching cells that are so characteristic of the anterior layer in 

 the black iris of many birds are, on the other hand, highly resistant to 

 the action of formalin; for 5 "jo and even stronger solutions have little or 

 no effect in dissipating the colour. The granules do not tend to coalesce 

 into larger globules nor do they show any signs of crystalloid change. 

 The brown pigment granules are intermediate in their chemical re- 

 actions between the yellow and the black, and are partially affected by 

 formalin. 



