Chemistry of Coat- Colour in Animals. 



57 



hair to be seen when the coat is lying flat. The blue appearance mnst 

 therefore be due to some other cause, and I believe the fact to be that in 

 blue hairs the pigment is entirely confined to the medulla, whereas in 

 black and other intense colours there are many hairs in which granules 

 are distributed throughout the fibrils of the cells of the cortex. These 

 granules within the cortex absorb the light which in dilute hairs passes 

 through and is reflected by the air-cells or vacuoles which occupy the 

 medulla, thus causing an increase in the white light and a consequent 

 dilution of the colour observed. Dilute colours therefore, and perhaps other 

 colours also, depend rather upon the distribution and intensity of the 

 pigment than upon its chemical composition. 



VIII. Summary and Conclusions. 



1. Miss Durham's evidence for the existence of a tyrosinase in the skins of 

 vertebrates is inconclusive. 



2. A peroxidase can, however, be extracted from the skins of certain 

 coloured rabbits and mice, which behaves like a tyrosinase towards tyrosine 

 in the presence of hydrogen peroxide. It can be precipitated from solution 

 by saturation with ammonium sulphate or by an excess of alcohol. 



3. The peroxidase present in agouti, chocolate, and blue rabbits is indis- 

 tinguishable in its reactions from that present in black rabbits ; but no 

 peroxidase could be extracted from yellow and orange rabbits. 



4. Spiegler's " white melanin " is not a pigment substance ; nor is it the 

 cause of dominant whiteness, which is due, as has been suggested by Gortner, 

 to the presence of an inhibitor or anti-tyrosinase in the skin. 



5. Dominant whiteness in the English rabbit is due to the presence of a 

 tyrosinase-inhibitor in the skin, which destroys the activity of tyrosinase ; and 

 the dominant white bellies of yellow and agouti rabbits are due to the same 

 cause. The inhibitor can be precipitated by saturation with ammonium 

 sulphate, and is destroyed by boiling or by standing for 48 hours. 



6. Recessive vjhiteness in rabbits and mice is due to the lack of the enzyme, 

 unit of the pigment-producing system, for no tyrosinase or anti-tyrosinase 

 could be extracted from their skins. There is not sufficient evidence to 

 decide, whether a chromogen is present or not. 



7. The presence of an unoxidised chromogen might, however, serve to 

 explain the occurrence of certain colourless granular particles which are found 

 in the medullary cells of the hairs of some white animals. These particles are 

 microscopically visible when stained, and in appearance very closely resemble 

 coloured pigment granules. 



8. The capacity of both white and coloured skin extracts to oxidise 

 vol. lxxxix. — B. jt 



