Chemistry of Coat-Colour in Animals. 



55 



this oxidation. More probably, both in this case and in that described by 

 Gortner, the poly -phenols are catalysed by some organic colloidal material in 

 the fluid extract and not by a specific enzyme at all. 



VII. A Possible Cause of Colour Variation. 



The foregoing experiments call attention to one or two points which are 

 worthy of further consideration, since they throw some light on the nature 

 of the cause determining the difference between the various coat-colours 

 of rabbits.* Chodat has shown how the colour due to the action of 

 tyrosinase upon tyrosine and certain phenols is modified by the presence of 

 amino-acids or polypeptides. He suggests, therefore, that the colour of a 

 given pigment depends on the action of a particular oxidase upon different 

 combinations of a phenol group, and an amino or colour-modifying group. 

 Before accepting such a theory it must be clearly shown that such variations 

 in colour are really due to a difference of the quality and not of the 

 quantity of the pigment present. The colours concerned in the case of 

 rabbits and mice are generally divided into black, chocolate and yellow, 

 many of the other varieties, such as agouti and blue, being caused by different 

 combinations and concentrations of these three colours. The so-called 

 black pigment, however, appears chocolate by strong transmitted light, and 

 bright yellow in very dilute solutions. Moreover, chocolate pigment appears 

 black in concentrated solutions and yellow in dilute solutions. It is 

 possible, therefore, that these differences of colour may be due to the 

 concentration of the pigment in the granules and to the manner of their 

 distribution rather than to any real qualitative difference in the pigment. 

 Now the properties of the tyrosinase extracted from chocolate rabbits 

 appeared identical with those of the tyrosinase from black rabbits, which 

 seems to support this view. On the other hand, no tyrosinase could be 

 extracted from yellow and orange rabbits, which suggests that an oxidase 

 of some other nature is present. This failure to find tyrosinase might 

 equally well be explained on the supposition that a very small amount of 

 tyrosinase is necessary to produce a yellow colour in the hair, and that this 

 was lost to such an extent during extraction that the remainder could not 

 give a yellow colour in vitro. The observation of Miss Durham'sf that the 

 pigment of black, chocolate, and yellow hairs showed a marked difference 

 in solubility when the entire hair was treated with alkali, appears at first 



* Chodat, ' Archives des Sciences physiques et naturelles,' 4th period, vol. 33, p. 70 

 (1912). 



t See paper by Bateson, 1 Proc. Zool. Soc.,' vol. 2, p. 71 (1903). 



