THE HOUSE MOUSE 643 



Some of the medullary chambers contain air only, but in the 

 majority, pigment granules occur as well. The pigment present varies 

 in amount, from a few isolated granules to thick clusters adhering to 

 one or other of the walls of the medullary chamber, and almost filling 

 its space. The medullary pigment of the " spines " and of the lower 

 parts of the other hairs is usually black and opaque ; but a few granules 

 are dark brown, and to some extent translucent. The base of the hair 

 and many tracts of the cortex are clear and devoid of pigment. In 

 other places, as at the expansions, and especially towards the distal 

 contractions, the cortex contains many scattered pigment granules, 

 which are occasionally arranged in two or three little longitudinal and 

 closely approximated rows. The cortical pigment of the longer hairs 

 is usually dark brown in their lower portions, but at these deep levels 

 a few yellowish granules may occasionally be seen.^ 



As mentioned above, the terminal segment of a long hair is usually 

 yellow except towards its tip, where the medullary pigment is absent 

 and the abundant dark cortical pigment gives a dusky hue. On care- 

 fully examining such a hair segment, it is frequently possible to 

 observe, firstly, that the medullary pigment is rather darker (brownish) 

 proximally, but becomes gradually lighter to golden or translucent 

 yellow distally ; and secondly, that the cortical pigment shows a similar 

 transition at the corresponding levels. 



Colour is now regarded by many ^ as resulting from the oxidation 

 of a chromogen by the action of a ferment ; the tint produced depend- 

 ing partly upon the nature of the chromogen and partly upon the degree 

 to which it is oxidised. The results of our work, confined as it has 

 been to a careful microscopic examination of the hairs of mice and 

 many other mammals, lead us to think that the transition from black 

 pigment through brown to yellow is a gradual one, and that it may well 

 be the result of progressive oxidation. The gaseous contents of the 

 medullary chambers perhaps play a part in this process, and the dark 

 hue of the cortical pigment of the hair tips may be due to the fact 

 that the medullary space dies out before reaching the hair tip. 



The local attenuations of the longer dorsal hairs of the House 

 Mouse probably assist in the coat change. The distal yellow segment 

 of the hair breaks off in due course at the contraction next below it ; 

 this contracted part becomes the new hair tip, and as growth pushes it 



' For another account of structure of hairs and distribution of pigment in House 

 Mouse, see Durham, in Bateson, Proc. Zool. Soc, 1903, ii., 72. 



^ For further information on the chemistry of colour, see— Cudnot, Arch. Zool. 

 Exper. et Gdn., Notes et Revue, l, xxxiii. ; Durham, Proc. Roy. Soc, 74, 310, 1904, 

 mAJourn. Physiol., ■^t,, May 1907 ; Mudge, /owrw. Physiol., 38, March and Oct. 1909, 

 and Nature, 14th April 1910, 18 ; SoUas, Nature, 24th March 1910, 96 ; Forbes, 

 iMd., 2ist April 1910, 217 ; Wheldale, Prog. Ret Botanica, iii., 457, 1910 ; Onslow, 

 Proc. Roy. Soc, B 1915, 36. 



