72 MR. W. BATESON ON COLOUR-HEREDITY, [May 26, 



methods have to be adopted for tracing the identity of the indi- 

 viduals, which in such work is indispensable. Another difficulty 

 arises from the fact that mice present few readily estimable features 

 of structure. Also, though there are now many types of colour, 

 few of them as yet exist as pure strains, and hence it is not easy 

 to obtain reliable material with which to begin the experiments. 

 Nevertheless, in spite of these drawbacks, the subject is a good one, 

 and there can be no doubt that our knowledge of heredity can be 

 rapidly extended by experiments on mice. As regards Rats the 

 case is similar, save in one respect, in which there is a very 

 remarkable difference, namely that the colour-types of fancy rats 

 are as yet extremely few. For this reason, though the scope of 

 experiment is reduced in the case of rats, some serious complica- 

 tions are eliminated, and certain fundamental questions, as, for 

 example, the relation of pied to self-coloured varieties, could 

 probably be studied more easily in rats than in mice. 



As a great deal of work on these species is now being done, it 

 has seemed to me useful to codify the chief information already 

 at our disposal, and to state as carefully as is yet possible some 

 of the more immediate problems presented by the existing facts. 



It would greatly assist discussion of these problems if uniform 

 names could be used for the colour- types. An attempt is there- 

 fore made to suggest such names, and to indicate how the types 

 may be recognised. The specimens examined for this purpose 

 have been obtained chiefly from Mr. J.Wilson Steer, of 45 Raleigh 

 Road, Hornsey, iST., and from Mr. Atlee, of Royston, Oambs., and 

 I am greatly indebted to both of these well-known breeders of 

 mice for information and assistance. The microscopical examina- 

 tion and discrimination of the types was carried out by Miss F. 

 M. Durham. This work is only in a preliminary stage and, it is 

 hoped, will form the subject of a separate communication. 



Microscopical examination shows the hairs of mice to contain 

 numerous minute medullary spaces separated from each other by 

 bridges of keratin. These spaces are arranged in longitudinal 

 rows, the number of which varies from one to four (perhaps 

 five), thick hairs having usually more rows than thin hairs. The 

 pigment is deposited in two ways:— (1) massed in the proximal 

 walls of the medullary spaces, and (2) scattered in the external 

 cortex. Since air bubbles out of the spaces when reagents are 

 applied, the spaces probably open to the exterior. 



The pigments in wild M. musculus or sylvaticus are readily seen 

 to be of three kinds:— (1) Densely opaque WacA;. (2) Less opaque 

 brown. (3) Transparent yeZZoio. The chemical nature of these 

 pigments and their possible relations to each other seem to be quite 

 obscure. If the hairs are cleared of air, the three kinds of pigment 

 can be recognised. On treating with 40 per cent, aqueous solution 

 of potash, the yellow dissolves at once. The brown disappears much 

 more slowly, but is rather more soluble than the black, which can 



