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Show 72 MR. W. BATESON ON COLOUR-HEREDITY [May 26, methods have to be adopted for tracing the identity of the individuals, 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 complications 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 m e 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 therefore 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, N., and from Mr. Atlee, of Royston, Cambs., and I a m greatly indebted to both of these well-known breeders of mice for information and assistance. The microscopical examination and discrimination of the types was carried out by Miss F. M. Durham. This work is only in a preliminary stage and, it is boped, 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 black. (2) Less opaque brown. (3) Transparent yellow. 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 |
