The present paper is based upon observations and experiments made on 300 mice, obtained in the laboratories of the Imperial Cancer Research Fund since the summer of 1908, and in which cancer had developed spontaneously. Excluding lung adenomata, lymphomata, and sebaceous adenomata there remain 288 mice in which 350 tumours were observed, microscopical examination showing that they were to be regarded as true new growths. When these tumours were first observed, i.e. at the time of entering them

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... n the laboratory register, 49 of the mice presented multiple tumours, and in an additional 77 mice, other subcutaneous tumour-nodules developed at a later date. Of the 350 tumours first observed, 42 had a structure quite distinctive from that of the well-known mammary tumours of the mouse, and of them 25 had certainly arisen outside the mamma; 22 were squamous-celled carcinomata with marked keratinisation, and of them 14 arose in the mammary region, and 8 entirely outside it (jaw, face, vulva, and anus); 5 were sebaceous carcinomata, and of them 3 arose in the mammary region, and 2 outside it (anus). The remaining tumours were :-