During the last six years, with the progressive increase in the amount and variety of the work being carried out in the laboratories of the Imperial Cancer Research Fund, a gradual division of labour has taken place. Thus it has come about that in addition to much routine work of the laboratory, certain branches of the investigations have devolved more and more on individual workers. Among other subjects on which I have been engaged during the past, three years Dr. Bashford has specially charged me with the investigation of the new growths forwarded to the laboratory from animals other than man, and, with the care and initial propagation of spontaneous mouse tumours. Hence it comes about that all the material on which this thesis is based has passed through my hands. The histological preparations on which my observations have been made, I have made personally, and the figures have been made by myself or I have been responsible for them when made by others, as stated in each case. The initial attempts at propagation of most of the strains of mouse cancer now growing in the laboratory, were performed personally, and only strains for which I am responsible are referred to in the following pages.