An account of variations in epidermal mitotic activity during the oestrous cycle of the female mouse was first given by H. F. Bullough [1943], and further observations on this phenomenon, and on variations induced by oestrone injections, were given by W. S. Bullough [1946] and H. F. Bullough [1947]. It was demonstrated that the epidermis undergoes cyclic growth similar to that of the reproductive organs, and that the epidermal mitosis rate can be stimulated by injections of oestrone. From these results, and from similar observations on other tissues, it appeared probable that a primary function of the oestrogenic and androgenic hormones is to stimulate mitosis throughout the body, and it was therefore suggested that these substances should be collectively called mitogenic hormones. All these observations were based on results obtained with the colchicine technique, the actual method adopted being that of Allen [1937]. Recently, evidence obtained with the earclip technique