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I. THE question as to the causes of heat has been frequently considered. While earlier authors assumed a nervous stimulus originating in the ovary to be responsible for the induction of heat, recent investigators are of the opinion that an ovarian hormone incites the readiness for mating which, as a rule, only occurs during certain stages of the cestrous cycle. This conception has gained prominence since ALLEN and DoisY showed that the typical morphological changes of the cestrous cycle in rat and mouse can be incited in ovariotomised animals by the injection of certain ovarian and placental extracts, and also reported that in some of the injected animals mating occurred. Many of the numerous investigators who have studied the effects of the ALLENDoisY factor have referred to it as the "female sex hormone," although they concerned themselves mainly or exclusively with its morphological actions, and did not consider the sexual behaviour of the animals. Moreover, several of the investigators who observed their animals for the occurrence of mating noticed that only a certain percentage mated. The percentage of copulation reported by PARKES was 7-6 (mice); in this percentage only those animals are included in which cornification had occurred under the influence of the extracts. It is obvious that cornification of the vaginal epithelium and cestrus are not only two essentially different phenomena, but also that the former is not necessarily an indication of the latter. The relatively small percentage of mating induced by injections of ovarian extracts made it, moreover, doubtful whether the same conditions are responsible for mating and the morphological changes accompanying it. One of the present authors (B. P. W.) observed that no case of mating occurred in the course of an experiment in which cornification was induced in forty-three ovariotomised rats by extracts of cow's placenta; HEMINGSEN, on the other hand, reported that three spayed mice mated without having received injections and without having regenerated ovaries. It was