Cclxxv. the Endocrine Factors Con-

IT is now generally believed that the nature and extent of the periodic changes occurring in the ovary are controlled by the anterior lobe of the pituitary. The study of this relationship has been seriously handicapped, however, because the gonad-stimulating properties of extracts and other preparations can only be adequately studied in the hypophysectomised animal. Since it is practically impossible to remove the anterior lobe of the mammal alone or even carry out total hypophysectomy with ease, progress is beset by serious obstacles. In a previous paper [Bellerby, 1933] it was pointed out that this difficulty can be overcome by making use of certain amphibia, and attention was drawn to the suitability of the African Clawed Toad (Xenopus laevis) for use in the study of the ovary-pituitary relationship particularly when used for the assay of a gonad-stimulating substance. Before the practical possibilities of Xenopus were ascertained experiments were carried out to determine the suitability of the common English frog (Rana temporaria) for the same purpose. As regards the removal of the pituitary Rana possesses the same main advantages as X. laevis [Hogben, 1923]. (a) It can be easily and rapidly hypophysectomised without injury. (b) One or both lobes of the pituitary can be removed with precision. (c) Completeness of removal can be checked by the characteristic colour response of the animal. The present investigation shows that Rana is much less satisfactory than Xenopus for use in the study of the gonad-stimulating properties of extracts. It can nevertheless be used with advantage to confirm conclusions derived from experiments on Xenopus and will be made use of in the present studies for that purpose. EXPERIMENTAL.