Whereas the influence of the follicular hormone on the oestrous phenomena and the cyclical changes in the female genital tract, and the provocation of the progestational phase of the endometrium by the corpus luteum hormone, seem to be perfectly clear, the function of the corpus luteum in the process of implantation of the egg has not yet been explained. This most important process, which can easily be regarded as the purpose and fulfilment of the whole genital function, is not entirely understood in its interactions of cause and effect. The corpus luteum 'protects the egg' [Fraenkel, 1903,1910], it directs the implantation, and its presence is indispensable for the accomplishment of the implantation [Bouin and Ancel, 1910; Ancel and Bouin, 1924; Corner and Allen, 1929; Courrier, 1935; Courrier and Kehl, 1938; Pincus and Werthessen, 1938, and others]. According to Corner [1928] the corpus luteum is also necessary for the nutrition of