My investigations on the changes undergone by the genital organs, and more particularly the uterus, on ovulation and its relation to œstrus and coition, and on the mode of development of the corpus luteum in the sheep, were commenced in the summer of 1900, and continued until the close of the sheep’s sexual season in the following February. A preliminary account was then communicated to the Royal Society, and afterwards published in the ‘Proceedings’ (vol. 68, 1901). The work was resumed in the autumn of 1901, and has been subsequently brought to a conclusion. The sheep employed were either Scotch Black-faced, or half-breeds between Cheviots and Border Leicesters, Cheviots and Black-faced, or Leicesters and Black-faced. Although belonging to typically mountain breeds, except in the case of the half-bred Border Leicesters, they were kept on good pasture under comparatively Lowland conditions, in the neighbourhood of Penycuik, Midlothian.