The corpus luteum was first clearly described by Regner de Graaf (1672). That it is an organ of internal secretion was suggested by Louis-Auguste Prenant in 1898 and by Gustav Born about the same time. Clues to its specific action upont the endometrium were discovered through experiments by Ludwig Fraenkel (1903 and 1910), by Leo Loeb (1907) and by Paul Ancel and Paul Bouin (1910). The control of uterine motility by the corpus luteum was demonstrated by Hermann Knaus (1927 ff.). G.W. Corner (1928) demonstrated the necessity of the corpus luteum for survival of the pre-implantation embryo. Using a test based upon Fraenkel and Ancel/ Bouin, Corner and W.M. Allen isolated the hormone progesterone (1929) and first maintained pregnancy with corpus luteum extracts after early ablation of the ovaries (rabbit, 1930). Purification of progesterone was the work of several investigators between 1928 and 1934. The structural formula was worked out by Adolf Butenandt (1933–1934).