absence of mitosis. In 1919, Lillie summarized his findings to that point in Problems of Fertilization, where he concluded that fertilization was a series of timed and irreversible events. Lillie was also a pioneer in the study of sex steroids in sexual differentiation. He published "The Theory of the Free-martin" in Science in 1917, which found that sex steroids in the blood controlled differentiation. Free-martins, sterile female cows born without sex organs, were a perplexing issue for cattle ranchers. Lillie found that free-martins formed when twins shared the same placenta. The hormones from the male twin would then be shared with the female, stunting the growth of her reproductive system. Eugenics Lillie was involved in the American eugenics movement at multiple levels: he was member of Chicago’s Eugenics Education Society, he was a committee member of the Second International Eugenics Congress, and he served on the advisory council for the Eugenics Committee of the United States. https://embryo.asu.edu/pages/frank-rattray-lillie-1870-1947 Lillie's status as a first-rate scientist likely helped to legitimize eugenics. https://www.tandfonline.com/doi/abs/10.1080/10999949.2016.1230821 References Further reading "Addresses at the Lillie Memorial Meeting, Woods Hole, August 11, 1948" Biological Bulletin, Vol 95, No2, 151–162, October 1948; "Moon Out of the Well; Reminscences," by Mary Prentice Lillie Barrows (daughter of F.R. Lillie), unpublished 1970; "The Development of the Chick. An Introduction to Embryology. Lillie, Frank R. New York, Henry Holt, 1908. Gilbert, S. F. 2003. Edmund Beecher Wilson and Frank R. Lillie and the relationship between evolution and development, Developmental Biology, Seventh edition, Sinauer External links National Academy of Sciences Biographical Memoir Guide to the Frank R. Lillie Papers 1899–1941 at the University of Chicago Special Collections Research Center 1870 births 1947 deaths American zoologists University of Chicago faculty Clark University fellows University of Michigan faculty Vassar College faculty Members of the United States National Academy of Sciences People from Old Toronto Scientists from Toronto Scientists from Chicago