Frank Rattray Lillie (June 27, 1870 – November 5, 1947) was an American zoologist and an early pioneer of the study of embryology.

Born in Toronto, Ontario, Canada, Lillie moved to the United States in 1891 to study for a summer at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts.

He identified the influence of potassium on cell differentiation and elucidated the biological mechanisms behind free-martins.

Lillie was instrumental in founding the Woods Hole Oceanographic Institution and served as its first president.

He accepted a summer position at the Marine Biological Laboratory (MBL) in Woods Hole, Massachusetts, then became a fellow in zoology at Clark University, where he studied under Charles Otis Whitman.

In 1894 (at the age of 24) he received the degree of Doctor of Philosophy in Zoology summa cum laude.

[6] In 1902, Lillie was able to convince representatives from the Rockefeller Foundation, Carnegie Corporation of New York, General Education Board, and brother-in-law Charles Richard Crane to provide the institution with financial support.

After Whitman retired in 1908 to chair the zoology department, Lillie took his place as the director of the laboratory.

With a $3 million grant from the Rockefeller Foundation, Lillie was able to establish the Woods Hole Oceanographic Institution in 1930.

[4] The American Society of Zoologists named Lillie the organization's representative to the United States National Research Council (NRC) Division of Biology and Agriculture in 1919.

He published "The Theory of the Free-martin" in Science in 1917, which found that sex steroids in the blood controlled differentiation.

[4] Free-martins, sterile female cows born without sex organs, were a perplexing issue for cattle ranchers.