<p>Frank Lillie’s discovery of the explanation of the development of the free-martin—a sexual abnormality in cattle—was an important contribution to studies of sexual differentiation in the early 20th century. Several historians have framed its significance in terms of a debate between geneticists and endocrinologists about what determines sex. I argue that this framing masks the real significance of the discovery. As an embryologist studying the physiology of development, Lillie studied whether and to what degree sexual development is plastic, not when and how it is initially determined. The important lesson he took away from the free-martin was that an embryo retains its ability to differentiate even after its sex has been initially determined. In other words, sex determination is not an “irreversible predestination.” Understood this way, the free-martin discovery did not help to distinguish the relative importance of chromosomes or hormones in determining sex. Rather, it showed that sexual development can proceed in opposition to an individual’s determined sex.</p>