Lillie 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." (Wikipedia article on Frank Rattray Lillie, accessed 3-2020).