A freemartin is a female bovine with a masculinized brain that behaves as would a bull, at least insofar as it will mount normal females. An 18th century physician, John Hunter, discovered that freemartins always had male twins. In 1916, several researchers independently discovered what happens when the chorion (the outer layer of the two membranes that completely envelope a fetus) of a male and the chorion of a female bovine fetus fuse in the uterus. The blood vessels in the chorions become interconnected and male hormones pass from the male twin to the female twin. The male hormones then masculinize the female twin, and the result is a freemartin.
