In the s the biologist Ronald Ericsson came up with a way to separate sperm carrying the male-producing Y chromosome from those carrying the X. He sent the two kinds of sperm swimming down a glass tube through ever-thicker albumin barriers. The sperm with the X chromosome had a larger head and a longer tail, and so, he figured, they would get bogged down in the viscous liquid. The sperm with the Y chromosome were leaner and faster and could swim down to the bottom of the tube more efficiently. In the late s, Ericsson leased the method to clinics around the U. Instead of a lab coat, he wore cowboy boots and a cowboy hat, and doled out his version of cowboy poetry.