Twentieth-century researcher Ernest John Christopher Polge studied the reproductive processes of livestock and determined a method to successfully freeze, thaw, and utilize viable sperm cells to produce offspring in animals. In 1949, Polge identified glycerol as a cryoprotectant, or a medium that enables cells to freeze without damaging their cellular components or functions. Several years later, Polge used glycerol in a freezing process called vitrification, which enabled him to freeze poultry sperm, thaw that sperm, and use it to fertilize vertebrate embryos. He later adapted those methods to be applied to several other species including goats, cows, and pigs, which enabled farmers to fertilize livestock with sperm or embryos after long-term storage. Additionally, Polge's development of methods to freeze and store living samples has equipped reproductive health researchers and medical professionals with the abilities to mass collect and store human sperm.