Theophilus Shickel Painter studied the structure and function of chromosomes in the US during in the early to mid-twentieth century. Painter worked at the University of Texas at Austin in Austin, Texas. In the 1920s and 1930s, Painter studied the chromosomes of the salivary gland giant chromosomes of the fruit fly (Drosophila melanogaster), with Hermann J. Muller. Muller and Painter studied the ability of X-rays to cause changes in the chromosomes of fruit flies. Painter also studied chromosomes in mammals. He investigated the development of the male gamete, a process called spermatogenesis, in several invertebrates and vertebrates, including mammals. In addition, Painter studied the role the Y-chromosome plays in the determination and development of the male embryo. Painter's research concluded that egg cells fertilized by sperm cell bearing an X-chromosome resulted in a female embryo, whereas egg cells fertilized by a sperm cell carrying a Y-chromosome resulted in a male embryo. Painter's work with chromosomes helped other researchers determine that X- and Y-chromosomes are responsible for sex determination.