Studies in Spermatogenesis is a two part monograph written by American geneticist Nettie Maria Stevens which highlighted her increasingly promising focus of sex-determination studies and chromosomal inheritance. She observed that male mealworms produced two kinds of sperm, one with a large chromosome and one with a small chromosome. When the sperm with the large chromosome fertilized eggs, they produced female offspring, and when the sperm with the small chromosome fertilized eggs, they produced male offspring. The pair of sex chromosomes that she studied later became known as the X and Y chromosomes.