Spermatogonium is a class made up of all of the spermatogenic cells. Each of these contains forty-six chromosomes in its nucleus. During early adolescence, certain hormones stimulate spermatogonia to become active. Some undergo mitosis (dividing into two daughter cells), giving rise to new spermatogonia and providing a reserve supply. Others enlarge and become primary spermatocytes that then divide by a special type of cell division called meiosis. In the course of meiosis, the primary spermatocytes each divide to form two secondary spermatocytes. Each of these cells, in turn, divides to form two spermatids, which mature into sperm cells. The process by which sperm cells are produced is called spermatogenesis.