Spermatogenesis occurs in the testes of males, and involves cell division by both mitosis and meiosis. Germinal epithelium cells divide by mitosis to produce diploid 1st degree spermatocytes which themselves divide by the first division of meiosis into haploid 2nd degree spermatocytes. These spermatocytes divide by meiosis II into haploid spermatids which mature into sperm cells (spermatozoa) by the process of differentiation. The sperm cells are nursed by Sertoli cells. Each 1∘ spermatocyte becomes 4 sperm cells, which are all the same size because the cytoplasm is shared evenly in each division. The process begins at puberty and occurs continuously producing as many as 300 million spermatozoa every day.