In human, primary spermatocytes are formed by

Spermatogonia undergo mitosis and differentiate into primary spermatocytes.

The spermatogonia present on the inside wall of seminiferous tubules multiply by mitotic division and increase in numbers. Some of the spermatogonia called primary spermatocytes periodically undergo meiosis. A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes. The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids. The spermatids are transformed into spermatozoa (sperms) by the process called spermiogenesis.