The adult testis consists largely of a series of tubules with a central cavity. Sperm cells are continuously maturing as they move from the outer edge of the tubule into the central lumen; the most primitive forms, called spermatogonia, differentiate first into spermatocytes and then spermatids. They eventually mature into spermatozoa and are released into the lumen. Spermatozoa travel through the tubular network to be stored in seminal vesicles and, finally, to be ejaculated with the semen. Interspersed among the seminiferous tubules are Sertoli cells, and in the area between tubules (interstitium) are located the hormone-secreting Leydig cells.