Sperm production (spermatogenesis) is a lengthy, but continuous process. Sperm are at different stages of their development in the testis. Within one testis, some sperm are at early stages and others are at later stages of development. 

It takes about 70 days for a single germ cell (precursors of sperm) to develop into the mature sperm found in semen that can fertilise an egg. Thousands of sperm are being made in the testis each day. 

When ejaculation starts, sperm are transported from the tail of the epididymis, through the vas deferens, to the urethra in the penis. As sperm pass through the different tubes, fluid from the seminal vesicles, Cowper’s and prostate glands is added to the sperm. The mixture is then called semen. Over 90% of the semen is produced from the prostate and seminal vesicles and not from the testis.