Human reproduction involves sexual reproduction and viviparity. The male reproductive system includes testes that produce sperm and testosterone. Spermatogenesis occurs in the seminiferous tubules of the testes. The female reproductive system includes ovaries that produce eggs and release them during ovulation, and other structures like the uterus that support gestation. Oogenesis occurs in the ovaries and involves the formation of eggs. Fertilization happens when sperm fuse with an egg, forming a zygote. If implantation occurs, gestation and birth will follow.