Some species reproduce both asexually and sexually, including yeast and plants. Plant sexual reproduction involves an alternation of generations and both haploid and diploid phases. Chlamydomonas undergoes cell fusion, which may have been a forerunner of sexual reproduction. In humans, gametes are produced in the male and female reproductive systems. These systems include paired gonads, where sperm and oocytes are manufactured, networks of tubes, and glands.
Gametogenesis includes meiosis and maturation and produces sperm and oocytes.
Developing sperm originate in seminiferous tubules within the paired testes, passing through different stages of maturation as they approach the lumen of the tubule. Sperm mature in the epididymis and vasa deferentia, and they exit the body through the urethra during sexual intercourse. The prostate gland, seminal vesicles, and bulbourethral glands add secretions to sperm. Oocytes originate in the ovaries. Each month after puberty, one ovary releases an oocyte into a fallopian tube, which leads to the uterus.