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. Meiosis halves the number of chromosomes in somatic cells to produce haploid gametes. A species' chromosome number stays constant because a gamete's DNA replicates once, but the cells divide twice. Meiosis provides genetic variability by partitioning different combinations of genes into gametes through independent assortment. Crossing-over increases the variability.