Ovaries and Testes both undergo meiosis, the generation of haploid cells. These are called the gametes and these "half cells" join together during the process of fertilization.
Ovaries are different, however, because these gametes, or the 'eggs', are generated while the female is still an embryo. That is why there is a limited amount and they are not fertile beyond about 40 or 50 years old.
Testes are different, because they are external, but also because the gametes are generated daily. Unlike eggs, sperm comes out of the body and into another. This process starts at about the age of 12 and will not stop until death.
