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Almost all vertebrates reproduce sexually, and sexual life cycles always entail two complementary processes: meiosis and fertilization. Meiosis is a type of cell division which produces genetically haploid (n) cells, or gametes, from diploid (2n) precursor cells. These gametes, the sperm and eggs (or ova) in males and females, respectively, unite in the process of fertilization to produce a new diploid individual, a zygote. This zygote then undergoes a series of changes which constitute its development into a mature adult form. The reproductive systems of males and females contain the structures and perform the functions that make these events possible.

Reproductive systems are unique in that they are not necessary to promote and facilitate the life of the individual organism. Their ultimate function is simply to ensure that new individuals can be produced. Specifically, this means that they are involved in the processes of gamete production and delivery, and the maintenance of the life of a developing individual. As usual, we will explore sexual reproduction and development in the context of the human process, realizing that while the fundamentals are similar in most vertebrates, differences exist in specific strategies between the vertebrate classes.

Please Note: some of the terminology used in this chapter, especially with reference to meiosis, presupposes a general knowledge of basic genetic principles and terms. Review Chapter 19 if you are uncomfortable with these concepts.

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