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The Evolution of Organ Systems$
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Andreas Schmidt-Rhaesa

Print publication date: 2007

Print ISBN-13: 9780198566687

Published to Oxford Scholarship Online: September 2007

DOI: 10.1093/acprof:oso/9780198566687.001.0001

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Reproductive organs

Reproductive organs

(p.240) CHAPTER 13 Reproductive organs
The Evolution of Organ Systems

A. Schmidt-Rhaesa

Oxford University Press

Animals can reproduce in two ways: asexually or sexually. Both modes have advantages and their evolutionary distribution and significance is discussed in this chapter. In sexually reproducing animals, sexes can be separate (gonochoristic) or combined in one individual (hermaphroditic). Gametes can be formed in a variety of ways, but not necessarily in particular structures called gonads. Sponges do not have anything like a gonad, and cnidarians and ctenophores concentrate gametes in particular regions called gametogenic areas. Most bilaterian animals have particular gonads, which can originate in different ways. The gametes also originate in different ways, either very early in development by ‘preformation’ or later by ‘epigenesis’. Gametes are transferred in different ways, producing numerous reproductive structures for gamete storage and transfer.

Keywords:   gonad, gamete, asexuality, sexuality, gonochorism, hermaphroditic, preformation, epigenesis, primary germ cell

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