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In oogamy the large [[female]] [[gamete]] (also known as [[Egg cell|ovum]]) is immotile, while the small [[male]] gamete (also known as [[sperm]]) is mobile.<ref>{{Cite book |last1=Fusco |first1=Giuseppe |url=https://books.google.com/books?id=AKGsDwAAQBAJ|title=The Biology of Reproduction |last2=Minelli |first2=Alessandro |date=2019-10-10 |publisher=Cambridge University Press |isbn=978-1-108-49985-9 |pages=111–112 |language=en |author-link2=Alessandro Minelli (biologist)|access-date=29 March 2021}}</ref> Oogamy is a common form of anisogamy, with almost all animals and land plants being oogamous.
Oogamy is found in most sexually reproducing species, including all vertebrates, land plants, and some algae. The ancestral state of sexual reproduction is believed to be isogamy, with oogamy evolving through anisogamy. Once oogamy evolves, males and females typically differ in various aspects. Internal fertilization may have originated from oogamy, although some studies suggest {{Citation needed|date=June 2023}} that oogamy in certain species may have evolved before the transition from external to internal fertilization. In streptophytes, oogamy occurred before the split from green algae.
== Occurrence ==
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