Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the mother.[1]
The term 'viviparity' and its adjective form 'viviparous' derive from the Latin vivus meaning "living" and pario meaning "give birth to".[2]
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Five modes of reproduction have been differentiated in animals[3] based on relations between zygote and parents. The five include two nonviviparous modes: ovuliparity, with external fertilisation, and oviparity, with internal fertilisation. In the latter, the female lays zygotes as eggs with a large yolk; this occurs in all birds, most reptiles, and some fishes.[4] These modes are distinguished from viviparity, which covers all the modes that result in live birth:
- Histotrophic viviparity: the zygotes develop in the female's oviducts, but find their nutriments by oophagy or adelphophagy(intra-uterine cannibalism of eggs or sibling embryos in some sharks or in the black salamander Salamandra atra).
- Hemotrophic viviparity: nutrients are provided by the female, often through some form of placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills. The skink Pseudemoia entrecasteauxii and most mammals exhibit a hemotrophic viviparity.
- Placental viviparity is arguably the most highly developed form of viviparity. Placental mammals, including humans, are the best-known example, but adaptations in some other animals also have incorporated this principle or close analogies. Other examples include some species of scorpions[5] and cockroaches,[6][7] certain genera of sharks and snakes, and velvet worms.
- Ovoviviparity, a less developed form of viviparity, occurs in most vipers, and in most live-bearing bony fishes (Poeciliidae). However, the term is poorly and inconsistently defined, and may be obsolete.[4] This term has been redefined and more commonly referred to as oviparous egg retention or prolonged egg retention.[1]
At least some transport of nutrients from mother to embryo appears to be common to all viviparous species, but those with fully developed placentas such as found in the Theria, some skinks, and some fish can rely on the placenta for transfer of all necessary nutrients to the offspring and for removal of all the metabolic wastes as well once it has been fully established during the early phases of a pregnancy. In such species, there is direct, intimate contact between maternal and embryonic tissue, though there also is a placental barrier to control or prevent uncontrolled exchange and the transfer of pathogens.
In at least one species of skink in the large genus Trachylepis, placental transport accounts for nearly all of the provisioning of nutrients to the embryos before birth. In the uterus, the eggs are very small, about 1 mm in diameter, with very little yolk and very thin shells. The shell membrane is vestigial and transient; its disintegration permits the absorption of nutrients from uterine secretions. The embryo then produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels. As it penetrates the lining, the embryonic tissue grows aggressively till it forms sheets of tissue beneath the uterine epithelium. They eventually strip it away and replace it, making direct contact with maternal capillaries.[8] In several respects, the phenomenon is of considerable importance in theoretical zoology. Blackburn & Flemming (2011)[8] remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.[8]
There is no relationship between sex-determining mechanisms and whether a species bears live young or lays eggs. Temperature-dependent sex determination, which cannot function in an aquatic environment, is seen only in terrestrial viviparous reptiles. Therefore, marine viviparous species, including sea snakes and, it now appears, the mosasaurs, ichthyosaurs, and plesiosaurs of the Cretaceous, use genotypic sex determination (sex chromosomes), much as birds and mammals do.[9] Genotypic sex determination is also found in most reptiles, including many viviparous ones (such as Pseudemoia entrecasteauxii), whilst temperature dependent sex determination is found in some viviparous species, such as the montane water skink (Eulamprus tympanum).[10]
https://en.wikipedia.org/wiki/Viviparity
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