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Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs. In plants, it means reproduction via embryos, such as buds, that develop from the outset without interruption, as opposed to germinating externally from a seed. == In animals == Five modes of reproduction can be differentiated in animals〔Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris〕 based on relations between zygote and parents: * Ovuliparity: fecundation (fertilisation) is external. It occurs in aquatic arthropods such as some Crustacea; also in many fishes and Amphibia * Oviparity: fecundation is internal, the female lays zygotes as eggs with large vitellus; occurs in all birds, most reptiles, some fishes. *Ovoviviparity: a defunct term used to explain oviparity with retention of zygotes in the female’s body or in the male’s body, without trophic interactions between zygote and parents. The term is currently not used to explain the reproductive biology of amniotes for multiple reasons, including strong evidence for placental transport of nutrients in all species studied.〔Blackburn, D. G. (2000). Classification of the reproductive patterns of amniotes.:" Herpetological Monographs", 371-377.〕 In seahorses, zygotes are retained in the male’s ventral "marsupium". In the frog ''Rhinoderma darwinii'', the zygotes develop in the vocal sac. In the frog ''Rheobatrachus'', zygotes developed in the stomach. *Histotrophic viviparity: the zygotes develop in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (intra-uterine cannibalism 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. The relatively less developed form of animal vivipary, ovoviviparity, occurs in most vipers for instance, and also in most live-bearing bony fishes (Poeciliidae); the more developed form of vivipary is called placental viviparity. Placental mammals are the best example, but other animals have also adapted by incorporating this principle. Examples include some species of scorpions〔Capinera, John L., ''Encyclopedia of entomology''. Springer Reference, 2008, p. 3311.〕 and cockroaches,〔Costa, James T., ''The Other Insect Societies''. Belknap Press, 2006, p. 151.〕 certain genera of sharks and snakes, and velvet worms. While incipient transport of nutrients appears to be common in all viviparous species. Some viviparous species receive substantial amounts of nutrients via the placenta (present in therians, some skinks and some fish). In such species there is direct juxtaposition of maternal and embryonic tissue. In at least one species of skink in the large genus ''Trachylepis'' placental transport accounts for nearly all provisioning of nutrients to embryos before birth. In the uterus the eggs are very small, about 1mm 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. In several respects the phenomenon is of considerable importance in theoretical zoology. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile.〔Blackburn, D. G. and Flemming, A. F. (2011), Invasive implantation and intimate placental associations in a placentotrophic african lizard, Trachylepis ivensi (scincidae). Journal of Morphology. 〕 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.〔Chris L. Organ et al. (2009) "Genotypic sex determination enabled adaptive radiations of extinct marine reptiles", ''Nature'' 461, 389-392 (17 September 2009)〕 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'').〔Robert, Kylie A., and Michael B. Thompson. "Sex determination: viviparous lizard selects sex of embryos." Nature 412, no. 6848 (2001): 698-699.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「vivipary」の詳細全文を読む スポンサード リンク
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