Fertilizatio interna est fusura ovi et seminis per reproductionem sexualem intra corpus parentis. Ut hoc fiat, mas semen in reproductivum feminae tractum introducere debet. Quod mammalibus, reptilibus, nonnullis avibus, nonnullis piscibus, et quibusdam aliis animalium gregibus per copulationem efficitur, cum penis vel aliud organum intromittens in vaginam aut cloacam introducitur. Plurimi aves osculo cloacali utuntur, binis animalibus earum cloacas cum semen transferant comprimentibus. Urodela, araneae, ac nonnulla insecta et mollusca fertilizationem internam efficiunt per spermatophoros, fascicula seminum, a mare ad feminam transferenda. Post fertilizationem, embryones ut ova in organismis oviparis ponuntur, aut in organismis viviparis iam crescunt intra reproductivum matris tractum, ut animalia viva denique gignantur.

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