Marsupials
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Marsupials' reproductive systems differ markedly from those of placental
mammals. The female develops a kind of yolk sac in her womb which
delivers nutrients to the embryo. Embryos of some marsupials
additionally form placenta-like organs that connect them to the uterine
wall, although it is not certain that they transfer nutrients from the
mother to the embryo. Pregnancy is very short, typically 4 to 5 weeks,
and the embryo is born at a very young stage of development.
