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Monotremes lay eggs, from which the baby emerges. Marsupial babies develop in a primitive placenta in the uterus, and are delivered very prematurely. The baby/babies must crawl up to the nipple and attach to it until they have developed further. Placental mammals have a more advanced placenta in the uterus, and the babies are kept in it until they are much more fully developed than in marsupials. They are then delivered.

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15y ago

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