Definition of placenta
1
: the vascular organ in mammals except monotremes and marsupials that unites the fetus to the maternal uterus and mediates its metabolic exchanges through a more or less intimate association of uterine mucosal with chorionic and usually allantoic tissues
also
: an analogous organ in another animal
2
: a sporangium-bearing surface
especially
: the part of the carpel bearing ovules
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Merriam-Webster unabridged
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Time Traveler for placenta
The first known use of placenta was in 1638
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