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Yoho National Park
yoicks
yoke
yokel
Yokohama
yokozuna
Yokuts
yolk
yolk sac
Yom Arafat
Yom Kippur
yom tov
yomp
yon
yond
yonder
yoni
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yolk sac
yolk sac (plural yolk sacs)


noun 
Definition:
 
membrane surrounding embryo: a thin membrane surrounding the embryo in birds, fish, reptiles, and mammals. In birds, fish, and reptiles, it encloses the yolk.
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