Webster's English Dictionary

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chorion

cho.ri.on \'ko-r-e--.a:n, 'ko.r-\ \.ko-r-e--'a:n-ik, .ko.r-\ n [NL, fr. Gk] 
   : the highly vascular outer embryonic membrane of higher vertebrates that  
   in placental mammals is associated with the allantois in the formation of 
   the placenta - cho.ri.on.ic aj