chorion

from WordNet (r) 3.0 (2006)

chorion
    n 1: the outermost membranous sac enclosing the embryo in higher
         vertebrates (reptiles, birds and mammals)

from The Collaborative International Dictionary of English v.0.48

Chorion \Cho"ri*on\, n. [NL., fr. Gr. ?.]
   1. (Anat.)
      (a) The outer membrane which invests the fetus in the
          womb; also, the similar membrane investing many ova at
          certain stages of development.
      (b) The true skin, or cutis.
          [1913 Webster]

   2. (Bot.) The outer membrane of seeds of plants.
      [1913 Webster]