Zona pellucida

from WordNet (r) 3.0 (2006)

zona pellucida
    n 1: thick membrane around the mammalian ovum; can be penetrated
         by one sperm in the fertilization process; usually remains
         around the fertilized egg until it is implanted in the wall
         of the uterus

from The Collaborative International Dictionary of English v.0.48

Zona \Zo"na\, n.; pl. {Zonae}. [L., a girdle. See {Zone}.]
   A zone or band; a layer.
   [1913 Webster]

   {Zona pellucida}. [NL.] (Biol.)
   (a) The outer transparent layer, or envelope, of the ovum. It
       is a more or less elastic membrane with radiating striae,
       and corresponds to the cell wall of an ordinary cell. See
       {Ovum}, and Illust. of {Microscope}.
   (b) The zona radiata.

   {Zona radiata} [NL.] (Biol.), a radiately striated membrane
      situated next the yolk of an ovum, or separated from it by
      a very delicate membrane only.
      [1913 Webster]