zona : Idioms & Phrases

• noun 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
  

WordNet

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

Webster 1913

• NL. (Biol.), a radiately striated membrane situated next the yolk of an ovum, or separated from it by a very delicate membrane only.

Webster 1913