The mature ovum or a female gamete is spherical in shape. Its cytoplasm is called ooplasm containing large nucleus. The cytoplasm is enveloped by cell membrane (plasma membrane). The membrane forming the surface layer of an ovum is called the vitelline membrane. The plasma membrane of the ovum is surrounded by inner thick, non cellular zona pellucida; probably secreted by the follicular cells and outer very thick cellular corona radiata. The latter is formed of radially elongated follicular cells.