During ovulation, the secondary oocyte released from the Graafian follicle is surrounded by protective layers. These layers include the cumulus oophorus, corona radiata, and zona pellucida.
Chorion is an extra embryonic membrane found outside the embryo. It forms the outermost membrane. It is not found outside the oocyte.
In the secondary oocyte, the outermost layer of granulosa cells in which the cells are found scattered throughout and not arranged properly into a layer, is called the cumulus oophorus.
Inner to the cumulus oophorus, lies another layer of granulosa cells, arranged perfectly into a cellular layer called corona radiata.
Inner to the corona radiata, lies the glyco proteinaceous, non cellular layer of zona pellucida.