Both of these cells are encased in a thick glycoprotein shell called the zona pellucida. Genetically, the secondary oocyte that arrives in the oviduct is in metaphase of the second meiotic division. The metaphase plate is located inside the oocyte immediately below the first polar body. The final structural feature of the egg that serves a critical function during fertilization is a set of cortical granules . During oogenesis, the oocyte develops thousands of small membrane-bound granules that accumulate in the cortical cytoplasm, just beneath the plasma membrane.
Cortical granules are the egg's homolog to the acrosome in sperm.
Their function is described in the following section on fertilization.