The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the oocyte, external but of essential importance to it. The zona pellucida first appears in uni-laminar primary oocytes. It is secreted by both the oocyte and the follicular cells. In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes to be replaced by the underlying layer of trophoblastic cells. The zona pellucida is essential for oocyte death and fertilization.