thin tissue surrounding the yolk and ovum, forming the yolk sac; consists of 4 layers, 2 formed in the ovary (oolemma and perivitelline membrane) and 2 in the infundibulum of the oviduct (middle continuous layer and extravitelline membrane); after the ovum is ovulated into the infundibulum, sperm penetrate the perivitelline membrane to fertilize the ovum, but the final, extravitelline membrane acts as a barrier to sperm, preventing fertilization after it surrounds the ovum; Latin vitellus = yolk