Egg jelly(extracellular layer, jelly coat) is a gelatinous layer that surrounds the oocytes of many organisms and releases species-specific chemoattractants that activate and guide sperm to the oocyte. The egg jelly is located immediately surrounding the vitelline envelope and consists primarily of a network of short peptides and sulfated fucan glycoproteins. These short peptides diffuse into the surrounding area and stimulate respiration and movement of the sperm to the egg. An example of such a peptide is resact which has been studied as the primary means of attracting and orientating sperm to the eggs in sea urchins. The sulfated fucan glycoproteins play an important role in binding to sperm receptors and triggering the acrosomal reaction.
Many other functions for the egg jelly have been proposed including sperm agglomeration, protection from mechanical stress and polyspermy, and increasing the size of the egg to improve its chances of colliding with sperm. In addition to the sea urchin, egg jelly appears in many species including invertebrates and mammals, and can vary in composition and complexity from the relatively homogenous single layer sea urchin egg to the three layer egg jelly in starfish.