Cortical Reaction

of an egg, a change in the cortical (surface) layer of an egg in response to an activating stimulus. The change spreads in wavelike fashion in all directions from the site of a sperm’s contact with the cellular membrane of the egg or from the site of application of an artificial stimulus (for example, a needle prick).

The visible manifestation of the cortical reaction is preceded by a latent period during which the wave of excitation (activating impulse) spreads through the cortical layer. In most animals whose eggs have cortical bodies the visible phase of the cortical reaction then appears. The contents of the cortical bodies are released outside the egg proper, whereupon this area fills with water, resulting in separation of the egg membrane from the surface of the ooplasm and in the formation of a perivitelline space. The cortical reaction covers the entire surface of the egg in 10–90 seconds in sea urchins and in 2–5 minutes in fish (depending on the temperature).

The cortical reaction plays an important role in protecting an egg against penetration by too many sperm (the sperm agglutinate on contact with the perivitelline fluid). Inhibition of the cortical reaction therefore leads to polyspermy. The properties of the egg membrane change as a result of the cortical reaction and the release of substances formerly localized in the deeper layers of the ooplasm. A medium is created around the fertilized egg that is favorable to its development.