Sea Urchin fertilization requires that sperm find egg, identify it as the correct species, and fuse wit it. Sperm first meet with the thick outermost structure generated by the egg called the - This interaction causes a depolarization that leads to the . Another species specification step occurs when (now located at the front of the sperm) subsequently bind to fertilizins with active sites protruding from the eggs The fast and slow blocks to polyspermy begin with interactions between sperm proteins and proteins located on the.