Polyspermy

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Abstract
Two mechanisms exist for the polyandrous syngamy of sperm and egg nuclei. One of these, Type I, consists in the prevention of all but one spermatozoon from entering the egg. In Type II, several spermatozoa enter the egg but only one sperm nucleus unites with the egg nucleus. In this type of inhibition a substance diffuses from the egg-, or, sperm nucleus causing degeneration of the supernumerary spermatozoa. Type I, called "block to polyspermy," consists of a change of the egg cortex starting at the point of sperm attachment and passing over the egg surface. It is not complete until after passage of cortical change, though there is a faster incomplete block believed to pass over the egg surface in about 2 seconds. Agents such as nicotine slow up the fast partial block. Fertilization can be reversed by appropriate treatment of the fertilized egg at particular times after fertilization. Sea urchin eggs may thereafter be re-fertilized after the 1st cleavage.