The fertilization envelope formed after a sperm contacts an egg prevents polyspermy by what 2 step sequence, one fast and one slow.

The prefix "poly-" means "more than one" and the suffix "-spermy" is clearly related to sperm. Accordingly, polyspermy is when more than one sperm fertilizes an egg, something that organisms try to avoid.

To avoid this extra fertilization, specific organisms can use "electrical polyspermy blocks" (the fast process), which changes the negative electrical charge of the egg to positive, thus repelling the also positively charged sperm. The slower prevention process is when an organism undergoes a secretion reaction, or a cortisol reaction, which contributes to envelopping the egg by changing its membrane.

The prevention of polyspermy consists of two steps:

 1. The "electrical polyspermy block" (fast block) which includes a change in the electrical charge across the surface of the egg. Unfertilized eggs have a negative charge, but after the fertilization it becomes positive. When a sperm encounters an egg with a positive charge, the fusion is blocked because sperm surface membrane is also positively charged and thus it is repelled by the positive charge at the egg surface.

2. The "cortical reaction" (slow block) is a process of the release of cortical granules from the egg and it modifies the extracellular coat of the egg. It happens during the fertilization. Modifications include removal of receptors for the other sperm and formation of the fertilization envelope from the cortical granules.

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