Final answer:
The block to polyspermy is triggered by a rise in calcium ion levels in the oocyte's cytoplasm, which initiates the cortical reaction to prevent additional sperm from penetrating the egg.
Step-by-step explanation:
In the block to polyspermy, entry of the sperm's contents causes calcium ion levels in the oocyte's cytoplasm to rise, triggering the cortical reaction. This process entails a rise in calcium ion concentration following sperm penetration, leading to the release of cortical granules from the oocyte which perform various functions like altering the membrane's structural integrity and establishing a barrier against subsequent sperm penetration, thus ensuring only one sperm fertilizes the egg.