Normally a sperm can easily penetrate the egg, but if another sperm has already entered, or the sperm and/or egg is defective. Ignore the other answer because it merely restates your question. So a sperm can always penetrate an egg unless it is too late or either it or the egg is defected. Hope this helps!
It means that there the egg already penetrated by another sperm
When sperm cannot penetrate the egg, it's often due to the inability to navigate the egg's protective layers. This can be caused by incomplete capacitation, failure of the acrosomal reaction, or other fertility issues. Successful penetration is essential for fertilization, and problems in this process can lead to infertility.
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