File:Sperm Entry.jpg

The first polar body (the smaller cell atop the oocyte) deforms the mammalian egg away from its encapsulating zona pellucida, creating a gap.

From: The Ins and Outs of Sperm Entry Chanut F PLoS Biology Vol. 4, No. 5, e160 doi:10.1371/journal.pbio.0040160

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