Scanning electron micrograph (SEM) of a single spermatozoon penetrating the layer of follicular cells (the corona radiata) that envelops an ovum (egg) during the initial stages of fertilisation. To continue its journey the sperm must penetrate an underlying glycoprotein membrane, the zona pellucida. Any competing sperm are prevented from entering the egg by rapid chemical changes that take place in the zona pellucida following penetration by the single successful spermatozoon. Fertilisation occurs when male & female nuclei fuse; a zygote thus formed begins to develop through dividing by binary fission.
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