FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Artwork illustrating the process of mammalian fertilisation, showing penetration of an ovum or egg by a single spermatozoon. The sperm has successfully penetrated the layers of follicular cells, the corona radiata, which envelop the egg & broken through the underlying glycoprotein membrane or zona pellucida to begin its journey towards the ovum's nucleus. Numerous competing sperms are prevented from participating by rapid changes in the chemical nature of the zona pellucida. Fertilisation occurs when male & female pronuclei fuse; a zygote thus formed begins to develop through dividing by binary fission.
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