ALFRED PASIEKA / SCIENCE PHOTO LIBRARY ALFRED PASIEKA / SCIENCE PHOTO LIBRARY
Fertilization. Artwork of sperm cells (white and black) surrounding a human egg cell (round). The egg cell is protected by an outer layer, the zona pellucida (black). If fertilisation has already occurred, then this layer will have thickened to prevent any further sperm entering the egg. The head of the successful sperm releases the genetic material that it contains, and this mixes with the genetic material carried by the egg. The resulting zygote then has the full set of genes required to develop into a new human. The sperm cell (spermatazoon, male) and the egg cell (ovum, female) are the human gametes (reproductive cells).
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