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The Endowment for Human Development
The Endowment for Human Development
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Fertilization Begins

Fertilization Begins [Click for next image]
From The Biology of Prenatal Development.Buy Now

Note:

Fertilization begins when a single spermatozoon penetrates the zona pellucida, which surrounds the oocyte. This event triggers a reaction called the zona reaction, which dramatically alters the outer membrane and prevents the entry of any additional spermatozoa into the oocyte.

Label Key:

1. principal piece of spermatozoon
2. zona pellucida
3. midpiece
4. neck
5. postacromial region
6. acrosomal cap
All ages referenced to fertilization, not last menstrual period.
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