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