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Slideshow: Slideshow 1

Fertilization Begins

#363, Slide 1 of 19

From The Biology of Prenatal Development.Buy Now


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.
Fertilization BeginsThree-Cell EmbryoEarly BlastocystImplantation Well UnderwayThe 4-Week EmbryoVitelline Vessels and Yolk SacHand Plate and Vital OrgansHand, 7 Weeks Pregnant7½-Week EmbryoRight Eye and Facial Profile9-Week Fetus, MRI AnimationExit Stage LeftPosterior Surface of Left ThighStill HidingThe MouthFace, 27-Week FetusFace and Left ForearmReady for Birth and Beyond
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