Sperm Penetrating Egg A sperm spends several hours in the female reproductive tract undergoing a process called capacitation, which enables it to penetrate an egg. here we see a sperm approaching an egg. Although several hundred sperm reach an egg, only one penetrates it. The egg then produces a hard protein covering to prevent penetration by another sperm.

6-Week-Old Embryo (Rear View) At 6 weeks, the embryo is just over half an inch long. As shown in this photograph, the arms and legs are just beginning to grow, and the head area is extremely large compared to its size after birth. The embryo is shown here in the amniotic sac, which is filled with fluid to protect it.

4½-Month-Old Fetus Sucking Thumb At about 4½ months of prenatal development, the fetus is about 10 inches long and weighs approximately 9 ounces. As shown in this photograph, the fetus shows the reflexive movement of sucking its thumb. This activity appears remarkably similar to thumb-sucking in neonates.

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