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Investigating Human Biology: Twins Separated at Birth

Related Article: Learn more about fetal development.

In rare instances, twins do not fully separate in the womb and are born conjoined. In these cases, doctors will attempt to separate them, though because they may share vital organs it is not always possible.

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Why Tell Me Why: Belly Button Beginnings (1:19)

Ever wonder why your belly has a button? This video examines where belly buttons come from and why they're all so different.

Organ Systems: The Reproductive System (0:56)

The male reproductive system is centered around the testes. The female reproductive system is centered around the ovaries.

Human Mutants: Fetal Skeletons (1:49)

On Discovery Channel's "Human Mutants: The Mystery of Growth," learn how the skeleton begins to grow in an embryo and how it continues to develop.

Birth Day: Natural Childbirth (7:58)

In this Birth Day clip, a condition of preeclampsia causes the doctor and patient to change their original plan on a completely natural childbirth.

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Basics of Biology: How Organ Systems Form (1:25)

All of the organ systems in a human develop from a single fertilized egg that differentiates itself based on the instructions in its DNA.

Reality Matters: Teen Sexuality (30:00)

Hormones, peers, the media and other factors all contribute to the pressure many teens feel to have sex. This series examines the realities of that pressure, the risks teens face today -- including STDs and AIDS -- and ways they can protect... More »

Reality Matters: Teen Sexuality and Teen Parenthood (3:57)

Teenage parents take on a lot of responsibility at a very young age, and in this segment they tell others not to take the chances they did.

Reality Matters: Teen Sexuality and The Waiting Game (3:18)

More and more teenagers are making the decision to wait to have sex until they are more mature. Learn more about the benefits of abstinence in this clip.

Reality Matters: Teen Sexuality, HIV and AIDS (6:44)

HIV/AIDS is not well understood by teenagers, and many think they can't be infected by the virus. This video explores the differing lives of two teenage AIDS victims.

Reality Matters: Teen Sexuality and STDs (5:58)

Sexually transmitted diseases can be passed along in a variety of ways. While some STDs are treatable, others are incurable, and can affect victims for the rest of their lives.

Reality Matters: Teen Sexuality and Statistics (3:22)

The percentage of teens who are sexually active today is much lower than most teenagers think. Take a look at this clip to learn more about the facts behind teen sexuality.

Fertilization and Sexual Reproduction: Part 6 (5:33)

Four phases of mitosis lead to cell duplication and division. Check out this clip to learn more about the process of mitosis.

Fertilization and Sexual Reproduction: Part 5 (1:25)

This clip examines how the nuclei of the sperm and egg combine to form the zygote.

Fertilization and Sexual Reproduction: Part 4 (2:06)

Watch as the sperm enters the egg in this video.