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Development - From Cell to Organism

Development - From Cell to Organism

Introduction: Human life starts out as a single cell. This cell then gets fertilized by a sperm to become a zygote. What prevents more than one sperm from fertilizing an egg? It turns out that the second a sperm enters the egg, a series of chemical reactions take place that create a shield around the egg that prevents other sperm from entering.
At times, two (or more) egg cells are released and fertilized. This is what results in fraternal twins. While the individuals produced from these eggs may share similar traits, it is not uncommon for them to look very differently or even be of different genders.

Once the zygote implants itself into the uterine wall, additional cell divisions take place. Over time, the single cell will become a ball of cells called a morula, then a blastula, then a gastrula. It is at this stage that neurulation takes place. This is the development of the neural tube and other nervous tissue.

In humans around the 4 month mark, additional cell specialization takes place. This involves the development of a stronger heart, the replacement of cartilage by bone in the skeleton, and the growth of soft hair and skin.

The final three months of pregnancy involve the fetus doubling in size and its internal organs change in order to prepare it for life outside of the mother's body.

It usually takes 9 months (in humans) from conception to birth.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about how an organism develops.

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