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An Introduction to Zoology:  Chapter 6
An Introduction to Zoology: Chapter 6
Published by The Lone Ranger
Default Amniotes and the Four Extraembryonic Membranes

Amniotes and the Four Extraembryonic Membranes:
Because their eggs arenít surrounded by protective membranes that would prevent desiccation, early vertebrates (fishes and amphibians) had no choice but to lay their eggs in water, or at least wet environments. Eventually, however, the amphibians gave rise to descendents known as the amniotes. In amniotes (reptiles, birds, and mammals), the egg is surrounded by four extraembryonic membranes that provide nutrition and protection for the developing embryo. In some amniotes, the embryo remains inside the motherís body to develop, but in most, the mother secretes a protective shell around the egg and then expels it from her body.

The four extraembryonic membranes that surround the amniotic egg are the yolk sac, the chorion, the amnion, and the allantois. The key membrane is the amnion, because it is more or less water-impermeable, which means that the egg does not have to be deposited into a wet environment.

The amniotic egg is water-impermeable but allows free exchange of oxygen and carbon dioxide between the developing embryo and the outside environment. It also contains yolk to nourish the developing embryo. As such, itís an effective life-support system that allows amniotes to lay their eggs on land, making them much more independent of water than are their amphibian ancestors.

The amniotic egg.


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By monruw on 03-30-2011, 01:55 PM
Default Re: An Introduction to Zoology: Chapter 6

why it's called animal-vegetal axis? sound like kinda food, meat and vegetable or what else~ any story behind this?
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By The Lone Ranger on 05-22-2011, 01:35 AM
Default Re: An Introduction to Zoology: Chapter 6

"Vegetal," is related to "vegetable." Many plants can reproduce asexually, whereas virtually all animals reproduce sexually.

Probably for this reason, "vegetal" came to refer to processes in living things that are "plant-like," especially processes that do not occur through sexual reproduction. More to the point, perhaps, plants generally grow much more slowly than do animals. So the "vegetal" pole of an egg gets its name for the fact that the cells in this region grow and divide much more slowly than do the cells in the "animal" pole.


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