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An Introduction to Zoology:  Chapter 6
An Introduction to Zoology: Chapter 6
Published by The Lone Ranger
08-07-2008
Default Egg Types


An egg with very little yolk that is distributed more or less evenly throughout the egg is known as an isolecithal egg (from the Greek “isos,” meaning “same” and “lekithos,” meaning “yolk”). Mammals typically have isolecithal eggs. Since the mother provides nutrients to the developing embryo through the placenta, most mammalian eggs require very little supporting yolk.

An egg with a moderate amount of yolk concentrated at the vegetal pole is a mesolecithal egg (from the Greek “mesos,” meaning “middle”). Amphibians such as frogs and salamanders typically have mesolecithal eggs.

An egg with a large amount of yolk that is densely concentrated at the vegetal pole is a telolecithal egg (from the Greek “telos,” meaning “end”). Telolecithal eggs are typical of birds and reptiles. Because so much yolk is present, the developing animal can remain inside the egg for an extended period of time, and so is hatched in a relatively well-developed state.

An egg with a relatively large amount of yolk that isn’t concentrated at one end but is instead found in the center of the egg is a centrolecithal egg. Many insects have centrolecithal eggs. As the embryo develops, it grows around the yolk, gradually absorbing it as it grows.

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  #1  
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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  #2  
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.


Cheers,

Michael
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