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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 Cleavage

Cleavage:
Soon after fertilization, the zygote begins to divide via cleavage. Typically, the first cleavage occurs about 90 minutes after a spermatozoan first contacts the ovum. Exactly how cleavage occurs is influenced by how much yolk the egg contains.

Before cleavage begins, the zygote has a visible animal-vegetal axis. This axis is visible because yolk, nutrition for the developing embryo, is concentrated at one end of the zygote. The end where the yolk is concentrated is the vegetal pole of the zygote, and the opposite end is the animal pole. The amount and distribution of yolk influences cleavage because yolk, being non-living, interferes with cleavage.



Salamander eggs (Phylum Chordata): Each egg has a
clearly-visible animal pole (dark) and vegetal pole (light).

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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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