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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 Meroblastic Cleavage


In eggs with more yolk, cleavage cannot cut through the large mass of yolk, and so the egg does not completely divide with each cleavage. This is known as meroblastic cleavage (from the Greek “meros,” meaning “part”). In meroblastic cleavage, what ultimately happens is that a clump of dividing cells sits atop the undivided yolk.

In birds, reptiles, most fishes, some amphibians, cephalopod mollusks, and monotreme mammals (all of which have telolecithal eggs), so much yolk is present that cleavage can occur only in a narrow disk at the extreme animal end of the egg. As the embryo grows, blood vessels penetrate into the yolk. The yolk is gradually broken down and absorbed, but it never divides. In insects with centrolecithal eggs, cleavage occurs in a ring surrounding the central, uncleaved yolk. As the embryo develops, the yolk is gradually broken down and absorbed.



A zebrafish (Phylum Chordata) has telolecithal eggs and meroblastic cleavage.
Two cleavage divisions have occurred in this egg. The animal pole (left) has
divided into four cells, but the vegetal pole (right) has not divided at all.

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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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