Go Back   Freethought Forum > The Library > Articles & Essays > Science

Comment
 
Article Tools Display Modes
An Introduction to Zoology:  Chapter 6
An Introduction to Zoology: Chapter 6
Published by The Lone Ranger
08-07-2008
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.

Contents

Article Tools

Featured Articles
<<  <    Next Page: The Yolk Sac (Page 18 of 24)    >  >>
  #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?
Reply With Quote
  #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
Reply With Quote
Comment

  Freethought Forum > The Library > Articles & Essays > Science


Currently Active Users Viewing This Article: 1 (0 members and 1 guests)
 
Article Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

 

All times are GMT +1. The time now is 09:19 AM.


Powered by vBulletin® Version 3.8.2
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.

Article powered by GARS 2.1.8m ©2005-2006
Page generated in 0.24426 seconds with 14 queries