The Amniotic Egg: An Adaptation to Land Life
Amphibians never succeeded in becoming fully terrestrial because amphibian eggs must be laid in water to avoid drying out. Reptiles were the first vertebrates to completely master the challenge of living on dry land because an adaptation had evolved (among others of course) that prevented their eggs from drying out: The amniotic egg
A Crocodile Egg as an Example of an Amniotic Egg: 1. eggshell 2. yolk sac 3. yolk (nutrients) 4. vessels 5. amnion 6. chorion 7. air space 8. allantois 9. albumin (egg white) 10. amniotic sac 11. crocodile embryo 12. amniotic fluid
The eggshell protects the embryo and keeps it from drying out, but it is flexible to allow gas exchange. Most reptiles lay watertight eggs that contain a food source (the yolk) and a series of four membranes: 1. the yolk sac, 2. the amnion, 3. the allantois, and the chorion. Each membrane plays a role in making the egg an independent life support system.
The outermost membrane of the egg is the chorion, which lies just beneath the porous shell. It allows oxygen to enter the egg and carbon dioxide to exit the egg, but retains water within the egg.
The amnion encases the developing embryo within a fluid-filled sac. It contains amniotic fluid which protects and cushions the embryo. It aids in osmoregulation and serves as a saltwater reservoir.
The yolk sac provides protein and fat rich nutrients from the yolk for the embryo via blood vessels connecting to the embryo’s gut.
The allantois collects the metabolic waste excreted by the embryo.
The amniotic egg became a bestseller: All modern reptiles, birds and mammals show this same pattern of membranes within the egg. These three vertebrate classes are called amniotes.
Science–Video: The game-changing amniotic egg
Author: Amelia P., User: Catsloveme207
Date: 20 May 2013