1. Amnion is a membranous sack within the shell that surrounds the embryo, and is filled with amniotic fluid that provides a protective environment for the organism, and reduces its risk of injury.
2. Chorion is the outermost membrane layer of the egg, in direct contact with the inside of the calcerous shell. This membrane absorbs oxygen that permeates through the porous shell from the outside environment, and transports it to the embryo. It also releases waste carbon dioxide from within the egg through the shell, and to the outside world.
3. The allantois is a membrane found between the amnion and chorion, and acts as a storage facility for nitrogenous waste excretions produced by the embryo; keeping it away from the developing organism.
4. The yolk sack contains the yolk; a highly nutritious and protein rich food source that the developing embryo requires to develop.
The cleidoic egg is a true feat of evolution. Millions of years ago after fish-like amphibians left the water and attempted to survive on land, the ability to lay eggs with tough outer coverings that protected the inside from dehydration, rather than the gelatinous eggs of amphibians and fish that require moist environments to stop desiccation, improved the ability for land colonisation. This allowed them to inhabit more arid environments such as deserts, and lead to a range of vertebrate organisms effectively inhabiting the planets variety of terrains.
-Thomas Glen
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