Evolution of eggs with a water-impermeable amniotic membrane surrounding a fluid-filled amniotic cavity permits embryonic development on land without danger of dessication. The main diagram shows the arrangement of membranes in a typical egg-laying oviparous vertebrates. In live-bearing viviparous vertebrates, the shell and chorionic membrane are absent and the allantoic and yolk sac membranes are incorporated into the connection with the maternal circulation
( http://www.mun.ca/biology/scarr/Amniotic_egg.htm )
The amniotes are a microphylum of tetrapod vertebrates that include the Synapsida (mammals) and Reptilia (reptiles and dinosaurs, including birds). They are defined by embryonic development that includes the formation of several extensive membranes, the amnion, chorion, and allantois. Amniotes develop directly into a (typically) terrestrial form with limbs and a thick stratified epithelium, rather than first entering a feeding larval tadpole stage followed by metamorphosis as in amphibians. In amniotes the transition from a two-layered periderm to cornified epithelium is triggered by thyroid hormone during embryonic development, rather than metamorphosis . The unique embryonic features of amniotes may reflect specializations of eggs to survive drier environments, or the massive size and yolk content of eggs designed for direct development to a larger size.
Features of amniotes designed for survival on land include a sturdy but porous leathery or hard eggshell, and an allantois designed to facilitate respiration while providing a reservoir for disposal of wastes. Their kidneys and large intestines are also designed to retain water. Most mammals do not lay eggs, but corresponding structures may be found inside the placenta.
The first amniotes looked like small lizards, for instance Casineria kiddi who lived for about 340 million years ago, and their eggs were small and covered with a membrane, not a hard shell like often seen today. Some modern amphibians lay eggs on land but without any protection to speak of, while others like some lungless salamanders and Amphiuma also lay small eggs on land, but these are covered by a rubber-like membrane even if they lack advanced traits like an amnion. This kind of eggs became possible with internal fertilisation. The outer membrane, a soft shell, evolved as a protection against the harsher environments on land. It was probably because the embryos were safer on land than in water that some species got the habit of laying them out of the water. One can assume the ancestors of the amniotes laid their eggs in moist places, as such animals wouldn't have too many difficulties in finding depressions under fallen logs or other suitable places in the ancient forests, and dry conditions were probably not the main reason why the soft shell emerged.
In fish and amphibians there is only one inner membrane, also called an embryonic membrane. In amniotes the inner anatomy of the egg has evolved further, new structures have developed to take care of the gas exchanges between the embryo and the atmosphere, as well as dealing with the waste problems. To grow a thicker and thougher shell there were no other alternatives than finding new ways to supply the embryo with oxygen, as diffusion alone wouldn't be enough any more. After the egg had gotten these structures, further sophistication of them allowed the amniotes to lay much bigger eggs in much drier habitats. Bigger eggs meant bigger offspring, and bigger adults meant bigger eggs, which meant the amniotes had gotten the opportunity to grow bigger than their ancestors. But real growth was not possible until they stopped relying on small invertebrates as their main food source, and started to eat plants or other vertebrates, or returned to the water. New habits and heavier bodies meant further evolution for the amniotes, both in behavior and anatomy.
There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of temporal fenestrae (openings) behind the eye. In anapsids there are none, in synapsids there is one, and in most diapsids there are two.
The skeletal remains of amniotes can be identified by their having at least two pairs of sacral ribs and an astragalus bone in the ankle.
( http://en.wikipedia.org/wiki/Amniotic_egg )
http://dictionary.reference.com/search?r=2&q=amniotic it means when i put my private into urs and add sperm into ur egg This link should help (with pic). Source(s):
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