Amniotes are creatures that can reproduce on land, thanks to the amniotic egg. Why are mammals considered to be amniotes, given that most mammals do not lay eggs? What structures do mammals have that are the same, or comparable to an amniotic egg?
An amniote is a tetrapod capable of producing an amniotic egg; the egg does not need to remain outside of the body, although this is typical. Mammals for example, contain the egg inside their body for the entire duration of fetal development. When the fetus has matured into an infant, the mother gives birth (as opposed to having laid an egg, and allowing it to mature outside of the body). The physical structure of the female human body during pregnancy clearly resembles an amniotic egg (a chicken egg for example). The amniotic egg has multiple functions; a fluid-filled sac for embryonic cushioning/protection, as well as moisture.
There are four basic extraembryonic membranes that develop outward from the embryo; the yolk sac, the chorion, the allantois, and the shell and albumen. These are crucial pieces of the amniotic structure. Pictured to the left is a diagram of a human fetus; to the right is a basic amniotic egg.
Amniotic Eggs. Digital image. Boundless.com. N.p., n.d. Web. 27 Apr. 2015. <https://www.boundless.com/biology/textbooks/boundless-biology-textbook/vertebrates-29/reptiles-174/characteristics-of-amniotes-670-11892/images/fig-ch29_04_01/>.
Gestation: Human Fetus in Uterus. Digital image. Encyclopedia Britannica Online. Encyclopedia Britannica, n.d. Web. 27 Apr. 2015. <http://www.britannica.com/EBchecked/topic/232124/gestation//images-videos/124247/gestation-human-fetus-in-uterus>.