Final answer:
The amnion is an essential membrane in the amniotic egg that provides a protective, fluid-filled environment for the embryo, aiding in its development and protection from external conditions.
Step-by-step explanation:
The amnion is a crucial membrane within the amniotic egg, acting as a protective sac that surrounds the embryo or fetus. This membrane forms the amniotic cavity, which fills with amniotic fluid, a substance that acts as an aquatic environment safeguarding the developing embryo from physical shocks and temperature fluctuations. During gestation, the amniotic fluid primarily contains a filtrate of maternal plasma, but later contains fetal urine, as the fetal kidneys begin to function. The amniotic fluid enables the embryo to move freely, and helps it prepare for swallowing and breathing outside the uterus.
Along with the amnion, there are other important extraembryonic membranes, including the yolk sac, which provides nutrients, the allantois, which facilitates waste removal and gas exchange, and the chorion, which encases the amnion and contributes to the formation of the placenta in mammals. In egg-laying amniotes, the shell provides an additional layer of protection and permits the retention of water and nutrients but varies between species in composition and structure.
The evolution of these extraembryonic membranes marked a pivotal moment in the divergence of amniotes from amphibians, allowing them to thrive in more diverse and drier habitats by decreasing their dependence on aquatic environments for reproduction.