359-299 million years ago


embryo with yolk sac

Amphibians, although terrestrial, are challenged by terrestrial environments because they must undergo a larval stage in water (although some complete this stage in eggs laid in a very moist environment). The first amniotes were fully adapted to terrestrial life because they did not require water for development and could lay their eggs on land. Before a developing amniote embryo forms a brain or heart, kidneys or intestines, it wraps itself in a number of membranes. These extraembryonic membranes are made of cells from the embryo but will not form any part of the infant at birth. In the following models, note that the relative size of the embryo and these embryonic membranes early in development. There are 4 extraembryonic membranes of amniotes, membranes which surround the embryo but do not form part of the future individual. The yolk sac provides nourishment for the developing embryo in the egg (it still present in placental mammals despite the presence of the placenta that is the source of nourishment).

embryo with chorion

The chorion is the layer inside the egg shell that performs gas exchange (and forms the fetal part of the placenta in placental mammals).

embryo with amnion

The amnion is filled with a fluid that absorbs shock and prevents temperature change. This fluid has about the same concentration of salts as seawater. Interestingly, even the embryos of terrestrial embryos develop in water similar to the ancient seas. The allantois collects the wastes which accumulate during embryonic development in the egg (wastes of amphibian embryos can dissolve into the surrounding water).