The female usually lays eggs in moist areas or water, as the clasping male fertilizes them externally during amplexus . The egg does not have a protective shell. An aquatic tadpole with a tail and gills swims until thyroid hormone control leads to metamorphosis into an adult. Given their dependence on moist habitats for their skin and their reproduction, it is not surprising that amphibians are not currently dominant land organisms. In contrast to the amphibians, the reptiles are able to live away from water.
Keratin is a protective epidermal protein.
Chromatophores are present in the skin, but not to the dramatic extent as amphibians. The yolk or nutrient of the egg is enclosed in a yolk sac , and other membranes form an inner amnion and outer chorion . The amnion is protective around the embryo and the chorion becomes hightly vascular and aids in gas exchange. Waste products are stored as nontoxic uric acid in the allantois , a ventral outgrowth from the GI tract that will fill up space between the amnion and the chorion, supplying many blood vessels to help oxygenate the embryo. But an eggshell needs pores for this gas exchange, which can lead to water loss.