 The egg of an amphibian is typically surrounded by a transparent gelatinous covering secreted by the oviducts and containing mucoproteins and mucopolysaccharides . This capsule is permeable to water and gases , and swells considerably as it absorbs water . The ovum is at first rigidly held , but in fertilised eggs the innermost layer liquefies and allows the embryo to move freely . This also happens in salamander eggs , even when they are unfertilised . Eggs of some salamanders and frogs contain unicellular green algae . These penetrate the jelly envelope after the eggs are laid and may increase the supply of oxygen to the embryo through photosynthesis . They seem to both speed up the development of the larvae and reduce mortality . Most eggs contain the pigment melanin which raises their temperature through the absorption of light and also protects them against ultraviolet radiation . Caecilians , some plethodontid salamanders and certain frogs lay eggs underground that are unpigmented . In the wood frog ( Rana sylvatica ) , the interior of the globular egg cluster has been found to be up to 6 ° C ( 11 ° F ) warmer than its surroundings , which is an advantage in its cool northern habitat . 
