Page "Amphibian" Paragraph 71

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 that lay eggs underground have unpigmented eggs.

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.