Frog eggs are coated with clear jelly that keeps moisture in and helps protect them from infection. Inside each fertilized egg, a tiny embryo develops.
In a few days, larvae wriggle out of the jelly and begin a free-swimming, fishlike life as tadpoles.
Hind legs appear first, accompanied by changes in the skeleton, circulatory system, and digestive system.
Later the front legs appear. About the same time, the tadpole loses its gills and starts to breathe with its lungs.
The adult frog uses its lungs to breathe and spends much of its time on land. It will return to water to reproduce.
The word amphibian means "double life." Explain how that relates to the frog's life cycle. An amphibian spends its life in two different places, part on land and part in the water.
Scientists consider amphibians good indicators of environmental change. Based on an amphibian's life cycle, think of a reason why that might be so. Amphibians are sensitive to environmental changes on land, in water, and in the atmosphere.
