Wisconsin Sea Grant Frogs: A Frog's Life - Metmorphosis

Toads and frogs are amphibians, and the word "amphibian" comes from Greek words meaning "double life." This refers to the fact that during a frog's life, it lives both in the water and on land -- one of the reasons we find amphibians so fascinating. The amazing change, or "metamorphosis" -- that happens in a frog's life follows this pattern:

1. In the spring, male frogs and toads move to watery breeding sites and start calling to attract females and, in some cases, to warn other males to keep away from their territories.

2. Once a male and female pair up, the male clasps the female in a piggyback position called amplexus, releasing his sperm as she releases her eggs. The eggs are fertilized outside the female's body.

3. This mass of soft, jelly-coated eggs, often numbering in the hundreds or thousands, often sticks to water plants or other vegetation. The eggs hatch into tiny fish-like tadpoles that have gills, like fish, to allow them to breathe while in the water.

The cell division in an egg:

1) The fertilized egg at the single cell stage.
2) Within an hour or two after being laid, the first cleavage (cell division) occurs. The one cell is separated into two embryo cells. It's thought that these two cells become the right and left sides of the tadpole.
3) Cleavage continues, with the two cells splitting into four.
4) And the four cells into eight (not pictured) and then 16. The embryo at the 16-cell stage is called the "morula" and looks somewhat like a raspberry.
5) More cell division occurs. At this stage, the embryo is called the "blastula."
6) At about 100 hours, the tadpole begins to take shape.
7) The tadpole's features are more discernible. The tail begins to move from side to side.
8) Tadpole hatching.
9) Soft, jelly-coated frog eggs.

4. The tadpole usually grows quickly, swimming around and eating algae or tiny organisms in the water.

5. Legs sprout from the tadpole's body, and the tadpole's tail becomes smaller, actually being absorbed into the body. The tadpole also develops lungs to allow it to breathe out of water. Its intestines change from a long coiled gut to a short gut, to accommodate the change in diet from a grazing tadpole to a meat-eating frog. It is thought during metamorphosis that the immune system is largely shut down to accommodate all the physical changes. Tadpoles are especially susceptible to disease and parasite attacks at this time.

6. Many tadpoles are eaten by fish, birds or other frogs -- but the ones that survive these early stages continue their transformation into adult frogs that can move out of the water and live on land.

7. In the spring, adult frogs move into the water again to mate and the cycle starts all over again.

copyright 2001 University of Wisconsin Sea Grant Institute