Life Cycle

The Natural Thyroid Diet

The Natural Thyroid Diet

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Within a few days of fertilization, the eggs hatch into tadpoles. A newly hatched tadpole lives off yolk stored in its body. It gradually grows larger and develops three pairs of gills. Eventually, the tadpole's mouth opens, allowing it to feed. The tadpole grows and slowly changes from an aquatic larva into an adult. This process of change is called metamorphosis. Legs grow from the body, and the tail and gills disappear. The mouth broadens, developing teeth and jaws, and the lungs become functional.

Biologists have long studied the process of metamorphosis and regeneration to learn what controls such dramatic physical changes. A hormone called thyroxine is produced by the thyroid gland and circulates throughout the bloodstream to stimulate metamorphosis.

The life cycles of many amphibians are similar to that of the frog shown in Figure 40-14. But there are a variety of alternative reproductive patterns among amphibians. For example, many amphibians do not lay their eggs in water. They select a moist place on land, such as under a rock, inside a rotting log, or in a tree. One or both parents may even construct a nest for the eggs. A number of frog species make a nest of mucus, whipping it into a froth by kicking their hind legs. And not all amphibians undergo metamorphosis. Some salamanders, such as the axolotl in Figure 40-15, remain in the larval stage for their entire life. The axolotl's thyroid does not produce thyroxine. Other amphibians bypass the free-living larval stage and hatch from the egg as a small version of the adult.

figure 40-14

The life cycle of a frog begins with mating. When the eggs hatch, a tadpole is released. One of the first developments of metamorphosis is the growth of hind legs. When the tadpole completes metamorphosis, a small adult emerges from the water onto the land.

figure 40-15

The axolotl does not produce thyroxine, and as a result, does not undergo metamorphosis. It retains its gills and lives a completely aquatic life.

The axolotl does not produce thyroxine, and as a result, does not undergo metamorphosis. It retains its gills and lives a completely aquatic life.

Rhinoderma darwiniimale frogs exhibit parental care by holding the maturing eggs and larvae in their vocal sacs. This frog has already released his offspring, whose tails are still visible. Not all frogs express parental care in this way, and some frogs express no parental behavior at all.

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