Most amphibians lay their eggs in water. Frogs’ eggs are called spawn. They are protected from predators by a thick layer of jelly. Inside this a tadpole develops. When it hatches out, it is able to swim, using its long tail, and breathes through gills. As the tadpole grows, first hind legs and then fore legs begin to grow. Lungs develop, and the young frog is able to begin to breathe with its head above water. Gradually, the tail shortens until the young frog resembles its adult parents.
In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.
Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers, and fins. They also make use of the lateral line organ. After metamorphosis, these organs become redundant and will be resorbed by controlled cell death, called apoptosis. The amount of adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.
Egg Stage: Amphibian eggs are fertilized in a number of ways. External fertilization, employed by most frogs and toads, involves a male gripping a female across her back, almost as if he is squeezing the eggs out of her. The male releases sperm over the female’s eggs as they are laid. Another method is used by salamanders, whereby the male deposits a packet of sperm onto the ground. The female then pulls it into her cloaca, a single opening for her internal organ systems. Therefore, fertilization occurs internally. By contrast, caecilians and tailed frogs use internal fertilization, just like reptiles, birds, and mammals. The male deposits sperm directly into the female’s cloaca.
Larval stage: When the egg hatches, the organism is legless, lives in water, and breathes with gills, resembling their evolutionary ancestors (Fish). During the larval stage, the amphibian slowly transforms into an adult by losing its gills and growing four legs. Once development is complete, it can live on land.
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