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Hatchling
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Most species of frogs lay their eggs in water. The eggs float in individual chambers of protective jelly-like material that forms into clumps or strings, depending on the species. After about a month in the egg, a tadpole hatches. At this point, it looks like a small black fish. It breathes through external gills.
A hatchling cannot swim but instead uses special suckers on its mouth to cling to plants or rocks. For food, it still relies on the nourishment in the yolk of its egg.
Utterly defenseless as eggs and hatchlings, the vast majority of tadpoles do not survive to the free-swimming stage. In compensation, many species of frogs will produce thousands of eggs per female, and lay them in communal masses to overwhelm predators.
Free-Swimming Tadpole
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As the hatchling continues to cling to its anchor, a tail grows. When the tail has completely developed, the tadpole will be ready to swim freely and seek out other sources of food.
As the tail grows, the oral suckers disappear, and a horny beak and toothlike rows of raised ridges form in the mouth to allow the tadpole to eat solid food. Most tadpoles eat tiny underwater plants, but some are carnivorous, and some will even eat other tadpoles.
At this stage, the tadpole usually has a relatively large round head that abruptly attaches to its long skinny tail. According to David Badger in "Frogs", the other name for a tadpole, polliwog, came from this appearance: poll is an archaic term for head, while wog meant wiggle.
Froglet
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As the tadpole continues to grow, small nubs appear that will develop into the powerful back legs. As the tail begins to shrink, absorbing back into the body, front legs appear as well. By now, the external gills have been replaced with internal gills (lungs), and the tadpole is now considered a froglet. Although it must keep its skin moist in order to breathe, at this point the froglet can live on land. The froglet becomes a frog when its tail is completely absorbed back into its body. For many species the entire metamorphosis from hatching to frog takes from about a week to a few months, but for some species this transition can take as long as two years.
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Stages of Tadpoles
Frogs are tailless, cold-blooded amphibians that live part of their life in water, and part on land. Frogs have four toes on each of their front feet, and five toes on each of their back feet. Closely related to salamanders and newts, frogs are fascinating for their variety in terms of size, the range of skin colors, the environments they inhabit, and the sounds of their calls. Perhaps most interesting of all, we can clearly observe the dramatic changes that take a frog from hatchling to free-swimming tadpole to froglet and then to the adult frog.