Lifecycle of the Frog
The egg begins as a single cell. There may be thousands of eggs laid at once. It becomes surrounded by a jellylike covering, which protects the egg. The female may stay with the eggs to take care of the young after she had laid them.
The mass of cells in the egg come to form an embryo. Organs and gills begin to form, and the embryo lives off the internal yolk. This supplies it with nutrients for approximately 21 days, give or take.
After its development period, the embryo leaves its jelly filled shell, and attaches itself to a weed in the water. This quickly becomes a tadpole. The tadpole has a long tail, and lives in the water. It has external gills like a fish to absorb oxygen and feeds on algae. It is extremely vulnerable, and must rely on its camouflage to protect it. Colors range from brown, green or gray.
The Tadpole Begins to Change
About five weeks, the tadpole begins to change. It start to grow hind legs, which are soon followed with forelegs. Lungs begin to develop, preparing the frog for its life on land.
Over time, the tadpole becomes even more like a frog. Its mouth widens, it loses its horny jaws, gills are lost. The tail becomes much smaller, and the legs grow. The lungs are almost functioning at this point.
Eleven weeks +/- after the egg was laid, a fully developed frog with lungs, legs, and no tail emerges from the water. This frog will live mostly on land, with occasional swims. Eventually, it finds a mate. The way this is done varies depending on the species. The female lays the eggs, the male fertilizes them, and the whole process begins again.
This metamorphosis chart varies based on the type of frog/toad species and its environment.