The metamorphosis of the frog is truly one of nature's wonders. What starts out as a simple head and tail with no discernable mouth parts or internal organs can end up a creature that can live on land and in water. How this comes about is not only a study in biology, but a lesson in survival.
First Comes the Egg
Prior to mating, hormones in the female signal the ovaries to begin the process of developing eggs and then the liver to begin producing the yolk proteins. Once the eggs have developed, they are fertilised by the male as the female excretes them into the water. The jelly sac that surrounds the eggs not only protects them and makes them appear larger to predators, it also activates the sperm.
Eggs are laid in water in bunches that may contain between 2,500 and 20,000 eggs depending on the species. Most will not survive. Those that do will hatch in about 6 to 21 days depending on the species and temperature of the water. Colder water will slow the process.
Using its long tail, a newborn tadpole will wriggle to the nearest substance it can attach itself to while it feeds on the yolk remaining inside its body. At this point, it isn't much more than the head and tail---gills haven't fully developed nor does it have any feeding mechanisms in the mouth. About 7 to 10 days after it hatches, it will begin to swim around and eat algae.
By the time the tadpole is about 4 weeks old, it will have developed tiny teeth and primitive intestines. It still relies on its tail to get around at this point, swimming and even schooling with other tadpoles.
When the tadpole is 6 to 9 weeks old, tiny legs will bud just in front of the tail. When those legs have formed and smaller ones have emerged near the front, the tadpole has assumed the shape of a tiny frog. Lungs develop, the gills disappear and there are changes to the digestive tract that will help the tadpole switch from an herbivorous to a carnivorous diet. During this time, the tail begins to be absorbed into the body as food and the tadpole---now called a froglet---relies on new legs to crawl out of the water to breathe.
Complete metamorphosis can take anywhere from a few weeks to a few months, depending on the species. As an example, bullfrogs remain in the tadpole stage for up to two years.
Survival of the Luckiest
Of the thousands of eggs laid each spring, only a small number will hatch. Some won't have been fertilised. Others might dry up in the sun or break in the water. Still more will be eaten by birds or small animals. Of those that do hatch, the tadpoles will risk death from poisons such as weed killer that leaches into ponds, drought, lack of food, extreme temperature or will become prey for larger animals.