Metamorphosis is a remarkable biological process in which an animal undergoes a complete and dramatic physical transformation after birth or hatching. The life cycle of a frog serves as a classic and well-known example of this phenomenon. It begins with the eggs, which are typically laid in water and hatch into a larval stage known as a tadpole. The tadpole is an aquatic creature, with a fish-like body, a long tail for propulsion, and gills that allow it to breathe underwater. During this initial phase, the tadpole’s diet consists mainly of algae and other plant matter, but some tadpoles are carnivores.

As the tadpole grows, a series of profound changes begins, leading to its transformation into a frog. Hind limbs emerge first, followed by forelimbs. Internally, lungs develop, replacing the gills, which allows the animal to breathe air. The long tail gradually shortens and is absorbed into the body, providing nutrients for the final stages of growth. These changes prepare the frog for life on land, shifting its primary habitat and its diet from a plant-based one to one of insects and other small animals. This complete metamorphosis is a beautiful and complex transition that bridges 2 different worlds.

The Egg

A frog egg is a single-celled reproductive unit that is typically laid in a gelatinous mass in freshwater. Each egg, or ovum, is spherical and surrounded by protective jelly layers that swell upon contact with water, providing a cushion and helping to deter predators. The egg itself has a distinct polarity: a dark, pigmented animal pole at the top, which contains the nucleus and cytoplasm, and a lighter, yolk-rich vegetal pole at the bottom. This yolk serves as the primary source of nutrition for the developing embryo. Frog eggs are fertilized externally, and once fertilized, the zygote begins the process of cleavage, leading to the formation of a new individual.

The Embryo

The frog embryo undergoes a series of developmental stages following fertilization, starting with cleavage, where the zygote rapidly divides into smaller cells called blastomeres. This leads to the formation of a hollow ball of cells known as the blastula. The embryo then proceeds to gastrulation, a crucial process where the blastula reorganizes into a multi-layered structure with three distinct germ layers: the ectoderm, mesoderm, and endoderm. These layers are the precursors to all the frog’s tissues and organs. Subsequent development, called neurulation, involves the formation of the neural tube from the ectoderm, which will eventually become the central nervous system. This entire process transforms the single-celled zygote into a recognizable tadpole larva, ready to hatch and begin its free-swimming life.

The Tadpole

After its development period, the embryo leaves its jelly filled shell, and attaches itself to plant vegetation or a rock 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 and sometimes preys on aquatic invertebrates (and sometimes each other). 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

In around 5 weeks (give or take), 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.

The Froglet

The froglet phase is a critical and transitional stage in the life cycle of a frog, marking the final steps of metamorphosis from an aquatic tadpole to a terrestrial adult. During this time, the tadpole’s body undergoes dramatic changes. Its legs are fully developed, and its long tail begins to shrink and is gradually absorbed into the body, serving as a source of nutrients. The froglet also develops lungs, allowing it to breathe air for the first time and venture out of the water onto land. While it now resembles a miniature frog, it is still very small and often remains near the water’s edge as it completes its transformation and adapts to its new environment.

The Frog

The final adult phase marks the completion of the frog’s life cycle, a stage where the fully developed frog is capable of living on land, though it often remains near water. An adult frog possesses all the characteristics of its species, including a strong, tailless body, powerful hind legs for jumping, and well-developed lungs. Its skin is typically moist and permeable, aiding in respiration, and its diet consists of insects and other small animals. The primary purpose of this final stage is reproduction. Adult frogs, now sexually mature, will seek out mates, with females laying eggs and males fertilizing them externally. This reproductive act initiates the entire life cycle anew, beginning with the development of a new generation of eggs, which will eventually hatch into tadpoles, undergo metamorphosis, and ultimately become adult frogs themselves.

This metamorphosis description varies based on the type of frog species and its environmental conditions.