Frogs embryos are typically surrounded by several layers of gelatinous material. When several eggs are clumped together, they are collectively known as frogspawn. The jelly provides support and protection while allowing the passage of oxygen, carbon dioxide and ammonia. It absorbs moisture and swells on contact with water. After fertilization, the innermost portion liquifies to allow free movement of the developing embryo. Most eggs are black or dark brown and this has the advantage of absorbing warmth from the sun which the insulating capsule retains. The shape and size of the egg mass is characteristic of the species. The length of the egg stage depends on the species and the environmental conditions. Aquatic eggs normally hatch within one week when the capsule splits as a result of enzymes released by the developing larvae.
The larvae that emerge from the eggs, known as tadpoles (polliwogs), typically have oval bodies and long, vertically flattened tails. Tadpoles lack eyelids and have cartilaginous skeletons,lateral line systems, gills for respiration, and vertically flattened tails they use for swimming. From early in its development, a gill pouch covers the tadpole's gills and front legs. The lungs soon start to develop and are used as an accessory breathing organ. Tadpoles lack true teeth, but the jaws in most species have two elongated, parallel rows of small, keratinized structures called keradonts in their upper jaws. Tadpoles are typically herbivorous, feeding mostly on algae, including diatoms filtered from the water through the gills. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles, and fish.
At the end of the tadpole stage, a frog undergoes metamorphosis in which its body makes a sudden transition into the adult form. This metamorphosis typically lasts only 24 hours, and is initiated by production of the hormone thyroxine. This causes different tissues to develop in different ways. The principal changes that take place include the development of the lungs and the disappearance of the gills and gill pouch, making the front legs visible. The lower jaw transforms into the big mandible of the carnivorous adult, and the long, spiral gut of the herbivorous tadpole is replaced by the typical short gut of a predator. The nervous system becomes adapted for hearing and stereoscopic vision, and for new methods of locomotion and feeding. The eyes are repositioned higher up on the head and the eyelids and associated glands are formed. The eardrum, middle ear, and inner ear are developed. The skin becomes thicker and tougher, the lateral line system is lost, and skin glands are developed.The final stage is the disappearance of the tail, but this takes place rather later, the tissue being used to produce a spurt of growth in the limbs. Frogs are at their most vulnerable to predators when they are undergoing metamorphosis.















