Among any vertebrate animals, amphibians showed the most diversity in reproduction; the method of fertilization can be either internal or external. Most primitive families (Cryptobranchus and Asiatic Salamander) have external fertilization. Sperm is sprinkled into the water around the eggs. However, most salamanders transfer their sperm through a small pouch called spermatophore which is received by female cloaca chamber during courtship. Sperm can be used immediately, or can be stored in a particular gland (spermatheca) for use the next season as most species would have done. The mole salamanders (genus Ambystoma) of North America are parthenogenesis, and they use their sperm only for the development process; the sperm have no contribution role in genetics.
Other than a few exceptions, frogs have external fertilization. After the eggs are laid, the sperm are released when males clasp onto the females (amplexus). Amplexus is absent in some poisonous frogs, the eggs are fertilized after the sperm are released by the males. The male and female cloacae of certain species would clench together during sperm transfer, this insemination can be regarded as internal fertilization without an inserter. The tail of the Tailed Frog in North America is actually an extension of cloaca chamber to be inserted into female cloaca chamber during the courtship. All caecilian have internal fertilization; males would evert and invert their cloaca to be used as a hemipenis for insertion purposes.
Most amphibians are oviparous, they lay their eggs in clean water or on land. Some are viviparous where the eggs remain inside the mother and the fetus is able to access nutrients through the food stored in the yolk sac or can be withdrawn directly from the mother. The number of eggs laid by frogs varies from a single egg to 25000 eggs. The eggs salamanders lay usually do not exceed several dozen but some would lay up to 400 eggs.
There may be only one fertilized egg in the entire spawn, or be a string of eggs entirely wrapped inside a gel-like seal. The larvae have gills and can live in water if the eggs are laid in water (or close enough to a water source so that the larvae can crawl or be washed into the water), and finally metamorphose into small adults.
Amphibians utilize various locations to lay eggs, including still or moving water, muddy ditches made by the males, or holes in logs or under rocks, debris or caves, leaves floating above water, or branches and leaves filled with water. Nonetheless, each species has a preferred location. Species laying eggs on land usually do not have a larvae stage, and the young usually live independently and develop into adults directly. This phenomenon does occur in many tropical frogs and almost all terrestrial salamanders. The eggs of many frogs and salamander would be protected by one of the parents, while some species of frogs carry their eggs or tadpoles.
Some frogs are viviparous. The yolk in the fetus of viviparous frogs from Puerto Rico provides the nutrients required for development, while the tail of the fetus is small and is heavily vascularized to provide gas exchange. Certain species of African toad (genera Nectophrynoides and Nectophryne) are viviparism. The fetus of a vivaparous toad in eastern Africa is nourished by ingesting 'uterine milk,' a mucoprotein secreted by the oviduct to support its life after the yolk is depleted.
Four European species and salamanders in southwest Asia are usually viviparous. A salamander from Turkey (Mertensiella luschani) and two high-altitude Alpine salamanders (gold Alpine salamander and Lanza's Alpine salamander) usually give birth to one or two fully developed offspring. Only 1~2 young may survive from up to 30 fertilized eggs. The survived juvenile will cannibalize its siblings, and is completely metamorphosed after 2~4 years of pregnancy period before birth. Fire salamander in Europe is under the same genus. The larvae are born under water, but are fully metamorphosed for those born in the mountainous region. In addition, olm from southeastern Europe may be viviparous if the water temperature is high enough; they may give birth to two juveniles with gills. Although most female salamanders protect their eggs, but almost half of the species are viviparism. After the yolk is depleted, these large fetuses will switch to “uterine milk” and will scrap for the materials on the oviduct for food with specialized teeth. Gas exchange occurs in extremely enlarged gills and between the walls of oviduct. The gills will be absorbed and disappear before birth. The amphibians described above are currently confirmed to be viviparous.