<P> Many reptiles and the vast majority of invertebrates, most fish, amphibians and all birds are oviparous, that is, they lay eggs with little or no embryonic development taking place within the mother . In aquatic organisms, fertilization is nearly always external with sperm and eggs being liberated into the water (an exception is sharks and rays, which have internal fertilization). Millions of eggs may be produced with no further parental involvement, in the expectation that a small number may survive to become mature individuals . Terrestrial invertebrates may also produce large numbers of eggs, a few of which may avoid predation and carry on the species . Some fish, reptiles and amphibians have adopted a different strategy and invest their effort in producing a small number of young at a more advanced stage which are more likely to survive to adulthood . Birds care for their young in the nest and provide for their needs after hatching and it is perhaps unsurprising that internal development does not occur in birds, given their need to fly . </P> <P> Ovoviviparity is a mode of reproduction in which embryos develop inside eggs that remain in the mother's body until they are ready to hatch . Ovoviviparous animals are similar to viviparous species in that there is internal fertilization and the young are born in an advanced state, but differ in that there is no placental connection and the unborn young are nourished by egg yolk . The mother's body provides gas exchange (respiration), but that is largely necessary for oviparous animals as well . In many sharks the eggs hatch in the oviduct within the mother's body and the embryos are nourished by the egg's yolk and fluids secreted by glands in the walls of the oviduct . The Lamniforme sharks practice oophagy, where the first embryos to hatch consume the remaining eggs and sand tiger shark pups cannibalistically consume neighbouring embryos . The requiem sharks maintain a placental link to the developing young, this practice is known as viviparity . This is more analogous to mammalian gestation than to that of other fishes . In all these cases, the young are born alive and fully functional . The majority of caecilians are ovoviviparous and give birth to already developed offspring . When the young have finished their yolk sacs they feed on nutrients secreted by cells lining the oviduct and even the cells themselves which they eat with specialist scraping teeth . The Alpine salamander (Salamandra atra) and several species of Tanzanian toad in the genus Nectophrynoides are ovoviviparous, developing through the larval stage inside the mother's oviduct and eventually emerging as fully formed juveniles . </P> <P> A more developed form of viviparity called placental viviparity is adopted by some species of scorpions and cockroaches, certain genera of sharks, snakes and velvet worms . In these, the developing embryo is nourished by some form of placental structure . The earliest known placenta was found recently in a group of extinct fishes called placoderms, which are ancestral to mammals . A fossil from Australia's Gogo Formation, laid down in the Devonian period, 380 million years ago, was found with an embryo inside it connected by an umbilical cord to a yolk sac . The find confirmed the hypothesis that a sub-group of placoderms, called ptyctodontids, fertilized their eggs internally . Some fishes that fertilize their eggs internally also give birth to live young, as seen here . This discovery moved our knowledge of live birth back 200 million years . The fossil of another genus was found with three embryos in the same position . Placoderms are a sister group of the ancestor of all living jawed fishes (Gnathostomata), including both chondrichthyians, the sharks & rays, and Osteichthyes, the bony fishes . </P> <P> Among lizards, the viviparous lizard Zootoca vivipara, slow worms and many species of skink are viviparous, giving birth to live young . Some are ovoviviparous but others such as members of the genera Tiliqua and Corucia, give birth to live young that develop internally, deriving their nourishment from a mammal - like placenta attached to the inside of the mother's uterus . In a recently described example, an African species, Trachylepis ivensi, has developed a purely reptilian placenta directly comparable in structure and function to a mammalian placenta . Vivipary is rare in snakes, but boas and vipers are viviparous, giving birth to live young . </P>

How animals give birth to their young ones