In animals, three types of ontogenesis are distinguished: larval, ovipositor, and intrauterine.
The larval type of development is common in insects, amphibians and some other animals. The eggs of these animals contain little yolk; larvae emerge from their eggs.
The larva actively feeds and increases in size. The next stage of development in the larval type of ontogenesis: metamorphosis – transformation of the larva into an adult organism.
The larval stage provides animals with the ability to live in different environments and use different foods. For example, caterpillars feed on leaves, and butterflies feed on nectar; tadpoles feed on zooplankton, and frogs feed on insects.
In sedentary, attached and parasitic species, free-living larvae contribute to the dispersal of the species and the expansion of the range. This avoids overpopulation and weakens intraspecific competition for food and other resources.
The ovipositor type of individual development is found in reptiles, birds and first animals (platypus and echidna). Their egg cell contains a large supply of nutrients, the embryo develops in the egg. Such animals do not have a larval stage.
The intrauterine type of ontogenesis is observed in most mammals, including humans. In this case, the development of the embryo takes place in the maternal organism. The mother’s body provides all the needs of the growing embryo (respiration, nutrition, excretion) through a special organ – the placenta. Intrauterine development ends with the birth process.