In animals, three types of ontogenesis are distinguished: larval, ovipositor and intrauterine.
The larval type of development is found, for example, in insects, fish, and amphibians. The yolk in their eggs is small, and the zygote quickly develops into a larva, which independently eats and grows. Then, after some time, metamorphosis occurs – the transformation of the larva into an adult. In some species, there is even a whole chain of transformations from one larva to another and only then – into an adult. The meaning of the existence of larvae may lie in the fact that they feed on other foods than adults, and thus the food base of the species expands. Compare, for example, the nutrition of caterpillars (leaves) and butterflies (nectar) or tadpoles (zooplankton) and frogs (insects). In addition, in the larval stage, many species actively inhabit new territories. For example, larvae of bivalves are capable of swimming, while adults are practically motionless.
The egg-laying type of ontogenesis is observed in reptiles, birds, and oviparous mammals, whose eggs are rich in yolk. The embryo of such species develops inside the egg; the larval stage is absent.
The intrauterine type of ontogenesis is observed in most mammals, including humans. At the same time, the developing embryo is delayed in the mother’s body, a temporary organ is formed – the placenta, through which the mother’s body provides all the needs of the growing embryo: respiration, nutrition, excretion, etc. Intrauterine development ends in the process of procreation.