Types of Development of Animals
Different animals have evolved various methods of development. These methods can be broadly categorized into two categories
(i) Direct development
(ii) Indirect development.
Direct development
In a number of animals whose eggs have little or no yolk, such as the placental mammals, the embryos develop and grow in the womb of the mother from that they also obtain nourishment. Once, the offspring has developed to a stage in which it looks like a miniature adult, the mother gives birth to it. This young one acquires its adult size and sexual maturity after birth over a period of time via gradual growth. In animals whose eggs have a large amount of yolk such as those of the birds and reptiles, the eggs grow oviparously outside the mother's body. Young ones resembling the miniature adults hatch out. The young individual then attains adult size and sexual maturity by gradual growth over a length of time.
Indirect development
In many animals like frog Amphioxus, Herdmania, insects and several other invertebrates and vertebrates, the offspring which hatches out from the egg looks extremely different from the adult form, and is called a larva. The larva leads an independent presence for some time that varies from species to species, and then transforms into a miniature adult by the process of metamorphosis.