Direct and indirect development are terms that describe different processes of animal development. Animal development begins with a fertilised egg. The difference between direct and indirect development lies chiefly in the progression through the juvenile phase of life. The path from conception to a sexually mature adult creature is very different in these two processes.
After an egg is fertilised, the resulting cell begins to divide. This division causes cells to replicate and then to specialise in the zygote. Specialisation or differentiation of cells is caused by genes that are activated, translated or transcribed. Cells can also be differentiated due to chemicals outside the cells: alcohol, pollutants and so forth. The animal inside the egg is nourished by a rich yolk full of fats and proteins. The size of the yolk depends upon the animal's developmental type, direct or indirect.
Direct development refers to the process of development in which an animal is born in a smaller version of its adult form. There is no major transition in the form of the animal from infancy to maturity. Animals who experience direct development may have a large amount of yolk in order to nourish the young, or the young may be fed directly by the mother's body. Both these methods of nourishing the young require a great deal of energy from the mother. Therefore, the number of offspring must necessarily be small.
With indirect development, an animal's birth form is very different from the adult form. The embryo hatches from the egg in a larval form. The larva undergoes a drastic metamorphosis in order to achieve its adult stage. Animals that undergo indirect development lay numerous eggs. Because the eggs are small, they have relatively little yolk. Due to the small amount of yolk, the larva develops and hatches rapidly.
Animals Associated With Direct Development
Reptiles, birds and mammals tend to experience direct development. This means that the young of these species are hatched looking like a miniature version of their adult parents. The survival of these creatures on land is due in part to the construction of the egg from which the offspring come. The series of four extra-embryonic membranes of the egg allow for exchange of gasses, containment of waste material and protection of the foetus.
Animals Associated With Indirect Development
Some echinoderms, amphibians and insects undergo indirect development: butterflies, dragonflies, frogs and so forth. The larva or young form of these creatures often fulfil a different ecological niche than the adult animal does. Therefore, more young can exist and thrive together than would be possible for the adult form.