After the body was born, it begins its postembryonic development, which can last from 1-2 days to several hundred years - it all depends on species. It follows that the life span is a species characteristic of all organisms, regardless of the level of organization of such organisms. Postembryonic ontogeny consists of such periods: juvenile, puberty and senile, which ends in death. All multicellular organisms are subject to a direct or indirect type of development.
Principles of direct development of
Post-embryonic development in direct form is characteristic of mammals, reptiles, birds, some insects and, of course, humans. In the development of the latter, the following periods are noted:
- childhood;
- adolescence;
- youth;
- the phase of youth;
- stage of maturity;
- old age.
Each such period is accompanied by certain changes, which ultimately lead to aging and death of the body. It should be noted that in the old age there is a lot of physiological and morphological processes that lead to a decrease in the vitality and stability of the organism to the negative impact of external and internal factors. Such mechanisms, unfortunately, have not yet been fully understood, so they can not be prevented by artificial means.
Death completes not only post-embryonic development, but also the individual existence of the organism. It can be of a physiological nature, that is, it may occur due to aging, but also as a result of pathological changes that often result in various diseases or traumas.
Indirect development of
Indirect postembryonic development occurs exclusively in multicellular animals and is characterized by the appearance of a larva from an egg-embryo, which differs in its structure from adult individuals, although it is able to feed on its own. Outwardly, the larva, of course, may have a distant similarity with its ancestors, but its structure is much simpler, and the dimensions are much smaller. The fetus has special internal organs, which allows it to lead a way of life different from that of adult individuals of the same species. However, in this case, the larva completely lacks rudimentary sexual characteristics, so at this stage it is impossible to determine in whom it will turn - into a male or female.
Indirect postembryonic development implies deep changes in the body that occur throughout the period. In animals, such processes affect not only certain parts of the body, but the entire body as a whole. Over time, the organs of the larvae disappear, and in their place organs appear that are characteristic of adult animals. Postembryonic development of animals can be of two types: incomplete and complete metamorphosis. In the first case, the insect passes through such stages: the egg, the larva, the imago, and in the second - the transformation of the larva into a full-grown adult occurs through the pupal stage.