After the body was born, its embryonic development begins, which can last from 1-2 days to several hundred years - it all depends on the species. It follows that the duration of life is a species sign of all organisms, regardless of the level of organization of those. Postembryonic ontogenesis 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
Postembryonic development in a direct form is characteristic of mammals, reptiles, birds, some insects, and, of course, humans. In the development of the latter, such periods are noted:
- childhood;
- adolescence ;
- youth;
- 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 senile period there are many physiological and morphological processes that lead to a decrease in vitality and body resistance to the negative effects of external and internal factors. Unfortunately, such mechanisms have not yet been fully studied; therefore, they cannot be prevented by artificial methods.
Death completes not only postembryonic development, but also the individual existence of the organism. It can have a physiological nature, that is, occur due to aging, and also occur as a result of pathological changes, which often result in various diseases or injuries.
Features of indirect development
Indirect postembryonic development occurs exclusively in multicellular animals and is characterized by the appearance of a larva from an egg - an embryo, which is significantly different in structure from adult individuals, although it is already 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 embryo has special internal organs, which allows it to lead a lifestyle different from that of adults of the same species. However, at the same time, the larva has completely no rudimentary sexual characteristics; therefore, at this stage it is impossible to determine whether it will turn into a male or a female.
Indirect postembryonic development implies deep transformations of the body that occur throughout the entire period. In animals, such processes affect not only certain parts of the body, but the whole organism. Over time, the organs of the larva disappear, and in their place appear organs 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 goes through such stages: an egg, a larva, an imago, and in the second, the larva transforms into a full-fledged adult individual through the pupal stage.