Ontogenesis is the development of a specific, individual individual. This process covers the period from the moment the gametes are combined until the death of the organism. Studying all this time period is developmental biology. Ontogenesis of plants and animals proceeds unequally in their different systematic groups.
This article discusses the development of multicellular animals. Although the main stages of ontogenesis, its general laws are also characteristic of plants.
It starts from the moment when the egg is fertilized by a sperm or activated by another effect. Then it begins to fragment, that is, division into several small cells occurs. When their number reaches several hundred or even thousands, the shape of the embryo changes. The rudiments of various organs appear in it. In vertebrate embryos, the intestine, central nervous system, chord, muscles, etc. are primarily formed. The complex of these processes is called morphogenesis.
Following it, or simultaneously with it, the embryonic cells acquire persistent features that make it possible to distinguish them from each other: chemical composition, structure, shape.
As already mentioned, ontogenesis is a process that continues after the embryo is born (or hatch). It manifests itself at this stage in the form of metamorphosis (for those individuals that go through the larval stage), growth, constant renewal of cells of the intestinal wall , lymphoid organs, and blood. The aging processes are not all scientists refer to ontogenesis.
What forces drive such a complex process? There is no comprehensive answer to this question. However, some important regularities of ontogenesis are still established.
First, absolutely all cells of the embryo closely interact with each other. Each of them “knows” about exactly where it is located. For example, if you remove a small part from the whole embryo, then the remaining cells will immediately begin to develop differently, in accordance with the current situation. In this way, the defect is at least partially eliminated and further the embryo can develop normally. Perhaps the cells "communicate" with each other through contacting membranes.
Secondly, the very first embryos in the embryo already influence each other, while encouraging them to develop in a given direction. For example, in vertebrates this happens as follows. The embryo of the chord (dorsal string) affects the surface tissues of the embryo that come into contact with it and induces them to form the primordium of the central nervous system. Such an interaction in biology is called embryonic induction. Some of these influences, including the one mentioned above, are caused by substances that diffuse from one germ to another. Others are impossible without direct contacts between the cells of those primordia that interact. For example, the induction of another limb in amphibians is caused by abduction of a nerve ending or transplantation of a foreign body under the skin of an embryo.
In addition, the specialization of cells is affected by the synthesis of proteins in them, the sequence of amino acids in which is determined by the gene set. Why not all of them are active is also not exactly known.
So, ontogenesis is a process that still attracts the attention of scientists, not only biologists, but also mathematicians, chemists, physicists. They strive to create a theory of the development of a living organism that would subsequently help to make a number of other discoveries. Indeed, to explore ontogenesis is not only theorizing. This work is of great applied value, since the data obtained can help in the treatment of malignant tumors and other deviations from normal development, in managing the development of animals and plants, and in the fight against harmful environmental influences.