Ontogenesis is a process of individual development of various organisms from the beginning of existence to the very end of life. This term was proposed by E. Haeckel, a German scientist, in 1886. In the article, we briefly consider ontogenesis, its types, and their specificity in various species.
Ontogenesis of single and multicellular
In protozoa and bacteria, it almost coincides with the cell cycle. In these organisms, ontogenesis begins with the appearance of a unicellular organism by dividing the mother cell. This process ends with death occurring as a result of adverse effects, or another division.
Ontogenesis of multicellular species that reproduces asexually begins with the separation of a group of cells from the mother’s body (remember, for example, the process of hydra budding). By sharing mitosis, these cells form a new individual with all organs and systems. In species that reproduce sexually, the process of ontogenesis begins with fertilization of the egg, after which a zygote is formed, which is the first cell of a new individual.
Is ontogenesis the transformation of an organism into an adult?
We hope that you answered this question correctly, as the concept of interest to us is revealed at the beginning of the article. And the types of ontogenesis, and this process itself, as you remember, apply to the whole life of the organism. They cannot be reduced to the growth of an individual until it becomes an adult. Ontogenesis is a chain of complex processes that occur at all levels of the body. Their result is the formation of vital functions, structural features inherent in individuals of this species, and the ability to reproduce. Ontogenesis ends with processes that lead to aging, and then to death.
The following 2 main periods are distinguished in ontogenesis - embryonic and postembryonic. On the first of these, an embryo is formed in animals. He formed the basic organ systems. Next comes the postembryonic period. During it, formative processes end, then puberty occurs, then reproduction, aging and, finally, death.
Implementation of hereditary information
The new individual receives a kind of instructions with the genes of the parents, which indicate what changes will occur in the body for him to successfully pass the life path. Therefore, the process we are interested in is the realization of hereditary information. Next, we will examine in more detail ontogenesis (types and their features).
Direct and indirect ontogenesis
With a direct type, an organism that was born is basically similar to an adult, there is no stage of metamorphosis. With an indirect type, a larva appears, which differs in its internal and external structure from the adult organism. It differs in the way of movement, the nature of food, and also has a number of other features. The larva turns into an adult as a result of metamorphosis. It gives organisms great benefits. This type of development is sometimes called larval. The direct type is found in utero and in non-larval form.
Let us consider in more detail each of them.
Indirect ontogenesis: types, periods
It is observed in many species of invertebrates, as well as in some vertebrates (amphibians, snakes). In the process of development, they single out one or several larval stages. Its presence is due to rather small reserves of yolk contained in the eggs of these animals. In addition, it is explained by the need to change the habitat during development or to settle for species that lead a parasitic, sedentary or sedentary lifestyle.
Complete and incomplete development
Larvae born on their own live. They actively feed, develop and grow. They have a number of special temporary (provisional) organs that are absent in adults. The larval (indirect) type of development happens with complete or incomplete transformation. This division is carried out on the basis of the characteristics of metamorphosis, which is characterized by one or another ontogenesis. Its types require a more detailed consideration, so we will tell you more about them.
If we are talking about incomplete transformation, the larva that was born over time loses its larval organs and instead receives the constant ones that are characteristic of adult organisms (remember, for example, grasshoppers). If development is carried out with complete transformation, then the larva first becomes a motionless doll. Then an adult comes out of it, which is very different from the larva (remember the butterflies).
Why are larvae needed?
The meaning of their existence, perhaps, lies in the fact that they do not use such food as adults, due to which the food base of this species expands. You can compare, for example, the nutrition of caterpillars and butterflies (leaves and nectar, respectively) or tadpoles and frogs (zooplankton and insects). In addition, many species, being in the larval stage, are actively exploring new territories. Larvae of bivalve mollusks, for example, are capable of swimming, which cannot be said of adult individuals that are practically motionless.
Development with metamorphosis in amphibians and fish
The types of development (ontogenesis) that occur with metamorphosis are characteristic of vertebrates such as amphibians and fish. For example, a tadpole (larva) is formed from the eggs of a frog, which in its structure, habitat and lifestyle is very different from adult individuals. The tadpole has gills, tail, lateral line organ, two-chamber heart. Like fish, it has one circulation. When the larva reaches a certain level of development, its metamorphosis occurs, during which signs characteristic of an adult organism appear. This is how the tadpole eventually turns into a frog.
In amphibians, the existence of the larval stage provides the opportunity to live in a different environment, as well as use different foods. The tadpole, for example, lives in water and eats plant foods. The frog, on the other hand, feeds on animal food and leads for the most part a terrestrial lifestyle. Many insects have a similar phenomenon. A change in the habitat, and hence the lifestyle, when moving from the larval stage to the adult stage, reduces the intensity of the struggle for survival within this species.
Direct type of development
We continue to describe the main types of ontogenesis and move on to the next - direct. It is also called non-larval. It happens in utero and ovipositor. We briefly characterize these types, the stages of ontogenesis of which are significantly different from each other.
Ovipositor type
It is observed in a number of vertebrates, as well as in birds, reptiles, fish and some mammals in which eggs are rich in yolk. In this case, the embryo develops inside the egg for a long time. The main vital functions are carried out by the germinal membranes - special provisional organs.
Mammals laying eggs
There are 3 species of mammals that lay eggs, which is generally not characteristic of this class. However, in this case, the cubs are fed milk. This is characteristic of mammals in general. Platypus eggs are laid (pictured above), long-nosed and short-nosed echidna. They live in Australia, Tasmania and New Guinea and belong to the one-pass order.
These animals resemble reptiles not only by laying eggs, but also by the structure of the excretory, reproductive and digestive systems, as well as by many anatomical features (structure of the spine, ribs and shoulder girdle, eye structure). Monotremes, however, are classified as mammals, since their heart has 4 chambers, they are warm-blooded, covered in fur, and feed their young with milk. In addition, a number of structural features of their skeleton are also characteristic of mammals.
Intrauterine type
The topic "Types of ontogenesis and their characteristics" has been practically revealed by us. However, we have not yet talked about the last, intrauterine type. It is characteristic of humans and higher mammals, in whose eggs there is practically no protein. In this case, all the vital functions of the resulting embryo are realized through the mother's body. For this purpose, the placenta, a special provisional organ, develops from the tissues of the embryo and mother.
Placenta
This organ exists only during pregnancy. The placenta in a person is located in the body of the uterus more often along its posterior wall, less often - along the front. It is fully formed at about 15-16 weeks of gestation. At the 20th week, an active exchange begins to occur through the placental vessels.
The human placenta is a round flat disk. Its mass at the time of birth is about 500-600 g, thickness is 2-3 cm, and diameter is 15-18 cm. There are 2 surfaces in the placenta: fruit and maternal.
At the end of pregnancy, physiological aging of the placenta occurs . It is accompanied by the occurrence of salt deposition sites, a decrease in the exchange surface area. The process of procreation continues ontogenesis.
The types that we examined were only briefly described. We hope you find all the necessary information in this article. The definition and types of ontogenesis should be well known if you are preparing for an exam in biology.