What is the individual development of an organism. Individual development of organisms - ontogenesis. Stages of individual development

The individual development of organisms or ontogenesis is a long and complex process of the formation of organisms from the moment of formation of germ cells and fertilization (with sexual reproduction) or individual groups of cells (with asexuality) until the end of life.

From the Greek "ontos" - being and genesis - occurrence. Ontogenesis is a chain of strictly defined complex processes at all levels of the organism, as a result of which structural features, life processes, and the ability to reproduce inherent only to individuals of a given species are formed. Ontogenesis ends with processes that naturally lead to aging and death.

With the genes of the parents, the new individual receives a kind of instructions about when and what changes should occur in the body so that it can successfully go through the entire life path. Thus, ontogeny is the realization of hereditary information.

Historical reference

The process of the emergence and development of living organisms has been of interest to people for a long time, but embryological knowledge has been accumulating gradually and slowly. The great Aristotle, observing the development of the chicken, suggested that the embryo is formed as a result of the mixing of fluids belonging to both parents. This opinion lasted for 200 years. In the 17th century, the English physician and biologist W. Harvey made some experiments to test Aristotle's theory. As court physician to Charles I, Harvey received permission to use the deer that live in the royal estates for experiments. Harvey studied 12 female deer that died at different times after mating.

The first embryo, removed from a female deer a few weeks after mating, was very small and did not at all look like an adult animal. The deer that died at a later date had larger embryos, they had a great resemblance to small, newly born fawns. This is how the knowledge of embryology was accumulated.

The following scientists have made significant contributions to embryology.

· Anthony van Leeuwenhoek (1632-1723) discovered spermatozoa in 1677, he was the first to study parthenogenesis in aphids.

· Jan Swammerdam (1637-1680) was the first to study the metamorphosis of insects.

· Marcello Malpighi (1628-1694) was the author of the first studies on the microscopic anatomy of the development of the organs of the embryo of a chicken.

· Kaspar Wolf (1734–1794) is considered the founder of modern embryology; more precisely and in more detail than all his predecessors, he studied the development of the chick in the egg.

· The real creator of embryology as a science is the Russian scientist Karl Baer (1792–1876), a native of the Estland province. He was the first to prove that during the development of all vertebrates, the embryo is first formed from two primary cell layers, or layers. Baer saw, described, and then demonstrated at the congress of naturalists the egg cell of mammals from the dog he had opened. He discovered a way of developing the axial skeleton in vertebrates (from the so-called dorsal chord). Baer was the first to establish that the development of any animal is a process of the unfolding of something preceding, or, as they would now say, the gradual differentiation of more and more complex formations from simpler primordia (the law of differentiation). Finally, Baer was the first to appreciate the importance of the importance of embryology as a science and laid it as the basis for the classification of the animal kingdom.

A.O. Kovalevsky (1840-1901) is known for his famous work "The history of the development of the lancelet." Of particular interest are his works on the development of ascidians, ctenophores and holothurians, on the postembryonic development of insects, etc. Studying the development of the lancelet and extending the data obtained to vertebrates, Kovalevsky once again confirmed the correctness of the idea of the unity of development in the entire animal kingdom.

I.I. Mechnikov (1845–1916) gained particular fame for his research on sponges and jellyfish, ie. lower multicellular. A prominent idea of Mechnikov was his theory of the origin of multicellular organisms.

A.N. Severtsov (1866-1936) is the largest of modern embryologists and comparative anatomists, the creator of the theory of phylembryogenesis.

Individual development of unicellular organisms

In the simplest organisms, the body of which consists of one cell, ontogeny coincides with the cell cycle, i.e. from the moment of appearance, by division of the mother cell, until the next division or death.

Ontogenesis of unicellular organisms consists of two periods:

- maturation (synthesis of cell structures, growth).

- maturity (preparation for division).

- the fission process itself.

Ontogenesis in multicellular organisms is much more complicated.

For example, in various parts of the plant kingdom, ontogeny is represented by complex developmental cycles with a change in sexual and asexual generations.

Ontogenesis in multicellular animals is also a very complex process and much more interesting than in plants.

In animals, three types of ontogenesis are distinguished: larval, ovipositor, and intrauterine. The larval type of development is found, for example, in insects, fish, and amphibians. There is little yolk in their eggs, and the zygote quickly develops into a larva, which feeds and grows on its own. Then, after some time, metamorphosis occurs - the transformation of the larva into an adult. In some species, there is even a whole chain of transformations from one larva to another and only then into an adult. The meaning of the existence of larvae may lie in the fact that they eat a different food than adults, and, thus, the food base of the species expands. Compare, for example, feeding caterpillars (leaves) and butterflies (nectar), or tadpoles (zooplankton) and frogs (insects). In addition, many species actively colonize new territories during the larval stage. For example, the larvae of bivalve molluscs are capable of swimming, while the adults are practically immobile. The ovipositor type of ontogeny is observed in reptiles, birds and oviparous mammals, the eggs of which are rich in yolk. The embryo of such species develops inside the egg; there is no larval stage. The intrauterine type of ontogenesis is observed in most mammals, including humans. In this case, the developing embryo is retained in the mother's body, a temporary organ is formed - the placenta, through which the mother's body provides all the needs of the growing embryo: respiration, nutrition, excretion, etc. Intrauterine development ends in the process of childbirth.

Direct development , in which an individual emerges from the mother's body or egg membranes, which differs from the adult only in its smaller size (birds, mammals). Distinguish between: non-larval (ovipositor) type, in which the embryo develops inside the egg (fish, bird), and intrauterine type, in which the embryo develops inside the mother's body - and is associated with it through the placenta (placental mammals).

Ontogenesis - the individual development of an organism

1. What is ontogeny?
2. What is the set in the zingot?

Ontogenesis.

The process of individual development of an individual from the beginning of its existence to the end of life is called ontogenesis. Have bacteria and the simplest ontogenesis practically coincides with cell cycle and begins with the emergence of a unicellular organism as a result of the division of the maternal, and ends with the next division of this organism or death from adverse influences.

In multicellular species that reproduce asexually, ontogenesis begins with the isolation of a group of cells of the mother's body (remember, for example, hydra budding), which, sharing mitosis, form a new individual with all its systems and organs.

In those species that reproduce sexually, ontogenesis begins from the moment fertilization ovum and the formation of a zygote - the first cell of a new organism.

Ontogeny is not just the growth of a small individual until it turns into a large one. This is a chain of strictly defined complex processes at all levels of the organism, as a result of which the structural features, life processes, and the ability to reproduce inherent only to individuals of this type are formed. Ontogenesis ends with processes that naturally lead to aging and death.

With the genes of the parents, the new individual receives a kind of instructions about when and what changes must occur in the body so that it can successfully go through the entire life path. Thus, ontogeny is the realization of hereditary information.

Types of ontogenesis.

In animals, three types of ontogenesis are distinguished: larval, ovipositor and intrauterine development.

The larval type of development is found, for example, in insects, fish, and amphibians. There is little yolk in their eggs, and the zygote quickly develops into a larva, which feeds and grows on its own. Then, after some time, metamorphosis occurs - the transformation of the larva into an adult (Fig. 54). In some species, there is even a whole chain of transformations from one larva to another and only then into an adult.

The meaning of the existence of larvae may lie in the fact that they eat a different food than adults, and, thus, the food base of the species expands. Compare, for example, the nutrition of caterpillars (leaves) and butterflies (nectar) or tadpoles (zooplankton) and frogs (insects). In addition, in the larval stage, many species actively colonize new territories. For example, the larvae of bivalve molluscs are capable of swimming, while the adults are practically immobile.

The ovipositor type of ontogeny is observed in reptiles, birds and oviparous mammals, the eggs of which are rich in yolk. The embryo of such species develops inside the egg; no larval stage.

The intrauterine type of ontogenesis is observed in most mammals, including humans. In this case, the developing embryo is retained in the mother's body, a temporary organ is formed - the placenta, through which the mother's body provides all the needs of the growing embryo: respiration, nutrition, excretion, etc. Intrauterine development ends in the process of childbirth.

Periods of ontogenesis.

Any type of ontogenesis in multicellular animals is usually divided into two periods: embryonic and postembryonic.

The embryonic period begins with fertilization and is the formation of a complex multicellular organism in which all organ systems are represented. This period ends with the release of the larva from its membranes (with the larval type), the release of the individual from the egg (with the ovipositor type) or the birth of an individual (with the intrauterine type of ontogenesis).

The postembryonic period begins with the completion of the embryonic period. It includes puberty, adulthood, aging and ends in death.

The periods and terms of ontogenesis are very different for different groups of living organisms. For example, in very many vertebrates, the individual is in an adult state for most of its existence. In contrast, in many insects, the adult stage is the shortest and lasts only a few hours, necessary for the reproduction of offspring. There are great differences in the life cycles of animals, plants and fungi.
Ontogenesis. Types of ontogenesis. Metamorphosis. Placenta.

1. What is the difference between ontogeny of unicellular organisms and ontogeny of multicellular organisms?
2. What types of ontogenesis are distinguished in animals? What are their features?
3. How does the embryonic period of embryogenesis in a crocodile end?
4. What are the functions of the placenta?

The ability of some animals to reproduce sexually in the early stages of ontogenesis, for example, in the larval stage, is called neoteny. Neotenia is characteristic, for example, of an amphibian animal - the Mexican ambistoma, which under natural conditions can remain in a larval state all its life. The larva lives in the water, where it reproduces. This larva is called an axolotl, and it turns into an ambistome under the action of the thyroid hormone.

Kamensky A.A., Kriksunov E.V., Pasechnik V.V. Biology Grade 10
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Remember!

What are the periods of the individual development of the organism?

What is development with metamorphosis?

The indirect, or larval, type of development is characteristic of many invertebrates and some vertebrates (fish and amphibians). This presupposes the birth of an individual, sometimes completely unlike an adult organism. In the process of indirect development, an individual passes through one or several larval stages (a tadpole in a frog, a caterpillar in a butterfly). The larvae lead an independent life, actively feed, grow and develop. After a certain time, the larva turns into an adult - metamorphosis occurs, therefore, sometimes this type of development is called development with metamorphosis. During metamorphosis, larval organs are destroyed and organs inherent in adult animals arise. For many species, the presence of a larval stage in the development process is the possibility of dispersal and the absence of competition between individuals of different ages for habitat and food.

What organisms are characterized by this type of development?

Fish, amphibians, some insects

Review questions and assignments

1. What is called the individual development of an organism? Give a definition of this concept.

The individual development of an individual, the entire set of its transformations from emergence to the end of life is called ontogenesis.

2. List the periods of ontogenesis.

In organisms that reproduce sexually, individual development begins from the moment of fertilization and the formation of a zygote and is divided into two periods: embryonic (the period of embryonic development) and postembryonic (the period of post-embryonic development). The ratio of the duration of these periods in organisms of different species can be very different.

3. What development is called embryonic and what is postembryonic?

Embryonic period (embryogenesis). This period lasts from the moment of the formation of the zygote until the release of the embryo from the egg or birth. It proceeds in several stages. Postembryonic period. This period begins from the moment the organism is born and ends with its death. Distinguish between indirect and direct types of postembryonic development.

4. What are the types of postembryonic development of the organism? Give examples.

5. What is the biological significance of metamorphosis?

After a certain time, the larva turns into an adult - metamorphosis occurs, therefore, sometimes this type of development is called development with metamorphosis. During metamorphosis, larval organs are destroyed and organs inherent in adult animals arise. For many species, the presence of a larval stage in the development process is the possibility of dispersal and the absence of competition between individuals of different ages for habitat and food, since larvae and adults can live in different environments and have a different type of nutrition.

Embryonic layers (embryonic layers, lat.folia embryonalia) are the layers of the body of the embryo of multicellular animals that are formed during gastrulation and give rise to various organs and tissues.

7. What is cell differentiation? What do you think can cause the disruption of cell differentiation during development?

Differentiation is the process of transformation of non-specialized germ cells into various cells of the body, differing in structure and performing certain functions. Differentiation does not begin immediately, but at a certain stage of development and is carried out through the interaction of germ layers (at an early stage) and organ rudiments (at a later stage). Differentiation of embryonic cells does not arise immediately, but at a certain stage of development. In the early stages of cleavage, the cells of the embryo are not yet specialized, so each of them can give rise to a whole organism. If, for any reason, these cells separate, two identical embryos are formed, containing identical genetic information, each of which develops into a full-fledged individual. As a result, identical, or monozygotic, twins are born. In the human population, these are the only people with an identical genotype and are copies of each other. Yes, a violation of differentiation can occur at any stage of embryogenesis, for various reasons.

8. Describe the concept of "growth". What is a certain height; indefinite growth? What type of growth is typical for plant organisms?

Growth - an increase in the mass and linear dimensions of an individual due to an increase in the number and size of cells and non-cellular formations. With the type of growth, which is called a certain, the body, having reached a certain level of maturity, ceases to grow in size. Most animals, growing up, grow more and more slowly and, having reached a certain age, stop growing. This type of growth is called definite. This type of growth is typical for most animals. If an organism grows all its life, then one speaks of an indefinite type of growth. It is characteristic of plants, fish, molluscs, amphibians. With an indeterminate type of growth, organisms grow throughout their life, such as mollusks, fish, and amphibians. After the completion of active growth, the body enters the stage of maturity, which is associated with childbirth. The process of individual development ends with aging and death. Plant organisms are characterized by unlimited growth - an indefinite type.

Think! Remember!

1. Why are different tissues and organs formed from germ cells of equal value at the beginning of development?

Differentiation does not begin immediately, but at a certain stage of development and is carried out through the interaction of germ layers (at an early stage) and organ rudiments (at a later stage). Differentiation of embryonic cells does not arise immediately, but at a certain stage of development. In the early stages of cleavage, the cells of the embryo are not yet specialized, so each of them can give rise to a whole organism. If, for any reason, these cells separate, two identical embryos are formed, containing identical genetic information, each of which develops into a full-fledged individual.

2. Do you know the types of organisms whose period of embryonic development is longer than postembryonic? If so, please explain what these features are related to.

3. How do you understand the following statement: "Derivatives of several germ layers take part in the construction of any organ or part of the body"? Give examples to prove this statement.

An organism or an organ cannot be built from one leaf; all three leaves are needed for this, since each of them is responsible for the formation of specific tissues. For example, the epithelium of the skin is formed by the ectoderm, and the intestinal epithelium by the endoderm.

4. Remember from previous courses in biology, what is the difference between the development of oviparous mammals (subclass of the First Beast) and representatives of the infraclass Lower animals (order Marsupials) from the development of other mammals (infraclass Higher animals).

In the lesson, we go-to-roam about how the or-ga-niz-we-are-unicellular and multicellular, we will consider their individual development - ontogenesis, we will learn the important stages in the life of multicellular organisms.

The new cell does not have enough cell structures and all the proteins are not formed for its normal life. but-sti. Therefore, the cell cycle can be divided into several stages or phases (Fig. 2).

Rice. 2. Stages of development of a unicellular organism ()

The first stage is the stage of maturation. When the necessary cellular structures are formed, the cell enters the next phase - maturity. In this phase, the cell completes all the functions it needs. Maturity for-kan-chi-va-is-Xia new de-le-ni or death of the cell.

With many-kle-to-us-ga-niz-ma-mi-si-tu-a-tion pro-out-goes-dit is much more difficult. In the life of such or-ga-niz-mo, two important stages can be identified (Fig. 3).

Rice. 3. Ontogenesis ()

The first per-ri-od - um-brie-o-nal-ny, in the mil-co-pi-ta-yu-um-bri-o-nal-nal-ny-nal-ny-nal-nal-ny-ri-o-oo -dit inside ma-terin-sko-ga-niz-ma (in-uterine development). The second per-ri-od na-chi-na-et-sya with the mo-men-that birth-de-nia or exit from the eggs-tse-lo-check - in-st- em-brie-o-nal-development.

Em-bri-o-nal-ny per-od includes 3 main stages:

1 fractional stage(Fig. 4): pro-out-goes the division of the cell into the add-on - bla-sto-me-ry.

Rice. 4. Stage of crushing ()

In just 4 hours from one cell for-mi-ru-is-Xia 64 bla-sto-me-ra, but their growth does not come out. Za-kan-chi-va-et-sya stages of fraction-lening for-mi-ro-va-ni-em bla-stu-ly (za-ro-dy-she-vy pu-zy-rek ). It consists of one layer of cells with a cavity inside;

2 stage ha-stru-la-tsion(Fig. 5) - the formation of the ro-dy-she-leaves.

In more primitive many-cell-exact-or-ha-niz-mov, for example, ki-shech-but-hollow, for-mi-ru-e - there are only two leaves: outside - ek-to-der-ma - and inside - en-to-der-ma. The more you-so-ko-or-ha-ni-zo-van-zo-zo-zo-in for-mi-ru-e-xia has a third for-ro-dy-she-yi-stock - me -zo-der-ma (between ek-to-der-my and en-to-der-my).

Rice. 5. Gastrulation ()

3 stage - or-ha-no-ge-nez(Fig. 6) - this is a period of inter-and-mo-action for-ro-d-she-she-leaves, of which all are or-ga-ny and fabrics or-ga-niz-ma.

Rice. 6. Organogenesis ()

In man, the first to isolate the head-brain, this goes out in the third week after part-time. The size of the em-bri-o-na at this moment is only 2 mil-li-meters (Fig. 7).

Rice. 7. Organogenesis, human embryo ()

Ek-to-der-ma gives na-cha-lo with skin-cuts, as well as epi-te-li-al-tk-yum (hair-lo-sy, zhe-le-zy external her secretions, nails), a nervous system develops from the ectoderm. Me-zo-der-ma gives na-cha-lo the main inner-nim or-ga-nam - you-de-li-tel-noy and in-lo-voy system-me. En-to-der-ma ob-ra-zu-et or-ga-ny pi-shche-va-ri-tel-noy, d-ha-tel-noy-system-system, as well as Threat of internal secret-reaction.

Already from the first days of its development, for-breathing or-ga-niz-ma feeling-te-len to the influence of damaging factors moat. To such factors, there are various chi-mi-ch-stuff: al-ko-gol, ni-ko-tin, le-kar-est- nye means, salts of heavy metals and nar-ko-ti-pre-para-para-you. Radiation and various infections are very dangerous for the development of a living organism.

The influence on the or-ga-nism of the mother of these factors can lead to the fact that the further development of the fetus will not pass and will come to death, or in the ro-div-she-go, the re-ben-ka will manifest because of the change, which biologists have Shy-va-yut ugliness.

After birth, de-nia, na-stu-pa-et, next-du-yu-shi-per-ri-de-development of life-in-go, or-ga-niz-ma - in st-um-brie -o-nal-ny (Fig. 8).

Rice. 8. Postembryonic development ()

Direct-my-development- development without interruption, with a gradual growth (Fig. 9).

Rice. 9. Direct development ()

An individual is like a ro-di-tel-skiy or-ga-nizm. Direct development harak-ter-no for fish, pre-wash-out, birds and babies.

Indirect development(with met-mor-pho-zom) - the process of converting the or-ga-niz-ma at the li-chi-night stage into an adult individual (Fig. 10 ).

Rice. 10. Indirect development ()

It co-lead-yes-em-sya ana-to-mi-che-ski-mi and fi-zio-lo-gi-che-ski-mi pe-re-build-ka-mi or-ha- bottom-ma. This is a way of developing ha-rak-te-ren for terrestrial-water-and-on-earth.

Raz-li-cha-yut full me-ta-morphoz and incomplete met-ta-morphosis... With full me-ta-mor-pho-ze or-ga-nizm, a number of stages go through, sharply from each other in a way of life and ha-rak-te-rum pi-ta-niya (fig. 11).

Rice. 11. Complete metamorphosis ()

These are the stages of an egg, li-chin-ki, ku-kol-ki, an adult individual (imago). Such a development is ha-rak-ter-no for ba-bo-chek (four-wing-wing) and beetles (hard-wing-wing).

With incomplete met-ta-mor-pho-ze (Fig. 12), the stages of ku-kol-ki are from-day-to-no, but if-chin-ka is not enough from an adult. This can be found at Kuz-no-chi-kov and sa-ran-chi.

Rice. 12. Incomplete metamorphosis ()

Independence-vi-si-mo- from the type of development in all living or-ga-niz-mov you-de-la-yut three stages: youth, maturity and old-age ... Each of the stages ha-rak-te-ri-zu-is-Xia determined-de-len-mi-zio-lo-gi-che-mi from-me-not-no- i-mi.

Ying-di-vi-doo-al-development-development is one of the most interesting processes that go into living or-ga-niz-me, when from a single cell emerges a complex living or-ga-nizm and in the process of life-de-i- tel-ness pre-ter-pe-va-et a number of-me-niy. Each or-ga-nizm uses its main function - to leave offspring, the life of the or-ga-niz-ma za-kan-chi-va-et-sya it natural death.

Bibliography

Mamontov S.G., Zakharov V.B., Agafonova I.B., Sonin N.I. Biology. General patterns. - Bustard, 2009.
Ponomareva I.N., Kornilova O.A., Chernova N.M. Fundamentals of General Biology. Grade 9: Textbook for 9th grade students of educational institutions / Ed. prof. I.N. Ponomareva. - 2nd ed., Rev. - M .: Ventana-Graf, 2005.
Pasechnik V.V., Kamensky A.A., Kriksunov E.A. Biology. Introduction to General Biology and Ecology: Textbook for Grade 9, 3rd ed., Stereotype. - M .: Bustard, 2002.

Blgy.ru ().
Sbio.info ().
Estnauki.ru ().

Homework

What is ontogeny and what stages does it consist of in multicellular organisms?
What are the stages of embryonic development?
What are the stages of development of the postembryonic period?

Ontogenesis called the set of processes occurring in the body, from the moment of formation of the zygote to death. It is divided into two stages: embryonic and postembryonic.

The embryonic period
The embryonic period is considered the period of embryonic development from the moment of formation of the zygote to exit from the egg membranes or birth; in the process of embryonic development, the embryo goes through the stages of cleavage, gastrulation, primary organogenesis and further differentiation of organs and tissues.

Crushed. Crushing is the process of formation of a multicellular single layer aarodysh - blastula. Cleavage is characterized by: 1) cell division by mitosis with preservation of the diploid set of chromosomes; 2) a very short mitotic cycle; 3) blastomeres are not differentiated, and hereditary information is not used in them; 4) blastomeres do not grow and subsequently become smaller and smaller; 5) the cytoplasm of the zygote does not mix and does not move.

The first cleavage furrow runs in the meridional llrrness, connecting both poles - vegetative and ayimal, - and divides the zygote into two identical cells. This is the stage of two blastomeres. The second groove is also meridional, but perpendicular to the first. It divides both blastomeres resulting from the first division in two - four similar blastomeres are formed. The next, third, cleavage furrow is latitudinal. It lies slightly above the equator and divides all four blastomeres into eight cells at once. Subsequently, the cleavage furrows alternate. As the number of cells increases, their division becomes non-simultaneous. Blastomeres move further and further from the center of the embryo, forming a cavity. At the end of cleavage, the embryo takes the form of a bubble with a wall formed by one layer of cells closely adjacent to each other. The internal cavity of the embryo, which initially communicated with the external environment through the gaps between the blastomeres, becomes completely isolated as a result of their tight closure. This cavity is called the primary body cavity - the blastocoel. Cleavage ends with the formation of a single-layer multicellular embryo - blastula

The fragmentation of a fertilized egg can take place in different ways. The lancelet egg is crushed completely and has blastomeres of equal size. This type of crushing is called complete, uniform. In fish, amphibians and some other animals, cleavage is also complete, but uneven: blastomeres at the vegetative pole (where the yolk is concentrated) are larger than at the opposite animal pole (where the nucleus is located surrounded by cytoplasm)

The third type of cleavage is characteristic of the eggs of birds, reptiles, which have a lot of yolk, and is called discoidal. Here, only the nucleus and a thin section of the cytoplasm are involved in cleavage, as a result of which an embryonic disc is formed (the yolk of the egg is not cleaved in this case). In arthropod eggs (the yolk is concentrated in the center of the egg), crushing superficial - blastomeres are located along the periphery of the egg, where the cytoplasm covering the yolk lies in a narrow strip.

With complete cleavage (for example, in a lancelet at the stage of 32 blastomeres), the embryo looks like a mulberry berry and is called morula. At approximately the 64 blastomere stage, a cavity is formed in it, and the blastomeres are arranged in one layer, forming the embryo wall. This stage of the embryo is called blastula . Soon the process of the emergence of a two-layer embryo begins - gastrulation. The embryo at this stage consists of clearly separated layers of cells, the so-called germ layers: external, or ectoderm and internal, or endoderm. Gastrulation is characterized by: 1) movement of cell masses; 2) the beginning of the use of the hereditary material of the embryonic cells and the appearance of the first signs of cell differentiation; 3) cell division is poorly expressed; 4) the appearance of the first tissues

There are several ways to gastrulate. First -immigration - observed in coelenterates: after the formation of blastula, some cells of the body wall of the embryo immigrate deep into the cavity and gradually fill it. Then they adjoin from the inside to the outer layer of cells and a two-layer germ-gastrula. Gastrulation in the lancelet and some other animals proceeds by intussusception. Following the formation of blastula, the entire vegetative pole invades inward, adjoins the animal pole, and the embryo becomes two-layered: the outer germ layer is called ectoderm, internal - endoderm. This stage of the embryo has a primary mouth, the blastopore, which leads to the primary intestine. Two-layer animals - sponges and coelenterates - end their embryonic development here. Subsequently, the cells of their ectoderm and endoderm differentiate and several cell types arise.

In amphibians, gastrula is formed in a different way: smaller blastomeres from the animal pole creep over the large blastomeres of the vegetative pole, so that a two-layer embryo is obtained by fouling small blastomeres of large ones. In arthropods, during cleavage, blastomeres separate daughter cells from themselves into the cavity, where they form the second layer of the embryo - endoderm. This method of gastrula occurrence is called splitting. Different ways of forming a two-layered embryo in different species of animals are due to the amount and nature of the distribution of the yolk in the egg. However, strictly separate types of gastrulation are not observed, their subdivision is conditional.

Primary organogenesis. After the completion of gastrulation, a complex of axial organs is formed in the embryo: the neural tube, notochord, intestinal tube. Beginning with flatworms, a major complication began in the evolution of the animal world: a third embryonic leaf is laid in the embryo - mesoderm. In chordates, this occurs by detaching mesodermal pockets from the endoderm, which grow between the first and second germ layers, forming a secondary body cavity.

Further differentiation of the embryonic cells leads to the emergence of numerous derivatives of embryonic layers-organs and tissues.

Differentiation ordifferentiation - it is the process of the emergence and growth of structural and functional differences between individual cells and parts of the embryo. From a morphological point of view, differentiation is expressed in the fact that several hundred types of cells of a specific structure, differing from each other, are formed. From a biochemical point of view, the specialization of cells consists in the synthesis of certain proteins that are characteristic only of this type of cell. Biochemical specialization of cells is provided differential activity re new, that is, in the cells of different germ layers - the rudiments of certain organs in systems - different groups of genes begin to function. With further differentiation ke cells included part germ layers, from ectoderm formed: the nervous system, sensory organs, skin epithelium, tooth enamel; from endoderm - the epithelium of the midgut, the digestive glands - the liver and pancreas, the epithelium of the gills and lungs; from mesoderm- muscle tissue, connective tissue, circulatory system, kidneys, sex glands, etc. In different animal species, the same germ layers give rise to the same organs and tissues. This means that they homologous.

In chordates, soon after gastrulation, a small area of the dorsal ectoderm in the form of a plate plunges deep into the embryo, bends and forms a neural tube with a cavity inside filled with fluid. From the cells of the ectoderm, the skin with their derivatives (hair, nails, feathers, hooves) and sense organs develop. From the upper part of the endoderm, a notochord is formed, from the lower part - the epithelium lining the middle sections of the intestine, digestive glands and respiratory organs. From the ectoderm, located above the notochord, a neural tube develops. From the mesoderm, muscles, skeleton, circulatory system, sex glands, excretory organs and the skin itself - the dermis - are formed.

The embryonic development of animals takes place either in the maternal organism or in the external environment.

The homology of the germ layers of the overwhelming majority of animals is one of the proofs of the unity of the animal world.

Embryonic induction. Embryonic induction can be defined as a phenomenon in which, in the process of embryogenesis, one primordium affects another, determining the path of its development, and, in addition, itself is subjected to an inducing effect from the first primordium.

Germ layers, their derivatives (T.A. Kozlova, V.S.Kuchmenko. Biology in tables. M., 2000)

embryonic development (T.A. Kozlova, V.S.Kuchmenko. Biology in tables. M., 2000)

Postembryonic period of development

At the moment of birth or exit of the organism from the egg membranes, the embryonic ends and begins postembryonic period of development. Postembryonic development can be direct sumindirect and accompanied by transformation (metamorphosis). With direct development, an organism of small size emerges from the egg membranes or from the mother's body, but it contains all the main organs characteristic of an adult animal (reptiles, birds, mammals). Postembryonic development in these animals is mainly reduced to growth and puberty - pre-reproductive period; reproduction - reproductive period and aging - postreductive period.

In organisms with a low yolk content in the egg, indirect development is accompanied by the formation of the larval stage. A larva emerges from the egg, usually simpler than an adult animal, with special larval organs that are absent in an adult state. The larva feeds, grows, and, over time, the larval organs are replaced by organs characteristic of adult animals. At incomplete metamorphosis replacement of larval organs occurs gradually, without cessation of active feeding and movement of the organism (locusts, amphibians). Complete metamorphosis includes the pupal stage, in which the larva transforms into an adult animal - imago (butterflies).