After the birth of an organism, its postembryonic development begins (postnatal for humans), which in different organisms lasts from several days to hundreds of years, depending on their species. Consequently, life expectancy is a species characteristic of organisms that does not depend on the level of their organization.
In postembryonic ontogenesis, a distinction is made between the juvenile and pubertal periods, as well as the period of old age, ending with death.
Juvenile period. This period (from the Latin juvenilis - young) is determined by the time from the birth of the organism to puberty. It occurs differently in different organisms and depends on the type of ontogenesis of the organisms. This period is characterized by either direct or indirect development.
In the case of organisms characterized by direct development (many invertebrates, fish, reptiles, birds, mammals, humans), those hatched from egg membranes or newborns are similar to adult forms, differing from the latter only in smaller sizes, as well as underdevelopment of individual organs and imperfect proportions bodies.
A characteristic feature of the growth in the juvenile period of organisms subject to direct development is that the number and size of cells increase, and the proportions of the body change. Human growth in different periods of his ontogenesis. The growth of different human organs is uneven. For example, head growth ends in childhood, legs reach proportional size by about 10 years. The external genitalia grow very quickly between the ages of 12 and 14 years. A distinction is made between definite and indefinite growth. A certain growth is characteristic of organisms that stop growing at a certain age, for example, insects, mammals, humans. Indefinite growth is characteristic of organisms that grow throughout their lives, for example, mollusks, fish, amphibians, reptiles, and many types of plants.
In the case of indirect development, organisms undergo transformations called metamorphoses (from the Latin metamorphosis - transformation). They represent modifications of organisms during development. Metamorphoses are widely found in coelenterates (hydra, jellyfish, coral polyps), flatworms (fasciola), roundworms(roundworms), mollusks (oysters, mussels, octopuses), arthropods (crayfish, river crabs, lobsters, shrimp, scorpions, spiders, ticks, insects) and even some chordates (tunicates and amphibians). In this case, complete and incomplete metamorphoses are distinguished. The most expressive forms of metamorphosis are observed in insects that undergo both incomplete and complete metamorphosis.
Incomplete transformation is a development in which an organism emerges from the egg shells, the structure of which is similar to the structure of an adult organism, but its size is much smaller. Such an organism is called a larva. During the process of growth and development, the size of the larvae increases, but the existing chitinized cover prevents a further increase in body size, which leads to molting, i.e., the shedding of the chitinized cover, under which there is a soft cuticle. The latter straightens out, and this is accompanied by an increase in the size of the animal. After several moults, the animal reaches maturity. Incomplete transformation is typical, for example, in the case of the development of bedbugs.
Complete metamorphosis is a development in which a larva is released from the egg shells, significantly different in structure from the adults. For example, in butterflies and many insects the larvae are caterpillars. Caterpillars are subject to molting, and can molt several times, then turning into pupae. From the latter, adult forms (imago) develop, which do not differ from the original ones.
In vertebrates, metamorphosis occurs among amphibians and bony fishes. The larval stage is characterized by the presence of provisional organs, which either repeat the characteristics of the ancestors or have a clearly adaptive significance. For example, a tadpole, which is the larval form of a frog and repeats the characteristics of the original form, is characterized by a fish-like shape, the presence of gill breathing, and one circle of blood circulation. The adaptive features of tadpoles are their suckers and long intestines. It is also characteristic of larval forms that, in comparison with adult forms, they are adapted to life in completely different conditions, occupying a different ecological niche and a different place in the food chain. For example, frog larvae have gill breathing, whereas adult forms are pulmonary. Unlike adult forms, which are carnivores, frog larvae feed on plant foods.
The sequence of events in the development of organisms is often called life cycles, which can be simple or complex. The simplest development cycles are characteristic, for example, of mammals, when an organism develops from a fertilized egg, which again produces eggs, etc. Complex biological cycles are cycles in animals, which are characterized by development with metamorphosis.
Development and differentiation associated with metamorphosis are the result of natural selection, due to which many larval forms, for example, insect caterpillars and frog tadpoles, are better adapted to the environment than adult sexually mature forms.
Puberty. This period is also called mature, and it is associated with the sexual maturity of organisms. The development of organisms during this period reaches its maximum.
On growth and development in the postembryonic period big influence have environmental factors. For plants, the decisive factors are light, humidity, temperature, quantity and quality nutrients in the soil. For animals, proper feeding is of paramount importance (the presence of proteins, carbohydrates, lipids, mineral salts, vitamins, microelements in the feed). Oxygen, temperature, light (synthesis of vitamin D) are also important.
The growth and individual development of animal organisms are subject to neurohumoral regulation by humoral and nervous regulatory mechanisms. A hormone similar to this has been discovered in plants. active substances, called phytohormones. The latter affect the vital functions of plant organisms.
In animal cells, during their life processes, chemically active substances are synthesized that affect life processes. Nerve cells of invertebrates and vertebrates produce substances called neurosecretions. The endocrine, or internal, secretion glands also secrete substances called hormones. Endocrine glands, particularly those related to growth and development, are regulated by neurosecretions. In arthropods, the regulation of growth and development is very well illustrated by the effect of hormones on molting. The synthesis of larval secretions by cells is regulated by hormones that accumulate in the brain. A special gland in crustaceans produces a hormone that inhibits molting. The levels of these hormones determine the frequency of molting. In insects, hormonal regulation of egg maturation and diapause has been established.
In vertebrates, the endocrine glands are the pituitary gland, pineal gland, thyroid, parathyroid, pancreas, adrenal glands and gonads, which are closely related to one another. The pituitary gland in vertebrates produces gonadotropic hormone, which stimulates the activity of the gonads. In humans, the pituitary hormone affects growth. With a deficiency, dwarfism develops; with an excess, gigantism develops. The pineal gland produces a hormone that affects seasonal fluctuations in the sexual activity of animals. Thyroid hormone influences the metamorphosis of insects and amphibians. In mammals, underdevelopment of the thyroid gland leads to growth retardation and underdevelopment of the genital organs. In humans, due to a defect in the thyroid gland, ossification and growth are delayed (dwarfism), puberty does not occur, and the mental development(cretinism). The adrenal glands produce hormones that influence metabolism, growth and differentiation of cells. The gonads produce sex hormones that determine secondary sexual characteristics. Removal of the gonads leads to irreversible changes in a number of characteristics. For example, in castrated roosters, the growth of the comb stops and the sexual instinct is lost. A castrated man acquires an external resemblance to a woman (a beard and hair on the skin do not grow, fat is deposited on the chest and pelvic area, the timbre of the voice is preserved, etc.).
Plant phytohormones are auxins, cytokinins and gibberellins. They regulate cell growth and division, the formation of new roots, flower development and other properties in plants.
At all periods of ontogenesis, organisms are capable of restoring lost or damaged body parts. This property of organisms is called regeneration, which can be physiological and reparative.
Physiological regeneration is the replacement of lost body parts during the life of the body. Regenerations of this type are very common in the animal world. For example, in arthropods it is represented by molting, which is associated with growth. In reptiles, regeneration is expressed in the replacement of the tail and scales, in birds - feathers, claws and spurs. In mammals, an example of physiological regeneration is the annual shedding of antlers by deer.
Reparative regeneration is the restoration of a body part of an organism that has been violently removed. Regeneration of this type is possible in many animals, but its manifestations vary. For example, it is common in hydras and is associated with the reproduction of the latter, since the entire organism regenerates from a part. In other organisms, regeneration manifests itself in the form of the ability of individual organs to recover after the loss of any part. In humans, epithelial, connective, muscle and bone tissues have a fairly high regenerative ability.
Plants of many species are also capable of regeneration. Regeneration data has great importance not only in biology. They are widely used in agriculture, in medicine, in particular in surgery.
Old age as a stage of ontogenesis. Old age is the penultimate stage of animal ontogenesis, and its duration is determined by the total lifespan, which is a species characteristic and which varies among different animals. Old age has been studied most accurately in humans.
There are a variety of definitions of human old age. In particular, one of the most popular definitions is that old age is the accumulation of successive changes that accompany an increase in the age of an organism and increase the likelihood of its illness or death. The science of human aging is called gerontology (from the Greek geron - old man, old man, logos - science). Its task is to study the patterns of age transition between maturity and death.
Chordates are one of the largest types of the animal kingdom, whose representatives have mastered all habitats. This type includes three groups (subtypes) of organisms: tunicates (including bottom-dwelling sea sessile organisms - ascidians), skullless (small fish-like sea creatures - lancelets), vertebrates (cartilaginous and bony fish, amphibians, reptiles, birds and mammals). Man is also a representative of the chordate phylum. The origin of the phylum chordates is the most important stage V historical development animal world, meaning the emergence of a group of animals with a unique structure plan, which made it possible in further evolution to achieve the maximum complexity of structure and behavior among living beings. In essence, this means reconstructing the first steps on the path that led to man. That is why the problem of the origin of chordates has been of great interest to biologists for more than a century and a half.
The origin of chordates dates back to a period in the evolution of the animal kingdom from which very few paleontological remains remain. It is only known that in the Middle Cambrian (about 550 million years ago) skullless species, related to modern lancelets, already existed. The earlier stages of the evolution of chordates have to be reconstructed on the basis of the study of modern organisms, mainly using comparative anatomical and embryological methods, drawing on data from other areas of biology.
MAIN HYPOTHESES FOR THE ORIGIN OF CHORDATES
One of the first concepts linking the body plan of chordates with the body plans of other groups of invertebrates was developed by the outstanding French comparative anatomist Geoffroy Saint-Hilaire back in the first half of the 19th century. He believed that segmented animals (annelids and arthropods) could be considered inverted vertebrates. “Look at the crayfish, overturned on its back,” he said, “and you will see that various systems it is located in exactly the same way as in higher vertebrates." It was this concept that was at the center of the public dispute between Geoffroy Saint-Hilaire and Georges Cuvier, which took place in Paris in 1830 during the July Revolution that overthrew the Bourbons. As is known, in that Georges Cuvier won the famous debate.However, zoologists returned to the idea of comparing chordates (and vertebrates) and inverted annelids or arthropods repeatedly throughout the 19th century.
This point of view was defended by the outstanding zoologist and, as we would now say, “organizer of science” Anton Dorn, who built the world’s first and still successfully operating Neapolitan Zoological Station with his personal funds. At the beginning of the 20th century, the idea of the origin of chordates from arthropods that turned over on their dorsal side was defended by Gaskell, Patten and other biologists.
Ingenious and well-explained hypotheses of the structure of chordates of their origin from inverted annelids or arthropods have lost their popularity primarily due to the fact that the fundamental embryological differences between annelids and arthropods, on the one hand, and chordates, on the other.
At the beginning of this century, hypotheses were developed about the origin of chordates from intestinal-breathing marine worm-like organisms living in the thickness of the soil. These animals (like chordates) have gill slits, and in the anterior part of the body there is a supporting outgrowth of the intestine (stomochord), which has been compared to a notochord. However, intestinal breathers differed sharply from chordates in the reverse direction of blood flow, the location of the heart, and the structure nervous system and other important features of the organization. Attempts have been made to breed chordates from echinoderms, nemerteans, mollusks and other groups of invertebrates, but none of them were successful.
A peculiar reaction to the failures of zoologists to solve the problem of the origin of chordates using traditional methods was the development of so-called neotenic hypotheses, which derive chordates from ascidian larvae. Adult ascidians are attached marine animals that filter water through a voluminous gill sac and trap tiny organic particles suspended in the water. They have neither a notochord nor a neural tube and are not at all similar to chordates. But, as has long been known (and this was first shown at the end of the last century by the outstanding Russian biologist A.O. Kovalevsky), ascidian larvae have both a notochord and a neural tube, which are lost during metamorphosis. Based on this, many biologists (Beryl, Wither, Bone, etc.) suggest that chordates descended from ascidian larvae that became sexually mature. True, in this case it remains unclear where the ascidians themselves came from. And it is difficult to imagine that the notochord and neural tube would arise in larvae that live for 1–2 days and do not feed. It is more likely that these features are the legacy of the free-living ancestors of ascidians, about whose organization neotenic hypotheses say nothing. None of the hypotheses considered can currently be considered true, but each of them contains individual elements that may be useful in reconstructing the origins of chordates.
Cell life cycle: prosynthetic, synthetic and postsynthetic phases. The significance of these phases in the life of a cell.
Life cycle of cell – this is the time of existence of a cell from division to division or until death. In a single cell, the cell cycle coincides with the life of the individual. In continuously reproducing tissue cells, the life cycle of the cell coincides with the life of the individual. In continuously reproducing tissue cells, the life of the cell coincides with the mitotic cycle (mitosis). Consists of 3 periods: presynthetic period (PP) - after completion of mitosis, the cell can enter a period of preparation for DNA synthesis. PP designated by the symbol G1 (interval). During this period, RNA and proteins are formed in the cell, the activity of enzymes that participate in DNA biosynthesis increases, organelles and cell growth occur. After completion of the phase, the cell begins DNA synthesis, i.e. its replication – doubling (in S phase). Synthetic period (SP) - the replication process begins simultaneously at many points on each chromosome. The 2 strands of the original DNA molecule diverge, and each becomes a template for new strands. Each daughter molecule contains one old and one new chain of nucleotides connected in series. The duration of the S phase varies in different cells and is 6-12 hours (in mammalian cells). As a result of DNA doubling, each chromosome contains 2 more DNA than it had before the start of the S phase, but the number of chromosomes does not change, therefore, after DNA reduplication is completed chromosome set cells become 2n4c. Postsynthesis period - after DNA synthesis is completed - S phase - the cell does not immediately begin to divide. The time from the end of DNA synthesis to the beginning of mitosis is the postsynthetic period - G2. During this period, the cell’s preparation for mitosis is completed, proteins are doubled, cell growth is completed, and ATP accumulates.
Linked inheritance and crossing over. Genetic maps, the principle of their construction in eukaryotes.
Linked heritage. Crossing over. Genetic maps. Cat genes. are part of one chromosome are called linked and form a group. clutch. They are inherited as a single whole because this is determined by the behavior of the chromosome in meiosis. In this case, segregation according to linked characteristics does not obey the law of independent inheritance. If genes are located close to each other, then they are always preserved in their original combinations. More common phenomena. situations when genes are located on certain. moving each other away from the other. In this case of partial linkage, they can separate as a result of a process - crossing over - this is a type of genetic recombination. Crossing over occurs in prophase of the first meiotic division at the time of chromosome conjugation (at this time, chromatids of homologous chromosomes exchange fragments hereditary material and new combinations of genes appear). The amount of crossing over, calculated as % of the ratio of recombinants to the total number of offspring, yavl. an indicator of the distance between genes and is used for chromosome mapping - the location of genes on a chromosome map in a strictly defined order and at fixed distances.
Genetics. Mutations as the basis for their occurrence hereditary variability organisms. Types of mutations (gene, chromosomal and genomic). Hereditary variability is the process of occurrence of hereditary changes in the body - mutations. (in 1899 by the Russian scientist S.I. Korzhinsky and in 1900-1901 by the Dutch geneticist G. de Vries, who, in particular, introduced the terms<мутация» (лат. изменение) и «мутагенез». Способность мутировать присуща всем формам жизни на Земле и лежит в основе фундаментального свойства живого - изменчивости. Сущность мутаций состоит в изменении структуры ДНК, являющейся носителем генетической информации. Мутации приводят к возникновению нового признака или изменению (утрате) старого. Различают спонтанный (возникающий в естественных условиях без четко регистрируемых внешних воздействий) и индуцированный (в результате искусственных воздействий) М.
Types of mutations: Genomic: - polyploidization (the formation of organisms or cells whose genome is represented by more than two (3n, 4n, 6n, etc.) sets of chromosomes) and aneuploidy (heteroploidy) - a change in the number of chromosomes that is not a multiple of the haploid set (see Inge-Vechtomov , 1989). Depending on the origin of chromosome sets among polyploids, a distinction is made between allopolyploids, which have chromosome sets obtained by hybridization from different species, and autopolyploids, in which the number of chromosome sets of their own genome increases by a multiple of n.
At chromosomal mutations major rearrangements of the structure of individual chromosomes occur. In this case, there is a loss (deletion) or doubling of a part (duplication) of the genetic material of one or more chromosomes, a change in the orientation of chromosome segments in individual chromosomes (inversion), as well as a transfer of part of the genetic material from one chromosome to another (translocation) (an extreme case - unification of entire chromosomes, the so-called Robertsonian translocation, which is a transitional variant from a chromosomal mutation to a genomic one).
On gene level of changes in the primary DNA structure of genes under the influence of mutations are less significant than with chromosomal mutations, however, gene mutations are more common. As a result of gene mutations, substitutions, deletions and insertions of one or more nucleotides, translocations, duplications and inversions of various parts of the gene occur. In the case when only one nucleotide changes under the influence of a mutation, they speak of point mutations.
Zoology of vertebrates. Origin and evolution of the class of fish. The oldest known jawless fish-like animals are known from the Early Ordovician (about 450-470 million years ago). With the development of jaws, the first fish arose from one of the gill arches. In addition to jaws, fish have paired fins, an inner ear with three semicircular canals and gill arches. Despite the appearance of the first gnathostomes in the Ordovician, they occupied a subordinate position until the Devonian. Thus, fish and agnathans existed for more than 100 million years in conditions where agnathans predominated, in contrast to the present time. Cartilaginous fish appeared at the turn of the Silurian and Devonian, about 420 million years ago, and reached their peak in the Carboniferous. Cartilaginous fish. Armored fish appeared for the first time. 450 million years ago. The first chordates in which there were 10 or more gill apparatuses near the head - movably connected to each other (to the very end of the head). Of 3-4 pairs of jaw apparatus (grasping). Flaxid scales appear on the skin, appearing on 3.4 gill arches - food retention, 1-2 arches - grasping and retaining - this is how the hyoid apparatus appeared from other arches.
The first fish were acanthodia-periodonts, from which bony and cartilaginous fish evolved. Cartilaginous (ancient) origin in fresh water bodies. The skeleton is cartilaginous, there is a chord, a jointed skeleton. Bony fish have been present in the world's oceans since at least the Devonian; it is possible that they already existed in the Silurian. Guiyu oneiros is the earliest known bony fish. Bony fish have a bony skeleton. Ossification, formation of bone tissue, articulated column. The structure of the bone skeleton is progressive. Bony fish live in any environment (fresh, sea, cave).
Cytology and histology. Cell membrane systems and the membrane principle of its organization. Structure and properties of biological membranes and their properties. Cell organelles: 1) - obligatory - ensure the vital activity of the cell --> mitochondria, ribosomes, plasma membrane, Golgi apparatus, genetic apparatus.
-->membrane (single-membrane - Golgi apparatus, vacuole, granular ER, lysosomes, microsomes) - (double-membrane - mitochondria, plastids, nucleus)
à non-membrane (ribosomes, cell center, microtubules, microfilaments, microfibrils)
2) facultative-highly specialized are optional for all cells àchloroplasts, vacuole, cell center
The endoplasmic reticulum is a branched network formed by a membrane (single membrane) on which enzymes are located.
EPS: smooth (agrocular), rough (granular)
On the membranes, the rough EPS contains ribosomes. Functions: protein synthesis.
The Golgi apparatus is a collection of membrane structures in the form of flattened cisterns and small vesicles. The tanks are connected to eps channels. Proteins are synthesized on eps membranes, polysaccharides and fats are transported to A.G. condense within its structures and are packaged as secretions or used by the cell itself.
Lysosomes are single-membrane organelles that break down proteins, polysaccharides, lipids, etc. Function of digestion of substances.
The nucleus is a double-membrane organelle. Consists of a nuclear membrane of chromatin (nucleolus) of karyoplasm (nuclear juice). On outer membrane a large number of lysosomes The nuclear membrane becomes rough eps. The core shell is permeated with pores. Pores are formed by the fusion of two membranes. Chromatin consists of DNA in complex with protein. Chromosomes are formed only in dividing cells due to the spiralization of chromatin threads. Kernel functions: information storage and its implementation, transmission.
Biological membrane- This is a thin lipoprotein semi-permeable film, consisting of a double layer of lipid molecules, which includes protein molecules.
Fluid mosaic model of biological membranes. The membrane forms a double layer of phospholipids. Phospholipids consist of: - head-polar (-) neurons, hydrophilic; tails are non-polar, uncharged, hydrophilic.
Protein molecules do not form; they are located in the lipid layer. Most proteins are not associated with lipids.
Types of proteins: -integral - penetrate the thickness of the membrane; -semi-integral proteins are half immersed in the membrane; - peripheral proteins are located on the surface of membranes.
Lipids account for 60%, proteins 40-75%. Many membranes contain carbohydrates that form a layer of glycocalyx, found only in animal cells.
Properties of the cellular (biological) membrane.
1) Closedness - during the process of lipid assembly, the locks close on themselves. This leads to the formation of closed compartments in the cell called compartments
2) Dynamism - some molecules of lipids and proteins can move in the membrane i.e. retain the ability to diffuse
3) Asymmetry - they differ in the presence of glycocalyx, in the composition of lipids and proteins.
4) Selective permeability - this property ensures the regulation of the penetration of necessary molecules into the cell, as well as the removal of metabolic products
Functions of the cell membrane
Barrier – delimits the contents of the cell from the external environment, i.e. maintaining integrity
Formation of cell membrane structures
Transport of nutrients, waste inside and outside
Receptor – perception of signals and transmission of them into the cell
They take part in the construction of special cell processes, such as microvilli, cilia, and receptor processes.
Tests
99-1. What is the sequence of stages individual development characteristic of the animal shown in the picture?
A) egg - adult insect
B) egg – larva – adult insect
C) egg – larva – pupa – adult insect
D) egg – pupa – larva – adult insect
Answer
99-2. The grasshopper, unlike the cabbage butterfly,
A) breathes through tracheas
B) develops with not complete transformation
B) has an open circulatory system
D) has three pairs of legs
Answer
99-3. What sequence of stages of individual development is characteristic of a butterfly?
A) sperm – larva – pupa – adult animal
B) blastula – zygote – pupa – adult animal
C) egg – larva – adult animal
D) egg – larva – pupa – adult animal
Answer
99-4. Are the judgments about the development of insects correct?
1. In postembryonic development, insects with complete transformation go through the stages of development: larva > pupa > adult insect.
2. Different nutrition of larvae and adults of one or another insect species eliminates competition between them.
A) only 1 is correct
B) only 2 is correct
C) both statements are correct
D) both judgments are incorrect
Answer
99-5. Which of the following insects develop with incomplete metamorphosis?
A) Coleoptera
B) Diptera
B) Lepidoptera
D) Orthoptera
Answer
99-6. What sequence of stages of individual development is characteristic of the cabbage white butterfly?
A) egg > butterfly
B) egg > butterfly > larva
B) egg > larva > pupa > butterfly
D) egg > pupa > larva > butterfly
Answer
99-7. Insects with complete metamorphosis include
A) locusts
B) aphids
B) grasshopper
D) cabbage butterfly
Answer
99-8. Which of the following insects develop with complete metamorphosis?
A) Diptera
B) Orthoptera
B) Homoptera
D) Hemiptera
Answer
99-9. Which of the following insects is characterized by the pupal stage?
A) honey bee
B) locusts
B) dragonfly
D) praying mantis
Answer
99-10. Are the judgments about the development of insects correct?
1. The pupa is a resting stage in the development of insects, since it does not feed, move or develop.
2. In insects with complete metamorphosis, the larva looks like an adult animal.
A) only 1 is correct
B) only 2 is correct
C) both statements are correct
D) both judgments are incorrect
Development is an integral factor of life. It begins with a fertilized egg and ends with puberty. The postembryonic period is characterized by direct and indirect development. Direct development is a biological process in which a multicellular organism grows and enlarges, increasing the complexity of its organization. This phenomenon is typical for humans, fish, birds and mammals.
Indirect development is the process by which the embryo develops into a mature individual involving the larval stage, which is accompanied by metamorphosis. This phenomenon is observed, for example, in most invertebrates and amphibians.
Features of the postembryonic period
Periods of postembryonic development are accompanied by changes morphological features, habits and habitats. For direct development characteristic feature is that after birth, the embryo is a reduced copy of the adult organism, it differs only in size and the absence of some characteristics that are acquired only over time. An example would be the development of humans, animals and some reptiles. Indirect development is typical for invertebrates, mollusks and amphibians. In this case, the embryo has significant differences compared to the adult animal. An example would be the common butterfly. Only after several stages of development have passed will the small larva be transformed beyond recognition.
Periods of development
The periods include juvenile stage, adulthood and senescence.
- The juvenile period covers the time from birth to puberty. This stage is accompanied by adaptation to the new environment. It is worth noting that many animals and reptiles, which are characterized by a direct path of postembryonic development, develop in approximately the same way. The only difference is the time frame. This one ends
- The period of maturity, called the reproductive stage, is characterized by cessation of growth. The body undergoes self-renewal of certain structures and their gradual wear and tear.
- The aging period is accompanied by a slowdown in recovery processes. As a rule, there is a decrease in body weight. If there was no violent intervention, then natural death occurs when vital systems cease to function as a result of the slowdown of all processes.
Indirect development: examples and stages
Let's look at how life begins in a new being. Direct and indirect development are terms that describe various processes life activity of animals, which begins with a fertilized egg. During postembryonic development, organ systems are finally formed, growth is observed, followed by procreation. Then aging occurs, and in the absence of external interventions, natural death occurs. 
- Immediately after birth, a series of transformations begin. At this time, the small organism differs from the adult both externally and internally.
- The second stage is the transformation into a completely new body. Metamorphosis is a postembryonic change in body shape with alternation of several stages.
- The third stage is the final stage, which ends with puberty and procreation.
Characteristics of indirect development
Indirect development is typical for multicellular organisms. A larva emerges from the laid egg, which is externally and internally not similar to the adult. In structure it is a simpler creature, usually smaller in size. In its appearance, it may be vaguely similar to its distant ancestors. An example would be the larva of an amphibian such as a frog.
Externally, the tadpole is very similar to a small fish. Thanks to the presence of special larval organs, it can lead a completely different life than sexually mature individuals. They do not even have rudimentary sexual differences, so it is not possible to determine the sex of the larva. In a certain number of animal species, this stage of development takes most their lives.
Radical metamorphoses
With indirect development, the newborn animal is very different from the mature form in a number of anatomical characteristics. The embryo hatches from the egg as a larva, which undergoes radical metamorphosis before reaching its adult stage. Indirect development is typical for animals that lay numerous eggs. These are some echinoderms, amphibians and insects (butterflies, dragonflies, frogs, and so on). The larvae of these creatures often occupy a completely different ecological space than the adult animal. They feed, grow and at some point transform into an adult animal. These global metamorphoses are accompanied by numerous physiological changes.
Pros and cons of direct development
The advantage of direct development is that growth requires much less energy and vital ingredients, since no global changes occur in the body. The disadvantage is that the development of the embryo requires large reserves of nutrients in the eggs or gestation in the womb.
A negative point is also that competition within the species may arise between young and adult animals, since their habitat and food sources coincide.
Pros and cons of indirect development
Due to the fact that organisms with an indirect type of development live in different places, competitive relationships between larvae and adults, as a rule, do not arise. Another advantage is that the larvae of sedentary creatures help the species expand its habitat. Among the disadvantages, it is worth pointing out that the indirect development of animals into adults often lasts a long period of time. For high-quality transformations, you need a large amount of nutrients and energy.
Types of indirect development
The following types of indirect development are distinguished: with complete and partial metamorphosis. With complete transformation, indirect development is characteristic of insects (butterflies, beetles, some hymenoptera). The hatched larvae begin to eat, grow, and then become immobile cocoons. In this state, all organs of the body disintegrate, and the resulting cellular material and accumulated nutrients become the basis for the formation of completely different organs characteristic of an adult organism. 
With partial metamorphosis, indirect postembryonic development is characteristic of all species of fish and amphibians, certain mollusks and insects. The main difference is the absence of a cocoon stage.
Biological role of the larval stage
The larval stage is a period of active growth and supply of nutrients. Appearance, as a rule, is very different from the adult form. They have their own unique structures and organs that an adult individual does not have. Their diet may also differ significantly. Larvae are often adapted to environment. For example, tadpoles live almost exclusively in water, but can also live on land, like adult frogs. Some species are immobile as adults, while their larvae move and use this ability to disperse and expand their habitat.
HUMAN ONTOGENESIS
Ontogenesis – full cycle individual development of the body. In the time interval, ontogenesis begins with the fertilization of the egg and ends with the death of the organism. And from a biological point of view, ontogenesis is the process of complete and step-by-step implementation of hereditary information at all stages of the existence of an organism, while the environment has a significant influence on the development of the organism.
Understanding the mechanisms of ontogenesis is one of the main problems modern biology, therefore, various biological disciplines are involved in the study of patterns of individual development: cytology, histology, molecular genetics, biochemistry, etc. There are two independent disciplines, studying directly the stages of ontogenesis: embryology and gerontology. Taking this approach into account, the modern synthetic theory of ontogenesis is often called developmental biology.
With all the diversity of the animal world, the following main types of ontogenesis can be distinguished:
Indirect development Direct development
(larval, with metamorphosis)
With complete metamorphosis - non-larval (fish, reptiles, birds)
With incomplete metamorphosis - intrauterine
The type of ontogenesis, its features and possible disorders are determined by the interaction of two main factors: hereditary information of a given organism and the peculiarities of its habitat conditions. And this interaction takes place at any stage of individual development.
Periodization of ontogeny. It is generally accepted to divide ontogenesis into two periods: embryonic (for humans - prenatal, prenatal) and postembryonic (postnatal). Each of them, in turn, is divided into shorter segments (stages), which are characterized by certain morphological and functional features.
Any organism can arise only in the presence of two full-fledged germ cells, therefore, it is more justified to distinguish another period of ontogenesis - progenesis (proembryonic period), which precedes ontogenesis itself. The proembryonic period coincides in time with gametogenesis and also includes insemination and fertilization.
I. Proembryonic period. The importance of gametogenesis for further development descendants:
Formation of haploid cells (ensures the constancy of the number of chromosomes)
The emergence of new combinations of hereditary material
Generative mutations (the cause of hereditary diseases)
Significant events of insemination and fertilization:
1. Sperm count. The ejaculate contains about 3x10 8 sperm (60-120 million in 1 ml) and they retain the ability to fertilize for 2 days.
2. Capacitation - activation of sperm during their movement through the female reproductive tract.
3. The sperm overcomes the membranes of the egg and binds to a specific receptor (receptors are species-specific!).
4. Acrosome reaction - acrosome enzymes (hyaluronidase, proteases, etc.) destroy the transparent membrane
5. The membranes of the egg and sperm are in contact, the head of the sperm is immersed in the cytoplasm of the egg. This is followed by the stages of internal fertilization.
6. Cortical reaction - changes in the transparent membrane make it impenetrable to other sperm. The transparent membrane protects the conceptus (the embryo in the morula stage) as it passes through the fallopian tube.
II. Prenatal period. The following periods are distinguished in human prenatal development:
- initial: first 2 weeks (developmental stage - conceptus)
- embryonic: 3-8 weeks (developmental stage – embryo)
- fetal (fetal): until the end of pregnancy (stage of development - fetus)
Initial period. After the formation of the zygote, the fragmentation stage begins - mitotic cell divisions without increasing their total volume. The human egg has an isolecithal type of structure (there are few nutrients and they are evenly distributed throughout the cell), therefore the type of fragmentation is holoblastic - the zygote is completely divided into two blastomeres. Subsequent crushing is asynchronous and somewhat uneven. After the third division, the morula stage is formed - a group of cells enclosed inside a transparent membrane. The central cells form gap junctions, and the peripheral cells form tight junctions among themselves and form a protective layer for the inner cells. With subsequent divisions, the blastocyst stage is formed. It clearly distinguishes the inner cell mass - the embryoblast (the embryo itself is formed from these cells, partial or complete separation of cells leads to the development of twins) and the outer layer - the trophoblast (participates in the penetration of the blastocyst into the uterine mucosa and the formation of the chorion). A fluid-filled cavity, the blastocoel, appears inside the blastocyst. The outer transparent shell becomes thinner and disappears. The events described take place in fallopian tubes. On days 6-7, the blastocyst appears in the uterine cavity and implantation occurs - penetration into the uterine mucosa.
The introduction of a blastocyst into the abdominal cavity leads to an ectopic pregnancy, and in the fallopian tubes - to a tubal pregnancy.
The next stage of development consists of coordinated, strictly regular mutual movements of extensive cell masses. These processes are called morphogenetic (shape-forming) movements, or morphogenesis. As a result of morphogenesis, the embryo acquires a two- or three-layer structure (gastrulation stage), the neural plate is formed, and then the neural tube (neurulation stage). Later, the specialization of embryonic cells (histogenesis) and the formation of individual organs (organogenesis) begin. These stages are discussed in detail at the Department of Histology.
Consider figures 93 and 94. What two types of development are characteristic of the animals depicted in the figures. What stages do locusts, butterflies, fish, frogs and humans go through in their development?
Rice. 93. Post-embryonic direct development
The individual development of an organism continues after its birth, when the embryo has already formed and can exist independently outside the egg or the mother’s body. The period of development of the body after birth is called post-embryonic, or post-embryonic (from the Latin post - after and embryo). This period occurs differently in different organisms. Therefore, a distinction is made between direct and indirect development.
Direct and indirect development. Direct development takes place without transformation. The born organism is similar to an adult individual and differs only in size, body proportions and underdevelopment of some organs. This development is mainly observed in fish, reptiles, birds and mammals (Fig. 93). So, a larva with a yolk sac emerges from a fish egg. It develops into a fry, similar to an adult, but differing from it in the underdevelopment of a number of organs.
During development with transformation (Fig. 94), a larva appears from the egg completely different from the adult organism. Such development is called indirect, or development with metamorphosis (from the Greek metamorphosis - transformation), i.e. with several larval stages of gradual transformation into an adult. The larvae actively feed and grow, but, with rare exceptions, are not capable of reproduction.
Rice. 94. Post-emergent indirect development (complete metamorphosis of the butterfly): 1 - egg: 2 - larva (caterpillar): 3 - pupa; 4 - adult insect
Development with metamorphosis is characteristic of insects and amphibians. Moreover, in insects metamorphosis can be complete or incomplete. During development with complete metamorphosis, insects go through a number of successive stages, which, as a rule, differ sharply from each other in their lifestyle and feeding pattern. For example, in a butterfly, a caterpillar emerges from an egg and has a worm-like body shape. Then, after several molts, the caterpillar turns into a pupa - a stationary stage that does not feed, but only develops into an adult insect. After some time, a butterfly emerges from the pupa. The food and feeding method of the larva and adult insect differ. The caterpillar eats plant leaves and has a gnawing mouthpart, while the butterfly feeds on the nectar of flowers and has a sucking mouthpart. Sometimes, in some species of insects, the adult does not feed at all, but immediately begins to reproduce (silkworm).
During development with incomplete metamorphosis, the pupal stage is absent and the larvae differ little from adult insects. Thus, in the locust, the larva that emerges from the egg is smaller in size compared to the adult stage and its wings are underdeveloped.
Among vertebrates, development with transformation is observed mainly in amphibians. For example, the larval stage of a frog is a tadpole. When it emerges from the egg, it resembles a fish fry. It has no limbs, has gills instead of lungs, and a tail, with which it actively swims in the water. After some time, the tadpole’s limbs form, its lungs develop, its gill slits become overgrown, and its tail disappears. Two months after hatching, the tadpole develops into an adult frog.
The transformation of a larva into an adult is associated with the production of special hormones by the endocrine glands. For example, to transform a tadpole into a frog, the thyroid hormone thyroxine is needed. In some cases, with a lack of hormones, the larval period can be prolonged for life and at this stage the body can begin to reproduce. Thus, the larva of the amphibian Ambystoma - axolotl, with a lack of thyroid hormone, does not turn into an adult and can reproduce (Fig. 95). When thyroxine is added to the water, development proceeds to completion and the axolotl turns into an ambistoma.
Rice. 95. Ambystoma (left) and its axolotl larva (right)
Height. Characteristic property individual development - the growth of the organism, i.e., an increase in its size and mass. According to the nature of growth, all animals can be divided into two groups - with indefinite and definite growth. With indefinite growth, the body size of an organism increases throughout its life. This is observed, for example, in mollusks, amphibians, fish and reptiles. Organisms with a certain height stop growing at a certain stage of development. These are insects, birds and mammals. Growth rates in animals vary throughout the entire period and are controlled by hormones. For example, in mammals (including humans), growth is regulated by the pituitary hormone somatotropin. It is actively produced in childhood, and after puberty the amount of the hormone gradually decreases and growth stops.
After an intensive period of growth, the body enters the stage of maturity, which is also characterized by changes in physiological processes in the body. This period is associated with childbirth.
Aging and death. Life expectancy depends on individual characteristics type of organism, but does not depend on the level of its organization. For example, mice live only 4 years, ravens live up to 70 years, and the freshwater pearl mussel mollusk lives up to 100 years.
The process of individual development of an organism ends with aging and death. Aging is a general biological pattern characteristic of all organisms. During the aging process, all organ systems change, their structure and functions are disrupted.
There are several theories of aging. One of the first was proposed by the Russian scientist Ilya Ilyich Mechnikov. According to this theory, the aging of the body is associated with an increase in the processes of intoxication and self-poisoning as a result of the accumulation of metabolic products and the activity of putrefactive bacteria.
Many modern theories suggest that the aging of the body is a consequence of changes in the genetic apparatus of cells, which lead to a decrease in the activity of protein biosynthesis processes. A significant reason for changes in genetic activity is the weakening of the work of enzyme proteins. With age, the frequency of chromosomal disorders increases. The restoration of damaged DNA sections proceeds more slowly, mutations accumulate, which manifest themselves in the structures of RNA and proteins.
Scientific hypotheses have been put forward that link the aging of the body with hormonal disorders, in particular with changes in the function of the thyroid gland.
In humans, the aging process is caused by the action of many biological factors. An important role in aging is played by social environment, surrounding a person. The science that deals with the problems of human aging is called gerontology (from the Greek hero - old man). Aging is an inevitable stage in the development of any organism. Next comes death, which is a necessary condition for the continuation of the life of other organisms.
Exercises based on the material covered
- What types after embryonic development do you know?
- What is the difference between direct and indirect development? Give examples of animals with different types of development.
- What is the advantage of development with transformation?
- How does development with complete metamorphosis differ from development with incomplete metamorphosis? Give examples of animals with different types of metamorphosis.
- What is the aging of the body? What theories of aging do you know? Which one is most likely in your opinion? Justify your answer.
- What is the biological meaning of the death of an organism?