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Few people think that each stage of life has its own name. Thus, science knows embryonic and postembryonic development. These periods are very different from each other and have different characteristics. The article will discuss what kind of development is called postembryonic. You will learn the types of this period and features.
What development is called postembryonic?
The moment when the embryo leaves the membranes is transitional from postembryonic. People call this time birth. It is worth noting that this process is very different for different living organisms.
Types of development of a living organism
The postembryonic period can be characterized by two types of development. Science knows the direct and indirect transformation of a living organism.
Direct development
This type includes the transformation of an organism that is very similar to an adult. Thus, human postembryonic development has a direct form. This also includes the development of animals and some reptiles.
After birth, the embryo is very similar to the adult organism. However, he is distinguished by his size and lack of skills.
Indirect development, or metamorphosis
The postembryonic period may have an indirect form (metamorphosis). In this case, an organism emerges from the membranes that is very different from the adult. This includes some invertebrates, mollusks and reptiles. As an example, we can take an ordinary butterfly. The postembryonic development of the organism in this case is characterized by being in the form of a larva. Only after this does the butterfly become like an adult.
Characteristics of the postembryonic period (direct development)
As mentioned above, immediately after the new organism leaves the membranes, a new stage in development begins. Postembryonic human development is divided into several stages. The final stage is Consider the main stages of the postembryonic period.
First stage of development: from birth to six months
At this stage, the baby adapts to the new environment. He learns to hear and focus his gaze on certain things. Nutrition at this stage of development is exclusively liquid. The baby cannot yet eat adult food on his own and is completely dependent on his mother. Some children develop more quickly than others. They move to the next stage of the postembryonic period faster. So, some babies can already stand, sit and crawl at six months. However, these are only physical skills. Science still does not know of a case where a child at this stage began to express himself in a language understandable to an adult.
It is worth noting that many animals and reptiles that go through a direct path of postembryonic development live in much the same way. However, for them this period can pass much faster than for humans.
Second stage of development: up to one year
At this stage, the little person learns to sit, crawl and walk. This stage is characterized by the acquisition of physical skills. Also, the young body begins to gradually get used to the new food. Many babies at this stage begin to make new sounds. This is how they learn to speak a language that adults can understand. If the child develops faster, then at this stage he may already speak some words or even simple sentences.
Reptiles and animals during this period are completely ready for adult life. They can get their own food and no longer depend on adults.
Third period: growth and learning
The human body is characterized by the longest postembryonic development. At this stage, the child can already take care of himself, but it is still very difficult for him to live without his parents. In ordinary life, children at this stage attend preschool and then school institutions. They grow, develop and learn everything that will be useful to them in adulthood.
Mammals and animals at this stage have already completely passed the postembryonic period and are considered adults.
Fourth period: puberty
So, you already know what development is called postembryonic. In humans, it ends at about 16-20 years of age. At this stage, important parts of the body begin to work. The pituitary and adrenal glands produce certain hormones. In women, the ovaries begin to function and menstruation appears. Men, on the other hand, acquire sexual maturity and are able to continue their race.
The postembryonic period ends with the transition to adulthood. A person is already able to live independently and create his own family.
Characteristics of indirect postembryonic development
As in the previous case, this development is divided into several main stages. Let's look at them in detail.
First stage: after birth
Immediately after the body leaves the membranes, its further transformation begins. At this stage, the new individual is completely different from the adult. It is very different in appearance and internal development.
Second stage: transformation
At this stage, a strong transformation of the new organism occurs. It occurs differently in each species of living creature. The individual becomes similar to an adult animal or mammal. However, many organisms at this stage still require care and protection. They cannot yet provide for themselves.
Stage three: acquiring skills
Indirect postembryonic development has a final stage, which is characterized by the acquisition of certain skills. They will help the future organism to live independently and not depend on outsiders.
Summary and conclusion
Now you know what development is called postembryonic. It is worth noting that indirect transformation occurs much faster than the direct postembryonic period.
A person undergoes the longest course of this stage. However, human skills make it possible to survive even in the most difficult conditions. The same cannot be said about reptiles or animals. Some doctors monitor human development in the postembryonic period: pediatrician, neurologist and others. The baby’s parents also watch especially closely.
Every person should know what the postembryonic period is. Initial data about this time are known from biology lessons. It is this kind of information that helps develop your own world.
During this lesson, you will continue to study the topic “Individual Development”, consider the post-embryonic period of ontogenesis of living organisms, which begins after the animal leaves the embryonic membranes or the mother’s body (in viviparous animals). Familiarize yourself with the features of the juvenile, pubertal (adult) and senile (aging) periods in the life of humans and animals. You will learn about direct and indirect development during the juvenile period, what hormones influence the development processes of the body, what accelerates and what slows down the aging process. What animals live unusually long and do not age with age, and how they have learned to control the genetic aging program. Can a person be forever young m.
The postembryonic period begins from the moment a new organism emerges from the egg membranes or from the mother’s body. It is divided into three periods and includes the juvenile period, puberty and the aging period.
The first juvenile period lasts until the end of puberty and can occur along two different paths. Direct development occurs if a new individual emerges from the egg or the mother’s body, outwardly similar to the adult, but smaller in size and with an unformed reproductive system. This type of development includes the development of birds, reptiles, and mammals (Fig. 1).
Rice. 1. Examples of direct development of animals. From top to bottom: birds (chicken), reptiles (crocodile) and mammals (humans)
Another type of development is called indirect and occurs with metamorphosis (transformation). However, the larva does not resemble the adult. This type of development is characteristic of frogs, some insects and worms (Fig. 2).
The juvenile period is always accompanied by the growth of the organism; on the one hand, the growth process is programmed genetically, and on the other hand, it depends on the conditions of existence.
Rice. 2. Examples of indirect development of animals: amphibians (frog), lepidoptera (butterfly)
In a small aquarium, fish will never reach the size to which they grow in natural conditions, but if, during the juvenile period, fish from a small aquarium are transplanted into a large aquarium, then such fish will grow larger than those that remained in a small aquarium.
In humans, growth is controlled by a number of hormones secreted by the hypothalamus, pituitary gland, thyroid gland and gonads.
The second period of postembryonic development - the so-called puberty period, or the period of maturity (Fig. 3), in vertebrates, takes up most of life.
Rice. 3. The period of maturity in humans
The period of maturity smoothly flows into the next period - old age.
Hormones and growth in mammals
The growth and development of mammals is influenced by hormones produced by the thyroid gland, adrenal cortex and gonads (Fig. 4). Their activity is regulated by the pituitary gland, which has been proven in numerous experiments.
Rice. 4. Glands affecting growth and development
A certain substance was extracted from the pituitary gland, which, when administered to animals, increased their body weight. This substance was called growth hormone. In further studies it was shown that this is one of the tropic hormones of the pituitary gland, which is called somatotropin.
Human somatotropin is a protein consisting of 190 amino acids. It enhances growth processes throughout the body, accelerating the synthesis of nucleic acids in preparation for mitosis, promoting the absorption of amino acids into cartilage and muscle tissue.
Aging is a general biological pattern characteristic of all living organisms. At a certain age for each organism, changes begin in the body that reduce the ability of this organism to adapt to changing conditions of existence (Fig. 5).
Rice. 5. Old man
Improving living conditions, reducing child mortality, defeating many diseases - all this leads to an increase in life expectancy.
If in the 16th and 17th centuries the average life expectancy was about 30 years, now in prosperous countries the average life expectancy for men reaches 75 years (Fig. 4), and for women 80.
Obviously, this is not the limit, and victory over cardiovascular and cancer diseases can extend a person’s life to 120-140 years, but at the same time, naturally, a person must lead a so-called healthy lifestyle and not abuse alcohol and nicotine.
Death is the cessation of human life, but death is necessary for the evolutionary process, since without it the change of generations would not occur.
The aging process is programmed genetically, but there is still no single theory to explain this process.
Some researchers believe that the aging process is triggered by a group of genes that initiates the so-called aging program - this point of view is confirmed by the existence of a rare human disease, progeria.
Types of human aging
Doctors say that the aging process is a complex, multifaceted and genetically determined process, which cannot be prevented, but it can be slowed down. There are two types of aging - physiological and pathological.
With physiological aging, a person’s age is similar to the passport age, and with pathological aging, accelerated aging of the body is observed, associated in most cases with various stress factors or with a genetic predisposition.
Long-lived animals
Old age and decrepitude are not a mandatory attribute of a long life. There are species that can live for centuries. These species include the pearl oyster mollusk (Fig. 6), which can live about 200 years without aging. Every year, the pearl mussel becomes more resistant to environmental conditions and even reproduces better.
Rice. 6. Bivalve mollusk pearl mussel. With age, the pearl oyster does not age, but becomes more viable.
But, nevertheless, even the pearl oyster turns out to be not immortal. She dies due to uncoordinated growth of the shell and the muscular leg that holds it in an upright position. The huge shell overweight and falls to the bottom, plankton filtration stops, and the pearl oyster dies of hunger.
The same thing haunts large turtles (Fig. 7), whose shell becomes too heavy.
Rice. 7. Giant turtles
As for mammals, there are also long-lived species among them. Such species include the bowhead whale (Fig. 8) - it can live for more than 200 years, and when it dies, it does not show signs of aging, and what it dies from is not yet known.
Rice. 8. Bowhead whale
Not long ago, a naked mole rat creature was discovered (Fig. 9), which lives in Africa. The lifespan of this creature is 28 years, and it is no larger in size than a vole. So, the naked mole rat managed to reverse the aging program. This animal does not have cardiovascular diseases, diabetes, or cancer - it has a powerful immune system that will remain active throughout its life. The mole rat has two teeth; a third of all the muscles of the body provide the work of these incisors. Excavators gnaw labyrinths underground with an area approximately the size of two football fields.
In the center of this labyrinth are the onto-queens, around there are 250 warriors. The queen has from one to three husbands; many males die in the fight against enemies, snakes and diggers from neighboring flocks. Scientists have not yet established what causes the natural death of mole rats in the absence of any other changes.
Rice. 9. Naked mole rat in a hole
Thus, the process of individual development is controlled by the genetic program of the organism, but can be regulated and corrected by environmental factors that the organism encounters.
A child with progeria shows obvious signs of old age; at 10-12 years old he looks like a very old person (Fig. 10).
Rice. 10. Children suffering from progeria
It has also been proven that DNA in any organism is constantly damaged by various physical and chemical influences. At a young age, special enzymes actively work to restore the normal structure of DNA, but in old age the enzymes function less and less, and the accumulation of errors in the DNA structure leads to cancer and metabolic disorders.
Bibliography
- A.A. Kamensky, E.A. Kriksunov, V.V. Beekeeper. General biology, grades 10-11. - M.: Bustard, 2005. Download the textbook from the link: ().
- D.K. Belyaev. Biology 10-11 grade. General biology. A basic level of. - 11th edition, stereotypical. - M.: Education, 2012. - 304 p. ().
- V.B. Zakharov, S.G. Mamontov, N.I. Sonin, E.T. Zakharova. Biology 11th grade. General biology. Profile level. - 5th edition, stereotypical. - M.: Bustard, 2010. - 388 p. ().
in English:
- Wikipedia ().
Homework
- What is ontogeny? What stages does it consist of?
- What is called postembryonic development of an organism? What are the stages of postembryonic development?
- What types of juvenile development occur in animals? What is their difference?
- Why does adulthood follow the juvenile period?
- What is old age? What do you know about the mechanisms of aging?
- Can a person live forever? What disease is associated with the onset of early old age?
- What hormones influence the transition from one stage of ontogenesis to another?
- What long-lived animals do you know? How do they fight aging? Why do they still die?
Ontogenesis is the individual development of an organism from the moment of its birth to death. Ontogenesis begins with fertilization(fusion of sperm and egg). In this case, a zygote is formed, which combines the hereditary material of the father and mother.
Germinal (embryonic) development
This is the period from the moment of fertilization until the child emerges from the egg (birth). Includes the stages of cleavage, gastrulation, and organogenesis.
Splitting up is a series of divisions of the zygote through mitosis. The interval between divisions is very short, during which only DNA doubling occurs, and cell growth does not occur (the egg was already very large). During the process of fragmentation, the cells gradually decrease in size until they reach normal size. After crushing it is formed morula(a ball of cells) and then blastula(a hollow ball of cells; the cells are blastomeres, the cavity is blastocoel, the wall is single-layered).
The blastula then turns into gastrulu- double-layer ball. The outer layer of gastrula cells is called ectoderm, internal - endoderm, the opening in the gastrula is called the primary mouth, it leads into the intestinal cavity.
Organogenesis(organ formation) begins with the formation of the neural plate in the ectoderm on the dorsal side of the embryo. Further
- the nervous system and skin are formed from the ectoderm;
- from the endoderm - the digestive and respiratory system;
- from the mesoderm - everything else (skeleton, muscles, circulatory, excretory, reproductive systems).
Postembryonic development
Lasts from birth (hatching) until the start of reproduction (puberty). There are two types:
- Direct- when the child is similar to the parent, only smaller in size and some organs are underdeveloped (mammals, birds).
- Indirect (with transformation, with metamorphosis)- when the child (larva) is very different from the parent (frogs, insects). The advantage of indirect development is that parents and children do not compete with each other for food and territory.
1. All the terms below are used to describe the embryo of an animal of the phylum Chordata. Identify two terms that “drop out” from the general list and write down the numbers under which they are indicated.
1) crushing
2) gastrulation
3) chitinization
4) organogenesis
5) mesoglea
Answer
2. All the terms below, except two, are used to describe the ontogenesis of the lancelet. Identify two terms that “fall out” from the general list and write down the numbers under which they are indicated.
1) blastopore
2) blastomere
3) crushing
4) parthenogenesis
5) metamorphosis
Answer
3. All the processes listed below, except two, occur during organogenesis in the embryonic development of the lancelet. Identify two processes that “fall out” from the general list and write down the numbers under which they are indicated.
1) formation of the skin
2) formation of blastomeres
3) the appearance of two germ layers
4) formation of the neural tube
5) laying the chord
Answer
Establish a correspondence between the process and the stage of lancelet embryogenesis: 1) blastula, 2) gastrula. Write numbers 1 and 2 in the correct order.
A) increase in the number of blastomeres
B) mitosis of the zygote
B) formation of germ layers
D) formation of blastocoel
D) intensive movements of cells inside the embryo
Answer
1. Establish the sequence of stages of individual human development, starting from the zygote. Write down the corresponding sequence of numbers.
1) formation of mesoderm
2) formation of a two-layer embryo
3) formation of the nervous system
4) formation of blastomeres
5) formation of a four-chamber heart
Answer
2. Establish the sequence of stages of embryonic development of animals
1) the appearance of mesoderm
2) formation of two germ layers
3) formation of blastomeres
4) formation of tissues and organs
Answer
3. Establish the sequence in which the processes of embryogenesis occur in the lancelet. Write down the corresponding sequence of numbers.
1) formation of a single-layer embryo
2) formation of mesoderm
3) formation of endoderm
4) organ differentiation
5) formation of blastomeres
Answer
4. Establish the correct sequence of ontogenetic processes in the lancelet.
1) zygote
2) blastula
3) organogenesis
4) neurula
5) gastrula
Answer
5. Establish the sequence of stages of embryonic development of the lancelet. Write down the corresponding sequence of numbers.
1) formation of a single-layer embryo
2) fragmentation of the zygote
3) formation of internal organs and organ systems
4) formation of the neural plate and notochord
5) formation of a two-layer embryo
Answer
Choose one, the most correct option. At the first stages of its development, the multicellular two-layer embryo is
1) neurulu
2) gastrula
3) morula
4) blastula
Answer
A) brain
B) skeleton
B) muscles
D) skin glands
D) lungs
E) liver
Answer
Choose one, the most correct option. In the embryonic development of a chordate animal, organogenesis begins with
1) mitotic division of the zygote
2) formation of a single-layer embryo
3) formation of the neural tube
4) the appearance of the first cleavage furrow
Answer
Establish a correspondence between developmental features and type: 1) Incomplete transformation, 2) Complete transformation. Write numbers 1 and 2 in the correct order.
A) The larva differs from the imago in structure
B) The habitat and food of the larva and imago are similar
C) The larva is similar to the adult
D) Passes in 3 stages
D) Goes through 4 stages
E) The larva has an underdeveloped reproductive system
Answer
Establish a correspondence between the characteristics of the process of vertebrate embryogenesis and the stage for which it is characteristic: 1) blastula, 2) gastrula
A) formation of endoderm
B) formation of a spherical embryo with a cavity inside
B) fragmentation of the zygote
D) formation of a single-layer embryo
D) formation of a two-layer embryo
E) formation of two germ layers
Answer
Choose one, the most correct option. In embryogenesis, the difference between blastula and gastrula is
1) in the formation of a two-layer embryo
2) in the development of germ layers
3) in the active movement of cells
4) in the formation of a single-layer embryo
Answer
Choose one, the most correct option. During the individual development of an animal, a multicellular organism is formed from the zygote as a result
1) gametogenesis
2) fertilization
3) meiosis
4) mitosis
Answer
Choose one, the most correct option. The development of the organism contributes to the weakening of competition between parents and offspring
1) germinal
2) historical
3) direct
4) indirect
Answer
Choose one, the most correct option. Individual development of the human body begins with the process
1) birth
2) formation of germ cells
3) blastula formation
4) fertilization
Answer
Establish a correspondence between the organ, tissue of a vertebrate animal and the germ layer from which they are formed: 1) endoderm, 2) mesoderm
A) intestines
B) blood
B) kidneys
D) lungs
D) cartilage tissue
E) heart muscle
Answer
1. Establish a correspondence between the structure of the human body and the germ layer from which it was formed: 1) ectoderm, 2) mesoderm
A) pain receptors
B) hair
B) lymph and blood
D) adipose tissue
D) nail plates
Answer
2. Establish a correspondence between the organ, tissue of a vertebrate animal and the germ layer from which they are formed during embryogenesis. Write the numbers in order corresponding to the letters: 1) ectoderm, 2) mesoderm
A) blood
B) tooth enamel
B) cartilage tissue
D) heart muscle
D) skin glands
Answer
Establish a correspondence between the characteristics and the stage of formation of the lancelet embryo: 1) single-layer embryo, 2) two-layer embryo. Write numbers 1 and 2 in the correct order.
A) invagination of a group of cells into the blastula
B) mitosis of the zygote
B) formation of the walls of the primary intestine
D) formation of blastocoel
D) formation of blastomeres
Answer
1. Establish a correspondence between the structures and germ layers indicated by numbers in the figure. Write numbers 1 and 2 in the correct order.
A) internal mucous membranes
B) adrenal glands
B) tissue fluid
D) lymph
D) epithelium of the alveoli
Answer
2. Establish a correspondence between the structures of the embryo and the germ layers, indicated in the figure by numbers 1 and 2, from which these structures are formed: write numbers 1 and 2 in the order corresponding to the letters.
A) muscles
B) alveoli
B) liver
D) blood vessels
D) heart
Answer
Establish a correspondence between the structures and germ layers indicated in the figure by numbers 1, 2: 1) ectoderm, 2) endoderm. Write numbers 1 and 2 in the correct order.
A) epidermis of the skin
B) nerve tissue
B) liver
D) pancreas
D) tooth enamel
Answer
Establish a correspondence between the organs and the germ layers from which they were formed: 1) ectoderm, 2) endoderm, 3) mesoderm. Write the numbers 1, 2 and 3 in the correct order.
A) brain
B) liver
B) blood
D) bones
D) pancreas
E) skin
Answer
Establish the sequence of processes of embryogenesis in the lancelet
1) blastula formation
2) fragmentation of the zygote
3) formation of three germ layers
4) gastrula formation
Answer
Choose three options. What tissues and organs of a vertebrate animal are formed from the cells indicated in the figure by number 1?
1) sweat glands
2) bone tissue
3) nail plates
4) connective tissue
5) skin epidermis
6) smooth muscle tissue
Answer
Establish the sequence of processes of embryonic development of a chordate animal. Write down the corresponding sequence of numbers.
1) gastrulation
2) formation of neurula
3) formation of morula
4) formation of mesoderm
5) blastula formation
6) fragmentation of the zygote
7) histogenesis
Answer
ECTODERM AND MESODERM
1. Establish a correspondence between the structures and the germ layers, indicated in the figure by numbers 1, 2, from which these structures are formed. Write numbers 1 and 2 in the correct order.
A) nerve tissue
B) blood
B) skeleton
D) smooth muscle tissue
D) skin epidermis
Answer
2. Establish a correspondence between the structures of the embryo and the germ layers, indicated in the figure by numbers 1, 2, from which these structures are formed. Write numbers 1 and 2 in the correct order.
A) nails
B) sense organs
B) blood
D) lymph
D) bone skeleton
Answer
3. Establish a correspondence between the structures and germ layers indicated in the figure by numbers 1 and 2. Write numbers 1 and 2 in the correct order.
A) nerve tissue
B) blood
B) bone
D) kidney
D) tooth enamel
E) muscle
Answer
4. Establish a correspondence between the structures and germ layers indicated in the figure by numbers 1, 2. Write numbers 1 and 2 in the order corresponding to the letters
A) epidermis of the skin
B) skeletal muscles
B) blood
D) retinal receptors
D) cartilage tissue
E) cerebral cortex
Answer
Establish a correspondence between the organs and the germ layers from which they develop: 1) ectoderm, 2) endoderm, 3) mesoderm. Write numbers 1-3 in the correct order.
A) brain
B) small intestine
B) cartilage
D) muscles
D) pancreas
E) hair
Answer
Establish a correspondence between the characteristic and the type of germ layer: 1) endoderm, 2) mesoderm. Write numbers 1 and 2 in the correct order.
A) is formed at the neurula stage
B) formed as a result of gastrulation
C) participates in the formation of the digestive organs
D) absent in the ontogenesis of hydra
D) participates in the formation of the circulatory system
Answer
Choose one, the most correct option. Are the following judgments about the individual development of organisms true? A) In the embryonic period of animal development, an increase in the number of cells occurs, and then their differentiation. B) The process of formation of a two-layer embryo occurs during the period of fragmentation during the division of blastomeres.
1) only A is correct
2) only B is correct
3) both judgments are correct
4) both judgments are incorrect
Answer
Select two of the processes listed below that relate to fertilization. Write down the numbers under which they are indicated.
1) fusion of male and female gametes
2) formation of gametes
3) formation of a zygote
4) fragmentation of the zygote
5) formation of a colony
Answer
Select two of the processes listed below that relate to embryogenesis. Write down the numbers under which they are indicated.
1) formation of gametes
2) formation of a zygote
3) blastulation
4) postembryonic development
5) gastrulation
Answer
Establish a correspondence between the processes and stages of the embryonic period: 1) fragmentation, 2) gastrulation. Write numbers 1 and 2 in the order corresponding to the letters.
A) directed movement and differentiation of cells is carried out
B) germ layers are formed
C) a multilayer embryo is formed
D) cellular material accumulates for further development
D) a blastula is formed
Answer
All but two of the processes below occur during gastrulation of the lancelet embryo. Identify two processes that “fall out” from the general list and write down the numbers under which they are indicated.
1) fragmentation of the zygote
2) invagination of a layer of cells into the cavity of the blastula
3) formation of the cavity of the primary intestine
4) formation of the neural tube
5) formation of two germ layers
Answer
All of the characteristics below, except two, refer to the indirect type of postembryonic development. Identify two characteristics that “drop out” from the general list and write down the numbers under which they are indicated.
1) helps to weaken the struggle for existence between parents and offspring
2) the birth of offspring that are similar in appearance to an adult organism
3) a born organism may differ from an adult organism in morphological characteristics, lifestyle (type of nutrition, nature of movement)
4) can be oviparous and intrauterine
5) metamorphosis occurs
Answer
Look at the drawing. Determine A) the stage of embryogenesis of a chordate animal, B) the germ layer, indicated in the figure by a question mark, and C) the organs that develop from it. For each letter, select the corresponding term or concept from the list provided.
1) gastrula
2) neurula
3) blastula
4) ectoderm
5) endoderm
6) mesoderm
7) kidneys, muscles, heart
8) liver, pancreas
Answer
Establish a correspondence between the characteristics and stages of lancelet ebryogenesis: 1) blastula, 2) gastrula, 3) neurula. Write numbers 1-3 in the order corresponding to the letters.
A) formation of mesoderm
B) the presence of undifferentiated cells
B) invagination of blastula cells into the cavity
D) formation of the cavity of the primary intestine
D) three-layer embryo
Answer
Establish a correspondence between the processes and stages of lancelet embryogenesis: 1) gastrula, 2) neurula. Write numbers 1 and 2 in the order corresponding to the letters.
A) formation of two germ layers
B) formation of the neural tube
B) formation of the chord
D) differentiation of cells into ectoderm and endoderm
D) formation of the cavity of the primary intestine
Answer
© D.V. Pozdnyakov, 2009-2019
The period of postembryonic development of multicellular animals consists of three stages.
Juvenile(from birth to puberty of an individual) can occur in different animals as direct development or with metamorphosis. In the first case (for example, in reptiles, birds, mammals), a new organism emerges from the egg shells or from the mother’s body, having all the main organs characteristic of an adult animal. The difference lies only in the smaller size, different proportions of the body of the newborn individual and the underdevelopment of the reproductive system. However, in a considerable number of invertebrates (coelenterates, worms, mollusks, insects) and some chordates (ascidians, amphibians) development occurs with metamorphosis (with “transformation”). A larva emerges from the egg, which in its structure, lifestyle, diet, and habitat is significantly different from the adult animal. Suffice it to recall a caterpillar and a butterfly, a tadpole and an adult frog. Many organs of the larva are fundamentally different from those of the adult animal and function only during the juvenile stage. “Transforming” (performing metamorphosis), the larva loses them (for example, in a tadpole - gills, tail, lateral line, etc.) and acquires organs characteristic of an adult (in a frog - limbs, lungs, second circle of blood circulation) (Fig. . 1).
Rice. 1.Sequence of developmental stages of a frog tadpole
In insects, there are two forms of metamorphosis - incomplete And full(Fig. 2) .
Rice. 2.Comparison of insect development with incomplete transformation: a - grasshopper; b - and an insect with complete transformation - the butterfly Platysamia cecropia
With incomplete metamorphosis, characteristic of primitive insects (cockroaches, grasshoppers, crickets), the larva and adult animal are similar in structure, nutrition, and lifestyle. The larva differs only in its smaller size, underdeveloped reproductive system and (in winged insects) wings. Through a series of molts, it grows, develops, and at the last molt turns into an adult insect.
In the case of complete metamorphosis (butterflies, beetles, Hymenoptera, Diptera), in addition to the larval stage, there is also a stage pupae. An insect larva with complete metamorphosis differs sharply from an adult in its structure, habitat, and method of feeding. Their transformation into an adult insect occurs precisely at the pupal stage. Outwardly, it is passive (does not feed, most often is motionless), but very complex processes occur inside it. At the first stage, the tissues and organs of the pupated larva are almost completely destroyed. And then, from special undifferentiated cells (still in the body of the larva), due to their intensive division and differentiation, the organs of an adult insect are formed. It is released from the shells of the pupa and moves on to active existence.
At the juvenile stage of development, there is a decrease in the rate of growth and differentiation, although they are still clearly expressed.
Having reached sexual maturity, the animal enters reproductive stage(maturity stage). It multiplies, and its growth and formation of new organs completely stops. The body now only undergoes processes of self-renewal of structures that wear out in the process of life, as well as their traumatic damage.
From a certain (for each species) moment in ontogenesis begins aging stage. Restorative processes (self-renewal of structures) in the body slow down. As a result, there is a decrease in the mass and size of organs and the entire body. Over time, the “inhibition” of processes in the body increases, and at some point the functioning of vital systems stops and natural death occurs.
The concept of natural death implies the absence of any disease or damage that caused the death of the body. According to modern concepts, the lifespan of individuals of each species is genetically programmed. And therefore the cessation of individual life is a natural event.
