Stages of embryonic development of vertebrates. Embryonic development

Regardless of the method of reproduction, the beginning of a new organism is given by one cell containing hereditary inclinations and possessing all characteristic features and the properties of the whole organism.

Individual development consists in the gradual implementation of hereditary information received from parents.

Evolutionary embryology was initiated by the Russian scientists A.O. Kovalevsky and I. I. Mechnikov. They first discovered three germ layers and established the principles of development of invertebrates and vertebrates. Ontogeny, or individual development, is the entire period of an individual's life from the moment the zygote is formed to the death of the organism. Ontogenesis is divided into two periods:

Embryonic period: from the formation of the zygote to birth or release from the egg membranes;
- postembryonic period: from the release of the egg membranes or birth to the death of the organism.

The embryonic development of chordates takes place in a number of stages:
splitting up;
gastrulation;
laying of axial organs and neurulation;
histo- and organogenesis

The table presents comparative analysis all stages of embryogenesis in various groups of chordates.

	*Lancelet*	*Amphibians*	*Birds, reptiles*	*Mammals*
Egg type	Isolecital	Moderate telolecital	Telolecital	Alecital
Fertilization	Outdoor	Outdoor	Internal	Internal
Splitting up	Full uniform synchronous	Complete uneven, asynchronous	Incomplete discoidal	Full, uniform, asynchronous
Blastula	Celloblastula	Amphiblastula	Discoblastula	Blastocyst
Gastrulation	Invagination invagination	Epibolia fouling	Imigration delimation penetration	Delamination bundle
Axial organs complex	All are formed - notochord, neural tube, intestinal tube
Histogenesis	Everyone has 3 germ layers - ectoderm, mesoderm, endoderm
Organogenesis	From ectoderm - skin epidermis, tooth enamel, nervous system. An epithelium of different forms is formed from the endoderm. organs: the midgut and its outgrowths. From the mesoderm are formed: connective tissue (bones, tendons, lymph node, blood); dentin of teeth, muscle tissue, epithelium of the genitourinary system.
Embryonic membranes	Not formed	Not formed	Amniotic, serous, allantois, yolk sac	Placenta, chorion
Post type embryonic development		Indirect

The stage of cleavage depends on the structural features of the egg.

The amount of yolk contained in an egg varies considerably; it serves as the main factor determining the size of the egg and the type of zygote cleavage:

and) oligolecital eggs - they contain little yolk, the nuclei are usually located in the center of the ovum; this type of eggs is typical for lancelet and humans, since a small amount of yolk is evenly distributed in the cytoplasm, therefore this type of eggs is also called isocytal.

b) mesolecital eggs contain a moderate amount of yolk, most often unevenly distributed in the cytoplasm. Mesolecital eggs are common among primitive aquatic forms, which indicates that they were characteristic of ancestral vertebrates. Since most anamnias are aquatic forms, their eggs contain a yolk located in the lower half of the egg, such eggs are called telolectile.

in) Sharks and rays on the one hand, and reptiles and birds on the other, have large eggs. They are called polylecital, because most of the cell is occupied by the yolk, and the cytoplasm, which is relatively small, is concentrated at one pole. Since the polarity is sharply expressed in these oocytes, these cells are also called sharply lecid cells.

Crushing is the process of division, as a result of which blastula is formed.

The nature of oocyte cleavage depends on the amount of yolk in the oocyte. The yolk, being inert, does not play an active role in cleavage, which is carried out by the nucleus and cytoplasm of the cell; it exhibits a local inhibiting effect by mechanically inhibiting this process.

The following types of cleavage are distinguished: complete - holoblastic, when the entire cytoplasm of the zygote undergoes cleavage and meroblastic or incomplete, when the cytoplasm undergoes cleavage only in the animal pole - this type of cleavage (sharp telolecital eggs) is called discoidal. According to the crushing time, uniform and uneven are distinguished. Crushing in animals with isocyte eggs is holoblastic.

Anamnias are characterized by incomplete cleavage, discoidal in birds and reptiles; complete, uniform, asynchronous - in mammals.

The next stage of embryonic development is gastrulation. At this time, blastomeres, which continue to divide rapidly, acquire locomotor activity and move relative to each other, forming layers of cells - germ layers. Gastrulation can occur either by invagination (invagination) of one of the blastula walls into the blastocoel cavity, by immigration of individual cells, epibolism (fouling), or delamination (splitting into two plates).

As a result, the outer germ layer is formed - the ectoderm, and the inner one - the endoderm. In most multicellular animals, a third, middle germ layer is formed between them - the mesoderm, formed from cells lying on the border between the outer and inner layers.

The embryo in the period of neurulation following gastrulation is called neurula. Neurulation begins with a thickening of the ectoderm on the dorsal side of the embryo - the neural plate, which is determined under the inducing influence of the chordomesoderm during gastrulation. Folds rise along the edges of the neural plate - nerve rollers, its middle part gradually deepens, the roller approaches, merging along the middle dorsal line, etc. the neural plate becomes a neural tube. The latter is separated from the rest of the ectoderm, which is transformed into the integumentary epithelium; between the dorsal side of the neural tube and the integumentary epithelium is a derivative of the nerve rollers - the neural crest.

During the period of neurulation, formation processes also occur in other germ layers. In animals with complete cleavage of the endoderm, during this period, the gastrocoel is completely surrounded, which turns into the cavity of the definitive intestine. The induction interaction between the parts of the embryo continues during neurulation, determining the further dissection of the neural tube into sections of the central nervous system, as well as further differentiation of the mesodermal and endodermal organs.
By the end of neurulation, the embryo acquires a plan of the structure of an adult organism: on the dorsal side, under the epithelium, there is a neural tube, under it is the notochord, below the intestine, the front and rear ends of the body become distinguishable. In embryology there is a term - embryonic induction - mutual influence parts of the embryonic organism.

Further, the processes of histogenesis (tissue formation) and organogenesis (organ formation) in the embryo (embryo) begin. As a result of differentiation of the cells of the germ layers, various tissues and organs of the developing organism are formed. Integuments and the nervous system are formed from the ectoderm. Due to the endoderm, the intestinal tube, liver, pancreas, and lungs are formed. The mesoderm produces all the other systems: musculoskeletal, circulatory, excretory, reproductive. The discovery of the homology (similarity) of three germ layers in almost all animals served as an important argument in favor of the point of view of the unity of their origin.

By the end of the embryonic period, the embryo already has all the main organs and systems that ensure viability in the external environment.
The embryonic period ends with the birth of a new individual, capable of independent existence.

The period of embryonic development is the most difficult in higher animals and consists of several stages:

1 the formation of a zygote

2. Crushing

3.Education of blastula

4. Gastrulation

5. Histo- and organogenesis

The first stage of the embryonic period is the formation of a zygote. Zygote is a unicellular embryo or unicellular stage of development of an organism.

A number of processes take place in the zygote:

a) movement of the cytoplasm (cytoplasmic structures) - this leads to the formation of bilateral symmetry and polarity.

b) reconstruction of the CPM. A superficial (cortical) layer appears, which excludes the fusion of the zygote with other male reproductive cells.

c) the formation of a nuclear envelope around the fused nuclei (syncarion)

d) RNA synthesis, protein synthesis is carried out.

Cleavage is accompanied by mitosis, as a result of which the unicellular embryo becomes multicellular. However, the embryo does not increase in size, there is no cell growth, the volume of the embryo does not change, the interphase is very short, G1 is absent.

The cells that form during the cleavage process are called blastomeres. The size of the cells becomes smaller with each division. The nature of cleavage is not the same in different animals and depends on the amount of yolk and its distribution in the cytoplasm. The larger the yolk, the slower this part of the cytoplasm divides.

Crushing ends with the formation of blastula. Blastula Is a multicellular single-layer embryo.

Blastula has a wall (layer of cells) - blastoderm. Inside the blastula there is a cavity - the blastocoel or the primary body cavity, filled with fluid. The fluid is secreted by blastomeres. In the blastula, the roof (where the animal pole of the ovum was) and the bottom (the vegetative pole of the cell) are distinguished, and the marginal zone between them.

Gastrulation is the period of formation of germ layers. Gastrulation is a complex process of chemical and morphological changes, which are accompanied by cell division, cell growth, directional movement and differentiation of cells. As a result of these processes, a two-layer embryo is first formed - the gastrula, consisting of the outer embryo layer - the ectoderm and the inner - endoderm. This stage is called early gastrula. At the stage of late gastrula, the third germ layer, the mesoderm, is formed.

The germ layers differ from each other not only in their location, but also in the size and shape of the cells. Each germ layer subsequently gives rise to certain tissues and organs. It was at the stage of gastrulation that for the first time it was possible to detect in high concentrations proteins specific for some areas of cellular differentiation of an adult organism (for example, muscle tissue protein - myosin).

Histogenesis - the process of tissue formation in embryogenesis. Organogenesis - the process of forming organ systems in embryogenesis.

At this stage of embryonic development, two phases are distinguished.

1. Neurulation - the formation of axial organs: neural tube, chord. The embryo at this stage is called neurula.

This phase proceeds as follows: a group of cells is flattened from the ectoderm on the dorsal side of the embryo and a neural plate is formed. The edges of the neural plate are raised and nerve ridges are formed. Along the midline of the neural plate, cells move and a depression appears - the neural groove. The edges of the neural plate are closed.

As a result of these processes, a neural tube with a cavity - a nervecoel - appears. The neural tube sinks under the ectoderm. The anterior neural tube forms the brain, and the rest of the neural tube forms the spinal cord.

The process of neural tube formation can be conditionally divided into 3 stages:

Neural plate formation

Formation of the neural groove,

Fusion of the edges of the neural plate with the formation of a neural tube.

Part of the cells of the ectoderm of the dorsal side of the embryo is not part of the neural tube and forms an accumulation of cells along the neural tube, called the ganglion plate. From which pigment cells of the epidermis of the skin, hair, feathers, nerve cells of the spinal and sympathetic nerve nodes are formed.

Notochord formation also occurs at an early stage of neurulation from the endomesodermal (common with endoderm and mesoderm) anlage of the primary intestinal wall. The chord is located under the neural tube

The second phase of histo - and organogenesis of embryonic development is associated with the development of individual organs and tissues.

From the material of the endoderm, the epithelium of the esophagus, stomach and intestines, liver cells, part of the cells of the pancreas, the epithelium of the lungs and airways, secreting cells of the pituitary and thyroid gland are formed.

From the material of the ectoderm, the epidermis of the skin and its derivatives develop - feather, claws, hair, mammary glands, skin glands (sebaceous and sweat), nerve cells of the organs of vision, hearing, smell, oral epithelium, tooth enamel.

8. Embryonic development of animals

Embryogenesis - the development of the embryo - begins from the moment of fertilization and the formation of a zygote and ends with the birth of an organism or its release from an egg. This process takes place in several stages.

Splitting up

After the fusion of the nuclei of two gametes and the formation of a zygote, the development of the embryo begins. The first stage of development is called fragmentation. As a result of mitosis, the egg begins to divide rapidly into two, then into four, and the second cleavage groove runs perpendicular to the first. 4 cells are formed, which are called blastomeres. As a result of subsequent cleavage, 8, 16, 32, etc. blastomeres are formed. Cleavage differs from ordinary mitosis in that cells practically do not increase in size, do not grow. The process is very fast. For example, 64 cells are formed from the zygote cell in 4 hours from the moment of fertilization. The interphase between divisions is very short and consists only of the DNA replication stage. The presynthetic period is absent, i.e., protein synthesis does not occur, the cleavage embryo lives on the substances accumulated in the egg. The total weight of the embryo does not change at this stage.

The nature of cleavage depends on the type of animal and the type of egg (Fig. 16).

Figure: 16. The initial stages of egg breaking: A - lancelet, B - frogs, C - birds

It can be uniform or complete, when the egg is completely divided into blastomeres (lancelet, sea urchin, mammals), or it can be incomplete when there is a lot of yolk in the egg and only the upper disk of the egg is crushed (birds, reptiles, fish).

Blastula stage

Crushing ends in education blastula - a single-layer embryonic vesicle with a cavity inside. The walls of the vesicle are formed by a single layer of cells (Fig. 17, A).

Figure: 17. Stages of development of the embryo. A - blastula; B - gastrula; B - neurula: 1 - ectoderm; 2 - endoderm, from which the intestinal tube is formed; 3 - gastric cavity - gastrocoel; 4 - blastopore; 5 - mesoderm; 6 - neural plate (tube); 7 - chord

Gastrulation

After the formation of the blastula, the second stage of embryo development begins - gastrula(Fig. 17, B). Gastrulation begins with the invagination of the lower blastula cells into the cavity. As a result, two layers of cells and a cavity with a hole are formed - blastopore. Gastrula cavity - gastric cavityfurther turns into an intestinal cavity.

Gastrula is a two-layer embryo sac, the outer outer layer of cells of which is called ectoderm, and the inner layer - endoderm. At the stage of two germ layers, development ends in sponges and coelenterates. In the rest of the animals, the formation of the third germ layer goes on - mesoderm.

Neurula stage

The stage following the gastrula is called neurula and is characterized by the formation of the third germ layer and neural tube. From the side of the lower part of the embryo, cell migration occurs. These cells then give rise to another layer of cells - mesoderm. Between the ectoderm and the endoderm, the third germ layer is laid. On both sides of the primary intestine - the gastrocoel, mesoderm cells are formed, one layer of cells of which borders on the endoderm, and the other adjoins the ectoderm. A three-layer embryo is formed. The subsequent development of the embryo is associated with the interaction of three germ layers, from the cells of which the tissues and organs of the future organism develop.

At the same time, the upper cells of the ectoderm thicken, move inward, forming the so-called neural plate. The edges of the neural plate roll up into a tube that separates from the ectoderm and forms the neural tube. Later, the brain and spinal cord of vertebrates is formed from it. Another axial organ, the chord, is formed from the cells of the mesoderm under the neural tube along the longitudinal axis. The digestive tube is located under the chord.

At the end of the neurula stage, an axial complex is formed: the neural tube, notochord, and digestive tube. On both sides of the neural tube and notochord are large areas of the mesoderm, from which the skeleton, muscles and other organs are subsequently formed.

Organogenesis

All tissues and organs of the future organism develop from the three germ layers. The laying and development of organs is called organogenesis.

From the ectoderm, the skin develops - the epidermis and its derivatives (nails, hair, sebaceous and sweat glands, tooth enamel), the nervous system, sense organs, as well as some of the endocrine glands.

From the endoderm, epithelial tissue develops, lining the organs of the digestive, respiratory (alveoli), genitourinary system, as well as the digestive glands: liver, pancreas. All internal mucous membranes are formed from endoderm. Thus, all types of epithelial tissue are formed from the ectoderm and endoderm.

Muscle and all types of connective tissue are formed from the mesoderm. The cartilaginous and bone skeleton is subsequently formed from the notochord, and muscles, the circulatory system, the heart, the kidneys, and the reproductive system are formed from the lateral sections of the mesoderm.

The endocrine glands have different origins: some of them develop from the neural tube (pituitary gland, pineal gland), others directly from the ectoderm (thyroid gland). The adrenal glands and sex glands are formed from the mesoderm.

The interaction of parts of the embryo

The result of the development of an organism from an egg is determined by the set of chromosomes and genes of the given organism. All cells of the embryo develop from one original cell - the zygote, have the same set of chromosomes and genetic information... However, different sets of genes function in different germ layers, which leads to the formation of different tissues and organs. Thus, in the course of development, with a constant set of genes of all cells, their activity changes.

To study this process, an experiment was carried out on the transplantation of a frog skin nucleus into an unfertilized egg, in which its own nucleus was previously destroyed. The egg cell was stimulated to develop by a special injection with a micropipette. A normal blastula, gastrula and then a tadpole developed from an egg cell with a transplanted diploid nucleus. The result of the experiment proves that the constancy of the set of genes is preserved in all cells, and their specialization in the development process is the result of the action of certain factors.

The specificity of the work of cells does not arise immediately, but at a certain stage of embryogenesis. It has been established that at the stage of 4–16 blastomeres (depending on the animal species), each cell can develop into a normal organism, ie, it has equal heredity. Further, this ability is gradually lost. In the rabbit, equal heredity is preserved at the stage of 4 blastomeres, in the newt - 16 blastomeres, in humans - at stage 4, less often 6 blastomeres, which is confirmed by the birth of 4, rarely 6 identical twins. Further, the blastomeres lose the property of equal heredity and differentiate. Regulation of gene activity occurs on molecular level due to regulatory proteins. Specific substances, hormones, enter the nucleus from the cytoplasm, which act on regulatory proteins and thereby activate or suppress the activity of the corresponding genes. In the process of development, the specialization of cells is the result of the interaction of the nucleus and the cytoplasm, as well as the action of environmental factors.

Cell differentiation is the basis for the formation of tissues and organs. Substances or a group of cells that stimulate the development of organs and tissues of the embryo are called inductors or organizers, and the phenomenon of stimulation - embryonic induction.

So, the organizers directing the development of the neural tube are the cells of the mesoderm and notochord. They secrete special substances that act on the ectoderm and stimulate the development of the neural tube. If a part of the ectoderm at the stage of early gastrula is transplanted from the upper dorsal part down to the abdominal side, then the abdominal skin will develop from it. If, on the contrary, the lower part of the ectoderm is transplanted to the upper side, then a neural plate will develop from it. Experiments on the transfer of different parts of the embryo have made it possible to determine the role of each part in embryonic induction.

It has been established that there are critical periods in the development of the embryo when a violation can occur normal development... Such periods are, for example, the middle of cleavage, the beginning of gastrulation, and the formation of axial organs. At this time, the embryo is especially sensitive to a lack of oxygen, temperature changes, and mechanical stress. Critical periods coincide with the differentiation of tissues and organs. The better the egg is protected, the less susceptible it is to external influences. For example, the death of fish eggs is several times higher than that of embryos in bird eggs. In mammals, the development of the embryo occurs in the mother's body, so the likelihood of embryonic death is much less.

The prenatal development of the fetus is influenced by the mother's living conditions. Primary oocytes can be adversely affected even before pregnancy. It is known that oocytes of the 1st order are laid in embryonic development and then periodically mature throughout the entire childbearing period of a woman. But the older the woman, the older the oocytes are, which means that they are more susceptible to change under the influence various factors, the likelihood of any anomaly occurring in them is increased. Statistics show that the older a woman is, the higher the likelihood of having a child with abnormalities. Various viral diseases, the use of certain medications (antibiotics, hormonal drugs) also have a negative effect on the development of the embryo, narcotic substances, alcohol. X-rays and other ionizing radiation are powerful factors causing abnormalities in the development of the embryo.

Questions for self-control

1. Compare the different types of egg cleavage shown in Figure 16. Explain the differences in cleavage in different organisms.

2. What are the names of the cells formed as a result of cleavage?

3. What is the difference between crushing and ordinary division?

4. What are the main stages of development of the embryo.

5. At what stage does cell differentiation take place?

6. Fish, amphibians, reptiles, birds have very large eggs. In mammals, they are much smaller. What is the reason for this?

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The development of the human body begins from the very first day of fertilization of an egg with a sperm. The stages of embryogenesis are counted from the moment the cell begins to develop, which subsequently forms an embryo, and a full-fledged embryo appears from it.

The development of the embryo fully begins only from the second week after fertilization, and starting from the 10th week, the fetal period is already carried out in the mother's body.

The first stage of the zygote

Absolutely all somatic cells of the human body have a double set of chromosomes, and only sex gametes contain a single set. This leads to the fact that after fertilization and fusion of the male and female germ cells, the set of chromosomes is restored and becomes double again. The resulting cell is called a "zygote".

The characteristic of embryogenesis is such that the development of the zygote is also divided into several stages. Initially, the newly formed cell begins to divide into new cells of different sizes, called morula. The intercellular fluid is also unevenly distributed. A feature of this stage of embryogenesis is that morula formed as a result of division do not grow in size, but only increase in number.

Second phase

When cell division ends, blastula is formed from them. It is a single-layered embryo the size of an egg cell. Blastula already carries all the necessary DNA information and contains cells of unequal size. This happens already on the 7th day after fertilization.

After that, the single-layer embryo passes through the stage of gastrulation, which is the movement of existing cells into several germ sheets - layers. First, 2 of them are formed, and then a third appears between them. During this period, a new cavity is formed in the blastula, called the primary mouth. The previously existing cavity disappears completely. Gastrulation enables the future embryo to clearly distribute cells for the further formation of all organs and systems.

In the future, all skin, connective tissues and the nervous system are formed from the first formed outer layer. The lower, formed by the second, layer becomes the basis for the formation of the respiratory organs, excretory system. The last, middle cell layer is the basis for the skeleton, circulatory system, muscles and others internal organs.

The layers in the scientific environment are named accordingly:

ectoderm;
endoderm;
mesoderm.

Third stage

After all the above stages of embryogenesis have passed, the embryo begins to grow in size. In a short time, it begins to represent a cylindrical organism with a clear distribution on the head and tail ends. The growth of the finished embryo continues until day 20 after fertilization. At this time, the plate formed earlier from the cells, the precursor of the nervous system, is transformed into a tube, which later represents the spinal cord. Other nerve endings gradually grow from it, filling the entire embryo. Initially, the processes are divided into dorsal and abdominal. Also at this time, the cells are also distributed for further division between muscle tissues, skin and internal organs, which are formed from all cell layers.

Extraembryonic development

Everything initial stages embryogenesis occurs in parallel with the development of extraembryonic parts, which will further provide the embryo and the fetus with nutrition and support life.

When the embryo is already fully formed and out of the tubes, the embryo is attached to the uterus. This process is very important, since the future activity of the fetus depends on the correct development of the placenta. It is at this stage that embryos are transferred during IVF.

The process begins with the formation of a nodule around the embryo, which is a double layer of cells:

embryoplast;
trophoblast.

The latter is the outer shell, therefore, is responsible for the effectiveness of the embryo's attachment to the walls of the uterus. With its help, the embryo penetrates into the mucous membranes of the female organ, implanting itself directly into their thickness. Only a reliable attachment of the embryo to the uterus gives rise to the next stage of development - the formation of a child's place. The development of the placenta is carried out in parallel with its separation from the litter. The process is ensured by the presence of a trunk fold, which, as it were, pushes the walls away from the body of the embryo. At this stage of development of the embryo, the only connection with the placenta is the umbilical stalk, which later forms a cord and provides nutrition for the baby for the rest of the intrauterine period of his life.

Interestingly, the early stages of embryogenesis in the region of the umbilical stem also have a yolk duct and a yolk sac. In non-placental animals, birds, and reptiles, this sac represents the yolk of the egg through which the embryo receives nutrients during its formation. In humans, this organ, although it is formed, has no effect on the further embryonic development of the body, and over time it simply decreases.

The umbilical cord contains blood vessels that carry blood from the embryo to the placenta and back. Thus, the embryo receives nutrients from the mother and removes metabolic products. This part of the connection is formed from the allantois or part of the urinary sac.

The embryo developing inside the placenta is protected by two membranes. In the inner cavity there is a protein liquid, which is a water shell. The baby swims in it before its birth. This sac is called the amnion, and its filling is amniotic fluid. All are enclosed in one more shell - the chorion. It has a villous surface and provides the embryo with breathing and protection.

Step by step consideration

In order to analyze in more detail human embryogenesis in a language understandable to most, it is necessary to start with its definition.

So, this phenomenon is the intrauterine development of the fetus from the day of fertilization until birth. This process begins only after 1 week after fertilization, when the cells have already finished dividing and the finished embryo moves into the uterine cavity. It is at this time that the first critical period begins, since its implantation should be as comfortable as possible for the mother's body and for the embryo itself.

This process is carried out in 2 stages:

tight attachment;
penetration into the thickness of the uterus.

The embryo can be attached to any part of the uterus, except for the lower part. It is important to understand that this entire process is carried out for at least 40 hours, since only gradual actions can ensure complete safety and comfort for both organisms. The place of attachment of the embryo after attachment is gradually filled with blood and overgrown, after which the most important period of development of the future person begins - the embryonic one.

First organs

The embryo attached to the uterus already possesses organs that somewhat resemble the head and tail. The very first, after the successful attachment of the embryo, a protective organ develops - the chorion. To more accurately imagine what he is, you can draw an analogy with a thin protective film of a chicken egg, which is located directly under the shell and separates it from the protein.

After this process, organs are formed that provide further nutrition for the crumb. Already after the second week of pregnancy, you can observe the appearance of allantois, or the umbilical cord.

Third week

The transfer of embryos to the stage of the fetus is carried out only after the completion of its formation, but already in the third week you can notice the appearance of clear outlines of future limbs. It is during this period that the body of the embryo is isolated, the trunk fold becomes noticeable, the head stands out and, most importantly, the future baby's own heart begins to beat.

Power change

This period of development is marked by another important stage. Starting from the third week of life, the embryo stops receiving nutrition according to the old system. The fact is that the reserves of the egg are depleted by this moment, and for further development the embryo needs to receive the substances necessary for further formation from the mother's blood. At this point, to ensure the efficiency of the entire process, allantois begins to transform into the umbilical cord and placenta. It is these organs that will provide the fetus with nutrition and free it from waste products for the rest of the intrauterine time.

Fourth week

At this time, it is already possible to clearly determine the future limbs and even the places of the eye sockets. Externally, the embryo changes slightly, since the main emphasis of development is given to the formation of internal organs.

Sixth week of pregnancy

At this time, the expectant mother should pay special attention to her own health, since during this period the thymus gland of her future baby is being formed. It is this organ that will be responsible for the performance of the immune system throughout its life. It is very important to understand that the mother's health will also affect the ability of her child to resist external stimuli throughout her independent life. You should not only pay attention to the prevention of infections, but also to warn yourself against nervous situations, to monitor the emotional state and the environment.

Eighth seven-day

Only starting from this time threshold, the expectant mother can find out the gender of her child. Exclusively at 8 weeks, the sexual characteristics of the fetus and the production of hormones begin to form. Of course, you can find out the gender if the child himself wants it and turns the right side on the ultrasound.

The final stage

Starting from the 9th week, the fetal ends and begins. By this time, a healthy baby should already have formed all the organs - they just have to grow. At this time, the child's body weight is actively gaining, his muscle tone increases, the hematopoietic organs are actively developing; the fetus begins to move erratically. Interestingly, the cerebellum by this time is usually not yet formed, so the coordination of fetal movements occurs over time.

Dangers during development

Different stages of embryogenesis have their weak points. To understand this, you need to consider them in more detail. So, in some periods, human embryogenesis is sensitive to infectious diseases of the mother, and in others - to chemical or radiation waves from the external environment. If problems arise during such a critical period, the risk of developing birth defects in the fetus will increase.

To avoid this phenomenon, you should know all the stages of embryo development and the dangers of each of them. So, the period of blastula is a special sensitivity to all external and internal stimuli. At this time, most of the fertilized cells die, but since this stage passes in the first 2, most women do not even know about it. The total number of embryos dying at this time is 40%. at the moment it is very dangerous, since there is a risk of rejection of the embryo by the mother's body. Therefore, during this period, you need to take care of yourself as much as possible.

The transfer of embryos into the uterine cavity marks the beginning of the period of the greatest vulnerability of the embryo. At this time, the risk of rejection is no longer so great, but from the 20th to the 70th days of pregnancy, all vital organs are laid, for any negative impacts on the maternal body at this time, the likelihood of developing congenital health abnormalities in the unborn baby increases.

Usually, by the end of the 70th day, all organs are already formed, but there are also cases of delayed development. In such situations, with the onset of the fetal period, there is a danger for these organs. Otherwise, the fetus is already fully formed and begins to actively increase in size.

If you want your unborn child to be born without any pathologies, then monitor your health both before and after the moment of conception. Lead the right lifestyle. And then no problems should arise.

Ontogenesis is the individual development of an organism from the moment of its inception to death. Ontogenesis begins with fertilization (fusion of a sperm and an egg). In this case, a zygote is formed, in which the hereditary material of the father and mother is combined.

Embryonic (embryonic) development

This is the period from the moment of fertilization until the baby comes out of the egg (birth). It includes the stages of cleavage, gastrulation, organogenesis.

Splitting up is a series of divisions of the zygote by mitosis. The interval between divisions is very short, only DNA duplication occurs in it, and cell growth does not occur (the egg was already very large). In the process of cleavage, cells gradually shrink until they reach normal size. After crushing, it forms morula (ball of cells) and then blastula (hollow ball of cells; cells - blastomeres, cavity - blastocoel, single-layer wall).

Then the blastula turns into gastrulu - two-layer ball. The outer layer of gastrula cells is called ectoderm, internal - endodermThe opening in the gastrula is called the primary mouth, and it leads into the intestinal cavity.

Organogenesis (organ formation) begins with the formation of a 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 systems;
from the mesoderm - everything else (skeleton, muscles, circulatory, excretory, reproductive systems).

Postembryonic development

It lasts from birth (hatching) to the start of reproduction (the onset of puberty). There are two types:

Direct - when the child is similar to the parent, only smaller in size and some organs (mammals, birds) are underdeveloped.
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 of the following terms are used to describe the Chordate animal embryo. Define two terms "falling out" of 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 of the terms listed below, except two, are used to describe the embryonic stage of ontogenesis. Identify two terms that "fall out" from the general list, and write down the numbers under which they are indicated.
1) larva
2) blastula
3) gastrula
4) imago
5) mesoderm

Answer

3. All of the following terms, except two, are used to describe the ontogeny 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

Establish a correspondence between the process and the stage of lancelet embryogenesis: 1) blastula, 2) gastrula. Write down the numbers 1 and 2 in the correct order.
A) an increase in the number of blastomeres
B) zygote mitosis
C) the formation of germ layers
D) blastocoel formation
E) intensive movement 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) the formation of mesoderm
2) the formation of a two-layer embryo
3) the formation of the nervous system
4) the formation of blastomeres
5) the formation of a four-chambered heart

Answer

2. Establish the sequence of stages of embryonic development of animals
1) the appearance of mesoderm
2) the formation of two germ layers
3) the formation of blastomeres
4) the 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) the formation of a single-layer embryo
2) the formation of mesoderm
3) the formation of endoderm
4) organ differentiation
5) the 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) the formation of a single-layer embryo
2) crushing the zygote
3) the formation of internal organs and organ systems
4) the formation of the neural plate and chord
5) the formation of a two-layer embryo

Answer

Choose the one that is most correct. At the first stages of its development, a multicellular two-layer embryo is
1) neurulu
2) gastrulu
3) morulu
4) blastulu

Answer

A) brain
B) skeleton
B) muscles
D) skin glands
D) lungs
E) liver

Answer

Choose the one that is most correct. In the embryonic development of the chordate animal, organogenesis begins with
1) mitotic division of the zygote
2) the formation of a single-layer embryo
3) neural tube formation
4) the appearance of the first furrow of division

Answer

Establish a correspondence between developmental features and type: 1) Incomplete transformation, 2) Complete transformation. Write down the numbers 1 and 2 in the correct order.
A) The larva differs from the adult in structure
B) The habitat and food of the larva and the imago are similar
C) The larva looks like an imago
D) It takes place in 3 stages
E) It 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) the formation of endoderm
B) the formation of a spherical embryo with a cavity inside
C) crushing the zygote
D) the formation of a single-layer embryo
E) the formation of a two-layer embryo
E) the formation of two germ layers

Answer

Choose the one that is most correct. 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 the one that is most correct. With the individual development of an animal from a zygote, a multicellular organism is formed as a result
1) gametogenesis
2) fertilization
3) meiosis
4) mitosis

Answer

Choose the one that is most correct. The development of the body contributes to the weakening of competition between parents and offspring
1) embryonic
2) historical
3) direct
4) indirect

Answer

Choose the one that is most correct. The 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 the vertebrate animal and the germ layer from which they are formed: 1) endoderm, 2) mesoderm
A) intestines
B) blood
C) kidneys
D) lungs
E) 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) hairline
C) lymph and blood
D) adipose tissue
D) nail plates

Answer

2. Establish a correspondence between the organ, tissue of the vertebrate animal and the germ layer, from which they are formed during embryogenesis. Write down the numbers in the 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 characteristic and the stage of formation of a lancelet embryo: 1) a single-layer embryo, 2) a two-layer embryo. Write down the numbers 1 and 2 in the correct order.
A) invagination of a group of cells into the blastula
B) zygote mitosis
C) the formation of the walls of the primary intestine
D) blastocoel formation
E) the formation of blastomeres

Answer

1. Establish a correspondence between the structures and germ layers, indicated in the figure by numbers. Write down the numbers 1 and 2 in the correct order.
A) internal mucous membranes
B) adrenal glands
B) interstitial fluid
D) lymph
E) alveolar epithelium

Answer

2. Establish the correspondence between the structures of the embryo and the germ layers, indicated in the figure by the numbers 1 and 2, from which these structures are formed: write down the numbers 1 and 2 in the order corresponding to the letters.
A) muscles
B) alveoli
C) liver
D) blood vessels
E) heart

Answer

Establish a correspondence between the structures and germ layers, indicated in the figure by the numbers 1, 2: 1) ectoderm, 2) endoderm. Write down the numbers 1 and 2 in the correct order.
A) skin epidermis
B) nervous tissue
C) 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 down the numbers 1, 2 and 3 in the correct order.
A) brain
B) liver
C) blood
D) bones
E) pancreas
E) leather

Answer

Establish the sequence of the processes of embryogenesis in the lancelet
1) the formation of blastula
2) crushing the zygote
3) the formation of three germ layers
4) the formation of gastrula

Answer

Choose three options. What tissues and organs of a vertebrate animal are formed from the cells indicated in the figure with the number 1?
1) sweat glands
2) bone tissue
3) nail plates
4) connective tissue
5) cutaneous epidermis
6) smooth muscle tissue

Answer

Establish the sequence of the processes of embryonic development of the chordate animal. Write down the corresponding sequence of numbers.
1) gastrulation
2) the formation of neurula
3) the formation of morula
4) the formation of mesoderm
5) blastula formation
6) crushing the zygote
7) histogenesis

Answer

1. Establish a correspondence between the structures and germ layers, indicated in the figure by the numbers 1, 2, from which these structures are formed. Write down the numbers 1 and 2 in the correct order.
A) nervous tissue
B) blood
B) skeleton
D) smooth muscle tissue
D) cutaneous epidermis

Answer

2. Establish a correspondence between the structures of the embryo and the germ layers, indicated in the figure by the numbers 1, 2, from which these structures are formed. Write down the numbers 1 and 2 in the correct order.
A) nails
B) senses
C) blood
D) lymph
D) bone skeleton

Answer

3. Establish a correspondence between the structures and germ layers indicated in the figure with numbers 1 and 2. Write down the numbers 1 and 2 in the correct order.
A) nervous tissue
B) blood
B) bone
D) kidney
D) tooth enamel
E) muscle

Answer

Establish a correspondence between the organs and the germ layers from which they develop: 1) ectoderm, 2) endoderm, 3) mesoderm. Write down the numbers 1-3 in the correct order.
A) brain
B) small intestine
B) cartilage
D) muscles
E) pancreas
E) hair

Answer

Establish a correspondence between the characteristic and the type of germ leaf: 1) endoderm, 2) mesoderm. Write down the 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 system
D) absent in the ontogeny of hydra
D) participates in the formation of the circulatory system

Answer

Choose the one that is most correct. Are the following judgments about the individual development of organisms correct? A) In the embryonic period of animal development, the number of cells increases, and then their differentiation. B) The process of formation of a two-layer embryo occurs during the cleavage period during division of blastomeres.
1) only A is true
2) only B is true
3) both statements are true
4) both judgments are wrong

Answer

Choose from the following two processes related to fertilization. Write down the numbers under which they are indicated.
1) fusion of male and female gametes
2) the formation of gametes
3) the formation of a zygote
4) crushing the zygote
5) colony formation

Answer

Choose from the following two processes related to embryogenesis. Write down the numbers under which they are indicated.
1) the formation of gametes
2) the 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) cleavage, 2) gastrulation. Write down the numbers 1 and 2 in the order corresponding to the letters.
A) directed movement and differentiation of cells
B) germ layers are formed
C) a multilayer embryo is formed
D) cellular material accumulates for further development
D) blastula is formed

Answer

All the processes listed below, except for two, 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) crushing the zygote
2) invagination of the cell layer into the blastula cavity
3) the formation of the cavity of the primary intestine
4) neural tube formation
5) the formation of two germ layers

Answer

All but two of the characteristics listed below refer to the indirect type of postembryonic development. Identify two characteristics that "fall 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 looks like an adult organism
3) a born organism may differ from an adult organism in morphological characteristics, lifestyle (type of food, nature of movement)
4) can be ovipositor and intrauterine
5) metamorphosis occurs

Answer

Consider the drawing. Determine A) the stage of embryogenesis of the chordate animal, B) the germ layer indicated in the figure question mark, and B) the organs that develop from it. For each letter, select the appropriate term or corresponding 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 down the numbers 1-3 in the order corresponding to the letters.
A) the formation of mesoderm
B) the presence of undifferentiated cells
C) invagination of blastula cells into the cavity
D) the formation of the cavity of the primary intestine
E) three-layer embryo

Answer