Embryonic development of birds. Bird egg What are the stages of egg formation before it is laid
Reproductive organs. In birds, as in other vertebrates, the reproductive organs are the testes in males, and the ovaries in females (see Fig. 165). They are located in the body cavity. Bean-shaped paired testicles are located in the region of the sacrum. By the time of reproduction, their size increases a thousand times. From the testes, the vas deferens open into the cloaca.
In females, only one - the left - ovary develops. It is located at the top of the left kidney. Reduction (disappearance of the organ due to loss of function) of the right ovary is associated with the laying of large hard-shelled eggs. Through a narrow pelvis, only one egg is possible to advance.
Rice. 166. The structure of the egg: 1 - protein; 2 - yolk; 3 - air chamber; 4 - shell membrane; 5 - halazy; 6 - shell
Egg development. The eggs of birds are large, rich in yolk. The mature egg enters the oviduct. Fertilization takes place in the upper part of it. The walls of the oviduct contract, pushing the egg (fertilized egg) towards the cloaca. When moving, it is covered with egg shells, which are formed from the secretions of the glands of the walls of the oviduct. First, the egg is covered with protein, then with two fibrous (undershell) and then with shell membranes.
The egg enters the cloaca and is laid out. The formation of an egg in the oviduct in birds of different species takes from 12 to 48 hours.
Bird eggs are large, contain a lot of nutrients and water in the protein and yolk (Fig. 166). By the time the egg is laid, the germinal disc is visible on top of the yolk - the result of crushing (division) of the fertilized egg. The yolk, suspended on flagella - chalaz, is located in the center of the egg. The lower part of the yolk is heavier, so when the egg is turned over, the germinal disc is always on top, in the best conditions for heating during incubation.
Outside, the egg is protected by a calcareous shell, which has numerous microscopic pores. Through them, gas exchange occurs between the developing embryo and external environment. Shell lime is partly used to form the skeleton of the developing embryo. On top of the calcareous shell, the egg has a thin shell membrane that protects it from the penetration of microbes. The egg shell of open nesting birds has a protective coloration. The shell of the eggs of hollow nesters and norniks is light or pure white.
Embryo development. The embryo in the egg develops very quickly at high temperature (37-38 ° C) and a certain humidity. These conditions are provided by the bird incubating the clutch. The mother hen regularly turns her eggs over, changes the density of incubation: if the air temperature is too high, the bird rises in the nest, cools the clutch, periodically wetting the plumage, and protects it from the sun's rays with its own shadow.
Rice. 167. Chicken development: 1 - embryo; 2 - yolk; 3 - protein; 4 - air chamber; 5 - germinal membranes
The development of the embryo has been well studied in domestic chickens (Fig. 167). On the second or third day, the circulatory and nervous systems are laid in the chicken embryo, eye bubbles are clearly visible. At the beginning of development, the forelimbs of the embryo are similar to the hind ones, there is a long tail, gill slits are visible in the cervical region. This indicates that the ancestors of birds had gills. On the fifth or sixth day, the embryo acquires bird features. By the end of development, the chick fills the entire internal cavity of the egg.
When hatching, the chick breaks through the shell (parchment) shell, sticks its beak into the air chamber and begins to breathe. With the help of an egg tooth (a tubercle on the mandible), the chick breaks the shell and gets out of it.
Rice. 168. Nestlings of brood (1) and chicks (2) birds
Brood and nesting birds
In chickens, ducks, geese, swans, the chicks hatch from the egg covered with thick fluff, with open eyes. After drying, after a few hours they leave the nests and follow their parents. Birds with this type of development are called brood (Fig. 168, 1). Nestlings of brood birds are able to feed on their own, but at first they need protection from enemies and heating by their parents.
In songbirds, pigeons, woodpeckers, parrots, chicks hatch helpless, with their eyes closed. Their body is covered with sparse fluff or naked. They are helpless, need to be fed, warmed and protected by their parents. Birds with this type of development are called chicks or nesting birds. Parents feed such chicks in the nests for a long time, supplement them after leaving the nest until the young become independent.
As a rule, nesting birds lay fewer eggs than brood birds.
Egg laying and parental care in birds have reached perfection, providing the same high reproductive efficiency as live birth and nursing of young with milk in mammals.
Lesson learned exercises
- Tell us about the structural features of the reproductive organs of birds, noting the features associated with flight.
- What are the main stages in the formation of an egg before it is laid?
- How does a chick develop in an egg?
- How are brood chicks different from nestlings? Give examples using Figure 168.
Fertilization and stages of development of the embryo
After natural mating or artificial insemination, sperm pass up the oviduct. A lot of them accumulate in the lumen of the tubular glands of the utero-vaginal joint and the cervical part of the funnel of the oviduct. The ovulated egg enters the funnel of the oviduct, where it meets with the sex cells of the male. The head of the sperm is asymmetric, so its movement is rectilinear, it continuously rotates around its longitudinal axis, which ensures its meeting with the egg. Sperm penetrates the egg and merges with it, fertilization occurs. In s.-x. In birds, more than 300 sperm enter the egg. However, the nucleus of the female fuses only with the nucleus of one sperm. The rest of the sperm are assimilated by the egg.
After fertilization, the egg enters the stage of crushing (segmentation). This process begins in the isthmus of the oviduct 4-5 hours after ovulation. First, the first furrow is formed, then after 20-25 minutes. - the second. When it enters the uterus, the number of segments (blastomeres) reaches 4-8. Here, cleavage continues, and within 24 hours from the start of fertilization, a blastoderm (with 256 blastomeres) is formed.
If the eggs are placed in appropriate conditions, the development of the embryo continues. In the absence of the necessary external conditions, its development stops, viability gradually decreases, and within 25-30 days after laying the egg, the embryo dies. Therefore, the sooner the egg after the demolition enters the incubator, the better its further development proceeds. As cleavage proceeds, the germinal disc becomes multilayered. In a laid egg, the blastoderm already consists of two germ layers: the outer (ectoderm) and the inner (endoderm). The ectoderm is represented by high cells, tightly adjacent to each other. Endoderm cells lie loosely and have an irregular shape. Thus, the germinal disc becomes two-layered. The process of formation of these layers is called gastrulation. In the center of the germinal disc, the cells of the outer leaf are arranged in one layer, and accumulate in large numbers along the edges of the disc. Therefore, the center is called a light or transparent field through which the yolk is visible; it is surrounded by an opaque field through which the yolk is not visible. In this state of the germinal disc, the laying hen lays an egg. The development of the embryo in the oviduct lasts 24-27 hours.
The development of the embryonic disc in the mother's body occurs at a temperature of 40.5-41 ° C. After laying, the egg cools, the development of the embryo slows down, water begins to evaporate from the egg.
During incubation (or under a hen) the development of the embryo resumes in the egg. Therefore, the main task of incubation is to create the most favorable intraegg conditions for the developing embryo. The embryo grows and develops very quickly.
In the first 12 hours of incubation in a bright field, an accumulation of cells in the form of a strand is observed - the primary strip. From it, on both sides, between the two germ layers - outer and inner - the middle germ layer (mesoderm) grows. All the tissues and organs of the bird are formed from these three leaves. Ectoderm gives rise to the nervous system, skin and their derivatives (feathers, claws); endoderm - lungs, digestive tract, pancreas, thyroid, goiter and liver. From the mesoderm, cartilage, bones, muscles, blood and lymphatic vessels, the excretory system and the sex glands are formed. The laying of the main organs and tissues (nervous, circulatory and excretory systems) occurs in the period up to 48 hours of incubation.
After 12 h, the light field stretches in the direction of the minor axis of the egg and gradually assumes a pear-shaped shape.
The primary streak grows from the narrow part of the light field to the wide one. In its front part, a recess is formed - Hensen's knot. Ahead of this recess appears, as it were, a continuation of the primary strip - the head process, from which the primary axial skeleton - the notochord - grows later.
In the dark field, blood processes appear in the mesoderm, which begin to merge with each other and form a vascular network. Blood processes are a mass of cells from which erythrocyte precursors, blood plasma and blood vessels are formed. Shortly after formation, they turn red, because. hemoglobin appears.
The blood vessels in the yolk unite into two yolk veins, from both sides they go to the embryo, merging with its vessels and forming a loop. Complete circulation occurs by 49 hours of incubation.
The yolk veins carry blood enriched with nutrients and oxygen to the embryo. From the embryo, depleted blood flows through the vitelline arteries. The latter branch into capillaries, which again gather into veins, through which blood returns to the heart and body of the fetus. Subsequently, the allantois vessels join the circulatory system of the embryo.
Somites are formed at the end of the first day of incubation, as segments of accumulation of mesoderm cells along the notochord and neural tube. Three main parts arise from each somite, which are the rudiments of the axial skeleton (sclerotome), muscles (myotome) and the dermis of the skin.
germinal membranes(yolk sac, amnion and allantois with a serous membrane) are organs that play an important role in the development of the embryo outside the mother's body.
By the 6th day, allantois reaches the inner surface of the shell. The embryo begins to use the oxygen of the air of the incubator through the shell through the circulatory system of the allantois. Since that time, the allantois has become the main respiratory organ of the embryo. The circulatory system of the allantois is connected with the circulatory system of the embryo by one allantoid artery and one allantoid vein.
Lining the shell from the inside of the egg, allantois takes part in the use of shell substances by the embryo. These substances, penetrating through the shell membranes into the circulatory systems of the allantois, enter the embryo. By the end of incubation, the allantois fluid evaporates in a significant amount and is partially absorbed. Allantois begins to dry out, gradually atrophy, blood vessels become empty. The respiratory function passes to the lungs, the connection of the circulatory allantois with the circulatory system of the embryo gradually stops. After hatching, the allantois remains in the shell.
Embryo position. The fetus turns on its left side and bends its head and body from its prone position. If the embryo turns not to the left, but to the right side and its bend is broken, then development goes on incorrectly
Starting from the 11th day of incubation, the embryo changes its position again. Until this time, the head of the embryo grew faster than the body. From this time on, the body of the embryo begins to grow faster than the head.
By the time of hatching, the embryo is located along the major axis of the egg, the head is at the blunt end, the legs are pressed to the body, between them is the yolk drawn into the body cavity. The body of the embryo fills the entire egg in half of the sharp end. The head and neck are in constant motion, which first cause the rupture of the shells, and then the destruction (peening) of the shell. The movements of the neck and head and the simultaneous pushing away from the shell with the legs lead the embryo into a counterclockwise rotational movement. At the same time, with its beak, the embryo breaks off small pieces shells. The shell breaks into two parts - a smaller one from the side of the blunt end and a larger one from the side of the sharp one.
Yolk. Until about the 7th day of incubation, the weight of the yolk increases, and then gradually decreases. This decrease occurs especially intensively after the complete use of the protein.
The substances of the yolk are used by the embryo through the circulatory system of the vascular field (hereinafter the yolk sac). First, protein substances enter the yolk in a larger amount than are consumed by the embryo, so the weight of the yolk increases. At this time, liquefaction of the yolk occurs, as a result, a "new plasma" is formed under the embryo with the participation of protein substances, into which the embryo is immersed along with the amnion. "New plasma" in all its properties differs significantly from the yolk and protein. It is rich in nutrients in an easily digestible form, it provides a favorable environment for the embryo: its reaction is less alkaline than the reaction of protein, and less acidic compared to the yolk. By the 6th day, the weight of the yolk begins to decrease. By the end of incubation, less than 50% of the original weight remains.
In the first five days, water from the protein penetrates the yolk, enters the embryo through the circulatory system of the yolk sac and takes part in metabolism, building the body of the embryo, and the unused part of it is excreted along with metabolic products. The egg loses water as a result of evaporation from the protein. The greater the evaporation of water, the less of it and the nutrients dissolved in it will pass into the yolk, which will worsen the nutrition of the embryo these days.
From the 6th to the 11th day, allantois gradually covers the entire contents of the egg, including the albumen. The amount of water evaporated from the protein gradually decreases, while the amount of water evaporated from the allantois increases. Water evaporates from the allantois, which has already taken part in the metabolism and is excreted by the embryo as unnecessary.
From the 11th day of incubation until the hatching of the shell, water enters the embryo from the yolk and albumen and penetrates through the mouth and digestive tract, and evaporation occurs only due to the allantois fluid, which causes a constant flow of it and the nutrients dissolved in it to the embryo from the yolk and squirrel. The delay in the evaporation of water from the allantois worsens the nutritional conditions of the embryo, stops their growth and development. After pipping the shell and during hatching, the water evaporates when the chick dries and when breathing with lungs.
Use of nutrients by the embryo. Not all substances of the yolk and protein immediately become available for use by the embryo. First, more digestible carbohydrates are used, then more complex ones that require preliminary splitting - proteins, fats.
mineral exchange.
The minerals of the yolk are used by the embryo at the earliest stages of development by diffusion. From the yolk come mainly calcium, phosphorus, manganese and iron. During the incubation period, about 30% of the mineral reserves are used from the yolk.
Calcium is mainly used from the shell, which makes up 75% of the total calcium in the bones of a day-old chick. Approximately by the 12-13th day of incubation under the influence of water and carbon dioxide. Calcium shell from an insoluble form becomes soluble. The circulatory system of the allantois washes calcium out of the shell and transfers it to the embryo. At this time, the weight of the shell decreases, along with it, its strength decreases.
carbohydrate metabolism.
In the embryo, carbohydrates begin in the form of glycogen and lactic acid. Carbohydrates are of great importance in the nutrition of the embryo in the first days of incubation, when it cannot intensively use more complex substances. The sugar content in the embryo rises until the 11th day of incubation.
Glycogen stores in the muscles and liver serve as a source of energy and ensure the movement of the embryo. The formation of glycogen occurs from the first day in the yolk sac, and from 7-8 days - in the liver. Its reserves are increasing before the start of the withdrawal. Pitting of the shell, circular movements of the embryo in the process of hatching and release from the shell require a lot of energy. If development occurs with insufficient accumulation of glycogen, then the embryos, being outwardly completely ready for hatching, will not be able to free themselves from the shell and remain alive in it for a long time.
Protein exchange. Proteins are the main component of the embryo and its embryonic membranes. Proteins also serve as a source of energy.
Fat exchange. In the last days of incubation, fats serve as the main source of nutrition and energy. During incubation, 28% of the fatty acid reserves of the eggs pass into the embryo and 32% remains in the yolk, which is then drawn into the body cavity of the embryo. The remaining 409% is oxidized, as a result of which heat is released. When oxidized, fat gives more than 2 times more heat than carbohydrates and proteins. The main source of energy in the process of embryonic development of birds are fats: they account for 80% of all energy released. When fats are oxidized, a large amount of water is released (100 g of fat gives 107.1 g of water). This water replenishes reserves, which by this time are decreasing as a result of evaporation.
1. Tell us about the structural features of the reproductive organs of birds, noting the features associated with flight.
In birds, as in other vertebrates, the reproductive organs are the testes in males, and the ovaries in females.
They are located in the body cavity. Bean-shaped paired testicles are located in the region of the sacrum. By the time of reproduction, their size increases a thousand times. From the testes, the vas deferens open into the cloaca.
In females, only one - the left - ovary develops. It is located at the top of the left kidney. Reduction (disappearance of the organ due to loss of function) of the right ovary is associated with the laying of large hard-shelled eggs. Through a narrow pelvis, only one egg is possible to advance.
2. What are the main stages in the formation of an egg before it is laid?
The mature egg enters the oviduct. Fertilization takes place in the upper part of it. The walls of the oviduct contract, pushing the egg (fertilized egg) towards the cloaca. When moving, it is covered with egg shells, which are formed from the secretions of the glands of the walls of the oviduct. First, the egg is covered with protein, then with two fibrous (undershell) and then with shell membranes. The egg enters the cloaca and is laid out. The formation of an egg in the oviduct in birds of different species takes from 12 to 48 hours. By the time the egg is laid, the embryonic disk is visible on top of the yolk - the result of crushing (division) of the fertilized egg
3. How does the chick develop in the egg?
The embryo in the egg develops very quickly at high temperature (37-38 ° C) and a certain humidity. On the second or third day, the circulatory and nervous systems are laid in the chicken embryo, eye blisters are clearly visible. At the beginning of development, the forelimbs of the embryo are similar to the hind ones, there is a long tail, gill slits are visible in the cervical region. This indicates that the ancestors of birds had gills. On the fifth or sixth day, the embryo acquires bird features. By the end of development, the chick fills the entire internal cavity of the egg.
4. How are brood chicks different from nestlings? Give examples using Figure 168.
Figure 168: Nestlings of brood (1) and nestling (2) birds.
Chicks of brood birds are able to feed on their own, but at first they need protection from enemies and heating by their parents. For example, in chickens, ducks, geese, swans, chicks hatch from eggs covered with thick fluff, with open eyes. After drying, after a few hours they leave the nests and follow their parents.
Nesting birds are fed by their parents for a long time, they are supplemented after leaving the nest until the young become independent. For example, in songbirds, pigeons, woodpeckers, parrots, chicks hatch helpless, with their eyes closed. Their body is covered with sparse fluff or naked.
Good afternoon, dear readers! Today we will give a description, show photos and videos about the development of a chicken in an egg by day during incubation at home and in poultry farms. confidently practiced both on a factory scale and in private courtyards.
But, despite the wide distribution, few people think about the complex mechanism laid down at the genetic level, which ensures the growth and development of the chicken.
Until now, there is an opinion that the chick grows from the yolk. In this article, you will learn all the secrets hidden under, as well as what kind of “terrible” meaning is hidden under the words allantois in a chicken and amnion in a chicken, and what function they perform.
The development of a chicken in an egg by day photo
Blastodisc
Chick development begins with the blastodisc. Blasodisk is a small clot of cytoplasm located on the surface of the yolk. At the location of the blastodisc, the density of the yolk is much lower, which contributes to the constant floating of the yolk with the blastodisk upwards.
This feature provides better heating during the incubation process. The fertilized blastodisc begins dividing while still in the body of the chicken, and by the time of demolition it is already completely surrounded by the blastoderm. The blastodisc looks like a small white spot about 2 mm in size.
The light halo surrounding the germinal disc is the blastoderm.
When the egg enters favorable environmental conditions, which stopped after laying, cell division continues.
You should know: Contrary to the popular belief that candling can be carried out only from the 6th day of incubation, the development of the blastoderm is clearly visible after 18-24 hours from the start of incubation. By this time, a blackout 5–6 mm in diameter is clearly visible, easily moving when the egg is turned over.
On the 2nd - 3rd day of incubation, the development of provisional shells begins:
- Amnion in a chicken
- Allantois in chicken
All of them are, in fact, temporary organs designed to perform the functions of ensuring the vital activity of the embryo until the moment of its final formation.
Amnion in a chicken
It is a shell that protects the embryo from physical impact and drying, due to filling with liquid. The amnion in the chicken regulates the amount of fluid depending on the age of the embryo.
The epithelial surface of the amniotic sac is able to fill the cavity with the embryo with water, and also ensures the outflow of fluid as it grows.
Allantois in chicken
One of the temporary organs that performs many functions:
- oxygen supply to the embryo;
- isolates waste products from the embryo;
- participates in the transport of fluid and nutrients;
- delivers minerals and calcium from the shell to the embryo.
The allantois in the chick, in the process of growth, creates an extensive vascular network that lines the entire inner surface of the egg and is connected to the chick through the umbilical cord.
Chicken breath in an egg
Oxygen exchange in the egg, depending on the stage of development of the chicken, has a different mechanism. At the initial stage of development, oxygen comes from the yolk directly to the cells of the blastoderm.
With the advent of the circulatory system, oxygen enters the blood already, still from the yolk. But the yolk cannot fully ensure the respiration of a rapidly growing organism.
Starting from the 6th day, the function of providing oxygen is gradually transferred to the allantois. Its growth begins in the direction of the air chamber of the egg and, having reached it, covers an ever larger internal area of the shell. The larger the chicken grows, the larger the area covered by allantois.
When candling, it looks like a pinkish network, covering the entire egg and closing on its sharp side.
Eating a Chicken in an Egg
In the first days of development, the embryo uses the nutrients of protein and yolk. Since the yolk contains a whole complex of minerals, fats and carbohydrates, it is able to provide all the initial needs of a growing organism.
After the closure of the allantois (day 11 of development), a redistribution of functions occurs. The embryo becomes larger and assumes a position along the long axis of the egg, head towards the blunt end. Protein at this point is concentrated in the sharp end of the egg.
The weight of the chick, coupled with the pressure of the allantois, ensures the displacement of the protein and its penetration through the amnion into the mouth of the embryo. This continuous process ensures the rapid growth and development of the chick in the egg day by day during incubation.
From day 13, the minerals that the chicken uses to further development are delivered by allantois from the shell.
You should know: The normal nutrition of a chicken is able to provide only a timely closed allantois in a chicken. If, when it closes, at the sharp end of the egg, there remains a protein not covered with vessels, the chicken will not have enough nutrients for further growth.
Egg position and chick development
AT recent times Incubation of chicken eggs in a vertical position is increasingly practiced. But this method does not have the best effect on the development of the chicken.
When upright, the maximum inclination when turning is 45°. This inclination is not enough for the normal growth of the allantois and its timely closing. This is especially true for large eggs.
When incubated in a horizontal position, the rotation is provided by 180°, which has a positive effect on the growth of the allantois and, as a result, the nutrition of the chick.
As a rule, fluffies hatched with eggs upright have a weight 10% lower than those hatched in a horizontal position.
The importance of egg turning for chick development
Turning eggs during incubation is necessary at all stages of development, except for the first day and the last two. On the first day, intensive heating of the blastodisk is necessary, and on the last day, the little squeaker has already taken a position to break through the shell.
The content of the article
EGG, the female sex cell that is produced in the ovaries of the female. To the layman, the word "egg" usually means egg, covered with a hard shell and eaten. However, for a biologist, an egg is a specialized cell from which almost all organisms develop, including plants. Even some single-celled protists, in which two cells fuse during reproduction, function like a spermatozoon or an egg. In relation to the microscopic egg of plants, as well as mammals and many other animals, the term "ovum" is often used.
VARIETY OF EGGS
eggs of animals belonging to different groups, extremely diverse in size, shape and color; no less differences are observed in the number of eggs produced different types. Yes, a mature egg sea urchin red, reaches 70–80 microns in diameter, and one female produces millions of eggs; the female mosquito lays from 100 to 200 eggs, and the freshwater Japanese fish orysia, or medaka ( Orysius latipes), – only 10–30. The size and number of eggs depend little on the size of the animal, but are determined mainly by the breeding strategy.
Among mammals, the largest eggs are characteristic of oviparous - platypus and echidna. The diameter of the platypus egg is 4.4 mm, that of the echidna is 3 mm. A mature human egg is approximately 100 microns (0.1 mm) in diameter, a rhesus monkey is 118 microns, a guinea pig is 76 microns, a rabbit is 160 microns, and a mouse is 80 microns.
The size of bird eggs is usually estimated by their mass (more precisely). Most small egg- only 0.5 g - in hummingbirds Trochilus colubris, and the largest egg in the modern animal world is in the ostrich Struthio camelus: it goes back to 1400. Indigenous Africans used ostrich egg shells as vessels for water. However, apparently, the largest egg belonged to an extinct bird - epiornis ( Aepyornis), who lived in Madagascar; its capacity exceeded 9 liters. The leghorn chicken egg has a mass of 58 g. Eggs are spherical, ellipsoidal, conical and oblong in shape.
The number of eggs in a clutch also varies. For example, penguins lay one egg each, pigeons - two, partridges - up to 20 eggs per clutch.
Thrush eggs are bluish-green. Domestic chickens have eggs that are white, yellow, or various shades of brown. A breed of hens has been reported to lay blue-green eggs. The size, shape and color of eggs sometimes vary among different members of the same species.
STRUCTURE AND DEVELOPMENT
The process leading to the formation of the female gamete, or mature egg, is called oogenesis. It is divided into two phases: generative and vegetative. The generative phase begins with the reproduction of primary germ cells - they are isolated in the early stages of embryonic development and are intended for the formation of gametes. These cells give rise to oogonia, each of which then forms the so-called. oocyte.
In the vegetative phase, the oocyte enters a period of growth characterized by an increase in the mass of its cytoplasm. Then it accumulates yolk and undergoes a special cell division - meiosis. Meiosis ends with the formation of a mature egg.
In mammals, the vegetative phase is initiated by follicle-stimulating hormone produced by the pituitary gland. In insects, oogenesis is stimulated by juvenile hormone, which is produced by adjacent bodies - paired glands located in the head.
During the generative phase and in the early period of the vegetative phase, the future egg differs little from any other type of cell, i.e. it does not have those specific features that are characteristic of the egg. At this stage, the young oocyte is surrounded by a membrane called the oolemma. Its nucleus is immersed in the cytoplasm containing specialized structures - organelles. In many organisms, oogenesis proceeds with the participation of follicular cells and trophocytes.
Core.
A young oocyte contains a nucleus with a large nucleolus and a diploid set of chromosomes; It has as many chromosomes as any other cell in the organism. The transition from a diploid set of chromosomes to a haploid (i.e., halved) set occurs as a result of meiosis. The haploid number of chromosomes is characteristic only of gametes.
In all the eggs studied, the nucleus is surrounded by a nuclear membrane pierced by pores located at some distance from each other. In many animals, during oogenesis, a membrane system is formed in the egg, known as the annulate lamella: it arises from the nuclear membrane.
Cytoplasm.
Oocytes contain a large amount of cytoplasm, which has a complex structure. It contains many mitochondria necessary to provide the cell with energy; the membrane system of the endoplasmic reticulum and numerous ribosomes on which protein synthesis occurs; the Golgi complex and lysosomes - the enzymes of the latter carry out intracellular digestion and can even initiate the destruction of the egg.
In young insect oocytes, microtubules were also found, which, apparently, are involved in the movement of the cytoplasm. They are rare in the eggs of other invertebrates and in vertebrates.
In addition to this set of organelles, which are also characteristic of other cells, the cytoplasm of the egg in many cases contains the so-called. cortical granules, or bodies, which in a number of animals play an important role in fertilization. However, its most important feature is the presence of yolk, which is necessary for the nutrition of the embryo.
There are at least three possible ways in which the yolk is formed. First, it can be produced by oocyte organelles. Secondly, the precursors of the yolk, i.e. the substances from which it is formed can be produced not in the oocyte, but in other cells and enter the oocyte by endocytosis. Finally, a combination of these two processes is possible.
Oolemma.
In the early stages of development, the oolemma is smooth, but later finger-like outgrowths called microvilli form on it. The outer surface of the oolemma is covered with a loose layer, which is considered part of this shell.
follicular cells.
In many organisms, the egg is surrounded by a layer of follicular cells, in the cytoplasm of which there are organelles similar to those of the oocyte. While the oocyte is developing, the cytoplasm of the follicular cells forms outgrowths, which sometimes merge with the microvilli of the oocyte. The function of follicular cells in many animals remains unknown. However, in insects such as dragonflies and fruit flies, follicular cells produce the material used to form the secondary membrane around the egg.
Trophocytes, or feeding cells.
In some invertebrates, such as ctenophores and insects, a group of trophocytes is located at one of the poles of the egg. It has been established that the synthesis of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) begins in trophocytes, and RNA, together with ribosomes, is transferred to the oocyte through cytoplasmic bridges. In sponges, the oocyte completely absorbs (phagocytes) these cells.
Maturation.
An egg can leave the ovary while on different stages maturation; this means that its nucleus can be either diploid (in this case, the process of meiosis is completed during fertilization), or already haploid. So, in many worms and mollusks, as well as in a number of mammals (dogs, foxes, horses), meiosis by the time of fertilization is at the prophase stage, i.e. a large diploid nucleus (embryonic vesicle) is still preserved in the egg. In other molluscs, such as the common mussel ( Mytilus edulis), and in many insects the mature egg is in the metaphase of the first mitotic division; in most vertebrates - in the metaphase of the second meiotic division; in coelenterates and sea urchins, meiosis in a mature egg is completed and the nucleus is haploid. A number of animals are difficult to attribute to any of these four groups. For example, starfish eggs Asterias under certain conditions, it is possible to fertilize at different times after their laying, when they are at different stages of maturation.
EGG OF BIRDS
The structure of the bird egg is entirely consistent with its purpose - the egg contains everything necessary for the full development of a new organism. Immediately before entering the oviduct, it is a single cell filled with liquid material - the yolk; its nucleus is located in a region called the blastodisc. After the egg has entered the oviduct, fertilization becomes possible. As the egg moves through the oviduct, glands located in the wall of the oviduct secrete substances from which auxiliary structures are formed, including albumen, shell membranes and shells. The passage of the egg through the oviduct takes approximately 22 hours. If the egg has been fertilized, then by the time of laying it cannot be considered a single cell, since crushing has already begun in it and a flat double layer of cells has formed, called the blastoderm.
The nutrition of the embryo is provided by the yolk. There are two types of yolk - white and yellow; they are arranged in the egg in alternating concentric layers. Most of the yolk is yellow yolk, which contains at least two proteins, phosphovitin and lipovitellin, as well as some lipids and carbohydrates. The main part of the white yolk, called the latebra, is located in the center of the egg; it looks like a flask, the neck of which extends to the surface of the yolk. The superficial area of the white yolk is called the Pander nucleus; directly above it lies the blastoderm.
The yolk is enclosed in the so-called. vitellin membrane and surrounded by protein. The white of the egg has a yellowish hue created by the pigment ovoflavin, but after coagulation (coagulation) it becomes white. Part of the protein forms a spiral structure around the yolk - chalaza, which maintains the yolk in suspension.
The contents of the egg are surrounded by two shell membranes, inner and outer, similar to parchment. Above them lies a shell, consisting mainly of calcium carbonate. After the egg is laid at its blunt end, the shell membranes begin to separate from one another, and an air chamber forms in this place. By the size of the chamber, one can usually judge the freshness of the egg: if a fresh egg is placed in a weak saline solution, it will sink to the bottom, since the air chamber is small, and a stale egg will float, since this chamber has increased in volume.
There are times when two or three eggs mature at once. As they pass through the oviduct at the same time, they may be covered with albumen and shell together, so that an egg containing two or three yolks is obtained.
FERTILIZATION
Fertilization is a multi-stage process. It begins with the interaction and subsequent fusion of the egg and sperm, and ends with the union of two sets of chromosomes - one from the mother and the other from the father. This combination not only restores the diploid number of chromosomes, but also creates new genetic combinations. Fish and many amphibians release spermatozoa and eggs (caviar) into the water, so that their fertilization is external, i.e. occurs outside the body of the animal; the same is true of many marine invertebrates. In terrestrial invertebrates, as well as in other vertebrates, fertilization is internal, i.e. the fusion of the sperm with the egg occurs in the reproductive system of the female.
It remains unknown how the spermatozoa of a given species come into contact with the eggs of their own and not some other species, even in cases where the male releases sperm into vast expanses of water. According to some researchers, the egg secretes a species-specific substance that attracts the appropriate spermatozoa due to their ability to chemotaxis - movement along the concentration gradient of the recognized chemical. Some spermatozoa actively search for the egg, moving towards it with the help of a long flagellum. In a number of invertebrates, spermatozoa move like amoeba.
In many animals, the sperm enters the egg at any point on its surface, but in insects and fish, only through a special hole (micropyle). Apparently, spermatozoa, which are able to enter the egg anywhere, do so by softening the area of the egg membranes with the help of enzymes contained in their acrosome. As a result of direct contact between the spermatozoon and the egg, their membranes merge, forming one continuous membrane that unites these two cells.
At this stage of the fertilization process, in very many animals, a change occurs in the surface layer of the egg due to the fact that the cortical granules contained in the cytoplasm of the egg quickly release their contents under the egg membrane; the released substances are hydrated, increasing the occupied volume, which leads to the separation of the membrane from the cytoplasm: between them appears the so-called. perivitelline space, and, in addition, the properties of the egg membrane change. As a result, a favorable environment arises around the fertilized egg and an obstacle is created for the penetration of additional spermatozoa. However, the activity of the cortical granules is not the only factor responsible for the fact that in most animals only one sperm can enter the egg.
After the sperm enters the egg, the shell of its nucleus disintegrates, and the released chromatin (the substance that makes up the chromosomes) is in the cytoplasm of the egg and has been under its control ever since.
Further events may proceed in different ways. For example, the sea urchin Arbacia the nuclear membrane of the sperm disintegrates immediately after its penetration into the egg, and this is followed by dispersion of the compact mass of chromatin. The chromatin then separates again from the cytoplasm of the egg as a result of the repair of the nuclear envelope.
In some animals, the nuclei of the sperm and eggs, once in the common cytoplasm, immediately come into contact; their membranes merge, and a single diploid nucleus is formed in a single cell - the zygote.
In other animals, such as the rabbit, the nuclei of the spermatozoon and the egg come together, after which both nuclear membranes are destroyed. The two haploid sets of chromosomes then line up so that the zygote can begin to divide; the diploid number of chromosomes in it was restored.
After fertilization, external or internal, the process of crushing the zygote and the development of the embryo begins.
PARTHENOGENESIS
Many invertebrates and lower vertebrates are characterized by parthenogenetic (virgin) reproduction, i.e. their eggs can develop without fertilization. In some cases, for example in fish, this requires prior contact of eggs with spermatozoa of individuals of another species: in this case, the egg is activated (inducing it to crush), but not fertilization. A similar activation of eggs (both invertebrates and lower vertebrates) can be induced under laboratory conditions. To do this, use methods such as pricking with a needle moistened with blood, keeping eggs at elevated or low temperature, either in an acidic or alkaline environment, or in a hypertonic saline solution (i.e., in a solution with a higher salt concentration than in a cell), or in a solution of strychnine or saponin. If as a result of such influences it is possible to obtain a diploid organism, then this usually occurs due to the suppression of one of the divisions of meiosis or one of the first divisions of the egg. However, with artificial parthenogenesis, it is far from always possible to achieve the full development of a new organism - most often, the development of the embryo stops in the early stages. Therefore, in most cases it remains unclear whether these artificially induced processes correspond to normal development. It has been shown, however, that the sea urchin Arbacia punctulata activation of eggs with a hypertonic solution, namely, sea water with a high content of some salts, induces processes similar to those observed during fertilization.
It was also possible to obtain complete and massive (from the vast majority of eggs) parthenogenetic development of the silkworm, using various physical (in particular, temperature) and chemical influences. It turned out that with a sufficiently strong impact on unfertilized eggs, meiotic division is inhibited in them, and later only females hatch from such eggs. The same, but weaker effect, which does not inhibit meiosis, but activates eggs, leads to the development of only males. Thus, with the help of artificial parthenogenesis, it is possible not only to cultivate this species, but also to regulate the sex ratio in the breeding population, which is important, since males produce more silk than females. This method of parthenogenetic breeding of the silkworm has received practical application.
Curious experiments were carried out on frogs. The nucleus was removed from the frog egg and the somatic cell nucleus was introduced instead. As already mentioned, the nuclei of all somatic cells, both embryonic and taken from an adult organism, contain a diploid set of chromosomes, in contrast to the nucleus of haploid eggs. In a series of such experiments in clawed frog oocytes ( Xenopus laevis) transferred diploid nuclei from cells of the blastula, gastrula, or from the brain of an adult. It turned out that the oocyte cytoplasm is able to change the nature of the activity of the transplanted nucleus, regulating it in such a way that it corresponds to the activity of the cytoplasm. As a result, an adult frog can develop from an oocyte with a transplanted diploid nucleus.
