The structure of the lancelet blastula. Blastula structure. The history of the discovery of the lancelet

Crushing leads to the formation of a spherical embryo - blastula... If a solid ball is formed without a cavity inside, then such an embryo is called a morula. The formation of blastula or morula depends on the properties of the cytoplasm. Blastula is formed with sufficient viscosity of the cytoplasm, morula - with low viscosity. With sufficient viscosity of the cytoplasm, the blastomeres retain their rounded shape and only slightly flatten at the points of contact. As a result, a gap appears between them, which increases as it cleaves, fills with fluid and turns into a blastocoel. With a low viscosity of the cytoplasm, the blastomeres are not rounded and are located closely to each other, there is no gap and a cavity is not formed. Blastula different in their own way structure and depend on.

Blastula types

There are five types of blastul: celloblastula, amphiblastula, sterroblastula, discoblastula and periblastula. Celloblastula is formed when completely uniform from oocytes of the homolecital type (lancelet). The blastoderm of the celloblastula consists of one row of more or less identical blastomeres; inside there is a large cavity - the blastocoel.

Blastoderm amphiblastula consists of several rows of cells. The blastoderm in the animal part is thinner than in the vegetative one. The blastocoel is smaller than that of the blastocoel and is displaced toward the animal pole. This type of blastula is formed with complete uneven crushing and is characteristic of cyclostomes and.

Sterroblastula consists of one row of large blastomeres, which deeply enter the blastula cavity, the blastocoel is therefore either very small or absent (some arthropods).

Discoblastula formed with incomplete discoidal cleavage. The blastocoel in the form of a narrow slit is located between the embryonic disc and the yolk. The roof of the blastula is represented by the blastoderm, and the bottom is represented by the yolk. Such blastula is typical for bony, reptiles and birds. The blastoderm of the periblastula consists of a single row of cells that surround the yolk. There is no cavity in it. Periblastula is observed in some insects.

Topic 4

Embryogenesis history

1. General characteristics of anamnias and amniotes.

2. Embryogenesis history.

3. Embryogenesis of the lancelet.

4. Embryogenesis of amphibians, lampreys.

5. Embryogenesis of cartilaginous and teleost fishes.

1. Antipchuk, Yu.P. Histology with the basics of embryology / Yu.P. Antipchuk. - M .: Education, 1983 .-- 240 p.

2. Almazov, I. V., Sutulov L.S. Atlas on histology and embryology / I.V. Almazov, L.S. Sutulov. - M .: Medicine, 1978 .-- 148 p.

3. Histology / ed. Yu.I. Afanasyev. - M: Medicine, 1989 .-- 361 p.

4. Ryabov, K.P. Histology with the basics of embryology / K.P. Ryabov. - Mn .: Higher. shk., 1991 .-- 289 p.

5. Biological encyclopedic dictionary / ed. M.S. Gilyarov. - M .: Sov. Encycl., 1989 .-- 864 p.

6. Workshop on histology, cytology and embryology / ed. ON THE. Yurina, A.I. Radostina. - M .: Higher. shk., 1989 .-- 154 p.

Ham A., Cormik D. Histology / A. Ham, D. Cormik. - M .: Mir, 1983 .-- 192

1. Features of the embryonic development of mammals.

2. Embryogenesis of oviparous mammals.

3. Embryogenesis of marsupial mammals.

4. Embryogenesis of placental mammals.

5. Human embryogenesis.

General characteristics of anamnias and amniotes

General characteristics of anamnias

Based on the characteristics of embryonic development, all chordates are divided into two groups: anamnias and amniotes. Anamnia- these are animals in which, in the process of embryonic development, such embryonic membranes as amnion, or water membrane, and allantois are not formed. Anamnias include chordates, leading a primary aquatic lifestyle, as well as lower chordates, closely related to the aquatic environment during the period of reproduction and embryonic development of embryos - jawless, fish and amphibians. In connection with the embryonic development of these chordates in the aquatic environment, they lack an aqueous membrane and allantois, since the functions of respiration, excretion and nutrition of the developing embryo are provided by the surrounding aquatic environment.

Chordates belonging to anamnias by the nature of embryonic development can be divided into three groups:

1) lancelet, the eggs of which contain little yolk;

2) some cyclostomes, fish (cartilaginous ganoids) and amphibians, the eggs of which contain an average amount of yolk;

3) Selahia and bony fish, eggs contain a lot of yolk.

Lancelet embryogenesis

After fertilization, the redistribution of the yolk begins in the ovum of the lancelet, which is concentrated mainly on one side of the ovum, corresponding to the vegetative pole. The animal pole of the ovum is identified by the second polar body located above it. Oocyte fragmentation is complete, uniform (Figure 1).

/ - animal pole; 2 - vegetative pole; 3 - accumulation of yolk; 4 - celloblastula; 5 - cells of the blastoderm.

Drawing–1. Sequence (I –Vi) lancelet ovum cleavage

The first two splits are meridional, the third is equatorial. Further fragmentation takes place alternately in one direction or the other, and the number of cells increases exponentially. After the formation of a single-layer embryo - blastula, it becomes noticeable that the cells of the animal pole are smaller than the cells of the vegetative pole. In the spherical celloblastula of the lancelet, a flattened part of the vegetative pole is distinguished, called the bottom of the blastula, and the opposite part corresponding to the animal pole is called the roof of the blastula. The cells that form the roof of the blastula will differentiate into cells of the outer germ layer, or ectoderm, and the cells of the bottom of the blastula, into the endoderm.

Gastrulation is carried out by invagination of the blastoderm of the vegetative pole into the blastocoel. The invagination continues until the cells of the vegetative pole come in contact with the cells of the animal pole, in connection with which the blastocoel cavity narrows and disappears (Figure 2).

I - celloblastula; II - IV - gastrulation; V-neurula;

1 - ectoderm; 2 – endoderm; 3 - chord; 4 – mesoderm; 5 - neural plate; 6 - upper and 7 - lower lip of the blastopore; 8 - blastopore; 9 – cavity of the primary intestine; 10 - cavity of the secondary intestine; 11 - in general.

Picture 2–Lancelet embryogenesis

With the completion of the first stage of gastrulation, a two-layer embryo, or gastrula, appears, consisting of cells of the outer germ layer - the ectoderm and the inner embryonic layer - the endoderm. As a result of invagination, a cavity of the primary intestine is formed, lined with endoderm cells, which communicates with the external environment by the blastopore. The cellular composition of the endoderm is heterogeneous, since it also includes the cellular material of the future notochord and mesoderm. With the formation of the cavity of the primary intestine, the embryo begins to grow rapidly and lengthens, but the most intense morphogenetic processes are carried out in the region of the upper, or dorsal, lip of the blastopore. Immediately behind the upper lip of the blastopore, on the dorsal surface of the embryo, the ectoderm thickens and consists of tall prismatic cells called the medullary or neural plate. The ectoderm surrounding the neural plate is represented by small cells that form the skin. Under the neural plate, the endoderm cells, which represent the material of the future notochord, undergo the same changes. In the future, the neural plate begins to bend, forming a neural groove, and the cells of the skin ectoderm intensively crawl onto it. Subsequently, the neural groove deepens, its edges close, and it turns into a neural tube, the cavity of which is called a neural canal. The cells of the cutaneous ectoderm are closed, and the neural tube is under them. At the same time, the cells of the endoderm, adjacent to the neural plate, bend towards the latter, twist and separate into a dense cord - a chord, which looks like a solid cylinder. On the sides of the chordal primordium, the endoderm invaginates towards the ectoderm, forming mesodermal protrusions, or mesodermal sacs, which subsequently detach from the endoderm and begin to grow between the ectoderm and the endoderm. The cavity of the mesodermal sacs arising from the gastrocoel turns into a secondary body cavity, or the whole. Thus, in the process of gastrulation, a three-layered embryo arises.

After separation of the notochord and lacing of the mesodermal sacs, the edges of the endoderm gradually converge in the dorsal part of the embryo and, closing, form a closed intestinal tube. Following gastrulation in the embryo, a complex of axial organs arises, characteristic of representatives of the chordate type. It consists of a notochord, on the sides of which there are clusters of segmented mesoderm - somites.

The formation of axial organs occurs at the neurula stage. The neural tube of the lancelet in the front and back of the embryo remains open for some time. Later, on the posterior part of the body of the embryo, the ectoderm grows onto the blastopore and closes it so that the cavity of the neural tube communicates with the intestinal cavity by the neuro-intestinal canal, which quickly overgrows. The mouth opening in the embryo of the lancelet is formed a second time at the anterior end of the body due to the thinning and breakthrough of the ectoderm.

The third germ layer, or mesoderm, of the embryo of the lancelet is segmented throughout. The mesodermal segments are further divided into the dorsal part - somites and the abdominal part - splanchnotomes. Somites remain segmented, while splanchnotomas on each side of the body lose their primary segmentation, merge and form, splitting into two sheets, right and left coelomic cavities. The latter are combined under the intestinal tube into a common secondary body cavity. When the tail begins to form in the lancelet, the neurointestinal canal disappears, and at the posterior end of the embryo, at the site of the blastopore, due to the thinning and breakthrough of the body wall, an anal opening appears. Having passed the described stages of development, the lancelet becomes a free-swimming larva. During the period of larval development, organogenesis and histogenesis are completed and the larva turns into an adult animal.

The result of active cell division, growth and directional movements (migrations) of cell flows with the formation of a multilayer embryo, or gastrula, (the emergence of layer-by-layer, separated from each other by a distinct gap, germ layers: external - ectoderm, middle - mesoderm, internal - endoderm). The movement of cells occurs in a strictly defined area of ​​the embryo - in the area of ​​the gray sickle. The latter was described by W. Roux in 1888. In a fertilized amphibian egg, a gray sickle appears as a colored area on the side opposite to the penetration of sperm. This site is believed to be the location of the factors necessary for gastrulation.

In different representatives of vertebrates, it occurs in several main ways: by invagination (invagination), immigration (moving part of the cells into the embryo), epiboly (fouling), delamination (splitting). The methods of gastrulation depend on the type of ovum. With any method of gastrulation, the leading forces are uneven cell proliferation in different parts of the embryo, the level of metabolic processes in cells located in different parts of the embryo, the activity of amoeboid cell movements, as well as inductive factors (proteins, nucleoproteins, steroids, etc.). As a result of gastrulation, the main rudiments of organs and tissues are isolated.

The next period embryogenesis is histo- and organogenesis - the differentiation of various tissues and organs of the body from the material of the germ layers and embryonic rudiments.

As a result of gastrulation, a multi-layered embryo... Despite the different methods of gastrulation, after isolation of the material of the germ layers, along the axis of the embryo, there is the material of the notochord, which underlies the neural plate; to the left and right of the notochord, the material of the mesoderm is located. All this characterizes the axial complex of primordia. In the future, the formation of organ rudiments occurs, which are spatially localized groups of stem cells - sources of tissue development. The patterns of differentiation of the cellular material of the primordia can be traced in the embryogenesis of the most studied animals.

Lancelet. Lancelet development.

The classic object of embryological research lancer studied in detail by A.O. Kovalevsky. The lancelet is a representative of the class of chordates of the subtype of skullless, up to 8 cm in size and lives on the sandy bottom in warm seas. It got its name from its shape resembling a lancet (a surgical instrument with a double-edged blade, a modern scalpel).

Egg lancelet is oligo- and isocytal, 110 µm in size, the core is located closer to the animal pole. Fertilization is external. The fragmentation of the zygote is complete, almost uniform, synchronous and ends with the formation of a blastula. As a result of the alternation of meridian and latitudinal cleavage furrows, a single-layer blastula is formed with a cavity filled with a liquid - a blastocele. The blastula retains its polarity, its bottom is vegetative, and the roof is the animal part; there is an edge zone between them.

At gastrulation there is an invagination of the vegetative part of the blastula into the animal part. The invagination gradually deepens and, finally, a double-walled bowl with a wide gaping opening leading into the newly formed cavity of the embryo is formed. This type of gastrulation is called intussusception. This is how blastula turns into gastrula. In it, the material of the embryo is differentiated into the outer layer, the ectoderm, and the inner layer, the endoderm. The cavity of the bowl is called the gastrocele, or the cavity of the primary intestine, which communicates with the external environment through the blastopore, which corresponds to the anus. In the blastopore, the dorsal, ventral, and two lateral lips are distinguished. As a result of invagination, the center of gravity of the embryo shifts, and the embryo turns upward with the blastopore. The edges of the blastopore gradually close and the embryo elongates. The topography of the cells in the blastopore lips determines the development of different parts of the embryo. During gastrulation, the notochord and mesoderm are separated from the inner leaf of the gastrula, which are located between the ecto- and endoderm. Gastrulation ends with the formation of an axial complex of primordia and further - with the isolation of organ primordia. The chorda induces the development of the neural tube from the material of the dorsal ectoderm. This part of the ectoderm thickens, a neural plate (neuroectoderm) is formed, which sags along the midline and turns into a groove.

The edges of the groove gradually close into the neural tube. Remaining part ectoderm- cutaneous, grows together over the neural tube. However, at the very anterior and posterior ends of the embryo, the neural tube communicates with the external environment for some time through two holes - neuropores. Subsequently, the mesoderm is divided into dorsal segments - somites, the number of which increases from 15 pairs to 60-65 pairs in an adult lancelet. Part of the laterally located mesoderm is not segmented and splits into external (parietal) and internal (visceral) splanchnotome sheets. These sheets grow between the ecto- and endoderm and, reaching the middle on the ventral side of the embryo under the intestinal tube, grow together, forming a single secondary cavity - the whole. At the anterior end of the embryo, a depression (mouth bay) appears, growing towards the anterior part of the intestinal tube. When the ectoderm of the mouth bay and the blind end of the intestinal tube come into contact, apoptosis of cells occurs and communication of the intestine with the external environment occurs. A similar process takes place at the posterior end of the embryo. On the sides of the head section of the embryo, contact between the cutaneous ectoderm and the intestinal endoderm also occurs. A breakthrough occurs at the site of this contact. So the cavity of the anterior intestine communicates with the external environment (the branchial apparatus is formed). After that, the embryo emerges from the egg shell into the external environment in the form of a larva.

Labeling techniques for studying migration processes blastomeres made it possible to identify certain areas of the embryo in the early stages of development (zygotes - blastula), which later develop into germ layers and embryonic rudiments of organs and tissues. These areas have been called presumptive (putative) sites, or primordia.

The small amount of yolk explains the ease of crushing and gastrulation. The crushing is complete, almost uniform, of the radial type; as a result, celloblastula is formed.

Crushing of the lancelet egg (according to Almazov, Sutulov, 1978):

A - zygote; B, C, D - formation of blastomeres

(the location of the dividing spindle is shown)

The animal pole corresponds approximately to the future anterior end of the larva's body. The fertilized ovum (zygote) is split into blastomeres in its entirety in the correct geometric progression. Blastomeres are almost the same size, animal ones only slightly smaller than vegetative ones. The first cleavage furrow is the meridinal one, which passes through the animal and vegetative pole. It divides the globular egg into two perfectly symmetrical halves, but the blastomeres are rounded. They are spherical, have a small area of ​​contact. The second cleavage groove is also meridional, the first

is pendicular to the first, and the third is latitudinal.

As the number of blastomeres increases, they diverge more and more from the center of the embryo, forming a large cavity in the middle. In the end, the embryo takes the form of a typical celloblastula - a bubble with a wall formed by one layer of cells - blastoderm and with

a lobe filled with a liquid - a blastocele.

Blastula cells, at first rounded and therefore not tightly closed, then acquire the shape of prisms and close tightly. Therefore, late blastula, as opposed to early, is called epithelial. The late blastula stage completes the cleavage period. By the end of this period, the size of the cells reaches a minimum, and the total mass of the embryo does not increase in comparison with the mass of the fertilized egg.

Crushing in amphibians.

Fragmentation in amphibians is holoblastic (complete), uneven and asynchronous. The first furrow of cleavage is meridional; it runs from the antimal pole to the vegetative one. Since in amphibians the vegetative pole is overloaded with yolk, the closer to it, the lower the speed of incising the furrow. She has not yet managed to reach the vegetative pole when the second furrow begins to cut.

Rice. 3. Crushing (A - E) of the zygote and cross section of the blastula (G) of the frog

(after Gilbert, 1993)

The second groove is also meridional, running perpendicular to the first. After passing the first groove, two are formed, after the second - four blastomeres. At the stage of 4 blastomeres, two animal blastomeres receive ½ of the gray sickle material, while in 2 vegetative blastomeres there is no gray sickle material.

The third cleavage furrow is latitudinal and runs closer to the animal pole. As a result of its passage, 8 blastomeres are formed: 4 animal and 4 vegetative, significantly different in size. This is followed by two meridional grooves. Due to the difference in the size of blastomeres and the amount of yolk, at first 4 animal blastomeres (short-term stage of 12 blastomeres) are divided, and then 4 vegetative blastomeres (stage of 16 blastomeres).

The next two simultaneously occurring furrows are latitudinal. Faster passage of the upper latitudinal furrow determines the 6 short-term stage of 24 blastomeres. Completion of the passage of the lower latitudinal furrow leads to the formation of 32 blastomeres. After the 64 blastomere stage, despite the fact that the sequence of grooves is preserved, the geometric sequence of the row (2 - 4 - 8 - 12 - 16 - 24 - 32 - 64) is disrupted. In parallel with the passage of the meridional and latitudinal cleavage grooves, a tangential groove is also formed, as a result of which the embryo acquires multilayerness.

An amphibian embryo containing from 16 to 64 cells is usually called a morula because of its distant external resemblance to a mulberry berry (Latin morum). At the 128-cell stage, a well-distinguishable blastocoel appears and it is generally accepted that at this time the embryo reaches the blastula stage, although the formation of the blastocoel can be traced from the very first cleavage division. (Golichenkov V.A.)

Crushing in fish

Polylecital eggs,

telolecital. It is noted

polyspermia. It is believed that

sperm nuclei not fused with

female pronucleus, long

preserved in the vitelline layer

eggs bordering blastoderm,

and even participate in processing

yolk. The fragmentation is discoidal.

Fifth division of crushing

separates a number of blastomeres from

yolk. Marginal blastomeres

and the blastomeres at the base of the blastoderm retain their connection with the yolk. Six divisions of fragmentation are more or less synchronous, after which the synchrony is broken. As a result, a periblast or a layer of merocytes is formed at the base of the blastoderm. As a result of cleavage, a discoblastula is formed, externally limited by a layer of tightly connected integumentary cells. Over time, the slit-like cavity of the blastula increases, and to a greater extent at one of the edges of the blastodisc. It is believed that differences in the size of the blastocoel (or, in other words, the density of cell packing in different parts of the blastoderm) determine the axes and bilateral symmetry of the future embryo - the area of ​​the expanded blastocoel corresponds to the caudal end of the embryo.

Crushing in reptiles

Ovum fragmentation in reptiles is incomplete, discoidal. Since fertilization of the egg takes place in the upper third of the oviduct, cleavage begins during the movement of the egg through the oviduct, and when the egg is laid, the embryo is either at the stage of discoblastula or early gastrula. In a number of reptiles, due to the long-term presence of the egg in the oviduct, a new organism is formed, which is born, or leaves the egg immediately after its laying (viviparous lizard, vipers).

Discoidal cleavage results in discoblastula. The roof of the blastula consists of small cells of the blastoderm, the aggregate of which is called the blastodisc, and the bottom of the blastula is formed by an uncrushed mass of yolk. Part of the yolk under the embryonic disc is resorbed and a sub-embryonic cavity is formed in the form of a gap between the outer and inner layers of the blastodisc.

Crushing in birds

The fragmentation and formation of blastula in birds occurs when the egg moves down the oviduct, i.e. simultaneously with its surroundings by tertiary shells. The cleavage period for a chicken embryo is 22 hours.

The fragmentation of the zygote in birds is incomplete, of the discoidal type. Only the embryonic disc located at the animal pole, which has an insignificant surface and volume in comparison with the non-crushing mass of the yolk, undergoes crushing. The first three cleavage furrows are radial, corresponding to the meridional furrows of the lancelet and amphibians, then latitudinal and tangential furrows appear (Fig. 2).

Rice. 2. Discoidal cleavage in chicken (according to Dondua, 2005):

A - 2 blastomeres; B - 4 blastomeres; B - 8 blastomeres; D - 16 blastomeres

With discoidal cleavage, cells (periblast), incompletely separated from the yolk, remain all the time at the edge of the blastodisc and under it, from which the central blastomeres are separated. The fragmentation in birds is sharply uneven and irregular. Starting from the passage of the third and fourth grooves, cells of different sizes are formed, without any correctness and constancy in their arrangement.

By the time of oviposition, the egg goes through about 14 divisions of the zygote, as a result of which the blastoderm of the newly laid egg contains about 60 thousand cells. As a result, a multilayer blastodisc is formed, consisting of cells of irregular shape, closely adjacent to each other and overlying the unbroken yolk (Fig. 3).

Rice. 3. Formation of the embryonic disc of the chicken (according to Dondua, 2005):

A - late stage of crushing; B - before the beginning of gastrulation:

1 - blastomeres; 2 - epiblast; 3 - sub-embryonic cavity; 4 - yolk

Under the influence of blastodisc cells and merocytes, part of the yolk under the embryonic disc liquefies, a small space is formed,

filled with liquid - the sub-embryonic cavity. At this stage, two areas of the blastodisc are clearly distinguishable: a light field in the center (area pellucida) and a dark one along the periphery (area opaca). ( Golichenkov, V.A. Workshop on)

The appearance of a bright field is due to the fact that in the center of the blastodisc

The yolk is used by the embryo first of all, as a result of which a sub-embryonic gap is formed and this entire area looks transparent. The cells of the peripheral zone along the edge of the blastodisc overlap the yolk, so this area of ​​the embryo looks dark. ( Golichenkov, V. A)

Crushing of mammals.
Soon after the formation of the zygote, a series of mitotic divisions begins, strictly controlled by the genome and called cleavage; this process begins a few hours after fertilization, even in the oviducts. Each division lasts from 12 to 24 hours. The zygote divides into blastomere cells in a specific sequence. The first division occurs in a plane passing through both poles of the egg, thus the spherical zygote is divided into two hemispherical blastomeres. The resulting division groove is not located randomly, but depending on the place of penetration of sperm and the subsequent redistribution of the cytoplasm. The second crushing groove runs perpendicular to the previous one. Four blastomeres are obtained, grouped according to the principle of radial symmetry, then they again divide in half to form eight blastomeres. Continuing divisions of blastomeres are characterized by alternating division planes - one of the two blastomeres divides in the equatorial plane, the other in the meridional plane. This type of division, typical of mammals, is called alternating. Moreover, the blastomeres do not divide simultaneously, and as a result, there is no clear increase in the number of cells from 2 to 4, and then to 8. At certain points in time, the embryos contain an odd number of blastomeres.
Each of the cells formed during cleavage is about 2 times smaller than the maternal one. In the period between cleavage divisions, interphase cell growth does not occur, and therefore the total mass of all cells remains approximately equal to the mass of the egg cell.

Rice. 1. Early stages of embryonic development in mammals:

I - beginning of crushing;
A - fertilization; B, C, D - blastomeres;
II - formation of morula - gradual overgrowth of dark cells with light;
III - blastocyst formation:
A - the beginning of the formation of the cavity;
B - separation of the embryonic nodule from the trophoblast;
C - transformation of the embryonic nodule into the embryonic flap (discoblastula);
1 - trophoblast; 2 - embryoblast.

As a result of divisions, a multicellular embryo is formed, which resembles a raspberry in appearance and is called a morula. At the stage of 8-cell morula, which is characteristic only for mammals, there is a significant convergence of 6astomeres. In this case, tight contacts arise between cells, allowing small molecules and ions to pass from cell to cell. This phenomenon is called compaction and contributes to the further differentiation of the embryo and the separation of the trophoblast, which takes over the nutritional function, and the inner cell mass, which gives rise to the embryo itself. In the process of compaction, individual parts of the plasma membrane of the embryonic cells begin to move in different, strictly genetically determined directions. These processes involve proteins that make up the cell membranes, and with the appearance of microvilli that attach blastomeres to each other, their cytoskeleton changes.
In the 32-cell morula stage, the dog's embryo enters the uterus. This occurs on the 7-8th day, which is much later than in other mammals. During this time, development stops and the death of embryos with dysfunctions caused by both hereditary and environmental factors occurs. Thus, hereditary and environmental factors are the acting factors of natural selection.
Blastomeres obtained in the process of division are located on the periphery, and gradually in the middle of the dense cell mass of the morula, a cavity (blastoblast) is formed and the actual embryonic cluster of cells - the embryoblast, otherwise called the embryonic node, and the surrounding layer of feeding cells - trophoblast, is separated. The body of the embryo is later formed from the embryoblast. The trophoblast serves as a nourishing leaf for the early embryo. This stage of development of the embryo is called the blastocyst.
The blastocyst at this stage consists of the outer layer of trophoblast cells and the inner cell mass, a globular cluster of cells attached from the inside to one of the trophoblast poles and which is the material for building the embryo. The future location of the cell in the embryo or trophoblast is determined in the process of compaction, when the cells are either on the surface or inside the embryo.

Conclusion

Bibliography

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3. Golichenkov, V. A. Workshop on embryology / V. A. Golichenkov,

M.L.Semenova. M.: Academa, 2004.

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E.N. Nikeriasova. M.: Academa, 2004.

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S.-Pb .: Publishing house of St. Petersburg State University, 2005.Vol. 1.

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An egg fertilized outside the mother's body undergoes complete and almost uniform cleavage. The result is a typical globular blastula. Larger cells vegetativelyAt the second pole, the blastula begin to invaginate inward, and a typical invagination gastrula is formed.

Then the gastrula is pulled out, the gastropore (blastopore) is reduced, and the ectoderm along the dorsal side to the very gastrothe time begins to deepen, forming a neural plate. Subsequently, the neural plate is separated from the cells of the neighboring ectoderm, and the ectoderm grows together over the neural plate and over the gastropore. Still later, the edges of the neural plate turn upward and grow together, so that the plate turns into a neural tube. Since the neural plate continues back to the gastropore, at this stage of development at the posterior end of the embryo, the intestinal cavity is connected to the cavity of the central nervous system through the neuro-intestinal canal (canalis neuroentericus). At the anterior end, the nerve folds close the last, so that here the nerve canal for a long time communicates with the external environment through the opening — neuroporus. In the future, an olfactory fossa forms at the site of the neuropore.

(according to Schmalhausen). I - whole tubule with many nephrostomes and solenocytes; II - part of the renal tubule with seven solenocytes sitting on it:

1 - the upper end of the branchial cleft, 2 - the opening of the renal tubule into the perigabranial cavity

(schematically). I — blastula; II, III, IV - gastrulation; V and VI - the formation of the mesoderm, chord and nervous systems:

1 - animal pole, 2 - vegetative pole, 3 - gastric cavity, 4 - gastropore (blastopore), 5 - neural canal, 6 - neurointestinal canal, 7 - neuropore, - 8 - mesoderm fold, 9 - coelomic sacs, 10 - chord, 11 - the place of the future mouth, 12 - the place of the future anus

(according to Parker):

1 - ectoderm, 2 - endoderm, 3 - mesoderm, 4 - intestinal cavity, 5 - neural plate, 6 - central nervous system, 7 - neurocoel, 8 - chord, 9 - secondary body cavity, 10 - parietal peritoneal leaf, 11 - visceral peritoneum

(according to Delage):

I - endostyle, 2 - oral opening, 3 - right and 4 - left metapleural folds, 5 - left branchial slits, 6 - right branchial slits

Simultaneously with the development of the central nervous system, differentiation of the endoderm occurs. First, from above, along the sides of the primary intestine, longitudinal protruding folds begin to form - the rudiments of the future mesoderm, while the endoderm strip between these folds begins to thicken, fold and, finally, split off from the intestine and turns into the rudiment of the notochord. Further development of the mesoderm proceeds as follows. First, the folds of the primary intestine, lying on the sides of the rudimentary chord, are separated from the intestine and turn into a series of closed, segmented coelomic sacs. Their walls represent the mesoderm, and the cavities represent the secondary body cavity, or the whole. Subsequently, the coelomic sacs grow up and down, and each sac is subdivided into the dorsal region, located on the side of the notochord and neural tube, and into the abdominal region, located on the sides of the intestine. The dorsal regions are called somites, the abdominal regions are called lateral plates. Mainly muscle segments are formed from somites - myotomes, which are worn in an adultthe name of the animal myomers, and the skin itself (corium), while the leaves of the peritoneum are formed from the lateral plates, and the whole of an adult animal is formed from the cavities of the lateral plates, which merge with each other. Finally, by invagination, a mouth is formed at the front end of the body, and an anus at the back.