Gastrulation. Lancelet

The individual development of the lancelet is the simplest initial scheme of embryogenesis, through the gradual complication of which in the course of evolution more complex developmental systems of chordates, including humans, arose.

STRUCTURE OF THE EGG. FERTILIZATION

Lancelet eggs are poor in yolk and microscopically small (100-120 microns), and belong to the isolecithal type. The yolk grains are small and distributed almost evenly in the cytoplasm. However, the egg cell has an animal and a vegetative pole. In the area of ​​the animal pole, when the egg matures, the reduction bodies are separated. The nucleus in a fertilized egg is located closer to the animal pole due to the not entirely uniform distribution of the yolk, being located in the part of the cell free from yolk inclusions. The maturation of the egg occurs in water. The first reduction body is separated at the animal pole of the oocyte even before fertilization. It is washed off with water and dies.

Lancelet females spawn eggs into the water, and males release sperm here - fertilization is external, monospermic. After penetration of the sperm, a fertilization membrane is formed around the egg, which prevents others from penetrating the egg.

excess sperm. Following this, the second reduction body is separated, which is located between the vitelline membrane and the egg.

All further development also takes place in water. After 4-5 days, a microscopic larva hatches from the egg shell and begins to feed on its own. First it floats, and then settles to the bottom, grows and metamorphoses

SPLITTING UP. BLASTULA

The small amount of yolk explains the ease of crushing and gastrulation. The fragmentation is complete, almost uniform, of the radial type, resulting in the formation of a coeloblastula (Fig. 1).

Rice. 1.Crushing the lancelet egg (according to Almazov, Sutulov, 1978):

A- zygote; B, C, D- formation of blastomeres (location of the spindle is shown)

The animal pole approximately corresponds to the future anterior end of the larval body. The fertilized egg (zygote) is completely divided into blastomeres in the correct geometric progression. Blastomeres are almost identical in size, animal only slightly

how many smaller than vegetative ones. The first cleavage furrow is meridional and passes through the animal and vegetative poles. It divides the spherical egg into two perfectly symmetrical halves, but the blastomeres are rounded. They are spherical, have a small area of ​​co-

touch. The second crushing furrow is also meridional, perpendicular to the first, and the third is latitudinal.

As the number of blastomeres increases, they diverge further and further from the center of the embryo, forming a large cavity in the middle. In the end, the embryo takes the form of a typical coeloblastula - a vesicle with a wall formed by one layer of cells - the blastoderm and a cavity filled with fluid - the blastocoel (Fig. 2).

The blastula cells, initially round and therefore not tightly closed, then take on the shape of prisms and close tightly. Therefore, the late blastula, as opposed to the early one, is called epithelial.

The late blastula stage completes the period of cleavage. By the end of this period, cell sizes reach a minimum, and the total mass of the embryo does not increase compared to the mass of the fertilized egg.

Rice. 2. Lancelet blastula (according to Almazov, Sutulov, 1978):

A - appearance; B - cross-section (the arrow shows the postero-anterior direction of the body of the future embryo); B - location of materials of future organs on a sagittal section of the blastula

GASTRULATION

Gastrulation occurs by invagination - invagination of the vegetative hemisphere of the blastula inward, towards the animal pole (Fig. 3). The process proceeds gradually and ends with the entire vegetative hemisphere of the blastula moving inward and becoming the internal germ layer - the primary endoderm of the embryo. The factor causing intussusception is the difference in the rate of cell division in the marginal zone and in the vegetative part of the blastula, leading to active movement of cellular material. The animal hemisphere becomes

Rice. 3. Initial stages of lancelet gastrulation (according to Manuilova, 1973):

the outer germ layer is the primary ectoderm. The embryo takes the form of a two-layer cup with a wide gaping opening - the primary mouth or blastopore. The cavity into which the blastopore leads is called the gastrocoel (the cavity of the primary intestine). As a result of invagination, the blastocoel is reduced to a narrow gap between the outer and inner germ layers. At this stage, the embryo is called a gastrula (Fig. 4 A, B).

The primary intestine (archenteron), represented by the internal germ layer surrounding the gastrula cavity, is the rudiment of not only the digestive system, but also other organs and tissues of the larva. The blastula, like the egg, floats with its animal pole upward inthe force of the greater weight of the vegetative hemisphere.

As a result of invagination, the center of gravity of the embryo moves and the gastrula turns upward with the blastopore.

The blastopore is surrounded by dorsal, ventral and lateral lips. Next, concentric closure of the edges of the blastopore occurs and the embryo elongates. In the lancelet, a representative of deuterostomes, the blastopore corresponds not to the mouth, but to the anus, indicating

posterior end of the embryo. As a result of the closure of the edges of the blastopore and protrusion of the body in the anteroposterior direction, the embryo elongates. At the same time, the diameter of the gastrula decreases - the total mass of the cells composing the embryo cannot increase while development proceeds under the cover of the egg membranes. The embryo acquires bilateral symmetry.

The location of the primordia in the late gastrula is best seen in a cross section of the embryo (Fig. 4 C, D).

Its outer wall is formed by ectoderm, heterogeneous in its composition. In the dorsal part, the ectoderm is thickened and consists of tall cylindrical cells. This is the rudiment of the nervous system, which remains

Rice. 4.

Gastrula of the lancelet (according to Manuilova, 1973):

A- early stage;B- late stage; IN- transverse section through the late gastrula; G- gastrula turning into neurula (transverse section)

still on the surface andforms the so-called medullary orneural plate. The rest of the ectoderm consists of small cells and is the rudiment of the animal's integument. Under the neural plate in the inner germ layer there is a notochord rudiment, on both sides of which there is mesoderm material in the form of two cords. Located in the abdominal partendoderm, forming the base of the primary gut, the roof of which is made up of the rudiments of the notochord and mesoderm.

The material of the future internal organs, being in the blastula from the outside, during the process of gastrulation moves inside the embryo and is located at the sites of the organs developing from them. Only the rudiment of the nervous system remains on the surface. It plunges into the embryo at the stage following the gastrula.

NEURULATION AND FORMATION OF AXIAL ORGANS

At the end of gastrulation, the next stage in the development of the embryo begins - the differentiation of the germ layers and the laying of organs. The presence of a complex of spinal organs: the neural tube, notochord and axial muscles, also known as axial muscles, is one of them

characteristic features of the chordate phylum.

The stage at which the formation of axial organs occurs is called neurula. Externally, it is characterized by changes occurring in the rudiment of the nervous system.

They begin with the growth of ectoderm along the edges of the neural plate. The resulting neural folds grow towards each other and then close together. The plate sinks inward and bends greatly (Fig. 5).

Rice. 5.Neurula of lancelet (according to Manuilova, 1973):

A- early stage (transverse section); B- late stage (transverse

section), letter “ C ” indicates the secondary body cavity (coelom)

This leads to the formation of a groove, and then a neural tube, which remains open for some time in the anterior and posterior parts of the embryo (these changes are most conveniently observed in a cross-section of the embryo). Soon, in the posterior part of the body, the ectoderm grows onto the blastopore and the opening of the neural tube, closing them in such a way that the neural tube remains connected to the intestinal cavity - the neurointestinal canal is formed.

Simultaneously with the formation of the neural tube, significant changes also occur in the inner germ layer. The materials of future internal organs are gradually separated from it. The notochord rudiment begins to bend, separates from the common plate and turns into a separate cord in the form of a solid cylinder. At the same time, the mesoderm separates. This process begins with the appearance of small pocket-like outgrowths on both sides

inner leaf. As they grow, they separate from the endoderm and, in the form of two cords with a cavity inside, are located along the entire length of the embryo. In addition to the longitudinal grooves, two more pairs of coelomic sacs are successively separated from the anterior end of the primary intestine.

Thus, in the development of the lancelet there is a stage characterized by the presence of three pairs of segments and indicating the evolutionary relationship of the lancelet with the three-segmented larvae of hemichordates and echinoderms. The lancelet has a pronounced enterocoelous mode

formation of the coelom - its detachment from the primary intestine. This method is the original one for all deuterostome animals, but in almost none of the higher vertebrates, with the exception of cyclostomes, is it represented with such clarity. After separation of notochord and mesoderm

the edges of the endoderm gradually come closer together in the dorsal part and eventually close, forming a closed intestinal tube.

During further development, the mesoderm is segmented: the strands are divided transversely into primary segments or somites. These form three main bookmarks:

The dermatome is formed from the outer somite, facing the ectoderm, - from its cells the connective part of the skin, represented mainly by fibroblasts, subsequently arises;

The sclerotome is formed from the inner part of the somite, adjacent to the notochord (lower vertebrates) or to the notochord and neural tube (higher vertebrates) - it represents the rudiment of the axial skeleton;

The myotome is the part of the somite located between the dermatome and the sclerotome - it is the rudiment of all striated muscles.

Somite differentiation in the lancelet proceeds differently than in vertebrates. This difference is expressed in the fact that in vertebrates only the dorsal part of the mesodermal strands is segmented, while in the lancelet they are completely divided into segments. The latter are soon divided into a dorsal part - somites, and an abdominal part - splanchnotome.

The somites, from which the trunk muscles develop, remain separate from each other, while the splanchnotomes merge on each side, forming the left and right cavities, which then unite under the intestinal tube into a common secondary body cavity (coelom).

In the development of the lancelet, on the one hand, the features of typical vertebrates are clearly presented (the characteristic arrangement of the primordia during gastrulation, the formation of the notochord from the dorsal wall of the primary intestine and the neural plate from the dorsal ectoderm), and on the other hand, the features of invertebrate deuterostome animals (coeloblastula, invagination gastrula, three-segment stage, enterocoelous anlage of mesoderm and coelom formation).

Subsequently, due to the formation of the tail, the neurointestinal canal disappears. An oral opening breaks through in the head part of the intestinal tube, and an anal opening is formed at the rear end, under the tail, by a secondary breakthrough of the animal’s body wall in place of the closed blastopore. The embryo enters the stagefree-swimming larva.

loblastula (lancelet). With complete uneven fragmentation of oligolecithal eggs, a blastula with a very small blastocoel is formed - steroblastula (mammals). From the mesolecithal eggs of amphibians, a blastula with a thick bottom and a small blastocoel shifted to the roof is formed - an amphiblastula, and with partial discoidal crushing of polytelolecithal eggs - a discoblastula, in which the blastocoel has the appearance of a slit (bird).

IN In the formation of a blastula, not only the amount of yolk matters

And crushing intensity, but also types of adhesion

between blastomeres, the size of the spaces between them, the phenomena of mutual sliding, the formation of outgrowths such as pseudopodia and other insufficiently studied phenomena. At any stage of development, the embryo is an integrated whole. For example, at the stage of two blastomeres, when they are separated, a full-fledged organism (identical twins) is formed from each blastomere. If you kill one of the blastomeres (by puncture), but do not separate it from the other, only half of the embryo will develop from the remaining blastomere. Integration increases as the number of blastomeres increases, and in an 8-cell embryo, each cell loses the ability to develop into a whole organism. In the vast majority of animals, the blastula without stopping passes into the next stage of development - gastrula. The exception is birds, and among mammals, animals of the mustelidae family. In birds, the embryo freezes at the blastula stage in the interval from laying the egg to incubation (or incubation). This time should not exceed 2-3 weeks, since the viability of the embryo decreases. In mustelids, developmental delay can last 2-3 months and is associated with the regulation of the timing of the birth of puppies (in the spring), regardless of the timing of fertilization.

GASTRULATION

Gastrulation is a set of complex processes leading to the formation of a gastrula - an embryo consisting of several layers of cells. It is associated with the active movement of cellular material, as a result of which the embryo turns from a single-layer (blastula) into a two-layer one. The processes of gastrulation have been under the close attention of embryologists for decades now, trying to unravel the reasons for the active movement and differentiation of cells and entire cell layers. There are many theories that explain these morphogenetic movements by differences in the activity of mitoses in different parts of the embryo, in the ability of cells to change shape, stick together, carry out amoeboid, sliding, translational movements, in the release of biologically active substances by cells that specifically affect their microenvironment, in changes in cell metabolism, etc. However, the existing theories speak only about individual points and do not provide a general cause-and-effect characteristic of the morphogenetic processes of the gastrulation period, the causes of which still remain unclear in many respects.

There are four types of gastrulation (Fig. 19): 1. Invagination - invagination. In this way, the gastrula is formed from the coeloblastula. The bottom cells invaginate into the blastocoel and reach the roof cells of the blastula from within. The embryo changes from round to cup-shaped or sac-shaped, from single-layered to double-layered. The layers are called germ layers. The outer germ layer is called ectoderm, the inner one is called endoderm. The sac-like cavity of the gastrula is the primary intestine - the gastrocoel, and the opening through which it communicates with the external environment is the primary mouth is a blastopore. This type of gastrulation is characteristic of the lancelet.

Rice. 19. Types of gastrulation:

A - intussusception; B - epiboly; B - migration; G - delamination.

In worms, mollusks and arthropods, the definitive mouth is formed at the base of the blastopore, which is why they are called protostomes. In deuterostomes (chaete-maxillary, brachiopods, echinoderms, intestinal-breathing and chordates), the mouth appears on the ventral side of the head end, and the blastopore turns into the anus or neurointestinal canal.

2. Immigration - settlement. In this case, from the blastoderm, especially from the bottom of the blastula, cells move into its blastocoel, located under the outer layer. The outer layer of cells turns into ectoderm, and the underlying one into endoderm. This type of gastrulation is characteristic of coelenterates.

3. Delamination - separation. Occurs when synchronous division of blastomeres occurs parallel to the surface of the blastula. In this case, one layer of cells will lie on the outside, and the other on the inside. This type of gastrulation occurs in coelenterates (jellyfish).

4. Epiboly - fouling. Occurs when the vegetative pole of the zygote is overloaded with yolk or does not participate in crushing

(partial crushing), or during crushing very large blastomeres are formed here, filled with yolk and therefore dividing slowly. The smaller, rapidly dividing cells of the blastula roof overgrow them. This type of gastrulation occurs in oligochaete worms.

This or that type of gastrulation is rarely found in its pure form in the animal world. Much more often, mixed gastrulation is observed, including elements of two or even three of the listed types. In any case, gastrulation leads to the formation of the same germ layers: ectoderm, endoderm and mesoderm (it is not present in sponges and coelenterates).

Differentiation of germ layers. Following the formation of two germ layers or in parallel with them, a third germ layer, the mesoderm, is formed between the ectoderm and endoderm. During the normal development of the embryo, no matter what class and type it belongs to, the leaves, interacting with each other, differentiate in a strictly defined direction: from each germ layer the rudiments of certain homologous tissues are formed - histogenesis and organs - organogenesis. In chordates, the so-called axial organs: neural tube, notochord and intestinal tube(see color table I). With further organogenesis, the skin epithelium and its derivatives (horn formations, skin glands), the nervous system, and the anterior and posterior ends of the intestinal tube develop from the ectoderm. Derivatives of endoderm are the epithelial lining of the digestive canal and its glands, the respiratory system, and mesoderm is the muscular system, skeleton, organs of the genitourinary system, walls of serous cavities of the body, etc. Mesoderm also gives rise to mesenchyme - embryonic connective tissue, which is part of most organs.

IN In the process of organogenesis, the relationship and interpenetration of the germ layers is so close that the cellular elements of two, or even all three, germ layers take part in the formation of almost every organ. In addition, during the period of active differentiation of the embryonic material, the phenomenon of induction is widespread, when one any bookmark influences the nature of the development of another. Thus, the material of the dorsal lip of the blastopore induces the development of the neural tube, as a result of which the neural tube can arise in any other place where the material of the dorsal lip has been transferred.

IN In the process of embryogenesis, signs characteristic of an animal appear gradually. Thus, in the embryo of any vertebrate animal, it is first possible to determine the features characteristic of the chordate type, for example, the nature of the appearance and location of the axial organs. Then the features inherent in the class become visible, for example the nature of the skin, later

- order, family, genus, species, breed and, finally, individual.

Therefore, before starting to study the development of such complexly organized animals as mammals, it is advisable to become familiar with the more primitively organized chordates. The most convenient and well-studied object is the lancelet.

Questions for self-control. 1. How does cleavage differ from ordinary cell division and how does it depend on the structural features of the egg? 2. What are blastula and gastrula, what types of blastula and gastrulation do you know? 3. How does germ layer differentiation proceed?

Chapter 6. EMBRYONAL DEVELOPMENT OF ANIMALS OF DIFFERENT CLASSES OF CHORDATE TYPE

DEVELOPMENT OF THE LANCELATE

The lancelet is a primitive chordate animal, 2-5 cm long, that lives in coastal waters. Its development was studied in detail by A. O. Kovalevsky. The lancelet egg is 100-120 µm in diameter, iso-oligolecithal, external fertilization and development (color table I). Crushing is complete and uniform. The first ten divisions occur synchronously. The first groove dividing the zygote into two blastomeres is meridian. It passes through three points (the apical pole, the site of sperm penetration and the vegetative pole) and divides the embryo into right and left halves. The second groove is also meridian, but runs perpendicular to the first and divides the embryo into dorsal (dorsal) and ventral (ventral) halves. The third furrow is latitudinal, runs almost along the equator and divides the embryo into anterior and posterior parts, after which it consists of eight blastomeres. Crushing is a fast process. Within two hours after fertilization, a coeloblastula is formed, which consists of more than 1000 cells.

Gastrulation begins by intussusception. The bottom of the blastula is flattened and then pressed inward. After 11 hours of fertilization, a two-layer sac-like gastrula with a wide blastopore. Ectoderm it consists of flattened cells, and the endoderm consists of taller and larger cells. Then the embryo elongates somewhat in length. At the same time, its dorsal side is flattened, and the blastopore narrows. It distinguishes the dorsal lip

The edge of the blastopore adjacent to the dorsal side of the embryo, opposite to it - ventral lip and lateral lips located between them. In the area of ​​the dorsal lip of the blastopore, particularly active processes of cell reproduction and movement occur. At the same time, the blastopore becomes smaller and smaller, and the growing cellular material moves in the head direction. The strand of ectodermal cells lying on the dorsal side along the entire embryo turns into neural plate. The areas of ectoderm located on the sides of the neural plate rise, forming two longitudinal ridges, and the neural plate deepens in the form of a groove. The longitudinal ectodermal ridges are becoming increasingly closer to each other, and the groove of the neural plate is deepening. By the time the edges of the neural groove close and it becomes the neural tube, the ectodermal folds also fuse with each other and the neural tube appears under the ectoderm, which now becomes cutaneous ectoderm. Ectoder-

Features of the embryonic development of anamnia are studied using the example of lancelet, fish and amphibians.

The eggs of the lancelet are primary isolecithal , fertilization takes place in water, i.e. external After fertilization, a zygote is formed, which undergoes complete and uniform fragmentation - development holoblastic . The zygote is divided first by two successive mitoses in mutually perpendicular meridional planes into four, then by the equatorial furrow into eight blastomeres, etc. The cleavage planes alternate, and after the seventh division a blastula of the type appears coeloblastula .

Blastomeres, forming blastoderm vary in size and quality, because... there is a distribution of different quality material in the cytoplasm of the zygote, which undergoes internal differentiation. The resulting coeloblastula consists of large-cell yolk blastomeres forming the bottom (future intestinal endoderm), medium-sized blastomeres located dorsally above them - the material of the dorsal falx (future notochord) and small blastomeres surrounding the bottom of the blastula - the material of the central falx (future mesoderm). All this is surrounded by ectoderm.

Using the intravital staining method, it was found that all of the listed areas of the blastula move by tucking through the lips of the blastopore, are located around the gastrocoel and create the basis for the organotypic period of lancelet development - the period of differentiation of tissues and organs.

The blastula has a cavity - blastocoel . The blastocoel is filled with liquid - a waste product of blastoderm cells.

By intussusception , i.e. retraction of the vegetative hemisphere into the animal one, the blastula is transformed into gastrulu , the wall of which becomes two-layer and consists of ectoderm outside and endoderm inside . These are the primary germ layers.

The cavity of the primary intestine is formed in the gastrula - gastrocel , which communicates with the external environment through blastopore . Due to the movement of the center of gravity towards the animal pole, the embryo turns 180° with the blastopore upward and continues to float in the water.

Later, the embryo elongates. It is released from the primary endoderm in the dorsal direction. chordal plate, and in the dorsolateral two mesodermal plates. From the primary ectoderm along the midline of the body stands out nervous a plate consisting of higher cells than the rest of the ectoderm. The neural plate is detached from the ectoderm and plunges under it, first turning into philtrum , and then in neural tube , the rest of the skin ectoderm closes over the neural tube. Simultaneously with the formation of the neural tube, the notochordal plate is transformed into a round cellular cord - chord , the mesodermal plates curl into hollow tubes lying between the notochord and the cutaneous ectoderm, and the remaining endoderm closes into secondary colon . This creates a complex of axial organs that characterize the type of chordate animal.

The mesoderm is metamerically (from the head and tail of the embryo) divided into segments, and no segmentation occurs in the tail of the embryo. In addition, the first two segments develop independently, reproducing the ancient three-segmented larval form of skullless - Dipleurula . Each segment of the mesoderm, excluding the first two (“ancient”) segments, grows in the dorsoventral direction and is divided into three parts: somite (dorsally), splanchnotoma (ventrally) and segmental leg between them.

Somites differentiate into dermatome – skin sheet (lateral), sclerotome – skeletal rudiment (central) and myotome – muscle leaf (remnant after isolation of the first two). Skeletal (somatic) muscles subsequently develop from the myotome. The splanchnotome splits into two leaves: visceral (internal) and parietal (parietal), between them there is a secondary body cavity - in general . From both leaves of the splanchnotome, a network-shaped tissue stands out - mesenchyme , which is also formed from the sclerotome and dermotome of the somite. Mesenchyme (embryonic connective tissue) fills the entire space between the three germ layers. From the remaining part of both layers of the splanchnotome, the lining of the coelom arises - mesothelium . Finally, the segmental stalk is transformed into nephrogonothome – epithelial lining of the excretory system and the germ of the reproductive system.

The period of differentiation of tissues and organs ends the larval period of development of the lancelet, which lasts about three months, and a sexually mature animal emerges from the larva.

The result of active cell division, growth and directed movements (migration) of cell flows with the formation of a multilayer embryo, or gastrula (the appearance of layer-by-layer germ layers, separated from each other by a distinct gap: outer - ectoderm, middle - mesoderm, inner - endoderm). The movement of cells occurs in a strictly defined area of ​​the embryo - in the gray crescent area. The latter was described by V. Roux in 1888. In a fertilized amphibian egg, the gray sickle is revealed as a colored area on the side opposite to the penetration of the sperm. Factors necessary for gastrulation are believed to be localized in this location.

In different representatives of vertebrates it occurs in several main ways: by invagination (invagination), immigration (movement of some cells inside the embryo), epiboly (fouling), delamination (splitting). The methods of gastrulation depend on the type of egg. With any method of gastrulation, the leading forces are the uneven proliferation of cells 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 separated.

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

As a result of gastrulation, a multilayered embryo. Despite the different methods of gastrulation, after the material of the germ layers is released, 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 is the material of the mesoderm. All this characterizes the axial complex of rudiments. Subsequently, 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. Development of the lancelet.

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

Egg lancelet is oligo- and isolecithal, 110 µm in size, the nucleus 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 fluid - the blastocoel. The blastula retains polarity, its bottom represents the vegetative part, and the roof represents the animal part; between them there is a marginal zone.

At gastrulation the vegetative part of the blastula invaginates into the animal part. The invagination gradually deepens and, finally, a double-walled cup is formed with a wide gaping hole leading into the newly formed embryonic cavity. This method of gastrulation is called intussusception. This is how the blastula turns into a gastrula. In it, the material of the embryo turns out to be differentiated into an outer layer - ectoderm, and an inner layer - endoderm. The cavity of the cup is called the gastrocoel, or the cavity of the primary intestine, which communicates with the external environment through a blastopore, which corresponds to the anus. The blastopore has a dorsal, ventral and two lateral lips. As a result of invagination, the center of gravity of the embryo shifts, and the embryo turns upward with its blastopore. Gradually, the edges of the blastopore close and the embryo elongates. The topography of cells within the blastopore lips determines the development of different parts of the embryo. During gastrulation, the notochord and mesoderm are separated from the inner layer of the gastrula, which are located between the ecto- and endoderm. Gastrulation ends with the formation of the axial complex of rudiments and then with the separation of organ rudiments. The notochord induces the development of a neural tube from dorsal ectoderm material. This part of the ectoderm thickens, forming a neural plate (neuroectoderm), which bends 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 for some time with the external environment through two openings - 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 outer (parietal) and inner (visceral) layers of the splanchnotome. These leaves grow between the ecto- and endoderm and, having reached the middle on the ventral side of the embryo under the intestinal tube, grow together, forming a single secondary cavity - the coelom. At the anterior end of the embryo, a depression (oral bay) appears, growing towards the anterior section of the intestinal tube. When the ectoderm of the oral bay comes into contact with the blind end of the intestinal tube, cell apoptosis occurs and communication between the intestine and the external environment occurs. A similar process occurs at the posterior end of the embryo. On the sides of the head section of the embryo, contact between the skin ectoderm and the intestinal endoderm also occurs. A breakthrough occurs at the point of this contact. This is how the cavity of the foregut communicates with the external environment (the gill apparatus is formed). After this, the embryo emerges from the egg shell into the external environment in the form of a larva.

Labeling methods 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 were called presumptive areas, or rudiments.

An egg fertilized outside the mother's body undergoes complete and almost uniform fragmentation. The result is a typical spherical blastula. Larger cells vegetativelyThe th pole of the blastula begins to invaginate inward, and a typical invagination gastrula is formed.

Then the gastrula elongates, the gastropore (blastopore) decreases, and the ectoderm along the dorsal side to the gastroporethe pore begins to deepen, forming the neural plate. Subsequently, the neural plate separates from the cells of the neighboring ectoderm, and the ectoderm grows together over the neural plate and over the gastropore. Even 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 using the neurointestinal canal (canalis neuroentericus). At the anterior end, the nerve folds close last, so that here the nerve canal communicates with the external environment for a long time through an opening called the neuropore (neuroporus). Subsequently, an olfactory fossa is formed in place 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 - upper end of the gill slit, 2 - opening of the renal tubule into the peribranchial cavity

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

1 - animal pole, 2 - vegetative pole, 3 - gastric cavity, 4 - gastropore (blastopore), 5 - nerve canal, 6 - neurointestinal canal, 7 - neuropore, - 8 - mesoderm fold, 9 - coelomic sacs, 10 - chord, 11 - place of the future mouth, 12 - 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 - notochord, 9 - secondary body cavity, 10 - parietal layer of peritoneum, 11 - visceral peritoneum

(according to Delage):

I - endostyle, 2 - oral opening, 3 - right and 4 - left metapleural folds, 5 - left gill slits, 6 - right gill 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 strip of endoderm contained between these folds begins to thicken, curl up 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 notochord, are separated from the intestine and turn into a series of closed, segmentally located coelomic sacs. Their walls represent the mesoderm, and the cavities represent the secondary cavity of the body, or coelom. Subsequently, the coelomic sacs grow up and down, and each sac is divided into a dorsal section, located on the side of the notochord and neural tube, and an abdominal section, located on the sides of the intestine. The dorsal sections are called somites, the ventral sections are called lateral plates. Somites form mainly muscle segments - myotomes, which are worn in adultsThe animal name is myomeres, and the skin itself (corium), while the leaves of the peritoneum are formed from the lateral plates, and the whole of the 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 anterior end of the body, and an anus at the posterior end.