General patterns of embryonic development of chordates.

Embryogenesis of lancelets and amphibians

• 1. Definition of the concept of embryogenesis.
• 2. General patterns of embryonic development of chordates. The essence of the basic biogenetic law.
• 3. Characteristics of the stages of development of the embryo.
• 4. Embryonic development of the lancelet.
• 5. Features of the embryogenesis of fish and amphibians.
• 6. Differentiation of the mesoderm in representatives of the type of chordates.

Embryogenesis is a chain of complex interrelated transformations leading to the emergence of multicellular organisms capable of existing in the external environment.

The phenomena observed in this case are reduced to two groups: the processes of differentiation and the processes of growth.

The processes of differentiation represent true development. They lead to the appearance of cells, tissues and organs characteristic of an organism of a certain type, class and species.

The progressive development and differentiation of embryonic cells are due to the differential action of genes. This means that in the early stages of embryogenesis, only individual genes are actively functioning, then all large groups of them. In this case, a strictly ordered change of these active states occurs, programmed by the hereditary basis itself (genetic determinacy - determinatio - restriction), which directs ontogeny along a certain path. The hereditary basis has developed over the centuries-old history of the development of the species, i.e. the entire previous evolution of animals - phylogenesis (files - tribe). F. Müller and E. Haeckel laid this main pattern of development in the basis of the biogenetic law they formulated (1872 - 1874), the essence of which can be expressed in the form of a simple aphorism: ontogenesis there is a shortened form phylogenesis.

Due to phylogenetic relationship, in early embryogenesis, animals go through common stages that reflect the main stages of the evolution of the animal world:

• 1) the formation of a zygote (fertilization) - a unicellular level of organization of living beings;
• 2) crushing of the zygote - transition to the multicellular level of organization;
• 3) the formation of germ layers (gastrulation) - the transition to a multilayer type of animal structure;
• 4) differentiation of the germ layers with the processes of organogenesis and histogenesis, as a result of which at first the signs inherent in the type of animal appear, and then gradually the features characteristic of the class, genus, family, species, breed and, finally, the individual are revealed.

In development, the factors of mutual influence of germinal rudiments on each other (induction) are not excluded, due to which some of them play the role of germinal organizers.

Fertilization- a complex process of mutual assimilation of the egg and sperm, as a result of which a new organism is formed - zygote(zygotes - joined together). The zygote is the book of heredity, written in letters of maternal and paternal genes. The combination of two hereditary bases provides an increased vitality of the developing individual.

In animals whose development takes place in an aquatic environment, fertilization is external, while in representatives of the majority of terrestrial vertebrates, it is internal.

Cleavage of the zygote- this is the process of repeated mitotic division of the zygote without the growth of the resulting blastomeres, as a result of which the embryo acquires the simplest multicellular form, called blastula(blastos - sprout, germ). It can be complete holoblastic(holos - whole, whole), in which the entire zygote is crushed, and partial - meroblastic(meros - part), with fragmented animal pole only. Complete crushing, in turn, is uniform and uneven.

gastrulation- the stage of formation of a two-layer embryo. Its superficial cell layer is called the outer germ layer - ectoderm(ecto - external, outside; derma - skin), deep - internal, endoderm(endon - inside).

In primitive chordates, such an embryo in its shape resembles a single-chamber stomach (gaster), which served as the basis for designating all varieties of embryos at the stage of formation of germ layers by the term gastrula.

Differentiation germ layers ensures the appearance in a strictly defined sequence of the entire variety of cells, tissues and organs of animals of a certain type, class and species, i.e. complete organo- and histogenesis. In this case, each time, axial organs first appear (neural tube, chord and primary intestine) and the third, middle in position, germinal layer - mesoderm.

Embryogenesis of the lancelet.

Lancelets are small (up to 5 cm long), rather primitively arranged non-cranial animals of the chordate type, living in warm seas (including the Black Sea), passing through the larval stage in development, capable of independently existing in the external environment.

The first complete description of their development was presented by A.O. Kovalevsky. It is a classic example of initial forms, which are used as basic models for studying the features of embryogenesis in representatives of other classes of chordates.

The conditions and nature of the development of the lancelet do not require a significant accumulation of a reserve of nutrient material, therefore their eggs are of the oligolecital type. Fertilization is external.

Cleavage of the zygote is complete, uniform and synchronous. With each round of division of the zygote, an even number of approximately equal in size blastomeres (blastula particles) is formed, the number of which increases exponentially.

The first division furrow runs in the sagittal plane meridian. It forms the left and right halves of the embryo. The second furrow, also meridian, runs perpendicular to the first (frontal plane) and marks the future dorsal and abdominal parts of the body. The third furrow is latitudinal. Divides blastomeres into anterior and posterior, providing segmentation of the future trunk.

In further periods of development, the meridian and latitudinal cleavage furrows replace each other in a strictly regular sequence. The blastomeres formed as a result of such crushing become progressively smaller in size. The progressive increase in their number leads to the fact that the blastomeres force each other outward, due to which space is released in the central part of the embryo, and the dividing cells themselves form a single-layer wall - blastoderm. Thus, a spherical blastula appears with a cavity enclosed inside - blastocoele. This type of blastula is called coeloblastula(caelum - vault of heaven).

In the whole blastula, it is customary to distinguish roof(animal pole of the egg), bottom(vegetative pole of the egg) and edge zones. The bottom blastomeres are characterized by some increase in size due to the natural displacement of the yolk to the base of the vegetative pole of the oocyte.

The presence of a large blastocoel and a single-layer blastoderm predetermines the simplest way of gastrulation in the lancelet embryo - invagination of the bottom blastomeres towards the roof ( intussusception). Tightly adjacent to the dorso-lateral parts of the blastula, the invaginating blastomeres displace blastocoel, forming the inner germ layer of the endoderm and a new cavity of the embryo - gastrocoel, which through the primary oral opening ( blastopore) communicates with the environment.

The blastomeres of the roof and lateral zones make up the outer germ layer.

The resulting two-layer embryo (gastrula) feeds on its own due to the ingress of water enriched with plankton into the gastrocoel.

At the next stage of development, a strand of intensely dividing cells differentiates from the median dorsal ectoderm, which separates from the cells of other zones of the outer germ layer, descends slightly and becomes the neural plate, which subsequently forms the first axial organ of the lancelet larva - neural tube. The remaining part of the ectoderm, being the outermost layer of the body, turns into the integumentary epithelium of the skin - epidermis.

The rest of the axial organs and mesoderm develop by differentiation of various parts of the inner germ layer.

So, from the most dorsal middle part of it (as in the case of isolation of the neural plate), the notochordal plate stands out, which then twists into a dense cell cord - chord(the second axial organ of the larva), which in lancelets remains as the main supporting organ - the dorsal string.

On both sides of the chordal plate, in the dorso-lateral sections of the endoderm, paired rudiments of the third germ layer are differentiated - mesoderm, which ensures the bilateral symmetry of the body, the metamerism of its structure (segmentation) and the development of many organs and tissues.

The ventral part of the endoderm serves as the basis for the formation of the third axial organ - primary colon. The cells of the rudiments of the mesoderm are characterized by the strongest energy of division, the most intensive increase in their number, due to which the growing ribbon-like plates are forced to protrude towards the ectoderm and form folds. Resting with the tops of the folds against the dorsal ectoderm, with the inner edges against the chordal plate, and with the outer edges against the remaining ventral part of the endoderm, each mesodermal rudiment wraps down with further growth, is introduced between the outer and inner germ layers, helping the notochordal plate close into a string, the neural groove becomes a tube, and the ventral endoderm form the primary gut.

In turn, in each rudiment of the mesoderm, their basal edges also close, as a result of which these rudiments take the form of closed sac-like formations with a cavity inside. One of the leaves is adjacent to the ectoderm (the outer wall of the body of the larva) and therefore receives the name parietal(wall), the other - to the primary internal organ (gut), which gives reason to call it visceral. With subsequent development, both rudiments of the mesoderm ventrally, below the primary intestine, grow together. As a result, a single secondary body cavity appears in the body of the lancelet - in general, enclosed between the parietal and visceral sheets of its mesoderm.

Features of embryogenesis of fish and amphibians.

Fish and amphibians are characterized by a fairly high level of morphofunctional organization of the body, close phylogenetic relationship and the presence of stages of larval metamorphosis occurring in the aquatic environment, which determines the similarity in the structure of their eggs and the course of the main stages of embryonic development.

In connection with the intermediate position of the class of amphibians between pure inhabitants of the aquatic environment and representatives of animals leading a terrestrial lifestyle, it is most expedient to focus on the main features of the prelarval embryogenesis of amphibians.

Amphibian eggs accumulate a significant amount of yolk inclusions, which provide early stages of development (mesotelecithal type). The yolk occupies most of the cell (vegetative pole). The smaller animal pole is distinguished by a black or dark gray color due to the accumulation of black pigment, which accumulates in itself the thermal energy of the sun, which is not yet hot, in the initial spring time. Fertilization is external. Cleavage of the amphibian zygote is complete, uneven, slowed down due to the yolk. The first two cleavage furrows run meridian, as in the lancelet, dividing the zygote into 4 equal blastomeres. But already the first latitudinal furrow transforms fragmentation into an uneven form, since it passes in the border zone, between the animal and vegetative poles, which is why the upper blastomeres are smaller ( micromeres) compared to the lower ones loaded with yolk in large quantities ( macromers). During subsequent rounds of crushing, small blastomeres divide faster, releasing a small cavity (blastocoel) in the roof area, and large ones more slowly. They are inactive, which is why they form a multilayer bottom of the blastula and, to a lesser extent, its marginal zones. This type of blastula is called amphiblastula.

Due to the fact that most of the amphiblastula is formed by large blastomeres rich in yolk, its bottom and marginal zones are ready-made endoderm, which later completely turns into a trophic organ - the primary intestine.

The ectoderm, therefore, in the embryos of amphibians should appear, in contrast to the lancelets, anew. Only rapidly dividing micromeres of the roof can act as a source of its formation. Constantly accumulating in large numbers in this area, the indicated small blastomeres slide down and gradually overgrow the marginal zones and the bottom, forming a kind of outer wrap around them ( ectoderm), which by its nature resembles the process of manual production of large dosage forms - boluses. This was the basis for assigning such a peculiar type of gastrulation in amphibians the name epiboly, but translated by its essential meaning as fouling.

At the stage of differentiation of the germ layers, the neural plate, like in the lancelet, appears on the basis of the dorsal median ectoderm, but the formation of the rudiments of the notochord and mesoderm undergoes significant changes and is also transferred to the outer germ layer - to the region of its marginal zone on the side of the future caudal part of the body of the embryo (grey crescent zone).

Differentiating cells of the initially common chordomesodermal anlage actively multiply and migrate in a powerful stream into the depth of the gastrula, invaginating into the blastocoel. The middle part of this cell stream moves in a cranial direction above the endoderm, forming the notochordal plate. Its lateral branches represent the beginnings of the paired mesoderm. Separating from the chordal plate, the mesodermal cells go to the left and right of the central plane of the embryo, wrap themselves ventrally over the upper edges of the endoderm, and, continuing to divide and grow intensively, penetrate between the endoderm and ectoderm, helping the said endoderm close into the primary intestinal tube.

The resulting mesoderm, by moving and stratifying cells, forms parietal and visceral sheets with a secondary body cavity enclosed between them - as a whole.

Subsequent morphogenetic processes mesoderm differentiation proceed similarly in representatives of all classes of vertebrates.

In the dorsal parts of the left and right mesoderm, the cells multiply intensively, as a result of which the cavity between the sheets disappears, and both halves of it are successively divided into segments (provide the metamerism of the animal's structure). Each such segment is involved in the formation of the corresponding parts of the body, which is why they are given the name somites(soma - body).

Protruding medially under each somite, the middle sections of the mesoderm form tubular outgrowths - segment legs, which are the basis for the subsequent formation of urinary and reproductive organs from them. The kidneys are the first to develop on their basis, which is why the segmental legs can also be called nephrotomes(nephros - kidney).

The vast lower parts of the left and right mesoderm remain unsegmented, continue their ventral growth towards each other and, growing together, now form a single secondary body cavity in which the internal organs are located, which predetermines the assignment of their name splanchnotomes(splanchna - insides).

In the somites of the mesoderm, the cellular material, differentiating, is divided into three sagittal plates. The outer plate serves as a base for the formation of the connective tissue base of the skin ( dermatome), medium - skeletal muscles ( myotome), and the inner one - a strong support for the body - the skeleton ( sclerotome).

The left and right halves of the splanchnotome are actively evicted in the spaces between the germ layers and axial organs, cellular elements that form a temporary germ tissue - mesenchyme from which all types of supporting-trophic tissues, the endothelium of blood and lymphatic vessels, as well as smooth muscle tissue of internal organs will subsequently be formed.

The cells of the parietal and visceral sheets of the mesoderm remaining after the separation of the mesenchyme are transformed into a single-layer squamous epithelium of the serous membranes - mesothelium.

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

AT the formation of the blastula is important not only the amount of yolk

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. In the event that one of the blastomeres is killed (by a puncture), but not separated 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 passes into the next stage of development, the gastrula, without stopping. The exceptions are birds, and from mammals, animals of the weasel family. In birds, the embryo freezes at the blastula stage in the interval from egg laying to incubation (or incubation). This time should not exceed 2-3 weeks, as the viability of the embryo decreases. In mustelids, a 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 from a single-layer (blastula) turns into a two-layer one. For decades, the processes of gastrulation have been under the close attention of embryologists who are trying to unravel the causes of 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 with each other, to carry out amoeboid, sliding, translational movements, in the release of biologically active substances by cells that directly affect their microenvironment, in changes in cell metabolism, etc. However, the current theories speak only of individual moments and do not give a general causal characteristic of the morphogenetic processes of the gastrulation period, the causes of which are still largely unclear.

There are four types of gastrulation (Fig. 19): 1. Invagination - invagination. In this way, the gastrula is formed from the coeloblastula. The fundus cells invaginate into the blastocoel and reach from within the cells of the roof of the blastula. The embryo from round becomes cup-shaped or sac-shaped, from one-layer to two-layer. The layers are called germ layers. The outer germ layer is called the ectoderm, the inner one is the endoderm. The saccular 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 the blastopore. This type of gastrulation is characteristic of the lancelet.

Rice. 19. Types of gastrulation:

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

In worms, mollusks and arthropods, the definitive mouth is formed on the basis of the blastopore, therefore they are called protostomes. In deuterostomes (chaetognaths, brachiopods, echinoderms, enteropneathers, and chordates), the mouth arises on the ventral side of the head end, and the blastopore turns into an anus or neurointestinal canal.

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

3. Delamination - stratification. Occurs if there is a synchronous division of blastomeres parallel to the surface of the blastula. In this case, one layer of cells will be lying outside, and the other - 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. Smaller, rapidly dividing blastula roof cells overgrow them. This type of gastrulation occurs in oligochaetes.

This or that type of gastrulation is rarely found in the animal kingdom in its pure form. Much more often, mixed gastrulation is observed, which includes 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 (sponges and coelenterates do not have it).

Germ layer differentiation. 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. With 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 germinal leaf, the rudiments of certain homologous tissues are formed - histogenesis and organs - organogenesis. In chordates, before others, 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 the endoderm are the epithelial lining of the alimentary canal and its glands, the respiratory system, and the mesoderm is the muscular system, skeleton, organs of the genitourinary system, the walls of the serous body cavities, etc. The mesoderm also gives rise to mesenchyme - embryonic connective tissue, which is part of most organs.

AT In the process of organogenesis, the interrelation 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 germinal material, the phenomenon of induction is widespread, when one any bookmark affects 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, the neural tube can arise in any other place where the material of the dorsal lip has been transferred.

AT In the process of embryogenesis, signs characteristic of an animal appear gradually. So, in the embryo of any vertebrate animal, one can first determine the features characteristic of the chordate type, for example, the nature of the appearance and location of the axial organs. Then the traits inherent in the class become visible, for example, the nature of the skin, later

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

Therefore, before proceeding to the study of 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 normal 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 differentiation of germ layers proceed?

Chapter 6

DEVELOPMENT OF THE LANCELE

The lancelet is a primitive chordate animal 2-5 cm long, lives in coastal waters. Its development has been studied in detail by A. O. Kovalevsky. Ovum of a lancelet with a diameter of 100-120 microns, isooligolecithal, fertilization and development are external (color table I). Crushing is complete, uniform. The first ten divisions occur synchronously. The first furrow, dividing the zygote into two blastomeres, is meridional. 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 furrow is also meridional, 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 invagination. The bottom of the blastula flattens, and then is pressed inward. After 11 hours from fertilization, a two-layer saccular gastrula with a wide blastopore. ectoderm it consists of flattened cells, and the endoderm - of taller and larger ones. Then the embryo is somewhat elongated in length. At the same time, its dorsal side flattens, 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 region of the dorsal lip of the blastopore, especially active processes of cell reproduction and movement take place. In this case, the blastopore becomes smaller, and the growing cellular material moves in the head direction. A strand of ectodermal cells lying on the dorsal side along the entire embryo turns into neural plate. Parts of the ectoderm located on the sides of the neural plate rise, forming two longitudinal ridges, and the neural plate deepens in the form of a gutter. The longitudinal ectodermal ridges are increasingly approaching each other, and the groove of the neural plate is deepening. By the time the edges of the neural trough close and it turns into a neural tube, the ectodermal ridges have also fused with each other and the neural tube is under the ectoderm, which now becomes skin ectoderm. Ectoder-

Ontogeny, or individual development, is called the entire period of an individual's life from the moment the spermatozoa merge with the egg and the formation of a zygote until the death of the organism. Ontogeny is divided into two periods: 1) embryonic - from the formation of a zygote to birth or exit from the egg membranes; 2) postembryonic - from the exit from the egg membranes or birth to the death of the organism.

In most multicellular animals, the stages of embryonic development that the embryo goes through are the same. In the embryonic period, three main stages are distinguished: crushing, gastrulation and primary organogenesis.

The development of an organism begins with a unicellular stage. As a result of repeated divisions, a unicellular organism turns into a multicellular one. The resulting cells are called blastomeres. When dividing blastomeres, their size does not increase, so the division process is called crushing. During the period of crushing, cellular material accumulates for further development.

As the number of cells increases, their division becomes non-simultaneous. Blastomeres move further and further away from the center of the embryo, forming a cavity - the blastocoel. The cleavage is completed with the formation of a single-layer multicellular embryo - the blastula.

A feature of crushing is an extremely short mitotic cycle of blastomeres compared to the cells of an adult organism. During a very short interphase, only DNA duplication occurs.

Blastula, as a rule, consisting of a large number of blastomeres (in the lancelet - from 3000 cells), in the process of development passes into a new stage, which is called gastrula. The embryo at this stage consists of separated layers of cells, the so-called germ layers: the outer, or ectoderm, and the inner, or endoderm. The set of processes leading to the formation of a gastrula is called gastrulation. In the lancelet, gastrulation is carried out by pushing a part of the blastula wall into the primary body cavity.

After completion of gastrulation, a complex of axial organs is formed in the embryo: neural tube, notochord, intestinal tube. The ectoderm bends, turning into a groove, and the endoderm, located to the right and left of it, begins to grow on its edges. The groove plunges under the endoderm, and its edges close. The neural tube is formed. The rest of the ectoderm is the rudiment of the skin epithelium. At this stage, the embryo is called a neurula.

The dorsal part of the endoderm, located directly under the nerve bud, separates from the rest of the endoderm and folds into a dense cord - a chord. From the rest of the endoderm, the mesoderm and intestinal epithelium develop. Further differentiation of germ cells leads to the emergence of numerous derivative germ layers - organs and tissues.

From ectoderm the nervous system, the epidermis of the skin and its derivatives, the epithelium lining the internal organs develops. From endoderm develop epithelial tissues lining the esophagus, stomach, intestines, respiratory tract, liver, pancreas, epithelium of the gallbladder and bladder, urethra, thyroid and parathyroid glands.

Derivatives mesoderm are: the dermis, the entire connective tissue itself, the bones of the skeleton, cartilage, the circulatory and lymphatic systems, the dentin of the teeth, the kidneys, the gonads, the muscles.

The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. At the same time, one germ influences the other, to a large extent determining the path of its development. In addition, the rate of growth and development of the embryo is influenced by external and internal conditions.

1.Embryonic development of the lancelet, gastrulation, histo- and organogenesis. The concept of presumptive rudiments and their location at the lancelet blastula stage.

The lancelet is a representative of the class of chordates of the non-cranial subtype, up to 8 cm in size and lives on a sandy bottom in warm seas. It got its name because of the shape resembling a lancet (a surgical instrument with a double-edged blade, a modern scalpel).

The ovum of the lancelet is oligo- and isolecithal, 110 µm in size, the nucleus is located closer to the animal pole. Fertilization is external. Cleavage of the zygote is complete, almost uniform, synchronous and ends with the formation of the blastula. As a result of the alternation of meridional and latitudinal cleavage furrows, a single-layer blastula with a cavity filled with fluid, the blastocoel, is formed. The blastula retains polarity, its bottom is the vegetative part, and the roof is the animal part; between them is the marginal zone.

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 is formed with a wide gaping hole leading into the newly formed cavity of the embryo. This type of gastrulation is called invagination. This is how the blastula turns into a gastrula. In it, the material of the embryo is differentiated into the outer leaf - ectoderm, and the inner - endoderm. The cavity of the bowl is called the gastrocoel, or the cavity of the primary intestine, which communicates with the external environment through the blastopore, which corresponds to the anus. In the blastopore, 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 cells in the lips of the blastopore determines the development of different parts of the embryo. During gastrulation, the notochord and mesoderm separate 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 - the isolation of organ rudiments. The notochord induces the development of the neural tube from the material of the dorsal ectoderm. This part of the ectoderm thickens, forming the neural plate (neuroectoderm), which bends along the midline and turns into a groove.

Histo- and organogenesis: In the lancelet, a neural tube is formed from the ectoderm on the dorsal side of the embryo. The rest of the ectoderm forms the skin epithelium and its derivatives. The notochord is formed from the ento- and mesoderm under the neural tube. Under the notochord is the intestinal tube, on the sides of the notochord is the somite mesoderm. The outer part of the somite adjacent to the ectoderm is called the dermotome. It forms the connective tissue of the skin. The inner part - the sclerotome - gives rise to the skeleton. Between the dermotome and the sclerotome is the myotome, which gives rise to striated muscles. Under the somites are the legs (nephrogonotome), from which the genitourinary system is formed.

Coelomic bags are formed symmetrically on the sides. The walls of the coelomic sacs facing the intestines are called splanchnopleura, and those facing the ectoderm are called somatopleura. These sheets are involved in the formation of the cardiovascular system, pleura, peritoneum, pericardium.

Characteristic features of the biology and anatomy of Acrania Cranial - a small group of marine benthic animals, a typical representative of which is the lancelet (Branchiostoma lanceolatum, formerly called Amphioxus lanceolatum) A A

Anterior part of the body of Branchiostoma lanceolatum Anterior part of the body of the lancelet (appearance - on the left and scheme - on the right). The left photo shows the preoral opening facing forward and downward, framed by a border of tentacles. Myomers in the form of chevrons make up the musculature from the most anterior end to the most posterior point of the body. The first two chambers of the segmented ovary are visible. The diagram (left) shows a chord reaching the most anterior point of the body and a pharynx perforated by numerous gill slits.

Fertilized ovum of Branchiostoma belcheri Photos were taken using a scanning electron microscope. The arrows indicate the second polar body. In the right photo, at higher magnification, on the surface of the zygote, one can see the polar body and numerous microvilli

Stage 8 of the blastomere is the result of the third “latitudinal” division (the left picture is a view from the animal pole, the right one is from the side, at an angle). The photo clearly shows the difference in size between animal and vegetative blastomeres. As before, the surface of adjoining blastomeres to each other is small.

Branchiostoma belcheri. Stage 16 blastomeres. When viewed from the animal pole (left image), differences in size between animal and vegetative blastomeres are clearly visible. It should be noted the strict alignment between the blastomeres of the animal and vegetative layers.

Branchiostoma belcheri. Late gastrula stage (8 hours 50 minutes) after fertilization. Typical two-layer embryo. In the photo on the left, one can see a pronounced narrowing of the blastopore (Bp). Differences between the ectoderm (a layer of cuboidal cells) and the endodermis (a layer of cylindrical cells) are visible on the cleavage on the right. Ac - archenteron (gastrocoel).

19 Notogenesis in the lancelet Formation of the axial complex of rudiments (transverse sections). Bending of the neural plate and chordal plate in opposite directions. Protrusion and separation of the mesodermal ridges (7) from the fornix of the archenteron, followed by their segmentation.

Prelarval stages of development. There is a closure of the neural ridges (6), separation of the notochord (5), formation of somites and then coelomic sacs (9). 10 - myotome, 11 - myocoel, 12 - dermatome, 13 - anlage of ventral mesenterium, 14 - subintestinal vein (black), 15 - visceral leaf of splanchnotome, 16 - parietal leaf of splanchnotome, 17 - splanchnotome (lateral plate of mesoderm ), 18 – splanchnocoel.