The germ layers of the lancelet. Evolutionary features of embryogenesis of primitive chordates on the example 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 blastomeres displace 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 - the 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 intensively dividing cells differentiates from the median dorsal ectoderm, which separates from the cells of other zones of the outer germ layer, descends somewhat downward 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 (intestine), 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.

The ovule of the lancelet is oligolecithal with an isolecithal distribution of the yolk. The cleavage is complete, uniform, synchronous (the cleavage furrows are meridional or latitudinal); after the 7th cleavage (128 blastomeres), the cleavage ceases to be synchronous). When the number of cells reaches 1000, the embryo becomes a blastula (i.e., a single-layer embryo with a cavity (blastocoel) filled with a gelatinous mass, the blastula wall is called a blastoderm. The type of lancelet blastula is a uniform coeloblastula, it has a bottom, a roof.

Then gastrulation begins by invagination, i.e. invaginations on the blastula inside.

In total, there are 4 ways of gastrulation, they are usually combined, but one of them prevails:

1. Intussusception (invagination)

2. Immigration (movement of cells with their immersion inside the bastula)

3. Epiboly (fouling)

4. Delamination (splitting of the blastula wall into 2 sheets).

As a result of gastrulation, a 2-layer embryo is formed - gastrula, it has a cavity (in fact, this is the cavity of the primary intestine) and a hole leading to the cavity (blastopore - primary mouth). The blastopore is surrounded by 4 lips (the organizers of organogenesis). The material passing through the dorsal lip becomes the notochord, and through the rest of the lips it becomes the mesoderm.

After the end of gastrulation, the embryo begins to grow rapidly in length. The dorsal part of the outer leaf flattens and turns into the neural plate. The rest of this plate is laterally overgrown with the neural plate and becomes the outer lining of the skin, thus. the neural plate is inside, a groove first forms in it, then it twists into a tube with a central channel inside (neurocoel).

The inner sheet is split into several parts. The dorsal region is flattened. It is wrapped in a cylindrical cord, separated from the intestine and turns into a chord. The areas adjacent to the notochord are also separated and form 2 sacs (mesoderm, in fact), thus. the mesoderm is formed by the enterocele route, i.e. lacing bags.

Parts of the mesoderm:

Segmental pedicles (nephrogonadotomes)

Splanchnot (a whole is formed between its leaves).

After the chord, neural tube and mesoderm have formed, the endoderm folds and forms the 3rd axial organ - the primary intestine (all other sections of the gastrointestinal tract with glands subsequently develop from it).

Embryogenesis of amphibians.

The amphibian ovum is mesolecithal with a telolecithal yolk distribution. Crushing is complete, uneven, asynchronous. After the 3rd crushing, 4 small cells (micromeres) are concentrated at the animal pole and 4 large cells (macromeres) are concentrated at the vegetative pole. In addition to the latitudinal and meridional division furrows, tangential furrows also appear (they run parallel to the surface of the egg and divide the blastomeres in a parallel plane), thus. the wall of the blastula is multilayered. Blastomeres begin to divide asynchronously, and the cells of the bottom of the blastula (filled with yolk) are much larger than the cells of the roof of the blastula. The blastoderm consists of several layers, and the blastocoel (compared to the lancelet) is significantly reduced. Thus, an uneven multi-layered coeloblastula (amphiblastula) arises.

Gastrulation occurs by partial invagination and epiboly (see question 21). More active cells from the animal half “crawl” onto the vegetative half of the embryo, and subsequently penetrate into the embryo. During gastrulation, the cells multiply weakly, mainly stretching and displacement of the cells of the surface layer occurs. After the end of gastrulation, the embryo begins to actively grow.

The mesoderm material is laid during gastrulation, and the mesoderm immediately detaches from the primary intestine, moves forward and ventrally, and spreads out between the ecto- and endoderm (this method of laying the mesoderm is called proliferative). At first, there are no cavities in the mesoderm, it consists of 2 bags, then somites separate from it; the dorsal part of the mesoderm turns into myotomes, the ventral part - into splanchnotomes. First, myotomes are connected with splanchnotomes by segmental legs (give rise to excretory organs), then they diverge.

The muscles of the body develop from the myotomes (from the medial side of the myotome), the sclerotome (the skeletal sheet develops from it - the rudiment of the mesenchyme, from which, accordingly, all supporting-trophic tissues develop), the dermatome (the lateral part of the myotome) - becomes the mesenchyme, from which the deep layers develop skin.

Notochord, neural tube, primary gut develops approximately like a lancelet (passing through the dorsal lip).

Embryogenesis of birds.

The ovum is polylecital with a telolecithal distribution of the yolk. Cleavage - partial (meroblastic), discoidal. The first 2 furrows of division are meridional, then radial and tangential furrows appear. As a result, a discoblastula (blastodisc) is formed - one layer of cells lying on the yolk, where the cells are adjacent to the yolk - a dark field, where they are not adjacent - a light field. In a freshly laid egg, the embryo is at the stage of discoblastula or early gastrula.

Gastrulation occurs by immigration and delamination, i.e. a number of cells simply leave the outer layer (immigration), and some of the blastomeres begin to divide in such a way that the mother cell remains on the outer wall, and the daughter cell passes into the second layer (delamination).

After 12 hours of incubation, due to cell migration along the edges of the light field, a primary streak and Hensen's knot are formed, which act as blastopore lips.

In the early gastrula, 2 leaves are distinguished: the epiblast - the outer leaf and the hypoblast - the inner leaf.

The epiblast contains presumptive rudiments of the skin ectoderm, neural plate, notochord cells, and mesoderm cells; in the hypoblast there are only rudiments of the endoderm.

Notochord cells are invaginated from the epiblast through the Hensen's node, and mesoderm cells are invaginated through the primary streak; a 3-layer embryo is formed. The method of laying the mesoderm is late invagination. It differentiates like amphibians.

Further development of the embryo is associated with the formation of extra-embryonic (fetal) membranes. Shells are formed due to the growth of cells of the germ layers outside the body of the embryo. First, a trunk fold is formed (with the participation of all 3 sheets), which lifts the embryo above the yolk, then 2 amniotic folds close over the embryo with the participation of the ectoderm and the parietal sheet of the mesoderm - the amnion and the serous membrane (chorion) are formed. A little later, the cells of the endoderm and the cells of the visceral sheet of the mesoderm form the wall of the yolk sac and allantois.

Embryogenesis of mammals.

The ovum is secondary oligolecithal (alicetal). Cleavage is complete, uneven, asynchronous; as a result of the first crushing, 2 types of blastomeres are formed: small and light (crush faster) and large dark (crush slowly). A trophoblast (vesicle) is formed from the light ones, the dark ones are inside the vesicle and are called the embryoblast (the embryo itself will develop from them). The trophoblast, within which the embryoblast is located, is called the blastoderm vesicle or blastocyst. The type of blastula is sterroblastula (i.e., the embryoblast is inside the trophoblast, there is no cavity). The blastocyst enters the uterus from the oviduct, where it feeds on royal jelly, its cells actively grow, then the first attachment of the trophoblast to the uterine wall occurs - implantation.

Gastrulation occurs by delamination (splitting), an epiblast is formed (contains the rudiments of the notochord, neural plate, mesoderm, ectoderm) and a hypoblast (it contains only the rudiments of the mesoderm).

The laying of the mesoderm occurs in the same way as in birds - late invagination through the Hensen's knot and the primary streak.

The mesoderm differentiates into 3 buds (primary differentiation): somites, nephrogonadotomes, and splanchnotomes/lateral plates.

Later (secondary differentiation) - each somite is divided into:

dermatome (later - the mesh layer of the skin),

myotomes (subsequently - striated skeletal muscle tissue),

sclerotomes (subsequently - cartilaginous and bone tissue of the skeleton);

The splanchnotome is divided into visceral and parietal sheets, between which the whole is laid.

From the nephrogonadotome, the epithelium of the excretory and reproductive systems is laid.

The embryonic development of mammals is also associated with the formation of extraembryonic membranes. Just as in birds, the beginning of the formation of membranes is associated with the formation of the trunk, and then the amniotic fold.

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 systems of development of chordates, including humans, arose.

STRUCTURE OF THE EGG. FERTILIZATION

The eggs of the lancelet are poor in yolk and microscopically small (100-120 microns), are of the isolecithal type. Yolk granules are small and distributed in the cytoplasm almost evenly. However, the animal and vegetative poles are distinguished in the egg. In the region of the animal pole, when the ovum matures, the separation of the reduction bodies occurs. The nucleus in a fertilized egg is closer to the animal pole due to the uneven 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 separates at the animal pole of the oocyte before fertilization. It is washed away with water and dies.

The females of the lancelet spawn their eggs into the water, and the males release spermatozoa here - fertilization is external, monospermic. After the penetration of the sperm around the egg, a fertilization membrane is formed, which prevents the penetration of other eggs into the egg.

excess sperm. This is followed by the separation of the second reduction body, which is located between the yolk membrane and the egg.

All further development also takes place in water. After 4-5 days, a microscopic larva hatches from the egg shell, which proceeds to independent feeding. First, it floats, and then settles to the bottom, grows and metamorphoses.

SPLITTING UP. BLASTULA

A small amount of yolk explains the ease of crushing and gastrulation. Cleavage is complete, almost uniform, of a radial type; as a result, a coeloblastula is formed (Fig. 1).

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

A- zygote; B, C, D- the formation of blastomeres (the location of the fission 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 of almost the same size, animal only slightly

how much smaller than the 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

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 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 coeloblastula - a vesicle with a wall formed by one layer of cells - the blastoderm and with a cavity filled with fluid - the blastocoel (Fig. 2).

Blastula cells, initially rounded and therefore not tightly closed, then take the form of prisms and close tightly. Therefore, the late blastula, in contrast to the early one, is called epithelial.

The late blastula stage completes the cleavage period. 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. Blastula lancelet (according to Almazov, Sutulov, 1978):

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

GASTRULATION

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

Rice. 3. The initial stages of gastrulation of the lancelet (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 (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 an internal germ layer surrounding the gastrula cavity, is the rudiment not only of the digestive system, but also of other organs and tissues of the larva. Blastula, like the egg, floats with the animal pole up 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. Then there is a concentric closing of the edges of the blastopore and elongation of the embryo. In the lancelet, a representative of deuterostomes, the blastopore corresponds not to the mouth, but to the anus, denoting

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

The location of the rudiments in the late gastrula is best seen in the transverse 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 high cylindrical cells. It is the germ of the nervous system, which remains

Rice. 4.

Lancelet gastrula (according to Manuilova, 1973):

A- early stage;B- late stage; V- cross section through the late gastrula; G- gastrula passing 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 primordial integument of the animal. Under the neural plate in the inner germ layer is the rudiment of the notochord, on both sides of which there is mesoderm material in the form of two strands. Located in the abdomenendoderm, which forms the base of the primary intestine, the roof of which is formed by the rudiments of the notochord and mesoderm.

The material of the future internal organs, being in the blastula from the outside, in the process of gastrulation moves inside the embryo and is located in the places 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, chord and axial muscles, also known as axial, is one of them.

characteristic features of the chordate type.

The stage at which the laying of axial organs occurs is called neurula. Outwardly, it is characterized by changes that occur with 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. The plate, on the other hand, sinks inwards and bends strongly (Fig. 5).

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

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

section), the letter “ C ” denotes the secondary cavity of the body (whole)

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

Simultaneously with the formation of the neural tube, significant changes occur in the inner germ layer. Materials of future internal organs are gradually separated from it. The rudiment of the chord begins to bend, stands out from the common plate and turns into a separate strand in the form of a solid cylinder. At the same time, the separation of the mesoderm occurs. This process begins with the appearance of small pocket-like outgrowths on two sides

inner sheet. As they grow, they separate from the endoderm and, in the form of two strands 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 successively separate 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 enterocelous method

the formation of the coelom - its lacing from the primary intestine. This method is the starting point for all deuterostomes, but almost none of the higher vertebrates, with the exception of cyclostomes, is presented with such clarity. After separation of the notochord and mesoderm

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

In the course of further development, the mesoderm is segmented: the strands are divided transversely into primary segments or somites. Three main bookmarks are formed from them:

The dermatome is formed from the outer somite facing the ectodermis, from which the connective part of the skin subsequently arises, represented mainly by fibroblasts;

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

The myotome is a part of the somite located between the dermatome and the sclerotome - it is the rudiment of the entire striated muscle.

The differentiation of somites 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 cords is segmented, while in the lancelet they completely break up into segments. The latter are soon divided into the dorsal part - somites, and the abdominal part - splanchnot.

The somites from which the trunk musculature develops 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 represented (the characteristic location of the rudiments during gastrulation, the formation of a chord 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 deuterostomes (coeloblastula, invaginated gastrula, three-segmented stage, enterocele anlage of mesoderm and coelom formation).

In the future, due to the formation of the tail, the neuro-intestinal canal disappears. In the head part of the intestinal tube, a mouth opening breaks through, and at the posterior end, under the tail, an anal opening is formed - by a secondary breakthrough of the wall of the animal's body at the site of the closed blastopore. The embryo enters the stagefree-swimming larvae.

Theme 4

Embryogenesis anamnios

1. General characteristics of anamnia and amniotes.

2. Embryogenesis anamnia.

3. Embryogenesis of the lancelet.

4. Embryogenesis of amphibians, lampreys.

5. Embryogenesis of cartilaginous and bony fishes.

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

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

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

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

5. Biological encyclopedia / 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. school, 1989. - 154 p.

Ham A., Cormick D. Histology / A. Ham, D. Cormick. - 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 anamnia and amniotes

General characteristics of anamnia

Based on the characteristics of embryonic development, all chordates are divided into two groups: anamnia and amniotes. Anamnii- these are animals in which, in the process of embryonic development, such embryonic membranes as the amnion, or water membrane, and allantois are not formed. Anamnias include chordates, leading a primary aquatic lifestyle, as well as lower chordates, closely associated with 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 aquatic membrane and allantois, since the functions of respiration, excretion and nutrition of the developing embryo are provided by the surrounding aquatic environment.

Chordates related to anamnia according to the nature of embryonic development can be divided into three groups:

1) lancelet, whose eggs contain little yolk;

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

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

Embryogenesis of the lancelet

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 egg is determined by the second polar body located above it. Cleavage of the egg is complete, uniform (Figure 1).

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

Drawing–1. Consistency (I –vi) cleavage of the ovule of the lancelet

The first two divisions are meridional, the third is equatorial. Further fragmentation goes alternately in one direction or the other, and the number of cells increases exponentially. After the formation of a single-layer embryo - the blastula, it becomes noticeable that the cells of the animal pole are smaller than the cells of the vegetative pole. In the spherical coeloblastula 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 blastula roof. The cells that form the roof of the blastula will differentiate into the cells of the outer germ layer, or ectoderm, and the cells of the bottom of the blastula will differentiate into the endoderm.

Gastrulation occurs by invagination of the vegetative pole blastoderm into the blastocoel. The invagination continues until the cells of the vegetative pole come into contact with the cells of the animal pole, due to which the blastocoel cavity narrows and disappears (Figure 2).

I - coeloblastula; 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.

Figure 2–Embryogenesis of the lancelet

With the completion of the first stage of gastrulation, a two-layer embryo, or gastrula, arises, consisting of cells of the outer germ layer - the ectoderm and the inner germ 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 chord and mesoderm. With the formation of the cavity of the primary intestine, the embryo begins to grow rapidly and lengthens, but the most intensive shaping processes are carried out in the region of the upper, or dorsal, lip of the blastopore. Just 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, endoderm cells, which represent the material of the future notochord, undergo the same changes. Subsequently, the neural plate begins to sag, 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 the nerve canal. The cells of the skin ectoderm close up, and the neural tube is under them. At the same time, endoderm cells adjacent to the neural plate bend towards the latter, twist and separate into a dense strand - a chord, which looks like a solid cylinder. On the sides of the chordal anlage, 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 endoderm. The cavity of the mesodermal sacs, arising from the gastrocoel, turns into a secondary body cavity, or coelom. Thus, in the process of gastrulation, a three-layer embryo appears.

After isolation of the notochord and lacing of the mesodermal sacs, the edges of the endoderm gradually approach each other in the dorsal part of the embryo and, closing, form a closed intestinal tube. Following gastrulation, a complex of axial organs appears in the embryo, which is characteristic of representatives of the chordate type. It consists of a chord, on the sides of which are clusters of segmented mesoderm - somites.

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

The third germ layer, or mesoderm, of the lancelet embryo is segmented throughout. The mesodermal segments are further divided into the dorsal part - somites and the abdominal part - splanchnotomes. The somites remain segmented, and the splanchnotomes on each side of the body lose their primary segmentation, merge and form, splitting into two sheets, the right and left coelomic cavities. The latter unite under the intestinal tube into a common secondary body cavity. When the tail begins to form in the lancelet, the neurointestinal canal disappears, and an anus appears at the posterior end of the embryo in place of the blastopore due to thinning and rupture of the body wall. 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 egg, fertilized outside the mother's body, undergoes complete and almost uniform crushing. The result is a typical globular blastula. Larger cells vegetativelythe blastula begin to bulge inwards at the poles of the blastula, and a typical invaginated gastrula is formed.

Then the gastrula stretches, the gastropore (blastopore) decreases, and the ectoderm along the dorsal side to the very gastrothe time begins to deepen, forming the neural plate. Subsequently, the neural plate separates from the cells of the neighboring ectoderm, and the ectoderm fuses over the neural plate and over the gastropore. Still later, the edges of the neural plate fold up and fuse, 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 neuro-intestinal canal (canalis neuroentericus). At the anterior end, the nerve folds close last of all, so that here the nerve canal communicates with the external environment for a long time through an opening, the neuroporus. In the future, an olfactory fossa is formed at the site of the neuropore.

(according to Schmalhausen). I - a 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 gill slit, 2 - the opening of the renal tubule into the peribranchial cavity

(schematically). I—blastula; II, III, IV - gastrulation; V and VI - the formation of mesoderm, chord 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 - 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 - notochord, 9 - secondary body cavity, 10 - parietal sheet 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, endoderm differentiation 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 band 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 gut, lying on the sides of the rudimentary chord, are separated from the gut and turn into a series of closed, segmentally arranged coelomic sacs. Their walls are the mesoderm, and the cavities are the secondary cavity of the body, or coelom. Subsequently, the coelomic sacs grow up and down, and each sac is subdivided into a dorsal region, located on the side of the notochord and neural tube, and an abdominal region, located on the sides of the intestine. The dorsal sections are called somites, the abdominal sections are called lateral plates. From somites, mainly muscle segments are formed - myotomes, which are worn in an adultthe name of the animal is the myomeres, and the skin itself (corium), while the sheets of the peritoneum are formed from the lateral plates, and the whole 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.