Initial stages of embryonic development of the lancelet. Development of the lancelet. Reproduction and development

The development of the lancelet was first studied by A. O. Kovalevsky. This question is of great interest, since analysis of the stages of development of the most primitive of modern chordate animals provides some basis for judging the early stages of chordate phylogeny. In addition, lancelet development provides a simplified schematic picture of embryonic development in other chordates. Figure 4 and Figure 5 depict the successive stages of embryonic development of the lancelet up to the formation of the larva. The fragmentation of the fertilized egg is complete and almost uniform: when a blastula is formed, it is clear that on its lower side, corresponding to the vegetative ("plant") part of the egg, the cells are larger than on the upper. Because of this, the inner layer of the next gastrula stage is represented by larger cells. Crushing occurs very quickly. In the ectoderm of the upper side of the embryo, a medullary plate is separated, the edges of which curl up and then close. The neural tube arising in this way maintains communication with the external environment at the anterior end (through the neuropore), and at the posterior end (through the neurointestinal canal) with the cavity of the gastrula, i.e. with the primary gut. Subsequently, the neurointestinal canal disappears completely, and the olfactory fossa remains in place of the neuropore.

From the lateral plate the peritoneum, mesenteries (in which the main blood vessels arise in the form of longitudinal canals), and intestinal muscles develop. Nephridial tubules develop as finger-like protrusions of the walls of the secondary body cavity. The gonads develop as protrusions of that part of the walls of the body cavity that corresponds to the place of separation of the somite and the lateral plate of the gonotome. The mouth is formed by protrusion of the primary intestine at the end opposite the gastropore (primary mouth) and counter-invagination of the ectoderm. A breakthrough occurs at the meeting point of these formations. The formation of the mouth and gill slits occurs asymmetrically. The oral opening is formed on the lower left side of the embryo. The left gill slits (there are 14 of them) initially appear on the ventral side and then move to the right side of the embryo. Then another row of slits appears here (there are 8 of them), located above the previously mentioned 14 slits. Subsequently, the lower row of slits shifts to the abdominal side and only after that to the left side of the body. Their number is reduced from 14 to 8. The number of gill slits on both sides then increases sharply. Subsequently, the mouth moves to the ventral side. The atrial cavity appears initially in the form of a groove on the lower surface of the body. The metapleural folds that form this groove grow towards each other and, closing, form a cavity that opens outward only in its posterior part, where the mentioned folds do not grow together. In general, the larval development of the lancelet lasts about three months.

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 midline 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 excretory system and the sexual primordium.

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.

Topic 4

Embryogenesis anamnia

1.general characteristics anamnia and amniote.

2. Embryogenesis of anamnia.

3. Embryogenesis of the lancelet.

4. Embryogenesis of amphibians, lampreys.

5.Embryogenesis of cartilaginous and bony fish.

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

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

3. Histology / ed. Yu.I. Afanasyeva. – 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 encyclopedic dictionary / ed. M.S. Gilyarov. – M.: Sov. Encycl., 1989. – 864 p.

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

Ham A., Cormick D. Histology / A. Ham, D. Cormick. – M.: Mir, 1983. – 192

1. Features of embryonic development of mammals.

2. Embryogenesis of oviparous mammals.

3. Embryogenesis of marsupial mammals.

4. Embryogenesis of placental mammals.

5. Human embryogenesis.

1. General characteristics of anamnias and amniotes

1. General characteristics of anamnesia

Based on the featuresembryonic development, all chordates subdividingThey are divided into two groups: anamnias and amniotes. Anamnesia – these are animals in which, during embryonic development, embryonic membranes such as the amnion or aqueous membrane are not formedka, and allantois. The anamnias include chordates leading feathers vicious lifestyle, as well as lower chordates, closely associated with the aquatic environment during the breeding and embryonic periodsnational development of embryos–jawless, fish and terrestrialaquatic Due to the embryonic development of these chordatesV aquatic environment, they lack watermembrane and allantois, since the functions of respirationThe formation, secretion and nutrition of the developing embryo is provided by the surrounding aquatic environment.

Chordates belonging to anamnia by character terus of embryonic developmentcan be divided into three groups:

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

2) some cyclostomes, fish (cartilaginous hanoids) and amphibians, the eggs of which contain a mediumamount of yolk;

3) selyachia and bony fishes,eggs contain a lot of yolk.

2. Embryogenesis of lancelet

After fertilization inredistribution of the yolk begins in the lancelet egg,which concentrates mainly on one side of the eggcells corresponding to the vegetative pole. Animalthe pole of the egg is determined by the one located above itsecond polar body. The fragmentation of the egg is complete and uniform (Figure 1).

/ – animal pole; 2 – vegetative pole; 3 – accumulation of yolk; 4 – whole stool; 5 – blastoderm cells.

Drawing– 1. Consistency ( I –VI)lancelet egg fragmentation

The first two crushings occur meridionally, third - equatorial. Further crushing proceeds acrossexactly in one direction, then in the other, and the amount of gluecurrent increases in geometric progression. After the imageformation of a single-layer embryo– blastula, it becomes noticeable that the cells of the animal pole are smaller than the vege cellsative pole. In the spherical coeloblastula of the lanceletdistinguish the flattened part of the vegetative pole, calledI think bottom of the blastula, and the opposite part correspondscorresponding to the animal pole is called roof of the blastula. Cellki forming the roof of the blastula will differentiate intocells of the outer germ layer, or ectoderm, and cells of the bottom of the blastula- into the endoderm.

Gastrulation occurs by invagination of the blastodereswe vegetative pole inside the blastocoel. Invagination aboutlasts until the cells of the vegetative poletouch the cells of the animal pole, and thereforethe 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– top and 7 – lower lip of blastopore; 8– blastopore; 9 – cavity of the primary intestine; 10 – cavity of the secondary intestine; eleven– in general.

Figure 2–Embryogenesis of lancelet

Completed After the first stage of gastrulation, a two-layer membrane appearsembryo, or gastrula, consisting of cells of the outer germ layer– ectoderm and inner germ layer– 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 a blastopore. Cellularthe composition of the endoderm is heterogeneous, since it also includescellular material of the future notochord and mesoderm. With the formation of the cavity of the primary intestine, the embryo begins to grow rapidlyand lengthens, but the most intense formativeprocesses are carried out in the area of ​​the upper, or dorsal,blastopore lips. Directly behind the upper lip of the blastopopa, on the dorsal surface of the embryo, the ectoderm thickens and consists of tall prismatic cells called medulespolar or neural plate. Ectoderm surrounding the nervelamina, is represented by small cells that formcut the skin. Under the neural plate the same changesendoderm cells, which representmaterial of the future chord. Subsequently, the neural platebegins to sag, forming a neural groove, and the cellsthe skin ectoderm intensively creeps onto it. SubsequentlyAs time passes, the neural groove deepens, its edges close, and itturns into a neural tube, the cavity of which is callednerve canal. The cells of the skin ectoderm close together andthe neural tube appears underneath them. At the same time the cellsendoderms adjacent to the neural plate sag intoside of the latter, curl and separate into a dense cord - chord, which looks like a solid cylinder. One hundred eachFrom the notochordal primordium, the endoderm invaginates to the sidewell, ectoderm, forming mesodermal protrusions, or mesodermal bags, which are subsequently laced fromendoderm and begin to grow between the ectoderm and endoderm. The cavity of mesodermal sacs arising from the gastrocoel turns into the secondary body cavity, or coelom.Thus, during the process of gastrulation, a three-layered embryo

After detachment of the notochord and unlacing of the mesodermalof these sacs, the edges of the endoderm gradually come closer together in the dorsalparts of the embryo and, closing, form a closed intestinalhandset. Following gastrulation, the embryo develops a complexaxial organs, characteristic of representatives of the chordo typeexit It consists of a notochord, on the sides of which there are clusters of segmented mesoderm– somites.

The formation of axial organs occurs at the neurula stage.The neural tube of the lancelet in the anterior and posterior parts of the embryo remains open for some time. In the future onof the posterior part of the body of the embryo, the ectoderm grows onto the blastoporeand closes it so that the cavity of the neural tube communicateswith the intestinal cavity, the neurointestinal canal, which quicklythe rho is overgrown. The mouth opening of the lancelet embryo is appears secondarily at the anterior end of the body due to thinningand breakthrough of the ectoderm.

The third germ layer, or mesoderm, of the lancelet embryo is segmented throughout. Mesodermalthe segments are further divided into the dorsal part– with mita and ventral part– splanchnotomes. Somites remain todaymented, and splanchnotomes on each side of the body of the morning undergo primary segmentation, merge and form,splitting into two leaves, right and left coelomic layerssti. The latter unite under the intestinal tube into a commonsecondary body cavity. When does the lancelet begin to form?the tail breaks, then the neurointestinal canal disappears, and on the backat the end of the embryo at the site of the blastopore due to thinningand a breakthrough in the body wall, an anus appears. Having gone through the described stages of development, the lancelet becomes free floating larva. During the period of larval development Organogenesis and histogenesis occur and the larva turns into adult animal.

3. Embryogenesis of lampreys, cartilaginous ganoids and amphibians

These groups of animals have commonfeatures of cleavage, gastrulation and neurulation. The fragmentation of the egg is complete and uneven (Figure 3).

1 – blastomeres; 2 – amphiblastula; 3 – blastocoel; 4 – blastoderm; 5 - roof of blastula; 6– bottom of the blastula.

Figure 3– Subsequence ( I– VI ) crushing the lamprey egg

The first two furrowscrushing occurs meridianally, starting from anithe small pole, and the third groove runs close, but equatorially. Blastomeres of the animal polesmaller than the blastomeres of the vegetative pole. Droblereduction of animal and vegetative blastomeres to the seventh fractionlenition takes place almost synchronously, then bothhalves of the embryo begin to split asynchronously. Exceptindicated crushing furrows, tangential bores appeardischarge, therefore the wall of the resulting blastula consistsfrom several rows of cells.

As a result of uneven fractionsleniya blastomeres of the vegetative pole containing manyyolk, form the blast wall ly – blastoderm. Blastocoel raceslies closer to the animal pole. The resulting blastula is called amphiblastula.

The amphiblastula has wingsshu, corresponding to the animal pole, consists of 1 – 3 rows of cells, bottom corresponding to the vegetative fieldsou, totals 11– 13 rows of cells, and an equatorialzone containing 3– 5 rows of cells.

Gastrulation occurs through intussusception and epiboly. Invagination of the blastoderm beginsV equatorial zone, Nothow much below the bottom of the blastocoel. Invagination occurs after the appearance of a small crescent-shaped depression, orfalciform groove, which is convexly directed to one hundredthe ron of the animal pole. The crescentic groove formsdorsal lip of the blastopore. Animal blastoderm cellspoles, i.e. the future ectoderm, multiply intensively andbegin to creep onto the cells of the vegetativelyus, overgrowing them from the surface, with the exception of cellsblastoderm in the region of the falciform groove and below the placentaher. Intensive proliferation of blastoderm cells in the areathe animal pole also ensures the movement of cellsmaterial from the surface into the embryo in the processintussusception. Through the dorsal lip of the blastoporecellular material is invaginated firsttoderm and prechordal plate, i.e. the material thatry is located in front of the cellular material of the notochordalrudiment. Next, it invaginates the material of the notochord and on the sides of the entodermis. The bottom of the falciform groove in the form of a double fold is invaginated into the blastocoel in the direction of the animal poleparallel to the blastoderm. Cavity of the primary intestine, boundariesmade by endoderm cells, enlarges and sharply narrowsblastocoel. Blastocoel is a thin cell septum insidethe third germ layer is first separated from the gastrocoel,then the endoderm cells diverge, and both cavities are connectedmerge into a single cavity of the primary intestine.

As cellular material invaginates into the blastocoelthe crescent-shaped fissure enlarges and becomes horseshoe-shapednew form, i.e., the lateral lips of the blastopore are formed. Thenthe blastopore becomes ring-shaped– a vein appears tragal, or ventral, lip of the blastopore. Ring shapeblastopore is due to the fact that in its central part there are raceslarge, yolk-rich vegetative blastomeres relythe opposite pole of the blastula, which, due to its size, cannotinvaginate into the blastocoel. Therefore, invagination of materialis carried out only along their periphery, and the blastopore has the formnarrow annular fissure. By the time the ventrallips of the blastopore, almost the entire endoderm invaginates and only a small part of it is on the surface in the centerblastopore. Blastomeres located in the central partblastopore, very rich in yolk, which is why they got the name yolk plug(Figure 4).

I – amphiblastula; II – III – gastrulation; IV – neurula;

1 – ectoderm; 2 – endoderm; 3 – chord; 4 – mesoderm; 5 - neural plate;

6 – upper and 7 – lower lip blastopore; 8 – blastopore; 9 – gastrocoel; 10 – neural tube; eleven – nerve canal; 12 – segmented mesoderm; 13 – unsegmented mesoderm; 14 – vitelline endoderm (yolk plug)

Figure 4– Embryogenesis of amphibians

Segmented mesoderm material– the somites invaginate through the lateral lips, and the cellular material of the unsegmented mesoderm– splanchnotomes – through the lower lip.Due to the invagination of a large amount of cellular material, cage ectoderm cells change their original position. Clethe exact material of the future neural plate is stretchedover the entire animal surface of the embryo, and the animal alonglus appears at the anterior end of the embryo, opposite blasaxe. On early stages intussusception cellular materialthe future notochord is separated from the endoderm and the notochordal plate immediately folds into a longitudinal cord– chord, which paradise is torn off from the primary intestine, and the latter is on the upperside remains open for some time. Availablethe edges of the intestinal endoderm quickly restore the defect, oncemelting under the notochord, and the wall of the primary intestine becomes solid.

From the very beginning of invagination, the segmented cellzoderms are not part of the cellular material of the primaryintestines, but invaginate through the blastopore independently, locatedlying between the ectoderm and the wall of the primary intestine. Segmented mesoderm forms clusters on the sides of the notochord cells – somites. Unsegmented mesoderm is also includedlies between the ectoderm and the wall of the primary intestine, formszuya splanchnotomes, which lack segmentation. Connection betweensegmented and non-segmented mesoderm carrying outis performed using segmental legs, or nephrotomes. Nesegmen tified mesoderm on both sides grows under the endodermprimary intestine, then unites to form a common wholemic cavity. After this, the embryo becomes three layered.

The formation of axial organs in lampreys, cartilaginous ganoids and amphibians begins already at the end of the gastrulation process with the separation of notochord material. Simultaneously with the emergencechord ectoderm forms a neural plate, along the edges of whichswarm, thickenings appear in the form of neural ridges. Restpart of the ectoderm is the cutaneous ectoderm. Thenthe nerve forms the neural groove, andthe neural folds rise, come closer together and when formedneural tubes merge into a single unpaired ganglion tuberecord. Neural tube and ganglion plate submergedgrow inside the embryo, and a cutaneous ecto grows on top of them dermis.

When somites separate, the third pair of somites appears first, then the segmentation process spreads from front to back, and the first two pairs of somites appear later. central part The somite differentiates into a muscular plate, or myotome, from which striated muscle tissue of the skeletal type subsequently develops. The part of the somite adjacent to the notochord and neural tube differentiates into a skeletal layer, or sclerotome, from which the axial skeleton and the skeleton of the limbs develop. The upper lateral part of the somite, which is adjacent to the ectoderm, turns into a skin plate, or dermatome, which forms the basis of the skin.

Nephrotomes participate in the formation of kidney tubules, and splanchnotomes, splitting into two leaves– parietal and visceral, form bilateral coelomic cavities, which then merge into a common secondary body cavity. The visceral layer of the splanchnotome takes part in the formation of the intestinal wall and heart; it also forms the visceral layer of the peritoneum, pleura, cardiac sac, and the parietal layer– parietal leaf of the serous membranes of the indicated body cavities,

4. Embryogenesis of cartilaginous and bony fish

The fragmentation of the egg is partial, uneven, or discoidal. The crushing process covers only a small part of the animal pole and leads to the formation of a discoblastula. The discoblastula blastoderm in these animals is called blastodiscom or germinal disc, and the bottom of the blastula is formed by a superficial layer of uncrushed yolk– periblast. Blastodisc cells, multiplying, form a multilayer blastodisc, which turns from round to oval, and upper layer its cells acquire an epithelial-like shape (Figure 1.5).

1 – blastomeres; 2 – periblast; 3 – merocytes; 4 – yolk; 5 – blastocoel

Figure 1.5– Subsequence ( I– V ) fragmentation of the stingray embryo

The formation of a two-layer embryo occurs through intussusception. Gastrulation begins with the movement of cells to the posterior edge of the blastodisc, which thickens and begins to fold over its own edge, forming endoderm and ectoderm. The edge of the blastodisc through which the cellular material is tucked in, or intussusception, is called edge notch. The latter is the blastopore. The middle part of the marginal notch corresponds to the upper, or dorsal, lip, and its lateral parts– lateral lips of the blastopore. The invagination cavity, located between the endoderm and the uncrushed yolk, corresponds to the cavity of the primary intestine. The endoderm in its middle part contains the cellular material of the notochordal plate, and on the sides– mesoderm material, initially segmented, and unsegmented at the edges of the marginal notch. Thus, the mesoderm arises by invagination, to which immigration is added.

During the process of invagination, only that part of the endoderm is formed, which subsequently forms the intestinal tube, more precisely, its epithelial lining. The remaining endoderm, which then grows over the yolk, arises from the deep layers of blastodisc cells by delamination of the outer layer of blastodisc cells or from the periblast. It is called the vitelline endoderm. In many fish, one of the listed methods for the formation of endoderm or a combination of them occurs. Subsequently, the intestinal endoderm unites with the vitelline endoderm into a single internal germ layer. This completes gastrulation (Figure 1.6).

I – discoblastula; II – the beginning of blastoderm invagination; III – blastodisc; IV – gastrula; V – formation of mesoderm;

1 – outer layer of blastodisc cells; 2– cellular material of the future vitelline endoderm; 3 – periblast; 4 – merocytes; 5 – yolk; 6 – blastocoel; 7 – edge notch; 8 – gastrocoel; 9 – cellular material of the notochord; 10 – mesoderm; eleven – intestinal endoderm; 12 – ectoderm; 13 – cellular material of the neural plate

Figure 1.6. Embryogenesis of cartilaginous fish

The formation of axial organs occurs in approximately the same way as in amphibians, however, unlike the latter, in fish the formation of the intestinal tube occurs differently due to the presence of large reserves of yolk in the egg. During development, the fish embryo remains spread out on the uncrushed yolk for a long time. At first, the embryo does not have an abdominal wall. The closure of endoderm cells into a tube occurs when all three germ layers of the reserve yolk become overgrown and a yolk sac is formed. Intensively multiplying, the cells of the three germ layers from the body of the embryo begin to spread to the periphery and move towards the yolk. This process is called the yolk fouling process. It is most intense in the front and sides of the embryo. In the posterior part of the embryo, where the material was folded in during gastrulation, the fouling of the yolk proceeds more slowly due to the intensive growth of the tail part of the embryo. Next, the lateral lips of the blastopore come together and grow together, thereby forming the abdominal wall of the embryo's body, the tail part of the embryo is detached from the yolk, and the embryo itself moves to the center of the embryonic disc. After the tail part of the embryo separates from the yolk, fouling of the yolk also begins from the posterior part of the blastodisc, or germinal disc.

Between the head and torso of the embryo, on the one hand, and the extraembryonic ectoderm, mesoderm and endoderm– on the other hand, there is a narrowing– interception, called trunk fold. Thanks to the trunk fold, the head end of the embryo also detaches from the yolk. Lastly, the body of the embryo separates from the yolk. The trunk fold promotes the folding of the endoderm into a tube and the formation of the abdominal wall of the embryo. However, the process of folding the endoderm into a tube does not cover the entire intestine and in the middle part of the body the intestinal tube remains open. At this point in the intestinal cavity there is a duct called yolk stalk, communicates with the cavity of the yolk sac,

With the formation of the vitelline stalk, the endoderm is clearly divided into intestinal endoderm and vitelline, or extraembryonic, endoderm. Extraembryonic ectoderm, mesoderm and endoderm, completely overgrown with the yolk, form yolk sac, which is temporary, or provisional authority embryo (Figure 1.7).

I – fish embryo with yolk sac: 1 – fish body; 2 – yolk sac, 3– yolk;

II – yolk sac wall: 1– extraembryonic ectoderm; 2 – out germinal mesoderm; 3 – extraembryonic (yolk) endoderm; 4– yolk grains; 5 - nuclei of vitelline endoderm cells; 6 – blood vessels extraembryonic mesoderm; 7 – epithelial covers and 8 – goblet cells extraembryonic ectoderm.

Figure 1.7– Structure of the yolk sac of bony fishes

The endoderm of the yolk sac ferments the yolk and absorbs nutrients. The mesoderm of the yolk sac, thanks to a well-developed system of blood vessels, transports nutrients to the body of the embryo, and the ectoderm covering it performs protective functions. In addition to its trophic function, the yolk sac performs respiratory and hematopoietic functions. At the end of embryonic development, when yolk reserves are depleted, the yolk sac either falls off or becomes part of the intestinal wall and abdominal wall of the body.

Development of the lancelet and its systematic position remained a mystery for a long time. Now scientists know for sure that this representative of the Chordata type has indirect development.

General characteristics of the type Chordata

Fish, amphibians, reptiles, birds, mammals - all these animals are representatives of what unites such different organisms? It turns out that they all have overall plan buildings.

At the base of their body is what is called the notochord. In the lancelet it persists throughout its life. The neural tube is located above the notochord. During metamorphosis, in most representatives of the type, the spinal cord and brain are formed from it. Under the axial skeleton there is a tube-shaped intestine. The pharynx of chordates contains gill slits. In species that live in water, this trait is preserved, but in terrestrial species it is characteristic only of embryonic development.

History of the discovery of the lancelet

Why has the development of the lancelet caused a lot of controversy and questions for a long time? The fact is that for a long time it was considered a mollusk. Lancelet (photo below illustrates it external structure) really resembles these animals. It has a soft translucent body and lives in an aquatic environment - in the shallow waters of seas and oceans. But the peculiarities of the internal organization made it possible to distinguish them into a separate systematic unit.

In addition, thanks to the works of Peter Pallas, it was established that these animals are the ancestors of modern vertebrates. Scientists call these organisms living fossils. It is believed that the lancelet has not evolved because it has perfectly adapted to its habitat and lifestyle in the complete absence of competitors.

Features of the external structure

Due to the shape of the body, this animal has an unusual name - lancelet. The photo shows that this organism resembles an ancient surgical instrument, which is sharpened on both sides. It's called a lancet. This similarity perfectly illustrates the features of the external structure.

The body of the lancelet reaches a maximum length of 8 cm. It is compressed on the sides and pointed at the ends. On one side, a longitudinal fold of the body forms fins - dorsal and caudal. The rear end of the body of the lancelet is buried in the sand. At the front there is a preoral funnel, surrounded by tentacles.

Skeleton and musculature

The development of the lancelet is characterized by the preservation of the notochord throughout its life. In the form of a cord, it stretches along the entire body from the anterior to the posterior end. On both sides of the chord there are a number of muscles. This structure of the musculoskeletal system allows the lancelet to move uniformly. Muscle contractions lead to bending of the body, and with the help of the chord, it straightens.

Internal structure

The organs of the lancelet form all physiological systems. The digestive system is represented by the mouth, pharynx and a through tubular intestine with a hepatic outgrowth, which performs the function of a gland. By type of nutrition, lancelets are heterotrophic filter feeders. This process is closely related to respiration, which occurs through the gills and the entire surface of the body.

The excretory organs also open into the peribranchial cavity. They are represented by numerous paired tubes - nephridia. open It consists of abdominal and dorsal vessels.

The reproductive organs of the lancelet are called gonads. These are paired glands, the number of which can reach up to 25. Lancelets are dioecious animals. Therefore, they develop ovaries or testes. These animals do not have reproductive ducts. Therefore, cells enter the peribranchial cavity when the gonads or body walls rupture.

Reproduction and development

The reproductive organs of lancelets ensure their external fertilization. The gametes are released into the water, where they merge. Females spawn after sunset in all seasons except winter. Their reproductive cells contain very little yolk and are characterized by small sizes - about 100 microns.

Even before the start of crushing, the contents of lancelet eggs are differentiated into three ectoderm, meso- and endoderm. During subsequent divisions, each of them forms corresponding organ systems.

The development of the lancelet gives an idea of ​​the features of this process in chordates. It consists of a number of sequential processes: fertilization, crushing, gastro- and neurulation, organogenesis. The reproduction of lancelets, as well as their further development, is closely related to water. A larva develops from a fertilized egg after 4-5 days. It has a size of up to 5 mm and floats freely in the water column thanks to its numerous cilia. The larval stage lasts about 3 months. At night it rises to the surface of the water, and during the day it sinks to the bottom.

Amphioxides are the name given to giant lancelet larvae, which are a phenomenon of the animal world. At first they were mistaken for adults. But in the course of numerous studies it was found that they live only on the surface of the water as part of plankton. Amphioxides, which can reach 11 mm, retain all the features of the larval structure. Their body is covered with cilia; their oral tentacles, peribranchial cavity and gonads are practically undeveloped.

So, lancelets are primitive marine chordates. They belong to the subphylum Cephalochordates, class Cephalochordates. Lancelets are characterized by a sedentary lifestyle, being dioecious animals with external fertilization and an indirect type of development.

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. More later edge The neural plate folds upward and fuses, so that the plate becomes the 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 connects with the central cavity 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.