Scheme of the stages of development of the lancelet embryo. Type Chordata: structure and development of the lancelet

Ontogenesis, or individual development, is the entire period of an individual’s life from the moment the sperm merges with the egg and the formation of a zygote until the death of the organism. Ontogenesis is divided into two periods: 1) embryonic - from the formation of the zygote to birth or exit from the egg membranes; 2) postembryonic - from exit from the egg membranes or birth to 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, there are three main stages: cleavage, gastrulation and primary organogenesis.

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

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

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

The blastula, usually consisting of a large number of blastomeres (in the lancelet - 3000 cells), during development passes into a new stage called gastrula. The embryo at this stage consists of separated layers of cells, the so-called germ layers: the outer layer, or ectoderm, and the inner layer, or endoderm. The set of processes leading to the formation of a gastrula is called gastrulation. In the lancelet, gastrulation occurs by invagination of part of the blastula wall into the primary body cavity.

After completion of gastrulation, the embryo forms a complex of axial organs: neural tube, notochord, and 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 sinks under the endoderm, and its edges close. A 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 rudiment, is separated from the rest of the endoderm and folds into a dense cord - the notochord. From the remaining part of the endoderm, mesoderm and intestinal epithelium develop. Further differentiation of the embryonic cells leads to the emergence of numerous derivatives of the germ layers - organs and tissues.

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

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

The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. In this case, one rudiment influences the other, largely determining the path of its development. In addition, the rate of growth and development of the embryo is influenced by external and internal conditions.

Topic 4

Embryogenesis anamnia

1. General characteristics of anamnias and amniotes.

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 chordatesin an aquatic environment, they lack aquaticmembrane 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 gluethe current increases exponentially. 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 the 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

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 beginsin the 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 area 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. In the early stages of 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 outIt is 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. The central part of the somite differentiates into a muscular plate, or myotome, from which skeletal-type striated muscle tissue 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 the upper layer of its cells acquires 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, 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 torn off 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.

Topic 4

Embryogenesis anamnia

1. General characteristics of anamnias and amniotes.

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.

General characteristics of anamnias and amniotes

General characteristics of anamnesia

Based on the characteristics of embryonic development, all chordates are divided into two groups: anamnia and amniotes. Anamnesia- these are animals in which, during embryonic development, embryonic membranes such as the amnion, or aqueous membrane, and allantois are not formed. Anamnias include chordates that lead a primary aquatic lifestyle, as well as lower chordates that are closely associated with the aquatic environment during the period of reproduction and embryonic development of embryos - jawless fish and amphibians. Due to the embryonic development of these chordates in an 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 aquatic environment surrounding it.

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

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

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

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

Embryogenesis of lancelet

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

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

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

The first two fragmentations occur meridionally, the third - equatorially. Further fragmentation occurs 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 bottom of the blastula, and the opposite part, corresponding to the animal pole, is called roof of the blastula. The cells forming the roof of the blastula will differentiate into cells of the outer germ layer, or ectoderm, and the cells at the bottom of the blastula will differentiate into endoderm.

Gastrulation occurs by invagination of the blastoderm of the vegetative pole into the blastocoel. Invagination continues until the cells of the vegetative pole come into contact with the cells of the animal pole, and therefore 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 – overall.

Figure 2–Embryogenesis of lancelet

With the completion of the first stage of gastrulation, a two-layer embryo, or gastrula, appears, consisting of cells of the outer germ layer - ectoderm and the 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. The cellular composition of the endoderm is heterogeneous, since it also includes the cellular material of the future notochord and mesoderm. With the formation of the cavity of the primary intestine, the embryo begins to grow rapidly and lengthen, but the most intense formation processes take place in the area 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 bend, forming a neural groove, and the cells of the skin ectoderm intensively creep onto it. Subsequently, the neural groove deepens, its edges close, and it turns into a neural tube, the cavity of which is called the neural canal. The cells of the skin ectoderm close together, and the neural tube appears under them. At the same time, the endoderm cells adjacent to the neural plate bend towards the latter, twist and separate into a dense cord - a notochord, which looks like a solid cylinder. On the sides of the notochordal primordium, the endoderm invaginates towards the ectoderm, forming mesodermal protrusions, or mesodermal sacs, which subsequently become detached from the endoderm and begin to grow between the ectoderm and endoderm. The cavity of the mesodermal sacs arising from the gastrocoel turns into the secondary body cavity, or coelom. Thus, during the process of gastrulation, a three-layer embryo appears.

After the separation of the notochord and the 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, the embryo develops a complex of axial organs characteristic of representatives of the chordate phylum. 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. Subsequently, on the posterior part of the body of the embryo, the ectoderm grows onto the blastopore and closes it so that the cavity of the neural tube communicates with the intestinal cavity through the neurointestinal canal, which quickly becomes overgrown. The oral opening in the lancelet embryo is formed secondarily at the anterior end of the body due to thinning and breakthrough of the ectoderm.

The third germ layer, or mesoderm, of the lancelet embryo is segmented throughout. The mesodermal segments are further divided into a dorsal part - somites and an 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 leaves, the right and left coelomic cavities. The latter unite under the intestinal tube into a common secondary body cavity. When the lancelet's tail begins to form, the neurointestinal canal disappears, and at the posterior end of the embryo, in place of the blastopore, due to thinning and breakthrough of the body wall, an anal opening appears. Having gone through 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 development of the lancelet and its systematic position have long remained a mystery. 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 a common structure plan.

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. The lancelet (the photo below illustrates its 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.

Vertebrates evolved from skullless animals. The modern representative of the skullless subtype is the lancelet (Fig. 34). In the development of the lancelet we see the simplest scheme of embryonic development of chordates, which became significantly more complex during the process of evolution in vertebrates and especially in humans.

The lancelet is a marine animal. The female and male release reproductive cells (eggs and sperm) directly into the water, where fertilization and development of the embryo occurs.

Following fertilization, the zygote enters a period of cleavage (Fig. 35); the number of blastomeres increases rapidly (2, 4, 8, 16, etc.).

During the process of division, blastomeres gradually move away from the center of the embryo to the periphery, forming an ever-increasing cavity in the center.

In this regard, by the end of the crushing period, a typical blastula appears, the wall of which is formed by one layer of cells (blastoderm), and its cavity (blastocoel) is filled with fluid. The next period (gastrulation) is associated with invagination, i.e., invagination of one (vegetative) half of the blastula into the other (animal) * (Fig. 35). As a result, a gastrula ** is formed, which has an internal germ layer (primary endoderm), a gastrocoel (primary gut cavity) and a blastopore (primary mouth).

* (In a lancelet egg, one half contains more yolk than the other. As a result of studying the development of embryos, it was established that the part of the egg, supplied with a large amount of yolk, when crushed, forms that half of the blastoderm, which invaginates during the gastrulation period and forms the internal germ layer - the endoderm. It is known that the digestive and other systems of the so-called plant (vegetative) life are formed from the endoderm. Therefore, both the part of the egg containing a larger amount of yolk and the part of the blastula formed from it as a result of crushing are called vegetative parts. From the opposite part of the egg and the corresponding part of the blastula, the ectoderm and then the organs of the animal part develop, in particular the nervous system, etc. Therefore, these parts of the egg and blastula are called animal.)

** (Greek, gaster - stomach. Hence the name “gastrula” to emphasize that the embryo at this stage is equipped with the rudiment of the digestive system in the form of the primary intestine.)

During the period of invagination, the blastocoel (primary body cavity) remains for some time in the form of a narrow gap between the outer and inner germ layers, and then disappears. After formation, the gastrula begins to increase in length; at the same time, concentric closure of the edges of the blastopore (primary mouth) * occurs.

* (The blastopore (primary mouth), connecting the primary gut with the external environment, in some animals at subsequent stages of development remains as an oral opening (protostomes), while in others it becomes an anal opening (deuterostomes). In the latter case, the oral opening is formed at the opposite end. Protostomes include worms, mollusks and arthropods, deuterostomes include echinoderms and chordates, in particular the lancelet and vertebrates, including humans.)

The end of gastrulation coincides with the beginning of the period of separation of the main rudiments of organs and tissues (Fig. 36). At this time, the thickened dorsal portion of the primary ectoderm turns into the neural plate, from which the neural tube arises, passing through the neural groove stage (Fig. 36, 37). The neural tube is the rudiment of the nervous system.

The other part of the outer germ layer, during further development, closes over the neural tube and is the rudiment of the skin epithelium (epidermis).

The inner germ layer undergoes a number of changes in cellular composition, then the following formations separate from it: in the area of ​​the middle part of its roof - a notochordal plate, from which the notochord rudiment is formed; in the area of ​​the lateral parts of the roof of the primary intestine there are pocket-shaped protrusions, which are then separated from the wall of the primary intestine. The cellular material of the detached pocket-like protrusions of the primary intestine fills the primary body cavity (located between the ectoderm and endoderm) and represents the rudiment of the middle germ layer (mesoderm). In the center of the isolated area there is a space detached from the cavity of the primary intestine, which is the secondary body cavity. The remaining part of the primary endoderm (bottom of the primary intestine) forms the intestinal tube, which is the rudiment of the secondary (definitive) intestine and from which the intestinal epithelium subsequently arises.

Thus, by the end of gastrulation, during the period of separation of the rudiments of organs and tissues, the neural tube turns out to be located on the dorsal side of the embryo in the middle position, and under it the notochord and the intestinal tube are successively located. Bilateral symmetry is finally revealed. Mesodermal pockets appear. The rudiments of mesoderm with cavities inside them grow on the right and left into the gap between the skin ectoderm and the intestinal tube and connect under the latter. At the same time, the mesoderm, extended along the body between the cutaneous ectoderm, on the one hand, and the notochord and intestinal tube, on the other hand, is divided into a number of separate sections (segments), located along the length of the body near each other. The left and right mesodermal pockets are subject to segmentation. At the same time, each mesodermal sac is divided throughout its entire length into a dorsal section (somite) and a ventral section (splanchnotome). The somites lose their cavity, become dense and serve as the starting material for the trunk muscles. Splanchnotomes retain a cavity. They remain separated from each other for some time (as a result of segmentation), and then the isolated cavities contained in separate splanchnotomes merge, so that a coelomic cavity (secondary body cavity, coelom) is formed for all segments of the body. The material of the walls of splanchnotomes is the original material of the epithelial lining of the secondary body cavity *.

* (The secondary body cavity (coelom) arises in the thickness of the middle germ layer. Various cavities are formed from it during embryonic development. In humans, these are, in particular, the peritoneal and pleural cavities and the pericardial cavity. The primary body cavity (blastocoel) disappears during gastrulation and the formation of mesoderm.)

The development of the lancelet larva ends with the appearance of the oral and anal openings, gill slits, the formation of organs, etc.