A zygote is formed, capable of further development. The division of a zygote is called cleavage. Splitting up- This is the repeated division of the zygote after fertilization, as a result of which a multicellular embryo is formed.

The zygote divides very quickly, the cells decrease in size and do not have time to grow. Therefore, the embryo does not increase in volume. The resulting cells are called blastomeres, and the constrictions separating them from each other are called cleavage furrows.

The following crushing furrows are distinguished in direction: meridional - these are furrows that divide the zygote from the animal to the vegetative pole; the equatorial furrow divides the zygote along the equator; latitudinal furrows run parallel to the equatorial furrow; tangential grooves run parallel to the surface of the zygote.

The equatorial furrow is always one, but there can be many meridional, latitudinal and tangential furrows. The direction of the crushing furrows is always determined by the position of the division spindle.
Crushing always takes place according to certain rules:

The first rule reflects the location of the cleavage spindle in the blastomere, namely:
- the cleavage spindle is located in the direction of the greatest extent of the cytoplasm, free from inclusions.

The second rule reflects the direction of the crushing furrows:
- crushing furrows always run perpendicular to the fission spindle.

The third rule reflects the speed of crushing furrows:
- the speed of passage of the cleavage furrows is inversely proportional to the amount of yolk in the egg, i.e. in that part of the cell where there is little yolk, the furrows will pass at a higher speed, and in the part where there is more yolk, the speed of passage of the cleavage furrows slows down.

Cleavage depends on the amount and location of the yolk in the egg. With a small amount of yolk, the entire zygote is crushed, with a significant amount, only a part of the zygote free of yolk is crushed. In this regard, the eggs are divided into holoblastic (completely crushed) and meroblastic (with partial crushing). Consequently, crushing depends on the amount of yolk and, taking into account a number of features, is subdivided: according to the completeness of the coverage of the zygote material by the process, into complete and incomplete; according to the ratio of the sizes of the formed blastomeres to uniform and uneven, and according to the consistency of blastomere divisions - synchronous and asynchronous.

Complete crushing can be uniform and uneven. Complete uniform is characteristic of eggs with a small amount of yolk and its more or less uniform arrangement in. This type of crushed egg. In this case, the first furrow runs from the animal to the vegetative pole, two blastomeres are formed; the second furrow is also meridional, but runs perpendicular to the first, four blastomeres are formed. The third is equatorial, eight blastomeres are formed. After this, there is an alternation of meridional and latitudinal crushing furrows. The number of blastomeres after each division increases by a factor of two (2; 4; 16; 32, etc.). As a result of such crushing, a spherical embryo is formed, which is called blastula. The cells that form the wall of the blastula are called the blastoderm, and the cavity inside is called the blastocoel. The animal part of the blastula is called the roof, and the vegetative part is called the bottom of the blastula.

Complete uneven crushing is typical for eggs with an average content of yolk located in the vegetative part. Such eggs are characteristic of cyclostomes and. Wherein type of crushing blastomeres of unequal sizes are formed. In the animal pole, small blastomeres are formed, which are called micromeres, and in the vegetative pole - large ones - macromeres. The first two furrows, like those of the lancelet, run meridionally; the third furrow corresponds to the equatorial furrow, but is shifted from the equator to the animal pole. Since the cytoplasm free from yolk is located in the animal pole, fragmentation occurs faster here and small blastomeres are formed. The vegetative pole contains the bulk of the yolk, so the cleavage furrows pass more slowly and large blastomeres are formed.

Incomplete cleavage is characteristic of telolecithal and centrolecithal oocytes. Only the part of the egg that is free of yolk takes part in crushing. Incomplete crushing is divided into discoidal (bony, reptiles, birds) and superficial (arthropods).

Telolecithal oocytes are divided by incomplete discoidal cleavage, in which a large amount of yolk is concentrated in the vegetative part. In these eggs, the yolk-free part of the cytoplasm in the form of a germinal disc is spread out on the yolk at the animal pole. Cleavage occurs only in the region of the germinal disc. The vegetative part of the egg, filled with yolk, does not take part in crushing. The thickness of the germinal disc is negligible, so the cleavage spindles in the first four divisions are horizontal, and the cleavage furrows run vertically. One row of cells is formed. After several divisions, the cells become high and the cleavage spindles are located in them in a vertical direction, and the cleavage furrows run parallel to the surface of the egg. As a result, the germinal disc turns into a plate consisting of several rows of cells. Between the germinal disc and the yolk, a small cavity appears in the form of a gap, which is similar to the blastocoel.

Incomplete superficial crushing is observed in centrolecithal eggs with a large amount of yolk in its middle. The cytoplasm in such eggs is located along the periphery and a small part of it is in the center near the nucleus. The rest of the cell is filled with yolk. Thin cytoplasmic strands pass through the mass of the yolk, connecting the peripheral cytoplasm with the perinuclear one. Fragmentation begins with the fission of nuclei, as a result, the number of nuclei increases. They are surrounded by a thin rim of the cytoplasm, move to the periphery and are located in the yolk-free cytoplasm. As soon as the nuclei enter the surface layer, it divides into blastomeres according to their number. As a result of such crushing, the entire central part of the cytoplasm moves to the surface and merges with the peripheral one. Outside, a continuous blastoderm is formed, from which the embryo develops, and inside is the yolk. Superficial crushing is characteristic of arthropod eggs.

The nature of crushing is also influenced by the properties of the cytoplasm, which determine the relative position of the blastomeres. On this basis, radial, spiral and bilateral crushing are distinguished. With radial crushing, each upper blastomere is located exactly under the lower one (coelenterates, echinoderms, lancelet, etc.). During spiral crushing, each upper blastomere is displaced relative to the lower one by half, i.e. each upper blastomere is located between the two lower ones. In this case, the blastomeres are arranged as if in a spiral (worms, molluscs). With bilateral crushing, only one plane can be drawn through the zygote, on both sides of which identical blastomeres (roundworms, ascidians) will be observed.

EMBRYO DEVELOPMENT

The essence of the crushing stage. Splitting up - this is a series of successive mitotic divisions of the zygote and further blastomeres, ending in the formation of a multicellular embryo - blastula. The first cleavage division begins after the union of the hereditary material of the pronuclei and the formation of a common metaphase plate. Cells formed during cleavage are called blastomeres(from Greek. blaste- sprout, germ). A feature of mitotic divisions of crushing is that with each division the cells become smaller and smaller until they reach the ratio of the volumes of the nucleus and cytoplasm that is usual for somatic cells. In a sea urchin, for example, this requires six divisions and the embryo consists of 64 cells. Between successive divisions, cell growth does not occur, but DNA is necessarily synthesized.

All DNA precursors and necessary enzymes are accumulated during oogenesis. As a result, mitotic cycles are shortened and divisions follow each other much faster than in ordinary somatic cells. First, blastomeres are adjacent to each other, forming a cluster of cells called morula. Then a cavity is formed between the cells - blastocoel, filled with liquid. Cells are pushed to the periphery, forming the wall of the blastula - blastoderm. The total size of the embryo by the end of cleavage at the blastula stage does not exceed the size of the zygote.

The main result of the crushing period is the transformation of the zygote into multicellular one-shift embryo.

Morphology of crushing. As a rule, blastomeres are arranged in a strict order relative to each other and the polar axis of the egg. The order, or method, of crushing depends on the amount, density and distribution of the yolk in the egg. According to the rules of Sachs-Hertwig, the cell nucleus tends to be located in the center of the cytoplasm free from yolk, and the spindle of cell division - in the direction of the greatest extent of this zone.

In oligo- and mesolecithal eggs, cleavage complete, or holoblastic. This type of crushing is found in lampreys, some fish, all amphibians, as well as in marsupials and placental mammals. With complete division, the plane of the first division corresponds to the plane of bilateral symmetry. The plane of the second division runs perpendicular to the plane of the first. Both furrows of the first two divisions are meridian, ᴛ.ᴇ. start at the animal pole and spread to the vegetative pole. The egg cell is divided into four more or less equal in size blastomeres. The plane of the third division runs perpendicular to the first two in the latitudinal direction. After that, in mesolecithal eggs at the stage of eight blastomeres, uneven crushing is manifested. At the animal pole there are four smaller blastomeres - micrometers, on the vegetative - four larger ones - macromers. Then the division again goes in the meridian planes, and then again in the latitudinal ones.

In polylecithal oocytes of bony fish, reptiles, birds, and also monotreme mammals, cleavage partial, or meroblastic,ᴛ.ᴇ. covers only the cytoplasm free from yolk. It is located in the form of a thin disk at the animal pole, in connection with this, this type of crushing is called discoidal.

When characterizing the type of crushing, the relative position and rate of division of blastomeres are also taken into account. If the blastomeres are arranged in rows one above the other along the radii, crushing is called radial. It is typical of chordates and echinoderms. In nature, there are other variants of the spatial arrangement of blastomeres during crushing, which determines such types of it as spiral in mollusks, bilateral in ascaris, anarchic in jellyfish.

A relationship was noted between the distribution of the yolk and the degree of synchronism in the division of animal and vegetative blastomeres. In oligolecithal eggs of echinoderms, cleavage is almost synchronous; in mesolecithal egg cells, synchrony is disturbed after the third division, since vegetative blastomeres divide more slowly due to the large amount of yolk. In forms with partial cleavage, divisions are asynchronous from the very beginning, and blastomeres occupying a central position divide faster.

Rice. 7.2. Cleavage in chordates with different types of eggs.

BUT - lancelet; B - frog; AT - bird; G - mammal:

I-two blastomeres II- four blastomeres, III- eight blastomeres, IV- morula, V- blastula;

1 - crushing furrows, 2 -blastomeres, 3- blastoderm, 4- blastoiel, 5- epiblast, 6- hypoblast, 7-embryoblast, 8- trophoblast; the sizes of the nuclei in the figure do not reflect the true size ratios

Rice. 7.2. Continuation

By the end of crushing, a blastula is formed. The type of blastula depends on the type of crushing, and therefore on the type of egg. Some types of crushing and blastula are shown in Fig. 7.2 and scheme 7.1. For a more detailed description of cleavage in mammals and humans, see Sec. 7.6.1.

Features of molecular-genetic and biochemical processes during crushing. As noted above, mitotic cycles during the cleavage period are greatly shortened, especially at the very beginning.

For example, the entire fission cycle in sea urchin eggs lasts 30-40 minutes, while the duration of the S-phase is only 15 minutes. gi- and 02-periods are practically absent, since the necessary supply of all substances has been created in the cytoplasm of the egg cell, and the greater, the larger it is. Before each division, the synthesis of DNA and histones occurs.

The rate at which the replication fork moves along the DNA during cleavage is normal. At the same time, there are more points of initiation in the DNA of blastomeres than in somatic cells. DNA synthesis occurs in all replicons simultaneously, synchronously. For this reason, the time of DNA replication in the nucleus coincides with the doubling time of one, moreover, shortened, replicon. It was shown that when the nucleus is removed from the zygote, cleavage occurs and the embryo in its development reaches almost the blastula stage. Further development stops.

At the beginning of cleavage, other types of nuclear activity, such as transcription, are practically absent. In different types of eggs, gene transcription and RNA synthesis begin at different stages. In cases where there are many different substances in the cytoplasm, as, for example, in amphibians, transcription is not activated immediately. RNA synthesis in them begins at the stage of early blastula. On the contrary, in mammals, RNA synthesis already begins at the stage of two blastomeres.

During the cleavage period, RNA and proteins are formed, similar to those synthesized during oogenesis. These are mainly histones, cell membrane proteins and enzymes necessary for cell division. These proteins are used immediately along with the proteins stored earlier in the cytoplasm of the oocytes. Along with this, during the period of crushing, the synthesis of proteins is possible, which was not there before. This is supported by data on the presence of regional differences in the synthesis of RNA and proteins between blastomeres. Sometimes these RNAs and proteins come into action at a later stage.

An important role in fragmentation is played by the division of the cytoplasm - cytotomy. It has a special morphogenetic significance, since it determines the type of crushing. In the process of cytotomy, a constriction is first formed with the help of a contractile ring of microfilaments. The assembly of this ring takes place under the direct influence of the poles of the mitotic spindle. After cytotomy, the blastomeres of oligolecithal eggs remain connected to each other only by thin bridges. It is at this time that they are easiest to separate. This is because cytotomy leads to a reduction in the area of ​​contact between cells due to the limited surface area of ​​the membranes.

Immediately after cytotomy, the synthesis of new sections of the cell surface begins, the contact zone increases, and the blastomeres begin to tightly touch. Cleavage furrows run along the boundaries between individual sections of the ovoplasm, reflecting the phenomenon of ovoplasmic segregation. For this reason, the cytoplasm of different blastomeres differs in chemical composition.

Crushing - concept and types. Classification and features of the category "Crushing" 2017, 2018.

- Political fragmentation of the empire.

At the end of the XII - beginning of the XIII century. on the basis of the general social and economic revival of Germany, important changes were made in the political structure of the empire: the former feudal regions (duchies, archbishoprics) turned into almost completely independent states .... .

- Fertilization. Splitting up.

FERTILIZATION Lecture 8 Fertilization is the stimulation of an egg by a spermatozoon to develop while simultaneously transferring the father's hereditary material to the egg. In the process of fertilization, the spermatozoon fuses with the egg, while the haploid nucleus ....

- Crushing minerals

PREPARATORY PROCESSES LECTURE No. 4 Washing of minerals Washing is used in the enrichment of alluvial deposits of rare and precious metals, ferrous metal ores, phosphorites, kaolins, building materials (sand, crushed stone), ...

Fragmentation in biology, the process of successive mitotic divisions of a fertilized egg (zygote) into daughter cells (blastomeres), each of which retains its original volume after division; the first stage in the development of all multicellular animals and humans. The total volume of the cytoplasm of blastomeres formed in the process of crushing does not exceed the total volume of the cytoplasm of the zygote. At the same time, the amount of nuclear substance (DNA) in blastomeres doubles before each subsequent cleavage division. Therefore, the ratio of the amount of DNA to the amount of cytoplasm in the egg (the so-called nuclear-cytoplasmic ratio) doubles after each division during crushing; without this, subsequent developmental processes (morphogenesis and differentiation) are impossible, and this is precisely the biological meaning of fragmentation. Cleavage continues until the nuclear-cytoplasmic ratio returns to the values ​​it had before the oocyte growth period (see Oogenesis). There are successive periods of synchronous (the number of blastomeres is equal to 2n, where n is the number of divisions) and asynchronous (the number of blastomeres is not equal to 2n) crushing. The duration of each cleavage period is different in different animal species. So, in mammals (including humans), there is no period of synchronous cleavage at all, and in amphibians, the number of synchronous cleavage divisions reaches 10. At the end of cleavage, a multicellular embryo is formed - the blastula, the structure of which depends on the type of cleavage.

Only in rare cases, in particular in lower invertebrates (some cnidarians and flatworms), blastomeres are arranged randomly during cleavage (the so-called anarchic cleavages). In other animals, several types of ordered fragmentation are distinguished, depending both on the amount and location of the yolk in the eggs, and on the properties of their cortical (surface) layer. According to the so-called cleavage rules, the nuclei of the zygote and blastomeres are located in the center of the cytoplasm free from yolk, and the division spindles are located along the greatest extent of the latter. In eggs with a large amount of yolk, fragmentation is partial, or meroblastic (the yolk is not involved in it). The main types of meroblastic cleavage are: discoidal, in which blastomeres form a disc in the animal (yolk-poor) part of the egg, and the non-crushing yolk is located in the vegetative (yolk-rich) part (bony fish, birds, reptiles); superficial, in which the descendants of the zygote nucleus move along cytoplasmic bridges from the center of the egg to its surface, participating in the formation of the cell wall of the blastula - blastoderm, while the inner part of the blastula is filled with yolk (in most arthropods). In eggs with a smaller amount of yolk (round and annelids, molluscs, echinoderms, amphibians, etc.), as well as in its complete absence (mammals), fragmentation is complete (holoblastic). Moreover, if the yolk is still present in the vegetative region of the egg, the cleavage is uneven (the blastomeres formed in this part are larger in size than the blastomeres formed in the animal part), and if there is very little or no yolk, the cleavage is uniform, i.e. blastomeres are more or less equal in size.

In eggs with a small amount of yolk, spiral, radial, and several varieties of bilateral crushing are distinguished. During spiral crushing (molluscs, annelids), individual blastomeres or their rows (tiers) located one above the other during each division rotate relative to each other either clockwise or counterclockwise (dextral and sinistral crushing, respectively). The direction of twisting is determined by the mother's genotype, as it is related to the properties of the zygote's cortical layer. With radial crushing (echinoderms, chordates), such turns do not occur and the blastomeres are located exactly one above the other. Bilateral crushing (roundworms, ctenophores) is associated with active movements of blastomeres relative to each other. As a rule, during spiral and bilateral crushing, an early and irreversible determination of the morphogenetic fate of blastomeres is observed, associated both with the redistribution of the components of the egg cytoplasm (ooplasmic segregation) and with contact interactions of blastomeres. With radial crushing, determination occurs later.

Lit .: Belousov L. V. Fundamentals of general embryology. M., 2005; Dondua A.K. Developmental biology. SPb., 2005. T. 1-2.

As this fragmentation occurs, embryonic cells, called blastomeres, become smaller (palintomy division). This process is called crushing. It continues until the formation of a small cell embryo.

The types of crushing of fertilized eggs are determined by their structure. Crushing can be complete and partial(fig. 26), superficial, radial, spiral and bilateral.

At complete, or holoblastic, crushing the entire zygote or ovum is divided into blastomeres, and cleavage furrows penetrate into the deepest parts of the egg. In other words, the cleavage furrows completely separate the egg into blastomeres. Depending on the size of the resulting blastomeres, crushing can be equal-numbered and uneven. With uniform crushing, homolecithal eggs maintain synchrony of crushing of blastomeres for a long time, therefore they have the same size and doubling of their number is observed: 2, 4, 8, 16, 32, 64. With uneven crushing, the synchronism of crushing is disturbed, since poor yolk-complex blastomeres are crushed faster than yolk-rich blastomeres. As a result, they turn out to be of different sizes and there is no consistent doubling of their number.

At partial, or meroblastic, crushing the furrows do not separate the egg completely, so most of it is not crushed. There are also options here. For example, in birds and reptiles, the embryo develops due to a small amount of cytoplasm located at the animal (upper) pole. The rest of the egg is filled with yolk and is not crushed. A cytoplasmic disk is formed, along which crushing furrows pass.

Surface crushing characteristic of insects that have centrolecithal eggs. In this case, the first few divisions of the nucleus, located in the center of the egg, occur without division of the cytoplasm. Then the nuclei are surrounded by a small layer of cytoplasm and, as it were, float up from the mass of the yolk to the surface, uniting there with the surface layer of the cytoplasm. In this zone, the formation of blastomeres takes place, while an undivided yolk remains inside.

Depending on the location of blastomeres relative to each other, three types of crushing are distinguished: radial, spiral and bilateral.

At radial crushing the first two divisions are meridional, then equatorial divisions follow, when the division plane is perpendicular to the planes of the first two divisions. In this regard, four blastomeres of the animal hemisphere lie above four blastomeres of the vegetative hemisphere, which creates radial symmetry (Fig. 27, a).

Radial crushing of eggs is characteristic of such groups of animals as echinoderms. In a sea urchin, even at the stage of 64 blastomeres, it is impossible to identify individual of them, since the first stages of crushing give identical cells.

At spiral crushing(Fig. 27, b) animal cells are displaced in relation to the animal-vegetative (anterior-posterior) axis of the egg. The crushing plane runs at an angle to it and to the equator of the egg. The axes of the mitotic spindles form spirals, and the blastomeres seem to alternate with each other. Spiral fragmentation is characterized by a rigid order, which makes it possible to trace the further fate of blastomeres, or rather, their derivatives. After the first two cleavages, four blastomeres are formed A, B, C and D. Subsequent divisions give rise to smaller blastomeres (micromeres), 1 a, 1 b, 1 c and 1 d. The following micromeasures are 2 a, 2 b, 2 c and 2 d etc. It has been established that in a number of animals whose eggs are crushed according to the spiral type, the entire ectoderm comes from three quartets of micromeres that are separated from the first four blastomeres during successive stages of crushing.

Many groups of invertebrates, such as nemerteans, annelids, and mollusks, are characterized by spiral crushing. Sometimes the opinion is expressed that this type of fragmentation is a sign of phylogenetic relationship between them. It can only be stated with certainty that the spread of spiral fragmentation in several types only indicates that this is a conservative sign. material from the site

Splitting up I Splitting up

in engineering, the process of breaking pieces of solid material to reduce their size. The pieces are destroyed by external forces that overcome the forces of adhesion between the particles of the material. D. does not fundamentally differ from grinding (See Grinding). Conventionally, it is believed that with D., larger products are obtained, and when crushed, finer 5 mm. Ways D. ( rice. one ): crushing, splitting, abrasion and impact. Strong and abrasive materials are crushed mainly by crushing, strong and viscous - by crushing with abrasion, soft and brittle - by splitting and impact. D.'s work is spent on the deformation of the piece and on the formation of a new surface of small pieces. Most of the expended energy is dissipated in the form of heat, and only a small fraction is converted into the free surface energy of the solid. The total work of D. is equal to the sum of the work on deformation and on the formation of new surfaces. This generalized formula was proposed by P. A. Rebinder (1944). For approximate calculations, it is assumed that the work on D. piece size D at a given degree, D. is directly proportional D 2.5. D. is characterized by the degree of D., that is, by the ratio of the sizes of the largest pieces in the material before and after D. Dr. indicator - specific energy consumption, i.e. the amount kW· h for 1 t crushed material. D. is combined, as a rule, with screening m. Distinguish D. in the open ( rice. 2 , a) and closed ( rice. 2 , b) cycle. In the 1st case, the finished product is sieved on a screen before the crusher, and is also obtained after crushing; in the 2nd - the material after the crusher is sifted on a screen into large and small (ready); coarse material is returned to the same crusher for re-crushing. To obtain high degrees of D., several methods (stages) of D. are used in succession. When enriching ore, they are crushed in 2, 3, or 4 stages, the specific energy consumption for D. from pieces with sizes of 900-1200 mm up to 25 pieces mm - 1,5-3 kW· h for 1 t ores.

D. manual and fire was known for 3000 years BC. e. The simplest machines - falling pestles (crushes), driven by a water wheel, were already used in the Middle Ages and are described by G. Agricola. Machine D. has been developing since the beginning of the 19th century. (see Crusher).

From the 50s. In the USSR and other countries, hydroexplosive, thermal, electrothermal, and other methods of D. are being investigated, but for the next decades, the described mechanical methods will remain the main ones.

D. is used in the mining, metallurgical, chemical, food industries, in construction and agriculture.

Lit.: Levenson L. B., Klyuev G. M., Production of crushed stone, M., 1959; Andreev S. E., Zverevich V. V., Perov V. A., Crushing, grinding and screening of minerals, 2nd ed., M., 1966; Proceedings of the European meeting on grinding, trans. from German, M., 1966; Arsh E. I., Vitort G. K., Cherkassky F. B., New methods of crushing hard rocks, K., 1966; Ponomarev I.V., Crushing and screening of coals, M., 1970.

V. A. PEROV

II Splitting up

eggs, its segmentation, a series of successive divisions of the egg, as a result of which it is divided into ever smaller cells (blastomeres). D. is an indispensable stage in the development of all multicellular animals. It usually begins after the convergence of the male and female pronucleuses (see Fertilization) and the union of their chromosomes on the spindle of the 1st division of D. In some animals, D. of unfertilized eggs occurs (see Parthenogenesis). Sometimes fertilized eggs are at rest for some time (see Diapause) and are stimulated to develop by changes in external conditions (for example, ambient temperature). Initially, during the period of synchronous divisions, the nuclei in all blastomeres divide with the same and constant rhythm, the nuclear cycle is short; in different groups of animals, this period is not the same in duration, and in mammals it is absent. Then, during the period of asynchronous divisions, or blastulation (See Blastulation), the nuclear cycle lengthens, synchrony in the division of different nuclei is disrupted, at the interphase stage, the synthesis of ribonucleic acid (RNA) begins in them, and their morphogenetic function is detected. The division of the cytoplasm (cytotomy) follows but usually lags behind nuclear division (karyotomy). D. is not accompanied by growth, and the embryo retains the original dimensions of the egg. At the end of D., the embryo reaches the blastula stage (See Blastula).

The quantity and distribution of the yolk in the cytoplasm of the eggs influence the character of D.. Homolecithal eggs, which contain relatively little evenly distributed yolk, undergo complete uniform D. More often, the yolk is unevenly distributed in the cytoplasm of the egg (telolecithal and centrolecital eggs). An area containing more yolk is divided more slowly by a poor yolk - complete uneven D., or does not divide at all - partial D. Eggs that undergo complete D. are called holoblastic, partial D. are called meroblastic. Holoblastic include homolecithal (for example, the eggs of many invertebrates, lancelet, mammals) and part of telolecithal (for example, the eggs of some arthropods, most amphibians), which undergo complete but uneven D. (small blastomeres are called micromeres, medium ones are called mesomeres, large ones are macromeres ). Meroblastic eggs include part of telolecithal and centrolecithal with a large amount of yolk. In such telolecithal eggs, only the animal part of the egg, which is poor in yolk, is divided, which is successively divided into 2, 4 and more blastomeres that form a disk of cells on the surface of the non-crushing yolk - discoidal D. It is typical for eggs of scorpions, cephalopods, sharks and bony fish, birds, reptiles and lower mammals. As a result of discoidal D., a discoblastula is formed, the cavity of which is limited by the size of the blastoderm. Partial D. is also characteristic of the centrolecithal eggs of most arthropods. After fertilization, the nucleus begins to divide. After several synchronous divisions, the nuclei with the surrounding cytoplasm move along the cytoplasmic bridges to the surface layer of the cytoplasm, which at first is a symplast, then a separate cell separates around each nucleus. As a result, an embryo is formed, the wall of which consists of one layer of cells (blastoderm), and the central part is occupied by undivided yolk with cells located in it (vitellophages); such an embryo is called a periblastula, and D. is called superficial, or syncytial.

The character of D. is also affected by the properties of the cytoplasm of the egg, which determine the position of the division spindles and, as a consequence, the position of the blastomeres relative to each other, since the cleavage plane is always perpendicular to the axis of the spindle. On the basis of the relative position of blastomeres in complete D., radial, spiral, bilateral, and bisymmetric D. are distinguished. - the vegetative axis of the egg is the plane of symmetry. The first 2 furrows usually run meridional, and the 3rd - equatorially; then there is an alternation of meridional and equatorial divisions. As a result of radial D., a multicellular vesicle with a cavity is formed - a coeloblastula.

In spiral D., which is characteristic of most turbellarians, annulus, nemerteans, molluscs, and others, the micromeres that separate from the first four blastomeres (macromeres) are located in the spaces between them. The blastomeres of the upper tier are shifted relative to the lower tier to the right - dexiotropic D., or to the left - leotropic D. In spiral D., the embryo at the blastula stage has a cavity (uneven coeloblastula) or does not have it (sterroblastula). With bilateral D. (in roundworms, ascidians), as well as in the later stages of spiral D., divisions occur in such a way that the embryos have only one plane of symmetry. Bisymmetrical D. is observed very rarely (ctenophores) and is characterized by the presence of two planes of symmetry. Cm. a diagram of the structure of eggs, types of their crushing and types of blastula. One type or another of D. is usually inherent in most representatives of one or another class of animals, but sometimes different types of D. are observed within the class. Thus, among amphibians, most of which are characterized by complete uneven D., legless amphibians have discoidal D.; in mammals, both discoidal (single-pass) and complete D. (all higher mammals) occur. The latter, according to a number of features (separation of the germinal disc and the extra-embryonic part), approaches the discoidal, from which it originated. As a result of full D. there is a blastocyst; part of its wall, represented by a dense accumulation of cells, forms the germinal disk, the rest is Trophoblast.

In the process of D., the nuclei divide evenly (the nuclei of all blastomeres carry the full amount of genetic information and are equivalent both to each other and to the nucleus of the zygote), and the cytoplasm is divided unevenly. Differences in the properties of the cytoplasm of the first blastomeres in different animals are expressed to a different degree and depend on the level of its differentiation in oogenesis (see Ooplasmic segregation). In some animals, during the artificial separation of the first two blastomeres, a whole embryo is formed from each, in others - only a part of it, because. in the eggs of different animals, by the beginning of D., the cytoplasm reaches a different degree of differentiation (see Differentiation) (the earliest differentiation is characteristic of eggs with spiral, bilateral, and superficial D.). On this basis, regulatory and mosaic eggs are sometimes distinguished.

In the process of D., nuclei equivalent in genotype come into interaction with the cytoplasm, which is qualitatively different in different blastomeres, which is a condition for the differential realization of genetic information in them (see Embryonic development).

Lit.: Ivanov P. P., Guide to General and Comparative Embryology, L., 1945; Tokin B.P., General Embryology, Moscow, 1970.

T. A. Detlaf.

Scheme of the structure of eggs, types of their crushing and types of blastula: A - coeloblastula (1 - uniform, 2 - uneven: a - blastocoel); B - sterroblastula; C - discoblastula (a - blastocoel, b - yolk); G - periblastula.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

Synonyms:

Antonyms:

See what "Crushing" is in other dictionaries:

See division ... Dictionary of Russian synonyms and expressions similar in meaning. under. ed. N. Abramova, Moscow: Russian dictionaries, 1999 delimitation, lithotripsy, chiseling, ... ... Synonym dictionary

- (a. breaking, crushing; n. Brechen, Zerkleinerung, Quetschen; f. broyage, concassage; i. molienda) the process of breaking up pieces of ore, coal and other solid material in order to obtain the required size (more than 5 mm), granulometric ... … Geological Encyclopedia

Crushing: Crushing (technology) crushing a solid to a certain size; Crushing (printing) reproduction on the print of the same printing element twice, with an offset; Crushing (embryology) series ... ... Wikipedia

crushing, crushing, pl. no, cf. (book). 1. Action according to Ch. crush and crush. Stone crushing. Crushing the topic. 2. The process of dividing a fertilized egg into individual cells (biol.). Explanatory Dictionary of Ushakov. D.N. Ushakov. 1935 1940 ... Explanatory Dictionary of Ushakov

Splitting up- (grinding, grinding) - (ceramic) reduction in the size of materials depending on their hardness. [GOST R 54868 2011] Crushing of refractory raw materials [unshaped refractory] - grinding pieces of refractory raw materials [unshaped ... ... Encyclopedia of terms, definitions and explanations of building materials

In engineering, the process of breaking pieces of solid material into smaller ones. Depending on the size of the source material, there are: coarse (from 1000 to 100 mm), medium (from 100 to 40 mm), fine (30-5 mm) crushing ... Big Encyclopedic Dictionary

Eggs, a series of consecutive mitotic. divisions of a fertilized egg, as a result of which, without increasing in size, it is divided into ever smaller blastomere cells. D. an indispensable period of ontogeny for all multicellular animals. Usually… … Biological encyclopedic dictionary