The ability to produce offspring, to continue one’s generation from generation to generation, from century to century is one of those “wonderful gifts” that all living beings possess. Spring brings a lot of trouble to the inhabitants of forests, fields, meadows, gardens and vegetable gardens, ponds, lakes, rivers, seas and oceans. Everything from a nondescript insect to a whale giant surrenders to the care of posterity, which should come into the world.
Foxes, hares, rabbits, wolves, jackals, bears, tigers and lions adapt their homes for this purpose - burrows, lairs and lairs. In the forest and in the field, in the thicket of trees and bushes, under the shadow of thick, tall grass, representatives of the feathered kingdom bustle and tinker - crows, rooks, blackbirds, larks, woodpeckers, titmouse, robins, oatmeal, carduelis. They make nests, incubate eggs, from which the chicks hatch in due time. Lizards, snakes and turtles do not make nests, but lay eggs in grass, sand or simply on bare ground; the young generation hatch from eggs.
Meanwhile, in swamps and ponds there are a lot of eggs laid by frogs, and rivers and seas are teeming with eggs of various fish species. Each egg can become a frog or a fish over time.
True to the laws of nature and insects - beetles, flies, bees, bogs, dragonflies, grasshoppers and moths. Some of them build minks and nests, others use the first suitable place on the stems and leaves, in the cortex, in the buds or on the roots to lay eggs, from which a few weeks and sometimes months later, juveniles hatch.
In a word, the same thing is observed among living beings. They not only feed and grow, but also multiply. A plant arises from a plant. An animal is born from an animal. This is an immutable law of nature.
In humans, there are two sexes: male and female. We find the same thing in most animals. Lion and lioness, bull and cow, gander and goose, rooster and chicken - these are “men and women” in mammals and birds. They are called males and females.
It is not difficult to distinguish a male from a female. Usually the male is larger and more elegant than the female, especially in birds. Who does not distinguish a drake from a duck or a turkey from a turkey? Not so noticeable is the difference between males and females in reptiles (snakes, lizards, turtles), amphibians (frogs) and fish. But in insects, it again appears quite sharply: males are usually more elegant than females, more brightly colored and often have some kind of jewelry on their bodies.
However, even in cases where the male does not seem to be any different from the female, there is a significant difference between them.
Take two frogs, a male and a female. Let's open both the belly. Both the male and the female have a heart, lungs, stomach, liver, intestines - the organs necessary for the life of the animal. Before you are two figures (Fig. 1). On the left is one of the internal organs of the male frog, on the right is one of the internal organs of the female. The first is available only to the male, the second - only to the female.
What are these organs? And what are they for? They serve the cause of reproduction. Hence their name: reproductive organs. These are the very organs by which one sex is different from the other. That is why there is another name for the reproductive organs: genitals. The main part of the male genital organs is the seminal glands, or testes, and the most important part of the female genital organs is the egg glands, or ovaries.
The gland is called such an organ that produces some product necessary for the life of the animal. For example, the liver is called the digestive gland, because it secretes the bile necessary for the processing of fat, which, along with food, enters the intestines of the animal. What do the frog's sex glands produce - the egg and seminal glands? Eggs secrete eggs, and seminal - seminal fluid.
Thousands of tiny big-headed and tailed bodies, very nimble, like frog tadpoles, are floating in a drop of human seminal fluid, if you look at it under a microscope. These are the seminal bodies, or gum (sperm). Each of them can easily distinguish the head, neck and tail (Fig. 2). Working tail, like a screw, and spinning with his whole body, the zipper moves. In the seminal glands of various animals — a bull, a rooster, a frog, a lizard, a fish, a beetle, etc. — there are special live piles. They are diverse in form and move at different speeds.
In the ovarian glands, or ovaries, eggs mature. Usually they think that only birds, lizards, snakes, turtles, frogs, fish and insects have eggs. They think so because in these animals the eggs are quite large, especially in birds. But this is not true. Any animal that has ovaries matures eggs. At a certain time in life, a cat and a horse have eggs, a hare and a cow, a monkey and a woman. Only these eggs are very small: you need a microscope to see them.
Here are two such eggs (Fig. 3). One of them was taken from the ovary of a sea urchin, the other from a person’s ovary. Each of them resembles a tiny ball. Outside, it is covered with a delicate shell. Under the shell lies protoplasm, a semi-liquid substance similar to chicken egg protein (Fig. 4). In the protoplasm there is a large body, which is called the nucleus; and in the core - a smaller ball, nucleolus.
Ostrich, chicken, and even sparrow eggs are real giants compared to eggs of a sea urchin, human, frog, fish, and fly. Nevertheless, there is no significant difference between them. The ostrich’s heroic egg, an egg the size of thirty chicken eggs, originates in the ovaries of a bird in the form of the same tiny ball as a human egg. In it, as in a human egg, there is a shell, and protoplasm, and a nucleus with a nucleolus. But then it grows: it accumulates large reserves of water, salts, fat and protein - the reserves that go to the development and nutrition of the future ostrich. In addition, the ostrich egg is covered with a hard lime shell. But break the shell, drain the protein and take a closer look at the surface of the yolk: you will find a small speck. This is an essential part of an ostrich egg; the rest - protein and yolk - are just supplies of building and nutritional material. The same should be said about the eggs of other birds, lizards, snakes and turtles.
The offspring develops from eggs: a young fly from a fly egg, a chicken from a chicken egg, a puppy from a dog’s egg, a child from a woman’s egg. But eggs cannot develop without the help of relatives of their livestock: a chicken egg needs the help of the seed bodies of a rooster, a woman’s egg needs a man’s livestock, etc. Only when the egg connects to its related livestock does it begin to develop and can become the embryo of a new animal. Otherwise, it will die ^ not fulfilling its purpose.
The combination of an egg with a seed body is called fertilization. In some animals, fertilization occurs in the womb of the female. In others, for example in fish, eggs and gum are released into the water and here they are already connected. But. wherever fertilization takes place, it proceeds generally the same.
How does egg fertilization occur? Swim to the egg, the livestock surround it, working hard with tails, just trying to get ahead of each other. One of them manages to get ahead of everyone, he rests his head against the egg shell, boring it to get inside *. Protoplasm is extended by a small tubercle towards the gum. A few more minutes - and the goal is achieved: the gum inside the egg (Fig. 5).
* (In some animal species, not one, but many spermatozoa penetrate the egg. This phenomenon is called polyspermia. - Approx. open ed.)
What is the fate of the gum that has invaded the egg? Once inside the egg, it transforms: the movable tail disappears, dissolves in the protoplasm of the egg, and the head swells, becomes larger, much like the egg core. The head moves deep into the egg. To meet her, slowly squeezing through the grains of protoplasm, is the egg core. Finally, they converge almost in the middle of the egg and stop. Soon it is no longer possible to distinguish between the head of the gum (seed nucleus) and the egg core. Instead, they have one double core. Fertilization has occurred. All that was needed was done so that the embryo of a new animal began to develop from the egg.
The fertilized egg of a beetle, fish, frog, chicken, dog, human, etc. does not at all look like an adult animal. Before fertilization, it has the shape of a ball; it also looks after fertilization for several hours, days, and even months. But it can no longer remain unchanged forever. A buzzard that has crept inside an egg gives an impetus to the fact that it begins to change. And the egg is actually transformed: it is split in half, then into 4, and then into 8 and 16 parts (Fig. 6). This is not an egg, but the original germ of a new animal. The cells of which it consists continue to fragment. Now there are hundreds of them, thousands ... finally, millions. As before, they all remain connected to each other. From these cells various parts of the body of the new animal gradually develop. At first, they are barely outlined: you can’t immediately understand what it is - a head or a body, a leg or a hand. But later, individual parts of the body separate, and the embryo eventually acquires the appearance of an adult animal. It is born completely unlike that tiny, ugly-looking "ball" that we called a fertilized egg. This is how fertilization and subsequent development of the egg follow fertilization in all animals where there are males and females.
The animal world is infinitely diverse. There are organisms in which reproduction occurs somewhat differently. These animals include ciliates - tiny, brisk creatures that are found in hundreds in a drop of rotten water. Each ciliator is a very simple animal: it consists of only one cell, has, like an egg, a shell, protoplasm and a nucleus. Ciliates do not have males and females in the true sense of these words. They do not have such reproductive organs as a frog or a fly. And yet, under special conditions, they marry, and they observe what is commonly called fertilization.
Here two ciliates stopped, came close to each other and began to merge (Fig. 7). Each of them has a moving harness, there is protoplasm, there is a core. During fusion, the tourniquets disappear, the protoplasm mixes, and the nuclei join together. Some time passes, and instead of two ciliates, one is obtained. However, this is not an infusoria, but a round body - motionless, dressed in a dense shell, similar to a fertilized egg.
Further, the round body obtained from the merger of the two ciliates is divided in half. In other words, the same thing happens with a fertilized egg. There is, however, a difference. The halves of the divided egg remain connected to each other, and the halves of the fused ciliates are disconnected: a tourniquet appears in each of them, they become real ciliates. These ciliates usually breed by division: from one in a few days, a whole tribe of ciliates is obtained. But here comes a moment when the ciliates lose their ability to share. The situation is serious. Ciliates are in danger of becoming extinct. Here they are connected in pairs, get married. Two will merge into one, their protoplasms will mix, the nuclei will merge into one new core, and the ability to reproduce is regained.
Do all ciliates merge completely when they get married? No, not all. There is an infusoria named shoe. In appearance, it really resembles a tiny shoe. Shoes usually breed by division. But in their life there comes a time when they can no longer share. As soon as the "marriage season" comes, the shoes converge in pairs, but do not merge (Fig. 8). Two shoes, forming a pair, having spent some time near each other, again diverge. After that, each of them can again share. Before marriage, she temporarily lost the ability to reproduce. After marriage, this ability is restored.
What happened? Mating shoes exchanged parts of their cores. Each of them gave a different part of its nuclear material. Before marriage, each such shoe had a simple core. After marriage, it became mixed. A mixture of nuclear substances - this is the essence of fertilization in shoes. The same thing happens in other ciliates and in all animals in which reproduction is associated with fertilization.
There is a core in the animal’s egg. The head of the ginger is the same core. When fertilization occurs, the egg nucleus is connected to the seed nucleus (with the nucleus of the gum). During fertilization in fish, frogs, birds, mammals and other animals, the same thing happens as when pairing shoes: the mixing of nuclear substances. After mixing the nuclear substances, the egg begins to break up, develop, and becomes the embryo of a new animal.
How is the reproduction of plants? In spring, trees, bushes and grasses dress with flowers. Fruits are tied in flowers, and seeds develop in fruits. Seeds, once in suitable soil, germinate - they become new trees, bushes and grasses. Flowers, fruits, seeds, new plants - all these are links of the same chain. Let’s see how they are related.
In fig. 9 shows a flower. It consists of a short leg (pedicel), on which a cup composed of sepals and a corolla of several petals sit. There should be five petals on the flower. Only three are presented in the figure, so that you can properly see what is inside the corolla. For this is where the most important parts of the flower are hidden. Pay attention to the threads attached to the base of the corolla. These are stamens. At the top of each of them - two bags (anther), and in the bags - pollen. Stamens are male reproductive organs of plants. The stamen plays the same role as the seminal gland of an animal, and the pollen contained in the sacs of the stamen is essentially the same as the seminal fluid of animals.
Another thread protrudes from the middle of the flower, thicker than the others. This is a pestle. Like a stamen, a pistil is a reproductive organ, but not male, but female. Three parts are usually distinguished in it: the upper is called the stigma, the middle is called the column, and the lower is the ovary. In the ovary is one or more small bodies - the ovule (ovule, or seed kidney, that is, the kidney from which the seed is obtained). The ovary is the most essential part of the pestle. It can be compared with the egg gland of an animal. Such a flower is called bisexual, since it has both stamens and pestle, that is, the reproductive organs of both sexes, both male and female. Pears, apple trees, potatoes and cloves have bisexual flowers, while oak, alder, willow, hemp and hop flowers are unisexual. In an oak, for example, some of the flowers have only pistils: these are female flowers; and the other part is devoid of pistils and has only stamens: these are male flowers. However, no matter whether the flower is bisexual or same-sex, it usually does not fulfill its purpose, does not turn into a fruit with seeds, if it remains celibate *.
* (There are cases when a flower gives birth without “marriage”; but there is usually no seed in such a fruit. This has been observed more than once in some varieties of apple trees, pears and other plants.)
Marriage is a common phenomenon in plants. When the flower has fully blossomed and its stamens mature, then the sacs sticking out at the end of the stamens open and flower pollen falls out - thousands of dust particles, round, ovoid, smooth or patterned, depending on the plant. A few specks of dust fall on the stigma of a mature pistil - either in the same flower, or in the neighboring one. When this happens, the flower is pollinated. Finding themselves on the stigma of the pestle, dust particles sprout, that is, each of them expels a tube (Fig. 9, a, b). The tube pierces the stigma and, stretching more and more, makes its way along the column into the lower part of the pestle, into the ovary, where, as we already know, the ovules sit. So dust particles are sent to them. Inside each ovule, in a special pouch - it is called an embryo pouch - lies a tiny egg: a plant egg, from which a new plant should be formed. However, in order for this egg to begin to develop and become the embryo of a new plant, it must be fertilized: merge with the contents of the flower dust. From that, a speck of dust sprouts and its tubules go to where the ovules lie. The tip of the tube rests on the ovule in the place where the small hole is located. He penetrates this hole and finally gets to the egg: the contents of a flower speck, going down the tube, connects to the egg. Now the egg is fertilized. It will begin to crumble and turn into the embryo of a new plant. Both the ovule and the ovary itself will begin to grow. The ovary will become the fruit, and the ovule will become the seed. Inside this seed, when it ripens, you will find the embryo of a small plant with an embryonic stem, spine and leaves. It is enough for such a seed to get into the soil well warmed up by the sun, to be saturated with moisture, and it will sprout: it will put a root in the ground, drive out the stem and fresh leaves, in a word, become a young plant. But if the flower was not pollinated, if the egg hidden in its ovule remained unfertilized, then it itself will remain a hollow flower: it will fade without tying the fruit with seeds, without leaving posterity.
As you can see, the emergence of a new plant embryo in the plant world is essentially no different from the origin of living things in the animal world. True, you can say: well, where are the lively lively animals in plants? Is the flower speck of dust the same as the seed body? No, not the same. But the content of a speck of dust plays the same role in plants as the seed body (gum) in animals.
Scientists made a remarkable discovery: they found that in some plants motile vibrants form inside dust particles, just like seed bodies in animals. These gummies, having descended a dust particle, make their way into the ovule, from there into the embryo sac and connect to the egg. In these cases, the similarity between fertilization in plants and fertilization in animals is complete.
Apple tree, pear, potato, oak, hops, willow and other plants that give flowers are called flowering. But has anyone seen mosses, ferns, mushrooms and algae bloom? I did not see, of course, for it is a flowerless plant. However, they still multiply, and therefore have reproductive organs. Take a small segment of seaweed - Fucus vesiculosus. On it in places swellings filled with air are visible. This is a kind of swimming bladders, with the help of which this plant is kept in water. In addition to bubbles on the branch, there are small pits. We arm ourselves with a magnifying glass and take a closer look at one of these dimples. Inside, it is densely planted with hairs; some bags are sitting in front of the hairs, and in each of them there are eight round bodies. These are the female organs of reproduction of Fucus with eggs enclosed in them.
Fucus is a dioecious alga. Among Fucus species of the same species, there are two kinds of individuals. In some, on the branches, in the dimples, only female reproductive organs sit, in others - only male. The reproductive organs are placed, as it were, in two different houses: the female ones on the branches of the female Fucus, and the male ones on the branches of the male Fucus.
Take a branch of male Fucus, arm ourselves with a magnifying glass and consider one of the pits located on the branch. Branched hairs, like felt, fill the hole, on the hairs there are bags, the size is much smaller than those in which Fucus eggs are placed. These are the male reproductive organs of Fucus. They are filled with moving livelings.
When the time comes for reproduction, the sacs with eggs and gum torn. The eggs fall into the water. Gingerbirds also get enough sleep there. There are many, many more than eggs. And they are much smaller than an egg: just like a poppy seed near a large Antonovka. Gum swims to the eggs, surround them on all sides, trying to get inside. For what - you already know. You know what happens next. Therefore, I only recall that after the egg merges with one of the gum, it begins to crush, and then turns into a young seaweed - fucus.
Fertilization in plants is essentially no different from fertilization in animals. Yes, it should be so. The means that are available in nature for reproduction are the same. They are used by plants, animals and humans. All of them are the brainchild of nature and obey its laws.
Now let's turn to insects. Let's get to know the beehive population. It is densely populated: it has several thousand female workers, several hundred drones and one "queen" - the uterus. Drones are males, the uterus is a female, which can lay up to 4,000 eggs per day, and for its entire short life - about half a million. Female workers are also females, but they never mate with males. Workers, like the uterus, have ovaries; but compared with the queen’s reproductive organs, their ovaries are poorly developed and do not produce eggs.
But here's the curious thing. It happens that the hive is left without a uterus. Then some female workers begin to lay their eggs. Of these eggs, only drones develop. That is why such prolific female worker bees are called tinder. Tinder marriage does not enter into a marriage with a drone; it lays unfertilized eggs. And yet, from such eggs young bees are born - drones.
The general rule is this: eggs develop only after they connect with the gum. But this rule has exceptions. Eggs do not always need the help of gum. Sometimes they develop without fertilization. Sometimes the female does not marry, remains a virgin for life and still gives offspring - without the participation of the male, without the assistance of the livestock. The way in which these females breed is commonly called virgin reproduction (parthenogenesis).
Let's go to the silkworm. Mulberry worms, that is, caterpillars of the silkworm butterfly, are raised on special shelves. These caterpillars hatched from eggs that were laid by a female silkworm after they were fertilized by a male. However, it also happens that a female silkworm does not mate with a male, but still lays eggs. From these unfertilized eggs, caterpillars hatch, which eventually turn into butterflies. In short, not only tinder bees, but also silkworm butterflies can breed virgin.
This method of reproduction is also found in other animals: wasps, ants, butterflies, worms. But in all these animals, virgin reproduction is an accidental matter: it is observed as an exception, and not as a general rule. That is why it is much more interesting to talk about those animals in which virgin reproduction is a common and even inevitable phenomenon. To get to know them, we will go to the gardener.
There are many fruit trees in the garden - apple trees, pears, plums, peaches. There are beautiful rose bushes in it. One misfortune: some trees and rose bushes suffer from the invasion of small insects - aphids (they are also called grass lice).
Ask the gardener how these pests breed on fruit trees, and he will tell a lot of interesting things.
When damp and cold autumn sets in — a tough time for all insects, including aphids — the winged males and female aphids mate. After marriage, females lay fertilized eggs and die, males die, and eggs hibernate.
Spring comes, and with it warm days. Warmed by the rays of the sun, eggs come to life. From them young wingless aphids hatch. They are all females: not a single male! Males are not needed for these females, as they give birth without fertilization. They do not lay eggs, but give birth to live cubs, the same wingless females capable of breeding. So aphids breed throughout the spring and summer. One generation replaces another. While the sun warms and plenty of food, they are born in the thousands. Aphids cover the trunks and branches of fruit trees, leaves and flower buds of rose bushes. But these are only females - the whole female kingdom.
Summer is running out. Autumn is approaching again, and with it a change in the behavior of aphids. Aphids give birth not to wingless females, but to winged males and same females. Males marry females that lay fertilized eggs. The eggs hibernate and in the spring give again wingless virgin females. Then everything repeats first ...
As you know, sea urchins and starfish live in the sea. They breed with eggs. Eggs begin to develop only after fertilization. Does this mean that a starfish and a sea urchin can never, under any circumstances, reproduce virgin, that is, with the help of unfertilized eggs?
Two prominent scientists - the American Loeb and the French Delage - found that the eggs of a starfish or sea urchin can develop even when they are not fertilized. Delage showed this especially clearly.
He took the eggs of the sea urchin and placed them for about an hour in a weak solution of water with ammonia, sugar (sucrose) and tannin (tannic acid); then he took out the eggs from the solution, washed several times and immersed in a vessel with clean sea water. Then something unexpected happened: the unfertilized eggs began to break up and turned into the larvae of a sea urchin. True, most of the larvae died. But the survivors continued to develop and turned into real sea urchins.
This interesting experience leads to serious reflection on the power of man over nature and the "whims" of nature itself. The scientist made the seemingly impossible: the egg of the sea urchin turned into a hedgehog not with the assistance of a live bait, but under the influence of a solution of a certain composition.
Sea urchin is an invertebrate animal. And we already know that virgin reproduction is often observed in moths, wasps, bees, aphids and other invertebrate animals. Vertebrate animals are quite another matter — fish, snakes, birds, etc. Therefore, the experience of the French scientist Battalion, who managed to grow a vertebrate animal from an unfertilized egg, is even more interesting.
The battalion took the unfertilized frog eggs and pierced them with a thin glass needle. The puncture had a truly magical effect on the eggs - it’s just not a needle, but the livestock penetrated inside: the egg crushing started. Of many eggs, tadpoles developed. True, almost all the tadpoles then died. Only three tadpoles grew, and one even began to turn into a frog.
The experience of the Battalion, which managed to make the vertebrate reproduce virgin, even brighter than the experience of Delage, showed man’s power over nature *. It is not necessary, however, to exaggerate the size of this power. You should not think that a person can do anything. Contrary to nature, contrary to its laws, man cannot do anything. His strength lies in the knowledge of nature, in the understanding of its laws, in the ability to use them. Thousands of Battalions would not be able to do anything with frog eggs, if these eggs by nature did not have the ability to develop without the help of livestock.
* (The American scientist Olsen recently managed to obtain live turkey poultry without fertilization - Note. open ed.)
Wildlife is one. Unity can be observed everywhere and in everything, both small and large. The basic building material from which the bodies of all animals and plants inhabiting the earth are made is the same. The basic properties of this material are the same. Equally, finally, are those basic forces and abilities that are endowed with all living creatures without exception. The ability to reproduce is one such common gift. It affects the ability to reproduce virgin. Only not all this ability is revealed; and when revealed, it is not with equal strength. In aphids, it affects brightly, for them it is mandatory. In bees and silkworms, this ability is less pronounced, appears by chance. In sea urchins and frogs, virgin reproduction is not detected: it is as if in a hidden form and is found only in conditions that man creates artificially.
If wildlife is one, then virgin reproduction should also occur in plants, for a plant egg is essentially no different from an animal egg. I will give an example confirming this idea.
There is a milk plant called chickens potion. It grows in the garden and near fences like weed grass. Flowers at hen-potions are same-sex: some with stamens only, others with pestles only. These flowers sit on different bushes: on some men (stamen), and on others women (pistil). Dust particles from male flowers fall on the female pistils and thus fertilize them. But if the female flower of chickens-potions remains unfertilized, then it always dies, like a barren barren flower. Sometimes such a virgin (unfertilized) flower sets a fruit, and its ovules turn into seeds that expel new plants. This is virgin reproduction.
Most flowering plants have both stamens and pestle. They are bisexual, that is, they are hermaphrodites *. Among animals, hermaphrodites are less common. An ordinary snail is a real hermaphrodite. Each snail has in its body both gum and eggs. For this, she also has an appropriate organ: it cannot be said that this is an ovary; but you cannot call it the seminal gland, because it produces both male and female sex cells (eggs and gum). Leech is also a hermaphrodite. She, like a snail, has eggs and seed bodies in her body. But only she has not one, but two organs - and the seminal glands and ovaries.
* (Hermaphrodite is a Greek word, made up of two names: Hermes and Aphrodite, that is, a man and a woman - a creature with signs of both sexes. Hermes - in Greek mythology, the god of crafts and trade, Aphrodite - the goddess of beauty.)
Hermaphrodites are sometimes found among people, But these are more likely people with incorrect or ugly developed reproductive organs than real hermaphrodites.
The ancient Greeks had a myth that spoke of the time when there were no people: the Earth was inhabited by special humanoid creatures with four legs and arms and two faces. Each such monster had double breeding organs. These were hermaphrodites. Nature endowed them with heroic strength and a remarkable mind. These fabulous creatures wanted to enter Olympus, into the home of the gods. Then Zeus decided to punish the proud. To deprive them of bodily and spiritual power, he cut everyone in half. There are no bisexual and two-faced monsters. Instead, they appeared same-sex and unisexual creatures: men and women. Since then, each half has been looking for the other half that it lacks. Hence the attraction of one sex to another, the love of a man for a woman and a woman for a man.
Of course, this is a myth. There were never bisexual and two-faced creatures on earth with four legs and two pairs of arms. But the legend is not without some meaning. It expressed the idea that creatures of the same sex, that is, males and females, descended from bisexual creatures, that is, from hermaphrodites. It is difficult to say with certainty who first appeared on earth — bisexual or same-sex organisms. There are facts that suggest that same-sex beings descended from hermaphrodites.
Even more clearly, the craving for sex separation is observed on the example of barnacle crayfish; they were perfectly studied by Charles Darwin. Barnacle crayfish look a little like the famous crayfish. They sit motionless in their calcareous bivalve chambers, which are attached with a special “handle” to some underwater object. Their legs - thin, composed of many segments - look like a thick, bristly mustache, which either moves outward from the chamber, or goes inside; hence the name of these crayfish is barnacle.
One type of barnacle cancers is hermaphrodite: in each of them you will find both ovaries and seminal glands. Another species of barnacle crayfish has with it real males (one or several). These additional males - creatures underdeveloped and helpless - are attached to the body of hermaphrodites. Why are they needed here?
It turns out that these barnacles are hermaphrodites only in structure. But the seminal glands remain unused. Their role is played by "additional males". In other words: the eggs of such a barnacled hermaphrodite are not fertilized by the seed bodies of the hermaphrodite itself, but by the livestock of the "extra males". In this case, the hermaphrodite acts only as a female, that is, she behaves as a same-sex creature, and not bisexual. It is unlikely that he will lose anything if his seminal glands simply disappear as unnecessary, and he himself becomes not only in behavior, but also in structure in the same-sex animals, that is, a female. This assumption is supported by facts. Among the barnacle cancers there are also those in which instead of hermaphrodites we find females with males attached to them. There is a real division of the sexes into female and male.
It is often said: nature does not make leaps. This means that in nature everything changes step by step, everything arises gradually - perfect from imperfect, complex from simple, and vice versa: simple from complex, isolated from non-isolated. So stars arose. Sun and Earth. So the living beings inhabiting the earth arose: first simple and monotonous, and then, after millennia, more and more complex and diverse.
In the animal world, the development, apparently, went like this: at first there were simple animals, akin to amoebas; then sponges, polyps, worms, mollusks appeared, and even later - insects, fish and amphibians; after amphibians, again millennia later, reptiles, birds and finally mammals developed. Man, "the crown of creation and the ruler of the earth," appeared later than all. Invertebrate animals that arose much earlier than vertebrates are, as it were, their ancestors.
Among invertebrates there are many hermaphrodites. Among vertebrates there are almost none: there is a strict separation of the sexes. Since invertebrates are considered the ancestors of vertebrates, it can be assumed that the ancestors of animals of dioecious animals were bisexual, that is, that males and females descend from hermaphrodites.
In nature, reproduction is observed without flowers, stamens and pistils, without fertilization, but simply with the help of layering and cuttings. Strawberries and strawberries drive out, in addition to a short stem and leaves, also long lashes. Each such whip can let out roots, expand a bunch of leaves and thus give rise to new bushes of strawberries and strawberries. In the mountains you can find one interesting plant, it is called - youngsters (Fig. 10). Young growth grows on the ledges of rocks and has thick, fleshy leaves collected in a dense spherical head of cabbage. In the sinuses of these leaves, several thin processes are born, and at the end of each such process a bunch of small leaves are collected, collected in a ball. At first, the young shoot feeds on the juices of the mother's body. Then it comes off. A light blow of the breeze - and the young ball falls to the nearest ledge of the rock. Here he strengthens in the ground, driving out roots, feeds, grows and, over time, "gives birth" to offspring in the same way as he "was born". The ledges of the rock, meanwhile, are covered by a close family of youngsters, where "fathers", "children", and even "grandchildren" live nearby.
Various onion plants - common onions. tulip, hyacinth, etc. - multiply not only by seeds but also with the help of bulbs. Some plants can propagate by tubers. So propagates, for example, potatoes.
On the potato tuber in the dimples are "eyes", each such eye is a kidney, the germ of a new potato plant. Cut the potato tuber into several pieces, but so that each of them has at least one eye, bury them in the ground and you will grow several young potato bushes.
Usually they think that only plants can reproduce by buds. This is not true. Reproduction by budding is very common in animals.
Coral polyps breed with eggs and gum. However, they can also multiply by budding. On the body of the polyp, protrusions form. The protrusions grow, and gradually each such "kidney" becomes a new polyp (Fig. 11). There are polyps that reproduce not only in eggs and kidneys, but also in division. The very name "division" shows what is the matter. A polyp is divided by a constriction into two parts, and two polyps are obtained from it. They, in turn, are divided, and instead of two, four polyps are already evident. Then they get 8, 16, 32, 64, etc. to infinity. It is not surprising that so many corals accumulate in places in the sea that whole islands, underwater ridges and shallows are composed of their calcareous dwellings.
Other animals reproduce by division, for example starfish, some worm breeds, etc. But basically, the simplest microorganisms that live in air, in water and in soil reproduce in this way. Some of these living dust particles are plants, others are animals. Among animals, the most interesting are ciliates; among plants, bacteria are especially famous. Both are extremely prolific: in a few days they can produce millions of their own kind by simple division.
An infusoria shoe that you are already familiar with is divided by a transverse hauling in half. Each half grows and turns into a new shoe. Other ciliates also reproduce. Moreover, those that are covered with cilia are divided across, and those that are equipped with a tourniquet are divided along. Bacteria have the ability to crush into parts. Crushing is sometimes so fast that several millions of new sticks are obtained from one stick in 24 hours.
There is one circumstance that you should pay attention to when it comes to the reproduction of bacteria.
There is a bacterium that develops superbly in an infusion of hay dust. This is why it is called: hay bacteria, or hay bacillus. In good living conditions, when there is enough food, moisture and heat, the hay stick multiplies perfectly: it is divided into halves, and it seems that there will be no end to crushing it. But here come hard days for the hay stick: food is running out, there is not enough water. Crushing then goes slower and slower. It was as if it was about to completely stop, and then the end of the prolific tribe would come to an end: the old would die, and the new ones would be gone. But vain alarm. Take a closer look at them at this time with a microscope, and you will see a roundish shiny body inside each hay bacteria. This is a debate. They weren’t there before. They arose with the onset of adverse conditions.
Some time has passed since the moment when disputes formed. The shell of the sticks is torn, spores go outside. Now there are no hay bacteria: there are spores. Disputes endure all adversities until they fall into a favorable environment for development. Then they germinate, become hay sticks again. So spores save many bacteria from death.
However, there are plants for which spores are the same as seeds for flowering plants. Such, for example, are mushrooms - large, such as fly agaric, and tiny, such as those that form mold. Examining with a microscope a small piece of mold, it is easy to notice that it consists of thin jointed, tangled threads, among which columns with balls or tassels at the top rise. This is the body of the mold, the heads and tassels are fruit-bearing plants. Each head (or brush) is filled with spores. Each spore, germinating, gives rise to a new fungus. Large mushrooms also have disputes - mushrooms, mushrooms, boletus, mushrooms, mushrooms, etc. Spores develop in the caps of these mushrooms. Ripening, they scatter in all directions, and then sprout and give new mushrooms.
In addition to mushrooms, spores reproduce mosses, ferns, algae, lichens. In all these colorless plants, spore has the same meaning as the ovule or, more precisely, the egg in flowering plants. The difference is as follows: in flowering plants, the ovule turns into a seed and into a new plant only when the egg enclosed in the ovule is fertilized. Fertilization disputes usually do not need. They sprout without the assistance of gum.
There are organisms in nature that combine both methods of reproduction: asexual and sexual. Take a look at the underside of a common cinnamon fern leaf. Along the veins of the leaf on either side are sitting in rows some dark plaques the size of hemp seed. Each such plaque is a collection of bags filled with spores. Mature spores fall out of the sacs, scatter in all directions, fall into the soil and germinate. From each spore, a plant is formed that is not at all like a fern. It looks like a flat, heart-shaped leaf that lies on the ground and pulls food out of it with small roots. This is a preteen, the forerunner of a real fern. The time will come, the fern itself will grow. Here's how it goes.
On the undergrowth, on the underside, the reproductive organs develop - male and female. Usually, both appear on the same preteen. However, it also happens that only male reproductive organs appear on one teenager and only female organs on the other. In male organs, lively animals develop - mobile, with a bunch of cilia; and in female eggs are formed - round, motionless and much larger than gum. Gingerbirds merge with eggs. Real ferns develop from fertilized eggs. At first they are very small, but gradually become larger, take root, expel cirrus leaves.
To the question of how fern propagates, sexually or asexually, one must answer: both. On the leaves of fern many spores arise. Out of spores, prejudices arise. This is the first generation of fern “children”. They were born asexually. On a preteen, eggs and seed bodies develop in special organs, which, when merged, produce real ferns. This is the second generation of "children." They were born through sexual reproduction (Fig. 12). Such alternating propagation is not at all the exclusive property of ferns. So mosses multiply, for example. So many animals breed, especially marine ones, for example jellyfish.
In jellyfish, eggs are formed (in females) and seed bodies in males. Both are thrown into the water. Moving livestock find eggs and fertilize them. From each fertilized egg, an embryo is obtained, covered with movable cilia. He, living for some time at large, loses cilia, plunges deeper into the water and sits on a pebble or on a branch of algae. Having strengthened, it stretches out, takes a pear-shaped form and expels tentacles on the upper end of its body (Fig. 13).
Fig. 13. Reproduction and development of jellyfish: a - ciliary embryo; b, c - a germ without eyelashes; g - the germ drove out the tentacles; d - live "bump"; e - a grown "bump"; Well - the "bump" broke into saucers; h - the embryo of jellyfish; and - a young jellyfish; k - adult jellyfish
Next is a series of new transformations. The embryo grows, becomes much larger. On his body, several transverse annular constrictions are indicated. Now it resembles a fir cone. This, of course, is not a jellyfish.
We follow the fate of the "bumps". The constrictions on her body are becoming deeper and deeper. Soon, she begins to look like a bunch of saucers set on top of each other with serrated edges. All the "saucers" are interconnected by a thin jumper, as if strung on it. This jumper breaks over time, and the scattered "saucers" turn into a group of underdeveloped young jellyfish. Over time, each such jellyfish will grow, develop completely and become a real jellyfish.
* (It is mentioned in less detail in the essay "Invisible enemies and friends of man.")
Plasmodium lives in human blood. Blood, as you know, consists of blood fluid in which blood cells (red and white blood balls) float. Once in the blood, plasmodium is located on a red blood ball, and then sneaks into it. The blood ball serves as a plasmodium and temporary shelter and food. Here it eats and grows until it fills almost the entire blood cell with itself (see Fig. 14, 1-4).
Then it begins to multiply, that is, it crumbles into a whole bunch of small plasmodia. A lot of them. And so they tear the walls of the blood ball, fall into the blood fluid (Fig. 14, 56-96) and get down to business: they make their way inside the red blood cells, destroy them, and then again crumble into a bunch of young plasmodia. So one generation of them is replaced by another, multiplying by division. Finally, plasmodia appear in the same asexual way, which will not reproduce further.
Swamp fever is therefore called swamp fever because it is common in areas where there are many swamps. And where there are many swamps, there are many mosquitoes, among which there are malaria. Such a mosquito sits on the face or on the arm of a patient suffering from swamp fever, launches his proboscis into the skin and begins to suck out blood. Together with the blood, several pieces of plasmodia enter his intestines. Once in the intestines of a mosquito, plasmodia grow, become large and round, like a ball. Some of them are so round and remain (Fig. 14, 7 a). These are eggs. Others are divided into small areas, form heaps of gum (Fig. 14; 6-9). In the intestines of the mosquito, the ginger fuses with the eggs (Fig. 14; 10). After fertilization, the egg enters the intestinal wall and is divided into parts, thus giving many new plasmodia. The egg shell breaks, plasmodia exit from it and eventually end up in the salivary glands of the mosquito (Fig. 14; 13, 14). Now for the further development of plasmodium again must be in the human body.
Fig. 14. Life path of the causative agent of swamp fever: 1 - plasmodium in human blood; 2 - a red blood ball, to which a group of plasmodia rushes; 3 and 4 - plasmodium grows; 56-86 - shows how asexual propagation of plasmodium proceeds; 96 - young plasmodia emerging from a red blood ball. The rest of the figure (below the dotted line) shows how the sexual reproduction of plasmodium in the mosquito body occurs; 5, 6a-9a - plasmodium turns into an egg; 6-9 - plzamodia breaks up into a LOT of "zinger", 10- "egg" merges with the "zinger" (fertilization); 1-14 - the plasmodium that has arisen after fertilization grows, crushes, and gives a whole bunch of young plasmodia, which 7) again penetrate the red ball of a healthy person after a mosquito bite
Every living creature multiplies. Reproduction is one of the main phenomena of life. Reproduction is the "law of life." However, no one can reproduce when he wants and how he wants. The ability to reproduce has a limit. A lion and a tiger, living in freedom, give birth to children on time. In captivity, locked in pairs in the cells of the zoo, they only occasionally give offspring. Parrots breed well in the native forests of America and Australia. In captivity, they can live in pairs for two to three decades and never sit chicks.
The same is true among plants. What sometimes beautiful palm trees can be found in greenhouses! But they rarely bloom and almost always give empty flowers, they do not give fruit, and if they do, they never ripen. Meanwhile, in their homeland, in the forests of India or South America, in the steppes of Arabia or in the oases of the Sahara, the same palm trees are covered with color and give an abundant harvest of fruits. It is clear that not always and not under any conditions living things can multiply. It is clear that with a change in the situation, the ability of reproduction also changes.
We know that some organisms reproduce quickly and produce numerous offspring; others slowly and give a small offspring. We also know how different are the methods of reproduction that various organisms possess. Are these differences random or do they obey any rules?
The dog and the wolf are close relatives. Both in growth and structure are very similar to each other. However, a dog is more fertile than a wolf. She sometimes gives 12 puppies a year. A wolf is born no more than six wolf cubs per year. A domestic cat is much more fertile than a wild one: it usually gives birth to calves twice a year, each giving five to six kittens; and the wild gives birth only once a year to no more than five. Even more significant is the difference between the offspring of domestic and wild pigs. A wild pig gives six to eight pigs per year, while a domestic pig can give its owner as many as two dozen at the same time. We will find the same difference when comparing the fecundity of domestic goose and domestic duck with the fecundity of their wild relatives.
What explains this difference? Pets live in the care of man. Good nutrition promotes reproduction. Otherwise, the life of wild relatives develops. They do not always have plenty of food: and hunger is a poor ally of reproduction. Can a she-wolf raise many cubs in her womb if she does not have enough food? Can a wild goose give as much nutrients to the formation of eggs as a domestic goose usually spends on it?
Nutrition and reproduction are interconnected. Of two identical animals with the same ability to breed, the one that feeds better will be more prolific. The relationship between nutrition and fertility is particularly evident in plants. Transplant the plant from the infertile soil to the well-groomed soil, rich in food supplies, and it will tie a lot of new buds, each of which will become a leaf-stem shoot. And what is leafy shoots for derez, if not the offspring born asexually with the help of the kidneys?
On plants, you can check another breeding rule. It is well known to every gardener.
Young apple trees are brought up in the nursery. They sit in oily soil, eat plenty and expel many young shoots, that is, multiply by the kidneys, asexually. The gardener, however, wanted his apple trees to bloom as soon as possible and to make more fruit. In other words, he wants the apple trees to move from asexual to sexual reproduction, to reproduction using eggs and dust particles. The gardener either replants the apple trees on soil that is poor in nutrients, or, leaving the apple trees in their old place, cuts off part of their roots. In both cases, young trees begin to receive much less food than they received before. They cease to give green shoots, and are covered with flower buds, which, having blossomed, tied the fruits. While the apple trees received plentiful food, they propagated asexually; when food became less, they switched to sexual reproduction. The same can be said of insects.
Recall aphids. In spring and summer, they breed virgin - without fertilization, that is, asexually; at this time they have plenty of food. In the fall, food decreases, and then the aphids go on to sexual reproduction.
Another example. With good nutrition, ciliates, shoes can multiply for a long time by division. In poor conditions, they stop sharing and begin to mate. And here, as you see, asexual reproduction depends on abundant nutrition.
There is another “breeding rule”. The more the body spends food to warm itself and replace the losses caused by work in it, the less building material left for it to form eggs and young. Or the more waste of the body, the less its fertility.
Suppose that some animal eats abundantly, but at the same time does almost nothing. You can say in advance that it must be very prolific. Such an animal is, for example, a queen bee. Already in childhood, when she still does not look like a bee, but like a worm, she is fed very abundantly. When she grows up, she becomes the "queen", then the working bees are eager to rush to supply her with tasty and satisfying food. She eats literally for ten and does nothing. The whole hive is held by the labors of workers. And the queen has only one thing to do with her duties: lay eggs. And she does this job brilliantly: she lays up to four thousand eggs daily. The worker bee, on the contrary, and the toiler are exemplary - all day busy with something - and generally does not eat well. In addition, in childhood, during the larval season, she was not very spoiled with food. She usually does not form or lay eggs, that is, she is barren. To show that both the great fecundity of the “queen” and the sterility of the worker bee are closely related to their living and nutritional conditions, I recall the following facts.
The egg from which the working bee is born is no different from the egg from which the "queen" must arise. There is no difference between the newly hatched workers and the "royal" larvae. Place the working larva in the same large, spacious cell in which the queen is usually brought up, feed it as abundantly as the royal larva is fed - and it will become the "queen", that is, the prolific womb. And vice versa: transfer the newly hatched tsarist larva from its “choir” to an ordinary “proletarian” cell, transfer it to the same “table” used by all working larvae - and there will be no trace of the “regality”: it will turn into the most ordinary, barren worker-bee.
Let us see, however, on what else the great fecundity of a living organism may depend.
The elephant only in the thirtieth year of her life for the first time gives birth to only one cub. And the mouse, which has not yet "dried milk on its lips," becomes a mother, giving two to three times a dozen mice annually. A stunted dandelion two to three months after birth expels a whole "basket" of flowers with a number of several hundred pieces and forms about the same amount of seeds. A tall coconut palm only in the tenth year of life begins to bloom and tie fruit properly. While large animals and plants produce tens, hundreds, at most thousands of descendants in several years, ciliates and bacteria invisible without a microscope produce millions and tens of millions of similar ciliates and bacteria in a few days.
All these facts make us think that there is a connection between the sizes of living creatures and their fertility, which can be expressed as follows: the larger the organism, the fewer its offspring - if all other conditions are equivalent. The last reservation is very important. And here is how to understand it.
The frog and sparrow are about the same size. If the size of the animal affects fertility, it must be assumed that the frog and sparrow are equally fertile. In fact, the frog is incomparably more fertile than a sparrow: a sparrow displays only five to six chicks, the frog gives several hundred tadpoles, from which about the same number of frogs can turn out. What does this show? Does the above rule on the relationship between size and fertility be incorrect?
A sparrow and a frog, approximately the same size, would be equally prolific if, in everything else - in structure, in character, in their way of life - they were similar. But this is just not there. The frog spends comparatively little food on warming its body: it has “cool” blood and body. And the sparrow and the blood and body are "hot". Compared with a sparrow, a frog is an inactive and inactive animal. Sparrow, on the contrary, is very active and very mobile. And mobility and activity require, as you know, a large waste of food material. If the frog and sparrow of the same size are even equally good at eating, then even the "cold-blooded" and inactive frog will have much more nutrients and building substances left on the offspring than the "warm-blooded" and very mobile sparrow.
Birds are superior in structure to amphibians. In this sense, a sparrow is a more “developed” creature, more “perfect” than a frog. And life shows that of two organisms of the same growth, equally active and equally eating, the one that is simpler, that is, less developed, less perfect, is more fertile. The sparrow’s body has a more complex structure, so the sparrow should give less offspring even if its lifestyle was no different from the frog’s lifestyle.
These are the basic laws of the conditions for the reproduction of animals and plants, to which the world of living beings obeys. Only man is able to a greater or lesser extent to change (to the extent of knowledge of the laws of nature) the conditions of his existence, as well as animals and plants.
Asexual reproduction occurs with the participation of only one individual. It can be called growth, which goes beyond the usual measure of the volume of an individual. Its essence lies in the fact that some part is separated from the body of an adult mother’s body in one way or another, which then grows and reaches the size of an adult.
Usually asexual reproduction occurs in such a way that the cell that makes up the whole organism is divided into two halves. This method is called division. It is found only in lower animals: the rhizopod, ciliates, and others. The very process of division is that a groove appears on the body of the animal, in the form of a ring-shaped groove, which gradually deepens and finally divides the whole body into two parts, connected by a small jumper; the latter all becomes thinner and finally torn, so that the animal breaks up into two independent parts. They begin to grow, acquire the missing shells: flagella, cilia, etc., and turn into an adult animal.
This is the process of division only in the lowest animals, in the body of which there is almost no differentiation, and each part of the body is like the others. In more complex unicellular organisms, such as, for example, ciliates, this process is greatly complicated. In other animals, especially in lower multicellular organisms, asexual reproduction occurs in such a way that small outgrowths, so-called kidneys, form in certain places of their body, which are gradually fenced off from the rest of the body by a septum and then separated. This method of reproduction is called budding.
The essence of this process is that a new individual is formed from a small particle of the mother’s body, and not from half, as happens with division. The kidney in multicellular animals always consists of several cells, which makes it significantly different from the egg or spores, which initially always consist of one cell. Kidney reproduction is widespread in the animal kingdom. It is found in worms and in intestinal ones.
There are two types of budding: external and internal. External budding is more common; this is how it happens, for example, in polyps. On the surface of the body of this animal in a certain place, a small tubercle first forms. Soon, a small hollow process of the trunk cavity of the maternal organism penetrates inside it; the tubercle grows and takes the form of an adult animal. A hole forms at its outer end and a certain number of tentacles grow around it. In this state, the kidney is completely similar to the maternal organism and differs from it only in smaller size.
The fate of the kidneys is different. Sometimes it breaks open, begins to live independently and gradually reaches the size of an adult animal, but in some animals the mature kidneys do not fall away, but remain attached to the mother's body for life.
In this way, animal colonies form a complex system, for example, in the form of a tree branch.
In annelids, budding occurs somewhat differently. Some of the segments lying in the middle change in such a way that they become completely similar to the front segment of the animal, with the head. For some time, the worm lives in this form, but gradually other parts of the body grow in the changed segments, and then the animal breaks up into parts, and each of them, living independently, turns into an adult and then it splits into several parts.
Internal budding deserves special attention, as it constitutes a transitional stage to sexual reproduction. This method of reproduction consists in the fact that the kidneys are formed inside the body on the inner surface of its cavity. Separated from the walls of the body, the kidneys in the form of separate lumps are placed in the cavity of the mother's body and turn into new individuals. Thus, internal budding differs from sexual reproduction, which we will move on to, only in that the kidneys are composed of many cells, and not of one, which are the testes and spores.
Internal budding is observed only in flatworms, and the details of this process will be considered later, in due time, and now we turn to the third form of asexual reproduction.
This is the rarest form of asexual reproduction called sporogony, i.e. reproduction through spores. Sporogonia is observed in gregarians and some ciliates, it is not uncommon to see them., Also in some radios and radiolarians. The essence of this method of asexual reproduction is that the entire body of the animal breaks up into many small particles called spores; the animal itself ceases to live, and from each spore, under favorable conditions, a new animal can develop.
A very interesting form of asexual reproduction is copulation. Its essence lies in the fact that the two individuals are connected together so completely that their bodies after that are one, the so-called zizhot or zygote. After some time, the zygote begins to multiply intensively by division or snorogony. Copulation is very interesting because it represents a transitional stage to sexual reproduction. In most cases, copulating organisms are no different from each other, but in some animals, individuals that significantly differ from each other join; one of them strongly resembles the female reproductive cell of lower animals and plants - the egg, and the other male seed-seeded body, and this case of copulation is essentially no different from sexual reproduction of lower animals.
Finally, in some animals there is still a special form of mutual relationship, which is very close to copulation and is observed in some ciliates. This process, called conjugation, is performed as follows: two ciliates stick to one another and fuse at the site of contact; after a while, they again diverge and each continues to continue an independent life. Studying this process under a microscope, scientists noticed that during their connection in the internal organization of each conjugating ciliates, very complex changes occur. Their internal organs are destroyed, the nucleus breaks up into many small particles, which both conjugates exchange, so that, having separated at the end of conjugation, each of them carries away a particle of the other individual; the internal organs of each of them are subsequently restored.
Conjugation, just like copulation, is of great importance in the life of animals that begin this process. The ciliates are conjugated always after prolonged reproduction by division, and their organism is exhausted, it becomes senile, so further reproduction by division becomes already impossible. Conjugation, on the other hand, restores their strength, and after that both ciliates again for a long time gain the ability for asexual reproduction (division).
Of all the types of asexual reproduction that we have examined: division, budding, sporogony, copulation and conjugation, the first two forms should undoubtedly be recognized as the most important, and the rest are only different transitional stages to sexual reproduction. Asexual reproduction is closely related to the growth of the body and differs from it mainly in that the newly formed particles do not always remain with the mother's body, but only in very rare cases (colonies). Such a relationship of asexual reproduction with growth can be given a theoretical explanation, based on nutritional conditions in relation to the body volume.
Animal reproduction methods
Asexual reproduction.
In the animal world there are a number of methods of reproduction, for example, direct division or budding, characteristic of lower invertebrates, as well as parthenogenesis, observed even in higher vertebrates. It is clear that it is asexual reproduction that represents the simplest and least energy-intensive way to increase the number of individuals. However, for some reason, in the process of evolution, a complex process of sexual reproduction, associated with many problems and conventions, arose.
Sexual reproduction.
1. Conjugation. Sexual reproduction appears in animals already at the lowest levels of the evolutionary ladder. So, already in the simplest unicellular microorganisms - ciliates propagating by direct division, the so-called conjugation is observed, which is a kind of analogue of the sexual process. In the process of conjugation, two ciliates seem to grow together, for a time during which hereditary information is exchanged. Then the ciliates are disconnected, and then each continues to share on its own.
2. Parthenogenesis. In a number of invertebrates, as well as in many vertebrates, there is a phenomenon such as parthenogenesis, in which females lay their eggs or give birth to live cubs without the participation of males. Only females hatch from these eggs or are born in this way, and it is interesting that in nature there are whole local populations of similar species. A similar population of rock lizards has been discovered in Armenia. In the rest of its habitats, this very ordinary species reproduces in the usual way.
Parthenogenesis in experimental conditions is possible even in mammals. To do this, it is necessary to somehow stimulate the unfertilized egg to divide, which is possible in different ways, for example, simply by applying an injection with a microneedle.
3. Hermaphroditism. A number of invertebrate animals have hermaphroditism, in which each individual has both male and female sex glands. Among the most famous animals, hermaphrodites are, for example, earthworms, leeches and many types of snails. However, despite the fact that, in such animals, each individual produces both eggs and sperm, they mate with each other, producing a mutual exchange of sex cells. In large, shellless, aplisia mollusks or sea hares living in the coastal zone of the sea, up to 10-12 individuals can simultaneously participate in the process of fertilization, playing both the roles of males and the roles of females.
However, despite the presence of germ cells of both types in hermaphrodite animals, they rarely resort to self-fertilization, and, on the contrary, usually tend to mate with other, preferably non-related, individuals.
4. Sex change. In some animals, including a number of fish species, sex change occurs with age. So, in well-known aquarists, swordfish fish, it is quite common to turn middle-aged females into males. At the same time, the xiphoid process on the tail grows only in males, and they begin to exhibit typical sexual behavior, successfully fertilizing the females. A similar phenomenon is observed in small fish of marine junkers living in the coastal zone of the Black Sea. In some fish species, the reverse process was also noted: the transformation of males into females.
5. The alternation of sexual and asexual types of reproduction. In invertebrates such as intestinal, worms of various types, echinoderms, in parallel there are both sexual and asexual reproduction. Many invertebrates are characterized by the alternation of sexual and asexual generations. So, for example, coral polyps, which are very actively growing due to vegetative propagation, periodically produce a huge number of motile male and female germ cells. Fertilization occurs in the water column. From the resulting zygotes, motile larvae develop that settle on a suitable substrate and give rise to new coral colonies. In a similar way, all attached forms propagate: sponges, bryozoans, hydroid polyps, etc.
In some species, an alternation of parthenogenesis with normal sexual reproduction is observed. So, for example, aphids breed throughout the summer by live birth, in which unfertilized females give birth to females alone. Young female aphids begin to give birth to the following females at the age of several hours.
Thus, during the summer there passes a huge series of asexual generations. Zoologists have calculated that if all the offspring of one aphid survived, then in one summer it could cover the entire globe with a continuous blanket. Fortunately, since many other animals feed on aphids, this does not happen. It would seem that this method of breeding aphids is very successful and does not require much to be desired. However, in the fall, many aphids lay eggs, males that fertilize a relatively small number of females are hatched.
While in the fall, most aphids, having completed their life cycle, die.
Reproduction is one of the most common properties of living things, expressed in an increase in the number of individuals. In the process of reproduction, organisms reproduce their own kind and thereby ensure the continuity of life.
Reproduction of protozoa
The protozoa are characterized by asexual reproduction, which proceeds either as a monotomy (cell division in two with the subsequent growth of each daughter, for example, Amoeba proteus), or in the form of palynctomy (multiple division of the mother cell into several daughter, without their subsequent growth, for example, Plasmodium vivax ) In both cases, cell division is preceded by mitotic nuclear fission. In some ciliates (rootless and sucking), asexual reproduction proceeds by the type of budding.
Many of the simplest have a sexual process (not to be confused with sexual reproduction). It proceeds in two forms: copulation and conjugation. Copulation is the fusion of gametes - haploid germ cells. Gametes of protozoa can have a different structure and differ in the degree of mobility. If gametes of the same size, structure, and mobility are copied, then they speak of isogamous (equally homogeneous) copulations. During the merger of mobile gametes of various sizes, anisogamous (unequal) copulation occurs. Moreover, a larger cell is called a macrogamet (or female gamete), and a small one is called a microgamet (or male gamete). The extreme expression of anisogamy is oogamy - when a macrogamet is much larger than a microgamet and is motionless. Conjugation, or the exchange of genetic material between two individuals, is characteristic of ciliates. When conjugation, gametes are not formed, and individuals exchange vagus pronuclei, which, once in the partner’s cell, merge with their own stationary pronuclei (not to be confused with conjugation in algae).
During the sexual process, protozoa do not increase the number of individuals, but an increase in genetic diversity is achieved.
When describing the life cycles of protozoa, it is important to determine the position of the reduction division.
1. Gametic reduction. It occurs before the formation of gametes in organisms having diploid somatic cells (typical of all multicellular animals and some protozoa).
2. Zygotic reduction. It occurs in organisms with a haploid set of chromosomes after gamete fusion, that is, after the formation of a zygote (sporozoans, flagella, fungi).
3. Intermediate reduction. It was noted in organisms in the life cycle of which there is a change in diploid and haploid phases. In protozoa, this type of reduction division is characteristic of foraminifera. Among multicellular organisms - for all higher plants, and secondly - for some multicellular animals - rotifers.
Reproduction of multicellular animals
Asexual reproduction
Multicellular animals reproduce predominantly through sexual contact, but there are groups (especially among the lower invertebrates) that reproduce very successfully asexually.
Asexual multiplication of multicellular organisms is an increase in the number of individuals formed from somatic (non-asexual) cells. Among animals, it is completely absent in primary celiac worms and mollusks. In arthropods, vertebrates, asexual reproduction can include polyembryony, that is, asexual reproduction at the stages of embryonic development. This phenomenon was discovered by I.I. Mechnikov. In insects, polyembryony is described for riders - division at the morula stage. In mammals (armadillos), division occurs at the blastocyst stage. The appearance of identical twins in humans can be attributed to polyembryony.
Asexual reproduction plays a particularly important role in the life cycles of sponges, gastrointestinal, some worms, bryozoans, and tunicates. As a result of asexual reproduction, colonies appear in these animals. Reproduction proceeds by the type of budding. Sponges and bryozoans have peculiar internal kidneys (gemmules and statoblasts, respectively), at the stage of which there is an experience of adverse environmental conditions. In the intestinal and membranes, an alternation of asexual and sexual generation is observed. This phenomenon is called metagenesis. So, intestinal polyps are propagated by budding and represent an asexual stage in the life cycle, and jellyfish, which are formed on a polyp as a result of asexual reproduction, are the sexual stage, since they can only reproduce sexually.
Sexual reproduction
Sexual reproduction in animals exists in several forms. First, bisexual reproduction, which exists in the form of dioecious and hermaphroditism, can be distinguished, and secondly, virgin reproduction, or parthenogenesis.
Bisexual reproduction
With bisexual reproduction, fertilization necessarily occurs, that is, the fusion of female and male germ cells. In dioecious multicellular animals, sex cells are formed in different organisms - female in the body of the female, male in the body of the male. Gamete formation is preceded by meiosis.
During fertilization, a zygote is formed, the first cell of the body. In hermaphrodite animals, female and male germ cells are formed in the body of one individual. Hermaphroditism can be divided into natural and abnormal. Natural hermaphroditism is a very widespread phenomenon in the animal kingdom. It is found in sponges, intestinal, flatworms, annelids, mollusks, crustaceans and some fish. Natural hermaphroditism exists in various forms. So, in some animals, male and female germ cells are produced simultaneously, in others, one type of gamete is first produced, and then another. In the case when the male glands develop first, they speak of protandric hermaphroditism, and if the female sex glands begin to function first - about proterogynic hermaphroditism. Hermaphrodites develop various devices that prevent self-fertilization. It can be different maturities of female and male reproductive products, structural features of the reproductive apparatus, physiological barriers, etc.
In normally dioecious animals and humans, abnormal hermaphroditism occurs. Usually it arises as a result of genomic disorders - that is, the number of sex chromosomes changes with respect to autosomes. However, the cause of abnormal hermaphroditism may be hormonal disorders. In some cases, the animals develop female and male sex glands, in other cases, the sex glands belong to one sex, and secondary sexual characteristics demonstrate belonging to the other sex. As a result, females develop masculinity (masculinization), and males develop effemination (feminization).
Parthenogenesis
Parthenogenesis is a unique form of sexual reproduction. The egg begins to develop without the participation of the sperm, that is, without fertilization. This is same-sex reproduction. Natural parthenogenesis is common among all types of invertebrates, as well as in vertebrates, with the exception of mammals. In invertebrates, flatworms, rotifers, crustaceans, insects, mollusks can multiply in parthenogenetics. In some animals, eggs can develop only parthenogenetically, while in others (rotifers, bees) eggs can develop both parthenogenetically and as a result of fertilization. The uterus of bees lays fertilized eggs, from which working bees and future uterus develop, and dams - males - develop from unfertilized eggs. In flukes, parthenogenetic reproduction occurs at the larval stages (miracidia, sporocyst, redium). This type is called pedogenesis.
Particular forms of parthenogenesis are androgenesis and gynogenesis. In androgenesis, the embryo develops from the male nucleus, which is brought into the cell by the sperm, and the female nucleus is not involved in the development. This type of development is noted in some species of riders. During gynogenesis, the sperm penetrates the egg and activates its development, but its nucleus does not merge with the nucleus of the egg and does not participate in the further development of the embryo. Gynogenesis exists in amphibians, fish, and other animals. For example, in some fish, egg activation to development can be carried out by spermatozoa of other fish. Populations of such fish consist only of females (crucian carp).
Experiments on artificial parthenogenesis were begun at the end of the 19th century. on unfertilized silkworm eggs. In the middle of the XX century. B. L. Astaurov developed an industrial method of activating and developing unfertilized silkworm eggs under the influence of high temperature and other physical and chemical factors. As a result, he obtained parthenogenetic female butterflies.
Sexual and somatic cells
Somatic cells make up the bulk of the cells of a multicellular organism. Sex cells (gametes) are formed only at a certain stage of ontogenesis. When fused, gametes form a zygote - the first cell of a new organism. Sexual and somatic cells differ from each other in a number of ways. So, sperm and eggs are haploid, and body cells are diploid, that is, each gene is represented by two alleles. For example, human somatic cells have 46 chromosomes, and gametes have 23 chromosomes. Gametes and somatic cells have various nuclear-plasma relationships.
Particularly brightly this phenomenon is demonstrated by organisms with large eggs, for example, birds. The egg egg of the birds is the yolk. Its volume exceeds the volume of the original cell (from which it was formed) millions of times. The core volume remains virtually unchanged. With the development of the embryo (fragmentation), the nuclear-plasma relations of dividing cells acquire indicators characteristic of somatic.
In contrast to the ovum, sperm are very small. In humans, 50-70 microns. These reductions occur due to a sharp reduction in the volume of the cytoplasm, and the nucleus has a size corresponding to the nucleus of a somatic cell. In fact, the sperm head is represented only by a nucleus surrounded by a cell membrane. The metabolism of sex and somatic cells is different. In males, the sperm in the reproductive ducts are in a stationary, inactive state. Outside the body, they usually live a short time - in trout in sperm they die after 30 seconds, and in humans in seminal fluid - after 2-3 hours. In the genital tract of females, sperm can be alive for a longer time. For example, sperm live in the female genital tract for 5-8 days, and they remain viable for more than two years in the uterus of the bee.
Mature male germ cells are called sperm, or sperm. They were first discovered and described from sperm of mammals in 1667 by A. Levenguk. The spermatozoa of all vertebrates and most invertebrates consist of a head and flagellum, with the help of which they move in a liquid medium: with external fertilization - in water, with internal fertilization - in the fluid of the genital tract. Flagella of flagellate sperm have a structure typical of eukaryotes. Spermatozoa without a flagellum are called flagellate and are characteristic of roundworms and some arthropods. Such sperm are capable of amoeboid movement.
Female reproductive cells of animals are called eggs or ova. The eggs were discovered in 1827 by K. M. Baer. Usually the eggs are round or oval in shape, in a mature state they are motionless. In lower invertebrates (sponges, hydra), the eggs are capable of amoeboid movements. The egg cytoplasm contains yolk - a reserve nutrient necessary for the development of embryos. This is the specialization of the egg. Depending on the amount of yolk, egg sizes vary. Eggs lacking yolk (for riders) have a size of 6x10 microns. Eggs, poor in yolk, are large - from 50 to 90 microns. In mollusks, crustaceans and many other animals, eggs are large, contain a lot of yolk and reach 1.5 mm; shark egg - 70 mm. The largest eggs are in birds; an ostrich egg (without a protein shell) has a length of 80 mm, and with shells - 150 mm.
Egg shells are an indicator of the specialization and differentiation of germ cells. Primary membranes are formed due to the release of substances by the oocyte itself. The primary membrane is a film in contact with the oocyte membrane. It is also called the vitelline membrane. The secondary membrane occurs due to the secretion of certain substances by the ovary cells and is called chorion. Chorion is found in some invertebrates, fish and birds. The tertiary membrane is formed when the eggs pass through the oviducts. For example: gelatinous shell of amphibian eggs, protein, subshell and shell shells of bird eggs, cocoons of worms and mollusks, etc. An egg can have both all three shells and two of them (the chorion may be absent). The main function of the egg membranes is protective.
Gametogenesis
Gametogenesis is the process of formation of gametes, or germ cells. In primitive animals (sponges, some intestinal and flatworms), gametes can form anywhere in the body (for example, in sponges in the mesocile), and then they are brought out in one way or another. In the vast majority of animals, gametes are formed in the gonads, or gonads. The male gonad is called the testis, and the female is called the ovary. Gastropod mollusks have a hermaphroditic gland, this is a gonad in which eggs and sperm are produced simultaneously.
The process of sperm formation is called spermatogenesis. Spermatogenesis is divided into four stages: reproduction, growth, reduction division (division - maturation - meiosis) and spermiogenesis. The original cells, spermatogonia, are small in size. They have the ability to consistently mitotically divide, as a result of which their number increases significantly. This is the breeding season.
Then spermatogonia enter a period of growth and turn into first-order spermatocytes. Then they enter into a reduction division - meiosis, which includes two divisions - ripening. As a result of the first division, two spermatocytes of the second order are formed, and as a result of the second division, four spermatids. Spermatids are already haploid cells, they are not able to divide and differ from the original ones in smaller sizes. Spermatids enter the last, fourth stage of spermatogenesis, namely, spermiogenesis. It consists of successive complex transformations of spermatids. These transformations include: the formation of an acrosome, flagellum, rejection of part of the cytoplasm with the endoplasmic reticulum (EPS) and the Golgi complex, etc., which ends with the formation of mature sperm.
The process of egg formation is called oogenesis. Oogenesis consists of the same four stages as spermatogenesis. The initial cells, oogonia, are relatively small, with large nuclei. These cells begin to mitotically divide, that is, enter the stage of reproduction. In many animals, it occurs at the earliest stages of ontogenesis. For example, in mammals, even before birth, in an embryo. As a result, cells are formed - first-order oocytes. Many of them die or become trophocytes (nourishing cells).
Then the oocytes enter the period of growth. First comes cytoplasmic growth - the number of organoids increases. Then begins the formation of the yolk - vittellogenesis. When growth ends, in some animals, fertilization (roundworm) may occur, and the oocyte immediately enters the maturation period. In a lancelet, fertilization occurs after the first division-maturation. However, in most animals, fertilization occurs after the second division-maturation. As a result of the first division of meiosis, the first-order oocyte turns into a second-order oocyte (almost the same size as the first-order oocyte) and isolates the first polar body. In the second division of meiosis, one mature haploid egg and a second polar body are formed. The first polar body as a result of the second meiotic division forms two secondary polar bodies.
With the occurrence of processes of oogenesis and spermatogenesis, some differences are observed. Spermatogonia will multiply longer and more intensively than oogonia. Spermatocyte growth occurs faster than oocyte growth. A spermatocyte as a result of maturation gives four spermatozoa, and an oocyte - one mature egg.
Egg crushing
As a result of the fusion of female and male gametes, a zygote occurs - a diploid cell that can be considered as an organism at an early stage of development. After the fusion of female and male pronuclei, the cell begins to mitotically divide. Smaller cells appear - blastomeres, and the process itself is called crushing. After each division of the fragmentation, the embryonic cells become smaller, that is, the nuclear-plasma relations change: the nucleus remains the same, and the volume of the cytoplasm decreases. The process proceeds until these indicators reach the values \u200b\u200bcharacteristic of somatic cells.
The type of crushing depends on the quantity and nature of the distribution of the yolk in the egg. If the yolk is small and it is evenly distributed in the cytoplasm (echinoderms, mammals), then the crushing proceeds as uniform: the blastomeres are the same in size, the whole egg is crushed. If the yolk is unevenly distributed (there are more eggs at one pole), then the crushing proceeds according to the type of complete non-uniform: blastomeres - of different sizes, those that contain the yolk - are larger, the egg is crushed whole (roundworms, mollusks, etc.). Partial crushing - the part of the egg containing the yolk does not divide (insects, birds).
The types of crushing can also differ on the basis of the mutual spatial arrangement of the blastomeres: radial fragmentation (frogs), spiral (mollusks), bilateral (ascidia), two-symmetric (ctenophores).
Blastula formation
The crushing period ends with the formation of a blastula, an essential stage in the development of a multicellular organism. A typical blastula is a hollow ball formed by cells. The blastula cavity increases as it is crushed. It is filled with liquid, which is a product of the vital functions of cells. The cavity of the formed blastula is called the blastocele, or the cavity of the embryo. The cells forming the blastula wall can be the same (frog) or different (sponges, sea urchins, etc.). The types of blastula are very different in different animals. In some organisms, a ball without a cavity is formed in which the blastomeres are interconnected. Such an embryo is called morula. It is believed that morula is a type of blastula.
Education gastrula
The next stage in animal embryogenesis is gastrula. This is a two-layer embryo consisting of the outer leaf - ectoderm - and the inner leaf - endoderm. A bilayer embryo can be formed from a single-layer blastula by poking its wall into the blastocoel (invagination). The internal germinal leaf (endoderm) thus forms the primary intestine. It communicates with the external environment through the primary mouth or blastopore. In some animals, the remains of the blastocoel are preserved. In primary animals (intestinal, flat, round, annelids, etc.), the blastopore turns into the mouth of an adult organism. In secondary animals (echinoderms, brachopods, chordates), the blastopore turns into the anus, and the mouth reappears on the abdominal side of the anterior end of the body.
Gastrul can form when the cells creep out of the blastula wall into the embryo. In this case, it is called an immigration gastrula, characterized by the absence of a blastocele displaced by immigrant cells, and is typical of many intestinal cavities. In a number of intestinal cavities, delamination gastrula occurs. It is formed as a result of the splitting of one layer of cells into two. Epibolic gastrula is formed as a result of fouling of small ectoderm cells of larger endoderm cells located on the surface of eggs rich in yolk.
Germ leaves
In the animal kingdom, at the stage of gastrula, the development of two types of animals — sponges and coelenterates — stopped. These are bilayer organisms, that is, their tissues and cells are formed as a result of the differentiation of two germ layers - the primary ectoderm and primary endoderm. In other animals (beginning with flatworms), in the late stages of gastrulation, a third germinal leaf, the mesoderm, appears. There are differences in the types of bookmarks of the mesoderm. In the primary ones, between the ectoderm and the endoderm two or more cells are laid - teloblasts, of which the mesoderm is formed by further divisions. This method is called teloblastic. In the secondary ones, the third germinal layer is laid enterocoelally: that is, protrusions in the form of pockets are separated from the primary intestine. The cavity of these sacs then turns into a special cavity of the body - the secondary, or the whole.
Animal cell
Differences of multicellular representatives of the Animalia kingdom from organisms of other kingdoms (Fungi, Plantae) can be traced already at the cellular level. Animal cells have the following distinctive features:
1. The cell is covered only with a cytoplasmic membrane (the plant cell outside the membrane has a membrane of cellulose, and fungal cells of chitin).
2. There is no central vacuole in the animal cell (it is in the plant cell and is filled with cell juice).
3. There is centriole in the animal cell, but not in the plant cell.
4. The reserve nutrient of the animal cell is glycogen, and the plant cell is starch.
5. An animal cell - a heterotroph, it lacks plastids, while plastids are present in a plant cell.
Characteristic of all multicellular animals is their life cycle with a predominance of the diploid stage. In the life cycle of multicellular haploids, only gametes are hapless, at the fusion of which a diploid zygote occurs - the first cell of the future multicellular organism.