Who is characterized by internal fertilization, what is the essence of this process, and what is its biological significance? You can answer these and many other questions when you get acquainted with our article.
What is sexual reproduction
Reproduction is one of the properties of all living organisms. This process ensures the continuity of generations. Sexual reproduction creates new combinations of genetic material, and hence the signs of organisms. It is this process that underlies heredity and variability.
Sex is called reproduction, during which gametes take part. These are specialized cells containing a haploid chromosome set. In nature, both plants and animals can carry it out.
Gamete structure
The process of merging gametes is fertilization. Internal or external fertilization is carried out only by sex cells. Distinguish between male and female gametes - sperm and egg cells. They have significant differences in structure. So, female germ cells are not capable of movement and contain a sufficient supply of nutrients. This is due to the fact that it is on the basis of female gametes that the future organism develops. Male reproductive cells of plants are also not capable of movement, therefore, the process of fertilization in these organisms is preceded by pollination.
Gametes are structures that have a single, or haploid, set of chromosomes. And such a structure is not at all accidental. The fact is that an adult organism must have a double (diploid) chromosome set. This is only possible with the fusion of haploid gametes.
External and internal fertilization
Fertilization refers to the combination of the genetic material of germ cells. Depending on the place in which this process takes place, several types are distinguished. External fertilization proceeds outside the body of the female. In nature, it is found in amphibians and fish. Inner fertilization is characteristic of typically terrestrial animals: reptiles, birds, mammals.
Features of external fertilization
External or external fertilization begins with the removal of germ cells outside. Therefore, the convergence of organisms is not necessary at the same time. Despite this, an accumulation of breeding individuals is often found in nature. For example, fish or frogs during spawning.
External fertilization, internal or its intermediate type begins with the process of insemination. Its essence is the convergence of germ cells. Scientists have found that with external fertilization, almost immediately after the contact of the cells, changes occur in the electrical impulses of the egg shells. And after 7 seconds, the contents of the gametes are already connected, resulting in the formation of a zygote. It is repeatedly divided and a multicellular embryo is gradually formed.
Female animals, which are characterized by external fertilization, simultaneously release a large number of eggs into the water. For example, fish toss several thousand eggs at once. Only a small part of them will be fertilized and turn into fry. The rest will become prey for aquatic animals.
What is the advantage of internal fertilization?
Internal fertilization of animals occurs in the female reproductive ducts. There is a fixed egg. A sperm approaches her as a result of sexual intercourse. It has already been established that in most cases only the nuclear material of the male gamete penetrates the egg. Its cytoplasm practically does not participate in the process of formation of a new organism.
The main advantage of internal fertilization is that the fetus is relatively independent of adverse environmental conditions. Its development a certain time occurs inside the mother's body. It provides the embryo with everything necessary: heat, moisture, oxygen, nutrients. In addition, during fertilization inside the body, the probability of gamete fusion increases significantly, which determines the stability of the reproduction process in such individuals. For these reasons, the number of female gametes capable of fertilization is much less than animals that release them into the environment.
A single egg is fertilized by one sperm. But why do many organisms give birth to several individuals, or even tens? This is possible in two ways. In the first case, several eggs go to fertilization at once, each of which connects to a separate male gamete. In this case, a man with opposite twins is born . They can be of the same or different sexes, and they are no more similar to each other than siblings. The identical twins appear as a result of dividing the zygote into several parts. In this case, a person gives birth to children of the same sex, similar to each other as two drops of water.
Sexual reproduction of plants
In flowering plants, fertilization also occurs - internal. Representatives of this systematic unit have a number of features that appear during the sexual process. It is carried out by a generative organ - a flower. The process of merging gametes is preceded by pollination. Its essence lies in the transfer of male germ cells to the stigma of a pestle using wind, insects, water or humans.
Double fertilization
Next, two sperm together with a germinating germ tube fall into the lower expanded part of the pestle - the ovary. This is where the fusion of one sperm with a female gamete occurs, and the other with a central germ cell. Therefore, such fertilization is called double. As a result, an embryo is formed surrounded by a reserve nutrient endosperm and membrane. In other words, the seed.
This process has provided modern flowering plants with a dominant position on the planet. The egg and the germ are protected by the walls of the ovary, and the seed contains the necessary supply of nutrients and water necessary for the growth and development of an adult plant.
Type of fertilization and animal habitat
It is easy to trace the dependence of the living environment of organisms and the type of fertilization. Thus, the fusion of gametes in the external environment occurs in water, where the embryo of organisms with external fertilization initially develops. Moreover, this process is possible only in a neutral or alkaline environment, and in acidic it becomes impossible.
The emergence of internal fertilization in the process of evolution is associated with the emergence of chordates on land. The life of representatives of this type outside the water became possible precisely because of this sign. In vitro fertilization in reptiles takes place inside the body of the female, where the embryo initially develops. It is located in an egg, which contains a rich supply of substances and in most is covered with dense shells. An increase in the amount of yolk compensates for the absence of the larval stage in the ontogenesis of reptiles. And the appearance of dense shells makes possible the development of eggs on land and reliably protects from drying out and mechanical damage.
Ontogenesis of multicellular animals
The zygote, which is formed as a result of fertilization, begins to repeatedly divide. After a certain time, it already consists of a number of cells - blastomeres. Next, the stage of gastrula begins, which is characterized by the laying of embryonic leaves. The development of the embryo continues with the formation of organs and their systems.
Individual development of multicellular animals includes embryonic and postnatal periods. In organisms with internal fertilization, the first of them occurs in the mother's body or inside the egg. This provides a higher level of development of animals, as well as their ability to live independently after birth. From the moment of birth, the postembryonic period begins. Fertilization, internal or external, determines the future type of development of organisms. In the first case, it occurs without a larval stage. In this case, the newborn individual is slightly different from the mature one. This type of development is called direct. But fish and amphibians go through the larval stage, during which they additionally develop, reaching the level of organization of adult representatives.
So, internal fertilization is the process of fusion of germ cells within the body of a female. Compared to the outside, it has a number of significant advantages: a higher probability of gamete fusion, their independence from external conditions and ensuring a higher viability of future individuals.