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 read 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 characteristics of organisms. It is this process that underlies heredity and variability.
Sexual reproduction is called, during which gametes take part. These are specialized cells containing a haploid chromosome set. In nature, both plants and animals can carry it out.
Structure of gametes
The process of gamete fusion is fertilization. Internal or external fertilization is carried out only by germ cells. There are male and female gametes - sperm and eggs. They have significant differences in structure. Thus, the female germ cells are notable to move 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 sex cells of plants are also not capable of movement, so 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 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 is the connection of the genetic material of germ cells. Depending on the place in which this process takes place, there are several types of it. External fertilization takes place outside the body of the female. In nature, it is found in amphibians and fish. Internal 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 to the outside. Therefore, the convergence of organisms in this case is not at all necessary. Despite this, in nature, an accumulation of breeding individuals is often found. For example, fish or frogs during spawning.
External fertilization, internal or intermediate type begins with the process of insemination. Its essence lies in the convergence of germ cells. Scientists have found that during external fertilization, almost immediately after the contact of the cells, changes occur in the electrical impulses of the egg membranes. And after 7 seconds, the contents of the gametes are already combined, resulting in the formation of a zygote. It divides many times and gradually forms a multicellular embryo.
Female animals, which are characterized by external fertilization, simultaneously release a large number of eggs into the water. For example, fish throw several thousand eggs at once. Only a small part of them will be fertilized and turn into fry. The rest will become the prey of aquatic animals.
What is the advantage of internal fertilization
Internal fertilization of animals occurs in the genital ducts of the female. This is where the immobile egg is located. The spermatozoon approaches her as a result of sexual intercourse. It has already been established that in most cases only the nuclear substance 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 unaffected by adverse environmental conditions. Its development for 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. ByFor these reasons, the number of female gametes capable of fertilization is much smaller compared to animals that release them into the environment.
A specific egg is fertilized by one sperm. But why do many organisms give birth to several individuals at once, or even dozens? This is possible in two ways. In the first case, several eggs come out for fertilization at once, each of which is connected to a separate male gamete. In this case, fraternal twins are born in a person. They can be of the same or different sexes, and they are no more similar to each other than siblings. Identical twins result from the division of the zygote into several parts. In this case, a person gives birth to children of the same sex, similar to each other like two drops of water.
Sexual reproduction of plants
Flowering plants also undergo fertilization - internal. Representatives of this systematic unit have a number of features that manifest themselves during the sexual process. It is carried out by a generative organ - a flower. The process of fusion of gametes is preceded by pollination. Its essence lies in the transfer of male germ cells on the stigma of the pistil with the help of wind, insects, water or a person.
Double fertilization
Further, two spermatozoa, together with the germinating germ tube, descend into the lower expanded part of the pistil - the ovary. This is where the fusion of one sperm with the female gamete occurs, and the other with the central germ cell. Therefore, suchfertilization is called double. As a result, an embryo is formed, surrounded by a reserve nutrient, the endosperm and the shell. In other words, a seed.
This process has given modern flowering plants a dominant position on the planet. The egg and embryo are reliably 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 habitat of animals
It is easy to trace the dependence of the habitat of organisms and the type of their 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 an acidic one 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 thanks to this feature. Internal fertilization in reptiles occurs inside the body of the female, where the embryo initially develops. It is found in the egg, which contains a rich supply of substances and most are covered with dense shells. The increase in the amount of yolk compensates for the absence of the larval stage in the ontogeny of reptiles. And the appearance of dense shells makes it possible to develop the egg on land and reliably protects against drying and mechanical damage.
Ontogeny of multicellular animals
The zygote, which is formed as a result of fertilization, begins to divide many times. After a certain time, it already consists of a number of cells - blastomeres. Next, the gastrula stage begins, which is characterized by the laying of germ layers. The process of development of the embryo continues with the formation of organs and their systems.
The individual development of multicellular animals includes the embryonic and post-embryonic periods. In organisms with internal fertilization, the first fertilization occurs in the mother's organism or inside the egg. This ensures 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. At the same time, the newborn individual differs slightly from the mature one. This type of development is called direct. But fish and amphibians go through the larval stage, during which they further develop, reaching the level of organization of adult representatives.
So, internal fertilization is the process of fusion of germ cells inside the body of the 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.
