The life of every living being is made up of certain biological periods and stages. In general, there are two main ones - embryonic and postembryonic. What each of them is, what it is based on and how it is characterized, will be discussed in this article.
Understanding Germ Education
Before an organism is born and its life begins, it is necessary that it be formed in the womb of the parent being (most often maternal, there are also exceptions - hermaphrodites).
For this there is a natural process - fertilization. It consists in the confluence of male and female sexual gametes (cells), as a result of which a zygote will be formed - an undecided embryo. This fertilized egg will be the beginning of the embryonic period in the development of a new life.
Fertilization options can be different, because there are a lot of living beings, and all of them are so unique, unusual and variable in external and internal features, that it remains only to be surprised by nature.
There are three main ways:
- internal fertilization - occurs strictly inside the body of the female;
- external - sex products are carried to the external environment and are found there;
- the hermaphroditic genus - both male and female genital ducts are poured into a single channel within a single organism without a specific genus.
For plants, the process of fertilization is generally special. In flowers, it is called double and was discovered by the Russian scientist Navashin in 1898. The spore ones are necessarily looking for water to bring about this phenomenon, and the gymnosomes are not in a hurry with it at all. Their germ is formed about two years.
However, as soon as the zygote was formed, a new stage in the development of the future individual, the embryonic, began. It is also heterogeneous and consists of several stages.
Embryonic stage of development: a characteristic
This is a very important period, because it is in it that all organs and systems are laid down, the features of the future organism are determined. It begins with the formation of zygotes, and ends with the moment of birth to light.
During this time, the embryo goes through a series of successive stages:
- cleavage of the zygote by the mitotic pathway;
- blastula - a hollow ball inside of two layers of cells;
- gastrulation - the formation of the embryo itself with two layers of cells lining it inside and outside;
- neurula - the formation of three germ layers and the laying of the neural tube;
- the formation of organs and their systems in the future individual.
Obviously, then comes the time of growth and development of the embryo itself. This continues until he reaches a certain size, gains strength for birth, is ready for life in a different, outdoor environment. After all, further it is awaited by postembryonic development, which proceeds in completely different conditions and according to different rules.
Birth and its types
The process of birth, or childbirth, is complex and very difficult, but quite natural. This is the transitional moment from the embryo to the time when the postembryonic stage begins. The bottom line is to push the fetus out of the mother's womb together with the afterbirth, in which it has been all this time. If we talk about animals, then here we can also include the ejection of eggs from the cloaca, live birth following the example of man, the laying of larvae by insects.
Types of birth in different organisms:
- egg laying;
- ejection of the capsule with the larvae;
- the introduction of the larva under the skin of the intermediate host;
- live birth
Each species has its own way. However, it is most often found live birth - in birds, mammals, some reptiles, humans.
Postembryonic period of development
As already mentioned above, it originates from the moment the baby is born into the light or from its release from the egg walls, larval film, cocoon, and other things. Since this time, postembryonic development can go in different ways. In total, two main paths are indicated:
Which of them will be chosen depends on the genetic characteristics of each particular species. For example, many insects choose the second path, since it allows them to overcome major environmental and biological problems.
For mammals, birds, humans, the first option is more acceptable, which includes the future care of emerging offspring.
The direct postembryonic development of an organism consists in the appearance after birth of an individual, completely repeating the maternal in external and internal features, only smaller in size and with not fully developed organ systems.
In other words, a reduced copy of the original individual is born. The cub leads the same lifestyle, eats the same foods as the parents. They are the ones who in the course of life will teach the baby everything that is useful to him for maximum adaptability to environmental conditions. And also in order to occupy a niche in nature.
Such postembryonic development of characteristic birds, reptiles, mammals. One can cite many examples to prove it. No lioness will leave her cubs until, until they begin to independently produce their own food. Birds are in their nests with food for the Chicks that just hatched from the nest, just want to eat. Before they hatch the offspring, providing the eggs for protection and optimum temperature.
Indirect postembryonic development also has another name - with metamorphosis. This word translates as “transformation”, “transformation”. This is what happens. An appearing animal does not immediately become the same as its parents. At first it is in a completely different, larval stage. Most often, such transformations are characteristic of different types of insects. Rarely when an organism identical to an adult leaves the larva immediately.
Most often, it is completely different, not only in terms of appearance, but also in the way of life, the way and type of food, and the occupied territory. The reasons for this phenomenon are many. One of the main ones are:
- preservation of the number of species, as the competition for food goes away;
- different habitats and living conditions, which contributes to a wider area of settlement.
Indirect postembryonic development of organisms is characteristic of:
- invertebrates (insects, sponges, intestinal cavities) animals;
- cyclostomata (lamprey, for example);
- some types of fish;
This is a very convenient and profitable transformation created by nature. At a certain stage of development, the larva turns into the original parental form. But before that, its main task is nutrition and accumulation of substances for a successful metamorphosis.
A dramatic example of this is dragonflies, in which the imago and the larva are very different from each other. It is known that it is difficult to meet a more cruel and voracious predator than the young cubs of these insects. They are capable of killing individuals that exceed them in size and mass.
Amphibian larvae have gills and live only in the aquatic environment, while adults breathe with the integuments of the body and lungs, and their living environment is twofold.
Butterflies are generally radically different in the stages of their postembryonic development. After all, the caterpillars look completely different than the beautiful adult insects that everyone admires.
Postembryonic period: juvenile
In addition to the classification that we have already given, one more should be indicated. The postembryonic development of organisms includes three stages:
The first, juvenile, begins from the moment of birth and continues until full maturation, including sexual. The main feature is growth and development, and it is very active. If we talk about people, this is the period of childhood, which lasts until the end of adolescence.
It is very important at this stage to obtain a sufficient amount of nutrients. In the wild, this is also the moment of gaining valuable knowledge from parents and how to live in the outside world. Mammals teach cubs to walk, to hunt, to choose food, to care for themselves.
This postembryonic period occupies the main part of life in most species of creatures. After all, it includes the entire conscious and meaningful stage of maturity. In humans, this is the age within about 16 to 55-60 years. Of course, each is different. After all, each individual has its own genetic features, so it is difficult to define clear boundaries.
During this period, the main life stage passes, including the reproduction of offspring and its upbringing and cultivation.
Aging and death
This is the final stage in the life of every living being. If we talk about invertebrates, reptiles and other primitive animals, then for many people aging is a period immediately after the birth of offspring. After all, the purpose of existence is in this process.
Mammals and people fully understand the essence of this stage. After all, aging occurs gradually, death also occurs naturally, not instantaneously (if we talk about normal natural options). What happens in the body during this period?
- The metabolic process is changing.
- The work of the brain and the sense organs is getting worse.
- Exacerbated chronic and other diseases.
- The body is preparing for the cessation of life.
This period completes the postembryonic development of animals. The same applies to man.
The life cycle of different types of insects
Indirect postembryonic development, as we have said, is most characteristic of insects. Among invertebrates you can find many examples confirming this phenomenon:
In this case, the transformation into an adult is not the same for everyone. There are creatures that do not fully implement it. Such a metamorphosis is called incomplete. The majority turns into parents rather quickly and completely.
The development of animals after birth
Mammals – particular in relation to post-embryonic development. They are able, as people, to care for offspring, to feed him milk, to train and educate. One can think of the kangaroo, hiding in mom's purse, when he was already quite skilled runner and jumper.
Among the bears, elephants, wolves, the unspoken law is always preserved - none of his own will touch the kid, and if he loses his parent, it means that his relatives will replace him. This confirms the fact that nature is very wise, correctly and correctly sets priorities. After all, if we talk about large mammals, the offspring rarely reaches five pups. Therefore, it is very important for them to preserve and grow them.
But amphibian females, fish, worms, insects can lay millions of eggs, eggs and so on. Therefore, it does not make sense to care. Of such a number of offspring, many survivors will remain.