The life of every living being consists of certain biological periods and stages. In general, there are two main ones - embryonic and postembryonic. What each is, what is based and how is characterized, we consider in this article.
General idea of the formation of the embryo
Before the organism is born and begins its life, it is necessary that it is formed in the womb of the parent (most often the mother, there are exceptions - hermaphrodites).
For this, there is a natural process - fertilization. It consists in the fusion of male and female sex gametes (cells), as a result of which a zygote is formed - a still undefined fetus. This fertilized egg will become the beginning of the embryonic period in the development of a new life.
Variants of fertilization can be different, because there are a lot of living things, and all of them are so unique, unusual and varied in their external and internal characteristics, which only surprises nature.
There are three main ways to distinguish:
- internal fertilization - occurs strictly within the body of the female;
- external - the sex products are taken out into the external environment and there are found;
- hermaphrodite genus - both male and female sexual ducts are poured into one channel inside a single organism without a certain genus.
For plants, the process of fertilization is special. In flowering plants it was called double and was discovered by the Russian scientist Navashin in 1898. Spore is necessarily looking for water for the implementation of this phenomenon, and gymnosperms with him generally do not hurry. Their embryo is formed about two years.
Nevertheless, as soon as the zygote was formed, a new stage in the development of the future individual is beginning - the embryonic one. It is also heterogeneous and consists of several stages.
Embryonic stage of development: characteristic
This is a very important period, as it is the place where all organs and systems are laid, the features of the future organism are determined. It begins with the formation of the zygote, and ends with the moment of birth.
During this time, the embryo passes through a series of successive stages:
- fragmentation of the zygote mitotic;
- blastula - hollow inside a sphere of two layers of cells;
- gastrulation - the formation of the fetus itself with two layers of cells lining it inside and out;
- neurula - formation of three embryonic leaves and laying of the neural tube;
- the formation of organs and their systems in a future individual.
Obviously, then comes the time of growth and development of the embryo itself. This continues until it reaches a certain size, builds up the strength for birth, will be ready for life in another, outdoor environment. After all, it is already awaited by post-embryonic development, which takes place in completely different conditions and according to other rules.
Birth and its types
The process of birth, or birth, is complex and very difficult, but quite natural. This is the transitional moment from the embryo by the time the postembryonic stage comes. The essence is to push the mother of the fetus out of the womb together with the afterbirth in which he was all this time. If we talk about animals, then here you can include pushing eggs out of the cloaca, live birth following the example of a man, larvae postponing insects.
Types of birth in different organisms:
- laying of eggs;
- ejecting capsule with larvae;
- introduction of the larva under the skin of the intermediate host;
- live birth.
Each species has its own way. However, the most common occurrence is live birth - in birds, mammals, some reptiles, humans.
Post-embryonic period of development
As already mentioned above, it originates from the moment of appearance of the cub to the light or from its exit from the egg walls, larval film, cocoon and other things. Since then, post-embryonic development can go differently. In total, there are two main ways:
Which of them will be chosen depends on the genetic characteristics of each particular species. For example, many insects choose the second way, as it allows to overcome the main ecological and biological problems.
For mammals, birds, humans, the first option is more acceptable, including the future care of the emerging offspring.
Direct postembryonic development of the organism consists in the appearance after birth of an individual completely repeating the external and internal signs of the mother, only smaller in size and with not fully developed organ systems.
In other words, a reduced copy of the original specimen appears. The baby leads the same lifestyle, consumes the same foods as the parents. It is they who in the course of life will teach the kid everything that is useful to him for maximum fitness to the conditions of the environment. And also to take your 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 is translated as "transformation", "transformation". This is what happens. An emerging animal does not immediately become the same as his parents. First it resides in a completely different, larval stage. Most often, such transformations are typical for different types of insects. Rarely, when the body immediately emerges from the larva, identical to the adult.
Most often, it is completely different, not only in appearance, but also in the way of life, the way and type of nutrition, the occupied territory. There are many reasons for this phenomenon. Some of the main ones are:
- preservation of the number of species, as competition for food goes away;
- different habitats and conditions of existence, which contributes to a wider area of settlement.
Indirect postembryonic development of organisms is typical for:
- invertebrates (insects, sponges, coelenterates) animals;
- cyclostomes (lampreys, for example);
- some species 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 successful metamorphosis.
Very bright example can serve as dragonflies, in which the imago and the larva are sharply different from each other. It is known that it is difficult to meet a more cruel and voracious predator than the small cubs of these insects. They are able to kill individuals that exceed them in size and weight.
The larvae of amphibians have gills and live only in the aquatic environment, while adult individuals breathe body covers and lungs, and the environment of their life is double.
Butterflies in general are radically different in the stages of their postembryonic development. After all, the caterpillars look quite different than the adult beautiful insects, which are so admired by everyone.
Post-embryonic period: juvenile
In addition to the classification that we have already mentioned, it is necessary to indicate one more. Postembryonic development of organisms includes three stages:
The first, juvenile, begins with the moment of birth and continues until full maturation, including sexual maturity. The main feature is growth and development, and very active. If we talk about people, then this is the period of childhood, which lasts until the very 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 obtaining valuable knowledge from parents and how to live in the surrounding world. Mammals teach young people to walk, hunt, choose food, take care of themselves.
The pubertal period
This postembryonic period in most species of creatures occupies the main part of life. After all, it includes the entire conscious and meaningful stage of maturity. In humans, this age ranges from about 16 to 55-60 years. Of course, everyone has different ways. After all, each individual has its own genetic characteristics, so it is difficult to identify clear boundaries.
During this period, the main life stage passes, including reproduction of the offspring and its upbringing and rearing.
Aging and death
This is the final stage in the life of every living being. Speaking of invertebrates, reptiles and other primitive animals, many have aging - the period immediately after the birth of the offspring. After all, the purpose of existence is precisely in this process.
Mammals and people fully understand the essence of this stage. After all, aging is gradual, death also comes naturally, not instantly (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, sensory organs is deteriorating.
- Chronic and other diseases worsen.
- The body is preparing to stop life.
This period completes the postembryonic development of animals. The same applies to man.
Life cycle of different insect species
Indirect postembryonic development, as we have already said, is most characteristic of insects. Among invertebrates, you can find many examples that support this phenomenon:
In this case, the transformation into an adult does not all occur in the same way. There are beings who do not fully realize it. This metamorphosis is called incomplete. Most of them turn into parents quite quickly and completely.
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, there is always an unspoken law - a kid will not be touched by any of his own, and if he loses his parent, then he will be replaced by his relatives. This is confirmed by the fact that nature very wisely, correctly and competently sets priorities. After all, if we talk about large mammals, the offspring rarely reaches five cubs. Therefore it is very important for them to preserve and grow them.
But female amphibians, fish, worms, insects can lay millions of eggs, eggs, and so on. Therefore, care has no meaning. Of this number of offspring, there will still be many survivors.