The embryonic period, called embryogenesis, originates from the connection of the nuclei of the female and male germ cells and is a process of fertilization. So, in those organisms that are characterized by intrauterine development, embryogenesis ends in birth, in organisms with a larval type development ends in exit from the embryonic membranes.
The embryonic period of development has several stages:
1.Zygote. During fertilization, the male reproductive cell, reaching the egg, provokes its development. Chemical and physical processes begin to occur in it, which contribute to the formation of egg symmetry, the elimination of the membranes of the nuclei, as a result of which, the nuclei of two cells are connected, and DNA is formed.
2. Crushing (the first stage of development of the zygote) - division of the zygote begins. In the ovum, which moves along the fallopian tube, grooves form, due to which cell division occurs. The cells formed in this way are called morula. All multicellular organisms that reproduce sexually pass through this stage; only the process of cell division (radial, bilateral, spiral) is different.
A feature of cell division is that they do not grow. This process involves the formation of a single large cell (egg) of a large number of small cells, with less cytoplasm near the nuclei.
The embryonic period does not end there, consider the following stages of embryo development .
3. Blastula (the formation of a multicellular structure in the form of a bubble) - consists of a layer of cells called embryonic. The size of the blastula approaches the size of the egg, therefore, when dividing cells, the number of nuclei and DNA increases.
4. Gastrulation - the stage of movement of embryonic cells, resulting in the formation of three layers of germ layers . This stage is characterized by an increase in the synthesis of proteins and ribosomes, during this period the pole (vegetative) extends into the blastula, the opposite poles are connected, and the blastula cavity is eliminated. In this case, a new cavity is formed, which is called the blastopore or primary mouth.
Thus, gastrulation is a necessary moment in the development of the embryo, since the embryonic period at this stage allows the formation of its organs and tissues, as well as body systems.
It should be noted that the formation of tissues and organs of the embryo in different periods have different sensitivity to the damaging effects of the environment, for example, to infections, radiation or chemical agents. These periods of increased sensitivity are called critical, there is an increased likelihood of deviations.
So, the embryonic period has several critical points. Consider them in more detail:
1. The blastula period (the first two weeks after conception) - the embryo either dies or continues to develop without deviations. At this time, a large number of embryos die (40%), which began their development from mutated germ cells.
2. From the twentieth to the seventieth day after fertilization - the period of greatest vulnerability of the embryo, since all vital organs begin to form and form.
3. The fetal period is characterized by rapid growth of the fetus. Here quite often violations of its development can occur only in those organs that have not completed their formation.
Thus, the embryonic period of ontogenesis is characterized by the formation and development of the embryo through cell division, the formation of tissues, organs and systems. In various living organisms, this period varies in time, but in any case, it begins from the moment of conception and ends with the birth of a new life.