Embryonic period of development

The embryonic period, called embryogenesis, originates from the union of the nuclei of the female and male germ cells and represents the process of fertilization. Thus, in those organisms that are prone to intrauterine development, embryogenesis ends in birth, in organisms with a larval type of development - an exit from the embryonic membranes.

The embryonic period of development has several stages:

1. The death. When fertilized, the male reproductive cell, reaching the ovum, provokes its development. It begins to occur chemical and physical processes that contribute to the formation of symmetry of the egg, the elimination of nuclear membranes, resulting in the nucleus of two cells are connected, and DNA is formed.

2. Crushing (the first stage of the development of the zygote) - begins the division of the zygote. In the ovule, which moves along the fallopian tube, grooves are formed, due to which the cells are divided. The cells formed in this way are called morulae. This stage is passed by all multicellular organisms that reproduce sexually, 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 large cell (egg) a large number of small cells, with less cytoplasm near the nuclei.

The embryonic period does not end there, we will 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, which are called embryonic. The size of the blastula is close to the size of the egg, so when dividing the cells, the number of nuclei and DNA increases.

4. Gastrulation - the stage of the movement of embryonic cells, resulting in the formation of three layers of embryonic sheets. This stage is characterized by an increase in the synthesis of proteins and ribosomes, during this period, the pole (vegetative) extends inside the blastula, the opposite poles unite, and the blastula cavity is eliminated. This creates a new cavity, which is called the blastopore or primary mouth.

Thus, gastrulation is a necessary moment for embryo development, since The embryonic period at this stage allows the formation of its organs and tissues, as well as the systems of the body.

It should be noted that the formation of tissues and organs of the embryo at different periods have different sensitivity to the damaging effects of the environment, for example, to infections, radiation or chemical agents. These periods of hypersensitivity are called critical, here the probability of developing deviations increases.

So, the embryonic period has several critical moments. Let's consider them in more detail:

1. Period of blastula (the first two weeks after conception) - the embryo either perishes, or continues to develop without deviations. At this time, a large number of embryos (40%) die, which began to develop from mutated germ cells.

2. From the twentieth to the seventieth day after fertilization, the period of the most vulnerable embryo, as all the vital organs begin to be laid and formed.

3. Fertile period is characterized by rapid growth of the fetus. Here, quite often, violations of its development can occur only in those bodies that have not completed their formation.

Thus, the embryonic period of ontogenesis is characterized by the formation and development of the embryo by dividing the cells, forming tissues, organs and systems in it. In different 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.