Embryonic period, called embryogenesis originates from the compound nuclei of female and male germ cells and is the process of fertilization.For those organisms that tend to intrauterine development, embryogenesis ends with the birth, in organisms with the type of larval development - leaving the embryonic membranes.
Embryonic development period has several stages:
1.Zigota.At fertilization male germ cells, reaching the egg, provoking its development.It begin to happen chemical and physical processes that contribute to the formation of the symmetry of the egg, the elimination of nuclear membranes, resulting in the nuclei of the two cells are connected and formed DNA.
2. Crushing (the first stage of the zygote) - begins the division of the zygote.The egg that is moving through the fallopian tube, formed furrows, making cell division occurs.Formed in this way cells called morula.This stage are all multicellular organisms that reproduce sexually, only different is the process of cell division (radial, bilaterally spiral).
Feature dividing cells is that they do not grow.This process involves the formation of one large cell (egg) a large number of small cells with fewer cytoplasm near the nucleus.
Embryonic period does not end, consider the following stages of embryo development.
3. Blastula (formation of multicellular structures in the form of a bubble) - consists of a layer of cells, which are called embryonic.Size blastula approaching egg size, so during cell division, it increases the number of nuclei and DNA.
4. Gastrulation - stage movement embryonic cells, resulting in formation of the three germ layers of sheets.This stage is characterized by an increase in the synthesis of proteins and ribosomes, in this period there is protrusion of the pole (vegetative) into the blastula, the opposite poles are connected, and the cavity of the blastula is liquidated.At the same time, a new cavity, called the blastopore or primary mouth.
Thus, gastrulation is a necessary factor in the development of the embryo, as embryonic period at this stage allows the formation of its organs and tissues, and body systems.
Note the formation of tissues and organs of the embryo at different periods have different sensitivity to damaging environmental effects, such as infection, radiation, or chemical agents.These periods of hypersensitivity known as critical, are more likely to develop abnormalities.
So, embryonic period has several critical moments.Let us examine them in more detail:
1. blastula period (the first two weeks after conception) - or the embryo dies, or continues to evolve without deviation.At this time, killed a large number of embryos (40%), which began its development of mutated germ cells.
2. With seventy twentieth day after fertilization - the period of greatest vulnerability of the embryo as starting to take shape, and laid all the vital organs.
3. The fetal period is characterized by the rapid growth of the fetus.It often can occur violation of its development only in those bodies who have not completed their formation.
Thus, the embryonic period of ontogenesis is characterized by the formation and development of the embryo by cell division, the formation of his tissues, organs and systems.In different organisms different in this period of time, but in any case, it begins from the moment of conception and ends with the birth of a new life.
