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Development of the human organism

Development of the human organism - ontogenesis - is divided into two periods: prenatal (intrauterine) and postnatal (extrauterine). Utero period lasts from the moment of fertilization and to the birth of a child and also consists of two phases: embryonic (first 2 months) and fetal (3 to 9 months). At the moment of fertilization male germ cell - spermatozoon (sperm) - penetrates into the female sex cell - ovule (oocyte), causing appears a zygote. Fertilization usually occurs in the fallopian tube. Zygote (single-celled embryo) has all the properties of both gametes.

1st week of embryonic development - is crushing, in which one of the cells formed many cells - blastomeres that form multicellular blastula. Crushing lasts 3-4 days in the fallopian tube, and then continues in the uterus. By the end of the 1st week formed a bubble - blastocyst, which consists of internal group of cells - Embryoblast and peripheral group of cells - the trophoblast (lines the surface of Embryoblast). Between embryoblast trophoblast and a cavity filled with mesenchyme. The embryo begins to take root in the mucous membrane of the uterus (implantation). Trophoblast cells secrete the enzyme, loosening the surface layer of the endometrium and give outgrowths - villi and turn into villous membrane - chorion. From the chorion and endometrium neighboring to it developing placenta. From embryoblast formes the embryo.

At the 2nd week of the germ life the cells of embryoblast divided into two layers, from which are formed two bubbles: the outer layer of cells formed ectoblastic (amniotic) bubble; the inner layer of cells - entoblastic (yolk) bubble. During this period, the embryo consists of two layers: the outer - ectoderm and inner endoderm. 3rd week of the embryo life – is a period of gastrulation and formation of mesoderm and also chord formation.

After 4 week of of the embryo life begins the formation of organs primordia. Entodermal layer of the embryo forms a tube - initial intestine closed in the front and rear sections. Outside of the embryo remains the yolk sac connected to the initial intestine.

After 8-th week organs ending laying. On the 9th week embryo takes the form of a man. A prolific period lasts until such time as baby is born. This period is characterized by the organs formation (organogenesis).

Postnatal period lasts from the birth to the death of a man and is divided into periods:
- Newborn (1-10 days);
- Chest (10 days - 1 year);
- Early childhood (1-3 years);
- First childhood (4-7 years);
- The second childhood (8-12 years);
- Teenage (puberty) (13-16 years);
- Youth (17-21 years);
- Mature - 1st period (22-35 years)
- 2nd period (36-60 years)
- Elderly (61-74 years);
- Old (75-90 years);
- Longevity (90 years and over).
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