The development of the fetus inside the womb of a woman consists of three periods.
Stages of embryonic development.
The first period begins after penetration into the uterine cavity of the ovum and continues throughout the first week. At this time, human embryonic development includes the laying of primary organs, the formation of extra-embryonic systems, which are subsequently called upon to ensure the existence of the fetus until the complete formation of all systems and organs.
The primary fixation in the uterus of the fetal egg is followed by its active growth. Cell layers are formed - the basis from which internal organs will subsequently be formed. An egg receives nutrition by dissolving cells in the uterine mucosa by superficial embryonic cells. Thus, the fetus moves deeper into the cavity and is fixed inside it. Full accession, as a rule, ends on the twelfth day after fertilization.
Further, the cell mass begins to increase significantly, it is divided into two sheets: the inner (endoderm) and the external (ectoderm) germinal leaf. After the third week of gestation, a mesoderm is formed - the third leaf, located between the other two. The development of the human embryo, in particular its internal organs, will subsequently occur from these embryonic leaves.
Ectoderm is the basis for the formation of tooth enamel, upper skin layers, peripheral (nodes) and central (spinal and brain) nervous systems. The sensory organs (vision and hearing, olfactory and taste areas of the body) are also laid.
The inner cell layer (endoderm) is the basis for the formation of the respiratory and digestive systems.
Mesoderm is involved in the formation of musculoskeletal, cardiovascular, excretory and reproductive systems.
Human embryonic development in the first period includes cell distribution. One part of them forms the chorion - the primary fetal membrane. The chorion is covered with small villi, which subsequently grow into the uterine mucosa and, thus, form the placenta. Another part of the cells is involved in the creation of extra-germ organs.
Human embryonic development passes into the second phase from the second week. This period is called germinal. This stage continues after two months of gestation. During this period, the formation of all vital internal organs takes place.
Primary blood vessels also form at this time. Cells move from the head region of the embryo to the part between the inner and outer cell layer. As a result, the primordium of the chord is formed.
An important point during the second period is the formation of the placenta from the chorion.
From the ninth week of gestation and until the very birth of the child, the third - fetal - period lasts. Human embryonic development at this time is characterized by the achievement of full maturation of all formed structures of the body.
From the ninth week, the endocrine system begins to develop . Enhanced production of hormones necessary for growth begins. Especially important is insulin. After the third month, the maturation of the placenta ends, which also begins to produce its own hormones.
There is a development of the brain, the ripe sections of which are involved in the first movements of the fetus.
The formation of the immune system ends by the eighteenth week of fetal development. In the child's body, the production of white blood cells is activated. The components that make up the innate immune system of the fetus are formed - interferon and immunoglobulin. The spleen begins to work.
The most important changes in the pulmonary system occur at the twenty-fourth week. Terminal sacs are formed, with the help of which breathing becomes possible.
At the seventh month of gestation, the muscles and bones of the body grow, thus, the child’s body becomes larger than his head.
During the ninth month, an increase in body weight occurs actively. The child in this period becomes completely full-term and mature.