Prenatal development is the process through which the fetus develops from conception to become a complete organism.
When fertilization took place a zygote is formed about 12 hours after the cell has been divided first. After a number of cell divisions a ball of cells called morula is formed, which then develops into a blastula.
Already in the first days cells divide differently and for different cleavage patterns, some do it quickly to become a placenta and other not as fast for an embryo. Then blastula becomes a round disc with two layers form a group of cells with a growth center between the layers. From this center, a cell sent string out right over the plate. The embryo gets a spine string or prospective spine. The skin above the hardens and fold into a ditch, a nerve ditch.
The second layer begins to divide into body segments. Arms and legs begin to arrive at its sides. Then the tissue inside the body swells out and thickens into the skin. This skin will come later, according to a definite order to form the inner tissues of the fingers, toes, underarm, and more. The design of the bar troa come from a particular model in a gelatinous mass, ground substance, which fills all space. The molecules in the ground substance control the joints, for example, nose, and forehead.
In twelve streams along the trunk sides, there are the cells that will become ribs. They meet in the middle of the chest, where they form a sternum. Cells to become muscle grow between the ribs and in the body wall below. Embryo’s outermost layer of cells begins to make epidermis, where it will eventually become hair roots, sebaceous glands, and sweat glands.
The embryonic period in humans is calculated from second to eighth week, when the fetal stage begins.
At this stage the organogenesis (organ formation) and embryogenesis mainly occurs. After embryogenesis primarily growth occurs, but also the refining of the system. Between the eleventh and twelfth week of pregnancy the fetal weight increases by almost 60%.
Some early organ, such as Arteria Hyaloidea eyes, regresses later during fetal development.
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