Development of the Embryo
Although the embryonic period comes before a bulging belly, kicking fetus and other exciting happenings of later pregnancy, it sees some of the most amazing milestones of infant development in the womb. Starting when the developing embryo implants onto the wall of the uterus and lasting until the start of the fetal period at three months, the embryonic stage is a crucial period for the development of many body organs.
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Days After Implantation
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During the beginning of the embryonic period, a membrane called the amnion forms and surrounds the embryo. The amnion contains cushioning liquid that allows the embryo to move and supports it. The chorion, another membrane, lies right outside of the amnion. It will later form the placenta, an organ responsible for separating the infant from the mom and nourishing it. Right after the embryo implants into the uterine wall, its cells separate into three layers: the ectoderm, mesoderm and endoderm. Each layer will later form into different parts of the body. In fact, the beginnings of the brain and spinal chord begin developing from the ectoderm almost right away.
Week Three
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An infant develops very rapidly during the embryonic phase. By the third week after conception, the heart has already formed and begins beating. At this point, the embryo contains the beginnings of blood vessels and blood cells. In addition, the brain begins to divide into its three distinct sections: the hindbrain, the midbrain and the forebrain. The embryo develops according to two patterns: body parts closer to the head form before those lower down, and systems in the middle form before systems on the outside.
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Week Four
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By the fourth week, arm buds and leg buds can be seen and the embryo has developed vertebrae of the spine. The major veins and arteries have finished, but the eyes, ears and digestive system are just beginning to form.
Week Five
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As the chorion turns into the placenta, the umbilical chord develops and becomes a gateway between the placenta and the embryo. The hand plates are formed during week five, and the precursors to lungs, called "bronchial buds" take shape. Masses which will eventually become muscles are present in the head, middle of the body and limbs of the embryo.
Week Six
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By the sixth week, the head becomes the largest part of the developing body. Ears on the outside of the head are visible for the first time, and the division of the brain into its three parts is completed. Up until this time, the lower jaw had been composed of two halves, but they now join together.
Week Seven
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Week seven sees some major changes. For instance, both the face and the neck start to take shape. In addition, the embryo's eyelids begin to form. Muscle tissue rapidly separates into different types of muscle, and these muscles take on their ultimate shapes and functions throughout the body. The brain grows thousands of nerve cells each minute, and the stomach assumes its final shape and position.
Week Eight
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Up until the eighth week, embryos have not had sex characteristics or organs--both boys and girls have "gonadal ridges." However, at this time, genetics cause the gonadal ridges to turn into male gonads in boys. These male gonads begin producing male hormones, such as testosterone. The testosterone, in turn, suppresses a gland called the pituitary and begins to create male sex organs and features. On the other hand, the absence of testosterone in girls allows the pituitary gland to begin creating a rhythmic cycle that will eventually become her menstrual cycle. By the end of the eighth week, the fetus can move a little, reacts to mouth stimulation, and has ears that are fully formed.
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