By the 19th day of human embryonic development, ectoderm along the mid-back portion of the elongated embryo becomes thicker than adjoining ectoderm. Unequal cell growth causes the edges of the thickened plate to roll upward as the center buckles. The neural tube forms when the edges meet. A mass of cells called the neural crest is pinched off the top of the tube. These cells migrate to other parts of the embryo, where they give rise to neural and other structures. Eventually, the front part of the neural tube bends, thickens, and bulges to form the brain. The rest of the tube becomes the spinal cord.
Meanwhile, cells of the endoderm roll to form the archenteron—the primitive digestive tube, or “gut.” The archenteron later gives rise to the lungs, the liver, and the lining of the digestive tract. The mesoderm also begins to spread around the sides of the embryo. As it does, its top portion separates from the lateral segments and rounds into the long, solid notochord. The lateral sheets of mesoderm move down each side of the embryo until they meet at mid-bottom. At the same time, they split into two layers separated by a cavity. The inner layer, splanchnic mesoderm, lies next to the archenteron; the outer layer, somatic mesoderm, next to the ectoderm. Between these layers is the coelom, in which the vital organs will lie.
On each side of the embryo the mesoderm becomes organized into three distinct regions. Topmost, flanking the notochord and neural tube, is the epimere. At the bottom, next to the archenteron, is the hypomere. The narrow mesomere lies between them. At this stage, a human embryo is in its fourth week.
The epimere undergoes further differentiation. The lateral wall of each epimere, called the dermatome, provides the dermis of the skin. The upper wall, the sclerotome, produces the vertebrae (see skeleton). At about the ninth week of human embryogeny, the bony vertebrae begin to form around the notochord, and the skull starts to encase the developing brain. The lower wall, the myotome, gives rise to the muscles of the body wall and the appendages.
From the fifth through the eighth week of embryogeny, cells derived from the primitive tissues are organized into the organs and body structure that mark an embryo as human. The ears, eyes, and nose can be distinguished. Limbs and other external body parts take form. Beginning with the ninth week, the now recognizable individual is called a fetus. From then until birth most of the already well-established body structures continue to grow and develop.
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