The embryo develops from an egg that has been fertilized by a sperm (see fertilization). It starts as a single cell, but divides several times as it travels along the fallopian tube to the uterus to form a spherical mass of cells. About 6 days after conception, this mass becomes embedded in the uterus lining. At the site of attachment, the outer layer of cells obtains nourishment from the woman’s blood; this part will later become the placenta. In the cell mass, a flat disc forms, consisting of layers of cells from which all the baby’s tissues will form. The amniotic sac develops around the embryo.
Early in the 3rd week, the head of the embryo forms and the neural tube, which will later become the brain and spinal cord, forms along the embryo’s back. In the 4th week, the neural tube extends towards the head, where a fold becomes visible that will eventually form the brain. Developing ears appear as pits. Rudimentary eyes form as stalks. Within the embryo, buds of tissue form that will become the lungs,pancreas, liver, and gallbladder. A heart starts to develop in the form of a tube. Outer layers of the embryo begin to form the limb buds.
During the 5th week, the external ears become visible, pits mark the position of the nose, the jaws form, and the limb buds extend. Folds of tissue fuse to form the front wall of the chest and abdomen. The umbilical cord develops.
During weeks 6–8, the face becomes recognizably human, the neck forms, the limbs become jointed, and fingers and toes appear. After 8 weeks, most of the internal organs have formed and all external features are present.
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