Embryonic Development

The events of the six-week embryonic stage include differentiation of germ layers into specific body cavities and the formation of the placenta, the umbilical cord, and the embryonic membranes which provide sustenance and protection to the embryo. The embryonic stage lasts from the beginning of the third week to the end of the eighth week. At this stage, the developing organism is referred to as an embryo. During the embryonic stage, all of the body will form. While the events are forming the body within the embryo, a complex system of extraembryonic membranes also develop. The amnion is derived from ectoderm and mesoderm around the embryo forming a sac filled with fluid. The yolk sac is established during the second week and although it has no nutritive value, it is essential for embryonic development. The umbilical cord forms as the yolk sac shrinks and usually attaches near the center of the placenta. It contains two umbilical arteries and one umbilical vein which twists because it is longer than the arteries. The arteries carry deoxygenated blood toward the placenta, and the umbilical vein carries oxygenated blood from the placenta to the embryo. Early in the third week, a linear band known as the primitive streak appears along the dorsal midline of the embryonic disc, and establishes a structural foundation for development along a longitudinal axis, and will eventually give rise to the notochord, a primitive beginning of skeletal development. During the fourth week, the embryo increases in size and develops a connecting body stalk from the embryonic body to the placenta. By this time, the heart is developed and beating, pumping blood to all parts of the embryo. The head and jaws are apparent, and the primitive tissue that will form the eyes, mouth, brain, spinal cord, lungs, and digestive tissue has developed. The arm and leg buds are recognizable as small swellings on lateral body walls. The developing brain and spinal cord comes from the neural tube and develops rudimentary segments called somites which will become the vertebral column. Branchial arches develop about the neck area resembling gills. The most precarious time of prenatal development is during the embryonic stage of development. For this reason, it is critical that the mother take extremely good care of herself if she believes she is pregnant.