During the forty-six-day embryo stage, the embryo grows to a length of more than one inch (2.5 centimeters). By the end of the embryo stage many body systems will be in operation, and the embryo will begin to appear human-like.
The embryo takes nourishment and oxygen and releases waste products through the umbilical cord, which links it with the placenta. The umbilical cord contains three blood vessels through which the embryo's blood circulates to and from the placenta.
The placenta is a disk-shaped mass of tissue six to eight inches (15.2 to 20.3 centimeters) long and one inch (2.5 centimeters) thick and weighs about one pound (.45 kilogram). Implanted in the inner wall of the uterus, it serves as a two-way filter between the bloodstream of the mother and the embryo. The placenta makes it possible for the mother to carry on life functions such as digestion, excretion, circulation, and respiration for the embryo. Into the placenta, by way of two arteries in the umbilical cord, the embryo deposits such waste material as carbon dioxide. The mass of blood vessels on the mother's side of the placenta then absorbs the wastes into her bloodstream. The embryo receives, through the vein in the umbilical cord, fresh nutrients (oxygen, amino acids, sugar, fats, and minerals) from the mother's bloodstream, and hormones, antibodies, and other necessary substances by the same route.
The placenta acts as a highly permeable membrane and as a natural screen to keep out many—but unfortunately, not all—harmful substances. Thus, the unborn child receives only materials with molecules that are small enough to pass through the screen. At the same time that the embryo is taking shape, the amniotic sac is developing into a protective chamber. By the end of the eighth week this sac completely surrounds the embryo. The watery fluid inside keeps the embryo from being jostled by any sudden movements of the mother or by accidents that may happen to her, such as a fall. The amniotic sac also keeps the embryo at a constant temperature.
During the embryonic period, three layers of cells are differentiated. The outer layer, or ectoderm, develops into sensory cells, skin, and the nervous system. The middle layer, or mesoderm, becomes the excretory system, muscles, and blood. The inner layer, the endoderm forms the digestive system, lungs, and thyroid gland.
By the end of the third week of development the embryo's heart is beating and its nervous system is forming rapidly. After the fourth week the legs are curled and the eyes have appeared as dark circles. During the fifth and sixth weeks arms and legs can be seen. After eight weeks all of the major body organs are present. The liver is making blood cells, and the kidneys are removing waste products. The mouth, nose, eyes, and head are clear and distinct. The head is roughly half the total body size at this time. Fingers and toes are blunt, and ribs show under the fetus' skin.
The eight-week time span of embryonic development is a particularly vulnerable period in human growth. Chemicals, drugs, hormones, or viruses present in the mother's system can very easily affect the embryo, as is described in more detail below in the section on "Prenatal Environmental Influences."
- Prenatal Development - Fetal Stage
- Prenatal Development - Ovum Or Germinal Stage
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