The developing human organism from the implantation of the fertilized ovum in the uterus, about two weeks after conception to the eighth week after conception.
The embryonic period of fetal development is one of rapid growth when the major organ systems begin forming and the main external features first appear. It is also a critical time for prenatal health and development—the most sensitive of all the stages of human development. During this period miscarriages or birth defects can be caused by a variety of factors, including maternal illness or use of alcohol and other drugs, and exposure to radiation. An enormous amount has been learned about embryos through ultrasonography and the study of miscarried embryos.
At the beginning of the embryonic period, a structure known as the embryonic disc is formed from the inner cells of the fertilized egg. The outer cells, collectively known as the trophoblast, have begun to form the protective substances, including the amniotic fluid, placenta, and umbilical cord, that will surround and nourish the developing organism during the prenatal period. Also formed by the trophoblast is the yolk sac, which produces blood cells for the embryo until it can manufacture its own and then disappears. The embryonic disc, which begins as a flat structure, separates into three distinct layers: the ectoderm, or outer layer, which will turn into the skin and nervous system; the mesoderm, or middle layer, from which the circulatory, excretory, and reproductive systems, as well as the muscles and bone, will be formed; and the endoderm, or inner layer, which will become the lungs and digestive system. The embryo's growth follows two general patterns of development: cephalo-caudal (from the head downward) and proximo-distal (inner to outer). In the third week, the head and blood vessels begin to develop, as does the notochord, a rod-like structure along the back of the embryo that will later become the spine, spinal cord, and brain. During the fourth week, the heart begins beating, and the eyes, ears, nose, and mouth start to form. The first signs of arms and legs appear, as do the tissues from which the lungs, liver, and pancreas will later be formed. By the end of the first month, the embryo is about 5 mm (0.2 in) long and weighs about 0.02 g (0.0007 oz). However, it is already 7,000 times the size of the zygote that was formed immediately upon conception four weeks earlier.
Between the fifth and eighth weeks, the brain begins to regulate the functioning of the internal organs, which continue to develop. The arms and legs begin to extend and take on a definite shape, developing according to the cephalo-caudal and proximo-distal patterns: first the arms, hands, and fingers, followed by the legs, feet, and toes. By the end of the fifth week, the embryo has doubled in size and has grown a tail-like structure that will become the coccyx, or lowermost tip of the backbone. By the seventh week it is about 2 cm (1 in) long and facial features are visible. By the eighth week, the embryo has grown to about 4 cm (1.5 in), weighs about 1 g (0.04 oz), and has begun to take on a recognizably human shape. The facial features are fully formed, the heart has assumed its final shape, and all the other basic organs have begun forming, except for the reproductive organs. The fingers and toes, which had been webbed, are separate, and elbows and knees have formed as well. From the end of the eighth week on, the developing infant is known as a fetus.
Embryology—the study of embryos—has yielded a substantial amount of information about human development, including important discoveries about congenital abnormalities. In recent times, important ethical questions have been raised by the scientific use of human embryos for research and such techniques as in vitro
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