The name given to the unborn child after the eighth week of development. Humans, like all animals, begin as a single cell, the OVUM, in the ovary (see OVARIES). After FERTILISATION with a SPERMATOZOON, the ovum becomes embedded in the mucous membrane of the UTERUS, its covering being known as the decidua. Increase in size is rapid, and development of complexity is still more marked. The original cell divides repeatedly to form new cells, and these become arranged in three layers known as the ectoderm, mesoderm and endoderm. The ?rst produces the skin, brain and spinal cord, and the nerves; the second the bones, muscles, blood vessels and connective tissues; while the third develops into the lining of the digestive system and the various glands attached to it.
The embryo develops upon one side of the ovum, its ?rst appearance consisting of a groove, the edges of which grow up and join to form a tube, which in turn develops into the brain and spinal cord. At the same time, a part of the ovum beneath this is becoming pinched o? to form the body, and within this the endoderm forms a second tube, which in time is changed in shape and lengthened to form the digestive canal. From the gut there grows out very early a process called the allantois, which attaches itself to the wall of the uterus, developing into the PLACENTA (afterbirth), a structure well supplied with blood vessels which draws nourishment from the mother’s circulation via the wall of the womb.
The remainder of the ovum – which within two weeks of conception has increased to about 2 mm (1/12 inch) in size – splits into an outer and inner shell, from the outer of which are developed two covering membranes, the chorion and amnion; while the inner constitutes the yolk sac, attached by a pedicle to the developing gut of the embryo. From two weeks after conception onwards, the various organs and limbs appear and grow. The human embryo at this stage is almost indistinguishable in appearance from the embryo of other animals. After around the middle of the second month, it begins to show a distinctly human form and then is called the fetus. The property of ‘life’ is present from the very beginning, although the movements of the fetus are not usually felt by the mother until the ?fth month.
During the ?rst few days after conception the eye begins to be formed, beginning as a cup-shaped outgrowth from the mid-brain, its lens being formed as a thickening in the skin. It is very soon followed by the beginnings of the nose and ear, both of which arise as pits on the surface, which increase in complexity and are joined by nerves that grow outward from the brain. These three organs of sense have practically their ?nal appearance as early as the beginning of the second month.
The body closes in from behind forwards, the sides growing forwards from the spinal region. In the neck, the growth takes the form of ?ve arches, similar to those which bear gills in ?shes. From the ?rst of these the lower jaw is formed; from the second the hyoid bone, all the arches uniting, and the gaps between them closing up by the end of the second month. At this time the head and neck have assumed quite a human appearance.
The digestive canal begins as a simple tube running from end to end of the embryo, but it grows in length and becomes twisted in various directions to form the stomach and bowels. The lungs and the liver arise from this tube as two little buds, which quickly increase in size and complexity. The kidneys also appear very early, but go through several changes before their ?nal form is reached.
The genital organs appear late. The swellings, which form the ovary in the female and the testicle (or testis) in the male, are produced in the region of the loins, and gradually descend to their ?nal positions. The external genitals are similar in the two sexes till the end of the third month, and the sex is not clearly distinguishable till late in the fourth month.
The blood vessels appear in the ovum even before the embryo. The heart, originally double, forms as a dilatation upon the arteries which later produce the aorta. These two hearts later fuse into one.
The limbs appear at about the end of the third week, as buds which increase quickly in length and split at their ends into ?ve parts, for ?ngers or toes. The bones at ?rst are formed of cartilage, in which true bone begins to appear during the third month. The average period of human gestation is 266 days – or 280 days from the ?rst day of the last menstrual period. The average birth weight of an infant born of a healthy mother (in the UK) is 3,200 g (see table).
The following table gives the average size and weight of the fetus at di?erent periods:
(See also PREGNANCY AND LABOUR.)... fetus
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