(From faetco, to become putrid. foul ulcer ; and an abscess with a cyst. Severinus. Foetida Tinctura. See Asafcetida. Foetus, (from feo, to bring forth). See Voss. Etymol.) Epicyema, and epigonion. The young of all viviparous animals whilst in the womb, and of oviparous animals before they are hatched. (See Con-ceftio). The name is transferred by botanists to the embryos of vegetables.

In the human fcetus are several peculiarities not to be found in the adult. 1. The ductus, or canalis, arterio-sus. (See Arteriosus ductus). 2. The arteries of the navel string, which are continuations of the hypogastrics, after the birth, are shrivelled up, and form the ligamenta umbilicalia inferiora. 3. The veins of the navel string, which are formed by the union of all the venal branches in the placenta, and passing into the abdomen, become the falciform ligament of the liver. 4. The ductus venosus, q. v. 5. The lungs, which, before being inflated with air, are compact and heavy ; but after one inspiration become light, and as it were spongy. When the lungs swim in water, it is supposed that the child has breathed; and the conclusion is drawn that it has been murdered. But the uninflated lungs become specifically lighter than water as soon as any degree of putrefaction comes on, which quickly happens after the death of the child ; and where the utmost care hath been taken to preserve the child, it hath breathed once or twice, and then died ; and on the other hand, they may sink, though the child has breathed, if tubercles have formed in them. (See Medicina foren-sis.) 6. The thymus gland is very large in the foetus, but lessens as years advance, 7: The foramen ovale in

4R the heart of a foetus is generally closed in an adult. 8. The circulation of the blood. See Circulatio.

These peculiarities of structure are connected with the state of the foetus, while yet in utero, when it has the adventitious support of the mother, on whom its existence for a time depends. The uterine foetus is inclosed in a bag, composed of different membranes, styled the ovum; but the whole, if we except the external membrane, the decidua, is a part of the foetal system, and connected with its growth and existence.

The ovum, at the earliest period of its existence, or rather as soon as it becomes the object of even the assisted sight, appears like a small vesicle, slightly attached to some part, generally the fundus of the uterus; and, on examination, all the organs of which it consists are confusedly blended. They by degrees appear more distinct; and in the more advanced periods we discover the membranes already described in the articles Amnios, Chorion, and Decidua, q. v. The decidua forms apparently, at first, the principal bulk of the ovum, and is, as we have said, double; the outer membrane lining the uterus, and the inner, reflected, covering the ovum : the double chorion is the only membrane of the ovum which is properly a part of the mother.

In the early period of gestation, a white speck is observed on the amnios, near the part where the umbilical vessels are inserted. It is a vesicle filled with a white liquor called vesicula umbilicalis, alba or lactea, and united with the umbilical cord by a funis, consisting of an artery and vein. It disappears in the progress of gestation, and we are unacquainted with its use.

Though the bag or external parts of the conception are at first large in proportion to the foetus, they are afterwards related in an inverted ratio; the foetus increasing more rapidly than its contents and accompaniments : the latter seldom become more bulky after the seventh month. The foetus is, for a time, invisible; and when at first seen, resembles, as we have remarked, a tadpole, with a rounded head, from which a tail projects. This tail does not consist of what are afterwards the inferior extremities, for these only appear to sprout after some months; nor is it the superior extremities in embryo, for these apparently are added at the sides. When we can perceive any addition, we find a very minute moving point, somewhat below the head, which expands into a heart, at first conveying an almost colourless fluid, and afterwards red blood. Two large black points, the eyes, seem to project, after some time, from the head of the tadpole, and almost to cover it: a chasm is seen below, the future mouth; and a disproportionately large abdomen is soon conspicuous. As we find all these important organs accessory, or rather evolved at a subsequent period, we must consider the early formation, or the parts first evolved, as the most important. These can be only the brain and spinal marrow, each essential to the existence of the animal, and the motion of the heart, on which life depends. All the other organs probably exist at the earliest period, but are invisible from their minuteness and pellucidity. The muscles and membranes have their primordial stamina in the nervous system, and the fibrils are probably convoluted and compressed. What we style evolution, then, is only extension; and the bulk or size of an organ is limited by their length, and their capacity of admitting the interposition of inorganized matter.

We can sec an obvious instance of increased bulk by the interposition of such matter, if we trace the fibres of the stem of an apple, expanding to' the bulk of the fruit.

A foetus of four weeks is near the size of a common fly; soft, mucilaginous, and, in appearance, suspended by the belly;- its bowels covered by a transparent membrane. At six weeks, it is of a somewhat firmer consistence, nearly the size of a small bee; the extremities then begin to shoot out. At three months its shape is tolerably distinct, and it is about three inches long. At four, five, and six months, it is five, near seven, and near nine inches respectively. In the successive months it increases in length to twelve, fifteen and twenty, or twenty-two inches, though varying in different women, and in different births. Indeed all these measurements are rather approximations than accurate representations.