The concave surface of the placenta is clothed by the chorion; the convex is also covered by a kind of soft membrane, easily torn, which seems to be a continuation of the chorion, and the foetus is included under the double coat of the chorion and the amnios. The form is globular, because the intervals between the membranes and the foetus are filled with a transparent liquor. This liquor is contained by the amnios, which is the internal membrane, it is thin and transparent; it folds round the umbilical cord at its insertion into the placenta, and covers it the whole length to the navel of the foetus. The chorion is the external membrane; it is thick and spongy, sprinkled with sanguinary vessels, and composed of many coats, the exterior of which covers the convex surface of the placenta. It follows the inequalities, and covers the papillæ, which spring from the placenta, and are received in the cavities found at the bottom of the matrix, called _lacunæ_. The foetus adheres to the matrix by these insertions.
Some anatomists have thought that the human form had, like those of certain quadrupeds; a membrane called _allantois_, destined to receive the urine; and they have pretended to have found it between the chorion and the amnios, or in the middle of the placenta at the root of the umbilical cord, under the form of a very large bladder, in which the urine entered by a long pipe that composed part of the chord, and which opened on one side into the bladder, and on the other in this allantois membrane, being similar to the urachus in other animals. They owned, however, that it was not near so large in the human foetus as in quadrupeds, but that it was divided into many tubes, so minute, that they could scarcely be perceived, and that the urine passed into their cavities.
The experience and observations of most anatomists are contrary to this supposed discovery. They admit there is a kind of ligament which adheres by one end to the external surface of the bottom of the bladder, and extends to the navel; but it becomes so delicate, on entering into the cord, as to be nearly reduced to nothing: in common this ligament is not hollow, and we can see no orifice at the bottom of the bladder.