The secret of the above paradoxical statement is made apparent when we understand the meaning of the scientific term "osmosis." Osmosis is "the passage of a fluid through a membrane"; it is a chemical process, caused by the chemical affinity between two liquids or gases separated one from the other by a porous diaphragm or substance. In the process of osmosis in the case before us, the fetal blood takes up nourishing substances and oxygen from the blood of the mother, and passes on to the latter the waste products of the fetal system, by means of passing these substances through the thin porous membranes which separate the two independent systems of blood vessels, i. e., the system of the fetus, and that of the mother. Before birth, in fact, the fetus has its blood nourished and oxygenated by means of the food partaken of by its mother, and the oxygen taken in by the mother in her breathing. After its birth, the infant eats and breathes for itself, and thus nourishes its blood supply directly, instead of receiving it indirectly from the mother. The Placenta begins to be formed about the third month of gestation, and continues to develop steadily from that time. At the time of the delivery of the child the Placenta covers nearly or quite one-third of the inner space of the distended Uterus of the mother. The total "afterbirth" consists of the Placenta, the umbillical cord, and the remaining membranes of the ovum, all of which are expelled after the birth of the child. THE AMNION. An important appendage contained in the Uterus in connection with the developing fetus is that known as "The Amnion." This is an inner sack which forms within the womb, and which serves to enclose the fetus, and also to sheath the umbillical cord. The Amnion encloses the embryo very snugly during the early stages of its development, but it gradually becomes distended with a pale watery fluid, known as "the amniotic fluid," the purpose of which is to "float" the fetus and to give it mechanical support on all sides. This fluid is composed of water carrying in solution small quantities of albumin, urea, and salt.