The ovum passes slowly through the Fallopian tubes, and during the twelve days or more that it is passing to the uterus, there is some development, some increase in size, and by the time it has reached the uterus it has become impossible to find in it either vesicle or germinal spot. It is probable that in its passage it is nourished by the granulations which accompany it, and by absorbing the liquid secreted by the oviduct. As the impregnated ovum is developed, the chorion, the amnion and the embryo may be observed. The _chorion_, which corresponds to the membrane lining in the shell of an egg, is found covering the ovum at the earliest period that it has been seen in the uterus. It is smooth internally, but externally it is covered with short round villi, which at a later period remain only where the placenta is developed. The chorion is enveloped in a great measure by the reflected decidua; there is at the outset considerable space between the two, mostly filled at first by the villi of the chorion, though there may be between the two an effusion of blood; these villi soon disappear, and the membranes come in contact. In that part of the chorion that is not covered by the decidua reflexa, the villi are more and more developed, and they contribute a most important part in the formation of the placenta. At the same time that the placenta is formed, the villi on the other portion of the chorion is obliterated, so that the principal part of the chorion is a thin, colorless, transparent membrane, united outwardly to the reflexed decidua by short, delicate filaments, and inwardly to the amnion by an albuminous layer called the _tunica media_. There is also between the two membranes the _vesicula alba_. This bears a perfect analogy to the yolk of an egg; it is the vitellus surrounded by the blastoderm. Its use is to contain nutriment for the fœtus before the development of the placenta. The amnion is the most internal membrane of the ovum; it is continuous with the margins of the ventral opening in the fœtus, and closely envelopes the embryo in the early period. Its internal surface exhales a liquid into its own internal cavity, and in this the embryo swims freely. As this membrane is more and more filled, it presses back the exterior liquid and thereby condenses it until the amnion comes in contact with the chorion. And since it adheres to the abdominal parietes of the fœtus, it furnishes as it extends a membranous sheath to the allantoid and umbilical vesicle, and these vessels, and all parts thus enclosed constitute the _umbilical_ cord.