=The Chorion and Placenta.= Very early in pregnancy, probably while the fertilized ovum is journeying down the tube, tiny, thread-like projections, called _villi_, appear over the surface of the primitive chorion, giving it the shaggy appearance of a chestnut burr. Shortly after this shaggy ovum reaches the uterus and is embedded in the lining, the chorion, or the outer fetal membrane, is formed, being partly derived from the ectodermal layer of cells growing within the blastodermal vesicle. The chorion grows rapidly in size and thickness, and the villi upon its surface increase in size, number and complexity by frequent branching. In so doing the villi push their way into the maternal tissues surrounding them, and destroy the capillary walls with which they come in contact. Maternal blood escapes through the destroyed walls, forming tiny hemorrhagic areas, or “lakes of blood.” The chorionic villi float freely in these pools of maternal blood, which is constantly being refreshed by an inflow of arterial and an outflow of venous blood through the mother’s vessels. Blood vessels soon appear in these chorionic villi, and fetal blood then circulates through them. It becomes apparent, therefore, that the maternal and fetal blood streams are in such close relation that they are separated by only the thin membrane which forms the walls of the vessels in the villi. (Fig. 20.) [Illustration: FIG. 20.—Diagram of fetus, cord, membranes and placenta in utero at an early stage of their development. ] This arrangement makes it possible for the steadily proliferating villi to discharge one of their functions, which is to receive from the maternal blood nourishment for the embryo, and give up to the parent waste products from the growing body. This exchange of nourishment and waste matter takes place by means of osmosis. But freely as the exchange of materials occurs, there is never any contact, or mixing of maternal and fetal blood, nor does maternal blood at any time flow through fetal vessels. It was believed at one time that the fetus was nourished by milk which was in some way secreted by the gravid uterus, but this is disproved by present knowledge of the placental function. The second function of the villi, particularly after they have developed to the placental stage, is to assist in securely attaching the embryo to the uterine wall. The villi are equally distributed over the surface of the chorion at first, but as the sac increases in size and pushes out into the uterine cavity, they gradually atrophy and disappear, excepting over the small area beneath the vesicle where the chorion is in contact with the decidua basalis. At this site the villi become much more abundant, and it is here that the placenta eventually develops. This part of the chorion is termed the _chorion frondosum_, while the remainder, which is in contact with the decidua capsularis, is the _chorion læve_.