The mucous membrane which lines the cavity of the uterus responds to the stimulus of pregnancy in a characteristic manner and with a single purpose, namely, to promote the development of the ovum. In connection with menstruation we noted that this membrane periodically prepares for the reception of an ovum. And if the expected ovum has been fertilized, its arrival is followed by arrangements for its protection and nutrition which are far more elaborate than the preparations for its reception. Within a few weeks the mucous membrane becomes half an inch thick, that is, about ten times thicker than it was; and all the elements entering into its composition, become unusually active. The blood-vessels are congested; the glands pour out a more elaborate secretion; and certain cells lay up a bountiful store of material to be drawn upon in the formation of the embryo and the building up of the structures that promote its development. The ovum is as likely to find a resting place at one spot as another upon the surface of the uterine mucous membrane. The whole of that surface has been made ready to receive it; yet the area actually required to imbed the tiny object is extremely small. As the ovum escapes from the oviduct and enters the womb, it is smaller, in all probability, than the head of a pin. For at least a week after its coming, diligent search is necessary to find the site of implantation. Insignificant as it is at first, however, the region of implantation later becomes very prominent, for it undergoes a transformation that the rest of the mucous membrane does not share. That is to say, it becomes the point of attachment of the Placenta, an organ that has the very important function of drawing upon the resources of the mother's blood. As the ovum sinks into this especially prepared bed, the villi are formed. They break open the adjacent capillaries of the mother, thus diverting her blood from its accustomed course. The blood collects in microscopic lakes in contact with the capsule of the ovum, and from them flows back into the mother's veins. Through the veins it returns to her heart, by which it is distributed through the arteries to the various regions of the body. The tiny lakes, in which the villi hang, are thus made a part of the mother's circulation and as such are regularly replenished with purified blood. By this means the ovum receives a rich supply of nutriment, and as a natural consequence its growth is rapid.