THE FUNCTION OF THE WOMB.--While these changes are going on in the ovary, the womb is also preparing itself for its share of the work. The lining or internal surface of the womb is composed of mucous membrane, much the same as the interior lining of the mouth and throat. This lining becomes congested with blood, and is so intensely swollen at the time when the ovule or egg reaches the womb, that it is ready to rupture and bleed all over its surface. Just whether it will rupture and bleed, depends upon whether the egg is going to grow into a child or not. If it is not going to grow into a child, it immediately bleeds freely, and continues to bleed, until the whole lining of the womb and egg is passed out into the outer world. This takes four or five days and constitutes "menstruation." After menstruation is over, the womb begins again to prepare itself for the coming of the next ovule or egg, and as this occurs every twenty-eight days, menstruation is commonly termed the "monthly periods." WHY MENSTRUATION OCCURS EVERY TWENTY-EIGHT DAYS.--The reason why the womb does this every twenty-eight days is because it is impossible to tell just when the womb will be called upon to nourish and support a child. If it did not get rid of the old blood, it would not be in a healthy condition to nourish and take care of a baby, nor would its interior be ready to supply new fresh blood for the growth of the infant. Hence nature constructs and builds a new "nest" in the interior of the womb each month. It very much resembles the new home into which the bride and groom, go to begin housekeeping. When you told your little girl the story of life, you particularly drew her attention to the important fact that every living thing is created by the union of a male and female principle, and, therefore, has a mamma and papa. This applies to trees, flowers, vegetables, fish, animals, birds, insects,--every living thing, including human beings. We have seen that the ovule from the ovary is the female egg, or principle. It is the part the female contributes toward the future child. Before a child is possible, however, the ovule must meet the egg from the male. THE MALE OR PAPA EGG.--The male or papa egg is called a "spermatozoa." It reaches the interior of the womb through the lower opening, which you will remember opens into the vagina. Emphasize to your daughter that the female ovule or egg, and the male egg, or spermatozoa, are minute objects, so microscopically small that a hundred million of them could comfortably lie upon a ten-cent piece.