=Conception, or Fertilization.=—This is the process wherein the male element (spermatozoon) meets and unites with the female egg. From what is known from investigations of lower animals, this meeting usually takes place in the Fallopian tube. [Illustration: Fig. 14.—Human spermatozoa. _h_, head; _c_, intermediate portion; _t_, tail. (Williams.)] The egg expelled from the ovary is carried into the open end of the tube by peritoneal currents and passed on toward the uterus by the waving action of the hair-like outgrowths of the cells (ciliæ) that line the tube, aided, possibly, by the tubal muscle. The spermatozoon makes its way upward from the vagina by means of its tail. This activity, like the tail of a fish, or snake, or as a boat is sculled, drives the cell forward through the thin layer of fluid that covers the mucous membranes. The arrow-shaped spermatozoon travels at a rate that completes the passage to the ovary in twenty-four hours, but spermatozoa may lie in wait for the egg a considerable time, as is shown by the fact that they have been found alive in Fallopian tubes removed three and a half weeks after copulation. As soon as the male and female elements approach each other, they exercise a powerful magnetic attraction, which draws them together, and as soon as they touch, the two cells unite and the spermatozoon almost immediately disappears. Only one spermatozoon is required for the fertilization of an egg, and hence enormous numbers must perish without achieving their destiny. The fertilized egg has become the _ovum_, and originally 1/125 of an inch in diameter, it now begins to grow, and filled with a new energy, it passes down the tube and enters the uterus. Here it comes into contact with the soft mucosa and digs a hole for itself—a nest, very much as a warm bullet might sink into ice or snow—and is soon completely surrounded by a proliferating tissue called the decidua. The woman is now pregnant. The menstrual flow does not appear, and local and systematic changes are inaugurated. The egg enlarges rapidly. Little glove-finger-like projections (the villi) appear on its surface and dip down into the maternal tissues. Through these villi the egg gets nourishment until about the twelfth week, when the placenta forms. Externally the ovum resembles a chestnut burr. As the egg grows, the villi on the surface find it more and more difficult to secure nutriment, and _except at one place_, all gradually shrink and disappear. At this significant point, they increase greatly in size, number, and complexity to form the thick, cake-like placenta.