As a result of copulation, then, the spermatozoon (or a number of spermatozoa) comes in contact with the female ovum or egg. Then one or more of them, by means of a furious lashing of the tiny tail, manages to penetrate the outer covering of the ovum, and enters the space between the outer covering and the real body of the egg. Several spermatozoa may effect an entrance into this outer space, BUT ONLY ONE IS PERMITTED TO ENTER THE REAL BODY OF THE EGG. [Twins are produced by the impregnation of two ova by two spermatozoa, at the same time. The presence of the two ova at the same time is unusual]. The moment that the real body of the ovum is penetrated by the successful spermatozoon, a tough covering or thick membrane forms around the ovum and thus prevents the entrance of other spermatozoa. The successful spermatozoon then loses its tail, and the remaining head and body become what is known as "the male pronucleus." The authorities are uncertain as to the exact nature of the change which occurs when the ovum is penetrated by the spermatozoon. The outward manifestations of the change and transformation arising from the blending of the male and female elements are of course well known, but the "life process" eludes the power of the microscope. When Nature forms the thick membranous coating over the impregnated ovum, she draws the veil over one of her most important secrets. The first segmentation-nucleus having been formed by the blending and forging together of the male and female pronuclei, the process of segmentation begins. Segmentation proceeds as follows: the impregnated egg splits into halves, forming two joined cells; then into quarters, forming four joined cells; then into sixteenths, then into thirty-seconds, sixty-fourths, and so on, until the ovum consists of a combined mass of very minute granular-like cells, the whole resembling a mulberry. The segmentation of the nucleus precedes and then continues with the segmentation of the yolk. After the egg has been divided into a great number of these cells, the latter begin a centrifugal action resulting in the formation of a complete inner lining of closely packed cells, with a central cavity filled with the yolk liquid. In the meantime, the Uterus has been prepared for the reception of the impregnated and transformed ovum. A thick, spongy, juicy, mucus membrane forms, into which the changing ovum passes and attaches itself; the mucus membrane soon enveloping it and shutting it off from the rest of the Uterus. There now appears at one point on the ovum an opaque streak, which is called "the primitive trace" of the embryo--the first beginning of the young living creature. The "primitive trace" then grows in length and breadth. At this point we must leave the history of the ovum, or human egg, for the present; its further development will be related in the succeeding lesson, the subject of which is "Gestation." LESSON IV GESTATION OR PREGNANCY