This journey of the ovum through the tube is of enormous consequence. During its course occur the events which decide whether the ovum shall, like most of its fellows, be simply swept along to no end and lost, or whether by chance it is to receive the mysterious impulse which begins the development of a new human being. The amazing power which enables this cell to reproduce itself, and to develop with unbelievable complexity, is acquired somewhere in the tube by meeting and fusing with a _spermatozoon_, the germinal cell of the male. (Fig. 17.) The spermatozoa look very much like microscopic tadpoles, with their flat, oval heads, tapering bodies and long tails. As these tails serve somewhat as propellers, the male cells are capable of very rapid motion. But in spite of their strange appearance, they are cells after all, and resemble the female cells in that each one contains a nucleus, or germinal spot. An almost inconceivably large number of spermatozoa, floating in the seminal fluid, is deposited in the vagina at the time of intercourse. Nature evidently supplies the male and female cells with equal lavishness, in order to provide for the large number of both kinds which must inevitably be lost, and still have enough survive to accomplish the high purpose of their creation. A very considerable number of spermatozoa enter the uterus, and are enabled through their powers of motility, to travel up into the tubes, in spite of the downward current created by the cilia. And in the tube, usually in the upper end, they meet a recently matured and discharged ovum which is being swept downward, and are attracted to it somewhat as bits of metal are drawn to a magnet. Although the ovum which is destined to be fertilized is surrounded by several spermatozoa, only one actually enters and fuses with it. This fusion is termed _impregnation_, _fertilization_, or, in lay parlance, _conception_, and the instant at which it occurs marks the beginning of pregnancy. The establishment of this fact is of no little importance, since it does away with any possible controversy concerning the time at which a new life begins. The origin of the child is exactly coincident with the fusion of the male and female germinal cells. And furthermore, the sex of the child and any inherited traits and characteristics are also established at this decisive instant. No amount of dieting, exercise nor mental effort on the part of the expectant mother can alter or influence them in the smallest degree, for the father has made his complete contribution toward the creation of the new being, and after this event the mother provides nourishment only. All told, probably more than five hundred theories have been advanced to explain what it is that decides of which sex the forthcoming child will be.