The present belief regarding the causation of sex is that although there is but one kind of ovum, there are two kinds of spermatozoa, one capable of producing a male and the other a female child, but the sex-determining form of the male cell that fertilizes any one ovum is a matter of the merest chance. Statistics show that more male than female babies are born, the usual proportion being about 105 boys to 100 girls among those who are carried to “term” or the end of pregnancy. Among abortions and prematurely born babies there is also a larger number of boys than girls, but as more boys than girls die in infancy, the two sexes about even up in the number of those living to adult life. Concerning the time of the month when conception is most likely to occur, there is a wide difference of opinion. Some doctors think that the most favorable period is just before or just after menstruation, while others believe that conception is most likely to take place about midway between the menstrual periods. [Illustration: FIG. 6.—Diagram indicating process of cell division. ] Returning to the ovum which meets a spermatozoon in the course of its journey down the tube, we find that as soon as a spermatozoon enters an ovum it disappears and is completely absorbed, and, as the ovum in turn is instantly possessed of new powers, through the presence of the male cell, the result of this union is an entirely new cell. But instead of continuing its existence as a single cell, the fertilized ovum divides into two cells; these two into four; the four into eight and so on until a clustering mass of cells is formed which looks something like a mulberry. If you will look at Fig. 6 you will see what happens as this cell division progresses and also that in time the cells rearrange themselves in such a way as to leave a space in the center of the mass so that it becomes a little sac with a cluster of cells at one point, which hangs toward the center, called the _internal cell mass_. This will interest you because it is from cells at one point in this little cluster that the baby begins to develop, together with the cord, bag of waters and afterbirth, to be described later. While these changes are taking place, the entire mass is being carried slowly down the tube toward the uterus by the sweeping motion of the soft little hairs on the lining of the tube. It is steadily growing, and by the time it reaches the uterus the mass is about the size of the head of a pin. As you will remember, the lining of the uterus prepares each month to receive the fertilized ovum, becoming soft and thick. The cell mass floats around for a little while after it reaches the uterine cavity and then resting at some point, sinks down into the soft lining and is completely buried.