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. These two kinds are evidently deposited in the vagina in about equal numbers, and the sex-determining form that fertilizes any one ovum is a matter of the merest chance. Statistics show, however, that more male than female babies are born, the usual proportion being about 105 boys to 100 girls among those that reach full term. Among abortions and premature births there is also a larger number of boys than girls, and in elderly primiparæ the ratio increases to about 130 boys to 100 girls. But as more boys die in infancy than girls, the two sexes about even up in the number of those living to adult age.
Apparently, then, there is some factor operating slightly in favor of the purposeful activities of the male-producing spermatozoa. But so far no accurate means has ever been found whereby it was possible to influence the development or discover the sex of a child before its birth.
There is a wide difference of opinion concerning the time of the month when fertilization is most likely to occur. Observations made upon the wives of sailors and under a variety of conditions suggest that the most favorable period is just before or just after menstruation which represents the second stage of the menstrual cycle.
Dr. Williams believes, however, that fertilization is most likely to occur about midway during the intermenstrual period. But since it is probable that spermatozoa are constantly present in the tubes of women who are exposed to the possibility of becoming pregnant, it is difficult to do more than speculate about the time of the month at which fertility is greatest.
Another moot question relates to the age of the woman at which it is most desirable that the first child shall be born. Recent observations made by Dr. John W. Harris upon a large number of pregnancies occurring in very young girls indicates that from a standpoint which considers solely the physical welfare of the mother and her infant, sixteen years is the most satisfactory age at which to bear the first child.
However, when motherhood is considered from all standpoints, social, ethical, spiritual as well as physical, the consensus of opinion seems to be that the twenty-third year is the most favorable age for motherhood to begin. Children have been born to little girls nine years old and to women of sixty-two, but the extremes of the reproductive years are not favorable periods for child-bearing.
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, the result of this union is a cell which was previously non-existent.
This new cell is not only capable of reproduction by means of segmentation or cell division, but in the course of its sub-division and proliferation, it forms groups of cells which develop into tissues and structures widely different from each other. The entire complex human body, in addition to the placenta, cord, and membranes, arises from the single, extraordinary cell.
It first divides into two; these two divide into four; the four into eight and thus the process of division and sub-division continues until a solid mass is formed, shaped something like a mulberry and called the morula. (Fig. [18].)
While these developmental changes are taking place, the morula is being carried down the tube toward the uterus, by the sweeping motion of the ciliated membrane. The time consumed by this journey has not been definitely ascertained and though possibly it may be made in a few hours, it probably takes from five days to a week. Since the embryo is constantly moving during this time, it quite evidently has no attachment to the mother and cannot, therefore, derive any great amount of nourishment directly from her. The growth and development to this point, then, must be due chiefly to inherent powers within the mass of cells itself.