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. In all probability, the embryo is still in the morula stage and is about the size of the head of a pin when it reaches the uterus, where it finds that the endometrium has been prepared for its reception by the premenstrual swelling. The mucosa has grown thicker, more velvety and vascular, and its glands have increased in number and activity. The columnar epithelium of the endometrium is replaced by a thick layer of large, vacuolated cells, called decidual cells, and the uterine lining from now on is termed the _decidua gravidatis_. While the normal uterine mucosa is thin, averaging from 1 to 3 millimetres (0.039 to 0.117 inch) in thickness, it increases to a thickness of about 1 centimetre (⅞ inch) during pregnancy. [Illustration: FIG. 18.—Diagram of segmenting rabbit’s ovum. ]