The Prospective Mother, a Handbook for Women During Pregnancy. J. Morris Slemons
and continually sway to and fro like a wheat field on a windy day; and by their motion they create a current in the direction in which the ovum should move, namely, toward the uterus. While passing through the oviduct, the ovum has no attachment whatever to the mother, yet development is going on all the time. It is thus made perfectly clear that development is not directed by the parent. This independence of the parent, though it continues to be one of the characteristic features of the development of the ovum, shortly becomes less evident, for communication is set up between the mother and the ovum as soon as it reaches the uterus. Unless we were warned, we might easily misinterpret the significance of this attachment to the parent. It does not permit the mother, for instance, to influence the mind or character which the child will have. The purpose of the attachment is twofold, namely, to anchor the ovum, and to arrange channels by which, on the one hand, nutriment may reach the embryo, and, on the other, its waste products may return to the mother. The mother may influence the nutrition of the fetus; but she cannot determine the kind of brain or liver her child will have; neither for that matter can she alter the development of any portion of the embryo.
After its entrance into the cavity of the uterus prepared to receive and protect it, the mass of cells sinks into the soft, velvety lining of the organ. Here it is entirely surrounded by tissue which belongs to the mother. But just before implantation takes place the architecture of the ovum is modified in such a way as to indicate the trend of its subsequent development. We left it, a hollow ball passing down the oviduct; had we examined the sphere more closely we should have found its wall composed of a single layer of cells. At one spot, however, the wall soon thickens. The thickening is due to a specialized group of cells which gradually grows toward the hollow center of the ball. A little later, if we study the structure as a whole, we find it a small, distended sac, from the inner surface of which hangs a tiny clump of tissue. The clump of cells within and the inclosing sac as well are both requisite to the ultimate object of pregnancy; yet they fulfill very different purposes. The clump within will mold itself into the embryo; the inclosing sac will make possible the continued existence and growth of the embryo by securing and conveying to it nourishment according to its needs. These two structures, which from now on constitute the ovum, can best be considered separately and in the order of their development. We shall therefore first study the sac and in the next chapter the embryo.
For a time after this sac, or ball, as you may choose to think of it, becomes implanted in the uterus, every part of its wall shares in the responsibility of procuring nourishment for the embryo. On this account the wall, or capsule, is for several weeks the most conspicuous part of the ovum. Its position is naturally advantageous, for, since it forms the outermost region of the structure and comes into immediate contact with the tissues of the mother, it has the first opportunity to seize and appropriate nutriment. Consequently, while there is still relatively little development in the embryo, the capsule of the ovum gives evidence of rapid extension; the wall becomes thicker, and the circumference of the sac increases. The significant thing about this growth, however, is the fact that it does not progress evenly. At some points cell-division is more active than at others, with the result that the surface of the ovum speedily loses its smooth, regular outline. Projections from the capsule appear; they increase in number and in length; and by the end of four weeks the ovum, as yet less than an inch in diameter, resembles a miniature chestnut-burr. To make the comparison more accurate, we must imagine such a burr covered with limp threads instead of rigid spines.
These projections, the so-called Villi, push their way into the mucous membrane of the uterus and serve a two-fold purpose. One of their functions is to fix the ovum in its new abode; and, though the attachment is not at first very secure, it becomes stronger in the course of time and is capable of withstanding whatever tendency the activity of daily life may have to loosen it. The other, and equally important, task of the villi, the majority of which dip into the mother's blood, is to transmit substances to and from the embryo.
We have traced thus far the earliest steps in the development of the ovum. One portion, we observed, was promptly set apart for the construction of the future child; this favored portion became inclosed by all the rest of the ovum, which has a more or less spherical form and is technically called the fetal sac. The first duty of the sac is to take root in the womb, and the second, no less vital, is to draw nourishment from the mother. But neither of these functions can be performed without the participation of the uterine mucous membrane, the soil, as it were, in which the ovum is planted. We must now learn how the maternal tissues assume the responsibility placed upon them.
THE REACTION OF THE UTERUS.—The womb, which is small before marriage, is converted by pregnancy into the largest organ of the body. The virginal uterus, shaped somewhat like a pear, and placed with apex downward, is carefully protected within the bony basin between the hips, which is commonly called the Pelvis. The upper and larger part of the organ, known as the body, lies at the bottom of the abdominal cavity; the lower part, the neck, projects into the vagina. The cavity inside the womb communicates above with the two oviducts and terminates below in a canal which runs through the neck and opens into the vagina by an orifice known as the mouth of the womb.
Pregnancy modifies every portion of the womb in one way or another; but the most profound alterations occur in the body, in the cavity of which the ovum has come to rest. During the forty weeks of gestation the organ grows in weight from two ounces to as many pounds; from three inches in length it increases to fifteen inches; and its capacity is multiplied 500 times.
The mucous membrane which lines the cavity of the uterus responds to the stimulus of pregnancy in a characteristic manner and with a single purpose, namely, to promote the development of the ovum. In connection with menstruation we noted that this membrane periodically prepares for the reception of an ovum. And if the expected ovum has been fertilized, its arrival is followed by arrangements for its protection and nutrition which are far more elaborate than the preparations for its reception. Within a few weeks the mucous membrane becomes half an inch thick, that is, about ten times thicker than it was; and all the elements entering into its composition, become unusually active. The blood-vessels are congested; the glands pour out a more elaborate secretion; and certain cells lay up a bountiful store of material to be drawn upon in the formation of the embryo and the building up of the structures that promote its development.
The ovum is as likely to find a resting place at one spot as another upon the surface of the uterine mucous membrane. The whole of that surface has been made ready to receive it; yet the area actually required to imbed the tiny object is extremely small. As the ovum escapes from the oviduct and enters the womb, it is smaller, in all probability, than the head of a pin. For at least a week after its coming, diligent search is necessary to find the site of implantation. Insignificant as it is at first, however, the region of implantation later becomes very prominent, for it undergoes a transformation that the rest of the mucous membrane does not share. That is to say, it becomes the point of attachment of the Placenta, an organ that has the very important function of drawing upon the resources of the mother's blood. As the ovum sinks into this especially prepared bed, the villi are formed. They break open the adjacent capillaries of the mother, thus diverting her blood from its accustomed course. The blood collects in microscopic lakes in contact with the capsule of the ovum, and from them flows back into the mother's veins. Through the veins it returns to her heart, by which it is distributed through the arteries to the various regions of the body. The tiny lakes, in which the villi hang, are thus made a part of the mother's circulation and as such are regularly replenished with purified blood. By this means the ovum receives a rich supply of nutriment, and as a natural consequence its growth is rapid.
Before very long the diameter of the ovum is greater than the depth of the mucous membrane which surrounds it. Consequently that part of the membrane which covers it is pushed into the uterine cavity, as the ground is raised by a sprouting seed. Growth continues, the bulging increases, and extensive alterations are wrought both in the womb and in the capsule of the ovum. One of these alterations will be more easily understood if we still think of the ovum as a seed, for it grows away from its roots just as plants do. Most of the capsule, therefore, is removed step by step farther from its source of nourishment, for the maternal blood-vessels do not follow the expanding sac but retain their original position at its base. Partly on account of the lack of nutriment thus occasioned and partly on account of the distention caused by the contents of