Fertilization

The Prospective Mother: A Handbook for Women During Pregnancy
by J. Morris Slemons

(Page 4 of 17)

Convincing observations upon the lower forms of life, especially upon fishes, have shown that when the germinal cells come near to each other, the ovum attracts the spermatozoon. The power of attraction which the ovum exerts may be likened, most simply, to the influence of a magnet upon iron-filings. While there has been no opportunity to observe such attraction between the parent cells of human beings, its existence is not open to doubt. And it is practically certain that these cells meet in the oviduct, even in that portion of it which receives the ovum just as it leaves the ovary. Thither a number of the male cells have traveled by their own activity; several come in contact with the ovum and one, but only one, actually enters it. Almost at the moment when they touch, the two cells unite so intimately that all trace of the spermatozoon is lost. Fertilization of the ovum, as this event is scientifically termed, has as its main purpose the uniting of the nucleus of a male germinal cell with the nucleus of the female germinal cell. This detail has been carefully studied; we know that the nuclei quickly blend into one, and that the particles of living matter contributed by the male animate the female cell with a new and wonderful activity.

In our every-day way of speaking, fertilization means conception; it is the instant in which a living being begins its existence. There is no longer the slightest excuse for confusion regarding the period at which the life of the unborn child begins. Before the significance of fertilization was understood, it was perhaps not unreasonable to believe that life began with quickening or about the time the fetal heart-sounds could be heard. But now we must acknowledge that both these ideas were incorrect. The animation of the ovum at the moment of conception marks the beginning of growth and development which constitutes its right to be considered as a human being.

Individuality, hereditary traits, sex - all these, we may be sure - are unalterably determined from the moment of conception. The germinal cell forms the total contribution of the male parent to pregnancy; therefore no other opportunity for him to influence his progeny presents itself, and the substance which enters the ovum at the time of fertilization must be the basis of inheritance from the father. It is equally true, as we shall see in the next chapter, that the nucleus of the ovum and the nucleus alone transmits maternal qualities. The material which conveys inheritable characters can be seen and has been identified in both germinal cells; from each of them the fertilized ovum derives equal amounts. As the parental nuclei unite, the material which they contain intermingles and establishes a new being; to attain full development, it requires nothing further from the father, and nothing save nourishment from the mother.

The First Steps in Development. Although the identity of the spermatozoon is lost at the moment of fertilization, its influence just then begins to be asserted. In the fertilized ovum the dawn of development is shown at first by unusual activity within and later by alterations upon the surface. Before very long the circumference of the cell becomes indented as if a knife had been drawn around it, and shortly two cells appear in place of one. These two cells in turn divide, yielding four cells which grow and divide into eight. In this manner division follows division until a multitude of cells have sprung into existence, all of which cling together in the shape of a ball. Development always proceeds in the same orderly way; evidently it is governed by fixed laws which decree that the mass shall remain for a while in the form of a ball, though the ball, at first solid, soon becomes hollow.

While these changes are taking place the growing ovum is carried down the oviduct a distance of four to six inches and finally comes to rest in the uterus, where it is to dwell during the months necessary to its complete development. The time consumed by this journey cannot be measured accurately; it may be as short as a few hours or as long as several days, but in all probability it is never longer than a week. Although the element of time is uncertain the method of transmission is well understood. Of its own accord the ovum can move after fertilization no better than before; it is never capable of moving itself. The active agent of transportation is the oviduct, which has been fitted for this purpose with millions of short, hair- like structures that project into its interior. These are closely set upon the inner surface of the oviduct; their outer ends are free 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.