The Embryo : Part 1

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

(Page 4 of 18)

The new human being begins existence, as I have shown, as soon as the ovum is fertilized, though at that moment it consists merely of a solitary cell formed by the union of the two parental cells. From a beginning relatively simple the human body develops into the most complex of living structures; and, startling as it may appear to be, it is demonstrably true that every one of the millions of cells which compose an adult has descended from the ovum. Furthermore, the individual himself is not the entire progeny of the ovum; the placenta and the membranes dealt with in the preceding chapter, we saw, were also derived from that same source. They possess only a transitory importance, to be sure, and to most persons they are less interesting than the embryo, yet we gave them consideration before discussing its growth because the manner in which the ovum becomes attached to the womb and draws nutriment from the mother primarily determines the fate of a pregnancy.

Now that we have become familiar with the arrangements for the protection of the embryo, we are prepared to learn how it develops, and may accept the phrase, embryonic development, to cover the whole period of existence within the womb. In a more technical sense, however, the use of the term embryo is limited to the first six weeks of pregnancy and designates the condition of the young creature before it has acquired the form and the organs of the infant; after that time the unborn child is called a fetus. Embryonic development, therefore, in the strictest sense of the term, chiefly involves the shifting of various groups of cells and the bestowal upon them of different kinds of activity. During this period comparatively slight growth takes place. By about the twentieth week, the house, it may be said, is set in order; and there follows a period marked by the rapid growth of the fetus.

The Development of Form. A very old explanation of embryonic development was that the process consisted altogether in growth. According to that view the embryo lay curled up in the egg; at the outset it was equipped with organs, limbs, features, and all the other bodily structures found in an adult. In order that the ovum might be transformed into a mature infant, only unfolding and growth were required. After the microscope came into use, however, so simple an explanation could no longer be accepted. Scientists soon realized that the embryo did not exist "ready made" in the ovum, which, even when magnified, failed to bear the faintest likeness to a human being.

Although the microscope made impossible this very simple explanation, it gave in return a truer, if more complex, account of the transformation from egg to offspring. By this means it has been definitely proved that the ovum multiplies rapidly after it has been fertilized, and becomes, as was explained in the preceding chapter, a sac-like structure within which hangs a tiny clump of tissue. This inner mass of cells forms the embryo.

It has proved a difficult task to secure very young human embryos, and many of the ideas we hold relative to the initial stages in the development of man are based upon what has been found true in certain mammals, the class of animals to which we belong. The youngest human ovum known at present has already undergone about two weeks' development, and there the embryo is represented by a flat disk. From this stage to the stage of complete development a satisfactory series of embryos has now been collected, but it is impossible to give here, even in outline, a description of the evolution of the human embryo. No one can understand this intricate subject without the aid of diagrams, models, and other material beyond the reach of all save laboratory workers.

By the end of the second month the development of the embryo has advanced so far that anyone could recognize its human shape. About that time, too, the external sexual organs make their appearance. At first these are quite similar in both sexes; and, if they are used as the criterion, it is possible only toward the end of the third month to say whether the embryo is a male or female.

The Determination of Sex. The fact that a number of months pass before the sex can be distinguished by an external examination of the fetus has led to the erroneous belief that it can be influenced during the early part of pregnancy or actually determined at will. Various means to accomplish this have been suggested; many of them depend upon modifying the mother's mode of living according as a boy or girl is desired. The most widely known of these doctrines, that of Schenck, was to the effect that the sex of the offspring is always that of the weaker parent. He suggested, therefore, that increasing the vigor of the mother by an appropriate diet would produce a male child, whereas a decrease in her strength would lead to the opposite result. His views, however, were incorrect. After studying extensive statistics Newcomb came to the conclusion that "it is in the highest degree unlikely that there is any way by which a parent can affect the sex of his or her offspring."

Moreover, the results of experimental research clearly indicate that we shall never possess the means by which a mother may control the sex of her child. In the main laboratory investigations have sought to answer two questions. First, at what time is the sex of the offspring determined? and, second, what accounts for the origin of a male in one instance and of a female in another? The study of these problems has been carried on chiefly in connection with insects, worms, and fowl; but as yet insurmountable difficulties have prevented similar investigations in higher animals. For this reason, it is not without the greatest caution that results thus far obtained may be assumed to apply to man.

Sufficient facts, however, have been collected to admit no doubt regarding the answer to the first question. In most animals it is definitely known that the sex of the offspring has been fixed when the male cell enters the female cell, in other words, at the instant the ovum is fertilized. Excellent reasons exist for believing that human beings conform to this rule, and that the sex of the child is unalterably determined at the moment conception occurs. Consequently, any attempt to influence it after that event must prove futile.