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.