Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III THE EMBRYO The Development of Form—The Determination of Sex— Twins—The Rate of Growth—The Newborn Infant—Heredity—Maternal Impressions. 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 k...