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. DEVELOPMENT OF THE EMBRYO. DEVELOPMENT. Development has been studied in vertebrates and the few embryos of the human species obtainable of uncertain ages. The youngest ovum of a primate is of a monkey in the blastodermic stage. This is the ovum of Selenka. The next in time is fortunately human, the celebrated ovum of Peters, about five days old. It is not entirely perfect, but is of the utmost value as showing the early steps in the formation of the amnion and the great development of the mesoderm. Of such value was it that it revolutionized old ideas on that subject, and it shows that the human development differs much from the sequence found in other sources. The following description will be a hypothetical one of the human embryo from analogy with other vertebrates, checked in by the few known human embryos of corresponding ages. It cannot be exactly correct. The subject of embryology might easily be gone into beyond the requirements of obstetrics or the character of this book. Prior to the rupture of the Graafian follicle, the ovum undergoes a ripening or maturing process by the extrusion of two polar globules, the result of a reduction division of the nucleus. This, moving cen- trifugally, the protoplasm at the same time shrinking and leaving a space between it and the zona pellucida, undergoes mitotic division. In the human species the spindle is a perfect diamond, and the chromosomes are arranged regularly around the center with the definite number of eight, and with its axis inclined at right angles to the radius of the cell. As it reaches the surface of the protoplasm, the spindle rights itself and the upper part projects into the subzonal space. The spindle then divides equatorially and the extruded half is the first polar globule. A second is...